Poly(ethylene glycol) modified activated carbon for high performance proton exchange membrane fuel cells

A high water retention membrane is developed by co-assembling poly(ethylene glycol) (PEG) grafted activated carbon (AC-PEG) with Nafion. The AC-PEG is prepared via a sol–gel process. The use of PEG as a transporting medium in AC-PEG shows a largely improved water retention ability, a higher proton conductivity and a reduced swelling ratio, making it well suited for proton exchange membrane fuel cells (PEMFCs). Further, the composite membranes show improved mechanical properties at high temperature, thus ensuring the structural stability of membranes during the fuel cell operation. Compositional optimized AC-PEG/Nafion composite membrane (15 wt% compared to Nafion) demonstrates a better performance than the commercially available counterpart, Nafion 212, in fuel cell measurements. To identify the key factor of the improved performance, current interrupt technique is used to quantitatively verify the changes of resistance under different relative humidity environment.     

The development of PTFE/Nafion/TEOS membranes for application in moderate and high temperature proton exchange membrane fuel cells

PTFE/Nafion (PN) and PTFE/Nafion/TEOS (PNS) membranes were fabricated for the application of moderate and high temperature proton exchange membrane fuel cells (PEMFCs), respectively. Membrane electrode assemblies (MEAs) were fabricated by PTFE/Nafion (and PTFE/Nafion/TEOS) membranes with commercially available low and high temperature gas diffusion electrodes (GDEs). The effects of relative humidity, operation temperature, and back pressure on the performance and durability test of the as-prepared MEAs were investigated. Incorporating TEOS into a PNS membrane and adding another layer of carbon onto a GDE would result in low membrane conductivity and low fuel cell performance respectively. However, in this work it is shown that HT-PNS MEAs demonstrate a higher performance than LT-PN MEAs in severe conditions - high temperature (118 °C) and low humidity (25% RH). The TEOS and additional carbon layer function as water retaining agents which are especially important for high temperature and low humidity conditions. The HT-PNS MEA showed good stability in a 50 h fuel cell test at high temperature, moderate relative humidity (50% RH) and back pressure of 14.7 psi.     

Degradation mechanism study of PTFE/Nafion membrane in MEA utilizing an accelerated degradation technique

Nafion membranes are widely used for commercial membrane electrode assemblies (MEAs) in proton exchange fuel cells (PEMFCs). The polytetrafluoroethylene (PTFE)/Nafion (PN) composite membrane has the advantages of being low in cost, high in mechanical strength, and does not swell excessively. This study focuses on the properties of PTFE/Nafion membranes and PTFE/Nafion MEAs by comparing the durability and performance of the PN MEAs to commercial Nafion 211 MEAs. In an accelerated degradation test (ADT), the characterization of PTFE/Nafion and Nafion MEAs were analyzed using in-situ electrochemical methods such as polarization curves, AC impedance, cyclic voltammetry (CV), and linear sweep voltammetry (LSV). The results demonstrate an increase in the internal resistance on the PTFE/Nafion MEA only. The three mechanisms behind this unique result were proposed to be: (a) Separation of the catalyst layer from the membrane due to creep deformation; (b) Separation of the outer Nafion layer film from the core PTFE/Nafion membrane due to creep deformation; (c) Degradation of the Nafion plane (or Nafion dissolution) from the PTFE surface.     

Preparation and characterization of a modified montmorillonite/sulfonated polyphenylether sulfone/PTFE composite membrane

Organically modified montmorillonites are valuable materials that have been used to improve the permeability, water retention, and proton conductivity of proton exchange membrane for fuel cells. A sulfonated montmorillonite/sulfonated poly (biphenyl ether sulfone)/Polytetrafluoroethylene (SMMT/SPSU-BP/PTFE) composite membrane was prepared for fuel cells. The thermal stability of the SMMT was tested by the thermogravimetry-mass spectrometry (TGA-MS) and its structure in the composite membrane was characterized by X-ray diffraction (XRD). It was found that SMMT was stable up to 205 °C and the interlayer distance of the nanoclay expanded from 1.43 nm to 1.76 nm after the organic sulfonic modification. The SMMT was completely exfoliated in the composite membranes. The properties of ion-exchange capacity, water uptake, swelling ratio, proton conductivity, and mechanical strength of the composite membranes were investigated as well. The good water retention of SMMT made the SMMT/SPSU-BP and SMMT/SPSU-BP/PTFE composite membranes have about 20% more bound water than the SPSU-BP membrane. Due to the reinforce effect of the PTFE porous film, the SMMT/SPSU-BP/PTFE composite membrane presented low swelling even at elevated temperature and high stress strength. All of the properties indicate that the SMMT/SPSU-BP/PTFE composite membrane is very promising as the PEM for medium temperature PEMFCs.     

