(Invited) Unravelling the Core of Fuel Cell Performance: Engineering the Electrolyte (ionomer)/Catalyst Interface

Abstract

The biggest challenge of the widespread implantation of the polymer electrolyte membrane fuel cells (PEMFCs) is the cost, primarily due to the use of platinum catalysts. The high intrinsic catalyst activity exhibited using a rotating disc electrode (RDE) is rarely realized in the membrane electrode assembly (MEA), which is the core of PEMFC, due to the difference on the electrolyte(ionomer)/catalyst interfaces. To translate the catalyst RDE performance into MEA, the design of an ideal ionomer/catalyst interface is proposed: a thin, conformal ionomer film covers the maximum surface of a Pt nanoparticle that simultaneously maximizes catalyst utilization, (i.e. high mass activity and electrochemical active surface area) and O2 diffusion (i.e. high current density performance) without compromising proton conduction. Building such an interface is a long-standing challenge due to no control of ionomer distribution over catalyst particle, resulting in large ionomer agglomerates and inhomogeneous ionomer coverage over the catalyst nanoparticle, consequently, poor fuel cell performance. In this work, this ionomer/catalyst interface has been constructed utilizing the electrostatic charge attraction between positively charged catalyst and negatively charged ionomer particles in a liquid and preserved into a solid catalyst layer. Consequently, this interface leads to the previously unachieved fuel cell performance on both the catalyst utilization (75% vs. 45%) and the rated/peak power density (Pt/C, 0.910/1.430 W/cm2 for H2/Air, Pt loading: 0.1 mgPt/cm2), matching that of Pt alloy catalysts. This work demonstrated the formation of the interface in the liquid phase (using ultra-small angle x-ray scattering in combination with cryo-TEM, isothermal‐titration‐calorimetry) and the preserved interface in the solid catalyst layer (using TEM) and estimated the effective coverage and thickness of the ionomer film (using the limiting current density, RDE and fuel cell performance).