Dapeng Cao

Targeted Synthesis of a Porous Aromatic Framework with High Stability and Exceptionally High Surface Area

Porous materials have been of intense scientific and technological interest because of their vital importance in many applications such as catalysis, gas separation, and gas storage. Great efforts in the past decade have led to the production of highly porous materials with large surface areas. In particular, the development of metal–organic frameworks (MOFs) has been especially rapid. Indeed, the highest surface area reported to date is claimed for a recently reported MOF material UMCM-2, which has a N2 uptake capacity of 1500 cm g at saturation, from which a Langmuir surface area of 6060 m g (Brunauer–Emmett–Teller (BET) surface area of 5200 m g) can be derived. Unfortunately, the high-surface-area porous MOFs usually suffer from low thermal and hydrothermal stabilities, which severely limit their applications, particularly in industry. These low stability issues could be resolved by replacing coordination bonds with stronger covalent bonds, as observed in covalent organic frameworks (COFs) or porous organic polymers. However, the COFs and porous organic polymers reported to date have lower surface areas compared to MOFs; the highest reported surface area for a COF is 4210 m g (BET) in COF103. Thus, further efforts are required to explore various strategies to achieve higher surface areas in COFs. Herein, we present a strategy that has enabled us to achieve, with the aid of computational design, a structure that possesses by far the highest surface area reported to date, as well as exceptional thermal and hydrothermal stabilities. We report the synthesis and properties of a porous aromatic framework PAF-1, which has a Langmuir surface area of 7100 m g. Besides its exceptional surface area, PAF-1 outperforms highly porous MOFs in thermal and hydrothermal stabilities, and demonstrates high uptake capacities for hydrogen (10.7 wt % at 77 K, 48 bar) and carbon dioxide (1300 mgg 1 at 298 K, 40 bar). Moreover, the super hydrophobicity and high surface area of PAF-1 result in unprecedented uptake capacities of benzene and toluene vapors at room temperature. It is well known that one of the most stable compounds in nature is diamond, in which each carbon atom is tetrahedrally connected to four neighboring atoms by covalent bonds (Figure 1a). Conceptually, replacement of the C C covalent bonds of diamond with rigid phenyl rings should not only retain a diamond-like structural stability but also allow sufficient exposure of the faces and edges of phenyl rings with the expectation of increasing the internal surface areas. By employing a multiscale theoretical method, which

ZIF-derived in situ nitrogen-doped porous carbons as efficient metal-free electrocatalysts for oxygen reduction reaction

We have successfully prepared nanoporous Carbon-L and -S materials by using ZIF-7 as a precursor and glucose as an additional carbon source. Results indicate that Carbon-L and -S show an appropriate nitrogen content, high surface area, robust pore structure and excellent graphitization degree. The addition of an environmentally friendly carbon source – glucose – not only improves the graphitization degree of samples, but also plays a key role in removing residual Zn metal and zinc compound impurities, which makes the resulting materials metal-free in situ nitrogen-doped porous carbons. By further investigating the electrocatalytic performance of these nitrogen-doped porous carbons for oxygen reduction reaction (ORR), we find that Carbon-L, as a metal-free electrocatalyst, shows excellent electrocatalytic activity (the onset and half-wave potentials are 0.86 and 0.70 V vs. RHE, respectively) and nearly four electron selectivity (the electron transfer number is 3.68 at 0.3 V), which is close to commercial 20% Pt/C. Moreover, when methanol was added, the Pt/C catalyst would be poisoned while the Carbon-L and -S would be unaffected. By exploring the current-time chronoamperometric response in 25 000 s, we found that the duration stability of Carbon-L is much better than the commercial 20% Pt/C. Thus, both Carbon-L and -S exhibit excellent ability to avoid methanol crossover effects, and long-term operation stability superior to the Pt/C catalyst. This work provides a new strategy for in situ synthesis of N-doped porous carbons as metal-free electrocatalysts for ORR in fuel cells.

