Isolated Binary Fe-Ni Metal-Nitrogen Sites Anchored on Porous Carbon Nanosheets for Efficient Oxygen Electrocatalysis through High-Temperature Gas-Migration Strategy

Atomically dispersed dual-site catalysts can regulate multiple reaction processes and provide synergistic functions based on diverse molecules and their interfaces. However, how to synthesize and stabilize dual-site single-atom catalysts (DACs) is confronted with challenges. Herein, we report a facile high-temperature gas-migration strategy to synthesize Fe-Ni DACs on nitrogen-doped carbon nanosheets (FeNiSAs/NC). FeNiSAs/NC exhibits a high half-wave potential (0.88 V) for the oxygen reduction reaction (ORR) and a low overpotential of 410 mV at 10 mA cm-2 for the oxygen evolution reaction (OER). As an air electrode for Zn-air batteries (ZABs), it shows better performances in aqueous ZABs and excellent stability and flexibility in solid-state ZABs. The high specific surface area (1687.32 m2/g) of FeNiSAs/NC is conducive to electron transport. Density functional theory (DFT) reveals that the Fe sites are the active center, and Ni sites can significantly optimize the free energy of the oxygen-containing intermediate state on Fe sites, contributing to the improvement of ORR and the corresponding OER activities. This work can provide guidance for the rational design of DACs and understand the structure-activity relationship of SACs with multiple active sites for electrocatalytic energy conversion.

Vacancy-Enhanced Sb-N4 Sites for the Oxygen Reduction Reaction and Zn-Air Battery

With the advantages of a Fenton-inactive characteristic and unique p electrons that can hybridize with O2 molecules, p-block metal-based single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) have tremendous potential. Nevertheless, their undesirable intrinsic activity caused by the closed d10 electronic configuration remains a major challenge. Herein, an Sb-based SAC featuring carbon vacancy-enhanced Sb-N4 active centers, corroborated by the results of high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure, has been developed for an incredibly effective ORR. The obtained SbSA-N-C demonstrates a positive half-wave potential of 0.905 V and excellent structural stability in alkaline environments. Density functional theory calculations reveal that the carbon vacancies weaken the adsorption between Sb atoms and the OH* intermediate, thus promoting the ORR performance. Practically, the SbSA-N-C-based Zn-air batteries achieve impressive outcomes, such as a high power density of 181 mW cm-2, showing great potential in real-world applications.

Enhancement of Electrocatalytic Oxygen Reduction Reaction and Oxygen Evolution Reaction by Introducing Lanthanum Species in the Carbon Shell

The development of cost-effective non-noble metal electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) opens up the possibility for sustainable energy systems. Herein, we report a surface overcoating strategy with lanthanum organic complex (La-OC) as the precursor to prepare lanthanum species (La-SPc) encapsulated in nitrogen, fluorine, and sulfur self-doped porous carbon (NFS-PC) composites (La-SPc@NFS-PC) for efficient ORR and OER. The La-SPc is introduced not only as a promoter to increase the electrochemical stability of the La-SPc@NFS-PC catalysts but also to tailor the electronic structure of NFS-PC due to the unique electrochemical properties of La-SPc. In addition, the integration of La-SPc and NFS-PC can improve the electronic conductivity of composites by inducing electron redistribution and lowering the band gap, which is advantageous in enhancing the kinetics of charge transfer. Simultaneously, benefiting from the optimized porous structure and positive cooperation of La-SPc with NFS-PC shells, the obtained La-SPc@NFS-PC-3 delivers robust bifunctional ORR/OER activities and stabilities. More importantly, the Zn-air battery (ZAB) assembled with La-SPc@NFS-PC-3 demonstrates an outstanding power density (181.1 mW cm-2) and long cycling life, outperforming the commercial Pt/C. This work offers a rational approach to preparing high-efficiency rare-earth-based catalysts and provides potential applications in ZABs.

Coordination Engineering Induced d-Band Center Shift on Single-Atom Fe Electrocatalysts for Enhanced Oxygen Reduction

Part of the performance of single-atom catalysts (SACs) is greatly influenced by the microenvironment around a single metal site, of which the oxygen reduction reaction (ORR) is counted as one of them. However, an in-depth understanding of the catalytic activity regulation by the coordination environment is still lacking. In this study, a single Fe active center with axial fifth hydroxyl (OH) and asymmetric N,S coordination embedded in a hierarchically porous carbon material (Fe-SNC) are prepared. Compared to Pt/C and most of the reported SACs, as-prepared Fe-SNC has certain advantages in terms of ORR activity and maintains sufficient stability. Furthermore, the assembled rechargeable Zn-air battery exhibits impressive performance. The combination of multiple findings revealed that the introduction of S atoms not only facilitates the formation of porous structures but also facilitates the desorption and adsorption of oxygen intermediates. On the other hand, with the introduction of axial OH groups, the bonding strength of the ORR intermediate is reduced, and even the central position of the Fe d-band is optimized. The developed catalyst is expected to lead to further research on the multiscale design of the electrocatalyst microenvironment.

