Metal-Organic Framework-Derived Graphene Mesh: a Robust Scaffold for Highly Exposed Fe-N4 Active Sites toward an Excellent Oxygen Reduction Catalyst in Acid Media

Rational design of Fe, N co-doped porous carbon derived from conjugated microporous polymer as an electrocatalytic platform for oxygen reduction reaction

Porous iron-nitrogen-doped carbons (FeNC) offer a great platform for construction of cathodic oxygen reduction reaction (ORR) catalysts in fuel cells. However, challenges still remain regarding with the collapse of carbon-skeleton during pyrolysis, uneven distribution of active sites and aggregation of metal atoms. In this work, we synthesized Fe, N co-doped conjugated microporous polymer (FeN-CMP) through a facile bottom-up strategy using 1,3,5-triethynylbenzene and iron-chelated 3,8-dibromo-1,10-phenanthroline as monomers, ensuring the uniform coordination of N with Fe element in network. Then, the resulting FeN-CMP was treated by pyrolysis without structural collapse to obtain porous FeNC electrocatalyst for ORR. The most active catalyst was fabricated under 900 °C, which exhibits remarkable ORR activity in alkaline medium with half-wave potential of 0.796 V (18 mV and 105 mV positive deviation from the commercial Pt/C catalyst and post-doping catalyst), high selectivity with nearly 4e- transfer process and excellent methanol tolerance. Our study first developed porous FeNC electrocatalysts derived from Fe, N-anchoring CMPs based on pre-functionalization of monomers, which exhibits great potential as an alternative to commercial Pt/C catalyst for ORR, and provides a feasible strategy of developing multi-atoms doping catalysts for energy storage and conversion as well as heterogeneous catalysis.

Vapor deposition strategy for implanting isolated Fe sites into papermaking nanofibers-derived N-doped carbon aerogels for liquid Electrolyte-/All-Solid-State Zn-Air batteries

Single-atom catalysts (SACs), with precisely controlled metal atom distribution and adjustable coordination architecture, have gained intensive concerns as efficient oxygen reduction reaction (ORR) electrocatalysts in Zn-air batteries (ZAB). The attainment of a monodispersed state for metallic atoms anchored on the carbonaceous substrate remains the foremost research priority; however, the persistent challenges lie in the relatively weak metal-support interactions and the instability of captured single atom active sites. Furthermore, in order to achieve rapid transport of O2 and other reactive substances within the carbon matrix, manufacturing SACs based on multi-stage porous carbon substrates is highly anticipated. Here, we propose a methodology for the fabrication of carbon aerogels (CA)-supported SACs utilizing papermaking nanofibers, which incorporates advanced strategies for N-atom self-doping, defect/vacancy introduction, and single-atom interface engineering. Specifically, taking advantages of using green and energy-efficient feedstocks, combining with a direct pore-forming template volatilization and chemical vapor deposition approach, we successfully developed N-doped carbon aerogels immobilized with separated iron sites (Fe-SAC@N/CA-Cd). The obtained Fe-SAC@N/CA-Cd exhibited substantially large specific surface area (SBET = 1173 m2/g) and a multi-level pore structure, which can effectively mitigate the random aggregation of Fe atoms during pyrolysis. As a result, it demonstrated appreciable activity and stability in catalyzing the ORR progress (E1/2 = 0.88 V, Eonset = 0.96 V). Furthermore, the assembled liquid electrolyte-state Zn-air batteries (LES-ZAB) and all-solid-state Zn-air battery (ASS-ZAB) also provides encouraging performance, with a peak power density of 169 mW cm-2 for LES-ZAB and a maximum power density of 124 mW cm-2 for ASS-ZAB.

Bifunctional oxygen reduction/evolution reaction electrocatalysts achieved by axial ligand modulation on two-dimensional porphyrin frameworks

Exploring efficient and low-cost oxygen reduction and oxygen evolution reaction (ORR/OER) bifunctional catalysts is essential for the development of energy storage and conversion devices. Herein, enlightened by the experimentally synthesized cobalt(II) meso-tetraethynylporphyrins (Co-TEP) molecule, we designed a novel 2D covalent organic framework (COF), namely a 2D Co-TEP monolayer, by dimensional expansion. The 2D Co-TEP monolayer, with Co atoms distributed separately and stabilized by uniform pyrrolic-N coordination, features metal-nitrogen-carbon single-atom catalyst activity and shows tunable catalytic activity for the electrochemical ORR/OER by axial ligand (O, OH, Cl, CN, CH3, NO, F) modulation. By means of the state-of-the-art constant-potential first-principles computations and microkinetic simulations, we demonstrated that 2D Co-TEP-CN exhibits good ORR/OER performance in both acidic and alkaline conditions. The difference between the onset-potential for the OER and the half-wave potential for the ORR is only 0.85 V at pH = 1, smaller than that of Pt/IrO2 electrocatalysts. The good electrocatalytic performance is maintained by replacing the center metal atoms with Mn, Fe and/or Ni. Our investigation highlights the role of the pyrrolic-N coordination and the ligands in improving the catalytic activity of 2D COFs and provides new insights into the rational design of efficient bifunctional ORR/OER catalysts.

