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Recent Advances in the Functionalisation of Polybenzimidazole and Polyetherketone for Fuel Cell Applications
Abstract
This article reviews progress made over the past years in the functionalisation of polybenzimidazole and polyetherketones with a view to increasing their proton conduction properties without detriment to their thermohydrolytic and chemical stability such that corresponding membranes may be employed in hydrogen oxygen (air) or direct methanol fuel cells. The approaches include complexation of polybenzimidazole with acids, grafting of groups containing sulfonic acid moieties on to polybenzimidazole by N-substitution, and direct sulfonation of polyetherketones. A further approach concerns the incorporation of inorganic proton conducting particles in the polymer matrix, and this is developed in detail for the case of hybrid sulfonated polyetheretherketone–metal(IV) phosphate membranes.
... The chemical structure of polybenzimidazole is presented in Figure 8. Its rigid structure gives excellent thermal and mechanical properties, such as a melting point above 600 °C [35] and a glass transition of 435 °C [36]. It has the highest tensile strength among highperformance polymers, up to 145 MPa, and offers good chemical resistance [35,[37][38][39]. ...
... Its rigid structure gives excellent thermal and mechanical properties, such as a melting point above 600 °C [35] and a glass transition of 435 °C [36]. It has the highest tensile strength among highperformance polymers, up to 145 MPa, and offers good chemical resistance [35,[37][38][39]. However, it has a large water uptake of 15 wt.%, and the imidazole ring in the repeat unit may be subjected to hydrolysis, reducing its lifetime in applications such as fuel cells [40]. ...
... A proton exchange membrane fuel cell transforms the energy liberated during the hydrogen and oxygen reaction from chemical to electrical energy. The sulfonation of PEEK allows its proton conductivity to be increased [35,40]. Electrochemical properties (i.e., water uptake, thermal stability, proton exchange, dielectric conductivity) are the most reported properties. ...
This review aims to report the status of the research on polyaryletherketone-based thermoplastic blends (PAEK). PAEK are high-performance copolymers able to replace metals in many applications including those related to the environmental and energy transition. PAEK lead to the extension of high-performance multifunctional materials to target embedded electronics, robotics, aerospace, medical devices and prostheses. Blending PAEK with other thermostable thermoplastic polymers is a viable option to obtain materials with new affordable properties. First, this study investigates the miscibility of each couple. Due to different types of interactions, PAEK-based thermoplastic blends go from fully miscible (with some polyetherimides) to immiscible (with polytetrafluoroethylene). Depending on the ether-to-ketone ratio of PAEK as well as the nature of the second component, a large range of crystalline structures and blend morphologies are reported. The PAEK-based thermoplastic blends are elaborated by melt-mixing or solution blending. Then, the effect of the composition and blending preparation on the mechanical properties are investigated. PAEK-based thermoplastic blends give rise to the possibility of tuning their properties to design novel materials. However, we demonstrate hereby that significant research effort is needed to overcome the lack of knowledge on the structure/morphology/property relationships for those types of high-performance thermoplastic blends.
... [6][7][8][9] They have also been utilized as ligands, [10] organocatalysts, [11] UV filters, [12] optical brighteners, [13] dyes [14] and monomers. [15] Typically, under harsh dehydration process, 1,2-disubstituted benzimidazoles were formed from o-phenylenediamine and aldehydes or carboxylic acid derivatives. [15][16][17][18] This traditional method has drawbacks like use of highly reactive acid derivatives, oxidizing agents and carbonyl compounds, need of high reaction temperatures, and low atomic efficiency. ...
... [15] Typically, under harsh dehydration process, 1,2-disubstituted benzimidazoles were formed from o-phenylenediamine and aldehydes or carboxylic acid derivatives. [15][16][17][18] This traditional method has drawbacks like use of highly reactive acid derivatives, oxidizing agents and carbonyl compounds, need of high reaction temperatures, and low atomic efficiency. [20,21] Subsequently, N/C-alkylation/arylation of benzimidazoles by catalytic methods was reported. ...
... Interestingly, the use of excess alcohol (1.7-2 mmol) allowed us to achieve a solvent-free reaction. Increase of time (18 to 24 h) as well as temperature (120 to 140°C) resulted in improved conversion in the neat reaction (Table 1, entries [14][15][16][17]. Then, the arene moiety was changed from p-cymene to benzene (5); Ru(II) complex with the former arene unit displayed better activity over its benzene counterpart (Figure 4). ...
The catalytic activity of Ru(II)‐arene complexes containing ferrocene thiosemicarbazone (Fc‐TSC) ligands was investigated towards the selective synthesis of 1,2‐disubstituted benzimidazoles via acceptorless dehydrogenative coupling of diamines with primary alcohols. A series of Ru(II)‐p‐cymene complexes (1–4) containing Fc‐TSC ligands (L1–L4) were synthesized and characterized. From single crystal X‐ray crystallographic studies, the molecular structures of L3 and 4 were confirmed. The influence of electronic effect of ligands on the catalytic activity of their complexes was studied. The activity of good performer i. e. 4 was compared with that of its benzene counterpart (5). The catalysis was extended to aromatic, aliphatic and heterocyclic substituted primary alcohols, and phenylenediamines with electron‐donating or ‐withdrawing substituents. Overall, synthesis of 1,2‐disubstituted benzimidazoles was accomplished with good to moderate yields, with hydrogen and water as only by‐products.
... Polymer electrolyte fuel cells (PEFCs) operating at elevated temperatures (>100 C) offer signicant improvements over lowtemperature PEFCs, such as no humidication of the feed gas, no water recirculation, a more efficient cooling of the cell and a higher tolerance against feed gas impurities. [1][2][3] The proton conductivity of NAFION®-based proton exchange membranes (PEMs), used in PEFCs for low operation temperatures, depends mainly on the polymer's water uptake. For operation at elevated temperatures (>100 C), the conductivity of a new membrane material should be maintained in anhydrous conditions. ...
... Increasing the stoichiometry x from The proton transport mechanism is further discussed, together with the 1 H-PFG-NMR/DOSY measurements and the self-diffusion coefficient in the next section. The mobile protonic charge carriers in the PIL/H 2 If there is only vehicular transport, this should principally lead to a decrease in the (total) conductivity with increasing x. If there is also cooperative transport, the presence of H 2 O, acting as a proton acceptor, will also accelerate the intermolecular proton transfer between the MTau and the [2-Sema] + cation, leading to faster cooperative transport. ...
In this study, Brønsted-acidic proton conducting ionic liquids are considered as potential new electrolytes for polymer membrane fuel cells with operating temperatures above 100 °C. N-Methyltaurine and trifluoromethanesulfonic acid (TfOH) were mixed at various stoichiometric ratios in order to investigate the influence of an acid or base excess. The proton conductivity and self-diffusion of the “neat” and with 6 wt% water samples were investigated by following electrochemical and NMR methods. The composition change in the complete species and the relative proton transport mechanism based on the NMR results are discussed in detail. During fuel cell operation, the presence of significant amounts of residual water is unavoidable. In PEFC electrolytes, the predominating proton transfer process depends on the cooperative mechanism, when PILs are fixed on the polymer matrix within the membrane. Due to the comparable acidity of the cation [2-Sema]⁺ and the hydroxonium cation, with excess N-methyltaurine or H2O in the compositions, fast proton exchange reactions between the protonated [2-Sema]⁺ cation, N-methyltaurine and H2O can be envisaged. Thus, an increasing ratio of cooperative proton transport could be observed. Therefore, for polymer membrane fuel cells operating at elevated temperatures, the highly acidic PILs with excess bases are promising candidates for future use as electrolytes.
... Polymer electrolyte fuel cells (PEFCs), operational at an elevated temperature above 100°C, have attracted much attention recently, due to their superiorities compare to low temperature (LT)-PEFCs: (i) no feed gas humidification, (ii) a more efficient cooling system (easier water and heat management), (iii) the possibility of recovering high-grade waste heat, and (iv) a higher tolerance against feed gas impurities [1,2]. Currently, (high temperature) HT-PEFC, based on phosphoric acid doped polybenzimidazole (PBI) membranes, cannot compete with the performance characteristics of NAFION-based LT-PEFCs [3]. ...
... Thus, a weight increase of about 98 wt.% can be predicted, assuming 2 TfOanions per m-PBI repeating unit. In the obtained ''polybenzimidazolium triflate'' m-PBI-H 2 + (TfO -) 2 The uptake of a PIL by m-PBI membrane in a swelling process is dependent on the acidity of cation. The protonation of m-PBI chain is obviously a prerequisite for the uptake of an electrolyte. ...
Proton conducting ionic liquids (PILs) are discussed as new electrolytes for the use as non‐aqueous electrolytes at operation temperatures above 100 °C. During fuel cell operation the presence of significant amounts of residual water is unavoidable. The highly Brønsted‐acidic PIL 2‐Sulfoethylmethylammonum triflate [2‐Sema][TfO] is able to perform fast proton exchange processes with H2O, resulting from 1H‐NMR and pulsed field gradient (PFG)/diffusion ordered spectroscopy (DOSY) self‐diffusion measurements. Proton conduction takes place by a vehicle mechanism via PIL cations or H3O+, but also by a cooperative mechanism involving both species. Thus, highly Brønsted‐acidic PILs are promising candidates for the use as non‐aqueous electrolytes. To use [2‐Sema][TfO] as electrolyte in a proton electrolyte fuel cell (PEFC) it has to be immobilized in a host polymer. There is a (slow) uptake of the PIL by polybenzimidazole (PBI) up to a weight increase of ∼130%, due to a swelling process. A protonation of the basic imidazole moieties takes place. NMR analysis was applied to elucidate the molecular interactions between PBI, PIL, and residual water. Proton exchange, respectively an interaction between the polar groups and water can be observed in spectra, indicating a network of H‐bonds in doped PBI. Therefore, highly acidic PILs are promising candidates for the use as non‐aqueous electrolytes.
... For this reason, the current tasks are to improve existing membrane materials 5 and to find alternative proton-conducting polymers that are stable at high temperatures. 6,7 Polybenzimidazole, [8][9][10] polyetherketone, 10,11 polyether ether ketone, [12][13][14] polyarylene ether sulfone, 15 chitosan, 16 polyimide, 17 polysulfone 18 and other polymers were previously considered as materials for proton-conducting membranes. In most cases, proton-conducting materials were obtained by treating these polymers with reagents, e.g. ...
... For this reason, the current tasks are to improve existing membrane materials 5 and to find alternative proton-conducting polymers that are stable at high temperatures. 6,7 Polybenzimidazole, [8][9][10] polyetherketone, 10,11 polyether ether ketone, [12][13][14] polyarylene ether sulfone, 15 chitosan, 16 polyimide, 17 polysulfone 18 and other polymers were previously considered as materials for proton-conducting membranes. In most cases, proton-conducting materials were obtained by treating these polymers with reagents, e.g. ...
Sulfonated poly(1,3,4‐oxadiazole‐2,5‐diyl‐1,4‐phenyleneoxy‐1,4‐phenylene), poly(1,3,4‐oxadiazole‐2,5‐diyl‐10,10‐dioxophenoxathiine‐2,8‐diyl) and their copolymers were one‐pot synthesized in fuming sulfuric acid with the use of 4,4′‐oxydibenzoic acid and hydrazine sulfate as initial reagents. These copolymers were non‐fusible and did not dissolve in individual liquids except sulfuric acid. However, it was possible to achieve their unlimited solubility using a mixed solvent that contained dimethyl sulfoxide, formamide and water. In dilute and semi‐dilute solutions, the copolymers behaved like polyelectrolytes, while in concentrated solutions they formed gels; moreover, in the case of high polymer content, the gels were in a liquid crystalline state. The degree of sulfonation, temperature and water content in the mixed solvent influenced the state of copolymer solutions, their viscosity and viscoelasticity. The finding of the mixed solvent made it possible to form polymer films whose strength and heat resistance reached 33 MPa and 470 °C, respectively. The ionic conductivity of the films under direct current conditions was 0.001–0.01 μS cm–1 for sodium cations and 0.3–1 mS cm–1 for protons. © 2020 Society of Chemical Industry
... Nevertheless, the water management system related to Nafion membranes is complex and expensive [3]. Thus, membrane-inspired polybenzimidazole (PBI) and its derivatives, with exceptional physicochemical properties have attracted significant interest in many applications, including in chemical and biological applications [4,5]. PBI exhibits high proton conductivity at low water content, excellent thermal stability up to 200 o C, and has good mechanical properties. ...
