Influence of CO₂‑Regenerative Film Properties in Enhancing C₂₊ Products Selectivity While Mitigating CO₂ Crossover

Zero-gap anion-exchange membrane electrode assembly (AEMEA) electrolyzers operating in alkaline media face challenges such as CO₂ crossover and salting-out. The bipolar membrane electrode assembly (BPMEA) electrolyzer, using DI water as the electrolyte, addresses both CO₂ crossover and salting-out issues. As cations, which are crucial in stabilizing the CO₂R intermediates, are absent in the electrolyte, cations embedded in the AEM play an important role in dictating the CO₂R selectivity and activity in BPMEA systems. So far, no systematic study has been conducted on the influence of cations embedded in AEMs on CO₂ selectivity and activity in BPMEA systems. Moreover, BPMEA systems impose an additional challenge: low stability due to delamination of the bipolar membrane caused by CO₂ regeneration at the membrane-membrane interface. To enhance the stability of the electrolyzer, a simple yet highly reproducible strategy for coating a porous CO₂ regenerative film on smooth Nafion 117 is demonstrated in this work, along with a systematic study of four different commercially available AEMs, composed of different cations and cation densities, for use in combination with Nafion 117 and copper catalyst at the cathode. We found that the PiperION membrane delivers selectivity and activity comparable to those of alkali-metal cations, owing to the enhanced local electric field resulting from the combined effects of a high positive charge on the N atom of the piperidinium cation and the high ion-exchange capacity. Further, we studied the influence of the thickness of the PiperION porous layer over Nafion 117 on CO₂R selectivity and found that 70 μm is the minimum thickness to achieve maximum C₂₊ products selectivity, reduced the CO₂ crossover to 5% from 25% at 4 SCCM and 150 mA cm^-2, and was stable for operation beyond 100 h. The selectivity of this system, compared with AEMEA, and stability outperformed both AEMEA and BPMEA. This study helps design more effective BPMs to inhibit CO₂ crossover while enabling stable and selective electrochemical CO₂ reduction to C₂₊ hydrocarbons.