Cobalt-polypyrrole-carbon black (Co-PPY-CB) electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells: Composition and kinetic activity

Electrocatalysts consisting of polypyrrole (PPY) and Co deposited on carbon black (CB) at several compositions were prepared and tested for the oxygen reduction reaction (ORR) in a HClO4 buffer (pH =1) using a rotating ring-disk electrode (RRDE). It was determined that the most favorable catalyst composition (prior to calcination) had a CB:PPY weight ratio of 2 and a pyrrole:Co (i.e., PY:Co) molar ratio of 4. This catalyst had an onset potential of 0.785 V (vs. RHE) and a mass activity of ca. 1 A/g(cata) at the fuel cell relevant voltage of 0.65 V. Furthermore, it was found that the number of electrons exchanged during the ORR with the catalyst was ca. 3.5 and resulted in 28% yield of H2O2 at 0.65 V. which hints to an indirect 4e(-) reduction of O-2 to H2O, with H2O2 as an intermediate. From energy dispersive spectroscopy (EDS) and extended X-ray absorption fine structure (EXAFS) analysis, it is proposed that a PY:Co ratio of 4 favors the formation, prior to calcination, in the catalyst precursor of Co-N complexes in which Co is coordinated to 3 or 4 N atoms, resulting in strong Co-N interactions that limit the formation upon calcination of low ORR activity Co nanoparticles. These Co-N complexes give rise upon calcination to CoNx-2 sites in which the coordination of Co could favor the adsorption on them of O-2, which would make those sites particularly active and selective. At the same mass activity of 1 A/gcata, the voltage yielded by the catalyst was 200 mV lower than that for a state-of-the-art Pt (10 wt.%) catalyst. whose H2O2 output at 0.85 V was 39% and involves the exchange of 3.2e(-), overall making our material an attractive substitute to noble metal ORR electrocatalysts. (C) 2011 Elsevier B.V. All rights reserved.