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Properties of selected sulfonated polymers as proton-conducting electrolytes for polymer electrolyte fuel cells
Abstract
Two kinds of polymers were fabricated and tested as candidates of proton-conducting membranes for polymer electrolyte fuel cell (PEFC) applications. Poly benzimidazole (PBI) and poly(4-phenoxybenzoyl-1,4-phenylene, Poly-X 2000) (PPBP) were sulfonated and characterized as proton-conducting membranes. PBI was sulfonated as PBI-PS (propanesultone) and PBI-BS (butanesultone). PPBP was prepared at various sulfonation levels. Proton conductivities were measured at 60–160 °C. Power output characteristics of both polymers were measured by using commercial Pt/C electrodes.
... To overcome this drawback, fluorine-containing PBI has been introduced [84]. Prior to the preparation of the PBI membrane, PBI is sulfonated for a stable proton conductivity of the membrane above 100 • C [85]. There have been other attempts to produce orthophosphoric acid (H 3 PO 4 )-PBI by doping PBI with phosphoric acid, which has an influence on the high chemical and thermal stability of the membrane [86,87]. ...
... There have been other attempts to produce orthophosphoric acid (H 3 PO 4 )-PBI by doping PBI with phosphoric acid, which has an influence on the high chemical and thermal stability of the membrane [86,87]. However, the PBI membrane series operated at temperatures exceeding 160 • C exhibited creep and a reduced proton conductivity below 100 • C due to a certain amount of dehydration [85,88]. ...
... PVOH membranes have the disadvantages of low thermal properties and poor proton conductivity [79][80][81][82]. The PBI membrane has good mechanical properties and thermal stability, but it has a low proton conductivity below 100 • C and, therefore, requires a high operating temperature exceeding 160 • C [84][85][86][87]. ...
The proton exchange membrane (PEM) is pivotal among the various components of proton exchange membrane fuel cells (PEMFCs). From the many PEMs, perfluorosulfonic acid and non-fluorinated hydrocarbon electrolyte membranes are used in PEMFC operation, but they have a limited performance above 90 °C and at a relative humidity (RH) below 50%. Hence, the incorporation of nanoclay, an inorganic filler, into polymer matrixes has been attempted to improve the performance of PEMs. Nanoclays, such as montmorillonite and laponite in a layered silicate morphology, sepiolite nanofibers and halloysite nanotubes, with their fiber morphologies, and layered double hydroxide are attractive for composite membranes because they improve the hydrophilicity, hygroscopicity, and thermal stability of composite membranes at intermediate temperatures and low RH. The introduction of nanoclays also improves the mechanical properties. Furthermore, nanoclays are cost-competitive among the nanomaterials, thereby offering the potential to reduce composite membranes costs. This review highlights the preparation of composite membranes containing sulfonic, perfluorosulfonic, and amine groups, and other types of functionalized nanoclays, as well as the characterization of the composite membranes and cell performances operating at low RH.
... To overcome this drawback, fluorine-containing PBI has been introduced [84]. Prior to the preparation of the PBI membrane, PBI is sulfonated for a stable proton conductivity of the membrane above 100 • C [85]. There have been other attempts to produce orthophosphoric acid (H 3 PO 4 )-PBI by doping PBI with phosphoric acid, which has an influence on the high chemical and thermal stability of the membrane [86,87]. ...
... There have been other attempts to produce orthophosphoric acid (H 3 PO 4 )-PBI by doping PBI with phosphoric acid, which has an influence on the high chemical and thermal stability of the membrane [86,87]. However, the PBI membrane series operated at temperatures exceeding 160 • C exhibited creep and a reduced proton conductivity below 100 • C due to a certain amount of dehydration [85,88]. ...
... PVOH membranes have the disadvantages of low thermal properties and poor proton conductivity [79][80][81][82]. The PBI membrane has good mechanical properties and thermal stability, but it has a low proton conductivity below 100 • C and, therefore, requires a high operating temperature exceeding 160 • C [84][85][86][87]. ...
This thesis introduces novel electrolyte membranes which can be operated at low relative humidity (below 50%) and intermediate temperature, i.e., 90℃. More specifically, the thesis takes benefit from hygroscopicity of microfibrous SEP (sepiolite) and tubular HNT (halloysite). Changes in Nafion membrane properties with blending time were studied. Moreover, these nanoclays are functionalized and pretreated to make them proton conductive and to improve their compatibility with short-side-chain PFSA (perfluorosulfonic acid) composite membranes based on Aquivion. To begin with, functionalized and pretreated clay nanoparticles are characterized prior to incorporation in polymer matrix: ATR-FTIR (attenuated total reflection-fourier transform infrared spectroscopy), Py-GC/MS (pyrolysis gas chromatography mass spectrometry), and TGA (thermogravimetric analysis). Composites membranes have them been prepared and characterized for proton conductivity, water uptake, swelling, thermo-mechanical strength and chemical stability. The dispersion state of SEP and HNT inside polymer phase was observed using SEM/EDS (field emission scanning electron microscopy/Energy dispersive X-ray spectroscopy). The properties of pretreated nanoclays are characterized using XRD (X-ray diffraction) and EDS. Chemical stability regarding radical attack against composite membranes is clarified using Ion meter through fluoride ion (F-) analysis. Proton conductivity of composite membranes is also measured under condition of different relative humidity and temperature. Following this, it is demonstrated by DMA (dynamic mechanical analysis) results that the particular elongated morphology of SEPs and HNTs participates to improving mechanical property of the composite membranes with decreased swelling ratio. MEAs (membrane electrode assembly) performance are evaluated to understand the advantage of the presence of nanoclays in the composite membranes regarding the relative humidity of the feeding gas, the operating temperature of the cell, and the hydrogen crossover. Detailed abstracts including main results were provided at the beginning of each chapter.
... Different strategies have been followed to overcome the limitation of Nafion®, the perfluorinated ionomer widely chosen as the standard for PEMFC [1,2,3]. Non-fluorinated polymers such as PEEK [4,5,6], PSU [7,8] or PBI [9,10,11,12] and their sulfonated counterparts have been extensively studied as alternatives. Another strategy consists in incorporating inorganic fillers such as metal oxides [13,14,15,16,17,18,19,20,21], acids [22,23,24,25,26], phosphates or phosphonates [27,28,29] into the Nafion® matrix to make the composite or hybrid membrane more mechanically stable, less permeable to reactant and more hygroscopic. ...
