Flexible Solid-state Zn-air Battery Research Articles

Flexible solid-state Zn-air battery based on polymer-oxygen-functionalized g-C3N4 composite membrane.

Personalized healthcare devices require an energy storage system that is flexible and has good mechanical strength and stability for long periods. Zn-air batteries show promise as an alternative to Li-air batteries for this purpose. Zn-air batteries with a high theoretical specific energy density of 1350 W h kg-1 have the potential to replace other metal-air batteries but faces the challenges, such as dendrite formation and Zn corrosion, hindering their successful commercialization. In this work, we report the design and performance optimization of a solid-state flexible Zn-air battery with superior performance and good mechanical property. In addition, we focused on the development of a gel-polymer composite membrane as the electrolyte. The main advantage of the flexible electrolyte is its optimum combination of good ionic conductivity and mechanical strength. Thus, we attempted to address the above-mentioned issues by modifying poly(vinyl alcohol) (PVA) with o-g-C3N4 through the in situ formation of a composite. The interaction between the functional groups of o-g-C3N4 and PVA increased the conductivity without compromising the mechanical behavior of the composite. According to the optimization of the composite composition, it was concluded that 0.32 wt% o-g-C3N4 in PVA showed the highest conductivity and excellent mechanical strength (increase from 25 MPa for pristine PVA membrane to 35 MPa for g-C3N4-PVA composite membrane). The performance of the solid-state battery was better (40 hours) than the standard PVA KOH (13 hours) membrane. Moreover, the stability of the battery was retained at various bending angles, demonstrating its potential to be used in flexible electronic devices.

Graphite-N modified single Fe atom sites embedded in hollow leaf-like nanosheets as air electrodes for liquid and flexible solid-state Zn-air batteries

The oxygen reduction procedure requires the development of an extremely effective, easily fabricated, and non-noble metal electrocatalyst. In this paper, we offer an unique method for manufacturing a defect-rich Fe single-atom catalyst loaded on the N-doped porous carbon nanosheets carrier (Fe-N4@NC-PCSs) by in-situ pyrolysis of a double-layered leaf-like compound (ZIF-L@PDA-FePhen) using dopamine hydrochloride aided precursor ZIF-L. The Fe-N4@NC-PCSs catalyst exhibits exceptional oxygen reduction reaction (ORR) activity across the pH-universal range, particularly in alkaline electrolytes, with a prominent half-wave potential of 0.938 V, outperforming practically previously described non-noble electrocatalysts as well as the commercial Pt/C catalyst (0.84 V). At pH-universal, the Fe-N4@NC-PCSs catalyst exhibits exceptional half-wave potential and an impressive electron/proton transfer ability resulting from its rich iron nitrogen active sites and a significant number of metal defects. Furthermore, the liquid zinc-air battery, as well as the flexible solid-state battery based on Fe-N4@NC-PCSs work excellently, with open circuit voltage of 1.465 V, maximum peak power density of 207 mV cm−2, and good charge–discharge endurance and mechanical flexibility. Calculations using density functional theory show that graphitic nitrogen can improve the local electronic redistribution for active sites and give them a lower free-energy barrier to ORR. This work serves as motivation for the creation of nitrogen moiety regulation extremely reactive atomic metal catalysts.

In-situ engineering of 3D porous Co nanoparticles incorporated in B/N Co-doped carbon nanotube/nanofiber networks as integrated cathode materials for Zn-air batteries

Rational design and construction of low-cost and highly efficient bifunctional catalysts toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is crucial for the commercialization of the rechargeable Zn-air batteries (ZABs). Herein, a 3D self-standing bifunctional catalytic electrode has been successfully achieved by in-situ confinement of Co nanoparticles into B/N co-doped carbon nanotube/nanofiber well-integrated networks (Co-B/N-CNTs@CFs). The purpose of designing such a 3D network structure is to optimize the electronic structure of the active sites for high electrocatalytic activity, ensure continuous and fast electron and charge transfer for high utilization of active sites, and avoid the abscission and aggregation of nanocatalysts for high electrocatalytic stability, which is confirmed by a series of experimental, characterization, and theoretical studies. As a result, the Co-B/N-CNTs@CFs as integrated cathode materials deliver efficient and stable OER/ORR performance with a ΔE of 0.59 V, largely outperforming the benchmark Pt/C-RuO2 electrode (0.71 V), and hence leading to appealing applications in both the assembled aqueous and flexible solid-state ZABs. This study forges a new path for the production of integrated cathode while enhancing their superiority, allowing for the construction of energy storage devices.

