Showing papers in "Journal of Materials Chemistry in 2017"

Carbon-based composite materials for supercapacitor electrodes: a review

Abstract: Electrochemical capacitors, so-called supercapacitors, play an important role in energy storage and conversion systems. In the last decade, with the increasing volume of scientific activity and publications in this field, researchers have developed better tools to improve electrode materials. Although carbonaceous materials seem the most suitable for supercapacitor applications, a large diversity of materials has been proposed and studied. Yet, in order to accomplish performance beyond the limitations of each material, mainly in terms of energy density and durability, composite materials have been implemented, most of them being the combinations of carbon-based materials and other components. In this review, we present the recent advances in the field of composite materials that include at least one carbon-based component for supercapacitor electrodes. We focus on cases in which a single material by itself suffers from a drawback that can be overcome by combining it with other components, enabling the fabrication of a composite material with enhanced performance. We present several important compositions as well as the major routes of synthesis, characterization and performance of composite materials in this field.

Recent progress in carbon quantum dots: synthesis, properties and applications in photocatalysis

Abstract: Carbon quantum dots (CQDs) as a rising star of carbon nanomaterials, by virtue of their unique physicochemical, optical and electronic properties, have displayed tremendous momentum in numerous fields such as biosensing, bioimaging, drug delivery, optoelectronics, photovoltaics and photocatalysis. In particular, the rich optical and electronic properties of CQDs including efficient light harvesting, tunable photoluminescence (PL), extraordinary up-converted photoluminescence (UCPL) and outstanding photoinduced electron transfer have attracted considerable interest in different photocatalytic applications for the sake of full utilization of the solar spectrum. This review aims to demonstrate the recent progress in the synthesis, properties and photocatalytic applications of CQDs, particularly highlighting the fundamental multifaceted roles of CQDs in photoredox processes. Furthermore, we discuss the challenges and future direction of CQD-based materials in this booming research field, with a perspective toward the ultimate achievement of highly efficient and long-term stable CQD-based photocatalysts.

Oil/water separation techniques: a review of recent progresses and future directions

Abstract: Oil/water separation is a field of high significance as it has direct practical implications for resolving the problem of industrial oily wastewater and other oil/water pollution. Therefore, the development of functional materials for efficient treatment of oil-polluted water is imperative. In this feature article, we have reviewed the recent progress of oil/water separation technologies based on filtration and absorption methods using various materials that possess surface superwetting properties. In each section, we present in detail representative work and describe the concepts, employed materials, fabrication methods, and the effects of their wetting/dewetting behaviors on oil/water separation. Finally, the challenges and future research directions of this promising research field are briefly discussed.

A comprehensive review of Pt electrocatalysts for the oxygen reduction reaction: Nanostructure, activity, mechanism and carbon support in PEM fuel cells

Abstract: The sluggish rate of the oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells has been a major challenge. Significantly increasing efforts have been made worldwide towards a highly active ORR catalyst with high durability. Among all the catalysts exploited, Pt electrocatalysts are still the best in terms of a comprehensive evaluation. The investigation of Pt-based ORR catalysts is necessary for a practical ORR catalyst with low Pt content. This paper reviews recent progress in the studies of the mechanism, nanostructure, size effect and carbon supports of Pt electrocatalysts for the ORR. The importance of the size and structure control of Pt ORR catalysts, related with carbon support materials, is indicated. The potential methods for such control are discussed. The progress in theoretical studies and in situ catalyst characterization are also discussed. Finally, challenges and future developments are addressed.

Recent progress in layered transition metal carbides and/or nitrides (MXenes) and their composites: synthesis and applications

Abstract: Since their inception in 2011, from the inaugural synthesis of multi-layered Ti3C2Tx by etching Ti3AlC2 with hydrofluoric acid (HF), novel routes with a myriad of reducing agents, etchants and intercalants have been explored and many new members have been added to the two-dimensional (2D) material constellation. Despite being endowed with the rare combination of good electronic conductivity and hydrophilicity, their inherent low capacities, for instance, temper their prospect for application for electrodes in energy storage systems. MXene-based composites, however, with a probable synergistic effect in agglomeration prevention, facilitating electronic conductivity, improving electrochemical stability, enhancing pseudocapacitance and minimizing the shortcomings of individual components, exceed the previously mentioned capacitance ceiling. In this review, we summarise the development and progress in the synthesis of various multi-layered carbides, carbonitrides and nitrides, and intercalants, as well as the subsequent processing in order to delaminate them into single- and/or few-layered and incorporate into MXene-based composites, focusing on their performance and application as transparent conductive films, environmental remediation, electromagnetic interference (EMI) absorption and shielding, electrocatalysts, Li-ion batteries (LIBs), supercapacitors and other electrochemical storage systems.

