carbon xerogel catalysts prepared by the Strong Electrostatic Adsorption method

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis and characterization of highly loaded Pt/carbon xerogel catalysts prepared by the Strong Electrostatic Adsorption method Nathalie Job,a Frédéric Maillard,b Marian Chatenet,b Cédric J. Gommes,a Stéphanie Lambert,a Sophie Hermans,c John R. Regalbuto,d Jean-Paul Pirarda a

Laboratoire de Génie Chimique, Université de Liège, B6a, B-4000 Liège, Belgium LEPMI, UMR 5631 CNRS/Grenoble-INP/UJF, BP75, F-38402 St Martin d’Hères Cedex, France c Unité de Chimie des Matériaux Inorganiques et Organiques, Université Catholique de Louvain, Place Louis Pasteur 1/3, B-1348 Louvain-la-Neuve, Belgium d Department of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton Street, Chicago, IL 60607, USA b

Abstract In order to decrease the mass transport limitations reported in classical PEMFC electrodes, Pt/carbon xerogel catalysts have great potential to replace Pt/carbon black catalysts. These nanostructured materials with well defined pore texture allow for better gas/water diffusion and better contact between the platinum particles and the ionomer (Nafion®). Pt/carbon xerogel catalysts with high metal content (~ 25 wt.%) and high metal dispersion (nanoparticles ca. 2 nm in size) were prepared via the ‘Strong Electrostatic Adsorption’ method; the impregnation-drying-reduction step with H2PtCl6 was repeated until the desired metal loading was achieved. However, both physicochemical and electrochemical characterization show that the use of H2PtCl6 leads to Pt catalysts poisoned with chlorine, especially if the reduction temperature is lower than 450°C. This induces a dramatic decrease of the Pt utilization ratio in the final PEMFC catalytic layer. Keywords: carbon xerogels, Pt/C catalysts, PEM fuel cells, electrochemistry

1. Introduction Pt/C catalysts with high metal weight percentage are classically used in Proton Exchange Membrane Fuel Cells (PEMFCs) [1]. Indeed, minimization of ohmic and transport losses within the electrode, and compensation for both the sluggish oxygen reduction reaction rate and the usual lack of contact between Pt and the ionomer result from a compromise: (i) the thickness of the catalytic layer must be low, and (ii) the metal loading of the electrode must be high. In order to decrease the mass transport limitations encountered in PEMFC electrodes, which are prepared with Pt/carbon black catalysts, it was recently proposed to replace the classical carbon black support by carbon xerogels [2], i.e. nanostructured materials with well defined pore texture prepared by evaporative drying and pyrolysis of organic gels. Carbon xerogels allow for better gas/water diffusion within the pore texture of the electrode and better contact between the platinum particles and the ionomer (Nafion®).

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In this study, a carbon xerogel with selected pore size in the macropore range was used as Pt catalyst support. The catalyst was designed for use as oxygen reduction catalyst at the cathode of a PEM fuel cell. So as to obtain Pt/carbon xerogel catalysts with high metal content, the ‘Strong Electrostatic Adsorption’ (SEA) method [3, 4] was applied; H2PtCl6 aqueous solution was used as metal precursor. This method consists of maximizing the electrostatic interactions between the metal precursor and the support by adjusting the pH of the slurry. The latter depends on the surface chemistry of the carbon and on the chemical nature of the metal precursor. The goal was to obtain highly loaded Pt/carbon xerogel catalysts while preserving high metal dispersion: indeed, following the literature, the optimal Pt particle size for efficient oxygen reduction is ca. 2-3 nm [5]. The obtained catalyst was reduced under flowing hydrogen at various temperature conditions and characterized using both physico-chemical (TEM, XPS, CO chemisorption) and electrochemical (CO stripping voltammograms, ORR measurements) techniques. The samples were used as Pt/C catalyst at the cathode side of an air/H2 PEM fuel cell. The goal of the study was to demonstrate the effect of Pt chlorine poisoning, originating from the decomposition of H2PtCl6, and the importance of the catalyst reduction treatment on the cell performance. Finally, physico-chemical and electrochemical characterization allowed us to study the effect of poisoning on the metal availability and on the oxygen reduction kinetics.

