Flooding revisited: electrolyte management ensures robust electrochemical CO2 reduction

Abstract

Flooding, one of the main performance fading mechanisms of CO₂ electrolysers, is vaguely defined, and often used for very different phenomena that cause cell/stack failure. The term itself is also controversial, as a fully wet electrode is often observed after high-performing zero-gap electrolyser cells are disassembled. To resolve this apparent contradiction, we investigated the cation balance in a zero-gap CO₂ electrolyser cell operated under different conditions, and also actively controlled cation concentration in the cathode compartment to study its effect on the electrolyser performance. While a given cation concentration is needed for high-rate CO-formation, its further increase boosts the hydrogen evolution rate and decreases the CO₂ reduction rate (through two different mechanisms). When the cation content in the cathode is too high, hydrogen evolution occurs also on the carbon cathode support and the availability of CO₂ decreases at the cathode catalyst. During continuous operation, the cation flux from the anolyte to the cathode might change, which is also reflected in the cell performance. We demonstrate that such changes in performance can be counteracted by actively controlling the anolyte composition. We also suggest descriptors of the “health” of the cell, to ensure durable operation via the active control of the cation concentration at the cathode.