Hollow Co3O4 Nanosphere Embedded in Carbon Arrays for Stable and Flexible Solid‐State Zinc–Air Batteries

Highly active and durable air cathodes to catalyze both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are urgently required for rechargeable metal–air batteries. In this work, an efficient bifunctional oxygen catalyst comprising hollow Co₃O₄ nanospheres embedded in nitrogen‐doped carbon nanowall arrays on flexible carbon cloth (NC‐Co₃O₄/CC) is reported. The hierarchical structure is facilely derived from a metal–organic framework precursor. A carbon onion coating constrains the Kirkendall effect to promote the conversion of the Co nanoparticles into irregular hollow oxide nanospheres with a fine scale nanograin structure, which enables promising catalytic properties toward both OER and ORR. The integrated NC‐Co₃O₄/CC can be used as an additive‐free air cathode for flexible all‐solid‐state zinc–air batteries, which present high open circuit potential (1.44 V), high capacity (387.2 mAh g^?1, based on the total mass of Zn and catalysts), excellent cycling stability and mechanical flexibility, significantly outperforming Pt‐ and Ir‐based zinc–air batteries.     

Nitrogen,Fluorine, and Boron Ternary Doped Carbon Fibers as Cathode Electrocatalysts for Zinc–Air Batteries

Zinc–air batteries with high‐density energy are promising energy storage devices for the next generation of energy storage technologies. However, the battery performance is highly dependent on the efficiency of oxygen electrocatalyst in the air electrode. Herein, the N, F, and B ternary doped carbon fibers (TD‐CFs) are prepared and exhibited higher catalytic properties via the efficient 4e⁻ transfer mechanism for oxygen reduction in comparison with the single nitrogen doped CFs. More importantly, the primary and rechargeable Zn–air batteries using TD‐CFs as air–cathode catalysts are constructed. When compared to batteries with Pt/C + RuO₂ and Vulcan XC‐72 carbon black catalysts, the TD‐CFs catalyzed batteries exhibit remarkable battery reversibility and stability over long charging/discharging cycles.     

Facilitated Oxygen Chemisorption in Heteroatom‐Doped Carbon for Improved Oxygen Reaction Activity in All‐Solid‐State Zinc–Air Batteries

Driven by the intensified demand for energy storage systems with high‐power density and safety, all‐solid‐state zinc–air batteries have drawn extensive attention. However, the electrocatalyst active sites and the underlying mechanisms occurring in zinc–air batteries remain confusing due to the lack of in situ analytical techniques. In this work, the in situ observations, including X‐ray diffraction and Raman spectroscopy, of a heteroatom‐doped carbon air cathode are reported, in which the chemisorption of oxygen molecules and oxygen‐containing intermediates on the carbon material can be facilitated by the electron deficiency caused by heteroatom doping, thus improving the oxygen reaction activity for zinc–air batteries. As expected, solid‐state zinc–air batteries equipped with such air cathodes exhibit superior reversibility and durability. This work thus provides a profound understanding of the reaction principles of heteroatom‐doped carbon materials in zinc–air batteries.     

Defect Engineering of Chalcogen‐Tailored Oxygen Electrocatalysts for Rechargeable Quasi‐Solid‐State Zinc–Air Batteries

A critical bottleneck limiting the performance of rechargeable zinc–air batteries lies in the inefficient bifunctional electrocatalysts for the oxygen reduction and evolution reactions at the air electrodes. Hybridizing transition‐metal oxides with functional graphene materials has shown great advantages due to their catalytic synergism. However, both the mediocre catalytic activity of metal oxides and the restricted 2D mass/charge transfer of graphene render these hybrid catalysts inefficient. Here, an effective strategy combining anion substitution, defect engineering, and the dopant effect to address the above two critical issues is shown. This strategy is demonstrated on a hybrid catalyst consisting of sulfur‐deficient cobalt oxysulfide single crystals and nitrogen‐doped graphene nanomeshes (CoO_0.87S_0.13/GN). The defect chemistries of both oxygen‐vacancy‐rich, nonstoichiometric cobalt oxysulfides and edge‐nitrogen‐rich graphene nanomeshes lead to a remarkable improvement in electrocatalytic performance, where CoO_0.87S_0.13/GN exhibits strongly comparable catalytic activity to and much better stability than the best‐known benchmark noble‐metal catalysts. In application to quasi‐solid‐state zinc–air batteries, CoO_0.87S_0.13/GN as a freestanding catalyst assembly benefits from both structural integrity and enhanced charge transfer to achieve efficient and very stable cycling operation over 300 cycles with a low discharge–charge voltage gap of 0.77 V at 20 mA cm^?2 under ambient conditions.     

