As far as viable electrocatalysts for the CO2-to-CO electrolyzer (cell 1) are concerned, oxidederived metallic Ag continues to be the state-of-art cathode catalyst under operando condition with good CO efficiency. In Ag-derived gas diffusion electrode (GDE) flow electrolyzers, CO activity has been repeatedly reported up to 600 mA cm -2 current densities, yet combined with low efficiencies. Given its critical materials nature, large-scale utilization of Ag at today's metal loadings faces stiff resource and cost challenges. More seriously, Ag suffers from cathodic corrosion at CO2RR electrode potentials, resulting in unstable catalyst films. Starting from the initial report in 2015, a new class of nickel-nitrogen-doped carbon (Ni-N-C) single metal site catalyst was developed to meet these challenges. In H-type liquid electrolyte reactors, these PGM-free catalysts deliver comparable CO reactivity as Ag-based ones, yet require only minute (< 1 wt%) metal doping vs thick sputtered or particle layers of Ag black metal loadings, offering a vast materials cost advantage. In addition, unlike bulk metal films, the Ni-N-C catalysts possess tunable pore structures ranging from micro to meso porosity combined with large surface areas, which benefit mass transport and raise mass-based catalytic activities. Despite these features, key performance metrics of Ni-N-C catalyst layers in membrane/gas diffusion electrode assemblies incorporated in flow electrolyzer have remained poor compared to commercial, state-of-art Ag electrodes. In addition, most electrolyzer reports to date operated the CO electrolyzers at differential flow conditions of large CO2 excess compared to CO2 consumption, equivalent to very large stoichiometric ratios stoich (= CO2, in / CO2, consumed), often using high alkaline electrolyte. This resulted in poor single pass conversions and favored carbon loss across the membrane. Viable "CO2-to-CO" electrolyzer cathodes need to not only show 100% faradaic CO efficiency and high partial current density, but they have to show this at very low  values to push the molar CO content in the exit feed toward unity.