through a chemical reaction with oxygen or another oxidizing agent.

Hydrogen produced from the steam methane reforming of natural gas is the most common fuel, but

for greater efficiency hydrocarbons can be used directly such as natural gas and alcohols

like methanol. Fuel cells are different from batteries in that they require a continuous

source of fuel and oxygen/air to sustain the chemical reaction whereas in a battery the

chemicals present in the battery react with each other to generate an electromotive force.

Fuel cells can produce electricity continuously for as long as these inputs are supplied.

The first fuel cells were invented in 1838. The first commercial use of fuel cells came

more than a century later in NASA space programs to generate power for probes, satellites and

space capsules. Since then, fuel cells have been used in many other applications. Fuel

cells are used for primary and backup power for commercial, industrial and residential

buildings and in remote or inaccessible areas. They are also used to power fuel-cell vehicles,

including forklifts, automobiles, buses, boats, motorcycles and submarines.

There are many types of fuel cells, but they all consist of an anode, a cathode and an

electrolyte that allows charges to move between the two sides of the fuel cell. Electrons

are drawn from the anode to the cathode through an external circuit, producing direct current

electricity. As the main difference among fuel cell types is the electrolyte, fuel cells

are classified by the type of electrolyte they use followed by the difference in startup

time ranging from 1 sec for PEMFC to 10 min for SOFC. Fuel cells come in a variety of

sizes. Individual fuel cells produce relatively small electrical potentials, about 0.7 volts,

so cells are "stacked", or placed in series, to increase the voltage and meet an application's

requirements. In addition to electricity, fuel cells produce water, heat and, depending

on the fuel source, very small amounts of nitrogen dioxide and other emissions. The

energy efficiency of a fuel cell is generally between 40–60%, or up to 85% efficient in

cogeneration if waste heat is captured for use.

The fuel cell market is growing, and Pike Research has estimated that the stationary

fuel cell market will reach 50 GW by 2020.

History

The first references to hydrogen fuel cells appeared in 1838. In a letter dated October

1838 but published in the December 1838 edition of The London and Edinburgh Philosophical

Magazine and Journal of Science, Welsh physicist and barrister William Grove wrote about the

development of his first crude fuel cells. He used a combination of sheet iron, copper

and porcelain plates, and a solution of sulphate of copper and dilute acid. In a letter to

the same publication written in December 1838 but published in June 1839, German physicist

Christian Friedrich Schönbein discussed the first crude fuel cell that he had invented.

His letter discussed current generated from hydrogen and oxygen dissolved in water. Grove

later sketched his design, in 1842, in the same journal. The fuel cell he made used similar

materials to today's phosphoric-acid fuel cell.

In 1939, British engineer Francis Thomas Bacon successfully developed a 5 kW stationary

fuel cell. In 1955, W. Thomas Grubb, a chemist working for the General Electric Company,

further modified the original fuel cell design by using a sulphonated polystyrene ion-exchange

membrane as the electrolyte. Three years later another GE chemist, Leonard Niedrach, devised

a way of depositing platinum onto the membrane, which served as catalyst for the necessary

hydrogen oxidation and oxygen reduction reactions. This became known as the "Grubb-Niedrach fuel

cell". GE went on to develop this technology with NASA and McDonnell Aircraft, leading

to its use during Project Gemini. This was the first commercial use of a fuel cell. In

1959, a team led by Harry Ihrig built a 15 kW fuel cell tractor for Allis-Chalmers, which

was demonstrated across the U.S. at state fairs. This system used potassium hydroxide

as the electrolyte and compressed hydrogen and oxygen as the reactants. Later in 1959,

Bacon and his colleagues demonstrated a practical five-kilowatt unit capable of powering a welding

machine. In the 1960s, Pratt and Whitney licensed Bacon's U.S. patents for use in the U.S. space

program to supply electricity and drinking water. In 1991, the first hydrogen fuel cell

automobile was developed by Roger Billings. UTC Power was the first company to manufacture

and commercialize a large, stationary fuel cell system for use as a co-generation power

plant in hospitals, universities and large office buildings. UTC Power continues to be

the sole supplier of fuel cells to NASA for use in space vehicles, having supplied fuel

cells for the Apollo missions, and the Space Shuttle program, and is developing fuel cells

for cell phone towers and other applications. Types of fuel cells; design

Fuel cells come in many varieties; however, they all work in the same general manner.

