Transition Metal-Based Materials for Electrochemical and Photoelectrochemical Carbon-Free Nitrogen Cycling as H-Carrier

Ammonia, as a carbon-free nitrogen-based hydrogen carrier, has attracted significant interest in addressing the approaching energy model innovation in light of its high hydrogen content, low cost, ease of storage, and transport. However, the additional energy consumption and environmental pollution caused by the traditional Haber–Bosch ammonia production and thermal ammonia catalytic cracking process enforce the exploration of clean and renewable ammonia cycling approaches. Electrochemical (EC) and photoelectrochemical (PEC) ammonia synthesis and oxidation for hydrogen generation have shown great potential for achieving an eco-friendly and sustainable green hydrogen economy. Exploring low-cost, highly active, and stable catalysts is pivotal for both EC and PEC systems to achieve efficient ammonia conversion properties. Transition metal-based catalysts (TMCs) have shown significant potential in EC and PEC catalytic systems because of their high catalytic activity, low cost, and excellent stability. We summarize the recent advanced progress of TMCs applied to EC and PEC ammonia synthesis and decomposition to hydrogen generation. Moreover, we discuss the challenges and perspectives on exploring transition metal-based materials in EC and PEC ammonia conversion. This review offers guidance for developing carbon-free nitrogen cycling as a hydrogen carrier.