Catalytic mechanism of oxygen reduction on two types of CoN ₄ ‐graphene : A density functional study

Abstract

Co-coordinated nitrogen-doped carbons have been reported as highly active catalysts to oxygen reduction reaction (ORR) in fuel cell. Two CoN 4 -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 CoN 4 -G models possessed ORR activity. However, CoN 4 -G(A) catalyst showed higher catalytic activity but the CoN 4 -G(B) showed higher 4-electrons selectivity. On CoN 4 -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 H 2 O 2(ads) was 0.61 eV which was easy to overcome. Then the 2-electrons pathway was also possible. On CoN 4 -G(B), only 4-electrons ORR pathway was favorable. The most favorable reaction channel was arranged as: O 2 ! OOH ! 2OH ! OH + H 2 O ! 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.

Atomically dispersed Fe/Co–N–C and their composites for proton exchange membrane fuel cells

“…109,110 Many calculated works about different ORR mechanisms on various types of active sites have been reported. [111][112][113][114] 117 They supposed that the conventional scaling relation, 2DG(OH*) = DG(O*), is suitable for nearcontinuous active sites but not perfectly applicable to singleatom sites. A different scaling relation, DG(2OH*) = DG(O*) + 1.5 eV, in which two OH* are confined to the same atom, was considered (Fig.…”

“…109,110 Many calculated works about different ORR mechanisms on various types of active sites have been reported. 111–114 Zhou et al constructed a Fe–N–C SAC model and disclosed a self-adjusting mechanism induced by its intrinsic intermediate. 115 Partial current density analysis reveals that the associative path dominates over the dissociative path on Fe–N–C SACs.…”

Carbon-based single-atom catalysts: impacts of atomic coordination on the oxygen reduction reaction

“…Although Co-N-C has a variety of coordination modes, CoN 4 is the most commonly used model. 143,144 Zhang et al 145 investigated in depth the CoN 4 -graphene catalysts with pyridine-N and pyrrole-N coordination by DFT calculations (Fig. 10a and b).…”

Axial ligand engineering for highly efficient oxygen reduction catalysts in transition metal–N4 doped graphene

“…38 Two CoN 4graphene structures can be obtained by Co atom embedding into pyridinic-N and pyrrolic-N doped graphenes, and they both have good ORR activity with different mechanisms. 39 MnN 4graphene also shows good ORR activity and the direct dissociation of the oxygen molecule is the most favorable pathway, 40 different from FeN 4 -graphene 38 and CoN 4 -graphene 39 in which the hydrogenation of oxygen molecule is more favorable.…”

“…37 Theoretically, many efforts have been made to investigate transition metals and nitrogen co-doped graphene. 29,[38][39][40] The FeN 4 doped graphene exhibits good kinetic properties, which has energy barrier smaller than pure Pt and the ORR follows a four-electron process. 38 Two CoN 4graphene structures can be obtained by Co atom embedding into pyridinic-N and pyrrolic-N doped graphenes, and they both have good ORR activity with different mechanisms.…”