A study of manganese dioxide-hydrogen insertion compounds produced by different chemical insertion methods

Alday, Javier (1999) A study of manganese dioxide-hydrogen insertion compounds produced by different chemical insertion methods. Masters thesis, Middlesex University. [Thesis]

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Abstract

The reduction of an electrolytic manganese dioxide (EMD) by two different
chemical insertion methods (propan-2-o1 and hydrazinc .hydrate) has been studied by
XRD (X-Ray Diffraction), FTIR (Fourier Transfonn infrared) and Electrode Potential
Me:lsurements.
The level of H insertion may be represented by MnOOH" w'here r goes fi-om :::: 0,1
in the starting material (due to non-stoichiometry) up to 1.0 in the most reduced material.
H inseniol1 into EMD led to approximate isotropic lattice expansion up to r values close
to 0.7 - O.S for the propan-2-01 reduction method. This observation was consistent with a
homogeneous solid state reduction with formation of a solid solution in which II:'" and eare
mobile. In the region r = 0,7-0,8 to LO, new XRD non-moving lines emerged. while
the original lines continued to move, indicating anisotropic lattice expansion. This is due
to the appe:lrance of microdomains of the end product within the solid solution, implying
that H- and e' were no longer mobile in the crystal structure bur located in position, which
has been supported by FTIR measurements looking carefully at wavenumber regions
where 0-H vibration occurred.
For the hydrazine hydrate reduced samples, the O-H bond formation takes place at
a much earlier stage of reduction. Examination of the XRD patterns indicated
heterogeneous solid state reduction had occurred after r = 0,4. Heterogeneous reduction
was presumed to have occurred by H location in the outside layers of the particles.
A study of the potential of the compounds obtained under both reduction methods
has been carried out. together with the study of the stability of the H inserted compounds
in KOH electrolyte. Potential measurements confilmed development of a heterogeneous
potential coinciding to the appearance of new peaks on X-ray diffi:action and the
f0l111ation of O-H bonds as shown in FTIR. This behaviour appears at Cl. much earlier level
of reduction for the hydrazine hydrate reduced samples than for the propan-2-01 reduced
samples. The results confilm previous findings that H inserted compounds are unstable at
a !evel related to the fOlmation of micro domains in KOH at concentrations similar to
those used in alkaline manganese batteries, which limits the capacity of those batteries.

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