Toward Self‐Supported Bifunctional Air Electrodes for Flexible Solid‐State Zn–Air Batteries

The demand for flexibility and rechargeability in tandem with high energy density, reliability, and safety in energy‐storage devices to power wearable electronics has translated to significant advances in flexible solid‐state Zn–air batteries (FSZABs) technology. FSZABs using self‐supported bifunctional air electrodes are currently one of the most attractive alternatives to Li‐ion battery technology for next‐generation wearable electronics. Unlike the conventional powder‐based air electrodes, self‐supported bifunctional air electrodes offer higher electron‐transfer rate, larger specific surface area (and catalyst–reactant–product interfacial contact area), mechanical flexibility, and better operational robustness. To realize their potential nonetheless, self‐supported bifunctional air electrodes should have high and stable bifunctional catalytic activity, low cost, and environmental compatibility. This review first summarizes the three typical configurations and working principles of FSZABs. Then, significant development of self‐supported bifunctional air electrodes for FSZABs and efficient synthesis strategies are emphasized. The review concludes by providing perspectives on how to further improve the electrochemical performance of FSZABs and their suitability for next‐generation wearable electronic devices.

Biomass-Derived Bifunctional Cathode Electrocatalyst and Multiadaptive Gel Electrolyte for High-Performance Flexible Zn-Air Batteries in Wide Temperature Range.

High-efficiency and low-cost bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), as well as gel electrolytes with high thermal and mechanical adaptability are required for the development of flexible batteries. Herein, abundant Setaria Viridis (SV) biomass is selected as the precursor to prepare porous N-doped carbon tubes with high specific surface area and the 900°C calcination product of SV (SV-900) shows the optimum ORR/OER activities with a small EOER -EORR of 0.734 V. Meanwhile, a new multifunctional gel electrolyte named C20E2G5 is prepared using cellulose extracted from another widely distributed biomass named flax as the skeleton, epichlorohydrin as the cross-linker and glycerol as the antifreezing agent. C20E2G5 possesses high ionic conductivity from -40 to + 60 °C, excellent tensile and compressive resistance, high adhesion, strong freezing and heat resistance. Moreover, the symmetrical cell assembled with C20E2G5 can significantly inhibit Zn dendrite growth. Finally, flexible solid-state Zn-air batteries assembled with SV-900 and C20E2G5 show high open circuit voltage, large energy density, and long-term operation stability between -40 and + 60°C. This biomass-based approach is generic and can be used for the development of diverse next-generation electrochemical energy conversion and storage devices.

Synergetic Nanostructure Engineering and Electronic Modulation of a 3D Hollow Heterostructured NiCo2O4@NiFe-LDH Self-Supporting Electrode for Rechargeable Zn–Air Batteries

Developing electrocatalysts that integrate the merits of the hollow structure and heterojunction is an attractive but still challenging strategy for addressing the sluggish kinetics of oxygen evolution reaction (OER) in many renewable energy technologies. Herein, a 3D hierarchically flexible self-supporting electrode with a hollow heterostructure is intentionally constructed by assembling thin NiFe layered double hydroxide (LDH) nanosheets on the surface of metal-organic framework-derived hollow NiCo2O4 nanoflake arrays (NiCo2O4@NiFe-LDH) for rechargeable Zn-air batteries (ZABs). Theoretical calculations demonstrate that the interfacial electron transfer from NiFe-LDH to NiCo2O4 induces the electronic modulation, improves the conductivity, and lowers the reaction energy barriers during OER, ensuring high catalytic activity. Meanwhile, the 3D hierarchically hollow nanoarray architecture can afford plentiful catalytic active sites and short mass-/charge-transfer pathways. As a result, the obtained catalyst exhibits remarkable OER electrocatalytic performance, showing low overpotentials (only 231 mV at 10 mA cm-2, 300 mV at 50 mA cm-2) and robust stability. When assembling liquid and flexible solid-state ZABs with NiCo2O4@NiFe-LDH as the OER catalyst, the ZABs achieve excellent power density, high specific capacity, superior cycle durability, and good bending flexibility, exceeding the RuO2 + Pt/C benchmarks and other previously reported self-supporting catalysts. This work not only constructs an advanced hollow heterostructured catalyst for sustainable energy systems and wearable electronic devices but also provides insights into the role of interfacial electron modulation in catalytic performance enhancement.

