Morphology and dispersion of nanostructured manganese-cobalt spinel on various carbon supports: the effect on the oxygen reduction reaction in alkaline media

In this work a model nanometric manganese-cobalt spinel was deposited on five selected carbon carriers (Vulcan XC-72, Printex85, MWCNT, mesoporous carbon CMK-1, and amorphous carbon C-am.) to examine their role in modifying the electrocatalytic properties of the supported active phase for the oxygen reduction reaction (ORR) in alkaline media. The synthesized materials were thoroughly characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy (RS). The results confirmed formation of a highly crystalline nanometric manganese-cobalt spinel and allowed for assessment of the amorphous phase content in the used carbon supports. The composition of the obtained catalysts was investigated by thermo-gravimetric analysis (TGA) and X-ray fluorescence (XRF) measurements. The electrocatalytic properties of the supported spinels were determined by the rotating disk electrode (RDE) and the rotating ring-disk electrode (RRDE) methods, and compared with a commercial platinum catalyst (20 wt.% Pt/Vulcan XC-72). TEM analysis revealed that the carbon support governs both the dispersion and morphology of the deposited spinel. In the case of mesoporous and amorphous carbon supports, the spinel nanocrystals exhibit the polyhedral shapes with almost equal abundance of the (111) and (100) facets. The shape of the spinel nanocrystals deposited on the Vulcan XC-72 and Printex85 supports is dominated by the (111) termination, whereas for the MWCNT support the (100) facet is the most abundant accompanied by the highest dispersion of the nanoparticles. The ORR measurements revealed that the amorphous phase fraction in the carbon support promotes 2e– reduction, leading to production of HO . This undesired pathway is inhibited by preferential exposition of the (100) facets. The superior performance of the MWCNT support in the 4e– reduction process results from three factors: lowest content of the amorphous component, best dispersion of the spinel active phase, and its ability to promote preferential (100) faceting of the nanocrystals.