Atomic Fe Dispersed on N‐Doped Carbon Hollow Nanospheres for High‐Efficiency Electrocatalytic Oxygen Reduction

Exploration of high‐efficiency, economical, and ultrastable electrocatalysts for the oxygen reduction reaction (ORR) to substitute precious Pt is of great significance in electrochemical energy conversion devices. Single‐atom catalysts (SACs) have sparked tremendous interest for their maximum atom‐utilization efficiency and fascinating properties. Therefore, the development of effective synthetic methodology toward SACs becomes highly imperative yet still remains greatly challenging. Herein, a reliable SiO₂‐templated strategy is elaborately designed to synthesize atomically dispersed Fe atoms anchored on N‐doped carbon nanospheres (denoted as Fe–N–C HNSs) using the cheap and sustainable biomaterial of histidine (His) as the N and C precursor. By virtue of the numerous atomically dispersed Fe–N₄ moieties and unique spherical hollow architecture, the as‐fabricated Fe–N–C HNSs exhibit excellent ORR performance in alkaline medium with outstanding activity, high long‐term stability, and superior tolerance to methanol crossover, exceeding the commercial Pt/C catalyst and most previously reported non‐precious‐metal catalysts. This present synthetic strategy will provide new inspiration to the fabrication of various high‐efficiency single‐atom catalysts for diverse applications.     

Biomass Derived N‐Doped Porous Carbon Supported Single Fe Atoms as Superior Electrocatalysts for Oxygen Reduction

Exploring sustainable and high‐performance electrocatalysts for the oxygen reduction reaction (ORR) is the crucial issue for the large‐scale application of fuel cell technology. A new strategy is demonstrated to utilize the biomass resource for the synthesis of N‐doped hierarchically porous carbon supported single‐atomic Fe (SA‐Fe/NHPC) electrocatalyst toward the ORR. Based on the confinement effect of porous carbon and high‐coordination natural iron source, SA‐Fe/NHPC, derived from the hemin‐adsorbed bio‐porphyra‐carbon by rapid heat‐treatment up to 800 °C, presents the atomic dispersion of Fe atoms in the N‐doped porous carbon. Compared with the molecular hemin and nanoparticle Fe samples, the as‐prepared SA‐Fe/NHPC exhibits a superior catalytic activity (E _1/2 = 0.87 V and J _k = 4.1 mA cm^?2, at 0.88 V), remarkable catalytic stability (≈1 mV negative shift of E _1/2, after 3000 potential cycles), and outstanding methanol‐tolerance, even much better than the state‐of‐the‐art Pt/C catalyst. The sustainable and effective strategy for utilizing biomass to achieve high‐performance single‐atom catalysts can also provide an opportunity for other catalytic applications in the atomic scale.     

Pseudocapacitive Ni‐Co‐Fe Hydroxides/N‐Doped Carbon Nanoplates‐Based Electrocatalyst for Efficient Oxygen Evolution

The oxygen evolution reaction (OER) catalytic activity of a transition metal oxides/hydroxides based electrocatalyst is related to its pseudocapacitance at potentials lower than the OER standard potential. Thus, a well‐defined pseudocapacitance could be a great supplement to boost OER. Herein, a highly pseudocapacitive Ni‐Fe‐Co hydroxides/N‐doped carbon nanoplates (NiCoFe‐NC)‐based electrocatalyst is synthesized using a facile one‐pot solvothermal approach. The NiCoFe‐NC has a great pseudocapacitive performance with 1849 F g^?1 specific capacitance and 31.5 Wh kg^?1 energy density. This material also exhibits an excellent OER catalytic activity comparable to the benchmark RuO₂ catalysts (an initiating overpotential of 160 mV and delivering 10 mA cm^?2 current density at 250 mV, with a Tafel slope of 31 mV dec^?1). The catalytic performance of the optimized NiCoFe‐NC catalyst could keep 24 h. X‐ray photoelectron spectroscopy, electrochemically active surface area, and other physicochemical and electrochemical analyses reveal that its great OER catalytic activity is ascribed to the Ni‐Co hydroxides with modular 2‐Dimensional layered structure, the synergistic interactions among the Fe(III) species and Ni, Co metal centers, and the improved hydrophily endowed by the incorporation of N‐doped carbon hydrogel. This work might provide a useful and general strategy to design and synthesize high‐performance metal (hydr)oxides OER electrocatalysts.     

