Limitations and Improvement Strategies for Early-Transition-Metal Nitrides as Competitive Catalysts toward the Oxygen Reduction Reaction

The poor catalytic activity of early-transition-metal nitrides has prevented them from being competitive catalysts toward the oxygen reduction reaction (ORR). In the present study, we first explored the limitations for early-transition-metal nitrides as competitive catalysts in the view of O2 dissociation, finding that the limitations include insufficient d electrons (in the case of ScN, TiN, and VN) and unsuitable surface geometric structure (in the case of CrN), both of which can result in no O2 dissociation on early-transition-metal nitrides. Then on the basis of the above knowledge, we took VN as an example and proposed a strategy to enhance its ORR activity by enriching its d electrons through doping with 3d transition metals. The doped VN showed greatly enhanced ORR activity, with Co-doped VN exhibiting the best performance; its ORR activity was close to that of JM 20 wt % Pt/C. X-ray photoelectron spectroscopy (XPS) clearly revealed that Co doping significantly increased the proportion of V in a low-valence state. O2 temperature-programmed desorption (O2-TPD) measurements also presented some very important information induced by doping. Our theoretical analysis and experimental studies indicated that early-transition-metal nitrides with insufficient d electrons can be promising ORR catalysts via the strategy of enriching their d electrons through doping elements with rich d electrons.