First Principles study of H-insertion in MnO2

We present an extensive First Principles study on proton intercalation in the pyrolusite and ramsdellite forms of MnO2. It is shown that protons are always covalently bonded to an oxygen atom in MnO2 In ramsdellite, the proton prefers the pyramidal oxygen to the planar coordinated oxygen as that site is farther away from the Mn cations. In both pyrolusite and manganite, the octahedral sites are unstable, but the two local minima on each side of the octahedron are connected by a barrier of only about 25 meV, so that protons may rapidly exchange between these sites. Proton diffusion in pyrolusite occurs by hopping along the 1 X 1 open tunnels with an activation barrier that increases from about 575 meV at the beginning of discharge to about 1 eV at high H concentration. Diffusion in ramsdellite takes place along the 2 X 1 open tunnels and occurs with much lower activation energy (respectively, 200 and 400 meV, at low and high H concentrations). Introduction of twinning defects has a large adverse effect on the proton diffusivity. Results indicate that direct H-H interactions are not that significant compared to oxygen-mediated-interactions. Experimental and calculated ramsdellite discharge curves deviate significantly at early stages of the reduction process. The calculations on defected structures indicate that a significant source of this discrepancy may be due to presence of proton-compensated Mn vacancies in real MnO2, which create local sites with higher discharge potential. The calculations suggest that the ordered phase, observed in experiments at mid-reduction (groutellite, MnOOH0.5), is due to the lattice remaining coherent during intercalation. (C) 2002 Elsevier Science (USA).