Surface Oxygen Coordination of Hydrogen Bond Chemistry for Aqueous Ammonium Ion Hybrid Supercapacitor

Abstract Aqueous ammonium ion hybrid supercapacitor (A‐HSC) combines the charge storage mechanisms of surface adsorption and bulk intercalation, making it a low‐cost, safe, and sustainable energy storage candidate. However, its development is hindered by the low capacity and unclear charge storage fundamentals. Here, the strategy of phosphate ion‐assisted surface functionalization is used to increase the ammonium ion storage capacity of an α‐MoO 3 electrode. Moreover, the understanding of charge storage mechanisms via structural characterization, electrochemical analysis, and theoretical calculation is advanced. It is shown that NH 4 + intercalation into layered α‐MoO 3 is not dominant in the A‐HSC system; rather, the charge storage mainly depends on the adsorption energy of surface “O” to NH 4 + . It is further revealed that the hydrogen bond chemistry of the coordination between “O” of surface phosphate ion and NH 4 + is the reason for the capacity increase of MoO 3 . This study not only advances the basic understanding of rechargeable aqueous A‐HSC but also demonstrates the promising future of surface engineering strategies for energy storage devices.