Proton-conducting copper-based MOFs for fuel cells

Metal-organic frameworks (MOFs) are emerging as promising alternatives for proton-conductive materials due to their high porosity, large surface area, stability, and relatively low cost. Among these, copper-based MOFs (Cu-MOFs) stand out with unique advantages, including open metal sites, variable valence states, and strongly electrophilic Cu centers. In this review, we discuss recent advances and developments in the use of Cu-MOFs as proton-conductive materials, with a particular focus on their application as proton exchange membranes (PEMs). We introduce the most common strategies employed to date and review the key features that have contributed to the construction of efficient proton transport pathways in Cu-MOFs. Additionally, we review PEMs fabricated via direct thin-film deposition or as mixed-matrix membranes (MMMs) incorporating Cu-MOF fillers. Finally, we address the challenges that must be overcome in the coming years to develop more robust Cu-MOFs and to create more efficient thin films and Cu-MOF-based MMMs.

Prominent Intrinsic Proton Conduction in Two Robust Zr/Hf Metal-Organic Frameworks Assembled by Bithiophene Dicarboxylate

To date, developing crystalline proton-conductive metal-organic frameworks (MOFs) with an inherent excellent proton-conducting ability and structural stability has been a critical priority in addressing the technologies required for sustainable development and energy storage. Bearing this in mind, a multifunctional organic ligand, 3,4-dimethylthiophene[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), was employed to generate two exceptionally stable three-dimensional porous Zr/Hf MOFs, [Zr6O4(OH)4(DTD)6]·5DMF·H2O (Zr-DTD) and [Hf6O4(OH)4(DTD)6]·4DMF·H2O (Hf-DTD), using solvothermal means. The presence of Zr6 or Hf6 nodes, strong Zr/Hf-O bonds, the electrical influence of the methyl group, and the steric effect of the thiophene unit all contribute to their structural stability throughout a wide pH range as well as in water. Their proton conductivity was fully examined at various relative humidities (RHs) and temperatures. Creating intricate and rich H-bonded networks between the guest water molecules, coordination solvent molecules, thiophene-S, -COOH, and -OH units within the framework assisted proton transfer. As a result, both MOFs manifest the maximum proton conductivity of 0.67 × 10-2 and 4.85 × 10-3 S·cm-1 under 98% RH/100 °C, making them the top-performing proton-conductive Zr/Hf-MOFs. Finally, by combining structural characteristics and activation energies, potential proton conduction pathways for the two MOFs were identified.

Anchoring of Fe-MIL-101-NH2 to the Polymer Membrane Matrix through the Hinsberg Reaction to Promote Conductivity of SPEEK Membranes

SCPEEK@MOF proton exchange membranes, where SCPEEK is sulfinyl chloride polyether ether ketone and MOF is a metal-organic framework, were prepared by doping Fe-MIL-101-NH2 into polymers. The amino group in the MOF and the -SOCl2 group in thionyl chloride polyether ether ketone cross-link to form a covalent bond through the Hinsberg reaction, and the prepared composite membrane has stronger stability than other electrostatic interactions and simple physical doping composite membranes. The formation of covalent bonds improves the water absorption of the composite membrane, which makes it easy for water molecules to form hydrogen bonds. Moreover, SPEEK as a proton conductive polymer and the synergy of MOFs improve the proton conductivity of composite membranes. The composite membranes were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The swelling rate, water absorption, mechanical stability, ion exchange capacity, and proton conductivity of the pure sulfonated polyether ether ketone (SPEEK) membrane were compared with those of the mechanically doped SPEEK/MOF membrane and the composite membrane SCPEEK@MOF doped with different ratios of Fe-MIL-101-NH2, and all of the SCPEEK@MOF showed superior performance. When the Fe-MIL-101-NH2 loading rate of the composite membrane is 2%, the proton conductivity of the composite membrane can reach 0.202 S cm-1 at 363 K and a 98% relative humidity, which is much higher than that of the SPEEK/MOF membrane obtained by simple physical doping under the same conditions.

