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

摘要

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 (delta-, beta-, and alpha-MnO2). Additionally, the electrochemical performance of alpha-MnO2 was further improved by introducing oxygen vacancies generated at high temperature. Electrochemical studies show that M-300 (alpha-MnO2 heat-treated at 300 degrees 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.