Alternative energy technologies are greatly hindered by significant limitations in materials science. From low activity to poor stability, and from mineral scarcity to high cost, the current materials are not able to cope with the significant challenges of clean energy technologies. However, recent advances in the preparation of nanomaterials, porous solids, and nanostructured solids are providing hope in the race for a better, cleaner energy production. The present contribution critically reviews the development and role of mesoporosity in a wide range of technologies, as this provides for critical improvements in accessibility, the dispersion of the active phase and a higher surface area. Relevant examples of the development of mesoporosity by a wide range of techniques are provided, including the preparation of hierarchical structures with pore systems in different scale ranges. Mesoporosity plays a significant role in catalysis, especially in the most challenging processes where bulky molecules, like those obtained from biomass or highly unreactive species, such as CO2 should be transformed into most valuable products. Furthermore, mesoporous materials also play a significant role as electrodes in fuel and solar cells and in thermoelectric devices, technologies which are benefiting from improved accessibility and a better dispersion of materials with controlled porosity.

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Mesoporous materials for clean energy technologies

Ni-based metal organic frameworks (Ni-MOFs) with unique hierarchical hollow ball-in-ball nanostructure were synthesized by solvothermal reactions. After successive carbonization and oxidation treatments, hierarchical NiO/Ni nanocrystals covered with a graphene shell were obtained with the hollow ball-in-ball nanostructure intact. The resulting materials exhibited superior performance as the anode in lithium ion batteries (LIBs): they provide high reversible specific capacity (1144 mAh/g), excellent cyclability (nearly no capacity loss after 1000 cycles) and rate performance (805 mAh/g at 15 A/g). In addition, the hierarchical NiO/Ni/Graphene composites demonstrated promising performance as anode materials for sodium-ion batteries (SIBs). Such a superior lithium and sodium storage performance is derived from the well-designed hierarchical hollow ball-in-ball structure of NiO/Ni/Graphene composites, which not only mitigates the volume expansion of NiO during the cycles but also provides a continuous highly conductive graphene matrix to facilitate the fast charge transfer and form a stable SEI layer.

Metal Organic Frameworks Derived Hierarchical Hollow NiO/Ni/Graphene Composites for Lithium and Sodium Storage

A graphitic carbon nitride/rGO/perylene diimide polymer (g-C3 N4 /rGO/PDIP) Z-scheme heterojunction is successfully constructed to realize high-flux charge transfer and efficient photocatalytic overall water splitting. A giant internal electric field in the Z-scheme junction is built, enabling the charge separation efficiency to be enhanced dramatically by 8.5 times. Thus, g-C3 N4 /rGO/PDIP presents an efficient and stable photocatalytic overall water splitting activity with H2 and O2 evolution rate of 15.80 and 7.80 µmol h-1 , respectively, ≈12.1 times higher than g-C3 N4 nanosheets. Meanwhile, a notable quantum efficiency of 4.94% at 420 nm and solar-to-hydrogen energy-conversion efficiency of 0.30% are achieved, prominently surpassing many reported g-C3 N4 -based photocatalysts. Briefly, this work throws light on enhancing the internal electric field by interface control to dramatically improve the photocatalytic performance.

Efficient Photocatalytic Overall Water Splitting Induced by the Giant Internal Electric Field of a g‐C 3 N 4 /rGO/PDIP Z‐Scheme Heterojunction

