CuCo2S4 Nanosheets@N-Doped Carbon Nanofibers by Sulfurization at Room Temperature as Bifunctional Electrocatalysts in Flexible Quasi-Solid-State Zn-Air Batteries

Tailored interface engineering of Co3Fe7/Fe3C heterojunctions for enhancing oxygen reduction reaction in zinc-air batteries

The rational construction of highly active and robust non-precious metal oxygen reduction electrocatalysts is a vital factor to facilitate commercial applications of Zn-air batteries. In this study, a precise and stable heterostructure, comprised of a coupling of Co3Fe7 and Fe3C, was constructed through an interface engineering-induced strategy. The coordination polymerization of the resin with the bimetallic components was meticulously regulated to control the interfacial characteristics of the heterostructure. The synergistic interfacial effects of the heterostructure successfully facilitated electron coupling and rapid charge transfer. Consequently, the optimized CST-FeCo displayed superb oxygen reduction catalytic activity with a positive half-wave potential of 0.855 V vs. RHE. Furthermore, the CST-FeCo air electrode of the liquid zinc-air battery revealed a large specific capacity of 805.6 mAh gZn-1, corresponding to a remarkable peak power density of 162.7 mW cm-2, and a long charge/discharge cycle stability of 220 h, surpassing that of the commercial Pt/C catalyst.

Electrospinning-derived transition metal/carbon nanofiber composites as electrocatalysts for Zn-air batteries

The sluggish kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) significantly impede the broader implementation of Zn-air batteries (ZABs), underscoring the necessity for advanced high-efficiency materials to catalyze these electrochemical processes. Recent advancements have highlighted the potential of transition metal/carbon nanofiber (TM/CNF) composite materials, synthesized via electrospinning technology, due to their expansive surface area, profusion of active sites, and elevated catalytic efficacy. This review comprehensively examines the structural characteristics of TM/CNFs, with a particular emphasis on the pivotal role of electrospinning technology in fabricating diverse structural configurations. Additionally, it delves into the mechanistic underpinnings of various strategies aimed at augmenting the catalytic activity of TM/CNFs. A meticulous discourse is also presented on the application scope of TM/CNFs in the realm of electrocatalysis, with a special focus on their impact on the performance of assembled ZABs. Lastly, this review encapsulates the challenges and future prospects in the development of TM/CNF composite materials via electrospinning, aiming to provide an exhaustive understanding of the current state of research in this domain and to foster further advancements in the commercialization of ZABs.

Rational design of bimetal phosphide embedded in carbon nanofibers for boosting oxygen evolution

The development of non-precious metal electrocatalysts for oxygen evolution reaction (OER) is crucial for generating large-scale hydrogen through water electrolysis. In this work, bimetal phosphides embedded in electrospun carbon nanofibers (P-FeNi/CNFs) were fabricated through a reliable electrospinning-carbonization-phosphidation strategy. The incorporation of P-FeNi nanoparticles within CNFs prevented them from forming aggregation and further improved their electron transfer property. The bimetal phosphides helped to weaken the adsorption of O intermediate, promoting the OER activity, which was confirmed by the theoretical results. The as-prepared optimized P-Fe1Ni2/CNFs catalyst exhibited very high OER electrocatalytic performance, which required very low overpotentials of just 239 and 303 mV to reach 10 and 1000 mA cm-2, respectively. It is superior to the commercial RuO2 and many other related OER electrocatalysts reported so far. In addition, the constructed alkaline electrolyzer based on the P-Fe1Ni2/CNFs catalyst and Pt/C delivered a cell voltage of 1.52 V at 10 mA cm-2, surpassing the commercial RuO2||Pt/C (1.61 V) electrolyzer. It also offered excellent alkaline OER performance in simulated seawater electrolyte. This demonstrated its potential for practical applications across a broad range of environmental conditions. Our work provides new ideas for the ration design of highly efficient non-precious metal-based OER catalysts for water electrolysis.

Cascade NH3 Oxidation and N2O Decomposition via Bifunctional Co and Cu Catalysts

The selective catalytic oxidation of NH3 (NH3-SCO) to N2 is an important reaction for the treatment of diesel engine exhaust. Co3O4 has the highest activity among non-noble metals but suffers from N2O release. Such N2O emissions have recently been regulated due to having a 300× higher greenhouse gas effect than CO2. Here, we design CuO-supported Co3O4 as a cascade catalyst for the selective oxidation of NH3 to N2. The NH3-SCO reaction on CuO-Co3O4 follows a de-N2O pathway. Co3O4 activates gaseous oxygen to form N2O. The high redox property of the CuO-Co3O4 interface promotes the breaking of the N-O bond in N2O to form N2. The addition of CuO-Co3O4 to the Pt-Al2O3 catalyst reduces the full NH3 conversion temperature by 50 K and improves the N2 selectivity by 20%. These findings provide a promising strategy for reducing N2O emissions and will contribute to the rational design and development of non-noble metal catalysts.

Flexible Solid-State Metal-Air Batteries: The Booming of Portable Energy Supplies

The rapid development of portable and wearable electronics has given rise to new challenges and provoked research in flexible, lightweight, and affordable energy storage devices. Flexible solid-state metal-air batteries (FSSMABs) are considered promising candidates, owing to their large energy density, mechanical flexibility, and durability. However, the practical applications of FSSMABs require further improvement to meet the demands of long-term stability, high power density, and large operating voltage. This Review presents a detailed discussion of innovative electrocatalysts for the air cathode, followed by a sequential overview of high-performance solid-state electrolytes and metal anodes, and a summary of the current challenges and future perspectives of FSSMABs to promote practical application and large-scale commercialization in the near future.

