Crystallization design of MnO2 via acid towards better oxygen reduction activity

Manganese dioxide is one of the important electrode materials used for oxygen reduction reaction (ORR) in fuel-cell and metal-air batteries, and its electrochemical activity is greatly influenced by its crystalline structure. To elucidate the crystalline structure effect on the ORR, herein, we designed a facile method to obtain alpha-MnO2 and gamma-MnO2 via adjusting the acid under hydrothermal conditions using the same starting materials and reagents. The prepared alpha-MnO2 and gamma-MnO2 have a similar morphology composed of 3D microscopic spheres made up of numerous nanorods. X-ray photoelectron spectroscopy, temperature programmed desorption of oxygen characterization and conductivity measurements demonstrate that the alpha-MnO2 material has a relatively higher ratio of surface oxygen species, higher oxygen mobility and higher electrical conductivity. When compared with gamma-MnO2, alpha-MnO2 displays superior ORR activity with a higher electron transfer number and lower yield of peroxide species in alkaline solutions. The obtained results shed new light on the crystalline structure-dependent ORR activity of MnO2.