New Pd-based electrocatalysts for the electrolytic

production of hydrogen

José Artur Serrano Barbeiro Cardoso

Thesis to obtain the Master of Science Degree in

Bioengineering and Nanosystems

Supervisors: Prof. César Augusto Correia de Sequeira,

Dr. Diogo Miguel Franco dos Santos

Examination Committee

Chairperson: Prof. Luís Joaquim Pina da Fonseca

Supervisor: Prof. César Augusto Correia de Sequeira

Members of the Committee: Prof. João Carlos Salvador Santos Fernandes

October 2015

ii

iii

Acknowledgments

First of all I want to thank my supervisors, Prof. César Sequeira and Dr. Diogo Santos, for

guiding me through this final step in obtaining my Master’s degree. Their extraordinary willingness to

help, even when swamped with other work, as was most of the time, made this work easier every day.

I also want to thank Dr. Luís Amaral, David Cardoso and Marta Martins who, even though it was not

their job, were always ready to help with anything I needed.

A special thank you has to go to Joana Lourenço, Raisa Paes and Eurico Moutinho, my fellow

thesis colleagues, with whom I shared all the laughs and the occasional despair at the laboratory and

made going to work a pleasure.

I also want to thank my parents, not only for their incessant support while I was pursuing my

degree, but also for instilling in me the love of science and curiosity that brought me to this point.

Last but not least, a big thank you to all the friends that I made in college who accompanied me

through this incredible journey.

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v

Resumo

O presente paradigma económico é extremamente dependente do acesso à energia e da

distribuição de combustíveis. Combustíveis fósseis, como o carvão e o petróleo, são os mais

comummente utilizados e estão presentes em todo o tipo de aplicações. Estes combustíveis não são

renováveis e estão a ser lentamente gastos de forma irreversível. Junto com os seus efeitos negativos

no meio ambiente, estes problemas resultam numa busca por combustíveis alternativos. Hidrogénio

tem um grande potencial como uma alternativa a estes combustíveis comuns.

De forma a implementar uma economia baseada no hidrogénio, todos os passos envolvidos

neste sistema têm de ser estudados e melhorados, desde a produção à infraestrutura. O foco deste

trabalho foi a produção do hidrogénio utilizando a eletrólise da água. Deste método resulta hidrogénio

com um alto grau de pureza, no entanto este é dispendioso pois necessita de grandes quantidades de

energia. Para minimizar este obstáculo utilizam-se electrocatalisadores que reduzem a energia que é

necessário despender para que a reacção de descarga do hidrogénio ocorra.

Foram testados dez electrocatalisadores baseados em paládio (Pd). Três PdM/rGO (M = Au,

Fe e FeAg), PdNi suportado em compósitos de KB600, KB300 e grafeno com SnO2 e em Vulcan XC72,

e, finalmente, Pd suportado em MEGSAK, MEVSAK e Vulcan XC72.

Os resultados mais promissores foram obtidos com PdAu/rGO, PdNi/grafeno e Pd/MEVSAK,

com valores de j0 máximos de 0,37, 1,52 e 5,02 mA cm-2, respectivamente. O último destes

electrocatalisadores utiliza um novo material de suporte de origem biológica que justifica estudos mais

completos.

Palavras-chave: Reação de descarga do hidrogénio, Métodos eletroquímicos, Electrocatalisadores

baseados em Paládio, Suportes em carbono

vi

vii

Abstract

The current economic paradigm is extremely dependent on the access to energy and the

distribution of fuel. Fossil fuels are the most commonly used and are ever present in most applications.

These fuels are non-renewable and, as such, are slowly being depleted. Together with their negative

impact on the environment, these problems have resulted in the search for alternative fuels. Hydrogen

as a fuel shows a great potential as an alternative to fossil fuels.

In order to implement a so-called hydrogen economy, all the steps of such a system have to be

studied and improved, from production to distribution infrastructure. The focus here was on the

production step by using water electrolysis. This technique results in high purity, clean hydrogen, but a

drawback is its cost due to large energy requirements. In order to minimize this obstacle electrocatalysts

can be used to lower the potential necessary for the hydrogen evolution reaction to take place.

Ten different palladium (Pd) based materials were tested as electrocatalysts. Three PdM/rGO

(with M = Au, Fe and FeAg), PdNi supported on composites of KB600, KB300 and graphene with SnO2

and Vulcan XC72, and finally Pd supported on MEGSAK, MEVSAK and Vulcan XC72.

Of all these, the most promising results were obtained with PdAu/rGO, PdNi/graphene and

Pd/MEVSAK, with values of j0 as high as 0.37, 1.52 and 5.02 mA cm-2, respectively. The latter, using a

new biobased support material, seems to be a good candidate for further studies.

Keywords: Hydrogen evolution reaction, Electrochemical methods, Pd-based electrocatalysts, Carbon

supports

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Contents

Acknowledgments ................................................................................................................................... iii

Resumo .................................................................................................................................................... v

Abstract................................................................................................................................................... vii

Contents .................................................................................................................................................. ix

List of Figures .......................................................................................................................................... xi

List of Tables ........................................................................................................................................... xi

1. Introduction - Hydrogen Energy .......................................................................................................1

1.1. Motivation - Hydrogen as a fuel .....................................................................................................1

1.2. Hydrogen production .....................................................................................................................2

1.3. Hydrogen storage and transportation ............................................................................................3

1.3.1. Physical hydrogen storage .....................................................................................................3

1.3.2. Chemical hydrogen storage ...................................................................................................4

1.4. Hydrogen applications ...................................................................................................................4

1.4.1. Ground, air and naval vehicles ...............................................................................................4

1.4.2. Hydrogen for domestic, commercial and industrial purposes ................................................6

1.4.3. Hydrogen for electrochemical power generation....................................................................6

1.5. Hydrogen safety and economy ......................................................................................................7

2. Electrolytic Hydrogen Production ....................................................................................................9

2.1. Electrolytic decomposition of water ...............................................................................................9

2.2. Present uses of electrolytic hydrogen ........................................................................................ 10

2.3. Advanced electrolysis cell modules ............................................................................................ 11

2.4. Commercial water electrolyser manufacturers .......................................................................... 12

3. Electrocatalysts for HER................................................................................................................. 13

3.1. What is an electrocatalyst? ......................................................................................................... 13

3.2. Electrocatalysts and energy conversion ..................................................................................... 13

3.3. Thermodynamic considerations for electrocatalysts .................................................................. 14

3.3.1. Equilibrium potentials .......................................................................................................... 14

3.4. Reaction kinetics involving H2O - H2 – O2 ................................................................................... 15

3.4.1. H2 evolution reaction mechanism ........................................................................................ 15

3.4.2. Tafel plots for various HER catalysts .................................................................................. 16

3.4.3. Volcano-type plots ............................................................................................................... 18

3.5. Current state of catalyst development ........................................................................................ 18

3.5.1. Platinum-based catalysts for the HER ................................................................................ 18

x

3.5.2. Nanostructured electrocatalysts .......................................................................................... 19

4. Experimental methods .................................................................................................................... 21

4.1. Materials and chemicals ............................................................................................................. 21

4.2. Experimental setup ..................................................................................................................... 21

4.3. Characterisation of the electrocatalysts ..................................................................................... 21

4.4. Electrochemical techniques ........................................................................................................ 23

4.4.1. Rotating disc electrode ........................................................................................................ 23

4.4.2. Linear sweep and cyclic voltammetry ................................................................................. 23

4.4.3. Chronoamperometry tests ................................................................................................... 25

5. Results and discussion .................................................................................................................. 27

5.1. PdM alloys supported on rGO .................................................................................................... 27

5.1.1. Characterisation of the electrocatalysts .............................................................................. 27

5.1.2. HER studies......................................................................................................................... 28

5.2. PdNi alloys supported on composite supports ........................................................................... 32

5.2.1. Characterisation of the electrocatalysts .............................................................................. 32

5.2.2. HER studies......................................................................................................................... 33

5.2.3. Activation energy ................................................................................................................. 37

5.3. Pd supported on MEGSAK, MEVSAK and Vulcan xc72 ............................................................ 41

5.3.1. Characterisation of the electrocatalysts .............................................................................. 42

5.3.2. HER studies......................................................................................................................... 42

5.3.3. Activation energy ................................................................................................................. 45

6. Conclusions ..................................................................................................................................... 49

References ............................................................................................................................................ 51

xi

List of Figures

Fig. 1 – Hydrogen production sources in th US in 2010 (source: University of Iowa) .............................2

Fig. 2 – Simple water electrolyser ............................................................................................................9

Fig. 3 – Tafel plot for the MoP nanosheet ............................................................................................. 16

Fig. 4 – Tafel plot for Cobalt-Boride electrode ...................................................................................... 17

Fig. 5 – Tafel plot for the microspheres, d – MoS2 ............................................................................... 17

Fig. 6 – Tafel plot for NG-Mo and Dry N-Mo ......................................................................................... 17

Fig. 7 – Values of log j0 in HER for different metals in acidic media ..................................................... 18

Fig. 8 – STEM bright-field image of the structure and schematic illustration of the structure .............. 19

Fig. 9 – Energy dispersive spectrum of the nanosheet structure.......................................................... 20

Fig. 10 – Schematic representation of a rotating disk electrode ........................................................... 23

Fig. 11 – Nernst diffusion layer model .................................................................................................. 24

Fig. 12 – Example of a linear sweep voltammogram for HER on carbon and WO3/C electrode in a 1 M

KOH solution at a sweep rate of 5 mV s-1 ............................................................................................ 24

Fig. 13 – Example of a cyclic voltammogram for a Cu/Ni electrode in a 1 M KOH solution at a sweep

rate of 100 mV s-1 ................................................................................................................................. 24

Fig. 14 – CV obtained with PdAu/rGO electrode, at 25 mV s-1, in 8 M KOH electrolyte at 25 ºC, showing

the oxygen desorption peak ................................................................................................................. 28

Fig. 15 – Polarisation curves at (a) 25 and (b) 35 ºC for the PdM/rGO electrocatalysts. Measurements

made in 8 M KOH solution at a scan rate of 0.5 mV s-1 .................................................................................................................. 29

Fig. 16 – Tafel plots at (A) 25 and (B) 35 ºC for the PdM/rGO electrocatalysts .................................. 30

Fig. 17 – CV obtained with Pd/MEVSAK electrode, at 5 mV s-1, in 8 M KOH electrolyte at 25 ºC, showing

the absence of the oxygen desorption peak ......................................................................................... 33

Fig. 18 – XRD measurements pertaining to the (a) PdNi/Graphene, (b) PdNi/KB600 and (c) PdNi/KB300

electrodes ............................................................................................................................................. 33

Fig. 19 – Polarisation curves taken at a scan rate of 1 mV s-1 in 8 M KOH for the (a) PdNi/KB600, (b)

PdNi/KB300, (c) PdNi/Vulcan, and (d) PdNi/Graphene electrocatalysts ............................................. 34

Fig. 20 – Tafel plots for the (a) PdNi/KB600, (b) PdNi/KB300, (c) PdNi/Vulcan, and (d) PdNi/Graphene

electrocatalysts ..................................................................................................................................... 35

Fig. 21 – Arrhenius plots for the PdNi/composite electrocatalysts ....................................................... 37

Fig. 22 – CA measurements for PdNi/KB600 electrocatalyst, in an 8 M KOH electrolyte at temperatures

ranging from 25 to 85 ºC, for an applied potential of (a) -1.3 V, (b) -1.4 V and (c) -1.5 V..................... 38

Fig. 23 – CA measurements for PdNi/KB300 electrocatalyst, in an 8 M KOH electrolyte at temperatures

ranging from 25 to 85 ºC, for an applied potential of (a) -1.3 V, (b) -1.4 V and (c) -1.5 V .................... 39

Fig. 24 – CA measurements for PdNi/Vulcan electrocatalyst, in an 8 M KOH electrolyte at temperatures

ranging from 25 to 85 ºC, for an applied potential of (a) -1.3 V, (b) -1.4 V and (c) -1.5 V .................... 39

