Branched sulfonated polyimide/s-MWCNTs composite membranes for vanadium redox flow battery application

A novel sulfonated multi-wall carbon nanotubes (s-MWCNTs) filler is synthesized by ring-opening reaction. And then, a series of branched sulfonated polyimide (bSPI)/s-MWCNTs composite membranes are also prepared for application in vanadium redox flow batteries (VRFBs). The optimized bSPI/s-MWCNTs-2% composite membrane has lower vanadium ion permeability (2.01 × 10⁻⁷ cm² min⁻¹) and higher proton selectivity (1.06 × 10⁵ S min cm⁻³) compared to those of commercial Nafion 212 membrane. Moreover, the VRFB with bSPI/s-MWCNTs-2% composite membrane exhibits higher coulombic efficiencies (CEs: 96.0–98.2%) and energy efficiencies (EEs: 79.7–69.5%) than that with Nafion 212 membrane (CEs: 86.5–92.5% and EEs: 78.5–67.6%) at 80–160 mA cm⁻². The VRFB with bSPI/s-MWCNTs-2% composite membrane has stable battery performance over 400 cycles at 100 mA cm⁻², whose EE value is in the top level among previously reported SPI-based composite membranes. The results show that the bSPI/s-MWCNTs-2% composite membrane has a great prospect in VRFB application.     

Proton exchange membranes containing densely alkyl sulfide sulfonated side chains for vanadium redox flow battery

Due to further increase the performance of aromatic sulfonated proton exchange membrane (PEM) and make it play a better role in vanadium redox flow battery (VRFB), a series of poly(aryl ether sulfone)s containing eight alkyl sulfide sulfonated side chains (8SPAES-xx) are designed and synthesized. Their molecular structure, phase morphology and some selective properties were investigated in detail, respectively. It is confirmed that 8SPAES-xx membranes have clear hydrophilic/hydrophobic phase separation morphology. These membranes with the ion exchange capacity values of 1.08–1.61 mmol/g exhibit excellent ionic conductivity as well as moderate water uptake and good dimensional stability, and their values are in the range of 25–96 mS/cm, 8–28% and 5–17% at 30 °C, respectively. Among them, the proton conductivity of 8SPAES-12 membrane is 82 mS/cm at 30 °C, which exceeds the ionic conductivity of Nafion 117 (79 mS/cm). The membrane also shows high ion selectivity and excellent battery performance. At current density of 60 mA/cm², the highest energy efficiency of VRFB with 8SPAES-12 membrane is 87.3%, which is higher than that of Nafion 117 (83.8%). Furthermore, the efficiency of VRFB with 8SPAES-12 membrane remains good cycle stability.     

Proton exchange membranes with ultra-low vanadium ions permeability improved by sulfated zirconia for all vanadium redox flow battery

Suppressing vanadium ions crossover is a top priority in the development of membranes for vanadium redox flow battery (VRFB). One method is to dope inorganic fillers into polymer matrix, which usually decreases membrane's ion conductivity. In this work, sulfated zirconia (SZrO₂) is synthesized as a novel additive doped in sulfated poly (ether sulfone) (SPES) to simultaneously enhance the proton conduction and inhibit vanadium migration of the membranes. Membrane characterizations including battery test are carried out to reveal the effects of SZrO₂ on the membrane performance. The SPES/SZrO₂ composite membranes show vanadium permeability one order of magnitude lower than that of Nafion 212 and enhanced proton conductivity, which lead to superior cell performance. The columbic efficiency and energy efficiency of the VRFB reach 98.89% and 86.78%, respectively, at 100 mA cm⁻². Cycling test is carried out to evaluate the chemical and electrochemical stability of the membrane. Energy efficiency above 86% is maintained after70 charge-discharge cycles at 100 mA cm⁻².     