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.

Electronic tuning of Ni-Fe-Co oxide/hydroxide as highly active electrocatalyst for rechargeable Zn-air batteries

As a bifunctional oxygen electrocatalyst (oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)), spinel copper cobaltite (CuCo₂O₄) is attracting significant research interest owing to the tailored Co, Cu electronic structure and ease of adjusting the electrochemically active area. However, its poor OER performance (>300 mV at 10 mA cm^-2) limits its practical application for rechargeable zinc-air batteries. Therefore, we construct a CuCo₂O₄/NiFe LDH oxide/hydroxide interface to tune the properties of Ni, Fe and Co for enhancing OER activity and decreasing the charging overpotential of rechargeable zinc-air batteries. The obtained electrocatalysts show a low overpotential of 251 mV (10 mA cm^-2), which is 91 mV lower than the overpotential (342 mV) of CuCo₂O₄. By in situ Raman, XPS and electrochemical analyses, we ascribe the enhanced OER activity to the increasing Ni/Fe oxidation state triggered by the charge transfer of Ni/Fe and Co, which prompts CuCo₂O₄/NiFe LDH to rapidly form an active surface layer. Benefiting from enhanced OER performance, zinc-air batteries with a CuCo₂O₄/NiFe LDH electrode display a high round-trip efficiency with a low voltage gap of ∼0.78 V (10 mA cm^-2) due to the obvious decrease in the charging overpotential. These results suggest the importance of tuning the charge transfer on interfaces for designing high-efficiency electrocatalysts.

Bimetallic Organic Framework-Decorated Leaf-like 2D Nanosheets as Flexible Air Cathode for Rechargeable Zn-air Batteries

Exploring highly active and robust self-supporting air electrodes is the key for flexible Zn-air batteries (FZABs). Therefore, we report a novel 3D structural bimetal-based self-supporting electrode consisting of hybrid Cu, Co nanoparticles co-modified nitrogen-doped carbon nanosheets on carbon cloth (Cu, Co NPs@NCNSs/CC), which displays excellent electrochemical activity and durability of the oxygen reduction/evolution reaction (ORR/OER). The Cu, Co NPs@NCNSs/CC exhibits a half-wave potential of 0.863 V toward ORR and an overpotential of 225 mV at 10 mA cm^-2 toward OER, owing to its exposed bimetallic sites accelerating the kinetic reaction. In addition, the density functional theory calculation proves that the synergistic effect of CuCo sites favors ORR and OER. Hence, the FZABs based on Cu, Co NPs@NCNSs/CC achieve a larger open-circuit potential (1.45 V), higher energy density (130.10 mW cm^-2 ), and outstanding cycling stability. All remarkable results demonstrate valuable enlightenment for seeking advanced energy materials of portable and wearable electronics.

Emerging 2D Copper-Based Materials for Energy Storage and Conversion: A Review and Perspective

2D materials have shown great potential as electrode materials that determine the performance of a range of electrochemical energy technologies. Among these, 2D copper-based materials, such as Cu-O, Cu-S, Cu-Se, Cu-N, and Cu-P, have attracted tremendous research interest, because of the combination of remarkable properties, such as low cost, excellent chemical stability, facile fabrication, and significant electrochemical properties. Herein, the recent advances in the emerging 2D copper-based materials are summarized. A brief summary of the crystal structures and synthetic methods is started, and innovative strategies for improving electrochemical performances of 2D copper-based materials are described in detail through defect engineering, heterostructure construction, and surface functionalization. Furthermore, their state-of-the-art applications in electrochemical energy storage including supercapacitors (SCs), alkali (Li, Na, and K)-ion batteries, multivalent metal (Mg and Al)-ion batteries, and hybrid Mg/Li-ion batteries are described. In addition, the electrocatalysis applications of 2D copper-based materials in metal-air batteries, water-splitting, and CO₂ reduction reaction (CO₂ RR) are also discussed. This review also discusses the charge storage mechanisms of 2D copper-based materials by various advanced characterization techniques. The review with a perspective of the current challenges and research outlook of such 2D copper-based materials for high-performance energy storage and conversion applications is concluded.

Designing High-Quality Electrocatalysts Based on CoO:MnO2@C Supported on Carbon Cloth Fibers as Bifunctional Air Cathodes for Application in Rechargeable Zn-Air Battery