2D arrays of hollow carbon nanoboxes: outward contraction-induced hollowing mechanism in Fe-N-C catalysts

Maximizing the utilization efficiency of monatomic Fe sites in Fe-N-C catalysts poses a significant challenge for their commercial applications. Herein, a structural and electronic dual-modulation is achieved on a Fe-N-C catalyst to substantially enhance its catalytic performance. We develop a facile multi-component ice-templating co-assembly (MIC) strategy to construct two-dimensional (2D) arrays of monatomic Fe-anchored hollow carbon nanoboxes (Fe-HCBA) via a novel dual-outward interfacial contraction hollowing mechanism. The pore engineering not only enlarges the physical surface area and pore volume but also doubles the electrochemically active specific surface area. Additionally, the unique 2D carbon array structure reduces interfacial resistance and promotes electron/mass transfer. Consequently, the Fe-HCBA catalysts exhibit superior oxygen reduction performance with a six-fold enhancement in both mass activity (1.84 A cm-2) and turnover frequency (0.048 e- site-1 s-1), compared to microporous Fe-N-C catalysts. Moreover, the incorporation of phosphorus further enhances the total electrocatalytic performance by three times by regulating the electron structure of Fe-N4 sites. Benefitting from these outstanding characteristics, the optimal 2D P/Fe-HCBA catalyst exhibits great applicability in rechargeable liquid- and solid-state zinc-air batteries with peak power densities of 186 and 44.5 mW cm-2, respectively.

Optimizing the Activation Energy of Reactive Intermediates on Single-Atom Electrocatalysts: Challenges and Opportunities

Single-atom catalysts (SACs) have made great progress in recent years as potential catalysts for energy conversion and storage due to their unique properties, including maximum metal atoms utilization, high-quality activity, unique defined active sites, and sustained stability. Such advantages of single-atom catalysts significantly broaden their applications in various energy-conversion reactions. Given the extensive utilization of single-atom catalysts, methods and specific examples for improving the performance of single-atom catalysts in different reaction systems based on the Sabatier principle are highlighted and reactant binding energy volcano relationship curves are derived in non-homogeneous catalytic systems. The challenges and opportunities for single-atom catalysts in different reaction systems to improve their performance are also focused upon, including metal selection, coordination environments, and interaction with carriers. Finally, it is expected that this work may provide guidance for the design of high-performance single-atom catalysts in different reaction systems and thereby accelerate the rapid development of the targeted reaction.

Ultrasensitive cortisol electrochemical immunosensor amplifying by Au single-atom nanozymes and HRP enzymes

Cortisol, a corticosteroid hormone as a primary stress hormone response to internal and external stress, has been regarded as a gold standard reliable biomarker to evaluate human mental stress. The double enzymes strategy, using nanozyme and enzyme amplifying the electrochemical signal, has been widely used to improve the performance of electrochemical biosensors. An ultra-sensitive electrochemical cortisol sensor based on Au single-atom nanozymes had been fabricated through HRP labeled anti-cortisol antibody binding with Au by Au-S bond. Based on the high catalytic activity of Au single-atom nanozymes and the high selectivity of HRP-labeled anti-cortisol antibodies, the cortisol electrochemical sensor-based Au single-atom nanozymes had an excellent response to cortisol, such as high electrochemical activity, high sensitivity, high selectivity, and wide linear range (0.15-300 ng mL-1) and low detection (0.48 pg mL-1) through the four-parameter logistic model with 95% confidence. The electrochemical cortisol sensor was used to determine the cortisol concentration of human saliva at different times.

Sharply expanding single-atomically dispersed Fe-N active sites through bidirectional coordination for oxygen reduction

For Fe-NC systems, high-density Fe-N sites are the basis for high-efficiency oxygen reduction reaction (ORR), and P doping can further lower the reaction energy barrier, especially in the form of metal-P bonding. However, limited to the irregular agglomeration of metal atoms at high temperatures, Fe-P bonds and high-density Fe-N cannot be guaranteed simultaneously. Here, to escape the random and violent agglomeration of Fe species during high-temperature carbonization, triphenylphosphine and 2-methylimidazole with a strong metal coordination capability are introduced together to confine Fe growth. With the aid of such bidirectional coordination, the high-density Fe-N site with Fe-P bonds is realized by in situ phosphorylation of Fe in an Fe-NC system (Fe-P-NC) at high temperatures. Impressively, the content of single-atomically dispersed Fe sites for Fe-P-NC dramatically increases from 2.8% to 65.3% compared with that of pure Fe-NC, greatly improving the ORR activity in acidic and alkaline electrolytes. The theoretical calculation results show that the generated Fe2P can simultaneously facilitate the adsorption of intermediates to Fe-N4 sites and the electron transfer, thereby reducing the reaction energy barrier and obtaining superior ORR activity.

One-Pot Etching Pyrolysis to Defect-Rich Carbon Nanosheets to Construct Multiheteroatom-Coordinated Iron Sites for Efficient Oxygen Reduction

Constructing multiheteroatom coordination structure in carbonaceous substrates demonstrates an effective method to accelerate the oxygen reduction reaction (ORR) of supported single-atom catalyst. Herein, the novel etching route assisted by potassium thiocyanate (KCNS) is developed to convert metal-organic framework to 2D defect-rich porous N,S-co-doped carbon nanosheets for anchoring atomically dispersed iron sites as the high-performance ORR catalysts (Fe-SACs). The well-designed KCNS-assisted etching route can generate spatial confinement template to direct the carbon nanosheet formation, etching condition to form defect-rich structure, and additional sulfur atoms to coordinate iron species. Spectral and microscopy analysis reveals that the iron element in Fe-SACs is highly isolated on carbon nanosheet and anchored by nitrogen and sulfur atoms in unsymmetrical Fe-S1N3 structure. The optimized Fe-SACs with large specific surface area could show remarkable alkaline ORR performances with a high half-wave potential of 0.920 V versus RHE and excellent durability. The rechargeable zinc-air battery assembled with Fe-SACs air electrodes delivers a large power density of 350 mW cm-2 and a stable voltage platform during charge and discharge over more than 1300 h. This work proposes a novel strategy for the preparation of single-atom catalysts with multiheteroatom coordination structure and highly exposed active sites for efficient ORR.

