Thesis - surface modification of polymer membranes for DMFC DRAFT 20110623 t Paula L

Surface Modification of Polymer Membranes for Direct Methanol Fuel Cells

Thesis for the Master of Science Degree

Oskar Tynelius Department of Chemistry, Polymer & Materials Chemistry

Lund University, Sweden Examiner: Prof. Patric Jannasch

Thesis Work Performed at the Danish Technological Institute,

Høje-Taastrup, Denmark, 2010-2011 Supervisor: Dr. Yihua Yu

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PREFACE This thesis aims to report the works conducted on developing an improved polymer electrolyte membrane

for a direct methanol fuel cell. The work was carried out at the division for micro technology and surface

analysis at Teknologisk Institut in Copenhagen, during a time span of ten months, and was part of a project

called MicroPower, aimed at creating a fuel cell in the size range of a small battery. This thesis focuses on

methanol permeability reduction of the commercially available and widely used Nafion membrane, by

surface modification.

Vital additions to the work were done by a number of colleagues at the division. I would like to thank my

supervisor Dr. Yihua Yu for answering my avalanche of questions regarding general chemistry, Katherine

Bjørneboe and Erik Wisæus for excellent SEM imaging, Christian Kallesøe for IV-curve measurements and

participation in conductivity measurements, Lillemor Hansson for continuous feedback and support relating

to the Quartz Crystal Microbalance measurements, Anne-Charlotte Johansson for general electrochemistry

feedback, Klaus Krogsgaard for electronics tutoring, Torsten Lund-Olesen for general discussions on

membrane electrode assembly, Kenneth Brian Haugshøj for XPS characterisation and Jan Hales for

discussions on the whole fuel cell project.

ABSTRACT Two methods for surface modifying Nafion membranes for utilisation in Direct Methanol Fuel Cells (DMFC)

are investigated. In situ polymerisation of dopamine-melanin is used as a simple yet effective way of

reducing methanol permeability of Nafion by almost 70%. The polymerised layer is found to be stable in

operating conditions. The fabrications method is however inflexible and the conductive ability of the Nafion

is impaired. Layer-by-layer deposition of polyionic polymers is found to decrease the methanol permeability

with number of bilayers added, to an experimental minimum of 39% of that of pristine Nafion. The

conductivity was reduced, but found to be independent of number of layers added. The reaction

conditions are likely suboptimal, which rendered a very uneven surface coverage. The inhomogeneous

nature of the layer was confirmed using X-ray photoelectron spectroscopy (XPS). Methanol permeability

was measured using Quartz Crystal Microbalance (QCM) and conductivity was measured using

Electrochemical Impedance Spectroscopy (EIS).

SAMMANFATTNING Två olika metoder användes för att ytmodifiera Nafionmembran i syfte att använda dessa i bränsleceller.

Den ena metoden var in-situ-polymerisation av dopamin på Nafions yta, så att dopamin-melanin bildas.

Detta resulterade i en sänkning av metanolpermeabiliteten med nästan 70%. Lagret visade sig också vara

kemiskt motståndskraftigt. Metoden visade sig också vara inflexibel. I den andra metoden, layer-by-layer

deposition, tillfördes alternerande lager av polykatjoniska och polyanjoniska polymerer från lösning till ett

Nafionmembran. Den minimala metanolpermeabiliteten som uppnåddes med denna metod vad 39% av

obehandlat Nafions permeabilitet. Protonkonduktiviteten gick ner, men fanns vara oberoende av antalet

tillförda lager polymer. Lagerformationen var väldigt ojämn, och troligen suboptimal.

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Metanolpermeabiliteten mättes m.h.a. Quartz Crystal Microbalance (QCM) och konduktiviteten m.h.a.

Electrochemical Impedance Spectroscopy (EIS).

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CONTENTS

1. INTRODUCTION .............................................................................................................................. 1

1.1. Background ................................................................................................................................................... 1

1.2. The aim and outline of the thesis .................................................................................................................. 2

2. THEORY ............................................................................................................................................. 3

2.1. Orientation ................................................................................................................................................... 3

2.2. Fuel Cells fundamentals ................................................................................................................................ 5

2.2.1. Introduction ...................................................................................................................................................... 5

2.2.2. Direct Methanol Fuel Cell ................................................................................................................................. 5

2.2.3. Proton Conductivity .......................................................................................................................................... 6

2.2.4. Methanol permeability ..................................................................................................................................... 6

2.2.5. DMFC membrane characteristics ..................................................................................................................... 7

2.3. Nafion membrane ......................................................................................................................................... 8

2.4. Approach specific theory .............................................................................................................................. 9

2.4.1. Dopamine self-polymerising nano layer ........................................................................................................... 9

2.4.2. Layer-by-layer deposition ............................................................................................................................... 10

2.5. Risk assessment .......................................................................................................................................... 12

3. METHOD ......................................................................................................................................... 12

3.1. Measurement methods and materials ........................................................................................................ 12

3.1.1. Methanol permeability ................................................................................................................................... 12

3.1.2. Proton conductivity ........................................................................................................................................ 15

3.1.3. Polarization Curve Reference Measurement ................................................................................................. 17

3.1.4. SEM imaging ................................................................................................................................................... 18

3.1.5. Preparation and treatment of Nafion membranes ........................................................................................ 18

3.1.6. Dimensional swelling in water........................................................................................................................ 18

3.1.7. X-ray photoelectron spectroscopy ................................................................................................................. 18

3.2. Approach specific method .......................................................................................................................... 19

3.2.1. Self-polymerising dopamine-melanin ............................................................................................................ 19


4. RESULTS & DISCUSSION ............................................................................................................ 20

4.1. Dimensional swelling in water .................................................................................................................... 20

4.2. Optical appearance ..................................................................................................................................... 21

4.2.1. Dopamine-melanin ......................................................................................................................................... 21

4.2.2. Layer by layer deposition ............................................................................................................................... 21

4.3. SEM imaging ............................................................................................................................................... 21


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4.4. QCM measurements ................................................................................................................................... 25



4.4.3. QCM measurement limitations ...................................................................................................................... 28

4.5. Conductivity ............................................................................................................................................... 28

4.5.1. Conductivity measurement limitations .......................................................................................................... 29

4.6. Selectivity ................................................................................................................................................... 30

4.7. Polarisation Curve Measurements .............................................................................................................. 30



4.7.3. PCRM measurement limitations..................................................................................................................... 32

4.8. Resistance to methanol for Dopamine-melanin and Layer-by-layer deposition .......................................... 32

4.9. Dopamine-melanin resistance to alkaline environment .............................................................................. 32

4.10. XPS results for layer by layer deposition ..................................................................................................... 33

5. CONCLUSIONS ............................................................................................................................... 34

5.1. Major conclusions for Dopamine-melanin and LbL ..................................................................................... 34

5.2. Dopamine-melanin ..................................................................................................................................... 34

5.3. Layer-by-layer deposition ........................................................................................................................... 35

5.4. Suggestions for improvements ................................................................................................................... 35

5.5. Future research........................................................................................................................................... 36

6. REFERENCES ................................................................................................................................. 38

7. APPENDICES .................................................................................................................................. 40

7.1. MATLAB code example ............................................................................................................................... 40

7.2. Mathematical model used for permeability calculations ............................................................................ 42

7.3. Conductivity setup resistances and example of measurement curve .......................................................... 43

7.4. XPS results .................................................................................................................................................. 43

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1. INTRODUCTION

1.1. Background The effort to create a commercially available fuel cell design has been on-going for decades. Since the

invention of fuel cells in 1838, the development has been severely hampered not only by technological

hinders. Likely the extensive use of fossil fuels have also been of great importance, as it offers a source of

energy so cheap that it still is far out of reach for most other sources of energy in terms of price per unit of

produced energy. Fuel cells do however have advantages and since the 1960s they have been used in the

NASA’s space shuttle program, where costs are less of an obstacle. The fuel cell principle can be applied in a

number of different designs, and can run on a wide variety of fuels. This has led to a somewhat diverse

research effort, where the hydrogen fuel cell is given a big share of the attention due to its possibilities in

the technically very important automotive industry. Since fuel cells do not give off any dangerous or

poisonous exhausts during operation, it sometimes is promoted as a “green” source of energy. The validity

of this argument is however very dependent on how the fuel is produced, an aspect that is not always

taken into consideration. One strong advantage of fuel cells is however that it is possible to choose where

to deploy the fuel production, and thus where the exhausts will be produced. In this way use of fuel cells

can lead to improvements in the local climate, as well as extending the amount of places where fuel

conversion can take place in a safe way. Examples of fuel cells can be seen in Figure 1.

Figure 1. Examples of different applications of fuel cells. A) DTI early prototype housing. Size ~ 10 mm. B) TOYOTA hydrogen fuel cell bus. C) Type 212 German submarine partly driven by a hydrogen fuel cell.

One sector where fuel cells seem likely to reach a commercial breakthrough is in consumer electronics.

Here the fuel cell performance is not matched against the combustion engine, but against batteries. In this

sector size, weight and recharging/replenishing are key characteristics and several of the fuel cell’s

advantages lie in these areas. Several consumer electronics giants, such as Toshiba and Fujitsu, have made

efforts in developing Direct Methanol Fuel Cells for use in for example laptops. 1

In later years, DMFCs have been given a lot of attention by the military industry, for example due to the

possibility of DMFC to provide additional electric power to soldiers in the field at a much reduced weight

compared to batteries. This might favour the position of DMFC applications at present, as the military

industry tends to put emphasis on performance, rather than price.

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It might be in the above mentioned sector, as a “range extender” that the DMFC has its strongest appeal. In

symbiosis with a small battery which handles peak loads, the DMFC can continuously recharge a battery,

which works as intermediate energy storage between the fuel cell and the application.

