The development of highly active and stable noble metal-free electrocatalysts of hydrogen evolution reaction (HER) under both acidic and basic conditions for renewable-energy conversion techniques is of great significance. Herein, a practical in situ synthesis strategy for a three-dimensional Ni2P polyhedron with a hierarchically porous structure was presented, which was efficiently obtained from a nickel centered metal–organic frameworks (MOF-74-Ni) by direct low-temperature phosphorization. The as-prepared Ni2P polyhedron showed a high BET surface area (175.0 m2·g–1), hierarchically porous property, and outstanding metal dispersion, which well inherited the morphology and porosity of its MOF precursor. Compared with Ni2P particles obtained from a nonporous precursor, the as-prepared Ni2P polyhedron used as electrocatalyst exhibited excellent electrocatalytic performance toward the HER, with a low overpotential of 158 mV to produce the cathodic current density of 10 mA cm–2. A small Tafel slope of 73 mV per decade is obtained for Ni2P polyhedron, which revealed a Volmer-Heyrovsky mechanism during the HER. In addition, benefiting from the structural stability, the porous Ni2P polyhedron used as a electrocatalyst showed satisfactory long-term durability for the HER in acidic media.Keywords: electrocatalyst; hierarchical pores; hydrogen evolution reaction; metal−organic frameworks; nickel phosphide;

Organic bulk heterojunction solar cells with a high fullerene content (larger than 70%) have been studied in this work. The device performances of this kind of solar cell could be tuned by adjusting the blend ratio in the active layer. An appropriate amount of p-type semiconductor in the high fullerene content active blend layer is beneficial for light absorbance and exciton dissociation. The proper energy alignment between the highest occupied molecular orbital of a p-type material and an n-type fullerene derivative has a strong influence on the exciton dissociation efficiency. In addition, the mechanism of photogenerated charge recombination in the solar cells has been identified through intensity-dependent current density–voltage (J–V) measurements and results show that the mechanisms governing the recombination are crucial for solar cell performance.Topics: Electric properties; Electric transport processes and properties; Electron transfer; Electronic structure; Exciton; Heterojunction solar cells; Luminescence; Luminescence; Magnetic processes; Organic solar cells; Semiconductors; Semiconductors; Spectra;

AbstractThe design of highly efficient, stable, and noble-metal-free bifunctional electrocatalysts for overall water splitting is critical but challenging. Herein, a facile and controllable synthesis strategy for nickel–cobalt bimetal phosphide nanotubes as highly efficient electrocatalysts for overall water splitting via low-temperature phosphorization from a bimetallic metal-organic framework (MOF-74) precursor is reported. By optimizing the molar ratio of Co/Ni atoms in MOF-74, a series of CoxNiyP catalysts are synthesized, and the obtained Co4Ni1P has a rare form of nanotubes that possess similar morphology to the MOF precursor and exhibit perfect dispersal of the active sites. The nanotubes show remarkable hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic performance in an alkaline electrolyte, affording a current density of 10 mA cm−2 at overpotentials of 129 mV for HER and 245 mV for OER, respectively. An electrolyzer with Co4Ni1P nanotubes as both the cathode and anode catalyst in alkaline solutions achieves a current density of 10 mA cm−2 at a voltage of 1.59 V, which is comparable to the integrated Pt/C and RuO2 counterparts and ranks among the best of the metal-phosphide electrocatalysts reported to date.

•Amorphous polymer is used as electron-donating material in high performance bulk heterojunction organic solar cells.•In-chain charge transport will help to reappraise the charge transport mechanism in organic semiconductors.•The improved charge carrier mobility of both holes and electrons are the major reason for the fill factor increase.•The carrier recombination during the SVA processing induces a loss in device open circuit voltage.•The carrier dynamics is well illustrated by impedance spectroscopy and capacitance-voltage measurement.The current manuscript examines the structure and property relationship of amorphous conjugated polymer based bulk heterojunction solar cells. Solvent vapor annealing (SVA) is used to optimize the morphology. It is seen that the structure order of the blended thin film does not change but phase separation becomes prominent. The space charge limited current and admittance spectroscopy reveals that the charge transport ability of the SVA-treated device is significantly strengthened. Furthermore, both the current-voltage analysis and impedance spectroscopy illustrates that SVA can effectively promote the fill factor (FF) and short circuit current density (Jsc) due to the enhanced charge transport properties. However, the increased mobility after the SVA treatment causes non-geminate charge carrier recombination and reduces geminate recombinations. In particular, the reduction in open circuit voltage (Voc) of the solvent vapor annealed devices is found to originate from the charge recombination induced by the quasi-Fermi level variation between the donor and acceptor materials.Download high-res image (432KB)Download full-size image

