Investigation of IrO2 Stability As a Cell-Reversal Mitigation Catalyst in PEMFC Anodes

Abstract

In the recent years, the use of fuel cells for the propulsion of passenger vehicles and heavy-duty vehicles has gained renewed interest as alternative to internal combustion engine-powered vehicles. Particularly for the latter, the long-term durability of proton-exchange-membrane fuel cells (PEMFCs) is still a concern, which in part is governed by the catalyst layer durability during system operating conditions.1 During the operation of a PEM fuel cell system, certain transient conditions can occur that can lead to rapid degradation of the catalyst layers. On one hand, the widely studied start-up/shut-down (SUSD) phenomenon leads to significant degradation of the cathode catalyst layer due to the exposure of the cathode electrode to high potentials (>>1.0 V vs the reversible hydrogen electrode potential (VRHE)) that results from a H2/air-front passing through the anode flow field.2 On the other hand, an intermittent under-supply of hydrogen to one or several cells of a PEMFC stack can lead to a so-called cell-reversal (CR) event, resulting in the oxidation of the carbon support of the anode catalyst and in the subsequent collapse of the anode catalyst layer structure. The latter can be mitigated by the addition of an anode co-catalyst that facilitates the oxygen evolution reaction (OER) at much lower potentials than the carbon-supported platinum catalyst (Pt/C) that serves as anode catalyst for the hydrogen oxidation reaction (HOR).3 However, as shown by our recent study, the stability of the anode co-catalyst for the OER during SUSD events must be considered.4 In the present study, we examine different accelerated stress tests (ASTs) for single-cell PEMFCs that are designed to assess the stability of an anode co-catalyst for the OER in a PEMFC that is subjected to cell-reversal and SUSD events. Thus, we evaluated the stability of an anode co-catalyst under four conditions: i) subjecting the cell to extended SUSD cycles; ii) a cell-reversal protocol where a constant current is being drawn when feeding N2/air to the anode/cathode until a given cut-off cell potential is reached; iii) a protocol where cell-reversal cycling is induced by switching the anode feed gas between hydrogen and nitrogen while applying a constant current and supplying air to the cathode (later on referred as CR cycling); and, iv) an anode potential cycling protocol where the anode is supplied with N2, and its potential is cycled potentiostatically to different upper potential limits, with the cathode purged with air and serving as a counter electrode. With these ASTs, we examine the effect of the different anode potential cycles during SUSD and CR transients in order to elucidate the dependence of the IrO2 dissolution rate on operating potential.The stability of the IrO2 based anode co-catalyst is investigated by electrochemical measurements (cyclic voltammetry and polarization curves) and additionally via ex-situ XPS analysis. For example, Figure 1 shows the ex-situ XPS analysis of pristine and aged membrane electrode assemblies (MEAs). The upper two panels show the XPS data in the Pt 4f (BE>67 eV) and the Ir 4f (BE<67 eV) regions of the pristine Pt/C anode with an IrO2-based anode co-catalyst (Figure 1a) as well as the pristine Pt/C cathode (Figure 1b). The lower two panels show the cathode XPS data of the MEAs after 500 SUSD cycles (Figure 1c) and after 500 CR cycles (Figure 1d). In the case of the SUSD degraded MEA, a higher amount of Ir is observed on the cathode when compared to the CR degraded MEA. The mechanistic implications of these data will be discussed. References USDRIVE Fuel Cell Technical Team Roadmap (2017): https://www.energy.gov/eere/vehicles/downloads/us-drive-fuel-cell-technical-team-roadmap (accessed on 28.10.2021).T. Mittermeier, A. Weiß , F.Hasché, G. Hübner, H. A. Gasteiger. Journal of The Electrochemical Society, 164 (2), F127-F137 (2016).T. R. Ralph, S. Hudson, and D. P. Wilkinson, ECS Transactions, 1 (8), 67-84 (2006).M. Fathi Tovini, A. M. Damjanović, 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). Figure 1