Energy Materials

Solid-state batteries (SSBs) based on inorganic solid electrolytes (ISEs) are considered promising candidates for enhancing the energy density and the safety of next-generation rechargeable lithium batteries. However, their practical application is frequently hampered by the high resistance arising at the Li metal anode/ISE interface. Herein, a review of the conventional solid-state electrolytes (SSEs) the recent research on quasi-solid-state battery (QSSB) approaches to overcome the issues of the state-of-the-art SSBs is reported. The feasibility of ionic liquid (IL)-based interlayers to improve ISE/Li metal wetting and enhance charge transfer at solid electrolyte interfaces with both positive and lithium metal electrodes is presented together with a novel generation of IL-containing quasi-solid-state-electrolytes (QSSEs), offering favourable features. The opportunities and challenges of QSSE for the development of high energy and high safety quasi-solid-state lithium metal batteries (QSSLMBs) are also discussed.

Heteroatom-doped carbon materials have high gravimetric potassium-ion storage capability because of their abundant active sites and defects. However, their practical applications toward potassium storage are limited by sluggish reaction kinetics and short cycling life owing to the large ionic radius of K+ and undesirable parasitic reactions. Herein, we report a new strategy that allows for bottom-up patterning of thin N/P co-doped carbon layers with a uniform mesoporous structure on two-dimensional graphene sheets. The highly porous architecture and N/P co-doping properties provide abundant active sites for K+, and the graphene sheets promote charge/electron transfer. This synergistic structure enables excellent K+ storage performance in terms of specific capacity (387.6 mAh g-1 at 0.5 A g-1), rate capability (over 5 A g-1), and cycling stability (70% after 3000 cycles). As a proof of concept, a potassium-ion capacitor assembled using this carbon anode yields a high energy density of 107 Wh kg-1, a maximum power densities of 18.3 kW kg-1, and ultra-long cycling stability over 40000 cycles.

Phase change materials (PCMs) are considered one of the most promising energy storage methods owing to their beneficial effects on a larger latent heat, smaller volume change, and easier controlling than other materials. PCMs are widely used in solar energy heating, industrial waste heat utilization, energy conservation in the construction industry, and other fields. To avoid leakage, phase separation, and volatile problems of PCMs, the encapsulation technique typically uses organic polymer materials as shell structures of microcapsules. Furthermore, using inorganic materials to enhance the thermal property of phase change microcapsules is a popular approach in recent research. Especially, graphene oxide (GO) with high thermal conductivity was used as a common thermal conducting additive to improve the thermal performance of phase change microcapsules. Due to its amphiphilic property, GO combined with PCM microcapsules can achieve a variety of nanostructures for thermal energy storage. In this paper, four aspects have been summarized: configuration of PCMs, methods of combining GO with phase change microcapsules, position and content of GO, and applications of PCM/GO microcapsules. This work attempts to discuss preparation methods and heat-conducting properties of the PCM/GO microcapsules, which helps to better promote the application-targeted design and greatly improve the thermal properties of PCM microcapsules for various applications.

The application of Li-S batteries (LSBs) is hindered by the undesired shuttle effect that leads to the fast consumption of active materials. The separator modification by using the carbon matrix with embedded metal nitride as catalyst can ease the problem. However, the previous synthesis processes of metal nitride catalysts are difficult to achieve a balance between their high-density production, homogenous distribution and excellent electronic contact with conductive substrates. Herein, we propose a bond scissoring strategy based on g-C3N4 to prepare NbN catalyst domains with high-density loading uniformly embedded in mesoporous thin-layer conductive carbon network (NbN/C) for durable LSBs. The molten salt reaction process is favorable for the diffusion of Nb cations into a porous g-C3N4 precursor to break the C-N bond and immobilize the N element. The residual monolithic carbon framework with space confinement effect limits the irregular growth and stacking of NbN precipitates. The NbN catalytic domains exhibit a strong adsorption effect on lithium polysulfides (LiPSs) and accelerate their liquid-solid conversion reactions. The LSBs utilizing an NbN/C-modified separator show superior cycling and rate performance, with a high-capacity retention of 72.7% after 1,000 cycles under 2 C and a high areal capacity of ~7.08 mA h cm-2 under a high sulfur loading of 6.6 mg cm-2. This g-C3N4-assisted strategy opens a new gate for the design of an integrated catalysis-conduction network for high-performance LSBs.

The rapid development of electronic technology and energy industry promotes the increasing desire for energy storage systems with high energy density, thus calling for the exploration of lithium metal anode. However, the enormous challenges, such as uncontrollable lithium deposition, side reaction, infinite volume change and dendrite generation, hinders its application. To address these problems, the deposition behavior of lithium must be exactly controlled and the anode/electrolyte interface must be stabilized. The deposition of lithium is a multi-step process influenced by multi-physical fields, where nucleation is the key to final morphology. Hence, increasing investigations have focused on the employment of lithiophilic materials that can regulate lithium nucleation in recent years. The lithiophilic materials introduced into the deposition hosts or solid electrolyte interphases can regulate the nucleation overpotential and facilitate uniform deposition. However, the concept of lithiophilicity is still undefined and the mechanism is still unrevealed. In this review, the recent advances in the regulation mechanisms of lithiophilicity are discussed, and the applications of lithiophilic materials in hosts and protective interphases are summarized. The in-depth exploration of lithiophilic materials can enhance our understanding of the deposition behavior of lithium and pave the way for practical lithium metal batteries.

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) for dual-functional non-precious metal electrocatalysts are promising alternatives for Pt/Ru-based materials in rechargeable zinc-air batteries (ZABs). However, how to achieve dual-functional oxygen electrocatalytic activity on single-component catalysts and identify the sites responsible for ORR and OER still face many challenges. Herein, an efficient and stable dual-functional electrocatalyst is fabricated by a two-step hydrothermal method with iron phthalocyanine (FePc) π-π stacking on nickel-iron selenide layered hydroxide derivatives (Se/Ni3Se4/Fe3O4). The as-prepared multi-component catalyst (named as FePc/Se@NiFe) exhibits better oxygen electrocatalytic properties than Pt/Ru-based catalysts, with a half-wave potential (E1/2) of 0.90 V and an overpotential of 10 mA cm-2 (Ej10) of 320 mV. More importantly, chronoamperometry (I-T) and accelerated durability tests (ADT) show the unordinary stability of the catalyst. Both physical characterization and experimental results verify that the Fe-N4 moieties and Ni3Se4 crystalline phase are the main active sites for ORR and OER activities, respectively. The small potential gap (ΔE = Ej10 - E1/2 = 0.622 V) represents superior dual-functional activities of the FePc/Se@NiFe catalyst. Subsequently, the ZABs assembled using FePc/Se@NiFe exhibit excellent performances. This study offers a promising design concept for promoting further development of high-performance ORR and OER electrocatalysts and their application in ZAB.

