V2 O3 /MnS Arrays as Bifunctional Air Electrode for Long-Lasting and Flexible Rechargeable Zn-Air Batteries

Exploring highly efficient, stable, and cost-effective bifunctional electrocatalysts is crucial for the wide commercialization of rechargeable Zn-air batteries. Herein, a vanadium-oxide-based hybrid air electrode comprising a heterostructure of V₂ O₃ and MnS (V₂ O₃ /MnS) is reported. The V₂ O₃ /MnS catalyst shows a decent catalytic activity that is comparable to Pt/C toward the oxygen reduction reaction and acceptable toward oxygen evolution. The extraordinary stability as well as the low cost set the V₂ O₃ /MnS among the best bifunctional oxygen electrocatalysts. In a demonstration of an assembled liquid-state Zn-air battery using V₂ O₃ /MnS as cathode, high power density (118 mW cm^-2 ), specific capacity (808 mAh g_Zn ^-1 ), and energy density (970 Wh kg_Zn ^-1 ), as well as the outstanding rechargeability and durability for 4000 cycles (>1333 h, i.e., >55 days) are enabled. The V₂ O₃ /MnS is also integrated into an all-solid-state Zn-air battery to demonstrate its great potential as a flexible power source for next-generation electronics. Density functional theory calculations further elucidate the origin of the intrinsic activity and stability of the V₂ O₃ /MnS heterostructure.

Alloying Co Species into Ordered and Interconnected Macroporous Carbon Polyhedra for Efficient Oxygen Reduction Reaction in Rechargeable Zinc-Air Batteries

Engineering non-precious transition metal (TM)-based electrocatalysts to simultaneously achieve an optimal intrinsic activity, high density of active sites, and rapid mass transfer ability for the oxygen reduction reaction (ORR) remains a significant challenge. To address this challenge, a hybrid composite consisting of Fe_x Co alloy nanoparticles uniformly implanted into hierarchically ordered macro-/meso-/microporous N-doped carbon polyhedra (HOMNCP) is rationally designed. The combined results of experimental and theoretical investigations indicate that the alloying of Co enables a favorable electronic structure for the formation of the *OH intermediate, while the periodically trimodal-porous structured carbon matrix structure not only provides highly accessible channels for active site utilization but also dramatically facilitates mass transfer in the catalytic process. As expected, the Fe_0.5 Co@HOMNCP composite catalyst exhibits extraordinary ORR activity with a half-wave potential of 0.903 V (vs reversible hydrogen electrode), surpassing most Co-based catalysts reported to date. More remarkably, the use of the Fe_0.5 Co@HOMNCP catalyst as the air electrode in a zinc-air battery results in superior open-circuit voltage and power density compared to a commercial Pt/C + IrO₂ catalyst. The results of this study are expected to inspire the development of advanced TM-based catalysts for energy storage and conversion applications.

Combining Multivariate Electrospinning with Surface MOF Functionalization to Construct Tunable Active Sites toward Trifunctional Electrocatalysis

The development of high-performance multifunctional electrocatalysts operating in the same electrolyte is key to reduce the material and process costs of renewable energy conversion and storage devices. Herein, the fabrication of freestanding integral electrodes by combining multivariate electrospinning with surface metal organic framework functionalization to arrest pyrolytic emissions from fiber interior is reported, resulting in the expression of rich active sites with controlled composition, for example, the tunable Co-P coordination. The as-fabricated electrode of CoP@CF-900, when used as both the cathode and anode for overall water splitting, is able to deliver 200 mA cm^-2 at a cell voltage of 1.89 V, significantly outshining the Pt/C‖RuO₂ couple; when used as the air cathode for a zinc-air battery, is able to operate more than 150 h at 10 mA cm^-2 with a nearly constant round-trip energy efficiency of ≈60%, also outperforming the Pt/C+RuO₂ benchmark. The activity and kinetics origin of the superb multi-functionality is further elucidated through extensive electroanalytical, post-mortem, and operando characterizations, which underscore the construction of robust integral electrodes through synergistic structure and composition engineering.

Preconstructing Asymmetric Interface in Air Cathodes for High-Performance Rechargeable Zn-Air Batteries

Rechargeable zinc-air batteries afford great potential toward next-generation sustainable energy storage. Nevertheless, the oxygen redox reactions at the air cathode are highly sluggish in kinetics to induce poor energy efficiency and limited cycling lifespan. Air cathodes with asymmetric configurations significantly promote the electrocatalytic efficiency of the loaded electrocatalysts, whereas rational synthetic methodology to effectively fabricate asymmetric air cathodes remains insufficient. Herein, a strategy of asymmetric interface preconstruction is proposed to fabricate asymmetric air cathodes for high-performance rechargeable zinc-air batteries. Concretely, the asymmetric interface is preconstructed by introducing immiscible organic-water diphases within the air cathode, at which the electrocatalysts are in situ formed to achieve an asymmetric configuration. The as-fabricated asymmetric air cathodes realize high working rates of 50 mA cm^-2 , long cycling stability of 3400 cycles at 10 mA cm^-2 , and over 100 cycles under harsh conditions of 25 mA cm^-2 and 25 mAh cm^-2 . Moreover, the asymmetric interface preconstruction strategy is universal to many electrocatalytic systems and can be easily scaled up. This work provides an effective strategy toward advanced asymmetric air cathodes with high electrocatalytic efficiency and significantly promotes the performance of rechargeable zinc-air batteries.

Electronic Metal-Support Interaction Modulation of Single-Atom Electrocatalysts for Rechargeable Zinc-Air Batteries

High-performance oxygen electrocatalysts play a key role in the widespread application of rechargeable Zn-air batteries (ZABs). Single-atom catalysts (SACs) with maximum atom efficiency and well-defined active sites have been recognized as promising alternatives of the present noble-metal-based catalysts for oxygen reduction reaction and oxygen evolution reaction. To improve their oxygen electrocatalysis activities and reveal the structure-activity relationship, many advanced synthesis and characterization methods have been developed to study the effects of 1) coordination and electronic structure of the metal centers and 2) morphology and stability of the conductive substrates. Herein, a detailed review of the recent advances of SACs with strong electronic metal-support interaction (EMSI) for rechargeable ZABs is provided. Great emphasis was placed on the EMSI forms and design strategies. Moreover, the importance and the impact of the atomic coordinating structure and the substrates on the oxygen electrocatalytic activity and stability are highlighted. Finally, future directions and perspectives on the development of SACs are also presented.

