Heterogeneous Fe-N-C Cocatalyst for Hydroquinone Oxidation Enables Aerobic Oxidation of Primary Alcohols to Aldehydes

Benzoquinone (BQ) is commonly used as a cocatalyst in multi-component catalyst systems for aerobic oxidation of organic molecules. Such methods often proceed by a redox cascade in which a secondary catalyst supports aerobic oxidation of hydroquinone (HQ) to BQ. The present study shows that a commercially available non-precious metal heterogeneous catalyst, PAJ-Fe-N-C, is much more efficient than established homogeneous catalysts for aerobic oxidation of HQ to BQ, and this improved reactivity is crucial to support aerobic oxidation of primary alcohols to aldehydes with the homogeneous iridium catalyst, [Ir(trop2DAD]OTf (trop2DAD=N,N-bis(5-H-dibenzo[a,d]cycloheptene-5-yl)-1,4-diazabuta-1,3-diene). The combination of this heterogeneous/homogeneous catalyst system operates with Ir catalyst loadings as low as 0.05 mol % and turnover frequencies ≥350 h-1.

A Potassium-Doped Perovskite-Based Nanocomposite as an Efficient Bifunctional Oxygen Electrocatalyst for Rechargeable Zn-Air Batteries

Bifunctional oxygen electrocatalysts play a crucial role in the performance of rechargeable zinc-air batteries (ZABs), directly impacting key parameters such as capacity, round-trip efficiency, and durability. The ideal electrocatalysts for ZAB air electrodes must exhibit high catalytic activity for both oxygen reduction and oxygen evolution reactions in alkaline medium. This study presents a potassium-ion doping strategy to engineer the electron and defect structures of the perovskite oxide main phase, promoting phase separation to form a nanocomposite consisting of a perovskite phase and a secondary phase with an intergrowth structure. The resulting nanocomposite catalyst exhibits increased concentrations of Co3+ and oxygen vacancies, enhanced hydrophilicity, and improved adsorption of oxygen intermediates. As a result, the catalyst with the optimized composition demonstrates exceptional bifunctional activity and superior durability, leading to extended cycling stability and improved energy conversion efficiency in ZABs. Notably, it achieves a 42% increase in power density compared to the potassium-free pristine catalyst, a reduced voltage gap (ΔE = 0.83 V), and an extended cycle life of over 250 h. This work introduces a novel design paradigm for advanced metal-air battery catalysts through potassium-promoted defect-engineered heterostructure manipulation of perovskite oxides.

Synergistic Effects of Fe3O4/N-Doped Porous Carbon Nanospheres for Enhanced Oxygen Reduction Reaction

Designing transition metal oxide (TMO)/porous carbon composite materials for the oxygen reduction reaction (ORR) is a promising strategy in high-performance fuel cell technology. In this study, we used the isolation effect and pore-creating properties of Zn2+ to fabricate a composite material comprising ultrasmall Fe3O4 particles anchored on hierarchically N-doped porous carbon nanospheres. This material, referred to as CPZ1.0-Fe0.1, serves as an efficient ORR electrocatalyst in alkaline fuel cells. CPZ1.0-Fe0.1 exhibits outstanding ORR catalytic activity with a half-wave potential of 0.86 V in a 0.1 M KOH solution, surpassing that of the Pt/C catalyst (0.82 V). Furthermore, CPZ1.0-Fe0.1 exhibited excellent stability with minimal current degradation after 18 h of continuous testing. The superior ORR catalytic performance can be attributed to the synergistic effect between the catalytic sites and the high conductivity of porous carbon. The porous carbon nanospheres effectively address the low conductivity of Fe3O4 nanoparticles, while the ultra-small Fe3O4 nanoparticles anchored on the carbon surface provide efficient catalytic sites for the ORR.

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.