Rechargeable lithium batteries have attracted great attention as next generation power systems for electric vehicles (EVs). Lithium ion batteries, lithium–sulfur batteries, and lithium–oxygen batteries are all suitable to be the power systems for next generation EVs, but their power densities and cycling performance still need to be improved to match the requirements of practical EVs. Thus, rational design and controllable synthesis of electrode materials with unique microstructure and outstanding electrochemical performance are crucially desired. Porous carbon-based composites have many advantages for energy storage and conversion owing to their unique properties, including high electronic conductivity, high structural stability, high specific surface area, large pore volume for efficient electrolyte flux, and high reactive electrode materials with controllable size confined by porous carbon frameworks. Therefore, porous carbon composites exhibit excellent performance as electrode materials for lithium ion batteries, lithium–sulfur batteries, and lithium–oxygen batteries. In this review, we summarize research progress on porous carbon composites with enhanced performance for rechargeable lithium batteries. We present the detailed synthesis, physical and chemical properties, and the innovation and significance of porous carbon composites for lithium ion batteries, lithium–sulfur batteries, and lithium–oxygen batteries. Finally, we conclude the perspectives and critical challenges that need to be addressed for the commercialization of rechargeable lithium batteries.

Porous Carbon Composites for Next Generation Rechargeable Lithium Batteries

Carbothermal reduction could be employed as a facile technology for the synthesis of various novel materials, especially transition-metal-functionalized nanostructures. In particular, energy materials, such as ZnO, MnO2, and LiFePO4, combined with different carbon nanostructures have been widely synthesized via carbothermal reduction, which could be well established industrially due to its low-cost starting materials. In addition, a variety of carbon sources can be employed as comparatively low-cost carbon precursors for the synthesis of carbonaceous functional materials, such as porous carbon-coated magnetic nanoparticles (e.g., Co3O4@C, Fe3O4@C, and FeC3–C). These functional materials have great potential for use in energy and environmental applications. Carbothermal reduction methods possess some incomparable advantages, such as convenience, relatively low cost, and good repeatability, for commercial applications. However, they normally require a relatively high temperature for sustaining carbothermic reaction. Consequently, novel structures can also be derived from renewable, abundant carbon precursors. Examples include lignocellulosic biomass or other biological products derived from food or agricultural wastes (carbohydrates, cellulose, hemicellulose, lignin, chitin, inorganics, and proteins). Thermochemical conversion of biomass, including pyrolysis in an inert environment, has been developed for the production of energy and carbon materials. Furthermore, it is still a potential challenge to simultaneously produce high-quality biofuel products and synthesize value-added functionalized materials via in situ carbothermal reduction. This review will be a powerful resource for stimulating the development of sustainable metal-functionalized nanostructured materials by carbothermal reduction integrated with other advanced technologies, particularly for strengthening efforts towards novel materials for clean energy and environmental applications in the future sustainable society.

Carbothermal synthesis of metal-functionalized nanostructures for energy and environmental applications

The rational design of high-performance tri-functional electrocatalysts plays an important role for the fast development of Zn-air batteries (ZABs) and water splitting and still remains a great challenge up to date. Herein, Co4N nanoparticles encapsulated in nitrogen doped carbon (Co4N@NC) composite has been conveniently fabricated which shows excellent stability over 750 h at 5 mA cm−2 and the voltage gap remain almost constant for long cycles. Flexible ZABs assembled based on this catalyst can be normally charged/discharged at any bent angles. Co4N@NC also can be used as HER catalyst to derive efficient splitting water under the ZABs which are assembled with Co4N@NC as ORR/OER catalyst. Density functional theory calculations reveal that N-doping in Co crystal could improve electronic conductivity, decrease the overpotential and accelerate reaction kinetics for OER/ORR. The above results indicate that Co4N@NC can be used as promising tri-functional high performance catalyst for Zn-air battery and overall water splitting.

