Polybenzimidazole gel membrane for the use in fuel cell

Thermoreversible gelation of polybenzimidazole (PBI) in phosphoric acid (PA) is investigated by studying the gel morphology, thermodynamics of the gelation, and gelation kinetics utilizing test tube tilting and UV-Vis spectroscopy techniques. Gelation kinetics studies reveal that both the gelation rate and critical gelation concentration (C^∗_t=∞) are function of gelation temperature (T_gel) and the molecular weight of PBI. Highly dense fibrillar network morphology with large number of longer and thinner fibrils is obtained for higher gel concentration and higher molecular weight PBI. Both the gel melting (T_gm) and gelation (T_gel) temperature depend upon the gelation concentration and molecular weight of PBI. The presence of self-assembled chains of PA molecules, which help to produce the PBI crystallites, is observed from the thermodynamical study. I.R. and Raman studies prove the presence of strong hydrogen bonding interaction between the PBI and the PA molecules, and the free PA molecules in the gel network. The gelation occurs in two-step processes which include a slow rate determining conformational transition from coil to rod and followed by aggregation of rod via crystallization. The PA loading of PBI membrane obtained from the PBI-PA gel is significantly high compared to the conventional imbibing process membrane. The PBI gel membrane displays very high thermal and mechanical stabilities. The high acid loading and superb thermo-mechanical stability are due to the gel network structure of the membrane. The proton conductivity of the membrane at 160 °C and 0% relative humidity (RH) is ∼0.1 S cm⁻¹, which is higher than the reported values in the literature for the PBI. The activation energy of the proton conduction is 14-15 kJ/mol indicating faster proton transfer by hopping process inside the gel network.     

High Molecular Weight Polybenzimidazole Membranes for High Temperature PEMFC

High temperature operation of proton exchange membrane fuel cells under ambient pressure has been achieved by using phosphoric acid doped polybenzimidazole (PBI) membranes. To optimize the membrane and fuel cells, high performance polymers were synthesized of molecular weights from 30 to 94 kDa with good solubility in organic solvents. Membranes fabricated from the polymers were systematically characterized in terms of oxidative stability, acid doping and swelling, conductivity, mechanical strength and fuel cell performance and durability. With increased molecular weights the polymer membranes showed enhanced chemical stability towards radical attacks under the Fenton test, reduced volume swelling upon the acid doping and improved mechanical strength at acid doping levels of as high as about 11 mol H₃PO₄ per molar repeat polymer unit. The PBI‐78kDa/10.8PA membrane, for example, exhibited tensile strength of 30.3 MPa at room temperature or 7.3 MPa at 130 °C and a proton conductivity of 0.14 S cm^–1 at 160 °C. Fuel cell tests with H₂ and air at 160 °C showed high open circuit voltage, power density and a low degradation rate of 1.5 μV h^–1 at a constant load of 300 mA cm^–2.     

A Quaternary Polybenzimidazole Membrane for Intermediate Temperature Polymer Electrolyte Membrane Fuel Cells

A quaternary ammonium polybenzimidazole (QPBI) membrane was synthesized for applications in intermediate temperature (100–200 °C) hydrogen fuel cells. The QPBI membrane was imbibed with phosphoric acid to provide suitable proton conductivity. The proton conductivity of the membrane was 0.051 S cm^–1 at 150 °C with the PA acid loading level of 3.5 PRU (amount of H₃PO₄ per repeat unit of polymer QPBI). The QPBI membrane was characterized in terms of composition, structure and morphology by NMR, FTIR, SEM, and EDX. The fuel cell performance with the membrane gave peak power densities of 440 and 240 mW cm^–2 using oxygen and air, respectively, at 175 °C.     

On a new degradation mode for high-temperature polymer electrolyte fuel cells: How bipolar plate degradation affects cell performance

The contribution of the bipolar plate material to the overall degradation of a high temperature membrane electrode assembly (HT MEA) for polymer electrolyte fuel cells (PEFCs) is studied in terms of performance decrease, phosphoric acid uptake in the bipolar plates and change of surface morphology of the bipolar plates. Two different high temperature graphite composites, a surface treated graphite and a gold coated stainless steel flowfield and the respective MEAs are compared after operation at 180 °C. Both graphite surface treatment and gold coating lead to negligible uptake of the electrolyte and ensure low degradation rates, whereas the composite plates exhibit high uptake of acid from the MEA into the surface near bulk. Apparent MEA degradation caused by acid redistribution from the MEA to the increasingly porous plates is observed in terms of increased ohmic cell resistances and reduction of catalyst utilization as consequence of acid loss from the catalyst layers.     

