FUEL CELLS

A fuel cell uses the chemical energy of hydrogen or another fuel to cleanly and efficiently produce electricity. If hydrogen is the fuel, electricity, water, and heat are the only products. Fuel cells are unique in terms of the variety of their potential applications; they can provide power for systems as large as a utility power station and as small as a laptop computer.

WHY STUDY FUEL CELLS

Fuel cells can be used in a wide range of applications, including transportation, material handling, stationary, portable, and emergency backup power applications. Fuel cells have several benefits over conventional combustion-based technologies currently used in many power plants and passenger vehicles. Fuel cells can operate at higher efficiencies than combustion engines, and can convert the chemical energy in the fuel to electrical energy with efficiencies of up to 60%. Fuel cells have lower emissions than combustion engines. Hydrogen fuel cells emit only water, so there are no carbon dioxide emissions and no air pollutants that create smog and cause health problems at the point of operation. Also, fuel cells are quiet during operation as they have fewer moving parts.

HOW FUEL CELLS WORK

Fuel cells work like batteries, but they do not run down or need recharging. They produce electricity and heat as long as fuel is supplied. A fuel cell consists of two electrodes—a negative electrode (or anode) and a positive electrode (or cathode)—sandwiched around an electrolyte. A fuel, such as hydrogen, is fed to the anode, and air is fed to the cathode. In a hydrogen fuel cell, a catalyst at the anode separates hydrogen molecules into protons and electrons, which take different paths to the cathode. The electrons go through an external circuit, creating a flow of electricity. The protons migrate through the electrolyte to the cathode, where they unite with oxygen and the electrons to produce water and heat. Learn more about:

Parts of a fuel cell

Polymer electrolyte membrane (PEM) fuel cells are the current focus of research for fuel cell vehicle applications. PEM fuel cells are made from several layers of different materials. The main parts of a PEM fuel cell are described below.

The heart of a PEM fuel cell is the membrane electrode assembly (MEA), which includes the membrane, the catalyst layers, and gas diffusion layers (GDLs). Hardware components used to incorporate an MEA into a fuel cell include gaskets, which provide a seal around the MEA to prevent leakage of gases, and bipolar plates, which are used to assemble individual PEM fuel cells into a fuel cell stack and provide channels for the gaseous fuel and air.

MEMBRANE ELECTRODE ASSEMBLY

The membrane, catalyst layers (anode and cathode), and diffusion media together form the membrane electrode assembly (MEA) of a PEM fuel cell.

Polymer electrolyte membrane. The polymer electrolyte membrane, or PEM (also called a proton exchange membrane)—a specially treated material that looks something like ordinary kitchen plastic wrap—conducts only positively charged ions and blocks the electrons. The PEM is the key to the fuel cell technology; it must permit only the necessary ions to pass between the anode and cathode. Other substances passing through the electrolyte would disrupt the chemical reaction. For transportation applications, the membrane is very thin—in some cases under 20 microns.

Catalyst layers. A layer of catalyst is added on both sides of the membrane—the anode layer on one side and the cathode layer on the other. Conventional catalyst layers include nanometer-sized particles of platinum dispersed on a high-surface-area carbon support. This supported platinum catalyst is mixed with an ion-conducting polymer (ionomer) and sandwiched between the membrane and the GDLs. On the anode side, the platinum catalyst enables hydrogen molecules to be split into protons and electrons. On the cathode side, the platinum catalyst enables oxygen reduction by reacting with the protons generated by the anode, producing water. The ionomer mixed into the catalyst layers allows the protons to travel through these layers.

Gas diffusion layers. The GDLs sit outside the catalyst layers and facilitate transport of reactants into the catalyst layer, as well as removal of product water. Each GDL is typically composed of a sheet of carbon paper in which the carbon fibers are partially coated with polytetrafluoroethylene (PTFE). Gases diffuse rapidly through the pores in the GDL. These pores are kept open by the hydrophobic PTFE, which

prevents excessive water buildup. In many cases, the inner surface of the GDL is coated with a thin layer of high-surface-area carbon mixed with PTFE, called the microporous layer. The microporous layer can help adjust the balance between water retention (needed to maintain membrane conductivity) and water release (needed to keep the pores open so hydrogen and oxygen can diffuse into the electrodes).

HARDWARE

The MEA is the part of the fuel cell where power is produced, but hardware components are required to enable effective MEA operation.