Four Lanthanide(III) Metal-Organic Frameworks Fabricated by Bithiophene Dicarboxylate for High Inherent Proton Conduction

Currently, it is still a challenge to directly achieve highly stable metal-organic frameworks (MOFs) with superior proton conductivity solely through the exquisite design of ligands and the attentive selection of metal nodes. Inspired by this, we are intrigued by a multifunctional dicarboxylate ligand including dithiophene groups, 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), and lanthanide ions with distinct coordination topologies. Successfully, four isostructural three-dimensional lanthanide(III)-based MOFs, [Ln2(DTD)3(DEF)4]·DEF·6H2O [LnIII = TbIII (Tb-MOF), EuIII (Eu-MOF), SmIII (Sm-MOF), and DyIII (Dy-MOF)], were solvothermally prepared, in which the effective proton transport will be provided by the coordinated or free solvent molecules, the crystalline water molecules, and the framework components, as well as a large number of highly electronegative S and O atoms. As expected, the four Ln-MOFs demonstrated the highest proton conductivities (σ) being 0.54 × 10-3, 3.75 × 10-3, 1.28 × 10-3, and 1.92 × 10-3 S·cm-1 for the four MOFs, respectively, at 100 °C/98% relative humidity (RH). Excitingly, Dy-MOF demonstrated an extraordinary ultrahigh σ of 1 × 10-3 S·cm-1 at 30 °C/98% RH. Additionally, the plausible proton transport mechanisms were emphasized.

Rationalizing Structural Hierarchy in the Design of Fuel Cell Electrode and Electrolyte Materials Derived from Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are arguably a class of highly tuneable polymer-based materials with wide applicability. The arrangement of chemical components and the bonds they form through specific chemical bond associations are critical determining factors in their functionality. In particular, crystalline porous materials continue to inspire their development and advancement towards sustainable and renewable materials for clean energy conversion and storage. An important area of development is the application of MOFs in proton-exchange membrane fuel cells (PEMFCs) and are attractive for efficient low-temperature energy conversion. The practical implementation of fuel cells, however, is faced by performance challenges. To address some of the technical issues, a more critical consideration of key problems is now driving a conceptualised approach to advance the application of PEMFCs. Central to this idea is the emerging field MOF-based systems, which are currently being adopted and proving to be a more efficient and durable means of creating electrodes and electrolytes for proton−exchange membrane fuel cells. This review proposes to discuss some of the key advancements in the modification of PEMs and electrodes, which primarily use functionally important MOFs. Further, we propose to correlate MOF-based PEMFC design and the deeper correlation with performance by comparing proton conductivities and catalytic activities for selected works.

Efficacy of MOF-199 in improvement of permeation, morphological, antifouling and antibacterial characteristics of polyvinylidene fluoride membranes

Metal–organic frameworks (MOFs) are widely explored for advances in hybrid membranes because of their bonding and fondness in polymers.

Effective removal of levofloxacin drug and Cr(VI) from water by a composed nanobiosorbent of vanadium pentoxide@chitosan@MOFs

Wastewaters is generally polluted with various inorganic and organic contaminants which require effective multipurpose purification technology. In this respect, a novel V₂O₅@Ch/Cu-TMA nanobiosorbent was constructed via encapsulation of nanoscale metal organic frameworks (Cu-TMA) into vanadium pentoxide-imbedded-chitosan matrix to comprehensively investigate its efficiency in removal of levofloxacin drug (LEVO) (e.g., organic pollutant) and chromium (VI) (e.g., inorganic pollutant) from water. Both LEVO drug and Cr(VI) adsorptions were correlated to pseudo-second order (R² = 1) and Langmuir isotherm (R² = 0.9924 for LEVO and R² = 0.9815 for Cr(VI)). Adsorption of Cr(VI) was confirmed to be spontaneous and endothermic reactions, while LEVO was found to proceed via spontaneous and exothermic reactions based on the thermodynamic parameters. The emerged V₂O₅@Ch/Cu-TMA is regarded as an excellent nanobiosorbent for removal of inorganic contaminant as Cr(VI) from all natural water samples (tap, sea and wastewater) with percentages range 92.43%-96.95% and organic contaminant as LEVO drug from tap and wastewater (91.99%-97.20%).

Sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene based membranes containing CuO@g-C3N4 embedded with 2,4,6-triphenylpyrylium tetrafluoroborate for fuel cell applications

New series of polymer composite membranes were prepared from sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (S-PSEBS) and copper oxide loaded in graphitic carbon nitride (CuO@N-C) embedded with an ionic liquid, 2,4,6-triphenylpyrylium tetrafluoroborate. The structural and physicochemical properties of the composite membranes were studied in detail. Electrolyte membrane loaded with 8.0 wt% of CuO@N-C exhibited the maximum ion-exchange capacity of 3.1 meq. g^-1, whereas that of the pristine membrane was restricted to 1.8 meq. g^-1. From the TGA profile of the composite membrane, it was found to exhibit adequate thermal stability to be employed as electrolyte in fuel cells. Proton conductivity of the composite membranes was found to be in the range between 0.0179 S cm^-1 and 0.0229 S cm^-1. Indeed, the substantial results achieved with the S-PSEBS/CuO@N-C composite membranes were indicative of the notable features of the membranes for use in fuel cells.

Dual-functional coordination polymers with high proton conduction behaviour and good luminescence properties

Two coordination polymers, [M(5-hip)(H2O)3]n (M = Cd2+ (1), Zn2+ (2), 5-hip = 5-hydroxyisophthalic acid), have been synthesized under hydrothermal conditions. The crystal structure reveals that complexes 1 and 2 have 1D chain structures by the coordination of metal ions and 5-hip. 1D chains are connected by hydrogen bonds to form a 3D structure. AC impedance analysis shows that the proton conductivity of complexes 1 and 2 comes up to 1.58 × 10-3 S cm-1 (98%RH, 343 K) and 5.27 × 10-4 S cm-1 (98%RH, 353 K), respectively. To further improve the proton conductivity, a hybrid membrane was prepared by the solution casting method with complexes as fillers and sulfonated polyether ether ketone (SPEEK) as the organic matrix. The proton conductivity of hybrid membranes 1@SPEEK-5 and 2@SPEEK-5 is 1.97 and 1.58 times higher than that of pure SPEEK membranes, respectively. Furthermore, the two complexes are excellent fluorescent sensors, which could detect Cr2O72- in aqueous solution with high sensitivity and selectivity. Both of them have low detection limits for Cr2O72- in aqueous solution, where the detection limit of complex 1 is 0.8 μM and that of complex 2 is 1 μM. The above work demonstrates that the two complexes are dual-functional materials with high proton conduction and good fluorescence properties.

Enhanced properties of sulfonated polyether ether ketone proton exchange membrane by incorporating carboxylic-contained zeolitic imidazolate frameworks

Carboxylic-containing zeolitic imidazolate frameworks (ZIF-COOH) showed an obvious improvement in the performance of sulfonated polyether ether ketone (SPEEK)-based proton exchange membranes.

New series of organic–inorganic polymer nanocomposite membranes for fuel cell applications

Flexible organic–inorganic polymer nanocomposite membranes with uniformly distributed metal oxide nanoparticles were prepared using sulfonated poly(ether ether ketone) (SPEEK) as a base material and praseodymium oxide (PSO) as an inorganic additive. The degree of sulfonation of SPEEK was determined by proton nuclear magnetic resonance (NMR) analysis and found to be 60%. The characteristic properties of the polymer nanocomposite membranes were examined by thermogravimetric analysis, X-ray diffraction, ion exchange capacity, water uptake ability, and proton conductivity. The incorporation of metal oxide into the polymer matrix was confirmed by scanning electron microscope with energy dispersive X-ray spectroscopy and X-ray diffraction analyses. The nanocomposite membrane exhibits good thermal stability when compared to that of the pristine membrane and SPEEK with 10 wt% of PSO loading was found to be stable up to 450°C. The assessment of polymer electrolyte membrane is accomplished by fabricating membrane electrode assemblies of pure SPEEK and SP-PSO-10 membranes and the latter produced maximum peak power density of 622 mW cm−2. The constructed SPEEK/PSO nanocomposite membranes offered superior physicochemical properties while applying these materials in an H2-O2 fuel cell.