Showing papers in "Journal of Materials Chemistry in 2020"

Iron-based phosphides as electrocatalysts for the hydrogen evolution reaction: recent advances and future prospects

Abstract: Electrochemical water splitting is the most promising process to produce carbon-neutral hydrogen as an energy carrier, and hydrogen evolution reaction (HER) electrocatalysts are essential to reduce the energy barrier and improve hydrogen production efficiency. This driving force necessitates the design of HER electrocatalysts with low-cost, high-abundance, high activity and stability. In this review, we systemically summarize recent publications with critical insight into the advances of iron-based phosphides as HER electrocatalysts. Various synthesis strategies corresponding to different phosphating strategies for guiding iron phosphide design are described, followed by illustrations of how to boost FeP HER catalytic performance by enriching the accessible active sites and modifying the electronic structure. Finally, we point out several challenges and prospects, which can open up opportunities to design next-generation FeP HER electrocatalysts.

Highly stable Zn metal anodes enabled by atomic layer deposited Al 2 O 3 coating for aqueous zinc-ion batteries

Abstract: Rechargeable aqueous zinc-ion batteries (ZIBs) have attracted increasing attention as an energy storage technology for large-scale applications, due to their high capacity (820 mA h g−1 and 5854 A h L−1), inherently high safety, and their low cost. However, the overall performance of ZIBs has been seriously hindered by the poor rechargeability of Zn anodes, because of the dendrite growth, passivation, and hydrogen evolution problems associated with Zn anodes. Herein, Al2O3 coating by an atomic layer deposition (ALD) technique was developed to address the aforementioned problems and improve the rechargeability of Zn anodes for ZIBs. By coating the Zn plate with an ultrathin Al2O3 layer, the wettability of Zn was improved and corrosion was inhibited. As a result, the formation of Zn dendrites was effectively suppressed, with a significantly improved lifetime in the Zn–Zn symmetric cells. With the optimized coating thickness of 100 cycles, 100Al2O3@Zn symmetric cells showed a reduced overpotential (36.5 mV) and a prolonged life span (over 500 h) at 1 mA cm−2. In addition, the 100Al2O3@Zn has been verified in Zn–MnO2 batteries using layered δ-MnO2 as the cathode and consequently exhibits superior electrochemical performance with a high capacity retention of 89.4% after over 1000 cycles at a current density of 1 mA cm−2 (3.33C for MnO2) was demonstrated. It is expected that the novel design of Al2O3 modified Zn anodes may pave the way towards high-performance aqueous ZIBs and shed light on the development of other metal anode-based battery systems.

Advances and challenges in electrochemical CO2 reduction processes: an engineering and design perspective looking beyond new catalyst materials

Abstract: Electrochemical CO2 reduction (CO2R) is one of several promising strategies to mitigate CO2 emissions. Electrochemical processes operate at mild conditions, can be tuned to selective products, allow modular design, and provide opportunities to integrate renewable electricity with CO2 reduction in carbon-intensive manufacturing industries such as iron and steel making. In recent years, significant advances have been achieved in the development of highly efficient and selective electrocatalysts for CO2R. However, to realize fully the potential benefits of new electrocatalysts in low cost, large scale CO2R electrolyzers requires advances in design and engineering of the CO2R process. In this review, we examine the state-of-the-art in electrochemical CO2R technologies, and highlight how the efficiency of CO2R processes can be improved through (i) electrolyzer configuration, (ii) electrode structure, (iii) electrolyte selection, (iv) pH control, and (v) the electrolyzer's operating pressure and temperature. Although a comprehensive review of catalytic materials is beyond this review's scope, we illustrate how other engineering and design decisions may also influence CO2R reaction pathways because of effects on mass transfer rates, the electrode surface chemistry, interactions with intermediate reaction species, and rates of charge transfer.

Semiconductor-based photocatalysts for photocatalytic and photoelectrochemical water splitting: will we stop with photocorrosion?

Abstract: The status of photocatalytic (PC)/photoelectrochemical (PEC) water splitting as a promising approach to solar-to-chemical energy conversion has increased significantly over the past several decades for addressing the energy shortage. However, the overall energy conversion efficiency is still relatively poor due to the severe photocorrosion in photosensitive semiconductors. Herein, the review begins with the discussion of the photocorrosion mechanism with several typical semiconductors as examples. Then the feasible characterization methods used to evaluate the stability of semiconductors are summarized. Notably, most studies regarding water splitting focus on achieving high efficiency by improving the charge separation and transfer efficiency within the semiconductors. This review focuses on the recent advances in effective strategies for photocorrosion inhibition of semiconductor-based composites with respect to their intrinsic properties and interface charge transfer kinetics, including morphology/size control, heteroatom doping, heterojunction construction, surface modification, and reaction environment regulation. Furthermore, an in-depth investigation of photocorrosion pathways and mechanisms is critical to accurately and effectively address the photocorrosion of semiconductor-based composites to improve PC/PEC water splitting performance in the future.

