ORIGINAL PAPER

Investigation of β-SiC as an anode catalyst support for PEMwater electrolysis

Jakub Polonský & Petr Mazúr & Martin Paidar &

Karel Bouzek

Received: 29 October 2013 /Revised: 8 January 2014 /Accepted: 9 January 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract Because iridium is both expensive and scarce, it isessential to reduce the amount of IrO2 in the anode catalysts ofpolymer electrolyte membrane water electrolysers(PEMWEs). The potential ofβ-SiC to act as a catalyst supportfor PEMWE anodes was evaluated. To do so, a modifiedversion of the Adams fusion method was used to preparecatalysts with IrO2 supported on β-SiC with a mass percent-age of IrO2 of 20, 40, 50, 60, 70, 80, 90, and 100 %. The thin-film method was used for the electrochemical characterizationof catalysts by cyclic and linear sweep voltammetry. Thecatalysts were further characterized by scanning electronmicroscopy/energy dispersive X-ray (SEM-EDX) analysis,X-ray diffraction, X-ray photoelectron spectroscopy, and N2

adsorption (BET). Gas diffusion electrodes with the synthe-sized catalysts were prepared for tests in a laboratoryPEMWE. A 10 % improvement over pure IrO2 was found ina supported catalyst with 80wt.% IrO2. However, such a smallimprovement is not statistically significant. Therefore, thesupport may not influence the electrocatalytic activity of IrO2.

Keywords Iridium . Silicon carbide . Supported catalyst .

Polymer electrolyte membrane .Water electrolysis

Introduction

Polymer electrolyte membrane (PEM) water electrolysis is amodern and efficient way of producing hydrogen, which canbe used as an energy carrier. It differs from the industriallyestablished alkaline electrolysis by employing a proton-

conductive membrane as the electrolyte, and thus the processproceeds at an acidic pH. The advantage of PEM waterelectrolysis is that the current density and efficiency are typ-ically higher than that in the alkaline process [1]. The disad-vantage is that PEM technology involves high capital costs.On account of the acidic environment, both electrodes have toemploy precious metals as electrocatalysts. Additionally, thepolymer electrolyte membrane is, itself, muchmore expensivethan a simple diaphragm used as the separator in the tradition-al alkaline process. Of the two electrodes, the anode is the onethat needs a higher loading of the precious metalelectrocatalyst because the oxygen evolution reaction (OER)at the anode is much slower than the hydrogen evolutionreaction at the cathode [2]. Therefore, this paper focuses onimproving the anode to increase the competitiveness of PEM.

The best known electrocatalyst for the OER in PEM waterelectrolysis is RuO2, but its lifetime on the anode is limited[3]. IrO2, the second-best choice, is often used as a morestable, but also more expensive alternative to RuO2. Onepossibility to increase the efficiency of utilization of IrO2 isto use a catalyst support [4–13]. That being said, the stronglyoxidative environment coupled with high anode redox poten-tial means that there is only a very limited selection of stablematerials. Carbon, frequently used in PEM fuel cells, oxidizesto CO2 and is, therefore, not an option for the anode of a PEMwater electrolyser.

Several ceramic materials have been tested that could act asa catalyst support for the anode catalyst in PEM waterelectrolysers. One of the materials most frequently studied isSnO2. Some researchers reported an improved performance ofthe anode when using this support [14, 15], but others reportedmixed results [16, 17]. Other oxidic materials have beeninvestigated, such as Ta2O5 [18] and TiO2 [10]. Mazúr et al.[10] found that using TiO2 support improved the performance,mostly because the larger particles of supported catalyst didnot penetrate into the electrode body, thus increasing their

J. Polonský (*) : P. Mazúr :M. Paidar :K. BouzekDepartment of Inorganic Technology, Faculty of ChemicalTechnology, Institute of Chemical Technology, Prague, Technická 5,166 28 Prague 6-Dejvice, Czech Republice-mail: polonskj@vscht.cz

