Hydrothermal MnO2: synthesis, structure, morphology and discharge performance

Digestion of Mn2O3 in a range of H2SO4 solutions (0.01-0 M), at a variety of temperatures (20-140 °C) has led to the formation of a series of kinetically stable manganese dioxide samples via a dissolution-precipitation mechanism that involves disproportionation of a soluble Mn(III) intermediate. The resultant manganese dioxide samples are characterized in terms of their domain of phase stability, chemical composition, structure, morphology and electrochemical performance. γ-MnO2 predominates at all but high H2SO4 concentrations (>5 M), where α-MnO2 is formed, and high temperatures (>80 °C) where β-MnO2 is formed. The structural variety of γ-MnO2 in this domain of stability is interpreted in terms of: (i) the fraction of De Wolff defects (Pr), which is found to increase as the H2SO4 concentration is decreased and the temperature is increased; (ii) microtwinning (Tw), which despite being less statistically significant, is found to follow a similar trend; (iii) the cation vacancy fraction; (iv) the Mn(III) fraction. Both the latter structural properties decrease as the temperature is increased; but decreasing the H2SO4 concentration leads to a decrease in cation vacancy fraction and an increase in Mn(III) fraction. These structural characteristics, in particular the De Wolff defects, are interpreted on a molecular level in terms of soluble Mn(III) intermediate condensation, in which the electrolyte conditions determine the relative proportions of equatorial-axial edge sharing (ramsdellite domains only), and equatorial-axial corner sharing (both ramsdellite and pyrolusite domains) that occurs. Morphological differentiation is easily established due to the different characteristics of each phase. γ-MnO2 exists as fine needles (250 nm × 50 nm), β-MnO2 is formed as much larger columns (1 μm × 100 nm), while α-MnO2 is present as small spheres of up to 400 nm in diameter. Electrochemical characterization by voltammetry in an aqueous 9 M KOH electrolyte demonstrates that the performance of γ-MnO2 samples is comparable with that of commercial EMD, whereas α- and β-MnO2 suffer from diffusional limitations which lower their operating voltage. For γ-MnO2, superior performance results when lower temperatures and H2SO4 concentrations are used. This corresponds to intermediate levels of De Wolff defects and microtwinning, and to a cation vacancy fraction minimum.