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Oxygen evolution on Ebonex-supported Pt-based binarycompounds in PEM water electrolysis

A. Stoyanova*, G. Borisov, E. Lefterova, E. Slavcheva

Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl.11, 1113 Sofia, Bulgaria

a r t i c l e i n f o

Article history:

Received 16 November 2011

Accepted 7 February 2012

Available online 23 March 2012

Keywords:

PEM water electrolysis

Oxygen evolution reaction

Pt

Fe

Co

Ebonex

* Corresponding author. Tel.: þ359 2 979 27 8E-mail addresses: stoyanova@bas.bg, ant

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2012.02.032

a b s t r a c t

Ebonex-supported Pt-based binary electrocatalysts (PteFe, PteCo) in different metal ratios

were prepared by wet solegel method using acetilacetonate precursors (M[(C5H7O2)n]m,

M ¼ Pt, Fe, Co) and deposited on nonstoichiometric titanium oxide support. The compo-

sition of the synthesized composites were studied by X-ray diffraction (XRD) and X-ray-

photoelectron spectroscopy (XPS) analysis. Their electrocatalytic activity toward oxygen

evolution in PEM water electrolysis was investigated using the common electrochemical

techniques of cyclic voltammetry and steady state polarisation. The XRD spectra registered

a formation of solid solution between the metallic components accompanied by decrease

in the lattice parameter and the crystal size. The effects observed resulted in enhanced

efficiency toward oxygen evolution reaction of the synthesized PteFe/Ebonex and PteCo/

Ebonex catalysts compared to pure Pt.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction alloys of Pt with Fe and Co offer good performance mainly in

PEM water electrolysis is an alternative to the classical alka-

line technology for generation of pure hydrogen. This method

is environmentally friendly and compatible with the usage of

renewable energy sources but is still expensive. The main

source of energy dissipation during the PEM water splitting is

the oxygen evolution reaction which has slow kinetics,

proceeds at high overvoltage and requires high noble metal

catalytic loadings. The best electrocatalyst toward this reac-

tion among the pure metals is platinum. One way to reduce

the cost of the catalysts is to alloy Pt with other less expensive

metals. The alloying of Pt with various transition metals have

been justified by theoretical considerations based on the

Brewer bonding theory, predicting the probability for inter-

atomic hyper-hypo-d-electron interactions [1,2]. Alloy cata-

lysts of Pt with various transitions metals are often used in

order to improve the catalytic activity toward the anodic

reaction. It has been proven that carbon supported binary

0; fax: þ359 2 971 11 61.onia.stoyanova@gmail.co2012, Hydrogen Energy P

direct methanol and polymer electrolyte fuel cells [3e9]. The

results obtained showed changes in particle size, distribution

and electronic structure of the catalyst that are expected to be

beneficial for catalytic activity of these materials toward the

OER in water electrolyses as well.

The realization of synergetic effect with the catalytic

support is another way to increase the activity [10,11]. In this

respect the titanium oxide Magneli phases, commercially

recognized under the trade name Ebonex is of particular

interest since it has a unique combination of electrical

conductivity approaching that of a metal and high corrosion

resistance approaching that of ceramics [12].

Recent study in our group is focused on solegel synthesis

of different combinations of mono- and bimetallic

compounds, oxides and composite materials in which Pt is

partly or totally replaced by cheaper elements such as V, Cr,

Mn [13e15]. The borohydride wet chemical reduction (BH)

method has been applied to prepare PteCo catalysts in various

m (A. Stoyanova).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 6 5 1 5e1 6 5 2 116516

ratios [16,17]. The results obtained by now show that the

synthesized PteM catalysts supported on mechanically acti-

vated Ebonex show enhanced efficiency toward OER

compared not only to pure Pt but also to the same composi-

tions deposited on active carbon substrate with at equal

catalytic loadings. An improved stability at high anodic

potentials was observed as well. This is related to the already

mentioned unique properties and the hypo-d-electron char-

acter of the oxide substrate which facilitates the hypo-hyper-

d-electron interactions with the constituent metals

[13e15,17].