2. Experimental 2.1. Catalyst preparation The carbon support chosen for this study was a micro-macroporous carbon xerogel with a macropore size ranging from 50 to 85 nm, a specific surface area of 640 m² g-1 and a total pore volume of about 2.1 cm³ g-1 (including 0.26 cm³ g-1 of micropores, i.e. pores smaller than 2 nm). Carbon xerogels are materials composed of interconnected spherical-like microporous nodules. So, they classically display a bimodal pore size distribution: micropores inside the carbon nodules, and larger pores identified as the voids located between the nodules [6]. The size of the nodules and that of the voids inbetween is controlled by the synthesis conditions of the material [6, 7]. The carbon support was synthesized by the drying and pyrolysis of a resorcinolformaldehyde aqueous gel, following a procedure developed in a previous study [8]: the R/C (resorcinol/sodium carbonate) molar ratio of the gel precursor solution was chosen equal to 1000, while all other synthesis variables (from gel preparation to pyrolysis conditions) were kept identical as in the above-mentioned study. In brief, resorcinol and sodium carbonate were solubilized in water and then formaldehyde was added. Gelation and ageing were performed at 85°C (72 h), and were followed by evaporative drying (60-150°C, 1 day) and pyrolysis (800°C, 2 h) under nitrogen flow. Pt/carbon xerogel catalysts were obtained via the ‘Strong Electrostatic adsorption’ (SEA) method. This consists of maximizing the electrostatic interactions between the metal precursor and the support by adjusting the pH of the slurry to the adequate value, which depends on the surface chemistry of the carbon and on the precursor chosen. Indeed, interaction between the support and the metal precursor depends on both the precursor nature (anion or cation, size, etc.) and on the carbon surface chemistry. At pH values lower than the Point of Zero Charge (PZC), i.e. the pH at which the surface is neutral in terms of charge, the surface is positively charged, and the adsorption of anions is favoured (Fig. 1a). At pH higher than the PZC, the surface is negatively charged, and the adsorption of cations is enhanced. The PZC of carbon xerogels after

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pyrolysis is about 9.5 [4], which leaves a large pH range for the adsorption of both Pt anions and cations. However, the Pt uptake is limited by steric effects: indeed, in the case of impregnation with H2PtCl6, for instance, the maximum surface density can be calculated as a close packed arrangement of chloroplatinic acid complexes which retain one hydration sheath [3]. As a result, the Pt uptake vs. final pH of the impregnation solution at equilibrium passes through a maximum (Fig. 1b): indeed, in the case of the impregnation of carbon xerogels with H2PtCl6 aqueous solutions (1000 ppmPt), the initial pH leading to the highest Pt uptake was found to be 2.5 (final pH at equilibrium ~ 3.0), and the corresponding maximum Pt surface density was found equal to 0.8-0.9 µmol m-2, which corresponds to a weight percentage ranging from 8 to 10 wt.% [4].

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Fig. 1. (a) Principles of the Strong Electrostatic Adsorption (SEA) method: depending on the PZC of the support and on the impregnation pH, the adsorption of ositively or negatively charged species is favoured; (b) due to steric effects, the Pt uptake is limited and the Pt surface density vs. pH curve presents a maximum (results from [4]).

The procedure used for preparing Pt/carbon xerogels via the SEA method is fully described in reference [9]. Briefly, the carbon support was contacted with the impregnation solution at the optimal pH value of 2.5 until the equilibrium was reached. The impregnated catalyst was then filtered, dried and reduced under flowing hydrogen. In order to increase the Pt weight percentage up to acceptable values for electrochemical applications, this impregnation-drying-reduction cycle was repeated up to three times using the same original catalyst batch. Note also that various final reduction temperatures (200, 350 and 450°C) were tested to evaluate the effect of Pt poisoning by chlorine.

