The influence of ethylene glycol-based solvents on the morphological and supercapacitive properties of hydrothermalized δ-Bi2O3 with additional Bi2O2CO3

A Bi2O3 is a promising material for a supercapacitor negative electrode. A typical surfactant for synthesizing Bi2O3 via a hydrothermal method is ethylene glycol (EG); however, polyethylene glycol (PEG) has not been employed. In this study, three different glycol-based surfactants including EG, PEG400, and PEG600 were compared based on their effects on the phase, morphology, and supercapacitive properties of hydrothermal Bi2O3 (160 °C, 3 h). Along with δ-Bi2O3, Bi2O2CO3 also formed. The as-synthesized δ-Bi2O3/Bi2O2CO3 (BiEG, BiPEG400, and BiPEG600) had different morphological features but these structures were altered after grinding. Increasing the molecular weight of the surfactant reduced the carbonate phase, providing better connectivity between the electrode material and the current collector as well as lowering a charge transfer resistance. As a result, the BiPEG600 electrode exhibited the best electrochemical properties in terms of capacitance and retention. This also highlighted the importance of charge transfer and morphology, which overshadowed the Brunauer–Emmett–Teller (BET) surface area. Specifically, the BiPEG600 capacitance was 969 F g−1 at 1 A g−1, and was retained at 46% after 2000 cycles at 50 A g−1. Promising specific capacitances  obtained at high discharge rates of 50 and 60 A g−1 were 436 and 381 F g−1, respectively.