Heterogeneous Fe-N-C Catalyst for Aerobic Dehydrogenation of Hydrazones to Diazo Compounds Used for Carbene Transfer

Organic diazo compounds are versatile reagents in chemical synthesis and would benefit from improved synthetic accessibility, especially for larger scale applications. Here, we report a mild method for the synthesis of diazo compounds from hydrazones using a heterogeneous Fe-N-C catalyst, which has Fe ions dispersed within a graphitic nitrogen-doped carbon support. The reactions proceed readily at room temperature using O2 (1 atm) as the oxidant. Aryl diazoesters, ketones, and amides are accessible, in addition to less stable diaryl diazo compounds. Initial-rate data show that the Fe-N-C catalyst achieves faster rates than a heterogeneous Pt/C catalyst. The oxidative dehydrogenation of hydrazones may be performed in tandem with Rh-catalyzed enantioselective C-H insertion and cyclopropanation of alkenes, without requiring isolation of the diazo intermediate. This sequence is showcased by using a flow reactor for continuous synthesis of diazo compounds.

A polymer tethering strategy to achieve high metal loading on catalysts for Fenton reactions

The development of heterogenous catalysts based on the synthesis of 2D carbon-supported metal nanocatalysts with high metal loading and dispersion is important. However, such practices remain challenging to develop. Here, we report a self-polymerization confinement strategy to fabricate a series of ultrafine metal embedded N-doped carbon nanosheets (M@N-C) with loadings of up to 30 wt%. Systematic investigation confirms that abundant catechol groups for anchoring metal ions and entangled polymer networks with the stable coordinate environment are essential for realizing high-loading M@N-C catalysts. As a demonstration, Fe@N-C exhibits the dual high-efficiency performance in Fenton reaction with both impressive catalytic activity (0.818 min-1) and H2O2 utilization efficiency (84.1%) using sulfamethoxazole as the probe, which has not yet been achieved simultaneously. Theoretical calculations reveal that the abundant Fe nanocrystals increase the electron density of the N-doped carbon frameworks, thereby facilitating the continuous generation of long-lasting surface-bound •OH through lowering the energy barrier for H2O2 activation. This facile and universal strategy paves the way for the fabrication of diverse high-loading heterogeneous catalysts for broad applications.

Two-Dimensional Covalent Framework Derived Nonprecious Transition Metal Single-Atomic-Site Electrocatalyst toward High-Efficiency Oxygen Reduction

Designing an active, stable, and nonprecious metal catalyst substitute for Pt in the oxygen reduction reaction (ORR) is highly demanded for energy-efficient and cost-effective prototype devices. Single-atomic-site catalysts (SASCs) have been widely concerning because of their maximum atomic utilization and precise structural regulation. Despite being challenging, the controllable synthesis of SASCs is crucial for optimizing ORR activity. Here, we demonstrate an ultrathin organometallic framework template-assisted pyrolysis strategy to synthesize SASCs with a unique two-dimensional (2D) architecture. Electrochemical measurements revealed that Fe-SASCs displayed an excellent ORR activity in an alkaline media, having a half-wave potential and a diffusion-limited current density comparable to those of commercial Pt/C. Remarkably, the durability and methanol tolerance of Fe-SASCs were even superior to those of Pt/C. Furthermore, Fe-SASCs displayed a maximum power density of 142 mW cm^-2 with a current density of 235 mA cm^-2 as a cathode catalyst in a zinc-air battery, showing its great potential for practical applications.

Rational Design of Atomically Dispersed Metal Site Electrocatalysts for Oxygen Reduction Reaction

Future renewable energy supply and a cleaner Earth greatly depend on various crucial catalytic reactions for the society. Atomically dispersed metal site electrocatalysts (ADMSEs) have attracted tremendous research interest and are considered as the next-generation promising oxygen reduction reaction (ORR) electrocatalysts due to the maximum atom utilization efficiency, tailorable catalytic sites, and tunable electronic structures. Despite great efforts have been devoted to the development of ADMSEs, the systematic summary for design principles of high-efficiency ADMSEs is not sufficiently highlighted for ORR. In this review, the authors first summarize the fundamental ORR mechanisms for ADMSEs, and further discuss the intrinsic catalytic mechanism from the perspective of theoretical calculation. Then, the advanced characterization techniques to identify the active sites and effective synthesis methods to prepare catalysts for ADMSEs are also showcased. Subsequently, a special emphasis is placed on effective strategies for the rational design of the advanced ADMSEs. Finally, the present challenges to be addressed in practical application and future research directions are also proposed to overcome the relevant obstacles for developing high-efficiency ORR electrocatalysts. This review aims to provide a deeper understanding for catalytic mechanisms and valuable design principles to obtain the advanced ADMSEs for sustainable energy conversion and storage techniques.