Carbonates/bicarbonates (FeCO3, CoCO3 and Ni(HCO3)2) supported on reduced graphene oxide (rGO) are prepared by a simple method and examined as anode materials for sodium-ion batteries for the first time. Although carbonates in the composite are of the order of micrometers, they show fair electrochemical activities, particularly for FeCO3/rGO. It delivers a capacity of 247 mA h g−1 after 500 cycles at 500 mA g−1, corresponding to a capacity retention of 87% relative to the capacity at the second cycle. It also shows a superior rate capability with a capacity of 176 mA h g−1 at 2 A g−1. Ex situ XPS spectra, HRTEM images and SAED patterns demonstrate that the sodium uptake/extraction in FeCO3, CoCO3 and Ni(HCO3)2 is via M0/M2+-engaged redox reaction.

A phosphonate-based metal–organic framework membrane was constructed on a porous anodic alumina membrane (PAAM) substrate for H2/CH4 separation for the first time. Owing to the ultra-micro pore windows, this membrane exhibited effective size exclusion for CH4 molecules but suitable permeance for H2 molecules, giving rise to an exceptional high H2/CH4 separation selectivity.

N-doped porous carbons have been considered as one of the most promising earth-abundant catalysts for the oxygen reduction reaction (ORR) owing to their high activity and excellent stability. Among various types of N-containing groups, pyridinic-N has been identified as the most effective catalytic sites for the ORR, as demonstrated by a recent study. However, the fabrication of porous carbons with a high density of exposed pyridinic-N sites has been rarely reported. In this work, the ORR catalytic properties of a series of pyridinic-N doped porous carbon (PNPC) which was derived by carbonization of a pyridyl-ligand based MOF were investigated. At different carbonization temperatures, this series of carbon exhibits different pyridinic-N contents and different degrees of carbonization. The ORR studies show that the graphitization degree of carbon has a significant impact on ORR catalytic activity besides N-groups. Electrochemical impedance spectroscopy (EIS) reveals that the electron transfer resistance in the ORR decreases significantly with the higher degree of graphitization, which gives rise to a higher ORR activity for these PNPCs. The synergistic effect of the high density of pyridinic-N sites and decreased electron impedance results in remarkably improved ORR activity which is comparable with that of the commercial Pt/C (10 wt%) catalyst. The result of this work could provide some guidance for designing or synthesizing highly efficient ORR catalysts.

A hierarchical porous zirconium metal–organic framework (UiO-66) was prepared continuously through a microdroplet flow reaction strategy for the first time. The existence of metal-node defects arising from incomplete coordination in UiO-66 was found to be the main reason for the formation of mesopores. The dimensions of mesopores could be facilely tuned by adjusting the residence time, and the portion of mesopores was linearly correlated with the missing-nodes. The surface acidity was enhanced due to a large amount of pendant coordinated-free carboxylate groups in pores. With increasing residence time, the missing-nodes among the frameworks were sequentially repaired by self-healing of coordination spheres. Also, these hierarchical porous MOFs demonstrate superior storage capacities for CO2 and CH4. The method of constructing mesopores and producing surface acids presented in this work may open up a new avenue for developing novel hierarchical porous MOFs with special functionalities.

•Rutile TiO2 hierarchical nanotubes are prepared by a hard-template method.•Rutile TiO2 hierarchical nanotubes exhibit superior sodium storage properties.•Rutile TiO2 hierarchical nanotubes deliver a capacity of 221 mA h g−1 at 0.3 C.Hierarchical tubular structures constructed from rutile TiO2 nanorods are fabricated, using MnO2 nanorods as sacrificial templates. The resultant rutile TiO2 hierarchical nanotubes show unique structural features (hierarchical, hollow) and a large surface area (180 m2 g−1), which substantially improve their electrochemical performance. This type of hierarchical structures with high porosity produced by neighboring building blocks, are desirable for the development of sodium ion batteries, because the large amounts of pores can enhance the contact areas between electrode and electrolyte, and faciliate the ion diffusion during the charge/discharge process. As an anode material for sodium ion batteries, these hierarchical nanotubes deliver a high reversible capacity of 221 mA h g−1 at 0.3 C (100 mA g−1), superior rate capability with a stable capacity of 71 mA h g−1 at 15C, and good long-term cycling stability with a capacity of 79 mA h g−1 after 1000 cycles at 3 C. The superior sodium storage performances make this rutile TiO2 hierarchical nanotube a promising anode metarial for sodium ion batteries.