Self-assembly of metal-organic framework onto nanofibrous mats to enhance proton conductivity for proton exchange membrane

Constructing consecutive proton-conducting nanochannels and optimizing nanophase-separation within proton exchange membrane (PEM) was of guiding significance for improving proton transfer. Metal organic framework (MOF), as a novel and functional material had drawn increasing attention in the research of proton PEM because of its flexible tunability and designability. Herein, a novel MOF-based nanofibrous mats (NFMs) were prepared by the self-assembly of zeolitic imidazole framework-67 (ZIF-67) onto polyacrylonitrile (PAN) NFMs. Subsequently, the ZIF-67 NFMs were incorporated into Nafion matrix to prepare ZIF-67@Nafion composite membrane which aimed at constructing consecutive proton-conducting channels. Especially, the acid–base pairs between N–H (ZIF-67 NFMs) and –SO₃H (Nafion) could promote the protonation/deprotonation and subsequent proton leaping via Grotthuss mechanisms. As expected, the ZIF-67@Nafion-5 composite membrane showed a promising proton conductivity of 288 mS/cm at 80 °C and 100% RH, low methanol permeability of 7.98 × 10⁻⁷ cm²s⁻¹, and superior power density of 298.68 mW/cm² at 80 °C and 100% RH. In addition, the resulting composite membrane exhibited considerable enhancement in thermal stability and dimensional stability. This promising strategy provided a valuable reference for designing high-performance PEMs.     

Novel branched sulfonated polyimide membrane with remarkable vanadium permeability resistance and proton selectivity for vanadium redox flow battery application

A series of novel branched sulfonated polyimide (bSPI-x) membranes with 8% branched degree are developed for application in vanadium redox flow battery (VRFB). The sulfonation degrees of bSPI-x membranes are precisely regulated for obtaining excellent comprehensive performance. Among all bSPI-x membranes, the bSPI-50 membrane shows strong vanadium permeability resistance, which is as 8 times as that of commercial Nafion 212 membrane. At the same time, the bSPI-50 membrane has remarkable proton selectivity, which is four times as high as that of Nafion 212 membrane. The bSPI-50 membrane possesses slower self-discharge speed than Nafion 212 membrane. Furthermore, the bSPI-50 membrane achieves stable VRFB efficiencies during 200-time charge-discharge cycles at 120–180 mA cm^?2. Simultaneously, the bSPI-50 membrane exhibits excellent capacity retention compared with Nafion 212 membrane. All results imply that the bSPI-50 membrane possesses good application prospect as a promising alternative separator of VRFB.     

Development of perfluorosulfonic acid polymer-based hybrid composite membrane with alkoxysilane functionalized polymer for vanadium redox flow battery

A new kind of alkoxy silane functionalized polymer (ASFP) is synthesized by selectively functionalized carboxyl groups as a novel inorganic precursor polymer to prepare organic-inorganic hybrid membrane for vanadium redox flow battery (VRFB) system. The novel hybrid membrane has been fabricated by interconnection between hydrophilic domains of Nafion and ASFP functional group. The effective concentration of ASFP for hybrid membrane is 25% (wt/wt). The proton conductivity and selectivity of the hybrid membrane are comparable with those of the Nafion212 membrane, which is mainly attributed by the presence of additional hydrophilic domains in the hybrid membrane. The proton conductivity and ion exchange capacity of the Nafion-ASFP (75:25) membrane is 0.061 S/cm and 0.68 meq/g, respectively. Remarkably, the Nafion-ASFP membrane shows a low vanadium permeability (1.259 × 10⁻⁷ cm²/min) and high selectivity, which is an excellent advantage. As a result, the hybrid membrane shows comparable efficiency performance with Nafion212 over 50 cycles. Notably, the VRFB unit cell with Nafion-ASFP membrane achieves higher coulombic efficiency than Nafion212. The hybrid membrane reveals a new route to develop an alternative fluorinated polymer membrane with numerous advantages especially cost-effectiveness, homogeneous dispersion of inorganic silica precursor materials in the hybrid membrane without deterioration of mechanical strength, and lower vanadium ion crossover for VRFB system.     

