Regulating the Coordination Geometry and Oxidation State of Single-Atom Fe Sites for Enhanced Oxygen Reduction Electrocatalysis.

FeNC catalysts demonstrate remarkable activity and stability for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells and Zn-air batteries (ZABs). The local coordination of Fe single atoms in FeNC catalysts strongly impacts ORR activity. Herein, FeNC catalysts containing Fe single atoms sites with FeN3 , FeN4 , and FeN5 coordinations are synthesized by carbonization of Fe-rich polypyrrole precursors. The FeN5 sites possess a higher Fe oxidation state (+2.62) than the FeN3 (+2.23) and FeN4 (+2.47) sites, and higher ORR activity. Density functional theory calculations verify that the FeN5 coordination optimizes the adsorption and desorption of ORR intermediates, dramatically lowering the energy barrier for OH- desorption in the rate-limiting ORR step. A primary ZAB constructed using the FeNC catalyst with FeN5 sites demonstrates state-of-the-art performance (an open circuit potential of 1.629 V, power density of 159 mW cm-2 ). Results confirm an intimate structure-activity relationship between Fe coordination, Fe oxidation state, and ORR activity in FeNC catalysts.

Double-Confinement Construction of Atomically-Dispersed-Fe Bifunctional Oxygen Electrocatalyst for High-Performance Zinc-Air Battery.

Simultaneously achieving high activity for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is the key to constructing rechargeable Zn-air batteries (ZABs). Here the complexation of 1,10-phenanthroline and the spatial confinement effect of closo-[B12 H12 ]2- are used to solidify metal-boron-cluster-organic-polymers on the surface of SiO2 microspheres to construct a bifunctional oxygen electrocatalyst (FeBCN/NHCS). Driven by FeBCN/NHCS, the half-wave-potential of ORR surpasses that of the Pt/C catalyst, reaching 0.893 V versus RHE, and the overpotential (η10 ) of OER is as low as 361 mV. The ZABs of FeBCN/NHCS as an air cathode not only have high power density and specific capacity, but also have charge-discharge durability. The FeBCN/NHCS is not only related to the high specific surface area, but also the high exposure rate of single-atom Fe and the doping of heteroatom B. This study provides an efficient oxygen electrocatalyst and also contributes wisdom to the acquisition of highly active oxygen electrocatalyst.

Atomically Dispersed Iron Regulating Electronic Structure of Iron Atom Clusters for Electrocatalytic H2 O2 Production and Biomass Upgrading.

The integration of highly active single atoms (SAs) and atom clusters (ACs) into an electrocatalyst is critically important for high-efficiency two-electron oxygen reduction reaction (2e- ORR) to hydrogen peroxide (H2 O2 ). Here we report a tandem impregnation-pyrolysis-etching strategy to fabricate the oxygen-coordinated Fe SAs and ACs anchored on bacterial cellulose-derived carbon (BCC) (FeSAs/ACs-BCC). As the electrocatalyst, FeSAs/ACs-BCC exhibits superior electrocatalytic activity and selectivity toward 2e- ORR, affording an onset potential of 0.78 V (vs. RHE) and a high H2 O2 selectivity of 96.5 % in 0.1 M KOH. In a flow cell reactor, the FeSAs/ACs-BCC also achieves high-efficiency H2 O2 production with a yield rate of 12.51±0.18 mol gcat -1 h-1 and a faradaic efficiency of 89.4 %±1.3 % at 150 mA cm-2 . Additionally, the feasibility of coupling the produced H2 O2 and electro-Fenton process for the valorization of ethylene glycol was explored in detail. The theoretical calculations uncover that the oxygen-coordinated Fe SAs effectively regulate the electronic structure of Fe ACs which are the 2e- ORR active sites, resulting in the optimal binding strength of *OOH intermediate for high-efficiency H2 O2 production.

Bimetallic iron complex constructed clusters and single atoms neighboring structure to enhance oxygen reduction reaction performance.

