Enhancement of Electrochemical Nitrogen Reduction Activity and Suppression of Hydrogen Evolution Reaction for Transition Metal Oxide Catalysts: The Role of Proton Intercalation and Heteroatom Doping

During the electrochemical nitrogen reduction reaction (eNRR) and hydrogen evolution reaction (HER), interstitial proton intercalation readily occurs in some transition metal oxide (TMO) catalysts and changes their d-band electronic structure. This work fabricated phosphorus (P)-doped tungsten oxide (WO3) with enriched oxygen vacancies (OVs) to study the impact of proton intercalation and heteroatom doping on eNRR and HER. Our results demonstrated that the electronic structure of the P-OV-WO3 catalyst was altered by in situ proton intercalation as indicated by the greater negative onset potential of eNRR at -0.05 V compared to the proton intercalation potential of 0.3 V versus reversible hydrogen electrode (RHE). Compared to the non-P-doped WO3, the introduction of P doping in WO3 (e.g., 4.8 at. %) led to a reduction of more than 36% in proton intercalation. As a result, the HER activity of the P-OV-WO3 was significantly suppressed, as demonstrated by a considerably negative shift of the onset HER potential from -0.06 to -0.15 V and a slower HER kinetics with the Tafel slope increased from 129.0 to 343.1 mV/dec. Density functional theory calculations revealed the synergy of the proton intercalation, substitutional P doping, and the associated OVs in the improvement of N-2 activation and hydrogenation in eNRR. The increased eNRR and the suppressed HER led to a high Faradaic efficiency (FE) of 64.1% and the NH3 yield of 24.5 mu g<middle dot>mg (-1)(cat) h(-1) at -0.15 V versus RHE in H2SO4 (pH = 2) as the electrolyte. The specific NH3 yield is more than 20 times higher than that of C-WO3 (1.1 mu g<middle dot>mg (-1)(cat) h(-1) with a FE of 20%). The results exceed most of the reported eNRR performances for TMO-based catalysts. Thus, the synergistic proton intercalation and P doping could lead to newer designs and applications of TMO-based catalysts for improved eNRR while suppressing the competing HER.