Excellent Proton Conductivity Research Articles

Efficient and stable proton conduction achieved by accommodation of the membrane-wide cross-linking and branching strategies

Fabrication of high-temperature proton exchange membranes (HTPEMs) with high and durable proton conductivity, and sufficient mechanical/oxidative stability is of great significance but challenging. Herein, a flexible branched polymer (broPBI), a cross-linkable water-soluble proton conductor (PBSA), and a multi-functional cross-linker with high content of imidazole rings (BrABPBI) were designed to fabricate HTPEMs with good comprehensive performances. The branching structure of broPBI provided large free volume, high content of terminal groups, and interconnected main-chains, which are conducive to efficient proton conduction and improved mechanical property. BrABPBI covalently crosslinked broPBI and PBSA to form membrane-wide cross-linking structure, improving the mechanical strength and oxidative resistance while avoiding leaching of PBSA. The secondary amines in broPBI and PBSA became tertiary amines after cross-linking, which have higher basicity and thus higher doping capacity of PBSA. However, overusing of cross-linking or branching can deteriorate membrane performances. To achieve comprehensive good performance, the two strategies were appropriately accommodated in broPBI-BrABPBI-PBSA membranes. At 180 °C, the proton conductivity of broPBI-BrABPBI(20%)-PBSA(50%) composite membrane reached 0.140, 0.0701, and 0.0310 S cm−1 at 100% RH, 50% RH, and 0% RH, respectively. The membrane has excellent mechanical properties, proton conductivity, and oxidative stability, with low leaching of PBSA, demonstrating promising application prospects.

Proton Conduction Properties of Intrinsically Sulfonated Covalent Organic Framework Composites

The long–term stability of proton conductors is one of the most important factors in evaluating materials. Guest molecules can act as “bridges” for proton conduction channels and reside in the channels of covalent organic frameworks, but they are prone to leakage. Therefore, it is important to develop proton conductors with intrinsic proton conductivity. In this paper, we synthesized an intrinsically sulfonated covalent organic framework, TpPa–SO3H, which has a more stable proton conducting performance than that of TpPa@H2SO4 by loading guest molecules. Meanwhile, the proton conductivity of TpPa–SO3H was further improved by coating a superabsorbent polymer through an in situ reaction to obtain PANa@TpPa–SO3H (PANa: sodium polyacrylate). As a result, the modified composite exhibits an ultrahigh proton conductivity of 2.33 × 10−1 S cm−1 at 80 °C under 95% relative humidity (RH). The stability of PANa@TpPa–SO3H makes it an efficient proton transport platform with excellent proton conductivity and long–term durability.

High Proton Conductivity of the UiO-66-NH2-SPES Composite Membrane Prepared by Covalent Cross-Linking.

A sulfonated poly(ethersulfone) (SPES)-metal-organic framework (MOF) film with excellent proton conductivity was synthesized by anchoring UiO-66-NH2 to the main chain of the aromatic polymer through the Hinsberg reaction. The chemical bond was formed between the amino group in MOFs and the -SO2Cl group in chlorosulfonated poly(ethersulfones) to conduct protons in the proton channel of the membrane, making the membrane have excellent proton conductivity. UiO-66-NH2 is successfully prepared as a result of the consistency of the experimental and simulated powder X-ray diffraction (PXRD) patterns of MOFs. The existence of absorption peaks of characteristic functional groups in Fourier transform infrared (FTIR) spectra proved the successful preparation of SPES, PES-SO2Cl, and a composite film. The results of the AC impedance test indicate that the composite film with a 3% mass fraction has the best proton conductivity of 0.215 S·cm-1, which is 6.2 times higher than that of the blended film without a chemical bond at 98% RH and 353 K. To our knowledge, there are rarely any reports on the preparation of a composite membrane by directly linking MOFs and the membrane matrix with chemical bonds. This work provides a good way to synthesize the highly conductive proton exchange film.

Molecular Complexation of Polymers and Metal Oxide Clusters for Semicrystalline, Thermoplastic Anhydrous Proton Exchange Membranes in Fuel Cells.