Tunable Heterogeneous FeCo Alloy-Mo0.82N Bifunctional Electrocatalysts for Temperature-Adapted Zn-Air Batteries

The practical applications of temperature-tolerant Zn-air batteries (ZABs) rely on highly active and stable bifunctional catalysts that accelerate cathodic oxygen reduction (ORR) and oxygen evolution (OER) reactions. Herein, we successfully integrated fascinating transition metal nitrides and FeCo alloys through a simple coordination assembly and pyrolysis process. Importantly, the alloy-to-nitride ratio in the heterogeneous catalyst can be carefully regulated through the subsequent etching process. Moreover, the composition-dependent ORR/OER performance of the FeCo-Mo_0.82N catalysts was revealed. Aqueous ZABs using the optimized FeCo-Mo_0.82N-60 as a cathode exhibit a high peak power density of 149.7 mW cm^-2 and an impressive stability of 600 h with a low charge-discharge voltage gap decay rate of 0.025 mV h^-1, which exceeds those of most of recent reports. Furthermore, the FeCo-Mo_0.82N-60-based flexible ZABs display a small specific capacity degradation (3%) from 40 to -10 °C, demonstrating excellent temperature tolerance.

Rational Design of Flexible Zn-Based Batteries for Wearable Electronic Devices

The advent of 5G and the Internet of Things has spawned a demand for wearable electronic devices. However, the lack of a suitable flexible energy storage system has become the "Achilles' Heel" of wearable electronic devices. Additional problems during the transformation of the battery structure from conventional to flexible also present a severe challenge to the battery design. Flexible Zn-based batteries, including Zn-ion batteries and Zn-air batteries, have long been considered promising candidates due to their high safety, eco-efficiency, substantial reserve, and low cost. In the past decade, researchers have come up with elaborate designs for each portion of flexible Zn-based batteries to improve the ionic conductivities, mechanical properties, environment adaptabilities, and scalable productions. It would be helpful to summarize the reported strategies and compare their pros and cons to facilitate further research toward the commercialization of flexible Zn-based batteries. In this review, the current progress in developing flexible Zn-based batteries is comprehensively reviewed, including their electrolytes, cathodes, and anodes, and discussed in terms of their synthesis, characterization, and performance validation. By clarifying the challenges in flexible Zn-based battery design, we summarize the methodology from previous investigations and propose challenges for future development. In the end, a research paradigm of Zn-based batteries is summarized to fit the burgeoning requirement of wearable electronic devices in an iterative process, which will benefit the future development of Zn-based batteries.

Doping Effect on Mesoporous Carbon-Supported Single-Site Bifunctional Catalyst for ZincAir Batteries

Rechargeable zinc-air batteries (ZABs) require bifunctional electrocatalysts presenting high activity in oxygen reduction/evolution reactions (ORR/OER), but the single-site metal-N-C catalysts suffer from their low OER activity. Herein, we designed a series of single-site Fe-N-C catalysts, which present high surface area and good conductivity by incorporating into mesoporous carbon supported on carbon nanotubes, to study the doping effect of N and P on the bifunctional activity. The additional P-doping dramatically increased the content of active pyridine-N and introduced P-N/C/O sites, which not only act as extra active sites but also regulate the electron density of Fe centers to optimize the absorption of oxygenated intermediates, thereby ultimately improving the bifunctional activity of Fe-N-C sites. The optimized catalyst displayed a half-wave potential of 0.882 V for ORR and a low overpotential of 365 mV at 10 mA cm^-2 for OER, which significantly outperforms the counterpart without P, as well as noble-metal-based catalysts. The ZABs with air cathodes containing the N,P-co-doped catalysts exhibited a high peak power density of 201 mW cm^-2 and a long cycling stability beyond 600 h. Doping has shown to be an effective way to optimize the performance of single-site catalysts in bifunctional oxygen electrocatalysis, which can be extended to other catalyst systems.

Co/Co-N/Co-O Rooted on rGO Hybrid BCN Nanotube Arrays as Efficient Oxygen Electrocatalyst for Zn-Air Batteries

Developing high-performance non-noble metal bifunctional oxygen reduction and evolution reaction electrocatalysts is a critical factor for the commercialization of rechargeable Zn-air batteries. Herein, Co/Co-N/Co-O rooted on reduced graphene oxide (rGO) hybrid boron and nitrogen codoped carbon (BCN) nanotube arrays (BCN/rGO-Co) is prepared by facile low-temperature precross-linking and high-temperature pyrolysis treatment. Benefit from the synergistic effect of its B/N codoping, Co/Co-N/Co-O bifunctional active sites, 3D hybrid porous structure of BCN nanotubes, and highly conductive rGO sheets. The obtained BCN/rGO-Co exhibits superior bifunctional oxygen catalytic activity with a positive ORR half-wave potential (0.85 V) and a low OER potential (1.61 V) at 10 mA cm^-2. Additionally, the BCN/rGO-Co-based liquid Zn-air batteries displays a large peak power density of 157 mW cm^-2, and a long charge/discharge cycle stability of 200 h, outdoing the commercial Pt/C+Ru/C catalyst.