An integrated oxygen electrode derived from a flexible single-walled carbon nanotube film for rechargeable Zn-air batteries produced by electropolymerization

The development of low-cost, high-activity, and durable integrated bifunctional flexible air electrodes for use in Zn-air batteries is both challenging and important. We report a simple and scalable electropolymerization method used to prepare an electrode material comprising heavily N-doped carbon covering single-walled carbon nanotube (N/C-SWCNT) networks. The resulting core/shell structure of the hybrid electrode enabled the flexibility, mechanics, and three-dimensional interconnected porous structure of SWCNT films while containing abundant pyridinic N, which provided excellent catalytic activity for both the oxygen reduction and evolution reactions (overpotential gap = 0.76 V). A binder-free Zn-air battery using the N/C-SWCNT film as an oxygen electrode was assembled and showed a high peak power density of 181 mW/cm2, a high specific capacity of 810 mAh/g and stable discharge‒charge cycling performance. We also constructed a flexible solid-state Zn-air battery featuring not only a high power density of 22 mW/cm2 but also good flexibility and stability.

Interfacial Engineering of Leaf-like Bimetallic MOF-Based Co@NC Nanoarrays Coupled with Ultrathin CoFe-LDH Nanosheets for Rechargeable and Flexible Zn-Air Batteries.

Exploring high-efficiency, low-cost, and long-life bifunctional self-supporting electrocatalysts is of great significance for the practical application of advanced rechargeable Zn-air batteries (ZABs), especially flexible solid-state ZABs. Herein, ultrathin CoFe-layered double hydroxide (CoFe-LDH) nanosheets are strongly coupled on the surface of leaf-like bimetallic metal-organic frameworks (MOFs)-derived hybrid carbon (Co@NC) nanoflake nanoarrays supported by carbon cloth (CC) via a facile and scalable method for rechargeable and flexible ZABs. This interfacial engineering for CoFe-LDHs on Co@NC improves the electronic conductivity of CoFe-LDH nanosheets as well as achieves the balance of oxygen evolution reduction (OER) and oxygen reduction reaction (ORR) activity. The unique three-dimensional (3D) open interconnected hierarchical structure facilitates the transport of substances during the electrochemical process while ensuring adequate exposure of OER/ORR active centers. When applied as an additive-free air cathode in rechargeable liquid ZABs, CC/Co@NC/CoFe-LDH-700 demonstrates high open-circuit potential of 1.47 V, maximum power density of 129.3 mW cm-2, and satisfactory specific capacity of 710.7 mAh g-1Zn. Further, the flexible all-solid-state ZAB assembled by CC/Co@NC/CoFe-LDH-700 displays gratifying mechanical flexibility and stable cycling performance over 40 h. More significantly, the series-connected flexible ZAB is further verified as a chain power supply for LED strips and performs well throughout the bending process, showing great application prospects in portable and wearable electronics. This work sheds new light on the design of high-performance self-supporting non-precious metal bifunctional electrocatalysts for OER/ORR and air cathodes for rechargeable ZABs.

Ordered mesoporous carbon fiber bundles with high-density and accessible Fe-NX active sites as efficient ORR catalysts for Zn-air batteries

Fe-NX/C electrocatalysts have aroused extensive interest in accelerating sluggish oxygen reduction reaction (ORR) kinetics as potential alternatives to platinum catalysts in rechargeable Zn-air batteries (ZABs). However, the low density and poor accessibility of Fe-NX sites have severely restricted the electrocatalytic performance of Fe-NX/C. Herein, Fe, N co-doped ordered mesoporous carbon fiber bundles are prepared through a ligand-assisted strategy with nitrogen-rich 1,10-phenanthroline as space isolation agent. 1,10-Phenanthroline reveals a six-membered heterocyclic structure containing abundant nitrogen species to tightly coordinate with Fe ions, which is conducive to achieving high-density Fe-NX sites. Meanwhile, the adoption of SBA-15 as hard-templates enables the catalysts with highly ordered channels and large specific surface areas, improving the accessibility of Fe-NX sites. The optimal catalyst (PDA-Fe-900) demonstrates a positive half-wave potential of 0.84 V (vs. RHE) in alkaline solution, outperforming the commercial Pt/C (0.83 V). In addition, PDA-Fe-900 delivers comparable ORR performance to commercial Pt/C in acidic electrolyte. Impressively, when PDA-Fe-900 is employed as an air cathode, it achieves large power densities of 163.0 mW/cm2 in liquid-state ZAB and 116.6 mW/cm2 in the flexible solid-state ZAB. This work provides an efficient ligand-assisted pathway for fabricating catalysts with dense and accessible Fe-NX sites as high-performance ORR electrocatalysts for ZABs.

