Recent advances in zinc-air batteries

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.

Flexible Zn-Ion Batteries: Recent Progresses and Challenges

To keep pace with the increasing pursuit of portable and wearable electronics, it is urgent to develop advanced flexible power supplies. In this context, Zn-ion batteries (ZIBs) have garnered increasing attention as favorable energy storage devices for flexible electronics, owing to the high capacity, low cost, abundant resources, high safety, and eco-friendliness. Extensive efforts have been devoted to developing flexible ZIBs in the last few years. This work summarizes the recent achievements in the design, fabrication, and characterization of flexible ZIBs. Representative structures, such as sandwich and cable type, are particularly highlighted. Special emphasis is put on the novel design of electrolyte and electrode, which aims to endow reliable flexibility to the fabricated ZIBs. Moreover, current challenges and future opportunities for the development of high-performance flexible ZIBs are also outlined.

Bio-Integrated Wearable Systems: A Comprehensive Review

Bio-integrated wearable systems can measure a broad range of biophysical, biochemical, and environmental signals to provide critical insights into overall health status and to quantify human performance. Recent advances in material science, chemical analysis techniques, device designs, and assembly methods form the foundations for a uniquely differentiated type of wearable technology, characterized by noninvasive, intimate integration with the soft, curved, time-dynamic surfaces of the body. This review summarizes the latest advances in this emerging field of "bio-integrated" technologies in a comprehensive manner that connects fundamental developments in chemistry, material science, and engineering with sensing technologies that have the potential for widespread deployment and societal benefit in human health care. An introduction to the chemistries and materials for the active components of these systems contextualizes essential design considerations for sensors and associated platforms that appear in following sections. The subsequent content highlights the most advanced biosensors, classified according to their ability to capture biophysical, biochemical, and environmental information. Additional sections feature schemes for electrically powering these sensors and strategies for achieving fully integrated, wireless systems. The review concludes with an overview of key remaining challenges and a summary of opportunities where advances in materials chemistry will be critically important for continued progress.

MnO/N-Doped Mesoporous Carbon as Advanced Oxygen Reduction Reaction Electrocatalyst for Zinc-Air Batteries

The development of alternative electrocatalysts exhibiting high activity in the oxygen reduction reaction (ORR) is vital for the deployment of large-scale clean energy devices, such as fuel cells and zinc-air batteries. N-doped carbon materials offer a promising platform for the design and synthesis of electrocatalysts due to their high ORR activity, high surface area, and tunable porosity. In this study, materials in which MnO nanoparticles are entrapped in N-doped mesoporous carbon (MnO/NC) were developed as electrocatalysts for the ORR, and their performances were evaluated in zinc-air batteries. The obtained carbon materials had large surface area and high electrocatalytic activity toward the ORR. The carbon compounds were fabricated by using NaCl as template in a one-pot process, which significantly simplifies the procedure for preparing mesoporous carbon materials and in turn reduces the total cost. A primary zinc-air battery based on this material exhibits an open-circuit voltage of 1.49 V, which is higher than that of conventional zinc-air batteries with Pt/C (Pt/C cell) as ORR catalyst (1.41 V). The assembled zinc-air battery delivered a peak power density of 168 mW cm^-2 at a current density of about 200 mA cm^-2 , which is higher than that of an equivalent Pt/C cell (151 mW cm^-2 at a current density of ca. 200 mA cm^-2 ). The electrocatalytic data revealed that MnO/NC is a promising nonprecious-metal ORR catalyst for practical applications in metal-air batteries.

Flexible and Wearable All-Solid-State Al-Air Battery Based on Iron Carbide Encapsulated in Electrospun Porous Carbon Nanofibers

In this work, electrospinning N-doped carbon nanofibers containing iron carbide (Fe₃C@N-CFs) are synthesized and employed as the cathode in the flexible Al-air battery. Benefiting from the excellent catalytic activity of the iron carbide which is uniformly encapsulated in the N-doped carbon matrix, as well as the large specific surface area of the cross-linked network nanostructure, the as-prepared Fe₃C@N-CFs show outstanding catalytic activity and stability toward oxygen reduction reaction. The as-fabricated all-solid-state Al-air batteries with Fe₃C@N-CF catalyst show a stable discharge voltage (1.61 V) for 8 h, giving a capacity of 1287.3 mA h g^-1.