2.2. Catalyst characterization 2.2.1. Physico-chemical characterization Catalysts were characterized using several complementary techniques. The Pt content was evaluated by ICP-AES after elimination of the carbon and solubilisation of the metal [10]. In order to measure the size of the metal particles, the catalysts were investigated by transmission electron microscopy, with a Jeol 2010 (200 kV) device (LaB6 filament). The samples were crushed and dispersed in ethanol and subsequently deposited onto a copper grid. Particle size distributions were obtained by image analysis performed on a set of at least 1000 particles: the procedure is described in [9]. The samples were analyzed by X-ray photoelectron spectroscopy (XPS), performed on an SSI-X-probe (SSX-100/206) spectrometer from Fisons. The samples were stuck onto troughs with double-sided adhesive tape then placed on an insulating home-made ceramic carrousel with a nickel grid 3 mm above the samples, to avoid differential

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charging effects. A floodgun set at 8 eV was used for charge stabilisation. The energy scale was calibrated by taking the Au 4f7/2 binding energy at 84 eV. The C1s binding energy of contamination carbon set at 284.8 eV was used as internal standard value. Data treatment was performed with the CasaXPS program (Casa Software Ltd). Finally, CO chemisorption was used to determine the accessible Pt surface, SPt-Chem. Isotherms were measured with a Fisons Sorptomatic 1990 equipped with a turbomolecular vacuum pump that allows vacuum of 10-3 Pa. The entire procedure, from the sample preparation to the adsorption measurement, is fully described elsewhere [10]. Briefly, a first CO adsorption isotherm was achieved so as to measure the total amount of adsorbed carbon monoxide (chemisorbed + physisorbed). The catalyst was then outgassed, and a second CO adsorption isotherm (physisorbed CO) was measured. The amount of CO forming the chemisorbed monolayer on surface Pt atoms, deduced by extrapolating the nearly horizontal difference curve to the uptake axis, was used to calculate SPt-Chem. 2.2.2. Electrochemical characterization All electrochemical measurements were carried out in sulphuric acid (1 M) at 25°C. The voltammetric experiments were performed using an Autolab-PGSTAT20 potentiostat with a three-electrode cell and a saturated calomel electrode (SCE) as a reference electrode (+0.245 V vs. normal hydrogen electrode, NHE). The catalyst sample was deposited on a rotating disk electrode (EDT 101, Tacussel), used as working electrode. All details, from sample preparation to experimental conditions, are extensively described in reference [9]. The electrochemically active Pt surface area of the catalysts, SPt-Strip, was determined by CO stripping. This electrochemical technique consists in the electrooxidation of a CO monolayer previously adsorbed on the Pt surface. It allows estimating the real Pt surface area, assuming that the electrooxidation of a COads monolayer requires 420μC per cm2 of Pt. Since CO stripping is performed in aqueous electrolyte, it implies 100% utilization of the Pt surface atoms and is influenced neither by contact problems between the metal and the electrolyte nor by mass-transport limitations. In addition, the electrooxidation of a COads monolayer is a structure-sensitive reaction and provides a wealth of information on the particle size distribution and the presence/absence of particle agglomeration [11, 12]. The CO stripping voltammograms were recorded at 0.02 V s-1 between +0.045 and +1.245 V vs. NHE, after saturation of the electrolyte by CO (6 min bubbling) and removal of the non-adsorbed CO from the cell by purging with Ar (39 min). The electrocatalytic activity for the ORR of the elaborated Pt/C nanoparticles was measured in O2-saturated liquid electrolyte. The quasi-steady-state voltammograms were recorded at 10-3 V s-1 from +1.095 to +0.245 V vs. NHE. To account for the reactants diffusion-convection in the liquid layer, the experiment was repeated at four RDE rotation speeds (42, 94, 168 and 262 rad s-1) [13]. 2.2.3. Fuel cell test The catalysts were tested as PEM fuel cell cathodic catalytic layers on a unit cell-test bench: 50 cm² Membrane-Electrode Assemblies (MEAs) were prepared by the decal method as described in reference [2]. The electrolyte was a Nafion® membrane, and the anode a commercial anode made from Pt-doped carbon black (40 wt.%, TKK) deposited by Paxitech onto a carbon felt (0.6 mgPt cm-2 mixed with Nafion®). The thickness of the cathode was kept constant by keeping constant the carbon mass in the catalytic layer. The Nafion®/carbon mass ratio of the ink used to prepare the MEAs was fixed at 0.5. After a standardized start-up procedure, polarization curves, i.e. the Ucell = f(jm) curves, were measured by setting the cell voltage at each desired value for 15 min, which

