Showing papers in "Journal of Materials Chemistry in 2019"

Lithium-ion batteries: outlook on present, future, and hybridized technologies

Abstract: Lithium-ion batteries (LIBs) continue to draw vast attention as a promising energy storage technology due to their high energy density, low self-discharge property, nearly zero-memory effect, high open circuit voltage, and long lifespan. In particular, high-energy density lithium-ion batteries are considered as the ideal power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs) in the automotive industry, in recent years. This review discusses key aspects of the present and the future battery technologies on the basis of the working electrode. We then discuss how lithium-ion batteries evolve to meet the growing demand on high charge capacity and electrode stability. An account of a stand-alone energy device (off-grid system) that combines an energy harvesting technology with a lithium-ion battery is also provided. The main discussion is categorized into three perspectives such as the evolution from the conventional to the advanced LIBs (e.g., Li-rich transition metal oxide and Ni-rich transition metal oxide batteries), to the state-of-the-art LIBs (e.g., Li–air, Li–sulfur batteries, organic electrode batteries, solid-state batteries, and Li–CO2 batteries), and to the hybridized LIBs (e.g., metal halide perovskite batteries).

Biomass-derived porous carbon materials with different dimensions for supercapacitor electrodes: a review

Abstract: The exploration of renewable, cost-effective, and environmentally friendly electrode materials with high adsorption, fast ion/electron transport, and tunable surface chemistry is urgently needed for the development of next-generation biocompatible energy-storage devices. In recent years, biomass-derived carbon electrode materials for energy storage have attracted significant attention because of their widespread availability, renewable nature, and low cost. More importantly, their inherent uniform and precise biological structures can be utilized as templates for fabricating electrode materials with controlled and well-defined geometries. Meanwhile, the basic elements of biomass are carbon, sulfur, nitrogen, and phosphorus. The special naturally ordered hierarchical structures as well as abundant surface properties of biomass-derived carbon materials are compatible with electrochemical reaction processes such as ion transfer and diffusion. To date, a series of novel porous carbon materials with different dimensions have been prepared by various methods using biomass as the raw material, which is an important field in the fabrication of supercapacitor electrode materials. Herein, we summarized recently reported biomass-derived carbon materials with one-dimensional, two-dimensional, and three-dimensional structures and their applications as carbon-based electrode materials for supercapacitors. Finally, the current challenges and future perspectives of the carbon-based electrode materials with respect to the supercapacitor's performance were closely highlighted.

Recent advances in transition metal-based electrocatalysts for alkaline hydrogen evolution

Abstract: With the increasing demands in energy consumption and increasing environmental concerns, it is of vital significance for developing renewable and clean energy sources to substitute traditional fossil fuels. As an outstanding candidate, hydrogen is recognized as a green energy carrier due to its high gravimetric energy density, zero carbon footprints, and earth-abundance. Currently, water splitting in alkaline electrolytes represents one of the most promising methods for sustainable hydrogen production, and the key challenge lies in the development of high-performance electrocatalysts for the hydrogen evolution reaction (HER). Given the rapid advances in the design and development of efficient catalysts towards the alkaline HER, especially capable transition metal (TM)-based materials, this review aims to summarise recent progress in the theoretical understanding of the alkaline HER and TM-based electrocatalysts. TM-based catalysts classified by their different anionic compositions (metals, alloys, oxides, hydroxides, sulfides, selenides, tellurides, nitrides, phosphides, carbides, and borides) are comprehensively showcased. Special attention is given to mainstream strategies that can improve the catalytic properties of each category, as well as the underlying structure–activity regimes. Additionally, the challenges for the future development of novel catalysts are also analyzed.

Recent advances in layered double hydroxide electrocatalysts for the oxygen evolution reaction

Abstract: The energy consumption of hydrogen production from electrolytic water splitting originates from the oxygen evolution reaction (OER). Development of efficient and cost-effective OER electrocatalysts has become a high-priority research task. In this regard, layered double hydroxides (LDHs) as one of the promising OER electrocatalysts have been intensely researched due to their unique 2D layered structure and excellent physicochemical properties. Herein, this review aims to summarize recent strategies to design LDHs, including nanostructuring, hybrid LDHs with conductive materials, partial substitution of cations, interlayer anion replacement, vacancy creation, and combination of computational methods and operando techniques. Specifically, a thorough literature overview in the developments of LDHs to improve OER performance is appraised in detail, based on the compositional difference of transition metal components. Challenges and future directions in designing LDHs as OER electrocatalysts are discussed. The provided discussion will be favorable to explore and develop better catalysts and device units for practical applications and will offer a basic understanding of the OER process along with key issues to evaluate the performance.

