Pyridinic‐Nitrogen‐Dominated Graphene Aerogels with Fe–N–C Coordination for Highly Efficient Oxygen Reduction Reaction

Here, pyridinic nitrogen dominated graphene aerogels with/without iron incorporation (Fe‐NG and NG) are prepared via a facile and effective process including freeze‐drying of chemically reduced graphene oxide with/without iron precursor and thermal treatment in NH₃. A high doping level of nitrogen has been achieved (up to 12.2 at% for NG and 11.3 at% for Fe‐NG) with striking enrichment of pyridinic nitrogen (up to 90.4% of the total nitrogen content for NG, and 82.4% for Fe‐NG). It is found that the Fe‐NG catalysts display a more positive onset potential, higher current density, and better four‐electron selectivity for ORR than their counterpart without iron incorporation. The most active Fe‐NG exhibits outstanding ORR catalytic activity, high durability, and methanol tolerance ability that are comparable to or even superior to those of the commercial Pt/C catalyst at the same catalyst loading in alkaline environment. The excellent ORR performance can be ascribed to the synergistic effect of pyridinic N and Fe‐N _x sites (where iron probably coordinates with pyridinic N) that serve as active centers for ORR. Our Fe‐NG can be developed into cost‐effective and durable catalysts as viable replacements of the expensive Pt‐based catalysts in practical fuel cell applications.     

Recent Advances in Non‐Noble Bifunctional Oxygen Electrocatalysts toward Large‐Scale Production

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial reactions in energy conversion and storage systems including fuel cells, metal–air batteries, and electrolyzers. Developing low‐cost, high‐efficiency, and durable non‐noble bifunctional oxygen electrocatalysts is the key to the commercialization of these devices. Here, based on an in‐depth understanding of ORR/OER reaction mechanisms, recent advances in the development of non‐noble electrocatalysts for ORR/OER are reviewed. In particular, rational design for enhancing the activity and stability and scalable synthesis toward the large‐scale production of bifunctional electrocatalysts are highlighted. Prospects and future challenges in the field of oxygen electrocatalysis are presented.     

Symmetric Electronic Structures of Active Sites to Boost Bifunctional Oxygen Electrocatalysis by MN4+4 Sites Directly from Initial Covalent Organic Polymers

Transition metal-nitrogen-carbon (M-N-C) catalysts with CoN₄ centers have attracted great attention as a potential alternative to precious metal catalysts for bifunctional oxygen electrocatalysis. However, the asymmetric charge environment of the active site of MN₄ obtained by conventional pyrolysis strategy makes the unbalanced adsorption of oxygen molecules, which restricts the activities of both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, a series of well-defined quasi-phthalocyanine conjugated 2D covalent organic polymer (COP_BTC-M) is developed with MN₄₊₄ active sites through a pyrolysis-free strategy. Compared to CoN₄ site, the additional subcentral N₄ atoms in MN₄₊₄ site in COP_BTC-Co catalyst balance the charge environment and form a symmetric charge distribution, which changes the antibonding orbital of the active metal and regulate the oxygen species adsorption, thus improving the activity of the bifunctional oxygen electrocatalysis. In Silico screening demonstrates that cobalt has the best ORR and OER activity for COP_BTC-M with MN₄₊₄ sites, which can be attributed to the fewer anti-bonding orbital below the Fermi level, which weakens the oxygen species adsorption. Both theoretical and experimental results verify that the COP_BTC-Co possesses unique CoN₄₊₄ active sites and the harmonious coordinating environment can lead to superior bifunctional oxygen catalytic activity with a high bifunctional oxygen catalytic activity (ΔE [Ej₁₀ – E_1/2] = 0.76 V), which is comparable with the benchmark Pt/C-IrO₂ pairs. Accordingly, the as-assembled Zn–air battery exhibits a maximum power density of 157.7 mW cm ⁻² with stable operation for >100 cycles under an electric density of 10 mA cm ⁻². This study provides a characteristic understanding of the intrinsic active species toward MN_x centers and could inspire new avenues for designation of advanced bifunctional electrocatalysts that catalyze ORR and OER processes simultaneously.     