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assured the stabilization of the current. The current, jm, was normalized to the Pt metal loading of the cathode.

3. Results and discussion Figure 2 shows examples of TEM micrographs and the corresponding Pt particle size distributions obtained by image analysis of two Pt/carbon xerogel catalysts: the first one (Fig. 2a) was prepared by single impregnation of the support, and the second one (Fig. 2b) was obtained by three consecutive impregnation-drying-reduction cycles. Results show that repeating the impregnation with H2PtCl6 as Pt precursor yields an increase of the catalyst metal content up to 22.3 wt.% while keeping homogeneously dispersed Pt nanoparticles ca. 2 nm in size.

(a)

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Fig. 2. TEM micrographs of Pt/carbon xerogel catalysts and particle size distributions obtained by image analysis. (a) Single impregnation, 7.5 wt.% Pt and (b) triple impregnation, 22.3 wt.%.

Table 1 regroups the characterization data of three catalysts, prepared from the same batch of double-impregnated sample (metal loading: 15.0 wt.%); the only difference is the temperature (200-450°C) and duration (1-5 h) of the reduction treatment, performed under flowing H2. The samples are denoted as follows: the letter ‘C’ is followed by the reduction temperature (in °C) and the reduction time (h). The average particle size, dTEM, obtained by image analysis, does not change when increasing the reduction temperature from 200°C (C-200-1) to 450°C (C-450-5). This agrees with a set of additional experiments performed under N2, which showed that Pt nanoparticles begin to sinter only at T ≥ 600°C. Interestingly, the Pt surface area detected by CO chemisorption, SCO-chem, increases from 92 (C-200-1) to 124 m² gPt-1 (C-450-5). In addition, chlorine is detected at the surface of every sample by XPS measurements. It seems thus that, when the reduction temperature is too low, the catalyst remains poisoned by chlorine coming from the metal precursor (H2PtCl6). The calculated Cl/Pt ratio decreases with increasing the reduction temperature, from 0.33 (C-200-1) to 0.07 (C-450-5): so, removing chlorine species completely from the catalyst proves difficult. One also notices that the CO chemisorption and XPS data do not match well. Indeed, the Cl/Pt ratio decreases by a factor 5 between C-200-1 and C-450-5 while the detected Pt surface shows an increase of 30% only; in addition, from sample C-350-3 to sample

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C-450-5, Cl/Pt is multiplied by 3 while SCO-chem remains almost constant (118 and 124 m² gPt-1, respectively). Table 1. Physico-chemical and electrochemical characterization of catalysts reduced under various conditions. Sample

C-200-1 C-350-3 C-450-5

TEM dTEM (nm) ± 0.2 1.8 1.8 1.8

CO chemisorption SCO-chem (m² gPt-1) ± 10% 92 118 124

XPS Cl/Pt (-) ± 10% 0.33 0.23 0.07

CO stripping SCO-strip (1) SCO-strip (7) (m² gPt-1) (m² gPt-1) ± 10% ± 10% 37 69 61 87 129 142

