In alkaline and neutral media CO2 to multicarbon electrolyzers, CO2 rapidly converts to (bi)carbonate, crosses over the membrane and mixes with the O2-rich anodic gas stream. This so-called CO2 crossover phenomenon limits the carbon efficiency in those systems to ~25%, unless an energy-intensive, chemical separation step is performed to recover CO2 from the anodic gas stream.
Bipolar membranes (BPMs), under reverse bias, are known to eliminate CO2 crossover and convert (bi)carbonate back to CO2 – by generating a proton flow towards the cathode. However, implementing BPMs in CO2 reduction electrolyzers is challenging: one must keep the cathode locally alkaline to enable the reaction and ensure the regenerated CO2 can participate in the cathodic reactions.
Assisted by a numerical simulation model (Fig. 1a, 1c and 1d), we sought to develop a BPM-based electrolyzer design (Fig. 1b) that meets the criteria mentioned above. We discovered that the cathodic alkalinity and mass transfer efficiency of the regenerated CO2 are greatly affected by the gap distance between cathode and BPM, and by the buffering capacity of the electrolyte filling in this gap.
The simulation results suggest the following design principles for the BPM-based CO2 reduction electrolyzers: the local cathode pH and the diffusion layer thickness of the regenerated CO2 increase as the electrolyte thickness increases; the buffering capacity of the catholyte increases the diffusion layer thickness and reduces transport. Precise control of the thickness of a non-buffering catholyte should thus offer a route to high carbon efficiency, CO2 reduction selectivity and reaction rate.
The experimental results verified the designed functions of the BPM-based CO2 reduction electrolyzer. We report the CO2 crossover < 0.5% with a single-pass carbon efficiency of up to 78%, with other key performance metrics close to the anion membrane-based counterparts.
Fig. 1 | The configuration and simulation of the electrolyzer design. (a) The CO2 (solid lines) and pH distributions (dashed lines) in the 65 μm-thick electrolyte layer. The positions where the (bi)carbonates revert to CO2 are marked (red for non-buffering and black for buffering electrolyte). (b) The schemes and the mass transfer in the BPM-based CO2 reduction electrolyzer. (c) The pH distribution inside the electrolyte layer. (d) The dissolved CO2 concentration profile inside the electrolyte layer.