They are made up of three adjacent segments: 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.

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 breaks down the fuel into

electrons and ions. The anode catalyst is usually made up of very fine platinum powder.

The cathode catalyst turns the ions into the waste chemicals like water or carbon dioxide.

The cathode catalyst is often made up of nickel but it can also be a nanomaterial-based catalyst.

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

Mass transport loss. To deliver the desired amount of energy, the

fuel cells can be combined in series to yield higher voltage, and in parallel to allow a

higher current to be supplied. Such a design is called a fuel cell stack. The cell surface

area can also be increased, to allow higher current from each cell. Within the stack,

reactant gases must be distributed uniformly over each of the cells to maximize the power

output. Proton exchange membrane fuel cells

In the archetypical hydrogen–oxide proton exchange membrane fuel cell design, a proton-conducting

polymer membrane separates the anode and cathode sides. This was called a "solid polymer electrolyte

fuel cell" in the early 1970s, before the proton exchange mechanism was well-understood.

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 are 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 because the membrane is electrically insulating. On the cathode catalyst, oxygen

molecules react with the electrons and protons to form water.

In addition to this pure hydrogen type, there are hydrocarbon fuels for fuel cells, including

diesel, methanol and chemical hydrides. The waste products with these types of fuel are

carbon dioxide and water, when hydrogen is used the CO2 is released when methane from

natural gas is combined with steam in a process called steam methane reforming to produce

the hydrogen, this can take place in a different location to the fuel cell potentially allowing

the hydrogen fuel cell to be used indoors for example in fork lifts.

The different components of a PEMFC are; bipolar plates,

electrodes, catalyst,

membrane, and the necessary hardware.

The materials used for different parts of the fuel cells differ by type. The bipolar

plates may be made of different types of materials, such as, metal, coated metal, graphite, flexible

graphite, C–C composite, carbon–polymer composites etc. The membrane electrode assembly

is referred as the heart of the PEMFC and is usually made of a proton exchange membrane

sandwiched between two catalyst-coated carbon papers. Platinum and/or similar type of noble

metals are usually used as the catalyst for PEMFC. The electrolyte could be a polymer

membrane. Proton exchange membrane fuel cell design

issues Costs. In 2013, the Department of Energy estimated

that 80-kW automotive fuel cell system costs of US$67 per kilowatt could be achieved, assuming

volume production of 100,000 automotive units per year and US$55 per kilowatt could be achieved,

assuming volume production of 500,000 units per year. In 2008, professor Jeremy P. Meyers

estimated that cost reductions over a production ramp-up period will take about 20 years after

fuel-cell cars are introduced before they will be able to compete commercially 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 has experimented with

a catalyst enhanced with carbon silk, which allows a 30% reduction in platinum usage without

reduction in performance. Monash University, Melbourne uses PEDOT as a cathode. A 2011

published study documented the first metal-free electrocatalyst using relatively inexpensive

doped carbon nanotubes, which are less than 1% the cost of platinum and are of equal or

superior performance. Water and air management. 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 like electroosmotic 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

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)

under extreme temperatures. Current service life is 7,300 hours under cycling conditions.

Automotive engines must also be able to start reliably at −30 °C and have a high power-to-volume

ratio. Limited carbon monoxide tolerance of some

cathodes. Phosphoric acid fuel cell

Phosphoric acid fuel cells were first designed and introduced in 1961 by G. V. Elmore and

H. A. Tanner. In these cells phosphoric acid is used as a non-conductive electrolyte to

pass positive hydrogen ions from the anode to the cathode. These cells commonly work

in temperatures of 150 to 200 degrees Celsius. This high temperature will cause heat and

energy loss if the heat is not removed and used properly. This heat can be used to produce

steam for air conditioning systems or any other thermal energy consuming system. Using

this heat in cogeneration can enhance the efficiency of phosphoric acid fuel cells from

