In this paper, we derive a mathematical model for the cathode catalyst layer of a polymer electrolyte fuel cell. The model explicitly incorporates the restriction placed on oxygen in reaching the reaction sites, capturing the experimentally observed fall in the current density to a limiting value at low cell voltages. Temperature variations and interfacial transfer of O 2 between the dissolved and gas phases are also in- cluded. Bounds on the solutions are derived from which we provide a rigourous proof that the model admits a solution. Of particular interest are the maximum and minimum attainable values. We perform an asymptotic analysis in several limits inherent in the problem by identifying important groupings of pa- rameters. This analysis reveals a number of key relationships between the solutions, including the current density, and the composition of the layer. A comparison of numerically computed solutions and asymp- totic solutions shows very good agreement. Implications of the results are discussed and future work is outlined.
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The PEMFC catalyst layer structure and electrochemical parameters were analyzed using a numerical model with a random catalyst particles (particle agglomerates) distribution in a polymer (ionomer) porous matrix. An electrochemical activity of the internal surface of agglomerates was taken into account in the calculations. Application of such Monte Carlo simulation method to the catalyst layer with particles of two different sizes predicted a possibility of the ionomer concentration increase and a decrease of the ohmic losses associated with ion transport in the catalyst layer together with an improvement of water balance and mass transport. The main attention was paid to the Pt-based catalyst supported on the Vulcan XC-72R and carbon nanofibres agglomerates. It was shown that the formation of the catalyst particle clusters (particle agglomerates) for small particles (Vulcan XC-72R) starts at the catalyst concentration in the layer of about 30 vol.%. With the further catalyst concentration increase the amount of active particles achieves maximum at about 60 vol.%, when practically all the catalyst particles are becoming a part of a quasi-infinite cluster with electronic conductivity. For large particles (nanofibres) the generation of active clusters starts at lower catalyst concentration (about 10 vol.%) and it permits to involve a larger amount of catalyst in the electrochemical process at a lower catalyst concentration. Mixtures of such particles also have a lower threshold for beginning of percolation which becomes significant already at 20 vol.% of large particles in the catalyst mixture. This effect is caused by the beginning of the formation of clusters containing 3-5 large particles which size is comparable to the catalyst layer thickness. In all cases the dependence of the catalyst active surface area and the current density upon the catalyst concentration have a maximum which appears at a lower catalyst volume concentration in the layer for larger particles and their mixture with smaller particles. The numerical estimations demonstrated a possibility of precious metal loading reduction up to 30% and/or fuel cell performance increasing (current density) up to 20% just due to the addition of nanofibers (large particles) to the catalyst composition. The fuel cell tests demonstrated that the dependence of the current density upon the catalyst concentration has a distinct maximum which appears at a lower catalyst concentration for nanofibers (large particles) and their mixtures. The combined application of the Vulcan XC-72R supported catalyst together with the nanofibers supported catalyst (about 20-30 vol.% of the catalyst composition) permitted to increase PEMFC current density for about 10%.
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Fuel cells are one of the most promising sources of renewable energy and can be considered as clean energy sources because of their low sulphur and nitrogen oxide emissions and low noise level operation. In addition, they are more efficient as compared to the conventional power sources as their efficiencies are not limited by the Carnot cycle. A fuel cell operates like a battery, but does not need to be recharged and gives continuous power when supplied with fuel and oxidant . Attempts to develop fuel cells as power sources have been made over many years. Initially they were developed mainly for space and defence applications. However, the recent drive for more efficient and less polluting electricity generation technologies has resulted in substantial resources being directed into fuel cell development. Fuel cells, because of their high efficiencies, low noise and pollutant output, promise to revolutionize the power generation industry with a shift from centrally located generating stations and long-distance transmission lines to dispersed power generation at load sites . Irrespective of the type of fuel cells, they all consist of an anode (negative side), a cathode (positive side) and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit or load, producing direct current. As the main difference among fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use along with the difference in the time required for them to start ranging from 1 sec for Proton Exchange Membrane Fuel Cells (PEMFC) to 10 min for Solid Oxide Fuel Cells (SOFC). Fuel cells come in a variety of sizes. Individual fuel cells produce relatively small electrical potentials, about 0.7 volts, so cells are "stacked", or placed in series, to increase the voltage and meet an application's requirements . Several fuel cell types are under various stages of development: Low temperature fuel cells such as the Solid Polymer Electrolyte Fuel Cell (PEMFC) and the Alkaline Fuel Cell (AFC) are mainly considered for transport applications.
