Recirculating carbonate and bicarbonate based electrolyzers cannot operate continuously without in-line separation due to electrolyte depletion and ion crossover

Abstract

The commercialization of (bi)carbonate-based electrolyzers for reactions such as CO2 reduction requires examination of their long-term operation. We studied the effects of electrolyte depletion and electrolyzer performance drop-off through experiments and simulations in a system with separated, recirculating electrolyte reservoirs. The effects of mass transport, electrochemical reactions, and equilibrium reactions form a combined picture that explains the behavior in these systems, which is characterized by insufficient Image 1001 transport for electrochemical reactions at the anode surface. The carbonate equilibrium shifts towards bicarbonate as Image 1002 anions are consumed at the anode, and the bicarbonate equilibrium shifts further to CO2 as physical gas stripping by O2 bubbles removes dissolved CO2 until the electrolyte is fully depleted. We reduce the system into a simplified 1-D numerical model that identifies the relevant phenomena of the system. We use this model in tandem with experiments to show the loss of dissolved carbon from the anolyte inventory due to physical gas stripping that transports CO2 to the atmosphere. We also observe and model electro-osmotic flow that causes net fluid motion from anolyte to catholyte and decreases the diffusion of dissolved carbon species from cathode to anode. The simulations agree with experimental measurements and show that migration is the dominant component of the ionic transport that sustains the current. Our results show that continuous operation of these systems is not possible without some strategy to improve Image 1003 transport to the anode, such as recombining electrolyte streams after separation, using novel separators, or operating at much lower current densities.