Showing papers in "Journal of Materials Chemistry in 2021"

A review on the recent advances in hybrid supercapacitors

Abstract: Presently, supercapacitors have gained an important space in energy storage modules due to their extraordinarily high power density, although they lag behind the energy density of batteries and fuel cells. This review covers recent approaches to not only increase the power density, rate capability, cyclic stability, etc. of supercapacitors, but also to increase their energy density using hybrid architectures. Electrodes are the most important component of a supercapacitor cell, and thus this review primarily deals with the design of hybrid supercapacitor electrodes offering a high specific capacitance, together with the elucidation of the mechanisms involved therein. The electrode performance significantly depends on the available surface area, porosity and conductivity of the component materials, and thus nano-structuring of the electrode is an elegant approach, which is discussed in the subsections for 0-, 1-, 2-, 3-dimensional hybrid materials, including some miscellaneous hybrids. The fabrication of different hybrid materials using metal oxides, metal sulfides, carbon materials, etc. with conducting polymers such as polyaniline and polypyrrole and their characterization are delineated from the literature data. Here, we primarily focus on the mechanism of energy storage by non-faradic electrical double-layer capacitance and faradaic pseudo-capacitance, discussing the contributions of different component mechanisms towards the total capacitance. In the hybrids, the impact of the component concentration operating via different mechanisms for charge storage on their final electrochemical performance is discussed. The specific capacitance, volumetric capacitance, charge–discharge cycles, Ragone plot, etc. of hybrid supercapacitors are described. Besides household and heavy-duty applications, the state-of-the-art future applications of supercapacitors in robotics, renewable and sustainable energy devices, wearable and self-healing supercapacitors, and biotechnology and their challenges in real-world applications with the scope of future work are elucidated.

C-, N-Vacancy defect engineered polymeric carbon nitride towards photocatalysis: viewpoints and challenges

Abstract: As an alluring metal-free polymeric semiconductor material, graphite-like carbon nitride (g-C3N4; abbreviated as GCN) has triggered a new impetus in the field of photocatalysis, mainly favoured from its fascinating physicochemical and photoelectronic structural features. However, certain inherent drawbacks, involving rapid reassembly of photocarriers, low specific surface area and insufficient optical absorption, limit the wide-range applicability of GCN. Generation of 0D point defects mainly by introducing vacancies (C and/or N) into the framework of GCN has spurred extensive consideration owing to their distinctive qualities to manoeuvre substantially, the optical absorption, radiative carrier isolation, and surface photoreactions. The present review endeavours to summarise a comprehensive study on vacancy defect engineered GCN. Starting from the basic introduction of defects and C/N vacancy modulated GCN, numerous advanced strategies for the controlled designing of vacancy rich GCN have been explored and discussed. Afterwards, light was thrown on the various substantial technologies which are useful for characterising and identifying the introduction of defects in GCN. The salient significance of defect engineering in GCN has been reviewed concerning its impact on optical absorption, charge isolation and surface photoreaction ability. Typically, the achievement of defect engineered GCN has been scrutinised toward various applications like photocatalytic water splitting, CO2 conversion, N2 fixation, pollutant degradation, and H2O2 production. Finally, the review ends with conclusions and vouchsafing future challenges and opportunities on the intriguing and emerging area of vacancy defect engineered GCN photocatalysts.

Recent advances in transition-metal-sulfide-based bifunctional electrocatalysts for overall water splitting

Abstract: Hydrogen produced via water electrolysis can act as an ideal clean chemical fuel with superb gravimetric energy density and high energy conversion efficiency, solving the problems of conventional fossil fuel exhaustion and environmental contamination. Transition metal sulfides (TMS) have been extensively explored as effective, widely available alternatives to precious metals in overall water splitting. Herein, recent advances, covering preparation methods, intrinsic electrocatalytic performance, and optimization strategies, relating to TMS-based bifunctional electrocatalysts have been summarized systematically and comprehensively. Firstly, a general introduction to the reaction mechanisms and key parameters of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is provided. Next, the physicochemical properties of TMS and typical synthesis methods are introduced to give guidance for fabricating TMS materials with well-defined structures, controllable compositions, and excellent performance. Importantly, the intrinsic activities of TMS-based electrocatalysts and several strategies for improving their bifunctional electrocatalytic performance during water electrolysis are discussed in detail. Finally, perspectives covering the challenges and opportunities related to the further development of TMS-based materials with high activity and long-term durability for overall water splitting are given. The aim herein is to provide guidelines for the design and fabrication of TMS-based bifunctional electrocatalysts with excellent performance and to accelerate their large-scale practical application in water electrolysis.

