Novel Composite PEM with Long-Range Ionic Nanochannels Induced by Carbon Nanotube/Graphene Oxide Nanoribbon Composites

Graphene oxide-based materials as proton-conducting membranes for electrochemical applications

The rapid advancements of graphene oxide (GO)-based membranes necessitate the understanding of their properties and application potential. Generally, proton (H+)-conducting membranes, including GO-based ones, are crucial components in various energy-relevant devices, significantly determining the transport process, selectivity, and overall efficiency of these devices. Particularly, GO-based membranes exhibit great potential in electrochemical applications owing to their remarkable conductivity and ease of undergoing further modifications. This review is aimed at highlighting recent functionalization strategies for GO with diverse substrates. It is also aimed at emphasizing how these modifications can enhance the electrochemical performances of GO-based membranes. Notably, key aspects, such as the enhanced H+-transfer kinetics, improved conductivity, functionalities, and optimization, of these membranes for specific applications are discussed. Additionally, the existing challenges and future directions for the field of functionalized GO are addressed to achieve precise control of the functionalities of these membranes as well as advance next-generation electrochemical devices.

Enhanced Proton Transfer in Proton-Exchange Membranes with Interconnected and Zwitterion-Functionalized Covalent Porous Material Structures

Excellent proton-conductive accelerators are indispensable for efficient proton-exchange membranes (PEMs). Covalent porous materials (CPMs), with adjustable functionalities and well-ordered porosities, show much promise as effective proton-conductive accelerators. In this study, an interconnected and zwitterion-functionalized CPM structure based on carbon nanotubes and a Schiff-base network (CNT@ZSNW-1) is constructed as a highly efficient proton-conducting accelerator by in situ growth of SNW-1 onto carbon nanotubes (CNTs) and subsequent zwitterion functionalization. A composite PEM with enhanced proton conduction is acquired by integrating CNT@ZSNW-1 with Nafion. Zwitterion functionalization offers additional proton-conducting sites and promotes the water retention capacity. Moreover, the interconnected structure of CNT@ZSNW-1 induces a more consecutive arrangement of ionic clusters, which significantly relieves the proton transfer barrier of the composite PEM and increases its proton conductivity to 0.287 S cm-1 under 95 % RH at 90 °C (about 2.2 times that of the recast Nafion, 0.131 S cm-1 ). Furthermore, the composite PEM displays a peak power density of 39.6 mW cm-2 in a direct methanol fuel cell, which is significantly higher than that of the recast Nafion (19.9 mW cm-2 ). This study affords a potential reference for devising and preparing functionalized CPMs with optimized structures to expedite proton transfer in PEMs.

Metal Organic Frameworks Modified Proton Exchange Membranes for Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) have received considerable interest due to their low operating temperature and high energy conversion rate. However, their practical implement suffers from significant performance challenge. In particular, proton exchange membrane (PEM) as the core component of PEMFCs, have shown a strong correlation between its properties (e.g., proton conductivity, dimensional stability) and the performance of fuel cells. Metal-organic frameworks (MOFs) as porous inorganic-organic hybrid materials have attracted extensive attention in gas storage, gas separation and reaction catalysis. Recently, the MOFs-modified PEMs have shown outstanding performance, which have great merit in commercial application. This manuscript presents an overview of the recent progress in the modification of PEMs with MOFs, with a special focus on the modification mechanism of MOFs on the properties of composite membranes. The characteristics of different types of MOFs in modified application were summarized.

Fast Proton Conduction in Denatured Bovine Serum Albumin-Coated Nafion Membranes

Bovine serum albumin (BSA) is a globular soluble protein, which has been extensively used in biochemical engineering. BSA materials possess abundant hydrophilic charged amino acids, H-bonded networks, and various secondary structures, which has great potential in facilitating proton transfer. Herein, BSA-N117 (BSA-Nafion 117) membranes are conveniently and eco-friendly prepared by utilizing the adsorption and denaturation of BSA on the Nafion 117 surface. The morphology and secondary structures of the BSA layer are studied with field-emission scanning electron microscopy, atomic force microscopy, and Fourier transform infrared spectroscopy. BSA-N117 membranes show highly increased proton conductivity under various conditions, which could be attributed to the improved wettability, water uptake, and the denaturation of BSA. The in-plane proton conductivity of BSA-N117-5 reaches 0.3 and 0.06 S cm^-1 under 80 °C-95% RH and 100 °C-40% RH, respectively. The denaturation of BSA leads to the unfolding of α-helix structures and the formation of β-sheet structures. β-Sheet structures are more beneficial to proton conduction since β-sheet structures have stronger interactions with water molecules and protons could transport more directly in the parallel H-bonded network. Moreover, the denatured BSA modification layer could effectively help BSA-N117 membranes to possess higher selectivity and overcome the "trade-off" effect between proton conductivity and methanol resistance. The methanol permeability of BSA-N117 membranes is 1 order of magnitude lower than that of Nafion 117.

