Abstract
Carbon xerogels characterized by different textural, structural and chemical properties were synthesized and used as supports for Pt catalysts for the application in polymer electrolyte fuel cells. Synthesis conditions were varied in order to synthesize carbon xerogels following the sol-gel method. These included the reactants ratio (precursor/formaldehyde), the catalyst concentration (precursor/catalyst ratio) and type (basic and acid), the precursor type (resorcinol and pyrogallol) and the solvent (aqueous or acetone based). Stoichiometric mixtures of resorcinol and formaldehyde yielded well polymerized gels and highly developed structures. Slow gelation, favored by the presence of acetone as solvent in the sol and low catalyst concentration, resulted in higher polymerization extent with a highly mesoporous or even macroporous texture and more ordered structure, as evidenced by XPS and Raman spectroscopy. Small Pt particles of ca. 3.5 nm were obtained by using carbon xerogels characterized by an ordered surface structure. The specific activity towards the oxygen reduction reaction, i.e., the limiting catalytic process in low temperature fuel cells, is significantly favored by highly ordered carbon xerogels due to a metal-support enhanced interaction. Nevertheless, surface defects favor the distribution of the metallic particles on the surface of carbon, which in the end influences the effectiveness of the catalyst. Accelerated degradation tests were conducted to evaluate catalyst stability under potential cycling conditions. The observed decay of performance was considerably lower for the catalysts based on ordered carbon xerogels stabilizing Pt particles in a higher extent than the other xerogels and the commercial carbon black support.
Highlights
Since their introduction by Pekala in 1989 [1], resorcinol–formaldehyde gels have been extensively studied
Influence of Synthesis Conditions in the Textural Properties of Carbon Xerogels. Though it will be further on described in detail within the experimental section, note that the synthesized carbon xerogels were labelled as follows: CX stands for carbon xerogel, followed by an identifying number and the main variable changed in its synthesis
Carbon xerogels followed by RC-800-W or RC-1500-W represent carbon materials synthesized with a resorcinol/catalyst molar ratio = 800 or 1500, and water (W) as solvent
Summary
Since their introduction by Pekala in 1989 [1], resorcinol–formaldehyde gels have been extensively studied. The uniqueness of the textural properties of these synthesis-tailored carbon materials include high porosity (>80%), high surface area (400–1200 m2 g−1) and controlled pore size distributions (1 to 50 nm) These properties are fundamental requires in the application of carbon materials as supports in the preparation of catalytic systems, especially within the field of fuel cell technologies [5,6]. Several studies point out the key importance of a macro- mesopore enriched porous structure in carbon materials for fuel cells mass transportation processes [9,10,11] This fact stands out as a unique advantage of using carbon gels as electrocatalyst supports, in comparison to the mostly microporous carbon blacks normally used in the preparation of the commercially available catalytic systems. The activity towards the oxygen reduction reaction (ORR), occurring in the cathode compartment of polymer electrolyte fuel cells, as well as the resistance to potential cycling degradation was evaluated for the carbon xerogel-supported Pt electrocatalysts
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