Long-Zhang Dong

Coupled molybdenum carbide and reduced graphene oxide electrocatalysts for efficient hydrogen evolution

Electrochemical water splitting is one of the most economical and sustainable methods for large-scale hydrogen production. However, the development of low-cost and earth-abundant non-noble-metal catalysts for the hydrogen evolution reaction remains a challenge. Here we report a two-dimensional coupled hybrid of molybdenum carbide and reduced graphene oxide with a ternary polyoxometalate-polypyrrole/reduced graphene oxide nanocomposite as a precursor. The hybrid exhibits outstanding electrocatalytic activity for the hydrogen evolution reaction and excellent stability in acidic media, which is, to the best of our knowledge, the best among these reported non-noble-metal catalysts. Theoretical calculations on the basis of density functional theory reveal that the active sites for hydrogen evolution stem from the pyridinic nitrogens, as well as the carbon atoms, in the graphene. In a proof-of-concept trial, an electrocatalyst for hydrogen evolution is fabricated, which may open new avenues for the design of nanomaterials utilizing POMs/conducting polymer/reduced-graphene oxide nanocomposites.

Hexagonal@Cubic CdS Core@Shell Nanorod Photocatalyst for Highly Active Production of H2 with Unprecedented Stability

A highly effective, low-cost strategy for improved photocatalytic efficiency and stability of CdS is described. Based on the integration of hexagonal-cubic core-shell architecture with nanorod morphology, the concentric CdS nanorod phase junctions (NRPJs) obtained demonstrate extremely high H2 production rate and unprecedented photocatalytic stability.

Effect of Imidazole Arrangements on Proton-Conductivity in Metal–Organic Frameworks

Imidazole molecules were frequently incorporated into porous materials to improve their proton conductivity. To investigate how different arrangements of imidazoles in metal-organic frameworks (MOFs) affect the overall proton conduction, we designed and prepared a MOF-based model system. It includes an Fe-MOF as the blank, an imidazole@Fe-MOF (Im@Fe-MOF) with physically adsorbed imidazole, and an imidazole-Fe-MOF (Im-Fe-MOF), which contains chemically coordinated imidazole molecules. The parent Fe-MOF, synthesized from the exchange of carboxylates in the preformed [Fe3(μ3-O)](carboxylate)6 clusters and multitopic carboxylate ligands, serves as a control. The Im@Fe-MOF was prepared by encapsulating free imidazole molecules into the pores of the Fe-MOF, whereas the Im-Fe-MOF was obtained in situ, in which imidazole ligands coordinate to the metal nodes of the framework. Proton-conductivity analyses revealed that the proton conductivity of Im-Fe-MOF was approximately two orders of magnitude greater than those of Fe-MOF and Im@Fe-MOF at room temperature. The high proton conductivity of 1.21 × 10-2 S cm-1 at 60 °C for Im-Fe-MOF ranks among the highest performing MOFs ever reported. The results of the density functional theory calculations suggest that coordinated imidazole molecules in Im-Fe-MOF provide a greater concentration of protons for proton transportation than do coordinated water molecules in Fe-MOF alone. Besides, Im-Fe-MOF exhibits steadier performance than Im@Fe-MOF does after being washed with water. Our investigation using the above ideal crystalline model system demonstrates that compared to disorderly arranged imidazole molecules in pores, the immobilized imidazole molecules by coordination bonds in the framework are more prone to form proton-conduction pathways and thus perform better and steadier in water-mediated proton conduction.

Polyoxometalate-based metal–organic framework-derived hybrid electrocatalysts for highly efficient hydrogen evolution reaction

The design and fabrication of electrocatalysts for HER, with superior activity and stability, still remain a significant challenge for clean and renewable energy technologies. Here we have synthesized Fe3C/Mo2C-containing N, P co-doped graphitic carbon derived from POM@MOF-100 (Fe) (denoted as Fe3C/Mo2C@NPGC) via a “killing three birds with one stone” strategy. The Fe3C/Mo2C@NPGC catalyst demonstrates excellent electrocatalytic activity and stability towards HER with a low onset overpotential of 18 mV (vs. RHE), small Tafel slope of 45.2 mV dec−1, as well as long-term durability for 10 h, which is one of the best non-noble metal HER catalysts in acidic media reported so far. Most importantly, this work opens up exciting opportunities for fabricating novel and highly efficient electrocatalysts to replace Pt or Pt-based catalysts utilizing POM-based metal–organic frameworks (MOFs) as precursors.

Derivation and Decoration of Nets with Trigonal-Prismatic Nodes: A Unique Route to Reticular Synthesis of Metal–Organic Frameworks

Quests for advanced functionalities in metal-organic frameworks (MOFs) inevitably encounter increasing complexity in their tailored framework architectures, accompanied by heightened challenges with their geometric design. In this paper, we demonstrate the feasibility of rationally exploiting topological prediction as a blueprint for predesigned MOFs. A new triangular frusta secondary building unit (SBU), {Zn4(tz)3}, was bridged by three TDC(2-) fragments to initially form a trigonal prismatic node, {Zn8(tz)6(TDC)3} (Htz = 1H-1,2,3-triazole and H2TDC = 2,5-thiophenedicarboxylic acid). Furthermore, the trigonal prism unit can be considered as a double SBU derived from triply bound triangular frusta. By considering theoretical derived nets for linking this trigonal-prismatic node with ditopic, tritopic, and tetratopic linkers, we have synthesized and characterized a new family of MOFs that adopt the decorated lon, jea, and xai nets, respectively. Pore sizes have also been successively increased within TPMOF-n family, which facilitates heterogeneous biomimetic catalysis with Fe-porphyrin-based TPMOF-7 as a catalyst.

