The crystal structures and electrochemical properties of α-, β-, γ-, and δ-MnO 2 , synthesized by a redox method under various conditions, were studied for the application of MnO 2 as a positive electrode in a fuel cell/battery (FCB) system. The effects of potassium ion concentration (0-10 M) and temperature (60-160 • C) on the morphology of synthesized MnO 2 were investigated by X-ray diffraction, scanning electron microscopy, and the Brunauer-Emmett-Teller method. In addition, the charge and discharge characteristics, and life cycle performance of MnO 2 as a positive electrode in an FCB system, were investigated by sweep voltammetry and potentiometry. The results indicate that four different crystal structures were obtained by different synthesis conditions: three tunnel structures (α-, β-, and γ-MnO 2 ) and one layered structure (δ-MnO 2 ). The effects of precipitation conditions were mapped and summarized in a phase diagram. Electrochemical testing showed that MnO 2 with small tunnel structures (i.e., βand γ-MnO 2 ) exhibit better life cycle performance than either large tunnel structure α-MnO 2 or layered δ-MnO 2 . Based on XRD analysis carried out after cycling, a schematic diagram is proposed to explain the degradation of the different MnO 2 compounds.

In this work we synthesized various types (α-, -, -, and δ-) of MnO 2 via redox method and investigated the effect of potassium ion content and synthesis temperature on the crystalloid structure formation. It was found that increasing the synthesis temperature and decreasing the potassium ion content leads to MnO 2 with small tunnel structures. It is thus assumed that the large potassium ions work as a template for bigger void spaces and that low temperatures can further increase the void space due to weaker Brownian movement of the molecules during synthesis. The samples were then analyzed with X-ray diffraction and scanning electron microscope and cycled in an alkaline electrolyte. From the results, it was found that MnO 2 with small tunnel structures ( and ) showed better stability than the large tunnel structures (α and δ) and thus an improved cycle life as secondary battery electrode.

The possibility of replacing λ-MnO2 with previously used γ-MnO2 in the positive electrodes of fuel cell/battery (FCB) systems was analyzed. Samples were evaluated in three different modes: In fuel cell mode, the oxygen reduction reaction rate was measured at an elevated pressure of 1.0 MPa. In battery mode, the electrodes were electrochemically cycled at different rates to assess their proton diffusion. Finally, in FCB mode, the discharged electrodes were chemically charged with oxygen for one hour to quantify the chemical charge rate. For all modes, X-ray diffraction analysis was conducted to assess the crystal stability of both species. λ-MnO2 was found to exhibit a considerably better catalytic capability for oxygen reduction reaction and could be chemically charged more quickly with oxygen than γ-MnO2. It is proposed that these results were caused by a lower charge transfer resistance in λ-MnO2. However, γ-MnO2 showed better proton diffusivity than λ-MnO2, mainly owing to its higher surface area. In terms of crystal stability, λ-MnO2 was found to be superior to γ-MnO2, thus making it a promising positive electrode material for FCB systems.

The fuel cell/battery (FCB) system combines the working principle of an alkaline battery with an alkaline fuel cell. As anode material, a metal hydride is chosen, which is also found in nickel-metal hydride batteries. This mischmetal can be charged both electrochemically and under hydrogen atmosphere. As a cathode material, manganese dioxide (MnO2) is utilized. We have shown previously that MnO2can, under the right conditions, be cycled electrochemically and recharged from its discharged form (MnOOH) via oxidation in an oxygen saturated alkaline solution. MnO2 can have different crystalloid structures, depending on the allocation of the MnO6 octahedra. In previous work, we have mostly focused on electrolytic manganese dioxide (EMD), also called γ-MnO2, in which the MnO6 octahedra are aligned to create a combination of 2x1 and 1x1 tunnel structures. In this work, however, we investigated the possibility of utilizing spinel MnO2, also known as λ-MnO2, as an alternative To obtain λ-MnO2, we first synthesized LiMn2O4 via a solid state reaction of EMD with LiOH. The result was a spinel structure with Li+ ions occupying the tetrahedral sites. The lithium ions can then be removed chemically in a diluted acid, thereby leaving the solid in a spinel structure. We then analyzed the performance of λ-MnO2 for the application in FCB and compared it with γ-MnO2. For this, we manufactured half-cells, cycled the samples electrochemically and recharged MnOOH in an oxygen-saturated alkaline solution in an autoclave. The cathodes were manufactured through a pasting method with a slurry consisting of  MnO2, carbon black (CB) and ethylene-vinyl acetate (EVA) in a weight ratio of 100:15:10 dissolved in xylene. Half-cells were assembled with nickel foam and Hg/HgO as counter and reference electrode, respectively, polypropylene as separator and a 6M KOH solution as electrolyte. Compared with γ-MnO2, λ-MnO2 shows a similar capacity during the first discharge, but loses a high capacity between the first and second discharge. Afterwards, capacity loss is considerably lower and at around the same rate as for γ-MnO2. However, in terms of rechargeability with oxygen, λ-MnO2 shows superior results. In an oxygen-saturated 6M KOH solution under 1MPa pressure, after 1 hour, the fully discharged λ-MnO2 recharged to 15% of its maximum theoretical capacity, compared to 10% of γ-MnO2. These are very promising results on our target of combining the advantages of fuel cells with batteries. Further research is thus being conducted, mainly focusing on improving rechargeability of λ-MnO2.

A novel MnO2/NiOOH fibrous positive electrode for Fuel Cell/Battery (FCB) systems has been proposed. FCB systems containing manganese dioxide and metal hydride (MH) as active materials for the positive and negative electrodes, respectively, have been found to be attractive energy storage and power generation systems because they can function as both fuel cells and secondary batteries with high energy and power density. Moreover, they can also be chemically charged by gaseous hydrogen and oxygen supply. However, the chemical charging rate of the positive electrode of the FCB systems is relatively low compared to the negative electrode. In our previous study, highly dissolved oxygen in an electrolyte was used in order to improve the chemical reaction rate of oxygen charging on a manganese dioxide electrode. The study showed that the charge rate will be 3.45 mAh/(g･min) if the oxygen is supplied with the limiting pressure. This result implies that the proton diffusion in the crystalline structure of the manganese dioxide or the dissociation of oxygen molecule on the manganese dioxide surface is the rate determining step in the oxygen charging. In the current study, therefore, we suggest two approaches for the fabrication of FCB positive electrodes to improve the oxygen charging rate: i) enlargement of active surface area by an electrodeposition method of active materials on carbon fibers, and ii) introduction of small amount of nickel oxyhydroxide to fibrous manganese dioxide electrodes in order to enhance the electrical conductivity of the positive electrode. Nickel oxyhydroxide is a well known positive electrode material of Ni-MH batteries and has higher electrical conductivity than manganese dioxide. Thus, the combination of nickel oxyhydroxide with manganese dioxide is expected to lead to a higher electrical conductivity of the FCB positive electrode. In the present paper, the characteristics of the proposed fibrous electrode will be studied by using galvanostatic measurement, electrochemical impedance spectroscopy, scanning electron microscopy analysis, and X-ray diffraction measurement.