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.

Single-Atom Iron Catalyst Based on Functionalized Mesophase Pitch Exhibiting Efficient Oxygen Reduction Reaction Activity

Designing highly efficient and low-cost electrocatalysts is of great importance in the fields of energy conversion and storage. We report on the facile synthesis of a single atom (SA) iron catalyst via the pyrolysis of a functionalized mesophase pitch. Monomers of naphthalene and indole underwent polymerization in the presence of iron chloride, which afterwards served as the pore-forming agent and iron source for the resulting catalyst. The SA-Fe@NC catalyst has a well-defined atomic dispersion of iron atoms coordinated by N-ligands in the porous carbon matrix, exhibiting excellent oxygen reduction reaction (ORR) activity (E1/2 = 0.89 V) that outperforms the commercial Pt/C catalyst (E1/2 = 0.84 V). Moreover, it shows better long-term stability than the Pt/C catalyst in alkaline media. This facile strategy could be employed in versatile fossil feedstock and develop promising non-platinum group metal ORR catalysts for fuel cell technologies.

Recent Progress on Fe-Based Single/Dual-Atom Catalysts for Zn-Air Batteries

As one of the most competitive candidates for large-scale energy storage, zinc-air batteries (ZABs) have attracted great attention due to their high theoretical specific energy density, low toxicity, high abundance, and high safety. It is highly desirable but still remains a huge challenge, however, to achieve cheap and efficient electrocatalysts to promote their commercialization. Recently, Fe-based single-atom and dual-atom catalysts (SACs and DACs, respectively) have emerged as powerful candidates for ZABs derived from their maximum utilization of atoms, excellent catalytic performance, and low price. In this review, some fundamental concepts in the field of ZABs are presented and the recent progress on the reported Fe-based SACs and DACs is summarized, mainly focusing on the relationship between structure and performance at the atomic level, with the aim of providing helpful guidelines for future rational designs of efficient electrocatalysts with atomically dispersed active sites. Finally, the great advantages and future challenges in this field of ZABs are also discussed.

Atomically Dispersed Iron Active Sites Promoting Reversible Redox Kinetics and Suppressing Shuttle Effect in Aluminum-Sulfur Batteries

Rechargeable aluminum-sulfur (Al-S) batteries have been considered as a highly potential energy storage system owing to the high theoretical capacity, good safety, abundant natural reserves, and low cost of Al and S. However, the research progress of Al-S batteries is limited by the slow kinetics and shuttle effect of soluble polysulfides intermediates. Herein, an interconnected free-standing interlayer of iron single atoms supported on porous nitrogen-doped carbon nanofibers (FeSAs-NCF) on the separator is developed and used as both catalyst and chemical barrier for Al-S batteries. The atomically dispersed iron active sites (Fe-N4) are clearly identified by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption near-edge structure. The Al-S battery with the FeSAs-NCF shows an improved specific capacity of 780 mAh g-1 and enhanced cycle stability. As evidenced by experimental and theoretical results, the atomically dispersed iron active centers on the separator can chemically adsorb the polysulfides and accelerate reaction kinetics to inhibit the shuttle effect and promote the reversible conversion between aluminum polysulfides, thus improving the electrochemical performance of the Al-S battery. This work provides a new way that can not only promote the conversion of aluminum sulfides but also suppress the shuttle effect in Al-S batteries.