3D Carbon Electrocatalysts In Situ Constructed by Defect‐Rich Nanosheets and Polyhedrons from NaCl‐Sealed Zeolitic Imidazolate Frameworks

The proper structure design and defect engineering are of essential importance to develop advanced electrocatalysts for the oxygen reduction reaction (ORR), which is a critical reaction in both fundamental science and industrial applications. Herein, a three‐dimensional carbon electrocatalyst is prepared by in‐situ linking carbon polyhedrons with nanosheets through high‐temperature pyrolysis of metal‐organic frameworks (MOFs) confined in a salt‐sealed reactor. In the transformation to polyhedrons, the organic species partially decompose and form carbon nanosheets due to being confined in the salt reactor. The in ‐situ‐formed carbon nanosheets surround the carbon polyhedrons to form a 3D carbon network. Due to the confinement effect, the transformation of MOFs to carbon networks in the salt reactor is of high yield without significant loss of active carbon species, which would enhance the electron and mass transport for electrocatalysis. More interestingly, the as‐prepared 3D nanosheet‐linked‐polyhedron carbon (NLPC) is defect‐rich with high N‐doping levels and enriched active sites for electrocatalysis. With enhanced mass, electron transport, and enriched active sites, the material shows excellent activity as ORR electrocatalyst which is even comparable with Pt/C. The primary zinc‐air batteries assembled by the NLPC as the cathode also show outstanding performance.     

Single‐Atomic‐Co Electrocatalysts with Self‐Supported Architecture toward Oxygen‐Involved Reaction

Single‐atomic electrocatalysts (SACs) have shown great promise in electrocatalysis fields owing to their theoretical maximum atom utilization (100%). Yet still, it is far from expectation in practical applications due to entrapping within supports and blocking by aggregation. Herein, self‐supported carbon nanosheet arrays consisting of single‐atomic Co electrocatalyst (SS‐Co‐SAC) toward oxygen‐involved reaction and zinc–air batteries are reported. Impressively, the as‐synthesized SS‐Co‐SAC gives a markedly enhanced utilization of active sites (≈22.3%@2.3 wt%) as a result of single‐atomic dispersion of Co within a unique nanosheet arrays architecture, which is the largest value among other reported results. Benefiting from the high utilization of active sites, the SS‐Co‐SAC electrode exhibits outstanding electrocatalytic performance for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Notably, the turnover frequency value for ORR is determined to be ≈9.26 s^?1, which stands for the highest level among noble metal‐free electrocatalysts reported previously. Moreover, as an air‐cathode for zinc–air batteries with SS‐Co‐SAC, a power density of 195.1 mW cm^?2 and a robust durability are achieved. It is believed that this study would guide the future design of highly active and durable single‐atom catalysts for both fundamental research and practical applications.     

Superelastic and Arbitrary‐Shaped Graphene Aerogels with Sacrificial Skeleton of Melamine Foam for Varied Applications

Elastic graphene aerogels are lightweight and offer excellent and electrical performance, expanding their significance in many applications. Recently, elastic graphene aerogels have been fabricated via various methods. However, for most reported elastic graphene aerogels, the fabrication processes are complicated and the applications are usually limited by the brittle mechanical properties. Thus, it still remains a challenge to explore facile processes for the fabrication of graphene aerogels with low density and high compressibility. Herein, arbitrary‐shaped, superelastic, and durable graphene aerogels are fabricated using melamine foam as sacrificial skeleton. The resulting graphene aerogels possess high elasticity under compressive stress of 0.556 MPa and compressive strain of 95%. Thanks to the superelasticity, high strength, excellent flexibility, outstanding thermal stability, and good electrical conductivity of graphene aerogels, they can be applied in sorbents and pressure/strain sensors. The as‐assembled graphene aerogels can adsorb various organic solvents at 176–513 g g^?1 depending on the solvent type and density. Moreover, both the squeezing and combustion methods can be adopted for reusing the graphene aerogels. Finally, the graphene aerogels exhibit stable and sensitive current responses, making them the ideal candidates for applications as multifunctional pressure/strain sensors such as wearable devices.     