With the manipulation of surface charges and loadings, 1 nm super-acidic metal oxide clusters can co-crystallize with poly(ethylene glycol) (PEG) at molecular scale for thermoplastic anhydrous proton exchange membranes (PEMs). The coexistence of crystalline and amorphous regions endows the PEMs with a high Young's modulus and high flexibility, while the noncovalent complex interactions enable facile preparation and (re)processing. Furthermore, the diffusive dynamics of PEG chains is slowed by the confinement effect, while the local segmental dynamics is accelerated due to the transition of the chain conformation from helix to zigzag when confined in the crystalline framework. This greatly facilitates proton transportation in the crystalline region for an excellent anhydrous proton conductivity of 4.5 × 10-3 S cm-1 at 90 °C. The balanced proton conductivity, mechanical strength, and processability of the PEMs contribute to the promising power density of H2/O2 fuel cells assembled with co-crystalline PEMs at high temperatures under dry conditions.

Preparation and performance study of ionic liquid @ boron nitride modified polyimide proton exchange membrane

AbstractProton exchange membrane (PEM) plays a relatively significant part in PEM fuel cells (PEMFCs). In this study, ionic liquid were grafted onto boron nitride (ILs@BN) were prepared by wet ball milling, and the polyimide (PI) composite films with different contents (1, 2, 3, 4, and 5 wt%) of ILs@BN were fabricated by solution‐casting method. Results showed that compared with the original polyimide films, the performance of the obtained composite films has been significantly improved. Tensile strength of film containing 4 wt% ILs@BN was enormously enhanced by 93.62% (from 47 to 91.8 MPa), respectively, in contrast to pure PI film. Compared with pure PI film, film containing with 4 wt% ILs@BN was reduced from 71.2° to 63.2°, which indicated the better hydrophilicity of the film and potential proton transport channels. The composite film exhibited excellent proton conductivity, which was tremendously elevated by 58.35% (from 0.1215 S cm−1 for pure PI to 0.1924 S cm−1 for film containing with 4 wt% ILs@BN). Comprehensively, the film containing with 4 wt% ILs@BN presenting excellent mechanical properties (91.8 MPa), contact angle (63.2°) and proton conductivity (0.1924 S cm−1) demonstrating strong potential applications in fuel cells.

Polydopamine-coated polyimide nanofiber to anchor HPW and construct the pocket-like composite membrane with excellent proton conductivity and stability

Solid heteropolyacids with high intrinsic proton conductivity, like phosphotungstic acid (HPW), were added to further enhance the proton conductivity of sulfonated poly ether ketone (SPEEK). The great solubility of solid acid within water, however, would produce a dramatic decline in stability and durability. In this study, we utilized polydopamine (PDA) coated polyimide (PI) nanofiber to form the acid-base pair between PDA@PI nanofiber and HPW/SPEEK to anchor the HPW. Additionally, a pocket was constructed in the sandwich-structure PDA@PI/SPEEK + HPW membrane to help better anchor the HPW. After 6 weeks of durability treatment during 60 °C and 100% RH, proton conductivity of PDA@PI/SPEEK + HPW-20 was 163.5 mS cm−1, which was only 23.0% lower than that of the initial state. By sharp contrast, it was 55.2% lower than the initial state for pristine SPEEK. And its swelling ratio is only 54% compared with 128% of pristine SPEEK under the same condition. We first revealed and investigated the acid-base pair created by PDA@PI and HPW/SPEEK, which was beneficial for proton conductivity and durability. This pocket-like PDA@PI/SPEEK + HPW membrane would inspire a wide application in PEM among excellent proton conductivity, better stability, with endurance.

Parallelogram 3d-4f-5d heterometallic clusters based on trilacunary tungstoantimonates with excellent proton conductivity

Two 3d-4f-5d heterometallic cluster-containing polyoxometalates, formulated as Na22{(SbW9O33)4[La3W6MO18(H2O)8(CH3COO)4]2}·nH2O (abbreviated as La6M2, M = Co/Mn) were synthesized and structurally characterized. Single-crystal X-ray diffraction analyses reveal that the polyanions of La6Co2 and La6Mn2 consist of the uncommon 3d-4f-5d clusters {La6W12Co2} and {La6W12Mn2}, which are encapsulated by four trilacunary Keggin tungstoantimonates to form the parallelogram-shaped title compounds. Additionally, the polyanions can be extended into a two-dimensional (2D) frame by the linkage of peripheral Na+ ions. The inner space of the 2D layer was filled with water molecules and thus an H-bonded network was formed, which is expected to exhibit a fascinating proton conductivity. The study of water-assisted proton conduction demonstrated that La6Co2 and La6Mn2 were temperature- and humidity-dependent proton conductors, respectively, and the proton conductivities could reach 1.3 × 10−2 and 2.3 × 10−2 S/cm at 65 °C and 90% RH conditions.