Enhanced high-temperature polymer electrolyte membrane for fuel cells based on polybenzimidazole and ionic liquids

Polybenzimidazole (PBI)/ionic liquid (IL) composite membranes were prepared from an organosoluble, fluorine-containing PBI with ionic liquid, 1-hexyl-3-methylimidazolium tri?uoromethanesulfonate (HMI-Tf). PBI/HMI-Tf composite membranes with different HMI-Tf concentrations have been prepared. The ionic conductivity of the PBI/HMI-Tf composite membranes increased with both the temperature and the HMI-Tf content. The composite membranes achieve high ionic conductivity (1.6 × 10⁻² S/cm) at 250 °C under anhydrous conditions. Although the addition of HMI-Tf resulted in a slight decrease in the methanol barrier ability and mechanical properties of the PBI membranes, the PBI/HMI-Tf composite membranes have demonstrated high thermal stability up to 300 °C, which is attractive for high-temperature (>200 °C) polymer electrolyte membrane fuel cells.     

Modelling and experimental validation of a high temperature polymer electrolyte fuel cell

A steady-state, isothermal, one dimensional model of a proton exchange membrane fuel cell (PEMFC), with a polybenzimidazole (PBI) membrane, was developed. The electrode kinetics were represented by the Butler–Volmer equation, mass transport was described by the multi-component Stefan–Maxwell equations and Darcy’s law and the ionic and electronic resistances described by Ohm’s law. The model incorporated the effects of temperature and pressure on the open circuit potential, the exchange current density and diffusion coefficients, together with the effect of water transport across the membrane on the conductivity of the PBI membrane. The polarisation curves predicted by the model were validated against experimental data for a PEMFC operating in the temperature range of 125–200 °C. There was good agreement between experimental and model data of the effect of temperature and oxygen/air pressure on cell performance. The model was used to simulate the effect of catalyst loading and the Pt/carbon ratio on cell performance and, in the latter case, a 40 wt.% Pt/C ratio gave the highest peak power density.     

Oxidative reforming of biomass derived ethanol for hydrogen production in fuel cell applications

Oxidative reforming of biomass derived ethanol over an inexpensive Ni–Cu/SiO₂ catalyst has been carried out with respect to solid polymer fuel cell (SPFC) applications. Two types of runs were performed, either under diluted conditions (with helium as diluent) or under conditions corresponding to an on-board reformer. Selectivities of ethanol reforming have been analyzed as a function of operating parameters: reaction temperature, H₂O/EtOH molar ratio and O₂/EtOH molar ratio of the feed to the reformer. The hydrogen content and the CO₂/CO_x molar ratio in the outlet gases were used as parameters to optimize the operating conditions in the reforming reactor. The tests carried out at on-board reformer conditions evidenced that an H₂O/EtOH molar ratio=1.6 and an O₂/EtOH molar ratio=0.68 at 973 K allow a hydrogen rich mixture (33%) that can be considered of high interest for SPFC. Furthermore, the use of oxygen decreases the production of methane and coke which increases in turn the lifetime of the catalyst. The stability of this catalyst has been fully demonstrated by long time runs.     

Preparation and Characterization of Semi‐IPN Fluorine Containing Polybenzimidazole/Nafion Composite Membrane for Fuel Cells