High-Density Atomic Fe–N4/C in Tubular,Biomass-Derived,Nitrogen-Rich Porous Carbon as Air-Electrodes for Flexible Zn–Air Batteries

Developing low-cost single-atom catalysts (SACs) with high-density active sites for oxygen reduction/evolution reactions (ORR/OER) are desirable to promote the performance and application of metal–air batteries. Herein, the Fe nanoparticles are precisely regulated to Fe single atoms supported on the waste biomass corn silk (CS) based porous carbon for ORR and OER. The distinct hierarchical porous structure and hollow tube morphology are critical for boosting ORR/OER performance through exposing more accessible active sites, providing facile electron conductivity, and facilitating the mass transfer of reactant. Moreover, the enhanced intrinsic activity is mainly ascribed to the high Fe single-atom (4.3 wt.%) loading content in the as-synthesized catalyst.Moreover, the ultra-high N doping (10 wt.%) can compensate the insufficient OER performance of conventional Fe N C catalysts. When as-prepared catalysts are assembled as air-electrodes in flexible Zn–air batteries, they perform a high peak power density of 101 mW cm⁻², a stable discharge–charge voltage gap of 0.73 V for >44 h, which shows a great potential for Zinc–air battery. This work provides an avenue to transform the renewable low-cost biomass materials into bifunctional electrocatalysts with high-density single-atom active sites and hierarchical porous structure.     

Molecular Engineering of Noble Metal Aerogels Boosting Electrocatalytic Oxygen Reduction

Developing robust oxygen reduction reaction (ORR) electrocatalysts with high activity and durability remains great challenging while noble metal aerogels (NMAs) hold great potential because of their hierarchically porous morphology, excellent activity, and self-supported characteristic. Herein, a general molecular engineering strategy to synthesize molecule-noble metal aerogels (M-NMAs) via 3D assembly of metal nanoparticles (e.g., Pt, Pd, Au, Ag, and PtPd NPs) induced by metalloporphyrin as cross-linkers is reported. Due to the well synergy of NMAs and porphyrin molecule in creating the facile reaction pathway for ORR catalysis, these M-NMAs demonstrate boosted ORR activity and durability in different electrolytes. Particularly, the best PtPd-based M-NMA delivers 1.47 A mg_Pt⁻¹ and 2.13 mA cm⁻² in mass and specific activities, which are 11.3 and 14.2 times higher than those of the commercial Pt/C catalyst, respectively. Thus, this work not only provides a simple and universal functional engineering approach of NMAs with catalytic molecules, but also opens an avenue of the rational design for superior ORR electrocatalysts.     

Ag Anchored Atomically Around Nanopores of Porous Co(OH)2 for Efficient Bifunctional Oxygen Catalysis

Various clean energy storage and conversion systems highly depend on rational design of efficient electrocatalysts for oxygen reactions. Increasing both gas molecular diffusion and intrinsic activity is critical to boosting its efficiency for bifunctional oxygen electrocatalysis. However, controllable synthesis of catalysts that combines gas molecular diffusion and intrinsic activity remains a fundamental challenge. Herein, a two-step synthetic strategy is adopted to fabricate a composite oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) bifunctional catalyst (P-Ag-Co(OH)₂), of which, atomic Ag is anchored in reactive oxygen atoms around nanopores of Co(OH)₂ nanosheets. Abundant nanopores provide enough gas molecular diffusion channels, and the special Ag-O-Co-OH catalytic groups around nanopores display high intrinsic catalytic activity, which jointly result in an excellent ORR/OER performance. In alkaline electrolyte, P-Ag-Co(OH)₂ displays a high half-wave potential (0.902 V versus RHE) for ORR, and a low overpotential (235 mV at 10 mA cm⁻²) for OER, which is superior to non-noble catalysts in previous studies and Pt/C (Ir/C) catalyst. At the same time, the single-cell zinc-air battery is prepared with an extremely high discharge peak power density of 435 mW cm⁻² and excellent discharge–charge cycle stability.     