Optimization of the ionomer-to-carbon ratio in the cathode catalyst layer of PtCoMn/C alloy catalysts

The optimization of membrane electrode assembly (MEA) structures based on alloy catalysts can further improve the properties of proton exchange membrane fuel cells and accelerate their commercialization. Herein, novel MEAs with ultra-low platinum loading and high performance were prepared by a decal transfer method using a highly active PtCoMn/C alloy catalyst. The effects of different I/C ratios on the performance of PtCoMn/C cathodes were investigated by polarization curves. The results suggested an optimal I/C ratio of 0.9, leading to a maximum power density of 1.51 W/cm2. The effects of the I/C ratio on catalyst activity and proton conductivity were studied by AC impedance and cyclic voltammetry, and the impact of the I/C ratio on local oxygen transport resistance was evaluated by the limiting current method. The data revealed that the MEA has the highest ECSA of CL combined with the lowest Rct and higher proton conductivity,when the I/C ratio is 0.9. An influencing mechanism of ionomer content on performance of low platinum loading MEA prepared by PtCoMn/C was then drawn, suggesting that The highest activity at the I/C ratio of 0.9 mainly depends on its excellent proton conductivity.

Enhanced proton conductivity and overall water splitting efficiency of dye@MOF by post-modification of MOF

Herein, dye@MOF composite was prepared by the encapsulation of 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (HPTS) into the pores of metal-organic framework (MOF). Firstly, the proton conductivities (σ) of MOF and dye@MOF were evaluated, the σ value of dye@MOF is 10 times that of MOF at 100 ​°C, which indicates dye@MOF has excellent proton conduction capacity. Secondly, MOF and dye@MOF show excellent catalytic performances for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under alkaline medium, that is, when the current density is 10 ​mA•cm−2, the overpotentials of MOF and dye@MOF are 225 ​mV and 194 ​mV for OER, and 511 ​mV and 497 ​mV for HER, respectively. Moreover, an asymmetric electrolyzer of dye@MOF/nickel foam (NF)||dye@MOF/NF was assembled to assess the efficiency of dye@MOF for overall water splitting, the result shows that the current density of dye@MOF is 35 ​mA•cm−2 at 1.98 ​V, which exceeds that of MOF-based electrode. The excellent catalytic activity and enhanced proton conductivity of dye@MOF is mainly attributed to encapsulating dye molecules into the channels of MOF. In view of superior catalytic activity and excellent stability, dye@MOF is expected to be a bifunctional electrocatalyst for HER and OER.

Augmentation in proton conductivity of crosslinked poly(vinyl alcohol) through the introduction of polyvinyl pyrrolidone

AbstractWe aimed to augment the proton conductivity of poly(vinyl alcohol) (PVA) by crosslinking polyvinyl pyrrolidone (PVP) with PVA without any crosslinkers for the first time to form polyvinyl ketal (PVKL). Several characterization techniques were used, including Fourier transform infrared (FT‐IR), proton nuclear magnetic resonance (1H‐NMR), X‐ray diffraction (XRD), and thermogravimetric analysis (TGA). To investigate the wettability and proton conductivity performances of the PVKL film, the contact angle, water uptake, and proton conductivity were also evaluated. The PVKL film exhibited excellent water uptake ability (>400%); its proton conductivity was 100 mS cm−1 at 110°C. The proton conductivity of PVKL film increases because PVKL film has a larger charge carrier concentration owing to ketal ring formation. The results point to the longer lifespan for the PVKL film and reveal that the film exhibits excellent proton conductivity rather than PVA film. Thus, it can be potentially used in various optoelectronic applications.

