Supramolecular gel-assisted synthesis of double shelled Co@CoO@N-C/C nanoparticles with synergistic electrocatalytic activity for the oxygen reduction reaction

Ultrafine Mo2C nanoparticles embedded in an MOF derived N and P co-doped carbon matrix for an efficient electrocatalytic oxygen reduction reaction in zinc-air batteries

Exploring high-activity electrocatalysts for an oxygen reduction reaction (ORR) is of great significance for a variety of renewable energy conversion and storage technologies. Here, ultrafine Mo₂C nanoparticles assembled in N and P-co-doped carbon (Mo₂C@NPC) was developed from ZIF-8 encapsulated molybdenum-based polyoxometalates (PMo₁₂) as a highly efficient ORR electrocatalyst and shows excellent performance for zinc-air batteries. The well distribution of the PMo₁₂ in ZIF-8 results in the formation of ultrafine Mo₂C nanocrystallites encapsulated in a porous carbon matrix after pyrolysis. Significantly, from experimental and theoretical investigations, the highly porous structure, highly dispersed ultrafine Mo₂C and the N and P co-doping in the Mo₂C@NPC lead to the remarkable ORR activity with an onset potential of ∼1.01 V, a half-wave potential of ∼0.90 V and a Tafel slope of 51.7 mV dec^-1 at 1600 rpm in 0.1 M KOH. In addition, the Mo₂C@NPC as an ORR catalyst in zinc-air batteries achieved a high power density of 266 mW cm^-2 and a high specific capacity of 780.9 mA h g^-1, exceeding that driven by commercial Pt/C. Our results revealed that the porous architecture and ultrafine Mo₂C nanocrystallites of the electrocatalysts could facilitate mass transport and increase the accessibility of active sites, thus optimizing their performances in an ORR. The present study provides some guidelines for the design and synthesis of efficient nanostructured electrocatalysts.

Interfacial engineering of Mo2C-Mo3C2 heteronanowires for high performance hydrogen evolution reactions

Non-precious metal-based electrocatalysts with high activity and stability for efficient hydrogen evolution reactions are of critical importance for low-cost and large-scale water splitting. In this work, Mo₂C-Mo₃C₂ heteronanowires with significantly enhanced catalytic performance are constructed from an MoAn precursor via an accurate phase transition process. The structure disordering and surface carbon shell of Mo₂C-Mo₃C₂ heteronanowires can be precisely regulated, resulting in an enlarged surface area and a defect-rich catalytic surface. Density functional theory calculations are used to identify the effect of the defective sites and carbon shell on the free energy for hydrogen adsorption in hydrogen evolution. Meanwhile, the synergistic effect between different phases and the introduced lattice defects of Mo₂C-Mo₃C₂ are considered to enhance the HER catalytic performance. The designed catalyst exhibits optimal electrocatalytic activity in both acidic and alkaline media: low overpotentials of 134 and 116 mV at 10 mA cm^-2, a small Tafel slope of 64 mV dec^-1, and a long-term stability for 5000 cycles. This work will provide new insights into the design of high-efficiency HER catalysts via interfacial engineering at the nanoscale for commercial water splitting.

MOF-derived two-dimensional N-doped carbon nanosheets coupled with Co-Fe-P-Se as efficient bifunctional OER/ORR catalysts

Developing highly efficient, low-cost and bifunctional electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) plays a pivotal role in the scalable applications of zinc-air (Zn-air) batteries. Herein, Co-Fe-P-Se nanoparticles supported on two-dimensional nitrogen-doped carbon (Co-Fe-P-Se/NC) to construct a three-dimensional nanostructure were obtained under the assistance of metal-organic frameworks (MOFs). The two-dimensional nanosheet facilitated the electron transfer rate and exposed abundant active sites. The three-dimensional morphology composed of nanosheets was favorable for electrolyte transport and provided abundant channels for gas diffusion during the catalytic process. Moreover, the coexistence of Co and Fe had important effects on promoting the catalytic performances. Lastly, the catalytic performances for OER and ORR could be promoted effectively after the introduction of selenium and phosphorous in the designed electrocatalyst. Benefiting from the above merits, the prepared Co-Fe-P-Se/NC exhibited excellent catalytic performances for OER (overpotential of 0.27 V at 10 mA cm^-2), ORR (half-wave potential of 0.76 V) and rechargeable batteries (a low voltage gap of 0.719 V, high power density of 104 mW cm^-2 at 200 mA cm^-2 and high energy density of 805 W h Kg_Zn^-1). Moreover, the prepared electrocatalyst possessed more stable long-term stability in all the conducted experiments. This work provides a novel approach to develop and construct high-performance bifunctional nanocatalysts for metal-air batteries.

