Effect of nickel-ion doping in MnO₂ nanoneedles as electrocatalyst for the oxygen reduction reaction

In this paper, nickel (Ni)-doped MnO₂ nanoneedles were synthesized via a facile hydrothermal method. The effects of nickel doping on structure, morphology and chemical composition of MnO₂ nanoneedles were investigated by X-ray diffraction (XRD) spectroscopy, transmission electron microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The XRD patterns proved that nickel doping facilitated the formation of α-MnO₂. The role of nickel doping in MnO₂ nanoneedles for the oxygen reduction reaction in alkaline media was investigated. The electrocatalytic activities of the nanoneedles were tested by cyclic voltammograms, electrochemical impedance spectroscopy and rotating disk electrode measurements. The electrochemical measurements were operated for the varying amount of Ni doping (Ni atomic ratio of 2.22, 2.76 and 2.90 %) in O₂-saturated 0.1 M KOH aqueous solution. The 2.22 % Ni doping MnO₂ exhibited higher current density, more positive half-wave potential, lower charge transfer resistance and faster reaction kinetic. Further analysis reveals that the excellent electrocatalytic activity of 2.22 % Ni doping MnO₂ is attributed to the increment of Mn(III) as electrochemical active sites.