Dual functionalized graphene oxide incorporated with sulfonated polyolefin proton exchange membrane for fuel cell application

This study presented a strategy of incorporating hydrocarbon-based proton exchange membrane (PEM) with dual-functionalized GO attached phosphonic and sulfonic functional groups to improve proton conductivity, ion exchange capacity (IEC) and methanol permeability for direct methanol fuel cell application. It was found that the proton conductivity, IEC of PEM and fuel cell power density enhanced remarkably through doping functionalized GO. The PEM doped with 8% PS-MGO exhibited the IEC value of 2.06 mmol g−1, proton conductivity of 0.084 S cm−1, and power density of 78 mW cm−2 at 25 °C. The methanol permeability of doped PEM was constrained effectively, reduced to 18.42 10−7 cm2 S−1. In addition, the results suggested the properties improvements were proportional to the functionalized GO contents and the improving effects of dual-functionalized GO on PEM were superior to the single functionalized GO. These improvements were attributed to the additional acidic sites offered by functionalized GO construct consecutive proton conduction pathways for facilitating protons transportation and the potential differences between phosphonic and sulfonic groups provided extra energy to drag water-connected protons through the membrane under the vehicular mechanism. This study proposed a promising strategy to use dual functionalized GO preparing PEM with excellent proton conductivity, IEC, power density and methanol permeability properties for fuel cell application.

Molecular Dynamics Study on Swelling and Exfoliation Properties of Montmorillonite Nanosheets for Application as Proton Exchange Membranes

Existing proton exchange membranes have the disadvantages of low proton conductivity at high temperature, high cost, and poor barrier properties. Montmorillonite (MMT) materials have excellent thermal and chemical stability, and two-dimensional nanostructures formed by exfoliated MMT show excellent proton conductivity. Therefore, they are expected to become new proton exchange membrane materials. However, the existing exfoliation methods still lack a unified theoretical guidance. In this work, we systematically investigate the detailed swelling and exfoliation properties of MMT with different intercalation cations via extensive molecular dynamics simulations. We find that the swelling properties are determined by the hydration capacity of interlayer cations, and the exfoliation mainly occurs via a lateral sliding process between adjacent layers. The exfoliation energy barrier strongly depends on the interaction strength between interlayer cations and MMT nanosheets, swelling degree, and surface charge density. Lower interaction strength and a higher swelling degree together with larger surface charge density can result in an easy exfoliation process. Our work unveiled the detailed mechanism of the exfoliation process of MMT nanosheets and may contribute to the search for efficient MMT exfoliation methods.

Phosphoric acid-doped Gemini quaternary ammonium-grafted SPEEK membranes with superhigh proton conductivity and mechanical strength for direct methanol fuel cells

Proton exchange membranes (PEMs) with excellent proton conductivity, high mechanical strength, and low methanol permeability are essential for their application in direct methanol fuel cells (DMFCs). Herein, flexible alkyl side chains containing Gemini quaternary ammonium (QA) ions were grafted onto sulfonated poly(ether ether ketone) (SPEEK) to obtain novel amphiphilic polymers (S-DMEAx-BPT), and then doped with phosphoric acid (PA) to prepare S-DMEAx-BPT/PA membranes. PA immobilized on flexible side chains can possess more mobility and higher proton conduction ability when compared with PA bound to rigid main chains, thus contributing to the extremely high proton conductivity of S-DMEAx-BPT/PA (295.4 mS cm-1, 80 °C, 100% R.H) even at low PA doping levels. Moreover, with the help of abundant ionic crosslinking, the membranes had excellent dry and wet mechanical properties. Benefiting from the negligible PA leaching, these membranes could be used as the electrolyte in DMFCs and output the peak power density of 110.7 mW cm-2, which was 2.1 times that of Nafion 212 (52.8 mW cm-2). Moreover, the open-circuit voltages (OCV) retention remained at 82.4% after 140 h. Therefore, the strategy combining Gemini QA cations grafting, flexible side chains, and low-level PA doping provides a new methodology for fabricating high-performance PEMs for DMFCs.