Polymer electrolyte membranes with high proton conductivity continue to pose a challenge especially in the fields of biomaterial, semiconductor, membrane separation, and ion conductive membrane. Here, an alternative of a new class of highly conductive ferrocene-modified polybenzimidazole (PBI/Fc) membranes was prepared by the solvent casting method after the amidation reaction of ferrocene carboxylic acid (FCA) with imidazole groups in PBI solution. The properties of the as-prepared membranes were characterized by varied spectroscopic measurements. For instance, in the attenuated total reflectance-Fourier transform infrared (ATR-FTIR) measurements the characteristic vibrational bands of ferrocenyl moieties, and amide bond formation were identified. Interestingly, in the electrochemical studies, membrane PBI/Fc-5 was found to exhibit low absolute impedance and high proton conductivity of 537 × 10 3 Ω and 0.0249 × 10-8 S/cm, respectively. Proton conducting polymers have been widely used in energy storage systems, particularly in powering electronic machines and devices [1]. With energy storage, renewable energy can be made more reliable and less expensive. So far, Nafion membranes, which are highly hydrated, remain one of the most powerful man-made proton conductors [2]. Nevertheless, the water management system related to Nafion membranes is complex and expensive [3]. Thus, membrane-inspired polybenzimidazole (PBI) and its derivatives, with exceptional physicochemical properties have attracted significant interest in many applications, including in chemical and biological applications [4,5]. PBI exhibits high proton conductivity at low water content, excellent thermal stability up to 200 o C, and has good mechanical properties. Problems for further enhancement may include improved proton conductivity, durability, stability, and mechanical strength. Different methods are being used to boost the proton conductivity of the PBI membrane without affecting its mechanical strength, for instance, with the addition of inorganic fillers, acids, and structural modifications via the formation of ionic cross-linking [2]. Herein, in this report, we focus our efforts on studying the preparation, characterization, and proton conductivity study of PBI membranes modified with ferrocene carboxylic acid (FCA). Primarily, FCA was covalently cross-linked to PBI by amide bond linkages through the imidazole groups of the polymer at room temperature, thus leading to the formation of ferrocene-modified PBI. The efficiency of these modified PBI membranes was examined in the proton conductivity test by using electrochemical impedance spectroscopy (EIS) at room temperature. MATERIALS AND METHODS Materials Celazole® polybenzimidazole (PBI) solution (26%, MW = 27000 g/mol, PBI Performance Products Inc (USA)) and ferrocene carboxylic acid, FCA (1 g, Merck) were all used as the precursor materials of PBI/Fc membrane. The PBI solution contained 26 wt% polymer solids and 1.5 wt% lithium chloride (stabilizer) dissolved in DMAc. Dimethylacetamide (DMAc) (2.5 L, Merck) was used without any further purification. Distilled water was used throughout the experiment. Preparation of 15 wt% PBI Solution The PBI solution preparation processes were carried out according to the previously reported procedure [6].
... However, the material's thermal stability is greatly reduced in an oxidative environment. The decomposition mechanism is also more complex in the air due to the presence of oxygen in the degradation process [35][36][37]. ...
This study aimed to evaluate the aging of poly(ether-ether-ketone) (PEEK) submitted to different media: in air, in water bubbled with nitrogen, in water bubbled with air, and in water at pH 4, for 90 days at temperatures of 120, 140 and 160 °C. The physical, thermal and mechanical properties of PEEK specimens were evaluated before and after aging. The density measurements showed that the aging conditions employed did not promote water absorption or mass loss; thermogravimetric analyses (TGA) showed that all aging media exerted the same effects on the material's thermal stability, with variation in the initial thermal degradation (Tonset) below 5 °C concerning the unaged polymer. The thermal history obtained by differential scanning calorimetry (DSC) did not show significant variations in thermal transition temperatures in the first heating cycle, indicating that the aging conditions did not cause internal degradation in PEEK samples within the studied period. The degree of crystallinity calculated by X-ray diffractometry (XRD) and DSC demonstrated a slight increase over time due to thermal annealing at temperatures above the Tg of the unaged PEEK. The mechanical properties of the aged PEEK showed low variations in Young’s modulus and greater variations in the tension at rupture and elongation at rupture, being more drastic in the aqueous media saturated in air and acid solution.
... The study of Franck-Lacaze et al. demonstrated the diffusion kinetics of acids through poly(4-vinylpyridine)based weak AEMs and found that proton leakage was significant in high concentration acid solution [24]. Considering these factors, numbers of approaches have been verified to inhibit the proton leakage of AEMs, for example, the introduction of hydrophobic groups, the increase of cross-linking degree and adopting weakly basic ion exchange groups [25][26][27]. ...
Electrodialysis (ED) technology equipped with proton blocking anion exchange membranes (AEMs) has great potential applications in recovering acidic wastewater. To develop high performance proton-blocking AEMs, in this work, a series of imidazolium functionalized poly(vinyl chloride) (PVC) AEMs are designed. In particular, the length of alkyl side chains tethered on the imidazole compound is optimized to adjust the micro-structure of AEM. Our research demonstrates that the density of 1-butylimidazole functionalized PVC AEM matrix is enhanced, which has been proved by water uptake and thermogravimetric analysis. Hence, the acid blocking performance of optimized AEM (PVC-Im-4C) is strengthened. The maximum concentration of H⁺ in concentrate chamber of electrodialyser enriched by the PVC-Im-4C AEM is 1.81 M through the 24 h ED process at a current density of 20 mA·cm⁻² (the initial concentration of hydrogen ions: 1.0 M). The result suggests that the optimized PVC-Im-4C AEM with long alkyl side chain on imidazole shows the excellent capacity in acid enrichment. This work should provide guidance for the follow-up design of the advanced proton blocking AEMs.
... Polybenzimidazole(PBI) mainly was prepared from organic synthesis method , high performance polymer with high thermal stability, high conducting properties when doped. The preparation of PBI can be achieved by condensation reaction of diphenylisophthalate and 3,3',4,4'-tetraaminodiphenyl [43][44][45][46][47][48][49]. Recently we have reported synthesis of nanofibrous cerium(iv) Phosphate/ polybenzimidazole nanocomposite membrane [50]. ...
Nanosized zirconium phosphate and nano fibrous cerium phosphate , Zr(HPO 4) 2 .H 2 O(nZrP) , Ce(HPO 4) 2 .2.9H 2 O (nCeP f), respectively, were prepared and characterized. Mixing slurry aqueous solution of (nZrP and nCeP f) in 25:75 wt/wt% mixing ratios) , respectively, lead to formation of novel zirconium phosphate-fibrous cerium phosphate nanocomposite membrane, [Zr(HPO 4) 2 ] 0.25 [Ce(HPO 4) 2 ] 0.75 .3.87H 2 O(nZrP-nCeP f), was characterized. Zirconium phosphate-fibrous cerium phosphate/ polybenzimidazole-/polybenzimidazolee-co-polyaniline-/polybenzimidazole-co-polypyrrole-/polybenzimidazole-co-polyindole nanocomposite membranes were prepared via in-situ chemical oxidation polymerization of the benzimidazole, and its co-monomers in alcohol, that was promoted by the reduction of Ce(iv) ions present in the inorganic matrix of (nZrP-nCeP f) nanocomposite membrane. A possible explanation is nCeP f , present on the surface of composite membrane, is attacked by benzimidazole and its co-monomers , converted to cerium(III) orthophosphate(CePO 4). The resultant materials were characterized by elemental (C,H,N) analysis , FT-IR, and scanning electron microscopy(SEM) .. SEM images of the resulting nanocomposites reveal a uniform distribution of the polybenzimidazole and its co-polymers on the inorganic matrix. From elemental (C,H,N) analysis the amount of organic material (PBI) present in (nZrP-nCeP f)/PBI composite found to be = 5.7% in wt.. T.he amount of organic materials present in copolymers found to be for (nZrP-nCeP f)/PBI-co-PANI (PBI = 9.33%, PANI= 13.32% in wt)., for (nZrP-nCeP f) /PBI-co-PPy (PBI = 12.85%, PPy = 7.1% in wt) , and for (nZrP-nCeP f) /PBI-co-PIn (PBI = 16.47% , PIn = 8.81% in wt).
... Ionomers are fascinating polymeric materials, in which electrolytic groups, such as sulfonic acid Jones and Roziere, 2001;Kreuer, 1997;Kreuer et al., 2004;Li et al., 2003) or quaternary ammonium (Arges and Zhang, 2018;Bauer et al., 1990;Couture et al., 2011;Elattar et al., 1998;Sun et al., 2018;Varcoe and Slade, 2005), are anchored on the polymer chains and dissociated counter-ions can mi-grate in hydrated nanometric channels inside the hydrophobic polymer matrix. The resulting ion-conducting materials are very useful for many applications, including acid-base or humidity sensors Ruzimuradov et al., 2018), water purification (Bauer et al., 1990) or separation membranes for electrochemical energy technologies, such as proton exchange membrane (PEM) Mehta et al., 2003) or anion exchange membrane (AEM) fuel cells (Sun et al., 2018;Varcoe et al., 2014). ...
Proton-conducting polymers, such as sulfonated poly(ether ether ketone) (SPEEK), are of great industrial interest. Such proton
exchange membranes show high tendencies for water and water vapor uptake.
The incorporation of water not only leads to mass and dimensional changes,
but also to changes in conductivity by several orders of magnitude. Both
properties highly impact the potential application of the materials and,
therefore, have to be known precisely. As hydration is diffusion controlled,
thin films may behave differently to bulk specimens. However, the
determination of small mass changes occurring in thin-film samples is very
challenging.
In this work, a new measurement setup is presented to simultaneously
characterize the mass change and the conductivity of thin polymer films. The
mass change is measured by resonant piezoelectric spectroscopy (RPS) with a
nanobalance, which is based on high-precision piezoelectric resonators operating in thickness-shear mode (TSM). The mass resolution of this
nanobalance is ±7.9 ng. Electrochemical impedance spectroscopy and
an interdigitated electrode array are used for conductivity measurements.
The approach is validated by comparing two SPEEK films with different
degrees of sulfonation (DS). The relative humidity (RH) in the measurement setup was changed stepwise within the range ∼ 2 % < RH < ∼ 85 %. For both material compositions,
DS = 0.5 and DS = 0.9, the mass uptake, the hydration number and the
proton conductivity are presented and discussed depending on RH.
This newly designed experimental setup allows for in situ characterization of the
properties mentioned above; it can monitor not only the data for the
stationary state, but also the dynamics of the hydration. To the authors'
knowledge this is the first simultaneous and in situ measurement device for
simultaneously sensing mass and conductivity change due to hydration of
polymeric thin-film materials.
... For DMFCs, using PBI-based membranes could greatly reduce the methanol permeability [16], as pure PBI membranes have extremely low methanol permeability (about 5 × 10 −9 S cm −1 ) [17]. However, the proton conductivity of the PBI membrane is also low, which will greatly reduce the performance of fuel cells and makes it difficult for fuel cells using PBI-based membranes to compete with traditional PEMs (such as Nafion membranes) in performance [18]. To promote the proton conductivity of PBI membranes without sacrificing the chemical stability and the mechanical strength, various attempts have been applied, such as doping with acid [19][20][21], synthesizing modified PBIbased structures [22][23][24], applying nanomaterials [25][26][27], combining other cross-linkers [28][29][30] and building porous microstructures [31][32][33]. ...
Hydrogen-air proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are excellent fuel cells with high limits of energy density. However, the low carbon monoxide (CO) tolerance of the Pt electrode catalyst in hydrogen-air PEMFCs and methanol permanent in DMFCs greatly hindered their extensive use. Applying polybenzimidazole (PBI) membranes can avoid these problems. The high thermal stability allows PBI membranes to work at elevated temperatures when the CO tolerance can be significantly improved; the excellent methanol resistance also makes it suitable for DMFCs. However, the poor proton conductivity of pristine PBI makes it hard to be directly applied in fuel cells. In the past decades, researchers have made great efforts to promote the proton conductivity of PBI membranes, and various effective modification methods have been proposed. To provide engineers and researchers with a basis to further promote the properties of fuel cells with PBI membranes, this paper reviews critical researches on the modification of PBI membranes in both hydrogen-air PEMFCs and DMFCs aiming at promoting the proton conductivity. The modification methods have been classified and the obtained properties have been included. A guide for designing modifications on PBI membranes for high-performance fuel cells is provided.
... The operation of polymer electrolyte fuel cells (PEFCs) at elevated temperatures above 100°C carries significant advantages, such as: (i) a simplified system setup because water recyclation and feed gas humidification is not necessary, (ii) a more efficient cooling of the cell, (iii) the possibility of recovering high-grade waste heat and (iv) a lower sensitivity to CO contamination [1][2][3]. This is especially the case for electric cars. ...
