ELECTROCHEMICAL-BEHAVIOR OF NATURAL AND SYNTHETIC RAMSDELLITE

Five forms of MnO2, two stoichiometric [beta-MnO2 and ramsdellite from New Mexico (NM)] and three synthetic ones [Sedema WSA, IBA 11 and a n ew synthetic (S) ramsdellite], were characterized by XRD, FTIR and slo w voltammetry. NM-ramsdellite is found to be a mixture of ramsdellite, pyrolusite and groutellite. The new S-ramsdellite has the lowest frac tion of pyrolusite defects (de Wolff disorder) compared with other syn thetic samples. Slow potentiostatic reduction in 1 mol l(-1) KOH shows that NM-ramsdellite and beta-MnO2 are reduced at a lower potential an d have a lower electrochemical activity than synthetic, defect compoun ds, while S-ramsdellite is reduced at an intermediate potential. The 3 400 cm(-1) OH bending mode associated with Mn3+ is sharp and well reso lved in groutellite; it extends progressively until 1200 cm(-1) with i ncreasing stoichiometric defects. Structural and chemical disorders ap pear to be necessary for electrochemical activity by (i) simultaneousl y increasing the Fermi level energy in the oxides, (ii) levelling the insertion sites energy, and (iii) increasing the mixed valence state i nstability, which improves the H(+)e(-) insertion/reduction kinetics.