Cobalt-Tannin-Framework-Derived Amorphous Co-P/Co-N-C on N, P Co-Doped Porous Carbon with Abundant Active Moieties for Efficient Oxygen Reactions and Water Splitting

It remains a tremendous challenge to develop a low-cost, earth-abundant, and efficient catalyst with multifunctional activities for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein, a facile and scalable avenue was developed to prepare amorphous Co-P/Co-N-C supported on N, P co-doped porous carbon (Co-P/Co-N-C/NPC) with a large specific surface area (1462.9 m²  g^-1 ) and abundant reactive sites including Co-P, Co-N and NPC. The prepared electrocatalyst exhibits outstanding catalytic performance for HER (η=234 mV at 10 mA cm^-2 ), OER (η=374 mV at 10 mA cm^-2 ), and ORR (E_1/2 =0.89 V, vs. reversible hydrogen electrode). Benefiting from the excellent HER performance and outstanding OER activity, the Co-P/Co-N-C/NPC delivers a current density of 10 mA cm^-2 for overall water splitting at a cell voltage of 1.59 V, which is comparable with the IrO₂ -Pt/C couple electrode.

Carved nanoframes of cobalt-iron bimetal phosphide as a bifunctional electrocatalyst for efficient overall water splitting

Water electrolysis for hydrogen production has long been regarded as an ideal tactic for renewable energy conversion and storage, but is impeded by the sluggish kinetics of both the hydrogen and oxygen evolution reactions, which are therefore in urgent need for high-performance but low-cost electrocatalysts. Herein, nanoframes of transition metal phosphides (TMPs) with the 3D framework carved open have been demonstrated as highly potent bifunctional catalysts for overall water splitting, reaching the benchmark performance of the Pt/C‖RuO2 couple, and are much superior to their nanocubic counterparts. This excellent water splitting behavior can be attributed to the enlarged active surface area, less obstructed electrolyte infiltration, promoted charge transfer, and facilitated gas release. Further through in-depth activity analysis and post-electrocatalysis characterization, special attention has been paid to the fate and role of phosphorus in the electrocatalytic process, suggesting that despite the chemical instability of the TMPs (especially under OER conditions), excellent electrocatalytic stability can still be achieved through the amorphous bimetallic hydroxides/oxides formed in situ.

Using lithium chloride as a medium to prepare N,P-codoped carbon nanosheets for oxygen reduction and evolution reactions

The N,P-codoped carbon nanosheets prepared using LiCl as the medium possess excellent bifunctional catalytic effects for ORR and OER due to the large specific surface area and hydrophilic surface.

Recent Advances of Cobalt-Based Electrocatalysts for Oxygen Electrode Reactions and Hydrogen Evolution Reaction

This review summarizes recent progress in the development of cobalt-based catalytic centers as the most potentially useful alternatives to noble metal-based electrocatalysts (Pt-, Ir-, and Ru-based) towards the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) in acid and alkaline media. A series of cobalt-based high-performance electrocatalysts have been designed and synthesized including cobalt oxides/chalcogenides, Co–Nx/C, Co-layered double hydroxides (LDH), and Co–metal-organic frameworks (MOFs). The strategies of controllable synthesis, the structural properties, ligand effect, defects, oxygen vacancies, and support materials are thoroughly discussed as a function of the electrocatalytic performance of cobalt-based electrocatalysts. Finally, prospects for the design of novel, efficient cobalt-based materials, for large-scale application and opportunities, are encouraged.