Fig. 25 – CA measurements for PdNi/Graphene electrocatalyst, in an 8 M KOH electrolyte at

temperatures ranging from 25 to 85 ºC, for an applied potential of (a) -1.4 V and (b) -1.5 V .............. 40

xii

Fig. 26 – Arrhenius plots obtained with applied potentials of -1.3, -1.4 and -1.5 V for the (a) PdNi/KB600,

(b) PdNi/KB300, (c) PdNi/Vulcan and (d) PdNi/Graphene electrocatalysts ......................................... 41

Fig. 27 – XRD measurements pertaining to the (a) Pd/Vulcan, (b) Pd/MEGSAK and (c) Pd/MEVSAK

electrodes ............................................................................................................................................. 42

Fig. 28 – Polarisation curves taken at a scan rate of 1 mV s-1 in 8 M KOH for the (a) Pd/MEGSAK, (b)

Pd/MEVSAK and (c) Pd/Vulcan electrocatalysts ................................................................................. 43

Fig. 29 – Tafel plots for the (a) Pd/MEGSAK, (b) Pd/MEVSAK and (c) Pd/Vulcan electrocatalysts ... 44

Fig. 30 – Arrhenius plots for the Pd/support electrocatalysts .............................................................. 45

Fig. 31 – CA measurements for Pd/MEGSAK electrocatalyst in 8 M KOH electrolyte at temperatures

ranging from 25 to 85 ºC for an applied potential of (a) -1.3 V, (b) -1.4 V and (c) -1.5 V ..................... 46

Fig. 32 – CA measurements for Pd/MEVSAK electrocatalyst in 8 M KOH electrolyte at temperatures

ranging from 25 to 85 ºC for an applied potential of (a) -1.3 V, (b) -1.4 V and (c) -1.5 V ..................... 47

Fig. 33 – CA measurements for Pd/Vulcan electrocatalyst in 8 M KOH electrolyte at temperatures

ranging from 25 to 85 ºC for an applied potential of (a) -1.3 V, (b) -1.4 V and (c) -1.5 V ..................... 47

Fig. 34 – Arrhenius plots obtained with applied potentials of -1.3, -1.4 and -1.5 V for the (a) Pd/MEGSAK,

(b) Pd/MEVSAK and (c) Pd/Vulcan electrocatalysts ............................................................................ 48

xiii

List of Tables

Table 1 – Electrolyser suppliers by 2014, PEM – Proton exchange membrane, AEM – Anion exchange

membrane.............................................................................................................................................. 12

Table 2 – Real surface area of the Pd-based electrodes .................................................................... 28

Table 3 – Kinetic parameters for the three tested PdM/rGO electrocatalysts ...................................... 30

Table 4 – Kinetic parameters for the four tested PdNi/composite electrocatalysts .............................. 36

Table 5 – Ea values for the PdNi/composite electrocatalysts for each applied potential ...................... 40

Table 6 – Kinetic parameters for the three tested Pd/C electrocatalysts ............................................. 44

Table 7 – Ea values for the Pd/C electrocatalysts for each applied potential ...................................... 48

xiv

1

1. Introduction – Hydrogen Energy

1.1. Motivation - Hydrogen as a fuel

The world’s economy, just like our own way of life, is completely reliant on the access to energy.

Since the Industrial Revolution the consumption of energy in the form of various types of fuel, like coal,

oil and natural gas, has become a part of our daily lives, resulting in a consumption of 3020, 4185, 3827

million tonnes, in 2013 only, of coal, oil and natural gas, respectively (of oil equivalent for coal and

natural gas), and this number is constantly growing every year [1]. One tonne of oil equivalent

corresponds to the energy produced by burning one tonne of crude oil.

These are all fossil, non-renewable fuels, which means that their reserves will eventually be

depleted as these fuels take millions of years to be produced and our extraction and consumption of

energy is at such a large scale that the natural process of production is not in any way capable of keeping

up [2]. Another issue that is extremely important to address is the pollution resulting from the

consumption of fossil fuels. Reports from all around the world show that this pollution has a huge

negative impact on health and quality of life for the populations that live in the areas with the most

emission density [3], resulting in very high external costs related to healthcare that are not usually taken

into account when determining the cost of using these fuels [4, 5]. The health problems resulting from

breathing polluted air are not the only problems resulting from the emission of CO2, SO2 and NOx though.

CO2 is also a greenhouse gas and the main result from the consumption of fossil fuels. This means that

the global temperature of our planet is very likely to rise as a direct result of the increasing concentration

of this gas in the atmosphere. More clearly, this phenomenon has happened in the past century with an

increase of 0.74 ºC and it is predicted that it will continue to increase alarmingly if we do not find cleaner

alternatives for our energy sources [6].

A very promising alternative is using hydrogen as a fuel, preferably produced using clean,

renewable energy sources like wind power, solar energy or hydroelectricity [7]. First of all the only waste

produced by the consumption of hydrogen with oxygen is water, thus avoiding any emission of carbon

dioxide and other pollutants. Other significant advantages of using hydrogen as a fuel are the fact that

it is the lightest and most abundant element in the universe and is also an extremely efficient fuel [8].

Also, while the versatility of fossil fuels is very limited, as the only way it can be used by the consumer

is by flame combustion, which is not even the most efficient method of energy conversion, hydrogen

can be consumed in a variety of ways other than flame combustion, like direct steam production,

chemical conversion and using fuel cells [9], the latter having a much higher efficiency than combustion

of fossil fuels, 65% against 25% [8]. The reason why it is not already used at a large scale as the main

fuel for our needs is that its production is very costly when compared to fossil fuels like oil and natural

gas [7], obviously not taking into account the costs of repairing or minimising the negative effects of

these fuels on the atmosphere [9]. All in all hydrogen may be a much better fuel than any of the currently

used fossil fuels.

2

In this study the advantages of hydrogen as a fuel will be explored, always in comparison with

the current energy system, and the necessary infrastructures necessary to leave the fossil fuel age

behind and enter a hydrogen economy will also be briefly covered. The focus of this work though is

hydrogen production by water electrolysis, the fundamentals of this method will be explained in detail,

while also covering the current state of the art of this technology. The experimental part of this work will

consist in testing new electrocatalysts in the hope of improving the efficiency of the whole process.

1.2. Hydrogen Production

There are numerous methods of hydrogen production, the most widely used are intimately linked

to fossil fuels. As can be seen in Fig.1, 96% of the worldwide hydrogen production in 2012 was derived

from steam reforming of natural gas with only 4% resulting from water electrolysis [10], the method on

which this work is focused. Since the idea behind a hydrogen economy is to move away from using

fossil fuels, the clean production techniques have to take hold of the majority of the market.

Figure 1 - Hydrogen production sources in the US in 2010 [11].

The most common raw materials for hydrogen production are hydrocarbons, water and biomass,

and the methods can be classified in three categories: thermochemical, electrochemical and biological

[12]. Thermochemical hydrogen is obtained by its separation from the raw material as a result of a

thermal energy input. Electrochemical hydrogen is a result of the passage of an electric current with the

objective of forcing chemical reactions that will lead to the separation of hydrogen from the feedstock,

usually water. Finally biological methods are based on the use of microorganisms to produce hydrogen.

Extensive studies have been made on the cost of hydrogen production by using the various

methods and different raw materials. Bartels et al. [13] have compiled these results and it can be seen

that the cost of producing hydrogen from coal and natural gas are the cheapest techniques, actually

being cheaper than producing gasoline. This is something expectable since fossil fuels have the most

advanced technologies in energy production. The costs of using alternative, cleaner methods are much

48%

30%

18%

4%

Natural Gas Oil Coal Electrolysis

3

more expensive, with water electrolysis being the most expensive at around 4.4-7.4 $ kg-1 (2007 US

Dollars) [13]. With the development of new technologies and the improvement of existing ones for the

production of hydrogen from alternative sources these costs are expected to decrease shortly.

Water electrolysis shows the most purity in the hydrogen obtained (>99%) [7] with the

disadvantage of being the most expensive method. That being said this is still a very promising method,

especially at small scale [14]. For instance in remote areas of the world where, even though there is

some degree of isolation from the electricity grid, there is an abundance of renewable energy sources,

the excess energy obtained from these sources can be stored in the form of extremely pure hydrogen,

the example of a remote Alaskan village has been given by Isherwood et al. [15].

The greatest challenge with this technique is the reduction of the activation energy necessary

for the reactions responsible for the separation of hydrogen and oxygen from water to take place. One

of the best ways to do this is to find electrode materials that are efficient, durable and that are relatively

inexpensive [16].

1.3. Hydrogen storage and transportation

1.3.1. Physical hydrogen storage

A major challenge for a hydrogen based economy is the storage and transportation of the fuel.

In order for the use of hydrogen as a fuel to become widespread and replace fossil fuels, it is crucial to

have extremely efficient storage and transportation systems [17]. This means that the objective is to be

able to pack the largest quantity of hydrogen in the smallest volume possible, but this is not an easy

task since 1 kg of hydrogen in ambient conditions of temperature and atmospheric pressure occupies

11 m3 [18]. The two possible methods for physical storage of hydrogen are compressed gas and by

liquefying it [19].

The most common method for storage of hydrogen is by compressing it, which can be done by

using high pressure gas cylinders [18]. The state of the art for this technology, as reported by the U.S.

Department of Energy, is the 350-bar compressed tank system which shows a gravimetric capacity of

5.5 wt%, a volumetric capacity of 17.6 g-H2 L-1 and an efficiency of 56.5 %. The drawback of these tanks

is the cost of $18.7 kWh-1, which represents around 5 times more than the target cost for it to become

a valid alternative to gasoline [20].

Liquid hydrogen is stored in cryogenic tanks at 21.2 K at ambient pressure [18], since the critical

temperature of hydrogen is 33 K. This means that there will be relevant losses due to boil-off since it

needs a very low temperature to be maintained in liquid form. Another negative factor is the enormous

amount of energy necessary for the liquefaction process. This limits the possible uses of hydrogen in

this form, as it will only be viable in cases where the cost is not an issue and the gas is consumed in a

short time, for instance for air and space craft [18].

.

4

1.3.2. Chemical hydrogen storage

Hydrogen storage in the solid state is done by hydriding a metal or alloy, where the absorbed

hydrogen forms MHx, and where by heating, the MHx is dissociated and hydrogen is liberated from the

solid phase. By cooling, the metal or alloy is capable of reabsorbing the gas [17]. This technology has

shown higher volumetric capacity than the rest. That being said, it also brings a significant challenge

related with the high mass of the system, as the gravimetric capacity of most suitable materials has

been found to be very low. In order for this method to be used in mobile applications this has first to be

addressed [21]. At this stage the only viable applications for this are stationary ones, like industry or

storage complexes

Magnesium has a very high gravimetric storage capacity of 7.6 wt %, which represents the

amount of hydrogen per unit of mass of the material, making it a very promising material for hydrogen

storage [22]. That being said, its widespread commercial use is very limited since it also has a highly

stable hydride state and its reaction kinetics are considerably slow, resulting in inefficient absorption and

desorption rates [23].

An approach to solving this issue has been to try to reduce the thickness of the Mg films used

for hydrogen storage in order to reduce its stability. Reports show that, by synthesizing films with a

thickness on the nano scale, superior hydrogen absorption properties can be achieved. For instance Qu

et al. [24] reported that using an Mg film with a thickness of 20 nm, a saturated hydrogen content of 5.5

wt % at 298 K can be achieved. These results show a significant improvement over the desorption

temperatures (in excess of 573 K) found for thicker films (>1 μm) [25].

Another approach that seems to improve the absorption kinetics is by using other materials to,

together with Mg, produce thin film alloys. Several reports have been made using double and triple layer

films. These exhibit superior hydrogen absorption and desorption rates [26], even achieving desorption

and absorption of hydrogen with around 5 wt % in a matter of minutes and seconds, respectively, after

a number of cycles, showing high stability in terms of maintaining these favorable kinetics [27].

A final advantage of Mg thin films for hydrogen storage is its lightweight when compared with all

the bulk storage methods described above, allowing for a much more efficient storage when it comes to

sheer quantity [28]. Also magnesium and the techniques necessary for the fabrication of the thin films

are not very costly.

1.4. Hydrogen applications

1.4.1. Ground, air and naval vehicles

The success of a hydrogen based economy lies in the acceptance of this energy carrier by the

vehicle industry. For that to happen there has to be a consumer demand for vehicles that use this

technology. Greene et al. identified the three main aspects responsible for the future market potential of

hydrogen vehicles, the progress of technology, public policy and market behaviour [29].

