What is a Fuel Cell?
A Publication of the
Fuel cells, although
known in concept for more than 150 years, now are poised to make
significant contributions to stationary power generation. This Fuel
Cell Commercialization Group (FCCG) fact sheet provides a
brief history of fuel cell development, explains the working concept
and basic electrochemistry, describes how fuel cells are combined
into stacks and modules to achieve megawatt-scale power production,
and outlines some of the unique considerations that go into making
up a complete fuel cell power plant.
Finally Coming of
Sir William Grove is widely attributed
to be the “Father of the Fuel Cell.” Grove’s insights were gained
from his experiments in 1839 on the electrolysis of water. Grove
reasoned that it should be possible to reverse the process, reacting
hydrogen with oxygen to generate electricity. The term “fuel cell”
was coined in 1889 by Ludwig Mond and Charles Langer, who attempted
to build the first practical device using air and industrial coal
Attempts in the early 20th Century to build fuel cells that could
convert coal or carbon directly to electricity continued to fail
because of a lack of understanding of materials and electrode
kinetics. Meanwhile, the internal combustion engine was developed,
whose process was well understood. Petroleum was discovered and
rapidly exploited; electric vehicles and other electrochemical
approaches to energy production were quickly supplanted.
The first successful fuel cell devices resulted from inventions
in 1932 by engineer Francis Bacon. He improved on the expensive
platinum catalysts employed by Mond and Langer with a
hydrogen-oxygen cell using a less corrosive alkaline electrolyte and
inexpensive nickel electrodes. However, the technical challenges
were daunting and it was not until 1959, a quarter of a century
later, that Bacon and his coworkers were able to demonstrate a
practical five-kilowatt system capable of powering a welding
machine. In October of that same year, Harry Karl Ihrig of
Allis-Chalmers Manufacturing Company demonstrated his famous
20-horsepower fuel cell-powered tractor.
In the late 1950s, a then little-known federal agency called the
National Aeronautics and Space Administration (NASA) began to search
in earnest for a compact electricity generator to provide onboard
power for an upcoming series of manned space missions. After
discarding nuclear reactors as too risky, batteries as too heavy and
short-lived, and solar power as too cumbersome, NASA turned to fuel
NASA eventually funded more than 200 research contracts into all
aspects of fuel cell technology. Today, after reliably supplying
electricity (and water) to the Apollo and Space Shuttle missions,
fuel cells have proven their role in space.
These successes led to predictions in the 1960s that fuel cells
would be the panacea to the world’s energy problems. The same
qualities that make fuel cells ideal for space exploration — small
size, high efficiency, low emissions, minimal water use or net water
production — appeal as well to stationary power producers. Yet
bringing the technology down to earth has proven to be a tricky
Nearly 30 years and US$1 billion in research have been devoted to
address the barriers to the use of fuel cells for stationary
applications. An alkaline electrolyte, such as used for space
applications, requires very pure hydrogen, creating problems with
the use of common fuels such as natural gas or coal. Common fuels
also shorten the lifetimes of electrochemical components in cells
akin to the NASA design.
Fortunately, during the ensuing decades, a number of
manufacturers, the Electric Power Research Institute, the American
Gas Association, the Gas Research Institute, committed groups of
electric and gas utilities, and various federal agencies have
supported numerous demonstration initiatives and ongoing research
and development into stationary applications. In parallel, efforts
in Europe and Japan also received increased support and now
constitute significant governmentally backed initiatives.
The technology with the earliest promise for central station
generation, phosphoric acid fuel cells, now is being offered
commercially. An 11-megawatt unit was demonstrated in Tokyo, Japan,
and more than one hundred 200-kilowatt units have been installed
worldwide. More advanced designs, such as carbonate fuel cells and
solid oxide fuel cells, are the focus of major electric utility
efforts to bring the technology to the market.
Full-sized (commercial) cells and full-height stacks have been
successfully demonstrated for the carbonate fuel cell design.
Commercialization efforts in support of FuelCell Energy’s carbonate
fuel cell design are well underway, with significant buyer interest
It has taken more than 150 years to develop the basic science and
to realize the necessary materials improvement for fuel cells to
become a commercial reality. The fuel cell is finally coming of age.