Electrospun MXene/carbon nanofibers as supercapacitor electrodes

Abstract: Free-standing Ti3C2Tx MXene/carbon nanofiber electrodes are prepared via electrospinning Ti3C2Tx MXene flakes with polyacrylonitrile (PAN) and carbonizing the fiber networks. Using this simple fabrication method, delaminated MXene flakes are embedded within carbon nanofibers and these fiber mats are used as electrodes without binders or additives. Unlike coated electrodes, which may suffer from the active material delaminating from the substrate during folding or bending, composite electrodes are stable and durable. Previous attempts to incorporate Ti3C2Tx MXene into electrospun fibers resulted in low mass loadings, ∼1 wt% Ti3C2Tx MXene. In this work, MXene flakes are added into PAN solutions at a weight ratio of 2 : 1 (MXene : PAN) in the spinning dope, producing fiber mats with up to 35 wt% MXene. Composite electrodes have high areal capacitance, up to 205 mF cm−2 at 50 mV s−1, almost three times that of pure carbonized PAN nanofibers (70 mF cm−2 at 50 mV s−1). Compared with electrospun nanofibers spray-coated with Ti3C2Tx, these composite fibers exhibit double the areal capacitance at 10 mV s−1. This method can be used to produce MXene composite fibers using a variety of polymers, which have potential applications beyond energy storage, including filtration, adsorption, and electrocatalysis, where fibers with high aspect ratio, accessible surface, and porosity are desirable.

Covalent triazine frameworks: synthesis and applications

Abstract: Covalent triazine frameworks (CTFs) represent an exciting new type of porous organic material (POP), which have some unique characteristics, i.e., aromatic CN linkage (triazine unit) and the absence of any weak bonds. In particular, the strong aromatic covalent bonds endow CTFs with high chemical stability and rich nitrogen content, which bring great value for many practical applications and the interesting heteroatom effect (HAE). The unique properties make CTFs attractive for various applications, such as separation and storage of gases, energy storage, photocatalysis and heterogeneous catalysis. Based on the current status of research, CTFs can be classified into two categories, i.e., amorphous and crystalline CTFs. Since 2008, a series of synthetic strategies have been developed, i.e., an ionothermal trimerization strategy, a phosphorus pentoxide (P2O5) catalyzed method, amidine based polycondensation methods, a superacid catalyzed method and a Friedel–Crafts reaction method. The advent of these methodologies has prompted researchers to construct well-defined crystalline CTFs. This critical review systematically summarizes the development and challenges in the synthesis and potential applications of CTFs.

Linear-like lead-free relaxor antiferroelectric (Bi0.5Na0.5)TiO3–NaNbO3 with giant energy-storage density/efficiency and super stability against temperature and frequency

Abstract: A novel lead-free polar dielectric ceramic with linear-like polarization responses was found in (1 − x)(Bi0.5Na0.5)TiO3–xNaNbO3 ((1 − x)BNT–xNN) solid solutions, exhibiting giant energy storage density/efficiency and super stability against temperature and frequency. High-resolution transmission electron microscopy, Raman scattering and Rietveld refinements of X-ray diffraction data suggest that these property characteristics can be derived from temperature and electric field insensitive large permittivity as a result of relaxor antiferroelectricity (AFE) with polar nanoregions. Additionally, this feature intrinsically requires a high driving field for AFE to ferroelectric (FE) phase transitions due to large random fields. Measurements of temperature-dependent permittivity and polarization versus electric field hysteresis loops indicate that the high-temperature AFE P4bm phase in BNT was gradually stabilized close to room temperature, accompanying a phase transition from relaxor rhombohedral FEs to relaxor tetragonal AFEs approximately at x = 0.15–0.2. A record high of recoverable energy-storage density W ∼ 7.02 J cm−3 as well as a high efficiency η ∼ 85% was simultaneously achieved in the x = 0.22 bulk ceramic, which challenges the existing fact that W and η must be seriously compromised. Furthermore, desirable W (>3.5 J cm−3) and η (>88%) with a variation of less than 10% can be accordingly obtained in the temperature range of 25–250 °C and frequency range of 0.1–100 Hz. These excellent energy-storage properties would make BNT-based lead-free AFE ceramic systems a potential candidate for application in pulsed power systems.