Graphene Oxide-Intercalated Microbial Montmorillonite to Moderate the Dependence of Nafion-Based PEMFCs in High-Humidity Environments

The most fatal disadvantage of Nafion-based proton-exchange membrane fuel cells (PEMFCs) is their significant performance losses at low relative humidities (RH ≤ 80%), making external humidification necessary. In this work, a graphene oxide (GO)-intercalated microbial montmorillonite (mMMT) layered stack (GO@mMMT) was prepared to enhance the proton conductivity of a Nafion membrane under a low humidity at elevated temperatures. The prepared mMMT had a high specific surface area and pore volume due to microbial mineralization, allowing GO to act as a spacer intercalated between MMT layers. This layered stack greatly enhanced the water absorbance and retention capacity of GO@mMMT/Nafion composite membranes. The GO@mMMT/Nafion composite membrane exhibited excellent proton conductivity under various humidities. Particularly, the 0.5GO@mMMT/Nafion sample achieved a proton conductivity of 36.4 mS·cm–1 at 80 °C/98% RH and 17.3 mS·cm–1 at 80 °C/20% RH, which was 82% and 188% higher than that of the recast Nafion membrane, respectively. The assembled single cell reached a peak power density of 546 mW·cm–2, which was 60% higher than that of the recast Nafion single cell. These results indicate that the Nafion composite membranes with GO@mMMT incorporated layered stacks show substantial potential for PEMFC applications with simplified water management.

Construction of high-density proton transport channels in phosphoric acid doped polybenzimidazole membranes using ionic liquids and metal-organic frameworks

Introducing metal-organic frameworks (MOFs) into phosphoric acid (PA) doped polybenzimidazole (PBI) membranes to form proton transfer channels is an effective strategy to reduce the dependence of proton conductivity on PA. However, excessive MOF loading inevitably leads to reduced mechanical properties and damaged membrane structure. Ionic liquids (ILs) are reported as good plasticizers that can obviously improve the interface performance. Along this line, IL is introducing into the branched poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (BOPBI) cross-linked by KH-560. The interface performance between the MOF and PBI in the cross-linked BOPBI (CBOPBI)@MOF composite system is improved. The high MOF loading (50 wt%) composite membranes are prepared with acceptable mechanical properties for the first time. High density proton transport channels are constructed based on the high MOF loading in the PA doped CBOPBI@MOF membranes. For instance, the CBOPBI@MOF50%-IL30 shows an excellent proton conductivity (0.135 S cm−1), and a power density about a 26.9% increase (736 mW cm−2) compared with that of the CBOPBI@MOF40% membrane. Furthermore, the load voltage of the membrane with 50 wt% MOF is also well maintained. The membranes with high density proton transport channels exhibit high potential application prospects as high-temperature proton exchange membranes (HT-PEMs).

Transfer-free in-situ synthesis of high-performance polybenzimidazole grafted graphene oxide-based proton exchange membrane for high-temperature proton exchange membrane fuel cells

The preparation of traditional graphene oxide (GO)-based proton exchange membrane (PEM) composites asks for multiple transfer steps due to the different solvent requirements of GO preparation and polymer synthesis. Moreover, GO is easy to agglomerate in polymer matrix, it is difficult to give full play to the excellent proton conductivity of single/few-layered nanosheets. Herein, a novel transfer-free in-situ method is developed to prepare poly(2,5-benzimidazole) (ABPBI)-grafted electrolytic graphene oxide (EGO) composites (ABPBI-EGO) for high-temperature (HT) PEM fuel cells applications. The process involves the in-situ electrochemical oxidation and exfoliation of graphite foil into single/few-layered EGO in Eaton's reagent and followed by in-situ synthesis of ABPBI-grafted-EGO composites in the same reagent. This strategy leads to a single or few-layered dispersion of EGO nanosheets in ABPBI matrix, thus significantly improving the physicochemical performance. By constructing fast proton-conducting channels through single/few-layered GO nanosheets, composite membranes without free phosphoric acid exhibit superior proton conductivities over the whole temperature range of 120–180 °C. ABPBI-0.5EGO composite membrane-based fuel cell achieves a power density of 349 mW/cm2 at 180 °C, which is nearly 2.3 times higher than the original ABPBI membrane. Thus, this transfer-free in-situ strategy shows great promise for developing high-performance HT-PEMs.

Covalent Pyrimidine Frameworks via a Tandem Polycondensation Method for Photocatalytic Hydrogen Production and Proton Conduction.

