Solid-state electrochemical synthesis of ammonia: a review

Ammonia is one of the most produced chemicals worldwide, and it is not only a major end product but also an important energy storage intermediate. The solid-state electrochemical synthesis of ammonia has the promise to overcome the limitations of the conventional catalytic reactors such as the limited conversion, severe environmental pollution and high energy consumption. Solid-state electrolytes either protonic or oxide ion conductors have been reviewed and particular emphasis is placed on their application to synthesise ammonia. The highest rate of ammonia formation according to the type of electrolyte utilised were in the following order; solid polymers > Ce_0.8Gd_0.2O_2−δ-(Ca₃(PO₄)₂-K₃PO₄) composites > fluorites > perovskites > pyrochlores although the catalysts in electrodes also play an important role. The highest rate reported so far is found to be 1.13 × 10⁻⁸ mol s⁻¹ cm⁻² at 80 °C with a potential of 2 V using Nafion membrane, SmFe_0.7Cu_0.1Ni_0.2O₃ (SFCN), and Ni-Ce_0.8Sm_0.2O_2−δ as solid electrolyte, cathode and anode, respectively. Synthesising ammonia from steam and N₂, by-passing H₂ stage offers many advantages such as reduction of device numbers and then the overall costs. The factors affecting the rate of ammonia formation have been discussed as well.