Phosphoric acid fuel cells (see Fig. 9.8) operate at intermediate tem­peratures (~200°C) and are very well developed and commercially avail­able today. Hundreds of PAFC systems are working around the world in hospitals, hotels, offices, schools, utility power plants, landfills and wastewater treatment plants, and so forth. Most of the PAFC plants are in the 50- to 200-kW capacity ranges, but large plants of 1- and 5-MW capacity have also been built; a demonstration unit has achieved 11 MW of grid-quality ac power [3]. PAFCs generate electricity at more than 40% efficiency and if the steam produced is used for cogeneration, efficien­cies of nearly 85% can be achieved. PAFCs use liquid phosphoric acid as the electrolyte. One of the main advantages to this type of fuel cell, besides high efficiency, is that it does not require pure hydrogen as fuel and can tolerate up to 1.5% CO concentration in fuel, which broadens the choice of fuels that can be used. However, any sulfur compounds present in the fuel have to be removed to a concentration of <0.1 ppmV. Temperatures of about 200°C and acid concentrations of 100% H3PO4 are commonly used, while operating pressure in excess of 8 atm has been used in an 11-MW electric utility demonstration plant [3, 22, 23].

Electrochemistry of PAFCs. The electrochemical reactions occurring in a PAFC are

At the anode:

H2 ^ 2H+ + 2e~

At the cathode:

—O2 + 2H+ + 2e S H2O The overall cell reaction:

2 O2 + H2 s H2O

The fuel cell operates on H2; CO is a poison when present in a concen­tration greater than 0.5%. If a hydrocarbon such as natural gas is used as a fuel, reforming of the fuel by the reaction

CH4 + H2O ^ 3H2 + CO

and shifting of the reformat by the reaction

CO + H2O ^ H2 + CO2

is required to generate the required fuel for the cell.

Electrolyte. The PAFC uses 100% concentrated phosphoric acid (H3PO4) as an electrolyte. The electrolyte assembly is a 0.1- to 0.2-mm-thick matrix made of silicon carbide particles held together with a small amount of PTFE. The pores of the matrix retain the electrolyte (phosphoric acid) by capillary action. At lower temperatures, H3PO4 is a poor ionic con­ductor and CO poisoning of the Pt electrocatalyst in the anode can become severe. There will be some loss of H3PO4 over long periods, depending upon the operating conditions. Hence, as a general rule, suf­ficient acid reserve is kept in the matrix at the beginning.

Electrode. The PAFC (similar to a PEMFC) uses gas diffusion electrodes. Platinum or platinum alloys are used as the catalyst at both electrodes. In the mid-1960s, the conventional porous electrodes were PTFE-bonded Pt black, and the loadings of Pt were about 9 mg/cm2. In recent years, Pt supported on carbon black has replaced Pt black in porous PTFE — bonded electrode structures. Pt loading has also dramatically reduced to about 0.25 mg Pt/cm2 in the anode and about 0.50 mg Pt/cm2 in the cathode. The porous electrodes used in a PAFC consist of a mixture of the electrocatalyst supported on carbon black and a polymeric binder to bind the carbon black particles together to form an integral structure. A porous carbon paper substrate provides structural support for the electrocatalyst layer and also acts as the current collector. The com­posite structure consisting of a carbon black/binder layer onto the carbon paper substrate forms a three-phase interface, with the electrolyte on one side and the reactant gases on the other side of the carbon paper. The stack consists of a repeating arrangement of a bipolar plate, the anode, electrolyte matrix, and cathode.

Hardware. A bipolar plate separates the individual cells and electri­cally connects them in a series in a fuel cell stack. A bipolar plate has a multifunction design; it has to separate the reactant gases in the adja­cent cells in the stack, so it must be impermeable to reactant gases; it must transmit electrons to the next cell (series connection), so it has to be electrically conducting; and it must be heat conducting for proper heat transfer and thermal management of the fuel cell stack. In some designs, gas channels are also provided on the bipolar plates to feed reactant gases to the porous electrodes and to remove the reaction products. Bipolar plates should have very low porosity so as to minimize phosphoric acid absorption. These plates must be stable and corrosion-resistant in the PAFC environment. Bipolar plates are usually made of graphite — resin mixtures that are carbonized and heat treated to 2700°C to increase corrosion resistance. For 100-kW and larger power generation systems, water cooling has to be used and cooling channels are provided in the bipolar plates to cool the stack.

Temperature and humidity management. Temperature and humidity management are essential for proper operation of a PAFC. The PAFC system has to be heated up to 130°C before the cell can start working. At lower temperatures, concentrated phosphoric acid does not get dis­sociated, resulting in a low availability of protons. Also, due to lower vapor pressure of the concentrated acid, the water generated will not come out with the reactant stream and the moisture retention dilutes the acid. This causes an increase in acid volume, which results in acid oozing out through the electrode. With the start of normal cell operation, its tem­perature increases and acid concentration gets back to its normal value that causes acid volume to shrink, resulting in drying of the electrolyte matrix pores if the acid is not replenished. Controlled stack heating at start-up is achieved by using an insertable heater system. During oper­ation, the temperature of the stack is maintained by controlling the air flow in the oxidant channel. At high loading conditions, insertable coolers may be used to remove excess heat from the stack. Large-power PAFC systems use a water-cooling system.

Moisture generated at the cathode dilutes the acid on the cathode side of the electrolyte matrix, causing higher vapor pressure. This results in more moisture out with the oxidant stream. With the movement of pro­tons from anode to cathode, moisture migration takes place at the cath­ode side also. This water evaporation results in an acid concentration gradient from anode to cathode, causing low availability of protons and a lower potential of the cell. Therefore, water management is needed to maintain humidity of the anode stream gas at a sufficient level so that the vapor pressure matches the acid concentration level at the operat­ing temperature.

Performance. For good performance, the normal operating tempera­ture range of a PAFC is 180°C < T < 250°C; below 200°C, the decrease in cell potential is significant. Although an increased temperature increases performance, higher temperatures also result in increased catalyst sin­tering, component corrosion, electrolyte degradation, and evaporation. PAFCs operate in the current density range of 100-400 mA/cm2 at 600-800 mV/cell. Voltage and power limitations result from increased corrosion of platinum and carbon components at cell potentials above approximately 800 mV. Since the freezing point of phosphoric acid is 42°C, the PAFC must be kept above this temperature once commis­sioned to avoid the thermal stresses due to freezing and thawing. Various factors affect the PAFC life. Acid concentration management by proper humidity control is very important to prevent acid loss and performance degradation. A PAFC has a life of 10,000-50,000 h, commercially avail­able (UTC Fuel Cells) PAFC systems operating at 207°C have shown a
life of 40,000 h with reasonable performance (degradation rate AVlifetime (mV) = —2 mV/1000 h) [3, 23].