Dead lithium: mass transport effects on voltage, capacity, and failure of lithium metal anodes

Abstract: Improvement of the performance of Li metal anodes is critical to enable high energy density rechargeable battery systems beyond Li-ion. However, a complete mechanistic understanding of electrode overpotential variations that occur during extended cycling of Li metal is lacking. Herein, we demonstrate that when using a Li metal electrode, the dynamic changes in voltage during extended cycles can be increasingly attributed to mass transport. It is shown that these mass transport effects arise as a result of dead Li accumulation at the Li metal electrode, which introduces a tortuous pathway for Li-ion transport. In Li–Li symmetric cells, mass transport effects cause the shape of the galvanostatic voltage response to change from “peaking” to “arcing”, along with an increase in total electrode overpotential. The continued accumulation of dead Li is also conclusively shown to directly cause capacity fade and rapid “failure” of Li–LCO full cells containing Li metal anodes. This work provides detailed insights into the coupled relationships between cycling, interphase morphology, mass transport and the overall cell performance. Furthermore, this work helps underscore the potential of Li–Li symmetric cells as a powerful analytical tool for understanding the effects of Li metal electrodes in full cell batteries.

Ultra-thin nanosheet assemblies of graphitic carbon nitride for enhanced photocatalytic CO2 reduction

Abstract: A two-dimensional layered polymeric photocatalyst, graphitic carbon nitride (g-C3N4), is becoming the rising star in the field of solar-to-fuel conversion. However, the performance of commonly prepared g-C3N4 is usually very weak because of the high recombination rate of photogenerated charge carriers and a small amount of surface active sites. Here we demonstrate simultaneous texture modification and surface functionalization of g-C3N4via a stepwise NH3-mediated thermal exfoliation approach. The resulting g-C3N4 photocatalyst possesses a hierarchical structure obtained by the assembly of amine-functionalized ultrathin nanosheets and thus exhibits remarkably enhanced light harvesting, a high redox ability of charge carriers, increased CO2 adsorption and a larger amount of surface active sites, as well as improved charge carrier transfer and separation. Therefore the aforementioned hierarchical g-C3N4 consisting of amine-functionalized ultra-thin nanosheets shows much better performance for photocatalytic CO2 reduction than unmodified conventional g-C3N4 photocatalysts.

Biomass derived carbon for energy storage devices

Abstract: Electrochemical energy storage devices are becoming increasingly more important for reducing fossil fuel energy consumption in transportation and for the widespread deployment of intermittent renewable energy. The applications of different energy storage devices in specific situations are all primarily reliant on the electrode materials, especially carbon materials. Biomass-derived carbon materials are receiving extensive attention as electrode materials for energy storage devices because of their tunable physical/chemical properties, environmental concern, and economic value. In this review, recent developments in the biomass-derived carbon materials and the properties controlling the mechanism behind their operation are presented and discussed. Moreover, progress on the applications of biomass-derived carbon materials as electrodes for energy storage devices is summarized, including electrochemical capacitors, lithium–sulfur batteries, lithium-ion batteries, and sodium-ion batteries. The effects of the pore structure, surface properties, and graphitic degree on the electrochemical performance are discussed in detail, which will guide further rational design of the biomass-derived carbon materials for energy storage devices.