Phase-Reconfiguration-Induced NiS/NiFe2 O4 Composite for Performance-Enhanced Zinc-Air Batteries

Constructing composite structures is an essential approach for obtaining multiple functionalities in a single entity. Available synthesis methods of the composites need to be urgently exploited; especially in situ construction. Here, a NiS/NiFe₂ O₄ composite through a local metal-S coordination at the interface is reported, which is derived from phase reconstruction in the highly defective matrix. X-ray absorption fine structure confirms that long-range order is broken via the local metal-S coordination and, by using electron energy loss spectroscopy, the introduction of NiS/NiFe₂ O₄ interfaces during the irradiation of plasma energy is identified. Density functional theory (DFT) calculations reveal that in situ phase reconfiguration is crucial for synergistically reducing energetic barriers and accelerating reaction kinetics toward catalyzing the oxygen evolution reaction (OER). As a result; it leads to an overpotential of 230 mV @10 mA cm^-2 for the OER and a half-wave potential of 0.81 V for the oxygen reduction reaction (ORR); as well as an excellent zinc-air battery (ZAB) performance with a power density of 148.5 mW cm^-2 . This work provides a new compositing strategy in terms of fast phase reconstruction of bifunctional catalysts.

Oxygen-Terminated Nb2CO2 MXene with Interfacial Self-Assembled COF as a Bifunctional Catalyst for Durable Zinc-Air Batteries

The desirable air cathode in Zn-air batteries (ZABs) that can effectively balance oxygen evolution and oxygen reduction reactions not only needs to adjust the electronic structure of the catalyst but also needs a unique physical structure to cope with the complex gas-liquid environment. In this work, first-principles calculations were carried out to prove that oxygen-terminated Nb₂CO₂ MXene played an active role in enhancing the sluggish reaction of oxygen intermediates. Nb₂CO₂ MXene could also stimulate the spatial accumulation of discharge products, which was beneficial to improve the stability of secondary ZABs. Molecular dynamics simulation was used to show that the confinement effect of COF could effectively regulate the concentration of O₂ on the surface of Nb₂CO₂@COF, which was conducive to an efficient and durable reaction. COF-LZU1 was self-assembled on the interface of Nb₂CO₂ MXene (Nb₂CO₂@COF) for the first time. The Nb₂CO₂@COF electrode had excellent OER/ORR overpotentials with the potential difference (ΔE) of 0.79 V. When applied to the configuration of ZABs, Nb₂CO₂@COF showed a power density of 75 mW cm^-2 and favorable long-term charge/discharge stability, so it could be used as a potential candidate cathode for noble-metal-based catalysts. This idea of combining MXenes and COFs sheds some light on the design of ZABs.

RuCoOx Nanofoam as a High-Performance Trifunctional Electrocatalyst for Rechargeable Zinc-Air Batteries and Water Splitting

Designing high-performance trifunctional electrocatalysts for ORR/OER/HER with outstanding activity and stability for each reaction is quite significant yet challenging for renewable energy technologies. Herein, a highly efficient and durable trifunctional electrocatalyst RuCoO_x is prepared by a unique one-pot glucose-blowing approach. Remarkably, RuCoO_x catalyst exhibits a small potential difference (ΔE) of 0.65 V and low HER overpotential of 37 mV (10 mA cm^-2), as well as a negligible decay of overpotential after 200 000/10 000/10 000 CV cycles for ORR/OER/HER, all of which show overwhelming superiorities among the advanced trifunctional electrocatalysts. When used in liquid rechargeable Zn-air batteries and water splitting electrolyzer, RuCoO_x exhibits high efficiency and outstanding durability even at quite large current density. Such excellent performance can be attributed to the rational combination of targeted ORR/OER/HER active sites into one electrocatalyst based on the double-phase coupling strategy, which induces sufficient electronic structure modulation and synergistic effect for enhanced trifunctional properties.

Metal-Organic Framework-Derived Trimetallic Nanocomposites as Efficient Bifunctional Oxygen Catalysts for Zinc-Air Batteries

Transition-metal-based multifunctional catalysts have attracted increasing attention owing to high possibilities of substituting the expensive noble-metal-based catalysts in various scenarios. Multivariate metal-organic frameworks (MTV-MOFs) are ideal precursors to prepare multimetallic nanocomposites with high catalytic activity since the uniform distribution and precise regulation of mixed metal centers, as well as the consequent strong synergistic effect, could be readily achieved. Herein, a Mn/Co/Ni trimetallic catalyst (MnCoNi-C-D) with a hollow rhombic dodecahedron shape was synthesized via pyrolysis of the corresponding trimetallic-based MTV-MOF. The catalyst shows outstanding electrochemical activity toward the oxygen reduction reaction including a half-wave potential of 0.82 V and superior tolerance against methanol as well as high stability in an alkaline medium, and its oxygen evolution reaction activity also surpasses a RuO₂ catalyst. Moreover, primary and rechargeable zinc-air batteries based on MnCoNi-C-D delivered preferable performances compared with commercial Pt/C-RuO₂, including higher peak power density (116.4 mW cm^-2), higher specific capacity (841.3 mAh g^-1), higher open-circuit potential (OCV) (1.46 V), and better stability for more than 180 h. A comprehensive comparison was also conducted to prove the necessity of employing the MTV-MOF as the precursor and investigate the intrinsic superiority of the catalyst.