To achieve the requirements of rechargeable Zn-air batteries (ZABs), designing efficient, bifunctional, stable, and cost-effective electrocatalysts is vital for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which still are struggling with unsolved challenges. The present research provides a concept based on the nanoscale composites which were engineered by using MnO₂@C, CoO@C, and CoO:MnO₂@C bifunctional electrocatalysts for fabrication of uniform carbon cloth (CC)-based electrodes. The CoO:MnO₂@C electrocatalyst represented more efficient electrochemical properties through ORR and OER processes with superior positive half-wave potential (E_1/2 = 0.78 V) and better limiting current density (i = 1.10 mA cm^-2) in comparison with MnO₂@C (E_1/2 = 0.71 V, i = 0.92 mA cm^-2) and CoO@C (E_1/2 = 0.69 V, i = 0.86 mA cm^-2) electrocatalysts. For the rechargeable ZABs fabricated by using CoO:MnO₂@C-CC as an O₂-breathing cathode, the specific capacity (SC), peak power density (P), open-circuit voltage (E_OCV), and gap of charge/discharge voltage resulted in values of 520 mAh g_Zn^-1, 210.0 mW cm^-2, and 1.45 and 0.45 V, respectively, that afforded greater electrochemical characters than what was obtained for ZABs based on MnO₂@C-CC (410 mAh g_Zn^-1, 195.0 mW cm^-2, 1.38 and 0.44 V) and CoO@C-CC (440 mAh g_Zn^-1, 165.0 mW cm^-2, 1.15 and 0.54 V). At the same time, lower E_i=10 (= 1.45 V) implied a more efficient OER in alkaline electrolyte solution for CoO:MnO₂@C than MnO₂@C (E_i=10 = 1.50 V) and CoO@C (E_i=10 = 1.39 V). Based on cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and X-ray photoelectron spectroscopy (XPS) results, it could be stated that the CoO:MnO₂@C catalytic surface could experience 30 and 32% lower charge transfer resistance (R_ct = 13.9 Ω) than MnO₂@C (R_ct = 20.1 Ω) and CoO@C (R_ct = 29.7 Ω), respectively, which empowers an enhancement in ORR/OER performance. Prominently, the design concept of proposed electrocatalysts could suggest clear horizon for the synthesis and development paradigms of bifunctional catalysts for energy storage materials and devices.

Iron Single Atoms-Assisted Cobalt Nitride Nanoparticles to Strengthen the Cycle Life of Rechargeable Zn-Air Battery

The development of nonprecious metal catalysts with both oxygen reduction and evolution reactions (ORR/OER) is very important for Zn-air batteries (ZABs). Herein, a Co_5.47 N particles and Fe single atoms co-doped hollow carbon nanofiber self-supporting membrane (H-CoFe@NCNF) is synthesized by a coaxial electrospinning strategy combined with pyrolysis. X-ray absorption fine spectroscopy analyses confirm the state of the cobalt nitride and Fe single atoms. As a result, H-CoFe@NCNF exhibits a superior bifunctional performance of E_onset  = 0.96 V for ORR, and E_{j = 10} = 1.68 V for OER. Density functional theory calculations show that H-CoFe@NCNF has a moderate binding strength to oxygen due to the coexistence of nanoparticle and single atoms. Meanwhile, the Co site is more favorable to the OER, while the Fe site facilitates the ORR, and the proton and charge transfer between N and metal atoms further lower the reaction barriers. The liquid ZAB composed of H-CoFe@NCNF has a charge-discharge performance of ≈1100 h and a peak power density of 205 mW cm^-2 . The quasi-solid-state ZAB assembled by the self-supporting membrane of H-CoFe@NCNF is proven to operate stably in any bending condition.

Hydrothermally Grown Dual-Phase Heterogeneous Electrocatalysts for Highly Efficient Rechargeable Metal-Air Batteries with Long-Term Stability

Metal-air batteries as alternatives to the existing lithium-ion battery are becoming increasingly attractive sources of power due to their high energy-cost competitiveness and inherent safety; however, their low oxygen evolution and reduction reaction (OER/ORR) performance and poor operational stability must be overcome prior to commercialization. Herein, it is demonstrated that a novel class of hydrothermally grown dual-phase heterogeneous electrocatalysts, in which silver-manganese (AgMn) heterometal nanoparticles are anchored on top of 2D nanosheet-like nickel vanadium oxide (NiV₂ O₆ ), allows an enlarged surface area and efficient charge transfer/redistribution, resulting in a bifunctional OER/ORR superior to those of conventional Pt/C or RuO₂ . The dual-phase NiV₂ O₆ /AgMn catalysts on the air cathode of a zinc-air battery lead to a stable discharge-charge voltage gap of 0.83 V at 50 mA cm^-2 , with a specific capacity of 660 mAh g^-1 and life cycle stabilities of more than 146 h at 10 mA cm^-2 and 11 h at 50 mA cm^-2 . The proposed new class of dual-phase NiV₂ O₆ /AgMn catalysts are successfully applied as pouch-type zinc-air batteries with long-term stability over 33.9 h at 10 mA cm^-2 .

Research Progress of Bifunctional Oxygen Reactive Electrocatalysts for Zinc-Air Batteries

Zinc-air batteries (ZABs) have several advantages, including high energy density, cheap price and stable performances with good application prospects in the field of power batteries. The charging and discharging reactions for the air cathode of ZABs are the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively, which play an important role in the whole performance of ZAB. Due to the cost and limited reserves of highly active precious metal catalysts, it is crucial to design alternative efficient and stable dual-functional non-precious metal catalysts. In the present review, we present a systematic summary of the recent progress in the use of transition metal-based electrocatalysts as alternatives to precious metals for the positive poles of ZAB air. Combined with state-of-the-art in situ characterization technologies, a deep understanding of the catalytic mechanism of OER/ORR provided unique insights into the precise design of excellent synthetic non-precious metal catalysts from the perspective of atomic structure. This review further shows that the hybrid electric battery is a new strategy to improve the efficiency of the hybrid electric battery, which could be available to alleviate the problem of resource shortage. Finally, the challenges and research trends for the future development of ZABs were clearly proposed.