Atomically Transition Metals on Self‐Supported Porous Carbon Flake Arrays as Binder‐Free Air Cathode for Wearable Zinc−Air Batteries

Metal single‐atom materials with their high atom utilization efficiency and unique electronic structures usually show remarkable catalytic performances in many crucial chemical reactions. Herein, a facile and easily scalable “impregnation‐carbonization‐acidification” strategy for fabricating a class of single‐atom‐anchored (including cobalt and nickel single atoms) monolith as superior binder‐free electrocatalysts for developing high‐performance wearable Zn–air batteries is reported. The as‐prepared single atoms, supported by N‐doped carbon flake arrays grown on carbon nanofibers assembly (M SA@NCF/CNF), demonstrate the dual characteristics of excellent catalytic activity (reversible oxygen overpotential of 0.75 V) and high stability, owing to the greatly improved active sites' accessibility and optimized single‐sites/pore‐structures correlations. Furthermore, wearable Zn–air battery based on Co SA@NCF/CNF air electrode displays superior stability under deformation, satisfactory energy storage capacity, and good practicality to be utilized as an integrated battery system. Theoretical calculations reveal a mechanism for the promotion of the catalytic performances on single atomic sites by lowering the overall oxygen reduction/evolution reaction barriers comparing to metal cluster co‐existing configuration. These findings provide a facile strategy for constructing free‐standing single‐atom materials as well as the engineering of high‐performance binder‐free catalytic electrodes.     

Oxidizing Vacancies in Nitrogen‐Doped Carbon Enhance Air‐Cathode Activity

Oxidizing vacancies in nitrogen‐doped carbon have recently been reported to enhance the oxygen reaction activity of air cathodes, but their specific role has remained elusive and controversial. Herein, the critical role of oxidizing the vacancies in enhancing the oxygen reduction reaction for metal–air battery is identified with density functional theory. Deliberate introduction of oxygen‐enriched vacancies in nitrogen‐doped carbon is shown experimentally to provide superior oxygen reduction activity. In situ X‐ray powder diffraction gives direct observation of the oxygen reactions in a zinc–air battery catalyzed by vacancy‐enriched oxidized carbon; the intensity changes of the carbon peak show continuous chemisorption of oxygen intermediates on the carbon cathode during discharge. The air‐cathode performance is shown to exceed that with Pt/C+IrO₂ catalysts.     

Co Nanoparticles Confined in 3D Nitrogen‐Doped Porous Carbon Foams as Bifunctional Electrocatalysts for Long‐Life Rechargeable Zn–Air Batteries

Proper design and simple preparation of nonnoble bifunctional electrocatalysts with high cost performance and strong durability for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is highly demanded but still full of enormous challenges. In this work, a spontaneous gas‐foaming strategy is presented to synthesize cobalt nanoparticles confined in 3D nitrogen‐doped porous carbon foams (CoNCF) by simply carbonizing the mixture of citric acid, NH₄Cl, and Co(NO₃)₂·6H₂O. Thanks to its particular 3D porous foam architecture, ultrahigh specific surface area (1641 m² g^?1), and homogeneous distribution of active sites (C–N, Co–N_x, and Co–O moieties), the optimized CoNCF‐1000‐80 (carbonized at 1000 °C, containing 80 mg Co(NO₃)₂·6H₂O in precursors) catalyst exhibits a remarkable bifunctional activity and long‐term durability toward both ORR and OER. Its bifunctional activity parameter (ΔE) is as low as 0.84 V, which is much smaller than that of noble metal catalyst and comparable to state‐of‐the‐art bifunctional catalysts. When worked as an air electrode catalyst in rechargeable Zn–air batteries, a high energy density (797 Wh kg^?1), a low charge/discharge voltage gap (0.75 V), and a long‐term cycle stability (over 166 h) are achieved at 10 mA cm^?2.     

High‐Performance Aqueous Zinc–Ion Battery Based on Layered H2V3O8 Nanowire Cathode

Rechargeable aqueous zinc–ion batteries have offered an alternative for large‐scale energy storage owing to their low cost and material abundance. However, developing suitable cathode materials with excellent performance remains great challenges, resulting from the high polarization of zinc ion. In this work, an aqueous zinc–ion battery is designed and constructed based on H₂V₃O₈ nanowire cathode, Zn(CF₃SO₃)₂ aqueous electrolyte, and zinc anode, which exhibits the capacity of 423.8 mA h g⁻¹ at 0.1 A g⁻¹, and excellent cycling stability with a capacity retention of 94.3% over 1000 cycles. The remarkable electrochemical performance is attributed to the layered structure of H₂V₃O₈ with large interlayer spacing, which enables the intercalation/de‐intercalation of zinc ions with a slight change of the structure. The results demonstrate that exploration of the materials with large interlayer spacing is an effective strategy for improving electrochemical stability of electrodes for aqueous Zn ion batteries.     