In this study, a protic ionic liquid (PIL), 2-Sulfoethylmethylammonium triflate [2-Sema][TfO] is considered as a potential new proton conducting electrolyte for future polymer membrane fuel cells capable of ambient air operation above 100 °C. The proton dynamics of the PIL with residual water are examined as a function of the hydration level on different time scales using pulsed field gradient nuclear magnetic resonance (PFG-NMR) and quasi-elastic neutron scattering (QENS). The separation of the different contributing relaxation processes enables a quantification of the proton fractions for the underlying hopping or vehicular motions. The hopping motion of the water in the time scale of picosecond and the vehicular motion in the time scale of nanosecond are detected by means of QENS. Such dynamic processes can be well described by the Chudley-Elliot jump model. This emphasised the presence of fixed jump lenghts. In the timescale of millisecond, the cooperative transport of the active protons of the acidic SO3H group and of the H2O molecules, as well as the vehicular transport of the PIL cations are detected by NMR. The different diffusion coefficients obtained by the NMR and QENS techniques are discussed in detail.
... Thus, phosphoric acid is frequently selected as a dopant due to its higher conductivity, outstanding thermal stability and low vapor pressure at high temperatures [25]. Jones and Rozière [26] also explained that the presence of free acids in the polymer structure and H 2 PO 4 − /HPO 4 2− anionic chains initiated higher proton conductivity of the PA-PBI polymer. The conductivity of the PA-PBI polymer depends on the amount of phosphoric aciddoped in the polymer. ...
Increasing world energy demand and the rapid depletion of fossil fuels has initiated explorations for sustainable and green energy sources. High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are viewed as promising materials in fuel cell technology due to several advantages, namely improved kinetic of both electrodes, higher tolerance for carbon monoxide (CO) and low crossover and wastage. Recent technology developments showed phosphoric acid-doped polybenzimidazole (PA-PBI) membranes most suitable for the production of polymer electrolyte membrane fuel cells (PEMFCs). However, drawbacks caused by leaching and condensation on the phosphate groups hindered the application of the PA-PBI membranes. By phosphate anion adsorption on Pt catalyst layers, a higher volume of liquid phosphoric acid on the electrolyte–electrode interface and within the electrodes inhibits or even stops gas movement and impedes electron reactions as the phosphoric acid level grows. Therefore, doping techniques have been extensively explored, and recently ionic liquids (ILs) were introduced as new doping materials to prepare the PA-PBI membranes. Hence, this paper provides a review on the use of ionic liquid material in PA-PBI membranes for HT-PEMFC applications. The effect of the ionic liquid preparation technique on PA-PBI membranes will be highlighted and discussed on the basis of its characterization and performance in HT-PEMFC applications.
... methanol, oxygen, etc.) (Savadogo, 1998;Mauritz and Moore, 2004). One common approach has been to sulfonate existing thermoplastics such as polystyrene (Ding et al., 2001;Roziere and Jones, 2003), poly (ether ketone)s (Zaidi 2000;Jones and Roziere, 2001) and poly (ether sulfone)s (Nolte et al., 1993;Wang et al., 2002). This has generated mostly PEM's with lower costs and improved thermal stability, but generally lower ionic conductivities at comparable ion exchange capacities than Nafion (Kreuer, 2001) and many of these thermoplastics-based PEMs are more susceptible to oxidative or acid-catalyzed degradation than Nafion (Hubner and Roduner, 1999). ...
Multi-phenyl structured random polymer was synthesized via condensation polymerization
reaction by applying different monomer ratios and characterized by various spectroscopic
methods (FT-IR, 1
H NMR). The prepared polymers showed good thermooxidative stability up
to 400 ºC. The surface morphology was studied by FESEM that showed the good linkage
among the polymer chains. The EDS data of poly(fluorenylene ether ketone), PFEK;
demonstrated that all the monomers participated in the copolymerization reaction. Inherent
viscosity values of the polymers were obtained in the range of 0.76∼1.12 dL g-1. The polymers’
yield was within 85~90%. The obtained results indicate that the multi-phenyl structured
polymer will be the good candidates to prepare the effective aromatic hydrocarbon polymer
electrolyte membrane
... In recent years, phosphoric acid (PA) shows a large interest in this connection due to its high intrinsic proton conductivity and low vapor pressure (Melchior et al., 2017). Therefore, PA doped with PBI membranes (PBI-PA) have been thoroughly researched (Wainright et al., 1995;Jones and Rozière, 2001;Li et al., 2004). However, the chemical stability of PBI-PA will be compromised because PA will be dehydrated when the system reaches 150°C or higher (Kim et al., 2014). ...
Fuel cells (FCs) are a chemical fuel device which can directly convert chemical energy into electrical energy, also known as electrochemical generator. Proton exchange membrane fuel cells (PEMFCs) are one of the most appealing FC systems that have been broadly developed in recent years. Due to the poor conductivity of electrolyte membrane used in traditional PEMFC, its operation at higher temperature is greatly limited. The incorporation of ionic liquids (ILs) which is widely regarded as a greener alternative compared to traditional solvents in the proton exchange membrane electrolyte shows great potential in high temperature PEMFCs (HT-PEMFCs). This review provides insights in the latest progress of utilizing ILs as an electrochemical electrolyte in PEMFCs. Besides, electrolyte membranes that are constructed by ILs combined with polybenzimidazole (PBI) have many benefits such as better thermal stability, improved mechanical properties, and higher proton conductivity. The current review aims to investigate the newest development and existing issues of ILs research in electrolyte and material selection, system fabrication method, synthesis of ILs, and experimental techniques. The evaluation of life cycle analysis, commercialization, and greenness of ILs are also discussed. Hence, this review provides insights to material scientists and develops interest of wider community, promoting the use of ILs to meet energy challenges.
... These developments are principally motivated to lower the material cost for low-temperature operation as recently reviewed. [17][18][19][20][21][22][23] Most of the sulfonated polymers were developed by postsulfonation of preformed polymers or copolymerization produced from sulfonated monomers. 24 Besides these, sulfonic acid groups may be activated functional groups. ...
As the world’s transportation is seeking to switch towards renewable and sustainable sources of energy, the research in fuel cell technology has gained momentum. Proton exchange membrane fuel cell (PEMFC) operating at temperature range 100–200°C (high-temperature proton exchange membrane fuel cells, HT-PEMFCs) has gained interest in their major application to electric power generation. The most promising material is polybenzimidazoles (PBI). Synthesis methods such as condensation polymerization, solid-state or melt polymerization, etc. give the polymer with different inherent viscosity. The monomer modifications both in tetramine and the diacid, reveal variations in glass transition value. Further insight into the membrane casting solvents and methods along with its proton conductivity has been reviewed. Review paper is comprising of Part 1: for the synthesis methods, structural changes, and applications of PBIs in HT-PEMFCs while, Part 2: for the various kinds of PBIs has been discussed.[Formula: see text]
... One of the current research focuses is to improve the protonic conductivity of PFSA membranes at high temperatures [220]. Three technical pathways have been widely investigated: (i) adding hydrophilic inorganic materials [221][222][223], (ii) using non-aqueous and non-volatile solvents to replace water as the proton acceptor [224], and (iii) adopting proton conductors in solid states [225]. The high operating temperature improves the electrode reaction kinetics [226,227], alleviates the co-poisoning problem at low temperatures [228][229][230], simplifies the fuel treatment process (as shown in Fig. 12) [231], and optimizes hydrothermal management [232]. ...
Membrane is one of the most important components in proton exchange membrane fuel cells (PEMFCs), which determines the transport phenomena, performance, and durability. With the rapid development of novel membranes, many transport coefficients in membranes applied in numerical studies are outdated due to the lack of experimental data for new membranes. In this review, the fundamentals of commercially available membranes are scrutinized, followed by the fundamental working mechanisms. A detailed examination of the transport phenomena within the membranes, including transport mechanisms, mathematical description, and experimental methods, is conducted for protonic conduction, electro-osmosis drag, diffusion, hydraulic permeation, and gas crossover, which are urgently needed for theoretical and numerical studies. It is found that various empirical or analytical correlations have been established to predict the transport coefficients of the membranes. However, empirical models may not be accurate for all types of membranes since there is no sufficient experimental data for a solid correlation and validation. The experimental methods reviewed in the present study can be applied for new membranes, which is essential to quantify the transport phenomena and its further impact on cell performance and durability. The key transport-phenomena-related factors that affect the performance and failure modes of membranes are also reviewed in this study, which helps to develop strategies in improving membranes’ performance and durability during operation. This review deepens the understanding of the short-term and long-term performance of the membrane in PEMFCs and provides important insights into the further design of novel membranes.
... Nevertheless, their notable drawbacks are viz, exorbitant, high methanol-crossover, low temperature operation and drop of conductivity at elevated temperatures (>80°C) have motivated an exhaustive search for a suitable alternative. 4 Several aromatic thermoplastics, 5-10 namely poly(aryl ether ketone)s (PAEKs), 5,7-12 poly(ether sulfone) (PES), [13][14][15] polysulfone (PSU), 16,17 polybenzimidazole (PBI), 18,19 etc., have excellent thermo-oxidative stability, better mechanical properties, high chemical resistance and inexpensive. Among them, PEEK with reasonable conductivity characteristics, better thermal stability and magnificent mechanical properties are the suitable candidate for fuel cell membrane application. ...
The solid electrolyte membrane for a hydrogen-oxygen fuel cell was prepared and investigated from the potential of sulfonated poly(ether ether ketone) (SPEEK) embedded together with montmorillonite (MMT). Acid hydrophilized poly(ether ether ketone) (PEEK) and MMT with varying sulfonation levels of 25-70% prepared through solvent casting were investigated for their performance. The nuclear magnetic resonance (1 H NMR) spectra at 7.5 ppm affirmed the occurrence of sulfonation reaction, and its degree was studied by both 1 H NMR and titration method. The reaction time was varied to achieve sulfonation levels from 25 to 70%. The effect of incorporation of pristine and sulfonated MMT into SPEEK was examined. The membranes, prepared using solvent casting technique, were involved for water uptake over the wide range of temperature, thermal stability, and proton conductivity measurements. The cross-sectional surface arrangement of the membrane and clay dispersion was deliberated using SEM. The proton exchange membrane fuel cell (PEMFC) single cell's tests revealed that sulfonated PEEK membrane demonstrated service performance comparable to that of Nafion, as validated using MATLAB software.
... The research system has developed from the early doping modification of commercial perfluorosulfonic acid-type proton exchange membranes [3][4][5] to the present inorganic hightemperature proton exchange membrane [6][7][8][9], phosphoric acid-doped high-temperature proton exchange membrane [1,[10][11][12][13], phosphonic polymer membrane [14], and other diverse membrane materials coexist. Phosphoric acid-doped high-temperature proton exchange membranes, such as phosphoric acid-doped polybenzimidazole (PBI), exhibit advantages such as high proton conductivity (4 × 10 −2~8 × 10 −2 S/cm −1 @ 150°C) and good chemical stability under high temperature, low humidity, or no water conditions, which has become a research hotspot of hightemperature proton exchange membrane materials [15,16], and are considered as one of the most promising hightemperature proton exchange membrane materials. ...
In the present work, a semi-interpenetrating network (semi-IPN) high-temperature proton exchange membrane based on polyethyleneimine (PEI), epoxy resin (ER), and polybenzimidazole (PBI) was prepared and characterized, aiming at their future application in fuel cell devices. The physical properties of the semi-IPN membrane are characterized by thermogravimetric analysis (TGA) and tensile strength test. The results indicate that the as-prepared PEI-ER/PBI semi-IPN membranes possess excellent thermal stability and mechanical strength. After phosphoric acid (PA) doping treatment, the semi-IPN membranes show high proton conductivities. PA doping level and volume swelling ratio as well as proton conductivities of the semi-IPN membranes are found to be positively related to the PEI content. High proton conductivities of 3.9∽7.8×10−2 S cm−1 are achieved at 160°C for these PA-doped PEI-ER/PBI series membranes. H2/O2 fuel cell assembled with PA-doped PEI-ER(1 : 2)/PBI membrane delivered a peak power density of 170 mW cm⁻² at 160°C under anhydrous conditions.
... For example,n umerous research works have been performed to develop polymeric protonc onductors such as polysulfones, [2] styrene-ethylene-butylene-styrene( SEBS) and poly(aryl ether ketone)s. [3,4] However,s uch aromatic-based polymers requiref urther functionalization with sulfonic acid groups for efficient proton conduction. [5b] Recently,m etal-organic frameworks( MOFs) and conjugated microporousp olymer (CMP) have been employeda sp otential candidates for proton conduction. ...