ORR b (V dec-1) ± 5% -0.074 -0.070 -0.066

SA90 (µA cm²Pt-1) ± 5% 10 11 12

CO stripping measurements allowed us to obtain complementary data about catalyst poisoning. Figure 3a shows the CO stripping voltammograms of two catalysts: C-200-1 and C-450-5, obtained by two different reduction treatments of the same initial batch (double impregnation, 15.0 wt.% metal). Globally, the surface of the peak detected between 0.7 and 1.0 V vs. ENH represents the current exchanged to electrooxidize the CO monolayer adsorbed on the Pt surface, and can be regarded as the electrochemically accessible Pt surface [9]. The curves in grey correspond to the voltammograms obtained on the fresh catalysts while the black curves are those measured after 7 consecutive CO strippings. One can observe that (i) the charge under the peak increases with the reduction temperature, (ii) the charge under the peak increases after several consecutive CO strippings, whatever the reduction temperature, and (iii) the peak is shifted toward lower potentials when the reduction temperature increases or when the number of consecutive CO strippings increases. These observations are in agreement with the hypothesis of catalyst poisoning. First, as already observed by CO chemisorption, the accessible Pt surface measured on the fresh catalyst increases with the reduction temperature: SCO-strip (1), increases from 37 (C-200-1) to 129 m² gPt-1 (C-450-5). Note that, contrary to CO chemisorption measurements, CO stripping data match perfectly the Cl/Pt ratios obtained by XPS: indeed, the relationship between Cl/Pt and SCO-strip (1) is close to linearity (not shown). Second, the detected surface increases after several CO strippings: this is due to the fact that chlorine species are progressively removed by repetitive adsorption/desorption processes. Indeed, Holscher and Sachtler [14] showed that CO is one of the strongest poisons adsorbed onto platinum: in the presence of CO, poisons originally adsorbed onto the Pt particle surface should be displaced. However, the kinetics of displacement may be too slow to be completed within a few minutes: this would explain why CO chemisorption in gaseous phase, during which equilibrium is reached prior to any further gas injection, leads to larger Pt surfaces than COads stripping in liquid phase; the differences in surface detected by CO chemisorption and CO stripping could then be due to slow Cl displacement kinetics. Finally, the positive shift of the onset of the CO electrooxidation peak may be ascribed to the competition between water and chloride species for the Pt adsorption sites. Indeed, previous studies [15, 16] have suggested that only a fixed number of active sites which are able to form OH species and to initiate the CO electrooxidation exist on the Pt surface. Competitive adsorption by Cl- species thus decreases artificially the number of active sites and shifts both the onset and the main CO electrooxidation peak towards positive potentials.

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Fig. 3. Electrochemical measurements on 15.0 wt.% Pt/C catalysts (double impregnation) reduced under H2 during 1h at 200°C, sample C-200-1 ({) and 5h at 450°C, sample C-450-5 (‘). (a) COads stripping voltammograms, measurements in H2SO4 1 M at 25°C, sweep rate of 0.02 V s-1. Curves in grey: first CO stripping; curves in black: voltammogram after 7 consecutive CO strippings. (b) Polarization curves at 70°C for Membrane-Electrode Assemblies with cathode processed with the same catalysts.

Oxygen Reduction Reaction (ORR) performed on the rotating disk electrode shows that the presence of chlorine has little effect on the intrinsic reactivity of the Pt sites. To evaluate the catalyst activity towards oxygen reduction, two parameters were evaluated: (i) the Tafel slope, b, and (ii) the specific activity at 0.90 V vs. NHE, SA90. This potential corresponds to 0.34 V ORR overpotential in 1 M sulphuric acid, a value classically monitored in a PEMFC cathode at low current densities, i.e. under kinetic control. Regarding samples C-200-1, C-350-3 and C-450-5, neither b nor SA90 changes significantly with the reduction treatment (Table 1), indicating that the reaction mechanism and kinetics remain the same. The only difference between the three samples is the accessible Pt surface, which decreases due to Cl poisoning when the reduction temperature is too low. Finally, Fig. 3b shows the impact of Cl poisoning on the functioning of an air/H2 fuel cell. The cathode of the Membrane-Electrode Assembly (MEA) was processed either with sample C-200-1, or with catalyst C-450-5. The catalyst poisoning dramatically decreases the current produced at a fixed potential. However, further calculation shows that the decrease of accessible Pt surface due to Cl coverage is not sufficient to explain the poor performance of catalyst C-200-1. Much probably, the presence of chlorine also hampers the contact between the Pt particles and the ionomer (i.e., Nafion®), and decreases thus further the amount of Pt atoms that are truly available for the oxygen reduction in the monocell. This was checked by in situ cyclic voltammetry: the detected Pt surface per mass unit of metal was lower in the processed MEA than in the initial catalyst powder.

