In Situ Self-Template Synthesis of Fe-N-Doped Double-Shelled Hollow Carbon Microspheres for Oxygen Reduction Reaction

Engineering of single atomic Fe-N4 sites on hollow carbon cages to achieve highly reversible MoS2 anodes for Li-ion batteries

Although the single atom electrocatalysts have been demonstrated as efficient catalysts for promoting Li2S/Na2S formation and decomposition in Li-S/Na-S batteries, the functional morphological and structural engineering capable of exposing more active sites is regarded as an essential factor to further enhance the catalytic activity. Here, we have synthesized a single atomically dispersed Fe sites embedded within hollow nitrogen doped carbon cages (Fe-N-HCN) using Fe3O4 spheres as an oxidant and sacrificial template, which is used as a high-efficiency catalyst for boosting the reversible capacity of MoS2 anode in lithium-ion batteries (LIBs). As expected, the electrochemical reaction of MoS2/Fe-N-HCN anode exhibits higher reversibility than pure MoS2 electrodes. Moreover, density functional theory is also employed to reveal that Fe-N-HCN can be effectively adsorbed and catalyze the rapid decomposition of Li2S. The hollow carbon cage structure can facilitate the exposure of the active Fe-N4 sites and favor the mass transfer during the electrochemical reactions, thus the synergistic effect of the Fe-N4 site and the hollow carbon cage structure together improve the catalytic activity for the conversion reaction of MoS2 anode.

ZIF-Co3O4@ZIF-Derived Urchin-Like Hierarchically Porous Carbon as Efficient Bifunctional Oxygen Electrocatalysts

Co3O4 nanoparticles were sandwiched into interlayers between ZIF-8 and ZIF-67 to form ZIF-Co3O4@ZIF precursors. Pyrolysis of ZIF-Co3O4@ZIF yielded an urchin-like hierarchically porous carbon (Co@CNT/NC), the thorns of which were carbon nanotubes embedded Co nanoparticles. With large specific surface area and hierarchically porous structure, as-prepared Co@CNT/NC exhibited excellent bifunctional oxygen electrocatalytic performances. It has good ORR performance with E1/2 of 0.85 V, which exceeds the Pt/C half-wave potential (E1/2=0.83 V). In addition, Co@CNT/NC has an OER performance close to that of RuO2. To further demonstrate the effect of Co modifying on the properties, the samples were subjected to acid washing treatment. Co-based nanoparticles were proved to After acid washing, there was obvious loss of Co particles in Co@CNT/NC, resulting in poor oxygen electrocatalysis. So, the pyrolysis products of ZIF-8-Co3O4@ZIF-67 retained large specific surface area and porous structure can be retained, and on the other hand, the carbon tube structure and original polyhedron framework. Besides, existence of Co nanoparticle@carbon nanotube provided more active sites and improved the ORR and OER performances.

Biomineralization of Sulfate-Reducing Bacteria In Situ-Induced Preparation of Nano Fe2O3-Fe(Ni)S/C as High-Efficiency Oxygen Evolution Electrocatalyst

Transition metal-based catalysts possess high catalytic activity for oxygen evolution reaction (OER). However, the preparation of high-performance OER electrocatalysts using simple strategies with a low cost still faces a major challenge. Herein, this work presents an innovative, in situ-induced preparation of the Fe2O3, FeS, and NiS nanoparticles, supported on carbon blacks (CBs) (denoted as Fe2O3-Fe(Ni)S/C) as a high-efficiency oxygen evolution electrocatalyst by employing biomineralization. Biomineralization, a simple synthesis strategy, demonstrates a huge advantage in controlling the size of the Fe2O3 and Fe(Ni)S nanoparticles, as well as achieving uniform nanoparticle distribution on carbon blacks. It is found that the electrocatalyst Fe2O3-Fe(Ni)S/C-200 shows a good OER electrocatalytic activity with a small loading capacity, and it has a small overpotential and Tafel slope in 1 m KOH solution with values of 264 mV and 42 mV dec-1, respectively, at a current density of 10 mA cm-2. Additionally, it presents good electrochemical stability for over 24 h. The remarkable and robust electrocatalytic performance of Fe2O3-Fe(Ni)S/C-200 is attributed to the synergistic effect of Fe2O3, FeS, and doped-Ni species as well as its distinct 3D spherical structure. This approach indicates the promising applications of biomineralization for the bio-preparation of functional materials and energy conversion.

Concave Structural Carbon Co-Doped with Iron Atom Pairs and Nitrogen as Ultra-High Performance Catalyst Toward Oxygen Reduction

It is crucial to rationally design and synthesize atomic-scale transition metal-doped carbon catalysts with high electrocatalytic activity to achieve a high-efficient oxygen reduction reaction (ORR). Herein, an electrocatalyst comprised of Fe-Fe dual atom pairs and N-doped concave carbon are reported (N-CC@Fe DA) that achieves ultrahigh electrocatalytic ORR activity. The catalyst is prepared by a gaseous doping approach, with zeolitic imidazolate framework-8 (ZIF-8) as the carbon framework precursor and cyclopentadienyliron dicarbonyl dimer as the Fe-Fe atom pair precursor. The catalyst exhibits high cathodic ORR catalytic performance in an alkaline Zn/air battery and proton exchange membrane fuel cell (PEMFC), yielding peak power densities of 241 mW cm-2 and 724 mW cm-2, respectively, compared to 127 mW cm-2 and 1.20 W cm-2 with conventional Pt/C catalysts as cathodes. The presence of Fe atom pairs coordinate with N atoms is revealed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analysis, and Density Functional Theory (DFT) calculation results show that the Fe-Fe pair structure is beneficial for adsorbing oxygen molecules, activating the O─O bond, and desorbing OH* intermediates formed during oxygen reduction, resulting in a more efficient oxygen reaction. The findings may provide a new pathway for preparing ultra-high-performance doped carbon catalysts with Fe-Fe atom pair structures.

