Alkali metal cations enhance CO ₂ reduction by a Co molecular complex in a bipolar membrane electrolyzer

The electrochemical reduction of CO₂ is a promising pathway for converting CO₂ into valuable fuels and chemicals. The local environment at the cathode of CO₂ electrolyzers plays a key role in determining activity and selectivity, but currently some mechanisms are still under debate. In particular, alkali metal cations have been shown to enhance the selectivity of metal catalysts, but their role remains less explored for molecular catalysts especially in high-current electrolyzers. Here, we investigated the enhancement effects of cations (Na⁺, K⁺, Cs⁺) on Co phthalocyanine (CoPc) in a state-of-the-art reverse-biased bipolar membrane electrolyzer. When added to the anolyte, these cations increased the Faradaic efficiency for CO, except in the case of Na⁺ in which the effect was transient, but the effects are convoluted with the transport process through the membrane. Alternatively, these cations can also be added directly to the cathode as chloride salts, allowing the use of a pure H₂O anolyte feed, leading to sustained improved CO selectivity (61% at 100 mA cm^-2 after 24 h). Our results show that cation addition is a simple yet effective strategy for improving the product selectivity of molecular electrocatalysts, opening up new avenues for tuning their local environment for CO₂ reduction.This article is part of the discussion meeting issue 'Green carbon for the chemical industry of the future'.