An enhanced proton conductivity and reduced methanol permeability composite membrane prepared by sulfonated covalent organic nanosheets/Nafion

Sulfonated covalent organic nanosheets (SCONs) with a functional group (−SO₃H) are effective at reducing ion channels length and facilitating proton diffusion, indicating the potential advantage of SCONs in application for proton exchange membranes (PEMs). In this study, Nafion-SCONs composite membranes were prepared by introducing SCONs into a Nafion membrane. The incorporation of SCONs not only improved proton conductivity, but also suppressed methanol permeability. This was due to the even distribution of ion channels, formed by strong electrostatic interaction between the well dispersed SCONs and Nafion polymer molecules. Notably, Nafion-SCONs-0.6 was the best choice of composite membranes. It exhibited enhanced performance, such as high conductivity and low methanol permeability. The direct methanol fuel cell (DMFC) with Nafion-SCONs-0.6 membrane also showed higher power density (118.2 mW cm⁻²), which was 44% higher than the cell comprised of Nafion membrane (81.9 mW cm⁻²) in 2 M methanol at 60 °C. These results enabled us to work on building composite membranes with enhanced properties, made from nanomaterials and polymer molecules.     

Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis

A copper chloride (CuCl) electrolyzer that constitutes of composite proton exchange membrane (PEM) that functions at medium-temperature (>100 °C) is beneficial for rapid electrochemical kinetics, and better in handling fuel pollutants. A synthesized polybenzimidazole (PBI) composite membrane from the addition of ZrO₂ followed with phosphoric acid (PA) is suggested to overcome the main issues in CuCl electrolysis, including the copper diffusion and proton conductivity. PBI/ZrP properties improved significantly with enhanced proton conductivity (3 fold of pristine PBI, 50% of Nafion 117), superior thermal stability (>600 °C), good mechanical strength (85.17 MPa), reasonable Cu permeability (7.9 × 10⁻⁷) and high ionic exchange capacity (3.2 × 10⁻³ mol g⁻¹). Hydrogen produced at 0.5 A cm⁻² (115 °C) for PBI/ZrP and Nafion 117 was 3.27 cm³ min⁻¹ and 1.85 cm³ min⁻¹, respectively. The CuCl electrolyzer efficiency was ranging from 91 to 97%, thus proven that the hybrid PBI/ZrP membrane can be a promising and cheaper alternative to Nafion membrane.     

Construction of low-cost and long-time proton exchange membranes by using slow-release radical scavenger

The bottlenecks of commercial application of proton exchange membranes (PEM) fuel cell are cost and oxidation stability of PEM. Hence, we encapsulate Resveratrol (Res, a kind of reductant) in hydroxypropyl-β-cyclodextrins (CDs) to prepare the inclusion complexes of Res and CDs (Res@CDs) under the guidance of theoretical arithmetic. Then the Res@CDs are evenly dispersed in Nafion emulsion, which is subsequently combined with porous polytetrafluoroethylene (PTFE) substrate by emulsion pouring method to form the antioxidative composite membrane (Res@CDs-Nafion/PTFE). The as-prepared Res@CDs-Nafion/PTFE shows the similar performance on proton conductivity (103.9 mS cm⁻¹) and hydrogen-air fuel cell (317.84 mW cm⁻²) compared to the Nafion/PTFE composite membrane. The content of Nafion in the Res@CDs-Nafion/PTFE is less than 30%, which dramatically reduces the production cost compared to pure Nafion membrane. The weight loss of Res@CDs-Nafion/PTFE and Nafion/PTFE immersed in Fenton's reagent after 36 h is 4.97% and 16.49%, respectively, which demonstrate that Res@CDs can enhance oxidation stability of composite membrane. The Res@CDs-Nafion/PTFE offer huge merits of low cost and enhanced oxidation stability, which greatly promotes the application process of long-lifetime PEM fuel cell.     

