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The oxygen reduction reaction (ORR) represents a cornerstone for many clean energy conversion technologies such as fuel cells and metal‐air batteries. Nevertheless, the commercialization of these technologies is largely impeded by the slow kinetics of ORR, for which active, durable and cost‐effective ORR catalysts are needed. In recent years, zeolitic imidazolate framework (ZIF) derived carbon materials emerge as a new class of non‐precious metal catalysts (NPMCs) toward ORR, largely benefiting from their high surface area, abundant porosity, tunable chemical/electronic structure, and superior ORR activity which is comparable or even surpasses those state‐of‐the‐art Pt‐based ORR catalysts. This review offers a comprehensive overview of the recent advances in ZIF‐derived carbons for ORR. The synthesis strategies and the key factors affecting the ORR performance of ZIF‐derived carbon materials are discussed. Future research directions and perspectives on exploring ZIF derived carbons as efficient ORR catalysts are highlighted, with a focus on the principles of rationally engineering the coordination structures of active sites. [ABSTRACT FROM AUTHOR]

Oxygen reduction reaction (ORR) is the bottleneck of metal‐air batteries and fuel cells. Strain regulation can change the geometry and adjust the surface charge distribution of catalysts, which is a powerful strategy to optimize the ORR activity. The introduction of controlled strain to the material is still difficult to achieve. Herein, we present a temperature‐pressure‐induced strategy to achieve the controlled lattice strain for metal coordination polymers. Through the systematic study of the strain effect on ORR performance, the relationship between geometric and electronic effects is further understood and confirmed. The strained Co‐DABDT (DABDT=2,5‐diaminobenzene‐1,4‐dithiol) with 2 % lattice compression exhibits a superior half‐wave potential of 0.81 V. Theoretical analysis reveals that the lattice strain changes spin‐charge densities around S atoms for Co‐DABDT, and then regulates the hydrogen bond interaction with intermediates to promote the ORR catalytic process. This work helps to understand the catalytic mechanism from the atomic level. [ABSTRACT FROM AUTHOR]

The high overpotential of oxygen reduction reaction (ORR) prevents the wide commercialization of fuel cells and metal‐air batteries. To accelerate the reaction rate, porous electrocatalysts draw attention to obtain a higher specific surface area. However, the effect of micropores on ORR kinetics has not been understood because of the non‐uniform pore sizes, length, and tortuosity of practical electrocatalysts. In this study, we evaluate ORR activity in a micropore using platinum model electrodes with arrays of cylindrical pores with a uniform pore diameter of 1.8 nm. The model electrodes having pore lengths of 45, 100, and 380 nm are fabricated, and their ORR performances are examined by electrochemical measurements and numerical simulations in 0.1 mol dm−3 KOH aqueous solution. The intrinsic ORR activity of the micropore is successfully obtained with the electrode having a pore length of 45 nm, where oxygen transport resistance in the micropore has little effect on ORR. It is found that the intrinsic ORR activity of the micropore is higher than that of a planar Pt surface. X‐ray photoelectron spectroscopy and CO stripping voltammetry indicate a downshift of the d‐band center, which can be the origin of the high intrinsic ORR activity. [ABSTRACT FROM AUTHOR]

Zinc-air batteries and zinc-air fuel cells (ZAFC) are types of metal-air batteries that use zinc and oxygen from the air to generate power. They have high energy densities and are cost-effective to manufacture. During discharge, zinc particles form a porous anode saturated with electrolyte, while oxygen reacts at the cathode to form hydroxyl ions that migrate into the zinc paste and release electrons to travel to the cathode. In this study, a ZAFC was designed and produced with polyvinyl chloride (PVC) thermoplastic without any separator. The performance of the prepared air cathode in various temperatures and concentrations of the KOH electrolyte was evaluated. The results showed high current density and low Tafel slope at a high and low current density at a temperature of 318 K and a concentration of 4M. The prepared ZAFC has an open-circuit potential (OCP) of 1.346 V. The current density of 166.5 mA · cm² was obtained at the onset potential of 0.73 V. The power density and discharge capacity of the cell were 205.5 mW, and 283 mAh, respectively. These results show a high-performance single cell. [ABSTRACT FROM AUTHOR]

The use of platinum-free (Pt) cathode electrocatalysts for oxygen reduction reactions (ORRs) has been significantly studied over the past decade, improving slow reaction mechanisms. For many significant energy conversion and storage technologies, including fuel cells and metal–air batteries, the ORR is a crucial process. These have motivated the development of highly active and long-lasting platinum-free electrocatalysts, which cost less than proton exchange membrane fuel cells (PEMFCs). Researchers have identified a novel, non-precious carbon-based electrocatalyst material as the most effective substitute for platinum (Pt) electrocatalysts. Rich sources, outstanding electrical conductivity, adaptable molecular structures, and environmental compatibility are just a few of its benefits. Additionally, the increased surface area and the simplicity of regulating its structure can significantly improve the electrocatalyst's reactive sites and mass transport. Other benefits include the use of heteroatoms and single or multiple metal atoms, which are capable of acting as extremely effective ORR electrocatalysts. The rapid innovations in non-precious carbon-based nanomaterials in the ORR electrocatalyst field are the main topics of this review. As a result, this review provides an overview of the basic ORR reaction and the mechanism of the active sites in non-precious carbon-based electrocatalysts. Further analysis of the development, performance, and evaluation of these systems is provided in more detail. Furthermore, the significance of doping is highlighted and discussed, which shows how researchers can enhance the properties of electrocatalysts. Finally, this review discusses the existing challenges and expectations for the development of highly efficient and inexpensive electrocatalysts that are linked to crucial technologies in this expanding field. [ABSTRACT FROM AUTHOR]

In light of the depletion of conventional energy sources, it is imperative to conduct research and development on sustainable alternative energy sources. Currently, electrochemical energy storage and conversion technologies such as fuel cells and metal-air batteries rely heavily on precious metal catalysts like Pt/C and IrO2, which hinders their sustainable commercial development. Therefore, researchers have devoted significant attention to non-precious metal-based catalysts that exhibit high efficiency, low cost, and environmental friendliness. Among them, perovskite oxides possess low-cost and abundant reserves, as well as flexible oxidation valence states and a multi-defect surface. Due to their advantageous structural characteristics and easily adjustable physicochemical properties, extensive research has been conducted on perovskite-based oxides. However, these materials also exhibit drawbacks such as poor intrinsic activity, limited specific surface area, and relatively low apparent catalytic activity compared to precious metal catalysts. To address these limitations, current research is focused on enhancing the physicochemical properties of perovskite-based oxides. The catalytic activity and stability of perovskite-based oxides in Oxygen Reduction Reaction/Oxygen Evolution Reaction (ORR/OER) can be enhanced using crystallographic structure tuning, cationic regulation, anionic regulation, and nano-processing. Furthermore, extensive research has been conducted on the composite processing of perovskite oxides with other materials, which has demonstrated enhanced catalytic performance. Based on these different ORR/OER modification strategies, the future challenges of perovskite-based bifunctional oxygen electrocatalysts are discussed alongside their development prospects. [ABSTRACT FROM AUTHOR]