Bipolar plates. Each individual MEA produces less than 1 V under typical operating conditions, but most applications require higher voltages. Therefore, multiple MEAs are usually connected in series by stacking them on top of each other to provide a usable output voltage. Each cell in the stack is sandwiched between two bipolar plates to separate it from neighboring cells. These plates, which may be made of metal, carbon, or composites, provide electrical conduction between cells, as well as providing physical strength to the stack. The surfaces of the plates typically contain a “flow field,” which is a set of channels machined or stamped into the plate to allow gases to flow over the MEA. Additional channels inside each plate may be used to circulate a liquid coolant.

Gaskets. Each MEA in a fuel cell stack is sandwiched between two bipolar plates, but gaskets must be added around the edges of the MEA to make a gas-tight seal. These gaskets are usually made of a rubbery polymer.

Fuel cell systems

The design of fuel cell systems is complex, and can vary significantly depending upon fuel cell type and application. However, several basic components are found in many fuel cell systems:

Fuel cell stack Fuel processor Power conditioners Air compressors Humidifiers

FUEL CELL STACK

The fuel cell stack is the heart of a fuel cell power system. It generates electricity in the form of direct current (DC) from electro-chemical reactions that take place in the fuel cell. A single fuel cell produces less than 1 V, which is insufficient for most applications. Therefore, individual fuel cells are typically combined in series into a fuel cell stack. A typical fuel cell stack may consist of hundreds of fuel cells. The amount of power produced by a fuel cell depends upon several factors, such as fuel cell type, cell size, the temperature at which it operates, and the pressure of the gases supplied to the cell. Learn more about the parts of a fuel cell.

FUEL PROCESSOR

The fuel processor converts fuel into a form usable by the fuel cell. Depending on the fuel and type of fuel cell, the fuel processor can be a simple sorbent bed to remove impurities, or a combination of multiple reactors and sorbents.

If the system is powered by a hydrogen-rich, conventional fuel, such as methanol, gasoline, diesel, or gasified coal, a reformer is typically used to convert hydrocarbons into a gas mixture of hydrogen and carbon compounds called "reformate." In many cases, the reformate is then sent to a set of reactors to convert carbon monoxide to carbon dioxide and remove any trace amounts of carbon monoxide remaining and a sorbent bed to remove other impurities, such as sulfur compounds, before it is sent to the fuel cell stack. This process prevents impurities in the gas from binding with the fuel cell catalysts. This binding process is also called "poisoning" because it reduces the efficiency and life expectancy of the fuel cell.

Some fuel cells, such as molten carbonate and solid oxide fuel cells, operate at temperatures high enough that the fuel can be reformed in the fuel cell itself. This process is called internal reforming. Fuel cells that use internal reforming still need traps to remove impurities from the unreformed fuel before it reaches the fuel cell. Both internal and external reforming release carbon dioxide, but due to the fuel cells’ high efficiency, less carbon dioxide is emitted than by internal-combustion engines, such as those used in gasoline-powered vehicles.

POWER CONDITIONERS

Power conditioning includes controlling current (amperes), voltage, frequency, and other characteristics of the electrical current to meet the needs of the application. Fuel cells produce electricity in the form of direct

current (DC). In a DC circuit, electrons flow in only one direction. The electricity in your home and workplace is in the form of alternating current (AC), which flows in both directions on alternating cycles. If the fuel cell is used to power equipment that uses AC, the direct current will have to be converted to alternating current.

Both AC and DC power must be conditioned. Current inverters and conditioners adapt the electrical current from the fuel cell to suit the electrical needs of the application, whether it is a simple electrical motor or a complex utility power grid. Conversion and conditioning reduce system efficiency only slightly, around 2%–6%.

AIR COMPRESSORS

Fuel cell performance improves as the pressure of the reactant gases increases; therefore many fuel cell systems include an air compressor, which raises the pressure of the inlet air to 2–4 times the ambient atmospheric pressure. For transportation applications, air compressors should have an efficiency of at least 75%. In some cases, an expander is also included to recover power from the high pressure exhaust gases. Expander efficiency should be at least 80%.

HUMIDIFIERS

The polymer electrolyte membrane at the heart of a PEM fuel cell does not work well when dry, so many fuel cell systems include a humidifier for the inlet air. Humidifiers usually consist of a thin membrane, which may be made of the same material as the PEM. By flowing dry inlet air on one side of the humidifier and wet exhaust air on the other side, the water produced by the fuel cell may be recycled to keep the PEM well hydrated.