... Similarly to palygorskite, the structure of sepiolite is highly porous. This hydrated magnesium silicate (Si 12 Mg 8 O 30 (OH) 4 (H 2 O) 4 , 8H 2 O) is based on SiO 4 tetrahedra layers, with an inversion of the apical ends every six units ( Figure 1). ...
... Similarly to palygorskite, the structure of sepiolite is highly porous. This hydrated magnesium silicate (Si 12 Mg 8 O 30 (OH) 4 (H 2 O) 4 , 8H 2 O) is based on SiO 4 tetrahedra layers, with an inversion of the apical ends every six units ( Figure 1). ...
... There are three primary ways to synthesize sulfonated PBI. The first is to chemically modify the PBI backbone using sulfonating reagents (e.g., 4-bromomethylbenzene sulfonic acid sodium salt and alkanesultone) [28,84]. The second is to directly polymerize sulfonated PBI with the sulfonated dicarboxylic acid monomers [49,74]. ...
... Although desired sulfonated PBI structures and control of the degree of sulfonation can be achieved through There are three primary ways to synthesize sulfonated PBI. The first is to chemically modify the PBI backbone using sulfonating reagents (e.g., 4-bromomethylbenzene sulfonic acid sodium salt and alkanesultone) [28,84]. The second is to directly polymerize sulfonated PBI with the sulfonated dicarboxylic acid monomers [49,74]. ...
Polymer electrolyte membrane fuel cells (PEMFCs) expect a promising future in addressing the major problems associated with production and consumption of renewable energies and meeting the future societal and environmental needs. Design and fabrication of new proton exchange membranes (PEMs) with high proton conductivity and durability is crucial to overcome the drawbacks of the present PEMs. Acid-doped polybenzimidazoles (PBIs) carry high proton conductivity and long-term thermal, chemical, and structural stabilities are recognized as the suited polymeric materials for next-generation PEMs of high-temperature fuel cells in place of Nafion® membranes. This paper aims to review the recent developments in acid-doped PBI-based PEMs for use in PEMFCs. The structures and proton conductivity of a variety of acid-doped PBI-based PEMs are discussed. More recent development in PBI-based electrospun nanofiber PEMs is also considered. The electrochemical performance of PBI-based PEMs in PEMFCs and new trends in the optimization of acid-doped PBIs are explored.
... The grafting of pendant side chains onto polymer backbones causes an increase in the polymer free volume [19,20], which leads to greater PA dop . In the literature, there are many reports of alkyl-sulfonate and aryl-sulfonate substituent-grafted-PBIs (PBI-R-SO 3 Hs) [2,9,11,[21][22][23][24][25][26][27][28][29][30]. Compared with PBIs, PBI-R-SO 3 Hs display higher PA dop , higher proton conductivity, and higher solubility, but a lower decomposition temperature due to desulfonation by heating [2,[28][29][30][31], lower brittleness and mechanical strength due to the greater free volume [20], and a lower degree of interpolymer imidazole >N-H…N= hydrogen bonds [32] by the presence of side chains, and higher degree of crosslinking due to the Lewis acid-base interactions of the grafted pendant -R-SO 3 H group with imidazole -C=N: and -NH groups (i.e., −R-SO 3 − … H-N-C and -SO 3 H … N=C-) [31,33]. ...
... Compared with PBIs, PBI-R-SO 3 Hs display higher PA dop , higher proton conductivity, and higher solubility, but a lower decomposition temperature due to desulfonation by heating [2,[28][29][30][31], lower brittleness and mechanical strength due to the greater free volume [20], and a lower degree of interpolymer imidazole >N-H…N= hydrogen bonds [32] by the presence of side chains, and higher degree of crosslinking due to the Lewis acid-base interactions of the grafted pendant -R-SO 3 H group with imidazole -C=N: and -NH groups (i.e., −R-SO 3 − … H-N-C and -SO 3 H … N=C-) [31,33]. The electrochemical and mechanical properties of the PBI-R-SO 3 H membranes strongly depend on the content and length of the grafted pendant side chains [2,20,21]. Though many PBI-R-SO 3 H PEMs for high-temperature PEMFCs (HT-PEMFCs) have been reported in the literature, few studies have investigated phosphoric acid pendant side chain-grafted PBIs (PBI-R-PO 3 H 2 s, including benzyl methyl phosphoric acid-grafted PBI (PBI-BPA; PBI-C 6 H 4 -CH 2 -PO(OH) 2 ) and ethyl phosphoric acidgrafted PBI (PBI-EPA; PBI-C 2 H 4 -PO(OH) 2 ), and their application to PEMFCs [2,[16][17][18]. Researchers have reported the synthesis of PBI-EPAs with various degrees of EPA grafting and membrane preparations, which showed weaker mechanical strength of PBI-EPAs than PBI [2,16,18]. ...
Our previous work illustrated that blending 10–30 wt.% of ethyl phosphoric acid-grafted PBI (PBI-EPA, 12 mol% degree of grafting) in the PBI membrane enhances the phosphoric acid doping level (PAdop) and proton conductivity σ of the membrane. On the other hand, the mechanical properties were decreased, and the membrane was highly dissolved in an 85 wt.% H3PO4 aqueous solution, while the PBI-EPA concentration in the PBI/PBI-EPA blend membrane was greater than 50 wt.%. To improve the mechanical properties of the PBI/PBI-EPA blend membrane, crosslinked PBI/PBI-EPA membranes with a PBI-EPA concentration higher than 50 wt.% were prepared by blending epoxy resin as a crosslinker. These crosslinked blend membranes demonstrated greater PAdop and σ and better fuel cell performance than the neat-PBI and epoxide-crosslinked PBI membranes.
... Sulfonated poly(phenylquinoxaline)s (SPPQs) polymer is obtained by grafting sulfonic acid groups onto PPQs by sulfonation reaction. SPPQs not only inherit the excellent properties brought by their rigid main chain structure but also have sulfonic groups for conducting protons [13,14]. It has been reported that SPPQs were used to prepare proton exchange membranes [15][16][17][18][19][20][21][22]. ...