Robust Carbonaceous Nanofiber Aerogels from All Biomass Precursors

Aerogels, a type of fascinating material with very low density and high surface area, show many unique properties and unlimited applications. To boost their practical applications, it is necessary to develop efficient, controllable, and low-cost methods to produce high-performance aerogels on a large-scale, preferably in a sustainable way. Here, a general strategy is reported for controllable fabrication of a family of carbonaceous nanofiber aerogels (CNFAs) by biomass-derived nanofibers template-directed hydrothermal carbonization method. Abundant functional groups are exposed on the surface of the prepared carbonaceous nanofibers. Importantly, in contrast to traditional nature biopolymer-based aerogels, a superior combination of good recoverability and high strength is achieved for the CNFAs by adjusting the synthetic parameters. The successful synthesis of such fascinating materials provides an excellent platform for design and construction of devices for fast water treatment. The synthetic strategy and sustainable concept presented in this work will open a new way to prepare advanced aerogels with unique properties for wide applications.     

Perovskite Oxides for Cathodic Electrocatalysis of Energy-Related Gases: From O2 to CO2 and N2

The oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO₂RR), and nitrogen reduction reaction (NRR) are important cathodic reactions for renewable fuel production and energy conversion technologies. However, the sluggish kinetics of these reactions hinders the practical applications of the relevant technologies. Perovskite oxides, a promising family of catalysts with great flexibility in composition and structure engineering, exhibit versatility in electrocatalysis of these reactions. In this review, beginning with a brief introduction on the fundamentals of ORR, CO₂RR, and NRR, the mechanistic understanding of the electrocatalysis by perovskite oxides is discussed based on both molecular orbital and band theories. The recent advances of perovskite oxide-based electrocatalysts for ORR, CO₂RR, and NRR are then highlighted. Strategies for developing perovskite oxide-based ORR catalysts are emphasized with representative examples, intending to draw on the experience of developing ORR catalysts to both CO₂RR and NRR catalysts. Finally, perspectives on the perovskite oxide-based electrocatalysis are discussed by paying particular attention to the practical applications and future development of perovskite oxide-based catalysts.     

Mechanical and Thermal Management Characteristics of Ultrahigh Surface Area Single‐Walled Carbon Nanotube Aerogels

Lightweight aerogels with large specific surface area (SSA) have numerous applications. Free‐standing aerogels are created from single‐walled carbon nanotubes (SWCNTs), and their SSA and pore characteristics, electrical conductivity, mechanical properties, and thermal management attributes are determined. The SSA of the aerogels is extraordinarily high and approaches 1291 m² g^?1 at a density of 7.3 mg mL^?1, which is close to the theoretical limit (≈1315 m² g^?1). Mechanical characterization shows that these aerogels have open‐cell structures and their Young's moduli are higher than other aerogels at comparable density. The aerogels also enhance heat transfer in a forced convective process by ≈85%, presumably due to their large porosity and surface area.     

Porphyrin Aerogel Catalysts for Oxygen Reduction Reaction in Anion-Exchange Membrane Fuel Cells

Platinum group metal (PGM)-free catalysts for oxygen reduction reaction have shown high oxygen reduction reaction activity in alkaline media. In order to further increase the power density of anion-exchange membrane fuel cells (AEMFCs), PGM-free catalysts need to have a high site density to reach high current densities. Herein, synthesis, characterization, and utilization of heat-treated iron porphyrin aerogels are reported as cathode catalysts in AEMFCs. The heat treatment effect is thoroughly studied and characterized using several techniques, and the best performing aerogel is studied in AEMFC, showing excellent performance, reaching a peak power density of 580 mW cm⁻² and a limiting current density of as high as 2.0 A cm⁻², which can be considered the state-of-the-art for PGM-free based AEMFCs.     