CuCo2S4 Nanoparticles Embedded in Carbon Nanotube Networks as Sulfur Hosts for High Performance Lithium-Sulfur Batteries

Rational design of sulfur hosts for lithium-sulfur (Li-S) batteries is essential to address the shuttle effect and accelerate reaction kinetics. Herein, the composites of bimetallic sulfide CuCo₂S₄ loaded on carbon nanotubes (CNTs) are prepared by hydrothermal method. By regulating the loading of CuCo₂S₄ nanoparticles, it is found that when Cu²⁺ and CNT are prepared in a 10:1 ratio, the CuCo₂S₄ nanoparticles loaded on the CNT are relatively uniformly distributed, avoiding the occurrence of agglomeration, which improves the electrical conductivity and number of active sites. Through a series of electrochemical performance tests, the S/CuCo₂S₄-1/CNT presents a discharge specific capacity of 1021 mAh g^-1 at 0.2 C after 100 cycles, showing good cycling stability. Even at 1 C, the S/CuCo₂S₄-1/CNT cathode delivers a discharge capacity of 627 mAh g^-1 after 500 cycles. This study offers a promising strategy for the design of bimetallic sulfide-based sulfur hosts in Li-S batteries.

Anchoring Ni/NiO heterojunction on freestanding carbon nanofibers for efficient electrochemical water oxidation

Rational design of low-cost and efficient electrocatalyst for the anodic oxygen evolution reaction (OER) to replace noble-metal-based catalysts is greatly desired for the large-scale application of water electrocatalysis. And compared with the conventional powdery catalysts, the freestanding electrode architecture is more attractive owing to the enhanced kinetics and stability. In this work, we report an electrospinning-carbonization-post oxidation strategy to develop the freestanding N-doped carbon nanofibers anchored with Ni/NiO nanoparticles (denoted as Ni/NiO-NCNFs) as efficient OER electrocatalyst. In the synthesized Ni/NiO-NCNFs, the conductive ultrathin carbon layer could promote electron transfer and thus improve the electrocatalytic activity. Meanwhile, the ratio between Ni and NiO could be regulated by tuning the oxidation duration, so as to optimize the adsorption energy of intermediates and improve the OER activity. The Ni/NiO-NCNFs prepared with the oxidation time of 3 h exhibit a promising OER activity and long-term operation durability in 0.1 M KOH solution, requiring an overpotential as small as 153 mV to achieve a current density of 10 mA cm^-2. Its overpotential is far lower than that of the reported OER catalysts. This work offers an efficient pathway to develop low-cost and highly active freestanding transitional metal-based OER electrocatalyst for potential renewable electrochemical energy conversion.

Janus Hollow Nanofiber with Bifunctional Oxygen Electrocatalyst for Rechargeable Zn-Air Battery

Zn-air battery technologies have received increasing attention, while the application is hindered by the sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In order to explore an efficient method to fabricate a high-performance electrocatalyst via modification of advanced nanostructure, a coaxial electrospinning method with in-situ synthesis and subsequent carbonization to construct 3D flexible Janus-like electrocatalysts is developed. The resulting Janus nanofibers have a unique core-shell hollow fiber structure, where NiFe alloy electrocatalysts supported by N-doped carbon nanobelt are located on the inner wall of the carbon layer, and leaf-like Co-N nanosheets are anchored on the outer wall of the carbon layer. As a result, the electrocatalyst exhibits excellent bifunctional catalytic performance for ORR and OER, demonstrating the small potential gap value of 0.73 V between the ORR half-wave potential and the OER potential at 10 mA cm^-2 , which is even comparable to the mixed commercial noble catalyst with 20% Pt/C and RuO₂ . The rechargeable Zn-air battery is constructed and displays a large open-circuit voltage of 1.44 V, high power density (130 mW cm^-2 ) and energy density (874 Wh kg^-1 ). This study provides a concept to synthesize and construct high performance bifunctional electrocatalysts.

Stabilizing Cobalt Single Atoms via Flexible Carbon Membranes as Bifunctional Electrocatalysts for Binder-Free Zinc-Air Batteries

Single-atom catalysts with high activity and efficient atom utilization have great potential in the electrocatalysis field, especially for rechargeable zinc-air batteries (ZABs). However, it is still a serious challenge to rationally construct a single-atom catalyst with satisfactory electrocatalytic activity and long-term stability. Here, we simultaneously realize the atomic-level dispersion of cobalt and the construction of carbon nanotube (CNT)-linked N-doped porous carbon nanofibers (NCFs) via an electrospinning strategy. In this hierarchical structure, the Co-N₄ sites provide efficient oxygen reduction/evolution electrocatalytic activity, the porous architectures of NCFs guarantee the active site's accessibility, and the interior CNTs enhance the flexibility and mechanical strength of porous fibers. As a binder-free air cathode, the as-prepared catalysts deliver superdurability of 600 h at 10 mA cm^-2 for aqueous ZABs and considerable flexibility and a small voltage gap for all-solid-state ZABs. This work provides an effective single-atom design/nanoengineering for superdurable zinc-air batteries.

Frontiers and Structural Engineering for Building Flexible Zinc-Air Batteries

With the development of flexible devices, the demand for wearable power sources has increased and gradually become imperative. Zinc-air batteries (ZABs) have attracted lots of research interest due to their high theoretical energy density and excellent safety properties, which can meet the wearable energy supply requirements. Here, the flexibility of energy storage devices is discussed first, followed by the chemistries and development of flexible ZABs. The design of flexible electrodes, the properties of solid-state electrolytes (SSEs), and the construction of deformable structures are discussed in depth. The researchers working on flexible energy storage devices will benefit from the work.

Electrospun one-dimensional electrocatalysts for boosting electrocatalysis

Electrocatalytic reaction plays a crucial role in determining the energy conversion efficiency in advanced technology. However, it is limited by the sluggish reaction kinetics and high energy barrier. These shortcomings...

