Degradation Mechanism of an IrO2 Anode Co-Catalyst for Cell Voltage Reversal Mitigation Under Transient Operation Conditions of a PEM Fuel Cell

Abstract

The cell voltage reversal which can occur during the transient operation of PEM (proton exchange membrane) fuel cells on account of a short under-supply of hydrogen to one or several cells in a fuel cell stack can lead to the substantial degradation of anode catalysts and anode electrodes. This damage can be substantial even if the cell reversal only occurs over time scales of tens of milliseconds, which puts a high requirement on the fuel cell controls system. During cell reversal, the anode potential rises to far above 1 V vs. the reversible hydrogen electrode potential (RHE), causing platinum dissolution and oxidation of the carbon support; the latter leads to a collapse of the anode catalyst layer and to cell failure. One strategy to mitigate the damages of H2-starvation is the addition of a co-catalyst to the anode electrode which catalyzes the oxygen evolution reaction (OER), so that the non-damaging OER rather than the damaging carbon oxidation reaction (COR) will take place. A commonly used anode co-catalyst to facilitate the OER over the COR during cell reversal events is iridium oxide (IrO2).1 Many studies have investigated the effect of IrO2 co-catalyst on mitigating cell reversal damages in PEMFC anodes.2 However, there is no comprehensive study on the effect of the strongly reducing hydrogen atmosphere in a PEMFC anode on the chemical stability of IrO2, which could ultimately affect the stability of an IrO2 anode co-catalyst particularly during the repetitive transitions between normal operating conditions where the anode potential is at ≈0 V vs. RHE and the high anode potentials (≥1 V vs. RHE) during cell voltage reversal and start-up/shut-down (SUSD) transients. That these voltage cycles might be damaging is indicated in a recent publication from our group, where we had shown that H2 permeating through the membrane from the hydrogen electrode to the oxygen electrode of a PEM water electrolyzer during open circuit conditions voltage (OCV) leads to a reduction of the surface of IrO2 OER catalyst to metallic Ir, which in turn gets oxidized to an amorphous IrOx during the subsequent normal operation period of the electrolyzer at the high oxygen evolution potentials. The repetitive cycling between reducing conditions during OCV periods and oxidizing conditions during operation were shown to cause the enhanced dissolution of Ir, which could result in a loss of active material.3 In this contribution, we will discuss the consequences of using an IrO2 based anode co-catalyst to mitigate cell voltage reversal degradation in PEMFCs. Thermal gravimetric analysis (TGA) in reductive atmosphere (5 volume% H2/Ar) is a powerful tool to simulate the chemical environment of PEMFC anode during normal operation. By utilizing this tool, it is shown that the near-surface region of IrO2 can be completely reduced to metallic Ir under the operating condition of PEMFC anode. The formation of metallic Ir has been proven by post characterization of the reduced powders after the TGA experiments using X-ray photoelectron spectroscopy (XPS) and by cyclic voltammetry, where the development of hydrogen under potential deposition (HUPD) features of metallic Ir reveals the time-dependent reduction of IrO2 upon extended exposure to PEMFC anode conditions. Finally, it will be shown that anode potential transients caused, e.g., by SUSD events, leads to the dissolution of iridium from the IrO2 based anode co-catalyst. The consequences of this on the long-term PEMFC performance will be discussed.