Open 1D channels found in covalent organic frameworks are unique and promising to serve as pathways for proton conduction; how to develop high‐rate yet stable transporting systems remains a substantial challenge. Herein, this work reports a strategy for exploring proton‐conducting frameworks by engineering pore walls and installing proton‐containing polymers into the pores. Amide‐linked and sulfonated frameworks were synthesized from imine‐linked precursors via sequentially engineering to oxidize into amide linkages and to further anchor sulfonic acid groups onto the pore walls, enabling the creation of sulfonated frameworks with high crystallinity and channel ordering. Integrating sulfonated polyether ether ketone chains into the open channels enables proton hopping to across the channels, greatly increases proton conductivity and enables a stable continuous run. These results suggest a way to explore proton‐conducting COFs via systematic engineering of the wall and space of the open nanochannels.
... The second type is the non-perfluorinated polymers such as poly (aryl ether ketone)s (PAEKs), poly (ether sulfone) (PES) and polybenzimidazole (PBI), which have despite marvelous benefits, such as very low cost, excellent Chemical resistance and mechanical strength, High Oxidative and Thermal stability, is less popular than Nafion, it's attributed to the high swelling when wet, huge brittle behavior when dry and only moderately high proton conductivity, etc. [13,14]. These polymers are mostly attachments of sulfonic acid groups, such as PAEKs [15,16], PES [17,18], Polyimides [19,20], Secondary amines containing sulfonated PBI [21,22]. ...
The new poly (arylene ether sulfone) (CPAEs) polymer, and carboxylated through simple Thiol Ene reaction, is characterized by FTIR, ¹H NMR. The SnO2 nanoparticles are synthesized via alkaline and template free, one-pot hydrothermal method and characterized using HRTEM analysis. SnO2 nanoparticles in dispersed CPAEs polymer is synthesized and examined by PXRD, SEM and TGA analyses. Further, the typical properties of bare CPAEs and 1%, 2% and 3% SnO2 NPs of dispersed CPAEs nanocomposite membranes such as water uptake, swelling ratio, ion exchange capacity, proton conductivity and oxidative stability are evaluated. The PXRD pattern suggests the successful formation of amorphous natured CPAEs polymer and tetragonal rutile structured in SnO2 NPs. It is observed that the SEM images indicate SnO2 NPs, bare CPAEs polymers as spherical and form wavelike morphology. It is also noted that the HR-TEM image has identified SnO2 NPs as non-uniform in size with an average particle size of 4 nm. 3% SnO2 NPs loaded with CPAEs nanocomposite membrane exhibits an IEC value at 0.78 mmol/g-1 and a proton conductivity value of around 1.49 × 10⁻³ S/cm⁻¹ at 100 °C. It shows excellent oxidative stability with a value of 12.3% degradation after being exposed to Fenton reagent at 68 °C for 8 h.
... [23][24][25] Within this subset of aromatic-hydrocarbon-based PEMs exists a multitude of synthetic strategies to obtain various chemical architectures from comparatively lowcost starting materials. Common examples include sulfonated derivatives of poly(arylene ether sulfone)s, [26][27][28][29] poly(ether ether ketone)s, 13,30 poly(phenylene oxide)s, 31 poly(benzimidazole) s, [32][33][34][35] poly(imide)s 36 and poly(phenylene)s. [37][38][39][40][41][42] Of the numerous hydrocarbon-based structures examined in the literature to date, recent developments on sulfonated poly(phenylene)s have been particularly promising. ...
The impact of incorporating additional steric restrictions into highly sterically encumbered sulfonated polyaromatic polymers was investigated. Copolymers possessing between 0 and 10% nonlinear ortho or meta biphenyl units in an otherwise linear para biphenyl‐containing sulfo‐phenylated poly(phenylene) were synthesized in yields >80% and evaluated on the basis of their physical and electrochemical properties. When incorporated into sulfonated copolymers in ≤5 mol%, ortho and meta linked biphenyl moieties reduced membrane swelling in water by up to 23 and 19 vol%, respectively, compared to strictly para biphenyl‐linked copolymers. Despite this, copolymers possessing nonlinear, biphenyl‐linked monomers displayed a decrease in proton conductivity and mechanical strength. This study reinforces the importance of considering restricted rotation, backbone flexibility, and chain entanglement in the design of polymers aimed at improving their physical and electrochemical properties. © 2020 Society of Industrial Chemistry
... methanol, oxygen, etc.) (Savadogo, 1998;Mauritz and Moore, 2004). One common approach has been to sulfonate existing thermoplastics such as polystyrene (Ding et al., 2001;Roziere and Jones, 2003), poly (ether ketone)s (Zaidi 2000;Jones and Roziere, 2001) and poly (ether sulfone)s (Nolte et al., 1993;Wang et al., 2002). This has generated mostly PEM's with lower costs and improved thermal stability, but generally lower ionic conductivities at comparable ion exchange capacities than Nafion (Kreuer, 2001) and many of these thermoplastics-based PEMs are more susceptible to oxidative or acid-catalyzed degradation than Nafion (Hubner and Roduner, 1999). ...
In our study, multi-phenyl structured random polymer was synthesized via condensation
polymerization reaction by applying different monomer ratios and characterized by various spectroscopic methods (FT-IR, 1H NMR). The prepared polymers showed good
thermooxidative stability up to 400 ºC. The surface morphology was studied by FESEM that showed the good linkage among the polymer chains. The EDS data of poly(fluorenylene ether ketone), PFEK; demonstrated that all the monomers participated in the copolymerization reaction. Inherent viscosity values of the polymers were obtained in the range of 0.76∼1.12 dL g-1. The polymers’ yield was within 85~90%. The obtained results indicate that the multi-phenyl structured polymer will be the good candidates to prepare the effective aromatic hydrocarbon polymer electrolyte membrane.
... Because the success of this technology is still limited, the focus is on membranes and their further development concerning optimization of stability, efficiency and lifetime. Especially working temperatures of up to 130 • C are desirable for large scale usage [2][3][4][5][6]. For the evaluation and further development of proton exchange membranes, measurements of proton conductivity are the decisive tool. ...
... The polymer surface chains are expected to have greater mobility than fully cross-linked chains, yet the polymer phase is stable even when contacted by liquid mixtures, while the rugged porous inorganic support provides the desired mechanical integrity for the coated polymer layer. The ceramic-polymer hybrid membranes can be suitable not only for the separation of chemical mixtures, but also for catalysis, nanoscience, and electrolyte membrane fuel cells [6,11,20]. ...
... Because the success of this technology is still limited, the focus is on membranes and their further development concerning optimization of stability, efficiency and lifetime. Especially working temperatures of up to 130 °C are desirable for large scale usage [2][3][4][5][6]. For the evaluation and further development of proton exchange membranes, measurements of proton conductivity are the decisive tool. ...
In this study, we introduce a through-plane electrochemical measurement cell for proton conducting polymer membranes (PEM) with the ability to vary temperature and humidity. Model Nafion and 3M membranes, as well as anisotropic composite membranes, were used to compare through plane and in plane conductivity. Electrochemical impedance spectroscopy (EIS) was applied to evaluate the proton conductivity of bare proton exchange membranes. In the Nyquist plots, all membranes showed a straight line with an angle of 60–70 degrees to the Z’-axis. Equivalent circuit modeling and linear extrapolation of the impedance data were compared to extract the membrane resistance. System and cell parameters such as high frequency inductance, contact resistance and pressure, interfacial capacitance were observed and instrumentally minimized. Material-related effects, such as swelling of the membranes and indentation of the platinum mesh electrodes were examined thoroughly to receive a reliable through-plane conductivity. The received data for model Nafion and 3M membranes were in accordance with literature values for in-plane and through-plane conductivity of membrane electrode assemblies. Anisotropic composite membranes underlined the importance of a sophisticated measurement technique that is able to separate the in-plane and through-plane effects in polymer electrolytes.
Vanadium redox flow batteries (VRFB) are a promising technology for large‐scale storage of electrical energy, combining safety, high capacity, ease of scalability, and prolonged durability; features which have triggered their early commercial implementation. Furthering the deployment of VRFB technologies requires addressing challenges associated to a pivotal component: the membrane. Examples include vanadium crossover, insufficient conductivity, escalated costs, and sustainability concerns related to the widespread adoption of perfluoroalkyl‐based membranes, e.g., perfluorosulfonic acid (PFSA). Herein, recent advances in high‐performance and sustainable membranes for VRFB, offering insights into prospective research directions to overcome these challenges, are reviewed. The analysis reveals the disparities and trade‐offs between performance advances enabled by PFSA membranes and composites, and the lack of sustainability in their final applications. The potential of PFSA‐free membranes and present strategies to enhance their performance are discussed. This study delves into vital membrane parameters to enhance battery performance, suggesting protocols and design strategies to achieve high‐performance and sustainable VRFB membranes.
This review provides a depth of knowledge on the synthesis, properties and performance of aryl ether-free anion exchange polymer electrolytes for electrochemical and energy devices.
This study demonstrates the use of 1,5-naphthalenedisulfonic acid as a suitable building block for the efficient and economic preparation of alternating sulfonated polyphenylenes with high ion-exchange capacity (IEC) via Suzuki polycondensation. Key to large molar masses is the use of an all-meta-terphenyl comonomer instead of m-phenyl, the latter giving low molar masses and brittle materials. A protection/deprotection strategy for base-stable neopentyl sulfonates is successfully implemented to improve the solubility and molar mass of the polymers. Solution-based deprotection of polyphenylene neopentyl sulfonates at 150 °C in dimethylacetamide eliminates isopentylene quantitatively, resulting in membranes with high IEC (2.93 mequiv/g) and high proton conductivity (σ = 138 mS/cm). Water solubility of these copolymers with high IEC requires thermal cross-linking to prevent their dissolution under operating conditions. By balancing the temperature and time of the cross-linking process, water uptake can be restricted to 50 wt %, retaining an IEC of 2.33 mequiv/g and a conductivity of 85 mS/cm. Chemical stability is addressed by treatment of the membranes under Fenton’s conditions and by considering barrier heights for desulfonation using density functional theory (DFT) calculations. The DFT results suggest that 1,5-disulfonated naphthalenes are at least as stable as sulfonated polyphenylenes against desulfonation.
Metal-organic frameworks (MOFs) are unique hybrid porous materials formed by combining metal ions or clusters with organic ligands. Thiol and thioether-based MOFs belong to a specific category of MOFs where...
The development of green technologies like fuel cell is need of the day because of their zero emission and as an efficient technology to produce electrical energy. Among the different varieties of fuel cells, enhancing the performance of proton exchange membrane (PEM) fuel cell is emphasized because of their numerous advantages such as easy portability, less corrosive nature, and leakage‐free convenient setup. Generally used Nafion membranes in PEM fuel cells show few limitations such as the inability to work at high temperature and low relative humidity. Nanocomposite membranes play an indispensable role in overcoming these flaws. Incorporating numerous nanoadditives like silica, titanium dioxide, zirconium dioxide, graphene oxide, zirconium phosphate, heteropolyacids, and metal‐organic frameworks into the variety of polymer matrix such as Nafion, sulfonated polybenzimidazole, polysulfone, sulfonated poly(ether ether ketone), and biopolymers involving polyvinyl alcohol, chitosan is assessed with its characteristic properties of proton conductivity, mechanical stability, oxidative stability, and power density. Nanocomposite membranes aid to increase the mechanical stability of the PEMs by the combination of two or more polymer layers and especially with a solid support layer. Development of natural, biodegradable polymer‐based PEMs with enhanced proton conducting ability and chemical stability was possible only because of the nanocomposite model; otherwise, it was not possible. Certain hygroscopic inorganic additives improved the water uptake capacity of the nanocomposite membranes even at elevated temperatures. A large pool of nanocomposite membranes that can meet the desired characteristics of PEMs for fuel cell applications is reviewed in detail.
This review summarizes the current status, operating principles, and recent advances in high-temperature polymer electrolyte membranes (HT-PEMs), with a particular focus on the recent developments, technical challenges, and commercial prospects of the HT-PEM fuel cells. A detailed review of the most recent research activities has been covered by this work, with a major focus on the state-of-the-art concepts describing the proton conductivity and degradation mechanisms of HT-PEMs. In addition, the fuel cell performance and the lifetime of HT-PEM fuel cells as a function of operating conditions have been discussed. In addition, the review highlights the important outcomes found in the recent literature about the HT-PEM fuel cell. The main objectives of this review paper are as follows: (1) the latest development of the HT-PEMs, primarily based on polybenzimidazole membranes and (2) the latest development of the fuel cell performance and the lifetime of the HT-PEMs.