1.2. The aim and outline of the thesis The aim of this thesis is to present the development and characterisation of Nafion based Polymer

Electrolyte Membranes (PEM) for a miniaturised DMFC. At Denmark’s Technological Institute (DTI), the

MicroPower project aims to achieve an effective DMFC of the size range of a small battery. The choice of a

DMFC for this purpose is due to the relatively simple design, which requires no pre-treatment of the fuel

inside the cell. Methanol also has a high energy density (MJ/L). This kind of fuel cell can be operated at low

temperatures, which is demanded. One of the essential components of a DMFC system is a well-designed

PEM.

At DTI the focus of the MicroPower project is to produce high performing DMFCs according to certain

design requirements. Since these are yet to be finally decided, general DMFC performance will be used for

evaluation. It is beyond the scope of this thesis to discuss commercial considerations such as price and

scalability of the production process. At present one major problem of the DMFC is the so called methanol

crossover. In this process, which is explained later in this report, methanol escapes from the fuel chamber

through the PEM to the cathode, where it reacts or evaporates. In this way a large proportion of the fuel in

the fuel container can be lost. This leads to both decreased service time, and decreased electric potential.

For this reason, the core focus of the work reported herein is to decrease the methanol permeability

of the membrane used in the DMFC, while maintaining proton conductivity and mechanical properties.

The objective of this report is to describe improvements of the performance of PEM according to the

criteria mentioned above. The evaluation of the membranes will be done through methanol permeability

tests and other methods. Hopefully this will demonstrate the applicability of some approaches for lowering

the methanol crossover in DMFCs.

Figure 2. Schematic structure of the factual membrane approaches followed.

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In most aspects, this report follows standard notation and structure. Since several approaches have been

used in this thesis, it might still be of benefit to the reader to get a short introduction to the structure of it

(Figure 2). To try and address the problem of high methanol permeability in PEMs, two distinctively

different approaches were followed:

1. Surface modification of commercially available Nafion membrane through self-polymerisation of

dopamine, a small naturally occurring organic molecule.

2. Surface modification of commercially available Nafion membrane through Layer-by-Layer

deposition (LbL) of cationic and ionic polymers in alternating layers.

2. THEORY

2.1. Orientation The field of fuel cell research is very big and diverse. This also holds true for the research on DMFC. Thus,

an attempt to overview the whole field of DMFCs will not made. Instead an effort to present this thesis in

its context will be made.

As of the beginning of 2011, there is no clear alternative to Nafion membranes (by DuPont), which have

been around for several decades. Instead there is a large and diversified research effort to find suitable

membrane materials that surpass Nafion performance or certain properties of Nafion. New membranes for

DMFCs are intensively investigated, and therefore there are a great number of previous results to consider

before conducting own experiments.

The possible gains of operating a DMFC with highly concentrated methanol as fuel are outlined by Zhao et

al.2 The specific energy of DMFCs compared to Lithium-ion batteries is represented in Figure 3. The higher

efficiency of the fuel cell and the higher fuel concentration, the better the DMFC compares to the lithium-

ion battery that currently is standard for some consumer electronics applications that might be powered by

a small fuel cell. Operating at high concentrations therefore seems essential in order to provide a

competitive technological alternative.

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Figure 3. Specific energy comparison between DMFC at different efficiencies and Li-ion batteries 4.

As reported by Ramya et al,3 the measured methanol crossover (methanol permeability) for Nafion is

strongly influenced by the concentration of methanol used. Ramya et al. therefore concludes that 1-2M of

methanol might be the optimum operating concentration for a DMFC. This is in strong contrast to Zhao et

al. and highlights the diversity in the approaches available in this area of research. Since the permeability of

methanol in Nafion shows this behaviour, it is not recommended to compare results achieved at different

concentrations of methanol. Instead the focus should be put on relative improvement, rather than absolute

results.

Decreased methanol permeability will allow for an increased methanol feed concentration. This could lead

to a higher cell voltage and power density as well as increased operation time.1 For these reasons low

methanol permeability is the focus of this thesis. There are a number of methods to improve DMFC

efficiency, but low methanol permeability and high conductivity are the two properties that are mainly

governed by the membrane.

When considering developing a Nafion based membrane, rather than synthesising a completely new one,

there are several other approaches available. In this thesis surface modification of precasted Nafion

membranes have been used as approach, for several reasons. First, the DMFC is supposed to be operated

below 80⁰C and thus one of the main disadvantages of Nafion, dehydration at high temperatures, is not a

concern. Second, the high mechanical strength of Nafion is desired, as a very thin membrane with sufficient

strength would be an advantage from fuel cell design perspective. Third, the cation conductivity of Nafion is

thought to be sufficient in the long run. Therefore it seems like a feasible approach to make use of Nafion’s

advantageous properties, while suppressing methanol permeability through a surface treatment. With

these limits in mind, a number of articles were deemed relevant for theoretical and practical guidance.

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2.2. Fuel cell fundamentals

2.2.1. Introduction

The fundamental function of a fuel cell is to convert chemical potential into electrical current. In practise

this can be achieved in a number of ways, but the core process stays the same. In Figure 4 the DMFC

process is overviewed, but the principle is similar to that of other fuel cell types. On the anode side of the

cell the fuel is split up into cations and electrons with the aid of a catalyst. While the cations travel to the

cathode side through a PEM, the electrons are directed through an external load before they reach the

cathode. The movement of the electrons is driven by the excess positive charge present at the cathode. At

the cathode the cations and electrons combine to form the end product, which varies depending on the

type of fuel cell. Unfortunately this idealised function of the fuel cell is not realistic, and losses in different

fuel cell parts lowers the efficiency of the cell.

Figure 4. Sketch of the fundamental fuel cell operation for a DMFC.

2.2.2. Direct Methanol Fuel Cell

The DMFC uses liquid methanol as fuel (often diluted with water), and this is fed to the anode side either

actively with pumps, or passively through diffusion from a fuel chamber. At the catalyst, the methanol is

split into its components, and these later recombine at the cathode together with oxygen to form water.

The electrode reactions:

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As shown in Figure 4, the fundamental fuel cell principle is quite simple. Fuel cell operation can however

require a number of auxiliary systems. These can include, but are not restricted to, pumps for replenishing

reactants at the anode, pumps for removing products from the cathode, as well as other equipment that

cooperates in order to achieve high efficiency. These systems require energy and add to the complexity of

the complete fuel cell. DMFCs are however well suited to be designed without most of these support

systems. In this way both energy and space can be saved, and DMFCs are thus well suited to be built in

small sizes, one of the goals of the MicroPower project.

2.2.3. Proton Conductivity

The proton conductivity of the membrane is one of the fundamental properties that govern the

performance of the membrane. Conduction of protons is generally believed to take place in a combination

of two ways.4

The vehicle mechanism conveys protons by transporting hydronium ions ((H2O)nH+) through the membrane.

Increasing proportions of hydrophilic domains in a membrane increases the probability of this type of

conduction.

The Grotthuss mechanism, also known as the hopping or jump mechanism, means that stationary possibly

ionic molecules are protonated, and then give away excess protons/cations to other nearby molecules

which can accept them. In this way molecules within proximity of each other work like a conveyor belt. A

certain minimum density of conducting sites are required.

Both the vehicle mechanism and the Grotthuss mechanism work in the opposite direction of an ion

concentration gradient. That is, they will transport ions from an area with high ion concentration to an area

with a low ion concentration.

A general approach for achieving high proton conductivity is to have a lot of water within the membrane.

This is achieved by synthesising a membrane which is highly hydrophilic. High hydrophilicity does however

potentially lead to other problems, such as high methanol permeability and low mechanical strength.

2.2.4. Methanol permeability

The methanol molecule is in its size and polarity similar to the water molecule, and therefore behaves in a

similar way in some circumstances (see Figure 5). The methanol fuel for a DMFC is commonly diluted with a

amount of water. In a DMFC the cathode is exposed to air, and therefore it is a flow of water molecules

from the diluted methanol fuel at the anode to the cathode where the liquid evaporates. This water

migration can drag along methanol molecules. In this way a fuel cell can experience a severe loss of fuel,

and thus a corresponding loss of both power life time. This is a large weakness in current DMFC designs, as

sufficient proton conductivity can be achieved in a number of ways, but with a high degree of methanol

crossover as well.1 If methanol reaches the cathode it lowers the reaction potential, so called cathode

poisoning.

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Figure 5. Methanol (A) and water molecule (B).

2.2.5. DMFC membrane characteristics

There are a number of properties of a DMFC membrane that are important. These are sometimes

conflicting, and in such cases it is important to find the optimal compromise. First and foremost, a

membrane must be strong enough so that it does not deform or break during the lifetime of the fuel cell.

This is of high relevance in a DMFC, where the membrane will be hydrated during operation, but also

possibly dry during non-operation, since water in the membrane evaporates. The requirements to

withstand these varying conditions of mechanical strain and water uptake are therefore high. A membrane

seemingly can have a high mechanical strength, but will fail when used under realistic conditions. When the

membrane is placed between two electrodes covered in catalysts, in a so called Membrane Electrode

Assembly (MEA), the combined strength might be increased. A mechanically weak membrane will still likely

crack between the electrodes, which will severely affect the performance of the MEA. The proton

conductivity of the DMFC we are considering in this work is not required to be very high. The focus of this

fuel cell design is to provide low power over an extended period of time. Thus the proton conductivity

should be adequate but it is a less dominating characteristic compared to for example hydrogen fuel cells,

where the power output might be the most important characteristic of the whole system. Instead low

methanol permeability is crucial for a passively fed DMFC. Since the fuel is stored in a small container, fuel

lost through the membrane to the cathode cannot be replaced during operation. Thus the methanol

permeability has a direct effect on the service length of a passive DMFC. Indirectly this affects the size of

the fuel cell, as it must carry additional fuel to compensate for the fuel that will be lost during operation.