J Solid State ElectrochemDOI 10.1007/s10008-014-2388-0

degree of utilization. None of the oxidic materials are conduc-tive, and because conductivity can help decrease the ohmicdrop in the electrode, attempts have been made to use con-ductive materials. One approach to making an oxidic materialconductive is by doping it. A prominent example is Sb-dopedSnO2 [5, 9, 19, 20]. Another approach is the chemical mod-ification of an oxidic material, such as TiO2, which can bereduced to a conductive mixture of non-stoichiometric Tisuboxides [6, 21]. Another approach is to start with a materialthat is inherently conductive, such as TiC [4], TaC [11], orSiC-Si [8]. Since carbon in carbides can be oxidized, thechemical stability of these materials may be limited. In thispaper, β-SiC was evaluated as a possible new electrocatalystsupport for anodes in PEM water electrolysers. This materialwas selected because silicon carbides are frequently used ascatalyst supports, thus are widely available and inexpensive.Moreover, β-SiC has a certain amount of electronic conduc-tivity [22]. The chemical stability of a similar material,α-SiC-Si, was investigated by Nikiforov et al. [8], and the materialwas found to passivate under the conditions of OER.

This paper is a continuation of three papers which investi-gated conductive and nonconductive electrocatalyst supportsfor PEM water electrolysers anodes: α-SiC-Si [8], TiO2 [10],and TaC [11]. Applying a suitable electrocatalyst support is atraditional approach to enhance the degree of utilization of acatalytically active material. Such support must be chemicallystable; moreover, any electronic conductivity of the support iswelcome since it can help reducing the ohmic drop in theelectrocatalyst layer. A disadvantage of an unsupportedelectrocatalyst is that the small particles of IrO2 tend to pen-etrate deep into the body of the electrode, which is usually aporous metallic felt. Such particles are in poor or no contactwith the membrane and thus cannot participate in the electrodereaction. Supported electrocatalyst particles can be larger andthus stay on top of the electrode in a good contact with themembrane. At the same time, however, they retain a highspecific surface of IrO2. The aim of this paper is to investigatewhether β-SiC as an electrocatalyst support offers a goodcompromise between the chemical stability and electronicconductivity.

Experimental

Catalyst preparation

Catalysts were synthesized using a modified Adams fu-sion method [23]. The iridium precursor (H2IrCl6.4H2OAlfa Aesar, 99 %) was dissolved in 10 mL of isopropanol.Subsequently, β-SiC (Alfa Aesar, 99.8 %) and NaNO3

(16.7× molar excess) were added, and the mixture wasstirred for 1 h to ensure complete mixing. Then, themixture was heated to 70 °C, and the isopropanol was

left to evaporate completely. The dried mixture wasplaced in an electric furnace and heated to 500 °C witha temperature ramp of 4 °C/min. It was kept at 500 °C for1 h before cooling to room temperature. The product waswashed several times with demineralized water to removeall chlorides, which can cause interference during mea-surements. Finally, the catalysts were dried at 90 °C.

A series of catalysts was prepared with a mass fraction ofIrO2 of 20, 40, 50, 60, 70, 80, 90, and 100 wt.%. The catalystsare referred to as IrxSiy, where x is the mass percentage of IrO2

and y is the mass percentage of β-SiC. The synthesis process(omitting the iridium precursor) was also applied to neat β-SiC to see whether the process had any effect on the support’sproperties.

Physicochemical characterization

Nitrogen adsorption was used to determine the specific sur-face area of catalysts using a Micromeritics Gemini 2375analyzer. The Brunauer–Emmett–Teller (BET) isotherm wasused for calculations.

Scanning electron micrographs were collected using aHitachi S4700 FE-SEM. Since all samples were conduc-tive, no coating was applied. The scanning electron mi-croscope (SEM) was equipped with an energy dispersiveX-ray (EDX) analyzer which was used to collect elementmaps.

