Composite S-PEEK membranes for medium temperature polymer electrolyte fuel cells

Sulphonated-PEEK polymers with two different sulphonation degrees (DS) were obtained by varying the sulphonation parameters. Ionomeric membranes were prepared as a reference. Composite membranes were obtained by mixing different percentage of 3-aminopropyl functionalised silica to the polymers dissolved in DMAc. The resulting membranes were characterised in terms of water uptake, IEC and proton conductivity in different conditions of temperature and relative humidity.     

A review of polymer electrolyte membranes for direct methanol fuel cells

This review describes the polymer electrolyte membranes (PEM) that are both under development and commercialized for direct methanol fuel cells (DMFC). Unlike the membranes for hydrogen fuelled PEM fuel cells, among which perfluorosulfonic acid based membranes show complete domination, the membranes for DMFC have numerous variations, each has its advantages and disadvantages. No single membrane is emerging as absolutely superior to others. This review outlines the prospects of the currently known membranes for DMFC. The membranes are evaluated according to various properties, including: methanol crossover, proton conductivity, durability, thermal stability and maximum power density. Hydrocarbon and composite fluorinated membranes currently show the most potential for low cost membranes with low methanol permeability and high durability. Some of these membranes are already beginning to impact the portable fuel cell market.     

On the temperature distribution in polymer electrolyte fuel cells

This paper presents 2D thermal model of a fuel cell to elucidate some of the issues and important parameters with respect to temperature distributions in PEM fuel cells. A short review on various properties affecting the temperature profile and the heat production in the polymer electrolyte fuel cell is included. At an average current density of 1 A cm⁻², it is found that the maximum temperature of the MEA is elevated by between 4.5 and 15 K compared to the polarisation plate temperature. The smallest deviation corresponds to one dimensional transport, while the largest corresponds to the two dimensional transport considering anisotropic thermal conductivity. The two dimensional thermal model further predicts increased lost work. While most of the heat generation is allocated in the cathode, it is shown that the heat effect may be balanced by the water phase change in the anode. The most significant factor in determining the temperature distribution is the gas channel geometry (width and channel type), followed by the thermal conductivity of the porous transport layer and state of the water in the cell.     

Flexible blend polymer electrolyte membranes with excellent conductivity for fuel cells

Polymer electrolyte membrane with high conductivity is an indispensable need in fuel cell applications due to some major drawbacks of the commonly used polymer electrolytes. Herein, we have synthesized two thermally and chemically stable polymer electrolytes; poly(2,5-dioxo-2,5-dihydro-1H-pyrrole-1-carbonyl)sulfamoyl fluoride-co-styrene) (PDDPCSFS), and sulfonated Pmax-1200 (SPmax-1200). PDDPCSFS and SPmax-1200 showed (70.50 and 75.44) mS/cm proton conductivity (σ), (30 and 48)% water uptake (WU), and (1.35 and 1.93) meq./g ion exchange capacity (IEC) value, respectively, at 80 °C. We also prepared blend polymer electrolytes by blending PDDPCSFS and SPmax-1200 polymer with different ratios and observed that the blend polymer electrolytes exhibited enhanced performances compared to their parent's compounds by forming an excellent ion-conducting channel. The blend polymers, Blend (9:1), and Blend (8:2) exhibited excellent IEC (2.09 and 2.19) meq./g, σ (93.14 and 117.50) mS/cm at 80 °C under 80% relative humidity (RH), mechanical, and chemical stability, which are higher or comparable to Nafion 117. Moreover, the maximum power density of fuel cells with Blend (9:1) and Blend (8:2) polymer electrolytes was ca. (0.55 and 0.59) W/cm², respectively, which are very close to the power density of fuel cell with commercial polymer electrolyte. Therefore, these blend polymer electrolytes can be used as effective proton conductive materials for fuel cell applications.     