Side-chain type sulfonated poly(phenylquinoxaline) (SPPQ)-based proton exchange membranes (PEMs) with different ionic exchange capacity (IEC) were successfully synthesized by copolymerization from 4,4′-bis (2-diphenyletherethylenedione) diphenyl ether, 4,4′-bis (2-phenylethylenedione) diphenyl ether and 3,3′,4,4′-tetraaminobiphenyl, and post-sulfonation process. The sulfonic acid groups were precisely grafted onto the p-position of phenoxy groups in the side chain of PPQ after the convenient condition of the post-sulfonation process, which was confirmed by 1H NMR spectra and FTIR. The sulfonic acid groups of side-chain type SPPQ degraded at around 325 °C, and their maximum stress was higher than 47 MPa, indicating great thermal and mechanical stability. The water uptake increased with the increasing IEC and temperature. The size change in their plane direction was shown to be lower than 6%, indicating the stability of membrane electrode assembly. The SPPQ PEMs displayed higher proton conductivity than that of main chain. In the single cell test, the maximum power density of side-chain type SPPQ-5 was 63.8 mW cm-2 at 20 wt% methanol solution and O2 at 60 °C, which is largely higher than 18.4 mW cm-2 of NR212 under the same conditions. The SPPQ PEMs showed high performance (62.8 mW cm-2) even when the methanol concentration was as high as 30 wt%.
... Oppositely, the diffusion of the oxidant in the cathode chamber to the anode chamber negatively influences the performance of the bacteria for electricity generation in MFC. For good MFCs performance, PEMs should have (i) good proton transportation rate (ii) fuel impermeability (iii) electric insulation between the anode and cathode, (iv) good mechanical and thermal properties, (v) sufficient durability, and (vi) cost-effective (Bae et al., 2002). ...
Microbial fuel cell (MFC) is a combined technology for simultaneous generation of electricity and wastewater treatment. In MFC, the proton exchange membrane (PEM) is an essential component affecting electricity generation. In the current study, two proton exchange membranes, namely sulfonated polyethersulfone (SPES) and graphene oxide/sulfonated polysulfone hybrid nanocomposite (GO-SPES), were prepared and characterized using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The collected information confirmed the successful preparation of the membranes. Moreover, contact angle measurements, ion exchange capacity and degree of sulfonation of the prepared membranes were determined. The results showed that the introduction of GO nanoparticles into SPES membrane improved its proton exchange capability and resulted in better performance. The power density and the current generated from SPES membrane were 60 mW/m² and 425 mA/m², respectively. For GO-SPES, the obtained power density was 101.2 mW/m² and the current was 613 mA/m². Both membranes showed comparable chemical oxygen demand (COD) removal efficiency of about 80%; suggesting that the prepared membranes are working efficiently in wastewater treatment as PEMs in MFCs. As a final point, the performance of GO-SPES membrane was compared to the performace of the well-known Nafion® 117 membrane and the results were promising. To conclude, the GO-SPES membrane is an outstanding membrane for use as PEM in MFCs for simultaneous generation of electricity and wastewater treatment.
... 24 Besides these, sulfonic acid groups may be activated functional groups. 24 For developing polymer electrolytes for fuel cells, the most widely investigated systems include various sulfonated polymers such as Polyetheretherketons (PEEK) [25][26][27][28][29] Polyimide (PI) [24][25] or Polyetheretherketones (PEEKK), 30 Polyethersulfone (PES), [31][32][33][34][35][36] Poly (4-Phenoxybenzoyl-1-4 Phenylene (PPBP), [37][38][39] Poly (p-Phenylenes) (PP). 38 High conductivity is achieved at high degrees of sulfonation, but unfortunately, high sulfonation results in higher swelling leading to poor mechanical properties. ...
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]
... Polymeric electrolyte membrane (PEM) to be used as proton exchange membrane, it should have higher proton conductivity at lower levels of hydration, slight fuel crossover, with higher chemical and mechanical stability, and steady performance at low cost. 12 Nafion (the perfluorosulfonic acid membrane) is the most successful membrane used in proton exchange membrane fuel cell (PEMFC) 13 because of its high conductivity and chemical stability called. However, at temperatures greater than 95 C, Nafion will be thermally and mechanically unstable and possess low proton conductivity at high temperature and low humidity. ...
Poly(vinyl alcohol) (PVA) with sulfosuccinic acid (SSA) membrane was prepared with different concentrations of SSA (wt.%) and thermally crosslinked successfully at 100 °C. Ion exchange capacity and proton conductivity were found to increase with increasing the SSA content in the meantime the water uptake and hydration number were declined with the rise in the degree of sulfonation. This anomalous behavior was discussed based on Nerst equation parameters which confirm the results discussion with enhancement with data of contact angle. Tensile strength results suggest the deterioration of the PVA/SSA membranes with increasing SSA content because of the chemical reaction of SSA and PVA chains. Positron annihilation results were found to enhance the electrochemical‐mechanical results of the membranes. The o‐Ps lifetime was found to increase with increasing degree of sulfonation while SO3 was working as an inhibitor agent of PS formation as o‐Ps Intensity was declined with an elevated percentage of SSA content. The o‐Ps lifetime as a function of temperature for different PVA/SSA membranes having different SSA content increased with increasing SSA content. Furthermore, the glass transition temperature of PVA/SSA membranes was found to decrease with increasing the SSA content. The results were discussed based on the free volume theory.
... To validate this hypothesis, another model reaction was carried out using different monomer structures bearing benzophenone, which have frequently been used to fabricate p-phenyl electrolyte membranes [44,45]. Product 2 was synthesized from a 3,5-DCDMPM monomer, as shown in Scheme 2. The synthetic procedure of 3,5-DCDMPM and structure confirmation by 1 H NMR are provided in Scheme S1 and Figure 3, respectively. ...
Several methods to synthesize poly(phenylene) block copolymers through the nickel coupling reaction were attempted to reduce the use of expensive nickel catalysts in polymerization. The model reaction for poly(phenylene) having different types of dichlorobenzene derivative monomers illustrated the potential use of cost-effective catalysts, such as NiBr2 and NiCl2, as alternatives to more expensive catalysts (e.g., bis(1,5-cyclooctadiene)nickel(0) (Ni(COD)2)). By catalyzing the polymerization of multi-block poly(phenylene) with NiBr2 and NiCl2, random copolymers with similar molecular weights could be prepared. However, these catalysts did not result in a high-molecular-weight polymer, limiting their wide scale application. Further, the amount of Ni(COD)2 could be reduced in this study by approximately 50% to synthesize poly(phenylene) multi-block copolymers, representing significant cost savings. Gel permeation chromatography and nuclear magnetic resonance results showed that the degree of polymerization and ion exchange capacity of the copolymers were almost the same as those achieved through conventional polymerization using 2.5 times as much Ni(COD)2. The flexible quaternized membrane showed higher chloride ion conductivity than commercial Fumatech membranes with comparable water uptake and promising chemical stability.