Aqueous electrocatalytic N2 reduction for ambient NH3 synthesis: recent advances in catalyst development and performance improvement

Abstract: Electrochemical N2 reduction has emerged as an environmentally benign alternative to the Haber–Bosch process for sustainable NH3 synthesis under ambient reaction conditions, and considerable recent attention has focused on electrocatalytic NH3 synthesis from N2 and H2O in aqueous media. In this Minireview, we summarize the recent advances in the development of electrocatalysts for the N2 reduction reaction (NRR). Strategies to boost the NRR performances are also discussed. Perspectives for further research directions are provided finally.

A review of aspects of additive engineering in perovskite solar cells

Abstract: Solar energy is a clean source of energy that can help fulfill the increasing global energy demand. Among light harvesting devices, perovskite solar cells (PSCs) have been a focus of interest among next-generation photovoltaic (PV) technologies due to their incredible conversion efficiency (certified PCE of 25.2%) along with their lower cost and ease of fabrication. However, the presence of many transport barriers and defect trap states at the interfaces and grain boundaries has negative effects on PSCs; it decreases their efficiency and stability and increases the hysteresis effect. Further controlling the morphology, grain boundary, grain size, charge recombination and density of defect states in the perovskite layer is necessary to enhance its photovoltaic performance and stability. In this review, we summarize the intensive research efforts in aspects of additive engineering in PSCs, including physical and chemical passivation, and the use of a wide variety of organic and inorganic additives to address these issues. Here, we mainly focus on passivation techniques in the perovskite active layer and their effects on the PV performance and stability of PSCs; the grain boundaries and surface defects in the perovskite layer play major roles in the recombination, carrier lifetime and charge transfer of PSCs.

A comprehensive review on oxidative desulfurization catalysts targeting clean energy and environment

Abstract: Harvesting clean energy from fuel feedstocks is of paramount significance in the field of environmental science. In this dynamic area, desulfurization provides a valuable contribution by eliminating sulfur compounds from fuel feedstocks to ensure the utilization of fuels without the emission of toxic sulfur oxides (SOx gases). Nonetheless, the inadequacy of the current industrial technique (hydrodesulfurization, HDS) in the removal of refractory sulfur (RS) compounds and the stringent rules imposed on the fuel sulfur level have kindled research on other desulfurization methods like oxidative desulfurization (ODS). With the capacity of eliminating RS compounds under mild conditions, ODS is endorsed as a suitable replacement or complementary to HDS. ODS, in general, consists of two steps: (i) oxidation and (ii) extraction. The oxidation of sulfur compounds is carried out using a suitable catalyst (hereafter termed as an ODS catalyst) in the presence of an oxidant. Choosing a suitable ODS catalyst for industrial applications is still a quest among the various types of catalysts reported so far. With this outline, herein, all the types of ODS catalysts along with their synthetic methods, reactivity and mechanistic insights are reviewed. The activity of ODS catalysts could be influenced by factors like the type of RS compound, solvent, fuel, etc. and those factors are reviewed. The effects of ionic liquids, light, and ultrasound on the performance of ODS catalysts are also briefly summarized. The opportunities and challenges for ODS catalysts are comprehensively explicated in the end. Through this review, systematic information about the types of ODS catalysts including the basic definition, preparative methods, reactivity and mechanism can be comprehended. Furthermore, this review reveals the merits and demerits related to highlighting catalytic ODS as a replacement or complementary to HDS.

Biomass-derived porous graphitic carbon materials for energy and environmental applications

Abstract: As we all know, environmental protection and sustainable energy utilization are significant challenges for us. Due to their many excellent characteristics, carbon materials have been playing a very important role in energy and environmental applications. Biomass is the only renewable carbon source and crucial precursor of carbonaceous materials and has the advantages of a unique structure, a wide range of sources, biodegradability, and low cost. Developing high-performance carbonaceous materials from biomass is a significant research subject. Biomass-derived porous graphitic carbon materials (BPGCs) have received extensive attention as novel high-performance sustainable carbon materials owing to its well-developed porous structure, good graphitic structure, and heteroatom doping. Here, this review firstly focuses on the principal synthesis methodologies of BPGCs. Next, three electrochemical energy storage and conversion systems that utilize BPGCs are intensively investigated, including supercapacitors (SCs), lithium-ion batteries (LIBs) and fuel cells (FCs). Then, BPGCs are further reviewed in terms of their application in the field of environmental protection, which is also the first systematic summary of BPGCs in environmental applications. Finally, this review points out the direction that is worthy of further research in the future and the essential issues that have not yet been resolved.