Recent Advances in Flexible Zn-Air Batteries: Materials for Electrodes and Electrolytes

Flexible Zn-air batteries (ZABs) draw much attention due to the merits of high energy density, stability, and safety, and show potential applications for wearable devices. However, the development of flexible ZABs with great energy density, high round-trip efficiency, and long cycle life for practical applications is highly restricted by the lack of highly active oxygen catalysts, high ion-conducting solid-state electrolytes, appropriate Zn anodes, and advanced battery configuration. Promising oxygen catalysts should possess both, superior oxygen reduction reaction and oxygen evolution reaction performance and can be directly used as self-supporting cathodes without loading catalysts on support materials such as carbon cloth. In addition, electrolytes play an important role in ZABs; a good electrolyte should be in all-solid state with high ion conductivity. Moreover, for an excellent Zn anode, it is required to stably contact the electrolyte interface during the bending process. Therefore, in this review, recent advances in ZABs are summarized, including: i) the powder and 3D self-supporting oxygen catalysts, ii) the species of solid-state electrolytes, and iii) the rational design of Zn anodes. Finally, the challenges and opportunities of this promising field are presented.

Toward Flexible Zinc-Air Batteries with Self-Supported Air Electrodes

The compelling demand for higher energy performance, flexibility, and miniaturization is the main driving force of the energy storage and conversion industry's quest for flexible devices based on new integration and fabrication process. Herein, the recent advances on the development of flexible zinc-air batteries based on self-supported air electrodes are summarized, focusing on the multiscale and systematic design principles for the design of flexible air electrodes. With the electrocatalytic activity regulation and structural engineering strategies, the rational design of self-supported air electrodes is discussed in integrated devices to underpin the good flexibility for wearable requirement. The perspectives on promising developments of flexible zinc-air batteries and the accumulated knowledge from other flexible devices are also addressed for promoting the advances on flexible zinc-air batteries.

Structural Design Strategy and Active Site Regulation of High-Efficient Bifunctional Oxygen Reaction Electrocatalysts for Zn-Air Battery

Zinc-air batteries (ZABs) exhibit high energy density as well as flexibility, safety, and portability, thereby fulfilling the requirements of power batteries and consumer batteries. However, the limited efficiency and stability are still the significant challenge. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are two crucial cathode reactions in ZABs. Development of bifunctional ORR/OER catalysts with high efficiency and well stability is critical to improve the performance of ZABs. In this review, the ORR and OER mechanisms are first explained. Further, the design principles of ORR/OER electrocatalysts are discussed in terms of atomic adjustment mechanism and structural design in conjunction with the latest reported in situ characterization techniques, which provide useful insights on the ORR/OER mechanisms of the catalyst. The improvement in the energy efficiency, stability, and environmental adaptability of the new hybrid ZAB by the inclusion of additional reaction, including the introduction of transition-metal redox couples in the cathode and the addition of modifiers in the electrolyte to change the OER pathway, is also summarized. Finally, current challenges and future research directions are presented.

FeCo Nanoparticles Encapsulated in N-Doped Carbon Nanotubes Coupled with Layered Double (Co, Fe) Hydroxide as an Efficient Bifunctional Catalyst for Rechargeable Zinc-Air Batteries

Low-cost bifunctional nonprecious metal catalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical for the commercialization of rechargeable zinc-air batteries (ZABs). However, the preparation of highly active and durable bifunctional catalysts is still challenging. Herein, an efficient catalyst is reported consisting of FeCo nanoparticles embedded in N-doped carbon nanotubes (FeCo NPs-N-CNTs) by an in situ catalytic strategy. Due to the encapsulation and porous structure of N-doped carbon nanotubes, the catalyst shows high activity toward ORR and excellent durability. Furthermore, to enhance the OER activity, CoFe-layer double hydroxide (CoFe-LDH) is coupled with FeCo NPs-N-CNTs by in situ reaction approach. As the air electrode for rechargeable ZABs, the cell with CoFe-LDH@FeCo NPs-N-CNTs catalyst exhibits high open-circuit potential (OCP) of 1.51 V, high power density of 116 mW cm^-2 , and remarkable durability up to 100 h, demonstrating its great promise for the practical application of the rechargeable ZABs.

Surface Phosphorus-Induced CoO Coupling to Monolithic Carbon for Efficient Air Electrode of Quasi-Solid-State Zn-Air Batteries

One challenge facing the development of air electrodes for Zn-air batteries (ZABs) is the embedment of active sites into carbon, which requires cracks and blends between powder and membrane and results in low energy efficiency during manufacturing and utilization. Herein, a surface phosphorization-monolithic strategy is proposed to embed CoO nanoparticles into paulownia carbon plate (P-CoO@PWC) as monolithic electrodes. Benefiting from the retention of natural transport channels, P-CoO@PWC-2 is conducive to the construction of three-phase interface structure for efficient mass transfer and high electrical conductivity. The electrode exhibits remarkable catalytic activities for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with a small overpotential gap (EOER - EORR  = 0.68 V). Density functional theory calculations reveal that the incorporation of P on P-CoO@PWC-2 surface adjusts the electronic structure to promote the dissociation of water and the activation of oxygen, thus inducing catalytic activity. The monolithic P-CoO@PWC-2 electrode for quasi-solid-state or aqueous ZABs has excellent specific power, low charge-discharge voltage gap (0.83 V), and long-term cycling stability (over 700 cycles). This work serves as a new avenue for transforming abundant biomass into high-value energy-related engineering products.

Recent progress of electrocatalysts for oxygen reduction in fuel cells

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.