... High conductivity, good mechanical 1 3 properties, and excellent thermal stability at a temperature of up to 200 °C and under ambient pressure have been reported. The two PBI polymer systems which have been thoroughly investigated as acid-imbibed systems are based on commercially available poly(2,2′-m-phenylene-5,5′bibenzimidazole) (m-PBI) and poly(2,5-benzimidazole) (AB-PBI) [10][11][12][13][14][15]. Zhang et al. [16] reported that better cell performance could be achieved using such systems as the operating temperature approached 200 °C. ...
The objective of this work is to study a high-temperature proton exchange membrane fuel cell using CO- and methane-containing hydrogen-rich gases because of the advantages of high operating temperature and the growing feasibility of using natural gases or methane as the sources of hydrogen-rich reformate gases. According to the experimental results, it is suggested that the fuel cell be operated at 180 °C under reformate gases with high CO concentrations to avoid not only a significant decrease in performance, but also severe potential oscillations. In addition, the anode oxidation reaction is more sensitive to the temperature than the cathode reduction reaction under CO-containing H2. On the other hand, the effects of methane in the reformate gas on the fuel cell can be ignored because the existence of methane causes neither a decrease in the cell performance nor an increase in the anodic charge transfer resistance. Thus, the CO concentration and operating temperature are still the two dominant parameters with regard to the cell performance under CO- and methane-containing hydrogen-rich gases.
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... Fundamental functions of such proton exchange membranes are as follows: (i) the transport of protons, (ii) the separation of reactants and (iii) electric insulation between the anode and cathode [22]. Common themes critical to all high-performance proton exchange membranes include (i) high ionic conductivity, (ii) low permeability to fuel and oxidants, (iii) a low electro-osmotic drag coefficient, (iv) oxidative, hydrolytic and thermal stability, (v) adequate mechanical properties, allowing to obtain as thin as possible membrane (thickness approximately 10-250 μm), (vi) long lifetime, (vii) low cost and (viii) the capability of fabrication into membrane electrode assemblies (MEAs) [60,61]. The publication list as shown in Figure 10.5 undoubtedly proves that the polymer electrolyte membranes for fuel cells are objects of intent interest. ...
This review presents the most important research on alternative polymer membranes with ionic groups attached, provides examples of materials with a welldefined chemical structure that are described in the literature. Furthermore, it elaborates on the synthetic methods used for preparing PEMs, the current status of fuel cell technology and its application. It also briefly discusses the development of the PEMFC market.
... Fundamental functions of such proton exchange membranes are as follows: (i) the transport of protons, (ii) the separation of reactants and (iii) electric insulation between the anode and cathode [22]. Common themes critical to all high-performance proton exchange membranes include (i) high ionic conductivity, (ii) low permeability to fuel and oxidants, (iii) a low electro-osmotic drag coefficient, (iv) oxidative, hydrolytic and thermal stability, (v) adequate mechanical properties, allowing to obtain as thin as possible membrane (thickness approximately 10-250 μm), (vi) long lifetime, (vii) low cost and (viii) the capability of fabrication into membrane electrode assemblies (MEAs) [60,61]. The publication list as shown in Figure 5 undoubtedly proves that the polymer electrolyte membranes for fuel cells are objects of intent interest. ...
This review presents the most important research on alternative polymer membranes with ionic groups attached, provides examples of materials with a well-defined chemical structure that are described in the literature. Furthermore, it elaborates on the synthetic methods used for preparing PEMs, the current status of fuel cell technology and its application. It also briefly discusses the development of the PEMFC market.
... However, a high PA doping level (PADL) may lead to poor mechanical properties of PA doped PBI membranes [6] . This problem can be overcome by modifying the polymer structure via copolymerization [7] , N-substitution [8] or cross-linking of the polymer [9][10][11][12][13] at the reactive benzimidazole N -H sites; or by incorporating inorganic particles such as hygroscopic oxides (i.e., SiO 2 and ZrO 2 ) [14][15][16][17][18] , perovskite-type oxides (i.e., strontium cerate and barium titanate) [19,20] , heteropolyacids [21] , zirconium phosphates (ZrP) [22] , carbides (i.e., silicon carbide (SiC)) [23] , and graphene oxide (GO) [24] , which can improve the mechanical properties and acid doping levels of composite membranes because of hydrogen-bond interaction between the oxygen atoms of hydroxyl groups and acid [20] . ...
Polybenzimidazole containing ether bond (OPBI) was reinforced with silicon carbide whisker (mSiC) modified by 3-aminopropyltriethoxysilane (KH550), and then doped with phosphoric acid (PA) to obtain OPBI/mSiC/PA membranes. These OPBI/mSiC/PA membranes have excellent mechanical strength and oxidative stability and can be used for high temperature proton exchange membrane (HT-PEM). The tensile strength of OPBI/mSiC/PA membranes ranges from 27.3 to 36.8 MPa, and it increases at first and then decreases with the increase of mSiC content. The high mSiC content and PA doping level contribute to improve the proton conductivity of membranes. The proton conductivity of PBI/mSiC-10/PA membrane is 27.1 mS•cm⁻¹ at 170 °C without humidity, with an increase of 55.7% compared with that of OPBI/PA membrane. These excellent properties make OPBI/mSiC/PA membranes promising membrane materials for HT-PEM applications.
... Examples of these membranes include sulfonated polymers, such as polysulfone (SPSU), poly (ether sulfone) (SPES), poly(ether ketone) (SPEK), poly(ether ether ketone) (SPEEK), poly(phenylene sulfide) (SPPS), poly-(phosphazene), or polyimide (SPI), among many others [22,23,47e49]. Moreover, sulfonated or phosphonated polymer membranes were used for proton conductors in cells, such as polybenzimidazoles [8,22,50], polybenzoxazoles, and polybenzothiazoles [51]. However, these sulfonated, perfluorinated, or non-fluorinated membranes still have some disadvantages: they must remain to keep in humidity to achieve high proton conductivity. ...
