Electrochemical Nitrate Reduction to Ammonia on Polycrystalline Copper Electrodes in Alkaline Solutions

Abstract

Ammonia has widely utilized as the raw material of fertilizer and feeds in fuel cells and hydrogen storage materials (Orens. S, Proc. Natl. Acad. Sci. U.S.A. 2021, 118 (49)). In general, the Haber-Bosch process has supplied the mass production of ammonia (N 2 + H 2 → 2NH 3 ). However, this chemical process requires high-temperature and high-pressure conditions to cleave the triple N 2 and produces undesired CO 2 gas, as CH 4 is added for H 2 generation. It is attractive to yield ammonia in milder conditions and more cost-effective methods. In addition, the usage of waste and eco-poisoned species, which is inevitably produced, is valuable as the reactant. For this purpose, nitrate (NO 3 - ) conversion using electrochemical methods has drawn attention. Nitrate has a high solubility in aqueous media and better reactivity than N 2 gas at ambient temperature and atmospheric conditions. Nonetheless, the nitrate reduction undergoes multiple electron-transfer processes (NO 3 - + 8e - + 9H + → NH 3 + 3H 2 O), causing low ammonia selectivity competing against byproducts such as hydrogen, nitrogen oxides, and hydroxylamine. Here, I show critical factors determining the conversion selectivity of nitrate using Cu catalysts. I prepared three Cu foils treated by different surface cleaning processes. Surface morphology and roughness of Cu relying on the surface treatments significantly altered the conversion efficiency. In particular, nitric oxide (NO) was a pivotal intermediate to determine the final products, which was sensitive to the Cu surface condition. I will discuss details of the electrochemical nitrate reduction process observed by in-situ and ex-situ gas and spectroscopic analyses and correlate the conversion efficiency with the surface conditions of Cu foils. Figure 1