A fuel cell is an electrochemical device that creates electricity from a fuel and an oxidant (usually air). There are several types of fuel cells depending on the fuel source. The fuel and oxidant react in the presence of an electrolyte. Fuel cells consume the reactant (fuel) therefore the fuel must be replenished. The reaction occurs at the electrodes of the fuel cell which are stable and catalytic (they do not get used up). Typical fuels are hydrogen, hydrocarbons, and alcohol. The oxidant is usually oxygen, though air, chlorine, and chlorine dioxide are also used. 

Basically the electrode separates the electrons and protons from the fuel, and makes the electrons travel through the electrical circuit. The catalyst is usually Pt metal or alloy. The electrons travel through the circuit to the oxidant side of the reaction. There the electrons, protons, and the oxidant combine to form waste (usually water).

Proton Exchange Membrane Fuel Cell - PEMFC

The electrolyte is a proton conducting membrane which separates the anode (fuel) from the cathode (oxidant) sides. The hydrogen fuel separates at the anode to a proton and electron. 

Since the electrolyte is electrically insulating, the electrons are force to travel to a circuit. The protons are conducted through the electrolyte to the cathode. At the cathode, the oxygen molecules react with the electrons and protons to form water. A typical PEM fuel cell produce voltage between 0.6-0.7 V. Voltage decreases as current increase because of

  • Activiation loss
  • Ohmic Loss
  • Mass transport Loss

Fuel cells are optimized by combing them in series called fuel cell stacks or making them larger (creating a stronger current).

Cons

  • Costs - $1000 per kW
  • Production Costs - Nafion €400/m² of PEM electrolyte membrane.  Replace with hydrocarbon membrane or porous polyethylene membrane.
  • Temperature- The water that is created must be evaporated as soon as it is produced. If heat is too high the system will dry, cracking the electrolytic membrane. If the heat is too low, the system will flood decrease the rate of the reaction. Electro-osmotic pumps are being developed to address this issue.
  • Durability - To compete with other technologies must have 5,000 hour life span. Must have high power output to volume ratio (about 2.5 kW/liter).
  • Carbon monoxide at anode

Hydrocarbon fuels are diesel, methanol, which produce carbon dioxide and water as waste.

Fuel Efficiency

The more power that is drawn from the fuel cell the lower the efficiency. The power drawn is directly related to the current. Since the efficiency of a cell is proportional to the voltage, it is useful to show the relationship of voltage vs. current (polarization curves). A PEM cell running at 0.7 V will have about 50% efficiency. This means that 50% of the energy content of hydrogen is converted into electricity, and the rest is dissipated as heat. Though there is no combustion, thermodynamic laws still apply to fuel cells. The maximum theoretical efficiency is 83% at 25°C. This is higher than the Otto cycle thermal efficiency which is at 60%. The electrical output of a fuel cell must be converted to mechanical power which decreases efficiency. The right way to look at Fuel cells is that the limitation by thermodynamic laws are less severe than the limitations on conventional conversion systems. Thus, fuel cells have a high efficiencies in converting chemical energy to electrical energy.

In practice

Since air is no pure oxygen and has some moisture content, efficiency is decreased to levels comparable to a combustion ignition engine.

Hydrogen Fuel Production

Steam reforming 95% of the hydrogen gas is made from steam reformation. Steam (H2O) and methane react over a catalyst at high temperatures. The products are carbon oxides. Also at high temperatures sulfur dioxide and iodine can be used as catalysts to break water into hydrogen gas. The method combines water, sulfur dioxide, and iodine at high temperatures to produce sulfuric acid and hydriodic acid. At high temperatures sulfuric acid decomposes back into water and sulfur dioxide and hydriodic acid decomposes back into iodine and hydrogen gas. Photolysis uses sunlight to split water into hydrogen (and oxygen). Titania nanotubes are able to use ultraviolet light to dissociate water (nanotechnology).

Hydrogen Economy

The benefit of economies of scale in hydrogen production are offset by the cost of transport. Keeping miles of pipes at high pressures or low tempertaures is very expensive, requires energy, must be safe, and reliable. For long distances, and when high volume is needed, pipelines have proven to be the most cost-effective method for transporting hydrogen.

 Hydrogen Fuel Storage

Compressing the gas at 5000 PSI. Liquid hydrogen can be stored at tempatures near absolute zero. The storage tanks must be well insulated to keep this temperature. A possible way to storage hydrogen is as a metal hydride, combine hydrogen with a light metal. Lithium or boron could be used as the metal, and the hyride power could be heated to release hydrogen when needed. Carbon nanotubes (with titanium), aerogels, and polymers could store hydrogen gas.


Other fuels 

Ammonia (NH3) might be better than hydrogen gas when using a tank. The ammonia can be efficiently decomposed in situ by the action of a nanostructured iridium metal catalyst.

 

Animations 

http://www.bigs.de/en/shop/anim/bz01.swf

 

Idea 

Make biodiesel, use it as a fuel for fuel cell, generate electricty

Combine high temperatures of combustion reaction with the high temperatures needed for a fuel cell. Use the same fuel.