The application of protonic ceramic electrolysis cells (PCECs) for ammonia (NH₃) synthesis has been evaluated over the past 14 years. While nitrogen (N₂) is the conventional fuel on the cathode side, various fuels such as methane (CH₄), hydrogen (H₂), and steam (H₂O) have been investigated for the oxygen evolution reaction (OER) on the anode side. Because H₂ is predominantly produced through CO₂-emitting methane reforming, H₂O has been the conventional carbon-free option thus far. Although the potential of utilizing H₂O and N₂ as fuels is considerable, studies exploring this specific combination remain limited. PCEC fabrication technologies are being developed extensively, thus necessitating a comprehensive review. Several strategies for electrode fabrication, deposition, and electrolyte design are discussed herein. The progress in electrode development for PCECs has also been delineated. Finally, the existing challenges and prospective outlook of PCEC for NH₃ synthesis are analyzed and discussed. The most significant finding is the lack of past research involving PCEC with H₂O and N₂ as fuel configurations and the diversity of nitrogen reduction reaction catalysts. This review indicates that the maximum NH₃ synthesis rate is 14 × 10⁻⁹ mol cm⁻² s^−1, and the maximum current density for the OER catalyst is 1.241 A cm⁻². Moreover, the pellet electrolyte thickness must be maintained at approximately 0.8–1.5 mm, and the stability of thin-film electrolytes must be improved.

Ammonia is one of the promising carriers for hydrogen and a critical ingredient in many industries including fertilizers and pharmaceuticals. In the KAAP process, ruthenium- (Ru-) based catalysts showed 10-20 more activity compared with iron- (Fe-) based catalysts. The modifications that are applied to Ru-based catalysts revolve around changing the material of its support and/or promoters. This study compares the performance of a Ru-based catalyst for ammonia synthesis supported by La₂Ce₂O₇ using barium (Ba), cesium (Cs), potassium (K), and lithium (Li) as promoters. Based on structural, physicochemical, adsorption, and electronic state analysis, the Cs-promoted catalyst is expected to perform best among all the promoted catalysts, while our findings suggest that the K-promoted catalyst performed the best in the actual catalytic reaction. This result will affect the development of Ru/La₂Ce₂O₇-based catalysts, especially in ammonia synthesis at different temperatures and pressures.

Ammonia (NH₃) is a carbon-free and hydrogen-rich (17.8wt H₂) chemical that has the potential to revolutionize the energy sector. Compared with hydrogen (H₂), NH₃ can be easily liquefied, stored, and transported globally. However, the conventional thermocatalytic process to synthesize NH₃ accounts for 2 of global energy consumption and 1.2 of CO₂ emissions annually. To make the process further efficient, new catalysts must be developed to allow for NH₃ synthesis in milder conditions with high thermal stability. To this end, we have developed ruthenium (Ru) supported on perovskite (BaCe_xO_y) via a ball-milling-assisted exsolution method that allows for a more tunable morphology. Reactivity is compared with the catalyst prepared via the conventional impregnation technique. The as-synthesized catalysts are characterized by XRD, H₂-TPR, TEM, XPS, and APT. The NH₃ synthesis is carried out in a packed-bed tube reactor thermochemically. Using N₂ instead of Ar as the carrier gas during exsolution can favour reactivity by increasing active sites and perhaps improving metal-support interaction. The impregnated sample shows higher reactivity than the exsolved catalyst; however, the long-term durability is slightly better using the exsolved catalyst. Finally, APT results interestingly show that the exsolved catalyst is more resistant to hydride formation and hydrogen poisoning, which is one of the main deactivation mechanisms for such metallic catalysts.

Periodic oscillations in cell voltage and CO2 reduction product selectivity are commonly observed in laboratory studies using membrane electrode assembly (MEA) CO2 electrolyzers with anion-exchange membranes (AEM). These oscillations hinder mechanistic studies and progress toward industrial deployment of CO2 electrolyzers. To understand how to mitigate these oscillations in laboratory experiments, we investigated the effects of three electrolyzer design parameters on system stability: (i) membrane type and thickness, (ii) gas diffusion layer (GDL) properties, and (iii) cell compression, characterized as a normalized thickness difference (NTD) between the GDL and gasket and assembly torque. In our system, we found pairing a higher ionic conductivity AEM with a GDL with a cracked microporous layer (MPL) at a medium cell compression (NTD: 40%, torque: 2 Nm), led to the most stable CO2 to CO electrolysis with pseudo-steady-state CO Faradaic efficiency of around 85% achieved at 100 mA cm–2 for over 50 h. In contrast, lower conductivity AEMs showed earlier and more frequent oscillations related to water flooding, enhanced K+ crossover, and eventual short-circuiting. A lower compression assembly (NTD: 0%, torque: 2 Nm) exhibited a higher initial CO selectivity (>95% for 50 h) but developed more frequent oscillations associated with salt accumulation in the cathode flow channel. Our hypotheses are further supported by tests of an ”over-compressed” cell (NTD: 40%, torque: 4 Nm), which exhibited poor water drainage, rapid salt accumulation in the cathode gas channel, and an increased risk of electrode short-circuiting. These observations lead to practical AEM electrolyzer design rules: pairing high-conductive AEMs with crack-rich MPLs and optimizing cell compression to balance membrane-electrode interfacial contact and water drainage to maintain selective and stable CO2 electrolysis.