Solid oxide fuel cells (SOFCs) represent a breed of eco-friendly, weather-independent, decentralized power generation technologies, distinguished for their broad fuel versatility and superior electricity generation efficiency. At present, SOFCs are impeded by a lack of highly efficient oxygen reduction catalysts, a factor that significantly constrains their performance. The double perovskites LnBaCo2O5+δ (Ln = Lanthanide), renowned for their accelerated oxygen exchange and conductivity features, are widely acclaimed as a promising category of cathode catalysts for SOFCs. This manuscript offers a novel perspective on the physicochemical attributes of LnBaCo2O5+δ accumulated over the past two decades and delineates the latest advancements in fine-tuning the composition and nanostructure for SOFC applications. It highlights surface chemistry under operational conditions and microstructure as emerging research focal points towards achieving high-performance LnBaCo2O5+δ catalysts. This review offers a comprehensive insight into the latest advancements in utilizing LnBaCo2O5+δ in the field of SOFCs, presenting a clear roadmap for future developmental trajectories. Furthermore, it provides valuable insights for the application of double perovskite materials in domains such as water electrolysis, CO2 electrolysis, chemical sensors, and metal–air batteries. [ABSTRACT FROM AUTHOR]

The oxygen reduction and evolution reactions are considered the bottleneck in many electrochemical devices, i. e. fuel cells, water electrolyzers, and metal‐air batteries. The continuous focus has been on inventing and exploring cost‐effective and robust electrocatalysts. Few developed non‐precious metal/metal‐free materials, in fact, outperformed state‐of‐the‐art catalysts during the half‐cell study. However, most of these materials show limited activity during the full cell demonstration, restricting their deployment in commercial energy devices. In this direction, transition metal nitrides (TMNs) have emerged as a potential alternative with peculiar electronic properties and the ease of tuning their intrinsic as well as extrinsic properties. High hardness, refractory nature, d‐band modulation ability and comparatively lower energy for the nitride formation are the other motivations to explore their effectiveness in oxygen electrocatalysis. Considering this, the minireview attempts first to present the properties of catalytic interest, followed by the most viable synthesis approaches in nanoengineering of the TMNs. Next, we provide key trends toward catalytic property modulation for oxygen electrocatalysis, the role of TMNs as potential catalytic support, followed by the effect of TMNs′ in situ autoxidation on the performance. Finally, we state the current limitations of TMNs toward oxygen electrocatalysis, followed by our vision for further advancements. [ABSTRACT FROM AUTHOR]

Graphene is a well-known two-dimensional material with a large surface area and is used for numerous applications in a variety of fields. Metal-free carbon materials such as graphene-based materials are widely used as an electrocatalyst for oxygen reduction reactions (ORRs). Recently, more attention has been paid to developing metal-free graphenes doped with heteroatoms such as N, S, and P as efficient electrocatalysts for ORR. In contrast, we found our prepared graphene from graphene oxide (GO) by the pyrolysis method under a nitrogen atmosphere at 900 °C has shown better ORR activity in aqueous 0.1 M potassium hydroxide solution electrolyte as compared with the electrocatalytic activity of pristine GO. At first, we prepared various graphene by pyrolysis of 50 mg and 100 mg of GO in one to three alumina boats and pyrolyzed the samples under a N2 atmosphere at 900 °C. The prepared samples are named G50-1B to 3B and G100-1B and G100-2B. The prepared GO and graphenes were also analyzed under various characterization techniques to confirm their morphology and structural integrity. The obtained results suggest that the ORR electrocatalytic activity of graphene may differ based on the pyrolysis conditions. We found that G100-1B (Eonset, E1/2, JL, and n values of 0.843, 0.774, 4.558, and 3.76) and G100-2B (Eonset, E1/2, and JL values of 0.837, 0.737, 4.544, and 3.41) displayed better electrocatalytic ORR activity, as did Pt/C electrode (Eonset, E1/2, and JL values of 0.965, 0.864, 5.222, and 3.71, respectively). These results display the wide use of the prepared graphene for ORR and also can be used for fuel cell and metal–air battery applications. [ABSTRACT FROM AUTHOR]

Low‐cost and high‐efficiency oxygen reduction reaction (ORR) electrocatalysts are the basis for large‐scale applications of electrochemical energy storage and conversion technologies such as fuel cells and metal‐air batteries. Herein, cobalt and oxygen vacancy‐rich cobalt oxide encapsulated in nitrogen‐doped carbon and carbon nanotubes (Co/CoO@NC‐CNTs) have been prepared by direct solid‐state pyrolysis of Co‐MOF. The carbon morphology, oxygen vacancy content and type of nitrogen can be finely tuned by mixing Co‐MOF with proper amounts of melamine. The optimized material has an interwoven CNT structure with abundant micro‐ and mesopores, suitable nitrogen type and certain number of oxygen vacancies and exhibiting outstanding ORR electrocatalytic performance in alkaline media with an onset potential of 0.96 V and a half‐wave potential of 0.88 V vs RHE, which is higher than 20 wt% Pt/C, as well as being highly stable and well resistant to methanol. [ABSTRACT FROM AUTHOR]

Oxygen reduction reaction (ORR) is a key step in many electrochemical devices such as fuel cells and metal-air batteries. However, the reaction proceeds at a significant overpotential requiring Pt-based catalysts. The scarcity and economical challenges associated with Pt is one of the major limitations for the commercialization of the devices. In this context, the electrochemical research community is constantly exploring other low-cost and earth-abundant materials as ORR catalysts. Carbon nanomaterials are identified as promising electrocatalysts due to their superb electronic conductivity together with high specific surface area. However, the low reactivity of carbon is the major limiting factor in the fabrication of ORR catalysts. Recent studies have proved that chemical modification of the carbon network (substitution of foreign atoms, Ex: N, S, B, F, P) could alter the reactivity of carbon nanomaterials for ORR. Many doping strategies have been proposed including single atom doping, co-doping and multi-atom doping. The heteroatom doped carbons have delivered promising results towards ORR in alkaline media. This review presents a rational approach of doping methods and the electrochemical properties of heteroatom doped carbons, and we believe that this review could be a guiding material to design advanced non-noble catalysts for ORR in the coming future. [Display omitted] • Overview of oxygen reduction reaction principle and mechanism. • Design and development of heteroatom doped carbon based electrocatalysts. • Half-cell studies of various heteroatom doped electrocatalysts. • AEMFC performance of heteroatom doped carbon catalysts. • Overview on the stability of heteroatom doped electrocatalysts. [ABSTRACT FROM AUTHOR]