Electrocatalytic Oxygen Reduction Reaction of Peripheral Functionalized Cobalt Porphyrins(2.1.2.1)

Two peripheral functionalized clamp-shaped cobalt porphyrin(2.1.2.1) complexes were synthesized, and their electrocatalytic ORR abilities were investigated. The crystal data and optical and redox properties of them were revised by peripheral modification. The ORR capacities and DFT calculations of F5PhCo and F5NCo suggest superior selectivity for the 4e- ORR pathway. This work further confirms the clamp-shaped cobalt porphyrin complexes are ideal Co-N4 ORR catalysts.

The nearby atomic environment effect on an Fe-N-C catalyst for the oxygen reduction reaction: a density functional theory-based study

Fe-N-C materials have emerged as highly promising non-noble metal catalysts for oxygen reduction reactions (ORRs) in polymer electrolyte membrane fuel cells. However, they still encounter several challenges that need to be addressed. One of these challenges is establishing an atomic environment near the Fe-N4 site, which can significantly affect catalyst activity. To investigate this, herein, we employed density functional theory (DFT). According to our computational analysis of the Gibbs free energy of the reaction based on the computational hydrogen electrode (CHE) model, we successfully determined two C-O-C structures near the Fe-N4 site (referred to as str-11) with the highest limiting potential (0.813 V). Specifically, in the case of O-doped structures, the neighboring eight carbon (C) atoms around nitrogen (N) can be categorized into two distinct types: four C atoms (type A) exhibiting high sensitivity to the limiting potential and the remaining four C atoms (type B) displaying inert behavior. Electronic structure analysis further elucidated that a structure will have strong activity if the valence band maximum (VBM) around its gamma point is mainly contributed by dxz, dyz or dz2 orbitals of Fe atoms. Constant-potential calculations showed that str-11 is suitable for the ORR under both acidic and alkaline conditions with a limiting potential of 0.695 V at pH = 1 and 0.926 V at pH = 14, respectively. Additionally, microkinetic simulations indicated the possibility of str-11 as the active site for the ORR under working potential at pH = 14.

The role of high-resolution transmission electron microscopy and aberration corrected scanning transmission electron microscopy in unraveling the structure-property relationships of Pt-based fuel cells electrocatalysts

Platinum-based fuel cell electrocatalysts are structured on a nano level in order to extend their active surface area and maximize the utilization of precious and scarce platinum. Their performance is dictated by the atomic arrangement of their surface layers atoms via structure-property relationships. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are the preferred methods for characterizing these catalysts, due to their capacity to achieve local atomic-level resolutions. Size, morphology, strain and local composition are just some of the properties of Pt-based nanostructures that can be obtained by (S)TEM. Furthermore, advanced methods of (S)TEM are able to provide insights into the quasi-in situ, in situ or even operando stability of these nanostructures. In this review, we present state-of-the-art applications of (S)TEM in the investigation and interpretation of structure-activity and structure-stability relationships.

High Power Density of a Hydrogen Peroxide Fuel Cell Using Cobalt Chlorin Complex Supported on Carbon Nanotubes as a Noncorrosive Anode

Hydrogen peroxide fuel cells (HPFCs) have attracted much attention due to their simple one-compartment structures and high availability under harsh conditions such as an anaerobic environment; however, catalysis improvement is strongly demanded for both anodes and cathodes in terms of activity and durability. Herein, we report the high catalytic activity of CoII chlorin [CoII(Ch)] for hydrogen peroxide (H2O2) oxidation with a low overpotential (0.21 V) compared to that of the CoII phthalocyanine and CoII porphyrin complexes, which have previously been reported as active anode catalysts. Linear sweep voltammograms and differential pulse voltammograms of the CoII complexes (CoIIL) and the corresponding ligands clearly showed that the CoIIIL species are the active species for H2O2 oxidation. Then, one-compartment HPFCs were constructed with CoII(Ch) supported on multiwalled carbon nanotubes (CNTs) as the anode together with FeII3[CoIII(CN)6]2 supported on CNTs as the cathode. The maximum power density of the HPFCs reached 151 μW cm-2 with an open circuit potential of 0.33 V when the coverage of CNT surfaces with CoII(Ch) exceeded ∼60% at the anode.

Engineering Electronic Structure of Nitrogen-Carbon Sites by sp3 -Hybridized Carbon and Incorporating Chlorine to Boost Oxygen Reduction Activity

Development of efficient and easy-to-prepare low-cost oxygen reaction electrocatalysts is essential for widespread application of rechargeable Zn-air batteries (ZABs). Herein, we mixed NaCl and ZIF-8 by simple physical milling and pyrolysis to obtain a metal-free porous electrocatalyst doped with Cl (mf-pClNC). The mf-pClNC electrocatalyst exhibits a good oxygen reduction reaction (ORR) activity (E1/2 =0.91 V vs. RHE) and high stability in alkaline electrolyte, exceeding most of the reported transition metal carbon-based electrocatalysts and being comparable to commercial Pt/C electrocatalysts. Likewise, the mf-pClNC electrocatalyst also shows state-of-the-art ORR activity and stability in acidic electrolyte. From experimental and theoretical calculations, the better ORR activity is most likely originated from the fact that the introduced Cl promotes the increase of sp3 -hybridized carbon, while the sp3 -hybridized carbon and Cl together modify the electronic structure of the N-adjacent carbons, as the active sites, while NaCl molten-salt etching provides abundant paths for the transport of electrons/protons. Furthermore, the liquid rechargeable ZAB using the mf-pClNC electrocatalyst as the cathode shows a fulfilling performance with a peak power density of 276.88 mW cm-2 . Flexible quasi-solid-state rechargeable ZAB constructed with the mf-pClNC electrocatalyst as the cathode exhibits an exciting performance both at low, high and room temperatures.

