Engineering PGM-Free Electrodes to Facilitate Improved Performance for the Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells

Abstract

With the aiming of reducing stack cost, the investigation of platinum group metal (PGM)-free catalysts for oxygen reduction reaction (ORR) has gained a lot of interest in the recent decades,1 with the majority of focus on improved electrocatalysis via various precursors and synthesis methods. The most promising PGM-free catalysts in terms of activity are carbon-based materials doped with nitrogen and active transition metals (like Fe, Co, and Mn), obtained by high temperature pyrolysis.2 The literature regarding synthesis, characterization, and rotating disk electrode (RDE) performance of this class of PGM-free materials is abundant, with many works reporting beginning of life performance testing in PEFC. Relative to the extensive literature on electrocatalyst fabrication, a less extensive and rigorous examination of PGM-free electrode design has been performed. A number of variables like ionomer content, catalyst deposition method (e.g. spray coating, blade coating, hand painting), ink concentration, ink solvent composition and ink evaporation rate may influence PEFC performance. All of these process variables, along with the properties of the electrocatalyst itself, can alter the electrode ionomer distribution,3 resulting is significant differences in gas transport and electrode proton conductivity.3–5 In this work, we examine the impact of ionomer content, ink deposition method, and ink solvent composition on the PEFC performance of a commercially available PGM-free catalyst (Pajarito Powder LLC). Electrode properties will be assessed using different electrochemical diagnostics techniques. The viability of H2 limiting current will be discussed as a means to extract the pressure dependent and independent mass transport resistance of the electrode layer.6,7 Cyclic voltammetry and electrochemical impedance spectroscopy will be used to determine electrode capacitance and subsequently related to ionomer coverage and proton conductivity8,9 and contrasted with SEM images of MEA cross sections. Through this methodology we will demonstrate how the electrode fabrication process plays a crucial role in electrode properties and the absolute performance.