Fundamental understanding of nitrogen in biomass electrocatalysts for oxygen reduction and zinc-air batteries

Exploring high-efficiency catalysts for oxygen reduction reactions (ORRs) is essential for the development of large-scale applications of fuel cell and metal-air batteries technology. The as-prepared Fe-NC-800 via polymerization-pyrolysis strategy exhibited superior ORR activity with onset potential of 1.030 V vs. reversible hydrogen electrode (RHE) and half-wave potential of 0.908 V vs. RHE, which is higher than that of the Pt/C catalyst and most of other Fe-based catalysts. The different d-band center values can be attributed to the influence of different N-doped carbon, leading to the adjustment in the ORR activity. In addition, Fe-NC-800-based Zn-air battery showed better electrochemical performance with a high discharge specific capacity of 806 mA h g-1 and a high-power density of 220 mW cm-2 than that of the Pt/C-based battery. Therefore, the biomass Fe-NC-800 catalyst may become a promising substitute for Pt/C catalysts in energy storage and conversion devices.

Bionic Fe-N-C catalyst with abundant exposed Fe-Nx sites and enhanced mass transfer properties for efficient oxygen reduction

Transition metal and nitrogen co-doped carbon electrocatalysts are promising candidates to replace the precious metal platinum (Pt) in oxygen reduction reactions (ORR). Unfortunately, the electrochemical performance of existing electrocatalysts is restricted due to limited accessibility of active sites. Inspired by jellyfish tentacles, we design an efficient ORR micro-reactor called Fe-Nx/HC@NWs. It features abundant exposed Fe-Nx active sites dispersed on nitrogen-doped cubic carbon cages, which have a hierarchically porous and hairy structure. The accessible, atomically dispersed Fe-Nx sites and the elaborate substrate architecture synergize to provide the catalyst withremarkable ORR catalytic activity, extraordinary long-term stability, and favorable methanol tolerance in an alkaline electrolyte; overall, its performance is comparable to that of commercial carbon-supported Pt. Our synthesis is facile and controllable, paving a new avenue toward advanced non-precious metal-based electrocatalysts for energy storage and conversion.

Porous polypyrrole with a vesicle-like structure for efficient removal of per- and polyfluoroalkyl substances from water: Crucial role of porosity and morphology

Herein, a vesicle-like and porous polypyrrole (pPPy) was fabricated by in suit self-template method to efficiently capture per- and polyfluoroalkyl substances (PFASs) and the important role of porosity and morphology in PFAS removal was explored. Compared to solid PPy (sPPy), the porosity and vesicle-like morphology of pPPy endowed it with excellent properties such as large specific surface area (108.9 m2/g vs. 22.3 m2/g), suitable pore sizes (17.4 nm), dispersity, and high hydrophilicity, which facilitated mass transfer and enhanced PFAS sorption performance. The estimated sorption capacities of pPPy for perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS) were 509 mg/g and 532 mg/g, respectively, which were ∼2 times higher than sPPy. Furthermore, pPPy demonstrated PFAS removal of ≥ 90% across a wide pH range (3-9) and varying humic acid concentrations (0-50 mg/L). In actual water matrices, pPPy efficiently removed 12 short-chain (C-F number: 3-6) and long-chain PFASs (>90% removal for major PFASs), outperforming sPPy by ∼1.2-2.5 times. Notably, the enlarged porosity and regular morphology of pPPy significantly enhanced the removal of short-chain PFASs by ∼2 times. The spent pPPy could be regenerated and reused over 5 times. This research provides valuable insights for designing efficient PFAS sorbents by emphasizing control over porosity and morphology.

Nanostructured Conducting Polymers and Their Applications in Energy Storage Devices

Due to the energy requirements for various human activities, and the need for a substantial change in the energy matrix, it is important to research and design new materials that allow the availability of appropriate technologies. In this sense, together with proposals that advocate a reduction in the conversion, storage, and feeding of clean energies, such as fuel cells and electrochemical capacitors energy consumption, there is an approach that is based on the development of better applications for and batteries. An alternative to commonly used inorganic materials is conducting polymers (CP). Strategies based on the formation of composite materials and nanostructures allow outstanding performances in electrochemical energy storage devices such as those mentioned. Particularly, the nanostructuring of CP stands out because, in the last two decades, there has been an important evolution in the design of various types of nanostructures, with a strong focus on their synergistic combination with other types of materials. This bibliographic compilation reviews state of the art in this area, with a special focus on how nanostructured CP would contribute to the search for new materials for the development of energy storage devices, based mainly on the morphology they present and on their versatility to be combined with other materials, which allows notable improvements in aspects such as reduction in ionic diffusion trajectories and electronic transport, optimization of spaces for ion penetration, a greater number of electrochemically active sites and better stability in charge/discharge cycles.

