Unraveling the Origin of Sulfur‐Doped Fe‐N‐C Single‐Atom Catalyst for Enhanced Oxygen Reduction Activity: Effect of Iron Spin‐State Tuning

Performance of Nitrogen-Doped Carbon Nanoparticles Carrying FeNiCu as Bifunctional Electrocatalyst for Rechargeable Zinc-Air Battery

Catalysts for zinc-air batteries (ZABs) must be stable over long-term charging-discharging cycles and exhibit bifunctional catalytic activity. In this study, by doping nitrogen-doped carbon (NC) materials with three metal atoms (Fe, Ni, and Cu), a single-atom-distributed FeNiCu-NC bifunctional catalyst is prepared. The catalyst includes Fe(Ni-doped)-N4 for the oxygen evolution reaction (OER), Fe(Cu-doped)-N4 for the oxygen reduction reaction (ORR), and the NiCu-NC catalytic structure for the oxygen reduction reaction (ORR) in the nitrogen-doped carbon nanoparticles. This single-atom distribution catalyst structure enhances the bifunctional catalytic activity. If a trimetallic single-atom catalyst is designed, it will surpass the typical bimetallic single-atom catcalyst. FeNiCu-NC exhibits outstanding performance as an electrocatalyst, with a half-wave potential (E1/2) of 0.876 V versus RHE, overpotential (Ej = 10) of 253 mV versus RHE at 10 mA cm-2, and a small potential gap (ΔE = 0.61 V). As the anode in a ZAB, FeNiCu-NC can undergo continuous charge-discharged cycles for 575 h without significant attenuation. This study presents a new method for achieving high-performance, low-cost ZABs via trimetallic single-atom doping.

Sulfur Modified Carbon-Based Single-Atom Catalysts for Electrocatalytic Reactions

Efficient and sustainable energy development is a powerful tool for addressing the energy and environmental crises. Single-atom catalysts (SACs) have received high attention for their extremely high atom utilization efficiency and excellent catalytic activity, and have broad application prospects in energy development and chemical production. M-N4 is an active center model with clear catalytic activity, but its catalytic properties such as catalytic activity, selectivity, and durability need to be further improved. Adjustment of the coordination environment of the central metal by incorporating heteroatoms (e.g., sulfur) is an effective and feasible modification method. This paper describes the precise synthetic methods for introducing sulfur atoms into M-N4 and controlling whether they are directly coordinated with the central metal to form a specific coordination configuration, the application of sulfur-doped carbon-based single-atom catalysts in electrocatalytic reactions such as ORR, CO2RR, HER, OER, and other electrocatalytic reaction are systematically reviewed. Meanwhile, the effect of the tuning of the electronic structure and ligand configuration parameters of the active center due to doped sulfur atoms with the improvement of catalytic performance is introduced by combining different characterization and testing methods. Finally, several opinions on development of sulfur-doped carbon-based SACs are put forward.

Unraveling the Electron Transfer Effect of Single-Metal Ce-N4 Sites via Mesopore-Coupling for Boosted Oxygen Reduction Activity

The development of cerium (Ce) single-atom (SA) electrocatalysts for oxygen reduction reaction (ORR) with high active-site utilization and intrinsic activity has become popular recently but remains challenging. Inspired by an interesting phenomenon that pore-coupling with single-metal cerium sites can accelerate the electron transfer predicted by density functional theory calculations, here, a facile strategy is reported for directional design of a highly active and stable Ce SA catalyst (Ce SA/MC) by the coupling of single-metal Ce-N4 sites and mesopores in nanocarbon via pore-confinement-pyrolysis of Ce/phenanthroline complexes combined with controlling the formation of Ce oxides. This catalyst delivers a comparable ORR catalytic activity with a half-wave potential of 0.845 V versus RHE to the Pt/C catalyst. Also, a Ce SA/MC-based zinc-air battery (ZAB) has exhibited a higher energy density (924 Wh kgZn -1 ) and better long-term cycling durability than a Pt/C-based ZAB. This proposed strategy may open a door for designing efficient rare-earth metal catalysts with single-metal sites coupling with porous structures for next-generation energy devices.

