Recent advances and perspectives in proton-conducting metal-organic framework membranes.

Several thousands of metal organic frameworks (MOFs) have been reported to date, but the information on H2/N2 separation performances of MOF membranes is currently very limited in the literature. We report the first large-scale computational screening study that combines state-of-the-art molecular simulations, grand canonical Monte Carlo (GCMC) and molecular dynamics (MD), to predict H2 permeability and H2/N2 selectivity of 3765 different types of MOF membranes. Results showed that MOF membranes offer very high H2 permeabilities, 2.5 × 103 to 1.7 × 106 Barrer, and moderate H2/N2 membrane selectivities up to 7. The top 20 MOF membranes that exceed the polymeric membranes’ upper bound for H2/N2 separation were identified based on the results of initial screening performed at infinite dilution condition. Molecular simulations were then carried out considering binary H2/N2 and quaternary H2/N2/CO2/CO mixtures to evaluate the separation performance of MOF membranes under industrial operating conditions. Lower H2 permeabilities and higher N2 permeabilities were obtained at binary mixture conditions compared to the ones obtained at infinite dilution due to the absence of multicomponent mixture effects in the latter. Structure–performance relations of MOFs were also explored to provide molecular-level insights into the development of new MOF membranes that can offer both high H2 permeability and high H2/N2 selectivity. Results showed that the most promising MOF membranes generally have large pore sizes (>6 Å) as well as high surface areas (>3500 m2/g) and high pore volumes (>1 cm3/g). We finally examined H2/N2 separation potentials of the mixed matrix membranes (MMMs) in which the best MOF materials identified from our high-throughput screening were used as fillers in various polymers. Results showed that incorporation of MOFs into polymers almost doubles H2 permeabilities and slightly enhances H2/N2 selectivities of polymer membranes, which can advance the current membrane technology for efficient H2 purification.

The low‐cost, high specific surface area and porosity, controlled pore size, and chemical properties of metal–organic framework (MOF) materials have attracted much attention in the exploration of proton conduction. The method of chemically modifying MOF structures or introducing conductive medium into the holes can effectively improve the proton conductivities of the materials. Here, the structural tunability of ionic liquid (IL) and flexible MOF (fle‐MOF) materials are matched to give full play to the conductivity of IL, the framework support, and the microporous effect of MOFs, which achieves the synergistic effect of performance and expands the temperature range of proton transfer. Three kinds of CS/IL@fle‐MOF membranes were prepared by combining three fle‐MOFs with 1‐carboxymethyl‐3‐methylimidazole (CMMIM) in different proportions to obtain 15 pieces of membranes. The comparative analyses show that CS/IL@fle‐MOF membranes have excellent proton conduction performance at a wider temperature range (263–353 K) and lower relative humidity (75% RH). Among them, the proton conductivities of CS/CMMIM@MIL‐88A‐25% and CS/CMMIM@MIL‐88B‐125% are up to 1.33 and 1.42 S cm−1 at 75% RH and 353 K, respectively; whereas those of CS/CMMIM@MIL‐53(Fe)‐75% and CS/CMMIM@MIL‐88B‐125% reach up to 2.1 × 10−3 and 1.28 × 10−3 S cm−1 at 75% RH and 263 K, respectively. The Ea of CS/CMMIM@fle‐MOFs is in the range of 0.1–0.5 eV, suggesting that the proton transport follows predominantly the typical Grotthuss transfer mechanism. The results of this study indicate that the CS/CMMIM@fle‐MOF membranes combinations offer great potential for the design of composite porous proton‐conducting materials.

Nanostructured Polymer Composite Electrolytes with Self-Assembled Polyoxometalate Networks for Proton Conduction

Smart proton conductive metal-organic framework (MOF) membranes with dynamic remote control over proton conduction show high potential for use in advanced applications, such as sensors and bioprocesses. Here, we report a photoswitchable proton conductive ZIF-8 membrane by coencapsulating polystyrene sulfonate and graphene quantum dots into a ZIF-8 matrix (GQDs-PSS@ZIF-8) via a solid-confined conversion process. The proton conductivity of the GQDs-PSS@ZIF-8 membrane is 6.3 times higher than that of pristine ZIF-8 and can be reversibly switched by light due to photoluminescence quenching and the photothermal conversion effect, which converts light into heat. The local increase in temperature allows water molecules to escape from the porous channels, which cuts off the proton transport pathways and results in a decrease in proton conductivity. The proton conductivity is restored when the light is off owing to regaining water molecules, which act as proton carriers, from the surroundings. The GQDs-PSS@ZIF-8 membrane responds efficiently to light and exhibits an ON/OFF ratio of 12.8. This photogated proton conduction in MOFs has potential for the development and application of MOF-based protonic solids in advanced photoelectric devices.