Robust proton exchange membrane for vanadium redox flow batteries reinforced by silica-encapsulated nanocellulose

High ion selectivity and mechanical strength are critical properties for proton exchange membranes in vanadium redox flow batteries. In this work, a novel sulfonated poly(ether sulfone) hybrid membrane reinforced by core-shell structured nanocellulose (CNC-SPES) is prepared to obtain a robust and high-performance proton exchange membrane for vanadium redox flow batteries. Membrane morphology, proton conductivity, vanadium permeability and tensile strength are investigated. Single cell tests at a range of 40–140 mA cm⁻² are carried out. The performance of the sulfonated poly(ether sulfone) membrane reinforced by pristine nanocellulose (NC-SPES) and Nafion® 212 membranes are also studied for comparison. The results show that, with the incorporation of silica-encapsulated nanocellulose, the membrane exhibits outstanding mechanical strength of 54.5 MPa and high energy efficiency above 82% at 100 mA cm⁻², which is stable during 200 charge-discharge cycles.     

Nafion/SiO2 hybrid membrane for vanadium redox flow battery

Sol–gel derived Nafion/SiO₂ hybrid membrane is prepared and employed as the separator for vanadium redox flow battery (VRB) to evaluate the vanadium ions permeability and cell performance. Nafion/SiO₂ hybrid membrane shows nearly the same ion exchange capacity (IEC) and proton conductivity as pristine Nafion 117 membrane. ICP-AES analysis reveals that Nafion/SiO₂ hybrid membrane exhibits dramatically lower vanadium ions permeability compared with Nafion membrane. The VRB with Nafion/SiO₂ hybrid membrane presents a higher coulombic and energy efficiencies over the entire range of current densities (10–80 mA cm⁻²), especially at relative lower current densities (<30 mA cm⁻²), and a lower self-discharge rate compared with the Nafion system. The performance of VRB with Nafion/SiO₂ hybrid membrane can be maintained after more than 100 cycles at a charge–discharge current density of 60 mA cm⁻². The experimental results suggest that the Nafion/SiO₂ hybrid membrane approach is a promising strategy to overcome the vanadium ions crossover in VRB.     

UiO-66-NH2 functionalized cellulose nanofibers embedded in sulfonated polysulfone as proton exchange membrane

Metal–organic frameworks (MOFs) exhibit high proton conductivity, thermal stability, and offer immense flexibility in terms of tailoring their size. Owing to their unique characteristics, they are desirable candidates for proton conductors. Nevertheless, constructing ordered MOF proton channels in proton exchange membranes (PEMs) remains a formidable challenge. Herein, blend nanofibers of cellulose and UiO-66-NH₂ (Cell–UiO-66-NH₂) obtained via the electrospinning process were embedded in a sulfonated polysulfone matrix to obtain high-performance composite PEMs with an orderly arrangement of UiO-66-NH₂. Comprehensive characterization and membrane performance tests reveal that composite membrane with 5 wt% (nominal) UiO-66-NH₂ have revealed high proton conductivity of 0.196 S cm^?1 at 80 °C and 100% relative humidity. Meantime, the composite membrane exhibits a low methanol permeability coefficient (~5.5 × 10^?7 cm² s^?1). Moreover, the composite membrane exhibits a low swelling ratio (17.3%) even at 80 °C. The Cell–UiO-66-NH₂ nanofibers exhibit strong potential for use as a proton-conducting nanofiller in fuel-cell PEMs.     

Synthesis of (Si-PWA)-PVA/PTFE high-temperature proton-conducting composite membrane for DMFC

The inorganic silica immobilized PWA based (Si-PWA)-PVA/PTFE composite membrane was developed by an amalgamation of pore filling and layer by layer (LBL) casting. The composition of the top layer was optimized to be 0.3 M PWA: 0.2 M TEOS: 0.15PVA concerning to proton-conductivity and methanol permeability of the membrane. Surface morphological studies and elemental analysis were carried out by using SEM-EDX. The FT-IR and XRD analysis had confirmed the intercalation of sol with PTFE. Thermal deformation of the membrane was studied by TGA and it is stable up to 180 °C. Ion exchange capacity and water uptake were determined to be 2.38 meq per gram and 21.7%. The membrane has exhibited maximum proton conductivity of 41.2 mS cm⁻¹ at 100 °C. The membrane has significantly lower methanol permeability of 3.2 × 10⁻⁷ cm² S⁻¹ compared to that of Nafion117 (7.9 × 10⁻⁷ cm² S⁻¹) at 28 °C and the same trend was observed at 40, 60, and 80 °C. The (Si-PWA)-PVA/PTFE composite membrane is showed enhanced proton conductivity and lower methanol permeability at elevated temperatures.     

