Investigating the nanostructures and proton transfer properties of Nafion-GO hybrid membranes

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.

Polymer Electrolyte Membranes Containing Functionalized Organic/Inorganic Composite for Polymer Electrolyte Membrane Fuel Cell Applications

To mitigate the dependence on fossil fuels and the associated global warming issues, numerous studies have focused on the development of eco-friendly energy conversion devices such as polymer electrolyte membrane fuel cells (PEMFCs) that directly convert chemical energy into electrical energy. As one of the key components in PEMFCs, polymer electrolyte membranes (PEMs) should have high proton conductivity and outstanding physicochemical stability during operation. Although the perfluorinated sulfonic acid (PFSA)-based PEMs and some of the hydrocarbon-based PEMs composed of rationally designed polymer structures are found to meet these criteria, there is an ongoing and pressing need to improve and fine-tune these further, to be useful in practical PEMFC operation. Incorporation of organic/inorganic fillers into the polymer matrix is one of the methods shown to be effective for controlling target PEM properties including thermal stability, mechanical properties, and physical stability, as well as proton conductivity. Functionalization of organic/inorganic fillers is critical to optimize the filler efficiency and dispersion, thus resulting in significant improvements to PEM properties. This review focused on the structural engineering of functionalized carbon and silica-based fillers and comparisons of the resulting PEM properties. Newly constructed composite membranes were compared to composite membrane containing non-functionalized fillers or pure polymer matrix membrane without fillers.

Improved Thermo-Mechanical Properties and Reduced Hydrogen Permeation of Short Side-Chain Perfluorosulfonic Acid Membranes Doped with Ti3C2Tx

Benefiting from its large specific surface with functional -OH/-F groups, Ti3C2Tx, a typical two-dimensional (2D) material in the recently developed MXene family, was synthesized and used as a filler to improve the properties of the short side-chain (SSC) perfluorosulfonic acid (PFSA) proton exchange membrane. It is found that the proton conductivity is enhanced by 15% while the hydrogen permeation is reduced by 45% after the addition of 1.5 wt% Ti3C2Tx filler into the SSC PFSA membrane. The improved proton conductivity of the composite membrane could be associated with the improved proton transport environment in the presence of the hydrophilic functional groups (such as -OH) of the Ti3C2Tx filler. The significantly reduced hydrogen permeation could be attributed to the incorporation of the impermeable Ti3C2Tx 2D fillers and the decreased hydrophilic ionic domain spacing examined by the small angle X-ray scattering (SAXS) for the composite membrane. Furthermore, improved thermo-mechanical properties of the SSC/Ti3C2Tx composite membrane were measured by dynamic mechanical analyzer (DMA) and tensile strength testing. The demonstrated higher proton conductivity, lower hydrogen permeation, and improved thermo-mechanical stability indicate that the SSC/Ti3C2Tx composite membranes could be a potential membrane material for PEM fuel cells operating above the water boiling temperature.

Ionic Covalent Organic Frameworks for Energy Devices

Covalent organic frameworks (COFs) are a class of porous crystalline materials whose facile preparation, functionality, and modularity have led to their becoming powerful platforms for the development of molecular devices in many fields of (bio)engineering, such as energy storage, environmental remediation, drug delivery, and catalysis. In particular, ionic COFs (iCOFs) are highly useful for constructing energy devices, as their ionic functional groups can transport ions efficiently, and the nonlabile and highly ordered all-covalent pore structures of their backbones provide ideal pathways for long-term ionic transport under harsh electrochemical conditions. Here, current research progress on the use of iCOFs for energy devices, specifically lithium-based batteries and fuel cells, is reviewed in terms of iCOF backbone-design strategies, synthetic approaches, properties, engineering techniques, and applications. iCOFs are categorized as anionic COFs or cationic COFs, and how each of these types of iCOFs transport lithium ions, protons, or hydroxides is illustrated. Finally, the current challenges to and future opportunities for the utilization of iCOFs in energy devices are described. This review will therefore serve as a useful reference on state-of-the-art iCOF design and application strategies focusing on energy devices.

A Sulfonated Polyimide/Nafion Blend Membrane with High Proton Selectivity and Remarkable Stability for Vanadium Redox Flow Battery

A sulfonated polyimide (SPI)/Nafion blend membrane composed of a designed and synthesized SPI polymer and the commercial Nafion polymer is prepared by a facile solution casting method for vanadium redox flow battery (VRFB). Similar molecular structures of both SPI and Nafion provide good compatibility and complementarity of the blend membrane. ATR-FTIR, ¹H-NMR, AFM, and SEM are used to gain insights on the chemical structure and morphology of the blend membrane. Fortunately, the chemical stability of the SPI/Nafion blend membrane is effectively improved compared with reported SPI-based membranes for VRFB applications. In cycling charge-discharge tests, the VRFB with the as-prepared SPI/Nafion blend membrane shows excellent battery efficiencies and operational stability. Above results indicate that the SPI/Nafion blend membrane is a promising candidate for VRFB application. This work opens up a new possibility for fabricating high-performance SPI-based blend membrane by introduction of a polymer with a similar molecular structure and special functional groups into the SPI polymer.

Sulfonated Polyimide Membranes Derived from a Novel Sulfonated Diamine with Pendant Benzenesulfonic Acid for Fuel Cells

For improving the hydrolytic stability of sulfonated polyimides consisting of five membered anhydrides, novel sulfonated polyimides (NSPIs) were prepared via polymerization of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), with a novel diamine monomer with a pendant sulfonic acid group and 4,4-oxydianiline. Water uptake of this NSPI with an excellent film-forming ability was almost equal to that of Nafion® 117, while their ion exchange capacity (IEC) was 22% higher than Nafion® 117. The loss in weight decreased by 53% and loss in IEC decreased by 66% compared to that of Nafion® 117; both were used to quantitatively measure hydrolytic stability, and radical oxidative stability also increased by 75% when compared with Nafion® 117. Mechanically, this NSPI was superior, and its proton conductivity was higher than Nafion® 117 at elevated temperatures. All these improvements were due to the introduction of this pendant group. Taken together, we herein report a promising renewable energy source based on SPIs capable of displaying proton conductivity and enhanced hydrophilicity.