Influence of coordination structure of Fe-585DV/NxC4-x on the electrocatalytic performance of oxygen reduction reactions

Fe-N-C material, known for its high efficiency, cost-effectiveness, and environmental friendliness, is a promising electrocatalyst in the field of the oxygen reduction reaction (ORR). However, the influence of defects and coordination structures on the catalytic performance of Fe-N-C has not been completely elucidated. In our present investigation, based on density functional theory, we take an Fe adsorbed graphene structure containing a 5-8-5 divacancy (585DV) defect as a research model and investigate the influence of the coordination number of N atoms around Fe (Fe-NxC(4-x)) on the ORR electrocatalyst behavior in alkaline conditions. We find that the Fe-N4 structure exhibits superior ORR catalytic performance than other N coordination structures Fe-NxC4-x (x = 0-3). We explore the reasons for the improved catalytic performance through electronic structure analysis and find that as the N coordination number in the Fe-NxC(4-x) structure increases, the magnetic moment of the Fe single atom decreases. This reduction is conducive to the ORR catalytic performance, indicating that a lower magnetic moment is more favorable for the catalytic process of the ORR within the Fe-NxC(4-x) structure. This study is of great significance for a deeper understanding of the structure-performance relationship in catalysis, as well as for the development of efficient ORR catalysts.

Mass Production of Sulfur-Tuned Single-Atom Catalysts for Zn-Air Batteries

Single-atom catalysts (SACs) show great potential for rechargeable Zn-air batteries (ZABs); however, scalable production of SACs from sustainable resources is difficult owing to poor control of the local coordination environment. Herein, lignosulfonate, a by-product of the papermaking industry, is utilized as a multifunctional bioligand for the mass production of SACs with highly active MN₄ S sites (M represents Fe, Cu, and Co) via strong metalnitrogen/sulfur coordination. This effectively adjusts the charge distribution and promotes the catalytic performance, leading to highly durable and excellent performance in oxygen reduction and evolution reactions for ZABs. This study paves the way for the industrial production of cost-effective SACs in a sustainable manner.

Atomically Dispersed Zn-Pyrrolic-N4 Cathode Catalysts for Hydrogen Fuel Cells

To achieve practical application of fuel cell, it is vital to develop highly efficient and durable Pt-free catalysts. Herein, we prepare atomically dispersed ZnNC catalysts with Zn-Pyrrolic-N₄ moieties and abundant mesoporous structure. The ZnNC-based anion-exchange membrane fuel cell (AEMFC) presents an ultrahigh peak power density of 1.63 and 0.83 W cm^-2 in H₂ -O₂ and H₂ -air (CO₂ -free), and also exhibits long-term stability with more than 120 and 100 h for H₂ -air (CO₂ -free) and H₂ -O₂ , respectively. Density functional calculations further unveil that the Zn-Pyrrolic-N₄ structure is the origin of high activity of as-synthesized ZnNC catalyst, while the Zn-Pyridinic-N₄ moiety is inactive for oxygen reduction reaction (ORR), which successfully explain the puzzle why most Zn-metal-organic framework -derived ZnNC catalysts in previous reports did not present good ORR activity because of their Zn-Pyridinic-N₄ moieties. This work offers a new route for speeding up development of AEMFCs.

Boosting Benzene Oxidation with a Spin-State-Controlled Nuclearity Effect on Iron Sub-Nanocatalysts

A fundamental understanding of the nature of nuclearity effects is important for the rational design of superior sub-nanocatalysts with low nuclearity, but remains a long-standing challenge. Using atomic layer deposition, we precisely synthesized Fe sub-nanocatalysts with tunable nuclearity (Fe₁ -Fe₄ ) anchored on N,O-co-doped carbon nanorods (NOC). The electronic properties and spin configuration of the Fe sub-nanocatalysts were nuclearity dependent and dominated the H₂ O₂ activation modes and adsorption strength of active O species on Fe sites toward C-H oxidation. The Fe₁ -NOC single atom catalyst exhibits state-of-the-art activity for benzene oxidation to phenol, which is ascribed to its unique coordination environment (Fe₁ N₂ O₃ ) and medium spin state (t_2g ⁴ e_g ¹ ); turnover frequencies of 407 h^-1 at 25 °C and 1869 h^-1 at 60 °C were obtained, which is 3.4, 5.7, and 13.6 times higher than those of Fe dimer, trimer, and tetramer catalysts, respectively.

Decorating Single-Atomic Mn Sites with FeMn Clusters to Boost Oxygen Reduction Reaction

The regulation of electron distribution of single-atomic metal sites by atomic clusters is an effective strategy to boost their intrinsic activity of oxygen reduction reaction (ORR). Herein we report the construction of single-atomic Mn sites decorated with atomic clusters by an innovative combination of post-adsorption and secondary pyrolysis. The X-ray absorption spectroscopy confirms the formation of Mn sites via Mn-N₄ coordination bonding to FeMn atomic clusters (FeMn_ac /Mn-N₄ C), which has been demonstrated theoretically to be conducive to the adsorption of molecular O₂ and the break of O-O bond during the ORR process. Benefiting from the structural features above, the FeMn_ac /Mn-N₄ C catalyst exhibits excellent ORR activity with half-wave potential of 0.79 V in 0.5 M H₂ SO₄ and 0.90 V in 0.1 M KOH as well as preeminent Zn-air battery performance. Such synthetic strategy may open up a route to construct highly active catalysts with tunable atomic structures for diverse applications.