Precise Design and Modification Engineering of Single-Atom Catalytic Materials for Oxygen Reduction

Due to their unique electronic and structural properties, single-atom catalytic materials (SACMs) hold great promise for the oxygen reduction reaction (ORR). Coordinating environmental and engineering strategies is the key to improving the ORR performance of SACMs. This review summarizes the latest research progress and breakthroughs of SACMs in the field of ORR catalysis. First, the research progress on the catalytic mechanism of SACMs acting on ORR is reviewed, including the latest research results on the origin of SACMs activity and the analysis of pre-adsorption mechanism. The study of the pre-adsorption mechanism is an important breakthrough direction to explore the origin of the high activity of SACMs and the practical and theoretical understanding of the catalytic process. Precise coordination environment modification, including in-plane, axial, and adjacent site modifications, can enhance the intrinsic catalytic activity of SACMs and promote the ORR process. Additionally, several engineering strategies are discussed, including multiple SACMs, high loading, and atomic site confinement. Multiple SACMs synergistically enhance catalytic activity and selectivity, while high loading can provide more active sites for catalytic reactions. Overall, this review provides important insights into the design of advanced catalysts for ORR.

Ternary Heteroatomic Doping Induced Microenvironment Engineering of Low Fe-N4-Loaded Carbon Nanofibers for Bifunctional Oxygen Electrocatalysis

Fabricating highly efficient and long-life redox bifunctional electrocatalysts is vital for oxygen-related renewable energy devices. To boost the bifunctional catalytic activity of Fe-N-C single-atom catalysts, it is imperative to fine-tune the coordination microenvironment of the Fe sites to optimize the adsorption/desorption energies of intermediates during oxygen reduction/evolution reactions (ORR/OER) and simultaneously avoid the aggregation of atomically dispersed metal sites. Herein, a strategy is developed for fabricating a free-standing electrocatalyst with atomically dispersed Fe sites (≈0.89 wt.%) supported on N, F, and S ternary-doped hollow carbon nanofibers (FeN4 -NFS-CNF). Both experimental and theoretical findings suggest that the incorporation of ternary heteroatoms modifies the charge distribution of Fe active centers and enhances defect density, thereby optimizing the bifunctional catalytic activities. The efficient regulation isolated Fe centers come from the dual confinement of zeolitic imidazole framework-8 (ZIF-8) and polymerized ionic liquid (PIL), while the precise formation of distinct hierarchical three-dimensional porous structure maximizes the exposure of low-doping Fe active sites and enriched heteroatoms. FeN4 -NFS-CNF achieves remarkable electrocatalytic activity with a high ORR half-wave potential (0.90 V) and a low OER overpotential (270 mV) in alkaline electrolyte, revealing the benefit of optimizing the microenvironment of low-doping iron single atoms in directing bifunctional catalytic activity.

Boosting Alkaline Hydrogen Oxidation Activity of Ru Single-Atom Through Promoting Hydroxyl Adsorption on Ru/WC1- x Interfaces

The sluggish kinetics of the hydrogen oxidation reaction (HOR) in alkaline conditions continue to pose a significant challenge for the practical implementation of anion-exchange membrane fuel cells. Developing single-atom catalysts can accelerate the pace of new HOR catalyst discovery for highly cost-effective and active HOR performance. However, single-atom catalysts (SACs) for the alkaline HOR have rarely been reported, and fundamental studies on the rational design of SACs are still required. Herein, the design of Ru SAC supported on the tungsten carbide (Ru SA/WC1- x ) via in situ high-temperature annealing strategy is reported. The resulting Ru SA/WC1- x catalyst exhibits remarkably enhanced HOR performance in alkaline media, a level of activity that can not be achieved with carbon-supported Ru SAC. Electrochemical results and density functional theory demonstrate that promoting the hydroxyl adsorption on Ru SA/WC1- x interfaces, which is derived from the low potential of zero charge of WC1- x support, has a significant effect on enhancing the HOR performance of SACs. This enhancement leads to 5.8 and 60.1 times higher Ru mass activity of Ru SA/WC1- x than Ru nanoparticles on carbon and Ru single-atom on N-doped carbon, respectively. This work provides new insights into the design of highly active SACs for alkaline HOR.

Selective Etching of Metal-Organic Frameworks for Open Porous Structures: Mass-Efficient Catalysts with Enhanced Oxygen Reduction Reaction for Fuel Cells

Fe-Nx-C-based single-atom (SA-Fe-N-C) catalysts have shown favorable oxygen reduction reaction (ORR) activity. However, their application in proton exchange membrane fuel cells is hindered by reduced performance owing to the thick catalyst layer, restricting mass transfer and the O2 supply. Metal-organic frameworks (MOFs) are a promising class of crystal materials, but their narrow pores exacerbate the sluggish mass-transport properties within the catalyst layer. This study developed an approach for constructing an open-pore structure in MOFs via chelation-assisted selective etching, resulting in atomically dispersed Fe atoms anchored on an N, S co-doped carbon framework. The open-pore structure reduces oxygen transport resistance in the membrane electrode assembly (MEA) with unprecedented ORR activity and stability, as evidenced by finite element simulations. In an acidic electrolyte, the OP-Fe-NC catalyst shows a half-wave potential of 0.89 V vs RHE, surpassing Pt/C by 20 mV, and a current density of 29 mA cm-2 at 0.9 ViR-free in the MEA. This study provides an effective structural strategy for fabricating electrocatalysts with high mass efficiency and atomic precision for energy storage and conversion devices.

