Nature‐Inspired Electrochemical Energy‐Storage Materials and Devices

Currently, tremendous efforts are being devoted to develop high‐performance electrochemical energy‐storage materials and devices. Conventional electrochemical energy‐storage systems are confronted with great challenges to achieve high energy density, long cycle‐life, excellent biocompatibility and environmental friendliness. The biological energy metabolism and storage systems have appealing merits of high efficiency, sophisticated regulation, clean and renewability, and the rational design and fabrication of advanced electrochemical energy‐storage materials and smart devices inspired by nature have made some breakthrough progresses, recently. In this review, we summarize the latest developments in the field of nature‐inspired electrochemical energy‐storage materials and devices. Specifically, the nature‐inspired exploration, preparation and modification of electrochemical energy‐storage related materials including the active materials, binders, and separators are introduced. Furthermore, nature‐inspired design and fabrication of smart energy‐storage devices such as self‐healing supercapacitors, supercapacitors with ultrahigh operating voltage, and self‐rechargeable batteries are also discussed. The review aims to provide insights and expanded research perspectives for further study in this exciting field based on our comprehensive discussions.     

2D Materials for Large‐Area Flexible Thermoelectric Devices

The rapid development of the concept of the “Internet of Things (IoT)” requires wearable devices with maintenance‐free batteries, and thermoelectric energy conversion based on large‐area flexible materials has attracted much attention. Among large‐area flexible materials, 2D materials, such as graphene and related materials, are promising for thermoelectric applications due to their excellent transport properties and large power factors. In this Review, both single‐crystalline and polycrystalline 2D materials are surveyed using the experimental reports on thermoelectric devices of graphene, black phosphorus, transition metal dichalcogenides, and other 2D materials. In particular, their carrier‐density dependent thermoelectric properties and power factors maximized by Fermi level tuning techniques are focused. The comparison of the relevant performances between 2D materials and commonly used thermoelectric materials reveals the significantly enhanced power factors in 2D materials. Moreover, the current progress in thermoelectric module applications using large‐area 2D material thin films is summarized, which consequently offers great potential for the use of 2D materials in large‐area flexible thermoelectric device applications. Finally, important remaining issues and future perspectives, such as preparation methods, thermal transports, device designs, and promising effects in 2D materials, are discussed.     

Unprecedented Synthesis of Holey 2D Layered Double Hydroxide Nanomesh for Enhanced Oxygen Evolution

Creating nanosized pores in 2D materials can increase the edge sites, improve the mass transfer, and contribute to different physical properties, which shows potential applications in many fields including filtration membranes, electronics and energy storage devices, and catalysts. An iconic member of this type of material is porous graphene. Herein, a unique 2D layered double hydroxide (LDH) nanomesh is designed and synthesized as a new class of 2D holey materials. It represents the first case of exfoliation method for preparing 2D holey materials among all published reports. As an oxygen evolution reaction electrocatalyst, the 2D CoCo‐LDH nanomesh has apparently lower onset overpotential (220 mV) than that of compact nanosheets without holes (270 mV) owing to the pores through the plane that offer more highly active edge sites with lower coordination number and promote the mass diffusion. This work opens up a new avenue for designing 2D porous materials for energy conversion and storage.     

Recent Development of Metallic (1T) Phase of Molybdenum Disulfide for Energy Conversion and Storage

The development of a feasible and inexpensive strategy to obtain and utilize sustainable energy is an important issue for the sustainable development of human society. Over the past decade, significant progress has been made in the development of novel functional materials for energy conversion and storage. Owing to their unique physico‐chemical properties, 2D layered materials, such as graphene and transition metal dichalcogenides, have attracted great interest in energy‐related research. 1T‐MoS₂ is a metallic phase of molybdenum disulfide (MoS₂) with extraordinary electronic conductivity, enlarged interlayer spacing, and more electrochemically active sites along the basal plane, which offers intriguing benefits for energy‐related applications compared to its semiconducting counterpart (2H‐MoS₂). This review summarizes the preparation and structure–property relationships of 1T‐MoS₂, as well as the underlying relations between the metallic (1T) and semiconducting (2H) phases of MoS₂. Recent progress in the preparation and stabilization of 1T‐MoS₂ materials and their applications for energy conversion and storage are discussed, including water splitting to form hydrogen via photo/electrocatalysis and electricity storage in lithium‐ion batteries, sodium‐ion batteries, magnesium‐ion batteries, and supercapacitors. Optimization strategies of 1T‐MoS₂ to obtain enhanced practical properties based on theoretical calculations are also presented.     

