Carbon xerogels as catalyst supports for PEM fuel cell cathode

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Abstract

Carbon xerogels with various pore textures were prepared by evaporative drying and pyrolysis of resorcinol–formaldehyde gels, and used as supports for Pt catalysts in PEM fuel cell cathodes. The goal of this study was to determine whether carbon xerogels could replace the carbon aerogels which were previously used as Pt catalyst supports in the same electrochemical system, and to determine how the pore texture influences the cell performances. Pt catalysts were prepared by impregnation of carbon supports with aqueous H2PtCl6 solution followed by reduction in aqueous phase with NaBH4. Fuel cell measurements show that the metal surface actually available for the oxygen reduction reaction and the voltage losses due to diffusion phenomena strongly depend on the carbon pore texture. Finally, some carbon xerogels yield similar performance than carbon aerogels.

Introduction

The catalytic layer of the PEMFC is a key element to the cell performance. The catalyst has to be supported, usually on a carbon black, to improve its dispersion. Carbon blacks are constituted of small primary carbon particles (10–30 nm). These particles are assembled together as aggregates and the voids between the carbon particles are smaller when the carbon particles are smaller. The smaller the pores, the slower the diffusion rate of reactants and products. At the air-fed cathode, where oxygen, proton and water transports are involved in the oxygen reduction reaction, high potential losses due to diffusion limitations offset the influence of the metal dispersion of the catalyst.

Catalyst designers overcome this problem by achieving very high platinum loadings on the carbon black (mass ratio mPt/(mPt + mC)  0.5 and more), which enables one to prepare thin catalytic layers with acceptable diffusion-induced potential losses (∼0.1 V at 1 A/cm2) despite their low porosity while reaching high active metal surfaces. However, reducing the Pt loading of PEMFC electrodes is one of the requirements to reach commercially viable PEMFCs. As detailed by Gasteiger et al. [1], the reduction of platinum loading must be led in two ways: (i) improvement of the catalyst activity and efficiency and (ii) improvement of the electrode structure in order to decrease the diffusion-induced potential losses. Improving the catalyst activity and efficiency needs either to optimise the metal dispersion, or to use appropriate metal alloys [1]. The increase of the reactants and products diffusion rate in the catalyst layer requires new catalyst layers structures. This could be achieved by replacing classical carbon blacks by nanostructured carbons issued from organic gels.

In previous studies [2], [3], [4], carbon aerogels were used as supports to prepare Pt/C catalysts designed for fuel cells. Carbon aerogels are nanostructured materials obtained by supercritical drying and pyrolysis of an organic gel, i.e. a 3D polymer. The polymer is usually a resorcinol–formaldehyde resin prepared in a solvent, water in most cases. Water removal is performed after several solvent exchanges (water to acetone or ethanol, then to CO2) under supercritical conditions in order to preserve at best the pore texture [5], [6]. Finally, pyrolysis leads to a porous carbon whose pore texture can be finely controlled. While carbon blacks are constituted of aggregates connected through Van der Waals bonds, carbon aerogel powders display monolithic structures at the micrometer scale: micron-sized monoliths are made of covalently bonded carbon particles, which preserves the pores located between the particles whatever the process applied to carbon aerogels. In particular, the pore texture of a carbon aerogel micromonolith remains identical in a catalytic layer of a Membrane-Electrode Assembly [4]: this implies that the porous structure of the catalytic layer does not depend on the carbon particles arrangement during the MEA preparation, as it is the case of carbon black aggregates. The cell performances obtained with MEAs prepared from carbon aerogel supported Pt catalysts are encouraging [4]: the results obtained with a measurement cell which gas feed system was not optimised are approaching those obtained with commercial cathodes. Moreover, modifying the pore texture of the carbon aerogel via the synthesis variables of the polymer induces significant changes of the cell performances essentially related, as suspected, to a modification of the mass transfer limitations.

However, due to its complexity, supercritical drying needs to be replaced by a simpler method to make these materials attractive for industrial and domestic uses. A few years ago, it was shown that very porous carbon materials with controllable pore texture can be obtained by simple evaporative drying and pyrolysis of resorcinol–formaldehyde gels when the values of the synthesis variables are correctly chosen [7], [8], [9]. The obtained materials are then called ‘carbon xerogels’. Although supercritical drying leads to the wider pore texture range [8], evaporative drying is sufficient in many cases: for example, diffusional limitations were completely avoided for catalyzed gas phase reactions by choosing carbon xerogel supports with an appropriate pore size [10]. Moreover, Pt can be easily dispersed on these materials: very high dispersions can be obtained by impregnation with H2PtCl6 aqueous solutions [11].

The aim of this paper is thus to test carbon xerogels as Pt catalyst supports for PEM fuel cell cathodes, and to compare their performances with those obtained by using carbon aerogels: the final target is to check whether or not carbon aerogels can be replaced by carbon xerogels in this particular application. A second objective is to determine how the pore texture of the electrocatalyst support influences the air-fed fuel cell cathode performances.

Section snippets

Support synthesis

Four carbon xerogels (labelled as X1–X4) and one carbon aerogel (A1, i.e. the reference) were prepared for this study. The materials were synthesized following procedures used in previous studies; since these procedures were selected from previous works, they are slightly different for carbon xerogels [8], [9] and the aerogel [2], [3]. As a general comment, gels were first obtained by polycondensation of resorcinol with formaldehyde in water, in the presence of Na2CO3. Table 1 regroups the

Supports and catalysts

The pore texture parameters of the carbon supports are reported in Table 1 and follow the results obtained in previous studies [7], [8], [9]. The synthesis variables of the gels were chosen so as to cover a very wide pore size range: from 25 nm to 7 μm (Fig. 1). The specific surface area, SBET, ranges from 565 m2/g to 716 m2/g; in parallel, the micropore volume, VDUB, ranges from 0.25 to 0.29 cm3/g. As expected, the total pore volume, Vv, and the bulk density, ρbulk, strongly depend on the value of

Catalyst design

From the active surface of Pt, APt,MEA, one can evaluate the fraction of Pt atoms that are actually active for the oxygen reduction reaction. Indeed, following Eq. (4), Pt particles about 1 nm in diameter display 100% dispersion: all the Pt atoms would be accessible for such a catalyst. Eq. (5) leads to the conclusion that such particles correspond to a Pt surface equal to 285 m2/gPt. One can then calculate a global Pt utilization ratio, uPt-tot = APt,MEA/285, which represents the fraction of Pt

Conclusions

This first study enabled us to evaluate the usefulness of carbon xerogels as Pt catalyst supports for PEM fuel cell cathodes. The objective was to determine whether these materials, which are very simple to prepare, could replace the carbon aerogels previously used in the same system without decreasing the cell performances, and to show how pore texture modifications influence the cell functioning. Carbon xerogels can lead to performances close to that of the carbon aerogels previously used

Acknowledgement

N.J. and S.L. are both postdoctoral researchers of the F.R.S.-F.N.R.S. (Belgium). The authors thank the Fonds de Bay, the Fonds de recherche Fondamentale Collective, the Ministère de la Région Wallonne, the Interuniversity Attraction Pole (IAP) and Renault for their financial support. The authors also acknowledge the involvement of their laboratory in the Network of Excellence FAME of the European Union sixth framework program.

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