Regulating Spin States in Oxygen Electrocatalysis

Developing efficient and stable transition metal oxides catalysts for energy conversion processes such as oxygen evolution reaction and oxygen reduction reaction is one of the key measures to solve the problem of energy shortage. The spin state of transition metal oxides is strongly correlated with their catalytic activities. In an octahedral structure of transition metal oxides, the spin state of active centers could be regulated by adjusting the splitting energy and the electron pairing energy. Regulating spin state of active centers could directly modulate the d orbitals occupancy, which influence the strength of metal-ligand bonds and the adsorption behavior of the intermediates. In this review, we clarified the significance of regulating spin state of the active centers. Subsequently, we discussed several characterization technologies for spin state and some recent strategies to regulate the spin state of the active centers. Finally, we put forward some views on the future research direction of this vital field.

Tuning iron spin states in single-atom nanozymes enables efficient peroxidase mimicking

The large-scale application of nanozymes remains a significant challenge owing to their unsatisfactory catalytic performances. Featuring a unique electronic structure and coordination environment, single-atom nanozymes provide great opportunities to vividly mimic the specific metal catalytic center of natural enzymes and achieve superior enzyme-like activity. In this study, the spin state engineering of Fe single-atom nanozymes (FeNC) is employed to enhance their peroxidase-like activity. Pd nanoclusters (PdNC) are introduced into FeNC, whose electron-withdrawing properties rearrange the spin electron occupation in Fe(ii) of FeNC-PdNC from low spin to medium spin, facilitating the heterolysis of H2O2 and timely desorption of H2O. The spin-rearranged FeNC-PdNC exhibits greater H2O2 activation activity and rapid reaction kinetics compared to those of FeNC. As a proof of concept, FeNC-PdNC is used in the immunosorbent assay for the colorimetric detection of prostate-specific antigen and achieves an ultralow detection limit of 0.38 pg mL-1. Our spin-state engineering strategy provides a fundamental understanding of the catalytic mechanism of nanozymes and facilitates the design of advanced enzyme mimics.

Tuning the Spin State of the Iron Center by Bridge-Bonded Fe-O-Ti Ligands for Enhanced Oxygen Reduction

Exploring functional substrates and precisely regulating the electronic structures of atomic metal active species with moderate spin state are of great importance yet remain challenging. Hereon, we provide an axial Fe-O-Ti ligand regulated spin-state transition strategy to improve the oxygen reduction reaction (ORR) activity of Fe centers. Theoretical calculations indicate that Fe-O-Ti ligands in FeN₃ O-O-Ti can induce a low-to-medium spin-state transition and optimize O₂ adsorption by FeN₃ O. As a proof-of-concept, the oriented catalyst was prepared from atomic-Fe-doped polymer-like quantum dots and ultrathin o-terminated MXene. The optimal catalyst exhibits an intrinsic activity that is almost 5 times higher than the control sample (without axial Fe-O-Ti ligands). It also delivers a superior performance in Zn-air batteries and H₂ /O₂ anion exchange membrane fuel cells in a wide-temperature range.

Space-Confined Anchoring of Fe-Nx on Concave N-Doped Carbon Cubes for Catalyzing Oxygen Reduction

Carbon-based electrocatalysts with atomically dispersed Fe-N-C display promising performance for oxygen reduction reaction (ORR) amongst non-precious electrocatalysts. Nonetheless, increasing the number and utilization of Fe-N-C active sites is challenging. Designing morphologies and adjusting the pore structure of carbon-based electrocatalysts would boost the mass transfer, enhance the utilization of the active sites, and increase the overall ORR performance. In this work, a concave N-doped carbon cubes structure adorned with highly external Fe-N_x was designed and produced by the space-confined induced strategy. The optimal electrocatalyst revealed excellent ORR activity in both alkaline and acidic electrolytes, with half-wave potentials of 0.86 and 0.75 V, respectively. The superior performance arose from its unique concave structure, possessing more accessible active sites with improved intrinsic activity, which holds promising potential for preparing advanced ORR electrocatalysts.