A facile approach of fabricating proton exchange membranes by incorporating polydopamine-functionalized carbon nanotubes into chitosan

Recently, fabricating low-cost and high-performance proton exchange membranes (PEMs) with carbon nanotubes (CNTs) and chitosan (CS) have been extensively studied. Herein, we reported a facile but effective method to prepare CS-based PEM with polydopamine (PDA) functionalized CNTs (PDA@CNTs). The CS/PDA@CNTs composite membranes were prepared by incorporating PDA@CNTs into CS matrix followed by ion cross-linking with sulfuric acid. Due to the hydrogen bonding between the CS matrix and PDA coating, the thermal, mechanical and oxidation stability of the CS/PDA@CNTs composite membranes were improved. Moreover, the proton conductivity was also improved owing to the improved dispersion of PDA@CNTs in the CS matrix and the electrostatic interaction between PDA@CNTs and CS matrix. The maximum proton conductivity can reach to as high as 0.028 S cm⁻¹ under 80 °C with 2 wt% loading of PDA@CNTs. These results indicate that the CS/PDA@CNTs composite membranes have great potential to be used as PEMs in fuel cells.     

Control of self-organization of drop-casted Nafion film for improving proton conduction in a polymer-electrolyte-membrane fuel cell to raise its output power density

A simple drop-cast method to directly deposit Nafion polymer electrolyte membrane (PEM) on nanostructured thin-film catalyst layer composed of stacked Pt nanoparticles prepared by pulsed laser deposition (PLD) was demonstrated. Through optimization of solvent composition and drying temperature of Nafion solution to control self-organization of Nafion, a uniform PEM with better bulk and interface microstructures could be produced, leading to a significant improvement in the output current density of a PEM fuel cell over that using reference commercial PEMs. The formation of facile proton conduction pathways in the bulk Nafion membrane resulted in a 35% reduction in ohmic resistance compared to that with the commercial membrane. Moreover, the infiltration of Nafion in the catalyst layer formed suitable proton transport network to render more catalyst nanoparticles effective and thus lower charge-transfer resistance. With the optimized PLD, drop-cast, and hot-pressing conditions, the current density of PEMFCs using drop-casted PEM reached 1902 mA cm⁻² at 0.6 V at 2 atm H₂ and O₂ pressures with a cathode Pt loading of 100 μg cm⁻², corresponding to a power density of 1.14 W cm⁻² and a cathode mass-specific power density of 11.4 kW g⁻¹.     

Constructing interconnected nanofibers@ZIF-67 superstructure in nafion membranes for accelerating proton transport and confining methanol permeation

A high-performance proton exchange membrane was successfully prepared by incorporating an interconnected PMIA nanofiber@ZIF-8 network (ZHNFs) into Nafion solution, in which the unique ZHNFs were fabricated via in-situ growth of ZIF-67 on the surface of hierarchical PMIA nanofibers (HNFs) with multiscale nanofibers and high hydrophilicity. The hybrid membrane presents an outstanding performance with a peak proton conductivity of 0.277 S cm⁻¹ at 80 °C, 100RH% and a decreased methanol permeability of 1.415 × 10⁻⁷ cm² s⁻¹, implying a promising application in direct methanol fuel cells. The superior performance of the membranes could be due to the interconnected structure, high specific surface area of 122.637 m² g⁻¹ and active chemical bond of (−N–H) of ZHNFs. Specifically, the 3D interconnected structure of ZHNFs provides consecutive conduction path for protons, ensuring the improvement of proton conductivity. The effective interfacial acid-base pairs (−N–H ^… −SO₃H) formed via the tight interactions between the –N–H bonds in ZHNFs and –SO₃H groups in Nafion matrix could effectively ameliorate the compatibility of the nanofiber fillers and Nafion matrix, further promoting the methanol barrier of the hybrid membranes. Moreover, the generated acid-base pairs are beneficial for the efficient and rapid proton transfer via providing abundant proton conducting sites.     

