Trading Off Initial PEM Fuel Cell Performance versus Voltage Cycling Durability for Different Carbon Support Morphologies

Abstract

Tailored design of carbon supports and their pore morphologies is crucial to achieve the ambitious durability and performance targets for future proton exchange membrane fuel cells (PEMFCs). We compared platinum catalysts supported on solid Vulcan carbon, porous Ketjenblack carbon, and accessible porous modified Ketjenblack carbon in a voltage cycling-based accelerated stress test (AST) with frequent intermittent characterizations. We derived how catalyst morphologies affect cell performance and electrochemical properties (electrode roughness factor, ORR activity, oxygen transport resistances) at beginning-of-life (BoL) and in various states of degradation up to 200,000 voltage cycles. We confirmed the enhanced Pt surface area retention of porous carbon-supported catalysts, ascribed to well-shielded Pt particles in internal pores, but find that this comes at the expense of lower initial high current density performance already at BoL. Accessible porous carbon-supported catalysts with wider pores mostly retain those durability benefits while, simultaneously, maximizing H2/air performance at all current densities due to improved oxygen transport. We also tracked changes in catalyst accessibility throughout voltage cycling by analyzing local oxygen transport resistances and relative humidity-dependent platinum utilization. We propose that catalysts with porous carbon supports undergo oxidative pore opening, followed by continuous migration of internal Pt particles to the external carbon surface.