Irreducible IrO2 Anode Co-Catalysts for PEM Fuel Cell Voltage Reversal Mitigation and Their Stability Under Transient Operation Conditions
Authors: Fathi Tovini Mohammad , Damjanovia Ana Marija , A. El-Sayed Hany , Friedrich Franziska , Strehle Benjamin , Speder Jozsef , Ghielmi Alessandro , Gasteiger Hubert Andreas 
Published: July 2022

The cell voltage reversal that can occur during the transient operation of a proton exchange membrane fuel cell (PEMFC) stack leads to a substantial degradation of the anode catalyst. During cell reversal, the anode potential increases (>>1 V vs. the reversible hydrogen electrode potential (RHE)), causing severe oxidation of the anode catalyst carbon support, which 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) takes place. A commonly used anode co-catalyst to favor the OER over the COR during cell reversal events is iridium oxide (IrO2).1

Recent findings show that the near-surface layer(s) of IrO2 can be completely reduced to metallic Ir upon exposure to H2, e.g., under the operating conditions of a PEMFC anode.2, 3 Such alteration of the near-surface layer(s) of IrO2 drastically affects its stability during the anode potential transients that occur during start-up/shut-down (SUSD) events, where our recent study showed that the dissolution of metallic Ir and crossover of the dissolved Irn+ species through the membrane to the cathode electrode cause iridium deposition on the Pt/C cathode catalyst.3 Such iridium-based contamination on the cathode catalyst surface deteriorates the oxygen reduction reaction (ORR) activity of Pt and results in a significant performance loss during the normal operation of the fuel cell. At the same time, SUSD transients also cause an OER activity loss of the anode co-catalyst, which was shown to be mainly due to the redeposition of Pt dissolved from the anode hydrogen oxidation reaction (HOR) catalyst onto the reduced IrO2 anode co-catalyst, blocking its OER active sites.3 Since these degradation mechanisms are caused by the chemical reduction of the typically employed IrO2 anode co-catalysts in the PEMFC anode, a reduction-resistant OER catalyst would be required.
In this contribution, we introduce an unprecedented approach to synthesize IrO2 catalysts that are not reduced in the PEMFC anode environment. The preparation of such irreducible IrO2 (Irr-IrO2) catalysts is based on an industrially scalable procedure consisting of a high-temperature (650-1000 ᵒC) heat treatment step followed by a deagglomeration step and a post annealing step to prepare catalyst powders with specific surface areas of ~25 m2/g. Figure 1 shows the thermogravimetric analysis (TGA) according to our previously proposed evaluation protocol3 under 3.3 vol.% H2/Ar of an as-received commercial benchmark catalyst (IrO2/TiO2 (Umicore)), a self-made IrO2 catalyst heat-treated near the conventional temperature of 500 °C (IrO2-500 °C), and a stabilized IrO2 catalyst prepared by a procedure introduced in this contribution (Irr-IrO2). It can be seen that the temperature that corresponds to the reduction of 20% of the IrO2 phase (α = 20%) of the Irr-IrO2 catalyst is ~38 °C and ~53 °C higher than those of the IrO2-500 °C and the IrO2/TiO2 (Umicore) catalysts, respectively. Considering the fact that all of these catalysts have a specific surface area of ~25 m2/g, the observed reductive stability improvement of Irr-IrO2 catalyst is due to its intrinsic structural distinctions from the typically synthesized IrO2 and the commercial benchmark IrO2 catalysts. As will be shown, this higher stability is reflected by the observation that SUSD cycling of MEAs with the Irr-IrO2 as anode co-catalyst does not result in iridium dissolution and the associated iridium poisoning of the ORR activity of the cathode catalyst. Furthermore, while its OER activity is lower than that of conventional IrO2 catalysts, it still dramatically increases the cell reversal tolerance time of a conventional Pt/C anode.

References

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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,
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Figure 1. TGA (5 K min-1) experiments under 3.3 vol.% H2/Ar with the as-received IrO2/TiO2 (Umicore), the IrO2-500 °C, and the Irr-IrO2 catalyst powder samples (following a drying/cleaning step; see Ref. 3 for further details). The y-axis represents the fraction of the IrO2 phase in the catalyst powder samples that is reduced to metallic Ir (α). The black horizontal dashed line illustrates a reaction extent of α= 20%, to compare the stability of different samples in a H2-containing atmosphere.

Figure 1