Free radicals promote electrocatalytic nitrogen oxidation

Abstract

In this work, we introduce hydroxyl radicals into the electrocatalytic nitrogen oxidation reaction (NOR) for the first time. Cobalt tetroxide (Co₃O₄) acts not only as an electrocatalyst, but also as a nanozyme (in combination with hydrogen peroxide producing ˙OH), and can be used as a high-efficiency nitrogen oxidation reaction (NOR) electrocatalyst for environmental nitrate synthesis. Co₃O₄ + ˙OH shows an excellent nitrogen oxidation reaction (NOR) performance among Co₃O₄ catalysts in 0.1 M Na₂SO₄ solution. At an applied potential of 1.7 V vs. RHE, the HNO₃ yield of Co₃O₄ + ˙OH reaches 89.35 μg h^-1 mg_cat ^-1, which is up to 7 times higher than that of Co₃O₄ (12.8 μg h^-1 mg_cat ^-1) and the corresponding FE is 20.4%. The TOF of Co₃O₄ + ˙OH at 1.7 V vs. RHE reaches 0.58 h^-1, which is higher than that of Co₃O₄ (0.083 h^-1), demonstrating that free radicals greatly enhance the intrinsic activity. Density functional theory (DFT) demonstrates that ˙OH not only can drive nitrogen adsorption, but also can decrease the energy barrier (rate-determining step) of N₂ to N₂OH*, thus producing great NOR activity.