Deciphering the bridge oxygen vacancy-induced cascading charge effect for electrochemical ammonia synthesis

Oxygen vacancy engineering has recently been gaining much interest as the charging effect it induces in a material can be used for varied applications. Usually, semiconductor materials act poorly in electrocatalysis, particularly in the nitrogen reduction reaction (NRR), owing to their inherent charge deficit and huge band gap. Vacancy introduction can be a viable material engineering route to make use of these materials for the NRR. However, a detailed investigation of the vacancy-type and its role for the structural reorientation and charge redistribution of a material is lagging in the field of NRRs. This work thus focuses on the synthesis of oxygen vacancy-engineered SnO₂ with a gradual structural transformation from in-plane (io_v) to bridge-type oxygen vacancy (bo_v) density. Consequently, the electron occupancy of the sp³d hybrid orbital changes, leading to an upshifted valence band maxima towards the Fermi level. This has a profound effect on the nature of N₂ adsorption and the extent of NN bond polarization. Sn atoms adjacent to the bo_v are found to have a fair density of dangling charges that accomplish the NRR process at a comparatively low overpotential and determine the binding strength of the intermediates on the active site. The obscured yet stable reaction intermediates are thereby identified with in situ ATR-IR studies. A restricted hydrogen evolution reaction Faradaic on the Sn-site (favored over O-atoms) results in a Faradaic efficiency of 48.5%, which is better than that reported in all the literature reports on SnO₂ for the NRR. This study thus unveils sufficient insights into the role of oxygen vacancies in a crystal as well as electronic structural alteration of SnO₂ and the effect of active sites on the rate kinetics of the NRR.