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

Abstract

Membrane electrode assemblies enable CO2 electrolysis at industrially relevant rates, yet their operational stability is often limited by formation of solid precipitates in the cathode pores, triggered by cation crossover from the anolyte due to imperfect ion exclusion by anion exchange membranes. Here we show that anolyte concentration affects the degree of cation movement through the membranes, and this substantially influences the behaviors of copper catalysts in catholyte-free CO2 electrolysers. Systematic variation of the anolyte (KOH or KHCO3) ionic strength produced a distinct switch in selectivity between either predominantly CO or C2+ products (mainly C2H4) which closely correlated with the quantity of alkali metal cation (K+) crossover, suggesting cations play a key role in C-C coupling reaction pathways even in cells without discrete liquid catholytes. Operando X-ray absorption and quasi in situ X-ray photoelectron spectroscopy revealed that the Cu surface speciation showed a strong dependence on the anolyte concentration, wherein dilute anolytes resulted in a mixture of Cu+ and Cu0 surface species, while concentrated anolytes led to exclusively Cu0 under similar testing conditions. These results show that even in catholyte-free cells, cation effects (including unintentional ones) significantly influence reaction pathways, important to consider in future development of catalysts and devices.

Cu/Fe3O4 Nanocomposites from Layered Double Hydroxides as Catalysts for Selective Electroreduction of Carbon Dioxide

“…The exact state of Cu in the reaction process is further illustrated by Cu LMM Auger electron spectroscopy (Figure c). As seen from Figure c, the Cu LMM spectrum of Cu/Fe 3 O 4 has peaks at 572.9, 569.9, and 568.0 eV, corresponding to Cu 2+ , Cu + , and Cu 0 , respectively. , The appearance of Cu 2+ is considered to be the result of a small part of the catalyst surface being oxidized by air or residue of incomplete liquid phase reduction. This conclusion is supported by the absence of Cu 2+ characteristic peaks in its XRD pattern (Figure e).…”

CO2 flow electrolysis – limiting impact of heat and gas evolution in the electrolyte gap on current density

“…Currently, there are both flooding-resistant GDEs and the approach of intermittently washing off GDEs during electrolysis to extend their lifetime. 30–32…”

In situ investigation of ion exchange membranes reveals that ion transfer in hybrid liquid/gas electrolyzers is mediated by diffusion, not electromigration

“…These investigations are relevant for the development and optimization of this type of electrolyzer, in particular catholyte-free (zero-gap) devices, which are promising for efficient electrochemical CO 2 reduction. 9 The substitution of the liquid catholyte with a hydrated gaseous phase circumvents the limited CO 2 solubility in aqueous electrolytes and the dilution of the reaction products, which requires subsequent separation steps for product recovery. [33][34][35] Consequently, production costs and overall device complexity could be reduced, hence facilitating the scalability of such technology.…”

“…Once transferred across the membrane, these ions can then further participate in the ongoing electrochemical process(es). 9 Cation-exchange membranes (CEMs) contain negatively-charged functional groups that promote the transfer of cations ( e.g. protons) while preventing the transfer of anions.…”