Electrochemistry Communications 12 (2010) 1585–1588

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Electrochemistry Communications

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Hydrogen oxidation reaction on thin platinum electrodes in the polymer electrolytefuel cell

M. Wesselmark a,⁎, B. Wickman b, C. Lagergren a, G. Lindbergh a

a Applied Electrochemistry, School of Chemical Science and Engineering, KTH, SE-100 44 Stockholm, Swedenb Competence Centre for Catalysis, Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

⁎ Corresponding author. Tel.: +46 8 790 65 06; fax: +E-mail address: maria.wesselmark@ket.kth.se (M. W

1388-2481/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.elecom.2010.08.037

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 August 2010Received in revised form 31 August 2010Accepted 31 August 2010Available online 8 September 2010

Keywords:Hydrogen oxidation reactionLimiting current densityExchange current densityPEMFCFuel cellThermal evaporation

A method for measuring the kinetics of the hydrogen oxidation reaction (HOR) in a fuel cell under enhancedmass transport conditions is presented. The measured limiting current density was roughly 1600 mA cmPt

−2,corresponding to a rate constant of the forward reaction in the Tafel step of 0.14 mol m−2 s−1 at 80 °C and90% RH. The exchange current density for the HOR was determined using the slope at low overvoltages andwas found to be 770 mA cmPt

−2. The high values for the limiting and exchange current densities suggest thatthe Pt loading in the anode catalyst can be reduced further without imposing measurable voltage loss.

46 8 10 80 87.esselmark).

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The hydrogen oxidation reaction (HOR) contributes very little tothe polarisation of the fuel cell under normal operating conditions.However, the need to further lower the catalyst loading has brought upthe question of howmuch the amount of platinum in the anode can bereduced without encountering too high voltage losses. Most studies ofthe HOR kinetics have been performedwith rotating disc electrodes inliquid electrolyte [1–8] but some measurements in fuel cells [9–11] orat fuel cell like conditions [12] exist. The exchange current density, i0,determined or estimated from fuel cell measurements at 80 °C rangefrom 100 to 600 mA cm−2 [10,11] and additional studies are essentialin order to better evaluate the i0 for hydrogen oxidation in fuel cells.Mass transport often impacts the kinetic measurements and alterna-tive approaches with reduced mass transport effects under realisticfuel cell conditions have been developed. Neyerlin et al. [11] used a so-called hydrogen pump cell with asymmetric Pt loadings and for state-of-the-art gas diffusion layers they expected no mass transportresistance up to 3 A cm−2, which was not exceeded with a catalystloading of 3 μgPtcm−2. With a well determined ohmic resistance theywere able to fit the Butler-Volmer equation to the experimental dataand obtained exchange current densities of 200–600 mA cmPt

−2.In this study, we use a similar fuel cell setup as Neyerlin et al. but

with a different type of working electrode. The electrode was

prepared by evaporation of platinum directly onto the gas diffusionlayer (GDL), which enables very low loadings and well definedstructures. Normally the ionomer covers the Pt particles, but in thiscase they are only pressed against the membrane and the hydrogendiffuses only through the GDL before it reaches the thin “twodimensional” catalyst layer which should enable better masstransport of hydrogen to the catalytic sites.

2. Experimental

2.1. Model electrode preparation

CARBEL CL GDL (Gore Technologies) were cleaned in a flowing N2

stream prior to platinum deposition, by thermal evaporation invacuum (AVAC HVC-600, at about 10−6 mbar). Different amounts ofplatinum, corresponding to 1.5, 3 and 6 nm (mean thicknesses on aflat surface) were evaporated onto the GDL.

2.2. Electrode morphology and chemistry characterisation

In order to visualise the electrode morphology, scanning electronmicroscopy (SEM) images of the samples were acquired using a ZeissSupra 60 operating at 10 kV in the secondary electron mode.

2.3. Fuel cell setup

Discs (14 mm Ø) of the model electrodes were placed onto pre-washed and dried Nafion™ 115 membranes as working electrodes

1586 M. Wesselmark et al. / Electrochemistry Communications 12 (2010) 1585–1588

(WE). Commercial porous ELAT electrodes (30% Pt on Vulcan XC-72,0.5 mgPtcm−2) were used as counter and reference electrode (CE/RE)and the MEAs were hot pressed at 135 °C for 30 s at 1 MPa. The MEAswere mounted in a laboratory PEEK fuel cell [13], with a clampingforce over the current collectors of 380 N. The humidifiers (Fuel CellTechnologies Inc.) were held at 77 °C, resulting in 90% relativehumidity (RH) in the fuel cell with a temperature of 80 °C.

The MEAs were activated by cycling 2000 times between 0.6 and0.9 V versus RHE in oxygen. Cyclic voltammetry was performed innitrogen with 5% H2 in Ar on the CE/RE to avoid crossover of hydrogento the WE. All measured potentials are referred to the reversiblehydrogen electrode (RHE) and the potentials in the cyclic voltammo-grams are therefore corrected for the 45.5 mV shift due to the lowerhydrogen partial pressure.