Co‐N‐Doped Mesoporous Carbon Hollow Spheres as Highly Efficient Electrocatalysts for Oxygen Reduction Reaction

Rational design of cost‐effective, nonprecious metal‐based catalysts with desirable oxygen reduction reaction (ORR) performance is extremely important for future fuel cell commercialization, etc. Herein, a new type of ORR catalyst of Co‐N‐doped mesoporous carbon hollow sphere (Co‐N‐mC) was developed by pyrolysis from elaborately fabricated polystyrene@polydopamine‐Co precursors. The obtained catalysts with active Co sites distributed in highly graphitized mesoporous N‐doped carbon hollow spheres exhibited outstanding ORR activity with an onset potential of 0.940 V, a half‐wave potential of 0.851 V, and a small Tafel slope of 45 mV decade^?1 in 0.1 m KOH solution, which was comparable to that of the Pt/C catalyst (20%, Alfa). More importantly, they showed superior durability with little current decline (less than 4%) in the chronoamperometric evaluation over 60 000 s. These features make the Co‐N‐mC one of the best nonprecious‐metal catalysts to date for ORR in alkaline condition.     

2D Nanoporous Fe−N/C Nanosheets as Highly Efficient Non‐Platinum Electrocatalysts for Oxygen Reduction Reaction in Zn‐Air Battery

It is an ongoing challenge to fabricate nonprecious oxygen reduction reaction (ORR) catalysts that can be comparable to or exceed the efficiency of platinum. A highly active non‐platinum self‐supporting Fe?N/C catalyst has been developed through the pyrolysis of a new type of precursor of iron coordination complex, in which 1,4‐bis(1H‐1,3,7,8–tetraazacyclopenta(1)phenanthren‐2‐yl)benzene (btcpb) functions as a ligand complexing Fe(II) ions. The optimal catalyst pyrolyzed at 700 °C (Fe?N/C?700) shows the best ORR activity with a half‐wave potential (E_1/2) of 840 mV versus reversible hydrogen electrode (RHE) in 0.1 m KOH, which is more positive than that of commercial Pt/C (E_1/2: 835 mV vs RHE). Additionally, the Fe?N/C?700 catalyst also exhibits high ORR activity in 0.1 m HClO₄ with the onset potential and E_1/2 comparable to those of the Pt/C catalyst. Notably, the Fe?N/C?700 catalyst displays superior durability (9.8 mV loss in 0.1 m KOH and 23.6 mV loss in 0.1 m HClO₄ for E_1/2 after 8000 cycles) and better tolerance to methanol than Pt/C. Furthermore, the Fe?N/C?700 catalyst can be used for fabricating the air electrode in Zn–air battery with a specific capacity of 727 mA hg^?1 at 5 mA cm^?2 and a negligible voltage loss after continuous operation for 110 h.     

Reduced Graphene Oxide‐Wrapped Co9–xFexS8/Co,Fe‐N‐C Composite as Bifunctional Electrocatalyst for Oxygen Reduction and Evolution

Searching for highly efficient bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) using nonnoble metal‐based catalysts is essential for the development of many energy conversion systems, including rechargeable fuel cells and metal–air batteries. Here, Co_9–xFe_xS₈/Co,Fe‐N‐C hybrids wrapped by reduced graphene oxide (rGO) (abbreviated as S‐Co_9–xFe_xS₈@rGO) are synthesized through a semivulcanization and calcination method using graphene oxide (GO) wrapped bimetallic zeolite imidazolate framework (ZIF) Co,Fe‐ZIF (CoFe‐ZIF@GO) as precursors. Benefiting from the synergistic effect of OER active CoFeS and ORR active Co,Fe‐N‐C in a single component, as well as high dispersity and enhanced conductivity derived from rGO coating and Fe‐doping, the obtained S‐Co_9–xFe_xS₈@rGO‐10 catalyst shows an ultrasmall overpotential of ≈0.29 V at 10 mA cm^?2 in OER and a half‐wave potential of 0.84 V in ORR, combining a superior oxygen electrode activity of ≈0.68 V in 0.1 m KOH.     

Silk‐Derived 2D Porous Carbon Nanosheets with Atomically‐Dispersed Fe‐Nx‐C Sites for Highly Efficient Oxygen Reaction Catalysts

Controlled synthesis of highly efficient, stable, and cost‐effective oxygen reaction electrocatalysts with atomically‐dispersed Me–N_x–C active sites through an effective strategy is highly desired for high‐performance energy devices. Herein, based on regenerated silk fibroin dissolved in ferric chloride and zinc chloride aqueous solution, 2D porous carbon nanosheets with atomically‐dispersed Fe–N_x–C active sites and very large specific surface area (≈2105 m² g^?1) are prepared through a simple thermal treatment process. Owing to the 2D porous structure with large surface area and atomic dispersion of Fe–N_x–C active sites, the as‐prepared silk‐derived carbon nanosheets show superior electrochemical activity toward the oxygen reduction reaction with a half‐wave potential (E_1/2) of 0.853 V, remarkable stability with only 11 mV loss in E_1/2 after 30 000 cycles, as well as good catalytic activity toward the oxygen evolution reaction. This work provides a practical and effective approach for the synthesis of high‐performance oxygen reaction catalysts towards advanced energy materials.     

