Role of Lewis acidity and optimum contribution of cobalt and iron on the adsorption and electrochemical reduction of N2 to NH3

Abstract

Electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions is expected to be an environment-friendly and sustainable pathway for ammonia production. However, the low conversion rate and poor Faradaic efficiency of ammonia production attributed to the difficulty in N₂ adsorption, NN bond activation and concurrent hydrogen evolution reaction (HER) in aqueous electrolyte medium, inhibit its practical applications. Hence, development of robust and efficient electrocatalyst is highly desirable. In this work, designing N₂ reduction catalyst is purely based on assembling metal atoms of different N₂ adsorption affinities, in the form of bimetallic borate. In addition, N₂ being Lewis base can preferentially adsorb on the Lewis acid boron over H⁺ in acidic medium and suppress HER. To test this hypothesis, a bimetallic cobalt iron borate (CoFeBO) is designed in the form of nanoflakes using high energy ball-milling method. The electrocatalytic N₂ reduction is carried out in a H-cell fitted with proton exchange nafion-211 membrane using 0.05 M H₂SO₄ aqueous electrolyte. After the electrocatalytic N₂ reduction, the resultant NH₄⁺ ion concentration in the electrolyte is quantitatively analysed with the help of indophenol-blue method. As synthesised CoFeBO nanoflakes show NH₃ yield of 11.2 μg h⁻¹ cm⁻² with significant faradic efficiency of 47.8 % at +0.05 V vs RHE. The DFT calculations show that NRR mechanism favours the associative alternating pathway. It is concluded that the cobalt-site in CoFeBO catalyst offers the least energy barrier of 0.85 eV for hydrogenation of nitrogen atoms.