From isolated Ti-oxo clusters to infinite Ti-oxo chains and sheets: recent advances in photoactive Ti-based MOFs

Abstract: Metal–organic frameworks (MOFs) have emerged as a significant class of porous crystalline materials constructed from metal nodes and multidentate linkers. Many research studies have shown the promising applications of MOFs in gas adsorption and separation, electrocatalysis, photocatalysis, biomedicine etc., mainly due to the advantages of high porosity, large surface areas and easily tunable optical and electronic structures. Particularly, light-sensitive Ti-oxo clusters in Ti-based MOFs result in promising photocatalytic activity. Recently, a few Ti–carboxylate MOFs based on infinite Ti-oxo chains and sheets were reported, which open a new horizon for the photocatalytic activity of Ti-MOF chemistry. This review highlights some typical Ti-MOFs based on discrete Ti-oxo clusters and recent progress in the fabrication of Ti-MOFs based on infinite Ti-oxo chains and sheets. Moreover, facile modification methods for improving the photocatalytic activity under visible light irradiation are exemplified in detail. We hope that the photocatalytic applications of Ti-MOFs in photocatalytic H2 generation, photocatalytic reduction of CO2, dye photodegradation, photocatalytic alcohol oxidation and photocatalytic polymerization will benefit researchers who are interested in this field.

Fused heterocycle-based energetic materials (2012–2019)

Abstract: Fused cyclic energetic materials, a unique class of large conjugate structures which contain two or more rings that share two atoms and the bond between the rings, have been identified as promising contenders to traditional energetic materials. With a coplanar polycyclic structure, fused heterocyclic ring-based energetic materials feature considerably higher heats of formation (HOF) and ring-strain energy stored in the molecules. These result in attractive features of good energetic performance, enhanced thermal stability and low sensitivity toward destructive mechanical stimuli, which increases the safety of the synthesis, transfer, and storage of high-energy density compounds. This review addresses the chemistry of fused heterocyclic compounds, which provide valuable scaffolds for more powerful and less sensitive eco-friendly energetic materials. The reactions that are selected and discussed illustrate the versatility of 64 different fused heterocycles designed as building blocks for the synthesis of a wide range of high-performance energetic materials.

Highly tough supramolecular double network hydrogel electrolytes for an artificial flexible and low-temperature tolerant sensor

Abstract: High-performance hydrogel electrolytes with eminent toughness, high conductivity and anti-freezing properties have extensive applications in wearable devices or implantable sensors. However, it is still difficult to integrate excellent mechanical properties and high conductivity into one hydrogel sample simultaneously. This work introduced NaCl into a poly(vinyl alcohol)/poly(acrylic amide) (PVA/PAM) double network hydrogel to prepare PVA/PAM/NaCl supramolecular hydrogel electrolytes via a one-pot method. NaCl introduces physical entanglement into the PVA/PAM/NaCl hydrogel electrolytes and provides a dense and wrinkled three dimensional (3D) network nanostructure. The PVA/PAM/NaCl hydrogel electrolytes not only showed excellent mechanical properties (tensile strength up to 477 kPa, elongation at break to 1072% and a fracture energy of 2.484 MJ m−3), but also had high conductivity (up to 6.23 S m−1). A strain sensor based on the PVA/PAM/NaCl hydrogel electrolytes exhibited very high sensitivity (gauge factor = 24.901) with the ability of precise and reliable detection of human motions. Hydrogels also showed excellent anti-freezing properties and maintained excellent mechanical properties and conductivity at −20 °C. Introducing a physically cross-linking network through the effects of a metal salt could promote the performance of the hydrogels. This work provides a new insight into the design of multifunctional materials with applications on electronic skin, wearable devices and biosensors.

Amidoxime-based materials for uranium recovery and removal

Abstract: Resources and the environment are two eternal themes of social development. Nuclear energy, a green source with high energy density, can greatly alleviate the pressure of the energy crisis in today's society. To guarantee the long-term sufficient supply of nuclear fuel, mining seawater uranium is imperative. Meanwhile, the great threats of uranium to ecological security and human health make the removal of uranium from the environment urgent. To achieve these ends, a large number of materials with specific functions have been born as a result. Among them, amidoxime-based materials serve as one of the most promising candidates and are the main tool used for uranium extraction from aqueous systems owing to their special affinity for uranium. However, there is still huge room for improvement in amidoxime-based materials in terms of their economic efficiency and performance. In this paper, we provide a comprehensive review of amidoxime-based materials for uranium recovery and removal, including synthesis strategies, characterization and types of amidoxime-based materials, the factors that influence uranium extraction, and the binding mechanisms between amidoxime ligands and uranyl ions, as well as the cost drivers in applications. Meanwhile, the shortcomings of current research as well as future development directions and research hotspots are also pointed out. Based on the in-depth analysis of the currently available literature, a demand-oriented strategy for fabricating a new generation of amidoxime-based adsorbents was proposed, and means to enhance the adsorbent performance were discussed with regards to four aspects, including adsorption capacity, selectivity, kinetics and regenerability. This paper aims to provide guidance for the purposeful design of novel amidoxime-based materials, and to provide advice on circumventing unfavorable factors and solving the technical problems relating to uranium recovery and removal.