Fullerene-Derived Carbon Nanotubes and Their Electrocatalytic Properties in Oxygen Reduction and Zn-Air Batteries.

Carbon-based materials with superior electrochemical performances have been prepared from fullerenes by releasing their intrinsic advantages such as pentagon defects and π-electron carbons. To the best of our knowledge, fullerene-derived carbon nanotubes (CNTs) and their electrochemical behavior have not been experimentally investigated. In this work, in situ growth of CNT composites from fullerene is realized via a self-catalyzed process by employing an Fe-decorated fullerene (ferrocenylpyrrolidine C60) as the precursor and NH3 as the pyrolysis atmosphere. The results show that the in situ Fe doping in fullerene, the self-assembly of fullerene molecules, the pyrolysis temperature, and the NH3 flow play essential roles in the generation of CNTs. The as-prepared MN7-10/3 CNT composite exhibits efficient oxygen reduction performance with E1/2 = 0.82 V and Eon = 1.02 V vs the RHE. The flexible solid-state Zn-air battery constructed based on MN7-10/3 exhibits a superior power density (109.3 mW cm-2 at 180.9 mA cm-2) and long-term durability (the voltage remains at 95.6% of the initial value after discharging for 5000 s) compared with the benchmark Pt/C catalyst. The transformation of the Fe-decorated fullerene to CNTs reveals a new function of fullerenes and demonstrates a new solid-state synthetic method for CNTs.

ZIF-8-derived N-doped porous carbon wrapped in porous carbon films as an air cathode for flexible solid-state Zn-air batteries

Metal-organic frameworks are a new type of catalyst precursor with high specific surface area and controllable composition, which can be modified by post-treatment and are suitable for use as cathode catalysts for Zn-air batteries (ZABs). Here, a self-doped nitrogen nanocatalyst (N-PC@CF) with a double-layered porous structure is rationally designed for flexible solid-state ZABs. The outer porous carbon shell of the N-PC@CF is highly hydrophilic and O2 permeable, while the layered porous structure exposes sufficient active sites to shorten the mass transfer distance, which would promote electrocatalytic performance and increase flexibility efficiently. The obtained N-PC@CF has an onset potential of 0.926 V and a half-wave potential of 0.843 V in the oxygen reduction reaction test, which is equivalent to commercial Pt/C. Most importantly, the maximum power density of the assembled ZAB is 134.7 mW cm−2 and it exhibits a specific capacity of 776.8 mA h g−1 at 10 mA cm−2, which is better than the 99.9 mW cm−2 of the Pt/C-based battery. An obvious improvement in the constant current discharge–charge cycle durability of the ZAB is found when compared with Pt/C. The specific capacities of ZAB with N-PC@CF as the air cathode at 5, 10 and 15 mA cm−2 are 842.7, 776.8 and 715.0 mAh g−1 (calculated by the mass of zinc consumed), respectively, corresponding to high energy densities of 1089.7, 977.3 and 842.2 Wh kg−1. A flexible solid-state battery is assembled with excellent flexibility and stability, even if the battery is folded into a large angle (160°). This work provides a new strategy for the design and synthesis of metal-free air cathodes.