Deciphering the Precursor-Performance Relationship of Single-Atom Iron Oxygen Electroreduction Catalysts via Isomer Engineering

Single atom Fe-nitrogen-carbon (Fe-N-C) catalysts have high catalytic activity and selectivity for the oxygen reduction reaction (ORR), and are possible alternatives for Pt-based materials. However, the reasonable design and selection of precursors to establish their relationship with Fe-N-C catalyst performance is still a formidable task. Herein, precursors with controllable structures are easily achieved through isomer engineering, with the purpose of regulating the active site density and microscopic morphology of the final electrocatalyst. As-proof-of-concept, phenylenediamine isomers-based polymers are used as precursors to fabricate Fe-N-C catalysts. The Fe-PpPD-800 derived from p-phenylenediamine shows that the best ORR activity with a half-wave potential (E_1/2 ) reaches 0.892 V vs reversible hydrogen electrode (RHE), which is better than the counterparts derived from o-phenylenediamine (Fe-PoPD-800) and m-phenylenediamine (Fe-PmPD-800), even surpassing commercial Pt/C (E_1/2  = 0.881 V vs RHE). Furthermore, the self-made zinc-air battery based on Fe-PpPD-800 achieves high power density and specific capacity up to 242 mW cm^-2 and 873 mA h g_Zn ^-1 respectively, a stable open circuit voltage of 1.45 V, and excellent cycling stability. This work not only proves the practicability of adjusting the catalytic activity of single-atom catalysts through isomer engineering, but also provides an approach to understand the relationship between precursors and target catalysts performance.

A Substrate-Induced Fabrication of Active Free-Standing Nanocarbon Film as Air Cathode in Rechargeable Zinc-Air Batteries

Designing cost-effective and high-efficiency bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurred at air electrodes is vitally significant yet challenging for Zn-air batteries (ZABs). In this work, a zinc substrate induced fabrication is reported of free-standing nanocarbon hybrid film which shows good bifunctional activity and can be directly used as the air electrode in the rechargeable ZABs. The designed nanocarbon film in Zn-air battery provides a satisfactory power density of 185 mW cm^-2 and cycling stability for 1200 h under the current density of 10 mA cm^-2 . This hybrid film also gives a solid-state ZAB excellent flexibility with a power density of 160 mW cm^-2 . The free-standing hybrid with abundant cobalt-nitrogen-carbon species coupled with porous architecture would be the original factor for its satisfactory performance of rechargeable ZABs. This work would pave an ideal way to design integrated electrode with high electrocatalytic performance towards electrochemical energy technologies.

High-Performance Zinc-Air Batteries Based on Bifunctional Hierarchically Porous Nitrogen-Doped Carbon

Active and durable bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the cathode are required for high-performance rechargeable metal-air batteries. Herein, the synthesis of hierarchically porous nitrogen-doped carbon (HPNC) with bifunctional oxygen electrocatalysis for Zn-air batteries is reported. The HPNC catalyst possesses a large surface area of 1459 m² g^-1 and exhibits superior electrocatalytic activity toward ORR and OER simultaneously with a low OER/ORR overpotential of 0.62 V, taking the difference between the potential at 10 mA cm^-2 for OER and half-wave potential for ORR in 0.1 m KOH. Adopting HPNC as the air cathode, primary and rechargeable Zn-air batteries are fabricated. The primary batteries demonstrate a high open-circuit potential of 1.616 V, a specific capacity of 782.7 mAh g_Zn ^-1 and a superb peak power density of 201 mW cm^-2 . The rechargeable batteries can be cycled stably for over 360 cycles or 120 h at the current density of 5 mA cm^-2 . As elucidated by density functional theory, N-doping is preferred on defective sites with pentagon configuration and on the edge in the form of pyridinic-N-type. The high content of these two motifs in HPNC leads to the superior ORR and OER activities, respectively.

Synergistic Binary Fe-Co Nanocluster Supported on Defective Tungsten Oxide as Efficient Oxygen Reduction Electrocatalyst in Zinc-Air Battery

Rational design of metal oxide supported non-precious metals is essential for the development of stable and high-efficiency oxygen reduction reaction (ORR) electrocatalysts. Here, an efficient ORR catalyst consisting of binary Fe/Co nanoclusters supported by defective tungsten oxide and embedded N-doped carbon layer (NC) with a 3D ordered macroporous architecture (3DOM Fe/Co@NC-WO2- x ) is developed. The oxygen deficient 3DOM WO2- x not only serves as a porous and stable support, but also enhances the conductivity and ensures good dispersion of the binary Fe/Co nanocluster, benefiting its ORR catalytic activity. Theoretical calculation shows that there exists a synergistic effect of electron transfer from Fe to Co in the supported binary Fe/Co cluster, promoting the ORR reaction energetics. Accordingly, the 3DOM Fe/Co@NC-WO2- x catalyst exhibits excellent ORR activity in alkaline medium with a half wave potential (E1/2 ) of 0.87 V higher than that of Pt/C (0.85 V). The zinc-air batteries assembled by 3DOM Fe/Co@NC-WO2- x cathode deliver a higher power density and specific capacity than that of Pt/C. A new strategy of combining synergistic binary-metal nanoclusters and conductive metal oxide support design is provided here to develop efficient and durable ORR electrocatalyst.

A Controllable Dual Interface Engineering Concept for Rational Design of Efficient Bifunctional Electrocatalyst for Zinc-Air Batteries

Searching for bifunctional noble-free electrocatalysts with high activity and stability are urgently demanded for the commercial application of zinc-air batteries (ZABs). Herein, the authors propose a controllable dual interface engineering concept to design a noble-metal-free bifunctional catalyst with two well-designed interfaces (Ni₃ FeN|MnO and MnO|CNTs) via a simple etching and wet chemical route. The heterointerface between MnO and Ni₃ FeN facilitates the charge transfer rate during surface reaction, and heterointerface between MnO and carbon nanotubes (CNTs) support provides effective electron transfer path, while the CNTs matrix builds free diffusion channels for gas and electrolyte. Benefiting from the advantages of dual interfaces, Ni₃ FeN/MnO-CNTs show superior oxygen reduction reaction and oxygen evolution reaction catalytic activity with an ultralow polarization gap (∆E) of 0.73 V, as well as preferable durability and rapid reaction kinetics. As proof of concept, the practical ZAB with Ni₃ FeN/MnO-CNT exhibits high power density of 197 mW cm^-2 and rate performance up to 40 mA cm^-2 , as well as superior cycling stability over 600 cycles, outperforming the benchmark mixture of Pt/C and RuO₂ . This work proposes a controllable dual interface engineering concept toward regulating the charge, electron, and gas transfer to achieve efficient bifunctional catalysts for ZABs.