Also, the leaking methanol will interrupt and hinder the cathode reaction and thus significantly lower the

operation potential of the DMFC, as mentioned earlier.

It must be emphasised that the whole MEA works as a complete unit, and therefore improvements in one

characteristic might not lead to improvement in the total performance, if the limiting factor lies elsewhere.

Efforts to simulate fuel cell systems are underway, and these might be very helpful when dealing with

system weaknesses. No such system was however in place during the writing of this thesis.

2.3. Nafion membrane Commercially available Nafion membranes from Dupont were extensively used during the experimental

part of this thesis, both as substrates and for reference measurements (Figure 6). The structure of Nafion is

based on that of polytetrafluoroethylene, more known as Teflon. Through copolymerisation, sulfonic

perfluoro monomers and tetrafluoroethylene monomers are copolymerised. The product can then be

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extruded into desired shapes. A Teflon chain with some minimum amount of sulfonic acid groups can

conduct cations. This movement of cations is dependent on the porous structure of Nafion. It does not

conduct anions or electrons. The amount of sulfonic groups is given in a measure called “equivalent

weight” (EW). This is defined as the weight of Nafion in grams per mole of sulfonic acid groups. A standard

Nafion membrane might have an EW of 1100/mole.

Figure 6. A) One proposed morphology of Nafion, the water channel model.5 B) Teflon backbone with sulfonic acid pendant

groups.6

The methanol permeation through a Nafion membrane occurs since Nafion is very hydrophilic. The

methanol diffuses into these regions, and if there is a flow of water it will be dragged along through the

membrane. The exact morphology of Nafion is however still unclear, largely due to the fact Nafion cannot

truly be dissolved.4

The commercially available Nafion membrane used for this thesis was from Ion Power Inc. The type was

Nafion Membrane N115 5 mil, which is 127 micrometre thick (5 mil = 5 thousands of an inch), with an EW

of 1100 g/mole. Nafion swells when fully hydrated, and according to Soboleva et al. this changes Nafion

115’s thickness from 127µm to 158µm.7 This can be important to consider, as calculations for methanol

permeability and ion conductivity are linearly affected by changes in thickness.

2.4. Theoretical background

2.4.1. Dopamine self-polymerising layer

The chemistry behind mussel’s ability to adhere to virtually any surface available was reported by Lee et al.8

They noticed that the adhesive plaques produced and used by mussels are made up of a web of catechol

and amino groups. This web was able to adhere to principally any surface, both organic and inorganic. They

utilised the dopamine molecule (Figure 7) in order to mimic the behaviour of the mussel plaque.

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Figure 7. Dopamine molecule, with catechol group on the left and amino group on the right.

In alkaline conditions the dopamine molecule self-polymerised onto a surface as a thin layer, approximately

50nm, after 20-25 hours. Longer reaction time did not increase the thickness. No exact reaction mechanism

was given but since dopamine is a well-researched substance, due to its presence and importance in the

human body, likely reaction paths have been proposed.

Wang et al.9 reported drastic decreases in methanol permeability in Nafion membranes treated with

dopamine. In this work they investigate a number of different parameters, such as reaction pH, immersion

time and dopamine concentration. In all, they achieved a selectivity increase three times that of pristine

Nafion at 12M methanol concentration, owing to a sharp decrease in methanol permeability but only a

small decrease in proton conductivity. In general their observations correspond well to those of Lee et al.8

Li et al.10 reported that two dopamine treatments of a microporous substrate led to an additional layer of

“polydopamine” being formed on top of the other. Positron Annihilation Lifetime Spectroscopy (PALS)

revealed that the layer became denser after the second treatment. No measurements were however

conducted to investigate how this increased density might affect methanol permeability.

Agreeing on the factual formation of a polydopamine layer, the explanations as to why the growth stops

after some time differs. Wang et al. speculated that the sulfonate groups on the surface of the Nafion

substrate facilitate the deprotonation, and therefore the polymerisation, of the dopamine. When the layer

becomes thicker the sulfonic groups become more distant, and instead the dopamine gathers in clusters on

the surface of the already polymerised layer, to minimise surface tension.

Bernsmann et al. on the other hand, focus on the consumption of oxygen in the reaction

↔

and emphasises that lack of oxygen can severely inhibit the reaction.11 They also point out that the reaction

seems to be purely solution-interface driven. Polymerisation on the surface of substrates only occurs if

there is contact between mono- and oligomers of dopamine, but not between longer chains. These must be

formed on the substrate in order to adhere. If the polymerisation takes place in the solution these

polymers will not later add to the layer at the substrate. They also conclude the final product to be a form

of dopamine-melanin, whose proposed reaction pathway can be seen in Figure 8 below. There also seems

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to be a saturation concentration of the dopamine solution at ≈1 mg/ml, above which the growth rate of

dopamine-melanin does not increase. The dopamine-melanin is according to Bernsmann et al. stable at pH

< 13, above which the degradation of dopamine-melanin rapidly increases.

Figure 8. Reaction scheme proposed by Li et al.12

5,6-indolequinone is the fundamental unit of melanin.13

2.4.2. Layer-by-layer deposition

LbL deposition refers to a thin film technique where layers of two alternating substances can be added to a

surface stepwise, instead of through continuous growth. This can be done if these substances are chosen so

that they have properties that attract and potentially bind the other substance. A typical example would be

a cationic and an anionic substance. These substances are added to a substrate from solution, which

facilitates for the dissolved molecules to self-arrange at binding sites. By repeating this process a growing

number of layers can be added, and therefore LbL gives a very precise control over surface properties as

the thickness added can be strictly controlled. A sketch of the end product can be seen in Figure 9. The

possible materials to be added in this way can be any of inorganic clusters, clay particles, proteins, organic

molecules or polymers.14

If LbL modifying a membrane for DMFC usage, it is likely that ionic substances must be used in order to

achieve any considerable amount of conductivity. In such a combination the fundamental attractive forces

between the substances would be the electrostatic attraction between opposite charges, and possibly also

hydrogen bonding. This combined bonding is strong and can alter the properties of the combined substrate

and added film considerably.

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Figure 9. Sketch of possible LbL deposition on a Nafion substrate.

A suitable method and suitable substances are suggested by Argun et al.15 The two polyions used with the

most promising results were sulfonated poly(2,6-dimethyl 1,4-phenylene oxide) (sPPO) and poly(diallyl

dimethyl ammonium chloride) (PDAC). In this thesis sPPO will be replaced with sulfonated PEEK (poly(ether

ether ketone), victrex 150PF) (sPEEK), due to its resemblance (see Figure 10). PEEK is also known to be a

mechanically strong polymer.16

Figure 10. A) PPO. B) sPEEK.

2.5. Risk assessment The use of dopamine, and eventually melanin, is associated with some risks if handled inappropriate. The

cause for concern regarding dopamine is mainly because it acts as a neurotransmitter and hormone in the

brain. Dopamine in itself cannot cross the blood-brain barrier, but it is sometimes given intravenous to

patients to affect the sympathetic nervous system. The maximum amount of dopamine dissolved at any

time during this thesis was 100mg. Considering this, and its inability to self-diffuse into the brain, it is still

without doubt so that the waste material must be treated with care. Melanin also has numerous functions

in different biological systems, and is therefore treated with similar care as the dopamine.

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3. METHOD

3.1. Measurement methods and materials This thesis focuses on reducing methanol permeability, and therefore it is of great importance to measure

this correctly. Beyond this, the general performance of the membrane is also important to evaluate,

including the trade-off between permeability and conductivity. The materials used are in general chemicals

that are easily accessible from major chemical vendors. During handling of the chemicals appropriate safety

measures were taken.

3.1.1. Methanol permeability

3.1.1.1. Quartz Crystal Microbalance principles

The Quartz Crystal Microbalance with Dissipation (QCM-D) is a piece of equipment able to measure very

small mass changes. The core of the device is a small (approx. 10 mm in diameter) circular quartz crystal

with gold electrodes. Onto this crystal, a constant current of a solution is driven using a pump. The crystal is

sensitive to its closest surroundings (approx. 250 nm in pure water at 20⁰C).

The behaviour is affected by the properties of the solution.17 This can affect two fundamental properties of

the crystal. The crystal is supplied with an oscillating movement in-plane, and the over-frequencies of this

oscillation are mass dependant according to the Sauerbrey equation below. This equation is however best

suited for stiff materials.

Depending on the density of the layer of solution closest to the crystal, these frequencies will be damped in

a varying degree. This dampening is called dissipation, and can be calculated using the equation

When measuring methanol concentrations with a QCM-D, we are utilising the fact that changes in viscosity

of the solution affect the frequency and dissipation response. No methanol molecules bind to the quartz

crystal. Water has higher viscosity than methanol and therefore we can measure the change.18 By

measuring the response of the crystal to a known methanol concentration of a solution, it is possible to first

calibrate the crystal used, and then at a later stage to measure unknown concentrations.

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Figure 11. A) QCM equipment setup. B) Dual chamber with membrane sample.

3.1.1.2. Measurement setup

The methanol permeability of the investigated membranes was measured with a two-chamber cell

connected to the Q-Sense QCM equipment. The measurement setup and dual chamber with lid are

depicted in Figure 11. Chamber 2 was initially filled with Millipore water (18,2MΩ/cm) and Chamber 1 was

filled with 12M methanol solution (sketch can be found in Figure 12). Both chambers were continuously

stirred and the concentration in each chamber was assumed to be homogenous. The concentration of

methanol in Chamber 2 was measured over time with a QSense E1 equipment. This consists of QE 401

Electronics unit and a QCP 101 Chamber platform. The standard measurements time was five hours.