FeN 4 single-atom catalysts are among the most promising electrocatalysts for oxygen reduction reaction (ORR), which has important roles in energy conversion systems, such as fuel cells and metal-air batteries. It has been investigated that the activity of FeN 4 single-atom catalysts is highly dependent on the density and distance of the FeN 4 active centers. However, controllable synthesis of FeN 4 single-atom catalysts with designed active center distance and density is still a remaining difficulty. In this work, a series of pyridyl conjugated microporous polymers with varying types of nitrogen are used as precursors to synthesize carbonaceous single-atom ORR catalysts with different amounts of FeN 4 active centers. The influence of the nature of the nitrogen groups in the precursors on the number of FeN 4 active sites and the ORR activity trend of the obtained FeN 4 –C catalysts was investigated. The catalyst with the highest density of FeN 4 sites showed a high efficiency for ORR, making its application in Zn-air batteries promising. FeN 4 –C catalysts with different active site densities are synthesized by a series of pyridyl conjugated microporous polymers with different amount and position of N as precursors. The FeN 4 –C single-atom catalysts carbonized from polymeric precursors show increasing ORR activity with the increasing of active site density. The optimized FeN 4 –C-4 catalysts with the highest density of FeN 4 active sites show superior ORR performance than Pt/C and having potential for applications in Zn-air battery. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights: A comprehensive discussion of monitoring the structure evolutions of catalysts during ORR via multiple in-situ techniques to identify the active sites is presented. The extensive applications of in-situ techniques in elucidating the oxygen reduction reaction (ORR) mechanisms/pathways are reviewed. The challenges and recommendations of current in-situ techniques for monitoring the various dynamic evolutions in ORR are pointed out. Electrocatalytic oxygen reduction reaction (ORR) is one of the most important reactions in electrochemical energy technologies such as fuel cells and metal–O2/air batteries, etc. However, the essential catalysts to overcome its slow reaction kinetic always undergo a complex dynamic evolution in the actual catalytic process, and the concomitant intermediates and catalytic products also occur continuous conversion and reconstruction. This makes them difficult to be accurately captured, making the identification of ORR active sites and the elucidation of ORR mechanisms difficult. Thus, it is necessary to use extensive in-situ characterization techniques to proceed the real-time monitoring of the catalyst structure and the evolution state of intermediates and products during ORR. This work reviews the major advances in the use of various in-situ techniques to characterize the catalytic processes of various catalysts. Specifically, the catalyst structure evolutions revealed directly by in-situ techniques are systematically summarized, such as phase, valence, electronic transfer, coordination, and spin states varies. In-situ revelation of intermediate adsorption/desorption behavior, and the real-time monitoring of the product nucleation, growth, and reconstruction evolution are equally emphasized in the discussion. Other interference factors, as well as in-situ signal assignment with the aid of theoretical calculations, are also covered. Finally, some major challenges and prospects of in-situ techniques for future catalysts research in the ORR process are proposed. [ABSTRACT FROM AUTHOR]

Oxygen reduction reaction (ORR), an essential reaction in metal–air batteries and fuel cells, still faces many challenges, such as exploiting cost-effective nonprecious metal electrocatalysts and identifying their surface catalytic sites. Here we introduce bulk defects, Frank partial dislocations (FPDs), into metallic cobalt to construct a highly active and stable catalyst and demonstrate an atomic-level insight into its surface terminal catalysis. Through thermally dealloying bimetallic carbide (Co3ZnC), FPDs were in situ generated in the final dealloyed metallic cobalt. Both theoretical calculations and atomic characterizations uncovered that FPD-driven surface terminations create a distinctive type of surface catalytic site that combines concave geometry and compressive strain, and this two-in-one site intensively weakens oxygen binding. When being evaluated for the ORR, the catalyst exhibits onset and half-wave potentials of 1.02 and 0.90 V (versus the reversible hydrogen electrode), respectively, and negligible activity decay after 30,000 cycles. Furthermore, zinc–air batteries and H2–O2/air fuel cells built with this catalyst also achieve remarkable performance, making it a promising alternative to state-of-the-art Pt-based catalysts. Our findings pave the way for the use of bulk defects to upgrade the catalytic properties of nonprecious electrocatalysts. [ABSTRACT FROM AUTHOR]

The oxygen reduction reaction (ORR) is of great importance for clean energy storage and conversion techniques such as fuel cells and metal–air batteries (MABs). However, the ORR is kinetically sluggish, and expensive noble metal catalysts are required. The high price and limited preservation of noble metal catalysts has largely hindered the wide application of clean power sources such as fuel cells and MABs. Therefore, it is important to prepare non-expensive metal catalysts (NPMC) to cut the price of the fuel cells and MABs for wide application. Here, we report the preparation of a Co3O4 carried on the N-doped carbon (Co/N-C) as the ORR NPMC with a facile Pharaoh's Snakes reaction. The gas generated during the reaction is able to fabricate the porous structure of the resultant carbon doped with heteroatoms such as Co and N. The catalyst provides a high electrocatalytic activity towards ORR via the 4-e pathway with an onset and half-wave potential of 0.98 and 0.79 V (vs. RHE), respectively, in an electrolyte of 0.1 M KOH. The onset and half-wave potentials are close to those of the commercial Pt/C. This work demonstrates the promising potential of an ancient technology for preparing NPMCs toward the ORR. [ABSTRACT FROM AUTHOR]

Graphene supported FeN5 active sites, exhibiting high activity and stability of oxygen reduction reaction, enable a H2-air fuel cell to decay little during the latter 20–100 h operation. [Display omitted] • Electrochemically-exfoliated graphene improves forming FeN 5 with NH 3. • Graphene benefits catalytic stability of oxygen reduction reaction. • Fuel cell keep stable with little decay in latter 80 h operation. • High half-wave potentials in alkaline medium (0.940 V). Metal-nitrogen-carbon catalytic material, generally thought as a promising low-cost electrocatalyst of oxygen reduction reaction (ORR), surfers from long-term catalytic durability for fuel cell and metal-air battery. Here, we describe an approach of synthesizing electrochemically exfoliated graphene supported penta-nitrogen coordinated iron exhibiting encouraging ORR catalytic activity and durability. The synthesized catalyst exhibits high half-wave potentials in alkaline medium (E 1/2 0.940 V) enabling high-performance zinc-air battery and in acidic condition for durable fuel cell with a high peak power density (630 mW cm−2). The current density of a proton exchange membrane fuel cell loading the catalyst as cathode tends to decay little after initial hours under H 2 -air measurement, remarkably inhibiting commonly decline tendency of iron–nitrogen-carbon catalysts. DFT calculation combined with extensive experimental investigations demonstrate that penta-nitrogen coordinated iron supported by graphene is thermodynamically stable, and thus benefits the catalytic stability of ORR. [ABSTRACT FROM AUTHOR]

The development of electrocatalysts for oxygen reduction reaction (ORR) is important for energy conversion devices, such as fuel cells, and metal‐air batteries. Here, we have developed a confined space strategy to prepare a two‐dimensional (2D) leaf‐like nitrogen (N)‐containing porous carbon as a single‐atom catalyst substrate. ZIF−L materials have been confined in a thin silica layer to regulate the pyrolysis. The obtained Fe single atoms doped N‐containing porous carbons (Fe SAs/N−C) maintain the 2D morphology and have Fe single‐atom active sites. Correspondingly, Fe SAs/N−C exhibits excellent ORR performance (E1/2 of 0.907 V), which is more positive than those of commercially available Pt/C (0.874 V) and most reported non‐noble metal catalysts. The durability test shows that Fe SAs/N−C exhibits good stability during electrocatalytic process. This rational design shows a new strategy to prepare 2D catalyst supports with single‐atom active sites. [ABSTRACT FROM AUTHOR]

The oxygen reduction reaction (ORR) plays a crucial role in green-energy-related technologies including fuel cells and metal-air batteries. The rational design of cost-effective and high-efficiency electrocatalysts is of paramount importance to accelerate the sluggish kinetics of ORR. Recently, owing to the structural flexibility, high porosity, and large surface area, metal-organic frameworks (MOF)-derived carbon-based electrocatalysts have drawn significant attention as competent ORR catalysts. This review systematically summarizes recent progress toward advanced MOF-derived carbon-based electrocatalysts for ORR from the aspects of composition engineering, structural engineering, and the practical application in fuel cells and metal-air batteries, with the emphasis on the synthetic methods and the engineering mechanisms to achieve the enhanced ORR performance. Finally, some existing challenges and future perspectives for MOF-derived carbon-based electrocatalysts are proposed. • Recent advance on MOF-derived carbon-based ORR electrocatalysts is reviewed. • ORR mechanism and design principles of catalysts are discussed. • Composition engineering and structure engineering of MOF-derived carbon-based electrocatalysts are mainly scrutinized. • Facing challenges and future prospects of carbon-based electrocatalysts are addressed. [ABSTRACT FROM AUTHOR]