High-performance lead-free bulk ceramics for electrical energy storage applications: design strategies and challenges

Abstract: Compared with fuel cells and electrochemical capacitors, dielectric capacitors are regarded as promising devices to store electrical energy for pulsed power systems due to their fast charge/discharge rates and ultrahigh power density. Dielectric materials are core components of dielectric capacitors and directly determine their performance. Over the past decade, extensive efforts have been devoted to develop high-performance dielectric materials for electrical energy storage applications and great progress has been achieved. Here, we present an overview on the current state-of-the-art lead-free bulk ceramics for electrical energy storage applications, including SrTiO3, CaTiO3, BaTiO3, (Bi0.5Na0.5)TiO3, (K0.5Na0.5)NbO3, BiFeO3, AgNbO3 and NaNbO3-based ceramics. This review starts with a brief introduction of the research background, the development history and the basic fundamentals of dielectric materials for energy storage applications as well as the universal strategies to optimize their energy storage performance. Emphases are placed on the design strategies for each type of dielectric ceramic based on their special physical properties with a summary of their respective advantages and disadvantages. Challenges along with future prospects are presented at the end of this review. This review will not only accelerate the exploration of higher performance lead-free dielectric materials, but also provides a deeper understanding of the relationship among chemical composition, physical properties and energy storage performance.

Recent progress of high-entropy materials for energy storage and conversion

Abstract: The emergence of high-entropy materials (HEMs) with their excellent mechanical properties, stability at high temperatures, and high chemical stability is poised to yield new advancement in the performance of energy storage and conversion technologies. This review covers the recent developments in catalysis, water splitting, fuel cells, batteries, supercapacitors, and hydrogen storage enabled by HEMs covering metallic, oxide, and non-oxide alloys. Here, first, the primary rules for the proper selection of the elements and the formation of a favorable single solid solution phase in HEMs are defined. Furthermore, recent developments in different fields of energy conversion and storage achieved by HEMs are discussed. Higher electrocatalytic and catalytic activities with longer cycling stability and durability compared to conventional noble metal-based catalysts are reported for high-entropy materials. In electrochemical energy storage systems, high-entropy oxides and alloys have shown superior performance as anode and cathode materials with long cycling stability and high capacity retention. Also, when used as metal hydrides for hydrogen storage, remarkably high hydrogen storage capacity and structural stability are observed for HEMs. In the end, future directions and new energy-related technologies that can be enabled by the application of HEMs are outlined.

Inverse desert beetle-like ZIF-8/PAN composite nanofibrous membrane for highly efficient separation of oil-in-water emulsions

Abstract: Purification of highly emulsified oily wastewater is of significance but challenging due to the critical limitations of low efficiency and serious membrane blocking. Herein, inspired by the desert beetle's capability of water-collection in air, we first constructed an inverse desert beetle-like membrane to achieve oil capture in water, thus improving the separation performances of the surfactant-stabilized oil-in-water emulsion. The as-prepared nanofibrous membrane consists of hydrophobic/oleophilic ZIF-8 bumps and underlying underwater superoleophobic polyacrylonitrile (PAN), and possesses a similar structure to desert beetles but with opposite wettability. In the process of emulsion demulsification, besides the sieving effect of the underlying traditional underwater superoleophobic PAN membrane, the bumpy oleophilic ZIF-8 particles of the composite membrane can capture oil, thus strengthening the demulsification ability of the membrane. Furthermore, the separation efficiency of the inverse desert beetle-like membrane is up to 99.92% for surfactant-stabilized oil-in-water emulsion, which is superior to that of traditional single underwater superoleophobic PAN membranes lacking the hydrophobic bump structure. Moreover, for light oil-in-water emulsion, the permeation flux also increases because the captured tiny oil droplets can aggregate subsequently and grow large enough to detach from the underlying surface which greatly reduces the incidence of filter cake formation and congestion. This inverse desert beetle-like membrane opens up new avenues for designing advanced membranes in the oil/water emulsion separation field.

A vast exploration of improvising synthetic strategies for enhancing the OER kinetics of LDH structures: a review

Abstract: In the last few decades, layered double hydroxide (LDH) based materials have been given a lot of attention to the researchers due to significant advances in enabling efficient oxygen evolution. Their exceptional layered structure, sole electronic structure, and high specific surface area, makes them to show good OER catalytic performance. The catalytic performances of LDHs are largely governed by their composition, morphology, and intercalated anions, which can be produced via exfoliation processes and different synthetic methods. Although LDH materials have enormous advantages in energy-related applications, there are still some inherent drawbacks in the OER process that need to be overcome for long-term usage. These drawbacks in the internal properties of LDHs are attributed to their low electrical conductivity and structural collapse, particularly during the exfoliation process, making the OER kinetics unpredictable. To overcome these drawbacks, researchers have developed many strategies recently, and these are (i) nano-structuring; (ii) the addition of cations; (iii) anion exchange; (iv) the synthesis of materials with conductive supports; and (v) vacancy creation. Hence, this review ultimately focuses on how to overcome the extensive issues faced by LDHs in terms of synthetic and exfoliation methods and discusses various strategies developed in recent years in the field of the electrocatalytic water-based oxygen evolution reaction.

