The development of aqueous ammonium-ion batteries (AAIBs) requires electrode materials that combine high NH4 + storage capacity with rapid and reversible ion transport. Herein, a metal-vacancy MXene/polyaniline (Mo4/3CTz/PANI) composite is reported, in which the pseudocapacitive response is synergistically activated by introducing 0.1 m H2SO4 into 1 m (NH4)2SO4 electrolyte. This proton-assisted modulation enables rapid and reversible NH4 +/H3O+ co-intercalation, in contrast to the negligible ion insertion observed in the absence of H2SO4. Combined experimental and density-functional theory (DFT) analyses reveal that proton doping significantly improves the electronic conductivity of PANI and induces a reversible Mo6+/Mo5+ redox transition during cycling, which dynamically modulates the NH4 + adsorption energy (from -4.155 to -4.567 eV), thus facilitating both intercalation and deintercalation of NH4 +. As a result, the composite achieves a high specific capacity of 245 mAh g-1 at 0.1 A g-1, with excellent capacity retention of 84.2% after 11,000 cycles at 1.0 A g-1. Furthermore, the MnO2/CNTs||M:P = 5:1 full cell delivers a high energy density of 81.6 Wh kg-1 and a power density of 16 000 W kg-1. This work highlights a promising strategy for advancing MXene-based electrodes via proton-enhanced ion storage mechanisms, paving the way for high-performance AAIBs.