High temperature proton exchange membrane fuel cells (HT-PEMFCs) are proficient clean energy conversion devices for automotive and stationary applications. HT-PEMFC could mitigate the CO poisoning, humidity and heat management, and sluggish of oxygen reduction reaction (ORR). Acid doped polybenzimidazoles (PBIs)/functionalized PBIs polymer electrolyte membranes are familiar uses for HT-PEMFC because of high proton conductivity with thermo-mechanical stability. Proton conductivity of PBI membranes is greatly promising by acid doping dimension and cell operating temperature. PBI reactive sites (=N. H) and acidic anions prominently contribute the proton transfer through the prolonged hydrogen bonding network. Coating and sprayed methods are prominent techniques for fabrication of gas diffusion electrodes (GDEs), although shrinkage and hairline surface cracks observed on GDEs. Multi walled carbon nanotubes (MWCNTs) has been compromising unique characteristics for steady carbon support materials. Moreover, PTFE and PVDF can be used as catalyst binder to reduce corrosion rate. In this review, it has been focused the PBIs membrane, acid doping, GDEs, MEA and durability of MEA.
... Fuel cell performance was also reported, but it did not compete with that of Nafion membrane. 146 They have synthesized phos phonated PPs by nickel coupling reactions, although the phosphonated PPs were less proton conductive than the sulfo nated analogs. 147 A research group in Sandia National Laboratories synthe sized unique poly(phenylene) ionomers by Diels-Alder polymerization followed by sulfonation. ...
Recent research on the hydrocarbon aromatic ionomer membranes by step growth polymerization are reviewed in this chapter. Several kinds of aromatic polymers such as sulfonated poly(arylene ether sulfone)s, poly(arylene ether ketone)s, polyimides, poly(. p-phenylene)s, and poly(benzimidazole)s are discussed from an historical viewpoint up to the most recent progress. The discussion includes synthetic methods and chemical, physical, and electrochemical properties for fuel cell applications. In particular, the authors focus on how to balance the conflicting properties: proton conductivity under high-temperature and low-humidity conditions and chemical and physical stability. The effective and promising approaches are membranes with very high ion exchange capacity, sequenced copolymer structure (hydrophilic/hydrophobic block copolymers), and highly dense sulfonic acid groups. Fuel cell performance with some of the membranes is also discussed. The chapter concludes with future prospects for the development of advanced membrane materials.
... Their proton conductivities, measured at room temperature, improved from 10 À4 S/cm to 10 À2 S/cm for the SPBIs with a high benzylsulfonic acid content. Bae et al. [45] prepared sulfonated SPIs bearing different lengths of flexible side chains. The butylsulfonated-PBI exhibited higher conductivity. ...
... The similar phenomenon is also found in PA-doped PBI and PA-doped sulfonated PBI. The conductivity of PA-doped PBI and PA-doped sulfonated PBI is 1 Â 10 À 4 and 1 Â 10 À 2 S cm À 1 at room temperature, respectively [33,34]. This is due to the formation of new proton transport pathways in the membranes containing both SA and PA [17,18]. ...
... PA45-CSiSPIBI70 is 0.0031 S cm À1 , which is approximately two orders of magnitude higher than that of PA-PBI (30 C/30% RH, 6.5 Â 10 À5 S cm À1 ). The similar phenomenon is also found in PA-doped PBI and sulfonated PBI [33,34]. According to the proton transport mechanisms, the conductivity is strongly dependent on the density of proton-conducting sites of the membrane. ...
Silane-cross-linked sulfonated poly(imide benzimidazole) (CSiSPIBI) membranes were prepared using γ-(2,3-epoxypropoxy) propyltrimethoxysilane (KH560) as a cross-linker, and the cross-linked structure was characterized using Fourier transform infrared spectroscopy and solubility test. The resulted cross-linked membranes were further doped with phosphoric acid (PA) by means of the acid–base interaction with the alkaline imidazole ring and the electrostatic interaction with siloxane. The results show that PA-doped CSiSPIBI membranes have high proton conductivity due to the formation of a new proton transport pathway between PA and sulfonic acid. Under high temperature and low humidity conditions, the proton conductivity of PA-doped sulfonated membranes is one to two orders of magnitude higher than that of non-PA-doped membranes and PA-doped non-sulfonated membranes. The silane-cross-linked membranes display improved chemical stability and mechanical strength, especially the oxidative stability. The complete dissolution time in Fenton's reagent increases from 510 min for the sulfonated polyimide/polybenzimidazole blend membrane to 1450 min for the silane-cross-linked membrane.
Sulfonated aromatic polymers (SAPs) feature highly-hydrophilic active functional groups and inherent ionic nano-channels that make them a potential membrane material for desalination. The high content of sulfonic groups favors water transport but causes excessive swelling to impact the separation properties of the membrane. In this work, a novel crosslinking strategy via the linkage of carbon atoms on aryl groups by the Friedel-Crafts reaction was proposed for a newly synthesized sulfonated polystyrene-grafted poly(styrene-ethylene/butylene-styrene) block copolymer (S-SEBS-g-PSt) membrane. Polystyrene was grafted to provide extended side chains of the polymer where the aryl rings were crosslinked using formaldehyde dimethyl acetal (FDA) as the crosslinker to form methylene bridges. Atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS) characterization show that the hydrophilic region of the membrane was mildly shrunken while more continuous after crosslinking. The sulfonic acid groups were retained upon the FDA crosslinking and the high level of ion exchange capacity (IEC) was not affected by the crosslinking. Depending on the crosslinking conditions, the tensile strength of the membrane could be increased by 40-120% and the swelling ratio was reduced by 50-80%. As high as 76.8 kg m⁻² h⁻¹ of water flux and over 99.95% of salt rejection were achieved in treating with 5 wt% hypersaline water at 75 °C. Water permeability reached an unprecedentedly high level of 1 500 000-3 500 000 barrer upon desalinating 3.5-20 wt% salt solutions. The high performance of the membrane makes it promising for potential application in desalination and treating hypersaline wastewater.