3D Nanostructures for the Next Generation of High‐Performance Nanodevices for Electrochemical Energy Conversion and Storage

Among the different nanostructures that have been demonstrated as promising materials for various applications, 3D nanostructures have attracted significant attention as building blocks for constructing high‐performance nanodevices. Particularly over the last decade, considerable research efforts have been devoted to designing, fabricating, and evaluating 3D nanostructures as electrodes for electrochemical energy conversion and storage devices. Although remarkable progress has been achieved, the performance of electrochemical energy devices based on 3D nanostructures in terms of energy conversion efficiency, energy storage capability, and device reliability still needs to be significantly improved to meet the requirements for practical applications. Rather than simply outlining and comparing different 3D nanostructures, this article systematically summarizes the general advantages as well as the existing and future challenges of 3D nanostructures for electrochemical energy conversion and storage, focusing on photoelectrochemical water splitting, photoelectrocatalytic solar‐to‐fuels conversion from nitrogen and carbon dioxide, rechargeable metal‐ion batteries, and supercapacitors. A comprehensive understanding of these advantages and challenges shall provide valuable guidelines and enlightenments to facilitate the further development of 3D nanostructured materials, and contribute to the achieving more efficient energy conversion and storage technologies toward a sustainable energy future.     

Applications of Phosphorene and Black Phosphorus in Energy Conversion and Storage Devices

The successful isolation of phosphorene (atomic layer thick black phosphorus) in 2014 has currently aroused the interest of 2D material researchers. In this review, first, the fundamentals of phosphorus allotropes, phosphorene, and black phosphorus, are briefly introduced, along with their structures, properties, and synthesis methods. Second, the readers are presented with an overview of their energy applications. Particularly in electrochemical energy storage, the large interlayer spacing (0.53 nm) in phosphorene allows the intercalation/deintercalation of larger ions as compared to its graphene counterpart. Therefore, phosphorene may possess greater potential for high electrochemical performance. In addition, the status of lithium ion batteries as well as secondary sodium ion batteries is reviewed. Next, each application for energy generation, conversion, and storage is described in detail with milestones as well as the challenges. These emerging applications include supercapacitors, photovoltaic devices, water splitting, photocatalytic hydrogenation, oxygen evolution, and thermoelectric generators. Finally the fast‐growing dynamic field of phosphorene research is summarized and perspectives on future possibilities are presented calling on the efforts of chemists, physicists, and material scientists     

Highly Efficient Materials Assembly Via Electrophoretic Deposition for Electrochemical Energy Conversion and Storage Devices

Featuring pronounced controllability, versatility, and scalability, electrophoretic deposition (EPD) has been proposed as an efficient method for film assembly and electrode/solid electrolyte fabrication in various energy storage/conversion devices including rechargeable batteries, supercapacitors, and fuel cells. High‐quality electrodes and solid electrolytes have been prepared through EPD and exhibit advantageous performances in comparison with those realized with traditional methods. Recent advances in the application of EPD materials in electrochemical energy storage and conversion devices are summarized. In particular, the parameters that influence the efficiency of an EPD process from colloidal preparation to deposition are evaluated with the aim to provide insightful guidance for realizing high‐performance electrochemical energy conversion materials and devices.     

Fiber‐Based Thermoelectric Generators: Materials,Device Structures,Fabrication, Characterization,and Applications

Fiber‐based flexible thermoelectric energy generators are 3D deformable, lightweight, and desirable for applications in large‐area waste heat recovery, and as energy suppliers for wearable or mobile electronic systems in which large mechanical deformations, high energy conversion efficiency, and electrical stability are greatly demanded. These devices can be manufactured at low or room temperature under ambient conditions by established industrial processes, offering cost‐effective and reliable products in mass quantity. This article presents a critical overview and review of state‐of‐the‐art fiber‐based thermoelectric generators, covering their operational principle, materials, device structures, fabrication methods, characterization, and potential applications. Scientific and practical challenges along with critical issues and opportunities are also discussed.     

