Collector/separator plate

Low current densities - Low partial load toleration - System dimensions and complexity (footprint) - Extensive gas purification - Corrosive liquid electrolyte - Corrosive environment ( acidic membrane) - High investment costs due to costly components (catalysts, current collectors, separator plates) - Laboratory and test phase - Long-term stability (mechanical) - Heat management... 

The heart of a fuel cell is a polymer, proton-conductive membrane. On both sides of the membrane there is a porous electrode. The electrodes must be porous because the reactant gases are fed from the back and must reach the interface between the electrodes and membrane, where the electrochemical reactions take place in the so called catalyst layer, or more precisely, on the catalyst surface. Technically, the catalyst layer may be a part of the porous electrode or part of the membrane, depending on the manufacturing process. The multilayer assembly of the membrane sandwiched between the two electrodes is commonly called the membrane electrode assembly or MEA. The MEA is then sandwiched between the collector/separator plates—"collector" because they collect and conduct electrical current and "separator" because in multicell configuration they separate the gases in the adjacent cells. At the same time, in multicell configuration they physically/electrically connect the cathode... 

The bipolar collector/separator plates have several functions in a fuel cell stack. Their required properties follow from their functions, namely ... 

Austenitic stainless steels like 31 OS, 316, or 316L are typically used for the construction of cathode and anode current collectors and bipolar separator plates. Corrosion of these steel components is a major lifetime-limiting factor in MCFC. The corrosion behavior of stainless steel components in molten carbonate conditions has been studied extensively during the past decade. Research is being aimed at increasing the corrosion resistance of these components by altering the alloy composition or by surface modification techniques. ... 

Finally, the stainless-steel bipolar plate consists of a separator and current collectors. The plate is exposed to the anodic environment on one side and the cathodic environment on the other. The low oxygen partial pressure on the anodic side of the bipolar plate prevents the formation of a protective oxide coating and, on the cathode side, the contact electrical resistance increases as an oxide scale builds up. Active research is focused on finding alloys for bipolar current-collector materials that function well in both anodic and cathodic environments, have a low cost and ohmic resistance, and have good corrosion resistance [15]. 

The stacks are assembled by building up layers of cells inserting current collectors and separator plates between one cell and the next. Once the cell or stack is assembled and mechanically clamped together, it is slowly heated up to above the melting temperature of the electrolyte. Once the electrolyte melts, it penetrates into the pores of the matrix. 

Electrolyte plate Cathode Current collector Separator... 

An AutoSpec-TOF mass spectrometer has a magnetic sector and an electron multiplier ion detector for carrying out one type of mass spectrometry plus a TOF analyzer with a microchannel plate multipoint ion collector for another type of mass spectrometry. Either analyzer can be used separately, or the two can be run in tandem (Figure 20.4). 

A fuller description of the microchannel plate is presented in Chapter 30. Briefly, ions traveling down the flight tube of a TOF instrument are separated in time. As each m/z collection of ions arrives at the collector, it may be spread over a small area of space (Figure 27.3). Therefore, so as not to lose ions, rather than have a single-point ion collector, the collector is composed of an array of miniature electron multipliers (microchannels), which are all connected to one electrified plate, so, no matter where an ion of any one m/z value hits the front of the array, its arrival is recorded. The microchannel plate collector could be crudely compared to a satellite TV dish receiver in that radio waves of the same frequency but spread over an area are all collected and recorded at the same time of course, the multichannel plate records the arrival of ions not radio waves. 

Diagram showing a flow of ions of m/z a, b, c, etc. traveling in bunches toward the front face of a microchannel array. After each ion strikes the inside of any one microchannel, a cascade of electrons is produced and moves toward the back end of the microchannel, where they are collected on a metal plate. This flow of electrons from the microchannel plate constitutes the current produced by the incoming ions (often called the ion current but actually a flow of electrons). The ion.s of m/z a, b, c, etc. are separated in time and reach the front of the microchannel collector array one set after another. The time at which the resulting electron current flows is proportional to V m/z). 

Bands of ions of different m/z values and separated in time in a broad ion beam traveling from left to right toward the front face of a microchannel assembly. The ions produce showers of electrons, and these are detected at the collector plate, which joins all the elements as one assemblage. 