A highly stable polyoxometalate-based metal–organic framework with π–π stacking for enhancing lithium ion battery performance

A novel polyoxometalate-based metal–organic framework (POMOF), [PMo8VMo4VIO37(OH)3Zn4][TPT]5·2TPT·2H2O (NNU-11, TPT = tris-(4-pyridyl)triazine), was synthesized. Zn-e-Keggin fragments were directly connected with TPT ligands generating 2D layers and further interdigitated with each other by π–π stacking interactions to pack into a 3D array. The compound exhibited excellent stability in air and different organic solvents and even in different pH (pH 1–11) aqueous solutions. It can be utilized as an anode material for lithium ion batteries (LIBs) due to the combination of the multi-electron redox property of POM units and the functionalization of MOFs. NNU-11 exhibited a highly reversible capacity of 750 mA h g−1 at a current density of 50 mA g−1 after 200 cycles along with excellent cycle stability and rate performance. More importantly, for the first time, we designed and synthesized a POMOF crystalline structure model supported by π–π stacking interactions to demonstrate that intermolecular π–π stacking interactions are beneficial to promote the performance of LIBs.

Engineering Zn1-xCdxS/CdS Heterostructures with Enhanced Photocatalytic Activity.

Various porous Zn1-xCdxS/CdS heteorostructures were achieved via in situ synthesis method with organic amines as the templates. Because of the larger radius of Cd(2+) than that of Zn(2+), CdS quantum dots are formed and distributed uniformly in the network of Zn1-xCdxS. The Zn1-xCdxS/CdS heterostructure with small Cd content (10 at%) derived from ethylenediamine shows very high H2-evolution rate of 667.5 μmol/h per 5 mg photocatalyst under visible light (λ ≥ 420 nm) with an apparent quantum efficiency of 50.1% per 5 mg at 420 nm. Moreover, this Zn1-xCdxS/CdS heterostructure photocatalyst also shows an excellent photocatalytic stability over 100 h.

Efficient Electrocatalyst for the Hydrogen Evolution Reaction Derived from Polyoxotungstate/Polypyrrole/Graphene

Efficient hydrogen evolution reaction (HER) from water by electrocatalysis using cost-effective materials is critical to realize the clean hydrogen production. Herein, with controlling the structure and composition of polyoxotungstate/conductive polypyrrole/graphene (PCG) precursor precisely and followed by a temperature-programmed reaction, we developed a highly active and stable catalyst: NC@Wx C/NRGO (NC: nitrogen-doped porous carbon, NRGO: nitrogen-doped reduced graphene oxide). The composite presents splendid performance towards HER in acidic media, with a small onset overpotential of 24 mV versus RHE (reversible hydrogen electrode), a low Tafel slope of 58.4 mV dec-1 , a low overpotential of 100 mV at 10 mA cm-2 , and remarkable long-term cycle stability. This is one of the highest HER catalysts among the tungsten carbide-based materials ever reported.

Bimetallic Carbides-Based Nanocomposite as Superior Electrocatalyst for Oxygen Evolution Reaction

The development of highly efficient and low-cost oxygen evolution electrocatalysts is extremely imperative for the new energy technology. Transition metal carbides have been investigated as remarkable hydrogen evolution reaction (HER) electrocatalysts but undesired oxygen evolution reaction (OER) electrocatalysts and need further study. Here, a cobalt-molybdenum-based bimetallic carbide coated by N-doped porous carbon and anchored on N-doped reduced graphene oxide film (Co6Mo6C2/NCRGO) is synthesized by directly carbonizing the Co-doped polyoxometalate/conductive polymer/graphene oxide (Co-PCG) precursors. The precise control of the Co/Mo molar ratio in the Co-PCG precursor is of critical importance to synthesize pure phase bimetallic carbide of Co6Mo6C2. As the highly active and robust OER electrocatalyst, the Co6Mo6C2/NCRGO composite exhibits excellent activity in alkaline solution, affording a low overpotential of 260 mV versus RHE at 10 mA cm-2, a small Tafel slope of 50 mV dec-1, as well as long-term stability. The superior OER performances are strongly associated with the active Co6Mo6C2 particles, polypyrrole (PPy)-derived N-doped porous carbon, and the conductive RGO films. Remarkably, it is the first evidence that the bimetallic carbides were used as the OER catalysts with such high OER activity.

Highly active Co–Mo–C/NRGO composite as an efficient oxygen electrode for water–oxygen redox cycle

The slow kinetics of the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) have hindered energy conversion and storage greatly. Design of a new class of low-cost and highly efficient electrocatalysts for the water–oxygen redox cycle (WORC) system including OER and ORR is considered a huge challenge. Controlled synthesis of unique and stable precursors as a perfect platform to synthesize target products with diverse compositions are of vital importance. Herein, we synthesized a metal/metal carbide-based composite (denoted as Co–Mo–C/NRGO-1) by directly carbonizing Co-doped polyoxometalate/conductive polymer/graphene precursor (Co-PCG) as an efficient bi-functional electrocatalyst. The resulting Co–Mo–C/NRGO-1 composite exhibited superior electrocatalytic activity for OER with an ultra-low Tafel slope of 42 mV dec−1, a small overpotential of 330 mV vs. RHE at the current density of 10 mA cm−2 and long-term stability in alkaline medium. The ORR performance was also investigated with a positive onset potential (∼−95 mV vs. Ag/AgCl), remarkable stability over 30 000 s and good tolerance to methanol crossover. Most importantly, the OER performance of Co–Mo–C/NRGO-1 was the best among all the reported carbide-based materials and was comparable to the best OER electrodes.