Catalytic mechanism of oxygen reduction on two types of CoN4-graphene: A density functional study

摘要

Co-coordinated nitrogen-doped carbons have been reported as highly active catalysts to oxygen reduction reaction (ORR) in fuel cell. Two CoN4-graphene catalysts with a hybrid structure of single Co atom embedded into pyridinic-N and pyrrolic-N doped graphenes, respectively, were studied. The detailed thermodynamic and kinetic behavior of the ORR on these two models was performed by DFT calculation. The results showed that both the two CoN4-G models possessed ORR activity. However, CoN4-G(A) catalyst showed higher catalytic activity but the CoN4-G(B) showed higher 4-electrons selectivity. On CoN4-G(A), both 2-electrons and 4-electrons pathways were feasible. Along the 4-electrons pathway, the OH reduction step was the thermodynamic and kinetic rate-determining step with the limiting potential of 0.43 V and a barrier of 0.42 eV. The formation barrier of H2O2(ads) was 0.61 eV which was easy to overcome. Then the 2-electrons pathway was also possible. On CoN4-G(B), only 4-electrons ORR pathway was favorable. The most favorable reaction channel was arranged as: O-2 -> OOH -> 2OH -> OH + H2O -> 2H(2)O. The limiting potential of 4-electrons pathway was 0.35 V. The second step was the kinetic rate-determining step with the barrier of 0.86 eV.