Metal–organic frameworks (MOFs) beyond crystallinity: amorphous MOFs, MOF liquids and MOF glasses

Abstract: The field of metal–organic frameworks (MOFs) has been incorrectly believed to be purely crystalline. However, non-crystalline MOFs (amorphous MOFs, MOF liquids, and MOF glasses) are starting to emerge as alternative materials, beyond the dictatorial domain of crystalline MOFs. Non-crystalline MOFs present many opportunities, either as novel functional materials themselves, or as vehicles to create other materials. In this extensive Review, we describe the two approaches to preparing amorphous MOFs: (1) the amorphization of crystalline MOFs and (2) the direct synthesis. Special attention is paid to the relationship between preparation method, properties and applications of amorphous MOFs. We also explore the field of MOF liquids and their applications, centering our attention to the phenomenon of melting. Finally, MOF glasses are explained. We highlight the properties and applications of the MOF glasses that are not usually found in crystalline MOFs. New related glass materials such as MOF-blends, flux melted MOFs, MOF crystal-glass composites, MOF and inorganic glass composites, and MOF glass membranes are also reviewed. We conclude the fields of amorphous MOFs, MOF liquids, and MOF glasses by presenting our thoughts on the possible future research directions.

Electrostatically self-assembled two-dimensional magnetized MXene/hollow Fe3O4 nanoparticle hybrids with high electromagnetic absorption performance and improved impendence matching

Abstract: Electromagnetic pollution often interferes with the normal use of sophisticated electric devices leading to the necessity of developing electromagnetic wave absorbers with light weight and strong absorption ability. Herein, we synthesized two-dimensional magnetized MXene hybrids by electrostatic assembly of negatively charged few-layered Ti3C2Tx (MXene) with positively charged hollow Fe3O4 nanoparticles (HFO). The few-layered MXene was obtained by etching Ti3AlC2via a modified LiF–HCl method followed by a sonication process, while HFO was fabricated by a facile hydrothermal process. The MXene/HFO hybrids were light weight and achieved a high EM wave absorption performance (RLmin of −63.7 dB at a thin thickness of 1.56 mm). Moreover, the strong EM wave attenuation resulted from the synergistic effect arising from dielectric loss, magnetic loss, interface polarization and improved impedance matching. Therefore, the as-prepared magnetized MXene hybrids are expected to be candidates for high performance electromagnetic microwave absorbers.

Chemical supercapacitors: a review focusing on metallic compounds and conducting polymers

Abstract: Capacitors began their journey in 1745, and to date have advanced in the form of supercapacitors. Supercapacitors are one of the advanced forms of capacitors with higher energy density, bridging capacitors and batteries. The energy storage through the formation of an electrical double layer is pivotal for supercapacitor technology. Accordingly, to further improve the energy density, surface faradaic (pseudocapacitive) processes are employed, and henceforth, the journey of chemical supercapacitors commenced. Herein, the materials, mechanisms and fabrication of chemical supercapacitors based on metallic compounds and conducting polymers are discussed in detail. The inherent limitations of these materials are addressed, and the feasible mitigation measures are identified. Poor conductivity, slow diffusion kinetics and rapid structural disintegration over cycling are the common constraints of metallic compounds, which can be overcome by preparing conductive nanocomposites. Thus, versatile conductive nanocomposites of metal oxides, hydroxides, carbides, nitrides, phosphides, phosphates, phosphites, and chalcogenides are elaborated. A lack of structural integrity is the prime obstacle for the realization of conducting polymer-based supercapacitors, which may be solved by forming composites with robust support from carbonaceous materials or metallic compounds. Consequently, the composites of polyaniline, polypyrrole, polythiophene and polythiophene-derivatives are discussed. The historical accounts of early stages of works are emphasised in order to review the developmental pathways of chemical supercapacitors. The construction of full cells and their performance data are presented herein, which synchronize the behaviour of practical scaled-up devices. To the best of our knowledge, this review is the first holistic description of chemical supercapacitors based on metallic compounds and conducting polymers from the first reports to recent advancements.

Recent advances in highly active nanostructured NiFe LDH catalyst for electrochemical water splitting

Abstract: Highly efficient, low-cost electrocatalysts having superior activity and stability are crucial for practical electrochemical water splitting, which involves hydrogen and oxygen evolution reactions (HER and OER). The sustainable production of hydrogen fuel from electrochemical water splitting requires the development of a highly efficient and stable electrocatalyst with low overpotential that drives electrochemical redox reactions. Electrochemical water splitting using highly active nickel-iron layered double hydroxide (NiFe LDH) catalyst having a very high turnover frequency and mass activity is considered as a potential contender in the area of electrocatalysis owing to the practical challenges including high efficiency and long durability at low overpotential, which shows great potential in future hydrogen economy. This review includes certain recommendations on enhancing the electrocatalytic performance of NiFe LDH-based electrocatalyst, particularly through morphology engineering, construction of hierarchical/core–shell nanostructures, and doping of heteroatoms through combined experimental assessment and theoretical investigations, which in turn improve the electrocatalytic performance. Finally, emphasis is made on the bifunctional activity of the NiFe LDH catalyst for overall water splitting. At the end, the conclusions and future outlook for the design of the NiFe LDH catalyst towards scale-up for their use as electrolyzer at the industrial level are also discussed.

