Failure Mode Diagnosis and Stabilization of an Efficient Reverse-Bias Bipolar Membrane CO2 to CO Electrolyzer

Efficient and stable CO2-to-CO electrolyzers are key process components for the generation of green synthesis gas and its downstream conversion and valorization to carbonaceous e-chemicals and e-fuels. While alkaline CO2 electrolyzers suffer from low CO2 utilization due to cathodic carbonate formation and crossover, acidic CO2 electrolyzers suffer from low CO faradaic efficiency. Reverse-bias Bipolar Membrane (BPM) cell architectures have been proposed to promote cathodic proton transport, yet resulted in limited cell lifetimes due to complex degradation and failure regimes. A thorough diagnosis of BPM cell dynamics is missing to date. Here, we build and diagnose an efficient zero-gap reverse-bias BPM CO2-to-CO electrolyzer cell deploying CO-selective single Ni atom cathode catalysts. We analyzed its key cell performance parameters and diagnosed the cell stability and failure regimes over 100 hours. The electrolyzer cell showed excellent performance up to 500 mA cm-2 with CO faradaic efficiency near 100 %. The proton-controlled ion transport in the cathode was directly confirmed by an experimental carbon cross-over coefficient (CCC) of zero, suggesting minimal carbon loss due to carbonate formation. This was coupled to a high single pass conversion of ~70% at the largest current densities and 60 vol% CO in the cell outlet, ideally suited for process cascade involving electro- or thermal catalytic steps. While use of a N2 bleed for internal reference has been known to be critical for accurate evaluation of cell performance, we now propose the experimental N2 vol% in the combined cell outlet and bleed flow to be also a valuable diagnostic tool to recognize and analyze cell failure regimes.