A series of flexible 3-fold interpenetrated lanthanide-based metal organic frameworks (MOFs) with the formula [Ln(HL)(DMA)2]·DMA·2H2O, where Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Er, DMA = dimethylacetamide, and H4L = 5,5′-(2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene)bis(azanediyl)diisophthalic acid, have been prepared. [Sm(HL)(DMA)2]·DMA·2H2O was studied as an exemplar of the series. The activated Sm(HL)(DMA)2 framework exhibited reversible single-crystal-to-single-crystal (SCSC) structural transformations in response to adsorption and desorption of guest molecules. X-ray single crystal structural analysis showed that activation of [Sm(HL)(DMA)2]·DMA·2H2O by heat treatment to form Sm(HL)(DMA)2 involves closing of 13.8 × 14.8 Å channels with coordinated DMA molecules rotating into the interior of the channels with a change from trans to cis Sm coordination and unit cell volume shrinkage of ∼20%, to a void volume of 3.5%. Solvent exchange studies with CH2Cl2 gave [Sm(HL)(DMA)2]·2.8CH2Cl2 which, at 173 K, had a structure similar to that of trans-[Sm(HL)(DMA)2]·DMA·2H2O. CH2Cl2 vapor sorption on activated cis-[Sm(HL)(DMA)2] results in gate opening, and the fully loaded structure has a similar pore volume to that of trans-[Sm(HL)(DMA)2]·2.8CH2Cl2 structure at 173 K. Solvent exchange and heat treatment studies also provided evidence for intermediate framework structural phases. Structural, thermodynamic, and kinetic aspects of the molecular gating mechanism were studied. The dynamic and structural response of the endothermic gate opening process is driven by the enthalpy of adsorption, entropic effects, and Fickian diffusion along the pores produced during framework structure development thus relating the structure and function of the material. Exceptionally high CO2 selectivity was observed at elevated pressure compared with CH4, H2, O2, and N2 due to molecular gate opening of cis-[Sm(HL)(DMA)2] for CO2 but not for the other gases. The CO2 adsorption induced the structural transformation of cis-[Sm(HL)(DMA)2] to trans-[Sm(HL)(DMA)2], and hysteretic desorption behavior allows capture at high pressure, with storage at lower pressure.

MnO nanorods encapsulated by N-doped carbon are prepared, using polypyrrole-coated MnOOH nanorods as both a template and a precursor. The resulting coaxial nanorods have a one-dimensional shape, nanoscale size and an N-doped carbon coating within one particle, which substantially improves their electrochemical performance. As an anode material for lithium-ion batteries, the coaxial nanorods of MnO/N-doped carbon deliver a specific capacity of 982 mA h g−1 at a current density of 500 mA g−1 after 100 cycles, higher than the values for pure MnO nanostructures and MnO/C nanocomposites. At a current density of 5000 mA g−1, the reversible capacity of the coaxial nanorods could be as high as 372 mA h g−1.

Metal–organic frameworks (MOFs) with constricted pores can increase the adsorbate density of gas and facilitate effective CO2 separation from flue gas or natural gas due to their enhanced overlapping of potential fields of the pores. Herein, an MOF with constricted pores, which was formed by narrow channels and blocks of functional groups, was fabricated from the assembly of a methyl-functionalized ligand and Zn(II) centers (termed NPC-7-Zn). Structural analysis of the as-synthesized NPC-7-Zn reveals a series of zigzag pores with pore diameters of ∼0.7 nm, which could be favorable for CO2 traps. For reinforcing the framework stability, a solvothermal metal metathesis on the pristine MOF NPC-7-Zn was performed, and a new Cu(II) MOF (termed NPC-7-Cu) with an identical framework was produced. The influence of the reaction temperatures on the metal metathesis process was investigated. The results show that the constricted pores in NPC-7-Zn can induce kinetic issues that largely slow the metal metathesis process at room temperature. However, this kinetic issue can be solved by applying higher reaction temperatures. The modified MOF NPC-7-Cu exhibits significant improvements in framework stability and thus leads to a permanent porosity for this framework. The constricted pore structure enables enhanced potential fields for these pores, rendering this MOF with high adsorbate densities for CO2 and high adsorption selectivity for a CO2/N2 gas mixture. The adsorption kinetic studies reveal that CH4 has a faster diffusion rate constant than CO2, showing a surface diffusion controlled mechanism for CO2 and CH4 adsorption.Keywords: adsorbate density; adsorption kinetics; constricted pores; gas selectivity; metal metathesis; metal−organic frameworks