Transition metal (TM) single atomic catalysts (MSAC-N-C) derived from doped zeolite imidazolate frameworks (ZIF-8) are considered attractive oxygen reduction reaction (ORR) catalysts for fuel cells and metal-air batteries due to their advantages of high specific surface area, more active catalytic sites, adjustable pore size, and coordination topology features. This review provides an updated overview of the latest advances of MSAC-N-C catalysts derived from ZIF-8 precursors in ORR electrocatalysis. Particularly, some key challenges, including coordination environments regulation of catalysis center in MSAC-N-C, the active sites loading optimization and synergistic effects between TM nanoclusters/nanoparticles and the single atoms on MSAC-N-C catalysis activity, as well as their adaptability in various devices, are summarized for improving future development and application of MSAC-N-C catalysts. In addition, this review puts forward future research directions, making it play a better role in ORR catalysis for fuel cells and metal air batteries. [ABSTRACT FROM AUTHOR]

Opening up electrocatalysts for oxygen reduction reaction (ORR) is essential for practical application in fuel cells and metal-air batteries; however, how to make the catalysts with both good performance and low cost is difficult. Recently, research on the ORR of molybdenum disulfide-based catalysts in alkaline electrolytes has been on the rise. However, the development of MoS2 catalyst for acidic ORR is still in its infancy. Herein, without using reductant and morphology control reagent, we firstly obtained flowerball-like MoS2/Vulcan XC-72R (VC) nanocomposites via hydrothermal method. The designed composite exhibits a nearly 4e− ORR process with 0.78 and 0.92 V onset potentials in 0.1 M KOH and HClO4, respectively. Furthermore, the flowerball-like composite shows utmost electrochemical stability judging by 87 and 80% current retention for about 5.5 h either in alkaline or acid media, long term durability for continuous 10,000 cycles, and stronger resistance to methanol than the commercial Pt/C catalyst. The abundant Mo edges as catalytic active centers of flowerball-like structure, high electron conductivity, and enhanced mass transport in either alkaline or acidic electrolyte are favorable for catalytic performance. The prepared catalyst provides great potential for the substitution of noble metal based catalysts in fuel cells and metal-air batteries. [ABSTRACT FROM AUTHOR]

Oxygen reduction reaction (ORR) is a vital electrochemical reaction for energy conversion that has fascinated the interest of materials researchers to promote active and cost-effective electrocatalysts with improved reaction kinetics. Numerous, researches have been undertaken to achieve the desired characteristic cathode materials to revolutionize energy conversion devices including fuel cells and metal-air batteries (where ORR occurs). More importantly, the evaluation of electrocatalyst efficiency for ORR using typical electrochemical experiments, data collection, and analysis must be performed systematically and uniformly. This review provides a systematic guide to assess the electrochemical efficiency because it includes standard and widely recognized experimental protocols for each electrochemical technique, as well as easy-to-follow lab procedures and a detailed analysis of essential kinetic ORR parameters with a theoretical framework. The major function of each electrode in cyclic voltammetry, hydrodynamic linear scan voltammetry with rotating disc electrodes, and electrochemical impedance spectroscopy for ORR were demonstrated, as well as the construction of a three-electrode electrochemical cell. By following standard protocols and experimental procedures, an aspiring electrochemist would be capable to enforce fundamental and advanced electrochemical techniques much efficiently in their understanding of ORR kinetics evaluation. [Display omitted] • This review provides crucial insights and information on electrochemistry of ORR. • Overview on electrochemical techniques, measurements and calculation procedures. • The desired experimental approach for evaluating ORR electrocatalyst efficiency. • A beginner's guide to perform cathodic ORR for fuel cell applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Oxygen reduction reaction (ORR) has been extensively used in fuel cells and rechargeable metal-air batteries. • This review details the latest developments in ORR electrocatalyst development. • These electrocatalysts specifically include platinum group metals (PGM), transition metals and carbon-based catalysts. • The preparation and ORR performance of various electrocatalysts are discussed in detail. • Speculation and discussion on the development prospects and challenges of ORR electrocatalysts. Oxygen reduction reaction (ORR) has gradually been in the limelight in recent years because of its great application potential for fuel cells and rechargeable metal-air batteries. Therefore, significant issues are increasingly focused on developing effective and economical ORR electrocatalysts. This review begins with the reaction mechanisms and theoretical calculations of ORR in acidic and alkaline media. The latest reports and challenges in ORR electrocatalysis are traced. Most importantly, the latest advances in the development of ORR electrocatalysts are presented in detail, including platinum group metal (PGM), transition metal, and carbon-based electrocatalysts with various nanostructures. Furthermore, the development prospects and challenges of ORR electrocatalysts are speculated and discussed. These insights would help to formulate the design guidelines for highly-active ORR electrocatalysts and affect future research to obtain new knowledge for ORR mechanisms. [ABSTRACT FROM AUTHOR]

Developing cost‐effective non‐precious metal electrocatalysts (NPMCs) for the sluggish oxygen reduction reaction (ORR) is critical for the large‐scale application of metal‐air battery and fuel cell technologies. Herein, we present an effective Mn/Zn bimetal organic frameworks (MOFs)‐based strategy to synthesize an efficient manganese (Mn) and nitrogen (N) co‐doped porous carbon electrocatalyst with atomically dispersed N‐coordinated single Mn moieties (Mn‐Nx) for ORR catalysis. This strategy involves the adsorption of Mn ions into the Mn/Zn bimetal MOFs followed by the subsequent pyrolysis in Ar and then activation in NH3. Owing to the atomically dispersed Mn‐Nx sites, the large specific surface area and the high content of graphitic‐N, the resulting electrocatalyst shows a superior ORR activity with a half‐wave potential of 0.885 V and an outstanding electrochemical durability in alkaline electrolyte outperforming commercial Pt/C, and it also exhibits a superior performance for an assembled primary Zn–air battery. This work offers a new prospect for the synthesis of MOFs‐derived highly efficient NPMCs for ORR. [ABSTRACT FROM AUTHOR]

[Display omitted] Designing efficient electrocatalysts for the oxygen reduction reaction (ORR) is crucial to enhance the energy efficiencies of metal-air batteries and fuel cells. Palladium (Pd) catalysts show great potential due to their high intrinsic activity towards ORR but suffer from inferior durability. Here, we aim to employ tin oxide (SnO 2) supports to tailor the lattice strain and electron density of Pd catalysts to enhance their ORR performance. By using electrospinning and solvothermal techniques, a hierarchical Pd/SnO 2 hybrid catalyst was facilely synthesized with Pd nanoparticles anchored onto both the inside and outside walls of nanotube-like SnO 2 supports. Owing to the SnO 2 supports and the endowing metal-support interactions, tensile-strain and electron-rich features were both verified for the Pd nanoparticles in the Pd/SnO 2 catalyst. In comparison, no such features were found for the Pd nanoparticles in the Pd/C catalyst. As a consequence, the Pd/SnO 2 hybrid catalyst exhibits 2.5-times higher mass activity than the Pd/C catalyst and greatly improved durability with a current decay of 4% loss over 50 h compared with that (18%) of the Pd/C catalyst. [ABSTRACT FROM AUTHOR]

