Unraveling the role of unpaired 3d electrons in eg orbital of CoO: Driving π feedback of N2 for efficient ammonia electrosynthesis

Abstract

Transition metal oxides (TMOs) have been emerged as efficient electrocatalysts for ammonia (NH3) electrosynthesis. However, the relationship between the d electron configuration of TMOs and their catalytical activity needs to be deeply understood towards for electrochemical nitrogen reduction reaction (ENRR). Herein, we focused on illustrating the effect of orbital occupation/distribution of d electrons on the ENRR performance of Cobalt oxides by deliberately selecting CoO (with a t2g5eg2 electron configuration of Co2+) and Co2O3 (with a t2g6eg0 electron configuration of Co3+) as modeling catalysts. Interestingly, the CoO catalyst exhibited a high ammonia yield of 33.46 μg h−1 mgcat−1 at −0.6 V versus reversible hydrogen electrode (RHE) and a prominent faradic efficiency (FE) of 5.1%, which apparently outperforms the Co2O3 catalyst. We used XAS, M-T and MFM to thoroughly characterize the electronic structure of the active center of the samples. Importantly, Theoretical calculations and In-situ techniques demonstrated that the unpaired electrons in eg orbital of Co2+ play crucial role in boosting the adsorption and activation of N2 on CoO. In specific, the t2g5eg2 electron configuration delivers more electron from Co2+ to N2 and the unpaired electrons in eg orbital of Co2+ site promote surprisingly the π-feedback process from CoO to N2, resulting a lower hydrogenation energy barrier for N2 activation. This work provides a deep understanding of the structure–function relationship of TMOs catalyst for ENRR.