The Fuel Cell
Fuel cells often are
described as continuously operating batteries or an electrochemical
engine. Like batteries, fuel cells produce power without combustion
or rotating machinery. Fuel cells make electricity by combining
hydrogen ions, drawn from a hydrogen-containing fuel, with oxygen
atoms. Batteries provide the fuel and oxidizer internally, which is
why they must be recharged periodically. Fuel cells, on the other
hand, utilize a supply of these key ingredients from outside the
system and produce power continuously, as long as the fuel supply is
The fuel cell uses these ingredients to create chemical reactions
that produce either hydrogen- or oxygen-bearing ions at one of the
cell’s two electrodes. These ions then pass through an electrolyte
(which conducts electricity), such as phosphoric acid or carbonate,
and react with oxygen atoms. The result is an electric current at
both electrodes, plus waste heat and water vapor as exhaust
products. This current is proportional to the size (area) of the
electrodes. The voltage is limited electrochemically to about 1.23
volts per electrode pair, or cell. These cells then can be “stacked”
until the desired power level is reached.
The challenge in fuel cell development for practical applications
has been to improve the economics through the use of low-cost
components with acceptable life and performance. Pure hydrogen and
oxygen reactants have been replaced with common fossil fuels and
air. Low-cost electrodes and electrolytes have been developed.
Engineering, materials improvements, and manufacturing processes are
now being developed to produce fuel cells with sufficiently high
power, acceptable lifetimes, and affordable costs.
As each of these challenges is met, the promise of a
factory-fabricated power generator scalable to virtually any size
range with highly automated operation is being realized.
FUEL CELL TYPE
Fuel Cell System
A major distinguishing characteristic
of different fuel cells is in the electrolyte used. For stationary
power generation, the three major fuel cells are phosphoric acid,
carbonate, and solid oxide. While other differences exist among
these fuel cells, it is the type of electrolyte used that gives them
The phosphoric acid approach is the most mature of the
technologies. Platinum is required as a catalyst for the electrodes.
Converting (or “reforming”) of the natural gas used as fuel to a
hydrogen-rich gas the system needs occurs outside the fuel cell
stacks. Due to system complexity, capital costs are higher and
efficiencies are lower than those projected for the two
Compared to the phosphoric acid type, the carbonate fuel cell
operates at higher temperatures, can operate at or slightly above
ambient pressure, and uses less expensive, nickel-based electrodes.
Reforming can occur inside the fuel cell stacks. FCE’s technology,
which is being supported by the FCCG, uses internal reforming and is
called the Direct Fuel Cell. DFCs are inherently more efficient
compared to external reforming fuel cell systems and can generate
power at 50 to 60 percent efficiency in a single cycle. This far
surpasses generation technologies such as gas turbines, internal
combustion engines, and steam turbines.
The solid oxide fuel cell approach is the least mature of the
three. It uses a coated zirconia ceramic as the electrolyte. The
electrochemical conversion process occurs at very high temperatures,
supporting internal reforming. The cells themselves may be either
flat plates or tubular. There are basic manufacturing challenges
with all-ceramic construction, as yet unsolved, in mass producing
the cells. Solid oxide fuel cells promise to operate at moderately
high efficiencies with a high-grade waste heat product.
A polymer electrolyte membrane fuel cell has been proposed for
submegawatt stationary power plant applications. This fuel cell
operates at 175°F, uses platinum catalyst, and is susceptible to
poisoning by carbon monoxide and other impurities. While this type
of cell may be attractive for vehicular applications, especially
when hydrogen is available as fuel, its deployment for stationary
power will require significant effort.
The remaining discussion in this Fact Sheet focuses on FCE’s
carbonate fuel cell design.
The operating principles
for a carbonate fuel cell are simple in concept. The reactants —
fuel and air (the oxidant) — are fed to the cell’s electrodes. Ions
are transported through the electrolyte sandwiched between the
electrodes, creating a current equal to the externally connected
load. The basic reactions are:
Natural Gas + Air
==> Steam + Carbon Dioxide + Electricity + Heat
Natural Gas + Steam
(water + heat) ==> Hydrogen + Carbon Monoxide
Hydrogen + Carbon
Monoxide + Carbonate Ion ==> Steam + Carbon Dioxide + Heat
+ Electrons (electricity)
Carbon Dioxide + Oxygen
+ Electrons ==> Carbonate Ion
While natural gas is the primary fuel, with appropriate cleanup
any hydrogen-rich gas — including gas from landfills, digesters,
coal mines, or liquid fuels — can be supplied to the fuel cell. Note
that electricity, heat, water vapor, and carbon dioxide are the
products of these basic reactions.
The construction of an individual fuel
cell resembles a sandwich. In the DFC, fuel and oxidant are fed
through separate manifolds to the anode and cathode compartments of
the cells, divided by a bipolar separator plate. The anode is bathed
with fuel, the cathode with oxidant (air and carbon dioxide). These
electrodes consist mainly of porous, sintered nickel (anode) or
nickel oxide (cathode). Layered between the electrodes is the
carbonate electrolyte contained in a porous (ceramic) matrix.