4. Conclusions So as to obtain Pt/carbon xerogel catalysts with high metal content, the ‘Strong Electrostatic Adsorption’ method was applied. By repeating the impregnation-dryingreduction step with H2PtCl6 as Pt precursor, it was possible to increase the catalyst metal content up to 22.3 wt.% while keeping homogeneously dispersed Pt nanoparticles ca. 2 nm in size. However, both physico-chemical and electrochemical characterization

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show that the use of H2PtCl6, which is a very classical impregnation precursor, yields Pt catalysts poisoned with chlorine species. Chlorine coming from the metal precursor decomposition appears difficult to remove completely: even after reduction under H2 at 450°C for 5 h, Cl poisoning is still detected via electrochemical methods. The presence of chlorine on the Pt particles leads obviously to decreasing the active Pt surface, which can dramatically reduce the Pt utilization ratio in the PEMFC catalytic layer. Further work is in progress to extend the SEA method to Cl-free Pt precursors.

References [1] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, 2005, Appl. Catal. B, 56 (1-2), 9-35. [2] N. Job, J. Marie, S. Lambert, S. Berthon-Fabry, P. Achard, 2008, Energ. Convers. Manage., 49 (9), 2461-2470. [3] J.R. Regalbuto, in: Catalyst Preparation: Science and Engineering, J.R. Regalbuto (ed.), CRC Press, Taylor & Francis Group, Boca Raton, 2007, p. 297. [4] S. Lambert, N. Job, L. D’Souza, M.F.R. Pereira, R. Pirard, J.L. Figueiredo, B. Heinrichs, J.P. Pirard, J.R. Regalbuto, 2009, J. Catal., 261 (1), 23-33. [5] K. Kinoshita, Electrochemical Oxygen Technology, Wiley, New York, 1992, p. 48. [6] N. Job, R. Pirard, J. Marien, J.-P. Pirard, 2004, Carbon, 42 (3), 619-628. [7] S.A. Al-Muhtaseb, J.A. Ritter, 2003, Adv. Mater., 15 (2) 101-114. [8] N. Job, A. Théry, R. Pirard, J. Marien, L. Kocon, J.-N. Rouzaud, F. Béguin, J.-P. Pirard, 2005, Carbon, 43 (12) 2481-2494. [9] N. Job , S. Lambert, M. Chatenet, C.J. Gommes, F. Maillard, S. Berthon-Fabry, J.R. Regalbuto, J.-P. Pirard, 2010, Catal. Today, in press. [10] N. Job, M.F.R. Pereira, S. Lambert, A. Cabiac, G. Delahay, J.-F. Colomer, J. Marien, J.L. Figueiredo, J.-P. Pirard, 2006, J. Catal., 240 (2), 160-171. [11] F. Maillard, M. Eikerling, O.V. Cherstiouk, S. Schreier, E. Savinova, U. Stimming, 2004, Faraday Discuss., 125, 357-377. [12] F. Maillard, S. Schreier, M. Hanzlik, E.R. Savinova, S. Weinkauf, U. Stimming, 2005, Phys. Chem. Chem. Phys., 7 (2) 385-393. [13] A.J. Bard, L.R. Faulkner, Electrochemical methods: fundamentals and applications, Wiley, New-York, 1992, p. 283. [14] H.H. Holscher, W.M.H. Sachtler, 1966, Discuss. Faraday Soc., 41, 29-42. [15] F. Maillard, E.R. Savinova, U. Stimming, 2007, J. Electroanalytical Chem., 599 (2), 221-232. [16] B. Andreaus, F. Maillard, J. Kocylo, E. R. Savinova, M. Eikerling, 2006, J. Phys. Chem. B, 110 (42), 21028-21040.