Engineering at Subatomic Scale: Achieving Selective Catalytic Pathways via Tuning of the Oxidation States in Functionalized Single-Atom Quantum Catalysts

Regulating the catalytic pathways of single-atom sites in single atom catalysts (SACs) is an exciting debate at the moment, which has redirected the research towards understanding and modifying the single-atom catalytic sites through various strategies including altering the coordination environment of single atom for desirable outcomes as well as increasing their number. One useful aspect concerning the tunability of the catalytic pathways of SACs, which has been overlooked, is the oxidation state dynamics of the single atoms. In this study, iron single-atoms (FeSA) with variable oxidation states, dependent on the precursors, are harnessed inside a nitrogen-rich functionalized carbon quantum dots (CQDs) matrix via a facile one-step and low-temperature synthesis process. Dynamic electronic properties are imparted to the FeSAs by the simpler carbon dots matrix of CQDs in order to achieve the desired catalytic pathways of reactive oxygen species (ROS) generation in different environments, which are explored experimentally and theoretically for an in-depth understanding of the redox chemistry that drives the alternative catalytic pathways in FeSA@CQDs. These alternative and oxidation state-dependent catalytic pathways are employed for specific as well as cascade-like activities simulating natural enzymes as well as biomarkers for the detection of cancerous cells.

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

This study demonstrates a special ultrathin N-doped graphene nanomesh (NGM) as a robust scaffold for highly exposed Fe-N₄ active sites. Significantly, the pore sizes of the NGM can be elaborately regulated by adjusting the thermal exfoliation conditions to simultaneously disperse and anchor Fe-N₄ moieties, ultimately leading to highly loaded Fe single-atom catalysts (SA-Fe-NGM) and a highly exposed morphology. The SA-Fe-NGM is found to deliver a superior oxygen reduction reaction (ORR) activity in acidic media (half-wave potential = 0.83 V vs RHE) and a high power density of 634 mW cm^-2 in the H₂/O₂ fuel cell test. First-principles calculations further elucidate the possible catalytic mechanism for ORR based on the identified Fe-N₄ active sites and the pore size distribution analysis. This work provides a novel strategy for constructing highly exposed transition metals and nitrogen co-doped carbon materials (M-N-C) catalysts for extended electrocatalytic and energy storage applications.

Boosting Electrochemical Oxygen Reduction Performance of Iron Phthalocyanine through Axial Coordination Sphere Interaction

Precise regulation of the electronic states of catalytic sites through molecular engineering is highly desired to boost catalytic performance. Herein, a facile strategy was developed to synthesize efficient oxygen reduction reaction (ORR) catalysts, based on mononuclear iron phthalocyanine supported on commercially available multi-walled carbon nanotubes that contain electron-donating functional groups (FePc/CNT-R, with "R" being -NH₂ , -OH, or -COOH). These functional groups acted as axial ligands that coordinated to the Fe site, confirmed by X-ray photoelectron spectroscopy and synchrotron-radiation-based X-ray absorption fine structure. Experimental results showed that FePc/CNT-NH₂ , with the most electron-donating -NH₂ axial ligand, exhibited the highest ORR activity with a positive onset potential (E_onset =1.0 V vs. reversible hydrogen electrode) and half-wave potential (E_1/2 =0.92 V). This was better than the state-of-the-art Pt/C catalyst (E_onset =1.00 V and E_1/2 =0.85 V) under the same conditions. Overall, the functionalized FePc/CNT-R assemblies showed enhanced ORR performance in comparison to the non-functionalized FePc/CNT assembly. The origin of this behavior was investigated using density functional theory calculations, which demonstrated that the coordination of electron-donating groups to FePc facilitated the adsorption and activation of oxygen. This study not only demonstrates a series of advanced ORR electrocatalysts, but also introduces a feasible strategy for the rational design of highly active electrocatalysts for other proton-coupled electron transfer reactions.

Atomically Dispersed Cu Anchored on Nitrogen and Boron Codoped Carbon Nanosheets for Enhancing Catalytic Performance

Development of high-performance heterogeneous catalytic materials is important for the rapid upgrade of chemicals, which remains a challenge. Here, the benzene oxidation reaction was used to demonstrate the effectiveness of the atomic interface strategy to improve catalytic performance. The developed B,N-cocoordinated Cu single atoms anchored on carbon nanosheets (Cu₁/B-N) with the Cu-N₂B₁ atomic interface was prepared by the pyrolysis of a precoordinated Cu precursor. Benefiting from the unique atomic Cu-N₂B₁ interfacial structure, the designed Cu₁/B-N exhibited considerable activity in the oxidation of benzene, which was much higher than Cu₁/N-C, Cu NPs/N-C, and N-C catalysts. A theoretical study showed that the enhanced catalytic performance resulted from the optimized adsorption of intermediates, which originated from the manipulation of the electronic structure of Cu single atoms induced by B atom coordination in the Cu-N₂B₁ atomic interface. This study provides an innovative approach for the rational design of high-performance heterogeneous catalytic materials at the atomic level.