Summary: The highly efficient oxygen reduction reaction (ORR) electrocatalysts are desirable for the future energy storage scenario. Herein, we report three‐dimensional (3D) nanostructured electrocatalysts constructed by in situ vertical assembly of few‐layer ReS2 nanosheets onto the surface of nitrogen‐doped reduced graphene oxide (ReS2/NRGO). The ReS2/NRGO hybrids are featured with the fully exposed active sites, interconnected conducting channels, and facilitated charge transfer. Accordingly, the as‐synthesized ReS2/NRGO hybrids achieve a relatively small Tafel slope of 71 mV dec−1 and high diffusion‐limited current density of 5.58 mA cm−2, which holds potential promise for practical applications in fuel cells and metal‐air batteries. [ABSTRACT FROM AUTHOR]

A considerable amount of platinum (Pt) is required to ensure an adequate rate for the oxygen reduction reaction (ORR) in fuel cells and metal‐air batteries. Thus, the implementation of atomic Pt catalysts holds promise for minimizing the Pt content. In this contribution, atomic Pt sites with nitrogen (N) and phosphorus (P) co‐coordination on a carbon matrix (PtNPC) are conceptually predicted and experimentally developed to alter the d‐band center of Pt, thereby promoting the intrinsic ORR activity. PtNPC with a record‐low Pt content (≈0.026 wt %) consequently shows a benchmark‐comparable activity for ORR with an onset of 1.0 VRHE and half‐wave potential of 0.85 VRHE. It also features a high stability in 15 000‐cycle tests and a superior turnover frequency of 6.80 s−1 at 0.9 VRHE. Damjanovic kinetics analysis reveals a tuned ORR kinetics of PtNPC from a mixed 2/4‐electron to a predominately 4‐electron route. It is discovered that coordinated P species significantly shifts d‐band center of Pt atoms, accounting for the exceptional performance of PtNPC. [ABSTRACT FROM AUTHOR]

Efficient electrocatalysis of the oxygen reduction (ORR) and evolution (OER) reactions is essential in numerous renewable energy conversion systems, such as fuel cells, metal‐air batteries, and water electrolyzers. Design and optimization of electrocatalytic materials for such systems primarily rely on understanding the nature of active centers on the catalyst surface. This review focuses on several important aspects of the experimental identification of active sites on various model bifunctional ORR/OER electrocatalytic surfaces. Applications of the state‐of‐the‐art experimental techniques are analyzed. In addition, approaches to investigate and understand the influence of some supporting electrolyte components on the ORR and OER activities are discussed. [ABSTRACT FROM AUTHOR]

Nonprecious transition metals [e.g., cobalt (Co), iron (Fe), and zinc (Zn)], and nitrogen doped carbon materials are considered to be the most attractive alternatives to precious metal catalysts. Herein, Cobalt decorated nitrogen-doped carbon nanotubes were synthesized via a facile hydrothermal method and calcination. Benefiting from the N-doping, etching of Co nanoparticles to carbon nanotubes and synergistic effect between the components, the as-prepared Co/nitrogen-doped carbon nanotubes (Co-NCNT) displays a half-wave potential of 0.88 V (vs. RHE), a limiting current density of 5.6 mA cm−2, and electron transfer number of 3.9 in 0.1 M KOH. When applied in metal-air batteries, it delivered maximum power densities of 130.0 mW cm−2, 117.3 mW cm−2 and 58.6 mW cm−2 in alkaline Zn-air, Al-air batteries and neutral Mg-air batteries respectively, outperforming the commercial Pt/C. These demonstrate that the synthesized Co-NCNT is a promising candidate for ORR in metal-air batteries with both alkaline and neutral electrolytes. [Display omitted] • Facile synthesis of cost-effective catalyst in Mass preparation. • The Co-NCNT shows Pt/C comparable activities for ORR. • Synergistic effect leading to enhancement in activity. • The Al/Mg/Zn-Air fuel cells displays superior discharge performances. [ABSTRACT FROM AUTHOR]

Carbonaceous materials originated from electrospinning fibers with hierarchical porosity and good conductivity have attracted intensive attention in the field of electrocatalysis, especially for the oxygen reduction reaction (ORR). Large batches of fibers with uniform sizes can be easily fabricated by electrospinning technology as the precursors of carbon fibers, manifesting the potential to develop a large‐scale, low‐cost, high‐activity ORR catalyst production line for new energy devices such as fuel cells and metal–air batteries. Herein, based on a simple understanding of the ORR mechanism and electrospinning technology, those investigations on synthesizing non‐noble‐metal ORR catalysts through electrospinning in recent years are reviewed. The advantages brought by applying electrospinning technology to the preparation of ORR catalysts are briefly explained. Electrospinning fibers with specific structures for ORR catalysis obtained by innovated spinnerets are displayed. Importantly, the strategies of improving ORR performance for electrospinning fibers through substrate functionalization and active site regulation are discussed in detail. Finally, the prospects and challenges of developing electrospinning technology applied to non‐noble metal ORR electrocatalysis are proposed. [ABSTRACT FROM AUTHOR]

The electrochemical oxygen reduction reaction (ORR) is the key energy conversion reaction involved in fuel cells, metal‐air batteries, and hydrogen peroxide production. Proliferation and improvement of the ORR requires wider use of new and existing high performance catalysts; unfortunately, most of these are still based on precious metals and become uneconomical in mass‐use applications. Recent progress suggests that low cost and durable carbon materials can potentially be developed as efficient ORR catalysts. Significant efforts have been made in discovering fundamental catalytic mechanisms and engineering techniques to guide and enable viable regulation of both the ORR activity and selectivity of these carbon catalysts. Starting from the fundamental understanding, this report reviews recent progress in engineering carbon materials from exotic chemical doping to intrinsic geometric defects for improved ORR. On the basis of both theoretical and experimental investigations reported so far in this area, future improvements in carbon catalysts are also discussed, providing useful pathways for more efficient and reliable energy conversion technologies. [ABSTRACT FROM AUTHOR]

The design and development of robust and efficient non-Fe doped transition metal-nitrogen doped carbon (TM-N/C) catalysts for oxygen reduction reaction is significant for promoting the applications of TM-N/C based catalysts in sustainable energy devices such as metal-air batteries and fuel cells. In this study, exclusive Cu-N x moieties implanted in metal-organic framework (MOF) polyhedrons are successfully fabricated by an adsorption-calcination approach. The optimal electrocatalyst (CuNC-1100) exhibits much higher stability and satisfactory catalytic activity with a half-wave potential (E 1/2) of 0.885 V, which is 40 mV higher than commercial Pt/C in 0.1 M KOH solution, and exhibits a dominant 4-electron direct reaction path. The comprehensive characterization data reveals that the high activity of the catalyst originates from the porosity and high surface area, high dispersion of Cu atoms, and high density of graphitic nitrogen content and Cu-N x species. [Display omitted] • Porous carbon polyhedrons catalyst is prepared by an adsorption-calcination approach. • High density of Cu-N x species and graphitic nitrogen efficiently catalyzed oxygen reduction reaction (ORR). • The optimal CuNC-1100 catalyst exhibited an excellent ORR performance in 0.1 M KOH. [ABSTRACT FROM AUTHOR]