Nitrogen-doped graphene nanosheets as anode materials for lithium ion batteries: a first-principles study

First-principles calculations are performed to investigate the effects of the electron-deficiency of N-doped graphenes on their application in lithium ion batteries (LIBs), where three different defect models, graphitic, pyridinic, and pyrrolic graphenes are used. First, we investigate adsorption of a single Li atom on various graphenes and explore the change of the electronic properties in order to understand the adsorption mechanism. Then, adsorption of multiple Li atoms is also performed to consider the lithium storage properties of N-doped graphene nanosheets. The results show that the pyridinic graphene is the most suitable for Li storage with a high storage capacity, while the graphitic structure is the weakest of the three types. Moreover, the average potential of Li intercalation in the graphene materials was also calculated, and results indicate that the reversible capacity of the pyridinic structure can reach 1262 mAh g−1, which is higher than the experimental data (1043 mAh g−1). Therefore, we recommend pyridinic graphene in the N-doped structures as anode materials of lithium ion batteries and the corresponding reversible capacity of LIBs would be improved significantly. It is expected that this work could provide helpful information for the design and fabrication of anode materials of LIBs.

Highly Efficient Electrocatalysts for Oxygen Reduction Based on 2D Covalent Organic Polymers Complexed with Non‐precious Metals

A class of 2D covalent organic polymers (COPs) incorporating a metal (such as Fe, Co, Mn) with precisely controlled locations of nitrogen heteroatoms and holes were synthesized from various N-containing metal-organic complexes (for example, metal-porphyrin complexes) by a nickel-catalyzed Yamamoto reaction. Subsequent carbonization of the metal-incorporated COPs led to the formation of COP-derived graphene analogues, which acted as efficient electrocatalysts for oxygen reduction in both alkaline and acid media with a good stability and free from any methanol-crossover/CO-poisoning effects.

Porous covalent–organic materials: synthesis, clean energy application and design

Porous covalent–organic materials (COMs) are a fascinating class of nanoporous material with high surface area and diverse pore dimensions, topologies and chemical functionalities. These materials have attracted ever-increasing attention from different field scientists, owing to their potential applications in gas storage, adsorptive separation and photovoltaic devices. The versatile networks are constructed from covalent bonds (B–O, C–C, C–H, C–N, etc.) between the organic linkers by homo- or hetero-polymerizations. To design and synthesize novel porous COMs, we first summarize their synthesis methods, mainly including five kinds of coupling reaction, i.e. boronic acid, amino, alkynyl, bromine and cyan group-based coupling reactions. Then, we review the progress of porous COMs in clean energy applications in the past decade, including hydrogen and methane storage, carbon dioxide capture, and photovoltaic applications. Finally, to improve their gas adsorptive properties, four possible strategies are proposed, and high-capacity COMs for gas storage are designed by a multiscale simulation approach.

Metal–Organic Frameworks with Incorporated Carbon Nanotubes: Improving Carbon Dioxide and Methane Storage Capacities by Lithium Doping

Reduction of the anthropogenic emission of CO2 is currently a top priority because CO2 emission is closely associated with climate change. Carbon capture and storage (CCS) and the development of renewable and clean energy sources are two approaches for the reduction of CO2 emission. One of the most promising alternative fuels is CH4, which is the major component of natural gas. The efficient storage of CH4 is still one of the main challenges for its widespread application. Accordingly, the development of more efficient approaches for CO2 capture and CH4 storage is critically important. Recently, metal–organic frameworks (MOFs, e.g., MOF210 and NU-100) have shown great potential for gas storage because of their high specific surface area (SSA) and functionalized pore walls. However, most MOF materials still show relatively low CO2 and CH4 uptakes. To enhance CO2 and CH4 adsorption, it is imperative to develop new materials, such as covalent organic frameworks (COFs), or to modify MOFs by using postsynthetic approaches. Herein, we focus on the latter strategy. One of the modification approaches is incorporation of carbon nanotubes (CNTs) into MOFs in order to achieve enhanced composite performance, because of the unusual mechanical and hydrophobicity properties of CNTs. Another approach is doping MOFs or COFs with electropositive metals. Recent studies indicate that the surface carboxylate functional groups of a substrate could act as nucleation sites to form MOFs by heterogeneous nucleation and crystal growth. Both experimental and theoretical investigations indicate that the H2 adsorption capacities of MOFs can be enhanced significantly by doping alkali-metal ions, in particular Li ions, to the frameworks, owing to the strong affinity of Li ions towards H2 molecules. [3d, 7] Similarly, Lan et al. also showed theoretically that doping of COFs with Li ions can significantly enhance the CH4 uptake of COFs. [8] Most recently, the multiscale simulations performed by Lan et al. indicate that Li is the best surface modifier of COFs for CO2 capture among a series of metals (Li, Na, K, Be, Mg Ca, Sc and Ti). Furthermore, their simulations show that the excess CO2 uptakes of the lithium-doped COFs can be enhanced by four to eight times compared to the undoped COFs at 298 K and 1 bar. Motivated by these experimental and theoretical results, we synthesized hybrid MOF materials by using the two modification techniques outlined above, that is, 1) incorporation of CNTs into [Cu3(C9H3O6)2(H2O)3]·x H2O ([Cu3(btc)2], HKUST-1; btc = 1,3,5-benzenetricarboxylate), which is an important MOF material owing to its open metal sides and high thermal stabilities, as well as its sorption properties, 10] and 2) doping [Cu3(btc)2] with Li + ions. We used lithium naphthalenide (LiC10H8 ) to introduce Li ions into the [Cu3(btc)2] frameworks. These frameworks have Cu 2+

