Abstract
Deploying single-site NiNC catalysts in cathode catalyst layers of bipolar electrolyzer cells enables catalytic CO 2 valorization to e-CO at industrially relevant yields and efficiencies. The performance of the cathode layers is controlled by the turnover frequency of the active sites as well as mass transfer to and from the active sites. While the atomic scale structure-reactivity relations of single-site NiNC catalysts have been extensively studied, the mass transfer characteristics of single atom catalyst layers were poorly discussed. In this work, we design, build, and test NiNC catalyst layers using a novel set of distinct ion exchange ionomer materials and correlate the performance of cathode catalyst layers with their reactivity and stability in full single MEA electrolyzer cells. The Sustainion anion exchange ionomer delivered optimal performance, yielding about 90% CO faradaic efficiency up to 300 mA cm 2 and 15 h stable performance at 200 mA cm 2. Our analysis attributes its favorable electrolyzer performance to its balanced conductivity and hydrophobicity, which mitigates electrode flooding while ensuring excellent ion and CO 2 transfer rates even at high current densities.