A facile way to prepare semi‐interpenetrating polymer network (semi‐IPN) membrane which adopted 1,3‐benzenedisulfonyl azide (1,3‐BDSA) to crosslink with fluorine containing polybenzimidazole (Aliphatic‐16F‐PBI) in the Aliphatic‐16F‐PBI/Nafion composite membranes was proposed. By means of Fourier transformed infrared (FTIR) spectra analysis, the possible crosslinking reaction mechanism was investigated. Results suggested that 1,3‐BDSA molecule loses a nitrogen and forms nitrene upon heating. Then this nitrene reacts with C–H bond of Aliphatic‐16F‐PBI. Scanning electron microscope (SEM) images showed that the compatibility of PBI and Nafion improved while hexadecafluoro‐octyl groups were implanted into Aliphatic‐16F‐PBI molecule. The properties of Aliphatic‐16F‐PBI/Nafion composite membranes for fuel cell applications were determined through tests of gel fraction, thermogravimetry (TG), dimensional stability, mechanical property and proton conductivity. The gel fraction could reach 27.9% when 7.4% 1,3‐BDSA was added into the composite membranes. The proton conductivity of the semi‐IPN Aliphatic‐16F‐PBI/Nafion composite membranes could reach 0.69 × 10^–2 S cm^–1 at 120 °C at 100% relative humidity. Such high crosslink degree resulted in the improvement of the tensile strength, dimensional stability and chemical oxidative stability of semi‐IPN Aliphatic‐16F‐PBI/Nafion composite membranes. Nonetheless, it had little effect on the thermal stability.     

Analysis of high temperature polymer electrolyte membrane fuel cell electrodes using electrochemical impedance spectroscopy

An electrochemical impedance spectroscopy (EIS) study of electrodes in a phosphoric acid loaded polybenzimidazole (PBI) membrane fuel cell is reported. Using EIS, the effect of electrode parameters such as Pt catalyst wt%, acid doping in PBI and PTFE baesd electrodes and catalyst heat treatment on kinetic and mass transport characteristics is characterised. The influence of cell parameters of current load, temperature and oxidant gas on response is demonstrated and interpreted using an equivalent circuit model. For polarisable electrodes under small to medium steady-state current operation, the model was capable of identifying electrodes with the best kinetic or mass transport behaviour and classifying behaviour in terms of relative performance.     

Performance improvement of a micro borohydride fuel cell operating at ambient conditions

In this study, aqueous borohydride solutions were employed to fuel a micro cell. Electrochemical performance of the micro borohydride fuel cell was tested at ambient conditions without any auxiliary facilities. Electrochemical impedance spectroscopy (EIS) analyses were performed to characterize the cell performance. Both anion and cation exchange membranes were tried to separate the fuel from the cathode. Membrane properties were found to be a decisive factor for cell performance. A maximum power density of 40 mW/cm² at room temperature was achieved when the Nafion NRE211 membrane was used. Hydrogen evolution at the anode side resulted from the competitive hydrolysis reaction influenced cell performance by obstructing transfer of the electrolyte. The cell also demonstrated promising performance even when an Ag cathode was used.     

Effects of membrane electrode assembly preparation on the polymer electrolyte membrane fuel cell performance

This paper presents results of recent investigations to develop an optimized in-house membrane electrode assembly (MEA) preparation technique combining catalyst ink spraying and assembly hot pressing. Only easy steps were chosen in this preparation technique in order to simplify the method, aiming at cost reduction. The influence of MEA fabrication parameters like electrode pressing or annealing on the performance of hydrogen fuel cells was studied by single cell measurements with H₂/O₂ operation. Toray paper and carbon cloth as gas diffusion layer (GDL) materials were compared and the composition of electrode inks was optimized with regard to most favorable fuel cell performance. Commercial E-TEK catalyst was used on the anode and cathode with Pt loadings of 0.4 and 0.6 mg/cm², respectively. The MEA with best performance delivered approximately 0.58 W/cm², at 65 °C cell temperature, 80 °C anode humidification, dry cathode and ambient pressure on both electrodes. The results show, that changing electrode compositions or the use of different materials with same functionality (e.g. different GDLs), have a larger effect on fuel cell performance than changing preparation parameters like hot pressing or spraying conditions, studied in previous work.     

Synthesis and Characterization of a New Fluorine‐Containing Polybenzimidazole (PBI) for Proton‐Conducting Membranes in Fuel Cells