The in situ crosslinked membranes for vanadium redox flow battery (VRFB) application, prepared from sulfonated poly (arylene ether sulfone) (SPAES) and poly (vinyl alcohol) (PVA) as well as sulfonated poly (vinyl alcohol) (SPVA), are detailedly evaluated in this paper. The results of scanning electron microscope (SEM) and X-ray diffraction (XRD) reveal that the cross reaction improved the compatibility between SPAES and SPVA, and reduced the crystallinity in the membrane. The highly homogeneous dispersed structures of the crosslinked membrane are found under the electronic microscope. The crosslinked structure effectively reduces the water uptake, swelling ratio, and vanadium ion permeability, and more importantly improved the mechanical performance and stability of the membranes. Due to the introduction of sulfonic groups into the PVA, the proton selectivity of SPAES crosslinked SPVA (SPAES-C-SPVA) reaches to 3.37×10⁵ S min cm⁻³ which is 8 times higher than that of Nafion117. The VRFB single cell tests show that the cell assembled with SPAES-C-SPVA membrane significantly displays higher energy efficiency (70.1% vs 60.9% at 100 mA cm⁻²) and much longer self-discharge time than Nafion117 (98 h vs 41 h). After 100 charge-discharge cycling test, the crosslinked membrane still maintains excellent stability. The above results indicate the crosslinked membranes could be a promising candidate used in VRFB applications.
Protic ionic liquids (PILs) are discussed as new candidates for the use as non-aqueous electrolytes for fuel cells operating at temperatures above 80 °C. The molecular interactions in Diethylmethylammonium triflate ([Dema][TfO]) doped polybenzimidazole (PBI) blend membranes and the proton transport mechanism were investigated by means of TGA, IR and NMR. The mobility of the PIL ions is restricted to the PBI host polymer. The [Dema]⁺ cations and [TfO]⁻ anions interact strongly via H bonds with the polar groups of the PBI chains. This will significantly confine the proton conductivity of the membrane to vehicular transport. The proton transport was investigated by comparing to an analogous liquid state model using the monomer benzimidazole (BIm) instead of the PBI polymer. During fuel cell operation, it is unavoidable that residual water is present in significant quantities. Resulting from ¹H-NMR and PFG self-diffusion measurements, proton transport in the liquid state model takes place via a cooperative mechanism involving all of the species NH[Dema]⁺/NHBIm/H2O depending on the water fraction. Thus, it is suggested that conductivity in the PIL–PBI membrane be mainly provided by the cooperative transport of the protons. This study is intended to broaden understanding of the structure and proton transport mechanism, as well as to give possible ways to optimize PIL electrolyte doped polymer blend membranes for intermediate operating temperatures.
This reported study describes an effort to produce durable membranes for high performance vanadium redox flow batteries (VRFB). The reported novel, positively-charged membranes are derived from pyridine-containing poly(aryl ether ketone ketone) (PyPEKK) and are prepared using an environmentally-friendly method. Basic pyridine groups on the polymer backbone serve as proton acceptor sites and these electron-rich, polar sites establish proton transport channels, which provide PyPEKK membranes with an area resistance as low as 0.15 Ω cm², which is comparable to Nafion 212 (0.13 Ω cm²). The novel PyPEKK membranes have low vanadium permeability due to the Donnan repulsion between the positively charged protonated pyridine groups and the vanadium ions. Consequently, the resulting dense structure of these PyPEKK membranes produce an impressive energy efficiency in VRFB (89.0% at 80 mA cm⁻², and 80.1% at 180 mA cm⁻²). The PyPEKK membrane exhibit stable performance of 1000 cycles and low oxidation. These PyPEKK membranes are highly promising candidates for VRFB application.
Acid–base interactions between N-heterocycles and sulfonic acid groups are known to mitigate the excessive swelling of hydrocarbon-based proton exchange polymers, but concurrently reduce the concentration of hydrated protons therein, which lowers proton conductivity. We report sulfonated phenylated poly(phenylene) homopolymers designed with similar architecture but with an increasing number of strategically-placed N-atoms in the form of pyridyl units. It is discovered that polymers with 2 or more pyridines per repeat unit do not stoichiometrically neutralize pendent sulfonic acids. For example, four pyridines per repeat unit neutralize the equivalent of ∼2 sulfonic acids, resulting in a reduced number of pyridinium (H+)–sulfonate cross links than anticipated. DFT calculations reveal that externally-exposed pyridine groups form a stronger interaction with protons than a sterically-hindered pyridine that affects mechanical properties and water sorption. Nonetheless, as the number of N-atoms is increased, the fraction of neutralized –SO3H protons is increased, and the material's ion exchange capacity, proton conductivity, liquid and vaporous water sorption, dimensional swelling, steady-state water permeability, and transient diffusivity all decrease. With four pyridyl groups per repeating unit, dimensional swelling of the fully hydrated polymer, steady-state water permeability are similar to the Nafion N211 reference material. However, proton conductivities under reduced RH are substantially reduced due to their low water sorption. This work provides insight into tailoring proton exchange membranes via acid–base, self-neutralization for the purpose of controlling their transport properties.
Several star-shaped sulfonated poly(ether triazole)s abbreviated as PTAPSH-XX was prepared by Cu (I) catalyzed click polymerization of a mixture of a new tri-azide monomer namely, 4,4-tris[3-trifluoromethyl-4(4-azidophenoxy)phenyl]biphenyl (PGZ) (B3) and 4,4′-diazido-2,2′-stilbene disulfonic acid disodium salt (DADSDB) with an equimolar amount 4,4′-(propane-2,2-diyl) bis((prop-2-ynyloxy)benzene) (BPEBPA). The degree of sulfonation of the prepared copolymers was varied by changing the molar ratio of DADSDB in the polymer feed. The monomer and copolymers were characterized by FTIR and NMR spectroscopic techniques. The salt form of the branched copolymers was converted into flexible membranes by solution casting technique using dimethyl sulfoxide (DMSO) as solvent and was converted to their acid form by acidification in 1.5 M H2SO4. The introduction of the branching unit largely governs the physicochemical properties and proton conductivity of the membranes. The polymers displayed desirable set of properties required proton exchange membrane applications like high thermal stability (Td10 ~ 259–290 °C), low water uptake and excellent dimensional integrity. The membranes also displayed improved mechanical properties (TS = 51 to 73 MPa, YM = 2.07 to 2.37 GPa), enhanced oxidative stability and high proton conductivity (44 to 102 mS cm⁻¹ at 80 °C). The TEM images of the PTAPSH-XX membranes revealed excellent phase segregated morphology with the formation of ionic clusters within 15 to 90 nm.
Developing functional membrane with high conductivity and excellent stability is critical for improving the performance of vanadium redox flow batteries. In this work, we propose and prepare an asymmetric porous polybenzimidazole membrane composed of an ultra‐thin dense layer and a sponge‐like porous layer. The dense layer takes the main role of inhibiting vanadium crossover while the porous layer provides not only a fast proton conduction channel but also a mechanical support to the dense layer. Hence, the asymmetric PBI membrane exhibits a high proton conductivity (36.4 mS cm⁻¹ at room temperature) and an extremely low vanadium permeability (0.26 × 10⁻⁷ cm² min⁻¹). The vanadium redox flow battery with this membrane achieves an energy efficiency of 83.17% at 400 mA cm⁻² and 78.59% at 500 mA cm⁻². Furthermore, the battery enables to be continuously cycled for 1600 times without significant degradation at 400 mA cm⁻². All these results prove that the PBI membrane with an asymmetric porous structure is an ideal membrane for redox flow batteries.
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Chemically modified graphene-reinforced polybenzimidazole (PBI) nanocomposites were prepared by liquid-phase exfoliation of graphene oxide (GO) and reduced graphene oxide (rGO) in methanesulphonic acid (CH4O3S), followed by in-situ polymerization using GO–CH4O3S and rGO–CH4O3S suspensions as reaction media. Various reducing agents were used to produce rGOs and their reducing efficiency was examined to attain highly graphitic structure and excellent electrical conductivity of the resulting rGOs. The results of Raman, Fourier transform infrared and X-ray photoelectron spectroscopy indicate higher extent of reduction of GO with hydrazine compared to other reducing agents. The PBI nanocomposite containing 10 wt% rGO derived from hydrazine reduction reaction (rGO–H) exhibits the highest dc conductivity of 2.77 × 10−3 S cm−1 at room temperature, which is 11 orders of magnitude higher than pure PBI. The thermal annealing treatment at 350°C resulted in a substantial increase in dc conductivity of the PBI/GO nanocomposite, whereas the enhancement of conductivity is much less for the PBI/rGO nanocomposites. Compared to pure PBI, both tensile strength and Young’s modulus enhanced by 3.4 times and 6.9 times, respectively, for the PBI nanocomposites with 10 wt% GO content, which is ascribed to strong interfacial interactions and subsequent effective stress transfer between the PBI matrix and GO. The PBI/rGO nanocomposites exhibited relatively lower tensile strength/modulus compared to the GO-reinforced nanocomposite. The thermal stability of PBI was significantly improved upon the incorporation of both GO and rGO nanosheets, whereas higher thermal stability was achieved for rGO-reinforced nanocomposites.
Protic ionic liquids (PILs) based on the anion bis(trifluoromethanesulfonyl)imide were confined in polybenzimidazole (PBI) matrices. Quasi-solidified ionic liquid membranes (QSILMs) were fabricated and examined for mechanical and thermal stability. After doping in phosphoric acid (PA), the QSILMs exhibited conductivities of 30–60 mS cm⁻¹ at 180 °C. Fluorescence microscopy was used to investigate the structure of the composite PBI membranes. Membrane-electrode assemblies, fabricated with PA doped QSILMs, were tested in a single fuel cell and exhibited a performance increase with increasing temperature up to 200 °C. The best performance was obtained for the membrane electrode assembly containing 50 mol% of diethyl-methyl-ammonium bis(trifluoromethylsulfonyl)imide confined in the phosphoric acid doped PBI matrix with closed porosity. It reached 0.32 W cm⁻² at 200 °C and 900 mA cm⁻² at which voltage? The catalyst layer of the gas diffusion electrode impregnated with protic ionic liquid exhibited better long-term stability than the gas diffusion electrode impregnated with phosphoric acid within 100 h of operation at 200 °C and anhydrous conditions.
The structure-property relationship of sulfonated phenylated poly(phenylene)s possessing either angled or linear backbone moieties was investigated. Polymers were synthesized using either bent (ortho or meta) or linear (para) biphenyl linkages and evaluated for differences in physical and electrochemical properties. Model compounds, structurally analogous to the polymers, were prepared and characterized using spectroscopic and computational methods to elucidate structural differences and potential impacts on the properties of the respective polymers. A highly angled ortho biphenyl linkage resulted in a sterically hindered, rotationally restricted molecule. When incorporated into a polymer, the angled ortho biphenyl moiety was found to prevent membrane formation. The angled meta biphenyl-containing polymer, while forming a membrane, exhibited a 74% increase in volumetric expansion, 31% reduction in tensile strength, and 72% reduction in elongation at break when compared to the linear para biphenyl-containing analogue. The differences observed are attributed to a rotationally restricted backbone in the angled biphenyl systems. Collectively, this study suggests that incorporating angled biphenyl linkages into sulfonated phenylated poly(phenylene)s leads to highly rigid, inflexible backbones that prevent chain entanglement and the formation of free-standing membranes.
Proton exchange membrane fuel cells (PEMFCs) have garnered considerable attention and applications in the field of transportation because they achieve eco-friendly electricity generation with water as the only by-product. As the preferred solid electrolyte in PEMFCs, Nafion possesses various desirable attributes and high proton conductivity, but its prohibitive cost and practical limitations on operation are problematic. Recently, several types of porous platforms, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), porous organic polymers (POPs), and hydrogen-bonded organic frameworks (HOFs) have been deployed to develop conducting systems. Post-synthetic modification for porous platforms is a flagship smart methodology in membrane electrolyte fabrication for fuel cells that concurrently combines original and other desirable features that are complementary to each other and induce enhanced conductivity. Additionally, the introduction of proton conductive mixed matrix membranes, which have recently received considerable attention as a practical method to fabricate membranes, has inspired recent research trends. This review discusses post-synthetic modification-based proton conductors and their membranes in terms of design strategies, conduction mechanisms, and diverse diagnostic modalities for future electrolyte materials in fuel cell technology.
Organic-inorganic proton electrolytes were investigated by the cross-linking of various organic monomers (PDMS, PTMO) and metal (Zr, Ti) alkoxides. Proton conductors of 12-phosphotungstic (PWA) and phosphoric acids interacted with inorganic phase of organic-inorganic hybrids. Some hybrids showed high mechanical and thermal stability up to 300°C. Moreover, high proton conducting hybrids were investigated a cell performance using single MEA cell. The organic-inorganic conducting hybrids, which have high proton conductivity and thermal stability, can be expected for applications in intermediate temperature PEFCs.
