Mechanisms of oxygen reduction reaction on B doped FeN 4 G and FeN 4 CNT catalysts for proton‐exchange membrane fuel cells

Density functional theory (DFT) was used to calculate the stability, oxygen reduction reaction (ORR) mechanism and activity of B-doped FeN4CNT (carbon nano-tube [CNT]) and FeN4G (G, graphene). The B-doped catalysts are more stable and active than that of the un-doped, especially for FeN4B2G and FeN4B2CNT. Based on the Mulliken charge and electrostatic potential surface of these catalysts, Fe atom is found to be the most active site for the adsorption of O-contained species. It is shown that their adsorption energies decrease in the range: O > OH > Co-ad OH > OOH > O2 > H2O > H2O2 on these catalysts. H2O2 will be directly dissociated into two co-adsorbed OH* or O* + H2O* instead of H2O2 on the graphene series catalysts, and the process of reaction (H2O2 + * → 2OH*) on the active sites of the CNT series catalysts is strongly exothermic. Hence, desorption of H2O2* into the solution is difficult to proceed during the oxygen reduction process. All the catalysts are expected to promote a single site four electron process through the reaction path of I (O2 → O2* → OOH* → O* → OH* → H2O) except for the catalyst of FeN4CNT. The rate-determining step (RDS) for ORR process on FeN4B2G is the first reduction step (O2* → OOH*), while the RDS is the fourth reduction step (OH* → H2O) for the other catalysts. FeN4B2G exhibits the largest on-set potentials of 0.53 V, which is larger than the on-set potential of un-doped B FeN4G catalyst (0.39 V). In addition, the B-doped FeN4CNT catalyst shows the better activity compared to the un-doped ones.