Fundamentals and mechanistic insights for ammonium-ion energy storage: spotlight on metal-organic frameworks

The development of efficient and sustainable energy storage technologies necessitates a deep understanding of charge storage mechanisms and rational materials design. Ammonium-ion (NH₄⁺) hybrid supercapacitors have recently emerged as promising candidates owing to the unique physicochemical characteristics of NH₄⁺, including its small ionic radius, hydrogen-bonding capability, and high ionic mobility, which enable rapid ion transport and reversible intercalation. Metal-organic Frameworks (MOFs), with their highly tunable porosity, diverse metal-ligand coordination environments, and structural adaptability, provide an exceptional platform for designing advanced electrode materials for NH₄⁺ storage. This review highlights the fundamental aspects of ammonium-ion storage mechanisms, including diffusion dynamics, lattice flexibility, and hydrogen-bond-mediated interactions within MOF architectures. Emphasis is placed on the structural and electronic design principles that govern charge storage behavior in pristine MOFs and their functional derivatives. We further discuss recent progress in tailoring framework dimensionality, electronic conductivity, and redox-active site engineering to enhance electrochemical performance. Finally, the key challenges and future opportunities for developing high-capacity, durable, and flexible MOF-based materials for ammonium-ion energy storage are outlined, providing a roadmap toward their integration into next-generation energy devices.