Fuel cells are electrochemical devices that convert the chemical energy of Hydrogen fuel into electrical energy to work with a by-product of heat and water. Electrochemical reactions are the most efficient means to convert chemical to electrical energy.
FUEL CELLS AND BATTERIES
In principal, a fuel cell operates like a battery, but does not run down or require recharging like a battery as long as fuel and oxygen are supplied.
WHO SAID BY-PRODUCTS ARE DIRTY?
For a proton conducting fuel cell, gaseous fuels (H2) is fed continuously to the anode and an oxidant (O2) fed to the cathode. With the help of a catalyst, the hydrogen atom oxides in to a proton (H+) and an electron (e-) that take a different path to the cathode while the proton passes through the electrolyte. The electrons generate a current and recombine on the cathode to produce water and heat.
FUEL CELLS 101
FUEL CELL TYPE VS. APPLICATION
Fuel cells as energy conversion devices have a number of advantages such as:
high energy conversion efficiency which is relatively independent of size
good part load characteristics
modular design and flexibility of size
low environmental impact and rejection of heat at high temperature in some of the fuel cell types, which is suitable for combined heat and power (CHP)
favorable environmental signature
quick response to load changes.
All fuel cells contain either solid or liquid Electrolytes sandwiched between two electrodes.
Different types of fuel cells operate at different temperatures and on a variety of fuels, including both gaseous fuels such as hydrogen, natural gas, propane and biogases, to liquid fuels such as methanol and ethanol. Low temperature fuel cells require pure hydrogen, whereas higher temperature fuel cells can operate directly on hydrocarbon fuels such as natural gas.
There are different fuel cell types depending upon the application and the source of hydrogen fuel. fuel, and operating temperature. Fuel cells are typically classified by application ,fuel and operating temperature. For example:
AUTOMOTIVE fuel cells are designed for LOW TEMPERATURE PEM fuel cells and compressed 10,000 PSIG hydrogen fuel is supplied by on-board storage tanks, and,
RESIDENTIAL fuel cells are grid connected, operate 24/7, and requires a sustainable hydrogen fuel supply such as natural gas and is more suitable for HIGH TEMPERATURE PEM or SOLID OXIDE fuel cells.
SEE FUEL CELL COMPARE BELOW.
FUEL CELL COMPARE
High Temperature PEM
High Temperature PEM Fuel Cells (HTPEM) are considered as the next generation PEM fuel cell operating at 140-160C as compared to Low temperature PEM (LTPEM) fuel cells operating at 60-80C.
The electrochemical kinetics for electrode reactions are enhanced by using Polybenzimidazole (PBI) membranes with a high proton conductivity.
As a result of the 160C operating temperature, HTPEM fuel cells gases do not need humidification and the water produced is a single phase (i.e. water vapor). Thereby the Balance-of-Plant cost is simplified with a reduced cost and reduction in parasitic losses.
Due to an internal high heat capacity oil based cooling medium, the GEIG HTPEM fuel cell provides for high power density (1-50kW) as compared to an air cooled HTPEM system (0.1-2kW).
The high heat capacity oil based cooling medium also allows for combined heat power (CHP) and supports hot water heating with an overall system efficiency of 91%+.
The HTPEM fuel cell can tolerate up to 3% (30,000ppm) CO and up to 20ppm of sulphur without permanent degradation. In comparison, LTPEM fuel cells normally can tolerate less than 30ppm CO and less than 1 ppm of Sulphur. The high CO tolerance further allows for extreme system efficiency and cost reductions when integrating with fuel reforming to extract hydrogen.
Low Temperature PEM
Low Temp Polymer Electrolyte Membrane (LTPEM) Fuel Cells operates at 60-80C and consists of a proton conducting membrane, such as a perfluorosulphonic acid polymer as the electrolyte which has good proton conducting properties, contained between two Pt impregnated porous electrodes. The back of the electrodes are coated with a hydrophobic compound such as Teflon forming a wet proof coating which provides a gas diffusion path to the catalyst layer. Within the cell, H2 at the anode provides protons and releases electrons which pass through the external circuit to reach the cathode. The protons solvate with water molecules and diffuse through the membrane to the cathode to react with the O2 while picking up electrons to form water and heat.
LTPEM fuel cells are very senstive to CO fuel poisioning and requires 99.999% pure hydrogen.
Solid Fuel Cells (SOFC) is viable for generating electricity from hydrocarbon fuels. They operate from 700-1000C and allows internal fuel reforming to extract hydrogen with fast kinetics with non-precious materials. The high temperature however, places stringent requirements on material requirements and is best suited for constant power operation, i.e. non-load following. The SOFC integrates well with large coal gasification plants that are able to take advantage of operational synergies to produce high system efficiencies.
Molten Carbonate Fuel Cells (MCFC) operates at 650C and has a narrow temperature operating range. The high operating temperature is required to achieve sufficient conductivity of its carbonate electrolyte wile allowing low cost components. Higher temperatures also allow higher system efficiencies and greater fuel flexibility.
On the other hand, the higher operating temperatures places severe demands on the corrosion stability and life of cell components in the aggressive environment of the molten carbonate electrolyte.
Phosphoric Acid Fuel Cells (PFAC) is similar to the PEM Fel Cell. PFAC uses a proton-conducting electrolyte and operates at 200C. The electrochemical reactions take place on highly dispersed electrocatalyst particles supported on carbon black. As with PEM fuel cells, platinum (Pt) alloys are used as the catalyst at both electrodes. The electrolyte is an INORGANIC ACID, concentrated phosphoric acid which conduct protons.
Phosphoric acid is the only common onorganic acid that has good enough thermal stability, chemica and elecrochemical stability and low enough volatility to be considered as an electrolyte for fuel cells.
Phosphoric acid is tolerant to CO2 in the fuel and oxidant, unlike Akaline fuel cells.
Alkaline Fuel Cells (AFC) were developed for NASA and the space program and requires pure hydrogen and oxygen as reactants. They operate at relatively high temperatures and pressure and produces the highest electrical conversion efficiency of all fuel cells. However, due to space, military and undersea applications, cost is not a driving factor which is a requirement for consumer and industrial markets.
AFCs are susceptible to electrolyte poisoning by CO2 which must be removed if "AIR" is used instead of pure OXYGEN.