Lithium‐Doped 3D Covalent Organic Frameworks: High‐Capacity Hydrogen Storage Materials

Quick on the uptake: A multiscale theoretical method predicts that the gravimetric adsorption capacities of H(2) in Li-doped covalent organic frameworks based on the building blocks shown (Li violet, H white, B pink, C green, O red, Si yellow) can reach nearly 7 % at T=298 K and p=100 bar, suggesting that these Li-doped materials are promising adsorbents for hydrogen storage.

Nitrogen‐Doped Holey Graphitic Carbon from 2D Covalent Organic Polymers for Oxygen Reduction

Using covalent organic polymer pre-cursors, we have developed a new strategy for location control of N-dopant heteroatoms in the graphitic porous carbon, which otherwise is impossible to achieve with conventional N-doping techniques. The electrocatalytic activities of the N-doped holey graphene analogues are well correlated to the N-locations, showing possibility for tailoring the structure and property of N-doped carbon nanomaterials.

Nanoparticle Dispersion and Aggregation in Polymer Nanocomposites: Insights from Molecular Dynamics Simulation

It is a great challenge to fully understand the microscopic dispersion and aggregation of nanoparticles (NPs) in polymer nanocomposites (PNCs) through experimental techniques. Here, coarse-grained molecular dynamics is adopted to study the dispersion and aggregation mechanisms of spherical NPs in polymer melts. By tuning the polymer-filler interaction in a wide range at both low and high filler loadings, we qualitatively sketch the phase behavior of the PNCs and structural spatial organization of the fillers mediated by the polymers, which emphasize that a homogeneous filler dispersion exists just at the intermediate interfacial interaction, in contrast with traditional viewpoints. The conclusion is in good agreement with the theoretically predicted results from Schweizer et al. Besides, to mimick the experimental coarsening process of NPs in polymer matrixes (ACS Nano 2008, 2, 1305), by grafting polymer chains on the filler surface, we obtain a good filler dispersion with a large interparticle distance. Considering the PNC system without the presence of chemical bonding between the NPs and the grafted polymer chains, the resulting good dispersion state is further used to investigate the effects of the temperature, polymer-filler interaction, and filler size on the filler aggregation process. It is found that the coarsening or aggregation process of the NPs is sensitive to the temperature, and the aggregation extent reaches the minimum in the case of moderate polymer-filler interaction, because in this case a good dispersion is obtained. That is to say, once the filler achieves a good dispersion in a polymer matrix, the properties of the PNCs will be improved significantly, because the coarsening process of the NPs will be delayed and the aging of the PNCs will be slowed.

Zeolitic imidazolate framework-8 as a luminescent material for the sensing of metal ions and small molecules

We present a rare example of Zeolitic Imidazolate Framework-8 (ZIF-8), Zn(MeIM)2•(DMF)•(H2O)3, as luminescent probes with multi-function sensitivity to detect metal ions and small molecules. Our results show that the luminescence intensity of ZIF-8 is strongly sensitive to Cu2+ and Cd2+ ions and small molecules such as acetone. In particular, the luminescence intensity of desolvated ZIF-8 proportionally decreases to the concentration of Cu2+, while increases to the concentration of Cd2+ owing to the recognition of element-imidazole nitrogen sites. The luminescence intensity increases gradually with the increase of amounts of acetone in the standard desolvated ZIF-8-emulsions. These results reveal that the ZIFs might be a good luminescent sensor for metal ions and small molecules.