In this research, polybenzimidazole/boron nitride (PBI/BN) based composite membranes have been prepared for high-temperature PEM fuel cell (HT-PEMFC). BN was preferred because of its superior thermal robustness, high chemical stability, non-conductor property, and high plasticizer characteristic. The loading of BN in the composite membrane was studied between 2.5 to 10 wt%. The composite membranes were characterized using TGA, DSC, XRD, SEM, mechanical tests, acid doping/leaching, and proton conductivity measurements. The highest conductivity of 0.260 S/cm was found for PBI/BN-2.5 membrane at 180°C. It has been determined that the PBI/BN-2.5 membrane has higher performance than the PBI membrane according to the HT-PEMFC tests performed with Hydrogen and dry air. The heightened HT-PEMFC performance can be ascribed to interactive effects between BN particles and the PBI polymer matrix. PBI/BN composite membranes show a good perspective in the high-temperature PEMFC applications.
Trifluoromethanesulfonylimide-grafted polybenzimidazole (PBI-TFSI) was synthesized for proton exchange membrane (PEM) applications. Its proton conductivity was (a) less dependent on humidity and (b) higher than that of conventional fluorine-based PEM (Nafion) and propanesulfonic acid-grafted PBI (PBI-PS) at a relative humidity of 40%. The chemical structure of PBI-TFSI was investigated using ¹H and ¹⁹F nuclear magnetic resonance and Fourier transform infrared spectroscopy. The membranes exhibited good transparency, flexibility, and thermal stability up to 350 °C. Membranes with different side chain grafting ratios were prepared, and the water uptake and hydration number of the PBI-TFSI membranes were lower than those of the PBI-PS membranes, most likely because of the hydrophobicity of the side chain. The higher proton concentration provided by TFSI with stronger acidity than PS might be the reason for the higher proton conductivities of PBI-TFSI.
High-performance hydrogels play a crucial role as solid electrolytes for flexible electrochemical supercapacitors (ESCs). More specifically, all solid-state ESCs based on renewable, biodegradable and/or biocompatible hydrogels doped with inorganic salts as electrolytes are attractive not only because of their contribution to reduce the resource consumption and/or the generation of electronic garbage, but also due to their potential applicability in the biomedical field. Here, computer simulations have been combined with experimental measurements to probe the outstanding capability as solid electrolyte of photo-crosslinked unsaturated polyesteramide hydrogels containing phenylalanine, butenediol and fumarate, and doped with NaCl (UPEA-Phe/NaCl). Atomistic molecular dynamics simulations have shown the influence of the hydrogel pore structure in Na+ and Cl– ions migration, suggesting that UPEA-Phe/NaCl hydrogels prepared without completing the photo-crosslinking reaction will exhibit better behavior as solid electrolyte. Theoretical predictions have been confirmed by potentiodynamic and galvanostatic studies on ESCs fabricated using poly(3,4-ethylenedioxythiophene) electrodes and UPEA-Phe/NaCl hydrogels, which were obtained using different times of exposure to UV radiation (i.e. 4 and 8 h for uncomplete and complete photo-crosslinking reaction). Moreover, the behavior as solid electrolyte of the UPEA-Phe/NaCl hydrogel prepared using a photo-polymerization time of 4 h has been found to be significantly superior to those exhibited by different polypeptide and polysaccharide hydrogels, which were analyzed using ESCs with identical electrodes and experimental conditions.
This chapter presents the selected main types of natural materials, including chitosan, cellulose, alginate, starch, pectin, agar, or gelatin, that have proven suitable for production of solid biopolymer-based electrolyte membranes for fuel cells. Their detailed properties and possible approaches to the synthesis are described.
New and emerging technologies for electrochemical energy conversion processes and challenging separations have been major drivers for the tremendous development of new polybenzimidazole chemistries and materials in recent years.
The novel sulfonated polybenzimidazole (sPBI)/amine functionalized titanium dioxide (AFT) composite membrane is devised and studied for its capability of the application of high temperature proton exchange membrane fuel cells (HT-PEMFCs), unlike the prior low temperature AFT endeavors. The high temperature compatibility was actualized because of the filling of free volumes in the rigid aromatic matrix of the composite with AFT nanoparticles which inhibited segmental motions of the chains and improved its thermal stability. Besides, amine functionalization of TiO2 enhanced their dispersion character in the sPBI matrix and shortened the interparticle separation gap which finally improved the proton transfer after establishing interconnected pathways and breeding more phosphoric acid (PA) doping. In addition, the appeared assembled clusters of AFT flourished a superior mechanical stability. Thus, the optimized sPBI/AFT (10 wt%) showed 65.3 MPa tensile strength; 0.084 S. cm⁻¹ proton conductivity (at 160 °C; in anhydrous conditions), 28.6% water uptake and PA doping level of 23 mol/sPBI repeat unit. The maximum power density peak for sPBI/AFT-10 met the figure of 0.42 W.cm⁻² at 160 °C (in dry conditions) under atmospheric pressure with 1.5 and 2.5 stoichiometric flow rates of H2/air. These results affirmed the probable fitting of sPBI/AFT composite for HT-PEMFC applications.
Polymer Electrolyte Membrane Fuel Cells (PEMFC) are often viewed as enablers of decarbonised energy systems as they transform hydrogen directly into electricity with water as the main by-product. The attraction for fuel cells is also in their versatility because they can be implemented across a wide range of applications from microelectronics to large scale power generation. Herein, we review recent progress on the design and fabrication of PEMFC with a special focus on their air-breathing planar configuration as this extend the possibility of PEMFC to thin and flexible designs. To date the deployment of planar PEMFC highly depends upon scientific progress and technological solutions for cost reduction and long-term durability including the development of better proton conducting membranes and platinum-free catalysts to drive the oxygen reduction and hydrogen oxidation reactions efficiently. Long term durability is another challenge that can be addressed through the advancement of inexpensive and lightweight current collectors and highly efficient gas diffusion electrodes for better distribution of the reactants while maintaining an optimum hydration of the proton conducting membrane. Innovative fabrication methods for the various components of planar PEMFC as well as effective stack design and assembly are also critical for efficiency maximization, reproducibility and overall cost reduction.