Proton Conduction of Nafion Hybrid Membranes Promoted by NH3-Modified Zn-MOF with Host-Guest Collaborative Hydrogen Bonds for H2/O2 Fuel Cell Applications

It is of great significance to develop creative proton exchange membrane materials for proton exchange membrane fuel cells (PEMFCs). The strategy of doping metal-organic frameworks (MOFs) with guest molecules into the Nafion matrix is adopted to improve the electrochemical performance of Nafion hybrid membranes. Various and abundant hydrogen bonds can make a tremendous contribution to the proton conduction of hybrid membranes. In this work, we used high proton-conducting Zn-MOFs with the characteristics of host-guest collaborative hydrogen bonds as the filler to prepare Zn-MOF/Nafion hybrid membranes. Alternating current (AC) impedance tests show that when the doping amount of Zn-MOF is 5%, the proton conductivity reaches 7.29 × 10^-3 S·cm^-1, being 1.87 times that of the pure Nafion membrane at 58% relative humidity (RH) and 80 °C. In an attempt to prove the promotion effect of guest NH₃ on proton conductivity of Nafion hybrid membranes, Zn-MOF-NH₃ was filled into the Nafion matrix. Under the same conditions, its proton conductivity reaches the maximum value of 2.13 × 10^-2 S·cm^-1, which is 5.47 times that of the pure Nafion membrane. Zn-MOF-NH₃/Nafion-5 was used to fabricate a proton exchange membrane for application in H₂/O₂ fuel cells. The maximum power density of 212 mW cm^-2 and a current density of 630 mA cm^-2 reveal a respectable single cell performance. This study provides a promising method for optimizing the structure of MOF proton conductors and inspires the preparation of high-performance Nafion hybrid membranes.

Precise Molecular-Level Modification of Nafion with Bismuth Oxide Clusters for High-performance Proton-Exchange Membranes

Fabricating proton exchange membranes (PEMs) with high ionic conductivity and ideal mechanical robustness through regulation of the membrane microstructures achieved by molecular-level hybridization remains essential but challenging for the further development of high-performance PEM fuel cells. In this work, by precisely hybridizing nano-scaled bismuth oxide clusters into Nafion, we have fabricated the high-performance hybrid membrane, Nafion-Bi₁₂ -3 %, which showed a proton conductivity of 386 mS cm^-1 at 80 °C in aqueous solution with low methanol permeability, and conserved the ideal mechanical and chemical stabilities as PEMs. Moreover, molecular dynamics (MD) simulation was employed to clarify the structural properties and the assembly mechanisms of the hybrid membrane on the molecular level. The maximum current density and power density of Nafion-Bi₁₂ -3 % for direct methanol fuel cells reached to 432.7 mA cm^-2 and 110.2 mW cm^-2 , respectively. This work provides new insights into the design of versatile functional polymer electrolyte membranes through polyoxometalate hybridization.

Characterization and Application of Agave salmiana Cuticle as Bio-Membrane in Low-Temperature Electrolyzer and Fuel Cells

This work describes the application of the Agave salmiana cuticle as a new protonic exchange biological membrane (0.080 ± 0.001 mm thickness). Different chemical, electrochemical and mechanical treatments were evaluated to stimulate the ionic exchange properties of the cuticle. Thermal treatment was adequate for its application in a two-chamber electrolyzer. Under optimal conditions an ionic conductivity value of 10 ± 3 mS cm−1 was obtained; this value is similar to the value achieved using a Nafion membrane. The thermally-activated bio-membrane was also evaluated in a fuel cell, where the highest potential was obtained using methanol and hydrogen (0.46 ± 0.01 V). This result makes the Agave salmiana cuticle a competitive choice to replace the commercial membrane. Its surface morphology and their functional groups were evaluated through scanning electron microscopy (SEM), infrared spectroscopy and impedance spectroscopy. This thermally-treated Agave salmiana cuticle is an ecofriendly alternative to replace Nafion membranes in electrolyzer and fuel cells.

Constructing Long-Range Transfer Pathways with Ordered Acid-Base Pairs for Highly Enhanced Proton Conduction

Acid-base pairs hold great superiority in creating proton defects and facilitating proton transfer with less or no water. However, the existing acid-base complexes fail in assembling into ordered acid-base pairs and thus cannot always take full advantage of the acid-base synergetic effect. Herein, polymer quantum dots with inherent ordered acid-base pairs are utilized and anchored on dopamine-coated graphene oxide, thus forming into long-range conducting pathways. The resultant building blocks ( _nPGO) are integrated in a sulfonated poly(ether ether ketone) matrix to fabricate composite membranes. The constructed long-range transfer highways with ordered acid-base pairs impart to the composite membrane significantly enhanced proton conduction ability. Under the hydrated state, the composite membrane attains 91% increase over the control membrane in conductivity, and the single-cell fuel based on the membrane achieves 71% promotion in maximum power density. Under anhydrous conditions, more striking augment in conduction is observed for the composite membrane, reaching 7.14 mS cm^-1, almost 10 times of the control membrane value (0.78 mS cm^-1). Remarkably, such anhydrous proton conduction performance is even comparable to that of the composite membrane impregnated with ionic liquids, which is hard to realize with conventional fillers. Collectively, these results endow composite membranes great potential for applications in hydrogen-based fuel cells, sensors, and catalysis.