Oxide–carbon balance in binary Bi2O3/MWCNT composite electrodes: A current-dependent crossover between low-current capacitance and high-rate retention

Bismuth oxide is a promising pseudocapacitive material, but its low electronic conductivity limits its rate capability in supercapacitor electrodes. This study investigates how the oxide–carbon balance influences the structural characteristics and electrochemical behavior of binary Bi 2 O 3 /MWCNT composites. Composites with Bi 2 O 3 mass ratios of 100:20, 100:50, and 100:100 were prepared by integrating sol–gel-derived Bi 2 O 3 with MWCNTs through hydrothermal treatment followed by calcination. Structural and electrochemical properties were evaluated using X-ray diffraction, scanning electron microscopy, Raman spectroscopy, cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. Increasing the MWCNT content transformed the composite from an oxide-rich morphology into a more carbon-dominated network, shifted the Bi 2 O 3 rod-diameter distribution toward smaller values relative to the 100:20 composition, and reduced the crystallite size from 20.660 to 13.852 nm. Electrochemical measurements were performed in a three-electrode configuration using 1 M H 2 SO 4 over 0–1.0 V versus Ag/AgCl. The results revealed a current-dependent crossover rather than a universally optimal composition. The 100:100 electrode delivered the highest specific capacitance of 678.09 F g −1 at 1 A g −1 but retained only 37.89% at 10 A g −1 . In contrast, the 100:50 electrode maintained 288.83 F g −1 at 10 A g −1 , corresponding to 63.09% retention. These findings demonstrate that the carbon loading that maximizes low-current charge storage differs from that which best preserves accessible charge at high current density. Therefore, 100:100 is preferable for maximum low-current capacitance, whereas 100:50 provides the best high-rate-oriented compromise among the investigated compositions.