Oxygen-Bridged Vanadium Single-Atom Dimer Catalysts Promoting High Faradaic Efficiency of Ammonia Electrosynthesis.

Abstract

Single-atom catalysts have already been widely investigated for the nitrogen reduction reaction (NRR). However, the simplicity of a single atom as an active center encounters the challenge of modulating the multiple reaction intermediates during the NRR process. Moving toward the single-atom-dimer (SAD) structures can not only buffer the multiple reaction intermediates but also provide a strategy to modify the electronic structure and environment of the catalysts. Here, a structure of a vanadium SAD (V-O-V) catalyst on N-doped carbon (O-V₂-NC) is proposed for the electrochemical nitrogen reduction reaction, in which the vanadium dimer is coordinated with nitrogen and simultaneously bridged by one oxygen. The oxygen-bridged metal atom dimer that has more electron deficiency is perceived to be the active center for nitrogen reduction. A loop evolution of the intermediate structure was found during the theoretical process simulated by density functional theory (DFT) calculation. The active center V-O-V breaks down to V-O and V during the protonation process and regenerates to the original V-O-V structure after releasing all the nitrogen species. Thus, the O-V₂-NC structure presents excellent activity toward the electrochemical NRR, achieving an outstanding faradaic efficiency (77%) along with the yield of 9.97 μg h^-1 mg^-1 at 0 V (vs RHE) and comparably high ammonia yield (26 μg h^-1 mg^-1) with the FE of 4.6% at -0.4 V (vs RHE). This report synthesizes and proves the peculiar V-O-V dimer structure experimentally, which also contributes to the library of SAD catalysts with superior performance.