Role of Electrolyte Concentration in Bicarbonate Electroreduction

Abstract

Direct electroreduction of bicarbonate, a typical CO2 capture medium, offers a route to close the carbon cycle without energy-intensive CO2 release. However, bicarbonate electroreduction is commonly investigated in concentrated electrolytes misaligned with practical capture streams, leaving the concentration effect unclear. Here, we show that Cu-based bicarbonate electroreduction reactivity exhibits a volcano-shaped dependence on KHCO3 concentration. Using a custom-designed MEA-type operando Raman electrolyzer, we probe interfacial speciation and identify two competing regimes: in dilute electrolytes, the increasing CO2 conversion is governed by in situ CO2 supply, whereas in concentrated KHCO3, the decreasing reactivity is governed by competitive adsorption of (bi)carbonate ions, blocking active sites for CO adsorption and C–C coupling, accompanied by enhanced hydrogen evolution with bicarbonate-mediated proton supply. Consequently, a moderate concentration optimizes CO2 reduction reactivity by balancing CO2 supply against anion site-blocking. These findings establish electrolyte concentration as a practical lever to engineer the interface for integrated CO2 capture.