In this paper, we present a comprehensive non-isothermal, one-dimensional model of the cathode side of a Polymer Electrolyte Fuel Cell. We explicitly include the catalyst layer, gas diffusion layer and the membrane. The catalyst layer and gas diffusion layer are characterized by several measurable microstructural parameters. We model all three phases of water, with a view to capturing the effect that each has on the performance of the cell. A comparison with experiment is presented, demonstrating excellent agreement, particularly with regard to the effects of water activity in the channels and how it impacts flooding and membrane hydration. We present several results pertaining to the effects of water on the current density (or cell voltage), demonstrating the role of micro-structure, liquid water removal from the channel, water activity, membrane and gas diffusion layer thickness and channel temperature. These results provide an indication of the changes that are required to achieve optimal performance through improved water management and MEA-component design. Moreover, with its level of detail, the model we develop forms an excellent basis for a multi-dimensional model of the entire membrane electrode assembly.
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Note that the ion specie and its transport direction can differ, influencing the site of water production and removal, a system impact. The ion can be either a positive or a negative ion, meaning that the ion carries either a positive or negative charge (surplus or deficit of electrons). The fuel or oxidant gases flow past the surface of the anode or cathode opposite the electrolyte and generate electrical energy by the electrochemical oxidation of fuel, usually hydrogen, and the electrochemical reduction of the oxidant, usually oxygen. Appleby and Foulkes (1) have noted that in theory, any substance capable of chemical oxidation that can be supplied continuously (as a fluid) can be burned galvanically as the fuel at the anode of a fuel cell. Similarly, the oxidant can be any fluid that can be reduced at a sufficient rate. Gaseous hydrogen has become the fuel of choice for most applications, because of its high reactivity when suitable catalysts are used, its ability to be produced from hydrocarbons for terrestrial applications, and its high energy density when stored cryogenically for closed environment applications, such as in space. Similarly, the most common oxidant is gaseous oxygen, which is readily and economically available from air for terrestrial applications, and again easily stored in a closed environment. A three phase interface is established among the reactants, electrolyte, and catalyst in the region of the porous electrode. The nature of this interface plays a critical role in the electrochemical performance of a fuel cell, particularly in those fuel cells with liquid electrolytes. In such fuel cells, the reactant gases diffuse through a thin electrolyte film that wets portions of the porous electrode and react electrochemically on their respective electrode surface. If the porous electrode contains an excessive amount of electrolyte, the electrode may "flood" and restrict the transport of gaseous species in the electrolyte phase to the reaction sites. The consequence is a reduction in the electrochemical performance of the porous electrode. Thus, a delicate balance must be maintained among the electrode, electrolyte, and gaseous phases in the porous electrode structure. Much of the recent effort in the development of fuel cell technology has been devoted to reducing the thickness of cell components while refining and improving the electrode structure and the electrolyte phase, with the aim of obtaining a higher and more stable electrochemical performance while lowering cost.
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Using a laboratory doctor blade casting machine set at appropriate thickness with the aid of feeler gauges, casting of sulphonated polyisoprene (SPI) and sulphonated polyisoprene impregnated with carbon nanotubes (SPICNTs) into a thin membrane was carried out according to the method described by Idibie et al . Here, both SPI and SPICNTs were dissolved in 150 ml of toluene at elevated temperature to form a casting solution, and thereafter cast onto a clean polymer paper support. The casting process involves drawing the casting head of the blade along the length of the substrate, and allowed to cure for 5 days by exposing it to air, and later ovum dried below 80°C for about 3 hours. To remove any trace of residual solvent the cast membrane was vacuum dried for 2 hours.
PVA is a polyhydroxy polymer, which is very common in practical applications because of its easy preparation and biodegradability . It has been selected as polymer matrix in view of its film- forming capacities, hydrophilic properties and high density of reactive chemical functions favorable for cross-linking by irradiation, chemical or thermal treatments [11 - 13]. It is reported that PVA gels prepared by freeze-thaw technique resulted in higher mechanical strength in comparison with cross- linking of PVA by chemical or irradiative methods . One of the major challenges with the development of SAAEMs is the availability of suitable ionic conductivity with high chemical stability under fuel cell operating conditions.
In this area there is a need for solid polymer electrolyte membrane materials that are cheaper and more conductive than the current perfluoro- sulfonic acid (PFSA) materials, and which are also durable for up to 10,000 hours. In particular, mem- branes are needed for use at temperatures above 100ºC where liquid water is no longer present for proton transport. For DMFCs, new membranes with reduced methanol permeability are required.
of PGM, and the energy consumption to manufacture bi- polar plates . Analysing the contribution of the stack production further, two components turn out to be of spe- cial relevance. The GDE is responsible for a large share of the total acidification and, to a lesser degree, the GHG emissions. The crucial materials causing the high acidi- fication are PGM used as catalysts, due to the emissions during the pyrometallurgical treatment of the material. The flow field plate (FFP) is the second important com- ponent, particularly because of the electricity input for resin impregnation of the plate. Fundamentally, the en- ergy required for fuel stack production is driven up by the use of graphite .