X-ray diffractograms of all catalysts were collected on aPANalytical X’Pert Pro diffractometer equippedwith a Cu-Kαsource (α=1.54060). The Scherrer equation [24] was used toestimate the average crystallite size of IrO2 in the catalysts.

X-ray photoelectron spectroscopy was used to determinethe surface concentration of iridium, silicon, and oxygen andthe oxidation states of iridium. Using argon sputtering, surfacelayers were removed to estimate the depth profile of theelements analyzed. The instrument used was an ESCAProbeP(Omicron Nanotechnology Ltd.).

Electrochemical characterization

The electrochemical performance of the catalysts was evalu-ated using cyclic voltammetry, linear sweep voltammetry onrotating disc electrode, and tests in a laboratory PEM waterelectrolyser. The linear sweep voltammetry measurementswere used for Tafel slope analysis.

Rotating disc electrodes were used for conveniencebecause, when rotated, gas bubbles are instantaneouslyremoved from the electrode surface upon their formation.Since the electroactive substance was water, there was noeffect of rotation on the mass transfer. The conductive partof the rotating disc electrode was made of glassy carbonand was coated with the catalyst in accordance with thethin-film method [25]. The diameter of the glassy carbon

J Solid State Electrochem

disc was 4 mm. The catalyst was dispersed in demineralizedwater in such a way as to obtain 0.25 mg IrO2/mL and wasultrasonically homogenized for 30 min. Then, a total of 20 μLof catalyst dispersion was deposited onto the glassy carbondisc in four layers, and each layer was dried before the nextwas deposited. Finally, a total of 4 μL of Nafion solution(1 mg dry basis/mL) in isopropanol was deposited in twolayers to fix the catalyst powder on the electrode surface.

Cyclic and linear sweep voltammetry were carried outin a thermostated (25 °C), double-walled, three-neckedelectrochemical cell with a platinum foil counter-electrode and a mercury/mercurous sulfate reference elec-trode (MSE) with a saturated K2SO4 electrolyte. Thereference electrode was connected to the cell via a liquidjunction. The potential of the reference electrode was +0.64 V vs. SHE, and all potentials regarding cyclic andlinear sweep voltammetry in this paper refer to this refer-ence electrode. The electrolyte was 0.5 M H2SO4. A HekaPG310 potentiostat was used for the cyclic and linearsweep voltammetry experiments. Cyclic voltammogramswere measured from −0.4 to 0.8 V at a scan rate of500 mV/s. Linear sweep voltammograms were measuredfrom 0.7 to 0.9 V at 1 mV/s to ensure steady-state condi-tions on the electrode.

Anodes for the laboratory PEM water electrolyser wereprepared from titanium felt backing (2×2 cm, Fumatech)sprayed with a catalyst dispersion. The dispersion consistedof the catalyst, Nafion binder, and isopropanol 30 times themass of the catalyst. Before spraying, the dispersion wasultrasonically homogenized for 30 min. Nafion concentrationin the resulting catalyst layer was 15 wt.%. Nafion served bothas a binder and as a proton-conductive phase. The sprayingwas done in several layers to obtain 1.3 mg Ir/cm2.

A single-cell laboratory PEMwater electrolyser was used tomeasure the performance of the prepared anodes. The temper-ature was set at 90 °C, and both anodic and cathodic compart-ments were pressurized to 300 kPa. Two Ta-coated stainlesssteel flow plates were employed as the end plates and flowfields. The active area for electrolysis was 4 cm2. As thecathode, a commercial gas diffusion electrode HT-ELAT(BASF) was used with a platinum loading of 0.5 mg/cm2.Nafion 117 (DuPont) membrane served as the solid polymerelectrolyte. The membrane was activated as follows: First, itwas washed in deionized water at 80 °C for 60 min, after whichit was kept in 3 wt.%H2O2 solution at 60 °C for 120min. Then,it was converted into the H+ state by immersing it in 0.05mol/LH2SO4 at 60 °C for 30 min. Finally, the membrane was washedin deionized water at 60 °C for 120 min. The membrane wassandwiched between the anode and cathode and compressed inthe electrolyser. Deionized water was circulated through theanode compartment. A load curve was recorded from 1.4 to1.8 V with 50-mV increments. A Statron stabilized powersource was used as the power supply.