Co–Fe Alloy/N‐Doped Carbon Hollow Spheres Derived from Dual Metal–Organic Frameworks for Enhanced Electrocatalytic Oxygen Reduction

Metal–organic framework (MOF) composites have recently been considered as promising precursors to derive advanced metal/carbon‐based materials for various energy‐related applications. Here, a dual‐MOF‐assisted pyrolysis approach is developed to synthesize Co–Fe alloy@N‐doped carbon hollow spheres. Novel core–shell architectures consisting of polystyrene cores and Co‐based MOF composite shells encapsulated with discrete Fe‐based MOF nanocrystallites are first synthesized, followed by a thermal treatment to prepare hollow composite materials composed of Co–Fe alloy nanoparticles homogeneously distributed in porous N‐doped carbon nanoshells. Benefitting from the unique structure and composition, the as‐derived Co–Fe alloy@N‐doped carbon hollow spheres exhibit enhanced electrocatalytic performance for oxygen reduction reaction. The present approach expands the toolbox for design and preparation of advanced MOF‐derived functional materials for diverse applications.     

Atomically Dispersed Binary Co‐Ni Sites in Nitrogen‐Doped Hollow Carbon Nanocubes for Reversible Oxygen Reduction and Evolution

With the inspiration of developing bifunctional electrode materials for reversible oxygen electrocatalysis, one strategy of heteroatom doping is proposed to fabricate dual metal single‐atom catalysts. However, the identification and mechanism functions of polynary single‐atom structures remain elusive. Atomically dispersed binary Co‐Ni sites embedded in N‐doped hollow carbon nanocubes (denoted as CoNi‐SAs/NC) are synthesized via proposed pyrolysis of dopamine‐coated metal‐organic frameworks. The atomically isolated bimetallic configuration in CoNi‐SAs/NC is identified by combining microscopic and spectroscopic techniques. When employing as oxygen electrocatalysts in alkaline medium, the resultant CoNi‐SAs/NC hybrid manifests outstanding catalytic performance for bifunctional oxygen reduction/evolution reactions, boosting the realistic rechargeable zinc–air batteries with high efficiency, low overpotential, and robust reversibility, superior to other counterparts and state‐of‐the‐art precious‐metal catalysts. Theoretical computations based on density functional theory demonstrate that the homogenously dispersed single atoms and the synergistic effect of neighboring Co‐Ni dual metal center can optimize the adsorption/desorption features and decrease the overall reaction barriers, eventually promoting the reversible oxygen electrocatalysis. This work not only sheds light on the controlled synthesis of atomically isolated advanced materials, but also provides deeper understanding on the structure–performance relationships of nanocatalysts with multiple active sites for various catalytic applications.     

Co Nanoparticles Confined in 3D Nitrogen‐Doped Porous Carbon Foams as Bifunctional Electrocatalysts for Long‐Life Rechargeable Zn–Air Batteries

Proper design and simple preparation of nonnoble bifunctional electrocatalysts with high cost performance and strong durability for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is highly demanded but still full of enormous challenges. In this work, a spontaneous gas‐foaming strategy is presented to synthesize cobalt nanoparticles confined in 3D nitrogen‐doped porous carbon foams (CoNCF) by simply carbonizing the mixture of citric acid, NH₄Cl, and Co(NO₃)₂·6H₂O. Thanks to its particular 3D porous foam architecture, ultrahigh specific surface area (1641 m² g^?1), and homogeneous distribution of active sites (C–N, Co–N_x, and Co–O moieties), the optimized CoNCF‐1000‐80 (carbonized at 1000 °C, containing 80 mg Co(NO₃)₂·6H₂O in precursors) catalyst exhibits a remarkable bifunctional activity and long‐term durability toward both ORR and OER. Its bifunctional activity parameter (ΔE) is as low as 0.84 V, which is much smaller than that of noble metal catalyst and comparable to state‐of‐the‐art bifunctional catalysts. When worked as an air electrode catalyst in rechargeable Zn–air batteries, a high energy density (797 Wh kg^?1), a low charge/discharge voltage gap (0.75 V), and a long‐term cycle stability (over 166 h) are achieved at 10 mA cm^?2.     