Compressive Strain Modulation of Single Iron Sites on Helical Carbon Support Boosts Electrocatalytic Oxygen Reduction

Designing and modulating the local structure of metal sites is the key to gain the unique selectivity and high activity of single metal site catalysts. Herein, we report strain engineering of curved single atomic iron-nitrogen sites to boost electrocatalytic activity. First, a helical carbon structure with abundant high-curvature surface is realized by carbonization of helical polypyrrole that is templated from self-assembled chiral surfactants. The high-curvature surface introduces compressive strain on the supported Fe-N₄ sites. Consequently, the curved Fe-N₄ sites with 1.5 % compressed Fe-N bonds exhibit downshifted d-band center than the planar sites. Such a change can weaken the bonding strength between the oxygenated intermediates and metal sites, resulting a much smaller energy barrier for oxygen reduction. Catalytic tests further demonstrate that a kinetic current density of 7.922 mA cm^-2 at 0.9 V vs. RHE is obtained in alkaline media for curved Fe-N₄ sites, which is 31 times higher than that for planar ones. Our findings shed light on modulating the local three-dimensional structure of single metal sites and boosting the catalytic activity via strain engineering.

Engineering of bionic Fe/Mo bimetallene for boosting the photocatalytic nitrogen reduction performance

The "FeMo cofactors" in biological nitrogenase play a decisive role in nitrogen reduction. Herein, a novel bionic Fe/Mo bimetallene was applied in photocatalytic nitrogen reduction. The surface coating Fe/Mo bimetallene of Bi₂Mo_0.3W_0.7O₆ (BMWO) nanocrystals could effectively promote the separation and transportation of photogenerated carriers by multi-electron redox reactions and deliver 2.8 times longer photo-carrier lifetime. Consequently, the nitrogen fixation activity of Fe/Mo bimetallene-coated BMWO nanocrystal photocatalyst was obviously enhanced (218.93 μmol g^-1h^-1), which was about 4.8 times that of unmodified BMWO nanocrystals. This work provides a novel approach to design bionic Fe/Mo bimetallene-modified inorganic semiconductor photocatalysts for nitrogen reduction.

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

The modulation effect has been widely investigated to tune the electronic state of single-atomic M-N-C catalysts to enhance the activity of oxygen reduction reaction (ORR). However, the in-depth study of modulation effect is rarely reported for the isolated dual-atomic metal sites. Now, the catalytic activities of Fe-N₄ moiety can be enhanced by the adjacent Pt-N₄ moiety through the modulation effect, in which the Pt-N₄ acts as the modulator to tune the 3d electronic orbitals of Fe-N₄ active site and optimize ORR activity. Inspired by this principle, we design and synthesize the electrocatalyst that comprises isolated Fe-N₄ /Pt-N₄ moieties dispersed in the nitrogen-doped carbon matrix (Fe-N₄ /Pt-N₄ @NC) and exhibits a half-wave potential of 0.93 V vs. RHE and negligible activity degradation (ΔE_1/2 =8 mV) after 10000 cycles in 0.1 M KOH. We also demonstrate that the modulation effect is not effective for optimizing the ORR performances of Co-N₄ /Pt-N₄ and Mn-N₄ /Pt-N₄ systems.

