A 

fuel cell is an electrochemical cell that converts a source fuel into an electric current. It generates
electricity inside a cell through reactions between a fuel and an oxidant, triggered in the presence of an
electrolyte. The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte
remains within it. Fuel cells can operate continuously as long as the necessary reactant and oxidant flows
are maintained.

Fuel cells are different from conventional electrochemical cell batteries in that they consume reactant from
an external source, which must be replenished[1] – a thermodynamically open system. By contrast,
batteries store electrical energy chemically and hence represent a thermodynamically closed system.

Many combinations of fuels and oxidants are possible. A hydrogen fuel cell uses hydrogen as its fuel
and oxygen (usually from air) as its oxidant. Other fuels include hydrocarbons and alcohols. Other
oxidants include chlorine and chlorine dioxide.[2]

Design
Fuel cells come in many varieties; however, they all work in the same general manner. They are made up
of three segments which are sandwiched together: the anode, the electrolyte, and the cathode. Two
chemical reactions occur at the interfaces of the three different segments. The net result of the two
reactions is that fuel is consumed, water or carbon dioxide is created, and an electric current is created,
which can be used to power electrical devices, normally referred to as the load.

At the anode a catalyst oxidizes the fuel, usually hydrogen, turning the fuel into a positively charged ion
and a negatively charged electron. The electrolyte is a substance specifically designed so ions can pass
through it, but the electrons cannot. The freed electrons travel through a wire creating the electric current.
The ions travel through the electrolyte to the cathode. Once reaching the cathode, the ions are reunited
with the electrons and the two react with a third chemical, usually oxygen, to create water or carbon
dioxide.
A block diagram of a fuel cell

The most important design features in a fuel cell are:

 The electrolyte substance. The electrolyte substance usually defines the type of fuel cell.
 The fuel that is used. The most common fuel is hydrogen.
 The anode catalyst, which breaks down the fuel into electrons and ions. The anode catalyst is
usually made up of very fine platinum powder.
 The cathode catalyst, which turns the ions into the waste chemicals like water or carbon dioxide.
The cathode catalyst is often made up of nickel.

A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load. Voltage decreases as current
increases, due to several factors:

 Activation loss
 Ohmic loss (voltage drop due to resistance of the cell components and interconnects)
 Mass transport loss (depletion of reactants at catalyst sites under high loads, causing rapid loss
of voltage).[3]

To deliver the desired amount of energy, the fuel cells can be combined in series and parallel circuits,
where series yields higher voltage, and parallel allows a higher current to be supplied. Such a design is
called a fuel cell stack. The cell surface area can be increased, to allow stronger current from each cell.

[edit]Proton exchange fuel cells

On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and
electrons. These protons often react with oxidants causing them to become what is commonly referred to
as multi-facilitated proton membranes. 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 this example, the only waste product,
either liquid or vapor.

In addition to this pure hydrogen type, there are hydrocarbon fuels for fuel cells,
including diesel, methanol (see: direct-methanol fuel cells and indirect methanol fuel cells) and chemical
hydrides. The waste products with these types of fuel are carbon dioxide and water.

Construction of a high temperature PEMFC: Bipolar plate as electrode with in-milled gas channel structure, fabricated from
conductive plastics (enhanced withcarbon nanotubes for more conductivity); Porous carbon papers; reactive layer, usually
on the polymer membrane applied; polymer membrane.
Condensation of water produced by a PEMFC on the air channel wall. The gold wire around the cell ensures the collection
of electric current.[4]

The materials used in fuel cells differ by type. In a typical membrane electrode assembly (MEA), 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.
[edit]Proton exchange membrane fuel cell design issues

 Costs. In 2002, typical fuel cell systems cost US$1000 per kilowatt of electric power output. In
2009, the Department of Energy reported that 80-kW automotive fuel cell system costs in volume
production (projected to 500,000 units per year) are $61 per kilowatt.[5] The goal is $35 per kilowatt. In
2008 UTC Power has 400 kW stationary fuel cells for $1,000,000 per 400 kW installed costs. 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 mg/cm² to 0.7 mg/cm²) in platinum usage without reduction in performance.[6] Monash
University, Melbourne uses PEDOT as a cathode.[7]
 The production costs of the PEM (proton exchange membrane). The Nafion membrane currently
costs $566/m². In 2005 Ballard Power Systems announced that its fuel cells will use Solupor, a
porouspolyethylene film patented by DSM.[8][9]
 Water and air management[10] (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 likeelectroosmotic pumps focusing on 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. This is particularly challenging as the
2H2 + O2 -> 2H2O reaction is highly exothermic, so a large quantity of heat is generated within the fuel
cell.
 Durability, service life, and special requirements for some type of cells. Stationary fuel cell
applicationstypically require more than 40,000 hours of reliable operation at a temperature of -35 °C
to 40 °C (-31 °F to 104 °F), while automotive fuel cells require a 5,000 hour lifespan (the equivalent of
150,000 miles) under extreme temperatures. Current service life is 7,300 hours under cycling
conditions.[11] Automotive engines must also be able to start reliably at -30 °C (-22 °F) and have a high
power to volume ratio (typically 2.5 kW per liter).
 Limited carbon monoxide tolerance of the cathode.