Yingxiang Ye

High Anhydrous Proton Conductivity of Imidazole-Loaded Mesoporous Polyimides over a Wide Range from Subzero to Moderate Temperature

On-board fuel cell technology requires proton conducting materials with high conductivity not only at intermediate temperatures for work but also at room temperature and even at subzero temperature for startup when exposed to the colder climate. To develop such materials is still challenging because many promising candidates for the proton transport on the basis of extended microstructures of water molecules suffer from significant damage by heat at temperatures above 80 °C or by freeze below -5 °C. Here we show imidazole loaded tetrahedral polyimides with mesopores and good stability (Im@Td-PNDI 1 and Im@Td-PPI 2) exhibiting a high anhydrous proton conductivity over a wide temperature range from -40 to 90 °C. Among all anhydrous proton conductors, the conductivity of 2 is the highest at temperatures below 40 °C and comparable with the best materials, His@[Al(OH)(1,4-ndc)]n and [Zn3(H2PO4)6(H2O)3](Hbim), above 40 °C.

Metal–organic frameworks with a large breathing effect to host hydroxyl compounds for high anhydrous proton conductivity over a wide temperature range from subzero to 125 °C

It is important but still challenging to develop high-performance proton conducting materials for proton exchange membrane fuel cells (PEMFCs), as such materials should meet the following requirements: stable proton-transport pathway over a wide temperature range, high conductivity at work temperature, and small activation energy Ea to maintain high conductivity at start temperature. Here, we firstly demonstrated that flexible metal–organic frameworks (FMOFs) are good hosts to seek out better proton carriers for such high-performance proton conducting materials. A FMOF [Zn3(tz)2(bdc)2]n (FJU-31, Htz = 1H-1,2,3-triazole, H2bdc = terephthalic acid) with high thermal stability up to 400 °C, which can be readily synthesized from the Zn5(tz)6(NO3)4 precursor and H2bdc, has been employed to host various organic hydroxyls as new proton carriers. Three resulting FMOFs Zn3(tz)2(bdc)2@G (FJU-31@G, G = hydroquinone (Hq), cyclohexanol (Ch) or butanol (Bu)) show large breathing effect amplitudes up to 65% and guest-related single-crystal to single-crystal structural transformations by temperature stimulus. Most importantly, FJU-31@Hq hosting hydroquinone with a high melting point and small pKa exhibits a high anhydrous proton conductivity of 2.65 × 10−4 S cm−1, low activation energy Ea of 0.18 eV, and the widest temperature range from −40 to 125 °C for stable proton conduction among the crystalline porous materials.

Cobalt–citrate framework armored with graphene oxide exhibiting improved thermal stability and selectivity for biogas decarburization

A series of metal–organic framework (UTSA-16)–graphene oxide composites was synthesized. These composites are the first reported examples of core–shell type metal–organic framework composites armored with graphene oxide film. The parent materials (UTSA-16 and graphene oxide) and the nanocomposites were characterized using XRD, SEM, TEM, TGA and gas adsorption. The composites showed a greatly improved thermal stability compared with their parent materials. The UTSA-16–GO19 composite has a CO2/CH4 selectivity of 114.4, which is three times greater than that of UTSA-16 alone; of the previously reported metal–organic frameworks, only the polyamine-incorporated amine-MIL-101(Cr) has a higher CO2/CH4 selectivity. These graphene oxide composites provide a new direction for practical high-performance metal–organic framework materials.

Straightforward Loading of Imidazole Molecules into Metal–Organic Framework for High Proton Conduction

A one-step straightforward strategy has been developed to incorporate free imidazole molecules into a highly stable metal-organic framework (NENU-3, ([Cu12(BTC)8(H2O)12][HPW12O40])·Guest). The resulting material Im@(NENU-3) exhibits a very high proton conductivity of 1.82 × 10-2 S cm-1 at 90% RH and 70 °C, which is significantly higher than 3.16 × 10-4 S cm-1 for Im-Cu@(NENU-3a) synthesized through a two-step approach with mainly terminal bound imidazole molecules inside pores. Single crystal structure reveals that imidazole molecules in Im-Cu@(NENU-3a) isolate lattice water molecules and then block proton transport pathway, whereas high concentration of free imidazole molecules within Im@(NENU-3) significantly facilitate successive proton-hopping pathways through formation of hydrogen bonded networks.

40-Fold Enhanced Intrinsic Proton Conductivity in Coordination Polymers with the Same Proton-Conducting Pathway by Tuning Metal Cation Nodes.

Three isostructural imidazole-cation-templated metal phosphates (FJU-25) are the first examples to demonstrate that the tuning of metal cation nodes can be an efficient strategy to significantly improve the proton conductivity without changing the structure of the proton-conducting pathway.