Nanocomposite hydrogel-based strain and pressure sensors: a review

Abstract: Recently, flexible and wearable electronics have gained considerable research interest due to their potential applications in wearable devices, energy storage materials, electronic skins, sensors, etc. Compared to elastomers, hydrogels demonstrate more potential for flexible electronics because of their biomimetic structures, suitable mechanical properties and excellent biocompatibility. Among all the designs, nanocomposite hydrogel-based strain and pressure sensors which can transmit external stimulus to electrical signals have been intensely investigated due to their high mechanical strength, considerable conductivity and outstanding sensitivity. Numerous reports have been dedicated to the designs, preparations and applications of nanocomposite hydrogels. This review provides an up-to-date and comprehensive summary of research progresses of nanocomposite hydrogel-based strain and pressure sensors including designing strategies, preparing methods and applications of the five nanofiller based hydrogel sensors including carbon nanotube based, graphene oxide based, MXene based, polymer nanofiller based and other nanofiller based sensors. Representative cases are carefully selected and discussed regarding the fabrication, merits and demerits, respectively. Finally, perspectives and challenges are presented for the designs of future nanocomposite hydrogel-based strain and pressure sensors.

Anti-freezing flexible aqueous Zn–MnO2 batteries working at −35 °C enabled by a borax-crosslinked polyvinyl alcohol/glycerol gel electrolyte

Abstract: Flexible aqueous zinc-ion batteries (AZIBs) are promising to satisfy the emerging wearable electronics. However, conventional hydrogel electrolytes are unable to work at subzero temperatures because they inevitably freeze. In this work, a borax-crosslinked polyvinyl alcohol (PVA)/glycerol gel electrolyte is developed, in which glycerol can strongly interact with PVA chains, thus effectively prohibiting the formation of ice crystals within the whole gel network. Thanks to this, the freezing point of this gel electrolyte is below −60 °C, which allows it to work in extremely cold environments. Even at −35 °C, it still exhibits a high ionic conductivity of 10.1 mS cm−1 and great mechanical properties. On the basis of this anti-freezing gel electrolyte, a flexible quasi-solid-state aqueous Zn–MnO2 battery is assembled and realizes an impressive energy density of 46.8 mW h cm−3 (1330 μW h cm−2) at a power density of 96 mW cm−3 (2.7 mW cm−2) at 25 °C, outperforming nearly all the reported AZIBs. More importantly, when the temperature is reduced to −35 °C, a rather high energy density (25.8 mW h cm−3, 732 μW h cm−2) can still be achieved, and 53.3% of that value can be retained when the power density is increased to about 10-fold. This battery also shows excellent cycling durability (around 90% capacity retention over 2000 cycles) and great tolerance to various extreme conditions even when the temperature is down to −35 °C. These findings provide valuable insights into designing aqueous batteries/supercapacitors that can work in cold climates and high-altitude areas.

Current progress in electrocatalytic carbon dioxide reduction to fuels on heterogeneous catalysts

Abstract: As a promising and important carbon source, utilization of carbon dioxide (CO2) can effectively solve the energy crisis caused by fossil resource consumption and the environmental problems arising from the emission of CO2. The electrocatalytic method is currently a promising research method for CO2 reduction; however, it faces the problems of low product selectivity and poor faradaic efficiency (FE). Therefore, the design of effective catalysts with lower overpotential, high FE, and product selectivity is key consideration for the development of electrochemical CO2 reduction (CO2RR). Herein, we summarize the current research progress in the electrocatalytic reduction of CO2 to fuels on heterogeneous catalysts. Progress in the electrocatalytic reduction of two types of products, C1 products (CO, HCOOH, CH3OH, CH4) and C2 products (CH3CH2OH and C2H4), are discussed. The catalytic activity, FE, product selectivity, electrocatalytic mechanisms, and challenges faced in terms of product selectivity and catalytic activity stability in electrochemical CO2 reduction are discussed. This review can provide effective guidance for the design of effective catalysts with high activity, product selectivity, FE, and stability.