Research on lithium metal as a high-capacity anode for future lithium metal batteries (LMBs) is currently at an all-time high. To date, the different influences of a highly pure argon glovebox (GB) and an industry-relevant ambient dry room (DR) atmosphere have received little attention in the scientific community. In this paper, we report on the impact of in coin cell atmosphere (ICCA) on the performance of an LMB as well as its interphase characteristics and properties in combination with three organic carbonate-based electrolytes with and without two well-known interphase-forming additives, namely fluoroethylene carbonate (FEC) and vinylene carbonate (VC). The results obtained from this carefully executed systematic study show a substantial impact of the ICCA on solid electrolyte interphase (SEI) resistance (RSEI) and lithium stripping/plating homogeneity. In a transition metal cathode (NMC811) containing LMBs, a DR ICCA results in an up to 50% increase in lifetime due to the improved chemical composition of the cathode electrolyte interphase (CEI). Furthermore, different impacts on electrode characteristics and cell performance were observed depending on the utilized functional additive. Since this study focuses on a largely overlooked influential factor of LMB performance, it highlights the importance of comparability and transparency in published research and the importance of taking differences between research and industrial environments into consideration in the aim of establishing and commercializing LMB cell components.

The energy density of lithium-ion batteries based on intercalated electrode materials has reached its upper limit, which makes it challenging to meet the growing demand for high-energy storage systems. Electrode materials based on conversion reactions such as sulfur, organosulfides, and oxygen involving breakage and reformation of chemical bonds can provide higher specific capacity and energy density. In addition, they usually consist of abundant elements, making them renewable. Although they have the aforementioned benefits, they face numerous challenges for practical applications. For example, the cycled products of sulfur and molecular organosulfides could be soluble in a liquid electrolyte, resulting in the shuttle effect and significant capacity loss. The discharged product of oxygen is Li2O2, which could result in high charge overpotential and decomposition of the electrolyte. In this review, we present an overview of the current strategies for improving the performances of lithium-sulfur, lithium-organosulfide, and lithium-oxygen batteries. First, we summarize the efforts to overcome the issues facing sulfur and organosulfide cathodes, as well as the strategies to increase the capacity of organosulfides. Then, we introduce the latest research progress on catalysts in lithium-oxygen batteries. Finally, we summarize and provide outlooks for the conversion of electrode materials.

Single-atom catalysts (SACs) with high activity, unique selectivity, and nearly 100% atom utilization efficiency are promising for broad applications in many fields. This review aims to provide a summary of the current development of SACs and point out their challenges and opportunities for commercial applications in the energy process. The discussion starts with an introduction of various types of SACs materials, followed by typical SACs synthetic methods with concrete examples and commonly used characterization methods. The state-of-the-art synthesis methods, whereby SACs with stabilized single metal atoms on the substrate without migration and agglomeration could be obtained, are emphasized. Next, we give an overview of different types of substrates and discuss the effects of substrate species on the structure and properties of SACs. Then we highlight the typical applications of SACs and the remaining challenges. Finally, a perspective on the opportunities for the development of SACs for future commercial applications is provided.

The lithium-sulfur (Li-S) battery has been attracting much more attention in recent years due to its high theoretical capacity and low cost, although various issues, such as the “shuttle effect” and the low use ratio of active materials, have been hindering the development and application of Li-S batteries. The separator is an important part of Li-S batteries, and its modification is a simple and effective strategy to improve the electrochemical performance of Li-S batteries. In this work, we explore separators with different functions on their two sides that have been produced by a step-by-step electrospinning method. The multifunctional separator on one side is pure gelatin, and the other side is zeolitic imidazolate framework-67 (ZIF-67)-C60-gelatin. The ZIF-67-C60-gelatin layer on the cathode side is of great importance. The chemisorption sites on it are provided by ZIF-67, and the transformation sites of lithium polysulfide are provided by C60. Gelatin, which is on the anode side, as an admirable separator material, makes the lithium flux uniform and thus prevents the generation of lithium dendrites. This type of multifunctional nanofiber separator based on double gelatin layers plays an important role in the adsorption and conversion of polysulfides, and it improves the overall performance of the Li-S battery. As a result, the Li-S batteries assembled with the prepared separator can still maintain the capacity of 888 mAh g-1 after 100 cycles at 0.2 C, and the capacity retention rate of the Li-S batteries is 72.9% after 400 cycles at 2 C. This simple preparation method and high-performance bilayer membrane structure provide a new route for commercial application.

Although Ni-rich layered materials with the general formula LiNi1-x-yCoxMnyO2 (0 < x, y < 1, NCM) hold great promise as high-energy-density cathodes in commercial lithium-ion batteries, their practical application is greatly hampered by poor cyclability and safety. Herein, a LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode modified with a surface self-assembling LiLaO2 coating and subsurface La pillars demonstrates stabilized cycling at 4.6 V. The LiLaO2-coated NCM622 benefits from the suppression of interfacial side reactions, which relieves the layer-to-rock salt phase transformation and therefore improves the capacity retention under high voltages. Moreover, the La dopant, as a pillar in the NCM622 lattice, plays a dual role in expanding the c lattice parameter to enhance the Li-ion diffusion capability, as well as suppressing Ni antisite defect formation upon cycling. Consequently, the dual-modified NCM622 cathode exhibits an initial Coulombic efficiency of over 85% and a high capacity of over 200 mAh g-1 at 0.1 C. A specific capacity of 188 mAh g-1 with a capacity retention of 76% is achieved at 1 C after 200 cycles within a voltage range of 3.0-4.6 V. These findings lay a solid foundation for the materials design and performance optimization of high-energy-density cathodes for Li-ion batteries.