Efficient Nitrogen‐Doped Carbon for Zinc–Bromine Flow Battery

The zinc–bromine flow battery (ZBFB) is one of the most promising technologies for large‐scale energy storage. Here, nitrogen‐doped carbon is synthesized and investigated as the positive electrode material in ZBFBs. The synthesis includes the carbonization of the glucose precursor and nitrogen doping by etching in ammonia gas. Physicochemical characterizations reveal that the resultant carbon exhibits high electronic conductivity, large specific surface area, and abundant heteroatom‐containing functional groups, which benefit the formation and exposure of the active sites toward the Br₂/Br^? redox couple. As a result, the assembled ZBFB achieves a voltage efficiency of 83.0% and an energy efficiency of 82.5% at a current density of 80 mA cm^?2, which are among the top values in literature. Finally, the ZBFB does not yield any detectable degradation in performance after a 200‐cycle charging/discharging test, revealing its high stability. In summary, this work provides a highly efficient electrode material for the zinc–bromine flow battery.     

Atomically Thin Mesoporous Co3O4 Layers Strongly Coupled with N‐rGO Nanosheets as High‐Performance Bifunctional Catalysts for 1D Knittable Zinc–Air Batteries

Under development for next‐generation wearable electronics are flexible, knittable, and wearable energy‐storage devices with high energy density that can be integrated into textiles. Herein, knittable fiber‐shaped zinc–air batteries with high volumetric energy density (36.1 mWh cm^?3) are fabricated via a facile and continuous method with low‐cost materials. Furthermore, a high‐yield method is developed to prepare the key component of the fiber‐shaped zinc–air battery, i.e., a bifunctional catalyst composed of atomically thin layer‐by‐layer mesoporous Co₃O₄/nitrogen‐doped reduced graphene oxide (N‐rGO) nanosheets. Benefiting from the high surface area, mesoporous structure, and strong synergetic effect between the Co₃O₄ and N‐rGO nanosheets, the bifunctional catalyst exhibits high activity and superior durability for oxygen reduction and evolution reactions. Compared to a fiber‐shaped zinc–air battery using state‐of‐the‐art Pt/C + RuO₂ catalysts, the battery based on these Co₃O₄/N‐rGO nanosheets demonstrates enhanced and stable electrochemical performance, even under severe deformation. Such batteries, for the first time, can be successfully knitted into clothes without short circuits under external forces and can power various electronic devices and even charge a cellphone.     

Core–Shell Nitrogen‐Doped Carbon Hollow Spheres/Co3O4 Nanosheets as Advanced Electrode for High‐Performance Supercapacitor

Co₃O₄/nitrogen‐doped carbon hollow spheres (Co₃O₄/NHCSs) with hierarchical structures are synthesized by virtue of a hydrothermal method and subsequent calcination treatment. NHCSs, as a hard template, can aid the generation of Co₃O₄ nanosheets on its surface; while SiO₂ spheres, as a sacrificed‐template, can be dissolved in the process. The prepared Co₃O₄/NHCS composites are investigated as the electrode active material. This composite exhibits an enhanced performance than Co₃O₄ itself. A higher specific capacitance of 581 F g^?1 at 1 A g^?1 and a higher rate performance of 91.6% retention at 20 A g^?1 are achieved, better than Co₃O₄ nanorods (318 F g^?1 at 1 A g^?1 and 67.1% retention at 20 A g^?1). In addition, the composite is employed as a positive electrode to fabricate an asymmetric supercapacitor. The device can deliver a high energy density of 34.5 Wh kg^?1 at the power density of 753 W kg^?1 and display a desirable cycling stability. All of these attractive results make the unique hierarchical Co₃O₄/NHCS core–shell structure a promising electrode material for high‐performance supercapacitors.     

An Oxygen‐Vacancy‐Rich Semiconductor‐Supported Bifunctional Catalyst for Efficient and Stable Zinc–Air Batteries

The highly oxidative operating conditions of rechargeable zinc–air batteries causes significant carbon‐support corrosion of bifunctional oxygen electrocatalysts. Here, a new strategy for the catalyst support design focusing on oxygen vacancy (OV)‐rich, low‐bandgap semiconductor is proposed. The OVs promote the electrical conductivity of the oxide support, and at the same time offer a strong metal–support interaction (SMSI), which enables the catalysts to have small metal size, high catalytic activity, and high stability. The strategy is demonstrated by successfully synthesizing ultrafine Co‐metal‐decorated 3D ordered macroporous titanium oxynitride (3DOM‐Co@TiO_xN_y). The 3DOM‐Co@TiO_xN_y catalyst exhibits comparable activities for oxygen reduction and evolution reactions, but much higher cycling stability than noble metals in alkaline conditions. The zinc–air battery using this catalyst delivers an excellent stability with less than 1% energy efficiency loss over 900 charge–discharge cycles at 20 mA cm^?2. The high stability is attributed to the strong SMSI between Co and 3DOM‐TiO_xN_y which is verified by density functional theory calculations. This work sheds light on using OV‐rich semiconductors as a promising support to design efficient and durable nonprecious electrocatalysts.