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

The electrochemical N 2 reduction reaction (NRR) represents a green and sustainable route for NH 3 synthesis under ambient conditions. However, the mechanism of N 2 activation in the electrocatalytic NRR remains unclear. Herein, we found that the high spin state Mn 3+ -Mn 3+ pairs induced by oxygen vacancy in MnO 2 nanosheets greatly enhance the catalytic activities. The strong electron transfer between d orbital of Mn and orbital of N 2 forces the N 2 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 MnO 2 nanosheets with rich oxygen vacancies via using methyltriphenylphosphonium bromide (MPB) to induce more Mn 3+ -Mn 3+ pairs (Mn 3-3 -MnO 2 ), which can achieve a fairly high ammonia yield of up to 147.2 µg·h −1 ·mg cat −1 . at -0.75 V vs. reversible hydrogen electrode (RHE) and a high Faradaic efficiency (FE) of 11%. Furthermore, these mesoporous MnO 2 nanosheets exhibit the superior durability with negligible changes in both NH 3 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.