Figure 12. Diffusion cell sketch.

Prior to the measurements the equipment was run with Millipore water for several hours in order to

observe a stable baseline. The data was recorded with QSoft401 2.0.0.275, manipulated with QTools

3.0.5.198, exported to Excel 2007/2010 and finally used for calculations in MATLAB 7. The input data used

was the third, fifth and seventh overtone of the supplied oscillation frequency (F3, F5 & F7) and the third,

fifth and seventh overtone of the dissipation (D3, D5 & D7). From this, two separate values for P was

obtained, one from the F-data and one from the D-data. For further explanation of the QCM-D principle,

B A

14

the homepage of Q-Sense is highly recommended.19 A semi-empirical formula was then used to calculate

the methanol permeability. This can be found in the appendices.

3.1.1.3. Partial molar volume

When two volumes of methanol and water are mixed, the combined volume is smaller than the two

original volumes added to each other. This is due to a phenomenon called partial molar volume, where

methanol molecules and water molecules interact and pack more densely than in pure concentrated

solutions. Therefore mixing of methanol at different concentrations must be done with care, and for these

reasons molar concentration (mole/L) is used, rather than for example volume percentage. The reference

concentration used in all methanol permeability measurements was chosen to be 12M. This concentration

was used in several articles referred to in this thesis, and therefore gave possibilities for comparison. Also, a

high concentration was suitable for the measurements, as it would correspond better to the conditions

under which we expect a future DMFC to operate under.

3.1.2. Proton conductivity

The ionic conductivity of the membranes was measured by Electrochemical Impedance Spectroscopy (EIS).

The method and settings used in this thesis is mainly adapted from Sobolova et al.7 but a brief overview will

be given here. Information not present in this text can be found in the referred article.

Figure 13. A) Theoretical shape of the acquired Nyquist plot from an EIS measurement of a Nafion membrane, using an extremely high frequency interval.

7 B) Example of experimentally collected data plot with slope fitting.

7 C) Equivalent circuit used with the

Gamry software.

B A

C

15

In EIS an alternating voltage is applied across the membrane, to determine the resistance of the membrane

over a frequency interval. The EIS was carried out using a Gamry Reference 600 Potentiostat/Galvanostat,

Gamry Instrument framework software for acquisition and Gamry Echem Analyst for data modification and

analysis. The membrane was sandwiched in the measurement cell seen in Figure 13Figure 14 A. The

frequency range was 100KHz – 1MHz, and the AC voltage was 10 mV rms (root mean square), which

corresponds to 28.3 mV peak-to-peak amplitude. The temperature of the ionised water was 21⁰C at all

times. The parallel resistance of the system was measured by open-circuit, and the serial resistance was

measured by short-circuit. Each measurement was carried out ten times, in order to observe a stable

average value, which was later subtracted from the membrane measurements using the Gamry software. It

should be noted that the parallel resistance is expected to be very high, but has a very small influence on

the measurements. The serial resistance is small, but has a more significant influence.

Figure 14. A) EIS sample cell. B) Measurement setup. C)Sketch over the estimate of the x-axis intercept from the measurement data acquired in EIS.

B A

C

16

The electrodes were of copper and the connecting wires were soldered directly on the copper before gold

was evaporated onto the whole electrode. The measurement cell was immersed in water. The resistance of

the measurement setup was measured after both short-circuiting and open-circuiting the cell. These values

were later subtracted, in series and parallel respectively, from the measurement data.

The slope of the linear low frequency region corresponds to the bulk resistance according to

( ) (

)

( )

( ( ))

Calculations of the slope in the low frequency region can yield the x-axis intercept, and thereby the bulk

resistance when the imaginary part of the impedance is zero. This was done using the built in Simplex

model feature of the Gamry Echem Analyst. These results were controlled using linear fit tools.

The bulk resistance was then used in the equation below together with membrane thickness (L, cm) and

active membrane area (A, cm²) to find the conductivity (σ, S/cm).

3.1.3. Polarization Curve Measurement

For a quick feedback on the general trends in the performance, a polarization curve measurement was

conducted. This is a quick measurement of voltage and current generated in a MEA during a sample

operation. The mechanisms that govern the shape of the IV-curve obtained in these measurements were

complex. However as long as all other variables was kept constant, it could still give an interesting insight

into the behaviour of the membranes tested. Measurements were always compared to a reference

measurement conducted at the same occasion, using the same electrodes. Thus it was possible to

determine general trends in the performance of a new membrane. It was not reasonable to use the

voltages and currents measured for any calculations, but the values were always compared to the known

performance of the reference. The reference used was pre-treated Nafion 115. By using these kinds of

measurements it was easy to evaluate the effects of changes in membrane fabrication under realistic

conditions. This gave fast feedback in a way which otherwise would not have been possible. The

measurement cell is seen in Figure 15.

17

Figure 15. MEA setup with attached electrodes before measurement.

3.1.4. SEM imaging

Two different microscopes were used to characterise the membranes. A Carl Zeiss G34-1540 XB SEM/FIB

was used for the pictures of LbL, and FEG-SEM Ultra55 from Zeiss was used for pictures of dopamine

modified Nafion membranes. The preparation process for the SEM/FIB samples was:

Plunge freezing of sample in liquid nitrogen

Transfer to vacuums in the preparation chamber

Sublimation for 5 minutes at -90⁰C

Sputter coating with Platinum to an estimated thickness of 3-4 nm

Transfer to the main chamber and kept at -120⁰C

Imaging And for the FEG-SEM samples it was

Plunge freezing in liquid nitrogen

Fracturing

Vacuum in microscope chamber

Imaging

3.1.5. Preparation and treatment of Nafion membranes

All Nafion-based membranes were pre-treated in the same way.

30 min in deionised water at 80⁰C

30 min in 3% at 80⁰C


30 min in 1M at 80⁰C


These steps were taken in order to clean the membrane, and then charge it with as many protons as

possible, in order to increase proton conductivity. Before each methanol permeability measurement, the

18

membranes were put in Millipore water for several hours in order to fully hydrate it using the water as in

the measurement itself. Thereby a stable QCM baseline was more easily established.

3.1.6. Dimensional swelling in water

The through-plane (transversal) hydrated thickness of Nafion 115 and Nafion 115 modified with either

dopamine-melanin or 30 bilayers LbL was measured using SEM.

3.1.7. X-ray photoelectron spectroscopy

X-ray Photoelectron Spectroscopy (XPS) was used to investigate the chemical composition of the LbL

treated Nafion membranes. By irradiating a sample with X-rays, electron emission from the surface of the

sample is initiated. The energy of these electrons is then measured to find the relative intensities of

different chemical bonds in the sample surface. Through this it is possible to calculate chemical

composition, empirical formula etc.

For measurements in this thesis, a K-Alpha from Thermo Scientific was used, with depth profiling. Three

different spot measurements were done in different areas of the same LbL 15 bilayer sample.

3.2. Approach specific method

3.2.1. Self-polymerising dopamine-melanin

The procedure for the formation of a self-polymerising dopamine-melanin layer on Nafion substrates was

adapted primarily from Wang et al.9 but many variations were tried. The dopamine hydrochloride was

acquired from Sigma-Aldrich (CAS: 62-31-7). The standard procedure was:

1. Pre-treatment of Nafion membrane pieces, approximately 2*3 cm in size, according to section

3.1.5.

2. Dissolution of dopamine hydrochloride in either 20 ml or 50 ml Millipore water or deionised water

3. Addition of Tris (hydroxymethyl)aminomethane (Tris, Sigma-Aldrich CAS: 77-86-1) during stirring

until pH 8,5 was reached.

4. Immediate immersion of substrate membrane.

5. Storage on vibration table during the reaction time.

6. Removal from dopamine solution and cleaning in deionised water or Millipore water.

7. Ultrasonic cleaning 5+5 minutes.

8. Storage in deionised water or Millipore water.

Above is given the general case for 1 dopamine treatment. Variations on this method included:

Concentration 1-4mg/ml

Reaction time 1 to 48 h

Number of consecutive treatments

Solution access to air during reaction

3.2.1.1. Chemical resistance of dopamine-melanin

To investigate the chemical resistance of the dopamine-layer, two experiments were conducted. First, a

dopamine-melanin covered piece of Nafion was partly submerged at pH 13 for 24 hours, and the change in

the optical appearance was observed. Then another piece of dopamine-melanin covered Nafion was

19

prepared, and cut in into small pieces. The methanol permeability of the first piece, Piece A, was measured

while piece B was immersed at pH 13 for 24 hours, before measuring the permeability.

3.2.1.2. Methanol resistance of dopamine-melanin

The resistance to methanol was investigated, because of its relevance for the service length of a dopamine-

melanin covered membrane in a functioning DMFC. It could not be ruled out beforehand that methanol

could not dissolve the dopamine layer. To test this, a piece of Dopamine-melanin covered Nafion 115

repeatedly had its methanol permeability measured.


Immersion of a pre-treated Nafion 115 piece into a cationic solution of 1/10 monomole (one mole of the

monomer unit of the polymer) poly(diallyl dimethyl ammonium chloride) (PDAC) during 20 minutes was

followed by rinsing in deionised water for 10 min. After that the sample was immersed into an anionic

solution of DMF (dimethylformamide) and sulfonated poly(ether ether ketone) (sPEEK) followed by another

rinsing. This procedure corresponds to one treatment (= one bilayer), and the whole process was repeated

for as many times as desired. During processing, the modified membranes were stored in Millipure water.