A review on special wettability textiles: theoretical models, fabrication technologies and multifunctional applications

Abstract: Inspired by the superhydrophobic lotus surface in nature, special wettability has attracted a lot of interest and attention in both academia and industry In this review, theoretical models and fabrication strategies of superhydrophobic textiles have been discussed in detail The strategies for constructing fabric surfaces with an anti-wetting property are categorized and discussed based on the morphology of particles coated on the textile fibre Such special wettability textile surfaces are demonstrated with self-cleaning, oil/water separation, self-healing, UV-blocking, photocatalytic, anti-bacterial, and flame-retardant performances Correspondingly, potential applications have been illustrated for self-cleaning, oil/water separation, asymmetric/anisotropic wetting janus fabric, microfluidic manipulation, and micro-templates for patterning In each section, representative studies are highlighted with emphasis on the special wetting ability and other relevant properties Finally, the difficulties and challenges for practical application were briefly discussed

Characterization and properties of Zn/Co zeolitic imidazolate frameworks vs. ZIF-8 and ZIF-67

Abstract: A novel Zn/Co zeolitic imidazolate framework (ZIF) has been constructed by an easy and straightforward room temperature technique Several characterization techniques such as SEM, TEM-EDX, single-crystal XRD and ICP have been applied to confirm that the structure formed is a sodalite (SOD) cage type structure The Zn/Co-ZIF possesses a high nano-crystallinity and porosity with a large surface area By tuning the amount of Co and Zn in the Zn/Co zeolitic imidazolate framework, the physical and chemical properties have been improved compared with those of the single metal frameworks (ZIF-8 and ZIF-67) Consequently, the Zn/Co-ZIF was investigated for two different applications; gas adsorption (CO2, CH4 and N2) and catalysis (CO2 conversion to cyclic carbonates) and the obtained results were compared with the performance of previously reported single metal frameworks (ZIF-8 and ZIF-67) Additionally, hydrolytic stability tests under ambient conditions and immersed in water at 75 °C were performed and pointed out that Zn/Co-ZIF exhibits a higher stability Moreover, based on these results, the Zn/Co-ZIF demonstrates better properties compared with ZIF-8 and ZIF-67

Highly stable, phase pure Cs2AgBiBr6 double perovskite thin films for optoelectronic applications

Abstract: Hybrid lead halide perovskites have emerged as high-performing semiconductors for optoelectronic applications such as photovoltaics. However, their toxicity and stability issues represent significant challenges. Recently, double perovskites have been suggested as an alternative and, especially, Cs2AgBiX6 (X = Cl, Br) has proved to be a promising material as it is non-toxic and highly stable. However, the low solubility of precursors has so far hampered the fabrication of high quality films. Here, we demonstrate for the first time the fabrication of Cs2AgBiBr6 films and incorporate them into working devices. Powder X-ray diffraction measurements revealed that high annealing temperatures of at least 250 °C are required to fully convert the precursors into Cs2AgBiBr6. After the optimization of the synthesis conditions, photovoltaic devices comprising our Cs2AgBiBr6 films show power conversion efficiencies (PCE) close to 2.5% and a open circuit voltage (Voc) exceeding one volt, which is currently the highest Voc reported for a bismuth halide based perovskite, showing the potential of double perovskites as the absorber material. Furthermore, our results revealed excellent stability of the devices upon exposure to working conditions without encapsulation. Our work opens the way to a new class of perovskites with significant potential for optoelectronic applications.

Potassium–sodium niobate based lead-free ceramics: novel electrical energy storage materials

Abstract: The development of lead-free bulk ceramics with high recoverable energy density (Wrec) is of decisive importance for meeting the requirements of advanced pulsed power capacitors toward miniaturization and integration. However, the Wrec (<2 J cm−3) of lead-free bulk ceramics has long been limited by their low dielectric breakdown strength (DBS < 200 kV cm−1) and small saturation polarization (Ps). In this work, a strategy (compositions control the grain size of lead-free ceramics to submicron scale to increase the DBS, and the hybridization between the Bi 6p and O 2p orbitals enhances the Ps) was proposed to improve the Wrec of lead-free ceramics. (K0.5Na0.5)NbO3–Bi(Me2/3Nb1/3)O3 solid solutions (where Me2+ = Mg and Zn) were designed for achieving large Ps, and high DBS and Wrec. As an example, (1 − x)(K0.5Na0.5)NbO3–xBi(Mg2/3Nb1/3)O3 (KNN–BMN) ceramics were prepared by using a conventional solid-state reaction process in this study. Large Ps (41 μC cm−2) and high DBS (300 kV cm−1) were obtained for 0.90KNN–0.10BMN ceramics, leading to large Wrec (4.08 J cm−3). The significantly enhanced Wrec is more than 2–3 times larger than that of other lead-free bulk ceramics. The findings in this study not only provide a design methodology for developing lead-free bulk ceramics with large Wrec but also could bring about the development of a series of KNN-based ceramics with significantly enhanced Wrec and DBS in the future. More importantly, this work opens a new research and application field (dielectric energy storage) for (K0.5Na0.5)NbO3-based ceramics.