Effects of Reduction Methods on the Performance of Shewanella oneidensis MR-1 Palladium/Carbon Catalyst for Oxygen Reduction Reaction

The influence of the reduction method on the morphology and performance of the catalyst still controversial. In this study, hydrogen, Shewanella oneidensis MR-1 (MR-1), MR-1 and hydrogen coreduction are used to reduce the palladium ions adsorbed by MR-1 to obtain Pd/CH2, Pd/CM, and Pd/CH2+M catalyst, respectively. It is found that the palladium nanoparticles (Pd NPs) in Pd/CH2+M are the largest, while the Pd NPs in Pd/CM are the smallest. This is due to the reduction of Pd NPs in Pd/CH2+M under anaerobic conditions to form smaller Pd NPs that will further aggregate and grow in H2. In addition, Pd/CM exhibited the best catalytic performance with a mass activity of 0.31 A mg-1, better than that of Pd/CH2 (0.06 A mg-1) and Pd/CH2+M (0.13 A mg-1). This study provides a meaningful reference for the selection of reduction methods in metal catalysts.

Boosting oxygen reduction reaction kinetics through perturbating electronic structure of single-atom Fe-N3S1 catalyst with sub-nano FeS cluster

Single atomic Fe-N4 catalyst exhibits a great prospect for oxygen reduction reaction (ORR) and adjusting the intrinsic coordination structure and the carbon matrix structure effectively improves the catalytic activity. However, controlling the active site coordination structure and its surrounding environment at atomic level remains a challenge. In this paper, Fe-N3S1 and FeS sub-nano cluster were innovatively concatenated on S, N co-doped carbon matrix (SNC), denoted as FeS/FeSA@SNC catalysts, for modulating ORR catalysis performance. Both experimental measurements and theoretical calculations have confirmed that the local electron configuration of Fe center is modulated by this unique structure combination leading to optimized ORR kinetics. Based on this design, the synthesized FeS/FeSA@SNC delivers ORR activity with a half-wave potential of 0.9 V (vs. RHE), excelling that of commercial Pt/C (0.87 V) and the Zn-air battery (ZAB) with this cathode catalyst delivers a peak power density of 126 mW cm-2. This work presents a novel strategy for manipulating the single-atom active sites through control the local coordination structure and provides a reference for the development of novel efficient ORR electrocatalysts.

Bioinspired porous three-coordinated single-atom Fe nanozyme with oxidase-like activity for tumor visual identification via glutathione

Inspired by structures of natural metalloenzymes, a biomimetic synthetic strategy is developed for scalable synthesis of porous Fe-N3 single atom nanozymes (pFeSAN) using hemoglobin as Fe-source and template. pFeSAN delivers 3.3- and 8791-fold higher oxidase-like activity than Fe-N4 and Fe3O4 nanozymes. The high catalytic performance is attributed to (1) the suppressed aggregation of atomically dispersed Fe; (2) facilitated mass transfer and maximized exposure of active sites for the created mesopores by thermal removal of hemoglobin (2 ~ 3 nm); and (3) unique electronic configuration of Fe-N3 for the oxygen-to-water oxidation pathway (analogy with natural cytochrome c oxidase). The pFeSAN is successfully demonstrated for the rapid colorimetric detection of glutathione with a low limit of detection (2.4 nM) and wide range (50 nM-1 mM), and further developed as a real-time, facile, rapid (~6 min) and precise visualization analysis methodology of tumors via glutathione level, showing its potentials for diagnostic and clinic applications.

Insight into Defect Engineering of Atomically Dispersed Iron Electrocatalysts for High-Performance Proton Exchange Membrane Fuel Cell

Atomically dispersed and nitrogen coordinated iron catalysts (Fe-NCs) demonstrate potential as alternatives to platinum-group metal (PGM) catalysts in oxygen reduction reaction (ORR). However, in the context of practical proton exchange membrane fuel cell (PEMFC) applications, the membrane electrode assembly (MEA) performances of Fe-NCs remain unsatisfactory. Herein, improved MEA performance is achieved by tuning the local environment of the Fe-NC catalysts through defect engineering. Zeolitic imidazolate framework (ZIF)-derived nitrogen-doped carbon with additional CO2 activation is employed to construct atomically dispersed iron sites with a controlled defect number. The Fe-NC species with the optimal number of defect sites exhibit excellent ORR performance with a high half-wave potential of 0.83 V in 0.5 M H2 SO4 . Variation in the number of defects allows for fine-tuning of the reaction intermediate binding energies by changing the contribution of the Fe d-orbitals, thereby optimizing the ORR activity. The MEA based on a defect-engineered Fe-NC catalyst is found to exhibit a remarkable peak power density of 1.1 W cm-2 in an H2 /O2 fuel cell, and 0.67 W cm-2  in an H2 /air fuel cell, rendering it one of the most active atomically dispersed catalyst materials at the MEA level.

Similar electronic state effect enables excellent activity for nitrate-to-ammonia electroreduction on both high- and low-density double-atom catalysts

Double-atom catalysts (DACs) for harmful nitrate (NO3-) electroreduction to valuable ammonia (eNO3RR) is attractive for both environmental remediation and energy transformation. However, the limited metal loading in most DACs largely hinders their applications in practical catalytic applications. Therefore, exploring ultrahigh-density (UHD) DACs with abundant active metal centers and excellent eNO3RR activity is highly desired under the site-distance effect. Herein, starting from the experimental M2N6 motif deposited on graphene, we firstly screened the low-density (LD) Mn2N6 and Fe2N6 DACs with high eNO3RR activity and then established an appropriate activity descriptor for the LD-DAC system. By utilizing this descriptor, the corresponding Mn2N6 and Fe2N6 UHD-DACs with dynamic, thermal, thermodynamic, and electrochemical stabilities, are identified to locate at the peak of activity volcano, exhibiting rather-low limiting potentials of -0.25 and -0.38 V, respectively. Further analysis in term of spin state and orbital interaction, confirms that the electronic state effect similar to that of LD-DACs enable the excellent eNO3RR activity to be maintained in the UHD-DACs. These findings highlight the promising application of Mn2N6 and Fe2N6 UHD-DACs in nitrate electroreduction for NH3 production and provide impetus for further experimental exploration of ultrahigh-density DACs based on their intrinsic electronic states.