Electrospun One-Dimensional Electrocatalysts for Oxygen Reduction Reaction: Insights into Structure-Activity Relationship

Oxygen reduction reaction (ORR) is an efficiency-determining process at the cathode in several energy storage and conversion devices, typically such as metal-air batteries and fuel cells. To date, a considerable amount of ORR electrocatalysts have been purposely exploited to address the key issues of high overpotentials and sluggish electrochemical kinetics. Electrospinning is a popular additive manufacturing technology, enabling the production of one-dimensional (1D) electrocatalysts with outstanding chemical stability and structural diversity. However, compared with the well-studied composite/structural design as well as performance advancement, insights into structure-activity relationship are yet to be settled. To clarify this key issue, herein, a dedicated review on the structure-activity relationship between the 1D architectures of electrospun electrocatalysts and their catalytic ORR property is presented. First, the development and principles of electrospinning technique, the composition regulation- and structure design-oriented fundamentals are summarized by imputing the perspectives of mechanistic understanding. Then, the typical examples of nanofiber-shaped and nanofiber-supported electrocatalysts with different compositions and structures for ORR are implemented to establish different structure-activity relationship by comparative studies. Finally, we also identify some ongoing challenges and present future perspectives to direct the precise manipulation of structure-activity relationship for further activation and optimization of electrospun 1D electrocatalysts.

Recent advance in the fabrication of carbon nanofiber-based composite materials for wearable devices

Carbon nanofibers (CNFs) exhibit the advantages of high mechanical strength, good conductivity, easy production, and low cost, which have shown wide applications in the fields of materials science, nanotechnology, biomedicine, tissue engineering, sensors, wearable electronics, and other aspects. To promote the applications of CNF-based nanomaterials in wearable devices, the flexibility, electronic conductivity, thickness, weight, and bio-safety of CNF-based films/membranes are crucial. In this review, we present recent advances in the fabrication of CNF-based composite nanomaterials for flexible wearable devices. For this aim, firstly we introduce the synthesis and functionalization of CNFs, which promote the optimization of physical, chemical, and biological properties of CNFs. Then, the fabrication of two-dimensional and three-dimensional CNF-based materials are demonstrated. In addition, enhanced electric, mechanical, optical, magnetic, and biological properties of CNFs through the hybridization with other functional nanomaterials by synergistic effects are presented and discussed. Finally, wearable applications of CNF-based materials for flexible batteries, supercapacitors, strain/piezoresistive sensors, bio-signal detectors, and electromagnetic interference shielding devices are introduced and discussed in detail. We believe that this work will be beneficial for readers and researchers to understand both structural and functional tailoring of CNFs, and to design and fabricate novel CNF-based flexible and wearable devices for advanced applications.

Transition metal sulfides meet electrospinning: versatile synthesis, distinct properties and prospective applications

One-dimensional (1D) electrospun nanomaterials have attracted significant attention due to their unique structures and outstanding chemical and physical properties such as large specific surface area, distinct electronic and mass transport, and mechanical flexibility. Over the past years, the integration of metal sulfides with electrospun nanomaterials has emerged as an exciting research topic owing to the synergistic effects between the two components, leading to novel and interesting properties in energy, optics and catalysis research fields for example. In this review, we focus on the recent development of the preparation of electrospun nanomaterials integrated with functional metal sulfides with distinct nanostructures. These functional materials have been prepared via two efficient strategies, namely direct electrospinning and post-synthesis modification of electrospun nanomaterials. In this review, we systematically present the chemical and physical properties of the electrospun nanomaterials integrated with metal sulfides and their application in electronic and optoelectronic devices, sensing, catalysis, energy conversion and storage, thermal shielding, adsorption and separation, and biomedical technology. Additionally, challenges and further research opportunities in the preparation and application of these novel functional materials are also discussed.

High-Quality CoFeP Nanocrystal/N, P Dual-Doped Carbon Composite as a Novel Bifunctional Electrocatalyst for Rechargeable Zn-Air Battery

A novel composite catalyst (CoFeP@C) was constructed by high-quality CoFeP nanoparticles embedded in a N, P dual-doped carbon matrix. These CoFeP nanoparticles are rich in active sites of the oxygen evolution reaction (OER) at surfaces and provide metallic conductivity in their bulk phases. The N, P dual-doped carbon matrix provided abundant active sites of the oxygen reduction reaction (ORR) and formed a conductive network substrate. The ideal composite structure endowed CoFeP@C with highly efficient bifunctional performance for catalyzing both OER and ORR, accordingly making CoFeP@C an ideal catalyst for rechargeable Zn-air batteries. The liquid Zn-air battery of CoFeP@C has achieved a large power density of 143.5 mW/cm² and can be charged and discharged stably for 200 h (1200 cycles). The solid-state Zn-air battery of CoFeP@C has achieved a power density of 72.6 mW/cm² and can stably run for 20 h. This work has deepened the understanding of synergistic catalysis and paved one way for the development of high-performance bifunctional catalysts.

Confined Selenium in N-Doped Mesoporous Carbon Nanospheres for Sodium-Ion Batteries

In this paper, we have adopted a simple and etching-free method to prepare mesoporous carbon spheres in one step. Selenium can be deposited in the internal cavity, which can avoid pulverization due to the combined effect of volume expansion and a solid-electrolyte interphase (SEI) film while charging and discharging. Therefore, the as-prepared selenium and nitrogen codoped mesoporous carbon nanosphere (Se@NMCS) composites can deliver an outstanding sodium-storage performance of 336.6 mAh g^-1 at a present density of 200 mA g^-1 and great long-cycling performance. For a further understanding of the Na⁺ storage mechanism of the Se@NMCS anode in sodium-ion batteries (SIBs), the phase evolution of the Se@NMCS anode has been explored during the charge/discharge process by conducting in situ Raman investigation.

