Due to the global agenda to achieve net zero carbon emission, the needs to develop carbon capture and utilization technology has been sharply emerging. Although electrochemical CO₂ reduction (CO₂R) to useful carbon chemicals is challenging to meet the industrial requirements, recent studies have made a significant improvement in the performances both of the product selectivity and current density. Gas-diffusion electrode configurations allow direct CO₂ gas-feeding to the catalyst layer and several hundred mA/cm² of current density have been demonstrated with a zero-gap type of membrane electrolyzer assembly (MEA) electrolyzer. However, many underlying phenomena are still not clearly understood.

Among various components of MEA, here, I will focus on the cathode catalyst, and balanced CO₂ / water supply at the catalyst/membrane boundary to understand the CO₂R activity. We demonstrate the importance of the water supply from the anolyte for the selective CO₂R in the zero-gap MEA electrolyzer. We confirm that hydrogen is mostly originated from the aqueous anolyte for the production of the ethylene from CO₂R with controlled labelling experiments. In terms of the Cu catalyst morphology, the alkaline environment near the catalyst layer leads a significant morphology changes during the pre-treatment steps for the ionomer activation, which contributes to increase in the CO₂R activity. The microenvironment behavior is simulated and compares with the experimental CO₂R MEA performance to understand the mass flow and CO₂R activity, which also support the catalyst thickness dependence. For the practical application consideration, we also study the CO₂R activity for CO production with the low concentration (~ 10%) of CO₂ feeding and amine-captured direct CO₂ reduction condition. Weak HER activity is important especially with the low concentration of CO₂ because the slower CO₂ reduction rate lead to more hydrogen compared to 100 % of CO₂ gas feeding. These studies will give insight to the intrinsic and extrinsic factors determining the activities of CO₂R MEA.