Water electrolysis has the potential to become a key element in coupling the electricity, mobility, heating and chemical sector via power-to-liquids or power-to-gas in a future sustainable energy systems [1]. While proton exchange membrane (PEM) water electrolysis offer several advantages over traditional electrolysis technology, advances are still needed in catalyst stability against corrosion and fluctuated loads for PEM water electrolysis [2].

In particular, the anode requires improvement in the activity and stability of the catalyst due to the sluggish kinetics of oxygen evolution reaction (OER) and the corrosive environment due to high overpotentials. The state-of-the-art anode catalyst in conventional PEMWEs is iridium oxide. 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]. Furthermore, platinum degradation on the cathode has been reported where platinum nanoparticles were coarsened after long-term operation [6, 7]. Although the platinum coarsening is not directly related to the cell potential increase 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 (Figure 1a, b). In addition, metallic iridium deposits are found in the electrolyte membrane adjacent to the anode catalyst for the post mortem MEA cross section (Figure 1c, d). EDX mapping of the deposits confirm the metallic state of iridium (Figure 1 e, f). Further analysis of the post mortem MEA cross section will elucidate through-plane catalyst loading distribution using high-resolution TEM and EDX analysis, for both anode and cathode.

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, 26, 120-128.

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

Figure 1. (a) SEM images of anode catalyst layer cross section before MEA test; (b) TEM image of anode catalyst layer cross section after MEA test; (c) Membrane region adjacent to anode catalyst layer; (d) dendritic iridium deposit in (c); (e) and (f) shows the EDX mapping of the iridium deposit.