Hollow Spherical Superstructure of Carbon Nanosheets for Bifunctional Oxygen Reduction and Evolution Electrocatalysis

The pyrolysis of metal-organic frameworks (MOFs) is an ingenious way to synthesize carbon-based materials with unique morphology for various applications including electrocatalysis. In this work, we reported a facile morphology regulation strategy for the synthesis of a spherical superstructure of MOF nanosheets. The use of metal hydroxide nanosheets on Zn particles as precursors/templates allowed MOFs with general polyhedron shape to form nanosheets and assemble into a spherical superstructure in the ligand solution. Further, a hollow spherical superstructure of carbon nanosheets decorated with metal-based nanoparticles was fabricated through the pyrolysis of MOF nanosheet superstructures at 950 °C, where the substrate/template Zn particle cores were evaporated away. The obtained composites possess carbon-based superstructures with abundant mesopores and metal-based nanoparticles with rich alloy/oxide interfaces. These features endow this MOF-derived carbon-based material with outstanding bifunctional activity for oxygen reduction/evolution reactions and great performances in Zn-air batteries.

Atomic layer deposited nickel sulfide for bifunctional oxygen evolution/reduction electrocatalysis and zinc-air batteries

Rechargeable Zn-air batteries are a promising type of metal-air batteries for high-density energy storage. However, their practical use is limited by the use of costly noble-metal electrocatalysts for the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurred at the air electrode of the Zn-air batteries. This work reports a new non-precious bifunctional OER/ORR electrocatalyst of NiS_x/carbon nanotubes (CNTs), which is made by atomic layer deposition (ALD) of nickel sulfide (NiS_x) on CNTs, for the applications for the air electrode of the Zn-air batteries. The NiS_x/CNT electrocatalyst on a carbon cloth electrode exhibits a low OER overpotential of 288 mV to reach 10 mA cm^-2in current density, and the electrocatalyst on a rotating disk electrode exhibits a half-wave ORR potential of 0.81 V in alkaline electrolyte. With the use of the NiS_x/CNT electrocatalyst for the air electrode, the fabricated aqueous rechargeable Zn-air batteries show a fairly good maximum output power density of 110 mW cm^-2, which highlights the great promise of the ALD NiS_x/CNT electrocatalyst for Zn-air batteries.

Bimetallic Metal-Organic Framework-Derived Graphitic Carbon-Coated Small Co/VN Nanoparticles as Advanced Trifunctional Electrocatalysts

The rational design and construction of multifunctional electrocatalysts with high activity, low cost, and outstanding stability are highly desirable for the development of renewable energy but are still a big challenge. Bimetallic catalysts are a kind of promising candidates, like the hybrids of Co and VN nanoparticles (Co/VN). However, the inevitable aggregation during the preparation and electrochemical process lowers their reactivity and durability. Herein, small Co/VN nanoparticles (4-8 nm) embedded in porous graphitic carbon layers (Co/VN NPs@C) were obtained through the pyrolysis of metal-organic frameworks (MOFs). The synergistic effect of in situ generated Co and VN NPs together with fast electron transfer from graphitic carbon layers renders this catalyst to possess excellent trifunctional performance. More attractively, Co/VN NPs@C as both the anode and the cathode shows a low voltage of 1.58 V when the current density is up to 10 mA cm^-2, exceeding most electrocatalysts based on non-noble metals. The rechargeable Zn-air batteries constructed by Co/VN NPs@C deliver high round-trip efficiency together with a peak power density of 130 mW cm^-2, a specific capacity of 757 mAh g^-1, and desirable stability, outperforming the traditional Zn-air batteries based on the Pt/C and RuO₂ pair. This work opens a promising avenue toward constructing highly effective multifunctional electrocatalysts by designing small-sized nanoparticles with various active sites derived from MOFs.

Strongly Coupled NiCo2O4 Nanocrystal/MXene Hybrid through In Situ Ni/Co-F Bonds for Efficient Wearable Zn-Air Batteries

Recently, owing to the high energy density and excellent security, wearable Zn-air batteries (ZABs) have been known as one of the most prominent wearable energy storage devices. However, sluggish oxygen reaction kinetics of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in the air-breathe cathode seriously has limited further practical applications. In this work, we synthesize a NiCo₂O₄ nanocrystal/MXene hybrid with strong Ni/Co-F bonds. The prepared MXene-based hybrid composites show remarkable ORR and OER electrocatalytic activity, which results in the fabricated solid-state ZAB device to achieve an open-circuit voltage of 1.40 V, peak power density of 55.1 mW cm^-2, and energy efficiency of 66.1% at 1.0 mA cm^-2; to the best of our knowledge, this is the record performance among all reported flexible ZABs with MXene-based air cathodes and comparable with some noble metal catalysts. Moreover, even after cutting and suturing, our flexible solid-state ZAB devices are tailorable with high rate of performance.

