Improvement of O₂ adsorption for α-MnO₂ as an oxygen reduction catalyst by Zr⁴⁺ doping

Zr⁴⁺ doped α-MnO₂ nanowires were successfully synthesized by a hydrothermal method. XRD, SEM, TEM and XPS analyses indicated that Mn³⁺ ions, Mn⁴⁺ ions, Mn⁴⁺ ions and Zr⁴⁺ ions co-existed in the crystal structure of synthesized Zr⁴⁺ doped α-MnO₂ nanowires. Zr⁴⁺ ions occupied the positions originally belonging to elemental manganese in the crystal structure and resulted in a mutual action between Zr⁴⁺ ions and Mn³⁺ ions. The mutual action made Mn³⁺ ions tend to lose their electrons and Zr⁴⁺ ions tend to get electrons. Cathodic polarization analyses showed that the electrocatalytic activity of α-MnO₂ for oxygen reduction reaction (ORR) was remarkably improved by Zr⁴⁺ doping and the Zr/Mn molar ratio notably affected the ORR performance of the air electrodes prepared by Zr⁴⁺ doped α-MnO₂ nanowires. The highest ORR current density of the air electrodes prepared by Zr⁴⁺ doped α-MnO₂ nanowires in alkaline solution appeared at Zr/Mn molar ratio of 1 : 110, which was 23% higher than those prepared by α-MnO₂ nanowires. EIS analyses indicated that the adsorption process of O₂ molecules on the surface of the air electrodes prepared by Zr⁴⁺ doped α-MnO₂ nanowires was the rate-controlling step for ORR. The DFT calculations revealed that the mutual action between Zr⁴⁺ and Mn³⁺ in Zr⁴⁺ doped α-MnO₂ nanowires enhanced the adsorption process of O₂ molecules.