Impact of Current Density, Anolyte Salts and Pulsing on Water Crossover in Zero Gap CO₂ Electrolyzers Revealed by Operando Neutron Radiography

The major barriers to practical implementation of CO₂ electrolyzers are alleviating failure modes induced by water and alkali cation crossover from the anolyte. Electrolyzer failure caused by flooding of the gas diffusion electrode (GDE) or salt precipitation typically occur within the first 24 hours of electrolysis. Water and salt crossover are partially driven by electro-osmotic diffusion, and are thus dependent on the identity of the cations in the anolyte. To understand water crossover and its consequent effects on electrolyzer durability and selectivity we used operando neutron radiography to directly image water crossover of CO₂ to CO and CO₂ to C₂H₄ electrolyzers. In the case of the CO₂ to CO electrolyzer, we observed the water crossover with different alkali cations under working conditions from 100 to 1000 mA cm^-2. We found that the tighter hydration sphere of Cs⁺ enables crossover at lower current densities which is observed as water flooding through the GDE into the flow fields. The larger hydration sphere of K⁺ leads to the need for larger current densities to drive water crossover, but solubility differences between the corresponding carbonate salts lead to more catastrophic flooding/precipitation while using KOH anolyte at high current densities. With the CO₂ to C₂H₄ electrolyzer, we imaged the effect of minutes long regenerative pulses on water crossover. We found that these low current density pulses improved electrolyzer lifetime by allowing water through the back of the gas diffusion electrode and subsequently drying it back out. These results demonstrate the importance of direct operando understanding water crossover in order to tailor the catalyst microenvironment for electrolyzer scale-up and durability.

This work was conducted under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory (LLNL) under Contract DE-AC52-07NA27344.