The importance of proton conductivity is enormous for biological systems and in devices such as electrochemical sensors, electrochemical reactors, electrochromic devices, and fuel cells. In the book chapter, the phenomenon of proton conductivity in materials was discussed with a special emphasis on five different types of conductive materials, namely, perfluorinated ionomers, partially fluorinated, aromatic polymers, acid-base complexes, non-fluorinated ionomers, and hydrocarbon. In a fuel cell, the proton exchange membranes (PEMs) have a profound influence on its performance. Many researchers have investigated the functionalization methods to solve the methanol crossover problem and to obtain low electronic conductivity, low electroosmotic drag coefficient, good mechanical properties, good chemical stability, good thermal stability, and high proton conductivity. The way forward of developing high-performance proton-conductive polymeric membrane via electrospinning for as fuel cells was also addressed.
The overall aim of this PhD thesis was to characterize the properties of commercial Nafion N115 and Nafion NRE212 membranes in term of water sorption, transport, and mechanical properties over a wide range of experimental conditions. Because of the high dispersion of the data in the literature, our primary objective was to gather a comprehensive set of experimental measurements and to compare them with published results. Simple and reproducible protocols allowed us to measure the membrane properties over a wide range of experimental conditions and to study the influence of certain parameters on their evolution. For example, the samples were heat-treated at different temperatures and the effect of thermal history on water sorption, transport and mechanical properties was investigated. Nafion membranes were also exposed to moderate temperature (60°C - 80°C) and constant relative humidity (RH = 0.3 to 0.95) for long periods of time, which is known to cause a so-called ?hygrothermal aging? resulting in a decrease in their sorption capacity and proton conductivity. Such effects were observed but they appeared to be reversible and without noticeable consequences in term of fuel cell performance. Our experimental results can be used in studies involving water transport, water management and durability of fuel cells, especially for numerical simulation or modelling. More fundamentally, they can help understanding the thermodynamics of sorption and transport phenomena in PFSA membranes
After approximately 15 years of development, polybenzimidazole (PBI) chemistries and the concomitant manufacturing processes have evolved into commercially produced membrane electrode assemblies (MEAs). PBI MEAs can operate reliably without complex water humidification hardware and are able to run at elevated temperatures of 120-180 OC due to the physical and chemical robustness of PBI membranes. These higher temperatures improve the electrode kinetics and conductivity of the MEAs, simplify the water and thermal management of the systems, and significantly increase their tolerance to fuel impurities. Membranes cast by a newly developed polyphosphoric acid (PPA) Process possessed excellent mechanical properties, higher phosphoric acid (PA)/PBI ratios, and enhanced proton conductivities as compared to previous methods of membrane preparation. p-PBI and m-PBI are the most common polymers in PBI-based fuel cell systems, although AB-PBI and other derivatives have been investigated.
This work describes for the first time a novel functionalization of poly[2,2′-(m-phenylen)-5,5′-bisbenzimidazole] (PBI), a polymer stable in a wide range of solvents, with a DNA base, adenine (PBI-Ax%). Functionalization was achieved through alkylation of N4 benzimidazole nitrogen with 9-(3-bromopropyl) adenine (9-BPA). The N9 adenine nitrogen that binds to deoxyribose in DNA molecule was inhere used for covalent binding to the PBI, leaving free the remaining adenine positions, as the ones available in a natural DNA molecule. Therefore, for the first time, the use of adenine based polymer is here suggested as a biomimicry strategy for degenotoxification of post-reaction synthetic active pharmaceutical ingredient (API) streams. The very same interactions responsible by in vivo genotoxicity, present in the novel adenine based polymer, PBI-Ax%, were used for removal of a genotoxic impurity (GTI). Methyl p-toluenesulfonate (MPTS) at 100 ppm in dichloromethane was selected as a representative sulfonate GTI, which is present in a wide range of API post-reaction streams. The functionalization reaction efficiency was assessed using NMR at yields of 40–69% or 27–31%, for addition of 0.13–0.25 or 0.5–1.0 mol eq of 9-BPA to N4 PBI nitrogen, respectively. Synthesis and purification of 9-BPA was first optimized, increasing yields from previously reported values of 38–61%. The maximum removal of genotoxic impurity was obtained for PBI-A12% polymer, obtained at 0.25 M equivalents of 9-BPA to N4 PBI nitrogen, at a value of (96.6 ± 0.2)%, against (10.2 ± 2.8)% of the original non modified PBI.
The study conducted a molecular dynamics simulation based on a condensed-phase optimised molecular potentials for atomistic simulation studies force field model to investigate an anhydrous system of phosphoric acid-doped polybenzimidazole (poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole], PBI). Intermolecular pair correlation functions and corresponding coordination numbers were calculated to research the strengths for various types of hydrogen bonding. The results display that the strengths of the hydrogen bonding interactions are in the order of o1–h pair > o2–h pair > n2a–h pair > n3a–h pair, and most protons are located around the neighbourhood of H2PO4– rather than that of PBI. The proton conductivities are 3.86 × 10−3 S cm−1 at 298 K and 8.50 × 10−3 S cm−1 at 413 K. Moreover, the value obtained from our simulation system at 413 K is within the same order of magnitude as the experimentally measured value 0.012 S cm−1 at 420% doping level. The distribution of proton displacement exhibits that the displacement of most protons is about 1.25–2.5 Å. The displacement is over 3.0 Å only for a fraction of protons. In addition, the greatest displacement can approach 4.595 Å. The trajectory analyses of protons show that the most possible mechanisms of proton transfer come from three ways: (a) between two H2PO4– anions, (b) between H2PO4– anions and benzimidazole moieties and (c) between two benzimidazole moieties. The dynamics of polymer motion was studied by the trajectory analyses of ring flips. The large amplitude flips of rings in the polymer chains were found in the system. The flips between benzene and benzimidazole are more frequent than that between benzimidazole moieties.
Recent progress in the synthesis of polybenzimidazole (PBI) derivatives is summarized for application as high temperature polymer electrolyte membrane in fuel cells. Various designs in the polymer structure are described aiming at improvement of the membrane performance. The ways to produce PBI derivatives containing different functional groups, segments, or blocks of other macromolecules are classified as main-chain modification, copolymerization, and side-chain grafting. The synthetic routes and associated characterization methods particularly with respect to the polymer structures are also addressed.