Electrocatalysts are useful in lowering the energy barrier in oxygen reduction reaction (ORR). In this study, a catalyst with neighboring Fe single-atom and cluster is created by adsorbing a bimetallic Fe complex onto N-doped carbon and then pyrolyzing it. The resulting catalyst has good performance and a half-wave potential of 0.89 V. When used in Zn-air batteries, the voltage drops by only 8.13 % after 145 h of cycling. Theoretical studies show that electrons transfer from neighboring clusters to single atoms and the catalyst has a lower d-band center. These reduce intermediate desorption energy, hence improving ORR performance. This work demonstrates the capacity to adjust the catalytic properties through the interaction of diverse metal structures, which helps to design more efficient catalysts.

Iron single atoms anchored on ultrathin carbon nitride photocatalyst for visible light-driven water decontamination.

Graphitic carbon nitride has gained considerable attention as a visible-light photocatalyst. However, its photocatalytic efficiency is restricted by its limited capacity for absorbing visible light and swift recombination of charge carriers. To overcome this bottleneck, we fabricated an atomic Fe-dispersed ultrathin carbon nitride (Fe-UTCN) photocatalyst via one-step thermal polymerization. Fe-UTCN showed high efficiency in the photodegradation of acetaminophen (APAP), achieving > 90 % elimination within 60-min visible light irradiation. The anchoring of Fe atoms improved the photocatalytic activity of UTCN by narrowing the bandgap from 2.50 eV to 2.33 eV and suppressing radiative recombination. Calculations by density functional theory revealed that the Fe-N4 sites (adsorption energy of - 3.10 eV) were preferred over the UTCN sites (adsorption energy of - 0.18 eV) for the adsorption of oxygen and the subsequent formation of O2•-, the dominant reactive species in the degradation of APAP. Notably, the Fe-UTCN catalyst exhibited good stability after five successive runs and was applicable to complex water matrices. Therefore, Fe-UTCN, a noble-metal-free photocatalyst, is a promising candidate for visible light-driven water decontamination.

N and S dual-coordinated Fe single-atoms in hierarchically porous hollow nanocarbon for efficient oxygen reduction.

Fe-, and N-co-doped carbon (FeNC) electrocatalysts are promising alternatives to Pt-based catalysts for oxygen reduction reaction (ORR); however, simultaneously enhancing their intrinsic activity and exposure of Fe active sites remains challenging. Herein, we report S-modified Fe single-atom catalysts (SACs) anchored on N,S-co-doped hollow porous nanocarbon (Fe/NS-C) for ORR. The unique hollow structure and large surface area of the SACs are favorable for mass/electron transport and exposure of Fe single-atom active sites. The as-prepared Fe/NS-C electrocatalysts display a high-efficiency ORR activity with a half-wave potential of 0.893 V versus the reversible hydrogen electrode and exceed that of the benchmark commercial Pt/C catalyst as well as most reported transition-metal based SACs. Impressively, the Fe/NS-C-based Al-air battery (AAB) displays a high open circuit voltage of 1.48 V, a maximum power density of 140.16 mW cm-2, and satisfactory durability, outperforming commercial Pt/C-based AAB. Furthermore, Fe/NS-C exhibits considerable potential as a cathode catalyst for application in direct methanol fuel cells. Experimental and theoretical calculation results reveal that the excellent ORR performance of Fe/NS-C can be contributed to the highly active FeN3S sites and the unique hollow structure. This work provides new insights into the rational design and synthesis high-performance ORR electrocatalysts for energy conversion and storage devices. of employing ZIF-8 as precursors.

Fe Single Atoms and Fe2O3 Clusters Liberated from N-Doped Polyhedral Carbon for Chemoselective Hydrogenation under Mild Conditions.

In the area of catalysis, selective reduction of nitro compounds to amino compounds is a colossal challenge due to the existence of competitive reducible functional groups. Herein, an Fe-based catalyst FeSAs/Fe2O3ACs/N-doped polyhedral carbon (NPC) has been designed and synthesized. As we expected, compared with FeSAs and FeNPs, FeSAs/Fe2O3ACs/NPC shows excellent catalytic performance (turnover frequency up to 1923 h-1, calculated with nitrobenzene), chemoselectivity, and tolerance during the hydrogenation reaction of nitro compounds under room temperature because of the synergistic effects between FeSAs and Fe2O3ACs. The theoretical calculations show that FeSAs prefers to undergo hydrazine decomposition to generate hydrogen and the Fe2O3ACs surface is more active toward the nitrobenzene reduction to aniline.