Influence of silicotungstic acid on activity and durability of cathode electrocatalysts for proton exchange membrane fuel cells

Abstract

Proton exchange membrane fuel cells are nearing commercialization, but they still suffer from high costs, largely due to high Pt loadings in the cathode, and low durability due to Pt dissolution and carbon corrosion. This work attempts to address both issues through the development and testing of a novel catalyst. Colloidal Pt prepared by an ethylene glycol reduction method was deposited onto Ketjen black carbon supports functionalized with (0, 3.2, 7.1, and 15.9wt%) 11-silicotungstic acid (Pt/SiW11-C). Electrochemical characterization of the catalysts was performed using rotating disk electrodes (RDE) in electrolytes of 0.1 M HClO4 and 0.5 M H2/SO4. XRD and TEM respectively showed smaller crystallite size and more uniform deposition of Pt nano-particles for Pt/SiW11-C catalysts. A maximum in the ORR mass activity of 373 mA/mgPt was observed for the 3.2wt% SiW11 catalyst, an 18% improvement over Pt/C. An increase in the electrochemical area (ECA) due to lower Pt particle size and more narrow size distribution is attributed to providing the mass activity enhancement. After 30,000 durability cycles in the potential range 0.6-1.0 V, Pt/SiW11-C showed less Pt particle growth (TEM), and a factor of 1.4 improvement in terms of mass activity retention. After 6,000 durability cycles in the potential range 1.0-1.6 V, Pt/SiW11-C showed a factor of 2 increase in mass activity retention compared to Pt/C. The improvement is attributed to a slower rate of carbon corrosion. The optimal 3.2wt% SiW11 catalyst and the baseline Pt/C catalyst were scaled up and constructed into membrane electrode assemblies (MEAs). Fuel cell testing of the MEAs in an H2/O2 environment at 100% RH and 80 degrees C showed a similar improvement in ORR mass activity for the Pt/SiW11-C catalyst relative to Pt/C. Recommended future work includes further fuel cell optimization, fuel cell durability testing, use of carbons with different geometries, use of alternate heteropoly acids, and extension to platinum alloys.