In this paper, a phosphoric acid fuel cell integrated with reformer and evaporator is demonstrated. Oxidative steam reforming of methanol (OSRM) process is employed in this system in cooperated with a high efficient evaporator, and the reacted gas is sent into a phosphorus-acid fuel cell (PAFC) for direct power generation after surplus methanol/water filtration. The results show that the maximum power density of this hybrid system achieves 277 mW/cm² without CO2 removal, while it achieves 485 mW/cm² when employing pure hydrogen as the fuel.
Proton-conducting ionic liquids (PILs) are discussed herein as potential new electrolytes for polymer membrane fuel cells, suitable for operation temperatures above 100 °C. During fuel cell operation, the presence of significant amounts of residual water is unavoidable, even at theses elevated temperatures. By performing electrochemical and NMR methods, the impact of residual water on 2-Sulfoethylmethylammo-nium triflate [2-Sema][TfO], 1-Ethylimidazolium triflate [1-EIm][TfO] and Diethyl-methylammonium triflate [Dema][TfO] are analyzed. The cationic acidity of these PILs varies by over ten orders of magnitude. Appropriate amounts of the PIL and H2O were mixed at various molar ratios to obtain compositions, varying from the neat PIL to H2O-excess conditions. The conductivity of [2-Sema][TfO] exponentially increases depending on the H2O concentration. The results from 1H-NMR spectroscopy and self-diffusion coefficient measurements by 1H field-gradient NMR indicate a fast proton exchange process between [2-Sema]+ and H2O. Conversely, [1-EIm][TfO] and [Dema][TfO] show only a very slow or non-significant, respectively, proton exchange with H2O during the time-scale relevant for transport. The proton conduction follows a combination of vehicle and cooperative mechanisms in high acidic PIL, while a mostly vehicle mechanism in medium and low acidic PIL occurs. Therefore, high acidic ionic liquids are promising new candidates for polymer electrolyte fuel cells at an elevated temperature.
When complexed with alkaline such as potassium hydroxide, sodium hydroxide or lithium hydroxide, films (40 μm thick) of polybenzimidazole (PBI) show conductivity in the 5 × 10−5–10−1 S/cm−1 range, depending on the type of alkali, the time of immersion in the corresponding base bath and the temperature of immersion. It has been shown that PBI has a remarkable capacity to concentrate KOH, even in an alkaline bath of concentration 3 M. The highest conductivity of KOH-doped PBI (9×10−2 S cm−1) at 25°C obtained in this work is higher than the we had obtained previously as optimum values for H2SO4-doped PBI (5 × 10−2 S cm−1 at 25°C) and H3PO4-doped PBI ( 2 × 10−3 S cm−1 at 25°C). PEMFCs based on an alkali-doped PBI membrane were demonstrated, and their characteristics exhibited the same performance as those of PEMFCs based on Nafion® 117. Their development is currently under active investigation.
Solid Polymer Electrolyte Membranes (SPEMs) play a vital role in polymer electrolyte fuel cell systems. The high cost (US$70-150/ft2) and/or poor performance of the existing membranes prompted many industrial and university research groups world-wide to develop very specific and low cost (US$2-50/ft2) polymer-based electrolyte membranes. This paper is discussing progress on these key topics and the openings for the future. The advantages and disadvantages of the already developed perfluorinated ionomer membranes (Nafion, Flemion, Aciplex, Dow or Asahi Chemical) are described. The development of SPEMs during the last ten years is investigated and analysed in terms of methods of preparation, properties and potential uses in polymer electrolyte fuel cell systems. The SPEMs, which are in development, has been classified as perfluorinated polymer, partially perfluorinated polymer and non-perfluorinated membranes. The advantages and disadvantages for each type of SPEMs are addressed. The openings for industrial applications of the various SPEMs are discussed including their limits and the future studies which may be done to improve their performance in practical systems.
Anhydrous mixtures of PolyBenzImidazole (PBI) with H3PO4, H2SO4 and HBr have been investigated by Infra Red spectroscopy and impedance measurements. The IR study indicates that the nitrogen of the imide is protonated by the acids. The anions are linked to the polymer by rather strong hydrogen bonding. The conductivity of the acid doped PBI is governed by an activated mechanism (Arrhenius law). Strong correlations between the conductivity and the nature of the associated anions can be established. The best conductivity is achieved with H2SO4. Typically, PBI–3H2SO4 exhibits an anhydrous conductivity of 10−4 Scm−1, at 300 K.
A state-of-the-art in radiation-induced graft copolymerization of styrene and acrylic acid monomers into Teflon-FEP films is presented with a view to develop proton exchange membranes for various applications. This process offers an easy control over the composition of
a membrane by careful variation in radiation dose, dose rate, monomer concentration, and temperature of the grafting reaction. By varying the nature and the amount of the grafted content, it is possible to achieve a membrane with desired physico-chemical properties. In this paper, a correlation
among the degree of grafting, structural changes, and properties of graft copolymer membranes is discussed.
Protonic conduction in liquid electrolytes is commonplace but is relatively rare in solids. There is much interest worldwide in proton conducting solids, both from the scientific aspect, as materials with novel properties, but also for their possible applications in high-density solid-state batteries, sensors and other electrochemical devices. This book gives a comprehensive review of proton conductors, including theory, techniques, the materials themselves and applications.
Protonic conduction in liquid electrolytes is commonplace but is relatively rare in solids. There is much interest worldwide in proton conducting solids, both from the scientific aspect, as materials with novel properties, but also for their possible applications in high-density solid-state batteries, sensors and other electrochemical devices. This book gives a comprehensive review of proton conductors, including theory, techniques, the materials themselves and applications.
A new process has been developed for the sulfonation of arylene polymers which can be lithiated, like polysulfone Udel®. The sulfonation process consists of the following steps: (1) lithiation of the polymer at temperatures from −50 to −80°C under argon, (2) gassing of the lithiated polymer with SO2; (3) oxidation of the formed polymeric sulfinate with H2O2, NaOCl, or KMnO4; (4) ion-exchange of the lithium salt of the sulfonic acid in aqueous HCl. The advantages of the presented sulfonation procedure are: (1) in principle all polymers which can be lithiated can be subjected to this sulfonation process; (2) by this sulfonation procedure the sulfonic acid group is inserted into the more hydrolysis-stable part of the molecule; (3) this process is ecologically less harmful than many common sulfonation procedures. The sulfonated polymers were characterized by NMR, titration and elemental analysis, by IR spectroscopy, and by determination of ionic conductivity. Also the hydrolytic stability of the sulfonated ion-exchange polymers was investigated. Polymers with an ion-exchange capacity of 0.5 to 3.2 mequiv SO3H/g Polymer have been synthesized and characterized. The following results have been achieved: membranes made from the sulfonated polymers show good conductivity, good permselectivity (>90%), and good hydrolytic stability in 1N HCl and water at temperatures up to 80°C. © 1996 John Wiley & Sons, Inc.
ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY
Volume 227
Pages U370-U370
Non-aqueous proton conducting polymer gel electrolytes containing different weak carboxylic acids viz. ortho-, meta-, parahydroxybenzoic acid (o-HBA, m-HBA, p-HBA) and succinic acid (SA) gelled with polyvinylidenefluoride (PVdF) and polyvinylidenefluoride-cohexafluoropropylene (PVdF-HFP) have been synthesized and characterized in the present study. The conductivity of these electrolytes has been found to be of the order of 10 -4 S/cm at room temperature and they show excellent stability over the operational temperature range. The conductivity of these electrolytes has been found to depend on dissociation constant of acid, nature of the solvent and the concentration of the polymer used. The addition of polymer to these electrolytes results in change in the electrical properties which has been explained to be due to the active role played by the polymer.
The protonic conductivity of polybenzimidazole (PBI) in various electrolytes was systemically studied. It was shown that blank PBI is a protonic insulator. Its conductivity measured in acid electrolyte is ∼10-10 to 10-9 S- cm-1. The same conductivity was obtained for measurements taken on PBI used in a PEM fuel cell configuration. After doping in H2SO4, HCl, HNO3, HClO4 or H3PO4 it exhibited high protonic conductivity. The conductivity depends on the type of doping acid agent and its concentration. The highest conductivity obtained (i.e. 0.0601 S · cm-1 for PBI doped with 16 mol/L H2SO4) was as good as that of Nafion® 117. The conductivity changes in the order H2SO4 > H3PO4 > HClO4 > HNO3 > HCl for high concentrations (11-16 mol/L) of the doping acid. It was shown that the high conductivities of PBI films were obtained after doping them with H2O4, H3PO4 or HClO4. These high values make them suitable candidates for fuel cell applications. From these results, it was shown that the conductivity of blank PBI in acid media must be measured before the film is doped with acid.
In 1983, Celanese Corporation commercialized PBI fiber, spun from solutions of poly left bracket 2,2 prime -(m-phenylene)-5,5 prime -bibenzimidazole right bracket , for a wide range of textile applications. And, with a unique, new polymer commercially available for the first time, Celanese also undertook the development and evaluation of other forms of the polymer. Process and application development of PBI films, fibrids, papers, microporous resin, sizing, coatings and molding resins have been started. Applications for PBI utilize its unique chemistry, a polymeric secondary amine, as well as its thermal and chemical stability. A historical review of the development of Polybenzimidazoles is presented in this paper.
Recently, polybenzimidazole membrane doped with phosphoric acid (PBI) was found to have promising properties for use as a polymer electrolyte in a high temperature (ca. 150 to 200 C) proton exchange membrane direct methanol fuel cell. However, operation at 200 C in strongly reducing and oxidizing environments introduces concerns of the thermal stability of the polymer electrolyte. To simulate the conditions in a high temperature fuel cell, PBI samples were loaded with fuel cell grade platinum black, doped with ca. 480 mole percent phosphoric acid (i.e., 4.8 H{sub 3}PO{sub 4} molecules per PBI repeat unit) and heated under atmospheres of either nitrogen, 5% hydrogen, or air in a thermal gravimetric analyzer. The products of decomposition were taken directly into a mass spectrometer for identification. In all cases weight loss below 400 C was found to be due to loss of water. Judging from the results of these tests, the thermal stability of PBI is more than adequate for use as a polymer electrolyte in a high temperature fuel cell.
Gas permeation properties for Nafion membranes and their composites were investigated under variou conditions. The permeability coefficients of Nafion depended greatly on the water content, the cation form, an the ion-exchange capacity. The gas permeation rate through a same sample varied with temperature, pressure, and membrane thickness. The permeability of hydrogen was about twice as great as that of oxygen. The electrocatalyst plated on the membrane did not serve as a barrier for gas permeation, but the structure of the catalyst layer played an important role in gas permeation during water electrolysis.
The thermal, mechanical and electrochemical characterisation of sitlfonated polyetherketone, including fuel cell tests in hydrogen/ oxygen and hydrogen/air are described. In thermogravimetric analysis, PEEK-S membranes lose water up to 150°C and degradation of the sulfonic acid groups takes place at ca. 240°C. Thermomechanical analysis of a PEEK-S membrane of 60 μm thickness and equivalent weight 625 g/mole shows that the membrane undergoes a shrinkage of 1.5 % up to 140°C. Reversible elongation of 0.6 % occurs thereafter up to 180°C. The conductivity, measured by impedance spectroscopy, on non-reinforced and on woven-polymer reinforced PEEK-S, is reported as a function of temperature and of relative humidity (RH), and compared with that of Nafwn®-117. At 100°C and 100% RH the conductivity of PEEK-S is 2 5.10-2 Scm-1 (depending on thermal history), increasing to 0.11 Scm-1 at 150°C. Polarisation characteristics of a non-reinforced PEEK-S membrane of 18 ®m thickness at temperatures up to 110°C under conditions of hydrogen/air and hydrogen/oxygen are compared. The results of fuel cell (H2-O2) tests on composite, reinforced membranes are reported.
Sulfonated polyetheretherketone has been used as polymer matrix for hybrid membrane formation with inorganic proton conductors. Membranes incorporating up to 40 % w/w of inorganic proton conductor, including amorphous silica, zirconium phosphate sulfophenylphosphonate and zirconium phosphate, have been prepared. The membranes have been characterised using powder X-ray diffraction and MAS NMR spectroscopy where appropriate, and conductivity measurements performed as a function of relative humidity (RH) at 100 °C and as a function of temperature at 100 % RH. In all cases, the presence of the inorganic particles leads to an increase in conductivity of the polymer membrane, and is without detriment to its flexibility. For example, PEEK-S-based membranes containing 10 % amorphous silica, 30 % zirconium phosphate or 40 % amorphous zirconium phosphate sulfophenylphosphonate present conductivities in the range 0.03 - 0.09 Scm-1 at 100 °C/100 % RH. Polarisation characteristics for PEEK-S membranes containing 25 and 13 % w/w of zirconium phosphate under conditions of hydrogen/ oxygen and hydrogen/air respectively are reported, up to 120°C.