A ΔE = 0.63 V Bifunctional Oxygen Electrocatalyst Enables High-Rate and Long-Cycling Zinc-Air Batteries

Rechargeable zinc-air batteries constitute promising next-generation energy storage devices due to their intrinsic safety, low cost, and feasibility to realize high cycling current density and long cycling lifespan. Nevertheless, their cathodic reactions involving oxygen reduction and oxygen evolution are highly sluggish in kinetics, requiring high-performance noble-metal-free bifunctional electrocatalysts that exceed the current noble-metal-based benchmarks. Herein, a noble-metal-free bifunctional electrocatalyst is fabricated, which demonstrates ultrahigh bifunctional activity and renders excellent performance in rechargeable zinc-air batteries. Concretely, atomic Co-N-C and NiFe layered double hydroxides (LDHs) are respectively selected as oxygen reduction and evolution active sites and are further rationally integrated to afford the resultant CoNC@LDH composite electrocatalyst. The CoNC@LDH electrocatalyst exhibits remarkable bifunctional activity delivering an indicator ΔE of 0.63 V, far exceeding the noble-metal-based Pt/C+Ir/C benchmark (ΔE = 0.77 V) and most reported electrocatalysts. Correspondingly, ultralong lifespan (over 3600 cycles at 10 mA cm^-2 ) and excellent rate performances (cycling current density at 100 mA cm^-2 ) are achieved in rechargeable zinc-air batteries. This work highlights the current advances of bifunctional oxygen electrocatalysis and endows high-rate and long-cycling rechargeable zinc-air batteries for efficient sustainable energy storage.

Recent advances of noble-metal-free bifunctional oxygen reduction and evolution electrocatalysts

Bifunctional oxygen reduction and evolution constitute the core processes for sustainable energy storage. The advances on noble-metal-free bifunctional oxygen electrocatalysts are reviewed.

Aluminum-doping-based method for the improvement of the cycle life of cobalt-nickel hydroxides for nickel-zinc batteries

The unsatisfactory cycle life of nickel-based cathodes hinders the widespread commercial usage of nickel-zinc (Ni-Zn) batteries. The most frequently used methods to improve the cycle life of Ni-based cathodes are usually complicated and/or involve using organic solvents and high energy consumption. A facile process based on the hydrolysis-induced exchange of the cobalt-based metal-organic framework (Co-MOF) was developed to prepare aluminum (Al)-doped cobalt-nickel double hydroxides (Al-CoNiDH) on a carbon cloth (CC). The entire synthesis process is highly efficient, energy-saving, and has a low negative impact on the environment. Compared to undoped cobalt-nickel double hydroxide (Al-CoNiDH-0%), the as-prepared Al-CoNiDH as the electrode material displays a remarkably improved cycling stability because the Al-doping successfully depresses the transition in the crystal phase and microstructure during the long cycling. Benefiting from the improved performance of the optimal Al-CoNiDH electrode (Al-CoNiDH-5% electrode), the as-constructed aqueous Ni-Zn battery with Al-CoNiDH-5% as the cathode (Al-CoNiDH-5%//Zn) displays more than 14% improvement in the cycle life relative to the Al-CoNiDH-0%//Zn battery. Moreover, this Al-CoNiDH-5%//Zn battery achieves a high specific capacity (264 mAh g^-1), good rate capability (72.4% retention at a 30-fold higher current), high electrochemical energy conversion efficiency, superior fast-charging ability, and strong capability of reversible switching between fast charging and slow charging. Furthermore, the as-assembled quasi-solid-state Al-CoNiDH-5%//Zn battery exhibits a decent electrochemical performance and satisfactory flexibility.

Phase Engineering of Iron-Cobalt Sulfides for Zn-Air and Na-Ion Batteries

Rechargeable batteries are promising platforms for sustainable development of energy conversion and storage technologies. Highly efficient multifunctional electrodes based on bimetallic sulfides for rechargeable batteries are extremely desirable but still challenging to tailor with controllable phase and structure. Here, we report a colloidal strategy to fabricate FeCo-based bimetallic sulfides on reduced graphene oxide (rGO), which are expected to display highly efficient oxygen electrocatalysis and sodium storage performances. Specifically, as-screened FeCo₈S₈ nanosheets (NSs) on rGO originating from suitable tailoring of the Co₉S₈ matrix with Fe at the atomic level exhibited a very low potential difference (0.77 V) at 10 mA cm^-2 and negligible voltage loss after 200 cycles as an air electrode for Zn-air batteries. For Na-ion batteries, FeCo₈S₈ NS/rGO demonstrated a superior high-rate capability (188 mAh g^-1 at 20 A g^-1) with long-term cycling stability. The bifunctional electrocatalytic property and sodium storage performance are attributed to not only the synergistic effect of Fe/Co but also the optimized catalytic activity and ion transport ability by the in situ rGO hybrid. This work demonstrates the potential applications of FeCo-based bimetallic sulfides as efficient electrode materials for both rechargeable Zn-air and Na-ion batteries.

