Pengcheng Dai

High oxygen reduction activity on a metal–organic framework derived carbon combined with high degree of graphitization and pyridinic-N dopants

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.

In Situ Synthesis Strategy for Hierarchically Porous Ni2P Polyhedrons from MOFs Templates with Enhanced Electrochemical Properties for Hydrogen Evolution

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.

Nickel metal–organic framework implanted on graphene and incubated to be ultrasmall nickel phosphide nanocrystals acts as a highly efficient water splitting electrocatalyst

The development of low-cost, efficient, and stable electrocatalysts with bifunctional catalytic activity for overall water splitting is desirable but remains a great challenge. Here, a template-confinement strategy is presented with nickel metal–organic framework (MOF-74-Ni) implanted on graphene oxide and incubated by low temperature phosphorization to become ultrasmall nickel phosphide nanocrystals anchored on reduced graphene oxide (termed Ni2P/rGO). The size-controlled synthesis of ultrasmall metal-based catalysts is of vital economic interest and scientific importance for chemical conversion technologies. The Ni2P/rGO guarantees large active surface area and perfect dispersity of the active sites with ultrasmall particle sizes (average about 2.6 nm), which can serve as a highly efficient electrocatalyst for overall water splitting. In 1.0 M KOH, the Ni2P/rGO exhibited remarkable electrocatalytic performance for both HER and OER, affording a current density of 10 mA cm−2 at overpotentials of 142 mV for HER and 260 mV for OER with small Tafel slope. Furthermore, an electrolyzer employed with Ni2P/rGO as a bifunctional catalyst in both the cathode and anode in 1.0 M KOH generated 10 mA cm−2 at a voltage of 1.61 V with excellent stability, comparable to the integrated Pt/C and RuO2 counterparts, which is among the best performances of transition metal phosphides (TMPs).

Missing-node directed synthesis of hierarchical pores on a zirconium metal–organic framework with tunable porosity and enhanced surface acidity via a microdroplet flow reaction

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.

Carbonates (bicarbonates)/reduced graphene oxide as anode materials for sodium-ion batteries

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.

Boosting ORR Catalytic Activity by Integrating Pyridine-N Dopants, a High Degree of Graphitization, and Hierarchical Pores into a MOF-Derived N-Doped Carbon in a Tandem Synthesis

N-doped carbon materials represent promising metal-free electrocatalysts for the oxygen reduction reaction (ORR), the cathode reaction in fuel cells, metal-air batteries, and so on. A challenge for optimizing the ORR catalytic activities of these electrocatalysts is to tune their local structures and chemical compositions in a rational and controlled way that can achieve the synergistic function of each factor. Herein, we report a tandem synthetic strategy that integrates multiple contributing factors into an N-doped carbon. With an N-containing MOF (ZIF-8) as the precursor, carbonization at higher temperatures leads to a higher degree of graphitization. Subsequent NH3 etching of this highly graphitic carbon enabled the introduction of a higher content of pyridine-N sites and higher porosity. By optimizing these three factors, the resultant carbon materials displayed ORR activity that was far superior to that of carbon derived from a one-step pyrolysis. The onset potential of 0.955 V versus a reversible hydrogen electrode (RHE) and the half-wave potential of 0.835 V versus RHE are among the top ranks of metal-free ORR catalysts and are comparable to commercial Pt/C (20 wt %) catalysts. Kinetic studies revealed lower H2 O2 yields, higher electron-transfer numbers, and lower Tafel slopes for these carbon materials compared with that derived from a one-step carbonization. These findings verify the effectiveness of this tandem synthetic strategy to enhance the ORR activity of N-doped carbon materials.