Performance comparison of long and short-side chain perfluorosulfonic membranes for high temperature polymer electrolyte membrane fuel cell operation

A new Aquivion™ E79-03S short-side chain perfluorosulfonic membrane with a thickness of 30 μm (dry form) and an equivalent weight (EW) of 790 g/equiv recently developed by Solvay-Solexis for high-temperature operation was tested in a pressurised (3 bar abs.) polymer electrolyte membrane (PEM) single cell at a temperature of 130 °C. For comparison, a standard Nafion™ membrane (EW 1100 g/equiv) of similar thickness (50 μm) was investigated under similar operating conditions. Both membranes were tested for high temperature operation in conjunction with an in-house prepared carbon supported Pt electrocatalyst. The electrocatalyst consisted of nanosized Pt particles (particle size ∼2 nm) dispersed on a high surface area carbon black. The electrochemical tests showed better performance for the Aquivion™ membrane as compared to Nafion™ with promising properties for high temperature PEM fuel cell applications. Beside the higher open circuit voltage and lower ohmic constraints, a higher electrocatalytic activity was observed at high temperature for the electrocatalyst-Aquivion™ ionomer interface indicating a better catalyst utilization.     

SPEEK/functionalized silica composite membranes for polymer electrolyte fuel cells

Silica and sulfonic acid functionalized silica were synthesized by condensation of appropriate precursors through a sol–gel approach. SPEEK with three different ion exchange capacities (1.35, 1.75 and 2.1 mequiv. g⁻¹) were prepared by sulfonation of PEEK. Composite membranes with 5% and 10% additive loadings were prepared by solvent casting. Characterization by FTIR spectroscopy confirmed the presence of sulfonic acid groups in the functionalized silica additives. The agglomerate size of the additives was estimated by scanning electron microscopy to be between 2 and 5 μm. The room temperature liquid water uptake of the membranes was evaluated. Water uptake increased with SPEEK IEC. Composite membranes exhibited lower water uptakes when compared to pure SPEEK. Proton conductivities of up to 0.05 S cm⁻¹ at 80 °C and 75% relative humidity and 0.02 S cm⁻¹ at 80 °C and 50% relative humidity were recorded for SPEEK composite membranes prepared using sulfonic acid functionalized silica. Hydrogen crossover through the membrane was determined through linear sweep voltammetry on membrane electrode assemblies (MEAs). Hydrogen crossover current densities for all the MEAs were on the order of 1–2 mA cm⁻². MEAs tested showed reasonable performance at 80 °C and 75% and 50% relative humidities.     

Nano-TiO2-coated polymer electrolyte membranes for direct methanol fuel cells

Composite polymer electrolyte membranes with nano-TiO₂ films are fabricated by deposition of titania nanoparticles from a sol solution. Measurements of ion conductivity, methanol permeability and single-cell performance of the modified Nafion membranes are conducted. The TiO₂ films adhere well and are crack-free. The protonic conductivity of the composite membranes decreases with increasing titania content, but methanol permeability is reduced. Preliminary tests on a single-cell of a direct methanol fuel cell (DMFC) indicate that a titania-coated membrane with 0.009 mg cm⁻² content gives the highest cell voltage and maximum power density.     

Preparation of radiochemically pore-filled polymer electrolyte membranes for direct methanol fuel cells

Polymer electrolyte pore-filled membranes, for possible use in direct methanol fuel cells (DMFCs), have been prepared by radiochemical grafting of styrene into porous poly(vinylidene fluoride) (PVDF) films using simultaneous irradiation with an electron beam (EB) followed by a sulfonation reaction. The physico-chemical properties of the obtained polystyrene sulfonic acid pore-filled membranes are evaluated using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), ac impedance, and a methanol diffusion cell. The effects of variation of the grafting yield (Y%) on the ionic conductivity and the methanol permeability of the membranes are investigated. The ionic conductivity of the membranes increases with increase in Y% and exceeds that of the perfluorinated ionomer membrane, Nafion 117, at a grafting yield of 46%. The methanol permeability of 40 and 46% pore-filled membranes is lower than that of Nafion 117 by 53 and 71%, respectively. The performance characteristic factor suggests that these membranes are potential candidates for DMFC applications.     