Using dopamine interlayers to construct Fe/Fe3C@FeNC microspheres of high N-content for bifunctional oxygen electrocatalysts of Zn-air batteries

High activity bifunctional oxygen electrocatalysts are crucial for the development of high performing Zn-air batteries. Fe-N-C systems decorated with Fe/Fe₃C nanoparticles have been identified as prospective candidates in which almost all the active sites need the presence of N. To anchor more N, an Fe₂O₃ microsphere template was covered by a thin layer of polymerized dopamine (PDA) before it was mixed with a high N-content source of g-C₃N₄. The PDA interlayer not only provides a part of C and N but also serves as a buffer agent to hinder fast reactions between Fe₂O₃ and g-C₃N₄ during pyrolysis to avoid the destruction of the microsphere template. The prepared Fe/Fe₃C@FeNC catalyst showed superior electrochemical performance, achieving a high half-wave potential of 0.825 V for ORR and a low overpotential of 1.450 V at 10 mA cm^-2 for OER. The rechargeable Zn-air battery assembled with the as-obtained Fe/Fe₃C@FeNC catalyst as a cathode offered a high peak energy density of 134.6 mW cm^-2, high specific capacity of 856.2 mA h g_Zn^-1 and excellent stability over 180 h at 5 mA cm^-2 (10 min per cycle) with a small charge/discharge voltage gap of ∼0.851 V. This work presents a practical strategy for constructing nitrogen-rich catalysts with stable 3D structures.

Nanosized FeS/ZnS heterojunctions derived using zeolitic imidazolate Framework-8 (ZIF-8) for pH-universal oxygen reduction and High-efficiency Zn-air battery

Low-cost, stable, and highly active electrocatalysts for oxygen reduction reaction (ORR), especially for pH-universal ORR, are vital for developing numerous renewable energy devices. Herein, a hierarchical N, S-codoped porous carbon-based catalyst (ZFP-800) coupled with abundant FeS/ZnS heterojunctions was facilely prepared via direct pyrolysis of a Ferrocene-crosslinked pyrrole hydrogel composited with zeolitic imidazolate framework-8 (ZIF-8) templates. Compared with the heterojunction-free catalytic activity, the ZFP-800 catalytic activity was significantly higher in pH-universal ranges. Moreover, the ZFP-800 exhibited competitive ORR performance to commercial Pt/C (20%) in various electrolytes, in terms of onset (E_onset), half-wave potentials (E_1/2), limiting current density (J_L), durability, and methanol immunity. For instance, it exhibited super ORR catalytic activity on E_onset and E_1/2, and exceeded that of the benchmark Pt/C in both the alkaline and neutral media. Furthermore, the application of ZFP-800 as a cathode catalyst in a home-made Zn-air battery demonstrated its operation capability in ambient conditions with a competitive performance on the specific energy density (828 mA·h·g_Zn^-1), maximum discharge power density (205.6 mW·cm^-2), rate performance, and the long-term stability (188 h at 5 mA·cm^-2). This study can facilitate the development of advanced heterojunction-based materials for renewable energy applications.

Ascorbic acid-modified dual-metal–organic-framework derived C-Fe/Fe3O4 loaded on a N-doped graphene framework for enhanced electrocatalytic oxygen reduction

A three-dimensional conductive network-based carbon nanostructured electrocatalyst for the ORR.

Stabilizing Fe-N-C Catalysts as Model for Oxygen Reduction Reaction

The highly efficient energy conversion of the polymer-electrolyte-membrane fuel cell (PEMFC) is extremely limited by the sluggish oxygen reduction reaction (ORR) kinetics and poor electrochemical stability of catalysts. Hitherto, to replace costly Pt-based catalysts, non-noble-metal ORR catalysts are developed, among which transition metal-heteroatoms-carbon (TM-H-C) materials present great potential for industrial applications due to their outstanding catalytic activity and low expense. However, their poor stability during testing in a two-electrode system and their high complexity have become a big barrier for commercial applications. Thus, herein, to simplify the research, the typical Fe-N-C material with the relatively simple constitution and structure, is selected as a model catalyst for TM-H-C to explore and improve the stability of such a kind of catalysts. Then, different types of active sites (centers) and coordination in Fe-N-C are systematically summarized and discussed, and the possible attenuation mechanism and strategies are analyzed. Finally, some challenges faced by such catalysts and their prospects are proposed to shed some light on the future development trend of TM-H-C materials for advanced ORR catalysis.

Single-Atom-like B-N3 Sites in Ordered Macroporous Carbon for Efficient Oxygen Reduction Reaction

On the premise of cleanliness and stability, improving the catalytic efficiency for the oxygen reduction reaction in the electrode reaction of fuel cells and metal-air batteries is of vital importance. Studies have shown that heteroatom doping and structural optimization are efficient strategies. Herein, a single-atom-like B-N₃ configuration in carbon is designed for efficient oxygen reduction reaction catalysis inspired by the extensively studied transition metal M-N_x sites, which is supported on the ordered macroporous carbon prepared by utilizing a hydrogen-bonded organic framework as carbon and nitrogen sources and SiO₂ spheres as a template. The co-doping of B/N and ordered macroporous structures promote the metal-free material high oxygen reduction catalytic performance in alkaline media. DFT calculations reveal that the B-N₃ structure played a key role in enhancing the oxygen reduction activity by providing rich favorable *OOH and *OH adsorption sites on the B center. The promoted formation of *OH/*OOH intermediates accelerated the electrocatalyst reaction. This study provides new insights into the design of single-atom-like nonmetallic ORR electrocatalysts and synthesis of ordered macroporous carbons based on hydrogen-bonded organic frameworks.