Atomically Dispersed FeN2 P2 Motif with High Activity and Stability for Oxygen Reduction Reaction Over the Entire pH Range

The past decade has witnessed the great potential of Fe-based single-atom electrocatalysis in catalyzing oxygen reduction reaction (ORR). However, it remains a grand challenge to substantially improve their intrinsic activity and long-term stability in acidic electrolytes. Herein, we report a facile chemical vapor deposition strategy, by which high-density Fe atoms (3.97 wt%) are coordinated with square-planar para-positioned nitrogen and phosphorus atoms in a hierarchical carbon framework. The as-crafted atomically dispersed Fe catalyst (denoted Fe-SA/PNC) manifests an outstanding activity towards ORR over the entire pH range. Specifically, the half-wave potential of 0.92 V, 0.83 V, and 0.86 V vs. reversible hydrogen electrode (RHE) are attained in alkaline, neutral, and acidic electrolytes, respectively, representing the high performance among reported catalysts to date. Furthermore, after 30,000 durability cycles, the Fe-SA/PNC remains to be stable with no visible performance decay when tested in 0.1 M KOH and 0.5 M H2 SO4 , and only a minor negative shift of 40 mV detected in 0.1 M HClO4 , significantly outperforming commercial Pt/C counterpart. The coordination motif of Fe-SA/PNC is validated by density functional theory (DFT) calculations. This work provides atomic-level insight into improving the activity and stability of non-noble metal ORR catalysts, opening up an avenue to craft the desired single-atom electrocatalysts.

Double Riveting and Steric Hindrance Strategy for Ultrahigh-Loading Atomically Dispersed Iron Catalysts Toward Oxygen Reduction

Atomically dispersed iron on nitrogen doped carbon displays high intrinsic activity toward oxygen reduction reaction, and has been identified as an attractive candidate to precious platinum catalysts. However, the loading of atomic iron sites is generally limited to below 4 wt% due to the undesired formation of iron-related particles at higher contents. Herein, this work overcomes this limit by a double riveting and steric hindrance strategy to achieve monodispersed iron with a high-loading of 12.8 wt%. Systematic study reveals that chemical riveting of atomic iron in ZIF-8 framework, chelation of Fe ions with interconfined 1,4-phenylenebisboronic, and physical hindrance are essential to obtain high-loading monodispersed Fe moieties. Resultantly, designed Fe-N-C-PDBA exhibits superior catalytic activity and excellent stability over commercial platinum catalysts toward oxygen reduction reaction in both half-cells and zinc-air fuel cells (ZAFCs). This provides an avenue for developing high-loading single-atom catalysts (SACs) for energy devices.

Nitrogen-doped hierarchical porous carbons derived from biomass for oxygen reduction reaction

Nowadays biomass has become important sources for the synthesis of different carbon nanomaterials due to their low cost, easy accessibility, large quantity, and rapid regeneration properties. Although researchers have made great effort to convert different biomass into carbons for oxygen reduction reaction (ORR), few of these materials demonstrated good electrocatalytical performance in acidic medium. In this work, fresh daikon was selected as the precursor to synthesize three dimensional (3D) nitrogen doped carbons with hierarchical porous architecture by simple annealing treatment and NH₃ activation. The daikon-derived material Daikon-NH₃-900 exhibits excellent electrocatalytical performance towards oxygen reduction reaction in both alkaline and acidic medium. Besides, it also shows good durability, CO and methanol tolerance in different electrolytes. Daikon-NH₃-900 was further applied as the cathode catalyst for proton exchange membrane (PEM) fuel cell and shows promising performance with a peak power density up to 245 W/g.

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.

Coupling Single-Ni-Atom with Ni-Co Alloy Nanoparticle for Synergistically Enhanced Oxygen Reduction Reaction

Developing nonprecious metal-based oxygen reduction reaction (ORR) electrocatalysts with superior activity and durability is crucial for commercializing proton-exchange membrane (PEM) fuel cells. Herein, we report a metal-organic framework (MOF)-derived unique N-doped hollow carbon structure (NiCo/hNC), comprising of atomically dispersed single-Ni-atom (NiN₄) and small NiCo alloy nanoparticles (NPs), for highly efficient and durable ORR catalysis in both alkaline and acidic electrolytes. Density functional theory (DFT) calculations reveal the strong coupling between NiN₄ and NiCo NPs, favoring the direct 4e^- transfer ORR process by lengthening the adsorbed O-O bond. Moreover, NiCo/hNC as a cathode electrode in PEM fuel cells delivered a stable performance. Our findings not only furnish the fundamental understanding of the structure-activity relationship but also shed light on designing advanced ORR catalysts.