The separator plates, electrodes, and electrolyte layers are
known as “repeating” components. The separator plates can be cut
inexpensively and bent to size. The electrodes and electrolyte
layers are produced on continuous tape-casting
machines from relatively inexpensive materials and cut to length.
Cell production lends itself well to automated factory
Individual cells generate a relatively small voltage, on the
order of 0.7 to 1.0 Volt each (after accounting for resistance
Scaling Up The
The current produced by an individual
fuel cell is approximately a linear function of cell surface area.
For FCE’s commercial cells, the two-foot by four-foot cell area is a
trade-off between acceptable current (amperage) levels and
manufacturing and transportation constraints.
A 100-Kilowatt Stack
To develop higher voltages, cells are “stacked” and connected in
series. As the diagram indicates, stack design considerations
include manifolds for uniform gas distribution to each cell and to
maintain cell compression and mechanical integrity at the stack’s
high operating temperatures. For FCE’s commercial megawatt-class
power plants, individual stacks contain about 340 cells. Between
each 10-cell grouping is a special catalyst-containing cell to
improve internal reforming. Each stack has interconnections for
fuel, air, and electricity.
Several stacks then are combined into a truck-transportable
“module,” fabricated at the factory with all relevant connections,
for shipping and installation at the site. The desired output from
the power plant is obtained by combining a number of modules at the
site. FCE’s megawatt-class power plants will contain one or two
modules, each containing four stacks.
DFC power plants can achieve high electrical efficiencies at
small sizes without the need for a bottoming cycle. The waste heat
produced is well-suited to cogeneration or process heat
FCE’s design is significant
because the fuel is “reformed” to hydrogen-rich gas internally in
the stack, hence the name Direct Fuel Cell (DFC); this eliminates
the fuel processing unit required by phosphoric acid fuel cells and
other designs. Three significant advantages result from internal
reforming: (1) costly separate equipment to process the fuel is
eliminated, leading to lower overall capital costs; (2) equipment
count is lower, leading to simpler operation and higher reliability;
and (3) efficiency of the system is increased.
The diagram indicates FCE’s
“simplified” power plant design.
(1) Sulfur and other impurities are removed from the natural gas
(CH4) in a cleanup
(2) Fuel and steam are fed to the cell’s anode section. The fuel
is internally reformed and electrochemically oxidized by carbonate
ions formed at the cathode by the reaction of oxygen and carbon
(3) The anode exhaust stream is mixed with air and fed to the
cathode. This is the source of the oxygen and carbon dioxide in (4).
The cathode exhaust, resembling flue gas, is cooled with the
extracted heat used to preheat and vaporize the water. Thermal
energy at ~800°F is available for cogeneration (5).
(6) DC power produced by the fuel cells is conditioned by a
high-efficiency inverter to meet AC electrical grid
Power output is controlled by varying fuel and oxidant feeds to
the fuel cells. The inverter controls real and reactive power
The stacks have a projected commercial life of 40,000 hours. The
stacks degrade gradually over their projected life and must be
replaced periodically. The stack replacement decision is in reality
an economic one, trading off performance loss versus fuel costs,
within the thermal management limits of the balance-of-plant (BOP).
The degree of stack or module flanging or valving dictates how much
of the plant needs to be taken off-line when a stack is
Power plant heat rate and output are functions of the current
density at which the cells operate. As cells age, heat rate can be
maintained by allowing the cells to operate at a lower current
density (thus lowering plant output), or rated output can be
maintained by operating the cells at a higher current density (with
an increase in heat rate). The FCE design assumes two shutdowns for
planned and unplanned maintenance.
FCE’s commercialization program consists of a series of
progressively linked steps to scale-up the power plant. Particular
emphasis is being placed on BOP design, integration, operability,
The fuel cell’s mature capital costs
will be competitive economically with other technologies, especially
where strict environmental compliance is required. Due to short lead
times (manufacture to installation), investments are incurred only
when the capacity is needed. Because of the plant’s high
efficiencies and reliability, particularly when operating at partial
load, operating costs are competitive. Siting and operating
flexibilities and benefits unique to fuel cells can lead to
additional, site-specific dollar savings.
With their minimal environmental impact, competitive costs, and
unsurpassed operational benefits and flexibility, fuel cells are
truly a winning combination.
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For information on the FCCG
commercialization program, contact:
Fuel Cell Commercialization
1800 M Street, N.W., Suite 300
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H2 Solutions, Inc.