In this work Ebonex-supported Pt-based binary electro-

catalysts (PteFe and PteCo) in different metal ratios were

prepared by wet solegel method using acetilacetonate

precursors (M((C5H7O2)n)m or M-acac, M ¼ Pt, Fe, Co) to

investigate their activity toward oxygen evolution reaction in

PEM water electrolysis. The structure of the synthesized

composite catalysts were studied by XRD analysis. The elec-

trocatalytic activity was investigated using common electro-

chemical techniques of cyclic voltammetry and steady state

polarisation. Their properties are compared with those of Co-

containing compositions synthesized by BH method.

2. Experimental

2.1. Catalysts preparation

The synthesis of the chosen composite materials consisted in

direct selective grafting of the metals from acetilacetonate

precursors M((C5H7O2)n)m or M-acac (M ¼ Pt, Fe, Co). The

substrate used was a commercial Ebonex powder (average

particle size 5 mm) which before synthesis was subjected to

mechanical treatment in a planetary ball mill for 40 h. The

metallic part in each of the catalyst was 20 wt.%, with

different weight ratios of precursors (Pt:M ¼ 1:1 and 2:3). The

preparation procedure included two steps. The first one was

the pretreatment of the support and the precursor using

magnetic stirrer and ultrasonic bath, their mixing and heating

at temperature 60 �C until a fine gel was obtained. In the

second step of the synthesis, the mixture was heated in inert

atmosphere at temperature 240 �C, reduced in H2 atmosphere

and then gradually cooled [18].

2.2. Physical characterization

The phase composition, morphology and surface structure of

the catalysts under study were investigated by methods of X-

Ray diffraction (XRD).

XRD spectra were recorded by X-ray diffractometer Philips

APD15. The diffraction data were collected at a constant rate

of 0.02�s�1 over an angle range of 2q ¼ 10e90�. The size of Pt

crystallites was determined by Scherrer equation [19].

D ¼ kl=bcosQ (1)

where D is the average dimension of crystallites, k is the

Scherrer constant in the range 0.85e1.0 in dependence on the

crystal type (is usually assumed to be k z 9); l is the X-ray

wavelength,Q is the Bragg angle and b is the peak broadening

in radians.

The XPS of the samples were recorded with an ESCALAB

MK II (VG Scenific, England) electron spectrometer. The

photoelectrons were excited with a twin anode X-ray source

using Al Ka (hn ¼ 1486.6 eV) radiation. C 1 s photoelectron line

at 284.8 eV was used as a reference for calibration. Curve

fitting of the core-level XPS lines was carried out using

CasaXPS software with a GaussianeLorentzian product func-

tion and a non-linear Shirley background.

2.3. Laboratory PEM cell and test procedure

The electrochemical tests were performed on membrane

electrode assemblies (MEAs) using a polymermembrane as an

electrolyte. MEA was prepared by hot pressing of electrodes

for hydrogen and oxygen evolution on both sides of

a commercial Nafion 117 membrane (Alfa Aesar), using a 5%

Nafion solution (Alfa Aesar) as a binder. The electrodes had

a double layered structure, consisting of a hydrophobic

backing layer and an active catalytic one, and geometric area

of 0.5 cm2. The hydrogen electrode (HE) was a magnetron

sputtered Pt film with a loading of 0.27 mgPt cm�2. A

commercial E-TEK catalyst containing 20% Pt on carbon

support was used to prepare the reference electrodes (RE).

The synthesized catalysts were used to prepare the elec-

trode for the oxygen evolution reaction (OER). The backing

layer was made from a mixture of carbon particles (Shawini-

gan Acetylene Black) and 30 wt.% of PTFE suspension,

deposited on thin carbon cloth. The catalytic layer was spread

upon the backing one as an ink (catalyst particles mixed with

diluted NafionR ionomer) at several steps as after each one the

electrode was dried for 30 min at 80 �C. The procedure was

repeated until a metal loading of 0.5 mg cm�1 was reached.

