Fuel Cells

The principle of the fuel cell (FC) is similar to that of the electrical storage battery. However, whereas the battery has a fixed stock of chemical reactants and can ‘‘run down,’’ the fuel cell is continuously supplied (from a separate tank) with a stream of oxidizer and fuel from which it generates electricity. Electrolysis—in which passage of an electrical current through water decomposes it into its constituents, H2 and O2—was still novel in 1839 when a young Welsh lawyer–scientist, William Grove, demonstrated that it could be made to run in reverse. That is, if the H2 and O2 were not driven off but rather allowed to recombine in the presence of the electrodes, the result was water— and electrical current.

The mechanism of electrical generation was not fully understood until development of atomic theory in the twentieth century. Like a battery, an FC has two electrodes connected via an electrical load and physically separated by an electrolyte—a substance that will selectively pass either positive ions (acidic electrolyte) or negative ions (alkaline electrolyte). Oxidizer (e.g., O2) enters at one electrode and fuel (e.g., H2) at the other. At the cathode (the electrically positive electrode) atoms of fuel occasionally ionize, forming positively charged ions and free electrons. This is accelerated by a catalyst at the electrodes as well as suitable conditions of temperature and pressure. Similarly at the negative electrode (anode), oxidizer atoms spontaneously form negative ions. Then, depending on electrolyte, either positive or negative ions migrate through it to combine with ions of opposite polarity, while electrons (unable to pass through the electrolyte) flow through the electrical load from anode to cathode.

While the Second Law of Thermodynamics severely limits the efficiency of heat engines operating at practical temperatures, fuel cells can extract more power out of the same quantity of fuel compared to traditional combustion, since the hydrogen fuel is not converted to thermal energy, but used directly to produce mechanical energy. In principle, an FC consuming pure hydrogen and oxygen and producing liquid water can achieve an efficiency of 94.5 percent with an open-circuit potential of 1.185 volts. In practice a variety of losses cannot altogether be eliminated, but achievable efficiencies are considerably greater than those of most heat engines. Multiple cells connected in series supply higher voltages.

Practical challenges include electrolyte–electrode chemistry and physical properties, catalysts, fuel and oxidizer composition and purity, internal electrical losses, corrosion and other destructive chemical reactions, and a host of mechanical issues. Efficiency, silence, and lack of polluting emissions stimulated great interest, however. In searching for suitable systems, twentieth century researchers developed various approaches, most named for their electrolytes. Most of the developments in the field have come as a result of proprietary interests and the work of corporate
teams of researchers.

Alkaline FCs (AFCs)
From the 1930s Francis Bacon of the U.K., followed later by U.S. researchers, turned from acidic electrolytes to more tractable potassium hydroxide (KOH). Bacon also pioneered use of porous electrodes through which gaseous reactants diffused. The first major application of FC technology was in the U.S. space program where AFCs provide power (and potable water) for the space shuttles. The need for extremely pure H2 and O2 made AFCs uneconomic for more mundane applications.
Molten carbonate FCs (MCFCs)
MCFCs grew out of SOFC research (see below). In the 1950s Dutch investigators G.H.J. Broers and J.A.A. Ketelaar turned to molten lithium-, sodium, and potassium carbonates as electrolytes, as did Bacon in the U.K. At typical 650_C operating temperatures, MCFCs produce waste heat in a form useful for industrial purposes or to power turbines for added electrical output. High temperatures relax the need for costly catalysts while their carbonate chemistry is tolerant of carbon monoxide (CO), which as an impurity is problematic for alkaline fuel cells. However chemical and mechanical problems have thus far impeded wide application.

Phosphoric Acid FCs (PAFCs)
Interest in phosphoric acid as an electrolyte emerged slowly until the mid-1960s, after which PAFCs rapidly became the first FCs to see significant commercialization. At around 200_C, PAFC waste heat may be used to reform (convert) hydrocarbons or coal to H2 for fuel or power an auxiliary turbine. PAFCs are relatively tolerant of CO but sulfur must be separated. Units up to 250 kilowatts output are sold for fixed-site power applications and experimental units have shown promise in buses. Problems center on internal chemical reactions and corrosion.

Proton-Exchange Membrane FCs (PEMFCs)
In the early 1960s Thomas Grubb and Leonard Niedrach of General Electric in the U.S. developed a polymer membrane which, moistened with water, served as an effective and stable electrolyte. Initial space application attempts in the 1960s revealed reliability problems (and led to adoption of AFCs) but further development has held out strong promise for ground and marine vehicle applications as well as small fixed generators. Operating temperatures of less than 100_C and high power relative to size and weight, suit PEMFCs especially. Platinum catalysts are necessary and the H2 fuel must be essentially free of CO. By century’s end PEMFCs had shown some promise of being adaptable to liquid methanol in place of H2 fuel.

Solid-Oxide FCs (SOFCs)
Beginning in the 1930s experimenters first in Switzerland and Russia sought high-temperature (around 1000_C) solid ceramic electrolytes. At such temperatures, reactions proceed rapidly without costly catalysts and many fuel stocks can be reformed to produce H2 within the SOFC, while the waste heat can be used for many purposes. But physical and chemical problems of high-temperature operation have been difficult and it remained uncertain at century’s end how well the promise of SOFCs could be realized.

Metal FCs (MFCs)
To avoid problems of H2 supply or conversion some FC developers turned, late in the century, to metal fuels, usually zinc or aluminum. Electrolytes include liquid KOH and proton-exchange membranes. Waste products are metal oxides.

Conclusion
Over the twentieth century FCs moved from laboratory curiosity to practical application in limited roles and quantities. It is very possible that the twenty-first century will see them assume a major or even dominant position as power sources in a broad array of applications. Obstacles are largely economic and the outcome will be influenced by success in development of competing systems as well as FCs themselves.

Related Video: How Fuel Cells Work