Selective and quantitative nitrate electroreduction to ammonium using a porous copper electrode in an electrochemical flow cell

• A novel porous copper electrode of high surface areas and low densities was prepared.
• The copper cathode was examinated for electroreduction of nitrate to ammonium.
• Selective and quantitative reduction of nitrate was obtained in neutral solution.
• High chemical yields and current efficiencies toward ammonium formation was recorded.
• Recycling of nitrates as an abundant and inexpensive source of ammonium.

The aim of this work was to set up a novel electrochemical system allowing an efficient transformation of concentrated nitrate solutions to ammonium and which can be subsequently implemented on a large scale application. First, this paper describes the preparation of a porous copper modified electrode by successive electrodeposition of nickel then copper on a graphite felt of large specific surface area. Homogeneous Cu coating of all fibers in the 3D porous structure was successfully obtained using low concentrations of copper salts and high applied current intensities. The porous copper electrode was then used in a flow electrochemical process to achieve a selective and quantitative transformation of concentrated nitrate into ammonium. Different electrolytic solutions, slightly acid (acetate buffer) or neutral (phosphate buffer), and flow rates were investigated. The nitrate solution was quantitatively reduced into NH4+ with high selectivity in only one pass through the electrode. When the applied current was similar to the theoretical one, the maximum selectivity (96%) and the best current efficiency (72%) for NH4+ formation were reached at pH 7.2 with a flow rate of 2 mL min−1. The obtained ammonium solution can be subsequently used either as a potential nitrogen source during microbial culture or simply as a fertilizer.