Effect of Thermally Induced Oxygen Vacancy of α-MnO2 Nanorods toward Oxygen Reduction Reaction.

Abstract

MnO2 has been explored for various applications in environmental and energy aspects. However, the thermal sensitivity of the MnO2 crystal structure never been studied. As a potential cathode material for fuel cell, α-MnO2 has a higher specific activity than Pt/C based on per metals cost. In this work, the physical and electrochemical properties of α-MnO2 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-MnO2 materials. From EELS and X-ray photoelectron spectroscopy (XPS) analysis, the oxygen vacancies on the α-MnO2 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-MnO2 nanorods tended to become longer and thinner with increasing temperature. This work suggests that the α-MnO2 nanorods are thermal sensitive materials and their performance for ORR can be boosted under certain temperatures.