Types of fuel cells.

Fuel cells are classified primarily by the kind of electrolyte they employ. This classification determines the kind of electro-chemical reactions that take place in the cell, the kind of catalysts required, the temperature range in which the cell operates, the fuel required, and other factors. These characteristics, in turn, affect the applications for which these cells are most suitable. There are several types of fuel cells currently under development,

each with its own advantages, limitations, and potential applications. Learn more about the following types of fuel cells:

Polymer electrolyte membrane (PEM) fuel cells Direct methanol fuel cells Alkaline fuel cells Phosphoric acid fuel cells Molten carbonate fuel cells Solid oxide fuel cells Reversible fuel cells. See our comparison of fuel cell technologies to learn more about the advantages and disadvantages of each fuel cell type, or find information about fuel cell system R&D targets and status.

POLYMER ELECTROLYTE MEMBRANE FUEL CELLS

Polymer electrolyte membrane (PEM) fuel cells—also called proton exchange membrane fuel cells—deliver high power density and offer the advantages of low weight and volume compared with other fuel cells. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum or platinum alloy catalyst. They need only hydrogen, oxygen from the air, and water to operate. They are typically fueled with pure hydrogen supplied from storage tanks or reformers.

PEM fuel cells operate at relatively low temperatures, around 80°C (176°F). Low-temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. However, it requires that a noble-metal catalyst (typically platinum) be used to separate the hydrogen's electrons and protons, adding to system cost. The platinum catalyst is also extremely sensitive to carbon

monoxide poisoning, making it necessary to employ an additional reactor to reduce carbon monoxide in the fuel gas if the hydrogen is derived from a hydrocarbon fuel. This reactor also adds cost.

PEM fuel cells are used primarily for transportation applications and some stationary applications. Due to their fast startup time and favorable power-to-weight ratio, PEM fuel cells are particularly suitable for use in passenger vehicles, such as cars and buses.

DIRECT METHANOL FUEL CELLS

Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel cells (DMFCs), however, are powered by pure methanol, which is usually mixed with water and fed directly to the fuel cell anode.

Direct methanol fuel cells do not have many of the fuel storage problems typical of some fuel cell systems because methanol has a higher energy density than hydrogen—though less than gasoline or diesel fuel. Methanol is also easier to transport and supply to the public using our current infrastructure because it is a liquid, like gasoline. DMFCs are often used to provide power for portable fuel cell applications such as cell phones or laptop computers.

ALKALINE FUEL CELLS

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the U.S. space program to produce electrical energy and water on-board spacecraft. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100°C and 250°C (212°F and 482°F). However, newer AFC designs operate at lower temperatures of roughly 23°C to 70°C (74°F to 158°F). In recent years, novel AFCs that use a polymer membrane as the electrolyte have been developed. These fuel cells are closely related to conventional PEM fuel cells, except that they use an alkaline membrane instead of an acid membrane. The high performance of AFCs is due to the rate at which electro-chemical reactions take place in the cell. They have also demonstrated efficiencies above 60% in space applications. The disadvantage of this fuel cell type is that it is easily poisoned by carbon dioxide (CO2). In fact, even the small amount of CO2 in the air can affect this cell's operation, making it necessary to purify both the hydrogen and oxygen used in the cell. This purification process is costly. Susceptibility to poisoning also affects the cell's lifetime (the amount of time before it must be replaced), further adding to cost. Alkaline membrane cells have lower susceptibility to CO2 poisoning than liquid-electrolyte AFCs do, but performance still suffers as a result of CO2 that dissolves into the membrane.

Cost is less of a factor for remote locations, such as in space or under the sea. However, to compete effectively in most mainstream commercial markets, these fuel cells will have to become more cost-effective. To be economically viable in large-scale utility applications, AFCs need to reach operating times exceeding 40,000 hours, something that has not yet been

achieved due to material durability issues. This obstacle is possibly the most significant in commercializing this fuel cell technology.

PHOSPHORIC ACID FUEL CELLS

Phosphoric acid fuel cells (PAFCs) use liquid phosphoric acid as an electrolyte—the acid is contained in a Teflon-bonded silicon carbide matrix—and porous carbon electrodes containing a platinum catalyst. The electro-chemical reactions that take place in the cell are shown in the diagram to the right.