Both electrodeswere hot pressed onto the PEM electrolyte [15].

The performance characteristics of the preparedMEAwere

investigated in a self made laboratory PEM electrolytic cell,

consisting of two gas compartments where hydrogen and

oxygen evolution take place, separated by the membrane

electrode assembly under study. A reference electrode is sit-

uated in the hydrogen evolution compartment. The catalytic

activity of the prepared catalysts was studied using the tech-

niques of cyclovoltammetry and steady state polarization at

temperature of 20 �C and 80 �C (a typical operating tempera-

ture for PEMWE). All electrochemical measurements were

carried out with a commercial Galvanostat/Potentiosat POS 2

Bank Electronik, Germany.

3. Result and discussion

The XRD spectra of the synthesized PteFe/Ebonex and PteCo/

Ebonex composite catalysts are presented in Fig. 1 and Fig. 2.

For easier phase identification the spectrum of Fe/Ebonex and

Co/Ebonex catalyst is presented together with that of the

Ebonex support. In Table 1 the cell parameters and crystallite

size calculated from these spectra are summarized.

In the spectra of all samples the characteristic peaks of

the Magneli phases titanium oxide are registered. The typical

fcc Pt peaks that appear on the spectrum of the pure Pt/

Ebonex catalyst shift significantly to higher diffraction

angles with Fe and Co addition. The new positions are closer

Fig. 1 e XRD spectra of the studied PteFe/Ebonex catalysts.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 6 5 1 5e1 6 5 2 1 16517

to the M3Pt crystal phases than to Pt:M (M ¼ Fe, Co) [20e22].

The cell parameter decreases from 3.916 A for Pt/Ebonex to

3.756 A for 2Pt:3Fe/Ebonex and to 3.746 A for 2Pt:3Co/Ebonex.

The results indicate that the most of the metals atoms are

incorporated in the Pt-M crystal cell. Additionally, Fe3O4

phase is identified in Fe and PteFe samples (Fig. 1). In Co/

Ebonex catalyst the diffraction peaks of the metallic Co

(cubic and hexagonal) are registered. Although no Co oxides

are found, they could present in amorphous state. The size

of Pt:M (M ¼ Fe, Co) crystallites decreases more than twice

compared to pure Pt.

The XPS analysis show that platinum exists mainly in

metallic form but when used in alloys the binding energy

increases (with w0.5 eV). A presence of small amount OH�

groups is also registered on the surface Fig. 3.

A problem in the analysis of Fe2p and Co2p photoelectron

spectra arises from their complex nature due to spin-orbital

coupling, satellite structure and multiplet splitting of the

oxidation states, as well as the low difference between M2þ

and M3þ binding energies. But energy separation of the shake

up satellites, and spin-orbital splitting are very sensitive to the

chemical state. Therefore, the satellite structure can be used

to identify the oxide phases [23e25].

Fig. 2 e XRD spectra of the studied PteCo/Ebonex catalysts.

In iron oxides the satellite located atw8 eV above themain

line (around 719 eV) is connected with Fe3þ while the one at

714e715 eV is prescribed to the existence of Fe2þ. The blur of

the satellite structure between 714 and 720 eV is an indication

for the presence of iron in the second and third valence,

particularly as Fe3O4 [23]. The Fe 2p high-resolution spectrum

of the Pt:Fe/Ebonex catalyst is in accordance with the fore-

going and confirms XRD results for Fe3O4 phase (Fig. 4).

Additional information is obtained by decomposition of the 2p

spectrum with 3 doublets corresponding to Fe� (w706 eV),

Fe2þ(w709 eV), Fe3þ (w711 eV) and another two doublets for

Fe2þsat. (w714.5 eV) and Fe3þsat (w719 eV). An asymmetric tail

function was used for the Fe�, Fe2þ and Fe2þcomponents

[24,25]. The ratio Fe3þ: Fe2þ <2:1 means that there is an excess

of Fe2þ which may be presents as FeO or Fe(OH)2. Small

amount of Fe0 is registered too.