The proton conductivity of polymer membranes were investigated and of all the prepared composite membranes PAC3 shows maximum proton conductivity of 1.08×10 -1 S cm -1 at a temperature of 70 °C and 100% RH. By looking at the water uptake and the proton conductivity results, it can be seen that water uptake results correlate with the conductivity of the composite membranes. The membrane having higher conductivity has also higher mechanical stability; from these results suggest that the developed composite membranes offer great promise for DMFC applications and are well suited for polymer electrolyte membranes.
More studies only use I-V curves and impedance spectroscopy to study their fuel cell rather than more specific methods such as pressure drop analysis , neutron radiography  and limiting current method [3, 14]. Caulk and Baker studied the water transport in hydrophobic GDL by limiting current method [35-37] and they reported that there were three regions in the GDLs dry region, transition region and wet region respectively. The saturated situation in GDLs was mainly regarded for evaluating the water management ability in PEMFC . However, only Toray series carbon papers were released in their work.
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increase in its macropore volumes. From the view of effective porosity, which is seen as a more practical structure parameter than the total porosity as it only reflects the pore spaces interconnected with each other and open to the surface , the MPL-free GDE should also has higher effective porosity owing to its 3D-interlaced micropore/macropore CL structure that makes more micropores between the catalyst particles open to or interconnected to the surfaces of the macropores within the CL. Theoretically, the operating current of the fuel cell relies on the gas diffusional flux . It is anticipated that the MPL-free GDE possesses larger gas diffusional flux due
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In subsequent experim ents this direct reproduction of the known lithium content by the AA spectrometer did not occur, requiring the production of a calibration graph to relate the experim entally determined lithium concentra tions to known lithium concentrations. As well as using the data from the reference compartments, electrolyte was taken from the remains of the disc from which the electrolyte slab was taken; this allowed samples of a wide weight range to be taken, producing a wider range of lithium con tent data and consequently a more reliable fit between the indica ted and known lith ium contents. A fresh calibration graph was needed for each experiment, and on occasion the calibration chart was curved rather than linear. The reasons why this was so and why only one exact set of data were obtained are not known, and have not been investigated further, because the calibration graphs proved to be a satisfactory solution to the'discrepancy.
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promising approaches in the process of tuning the microphase separation nature of ion exchange membranes [77-84]. The filler nanoparticles that consist of high-density tends to alter the hydrophilic conductive groups, thus producing strong attraction to the hydrophilic pendant chains and strong repulsion to the hydrophobic backbones of polymer, which is shown to behave as ’seeds’ that guide the microphase aggregation process at the filler/polymer interface and sequentially generate well-defined channels to facilitate ion mobility. Li et al.  introduced a novel filler for imPEEK based on octaphenyl polyhedral oligomeric silsesquioxane (O-POSS). The O- POSS is a type of hybrid material that is composed of inorganic cage-like Si- O cores which is surrounded by 8 phenyl groups. On a more important note, the unique features provide beneficial influence on membrane characteristics as follows: (1) O-POSS particles exhibit precise tuning of the nanoscale (small) ion channels that results in the reduction of gas-crossover, (2) O- POSS increases the amount of ion channels formation at the polymer/filler interface, and (3) O-POSS existence minimizes the aggregation or sedimentation during casting process . The presence of pristine O-POSS and functionalized O-POSS by imidazole group in imPEEK is investigated in order to determine their compatibility state. The incorporation of pristine O- POSS in ImPEEK membrane reduces the homogeneity of membranes, thus leading to the formation of defects. The presence of imidazolium groups in O- POSS (Im-O-POSS) also increases its compatibility with ImPEEK and appears to be very homogenous without any defects. The ImPEEK composited with Im-O-POSS consists of enhanced microphase separation structures which is responsible in forming the hydrophilic clusters in order to facilitate the mobility of hydroxide ion inside the membrane. The hydroxide conductivity shows that the ImPEEK composited with Im-O-POSS (44.11 mS cm -1 ) produces high hydroxide conductivity compared to the control ImPEEK
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 Caisheng Wang, M. Hashem Nehrir, and Steven R. Shaw, Dynamic Models and Model Validation for PEM Fuel Cells Using Electrical Circuits, IEEE Transactions On Energy Conversion, Vol. 20, NO. 2, June2005  J.C.Amphlett, R.M. Baumert, R.F. Mann, B.A. Peppley, and P.R.Roberge, Performance mo- deling of the Ballard Mark IV solid polymer electrolyte fuel celt, Journal of Electro- chemical Society, vol. 142, 1995.
