On the operation of switchable oxygen depolarized cathodes

Abstract

The electrochemical chlor-alkali electrolysis remains an energy-intensive production process even though many improvements have been developed over the last decades. Oxygen depolarized cathodes (ODC) reduce energy consumption by approximately 25% while oxygen is consumed instead of hydrogen being evolved. The switchable ODC (sODC) facilitates the oxygen-consuming and the conventional hydrogen-evolving modes with the same electrode. In this study, we investigate sODCs for demand-side electrolysis by switching between modes. Experimental investigations covered current densities ranging from 50mAcm−2 to 525mAcm−2, and we evaluated the long-term effect of switching between the two modes on the system's stability. In addition, cell potential, Faraday efficiency and contact angle measurements were compared for pristine and used sODCs after up to 1600 switching cycles. The lab cell was implemented in 3D-CFD simulations to investigate a nitrogen flushing process between the two modes to prevent the formation of explosive mixtures in the electrolyzer. sODCs showed a stable and continuously high Faraday efficiency. However, an increase in cell potential over 1000 switching cycles of up to 7.8% was observed, which was attributed to electrolyte flooding. Still, the sODC is well comparable to the conventional ODC with a marginal difference in cell potential (0.07V), which demonstrates its high potential for industrial application. 3D-CFD simulations were compared to experimental flushing time measurements. We implemented the resulting flushing times for a safe operation in the subsequent switching cycles. The verified CFD simulations can help to further optimize the flushing procedure for an economically feasible switching process in industry-sized cells. Switchable electrodes enable flexibility to react to fluctuating boundary conditions such as the electricity price and can be implemented in many processes beyond the chlor-alkali electrolysis.