Fig. 4 shows deconvoluted Co2p spectrum for catalyst

PteCo/Ebonex. The high spin Co2þ compounds such as CoO

(780eV), Co(OH)2 (780e781 eV) exhibit strong satellite lines

which are located at about 5e6 eV above the main line, in

contrast to Co3þ-2p that exhibits a weak satellite shifted to

higher binding energies (Co3O4, CoOOH) [26e29]. The separa-

tion of the Co2p3/2e2p1/2 spin-orbit components is larger by

about 1 eV for the high spin Co2þ compared to the low spin

Co3þ valence states. The splitting is usually 16.0 eV for Co2þ

and 15.0 eV for Co2þ and Co0. As result, the less intensive

Co2p1/2 peak exhibits a higher differences between of Co2þ

and Co3þ species and can be used for Co valence states iden-

tification. The Co2p spectrum was deconvoluted into six

doublets, corresponding to Co0 at w778 eV, Co2þ at w781 eV

and w782, Co3þ at w780 eV [26] and two satellites:Co2þSat. at

w787 eV and Co3þSat. at w791 eV [29]. The component at

w781 eV and strong satellite peak at 786.5 eV as well as the

value 15.9 eV for spin-orbital splitting means that cobalt is

present mainly in second valence state. The spectrum of

metallic cobalt gives rise at 778 eV Co 2p3/2. The widening of

the Co2p1/2 peak toward lower binding energies may due to

Co0, and Co3þ.The O1s spectra are fitted with three peaks corresponding

to different oxygen bonds (Fig. 5). The component situated at

lower binding energy (about 529e530 eV) is due to the oxygen

in the MeeO bonds MeOx (Me ¼ Fe, Co, Pt, Ti). The O1s

component around 531.5e532 eV corresponds to OH� groups.

The high energy component (533e534 eV) can be assigned to

adsorb H2O [29,30]. According to O1s, and Co2p XPS core-level

spectra, one can conclude that a majority part of Co2þ state

exist as hydroxide, i.e. as Co(OH)2.The cyclovoltammetry tests were performed on all

synthesized catalysts in order to obtain qualitative informa-

tion about the electrochemical activity and the nature of the

processes occurring on the catalyst surface. To illustrate the

influence of the introduced second metal on the catalytic

performance, in Fig. 6 are shown the CV-curves of MEAs

containing all catalyst under study.

The CV-curves show that the OER on the bimetallic

Ebonex-supported catalysts starts at lower potentials

compared to pure metals (Co, Fe, and Pt). The evolution of

oxygen is most intensive in the case of 2Pt:3Co catalyst. For

this sample the oxygen offset potential has the lowest value

(about 1.45 V) and the reaction reaches much higher current

Table 1 e Pt cell parameter and crystal size of the synthesized mono and bimetallic compounds.

Sample Initial composition,Pt:M (M ¼ Fe, Co) ratio, wt. %

Crystallite size,D111, nm

Crystallite size,D200, nm

Pt cellparameter. A

Pt/Ebonex (20%) 1:0 16 12 3.916

Pt:Fe/Ebonex 1:1 6 6 3.769

2Pt:3Fe/Ebonex 2:3 8 7 3.756

Pt:Co/Ebonex 1:1 4 5 3.747

2Pt:3Co/Ebonex 2:3 6 6 3.746

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 6 5 1 5e1 6 5 2 116518

densities compared to all other catalysts (at 1.8 V e about

100 mA cm�2). On the CV of the PteFe samples distinct nearly

reversible anodic and cathodic peaks situated in the potential

range 0.7e0.75 V are observed. These peaks are due to the

redox transition Fe3þ/Fe2þ and correspond well with the XRD

and XPS results, indicating an existence of Fe3O4 phase.

The CV-curves of the Pt-free Fe/Ebonex and Co/Ebonex

catalysts show very low current density.

