Controllable synthesis of a hybrid mesoporous sheets-like Fe0.5NiS2@ P, N-doped carbon electrocatalyst for alkaline oxygen evolution reaction.

Owing to the high cost of precious metal catalysts for the oxygen evolution reaction (OER), the production of highly efficient and affordable electrocatalysts is important for generating pollution-free and renewable energy via electrochemical processes. A facile hydrothermal approach was employed to synthesize hybrid mesoporous iron-nickel bimetallic sulfides @ P, N-doped carbon for the OER. The prepared Fe0.5NiS2@C exhibited an overpotential (η) of 250 mV at 10 mA/cm2. This exceeded the overpotentials recently reported for surface-modified P, N-doped carbon-based catalysts for the OER in a 1 M KOH medium. Moreover, the Fe0.5NiS2@C catalyst showed a notable Tafel slope of 90.5 mV/dec with long-dated stability even after 24 h at 10 mA/cm2. The superior OER performance of the Fe0.5NiS2@C catalysts may be due to their large surface area, sheet-like morphology with abundant active sites, fast transfer of mass and electrons, control of the electronic structure by co-treatment with heteroatoms (e.g., P and N), and the synergistic effect of bimetallic sulfides, making them favorable catalysts for the oxygen evolution reaction. Density functional theory (DFT) calculations showed that the Fe0.5NiS2@C catalyst exhibited strong H2O-adsorption energy. The enhanced OER activity of Fe0.5NiS2@C was attributed to its higher surface area, favorable H2O adsorption energy, improved electron transfer efficiency, and lower Gibbs free energy compared to those of the other proposed catalysts.

Ultrasound-Assisted Preparation and Performance Regulation of Electrocatalytic Materials.

With the advancement of scientific research, the introduction of external physical methods not only adds extra freedom to the design of electro-catalytical processes for green technologies but also effectively improves the reactivity of materials. Physical methods can adjust the intrinsic activity of materials and modulate the local environment at the solid-liquid interface. In particular, this approach holds great promise in the field of electrocatalysis. Among them, the ultrasonic waves have shown reasonable control over the preparation of materials and the electrocatalytic process. However, the research on coupling ultrasonic waves and electrocatalysis is still early. The understanding of their mechanisms needs to be more comprehensive and deep enough. Firstly, this article extensively discusses the adhibition of the ultrasonic-assisted preparation of metal-based catalysts and their catalytic performance as electrocatalysts. The obtained metal-based catalysts exhibit improved electrocatalytic performances due to their high surface area and more exposed active sites. Additionally, this article also points out some urgent unresolved issues in the synthesis of materials using ultrasonic waves and the regulation of electrocatalytic performance. Lastly, the challenges and opportunities in this field are discussed, providing new insights for improving the catalytic performance of transition metal-based electrocatalysts.

Ultrasonic-assisted preparation of two-dimensional materials for electrocatalysts.

Developing green, environmental, sustainable new energy sources is an important problem to be solved in the world. Among the new energy technologies, water splitting system, fuel cell technology and metal-air battery technology are the main energy production and conversion methods, which involve three main electrocatalytic reactions, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). The efficiency of the electrocatalytic reaction and the power consumption are very dependent on the activity of the electrocatalysts. Among various electrocatalysts, the two-dimensional (2D) materials have received widespread attention due to multiple advantages, such as their easy availability and low price. What' important is that they have adjustable physical and chemical properties. It is possible to develop them as electrocatalysts to replace the noble metals. Therefore, the design of two-dimensional electrocatalysts is a focus in the research area. Some recent advances in ultrasound-assisted preparation of two-dimensional (2D) materials have been overviewed according to the kind of materials in this review. Firstly, the effect of the ultrasonic cavitation and its applications in the synthesis of inorganic materials are introduced. The ultrasonic-assisted synthesis of representative 2D materials for example transition metal dichalcogenides (TMDs), graphene, layered double metal hydroxide (LDH), and MXene, and their catalytic properties as electrocatalysts are discussed in detail. For example, the CoMoS4 electrocatalysts have been synthesized through a facile ultrasound-assisted hydrothermal method. The obatined HER and OER overpotential of CoMoS4 electrode is 141 and 250 mV, respectively. This review points out some problems that need to be solved urgently at present, and provides some ideas for designing and constructing two-dimensional materials with better electrocatalytic performance.

