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.

Over the last decade, significant effort has been devoted to the applications of hierarchically structured porous materials owing to their outstanding properties such as high surface area, excellent accessibility to active sites, and enhanced mass transport and diffusion. The hierarchy of porosity, structural, morphological and component levels in these materials is key for their high performance in all kinds of applications. The introduction of hierarchical porosity into materials has led to a significant improvement in the performance of materials. Herein, recent progress in the applications of hierarchically structured porous materials from energy conversion and storage, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine is reviewed. Their potential future applications are also highlighted. We particularly dwell on the relationship between hierarchically porous structures and properties, with examples of each type of hierarchically structured porous material according to its chemical composition and physical characteristics. The present review aims to open up a new avenue to guide the readers to quickly obtain in-depth knowledge of applications of hierarchically porous materials and to have a good idea about selecting and designing suitable hierarchically porous materials for a specific application. In addition to focusing on the applications of hierarchically porous materials, this comprehensive review could stimulate researchers to synthesize new advanced hierarchically porous solids.

Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine

Carbon-based nanomaterials have been widely used as catalysts or catalyst supports in the chemical industry or for energy or environmental applications due to their fascinating properties. High surface areas, tunable porosity, and functionalization are considered to be crucial to enhance the catalytic performance of carbon-based materials. Recently, the newly emerging metal–organic frameworks (MOFs) built from metal ions and polyfunctional organic ligands have proved to be promising self-sacrificing templates and precursors for preparing various carbon-based nanomaterials, benefiting from their high BET surface areas, abundant metal/organic species, large pore volumes, and extraordinary tunability of structures and compositions. In comparison with other carbon-based catalysts, MOF-derived carbon-based nanomaterials have great advantages in terms of tailorable morphologies and hierarchical porosity and easy functionalization with other heteroatoms and metal/metal oxides, which make them highly efficient as...

Development of MOF-Derived Carbon-Based Nanomaterials for Efficient Catalysis

Graphitic carbon nitride (g-C3N4) has been deemed a promising heterogeneous metal-free catalyst for a wide range of applications, such as solar energy utilization toward water splitting, and its photocatalytic performance is reasonably adjustable through tailoring its texture and its electronic and optical properties. Here phosphorus-doped graphitic carbon nitride nanostructured flowers of in-plane mesopores are synthesized by a co-condensation method in the absence of any templates. The interesting structures, together with the phosphorus doping, can promote light trapping, mass transfer, and charge separation, enabling it to perform as a more impressive catalyst than its pristine carbon nitride counterpart for catalytic hydrogen evolution under visible light irradiation. The catalyst has low cost, is environmentally friendly, and represents a potential candidate in photoelectrochemistry.

Mesoporous Phosphorus-Doped g-C3N4 Nanostructured Flowers with Superior Photocatalytic Hydrogen Evolution Performance

Electrocatalysts are required for clean energy technologies (for example, water-splitting and metal-air batteries). The development of a multifunctional electrocatalyst composed of nitrogen, phosphorus, and fluorine tri-doped graphene is reported, which was obtained by thermal activation of a mixture of polyaniline-coated graphene oxide and ammonium hexafluorophosphate (AHF). It was found that thermal decomposition of AHF provides nitrogen, phosphorus, and fluorine sources for tri-doping with N, P, and F, and simultaneously facilitates template-free formation of porous structures as a result of thermal gas evolution. The resultant N, P, and F tri-doped graphene exhibited excellent electrocatalytic activities for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The trifunctional metal-free catalyst was further used as an OER–HER bifunctional catalyst for oxygen and hydrogen gas production in an electrochemical water-splitting unit, which was powered by an integrated Zn–air battery based on an air electrode made from the same electrocatalyst for ORR. The integrated unit, fabricated from the newly developed N, P, and F tri-doped graphene multifunctional metal-free catalyst, can operate in ambient air with a high gas production rate of 0.496 and 0.254 μL s−1 for hydrogen and oxygen gas, respectively, showing great potential for practical applications.