"Ship in a Bottle" Design of Highly Efficient Bifunctional Electrocatalysts for Long-Lasting Rechargeable Zn-Air Batteries

The poor durability of bifunctional oxygen electrocatalysts is one main bottleneck that suppresses the widespread application of rechargeable metal-air batteries. Herein, a "ship in a bottle" design is achieved by impregnating fine transition metal dichalcogenide nanoparticles into defective carbon pores that act as interconnected nanoreactors. The erected 3D porous conductive architecture provides a "highway" for expediting charge and mass transfer. This design not only delivers a high surface-to-volume ratio to increase numbers of exposed catalytic sites but also precludes nanoparticles from aggregation during cycling owing to the pore spatial confinement effect. Therefore, the long-term plague inherent to nanocatalyst stability can be solved. Moreover, the synergistic coupling effects between defect-rich interfaces and chemical bonding derived from heteroatom-doping boost the catalytic activity and prohibit the detachment of nanoparticles for better stability. Consequently, the developed catalyst presents superior bifunctional oxygen electrocatalytic activities and durability, out-performing the best-known noble-metal benchmarks. In a practical application to rechargeable Zn-air batteries, long-term cyclability for over 340 h is realized at a high current density of 25 mA cm^-2 in ambient air while retaining an intact structure. Such a universal "ship in a bottle" design offers an appealing and instructive model of nanomaterial engineering for implementation in various fields.

Sponge Effect Boosting Oxygen Reduction Reaction at the Interfaces between Mullite SmMn2O5 and Nitrogen-Doped Reduced Graphene Oxide

Exploring the effect of interfacial structural properties on catalytic performance of hybrid materials is essential in rationally designing novel electrocatalysts with high stability and activity. Here, in situ growth of mullite SmMn₂O₅ on nitrogen-doped reduced graphene oxide (SMO@NrGO) is achieved for highly efficient oxygen reduction reaction (ORR). Combining X-ray photoelectron spectroscopy and density functional theory calculations, interfacial chemical interactions between Mn and substrates are verified. Interestingly, as revealed by charge density difference, the interfacial Mn-N(C) bonds display a sponge effect to store and compensate electrons to boost the ORR process. In addition, bidentate adsorption of oxygen intermediates instead of monodentate ones is observed in hybrid materials, which facilitates the interactions between intermediates and active sites. Experimentally, the hybrid catalyst SMO@NrGO exhibits a half-wave potential as high as 0.84 V, being comparable to benchmark Pt/C and higher than that of the pure SMO (0.68 V). The Zn-air battery assembled with SMO@NrGO shows a high discharge peak power density of 244 mW cm^-2 and superior cycling stability against noble metals.

Bimetallic Metal-Organic-Framework/Reduced Graphene Oxide Composites as Bifunctional Electrocatalysts for Rechargeable Zn-Air Batteries

The most challenging issue in the development of metal-air batteries is the insufficient catalytic activity of the cathode toward oxygen evolution and reduction reactions (OER/ORR). Metal-organic frameworks (MOFs) and MOF-based electrocatalysts have drawn considerable attention for the replacement of noble-metal electrocatalysts. Here, the rational design and synthesis of bimetallic CoNi-MOF nanosheets/reduced graphene oxide (rGO) hybrid electrocatalysts is reported. The CoNi-MOF nanosheets were in situ grown onto rGO assisted by the surfactant modulation. The newly developed CoNi-MOF/rGO hybrids, consisting of homogeneously distributed nanosheets encapsulated by rGO, display excellent electrocatalytic activities toward OER and ORR. The much improved bifunctional catalytic performance is ascribed to the synergy among the CoNi-MOF nanosheets and rGO, the abundant exposed active sites, and the enhanced electron conductivity. Moreover, the rechargeable Zn-air batteries with CoNi-MOF/rGO-based air electrodes display high energy density and cycling stability, demonstrating the great potential as advanced bifunctional electrocatalysis in electronic devices.

Three-Dimensional Graphene-Supported Ni3Fe/Co9S8 Composites: Rational Design and Active for Oxygen Reversible Electrocatalysis

The development of low-cost and efficient electrocatalysts with a bicomponent active surface for reversible oxygen electrode reactions is highly desirable and challenging. Herein, we develop an effective calcination-hydrothermal approach to fabricate graphene aerogel-anchored Ni₃Fe-Co₉S₈ bifunctional electrocatalyst (Ni₃Fe-Co₉S₈/rGO). The mutually beneficial Ni₃Fe-Co₉S₈ bifunctional active components efficiently balance the performance of oxygen reduction and oxygen evolution reactions (ORR/OER), in which Co₉S₈ promotes the ORR and Ni₃Fe facilitates the OER. This balance behavior has an obvious advantage over that of monocomponent Ni₃Fe/rGO and Co₉S₈/rGO catalysts. Meanwhile, the additional synergy between porous rGO aerogels and Ni₃Fe-Co₉S₈ endows the composite with more exposed active sites, faster electrons/ions transport rate, and better structural stability. Benefiting from the reasonable material selection and structural design, the Ni₃Fe-Co₉S₈/rGO exhibits not only outstanding ORR activity with the high onset- and half-wave potentials ( E_onset = 0.91 V and E_1/2 = 0.80 V) but also satisfactory OER activity with a low overpotential at 10 mA cm^-2 (0.39 V). Moreover, rechargeable Zn-air cells equipped with Ni₃Fe-Co₉S₈/rGO exhibit excellent rechargeability and a fast dynamic response.