Flexible Zinc-Air Battery with High Energy Efficiency and Freezing Tolerance Enabled by DMSO-Based Organohydrogel Electrolyte

With the emergence of various flexible electronics, the flexible zinc-air battery (ZAB) is considered a promising energy source with low cost, high energy density, and safety. However, gel electrolytes that improve the freezing tolerance and energy efficiency of ZABs are rarely explored. Herein, an organohydrogel electrolyte (OHE) is fabricated by soaking poly(2-acrylamido-2-methylpropanesulfonic acid)/polyacrylamide (PAMPS/PAAm) double-network hydrogel in aqueous KOH electrolyte with dimethyl sulfoxide (DMSO) additive. The prepared OHE exhibits high mechanical strength and excellent ionic conductivity. In addition, the introduction of DMSO effectively improves freezing tolerance and electrochemical performance especially in energy efficiency of ZABs due to that DMSO can break the hydrogen bonds between water molecules and alter the path of the conventional oxygen evolution reaction in ZAB simultaneously. Compared with the control hydrogel electrolyte, the optimized OHE enables flexible ZABs to not only exhibit an exceptionally low charge voltage of 1.63 V, high energy efficiency of 74.2%, and long cycling life of 177 cycles, but also to operate with an excellent specific capacity of 562 mAh g^-1 and energy density of 523.4 Wh kg^-1 at -40 °C. Moreover, the obtained flexible ZABs keep a stable output under deformations and extreme low temperature, manifesting a great potential for functional wearable devices.

Metal-Triazolate-Framework-Derived FeN4 Cl1 Single-Atom Catalysts with Hierarchical Porosity for the Oxygen Reduction Reaction

The construction of single-atom catalysts (SACs) with high single atom densities, favorable electronic structures and fast mass transfer is highly desired. We have utilized metal-triazolate (MET) frameworks, a subclass of metal-organic frameworks (MOFs) with high N content, as precursors since they can enhance the density and regulate the electronic structure of single-atom sites, as well as generate abundant mesopores simultaneously. Fe single atoms dispersed in a hierarchically porous N-doped carbon matrix with high metal content (2.78 wt %) and a FeN₄ Cl₁ configuration (FeN₄ Cl₁ /NC), as well as mesopores with a pore:volume ratio of 0.92, were obtained via the pyrolysis of a Zn/Fe-bimetallic MET modified with 4,5-dichloroimidazole. FeN₄ Cl₁ /NC exhibits excellent oxygen reduction reaction (ORR) activity in both alkaline and acidic electrolytes. Density functional theory calculations confirm that Cl can optimize the adsorption free energy of Fe sites to *OH, thereby promoting the ORR process. The catalyst demonstrates great potential in zinc-air batteries. This strategy selects, designs, and adjusts MOFs as precursors for high-performance SACs.

Robust wrinkled MoS2/N-C bifunctional electrocatalysts interfaced with single Fe atoms for wearable zinc-air batteries

The ability to create highly efficient and stable bifunctional electrocatalysts, capable of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the same electrolyte, represents an important endeavor toward high-performance zinc-air batteries (ZABs). Herein, we report a facile strategy for crafting wrinkled MoS₂/N-doped carbon core/shell nanospheres interfaced with single Fe atoms (denoted MoS₂@Fe-N-C) as superior ORR/OER bifunctional electrocatalysts for robust wearable ZABs with a high capacity and outstanding cycling stability. Specifically, the highly crumpled MoS₂ nanosphere core is wrapped with a layer of single-Fe-atom-impregnated, N-doped carbon shell (i.e., Fe-N-C shell with well-dispersed FeN₄ sites). Intriguingly, MoS₂@Fe-N-C nanospheres manifest an ORR half-wave potential of 0.84 V and an OER overpotential of 360 mV at 10 mA⋅cm^-2 More importantly, density functional theory calculations reveal the lowered energy barriers for both ORR and OER, accounting for marked enhanced catalytic performance of MoS₂@Fe-N-C nanospheres. Remarkably, wearable ZABs assembled by capitalizing on MoS₂@Fe-N-C nanospheres as an air electrode with an ultralow area loading (i.e., 0.25 mg⋅cm^-2) display excellent stability against deformation, high special capacity (i.e., 442 mAh⋅g^-1 _Zn), excellent power density (i.e., 78 mW⋅cm^-2) and attractive cycling stability (e.g., 50 cycles at current density of 5 mA⋅cm^-2). This study provides a platform to rationally design single-atom-interfaced core/shell bifunctional electrocatalysts for efficient metal-air batteries.

Encapsulating Cobalt Nanoparticles in Interconnected N-Doped Hollow Carbon Nanofibers with Enriched CoNC Moiety for Enhanced Oxygen Electrocatalysis in Zn-Air Batteries

Rational design of bifunctional efficient electrocatalysts for both oxygen reduction (ORR) and oxygen evolution reactions (OER) is desirable-while highly challenging-for development of rechargeable metal-air batteries. Herein, an efficient bifunctional electrocatalyst is designed and fabricated by encapsulating Co nanoparticles in interconnected N-doped hollow porous carbon nanofibers (designated as Co@N-C/PCNF) using an ultrafast high-temperature shock technology. Benefiting from the synergistic effect and intrinsic activity of the CoNC moiety, as well as porous structure of carbon nanofibers, the Co@N-C/PCNF composite shows high bifunctional electrocatalytic activities for both OER (289 mV at 10 mA cm-2 ) and ORR (half-wave potential of 0.85 V). The CoNC moiety in the composite can modulate the local environmental and electrical structure of the catalysts, thus optimizing the adsorption/desorption kinetics and decreasing the reaction barriers for promoting the reversible oxygen electrocatalysis. Co@N-C/PCNF-based aqueous Zn-air batteries (AZAB) provide high power density of 292 mW cm-2 , and the assembled flexible ZAB can power wearable devices.

Epitaxially Grown Heterostructured SrMn3 O6-x -SrMnO3 with High-Valence Mn3+/4+ for Improved Oxygen Reduction Catalysis

Heterostructured catalysts show outstanding performance in electrochemical reactions owing to their beneficial interfacial properties. However, the rational design of heterostructured catalysts with the desired interfacial properties and charge-transfer characteristics is challenging. Herein, we developed a SrMn₃ O_6-x -SrMnO₃ (SMO_x -SMO) heterostructure through epitaxial growth, which demonstrated excellent electrocatalyst performance for the oxygen reduction reaction (ORR). The formation of high-valence Mn^3+/4+ is beneficial for promoting a positive shift in the position of the d-band center, thereby optimizing the adsorption and desorption of ORR intermediates on the heterojunction surface and resulting in improved catalytic activity. When SMO_x -SMO was applied as an air-electrode catalyst in a rechargeable zinc-air battery, a high output voltage and power density was achieved, with performance comparable to a battery prepared with Pt/C-IrO₂ air-electrode catalysts, albeit with much better cycling stability.

Can Aqueous Zinc-Air Batteries Work at Sub-Zero Temperatures?

Efficient energy storage at low temperatures starves for competent battery techniques. Herein, inherent advantages of zinc-air batteries on low-temperature electrochemical energy storage are discovered. The electrode reactions are resistive against low temperatures to render feasible working zinc-air batteries under sub-zero temperatures. The relatively reduced ionic conductivity of electrolyte is identified as the main limiting factor, which can be addressed by employing a CsOH-based electrolyte through regulating the solvation structures. Accordingly, 500 cycles with a stable voltage gap of 0.8 V at 5.0 mA cm^-2 is achieved at -10 °C. This work reveals the promising potential of zinc-air batteries for low-temperature electrochemical energy storage and inspires advanced battery systems under extreme working conditions.