Numerical investigation on the performance of proton exchange membrane fuel cells with channel position variation

Such factors as mole fractions of species, water generation, and conductivity influence the performance of proton exchange membrane fuel cells (PEMFCs). The geometrical shape of the fuel cells also should be considered a factor in predicting the performance because this affects the species' reaction speed and distribution. Specifically, the position between the channel and rib is an important factor influencing PEMFC performance because the current density distribution is affected by the channel and rib position. Three main variables that decide the current density distribution are selected in the paper: species concentration, overpotentials, and membrane conductivity. These variables should be considered simultaneously in deciding the current density distribution with the given PEMFC cell voltage. In addition, the inlet relative humidity is another factor affecting current density distribution and membrane conductivity. In this paper, two channel‐to‐rib models, namely, channel‐to‐channel and the channel‐to‐rib, are considered for comparing the PEMFC performance. Thorough performance comparisons between these two models are presented to explain which is better under certain parameters. A three‐dimensional numerical PEMFC model is developed for obtaining the current density distribution. Water transfer mechanism because of electro osmotic drag and concentration diffusion also is presented to explain the PEMFC performance comparison between the two models. Copyright © 2012 John Wiley & Sons, Ltd.     

Highly ordered Nafion‐silica‐HPW proton exchange membrane for elevated temperature fuel cells

Well‐ordered Nafion‐silica‐HPW proton exchange membranes with Nafion ionomers as co‐surfactant have been synthesized through a facile self‐assembly between the positively charged silica, negatively charged HPW acids, and Nafion ionomers. The results exhibited uniform nanoarrays with long‐range order when Nafion content in the complex is lower than 30 wt%. The electrolyte stripe textures were clearly presented with an interval channel of 5–6 nm. The well‐ordered proton conducting sites made the proton move through the membrane freely with low humidity dependence of proton transportation through the Nafion‐silica‐HPW electrolyte. The proton conductivities of the Nafion‐silica‐HPW electrolyte with Nafion content of 10–30 wt% were 0.018–0.022 Scm^?1 at 25 °C without humidification, and the conductivities increased to 0.043–0.05 Scm^?1 when the temperature increased to 200 °C. The capillary condensation of the ordered structure also improved the water uptake of the Nafion‐silica‐HPW electrolyte at low humidity. With external humidifying of 25 RH%, the water uptake of the Nafion‐silica‐HPW electrolyte with Nafion content of 10–30 wt% reached to 15–23 wt% at elevated temperature of 100–200 °C. The improvement of the water uptake facilitated proton transport through the Nafion‐silica‐HPW electrolytes, resulting in proton conductivities of 0.082–0.095 Scm^?1 at temperature of 150–200 °C, 25 RH%. Copyright © 2012 John Wiley & Sons, Ltd.     

Functionalized titania nanotube composite membranes for high temperature proton exchange membrane fuel cells

In this study, functionalized titania nanotubes (F-TiO₂-NT) were synthesized by using 3-mercaptopropyl-tri-methoxysilane (MPTMS) as a sulfonic acid functionalization agent. These F-TiO₂-NT were investigated for potential application in high temperature hydrogen polymer electrolyte membrane fuel cells (PEMFCs), specifically as an additive to the proton exchange membrane. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) results confirmed that the sulfonic acid groups were successfully grafted onto the titania nanotubes (TiO₂-NT). F-TiO₂-NT showed a much higher conductivity than non-functionalized titania nanotubes. At 80 °C, the conductivity of F-TiO₂-NT was 0.08 S/cm, superior to that of 0.0011 S/cm for the non-functionalized TiO₂-NT. The F-TiO₂-NT/Nafion composite membrane shows good proton conductivity at high temperature and low humidity, where at 120 °C and 30% relative humidity, the proton conductivity of the composite membrane is 0.067 S/cm, a great improvement over 0.012 S/cm for a recast Nafion membrane. Based on the results of this study, F-TiO₂-NT has great potential for membrane applications in high temperature PEMFCs.     