Combined Electron and Structure Manipulation on Fe-Containing N-Doped Carbon Nanotubes To Boost Bifunctional Oxygen Electrocatalysis

It is a challenge to synthesize highly efficient nonprecious metal electrocatalysts with a well-defined nanostructure and rich active species. Herein, through electron engineering and structure manipulation simultaneously, we constructed Fe-embedded pyridinic-N-dominated carbon nanotubes (CNTs) on ordered mesoporous carbon, showing excellent oxygen reduction reaction activity (half-wave potential, 0.85 V) and an overpotential of 420 mV to achieve 10 mA cm^-2 for oxygen evolution reaction in alkaline media (potential difference, 0.80 V). Density functional theory calculation indicates those Fe@N₄ clusters improve charge transfer and further promote the electrocatalytic reactivity of the functionalized region in CNTs. Rechargeable Zn-air batteries were assembled, displaying robust charging-discharging cycling performance (over 90 h) with voltage gap of only 0.08 V, much lower than that of the Pt/C + Ir/C electrode (0.29 V). This work presents a highly active nonprecious metal-based bifunctional catalyst toward air electrode for energy conversion.

Edge Defect Engineering of Nitrogen-Doped Carbon for Oxygen Electrocatalysts in Zn-Air Batteries

Metal-free bifunctional oxygen electrocatalysts are extremely critical to the advanced energy conversion devices, such as high energy metal-air batteries. Effective tuning of edge defects and electronic density on carbon materials via simple methods is especially attractive. In this work, a facile alkali activation method has been proposed to prepare carbon with large specific surface area and optimized porosity. In addition, subsequent nitrogen-doping leads to high pyridinic-N and graphitic-N contents and abundant edge defects, further enhancing electrochemical activities. Theoretical modeling via first-principles calculations has been conducted to correlate the electrocatalytic activities with their fundamental chemical structure of N doping and edge defect engineering. The metal-free product (NKCNPs-900) shows a high half-wave potential of 0.79 V (ORR). Furthermore, the assembled Zn-air batteries display excellent performance among carbon-based metal-free oxygen electrocatalysts, such as large peak power density up to 131.4 mW cm^-2, energy density as high as 889.0 W h kg^-1 at 4.5 mA cm^-2, and remarkable discharge-charge cycles up to 575 times. Preliminarily, the rechargeable nonaqueous Li-air batteries were also investigated. Therefore, our work provides a low-cost, metal-free, and high-performance bifunctional carbon-based electrocatalyst for metal-air batteries.

Flexible/Rechargeable Zn-Air Batteries Based on Multifunctional Heteronanomat Architecture

The increasing demand for advanced rechargeable batteries spurs development of new power sources beyond currently most widespread lithium-ion batteries. Here, we demonstrate a new class of flexible/rechargeable zinc (Zn)-air batteries based on multifunctional heteronanomat architecture as a scalable/versatile strategy to address this issue. In contrast to conventional electrodes that are mostly prepared by slurry-casting techniques, heteronanomat (denoted as "HM") framework-supported electrodes are fabricated through one-pot concurrent electrospraying (for electrode powders/single-walled carbon nanotubes (SWCNTs)) and electrospinning (for polyetherimide (PEI) nanofibers) process. Zn powders (in anodes) and rambutan-shaped cobalt oxide (Co₃O₄)/multiwalled carbon nanotube (MWCNT) composite powders (in cathodes) are used as electrode active materials for proof of concept. The Zn (or Co₃O₄/MWCNT) powders are densely packed and spatially bound by the all-fibrous HM frameworks that consist of PEI nanofibers (for structural stability)/SWCNTs (for electrical conduction) networks, leading to the formation of three-dimensional bicontinuous ion/electron transport channels in the electrodes. The HM electrodes are assembled with cross-linked polyvinyl alcohol/polyvinyl acrylic acid gel polymer electrolytes (acting as zincate ion crossover-suppressing, permselective separator membranes). Benefiting from its unique structure and chemical functionalities, the HM-structured Zn-air cell significantly improves mechanical flexibility and electrochemical rechargeability, which are difficult to achieve with conventional Zn-air battery technologies.

Controllable Synthesis of Ni xSe (0.5 ≤ x ≤ 1) Nanocrystals for Efficient Rechargeable Zinc-Air Batteries and Water Splitting

