Mo-Doped MnO2 for Enhanced H+/NH4+ Co-Intercalation in Aqueous Ammonium-Ion Hybrid Supercapacitors

Abstract

MnO₂-based electrode materials have attracted extensive research interest in aqueous ammonium-ion hybrid supercapacitors (A-HSCs) due to their low cost, environmental benignity, and high theoretical capacitance. However, their practical capacitance remains inferior to that of lithium-ion battery electrodes, largely due to the limitations of a pure NH₄⁺ storage mechanism. In this work, Mo⁶⁺-doped MnO₂ (MMN-2) was synthesized via a facile hydrothermal method. Experimental results demonstrate that Mo⁶⁺ doping promotes a H⁺/NH₄⁺ co-intercalation/deintercalation mechanism, substantially boosting the specific capacitance of MnO₂. Moreover, Mo⁶⁺ doping optimizes the microstructure and pore size distribution of MnO₂, improving electrolyte ion transport and structural stability during cycling. The MMN-2 electrode delivers a high specific capacitance of 586.2 F g^-1 at 2 mA cm^-2 and exhibits remarkable cycling stability, retaining 91.9% of its capacitance after 17,200 cycles. When assembled with activated carbon cloth (ACC) into an MMN-2//ACC A-HSC device, it achieves a maximum energy density of 0.787 mWh cm^-2 at a power density of 0.5 mW cm^-2. This work highlights Mo doping as an effective strategy to enable H⁺/NH₄⁺ co-intercalation in MnO₂, providing valuable guidance for designing high-performance ammonium-ion storage materials.