Mesoporous MnO2 nanosheets for efficient electrocatalytic nitrogen reduction via high spin polarization induced by oxygen vacancy

The electrochemical N2 reduction reaction (NRR) represents a green and sustainable route for NH3 synthesis under ambient conditions. However, the mechanism of N2 activation in the electrocatalytic NRR remains unclear. Herein, we found that the high spin state Mn3+-Mn3+ pairs induced by oxygen vacancy in MnO2 nanosheets greatly enhance the catalytic activities. The strong electron transfer between d orbital of Mn and orbital of N2 forces the N2 to be of radical nature, which activates the hydrogenation process and weakens the N≡N bond. Based on the density functional theory (DFT) calculation results, we precisely designed mesoporous MnO2 nanosheets with rich oxygen vacancies via using methyltriphenylphosphonium bromide (MPB) to induce more Mn3+-Mn3+ pairs (Mn3-3-MnO2), which can achieve a fairly high ammonia yield of up to 147.2 µg·h−1·mgcat−1. at -0.75 V vs. reversible hydrogen electrode (RHE) and a high Faradaic efficiency (FE) of 11%. Furthermore, these mesoporous MnO2 nanosheets exhibit the superior durability with negligible changes in both NH3 yield and FE after a consecutive 6-recycle test and the current density electrolyzed over a 24-hour period. Our findings offer an approach to designing highly active transition metal catalysts for electrocatalytic nitrogen reduction.