Self-Catalyzed Growth of Co-N-C Nanobrushes for Efficient Rechargeable Zn-Air Batteries

Highly efficient and stable bifunctional electrocatalysts for oxygen reduction and evolution are essential for aqueous rechargeable Zn-air batteries, which require highly active sites as well as delicate structural design for increasing effective active sites and facilitating mass/electron transfer. Herein, a scalable and facile self-catalyzed growth strategy is developed to integrate highly active Co-N-C sites with 3D brush-like nanostructure, achieving Co-N-C nanobrushes with Co,N-codoped carbon nanotube branches grown on Co,N-codoped nanoparticle assembled nanowire backbones. Systematic investigations suggest that nanobrushes deliver significantly improved electrocatalytic activity compared with nanowire or nanotube counterparts and the longer nanotube branches give the better performance. Benefiting from the increase of accessible highly active sites and enhanced mass transfer and electron transportation, the present Co-N-C nanobrush exhibits superior electrocatalytic activity and durability when used as a bifunctional oxygen catalyst. It enables a rechargeable Zn-air battery with a high peak power density of 246 mW cm^-2 and excellent cycling stability. These results suggest that the reported synthetic strategy may open up possibilities for exploring efficient electrocatalysts for diverse applications.

Nitrogen-doped carbon fibers embedding CoOx nanoframes towards wearable energy storage

As continuous consumption of the world's lithium reserves is causing concern, alternative energy storage solutions based on earth-abundant elements, such as sodium-ion batteries and zinc-air batteries, have been attracting increasing attention. Herein, nanoframes of CoOx are encapsulated into carbonized microporous fibers by electrospinning zeolitic imidazolate frameworks to impart both a sodium-hosting capability and catalytic activities for reversible oxygen conversion. The ultrahigh rate performance of sodium-ion batteries up to 20 A g-1 and ultrastable cycling over 6000 cycles are attributed to a dual-buffering effect from the framework structure of CoOx and the confinement of carbon fibers that effectively accommodates cyclic volume fluctuation. Both in situ Raman and ex situ microscopic analyses unveil the reversible conversion of CoOx during the sodiation/desodiation process. The excellent ORR activity, superior to that of commercial Pt/C, is mainly ascribed to the abundant Co-N-C species and the full exposure of active sites on the microporous framework structure. Flexible and rechargeable sodium-ion full batteries and zinc-air batteries are further demonstrated with great energy efficiency and cycling stability, as well as mechanical deformability.

Hierarchical Carbon Microtube@Nanotube Core-Shell Structure for High-Performance Oxygen Electrocatalysis and Zn-Air Battery

Zinc-air batteries (ZABs) hold tremendous promise for clean and efficient energy storage with the merits of high theoretical energy density and environmental friendliness. However, the performance of practical ZABs is still unsatisfactory because of the inevitably decreased activity of electrocatalysts when assembly into a thick electrode with high mass loading. Herein, we report a hierarchical electrocatalyst based on carbon microtube@nanotube core-shell nanostructure (CMT@CNT), which demonstrates superior electrocatalytic activity for oxygen reduction reaction and oxygen evolution reaction with a small potential gap of 0.678 V. Remarkably, when being employed as air-cathode in ZAB, the CMT@CNT presents an excellent performance with a high power density (160.6 mW cm-2), specific capacity (781.7 mAhg Zn -1 ) as well as long cycle stability (117 h, 351 cycles). Moreover, the ZAB performance of CMT@CNT is maintained well even under high mass loading (3 mg cm-2, three times as much as traditional usage), which could afford high power density and energy density for advanced electronic equipment. We believe that this work is promising for the rational design of hierarchical structured electrocatalysts for advanced metal-air batteries.

Rechargeable Zn-MnO2 batteries: advances, challenges and perspectives

As one type of advanced alternative batteries, zinc-ion batteries (ZIBs) have attracted increasing attention because of their advantages of cost-effectiveness, high safety and environmentally benign features. However, the performance of cathode materials has become a bottleneck for the future application of ZIBs. In recent years, manganese dioxide (MnO₂)-based materials as cathodes for ZIBs have been intensively explored. In this review, recent advances in MnO₂-based cathode materials for ZIBs are comprehensively reviewed with a discussion about the reaction mechanisms for the fundamental understanding of the electrochemical processes. Furthermore, several challenges hindering the technology maturity are also analyzed with corresponding strategies to further improve the electrochemical performance of such Zn-MnO₂ batteries.

RuNi Nanoparticles Embedded in N-Doped Carbon Nanofibers as a Robust Bifunctional Catalyst for Efficient Overall Water Splitting

Developing high-performance, low-cost, and robust bifunctional electrocatalysts for overall water splitting is extremely indispensable and challenging. It is a promising strategy to couple highly active precious metals with transition metals as efficient electrocatalysts, which can not only effectively reduce the cost of the preparation procedure, but also greatly improve the performance of catalysts through a synergistic effect. Herein, Ru and Ni nanoparticles embedded within nitrogen-doped carbon nanofibers (RuNi-NCNFs) are synthesized via a simple electrospinning technology with a subsequent carbonization process. The as-formed RuNi-NCNFs represent excellent Pt-like electrocatalytic activity for the hydrogen evolution reaction (HER) in both alkaline and acidic conditions. Furthermore, the RuNi-NCNFs also exhibit an outstanding oxygen evolution reaction (OER) activity with an overpotential of 290 mV to achieve a current density of 10 mA cm-2 in alkaline electrolyte. Strikingly, owing to both the HER and OER performance, an electrolyzer with RuNi-NCNFs as both the anode and cathode catalysts requires only a cell voltage of 1.564 V to drive a current density of 10 mA cm-2 in an alkaline medium, which is lower than the benchmark of Pt/C||RuO2 electrodes. This study opens a novel avenue toward the exploration of high efficient but low-cost electrocatalysts for overall water splitting.