MXene-based Materials for Water Splitting: Synthesis and Modification.

With abundant metal site and tunable electronic structure, MXene is considered as a promising electrocatalyst for the conversion of energy molecules. In this review, the latest research progress on inexpensive MXene-based catalysts for water electrolysis is summarized. Typical preparation and modification methods and their advantages and disadvantages are briefly discussed, with a focus on the regulation and design of the surface interface electronic states, which improve the electrocatalytic performance of MXene-based materials. The main strategies for the electronic state modification include end-group modification, heteroatom doping, and heterostructure construction. Some limitations of MXene-based materials, which should be considered in the rational design of advanced MXene-based electrocatalyst, are also discussed. Finally, prospects for the rational design of Mxene-based electrocatalysts is proposed.

Atomic understanding of the strain-induced electrocatalysis from DFT calculation: progress and perspective.

Catalyst activity affects the reaction rate, and an increasing number of studies have shown that strain can significantly increase the electrocatalytic activity. Catalysts such as alloys and core-shell structures can modulate their properties through strain effects. Reasonable simulation techniques can be used to predict and design the catalytic performance based on understanding the strain action mechanism. Therefore, the methodological flow of theoretical simulations is summarised in this review. The mechanism underlying the strain-adsorption-reaction relationship is discussed using density functional theory (DFT) calculations. An introduction to DFT is given first, followed by a quick rundown of the strain classification and application. Typical electrocatalytic reactions, namely, the hydrogen and oxygen evolution reactions and oxygen reduction reaction, are taken as examples. After briefly explaining these reactions, the relevant studies on simulating the strain to tune the catalyst performance are covered. The simulation methods are summarised and analysed to observe the effects of strain on electrocatalytic properties. Finally, a summary of the issues with simulated strain-assisted design and a discussion on the perspectives and forecasts for the future design of effective catalysts are provided.

Core-shell NiS2@C encased by thin carbon layer as high-rate and durable electrode for aqueous rechargeable battery.

Nickel sulfides, as promising candidate for aqueous rechargeable battery, have aroused broad attention on account of abundant natural resources, rich phases, moderate price and high theoretical capacity. Nevertheless, tremendous volume expansion during repeated charging-discharging procedure leads to the poor rate capability and cycling stability of nickel sulfide electrodes. Therefore, in this work, core-shell NiS2@C encapsulated by thin hydrothermal carbon (HC) layer (NiS2@C/HC) has been designed and prepared without any surfactants or templates assistance, which avoid tedious process and shorten preparation cycle greatly. When matched with the treated iron powder (TIP) electrode to form NiS2@C/HC//TIP aqueous rechargeable battery, the NiS2@C/HC//TIP battery exhibits a high discharge capacity of 205.1 mAh g-1 at 1 A g-1, remarkable rate ability (176.4 mAh g-1 at 5 A g-1, about 86% capacity conversation) and superiorly durable stability (80.8 % capacity retention after 10,000 cycles at ultra-high current density of 15 A g-1). The outstanding high-rate capability and cycling stability for aqueous rechargeable battery can be ascribed to the distinct cowpea-like architecture and intrinsic properties of NiS2@C/HC. Specifically, the interior porous carbon provides a space to tolerate the volume expansion of the NiS2 nanoparticles and prevent NiS2 nanoparticles from aggregation, guaranteeing its high-rate capability. Meanwhile, the exterior HC layer is conducive to improve the electric conductivity to facilitate the electrons transfer and promote the mechanical strength of the whole active materials, ensuring its robust cycling stability.

High-loading LiBH4 Confined in Structurally Tunable Ni Catalyst-decorated Porous Carbon Scaffold for Fast Hydrogen Desorption.

