Highly efficient Electrodes:
Using the Vacuum Plasma Spray coating, electrodes are developed with low-cost materials that have a high efficiency / low overpotential and little degradation in intermittent operation.
For the cathode (hydrogen production electrode) the overpotential can be reduced by 210 mV due to the coating.
For the anode (oxygen production electrode) the overpotential can be reduced by up to 161 mV due to the coating.
Details: Electrodes for the electrolyser were prepared by coating plain nickel electrodes with an active layer by using the plasma spray technique. The active layer did not contain any noble metals but only inexpensive material. By coating the cathode with an active layer of NiAlMo and the anode with an active layer of NiAl the accumulated electrode overpotential could be reduced from 705 mV for the uncoated electrodes to 345 mV for the coated electrodes at 750 mA/cm2, i.e. the electrode losses could be reduced by more than 50% without expensive noble metals.Before using the electrodes in the electrolyser they are activated by leaching out the Al components in KOH with the addition of a complex former. As a result, the coated layer exhibits a high specific area, the active Raney-nickel electrode surface. Technical size electrodes (300 cm2) could be prepared by vacuum plasma spraying (VPS) and are made available for integration into a stack of 10 kW nominal power. Long term tests of the electrodes in on-off-operation are being presently performed. For the cathode a further overpotential reduction can be achieved when mixing the NiAl with active oxides like Co3O4 and LSCF (Lathanum Strontium Cobalt Iron oxide) with perovskite structure. The mixed valence structure of the oxides improves the catalytic activity while the high surface nickel obtained after activation is responsible for the electric conductivity of the active layer. However, the long term stability of coating layers prepared of oxides and NiAl by plasma spraying still needs further improvement.
To obtain a better understanding of the pore systems in the electrode active layers and their changes and effects in aging microstructure investigation with electron microscopy has been performed. For this purpose, the surface and cross-sections of vacuum plasma sprayed Raney Ni cathodes are examined. The conventional two dimensional investigations provide valuable structural insight to guide the planned three dimensional characterisation of structural changes of the pore space during long term alkaline electrolysis operation. Imaging by scanning electron microscopy reveals heterogeneous surface and cross-sectional morphology. The activated electrode surface can be described as rough, complex and heterogeneous with multi-scale pores and features. In the cross-section, the electrode consists of two ~60 µm thick layers, a NiAl adhesion layer and a NiAlMo functional layer.