Improving the performance of sulfonated polymer membrane by using sulfonic acid functionalized hetero-metallic metal-organic framework for DMFC applications

Proton transport played a crucial part in the fuel cells, sensors, and batteries. The electrolyte used in fuel cells should possess high proton conductivity and good chemical stability. Herein, taking advantage of the high proton conductivity of metal-organic framework (MOF) and the good chemical stability of branched polymers, a new heterometallic mediated MOF (Zr-Cr-SO₃H) is synthesized and utilized as a filler in the highly branched sulfonated polymer (BSP). In addition, Zr-SO₃H MOF is also prepared for comparison. Transmission electron microscope study shows that the prepared MOF particles are spherical in size and interconnected through nanosheets. The optimized quantity of MOFs inside the polymer matrix improves the water sorption, mechanical property, and proton conductivity. The composite membranes display an improved open-circuit voltage than the pristine BSP membrane. By comparing the Zr-SO₃H MOF incorporated composite membrane, Zr-Cr-SO₃H MOF incorporated composite membranes display higher proton conductivity and peak power density in a single-cell test. In particular, the single-cell fabricated with Zr-Cr-SO₃H MOF incorporated composite membrane is able to reach the peak power density of 64.6 mWcm⁻² at 60°C, which is 26% greater than the Nafion 212 membrane. Furthermore, this work offers a new strategy for the utilization of hetero-metal MOF as a filler for proton exchange membrane applications.     

Properties and fuel cell performance of proton exchange membranes prepared from disulfonated poly(sulfide sulfone)

A series of disulfonated poly(sulfide sulfone)s (SPSSF)s copolymers are synthesized via direct aromatic nucleophilic substitution polycondensation of 4,4′-dichlorodiphenylsulfone (DCDPS), 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone (SDCDPS) and 4,4′-thiobisbenzenethiol at various molar ratios. Tough and flexible membranes with 30 mol% (SPSSF30) to 50 mol% (SPSSF50) SDCDPS monomers are obtained by casting from DMAc solution. Their physicochemical properties including thermal properties, mechanical properties, water uptake, swelling ratio and oxidative stability are fully investigated. And the fuel cell performance of SPSSF membranes at different temperature and relative humidity is evaluated comprehensively for the first time. It is found that the SPSSF40 membrane exhibited low dimensional change in the temperature range of 20–100 °C, good mechanical properties, high oxidative stability and comparable fuel cell performance to Nafion 212 membrane. Besides, the H₂ crossover density of the SPSSF40 membrane is only 50% of that of Nafion 212 membrane. Consequently, SPSSF40 membranes prove to be promising candidates as new polymeric electrolyte materials for proton exchange membrane (PEM) fuel cells operated at medium temperatures.     

Coated stainless steel as bipolar plate material for anion exchange membrane fuel cells (AEMFCs)

Low-temperature anion-exchange membrane fuel cells (AEMFCs) are gathering increased attention because they facilitate the use of non-precious metal catalysts, which might drastically reduce costs compared to low-temperature proton exchange membrane fuel cells (PEMFCs). Metallic, e.g., stainless steel, bipolar plates (BPPs) present a cost-efficient solution for this type of cell. However, anodic film formation at high positive potentials (approx. +1.5 V vs. RHE) on uncoated metals/stainless steels leads to high interfacial contact resistance (ICR) of the BPP once exposed to such a potential. A potential of +1.5 V vs. RHE is commonly reached under local and global hydrogen starvation, which rules out the use of uncoated metals in AEMFCs. We have investigated the ICR change and oxide film formation of several materials under simulated fuel cell operating conditions and found that suppression of anodic film formation and, in turn, low ICRs can be achieved by carbon coating of stainless steels.     

Preparation and characterization of phosphotungstic acid-derived salt/Nafion nanocomposite membranes for proton exchange membrane fuel cells

Nafion/Cs_2.5H_0.5PW₁₂O₄₀ nanocomposite membranes are prepared and characterized as alternate materials for PEMFC operation at high temperature/low humidity. The Cs_2.5H_0.5PW₁₂O₄₀ solid acid particles (hereafter CsPWA) have the high surface area, the high hygroscopic property and the ability to generate proton in the presence of water molecules. The results of prepared membranes at three levels (0, 10 and 15%) indicate that the CsPWA particles have influence on the water content, ion exchange capacity, thermal properties (TGA and DSC), proton conductivity and PEM fuel cell performance. Particles agglomeration and Nafion active sites (sulfonic groups) covering are seen in the nanocomposite membranes. The conductivity of nanocomposite membranes at high temperatures (110 and 120 °C) is higher than plain Nafion and may be related to the additional water within the nanocomposite membrane and/or the additional surface functional site provide by CsPWA. The fuel cell responses show that in the fully hydrated state and at the higher current densities, the prepared MEAs with nanocomposite membranes possess better response compared with the plain Nafion. In partially hydrated cell, at both low and high current densities, the superior performance of the MEA prepared by nanocomposite membranes is observed.     

Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells

Graphite oxide/polybenzimidazole synthesized by 3, 3′-diaminobenzidine and 5-tert-butyl isophthalic acid (GO/BuIPBI) and isocyanate modified graphite oxide/BuIPBI (iGO/BuIPBI) composite membranes were prepared for high temperature polymer proton exchange membrane fuel cells (PEMFCs). All membranes were loaded with different content of phosphoric acid to provide proton conductivity. The GO/BuIPBI and iGO/BuIPBI membranes were characterized by SEM which showed that the filler GO or iGO were well dispersed in the polymer matrix and had a strong interaction with BuIPBI, which can improve the chemical stability of BuIPBI membrane and support a higher acid content. The proton conductivities of the GO/BuIPBI and iGO/BuIPBI with high acid loading were 0.016 and 0.027 S/cm, respectively, at 140 °C and without humidity.     

Operation of a proton exchange membrane fuel cell under non-humidified conditions using a membrane–electrode assemblies with composite membrane and electrode

Composite membranes with hydrophilic substances can retain water and allow the operation of proton exchange membrane fuel cells (PEMFCs) under non-humidified conditions. In this work, thin Nafion composite membranes with silica are prepared to operate a PEMFC with dry fuel and oxidant. In addition, the role of silica in the catalyst layer as a water retainer is studied. In particular, the anode and the cathode are modified separately to elucidate the effect of silica. The incorporation of silica in the membrane and the catalyst layer enhances single-cell performance under non-humidified operation. The cell performance of membrane–electrode assemblies using the composite membrane and electrode is higher than that of a MEA using commercial Nafion 111 membrane under non-humidified conditions.     

Sulfonated PEEK-based polymers in PEMFC and DMFC applications: A review

Nowadays, polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are devices known for using proton conducting membranes. From a conceptual point of view, DMFC and PEMFC systems are very similar, except for being supplied by different fuels such as methanol and hydrogen, respectively. Commonly, PEMFCs are performed at temperature lower than 100 °C owing to the failure of the electrochemical performances of Nafion. Nevertheless, taking into account the poisoning effect of CO on the fuel cell catalyst (conventionally based on Pt), the ideal working temperature of the PEMFCs should be above 100 °C, where CO poisoning could be drastically reduced or avoided. Today, Nafion is recognized as the most used proton exchange membrane in the market, useful for both PEMFC and DMFC applications. It is based on a perfluorinated polymer and shows good thermal stability and high proton conductivity as main benefits. On the contrary, Nafion is an expensive material and suffers high fuel crossover (particularly, methanol crossover in DMFC applications) besides the proton conductivity loss above 100 °C. Therefore, in the last decades many scientists paid special attention on the development of new materials based on non-fluorinated polymers as an alternative to Nafion. One of the most promising class of is represented by the polyetheretherketone (PEEK). According to the specialized literature, interesting performances in terms of proton conductivity and thermo-chemical properties as well as low fuel crossover and costs are noticeable for sulfonated PEEK-based polymers. Indeed, many scientific applications are devoted to modify PEEK polymer for manufacturing membranes alternative to Nafion for both PEMFC and DMFC applications. Among them, important methods are exploited for preparing electrolyte membranes from PEEK such as: a) PEEK electrophilic sulfonation (S-PEEK); b) S-PEEK and non-functional polymers blending; c) S-PEEK, heteropolycompounds and poly-ether-imide doping with inorganic acids, etc.     

Composite membranes based on sulfonated poly(aryl ether ketone)s containing the hexafluoroisopropylidene diphenyl moiety and poly(amic acid) for proton exchange membrane fuel cell application

Composite membranes based on sulfonated poly(aryl ether ketone)s containing the hexafluoroisopropylidene diphenyl moiety and poly(amic acid) with oligoaniline in the main chain have been prepared and immersed in H₃PO₄ to obtain acid-doped composite films. As expected, the water uptake values and methanol permeability of the composite membranes decrease with the increase of the weight fraction of PAA in the membrane matrix. Notably, the SPEEK-6F/PAA-15 shows a water uptake of 13.2% and a methanol permeability of 0.9 × 10⁻⁷ cm² s⁻¹, which are much lower than those of the Nafion (28.6% and 15.5 × 10⁻⁷ cm² s⁻¹, respectively). Although the proton conductivities decrease after the addition of PAA, higher selectivity values are obtained with the composite membranes. Therefore, the SPEEK-6F/PAA blend membranes, with the improved proton conductivity, methanol resistance and good thermal stability, can be used as a good alternative for proton conductive membranes with potential application in proton exchange membrane fuel cells (PEMFCs).     