Results and discussion

Physicochemical characterization

Specific surface

According to the manufacturer, the support had a specificsurface area of 11.5 m2/g. A value of 13.8 m2/g was obtainedby the initial characterization of the as-received material. Afterthe support had been subjected to the synthesis procedure, itssurface area dropped to 6.3 m2/g. The highest temperatureduring synthesis was 26 % of the melting point of siliconcarbide. Thus, not the sintering of the material, but possiblythe elevated temperature may have facilitated some particleagglomeration, which decreased the specific surface area.

Pure IrO2 had a much higher specific surface area of121 m2/g, indicating very fine particles (Fig. 1). The specificsurface area of the supported catalysts increased to that of theIr90Si10 catalyst (Fig. 1), which exhibited 137 m2/g, whichwas more than the pure IrO2 had. The addition of a supporthad the positive effect of increasing the surface area. Duringsynthesis, IrO2 has to overcome the nucleation barrier. Thepresence of a support results in additional crystallization cen-ters, which lower the barrier. The IrO2 can thus start tocrystallize on more sites than without the presence of a sup-port. This results in better dispersion, smaller particles, and,ultimately, a higher specific surface area of IrO2.

The specific surface area of the IrO2 part was calculatedfrom the composition of samples by subtracting the β-SiCshare. Theβ-SiC surface area was assumed to remain constantand to be equal to the surface area of a blank sample after thesynthesis route (6.3 m2/g). It was assumed that theelectrocatalyst was relatively porous and thus permeable tothe N2. Nevertheless, certain amount of support surface mayeventually become inaccessible after IrO2 deposition. In such

Fig. 1 BET specific surface area (SSA) of the catalysts and support aftersynthesis procedure, documenting enhancement of SSA for supportedcatalysts

J Solid State Electrochem

case, the BET surface area of IrO2 may be underestimated.That being said, the ratio of BET area of β-SiC to IrO2 wasaround 0.05. Therefore, the uncertainty of measurement wasnot influenced significantly by this assumption. All the sup-ported catalysts had larger surface area of the IrO2 part thanthe pure IrO2. Catalysts with 40–80 wt.% of IrO2 had over160 m2/g, which was a substantial, around 35 % increase overpure IrO2.

SEM and EDX analysis

Scanning electron micrographs of Ir80Si20, Ir40Si60, andIr0Si100 (for comparison) were obtained to assess the surfacedistribution of IrO2 on the support (Fig. 2). IrO2 appears assmall brighter dots on the surface. Visibility was, however,limited because the size of the “dots” was only several nano-meters, getting close to the physical resolution of the instru-ment. Nevertheless, a definite surface roughness can be seenas compared to the pure support. From the SEM images, itappears that the distribution of IrO2 was even on the supportsurface, and the size of the IrO2 particles was in the order ofnanometers. The uniformity of distribution was also con-firmed by EDX element maps (Fig. 3).

X-ray diffraction

The X-ray diffraction data served two purposes: first, to seewhether any new phase formed during the synthesis and,second, to estimate the average crystallite size of the IrO2.The diffractograms showed no phase other than IrO2 and SiC.The sensitivity of the measurement is about 1 wt.%; since nonew phases were identified, it can be concluded that thesynthesis did not influence the bulk composition of thecatalysts.

The average crystallite size of IrO2 was highest in the pureIrO2 sample—8.3 nm (Fig. 4). In all the supported catalysts, itwas lower around 3 nm (except Ir20Si80 where it attained5.3 nm). These values are comparable to those of Nikiforovet al. [8] who evaluated α-Si-SiC composite as a supportmaterial. The support did positively influence the size of thecrystallites. As discussed in “SEM and EDX analysis” section,it facilitates better dispersion of IrO2, resulting in smallercrystallite sizes tending to have higher catalytic activity be-cause of the higher specific surface area. The largest crystal-lites in supported catalysts were found in Ir20Si80.