5

The progress of technology can be translated into the cost of hydrogen fuel when compared

with the cost of gasoline and other fossil fuels. This cost has to take into account all aspects of using

each fuel, including storage, infrastructure costs and the cost of the actual vehicle. A systematic analysis

of the issues regarding the utilization of hydrogen as a fuel to power ground vehicles was made by

Zhang & Hu [30]. The most important advantage of this fuel, besides all the environmental and health

benefits it brings, is its efficiency. Since the electrode reaction on a fuel cell does not involve a

combustion process and heat engine working, the energy efficiency is not affected by the Carnot cycle,

being able to reach a maximum of 70%. The drawbacks are the fact that its storage will either occupy

much more space (gaseous and liquid storage) or weight a lot more (solid state). These issues might

be solved in the future by using thin films to store hydrogen, as was mentioned in the previous chapter.

A final issue with the usage of hydrogen as a fuel is its safety, this aspect will be investigated in section

1.5.

All over the world studies are being carried out in order to ascertain the viability of using

hydrogen fuel in the transportation systems. For instance in Algeria it is understood that the economy is

dependent on having a strong transport sector, as such alternative fuels, hydrogen in this case, are

being studied [31]. This fuel presents many opportunities since it permits exploitation of the huge solar

energy availability in this country and it also allows the development of more isolated areas, stopping

desertification. This study shows that hydrogen fuel has a very high potential of being competitive with

the current fossil fuels in the future. Another very interesting study is being carried out globally by testing

fuel cell electric buses in a variety of countries. This study shows that the efficiency in miles per diesel

equivalent is much higher for hydrogen fuel than for the conventional fuels, and they have also been

proven to operate with zero local emissions and reduced noise [32]. The main barriers identified for a

widespread usage of these buses were the high cost of the vehicles, the lack of a refueling infrastructure

and the price of hydrogen fuel.

It is not only for ground vehicles that hydrogen fuel can be used as an alternative. In aviation,

studies have also been carried out showing that using this new fuel results in less energy consumption,

both in long range flights and in short range flights, for a variety of aircraft vehicles [33]. Another report

shows that the performance of liquid hydrogen is slightly better than kerosene as a fuel for aircraft, with

extremely lower emissions, resulting in a highly reduced negative environmental impact [34]. Once more

the problems identified with the change to hydrogen fuel are the lack of infrastructure and the high cost

of this fuel.

For the case of naval applications, the studies on the viability of hydrogen as a fuel are not

progressing as much as with the other means of transport and the scope of these studies is more limited.

Austrian companies Fronius International, Bitter GmbH and Frauscher have developed a new electric

boat powered by hydrogen fuel cells, with the aim of competing with the conventional battery-powered

boats [35]. It was reported that this boat has a range of 80 km running at a 4kW output, which is twice

that of the conventional boats. A great advantage is the fact that it does not require charging time, since

it uses hydrogen fuel, the only thing that needs to be done is change the fuel cartridge. Another example

of a hydrogen fuel boat is the one developed by the Istanbul Technical University which operates in the

Golden Horn estuary with the purpose of serving as a shuttle between the various wharfs [36]. In this

6

case the boat is powered by an 8 kW fuel cell with storage for 20 kg of hydrogen and has an autonomy

of 10 hours for 5 kg of hydrogen at a speed of 13 km h-1. It should be noted that, in both of these cases,

the refueling stations are producing hydrogen by water electrolysis, with the Hydrogenics Turkish station

being able to produce up to 65 kg of hydrogen fuel per day.

1.4.2. Hydrogen for domestic, commercial and industrial purposes

In order to change the paradigm of the current energy system, hydrogen as a fuel has to be

used in every sector. For domestic and commercial purposes it can be used to provide heating and

electricity. These are extremely important applications where the energy sources need to be substituted

as they represent 40% of the final energy use in the world [37].

An interesting study was carried out in the United Kingdom in order to access the viability of this

type of fuel in covering the energy needs of houses, as well as investigating the capacity for

decarbonisation since the UK government established the goal of reducing CO2 emissions by 80% until

2050 [38]. The study contemplated two different scenarios, the first with hydrogen produced by using a

natural gas boiler to support the fuel cell system, and the second with hydrogen produced by renewable

and clean energies and using a heat pump as support. The authors reported that the first scenario is

not a considerable change since only 938 kg of CO2 were saved [38]. The second scenario not only

shows that hydrogen powered fuel cells can indeed support the electricity and heating demands of a

house using a heat pump, but also shows that the electricity produced on site is higher, increasing the

amount of electricity generated by 128%. The total consumption of CO2 is much lower since a part of

the hydrogen is produced locally and the rest of the necessary hydrogen is obtained by external clean

sources.

Together with other reports on this subject [39], it can be concluded that with the current state

of technology it is viable to use hydrogen as a fuel for domestic and commercial purposes, reducing

CO2 in the process. The drawback of this change is, as always, the cost of changing the infrastructure.

To add to the use of hydrogen as a fuel in industry, there are a variety of other uses for hydrogen.

For instance it is widely used as a reactant in the chemical industry in hydrogenation processes. Other

uses include O2 scavenging to prevent oxidation and corrosion, and as a coolant in electrical generators

[40].

1.4.3. Hydrogen for electrochemical power generation

As mentioned before, hydrogen fuel can be consumed in a variety of ways in order to produce

electrical energy. By flame combustion it can produce steam, which can then be used for the production

of electrical energy. Other methods consist on chemical conversion or by using fuel cells, the latter being

an extremely efficient and clean method.

A fuel cell is composed of an anode and a cathode separated by a solid or liquid electrolyte. By

supplying the anode with hydrogen and the cathode with air, the electrochemical reactions (1) and (2)

7

take place leading to the overall reaction (3) [41]. The ions involved in these reactions are constantly

flowing through the electrolyte. This flow of ionic charge has to be balanced by the flow of electronic

charge along the external circuit that connects the two electrodes. This flow of electronic charge, which

is represented in the electrode process by the electrons, results from the conversion of the chemical

energy into electrical energy that occurs at the electrodes during the cell operation.

𝐶𝑎𝑡ℎ𝑜𝑑𝑒: 𝐻2 + 2𝑂𝐻− → 2𝐻2𝑂 + 2𝑒− (1)

𝐴𝑛𝑜𝑑𝑒: 1

2𝑂2 + 𝐻2𝑂 + 2𝑒− → 2𝑂𝐻− (2)

𝑂𝑣𝑒𝑟𝑎𝑙𝑙: 𝐻2 +1

2𝑂2 → 𝐻2𝑂 + 𝐸𝑛𝑒𝑟𝑔𝑦 (3)

Hydrogen fuel cells are not limited by the Carnot cycle, as is the case with combustion engines.

As such, much higher efficiencies can be obtained, reaching up to 65% compared to the 25% of the

current car engines. This number can be increased even further to 85% if the heat produced by the fuel

cell can be used to harvest more energy [42].

Fuel cell technology is clearly a very interesting alternative to the lithium-ion batteries which are

used in electrical vehicles, since the latter have a very high cost and other issues, while fuel cells have

a high energy density and the refueling time is much lower. This technology is also becoming

increasingly cheaper as there is a lot of research and development being made in this area. For instance,

fuel cells using cheap materials (as opposed to platinum) like zinc are already showing promising results

[43].

1.5. Hydrogen safety and economy

Hydrogen as a fuel is clearly an alternative to non-renewable fossil fuels currently being used.

It is renewable, extremely efficient, versatile, and its consumption has near zero emissions of

greenhouse gases. The only waste that results from its consumption is water.

The two major issues to implementing a hydrogen based economy and leaving the standard

fuels behind are the cost of changing the whole energy infrastructure, and the safety of its widespread

use.

The dangers in case of accident when using hydrogen include physiological hazards like

frostbite and suffocation, physical damage to the systems responsible for its correct use by

embrittlement (loss of ductility) and component failure, and there is always the risk of burning or

explosion, as hydrogen together with air produces a flammable and explosive mixture [44].

The main challenges for the safe utilization of hydrogen fuel that have to be addressed before

its widespread use can be summarised as its ease of leaking, low ignition energy, the variety of

8

combustible gases which can be obtained by mixing hydrogen with air, buoyancy, and the embrittlement

of metals [44].

In order to reduce the cost of replacing fossil fuels with hydrogen it is necessary to establish a

system of economic incentives, so that the building of the necessary infrastructure and the development

of this market can take place. Lowering the price at the end user requires improvements at every step

of the whole energy system: production, storage, transportation and processing [45]. The greatest

challenge here is in the storage and transportation sectors, which are both closely interconnected, as

reported before in section 1.3.

9

2. Electrolytic Hydrogen Production

2.1. Electrolytic decomposition of water

The production of hydrogen and oxygen can be made by separating the two species during the

water (H2O) splitting. This method produces the purest form of these gases since no other elements are

present in the reactions involved [7]. The splitting of water can be described in a simple approach by the

following reaction [46]:

𝐻2𝑂 + 𝐸𝑛𝑒𝑟𝑔𝑦 → 𝐻2 +1

2𝑂2 (4)

The energy required for the H2 production can have any origin. This is why hydrogen is

considered an energy carrier, it is only a method of storing energy so that it can be transported and used

elsewhere. Ideally the source of this energy should be renewable, for instance solar energy, so that a

zero emission complete energy system can be obtained.

The setup used for water electrolysis is very similar to a fuel cell. As is illustrated in Fig.2, there

are two electrodes, an anode and a cathode, submerged in an electrolyte, which are connected by an

external electrical circuit. Usually a membrane is also used to separate the two electrodes so that there

is no mixture between hydrogen and oxygen gases [47], which would reduce the purity of the produced

hydrogen and could also lead to the risk of explosion [44].

Figure 2 - Simple water electrolyser.

The energy applied to the system takes the form of a direct current (DC Power). As a result of

the electronic charge gradient established between the electrodes, consequence of the DC power

applied to the unit, the two ions OH- and H+ migrate to the electrode/electrolyte interfaces, where they

10

discharge, leading to O2 and H2 gases produced at the anode and cathode, respectively. The hydrogen

gas resultant of this process (H2) [7] can then be stored and used later.

This process can be made more or less efficient by varying the electrolyte or the material and

structure of the electrodes, by changing the material of the membrane, by varying the distance between

the two electrodes, by operating under dynamic conditions instead of static ones, and also by changing

the geometric design of the electrodes. The focus of the experimental part of this work is on testing

different electrodes/electrocatalysts for this process. As such, the Hydrogen Evolution Reaction (HER)

mechanism are explored with a higher level of detail in sections 3.4 and 3.5.

2.2. Present uses of electrolytic hydrogen

As mentioned above, only 4% of the total hydrogen production is made using water electrolysis.

This is a direct result of this method of production being the most expensive [13]. For this reason

industrial hydrogen produced by electrolysis is not used for specific applications, it is used when the

conditions are such that it becomes the most cost-effective method [48]. For instance, in the

“Development of Water Electrolysis in the European Union – Final Report” of 2014, the contrast between

the origin of the hydrogen used in the food industry in Canada and Europe is shown. In the first case

the origin is water electrolysis as in Quebec there are very large plants that use hydroelectricity, but in

the case of Europe hydrogen comes from steam reforming of natural gas.

Apart from the food industry it was also reported that electrolytic hydrogen is used in the fertilizer

industry, in making other chemicals, metallurgy, glass production, manufacture of electronic devices and

for cooling of generators in power plants.

As for energy generation, only a very small fraction of the hydrogen produced by water

electrolysis is used, including also for vehicles and renewable energy storage. These cases only happen

when there is an advantage for this technology in terms of cost, for instance when it is possible to benefit

from having the hydrogen being produced at the point of use.

A sector where it seems that hydrogen is gaining a lot of strength is in the power-to-gas

technology. These are basically plants where energy generated from renewable sources is converted

to hydrogen and is then stored in pressurised tanks so that it can be used when needed [49]. By 2013

almost 50 of these pilot plants were running, with most of them being situated in Europe and North

America. The hydrogen produced by these plants is used in electricity generation (fuel cells and

combustion), fuel for hydrogen vehicles and industry, gas distribution systems and also for the

production of methanol [50].