•MnOx hierarchical microspheres are prepared by a template method.•MnOx hierarchical microspheres exhibit superior electrochemical properties.•MnO anode delivers a capacity of 810 mAh g-1 at 0.5 C after 100 cycles.Four type of MnOx (MnO2, Mn2O3, Mn3O4, MnO) hierarchical microspheres assembled by rod-like building blocks are synthesized by a facile hydrothermal process with/without a consequent calcination. The morphology and structure of these hierarchical microspheres are confirmed by XRD, SEM, TEM, HRTEM, XPS and BET measurements. The electrochemical properties of the four hierarchical microspheres are investigated in terms of cycling stability and rate capability. Specific capacities of 240, 396, 271 and 810 mAh g−1 can be achieved after 100 cycles at 0.5C for MnO2, Mn2O3, Mn3O4 and MnO, respectively. Even at a high rate of 2C, MnO microspheres can still deliver a reversible capacity of 406 mA h g−1. Their superior electrochemical properties might be attributed to the secondary nanostructure in the MnOx microspheres, which could effectively shorten the diffusion pathway of Li+, tolerate the structural stress caused by Li+ insertion/extraction, reduce the side reactions with electrolyte, and restrain the self-aggregation of nanomaterials.

A novel nanoporous metal–organic framework NPC-4 with excellent thermal stability was assembled from 2,3,5,6-tetramethylbenzene-1,4-diisophthalate (TMBDI) and the paddle-wheel secondary building unit (Cu2(COO)4). The porous structure comprises a single type of nanoscale cage (16 Å diameter) interconnected by windows (5.2 × 6.3 Å), which give a high pore volume. CH4 (195–290 K), CO2 (198–303 K), N2 (77 K), and H2 (77 K) adsorption isotherms were studied for pressures up to 20 bar. NPC-4 exhibits excellent methane and carbon dioxide storage capacities on a volume basis with very high adsorbate densities, under ambient conditions. Isobars were investigated to establish the relationship for adsorption capacities over a range of storage temperatures. The isosteric enthalpies of adsorption for both CH4 and CO2 adsorption did not vary significantly with amount adsorbed and were ∼15 and ∼25 kJ mol–1, respectively. The adsorption/desorption kinetics for CH4 and CO2 were investigated and activation energies, enthalpies of activation, and diffusion parameters determined using various kinetic models. The activation energies for adsorption obtained over a range of uptakes from the stretched exponential kinetic model were 5.1–6.3 kJ mol–1 (2–13.5 mmol g–1) for CO2 and 2.7–5.6 kJ mol–1 (2–9 mmol g–1) for CH4. The activation energies for surface barriers and diffusion along pores for both CH4 and CO2 adsorption obtained from a combined barrier resistance diffusion model did not vary markedly with amount adsorbed and were <9 kJ mol–1. Comparison of kinetic and thermodynamic parameters for CH4 and CO2 indicates that a surface barrier is rate determining at high uptakes, while intraparticle diffusion involving diffusion through pores, consisting of narrow windows interconnecting with nanocages, being rate determining at very low uptakes. The faster CH4 intraparticle adsorption kinetics compared with CO2 for NPC-4 was attributed to faster surface diffusion due to the lower isosteric enthalpy of adsorption for CH4.

A new methylol and methyl functionalized metal–organic frameworks (MOFs) QI-Cu has been designed and synthesized. As a variant of NOTT-101, this material exhibits excellent CO2 uptake capacities at ambient temperature and pressure, as well as high CH4 uptake capacities. The CO2 uptake for QI-Cu is high, up to 4.56 mmol g–1 at 1 bar and 293 K, which is top-ranked among MOFs for CO2 adsorption and significantly larger than the nonfunctionalized NOTT-101 of 3.93 mmol g–1. The enhanced isosteric heat values of CO2 and CH4 adsorption were also obtained for this linker functionalized MOFs. From the single-component adsorption isotherms, multicomponent adsorption was predicted using the ideal adsorbed solution theory (IAST). QI-Cu shows an improvement in adsorptive selectivity of CO2 over CH4 and N2 below 1 bar. The incorporation of methylol and methyl groups also greatly improves the hydrostability of the whole framework.Keywords: CO2 uptake; functionalization; metal−organic frameworks (MOFs); selectivity