Fe, B, and N Codoped Carbon Nanoribbons Derived from Heteroatom Polymers as High-Performance Oxygen Reduction Reaction Electrocatalysts for Zinc-Air Batteries

For zinc-air batteries, it is of great importance to heighten the oxygen reduction reaction (ORR) activity of cathode electrocatalysts. Herein, we synthesized carbon nanoribbons doped with Fe, B, and N as high-activity ORR electrocatalysts by a templating method. Benefiting from the melamine fiber (MF) and B doping, the as-prepared carbon nanoribbon has a high specific surface area, and the improved turnover frequency of Fe sites increases the ORR activity. The as-synthesized Fe-B-N-C electrocatalyst shows an improved half-wave potential and limited current density compared to Fe-N-C, B-N-C, and N-C. Moreover, zinc-air batteries with the Fe-B-N-C electrocatalyst exhibit a higher specific capacity and better long-term durability compared to those with commercial Pt/C. This work provides an effective strategy to synthesize noble-metal-free electrocatalysts for wide applications of zinc-air batteries.

Heterostructural Interface in Fe3C-TiN Quantum Dots Boosts Oxygen Reduction Reaction for Al-Air Batteries

Oxygen reduction electrocatalysts play important roles in metal-air batteries. Herein, Fe₃C-TiN heterostructural quantum dots loaded on carbon nanotubes (FCTN@CNTs) are prepared as electrocatalysts for the oxygen reduction reaction (ORR) through a one-pot pyrolysis. The Fe₃C-TiN quantum dots with a diameter of 2-5 nm show the unique characteristic of heterostructural interface. The as-prepared FCTN@CNTs display Pt/C comparable ORR performance (E_onset 1.06 and E_1/2 0.95 V) in alkaline medium, which is ascribed to the heterostructural interface between TiN and Fe₃C. Furthermore, the Al-air batteries with the FCTN@CNT catalyst display superior discharge performance, demonstrating good feasibility for practical application. This work provides an effective new method to synthesize affordable and efficient oxygen reduction reaction catalysts.

Simultaneously Engineering the Coordination Environment and Pore Architecture of Metal-Organic Framework-Derived Single-Atomic Iron Catalysts for Ultraefficient Oxygen Reduction

Designing highly efficient and durable electrocatalysts that accelerate sluggish oxygen reduction reaction kinetics for fuel cells and metal-air batteries are highly desirable but challenging. Herein, a facile yet robust strategy is reported to rationally design single iron active centers synergized with local S atoms in metal-organic frameworks derived from hierarchically porous carbon nanorods (Fe/N,S-HC). The cooperative trithiocyanuric acid-based coating not only introduces S atoms that regulate the coordination environment of the active centers, but also facilitates the formation of a hierarchically porous structure. Benefiting from electronic modulation and architectural functionality, Fe/N,S-HC catalyst shows markedly enhanced ORR performance with a half-wave potential (E_1/2 ) of 0.912 V and satisfactory long-term durability in alkaline medium, outperforming those of commercial Pt/C. Impressively, Fe/N,S-HC-based Zn-air battery also presents outstanding battery performance and long-term stability. Both electrochemical experimental and density functional theoretical (DFT) calculated results suggest that the FeN₄ sites tailored with local S atoms are favorable for the adsorption/desorption of oxygen intermediate, resulting in lower activation energy barrier and ultraefficient oxygen reduction catalytic activity. This work provides an atomic-level combined with porous morphological-level insights into oxygen reduction catalytic property, promoting rational design and development of novel highly efficient single-atom catalysts for the renewable energy applications.

Encapsulating Cobalt Nanoparticles in Interconnected N-Doped Hollow Carbon Nanofibers with Enriched CoNC Moiety for Enhanced Oxygen Electrocatalysis in Zn-Air Batteries

Rational design of bifunctional efficient electrocatalysts for both oxygen reduction (ORR) and oxygen evolution reactions (OER) is desirable-while highly challenging-for development of rechargeable metal-air batteries. Herein, an efficient bifunctional electrocatalyst is designed and fabricated by encapsulating Co nanoparticles in interconnected N-doped hollow porous carbon nanofibers (designated as Co@N-C/PCNF) using an ultrafast high-temperature shock technology. Benefiting from the synergistic effect and intrinsic activity of the CoNC moiety, as well as porous structure of carbon nanofibers, the Co@N-C/PCNF composite shows high bifunctional electrocatalytic activities for both OER (289 mV at 10 mA cm-2 ) and ORR (half-wave potential of 0.85 V). The CoNC moiety in the composite can modulate the local environmental and electrical structure of the catalysts, thus optimizing the adsorption/desorption kinetics and decreasing the reaction barriers for promoting the reversible oxygen electrocatalysis. Co@N-C/PCNF-based aqueous Zn-air batteries (AZAB) provide high power density of 292 mW cm-2 , and the assembled flexible ZAB can power wearable devices.