Stakeholders in the industry of hydrogen expect that in the future the uses of electrolytic

hydrogen will mainly be in transport and in energy storage, leaving the conventional industry use behind

[48]. It is expected that, by 2030, the scale of hydrogen energy will be on the GW, with thousands of

refueling stations worldwide. As for its applications as an industrial chemical it is expected that it may

still be relevant for potential emerging uses.

11

2.3. Advanced electrolysis cell modules

There are two different designs for the cell configuration, unipolar and bipolar. In the unipolar

configuration the anodes and cathodes are submerged alternatively in a container filled with the

electrolyte. This means that we have a system of anode, cathode, anode, cathode, and so forth. The

advantage of this method is that it is very easy to build and maintain, with the drawbacks of usually only

being able to operate at low temperatures and low current densities [51].

The bipolar configuration uses bipolar electrodes, which behave as cathodes to one cell and as

anodes to the other cell. These are all clamped together, as such one of the main advantages of this

configuration is the size of the stack, which is much smaller than in the unipolar design. The other

advantages are a higher current density and the fact that it produces higher pressure hydrogen [51].

The main disadvantage of this configuration is that, when a single unit in the cell has to be repaired, the

whole process has to be stopped since the cells are connected in series, whereas in the unipolar

configuration the units can be connected in parallel [52].

Instead of using a liquid electrolyte, a solid polymer electrolyte membrane, namely Proton

Exchange Membrane (PEM), can be used, with the two electrodes bonded to it in each side. This

membrane, a perfluorosulfonic acid polymer, is generally used in fuel cells [53]. The advantages that

have been found to using this technology are a greater energy efficiency, higher rates of production and

a more compact design [54].

The issue with PEM based electrolysers is their cost, as they are much more expensive than

the usual alkaline water electrolysers. In order to solve this problem without losing most of the

advantages of the PEM system, an anion exchange membrane (AEM) can also be used [55]. This

technology adds another advantage to the electrolysis process beside the low cost, it also mitigates the

precipitation of carbonates as there is no presence of metal cations, which is a disadvantage of the PEM

system in the case of cationic impurities in the feedwater. The main disadvantage of AEM is that the

performance is lower than with the PEM due to ionic conduction losses.

12

2.4. Commercial water electrolyser manufacturers

At the time of 2014 the main manufacturers and suppliers of water electrolysers can be

summarised in Table 1.

Table 1 - Electrolyser suppliers by 2014, PEM – Proton exchange membrane, AEM – Anion exchange membrane, Alkaline refers to a liquid alkaline electrolyte [48].

Company Country Technology H2 Purity (%) Efficiency (%)

Acta Italy AEM 99.94 63

AREVA France PEM 99.9995 60

CETH2 France PEM 99.9 61

ELT Elektrolyse

Technik Germany Alkaline 99.85 65

Erredue s.r.l Italy Alkaline 99.5 56

H2 Nitidor Italy Alkaline 99.9 64

H-TEC SYSTEMS Germany PEM N/A 60

Hydrogenics Belgium and

Canada Alkaline 99.998 58

Idroenergy Italy Alkaline 99.5 64

IHT Industrie Haute

Technologie Switzerland Alkaline N/A 65

ITM Power United Kingdom PEM 99.99 62

NEL Hydrogen Norway Alkaline >99.8 67

McPhy Germany Alkaline >99.3 58

Proton OnSite USA PEM 99.9998 52

Siemens Germany PEM N/A ~55

Teledyne Energy

Systems USA Alkaline 99.9998 N/A

Wasserelektrolyse

Hydrotechnik Germany Alkaline 99.9 57

As can be seen from Table 1, the majority of electrolysis cell manufacturers are situated in

Europe. All of them show extremely high values of purity with efficiencies varying between 52% and

67%. The predominant technologies being used for the cells are alkaline, which basically means that

the electrolyte used is alkaline, and PEM.

All of these values were taken from the technical data sheets supplied by the manufacturers, so

these should be seen as optimal values.

13

3. Electrocatalysts for HER

3.1. What is an electrocatalyst?

A catalyst is a substance used to increase the rate of a specific chemical reaction. This is

achieved by reducing the activation energy necessary for the reaction to occur or, in other words, the

formation of the products is facilitated by the addition of the catalyst.

In electrochemical reactions we can have electrocatalysts, which have the same purpose as

catalysts. These are usually in the solid state on the surface of the electrodes or they can be the

electrode itself. Electrocatalysts work by changing the kinetics of the reaction or even by changing the

mechanisms through which the reaction takes place [56]. For instance the electrocatalysts which are

considered to be the best right now are Platinum (Pt)-based. This is due to a variety of characteristics

like their high exchange current density, stable electrical properties, and the fact that they work

extremely well at high temperatures and are highly resistant to chemical degradation [57]. The problem

with using these electrodes is their cost, as Pt is a rare noble metal. As such, the electrodes are generally

based on cheaper materials, like Nickel (Ni), and then Pt is dispersed in its surface as small particles.

Therefore electrocatalysis is crucial in the field of energy conversion, where efficiency is highly

required.

3.2. Electrocatalysts and energy conversion

When converting electrical energy to chemical energy, for the synthesis of a fuel, in this case

Hydrogen, it is essential to have the highest efficiency possible. This means that the highest percentage

possible of energy should be stored in the fuel with only minimal wasted energy. For the case of water

electrolysis we have a redox reaction (Eq. 6). In order for this reaction to take place a potential difference

(ΔE) between the cathode and the anode has to be applied. This potential difference needs to be enough

to overcome the equilibrium potential (ΔEe) and the overpotential (η), and can thus be defined by Eq. 5

[58].

∆𝐸 = ∆𝐸𝑒 + 𝜂 (5)

Since the equilibrium energy is a characteristic of the reaction it cannot be decreased, as such

the focus is on decreasing the overpotential. A great fraction of this overpotential is a result of the energy

required to overcome the activation energies of the product formation [7]. By decreasing the value of

the overpotential a lower ΔE is needed, and consequently less energy has to be supplied to the

electrolyser, thus increasing the efficiency of the process. This is where the electrocatalyst has an

important role, by reducing the activation energy for the formation of hydrogen and oxygen the

overpotential is significantly reduced. The objective is then to find electrocatalysts that allow for the

14

lowest overpotential and with the longest working lifetime, while also being feasible economically at

large scale.

3.3. Thermodynamic considerations for electrocatalysts

3.3.1. Equilibrium potentials

In order to better understand how the use of an electrocatalyst will benefit the rate of hydrogen

production during the HER, we must first take a look at the thermodynamics of electron transfer at the

surface between the electrode and the solution. This process can be simply described by a redox

reaction.

𝑜𝑥 + 𝑛𝑒− ↔ 𝑟𝑒𝑑 (6)

Since the objective is to have a higher yield of hydrogen and a lower overpotential, the relevant

factor here is the electron-transfer rate (kc) which is dependent on the potential difference between the

electrode and the solution (E). The quantity of oxidation material (m) reduced at the electrode is given

by Eq. 7, where Q is the electrical charge (𝑄 = ∫ 𝑖𝑑𝑡 = 𝑖𝑡) and F is the Faraday constant.

𝑚 =𝑄

𝑛𝐹 (7)

The relevant factor to improve is the rate at which the material is reduced by area, which will

then be given by Eq. 8 [58],

𝑁 =𝑑𝑚

𝐴. 𝑑𝑡=

𝑗

𝑛𝐹 (8)

with the current density 𝑗 =𝑖

𝐴. Since we want to determine kc which is related to the concentration (𝑘𝑐 =

𝑁

𝑐), Eq. 8 has to be changed to Eq. 9.

𝑘𝑐 =𝑗

𝑛𝐹𝑐 (9)

The rate of electron transfer, both for the cathodic and the anodic reactions, has been empirically

shown to be given by Eqs. 10 and 11, respectively [7],

𝑘𝑐 = 𝑘𝑐0𝑒−𝛼𝑐𝑛𝐹𝜂

𝑅𝑇 (10)

𝑘𝑎 = 𝑘𝑎0𝑒𝛼𝑎𝑛𝐹𝜂

𝑅𝑇 (11)

15

with kc0 and ka0 being the electron transfer rate for the cathodic and anodic reactions when η = 0. The

fraction of E effective in increasing the rate of reduction is given by a constant α, which is the transfer

coefficient. When the concentrations of the oxidants and the reductants do not change, in Eq. 6, the

case when the current densities at the anode and at the cathode are the same and, as such, the total

net current is 0, ΔE = ΔEe, which is the equilibrium potential. This value can be determined

experimentally or by using the Nernst equation (Eq. 12) [7],

𝛥𝐸𝑒 = 𝛥𝐸0 +𝑅𝑇

𝑛𝐹ln

𝑐𝑜

𝑐𝑟

(12)

where ΔE0 is the formal potential for the specific electrode that is being used, which is the value when

the concentration of oxidant (co) is the same as the concentration of reductant (cr). This is a dynamic

equilibrium; there is still conversion taking place. Despite that, the current densities of the cathodic and

anodic reactions are the same, which results in a total net current of 0. Thus, at this equilibrium jc = ja =

j0, where j0 is called the exchange current density. This value, dependent on the electrode material,

represents an important aspect of the electrocatalytic properties of the electrode since it is related to the

overpotential by the Butler-Volmer equation (Eq. 13) [7]. As can be seen, the higher the exchange

current density is, the lower the overpotentials have to be to achieve the same current densities.

𝑗 = 𝑗0 [𝑒𝛼𝑎𝑛𝐹𝜂

𝑅𝑇 − 𝑒−𝛼𝑐𝑛𝐹𝜂

𝑅𝑇 ] (13)

3.4. Reaction kinetics involving H2O – H2 – O2

3.4.1. H2 evolution reaction mechanism

To understand the kinetics involving the production of hydrogen by water electrolysis it is crucial

to comprehend the mechanisms through which the hydrogen evolution reaction (HER) takes place. For

the case of a high alkaline media the three steps involved have been found to be the following [59]:

Volmer step: 𝑀 + 𝐻2𝑂 + 𝑒− ↔ 𝑀𝐻𝑎𝑑𝑠 + 𝑂𝐻− (14)

Heyrovsky step: 𝑀𝐻𝑎𝑑𝑠 + 𝐻2𝑂 + 𝑒− ↔ 𝑀 + 𝐻2 + 𝑂𝐻− (15)

Tafel step: 2𝑀𝐻𝑎𝑑𝑠 ↔ 2𝑀 + 𝐻2 (16)

The Volmer step is always the first step since it is by this reaction (Eq. 14) that the first interaction

between the aqueous solution and the electrode surface, or in other words, the hydrogen adsorption

takes place. After this initial step, any of the other two may take place, to produce the hydrogen gas

16

(H2). In the Heyrovsky step an electrochemical desorption of the gas takes place following Eq. 15. In the

Tafel step chemical desorption takes place according to Eq. 16. There are then two pathways through

which the HER can go through: Volmer - Heyrovsky and Volmer – Tafel. By determining the step that is

slower during the whole process it is possible to ascertain the rate determining step of the HER.

3.4.2. Tafel plots for various HER catalysts

A linear Tafel plot describes how the logarithm of the current density varies with the electrode

overpotential assuming that the electrode reaction is uniquely controlled by charge transfer and that n

is relatively high [60]. In this section an overview of the state of the art of the electrocatalysts with the

best Tafel plots will be made.

A group of researchers from China have built a novel electrocatalyst for the HER based on MoP

nanosheets supported on biomass-derived carbon flakes (CFs) (nanotechnology materials in electrodes

are increasingly taking over the usual electrode materials like metal alloys) [61]. The Tafel plot obtained

using this electrocatalyst was the one represented in Fig. 3:

Figure 3 - Tafel plot for the MoP nanosheet [61].

Obviously the lower the slope (overpotential vs current) the better, since it will mean that a lower

variation of the overpotential is needed in order to obtain a significant increase of the current density.

The lowest Tafel slope for the four cases studied in this report was for the MoP/CF, with a value of 56.4

mV dec-1.