Co4N nanoparticles encapsulated in N-doped carbon box as tri-functional catalyst for Zn-air battery and overall water splitting

Novel Designed MnS-MoS2 Heterostructure for Fast and Stable Li/Na Storage:Insights into the Advanced Mechanism Attributed to Phase Engineering

From a thermodynamic point of view, electrocatalytic nitrogen reduction reaction (NRR) is possible for the carbon-based metal-free catalysts, which are gradually becoming a class of potential alternatives to metal-based catalysts. However, the clarification of true active sites and corresponding exact catalytic mechanism of metal-free catalysts is urgently required. Taking full advantage of boron “Lewis acid”, a series of boron carbonitride (BCN) materials were designed and synthesized and the “Lewis acid catalysis” sites were tuned easily by adjusting the relative contents of boron and nitrogen atoms. Boron-enriched BCN exhibited outstanding NRR performance with an ammonia yield of −8.39 μg h−1.cm−2cat. (−41.9 μg h−1.mg−1cat.) and Faradaic efficiency of −9.87 %, together with excellent stability. Density functional theory calculations indicate that the boron sites of BCN enable the low energy barrier of the NRR rate-determining steps and the spontaneity of nitrogen adsorption. Our philosophy opens a new possible avenue to explore high-performance metal-free materials for nitrogen activation.

Metal-free boron carbonitride with tunable boron Lewis acid sites for enhanced nitrogen electroreduction to ammonia

As a metal-free polymeric semiconductor, graphitic carbon nitride (g-C3N4) is a promising candidate for photocatalytic water splitting but suffers from the insufficient optical absorption and sluggish utilization of photocarriers. Herein, based on the energy band engineering theory, a new design philosophy of tunable p-n homojunction is proposed and realized in a novel P-doped g-C3N4 (PCN) material. Through regulating the tendency of transformation in p-n homojunction, a strong internal electric field is built and integrated with two secondary internal electric fields by further decorating with Ti3C2, leading to the formation of a triple-internal electric field system. Thus the spatial separation efficiency of photocarriers is improved and their lifetime is prolonged. The specific photocatalyst exhibits excellent overall water splitting performance in pure water with a notable quantum yield. This work presents a peculiar concept of adjustable p-n homojunction and holds great significance for the rational design of advanced photocatalyst in water splitting.

Rational modulation of p-n homojunction in P-doped g-C3N4 decorated with Ti3C2 for photocatalytic overall water splitting

Luminescence glass is a potential candidate for the light-emitting diodes (LEDs) applications. Here, we study the structural and optical properties of the Eu-, Tb-, and Dy-doped oxyfluoride silicate glasses for LEDs by means of X-ray diffraction, photoluminescence spectra, Commission Internationale de L’Eclairage (CIE) chromaticity coordinates, and correlated color temperatures (CCTs). The results show that the white light emission can be achieved in Eu/Tb/Dy codoped oxyfluoride silicate glasses under excitation by near-ultraviolet light due to the simultaneous generation of blue, green, yellow, and red-light wavelengths from Tb, Dy, and Eu ions. The optical performances can be tuned by varying the glass composition and excitation wavelength. Furthermore, we observed a remarkable emission spectral change for the Tb 3+ single-doped oxyfluoride silicate glasses. The 5 D3 emission of Tb 3+ can be suppressed by introducing B2O3 into the glass. The conversion of Eu 3+ to Eu 2+ takes place in Eu single-doped oxyfluoride aluminosilicate glasses. The creation of CaF2 crystals enhances the conversion efficiency. In addition, energy transfers from Dy 3+ to Tb 3+ and Tb 3+ to Eu 3+ ions occurred in Eu/Tb/Dy codoped glasses, which can be confirmed by analyzing fluorescence spectra and energy level diagrams.

Jianxing Shen

Papers

Co4N nanoparticles encapsulated in N-doped carbon box as tri-functional catalyst for Zn-air battery and overall water splitting

Novel Designed MnS-MoS2 Heterostructure for Fast and Stable Li/Na Storage:Insights into the Advanced Mechanism Attributed to Phase Engineering

Metal-free boron carbonitride with tunable boron Lewis acid sites for enhanced nitrogen electroreduction to ammonia

Rational modulation of p-n homojunction in P-doped g-C3N4 decorated with Ti3C2 for photocatalytic overall water splitting

Eu-, Tb-, and Dy-Doped Oxyfluoride Silicate Glasses for LED Applications