Research in membranes is oriented to thermostable polymers polyetheretherketone, polysulphone, etc. Regarding electrodes, most efforts are centred on reducing the amount of platinum required in catalysts and improving gas diffusion layers. Advances in the production, transport and storage technologies are also necessary, to make hydrogen an economically viable alternative .
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Cell Dynamics Modelling: better understanding of the processes involved in fuel cell operation, both at the membrane level and the auxiliary subsystems compressors, line heaters, humidiers, etc. This is also widening the spectrum of applications of PEM fuel cells, making them attractive alternatives in elds traditionally reserved to other types of cells, for instance, in stationary, high-power installations. On the other hand, accurate models of gas distribution and uid dynamics inside cells are improving the predictability of electrochemical processes, with direct impact on design strategies.
From the automatic control perspective, efforts are conducted to the development of robust, nonlinear strategies capable to improve efciency and reliability of PEM cells, avoiding permanent damage to membranes. Robustness is required. Additionally, nonlinear control strategies usually allow wider operation ranges than local approaches that linearise the system around equilibrium points.
For commercial success, it is also important to develop models and strategies to diagnose and predict failure situations. Additionally, they must be chemically stable for the entire operation range. Standard membranes are made of peruorosulfonic acid PFSA.
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This material is, essentially, a copolymer of tetrauoroethylene TFE and several sulfonated peruor monomers. Dow Chemical has developed a composite membrane called GoreSelect , using a Teon-like material. Important features of membranes suited for PEM fuel cells are protonic conductivity, water transport properties, gas permeability, mechanical resistance and dimensional stability.
These parameters strongly depend on the membrane water content. The water content is usually expressed as the weight rate of water and dry polymer or, alternatively, the rate between the number of water molecules per sulfonic groups present in the polymer. The maximum amount of water in a given membrane depends heavily on its previous preparation . Regarding its critical role on protonic conduction, it is important to keep a proper membrane water content at all possible operating conditions.
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Several mechanisms affecting water transport are present in a PEM fuel cell, and their combined effects determine the amount of water present. Among them, the most relevant are the following: Water generation on the cathode side, at a rate proportional to the electric current produced. Electro-osmotic drag, produced by water molecules dragged by the proton ow from anode to cathode. Diffusion, due to the water concentration gradient across the membrane.
Water permeability, due to the pressure difference between anode and cathode channels. In thin membranes, diffusion can be compensated by the drying effect produced by electro-osmotic drag at the anode. In thick membranes this drying effect can be. Effective area is 50 cm2. An ideal electrolyte should be impermeable to reactant gases, to avoid gas mixing inside the cell.
However, due to its porous structure, water content, and the solubility of hydrogen and oxygen in water, small amounts of reactant gases can pass through the membrane. It is worth noting that hydrogen has a permeability one order of magnitude higher than oxygen . This is where the electrochemical reactions take place. The later can be accomplished by painting the electrode with a PFSA solution in a mix of alcohol and water, or simply mixing the ionomer and catalyst in the preparation of the catalytic layer.
A 30 percent ionomer content in the catalytic layer is typical. The catalyst most usually employed in both electrodes of PEM fuel cells is platinum. By the end of the s this amount was reduced to 0. Considering that the effective area of catalyst is of paramount importance, it is crucial to achieve a ne dispersion of catalyst particles in the support material, usually carbon powder .
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The combination of electrodes and polymeric membrane is known as Membraneelectrode assembly MEA. There are basically two different approaches for its con-. In the rst one, the catalyst is deposited on a porous substrate called gas diffusion layer, which is typically a carbon bre paper.
These are then placed at both sides of the membrane using heat and pressure to ensure proper contact. In the second approach, the catalyst is deposited directly on the membrane, an arrangement known as catalysed membrane. A porous substrate is then added, resulting in a ve-layer MEA Fig. Several techniques can be used for deposition of catalyst on diffusion layers and membranes spreading, spraying, sputtering, painting, screen printing, decaling, electro-deposition, evaporative deposition, impregnation reduction, etc.
They are typically made of porous materials. Regarding desirable properties of diffusion layers, the following aspects must be considered: Porosity must be such that the ux of reactants and water is efcient. Note that both ows are in opposite directions. Electrical and thermal conductivity must be high. The contact resistance or interface is typically dominant versus the volume conductivity.