The exploration of solid polymer-based composite electrolytes (SCPEs) that possess good safety, easy processability, and high ionic conductivity is of great significance for the development of advanced all-solid-state lithium-metal batteries (ASSLMBs). However, the poor interfacial compatibility between the electrode and solid electrolyte leads to a large interfacial impedance that weakens the electrochemical performance of the battery. Herein, an interpenetrating network polycarbonate (INPC)-based composite electrolyte is constructed via the in-situ polymerization of butyl acrylate, Li7La3Zr2O12 (LLZO), Lithium bis(trifluoromethanesulphonyl)imide, succinonitrile and 2,2-azobisisobutyronitrile on the base of a symmetric polycarbonate monomer. Benefiting from the synergistic effect of each component and the unique structure features, the INPC&LLZO-SCPE can effectively integrate the merits of the polymer and inorganic electrolytes and deliver superior ionic conductivity (3.56 × 10-4S cm-1 at 25 °C), an impressive Li+ transference number [t(Li+) = 0.52] and a high electrochemical stability window (up to 5.0 V vs. Li+/Li). Based on this, full batteries of LiFePO4/INPC&LLZO-SCPE/Li and LiNi0.6Co0.2Mn0.2O2/INPC&LLZO-SCPE/Li are assembled, which exhibit large initial capacities of 156.3 and 158.9 mAh g-1 and high capacity retention of 86.8% and 95.4% over 500 and 100 cycles at 0.2 and 0.1 C, respectively. This work offers a new route for the construction of novel polycarbonate-based composite electrolytes for high-voltage ASSLMBs.

Based on density functional theory, a new two-dimensional boron nitride, Pmma BN, is proposed and studied in detail for the first time. The stability of Pmma BN is demonstrated by phonon spectra, ab initio molecular dynamics simulations at 300 and 500 K, and in-plane elastic constants. The orientation dependences of the Young’s modulus and Poisson’s ratio show that Pmma BN has large mechanical anisotropy. Pmma BN is an indirect band gap semiconductor material with a band gap of 5.15 eV, and the hole and electron effective mass have high anisotropy. The electron carrier mobility of Pmma BN along the x and y directions is similar, while the hole carrier mobility along the y direction is more than twice that along the x direction. By studying the effect of uniaxial tensile strain on Pmma BN, the band gap of Pmma BN remains indirect under the uniaxial strain, and its adjustable range reaches 0.64 eV at uniaxial strain along the x direction. When uniaxial strain is applied along the y direction, the positions of the conduction band minimum and valence band maximum change. Pmma BN under uniaxial strain show strong optical absorption capacity in the ultraviolet region. To explore clean energy applications, the thermoelectric properties are also investigated.

A major challenge in developing zinc-air batteries (ZABs) is to exploit suitable cathodes to efficiently accelerate the key electrocatalytic processes involved. Herein, a bifunctional oxygen catalytic self-supported MnO2-based electrode is designed that displays superior oxygen reduction and evolution reaction performance over noble metal electrodes with a total overpotential of 0.69 V. In addition, the as-synthesized NiCo2O4@MnO2/carbon nanotube (CNT)-Ni foam self-supported electrode can be directly used as an oxygen electrode without externally adding carbon or a binder and shows reasonable battery performance with a high peak power density of 226 mW cm-2 and a long-term charge-discharge cycling lifetime (5 mA for 160 h). As expected, the rapid oxygen catalytic intrinsic kinetics and high battery performance of the NiCo2O4@MnO2/CNTs-Ni foam electrode originates from the unique three-dimensional hierarchical structure, which effectively promotes mass transfer. Furthermore, the CNTs combined with Ni foam form a unique “meridian” conductive structure that enables rapid electron conduction. Finally, the abundant Mn3+ active sites activated by bimetallic ions shorten the oxygen catalytic reaction distance between the active sites and reactant and reduce the surface activity of MnO2 for the O, OH, and OOH species. This work not only offers a high-performance bifunctional self-supported electrode for ZABs but also opens new insights into the activation of Mn-based electrodes.

Alloying materials (e.g., Si, Ge, Sn, Sb, and so on) are promising anode materials for next-generation lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to their high capacity, suitable working voltage, earth abundance, environmental friendliness, and non-toxicity. Although some important breakthroughs have been reported recently for these materials, their dramatic volume change during alloying/dealloying causes severe pulverization, leading to poor cycling stability and safety risks. Although the nanoengineering of alloys can mitigate the volumetric expansion to some extent, there remain other drawbacks, such as low initial Columbic efficiency and volumetric energy density. Porous microscale alloys comprised of nanoparticles and nanopores inherit micro- and nanoproperties, so that volume expansion during lithiation/sodiation can be better accommodated by the porous structure to consequently release stress and improve the cycling stability. Herein, the recent progress of porous microscale alloying-type anode materials for LIBs and SIBs is reviewed by summarizing the Li and Na storage mechanisms, the challenges associated with different materials, common fabrication methods, and the relationship between the structure and electrochemical properties in LIBs and SIBs. Finally, the prospects of porous microscale alloys are discussed to provide guidance for future research and the commercial development of anode materials for LIBs and SIBs.

Dealloying has been an essential technique for developing nanostructured catalysts for the oxygen evolution reaction (OER). Self-supported active catalysts can be fabricated through an alloying-dealloying process on metal foil surfaces. This study uses a Ga-assisted alloying-dealloying strategy combined with electrooxidation and heteroatom doping to fabricate a Fe-doped Ni(OH)2/Ni self-supported OER catalyst. We find that the surface phase compositions and dealloyed structures can be adjusted by controlling the reaction-diffusion temperature and time. The optimized O-Ni-Fe/200-3 catalyst shows an overpotential of 318 mV to activate a 10 mA cm-2 current density with a Tafel slope of 60.60 mV dec-1. Ex-situ characterization of the catalyst proves that Fe doping promotes the formation of active NiOOH, which contributes to the excellent OER activity. This study extends the Ga-assisted alloying-dealloying strategy and demonstrates the possibility of controlling the microstructure of dealloyed materials by changing the reaction-diffusion conditions.