The PAFC is considered the "first generation" of modern fuel cells. It is one of the most mature cell types and the first to be used commercially. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses.

PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than PEM cells, which are easily "poisoned" by carbon monoxide because carbon monoxide binds to the platinum catalyst at the anode, decreasing the fuel cell's efficiency. PAFCs are more than 85% efficient when used for the co-generation of electricity and heat but they are less efficient at generating electricity alone (37%–42%). PAFC efficiency is only slightly more than that of combustion-based power plants, which typically operate at around 33% efficiency. PAFCs are also less powerful than other fuel cells, given the same weight and volume. As a result, these fuel cells are typically large and heavy. PAFCs are also expensive. They require much higher loadings of expensive platinum catalyst than other types of fuel cells do, which raises the cost.

MOLTEN CARBONATE FUEL CELLS

Molten carbonate fuel cell s (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide matrix. Because they operate at high temperatures of 650°C (roughly 1,200°F), non-precious metals can be used as catalysts at the anode and cathode, reducing costs.

Improved efficiency is another reason MCFCs offer significant cost reductions over phosphoric acid fuel cells. Molten carbonate fuel cells, when coupled with a turbine, can reach efficiencies approaching 65%, considerably higher than the 37%–42% efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be over 85%.

Unlike alkaline, phosphoric acid, and PEM fuel cells, MCFCs do not require an external reformer to convert fuels such as natural gas and biogas to hydrogen. At the high temperatures at which MCFCs operate, methane and other light hydrocarbons in these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces cost.

The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life.

Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that double cell life from the current 40,000 hours (~5 years) without decreasing performance.

SOLID OXIDE FUEL CELLS

Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte. SOFCs are around 60% efficient at converting fuel to electricity. In applications designed to capture and utilize the system's waste heat (co-generation), overall fuel use efficiencies could top 85%.

SOFCs operate at very high temperatures—as high as 1,000°C (1,830°F). High-temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system.

SOFCs are also the most sulfur-resistant fuel cell type; they can tolerate several orders of magnitude more sulfur than other cell types can. In addition, they are not poisoned by carbon monoxide, which can even be used as fuel. This property allows SOFCs to use natural gas, biogas, and gases made from coal. High-temperature operation has disadvantages. It results in a slow startup and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for utility applications but not for transportation. The high operating temperatures also place stringent durability requirements on materials. The development of low-cost materials with high durability at cell operating temperatures is the key technical challenge facing this technology.

Scientists are currently exploring the potential for developing lower-temperature SOFCs operating at or below 700°C that have fewer durability problems and cost less. Lower-temperature SOFCs have not yet matched the performance of the higher temperature systems, however, and stack materials that will function in this lower temperature range are still under development.

REVERSIBLE FUEL CELLS

Reversible fuel cells produce electricity from hydrogen and oxygen and generate heat and water as byproducts, just like other fuel cells. However, reversible fuel cell systems can also use electricity from solar power, wind power, or other sources to split water into oxygen and hydrogen fuel through a process called electrolysis. Reversible fuel cells can provide power when needed, but during times of high power production from other technologies (such as when high winds lead to an excess of available wind power), reversible fuel cells can store the excess energy in the form of hydrogen. This energy storage capability could be a key enabler for intermittent renewable energy technologies. View the Fuel Cell Technologies Office's fuel cell animation to see how a fuel cell operates.

RESEARCH AND DEVELOPMENT GOALS

The U.S. Department of Energy (DOE) is working closely with its national laboratories, universities, and industry partners to overcome critical technical barriers to fuel cell commercialization. Cost, performance, and durability are still key challenges in the fuel cell industry. View related links that provide details about DOE-funded fuel cell activities. Cost—Platinum represents one of the largest cost components of a fuel

cell, so much of the R&D focuses on approaches that will increase activity and utilization of current platinum group metal (PGM) and PGM-alloy catalysts, as well as non-PGM catalyst approaches for long-term applications.

Performance—To improve fuel cell performance, R&D focuses on developing ion-exchange membrane electrolytes with enhanced efficiency and durability at reduced cost; improving membrane electrode assemblies (MEAs) through integration of state-of-the-art MEA components; developing transport models and in-situ and ex-situ experiments to provide data for model validation; identifying degradation mechanisms

and developing approaches to mitigate their effects; and maintaining core activities on components, sub-systems, and systems specifically tailored for stationary and portable power applications.

