Morphology and Electrochemical Properties of α-, β-, γ-, and δ-MnO₂Synthesized by Redox Method

The crystal structures and electrochemical properties of α-, β-, γ-, and δ-MnO2, synthesized by a redox method under various conditions, were studied for the application of MnO2 as a positive electrode in a fuel cell/battery (FCB) system. The effects of potassium ion concentration (0–10 M) and temperature (60–160°C) on the morphology of synthesized MnO2 were investigated by X-ray diffraction, scanning electron microscopy, and the Brunauer-Emmett-Teller method. In addition, the charge and discharge characteristics, and life cycle performance of MnO2 as a positive electrode in an FCB system, were investigated by sweep voltammetry and potentiometry. The results indicate that four different crystal structures were obtained by different synthesis conditions: three tunnel structures (α-, β-, and γ-MnO2) and one layered structure (δ-MnO2). The effects of precipitation conditions were mapped and summarized in a phase diagram. Electrochemical testing showed that MnO2 with small tunnel structures (i.e., β- and γ-MnO2) exhibit better life cycle performance than either large tunnel structure α-MnO2 or layered δ-MnO2. Based on XRD analysis carried out after cycling, a schematic diagram is proposed to explain the degradation of the different MnO2 compounds.