Zinc-ion supercapacitors (ZISCs) are recognized as one of the most promising types of energy storage devices with the advantages of high theoretical capacity and safety, nontoxicity, low cost, abundant resources (~300 times higher than lithium), and lightweight. So far, multifunctional integrated ZISCs have greatly broadened their application scenarios. In addition to enhancing the electrochemical performance via the design of advanced electrodes and electrolytes, the complex application scenarios and in-depth development of energy storage devices have resulted in higher requirements for ZISCs with multifunctional integrated applications. However, to the best of our knowledge, there is no relevant review about summarizing advanced multifunctional ZISCs. In this review, various advanced multifunctional ZISCs, including micro, self-powered integrated, antifreezing, and stretchable ZISCs, are comprehensively presented to fully understand the advanced evolution of multifunctional ZISCs. The working principles and challenges of ZISCs are analyzed and the future development directions and expectations of advanced multifunctional ZISCs are discussed. This review provides significant guidance for the multifunctional development of ZISCs for future studies.

To address the fossil energy crisis and environmental problems, the urgent demand for clean energy has promoted the rapid development of advanced rechargeable metal-air batteries based on the redox reaction couples of gases, such as the oxygen reduction, oxygen evolution, carbon dioxide reduction and carbon dioxide evolution reactions. High-efficiency electrocatalysts are highly desirable to enhance the conversion efficiency of these reactions for enhancing battery performance. Significant advances in single-atom catalysts (SACs) on carbon matrices have been witnessed in recent years as attractive and unique systems to improve the electrocatalytic activities for high-performance rechargeable Zn-and Li-air batteries. This review summarizes the latest achievements in the applications of carbon-supported SACs in metal-air batteries, with a particular focus on the rational design of SACs and their fundamental electrocatalytic mechanism at the atomic level. The future development and perspectives of SACs in the field of metal-air batteries are also discussed.

Electroactive organics have attracted significant attention as electrode materials for next-generation rechargeable batteries because of their structural diversity, molecular adjustability, abundance, flexibility, environmental friendliness and low cost. To date, a large number of organic materials have been applied in a variety of energy storage devices. However, the inherent problems of organic materials, such as their dissolution in electrolytes and low electronic conductivity, have restricted the development of organic electrodes. In order to solve these problems, many groups have carried out research and remarkable progress has been made. Nevertheless, most reviews of organic electrodes have focused on the positive electrode rather than the negative electrode. This review first provides an overview of the recent work on organic anodes for Li-and Na-ion batteries. Six categories of organic anodes are summarized and discussed. Many of the key factors that influence the electrochemical performance of organic anodes are highlighted and their prospects and remaining challenges are evaluated.

As a result of the extensive research and application of LiFePO4 (LFP) in the past > 20 years, there is now a relatively in-depth understanding of its structural stability, phase transition mechanism and electrochemical properties. However, the difficulties faced by further improving the performance of LFP due to its intrinsic low electronic and ionic conductivity have not yet been effectively solved. In order to unlock the effect of transition metal doping on the physicochemical properties of LFP, we establish doping models for all 3d, 4d and 5d transition metals in LFP and compare and analyze their structural properties, band gaps, formation energies, elastic properties, anisotropies and lithiation/delithiation voltages using ab-initio computational screening. According to our screening results, the V-, Mn-, Ni-, Rh- and Os-doped LFP structures have excellent electrochemical properties and can be used as high-performance cathode materials for Li-ion batteries.

Conventional lithium-ion batteries with inflammable organic liquid electrolytes are required to make a breakthrough regarding their bottlenecks of energy density and safety, as demanded by the ever-increasing development of electric vehicles and grids. In this context, solid-state lithium batteries (SSLBs), which replace liquid electrolytes with solid counterparts, have become a popular research topic due to their excellent potential in the realization of improved energy density and safety. However, in practice, the energy density of SSLBs is limited by the cathode mass loading, electrolyte thickness and anode stability. Moreover, the crucial interfacial issues related to the rigid and heterogeneous solid-solid contacts between the electrolytes and electrodes, including inhomogeneous local potential distributions, sluggish ion transport, side reactions, space charge barriers and stability degradation, severely deteriorate the cycle life of SSLBs. Solidification, which converts a liquid into a solid inside a solid battery, represents a powerful tool to overcome the aforementioned obstacles. The liquid precursors fully wet the interfaces and infiltrate the electrodes, followed by in-situ conformal solidification under certain conditions for the all-in-one construction of cells with highly conducting, closely contacted and sustainable electrode/electrolyte interfaces, thereby enabling high energy density and long cycle life. Therefore, in this review, we address the research progress regarding the latest strategies toward the solidification of the electrolyte layers and the interfaces between the electrodes and electrolytes. The critical challenges and future research directions are proposed for the solidification strategies in SSLBs from both science and engineering perspectives.

Rechargeable aqueous Zn-ion batteries (AZIBs) are considered alternative stationary storage systems for large-scale applications due to their high safety, low cost, and high power density. However, Zn anode issues including dendrite formation and side reactions greatly hinder the practical application of AZIBs. To solve the Zn anode issues, various strategies based on material designs have been developed. It is necessary to analyze and classify these strategies according to different materials, because different properties of materials determine the underlying mechanisms. In this review, we briefly introduce the fundamental issues in Zn anodes. Furthermore, this review highlights the material designs for the protection of Zn anodes in mild AZIBs. Finally, we also offer insight into potential directions in the material designs to promote the development of AZIBs in the future.

In response to the global energy crisis, water splitting has become one of the most efficient methods to produce hydrogen as an excellent substitute for fossil fuels. The diffusion coefficient of hydrogen and its interaction with iron have granted carbon steel (CS) the susceptible nature to hydrogen, and therefore CS is considered a promising electrocatalyst in the hydrogen evolution reaction. Compared to many traditional alkaline electrolytes, simulated seawater exhibits reasonable performance that facilitates an effective hydrogen evolution reaction. In the electrolysis of simulated seawater, the lowest overpotential of strained CS samples (-391.08 mV) is comparable to that of Pt plate electrodes (-377.31 mV). This is the result of the plane strain introduced to CS samples by a hydraulic press and indentation, which help to facilitate mass transport through diffusion for hydrogen evolution. The susceptibility of CS is verified by the formation of nanoscale hydrogen blisters that form in the proximity of grain boundaries. These blisters are the result of hydrogen gas pressure that is built up by the absorbed atomic hydrogen. These hydrogen atoms are believed to accumulate along the CS {1 1 0} planes adjacent to grain boundaries. CS has so far not been studied for the catalysis of water splitting. In this study, CS is used as an electrocatalyst for the first time as a cost-effective method for the utilization of seawater that further contributes to the promotion of green energy production.