MIL-101-Derived Mesoporous Carbon Supporting Highly Exposed Fe Single-Atom Sites as Efficient Oxygen Reduction Reaction Catalysts

Fe single-atom catalysts (Fe SACs) with atomic FeN_x active sites are very promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR). The pyrolysis of metal-organic frameworks (MOFs) is a common approach for preparing Fe SACs, though most MOF-derived catalysts reported to date are microporous and thus suffer from poor mass transfer and a high proportion of catalytically inaccessible FeN_x active sites. Herein, NH₂ -MIL-101(Al), a MOF possessing a mesoporous cage architecture, is used as the precursor to prepare a series of N-doped carbon supports (denoted herein as NC-MIL101-T) with a well-defined mesoporous structure at different pyrolysis temperatures. The NC-MIL101-T supports are then impregnated with a Fe(II)-phenanthroline complex, and heated again to yield Fe SAC-MIL101-T catalysts rich in accessible FeN_x single atom sites. The best performing Fe SAC-MIL101-1000 catalyst offers outstanding ORR activity in alkaline media, evidenced by an ORR half-wave potential of 0.94 V (vs RHE) in 0.1 m KOH, as well as excellent performance in both aqueous primary zinc-air batteries (a near maximum theoretical energy density of 984.2 Wh kg_Zn ^-1 ) and solid-state zinc-air batteries (a peak power density of 50.6 mW cm^-2 and a specific capacity of 724.0 mAh kg_Zn ^-1 ).

Interfacial Covalent Bonds Regulated Electron-Deficient 2D Black Phosphorus for Electrocatalytic Oxygen Reactions

Developing resource-abundant and sustainable metal-free bifunctional oxygen electrocatalysts is essential for the practical application of zinc-air batteries (ZABs). 2D black phosphorus (BP) with fully exposed atoms and active lone pair electrons can be promising for oxygen electrocatalysts, which, however, suffers from low catalytic activity and poor electrochemical stability. Herein, guided by density functional theory (DFT) calculations, an efficient metal-free electrocatalyst is demonstrated via covalently bonding BP nanosheets with graphitic carbon nitride (denoted BP-CN-c). The polarized PN covalent bonds in BP-CN-c can efficiently regulate the electron transfer from BP to graphitic carbon nitride and significantly promote the OOH* adsorption on phosphorus atoms. Impressively, the oxygen evolution reaction performance of BP-CN-c (overpotential of 350 mV at 10 mA cm^-2 , 90% retention after 10 h operation) represents the state-of-the-art among the reported BP-based metal-free catalysts. Additionally, BP-CN-c exhibits a small half-wave overpotential of 390 mV for oxygen reduction reaction, representing the first bifunctional BP-based metal-free oxygen catalyst. Moreover, ZABs are assembled incorporating BP-CN-c cathodes, delivering a substantially higher peak power density (168.3 mW cm^-2 ) than the Pt/C+RuO₂ -based ZABs (101.3 mW cm^-2 ). The acquired insights into interfacial covalent bonds pave the way for the rational design of new and affordable metal-free catalysts.

Hydrothermal-Induced Formation of Well-Defined Hollow Carbons with Curvature-Activated N-C Sites for Zn-Air Batteries

Metal-free carbons have been regarded as one of the promising materials alternatives to precious-metal catalysts for oxygen reduction reaction (ORR) due to their high activity and stability. In this paper, well-defined N-doped hollow carbons (NHCs) are firstly synthesized by using an ammonia-based hydrothermal synthesis that is environmentally friendly and suitable for mass production in industry and a commercial black carbon as raw material. Moreover, the shell thickness of the NHCs can be easily tuned by this hydrothermal strategy. Zn-air battery test results reveal shell thickness-dependent activity and durability for ORR over the NHCs, which exceeds that obtained by commercial Pt/C (20 wt %). The enhanced battery performance can be attributed to the curvature-activated N-C moieties on the hollow carbon surface, which served as the main active sites for ORR as evidenced by DFT calculations. The proposed approach may open a way for designing curved hollow carbons with high graphitization degree and dopant nitrogen level for metal-air batteries or fuel cells.

Nonprecious Bimetallic Sites Coordinated on N-Doped Carbons with Efficient and Durable Catalytic Activity for Oxygen Reduction

Developing efficient, inexpensive, and durable electrocatalysts for the oxygen reduction reaction (ORR) is important for the large-scale commercialization of fuel cells and metal-air batteries. Herein, a hierarchically porous bimetallic Fe/Co single-atom-coordinated N-doped carbon (Fe/Co-N_x -C) electrocatalyst for ORR is synthesized from Fe/Co-coordinated polyporphyrin using silica template-assisted and silica-protection synthetic strategies. In the synthesis, first silica nanoparticles-embedded, silica-protected Fe/Co-polyporphyrin is prepared. It is then pyrolyzed and treated with acidic solution. The resulting Fe/Co-N_x -C material has a large specific surface area, large electrochemically active surface area, good conductivity, and catalytically active Fe/Co-N_x sites. The material exhibits a very good electrocatalytic activity for the ORR in alkaline media, with a half-wave potential of 0.86 V versus reversible hydrogen electrode, which is better than that of Pt/C (20 wt%). Furthermore, it shows an outstanding operational stability and durability during the reaction. A zinc-air battery (ZAB) assembled using Fe/Co-N_x -C as an air-cathode electrocatalyst gives a high peak power density (152.0 mW cm^-2 ) and shows a good recovery property. Furthermore, the performance of the battery is better than a corresponding ZAB containing Pt/C as an electrocatalyst. The work also demonstrates a synthetic route to a highly active, stable, and scalable single-atom electrocatalyst for ORR in ZABs.

Efficient Bifunctional Catalytic Electrodes with Uniformly Distributed NiN2 Active Sites and Channels for Long-Lasting Rechargeable Zinc-Air Batteries

Freestanding bifunctional electrodes with outstanding oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) properties are of great significance for zinc-air batteries, attributed to the avoided use of organic binder and strong adhesion with substrates. Herein, a strategy is developed to fabricate freestanding bifunctional electrodes from the predeposited nickel nanoparticles (Ni-NCNT) on carbon fiber paper. The steric effect of monodispersed SiO₂ nanospheres limits the configuration of carbon atoms forming 3D interconnected nanotubes with uniformly distributed NiN₂ active sites. The bifunctional electrodes (Ni-NCNT) demonstrate ideal ORR and OER properties. The zinc-air batteries assembled with Ni-NCNT directly exhibit extremely outstanding long term stability (2250 cycles with 10 mA cm^-2 charge/discharge current density) along with high power density of 120 mV cm^-2 and specific capacity of 834.1 mA h g^-1 . This work provides a new view to optimize the distribution of active sites and the electrode structure.