A review on recent developments and challenges of cathode materials for rechargeable aqueous Zn-ion batteries

Abstract: Owing to their high cost and safety hazards, and the low abundance of Li in natural resources, the future of Li-ion batteries is becoming difficult. To replace Li-ion batteries, aqueous multivalent ion batteries are considered to be a worthwhile choice. In particular, aqueous zinc ion batteries are becoming an attractive option due to the natural abundance and unique properties of zinc. However, as for the Li-ion battery, the Zn-ion battery also has its own inadequacies in terms of cathodes. Finding a suitable cathode material for Zn-ion batteries with adequate structural stability and high capacity is an uphill task for researchers. This review presents the recent developments of various cathode materials in zinc ion batteries and their effectiveness towards the advancement of Zn-ion batteries. Based on the collected literature, various strategies adopted for enhancing the performance of Zn-ion batteries are also briefly discussed in this review. Furthermore, the explicit progress and future perspectives of Zn-ion batteries are also discussed.

Review of high entropy ceramics: design, synthesis, structure and properties

Abstract: High entropy ceramics are novel materials with no less than four different cations or anions. The development of high entropy ceramics follows the ‘configurational entropy stabilized single phase’ concept, which was first demonstrated for high entropy metal alloys in 2004. The advantages of high entropy ceramics are their compositional and structural diversity, and many of them have a band gap, which makes them potential functional materials for a wide range of applications. They have recently generated significant interest with the publication of 70+ related papers since 2015. In this review we have summarized the recent progress in this rapidly growing field. We emphasize the progress by researchers to answer the following three fundamental questions for high entropy ceramics: (1) which combinations of cations or anions can be synthesized as single-phase materials; (2) are the component elements truly random down to the atomic scale; and (3) what new physics, properties and applications will the incorporation of multi-elements elements bring. These fundamental questions are still open at this stage and warrant further studies. The objective of this review is to give a comprehensive overview of the literature to date on high entropy ceramics and to guide further investigation in this emerging field.

Zinc ion stabilized MnO2 nanospheres for high capacity and long lifespan aqueous zinc-ion batteries

Abstract: Rechargeable zinc-ion batteries based on Zn/MnO2 in neutral aqueous electrolytes are promising for grid-scale energy storage applications owing to their favorable merits of high safety, low cost and environmental benignity. However, MnO2 cathodes are subjected to the challenging issues of poor cyclability and low rate capability. Herein, we report a facile chemical method for the preparation of mesoporous MnO2 flower-like nanospheres with the layered framework stabilized by hydrated Zn2+ pillars. The MnO2 cathode could deliver a reversible specific capacity of 358 mA h g−1 at 0.3 A g−1 after 100 cycles, a high rate capacity of 124 mA h g−1 at 3.0 A g−1, and excellent operating stability over 2000 cycles. Structural and morphological investigations demonstrate an energy storage mechanism of co-insertion/extraction of H+ and Zn2+ accompanied by deposition/dissolution of zinc sulfate hydroxide hydrate flakes on the electrode surface. The superior electrochemical performance makes the zinc ion stabilized MnO2 promising for high capacity and long lifespan zinc-ion batteries.

Recent progress in electrocatalytic nitrogen reduction

Abstract: Ammonia (NH3) is an activated nitrogen building block for the manufacture of modern fertilizers, plastics, fibers, explosives, etc.; however, its production is limited to the traditional Haber–Bosch process. Very recently, electrocatalytic reduction of nitrogen (N2) has been demonstrated to be a clean and sustainable approach to produce NH3, which has aroused widespread attention. All types of electrocatalysts have been developed and designed for the electrochemical nitrogen reduction reaction (NRR). To achieve both high catalytic performance and selectivity, electrocatalysts must be rationally designed to optimize the mass transport, chemi(physi)sorption, and transfer of protons and electrons. In this review, we summarize the recent developments on electrocatalysts for N2 fixation. First, we discuss the reaction mechanisms of the NRR. Second, three categories of electrocatalysts according to their chemical compositions are surveyed. Then, the effects of different types of electrolytes on NRR activity and electrolyte choice are also summarized. Finally, the existing challenges and future perspectives are discussed.