High-Density Cobalt Nanoparticles Encapsulated with Nitrogen-Doped Carbon Nanoshells as a Bifunctional Catalyst for Rechargeable Zinc-Air Battery

High efficient electrocatalytic activity and strong stability to both oxygen reduction reaction (ORR) and oxygen evolution (OER) are very critical to rechargeable Zn-air battery and other renewable energy technologies. As a class of promising catalysts, the nanocoposites of transition metal nanoparticles that are encapsulated with nitrogen-doped carbon nanoshells are considered as promising substitutes to expensive precious metal based catalysts. In this work, we demonstrate the successful preparation of high-density cobalt nanoparticles encapsulated in very thin N-doped carbon nanoshells by the pyrolysis of solid state cyclen-Co-dicyandiamide complex. The morphologies and properties of products can be conveniently tuned by adjusting the pyrolysis temperature. Owing to the synergetic effect of hybrid nanostructure, the optimized Co@N-C-800 sample possesses outstanding bifunctional activity for both ORR and OER in alkaline electrolyte. Meanwhile, the corresponding rechargeable zinc-air battery that is based on Co@N-C-800 air cathode also has excellent current density, low charge-discharge voltage gap, high power density, and strong cycle stability, making it a suitable alternative to take the place of precious metal catalysts for practical utilization.

In Situ Coupling Strategy for Anchoring Monodisperse Co9S8 Nanoparticles on S and N Dual-Doped Graphene as a Bifunctional Electrocatalyst for Rechargeable Zn-Air Battery

An in situ coupling strategy to prepare Co₉S₈/S and N dual-doped graphene composite (Co₉S₈/NSG) has been proposed. The key point of this strategy is the function-oriented design of organic compounds. Herein, cobalt porphyrin derivatives with sulfo groups are employed as not only the coupling agents to form and anchor Co₉S₈ on the graphene in situ, but also the heteroatom-doped agent to generate S and N dual-doped graphene. The tight coupling of multiple active sites endows the composite materials with fast electrochemical kinetics and excellent stability for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The obtained electrocatalyst exhibits better activity parameter (ΔE = 0.82 V) and smaller Tafel slope (47.7 mV dec^-1 for ORR and 69.2 mV dec^-1 for OER) than commercially available Pt/C and RuO₂. Most importantly, as electrocatalyst for rechargeable Zn-air battery, Co₉S₈/NSG displays low charge-discharge voltage gap and outstanding long-term cycle stability over 138 h compared to Pt/C-RuO₂. To further broaden its application scope, a homemade all-solid-state Zn-air battery is also prepared, which displays good charge-discharge performance and cycle performance. The function-oriented design of N₄-metallomacrocycle derivatives might open new avenues to strategic construction of high-performance and long-life multifunctional electrocatalysts for wider electrochemical energy applications.

Three-Dimensional Fe,N-Decorated Carbon-Supported NiFeP Nanoparticles as an Efficient Bifunctional Catalyst for Rechargeable Zinc-O2 Batteries

The electro-catalyzed oxygen reduction and evolution reactions (ORR/OER) are the key elements of many energy conversion systems, such as fuel cells, water electrolyzers, and rechargeable metal-air batteries. Structural design of durable non-noble nanomaterials as bifunctional OER/ORR catalysts is a major drawback to commercial applications. Herein, we exposed a strongly coupled hybrid material comprising of NiFeP-cubes nanoparticles supported on three-dimensional interconnected Fe,N-decorated carbon (3D-FeNC) as a robust bifunctional ORR/OER catalyst. The strongly coupled NiFeP@3D-FeNC catalyst shows better electron and mass transfer capability, exposure of abundant ORR/OER active sites on the surface, and strongly coupled effects. Accordingly, the as-prepared NiFeP@3D-FeNC catalyst exhibits robust ORR activity (half-wave potential of 0.84 V vs reversible hydrogen electrode) and OER performance (over-potential 0.25 V@10 mA cm^-2) in alkaline media. Significantly, the oxygen electrode prepared from the NiFeP@3D-FeNC catalyst further demonstrated superior charge/discharge behavior and long-lasting rechargeability than the benchmark Pt/C + IrO₂ catalyst in rechargeable zinc-O₂ batteries. This approach opens up a new avenue for the synthesis and advanced the hybrid nanomaterials for various applications.