Cellulose‐based Supercapacitors: Material and Performance Considerations

One of the biggest challenges we will face over the next few decades is finding a way to power the future while maintaining strong socioeconomic growth and a clean environment. A transition from the use of fossil fuels to renewable energy sources is expected. Cellulose, the most abundant natural biopolymer on earth, is a unique, sustainable, functional material with exciting properties: it is low‐cost and has hierarchical fibrous structures, a high surface area, thermal stability, hydrophilicity, biocompatibility, and mechanical flexibility, which makes it ideal for use in sustainable, flexible energy storage devices. This review focuses on energy storage applications involving different forms of cellulose (i.e., cellulose microfibers, nanocellulose fibers, and cellulose nanocrystals) in supercapacitors, with particular emphasis on new trends and performance considerations relevant to these fields. Recent advances and approaches to obtaining high capacity devices are evaluated and the limitations of cellulose‐based systems are discussed. For the first time, a combination of device‐specific factors such as electrode structures, mass loadings, areal capacities, and volumetric properties are taken into account, so as to evaluate and compare the energy storage performance and to better assess the merits of cellulose‐based materials with respect to real applications.     

Advanced Composite 2D Energy Materials by Simultaneous Anodic and Cathodic Exfoliation

Composite materials based on graphene and other 2D materials are of considerable interest in the fields of catalysis, electronics, and energy conversion and storage because of the unique structural features and electronic properties of each component and the synergetic effects brought about by the compositing. Approaches to the mass production of 2D materials and their composites in a facile and affordable way are urgently needed to enable their implementation in practical applications. Here a novel electrochemical exfoliation approach to prepare 2D composites is proposed, which combines simultaneous anodic exfoliation of graphite and cathodic exfoliation of other 2D materials (namely MoS₂, MnO₂, and graphitic carbon nitride). The synthesis is carried out in a single‐compartment electrochemical cell to in situ produce functional 2D composite materials. Applications of the as‐prepared 2D composites are demonstrated as (i) effective hydrogen evolution catalysts and (ii) supercapacitor electrode materials. The method enables the compositing of semiconductive, or even insulating, 2D materials with conductive graphene in an easy, cheap, ecofriendly, yet efficient way, liberating the intrinsic functions of 2D materials, which are usually hindered by their poor conductivity. The method is believed to be widely applicable to the family of 2D materials.     

Metal‐Air Batteries: Metal–Air Batteries with High Energy Density: Li–Air versus Zn–Air (Adv. Energy Mater. 1/2011)

In the past decade, there have been exciting developments in the field of lithium ion batteries as energy storage devices, resulting in the application of lithium ion batteries in areas ranging from small portable electric devices to large power systems such as hybrid electric vehicles. However, the maximum energy density of current lithium ion batteries having topatactic chemistry is not sufficient to meet the demands of new markets in such areas as electric vehicles. Therefore, new electrochemical systems with higher energy densities are being sought, and metal‐air batteries with conversion chemistry are considered a promising candidate. More recently, promising electrochemical performance has driven much research interest in Li‐air and Zn‐air batteries. This review provides an overview of the fundamentals and recent progress in the area of Li‐air and Zn‐air batteries, with the aim of providing a better understanding of the new electrochemical systems.     

Metal–Air Batteries with High Energy Density: Li–Air versus Zn–Air

In the past decade, there have been exciting developments in the field of lithium ion batteries as energy storage devices, resulting in the application of lithium ion batteries in areas ranging from small portable electric devices to large power systems such as hybrid electric vehicles. However, the maximum energy density of current lithium ion batteries having topatactic chemistry is not sufficient to meet the demands of new markets in such areas as electric vehicles. Therefore, new electrochemical systems with higher energy densities are being sought, and metal‐air batteries with conversion chemistry are considered a promising candidate. More recently, promising electrochemical performance has driven much research interest in Li‐air and Zn‐air batteries. This review provides an overview of the fundamentals and recent progress in the area of Li‐air and Zn‐air batteries, with the aim of providing a better understanding of the new electrochemical systems.     

