Effect of Thermally Induced Oxygen Vacancy of α-MnO Nanorods toward Oxygen Reduction Reaction.

MnO has been explored for various applications in environmental and energy aspects. However, the thermal sensitivity of the MnO crystal structure never been studied. As a potential cathode material for fuel cell, α-MnO has a higher specific activity than Pt/C based on per metals cost. In this work, the physical and electrochemical properties of α-MnO nanorods were explored for the first time under thermal treatment with different temperatures (300, 400, and 500 °C). Under thermal treatment, oxygen vacancies were induced. The high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) have been taken to explore oxygen vacancies of α-MnO materials. From EELS and X-ray photoelectron spectroscopy (XPS) analysis, the oxygen vacancies on the α-MnO nanorods were strengthened with the temperature increasing. The sample with 400 °C treatment exhibited the best performance toward ORR, excellent methanol tolerance and higher stability compared to commercial Pt/C in alkaline media due to its combination of preferable growth on (211) plane and moderate oxygen vacancies as well as coexistence of Mn (IV)/ Mn (III) species. It was also observed the α-MnO nanorods tended to become longer and thinner with increasing temperature. This work suggests that the α-MnO nanorods are thermal sensitive materials and their performance for ORR can be boosted under certain temperatures.