Synthesis and Evaluation of RhxSy Catalyst with High Affinity for Nafion Ionomer for HOR/HER in a H2-Br2 Regenerative Fuel Cell

Abstract

Synthesis and evaluation of RhxSy with high mass-specific electrochemical surface area (ECSA/mass) and high affinity to Nafion polymer are reported. Besides the previously reported supporting carbon functionalization, the nanoparticle size was optimized by a diffusion-controlled growth process to control Rh2S3 monomer concentration, which reduced the average particle size from 13.2 to 5.2 nm and increased the ECSA/mass from 9.1 to 36.5 m2/g-Rh. The affinity issue between the surface ketone functional group and the Nafion ionomer in the catalyst ink was resolved by using the Baeyer-Villiger reaction and ester hydrolysis to convert it to the Nafion-friendly carboxylic group with no adverse effect on the precipitated catalysts. The H2-Br2 fuel cell tests confirmed that the high mass transfer issue observed with the RhxSy catalysts on ketone-dominated carbon substrate was resolved with the new catalysts, validating the hypothesis that the low affinity to Nafion polymer of the surface ketone group was the cause for the formation of thick Nafion polymer layer on the catalyst surface and high mass transportation resistance. Using the new catalyst, the discharge performance of the H2-Br2 fuel cell improved by 5.6 times (0.15 vs. 0.027 A/cm2 at 0.2 V below OCV) over that of the fuel cell with the RhxSy/untreated carbon catalyst.