Nine metal diphosphonates, [Co(H2L)(pyz)(H2O)][(H2O)0.3] (1), [Ni(H2L)(pyz)(H2O)2] (2), [Ni(H2L)(2,2′-bipy)2][(H2O)2] (3), [Ni(H2L)(4,4′-bipy)(H2O)][(H2O)2] (4), [Ni(H2L)(dpe)(H2O)2][(H2O)2] (5), [Cu(H2L)(pyz)(H2O)2] (6), [Cu(H2L)(4,4′-bipy)][(H2O)2] (7), [Zn(H2L)(2,2′-bipy)(H2O)2] (8) and [Cd(H2L)(pyz)(H2O)2] (9), have been synthesized from a diphosphonate ligand 2,5-dimethyl-1,4-phenylenediphosphonic acid (H4L) and four N-donor auxiliary ligands (pyz = pyrazine, 2,2′-bipy = 2,2′-bipyridine, 4,4′-bipy = 4,4′-bipyridine, dpe = 1,2-di(4-pyridyl)ethylene). In compound 1, pyrazine molecules behave as pillars which connect the Co(H2L) layers into a 3D network structure. Compounds 2, 6 and 9 are isostructural and also show 3D framework structures, in which metal centers are linked by the bidentate diphosphonate ligands into 1D infinite chains and are connected by the pyrazine linkers. In compounds 3 and 8, the diphosphonate ligands, showing bidentate or tetradentate coordination modes, bridge the respective metal ions (Ni2+ and Zn2+) into a 1D infinite chain or 2D layer structure, respectively, in which the 2,2′-bipy ligands chelate to the central metal ions and complete the coordination spheres. In compound 4, the 4,4′-bipy molecules also behave as pillars between the 2D layers, which are constructed from tridentate bridging diphosphonate ligands and six-coordinated Ni2+ ions. Compounds 5 and 7 have similar square grid layered structures which are constructed from bridging bidentate diphosphonate ligands and 4,4′-bipy or dpe linkers. Photophysical measurements indicate that compounds 8 and 9 display ligand centered emissions. Magnetic studies reveal that dominant antiferromagnetic interactions are propagated in compounds 1–3 between the magnetic centers.

N-doped porous carbons have been considered as one of the most promising earth-abundant catalysts for the oxygen reduction reaction (ORR) owing to their high activity and excellent stability. Among various types of N-containing groups, pyridinic-N has been identified as the most effective catalytic sites for the ORR, as demonstrated by a recent study. However, the fabrication of porous carbons with a high density of exposed pyridinic-N sites has been rarely reported. In this work, the ORR catalytic properties of a series of pyridinic-N doped porous carbon (PNPC) which was derived by carbonization of a pyridyl-ligand based MOF were investigated. At different carbonization temperatures, this series of carbon exhibits different pyridinic-N contents and different degrees of carbonization. The ORR studies show that the graphitization degree of carbon has a significant impact on ORR catalytic activity besides N-groups. Electrochemical impedance spectroscopy (EIS) reveals that the electron transfer resistance in the ORR decreases significantly with the higher degree of graphitization, which gives rise to a higher ORR activity for these PNPCs. The synergistic effect of the high density of pyridinic-N sites and decreased electron impedance results in remarkably improved ORR activity which is comparable with that of the commercial Pt/C (10 wt%) catalyst. The result of this work could provide some guidance for designing or synthesizing highly efficient ORR catalysts.

MnO nanorods encapsulated by N-doped carbon are prepared, using polypyrrole-coated MnOOH nanorods as both a template and a precursor. The resulting coaxial nanorods have a one-dimensional shape, nanoscale size and an N-doped carbon coating within one particle, which substantially improves their electrochemical performance. As an anode material for lithium-ion batteries, the coaxial nanorods of MnO/N-doped carbon deliver a specific capacity of 982 mA h g−1 at a current density of 500 mA g−1 after 100 cycles, higher than the values for pure MnO nanostructures and MnO/C nanocomposites. At a current density of 5000 mA g−1, the reversible capacity of the coaxial nanorods could be as high as 372 mA h g−1.