Self-sacrificial template method to MnO2 microspheres as highly efficient electrocatalyst for oxygen evolution reaction

epsilon-MnO2 microspheres have been synthesized using a self-sacrificial template method by annealing MnCO3 microspheres. The transformation process from MnCO3 to MnOx at four different annealing temperatures has been investigated. XRD data show that MnOx samples obtained at 250, 350, 450, and 550 A degrees C are corresponding to MnCO3, the mixture of MnCO3 and epsilon-MnO2, pure epsilon-MnO2 and pure Mn3O4, respectively. SEM images show that the spherical morphology of epsilon-MnO2 microspheres is kept with the diameter about 4 mu m. The electrocatalytic measurements of different MnOx samples for oxygen evolution reaction (OER) have been systematically studied. The results show that epsilon-MnO2 microspheres exhibit excellent OER activity, with an overpotential of 0.42 V (at j = 10 mA cm(-2)) and a low Tafel slope of 75.7 mV dec(-1), which is much better than the other MnOx samples. The enhanced activity may be attributed to the crystal phase and the spherical morphology of epsilon-MnO2.