The capacitive behaviours of MnO₂/carbon fiber composite electrode prepared in the presence of sodium tetraborate

The preparation and capacitive behaviour of boron-doped birnessite-type manganese dioxide is described. The MnO₂ has been deposited directly onto carbon fiber substrate via the in-situ redox reaction between KMnO₄ and carbon fiber in the presence of sodium tetraborate. A combination of physical techniques (scan electron microscopy, transmission electron microscope, X-ray diffraction and X-ray photoelectron spectra) and electrochemical methods (cyclic voltammetry, electrochemical impedance spectra and galvanostatic charge/discharge) is used to study the effects of the presence of sodium tetraborate on the deposition process, morphology, microstructure, and capacitive properties. The repulsions among "hunter" ion [B(OH)₄]^– resulting from the hydrolysis of Na₂B₄O₇ lead the nucleation points to grow outward individually and thus the formation of MnO₂ films that are covered with large amounts of isolated nanoparticles about 50–100 nm in diameter. The increasing binding energies of manganese and oxygen should be a result of interstitial boron ions in MnO₂ lattice. The electrochemical measurements indicate that the boron-doped MnO₂ electrode has larger specific capacitance, better rate capacity as well as superior cycle stability. - Highlights: • Boron doping by using sodium tetraborate as dopant. • Boron-doped birnessite-type MnO₂/carbon fiber composite electrode. • Enhanced capacitive properties through nonmetal element doping.