Degradation of alkaline electrolyser electrodes

Kurzfassung

The RESelyser project has developed high pressure, highly efficient, low cost alkaline water electrolysers (AWE) that can be integrated with renewable energy power sources incorporating Raney Ni coated electrodes, an advanced membrane concept and a new cell design. These aspects demonstrated on the electrode, cell and stack level in the RESelyser project, also at elevated system pressure improve: electrode efficiency and stability for intermittent operation, mass transfer and hydrogen and oxygen gas purity at partial load and high purity. Raney Ni coatings are applied to expanded Ni metal sheets using vacuum plasma spraying (VPS) of powders to produce low cost efficient AWE electrodes. Hydrogen evolution cathodes were produced using a bi-layer coating of Al-Ni (intermediate bonding) and Al-Ni-Mo alloys (cathode). Oxygen evolution anodes were produced using only the Al-Ni alloy. Compared to uncoated Ni electrodes the coatings significantly reduce electrode overpotentials. Cyclic on-off testing of anodes and cathodes in excess of 2000 h and 1000 cycles revealed high efficiency retention. Two and three dimensional cathode microstructure investigations reveal that the VPS process produces complex electrode microstructures containing a mixture of intermetallic and solid solution phases present in the raw powders modified by partial melting during spraying. Activation of the electrodes by leaching of the Al rich phases leaves a correspondingly complex and heterogeneous porous cathode consisting of mostly Ni. The surfaces of the activated cathodes exhibit a large roughness throughout the nanometer to micrometer length scale. Correspondingly the internal porosity of the cathode spans the same length scale where large pores appear as distorted pancakes parallel to the cathode surface that are connected to nanometer scale porous networks corresponding to the original dendritic structures removed by the activation process. Cathode microstructures before and after electrolysis operation and after storage in water were investigated. Analyses of cross sections and electrode surfaces revealed extensive formation of nanometer scale desert rose like structures on the surface and in the pores on several cathodes. The formation to be strongly related to electrolysis operation and to a lesser degree on distilled water storage duration. TEM and X-ray diffraction identified the desert rose structures to be theophrastite, -Ni(OH)2. The possible implications on cathode performance and degradation are discussed in terms of pore shape and blockage, surface area increase and the extent that the hydrogen evolution reaction occurs within the porous electrode below the free surface. Acknowledgements: This work is funded by the European Union’s Seventh Framework Programme for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n° [278732] and Energinet.dk (ForskEl) via contract: 2012-1-10818.