Cellulose Silica Hybrid Nanofiber Aerogels: From Sol–Gel Electrospun Nanofibers to Multifunctional Aerogels

Aerogels are considered ideal candidates for various applications, because of their low bulk density, highly porous nature, and functional performance. However, the time intensive nature of the complex fabrication process limits their potential application in various fields. Recently, incorporation of a fibrous network has resulted in production of aerogels with improved properties and functionalities. A facile approach is presented to fabricate hybrid sol–gel electrospun silica‐cellulose diacetate (CDA)‐based nanofibers to generate thermally and mechanically stable nanofiber aerogels. Thermal treatment results in gluing the silica‐CDA network strongly together thereby enhancing aerogel mechanical stability and hydrophobicity without compromising their highly porous nature (>98%) and low bulk density (≈10 mg cm^?3). X‐ray photoelectron spectroscopy and in situ Fourier‐transform infrared studies demonstrate the development of strong bonds between silica and the CDA network, which result in the fabrication of cross‐linked structure responsible for their mechanical and thermal robustness and enhanced affinity for oils. Superhydrophobic nature and high oleophilicity of the hybrid aerogels enable them to be ideal candidates for oil spill cleaning, while their flame retardancy and low thermal conductivity can be explored in various applications requiring stability at high temperatures.     

Recent Developments of Microenvironment Engineering of Single-Atom Catalysts for Oxygen Reduction toward Desired Activity and Selectivity

Oxygen reduction reaction (ORR) is an essential process for sustainable energy supply and sufficient chemical production in modern society. Single-atom catalysts (SACs) exhibit great potential on maximum atomic efficiency, high ORR activity, and stability, making them attractive candidates for pursuing next-generation catalysts. Despite substantial efforts being made on building diversiform single-atom active sites (SAASs), the performance of the obtained catalysts is still unsatisfactory. Fortunately, microenvironment regulation of SACs provides opportunities to improve activity and selectivity for ORR. In this review, first, ORR mechanism pathways on N-coordinated SAAS, electrochemical evaluation, and characterization of SAAS are displayed. In addition, recent developments in tuning microenvironment of SACs are systematically summarized, especially, strategies for microenvironment modulation are introduced in detail for boosting the intrinsic 4e⁻/2e⁻ ORR activity and selectivity. Theoretical calculations and cutting-edge characterization techniques are united and discussed for fundamental understanding of the synthesis–construction–performance correlations. Furthermore, the techniques for building SAAS and tuning their microenvironment are comprehensively overviewed to acquire outstanding SACs. Lastly, by proposing perspectives for the remaining challenges of SACs and infant microenvironment engineering, the future directions of ORR SACs and other analogous procedures are pointed out.     

Engineering Synergistic Edge-N Dipole in Metal-Free Carbon Nanoflakes toward Intensified Oxygen Reduction Electrocatalysis

Nitrogen doping represents an effective way to induce charge/spin polarization in nanocarbons for promoting oxygen reduction reaction (ORR) activity. However, it remains elusive to define the dominant active sites with respect to two critical N-configurations of pyridinic-N and graphitic-N. Herein, a tandem catalytic graphitization and nitrogen modification strategy for the synthesis of metal-free nitrogen-doped carbon nanoflakes (NCF) featuring the edge-suffused and graphite-analogous structure is presented. NCF exhibits superb Pt-like ORR activity (0.85 V for half-wave potential and 5.9 mA cm⁻² for diffusion-limited current density) but much stronger robustness in the alkaline medium. The experimental and theoretical studies suggest the key role of graphitic-N in ORR. Furthermore, it unveils that the high activity of NCF should be traced to a synergistic polarization of the edge-type pyridinic-N/graphitic-N dipole spaced by one edge peak carbon atom on the armchair edges. This study sheds light on the understanding of ORR active sites in the nitrogen-doped nanocarbons for ORR.     

A Type of 1 nm Molybdenum Carbide Confined within Carbon Nanomesh as Highly Efficient Bifunctional Electrocatalyst

Highly efficient platinum‐alternative bifunctional catalysts by using abundant non‐noble metal species are of critical importance to the future sustainable energy reserves. Unfortunately, current electrocatalysts toward hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are far from satisfactory because of lacking reasonable design and assembly protocols. A type of 1‐nm molybdenum carbide nanoparticles confined in mesh‐like nitrogen‐doped carbon (Mo₂C@NC nanomesh) with high specific surface area is reported here. In addition to the superior ORR performance comparable to platinum, the catalyst offers a high HER activity with small Tafel slope of 33.7 mV dec^?1 and low overpotential of 36 mV to reach ?10 mA cm^?2. Theoretical calculations indicate that the active sites of the catalyst are mainly located at Mo atoms adjacent to the N‐doped carbon layer, which contributes the high HER activity. These findings show the great potential of Mo₂C species in wide electrocatalysis applications.     

