Enhanced Proton Conductivity Research Articles

Lyotropic ionic liquid crystal gels derived from polyimidazolium hydrogen sulfate for enhanced proton conductivity

AbstractAnisotropic gels developed from polymer networks accommodating lyotropic liquid crystals are promising electrolytes but electrochemically inert polymers prevent ions from being conducted efficiently. In this work, we synthesized protic ionic liquid polymer in lyotropic ionic liquid crystal to improve proton conduction across the polymer skeleton in anisotropic gels. In situ polymerization of 1‐vinylimidazolium hydrogen sulfate ionic liquid dissolved in aqueous 1‐tetradecyl‐3‐methylimidazolium hydrogen sulfate lyotropic liquid crystal gave chemical liquid crystal gels exhibiting both dynamic order and mechanical strength. The protic ionic liquid polymer bridged the hydrogen bond chains between neighboring ionic lyotropic liquid crystal domains to give a proton conductivity of 117 mS cm−1 at 25°C. Macroscopic alignment of the liquid crystal gel achieved by mechanical shearing promoted proton conductivity to 137 mS cm−1. The macroscopically aligned liquid crystal gel acted stably in a fuel cell device, delivering a peak power density 2.95 times higher than the unaligned one. Gelation of lyotropic ionic liquid crystals with protic ionic liquid polymer provides an effective strategy to develop anisotropic gel electrolytes for electrochemical devices.

Construction of Successive Proton Conduction Channels to Accelerate the Proton Conduction Process in Flexible Proton Exchange Membranes.

Successive proton conduction channels are constructed with the spin coating method in flexible proton exchange membranes (PEMs). In this research, phosphoric acid (PA) molecules are immobilized in the multilayered microstructure of Kevlar nanofibers and polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) polymer molecular chains. As a result, successive proton conduction channels can accelerate the proton conduction process in the prepared membrane with the multilayered microstructure. Additionally, the microstructure fractures of the composite membranes from the external force of folding and stretching operations are modified by the inner PA molecules. Notably, numerous PA molecules are further combined through formed intermolecular hydrogen bonding. The stretched membrane absorbs more PA molecules owing to the arrangement of PA molecules, Kevlar nanofibers, and SEBS molecular chains. The stretched membrane thus exhibits the enhanced proton conduction ability, such as the through-plane proton conductivity of 1.81 × 10-1 S cm-1 at 160 °C and that of 4.53 × 10-2 S cm-1 at 120 °C lasting for 600 h. Furthermore, the tensile stress of PA-doped stretched membranes reaches (3.91 ± 0.40)-(6.15 ± 0.43) MPa. A single proton exchange membrane fuel cell exhibits a peak power density of 483.3 mW cm-2 at 120 °C.

Comparative Analysis of Proton Conductivity in Two Zn-Based MOFs Featuring Sulfate and Sulfonate Functional Groups.

Metal-organic frameworks (MOFs) have shown promising potential as proton-conducting materials due to their tunable structures and high porosity. In this study, two novel MOFs had been successfully synthesized, one containing sulfate groups (MOF-1; [Zn4(TIPE)2(SO4)4(H2O)]·5H2O) and the other containing sulfonate groups (MOF-2; [Zn2(TIPE)(5-sip)(NO3)0.66]·0.34NO3·17.5H2O) (TIPE = 1,1,2,2-tetrakis(4-(1H-imidazole-1-yl)phenyl)ethene, H35-sip = 5-sulfoisophthalicacid), and the effect of the two groups on the proton conductivity of Zn-based MOFs had been investigated and compared for the first time. The proton conductivity of these MOFs was systematically measured at different temperatures and humidity conditions. Remarkably, the results revealed significant differences in proton conductivity between the two sets of MOFs. At 90 °C and 98% RH, MOF-1 and MOF-2 achieved optimal proton conductivity of 4.48 × 10-3 and 5.69 × 10-2 S·cm-1, respectively. This was due to the structural differences arising from the presence of different functional groups, which subsequently affected the porosity and hydrophilicity, thereby influencing the proton conductivity. Overall, this comparative study revealed the influence of sulfate and sulfonate groups on the proton conductivity of Zn-based MOFs. This research provided a feasible idea for the development of advanced MOF materials with enhanced proton conductivity and opened up new possibilities for their application in proton devices.