During the synthesis of Ir20Si80, the concentration of Irprecursor was relatively low compared to the other catalysts.This means that the nucleation barrier was comparativelyhigher, and the relative number of crystallization centers waslower than in the more concentrated catalysts, which resultedin the formation of larger crystallites.

X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy was used to study the oxi-dation states of iridium and the depth profiles of elements. Theoxidation states of iridium were estimated from thedeconvolution of peaks located between 60 and 68 eV (Ir4f). Ir50Si50 was chosen as a representative sample (Fig. 5).Three doublets were observed before sputtering. They wereattributed to Ir (0), Ir (IV), and Ir (VI). The occurrence of ahigher oxidation state of iridium was also noted by Siracusanoet al. [6] and Chen et al. [26]. However, some authors considerthe main component to be Ir (III) [27, 28]. Judging from theoxygen-to-(Ir+Si) ratio, it seems more plausible that in ourcase, the third oxidation state was (VI). After 1 min ofsputtering, the oxidation state (VI) disappeared completely,and the spectrum was deconvoluted into two doublets

Fig. 2 SEM images of a Ir80Si20, b Ir40Si60, and c Ir0Si100 showing bright IrO2 particles (a, b) and neat uncoated surface (c); acceleration voltage,15 kV; working distance 4.6–5.0 mm

Fig. 3 EDX element mapshowing even distribution of Irover SiC support surface

J Solid State Electrochem

pertaining to the oxidation state (0) and (IV), as discussedabove.

Depth profiling was estimated from three measurements. Arepresentative example is given in the inset in Fig. 5. The firstmeasurement was acquired from an unmodified sample, andthe next two were acquired after 5 and 10 min of sputteringusing an argon ion beam. No other element, besides theexpected Ir, Si, O, and C, was found in the samples. Sincecarbon is the main contamination source, its concentrationwould be exaggerated and is therefore not shown in thegraphs. The surface of the catalysts showed them to be highlyoxidized. The oxygen-to-(Ir+Si) ratio would be 2 if the sur-face composition were IrO2+SiO2. For samples with 40–90 wt.% IrO2, this ratio was between 1.91 and 2.76, indicatingthat the surface was indeed highly oxidized and in some caseseven more than corresponding to the assumed stoichiometry.This may be explained by iridium being oxidized to a higheroxidation state than (IV). The Ir20Si80 and Ir0Si100 samplesshowed a markedly lower content of surface oxygen (1.52 and1.11, respectively). This suggests that the principal oxygenbinding partner in the sample was iridium.

The oxygen-to-(Ir+Si) ratio dropped dramatically aftersputtering, averaging 0.37 after 5 min and 0.29 after 10 minfor catalysts with 40–90 wt.% IrO2. The high concentration of

oxygen was thus limited to a very thin surface layer. Beneaththis surface, iridium was present in the sample both in anoxidized and a reduced, metallic state. The authors suggest thatthe reduction of IrO2 to Ir follows reaction (1). Metallic iridiumis also an effective electrocatalyst that may become oxidized onthe electrode. Therefore, the reduction should not influence theelectrocatalytic activity of the prepared composite.

2 IrO2 þ SiC→2 Ir þ SiO2 þ CO2

or3 IrO2 þ 2 SiC→3 Ir þ 2 SiO2 þ 2 CO

ð1Þ

Electrochemical characterization

Cyclic voltammetry

Cyclic voltammograms of IrO2 do not have any clearly de-fined peaks [5, 29]. There is only one broad pseudocapacitivepeak in both directions. The total anodic charge has been

Fig. 4 IrO2 crystallite sizes in dependence of the load on the supportsurface

Fig. 5 Deconvolution of XPSspectra for Ir50Si50 before andafter sputtering; the inset shows adepth profile of elementalcomposition; the balance to100 % is carbon both from β-SiCand contamination

