Fuel Cells

Fuel Cells

Petroleum, the worlds most prolific fuel, is becoming more scarce and its burning produces emissions which shoulder much of the responsibility for air pollution (Fig. 1). Contributions also come from deforestation, carbon dioxide from the burning of coal, and methane release. In order to reverse the trend of destroying the environment, a change to a more ecologically mundane resource, or method of producing energy such as hydrodynamic, wind, geothermal, solar and tidal is desirable. These methods are presently employed in a somewhat small scale, but require specific environments in order to work effectively. Fuel cells need no particular environment to work well (other than a heat sink) and is highly efficient both electrically and physically (without sound and with far fewer harmful air pollutants).

A fuel cell is an electrochemical device which brings together hydrogen and oxygen, or air in the midst of a catalyst to produce electricity, heat and water. (Fig. 2) or (Fig. 2a) The single cell fixture consists a single electrolyte sandwiched between electrodes. This inner sandwich is then placed in-between current collectors which usually serve as the poles of the cell. A fuel cell generates current by transforming (usually by using the catalyst in the electrodes) hydrogen gas into a mixture of hydrogen ions and electrons on the anode side of the cell. Because of the insulating nature of the electrolyte, the anions transfer through the electrolyte to the cathode side of the cell while the electrons are conducted to the current collectors and through a load to do work. The electrons then travel to the cathode side current collector where they disperse onto the electrodes to combine with incoming hydrogen anions, oxygen, or air in the presence of a catalyst to form water completing the circuit.

This process occurs in all types of fuel cells (alkaline, solid polymer, phosphoric acid and solid oxide) except for molten carbonate. The molten carbonate cell transfers the carbonate ions formed by the reaction of oxygen and carbon monoxide in the presence of electrons from the cathode side to the anode side to react with hydrogen and form water and two electrons for current. Thus the net flow of ions in the electrolyte is opposite of that in all other fuel cells, but since the current flows in the same direction as the other fuel cell types, the anode and cathode keep their polarity.

The fuel cell was first invented in 1839 by Sir William Grove, a professor of experimental philosophy at the Royal Institution in London. He tested what turned out to be the precursor to the phosphoric acid fuel cell by enclosing platinum in tubes of hydrogen and oxygen gas while submerging the tubes in sulfuric acid. (Fig. 3) Unfortunately, he was hampered by the inconsistency of cell performance (a common feature of cells today), but realized the importance of the three phase contact (gas, electrolyte and platinum) to energy generation. He spent most of his time searching for an electrolyte that would produce a more constant current. He found several electrolytes which produced current, but still struggled with consistent results. He also noted the potential of the energy production method commercially if hydrogen could replace coal and wood as energy sources (1).

Since that time, researchers world wide have attempted to increase cell performance electrically, chemically as well as physically. Their experiments ranged from an improved three phase contact to smart materials and the adoption of off gases from other power sources. After over 150 years of research, fuel cells can be divided into five major categories named after the electrolyte used in each; alkaline, solid polymer, phosphoric acid, molten carbonate and solid oxide. The five types resulted from the knowledge that heat accelerates chemical reaction rates and thus the electrical current. The materials used for electrolytes have their best conductance only within certain temperature ranges and thus other materials must be used in order to take advantage of the temperature increase (2).

Solid oxide fuel cells (SOFC) which operate at the highest temperature (1000 - 1100 degrees Celsius) are not the most reactive because of the low conductivity of its ionic conducting electrolyte (yttria-stabilized zirconia). (Fig. 4) Many advances have been made in solid oxide fuel cell (SOFC) research to increase the chemical to electrical efficiency to 50%, but because of the conductivity and the heat, it has been used mainly in large power plants which can use the cogeneration of steam for additional power. Because of the high temperature, the cell requires no expensive catalysts, or additional humidification and fuel treatment equipment which excludes the cost of these items. The primary drawback to this type of fuel cell is the cost of the containment which requires exotic ceramics which must have similar expansion rates. SOFCs are now being considered for large power plants and for industrial applications because of its electrolytic resistance to poisoning which allows internal reforming of many carbon compounds into hydrogen to create power. (1, 2)

The molten carbonate, which operates at 600 degrees Celsius can use CO as a fuel input on the cathode side but needs hydrogen on the anode. Although the high temperature allows carbon in the cell, sulfur can poison the cell in small quantities (~1ppm). Carbonate ions are produced at the cathode and flow across the membrane to react with hydrogen and form two electrons, water and carbon dioxide. (Fig. 5) In an actual system, because of the internal heat, the cell can reform methanol into hydrogen for the anode reaction and use the carbon dioxide and extra hydrogen (burned in the presence of air) as fuel for the cathode reaction. The temperature is high enough for additional power production through cogeneration of steam and low enough eliminate the need of expensive catalysts and containment required in the SOFC. A MCFC operates nominally at 0.16 A/cm^2 and 0.75 volts per cell with better performance under pressurized conditions. Nickel compounds are used for the electrodes while the electrolyte contains a mixture of 68% lithium carbonate and 32% potassium carbonate in a porous gamma-lithium-aluminum oxide matrix. The efficiency using this system has risen to 50% in a combined (electrical and steam) cycle. MCFC, like the SOFC, is also used for mega-watt size power plants because of its heat (1, 2).

Phosphoric acid fuel cells (PAFC) are the oldest type whose origins extend back to the creation of the fuel cell concept. Many different acids have been used in order to boost performance such as sulfuric and perchloric acids, but when the temperature increases above