The quasi steady state polarization tests of the anodic

partial electrode reaction involved in the electrolytic water

splitting are performed for all catalysts under study. The

results are presented in Fig. 7. For better illustration of the

influence of Fe and Co on the catalyst efficiency and utilization

of Pt the obtained current densities are normalized to the

catalytic loading and presented in Fig. 8 as mass activity. The

Pt-free Fe/Ebonex and Co/Ebonex samples are not included

because they showed an insignificant catalytic activity.

Fig. 3 e High resolution Pt4f photoelectron spectra of

Pt:M/Ebonex catalysts.

The results obtained are in good accordance with our

previous findings for OER catalytic activity of PteCo/Ebonex,

synthesized by BH method [18].

The investigated 2Pt:3Fe and 2Pt:3Co catalysts deposited on

Ebonex possess enhanced efficiency toward oxygen evolution

reaction compared to pure Pt in the whole potential range,

while Pt:Fe and Pt:Co are more active at high overvoltages

(above 1.65 V). The composition 2Pt:3Co/Ebonex exhibits the

best catalytic properties as the OER reaches current densities

up to 230 mA cm�2 (at 1.9 V). These values are higher than

those obtained with bimetallic Co-containing Pt/Ebonex

catalyses synthesized by the BH method and investigated at

the same experimental conditions [17].

Ebonex as a supporting material has stable behavior and

good corrosion resistance at the high anodic potentials of

intensive oxygen evolution. This prevents the decrease in the

electrode electrical conductivity and the MEA degradation.

Fig. 4 e High resolution Fe2p and Co2p core-level

photoelectron spectra of Pt:M/Ebonex catalysts.

Fig. 5 e High resolution O1s photoelectron spectra of Pt:M/

Ebonex catalysts.

Fig. 7 e Polarization curves of catalysts under study at 80 �Cand scanning rate 1 mV sL1.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 6 5 1 5e1 6 5 2 1 16519

This is confirmed by the galvanostatic experiments carried

out at 80 �C which showed stable catalytic properties of Pt:Fe

and Pt:Co catalysts (Fig. 9).

The enhanced catalytic activity of Fe- and Co-containing

Pt-compounds supported on Ebonex can be explained with

Fig. 6 e Cyclovoltametric curves of catalysts under study at

20 �C and scanning rate 100 mV sL1.

the formation of solid solution between the metallic compo-

nents. The alloying proven by the XRD data causes some

structural effects such as reduction of the lattice parameter

and the crystallite size (Table 1) which in turn, leads to

increase of the surface area and to enhanced catalytic activity.

The PteCo samples have lower Pt-cell parameters and exhibit

higher efficiency.

The stable behavior and good corrosion resistance at the

high anodic potentials of Ebonex has been already demon-

strated during oxygen evolution, where Magneli phases serve

not only as a supporting material, but also contribute to the

efficiency of the composite catalyst [31]. For the compositions

studed herein, the XPS analysis showed a realization of elec-

tronic hypo-hyper-d-metal-support interactions and Pt-M

alloying leading to changes in the electronic density of the

atoms and the surface-intermediate bond strength as well.

Fig. 8 e Polarization curves of catalysts under study,

presented as mass activity of Pt in the composite, at 80 �Cand scanning rate 1 mV sL1.

Fig. 9 e Galvanostatic experiments of 2Pt:3Fe/Ebonex and

2Pt:3Co/Ebonex catalysts at 80 �C, i [ 50 mA cmL2.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 6 5 1 5e1 6 5 2 116520

4. Conclusions

The results demonstrate that PteFe and PteCo catalysts sup-

ported on Ebonex possess enhanced efficiency toward oxygen

evolution reaction in PEMwater electrolysis compared to pure

Pt. The observed effects are explained with formation of solid

solution between the metallic components. The alloying

between these metals reduces the lattice parameter and the

size of the crystals, thus increasing the active surface sites

available for the reaction. A realization of synergetic effect as

a result of hypo-hyper-d-electronic interactions between

catalyst and support, which further increases the catalytic

effect, is also assumed.

Acknowledgments

This research has been financially supported by the National

Science Found at BulgarianMinistry of Education and Science,

contract DTK 02/68.

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