Emerging applications of porous organic polymers in visible-light photocatalysis

Abstract: Porous organic polymers (POPs) are emerging porous solids featuring high porosity, low density, diverse composition, facile functionalization, and high thermal/chemical stability. POPs are proven competitive candidates in various applications, such as sorption/separation, energy storage, biomedical applications, optical devices, and catalysis. Photocatalysis enables multifarious transformations of chemicals under mild conditions without enormous energy consumption and contaminant generation. Catalytically active species can be incorporated into POPs by means of covalent bonding or metal–ligand coordination, and the catalytic power of POPs can be fine tuned by modulating the electronic properties of monomers and pore structures of the solids, which make POP-based heterogeneous photocatalysts designable for task-specific applications. The outstanding performance and reusability distinguish POPs from their homogeneous analogues. As a result, their catalytic efficiency can be significantly enhanced and cost would be considerably reduced. In this review, we aim to highlight the recent research of POPs in visible-light-driven photocatalysis, including organic synthesis, hydrogen evolution, carbon dioxide reduction, and degradation of organic pollutants. Technological concerns over the hindrance of preparation, post-processing, performance optimization, cost and energy consumption are also discussed.

Oxidation-resistant titanium carbide MXene films

Abstract: Two-dimensional transition metal carbides (MXenes) have attracted much attention due to their excellent electrical conductivity and outstanding performances in energy storage, telecommunication, and sensing applications. It is known that MXene flakes are readily oxidized in either humid air or aqueous environments. While the chemical instability of MXenes may limit their use in applications involving ambient environments and long-term operation, oxidation behaviour of MXene films has not been addressed. In this work, we demonstrate a hydrogen annealing method to increase the oxidation stability of Ti3C2 MXene in two different aspects: (1) dramatic improvement in the oxidation stability of pristine MXene films against harsh conditions (100% relative humidity, 70 °C), and (2) large recovery in the electrical conductivity of previously oxidized Ti3C2 MXene films. We also demonstrate an electric-field-induced heater capable of stable operation under highly oxidizing conditions, based on the oxidation-resistant MXene film. A total loss of heat generation ability was observed for the as-prepared MXene film, while the hydrogen-annealed one maintained its bright infrared radiation, under the highly oxidizing conditions. This work offers a solution to industrial applications of unprotected MXene films, securing their stable and long-term operation in humid conditions.

Bifunctional electrocatalysts for Zn–air batteries: recent developments and future perspectives

Abstract: As one of the most promising alternatives for future energy systems, the rechargeable Zn–air battery (ZAB) has attracted extensive attention due to its extraordinarily high theoretical specific energy density. However, several obstacles restrict its practical application. One challenge is the sluggish kinetics of oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER) in the discharging and charging processes of ZABs. In addition, when using unifunctional ORR or OER electrocatalysts as air electrodes, like noble metal catalysts (Pt/C or Ru/IrO2), there are the disadvantages of high cost and poor stability. Therefore, rational design of non-noble metal bifunctional ORR/OER electrocatalysts with high activity and stability is essential for the development of ZABs. In this review, we discuss the latest developments of non-noble metal bifunctional ORR/OER electrocatalysts for ZABs. Firstly, the related reaction mechanisms of ORR and OER are introduced. Then, the latest developments of bifunctional ORR/OER materials for ZABs are discussed in detail from three aspects: (i) MOF-based catalysts, including pristine MOFs and their derivatives; (ii) metal-free-based carbon catalysts, including heteroatom-doped carbon and defective carbon; (iii) metal-based catalysts, including metal–nitrogen–carbon materials (such as metals/alloys, single-atom) and metal compound materials. Finally, some challenges and outlooks for the optimal design of bifunctional air electrodes for rechargeable ZABs with high activity and ultra-long lifetime are put forward.

Air-permeable, multifunctional, dual-energy-driven MXene-decorated polymeric textile-based wearable heaters with exceptional electrothermal and photothermal conversion performance

Abstract: Multifunctional, high-performance wearable heaters are highly desired for future human health-related applications but are generally hindered by the absence of flexibility, air-permeability, and clothes-knittability. Here, polymeric textile-based wearable heaters are constructed by decorating an MXene on the fiber surface via a simple solution dip coating technique. Alkali pretreatment of textiles enables strong interaction between the MXene and textiles through the synergistic effect of hydrogen bonds and physical rivet action. These MXene-decorated textiles (M-textiles) not only maintain the innate flexibility, comfort, light-weight and permeability characteristics of the textile substrates, but also exhibit exceptional heating performance including dual-driven energy conversion (electrothermal and photothermal), wide temperature range (40–174 °C in electrothermal and 40–204 °C in photothermal conversion), safe operating conditions (1–3.5 V for electrothermal, and NIR/FIR or abundant sunlight for photothermal conversion), and fast thermal response (reaching over 100 °C within 25 s at 2.5 V or within seconds in photothermal conversion). Impressively, the M-textiles integrate superb resistance to fire and bacteria, and a high electromagnetic interference (EMI) shielding efficiency of 42.1 dB in the X-band. These multifunctional M-textile wearable heaters are highly promising for applications in warmth-keeping, thermotherapy, deicing, heating water, EMI shielding, and antibacterial and fire protection, and are ideal candidates for future health management and protection.