A "Catalysis-Immobilization-Deposition" Stepwise Strategy toward Dynamic Equilibrium of Polysulfides Conversion and Diffusion in Lithium-Sulfur Batteries

Stepwise electrocatalysis can remarkably accelerate the kinetics of two consecutive reactions in sulfur electrochemistry. However, the significant difference between the catalysis and diffusion rates of polysulfides results in persistent shuttling in the stepwise electrocatalysts. Here, a stepwise electrocatalytic strategy of catalysis-immobilization-deposition is proposed for achieving the consistency of diffusion and catalysis of polysulfides. Accordingly, a sandwich-like stepwise electrocatalyst is designed, which is composed of Co nanoparticles (Co-NP), mesoporous SiO₂ , and iron single atom (Fe-SA) (denoted as Co-NP@SiO₂ @Fe-SA), serving as catalysis core, immobilization interlayer, and deposition shell, respectively. Benefitting from the dynamic equilibrium between production and consumption of polysulfides achieved by the spatial synergistic effect of the triple sites, the S/Co-NP@SiO₂ @Fe-SA cathode delivers a high reversible capacity of 731 mAh g^-1 over 500 cycles at 1 C with a small capacity decay of 0.039% per cycle. Moreover, a high areal capacity of 3.8 mAh cm^-2 at a sulfur loading of 4.5 mg cm^-2 is achieved with a low electrolyte/sulfur ratio of 5.9. This work sheds light on a new host design concept with high catalytic activity, stability, and selectivity to enable high performance lithium-sulfur batteries.

Recent Advances of Electrocatalyst and Cell Design for Hydrogen Peroxide Production

Electrochemical synthesis of H2O2 via a selective two-electron oxygen reduction reaction has emerged as an attractive alternative to the current energy-consuming anthraquinone process. Herein, the progress on electrocatalysts for H2O2 generation, including noble metal, transition metal-based, and carbon-based materials, is summarized. At first, the design strategies employed to obtain electrocatalysts with high electroactivity and high selectivity are highlighted. Then, the critical roles of the geometry of the electrodes and the type of reactor in striking a balance to boost the H2O2 selectivity and reaction rate are systematically discussed. After that, a potential strategy to combine the complementary properties of the catalysts and the reactor for optimal selectivity and overall yield is illustrated. Finally, the remaining challenges and promising opportunities for high-efficient H2O2 electrochemical production are highlighted for future studies.

Rational Fabrication of Defect-Rich and Hierarchically Porous Fe-N-C Nanosheets as Highly Efficient Oxygen Reduction Electrocatalysts for Zinc-Air Battery

The rational design of morphology and structure for oxygen reduction reaction (ORR) catalysts still remains a critical challenge. Herein, we successfully construct defect-rich and hierarchically porous Fe-N-C nanosheets (Fe-N-CNSs), by taking advantage of metal-organic complexation and a mesoporous template. Benefiting from the advantages of high density of active sites, fast mass transfer channels, and sufficient reaction area, the optimal Fe-N-CNSs demonstrate satisfactory ORR activity with an excellent half-wave potential of up to 0.87 V, desirable durability, and robust methanol tolerance. Noteworthy, the Fe-N-CNSs based zinc-air battery shows significant performance with a peak power density of 128.20 mW cm^-2 and open circuit voltage of 1.53 V, which reveals that the Fe-N-CNSs catalysts present promising practical application prospects. Therefore, we believe that this research will provide guidance for the optimization of Fe-N-C materials.

High-performance atomic Co/N co-doped porous carbon catalysts derived from Co-doped metal-organic frameworks for oxygen reduction

Improving the activity and durability of carbon-based catalysts is a key challenge for their application in fuel cells. Herein, we report a highly active and durable Co/N co-doped carbon (CoNC) catalyst prepared via pyrolysis of Co-doped zeolitic-imidazolate framework-8 (ZIF-8), which was synthesized by controlling the feeding sequence to enable Co to replace Zn in the metal-organic framework (MOF). The catalyst exhibited excellent oxygen reduction reaction (ORR) performance, while the half-wave potential decreased by only 8 mV after 5,000 accelerated stress test (AST) cycles in an acidic solution. Furthermore, the catalyst exhibited satisfactory cathodic catalytic performance when utilized in a hydrogen/oxygen single proton exchange membrane (PEM) fuel cell and a Zn-air battery, yielding maximum power densities of 530 and 164 mW cm^-2, respectively. X-ray absorption spectroscopy (XAS) and high-angle annular dark field-scanning transmission electron microscopy (HAAD-STEM) analyses revealed that Co was present in the catalyst as single atoms coordinated with N to form Co-N moieties, which results in the high catalytic performance. These results show that the reported catalyst is a promising material for inclusion into future fuel cell designs.

Atomic coordination structural dynamic evolution of single-atom Mo catalyst for promoting H2 activation in slurry phase hydrocracking

Development of efficient catalysts with high atomic utilization and turnover frequency (TOF) for H₂ activation in slurry phase hydrocracking (SPHC) is crucial for the conversion of vacuum residue (VR). Herein, for the first time, we reported a robust and stable single atoms (SAs) Mo catalyst through a polymerization-pyrolysis-in situ sulfurization strategy for activating H₂ in SPHC of VR. An interesting atomic coordination structural dynamic evolution of Mo active sites was discovered. During hydrocracking of VR, the O atoms that coordinated with Mo were gradually replaced by S atoms, which led to the O/S exchange process. The coordination structure of the Mo SAs changed from pre-reaction Mo-O₃S₁ to post-reaction Mo-O₁S₃ coordination configurations, promoting the efficient homolytic cleavage activation of H₂ into H radical species effectively. The evolved Mo SAs catalyst exhibited robust catalytic hydrogenation activity with the per pass conversion of VR of 65 wt%, product yield of liquid oils of 93 wt%, coke content of only 0.63 wt%, TOF calculated for total metals up to 0.35 s^-1, and good cyclic stability. Theoretical calculation reveals that the significant variation of occupied Mo 4d states before and after H₂ interaction has a direct bearing on the dynamic evolution of Mo SAs catalyst structure. The lower d-band center of Mo-O₁S₃ site indicates that atomic H diffusion is easy, which is conducive to catalytic hydrogenation. The finding of this study is of great significance to the development of high atom economy catalysts for the industrial application of heavy oil upgrading technology.