Graphitic carbon nitride (g-C3N4)-based photocatalysts for solar hydrogen generation: recent advances and future development directions

Abstract: Graphitic carbon nitride (g-C3N4) is a metal-free conjugated polymer constructed from two-dimensional sheets with a bandgap energy of 2.7 eV, which makes it an applicable and efficient visible-active photocatalyst for H2 production. In the present study, the basic concepts and principles of photocatalytic water splitting have been discussed, and a guide for the selection of appropriate photocatalysts, focusing on the g-C3N4 nanomaterials, has been proposed. Our approach is mainly concentrated on evaluating two factors, namely the solar-to-hydrogen (STH) conversion and apparent quantum yield (AQY) for different photocatalysts, to provide an in-depth analysis and a framework for solar H2 production for future research directions. We compared hydrogen production from an economic viewpoint and performance of g-C3N4 nanomaterials through photochemical (PC) and photoelectrochemical (PEC) methods. Various approaches for efficient solar H2 generation over a modified g-C3N4 surface with the possibility for commercialization have been introduced. The promising approaches for the effective utilization of g-C3N4 are categorized into three proposed methods: electronic structure tuning, hybrid and nanocomposite fabrication, and finally geometric structure manipulation. Finally, we compared the recent findings and key achievements for g-C3N4-based photocatalysts modified based on the abovementioned three approaches to propose two possible scenarios for their use in the future development of efficient solar H2 generation.

A critical review on tin halide perovskite solar cells

Abstract: APbI3−xBrx perovskite solar cells with the bandgap of ∼1.5–1.6 eV, where A represents caesium, methylammonium, formamidinium and mixtures thereof, currently present certified efficiencies very close to those of established thin film technologies (such as CIGS and CdTe) and are thus one of the most important optoelectronics. To restrict the use of lead, as well as to tune the band gap of the material close to the optimum according to the Shockley–Queisser limit (being 1.34 eV), substitution (total or partial) of Pb2+ by Sn2+ should take place. In this review, we present results on single junction solar cells, utilizing CsSnI3−xBrx, CH3NH3SnI3−xBrx or NH2CHNH2SnI3−xBrx perovskites as absorbers, as well as a mixture of Sn2+/Pb2+ being adopted as the metal binary cation, reducing the bandgap to 1.2–1.4 eV. We also highlight very recently recorded efficiencies of perovskite-on-perovskite tandem solar cells, produced by the combination of the above low band gap materials with typical highly performing semi-transparent APbI3−xBrx perovskites of a higher band gap (close to 1.6–1.8 eV). We discuss these fascinating results, focusing on some key points such as, among others, the role of the tin compensator/reducing agent (usually SnF2) during perovskite crystallization. In addition, we present the critical challenges that currently limit the efficiency/stability of these systems and propose prospects for future directions.

Revitalizing carbon supercapacitor electrodes with hierarchical porous structures

Abstract: Carbon materials, owing to their excellent electrical conductivity, tailorability, inexpensiveness and versatility, have been extensively studied as electrode materials for supercapacitors. The capacitance of carbon-based supercapacitor electrodes has remained at a mediocre level between 100 and 200 F g−1 for decades. Until recently, a new family of carbon materials termed hierarchical porous carbons has pushed the capacitance to new benchmark values beyond 300 F g−1, and has revitalized the exploration of carbon materials for supercapacitors. Hierarchical porous carbons contain different scales of pores (from micropores to macropores) inter-connected together and assembled in hierarchical patterns. Experimental studies coupled with theoretical investigations have elucidated that the presence of micropores is responsible for offering a large surface area to enhance charge storage capability, whilst mesopores, macropores and the hierarchical structure improve electrolyte infiltration and facilitate ion diffusion. This review will start by introducing different pore types and the definition of hierarchical porous structures, followed by discussion and exemplification of major synthesis strategies. In addition, recent molecular-level understanding of the relationship between pore size, functionalities inside pores, pore spatial distribution and capacitive performance is presented. Finally, challenges and future opportunities associated with hierarchical porous carbons for supercapacitors are discussed.