Two-Dimensional Zeolitic Imidazolate Framework/Carbon Nanotube Hybrid Networks Modified Proton Exchange Membranes for Improving Transport Properties

Metal-organic framework (MOF)/polymer composite proton exchange membranes (PEMs) are being intensively investigated due to their potentials for the systematic design of proton-conducting properties. However, the development of MOF/polymer composite PEMs possessing high selectivity remains exceedingly desirable and challenging for practical application. Herein, two-dimensional (2D) zeolitic imidazolate framework (ZIF-8)/carbon nanotube (CNT) hybrid cross-linked networks (ZCN) were synthesized via the rational design of the physical form of ZIF-8, and then a series of composite PEMs were prepared by hybridizing ZCN with sulfonated poly(ether ether ketone) (SPEEK) matrix. The effect of the incorporation of zero-dimensional (0D) raw ZIF-8 nanoparticles and 2D ZCN on the proton conduction and methanol permeability of the composite membranes was systemically studied. Benefiting from the morphological and compositional advantages of ZCN, the SPEEK/ZCN composite membranes displayed a significant enhancement in proton conductivity under various conditions. In particular, the proton conductivity of SPEEK/ZCN-2.5 membrane was up to 50.24 mS cm^-1 at 120 °C-30% RH, which was 11.2 times that of the recast SPEEK membrane (4.50 mS cm^-1) and 2.1 times that of SPEEK/ZIF membrane (24.1 mS cm^-1) under the same condition. Meanwhile, the methanol permeability of the SPEEK/ZCN composite membranes was greatly reduced. Therefore, novel MOF/polymer composite PEMs with high selectivity were obtained. Our investigation results reveal that the proton conductivity and methanol permeability of the MOF/polymer composite membranes can be effectively tailored via creating more elaborate superstructures of MOFs rather than altering the chemical component. This effective strategy may provide a useful guideline to integrate with other interesting MOFs to design MOF/polymer composite membranes.

Poly(2,5-benzimidazole)-Grafted Graphene Oxide as an Effective Proton Conductor for Construction of Nanocomposite Proton Exchange Membrane

To improve proton conduction properties of conventional sulfonated poly(ether ether ketone) (SPEEK), poly(2,5-benzimidazole)-grafted graphene oxide (ABPBI-GO) was prepared to fabricate nanocomposite membranes, which then were further doped with phosphoric acid (PA). The ABPBI-GO was synthesized through the reaction of 3,4-diaminobenzoic acid with the carboxyl acid groups present on the GO surface. The simultaneous incorporation of ABPBI-GO and PA into SPEEK did not only improve the physicochemical performance of the membranes in terms of thermal stability, water uptake, dimensional stability, proton conductivity, and methanol permeation resistance but also relieve PA leaching from the membranes though acid-base interactions. The resulting composite membranes exhibited enhanced proton conductivities in extended humidity ranges thanks to the hygroscopic character of PA and the increased water uptake. Moreover, the unique self-ionization, self-dehydration, and nonvolatile properties of PA improved the high-temperature proton conductivities (σ) of PA-doped membranes. The PA-doped SPEEK/ABPBI-GO-3.0 delivered a σ of 7.5 mS cm^-1 at 140 °C/0% RH. This value was fourfold higher than that of pristine SPEEK membranes. The PA-doped SPEEK/ABPBI-GO-3.0 based fuel cell membranes delivered power densities of 831.06 and 72.25 mW cm^-2 at 80 °C/95% RH and 120 °C/0% RH, respectively. By contrast, the PA-doped SPEEK membrane generated only 655.63 and 44.58 mW cm^-2 under the same testing conditions.