Fig. 6 Typical cyclic voltammogram of IrO2 (obtained from Ir50Si50);scan rate 500 mV/s, glassy carbon electrode, deaerated 0.5 M H2SO4 at25 °C

J Solid State Electrochem

suggested to be proportional to the total number of active sites[30]. However, there is no generally accepted method ofcalculating the number of active sites from the anodic charge,as in, for example, the case of platinum. The voltammogramswere mostly symmetrical along the abscissa from around 0 to0.8 V, exhibiting pseudocapacitive behavior (Fig. 6). This isan indicator of surface redox processes between Ir (III) and Ir(IV) species. This behavior results in ratios of anodic-to-cathodic charge (Qa/−Qc) close to unity. Deviations fromunity indicate the presence of other reactions. In our experi-ments, other reactions could only come from the catalystsupport, which is not desired because surface coverage byIrO2 not only protects the support from corrosion, but alsoensures the high conductivity of the catalyst.

The scan rate chosen was high (500 mV/s) to obtain highercurrent densities which improve the signal-to-noise ratio. Fur-thermore, Marshall et al. [31] observed that a higher scan rateresulted in greater differences between the charge ratios. Thevoltammograms were recorded in the region of water stability,between −0.2 and 0.8 V.

The anodic-to-cathodic charge ratios were close to unity(1.01–1.05) for all but the Ir20Si80 catalyst, which showed aratio of 1.22. This indicates that with 40wt.% IrO2, the surface

was evenly covered. Pronounced support reactions only tookplace when the IrO2 loading was low.

Tafel analysis

Linear sweep voltammograms were collected under pseudo-steady-state conditions using 1 mV/s of scan rate from 0.7 to0.9 V. This region covers the onset of oxygen evolutionreaction. The current that can be applied to a thin-film elec-trode is limited to approximately 1 mA before the layer startsto disintegrate due to the gas bubbles evolved. Tafel slopeswere calculated from the linear parts of the graph of log(I)versus potential. Two regions are typical, where such relation-ship is linear [12, 32]. The first region occurs at lower currentdensities, and the Tafel slope is lower than in the secondregion located at higher current loads. We investigated thefirst region (inset in Fig. 7) because of the current densitylimitations of the thin-film electrode.

The Tafel slopes did not differ much apart from theIr20Si80 catalyst, which showed a markedly higher Tafelslope (42 mV/dec) than the other catalysts (averaging at37 mV/dec) (Fig. 7). The reproducibility of measurementwas approximately ±2 mV/dec. The values are almost thesame as those found by Marshall et al. for IrO2-SnO2

catalysts [31] including the higher slope for the catalystwith low IrO2 loading. Tafel slopes on DSA electrodes areusually around 60 mV/dec for catalysts with ≥10 wt.%IrO2 [32, 33].

In acidic media, oxide path and electrochemical oxide pathhave been proposed as the mechanism for the OER [31, 32,34, 35] for Tafel slopes between 30 and 40 mV/dec. A Tafelslope of 30 mV/dec points to the second step in the oxide path(3) as being the rate-determining step.

The oxide path is as follows:

Sþ H2O→S‐OHþ Hþ þ e− ð2Þ

S‐OHþ S‐OH→S‐Oþ Sþ H2O ð3Þ

S‐Oþ S‐O→2 Sþ O2 ð4Þ

If the Tafel slope is 40 mV/dec, it suggests that thesecond step in the electrochemical oxide path (5) is therate-determining step. Electrochemical oxide path differsfrom the oxide path only in the second step, which isdescribed in reaction (5):

S‐OH→S‐Oþ Hþ þ e− ð5Þ

Fig. 7 Tafel slopes for the catalysts obtained from linear sweep voltam-mograms; scan rate 1 mV/s, glassy carbon electrode, deaerated 0.5 MH2SO4 at 25 °C; the inset shows Tafel diagram for Ir50Si50 with markedlinear region

