Hydrated Manganese-Doped Calcite as an Oxygen-Reduction Reaction Catalyst

Abstract

Electrolytic MnO2 (EMD) is a widely known cathode material for alkaline batteries and is being researched as an inexpensive catalyst for oxygen-reduction reactions (ORR) for application in fuel cells and Zn/Air batteries. In previous independent work, Mn-doped metal carbonates were explored as competitive alternatives to EMD. Hydrated Mn-doped CaCO3 (MDC) exhibited anomalous capacity and its cathode displayed air-breathing behavior, leading to the hypothesis that MDC, like EMD, may show electrocatalytic activity towards ORR. This report aimed to test this hypothesis, quantitatively evaluate the effectiveness of MDC as an ORR catalyst and compare its performance to EMD under similar conditions using rotating disk electrode (RDE) voltammetry and Koutecky-Levich analysis. For both EMD and MDC, the ORR was found to occur roughly around 0.0V v/s Hg/HgO and the number of electrons transferred was 3.60 for EMD and 1.58 for MDC. A 2+2 e− pathway and a 1+1 e− pathway involving superoxides were proposed for the ORR in EMD and MDC respectively. Since a direct 4 e− reduction pathway is most preferred for ORR in practical applications, EMD was found to be electrochemically superior to MDC as an ORR catalyst. This was, however, found to seemingly contradict the behavior observed in the beaker cells made with Ni foam cathodes coated with MDC and EMD. While the MDC cathode displayed air-breathing behavior and spontaneously wicked in alkaline electrolyte, EMD did not behave similarly. In order to better understand and reconcile these results, the structures of these materials and the preparation of cathode material using MDC or EMD with conductive additive (carbon black) were studied under SEM. It was concluded that using the current cathode formulation, the relative difference in particle size and shape between the EMD and the carbon black led to less surface contact and hence, a lower surface area being electrochemically active for EMD. MDC, on the other hand, was synthesized using acetylene black with similar particle size and form factor as the conductive additive and hence, exhibited air-breathing behavior more easily. Using this cathode formulation for MDC, a novel design for a Zn/Air battery was also attempted and will be the subject of future research. Moreover, while cycling MDC in water and on saturating it with air, the distinct smell of ammonia was detected leading to the hypothesis that this material, in addition to being an ORR catalyst, may also reduce nitrogen to ammonia. Research is currently underway to prove this hypothesis.