Polarisation curves for H2 oxidation were performedwith H2 fed inexcess at 2.75 ml s−1 on bothWE and CE/RE. The potential was sweptbetween −0.1 and 0.5 V, at 10 mV s−1 and between 0 and 0.5 V at1 mV s−1. The cell resistance was determined by the real axisintercept of the impedance spectra performed from 100 kHz to 1 Hz,both in N2 and H2, at several potentials (Autolab PGSTAT302N).

3. Results and discussion

Fig. 1 shows cyclic voltammograms of the electrodes with differentPt loadings. The electrochemically active surface area (ECSA) wasdetermined by the hydrogen desorption charges, assuming210 μC cmPt

−2, and was 0.37, 0.30, and 0.09 cmPt2 for the samples

with 6, 3, and 1.5 nm Pt, respectively. This is roughly 20 times lowerthan the ECSA determined in liquid electrolyte, confirming that onlythe upper part of the electrode is in contact with the Nafionmembrane. However, the relation between the ECSAs of the differentsamples is similar in both the fuel cell and liquid environment. Theincrease in ECSA with increasing Pt thickness can be explained by thephysical deposition and the structure of the GDL [14]. Firstly, the filmgrowth proceeds via the formation of isolated nanoparticles due to Ptatom surface diffusion on the carbon. Secondly, the high porosity ofthe GDL support implicates that the exposed area where Pt isdeposited is significantly larger than the projected area. Thus, theevaporated Pt will not form a continuous film, even at the highestamount of Pt deposited in this study (6 nm). This can also be seen inFig. 2 where SEM images of the three electrodes are shown. For the1.5 nm Pt sample, the Pt nanoparticles were about 2 nm in size andseparated by a few particle diameters. On the higher loaded samples,both the nanoparticle density and the size of the individual particlesincreased and on the 6 nm Pt sample, the nanoparticles started toform a network structure.

Fig. 1. Cyclic voltammetry on 6 nm (·······), 3 nm (– – – –) and 1.5 nm (—) Pt innitrogen at 80 °C, 90% RH and 200 mV s−1.

Fig. 2. SEM images of 1.5 (top), 3 (middle), and 6 nm (bottom) Pt evaporated on GDL.High magnification images are shown as insets.

The non iR-corrected polarisation curves for HOR, displayed asgeometric current density versus potential, are shown in Fig. 3A. Allelectrodes reach a limiting current which should not be diffusioncontrolled, as mentioned in the introduction. This is further supportedby the fact that the three samples of different Pt loading situated onthe same exposed area, reach different limiting current densities (inthe same order as the amount of Pt). The only step in the HORwhich isnot potential dependent is the dissociative adsorption of hydrogen inthe Tafel step. This step is followed by the electrochemical oxidationof H ad-atoms in the Volmer step according to:

H2 + 2Pt↔2H−Pt Tafel reaction ð1Þ

H−Pt↔Pt + H + + e� Volmer reaction: ð2Þ

Fig. 3. Hydrogenoxidation reaction on 6 nm(·······), 3 nm(– – – –) and 1.5 nm(—)Pt. A) non iR-corrected geometric current density B) iR-corrected specific current density(i.e. current divided by theECSA) versus potential at 80 °C, 90%RHand1 mV s−1. The insetin B displays the specific current densities for the same electrodes at low overpotentialsand 10 mV s−1.

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The Tafel–Volmer mechanism is apparently operational with theTafel step as the rate determining step at higher potentials, which isconsistent with other publications [1,4]. The lack of further increase incurrent density at higher potentials rules out the possibility of theHeyrovsky–Volmer mechanism being active at higher potentials,suggested by others [6]. The dip in current density when increasingpotential is proposed to be an effect of adsorption of impurities onplatinum or small variations in the interaction of platinum, Nafion,and water. Since the adsorption rate is limiting the current density,even small variations in accessible Pt surface sites will have a greatimpact on the current density.

Parameters such as temperature, gas flow rate, and relativehumidity are known to have a significant impact on the three phaseinterface of the reactive sites [15,16]. We consider that the mostpropermeasure of the ECSA is made at the same conditions as those ofthe studied reaction. The only parameter which is not a true effect ofthe cell conditions is the nitrogen gas flow rate which affects theamount of adsorbed hydrogen [15]. By varying the nitrogen flowduring cyclic voltammetry, we concluded that the gas flow rate usedin Fig. 1, resulted in an approximately 30% too low hydrogendesorption charge. The ECSA was therefore corrected for this whennormalising the current in the hydrogen oxidation polarisation curvesin Fig. 3B.

All samples reach almost the same limiting current density for theiR-corrected specific current densities (i.e. current normalised to theECSA) in Fig. 3B. The values used for iR correction were 260, 290 and

230 mΩ cm2 for 6, 3 and 1.5 nm Pt, respectively. A limiting currentdensity of 1600 mA cmPt

−2, is significantly higher than the previouslypresented limiting currents [1–6,12]. The limiting current was used todetermine the rate constant for the forward reaction of the Tafel stepfrom the rate expression of the Tafel step below.

vT = kT;f 1−θð Þ2pH2−kT;bθ

2 ð3Þ

where kT,f are the rate constants, θ is the hydrogen coverage on Pt, andpH2

is the hydrogen partial pressure. When the limiting current isreached, the hydrogen coverage is close to zero and the reactionlimited current for the Tafel–Volmer mechanism can be written as:

ilim;T = 2FvT = 2FkT;f pH2: ð4Þ

The hydrogen partial pressure was 0.6, which resulted in a valuefor the rate constant, kT,f, around 0.14 mol m−2 s−1.