Restricting Growth of Ni3Fe Nanoparticles on Heteroatom-Doped Carbon Nanotube/Graphene Nanosheets as Air-Electrode Electrocatalyst for Zn-Air Battery

Exploring bifunctional oxygen electrode catalysts with efficient and stable oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) performance is one of the limitations for high-performance zinc-air battery. In this work, Ni₃Fe alloy nanoparticles incorporated in three-dimensional (3D) carbon nanotube (CNT)/graphene nanosheet composites with N and S codoping (Ni₃Fe/N-S-CNTs) as bifunctional oxygen electrode electrocatalysts for zinc-air battery. The main particle size of Ni₃Fe nanoparticles could be well restricted because of the unique 3D structure of carbon nanotube/graphene nanosheet composites (N-S-CNTs). The large specific area of N-S-CNTs is conducive to the uniform dispersion of Ni₃Fe nanoparticles. On the basis of the synergistic effect of Ni₃Fe nanoparticles with N-S-CNTs, and the sufficient exposure of reactive sites, the synthesized Ni₃Fe/N-S-CNTs catalyst exhibits excellent OER performance with a low overpotential of 215 mV at 10 mA cm^-2, and efficient ORR activity with a half-wave potential of 0.877 V. When used as an electrocatalyst in zinc-air battery, the device exhibits a power density of 180.0 mW cm^-2 and long term durability for 500 h.

Co2B and Co Nanoparticles Immobilized on the N-B-Doped Carbon Derived from Nano-B4C for Efficient Catalysis of Oxygen Evolution, Hydrogen Evolution, and Oxygen Reduction Reactions

A novel hybrid electrocatalyst of Co₂B and Co nanoparticles immobilized on N-B-doped carbon derived from nano-B₄C (Co₂B/Co/N-B-C/B₄C) is in situ synthesized by pyrolysis of nano-B₄C supporting Co(OH)₂ nanoparticles with melamine. The Co₂B and Co nanoparticles are formed and anchored on the generated N and B codoped carbon and undecomposed B₄C. The hybrid exhibits remarkable catalytic performances toward the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR)-a very small potential of 1.53 V at 10 mA cm^-2 for the OER and a high catalytic kinetics and superior durability for the ORR-which are superior to the RuO₂ and Pt/C catalyst, respectively. Most impressively, the hybrid delivers a very small potential gap of 710 mV, which is lower than those of most bifunctional electrocatalysts reported. In addition, the hybrid also shows a satisfying hydrogen evolution reaction performance offering a small overpotential of 220 mV at 10 mA cm^-2 and wonderful stability. The excellent trifunctional catalytic performances issue from synergetic effects of Co₂B, metal Co, Co/N-doped carbon, and B self-doped carbon coexisting in the hybrid with good interaction mutually. This work provides a new-type efficient multifunctional catalyst for regenerative fuel cell and overall water-splitting technologies.

N,S-Atom-coordinated Co9S8 trinary dopants within a porous graphene framework as efficient catalysts for oxygen reduction/evolution reactions

The intrinsic instability and difficulty in controlling the uniform size distributions of cobalt sulfides greatly restrict their application for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as a bifunctional electrocatalyst in regenerative fuel cells and rechargeable metal-air batteries. Herein, we synthesize a stable electrocatalyst of N,S-atom-coordinated Co9S8 trinary dopants within a porous graphene framework (Co9S8@NS-3DrGO), in which Co9S8 nanoparticles show uniform sizes and distributions. The stable Co9S8-based composites are fabricated by a facile soft template-assisted strategy, and the attraction of this method is that the intermediate of melamine formaldehyde resin (MFR) plays trifunctional roles, including (i) it acts as the templated bonding material to crosslink GO sheets together, (ii) it facilitates the formation of a core-shell architecture, and (iii) it acts as the N source for doping. Catalyst composition and performance are largely dependent on the pyrolysis temperature. Extensive investigation elucidates that the mechanism of electrocatalytic activity is attributed to: (i) the unique core-shell structure of the composites, as well as uniform particle sizes and distributions of Co9S8, (ii) the active nitrogen (pyridinic N and graphitic N) contents, and (iii) the large surface area and porous architecture. The composite pyrolyzed at 850 °C exhibits the best electrocatalytic performance, which shows a positive ORR half-wave potential (0.826 V), a small OER overpotential (317 mV) at 10 mA cm-2, and high stability, comparable to the commercial noble catalysts Pt/C and RuO2 in alkaline media. Furthermore, when applied in zinc-air batteries, it also displays a comparable performance to a Pt/C + RuO2 mixture catalyst. This work provides an approach to stabilize cobalt sulfides and control their particle sizes and distributions by ingeniously employing the soft template of MFR and pyrolyzing them at various temperatures.

