Fuel cell

Fuel cells are electrochemical devices that convert the chemical energy of a fuel directly into electrical energy through controlled redox reactions. Unlike batteries, which store finite chemical energy internally, fuel cells require a continuous supply of fuel—commonly hydrogen—and an oxidising agent, typically oxygen from the air. As long as these reactants are constantly delivered, fuel cells can generate electricity, heat and small quantities of by-products such as water vapour.

Principles of Operation

All fuel cells consist of three essential components: an anode, an electrolyte and a cathode. Their operation involves two half-reactions occurring at the electrode–electrolyte interfaces.
At the anode, a catalyst promotes oxidation of the fuel. In hydrogen fuel cells, the hydrogen molecules split into positively charged protons and negatively charged electrons. The electrolyte allows the ions—usually protons—to pass, while acting as an electrical insulator to prevent electrons from crossing internally.
Electrons instead travel through an external circuit from anode to cathode, producing direct current electricity. At the cathode, another catalyst enables the reaction between oxygen, the incoming ions and electrons, resulting in the formation of water or other products depending on the fuel.
Individual cells typically yield a voltage of about 0.7 volts under load; therefore, they are arranged in series or parallel combinations—forming a fuel cell stack—to attain the required power output.

Historical Development

Early concepts of fuel cells originated in the nineteenth century.

• In 1838, Sir William Robert Grove demonstrated the first crude fuel cell using iron, copper and porcelain plates with copper sulphate and dilute acid electrolytes.
• In 1839, Christian Friedrich Schönbein described a related hydrogen–oxygen system.
• In 1842, Grove published a more refined design resembling later phosphoric acid fuel cells.

The twentieth century witnessed major advances:

• In 1932, Francis Thomas Bacon developed a practical 5 kW hydrogen–oxygen alkaline fuel cell.
• By the 1950s, W. Thomas Grubb and Leonard Niedrach at General Electric improved membrane-based systems using sulphonated polystyrene and platinum catalysts, creating the Grubb–Niedrach cell.
• In 1959, fuel cells powered an experimental Allis-Chalmers tractor and a welding unit.
• NASA adopted the alkaline fuel cell during Project Gemini and later missions, using it both for electricity generation and water production.
• In the 1960s, Pratt & Whitney licensed Bacon’s patents for space applications.
• Large stationary cogeneration systems were later commercialised for hospitals, universities and office buildings.
• In 2015, the United States Senate designated 8 October as National Hydrogen and Fuel Cell Day, coinciding with hydrogen’s atomic weight (1.008).

Fuel cell technology has since expanded into transport, industrial power, remote energy systems and residential applications.

Types of Fuel Cells

Fuel cells are primarily classified by their electrolyte, which determines operating temperature, efficiency and suitable applications.
Proton-exchange membrane fuel cells (PEMFCs)PEMFCs use a polymer electrolyte membrane—commonly Nafion—that conducts protons.

• They operate at relatively low temperatures (around 70–100°C).
• Hydrogen diffuses to the anode, where it splits into protons and electrons.
• The membrane allows protons to reach the cathode, while electrons generate an external electric current.
• Oxygen at the cathode combines with electrons and protons to form water.
• PEMFCs are noted for fast start-up times (approximately one second), low emissions and suitability for vehicles and small-scale power.

Alkaline fuel cells (AFCs)AFCs employ an alkaline electrolyte such as potassium hydroxide and achieve high efficiency. Historically important in space missions, they demonstrate rapid response and reliable water production.
Phosphoric acid fuel cells (PAFCs)These use liquid phosphoric acid as an electrolyte, operate at about 150–200°C and are used in stationary cogeneration systems.
Solid oxide fuel cells (SOFCs)SOFCs use ceramic electrolytes that conduct oxide ions, enabling operation at high temperatures (600–1000°C). Their high operating temperature allows internal reforming of hydrocarbon fuels and increases tolerance to fuel impurities. However, they feature longer start-up times—up to ten minutes.
Molten carbonate fuel cells (MCFCs)MCFCs employ molten carbonate electrolytes at temperatures exceeding 600°C. They support hydrocarbon fuels and are suited to large-scale industrial power.
Other specialised forms include direct methanol fuel cells, regenerative fuel cells and systems incorporating flow battery concepts.

Efficiency and By-products

Fuel cells offer electrical efficiencies of around 40–60%. When their waste heat is captured for heating or industrial processes, combined heat and power configurations can reach efficiencies as high as 85%.Their emissions depend on fuel type: hydrogen-fed PEMFCs produce primarily water and heat, while hydrocarbon-fuelled cells may emit small quantities of carbon dioxide or nitrogen oxides. Because PEM membranes limit nitrogen crossover, PEMFCs typically generate fewer nitrogen oxide emissions than high-temperature systems such as SOFCs.

Design and Material Considerations

Fuel cell design incorporates several engineered components:

• Electrolytes may be ceramics, polymer membranes, molten salts or liquid acids.
• Catalysts at the anode—often finely dispersed platinum—facilitate oxidation of the fuel.
• Cathode catalysts, frequently nickel-based at higher temperatures, accelerate oxygen reduction.
• Gas diffusion layers regulate gas flow and resist oxidation.
• Bipolar plates provide electrical conduction and control gas distribution through channel structures.

Voltage losses occur through resistance in cell materials (ohmic losses), electrode reaction limitations (activation losses) and reduced reactant availability at high loads (mass transport losses).
Fuel cells can be engineered in various configurations, adjusting membrane surface area, stacking arrangements and flow designs to meet required power levels.

Applications and Related Technologies

Fuel cells support a diverse range of uses:

• Transport, including cars, buses, trains, boats, forklifts and submarines.
• Stationary power, supplying hospitals, universities and industrial sites.
• Remote or off-grid systems, where reliability and low maintenance are essential.
• Space exploration, where fuel cells provide both electricity and potable water.