Carbonation in Low-Temperature CO2 Electrolyzers: Causes, Consequences, and Solutions

Abstract

Electrochemical reduction of carbon dioxide (CO2) to useful products is an emerging power-to-X concept, which aims to produce chemicals and fuels with renewable electricity instead of fossil fuels. Depending on the catalyst, a range of chemicals can be produced from CO2 electrolysis at industrial-scale current densities, high Faraday efficiencies, and relatively low cell voltages. One of the main challenges for up-scaling the process is related to (bi)carbonate formation (carbonation), which is a consequence of performing the reaction in alkaline media to suppress the competing hydrogen evolution reaction. The parasitic reactions of CO2 with the alkaline electrolytes result in (bi)carbonate precipitation and flooding in gas diffusion electrodes, CO2 crossover to the anode, low carbon utilization efficiencies, electrolyte carbonation, pH-drift in time, and additional cost for CO2 and electrolyte recycling. We present a critical review of the causes, consequences, and possible solutions for the carbonation effect in CO2 electrolyzers. The mechanism of (bi)carbonate crossover in different cell configurations, its effect on the overall process design, and the economics of CO2 and electrolyte recovery are presented. The aim is to provide a better understanding of the (bi)carbonate problem and guide research directions to overcome the challenges related to low-temperature CO2 electrolysis in alkaline media.