Nanocellulose-based foams and aerogels: processing, properties, and applications

Abstract: Nanocellulose is a renewable and biocompatible nanomaterial with a high strength low density and tunable surface chemistry This review summarizes the main processing routes and significant properties of nanocellulose-based foams and aerogels Challenges, such as how to produce long-term stable wet foams or how to avoid structural collapse of the material during solvent removal using eg supercritical drying, are discussed Recent advances in the use of ice templating to generate iso- or anisotropic foams with tunable mechanical and thermal properties are highlighted We illustrate how the porous architecture and properties of nanocellulose-based foams and aerogels can be tailored for applications in eg thermal insulation and energy storage

MXene: an emerging two-dimensional material for future energy conversion and storage applications

Abstract: The development of two-dimensional (2D) high-performance electrode materials is the key to new advances in the fields of energy conversion and storage. MXenes, a new intriguing family of 2D transition metal carbides, nitrides, and carbonitrides, have recently received considerable attention due to their unique combination of properties such as high electrical conductivity, hydrophilic nature, excellent thermal stability, large interlayer spacing, easily tunable structure, and high surface area. In this review, we discuss how 2D MXenes have emerged as efficient and economical nanomaterials for future energy applications. We highlight the promising potential of these materials in energy conversion and storage applications, such as water electrolyzers, lithium ion batteries, and supercapacitors. Finally, we present an outlook of the future development of MXenes for sustainable energy technologies.

Dual-action smart coatings with a self-healing superhydrophobic surface and anti-corrosion properties

Abstract: This work introduces a new self-healing superhydrophobic coating based on dual actions by the corrosion inhibitor benzotriazole (BTA) and an epoxy-based shape memory polymer (SMP). Damage to the surface morphology (e.g., crushed areas and scratches) and the corresponding superhydrophobicity are shown to be rapidly healed through a simple heat treatment at 60 °C for 20 min. Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) were used to study the anti-corrosion performance of the scratched and the healed superhydrophobic coatings immersed in a 3.5 wt% NaCl solution. The results revealed that the anti-corrosion performance of the scratched coatings was improved upon the incorporation of BTA. After the heat treatment, the scratched superhydrophobic coatings exhibited excellent recovery of their anti-corrosion performance, which is attributed to the closure of the scratch by the shape memory effect and to the improved inhibition efficiency of BTA. Furthermore, we found that the pre-existing corrosion product inside the coating scratch could hinder the scratch closure by the shape memory effect and reduce the coating adhesion in the scratched region. However, the addition of BTA effectively suppressed the formation of corrosion products and enhanced the self-healing and adhesion performance under these conditions. Importantly, we also demonstrated that these coatings can be autonomously healed within 1 h in an outdoor environment using sunlight as the heat source.

Transition metal oxides with one-dimensional/one-dimensional-analogue nanostructures for advanced supercapacitors

Abstract: With the increasing energy demand and the overconsumption of fossil fuels, renewable energy-storage devices with higher efficiency are of great interest. In particular, supercapacitors have recently gained significant attention due to their excellent charge–discharge performance, long-term cycle lifetimes, and high specific power. In addition, supercapacitors could also make up the difference in energy and power between batteries and traditional capacitors. In the future, the promising family of transition metal oxides (TMOs) will play a significant role in environmentally friendly, low-cost, and high-powered energy storage. Furthermore, one-dimensional (1D) and one-dimensional-analogue nanostructures could remarkably enhance the characteristic properties of TMOs. In this review, we focused on the recent progress in the preparation and electrochemical properties of the next-generation supercapacitors.

