Performance of PAFC

For PAFC, the polarisation contribution of the two half cells is shown in Fig. 2.4. Hydrogen is taken to be the reference point/zero polarisation. 

At higher pressure and current density, the diffusion polarisation at the cathode decreases and reversible cell potential increases. At the cathode, increased oxygen and water pressures decrease activation polarisation. As shown in Fig. 2.5, an increase of 44 mV is observed when pressure and temperature are increased by 2.9 atm and 15°C, respectively. An increase in temperature exhibits a beneficial effect on cell performance because the polarisation losses diminish with increased temperature. The relationship between voltage gain and temperature change is given by  

5 The increase in PAFC cell performance with pressure increase 

The cathode performance is also affected by the utilisation and composition oxidant. The use of 21 % O2 instead of pure O2 decreases the current density. As the oxygen is utilised, the cathode polarisation also increases. The voltage loss due to change in oxidant utilisation at cathode is given by 

Similarly, the voltage loss due to change in hydrogen partial pressure at the anode is given by 

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It is well known that an increase in the cell operating pressure enhances the performance of PAFCs (11, 33, 34). The theoretical change in voltage (AVp) as a function of pressure (P) is expressed as... 

The ideal performance of a fuel cell depends on the electrochemical reactions that occur with different fuels and oxygen as summarized in Table 2-1. Low-temperature fuel cells (PEFC, AFC, and PAFC) require noble metal electrocatalysts to achieve practical reaction rates at the anode and cathode, and H2 is the only acceptable fuel. With high-temperature fuel cells (MCFC, ITSOFC, and SOFC), the requirements for catalysis are relaxed, and the number of potential fuels expands. Carbon monoxide "poisons" a noble metal anode catalyst such as platinum (Pt) in low-temperature... 

As in the case of PAFC discussed in the previous section, an increase in cell operating pressure enhances performance of AFC s. Figure 4-3 presents a plot of the increase in the reversible e.m.f of alkaline cells with pressure over a wide range of temperatures (14). The increase in cell open circuit voltage will be somewhat less because of the greater gas solubility at increasing pressure which produces higher lost currents. 

As in the case with PAFC s, voltage obtained from an AFC is affected by ohmic, activation, and concentration losses. Figure 4-7 presents data obtained in the 1960 s (18) which summarizes these effects, excluding ohmic losses, for a catalyzed reaction (0.5-2.0 mg noble metal/cm ) with carbon-based porous electrodes for H2 oxidation and O2 reduction in 9 N KOH at 55-60 C. The electrode technology was similar to that employed in the fabrication of PAFC electrodes. Performance of AFC s with carbon-based electrodes has not changed dramatically since these early results were obtained. 

Figure 5-1 Improvement in the Performance of Hi-Rich Fuel/Air PAFCs...

Mitsubishi Electric Corporation investigated alloyed catalysts, processes to produce thinner electrolytes, and increases in utilization of the catalyst layer (20). These improvements resulted in an initial atmospheric performance of 0.65 mV at 300 mA/cm or 0.195 W/cm, which is higher than the IFC performance mentioned above (presented in Table 5-2 for comparison). Note that this performance was obtained on small 100 cm cells and may not yet have been demonstrated with full-scale cells in stacks. Approaches to increase life are to use series fuel gas flow in the stack to alleviate corrosion, provide well-balanced micro-pore size reservoirs to avoid electrolyte flooding, and use a high corrosion resistant carbon support for the cathode catalyst. These improvements have resulted in the lowest PAFC degradation rate publicly acknowledged, 2 mV/1000 hours for 10,000 hours at 200 to 250 mA/cm in a short stack with 3600 cm area cells. 

One of the primary areas of research is in extending cell life. The goal is to maintain the performance of the cell stack during a standard utility application (-40,000 hours). Current state-of-the-art PAFCs (46, 47, and 48) show the following degradation over time ... 

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/... 

What are the lessons learned from the failure of PAFC to become a commercial success and how do these lessons apply to other stationary fuel cell systems in development and demonstration Was the cause of failure only the high cost relative to the other DG systems The PAFC systems appeared to perform well. The federal government had spent more than 411 million on PAFC. Should it have continued... 

The operating temperatures and acid concentrations of PAFCs have increased to achieve higher cell performance temperatures of about 200 °C (392 °F) and acid concentrations of 100 percent H3PO4 are commonly used today. Although the present practice is to operate at atmospheric pressure, the operating pressure of PAFCs surpassed 8 atm in the 11 MW electric utility demonstration plant, confirming an increase in power plant efficiency. However, a number of... 

At a temperature of around 200 °C the fuel cell reaction should proceed much more quickly than it does at the lower temperatures typical for PEM fuel cells (PEMFCs). However, since both H3PO4 and H2P04 can adsorb onto the surface of the Pt catalyst, they significantly lower the reaction kinetics, especially for the oxygen reduction reaction (ORR). Consequently, a PAFC operated at about 200 °C proceeds much more slowly than a PEMFC operated at less than 90 °C. As shown in Figure 11.2, the performance of the state-of-flie-art PAFC operated at 160 °C is about 150 mV lower than that of a PEMFC operated at 70 °C. 

Sugano et al. [46] reported the analysis of the dynamic behavior of a PAFC stack cooling systems. Miki and Shimizu [47] reported the results of the dynamic characteristics of a fuzzy control based stack cooling system. An analytical, exergetic, and thermoeconomic analysis of a 200 kWel PAFC power plant was presented by Kwak et al. [48]. In [49], a novel optimization tool was developed that realistically described and optimized the performance of a PAFC system. Zhang et al. [50] presented an analytical model to optimize several parameters using a thermodynamic-electrochemical analysis. In [51], a dynamic model was developed to simulate a PAFC system and associated components. 

Brenscheidt T., Janowitz K., Salge H.J., Wendt H., and Brammer F. (1998) Performance of ONSI PC25 PAFC cogeneration plant , International Journal of Hydrogen Energy, 23(1), 53-56. 

There are now some 65 MW of PAFC systems worldwide. Most of the plants are in the 50-200 kW capacity range, but large plants of 1 and 5 MW have been built, including an 11 MW plant for Tokyo Electric Power. Details on performance, reliability, and cost reduction initiatives are contained in several reviews. 

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