High‐Performance GeTe‐Based Thermoelectrics: from Materials to Devices

High‐performance GeTe‐based thermoelectrics have been recently attracting growing research interest. Here, an overview is presented of the structural and electronic band characteristics of GeTe. Intrinsically, compared to low‐temperature rhombohedral GeTe, the high‐symmetry and high‐temperature cubic GeTe has a low energy offset between L and Σ points of the valence band, the reduced direct bandgap and phonon group velocity, and as a result, high thermoelectric performance. Moreover, their thermoelectric performance can be effectively enhanced through either carrier concentration optimization, band structure engineering (bandgap reduction, band degeneracy, and resonant state engineering), or restrained lattice thermal conductivity (phonon velocity reduction or phonon scattering). Consequently, the dimensionless figure of merit, ZT values, of GeTe‐based thermoelectric materials can be higher than 2. The mechanical and thermal stabilities of GeTe‐based thermoelectrics are highlighted, and it is found that they are suitable for practical thermoelectric applications except for their high cost. Finally, it is recognized that the performance of GeTe‐based materials can be further enhanced through synergistic effects. Additionally, proper material selection and module design can further boost the energy conversion efficiency of GeTe‐based thermoelectrics.     

Holey 2D Nanomaterials for Electrochemical Energy Storage

2D nanomaterials provide numerous fascinating properties, such as abundant active surfaces and open ion diffusion channels, which enable fast transport and storage of lithium ions and beyond. However, decreased active surfaces, prolonged ion transport pathway, and sluggish ion transport kinetics caused by self‐restacking of 2D nanomaterials during electrode assembly remain a major challenge to build high‐performance energy storage devices with simultaneously maximized energy and power density as well as long cycle life. To address the above challenge, porosity (or hole) engineering in 2D nanomaterials has become a promising strategy to enable porous 2D nanomaterials with synergetic features combining both 2D nanomaterials and porous architectures. Herein, recent important progress on porous/holey 2D nanomaterials for electrochemical energy storage is reviewed, starting with the introduction of synthetic strategies of porous/holey 2D nanomaterials, followed by critical discussion of design rule and their advantageous features. Thereafter, representative work on porous/holey 2D nanomaterials for electrochemical capacitors, lithium‐ion and sodium‐ion batteries, and other emerging battery technologies (lithium‐sulfur and metal‐air batteries) are presented. The article concludes with perspectives on the future directions for porous/holey 2D nanomaterial in energy storage and conversion applications.     

Graphene‐Based Nanocomposites for Energy Storage

Since the first report of using micromechanical cleavage method to produce graphene sheets in 2004, graphene/graphene‐based nanocomposites have attracted wide attention both for fundamental aspects as well as applications in advanced energy storage and conversion systems. In comparison to other materials, graphene‐based nanostructured materials have unique 2D structure, high electronic mobility, exceptional electronic and thermal conductivities, excellent optical transmittance, good mechanical strength, and ultrahigh surface area. Therefore, they are considered as attractive materials for hydrogen (H₂) storage and high‐performance electrochemical energy storage devices, such as supercapacitors, rechargeable lithium (Li)‐ion batteries, Li–sulfur batteries, Li–air batteries, sodium (Na)‐ion batteries, Na–air batteries, zinc (Zn)–air batteries, and vanadium redox flow batteries (VRFB), etc., as they can improve the efficiency, capacity, gravimetric energy/power densities, and cycle life of these energy storage devices. In this article, recent progress reported on the synthesis and fabrication of graphene nanocomposite materials for applications in these aforementioned various energy storage systems is reviewed. Importantly, the prospects and future challenges in both scalable manufacturing and more energy storage‐related applications are discussed.     

Emerging 3D‐Printed Electrochemical Energy Storage Devices: A Critical Review

Three‐dimensional (3D) printing, a layer‐by‐layer deposition technology, has a revolutionary role in a broad range of applications. As an emerging advanced fabrication technology, it has drawn growing interest in the field of electrochemical energy storage because of its inherent advantages including the freeform construction and controllable 3D structural prototyping. This article focuses on the topic of 3D‐printed electrochemical energy storage devices (EESDs), which bridge advanced electrochemical energy storage and future additive manufacturing. Basic 3D printing systems and material considerations are described to provide a fundamental understanding of printing technologies for the fabrication of EESDs. The performance metrics of 3D‐printed EESDs are then given and the related performance optimization strategies are discussed. Next, the recent advances of 3D‐printed EESDs, including sandwich‐type and in‐plane architectures, are summarized. Conclusions and future perspectives with some unique challenges and important directions are then discussed. It can be expected that, with the help of 3D printing technology, the development of advanced electrochemical energy storage systems will be greatly promoted.     