High-Loading Co Single Atoms and Clusters Active Sites toward Enhanced Electrocatalysis of Oxygen Reduction Reaction for High-Performance Zn–Air Battery

The development of precious-metal alternative electrocatalysts for oxygen reduction reaction (ORR) is highly desired for a variety of fuel cells, and single atom catalysts (SACs) have been envisaged to be the promising choice. However, there remains challenges in the synthesis of high metal loading SACs (>5 wt.%), thus limiting their electrocatalytic performance. Herein, a facile self-sacrificing template strategy is developed for fabricating Co single atoms along with Co atomic clusters co-anchored on porous-rich nitrogen-doped graphene (Co SAs/AC@NG), which is implemented by the pyrolysis of dicyandiamide with the formation of layered g-C₃N₄ as sacrificed templates, providing rich anchoring sites to achieve high Co loading up to 14.0 wt.% in Co SAs/AC@NG. Experiments combined with density functional theory calculations reveal that the co-existence of Co single atoms and clusters with underlying nitrogen doped carbon in the optimized Co₄₀SAs/AC@NG synergistically contributes to the enhanced electrocatalysis for ORR, which outperforms the state-of-the-art Pt/C catalysts with presenting a high half-wave potential (E_1/2 = 0.890 V) and robust long-term stability. Moreover, the Co₄₀SAs/AC@NG presents excellent performance in Zn–air battery with a high-peak power density (221 mW cm⁻²) and strong cycling stability, demonstrating great potential for energy storage applications.     

Reversibly Compressible,Highly Elastic,and Durable Graphene Aerogels for Energy Storage Devices under Limiting Conditions

High porosity combined with mechanical durability in conductive materials is in high demand for special applications in energy storage under limiting conditions, and it is fundamentally important for establishing a relationship between the structure/chemistry of these materials and their properties. Herein, polymer‐assisted self‐assembly and cross‐linking are combined for reduced graphene oxide (rGO)‐based aerogels with reversible compressibility, high elasticity, and extreme durability. The strong interplay of cross‐linked rGO (x‐rGO) aerogels results in high porosity and low density due to the re‐stacking inhibition and steric hinderance of the polymer chains, yet it makes mechanical durability and structural bicontinuity possible even under compressive strains because of the coupling of directional x‐rGO networks with polymer viscoelasticity. The x‐rGO aerogels retain >140% and >1400% increases in the gravimetric and volumetric capacitances, respectively, at 90% compressive strain, showing reversible change and stability of the volumetric capacitance under both static and dynamic compressions; this makes them applicable to energy storage devices whose volume and mass must be limited.     

Dual-Engineering of Porous Structure and Carbon Edge Enables Highly Selective H2O2 Electrosynthesis

Edge engineering has emerged as a powerful strategy to activate inert carbon surfaces, and thus achieve a notable enhanced electrocatalytic activity. However, the rational manipulation of carbon edges to achieve enhanced catalytic performance remains a formidable challenge, primarily hindered by immature synthesis methods and the obscured understanding of the structure-activity relationship. Herein, an organic–inorganic hybrid co-assembly strategy is used to fabricate a series of mesoporous carbon nanofibers (MCNFs) with controllable edge site densities and the impact of edge population on electrochemical oxygen reduction reaction (ORR) pathways is investigated. The optimized MCNFs catalyst exhibits a remarkable 2e⁻ ORR performance, as evidenced by a high H₂O₂ selectivity (>90%) across a wide potential window of 0.6 V and a large cathodic current density of −3.0 mA cm⁻² (at 0.2 V vs. reversible hydrogen electrode). Strikingly, the density of carbon edge sites can be changed to tune the ORR activity and selectivity. Experimental validation and density functional theory calculations confirm that the presence of edge defects can optimize the adsorption strength of *OOH intermediates and balance the selectivity and activity of the 2e⁻ ORR process. This study provides a new path to achieve high ORR activity and 2e⁻ selectivity in carbon-based electrocatalysts.