It is of vital importance to boost the intrinsic activity and augment the active sites of expensive and scarce platinum-based catalysts for advancing a variety of electrochemical energy applications. We herein report a mild electrochemical bottom-up approach to deposit ultrafine, but stable, Pt 8 Ag 4 alloy clusters on carbon nanotubes (CNTs) by elaborately designing bimetallic organic cluster precursors with four silver and eight platinum atoms coordinated with µ,σ-bridged ethynylpyridine ligands, i.e., [Ag 4 (C 24 H 16 N 4 Pt) 8 (BF 4) 4 ]. The Pt 8 Ag 4 cluster/CNT hybrids present impressively high platinum mass activity that is threefold that of commercial Pt/C toward the hydrogen evolution reaction, as a result of the cooperative contributions from the Ag atoms that enhance the intrinsic activity and the CNT supports that increase the activity sites. The present work affords an attractive avenue for engineering and stabilizing Pt-based nanoclusters at the atomic level and represents a promising strategy for the development of high-efficiency and durable electrocatalysts.

Blue energy harvesting based on the ion flow obtained from seas and rivers provides a clean, stable and continuous electric output that is highly dependent on ion-selective membranes (ISMs) that conduct single ions. In recent years, ISMs constructed based on two-dimensional (2D) nanofluidics have demonstrated promising application prospects in blue energy harvesting due to their facile fabrication, excellent ion selectivity and high ion flux. In this review, the principles of 2D nanofluidics in regulating ionic transport are firstly proposed and discussed, including ion selectivity and ultrafast ion transmission, which are considered as two critical factors for achieving highly efficient blue energy harvesting. The advantages of 2D nanofluidics towards blue energy harvesting are analyzed to reveal the necessity of this review. The construction of 2D nanofluidic membranes based on several typical materials and their recent research advances in salinity gradient-and pressure-driven blue energy harvesting are also summarized in detail. Finally, the existing challenges of 2D nanofluidic membranes regarding blue energy harvesting applications are discussed to provide new insights for the development of high-performance blue energy harvesting systems based on 2D nanofluidics.

The scarcity of lithium resources and the unsafety of organic electrolytes limit the further application of lithium-ion batteries (LIBs) in electric vehicles and grid-scale energy storage. Aqueous zinc-ion batteries (AZIBs) are potential complements for LIBs for large-scale grid energy storage because of their abundant resources, environmental friendliness, intrinsic safety and low cost. However, current AZIBs are mainly based on intercalation-type cathodes and their energy densities are not competitive with LIBs. Fortunately, conversion-type cathodes, with higher specific capacity and lower price, endow AZIBs with excellent potential for practical applications. In this review, the mechanism of energy storage and the progress in developing AZIBs based on conversion-type cathodes are summarized. Perspectives on critical scientific issues and the potential developmental directions of AZIBs are also proposed.

Lithium-rich manganese-based cathode materials are expected to promote the commercialization of lithium-ion batteries to a new stage by virtue of their ultrahigh specific capacity and energy density advantages. However, they are still restricted by complex phase transitions and electrochemical performance degradation caused by labile anion charge compensation. A deep understanding of the electrochemical properties contained in their intrinsic structures and the key driving factors of structural deterioration during cycling are crucial to guide the preparation and optimization of lithium-rich materials. Considering recent progress, this review introduces the intrinsic properties of Li-rich manganese-based cathode materials from interatomic interactions to particle morphology at multiple scales in the spatial dimension. The charge compensation mechanism and energy band reorganization of the initial charge and discharge, the structural evolution during cycling and the electrochemical reaction kinetics of the materials are analyzed in the temporal dimension. Based on the relationship between structure and electrochemical performance, preparation methods and modification methods are introduced to guide and design cathode materials. Effective characterization methods for studying anion charge compensation behavior are also demonstrated. This review provides important guidance and suggestions for making full use of the high specific capacity in these materials derived from anion redox and the maintaining of its stability.

Solid electrolytes are recognized as being pivotal to next-generation energy storage technologies. Sulfide electrolytes with high ionic conductivity represent some of the most promising materials to realize high-energy-density all-solid-state lithium batteries. Due to their soft nature, sulfides possess good wettability against Li metal and their preparation process is relatively effortless. High cell-level sulfide-based all-solid-state lithium batteries have gradually been realized in recent years. However, there are still several disadvantages that sulfide electrolytes need to overcome, including their sensitivity to humid air and instability to electrodes. Herein, the recent progress for sulfide electrolytes, with particular attention given to electrolyte synthesis mechanisms, electrochemical and chemical stability, interphase stabilization and all-solid-state lithium batteries with high cell-level energy density, is presented.

The future development of lithium-ion batteries for electric vehicles requires significantly higher energy density and this is largely dependent on the application of novel active materials with high specific capacity. Recently, Sn-Si hybrid materials have been shown to exhibit both high specific capacity and good cycle stability. In practice, Sn-Si materials are mixed with graphite to form composite anodes to further improve the stability. However, detailed investigations of Sn-Si/graphite anodes are scarce. This study examines the electrochemical and expansion performance of Sn-Si/graphite anodes and features a morphological, structural and chemical analysis. The percolation and lattice expansion models are shown to fit well for the capacity and expansion evolution law of the composite anodes, respectively, as a function of Sn-Si hybrid content. Based on a comparison with a conventional graphite anode, efficient Sn-Si/graphite composite anodes could be formed that achieve a high reversible capacity (450 mAh g-1), a promising 1st Coulombic efficiency (75%) and stable cycling (cycling coulombic efficiency > 98%), thereby making them some of the most promising Sn-based anodes for industrial applications.

Li-S batteries, as the most promising post Li-ion technology, have been intensively investigated for more than a decade. Although most previous studies have focused on liquid systems, solid electrolytes, particularly all-solid-state polymer electrolytes (ASSPEs) and quasi-solid-state polymer electrolyte (QSSPEs), are appealing for Li-S cells due to their excellent flexibility and mechanical stability. Such Li-S batteries not only provide significantly improved safety but are also expected to augment the all-inclusive energy density compared to liquid systems. Therefore, this perspective briefly summarizes the recent progress on polymer-based solid-state Li-S batteries, with a special focus on electrolytes, including ASSPEs and QSSPEs. Furthermore, future work is proposed based on the existing development and current challenges.

