Defective electrocatalysts, especially for intrinsic defective carbon, have aroused a wide concern owing to high spin and charge densities. However, the designated nitrogen species favorable for creating defects by the removal of nitrogen, and the influence of defects for the coordination structure of active site and oxygen reduction reaction (ORR) activity have not been elucidated. Herein, we designed and synthesized a pair of electrocatalysts, denoted as Fe-N/C and Fe-ND/C for coordination sites of atomic iron-nitrogen and iron-nitrogen/defect configuration embedded in hollow carbon spheres, respectively, through direct pyrolysis of their corresponding hollow carbon spheres adsorbed with Fe(acac)3. The nitrogen defects were fabricated via the evaporation of pyrrolic-N on nitrogen doped hollow carbon spheres. Results of comparative experiments between Fe-N/C and Fe-ND/C reveal that Fe-ND/C shows superior ORR activity with an onset potential of 30 mV higher than that of Fe-N/C. Fe-ND sites are more favorable for the enhancement of ORR activity. Density functional theory (DFT) calculation demonstrates that Fe-ND/C with proposed coordination structure of FeN4−x (0<x<4) anchored by OH as axial ligand during ORR, weakens the strong binding of OH* intermediate and promotes the desorption of OH* as rate-determining step for ORR in alkaline electrolyte. Thus, Fe-ND/C electrocatalysts present much better ORR activity compared with that of Fe-N/C with proposed coordination structure of FeN4.

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This study reports the potential application of Ni2P as highly effective catalyst for chemical CO2 recycling via dry reforming of methane (DRM). Our DFT calculations reveal that the Ni2P (0001) surface is active toward adsorption of the DRM species, with the Ni hollow site being the most energetically stable site and Ni–P and P contributing as coadsorption sites in DRM reaction steps. Free-energy analysis at 1000 K found CH–O to be the main pathway for CO formation. The competition of DRM and reverse water gas shift (RWGS) is also evidenced with the latter becoming important at relatively low reforming temperatures. Very interestingly, oxygen seems to play a key role in making this surface resistant toward coking. From microkinetic analysis, we have found greater oxygen surface coverage than that of carbon at high temperatures, which may help to oxidize carbon deposits in continuous runs. The tolerance of Ni2P toward carbon deposition was further corroborated by DFT and microkinetic analysis. Along with the higher probability of C oxidation, we identify poor capacity of carbon diffusion on the Ni2P (0001) surface with very limited availability of C nucleation sites. Overall, this study opens avenues for research in metal-phosphide catalysis and expands the application of these materials to CO2 conversion reactions.