A Zeolitic-Imidazole Frameworks-Derived Interconnected Macroporous Carbon Matrix for Efficient Oxygen Electrocatalysis in Rechargeable Zinc-Air Batteries

Nanostructures derived from zeolitic-imidazole frameworks (ZIFs) gain much interest in bifunctional oxygen electrocatalysis. However, they are not satisfied well for long-life rechargeable zinc-air batteries due to the limited single particle morphology. Herein, the preparation of an interconnected macroporous carbon matrix with a well-defined 3D architecture by the pyrolysis of silica templated ZIF-67 assemblies is reported. The matrix catalyst assembled zinc-air battery exhibits a high power density of 221.1 mW cm^-2 as well as excellent stability during 500 discharging/charging cycles, surpassing that of a commercial Pt/C assembled battery. The synergistic effect from the interconnected macroporous structure together with abundant cobalt-nitrogen-carbon active sites justify the excellent electrocatalytic activity and battery performance. Considering the advanced nanostructures and performance, the as-synthesized hybrid would be promising for rechargeable zinc-air batteries and other energy technologies. This work may also provide significant concept in the view of electrocatalysis design for long-life battery.

Self-Phosphorization of MOF-Armored Microbes for Advanced Energy Storage

Utilization of microbes as the carbon source and structural template to fabricate porous carbon has incentivized great interests owing to their diverse micromorphology and intricate intracellular structure, apart from the obvious benefit of "turning waste into wealth." Challenges remain to preserve the biological structure through the harsh and laborious post-synthetic treatments, and tailor the functionality as desired. Herein, Escherichia coli is directly coated with metal-organic frameworks (MOFs) through in situ assembly to fabricate N, P co-doped porous carbon capsules expressing self-phosphorized metal phosphides. While the MOF coating serves as an armoring layer for facilitating the morphology inheritance from the bio-templates and provides metal sources for generating extra porosity and electrochemically active sites, the P-rich phospholipids and N-rich proteins from the plasma membrane enable carbon matrix doping and further yield metal phosphides. These unique structural and compositional features endow the carbon capsules with great capabilities in suppressing polysulfide shuttling and catalyzing reversible oxygen conversion, ultimately leading to the superb performance of lithium-sulfur batteries and zinc-air batteries. Combining the bio-templating strategy with hierarchical MOF assembly, this work opens a new avenue for the fabrication of highly porous and functional carbon for advanced energy applications.

A Humidity-Induced Nontemplating Route toward Hierarchical Porous Carbon Fiber Hybrid for Efficient Bifunctional Oxygen Catalysis

Hierarchical porous carbons (HPCs) are highly efficient supports for various remarkable catalytic systems. However, templates are commonly utilized for the preparation of HPCs, and the postremoval of the templates is uneconomical, time-consuming, and harmful for the environment in most cases. Herein, a new humidity-induced nontemplating strategy is developed to prepare 1D HPC with rich topologies and interconnected cavities for catalysis and energy storage applications. Porous electrospun nanofibers as calcination precursors are prepared via a humidity-induced phase separation strategy. A nitrogen-doped hierarchical porous carbon nanofiber (HPCNF), loading Co/Co₃ O₄ hetero-nanoparticles as exemplary nonprecious-metal active substance (Co/Co₃ O₄ @HPCNF), is fabricated through the subsequent hydrothermal and pyrolysis treatment. The internal mesopore and cavity structure can be simply controlled by varying environment humidity during the electrospinning process. Benefiting from the unique topology, Co/Co₃ O₄ @HPCNF exhibits superior bifunctional activity when being used as electrocatalysts for oxygen reduction/evolution reactions. Moreover, the hybrid catalyst also demonstrates a remarkable power density of 102.5 mW cm^-2 , a high capacity of 748.5 mAh g_Zn ^-1 , and long cycle life in Zinc-air batteries. The developed approach offers a facile template-free route for the preparation of HPCNF hybrid and can be extended to other members of the large polymer family for catalyst design and energy storage applications.

Hierarchical 3D Oxygenated Cobalt Molybdenum Selenide Nanosheets as Robust Trifunctional Catalyst for Water Splitting and Zinc-Air Batteries

The development of hierarchical nanostructures with highly active and durable multifunctional catalysts has a new significance in the context of new energy technologies of water splitting and metal-air batteries. Herein, a strategy is demonstrated to construct a 3D hierarchical oxygenated cobalt molybdenum selenide (O-Co_{1- x} Mo_x Se₂ ) series with attractive nanoarchitectures, which are fabricated by a simple and cost-effective hydrothermal process followed by an exclusive ion-exchange process. Owing to its highly electroactive sites with numerous nanoporous networks and plentiful oxygen vacancies, the optimal O-Co_0.5 Mo_0.5 Se₂ could catalyze the hydrogen evolution reaction and oxygen evolution reaction effectively with a low overpotential of ≈102 and 189 mV, at a current density of 10 mA cm^-2 , respectively, and exceptional durability. Most importantly, the O-Co_0.5 Mo_0.5 Se₂ ||O-Co_0.5 Mo_0.5 Se₂ water splitting device only entails a voltage of ≈1.53 V at a current density of 10 mA cm^-2 , which is much better than benchmark Pt/C||RuO₂ (≈1.56 V). Furthermore, O-Co_0.5 Mo_0.5 Se₂ air cathode-based zinc-air batteries exhibit an excellent power density of 120.28 mW cm^-2 and exceptional cycling stability for 60 h, superior to those of state-of-art Pt/C+RuO₂ pair-based zinc-air batteries. The present study provides a strategy to design hierarchical 3D oxygenated bimetallic selenide-based multifunctional catalysts for energy conversion and storage systems.

2 D Hybrid of Ni-LDH Chips on Carbon Nanosheets as Cathode of Zinc-Air Battery for Electrocatalytic Conversion of O2 into H2 O2

It remains great challenge to develop precious-metal-free electrocatalysts to implement high-activity electrochemical conversion of O₂ into value-added hydroperoxide species (HO₂ ^- ), which are vulnerable when exposed to various transition-metal-based catalysts. A strategy based on steric hindrance and layered nickel-based layered double hydroxide (Ni-LDH) induction has been developed for one-pot inlaying high-density ultrathin 2 D Ni-LDH chips on in situ-grown carbon nanosheets (Ni-LDH C/CNSs). The resulting material exhibits high electrocatalytic selectivity with a faradaic efficiency up to 95 % for oxygen reduction into peroxide and attains a fairly high mass activity of approximately 22.2 A g^-1 , outperforming most metal-based catalysts reported previously. Systematic studies demonstrate that the greatly increased defect concentration at Ni edge sites of Ni-LDH chips results in more active sites, which contributes a favorable thermodynamically neutral adsorption of OOH* and adsorbed H₂ O₂ molecules relatively weakly. Additionally, the modified CNSs effectively suppress H₂ O₂ decomposition and avoid O-O bond cleavage to produce H₂ O by steric effects. The synergistic effect of CNSs and Ni-LDH chips therefore leads to high activity and high selectivity in a two-electron pathway. A proof-of-concept zinc-air fuel cell is proposed and set up to demonstrate the feasibility of green synthesis of peroxide, generating an impressive H₂ O₂ production rate of 5239.67 mmol h^-1  g_cat. ^-1 .