Solid oxide fuel cells (SOFCs) represent a next-generation energy platform technology. Lowering the operating temperature has become a hot topic in SOFC research and is pivotal to their commercialization, since lower temperatures improve the sealing and durability, as well as reducing costs. However, the lower oxide-ion diffusion and transport in the electrolyte and electrodes at low temperatures seriously inhibit the electrochemical performance and practical applications of SOFCs. Therefore, the design of new structural and functional materials with high ionic conductivity and high electrocatalytic activity is crucial to the development of next-generation low temperature fuel cells. To face this challenge, Zhu et al. [1] have developed semiconductor-ionic materials (SIMs), which enable the production of new advanced SOFCs known as semiconductor membrane fuel cells (SMFCs), which link semiconductor physics and fuel cell electrochemistry at the nanoscale. In contrast to traditional SOFCs, the SMFC concept is proposed to replace the traditional electrolyte by a SIM or semiconductor membrane and it can deliver superior performance even at a lower temperature range (300-500 °C).

The development of green and renewable energy is becoming increasingly more important in reducing environmental pollution and controlling CO 2 discharge. Photocatalysis can be utilized to directly convert solar energy into chemical energy to achieve both the conversion and storage of solar energy. On this basis, photocatalysis is considered to be a prospective technology to resolve the current issues of energy supply and environmental pollution. Recently, several significant achievements in semiconductor-based photocatalytic renewable energy production have been reported. This review presents the recent advances in photocatalytic renewable energy production over the last three years by summarizing the typical and significant semiconductor-based and semiconductor-like photocatalysts for H 2 production, CO 2 conversion and H 2 O 2 production. These reactions demonstrate how the basic principles of photocatalysis can be exploited for renewable energy production. Finally, we conclude our review of photocatalytic renewable energy production and provide an outlook for future related research.

Sodium-ion batteries (SIBs) and capacitors (SICs) have been drawing considerable interest in recent years and are considered two of the most promising candidates for next-generation battery technologies in the energy storage industry. Therefore, it is essential to explore feasible strategies to increase the energy density and cycling lifespan of these technologies for their future commercialization. However, relatively low Coulombic efficiency severely limits the energy density of sodium-ion full cells, particularly in the initial cycle, which gradually decreases the number of recyclable ions. Presodiation techniques are regarded as effective approaches to counteract the irreversible capacity in the initial cycle and boost the energy density of SIBs and SICs. Their cyclic stability can also be enhanced by the slow release of supplemental sodium and high-content recyclable ions during cycling. In this review, a general understanding of the sodium-ion loss pathways and presodiation process towards full cells with high Coulombic efficiency is summarized. From the perspectives of safety, operability and efficiency, the merits and drawbacks of various presodiation techniques are evaluated. This review attempts to provide a fundamental understanding of presodiation principles and strategies to promote the industrial development of SIBs and SICs.

Transition metal molybdates have been studied as anode materials for high-performance lithium-ion batteries, owing to their high theoretical capacity and low cost, as well as the multivalent states of molybdenum. However, their electrochemical performance is hindered by poor conductivity and large volume changes during charge and discharge. Here, we report lithium molybdate (Li2MoO4) composited with carbon nanofibers (Li2MoO4@CNF) as an anode material for lithium-ion batteries. Li2MoO4 shows a shot-rod nanoparticle morphology that is tightly wound in the fibrous CNF. Compared with bare Li2MoO4, the Li2MoO4@CNF composite demonstrates superior high specific capacity and cycling stability, which are attributed to the reversible Li-ion intercalation in the LixMoyOz amorphous phase during charge and discharge. The capacity of the Li2MoO4@CNF anode material can reach 830 mAh g-1 in the second cycle and 760 mAh g-1 after 100 cycles at a charge/discharge current density of 100 mA g-1, which is much better than the bare Li2MoO4. This work provides a simple method to prepare a high-capacity and stable lithium molybdate anode material for lithium-ion batteries.

Energy storage devices, e.g., supercapacitors (SCs) and zinc-ion batteries (ZIBs), based on aqueous electrolytes, have the advantages of rapid ion diffusion, environmental benignness, high safety and low cost. Generally, SCs provide excellent power density with the capability of fast charge/discharge, while ZIBs offer high energy density by storing more charge per unit weight/volume. Although the charge storage mechanisms are considered different, manganese dioxide (MnO2) has proven to be an appropriate electrode material for both SCs and ZIBs because of its unique characteristics, including polymorphic forms, tunable structures and designable morphologies. Herein, the design of MnO2-based materials for SCs and ZIBs is comprehensively reviewed. In particular, we compare the similarities and differences in utilizing MnO2-based materials as active materials for SCs and ZIBs by highlighting their corresponding charge storage mechanisms. We then introduce a few commonly adopted strategies for tuning the physicochemical properties of MnO2 and their specific merits. Finally, we discuss the future perspectives of MnO2 for SC and ZIB applications regarding the investigation of charge storage mechanisms, materials design and the enhancement of electrochemical performance.

All-solid-state lithium-sulfur batteries (ASSLSBs) exhibit huge potential applications in electrical energy storage systems due to their unique advantages, such as low costs, safety and high energy density. However, the issues facing solid-state electrolyte (SSE)/electrode interfaces, including lithium dendrite growth, poor interfacial capability and large interfacial resistance, seriously hinder their commercial development. Furthermore, an insufficient fundamental understanding of the interfacial roles during cycling is also a significant challenge for designing and constructing high-performance ASSLSBs. This article provides an in-depth analysis of the origin and issues of SSE/electrode interfaces, summarizes various strategies for resolving these interfacial issues and highlights advanced analytical characterization techniques to effectively investigate the interfacial properties of these systems. Future possible research directions for developing high-performance ASSLSBs are also suggested. Overall, advanced in-situ characterization techniques, intelligent interfacial engineering and a deeper understanding of the interfacial properties will aid the realization of high-performance ASSLSBs.

Benefiting from the creation of new photovoltaic materials and innovations in device architectures, organic photovoltaic (OPV) cells are booming. Nonetheless, their prosperity is also accompanied by challenges, such as tedious synthetic routes, increasing costs and insufficient operational stability under practical stresses. Polythiophene, with a simple chemical structure, high scalability and excellent charge transport ability, is expected to be the most promising candidate among all kinds of polymer donors. Ternary mixing, as a simple and effective method for improving the efficiency and stability of OPVs, has attracted significant attention in recent decades. This review provides an overview of the recent advances in ternary OPVs based on polythiophene and discusses the role of various third components in three types of OPV active layers, where polythiophene serves as either the host material or additive, and also clarifies how the third component plays a role in determining morphology and device performance, and finally proposes future research directions for ternary OPVs featuring polythiophene. In short, this review provides insights into polythiophene-based multicomponent systems and helps readers better understand the relationships between morphology, efficiency and stability.