Ultrathin NiFe-LDH nanosheets strongly coupled with MOFs-derived hybrid carbon nanoflake arrays as a self-supporting bifunctional electrocatalyst for flexible solid Zn-air batteries

The construction of a hybrid hierarchical architecture is a promising strategy for optimizing the electrocatalytic activity and the performance of flexible Zn-air battery. However, it still remains a challenging task. This study reports on the use of a simple approach in the preparation of a flexible 3D self-supporting bifunctional catalyst. Ultrathin NiFe-based layered double hydroxide nanosheets (NiFe-LDH) have in this approach been strongly coupled to a metal-organic framework (MOF)-derived carbon nanoflake array, to be used in flexible Zn-air batteries. Importantly, the introduction of Co nanoparticles that were anchored onto the nitrogen-doped porous carbon (Co-NC) nanoflakes provided an abundance of active sites for the oxygen reduction reaction (ORR). Moreover, the catalytic oxygen evolution reaction (OER) process was improved by altering the local electronic structure of the Ni and Fe species in NiFe-LDH. The 3D interconnected conductive network structure, in addition to the strong coupling between NiFe-LDH and Co-NC, was found to give a catalyst with superior electrocatalytic OER performances. The overpotential was only 284 mV at 50 mA cm−2 in an alkaline medium, which is a result that outperforms the commercial RuO2 catalyst. This bifunctional catalyst did also exhibit a good catalytic performance that is comparable to that of commercial Pt/C towards ORR. Interestingly, when this catalyst was used as binder-free air cathode, the assembled flexible solid-state Zn-air battery demonstrated a favorable power density, cycling stability, and mechanical flexibility. The present work offers a facile and efficient strategy for the development and construction of self-supporting bifunctional OER/ORR electrocatalysts, to be used in wearable and flexible electronic devices.

Controllable construction of FeCo nanoparticles embedded 3D porous N-doped carbon nanonetworks as high efficiency bifunctional electrocatalysts for Zn-air batteries

Construction of efficient and low-cost bifunctional electrocatalysts toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) for rechargeable Zn-air batteries is still a great challenge but highly desirable. Herein, a series of low-cost bifunctional electrocatalysts including Fe/C, Fe-NPCNs, FeCo-NPCNs, and FeCo-CNTs are prepared by simply carbonizing the mixture of Fe-based metal-organic frameworks (Fe-MIL-53), melamine, and CoCl2.6 H2O with different contents. Structure and electrocatalytic performance of the as-fabricated four samples are systematically investigated and the FeCo-NPCNs catalyst with a structure of FeCo nanoparticles uniformly embedded 3D porous N-doped carbon nanonetwork exhibits the highest electrochemical activity with a positive half-wave potential of 0.87 V vs. RHE for ORR and a low overpotential of 240 mV vs. RHE for OER with a ΔE of 0.60 V at 10 mA cm-2, together with excellent stability and methanol tolerance, even outperforming the noble-metal-based Pt/C-RuO2 catalysts. Besides, the results of density functional theory (DFT) calculations reveal the boosted intrinsic activity due to the reduced energy barrier both for ORR and OER. Furthermore, the assembled aqueous rechargeable Zn-air battery with FeCo-NPCNs catalyst manifests a large power density of 161.2 mW cm-2, a low charge-discharge gap of 0.78 V, and long-term ability. Likewise, the flexible solid-state Zn-air battery with FeCo-NPCNs catalyst also demonstrates high round-trip efficiency and excellent durability under various bending angles. This work may serve as a reference to designing advanced bifunctional electrocatalysts with 3D porous carbon nanonetworks for rechargeable Zn-air batteries.

Designing superior bifunctional electrocatalyst with high-purity pyrrole-type CoN4 and adjacent metallic cobalt sites for rechargeable Zn-air batteries

Developing cost-effective and high-efficient bifunctional electrocatalyst for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is essential for rechargeable Zn-air battery application. However, achieving high-active dual sites for bifunctional oxygen electrocatalysis in a single catalyst, as well as determining the decisive active sites for OER and ORR to explore the synergistically catalytic mechanism remain challenges. Herein, we propose a catalyst consisting of high-purity pyrrole nitrogen anchored cobalt single-atom (CoN4) and adjacent metallic cobalt sites (Cop@CoNC) for explicit research of dominated active sites. The obtained Cop@CoNC exhibits superior electrocatalytic activity with a small overpotential difference of 0.68 V between OER potential at 10 mA cm−2 and ORR half-wave potential, outperforming commercial noble-metal based catalysts. Electrochemical measurements and theoretical simulations verify the pyrrole-type CoN4 and adjacent metallic cobalt are separately decisive active sites for ORR and OER activities. Simultaneously, metallic cobalt regulates electron distribution of CoN4 moiety to improve ORR activity. Importantly, the rechargeable liquid and flexible solid-state Zn-air batteries using Cop@CoNC as air electrode catalysts possess high peak power densities of 188.8 and 121.8 mW cm−2, respectively. Our findings provide a feasible strategy to design high-performance multifunctional catalysts for rechargeable Zn-air batteries application.

