PhD Defence at DTU Mechanical Engineering

PhD Defence 5th September: Development of low-cost heterogeneous electrocatalysts

Tuesday 01 Sep 15
Sune Egelund from DTU Mechanical Engineering defends his PhD "Development of low-cost heterogeneous electrocatalysts for large scale advanced alkaline electrolysers – with focus on oxygen evolution and system performance" Saturday 5th September. The defence takes place at 10.00 in DTU Skylab, Building 373A, DTU Lyngby. Professor Per Møller is supervisor.

Abstract

One of the important components needing optimization in order for alkaline electrolysis to become more effective and competitive is the electrodes. This can be done by selection of improved lectrocatalysts for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). This thesis mainly presents results from the electrode optimization work packages in a HTF project. To some extend also glimpses of other parts of the project has been described as well.

The original focus on OER electrocatalysts was later expanded to cover the complete electrode system including the HER. Electrodes which were suitable for both reactions were therefore tested either individually or in combination as combined OER-HER catalysts, which could function for both half-cell reactions. It has throughout the project been of significant importance that the resulting electrodes were suitable for industrial electrolysis. This meant that the most suitable electrocatalytic surfaces had to fulfill the right compromise between cost, electrocatalytic activity, scalability and long-term stability. Based on the lowcost constraint it was quite early decided to focus mainly on electrocatalysts made from electrodeposition or thermal decomposition procedures. As for the scalability requirement it was important that the manufacturing equipment was available or realistically constructed within the project.

The final goal was to construct a MW electrolyser. The surface area required for each electrode was estimated to be 3020 cm2. The scalability of the best electrocatalyst candidates was successfully demonstrated towards the end of the project – however only cathode candidates were up-scaled. This decision was taken as anode candidates were not found to change the global cell potential significantly enough to warrant coating of these. Ranking of the OER catalyst were in the beginning of the project conducted in a conventional threeelectrode electrochemical cell operated at 25 °C, 1 M KOH. No long-term behavior was evaluated in these cells due to equipment availability.

Long-term testing was later seen as a high priority and most tests were carried out in full-cells at what was believed to be realizable and realistic conditions using 50 wt% KOH at 120 °C. The harsh environment was selected as an electrocatalyst stress enhancer. In the full-cell, where the global potential situation was considered, the anodes were initially ranked against non-activated and polished nickel as cathode material. Using this approach it was determined that the OER overpotential (or global full-cell potential reduction) with several OER electrocatalysts, was reduced by up to 0,3 V during long-term operation. Such cell potential reductions were however not successfully transferred to full-cells where activated cathodes were inserted instead of polished nickel. It was in this way found, that the OER overpotential reductions observed in the full-cell with polished cathodes were not directly transferable to full-cells with activated cathodes.

As for the major achievement in the laboratory it was demonstrated that in a cell, equipped with an optimized cathode (NiC-NiCoS) and a non-activated nickel anode, a global cell potential of approximately 1,67 V after 3000 hours at 0,4 Acm-2, 120 °C, 50 wt% KOH could be achieved. The optimized coatings were subsequently up-scaled and the results were transferred to test stacks hosted by the cooperating project partner. Here it was verified that a average cell potential of approximately 1,66 V ± 0,02 V was obtained at a temperature of only 95 °C. Approximately the same activity was determined for different optimized cathodes at 0,4 Acm-2 in 30 wt% KOH. The optimized coatings were in some of these cells used as a combined OER-HER coating while other cells were equipped with non-activated nickel anodes. The results confirmed that non-activated nickel as anode material was sufficient for achieving efficiencies up to 88 – 90 % vs. HHV at 0,4 Acm-2, 95 °C.