Highly active bifunctional oxygen electrocatalysts derived from nickel- or cobalt-phytic acid xerogel for zinc-air batteries

Developing highly efficient non-noble metal electrocatalysts for oxygen electrode reactions is highly desirable for industrial scale application in energy related devices. Herein, two new kinds of Ni (POxN3-x)2/NPC and Co (POxN3-x)2/NPC (NPC: N, P-co-doped carbon) are synthesized through a facile post-treatment of nickel- or cobalt-phytic acid xerogel, followed by an annealing procedure under an argon and ammonia atmosphere at 800 °C. The as-prepared catalysts exhibit outstanding catalytic activities for both the oxygen reduction and evolution reactions, which are comparable to those of Pt/C and IrO2. Furthermore, the primary zinc-air batteries assembled with Ni (POxN3-x)2/NPC and Co (POxN3-x)2/NPC as the cathodes show gravimetric energy densities of 894 and 836 W h kgZn-1, which are superior to that of Pt/C (793 W h kgZn-1). In addition, the rechargeable zinc-air battery assembled with Ni (POxN3-x)2/NPC exhibits an excellent round-trip efficiency, which is shown by a slight increase in the sum of the overpotentials for discharge-charge cycling at a current density of 20 mA cm-2, even after experiencing 33 h of testing. To the best of our knowledge, there are few reports on metaphosphate salts where oxygen is partially replaced by nitrogen as bifunctional oxygen electrode catalysts for zinc-air batteries. This work provides an easy, low-cost and scalable avenue to develop new kinds of catalyst for application in energy devices.

Beyond Covalent Crosslinks: Applications of Supramolecular Gels

Traditionally, gels have been defined by their covalently cross-linked polymer networks. Supramolecular gels challenge this framework by relying on non-covalent interactions for self-organization into hierarchical structures. This class of materials offers a variety of novel and exciting potential applications. This review draws together recent advances in supramolecular gels with an emphasis on their proposed uses as optoelectronic, energy, biomedical, and biological materials. Additional special topics reviewed include environmental remediation, participation in synthesis procedures, and other industrial uses. The examples presented here demonstrate unique benefits of supramolecular gels, including tunability, processability, and self-healing capability, enabling a new approach to solve engineering challenges.

Supramolecular gel assisted synthesis of Co2P nanosheets as an efficient and stable catalyst for oxygen reduction reaction

Co₂P/C, with nanosheet morphology, is prepared through a facile supramolecular-gel assisted strategy which presents excellent electrocatalytic performance for ORR.

Porous graphene doped with Fe/N/S and incorporating Fe3O4 nanoparticles for efficient oxygen reduction

The honeycomb-like structured graphene was used to synthesize a 3D porous graphene doped with Fe/N/S and incorporating Fe₃O₄ nanoparticles. It showed comparable ORR activity to Pt/C, in alkaline media.

Self-assembled fibrillar networks induced by two methods: a new unmodified natural product gel

The discovery of new NPGs and the study of their self-assembing properties.

Metallic cobalt modified MnO–C nanocrystalline composites as an efficient bifunctional oxygen electrocatalyst

An efficient bifunctional oxygen electrocatalyst, slight metallic cobalt modified manganese oxide nanocrystalline is successfully synthesized through rheological phase reaction method.