Urchin‐Like Fe3Se4 Hierarchitectures: A Novel Pseudocapacitive Sodium‐Ion Storage Anode with Prominent Rate and Cycling Properties

Transition metal chalcogenides have received great attention as promising anode candidates for sodium‐ion batteries (SIBs). However, the undesirable cyclic life and inferior rate capability still restrict their practical applications. The design of micro–nano hierarchitectures is considered as a possible strategy to facilitate the electrochemical reaction kinetics and strengthen the electrode structure stability upon repeated Na⁺ insertion/extraction. Herein, urchin‐like Fe₃Se₄ hierarchitectures are successfully prepared and developed as a novel anode material for SIBs. Impressively, the as‐prepared urchin‐like Fe₃Se₄ can present an ultrahigh rate capacity of 200.2 mAh g^‐1 at 30 A g^‐1 and a prominent capacity retention of 99.9% over 1000 cycles at 1 A g^‐1, meanwhile, a respectable initial coulombic efficiency of ≈100% is achieved. Through the conjunct study of in situ X‐ray diffraction, ex situ X‐ray absorption near‐edge structure spectroscopy, as well as cyclic voltammetry curves, it is intriguing to reveal that the phase transformation from monoclinic to amorphous structure accompanied by the pseudocapacitive Na⁺ storage behavior accounts for the superior electrochemical performance. When paired with the Na₃V₂(PO₄)₃ cathode materials, the assembled full cell enables high energy density and decent cyclic stability, demonstrating potential practical feasibility of the present urchin‐like Fe₃Se₄ anode.     

Cu,N‐Codoped Carbon Nanodisks with Biomimic Stomata‐Like Interconnected Hierarchical Porous Topology as Efficient Electrocatalyst for Oxygen Reduction Reaction

Metal,N‐codoped carbon (M‐N‐C) nanostructures are promising electrocatalysts toward oxygen reduction reaction (ORR) or other gas‐involved energy electrocatalysis. Further creating pores into M‐N‐C nanostructures can increase their surface area, fully expose the active sites, and improve mass transfer and electrocatalytic efficiency. Nonetheless, it remains a challenge to fabricate M‐N‐C nanomaterials with both well‐defined morphology and hierarchical porous structures. Herein, high‐quality 2D Cu‐N‐C nanodisks (NDs) with biomimic stomata‐like interconnected hierarchical porous topology are synthesized via carbonization of Cu‐tetrapyridylporphyrin (TPyP)‐metal–organic frameworks (MOFs) precursors and followed by etching the carbonization product (Cu@Cu‐N‐C) along with re‐annealing treatment. Such hierarchical porous Cu‐N‐C NDs possess high specific surface area (293 m² g^?1) and more exposed Cu single‐atom sites, different from their counterparts (Cu@Cu‐N‐C) and pure N‐C control catalysts. Electrochemical tests in alkaline media reveal that they can efficiently catalyze ORR with a half‐wave potential of 0.85 V (vs reversible hydrogen electrode), comparable to Pt/C and outperforming Cu@Cu‐N‐C, N‐C, Cu‐TPyP‐MOFs, and most other reported M‐N‐C catalysts. Moreover, their stability and methanol‐tolerant capability exceed Pt/C. This work may shed some light on optimizing 2D M‐N‐C nanostructures through bio‐inspired pore structure engineering, and accelerate their applications in fuel cells, artificial photosynthesis, or other advanced technological fields.     

α‐Ni(OH)2 Originated from Electro‐Oxidation of NiSe2 Supported by Carbon Nanoarray on Carbon Cloth for Efficient Water Oxidation

Development of effective oxygen evolution reaction (OER) electrocatalysts has been intensively studied to improve water splitting efficiency and cost effectiveness in the last ten years. However, it is a big challenge to obtain highly efficient and durable OER electrocatalysts with overpotentials below 200 mV at 10 mA cm^?2 despite the efforts made to date. In this work, the successful synthesis of supersmall α‐Ni(OH)₂ is reported through electro‐oxidation of NiSe₂ loaded onto carbon nanoarrays. The obtained α‐Ni(OH)₂ shows excellent activity and long‐term stability for OER, with an overpotential of only 190 mV at the current density of 10 mA cm^?2, which represents a highly efficient OER electrocatalyst. The excellent activity could be ascribed to the large electrochemical surface area provided by the carbon nanoarray, as well as the supersmall size (≈10 nm) of α‐Ni(OH)₂ which possess a large number of active sites for the reaction. In addition, the phase evolution of α‐Ni(OH)₂ from NiSe₂ during the electro‐oxidation process was monitored with in situ X‐ray absorption fine structure (XAFS) analysis.     

