As the second largest chemical product globally, ammonia plays a pivotal role in the economy. The conventional Haber-Bosch process urgently needs innovation due to its high energy consumption and environmental pollution. The high-temperature electrochemical ammonia synthesis system based on protonic ceramic cells, which uses electric energy to overcome the thermodynamic barrier of nitrogen activation and achieves green synthesis under ambient pressure conditions, has attracted much attention.

Proton-conducting ceramic cells consist of an electrolyte and two electrodes (i.e., ammonia electrode and hydrogen electrode). The electrolyte must simultaneously fulfill dual functions, i.e., proton conduction and gas isolation. Its material design requires a) dense microstructure to block gas permeation, b) high proton conductivity and negligible electronic conductivity, c) chemical stability under high-temperature conditions with exposure to H₂O, CO₂, and NH₃, and d) thermal expansion coefficient compatibility with electrode materials to avoid thermomechanical stress-induced damage. This review systematically represents the applications of perovskite, fluorite, and pyrochlore-type electrolytes in ammonia synthesis. SrCeO₃-based oxides are the reported perovskite materials with a proton conductivity, with their proton transport capacity that is enhanced through ionic doping strategies. Compared to other perovskite oxides, BaCeO₃-based perovskites with a higher conductivity are currently among the most extensively studied proton-conducting materials. Doping with rare-earth metals is a common strategy to improve the proton conductivity of BaCeO₃-based oxides. Alkaline-earth zirconate oxides (i.e., CaZrO₃, SrZrO₃, BaZrO₃) exhibit lower proton conductivity than cerium-based oxides but demonstrate superior chemical stability and mechanical strength. LaGaO₃-based oxides achieve a nearly pure proton conductivity in H₂ atmospheres, and LaGaO₃-based electrolytes doped with Ca²⁺, Sr²⁺, or Ba²⁺ demonstrate a good performance in ammonia synthesis. Beyond perovskites, non-perovskite oxides such as fluorite (i.e., AO₂) and pyrochlore (i.e., A₂B₂O₇) also exhibit a notable proton conductivity under high-temperature and hydrogen-containing conditions. Rare-earth-doped CeO₂ fluorite-type electrolytes outperform perovskite counterparts in ammonia generation rates. SDC electrolyte achieves a record-high ammonia formation rate of 8.2×10^-9 mol·s^-1·cm^-2 at 650 ℃ as the maximum reported value based on proton-conducting ceramic. Pyrochlore-structured electrolytes have a proton conductivity comparable to fluorite structures but with enhanced chemical stability.

The catalytic activity and stability of the ammonia electrode directly affect the rate of ammonia synthesis. Electrode materials require superior nitrogen adsorption/dissociation capabilities coupled with a high NH₃ selectivity. Also, ammonia electrodes must demonstrate adequate electronic/protonic conductivity and chemical stability. Noble metals are widely used in early-stage proton-conducting ceramic electrochemical ammonia synthesis due to their high electronic conductivity and catalytic activity. Ag-Pd bimetallic alloys are the most common electrode materials. Pd exhibits a high catalytic activity for NRR, enabling N≡N bond dissociation at elevated temperatures. However, Pd preferentially adsorbs H atoms over N atoms, leading to dominant HER over NRR. Ag-Pd alloy partially suppresses HER, while enhancing ammonia production rates and faradaic efficiency. Perovskite materials become promising alternatives for ammonia electrodes due to their tunable structure, high thermal stability and low cost. Cobalt-based perovskites possess a superior catalytic activity, and they are extensively studied for electrochemical ammonia synthesis. However, some challenges such as cobalt reduction/volatilization, high thermal expansion coefficients, and elevated costs restrict their practical application. Ru-doped perovskites are particularly attractive due to the superior catalytic activity for N₂ activation. Iron-containing perovskites have attracted much attention as cathode materials for protonic ceramic ammonia synthesis. The multivalent characteristics of Fe confer dual advantages in vacancy regulation and electronic structure optimization.