A General and Scalable Approach to Sulfur-Doped Mono-/Bi-/Trimetallic Nanoparticles Confined in Mesoporous Carbon

Metal nanoparticles confined in porous carbon materials have been widely used in various heterogeneous catalytic processes due to their enhanced activity and stability. However, fabrication of such catalysts in a facile and scalable way remains challenging. Herein, we report a general and scalable thiol-assisted strategy to synthesize sulfur-doped mono-/bi-/trimetallic nanoparticles confined in mesoporous carbon (S-M@MC, M = Pt, Pd, Rh, Co, Zn, etc.), involving only two synthetic steps, i.e., a hydrothermal process and pyrolysis. The strategy is based on coordination chemistry and hydro-phobic interaction that the metal precursors coordinated with the hydrophobic thiol ligands are located at the hydrophobic core of micelles, in situ confined in the hydrothermally prepared mesostructured polymer, and then converted into sulfur-doped metal nanoparticles confined in MC after pyrolysis. It is demonstrated that the S-PtCo@MC exhibits enhanced catalytic activity and improved durability toward acidic hydrogen evolution reaction due to the confinement effect and S-doping.

Role of the Support Effects in Single-Atom Catalysts

In recent years, single-atom catalysts (SACs) have received a significant amount of attention due to their high atomic utilization, low cost, high reaction activity, and selectivity for multiple catalytic reactions. Unfortunately, the high surface free energy of single atoms leads them easily migrated and aggregated. Therefore, support materials play an important role in the preparation and catalytic performance of SACs. Aiming at understanding the relationship between support materials and the catalytic performance of SACs, the support effects in SACs are introduced and reviewed herein. Moreover, special emphasis is placed on exploring the influence of the type and structure of supports on SAC catalytic performance through advanced characterization and theoretical research. Future research directions for support materials are also proposed, providing some insight into the design of SACs with high efficiency and high loading.

Carbon Nanocage with Maximum Utilization of Atomically Dispersed Iron as Efficient Oxygen Electroreduction Nanoreactor

As key parameters of electrocatalysts, the density and utilization of active sites determine the electrocatalytic performance toward oxygen reduction reaction. Unfortunately, prevalent oxygen electrocatalysts fail to maximize the utilization of active sites due to inappropriate nanostructural design. Herein, a nano-emulsion induced polymerization self-assembly strategy is employed to prepare hierarchical meso-/microporous N/S co-doped carbon nanocage with atomically dispersed FeN₄ (denoted as Meso/Micro-FeNSC). In situ scanning electrochemical microscopy technology reveals the density of available active sites for Meso/Micro-FeNSC reach to 3.57 × 10¹⁴ sites cm^-2 , representing more than threefold improvement compared to micropore-dominant Micro-FeNSC counterpart (1.07 × 10¹⁴ sites cm^-2 ). Additionally, the turnover frequency of Meso/Micro-FeNSC is also improved to 0.69 from 0.50 e^- site^-1 s^-1 for Micro-FeNSC. These properties motivate Meso/Micro-FeNSC as efficient oxygen electroreduction electrocatalyst, in terms of outstanding half-wave potential (0.91 V), remarkable kinetic mass specific activity (68.65 A g^-1 ), and excellent robustness. The assembled Zn-air batteries with Meso/Micro-FeNSC deliver high peak power density (264.34 mW cm^-2 ), large specific capacity (814.09 mA h g^-1 ), and long cycle life (>200 h). This work sheds lights on quantifying active site density and the significance of maximum utilization of active sites for rational design of advanced catalysts.

Linking Enhanced Kinetics of Electrocatalytic Oxygen Reduction Reaction with Increased Utilization of Active Sites in a Hierarchical Single-Atom Catalyst

Single-atom catalysts (SACs) are of tremendous current research due to maximized use of metal atoms and enhanced activity and selectivity for a great variety of chemical reactions. Hierarchically structured SACs have been explored to further increase the number and accessibility of active sites to realize the full potentials of SACs. However, though plausible-sounding, these supposed advantages of hierarchically structured SACs are largely untested. The assumed enhancing effects on the formation of intermediates on and the overall reaction kinetics remain largely unknown. Herein is reported a Fe-SAC with a hierarchical hollow structure (Fe/HH) that showed excellent activity in oxygen reduction reaction and proton exchange membrane fuel cell. Comparative experimental and computational studies with respect to Fe/SS-the counterpart of Fe/HH with a compact primary structure-reveal a significantly increased number of active sites and their utilization in Fe/HH as reflected by the facilitated formation of the rate-determining-step intermediate Fe-OOH*. This work thus establishes unambiguously the connection between the increased utilization of active sites and the enhanced kinetics of the electrocatalytic reduction of oxygen.

Three-Dimensional Fe Single-Atom Catalyst for High-Performance Cathode of Zn-Air Batteries

Designing cost-effective and highly active oxygen reduction reaction (ORR) catalysts is critical for the development of Zn-air batteries (ZABs). Iron-nitrogen-carbon (Fe-N-C) catalysts with single-atom Fe-N_x active sites are considered as one of the most promising alternatives to noble Pt but are hindered by unsatisfactory activity and durability. Herein, a NaCl template-assisted in situ pyrolysis technique is utilized to massively fabricate Fe-N-C single-atom catalysts (SACs) anchored on the three-dimensional open-pore carbon networks (denoted as 3D SAFe). The 3D SAFe catalyst exhibits ultrahigh activity with a half-wave potential of 0.90 V (vs RHE), benefiting from the enhanced mass diffusion and the increased amount of effective Fe-N₄ sites. Consequently, the ZABs assembled with 3D SAFe deliver high peak power density up to 156 mW cm^-2 and outstanding durability of 80 h, suggesting the application potential of the 3D SAFe catalyst. This work inspires the rational design and synthesis of highly efficient SACs for ZABs.