Impedance measurements on some doped MnO₂ electrodes in H₂SO₄ electrolyte

Abstract

A.c. impedance behaviour of βMnO₂ and doped β-MnO₂ electrodes in H₂SO₄ medium was assessed with a view to explaining the mechanism of the discharge behaviour of MnO₂ electrodes in 4m H₂SO₄ electrolyte. The electrodes used in this work appear to be intermediate cases of planar and porous electrodes as the angles, θ, made by the low frequency part with the real axis are found to be in the range (30–60°). The Nyquist plots and the Randle plots tend to reinforce the observation made by Tye that the capacity yield is essentially diffusion controlled. The depression and flattening of semi-circles observed reveals a link with the heterogeneity of the planar electrode and with the porosity of the pitted electrode. The deviation from a 45° angle made by the low frequency part with the real axis may either be explained by the roughness of the electrode surface or the shallow pores on the surface of the electrodes; in other words due to the difference between the apparent and true surface areas. The double layer capacitance values of the electrodes seem to subsume adsorption capacitances and diffusion factors. Hence, the relative increase in magnitude. The electrodes appear to behave like planar electrodes when 10 µF is introduced into the circuit as a parallel capacitance since angles 0 vary between 40–58°. The undoped β-MnO₂ electrode, as well as those prepared from Li-MnO₂, Ag-MnO₂, and I.C.8, appear to be planar electrodes.