Direct Electrochemical Synthesis of NH3 from N2 and H2O

The Haber process for producing NH₃ is the most important chemical process developed in the 20th century. At 400 to 500 °C and 150 to 200 bar, N₂ and H₂ react to form NH₃. The required H₂ is produced by the reformation of CH₄ or coal with H₂O. For each ton of NH₃ produced, approximately 2 t CO₂ are emitted. The elimination of this CO₂ emission as well as to omit the use of carbon sources for H₂ production are highly desired. Around 80 % of the produced NH₃ is converted to be used as fertilizer.

A potentially more sustainable NH₃ synthesis method is the direct reduction of N₂ using an electrochemical membrane reactor (ecMR). It avoids CO₂ emission, if renewable energy sources, such as wind or solar power, are used to power the process.

An ecMR consists of an anode for water oxidation, a proton exchange membranes (PEM), and a cathode for the desired reduction as well as parallel reactions. The workability of the ecMR for the electrochemical reduction of CO₂ to synthesize hydrocarbons was proved, however, at very low production rates and current efficiencies.

Matthias Wessling and colleagues, DWI – Leibnitz Institute for Interactive Materials, Aachen, Germany, have used an ecMR process for the simultaneous generation of NH₃ and H₂ (pictured). The process is carbon‐independent and CO₂‐free: H₂O is used as an abundant source of H₂, and renewable energy can potentially drive the reaction.

The core of the ecMR is the membrane electrode assembly (MEA). It combines (i) the oxygen evolution reaction (OER) at the anode (H₂O is oxidized to form O₂ and electrons), (ii) the migration of H⁺ ions across the polymer cation exchange membrane (CEM) to the cathode, and (iii) the reduction of N₂ to NH₃ at the cathode. The main (parallel) reaction at the cathode is a reduction of H+, i.e., the hydrogen evolution reaction (HER). The MEA presented here consisted of three elements: a) an iridium mixed metal oxide (IrMMO) anode, b) a polymer CEM, and c) a Ti cathodic catalyst

According to the researchers, the ecMR demonstrates the workability to co‐generate NH₃ and H₂ directly from nitrogen and water vapor at ambient conditions. An exceptional high NH₃‐specific current efficiency of up to 27.5% is reported. The co‐generation can be tuned by the balance of process parameters. This synthesis is a viable option to store energy and produce fertilizer precursors.

According to the researchers, their work encourages further research on catalysts for the electrochemical synthesis of NH₃ to make the process more efficient in terms of NH₃ selectivity and production rates. In their opinion, there are several ways to improve the achieved production rate and current efficiency of the ecMR.