A fuel cell is a device that changes the chemical energy from a fuel, such as hydrogen, and an oxidizing agent, like oxygen, into electricity using chemical reactions. Unlike batteries, which use stored chemicals to create energy, fuel cells need a constant supply of fuel and oxygen to keep working. As long as fuel and oxygen are provided, fuel cells can generate electricity continuously.

The first fuel cell was created by Sir William Grove in 1838. The first time fuel cells were used for commercial purposes happened nearly 100 years later, in 1932, when Francis Thomas Bacon developed the hydrogen–oxygen fuel cell. The alkaline fuel cell, named after Bacon, has been used in NASA space programs since the 1960s to provide power for satellites and space capsules. Since then, fuel cells have been used in many ways, such as for backup power in buildings, in remote areas, and to power vehicles like cars, buses, trains, and submarines.

Fuel cells are made of three main parts: an anode, a cathode, and an electrolyte. The electrolyte allows ions, such as hydrogen ions (protons), to move between the anode and cathode. At the anode, a catalyst helps the fuel react, creating ions and electrons. The ions travel through the electrolyte to the cathode, while the electrons move through an external circuit, producing electricity. At the cathode, another catalyst helps ions, electrons, and oxygen combine to form water and other products. Fuel cells are grouped based on the type of electrolyte they use and how quickly they can start working, ranging from 1 second for proton-exchange membrane fuel cells (PEMFC) to 10 minutes for solid oxide fuel cells (SOFC). A related technology is flow batteries, where fuel can be reused after recharging. Each fuel cell produces about 0.7 volts, so multiple cells are connected in series to create enough power for a task. In addition to electricity, fuel cells also produce water vapor, heat, and small amounts of nitrogen dioxide, depending on the fuel. PEMFCs usually create less nitrogen dioxide than SOFCs because they operate at lower temperatures and use hydrogen fuel. Fuel cells typically have an energy efficiency of 40% to 60%, but with systems that capture waste heat, efficiency can reach up to 85%.

History

The first mention of hydrogen fuel cells was in 1838. In a letter dated October 1838, which was published in December 1838 in The London and Edinburgh Philosophical Magazine and Journal of Science, Sir William Grove, a Welsh physicist and lawyer, described his early fuel cell design. He used sheet iron, copper, and porcelain plates, along with a solution of copper sulfate and dilute acid. In another letter written in December 1838 but published in June 1839, German physicist Christian Friedrich Schönbein shared details about his own early fuel cell. His letter explained how electricity was produced when hydrogen and oxygen dissolved in water reacted. In 1842, Grove drew a sketch of his design in the same journal. His fuel cell used materials similar to those found in today’s phosphoric acid fuel cells.

In 1932, English engineer Francis Thomas Bacon created a 5 kW stationary fuel cell. NASA used the alkaline fuel cell (AFC), also called the Bacon fuel cell, starting in the mid-1960s.

In 1955, W. Thomas Grubb, a chemist at General Electric (GE), improved the original fuel cell design by using a sulphonated polystyrene ion-exchange membrane as the electrolyte. In 1958, another GE chemist, Leonard Niedrach, developed a method to coat the membrane with platinum, which helped speed up the chemical reactions needed for the fuel cell. This design became known as the "Grubb-Niedrach fuel cell." GE worked with NASA and McDonnell Aircraft to develop this technology, which was first used commercially during Project Gemini. In 1959, a team led by Harry Ihrig built a 15 kW fuel cell tractor for Allis-Chalmers, which was shown at state fairs across the United States. This system used potassium hydroxide as the electrolyte and compressed hydrogen and oxygen as the fuel. That same year, Bacon and his team demonstrated a practical 5 kW unit that could power a welding machine. In the 1960s, Pratt & Whitney licensed Bacon’s U.S. patents for use in the U.S. space program to generate electricity and drinking water, as hydrogen and oxygen were available from spacecraft tanks.

UTC Power became the first company to produce and sell a large, stationary fuel cell system for use in hospitals, universities, and large office buildings as a cogeneration power plant.

In recognition of the fuel cell industry and the United States’ role in its development, the U.S. Senate declared October 8, 2015, as National Hydrogen and Fuel Cell Day by passing S. RES 217. The date was chosen because it matches the atomic weight of hydrogen (1.008).

Types of fuel cells; design

Fuel cells come in many types, but they all work in a similar way. They have three main parts: the anode, the electrolyte, and the cathode. At the anode and cathode, chemical reactions happen. These reactions use fuel, create water or carbon dioxide, and produce electricity that can power devices, called the load.