Core@shell structured Co–CoO@NC nanoparticles supported on nitrogen doped carbon with high catalytic activity for oxygen reduction reaction

A composite with a hierarchical structure consisting of nitrogen doped carbon nanosheets with the deposition of nitrogen doped carbon coated Co–CoO nanoparticles (Co–CoO@NC/NC) has been synthesized by a simple procedure involving the drying of the reaction mixture containing Co(NO₃)₂, glucose, and urea and its subsequent calcination. The drying step is found to be necessary to obtain a sample with small and uniformly sized Co–CoO nanoparticles. The calcination temperature has a great effect on the catalytic activity of the final product. Specifically, the sample prepared at the calcination temperature of 800 °C shows better catalytic activity of the oxygen reduction reaction (ORR). Urea in the reaction mixture is crucial to obtain the sample with the uniformly sized Co–CoO nanoparticles and also plays an important role in improving the catalytic activity of the Co–CoO@NC/NC. Additionally, there exists a strong electronic interaction between the Co–CoO nanoparticles and the NC. Most interestingly, the Co–CoO@NC/NC is highly efficient for the ORR and can deliver an ORR onset potential of 0.961 V vs. RHE and a half-wave potential of 0.868 V vs. RHE. Both the onset and half-wave potentials are higher than those of most catalysts reported previously and even close to those of the commercial Pt/C (the ORR onset and half-wave potential of the Pt/C are 0.962 and 0.861 V vs. RHE, respectively). This, together with its high stability, strongly suggests that the Co–CoO@NC/NC could be used as an efficient catalyst for the ORR.

A simple method has been developed for the synthesis of Co–CoO@NC/NC, which exhibits high and stable performance for the ORR.

Robust Catalysis on 2D Materials Encapsulating Metals: Concept, Application, and Perspective

Great endeavors are undertaken to search for low-cost, rich-reserve, and highly efficient alternatives to replace precious-metal catalysts, in order to cut costs and improve the efficiency of catalysts in industry. However, one major problem in metal catalysts, especially nonprecious-metal catalysts, is their poor stability in real catalytic processes. Recently, a novel and promising strategy to construct 2D materials encapsulating nonprecious-metal catalysts has exhibited inimitable advantages toward catalysis, especially under harsh conditions (e.g., strong acidity or alkalinity, high temperature, and high overpotential). The concept, which originates from unique electron penetration through the 2D crystal layer from the encapsulated metals to promote a catalytic reaction on the outermost surface of the 2D crystal, has been widely applied in a variety of reactions under harsh conditions. It has been vividly described as "chainmail for catalyst." Herein, recent progress concerning this chainmail catalyst is reviewed, particularly focusing on the structural design and control with the associated electronic properties of such heterostructure catalysts, and also on their extensive applications in fuel cells, water splitting, CO₂ conversion, solar cells, metal-air batteries, and heterogeneous catalysis. In addition, the current challenges that are faced in fundamental research and industrial application, and future opportunities for these fantastic catalytic materials are discussed.

Cobalt ion-coordinated self-assembly synthesis of nitrogen-doped ordered mesoporous carbon nanosheets for efficiently catalyzing oxygen reduction

The design and synthesis of a promising porous carbon-based electrocatalyst with an ordered and uninterrupted porous structure for oxygen reduction reaction (ORR) is still a significant challenge. Herein, an efficient catalyst based on cobalt-embedded nitrogen-doped ordered mesoporous carbon nanosheets (Co/N-OMCNS) is successfully prepared through a two-step procedure (cobalt ion-coordinated self-assembly and carbonization process) using 3-aminophenol as a nitrogen source, cobalt acetate as a cobalt source and Pluronic F127 as a mesoporous template. This work indicates that the formation of a two dimensional nanosheet structure is directly related to the extent of the cobalt ion coordination interaction. Moreover, the critical roles of pyrolysis temperature in nitrogen doping and ORR catalytic activity are also investigated. Benefiting from the high surface area and graphitic degree, high contents of graphitic N and pyridinic N, ordered interconnected mesoporous carbon framework, as well as synergetic interaction between the cobalt nanoparticles and protective nitrogen doped graphitic carbon layer, the resultant optimal catalyst Co/N-OMCNS-800 (pyrolyzed at 800 °C) exhibits comparable ORR catalytic activity to Pt/C, superior tolerance to methanol crossover and stability.