Polynorepinephrine-regulated filler sulfonation toward interfacial reformation and conductive promotion of nanocomposite proton exchange membrane

Interfacial compatibility between filler and matrix substantially affects the overall morphology and performance of nanocomposites. Compared with bare montmorillonite (MMT), surface modified product such as sulfonated MMT (SMMT) has been known to render optimized interfacial compatibility in composite proton exchange membranes (PEMs) for proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). Nevertheless, in-depth understanding and investigation on the sulfonation of MMT are seldom focused, such as uniformity and sufficiency. Herein, these broadly neglected issues are proposed and resolved using an exemplary polynorepinephrine (PNE)-regulated SMMT (SNMMT). Rather than direct sulfonation utilizing the inherent hydroxyl groups of MMT, a PNE coating with ample hydroxyl groups is deposited on MMT beforehand, which results in more homogeneous and sufficient sulfonation of SNMMT. Compared with normal SMMT, this alteration on surficial geography of filler functionalization brings significant benefits when composited with Nafion matrix, including harmonized filler-matrix compatibility, improved mechanical stability and enhanced proton conduction. Moreover, the antioxidative ability of PNE layer enables improved chemical stability of nanocomposite PEM. The Nafion/SNMMT composite PEM achieves 0.275 S cm−1 proton conductivity at 95 °C in water, evidently beyond any previously reported composite PEMs using normal SMMT. Equipped with this reformed nanocomposite PEM, both PEMFC and DMFC display competitive peak power densities (1.121 W cm−2 with H2/O2 at 80 °C/100 % relative humidity (RH) for PEMFC, and 0.228 W cm−2 with 1.6 M methanol/100 % RH O2 for DMFC), and simultaneously show minor performance deteriorations in constant current density operation and RH cycle tests. It would be instructive the specific focus and reformation on the uniformity and sufficiency of filler modification may render significant interface and performance advancements for nanocomposites.

Novel Fluoroboric Acid Additive for Blend Membrane to be Used in PEM Fuel Cell, Characterization Studies, and Performance Test

This study focuses on developing an alternative membrane for PEMFC due to the disadvantages of using Nafion. Fluoroboric acid (FBA) was used as an additive material to SPEEK-PVA blend membranes at different weight ratios (1%, 5%, 7.5%, 10%, and 12.5%), and a synthesis procedure was carried out with the solution-casting. Thermal crosslinking was performed with all membranes. Utilizing FBA, with its highly electronegative fluorine groups, is a novel approach expected to enhance proton conductivity. The structural, morphological, and thermal properties of the synthesized membranes were determined by FTIR, XRD, SEM, TGA-DTG, and DSC. Water uptake capacity (WUC), swelling property, area change, dynamic mechanical analysis, ion exchange capacity (IEC), AC impedance analysis, hydrolytic stability, and oxidative stability analyses were performed for fuel cell applications. Although FBA does not have a crystal structure, the synergy it created with the SPEEK-PVA membrane increased the crystallinity of the membrane and, accordingly, glass transition temperature. SEM images of membranes at a ratio above 7.5% show that agglomerations occur in the structure and this is supported by other analyses. It was determined that the membrane composition with the highest WUC (16.44%), IEC (1.55 meq/g), and proton conductivity (0.57 S/cm) values contained 7.5% FBA from the characterization studies, and a single-cell performance test was actualized with this. 418 mA/cm2 current density and 250.8 mW/cm2 power density were obtained at 0.6 V cell potential, with the membrane containing 7.5% FBA. This study shows that the synthesized membrane, especially the FBA, is a promising option for PEMFC application.Graphical

A highly-selective layer-by-layer membrane modified with polyethylenimine and graphene oxide for vanadium redox flow battery

