In times of an increasing demand for energy production through renewable but fluctuating energy sources, such as wind or solar energy, hydrogen as an energy carrier becomes more and more important. Proton-exchange membrane water electrolysis (PEM-WE) is a suitable and already quite advanced technique for sustainable production of hydrogen.¹ However, coupling a PEM-WE with intermittent renewable energy sources will induce frequent current interrupts of the PEM-WE system. These events can potentially lead to rapid degradation of the membrane electrode assemblies (MEAs) and hence, a thorough understanding of the underlying mechanisms is crucial to assess the stability and lifetime of a PEM-WE and to choose appropriate operating conditions.

In this work, we present a test protocol involving operation at high (3 Acm^-2_geo) and low (0.1 Acm^-2_geo) current density, alternating with current interrupts during which the system remains at the open circuit voltage (OCV). Previous studies in our lab revealed that the permeation of hydrogen through the membrane into the anode compartment during extended OCV periods can cause the reduction of IrO_x², the most commonly used anode catalyst for the oxygen evolution reaction (OER) owing to its decent activity and high stability. During a subsequent start-up of the PEM‑WE, metallic Ir is oxidized to a hydrous Ir-oxide. The transformation of the catalyst surface was probed by cyclic voltammetry (CV) during the degradation test. While the initial CV (Fig. 1, black curve) typical for crystalline IrO_x is essentially featureless, CVs recorded after ten current-interrupt cycles revealed the formation of hydrogen under-potential-deposition (H-UPD) features (region 1, blue curve), which are characteristic for metallic Ir electrodes.³ The redox-features evolving at ≈0.8 V are characteristic of an amorphous, hydrous Ir-oxide (region 2).⁴ The appearance of these hydrous Ir-oxide features indicates a change in hydration state as well as in surface chemistry, which is known to affect both the OER activity and the stability of IrO_x.⁵ Amorphous hydrous Ir-oxide exhibits higher OER activity but lower stability compared to crystalline thermally grown IrO_x. Interestingly, the polarization curve recorded directly after IrO_x reduction during an OCV period shows a lower cell voltage (i.e., improved OER activity), thus supporting the formation of a hydrous Ir-oxide. However, since this hydrous oxide is less stable, a rapid decay of cell performance over an extended number of OCV cycles due to Ir dissolution/re‑precipitation occurs.

In summary, this study will provide a better understanding of the MEA degradation mechanism occurring over an extended number of OCV cycles, which could result in a PEM‑WE system when operated with intermittent renewable energy sources. This implies that hybridization strategies are required to maximize PEM‑WE durability. In addition, PEM‑WE load cycles to OCV may serve as an accelerated aging test.

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. Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., A Comprehensive Review on Pem Water Electrolysis. International Journal of Hydrogen Energy 2013, 38, 4901-4934.
2. Weiß, A.; Bernt, M.; Siebel, A.; Rheinländer, P. J.; Gasteiger, H. A., ECS Meet. 232 2017, Abstr. # I01F-1648.
3. Woods, R., Hydrogen Adsorption on Platinum, Iridium and Rhodium Electrodes at Reduced Temperatures and the Determination of Real Surface Area. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1974, 49, 217-226.
4. Pickup, P. G.; Birss, V. I., A Model for Anodic Hydrous Oxide Growth at Iridium. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1987, 220, 83-100.
5. Reier, T.; Teschner, D.; Lunkenbein, T.; Bergmann, A.; Selve, S.; Kraehnert, R.; Schlögl, R.; Strasser, P., Electrocatalytic Oxygen Evolution on Iridium Oxide: Uncovering Catalyst-Substrate Interactions and Active Iridium Oxide Species. Journal of The Electrochemical Society 2014, 161, F876-F882.

Figure 1 Cyclic Voltammograms (CVs) recorded at 50mV/s during the accelerated degradation before cycling (black) and after 10 cycles (blue) at 80 °C, ambient pressure and 5 mL min^-1_H2O (anode)/ 50 mL min^-1 H₂ (cathode) for an MEA with ~1.6 mg_Ir cm^-²_MEA anode and ~0.3 mg_Pt cm^-²_MEA cathode loading using a Nafion^® 212 (~50 µm) membrane