Interfacial built-in electric field engineering in MnO2/MXene heterostructures for high-performance aqueous ammonium-ion hybrid supercapacitors

Aqueous ammonium-ion hybrid supercapacitors (AAHSCs) have attracted increasing interest owing to their environmental friendliness, low cost, and fast charge/discharge capability. However, the practical application of MXene electrodes is limited by their intrinsic self-stacking behavior, which hinders ion transport and electrochemical utilization. Herein, MnO2 nanosheets were in situ grown on MXene to construct a MnO2/MXene heterostructure with an interfacial built-in electric field (BIEF). The heterointerface effectively suppresses MXene restacking while providing abundant electrochemically active sites for NH4+ storage. Meanwhile, the BIEF induced by the Fermi-level (EF) difference between MnO2 and MXene accelerates charge transfer and facilitates ion diffusion. Benefiting from the synergistic effects of pseudocapacitive Mn2+/Mn4+ redox reactions, hydrogen-bond interactions between NH4+ and oxygen-containing groups, and BIEF-assisted charge transport, the MnO2/MXene electrode delivers a high specific capacitance of 725.5 F g−1 at 1 A g−1. The assembled AAHSC device achieves an energy density of 99.7 Wh kg−1 at 900.7 W kg−1, with 75.9% capacitance retention after 10 000 cycles. This work provides an effective interfacial engineering strategy for developing high-performance ammonium-ion energy storage systems.