Catalysts combined with nanoconfinement can improve the sluggish desorption kinetics and poor reversibility of LiBH4 . However, at high LiBH4 loading, their hydrogen storage performance is significantly reduced. Herein, a porous carbon-sphere scaffold decorated with Ni nanoparticles (NPs) was synthesised by calcining a Ni metal-organic framework precursor, followed by partial etching of the Ni NPs to fabricate an optimised scaffold with a high surface area and large porosity that accommodates high LiBH4 loading (up to 60 wt.%) and exhibits remarkable catalyst/nanoconfinement synergy. Owing to the catalytic effect of Ni2 B (formed in situ during dehydrogenation) and the reduced hydrogen diffusion distances, the 60 wt.% LiBH4 confined system exhibited enhanced dehydrogenation kinetics with >87% of the total hydrogen storage capacity released within 30 min at 375 °C. The apparent activation energies were significantly reduced to 110.5 and 98.3 kJ/mol, compared to that of pure LiBH4 (149.6 kJ/mol). Moreover, partial reversibility was achieved under moderate conditions (75 bar H2 , 300 °C) with rapid dehydrogenation during cycling.

Non-precious transition metal single-atom catalysts for the oxygen reduction reaction: progress and prospects.

The massive exploitation and use of fossil resources have created many negative issues, such as energy shortage and environmental pollution. It prompts us to turn our attention to the development of new energy technologies. This review summarizes the recent research progress of non-precious transition metal single-atom catalysts (NPT-SACs) for the oxygen reduction reaction (ORR) in Zn-air batteries and fuel cells. Some commonly used preparation methods and their advantages/disadvantages have been summarized. The factors affecting the ORR performances of NPT-SACs have been focused upon, such as the substrate type, coordination environment and nanocluster effects. The loading mass of a metal atom has a direct effect on the ORR performances. Some general strategies for stabilizing metal atoms are included. This review points out some existing challenges of NPT-SACs, and also provides ideas for designing and synthesizing NPT-SACs with excellent ORR performances. The large-scale preparation and commercialization of NPT-SACs with excellent ORR properties are prospected.

Electrocatalyst design for the conversion of energy molecules: electronic state modulation and mass transport regulation.

Electrocatalytic conversions of energy molecules are involved in many energy conversion processes. Improving the activity of electrocatalysts is critical for increasing the efficiency of these energy conversion processes. However, the tailored design of highly active electrocatalysts for practical applications remains challenging. In this regard, we present an overview of the general design principles for efficient electrocatalysts and application of these principles in different electrocatalytic processes. Specifically, enhancing the intrinsic activity of electrocatalysts by electronic state modulation through heteroatom doping, vacancy introduction, interfacial electronic transfer and strain engineering is introduced. In addition, improving the apparent performance of electrocatalysts by mass transport regulation, which is realized by morphological and wettability control, is also discussed. Finally, enlightenment from these studies is summarized and perspectives for the future development of electrocatalysts are provided. The important progress highlighted in this work will provide solid foundations for the tailored design of electrocatalysts toward practical applications.

Noble Metal-Based Catalysts with Core-Shell Structure for Oxygen Reduction Reaction: Progress and Prospective.

With the deterioration of the ecological environment and the depletion of fossil energy, fuel cells, representing a new generation of clean energy, have received widespread attention. This review summarized recent progress in noble metal-based core-shell catalysts for oxygen reduction reactions (ORRs) in proton exchange membrane fuel cells (PEMFCs). The novel testing methods, performance evaluation parameters and research methods of ORR were briefly introduced. The effects of the preparation method, temperature, kinds of doping elements and the number of shell layers on the ORR performances of noble metal-based core-shell catalysts were highlighted. The difficulties of mass production and the high cost of noble metal-based core-shell nanostructured ORR catalysts were also summarized. Thus, in order to promote the commercialization of noble metal-based core-shell catalysts, research directions and prospects on the further development of high performance ORR catalysts with simple synthesis and low cost are presented.

Te-S covalent bond induces 1T&2H MoS2 with improved potassium-ion storage performance.

