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

• Boron doping by using sodium tetraborate as dopant.
• Boron-doped birnessite-type MnO2/carbon fiber composite electrode.
• Enhanced capacitive properties through nonmetal element doping.

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.