(510g) Degradation Mechanisms of PEM Water Electrolysis MEA after Long-Term Operation (Invited)

The volatile character of renewable energy sources, such as wind and solar energy, requires an energy storage system to compensate for the fluctuating power production and seasonal imbalances [1]. Water electrolysis has become the key technology for energy storage to couple with renewable energy sources [1]. For this purpose, proton exchange membrane (PEM) water electrolysis offers several advantages over alkaline water electrolysis, such as fast dynamic responses, high-pressure operation, low gas crossover, and more compact design. However, advances are still needed for PEM water electrolysis in terms of reducing the catalyst loading and improving the catalyst stability and system lifetime [2].

Due to the sluggish kinetics of oxygen evolution reaction (OER) and the corrosive environment at high overpotentials, iridium oxide has been the state-of-the-art catalyst for the anode of PEM water electrolyzers. However, iridium suffers from dissolution at high anodic potentials [3] albeit being the most corrosion resistant metal known. The iridium dissolution in a MEA is evidenced by the iridium deposits in the electrolyte membrane [4, 5]. On the other hand, the cathode catalyst for PEM water electrolyzers is typically platinum supported on carbon. It has been reported that platinum nanoparticles were coarsened after long-term operation [6, 7]. Although the coarsening of platinum is not directly related to the cell potential degradation after long-term operation, the stability of the platinum nanoparticles is important in the context of reducing the platinum loading [6].

In this study, post mortem MEAs are analyzed with electron microscopy to investigate the degradation mechanisms and to provide guidance for future electrode design. The full MEA was fabricated by reactive spray deposition technology (RSDT) with ultra-low iridium and platinum loading. The anode catalyst is composed of IrOx nanoparticles embedded in Nafion ionomer with an iridium loading of 0.08 mg cm^-2; while the cathode is platinum supported on carbon black with a platinum loading of 0.3 mg cm^-2. These loadings are about 10% of the loadings that are currently used in state-of-the-art electrolyzers.

The full RSDT-derived MEA successfully demonstrated a long-term operation of 4500 hours. Preliminary study on the anode catalyst degradation shows that the anode catalyst layer thickness is reduced by 15% due to compression and/or loss of iridium catalyst. In addition, metallic iridium deposits are found in the electrolyte membrane adjacent to the anode catalyst for the post mortem MEA cross section. The metallic character of the iridium deposits are confirmed with HAADF EDX mapping. Further analysis of the post mortem MEA cross section will elucidate through-plane catalyst loading distribution to investigate the catalyst migration and redistribution phenomena during long-term electrolysis operation.

References

[1] Buttler, A., Spliethoff, H., Renewable and Sustainable Energy Reviews, 2018, 82, 2440-2454.

[2] Spori, C., Kwan, J.T.H., Bonakdarpour, A., et al., Angew. Chem. Int. Ed., 2017, 56, 5994-6021.

[3] Cherevko, S., Geiger, S., Kasian, et al., Journal of Electroanalytical Chemistry, 2016, 774, 102-110.

[4] Grigoriev, S.A., Bessarabov, D.G., Fateev, V.N., Russian Journal of Electrochemistry, 2017, 53, 318-323.

[5] Lettenmeier, P., Wang, R., Abouatallah, R., et al., Electrochimica Acta., 2016, 210, 502-511.

[6] Rakousky, C., Reimer, U., Wippermann, K., et al., Journal of Power Sources, 2016, Journal of Power Sources, 2016,26, 120-128.

[7] Siracusano, S., Baglio, V., Van Dijk, N., et al., Applied Energy, 2017, 192, 477-489.