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 (NH 4 + ), 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 NH 4 + 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 NH 4 + 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 NH 4 + 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.