Unintended cation crossover influences CO2 reduction activity in Cu-based zero-gap electrolysers

Abstract Gas-diffusion anion exchange membrane electrode assemblies enable CO2 reduction at industrially relevant rates, yet their long-term operational stability is often limited by the formation of solid precipitates in the cathode pores. This is a consequence of unintended cation crossover from the anolyte, and a detailed understanding of the factors enabling this crossover is lacking. Here we show that the anolyte concentration governs the flux of cation migration through the membrane, and this substantially influences the behaviors of copper catalysts in catholyte-free CO2 electrolysers. Systematic variation of the anolyte ionic strength (using aqueous KOH or KHCO3) correlated with drastic changes in the observed product selectivity – most notably, below a threshold ionic strength, Cu catalysts produced predominantly CO, in contrast to the mixture of C2+ products typically observed on Cu. Cation (K+) quantification at the zero-gap cathode revealed that the magnitude of K+ crossover depends on the anolyte concentration, but becomes significant only above the aforementioned threshold which closely correlates with the onset of C2+ product formation, suggesting cations play a key role in C-C coupling reaction pathways. Operando X-ray absorption spectroscopy and quasi in situ X-ray photoelectron spectroscopy were used to study how the catalyst is affected by operation conditions. Cu surface speciation was found to show a strong dependence on the anolyte concentration, wherein dilute anolytes or pure H2O resulted in a mixture of Cu+ and Cu0 surface species, while concentrated anolytes led to exclusively Cu0 under similar testing conditions. Overall, our results show that even in catholyte-free cells, cation effects (including unintentional ones) can significantly influence reaction pathways, which must be considered in future development of catalysts and devices.