ABSTRACT A selective composite membrane for vanadium redox flow battery (VRFB) was successfully prepared by layer-by-layer (LbL) technique using a perfluorosulfonic sulfonic acid or Nafion 117 (N117). The composite membrane referred as N117-(PEI/GO)n, was obtained by depositing alternating layers of positively charged polyethylenimine (PEI) and negatively charged graphene oxide (GO) as polyelectrolytes. The physicochemical properties and performance of the pristine and composite membranes were investigated. The membrane showed an enhancement in proton conductivity and simultaneously exhibited a notable 90% reduction in vanadium permeability. This, in turn, results in a well-balanced ratio of proton conductivity to vanadium permeability, leading to high selectivity. The highest selectivity of the LbL membranes was found to be 19.2 × 104 S.min/cm3, which is 13 times higher than the N117 membrane (n = 0). This was translated into an improvement in the battery performance, with the n = 1 membrane showing a 4–6% improvement in coulombic efficiency and a 7–15% improvement in voltage efficiency at current densities ranging from 40 to 80 mA/cm2. Furthermore, the membrane displays stable operation over a long-term stability at around 88% at a current density of 40 mA/cm2, making it an attractive option for VRFB applications using the LbL technique. The use of PEI/GO bilayers maintains high proton conductivity and VE of the battery, opening up possibilities for further optimization and improvement of VRFBs.

A [V6B20]-based architecture incorporating two types of organic amine ligands: innovative additives for enhancing the proton conductivity of Nafion membrane

The novel composite material VB-1/Nafion demonstrates a remarkable enhancement in proton conductivity when compared to pure Nafion.

Examination of Nafion membrane performance with acid-base functionalized graphene oxide.

Functionalized graphene oxide (GO) as a good additive can improve the performance of proton exchange membrane (PEM) via the introduction of various functional groups. How to balance the proton conductivity and durability of membrane based on functionalized GO is a key issue. In this work, benzoic-acid-functionalized GO(BAF-GO) and 1,2,4-triazole-functionalized GO(TF-GO) are employed as doping candidates, and the co-doping effect on membrane performance is investigated by means of experiment and molecular dynamics simulation. Meanwhile, the quantum chemistry method is implemented to explore the interaction between TF-GO, membrane and BAF-GO. The results reveal that the composite membrane exhibits high durability and enhanced proton conductivity. When the doping mass ratio of BAF-GO to TF-GO is 3 : 1, the proton conductivity can be greatly improved, especially under low-humidity conditions. Excessive addition of basic groups does not enhance proton transport.

An unexpected imidazole-induced porphyrinylphosphonate-based MOF-to-HOF structural transformation leading to the enhancement of proton conductivity.

Post-synthetic modification of proton-conducting metal-organic frameworks (MOFs) by loading small molecules capable of generating protons into pores is an efficient approach for developing a new type of material with improved ionic conductivity. Herein, the synthesis, characterization and proton conductivity of a novel electroneutral MOF based on palladium(II) meso-tetrakis(4-(phosphonatophenyl))porphyrinate, IPCE-1Pd, are reported. The exposure of the obtained framework to imidazole by the diffusion vapor method has surprisingly led to its complete crystal-to-crystal MOF-to-HOF transformation, resulting in the formation of a novel hydrogen-bonded organic framework (HOF) IPCE-1Pd_Im, which is the first example of such kind of structural change among all known MOFs. This modification has led to an almost 25-fold increase in the proton conductivity in comparison with the pristine MOF, reaching up to 6.54 × 10-3 S cm-1 at 85 °C and 95% relative humidity, which is one of the highest values among all known porphyrin-based HOFs.

Ionic liquid-functionalized metal organic frameworks and their composite membranes for enhanced proton transport

Post-synthetic modification with ionic liquids is gaining prominence as a highly effective strategy for enhancing proton conductivity in metal–organic frameworks without significant structural changes.