Controllable Hortensia-like MnO2 Synergized with Carbon Nanotubes as an Efficient Electrocatalyst for Long-Term Metal-Air Batteries

The exploitation of a high-activity and low-cost bifunctional catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as the cathode catalyst is a research priority in metal-air batteries. Herein, a novel bifunctional hybrid catalyst of hortensia-like MnO₂ synergized with carbon nanotubes (CNTs) (MnO₂/CNTs) is controllably synthesized by reasonably designing the crystal structure and morphology as well as electronic arrangement. On the basis of these strategies, the hybrid accelerates the reaction kinetics and avoids the change of structures. As expected, MnO₂/CNTs exhibit a remarkable ORR and OER activity [low ORR Tafel slope: 71 mV dec^-1, low OER Tafel slope: 67 mV dec^-1, and small potential difference (Δ E): 0.85 V] and a long-term stability, which should be attributed to its unique morphology, K⁺ ions in the 2 × 2 tunnels, and synergistic effect between MnO₂ and CNTs. Notably, in zinc-air batteries (ZABs), aluminum-air batteries (AABs), and magnesium-air batteries (MABs), the composite shows high power density (ZABs: 243 mW cm^-2, AABs: 530 mW cm^-2, and MABs: 614 mW cm^-2) and large specific capacities (793 mA h g_Zn^-1, 918 mA h g_Al^-1, and 878 mA h g_Mg^-1). Importantly, the rechargeable ZABs reveal small charge-discharge voltage drop (0.81 V) and strong cycle durability (500 h), which are better than the noble-metal Pt/C + IrO₂ mixture catalyst (the voltage drop: 1.15 V at first and 2 V after 100 h). These superior performances together with the simple synthetic method of the hybrid reveal great promise in large-power energy storage and conversion equipment.

Research Advances of Amorphous Metal Oxides in Electrochemical Energy Storage and Conversion

Amorphous metal oxides (AMOs) have aroused great enthusiasm across multiple energy areas over recent years due to their unique properties, such as the intrinsic isotropy, versatility in compositions, absence of grain boundaries, defect distribution, flexible nature, etc. Here, the materials engineering of AMOs is systematically reviewed in different electrochemical applications and recent advances in understanding and developing AMO-based high-performance electrodes are highlighted. Attention is focused on the important roles that AMOs play in various energy storage and conversion technologies, such as active materials in metal-ion batteries and supercapacitors as well as active catalysts in water splitting, metal-air batteries, and fuel cells. The improvements of electrochemical performance in metal-ion batteries and supercapacitors are reviewed regarding the enhancement in active sites, mechanical strength, and defect distribution of amorphous structures. Furthermore, the high electrochemical activities boosted by AMOs in various fundamental reactions are elaborated on and they are related to the electrocatalytic behaviors in water splitting, metal-air batteries, and fuel cells. The applications in electrochromism and high-conducting sensors are also briefly discussed. Finally, perspectives on the existing challenges of AMOs for electrochemical applications are proposed, together with several promising future research directions.

Bimetallic Ni–Co composites anchored on a wool ball-like carbon framework as high-efficiency bifunctional electrodes for rechargeable Zn–air batteries

A novel hydrothermal–pyrolysis strategy is proposed to synthesize high-efficiency NiCo@N–C bi-functional electrocatalysts for oxygen transformation.

High dispersion and oxygen reduction reaction activity of Co3O4 nanoparticles on platelet-type carbon nanofibers

In this study, platelet-type carbon nanofibers prepared by the liquid phase carbonization of polymers in the pores of a porous anodic alumina template were used to prepare the Co₃O₄/carbon electrocatalysts.

S,N co-doped reduced graphene oxide sheets with cobalt hydroxide nanocrystals for highly active and stable bifunctional oxygen catalysts

A Co(OH)₂ anchored on S,N co-doped rGO as a highly active and stable bifunctional oxygen catalyst was developed via an efficient strategy and its catalytic activity was comparable to that of the benchmarked noble metal-based oxygen catalysts.