Rational Design of S-UiO-66@GO Hybrid Nanosheets for Proton Exchange Membranes with Significantly Enhanced Transport Performance

Metal-organic frameworks (MOFs) are being intensively explored as filler materials for polymeric proton exchange membranes (PEMs) due to their potentials for the systematic design and modification of proton-conducting properties. S-UiO-66, a stable MOF with functional groups of -SO₃H in its ligands, was selected here to prepare S-UiO-66@graphene oxide (GO) hybrid nanosheets via a facile in situ growth procedure, and then a series of composite PEMs were prepared by hybridizing S-UiO-66@GO and sulfonated poly(ether ether ketone) (SPEEK). The resultant hybrid nanosheets not only possessed abundant -SO₃H groups derived from the ligands of S-UiO-66 but also yielded a uniform dispersion of S-UiO-66 onto GO nanosheets, thus effectively eliminating the agglomeration of S-UiO-66 in the membrane matrix. Thanks to the well-tailored chemical composition and nanostructure of S-UiO-66@GO, the as-prepared SPEEK/S-UiO-66@GO composite PEMs present a significant increase in their proton conductivity under various conditions. In particular, the proton conductivity of the SPEEK/S-UiO-66@GO-10 membrane was up to 0.268 S·cm^-1 and 16.57 mS·cm^-1 at 70 °C-95% RH and 100 °C-40% RH (2.6 and 6.0 times that of recast SPEEK under the same condition), respectively. Moreover, the mechanical property of composite membranes was substantially strengthened and the methanol penetration was well-suppressed. Our investigation indicates the great potential of S-UiO-66@GO in fabricating composite PEMs and also reveals that the high proton conductivity of MOFs can be fully utilized by means of MOF/polymer composite membranes.

Novel Slightly Reduced Graphene Oxide Based Proton Exchange Membrane with Constructed Long-Range Ionic Nanochannels via Self-Assembling of Nafion

A facile method to prepare high-performance Nafion slightly reduced graphene oxide membranes (N-srGOMs) via vacuum filtration is proposed. The long-range connected ionic nanochannels in the membrane are constructed via the concentration-dependent self-assembling of the amphiphilic Nafion and the hydrophilic-hydrophobic interaction between graphene oxide (GO) and Nafion in water. The obtained N-srGOM possesses high proton conductivity, and low methanol permeability benefitted from the constructed unique interior structures. The proton conductivity of N-srGOM reaches as high as 0.58 S cm^-1 at 80 °C and 95%RH, which is near 4-fold of the commercialized Nafion 117 membrane under the same condition. The methanol permeability of N-srGOM is 2.0 × 10^-9 cm² s^-1, two-magnitude lower than that of Nafion 117. This novel membrane fabrication strategy has proved to be highly efficient in overcoming the "trade-off" effect between proton conductivity and methanol resistance and displays great potential in DMFC application.

Novel Composite Proton Exchange Membrane with Connected Long-Range Ionic Nanochannels Constructed via Exfoliated Nafion-Boron Nitride Nanocomposite

Nafion-boron nitride (NBN) nanocomposites with a Nafion-functionalized periphery are prepared via a convenient and ecofriendly Nafion-assisted water-phase exfoliation method. Nafion and the boron nitride nanosheet present strong interactions in the NBN nanocomposite. Then the NBN nanocomposites were blended with Nafion to prepare NBN Nafion composite proton exchange membranes (PEMs). NBN nanocomposites show good dispersibility and have a noticeable impact on the aggregation structure of the Nafion matrix. Connected long-range ionic nanochannels containing exaggerated (-SO₃^-)_n ionic clusters are constructed during the membrane-forming process via the hydrophilic and H-bonding interactions between NBN nanocomposites and Nafion matrix. The addition of NBN nanocomposites with sulfonic groups also provides additional proton transportation spots and enhances the water uptake of the composite PEMs. The proton conductivity of the NBN Nafion composite PEMs is significantly increased under various conditions relative to that of recast Nafion. At 80 °C-95% relative humidity, the proton conductivity of 0.5 NBN Nafion is 0.33 S·cm^-1, 6 times that of recast Nafion under the same conditions.

Tuning the Surface Properties of Graphene Oxide by Surface-Initiated Polymerization of Epoxides: An Efficient Method for Enhancing Gas Separation

Here, we describe an in situ approach for growing polyepoxides from the surfaces of graphene oxide (GO) using a surface-initiated polymerization reaction. The polymerization methodology is facile and general as a broad range of epoxides carrying various functional groups have been successfully polymerized by simply adding GO powders in the epoxide monomers. The resultant polyepoxide grafted GO are found to show enhanced dispersibility in various common solvents and to exhibit increased d-spacing between the basal planes. In particular, grafting poly(2,3-epoxy-1-propanol) (PEP) to GO results in a composite (i.e., GO-g-PEP) that is dispersible in water and miscible with polyether block amide, i.e., Pebax MH 1657. Preliminary studies have indicated the membranes prepared using Pebax/GO-g-PEP composites exhibit enhanced CO₂ permeabilities and selectivities in comparison to H₂, O₂, or N₂. The excellent performance in gas separation is attributed to the layered structure of the GO-g-PEP sheets with enlarged d-spacing and the functional groups present on the PEP chains grafted to the surfaces of GO sheets.