Strategies for efficient CO2 electroreduction in acidic conditions

CO2 electroreduction is a promising technique to convert renewable electricity and CO2 to high-value fuels and chemicals. Selectivity, energy efficiency, carbon efficiency and sustainability are the criteria for CO2 electroreduction techniques suitable for industrial application. With alkaline and neutral electrolytes, carbonate formation from CO2 leads to low carbon efficiency. High energy consumption to regenerate alkaline electrolyte and high resistance of neutral electrolyte cause low energy efficiency. Recently, CO2 reduction with acidic electrolyte becomes a hot topic due to its potential to increase carbon efficiency and energy efficiency. Improving the selectivity towards CO2 reduction is challenging in acidic condition. Diverse approaches were proposed to suppress H+ reduction and promote CO2 reduction. However, fundamental issues about cation effect and local pH effect on CO2 reduction in acidic condition are still under debate. Moreover, bicarbonate precipitation in gas diffusion electrode limits the sustainability with acidic electrolyte. This review tries to rationalize the reported strategies to improve the selectivity towards CO2 reduction in acidic condition from mass transport and electrode reactions. Different approaches, including adding alkali cations, surface decoration, nanostructuring, and electronic structure modulation, are designed based on these two aspects. This review also introduces the recent progress in CO2 electroreduction with metal cation-free acidic electrolyte. This strategy is deemed to improve the sustainability.