PI (Otto Chemie), α-cyclodextrin (α-CD) (Alfa Aesar) were the chemicals (AR grade) used in this study. The PI/ α-CD polymer electrolyte membranes were prepared by solution casting technique. The solvent, m-Cresol (>99%), was supplied by Merck. An appropriate amount of PI was dissolved in m-Cresol solvent at a constant rate of stirring (500 rpm) at 100 °C for 6 hours and α-CD were dissolved separately in m-Cresol and the solutions were mixed together and casted in Petri dish then dried to form a blend membrane.
In 1947, Glenn Howatt was the first to describe the process known as tape casting (121). In the 1950s, the use of a porous casting surface was replaced by a moving polymer carrier (122). This was the turning point in tape casting because the process was continuous. Presently, tape casting is a well-established forming technique for fabricating large, thin, and flat ceramic or metallic sheets (123). It is also known as “doctor blading” and “knife coating” (124; 125; 126; 127). The doctor blade is the scraping blade that removes the excess slurry on the moving polymer carrier to produce a thin layer of tape. Tape casting is an easy and low cost process that is widely used in the plastic, paper, paint and electronic industries. The flexibility of the unfired tape allows it to be cut into different geometry to design a circular or rectangular cell. It can also be rolled to form a tubular or rolled fuel cell. In additional, a multi-layer ceramic composed of layers with different porosities can be prepared by laminating
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Poly(2,5-benzimidazole) (AB-PBI) is an aromatic heterocyclic polymer and is the simplest known polybenzimidazole as shown in Figure 3.1. The synthesis of AB-PBI was first published by Vogel and Marvel in 1961. 1-3 Shortly after, a Japanese group of scientists also reported the synthesis of AB-PBI. 4-8 In the 1980's and 1990's, significant work went into the investigation of the physical properties of AB-PBI blended with other performance polymers such as, polybenzoxazole (PBO), and polybenzothiazole (PBT). 9,10 These polymer blends were studied as material candidates for fiber applications. 11,12 In addition, AB-PBI has been extensively studied by several research groups. 13-17 AB-PBI was not the first polybenzimidazole to be developed, but was very attractive as it offered the desirable properties of polybenzimidazole and was synthetically produced from a commercially available inexpensive monomer. 18,19 AB- PBI can be polymerized by the self condensation of 3,4-diaminobenzoic acid, and like most polybenzimidazoles, has excellent thermal and mechanical properties. AB-PBI synthesis has been reported in various solvent media including, DMAc, Eaton's reagent, and polyphosphoric acid. 20-22
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reduction of Ohmic and activation polarization . The experiments best data show a performance of 0.648V at a current density of 52 mA/cm 2 , which corresponds to a power density of about 34 mW/cm 2 for 2-propanol as fuel and 0.41V at a current density of 68 mA/cm 2 , which corresponds to a power density of about 28 mW/cm 2 for methanol as fuel. The cathode appears less susceptible to poisoning by CO from 2-propanol at higher oxygen pressures .
Chapter 2 and Chapter 3 each focus on the synthesis and processing of three new series of polybenzimidazole copolymer membranes (3,5-pyridine-r-2OH-PBI, 3,5- pyridine-r-para-PBI, and 3,5-pyridine-r-meta-PBI; 2,5-pyridine-r-meta-PBI, 2,5-pyridine- r-para-PBI, and 2,5-pyridine-r-2OH-PBI, respectively) using the PolyPhosphoric Acid (PPA) Process. Monomer pairs with high and low solubility characteristics were used to define phase stability-processing windows for preparing membranes with high temperature membrane gel stability. Creep compliance of these membranes (measured in compression at 180°C) generally decreased with increasing polymer content. Membrane proton conductivities decreased in a relatively constant manner with increasing membrane polymer content. Fuel cell performances of some high-solids copolymer membranes (up to 0.66 V at 0.2 A cm -2 following break-in) were comparable to para-PBI (0.68 V at 0.2 A cm -2 ) despite lower phosphoric acid (PA) loadings in the high solids membranes. Long-term steady-state fuel cell studies showed these copolymer MEAs maintained a consistent fuel cell voltage of >0.6 V at 0.2 A cm -2 for over 9000 h. Phosphoric acid that was continuously collected from the long-term study demonstrated that acid loss is not a significant mode of degradation for these membranes. The PBI copolymer membranes’ reduced high-temperature creep and long-term operational stability suggests that they are excellent candidates for use in extended lifetime electrochemical applications.
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