Activity Promotion of Core and Shell in Multifunctional Core-Shell Co2 P@NC Electrocatalyst by Secondary Metal Doping for Water Electrolysis and Zn-Air Batteries

Developing cost-efficient multifunctional electrocatalysts is highly critical for the integrated electrochemical energy-conversion systems such as water electrolysis based on hydrogen/oxygen evolution reactions (HER/OER) and metal-air batteries based on OER/oxygen reduction reactions (ORR). The core-shell structured materials with transition metal phosphide as the core and nitrogen-doped carbon (NC) as the shell have been known as promising HER electrocatalysts. However, their oxygen-related electrocatalytic activities still remain unsatisfactory, which severely limits their further applications. Herein an effective strategy to improve the core and shell performances of core-shell Co₂ P@NC electrocatalysts through secondary metal (e.g., Fe, Ni, Mo, Al, Mn) doping (termed M-Co₂ P@M-N-C) is reported. The as-synthesized M-Co₂ P@M-N-C electrocatalysts show multifunctional HER/OER/ORR activities and good integrated capabilities for overall water splitting and Zn-air batteries. Among the M-Co₂ P@M-N-C catalysts, Fe-Co₂ P@Fe-N-C electrocatalyst exhibits the best catalytic activities, which is closely related to the configuration of highly active species (Fe-doping Co₂ P core and Fe-N-C shell) and their subtle synergy, and a stable carbon shell for outstanding durability. Combination of electrochemical-based in situ Fourier transform infrared spectroscopy with extensive experimental investigation provides deep insights into the origin of the activity and the underlying electrocatalytic mechanisms at the molecular level.

Fe-Ni Alloy Nanoclusters Anchored on Carbon Aerogels as High-Efficiency Oxygen Electrocatalysts in Rechargeable Zn-Air Batteries

In this work, Fe-Ni alloy nanoclusters (Fe-Ni ANCs) anchored on N, S co-doped carbon aerogel (Fe-Ni ANC@NSCA catalysts) are successfully prepared by the optimal pyrolysis of polyaniline (PANI) aerogels derived from the freeze-drying of PANI hydrogel obtained by the polymerization of aniline monomers in the co-presence of tannic acid (TA), Fe³⁺ , and Ni²⁺ ions. In addition, the optimal molar ratio of the TA, Fe³⁺ , and Ni²⁺ ions for synthesis of Fe-Ni ANC@NSCA catalysts are 1:2:5, which can guarantee the formation of carbon aerogel composed of quasi-2D porous carbon sheets and the formation of high-density Fe-Ni ANCs with an ultrasmall size between 2 to 2.8 nm. These Fe-Ni ANCs consisting of N₄ -Fe-O-Ni-N₄ moiety are proposed as a new type of active species for the first time, to the best of the authors' knowledge. Thanks to their unique features, the Fe-Ni ANC@NSCA catalysts show excellent performance in oxygen reduction reaction with a half-wave potential (E_1/2 ) of 0.891 V and oxygen evolution reaction (260 mV @ 10 mA cm^-2 ) in alkaline media as bifunctional catalysts, which are better than the state-of-the-art commercial Pt/C catalysts and RuO₂ catalysts. Moreover, Zn-air battery assembled with the Fe-Ni ANC@NSCA catalysts also shows a remarkable performance and exceptionally high stability over 500 h at 5 mA cm^-2 .

Topological defect-containing Fe/N co-doped mesoporous carbon nanosheets as novel electrocatalysts for the oxygen reduction reaction and Zn-air batteries

Developing effective electrocatalysts for the oxygen reduction reaction is of great significance for clean and renewable energy technologies, such as metal-air batteries and fuel cells. Defect engineering is the central focus of this field because the overall catalytic performance crucially depends on highly active defects. For the ORR, topological defects have been proven to have a positive effect. However, because preparation and characterization of such defects are difficult, a basic understanding of the relationship between topological defects and catalytic performance remains elusive. In this study, topological defect-containing Fe/N co-doped mesoporous carbon nanosheets were synthesized using azulene-based sandwich-like polymer nanosheets as the precursor. As electrocatalysts, such porous carbon nanosheets exhibited promising ORR activity, methanol tolerance ability, and stability with a half-wave potential of 841 mV under alkaline conditions, which is superior to those of most of the reported porous carbons. As the air cathode for Zn-air batteries, the catalyst exhibited a peak power density of 153 mW cm^-2 and a specific capacity of 628 mA h g^-1,which were higher than those of a Pt/C-based Zn-air battery. Density functional theory calculation further proved the positive effect of topological defects on the oxygen reduction activity. These results indicate that bottom-up topological defect engineering could be a new and promising strategy for developing high-performance electrocatalysts.