Asymmetric Air Cathode Design for Enhanced Interfacial Electrocatalytic Reactions in High-Performance Zinc-Air Batteries

The rechargeable zinc-air battery (ZAB) is a promising energy storage technology owing to its high energy density and safe aqueous electrolyte, but there is a significant performance bottleneck. Generally, cathode reactions only occur at multiphase interfaces, where the electrocatalytic active sites can participate in redox reactions effectively. In the conventional air cathode, the 2D multiphase interface on the surface of the gas diffusion layer (GDL) inevitably results in an insufficient amount of active sites and poor interfacial contact, leading to sluggish reaction kinetics. To address this problem, a 3D multiphase interface strategy is proposed to extend the reactive interface into the interior of the GDL. Based on this concept, an asymmetric air cathode is designed to increase the accessible active sites, accelerate mass transfer, and generate a dynamically stabilized reactive interface. With a NiFe layered-double-hydroxide electrocatalyst, ZABs based on the asymmetric cathode deliver a small charge/discharge voltage gap (0.81 V at 5.0 mA cm^-2 ), a high power density, and a stable cyclability (over 2000 cycles). This 3D reactive interface strategy provides a feasible method for enhancing the air cathode kinetics and further enlightens electrode designs for energy devices involving multiphase electrochemical reactions.

Nitrogen-Doped Ketjenblack Carbon Supported Co3O4 Nanoparticles as a Synergistic Electrocatalyst for Oxygen Reduction Reaction

Developing a highly active and cost-effective cathode electrocatalyst with strong stability for oxygen reduction reaction (ORR) is extremely necessary. In this work, we reported a facile synthetic path to prepare a hybrid nanostructure formed of nitrogen-doped Ketjenblack carbon (N-KC) supported Co₃O₄ nanoparticles (Co₃O₄/N-KC), which could be used as a promising and stable electrocatalyst for ORR. Compared with the physical mixture of Co₃O₄ and N-KC and pure N-KC samples, the resulting Co₃O₄/N-KC nanohybrid afforded remarkably superb ORR activity with a half-wave potential of 0.82 V (vs. reversible hydrogen electrode, RHE) and a limiting current density of 5.70 mA cm^-2 in KOH solution (0.1 M). Surprisingly, the Co₃O₄/N-KC sample possessed a similar electrocatalytic activity but better durability to the 20 wt% Pt/C catalyst. The remarkable ORR activity of the Co₃O₄/N-KC nanohybrid was mainly due to the strong coupling effect between Co₃O₄ and N-KC, the N species dopant, high electroconductivity, and the large BET surface area. Our work enlightens the exploitation of advanced Co₃O₄/carbon hybrid material alternative to the Pt-based electrocatalysts.

In Situ Transformation of Prussian-Blue Analogue-Derived Bimetallic Carbide Nanocubes by Water Oxidation: Applications for Energy Storage and Conversion

Using bimetallic Prussian blue analogue (PBA) as a precursor is effective for preparing electrocatalysts for the oxygen evolution reaction (OER); however, the role of these PBA-derived catalysts in the OER is still ambiguous. Herein, by simply controlling synthesis temperature, a bimetallic PBA-derived O,N-codoped Ni-Fe carbide, can be well tuned to optimize structure and OER performance. Importantly, by a series of ex situ and in situ investigations, real active species of NiFeO_x H_y are in situ formed on the surface during the OER, which reveals a "pre-catalyst" role of O,N-codoped Ni-Fe carbides. Furthermore, it has been successfully applied to highly efficient Zn-air batteries and outplays its RuO₂ counterpart. When applied to photoelectrocatalytic water oxidation as the co-catalyst, it improves the performance of the BiVO₄ photoanode by enhancing hole collecting and transporting ability. We believe this research not only provides a highly efficient and low-cost electrocatalyst for the OER, but also unveils the "pre-catalyst" role of PBA-derived materials in energy-storage and conversion devices.

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_x N_y ). The 3DOM-Co@TiO_x N_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_x N_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.

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.

Structural Modulation of Co Catalyzed Carbon Nanotubes with Cu-Co Bimetal Active Center to Inspire Oxygen Reduction Reaction

Rational design of highly efficient catalyst for ORR is critical for development of advanced air cathode in Zn-air cells and fuel cells. To optimize the ORR performance of Co based cathode, the structure of carbon nanotube from DCI-Co precursor could be controlled through modulate synthetic parameters. The optimized ORR catalyst Co@NCNT-700 exhibit larger BET area, higher content of Co-N _x and graphitic N, which performance could be improved in further through Cu doping. The experiment data approved that the activity of Co-N _x was enhanced by the synergistic effect with introduced Cu. Furthermore, the high-performance zinc-air batteries was fabricated with the bimetal catalyst CuCo@NCNT-700 as an air electrode. The high open-cycle potential (1.54 V) and peak power density (0.275 W.cm^-2 at 0.474 A.cm^-2) were achieved, which would be potentially used to develop next generation energy conversion devices.

Multiscale Structural Engineering of Ni-Doped CoO Nanosheets for Zinc-Air Batteries with High Power Density

Zinc-air batteries offer a possible solution for large-scale energy storage due to their superhigh theoretical energy density, reliable safety, low cost, and long durability. However, their widespread application is hindered by low power density. Herein, a multiscale structural engineering of Ni-doped CoO nanosheets (NSs) for zinc-air batteries with superior high power density/energy density and durability is reported for the first time. In micro- and nanoscale, robust 2D architecture together with numerous nanopores inside the nanosheets provides an advantageous micro/nanostructured surface for O₂ diffusion and a high electrocatalytic active surface area. In atomic scale, Ni doping significantly enhances the intrinsic oxygen reduction reaction activity per active site. As a result of controlled multiscale structure, the primary zinc-air battery with engineered Ni-doped CoO NSs electrode shows excellent performance with a record-high discharge peak power density of 377 mW cm^-2 , and works stable for >400 h at 5 mA cm^-2 . Rechargeable zinc-air battery based on Ni-doped CoO NSs affords an unprecedented small charge-discharge voltage of 0.63 V, outperforming state-of-the-art Pt/C catalyst-based device. Moreover, it is shown that Ni-doped CoO NSs assembled into all-solid-state coin cells can power 17 light-emitting diodes and charge an iPhone 7 mobile phone.

