Phosphoric acid fuel cell applications

Because of this extreme sensitivity, attention shifted to an acidic system, the phosphoric acid fuel cell (PAFC), for other applications. Although it is tolerant to CO, the need for liquid water to be present to facilitate proton migration adds complexity to the system. It is now a relatively mature technology, having been developed extensively for stationary power usage, and 200 kW units (designed for co-generation) are currently for sale and have demonstrated 40,000 hours of operation. An 11 MW model has also been tested. 

In applications where Nafion is not suitable, at temperatures above 200 °C with feed gas heavily contaminated with CO and sulfur species, a phosphoric acid fuel cell (PAFC)-based concentrator has been effective [15]. Treating the gas shown in Table 1, a H2 product containing 0.2% CO, 0.5%CO2 and only 6 ppm H2S was produced. The anode electrode was formed from a catalyst consisting basically of Pt-alloy mixed with 50% PTFE on a support of Vulcan XC-72 carbon. The cathode was... 

There are six different types of fuel cells (Table 1.6) (1) alkaline fuel cell (AFC), (2) direct methanol fuel cell (DMFC), (3) molten carbonate fuel cell (MCFC), (4) phosphoric acid fuel cell (PAFC), (5) proton exchange membrane fuel cell (PEMFC), and (6) the solid oxide fuel cell (SOFC). They all differ in applications, operating temperatures, cost, and efficiency. 

Close to a thousand systems that produce over 10 kilowatts each have been installed worldwide. Most of these are fueled by natural gas. Phosphoric acid fuel cells (PAFCs) have typically been used for large-scale applications, but molten carbonate and solid oxide units also compete with PAFCs. 

Phosphoric acid fuel cell (PAFC) working at 180-200 °C vfith a porous matrix of PTFE-bonded silicon carbide impregnated with phosphoric acid as electrolyte, conducting by the H cation. This medium-temperature fuel cell is now commercialized by ONSI (USA), mainly for stationary applications. 

Phosphoric acid fuel cells (PAFC) use liquid phosphoric acid as an electrolyte - the acid is contained in a Teflon-bonded silicon carbide matrix - and porous carbon electrodes containing a platinum catalyst. The PAFC is considered the "first generation" of modern fuel cells. It is one of the most mature cell types, the first to be used commercially, and features the most proven track record in terms of commercial applications with over 200 units currently in use. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses. 

Fuel cells can be broadly classified into two types high temperature fuel cells such as molten carbonate fuel cells (MCFCs) and solid oxide polymer fuel cells (SOFCs), which operate at temperatures above 923 K and low temperature fuel cells such as proton exchange membrane fuel cells (PEMs), alkaline fuel cells (AFCs) and phosphoric acid fuel cells (PAFCs), which operate at temperatures lower than 523 K. Because of their higher operating temperatures, MCFCs and SOFCs have a high tolerance for commonly encountered impurities such as CO and CO2 (CO c)- However, the high temperatures also impose problems in their maintenance and operation and thus, increase the difficulty in their effective utilization in vehicular and small-scale applications. Hence, a major part of the research has been directed towards low temperature fuel cells. The low temperature fuel cells unfortunately, have a very low tolerance for impurities such as CO , PAFCs can tolerate up to 2% CO, PEMs only a few ppm, whereas the AFCs have a stringent (ppm level) CO2 tolerance. 

Phosphoric acid fuel cell (PAFC)—Phosphoric acid electrolyte with platinum catalyst. It can use hydrocarbon fuel and is suited for stationary applications. It can generate both electricity and steam. As many as 200 units in sizes ranging from 200 kW to 1 mW are in operation. 

The 200 kW phosphoric acid fuel cell (PAFC) was introduced into the market in 1991 by International Fuel Cells/ ONSI, now called UTC Fuel Cells. It is the only commercialized fuel cell technology. PAFC units have been installed in various applications—commercial, small industrial, landfill, and military—and some are used for cooling, heating, and power. To date there have been 250 units sold, at roughly 4500/kW. The U.S. Department of Defense (DOD) has cost-shared the purchase of three-quarters of the units sold to date. The units have performed well they have operated at 95 to 98 percent availability and 99.99 to 99.9999 percent reliability and have served 4 million customers and accumulated 4 million hours of operation. The cost of PAFC units has not decreased and in fact has increased from 3500/... 

In Section 3, the slow rate of the ORR at the Pt/ionomer interface was described as a central performance limitation in PEFCs. The most effective solution to this limitation is to employ dispersed platinum catalysts and to maximize catalyst utilization by an effective design of the cathode catalyst layer and by the effective mode of incorporation of the catalyst layer between the polymeric membrane electrolyte and the gas distributor/current collector. The combination of catalyst layer and polymeric membrane has been referred to as the membrane/electrode (M E) assembly. However, in several recent modes of preparation of the catalyst layer in PEFCs, the catalyst layer is deposited onto the carbon cloth, or paper, in much the same way as in phosphoric acid fuel cell electrodes, and this catalyzed carbon paper is hot-pressed, in turn, to the polymeric membrane. Thus, two modes of application of the catalyst layer - to the polymeric membrane or to a carbon support - can be distinguished and the specific mode of preparation of the catalyst layer could further vary within these two general application approaches, as summarized in Table 4. 

UTC Fuel Cells has partnered with Shell Hydrogen to develop a variety of fuel processors for natural gas, gasoline, and diesel feed for PEMFC, phosphoric acid fuel cell (PAFC), and distributed H2 production applications.8... 

Based on the above properties, glass-like carbon has found applications as heating elements, as containers for chemical reactions and as containers for molten metals. They are also used as separators and electrodes in phosphoric acid fuel cells (Ovshinsky, 2000). 

The phosphoric acid fuel cell (PAFC) has a quite similar construction and components as the PEMFC the electrolyte is liquid phosphoric acid in an inert matrix. The operation temperature of 200°C avoids formation of liquid water and improves CO tolerance of the electrocatalyst. For the catalyst properties, the same requirements are valid as for the PEMFC - nanoparticles with a high surface area and a good dispersion on the carbon carrier material are required. The application of PAFC typically is the combined heat and power supply in the 200-kW power range. 

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