Figure 16. A) PDAC repeat unit. B) PEEK repeat unit, unsulfonated.

The PDAC was purchased from Sigma Aldrich (CAS 26062-79-3) and the sPEEK had already been synthesised

from PEEK using the method described by Wootthikanokkhan et al. by Dr. Yihua Yu.16 The repeat units can

be seen in Figure 16. The samples with 0, 5, 7, 9 and 15 bilayers were produced from the same Nafion

sample, where pieces were continuously taken and processed. The sample for 30 bilayers however had to

be produced from a separate sample.

Efforts were made to dissolve the sPEEK in water, base and strong acid without success. Therefore, sPEEK

was dissolved in DMF and used in LbL deposition.

4. RESULTS & DISCUSSION The results for each characterisation method are presented together in the same subsection.

4.1. Dimensional swelling in water The dimensional swelling in water was found to be between 140µm and 160µm, independent of surface

modification. Since the variation in thickness from the same sample thickness in the dry state was that

B A

20

large, the dry state thickness was used for calculations of e.g. methanol permeability. In this way the

absolute values in the results are comparable to other modifications of the same type of substrate.

Approximately 127µm dry thickness of Nafion 115 could be confirmed by SEM and this value was

subsequently used for calculations. These results are similar to those reported in literature, but with a

considerable larger variation in the measured thickness. The freezing prior to SEM imaging could be the

source of the large variation in thickness. However, in theory it should not. Another measurement

technique could have been employed in order to verify the thickness.

4.2. Optical appearance

4.2.1. Dopamine-melanin

The polymerisation of dopamine on a Nafion 115 substrate made the originally colourless and transparent

membrane to become brownish in appearance, which increased in intensity with increased reaction time

(Figure 17). In this way it was easy and fast to confirm that polymerisation had taken place, even before

measurements and SEM characterisation.

Figure 17. A) Pristine Nafion 115 membrane. B) Nafion membrane polymerised in dopamine during 24 hours.

4.2.2. Layer by layer deposition

The optical appearance of the sPEEK/PDAC LbL modified Nafion membrane was slightly changed in relation

to pristine Nafion. After 15 bilayers there was a tendency towards a whitish colour change on the surface of

the Nafion membrane. This was however not uniform and varyied in strength over the surface of the

membrane.

4.3. SEM imaging


Figure 18 shows the appearance of a dopamine-melanin covered Nafion membrane after two treatments.

As can be seen from the cross sectional picture B) a dense, approximately 90 nm thick, layer have formed

on the surface. From both A) and B) it looks to be uniform and provide 100% coverage of the substrate. On

top of this layer there are plenty of lumps of a similar substance. This is likely excess dopamine-melanin

that has formed in clusters to minimise surface tension.9

B A

21

Figure 18. A) The excess DA formed on the surface of the layer. B) 90 nm dopamine layer seen in cross section where the excess formations are also noticeable.

There does not seem to be any clear signs of a second dopamine-treatment in Figure 19, where a single

treated and double treated membrane is compared.

Figure 19. Single (A) and double (B) treatment comparison of the same original piece of membrane. No apparent difference in layer thickness is visible (90-100nm).


Investigations using SEM revealed what seemed to be a non-uniform coverage of the membrane surface.

There was coverage of the substrate in all regions investigated, but the character of this coverage varied

from smooth to highly porous, as seen in Figure 20.

B A

B A

22

Figure 20. Top view of a sample with 15 bilayers of sPEEK/PDAC. A) 1000x. B) 10000x. There is evidence of regions where the porous structure is more or less exposed.

In Figure 21 the porous nature of the LbL coating is clear. Why this structure is formed and not a more plain

formation is not known.

Figure 21. The highly porous nature of the cross section of 15 bilayers is evident.

B A

B A

23

One possible explanation is that the polymer chains are not oriented along the surface of the membrane

during treatment, but instead a proportion of the reacting molecules arrange themselves in “heaps”. This

might eventually lead to formations of the type that can be seen in Figure 22.

Figure 22. A) PDAC/sPEEK LbL modified Nafion membrane. B) Accumulation of LbL material shows the deviation from the neat layers proposed in the theory section (equal magnification as A).

Another possibility is that every time the hydrated membrane was immersed into the sPEEK/DMF solution,

it would locally dilute the DMF with water, and thereby forcing a minor precipitation of sPEEK in the region

close to the membrane. This might cause precipitated polymer to form the type of lumps on the surface of

the substrate as seen in the SEM pictures above.

Figure 23. LbL 30 bilayers.A) Delamination of the LbL from the Nafion substrade in a region subjected to fracture. B) Increasingly asymetric and porous formations compared to the sample with 15 bilayers.

In Figure 23 some peculiar characteristics of the 30 bilayer LbL treated Nafion 115 at 2.5 K X magnification

is shown. An odd feature is seen in Figure 23 A, where the whole surface layer is wrinkled. This is due to

external forces, since this phenomenon is only seen close to the fracture of the membrane. It indicates that

B A

B A

24

the cohesion within the 30 bilayer coating is much stronger than the adhesion between the coating and the

substrate. In B) we can see the very irregular formation of the LbL coating. Given this uneven distribution of

polymer material, it seems reasonable that improvements in methanol permeability are not directly

proportional with the added amount of polymer, at least not once an initial layer has been formed.

4.4. QCM measurements


The methanol permeability of dopamine modified Nafion membrane can be found in Table 4.1. As seen,

relatively little differs between the different treatments when it comes to methanol permeability. The best

sample, New DA, was achieved after fresh dopamine had been purchased. Recommended storage

conditions had not been followed for the first batch of dopamine, and thus degradation is a possible reason

why the initial results were not improved until fresh dopamine was used again.

Table 4-1. All membranes dopamine modified at pH 8,5..

DA conc. (mg/ml)

No. of treatments

Immersion time (h)

P for D3-D7 (cm

2/s)*

P for F3-F7 (cm

2/s)*

P

(%)

Comments

- - - 2,3*10-6

2,4*10-6

100 Value is an average from three

different measurements.

2 1 24 9,5*10-7

9,3*10-7

39 2 2 24 1,0*10

-6 1,1*10

-6 45

4 2 24 1,1*10-6

1,1*10-6

47 2 1 24 8,1*10

-7 8,3*10

-7 34 Fresh DA purchased

2 1 24 - 8,5*10-7

31 Membrane saturated with DA prior to treatment

2 1 1 8,7*10-7

8,6*10-7

36 Reaction time only 1 hour * These permeability values are calculated from the third, fifth and seventh overtone of the frequency (F) and dissipation (D) respectively.

The reference value for pre-treated Nafion 115 was averaged after several measurements. Variation of the

reference value was at most 0,2*10-6 cm2/s. The QCM is in itself a very precise measuring device. The

auxiliary equipment used includes pumps and dual chamber apparatus, which might introduce

uncertainties into the measurement. Also, it cannot be ruled out that Nafion in itself have a varying

performance

Returning to the values in Table 4.1, it is difficult to find evidence that varying concentration and number of

immersions had any significant impact on the results. This is in line with the results from SEM, where no

difference was evident between single and double treatments. The results did not improve with doubled

concentration, which might be explained by Bernsmann et al. investigations where concentrations above 1

mg/ml was deemed saturated and thus did not impact the growth rate of the dopamine-melanin layer.11

The immersion time was kept at 24 hours for most measurements. This was because there was no effective

method to make our own measurements of thickness over time. According to Bernsmann et al. the growth

25

rate reached zero after 15-20 minutes, while Lee et al. claim 15-20 hours before growth has ended. Wang et

al. support the view taken by Bernsmann et al., as they report no significant change in permeability after 1

hour of treatment, 1 hour being the first measurement interval. Li et al. on the other hand reports

continuous growth over 24 hours.10

It is of note to realise that Li et al. and Wang et al. does not necessarily contradict each other. A growth for

24 hours might be consistent with no change in permeability after one hour, as increasing thickness of the

layer does not necessarily decrease the permeability. A situation where growing thickness is not affecting

the permeability is possible. Bernsmann et al. does even discuss this difference between their own and the

results of Lee, without being able to provide any definitive explanation. From these reports it was initially

not possible to draw a definitive conclusion on the suitable reaction time. Therefore 24 hours was used as a

standard.

One important question is, why have not a second layer of dopamine been formed after a second

treatment? The method by Li et al. have been followed in detail. Yet the thickness of samples treated

several times still corresponds very well to the thickness Li et al. report for single treatments after 24 hours,

100-110 nm, as well as the results of the present study for single treatments. Bernsmann et al. polymerises

considerably thinner layers, and with the corresponding growth per treatment, a 100 nm layer would be

formed after 40 or more treatments. With a corresponding method, there is no result reported where

considerably thicker layer of dopamine-melanin are formed, indicating that there might be a maximum

thickness in the region of 100-150 nm. With the exception of Wang’s theory, where sulphur catalyses the

reaction at the Nafion interface, no theoretical ground for a maximum thickness hypothesis can be found.9

Of note here is that Li et al. and Bernsmann et al, among others, use substrates lacking sulphur at the

surface. Thereby evidence are shown that presence of sulphur is not a necessity to grow >100 nm thick

layers of dopamine-melanin.


The QCM results for PDAC/sPEEK based LbL treated Nafion substrates can be found in Table 4-2. Of

immediate concern is the 5 layer treatment, which does not seem to have led to any change in permeability

at all (1% is well within both the error margin and the normal variation of the substrate). Long polymer

chains might not arrange together as densely as smaller molecules such as dopamine, and therefore

require many more layers before a complete surface cover is achieved. As could be seen in the SEM results

section, 15 bilayers formed a complete coverage of the membrane, although with a varying quality. This

trend was also possible to see for the sample with 30 bilayers.