In situ semi-quantitative analysis of zinc dissolution within nanoporous silicon by X-ray absorption fine-structure spectroscopy employing an X-ray compatible cell

The in situ study of the discharge process in a zinc-based half-cell employing a porous electrode as a structural scaffold is reported. The in situ characterization has been performed by synchrotron X-ray absorption fine-structure spectroscopy and, for this purpose, an inexpensive, simple and versatile electrochemical cell compatible with X-ray experiments has been designed and described. The experimental results reported here have been employed to semi-quantify the dissolved and undissolved zinc species during the discharge, allowing the cell feasibility to be tested and to better understand the functioning of the zinc half-cell based on porous electrodes.

Molecule-level graphdiyne coordinated transition metals as a new class of bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions

Graphdiyne (GDY) could provide a unique platform for synthesizing uniform single-atom catalysts (SACs) with high catalytic activity toward oxygen reduction (ORR) and oxygen evolution (OER) reactions.

Bimetallic NiCo/CNF encapsulated in a N-doped carbon shell as an electrocatalyst for Zn–air batteries and water splitting

Bimetallic NiCo/CNF encapsulated in a N-doped carbon shell (NiCo/CNF@NC) catalyst for application in a Zn–air battery and water splitting has been reported in this work.

Applications of 2D MXenes in energy conversion and storage systems

This article provides a comprehensive review of MXene materials and their energy-related applications.

Photoactive Zn–air batteries using spinel-type cobalt oxide as a bifunctional photocatalyst at the air cathode

Spinel-type cobalt oxide (Co₃O₄) was synthesized and used as a photoactive bifunctional electrocatalyst towards the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) at the air cathode of zinc–air batteries (ZABs).

Ni/NiM2O4 (M = Mn or Fe) supported on N-doped carbon nanotubes as trifunctional electrocatalysts for ORR, OER and HER

The NCNT/Ni–NiM₂O₄ (M = Mn or Fe) exhibit excellent trifunctional electrocatalytic performance for ORR, OER and HER.

Highly Active Bifunctional Electrocatalysts for Oxygen Evolution and Reduction in Zn-Air Batteries

To realize the full performance of Zn-air batteries, the co-presence of a highly efficient oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER) in the system is critical. Although copper and nickel are known to be bifunctional catalysts for ORR and OER, sluggish reactions as a result of the exceptionally strong O=O bond on the metal surface make it difficult to achieve high system efficiency. In this study, a metal carbide layer (CuC_x and NiC_x ) on dendritic copper and nickel is fabricated by a facile electrodeposition process to provide efficient catalytic active sites with moderate binding energy for easy electron transfer in both the OER and the ORR. The dendritic structure provides an enriched catalytic surface and the protective metal carbide layer offers an appropriate O binding energy and durability of Zn-air batteries. Owing to the presence of the stable metal carbide surface on the dendritic metal, the CuC_x /Cu and NiC_x /Ni catalysts exhibited well-defined limiting current densities of -5.19 and -5.11 mA cm^-2 , respectively, and improved ORR and OER activities with lower polarization than the corresponding metal catalysts. Density functional theory revealed a 0.74 eV decrease in the overpotential of NiC_x /Ni-catalyzed OER reactions compared with Ni-catalyzed OER reactions. The experimental and theoretical results prove that carbide layers on dendritic metal surfaces can greatly improve the activity of ORR and OER bifunctional electrocatalysts for Zn-air batteries.

Inhibition of Zinc Dendrite Growth in Zinc-Based Batteries

Zinc deposition and dissolution is a significant process in zinc-based batteries. During this process, the formation of zinc dendrites is pervasive, which leads to the loss of efficiency and capacity of batteries. The continually growing dendrites will finally pierce the separator and cause the batteries to short circuit. Thus, employing effective methods to inhibit the formation and growth of zinc dendrites is vital for the practical application of zinc-based batteries. This Minireview first clarifies the formation and growth principles of zinc dendrites. Then, the research and development of methods to solve the problem of zinc dendrites are reviewed, including ways to suppress the further formation and growth of dendrites as far as possible, to minimize the adverse effects of dendrites, along with ways to produce dendrite-free deposition processes. The mechanisms, advantages, drawbacks, and perspectives of these methods are illustrated. Thus, this overview of these methods will aid understanding of the formation process of zinc dendrites and provide an extensive, comprehensive, and professional reference to resolve the problem of zinc dendrites completely.