Advanced research trends in dye-sensitized solar cells

Abstract: Dye-sensitized solar cells (DSSCs) are an efficient photovoltaic technology for powering electronic applications such as wireless sensors with indoor light. Their low cost and abundant materials, as well as their capability to be manufactured as thin and light-weight flexible solar modules highlight their potential for economic indoor photovoltaics. However, their fabrication methods must be scaled to industrial manufacturing with high photovoltaic efficiency and performance stability under typical indoor conditions. This paper reviews the recent progress in DSSC research towards this goal through the development of new device structures, alternative redox shuttles, solid-state hole conductors, TiO2 photoelectrodes, catalyst materials, and sealing techniques. We discuss how each functional component of a DSSC has been improved with these new materials and fabrication techniques. In addition, we propose a scalable cell fabrication process that integrates these developments to a new monolithic cell design based on several features including inkjet and screen printing of the dye, a solid state hole conductor, PEDOT contact, compact TiO2, mesoporous TiO2, carbon nanotubes counter electrode, epoxy encapsulation layers and silver conductors. Finally, we discuss the need to design new stability testing protocols to assess the probable deployment of DSSCs in portable electronics and internet-of-things devices.

Recent progress in the development of biomass-derived nitrogen-doped porous carbon

Abstract: This review offers a focused discussion on the recent progress of biomass-derived nitrogen-doped porous carbon (NPC) and its applications. Various synthesis methods for biomass-derived NPCs are introduced and critically reviewed. N-doping is a promising approach for further improving the physicochemical/electrochemical properties of carbon materials. Besides, NPC synthesis from inexpensive biomass for energy storage applications is a green and sustainable strategy. NPCs can be synthesized directly from algae, chitosan, and glucosamine without using any additional N precursor. The effect of synthesis methods on the physicochemical properties of NPCs offers a direction for optimizing the properties of NPCs for diverse applications. The utilization of NPCs in various applications, including catalysis and electrochemical energy storage (e.g., fuel cells, batteries, and supercapacitors), is reviewed. Besides, a discussion on the use of NPCs in oxidation and hydrogenation reactions, CO2 capture and reduction is provided. The factors controlling the electrocatalytic performance of NPC are evaluated, such as the effect of N-content and the type of N species in NPCs. Finally, to improve the rational design of biomass-derived NPCs for catalysis and energy storage applications, an outlook and conclusion are provided.

Advances in noble metal (Ru, Rh, and Ir) doping for boosting water splitting electrocatalysis

Abstract: Electrochemical water splitting has a promising future in producing high-density and green hydrogen, however, the sluggish H2O dissociation process, due to the low H2O adsorption on the catalyst surface, greatly hinders the industrial electrochemical water splitting on a large scale. Therefore, intensive efforts have been devoted to the exploration of efficient approaches for fabricating highly efficient electrocatalysts with appropriate H2O adsorption, such as defect engineering, interface engineering, and morphology design. Among them, metal doping, particularly noble metal (Ru, Rh, and Ir) doping, is essential to optimize the adsorption of reaction intermediates on the surface of catalysts, and has thus attracted increasing research interest. In order to uncover the significant role of noble metal doping in boosting water splitting electrocatalysis, this minireview showcases the most recent examples towards this endeavor, and begins by illustrating the mechanisms for water splitting and several advanced approaches for realizing noble metal doping. In the main text, we have also specifically highlighted the influences of noble metal doping on the electrocatalytic performance. Finally, some challenges and future outlooks are also presented to offer guidance for practical applications.

Recent advances in cellulose-based piezoelectric and triboelectric nanogenerators for energy harvesting: a review

Abstract: Cellulose is the most earth-abundant natural polymer resource, which with combined eco-friendly and extraordinary sustainable properties such as renewability, biodegradability, low cost and excellent biocompatibility has been widely used by humans for thousands of years. In the past few years, many novel cellulosic materials and their unique applications have been developed including the recent research focus on energy harvesting. The high crystallization and plentiful polar hydroxyl groups endow cellulose with a large number of dipoles and strong electron donating capacity, resulting in a promising potential of piezoelectric and triboelectric effects. However, there is no review about cellulose-based nanogenerators until now. In this paper, the most recent developments of designing, modification, processing and integration of cellulose-based piezoelectric nanogenerators (PENGs), triboelectric nanogenerators (TENGs) and hybrid piezo/triboelectric nanogenerators (PTENGs) for energy harvesting and other applications are reviewed in detail. For cellulose-based PENGs, representative basic piezoelectric cellulose and recent research on PENG devices are discussed. For cellulose-based TENGs, several effective strategies including rough modification of contact surface, addition of electronic functional fillers and chemical modification for improving the output performance are further summarized. Meanwhile, the latest cellulose-based hybrid PTENG is also introduced from the fundamental design to the investigations on enhanced strategies. The opportunities and challenges of these cellulose-based nanogenerator devices are put forward in the final part, which could enable this up-to-date and state-of-the-art review to be an effective guidance for the future research on cellulose-based nanogenerators in energy harvesting.

