Development of catalysts with high activity and durability is essential for the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) of water splitting. Three-dimensional catalysts have large surface areas and good mechanical and antipoisoning properties and can directly act as working electrodes; these are especially important factors from the point of view of practical applications. Here we review recent significant progress in the field of three-dimensional catalysts for water splitting. Various three-dimensional catalysts that can be applied in electrocatalytic water splitting are presented. The fabricating methods, electrocatalytic performances, and the catalytic mechanisms of these catalysts are introduced. Lastly, the major challenges in this particular field and their prospective solutions are also discussed.

Water splitting for the production of hydrogen and oxygen is an appealing solution to advance many sustainable and renewable energy conversion and storage systems, while the key fact depends on the innovative exploration regarding the design of efficient electrocatalysts. Reported herein is the growth of CoP mesoporous nanorod arrays on conductive Ni foam through an electrodeposition strategy. The resulting material of well-defined mesoporosity and a high specific surface area (148 m² g⁻¹) can be directly employed as a bifunctional and flexible working electrode for both hydrogen and oxygen evolution reactions, showing superior activities as compared with noble metal benchmarks and state-of-the-art transition-metal-based catalysts. This is intimately related to the unique nanorod array electrode configuration, leading to excellent electric interconnection and improved mass transport. A further step is taken toward an alkaline electrolyzer that can achieve a current density of 10 mA cm⁻² at a voltage around 1.62 V over a long-term operation, better than the combination of Pt and IrO₂. This development is suggested to be readily extended to obtain other electrocatalysis systems for scale-up water-splitting technology.

Chemical doping has been recognized as a promising route to achieve novel physicochemical functions of porous carbons as metal-free catalysts in renewable energy-related technologies, such as electrochemical oxygen reduction reaction (ORR). However, it remains challenging to exploit an effective method for the synthesis of metal-free carbonaceous electrocatalysts. Herein, we report the direct synthesis of phosphorus-doped mesoporous carbonaceous electrocatalysts for the first time through soft-templating method, in which organophosphonic acid serves as the phosphorus source. The resulting mesoporous carbon material possesses high doping level, large surface area, and an interconnected mesopore system, ensuring the sufficient exposure and availability of catalytic sites to realize considerable catalytic activity for ORR in alkaline media. More importantly, much better tolerance for methanol oxidation, higher durability, and comparable Tafel slopes as compared with the commercial Pt/carbon (Pt/C) catalyst are valuable for developing alternative fuel cells and metal–air batteries.

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.

Mesoporous zirconium phosphonate materials with bridged organic groups in the framework were hydrothermally synthesized with 1-hydroxyethylidene-1,1-diphosphonic acid as coupling molecule and cetyltrimethylammonium bromide as template. The mesostructure is wormhole-like with a specific surface area of 702 m²/g and a uniform pore size of 3.6 nm, and the organophosphonate groups are homogeneously integrated in the network of the obtained solids. The hydroxyethylidene-bridged mesoporous zirconium phosphonates can serve as solid-acid catalysts for the synthesis of methyl-2,3-O-isopropylidene-β-D-ribofuranoside from D-ribose and exhibit high catalytic activity with rapid reaction rate, which are comparable to the catalytic performance when liquid acid HCl or commercial ion-exchange resin NKC-9 is used as catalyst. Furthermore, mesoporous zirconium phosphonates show high stability with good reusability without any loss of catalytic activity after five cycles, which demonstrates their potential for industrial applications.

The synthesis of porous hybrid materials has been extended to mesoporous non-silica-based organic-inorganic hybrid materials, in which mesoporous metal phosphonates represent an important family. By using organically bridged polyphosphonic acids as coupling molecules, the homogeneous incorporation of a considerable number of organic functional groups into the metal phosphonate hybrid framework has been realized. Small amounts of organic additives and the pH value of the reaction solution have a large impact on the morphology and textural properties of the resultant hybrid mesoporous metal phosphonate solids. Cationic and nonionic surfactants can be used as templates for the synthesis of ordered mesoporous metal phosphonates. The materials are used as efficient adsorbents for heavy metal ions, CO₂, and aldehydes, as well as in the separation of polycyclic aromatic hydrocarbons. They are also useful photocatalysts under UV and simulated solar light irradiation for organic dye degradation. Further functionalization of the synthesized mesoporous hybrids makes them oxidation and acid catalysts, both with impressive performances in the fields of sustainable energy and environment.

