Ionomer Loading Effects on the Performance of Ni Single-Atom Catalyst Layers in a Forward-Bias Bipolar Membrane CO2 Electrolyzer

In this study, the effect of the ionomer loading on the performance of a nickel single-atom catalyst (Ni-SAC) implemented as the cathode catalyst layer (CL) of a forward-bias bipolar membrane (FB-BPM) zero-gap CO2-electrolyzer was systematically investigated. By varying the ionomer-to-catalyst (I/C) mass ratio at a fixed catalyst loading (1.0 mg·cm–2), we reveal a critical trade-off between ionic conductivity and mass transport within these Ni-SAC catalyst layers. While high ionomer loadings (i.e., I/C values >0.75) improve ionic transport, they simultaneously exacerbate pore blockage and water accumulation within the corresponding CLs, leading to mass transport limitations under full cell operation. As a result, Ni-SAC electrodes with low ionomer loadings (I/C ≤0.75) achieve CO Faradaic efficiencies (FECO) ≥80% at current densities ≤150 mA·cm–2, whereas higher ionomer contents lead to a decrease in FECO accompanied by an increase in cell voltage. Notably, all electrodes exhibit a sharp voltage increase at the latest when reaching a current density of 200 mA·cm–2, highlighting a system-level limitation that we associate with local CO2 depletion. These findings provide initial design guidelines for optimizing Ni-SAC/PiperION cathode CL-microenvironments in FB-BPM CO2-electrolyzers.