Another electrocatalyst that was conceptualised by a group of researchers from India was a

Cobalt-Boride electrode, with a Tafel slope of 75 mV dec-1, as can be seen in Fig. 4 [62].

17

Figure 4 - Tafel plot for Cobalt-Boride electrode [62].

Another group of researchers from China made an electrocatalyst from NixSy hybrid

microspheres, obtaining a Tafel slope of 36.7 mV dec-1 (Fig. 5) [63].

Figure 5 - Tafel plot for the microspheres, c – NixSy, adapted from [63].

Finally in Australia another group of researchers developed an electrocatalyst based on

molybdenum sulphide (MoS) clusters-nitrogen-doped-graphene hybrid hydrogel, achieving a value for

the Tafel slope of 105 mV dec-1, as represented in Fig. 6 [64].

Figure 6 - Tafel plot for NG-Mo and Dry N-Mo [64].

18

3.4.3. Volcano-type plots

When choosing the material to use as an electrocatalyst one of the most relevant aspects that

has to be taken into account is the value of j0 characteristic of said material. It has been shown that most

physical and chemical properties of materials are periodic, following the periodic table, and these results

can be summarised in Gschneidner plots (or volcano type plots). For the case of this particular property

that should be maximised in order to achieve higher efficiencies in water electrolysis. Typical volcano

plots can be seen in Fig. 7. The maximum values can be found at the materials with d8 electrons, which

makes sense since the HER requires two slots for the adsorption of the hydrogen ions [65].

Figure 7 - Values of log i0 in HER for different metals in acidic media [65].

As we can see the maximum values correspond to Platinum (Pt), Palladium (Pd) and other rare

metals, which are very expensive materials, they are then usually used in conjugation with another

cheaper metal, like Nickel (Ni) since it also has acceptable properties while being economically viable.

It is then expected that, by using Pd alloys, promising results can be obtained.

3.5. Current state of catalyst development

3.5.1. Platinum-based catalysts for the HER

As said before, platinum has been found to be the best material [57] to be used as electrocatalyst

for the HER. As such, cathodes based on Pt have been studied for a long time, usually with Pt being

only a fraction of the electrode.

A recent example of an electrocatalyst that uses Pt as the support, instead of as the active

material, is the one developed by Tang et al., where Tungsten carbide in spherical hollow

microstructures was synthesized on a Pt support [66]. Even though Tungsten has a lower performance

19

than Pt, its considerably lower price and high resistance to surface oxidation makes it a good alternative

when used together with Pt. The results for this study showed a j0 of 0.84 mA cm-2 and 0.78 mA cm-2 for

electrocatalysts of this type synthesized at 1223 K and 1273 K respectively. The first value is higher

than those of commercial Pt/C catalysts (0.815 mA cm-2 [66]), which shows that this type of

electrocatalysts have the potential to be used commercially.

Another study uses Pt as the material for hollow nanospheres with graphene sheets as support.

Solid Pt nanoparticles in the same type of supports were also tested by this group of researchers from

Iran. The results obtained showed values of j0 of 0.94 mA cm-2 and 0.019 mA cm-2 for the hollow

nanospheres and the solid nanoparticles, respectively. The higher value for the first case is likely due

to the much higher surface area since the nanospheres are hollow, resulting in a higher electrocatalytic

behaviour [67].

Another way of reducing the concentration of Pt on the electrocatalysts is by using alloys. An

example of this are the Platinum-Dysprosium alloys. With an alloy with 60 at.% Dy j0 values as high as

29.2 mA cm-2 were shown to be obtainable at 85 ºC, and 14.9 mA cm-2 with 50 at.% Dy. The Pt-Dy (50

at.% Dy) alloy also showed high current densities at low overpotentials, presenting high electrocatalytic

activity, even higher than the activity of Pt [68].

It is possible to see that there is a great variety in how Pt is used for the fabrication of

electrocatalysts. All these different ways are born from the necessity of lowering the amount of Pt used

since this is a very expensive material.

3.5.2. Nanostructured electrocatalysts

Nanotechnology has the potential to be used in every field of technology, and electrocatalysts

are no exception. This technology allows us to control every aspect of the fabrication, to the point where

even the structure at the atom level of the electrocatalyst surface can be controlled. As such,

electrocatalysts based on nanotechnology methods show a great promise, with a large variety of

designs.

Such a design is the one proposed by Gong, et al. [69] where Nickel Oxide/Nickel structures

similar to heterojunctions were attached to carbon nanotubes, as can be seen in Fig. 8.

Figure 8 – (a) STEM bright-field image of the structure and (b) schematic illustration of the structure [69].

20

The advantages of this design are its high surface area, due to the carbon nanotube “forest”,

and the fact that it does not require expensive materials like Pt in its fabrication, while still maintaining

good electrocatalytic activity for water electrolysis. The fabricated electrolyzer was capable of achieving

current densities of 20 mA cm-2 at a voltage of 1.5 V and had good stability, thus showing a possible

alternative to the Pt based electrocatalysts.

A non-precious material that is showing great promise for HER is MoS2. A way of enhancing

this activity in nanostructures composed by this metal is by having more exposed sulfur edges. As was

already referred to before, such an electrocatalyst was developed by using hybrid microspheres of NixSy-

MoS2 [63]. The results of this study show that, in order to achieve a current density of 20 mA cm-2, the

overpotential required is of 0.33 V. As for its stability after 3000 cycles, the current density actually

increased slightly.

Another report shows a design based on MoP nanosheets of easy preparation supported on

carbon flake, as can be seen in Fig. 9, with results that point to a good alternative as a HER

electrocatalyst [61]. A current density of 10.1 mA cm-2 was achieved, with an overpotential of 200 mV,

and a Tafel slope of 56.4 mV dec-1. One of the reasons found for the better behaviour of the nanosheets

when compared to the bulk material was the presence of the conductive carbon support, which also

prevented the sheets from aggregation.

Figure 9 – Transmission electron microscopy (TEM) image of the nanosheet structures [61].

In conclusion it is possible to see that there is an enormous variety on how nanotechnology can

be used both to improve the activity of electrocatalysts and lower their cost.

21

4. Experimental details

4.1. Materials and chemicals

Platinum (Pt) is the benchmark electrocatalyst for water electrolysis [57], but due to its high cost

and scarcity, alternatives have been sought. Pd seems a promising alternative electrocatalyst for water

electrolysis [70] because, albeit still an expensive material, is cheaper than Pt [71], while retaining some

of the qualities that make Pt such an efficient electrocatalyst [72]. As such, this work focuses on

investigating the capabilities of different Pd alloys on a variety of supports.

The tested electrocatalysts can be divided in three groups. The first group is composed of three

PdM alloys (M=Au, Fe, FeAg), supported on reduced Graphene Oxide (rGO). The second group has

always the same alloy, PdNi, supported on four different supports, (SnO2/KB600), (SnO2/KB300),

(SnO2/Graphene) and Vulcan XC72. The third group uses Pd as the material, with Vulcan XC72 and

two biobased high surface area carbons, MEVSAK and MEGSAK, which are synthesized from the vine

shoot and the grape stalk, respectively, as the supports. All of these electrocatalysts were composed of

20% metal and 80% support.

A phenomenon that should be taken into account when studying the HER at Pd is the absorption

of hydrogen into the cathode resulting in a hydride [73]. In other words the Hads is absorbed into the Pd

lattice. This reaction will compete with the Tafel and Heyrovsky reactions and can thus influence the

rate at which they take place. A way of mitigating the effect of this reaction is by allowing the cathode to

become saturated with hydrogen at an equilibrium stage, so that when the studies are made there is no

space left for the hydrogen to be absorbed into the electrode [73].

The electrolyte used during these studies was always the same, an 8M KOH (AnalaR

NORMAPUR, 87% assay) alkaline solution prepared with Millipore water, which represents a highly

alkaline medium, typical of industrial alkaline electrolysis.

The electrocatalyst nanoparticle powders were obtained via a partnership with a research group

in Turkey. The materials were synthesized there and then sent to our group so that their electrochemical

properties could be studied. The catalytic inks were prepared by mixing 5 mg of the Pd-based alloys

supported on their respective support, in the form of powder, in a 125 µL solution of 2% polyvinylidene

difluoride (PVDF) in N-methyl-2-pyrrolidone (NMP).

4.2. Experimental setup

For the testing of the first group of electrocatalysts, a conventional three-electrode setup was

used: the reference electrode was a saturated calomel electrode (SCE) from Hannah Instruments (HI

5412), the counter electrode was a platinum mesh of 50 cm2 area from Johnson Matthey, and the rGO-

supported Pd-based electrode was used as the working electrode. The measurements were carried out

using a PAR 273A potentiostat from Princeton Applied Research, Inc.

22

For the two remaining groups of materials, the setup was basically the same with some minor

differences, the measurements were carried out using an ALS/DY 2325 RRDE-3A Rotating Disk

Electrode Apparatus from BAS Inc., and the counter electrode was a Pt coil (from BAS Inc.). All potential

values in this work are relative to SCE.

In the first group 10 µL of the electrocatalytic ink were pipetted onto the surface of a glassy

carbon (GC) disk electrode of 0.2 cm2. On the two remaining groups of materials only 1 µL was pipetted

since the area of the GC electrode was much smaller, 0.07 cm2. The electrodes were then left to dry

overnight in the oven at a temperature of 80 ºC, so that the solvent would evaporate and the adhesion

of the electrocatalytic ink to the surface was promoted.

4.3. Characterisation of the electrocatalysts

An important aspect, when studying the electrocatalytic properties of a material, is its surface

area, so that the current density (by surface area) can be obtained, and thus comparisons between

different materials can be done. While for the last two groups of materials the real surface area could

not be estimated, for reasons explained ahead, for the case of the first group this was possible.

In order to determine the real surface area of each electrode of the first group, the oxygen

desorption peak was recorded [74]. This method is based on the assumption that oxygen is chemisorbed

in a monoatomic layer before the oxygen molecule is formed. The real surface area can thus be

determined by the following equation:

A =Q0

Q0∗ (17)

where Q0∗ is the reference charge (for Pd it is considered to be 424 µC cm-2 and for Au 420 µC cm-2)

[75]. The average of these two values was considered for the case of the PdAu alloy and, for the

remaining samples, the value of the reference charge of Pd was used. Q0 is the charge that can be

obtained by the area under the peak that corresponds to the chemisorption of oxygen on the electrode

surface.

For the other two groups of materials the real surface area was not determined since only 1 µL

could be deposited on the top of the 0.07 cm2 radius glassy carbon electrode and, as a result, no oxygen

desorption peak could be observed. Therefore the geometric area was considered when determining

the current density. That being said, for these materials XRD (X-Ray Diffraction) measurements were

carried out. XRD permits the identification of the atomic structure of a crystal by the diffraction caused

by this structure on incident X-Ray beams. The relevant properties here being the angle of diffraction

and the intensity of electrons. By knowing the XRD profile of each structure it is then possible to identify

them in different samples. This technique was used to identify the components of the electrocatalysts.

23

4.4. Electrochemical techniques

4.4.1. Rotating disc electrode

The rotating disc electrode (RDE) technique is widely used in electrochemistry, and initially it

was selected for application in the present study. Later it was not found necessary, but given that some

attention was laid to its fundamentals, it was decided to include this brief note about RDE on this

subsection.

In an electrolyser the reaction that occur always involve transport of electrons between a

solution phase (the electrolyte) and a solid phase (the electrode). As such, for these reactions to occur,

it is necessary that the reactants are permanently fed to the electrode surface. Of course this is

dependent on the mass transport processes. These processes can be diffusion, due to concentration

gradients, migration, due to potential gradients, and finally convection, due to external mechanical

energy. When carrying out an experiment it is important to understand the conditions on which it takes

place, so that the results can be interpreted correctly. A way to assure this is by using rotating disk

electrode (RDE), since almost all the mass transport is due to convection [76].

This type of electrode is basically a polished disc surrounded by a sheath of an insulating

material. The electrode is then rotated on an axis perpendicular to its surface, thus pulling the solution

towards it and then tossing it outward, as is represented in Fig.10.

Figure 10 – Schematic representation of a rotating disc electrode [76].