Modelling of start-up time for high temperature polymer electrolyte fuel cells

Start-up time is one of the important factors that limit the application of high temperature polymer electrolyte fuel cells in several areas. Present work involves the analysis of different warm-up methodologies to analyse the start-up time for phosphoric acid doped PBI membrane based fuel cells. With this objective a number of three dimensional thermal models have been developed. Different heating methodologies such as reactant heating, coolant heating and combined heating (reactant and ohmic) are simulated. The ohmic heating is implemented for generating heat in the membrane itself at high current densities. Hence, combining it with other heating techniques is found effective in reducing start-up times significantly.     

A dynamic model for high temperature polymer electrolyte membrane fuel cells

A dynamic one-dimensional isothermal phenomenological model was developed in order to describe the steady-state and transient behavior of high temperature polymer electrolyte membrane fuel cells (PEMFC). The model accounts for transient species mass transport at the bipolar plates and gas diffusion layers and the electric double layers charge/discharge. To record the impedance spectra, a small sinusoidal voltage perturbation was imposed to the simulator over a wide range of frequencies, and the resultant current density amplitude and phase were recorded.The steady-state behavior of the fuel cell, as well as the impedance spectra were obtained and compared to experimental data of two different fuel cells equipped with different MEAs based on phosphoric acid polybenzimidazole membrane. This approach is new and allows a deeper analysis of the controlling phenomena. The model fitted quite well the I-V curves for both systems, but fairly well the Nyquist plots. The differences observed in the Nyquist plots were attributed to proton resistance in the catalyst layer and the gas diffusion limitations to cross the phosphoric acid layer that coats the catalyst, phenomena not included in the proposed phenomenological model.     

GO-nafion composite membrane development for enabling intermediate temperature operation of polymer electrolyte fuel cell

Increasing Polymer Electrolyte Fuel Cells’ (PEFCs) operating temperature has benefits on the performance and the ease of utilisation of the heat generated; however, efforts for high temperature PEFCs have resulted in high degradation and reduced life time. In the literature, conventional low temperature (T < 80 °C) and high temperature (140–200 °C) regimes have been extensively studied, while the gap of operating at intermediate temperature (IT) (100–120 °C) has been scarcely explored.The main bottleneck for operating at IT conditions is the development of a suitable proton exchange membrane with comparable performance and lifetime to the commercially used Nafion operating at conventional conditions. In this work, composite membranes of Graphene Oxide (GO) and Nafion of varied thickness were fabricated, characterised and assessed for in-situ single cell performance under automotive operating conditions at conventional and intermediate temperatures.The material characterisation confirmed that a composite GO-Nafion structure was achieved. The composite membrane demonstrated higher mechanical strength, enhanced water uptake, and higher performance. It was demonstrated that by utilising GO-Nafion composite membranes, an up to 20% increase in the maximum power density at all operating temperatures can be achieved, with the optimum performance is obtained at 100 °C. Moreover, the GO-Nafion membrane was able to maintain its open circuit voltage values at increased temperature and reduced thickness, indicating better durability and potentially higher lifetime.     

High performance electrocatalysts supported on graphene based hybrids for polymer electrolyte membrane fuel cells

In this study, new electrocatalysts for PEM fuel cells, based on Pt nanoparticles supported on hybrid carbon support networks comprising reduced graphene oxide (rGO) and carbon black (CB) at varying ratios, were designed and prepared by means of a rapid and efficient microwave-assisted synthesis method. Resultant catalysts were characterized ex-situ for their structure, morphology, electrocatalytic activity. In addition, membrane-electrode assemblies (MEAs) fabricated using resultant electrocatalysts and evaluated in-situ for their fuel cell performance and impedance characteristics. TEM studies showed that Pt nanoparticles were homogeneously decorated on rGO and rGO-CB hybrids while they had bigger size and partially agglomerated distribution on CB. The electrocatalyst, supported on GO-CB hybrid containing 75% GO (HE75), possessed very encouraging results in terms of Pt particle size and dispersion, catalytic activity towards HOR and ORR, and fuel cell performance. The maximum power density of 1090 mW cm^?2 was achieved with MEA (Pt loading of 0.4 mg cm^?2) based on electrocatalyst, HE75. Therefore, the resultant hybrid demonstrated higher Pt utilization with enhanced FC performance output. Our results, revealing excellent attributes of hybrid supported electrocatalysts, can be ascribed to the role of CB preventing rGO sheets from restacking, effectively modifying the array of graphene and providing more available active catalyst sites in the electrocatalyst material.     