Lithium (Li) metal batteries (LMBs) have emerged as the most prospective candidates for post-Li-ion batteries. However, the practical deployment of LMBs is frustrated by the notorious Li dendrite growth on hostless Li metal anodes. Herein, a protonated Li manganese (Mn) oxide with a high Mn3+/Mn4+ ratio is used as a Li adsorbent for constructing highly stable Li metal anodes. In addition to the Mn3+ sites with high Li affinity that afford an ultralow Li nucleation overpotential, the decrease in the average Mnn+ oxidation state also induces a disordered adsorbent structure via the Jahn-Teller effect, resulting in improved Li transfer kinetics with a significantly reduced Li electroplating overpotential. Based on the mutually improved Li diffusion and adsorption kinetics, the Li adsorbent is used as a versatile host to enable dendrite-free and stable Li metal anodes in LMBs. Consequently, a modified Li||LiNi0.8Mn0.1Co0.1O2 (NMC811) coin cell with a high NMC811 loading of 4.3 mAh cm-2 delivers a high Coulombic efficiency of 99.85% over 200 cycles and the modified Li||NMC811 pouch cell also achieves a remarkable improvement in electrochemical performance. This work demonstrates a novel approach for the preparation of highly efficient Li protection structures for safe LMBs with long lifespans.

In recent years, energy storage and conversion have become key areas of research to address social and environmental issues, as well as practical applications, such as increasing the storage capacity of portable electronic storage devices. However, current commercial lithium-ion batteries suffer from low specific energy and high cost and toxicity. Conversion-type cathode materials are promising candidates for next-generation Li metal and Li-ion batteries (LIBs). Metal fluoride materials have shown tremendous chemical tailorability and exhibit excellent energy density in LIBs. Batteries based on such electrodes can compete with other envisaged alternatives, such as Li-air and Li-S systems. However, conversion reactions are typically multiphase redox reactions with mass transport phenomena and nucleation and growth processes of new phases along with interfacial reactions. Therefore, these reactions involve nonequilibrium reaction pathways and significant overpotentials during the charge-discharge process. In this review, we summarize the key challenges facing metal fluoride cathode materials and general strategies to overcome them in cells. Different synthesis methods of metal fluorides are also presented and discussed in the context of their application as cathode materials in Li and LIBs. Finally, the current challenges and future opportunities of metal fluorides as electrode materials are emphasized. With continuous rapid improvements in the electrochemical performance of metal fluorides, it is believed that these materials will be used extensively for energy storage in Li batteries in the future.

Although carbon-supported platinum (Pt/C) has been generally used as a catalyst for the oxygen reduction reaction (ORR) in fuel cells, its practical application is limited by the corrosion reaction of the carbon support. Therefore, it is essential to develop new self-supported catalysts for the ORR. Noble metal aerogels represent highly promising self-supported catalysts with large specific surface area and excellent electrocatalytic activity. Classic sol-gel processes for aerogel synthesis usually take days due to the slow gelation kinetics. Here, we report a straightforward strategy to synthesize platinum-copper (PtCu) aerogels by reducing the metal salt solution with an excess of sodium borohydride at room temperature. The PtCu aerogels are formed in a relatively short time of 1 h through a rapid nucleation mechanism. The obtained PtCu aerogels have a highly porous structure with an appreciable topological surface area of 33.0 m2/g and mainly exposed (111) facets, which are favorable for the ORR. Consequently, the PtCu aerogels exhibit excellent ORR activity with a mass activity of 369.4 mA/mgPt and a specific activity of 0.847 mA/cm2, which are 2.6 and 3.3 times greater than those of Pt/C, respectively. The PtCu aerogels show remarkable ORR catalysis among all the noble metal aerogels that have been reported. The porous morphology and outstanding electrocatalytic activities of the PtCu aerogels illustrate their promising applications in fuel cells.

Despite the low cost, safety and high theoretical capacity of metallic zinc, zinc anodes face chronic problems, including zinc dendrites, corrosion and side reactions in aqueous zinc-ion batteries (ZIBs). Herein, a nitrogen-doped carbon nanoparticle coating layer derived from discarded cigarette filters is constructed to suppress parasitic side reactions and zinc dendrite growth. The dense coating layer isolates water from the zinc anode, effectively inhibiting side reactions. Furthermore, the special micro-mesoporous structure and sufficient zincophilic groups guarantee uniform Zn stripping/plating. Consequently, durable cycle stability (2400 cycles at a current density of 1 mA cm-2) with a stable polarization potential is achieved for symmetrical cells. The coating layer derived in this study therefore has the potential to improve the electrochemical performance of ZIBs.

Commercial carbonate electrolytes with poor oxidation stability and high flammability limit the operating voltage of Li-ion batteries (LIBs) to ~4.3 V. As one of the most promising candidates for electrolyte solvents, sulfolane (SL) has received significant interest because of its wide electrochemical window, low flammability and high dielectric permittivity. Unfortunately, SL-based electrolytes with normal concentrations cannot achieve highly reversible Li+ intercalation/deintercalation in graphite anodes due to an ineffective solid electrolyte interface, thus undermining their potential application in LIBs. Here, a low-concentration SL-based electrolyte (LSLE) is developed for high-voltage graphite||LiNi0.8Co0.1Mn0.1O2 (NCM811) full cells. A highly reversible graphite anode can be achieved through the preferential decomposition of the dual-salt LiDFOB-LiBF4 in the LSLE. The addition of fluorobenzene further restrains the decomposition of SL, endowing uniform, robust and inorganic-rich interphases on the electrode surfaces. As a result, the LSLE with improved thermal stability can support the MCMB||NCM811 full cells at 4.4 V, evidenced by an excellent cycling performance with capacity retentions of 83% after 500 cycles at 25 ℃ and 82% after 400 cycles at 60 ℃. We believe that the design of this fluorobenzene-containing LSLE offers an effective routine for next-generation low-cost and safe electrolytes for high-voltage LIBs.