Molten-salt-assisted synthesis of onion-like Co/CoO@FeNC materials with boosting reversible oxygen electrocatalysis for rechargeable Zn-air battery

A melt-salt-assisted method is utilized to construct an onion-like hybrid with Co/CoO nanoparticles embedded in graphitic Fe-N-doped carbon shells (Co/CoO@FeNC) as a bifunctional electrocatalyst. The iron-polypyrrole (Fe-PPy) is firstly prepared with a reverse emulsion. Direct pyrolysis of Fe-PPy yields turbostratic Fe-N-doped carbon (FeNC) with excellent oxygen reduction reaction (ORR) electrocatalysis, while the melt salt (CoCl₂) mediated pyrolysis of Fe-PPy obtains onion-like Co/CoO@FeNC with a reversible overvoltage value of 0.695 V, largely superior to Pt/C and IrO₂ (0.771 V) and other Co-based catalysts reported so far. The ORR activity is mainly due to the graphitic FeNC and further enhanced by CoN_x bonds, whereas the oxygen evolution reaction (OER) activity is principally due to the Co/CoO composite. Concurrently, Co/CoO@FeNC as cathode catalyst enables Zn-air battery with a high open circuit voltage of 1.42 V, a peak power density of 132.8 mW cm^-2, a specific capacity of 813 mAh g_Zn^-1, and long-term stability.

Multi-Path Electron Transfer in 1D Double-Shelled Sn@Mo2 C/C Tubes with Enhanced Dielectric Loss for Boosting Microwave Absorption Performance

1D tubular micro-nano structural materials have been attracting extensive attention in the microwave absorption (MA) field for their anisotropy feature, outstanding impedance matching, and electromagnetic energy loss capability. Herein, unique double-shelled Sn@Mo₂ C/C tubes with porous Sn inner layer and 2D Mo₂ C/C outer layer are successfully designed and synthesized via a dual-template method. The composites possess favorable MA performance with an effective absorption bandwidth of 6.76 GHz and a maximum reflection loss value of -52.1 dB. Specifically, the rational and appropriate construction of Sn@Mo₂ C/C tubes promotes the multi-path electron transfer in the composites to optimize the dielectric constant and consequently to enhance the capacity of electromagnetic wave energy dissipation. Three mechanisms dominate the MA process: i) the conductive loss resulted from the rapid electron transmission due to the novel 1D hollow coaxial multi-shelled structure, especially the metallic Sn inner layer; ii) the polarization loss caused by abundant heterogeneous interfaces of Sn-Mo₂ C/C and Mo₂ CC from the precise double-shelled structure; iii) the capacitor-like loss by the potential difference between Mo₂ C/C nanosheets. This work hereby sheds light on the design of the 1D hierarchical structure and lays out a profound insight into the MA mechanism.

Metal-Organic-Framework-Derived Nanostructures as Multifaceted Electrodes in Metal-Sulfur Batteries

Metal-sulfur batteries (MSBs) are considered up-and-coming future-generation energy storage systems because of their prominent theoretical energy density. However, the practical applications of MSBs are still hampered by several critical challenges, i.e., the shuttle effects, sluggish redox kinetics, and low conductivity of sulfur species. Recently, benefiting from the high surface area, regulated networks, molecular/atomic-level reactive sites, the metal-organic frameworks (MOFs)-derived nanostructures have emerged as efficient and durable multifaceted electrodes in MSBs. Herein, a timely review is presented on recent advancements in designing MOF-derived electrodes, including fabricating strategies, composition management, topography control, and electrochemical performance assessment. Particularly, the inherent charge transfer, intrinsic polysulfide immobilization, and catalytic conversion on designing and engineering of MOF nanostructures for efficient MSBs are systematically discussed. In the end, the essence of how MOFs' nanostructures influence their electrochemical properties in MSBs and conclude the future tendencies regarding the construction of MOF-derived electrodes in MSBs is exposed. It is believed that this progress review will provide significant experimental/theoretical guidance in designing and understanding the MOF-derived nanostructures as multifaceted electrodes, thus offering promising orientations for the future development of fast-kinetic and robust MSBs in broad energy fields.

Co/CoOx heterojunctions encapsulated N-doped carbon sheets via a dual-template-guided strategy as efficient electrocatalysts for rechargeable Zn-air battery

Developing highly efficient oxygen electrocatalysts is of vital importance for rechargeable Zn-air batteries (ZABs). Herein, Co/CoO_x nano-heterojunctions encapsulated into nitrogen-doped carbon sheets (NCS@Co/CoO_x) are fabricated via a dual-template-guided approach by using zeolitic imidazolate frameworks (ZIFs) as templates. The synergistic integration of structural and compositional advantages endows such catalyst with superior catalytic properties to benchmark noble-metal catalysts. To be specific, the hierarchical micro/mesopores affords massive mass transport channels and maximizes the exposure of accessible active sites, whereas the NCS matrix accelerates electron transfer and prevents the self-aggregation of active species during the electrocatalytic reaction. Moreover, abundant and synergistic Co-based active sites (CoO, Co₃O₄, Co-N_x) greatly promote the catalytic activity. As the cathode of both liquid and flexible solid-state ZABs, excellent device properties are achieved, outperforming those assembled with commercial Pt/C+RuO₂ catalyst. This work presents a feasible and cost-effective strategy for developing oxygen electrocatalysts derived from ZIFs templates.

