Insight into the metal-free electrocatalysis of heteroatom-doped carbon nanocages in competitive CO₂ reduction and H₂ evolution

Metal-free carbon-based catalysts exhibit diverse electrocatalytic performances in CO₂ reduction reaction (CO₂RR), but the attributions and contributions of active sites are still confusing to date. Herein, the hierarchical carbon nanocages (hCNC) doped with different heteroatoms (B, N, P, S) are prepared to examine the impact of dopants on the competitive CO₂RR and hydrogen evolution reaction (HER). The hCNC and P-doped hCNC show little CO₂RR activity, B- and S-doped hCNC show weak CO₂RR activity, while N-doped hCNC presents high CO₂RR activity. The CO Faradaic efficiency (FE_CO) of N-containing hCNC increases almost linearly with increasing the N content, even with the co-existing B or P. S- and SN-doped hCNC more facilitate the HER. 16 doping configurations are constructed, and up to 53 sites are examined for the electrochemical activities with a constant potential modelling method. The pyridinic-N(N*) is the best active site for CO₂RR to CO, while CBO₂H₂-1(αC*), CBO₂H₂-2(γC*), NO-1(βC*), PO₂H-3(αC*) and SO₃H-3(δC*) are active for HER. The optimized FE_CO achieves 83.6% for N-doped hCNC with 9.54 at.% nitrogen, and S-doped hCNC reaches ca. 30 mA·cm⁻² current density for HER. This study unveils the structure-performance correlation of heteroatom-doped hCNC, which is conducive to the rational design of advanced metal-free carbon-based catalysts.