Unlike the array collector, with a microchannel plate all ions of only one m/z value are detected simultaneously, and instrument resolution does not depend on the number of elements in the micro-channel array or on the separation of one element from another. For a microchannel plate, resolution of m/z values in an ion beam depends on their being separated in time by the analyzer so that their times of arrival at the plate differ. 

In a beam of ions separated in time according to m/z value, the total time taken for ions of different m/z values to arrive at a microchannel plate is so short (about 30 psec) that the spectrum appears to have been obtained instantaneously. Thus, for practical purposes, the array and microchannel plate collectors produce an instantaneous mass spectrum, even though the first detects a spatially dispersed set of m/z values and the second detects a temporally dispersed set. 

After the analyzer of a mass spectrometer has dispersed a beam of ions in space or in time according to their various m/z values, they can be collected by a planar assembly of small electron multipliers. There are two types of multipoint planar collectors an array is used in the case of spatial separation, and a microchannel plate is used in the case of temporal separation. With both multipoint assemblies, all ions over a specified mass range are detected at the same time, or apparently at the same time, giving these assemblies distinct advantages over the single-point collector in the analysis of very small quantities of a substance or where ions are produced intermittently during short time intervals. 

In 1909 one company began selling solar water heaters with separate collectors and insulated storage tanks. The collectors were made of copper tubing soldered to a copper plate in a glass covered box. The hot water was transferred to the storage tank by thermosyphon action, so the insulated tank had to be installed above the collector, typically in the attic or on the roof It was often connected to an auxiliary heater on the stove or furnace to supplement solar... 

Instead of scraping and manual collection of the adsorbent, the band can be sucked off the plate with a Vacuum-cleaner -type apparatus. Dekker [50] described an apparatus for the isolation of compounds from layers by elution and direct Millipore filtration, and Platt [51] designed a zone collector that used vacuum to transfer separated zones from layers direcdy to vials for hquid scintillation counting of radioactivity. 

Linking TLC with a tandem instrument differs from combining GC or LC with an appropriate spectrometer. Hyphenation of planar chromatographic techniques represents a niche application compared to HPLC-based methods. Due to the nature of the development process in TLC, the combination is often considered as an off-line in situ procedure rather than a truly hyphenated system. True in-line TLC tandem systems are not actually possible, as the TLC separation must be developed before the spots can be monitored. It follows that all TLC tandem instruments operate as either fraction collectors or off-line monitoring devices. Various elaborate plate extraction procedures have been developed. In all cases, TLC serves as a cleanup method. 

He et al. (2002) used an off-line HPLC/CE method to map cancer cell extracts. Frozen ovarian cancer cells (containing 107 cells) were reconstituted in 300 pL of deionized water and placed in an ultrasonic bath to lyse the cells. Then the suspension was centrifuged and the solubilized proteins were collected for HPLC fractionation. The HPLC separation was carried out on an instrument equipped with a RP C-4 column, 250 mm x 4.6 mm, packed with 5-pm spherical silica particles. Extracted proteins were dissolved in 300 pL of DI water, and lOOpL was injected onto the column at a flow rate of 1 mL/min. Buffer A was 0.1% TEA in water and buffer B was 0.1% TFA in acetonitrile. A two-step gradient, 15-30% B in 15 min followed by 30-70% B in 105 min, was used. The column effluent was sampled every minute into a 96-well microtiter plate with the aid of an automatic fraction collector. After collection, the fractions were dried at room temperature under vacuum. The sample in each well was reconstituted before the CE analysis with 10 pL deionized water. The... 

In our tests, we used pasted mixtures of carbon-carbon electrode components with KOH solution having a density of 1,26 g em"3. Positive and negative electrodes were pasted onto the conductive polymer film, separated by ionoconductive separator, made out of special paper, pressed between external collectors of nickel-plated copper with pressure of 8 kgf-ern 2. 

Other important parts of the cell are 1) the structure for distributing the reactant gases across the electrode surface and which serves as mechanical support, shown as ribs in Figure 1-4, 2) electrolyte reservoirs for liquid electrolyte cells to replenish electrolyte lost over life, and 3) current collectors (not shown) that provide a path for the current between the electrodes and the separator of flat plate cells. Other arrangements of gas flow and current flow are used in fuel cell stack designs, and are mentioned in Sections 3 through 8 for the various type cells. 

Both horizontal and vertical cross-flow separators require spreaders and collectors to uniformly distribute water flow through the plates For this reason, it is suggested that Eq. 8 be modified to indude a 75% spreader efficiency term ... 

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