In operando identification of the V4+-site-dependent nitrogen reduction reaction of VSx

Abstract

The unique d orbital electron configuration of transition-metal elements endows the metal centers of transition metal-based materials (TMs) with multiple valence features. However, the metal valence state dependent activity of TMs has been rarely investigated for the electrocatalytic nitrogen reduction reaction (ENRR), which involves multiple proton-electron coupling steps. Herein, we prepared VS_2−x with hybrid valences (V²⁺ and V⁴⁺) and VS₂ (with pure V⁴⁺) through a typical hydrothermal method to unveil the effect of V²⁺ and V⁴⁺ on the ENRR activity. The as-prepared VS₂ sample exhibited a prominent FE of 35.52% and a high ammonia yield of 41.21 μg mg_cat⁻¹ h⁻¹ at a lower overpotential (−0.3 V vs. RHE), which apparently outperforms the VS_2−x counterpart. We used HRTEM, XRD, XPS, in situ EIS and XAS to characterize the evolution of V²⁺ and V⁴⁺ in the obtained samples. Importantly, in situ Raman and theoretical calculations demonstrated that the V⁴⁺ site is responsible for the obtained high ENRR performance. Surprisingly, we found that the adsorption of N₂ molecules is an exothermic process on the V⁴⁺ site, but an endothermic process on the V²⁺ site. This work provides new insight into the catalytic activity of TM based ENRR catalysts.