What is Hydrogen Technology?
In the archetypal example of a hydrogen/oxygen proton exchange membrane fuel cell (PEMFC), a proton-conducting polymer membrane, (the electrolyte), separates the anode and cathode sides.
On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons.
The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electrically insulating.
On the cathode catalyst, oxygen molecules react with the electrons (which have traveled through the external circuit) and protons to form water.
In addition to pure hydrogen, there are hydrocarbon fuels for fuel cells, including diesel, methanol (see: direct-methanol fuel cells) and chemical hydrides.
The waste products with these types of fuel are carbon dioxide and water.
The materials used in fuel cells differ by type.
The electrode/bipolar plates are usually made of metal, nickel or carbon nanotubes, and are coated with a catalyst (like platinum, nano iron powders or palladium) for higher efficiency. Carbon paper separates them from the electrolyte. The electrolyte could be ceramic or a membrane.
A typical fuel cell produces about 0.86 volt.
To create enough voltage, the cells are layered and combined in series and parallel circuits to form a fuel cell stack. The number of cells used is usually greater than 45 but varies with design.
Fuel cell design problems
Costs. In 2002, typical cells had a catalyst content of USD $1000 per kilowatt of electric power output. The goal is to reduce the cost in order to compete with current market technologies including gasoline internal combustion engines. Many companies are working on techniques to reduce cost in a variety of ways including reducing the amount of platinum needed in each individual cell. Ballard Power Systems have experiments with a catalyst enhanced with carbon silk which allows a 30% reduction (1 **/cm² to 0.7 **/cm²) in platinum usage without reduction in performance.
The production costs of the PEM (proton exchange membrane). The Nafion® membrane currently costs €400/m². This, and the Toyota PEM and 3M PEM membrane can be replaced with the ITM Power membrane (a hydrocarbon polymer), resulting in a price of ~€4/m². One of the bigger companies is using Solupor® (a porous polyethylene film).
Water management (in PEMFCs). In this type of fuel cell, the membrane must be hydrated, requiring water to be evaporated at precisely the same rate that it is produced. If water is evaporated too quickly, the membrane dries, resistance across it increases, and eventually it will crack, creating a gas 'short circuit' where hydrogen and oxygen combine directly, generating heat that will damage the fuel cell. If the water is evaporated too slowly, the electrodes will flood, preventing the reactants from reaching the catalyst and stopping the reaction. Methods to manage water in cells are being developed by fuel cell companies and academic research labs.
Flow control. Just as in a combustion engine, a steady ratio between the reactant and oxygen is necessary to keep the fuel cell operating efficiently.
Temperature management. The same temperature must be maintained throughout the cell in order to prevent destruction of the cell through thermal loading.
Durability, service life, and special requirements for some type of cells. Stationary applications typically require more than 40,000 hours of reliable operation at a temperature of -35 °C to 40 °C, while automotive fuel cells require a 5,000 hour lifespan (the equivalent of 150,000 miles) under extreme temperatures. Automotive engines must also be able to start reliably at -30 °C and have a high power to volume ratio (typically 2.5 kW per liter).
Limited carbon monoxide tolerance of the anode.
A new variant of fuel cell, the Unitized Regenerative Fuel Cell (URFC), has been developed by Lawrence Livermore National Laboratory and AeroVironment of Monrovia, California. The URFC integrates fuel cell and electrolyzer functions into a single unit. By reversing the direction of operation, water and electricity are converted into hydrogen and oxygen; when operating normally, hydrogen and oxygen are converted into electricity and water just as in a conventional fuel cell.
Currently Ovonic offers a version of the URFC that does not regenerate hydrogen and oxygen, but instead stores energy in the fuel cell stack.
Proton Energy Systems also markets a Regenerative Fuel Cell (RFC) system, UNIGEN, for backup power systems, as well as aerospace and military applications.