Mo-doped MnO2@CC electrode for high-performance 2.4 V aqueous asymmetric supercapacitors

Despite many advantages of supercapacitors based on aqueous electrolytes, the inherently low operating voltage severely limits their energy density and practical applicability to many applications. Thus, it is a key scientific challenge to broaden the potential window of electrode materials for supercapacitors. Here we report our findings in tailoring the Fermi level of MnO2-based electrode by doping with a high-valence Mo6+ ion, thus modulating its overpotential in aqueous electrolytes and potential window. For example, the redox potential of Mo-doped MnO2 is effectively increased, resulting in a wider potential window. When grown on a carbon support, the specific capacitance of Mo-doped MnO2@CC with a mass loading of 15.1 mg cm−2 reaches 3.22 F cm−2 in a wide potential window of 1.2 V. The enhanced performance is attributed to the additional redox behavior induced by the Mo dopant, as confirmed by in situ Raman and ex-situ XPS analyses. When coupled with a C-Ti3C2Tx/WO3@CC negative electrode to construct an ASC with a total mass loading of 35.8 mg cm−2, the ASC exhibits a wide voltage window of 2.4 V, demonstrating a remarkable energy density of 0.89 mWh cm−2 at a power density of 2.56 mW cm−2. In addition, the device is operational in a wide operating temperature range of −10 to 50 °C, and a voltage of ∼ 58% is retained after self-discharge for 48 h. This work provides some new insights into the development of aqueous ASC devices of commercial-grade mass loadings with high output voltage, high energy density, and slow self-discharge rates.