Rationally tuning host–guest interactions to free hydroxide ions within intertrimerically cuprophilic metal–organic frameworks for high OH− conductivity

Hydroxide-anion-exchange membrane fuel cells (HEMFCs) are now considered as one of the most promising green energy-conversion technologies for stationary and mobile applications, showing high fuel conversion efficiency at high pH and low cost due to their ability to operate under basic conditions using non-precious metal catalysts. But one key impediment to commercialization is insufficient hydroxide ion (OH−) conductivity of the central HEM component. Here, we report the development of free OH− anion-containing metal–organic frameworks (FOMOFs) with rationally tunable host–guest interactions for HEMs with high OH− conductivity. Among three solids obtained by post-synthesis treatment of ultrastable MOF FJU-66 with various bases, free OH− anions are observed in FJU-66·[EVIm]OH with strong host–guest interactions between the MOF backbone and guest cations. Despite the lowest OH− concentration, FJU-66·[EVIm]OH exhibits the highest OH− conductivity close to 0.1 S cm−1. The high OH− conductivity achieved suggests the potential application of the FOMOFs for practical HEMs of fuel cells.

High proton conductivity in an unprecedented anionic metalloring organic framework (MROF) containing novel metalloring clusters with the largest diameter

An attractive anionic metalloring organic framework (MROF-1) containing unprecedented sextuply interlocked nanocages made up of original metalloring clusters with the largest diameter of ca. 21 A shows a high proton conductivity up to 1.72 × 10−2 S cm−1 at 70 °C and 97% RH, owing to plenty of counter cations (Me2NH2)+ located in the 1D channels.

Mixed-Valence Cobalt(II/III) Metal–Organic Framework for Ammonia Sensing with Naked-Eye Color Switching

The construction of colorimetric sensing materials with high selectivity, low detection limits, and great stability provides a significant way for facile device implementation of an ammonia (NH3) sensor. Herein, with excellent alkaline stability and exposed N sites in molecule as well as with naked-eye color switching nature generated from changeable cobalt (Co) valence, a three-dimensional mixed-valence cobalt(II/III) metal-organic framework (FJU-56) with tris-(4-tetrazolyl-phenyl)amine (H3L) ligand was synthesized for colorimetric sensing toward ammonia. The activated FJU-56 demonstrates a limit of detection of 1.38 ppm for ammonia sensing, with high selectivity in ammonia and water competitive adsorption, and shows outstanding stability and reversibility in the cyclic test. The NH3 or water molecules binding to the exposed N sites with the hydrogen-bond are observed by single-crystal X-ray diffraction, determining that the attachment of guest molecules to the FJU-56 framework changes the valence of Co ions with a naked-eye color switching response, which provides an ocular demonstration for ammonia capture and a valuable insight into ammonia sensing.

Additive-Induced Supramolecular Isomerism and Enhancement of Robustness in Co(II)-Based MOFs for Efficiently Trapping Acetylene from Acetylene-Containing Mixtures

Although supramolecular isomerism in metal-organic frameworks (MOFs) would offer a favorable platform for in-depth exploring their structure-property relationship, the design and synthesis of the isomers are still rather a challenging aspect of crystal engineering. Here, a pair of supramolecular isomers of Co(II)-based MOFs (FJU-88 and FJU-89) can be directionally fabricated by rational tuning the additives. In spite of the fact that the isomers have the similar Co3 secondary building units and organic linkers, they adopt distinct networks with acs and snw topologies, respectively, which derive from the conformational flexibility of the organic ligands. It is noteworthy that the porous structure of FJU-88 would be collapsed after removal of the solvent from the pores. But FJU-89a shows permanent porosity accompanied with unusual hierarchical micro- and mesopores and superior gas selective adsorption performance. In addition, FJU-89a can efficiently trap C2H2 from C2H2/CO2 and C2H2/CH4 mixture gases through fixed-bed dynamic breakthrough experiments.

Facile synthesis of oxidized activated carbons for high-selectivity and low-enthalpy CO2 capture from flue gas

The prospect of a worsening climatic situation prompts us to develop energy-saving and cost-effective CO2 capture technologies. In this work, three oxidized activated carbons (ACO-n, n is 1–3) were prepared through a facile synthesis approach via oxidation in the presence of KMnO4 and concentrated H2SO4 for 0.5, 1.0 or 2.0 hours. Interestingly, these carbon materials ACO-n can exhibit high CO2 capacity with low adsorption enthalpy and the selectivity toward flue gas can be adjusted by altering the period of oxidation. Among the pristine activated carbon and ACO-n materials, ACO-2 can exhibit the highest CO2 capacity of 3.01 mmol g−1 under ambient conditions with an adsorption enthalpy of only 23.1 kJ mol−1, slightly larger than the CO2 vaporization enthalpy. Its selectivity of 48.5 is double the value of the pristine activated carbon. A column breakthrough experiment was conducted to evaluate the CO2 separation capability on ACO-2 toward a CO2/N2 (15 : 85 v/v) mixture under kinetic flow conditions, which suggests that the oxidized activated carbon made from sustainable sources is promising for CO2 capture.