A series of ZnO-CeO₂ binary oxides with a novel hydrangea-like morphology and meso-/macroporous hierarchical structure of high surface area (around 100 m²/g) were synthesized by a simple one-pot hydrothermal process in the presence of triblock copolymer F127. Homogeneous mixing of wurtzite ZnO and cubic phase CeO₂ and the coexistence of Ce³⁺ and Ce⁴⁺ on the surface of the synthesized ZnO-CeO₂ materials were confirmed. UV/Vis diffuse reflectance spectra show that the absorption edges of the binary oxides shift remarkably to the visible range relative to those of pure ZnO and CeO₂. Their photoactivities were tested by photodegradation of Rhodamine B under UV and visible light irradiation, and excellent photocatalytic performance for organic waste degradation was found in the synthesized hydrangea-like meso-/macroporous ZnO-CeO₂ materials. Furthermore, their application as heterogeneous catalysts for CO removal was evaluated, and the results indicate that the synthesized ZnO-CeO₂ materials exhibit efficient activity for removing CO by catalytic oxidation, demonstrating a promising potential in environmental remediation.

Organic-inorganic hybrid materials of meso-/macroporous titanium triphosphonate materials were synthesized by using amino tri(methylene phosphonic acid) as the coupling molecule. The preparation was accomplished by a hydrothermal process in the presence or absence of small amounts of the diblock copolymer EO₃₀PO₃₄ and β-cyclodextrin as the organic additives. The organic-additive-assisted preparation efficiently aided the enlargement of the surface areas and pore volumes of the resultant porous titanium triphosphonates and helped improve the wormhole-like mesoporosity. In addition to the large macrochannels, with a size of 500–1000 nm, observed in all the titanium phosphonates synthesized with or without organic additives, a secondary small-scaled, spherical macroporous structure with a diameter of 50–100 nm was obtained by the addition of cyclodextrin. These macroporous structures are proposed to be templated by the aggregation of cyclodextrin. The structural characterization confirms the integrity of organic groups inside the framework. Large adsorption capacities for heavy metal ions and CO₂ are demonstrated in these hybrid materials, which makes them promising adsorbents for practical applications.

Cubic mesoporous titanium phosphonate materials with bridged organic groups inside the framework were synthesized by means of a one-pot hydrothermal autoclaving process, with the assistance of cationic surfactant cetyltrimethylammonium bromide. 1-Hydroxyethylidene-1,1-diphosphonic acid was used as the coupling molecule. A typical cubic mesophase with surface area of 1052 m² g⁻¹ and pore size of 2.6 nm was confirmed by XRD, TEM, and N₂ sorption analysis. The organophosphonate groups were homogeneously incorporated in the network of the mesoporous solids, as revealed by FTIR and magic-angle spinning (MAS) NMR spectroscopy, and thermogravimetry and differential scanning calorimetry (TG-DSC) measurements. The synthesized hydroxyethylidene-bridged cubic mesoporous titanium phosphonates proved to be thermally stable up to 350 °C, with a well-preserved hybrid framework and cubic mesoporous architecture. The obtained cubic mesophase could be transformed into a hexagonal mesophase by simply adjusting the molar ratios of the added raw materials, namely, a Ti/P molar ratio of 1:4 and a CTAB/Ti molar ratio of 1.9–2.3 for the cubic phase and Ti/P molar ratio of 3:4 and CTAB/Ti molar ratio of 0.1–0.4 for the hexagonal phase. The cubic hybrid materials could be used as efficient photocatalysts for the photodegradation of rhodamine B. Moreover, they were also used for adsorption of CO₂ and heavy metal ions and exhibited a significant capture amount of around 1.0 mmol g⁻¹ for CO₂ molecules at 35 °C and high adsorption capacity of 28.5 μmol g⁻¹ for Cu²⁺ ions with good reusability, which demonstrated their promising potential in environmental remediation.

An organic–inorganic hybrid nanostructured material of titania–diphosphonate (Ti-HEDP) was prepared from a simple self-assembly process with the precursor tetrabutyl titanate and 1-hydroxyethane-1,1-diphosphonic acid (HEDP). The prepared hybrid Ti-HEDP has a semicrystalline anatase phase, exhibiting a hierarchical macroporous structure composed of mesostructured Ti-HEDP nanorods with a length of 80–150 nm and a thickness of 18–38 nm. The BET surface area is 257 m²/g. The 1-hydroxyethane-1,1-diyl-bridged organophosphonate groups were homogeneously incorported into the network of the hierarchical nanostructured/porous solid, as revealed by FT-IR spectroscopy, MAS NMR spectroscopy, XPS, and TGA measurements. The optical property, photocatalytic activity, and metal ion adsorption ability of the hierarchical Ti-HEDP materials were also investigated, suggesting that they have potential applications in catalysis and adsorption.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)