Accelerated Degradation Test in PEM Water Electrolysis

Abstract

Proton exchange membrane water electrolysis (PEM-WE) is a suitable technology for producing hydrogen via electricity generated from renewable but fluctuating energy sources, such as wind or solar energy. Due to their modest activity but sufficiently high stability in the acidic environment of a PEM system, iridium oxides (IrOx) are the most commonly used catalysts for the oxygen evolution reaction (OER).1 However, recent studies revealed a strong correlation between the OER activity and the stability of IrOx depending on its surface morphology and hydration state (i.e., between highly crystalline thermal IrO2 and amorphous, hydrous oxides). Bulk IrO2 provides the best compromise regarding long lifetime requirements, since its OER activity decreases with inreaseing IrOx crystallinity, whereas its stability improves 2,3. Recent studies from our lab revealed that IrOx can easily be reduced to metallic iridium (Ir) when IrOx based membrane electrode assemblies (MEAs) are held at open circuit voltage (OCV) in a PEM-WE, where crossover H2 from the cathode side reduces the surface of the IrOx catalyst at the anode. This reduction step is indicated by the formation of H-UPD features in the recorded cyclic voltammograms (CVs). Interestingly, the polarization curve recorded directly after this reduction shifts towards lower cell voltages, corresponding to an improved OER activity (Figure 1). Moreover, in a subsequent CV (recorded after the latter polarization curve), the H-UPD features disappeared while the characteristic oxide formation and reduction peaks evolved, pointing towards a change of the catalyst surface properties to a state closer to less crystalline, hydrous IrOx. Considering the fact, that hydrous IrOx exhibits less stability4 and that this partial IrOx reduction and re-oxidation can occur during cycles of extended OCV periods, these findings must be considered as potential degradation mechanism in PEM-WE operation. During the lifetime of an electrolyzer, especially if coupled with a fluctuating power supply, operation interruptions can be expected to occur frequently, thereby altering the form of the IrOx. Therefore, we apply an accelerated test protocol cycling between operation and OCV periods to investigate this degradation mechanism further as well as the effect of potential mitigation strategies. Acknowledgements: This work was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies) and by the German Ministry of Education and Research (funding number 03SFK2V0, Kopernikus-project P2X). References (1) C. Rozain, E. Mayousse, N. Guillet and P. Millet, Appl. Catal. B, 182, 123 (2016) (2) T. Reier, D. Teschner, T. Lunkenbein, A. Bergmann, S. Selve, R. Kraehnert, R. Schlögl, and P. Strasser, J. Electrochem. Soc., 161, F876 (2014). (3) S. Cherevko, T. Reier, A. R. Zeradjanin, Z. Pawolek, P. Strasser, and K. J. J. Mayrhofer, Electrochem. Commun., 48, 81 (2014). (4) S. Geiger, O. Kasian, B. R. Shrestha, A. M. Mingers, K. J. J. Mayrhofer and S. Cherevko, J. Electrochem. Soc., 163, F3132–F3138 (2016) Figure 1 Polarization curves (A) at initial state (black) and after the reduction step (blue); IV plots were recorded galvanostatically at 80 °C and ambient pressure on a MEA (Nafion 212) with 5 cm² active area, fed with 5 mLmin-1 liquid H2O to the anode. HFR values estimated from impedance spectra recorded during the IV plots are displayed in (B). Figure 1