Improving the stability, selectivity, and cell voltage of a bipolar membrane zero-gap electrolyzer for low-loss CO₂ reduction

Electrolyzers for CO₂ reduction containing bipolar membranes (BPM) are promising due to low loss of CO₂ as carbonates and low product crossover, but improvements in product selectivity, stability, and cell voltage are required. In particular, direct contact with the acidic cation exchange layer leads to high levels of H₂ evolution with many common cathode catalysts. Here, Co phthalocyanine (CoPc) is reported as a suitable catalyst for a zero-gap BPM device, reaching 53% Faradaic efficiency to CO at 100 mA cm⁻² using only pure water and CO₂ as the input feeds. It is also shown that the cell voltage can be lowered by constructing a customized BPM using TiO₂ water dissociation catalyst, however this is at the cost of decreased selectivity. Switching the pure-water anolyte to KOH improved both the cell voltage and CO selectivity (62% at 200 mA cm⁻²), but cation crossover could cause complications. The results demonstrate viable strategies for improving a BPM CO₂ electrolyzer toward practical-scale CO₂-to-chemicals conversion.