This kind of system has been described by the Nernst diffusion layer model, where a small layer

of thickness δ adjacent to the electrode surface has a behaviour based on the diffusion process of mass

transport, and after that we have the convection ruled layer (Fig.11). The current density at the RDE is

then given by the Levich equation (Eq. 18),

𝑗 = 0.62𝑛𝐹𝐷0.67𝑣−0.166𝑐𝜔0.5 (18)

24

with n being the number of electrons, F the Faraday constant, D the diffusion coefficient, 𝑣 the kinematic

viscosity, c is the concentration of the electroactive species in the bulk solution, and finally ω is the

rotation speed of the electrode.

Figure 11 – Nernst diffusion layer model [76].

4.4.2. Linear sweep and cyclic voltammetry

In linear sweep voltammetry (LSV) the value of the current on the electrode is measured for a

range of applied potentials. In Fig.12 an example of a linear sweep voltammogram can be seen. In this

method the most relevant aspect is the rate at which the measurements are made [77]. On one hand

when we use a faster rate the measurements will not have enough time to reach the most accurate

value of current density for that potential, but the procedure will take less time. On the other hand with

a slower rate the values will be more accurate but the measurements will take much more time. Also it

should be taken into account the electron transfer rate for that electrode, since that can limit the rate at

which the measurements should be made.

Figure 12 - Example of a linear sweep voltammogram for HER on carbon and WO3/C electrode in a 1 M KOH solution at a sweep rate of 5 mV s-1 [78].

25

Cyclic voltammetry is basically the same as LSV, with the difference that, when the sweep

reaches one of the two values of limitation of the fixed range, the scan is reversed and there is a new

potential sweep in the opposite direction until it reaches the other limit value [77]. This process continues

for a determined interval of time at a constant sweep rate. This allows for the graphical representation

of both anodic reactions (when going forward) and cathodic reactions (when going backwards) since we

are still going through all the equilibrium positions of the redox reaction for that electrode. An example

of a cyclic voltammogram can be seen in Fig.13.

Figure 13 - Example of a cyclic voltammogram for a Cu/Ni electrode in a 1 M KOH solution at a sweep rate of 100 mV s-1 [79].

4.4.3. Chronoamperometry tests

Chronoamperometry tests are used in a variety of situations, namely to verify the stability of an

electrode and its resistance to corrosion. These tests consist on maintaining a constant potential on the

electrode and measuring the current over time, so that it can be seen if a higher potential is necessary,

after some time, to maintain the same current [80].

26

27

5. Results and discussion

5.1. PdM alloys supported on rGO

It has been shown that Pd supported on an Au substrate shows good electrocatalytic efficiency

for the HER [81]. Ag also leads to a high synergistic effect when alloyed with Pd, [82] thus making it one

of the most promising elements for this application. PdAg is also being used for oxygen reduction

reaction (ORR), oxidation of ethylene, hydrogenation of acetylene, and many other applications. Savafi

et al. [83] studied the electrochemical behaviour of this alloy supported on carbon ionic liquid electrode

(CILE) for HER. As for PdFe, while it has been extensively studied for the ORR [84, 85] with favourable

results, no reports have been found regarding its behaviour for the HER.

The electrocatalyst material is not the only responsible for improving the efficiency of the

reaction; in fact, the electrocatalyst support is also a deciding factor when it comes to performance. The

electrocatalyst support material will influence the electrochemical surface area, the particle size, the size

distribution, or even the conductivity of the catalyst [86]. Graphene shows a great potential as a support

in improving the performance of electrocatalysts for water electrolysis, as it possesses excellent

electrical and surface properties. This is due to its 2D sheet structure, which allows for the deposition of

nanoparticles on its surface or even between two sheets, thus increasing the electrocatalytic area of the

electrode [87]. The Pd-based electrocatalysts used in this group, like most graphene composites, have

been supported in reduced graphene oxide (rGO).

5.1.1. Characterisation of the electrocatalysts

Each PdM/rGO electrode was submitted to an applied potential of 1.6 V, for 300 s, to ensure

that the production of oxygen took place. Then, cyclic voltammograms were recorded by starting at the

open circuit potential (OCP), going to 0.6 V, back to -0.4 V and finally returning to the OCP. The oxygen

desorption peak can be easily seen in Fig. 14 for the case of the PdAu/rGO electrode.

28

Figure 14 - CV obtained with PdAu/rGO electrode, at 25 mV s-1, in 8 M KOH electrolyte at 25 ºC, showing the oxygen desorption peak.

The value of Q0 is found by integrating the peak area and determining the total charge. Applying

Eq. 17 the real surface area was then determined. The obtained values can be found in Table 2, together

with the corresponding roughness factor, which represents the value of Ar/Ag, with the geometric area

(Ag) being 0.2 cm2.

Table 2 - Real surface area of the Pd-based electrodes

Electrode 𝑸𝟎 (µC) Real Surface Area (cm2) Roughness Factor

PdAu/rGO 103 0.25 1.29

PdFe/rGO 93 0.22 1.11

PdFeAg/rGO 197 0.46 2.36

5.1.2. HER studies

The electrochemical measurements were carried out at temperatures of 25 and 35 ºC, due to

experimental limitations with this setup. The setup which changed for the last two groups of materials in

order to be able to study their behaviour at higher temperatures. At each temperature a pre-activation

step was performed to stabilise the OCP and make sure that the hydride issue mentioned before will

not have an effect on the results. For this purpose a potential of -1.20 V was applied for 60 min. For

PdAu the OCP was found to be -1.17 V and -1.11 V, for 25 and 35 ºC, respectively. As for PdFe and

PdFeAg, the OCPs were of -1.16 V and -1.20 V, respectively, at both temperatures. Fig. 15 presents

the polarisation curves for the three tested electrocatalysts at temperatures of 25 and 35 ºC.

29

Figure 15 – Polarisation curves at (a) 25 and (b) 35 ºC for the PdM/rGO electrocatalysts. Measurements made in 8 M KOH solution at a scan rate of 0.5 mV s-1.

As expected, larger current densities were obtained with the increase in temperature for all

alloys. The polarisation curves for the PdFe/rGO electrode did not show the typical behaviour, as no

exponential increase in current density can be seen, as would be expected for hydrogen discharge,

instead, a linear tendency can be observed. This might also suggest an explanation as to why the results

with PdFeAg/rGO were worse than the ones obtained with PdAu/rGO, as the former also has Fe in its

composition.

Tafel plots for these electrodes were obtained by linearising the polarisation curves using the

logarithm of the current density. The overpotential region used to adjust the experimental data to the

classic Tafel equation (Eq. 19) ranged between -0.04 and -0.28 V,

η =2.3RT

αFlog j0 +

2.3RT

αFlog j (19)

where R is the universal gas constant and F is Faraday’s constant. From this equation the values of 𝛼,

which represent the charge transfer coefficient, and of b =2.3RT

αF, were determined. 𝑏 is the Tafel slope

and is an indicator of the rate at which the current density increases with the increase of the applied

potential.

The obtained experimental data always showed good adjustment to the Tafel equation since

the values of R2 were always higher than 0.99. The Tafel plots for the three PdM/rGO catalytic inks can

be observed in Fig. 16.

30

Figure 16 – Tafel plots at (a) 25 and (b) 35 ºC for the PdM/rGO electrocatalysts.

Using the Tafel plots to determine j0 values implies the extrapolation of its value from a potential

region far from equilibrium. Therefore, it was considered to be more accurate to calculate j0 values by

using the low overpotentials form of the Butler-Volmer equation (Eq. 20).

η =RT

j0Fj (20)

The potential region considered for these calculations were the first 50 mV of applied

overpotential, using a scan rate of 0.5 mV s-1. Table 3 shows the values obtained for α, b and j0 for the

three PdM/rGO.

Table 3 - Kinetic parameters for the three tested PdM/rGO electrocatalysts.

Electrode Temperature (ºC) α b (mV dec-1) j0 (mA cm-2)

PdAu/rGO 25 0.40 149 0.09

35 0.29 210 0.37

PdFe/rGO 25 0.16 370 0.42

35 0.14 448 0.68

PdFeAg/rGO 25 0.24 251 0.46

35 0.22 274 0.58

The values of the Tafel slope are not only relevant as an indicator of the rate at which the

reaction takes place, but can also be used to determine the rate determining step (RDS) of the process

[88]. It is considered that, in alkaline medium, at 25 ºC, the Tafel slopes for the Volmer, Heyrovsky and

Tafel steps as RDS are 120, 40 and 30 mV dec-1, respectively [89]. For the case of the PdM/rGO

electrocatalysts studied it is proposed that the RDS is the Volmer step, since it possesses the closest

value to the Tafel slopes obtained at 25 ºC. It can also be noted that the values of b are much higher

31

than would be expected. This might be due to factors like a potential dependence on intermediate

adsorption [89].

A negative aspect of these results that should be taken into account are the relatively low values

of α. These values mean that the percentage of the overpotential used for the production of H2 with

these electrodes is somewhat low. Higher values were obtained with the PdAu/rGO electrocatalyst, and

the lowest with the PdFe/rGO.

The j0 values for PdFe/rGO and PdFeAg/rGO are quite similar to each other, which can be

attributed to the fact that the only difference between the two is the inclusion of Ag in the latter. Further

comparison with literature results is not possible as, to the author’s best knowledge, there are no

previous reports on these materials for the HER. The polarisation curves obtained for these two

materials show considerably different tendencies, with the PdFe/rGO electrocatalyst revealing a worse

performance than PdFeAg/rGO. It can thus be concluded that Fe is not a promising element for Pd

alloys for the HER, since its inclusion clearly leads to worse results. This phenomenon could be

explained by the Sabatier principle [90], which states that higher electrocatalytic activity is achieved by

having a catalytic surface with intermediate binding energies. This is due to the fact that with weak

binding the surface will not be able to activate the intermediates, but if they bind too strongly all the

surface sites will be occupied, thus decreasing the rate of the process. It is then suggested that by

alloying metals with low and high binding energy a synergistic effect can be reached, thus obtaining

more intermediate energies. In this case Pd and Fe are low binding energy materials, while Au and Ag

have high binding energies [91]. Thus the poor performance of the PdFe alloy might be explained by

the lack of synergy between these two metals. While PdAu and PdFeAg have metals from the two sides

which balance each other, leading to better performance for the HER. Another possible explanation is

that, since Fe has a tendency to oxidize, the surface of the electrocatalyst could be passivated, thus

preventing the reduction of hydrogen.

Concerning the PdAu/rGO electrode, studies have been made on the behaviour of this material

in a variety of supports and configurations. The Tafel slopes obtained in this work show similar values

to those previously reported [81, 92]. For instance, for the case of PdAu alloys on carbon ceramic

electrodes, the Tafel slopes at room temperature ranged from 136 to 165 mV dec-1 [92], or from 120 to

130 mV dec-1 [81], while for the PdAu/rGO electrode it was of 149 mV dec-1. The values of j0 are also

similar to those found in literature, 0.4 mA cm-2 [92].

As mentioned before, Pt is currently considered the benchmark electrocatalyst material for HER.

Previously, our group studied the behaviour of Pt-Dy alloys for this same application [68]. At 25 ºC the

value of b obtained for a 50% Dy alloy was of 147 mV dec-1, with an α of 0.40. While these results are

better than the ones obtained with PdFe/rGO and PdFeAg, they are similar to what was obtained here

with the PdAu/rGO catalyst. As such this alloy shows a performance at the level of a Pt-based

electrocatalyst. When compared to a 60% Dy loaded PtDy alloy, it even has better results, as the former

shows a value of b of 183 mV dec-1 and an α of 0.32. The j0 values were higher and lower, respectively,

in these two cases when compared to the PdM/rGO electrodes tested here.

Another relevant comparison that should be made is with Ni-based electrocatalysts, as their

relatively low cost makes them the common choice when it comes to industrial level electrolysis [89]. As

32

such the obtained values were compared to previous results with Ni-RE alloys [93], where, even though

the values of b and α were higher, the values of j0 found were much lower than the ones obtained here.

With the highest, at 25 ºC, being 3.67 × 10-3 mA cm-2, still being lower than 0.09 mA cm-2 obtained with

PdAu/rGO.

