Phosphoric acid cells

Improvements in the state-of-the-art of phosphoric acid cells are illustrated by Figure 5-1. performance by the 1 m (10 ft ) short stack, (f), results in a power density of nearly 0.31 WW. 

According to the electrolyte and working temperature, one distinguishes the low-temperature fuel cell technologies (i) alkaline fuel cell, AFC (70 to 80°C), (ii) proton exchange membrane fuel cell, PEMFC (70 to 80°C), (iii) phosphoric acid cell, PAFC (200°C) from the high-temperature technologies, (iv) molten carbonate fuel cell, MCFC (650 to 700°C), and (v) solid oxide fuel cell, SOFC (1000°C). 

This type of stabilized alloy catalyst is used today in commercial fuel cell electrodes of phosphoric acid cells and contributes significantly to the technical success of these electricity generators, which are today produced in units from 50 kW to 11 MW size. 

The phosphoric acid cell has been under research for a longer time than that of any other kind of fuel cell. Alloys of Pt with Cr, V, and Ti and other non-noble metals are better than Pt (Appleby, 1986). The particle size of the catalyst has been reduced to that of tens of atoms (Stonehart, 1993).10 Much attention has been given to the search for non-noble (hence cheaper) catalysts that are stable in hot acids. The best are the porphyrins, the formulas for which are shown in Fig. 13.20. They are applied to a base of graphite. These electrocatalysts are more effective in alkaline fuel cells than in those with acid electrolytes. Curiously, these substances are more stable and give better catalysis after pyrolysis in He at 800 °C, a process that would decompose the organic part of the structure. Perhaps the only active part of the porphyrin catalyst is the central... 

One possibility for supplying household energy is to distribute electricity from central fuel cell-based power plants to houses in the surrounding area. However, it may become cheaper to store methanol in each plant and use it in the co-generation of heat and electricity.19 Such a scheme would also make possible advantages in the distribution of lighting in households via pipes from a central light source powered by fuel cells. This type of situation may provide an application for phosphoric acid cells. 

In Williams, 2002, the Edison Power Research Institute surveys the US market for all cell types, except Regenesys. High capital costs snuff out the phosphoric acid cell, reinforcing the author s decision to cut a chapter from this book on that fuel cell type. For the SOFC the EPRI neatly brackets the Australian estimate. Because the technical underlay... 

There are four types of fuel cells in development. They differ in the electrolyte they use, but the mechanical and chemical fundamentals are similar. The electrolytes under investigation are Phosphoric Acid, Molten Carbonate, Solid Oxide and Solid Polymer. The Phosphoric acid cells operate at temperatures of 180 to 210 degrees Celsius. Molten carbonate cells operate at 600 to 700 degrees Celsius. Solid oxide Cells operate at 650 to 1000 degrees Celsius. These temperatures are uncomfortably high for home use and impractically high for automotive use. Only the Solid Polymer cells operate at a temperature range, 80 to 100 Celsius, a suitable for use in the home or automobile. 

Comparison of the capacity in the year 2000 versus that in 2015 shows that the annual growth in fuel cell capacity is projected to be in the range of 500 to 4000 MW. At the cost of 1000 per kilowatt of capacity, the market value is 0.5 billion to 4 billion per year for domestic utilities. The international market is estimated to be two to three times larger. Market penetration will be assisted with the development of another fuel cell concept based on molten-salt electrolytes. In comparison to the phosphoric acid cell, the molten-salt fuel cell is a simpler engineering system, has a greater operating efficiency (50 to 60 percent versus 40 to 45 percent), but lags 5 to 10 years in commercialization status. 

The fuel cell has already proved its usefulness in space technology and there are excellent prospects for its commerical application. Application on a large scale is not expected during the 20th century. The alkaline cell and the phosphoric acid cell are technically well developed, but from a commerical point of view it is questionable whether or not they will be of interest when other types reach technical maturity. The molten carbonate cell and the solid oxide cell seem to have the best prospects. For mobile application the solid polymer cell is a strong candidate. 

To the present time, it has not been possible to obtain a reasonable performance from anodes with a primary fuel feed, and the candidate systems are based on H2 (as in phosphoric acid cells at 573K) or H2 + CO (molten carbonate cells at 973K). 

Figure 5-1 depicts the operating configuration of the phosphoric acid cell. The electrochemical reactions occurring in PAFCs are... 

Farooque M, Kush A, Abens S (1990) Novel electrochemical hydrogen separation device using phosphoric acid cell. Sep Sci Technol 25 1361-1373... 

---