Effective Fixation of Carbon in g‐C3N4 Enabled by Mg‐Induced Selective Reconstruction

The methodology of metal‐involved preparation for carbon materials is favored by researchers and has attracted tremendous attention. Decoupling this process and the underlying mechanism in detail are highly required. Herein, the intrinsic mechanism of carbon fixation in graphitic carbon nitride (g‐C₃N₄) via the magnesium‐involved carbonization process is reported and clarified. Magnesium can induce the displacement reaction with the small carbon nitride molecule generated by the pyrolysis of g‐C₃N₄, thus efficiently fixing the carbon onto the in situ template of Mg₃N₂ product to avoid the direct volatilization. As a result, the N‐doped carbon nanosheet frameworks with interconnected porous structure and suitable N content are constructed by reconstruction of carbon and nitrogen species, which exhibit a comparable photoelectric conversion efficiency (8.59%) and electrocatalytic performances to that of Pt (8.40%) for dye‐sensitized solar cells.     

Identifying the Activation of Bimetallic Sites in NiCo2S4@g‐C3N4‐CNT Hybrid Electrocatalysts for Synergistic Oxygen Reduction and Evolution

Hybrid materials composed of transition‐metal compounds and nitrogen‐doped carbonaceous supports are promising electrocatalysts for various electrochemical energy conversion devices, whose activity enhancements can be attributed to the synergistic effect between metallic sites and N dopants. While the functionality of single‐metal catalysts is relatively well‐understood, the mechanism and synergy of bimetallic systems are less explored. Herein, the design and fabrication of an integrated flexible electrode based on NiCo₂S₄/graphitic carbon nitride/carbon nanotube (NiCo₂S₄@g‐C₃N₄‐CNT) are reported. Comparative studies evidence the electronic transfer from bimetallic Ni/Co active sites to abundant pyridinic‐N in underlying g‐C₃N₄ and the synergistic effect with coupled conductive CNTs for promoting reversible oxygen electrocatalysis. Theoretical calculations demonstrate the unique coactivation of bimetallic Ni/Co atoms by pyridinic‐N species (a Ni, Co–N₂ moiety), which simultaneously downshifts their d‐band center positions and benefits the adsorption/desorption features of oxygen intermediates, accelerating the reaction kinetics. The optimized NiCo₂S₄@g‐C₃N₄‐CNT hybrid manifests outstanding bifunctional performance for catalyzing oxygen reduction/evolution reactions, highly efficient for realistic zinc–air batteries featuring low overpotential, high efficiency, and long durability, superior to those of physical mixed counterparts and state‐of‐the‐art noble metal catalysts. The identified bimetallic coactivation mechanism will shed light on the rational design and interfacial engineering of hybrid nanomaterials for diverse applications.     

Carbon‐Intercalated 0D/2D Hybrid of Hematite Quantum Dots/Graphitic Carbon Nitride Nanosheets as Superior Catalyst for Advanced Oxidation

Efficient charge separation and sufficiently exposed active sites are important for light‐driving Fenton catalysts. 0D/2D hybrids, especially quantum dots (QDs)/nanosheets (NSs), offer a better opportunity for improving photo‐Fenton activity due to their high charge mobility and more catalytic sites, which is highly desirable but remains a great challenge. Herein, a 0D hematite quantum dots/2D ultrathin g‐C₃N₄ nanosheets hybrid (Fe₂O₃ QDs/g‐C₃N₄ NS) is developed via a facile chemical reaction and subsequent low‐temperature calcination. As expected, the specially designed 0D/2D structure shows remarkable catalytic performance toward the removal of p‐nitrophenol. By virtue of large surface area, adequate active sites, and strong interfacial coupling, the 0D Fe₂O₃ QDs/2D g‐C₃N₄ nanosheets establish efficient charge transport paths by local in‐plane carbon species, expediting the separation and transfer of electron/hole pairs. Simultaneously, highly efficient charge mobility can lead to continuous and fast Fe(III)/Fe(II) conversion, promoting a cooperative effect between the photocatalysis and chemical activation of H₂O₂. The developed carbon‐intercalated 0D/2D hybrid provides a new insight in developing heterogeneous catalysis for a large variety of photoelectronic applications, not limited in photo‐Fenton catalysis.