Mixed-metallic MOF based electrode materials for high performance hybrid supercapacitors

Abstract: Metal–organic frameworks (MOFs) have obtained increasing attention as a kind of novel electrode material for energy storage devices. Yet low capacity in most MOFs largely thwarts their application. In this study, an effective strategy was developed to improve the conductivity of MOFs by partially substituting Ni2+ in the Ni-MOF with Co2+ or Zn2+. The mixed-metal organic frameworks (M-MOFs) showed excellent electrochemical performance, which is attributed not only to the favorable paths for charge transport due to the presence of free pores, but also to the raised electrochemical double-layer capacitance (EDLC) at the enlarged specific surface area of the material. Meanwhile, the cycling stability of the assembled hybrid supercapacitors (M-MOFs//CNTs–COOH) is enhanced due to the alleviation of phase transformation during electrochemical cycling tests. More interestingly, the Co/Ni-MOF//CNTs–COOH also exhibited an excellent energy density (49.5 W h kg−1) and power density (1450 W kg−1) simultaneously. These values demonstrated the better performance of all the MOF materials in supercapacitors at present. In addition to broadening the application of MOFs, our study may open a new avenue for bridging the performance gap between batteries and supercapacitors.

Multiscale-structuring of polyvinylidene fluoride for energy harvesting: the impact of molecular-, micro- and macro-structure

Abstract: Energy harvesting exploits ambient sources of energy such as mechanical loads, vibrations, human motion, waste heat, light or chemical sources and converts them into useful electrical energy. The applications for energy harvesting include low power electronics or wireless sensing at relatively lower power levels (nW to mW) with an aim to reduce a reliance on batteries or electrical power via cables and realise fully autonomous and self-powered systems. This review focuses on flexible energy harvesting system based on polyvinylidene fluoride based polymers, with an emphasis on manipulating and optimising the properties and performance of the polymeric materials and related nanocomposites through structuring the material at multiple scales. Ferroelectric properties are described and the potential of using the polarisation of the materials for vibration and thermal harvesting using piezo- and pyro-electric effects are explained. Approaches to tailor the ferroelectric, piezoelectric and pyroelectric properties of polymer materials are explored in detail; these include the influence of polymer processing conditions, heat treatment, nanoconfinement, blending, forming nanocomposites and electrospinning. Finally, examples of flexible harvesting devices that utilise the optimised ferroelectric polymer or nanocomposite systems are described and potential applications and future directions of research explored.

A review of Ni-based layered oxides for rechargeable Li-ion batteries

Abstract: The portable electronic market, vehicle electrification (electric vehicles or EVs) and grid electricity storage impose strict performance requirements on Li-ion batteries, the energy storage device of choice, for these demanding applications. Higher energy density than currently available is needed for these batteries, but a limited choice of materials for cathodes remains a bottleneck. Layered lithium metal oxides, particularly those with high Ni content, hold the greatest promise for high energy density Li-ion batteries because of their unique performance characteristics as well as for cost and availability considerations. In this article, we review Ni-based layered oxide materials as cathodes for high-energy Li-ion batteries. The scope of the review covers an extended chemical space, including traditional stoichiometric layered compounds and those containing two lithium ions per formula unit (with potentially even higher energy density), primarily from a materials design perspective. An in-depth understanding of the composition–structure–property map for each class of materials will be highlighted as well. The ultimate goal is to enable the discovery of new battery materials by integrating known wisdom with new principles of design, and unconventional experimental approaches (e.g., combinatorial chemistry).

Rechargeable zinc–air batteries: a promising way to green energy

Abstract: As a promising technology, electrically rechargeable zinc–air batteries have gained significant attention in the past few years. Herein, in this review, we focused on the main challenges of the electrically rechargeable zinc–air batteries in alkaline electrolytes and the up-to-date progress from materials to technologies towards overcoming these technical barriers. We first overviewed the design and working mechanism of the battery and classified the hindrances into dendritic growth at the anode, lack of higher performance bifunctional catalysts at the air electrode, and electrolyte-related problems. Then, detailed discussions have been provided on the latest progress to address these technical issues based on the nano/micro-materials. Flexible zinc–air batteries as a new development have also been discussed in a separate section. Finally, conclusions have been provided followed by future perspective.

