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

Abstract

Density functional theory (DFT) was used to calculate the stability, oxygen reduction reaction (ORR) mechanism and activity of B-doped FeN 4 CNT (carbon nano-tube [CNT]) and FeN 4 G (G, graphene). The B-doped catalysts are more stable and active than that of the un-doped, especially for FeN 4 B2 G and FeN 4 B2 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 > O 2 > H 2 O > H 2 O 2 on these catalysts. H 2 O 2 will be directly dissociated into two co-adsorbed OH* or O* + H 2 O* instead of H 2 O 2 on the graphene series catalysts, and the process of reaction (H 2 O 2 + * ! 2OH*) on the active sites of the CNT series catalysts is strongly exothermic. Hence, desorption of H 2 O 2 * 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 (O 2 ! O 2 * ! OOH* ! O* ! OH* ! H 2 O) except for the catalyst of FeN 4 CNT. The rate-determining step (RDS) for ORR process on FeN 4 B2 G is the first reduction step (O 2 * ! OOH*), while the RDS is the fourth reduction step (OH* ! H 2 O) for the other catalysts. FeN 4 B2 G exhibits the largest onset potentials of 0.53 V, which is larger than the on-set potential of un-doped B FeN 4 G catalyst (0.39 V). In addition, the B-doped FeN 4 CNT catalyst shows the better activity compared to the un-doped ones.

Theoretical Study on B‐doped FeN4 Catalyst for Potential‐Dependent Oxygen Reduction Reaction

“…To make clear the ORR activity of FeN 4 sites, we further conduct the free energy calculations as shown in Figure 4. It should be noted that, the calculated free energy diagram of FeN 4 in agreement with the free energy diagrams that has been reported in the literatures [34,35] . Under CPS of 1.23 V, the hydrogenation and resolution of OH* become accessible, and the step corresponding to the maximum free‐energy changes (ΔGmax) is the O 2 adsorption for III‐B, IV‐B, and V‐B or O 2 hydrogenation for I‐B and II‐B, respectively.…”

“…It should be noted that, the calculated free energy diagram of FeN 4 in agreement with the free energy diagrams that has been reported in the literatures. [34,35] Under CPS of 1.23 V, the hydrogenation and resolution of OH* become accessible, and the step corresponding to the maximum free- An example of the most stable adsorption configurations of the ORR intermediates on the I-B candidate is shown in Figure 5 and the detailed free energies are collected in Table 1 and Table 2. In CPS of 1.23 V, the maximum freeenergy changes (ΔG max ) of FeN 4 sites is 0.55 eV, and most candidates of B-doped FeN 4 /C exhibit superior ORR catalytic activity (ΔG max below 0.55 eV) compared to FeN 4 /C, except for II-B which has a similar ORR activity (0.62 eV) to undoped FeN 4 sites.…”

Coordination environment engineering of single-atom catalysts for the oxygen reduction reaction

“…Similar to P atoms, B atom doping is also a feasible strategy to adjust the electronic structures of M-N-C SACs. [126][127][128][129] Recently, Sun et al reported a B-doped Fe-N 4 catalyst with Fe single atoms anchoring on B, N-doped carbon nanotubes (Fe SA /B,N-CNT, see Fig. 5a-c).…”

Realizing a high OER activity in new single-atom catalysts formed by introducing TMNx (x = 3 and 4) units into carbon nanotubes using high-throughput calculations

“…40,41 It is worth mentioning that the CNT-based SAC systems have been used as effective electrocatalysts for both the HER [42][43][44] and ORR. [45][46][47][48] However, relevant research is quite scarce, especially on the OER electrocatalytic activity of SACs based on CNTs.…”