Synergetic Chemistry and Interface Engineering of Hydrogel Electrolyte to Strengthen Durability of Solid-State Zn-Air Batteries.

For the challenging pursuit of high energy efficiency and mechanical tolerance in flexible solid-state Zn-air batteries (FSZABs), a hydrogel electrolyte (HE) consisting of dual-network crosslinked polyacrylic acid-Fe3+ -chitosan (PAA-Fe3+ -CS) polymer host infiltrated with a mixed aqueous electrolyte of NH4 Cl and ZnCl2 is developed. The absorbed near-neutral electrolyte renders the HE high ionic conductivity but low corrosiveness to both electrocatalysts and Zn metal anode (ZMA), ensuring more stable Zn-OH-O2 chemistry compared to that in strong alkaline electrolyte and thus endowing the assembled FSZABs with a landmark cycle life up to 120 h (5 mA cm-2 ). More intriguingly, the CS molecular beams introduced into the PAA hydrogel backbone will precipitate and fold subjecting to the Hofmeister effect when saturated with the near-neutral electrolyte, which can effectively enhance the interfacial adhesion strength of the HE on both air cathode and ZMA, achieving reliable and robust bonding between them. Thus, the FSZABs simultaneously exhibited a superior tolerance to repeated mechanical deformation during operation, allowing more than 360 continuous bending-recovery cycles without any decline in voltage efficiency. The ingenious chemistry and interface synergetic engineering on the crucial HEs provides a rational methodology to realize boosted electrochemical and mechanical durability of FSZABs forward for future practical implementation.

Boosting oxygen-reduction catalysis over mononuclear CuN2+2 moiety for rechargeable Zn-air battery

Endowing the transition-metal-nitrogen-carbon (M-N-Cs) electrocatalysts with high oxygen reduction activity for sustainable energy conversion, is a field of intense research while remains elusive. Here we exploit the variable configuration of atomically dispersed Cu1 centers on carbon support via rationally constructing prototypical CuN2+2 active sites to boost the kinetically sluggish electrochemical oxygen reduction. Density functional theory reveals the modified electronic structure of active Cu site and its derived orbital overlap through right symmetry with the degenerate π* orbital of O2 molecule, which more effectively facilitates O2 activation compared with CuN4 counterpart. The targeted electrocatalyst delivers superior alkaline ORR activity with a half-wave potential of 0.88 V and superior kinetics with Tafel slope of 48 mV dec−1, outperforming those of benchmark Pt/C. Remarkably, further demonstrated excellent peak power density and long-term durability in both primary liquid and flexible solid-state Zn-air battery assemblies signify the potential for practical applications.

Co/CoOx heterojunctions encapsulated N-doped carbon sheets via a dual-template-guided strategy as efficient electrocatalysts for rechargeable Zn-air battery

Developing highly efficient oxygen electrocatalysts is of vital importance for rechargeable Zn-air batteries (ZABs). Herein, Co/CoOx nano-heterojunctions encapsulated into nitrogen-doped carbon sheets (NCS@Co/CoOx) are fabricated via a dual-template-guided approach by using zeolitic imidazolate frameworks (ZIFs) as templates. The synergistic integration of structural and compositional advantages endows such catalyst with superior catalytic properties to benchmark noble-metal catalysts. To be specific, the hierarchical micro/mesopores affords massive mass transport channels and maximizes the exposure of accessible active sites, whereas the NCS matrix accelerates electron transfer and prevents the self-aggregation of active species during the electrocatalytic reaction. Moreover, abundant and synergistic Co-based active sites (CoO, Co3O4, Co-Nx) greatly promote the catalytic activity. As the cathode of both liquid and flexible solid-state ZABs, excellent device properties are achieved, outperforming those assembled with commercial Pt/C+RuO2 catalyst. This work presents a feasible and cost-effective strategy for developing oxygen electrocatalysts derived from ZIFs templates.

Binder-Free Air Electrodes for Rechargeable Zinc-Air Batteries: Recent Progress and Future Perspectives.