Proton transport of porous triazole-grafted polysulfone membranes for high temperature polymer electrolyte membrane fuel cell

Introduction of porous structure to high temperature polymer electrolyte membranes is one of effective pathways to increase their proton conductivity under elevated temperature. However, the effect of the porous structure on the proton diffusion mechanism of these membranes is still unclear. In this work, the proton transport behaviour of a series of porous triazole-polysulfone (PSf) membranes under elevated temperature is comprehensively investigated. The functional triazole ring in the framework of porous triazole-PSf acts a proton acceptor to form acid-base pair with phosphoric acid (PA). In addition, the proton diffusion coefficient and proton conductivity of PA-doped porous triazole-PSf is an order of magnitude higher than that of the PA-doped dense triazole-PSf membrane. Percolation theory calculation convinces that the high proton conductivity of PA-doped porous triazole-PSf is due to the formation of continuous long-range proton diffusion channels under high pore connectivity and porosity. On the contrary, excessive pore connectivity also results in high gas permeability, leading to decrease of the open circuit voltage and cell performance of the single cell. Consequently, the optimum porosity for the PA-doped porous triazole-PSf membrane is 75% for fuel cell operating with the maximum peak power density of 550 mW·cm^?2 and great durability for 120 h under 140 °C.     

Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of high-temperature polymer electrolyte membrane fuel cells

High-temperature polymer electrolyte membrane fuel cells with phosphoric acid doped polybenzimidazole (PBI) are made with three-layered membranes. The central membrane layer is meant as an acid reservoir made from direct cast PBI with a higher acid content than the outermost layers, which are post doped membranes acting as barrier layers to limit the acid transport out of the central layer. Cells with three-layered membranes and others with normal single layered membranes are tested at 170 and 180 °C. At both temperatures, the cells with three-layered membranes show significantly lower voltage decay rates than the corresponding cells with single-layered membranes. Post doped PBI membranes based on linear or thermally crosslinked PBI are used for the barrier layers of the three-layered membranes and for the single-layered membranes in the test series at 180 °C. The acid loss rates assessed by acid collection at the fuel cell exhaust, are rather comparable. At 180 °C, the cells are tested for up to 10,000 h and voltage decay rates of 2.3 and 4.1 μVh^-1 are measured for the cells with three-layered membranes and 14 and 11 μVh^-1 for cells with single-layered membranes.     

Optimization of proton exchange membranes and the humidifying conditions to improve cell performance for polymer electrolyte fuel cells

The relationship between thickness of the proton exchange membrane and cell performance was measured. Use of cells with thinner membrane resulted in an apparent reduction in the volume of water transferred from the anode to the cathode, offering stable cell performance even under conditions of low-humid reaction gas supplied. Internal humidification, with its constant feeding of sufficient water to the anode, was found to provide an equivalent of the external humidification process, without needing humid air. On the basis of these findings, a fuel cell module was assembled featuring newly designed gas separators to feed water directly to the fuel flow field in the cells. The fuel cell module was then submitted to module performance testing under a broad spectrum of operating conditions and it demonstrated performance stability against load variations from startups at room temperature to operations under the rated load.     

Sulfonated polyimide hybrid membranes for polymer electrolyte fuel cell applications

