Improvement of O(2) adsorption for α-MnO(2) as an oxygen reduction catalyst by Zr(4+) doping

Zr(4+) doped α-MnO(2) nanowires were successfully synthesized by a hydrothermal method. XRD, SEM, TEM and XPS analyses indicated that Mn(3+) ions, Mn(4+) ions, Mn(4+δ) ions and Zr(4+) ions co-existed in the crystal structure of synthesized Zr(4+) doped α-MnO(2) nanowires. Zr(4+) ions occupied the positions originally belonging to elemental manganese in the crystal structure and resulted in a mutual action between Zr(4+) ions and Mn(3+) ions. The mutual action made Mn(3+) ions tend to lose their electrons and Zr(4+) ions tend to get electrons. Cathodic polarization analyses showed that the electrocatalytic activity of α-MnO(2) for oxygen reduction reaction (ORR) was remarkably improved by Zr(4+) doping and the Zr/Mn molar ratio notably affected the ORR performance of the air electrodes prepared by Zr(4+) doped α-MnO(2) nanowires. The highest ORR current density of the air electrodes prepared by Zr(4+) doped α-MnO(2) nanowires in alkaline solution appeared at Zr/Mn molar ratio of 1 : 110, which was 23% higher than those prepared by α-MnO(2) nanowires. EIS analyses indicated that the adsorption process of O(2) molecules on the surface of the air electrodes prepared by Zr(4+) doped α-MnO(2) nanowires was the rate-controlling step for ORR. The DFT calculations revealed that the mutual action between Zr(4+) and Mn(3+) in Zr(4+) doped α-MnO(2) nanowires enhanced the adsorption process of O(2) molecules.