Table 4-2. Methanol permeability results for various Layer-by-Layer modified Nafion membranes

No. of bilayers

Anion Conc.* Cation Conc.* MeOH perm. (cm²/s) D-series

MeOH perm. (cm²/s) F-series

% of ref.

Nafion ref. - - - - 2.5*10-6 2.6*10-6

100

5 PDAC 1/10 sPEEK 1/10 2.5*10-6 2.5*10-6 99 7 PDAC 1/10 sPEEK 1/10 2.0*10-6 2.1*10-6 80 9 PDAC 1/10 sPEEK 1/10 1.9*10-6 1.9*10-6 73 15 PDAC 1/10 sPEEK 1/10 1.3*10-6 1.2*10-6 43 30 PDAC 1/10 sPEEK 1/10 1.0*10-6 9.8*10-7 39 *Concentration in monomole.

26

A plot of the results in Table 4-2 can be seen in Figure 24, where the correlation between methanol

permeability and number of bilayers is shown. Note that when the sample with 30 bilayers was assembled,

it was not the previous sample with 15 bilayers that had another 15 bilayers added onto it. Instead a new

sample had to be produced.

Figure 24. Correlation between number of bilayers of sPEEK/PDAC and methanol permeability. Note the absence of permeability reduction for five bilayers, and the small decrease between 15 and 30 bilayers.

4.4.3. QCM measurement limitations

Within the method used for the measurement of permeability are limitations. The process of diffusion from

chamber B (12M methanol compartment) to chamber A (the water compartment) can lead to a significant

change in the volume of both chambers. In the mathematical model used, these volumes are assumed to

be constant. During a five hours special test, with Nafion 115 as membrane, the change in volume was 0.6

ml. Nafion is the worst membrane in this aspect utilised in this thesis, and thus this volume should be the

maximal volume change present in any of the herein presented measurements. From the equations used in

1.1, we can calculate that this means a modification of the result of less than 1%. The choice to use the

mathematical model, was largely based on stability and positive previous experience of this model,

together with the equipment used.

27

If the dual chamber failed to hold tight around the inserted membrane, an abnormal amount of methanol

might be able to leak over to the measurement cell. There were sometimes problems with applying even

pressure on the membrane sealing, as some of the four screws used for this did not function properly.

Another possible problem is that Nafion swells to a higher degree in several organic solvents, compared to

water.4 Nafion side chains and the backbone are affected differently by water and methanol. Therefore it is

possible that the membrane, clamped between the dual chambers in the measurement apparatus, would

deform when it became exposed to methanol. This would make the effective membrane area to deviate

from the value used for calculations, causing the final permeability value to change. But since the effect

should be similar for all samples and measurements, as we always used the same concentration of

methanol, this would not alter the ratio of permeability between the reference membrane and other

membranes measured.

The QCM equipment is very sensitive to bubbles in the tubing of the pump and the measurement cell.

These might arise from a number of sources. For example, in the methanol solution used or due to

turbulence at the inlet of the tube. Also, physical interaction with the table on which the equipment stood

on might have affected the measurements. All these sources of errors are however easily recognised and a

quick evaluation of the shape of the QCM curves revealed abnormal behaviour. Thereby the numbers of

potential errors affecting the end results are decreased.

Of note to the reader is that during the progress of this thesis, both the Nafion substrate and the QCM

crystal were changed. The QCM crystal initially used broke half way through just after finalising the

dopamine-melanin related measurements. New crystals purchased from Q-Sense are expected to have very

small variations in quality. When calibrating the new crystal, the calibration curve achieved was very similar

to the one previously used. In the case of Nafion, a new sheet had to be purchased. The new Nafion should

be identical to the first one. With a new crystal and new Nafion, a slight difference in the reference value

for methanol permeability in Nafion was however recorded. For this reason measurement results from

dopamine-melanin and from LbL should be seen as completely separate. They should not be compared

directly, but rather through their respective performance compared to their own reference value.

4.5. Conductivity The conductivity results are presented in Table 4-3. The parallel and serial resistances can be found in

Appendix 7.3.

The results for Nafion 115, dopamine-melanin and LbL are reasonable, as their respective differences are

on the same level as reported in the literature.

Table 4-3. The bulk resistance and calculated conductivity obtained from Simplex model estimates. All values are means from at least five samples.

Sample RB (Ω) Std (S/cm) % of Nafion 115

Nafion 115 4.28 0.256 0.0099 100 Dopamine-melanin 10.9 1.35 0.0039 39 LbL 5 bilayers 10.2 0.54 0.0042 42 LbL 30 bilayers 10.5 1.4 0.0040 41

28

Here it is interesting to note two different findings. First, the dopamine-melanin is less conductive

compared to the Nafion reference, than stated by Wang et al.9 This might be due to some difference in the

preparation of the sample, or due to differences in the conductivity measurement. Secondly, the decrease

in conductivity between 5 and 30 bilayers is hardly noticeable, indicating that the conductive behaviour is

not related to the layer thickness. Further investigations with more measurements will have to be carried

out before any definite conclusions can be drawn.

4.5.1. Conductivity measurement limitations

There are several inherent deficiencies in the measurement method employed. First, a through plane

measurement will render a very thickness sensitive result. Compared to an in-plane measurement, which

measures the conductivity along the plane, the error will be much higher. Also, the thickness is possibly

altered by the high pressure from the sandwich construction of the measurement cell.

Second, a so-called two-probe measurement utilised in this work has a disadvantage in that there is

significant polarisation at the electrodes. This is partly accounted for by subtracting contact resistance etc.

It is instead suggested to use a four-prove measurement, where the polarisation problems are nullified.

Such a measurement is however more complicated and requires a more complex measurement cell.

Third, the model and equivalent circuit used was the simplest possible. In general one should always use

the simplest possible equivalent circuit to fit the data. However, since it was not possible to supply a high

enough frequency, it is difficult to evaluate the suitability of the equivalent circuit used. This might lead to

systematic errors in the calculated conductivity.

The gold that was evaporated onto the copper slowly degraded, and had to be re-evaporated onto the

electrodes. A more solid gold coverage, or even better platinum electrodes, would have increased the

precision of the measurements.

Another issue of significance is the operator’s experience and knowledge of the setup and equipment used,

and how different settings alter the results. It was the first time a measurement like this was carried out on

the equipment at hand. A significant improvement is expected with increased usage and experience.

4.6. Selectivity The selectivity is given by the ratio

and is a measure used for comparing membrane properties. Some examples from earlier results are here

presented in Table 4-4.

29

Table 4-4. Selectivity of some samples.

Sample MeOH perm. (cm²/s) Conductivity (S/cm) Selectivity (S×s/cm3)

% of reference

Nafion 2.5*10-6 0.0099 3960 100 Dopamine-melanin

8,1*10-7 0.0039 4815 122

LbL 5 bilayers 2.5*10-6 0.0042 1680 42 LbL 30 bilayers 1.0*10-6 0.0041 4100 104

Of interest here is that the selectivity of both dopamine-melanin and LbL 30 bilayers benefits from its

decreased methanol permeability. Again, the importance of such values can be questioned, but it may

nevertheless be a promising observation.

4.7. Polarisation Curve Measurements


In experiments with dopamine-melanin, there was a large variation of how much current it was possible to

draw from the cell, which possibly was related to the amount of excess dopamine-melanin. In Figure 26 we

can see one example of an IV-curve. The Open Circuit Potential (OCP) is increased, probably due to the

decreased methanol permeability.

Figure 25. Example of polarisation curve of dopamine-melanin covered Nafion membrane compared to Nafion 115.


From Figure 26 some general points can be seen. LbL modification of Nafion seems to lead to a similar

reduction in maximum current as Dopamine-melanin. This is in agreement with the conductivity results

from EIS.

30

Figure 26. Polarisation curves comparing Nafion, Nafion modified with Dopamine-melanin and Nafion with 30 bilayers of polyionic polymers added.

4.7.3. Polarisation curve measurement limitations

There are a number of limitations for the polarisation curve measurements. Among them are varying

catalyst loadings, rapid degradation of the catalyst used, and uneven contact between membrane and the

electrodes. As mentioned earlier, all measurements have been compared to their own reference

measurement of a Nafion membrane. In this way, changes in catalyst loading and degradation of the

catalyst have been compensated for. From an IV-curve a wealth of information regarding the electric

behaviour can be harvested. However, due to the limitations brought up, it has only been used as guidance

to the performance of the membranes in this work. These polarisation curves have simply been one way to

get an indication of the conductive behaviour of the membranes modified. In this way it has worked as a

suboptimal, yet valuable tool for characterisation.

4.8. Resistance to methanol for Dopamine-melanin and Layer-

by-layer deposition From any three consecutive methanol permeability measurements, very similar results were obtained.

Therefore it was concluded that both types of surface layers are reasonable stable to methanol, and do not

readily degrade.

31

4.9. Dopamine-melanin resistance to alkaline environment Synthetic melanin is supposed to be chemically resistant to acid, and only treatment in pH 13 or higher

should dissolve it . In the present work, it was possible to remove polymerised dopamine-melanin from

glass at pH 13. Therefore the two experiments described in section 3.2.1.1 were conducted. As seen in

Figure 27, there was a distinct difference in colour between the part of the membrane that had been at pH

13 for 24 hours and the part that had not.

Figure 27. Dopamine-melanin covered membrane where the part to the right have been subjected to alkaline conditions.