Multivalent metal ion hybrid capacitors: a review with a focus on zinc-ion hybrid capacitors

Abstract: Multivalent metal ion hybrid capacitors have been developed as novel electrochemical energy storage systems in recent years. They combine the advantages of multivalent metal ion batteries (e.g., zinc-ion batteries, magnesium-ion batteries, and aluminum-ion batteries) with those of supercapacitors, and are characterized by good rate capability, high energy density, high power output and ultralong cycle life. Herein, after a brief introduction to supercapacitors and multivalent metal ion batteries, we reviewed the recent progress in research on multivalent metal ion hybrid capacitors, with a focus on zinc-ion hybrid capacitors, from the perspectives of design concept, configuration, electrochemical behavior and energy storage mechanism. An outlook of the future research regarding multivalent metal ion hybrid capacitors was also presented. This review will be beneficial for researchers around the world to have a better understanding of multivalent metal ion hybrid capacitors and develop novel electrochemical energy storage systems to meet the demands of rapidly developing electric vehicles and wearable/portable electronic products.

Metal–organic framework composites and their electrochemical applications

Abstract: Metal–organic frameworks (MOFs) are a new class of porous coordination polymers, which are distinguished among polymer materials due to their remarkable properties, such as large surface area, adjustable structure and high porosity, and therefore, have generated extensive interest. Nevertheless, the poor electrical conductivity and narrow micropores of MOFs have constrained their applications. Hence, MOF composites, in which MOFs are combined with a variety of functional materials, have been introduced to mitigate the disadvantages of individual components. Zero-dimensional materials such as nanoparticles and quantum dots, one-dimensional materials (nanorods, nanotubes and nanobelts), and two-dimensional nanosheet materials can be combined with MOFs, and three-dimensional MOF composites have also been reported, including core–shell and cubic composites. In this review, the recent development of MOF composites and their synthetic methods, and their electrochemical applications, including catalysts, sensors, supercapacitors and batteries, are discussed according to their dimensions.

Recent progress in metal–organic polymers as promising electrodes for lithium/sodium rechargeable batteries

Abstract: Metal organic polymers (MOPs), including metal coordination polymers (CPs, one-dimensional), metal–organic frameworks (MOFs, two-/three-dimensional), Prussian blue (PB) and Prussian blue analogues (PBAs), have recently emerged as promising electrochemically active materials for energy storage and conversion systems. Due to the tunability of their composition and the structural versatility, diverse electrochemical behaviors for multi-electron reactions, fast-ion diffusion, and small volume change of electrodes could be achieved upon charging and discharging. Because of these superiorities, MOPs are considered as effective substitutes for future advanced energy storage systems. Here, we summarize the recent progress in pristine MOPs as electrode candidates for rechargeable lithium and sodium ion batteries. The working mechanisms and strategies for enhancing the electrochemical performance in related advanced electrochemical energy storage (EES) applications are also highlighted in this review.

16.7%-efficiency ternary blended organic photovoltaic cells with PCBM as the acceptor additive to increase the open-circuit voltage and phase purity

Abstract: The field of organic photovoltaics (OPVs) has seen rapid development in the past few years, particularly, with reports on the use of a high performance nonfullerene electron acceptor (named Y6) in binary devices. In this paper, we demonstrate a simple yet effective ternary approach that can simultaneously increase the open-circuit voltage, short-circuit current-density, and fill factor of a binary device based on Y6 and a donor polymer named PM6. By adding a small amount of PCBM into the PM6:Y6 system, we achieved a high efficiency of 16.7%, which is the best value reported for an OPV device to date. Importantly, this ternary material approach has wide-ranging applicability, as we demonstrated the same beneficial effects in multiple systems, including PM6:IT-4F, PM7:IT-4F, PM6:Y6, and PM7:Y6. The LUMO energy (−3.9 eV) of PCBM lies between the LUMO (−3.6 eV) of PM6/PM7 and the LUMO (−4.1 eV) of IT-4F/Y6, which is one reason for the increased Voc. After blending with PCBM, the homogenous fine-film morphology and the π–π stacking patterns of the host binary structure are maintained, while the phase purity is increased, the hole and electron mobilities are increased, and monomolecular recombination is reduced, all of which, plus the visible absorption of PCBM, are the reasons for the concurrently improved fill-factor and increased short-circuit current density. This approach can be used in other OPV systems and should have wide applicability.