Cobalt-zinc nitride on nitrogen doped carbon black nanohybrids as a non-noble metal electrocatalyst for oxygen reduction reaction

Bimetallic nitrides are now being considered as one of the emerging advanced functional materials due to their characteristic features and remarkable physicochemical properties. Herein, we report a new crystalline bimetallic nitride (Co₃ZnN) that belongs to the cubic crystal phase, which was successfully synthesized through direct nitridation of metallic salts as precursors. Co₃ZnN nanoparticles were then supported on nitrogen-doped XC-72 carbon black (N-CB), and this typical Co₃ZnN/N-CB nanohybrid discovered can serve as an efficient non-noble metal electrocatalyst with a 4e^- reaction pathway for ORR, and demonstrated excellent electrocatalytic performance with high activity and stability.

Strengthened Synergistic Effect of Metallic Mx Py (M = Co, Ni, and Cu) and Carbon Layer via Peapod-Like Architecture for Both Hydrogen and Oxygen Evolution Reactions

The smooth electric transmission is crucial for the high-efficient electrocatalysis. Herein, a series of peapod-like metallic M_x P_y /C (M = Co, Ni, and Cu) composites is developed as bifunctional catalysts toward hydrogen and oxygen evolution reactions. For the first time, the metallic property of Cu₃ P is confirmed through the theoretical calculation. The in-depth composition, structural and catalytic mechanism analysis of M_x P_y /C discloses that the comparable activity and considerable durability of these catalysts mainly result from the strengthened synergistic effect between metallic M_x P_y and carbon layer based on the unique peapod-like architecture. Especially, the atomic contact between M_x P_y and carbon not only provides an open channel for electronic transmission but also ensures the integrity of peapod-like structure. Furthermore, the high electric conductivity of the inner metallic M_x P_y and the outer carbon layer endows the M_x P_y /C catalyst with rapid charge migration during the electrocatalytic pathway. These findings shed light on the origin of high catalytic activity of M_x P_y /C and open a path for purposefully rationally synthesizing superior electrocatalysts.

Ultrathin LiCoO2 Nanosheets: An Efficient Water-Oxidation Catalyst

Ultrathin cation-exchanged layered metal oxides are promising for many applications, while such substances are barely successfully synthesized to show several atomic layer thickness, owing to the strong electrostatic force between the adjacent layers. Herein, we took LiCoO₂, a prototype cation-exchanged layered metal oxide, as an example to study. By developing a simple synthetic route, we synthesized LiCoO₂ nanosheets with 5-6 cobalt oxide layers, which are the thinnest ever reported. Ultrathin nanosheets thus prepared showed a surprising coexistence of increased oxidation state of cobalt ions and oxygen vacancy, as demonstrated by magnetic susceptibility, X-ray photoelectron, electron paramagnetic resonance, and X-ray absorption fine spectra. This unique feature enables a higher electronic conduction and electrophilicity to the adsorbed oxygen than the bulk. Consequently ultrathin LiCoO₂ nanosheets provided a current density of 10 mA cm^-2 at a small overpotential of a mere 0.41 V and a small Tafel slope of ∼88 mV/decade, which is strikingly followed by an excellent cycle life. The findings reported in this work suggest that ultrathin cation-exchanged layered metal oxides could be a next generation of advanced catalysts for oxygen evolution reaction.

Hierarchically Porous Electrocatalyst with Vertically Aligned Defect-Rich CoMoS Nanosheets for the Hydrogen Evolution Reaction in an Alkaline Medium

Effective electrocatalysts for the hydrogen evolution reaction (HER) in alkaline electrolytes can be developed via a simple solvothermal process. In this work, first, the prepared CoMoS nanomaterials through solvothermal treatment have a porous, defect-rich, and vertically aligned nanostructure, which is beneficial for the HER in an alkaline medium. Second, electron transfer from cobalt to MoS₂ that reduces the unoccupied d orbitals of molybdenum can also enhance the HER kinetics in an alkaline medium. This has been demonstrated via a comparison of the catalytic performances of CoMoS, CoS, and MoS₂. Third, the solvothermal treatment time evidently impacts the electrocatalytic activity. As a result, after 24 h of solvothermal treatment, the prepared CoMoS nanomaterials exhibit the lowest onset potential (42 mV) and overpotential (98 mV) for delivering a current density of 10 mA cm^-2 in a 1 M KOH solution. Thus, this study provides a simple method to prepare efficient electrocatalysts for the HER in an alkaline medium.

