Platinum catalysts are widely used as electrocatalysts in polymer electrolyte membrane fuel cells (PEMFC). However, there are many factors that may lead to their degradation in PEMFC and one of them are impurities, which can penetrate the cell together with the feeding gases and then poison the catalyst [1]. Sulfur contaminants are strong undesirable poisons for the fuel cell even though they are usually present at very low levels (up to 5 ppm) in both fuel and air streams [2]. Hydrogen sulfide can adsorb onto platinum and affect the catalyst performance causing the loss of mass activity [3].

In this work the influence of H₂S contamination on commercially available Pt catalyst with carbon support was studied by means of rotating ring-disk electrode experiments in perchloric acid. Accelerated stress testing (AST) was performed in terms of potential cycling up to 10.000 cycles in the absence of contaminants and in the presence of H₂S for comparison. Cyclic voltammetry technique in terms of hydrogen under potential deposition (HUPD) and CO stripping was employed in order to determine the electrochemical surface area (ECSA). Oxygen reduction reaction (ORR) was used to identify kinetic parameters including kinetic limited current, Tafel slopes, mass and specific activity of the catalyst before and after degradation.

After AST the platinum degradation was observed in the presence of the contaminant as well as in the absence of H₂S. Highly reproducible measurements showed that derived from HUPD and CO methods ECSA in the presence of poisoning species was slightly higher in comparison to the ECSA obtained in the absence of contamination. In contrast, the calculated catalyst activity after stress test in the presence of contaminations dropped more significantly compared to the test without poisoning species. This fact may be attributed to the sulfur adsorption on the catalyst layer leading to morphology changes [3]. Moreover, the increased Tafel slopes after AST may be an indication of the changes of the ORR mechanism from a four electron reaction to a two electron process reaction.

[1] R. Borup, Chemical Reviews 107 (2007) 3904–3951.

[2] Fuel Cell Handbook, US Department of Energy (2008).

[3] R. Mohtadi et al., Electrochemical and Solid-State Letters 6 (2003) A272–A274.