Phase engineering of a multiphasic 1T/2H MoS2 catalyst for highly efficient hydrogen evolution

Abstract: Molybdenum disulfide (MoS2) has attracted much attention as a promising electrocatalyst for the hydrogen evolution reaction (HER). Although tremendous efforts have been made to enhance the HER performance of MoS2, the functional design of its intrinsic structures still remains challenging. In this work, a highly active and stable multiphasic catalyst (1T/2H MoS2) is developed through a facile hydrothermal route, in which the 1T phase is induced by the intercalation of guest ions and molecules, and the concentration of the 1T phase can be controlled by adjusting the preparation temperature. The existence of the 1T phase provides more active sites and better conductivity for the HER, resulting in an excellent activity with a small Tafel slope of 46 mV dec−1. More importantly, the integration with the 2H phase is beneficial to the stabilization of the metastable 1T phase, ensuring the excellent durability of 1T/2H MoS2.

Alternative strategies in improving the photocatalytic and photoelectrochemical activities of visible light-driven BiVO4: a review

Abstract: The research interest on bismuth vanadate (BiVO4) has heightened over the past decade due to its proven high activity for water oxidation and organic degradations under visible light. Although metal doping and water-oxidation cocatalyst loading have been widely demonstrated to be useful to overcome the poor electron transport and slow water oxidation kinetics of BiVO4, the efficiency of this material is still greatly limited by poor charge separation. Various efforts directed at modifying the surface and bulk properties to improve the performance of BiVO4-based materials have therefore been developed, including crystal facet engineering, coupling with graphitic carbon material, annealing treatment, and nanoscaling. This review aims to provide insights into the most recent progress in these strategies in regard to their influences on the charge separation, transport, and transfer aspects of BiVO4, all of which are crucial to govern photochemical conversion efficiency. Understanding of these charge kinetics in relation to the properties of BiVO4 is of fundamental importance for rational design of BiVO4 with optimum structures, which may serve as a general guideline for the fabrication of metal oxide photocatalysts.

Electrochemical etching of Ti2AlC to Ti2CTx (MXene) in low-concentration hydrochloric acid solution

Abstract: In this study, we successfully demonstrate the electrochemical etching of Al from porous Ti2AlC electrodes in dilute hydrochloric acid to form a layer of Ti2CTx MXene on Ti2AlC. This is the first report on etching of the A layer from the MAX phase in a fluoride-free solution as a less hazardous method to process and handle MXenes. In addition, these MXenes possess only –Cl terminal groups, as well as the common ones, such as –O and –OH. However, electrochemical etching can also result in subsequent over-etching of parent MAX phases to carbide-derived carbon (CDC). We propose a core–shell model to explain electrochemical etching of Ti2AlC to Ti2CTx and CDC. The proposed model suggests that a careful balance in etching parameters is needed to produce MXenes while avoiding over-etching. Our electrochemical approach expands the possible range of both etching techniques and resulting MXene compositions.

A novel K-ion battery: hexacyanoferrate(II)/graphite cell

Abstract: Recently, research on novel and low cost batteries has been widely conducted to realize large-scale energy storage systems. However, few of the battery systems have delivered performance equal to that of Li-ion batteries. Herein, we propose non-aqueous K-ion batteries by developing hexacyanoferrate(II) compounds (so-called Prussian blue analogues), K1.75Mn[FeII(CN)6]0.93·0.16H2O and K1.64Fe[FeII(CN)6]0.89·0.15H2O, as affordable positive electrode materials. In particular, K1.75Mn[FeII(CN)6]0.93·0.16H2O prepared by a simple precipitation process delivers a high capacity of 141 mA h g−1 at 3.8 V as the average operating potential, resulting in a comparable energy density to that of LiCoO2, with excellent cyclability and rate performance in K half-cells. Operando X-ray diffraction measurements reveal that the excellent electrochemical performance of this material is attributed to its open and flexible framework, which can realize fully reversible K+ extraction/insertion and a structural change from monoclinic to tetragonal via cubic phases. For the first time, we demonstrate an inexpensive high-voltage K-ion full cell with a K1.75Mn[Fe(CN)6]0.93·0.16H2O/graphite configuration to prove its feasibility as a new promising battery system for an environmentally friendly future.