Bioinspired layered proton-exchange membranes with high strength and proton conductivity

High-performance proton exchange membranes (PEMs) are crucial to the overall performance of PEM fuel cells. However, there always exists a conflict between the high ion conductivity and mechanical properties of traditional PEMs. Herein, we have demonstrated a simple bioinspired strategy for fabricating nacre-inspired layered membrane (GPS-X) based on GO sheets by crosslinking two kinds of long-chain polymers (PDA and SPVA) via vacuum filtration. Strikingly, the resulting GPS-50 membrane is endowed with high tensile strength (216.5 MPa) and high proton conductivity (0.303 S cm^?1 at 80 °C) due to the combination of strong interfacial interactions and well-developed 2D channels, which surpasses Nafion and many other reported GO-based PEMs. Furthermore, the membrane exhibits higher output-power but lower weight than Nafion 212. Considering the achieved excellent properties induced by the independently adjustable 2D channels and mechanical properties, the bioinspired strategy offers guidance for constructing advanced membrane materials with potential applications in fuel cells.     

Cross-linked membranes based on sulfonated poly (ether ether ketone) (SPEEK)/Nafion for direct methanol fuel cells (DMFCs)

A series of cross-linked membranes based on SPEEK/Nafion have been prepared to improve methanol resistance and dimension stability of SPEEK membrane for the usage in the direct methanol fuel cells (DMFCs). Sulfonated diamine monomer is synthesized and used as cross-linker to improve the dispersion of Nafion in the composite membranes and decrease the negative effect of cross-linking on proton conductivity of membranes. FT-IR analysis shows that the cross-linking reaction is performed successfully. The effects of different contents of Nafion on the properties of cross-linked membranes are investigated in detail. All the cross-linked membranes show lower methanol permeability and better dimensional stability compared with the pristine SPEEK membrane. SPEEK-N30 with the 30 wt % Nafion shows a methanol permeability of 0.73 × 10⁻⁶ cm² s⁻¹ and a water uptake of 24.4% at 25 °C, which are lower than those of the pristine membrane. Meanwhile, the proton conductivity of SPEEK-N30 still remains at 0.041 S cm⁻¹ at 25 °C, which is comparable to that of the pristine SPEEK membrane. All the results indicate that these cross-linked membranes based on SPEEK/Nafion show good prospect for the use as proton exchange membranes.     

Incorporating graphene oxide to improve the performance of Nafion-mordenite composite membranes for a direct methanol fuel cell

The proton exchange membrane is one of the critical parts of a direct methanol fuel cell. High proton conductivity and low methanol permeability are required. To enhance the performance of a direct methanol fuel cell, graphene oxide was incorporated to Nafion-mordenite composite membranes to enhance the compatibility and to decrease methanol permeability. It was found that the membrane with silane grafted on graphene oxide-treated mordenite with a graphene oxide content of 0.05% presented the highest proton conductivity (0.0560 S·cm⁻¹, 0.0738 S·cm⁻¹ and 0.08645 S·cm⁻¹ at 30, 50, and 70 °C, respectively). This was about 1.6-fold of the recast Nafion and commercial Nafion 117 and was about 1.5-fold of that without graphene oxide incorporation. Finally, the operating condition was optimized using response surface methodology and the maximum power density was investigated. Power density of about 4-fold higher than that of Nafion 117 was obtained in this work at 1.84 M and 72 °C with a %Error between the model prediction and the fuel cell experiment of 0.082%.