The modulation of the characteristics of an MoS2 anode via substitutional doping, particularly N, P and Se, is vital for promoting the potassium-ion storage performances. However, these traditional chalcogen doping can only take the place of a sulfur element and not essentially change the inherent electrical nature of MoS2. Herein, novel Te-MoS2 materials have been synthesized via a simple hydrothermal process under Te doping. A half-metallic Te occupies the position of an Mo atom to form Te-S bonds, which is different from the same group Se element. After theoretical modeling and electrochemical measurements, it was observed that the formation of Te-S bonds can increase the electrical conductivity (about 530 times increment) and mitigate the mechanical stress to ensure the whole structural stability during the repeated insertion/extraction of K-ions. Moreover, the insertion of Te into the lattice of MoS2 generated the fractional phase transformation from 2H to the 1T phase of MoS2 and 1T&2H in-plane hetero-junction. Benefiting from these advantages, the 1T&2H Te-MoS2 anode delivered high capacities of 718 and 342 mA h g-1 at 50 and 5000 mA g-1, respectively, and an ultra-stable cycling performance (88.1% capacity retention after 1000 cycles at 2 A g-1). Moreover, the potassium-ion full cell assembled with K2Fe[Fe(CN)6] as the cathode demonstrates its practical application.

Ultra-Thin SnS2-Pt Nanocatalyst for Efficient Hydrogen Evolution Reaction.

Transition-metal dichalcogenides (TMDs) materials have attracted much attention for hydrogen evolution reaction (HER) as a new catalyst, but they still have challenges in poor stability and high reaction over-potential. In this study, ultra-thin SnS2 nanocatalysts were synthesized by simple hydrothermal method, and low load of Pt was added to form stable SnS2-Pt-3 (the content of platinum is 0.5 wt %). The synergistic effect between ultra-thin SnS2 rich in active sites and individual dispersed Pt nanoclusters can significantly reduce the reaction barrier and further accelerate HER reaction kinetics. Hence, SnS2-Pt-3 exhibits a low overpotential of 210 mV at the current density of 10 mA cm-2. It is worth noting that SnS2-Pt-3 has a small Tafel slope (126 mV dec-1) in 0.5 M H2SO4, as well as stability. This work provides a new option for the application of TMDs materials in efficient hydrogen evolution reaction. Moreover, this method can be easily extended to other catalysts with desired two-dimensional materials.

The phase-change evolution from surface to bulk of MnO anodes upon cycling.

Transition-metal oxides with low valence states are promising candidates as anodes for advanced rechargeable Li-ion batteries. Surprisingly, the capacities of such anode materials initially decrease and then increase after long-term cycling. Herein, MnO is selected as a representative material to study the structure-function relationship and elucidate the above-mentioned phenomena during long-term cycling. To this end, the surface reconstruction to bulk transformation of MnO anode materials during the cycling procedures has been revealed. The atomic scanning transmission electron microscopy images and theoretical modeling results illustrate the formation of stable surface-phase Mn3O4 and Li2MnO4, which promote the migration of Li ions. The complete bulk-phase transformation of MnO is then revealed, during which Mn2+ was found to be initially oxidized to Mn4+ and subsequently reduced to a mixed valence of Mn2+ and Mn3+, correlating with the tendency of their discharge capacity variation upon cycling. These direct atomic-scale observations of the migration behavior of Li ions in the MnO anode offer an essential step toward understanding the electrochemical performance evolution of transition-metal oxide anodes and guide the anode preparation for Li-ion batteries.

Hierarchical Ti3C2@TiO2 MXene hybrids with tunable interlayer distance for highly durable lithium-ion batteries.