The development of earth-abundant, highly active, and corrosion-resistant electrocatalysts to promote the oxygen reduction reaction (ORR) and oxygen and hydrogen evolution reactions (OER/HER) for rechargeable metal-air batteries and water-splitting devices is urgently needed. In this work, Ni _xSe (0.5 ≤ x ≤ 1) nanocrystals with different crystal structures and compositions have been controllably synthesized and investigated as potential electrocatalysts for multifunctional ORR, OER, and HER in alkaline conditions. A novel hot-injection process at ambient pressure was developed to control the phase and composition of a series of Ni _xSe by simply adjusting the added molar ratio of the nickel resource to triethylenetetramine. Electrochemical analysis reveals that Ni_0.5Se nanocrystalline exhibits superior OER activity compared to its counterparts and is comparable to RuO₂ in terms of the low overpotential required to reach a current density of 10 mA cm^-2 (330 mV), which may benefit from the pyrite-type crystal structure and Se enrichment in Ni_0.5Se. For the ORR and HER, Ni_0.75Se nanoparticles achieve the best performance including lower overpotentials and larger apparent current densities. Further investigations demonstrate that Ni_0.75Se could not only provide an enhanced electrochemical active area but also facilitate electron transfer during the electrocatalytic process, thus contributing to the remarkable catalytic activity. As a practical application, the Ni_0.75Se electrode enables rechargeable Zn-air battery with a considerable performance including a long cycling lifetime (200 cycles), high specific capacity (609 mA h g^-1 based on the consumed Zn), and low overpotential (0.75 V) at 10 mA cm^-2. Meanwhile, the water-splitting cell setup with an anode of Ni_0.5Se for the HER and a cathode of Ni_0.75Se for the OER exhibits a considerable performance with low decay in activity of 12.9% under continuous polarization for 10 h. These results suggest the promising potential of nickel selenide nanocrystals as earth-abundant and high-performance electrocatalysts for metal-air batteries and alkaline water splitting.

Carbon Nanosheets Containing Discrete Co-Nx-By-C Active Sites for Efficient Oxygen Electrocatalysis and Rechargeable Zn-Air Batteries

Structural and compositional engineering of atomic-scaled metal-N-C catalysts is important yet challenging in boosting their performance for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Here, boron (B)-doped Co-N-C active sites confined in hierarchical porous carbon sheets (denoted as Co-N,B-CSs) were obtained by a soft template self-assembly pyrolysis method. Significantly, the introduced B element gives an electron-deficient site that can activate the electron transfer around the Co-N-C sites, strengthen the interaction with oxygenated species, and thus accelerate reaction kinetics in the 4e^- processed ORR and OER. As a result, the catalyst showed Pt-like ORR performance with a half-wave potential (E_1/2) of 0.83 V versus (vs) RHE, a limiting current density of about 5.66 mA cm^-2, and higher durability (almost no decay after 5000 cycles) than Pt/C catalysts. Moreover, a rechargeable Zn-air battery device comprising this Co-N,B-CSs catalyst shows superior performance with an open-circuit potential of ∼1.4 V, a peak power density of ∼100.4 mW cm^-2, as well as excellent durability (128 cycles for 14 h of operation). DFT calculations further demonstrated that the coupling of Co-N_x active sites with B atoms prefers to adsorb an O₂ molecule in side-on mode and accelerates ORR kinetics.

Co9S8@MoS2 Core-Shell Heterostructures as Trifunctional Electrocatalysts for Overall Water Splitting and Zn-Air Batteries

The development of efficient non-noble-metal electrocatalysts is of critical importance for clean energy conversion systems, such as fuel cells, metal-air batteries, and water electrolysis. Herein, uniform Co₉S₈@MoS₂ core-shell heterostructures have been successfully prepared via a solvothermal approach, followed by an annealing treatment. Transmission electron microscopy, X-ray absorption near-edge structure, and X-ray photoelectron spectroscopy measurements reveal that the core-shell structure of Co₉S₈@MoS₂ can introduce heterogeneous nanointerface between Co₉S₈ and MoS₂, which can deeply influence its charge state to boost the electrocatalytic performances. Besides, due to the core-shell structure that can promote the synergistic effect of Co₉S₈ and MoS₂ and provide abundant catalytically active sites, Co₉S₈@MoS₂ exhibits a superior hydrogen evolution reaction performance with a small overpotential of 143 mV at 10 mA cm^-2 and a small Tafel slope value of 117 mV dec^-1 under alkaline solution. Meanwhile, the activity of Co₉S₈@MoS₂ toward oxygen evolution reaction is also impressive with a low operating potential (∼1.57 V vs reversible hydrogen electrode) at 10 mA cm^-2. By using Co₉S₈@MoS₂ catalyst for full water splitting, an alkaline electrolyzer affords a cell voltage as low as 1.67 V at a current density of 10 mA cm^-2. Also, Co₉S₈@MoS₂ reveals robust oxygen reduction reaction performance, making it an excellent catalyst for Zn-air batteries with a long lifetime (20 h). This work provides a new means for the development of multifunctional electrocatalysts of non-noble metals for the highly demanded electrochemical energy technologies.

Electrospun Thin-Walled CuCo2O4@C Nanotubes as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries

Rational design of optimal bifunctional oxygen electrocatalyst with low cost and high activity is greatly desired for realization of rechargeable Zn-air batteries. Herein, we fabricate mesoporous thin-walled CuCo₂O₄@C with abundant nitrogen-doped nanotubes via coaxial electrospinning technique. Benefiting from high catalytic activity of ultrasmall CuCo₂O₄ particles, double active specific surface area of mesoporous nanotubes, and strong coupling with N-doped carbon matrix, the obtained CuCo₂O₄@C exhibits outstanding oxygen electrocatalytic activity and stability, in terms of a positive onset potential (0.951 V) for oxygen reduction reaction (ORR) and a low overpotential (327 mV at 10 mA cm^-2) for oxygen evolution reaction (OER). Significantly, when used as cathode catalyst for Zn-air batteries, CuCo₂O₄@C also displays a low charge-discharge voltage gap (0.79 V at 10 mA cm^-2) and a long cycling life (up to 160 cycles for 80 h). With desirable architecture and excellent electrocatalytic properties, the CuCo₂O₄@C is considered a promising electrocatalyst for Zn-air batteries.