40–50% to about 80%. Phosphoric acid, the electrolyte used in PAFCs, is a non-conductive

liquid acid which forces electrons to travel from anode to cathode through an external

electrical circuit. Since the hydrogen ion production rate on the anode is small, platinum

is used as catalyst to increase this ionization rate. A key disadvantage of these cells is

the use of an acidic electrolyte. This increases the corrosion or oxidation of components exposed

to phosphoric acid. High-temperature fuel cells

SOFC

Solid oxide fuel cells use a solid material, most commonly a ceramic material called yttria-stabilized

zirconia, as the electrolyte. Because SOFCs are made entirely of solid materials, they

are not limited to the flat plane configuration of other types of fuel cells and are often

designed as rolled tubes. They require high operating temperatures and can be run on a

variety of fuels including natural gas. SOFCs are unique in that negatively charged

oxygen ions travel from the cathode to the anode instead of positively charged hydrogen

ions travelling from the anode to the cathode, as is the case in all other types of fuel

cells. Oxygen gas is fed through the cathode, where it absorbs electrons to create oxygen

ions. The oxygen ions then travel through the electrolyte to react with hydrogen gas

at the anode. The reaction at the anode produces electricity and water as by-products. Carbon

dioxide may also be a by-product depending on the fuel, but the carbon emissions from

an SOFC system are less than those from a fossil fuel combustion plant. The chemical

reactions for the SOFC system can be expressed as follows:

Anode Reaction: 2H2 + 2O2− → 2H2O + 4e− Cathode Reaction: O2 + 4e– → 2O2−

Overall Cell Reaction: 2H2 + O2 → 2H2O SOFC systems can run on fuels other than pure

hydrogen gas. However, since hydrogen is necessary for the reactions listed above, the fuel selected

must contain hydrogen atoms. For the fuel cell to operate, the fuel must be converted

into pure hydrogen gas. SOFCs are capable of internally reforming light hydrocarbons

such as methane, propane and butane. These fuel cells are at an early stage of development.

Challenges exist in SOFC systems due to their high operating temperatures. One such challenge

is the potential for carbon dust to build up on the anode, which slows down the internal

reforming process. Research to address this "carbon coking" issue at the University of

Pennsylvania has shown that the use of copper-based cermet can reduce coking and the loss of performance.

Another disadvantage of SOFC systems is slow start-up time, making SOFCs less useful for

mobile applications. Despite these disadvantages, a high operating temperature provides an advantage

by removing the need for a precious metal catalyst like platinum, thereby reducing cost.

Additionally, waste heat from SOFC systems may be captured and reused, increasing the

theoretical overall efficiency to as high as 80%–85%.

The high operating temperature is largely due to the physical properties of the YSZ

electrolyte. As temperature decreases, so does the ionic conductivity of YSZ. Therefore,

to obtain optimum performance of the fuel cell, a high operating temperature is required.

According to their website, Ceres Power, a UK SOFC fuel cell manufacturer, has developed

a method of reducing the operating temperature of their SOFC system to 500–600 degrees

Celsius. They replaced the commonly used YSZ electrolyte with a CGO electrolyte. The lower

operating temperature allows them to use stainless steel instead of ceramic as the cell substrate,

which reduces cost and start-up time of the system.

MCFC

Molten carbonate fuel cells require a high operating temperature, 650 °C, similar to

SOFCs. MCFCs use lithium potassium carbonate salt as an electrolyte, and this salt liquefies

at high temperatures, allowing for the movement of charge within the cell – in this case,

negative carbonate ions. Like SOFCs, MCFCs are capable of converting

fossil fuel to a hydrogen-rich gas in the anode, eliminating the need to produce hydrogen

externally. The reforming process creates CO

2 emissions. MCFC-compatible fuels include natural gas, biogas and gas produced from

coal. The hydrogen in the gas reacts with carbonate ions from the electrolyte to produce

water, carbon dioxide, electrons and small amounts of other chemicals. The electrons