Phosphoric acid fuel cell electrolyte management

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Many different types of fuel-cell membranes are currently in use in, e.g., solid-oxide fuel cells (SOFCs), molten-carbonate fuel cells (MCFCs), alkaline fuel eells (AFCs), phosphoric-acid fuel cells (PAFCs), and polymer-electrolyte membrane fuel cells (PEMFCs). One of the most widely used polymers in PEMFCs is Nalion, which is basically a fluorinated teflon-like hydrophobic polymer backbone with sulfonated hydrophilic side chains." Nafion and related sulfonic-add based polymers have the disadvantage that the polymer-conductivity is based on the presence of water and, thus, the operating temperature is limited to a temperature range of 80-100 °C. This constraint makes the water (and temperature) management of the fuel cell critical for its performance. Many computational studies and reviews have recently been pubhshed," and new types of polymers are proposed at any time, e.g. sulfonated aromatic polyarylenes," to meet these drawbacks. 

However, the problem of water management in the electrolyte, which we consider in some detail in Section 4.4 below, was judged too difficult to manage reliably, and for the Apollo vehicles, NASA selected the rival alkaline fuel cell (Warshay, 1990). General Electric also chose not to pursue commercial development of the PEMFC, probably because the costs were seen as higher than other fuel cells, such as the phosphoric acid fuel cell then being developed. At that time catalyst technology was such that 28 mg of... 

Many types of electrolytes have been used in fuel cells. Water solutions of acids, such as phosphoric, sulfuric, and trifluoroacetic acids (acidic electrolytes), and bases such as sodium hydroxide or potassium hydroxide (alkaline electrolytes), can be incorporated into efficient cells. Cells using water solutions as electrolytes have complex problems of water management and electrolyte retention under conditions of severe physical motion. These will probably not be suitable for automobile service. For stationary applications described in Chapter 6 the water based electrolytes may offer advantages. 

Phosphoric acid is also used as the electrolyte in so called High Temperature PEFC. In this case, the phosphoric acid is imbibed in a polybenzimidazole polymer matrix. While management of the liquid electrolyte in PAFC requires careful differential pressure control, High Temperature PEFCs are more tolerant Furthermore, the basic nature of the polybenzimidazole matrix prevents electrolyte migration. High Temperature PEFCs therefore are an attractive alternative to PAFC. Nevertheless, phosphoric acid is washed out from PAFC and high temperature PEFC once liquid water can form inside the fuel cell. Therefore, a continuous mode of operation is preferred in both cases. 

PAFCs run at temperatures around 200 °C. Unlike PEM fuel cells, the electrolyte in PAFCs does not rely on water for proton conduction therefore water management is not a concern. PAFC systems usually are used in coupled heat and power (CHP) applications for improved efficiency. Due to corrosion caused by phosphoric acid-based electrolyte and dissolution/corrosion of Pt-based catalyst in PAFCs, the operating voltage conditions of PAFCs are limited to less than 0.8 V per ceU. The dissolution/corrosion problems also eliminate the option of hot idling of PAFCs. Borohydride energy density at 60 °C would allow a smaller, more adaptive system. 

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