Manganese dioxide electrocatalysts for borohydride fuel cell cathodes?

Electrocatalysts of manganese dioxide (MnO2) supported on activated carbon (AC) were prepared by low-temperature (ltMnO2/AC) and high-temperature (htMnO2/AC) procedures. Structure, morphology and composition of the synthesised materials were investigated by X-ray diffraction analysis, transmission electron microscopy and thermogravimetric analysis. Activity of the two prepared electrocatalysts for the oxygen reduction reaction (ORR) was investigated in alkaline media and ORR kinetic parameters evaluated from rotating disc electrode measurements. It was found that the ORR proceeds as two-electron process at ltMnO2/AC-based electrode, while it proceeds as four-electron process at htMnO2/AC-based electrode. The two electrocatalysts were further investigated for application as cathode materials in direct borohydride fuel cells. For that purposes the ORR at MnO2/AC was investigated in the presence of borohydride in alkaline solution and in the temperature range from 25 to 65 °C. It could be observed that the activity of ltMnO2/AC for the ORR is significantly suppressed in the borohydride-containing solution whereas htMnO2/AC still showed high activity for the ORR.

► Synthesis of composites of MnO2 with activated carbon via two different methods.
► “Low-temperature” composite reduces O2 in alkaline media through two-electron process.
► “High-temperature” composite reduces O2 in alkaline media through four-electron process.
► “High-temperature” composite highly active for O2 reduction even in presence of borohydride.
► Elevated temperatures significantly promote O2 reduction.