Given that the catalyst is a discrete material small particles , diffusion layer pore size cannot be excessively big.
They must provide proper mechanical support to the membrane. However, some degree of exibility is required to provide good electrical contact. Although conicting somewhat, these requirements are typically fullled with carbon bre-based papers and cloths Fig. These materials are usually made hydrophobic to avoid ooding in the structure. To achieve this, diffusion layers are usually treated with polytetrauorethylene PTFE , a polymer similar to polyethylene.
To improve electrical properties, a microporous layer made of carbon or. Their purpose is twofold: on one hand, minimising leaking of gases to the exterior of the cells and, secondly, avoiding the mixing of reactant gases near the catalyst areas Fig. However, it is worth noting that a certain amount of hydrogen is expected to pass through the membrane by diffusion.
This can be computed from the number of cells, their width and type, effective area, partial pressures and working temperature. Sealing gaskets avoid direct combination of reactants, which in turn can produce irreversible damages to cell components . Most sealing gaskets are made of silicone, neoprene or plastic polymers, with additional bre materials to improve their mechanical properties. Among others, the most relevant are the following: Electrical connection between individual cells of the stack.
Separation of gases among adjacent cells.
Structural support to the cell. Robustness and light weight are mandatory. Efcient heat conduction. In addition, bipolar plates must be corrosion resistant, which make them expensive. These environmental conditions discourage the use of traditional choices like aluminum, steel, titanium or nickel. Corrosion produces ions of metal which can diffuse through the membrane, affecting its ionic conductivity, reducing its lifetime and increasing its electrical resistance.
For this reason, metallic plates are usually coated with non-metallic conductive materials, such as graphite, diamond carbon, conductive polymers, organic polymers, noble metals, metallic nitrides, tindoped indium, etc. Bipolar plates can also be made of thermoplastic materials, such as polypropylene, polyethylene and polyvinyldene uoride. Thermosetting resins phenolic, epoxy, etc.
An important property of bipolar plates is their electrical conductivity. Note that the overall conductivity of the cell is always lower than the material conductivity, due to the contact resistance among components. In Fig. Fans are customarily used in open-cathode designs working at ambient pressure.
Air compressors give autonomy and exibility to the system, allowing precise control of working pressures. Usually, both compressors and fans are electrically connected to the stack, becoming part of its losses or parasitic loads and thus reducing the amount of energy available to external loads. This results in a signicant reduction of the overall performance of the system as an energy conversion device.
Hydrogen storage for PEM cells is a matter of active research, the main options being compressed gas, cryogenic liquids and solid metallic hydrides. For proper operation, membranes must be totally saturated of vapour but, in order to ensure optimum performance, excess water must be efciently removed especially from the cathode line. Ionic conductivity in the polymeric membrane is directly related to its water content, which in turn affects the conversion efciency.
The amount of liquid water and membrane humidication levels can be modied by controlling the relative humidity of the reacting gases, as well as their individual pressures and temperatures. Temperature is as important as water content, and both magnitudes are closely related. This small gap between operational and ambient temperatures makes it difcult to remove the 1. The small voltage of each individual cell is heavily inuenced by changes in electric current, partial gas pressures, reactants humidity level, gas speed and stoichiometry, temperature and membrane water content.
According to Eqs. Then, an electronic conditioning system is necessary to meet load requirements. Usually an auxiliary power source is necessary to start the stack operation. At moisture levels below recommended values, ionomer water content falls, reducing conductivity and adversely affecting the kinetics of reduction and oxidation reactions. Water content of an MEA depends on many factors and is closely related with operating conditions.
For instance, in open circuit and also at low current densities, the small water production by oxygen reduction causes the MEA humidication levels to decrease, although the reacting gases are saturated with vapour. This is because water absorption in peruoro-sulfonated membranes is lower when the membrane has been equilibrated with vapour instead of liquid water. Besides, at higher current densities, the electro-osmotic dragging of water through the membrane tends to dry the anode.
An additional problem in this case is the eventual cathode ooding due to the excess of water produced by the electrochemical reactions.
Related Sliding-Mode Control of PEM Fuel Cells (Advances in Industrial Control)
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