A number of different approaches have been used in the literatureto determine the exchange current density for the HOR. Based on theobservation of a kinetically limited current density in Fig. 3, the Tafel–Volmermechanismwas assumed to be operational with the Tafel stepas the rate determining step. An expression for the current wasderived from the rate expressions for the Tafel and Volmer reactions,similar to Vogel [1] and Chen et al. [4]. By assuming that the Volmerreaction is at equilibrium, an expression for θ is obtained, whichmakes the reaction rate of the Tafel step potential dependent. Withthe surface coverage, θ, close to the equilibrium coverage at lowoverpotentials, the expression for the current density can be line-arised:

i =i02FηRT

1+i0idiff

: ð5Þ

If idiff (diffusion limiting current density) ismuchhigher than i0, thesecond term in the denominator becomes negligible, which wasassumed since themass transport rate was very high.We find that theassumptions underlying Eq. (5) are reasonable, but realise that theassumption of the Volmer step being at equilibrium probably is asimplification. The micropolarisation region, shown in the inset inFig. 3B, reveals a slope between 48 and 53 AcmPt

−2V−1 of thepolarisation curves. This corresponds to an average i0 of 770 mA cmPt

−2,which is higher but in the same range as the values presented by Sun etal. [6] and Neyerlin et al. [11]. The value of i0 is of course highlydependent on the assumed mechanism and the assumptions madewhen deriving the expressions, which means that it is difficult tocompare the actual numbers of i0 from the literature.

4. Conclusions

The exchange current density and limiting current of the HOR wasdetermined using model electrodes with Pt directly deposited ontothe GDL. The limiting current density was around 1600 mA cmPt

−2,corresponding to a reaction rate for the forward reaction in the Tafelstep of 0.14 mol m− 2 s− 1. An exchange current density of770 mA cmPt

−2 was calculated from the slope in the micropolarisationregion. The high value of i0 found in this work, is higher, but inaccordance with recent publications on HOR [8,11] and would enablea lowering of the anode catalyst loading without encountering toohigh voltage losses. The model electrodes and the method used in thiswork were shown to be very useful for studying the HOR kinetics.

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Acknowledgements

MISTRA (Swedish Foundation for Strategic Environmental Support)is gratefully acknowledged. The Competence Centre for Catalysis ishosted by Chalmers University of Technology and financially supportedby the Swedish Energy Agency and the member companies.

References

[1] W. Vogel, J. Lundquist, P. Ross, P. Stonehart, Electrochim. Acta 20 (1975) 79.[2] R. Notoya, A. Matsuda, J. Phys. Chem. 93 (1989) 5521.[3] N.M. Markovic, B.N. Grgur, P.N. Ross, J. Phys. Chem. B 101 (1997) 5405.[4] S. Chen, A. Kucernak, J. Phys. Chem. B 108 (2004) 13984.[5] I. Esparbé, E. Brillas, F. Centellas, J.A. Garrido, R.M. Rodríguez, J. Power Sources 190

(2009) 201.

[6] Y. Sun, J. Lu, L. Zhuang, Electrochim. Acta 55 (2010) 844.[7] J.X. Wang, S.R. Brankovic, Y. Zhu, J.C. Hanson, R.R. Adzic, J. Electrochem. Soc. 150

(2003) A1108.[8] J.X. Wang, T.E. Springer, R.R. Adzic, J. Electrochem. Soc. 153 (2006) A1732.[9] H.A. Gasteiger, J.E. Panels, S.G. Yan, J. Power Sources 127 (2004) 162.

[10] C.J. Song, Y.H. Tang, J.L. Zhang, J.J. Zhang, H.J. Wang, J. Shen, S. McDermid, J. Li, P.Kozak, Electrochim. Acta 52 (2007) 2552.

[11] K.C. Neyerlin, W.B. Gu, J. Jorne, H.A. Gasteiger, J. Electrochem. Soc. 154 (7) (2007)B631.

[12] A.R. Kucernak, E. Toyoda, Electrochem. Commun. 10 (2008) 1728.[13] J. Ihonen, F. Jaouen, G. Lindbergh, G. Sundholm, Electrochim. Acta 46 (2001) 2899.[14] P. Brault, A. Caillard, A.L. Thomann, J. Mathias, C. Charles, R.W. Boswell, S.

Escribano, J. Durand, T. Sauvage, J. Phys. D Appl. Phys. 37 (2004) 3419.[15] I.A. Schneider, D. Kramer, A. Wokaum, G.G. Scherer, Electrochem. Commun. 9

(2007) 1607.[16] R.W. Lindström, K. Kortsdottir, M. Wesselmark, A. Oyarce, C. Lagergren, G.

Lindbergh, J. Electrochem. Soc. (in press).