The development of heteroaromatic conjugated porous polymers (H-CPPs) have received enormous research interests, because of the important functional roles of the heteroatoms in photocatalysis and proton conduction. However, due to the synthetic challenges deriving from the stable structures, the structural diversity and synthetic methods of them are still limited. Herein, a new type of H-CPPs, covalent pyrimidine frameworks (CPFs), via an efficient tandem polycondensation reaction between aldehyde, acetyl, and amidine monomers is reported. The resulting CPFs are bridged by pyrimidine units, rich of nitrogen atoms and can be structurally regulated on demand. The CPFs are shown to be active photocatalysts for hydrogen evolution from methanol via a photo-thermo-catalysis process, achieving an excellent hydrogen evolution rate of 5282.8µmolh-1 g-1 . The CPFs can be further processed into a mixed matrix membrane, displaying an excellent proton conductivity of 1.30×10-2 Scm-1 at 413K under anhydrous condition.

Investigation of sulfonation degree and temperature on structure, thermal and membrane's properties of sulfonated poly (ether ether ketone)

In this paper, the primary and secondary relationship among factors affecting the sulfonation of poly (ether ether ketone) (PEEK) was investigated in detail by a response surface method in order to solve the problem of performance deterioration caused by excessively high or low sulfonation. Acid-base titration, infrared analysis (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) proved that temperature had the greatest influence on PEEK sulfonation and the sulfonate group was successfully connected to the main chain and the proton conduction mechanism was in accordance with Arrhenius law. Under optimized conditions, 44% sulfonation degree of PEEK was selected to test the dimensional stability, microstructure and proton conductivity, and to explore the influence of temperature on the membrane's structure and performance. The result indicates that the microstructure has become more homogeneous at 60 °C, resulting in continuous proton conduction channels, demonstrating excellent proton conductivity (5.04 × 10−2 S/cm).

Proton exchange membrane based on interpenetrating polymer network structure for excellent cell performance and chemical stability

Chemical stability and proton conductivity are the most critical factors for proton exchange membranes (PEMs). Here, we designed and prepared a co-polymer containing radical scavenger groups and imidazole groups. The membrane's acid-base crosslinking and interpenetrating network structure offer the PEM excellent dimensional stability and gas permeation inhibition properties. The continuous acid-base pairs and imidazole groups, which have high water retention, effectively improve the proton conductivity and reduce the PEM's gas permeability. Hence, a membrane electrode assembly based on as-prepared composite membranes exhibits higher single-cell performance and lower impedance than other membranes used for comparison. In addition, the introduction of hindered amine scavenger groups effectively eliminates the free radicals generated in the process of the electrochemical reaction. Both in-situ and ex-situ stability experiments show that the prepared composite membrane has excellent chemical stability. Morphological changes according to degradation level are also systematically analyzed. This paper shows that the introduction of imidazole and HA groups in the PFSA matrix membrane is effective for improving fuel cell performance and lifetime and for constructing continuous proton transport networks and interpenetrating network structures, making this a promising approach for achieving excellent proton conductivity and chemical stability in PEMs.

High proton conduction in two highly stable phenyl imidazole dicarboxylate-based Cd(II)-MOFs

In recent years, carboxylic acid-based MOFs have gathered a lot of attention because of their high stability and excellent proton conductivity, and are likely to be promising candidates as proton-exchange membranes. In this work, we synthesized two MOFs by reference method, namely [Cd4(μ3-PhIDC)2(μ4-PhIDC)2(H2O)]n (1) (H2PhIDC ​= ​2-phenyl-4,5-imidazole dicarboxylic acid), [Cd2(μ2-PhIDC)2(phen)2]n (2) (phen ​= ​1,10-phenanthroline), which have high water stability. Therefore, this work focuses on their intrinsic proton conductivity. The results showed that under the conditions of 98% RH/100 ​°C, the proton conductivities of 1 and 2 can achieve 2.71 ​× ​10−3 and 2.67 ​× ​10−3 ​S ​cm−1, respectively. Finally, based on the structural analysis and calculations of activation energies, the principle of proton transfer is inferred, which will afford new inspiration for the design of the proton conduction crystalline materials.