This indicates some sort of chemical change, at least on the surface. Since the polymerisation process

results in significant amounts of excess dopamine on the surface of the polymerised dopamine-melanin

layer, we initially cannot rule out that the change in appearance originates purely from the removal of

excess dopamine, rather than having a real effect on the methanol-blocking layer itself.

Results in Table 4-5 do however support the view that a chemical change of importance has taken place. As

seen, the results for Piece A is well within what can be considered normal for dopamine-melanin treated

Nafion 115 reported in this work. Piece B, being originally from the same membrane as Piece A, has

severely worsened permeability after 24 hours immersion at pH 13.

Table 4-5. Dopamine-melanin treatment effect resistance to alkaline conditions

Sample name Treatment in pH 13 (hours) P from D-series (cm2/s)

Piece A - 9.3*10-7 Piece B 24 1.7*10-6

4.10. XPS results for layer by layer deposition Results obtained with the XPS from spot 1, spot 2 and spot 3 are presented in Table 4-6. Complete XPS

results from all points and measurements can be found in the Appendices.

The data in Figure 28 was harvested at spot 1. The material composition and general trends were identical

at all three points, with the exception of fluorine, whose signal strength varied.

32

From Figure 28 we can see that the amount of nitrogen is highest at low etching time, i.e. closest to the

surface. This is probably explained by the fact that the last layer to be added in the LbL treatment before

XPS characterisation was made of the nitrogen containing polymer PDAC.

Figure 28. Atomic percent profile for spot 1. The general trends were similar or identical at all three points.

In Table 4-6 the ratio of nitrogen to sulphur is presented, together with the fluorine atomic percentage. The

nitrogen/sulphur ratio is not changing much, which indicates similar chemical composition with respect to

the polyionic polymers at all points. Since the fluorine signal is changing nevertheless, this is likely not due

to the chemical composition of the LbL, but due to some sort of inhomogeneity of the surface. This might

be the sheer bulk amount of polymer present at the different spot, as this in fact was an obvious feature

when examining the sample with optical microscope as well as electron microscope.

Table 4-6. Ratio between N1s and S2p at spot 1, spot 2 and spot 3.

Name At.% spot 1 At.% spot 2 At.% spot 3

F1s 2.2 0.2 5.3 N1s 3.7 3.7 3.3 S2p 2.2 2.2 2.2 Ratio N1s/S2p 1.7 1.7 1.5

The fluorine is only present in the backbone of the Nafion substrate, and thus the varying signal strength at

the surface can either be attributed to varying layer thickness at point 1, point 2 and point 3, or a varying

porosity at these points. From SEM pictures, for example Figure 22, we know that the layer is of different

0,1

1

10

100

0 500 1000 1500 2000 2500 3000 3500 4000

Ato

mic

per

cen

t (%

)

Etch time (s)

Atomic percent profile

C1s

O1s

S2p

N1s

F1s

33

thickness. This does however not exclude the possibility that the porosity is varying. The measurement

spot size of the XPS was 400µm. Compared to the individual formation size seen for example in Figure 20,

Figure 21, Figure 22 and Figure 23, this is relatively big. Therefore the differences present must be to a

more general variation between the measurement regions, rather than single deviations from the norm.

Figure 22 A) and B) might be examples of this, showing a clear variation of the overall topography of the LbL

coating. The origin of this difference is unknown. It cannot be ruled out that there is a structural difference

present, even at similar chemical compositions.

5. CONCLUSIONS

5.1. Major conclusions for Dopamine-melanin and LbL Both dopamine-melanin modified Nafion and Layer-by-Layer modified Nafion has shown promise in

applications as the DMFC considered in this report, for several reasons. They have both been shown to

have more than 50% decreased methanol permeability, while relevant amounts of proton conductivity

remained. This was also shown with polarisation curve measurements, where similar amounts of currents

compared to Nafion could be drawn for some voltages. Decreased methanol permeability should result in

increased operation time of a fuel cell with limited fuel, which is important for the DMFC considered

herein. A further gain is that the decreased methanol permeability should allow higher concentrations of

fuel to be used in the DMFC, without the methanol cross over increasing to unacceptable levels.

5.2. Dopamine-melanin The decrease in methanol permeability after polymerisation of dopamine onto the Nafion surface is very

substantial, almost 70% as most. Given the ease of preparation, and that a 60% reduction of the

permeability was reached after the first effort, the results are promising. Herein lies also the limitation of a

dopamine-melanin cover, the method is very inflexible. As shown in this thesis as well as others , few

parameters, if any, seem to be able to significantly change the final properties. Thus it is much depending

on the requirements of the fuel cell to determine if this is a suitable method to be employed in a DMFC.

Given the resemblance of the dopamine-melanin produced in this thesis and that reported elsewhere,

there are doubts concerning the conductivity results reported in this thesis. These might have to be

confirmed with a different measurement setup. From experiments with dopamine-melanin treated

membranes at pH 13, we can draw the conclusion that the methanol resistant layer formed can be partly

dissolved at very alkaline conditions. Results indicate that a significantly reduced reaction time, and thereby

a further simplified preparation procedure, has no negative effect on the performance of the dopamine-

melanin modified Nafion membrane. Given its properties and the ease of preparation, polymerisation of

dopamine-like molecules should find use for fuel cells and elsewhere.

5.3. Layer-by-layer deposition The LbL deposited layer showed a correlation between decreased methanol permeability and number of

bilayers added. This relationship levelled out after 15-30 bilayers. XPS results shows that the properties of

the layers are inhomogeneous on a scale of hundreds of micrometres. The uneven thickness and irregular

shape of the surface layer is confirmed by SEM images, and shows that a large proportion of the polymer

material is not arranged in a planar bilayer structure. The layer formed is resistant to methanol at high

34

concentrations. The conductivity of the bilayers is significantly lower than for Nafion, but an increased

selectivity indicates that this trade-off might be favourable.

LbL deposition is a very versatile method to deposit polymer material (amongst other materials available

for this method) but further research into the mechanisms and variables that govern the layer formation

must be undertaken to reach a high level of control over the end result.

5.4. Suggestions for improvements The conclusions of this thesis could have been improved by a more accurate measurement of the

conductivity of the modified membranes. Therefore it is suggested that a four-probe measurement

cell should be constructed in order to improve the quality of the data. It might also be a good idea

to test different electrode materials to further reduce the electrode-membrane interface

resistance.

The dual chamber measurement cell used for methanol permeability measurements should be

redesigned so that the volume and concentration changes are reduced. This could be achieved by

increasing the chamber volumes while keeping the effective diffusion area constant. The QCM-D

should still be able to determine changes in concentration with an acceptable precision.

The degree of sulfonation of the PEEK was not known, but only assumed based on the preparation

method used. The degree of sulfonation should be measured directly, for example using titration.

Deviation between the assumed and true degree of sulfonation of the PEEK backbone should affect

the LbL formation. Mismatch between the concentration of ionic groups in the polyionic polymers

used could potentially lead to a reduced binding ability.

An effort to carry out the LbL process using the same solvent for both anion and cation would have

ruled out or explained some of the questions regarding the lump formation on the substrate.

PDAC is completely ionised in water, but sPEEK might not be in DMF. By changing solvent for sPEEK

the degree of ionisation might be improved. Efforts were made to dissolve sPEEK in acidic

conditions, without success. However some other solvent might be more suitable. Utilising sodium

chloride as proposed by Argun et al.15 and Yilmazturk et al.20 among others, might also improve the

solubility or conductivity.

5.5. Future research The dopamine-melanin forming process might ultimately be governed by the molecule used in the reaction,

rather than reaction conditions. Therefore it could be of interest to self-polymerise some of the many

existing similar molecules, for example norepinephrine or L-DOPA (see Figure 29). Another possibility is to

use a molecule which includes OH-groups, amino groups and a conductive group such as SO3-.

35

Figure 29. A) Norepinephrine. B) 3,4.dihydroxy-L-phenylalanin (also known as L-DOPA and Levodopa).

Of immediate interest in the LbL process would be to investigate how the immersion time could be

reduced, perhaps through increased reaction temperature. With the method employed in this thesis, the

formation of one bilayer takes a full hour. This time could be significantly reduced, which in the short run

would greatly reduce the effort of non-robot assisted development of various LbL coatings. In the long run

this might be required for the method to be viable for production. In general, the importance of a number

of reaction conditions and their effect on porosity, conductivity etc. should be investigated.

It could also be of interest to investigate the properties of this, or similar, LbL coatings on a substrate other

than Nafion. In a first step this should be a water permeable polymer membrane without conductive

groups. In the end this might remove the use of a separate substrate altogether.

The importance of the ionic concentration of the polymers could be investigated, as this may severely limit

the amount of polymers possible to use in LbL processes.

The importance of water management in a high fuel concentration DMFC (a so-called HC-DMFC) has been

pointed out by Li et al.21 amongst others. However, this is not often mentioned in articles on fuel cell

membrane formation. When approaching 100% pure methanol fuel, it is important to consider this aspect.

Therefore future investigations should include characterisation of water permeability as a separate issue, in

parallel with methanol permeability.

B A

36

6. REFERENCES

1. DeLuca, N.W. & Elabd, Y.A. Polymer electrolyte membranes for the direct methanol fuel

cell: a review. Journal of Polymer Science Part B: Polymer Physics 44, 2201–2225(2006).

2. Zhao, T.S. et al. Towards operating direct methanol fuel cells with highly concentrated fuel.

Journal of Power Sources 195, 3451-3462(2010).

3. Ramya, K. & Dhathathreyan, K.S. Direct methanol fuel cells: determination of fuel crossover

in a polymer electrolyte membrane. Journal of Electroanalytical Chemistry 542, 109-

115(2003).