A Confinement Strategy for Stabilizing ZIF-Derived Bifunctional Catalysts as a Benchmark Cathode of Flexible All-Solid-State Zinc-Air Batteries

Carbon composites with embedded metal/metal oxides represent a group of versatile electrochemical catalysts that has attracted extensive research attention. However, the beauty of this concept is marred by the severe carbon evaporation and the aggregation of metal species during their synthetic process, leading to the diminishment in active sites and catalytic durability. To address this issue, this study demonstrates the feasibility of utilizing Al₂ O₃ nanolayer to trap volatile carbon and nitrogen species and alleviate the aggregation of Co species during the pyrolysis of the Zn/Co-ZIFs (ZIF = zeolitic imidazolate framework). With the confinement effect of an Al₂ O₃ nanolayer, the derived Co₃ O₄ -embedded N-doped porous carbon grown on carbon cloth presents outstanding bifunctional catalytic activity with a small potential difference of 787 mV between the half-wave potential of the oxygen reduction reaction and an overpotential at 10 mA cm^-2 of the oxygen evolution reaction. More impressively, an advanced flexible rechargeable zinc-air battery in all-solid-state configuration is assembled, which achieves the maximum power density of 72.4 mW cm^-3 and good cycling stability. The insights produced in this work will provide guidance for the rational design of metal/carbon hybrid catalysts and low-cost renewable energy systems.

Recent Advances in Materials and Design of Electrochemically Rechargeable Zinc-Air Batteries

The century-old zinc-air (Zn-air) battery concept has been revived in the last decade due to its high theoretical energy density, environmental-friendliness, affordability, and safety. Particularly, electrically rechargeable Zn-air battery technologies are of great importance for bulk applications like electric vehicles, grid management, and portable electronic devices. Nevertheless, Zn-air batteries are still not competitive enough to realize widespread practical adoption because of issues in efficiency, durability, and cycle life. Here, following an introduction to the fundamentals and performance testing techniques, the latest research progress related to electrically rechargeable Zn-air batteries is compiled, particularly new key findings in the last five years (2013-2018). The strategies concerning the development of Zn and air electrodes are in focus. The design of other battery components, namely electrolytes and separators are also discussed. Poor performance of O₂ electrocatalysts and the lack of the long-term stability of Zn electrodes and electrolytes remain major challenges. Finally, recommendations regarding the testing routines and materials design are provided. It is hoped that this up-to-date account will help to shape the future research activities toward the development of practical electrically rechargeable Zn-air batteries with extended lifetime and superior performance.

Sulfuration of an Fe-N-C Catalyst Containing Fex C/Fe Species to Enhance the Catalysis of Oxygen Reduction in Acidic Media and for Use in Flexible Zn-Air Batteries

During the preparation of atomically dispersed Fe-N-C catalysts, it is difficult to avoid the formation of iron-carbide-containing iron clusters ("Fe_x C/Fe"), along with the desired carbon matrix containing dispersed FeN_x sites. As a result, an uncertain amount of the oxygen reduction reaction (ORR) occurs, making it difficult to maximize the catalytic efficiency. Herein, sulfuration is used to boost the activity of Fe_x C/Fe, forming an improved system, "FeNC-S-Fe_x C/Fe", for catalysis involving oxygen. Various spectroscopic techniques are used to define the composition of the active sites, which include Fe-S bonds at the interface of the now-S-doped carbon matrix and the Fe_x C/Fe clusters. In addition to outstanding activity in basic media, FeNC-S-Fe_x C/Fe exhibits improved ORR activity and durability in acidic media; its half-wave potential of 0.821 V outperforms the commercial Pt/C catalyst (20%), and its activity does not decay even after 10 000 cycles. Interestingly, the catalytic activity for the oxygen evolution reaction (OER) simultaneously improves. Thus, FeNC-S-Fe_x C/Fe can be used as a high-performance bifunctional catalyst in Zn-air batteries. Theoretical calculations and control experiments show that the original FeN_x active centers are enhanced by the Fe_x C/Fe clusters and the Fe-S and C-S-C bonds.

Atomic Layer Co3 O4 Nanosheets: The Key to Knittable Zn-Air Batteries

Flexible, wearable, and portable energy storage devices with high-energy density are crucial for next-generation electronics. However, the current battery technologies such as lithium ion batteries have limited theoretical energy density. Additionally, battery materials with small scale and high flexibility which could endure the large surface stress are highly required. In this study, a yarn-based 1D Zn-air battery is designed, which employs atomic layer thin Co₃ O₄ nanosheets as the oxygen reduction reaction/oxygen evolution reaction catalyst. The ultrathin nanosheets are synthesized by a high-yield and facile chemical method and show a thickness of only 1.6 nm, corresponding to few atomic layers. The 1D Zn-air battery shows high cycling stability and high rate capability. The battery is successfully knitted into clothes and it shows high stability during the large deformation and knotting conditions.

Colloidal Cobalt Phosphide Nanocrystals as Trifunctional Electrocatalysts for Overall Water Splitting Powered by a Zinc-Air Battery

Highly efficient and stable electrocatalysts, particularly those that are capable of multifunctionality in the same electrolyte, are in high demand for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). In this work, highly monodisperse CoP and Co₂ P nanocrystals (NCs) are synthesized using a robust solution-phase method. The highly exposed (211) crystal plane and abundant surface phosphide atoms make the CoP NCs efficient catalysts toward ORR and HER, while metal-rich Co₂ P NCs show higher OER performance owing to easier formation of plentiful Co₂ P@COOH heterojunctions. Density functional theory calculation results indicate that the desorption of OH* from cobalt sites is the rate-limiting step for both CoP and Co₂ P in ORR and that the high content of phosphide can lower the reaction barrier. A water electrolyzer constructed with a CoP NC cathode and a Co₂ P NC anode can achieve a current density of 10 mA cm^-2 at 1.56 V, comparable even to the noble metal-based Pt/C and RuO₂ /C pair. Furthermore, the CoP NCs are employed as an air cathode in a primary zinc-air battery, exhibiting a high power density of 62 mW cm^-2 and good stability.

Atomically Thin Mesoporous Co3 O4 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 Co₃ O₄ /nitrogen-doped reduced graphene oxide (N-rGO) nanosheets. Benefiting from the high surface area, mesoporous structure, and strong synergetic effect between the Co₃ O₄ 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 Co₃ O₄ /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.

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.

Cobalt Phosphide Coupled with Heteroatom-Doped Nanocarbon Hybrid Electroctalysts for Efficient, Long-Life Rechargeable Zinc-Air Batteries

Metal phosphides and heteroatom-doped carbons have been regarded as promising candidates as bifunctional catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). However, both have suffered from stability issues during repeated ORR and OER operations in zinc-air batteries (ZABs). Herein, this study reports a versatile cobalt-based hybrid catalyst with a 1D structure by integrating the metal-organic framework-derived conversion approach and an in situ crosslinking method. Among them, the 1D hybrid catalyst composed of ultrasmall cobalt phosphide nanoparticles supported by nitrogen-, sulfur-, phosphorus-doped carbon matrix shows remarkable bifunctional activity close to that of the benchmark precious-metal catalysts along with an excellent durability in the full potential range covering both the OER and ORR. The overall overpotential of the rechargeable ZABs can be greatly reduced with this bifunctional hybrid catalyst as an air-electrode, and the cycling stability outperforms the commercial Pt/C catalyst. It is revealed that the cobalt phosphide nanoparticles are in situ converted to cobalt oxide under the accelerated conditions during OER (and/or ORR) of the ZABs and reduces the anodic current applied to the carbon. This contributes to the stability of the carbon material and in maintaining the high initial catalytic properties of the hybrid catalyst.