Unveiling the Catalytic Origin of Nanocrystalline Yttrium Ruthenate Pyrochlore as a Bifunctional Electrocatalyst for Zn-Air Batteries

Zn-air batteries suffer from the slow kinetics of oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER). Thus, the bifunctional electrocatalysts are required for the practical application of rechargeable Zn-air batteries. In terms of the catalytic activity and structural stability, pyrochlore oxides (A₂[B_2-xA_x]O_7-y) have emerged as promising candidates. However, a limited use of A-site cations (e.g., lead or bismuth cations) of reported pyrochlore catalysts have hampered broad understanding of their catalytic effect and structure. More seriously, the catalytic origin of the pyrochlore structure was not clearly revealed yet. Here, we report the new nanocrystalline yttrium ruthenate (Y₂[Ru_2-xY_x]O_7-y) with pyrochlore structure. The prepared pyrochlore oxide demonstrates comparable catalytic activities in both ORR and OER, compared to that of previously reported metal oxide-based catalysts such as perovskite oxides. Notably, we first find that the catalytic activity of the Y₂[Ru_2-xY_x]O_7-y is associated with the oxidations and corresponding changes of geometric local structures of yttrium and ruthenium ions during electrocatalysis, which were investigated by in situ X-ray absorption spectroscopy (XAS) in real-time. Zn-air batteries using the prepared pyrochlore oxide achieve highly enhanced charge and discharge performance with a stable potential retention for 200 cycles.

Interconnected Hierarchically Porous Fe, N-Codoped Carbon Nanofibers as Efficient Oxygen Reduction Catalysts for Zn-Air Batteries

Developing porous carbon-based non-precious-metal catalysts for an oxygen reduction reaction (ORR) is a suitable approach to significantly reduce the costs of fuel cells or metal-air batteries. Herein, interconnected hierarchically porous carbon nanofibers simultaneously doped with nitrogen and iron (HP-Fe-N/CNFs) were fabricated by facile pyrolysis of polypyrrole-coated electrospun polystyrene/FeCl₃ fibers. The obtained carbon nanofibers have a high specific surface area (569.6 m²/g) and large pore volume (1.00 cm³/ g) as well as effective doping of N and Fe. Benefiting from the improved mass transfer and utilization of active sites attributed to interconnected hierarchical porous structures, HP-Fe-N/CNFs display excellent ORR catalytic activity in alkaline media, with a comparable onset potential and half-wave potential but superior selectivity, stability, and tolerance against methanol to commercial 30 wt % Pt/C. Particularly, when applied in an assembled Zn-air battery, HP-Fe-N/CNFs outperform 30 wt % Pt/C in power density and long-term stability, explicitly showing their promising practical application.

Fe-Cluster Pushing Electrons to N-Doped Graphitic Layers with Fe3C(Fe) Hybrid Nanostructure to Enhance O2 Reduction Catalysis of Zn-Air Batteries

Non-noble metal catalysts with catalytic activity toward oxygen reduction reaction (ORR) comparable or even superior to that of Pt/C are extremely important for the wide application of metal-air batteries and fuel cells. Here, we develop a simple and controllable strategy to synthesize Fe-cluster embedded in Fe₃C nanoparticles (designated as Fe₃C(Fe)) encased in nitrogen-doped graphitic layers (NDGLs) with graphitic shells as a novel hybrid nanostructure as an effective ORR catalyst by directly pyrolyzing a mixture of Prussian blue (PB) and glucose. The pyrolysis temperature was found to be the key parameter for obtaining a stable Fe₃C(Fe)@NDGL core-shell nanostructure with an optimized content of nitrogen. The optimized Fe₃C(Fe)@NDGL catalyst showed high catalytic performance of ORR comparable to that of the Pt/C (20 wt %) catalyst and better stability than that of the Pt/C catalyst in alkaline electrolyte. According to the experimental results and first principle calculation, the high activity of the Fe₃C(Fe)@NDGL catalyst can be ascribed to the synergistic effect of an adequate content of nitrogen doping in graphitic carbon shells and Fe-cluster pushing electrons to NDGL. A zinc-air battery utilizing the Fe₃C(Fe)@NDGL catalyst demonstrated a maximum power density of 186 mW cm^-2, which is slightly higher than that of a zinc-air battery utilizing the commercial Pt/C catalyst (167 mW cm^-2), mostly because of the large surface area of the N-doped graphitic carbon shells. Theoretical calculation verified that O₂ molecules can spontaneously adsorb on both pristine and nitrogen doped graphene surfaces and then quickly diffuse to the catalytically active nitrogen sites. Our catalyst can potentially become a promising replacement for Pt catalysts in metal-air batteries and fuel cells.