Fig. 8 Load curves of laboratory PEM water electrolyser; 1.3 mg Ir/cm2

anode, 0.5 mg Pt/cm2 cathode, Nafion 117, 90 °C, 300 kPa; the insetshows linear regression formulae for the linear part of load curves where jis the current density andΔE is the cell voltage

J Solid State Electrochem

The mechanism seems not to have changed until the con-centration decreased down to 20 wt.% IrO2 where the Tafelslope increased, indicating a shift in the mechanism, or rathera change in the ratio of rate-determining steps on differentreaction sites. The material was polycrystalline, so differentreaction sites were present, and the measured Tafel slope wasthen an average for all the sites.

PEM water electrolyser

The operating voltage of a PEMwater electrolyser is typicallybetween 1.6 and 1.8 V. Assuming a thermoneutral water-splitting voltage of 1.48 V, the operating voltage thus corre-sponds to 82–92 % efficiency. Catalysts with 70, 80, 90, and100 wt.% IrO2 were selected for measurements because theyhad the lowest Tafel slopes (“Tafel analysis”). Load curveswere collected from 1.4 to 1.8 V to determine also the onset ofthe OER. Temperature has a great effect on the kinetics of theOER, and it also increases the conductivity of Nafion [36].However, it cannot be arbitrarily high before parts of theelectrolyser start to corrode. The membrane is especiallyaffected by high operating temperatures because Nafion tendsto dry out above 100 °C and to lose its conductivity. Anoperating temperature of 90 °C was chosen because it repre-sents the best compromise between rapid kinetics and a longservice life of the membrane and other components of theelectrolyser.

Two regions existed in the load curves (Fig. 8). In the firstregion, up to around 1.6 V, the current density increasedexponentially with the voltage. In this region, the main sourceof overpotential was the electrode kinetics, especially theOER. In the second region, from around 1.6 V, the currentdensity increased linearly with the voltage. A substantialsource of overvoltage was the cell ohmic resistance.

The Ir80Si20 electrocatalyst had the highest current densi-ty. At 1.6 V, it was 10 % higher than for the second-bestcatalyst, Ir100Si0. The other two supported catalysts per-formed worse than the pure IrO2. At 1.6 V, the Ir90Si10exhibited 8 % lower current density than Ir100Si0, while theIr70Si20 showed a current load 33 % below the Ir100Si0.

The cell ohmic resistance is composed of membrane resis-tance and electrode resistances. The conductivity of the cata-lyst layer contributes to the overall resistance and was the onlyparameter that changed in the catalysts that were tested. Bycomparing the slopes of the linear part of the load curves (insetin Fig. 8), the relative conductivity of the catalyst layer can beestimated. The higher the slope, the faster the current densityincreases with the voltage and the better the conductivity ofthe catalyst layer. The Ir100Si0 and Ir80Si20 catalysts had thehighest slope equally. The total current density was slightlyhigher in the Ir80Si20 catalyst, but this could be attributed tothe uncertainty of the measurement.

Conclusions

We have shown that by using β-SiC as a catalyst support, thespecific surface area of IrO2 is increased, and the crystallitesize decreased. However, such improvements failed to signif-icantly enhance the kinetics of the oxygen evolution reaction.The electrocatalyst containing 80 wt.% IrO2 was identified ashaving the optimal composition. It showed a 10 % increase incurrent density over pure IrO2. A 90 wt.% IrO2 electrocatalystperformed about 8 % worse than the pure IrO2. Such valuesare, however, statistically insignificant and can be attributed tothe uncertainty of measurement. The composition with70 wt.% IrO2 performed significantly worse than the othercompositions. Despite the increase in specific surface area anddecrease in crystallite size, the effect of β-SiC support waseither small or none, and further measurements would benecessary to confirm the presence of any effects.

Acknowledgments Financial support is acknowledged from the Min-istry of Industry and Trade of the Czech Republic (project no. FR-TI2/442) and Specific University Research (MSMT No. 20/2013).

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