The impact of imidazolium with steric hindrance on the dissociation of phosphoric acid and the performance of high-temperature proton exchange membranes

This study proposes a strategy of blending polyimidazolium with strong steric effect in HT-PEMs in order to promote phosphoric acid dissociation and enhance proton conduction efficiency.

Negative impact of poly(acrylic acid) on proton conductivity of electrospun catalyst layers

The electrospun catalyst layers (E-spun CLs) have attracted great attention recently owing to their decent performance and durability compared to conventional coated CL fabricated by scrape coating. However, proton transport in E-spun CLs is poorly understood, hindering further performance improvements. We unravel herein that the high molecular weight (MW) polymer used to form nanofibers, usually Poly(acrylic acid) (PAA), is a key factor affecting the proton conductivity. Nuclear magnetic resonance (NMR) unravels strong interaction between the ionomer and PAA. The proton conductivity of ionomer membranes with varying PAA content under different ionomer volume fractions and relative humidity (RH) was quantitatively assessed using the four-probe electrochemical impedance spectroscopy (EIS). E-spun CL exhibited lower proton conductivity than coated CL or pseudo catalyst layer (PCL) at the same ionomer volume fraction due to the detrimental effects of PAA. The proton transport tortuosity of E-spun CL increased with RH, likely attributed to the significant swelling ratio of PAA. The proton transport tortuosity of E-spun CL displayed less sensitivity to the ionomer volume fraction, which can be attributed to the homogeneous distribution of the ionomer within the nanofibers. Our work implies that the key to enhancing proton conductivity of E-spun CLs is to remove PAA completely, or to replace PAA with alternatives that would not impair proton transport.

Novel chitosan-ionic liquid immobilized membranes for PEM fuel cells operating above the boiling point of water

Novel chitosan (CS) composite membranes modified with ionic liquids (ILs), namely, 1-Hexyl-3- methylimidazolium tricyanomethanide ([HMIM][TCM]), Diethylmethylammonium methanesulfonate ([DEMA][OMs]), were synthesized to be used as a polymer electrolyte membrane (PEM) in for PEM fuel cells. The ionic conductivities of the prepared membrane samples were evaluated using electrochemical impedance spectroscopy (EIS). The CS is one of the attractive choices of green and sustainable membrane materials in PEMs because of its chemical tunability and cost-effectiveness. The membranes were prepared by solution casting followed by a solvent evaporating procedure. The low proton conductivity of pristine CS biopolymer was enhanced by the incorporation of ILs in the membrane framework. Tensile testing confirmed the excellent membrane mechanical integrity. A significant enhancement in the membranes’ characteristics was observed upon the introduction of [DEMA][OMs] into its matrix. The experimental results showed enhanced proton conductivities of the prepared membranes along with enhanced flexibility. The proton conductivity of pristine CS membrane was increased from 8.4 × 10−4 to 1.25 × 10−2 S/cm and 1.1 × 10−2 S/cm when a 30 wt% [HMIM][TCM] and 100 wt% [DEMA][OMs] were added, respectively. Synergistic effects of amine and imidazolium-based ILs on the electrical, structural, water uptake, and thermal properties of CS-based biopolymer solid electrolyte membranes are reported for the first time in this work. The aforementioned characterization results demonstrate the potential of using these membranes in PEM fuel cells operating above the boiling point of water and at a lower cost. This work showed that there should be an optimization amount of the ILs content in the membrane to maintain its mechanical integrity.

Interfacial Water Molecules as Agents for Phase Change Control and Proton Conductivity Enhancement in the Ammonium Vanadyl Tartrate System.