In order to improve the proton conductivity and stability of proton exchange membranes based on sulfonated poly(arylene ether sulfone)(SPAES), a series of cross-linked block SPAES membranes(cbSPAES) was prepared. The block SPAES copolymers were synthesized through block copolymerization and subsequently subjected to cross-linking modification in the presence of P2O5, where the cross-linking reaction taken place between -SO3H groups and active hydrogen atoms attached to the polymer backbones. Proton conductivity of the cbSPAES membranes were obtained by electrochemical impedance spectroscopy(EIS), dimensional and hydrolytic stabilities were evaluated by the membrane size changes in the in-plane/through-plane directions and accelerating aging test in water, respectively. The results indicate: (1) cbSPAES membranes have obvious better dimensional and hydrolytic stabilities than SPAES membranes; (2) at the same cross-linking degree, water uptakes and proton conductivities of cbSPAES membranes increase with the increase in repeat units of sulfonated moieties. For example, the membrane of cbSPAES(30/10)-10 exhibits water uptake of 65%, size changes in the in-plane/through-plane directions of 0.16/0.18 and proton conductivity of 163 mS/cm in water at 60 °C.
Proton exchange membrane fuel cell (PEMFC) performance with a cross-linked poly (vinyl alcohol)/sulfosuccinic acid (PVA/SSA) polymer is compared with Nafion® N-115 polymer. In this study, PVA/SSA (≈5 wt. % SSA) polymer membranes are synthesized by a solution casting technique. These cross-linked PVA/SSA polymers and Nafion are used as electrolytes and ionomers in catalyst layers, to fabricate different membrane electrode assemblies (MEAs) for PEMFCs. Properties of each MEA are evaluated using scanning electron microscopy, contact angle measurements, impedance spectroscopy and hydrogen pumping technique. I-V characteristics of each cell are evaluated in a H2-O2 fuel cell testing fixture under different operating conditions. PVA/SSA ionomer causes only an additional ≈4% loss in the anode performance compared to Nafion ionomer. The maximum power density obtained from PVA/SSA based cells range from 99 to 117.4 mW cm-2 with current density range of 247 to 293.4 mA cm-2. Ionic conductivity of PVA/SSA based cells is more sensitive to state of hydration of MEA, while maximum power density obtained is less sensitive to state of hydration of MEA. Maximum power density of cross-linked PVA/SSA membrane based cell is about 35% that of Nafion® N-115 based cell. From these results, cross-linked PVA/SSA polymer is identified as potential candidate for PEMFCs.
In this study, we performed density functional theory (DFT) calculations to elucidate the effect of sulfonic acid functional group on the hydrophilicity of polybenzimidazole (PBI). We investigated the adsorption of H_2O molecules on sulfonated PBI (SPBI) and on disulfonated PBI (DSPBI) with cis- or trans-conformation. We analyzed electronic properties such as charge re-distribution and electronic band gap in terms of the optimized structure of PBI systems. We found that PBI with higher degree of sulfonation shows greater hydrophilicity and that trans-DSPBI shows greater hydrophilicity than cis-DSPBI.
To improve the thermal stability, mechanical strength and proton conductivities of proton exchange membranes at high temperature and low humidity, polybenzimidazole (PBI)/ zirconium sulfophenylphosphonate (ZrSPP) proton exchange hybrid membranes were prepared in this study. ZrSPP as novel proton conductor was introduced into the PBI organic matrix to form hybrid membrane. The microscopic morphology, thermal stability, mechanical properties and proton conductivity of hybrid membrane were characterized by SEM, tensile test, TGA and AC impedance, respectively. The effects of ZrSPP doping content on performance of hybrid membrane were investigated. The results showed that ZrSPP was uniformly distributed in the hybrid membranes. The thermal stability improved with the increase of ZrSPP to some extent. The PBI/5% ZrSPP hybrid membrane showed the highest proton conductivity of 38mS/cm at 160℃ under anhydrous conditions and exhibited a tensile strength of 43.0MPa, which was better than NafionⓇ117 (tensile strength of 26.6MPa).
Proton-conducting inorganic/organic composite membranes, which were mechanically flexible, were fabricated, and their conduction properties were characterised for use as polymer electrolyte fuel cells (PEFCs) operating at relatively higher temperatures (100 similar to 150 degrees C). SiO2/PEO (polyethylene oxide) membranes doped with acidic molecules (monododecylphosphate) were fabricated by using the sol-gel route, and the proton conductivities were measured using impedance spectroscopy. The composite electrolytes showed reasonable electrical-conductivity values (10(-3)similar to 10(-4) S/cm) up to 150 degrees C and differential thermal analysis and thermogravimetric analysis (DTA/TG) data showed that the membranes were thermally stable up to 350 degrees C, possibly due to the SiO2 backbone framework.
This paper presents a fit between model and experiments for well-humidified polymer electrolyte fuel cells operated to maximum current density with a range of cathode gas compositions. The model considers, in detail, losses caused by: (1) interfacial kinetics at the Pt/ionomer interface; (2) gas-transport and ionic-conductivity limitations in the catalyst layer; and (3) gas-transport limitations in the cathode backing. Our experimental data were collected with cells that utilized thin-film catalyst layers bonded directly to the membrane, and a separate catalyst-free hydrophobic backing layer. This structure allows a clearer resolution of the processes taking place in each of these distinguishable parts of the cathode. In our final comparison of model predictions with the experimental data, we stress the simultaneous fit of a family of complete polarization curves obtained for gas compositions ranging from 5 atoms O2 to a mixture of 5% O2 in N2, employing in each case the same model parameters for interracial kinetics, catalyst-layer transport, and backing-layer transport. This approach allowed us to evaluate losses in the cathode backing and in the cathode catalyst layer, and thus identify the improvements required to enhance the performance of air cathodes in polymer electrolyte fuel cells. Finally, we show that effects of graded depletion in oxygen along the gas flow channel can be accurately modeled using a uniform effective oxygen concentration in the flow channel, equal to the average of inlet and exit concentrations. This approach has enabled simplified and accurate consideration of oxygen utilization effects.
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.
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 .
High temperature protonic conducting polymer membranes provide new technological applications in the electrochemical devices including electrochromic displays, chemical sensors, fuel cells and others. Organic/inorganic nanocomposites membranes, consisting of SiO2/PEO (Polyethylene Oxides) hybrids, are a remarkable family of isotropic, amorphous polymer materials, which have been synthesized through sol-gel processes. The hybrid membrane doped with acidic surfactant molecules shows good protonic conductivities at high temperatures above 100°C. The membrane was found to be thermally stable at high temperatures because of the inorganic SiO2 framework in the composites matrix.
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