Durability—A key performance factor is durability, in terms of a fuel cell system lifetime that will meet application expectations. DOE durability targets for stationary and transportation fuel cells are 40,000 hours and 5,000 hours, respectively, under realistic operating conditions. In the most demanding applications, realistic operating conditions include impurities in the fuel and air, starting and stopping, freezing and thawing, and humidity and load cycles that result in stresses on the chemical and mechanical stability of the fuel cell system materials and components. R&D focuses on understanding the fuel cell degradation mechanisms and developing materials and strategies that will mitigate them.

Polarography

Polarography, also called polarographic analysis, or voltammetry, in

analytic chemistry, an electrochemical method of analyzing solutions of

reducible or oxidizable substances. It was invented by a Czech

chemist, Jaroslav Heyrovský, in 1922.

In general, polarography is a technique in which the electric potential (or

voltage) is varied in a regular manner between two sets of electrodes

(indicator and reference) while the current is monitored. The shape of a

polarogram depends on the method of analysis selected, the type of

indicator electrode used, and the potential ramp that is applied. The Figure

shows five selected methods of polarography; the potential ramps are

applied to a mercury indicator electrode, and the shapes of the resulting

polarograms are compared.

The majority of the chemical elements can be identified by polarographic

analysis, and the method is applicable to the analysis of alloys and to

various inorganic compounds. Polarography is also used to identify

numerous types of organic compounds and to study chemical equilibria and

rates of reactions in solutions.

The solution to be analyzed is placed in a glass cell containing two

electrodes. One electrode consists of a glass capillary tube from which

mercury slowly flows in drops, and the other is commonly a pool of mercury.

The cell is connected in series with a galvanometer (for measuring the flow

of current) in an electrical circuit that contains a battery or other source of

direct current and a device for varying the voltage applied to the electrodes

from zero up to about two volts. With the dropping mercury electrode

connected (usually) to the negative side of the polarizing voltage, the

voltage is increased by small increments, and the corresponding current is

observed on the galvanometer. The current is very small until the applied

voltage is increased to a value large enough to cause the substance being

determined to be reduced at the dropping mercury electrode. The current

increases rapidly at first as the applied voltage is increased above this

critical value but gradually attains a limiting value and remains more or less

constant as the voltage is increased further. The critical voltage required to

cause the rapid increase in current is characteristic of, and also serves to

identify, the substance that is being reduced (qualitative analysis). Under

proper conditions the constant limiting current is governed by the rates of

diffusion of the reducible substance up to the surface of the mercury drops,

and its magnitude constitutes a measure of the concentration of the

reducible substance(quantitative analysis). Limiting currents also result from

the oxidation of certain oxidizable substances when the dropping electrode

is the anode.

When the solution contains several substances that are reduced or oxidized

at different voltages, the current-voltage curve shows a separate current

increase (polarographic wave) and limiting current for each. The method is

thus useful in detecting and determining several substances simultaneously

and is applicable to relatively small concentrations—e.g., 10−6

up to about

0.01 mole per litre, or approximately 1 to 1,000 parts per 1,000,000.

cold fusion

Not to be confused with ColdFusion , a software product, cold fusion is a hypothetical process in which hydrogen fusion supposedly occurs at room temperature. The topic is controversial, because the notion appears to defy the laws of physics. Some scientists believe that cold fusion represents a real phenomenon and that it will someday form the basis for an abundant, cheap source of energy. Others maintain that cold fusion, like perpetual motion, is impossible.

Hydrogen fusion as it is currently known is the process responsible for the energy output of the sun and most other stars. It does not ordinarily take place unless there is extreme heat (millions of degrees Celsius ) and extreme pressure. The only officially documented examples of human-generated fusion involve the explosions of hydrogen bombs. In the hydrogen fusion process, the nuclei of hydrogen atom s are driven together to form helium nuclei. It takes four hydrogen nuclei to ultimately produce a single helium nucleus . Energy, and certain subatomic particles, are emitted as byproducts.

After the first hydrogen bombs were successfully tested, scientists and engineers began searching for a way to control hydrogen fusion reactions and harness the energy in a constructive manner. Hydrogen fusion generates no dangerous nuclear waste, is far more efficient than the fission processes currently used in nuclear reactors, and has as its basis the most abundant element in the universe (hydrogen). In 1989, Stanley Pons and Martin Fleischmann of the University of Utah claimed to have produced hydrogen fusion in a controlled experiment at room temperature. The news created a stir among scientists, engineers, government agencies, and the public. It also caused a controversy among physicists that has been going on ever since.