Efficient fabrication of metal organic framework (MOF) membranes is important for their broad applications in molecular separations. However, current approaches for MOF membrane fabrication are usually time‐consuming due to the slow, random nucleation and crystal growth, particularly the lack of in‐situ defect healing ability. Here, we report an additive‐accelerated electrodeposition method, which allows ultrafast fabrication of MOF membranes through the synergy of electric field and catecholamine additives. The strong electric field facilitates the directed nucleation of MOF on the substrates while the multifunctional additives accelerate the MOF crystallization, growth and grain‐boundary defect healing. Consequently, we fabricate well‐intergrown, uniform MOF (ZIF‐8) membranes with an ultrathin thickness of ∼180 nm in 30 s, which is the most rapid fabrication of MOF membranes till now. The membranes exhibit superior C3H6/C3H8 separation performance with a C3H6 permeance of 145 GPU and a C3H6/C3H8 separation factor of 151, as well as good stability at high pressure of 7 bar, and the ultrafast membrane fabrication can be achieved on commercial ceramic substrates, exhibiting the potential for practical applications. This work may establish a platform for fast and controllable fabrication of MOF membranes as well as many other membranes based on metal‐coordination chemistry.

Efficient fabrication of metal organic framework (MOF) membranes is important for their broad applications in molecular separations. However, current approaches for MOF membrane fabrication are usually time-consuming due to the slow, random nucleation and crystal growth, particularly the lack of in-situ defect healing ability. Here, we report an additive-accelerated electrodeposition method, which allows ultrafast fabrication of MOF membranes through the synergy of electric field and catecholamine additives. The strong electric field facilitates the directed nucleation of MOF on the substrates while the multifunctional additives accelerate the MOF crystallization, growth and grain-boundary defect healing. Consequently, we fabricate well-intergrown, uniform MOF (ZIF-8) membranes with an ultrathin thickness of ∼180nm in 30 s, which is the most rapid fabrication of MOF membranes till now. The membranes exhibit superior C3H6/C3H8 separation performance with a C3H6 permeance of 145 GPU and a C3H6/C3H8 separation factor of 151, as well as good stability at high pressure of 7bar, and the ultrafast membrane fabrication can be achieved on commercial ceramic substrates, exhibiting the potential for practical applications. This work may establish a platform for fast and controllable fabrication of MOF membranes as well as many other membranes based on metal-coordination chemistry.

Fabrication of metal–organic framework membranes using solid metal precursors for separation application

Metal-organic framework anchored sulfonated poly(ether sulfone) as a high temperature proton exchange membrane for fuel cells

Exploring the performance limits of MOF/polymer MMMs for O2/N2 separation using computational screening

Development of well-constructed metal-organic framework (MOF) membranes can bring about breakthroughs in nanofiltration (NF) performance for water treatment applications, while the relatively loose structures and inevitable defects usually cause low rejection capacity of MOF membranes. Herein, a confined interfacial polymerization (CIP) method is showcased to synthesize polyamide (PA)-modified NF membranes with MOF nanosheets as the building blocks, yielding a stepwise transition from two-dimensional (2D) MOF membranes to polyamide NF membranes. The CIP process was regulated by adjusting the loading amount of piperazine (PIP)-grafted MOF nanosheets on substrates and the additional content of free PIP monomers distributed among the nanosheets, followed by the reaction with trimesoyl chloride in the organic phase. The prepared optimal membrane exhibited a high Na2SO4 rejection of 98.4% with a satisfactory water permeance of 37.4 L·m-2·h-1·bar-1, which could be achieved by neither the pristine 2D MOF membranes nor the PA membranes containing the MOF nanosheets as the conventional interlayer. The PA-modified MOF membrane also displayed superior stability and enhanced antifouling ability. This CIP strategy provides a novel avenue to develop efficient MOF-based NF membranes with high ion-sieving separation performance for water treatment.

Metal organic framework membranes for separation applications

Proton-conducting metal-organic frameworks (MOFs) have attracted attention as potential electrolytes for fuel cells. However, research progress in utilizing MOFs as electrolytes for fuel cells has been limited, mainly due to challenges associated with issues such as the fabrication of MOF membranes, and hydrogen crossover through the MOF's pores. Here, proton conductivity and fuel cell performance of a self-standing membrane prepared from of a bismuth subgallateMOF nanosheets with non-porous structure are reported. The fabricated MOF nanosheet membrane with no binding agent exhibits structural anisotropy. The proton conductivity in the membrane thickness direction (4.4×10-3Scm-1) at 90°C and RH 100% is observed to be higher than that in the in-plane direction of the membrane (3.3×10-5Scm-1). The open circuit voltage (OCV) of a fuel cell with ≈120µm proton conducting membrane is 1.0V. The non-porous nature of the MOF nanosheets contributes to the relatively high OCV. A fuel cell using ≈40µm membrane as proton conducting electrolyte records a maximum of 25mWcm-2 power density and a maximum of 109mAcm-2 current density with 0.91V OCV at 80°C in humid conditions.

Metal-organic framework membranes for proton exchange membrane fuel cells: A mini-review

Efficient removal of Co(II) from aqueous solution by flexible metal-organic framework membranes

Development of ZIF-8 membranes: opportunities and challenges for commercial applications