Introduction
Conclusions
Stationary Fuel Cells
Source: www.appice.es
Phosphoric Acid Fuel Cell (PAFC)
Overview
The excess heat generated can be used for heating, but while the cogeneration efficiency can be up to 85%, on its own, the efficiency is relatively low at 35-45%. Currently, research is focused on increasing the operating temperatures and phosphoric acid concentrations to achieve better cell performance.
Design
The PAFC uses liquid phosphoric acid soaked in a Teflon bonded silicon carbide matrix. The matrix has a very small pore structure and uses capillary action to keep the acid in place. Both the anode and cathode sides of the electrolyte have platinum catalyzed, porous carbon electrodes. The fuel and oxidant gasses are supplied to the backs of the electrodes by grooves formed into carbon or carbon-composite plates. These plates are electrically conductive and electrons move from the anode to the cathode of the adjacent cell.
Water is removed as steam on the cathode by excess oxidant flowing across the backs of the electrodes, but only if the system is running around 190°C. At lower temperatures, the water dissolves in the electrolyte.
Since at significantly higher temperatures, the phosphoric acid decomposes, the excess heat is removed from the fuel cell stack by channels on carbon plates filled with a coolant, such as air or water. Liquid cooling requires complex structures and connections but is more effective than gas cooling. While gas cooling is simpler, more reliable, and relatively low cost, there is a size restraint on the cell because gas cooling requires larger channels.
Source: www.dodfuelcell.com
In most designs, the plates have grooves on both sides, so one side supplies fuel to the anode of one cell and the other side supplies air or oxygen to the cathode of the adjacent cell.
The higher efficiency designs use pressurized reactants but are also more expensive and complicated, so atmospheric operation is more economical. In addition, increased pressure promotes corrosion because the phosphoric acid electrolyte produces a corrosive vapor that damages locations other than the active cell area and increasing the total pressure and increases the partial pressure of the phosphoric acid vapor. Nevertheless, increased pressure also increases the oxygen and water partial pressures, leading to a lower acid concentration and increased ionic conductivity with higher exchange current density. In short, it reduces ohmic losses, so optimizing the pressure to balance performance and longevity is key to producing a highly efficient design.
The electrodes are a mixture of electro-catalysts on a carbon black support and a polymeric binder, usually polytetrafluoroethylene, that binds to the carbon black to form a porous structure on a porous graphite substrate. As the electrodes are in contact with the acid, they are subject to corrosion, so stability is improved with heat treatment. While all graphite plates are corrosion resistant without heat treatment, they are costly.
Over time, the electrolyte escapes from the cell in the air stream, so an electrolyte reservoir plate, made of porous graphite replenishes phosphoric acid without electrolyte replacement. This structure also prevents electrolyte flooding by accommodating excess electrolyte.
Fuel and Fuel Impurities
Reformed fuels contain CO, carbon dioxide and unreacted hydrocarbons. The unreacted hydrocarbons and carbon dioxide are chemically inert and do not strongly affect cell performance. However, both temperature and CO concentration affect hydrogen oxidation on platinum.
Carbon monoxide can poison the catalysts but increasing the operating temperature also increases the PAFC’s carbon monoxide tolerance.
Hydrogen sulfide and carbonyl sulfide impurities also reduce catalytic effectiveness. Sulfur has to be removed prior to fuel reforming because rapid cell failure occurs with greater than 50ppm. Above this concentration, it adsorbs on Pt and blocks the active sites for hydrogen oxidation.
Molecular nitrogen acts as a diluent, however, ammonia decreases the rate of oxygen reduction by forming a phosphate salt with phosphoric acid. Consequently, the molecular nitrogen has to be limited to 4%. It was found that low oxygen utilization resulted in higher performance but also poor fuel use.
Future
Before the phosphoric acid fuel cell can compete with other energy technologies, it needs to be cheaper, more efficient, and increase its lifespan. Approaches to increasing lifespan include using series fuel flow to alleviate corrosion, balancing the pore size in the reservoirs to prevent flooding, and using high corrosion resistant carbon support for the cathode catalyst.
High performance and longer lifetime of electrodes are intrinsically at odds. The PTFE and carbon black particles initially act as gas networks, however, they eventually flood. One approach suggested using a hydrophilic carbon black as a fine electrolyte network and a wet-proofed carbon black as a gas-supplying network.
Advances in catalyst development may decrease costs, also making the PAFC a more attractive energy source. For instance, transition metal organic macrocycles have promising stability after heat treatment and catalytic activity comparable to platinum. Also, platinum/nickel alloys make more effective catalysts than pure platinum, decreasing costs further. While the PAFC can accommodate different fuels, the anode is sensitive to contaminants that can preferentially absorb on the noble catalysts. Current practice is to purify the fuel stream before reforming to eliminate hydrogen sulfide, COS and CO. A catalyst that could tolerate CO and hydrogen sulfide would greatly simplify the system and decrease costs.
Sources:
1. Bellona Foundation. www.bellona.no
2. EG&G Technical Services Under Contract No. DE-AM26-99FT40575 for U.S. Department of Energy. Fuel Cell Handbook 7th Edition. November 2004.
3. Department of Defense. www.dodfuelcell.com
4. Fuel Cell Test and Evaluation Center. www.fctec.com
5. Fuel Cell Today. www.fuelcelltoday.com