Recent advances in structural engineering of MXene electrocatalysts

Abstract: Most recently, two-dimensional (2D) transition-metal carbides (MXenes) have been demonstrated to be promising electrocatalysts owing to their unique chemical and electronic properties, e.g., metallic conductivity, high hydrophilicity, and tunable surface terminations. Herein, representative progress achieved in MXenes as hydrogen evolution reaction electrocatalysts is reviewed both experimentally and theoretically. Briefly, termination modification and heteroatom incorporation are applied to optimize the chemical and electronic configurations of active sites for intrinsically enhanced catalytic kinetics while various nanostructures and hybridizations are fabricated to increase the density and accessibility of active sites. Then, the achievements of MXene-based catalysts in other electrocatalysis processes are also summarized, including the oxygen evolution/reduction reaction, carbon dioxide reduction reaction and nitrogen reduction reaction. Finally, current challenges and future research directions for MXene-based electrocatalysis are discussed.

Ultrahigh enhancement rate of the energy density of flexible polymer nanocomposites using core–shell BaTiO3@MgO structures as the filler

Abstract: Dielectric energy storage capacitors are critical components widely used in electronic equipment and power systems due to their advantages of ultrahigh power density and high voltage. Herein, a novel core–shell BaTiO3@MgO (BT@MO) nanostructure was fabricated, in which highly insulating MgO was directly coated on a continuous ferroelectric nanoparticle BaTiO3 shell through a chemical precipitation method to improve the breakdown strength and electric displacement under high electric field. A large electric displacement (D ≈ 9.8 μC cm−2 under 571.4 MV m−1) was observed along with a high discharge energy density (Ud ≈ 19.0 J cm−3) for BT@MO/P(VDF-HFP) composites, which was 187% higher than that for a P(VDF-HFP) film when the filler content was 3 wt%. The enhancement rate of Ud in this study achieved the highest level among the reported results. It was revealed that the highly insulating MgO shell can enhance the breakdown strength by preventing charge injection from electrodes and impeding the development of electrical stress during the breakdown process, as confirmed by the leakage current measurements and the finite element simulations. The core–shell BT@MO structured filler provided an effective way to improve the energy storage properties of the polymer-based dielectrics.

Recent progress in g-C3N4 quantum dots: synthesis, properties and applications in photocatalytic degradation of organic pollutants

Abstract: Graphitic-carbon nitride quantum dots (g-C3N4QDs), as a rising star in the carbon nitride family, show great potential in many fields involving bioimaging, fuel cells, and photo(electro)catalysis, due to their fascinating optical and electronic properties. Especially, the efficient light capture, tunable photoluminescence and extraordinary up-conversion photoluminescence properties of g-C3N4QDs may offer promising potential for full utilization of the solar spectrum, thus promoting their applications in photocatalytic reactions. Some reviews on g-C3N4 have been presented; while most of them have concentrated on g-C3N4 in 3-dimensional (3D) or 2D structures, few focused on g-C3N4QDs. Therefore, this review aims to summarize the recent advances in g-C3N4QDs regarding their synthesis, optical and electronic properties and photocatalytic applications for degrading organic pollutants. Moreover, crucial issues in g-C3N4QD future application in these flourishing research areas are discussed, with prospects towards the final realization of efficient and long-term stable g-C3N4QD-based photocatalysts.

Two-dimensional transition metal carbide and nitride (MXene) derived quantum dots (QDs): synthesis, properties, applications and prospects

Abstract: The progress of two-dimensional (2D) MXene-derived QDs (MQDs) is in the early stages, but the materials have aroused great interest due to their high electrical conductivity, abundant active catalytic sites, easily tunable structure, satisfactory dispersibility, remarkable optical properties, good biocompatibility, manifold functionalizations, and so on. However, up to now, there is still no review paper on MQDs. Herein, the research advances of MQDs, including their synthetic routes (top-down and bottom-up methods), properties (structural, electronic, optical and magnetic properties), functionalizations (surface modifications, heteroatom doping and the construction of composites) and applications (sensing, biomedical, catalysis, energy storage and optoelectronic devices etc.), are critically highlighted, and the future prospects and challenges of MQDs are discussed. This review will serve as a one-stop point for comprehending the most advanced developments of MQDs, and will hopefully enlighten researchers to employ MQDs for satisfying the growing requirements of the diverse applications.