Designing an efficient air electrode is of great significance for the performance of rechargeable zinc (Zn)-air batteries. However, the most widely used approach to fabricate an air electrode involves polymeric binders, which may increase the interface resistance and block electrocatalytic active sites, thus deteriorating the performance of the battery. Therefore, binder-free air electrodes have attracted more and more research interests in recent years. This article provides a comprehensive overview of the latest advancements in designing and fabricating binder-free air electrodes for electrically rechargeable Zn-air batteries. Beginning with the fundamentals of Zn-air batteries and recently reported bifunctional active catalysts, self-supported air electrodes for liquid-state and flexible solid-state Zn-air batteries are then discussed in detail. Finally, the conclusion and the challenges faced for binder-free air electrodes in Zn-air batteries are also highlighted.

Formation of sandwiched leaf-like CNTs-Co/ZnCo2O4@NC-CNTs nanohybrids for high-power-density rechargeable Zn-air batteries

Air cathodes with high-efficiency oxygen reduction and evolution reaction (ORR/OER) performances are essential but still challenging for the development of high-power-density rechargeable Zn-air batteries. We herein report the preparation of a bifunctional oxygen electrocatalyst, composed of Co/ZnCo 2 O 4 nanoparticles sandwiched in leaf-like nitrogen-doped carbon microplates interwoven with carbon nanotubes (denoted as Co/ZnCo 2 O 4 @NC-CNTs). Owing to large amount of metal-N x and Co 3+ active sites as well as the interwove CNTs on the surface of carbon microplates, the Co/ZnCo 2 O 4 @NC-CNTs catalyst reveals excellent ORR/OER activities manifested as a small potential gap (ΔE = 0.70 V), a superior high-power density of 305 mW cm −2 and a stable rechargeability of up to 103 h with the Zn-air battery. Meanwhile, the Co/ZnCo 2 O 4 @NC-CNTs-based flexible solid-state Zn-air battery also exhibits competitive peak power density (151 mW cm −2 ), robust flexibility and integrability. A sandwiched leaf-like bifunctional CNTs-Co/ZnCo 2 O 4 @NC-CNTs electrocatalyst was designed and synthesized, which exhibits a superior high-power density of 305 mW cm −2 and a stable rechargeability of up to 103 h with the Zn-air battery. • Co/ZnCo 2 O 4 nanoparticles sandwiched in leaf-like N-doped carbon microplates interwoven with CNTs is synthesized. • Large amount of metal-N x and Co 3+ active sites can be obtained in the final product. • The catalyst exhibits excellent bifunctional catalytic activities for ORR and OER with a small potential gap (ΔE = 0.70 V). • The assembled Zn-air battery exhibits superior high-power density of 305 mW cm −2 and stable rechargeability of up to 103 h.

Defects enriched hollow porous Co-N-doped carbons embedded with ultrafine CoFe/Co nanoparticles as bifunctional oxygen electrocatalyst for rechargeable flexible solid zinc-air batteries

The construction and design of highly efficient and inexpensive bifunctional oxygen electrocatalysts substitute for noble-metal-based catalysts is highly desirable for the development of rechargeable Zn-air battery (ZAB). In this work, a bifunctional oxygen electrocatalysts of based on ultrafine CoFe alloy (4-5 nm) dispersed in defects enriched hollow porous Co-N-doped carbons, made by annealing SiO2 coated zeolitic imidazolate framework-67 (ZIF-67) encapsulated Fe ions. The hollow porous structure not only exposed the active sites inside ZIF-67, but also provided efficient charge and mass transfer. The strong synergetic coupling among high-density CoFe alloys and Co-Nx sites in Co, N-doped carbon species ensures high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity. First-principles simulations reveal that the synergistic promotion effect between CoFe alloy and Co-N site effectively reduced the formation energy of from O* to OH*. The optimized CoFe-Co@PNC exhibits outstanding electrocatalytic stability and activity with the overpotential of only 320 mV for OER at 10 mA·cm−2 and the half-wave potential of 0.887 V for ORR, outperforming that of most recent reported bifunctional electrocatalysts. A rechargeable ZAB constructed with CoFe-Co@PNC as the air cathode displays long-term cyclability for over 200 h and high power density (152.8 mW·cm−2). Flexible solid-state ZAB with our CoFe-Co@PNC as the air cathode possesses a high open circuit potential (OCP) up to 1.46 V as well as good bending flexibility. This universal structure design provides an attractive and instructive model for the application of nanomaterials derived from MOF in the field of sustainable flexible energy applications device.