Defect passivation using ultrathin PTAA layers for efficient and stable perovskite solar cells with a high fill factor and eliminated hysteresis

Abstract: One-step anti-solvent deposition methods are widely applied in inverted perovskite solar cells (PSCs). However, anti-solvent processed films typically exhibit a small grain size and abundant defects. Herein, we propose a facile passivation approach using an ultrathin poly(triarylamine) (PTAA) layer to passivate the interfacial and grain boundary defects. An energy band alignment is realized by introducing an ultrathin PTAA layer sandwiched between the perovskite layer and the poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer to suppress interfacial recombination and accelerate hole transfer. By optimum design of the PTAA layer, we fabricate PSCs with a power conversion efficiency (PCE) of 19.04%, a fill factor (FF) of 82.59% and a short-circuit current density (Jsc) of 21.38 mA cm−2. To the best of our knowledge, the obtained FF value is the maximum among those reported for inverted chlorine-doped methyl perovskite solar cells (PSCs). After storage under ambient conditions for 10 days, the PCE of the PSCs with PTAA modification is still maintained at the 80% level, indicating substantially improved stability. Our work provides an effective method to boost PSCs with enhanced efficiency and stability.

A low-cost “water-in-salt” electrolyte for a 2.3 V high-rate carbon-based supercapacitor

Abstract: Aqueous electrolytes have shown extraordinary promise for safe electrochemical energy storage devices, but their widespread use is severely limited by the narrow electrochemical stability window (ESW). Here we demonstrate the great feasibility of using a superconcentrated sodium perchlorate aqueous solution as a low-cost “water-in-salt” electrolyte to construct a high-performance carbon-based supercapacitor (SC). The attractive features of this electrolyte including wide ESW and excellent conductivity enable the model SC to fully work at 2.3 volts with superior rate capability and outstanding cycling stability. This SC exhibits a comparable energy density, a higher power density and a much lower price compared to commercial non-aqueous SCs working at 2.7 volts, representing significant progress toward practical applications.

Nanostructured Co-based bifunctional electrocatalysts for energy conversion and storage: current status and perspectives

Abstract: Electrocatalytic hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are three typical reactions in energy conversion devices, such as water electrolyzers, metal–air batteries and fuel cells However, the sluggish kinetics of the HER/OER/ORR and their dependency on noble-metal-based catalysts (eg Pt, Ir and Ru) hinder the large-scale commercial applications of these devices Hence, the development of low-cost, efficient, stable and scalable electrocatalysts, especially bifunctional HER/OER or OER/ORR electrocatalysts that could simultaneously catalyze HER/OER or OER/ORR, is highly desired but full of great challenges To date, non-noble cobalt (Co)-based materials, including cobalt oxide, cobalt phosphides, cobalt chalcogenides (sulfide and selenides), Co-included layered double hydroxides, Co–N–C, Co-based single atoms and their composites, have been widely investigated as bifunctional electrocatalysts owing to their innate electrochemical capabilities and structural adjustability by mixing, doping or combining with other materials, such as carbon materials In this review, the fundamental mechanisms of the bifunctional electrocatalysis of ORR/OER and OER/HER are introduced and analyzed, and recent progress in the design, synthesis and emerging applications of Co-based materials as bifunctional electrocatalysts for ORR/OER or OER/HER in alkaline electrolytes are summarized and discussed Finally, the strategies and perspectives for designing novel highly efficient bifunctional cobalt-based electrocatalysts are analyzed This analysis is expected to highlight the challenges faced by bifunctional noble-metal-free electrocatalysts for the study and development of next-generation electrocatalysts for the HER/ORR/OER

Engineering the interplanar spacing of ammonium vanadates as a high-performance aqueous zinc-ion battery cathode

Abstract: We report the engineering of the interplanar spacing of ammonium vanadates to achieve the optimal electrochemical performance as cathodes for aqueous zinc-ion batteries (ZIBs) and provide insights into the origin of the enhanced electrochemical behaviors. The NH4V4O10 compound, with the largest interplanar spacing (9.8 A) and high diffusion coefficient, exhibits high energy density (374.3 W h kg−1) and power density (9000 W kg−1) as well as superior long-term cycling performance (a discharge capacity of 255.5 mA h g−1 can be maintained after 1000 cycles at 10 A g−1). With the merits of impressive energy density and long cycle life, the NH4V4O10 might be a new promising cathode for aqueous ZIBs for grid energy storage applications. This work provides a direction for choosing or designing the ideal high-performance cathode for aqueous ZIBs and other advanced battery systems.