A new type of fluorine‐containing polybenzimidazole, namely poly(2,2′‐(2,2′‐bis(trifluoromethyl)‐4,4′‐biphenylene)‐5,5′‐bibenzimidazole) (BTBP‐PBI), was developed as a candidate for proton‐conducting membranes in fuel cells. Polymerization conditions were experimentally investigated to achieve high molecular weight polymers with an inherent viscosity (IV) up to 1.60 dl g^–1. The introduction of the highly twisted 2,2′‐disubstituted biphenyl moiety into the polymer backbone suppressed the polymer chain packing efficiency and improved polymer solubility in certain polar organic solvents. The polymer also exhibited excellent thermal and oxidative stability. Phosphoric acid (PA)‐doped BTBP‐PBI membranes were prepared by the conventional acid imbibing procedure and their corresponding properties such as mechanical properties and proton conductivity were carefully studied. The maximum membrane proton conductivity was approximately 0.02 S cm^–1 at 180 °C with a PA doping level of 7.08 PA/RU. The fuel cell performance of BTBP‐PBI membranes was also evaluated in membrane electrode assemblies (MEA) in single cells at elevated temperatures. The testing results showed reliable performance at 180 °C and confirmed the material as a candidate for high‐temperature polymer electrolyte membrane fuel cell (PEMFC) applications.     

Fuel constituent effects on fuel reforming properties for fuel cell applications

The effect of different types of compounds commonly found in diesel fuel (e.g., paraffins, naphthenes, and aromatics), as well as their chemical structure (e.g., branched versus linear paraffins) on fuel reforming has been investigated. Diesel reforming is very complicated because diesel is a complex mixture of hundreds of compounds with greatly different reactivities. The syngas production rates at the same conditions were observed in this order: paraffins > naphthenes ? aromatics. Additionally, the type of reforming performed (OSR, CPOX, or SR) as well as the process parameters (space velocity and reaction temperature) significantly affected the syngas production rates as well as carbon formation. The reactivity of one fuel component can affect the conversion pattern of others, e.g., overall yields from the reforming of a fuel mixture are not additive of yields from individual fuel components, rather the more reactive component is consumed first. Furthermore, the type of substituent in aromatics and naphthenes, the carbon chain length in n-paraffins, branching in paraffins, and degree of aromatic saturation affect the overall hydrocarbon conversion, syngas selectivity, and carbon formation. The presence of sulfur compounds in the fuel caused significant drops in H₂ yields compared to CO yields.     

A Three‐Dimensional Non‐isothermal Model of High Temperature Proton Exchange Membrane Fuel Cells with Phosphoric Acid Doped Polybenzimidazole Membranes

High temperature proton exchange membrane fuel cells (HT‐PEMFCs) with phosphoric acid doped polybenzimidazole (PBI) membranes have gained tremendous attentions due to its attractive advantages over conventional PEMFCs such as faster electrochemical kinetics, simpler water management, higher carbon monoxide (CO) tolerance and easier cell cooling and waste heat recovery. In this study, a three‐dimensional non‐isothermal model is developed for HT‐PEMFCs with phosphoric acid doped PBI membranes. A good agreement is obtained by comparing the numerical results with the published experimental data. Numerical simulations have been carried out to investigate the effects of operating temperature, phosphoric acid doping level of the PBI membrane, inlet relative humidity (RH), stoichiometry ratios of the feed gases, operating pressure and air/oxygen on the cell performance. Numerical results indicate that increasing both the operating temperature and phosphoric acid doping level are favourable for improving the cell performance. Humidifying the feed gases at room temperature has negligible improvement on the cell performance, and further humidification is needed for a meaningful performance enhancement. Pressurising the cell and using oxygen instead of air all have significant improvements on the cell performance, and increasing the stoichiometry ratios only helps prevent the concentration loss at high current densities.     

Mass transportation in diethylmethylammonium trifluoromethanesulfonate for fuel cell applications

To use the protonic mesothermal fuel cell without humidification, mass transportation in diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), trifluoromethanesulfuric acid (TfOH)-added [dema][TfO], and phosphoric acid (H₃PO₄)-added [dema][TfO] was investigated by electrochemical measurements. The diffusion coefficient and the solubility of oxygen were ca. 10⁻⁵ cm² s⁻¹ and ca. 10⁻³ M (=mol dm⁻³), respectively. Those of hydrogen were a factor of 10 and one-tenth compared to oxygen, respectively. The permeability, which is a product of the diffusion coefficient and solubility, of oxygen and hydrogen were almost the same for the perfluoroethylenesulfuric acid membrane and the sulfuric acid solution; therefore, these values are suitable for fuel cell applications. On the other hand, a diffusion limiting current was observed for the hydrogen evolution reaction. The current corresponded to ca. 10⁻¹⁰ mol cm⁻¹ s⁻¹ of the permeability, and the diffusion limiting species was the hydrogen carrier species. The TfOH addition enhanced the diffusion limiting current of [dema][TfO], and the H₃PO₄ addition eliminated the diffusion limit. The hydrogen bonds of H₃PO₄ or water-added H₃PO₄ might significantly enhance the transport of the hydrogen carrier species. Therefore, [dema][TfO] based materials are candidates for non-humidified mesothermal fuel cell electrolytes.     

