Electrochemistry leads the way toward green ammonia synthesis

• Ammonia is a crucial chemical, with its production approaching 180 million tonnes per year.
• Traditionally, ammonia is produced from nitrogen in air and hydrogen from fossil resources through catalytic reactions that require high temperature and pressure.
• Ammonia can also be synthesised using the combination of water electrolysis with renewable electricity and catalytic reactions, but this combination isn’t suitable for conditions of fluctuating electricity.
• Professor Ryuji Kikuchi at Hokkaido University and Professor Jun Kubota at Fukuoka University, Japan are developing new electrochemical routes to produce ammonia sustainably.
• Their ammonia synthesis approach makes effective use of surplus renewable energy that would otherwise be thrown away. In this way, it can contribute to the establishment of a carbon-neutral society.

Ammonia plays a crucial role in numerous industries, from agriculture, where it serves as a key component in fertilisers, to the production of important chemicals, such as nitric acid and urea. Ammonia is also widely used in refrigeration, in the manufacturing of explosives and cleaning products, and as a precursor in the pharmaceutical and textile industries. In Japan, ammonia is expected to be used as a fuel for thermal power plants and large ships and demonstration experiments are underway.

Given the indispensable role of ammonia in global industries, its production significantly impacts the world economy. Innovations in ammonia production methods can lead to substantial advancements in sustainable practices, positively impacting the global economy and environmental sustainability. Since 2014, Professor Ryuji Kikuchi of Hokkaido University and Professor Jun Kubota of Fukuoka University have been exploring new electrochemical approaches that can overcome the limitations of current methods for ammonia synthesis.

How is ammonia produced?

Traditionally, ammonia production has been predominantly achieved through the Haber-Bosch process, in which nitrogen and hydrogen are combined under high temperature and pressure in the presence of a metal catalyst. This method, while effective, demands substantial energy consumption, contributing to significant greenhouse gas emissions. Furthermore, the hydrogen gas required during the Haber-Bosch process is obtained from fossil fuels, such as natural gas. This makes the use of ammonia as a fuel environmentally and economically unsustainable.

The traditional production of ammonia demands substantial energy consumption, contributing to significant greenhouse gas emissions.

Ammonia can also be produced through water electrolysis. In this process, an electrical current is passed through water. This causes water molecules to split into oxygen and hydrogen. Hydrogen can then react with nitrogen to form ammonia. This is a clean and efficient approach to ammonia production. However, it is a two-step process which requires a steady supply of electric power. The integration of this process into ammonia production plants, exploiting fluctuating electricity sources, such as wind, solar, and solar thermal generators, is therefore impractical.

Electrochemical synthesis of ammonia

In recent years, increasing effort has been devoted to developing new routes to ammonia synthesis as alternatives to the Haber-Bosch and electrochemical water decomposition. One of the most promising approaches is the electrochemical reduction of atmospheric nitrogen, which can be conducted ideally, at a temperature lower than 100^oC. However, all the methods developed so far have proven to be low in both efficiency and ammonia yield, rendering them unsuitable for industrial use.

The electrochemical reduction of nitrogen for ammonia synthesis can also be accomplished using lithium-mediated chemical process. This method uses non-aqueous electrolytes in which the conduction of electricity is linked to the migration of specific ions, rather than electrons (occurring in normal conductors).

Chemically reduced lithium can readily activate nitrogen molecules, and this method enhances the overall reaction efficiency. This method is not subject to the chemical equilibrium constraints of hydrogen, nitrogen, and ammonia like the Haber-Bosch method, and is thought to enable efficient ammonia synthesis. At present, however, the method is limited by the large electric potential required for the reduction of lithium within the electrolyte, which makes it unsuitable for industrial applications.

Introducing novel electrochemical synthesis routes

Kikuchi and Kubota, who have conducted extensive research on the synthesis of ammonia over the years, propose electrolysis with the use of phosphate ion-based electrolytes, specifically (caesium dihydrogen phosphate) CsH₂PO₄ / (silicon phosphate) SiP₂O₇, at moderate temperatures (200–250^oC). These conditions are far milder than those of the Haber-Bosch process, which requires temperatures of 400–500^oC and pressures of the order of 15–30 MPa.

Crucially, the methods proposed by Kikuchi and Kubota achieve yields and efficiencies comparable to those of traditional commercial processes, making them promising candidates for green ammonia synthesis at an industrial scale.

Electrocatalytic nitrogen reduction

Nitrogen is a highly stable, chemically inert, and extremely abundant molecule. In nature, its conversion to ammonia and other related nitrogenous compounds is carried by specific microorganisms in soil, known as diazotrophs. This process, which is usually referred to as ‘nitrogen fixation’, requires the involvement of protein complexes, called nitrogenases, which only a handful of microorganisms can synthesise.

Kikuchi and Kubota are creating new, sustainable, and green electrochemical routes for the synthesis of ammonia at industrial scales.

In Kikuchi’s approach to ammonia production, nitrogenase is replaced by a catalyst composed of iron oxide and yttrium-doped barium zirconate, combined with ruthenium oxide. This material, which can bind strongly to nitrogen molecules, is used as the cathode – the negative electrode – in electrolysis using CsH₂PO₄/SiP₂O₇ as electrolyte.

The electrons supplied by the cathode are responsible for the reduction of nitrogen to form ammonia. Kikuchi has shown that the ammonia yield and the Faradaic efficiency of the process, which describes the ability of an electrolyte to promote charge transfer, reach values that make it a competitive alternative to traditional methods.

Leveraging hydrogen-permeable membranes

While Kikuchi’s approach exploits the direct reduction of nitrogen, Kubota has proposed a process in which water first undergoes electrolysis, to produce oxygen and hydrogen cations at a platinum anode. These positive hydrogen ions diffuse through the CsH₂PO₄/SiP₂O₇ electrolyte to reach a palladium-silver membrane, which is permeable to hydrogen, before reacting with nitrogen on a ruthenium-based catalyst, to produce ammonia.

Effectively, the membrane splits the system into two parts: one, in which water is decomposed by electrolysis, and the other, in which hydrogen reacts with nitrogen to produce ammonia. This makes it possible to achieve high ammonia production efficiency at working temperatures of 250^oC. Kubota has also shown that the production of ammonia increases with the total pressure and has demonstrated that an optimal yield can be achieved at a pressure of 0.7 MPa. Due to its lower temperature compared to the Haber-Bosch process, it has been observed that the conversion rate of hydrogen is approximately equivalent to that of the commercial Haber-Bosch method.

These two innovative approaches have efficiencies nudging those of commercial ammonia synthesis, but under much milder working conditions. Kikuchi and Kubota are now collaborating with fellow researchers and industry pioneers to create new, sustainable, and green electrochemical routes for the synthesis of ammonia at industrial scales.