This study demonstrates the reversible structural transformation, single-crystal-to-single-crystal, of the ammonium vanadyl (L-tartrate) complex salt from the hydrate phase to the anhydrous phase. The transformation can be initiated by stimuli, such as temperature, humidity, or vacuum conditions. The hydrate and anhydrous phases exhibit a tetragonal structure (P41212), with marked differences in hydrogen bonding due to the presence or absence of one water molecule per asymmetric unit. The intricate relationship between crystal packing and intermolecular interactions in the hydrate phase was investigated by crystallographic charge density analysis revealing, at the molecular level, the reasons for the observed 5 orders of magnitude higher proton conductivity of the hydrate phase compared to that of the anhydrous phase. To gain further insight into the processes occurring at the surfaces of grain boundaries and the proton transfer mechanisms in this system, rehydration of the complex salt was carried out by using D2O instead of H2O and monitored by in situ ATR-FTIR spectroscopy. The results highlight the critical role of interfacial water molecules in driving structural transformations and influencing proton conductivity.

Remarkably enhancing proton conductivity by intrinsic surface sulfonation of a pyrazine-linked covalent organic framework

Remarkably enhancing proton conductivity by intrinsic surface sulfonation of a pyrazine-linked covalent organic framework

Phosphorylated poly(ethylene-co-vinyl alcohol) doping for enhanced proton conductivity and mechanical properties of side chain–type sulfonated poly(aryl ether ketone) proton-exchange membranes

Phosphorylated poly (ethylene-co-vinyl alcohol) (PEVOH) was synthesized and blended with side chain–type sulfonated poly (aryl ether ketone) (SPAEK) to prepare SPAEK proton-exchange membranes with improved physicochemical and electrochemical properties. The results indicate that PEVOH doping helps improve the mechanical properties and proton conductivity of the membranes. The addition of PEVOH not only enhances proton transport but also provides proton transport channels for the membranes at a high temperature and low humidity, leading to excellent fuel cell performance and low ohmic resistance. The interaction between sulfonic acid and phosphonic acid side chain groups leads to the excellent nano-phase separation under microscopic morphology. This forms a large hydrogen bonding network between the acidic and amphoteric groups, which facilitates proton transport at low relative humidities.

Poly(arylene ether ketone sulfone)s functionalized with adenosine triphosphate-modified graphene oxide as proton exchange membranes for fuel cells

In this study, composite proton exchange membranes (PEMs) based on functional graphene oxide (GO) and carboxyl-containing sulfonated poly(arylene ether ketone sulfone)s organic matrix (C-SPAEKS) were synthesized. Modified GO nanosheets (ATP@GO) was synthesized by using GO as a template, then chemically bonded adenosine triphosphate (ATP) on GO nanosheets. Modified ATP@GO and composite membranes were characterized by FT-IR, 1H NMR, SEM and XPS. These composite membranes exhibit outstanding proton conductivity, thermal stability and dimensional stability. Compared to C-SPAEKS membranes (32.3 mS cm−1 at 20 °C, 108.9 mS cm−1 at 90 °C), C-SPAEKS/ATP@GO-1 membrane showed enhanced proton conductivity (50.0 mS cm−1 at 20 °C, 184.8 mS cm−1 at 90 °C). Meanwhile, the open circuit voltage (OCV) and power density of C-SPAEKS/ATP@GO-1 were 0.9941 V and 466.33 mW/cm2, respectively, which were much higher than the C-SPAEKS (0.8973 V, 190.72 mW/cm2), showing good fuel cell performance. The results show that functionalized GO could make an important contribution to proton conduction and improvement of comprehensive performance in composite membrane, which indicate that these composite membranes have the potential to be used as PEMs.

Enhancement in proton conductivity and vanadium resistance of SPEEK-based hybrid membrane induced by incorporating amphoteric GO-DA nanofillers for vanadium flow battery

