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Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Hydrogen has been hailed as a clean and sustainable alternative to finite fossil fuels in many energy systems. Water splitting is an important method for hydrogen production in high purity and large quantities. To accelerate the hydrogen evolution reaction (HER) rate, it is highly necessary to develop high efficiency catalysts and to select a proper electrolyte. Herein, the performances of non-noble metal-based carbon composites under various pH values (acid, alkaline and neutral media) for HER in terms of catalyst synthesis, structure and molecular design are systematically discussed. A detailed analysis of the structure-activity-pH correlations in the HER process gives an insight on the origin of the pH-dependence for HER, and provide guidance for future HER mechanism studies on non-noble metal-based carbon composites. Furthermore, this Review gives a fresh impetus to rational design of high-performance noble-metal-free composites catalysts and guide researchers to employ the established electrocatalysts in proper water electrolysis technologies.

Non-Noble Metal-based Carbon Composites in Hydrogen Evolution Reaction: Fundamentals to Applications.

The number of research reports published in recent years on electrochemical water splitting for hydrogen generation is higher than for many other fields of energy research. In fact, electrochemical water splitting, which is conventionally known as water electrolysis, has the potential to meet primary energy requirements in the near future when coal and hydrocarbons are completely consumed. Due to the sudden and exponentially increasing attention on this field, many researchers across the world, including our group, have been exerting immense efforts to improve the electrocatalytic properties of the materials that catalyze the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode, aided by the recent revolutionary discovery of nanomaterials. However, the pressure on the researchers to publish their findings rapidly has caused them to make many unnoticed and unintentional errors, which is mainly due to lack of clear insight on the activity parameters. In this perspective, we have discussed the use and validity of ten important parameters, namely overpotential at a defined current density, iR-corrected overpotential at a defined current density, Tafel slope, exchange current density (j0), mass activity, specific activity, faradaic efficiency (FE), turnover frequency (TOF), electrochemically active surface area (ECSA) and measurement of double layer capacitance (Cdl) for different electrocatalytic materials that are frequently employed in both OER and HER. Experimental results have also been provided in support of our discussions wherever required. Using our critical assessments of the activity parameters of water splitting electrocatalysis, researchers can ensure precision and correctness when presenting their data regarding the activity of an electrocatalyst.

Precision and correctness in the evaluation of electrocatalytic water splitting: revisiting activity parameters with a critical assessment

Conjugated polymers and related processing techniques have been developed for organic electronic devices ranging from lightweight photovoltaics to flexible displays. These breakthroughs have recently been used to create organic thermoelectric materials, which have potential for wearable heating and cooling devices, and near-room-temperature energy generation. So far, the best thermoelectric materials have been inorganic compounds (such as Bi2Te3) that have relatively low Earth abundance and are fabricated through highly complex vacuum processing routes. Molecular materials and hybrid organic–inorganic materials now demonstrate figures of merit approaching those of these inorganic materials, while also exhibiting unique transport behaviours that are suggestive of optimization pathways and device geometries that were not previously possible. In this Review, we discuss recent breakthroughs for organic materials with high thermoelectric figures of merit and indicate how these materials may be incorporated into new module designs that take advantage of their mechanical and thermoelectric properties. Thermoelectrics can be used to harvest energy and control temperature. Organic semiconducting materials have thermoelectric performance comparable to many inorganic materials near room temperature. Better understanding of their performance will provide a pathway to new types of conformal thermoelectric modules.

Organic thermoelectric materials for energy harvesting and temperature control

Ultralight ceramic aerogels with the property combination of recoverable compressibility and excellent high-temperature stability are attractive for use in harsh environments. However, conventional ceramic aerogels are usually constructed by oxide ceramic nanoparticles, and their practical applications have always been limited by the brittle nature of ceramics and volume shrinkage at high temperature. Silicon carbide (SiC) nanowire offers the integrated properties of elasticity and flexibility of one-dimensional (1D) nanomaterials and superior high-temperature thermal and chemical stability of SiC ceramics, which makes it a promising building block for compressible ceramic nanowire aerogels (NWAs). Here, we report the fabrication and properties of a highly porous three-dimensional (3D) SiC NWA assembled by a large number of interweaving 3C-SiC nanowires of 20–50 nm diameter and tens to hundreds of micrometers in length. The SiC NWA possesses ultralow density (∼5 mg cm–3), excellent mechanical properties o...

