Zhonghua Xiang

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.

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+

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.

An amino group functionalized metal–organic framework as a luminescent probe for highly selective sensing of Fe3+ ions

In this work, metal–organic framework (MOF) UMCM-1 and amino functionalized MOF (i.e., UMCM-1-NH2) were synthesized and their performances as luminescent probes were investigated. It is found that both unmodified and amino functionalized MOFs exhibit a luminescence quenching effect on metal ions. In particular, the amino functionalized MOF (UMCM-1-NH2) possesses high sensitivity and selectivity for Fe3+ ions and the luminescence is completely quenched in 10−3 M DMF solution of Fe3+. Moreover, the regenerated UMCM-1-NH2 still has high selectivity for Fe3+ ions, which suggests that the functionalized UMCM-1-NH2 is a promising luminescent probe for selectively sensing iron ions.

Synthesis of Luminescent Covalent–Organic Polymers for Detecting Nitroaromatic Explosives and Small Organic Molecules

Three porous luminescent covalent--organic polymers (COPs) have been synthesized through self-polycondensation of the monomers of tris(4-bromophenyl)amine, 1,3,5-tris(4-bromophenyl)benzene, and 2,4,6-tris-(4-bromo-phenyl)-[1,3,5]triazine by using Ni-catalyzed Yamamoto reaction. All the COP materials possess not only high Brunauer-Emmett-Teller (BET) specific surface area of about 2000 m(2) g(-1) , high hydrothermal stability, but also graphene-like layer texture. Interestingly, COP-3 and COP-4 show very fast responses and high sensitivity to the nitroaromatic explosives, and also high selectivity for tracing picric acid (PA) and 2,4,6-trinitrotoluene (TNT) at low concentration (<1 ppm). In short, the COPs may be a new kind of material for detecting explosives and small organic molecules.

Selective adsorption of carbon dioxide by carbonized porous aromatic framework (PAF)

A series of carbonized PAF-1s were obtained with enhanced gas storage capacities and isosteric heats of adsorption (Qst for short). Especially, PAF-1-450 can adsorb 4.5 mmol g−1 CO2 at 273 K and 1 bar. Moreover, it also exhibits excellent selectivity over other gases. On the basis of single component isotherm data, the dual-site Langmuir–Freundlich adsorption model-based ideal adsorption solution theory (IAST) prediction indicates that the CO2/N2 adsorption selectivity is as high as 209 at a 15/85 CO2/N2 ratio. Also, the CO2/CH4 adsorption selectivity is in the range of 7.8–9.8 at a 15/85 CO2/CH4 ratio at 0 < p < 40 bar, which is highly desirable for landfill gas separation. The calculated CO2/H2 adsorption selectivity is about 392 at 273 K and 1 bar for 20/80 CO2/H2 mixture. Besides, these carbonized PAF-1s possess excellent physicochemical stability. Practical applications in capture of CO2 lie well within the realm of possibility.

Covalent-organic polymers for carbon dioxide capture

Reducing anthropogenic carbon dioxide emission has become an urgent environmental and climate issue of our age. Here, a series of covalent-organic polymers (COPs) are synthesized, and the adsorption properties of these COPs for H2, CO2, CH4, N2 and O2 are studied. The H2 uptake of COP-2 reaches 1.74 wt% at 77 K and 1 bar, which is among the highest reported uptakes in the field of microporous organic polymers under similar conditions, and CO2 and CH4 adsorption capacities are 594 mg g−1 and 78 mg g−1, respectively, at 298 K and 18 bar. Then, based on the single component isotherm, the dual-site Langmuir–Freundlich (DSLF)-based ideal adsorption solution theory (IAST) is used to predict the selectivity of the COP materials for post-combustion (CO2–N2) and pre-combustion (O2–N2) gas mixtures. The IAST predicted results indicate that COP-1 exhibits significantly higher selectivity compared to COP-2, 3 and 4, due to its smaller pore size. In particular, the adsorption selectivity of COP-1 for the CO2–N2 mixture reaches 91 at a CO2 : N2 ratio of 15 : 85 at 298 K and 1 bar, and 2.38 for the 21 : 79 O2–N2 mixture at 298 K and 1 bar. Furthermore, these COPs also show robust properties for the removal of CO2 from natural gas. The adsorption selectivity of COP-1 for CO2–CH4 is in the range of 4.1–5.0 at a CO2 : CH4 ratio of 15 : 85 at 0 < P < 40 bar.

Edge Functionalization of Graphene and Two‐Dimensional Covalent Organic Polymers for Energy Conversion and Storage

Edge functionalization by selectively attaching chemical moieties at the edge of graphene sheets with minimal damage of the carbon basal plane can impart solubility, film-forming capability, and electrocatalytic activity, while largely retaining the physicochemical properties of the pristine graphene. The resultant edge-functionalized graphene materials (EFGs) are attractive for various potential applications. Here, a focused, concise review on the synthesis of EFGs is presented, along with their 2D covalent organic polymer (2D COP) analogues, as energy materials. The versatility of edge-functionalization is revealed for producing tailor-made graphene and COP materials for efficient energy conversion and storage.

A porous diamond carbon framework: a new carbon allotrope with extremely high gas adsorption and mechanical properties

Since the C60 and graphene were discovered, carbon allotropes have attracted an increasing attention. Here we designed a porous diamond-like carbon framework (named as D-carbon) by inserting –CC– linkers into all the C–C bonds in a diamond, which is a new carbon allotrope formed by sp3–sp hybridized carbon atoms. Interestingly, the porous D-carbon exhibits a high bulk modulus of 91.7 GPa, which is one order of magnitude larger than other porous MOF and COF materials. Moreover, the D-carbon also shows an extremely high excess volumetric methane uptake of 255 v(STP)/v at 298 K and 35 bar, largely exceeding the target (180 v(STP)/v) of US DOE and all other porous materials.