Sulfonated poly(ether ether ketone)-based hybrid membranes (S/GO-DA) have been fabricated through the solution-mixing method, and the introduced graphene oxide (GO) nanofillers functionalized by 2, 4-diaminobenesulfonic acid (DA) afford outstanding physicochemical properties and excellent battery performance for the S/GO-DA membranes. At low loading 1 wt% GO-DA nanofillers, the S/GO-DA-1 membrane gives the highest ion selectivity (45.4 × 103 S min cm−3) and the lowest vanadium ion permeability (7.38 × 10−7 cm2 min−1), which is superior to Nafion 212 (3.0 × 103 S min cm−3, 42.5 × 10−7 cm2 min−1) and SPEEK (3.85 × 103 S min cm−3, 94.4 × 10−7 cm2 min−1), respectively. Moreover, the better energy efficiency (81.1–68.6 %) and longer self-discharge time (78.3 h) of S/GO-DA-1 at 100–200 mA cm−2 current density further prove its good structure stability and durability, elucidating the balanced proton conductivity and vanadium resistance is helpful to enhance the membrane performance. In addition, after cycling at 150 mA cm−2 current density for at least 200 cycles, 76.3 % energy efficiency and 26.9 % charge retention rate are given, illustrating the combined physical barrier of GO-DA nanofillers and the acid-base interaction contribute a synergistic promoting effect on enhancing the battery efficiency of the S/GO-DA hybrid membranes. Therefore, the present work provides a facile method to construct a high-performance proton exchange membrane by developing 2D nanostructured nanofillers containing various chemical covalent-bonded functional amphoteric molecular chains.

Fundamental insights into structure-performance relationship for proton conductivity enhancement through polymer electrolyte membrane in Proton Exchange Membrane Fuel Cell

The efficiency of a polymer electrolyte membrane fuel cell (PEMFC) is limited by the proton conductivity of polymer electrolyte membrane (PEM). Local structural variation with hydrophobic-hydrophilic zone segregation in PEM affects proton conductivity profoundly. There is a lack of elemental theoretical model to predict proton transport in novel PEMs with a wide range of compositions and structures. In this work, the first-principle based meso-scale modelling has been attempted to address localised structural variations. The membrane is divided into repetitive lattice of uniform structure. The segregated domains follow periodic or random arrangements. The applicable governing equations are solved numerically under appropriate boundary conditions. The system parameters are recognized and their effects are thoroughly investigated. For the futuristic PEMs, the effect of variation in zone arrangement, its size and its gradual change was investigated. The model was validated with reported data of Nafion membranes with successful prediction of conductivity at different operating temperature and humidity. A phase-space plot was generated for use as a straight-forward tool for prediction of proton conductivity at different operating environment for varying segregation level. The developed model can enlighten the transport mechanism through any existing and futuristic PEMs, scale-up and design them according to the application requirement.

An enhanced proton conductivity of proton exchange membranes by constructing proton-conducting nanopores using PVP-UiO-66-NH-SO3H nanoparticles

The performance of proton exchange membranes (PEMs) can be enhanced by creating continuous proton conduction channels. In this work, PVP-UiO-66-NH-SO3H nanoparticles (PUNSNPs) were prepared and filled into sulfonated poly (ether ether ketone) (SPEEK) to fabricate PUNSNPs/SPEEK composite PEMs. To further improve the performance of the PEMs, phosphoric acid (PA) was doped and a continuous nanopore network was formed in PA/PUNSNPs/SPEEK porous PEMs due to the dissolution of PUNSNPs. The PA/PUNSNPs/SPEEK porous PEMs have high PA uptake ratio (96%–117 %) and dimensional stability thanks to the anchoring effect of the nitrogen-containing heterocyclic ring on the PVP chains. The acid-base pairing interaction between the chains (residual ligands from PUNSNPs) inside the unique continuous nanopore structure allows PA/PUNSNPs/SPEEK = 20 % porous PEMs to have the highest proton conductivity of 0.350 S cm−1 (80 °C/100 % RH). Furthermore, the PA/PUNSNPs/SPEEK porous PEMs retained PA up to more than 80 % (80 °C, 40 % RH, 7 days). Similarly, PA/PUNSNPs/SPEEK = 20 % porous PEMs immersed in deionized water for 45 days were able to maintain a proton conductivity of more than 83 %. Finally, PA/PUNSNPs/SPEEK porous PEMs have low methanol permeability. Thus, this study provides a new method for the preparation of porous PEMs with a continuous nanopore network structure and excellent performance.