Fuel cells electrolyte requirements

Alkaline fuel cells (AFC) — The first practical -+fuel cell (FC) was introduced by -> Bacon [i]. This was an alkaline fuel cell using a nickel anode, a nickel oxide cathode, and an alkaline aqueous electrolyte solution. The alkaline fuel cell (AFC) is classified among the low-temperature FCs. As such, it is advantageous over the protonic fuel cells, namely the -> polymer-electrolyte-membrane fuel cells (PEM) and the - phosphoric acid fuel cells, which require a large amount of platinum, making them too expensive. The fast oxygen reduction kinetics and the non-platinum cathode catalyst make the alkaline cell attractive. 

In the quest to improve fuel cell performance, the concept of fuel cell reactions requiring a three-phase interface was first proposed by Grove. In his initial experiment, he noticed that the reaction sped up when the three-phase area was large. In 1923, Schmid [7] developed the first gas diffusion electrode, which significantly increased the electrode active surface area and revolutionized fuel cell electrodes. The electrode contained a coarse-pore graphite gas-side layer and a fine porous platinum electrolyte layer. 

Pure water would, of course, be an impractical fuel cell electrolyte, but at the equilibrium point there is zero current, and a shortage of ions in minimally ionised pure water makes no difference to the equilibrium parameters. For a current generating fuel cell, vigorous ion migration is a requirement. Hence, for example, the KOH in Bacon s fuel cell (Bacon, 1969 Adams etal., 1963). 

While an understanding of the molecular processes at the fuel cell electrodes requires a quantum mechanical description, the flows through the inlet channels, the gas diffusion layer and across the electrolyte can be described by classical physical theories such as fluid mechanics and diffusion theory. The equivalent of Newton s equations for continuous media is an Eulerian transport equation of the form... 

Table 1 presents the anode performance characteristics. Efficient fuel cell operation requires that the electrodes must have sufficient electrolytes to maintain... 

Sealing in fuel cell is required to separate gases entering into the cell. The materials for sealing should withstand the acidic environment of electrolyte and be durable for long-term operation. Silicone rubber based materials are widely employed because of their elasticity and excellent heat resistance. Silicone based materials, however, are degraded during the operation due... 

The performance of a fuel ceU is determined by the total surface area of the catalyst particles that participate in the reactions. Ideally, aU the surface of the Pt particles is used and the Pt achieves 100% dispersion (e.g., Pt exists as individual atoms). In reality, the ideal situation does not exist and only a small fraction of the Pt atoms can participate in the fuel cell reaction. The reasons are that the Pt can not achieve 100% dispersion and the fuel cell reactions require the so-called three-phase boundaries. It can be seen from Eqs. 1 and 2 that both the anode and the cathode reactions involve protons, electrons, and reactants. So, only the Pt surface that is accessible to protons, electrons, and the reactant is active, and such regions are often called catalyst-electrolyte-reactant three phase boundaries as illustrated in Fig. 2. All the other Pt surface area is basically wasted. For an electrode composed of Pt (or... 

In fuel cells, however, more expensive materials are employed even though their amount can be tremendously reduced. The total geometric surface area of the cells shows a 30-fold decrease due to higher current densities obtained with more conductive fuel cell electrolytes. Fuel cells can be built in a bipolar construction with cells stacked in series with the negative current collector of one cell serving as the positive current collector of the adjacent cell. Fuel cells require a hydrogen tank and an air compressor which makes their balance of plant more complex. 

The ohmic loss is relatively easy to understand because the electrical resistance of the cell components behaves as a cause of voltage loss. However, determination of overpotential from the electrochemical reaction resistance at the electrodes has been an interesting research topic. The fuel cell electrodes require a large surface area to increase the reaction rate, and thus porous materials are employed. In addition, the electrode surface is covered by thin electrolyte film to provide the three-phase boundary of gas-liquid-solid where the electrochemical reaction occurs. Thus, the electrochemical resistance in MCFC is comprised of charge-transfer resistance on the electrode surface and mass transfer through the liquid film and gas channel as shown in Fig. 8.2. 

PEMFC requires an ion exchange polymer in the form of a continuous pore free sheet. The properties which characterize the ideal ion exchange membrane fuel cell electrolyte will include the following high... 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

As with batteries, differences in electrolytes create several types of fuel cells. The automobile s demanding requirements for compactness and fast start-up have led to the Proton Exchange Membrane (PEM) fuel cell being the preferred type. This fuel cell has an electrolyte made of a solid polymer. 

A membrane ionomer, in particular a polyelectrolyte with an inert backbone such as Nation . They require a plasticizer (typically water) to achieve good conductivity levels and are associated primarily, in their protonconducting form, with solid polymer-electrolyte fuel cells. 

In solid electrolyte fuel cells, the challenge is to engineer a large number of catalyst sites into the interface that are electrically and ionically connected to the electrode and the electrolyte, respectively, and that is efficiently exposed to the reactant gases. In most successful solid electrolyte fuel cells, a high-performance interface requires the use of an electrode which, in the zone near the catalyst, has mixed conductivity (i.e. it conducts both electrons and ions). Otherwise, some part of the electrolyte has to be contained in the pores of electrode [1]. 

Electrolytes for Electrochromic Devices Liquids are generally used as electrolytes in electrochemical research, but they are not well suited for practical devices (such as electrochromic displays, fuel cells, etc.) because of problems with evaporation and leakage. For this reason, solid electrolytes with single-ion conductivity are commonly used (e.g., Nafion membranes with proton conductivity. In contrast to fuel cells in electrochromic devices, current densities are much lower, so for the latter application, a high conductivity value is not a necessary requirement for the electrolyte. 

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