suggest poor in-plane CO2 transfer to active sites and poor subsequent catalytic conversion, as seen for the NiNC-PANI electrolyzers: suboptimal microporous morphology caused poor site accessibility and flooded micropores result in low FECO, as confirmed by the microscopic analysis of the sprayed layer structure, and the loading-independent double-layer capacitance (Supplementary Figure ). The loading-indepent value of CCC ~ 1 is testament to sufficiently effective ionic through-plane channels in the catalyst layer. This regime calls for adjustment of the catalyst pore size at constant layer thickness. Operating electrolyzer conditions with CCC ~ 1 and high FECO, as seen for NiNC-IMI cells at 1 mg cm -2 (green in We note that experimental values of CCC > 1.25 reliably identified catastrophic GDE flooding phenomena, where OH-ions reacted with excess CO2 to bicarbonate due to a sudden blockage of active sites. Here, the CCC value again proofs as a valuable diagnostic failure analysis indicator. On the other hand, for electrolyzers operating under acidic conditions using Nafion membranes (not shown in this contribution), a consistent value of CCC = 0 ensued, as no anionic carbon species forms and crosses the cation exchange membrane. A value of CCC = 2, which would suggest a pure HCO3 2-transfer, is yet to be observed experimentally.