Metal–organic frameworks for lithium–sulfur batteries

Abstract: Lithium–sulfur (Li–S) batteries have gained popularity over recent decades due to their theoretically superior performance, which renders them a promising new energy storage technology. To cater to the increasing demand for energy, suitable and effective materials are desired to constitute Li–S batteries, especially as host materials, separators and interlayers. Metal–organic frameworks (MOFs), novel porous materials, have attracted public attention because of their controllable porous structure and ultrahigh porosity. Moreover, MOF composites and MOF derivatives exhibit even better performance and reduce the shortcomings of pure MOFs. These MOF-based materials are confirmed to be perfect candidates for several important parts in Li–S batteries. In this review, we aim to provide a comprehensive introduction to the excellent performance of pristine MOFs, MOF composites and MOF derivatives in Li–S batteries as host materials, separators or interlayers, including their working mechanism as different constituents. An outlook of possible research directions in the near future and difficulties in development is also given.

Conversion of dinitrogen to ammonia on Ru atoms supported on boron sheets: a DFT study

Abstract: The prevalent catalysts for natural and artificial N2 fixation are transition-metal (TM) atoms. By using density functional theory computations, several TM atoms embedded on boron sheets as N2 fixation electrocatalysts were investigated in this work. Our results revealed that single ruthenium (Ru) atom-doped boron sheets exhibited outstanding catalytic activity for ammonia synthesis at ambient conditions through the distal pathway with small activation barrier of 0.42 eV; this was less than half of that of the reported flat Ru (0001) catalysts (1.08 eV). These results highlight the value of boron as a substrate for the design of single-atom catalysts due to its unique electron-deficient features.

Anionic defect engineering of transition metal oxides for oxygen reduction and evolution reactions

Abstract: The development of cost-effective catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for enhancing the energy efficiency of many electrochemical energy conversion and storage devices. Owing to their low cost and high activity, transition metal oxides have attracted much attention as alternative electrocatalysts to replace the currently used noble metal-based catalysts. Anion defects (e.g., oxygen vacancies, interstitials, and anion dopants) can significantly change the electronic structure of oxides or the stability of adsorbed intermediates, thus greatly enhancing the electrocatalytic activities of the oxide surface. Anionic defect engineering represents a potential new direction for rational design of high-performance electrocatalysts. In this review, recent progress in manipulating the anion defects in transition metal oxides for enhancing their activity and stability is summarized and the proposed mechanisms for enhanced performance are discussed in detail. Challenges and prospects are also discussed in the development of a new generation of highly efficient ORR and OER electrocatalysts.

Harnessing MOF materials in photovoltaic devices: recent advances, challenges, and perspectives

Abstract: Metal–organic framework (MOF) materials have achieved significant research interest in the fields of gas storage and separation over the last two decades because of the need for hydrogen utilization and carbon dioxide reduction. Besides, recently numerous functional MOFs have been exploited and applied in the optoelectronic field owing to some unique properties of MOF materials in those photovoltaic devices with enhanced performance and stability. This review focuses on the comprehensive summary of recent representative progress in the applications of MOFs in solar cell devices, including dye-sensitized solar cells, organic–inorganic hybrid perovskite solar cells, and organic solar cells, aiming to portray their prospects in the future.

An ultra-high energy density flexible asymmetric supercapacitor based on hierarchical fabric decorated with 2D bimetallic oxide nanosheets and MOF-derived porous carbon polyhedra

