Experimental and theoretical investigation of gas purity in pressurized alkaline water electrolysis

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Water electrolysis is a key technology for the transformation of renewable electrical energy into chemical energy, which can be used in different forms or reconverted after storage. Alkaline electrolysis is the most mature and industrially most widespread option among the available technologies. A hitherto unresolved and little understood problem is the contamination of the produces gases hydrogen and oxygen with the respective foreign gas. Especially in part-load operation, the concentrations of the foreign gases can rise to such high values that shutdown of the electrolyzer for safety reasons (explosion limits) becomes inevitable. Besides the safety aspect, this contamination also decreases the efficiency of the electrolysis process. I preliminary work at atmospheric pressure, it could be shown that the main source of the contaminations is the dissolution of the gases in the electrolyte and the transport to the other side of the cell caused by electrolyte mixing, and not the transport through the separator as usually assumed. In the proposed project, a systematic experimental and theoretical study regarding the influencing factors on the quality of the product gases during pressurized alkaline electrolysis shall be conducted. For this purpose all relevant parameters such as current density, pressure, temperature, electrolyte concentration and volume flow rate, as well as electrolyte management strategies shall be taken into account. The role of the individual contamination processes (mixing of gas-saturated electrolyte streams, diffusion and convection through separator) will be quantitatively determined. Firstly, all measurements shall be carried out under steady-state conditions and subsequently described with an adequate mathematical model. For this purpose, it is also necessary to measure currently unknown data such as gas solubilities. Based on the experimental findings and modeling, dynamic strategies for efficiency improvement and extension of the part load range shall be developed. The effectiveness of these measures will be firstly tested under constant current density. Finally, the electrolysis cell will be subjected to typical dynamic electricity profiles and the measured performance will be described with a suitable dynamic electrolyzer model.