High-entropy alloys: emerging materials for advanced functional applications

Abstract: Accompanied by enhancements in the ability to fabricate materials for humans, alloy-based materials have advanced from binary alloy systems to complicated compositions along with affording newer applications, which can accelerate the evolution of civilization. Recently, high-entropy alloys (HEAs) have drawn enormous attention in diverse fields because of their distinctive concept and unique properties. The impressive mechanical properties, such as excellent strength, unforgettable corrosion resistance, and superior thermostability, are inherited and overwhelming compared with traditional alloys. Therefore, HEAs have become an emerging class of advanced materials leading to a new field. Based on the exceptional synthesis methods, HEAs surprisingly afford numerous energy and environmental properties, which have endowed HEAs with promising applications. In this paper, we review the salient features of HEAs and summarize their core effects, phase structures, unconventional synthesis methods, and novel energy and environmental applications. In addition, we also discuss the broad space waiting to be explored and overview fruitful pathways of future trends and prospects.

Advances in electromagnetic shielding properties of composite foams

Abstract: In recent decades, problems with electromagnetic interference (EMI) radiation problems have arisen, that can seriously reduce the performance of precision devices nearby and threaten human health. In consequence, it is important to seek high efficiency materials to suppress EMI pollution. Generally, high magnetic permeability or electrical conductivity is essential for efficient EMI shielding. Conventionally, various forms of metal construction (sheet/films, coatings, etc.) are used for EMI shielding. However, metallic shielding has drawbacks that include high density, lack of corrosion resistance and expensive processing, which restrict its use in the modern electronic world. By contrast, conductive polymer composites (CPCs), formed from insulative polymers and conductive fillers, have attracted more and more interest from both industry and academia. CPCs have properties that offer great potential for application in efficient EMI shielding. These include low density, high flexibility, good chemical stability and easy processing and forming. From a theoretical viewpoint, it is generally accepted that the shielding of EM waves is due to the three basic mechanisms of reflection, absorption and multiple internal reflections. However, it must be recognized that the SE of CPCs has a close relation to the reflection mechanism, which can cause secondary EMI pollution. Therefore, materials with charge carriers or magnetic/electric dipoles as well as cellular structure should be the focus for the development of EMI shielding materials with strong absorption properties. On that basis, the main aim of this paper is to review the current position in research of the design of inorganic based foams (metal, carbon or MXene) and polymer composite foams as EMI shielding materials. On the one hand, these composite foams have the merit of being lightweight, and on the other hand, the special porous structure can effectively harvest microwaves by prolonging the travel path. As a result, absorption dominates EMI shielding, which satisfies current requirements of EMI shielding applications. This review also points out the future challenges and gives guidelines for finding solutions for the next generation of shielding applications using composite foams.

Mo2B2 MBene-supported single-atom catalysts as bifunctional HER/OER and OER/ORR electrocatalysts

Abstract: Searching for highly efficient and cost-effective bifunctional electrocatalysts for the oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), which can be applied to water splitting, fuel cells and metal–air batteries, is critical for developing clean and renewable energies. Yet it remains a great challenge. By means of first-principles calculations, we have studied the OER, ORR and HER catalytic activity of Mo2B2 MBene-supported single-atom catalysts (SACs) by embedding a series of transition metal atoms in the Mo vacancy (TM@Mo2B2, TM = Ti, V, Cr, Mn, Fe, Co, Ni and Cu) as electrocatalysts. All TM@Mo2B2 SACs show excellent metallic conductivity, which would be favorable for the charge transfer in electrocatalytic reactions. Importantly, Ni@Mo2B2 can be used as a HER/OER bifunctional electrocatalyst with a lower |ΔGH| (−0.09 eV) for the HER under 1/4H coverage and a lower overpotential (ηOER = 0.52 V) than that of IrO2 (ηOER = 0.56 V) for the OER, while Cu@Mo2B2 can be used as an OER/ORR bifunctional electrocatalyst with a lower overpotential (ηOER = 0.31 V) than that of IrO2 (ηOER = 0.56 V) and RuO2 (ηOER = 0.42 V) for the OER and a lower overpotential of 0.34 V than that of Pt (ηORR = 0.45 V) for the ORR, for both of which the transition metal atoms serve as the active sites. This work could open up an avenue for the development of non-noble-metal-based bifunctional MBene electrocatalysts.

