Electrochemical behaviour of natural and synthetic ramsdellite

Abstract

Five forms of MnO₂, two stoichiometric [β-MnO₂ and ramsdellite from New Mexico (NM)] and three synthetic ones [Sedema WSA, IBA 11 and a new synthetic (S) ramsdellite], were characterized by XRD, FTIR and slow voltammetry. NM-ramsdellite is found to be a mixture of ramsdellite, pyrolusite and groutellite. The new S-ramsdellite has the lowest fraction of pyrolusite defects (de Wolff disorder) compared with other synthetic samples. Slow potentiostatic reduction in 1 mol^–1 KOH shows that NM-ramsdellite and β-MnO₂ are reduced at a lower potential and have a lower electrochemical activity than synthetic, defect compounds, while S-ramsdellite is reduced at an intermediate potential. The 3400 cm^–1 OH bending mode associated with Mn³⁺ is sharp and well resolved in groutellite; it extends progressively until 1200 cm^–1 with increasing stoichiometric defects. Structural and chemical disorders appear to be necessary for electrochemical activity by (i) simultaneously increasing the Fermi level energy in the oxides, (ii) levelling the insertion sites energy, and (iii) increasing the mixed valence state instability, which improves the H⁺e^–insertion/reduction kinetics.