Enhancing the Performance of Shape-Controlled Octahedral Rhodium-Doped PtNi Nanoalloys inside Hydrogen-Air Fuel Cell Cathodes Using a Rational Design of Catalysts, Supports, and Layering

Abstract

Octahedral PtNi alloy nanoparticles show a very high catalytic activity for the oxygen reduction reaction. However, their integration into membrane electrode assemblies (MEAs) is challenging, resulting in low fuel cell performance. We report the application of three strategies that are promising to improve the MEA-based fuel cell performance of octahedral PtNi alloy nanoparticles: (1) Rh surface doping to stabilize the morphology, (2) high Pt weight percentage loading on carbon to decrease the catalyst layer thickness (at parity of geometric-area-normalized Pt loading), and (3) N-functionalized carbon supports to more homogeneously distribute the ionomer. The surface chemistry of the Rh dopants is analyzed by in situ X-ray absorption spectroscopy (XAS) under applied potentials in a liquid half-cell. The Rh dopants are present at the catalyst surface with a local coordination to oxygen atoms as in Rh oxide and show potential dependent changes in the oxidation states. A rotating disk electrode (RDE) screening showed advantages in using Ketjen Black EC300J instead of carbon Vulcan XC72R to accommodate high Pt weight percentage loading (similar to 30 Pt wt %). Finally, a Rh-doped PtNi nanoparticle catalyst was grown on 3% nitrogen-doped Ketjen Black and tested in a MEA-based single cell after being annealed and acid washed. The results showed modest mass activity (MA), 0.35 A mg(Pt)(-1) at 0.9 V, but significantly high performance at high current density for octahedral PtNi nanoparticles, 1500 mA cm(-2) at 0.6 V, to our knowledge the highest to date for this class of catalysts. Despite this achievement, the full potential of N doping could not be utilized, with samples showing negligible differences with respect to undoped carbon in both high- and low-humidity MEA testing. Even though no enhancement of mass transport at high current density by better distribution of the ionomer on the N-doped carbon was seen in MEA, this could be due to the diffusion of Ni cations, affecting ionomer interaction and overwhelming the effect of nitrogen species on the support.