Crystallographic Structure and Morphology Transformation of MnO₂Nanorods as Efficient Electrocatalysts for Oxygen Evolution Reaction

MnO 2 nanostructures with different crystallographic structure and morphologies have been synthesized through a facile template-free hydrothermal method. The growth mechanisms of these MnO 2 nanostructures have been discussed. At 140°C, slim solid α-MnO 2 nanorods are obtained. At 160°C, the crystal phase begins to transform from α-MnO 2 to β-MnO 2 . And at 180°C, uniformly hollow β-MnO 2 nanorods are formed. The electrocatalytic properties for oxygen evolution reaction (OER) of these as-prepared MnO 2 samples have been systematically studied in alkaline media. It is obvious that the structure-function relationship of MnO 2 samples for OER activity is dependent on the crystallographic structure and morphology, following the order α-MnO 2 > α-MnO 2 /β-MnO 2 > broken β-MnO 2 > β-MnO 2 . The best OER electrocatalyst is α-MnO 2 , with an overpotential of 0.45 V (at j = 10 mA cm −2 ). The enhanced OER activity is owing to not only the highest BET surface area (45.6 m 2 g −1 ), but also the largest electrochemical active surface area (C dl = 3225.0 μF cm −2 ). In addition, the intrinsic activity of each Mn atom in α-MnO 2 (TOF = 0.0056 s −1 at η = 0.45 V) is larger than that of β-MnO 2 (TOF = 0.0003 s −1 at η = 0.45 V).