At the anode, a catalyst breaks the fuel into positively charged ions and negatively charged electrons. The electrolyte is a material that lets ions pass through but stops electrons. The electrons move through a wire, creating an electric current. The ions travel through the electrolyte to the cathode. At the cathode, the ions join with electrons and react with another chemical, usually oxygen, to form water or carbon dioxide.

Fuel cells have several key parts:
– The electrolyte, which determines the type of fuel cell and can be made from materials like potassium hydroxide, salt carbonates, or phosphoric acid.
– The most common fuel is hydrogen.
– The anode catalyst, often made of platinum, splits the fuel into ions and electrons.
– The cathode catalyst, usually nickel, helps create waste products like water.
– Gas diffusion layers that help prevent chemical reactions that cause damage.

A typical fuel cell produces about 0.6 to 0.7 volts of electricity. As more current is needed, the voltage decreases because of:
– Activation loss
– Ohmic loss (voltage drop from resistance in parts of the cell)
– Mass transport loss (reactants running out quickly at high loads)

To provide more energy, fuel cells can be connected in series to increase voltage or in parallel to increase current. This setup is called a fuel cell stack. Increasing the surface area of the cell also allows more current to be produced.

In a hydrogen-oxide proton-exchange membrane fuel cell (PEMFC), a special polymer membrane (like nafion) holds the electrolyte between the anode and cathode. This type was once called a solid polymer electrolyte fuel cell (SPEFC) before scientists fully understood how protons move through the membrane.

At the anode, hydrogen splits into protons and electrons. The protons move through the membrane to the cathode, while electrons travel through an external circuit to create electricity. At the cathode, oxygen reacts with protons and electrons to form water.

Other fuels used in fuel cells include diesel, methanol, and chemical hydrides. These produce carbon dioxide and water as waste. When hydrogen is made from natural gas using steam methane reforming, the process happens separately from the fuel cell, allowing hydrogen to be used indoors, like in forklifts.

A PEMFC has these parts:
1. Bipolar plates
2. Electrodes
3. Catalyst
4. Membrane
5. Hardware like current collectors and gaskets

Materials vary by part. Bipolar plates can be made of metal, graphite, or composites. The membrane electrode assembly (MEA) is the core of the PEMFC and includes a proton-exchange membrane between two catalyst-coated carbon papers. Platinum is often used as a catalyst but can be damaged by carbon monoxide, requiring pure hydrogen.

Phosphoric acid fuel cells (PAFCs) were first made in 1961. They use phosphoric acid as the electrolyte and work at high temperatures (150–200°C). This heat can be used to make steam, improving efficiency to about 80% when combined with other systems. Platinum is used as a catalyst, but the acidic electrolyte can damage parts of the cell.

Solid acid fuel cells (SAFCs) use a solid acid as the electrolyte. At higher temperatures, the acid becomes more conductive. These cells use cesium dihydrogen phosphate and can operate for thousands of hours.

Alkaline fuel cells (AFCs) were first tested in 1959 and used in the Apollo space program. They use a potassium hydroxide or sodium hydroxide solution as the electrolyte and produce water by combining hydrogen and oxygen. These cells work best at 70–140°C and can generate about 0.9 volts. A newer version, the alkaline anion exchange membrane fuel cell (AAEMFC), uses a solid polymer instead of liquid electrolyte.

Solid oxide fuel cells (SOFCs) use a ceramic material called yttria-stabilized zirconia as the electrolyte. They work at very high temperatures (800–1000°C) and can use fuels like natural gas. Unlike other fuel cells, SOFCs move oxygen ions from the cathode to the anode instead of protons.

Efficiency of leading fuel cell types

The energy efficiency of a system or device that changes energy is calculated by dividing the useful energy it produces ("output energy") by the total energy it uses ("input energy") or by expressing the useful output as a percentage of the total input. For fuel cells, useful output energy is the electrical energy they produce. Input energy is the energy stored in the fuel. According to the U.S. Department of Energy, fuel cells are usually between 40% and 60% efficient. This is more efficient than some other energy systems, such as the internal combustion engine in a car, which is about 43% efficient. Steam power plants typically have efficiencies of 30% to 40%, while combined cycle gas turbine and steam plants can reach efficiencies above 60%. In combined heat and power (CHP) systems, waste heat from the main energy process—whether from a fuel cell, nuclear fission, or combustion—is captured and used, increasing the system’s efficiency to up to 85% to 90%.

