Subsurface Graphitic Nitrogen Activates Protonated Pyridinic-N Sites for Acidic Oxygen Reduction

Protonation of pyridinic nitrogen (pyri-N) sites in nitrogen-doped carbon catalysts under acidic conditions converts pyri-N to pyri-NH⁺, leading to severe catalytic deactivation. Restoring activity requires transforming pyri-NH⁺ back to pyri-NH, an energetically demanding process that limits oxygen reduction reaction (ORR) efficiency. To overcome this challenge, we developed an N-doped porous carbon catalyst (NpC-7) featuring surface pyri-N active sites and a subsurface graphitic nitrogen (grap-N) layer. The 2D growth of cyanuric acid (CA)-functionalized 2-hydroxyethyl methacrylate (HEMA) polymer within graphene oxide layers ensures uniform N-doping which modulates π-electron and spin states. This engineered architecture delivers ORR activity approaching Pt/C and surpasses conventional nitrogen doped graphene oxide (NrGO) (E_onset = 0.86 V, E_1/2 = 0.70 V vs. 0.76 and 0.61 V, respectively). Enhanced performance arises from subsurface grap-N, which donates electron to protonated pyri-NH⁺, generating an active pyri - NH^{δ +}  state that promotes O₂ adsorption. In situ electrochemical Raman spectroscopy and density functional theory (DFT) calculations confirm efficient O₂ adsorption and intermediate formation, indicating a (2 + 2)e^- ORR pathway. These findings highlight the critical role of subsurface grap-N in overcoming acidic deactivation through electronic modulation, offering a blueprint for designing acid-stable carbon electrocatalysts.