Sulfonated Si-MCM-41 (SMCM) with an ion exchange capacity (IEC) of 2.3 mequiv. g⁻¹ was used as a hydrophilic and proton-conductive inorganic component. Sulfonated polyimide (SPI) based on 1,4,5,8-naphthalene tetracarboxylic dianhydride and 2,2′-bis(3-sulfophenoxy) benzidine was used as a host membrane component. The SMCM/SPI hybrid membrane (H1) with 20 wt% loading of SMCM and an IEC of 1.90 mequiv. g⁻¹ showed the high mechanical tensile strength and the slightly higher water vapor sorption than the host SPI membrane (M1) with an IEC of 1.86 mequiv. g⁻¹. H1 and M1 showed anisotropic membrane swelling with about 10 times larger swelling in thickness direction than in plane one. The proton conductivity at 60 °C of H1 was lower in water than that of M1, but comparable at 30% RH. At 90 °C, H1 showed the rather lower performance of polymer electrolyte fuel cell (PEFC) at 82% RH than M1 and fairly better performance at 30% RH. On the other hand, at 110 °C and low humidity less than 50% RH, H1 showed the much better PEFC performance than M1 and Nafion 112. This was due to the promoted back diffusion of produced water by the superior water-holding capacity of SMCM. The SMCM/SPI hybrid membranes have high potential for PEFCs at higher temperatures and lower humidities.     

Investigation of a cost-effective strategy for polymer electrolyte membrane fuel cells: High power density operation

Polymer electrolyte membrane (PEM) fuel cell technology needs to overcome the cost barrier in order to compete with the internal combustion engines (ICEs) for transportation application. A viable approach is to raise fuel cell's power output without increasing its size and Pt loading in the catalyst layers (CLs). In this strategy, the cost per kW power output can be proportionally reduced due to the increased power density. This paper examines this strategy by exploring several important aspects that influence fuel cell performance under high power or current density using a three-dimensional (3-D) fuel cell model. It is shown that local CLs may be subject to low oxygen concentration under a high current density of 2 A/cm², causing low reaction rate near the outlet, especially under the land. Additionally, the oxygen reduction reaction (ORR) rate may be subject to a large through-plane variation under 2 A/cm², raising ohmic voltage loss in the CL. Two additional cases are investigated to improve fuel cell performance under 2 A/cm²: one has a 5 times thinner CL with the same ORR kinetics per membrane electrode assembly (MEA) area and the other has a 5 times thinner CL with 5 times higher ORR kinetics. The results show the output voltage is raised approximately from 0.5 V to 0.554 V in the former CL case and further to 0.606 V for the latter CL. To enable high-efficiency operation (e.g. >50%), thinner CLs with high ORR kinetics and GDLs with better transport properties are one research and development (R&D) direction.     

A fluorinated polymer/inorganic composite electrolyte membrane for intermediate temperature fuel cells

Composite membranes based on polytetrafluoroethylene (PTFE) and silicon dioxide (PTFE/SiO₂ × HPO₃) are fabricated to act as a fuel cell membrane for operation at temperatures from 120 to 200°C. A porous PTFE membrane is used as the membrane supporting structure and SiO₂ × HPO₃ sol as the proton conductor. SEM and EDX show that the sol clusters are connected together and adhered to the PTFE polymer. This structure completely fills the pores of the PTFE and minimises the gas cross‐over. The PTFE/SiO₂ × HPO₃ membrane has a high proton conductivity, up to 0.14 S cm^?1 at a relative humidity lower than 0.5%. The PTFE/SiO₂ × HPO₃ composite membrane gives the modest performance when it is tested in a hydrogen fuel cell although it is a potential material for the intermediate‐temperature proton‐conducting membrane fuel cell. Copyright © 2011 John Wiley & Sons, Ltd.     

A nonlinear viscoelastic-viscoplastic constitutive model for ionomer membranes in polymer electrolyte membrane fuel cells

This paper describes a phenomenological constitutive model for ionomer membranes in polymer electrolyte membrane fuel cells (PEMFCs). Unlike the existing approaches of elasto-plastic, viscoelastic, and viscoplastic model, the proposed model was inspired by micromechanisms of polymer deformation. The constitutive model is a combination of the nonlinear visco-elastic Bergström-Boyce model and hydration-temperature-dependent empirical equations for elastic modulus of ionomer membranes. Experiment results obtained from an uniaxial tension test for Nafion NR-111 membrane under well controlled environments were compared with simulated results by the finite element method (FEM) and the proposed model showed fairly good predictive capabilities for the large deformation behavior of the Nafion membrane subjected to the uniaxial loading condition in a wide range of relative humidity and temperature levels including liquid water.