Vibrating Powders: Electrochemical Quartz Crystal Microbalance Study of IrO2 and Pt/C Catalyst Layers for Voltage Reversal Tolerant Anodes in Fuel Cells

摘要

A fundamental understanding of the interaction between Pt and IrO2 catalysts in proton exchange membrane fuel cell (PEMFC) anodes is important to mitigate degradation during cell voltage reversal events by catalyzing the oxygen evolution reaction (OER) instead of carbon corrosion and Pt dissolution. The potential-dependent mass changes of untreated and different heat-treated (350, 450, and 550 degrees C) IrO2 and IrO2 + Pt/C powders were investigated in a simulated PEMFC environment by using the electrochemical quartz crystal microbalance (EQCM) technique. During successive potential cycling the frequency changes associated with IrO2 were correlated to the potential-dependent uptake of water and formation of iridium oxyhydroxide species, confirmed by X-ray photoelectron spectroscopy (XPS). Heat treatment of the IrO2 powders increased crystallinity, which triggered changes in the frequency response during accelerated cyclic voltammetry stress tests. During anodic polarization, the untreated IrO2 exhibited a steady increase in frequency beginning at ca. 0.6 V vs RHE, whereas the heat-treated IrO2 first revealed a frequency decrease and then leveled off before increasing. These observations were explained based on differences in potential-dependent water uptake and Ir oxyhydroxide formation as a function of IrO2 structural variations from amorphous to crystalline and Ir3+/4+ ratio. Pt/C alone when subjected to potential hold at 1.8 and 2.0 V vs RHE experienced significant mass decrease due mainly to carbon corrosion. The addition of IrO2 to Pt/C in the catalyst layer did not show any mass decrease during the same potential hold. The OER catalyzed by IrO2 protected Pt/C from degradation (i.e., carbon corrosion and Pt dissolution) at high potentials and created an ex situ voltage reversal tolerant anode.