Fe-doping induced morphological changes, oxygen vacancies and Ce3+–Ce3+ pairs in CeO2 for promoting electrocatalytic nitrogen fixation

Abstract: Developing active and robust catalysts for the electrocatalytic N2 reduction reaction (NRR) represents a promising strategy for ambient NH3 production but remains challenging. Herein, CeO2 was modulated by Fe-doping for the NRR in neutral media. Fe-doping was found to induce the morphological change of CeO2 from crystalline nanoparticles to partial-amorphous nanosheets which contained abundant oxygen vacancies (VO), resulting in more exposed active sites and accelerated electron transport. Density functional theory (DFT) calculations further revealed that the coexistence of the Fe dopant and its adjacent VO enabled the creation of Ce3+–Ce3+ pairs which served as the most active centers for effectively catalyzing the NRR and suppressing the hydrogen evolution reaction. These Fe-doping induced synergistic effects led to a significantly enhanced NRR performance of Fe-CeO2 with an NH3 yield of 26.2 μg h−1 mg−1 (−0.5 V) and a faradaic efficiency of 14.7% (−0.4 V). Therefore, this metal-doping induced multifunctionality will open up new opportunities to design powerful NRR catalysts for N2 fixation.

Asymmetric conductive polymer composite foam for absorption dominated ultra-efficient electromagnetic interference shielding with extremely low reflection characteristics

Abstract: Absorption-dominated electromagnetic interference (EMI) shielding polymer composites with low reflection are greatly desired for next-generation electronic devices due to their minimization of secondary electromagnetic radiation pollution. However, realizing highly efficient EMI shielding with microwave absorption-dominated features remains a great challenge. Herein, a flexible, extremely low reflection and ultraefficient EMI shielding waterborne polyurethane (WPU) composite foam with a unique asymmetric conductive network and oriented porous structure is assembled through a special density-induced filler separation combined with a directional freeze-drying method. The asymmetric conductive network is constructed from the highly conductive Ag-coated expanded polymer bead (EBAg) particles aggregated on the top of the foam as a conductive layer and the low conductivity graphene-supported iron–cobalt (FeCo@rGO) magnetic nanoparticles deposited at the bottom as an impedance matching layer. Because of the asymmetric network with rational layout of the impedance matching layer and conductive shielding layer, the composite foam exhibits an outstanding EMI SE of nearly 90 dB (84.8 dB on average) in the X band. The average reflection efficiency is only 0.3 dB, and the reflectivity is as low as 0.08, which is the lowest value for EMI shielding materials ever reported. Furthermore, benefitting from this asymmetric filler network structure, the composite foam exhibits excellent compression recovery and satisfactory water-resistant shielding stability. Our work provides a new strategy for designing ultraefficient EMI shielding materials with reliable absorption-dominated features and is highly promising for applications in next-generation electronic devices.

Ultrahigh discharge efficiency and improved energy density in rationally designed bilayer polyetherimide–BaTiO3/P(VDF-HFP) composites

Abstract: Polymer dielectric composites are of great interest as film capacitors that are widely used in pulsed power systems. For a long time, huge efforts have been devoted to achieving energy densities as high as possible to satisfy the miniaturization and high integration of electronic devices. However, the discharge efficiency which is particularly crucial to practical applications has gained little attention. With the target of achieving concurrently improved energy density and efficiency, a class of rationally designed bilayer composites consisting of a pure polyetherimide layer and a BaTiO3/P(VDF-HFP) composite layer were prepared. Interestingly, the bilayer composites exhibit ultrahigh discharge efficiencies η (>95%) under external electric fields up to 400 kV mm−1 which are much higher than most of the so far reported results (η < 80%). Meanwhile, a low loss (tan δ < 0.05 @ 10 kHz) comparable to that of the pure polyetherimide is obtained. In addition, the bilayer composites show impressive improvements in breakdown strengths Eb, i.e., 285%, 363%, 366% and 567% for composites with 5 vol%, 10 vol%, 20 vol% and 40 vol% BaTiO3, compared to their single layer counterparts, resulting in obviously improved energy densities Ud. In particular, the bilayer composite with 10 vol% BaTiO3 displays the most prominent comprehensive energy storage performance, i.e., η ∼ 96.8% @ 450 kV mm−1, Ud ∼ 6 J cm−3 @ 450 kV mm−1, tan δ ∼ 0.025 @ 10 kHz, and Eb ∼ 483.18 kV mm−1. The ultrahigh discharge efficiencies and high energy densities, along with low loss and breakdown strengths, make these bilayer composites ideal candidates for high-performance dielectric energy-storage capacitors.