Electrocatalytic, Kinetic, and Mechanism Insights into the Oxygen-Reduction Catalyzed Based on the Biomass-Derived FeOx @N-Doped Porous Carbon Composites

A valid strategy for amplifying the oxygen reduction reaction (ORR) efficiency of non-noble electrocatalyst in both alkaline and acid electrolytes by decorated with a layer of biomass derivative nitrogen-doped carbon (NPC) is proposed. Herein, a top-down strategy for the generally fabricating NPC matrix decorated with trace of metal oxides nanoparticles (FeO_x NPs) by a dual-template assisted high-temperature pyrolysis process is reported. A high-activity FeO_x /FeNC (namely Hemin/NPC-900) ORR electrocatalyst is prepared via simply carbonizing the admixture of Mg₅ (OH)₂ (CO₃ )₄ and NaCl as dual-templates, melamine and acorn shells as nitrogen and carbon source, hemin as a natural iron and nitrogen source, respectively. Owing to its unique 3D porous construction, large BET areas (819.1 m² ∙g^-1 ), and evenly dispersed active sites (FeN_x , CN, and FeO parts), the optimized Hemin/NPC-900 catalyst displays comparable ORR catalytic activities, remarkable survivability to methanol, and preferable long-term stability in both alkali and acid electrolyte compared with benchmark Pt/C. More importantly, density function theory computations certify that the interaction between Fe₃ O₄ nanoparticles and arm-GN (graphitic N at armchair edge) active sites can effectually promote ORR electrocatalytic performance by a lower overpotential of 0.81 eV. Accordingly, the research provides some insight into design of low-cost non-precious metal ORR catalysts in theory and practice.

Synergistic Effect of N-Doped sp2 Carbon and Porous Structure in Graphene Gels toward Selective Oxidation of C-H Bond

N-doped carbon materials represent a type of metal-free catalyst for diverse organic synthetic reactions. However, single N-doped carbon materials perform insufficiently in the selective oxidation reaction of C-H bond compared with metal catalysts or multielement co-doped materials. There are a few reports on the application of three-dimensional (3D) carbon materials in such a reaction. Besides, the relationship between the well-developed porous structures, heteroatom doping, and their catalytic performance is unclear. In this study, 3D porous N-doped graphene aerogel catalysts with high activity and selectivity for the C-H bond oxidation under mild reaction conditions have been synthesized through a two-step method. Systematic studies on the dosage of N sources, pyrolysis temperature, and their influences on the catalytic performances have been evolved. Moreover, solid evidence of the synergistic effect of sp² C atoms adjacent to the N atoms and porous structure promoting the performance has been provided in this work.

In situ confinement pyrolysis of ZIF-67 nanocrystals on hollow carbon spheres towards efficient electrocatalysts for oxygen reduction

The design and preparation of metal-organic frameworks (MOFs) as self-sacrificed precursors/templates has been considered as a promising strategy in recent years for fabricating metal/carbon electrocatalysts with intriguing architectures and outstanding properties. However, the serious aggregation during the calcination and the poor electron conductivity are still obstacles for these electrocatalysts which need to be urgently solved. Herein, an in situ confinement pyrolysis protocol is reported to transform ZIF-67 nanocrystals on hollow carbon spheres (HCS) to cobalt and nitrogen-enriched carbon shell, resulting in the formation of hierarchical HCS@Co/NC. This is the first study of electrochemistry for HCS decorated with MOFs or MOFs derivatives. In the structure, metallic Co nanoparticles (NPs) and N species are strongly anchored and dispersed in the network of nanocarbon shell, which not only affords a boosting conductivity but also greatly alleviates the aggregation of active sites. Meanwhile, the unique structure with hollow feature provides an effective pathway for mass transport and shortens the transmission path of electrons. Thanks to the advantages of structure and composition, the HCS@Co/NC catalyst exhibits a superb performance of oxygen reduction reaction, which outperforms the commercial Pt/C benchmark.

Finely dispersed CuO on nitrogen-doped carbon hollow nanospheres for selective oxidation of sp3 C–H bonds

Selective oxidation of sp3 C–H bonds has been demonstrated using a novel nanocomposite, CuO/N-C-HNSs, as the catalyst.

Hollow Carbon-Based Nanoarchitectures Based on ZIF: Inward/Outward Contraction Mechanism and Beyond

Hollow carbon-based nanoarchitectures (HCAs) derived from zeolitic imidazolate frameworks (ZIFs), by virtue of their controllable morphology and dimension, high specific surface area and nitrogen content, richness of metal/metal compounds active sites, and hierarchical pore structure and easy exposure of active sites, have attracted great interests in many fields of applications, especially in heterogeneous catalysis, and electrochemical energy storage and conversion. Despite various approaches that have been developed to prepare ZIF-derived HCAs, the hollowing mechanism has not been clearly disclosed. Herein, a specialized overview of the recent progress of ZIF-derived HCAs is introduced to provide an insight into their preparation strategy and the corresponding hollowing mechanisms. Based on the fundamental understanding of the structural evolution of ZIF nanocrystals during the high-temperature pyrolysis process, the hollowing mechanisms of ZIF-derived HCAs are classified into four categories: i) inward contraction of core-shell template@ZIF composites or hollow ZIFs, ii) outward contraction of ZIF@shell composites, iii) special outward contraction of ZIF arrays, and iv) mechanism beyond inward/outward contraction of pure ZIF nanocrystals. Finally, an outlook on the development prospects and challenges of HCAs based on ZIF precursors, especially in terms of controlled synthesis and future electrochemical application, is further discussed.

