Aqueous Ammonium Ion Hybrid Supercapacitor: A Diffusion Dynamics Perspective

Aqueous ammonium-ion hybrid supercapacitors (A-HSCs) have emerged as a promising energy storage technology, offering intrinsic safety, low cost, environmental sustainability. However, the realization of these advantages hinges critically on the diffusion dynamics of the ammonium ion (NH₄ ⁺), which remain incompletely understood and require systematic elucidation of the underlying mechanisms and optimization strategies. This review provides a comprehensive overview of recent advances in NH₄ ⁺ diffusion kinetics within A-HSCs, approached from three interconnected perspectives: electrode materials, electrolyte engineering, and charge carrier structure. We systematically summarize the kinetic characteristics and structural design strategies for four representative classes of electrode materials-layered compounds, tunnel structures, Prussian blue analogs (PBAs), and organic frameworks-that govern NH₄ ⁺ transport. In parallel, we elucidate how concentrated electrolytes, additive electrolytes, and high-entropy electrolytes (HEEs) reconfigure the hydrogen-bond network of water to modulate the solvation environment. Furthermore, we explore the unique diffusion behavior arising from the intrinsic tetrahedral geometry of NH₄ ⁺ and the influence of hydrogen isotope substitution, which provides a powerful experimental lever for disentangling the coupled contributions of mass, rotational inertia, and hydrogen-bond strength to the diffusion dynamics. Finally, we identify current limitations and challenges, and outline future directions for the development of high-performance aqueous ammonium-ion energy storage devices.