Covalent organic frameworks: emerging high-performance platforms for efficient photocatalytic applications

Abstract: Covalent organic frameworks (COFs), as a new emerging class of highly crystalline advanced porous materials with fascinating structural tunability and diversity as well as the desired semiconductor properties, have gained significant attention as highly promising and efficient photocatalysts or designer platforms for a variety of photocatalytic applications in recent years; thus a comprehensive review is timely to summarize the advances of this field. In this review, a background and brief timeline concerning the developments and key achievements of COFs are provided. Afterwards, a systematic overview of the potential photocatalytic applications realized to date in the fast growing field of COFs is provided with the aim of presenting a full blueprint of COFs for possible photochemical energy conversion and reactions. Finally, the challenges remaining and personal perspectives on further development of this type of material for photocatalysis are presented.

Fabrication of hierarchical Co3O4@CdIn2S4 p–n heterojunction photocatalysts for improved CO2 reduction with visible light

Abstract: Hierarchical Co3O4@CdIn2S4 p–n heterojunction photocatalysts, composed of ultrathin CdIn2S4 nanosheets supported on Co3O4 nanocubes, have been assembled through a facile solvothermal method for efficient CO2 reduction with visible light. The solution-processed surface growth strategy is advantageous to form an intimately coupled interface in situ for the Co3O4@CdIn2S4 hybrids with easily controllable compositions. Diverse physicochemical characterization experiments reveal that the Co3O4@CdIn2S4 heterostructures have more catalytically active sites, increased CO2 adsorption, as well as hampered recombination and accelerated separation and mobility of photoexcited charge carriers. As a result, the Co3O4@CdIn2S4 p–n heterojunction photocatalysts display greatly improved activity compared to their nanosheet-assembled CdIn2S4 counterpart, affording an optimal CO generation rate of 53 μmol h−1 (i.e., 5300 μmol h−1 g−1) and a high CO selectivity of 93.3%. Besides, the optimized Co3O4@CdIn2S4 photocatalyst has a high apparent quantum efficiency (AQE) of 1.87% under monochromatic light irradiation at 420 nm. Importantly, such metal sulfide composite photocatalysts also exhibit excellent stability against photocorrosion, due to the efficient hole transfer from CdIn2S4 to Co3O4. On the basis of the band structures and catalytic evaluation results, a possible CO2 photoreduction mechanism is further proposed.

Progress in nickel chalcogenide electrocatalyzed hydrogen evolution reaction

Abstract: Electrochemical water splitting powered by electrical energy derived from renewable sources is a green and faster way of producing bulk hydrogen with the highest purity. Unfortunately, the cost-inefficiency associated with energy loss (as overpotential) and costs of electrode materials have been forbidding this technology to surpass the currently dominant industrial process (steam reforming of hydrocarbons). With the recent evolution of transition metal chalcogenides, efficient commercial electrochemical water splitting is not too far. Transition metal chalcogenides are better in the hydrogen evolution reaction (HER) than pristine metals as they have negatively polarized chalcogenide anions with relatively lower free energy for proton adsorption. Moreover, chalcogenides are relatively easy to prepare and handle. Several metal chalcogenides have been reported with good HER activity among which Ni chalcogenides are reported to be exceptional ones. In recent years, growth of the nickel chalcogenide catalysed HER is massive. This review is devoted to bringing out a comprehensive understanding of what had happened in the recent past of this field with highlights on future prospects. In addition, we have also briefed the key physico-chemical properties of these materials and highlighted what one should anticipate while screening an electrocatalyst for electrochemical water splitting.

Bismuth-based photocatalysts for solar energy conversion

Abstract: Semiconductor photocatalysis is a promising technology for solar fuel production and environmental remediation. However, the solar energy conversion efficiency is far from satisfactory, which restricts its practical application. The development of semiconductor materials is crucial to promote the solar energy conversion efficiency in photocatalytic systems. Owing to the Bi 6s and O 2p hybrid orbitals in the valence band, most of the bismuth-based photocatalysts possess a narrow bandgap for visible light utilization, which has attracted increasing attention. In the past few years, a rich family of bismuth-based photocatalysts have been developed, which are yet to be comprehensively reviewed. In this review article, the recent progress of bismuth-based nanomaterials for photocatalysis including pollutant degradation, water splitting, CO2 reduction, N2 fixation, and organic synthesis is critically reviewed. In particular, promising strategies for promoting the photocatalytic activity of each material system is discussed and the key challenges and prospects of bismuth-based materials for photocatalysis are presented, which are believed to promote the development of this important research field.