Hydroxide ion dependent α-MnO2 enhanced via oxygen vacancies as the negative electrode for high-performance supercapacitors

Abstract

Manganese dioxide with low-cost and high theoretical capacity plays an essential role in the development of high-performance supercapacitors. However, most of the research on the application of pure MnO₂ in supercapacitors is mainly focused on positive electrode materials. In this work, we aim at studying the applicability and energy storage mechanism of MnO₂ as a negative electrode material for supercapacitors, and compared three different crystalline MnO₂ (δ-, β-, and α-MnO₂). Additionally, the electrochemical performance of α-MnO₂ was further improved by introducing oxygen vacancies generated at high temperature. Electrochemical studies show that M-300 (α-MnO₂ heat-treated at 300 °C) electrode materials have a high specific capacitance of 736.3 F g⁻¹ at 1 A g⁻¹, and exhibit remarkable cycling stability. Impressively, hydroxide ion dependence experiments and research of the electron transfer mechanism during charge and discharge indicate that the charge storage process of MnO₂ as a negative electrode is realized by the participation of OH⁻ and the mutual conversion of Mn(II), Mn(III) and Mn(IV), absolutely different from the MnO₂ positive electrode. We also theoretically quantified the contribution of the diffusion-controlled process and surface capacitance effects to investigate its energy storage mechanism. The assembled M-300//H-NiCo₂O₄ asymmetric supercapacitor exhibits excellent energy density (34.9 W h kg⁻¹) and cycling stability (80.6% after 10 000 cycles). This work provides a promising negative electrode material for supercapacitor device fabrication, and helps to theoretically understand the energy storage process of negative electrode materials under alkaline conditions.