Particle Size Effect on Platinum Dissolution: Considerations for Accelerated Stability Testing of Fuel Cell Catalysts

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) are highly attractive for use in electric vehicles. In PEMFCs, small particle sizes of the Pt catalyst are required to increase Pt utilization, which lower costs and also increase Pt dispersion, which itself minimizes O2 mass transport losses. Reducing the particle size improves both the utilization and dispersion; however, stability of small particle sizes is an issue. Pt dissolution, as one of the major degradation mechanisms of PEMFC catalyst layers, is predicted to depend on the particle size. The particle size cannot be independently varied from loading in commercial Pt/C materials that have been used in numerous studies employing accelerated stability tests (ASTs). Therefore, in the study presented here, the Pt particle diameter was varied from 2 to 10 nm by depositing mass-selected nanoparticles on a flat glassy carbon substrate using magnetron sputtering. This allows exclusive control over particle density (interparticle distance) and particle size, which becomes difficult even with advanced synthetic techniques for applied Pt/C materials. Additionally, effects of the 3D porous support are eliminated. These model systems were subjected to an aggressive AST in order to cause significant Pt dissolution, which was monitored online using the scanning flow cell coupled to an inductively coupled plasma mass spectrometer. The results uncover a previously overlooked phenomenon: two competing trends in dissolution from (1) particle passivation caused by particle size-dependent shifts on oxophilicity and (2) electrochemically active surface area. Therefore, crucial impacts of the particle size on degradation may be overlooked in a variety of electrochemical studies, which compare catalytic materials. Finally, suggestions are given for improved ASTs for dissolution, which provide insights into the intrinsic stability of Pt nanoparticles toward dissolution.