To realize high-rate and long-term performance of rechargeable batteries, the most effective approach is to develop an advanced hybrid material with a stable structure and more reaction active sites. Recently, 2D MXenes have become an up-and-coming electrode owing to their high conductivity and large redox-active surface area. In this work, we firstly prepared Ti3C2 MXenes through the selective etching of silicon from Ti3SiC2 (MAX) using HF and an oxidant for highly durable lithium-ion batteries (LIBs). The interlayer distance of Ti3C2 MXenes can be controlled with the oxidizability of the oxidant and etching temperature. In addition, Ti3C2@TiO2 MXene hybrids with further expanded interlayer spacing were purposefully fabricated by a simple hydrothermal method. The hierarchical N-doped Ti3C2@TiO2 MXene hybrids show that the in situ synthesized nanoscale TiO2 particles are loaded homogeneously on the layered N-doped Ti3C2 surface. The interlayer distance of N-doped Ti3C2@TiO2 MXene can reach 12.77 Å when using HNO3 as the oxidant at room temperature. As an anode material, the N-doped Ti3C2@TiO2(HNO3-RT) hybrid displays a high reversible capacity of 302 mA h g-1 at 200 mA g-1 after 500 cycles and 154 mA h g-1 at 2000 mA g-1 after 1500 cycles, which indicates its long cycle lifetime and excellent stability in LIBs. This highly durable LIB anode performance is ascribed to synergetic contributions from the high capacitive contribution, high electrical conductivity, high-capacity of in situ formed nanoscale TiO2 and interlayer-expanded architecture of the N-doped Ti3C2@TiO2(HNO3-RT). This study provides a theoretical basis for the application of MXenes as high capacity anodes for advanced LIBs.

Visualization of crystal plane selectivity for irreversible phase transition in MnO@C anode.

Transition metal oxides are widely regarded as one of the most promising candidates for lithium-ion battery (LIB) anodes. However, the mechanisms of irreversible reactions occurring during the charging/discharging process are still controversial. In this study, the atomic structural transitions of the MnO@C anode upon lithiation/delithiation at the first cycle of charging and discharging are elucidated. Based on the quantities of Li embedded and released in different states, the anisotropy of the crystal plane of lithiation/delithiation in MnO is directly observed. We determine that lithium ions can be completely inserted into/extracted from MnO(220), while this cannot be achieved in MnO(200), which is the main reason for capacity degradation. This study reveals the reaction mechanisms and structural evolution in the electrochemical reactions of MnO@C anode materials during lithiation and delithiation. Additionally, it also provides guidance for the fabrication and optimization of MnO-based materials for LIBs in the future.

The encapsulation of MnFe2O4 nanoparticles into the carbon framework with superior rate capability for lithium-ion batteries.

Binary transition metal oxides (BTMOs) have been regarded as one of the most hopeful anode materials for lithium-ion batteries (LIBs) owing to their high theoretical capacity, excellent electrochemical activity and abundant electrochemical reactions. However, BTMOs still suffer from two main problems, which are poor conductivity and large volume expansion during the charge/discharge processes. In order to address the above-mentioned problems, mesoporous MnFe2O4@C nanorods have been successfully synthesized in this work. The synergistic effect of the cross-linked carbon framework and mesoporous structure greatly improves the electrochemical performances. As expected, the mesoporous MnFe2O4@C electrode manifests discharge capacities of 987.5 and 816.6 mA h g-1 at the current densities of 100 and 2000 mA g-1, respectively, with the capacity retention ratio of 82.7%, exerting distinguished rate capabilities for LIBs.

Controlled Synthesis of Ni-Doped MoS2 Hybrid Electrode for Synergistically Enhanced Water-Splitting Process.

The development of high-efficiency, low-cost, and earth-abundant electrocatalysts for overall water splitting remains a challenge. In this work, Ni-modified MoS2 hybrid catalysts are grown on carbon cloth (Ni-Mo-S@CC) through a one-step hydrothermal treatment. The optimized Ni-Mo-S@CC catalyst shows excellent hydrogen evolution reaction (HER) activity with a low overpotential of 168 mV at a current density of 10 mA cm-2 in 1.0 m KOH, which is lower than those of Ni-Mo-S@CC (1:1), Ni-Mo-S@CC (3:1), and pure MoS2 . Significantly, the Ni-Mo-S@CC hybrid catalyst also displays outstanding oxygen evolution reaction (OER) activity with a low overpotential of 320 mV at a current density of 10 mA cm-2 , and remarkable long-term stability for 30 h at a constant current density of 10 mA cm-2 . Experimental results and theoretical analysis based on density functional theory demonstrate that the excellent electrocatalytic performance can be attributed mainly to the remarkable conductivity, abundant active sites, and synergistic effect of the Ni-doped MoS2 . This work sheds light on a unique strategy for the design of high-performance and stable electrocatalysts for water-splitting electrolyzers.