Carbon fiber paper for fuel cell electrode

Carbon fiber paper (CFP) has many advantages to be used for fuel cell electrode. In this presentation, CFP was prepared from pitch-based carbon fiber through impregnation with resin, molding, and heat-treatment. Effects of heat-treatment on the properties and structure of resultant CFP were studied by means of electrical and mechanical property measurement, X-ray diffraction, and SEM. The results showed that the electrical resistance and tensile strength were decreased at higher heat-treatment temperature, and d₀₀₂ became smaller, while L_a and L_c got larger. CFP with thickness of 0.3 mm, bulk density of 0.47 g/cm³ and specific resistance of 200 μΩ m was produced after heat-treatment at 2773 K.     

Integrated fuel processors for fuel cell application: A review

This report documents the key technological progress made over last two decades in the field of development of integrated fuel processor for hydrogen generation. Studies on process optimization based on numerical simulation/calculation, mass and energy management, parametric adjustment have been reported. A number of these studies discuss the application of reforming process assisted by other technologies such as pressure swing adsorption and membrane separation to enhance the hydrogen productivity and/or purity. However, for such systems the extent of integration among and between components remains limited. Accordingly, the net efficiency is compromised due to the mass/heat transfer rate and reaction dynamics either in the individual units or the complete system. Process intensification technologies such as engineered catalysts, on-site heat production/removal and product purification can not only allow precise control of reaction and heat/mass transfer rates, but also help optimize the operation conditions, and, consequently, improve overall efficiency and mitigate the requirement for materials and capital investment. It seems that micro-scale technologies, possessing the typical characteristics of process intensification technologies, have potential for making the integrated fuel processor into practice.     

PEM fuel cell current regulation by fuel feed control

We demonstrate that the power output from a PEM fuel cell can be directly regulated by limiting the hydrogen feed to the fuel cell. Regulation is accomplished by varying the internal resistance of the membrane-electrode assembly in a self-draining fuel cell with the effluents connected to water reservoirs. The fuel cell functionally operates as a dead-end design where no gas flows out of the cell and water is permitted to flow in and out of the gas flow channel. The variable water level in the flow channel regulates the internal resistance of the fuel cell. The hydrogen and oxygen (or air) feeds are set directly to stoichiometrically match the current, which then control the water level internal to the fuel cell. Standard PID feedback control of the reactant feeds has been incorporated to speed up the system response to changes in load. With dry feeds of hydrogen and oxygen, 100% hydrogen utilization is achieved with 130% stoichiometric feed on the oxygen. When air was substituted for oxygen, 100% hydrogen utilization was achieved with stoichiometric air feed. Current regulation is limited by the size of the fuel cell (which sets a minimum internal impedance), and the dynamic range of the mass flow controllers. This type of regulation could be beneficial for small fuel cell systems where recycling unreacted hydrogen may be impractical.     

Sustainable fuel cell integrated membrane desalination systems

According to the United Nations, between two and seven billion people will face water shortages by the year 2050. Further, it is estimated that the amount of water available per person will shrink by a third during the next two decades. Inadequate supply of good-quality water coupled with higher water demand due to rapid population growth and industrialisation in developing countries are among the major reasons for the worsening water situation. Current shortages of potable water around the world and looming water scarcity especially in the developing countries is the driving force behind the implementation of membrane technologies for seawater and brackish water desalination. Typical energy consumption in seawater reverse osmosis (RO) plants operating at 40–45% product water recovery and with energy recovery from the high pressure reject stream currently is about 3–4 kWh/m³. The near-term goal of the industry is to reduce energy consumption to less than 2 kWh/m³ by using a combination of energy efficient RO pumps, more efficient energy recovery devices, high performance low energy RO membranes, hybrid membrane systems, advanced pretreatment technologies and alternate energy integrated membrane systems. The beneficial aspects of using alternate energy systems such as on-site distributed fuel cell systems integrated with membrane desalination units in remote locations are discussed.