Uniform Fe3O4/Nitrogen-Doped Mesoporous Carbon Spheres Derived from Ferric Citrate-Bonded Melamine Resin as an Efficient Synergistic Catalyst for Oxygen Reduction

Developing a facile strategy to synthesize an efficient and inexpensive catalyst for the oxygen reduction reaction (ORR) is critical to the commercialization of many sustainable energy storage and conversion techniques. Herein, a novel and convenient strategy was presented to prepare Fe₃O₄ embedded into nitrogen-doped mesoporous carbon spheres (Fe₃O₄/N-MCS) by the polycondensation between methylolmelamines and ammonium ferric citrate (AFC) and subsequent pyrolysis process. In particular, the polycondensation reaction was completely finished within a very short time (6.5 min), and the iron contents can be adjusted and had a great influence on the microstructure. Moreover, the Fe₃O₄/N-MCS can be used as a robust catalyst for the ORR in alkaline media, and the catalyst with the iron content of 3.35 wt % exhibited excellent electrochemical performance in terms of more positive onset potential (E₀ = 1.036 V vs RHE) and half-wave potential (E_1/2 = 0.861 V) and much better methanol tolerance and long-term durability, in comparison with that of 20% Pt/C. The remarkable performance was ascribed to the characteristics of large specific surface area, mesoporous structure, high contents of pyridinic N and graphitic N, as well as strong electronic interaction between Fe₃O₄ and protective N-doped graphitic layers.

In situ integration of CoFe alloy nanoparticles with nitrogen-doped carbon nanotubes as advanced bifunctional cathode catalysts for Zn-air batteries

Electrochemical catalysis of O₂-incorporated reactions is a promising strategy for metal-air batteries. The performance of metal-air batteries is determined by the catalytic activities of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Therefore, developing efficient catalysts with superior activities for the ORR and OER is of great significance to expand the application range of metal-air batteries. Herein, CoFe alloy nanoparticles adhered to the inside wall of nitrogen doped carbon nanotubes (CoFe@NCNTs) are synthesized and can function as a Janus particle to efficiently catalyze the ORR and OER with desirable activities in 0.1 M KOH solution. Specifically, the CoFe@NCNTs present an onset potential of 0.95 V and a half-wave potential of 0.84 V as an ORR catalyst. When used as an air-cathode catalyst for a Zn-air battery, the CoFe@NCNTs cathode performs better than a Pt/C cathode, showing a high open-circuit potential of 1.45 V, a maximum power density of 150 mW cm^-2 and an average specific capacity of 808 mA h g_zn^-1 at current densities from 2 mA cm^-2 to 10 mA cm^-2.

Nitrogen and sulfur co-doping of 3D hollow-structured carbon spheres as an efficient and stable metal free catalyst for the oxygen reduction reaction

Three-dimensional, hollow-structured carbon sphere nanocomposites (N,S-hcs) doped with nitrogen and sulfur were prepared using a soft template approach followed by a high-temperature treatment. The synthesized N,S-hcs nanomaterials exhibited favourable catalytic activity for the oxygen reduction reaction (ORR) compared to carbon spheres doped solely with nitrogen (N-hcs), polypyrrole (PPY) solid nanoparticles and irregular fragments of polyaniline (PAN). These results demonstrated the co-doping of N/S and the relatively large surface area of the mesoporous carbon structure that enhanced the catalytic activity of the resulting material. Notably, the prepared N,S-hcs electrocatalysts provided four electron oxygen reduction selectivity, long-term durability and high resistance to methanol poisoning, all of which represented improvements over the conventional Pt/C electrocatalyst. The progress represented by this reported work is of great importance in the development of outstanding non-metal based electrocatalysts for the fuel cell industry.