Ultralight, Recoverable, and High-Temperature-Resistant SiC Nanowire Aerogel.

Intermetallic phase formation in diffusion-bonded Cu/Al laminates prepared by plasma activated sintering (PAS) was investigated in the temperature range 673–773 K for 10–30 min. Three intermetallic phases, Al4Cu9, AlCu, and Al2Cu, were identified in all the samples. The formation of Al2Cu as the first phase was rationalized on the basis of the effective heat of formation (EHF) model. The thermodynamic driving force for the preferential appearance of Al4Cu9 at the α-Cu(Al)/Al2Cu interface was evaluated. The time and temperature dependences of the growth of the three intermetallic layers were determined, and their apparent activation energies were calculated. The growth kinetic of the layers conformed to the parabolic law, implying that the intermetallic phase formation was volume diffusion-controlled in the temperature range. The apparent activation energies calculated for the growth of the total intermetallic layer, Al4Cu9, AlCu, and Al2Cu layers were about 80.8, 89.8, 84.6, and 71.1 kJ/mol, respectively.

Intermetallic phase formation in diffusion-bonded Cu/Al laminates

Component-controllable synthesis of Co(SxSe1−x)2 nanowires supported by carbon fiber paper as high-performance electrode for hydrogen evolution reaction

Cobalt telluride branched nanostructures on carbon fiber paper (CFP) with two different morphologies were synthesized via solution-based conversion reaction. Both the CoTe2 with nanodendrite and CoTe with nanosheet morphologies on the CoTe2 nanotube (CoTe2 NDs/CoTe2 NTs and CoTe NSs/CoTe2 NTs) supported by CFP exhibit high activities toward hydrogen evolution reaction (HER). Particularly, the CoTe NSs/CoTe2 NTs only require an overpotential of 230.0 mV to deliver the current density of 100 mA cm–2 in acid solution. After cycling for 5000 cycles or 20 h continual electrolysis, only a small performance loss is observed.

Morphology-Controllable Synthesis of Cobalt Telluride Branched Nanostructures on Carbon Fiber Paper as Electrocatalysts for Hydrogen Evolution Reaction

Pyrite-type CoSe2 necklace-like nanowires (NWs) were successfully grown on carbon fiber paper (CFP) and proven to be an efficient electrocatalyst towards the hydrogen evolution reaction (HER). By combining the use of mesoporous CFP and the nano-structuring of the electrocatalyst, the highly active CoSe2 necklace-like NWs on CFP only need the modest overpotentials of 188 and 199 mV to afford the current densities of 50 and 100 mA cm−2, respectively. A small Tafel slope of 34 mV dec−1 and a charge transfer resistance of 7.05 Ω illustrate the prominent electrocatalytic performance. After continual electrolysis for 20 h or conducting cycling for 5000 times, the high activity of CoSe2 NWs toward the HER is still perfectly preserved. The outstanding electrocatalytic performance and stability make the CoSe2 necklace-like NWs on CFP a promising earth-abundant electrocatalyst for the HER and other renewable energy applications.

Zhongqi Shi

Papers

Ultralight, Recoverable, and High-Temperature-Resistant SiC Nanowire Aerogel.

Intermetallic phase formation in diffusion-bonded Cu/Al laminates

Component-controllable synthesis of Co(SxSe1−x)2 nanowires supported by carbon fiber paper as high-performance electrode for hydrogen evolution reaction

Morphology-Controllable Synthesis of Cobalt Telluride Branched Nanostructures on Carbon Fiber Paper as Electrocatalysts for Hydrogen Evolution Reaction

CoSe2 necklace-like nanowires supported by carbon fiber paper: a 3D integrated electrode for the hydrogen evolution reaction