Pseudo-capacitive behavior of multi-walled carbon nanotubes decorated with nickel and manganese (hydr)oxides nanoparticles

We report on the synthesis, characterization of nickel hydroxide and manganese oxide nanoparticles decorating multiwalled carbon nanotubes composite material, and electrochemical performance for energy storage pro-cesses in symmetric electrochemical capacitors. A large pseudocapacitive voltage range of 1.7 V is presented using a coin cell containing a 1.0 M Li2SO4 aqueous solution, resulting in a maximum specific capacitance of 420 F g(-1) verified at 1.67 A g(-1) during the discharge process. The symmetric coin cell was highly stable with a very high coulombic efficiency of 99% even after 70,000 cycles of charge-discharge. The high electrochemical stability of coin cell was attributed to a synergism between the nanostructured carbon support and metal (hydr) oxides nanoparticles, i.e., the transport of electrons and ions across the porous electrode structure enables with a high degree of reversibility for the solid-state redox transitions reactions, which is the main contribution to the whole electrode specific capacitance. Near-surface structural changes of the electrodes were monitored during dynamic polarization by Raman and XRD synchrotron measurements. The presence of the active Ni-O stretching mode was verified during the charge-discharge processes according to the reversible solid-state redox process. The Raman shifts were also correlated with the reversible intercalation-deintercalation processes of Li+-ions into the MnOx host material, as well as reversible adsorption of solvated ions on the surface defects of carbon nanostructures.