Green hydrogen: Why do certain catalysts improve in operation?

Crystalline cobalt arsenide is a catalyst that generates oxygen during electrolytic water splitting in the production of hydrogen. The material is considered to be a model system for an important group of catalysts whose performance increases under certain conditions in the course of electrolysis. Now a HZB-team headed by Marcel Risch has observed at BESSY II how two simultaneous mechanisms are responsible for this. The catalytic activity of the individual catalysis centres decreases in the course of electrolysis, but at the same time the morphology of the catalyst layer also changes. Under favourable conditions, considerably more catalysis centres come into contact with the electrolyte as a result, so that the overall performance of the catalyst increases.

As a rule, most catalyst materials deteriorate during repeated catalytic cycles – they age. But there are also compounds that increase their performance over the course of catalysis. One example is the mineral erythrite, a mineral compound comprising cobalt and arsenic oxides with a molecular formula of (Co3(AsO4)2∙8H2O). The mineral stands out because of its purple colour. Erythrite lends itself to accelerating oxygen generation at the anode during electrolytic splitting of water into hydrogen and oxygen.

Read more in the HZB website

Image: Schematic of the electrochemical restructuring of erythrite. The fine needle-like structure melts during the conversion from a crystalline material to an amorphous one, which is porous like a Swiss cheese.

Credit: © HZB