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