When compared to pure Pd electrodes the three rGO-supported Pd alloys show improvements

in their electrocatalytic activity. For the case of a Pd-modified carbon fibre, for instance, the value of j0

observed was of 0.17 mA cm-2 in a 0.1 M NaOH electrolyte [72]. Therefore, these studies show that

rGO-supported Pd-based alloys seem to be a valid alternative when designing novel electrocatalysts for

the HER in alkaline water electrolysis.

5.2. PdNi alloys supported on composite supports

In this group of electrocatalysts, instead of studying the effect that changing the Pd based alloy

has on the electrochemical properties of the catalyst, the impact of using different supports was

investigated while maintaining the same alloy, PdNi.

Ketjenblack EC-600JD and EC-300J, or KB600 and KB300, are superconductive carbon blacks

that show extremely high electrical conductivity at low concentrations. These material are used for a

variety of applications, including batteries, fuel cells and conductive coatings. The main advantage of

this family of carbon blacks is that smaller quantities of material are needed, to obtain the same

properties, than with other carbon blacks [94]. Vulcan XC72 is another conductive carbon black usually

used in power cables and electrostatic dissipation [95]. As for graphene, its advantageous properties

for this kind of applications have already been explained on the previous chapter.

All of these activated carbons were used together with Tin Oxide Powder (SnO2) to form the

composite supports for the electrocatalysts in this group. In order to simplify the way the results are

presented, only the type of carbon on the support will be mentioned when referring to the

electrocatalysts.

5.2.1. Characterisation of the electrocatalysts

As mentioned before, for these materials, and for the next group, the real surface area was not

determined and the geometric area was used when determining the current density. This was due to

the fact that, since the quantity of material used was so small, 1 µL, no oxygen desorption peak could

be observed on the CV studies. As can be seen in Fig. 17, the currents are much lower than the ones

obtained in Fig. 14 and no peak can be found.

33

Figure 17 - CV obtained with Pd/KB600 electrode, at 5 mV s-1, in 8M KOH electrolyte at 25 ºC, showing the absence of the oxygen desorption peak.

As mentioned before, XRD measurements were carried out that permit the identification of the

metals in the alloy. These results can be observed in Fig. 18.

Figure 18 – XRD measurements pertaining to the (a) PdNi/Graphene, (b) PdNi/KB600 and (c) PdNi/KB300 electrodes.

As can be seen from Fig.18, the support is the largest part of the material being studied as an

electrocatalyst. The peaks pertaining to the PdNi alloy were assumed to be the halfway point between

the peaks, for a certain structure, for both Pd and Ni. This fact might induce some error, as a variety of

different peaks can be observed at degrees distant from the peaks identified, which could be the actual

PdNi peaks.

5.2.2. HER studies

The electrochemical measurements were carried out at temperatures ranging from 25 to 85 ºC.

Before any measurements were made, a pre activation step was taken in order to stabilise the OCP.

This was done for 15 minutes at a potential of -1.200 V. The OCPs obtained for the PdNi alloy supported

34

on KB600, KB300, Vulcan XC72 and Graphene were, respectively, -1.180 V, -1.185 V, -1.187 V, and -

1.178 V. Fig. 19 shows the polarisation curves of the four electrocatalysts for each of the seven

temperatures.

Figure 19 – Polarisation curves taken at a scan rate of 1 mV s-1 in 8 M KOH for the (a) PdNi/KB600, (b) PdNi/KB300, (c) PdNi/Vulcan, and (d) PdNi/Graphene electrocatalysts.

The results obtained for these electrocatalysts cannot be directly compared with the ones from

the first group since not only the setup is different, but also the number of temperatures tested is much

higher.

As expected, the maximum current density increases with the increase of temperature. Between

the four electrocatalysts it can be clearly seen that PdNi/Graphene shows the highest current densities,

with a significant gap separating it from the second best, PdNi/KB600, which represents the benchmark

in superconductive carbon blacks, attesting thus to the high quality of graphene for this kind of

applications.

Once more the Tafel plots for these materials were determined using Eq. 19. The results can

be observed in Fig. 20.

35

Figure 20 – Tafel plots for the (a) PdNi/KB600, (b) PdNi/KB300, (c) PdNi/Vulcan, and (d) PdNi/Graphene electrocatalysts.

The values of R2 were always higher than 0.99, showing once more that the data adjust well to

the Tafel equation. The relevant electrochemical properties of these electrocatalysts can be found in

Table 4.

36

Table 4 - Kinetic parameters for the four tested PdNi/composite electrocatalysts.

Electrode Temperature (ºC) α b (mV dec-1) j0 (mA cm-2)

PdNi/KB600

25 0.46 129 0.03

35 0.46 133 0.09

45 0.43 146 0.14

55 0.40 163 0.16

65 0.40 166 0.20

75 0.42 165 0.34

85 0.44 162 0.47

PdNi/KB300

25 0.56 105 0.00

35 0.52 117 0.01

45 0.55 116 0.02

55 0.50 131 0.04

65 0.47 141 0.06

75 0.46 150 0.06

85 0.46 155 0.07

PdNi/Vulcan

25 0.45 131 0.00

35 0.44 140 0.01

45 0.42 149 0.03

55 0.41 160 0.06

65 0.40 167 0.09

75 0.41 170 0.10

85 0.42 169 0.10

PdNi/Graphene

25 0.40 149 0.03

35 0.37 167 0.07

45 0.37 172 0.15

55 0.38 171 0.32

65 0.42 158 0.53

75 0.42 163 0.96

85 0.50 142 1.52

Like in the case of the PdM/rGO electrocatalysts the RDS is shown to be the Volmer step since,

at 25 ºC, the value of the Tafel slope is always close to 120 mV dec-1.

Even though PdNi/Graphene shows the lowest values of α, the values of j0 are much higher

than for any of the other electrocatalysts, thus showing this to be the best support to use in conjunction

with the PdNi alloy. The kinetic parameters obtained for the PdNi/KB600, PdNi/KB300, and PdNi/Vulcan

electrocatalysts, are all relatively similar, which makes sense seeing as they all belong to the family of

carbon blacks. From these, KB600 shows the best results, as it is considered the benchmark for carbon

blacks.

To the best knowledge of this author, no studies on PdNi alloys being used as an electrocatalyst

for the HER have been carried out. That being said a variety of Ni based electrocatalysts have been

idealised. The most similar electrocatalyst found in the literature was a nanostructured Ni phosphide

supported on carbon nanospheres [96]. This showed a j0 value of 0.49 mA cm-2 at room temperature, a

much higher value than the values obtained with these four electrocatalysts at 25ºC. Those researchers

did not investigate how that electrocatalyst behaved in function of temperature, which might be an

advantage for the PdNi electrocatalysts. For instance, the PdNi/Graphene has a j0 value of 1.52 mA cm-

2 at 85 ºC, while a value of only 0.03 mA cm-2 while at 25 ºC.

37

5.2.3. Activation energy

The HER activation energies, which represent the energy necessary to apply to the system in

order for the reaction to take place, of these four electrodes was also determined. In other words, this

value represents the ease with which the reaction occurs. The activation energy (Ea) was not calculated

for the PdM/rGO electrocatalyst group since only two temperatures were used in the tests. For this

purpose the values of j0 were plotted as function of the reciprocal temperature, 1/T (Fig. 21). The

Arrhenius equation (Eq. 21) was then used to examine the results,

ln 𝑗0 = ln 𝐴𝑖 −𝐸𝑎

𝑅𝑇 (21)

where Ai represents the Arrhenius pre-exponential factor.

Figure 21 – Arrhenius plots for the PdNi/composite electrocatalysts.

As can be seen in Fig. 21, the adjustments to the Arrhenius equation were not as good as

expected, with R2 values only higher than 0.90. In two cases, PdNi/Vulcan and PdNi/Graphene, one

point had to be rejected since it did not follow the expected behaviour.

The values of Ea obtained were of 38, 43, 26 and 60 kJ mol-1 for PdNi/KB600, PdNi/KB300,

PdNi/Vulcan and PdNi/Graphene, respectively. These values are close to the ones for pure Ni, 51 kJ

mol-1 [93], and pure Pd, 30 kJ mol-1 [97]. PdNi/KB600 and PdNi/KB300 show the symbiotic nature of this

alloy as the Ea is between those for the pure metals. For PdNi/Vulcan the Ea is actually lower than the

one for pure Pd, while the Ea for PdNi/Graphene was shown to be higher than that of pure Ni.

These results show that, while at room temperature these electrocatalysts are not as good as

others that are currently available, they scale well with temperature and, at 85 ºC, high values of j0 can

38

be obtained. So the viability of these materials is dependent on whether the payoff of maintaining such

a high temperature is actually worth it when it comes to the amount of hydrogen produced.

Another option to determine the value of the activation energy is to use the results obtained with

the CA measurements at each temperature. Instead of using j0, in this case the current density value

used in Eq. 21 is the current measured when the CA measurements stabilize for each temperature, that

is, the diffusion limited current density. These CA measurements were carried out at potentials of -1.3,

-1.4, and -1.5 V, so that some conclusion could be drawn from the evolution of Ea with the increase in

applied potential. It is expected that its value should decrease with higher applied potentials since less

energy should be necessary for the reaction to take place. The CA measurements for this group of

materials can be observed in Figures 22 through 25.

Figure 22 – CA measurements for PdNi/KB600 electrocatalyst in 8 M KOH electrolyte at temperatures ranging from 25 to 85 oC for an applied potential of (a) -1.3 V, (b) -1.4 V and (c) -1.5 V.

39

Figure 23 - CA measurements for PdNi/KB300 electrocatalyst in 8 M KOH electrolyte at temperatures ranging from 25 to 85 oC for an applied potential of (a) -1.3 V, (b) -1.4 V and (c) -1.5 V.

Figure 24 - CA measurements for PdNi/Vulcan electrocatalyst in 8 M KOH electrolyte at temperatures ranging from 25 to 85 oC for an applied potential of (a) -1.3 V, (b) -1.4 V and (c) -1.5 V.

40

Figure 25 - CA measurements for PdNi/Graphene electrocatalyst in 8 M KOH electrolyte at temperatures ranging from 25 to 85 oC for an applied potential of (a) -1.4 V and (b) -1.5V.

As can be easily observed in these figures, as the temperature and the applied potential is

increased, more noise is present in the results, even though the effective current remains quite stable.

This might be due to the increased production of hydrogen in the surface, even though there was not a

great presence of bubbles during the measurements. This current j value was then, as was said before,

used to determine the value of Ea using Eq.21. These results are represented in Table 5.

Table 5 – Ea values for the PdNi/composite electrocatalysts for each applied potential.

Electrode Potential (V) Ea (kJ mol-1)

PdNi/KB600

-1.3 27

-1.4 16

-1.5 20

PdNi/KB300

-1.3 35

-1.4 21

-1.5 11

PdNi/Vulcan

-1.3 38

-1.4 28

-1.5 14

PdNi/Graphene -1.4 50

-1.5 31

41

As expected the value of Ea tends to decrease visibly with the increase in applied potential. Also

notable is the fact that the values are usually lower than those obtained using the j0 values. This can be

due to the fact that, while the latter values were obtained using the expected value of the current at

equilibrium, the former are determined using currents obtained at fixed higher applied potentials.

The Arrhenius plots for this calculations can be seen in Fig. 26. The data showed an acceptable

adjustment to to the Arrhenius equation, with R2 values higher than 0.90, 0.91, 0.94 and 0.98 for the

PdNi/KB600, PdNi/KB300, PdNi/Vulcan and PdNi/Graphene electrodes, respectively

Figure 26 – Arrhenius plots obtained with applied potentials of -1.3, -1.4 and -1.5 V for the (a) PdNi/KB600, (b) PdNi/KB300, (c) PdNi/Vulcan and (d) PdNi/graphene electrocatalysts.

5.3. Pd supported on MEGSAK, MEVSAK and Vulcan XC72

In this group of materials Pd supported on three different activated carbons was used as the

electrocatalyst material. The effects of using two new bio-based high surface area carbon supports were

studied in this group. MEGSAK is a low cost activated carbon prepared form grape stalks [98], while

MEVSAK is prepared from vine shoots.

42

5.3.1. Characterisation of the electrocatalysts

XRD measurements were also carried out and the results can be observed in Fig. 27.

