A TEM/SEM microscopic analysis of the catalyst layer morphologies of NiNC-IMI and NiNC-PANI revealed important differences (Figure ). Larger-scale morphological differences of the two sprayed Gas Diffusion Electrode (GDE) layers became more obvious at higherresolution SEM scales (Figure ,k) and cross-section SEM images ( Figure ). Spongelike channel structures with fine, dense, and homogeneously distributed pores and channels (around 3 um diameter) were revealed in the NiNC-IMI layer. By contrast, the NiNC-PANI catalyst layer showed larger but fewer channels (additional high resolution SEM images are shown in Supplementary Figure ). Moreover, other physical characterizations, such as catalyst layer thickness, hydrophobicity, through-plane (TP) and in-plane (IP) conductivity, and gas permeability, were conducted on the sprayed GDEs, and the corresponding data are listed in Supplementary Table . Volumetric electrolyzer outlet flow rates were tracked using a N2 bleed (Supplementary Figure , Supplementary Equation ) and used for subsequent reaction rate and carbon crossover diagnosis. NiNC-IMI and NiNC-PANI catalysts were tested at various geometric catalyst loadings. Prior to CO2 electrolysis, the electrochemical cell impedance (Supplementary Figure and) revealed values of 0.21 (± 0.01) Ohm, independent of the catalyst loading. The catalytic CO2-to-CO cell performance was screened from -50 mA cm -2 to -500 mA cm -2 (at 50 and 100 mA cm -2 increments). At he beginning of each break-in, a characteristic cathode potential overshoot up to -5.5 V appeared (Supplementary Figure and). This potential overshoot increased with catalyst loading, which is why we associate this with the ionic reinforcing and wetting process of the MEA and the bipolar junction between ionomer and membrane. ) and is around 0.4, implying that the outlet contains a record 70 mol% CO (balance CO2), a previously unachieved CO-rich outlet stream.