Unveiling hydrogen-source-dominated CO₂ electroreduction activity on nitrogen-doped carbon nanotubes

In general, the activation of CO₂-rather than the availability of hydrogen source-is considered to be the determining step in electrocatalytic CO₂ reduction reaction (eCO₂RR) processes. However, the dissociation of H₂O, which is the dominant hydrogen source in most metal-based catalysts for the eCO₂RR in alkaline/neutral and even acidic electrolytes, suffers from the challenges of high dissociation barrier and carbonate formation. Herein, we design nitrogen-doped carbon nanotubes (N-CNTs). N-CNTs possess weak H₂O dissociation ability and can switch the hydrogen source from the dissociation of H₂O to hydrated protons, thereby greatly enhancing the eCO₂RR activity. In situ characterization and theoretical calculations confirm that the protons, rather than H₂O, act as the hydrogen source on the N-CNTs catalyst in an acidic electrolyte, which boosts the proton-coupled electron transfer process of *COOH formation and lowers the eCO₂RR barrier. Impressively, N-CNTs exhibit a remarkable faradaic efficiency of CO (FE_CO) at current densities ranging from -50 to -350 mA cm^-2 and sustained FE_CO at 200 mA cm^-2, outperforming most reported carbon catalysts. These findings identify the hydrogen source pathway as a new activity-tuning parameter for the eCO₂RR, paving a new path for the design of efficient electrocatalytic systems.