Electrochemical ammonia oxidation using nickel copper hydroxide with H₂ recovery at high current density and selectivity

Abstract

Electrochemical conversion of ammonia has received increasing attention due to the potential applications for fertiliser production and wastewater treatment. This work demonstrates the application of a homogeneously copper-doped nickel hydroxide, prepared through an easily scalable precipitation method, as an electrochemical oxidation catalyst. A cation exchange membrane divides the cell and prevents re-reduction of the oxidation product. During chronopotentiometry Ni_0.8Cu_0.2(OH)₂ was able to perform ammonia oxidation, with limited oxygen evolution, from 2.5 mA cm⁻² up to 400 mA cm⁻². The faradaic efficiency for nitrite formation increased with the applied current density. At a high initial ammonia concentration of 1 M, Ni_0.8Cu_0.2(OH)₂ converted 77% of the ammonia in less than 3.5 hours, applying a high current density of 400 mA cm⁻². This resulted in a faradaic efficiency of 96% total, which is 91% NO₂⁻ and 5% NO₃⁻, which would be impossible in an undivided cell. Therefore, this work demonstrates the potential for efficient and selective ammonia oxidation towards nitrite under industrially relevant current density and conditions.