Electrochimica Acta, Vol.85, 399-410, 2012
Carbon catalyst codoped with boron and nitrogen for oxygen reduction reaction in acid media
A carbon matrix codoped with boron and nitrogen atoms was synthesized for use as an oxygen reduction reaction (ORR) catalyst in acid media. The sol gel route developed to produce CO2 adsorbents was modified for the synthesis of the present catalysts using boric acid, urea, activated carbon, and iron acetate precursors. The effects of doped boron and nitrogen atoms as well as iron species on ORR activity and selectivity were investigated by analyzing X-ray photoelectron spectra, field emission-transmission electron microscope images, X-ray diffraction patterns, Brunauer-Emmett-Teller surface areas, cyclic voltammograms, rotating disk electrode voltammograms, and rotating ring-disk electrode voltammograms. Both boron and nitrogen atoms were simultaneously doped into a carbon matrix by two heat-treatment steps, the first under N-2 gas at 1173 K and the second under NH3 gas at 1223 K with the remaining boric acid-derived oxygen species that limited ORR activity. The remaining oxygen atoms were primarily bonded to the boron atoms and removed by adding iron acetate in the precursor dispersion, resulting in increased ORR activity. The iron species also played a role in increasing edge sites in the carbon matrix, whereas boron doping increased the amount of pyridinic nitrogen. The ORR activity was maximized by the simultaneous doping of boron and nitrogen atoms into the carbon matrix in the presence of iron species. The optimum mass fraction of iron for both ORR activity and selectivity was 0.014, and the activity and selectivity levels were retained after the acid leaching step and reheat treatment under Ar gas. Based on the experimental results, the edge sites in the carbon matrix containing pyridinic-nitrogen atoms near the boron atoms and/or the boron atoms were suggested to be responsible for the ORR activity. (C) 2012 Elsevier Ltd. All rights reserved.