In situ characterization of the growth of CNx carbon nano-structures as oxygen reduction reaction catalysts

Abstract

The growth process of nitrogen-doped carbon nano-structures (CNx) was characterized using in situ (XANES, EXAFS, XRD) and ex situ (XPS, TEM) techniques. CNx nano-structures were grown on two different Co-doped substrates: Vulcan carbon and MgO. CNx formation was achieved by pyrolyzing a C- and N-containing compound, CH3CN, at high temperatures. The Co phase was seen to go through different transformations during the pyrolysis process, depending on the growth substrate used. The Co species, which started in an acetate matrix prior to pyrolysis, became partially reduced with heating and with CH3CN treatment. CNx fibers that formed were washed with acid. XRD, XAFS, and XPS analyses showed the Co phase left behind to be primarily metallic, regardless of the growth substrate used. TEM imaging showed CNx to be in the form of stacked-cup nano-structures, with metallic cobalt particles visibly encased in carbon. The structure of CNx obtained as well as the nitrogen content was significantly different on the two substrates, which led to activity differences as shown by RRDE.