Solid state electrolyte membranes for direct methanol fuel cells were prepared from proton conductors and a polymer binder. The most promising proton conductor was tin oxide-containing mordenite, the conductivity of which was retained in the membranes. The diffusion of methanol across the membranes and the solvent uptake were lower than for commercial electrolyte membranes.
The temperature dependence of the conductivity is shown to be governed by the segmental chain motion. The frequency dependence of the phosphorus spin-lattice relaxation rate is interpreted using a stretched exponential correlation function exp[−(t/τ)β] with β=0.277. The long-range diffusion coefficient of the molecule (measured by the pulsed field gradient NMR technique) is shown to be quite close to the short-range diffusion coefficient of the chain segments determined from NQES.
The electro-oxidation of formic acid was studied in a direct-oxidation polymer-electrolyte fuel cell at 170 C using real-time mass spectrometry. The results are compared with those obtained for methanol oxidation under the same conditions. Formic acid was electrochemically more active than methanol on both Pt-black and Pt-Ru catalysts. The polarization potential of formic acid oxidation was ca. 90 to 100 mV lower than that of methanol. The oxidation of formic acid was dependent on the water/formic acid mole ratio. The best anode performance was obtained using a water/formic acid mole ratio of â¼2. In addition, Pt/Ru catalyst was more active than Pt-black for formic acid oxidation. The mass spectrometric results showed that COâ is the only reaction product of formic acid oxidation. The results are discussed in terms of possible formic acid oxidation mechanisms.
When complexed with a strong inorganic acid such as sulfuric or phosphoric acid, films of polybenzimidazole (PBI) show two types of behaviour, depending on the time of immersion in the corresponding acid bath. The first type, prepared at shorter doping times, has conductivity in the range 10–5–10–4 S cm–1, whilst the second, of conductivity >10–3 S cm–1 is formed after more prolonged immersion. The ‘switch-over' from one state to the other is at 10–11 h in H3PO4, and 2–3 h in H2SO4. PBI has a remarkable capacity to concentrate H3PO4 and, even in an acid bath of concentration 3 mol dm–3, the acid concentration within a PBI membrane is ca. 14.5 mol dm–3. IR spectroscopy performed on hydrated PBI membranes as a function of temperature, and on acid-complexed membranes as a function of the amount of sorbed acid confirms proton transfer from H3PO4 to the imino groups of PBI and, at high doping levels, the presence of undissociated H3PO4.
The electro-osmotic drag coefficient of water in two polymer electrolytes was experimentally determined as a function of water activity and current density for temperatures up to 200 C. The results show that the electro-osmotic drag coefficient varies from 0.2 to 0.6 in Nafion{reg_sign}/HâPOâ membrane electrolyte, but is essentially zero in phosphoric acid-doped PBI (polybenzimidazole) membrane electrolyte over the range of water activity considered. The near-zero electro-osmotic drag coefficient found in PBI indicates that this electrolyte should lessen the problems associated with water redistribution in proton exchange membrane fuel cells.
Two complementary methods were developed to produce sulphonated poly(oxy-1,4-phenylene-oxy-1,4-phenylene-carbonyl-1,4-phenylene) (SPEEK) with random, homogeneous compositions over the range of zero to one sulphonate group per repeat unit. The sodium salts (Na-SPEEK) were prepared from about 5 to 100% sodium sulphonate. They displayed excellent thermal stability. The behaviour of Tg, ΔTg and ΔCp at the glass transition as a function of composition suggested the onset of ionic clustering below 25 to 30% sodium sulphonate—an observation confirmed by preliminary SAXS studies. In particular, Tg increased sigmoidally from about 150°C for 5% Na-SPEEK to 415°C for 100% Na-SPEEK. No evidence of crystallinity was observed by d.s.c. in melted and quenched samples above 9% sodium sulphonate. The equilibrium water content at room temperature and 58% relative humidity was four molecules of water per sodium sulphonate group for all compositions. For immersed films, this value increased from 8 molecules of water per sodium sulphonate group for 38% Na-SPEEK to an indeterminably large number for 100% Na-SPEEK, which slowly dissolved. Upon re-equilibration at 58% relative humidity, the water content of the films decreased to about 5.5 molecules per sodium sulphonate group. A low temperature (−80°C to −60°C) mechanical relaxation peak was present in the films conditioned at 58% relative humidity.
In this contribution an overview is given about the state-of-the-art at the membrane development for proton-conductive polymer (composite) membranes for the application membrane fuel cells, focusing on the membrane developments in this field performed at ICVT.For preparation of the polymers, processes have been developed for sulfonated arylene main-chain polymers as well as for arylene main-chain polymers containing basic N-containing groups, including a lithiation step. Covalently cross-linked polymer membranes have been prepared by alkylation of the sulfinate groups of sulfinate group-containing polymers with α,ω-dihalogenoalkanes. The advantage of the covalently cross-linked ionomer membranes was their dimensional stability even at temperatures of 80–90°C, their main disadvantage their brittleness when drying out, caused by the inflexible covalent network. Sulfonated and basic N-containing polymers (commercial polymers as well as self-developed ones) have been combined to acid–base blends containing ionic cross-links. The main advantage of these membrane type was its flexibility even when dried-out, its good to excellent thermal stability, and the numerous possibilities to combine acidic and basic polymers to blend membranes having fine-tuned properties. The main disadvantage of this membrane type was the insufficient dimension stability at T>70–90°C, caused by breakage of the ionic cross-links, where the ionic cross-links broke as easier as lower the basicity of the polymeric base was. Some of the acid–base blend membranes were applied to H2 membrane fuel cells and to direct methanol fuel cells up to 100°C, yielding the result that these membranes show very good perspectives in the membrane fuel cell application.
Phosphoric acid doped polybenzimidazole (PBI-poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole]) has been investigated for use in a H2O2 fuel cell. The prototype fuel cell test results show that the PBI fuel cell worked quite well at 150 °C with atmospheric pressure hydrogen and oxygen which were humidified at room temperature. No membrane dehydration was observed over 200 h operating. The maximum power density of this prototype fuel cell was 0.25 W cm−2 at current density of 700 mA cm2. Further improvement of the cell performance is to be anticipated by properly impregnating the electrode structure with the polymer electrolyte. The advantage of the H2O2 fuel cell using PBI as polymer electrolyte is that the cell design and the routine maintenance can be significantly simplified because of the low electro-osmotic drag number and good proton conductivity of the PBI membrane at elevated temperature.
Poly(oxy-1,4-phenylene-oxy-1,4-phenylene-carbonyl-1,4-phenylene) (PEEK) and poly(4-phenoxybenzoyl-1,4-phenylene, Poly-X 2000) (PPBP), were sulfonated with sulfuric acid by incorporating sulfonic acid moieties in order to convert these polymers to proton-conducting polymers. The sulfonated polymers containing 65 mol% sulfonic acid showed a high proton-conductivity of 10−2–10−4 Scm−1 at room temperature. Sulfonated PPBP showed a much higher and more stable proton conductivity than sulfonated PEEK, which is in agreement with the strong water absorption of the former compound.
Polyhydrated hydrogen-magnesium and hydrogen-chromium forms of α-zirconium phosphate (α-ZrHMg0.5(PO4)2·3H2O and α-ZrH0.5Cr0.5(PO4)2·5H2O, respectively) were investigated by X-ray and TG measurements to determine the relative humidity range in which the polyhydrated phases are stable. In both cases the X-ray patterns and water content kept unaltered for RH 22%. The electrical transport properties of these compounds were investigated by: (a) impedance measurements at different temperatures (from 20 to -20°C) and relative humidities (from 90 to 22%), to have information about bulk/surface conduction; (b) combined ac/dc measurements at 25°C and 75% RH; (c) measurements of the EMF of the cell: Pt, H2/HM/HH/Pt, H2, at 25°C and 75% RH, where HH is monohydrogen zirconium phosphate and HM the hydrogen-salt form. The last two techniques were employed to separate protonic, ionic and electronic contributions to the total conductivity. The conductivity of both compounds was found to be dependent on relative humidity and thus on surface hydration; the protonic component turned out to be dominant.
Series of sequenced sulfonated naphthalenic polyimides with improved solubility were prepared by polycondensation in m-cresol using aromatic diamines containing phenylether bonds and/or bulky groups. Sulfonated polyimides were characterized by NMR and IR spectroscopies. Membranes were prepared by solution casting method and characterized by determining the ion-exchange capacity, water swelling, proton conductivity whereas the morphology of polymer membranes was studied by small angle neutron scattering.
A new type of reinforced composite perfluorinated polymer electrolyte membrane, GORE-SELECT{trademark} (W.L. Gore & Assoc.), is characterized and tested for fuel cell applications. Very thin membranes (5-20 {mu}m thick) are available. The combination of reinforcement and thinness provides high membrane, conductances (80 S/cm{sup 2} for a 12 {mu}m thick membrane at 25{degrees}C) and improved water distribution in the operating fuel cell without sacrificing longevity or durability. In contrast to nonreinforced perfluorinated membranes, the x-y dimensions of the GORE-SELECT membranes are relatively unaffected by the hydration state. This feature may be important from the viewpoints of membrane/electrode interface stability and fuel cell manufacturability.
Polybenzimidazole films doped with phosphoric acid are being investigated as potential polymer electrolytes for use in hydrogen/air
and direct methanol fuel cells. In this paper, we present experimental findings on the proton conductivity, water content,
and methanol vapor permeability of this material, as well as preliminary fuel cell results. The low methanol vapor permeability
of these electrolytes significantly reduces the adverse effects of methanol crossover typically observed in direct methanol
polymer electrolyte membrane fuel cells.
A protonic polymer electrolyte based on polybenzimidazole is proposed for use in a hydrogen sensor, operating at room temperature,
in air. The potentiometric response in inert gas containing hydrogen obeys the Nernst equation. In air, the EMF is a mixed
potential which results from the hydrogen oxidation and the oxygen reduction. The response time is of the order of 30 s whatever
the hydrogen concentration step within the hydrogen partial pressure range from
to 1 atm.
Two new polyelectrolytes were synthesized by sulfonation of poly(2,2'-m-phenylene-5,5'-bibenzimidazole) with 1,3-propanesultone and sodium 4-(bromomethyl)benzene sulfonate, respectively. The new polymers exhibit a significantly higher solubility than the parent material but retain much of its thermal stability
A polymer, poly(2,5-trimethylene benzimidazole), the first of a new family of nonsymmetrical polymers, was synthesized via an eight-step synthetic route. The polymer, obtained by melt polymerization, is amorphous and in its neutral form behaves as a moderate insulator. It forms 1 : 1 HCl adducts. When cast from formic acid solution, it forms 1 : 1 formic acid adducts. The acid adducts are semiconductors with resistivities in the 106–108 ohm-cm range. Space-charge effects are generated in the adducts as carrier mobility rises.
Polyaromatic ether-ketones possessing high thermal stabilities were prepared by Friedel—Crafts polymerizations from 4-phenoxybenzoyl chloride and from 4,4′-dichloroformyldiphenyl ether and diphenyl ether. Sulfonation and subsequent sulfamidation of these polymers afforded polyaromatic ether-ketone sulfonamides with different degrees of substitution depending on the reaction conditions. Sulfonation reactions with chlorosulfonic acid did not cause much degradation on the polymers. The polysulfonamides were soluble in various organic solvents such as N,N-dimethylformamide, dimethylsulfoxide, or chloroform, and could be cast into transparent films. These polymers may be used as desalination membranes.
Wholly aromatic polybenzimidazoles were synthesized from aromatic tetraamines and difunctional aromatic acids and characterized as new thermally stable polymers. The melt polycondensation of aromatic tetraamines and the diphenyl esters of aromatic dicarboxylic acids was developed as a general procedure of wide applicability. Polybenzimidazoles containing mixed aromatic units in the chain backbone were prepared from 3,3′-diaminobenzidine, 1,2,4,5-tetraaminobenzene and a variety of aromatic diphenyl dicarboxylates. Phenyl 3,4-diaminobenzoate could also be polymerized by melt condensation to give poly-2,5(6)-benzimidazole. The polymers were characterized by a high degree of stability, showing great resistance to treatment with hydrolytic media and an ability to withstand continued exposure to elevated temperatures. Most of the polymers were infusible, but some had melting points above about 400°C. Many of the polymers exhibited no change in properties on being heated to 550°C. and showed a weight loss of less than 5% when heated under nitrogen for several hours to 600°C. The polymers were soluble in concentrated sulfuric acid and formic acid, producing stable solutions. Many of the polymers were soluble in dimethyl sulfoxide and some also in dimethylformamide. The inherent viscosities of a number of polymers in 0.5% dimethyl sulfoxide solution ranged from approximately 0.4 to 1.1. The higher polymers could be cast into stiff and tough films from formic acid and dimethyl sulfoxide solutions.