Recent advances in layered Ln2NiO4+δ nickelates: fundamentals and prospects of their applications in protonic ceramic fuel and electrolysis cells

Abstract: In the past decade, intensive research on proton-conducting oxide materials has provided a basis for the development of intermediate-temperature protonic ceramic electrochemical cells, which constitute a real alternative to conventional cells based on oxygen-conducting electrolytes. To achieve both high efficiency and excellent performance, not only electrolytes but also electrode materials should be carefully selected considering their functional properties. Compared to the traditional ABO3 perovskite electrode materials, Ln2NiO4+δ with a layered structure has unique advantages (high chemical stability, mechanical compatibility, improved oxygen transport, and hydration ability), and thus is now becoming a hot topic in this field, offering both scientific and practical interests. However, a comprehensive and in-depth review is still lacking in the literature to date. Accordingly, this work presents a comprehensive overview of the prospects of layered nickelates (Ln2NiO4+δ, where Ln = La, Nd, and Pr) as one of the most attractive oxygen (steam) electrode materials for protonic ceramic electrochemical cells. In particular, the crystalline features, defect structure, stability, chemical properties, and mechanical compatibility of this class of materials, contributing to their transport functionality, are discussed with the primary emphasis on revealing the relationship between the composition of the materials and their properties. The presented systematic results reveal the main strategies regarding the utilisation of Ln2NiO4+δ-based electrodes and existing gaps related to fundamental and applied research aspects.

Recent progress in ammonia fuel cells and their potential applications

Abstract: Conventional technologies are largely powered by fossil fuel exploitation and have ultimately led to extensive environmental concerns. Hydrogen is an excellent carbon-free energy carrier, but its storage and long-distance transportation remain big challenges. Ammonia, however, is a promising indirect hydrogen storage medium that has well-established storage and transportation links to make it an accessible fuel source. Moreover, the notion of ‘green ammonia’ synthesised from renewable energy sources is an emerging topic that may open significant markets and provide a pathway to decarbonise a variety of applications reliant on fossil fuels. Herein, a comparative study based on the chosen design, working principles, advantages and disadvantages of direct ammonia fuel cells is summarised. This work aims to review the most recent advances in ammonia fuel cells and demonstrates how close this technology type is to integration with future applications. At present, several challenges such as material selection, NOx formation, CO2 tolerance, limited power densities and long term stability must still be overcome and are also addressed within the contents of this review.

Deeply reconstructed hierarchical and defective NiOOH/FeOOH nanoboxes with accelerated kinetics for the oxygen evolution reaction

Abstract: Transition metal phosphides (TMPs) have been reported as efficient pre-catalysts for the oxygen evolution reaction (OER) in alkaline media. In situ generated metal oxyhydroxides on the surface of TMPs serve as real active sites. However, the reconstruction of most of the reported TMPs is incomplete and the active components cannot be fully used. Herein, hollow nanostructured Ni5P2/FeP4 nanoboxes (NiFeP NBs) are designed and synthesized as pre-catalysts. During the OER, the NiFeP NBs deeply reconstruct into low-crystalline and ultrathin NiOOH/FeOOH nanosheet assembled nanoboxes (NiOOH/FeOOH NBs). In situ Raman spectroscopy and ex situ characterization studies provide evidence that the hollow nanostructure facilitates the deep reconstruction of NiFeP NBs. Benefiting from the hierarchical hollow structure, the abundant interface between NiOOH and FeOOH, and plentiful defects, the reconstructed NiOOH/FeOOH NBs exhibit superior OER activity and excellent stability. Density functional theory (DFT) calculations reveal that the Fe–Ni dual sites in the NiOOH/FeOOH interface may be the possible active sites.

Performance of metal-organic frameworks in the electrochemical sensing of environmental pollutants

Abstract: Environmental pollution has been a known threat to our world due to the rapid urbanization, changing lifestyle of people, and modern industrialization. Therefore, there is an urgent need to develop novel sensing approaches having promising performance with high reliability and sensitivity for the precise monitoring of various pollutants. Metal–organic frameworks (MOFs) have been intensively investigated by many researchers as electrode modifiers for electrochemical sensing due to their excellent properties and efficiency. Diverse MOF-based electrochemical sensing systems are applied for environmental analysis for the sensitive, rapid and cost-effective determination of various analytes because of their unique structures, and properties, including the tunable pore size, high surface area, high catalytic activity, and high density of active sites. The aim of this review article is to evaluate the application of MOFs in the electrochemical sensing of environmental pollutants including heavy metal ions, pesticides, phenolic compounds, nitroaromatic compounds, antibiotics, nitrite, and hydrazine. Current limitations and future directions for the application of MOF-based electrochemical sensors for the detection of environmental pollutants are discussed.

Ti3C2Tx MXene for electrode materials of supercapacitors

Abstract: To promote the development of supercapacitors and their applications in modern electronics, it is crucial to explore novel supercapacitor electrode materials. As a representative member of the rising 2D MXenes, Ti3C2Tx MXene has shown tremendous potential for supercapacitor electrodes owing to its unique physicochemical properties. Here, the most recent advances in Ti3C2Tx-based supercapacitor electrodes are comprehensively reviewed, with an emphasis on the vital role that Ti3C2Tx MXene plays in the remarkable electrochemical performance and related mechanisms. The fabrication methods, electrode structures, working mechanisms, electrochemical performance and related influencing factors, mechanical properties and applications, as well as the associated advantages/disadvantages of Ti3C2Tx-based supercapacitor electrodes are thoroughly and exhaustively summarized and discussed. Based on the recent progress, the existing challenges along with the corresponding possible solutions, and the future prospects of Ti3C2Tx-based materials for supercapacitors are also outlined and discussed.