We herein propose a dual ligand coordination strategy for deriving puissant non-noble metal electrocatalysts to substitute valuable platinum (Pt)-based materials toward oxygen reduction reaction (ORR), a key reaction in metal-air batteries and fuel cells. In brief, cobalt ions are firstly double-coordinated with proportionate 2-methylimidazole (2-MeIm) and benzimidazole (BIm) to obtain drum-like zeolitic imidazolate frameworks (D-ZIFs), which are then carbonized to output the final Co, N co-doped porous carbon (Co–N–PC D) catalyst inheriting the drum-like morphology of D-ZIFs. The Co–N–PC D is featured by mesopores and exhibits superb electrocatalytic behavior for ORR. Impressively, the half-wave potential of Co–N–PC D catalysts is 0.886 V with finer methanol-tolerance and stability than those of commercial Pt/C. Additionally, a zinc-air battery assembled from the Co–N–PC D displays an open-circuit voltage of 1.413 V, comparable to that of commercial Pt/C (1.455 V), suggesting the potentials of Co–N–PC D in practical energy conversion devices. [Display omitted] • Drum-like ZIFs (D-ZIFs) are prepared by Co2+, 2-methylimidazole and benzimidazole. • Pyrolysis of D-ZIFs precursor yields Co, N, co-doped porous carbons (Co–N–PC D). • The porous channels allow facile diffusion of O 2 to active species. • Co–N–PC D exhibits high performance for ORR and in zinc-air batteries. [ABSTRACT FROM AUTHOR]

Although, metal-free electrocatalysts have been exhibiting attractive oxygen reduction reaction (ORR) performance comparable to Pt/C in alkaline electrolytes, their activity in acidic and neutral mediums is unsatisfactory, hindering their widespread application in energy devices, such as proton exchange membrane fuel cells (PEMFCs), neutral metal-air batteries, and biofuel cells. Herein, N,P,S tri-doped holey carbon (NPS-HC) nanomaterials have been prepared using a one-step method with large specific surface area (1656.0 m2 g−1), abundant holes and edges, and high heteroatom content (7.57 at.%). The optimized NPS-HC exhibits outstanding ORR performance in universal pH mediums, being closely comparable to the commercial Pt/C. The NPS-HC is also utilized as air cathode in alkaline (peak power density: 286.6 mW cm−2 @ 488.1 mA cm−2) and neutral (85.7 mW cm−2 @ 228.8 mA cm−2) electrolytic Zn-air batteries as well as the ORR cathode in a practical PEMFC (275.1 mW cm−2) with comparable or superior performances than the previous reports. Such excellent electrocatalytic performance is attributed to the N,P,S tri-doping induced synergistic charge transfer and spin redistribution along with the hierarchical holey structure with exposed active sites and facilitated mass transport. Thus, this earth-abundant carbon-based nanomaterial holds great potential for high-performance pH-universal ORR catalysis in various energy-related technologies. This article presents a metal-free carbon-based nanomaterial for outstanding ORR performance in universal pH mediums, and its utilization as the air cathode in alkaline and neutral Zn-air batteries as well as the ORR cathode in a practical PEM fuel cell for efficient energy storage and conversion. [Display omitted] • N,P,S tri-doped holey carbon (NPS-HC) is prepared through a one-step method. • NPS-HC has a unique structure with a large specific surface area and abundant holes. • NPS-HC exhibits an outstanding ORR performance in pH-universal mediums. • NPS-HC shows good performance in alkaline/neutral Zn-air batteries and fuel cells. [ABSTRACT FROM AUTHOR]

Developing metal-free electrocatalysts with high performance towards oxygen reduction reaction (ORR) is critical for the commercialization of fuel cell technologies and metal-air batteries. Herein, we develop an effective and rational strategy to synthesize an ultrathin two-dimensional (2-D) carbon nanosheet (P–NCNS) with more exposed N and P doping sites as ORR electrocatalyst. This strategy adopts ice/salt dual-template to construct an ultrathin 2-D porous carbon nanosheet using gelatin as C/N precursor, and employs red P as P dopant to achieve controllable doping of heteroatoms. Red P also acts as a pore-forming agent to create abundant porosity on 2-D carbon surface through the activation of P-containing species. The resulting carbon nanosheet exhibits an ultrathin thickness of 1–2 nm, a large specific surface area (499.5 m2 g−1) and a well-defined micro/mesoporous structure, showing more exposed and abundant N, P doping sites that are accessible to the ORR reactants. As a result, P–NCNS shows an outstanding ORR performance both in alkaline and acidic electrolyte. More attractively, P–NCNS was used as the cathode electrocatalyst for a primary zinc-air battery showing good performance and long-term stability. This strategy offers a new insight for the design and synthesis of high-performance non-precious metal electrocatalyst for ORR. Image 1 • N and P co-doped ultrathin two-dimensional (2-D) micro/mesoporous carbon nanosheet (P–NCNS) was synthesized. • Ice/salt dual-template constructs an ultrathin 2-D mesoporous carbon nanosheet structure. • Red P was employed as P dopant and pore-forming agent to create abundant porosity on 2-D carbon surface. • P–NCNS exhibits excellent ORR activity and catalytic stability. • The primary Zn–air battery assembled by the P–NCNS as the cathode demonstrates comparable performance to commercial Pt/C. [ABSTRACT FROM AUTHOR]

Summary: Exploring progressed activity and stability, electrocatalyst toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), has been the most considerable factor in extensive commercialization of metal‐air batteries and fuel cells. Herein, Co/N co‐doped 3D hierarchical porous bifunctional oxygen electrocatalyst was synthesized using F127 as soft template and PMMA nanosphere as hard template to build hierarchically ordered macro/mesoporous porous nanostructure. The as‐obtained macro/mesoporous Co‐N‐doped carbon endowing its 3D ordered hierarchical porosity and satisfactory surface area, leading to superior performance for ORR, OER, and rechargeable Zn‐air battery. For ORR, the catalyst showed superior ORR activity with an attractive half‐wave potential of 0.85 V (vs RHE), remarkable methanol tolerance, and robust long‐term stability under alkaline electrolyte. For OER, the as‐synthetized electrocatalyst exhibited low overpotential of 1.67 V (vs RHE) under the current density of 10 mA/cm2. Moreover, the Zn‐air battery assembled from the hierarchical porous catalyst displayed a high peak power density of 146 mW/cm2. [ABSTRACT FROM AUTHOR]

The ordinary intrinsic activity and disordered distribution of metal sites in zero/one‐dimensional (0D/1D) single‐atom catalysts (SACs) lead to inferior catalytic efficiency and short‐term endurance in the oxygen reduction reaction (ORR), which restricts the large‐scale application of hydrogen−oxygen fuel cells and metal−air batteries. To improve the activity of SACs, a mild synthesis method was chosen to conjugate 1D Fe SACs with 2D graphene film (Fe SAC@G) that realized a composite structure with well‐ordered atomic‐Fe coordination configuration. The product exhibits outstanding ORR electrocatalytic efficiency and stability in 0.1 M KOH aqueous solution. DFT‐D computational results manifest the intrinsic ORR activity of Fe SAC@G originated from the newly‐formed FeN4−O−FeN4 bridge structure with moderate adsorption ability towards ORR intermediates. These findings provide new ways for designing SACs with high activity and long‐term stability. [ABSTRACT FROM AUTHOR]

