Separators ceramic

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

Huang, K.L., Holsen, T.M., Chou, T.C., and Selman, ).R. (2003) Comparing nafion and ceramic separators used in electrochemical purification of spent... 

Ceramic separator. A rigid ceramic separator has been developed by Corning, Inc. [45], with the purpose of improving the cycle-life of VRLA batteries by... 

Additional cycle tests, with both separator versions under slightly modified conditions, confirmed these results, although with slightly better results for the separator with the lower pore size, see Fig. 7.6. The cycle tests were stopped after 80 cycles with 42% of the initial capacity, and after 110 cycles with 47% of the initial capacity, for the two types of ceramic separators, respectively. Although no shorts were found in these batteries, a considerable amount of softening of the positive active-material was evident. These results underline the need for a separator with small pore size (preferably microporous) in order to prevent shorts and the expansion of the active material into the separator. 

A similar ceramic separator was used in a different ALABC project [27]. This separator had a thickness of 3.39 mm, an average pore size of 100 pm, and a surface area of 0.36 m g Compared with the test described above, a higher stack pressure, between 58 and 130 kPa, was applied. The cell with this separator failed after only 33 cycles, i.e., gave less than 80% of its initial capacity. The failure was due to severe negative-plate compaction and subsequent capacity limitation of this plate. 

Fig. 7.6. Cycle test (discharge rate 1.25 A, 100% DoD) of 8 V batteries with two versions of ceramic separator [50].

Extensive testing of metal sulfide electrode materials was performed in cells of the type shown in Figure 1. The metal sulfide electrode case and current take-off rod were made of dense graphite to avoid any contact and possible contamination of the active material by metallic constituents. The active material (metal sulfide powder in a porous graphite matrix or a mixture of metal sulfide and graphite powder) was contained in the cavity (2.5 cm in diameter X 0.6 cm deep) of the dense graphite electrode case. A porous ceramic separator was placed over the active material and secured with high purity alumina pins. 

It is convenient to consider ceramics that are essentially silicates, called traditional ceramics, separately from all of the others. This latter group comprises engineering ceramics, with important mechanical properties, electroceramics, when... 

BENEFITS AND TEST RESULTS OF A CERAMIC SEPARATOR COMPONENT FOR MICRO FUEL CELLS... 

This research focuses effort on the development of the structure and features of the ceramic separator and the long-term test results. 

Using multilayer ceramic technology, the thickness of the fuel cell is reduced, in part by the use of ceramics fluidic channels and inherent insu-lative characteristics (Figure 6-2). The fuel channels are incorporated inside the ceramic substrates. This allows the fuel to be protected from contaminates as well as allow for sealing due to ceramics ability to be hermetic when designs require complete sealing. This quality provides a mechanical structure which can effectively supply fuel to the MEA as well as seal off the MEA to optimize efficiency and prevent contamination. In addition, the ceramic separator plates are coated with metals which allow for the interconnection between the cathode and anode sides of the MEA for purposes... 

Benefits and Test Results of a Ceramic Separator Component... 

Figure 6-1. Cross-section view of PEMFC with ceramic separators...

Benefits and Test Results of a Ceramic Separator Component Table 6-1. A comparison of material characteristics of separators... 

Figure 6-3 shows the actual resistance measurements under increasing load conditions. The data reveal that contact resistance of the ceramic separator plate is smaller than carbon B and equivalent to gold-plated... 

The results for contact resistance were measured comparing the glass epoxy fabricated separators to the ceramic fabricated separators. These materials were fastened with 6 MIO screws under increasing torque conditions. Figure 6-6 compares the results between each material under the increasing torque and plots the resistances observed. The contact resistances measured are for one cell under torque and include the GDL resistance. This test shows that the ceramic separator resistance is much lower than that of glass epoxy. The measured resistance for the ceramic was 77% to that of the glass epoxy separator. 

Figure 6-6. Comparison results of contact resistance between a ceramic separator and a glass epoxy separator...

Further comparative study reviewed the maximum power density for ceramic separators and glass epoxy materials (Figure 6-7). The results indicated that the planar PEMFC using ceramic separators achieved over 200 mW cm at maximum power density. Compared to glass epoxy, the ceramic separator maximum power density was 138% higher. 

Long-term stability tests were also conducted on the ceramic separators. These tests evaluated the corrosion resistance during continuous power generation of a single cell PEMFC. The separators used in the... 

Figure 6-8. Long-term stability results of ceramic separators...

These test cells were operated and monitored for 1,000 h. The test cells were maintained to have a current density of 200 mW cm" throughout the test period with testing conducted at room temperature. Voltage output was evaluated and observed to within 10% over the duration of 1,000 h. Long term testing results proved that ceramic separators were capable of generating power under acidic conditions. This test will be performed until 3,000 h are reached. 

The results for using ceramic materials as an integral component for micro fuel cells have conclusive evidence that ceramics make an excellent separator plate for micro fuel cells. The thin structure of ceramics allows for rigidity without sacrificing increased resistance. A lower contact resistance when compared to other common separator materials increases power output. Ceramic s inert composition also provides excellent reliability in acidic conditions of the fuel cell. Ceramic separator plates achieved the highest power density of comparable materials and subsequently produced less than 10% voltage variation over 1,000 h of testing. 

The ceramic separator provides groundwork for future power sources that enable small form factors for portable electronics. 

The vapor pressure of potassium is much higher than that of lithium, and potassium can form an undesirable gas phase. Lithium dissolved in the electrolyte migrates toward the positive electrode where it is consumed in an unproductive chemical reaction. For an interelectrode distance of 1 mm, the associated self-discharge is equivalent to a leakage current density of 1-10 mA/cm. Moreover, dissolved lithium causes disintegration of the ceramic separators. The solubility of lithium greatly increases with increasing temperature. 

In these conditions, the possibility for the onset of corrosion processes is high, and these wiU occur preferentially in places in contact with cathodic species, such as stainless steel or iron rich dust particles, and inside the artificial crevices. In this last case, when a region of a sample is undergoing a massive dissolution process, it tends to act as a sacrificial anode because the rest of the surface is needed for the cathodic reaction. As all the central portions of the coupons have undergone this type of attack, owing to the crevices formed with the ceramic separators, etc., it is possible that corrosion of the free portions has to some extent been inhibited by this effect. 

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