Engineering defects in 2D g-C3N4 for wideband, efficient electromagnetic absorption at elevated temperature

Abstract: Metal-free 2D nanomaterials such as graphitic carbon nitride (g-C3N4) nanosheets have attracted enormous attention due to their ultralow mass density, excellent chemical stability, high specific surface area, unique electronic structure and permittivity. However, the electromagnetic (EM) wave absorption performance of g-C3N4 cannot satisfy the requirements for addressing the ever-increasing occurrence of EM pollution. Herein, we demonstrate that the creation of pores in g-C3N4 nanosheets, combined with subsequent doping with phosphorus (P) and sulphur (S) atoms, give rise to a continuous frequency dispersive behaviour along with an enhanced conductive loss capability. As a result, the S/P-doped nanoporous g-C3N4 exhibit an efficient EM absorption over a wide frequency region (e.g., 6.0 GHz of >90% of absorption effectiveness at a sample thickness of 1.8 mm) at elevated temperatures (e.g., >4.0 GHz of >90% of absorption effectiveness at a thickness of 1.2 mm at 150 °C). Overall, our results reported in this work unmask new principles by which metal-free 2D nanomaterials can be modified to enable a significant enhancement in their EM absorption performance.

Multifunctional organic ammonium salt-modified SnO2 nanoparticles toward efficient and stable planar perovskite solar cells

Abstract: Bulk and interfacial nonradiative recombination hinder the further enhancement of the power conversion efficiency (PCE) and stability of SnO2-based planar perovskite solar cells (PSCs) To date, it is still a huge challenge to minimize the bulk and interfacial nonradiative recombination losses, and thus maximize the potentials of PCE and stability Herein, a novel and effective multifunctional modification strategy through incorporating Girard's Reagent T (GRT) molecules with multiple functional groups to modify SnO2 nanoparticles (NPs), which significantly reduces the bulk and interfacial nonradiative recombination losses through the simultaneous achievement of suppressing nanoparticle agglomeration, improving the electronic property of SnO2 films, facilitating the vertical growth and enlarging the grain size of perovskite crystals, and passivating interfacial defects is reported As a result, the device based on GRT modification delivers a much higher PCE of 2163%, along with significantly suppressed hysteresis, as compared to the control device (1977%) The device stability is ameliorated after GRT modification The unencapsulated device with GRT maintains 995% of its initial PCE after aging at 60 °C for 720 h and 585% after illumination for 672 h under one sun, respectively The present work provides guidance for the design of multifunctional modification molecules toward efficient and stable PSCs

Gd-induced electronic structure engineering of a NiFe-layered double hydroxide for efficient oxygen evolution

Abstract: Rare earth (RE) elements have drawn increased attention recently as an effective promoter in electrocatalysis because of their partially filled 4f orbitals. Herein, a new type of RE hybrid electrocatalyst, consisting of a gadolinium-doped hierarchal NiFe-layered double hydroxide in situ grown on carbon cloth (Gd-NiFe-LDH@CC), is designed and developed via a facile one-step hydrothermal approach. The Gd doping regulates the electronic structure of NiFe-LDH and increases the number of oxygen vacancies, thus tuning the adsorption energies of oxygen intermediate species (e.g. HOO*). With the presence of Gd species, Gd-NiFe-LDH@CC exhibits superior electrocatalytic activity for the OER in an alkaline medium, which requires an overpotential of only 210 mV to afford 10 mA cm−2 current density, better than that of NiFe-LDH@CC (250 mV) and commercial RuO2 (298 mV). The robust electrocatalytic stability and satisfactory selectivity (nearly 100% Faraday efficiency) of Gd-NiFe-LDH@CC for the OER are also demonstrated. We ascribe such outstanding OER performance of Gd-NiFe-LDH@CC to the optimized electronic structure, rich oxygen vacancies and hierarchal porous morphology. Theoretical calculation further demonstrates that the electronic disturbance caused by Gd doping enhances the activity of Ni sites, resulting in stronger binding strength of HOO* at the Ni sites during the OER.

Rationally designed hierarchical N-doped carbon nanotubes wrapping waxberry-like Ni@C microspheres for efficient microwave absorption

Abstract: Hierarchical microstructures are playing important roles in the design and fabrication of high-performance microwave absorbing materials (MAMs) owing to their unique advantages. In this work, a series of special “double-hierarchical” N-doped carbon nanotubes wrapping waxberry-like Ni@C microspheres (NC@NCNTs) have been rationally designed and successfully fabricated by two-step pyrolysis processes, where the loading amount of NCNTs on the waxberry-like Ni@C microspheres can be easily modulated by changing the dosage of melamine. Benefiting from sufficient attenuation ability and good impedance matching, NC@NCNTs-2, whose relative carbon content is 51.1 wt%, exhibits the best reflection loss (RL) characteristics among this series of composites, including the minimum RL intensity of −41.5 dB and an effective absorption bandwidth of 5.2 GHz with an absorber thickness of only 1.7 mm. This performance is superior to that of many homologous Ni/C composites reported previously. The investigation on EM properties indicates that the unique “double-hierarchical” architecture of NC@NCNTs can not only create stronger dipole orientation and interfacial polarization relaxation, but can also result in higher conductive loss as well as extra multiple reflection effects for incident electromagnetic waves. We believe that these results will provide some inspirations and pathways for the production of high-performance MAMs with senior microstructures in the future.