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

The sluggish kinetics of oxygen reduction reaction (ORR) seriously restrains the practical implementation of fuel cells and metal-air batteries (MABs). While, Pt-based catalysts are well proven to be effective for the ORR process, they are rather expensive. Therefore, it is critical to developing low-cost and advanced catalysts with a rationally designed structure and abundant reactive sites as alternatives for Pt-based catalysts. Herein, nitrogen-doped hollow carbon polyhedrons (NHCP) have been fabricated by directly pyrolyzing ZIF-8, templated by NaCl. Benefiting from their hollow porous structure, large surface area, high graphitization degree and desired nitrogen bonding type, the as-synthesized NHCP exhibits exceptional ORR electrocatalytic activity with a half-wave potential (E 1/2) of 0.86 V, high selectivity and outstanding long-term stability, exceeding most reported nitrogen-doped metal-free carbon electrocatalysts. Furthermore, the assembled Zn-air battery with NHCP cathode delivers a peak power density of 272 mW cm−2, a specific capacity of 740 mAh g−1 and an operation period of 160 h. This contribution provides a new avenue for designing high-performance electrocatalysts for ORR. Image 1 [ABSTRACT FROM AUTHOR]

of the high-efficiency multifunctional electrocatalysts is highly demanded for many electrochemical energy devices, such as water electrolyzers, metal-air batteries and fuel cells. Here, Rh–Sn bimetallic nanocrystal catalysts supported on single-walled carbon nanotubes (Rh x Sn y /SWNTs) are reported, and the Rh 1 Sn 2 /SWNTs electrocatalyst show best multifunctional electrocatalytic performance. Strong electron synergy between Rh and Sn is the source of high catalytic activity.Sn transfers electrons from Rh to Sn, forming a unique positive environment around Rh, which effectively optimizes the binding energy of the reaction intermediate and further improves the catalytic activity. An alkaline electrolyzer using Rh 1 Sn 2 /SWNTs catalyst demands an ultra-low cell voltage of 1.56 V at the current density of 10 mA cm−2 for overall water splitting. In addition, Rh 1 Sn 2 /SWNTs demonstrates good oxygen reduction reaction (ORR) performance, making it an excellent catalyst for long-life Zn-air batteries. This work provides a facile strategy for the development of multifunctional electrocatalysts for the highly demanded electrochemical energy technologies. Image 1 • Rh 1 Sn 2 /SWNTs exhibit high active OER, HER and ORR performance. • SWNTs act as the framework to prevent particle agglomeration and improve stability. • Sn induces electron transfer from Rh to Sn to provide a positive environment for Rh. [ABSTRACT FROM AUTHOR]

Transition metal- and N-codoped carbon nanotubes (CNTs) have superior catalytic activity because the curling surface enhances the bonding ability of atoms within CNTs to other species. However, it is a great challenge to prepare CNTs with transition metal- and N-doped at high level during the growth of CNTs. Here, (Fe,N-codoped CNT)/(Fe-based nanoparticle) (Fe,N-CNT/FeNP) hybrid nanostructures are for the first time prepared through the carbonization of Fe-doped g-C 3 N 4. The doping of Fe and N is simultaneously realized during the formation of CNTs. Meanwhile, the abundant and homogeneous Fe and N in Fe-doped g-C 3 N 4 ensure high-level and uniform doping of Fe and N in CNTs. The Fe,N-CNT/FeNP hybrid nanostructures have several types of active components, including homogeneously distributed coordinating Fe moieties (FeC x N y or FeN x) and CFe 15.1 nanoparticles embedded in Fe,N-CNTs, towards oxygen reduction reaction (ORR). A superior ORR electrocatalytic performance is therefore obtained on the Fe,N-CNT/FeNP nanohybrids. Our preparation method opens an avenue to preparation of CNTs with transition metal- and N-doping at high-level, and the superior performance of Fe,N-CNT/FeNP nanostructures for ORR will be very helpful to the development of fuel cells and metal-air batteries. [ABSTRACT FROM AUTHOR]

Developing high efficiency and durability catalysts for the electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is fundamental to achieve the large‐scale commercial application of fuel cells and metal‐air batteries. Herein, a series of Co−N doped bi‐functional hierarchical porous electrocatalysts were synthesized using polyvinyl pyrrolidone (PVP) as soft template to assist the Zn/Co bimetal Zeolitic Imidazolate Framework (BMZIF) to form micro‐/mesoporous carbon nanospheres via a convenient one‐step solution synthesis method, followed by pyrolysis to form carbon nanospheres catalysts. The as‐prepared Co/N co‐doped carbon exhibits outstanding performances in the ORR and OER due to the formed hierarchical pores architecture and the high surface area. The catalyst displays superior ORR performance with a high half‐wave potential of 0.89 V, and a remarkable long‐term stability in 0.1 M KOH. In addition, the electrocatalyst elicits a low voltage of 1.72 V under the current density of 10 mA cm−2. Moreover, the zinc‐air battery fabricated with P‐CNCo‐3 delivers a power density of 175.1 mW cm−2, a high specific capacity of 791 mAh g−1 and an energy density of 976 Wh kg−1 at 20 mA cm−2. [ABSTRACT FROM AUTHOR]

The design and synthesis of metal‐free catalysts with superior electrocatalytic activity, high durability, low cost, and under mild conditions is extremely desirable but remains challenging. To address this problem, a polymer‐assisted electrochemical exfoliation technique of graphite in the presence of an aqueous acidic medium is reported. This simple, cost‐effective, and mass‐scale production approach could open the possibility for the synthesis of high‐quality nitrogen‐doped graphene–polypyrrole (NG‐PPy). The NG‐PPy catalyst displays an improved half wave potential (E1/2=0.77 V) in alkaline medium compared with G‐PPy (E1/2=0.66 V). Most importantly, this catalyst demonstrates excellent stability with high methanol tolerance, and it outperforms the commercial Pt/C catalyst and other previously reported metal‐free catalysts. The content of graphitic nitrogen atoms is the key factor for the enhancement of electrocatalytic activity towards oxygen reduction reactions (ORR). Interestingly, the NG‐PPy catalyst can be used as a cathode material in a zinc–air battery, which demonstrates a higher peak power density (59 mW cm−2) than G‐PPy (36.6 mW cm−2), highlighting the importance of the low‐cost material synthesis approach towards the development of metal‐free efficient ORR catalysts for fuel cell and metal–air battery applications. Remarkably, the polymer‐assisted electrophoretic exfoliation of graphite with a high yield (≈88 wt %) of few‐layer graphene flakes could pave the way towards the mass production of high‐quality graphene for a variety of applications. [ABSTRACT FROM AUTHOR]

Transition metal-nitrogen-carbon catalysts obtained from pyrolysis of simple sources of metal and nitrogen or metal-organic frameworks are envisioned as promising replacement of Pt-based catalysts for oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. However, the lack of clarity on the active site structure is a fundamental problem in developing efficient catalysts for the complete reduction of oxygen with maximum active site density. In this study, we have synthesized a simple metal-organic complex, Co (bpy) 2 CO 3 NO 3 ⋅ 5 H 2 O , 1 (bpy = 2, 2′-bipyridine) and studied its ORR activity in 0.1 M KOH supporting on Ketjenblack EC-600JD, 1 /C. The crystal structure of 1 was unambiguously determined using single-crystal X-ray diffraction. The onset potential of ORR, Tafel slopes, kinetic current density, the fraction of oxygens reduced to peroxyl ion (χ HO 2 − ) and the electron transfer number (n) were compared with relevant literature and commercial 20 wt % Pt/C. Primarily, 1 /C reduces O 2 through the direct 4-electron pathway. ' n' and χ HO 2 − on 1 /C were found to be 3.87 and 7% at 0.7 V iR-free vs. RHE, respectively. The turn-over frequencies for the 4- and 2-electron process were found to be 0.124 and 0.001 electrons [Co]−1 s−1 at 0.7 V iR-free vs. RHE, respectively. Density-functional theory analysis reveals that the preferential activation of the side-on mode of adsorption on 1 is responsible for high selectivity in the direct 4-electron pathway. Image 1 • The complex, Co (bpy) 2 CO 3 NO 3 ⋅ 5 H 2 O , 1 exhibits high chemical stability in 0.1 N KOH. • Carbon supported 1 , 1 /C, reduces the oxygen mainly through the direct 4-electron pathway. • ' n' and χ HO 2 − at 0.8 V iR-free vs. RHE on 1 /C are 3.9 and 4%, respectively. • The 4-electron oxygen reduction turn-over frequency of 1 /C is more than 2-fold higher compared to that of the 2-electron process. [ABSTRACT FROM AUTHOR]

