A study around the improvement of electrochemical activity of MnO2 as cathodic material in alkaline batteries

An optimized combination of reduction by methane and sulfuric acid digestion was developed to improve the electrochemical activity of manganese dioxide at a battery set. Chemical manganese dioxide, CMD, and electrolytic manganese dioxide, EMD, which have been destroyed after discharge cycling process in potential window of 900–1650 mV versus Hg/HgO, were reduced in a furnace with a flow of methane at 300 and 250 °C correspondingly. Thereafter, the reduced samples, CMDr and EMDr, were digested in a solution of sulfuric acid with optimized concentration and temperature. It was found that digested samples, CMDro and EMDro, typically show more stability in cycling, higher capacity and more reversible redox reaction. Alternatively, we reported about the effect of digestion temperature on electrochemical and structural properties of the samples. Digestion at temperatures 60 and 98 °C in 1.5 M sulfuric acid as superior concentration was preferred after comparative experiments in the range 40–98 °C. The samples which were digested in 60 °C (CMDro1 and EMDro1) showed superior electrochemical activity at the early stages of discharge cycling. By contrast, the samples which were obtained at 98 °C (CMDro2 and EMDro2) showed more stability and were superior to the former samples in final stages of discharge cycling process. Afterward, the electrochemical behavior of the pretreated samples was investigated by means of cyclic voltammetry technique and discharge cumulative capacity profiles. Also X-ray diffraction was employed to verify the responses of voltammetric methods. In XRD patterns, peak at 2θ = 28.6° which is due to β-MnO2 type was the strongest signal as temperature 98 °C was selected for digestion. After digestion at 60 °C, the characteristic peaks at 2θ = 38° and 42° were amplified which are attributed to formation of γ-MnO2. Interestingly enough, the results according to the XRD patterns were in good agreement with the electrochemical approaches.