All-solid-state direct Z-scheme NiTiO3/Cd0.5Zn0.5S heterostructures for photocatalytic hydrogen evolution with visible light

Abstract: The construction of NiTiO3/Cd0.5Zn0.5S heterostructures is presented as all-solid-state direct Z-scheme photocatalysts for the efficient and stable hydrogen production under visible light. The NiTiO3/Cd0.5Zn0.5S hybrids are assembled by growing Cd0.5Zn0.5S nanoparticles on the surface of NiTiO3 nanorods via a co-precipitation and hydrothermal coupled method. The compositional and structural features of the NiTiO3/Cd0.5Zn0.5S composites are fully disclosed via diverse physicochemical characterizations. The NiTiO3/Cd0.5Zn0.5S heterostructures are revealed to effectively capture the optical spectrum in the visible region as well as enhance the transfer and separation of photogenerated charge carriers through the Z-schematic pathway. Consequently, the optimized NiTiO3/Cd0.5Zn0.5S photocatalyst shows a high H2 production rate of 1058 μmol h−1 (26.45 mmol h−1 g−1), which is independent of any cocatalysts (such as Pt), together with a high apparent quantum yield (AQY) of 34% under monochromatic light irradiation at 420 nm. Besides, the NiTiO3/Cd0.5Zn0.5S composites also exhibit a high stability for the H2 evolution photocatalysis mainly due to the fact that the Z-schematic charge separation and migration can enable the efficient consumption of light-induced holes of Cd0.5Zn0.5S to prevent the photocorrosion effect. Finally, a possible photocatalytic H2 evolution mechanism over the Z-schematic NiTiO3/Cd0.5Zn0.5S heterostructures is also presented based on the results of the band structures, density functional theory (DFT) calculations and electron spin resonance (ESR) tests.

Coordination tunes the activity and selectivity of the nitrogen reduction reaction on single-atom iron catalysts: a computational study

Abstract: Tuning the electronic structure of a single-atom catalyst (SAC) by controlling its coordination has been recently shown to be a rather promising strategy for further improving its catalytic performance in some electrochemical reactions. Herein, by means of density functional theory (DFT) computations, the impacts of the coordination structure of an Fe–N–C catalyst on its catalytic activity toward the nitrogen reduction reaction (NRR) were explored. Our results revealed that the NRR activity on the central Fe atom can be greatly improved by its coordination with a boron (B) dopant. In particular, the computed limiting potential of the NRR on Fe–B2N2 is −0.65 V, which is the lowest among all B doped Fe–N–C catalysts, suggesting its high NRR catalytic activity. Interestingly, the introduction of B coordination can effectively modulate the interaction of the single Fe atom with the N2H* species, thus improving its NRR catalytic performance. In addition, Fe–B2N2 exhibits high NRR selectivity by effectively suppressing the competing hydrogen evolution reaction (HER) both thermodynamically and kinetically. Therefore, the single Fe catalyst with N and B dual coordination can be utilized as a promising NRR electrocatalyst, which not only highlights the significant effect of local coordination on catalytic activity and selectivity for the NRR, but also provides a new opportunity to further develop more advanced single-atom catalysts for ammonia synthesis.

Synthesis of core–shell Co@S-doped carbon@ mesoporous N-doped carbon nanosheets with a hierarchically porous structure for strong electromagnetic wave absorption

Abstract: Electromagnetic wave absorbents with hierarchically porous and core–shell structures have significantly positive influence on the electromagnetic wave absorption because of the enhanced interfacial polarization. Furthermore, the core–shell structure also introduces components with strong dielectric loss and good resistance to chemical corrosion. Herein, the cobalt–metal–organic frameworks @mesoporous polydopamine (Co–MOF@MPDA) composites with a core–shell structure are prepared by the bottom-up monomicelle assembly. After calcination, the Co@S-doped carbon core and mesoporous N-doped carbon shell (Co@SC@MNC) were obtained. Through adjusting the calcination temperature, the dielectric and magnetic loss can be tuned, resulting in the strong absorption capability for the electromagnetic wave. The minimum reflection loss reaches −72.3 dB, while the effective absorption bandwidth is as broad as 6.0 GHz. The unique structure and the formation of internal cavity between Co@SC and MNC contribute to the interfacial polarization. The enhancement of the dipole polarization loss and conduction loss are ascribed to the S, N-doped hierarchically porous carbon. Importantly, the presence of Co nanoparticles facilitates the magnetic–dielectric synergy to improve the impedance matching due to the introduction of magnetic loss. The novel structural design has potential application in the electromagnetic wave absorption field.