A new process has been developed for the sulfonation of arylene polymers which can be lithiated, like polysulfone Udel®. The sulfonation process consists of the following steps: (1) lithiation of the polymer at temperatures from −50 to −80°C under argon, (2) gassing of the lithiated polymer with SO2; (3) oxidation of the formed polymeric sulfinate with H2O2, NaOCl, or KMnO4; (4) ion-exchange of the lithium salt of the sulfonic acid in aqueous HCl. The advantages of the presented sulfonation procedure are: (1) in principle all polymers which can be lithiated can be subjected to this sulfonation process; (2) by this sulfonation procedure the sulfonic acid group is inserted into the more hydrolysis-stable part of the molecule; (3) this process is ecologically less harmful than many common sulfonation procedures. The sulfonated polymers were characterized by NMR, titration and elemental analysis, by IR spectroscopy, and by determination of ionic conductivity. Also the hydrolytic stability of the sulfonated ion-exchange polymers was investigated. Polymers with an ion-exchange capacity of 0.5 to 3.2 mequiv SO3H/g Polymer have been synthesized and characterized. The following results have been achieved: membranes made from the sulfonated polymers show good conductivity, good permselectivity (>90%), and good hydrolytic stability in 1N HCl and water at temperatures up to 80°C. © 1996 John Wiley & Sons, Inc.
In the presented paper, the preparation and characterization of new ionomer blend membranes containing sulfonated poly(etheretherketone) PEEK Victrex® is described. The second blend components were Polysulfone Udel®-ortho-sulfone-diamine, polymide PA Trogamid P (producer: Hüls) and poly(etherimide) PEI Ultem (producer: General Electric). In the blend membranes swelling was reduced by specific interaction, in the case of the blend components PA and PEI hydrogen bonds, and in the case of the blend component PSU–NH2 (partial) polysalt formation, leading to electrostatic interaction between the blend component macromolecules, and hydrogen bonds. The acid–base interactions also led to decrease of ionic conductivity by partial blocking of SO−3 groups for cation transport, compared with the ionic conductivity of the hydrogen bond blends. The acid–base blends showed better ion permselectivities than the hydrogen bond blends, even at high electrolyte concentrations, and thus better performance in electrodialysis. The thermal stability of the investigated blends was very good and in the case of the acid–base blends even better than the thermal stability of pure PEEK–SO3H. DSC traces of the blend membranes showed only one Tg. In addition, the membranes are transparent to visible light. But therefrom it cannot be concluded that the blend components are miscible to the molecular level: at the acid–base blend blends, the Tg of PEEK–SO3H is very similar to the Tg of PSU–NH2, and in the investigated hydrogen bond blends, the portion of PA or PEI, respectively, might be too low to be detected by DSC. The investigated blend membranes showed similar performance as the commercial cation-exchange membrane CMX in electrodialysis (ED) application. The performance of the acid–base blend membrane is better than the performance of the hydrogen bonded PEEK–PA blend, especially in the ED experiment applying the higher NaCl concentration. This is mainly due to the lower swelling and thus better ion permselectivity of the acid–base blend membrane, compared with the PEEK–PA blend. To get a deeper insight into the microphase structure of the investigated blends, dynamic mechanical analyses and TEM investigations of the prepared blend membranes are planned. In addition, due to their promising properties, the preparation of arylene main-chain acid–base blends with other polymeric acidic and basic components is planned. Furthermore, the acid–base blend membranes will be tested in H2 polymer electrolyte fuel cells and direct methanol fuel cells, because preliminary tests have shown that they have a good perspective in this application.
Ionic groups incorporated into a polymer have a decided effect on its physical properties. A number of ionomers and polyelectrolytes have been widely applied. In particular, sulfonated bisphenol-A polysulfone (SPSF) has been used as a composite or single-component membrane for the desalination of water. In this article, the synthesis and physical characteristics of sulfonated polysulfone are addressed. A detailed synthesis route is provided and methods that yield determinable levels of sulfonation are described. These ion-containing polymers retain an excessive amount of residual salts, which, of course, are impurities to the system. Therefore, before any analyses were made the polymers were subjected to a thorough soxhlet extraction process with boiling water, which appeared to be quite effective. The degree of sulfonation was assessed by several methods such as 1H NMR and FT-IR. A new 1H NMR method was derived because the method cited in the literature proved to be too inconsistent for our work. The new 1H NMR method used a quaternary ammonium counterion [N(CH3)4]. These methyl protons are easily measured and may be ratioed against the isopropylidene protons in the polymer backbone that act as an internal standard. Characterization of the physical properties of SPSF consisted of water uptake, differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and solubility studies. Its physical appearance and mechanical behavior were improved by the solution procedure. Also addressed were the effects of different counterions (Na+ & Mg++) with SPSFs of low levels of sulfonation. The variation in physical properties between the divalent and monovalent counterions is dramatic, especially when observed by TMA in the rubber plateau above the apparent glass temperature.
A direct methanol/oxygen solid polymer electrolyte fuel cell was demonstrated. This fuel cell employed a 4 mg cm–2 Pt-Ru alloy electrode as an anode, a 4 mg cm–2 Pt black electrode as a cathode and an acid-doped polybenzimidazole membrane as the solid polymer electrolyte. The fuel cell is designed to operate at elevated temperature (200C) to enhance the reaction kinetics and depress the electrode poisoning, and reduce the methanol crossover. This fuel cell demonstrated a maximum power density about 0.1 W cm–2 in the current density range of 275–500 mA cm–2 at 200C with atmospheric pressure feed of methanol/water mixture and oxygen. Generally, increasing operating temperature and water/methanol mole ratio improves cell performance mainly due to the decrease of the methanol crossover. Using air instead of the pure oxygen results in approximately 120 mV voltage loss within the current density range of 200–400 mA cm–2 .
The transport properties and the swelling behaviour of NAFION and different sulfonated polyetherketones are explained in terms of distinct differences on the microstructures and in the pKa of the acidic functional groups. The less pronounced hydrophobic/hydrophilic separation of sulfonated polyetherketones compared to NAFION corresponds to narrower, less connected hydrophilic channels and to larger separations between less acidic sulfonic acid functional groups. At high water contents, this is shown to significantly reduce electroosmotic drag and water permeation whilst maintaining high proton conductivity. Blending of sulfonated polyetherketones with other polyaryls even further reduces the solvent permeation (a factor of 20 compared to NAFION), increases the membrane flexibility in the dry state and leads to an improved swelling behaviour. Therefore, polymers based on sulfonated polyetherketones are not only interesting low-cost alternative membrane material for hydrogen fuel cell applications, they may also help to reduce the problems associated with high water drag and high methanol cross-over in direct liquid methanol fuel cells (DMFC). The relatively high conductivities observed for oligomers containing imidazole as functional groups may be exploited in fully polymeric proton conducting systems with no volatile proton solvent operating at temperatures significantly beyond 100°C, where methanol vapour may be used as a fuel in DMFCs.
The selectivity coefficient (kij) strategy is employed for quantitative assessment of the selectivity of amperometric biosensors based on ‘class’-selective enzymes. The kij values for such devices reflect the preference of the sensor for the primary substrate (i) relative to secondary ones (j), and is related to the kinetic parameters of these species in accordance to: kij = imax,j Km,i/imax,i Km,j. Various experimental procedures are used to estimate the kij values in connection with the response of tyrosinase and peroxidase electrodes towards various phenolic and peroxide substrate, respectively. Theoretical and practical considerations are discussed along with future prospects and scope.
This project is an attempt to synthesize and fabricate proton exchange membranes for hydrogen production via water electrolysis that can take advantage of the better kinetic and thermodynamic conditions that exist at higher temperatures. Current PEM technology is limited to the 125–150 °C range. Based on previous work evaluating thermohydrolytic stability, several families of polymers were chosen as viable candidates: polyether ketones, polyether sulfones, polybenzimidazoles, and polyphenyl quinoxalines. Representatives of each were converted into ionomers via sulfonation and fashioned into membranes for evaluation. In particular, the sulfonated polyetheretherketone, or SPEEK, was examined by thermoconductimetric analysis and performance tested in an electrolysis cell. Results comparable to commercial perfluorocarbon sulfonates were obtained.
In this contribution novel acid–base polymer blend membranes are introduced. The membranes are composed of sulfonated poly(etheretherketone) sPEEK Victrex or poly(ethersulfone) sPSU Udel® as the acidic compounds, and of PSU Udel® diaminated at the ortho position to the sulfone bridge, or poly(4-vinylpyridine), poly(benzimidazole) PBI CELAZOLE®, or poly(ethyleneimine) PEI (Aldrich) as the basic compounds. The membranes showed good proton conductivities at ion-exchange capacities IEC of 1 (IEC=meq SO3H/g dry membrane), and they showed excellent thermal stabilities (decomposition temperatures >270°C). Two of the membranes were tested in a H2 membrane fuel cell and showed good performance. The specific interaction of the SO3H groups and of the basic N groups was investigated via FTIR for the sulfonated PSU/diaminated PSU and for the sulfonated PSU/poly(4-vinylpyridine) (Pyr) blend. It could be proved that in the dry membranes polysalt groups exist formed by the following acid–base reaction: PSU–SO3H+H2N–PSU→[PSU–SO3]−+[H3N–PSU], and PSU–SO3H+P→[PSU–SO3]−+[H–Pyr].
The availability of stable polymeric membranes with good proton conductivity at medium temperatures is very important for the development of methanol PEM fuel cells. In view of this application, a systematic investigation of the conductivity of Nafion 117 and sulfonated polyether ether ketone (S-PEEK) membranes was performed as a function of relative humidity (r.h.) in a wide range of temperature (80–160°C). The occurrence of swelling/softening phenomena at high r.h. values prevented conductivity determinations above certain temperatures. Nevertheless, when r.h. was maintained at values lower than 80%, measurements were possible up to 160°C. The results showed that Nafion is a better proton conductor than S-PEEK at low r.h. values, especially at temperatures lower than 120°C. The differences in conductivity were, however, leveled out with the increasing r.h. and temperature. While at 100°C and 35% r.h. the conductivity of S-PEEK 2.48 was about 30 times lower than the conductivity of Nafion, both membranes reached a comparable conductivity (4×10−2 S cm−1) at 160°C and 75% r.h. The effect of superacidity and crystallization of the polymers on the conductivity, as well as the possibility of using Nafion and S-PEEK membranes in medium temperature fuel cells, are discussed.
Sulfonated poly(arylene ether sulfones) with various sulfonation levels have been prepared and evaluated as solid polymer electrolytes in electrolysers and fuel cells. Solution and slurry sulfonation of poly(arylene ether sulfones) such as Udel® P-1700 (PSU) and Victrex® PES 5200P (PES) yield polyelectrolytes which have been characterized using FTIR and 1H-NMR spectroscopy, titration, thermal analysis, and electrochemical characterization such as resistivity, selectivity of ion permeation, current/voltage plot and life time test in an electrolysis cell. In contrast to the sulfonated PSU, the PES sulfonated in a slurry process was water insoluble, even at high sulfonation levels of 90 mol%, and gave significantly improved electrochemical properties similar to those of fluorine-containing polyelectrolytes used in commercial membrane systems. A versatile in-situ crosslinking technique has been developed to crosslink the sulfonated poly (arylene ether sulfone) electrolytes during membrane processing in order to substantially reduce water swelling without impairing other membrane properties such as proton conductivity.
The oxidation of trimethoxymethane (TMM) (trimethyl orthoformate) in a direct oxidation PBI fuel cell was examined by on-line mass spectroscopy and on-line FTIR spectroscopy. The results show that TMM was almost completely hydrolyzed in a direct oxidation fuel cell which employs an acid doped polymer electrolyte to form a mixture of methylformate, methanol and formic acid. It also found that TMM was hydrolyzed in the presence of water at 120°C even without acidic catalyst. The anode performance improves in the sequence of methanol, TMM, formic acid/methanol, and methylformate solutions. Since formic acid is electrochemically more active than methanol, these results suggest that formic acid is probably a key factor for the improvement of the anode performance by using TMM instead of methanol under these conditions.
Grafting of sulfonated aryl groups on to polybenzimidazole, PBI, leads to a proton conducting polymer. The synthetic route allows close control of the degree of sulfonation, and a range of samples have been prepared with degrees of sulfonation up to 75% of the available sites. Sulfonation increases the conductivity from ca. 10−4 S cm−1 in non-modified PBI to >10−2 S cm−1 at room temperature for highly sulfonated samples. The dependence of the conductivity both on the degree of sulfonation and on the conditions of membrane preparation are discussed.