Scalable Synthesis of Fe/N-Doped Porous Carbon Nanotube Frameworks for Aqueous Zn-Air Batteries

Aqueous Zn-air batteries are emerging to be ideal next-generation energy-storage devices with high safety and high energy/power densities. However, the rational design and fabrication of low-cost, highly efficient, and durable electrocatalysts on the cathode side remain highly desired. Herein, template-assisted, scalable Fe-implanted N-doped porous carbon nanotube networks (Fe-N-CNNs) have been synthesized based on an environmentally friendly template hydroxyapatite nanowires (HAP NWs). Thanks to the hierarchical meso/micropores, high specific surface area, and abundant active sites, the optimized Fe-N-CNNs exhibit excellent oxygen reduction activity. Furthermore, the Zn-air batteries based on the Fe-N-CNNs cathode deliver a high discharge voltage of 1.27 V at a current density of 20 mA cm^-2 and a large peak power density of 202.2 mW cm^-2 . More far-reaching, this HAP-based template strategy opens a new avenue toward the mass production of efficient, cost-effective electrocatalysts, and the Fe-N-CNNs with hollow interiors are expected to extend their other potential uses in energy storage, molecular sieves, adsorbents, and biomedical engineering.

Metal-organic frameworks-derived core-shell Fe3O4/Fe3N@graphite carbon nanocomposites as excellent non-precious metal electrocatalyst for oxygen reduction

Metal-organic frameworks (MOFs), as precursors for synthesizing new carbon materials, hold promise for applications in the oxygen reduction reaction (ORR) as efficient non-precious metal catalysts. Here, a facile template-assisted strategy was adopted to fabricate a core-shell structure derived from MIL-101(Fe) and polyaniline. MIL-101(Fe) nanoparticles obtained by microwave-assisted synthesis were combined with PAni in different ratios and carbonized at 900 °C under flowing N2. An optimized core-shell Fe3O4/Fe3N@graphite carbon structure was successfully prepared and exhibited attractive ORR activity, with a half-wave potential of 0.916 V vs. RHE and an electron transfer number of 4.0 at 0.4 V vs. RHE. Furthermore, the catalyst displayed excellent stability in an alkaline solution. The superior ORR performance of the catalyst is mainly attributed to its stable core-shell structure, large specific surface area and high content of electrocatalytically active N species.

3D N-doped carbon framework with embedded CoS nanoparticles as highly active and durable oxygen reduction and evolution electrocatalyst

Development of bifunctional non-metal electrocatalyst for oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) with high efficiency, durable stability and low cost is a crucial and challenging issue. However, the heteroatom-doped carbon material including a carbon-based conductive additive would be easily oxidized under the high potential needed for driving the OER. Besides, the interaction between the heteroatom-doped carbon material that possesses electrocatalyst activity and a carbon-based conductive additive is weak, affecting the performance of the electrocatalyst. In this context, we introduce CoS nanoparticles into a three-dimensional N-doped carbon framework (CoS/NCF) by a morphology-retaining pyrolysis of polyaniline/CoS framework precursor, in which the polyaniline framework provides abundant functional groups to nucleate and grow CoS nanoparticles while retaining its interconnected three-dimensional porous structure. Benefiting from (i) the lower OER potential of CoS nanoparticles than the electro-oxidation decomposition potential of a carbon material and (ii) the strong affinity of CoS nanoparticles for a N-doped carbon framework, higher stability than commercial Pt/C system and greater catalytic activity towards ORR with an onset potential of about 0.921 V versus reversible hydrogen electrode (RHE) are observed. Furthermore, only a potential of 1.515 V versus RHE is required for achieving a current density of 10 mA cm^-2.

Restricting Growth of Ni3Fe Nanoparticles on Heteroatom-Doped Carbon Nanotube/Graphene Nanosheets as Air-Electrode Electrocatalyst for Zn-Air Battery

Exploring bifunctional oxygen electrode catalysts with efficient and stable oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) performance is one of the limitations for high-performance zinc-air battery. In this work, Ni₃Fe alloy nanoparticles incorporated in three-dimensional (3D) carbon nanotube (CNT)/graphene nanosheet composites with N and S codoping (Ni₃Fe/N-S-CNTs) as bifunctional oxygen electrode electrocatalysts for zinc-air battery. The main particle size of Ni₃Fe nanoparticles could be well restricted because of the unique 3D structure of carbon nanotube/graphene nanosheet composites (N-S-CNTs). The large specific area of N-S-CNTs is conducive to the uniform dispersion of Ni₃Fe nanoparticles. On the basis of the synergistic effect of Ni₃Fe nanoparticles with N-S-CNTs, and the sufficient exposure of reactive sites, the synthesized Ni₃Fe/N-S-CNTs catalyst exhibits excellent OER performance with a low overpotential of 215 mV at 10 mA cm^-2, and efficient ORR activity with a half-wave potential of 0.877 V. When used as an electrocatalyst in zinc-air battery, the device exhibits a power density of 180.0 mW cm^-2 and long term durability for 500 h.

