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 (IrO_x) 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 IrO_x 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 IrO_x crystallinity, whereas its stability improves ^2,3.

Recent studies from our lab revealed that IrO_x can easily be reduced to metallic iridium (Ir) when IrO_x 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 IrO_x 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 IrO_x.

Considering the fact, that hydrous IrO_x exhibits less stability⁴ and that this partial IrO_x 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 IrO_x. 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

 

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(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).