Design and Regulation of Novel MnFe2O4@C Nanowires as High Performance Electrode for Supercapacitor.

Bimetallic oxides have been considered as potential candidates for supercapacitors due to their relatively high electric conductivity, abundant redox reactions and cheapness. However, nanoparticle aggregation and huge volume variation during charging-discharging procedures make it hard for them to be applied widely. In this work, one-dimensional (1D) MnFe2O4@C nanowires were in-situ synthesized via a simply modified micro-emulsion technique, followed by thermal treatment. The novel 1D and core-shell architecture, and in-situ carbon coating promote its electric conductivity and porous feature. Due to these advantages, the MnFe2O4@C electrode exhibits a high specific capacitance of 824 F·g-1 at 0.1 A·g-1 and remains 476 F·g-1 at 5 A·g-1. After 10,000 cycles, the capacitance retention of the MnFe2O4@C electrode is up to 93.9%, suggesting its excellent long-term cycling stability. After assembling with activated carbon (AC) to form a MnFe2O4@C//AC device, the energy density of this MnFe2O4@C//AC device is 27 W·h·kg-1 at a power density of 290 W·kg-1, and remains at a 10 W·h·kg-1 energy density at a high power density of 9300 W·kg-1.

Large-scale Co9S8@C hybrids with tunable carbon thickness for high-rate and long-term performances of an aqueous battery.

To realize high-rate and long-term performances of an aqueous rechargeable battery, the most effective approach is to build electrode materials with more reaction active sites and stable structures. Transition metal sulfides have become up-and-coming electrodes due to their high conductivity. Herein, we demonstrated the in situ construction of core-shell Co9S8@C materials with controlled carbon content and thickness. Nanorod-like cobalt-organic chelates were used as the precursors. The cobalt in cobalt-organic chelates reacted with sublimed sulfur to generate the Co9S8 core in situ; meanwhile, the organic chelates were converted into carbon shells, which coated the Co9S8 core and connected with each other to maintain the whole rod shape. Moreover, tunable thickness and content of the carbon shell in the Co9S8@C composite could be achieved by regulating the composition of the reaction solvent. In addition, when 20 mL of dimethylcarbinol was used, the obtained Co9S8@C composite (H1) exhibited the most excellent electrochemical performances, in particular outstanding cycling stability. When assembled with a treated iron powder (TIP) electrode, the Co9S8@C//TIP aqueous rechargeable battery delivered 220.7 mA h g-1 discharge capacity at 1 A g-1, which decreased to 152.8 mA h g-1 even when the current density was increased by a factor of ten (10 A g-1), indicating surprising high-rate performance. Also, after 5000 cycles at 10 A g-1, 74.8% of capacity retention was obtained, further illustrating its excellent long-term cycling stability. Suitable electrode materials with a tunable carbon content have direct impact on the overall performance of an aqueous rechargeable battery, which will guide us for obtaining high-rate and long-term aqueous batteries.

Metal oxide-based supercapacitors: progress and prospectives.

Distinguished by particular physical and chemical properties, metal oxide materials have been a focus of research and exploitation for applications in energy storage devices. Used as supercapacitor electrode materials, metal oxides have certified attractive performances for fabricating various supercapacitor devices in a broad voltage window. In comparison with single metal oxides, bimetallic oxide materials are highly desired for overcoming the constraint of the poor electric conductivity of single metal oxide materials, achieving a high capacitance and raising the energy density at this capacitor-level power. Herein, we investigate the principal elements affecting the properties of bimetallic oxide electrodes to reveal the relevant energy storage mechanisms. Thus, the influences of the chemical constitution, structural features, electroconductivity, oxygen vacancies and various electrolytes in the electrochemical behavior are discussed. Moreover, the progress, development and improvement of multifarious devices are emphasized systematically, covering from an asymmetric to hybrid configuration, and from aqueous to non-aqueous systems. Ultimately, some obstinate and unsettled issues are summarized as well as a prospective direction has been given on the future of metal oxide-based supercapacitors.