Surface-engineered MoO3 with oxygen-functionalized carbon for enhanced ammonium-ion storage

Aqueous ammonium-ion supercapacitors have attracted increasing attention due to their inherent safety, environmental friendliness, high energy density, and low cost. In this study, we propose a surface engineering strategy based on grafting terephthalic acid onto α-MoO 3 , followed by high-temperature carbonization to produce oxygen-functionalized, carbon-coated MoO 3 composites (MoO 3 @C). In a three-electrode system, MoO 3 @C achieves a high specific capacitance of 457.5 F g −1 at a current density of 1 A g −1 . When assembled into symmetric supercapacitor devices, the material delivers an energy density of 40.6 Wh kg −1 and a power density of 434.0 W kg −1 . These excellent electrochemical performances are attributed to the carbon coating, which introduces abundant active sites, while oxygen-containing functional groups improve surface wettability and enhance pseudocapacitance. Additionally, strong hydrogen-bonding interactions between NH 4 + ions and highly active oxygen atoms contribute to the improved ammonium-ion storage capacity and structural stability of MoO 3 @C. Density functional theory calculations further confirm that the carbon matrix enhances NH 4 + adsorption energy and improves the electrical conductivity of the composite. This work provides a robust surface modification approach to improve ammonium-ion storage performance in MoO 3 -based electrodes, offering valuable insights for the design of high-performance supercapacitor materials. Schematic illustration of the symmetric supercapacitor and it energy density. • Oxygen-functionalized, carbon-coated MoO 3 is fabricated via grafting and carbonization, enabling NH 4 + supercapacitors. • MoO 3 @C delivers a specific capacitance of 457.5 F g -1 (1 A g -1 ) and achieves 40.6 Wh kg -1 at 434.0 W kg -1 in devices. • The carbon coating provides active sites and forms NH 4 + -O hydrogen bonding, facilitating ion storage, as confirmed by DFT.