Interface engineering in planar perovskite solar cells: energy level alignment, perovskite morphology control and high performance achievement

Abstract: We report a simple and effective interface engineering method for achieving highly efficient planar perovskite solar cells (PSCs) employing SnO2 electron selective layers (ESLs). Herein, a 3-aminopropyltriethoxysilane (APTES) self-assembled monolayer (SAM) was used to modify the SnO2 ESL/perovskite layer interface. This APTES SAM demonstrates multiple functions: (1) it can increase the surface energy and enhance the affinity of the SnO2 ESL, which induce the formation of high quality perovskite films with a better morphology and enhanced crystallinity. (2) Its terminal functional groups form dipoles on the SnO2 surface, leading to a decreased work function of SnO2 and enlarged built-in potential of SnO2/perovskite heterojunctions. (3) The terminal groups can passivate the trap states at the perovskite surface via hydrogen bonding. (4) The thin insulating layer at the interface can hinder electron back transfer and reduce the recombination process at the interface effectively. With these desirable properties, the best-performing cell employing a APTES SAM modified-SnO2 ESL achieved a PCE over 18% and a steady-state efficiency of 17.54%. Impressively, to the best of our knowledge, the obtained VOC of 1.16 V is the highest value reported for the CH3NH3PbI3 (MAPbI3) system. Our results suggest that the ESL/perovskite interface engineering with a APTES SAM is a promising method for fabricating efficient and hysteresis-less PSCs.

MOF-derived bi-metal embedded N-doped carbon polyhedral nanocages with enhanced lithium storage

Abstract: To tackle the issue of the low specific capacity (372 mA h g−1) of graphite as the anode material for lithium-ion batteries (LIBs), an effective and controllable strategy was developed to construct porous bimetallic Co/Zn embedded N-doped carbon (Co–Zn/N–C) polyhedral nanocages via annealing a ZIF-8@ZIF-67 precursor at 800 °C under Ar atmosphere. The results clearly displayed that metallic Co and Zn particles are uniformly dispersed in the carbon matrix. Porous Co–Zn/N–C polyhedral nanocages have a large specific surface area of 349.12 m2 g−1 and contain plenty of micropores and mesopores, which benefit from the carbonization of organic ligands and the catalytic effect of cobalt in the calcination process. As anodes for LIBs, the porous Co–Zn/N–C polyhedral nanocages showed an initial discharge capacity of 809 mA h g−1 and a capacity retention of 702 mA h g−1 after 400 cycles at a current density of 0.2 A g−1. Furthermore, a reversible capacity of 444 mA h g−1 was obtained at a much higher current density of 2 A g−1. The improved electrochemical performance was attributed to the synergistic effect of Zn and Co, the unique porous hollow structure as well as N doping, which relieved the impact of volume changes, maintained perfect electrical conductivity throughout the electrode and enhanced the electrochemical activities of lithium storage.

High-voltage and free-standing poly(propylene carbonate)/Li6.75La3Zr1.75Ta0.25O12 composite solid electrolyte for wide temperature range and flexible solid lithium ion battery

Abstract: Solid electrolyte is regarded as a perfect way to enhance safety issues and boost energy density of lithium batteries. Herein, we developed a type of free-standing poly(propylene carbonate)/Li6.75La3Zr1.75Ta0.25O12 composite solid electrolyte for ambient temperature and flexible solid-state lithium batteries. The composite solid electrolyte exhibited excellent comprehensive performance in terms of high ionic conductivity (5.2 × 10−4 S cm−1) at 20 °C, a wide electrochemical window (4.6 V), high ionic transference number (0.75) and satisfactory mechanical strength (6.8 MPa). When evaluated as solid electrolyte for an ambient-temperature solid lithium battery, such a composite electrolyte delivered excellent rate capability (5C) at 20 °C. This superior performance can be comparable to a liquid electrolyte-soaked PP separator-based lithium battery at room temperature. To our knowledge, this is the best rate capability of a solid composite electrolyte for a solid lithium battery at ambient temperature. Moreover, such a composite electrolyte-based flexible LiFePO4/Li4Ti5O12 lithium ion battery delivered excellent rate capability and superior cycling stability. All these fascinating features make poly(propylene carbonate)/Li6.75La3Zr1.75Ta0.25O12 a very promising all-solid-state electrolyte for flexible solid lithium batteries. Our study makes a big step into addressing the challenges of ambient-temperature solid lithium batteries.