Factors influencing the structure of electrochemically prepared α-MnO2 and γ-MnO2 phases

The α- and γ-phases of MnO2 prepared by electrolysis of MnSO4 and MxSO4 (where M = Li+, Na+, K+, Rb+, Cs+ or Mg2+) in aqueous solutions at various pH and voltage Ev values under ambient conditions have been systematically studied. The structures of powdery MnO2 produced are found to depend on the radius of the Mz+ counter cation in addition to the pH and Ev conditions. In order to achieve the α-phase for MnO2 formation under neutral pH condition, the radius of counter cation must be equal to or greater than 1.41 Å, the size of the K+ cation. The relative concentration ratio of [MnO4−]transient/[Mn2+], which is related to the pH–Ev conditions, also affects the structure of MnO2 produced with counter ions smaller than K+. For samples prepared in acidified solution with the counter ions of Li+, Na+ or Mg2+ at 2.2 V, the electrolysis products display the γ-MnO2 phase while those prepared at 2.8 V electrolysis produce a mixture of γ-MnO2 and α-MnO2 phases. Single phase of α-MnO2 is identified in the 5 V electrolysis products. Furthermore, the valence state of manganese was found to decrease as the applied voltage was reduced from 5.0 to 2.2 V. This implies that the lower [MnO4−]transient/[Mn2+] ratio or the less oxidative condition is responsible for the non-stoichiometric MnO2 structure with oxygen deficiency.