Abstract
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.
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