The cold-fusion experiments conducted by Pons and Fleischmann involved deuterium, anisotope of helium in which the nucleus contains a neutron as well as a proton . (Ordinary hydrogen has a nucleus consisting of a single proton only.) The deuterium was packed into electrodes made of a metallic element known as palladium. Under certain conditions, it appeared that energy was produced along with helium nuclei at room temperature, in the same way, and according to the same mathematical formulae, as observed in hydrogen fusion at high temperatures. But these results have proven difficult to reproduce. Even in apparently successful cold-fusion experiments, no one has yet harnessed the energy and thereby built a functional reactor.

Cold fusion back on the menu

Most chemists would rather forget all about cold fusion. After the barrage of criticism dismissing Stanley Pons and Martin Fleischmann's sensational 1989 claims that nuclei could be forced to fuse and release excess energy

at room temperature, only a small core of researchers has kept the idea from fading away entirely.

Yet as Chemistry World went to press, preparations were under way for an invited symposium focusing on cold fusion and low-energy nuclear reactions at the American chemical society's (ACS) 2007 March conference in Chicago. Isolated presentations have been scattered around ACS meetings before, and the American physical society (APS) groups together a number of cold fusion researchers every year, but the last comparable session was 'so far off I can't remember', according to cold fusion advocate George Miley, of the University of Illinois, US. Even Fleischmann himself has a paper at the ACS, though the eighty-year old chemist will not be attending. 'I feel there is a strong rebirth of interest in cold fusion,' said Miley. He and other cold fusion supporters are taking their ACS presence as one more indication of the subject's growing respectability. Organiser Jan Marwan said he was very surprised at how easy it was to gain acceptance for the symposium. But Gopal Coimbatore, program chair of the ACS's division of environmental chemistry, felt that unless a forum was provided, the subject might never get discussed; and 'with the world facing an energy crisis, it is worth exploring all possibilities'.

The chances of cold fusion meeting that crisis may seem remote, but enthusiasts point to recent research from the US navy's Space and naval warfare systems center (Spawar) in San Diego, California. Here, Stanislaw Szpak and Pamela Mosier-Boss have claimed a ream of evidence for nuclear reactions occurring in a system similar to the 1989 reports.

Pons and Fleischmann suggested that electrolysis could pack deuterium nuclei into a palladium lattice so tightly that they were fusing together; Szpak and Boss now claim to have speeded up this process by co-depositing palladium and deuterium onto a thin wire subjected to an electric field. They have used plastic films - so-called CR-39 detectors - to track charged particles emerging from their reactions, publishing most recently in Naturwissenschaften

1. And, unlike the original 1989 experiments, the

researchers claim their results are easily reproducible, with other groups reportedly detecting products of nuclear reactions such as alpha particles and gamma rays.

Acceptance by the scientific community is still the main target for cold fusion advocates - hence the importance of replication, appearing at major

conferences, and publishing in peer reviewed journals. In this at least, success seems imminent: Miley says his cold fusion paper is the first to be accepted to the Journal of Fusion Energy, which normally covers 'hot' thermonuclear fusion or sonofusion (which uses pulses of sound to rapidly compress bubbles in liquids). Meanwhile, Scott Chubb, who chaired a cold fusion session at an APS meeting in March, feels that Physical Review Letters, one of the top physics journals, may finally start accepting papers in the field. Avowed critics of cold fusion don't see anything to shout about, though. Frank Close, of the University of Oxford, UK, says he sees no renewed interest, 'just the usual suspects recycling'. Indeed, Fleischmann's ACS report is a re-presentation of research from the 1990s, showing that his calorimetry measurements were accurate. Bob Park, at the University of Maryland, US, agrees, but concedes that 'there are some curious reports - not cold fusion, but people may be seeing some unexpected low-energy nuclear reactions'.

But will the flare-up of cold fusion excitement last? Chubb is sure of it, but Fleischmann himself is less bullish. He approves of the Spawar research, but, as he told Chemistry World, 'my optimism is tempered by realism'. And Close's opinion is clear: 'Let's not confuse noise with signal'.