An Ion-Imprinting Derived Strategy to Synthesize Single-Atom Iron Electrocatalysts for Oxygen Reduction

Carbon-based single-atom catalysts (CSACs) have recently received extensive attention in catalysis research. However, the preparation process of CSACs involves a high-temperature treatment, during which metal atoms are mobile and aggregated into nanoparticles, detrimental to the catalytic performance. Herein, an ion-imprinting derived strategy is proposed to synthesize CSACs, in which isolated metal-nitrogen-carbon (Me-N₄ -C_x ) moiety covalently binds oxygen atoms in Si-based molecular sieve frameworks. Such a feature makes Me-N₄ -C_x moiety well protected/confined during the heat treatment, resulting in the final material enriched with single-atom metal active sites. As a proof of concept, a single-atom Fe-N-C catalyst is synthesized by using this ion-imprinting derived strategy. Experimental results and theoretical calculations demonstrate high concentration of single FeN₄ active sites distributed in this catalyst, resulting in an outstanding oxygen reduction reaction (ORR) performance with a half-wave potential of 0.908 V in alkaline media.

Rational Fabrication of Low-Coordinate Single-Atom Ni Electrocatalysts by MOFs for Highly Selective CO2 Reduction

Single-atom catalysts (SACs) have attracted tremendous interests due to their ultrahigh activity and selectivity. However, the rational control over coordination microenvironment of SACs remains a grand challenge. Herein, a post-synthetic metal substitution (PSMS) strategy has been developed to fabricate single-atom Ni catalysts with different N coordination numbers (denoted Ni-N_x -C) on pre-designed N-doped carbon derived from metal-organic frameworks. When served for CO₂ electroreduction, the obtained Ni-N₃ -C catalyst achieves CO Faradaic efficiency (FE) up to 95.6 %, much superior to that of Ni-N₄ -C. Theoretical calculations reveal that the lower Ni coordination number in Ni-N₃ -C can significantly enhance COOH* formation, thereby accelerating CO₂ reduction. In addition, Ni-N₃ -C shows excellent performance in Zn-CO₂ battery with ultrahigh CO FE and excellent stability. This work opens up a new and general avenue to coordination microenvironment modulation (MEM) of SACs for CO₂ utilization.

Transition Metal and Nitrogen Co-Doped Carbon-based Electrocatalysts for the Oxygen Reduction Reaction: From Active Site Insights to the Rational Design of Precursors and Structures

Considering the urgent requirement for clean and sustainable energy, fuel cells and metal-air batteries have emerged as promising energy storage and conversion devices to alleviate the worldwide energy challenges. The key step in accelerating the sluggish oxygen reduction reaction (ORR) kinetics at the cathode is to develop cost-effective and high-efficiency non-precious metal catalysts, which can be used to replace expensive Pt-based catalysts. Recently, the transition metal and nitrogen co-doped carbon (M-N_x /C) materials with tailored morphology, tunable composition, and confined structure show great potential in both acidic and alkaline media. Herein, the mechanism of ORR is provided, followed by recent efforts to clarify the actual structures of active sites. Furthermore, the progress of optimizing the catalytic performance of M-N_x /C catalysts by modulating nitrogen-rich precursors and porous structure engineering is highlighted. The remaining challenges and development prospects of M-N_x /C catalysts are also outlined and evaluated.

Ink-Assisted Synthetic Strategy for Stable and Advanced Composite Electrocatalysts with Single Fe Sites

The oxygen evolution reaction is critical to the efficiency of many energy technologies that store renewable electricity in chemical form. However, the rational design of high-performance and stable catalysts to drive this reaction remains a formidable challenge. Here, a facile ink-assisted strategy to construct a series of stable and advanced composite electrocatalysts with single Fe sites for permitting seriously improved performance characteristics is reported. As revealed by a suit of characterization techniques and theoretical methods, the improved electrocatalytic performance and stability can be attributed to the unique coordination states of Fe in the form of distorted FeO₄ C and the interfacial effect in the composite system that optimize and stabilize single Fe sites in changing to better configurations for intermediates adsorption. The findings provide a novel strategy to in-depth understanding of practical guidelines for the electrocatalyst design for energy conversion devices.