Relevant Properties of Carbon Support Materials in Successful Fe-N-C Synthesis for the Oxygen Reduction Reaction: Study of Carbon Blacks and Biomass-Based Carbons

Fe-N-C materials are promising non-precious metal catalysts for the oxygen reduction reaction in fuel cells and batteries. However, during the synthesis of these materials less active Fe-containing nanoparticles are formed in many cases which lead to a decrease in electrochemical activity and stability. In this study, we reveal the significant properties of the carbon support required for the successful incorporation of Fe-N-related active sites. The impact of two carbon blacks and two activated biomass-based carbons on the Fe-N-C synthesis is investigated and crucial support properties are identified. Carbon supports having low portions of amorphous carbon, moderate surface areas (>800 m2/g) and mesopores result in the successful incorporation of Fe and N on an atomic level and improved oxygen reduction reaction (ORR) activity. A low surface area and especially amorphous parts of the carbon promote the formation of metallic iron species covered by a graphitic layer. In contrast, highly microporous systems with amorphous carbon provoke the formation of less active iron carbides and carbon nanotubes. Overall, a phosphoric acid activated biomass is revealed as novel and sustainable carbon support for the formation of Fe-Nx sites. Overall, this study provides valuable and significant information for the future development of novel and sustainable carbon supports for Fe-N-C catalysts.

N,S-Co-Doped Porous Carbon Nanofiber Films Derived from Fullerenes (C60 ) as Efficient Electrocatalysts for Oxygen Reduction and a Zn-Air Battery

The development of highly efficient metal-free electrocatalysts for the oxygen reduction reaction (ORR) has attracted great attention for the creation of electrochemical energy devices. In this study, one-dimensional (1 D) fullerene nanofibers prepared from liquid-liquid interfacial precipitation are first fabricated into fullerene-derived carbon nanofiber films (FCNFs) through a simple filtration procedure. Then, pyrolysis of the FCNFs in the presence of ammonia and sulfur produces N- and S-co-doped porous carbon nanofiber films (N,S-PCNFs). As excellent metal-free electrocatalysts for the ORR, N,S-PCNFs exhibit remarkable catalytic activity, superior stability, and excellent methanol tolerance in both alkaline and acidic solution. Such a high ORR performance benefits from the robust porous nanofiber network structure with high concentrations of active N- and S- groups and abundant defects. Notably, upon practical use of N,S-PCNFs as catalysts in Zn-air batteries, a high power density and a large operating voltage are achieved, with a performance comparable to that of the commercial Pt/C catalyst. This work presents a facile strategy for the creation of a new class of energy nanomaterials based on fullerenes, demonstrating their practical uses in electrocatalytic ORR processes and Zn-air batteries.

Bioinspired Assembly of Double Honeycomb-Like Hierarchical Capsule Confined Encapsulation with Functional Micro/Nanocrystals

Inspired by "micro/nanoreactor" effect of cellular organelle on specific biochemical reactions, a double honeycomb-like hierarchical capsule confined encapsulation with functional micro/nanocrystals is designed. The bioinspired hierarchical capsules derived from polymeric composite microspheres are successfully fabricated through a combination of selective chemical etching and pyrolysis. In situ introduction of functional guests (including organometallic molecules, tetraethoxysilane, or metal-organic frameworks (MOFs)) into internal cellular structure of microspheres is first put forward by phase inversion method. The development of selective etching creates honeycomb-like structure on the outside surface of capsule and allows sulfur to homogeneously distribute into matrix. With the novel approach, the hierarchical channels (micro-meso-macropore) of composite capsule enhance transportation of reactants and dispersion of active sites, and thus exhibit superior photocatalytic oxidation and electromagnetic absorbing. The promising strategy will be applied more generally to encapsulate different species into hierarchical capsule with tailored properties and functionalities.

Copper- and Nitrogen-Codoped Graphene with Versatile Catalytic Performances for Fenton-Like Reactions and Oxygen Reduction Reaction

Copper- and nitrogen-codoped reduced graphene oxide material (Cu/N-rGO) was prepared with a hydrothermal method. Its versatile catalytic performances were demonstrated toward the oxidative degradation of rhodamine B (RhB) and oxygen reduction reaction (ORR). The Cu and N codoping of graphene enhanced not only its activation ability toward H2O2, but also its electrocatalytic ability for ORR. It was observed that the use of 3%Cu/N-rGO together with 40 mmol·L−1 H2O2 and 4 mmol·L−1 Na2CO3 could remove more than 94% of the added RhB (30 mg·L−1) in 20 min through a catalytic Fenton-like degradation. Quenching experiments and electron paramagnetic resonance (EPR) measurements indicated that the main reactive species generated in the catalytic oxidation process were surface-bound •OH. The modified graphene also showed good electrocatalytic activity for ORR reaction in alkaline media through a four-electron mechanism. On the electrode of Cu/N-rGO, the ORR reaction exhibited an onset potential of −0.1 V and a half-wave potential of −0.248 V, which were correspondingly close to those on a Pt/C electrode. In comparison with a Pt/C electrode, the 3%Cu/N-rGO electrode showed much greater tolerance to methanol. Such outstanding catalytic properties are attributed to the abundant active sites and the synergism between Cu and N in Cu/N-rGO.