Spinel MnO₂ As a Rapidly Oxidizing Cathode Material for Fuel Cell/Battery Systems

Abstract

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.

λ-MnO2Positive Electrode for Fuel Cell/Battery Systems

“…We conducted preliminary tests with λ-MnO 2 and found that λ-MnO 2 has a slightly higher chemical charge rate than γ-MnO 2 , despite a lower surface area. 17,18 As summarized in Table I, its capabilities in terms of cyclability and capacity are similar to γ-MnO 2 and more promising than other structures. Thus, in this paper, we analyzed the possibility of using λ-MnO 2 in a FCB system by comparing its performance in terms of ORR rate (fuel cell mode), redox cycling (battery mode), and chemical charging (FCB mode) with that of γ-MnO 2 .…”