Superior high-voltage aqueous carbon/carbon supercapacitors operating with in situ electrodeposited polyvinyl alcohol borate gel polymer electrolytes
Electrodeposited polyvinyl alcohol borate (PVAB) aqueous gel polymer electrolytes (GPEs) have been found to possess excellent high-voltage stability and high ionic conductivity which are promising in building aqueous supercapacitors with high operating voltage and good electrochemical performance. In this study, PVAB GPEs were formed directly on activated carbon electrodes by in situ electrodeposition to serve as both electrolytes and separators for high-voltage aqueous carbon/carbon supercapacitors. The morphology and structure of the prepared PVAB GPE layers were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. The electrochemical performance of supercapacitors using PVAB GPEs was tested and compared with those of supercapacitors using neutral salt aqueous electrolytes and organic electrolytes. The results show that the PVA molecules in PVAB GPEs are in the amorphous state and crosslinked by O → B− coordination bonds. The supercapacitors using PVAB GPEs can operate stably at 2 V, and no drastic electrolysis is observed in these supercapacitors even at 4 V. Among all the PVABs, PVA potassium borate (PVAPB) GPEs possess the best ionic conductivity. The high energy densities of 12.47 and 7.14 W h kg−1 can be achieved for the supercapacitor using the PVAPB GPE at the current densities of 0.2 and 2 A g−1, respectively, which are better than those of supercapacitors using 1 M Li2SO4 aqueous electrolyte and 1 M LiPF6 organic electrolyte especially at high current densities. The reversible ionization of water molecules cooperating with the reversible formation of O → B− coordination bonds is considered to play a critical role in the ionic transportation of PVAB GPEs.