Improving the cycling stability of biochar electrodes by purification via ion exchange

Abstract

Excellent long-term cycling stability and safety are the basic qualities of electrode materials in electrochemical energy storage equipment. Nevertheless, the inferior cycling stability and device safety caused by impurity ions constrain their practical application. Thus, it is essential to purify biomass-based carbon materials when they are used as electrodes in the field of energy storage. Herein, four purification methods are studied, including water-washing, acid-pickling, ion exchange and ultrasonic wave, respectively. Particularly, trace elements are detected by inductive coupled plasma emission spectrometer and ion chromatography to explore the impurity removal effects. The results suggest that ion exchange possesses the best impurity-cleaning effect among the four methods. In addition, the supercapacitor employing the biochar electrode material with minimum impurities exhibits excellent ultra-long cycling stability. In symmetric supercapacitors, the capacitance retentions are 86.7% after 120000 cycles using 6 M KOH electrolyte and 70% after 30000 cycles using 1 M MeEt3NBF4/PC electrolyte at 4 A/g. Furthermore, we reveal the influence mechanism of metal ion impurities on cycling stability and impedance. In summary, the elaborate study provides a feasible method to solve the problem of impurity removal and designs suitable biomass carbon materials for commercial application in the energy storage field.