Oxygen reduction reaction (ORR) catalysts are critical components for the development of high‐efficiency fuel cells and metal‐air batteries. Supported PtCo alloy nanoparticles (NPs) with modified surface properties and reactivity through ligand, strain and/or ensemble effects are regarded as promising inexpensive low‐Pt candidates. Herein, we report an annealing‐free assembly of PtCo nanoparticles anchored on nitrogen‐doped mesoporous carbon (PtCo@NMC) for advanced ORR electrocatalysis in both alkaline and acidic electrolytes. Compared with the commercial Pt/C catalyst, the half‐wave potential (E1/2) values of PtCo@NMC increase to 0.93–0.94 V (Vs. RHE) and positively shift by approximately 45 mV. Moreover, the annealing‐induced degradation stems from agglomeration and growth of alloy NPs. Considering the excellent long‐term durability according to both chronoamperometry (CA) and accelerated durability tests (ADTs), we demonstrate practical application with enhanced performance in electrochemical energy devices, such as a primary Zn–air battery assembled with PtCo@NMC cathode. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The worldwide fossil fuel shortage and resultant environmental issues urgently require renewable and clean energy technologies. Electrocatalytic oxygen reduction/evolution reactions (ORR/OER) are the cornerstone for renewable energy conversion and storage devices, such as fuel cells, electrolyzers, unitized regenerative fuel cells, and metal‐air batteries. High‐performance electrocatalysts are required to improve the ORR and OER activity and stability, and thus the device performance. Therefore, appropriate strategies and methods are crucial for the rational design and synthesis of highly efficient ORR/OER electrocatalysts. On the other hand, the conventional platinum‐group‐metal‐based (PGM‐based) catalysts, such as Pt and Ir/Ru (oxides), have been facing great challenges, including limited resources and high cost, leading to them being less competitive in the market. Thus, a lot of effort has been devoted to developing alternative PGM‐free ORR/OER catalysts, which, however, still suffer from low activity and insufficient stability. In this review paper, the strategies for engineering high‐performance PGM‐free ORR and OER electrocatalysts are discussed by reviewing the most recent advances. At the end, perspectives on the methods to rationally design PGM‐free ORR and OER catalysts are provided. [ABSTRACT FROM AUTHOR]

With a rising energy demand and anabatic environmental crisis arising from the fast growth in human population and society economics, numerous efforts have been devoted to explore and design plentiful multifunctional materials for meeting high‐efficiency energy transfer processes, which happen in various developed energy conversion and storage systems. As a special kind of multi‐metal oxides, perovskite with attractive physical and chemical properties, is becoming a rapidly rising star on the horizon of high‐performance catalytic materials with substantial research behaviors worldwide. The porous nanostructure in targeted catalysts is favorable to the catalytic activity and thus improves the overall efficiency of these energy‐related installations. In this review paper, recent advances made in the porous perovskite nanostructures for catalyzing several anodic or cathodic reactions in fuel cells and metal–air batteries are comprehensively summarized. Plenty of general preparation methods employed to attain porous perovskite‐type oxides are provided, followed by a further discussion about the influence of various strategies on structures and catalytic properties of the porous perovskites. Furthermore, deep insights gathered in the future development of porous perovskite‐based materials for energy conversion and storage technologies are also provided. [ABSTRACT FROM AUTHOR]

The catalysts are often used in fuel cells and metal-air batteries to speed up electrochemical reactions. In this study, we prepared CoFe 2 O 4 nanoparticles with mainly inverse spinel structure and FeCo 2 O 4 nanoparticles with mainly spinel structure as bifunctional catalysts by hydrothermal method. After annealing at 350 °C, pure CoFe 2 O 4 and FeCo 2 O 4 nanoparticles with uniform size distribution have been obtained. The CoFe 2 O 4 nanoparticles showed high current density of 5.5 mA/cm2 at −0.8 V in the ORR test. It's low Tafel slope of 83.0 mV/dec further confirmed the excellent ORR catalytic properties of CoFe 2 O 4 nanoparticles. Furthermore, the CoFe 2 O 4 nanoparticles also showed good OER properties with satisfied current density of 35.7 mV/cm2 at l.0 V and low OER Tafel slope of 71.0 mV/dec. Both the ORR and OER properties of CoFe 2 O 4 nanoparticles showed good time stability which were compared with FeCo 2 O 4 nanoparticles. These results indicated that CoFe 2 O 4 nanoparticles with mainly inverse spinel structure had better electrocatalytic performance than FeCo 2 O 4 nanoparticles with mainly spinel structure. The CoFe 2 O 4 nanoparticles with mainly inverse spinel structure show a significant potential application in rechargeable battery. Image 1 • The work used a simple, convenient and cheap method to prepare pure spinel oxides. • The relationship between spinel structure and electrocatalytic properties was studied. • We confirmed normal spinel structure and inverse spinel structure can coexist in spinel nanoparticles. [ABSTRACT FROM AUTHOR]

In view of the clean and sustainable energy, metal–organic frameworks (MOFs) based materials, including pristine MOFs, MOF composites, and their derivatives are emerging as unique electrocatalysts for oxygen reduction reaction (ORR). Thanks to their tunable compositions and diverse structures, efficient MOF‐based materials provide new opportunities to accelerate the sluggish ORR at the cathode in fuel cells and metal–air batteries. This Minireview first provides some introduction of ORR and MOFs, followed by the classification of MOF‐based electrocatalysts towards ORR. Recent breakthroughs in engineering MOF‐based ORR electrocatalysts are highlighted with an emphasis on synthesis strategy, component, morphology, structure, electrocatalytic performance, and reaction mechanism. Finally, some current challenges and future perspectives for MOF‐based ORR electrocatalysts are also discussed. [ABSTRACT FROM AUTHOR]

Exploring efficient and durable non-precious metal catalysts for oxygen reduction reaction (ORR) has long been pursued in the field of metal-air batteries, fuel cells, and solar cells. Rational design and controllable synthesis of desirable catalysts are still a great challenge. In this work, a novel approach is developed to tune the morphologies and structures of Fe–N–C catalysts in combination with the dual nitrogen-containing carbon precursors and the gas-foaming agent. The tailored Fe–N 1 /N 2 –C-A catalyst presents gauze-like porous nanosheets with uniformly dispersed tiny nanoparticles. Such architectures exhibit abundant Fe-N x active sites and high-exposure surfaces. The Fe–N 1 /N 2 –C-A catalyst shows extremely high half-wave potential (E 1/2 , 0.916 V vs. RHE) and large limiting current density (6.3 mA cm−2), far beyond 20 wt% Pt/C catalyst for ORR. This work provides a facile morphological and structural regulation to increase the number and exposure of Fe-N x active sites. The tailored Fe–N–C catalyst presents plentiful Fe-N x active sites and highly exposed active surfaces, resulting in an ultra-high ORR activity far beyond the 20 wt% Pt/C catalyst. Image 1 • A novel approach is explored to tune morphology and structure of Fe–N–C catalyst. • The tailored catalyst presents abundant active sites and high exposure surfaces. • The catalyst shows ultrahigh ORR activity, far beyond the commercial Pt/C. [ABSTRACT FROM AUTHOR]