Nanostructured p‐n Junctions for Kinetic‐to‐Electrical Energy Conversion

Piezoelectric ZnO nanorods grown on a flexible substrate are combined with the p‐type semiconducting polymer PEDOT:PSS to produce a p‐n junction device that successfully demonstrates kinetic‐to‐electrical energy conversion. Both the voltage and current output of the devices are measured to be in the range of 10 mV and 10 μA cm^?2. Combining these figures for the best device gives a maximum possible power density of 0.4 mW cm^?3. Systematic testing of the devices is performed showing that the voltage output increases linearly with applied stress, and is reduced significantly by illumination with super‐band gap light. This provides strong evidence that the voltage output results from piezoelectric effects in the ZnO. The behavior of the devices is explained by considering the time‐dependent changes in band structure resulting from the straining of a piezoelectric material within a p‐n junction. It is shown that the rate of screening of the depolarisation field determines the power output of a piezoelectric energy harvesting device. This model is consistent with the behavior of a number of previous devices utilising the piezoelectric effect in ZnO.     

Highly Conductive,Lightweight, Low‐Tortuosity Carbon Frameworks as Ultrathick 3D Current Collectors

The growing demand for advanced energy storage techniques and devices has driven the energy storage market to strive for higher performance, longer cycling life, and better safety. Thick electrode design enabling more electroactive materials has the potential to significantly improve the energy density on device level yet faces major challenges of slow ion transport and high deformability. Here, inspired by natural wood materials with aligned channels along the tree growth direction, a highly conductive, lightweight, and low‐tortuosity carbon framework (CF) directly carbonized from natural wood as an ultrathick 3D current collector is demonstrated. Benefiting from the uniqueness of the multichanneled CF, an ultrathick 3D electrode of lithium iron phosphate filled carbon framework with a large thickness of 800 µm and active material mass loading of 60 mg cm^?2 delivers a rational capacity of 7.6 mAh cm^?2 (95 Ah L^?1 based on volume), long cycling life, and lower deformability with enhanced mechanical properties. This work presents a design concept for thick electrode toward high performance energy storage devices that are not limited to lithium‐ion batteries.     

Molten‐NaNH2 Densified Graphene with In‐Plane Nanopores and N‐Doping for Compact Capacitive Energy Storage

Capacitive carbons are attractive for energy storage on account of their superior rate and cycling performance over traditional battery materials, but they usually suffer from a far lower volumetric energy density. Starting with expanded graphene, a simple, multifunctional molten sodium amide treatment for the preparation of high‐density graphene with high capacitive performance in both aqueous and lithium battery electrolytes is reported. The molten sodium amide can condense the expanded graphene, lead to nitrogen doping and, what is more important, create moderate in‐plane nanopores on graphene to serve as ion access shortcuts in dense graphene stacks. The resulting high‐density graphene electrode can deliver a volumetric capacitance of 522 F cm^?3 in a potassium hydroxide electrolyte; and in a lithium‐ion battery electrolyte, it exhibits a gravimetric and volumetric energy density of 618 W h kg^?1 and 740 W h L^?1, respectively, and even outperforms commercial LiFePO₄.     

Partially Reduced Holey Graphene Oxide as High Performance Anode for Sodium‐Ion Batteries

The current Na⁺ storage performance of carbon‐based materials is still hindered by the sluggish Na⁺ ion transfer kinetics and low capacity. Graphene and its derivatives have been widely investigated as electrode materials in energy storage and conversion systems. However, as anode materials for sodium‐ion batteries (SIBs), the severe π–π restacking of graphene sheets usually results in compact structure with a small interlayer distance and a long ion transfer distance, thus leading to low capacity and poor rate capability. Herein, partially reduced holey graphene oxide is prepared by simple H₂O₂ treatment and subsequent low temperature reduction of graphene oxide, leading to large interlayer distance (0.434 nm), fast ion transport, and larger Na⁺ storage space. The partially remaining oxygenous groups can also contribute to the capacity by redox reaction. As anode material for SIBs, the optimized electrode delivers high reversible capacity, high rate capability (365 and 131 mAh g^?1 at 0.1 and 10 A g^?1, respectively), and good cycling performance (163 mAh g^?1 after 3000 cycles at a current density of 2 A g^?1), which is among the best reported performances for carbon‐based SIB anodes.