Abstract: Flexible supercapacitors (SCs) are an emergent and promising technology for next-generation energy storage devices. However, low energy densities hindered their practical applications. Two-dimensional (2D) nanosheets can exhibit excellent electrochemical charge storage properties due to their short ion-diffusion distance and rich electroactive sites with multiple valence states. Herein, we report the direct growth of mesoporous 2D zinc cobaltite nanosheets on a flexible carbon cloth substrate (Zn–Co–O@CC) with an average thickness of ∼45 nm by a facile hydrothermal method at low temperature. The Zn–Co–O@CC electrode displays a high capacitance of 1750, 1573.65 and 1434.37 F g−1 at a current density of 1.5 A g−1 in LiCl, NaCl and KCl neutral aqueous electrolytes, respectively, with excellent rate capabilities at high current densities and demonstrates good cycling stability (>94%) for up to 5000 cycles. Moreover, highly flexible asymmetric supercapacitor (ASC) devices have been fabricated using Zn–Co–O@CC as a positive electrode and bimetallic organic framework (MOF)-derived nanoporous carbon polyhedra (NPC@CC) as a negative electrode (Zn–Co–O@CC//NPC@CC). The as-fabricated ASC can operate at a large potential window of 0.0–2.0 V and shows outstanding energy storage performance by delivering an ultra-high energy density of 117.92 W h kg−1 at a power density of 1490.4 W kg−1 with a cycling stability of 94% after 5000 charge/discharge cycles. To the best of our knowledge, the achieved energy storage performance of the ASC device is very competitive and the highest among all binary metal oxides, carbonaceous materials, and MXene-based SCs and ASCs to date. The applied strategy to fabricate SCs is capable of enhancing both electrochemical activity and cycling stability, and can be readily applied to other metal oxide-based SCs.

Cobalt nanoparticle-embedded nitrogen-doped carbon/carbon nanotube frameworks derived from a metal–organic framework for tri-functional ORR, OER and HER electrocatalysis

Abstract: Developing active and stable electrocatalysts of earth-abundant elements towards the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) still remains a crucial challenge. Herein, a cobalt-containing metal-organic framework using adenine as a ligand was synthesized and pyrolyzed without any other precursors, forming a cobalt nanoparticle-embedded nitrogen-doped carbon/carbon nanotube framework (Co@N-CNTF). Due to the abundant active sites of homogeneously distributed cobalt nanoparticles within nitrogen-doped graphitic layers, the resultant Co@N-CNTF catalysts exhibit an efficient and stable electrocatalytic performance as a tri-functional catalyst towards the ORR, OER and HER, including a high half-wave potential of 0.81 V vs. RHE for the ORR, and a low overpotential at 10 mA cm -2 for the OER (0.35 V) and HER (0.22 V). As a proof-of-concept, the Co@N-CNTF as an OER/HER bifunctional catalyst for full water splitting affords an alkaline electrolyzer with 10 mA cm -2 under a stable voltage of 1.71 V. Moreover, an integrated unit of a water-splitting electrolyzer using the Co@N-CNTF catalysts, which is powered with a rechargeable Zn-air battery using the Co@N-CNTF as an ORR/OER bifunctional catalyst on air electrodes, can operate under ambient conditions with high cycling stability, demonstrating the viability and efficiency of the self-powered water-splitting system.

Atomic (single, double, and triple atoms) catalysis: frontiers, opportunities, and challenges

Abstract: The atomic dispersing of metal atoms supported on an optimal substrate exhibits an ideal strategy for maximizing metal utilization for catalysis, which is particularly significant for exploiting new catalysts with low cost and high catalytic efficiency. The dramatic development of atomic metal catalysts, including single atom catalysts (SACs), double atoms catalysts (DACs), and triple atoms catalysts (TACs), has spawned two remarkable platforms: (1) bridging homogeneous catalysts and heterogeneous catalysts; (2) linking theoretical calculations and experimental results. In this review, recent syntheses, characterizations, and applications of SACs, DACs, and TACs are highlighted through a focus on various applied substrates. We extensively discuss the synthetic strategies of successfully achieving SACs, DACs, and TACs. Moreover, the opportunities and challenges in developing SACs, DACs, and TACs are pointed out, together with the prospects for the development of atomic catalysis.

Two-dimensional Janus PtSSe for photocatalytic water splitting under the visible or infrared light

Abstract: Recently, two-dimensional Janus materials have attracted increasing research interest due to their particular structure and great potential in electronics, optoelectronics and piezoelectronics. Here, we propose 2D Janus PtSSe with compelling photocatalytic properties which were investigated by means of first-principles calculations. 2D Janus PtSSe exhibits high thermal, dynamic and mechanical stability. Most remarkably, single-layer PtSSe exhibits an indirect band gap of 2.19 eV, high absorption coefficients in the visible light region, appropriate band edge positions and strong ability for carrier separation and transfer, thus rendering it a promising candidate for photocatalytic water splitting. Moreover, double-layer PtSSe compounds with different stacking configurations are extraordinary photocatalysts for water splitting even under infrared light, owing to their small band gaps as well as the built-in electrical field. Our results reveal 2D PtSSe with high experimental feasibility as a new platform for the overall water splitting reaction.