Figure 27 - XRD measurements pertaining to the (a) Pd/Vulcan, (b) Pd/MEGSAK and (c) Pd/MEVSAK electrodes.

As can be clearly seen in Fig. 27 Pd is predominant in these three electrocatalysts.

5.3.2. HER studies

Since there was no change in the setup from the previous group to this one, the process was

exactly the same. A constant potential of -1.20 V (vs SCE) was applied for 15 min before the

measurements in order to stabilise the value of the OCP. The OCP values for Pd/MEGSAK,

Pd/MEVSAK and Pd/Vulcan were of -1.19 V, -1.120 V and -1.19 V, respectively. Fig. 28 shows the

polarisation curves obtained for these three electrocatalysts.

43

Figure 28 - Polarisation curves taken at a scan rate of 1 mV s-1 in 8 M KOH for the (a) Pd/MEGSAK, (b) Pd/MEVSAK and (c) Pd/Vulcan electrocatalysts.

The first conclusion that can be drawn from the polarisation curves is the higher current densities

when compared to the previous group. This should already be expected since Pd is a better

electrocatalyst than Ni, albeit much more expensive. As for the particular case of the Pd/Vulcan

electrocatalyst, it can be seen that there are no relevant differences when compared to the PdNi/Vulcan

electrocatalyst and, in both cases, these showed the worse results of their respective group. As such it

can be concluded that Vulcan XC72 is not a promising support for this kind of applications.

The bio-based carbons, MEGSAK and MEVSAK, in particular the latter, show high current

densities, showing promising capabilities as supports for electrocatalysts for HER, in contrast with

Vulcan XC72.

Once more the Tafel plots were drawn for this group of materials and can be seen in Fig. 29.

Again the data showing good fittings to the Tafel equation, Eq.19, with values of R2 always higher than

0.99.

44

Figure 29 - Tafel plots for the (a) Pd/MEGSAK, (b) Pd/MEVSAK and (c) Pd/Vulcan electrocatalysts.

Table 6 shows the relevant electrochemical properties obtained for this group of materials.

Table 6 - Kinetic parameters for the three tested Pd/C electrocatalysts.

Electrode Temperature (ºC) α b (mV dec-1) j0 (mA cm-2)

Pd/MEGSAK

25 0.36 163 0.13

35 0.36 168 0.17

45 0.35 178 0.29

55 0.34 190 0.40

65 0.31 213 0.71

75 0.30 230 1.16

85 0.34 208 1.48

Pd/MEVSAK

25 0.41 144 0.22

35 0.41 148 0.37

45 0.40 156 0.63

55 0.39 165 1.04

65 0.36 185 1.99

75 0.36 192 2.29

85 0.34 207 5.02

Pd/Vulcan

25 0.56 105 0.01

35 0.44 137 0.03

45 0.39 162 0.05

55 0.37 175 0.07

65 0.35 194 0.15

75 0.34 202 0.16

85 0.38 189 0.11

As well as for the previous two groups, the RDS in this case seems to be the Volmer step since

the Tafel slope at 25 ºC, for the three electrodes, is always close to 120 mV sec-1.

45

The first aspect of these three electrodes that can be noticed is the low value of α for all the

electrocatalysts, which shows that a high percentage of the applied overpotential is being wasted.

The j0 value for the Pd/Vulcan electrocatalyst is quite low, the results being similar to the ones

with PdNi/Vulcan in the second group, thus reinforcing that this is not a promising material to use for the

HER. As for the Pd/MEGSAK electrode the results were much better, reaching j0 values as high as the

ones obtained with the PdNi/graphene. The material with the highest values of j0 from all the three

groups was Pd/MEVSAK, with values as high as 5.02 mA cm-2 at 85 ºC.

MEGSAK and MEVSAK are new support materials and, as such, their behaviour requires further

studies for these applications. The closest materials found were two Pd-modified carbon fibre

electrodes, which showed a j0 of 1.7 x 10-2 mA cm-2 and 0.16 mA cm-2 at room temperature in an alkaline

0.1 M NaOH electrolyte [72], [99].

5.3.3. Activation energy

Since in this group the electrodes were tested for temperatures from 25 ºC to 85 ºC, like in the

last group, the activation energies could once more be determined using Eq. 21. The Arrhenius plots

obtained this way can be seen in Fig. 30.

Figure 30 - Arrhenius plots for the Pd/C electrocatalysts.

When compared to the plots obtained for the PdNi electrodes, these show much better fittings

to the Arrhenius equation, with values of R2 higher than 0.97.

The values of the activation energy determined were of 45, 38 and 44 kJ mol-1 for the

Pd/MEGSAK, Pd/MEVSAK and Pd/Vulcan electrodes, respectively. Unfortunately, all of these values

are higher than the 30 kJ mol-1 considered for pure Pd [97], which leads to the conclusion that the

46

supports are responsible for these higher values. Their usage, however, is extremely useful in reducing

the electrocatalyst cost since it means that less percentage of Pd is necessary.

The results obtained with the MEGSAK and MEVSAK bio-based carbons have shown

interesting results that make them promising candidates as supports for HER electrocatalysts. That

being said, since they are relatively novel materials, further study is necessary in order to better

understand their behaviour and properties.

Just like in the previous group of materials, activation energies were obtained using CA

measurements measured at fixed applied potentials of -1.3, -1.4 and -1.5 V, where the current density

used in the Arrhenius equation (Eq. 21) is the current at which these tests stabilize. Figures 31 through

33 show the CA measurements obtained for this last group of materials.

Figure 31 - CA measurements for Pd/MEGSAK electrocatalyst in 8 M KOH electrolyte at temperatures ranging from 25 to 85 oC for an applied potential of (a) -1.3 V, (b) -1.4 V and (c) -1.5 V.

47

Figure 32 - CA measurements for Pd/MEVSAK electrocatalyst in 8 M KOH electrolyte at temperatures ranging from 25 to 85 oC for an applied potential of (a) -1.3 V, (b) -1.4 V and (c) -1.5 V.

Figure 33 - CA measurements for Pd/Vulcan electrocatalyst in 8 M KOH electrolyte at temperatures ranging from 25 to 85 oC for an applied potential of (a) -1.3 V, (b) -1.4 V and (c) -1.5 V.

48

Just as before, the noise increases with the increase of temperature and potential applied. It

should also be noted that, in the Pd/Vulcan electrode, the current densities found at 85 ºC are not the

highest, which suggests that this material could achieve its best performance at 75 ºC. Table 7 shows

the values obtained for Ea, using this method, for this group of materials.

Table 7 - Ea values for the Pd/C electrocatalysts for each applied potential.

Electrode Potential (V) Ea (kJ mol-1)

Pd/MEGSAK

-1.3 30

-1.4 23

-1.5 16

Pd/MEVSAK

-1.3 34

-1.4 26

-1.5 16

Pd/Vulcan

-1.3 32

-1.4 19

-1.5 9

The three electrocatalysts show the expected behaviour as the value of Ea always decreases

with the increase of the applied potential. Once more it can be seen that the values of Ea are lower than

those obtained using the Arrhenius equation with j0. This can be due to the fact that those are determined

using the current density at equilibrium, as explained previously.

Once more the data showed a good fitting to the Arrhenius equation, as shown in Fig. 34, with

values of R2 higher than 0.98, 0.99 and 0.95 for the Pd/MEGSAK, Pd/MEVSAK and Pd/Vulcan

electrocatalysts, respectively.

Figure 34 - Arrhenius plots obtained with applied potentials of -1.3, -1.4 and -1.5 V for the (a) Pd/MEGSAK, (b) Pd/MEVSAK and (c) Pd/Vulcan electrocatalysts.

49

6. Conclusions

The relevance of hydrogen in the future of fuel technologies is undeniable. Whether for its high

energy density properties or its almost nonexistent waste, hydrogen seems to be the best alternative to

the current technologies of oil-based fuels. That being said, the state of the art of all the aspects

necessary to the implementation of such a makeover of the energy paradigm is still away from what is

desired. Mainly in the field of hydrogen storage and transport. As for the production of hydrogen fuel

itself, which was the focus of this work, a variety of methods exist. Water electrolysis produces hydrogen

with the highest purity and, using clean and renewable energy sources, is an extremely promising

technique. The drawback is the large amount of energy necessary to overcome the obstacles inherent

to the formation of H2. A way of reducing this energy is by using electrocatalysts which lower the

necessary overpotential to be applied in the electrolyser for the reaction to take place.

In the experimental part of this work, ten different Pd-based electrocatalysts were tested for their

capabilities as overpotential reducing agents in the process of alkaline water electrolysis. These were

divided in three groups: the first with three different PdM alloys, namely PdAu, PdFe and PdFeAg,

supported on the same material, rGO; the second was composed of four different composite supports,

KB600, KB300, Vulcan XC72 and graphene, in a composite with SnO2, with always the same alloy,

PdNi; finally the last group was composed of three electrocatalysts with Pd as the metal and MEGSAK,

MEVSAK and Vulcan XC72 as the supports.

In the first group the materials properties were determined for temperatures of 25 and 35 ºC due

to limitations of the experimental setup, which were solved for the remaining two groups of materials. In

this group it was found that Fe seems to have a negative effect on the electrocatalytic activity of Pd

since its inclusion resulted in an undesirable linear behaviour of its polarisation curve, not as noticeable

with the PdFeAg alloy but still noticeable. From this group the most promising material was the

PdAu/rGO electrocatalyst with values of j0 of 0.37 mA cm-2, which were not the highest of the group but

managed to achieve the highest current densities as, during the LSV studies, it would enter the Tafel

region at lower overpotentials than any of the other two electrocatalysts.

For the materials in the two remaining groups the experimental setup was always the same.

The measurements were carried out at temperatures ranging from 25 to 85 ºC. This allowed for the

determination of the activation energy (Ea) of these electrodes.

In the PdNi group the most promising material was the one supported on graphene, with current

densities higher than 50 mA cm-2 at 85 ºC and j0 values of 1.52 mA cm-2. The values of Ea, for the

materials in this group, determined by using the j0 values with the Arrhenius equation were between

those reported for Pd and Ni individual parent metals.

In the last group, the electrocatalyst that showed the best results, not only in this group but of

all the tested materials, was the Pd/MEVSAK. Here the high cost of using pure Pd is somewhat

compensated by MEVSAK, which is a carbon support obtained from vine shoots. This electrocatalyst

showed current density values as high as 80 mA cm-2 at 85 ºC and values of j0 of 5.02 mA cm-2. One

thing in common found in the last two groups was that Vulcan XC72, which is the benchmark carbon

50

support material, always showed the worst results in every case, which shows that the other supports

show significant promise in replacing it.

CA measurements were taken at all temperatures at three different applied potentials, -1.3, -1.4

and -1.5 V, for the two last groups of materials in order to study how the activation energy changed by

increasing that potential. It was found, for all materials, that the value of Ea decreased with the increase

in potential applied, which makes sense since more energy is being fed into the reaction, thus increasing

the rate at which that reaction takes place. These Ea values were always lower than the ones obtained

with the j0 values, which pertain to an equilibrium state while the ones obtained with the applied

potentials are from a region far from that equilibrium. It should also be noted that the last group, with

pure Pd, showed lower values of Ea than those in the second group, with the PdNi alloy, which should

be expected since Pd is considered the best material for this kind of applications after Pt, the latter not

being a good option since it is extremely costly.

All in all, while some materials did not show encouraging results (mainly those involving Fe

alloyed with Pd and those supported on Vulcan XC72), it can be considered that three of the ten

materials studied are very promising as electrocatalysts for the HER, namely PdAu/rGO, PdNi/graphene

and Pd/MEVSAK. The support on the latter material deserves further studies as it is a relatively new

material that has not been studied at length. That is not to say that none of the other materials show

promise, as, for instance, Pd/MEGSAK and PdNi/KB600 also show noteworthy results.

A hydrogen based economy, based on clean renewable energy sources, seems to be one of

the best case scenario for our future when it comes to both how we produce energy and how we store

and transport it. In order for this scenario to become a reality, incentives are necessary to push academic

researchers and companies into investing in this area. Massive improvements have to be made in all of

the steps of such a system - production, storage, transport and infrastructure - so that we might finally

leave the dangers of using fossil fuels to store all the energy we need as a society, and improve our

living standards.

51

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