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.

All-in-One Bifunctional Oxygen Electrode Films for Flexible Zn-Air Batteries

As a promising energy-storage device, rechargeable Zn-air batteries have attracted considerable interests. Herein, a bifunctional oxygen electrode film prepared by adhering NiCo₂ O₄ nanosheets to a nitrogen and oxygen dual-doped carbon nanotubes film in a large scale is reported. The resulting self-supporting film electrode is multifunctional, which integrates a porous conducting structure for air diffusion and charge transfer, high-performance catalysts for oxygen reduction and evolution, and novel structural flexibility. The composite film demonstrates excellent oxygen reduction/evolution reaction catalytic activities with low Tafel slopes (50 mV dec^-1 for oxygen reduction reaction; 92 mV dec^-1 for oxygen evolution reaction). Without any additional current collector, gas diffusion layer, or binder, the obtained bifunctional film performs as an "all-in-one" air electrode in a Zn-air battery. A 50-cm-long cable-shaped Zn-air battery based on such a film air electrode exhibits high operating potentials (≈1.2 V at 0.25 mA cm^-2 ), low charging-discharging overpotentials (≈0.7 V), and stable cycling performance. Moreover, the flexible cable Zn-air batteries show excellent stability under different deformation conditions. The proposed concept of constructing scalable, all-in-one, freestanding, and flexible air electrodes would pave the way to develop next-generation wearable and portable energy-storage devices.

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.

Lithium Electrochemical Tuning for Electrocatalysis

Electrocatalysis is of great importance to a variety of energy conversion processes, where developing highly efficient catalysts is critical. While common strategies involve screening a wide range of materials with new chemical compositions or structures, a different approach to continuously, controllably, and effectively tune the electronic properties of existing catalytic materials for optimized activities has been demonstrated recently. Inspired by studies in lithium-ion batteries, systematical lithium electrochemical tuning (LiET) methods such as Li intercalation, extraction, cycling, and strain engineering, are employed to effectively tune the electronic structures of different existing catalysts and thus improve their catalytic activities dramatically. Herein, the advantages of the LiET method in electrocatalysis are introduced, and then some recent representative examples in improving the performances of important electrochemical reactions are reviewed briefly. Lastly, a few promising directions on extending the applications of the LiET method in electrocatalysis are proposed.

Robust N-doped carbon aerogels strongly coupled with iron-cobalt particles as efficient bifunctional catalysts for rechargeable Zn-air batteries

The rational design of highly-active and stable reversible oxygen electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) plays a key role in rechargeable metal-air batteries, yet remains a great challenge. Herein, a novel dual-crosslinked hydrogel strategy is proposed to synthesize a new type of carbon aerogel that anchors the iron-cobalt (FeCo) particles as a bifunctional oxygen catalyst. The proposed hydrogel composed of an organic/inorganic network can be easily obtained by initiating sol-gel polymerization of cyanometalates, chitosan and graphene oxide. After pyrolysis, FeCo nanocrystals can be in situ uniformly immobilized within the N-doped "dual-network" carbon aerogels (FeCo/N-DNC) with a robust 3D porous framework. When used as an electrocatalyst, the newly developed FeCo/N-DNC aerogels exhibit a positive onset potential (0.89 V) and half-wave potential (0.81 V) for the ORR and a low overpotential (0.39 V) at 10 mA cm-2 for the OER, while presenting excellent electrochemical stability after being tested for 10 000 s. More importantly, the FeCo/N-DNC driven Zn-air battery reveals a smaller charge/discharge voltage gap, higher power/energy density and better cycling stability than the costlier Pt/C + RuO2 mixture catalyst. Our findings provide a facile and feasible synthetic strategy for obtaining highly active and stable electrocatalysts.