Synthesis of Mesoporous γ-Al2O3 with Spongy Structure: In-Situ Conversion of Metal-Organic Frameworks and Improved Performance as Catalyst Support in Hydrodesulfurization

Over the past decades, extensive efforts have been devoted to modulating the textural properties, morphology and microstructure of γ-Al2O3, since the physiochemical properties of γ-Al2O3 have close correlations with the performance of hydrotreating catalysts. In this work, a spongy mesoporous γ-alumina (γ-Al2O3) was synthesized using Al-based metal-organic frameworks (Al-MOFs) as precursor by two-step pyrolysis, and this Al-MOF-derived γ-Al2O3 was used as hydrodesulfurization (HDS) catalyst support, to explore the effect of support on the HDS performance. Compared with industrial γ-Al2O3, the spongy alumina displayed well-developed porosity with relatively high surface area, large pore volume, and abundant weak Lewis acid sites. Based on catalyst characterization and performance evaluation, sulfurized molybdenum and cobalt molecules were able to incorporate and highly disperse into channels of the spongy mesoporous alumina, increasing the dispersion of active catalytic species. The spongy γ-Al2O3 was also able to enhance the diffusion efficiency and mass transfer of reactant molecules due to its improved texture properties. Therefore, the corresponding catalyst presented higher activities toward HDS of dibenzothiophene (DBT) than that from industrial alumina. The spongy mesoporous γ-alumina synthesized by Al-MOFs provides a new alternative to further develop novel γ-alumina materials with different texture and various nanoporous structures, considering the diversity of MOFs with different compositions, topological structures, and morphology.

Highly dispersed Zn nanoparticles confined in a nanoporous carbon network: promising anode materials for sodium and potassium ion batteries

Highly dispersed Zn nanoparticles confined in a nanoporous carbon network (ZNP/C) are prepared by directly annealing a Zn-containing metal–organic framework in an inert atmosphere and investigated as an anode material for sodium and potassium ion batteries for the first time. ZNP/C has unique structural features, such as highly dispersed Zn nanoparticles, a nanoporous carbon network and a high surface area, which can efficiently enhance the reactivity, facilitate ion/electron transportation and buffer volume changes, and thus greatly improve its Na/K storage performance. As a sodium ion battery anode, ZNP/C-600 shows a high capacity of 361 mA h g−1 over 100 cycles at 0.1 A g−1, and an ultrahigh capacity of 227 mA h g−1 is sustained after 1000 cycles at 2 A g−1. When employed as a potassium ion battery anode, ZNP/C-600 exhibits a high capacity of 200 mA h g−1 over 100 cycles at 0.1 A g−1, and a stable capacity of 145 mA h g−1 over 300 cycles at 0.5 A g−1. Qualitative and quantitative analyses reveal that capacitance and diffusion mechanisms account for the superior Na/K storage performance, in which the capacitance plays a significant role.

Impact of moderative ligand hydrolysis on morphology evolution and the morphology-dependent breathing effect performance of MIL-53(Al)

In this paper, a facile route is reported for the synthesis of MIL-53(Al) particles with different sizes and morphologies via the addition of various ratios of an esterified ligand to realize the deferred release of the ligand. Detailed proof indicates that the hydrolysis and deferred release of the ligand play a crucial role in modulating the morphology and size of the obtained MIL-53 nanoparticles without disrupting the original structure and framework of MIL-53. A possible mechanism for the growth of MIL-53 crystals with different exposed facets is proposed based on the structure of MIL-53 and BFDH (Bravais, Friedel, Donnay and Harker) theory. In addition, the influence of the crystal morphologies or exposed surfaces on the breathing effect and gas adsorption performance was evaluated using N2 and CO2 adsorption, and the as-prepared MIL-53 particles showed different adsorption isotherms and adsorption behavior under the same conditions. The MIL-53 particles synthesized using an appropriate amount of an esterified ligand could present an enhanced surface area when compared to that of bulk MIL-53, and the breathing effect during CO2 adsorption was moderate, while excessive amounts of the esterified ligand resulted in a decrease in the porosity and the formation of boehmite. Based on the experiment, the discrepancy in the breathing effect is mainly attributed to the stability of the narrow pore form with different exposed facets, but not the decreased particle size.