Nitrogen, Phosphorus, and Fluorine Tri‐doped Graphene as a Multifunctional Catalyst for Self‐Powered Electrochemical Water Splitting

Metal phosphonate materials are promising non-siliceous inorganic–organic hybrids that are synthesized by combining metal joints and organophosphonic linkages at the molecular scale. The mild conditions for metal phosphonate synthesis, their homogeneous composition and the combined merits of inorganic units and organic groups have permitted the rational design and incorporation of various functionalities through constituent building units. In this critical review, we present the development and recent advances related to the field of metal phosphonates and the relevant nanocomposites. The possibility to integrate the functionalities from both inorganic and organic moieties is discussed. The incorporation of well-defined porosity and capacity for post-modification have extended the application potential to the area of adsorption, separation, catalysis, environmental intervention, energy storage and biology. Metal phosphonates thus present an unprecedented opportunity for the rational and precise design of sophisticated materials with multifunctionality.

Metal phosphonate hybrid materials: from densely layered to hierarchically nanoporous structures

Metal-free carbonaceous materials have attracted considerable interests as heterogeneous catalysts owing to their superior physiochemical properties over metal-based catalysts, such as low cost, no pollution, chemical and thermal stabilities, as well as readily tailorable porous structure and surface chemistry. This review article provides an overview of the fundamentals and recent advances in the field of metal-free carbon catalysts, including graphenes, carbon nanotubes, mesoporous carbons, graphitic carbon nitrides, and related composites. Special focus is placed on their controllable preparation and applications in gas phase, liquid phase, electrochemical, and photocatalytic reactions, as well as defect and surface chemistry related catalytic activities of carbon materials. Some perspectives are highlighted on the development of more efficient metal-free carbonaceous catalysts featuring high stability, low cost, optimized structures, and enhanced performance, which are the key factors to accelerate the designed preparation and commercialization of carbocatalysts.

Metal‐Free Carbonaceous Materials as Promising Heterogeneous Catalysts

Zinc sulfide (ZnS) nanomaterials with well-defined mesoporosity were synthesized with the assistance of sonochemistry in an ethanol system of Zn(NO3)2 and Na2S, without using any templates or surfactants. The prepared ZnS presented a crystalline structure of the cubic zinc-blende phase and possessed a high specific surface area of 263 m2 g−1 with a narrow pore size distribution around 5.1 nm. Abundant surface defects existed in the obtained mesoporous ZnS, thanks to the NaNO3 salt protection through the process of occasional precipitation in the alcohol system. Sonochemically synthesized ZnS nanomaterials showed higher activities for the photodegradation of Rhodamine B under UV light irradiation than the ones prepared in the absence of sonochemistry or in an aqueous system, which was mainly due to the well-structured mesoporosity and the surface defects.

Sonochemistry-assisted synthesis and optical properties of mesoporous ZnS nanomaterials

Insights into mesoporous metal phosphonate hybrid materials for catalysis

Hollow manganese phosphonate microspheres of an inorganic–organic hybrid with hierarchically porous shells were prepared through a template-free hydrothermal method using ethylene diamine tetra(methylene phosphonic acid) as the coupling molecule. The hollow structures with hierarchical porosity were confirmed by SEM, TEM and N2 sorption. FT-IR, XPS and TG-DSC measurements revealed that the organophosphonate linkers were homogeneously incorporated into the hybrid framework. The hierarchical manganese phosphonates could be used as efficient adsorbents for the removal of copper ions, showing fast binding kinetics due to the well-structured porosity. The adsorption process follows pseudo-second order reaction kinetics, as well as Langmuir isotherm, indicating that Cu2+ was monolayer adsorbed on the hybrid by chemical complexation. Furthermore, the synthesized manganese phosphonates with peculiar porosity exhibited excellent size selectivity for protein adsorption in a complex solution, presenting the promising potential as candidates for biomaterials.

Zhong-Yong Yuan

Papers

Metal phosphonate hybrid materials: from densely layered to hierarchically nanoporous structures

Metal‐Free Carbonaceous Materials as Promising Heterogeneous Catalysts

Sonochemistry-assisted synthesis and optical properties of mesoporous ZnS nanomaterials

Insights into mesoporous metal phosphonate hybrid materials for catalysis

Hollow manganese phosphonate microspheres with hierarchical porosity for efficient adsorption and separation