Experimental study of proton exchange membrane fuel cells using Nafion 212 and Nafion 211 for portable application at ambient pressure and temperature conditions

Low humidification, large air stoichiometry, dry hydrogen and low operational temperature makes open-cathode proton exchange membrane fuel cells (PEMFCs) with forced-air convection, which is designed for portable applications, quite different from that used in automobile vehicles. In this paper, PEMFCs humidified at 30 °C using Nafion 212 and Nafion 211 as electrolytes were systematically investigated under simulating conditions. These conditions included air stoichiometry from 3 to 100 and cell temperature from 30 °C to 60 °C. The results indicate that the thinner membrane (Nafion 211) had better performance and more stable voltage output under air dual-function configuration than Nafion 212. Furthermore, the dynamic response of the voltage with cell temperature was also studied during rising and cooling procedure between 30 °C and 60 °C.     

Functionalizing carbon nanotubes for proton exchange membrane fuel cells electrode

In recent years, carbon nanotubes (CNTs) have been increasingly considered as an advanced metal catalyst support for proton exchange membrane fuel cells (PEMFCs), owing to their outstanding physical and mechanical characteristics. However, the effective attachment of metal catalysts, uniformly dispersed onto the CNT surface, remains a formidable challenge because of the inertness of the CNT walls. Therefore, the surface functionalization of CNTs seems necessary in most cases in order to enable a homogeneous metal deposition. This review presents the different surface functionalization approaches that provide efficient avenues for the deposition of metal nanoparticles on CNTs, for the application of catalyst supports in PEMFCs with improved reactivity.     

Approaches towards the development of heteropolyacid-based high temperature membranes for PEM fuel cells

Operating proton exchange membrane fuel cells (PEMFCs) at higher temperatures (above the boiling point of water) offer several advantages. It enhances the electrodes' kinetics, allows the recovery of useful heat, and offers better water management due to the formation of water in the vapor phase. There is a crucial need to either, modify the existing perfluorosulfonic acid membranes (i.e. Nafion) or develop a new class of membranes that can withstand higher temperature operation. Heteropolyacids (HPAs) represent a class of inorganic materials that have been investigated as additives in PEMFCs membranes for the purpose of: 1) enhancing the proton conductivity and, 2) reducing the fuel crossover. This review focuses on discussing the recent developments attained upon the introduction of HPAs in proton exchange membranes. The review summarized the various efforts made on either modifying the existing Nafion membranes with HPAs, or by immobilizing them in other polymers such as PBI and SPEEK. Remarkable enhancements in proton conductivities, as well as a significant reduction in fuel crossover, were reported. However, the leaching of HPAs is still a major obstacle. The current review concludes that the successful implementation of HPAs in PEMFCs membranes can be achieved upon developing proper immobilization techniques within the polymers' matrix.     

Novel cross-linked sulfonated poly (arylene ether ketone) membranes for direct methanol fuel cell

To prepare a cross-linked proton exchange membrane with low methanol permeability and high proton conductivity, poly (vinyl alcohol) is first blended with sulfonated poly (arylene ether ketone) bearing carboxylic acid groups (SPAEK-C) and then heated to induce a cross-linking reaction between the carboxyl groups in SPAEK-C and the hydroxyl groups in PVA. Fourier transform infrared spectroscopy is used to characterize and confirm the structure of SPAEK-C and the cross-linked membranes. The proton conductivity of the cross-linked membrane with 15% PVA in weight reaches up to 0.18 S cm⁻¹ at 80 °C (100% relative humidity), which is higher than that of Nafion membrane, while the methanol permeability is nearly five times lower than Nafion. The ion-exchange capacity, water uptake and thermal stability are investigated to confirm their applicability in fuel cells.     

质子交换膜燃料电池的水热管理

质子交换膜燃料电池电化学反应生成电能、热能和水。质子交换膜燃料电池中水管理与热管理是紧密关联互相耦合的,有效的水热管理对于提高电池的性能和寿命起着关键作用。本文对膜中水的迁移机理及影响水平衡的主要因素进行了分析,对目前较为有效的水管理方法进行了综述。另外,分析了在微重力条件下燃料电池水管理问题的重要性。燃料电池中约有40%~50%的能量耗散为热能,必须采取有效的散热方式及时排除这些热量。本文对质子交换膜燃料电池的温度分布、局部换热系数及散热等燃料电池热管理相关问题进行了分析。