Defect Engineering toward Atomic Co-Nx -C in Hierarchical Graphene for Rechargeable Flexible Solid Zn-Air Batteries

Rechargeable flexible solid Zn-air battery, with a high theoretical energy density of 1086 Wh kg^-1 , is among the most attractive energy technologies for future flexible and wearable electronics; nevertheless, the practical application is greatly hindered by the sluggish oxygen reduction reaction/oxygen evolution reaction (ORR/OER) kinetics on the air electrode. Precious metal-free functionalized carbon materials are widely demonstrated as the most promising candidates, while it still lacks effective synthetic methodology to controllably synthesize carbocatalysts with targeted active sites. This work demonstrates the direct utilization of the intrinsic structural defects in nanocarbon to generate atomically dispersed Co-N_x -C active sites via defect engineering. As-fabricated Co/N/O tri-doped graphene catalysts with highly active sites and hierarchical porous scaffolds exhibit superior ORR/OER bifunctional activities and impressive applications in rechargeable Zn-air batteries. Specifically, when integrated into a rechargeable and flexible solid Zn-air battery, a high open-circuit voltage of 1.44 V, a stable discharge voltage of 1.19 V, and a high energy efficiency of 63% at 1.0 mA cm^-2 are achieved even under bending. The defect engineering strategy provides a new concept and effective methodology for the full utilization of nanocarbon materials with various structural features and further development of advanced energy materials.

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_0.87 S_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_0.87 S_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_0.87 S_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.

"Wiring" Fe-Nx -Embedded Porous Carbon Framework onto 1D Nanotubes for Efficient Oxygen Reduction Reaction in Alkaline and Acidic Media

This study presents a novel metal-organic-framework-engaged synthesis route based on porous tellurium nanotubes as a sacrificial template for hierarchically porous 1D carbon nanotubes. Furthermore, an ultrathin Fe-ion-containing polydopamine layer has been introduced to generate highly effective FeN_x C active sites into the carbon framework and to induce a high degree of graphitization. The synergistic effects between the hierarchically porous 1D carbon structure and the embedded FeN_x C active sites in the carbon framework manifest in superior catalytic activity toward oxygen reduction reaction (ORR) compared to Pt/C catalyst in both alkaline and acidic media. A rechargeable zinc-air battery assembled in a decoupled configuration with the nonprecious pCNT@Fe@GL/CNF ORR electrode and Ni-Fe LDH/NiF oxygen evolution reaction (OER) electrode exhibits charge-discharge overpotentials similar to the counterparts of Pt/C ORR electrode and IrO₂ OER electrode.

Hollow Carbon Nanopolyhedra for Enhanced Electrocatalysis via Confined Hierarchical Porosity

A novel strategy for the fabrication of hollow Co and N-codoped carbon nanopolyhedra (H-CoNC) from metal-organic framework (MOF) using in situ evaporation of ZnO nanosphere templates is proposed. The excess Zn supply during the pyrolysis process is found beneficial in terms of high nitrogen (≈9.75 at%), relatively homogenous CoN bonding, and the electrochemically accessible hierarchical porous system. Compared with other reported "solid" CoNC of identical surface areas, the newly developed H-CoNC shows enhanced kinetic current in 0.1 m KOH electrolyte and elevated oxygen reduction reaction (ORR) performance in 6 m KOH. The latter exceeds results obtained with the benchmark 20 wt% Pt/C, which is related to the strong confinement of O₂ molecules in the H-CoNC hierarchical porous system. Furthermore, the H-CoNC displays great tolerance toward the methanol crossover and KSCN poisoning. Finally, the assembled Zn-air batteries with H-CoNC yield a record open circuit potential (1.59 V vs Zn, stabilized at 1.52 V), high power density (331.0 mW cm^-2 ), and promising rate performance. This work provides a new guideline for the design of MOF-derived carbon materials, as well as novel insights into spatial confinement effect toward the ORR activity.

Electrically Rechargeable Zinc-Air Batteries: Progress, Challenges, and Perspectives

Zinc-air batteries have attracted much attention and received revived research efforts recently due to their high energy density, which makes them a promising candidate for emerging mobile and electronic applications. Besides their high energy density, they also demonstrate other desirable characteristics, such as abundant raw materials, environmental friendliness, safety, and low cost. Here, the reaction mechanism of electrically rechargeable zinc-air batteries is discussed, different battery configurations are compared, and an in depth discussion is offered of the major issues that affect individual cellular components, along with respective strategies to alleviate these issues to enhance battery performance. Additionally, a section dedicated to battery-testing techniques and corresponding recommendations for best practices are included. Finally, a general perspective on the current limitations, recent application-targeted developments, and recommended future research directions to prolong the lifespan of electrically rechargeable zinc-air batteries is provided.

Co@Co3 O4 @PPD Core@bishell Nanoparticle-Based Composite as an Efficient Electrocatalyst for Oxygen Reduction Reaction

Durable electrocatalysts with high catalytic activity toward oxygen reduction reaction (ORR) are crucial to high-performance primary zinc-air batteries (ZnABs) and direct methanol fuel cells (DMFCs). An efficient composite electrocatalyst, Co@Co3 O4 core@shell nanoparticles (NPs) embedded in pyrolyzed polydopamine (PPD) is reported, i.e., in Co@Co3 O4 @PPD core@bishell structure, obtained via a three-step sequential process involving hydrothermal synthesis, high temperature calcination under nitrogen atmosphere, and gentle heating in air. With Co@Co3 O4 NPs encapsulated by ultrathin highly graphitized N-doped carbon, the catalyst exhibits excellent stability in aqueous alkaline solution over extended period and good tolerance to methanol crossover effect. The integration of N-doped graphitic carbon outer shell and ultrathin nanocrystalline Co3 O4 inner shell enable high ORR activity of the core@bishell NPs, as evidenced by ZnABs using catalyst of Co@Co3 O4 @PPD in air-cathode which delivers a stable voltage profile over 40 h at a discharge current density of as high as 20 mA cm(-2) .

Scalable Fabrication of Nanoporous Carbon Fiber Films as Bifunctional Catalytic Electrodes for Flexible Zn-Air Batteries

A flexible nanoporous carbon-fiber film for wearable electronics is prepared by a facile and scalable method through pyrolysis of electrospun polyimide. It exhibits excellent bifunctional electrocatalytic activities for oxygen reduction and oxygen evolution. Flexible rechargeable zinc-air batteries based on the carbon-fiber film show high round-trip efficiency and mechanical stability.

All-solid-state cable-type flexible zinc-air battery

A cable-type flexible Zn-air battery with a spiral zinc anode, gel polymer electrolyte (GPE), and air cathode coated on a nonprecious metal catalyst is designed in order to extend its application area toward wearable electronic devices.