Porous Organic Polymers as Fire-Resistant Additives and Precursors for Hyperporous Carbon towards Oxygen Reduction Reactions

Cyclotriphosphazene (CP) based porous organic polymers (POPs) have been designed and prepared. The introduction of CP into the porous skeleton endowed special thermal stability and outstanding flame retardancy to prepared polymers. The nonflammable level of PNK-CMP fabricated via the condensation of 2,2'-(1,4-phenylene)diacetonitrile (DAN) and hexakis(4-acetylphenoxy)cyclotriphosphazene (HACTP) through Knoevenagel reaction, in vertical burning tests reached V-2 class (UL-94) and the limiting oxygen index (LOI) reached 20.8 %. When used as additive, PNK-CMP could suppress the dissolving out of PEPA effectively, reducing environment pollution and improving the flame retardant efficiency. The POP and PEPA co-added PU (mPOP%: mPEPA%=5.0 %: 5.0 %) could not be ignited under simulated real-scale fire conditions. The nonflammable level of POP/PEPA/PU in vertical burning tests (UL-94) reached V-0 class with a LOI as high as 23.2 %. The smoke emission could also be suppressed, thus reducing the potential for flame spread and fire hazards. Furthermore, carbonization of PNK-CMP under the activation of KOH yield a hyperporous carbon (PNKA-800) with ultrahigh BET surface area (3001 m2 g-1) and ultramicropore size showing excellent ORR activity in alkaline conditions.

Preparation of Iron- and Nitrogen-Codoped Carbon Nanotubes from Waste Plastics Pyrolysis for the Oxygen Reduction Reaction

A novel method for the preparation of iron- and nitrogen-codoped carbon nanotubes (Fe-N-CNTs) is proposed, based on the catalytic pyrolysis of waste plastics. First, carbon nanotubes are produced from pyrolysis of plastic waste over Fe-Al₂ O₃ ; then, Fe-CNTs and melamine are heated together in an inert atmosphere. Different co-pyrolysis temperatures are tested to optimize the electrocatalyst production. A high doping temperature improves the degree of graphite formation and promotes the conversion of nitrogen into a more stable form. Compared with commercial Pt/C, the electrocatalyst obtained from pyrolysis at 850 °C shows remarkable properties, with an onset potential of 0.943 V versus RHE and a half-wave potential of 0.811 V versus RHE, and even better stability and anti-poisoning properties. In addition, zinc-air battery tests are performed, and the optimized catalyst exhibits a high maximum power density.

Rational Design of Hierarchical, Porous, Co-Supported, N-Doped Carbon Architectures as Electrocatalyst for Oxygen Reduction

Developing highly active nonprecious-metal catalysts for the oxygen reduction reaction (ORR) is of great significance for reducing the cost of fuel cells. 3D-ordered porous structures could substantially improve the performance of the catalysts because of their excellent mass-diffusion properties and high specific surface areas. Herein, ordered porous ZIF-67 was prepared by forced molding of a polystyrene template, and Co-supported, N-doped, 3D-ordered porous carbon (Co-NOPC) was obtained after further carbonization. Co-NOPC exhibited excellent performance for the ORR in an alkaline medium with a half-wave potential of 0.86 V vs. reversible hydrogen electrode (RHE), which is higher than that of the state-of-the-art Pt/C (0.85 V vs. RHE). Moreover, the substantially improved catalytic performance of Co-NOPC compared with Co-supported, N-doped carbon revealed the key role of its hierarchical porosity in boosting the ORR. Co-NOPC also exhibited a close-to-ideal four-electron transfer path, long-term durability, and resistance to methanol penetration, which make it promising for large-scale application.

Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction

Metal-air batteries (MABs) and reversible fuel cells (RFCs) rely on the bifunctional oxygen catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Finding efficient bifunctional oxygen catalysts is the ultimate goal and it has attracted a great deal of attention. The dilemma is that a good ORR catalyst is not necessarily efficient for OER, and vice versa. Thus, the development of a new type of bifunctional oxygen catalysts should ensure that the catalysts exhibit high activity for both OER and ORR. Composites with multicomponents for active centers supported on highly conductive matrices could be able to meet the challenges and offering new opportunities. In this Review, the evolution of bifunctional catalysts is summarized and discussed aiming to deliver high-performance bifunctional catalysts with low overpotentials.