Rational design of free-standing 3D porous MXene/rGO hybrid aerogels as polysulfide reservoirs for high-energy lithium–sulfur batteries

Abstract: Lithium–sulfur (Li–S) batteries with a high theoretical energy density are attracting increasing attention as promising candidates for next-generation energy storage systems. However, the insulating nature and undesirable shuttle effect of sulfur species dramatically impede their practical applications. Herein, a unique 3D porous Ti3C2Tx MXene/rGO (MX/G) hybrid aerogel is rationally designed and applied for the first time as a free-standing polysulfide reservoir to improve the overall performance of Li–S batteries. In this strategy, highly conductive MXene and rGO are integrated into a 3D interconnected porous aerogel structure with efficient 2D polar adsorption interfaces, enabling fast Li+/electron transport and strong chemical anchoring of lithium polysulfides as well as enhanced redox reaction kinetics. The robust MX/G aerogel electrodes deliver excellent electrochemical performances including a high capacity of 1270 mA h g−1 at 0.1C, an extended cycling life up to 500 cycles with a low capacity decay rate of 0.07% per cycle, and a high areal capacity of 5.27 mA h cm−2.

A flexible solid-state zinc ion hybrid supercapacitor based on co-polymer derived hollow carbon spheres

Abstract: High electrochemical performance energy storage devices coupled with low cost and high safety operation are in urgent need due to the increasing demand for flexible and wearable electronics. For these applications lithium-ion and sodium-ion batteries are vastly limited due to their relatively low power density and security risks. On the other hand, conventional supercapacitors are suitable for flexible and wearable electronics due to their high power density while their low energy density has hindered their wide applications. Lithium or sodium ion hybrid supercapacitors are promising energy storage devices that benefit from the combined high energy density of batteries and high power density of supercapacitors. However, the use of organic electrolytes and shortage of lithium resources are expected to limit their widespread commercialization for flexible and wearable electronics. Here, for the first time, we introduce a safe and flexible solid-state zinc ion hybrid supercapacitor (ZHS) based on co-polymer derived hollow carbon spheres (HCSs) as the cathode, polyacrylamide (PAM) hydrogel as the electrolyte and Zn deposited on carbon cloth as the anode. Owing to the high surface area of the HCSs and the hollow structure which improves the ion adsorption and desorption kinetics of the cathode, the flexible solid-state ZHS delivers a highest capacity of 86.8 mA h g−1 and a maximum energy density of 59.7 W h kg−1 with a power density of 447.8 W kg−1. Besides, it displays excellent cycling stability with 98% capacity retention over 15 000 cycles at a current density of 1.0 A g−1. Moreover, the solid-state ZHS is flexible enough to sustain various deformations including squeezing, twisting and folding due to the use of flexible electrodes and electrolytes. Our study unveils a pioneering flexible solid-state ZHS with high safety, which is a promising candidate for flexible and wearable energy storage devices.

3D printed electrochemical energy storage devices

Abstract: 3D printing technology, which can be used to design functional structures by combining computer-aided design and advanced manufacturing procedures, is regarded as a revolutionary and greatly attractive process for the fabrication of electrochemical energy storage devices. In comparison to traditional manufacturing methods, 3D printing possesses unique advantages in geometrical shape design as well as rapid prototyping, especially with high surface area complex 3D structure constructions. Recently, a number of 3D-printed electrochemical energy storage devices have been reported, showing an increased interest of the scientific community. To further advance material design and technology development, comprehensive understanding of the strengths and weaknesses of each 3D printing technique and knowledge of recent progress in 3D-printed electrochemical energy storage are indispensable. To that end, a literature review of recent advances of 3D printing technology for capacitive energy storage is provided herein. Emphasis is given on the design of printing materials, printing process and electrochemical performance of printed devices. Some 3D printed structural solid electrolytes for energy storage applications are also summarized. Discussion and outlook on the potential future designs and development of 3D printing for electrochemical energy storage devices are provided in the text.