Electrochemical performance of Bi2O3 supercapacitors improved by surface vacancy defects

Abstract

Vacancy defects have important impacts on the physical-chemical properties of metal oxides. In the present work, two types of Bi2O3 powders with different morphologies were synthesized for supercapacitor electrodes by calcination (denoted as c-Bi2O3) and two-step hydrothermal methods (donated as h-Bi2O3). Vacancy clusters of VBi‴VO··VBi‴ with different concentrations were observed in these two samples by positron annihilation lifetime spectroscopy. The results of electrochemistry revealed that the charge storage behavior of h-Bi2O3 with more VBi‴VO··VBi‴ defects obeyed both semi-infinite diffusion-controlled battery-type mechanism and surface-controlled pseudo-capacitance mechanism, while that of the calcined c-Bi2O3 electrode with a lower concentration of VBi‴VO··VBi‴ defects followed only the semi-infinite diffusion-controlled battery-type mechanism. The pseudo-capacitance of h-Bi2O3 could be attributed to the insertion/extraction process of more K+ ions in the VBi‴VO··VBi‴ surface defects. Due to the partial pseudo-capacitance and improved conductivity caused by more VBi‴VO··VBi‴ defects, the h-Bi2O3 electrode had a larger capacitance (1043 F g-1 at 1 A g-1), a higher rate performance (560 F g-1 at 60 A g-1), and better cycle stability (93% retention at 50 A g-1 after 2000 cycles). Furthermore, because of the high-concentration of VBi‴VO··VBi‴ defects, the Ni/Co-MOF//h-Bi2O3 asymmetric supercapacitor delivered a relatively higher specific energy density of 47 Wh kg-1 at 1125 W kg-1. Taken together, these results indicate that surface vacancy clusters play an important role in boosting the electrochemical performance of metal oxide electrodes.