Semiconductor membrane fuel cells are a new promising R&D for solid oxide fuel cells and proton ceramic fuel cells. There is a challenge of the electronic short circuit issue by using semiconductor to replace conventional electrolyte membrane. In this work, type II band alignment of the semiconductor heterostructure based on Mg-doped ZnO and ZnO can, on one hand, block electrons passing through the junction, and on the other hand, trigger the ionic properties of membrane to boost the fuel cell performance. The Mg doping into ZnO creates more oxygen vacancies at the surface of ZnO, leading to enhanced ionic transport, and meaningful fuel cell performance of 673 mW/cm2; while the Mg-doped ZnO/ZnO heterostructure fuel cell has delivered 997 mW/cm2 and OCV 1.04 V at 520 oC. It is worth highlighting that the constructed heterostructure interface, especially the band bending and constituted build-in electric field, plays a pivotal role in enhancing the ionic transport and suppressing the electron passing through the internal device. First principal calculations using density functional theory confirmed the doping of Mg and the formation of heterostructure with ZnO to help for enhancing charge carriers and separations. This work suggests that the constructed type II band alignment or the semiconductor heterostructure is useful for developing advanced fuel cells.

Hydrogen peroxide (H2O2) has been widely used in environmental cleaning, hospital disinfecting and chemical engineering. Compared to the traditional anthraquinone oxidation method, the electrocatalytic two-electron oxygen reduction reaction (2e-ORR) to produce H2O2 has become a promising alternative due to its green, safety and reliability. However, its industrial application is still limited by the slow reaction kinetics and low selectivity due to the competitive reaction of the 4e-ORR to H2O. Herein, we prepare a novel photoresponsive metal-free electrocatalyst based on oxidized g-C3N4/carbon nanotubes (OCN/CNTs) and introduce an external light field to realize the high-performance electrocatalytic 2e-ORR to produce H2O2. Impressively, the OCN/CNT electrocatalyst exhibits an outstanding H2O2 productivity of 30.7 mmol/gcat/h with a high faradaic H2O2 efficiency of 95%. The oxygen-containing groups of the OCN/CNTs promote the adsorption of oxygen intermediates and the photo-coupled electrocatalysis simultaneously improves the electron transport efficiency and enhances the electrocatalytic selectivity.

The oxygen evolution reaction (OER) is of fundamental importance as a half reaction and rate-controlling step that plays a predominant function in improving the energy storage and conversion efficiency during the electrochemical water-splitting process. In this review, after briefly introducing the fundamental mechanism of the OER, we systematically summarize the recent research progress for nonprecious-metal-based OER electrocatalysts of representative first-row transition metal (Fe, Co and Ni)-based composite materials. We analyze the effects of the physicochemical properties, including morphologies, structures and compositions, on the integrated performance of these OER electrocatalysts, with the aim of determining the structure-function correlation of the electrocatalysts in the electrochemical reaction process. Furthermore, the prospective development directions of OER electrocatalysts are also illustrated and emphasized. Finally, this mini-review highlights how systematic introductions will accelerate the exploitation of high-efficiency OER electrocatalysts.

Ambient heat, slightly above or at room temperature, is a ubiquitous and inexhaustible energy source that has typically been ignored due to difficulties in its utilization. Recent evidence suggests that a class of azobenzene (Azo) photoswitches featuring a reversible photoinduced crystal-to-liquid transition could co-harvest photon energy and ambient heat. Thus, a new horizon has been opened for recovering and regenerating low-grade ambient heat. Here, a series of unilateral para-functionalized photoinduced liquefiable Azo derivatives is presented that can co-harvest and convert photon energy and ambient heat into chemical bond energy and latent heat in molecules and eventually release them in the form of high-temperature utilizable heat. A straightforward crystalline-to-liquid phase transition achieved with ultraviolet light irradiation (365 nm) is enabled by appending a halogen/alkoxy group on the para-position of the Azo photoswitches, and the release of thermal energy is triggered by short-wavelength visible-light irradiation (420 nm). The phase transition properties of the trans- and cis-isomers and the energy density, storage lifetime and heat release performance of the cis-liquid are investigated with differential scanning calorimetry, ultraviolet-visible absorption spectroscopy, and an infrared (IR) thermal camera. The experimental results indicate a high energy density of 335 J/g, a long lifetime of 5 d and a heat release of up to 6.3 °C due to the coupled photochemical-thermophysical mechanism. This work presents a new model for utilizing renewable energy, i.e., the photoinduced conversion of ambient thermal energy.

Mg-air batteries, with their intrinsic advantages such as high theoretical volumetric energy density, low cost, and environmental friendliness, have attracted tremendous attention for electrical energy storage systems. However, they are still in an early stage of development and suffer from large voltage polarization and poor cycling performance. At present, Mg-air batteries with high rechargeability remain difficult to achieve, mainly because the discharge products [Mg(OH)2, MgO and MgO2] are thermodynamically and kinetically difficult to decompose at moderate voltage ranges. Therefore, it is crucial to optimize the reaction paths and kinetics from the electrodes to the batteries via the combination of materials design and first-principles calculations. In this review, remarkable progress is highlighted regarding the currently used materials for Mg-air batteries, including anodes, electrolytes, and cathodes. In addition, the corresponding reaction mechanisms are comprehensively surveyed. Finally, future perspectives for rechargeable Mg-air batteries with decreased voltage polarization and improved cycling performance are also described for further practical applications.

Lithium oxides are the most promising cathode candidates for high-performance lithium-ion batteries (LIBs), owing to their high theoretical capacity and average working voltage, which are conducive to achieving the ultimate goal of upgrading energy density. By raising the upper limit of the cutoff voltage, we may be able to further improve both the practical capacity and average voltage of lithium oxide cathodes. Unfortunately, the high-voltage operation of these cathodes results in significant challenges, namely, reduced surface structural stability and interfacial stability with electrolytes, thus degrading the electrochemical performance. Accordingly, surface/interface modification strategies, including surface coating, electrolyte regulation, binder design, and special surface treatments, are systematically summarized and comprehensively analyzed for high-voltage lithium oxide cathode materials in this review. Furthermore, the corresponding modification mechanisms are discussed in detail to better grasp the internal mechanisms for the enhanced electrochemical performance. Based on recent progress, we further propose predictable development directions for high-performance LIBs in future practical applications. This review provides new insights into various high-voltage lithium oxide cathodes and their universal surface/interface modification strategies towards advanced next-generation LIBs with high energy and power density and long cycle life.