Stability of IrO2 Oxygen Evolution Reaction Anode Co-Catalysts Under Transient Operation and Its Effect on PEM Fuel Cell Durability

Abstract

Among various degradation phenomena in polymer-electrolyte-membrane fuel cells (PEMFCs), cell reversal (CR) occurring due to an under-stoichiometric supply of H2 to the anode is known to cause the irreversible corrosion of the carbon support of carbon-supported platinum (Pt/C) anode catalysts via the carbon oxidation reaction (COR). Several strategies to mitigate the damages of cell reversal have been documented in the literature, from which the addition of an oxygen evolution reaction (OER) catalyst (e.g., IrO2) to the anode has attracted great attention. The lower OER onset potential in the presence of IrO2 impedes the increase of the anode potential during cell reversal events, thus reducing the COR rate.A recent study from our group showed that in spite of the high OER activity of an IrO2-based anode co-catalyst, its stability under transient PEMFC operation conditions, e.g., during unmitigated start-up/shut-down (SUSD) events, should be considered for long-term durability (1). The near-surface layer(s) of IrO2 can be completely reduced to metallic Ir upon exposure to H2 under typical operating conditions in a PEMFC anode, leading to Ir dissolution during the anode potential transients associated with SUSD cycles. The diffusion of dissolved Irn+ species through the membrane towards the cathode and its further deposition on the Pt/C catalyst reduces its oxygen reduction reaction (ORR) activity and results in a significant performance loss during normal PEMFC operation. Since the origin of such degradation mechanism is the chemical reduction of the IrO2 anode co-catalysts in the hydrogen environment of the anode, a reduction-resistant IrO2 would be required for long-term to stabilize such an anode co-catalyst (1). In a recent contribution from our group, we have introduced a novel approach to prepare IrO2 anode co-catalysts that are irreducible (proven by TGA analyses and observed after extended PEMFC operation). In contrast to typically employed IrO2 catalysts (usually prepared by heat treatment of an Ir precursor at T ≈ 400 °C), the stability of these irreducible IrO2 (irr-IrO2) catalysts is highlighted by the application of SUSD transients, where Ir dissolution is drastically reduced, so that no iridium poisoning of the Pt/C cathode catalyst is observed (2).In this work, we investigate the stability of a state-of-the-art IrO2 catalyst and this novel irr-IrO2 catalyst when used as anode co-catalysts, applying two different accelerated stress tests (ASTs): a) SUSD transients + CR cycles (relevant for PEMFC systems); and b) CR cycles only (commonly used as test to evaluate the effectiveness anode co-catalysts). The application of both ASTs allows the separation and comparison of trends in the activity of the OER catalyst (i.e., its ability to mitigate the COR at the anode) and in its stability (i.e., its stability towards Ir dissolution). The later is particularly important when considering the applicability of anode co-catalysts for actual PEMFC applications. References M. Fathi Tovini, A. M. Damjanovic, H. A. El-Sayed, J. Speder, C. Eickes, J.-P. Suchsland, A. Ghielmi, and H. A. Gasteiger, Journal of The Electrochemical Society, 168 (6), 064521 (2021). M. Fathi Tovini, A. M. Damjanović, H. A. El-Sayed, F. Friedrich, B. Strehle, J. Speder, A. Ghielmi and H. A. Gasteiger, : I03-1466 Irreducible IrO2 Anode Co-Catalysts for PEM Fuel Cell Voltage Reversal Mitigation and Their Stability Under Transient Operation Conditions, in 241st Electrochemical Society Meeting, Vancouver, BC, Canada (2022).