Regulating Fe-spin state by atomically dispersed Mn-N in Fe-N-C catalysts with high oxygen reduction activity

Abstract

As low-cost electrocatalysts for oxygen reduction reaction applied to fuel cells and metal-air batteries, atomic-dispersed transition metal-nitrogen-carbon materials are emerging, but the genuine mechanism thereof is still arguable. Herein, by rational design and synthesis of dual-metal atomically dispersed Fe,Mn/N-C catalyst as model object, we unravel that the O2 reduction preferentially takes place on FeIII in the FeN4 /C system with intermediate spin state which possesses one eg electron (t2g4eg1) readily penetrating the antibonding π-orbital of oxygen. Both magnetic measurements and theoretical calculation reveal that the adjacent atomically dispersed Mn-N moieties can effectively activate the FeIII sites by both spin-state transition and electronic modulation, rendering the excellent ORR performances of Fe,Mn/N-C in both alkaline and acidic media (halfwave positionals are 0.928 V in 0.1 M KOH, and 0.804 V in 0.1 M HClO4), and good durability, which outperforms and has almost the same activity of commercial Pt/C, respectively. In addition, it presents a superior power density of 160.8 mW cm−2 and long-term durability in reversible zinc–air batteries. The work brings new insight into the oxygen reduction reaction process on the metal-nitrogen-carbon active sites, undoubtedly leading the exploration towards high effective low-cost non-precious catalysts.

Engineering local coordination environments and site densities for high‐performance Fe‐N‐C oxygen reduction reaction electrocatalysis

“…Furthermore, both metal sites can serve as active centers during ORR, leading to new reaction pathways with lower reaction barriers. Various dual metal sites have been reported with enhanced ORR performance, including Fe‐Co, 59–63 Fe‐Mn, 64,65 and Fe‐Fe 66 …”

“…As a representative example, consider Fe‐Co dual‐atom sites (FeCoN 5 ) as a new type of active site with optimized structure for ORR 65 5A,B) demonstrated that Fe 2+ site was the dominant iron state in the FeCoN 5 catalyst up to 0.8 V, whereas almost negligible Fe 2+ was found for the FeN x catalyst at 0.4 V. This indicates that the FeCoN 5 catalyst had a higher Fe 3+ /Fe 2+ redox potential than FeN 4 sites during ORR.…”

“…These advantages resulted in FeCoN 5 being a much more active catalyst site than FeN 4 sites, displaying an E onset of only 1.02 V and an E 1/2 of 0.86 V (vs. RHE) in 0.1 M HClO 4 (Figure 5D). Zhang et al 65 reported that the neighboring MnN 4 sites can activate the spin state of Fe III sites, that is, changing from the low spin state (t 2g 5e g 0) to intermediate spin state (t 2g 4 e g 1). As evidenced by the magnetic susceptibility results (Figure 5E,F), the effective magnetic moment for Fe,Mn/N‐C and Fe/N‐C was 3.75 µ eff and 2.16 µ eff , thus corresponding to the number of unpaired d electron (n) of Fe III ion to be 1 and 0, respectively.…”

“…Magnetic susceptibility data for (E) Fe,Mn/N‐C and (F) Fe/N‐C. Reproduced with permission: Copyright 2021, Springer Nature 65 …”

An Adjacent Atomic Platinum Site Enables Single‐Atom Iron with High Oxygen Reduction Reaction Performance

“…Meanwhile, considering the much lower competitiveness of Pt‐N 4 to snatch O 2 than Fe‐N 4 active centers, [3e] O 2 molecules bonded with Pt‐N 4 center was not considered in the calculated reaction pathways. Based on these reasons, Fe‐N 4 is demonstrated as the only active center, while Pt‐N 4 is considered as the modulator (not active center) to tune the electronic state of Fe‐N 4 for the reaction pathway, which is completely different from the previously reported “synergetic effect” on the binary metal active centers, for example, Co‐Pt‐NC, [18] Fe‐Pt‐NC, [19] Fe‐Mn‐NC, [20] Fe‐Ni‐NC [21] and Cu‐N 4 /Zn‐N 4 , [22] Fe‐N 4 /Mn‐N 4 , [10b] Fe‐N 4 /Co‐N 4 [23] and Fe‐N 4 /Ni‐N 4 [2a, 11] et al. The theoretical η ORR of each catalyst can be determined by examining the reaction Gibbs free energies of the different mechanistic steps for Fe‐N 4 and Fe‐N 4 /Pt‐N 4 .…”