4. Mauritz, K.A. & Moore, R.B. State of Understanding of Nafion. Chemical Reviews 104,

4535-4586(2004).

5. Chen, S.-R.& Nafion water channel model. Nature Materials 7, 75(2008).

6. Heitner-Wirguin, C. Recent advances in perfluorinated ionomer membranes: structure,

properties and applications. Journal of Membrane Science 120, 1-33(1996).

7. Soboleva, T. et al. Investigation of the through-plane impedance technique for evaluation of

anisotropy of proton conducting polymer membranes. Journal of Electroanalytical

Chemistry 622, 145-152(2008).

8. Lee, H. et al. Mussel-inspired surface chemistry for multifunctional coatings. Science (New

York, N.Y.) 318, 426-30(2007).

9. Wang, J. et al. A facile surface modification of Nafion membrane by the formation of self-

polymerized dopamine nano-layer to enhance the methanol barrier property. Journal of

Power Sources 192, 336-343(2009).

10. Li, B., Liu, W. & Jiang, Z. Ultrathin and Stable Active Layer of Dense Composite Membrane

Enabled by Poly(dopamine). Langmuir 25, 7368-7375(2009).

11. Bernsmann, F. et al. Characterization of Dopamine−Melanin Growth on Silicon Oxide. The

Journal of Physical Chemistry C 113, 8234-8242(2009).

12. Li, Y. et al. Electrochemical quartz crystal microbalance study on growth and property of the

polymer deposit at gold electrodes during oxidation of dopamine in aqueous solutions. Thin

Solid Films 497, 270-278(2006).

13. Zajac, G. et al. The fundamental unit of synthetic melanin: a verification by tunneling

microscopy of X-ray scattering results. Biochimica et Biophysica Acta (BBA)-General

Subjects 1199, 271–278(1994).

37

14. Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science

277, 1232-1237(1997).

15. Argun, A. a, Ashcraft, J.N. & Hammond, P.T. Highly Conductive, Methanol Resistant

Polyelectrolyte Multilayers. Advanced Materials 20, 1539-1543(2008).

16. Wootthikanokkhan, J. & Seeponkai, N. Methanol permeability and properties of DMFC

membranes based on sulfonated PEEK/PVDF blends. Journal of Applied Polymer Science

102, 5941-5947(2006).

17. Hansson, L. Molecularly imprinted polymer integrated with a QCM.D as a melamine sensor

(Lunds University). (2009).

18. (Editor), W.M.H. Handbook of physics and chemistry 91st edition. CRC Press.

ISBN:9781439820773. (CRC Press: ).

19. Q-Sense. at <http://www.q-sense.com/qcm-d-technology>

20. Yılmaztürk, S. et al. A novel approach for highly proton conductive electrolyte membranes

with improved methanol barrier properties: Layer-by-Layer assembly of salt containing

polyelectrolytes. Journal of Membrane Science 343, 137-146(2009).

21. Li, X., Faghri, A. & Xu, C. Water management of the DMFC passively fed with a high-

concentration methanol solution. International Journal of Hydrogen Energy 35, 8690-

8698(2010).

22. Effective diffusivity in porous media. at <http://en.wikipedia.org/wiki/Mass_diffusivity>

38

7. APPENDICES

7.1. MATLAB code example The purpose of this code is to calculate the methanol permeability using the data file from the QCM.

clear all; %Constants V1 = 7; %ml V2 = 7; %ml C0 = 12344; %concentration at time=0 in mM d = 137; %membrane thickness in micrometer Dgluc = 6.73e-6; %cm²/s A = 0.785; %area of exposed membrane in cm² convMatrix = xlsread('Y:\Organization\OTS\Samples\calibrationmatrix.xls','A3:B8'); matrix = xlsread('Avni9layers20110126.xls','D2:I65536'); timeRow = xlsread('Avni9layers20110126.xls','A2:A65536'); nbrOfHours = 5; %number of hours to include in calculation startT = 0; %number of minutes from start of measurement until MeOH put into container ts = (1:1:nbrOfHours)*3600; %if nbrOfHours > (matrix(65535,1)/3600) % nbrOfHours = round(timeRow(65535,1)/3600); %end for i=1:nbrOfHours [min_difference,array_position] = min(abs(timeRow(:,1)-3600*i)); places(i)=array_position; end for j=1:6 placeMatrix(:,j)=matrix(places,j); end for k=1:6 molarConv(:,k) = (placeMatrix(:,k)-convMatrix(k,2))/convMatrix(k,1); end %determining through D-series mMolarD(:,1)=((molarConv(:,2)+molarConv(:,4)+molarConv(:,6))/3).*1e3; C1d = C0-mMolarD*V2/V1; reld = log((C1d-mMolarD)/C0);

39

%above is understood, below is not pd = polyfit(ts',reld,1); clf plot(ts,reld,'diamond'), hold on, grid on x = linspace(0,ts(nbrOfHours)); yd = polyval(pd,x); plot(x,yd) porosityd = (abs(pd(1))*d*0.0001)/(A*Dgluc*(1/V1+1/V2)); permD = porosityd*Dgluc %determining through F-series mMolarF(:,1)=((molarConv(:,1)+molarConv(:,3)+molarConv(:,5))/3).*1e3; C1f = C0-mMolarF*V2/V1; relf = log((C1d-mMolarF)/C0);

%above is understood, below is not pf = polyfit(ts',relf,1); plot(ts,relf,'square'), hold on, grid on x = linspace(0,ts(nbrOfHours)); yf = polyval(pf,x); plot(x,yf,'green') porosityf = (abs(pf(1))*d*0.0001)/(A*Dgluc*(1/V1+1/V2)); permF = porosityf*Dgluc

40

7.2. Mathematical model used for permeability calculations In Cell 1, the initial concentration C1(t=0) of methanol is C0, and in Cell 2, C2 (t=0) = 0.

( )

( )

( ) ( ) ( )

( )

( ( ) ( ))

Making use of several assumptions, as well as Laplace and inverse Laplace transform, we arrive at

In general it is given that the permeability can be expressed as the product of the diffusion coefficient

and the so called partition coefficient

The partition coefficient can be assumed K = 1 for Nafion , and the effective diffusivity is related to the

diffusion coefficient of the material through (“Effective diffusivity in porous media,”)

From earlier we know that

| |

By making use of some empirical relationships and assumptions regarding the porosity, we can calculate

P from this

| |

| |

(

)

41

7.3. Conductivity setup resistances and example of

measurement curve

Table over EIS measurements setup resistances

Mean Standard deviation

Parallel resistance x-intercept 29.4 (kΩ) 6.3 (kΩ)

Serial resistance x-intercept 99.8 (mΩ) 11.0 (mΩ)

Example of curve fitting for Nafion membrane.

7.4. XPS results Experiment Descriptions Table

15 bilayers\X-Ray015 400um - FG ON\Point 1 - before depth profiling

PDAC/SPEEK

Common Acquisition Parameters Table

Parameter

Total acq. time 5 mins 40.3 secs

No. Scans 5

Source Type Al K Alpha

Spot Size 400 µm

Lens Mode Standard

Analyser Mode CAE : Pass Energy 200.0 eV

Energy Step Size 1.000 eV

No. of Energy Steps 1361

42

Elemental ID and Quantification

Name Peak BE

Height CPS

FWHM eV

Area (P) CPS.eV

Area (N) KE^0.6

At. % SF Al Scof

Backgnd

C1s 285.08 261718.83 3.15 909525.58 12910.99 74.08 1.000 Smart

O1s 531.99 137831.19 3.72 537808.70 2991.18 17.16 2.930 Smart

F1s 688.74 24875.65 3.14 94082.61 385.41 2.21 4.430 Smart

N1s 400.79 15184.19 4.82 76303.72 639.44 3.67 1.800 Smart

S2p 167.74 13900.52 2.93 47072.85 378.37 2.17 1.670 Smart

Si2p ??? 102.09 2208.80 3.07 7714.85 123.12 0.71 0.817 Smart

??? = Lower confidence assignment

Experiment Descriptions Table


PDAC/SPEEK


Parameter


No. Scans 5


Spot Size 400 µm

Lens Mode Standard





Name Peak BE

Height CPS

FWHM eV

Area (P) CPS.eV

Area (N) KE^0.6

At. % SF Al Scof

Backgnd

N1s 400.52 17605.33 3.25 86690.65 726.38 3.66 1.800 Smart

C1s 285.16 300452.97 3.22 1059521.90 15040.82 75.72 1.000 Smart

O1s 532.06 169678.33 3.68 652317.83 3628.22 18.27 2.930 Smart

S2p 167.81 15617.86 2.96 52819.58 424.58 2.14 1.670 Smart

F1s 689.06 3393.29 3.56 10594.05 43.41 0.22 4.430 Smart

Experiment Descriptions Table


PDAC/SPEEK


Parameter


No. Scans 5


Spot Size 400 µm

Lens Mode Standard





Name Peak BE

Height CPS

FWHM eV

Area (P) CPS.eV

Area (N) KE^0.6

At. % SF Al Scof

Backgnd

C1s 285.23 233170.82 3.21 847537.16 12031.93 72.26 1.000 Smart

O1s 532.21 124305.41 3.70 493659.67 2746.01 16.49 2.930 Smart

F1s 688.99 62446.19 3.09 215214.02 881.79 5.30 4.430 Smart

N1s 401.63 13175.53 3.20 66289.23 555.78 3.34 1.800 Smart

S2p 168.01 13691.07 3.00 45501.19 365.78 2.20 1.670 Smart

Si2p ??? 101.88 1474.61 2.60 4413.80 70.43 0.42 0.817 Smart

??? = Lower confidence assignment

Complete data files can be supplied upon requiest.

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