The theoretical maximum efficiency of any power generation system is never fully achieved in practice. This calculation does not include other steps in energy production, such as fuel production, transportation, storage, or converting electricity into mechanical power. However, this method helps compare different types of power generation. The theoretical maximum efficiency of a fuel cell can get very close to 100%, while the theoretical maximum for internal combustion engines is about 58%.

Fuel cell efficiency varies by type. Acidic fuel cells are about 40% efficient, molten carbonate fuel cells are about 50% efficient, and alkaline, solid oxide, and PEM fuel cells are about 60% efficient.

Fuel cells cannot store energy like batteries, except as hydrogen. However, in some cases, such as stand-alone power plants using renewable energy like solar or wind, fuel cells are paired with electrolyzers and storage systems to create an energy storage system. As of 2019, 90% of hydrogen was used in oil refining, chemicals, and fertilizer production (which requires hydrogen for the Haber–Bosch process). By 2024, over 95% of hydrogen was still produced using steam methane reformation, a process that releases carbon dioxide. About 95% of this hydrogen is "grey hydrogen," most of the rest is "blue hydrogen," and only about 1% is "green hydrogen." The overall efficiency of converting electricity to hydrogen and back to electricity in such systems (called "round-trip efficiency") can range from 35% to 50%, depending on conditions. Electrolyzer/fuel cell systems can store large amounts of hydrogen, making them suitable for long-term energy storage.

Solid-oxide fuel cells produce heat when oxygen and hydrogen recombine. These cells can reach temperatures as high as 800°C (1,470°F). This heat can be used to warm water in a micro combined heat and power (m-CHP) system. When heat is captured, the total efficiency can reach 80% to 90% at the unit, but this does not include losses from production and distribution. CHP systems are being developed for home use in Europe.

Professor Jeremy P. Meyers wrote in the Electrochemical Society journal Interface in 2008, "Fuel cells are more efficient than combustion engines but less efficient than batteries, mainly because of inefficiencies in the oxygen reduction reaction (and the oxygen evolution reaction if hydrogen is made by splitting water). They work best when disconnected from the grid or when fuel is available continuously. For applications needing frequent starts, zero emissions, and where hydrogen is acceptable, a PEM fuel cell is becoming a better choice if replacing batteries is inconvenient." In 2013, military groups tested fuel cells to see if they could reduce the weight of batteries carried by soldiers.

In fuel cell vehicles, the "tank-to-wheel efficiency" is more than 45% at low power levels and averages about 36% when tested using the NEDC (New European Driving Cycle) method. A Diesel car has a NEDC efficiency of 22%. In 2008, Honda released a demonstration fuel cell car (the Honda FCX Clarity) that claimed a 60% tank-to-wheel efficiency.

It is important to consider losses from fuel production, transportation, and storage. Fuel cell vehicles using compressed hydrogen may have a "power-plant-to-wheel efficiency" of 22% if hydrogen is stored as high-pressure gas and 17% if stored as liquid hydrogen.

Applications

Stationary fuel cells are used to provide power for homes, businesses, and industries, both as main power sources and backup. These systems are especially helpful in remote areas, such as space stations, weather monitoring stations, large parks, communication centers, rural research areas, and some military operations. Fuel cells that use hydrogen can be small and light, with no large moving parts. Because they do not have moving parts and do not burn fuel, they can operate with very high reliability, up to 99.9999%, meaning less than one minute of power failure in six years.

Fuel cell systems do not store fuel inside themselves but instead use external storage. This makes them useful for large energy storage projects, including in rural areas. Different types of stationary fuel cells have varying efficiency levels, usually between 40% and 60%. However, when waste heat from fuel cells is used to heat buildings in a combined heat and power (CHP) system, efficiency can increase to 85%. This is much more efficient than traditional coal power plants, which are only about one-third efficient. If produced at scale, fuel cells used in CHP systems could save 20–40% on energy costs. Fuel cells also create less pollution than traditional power sources. For example, a fuel cell using natural gas would produce less than one ounce of pollution (other than carbon dioxide) for every 1,000 kilowatt-hours of energy, compared to 25 pounds of pollution from traditional systems. Fuel cells also produce 97% less nitrogen oxide emissions than coal-fired power plants.

A pilot project on Stuart Island, Washington, uses solar panels to power an electrolyzer that makes hydrogen. The hydrogen is stored in a tank and used by a fuel cell to provide backup power for a home not connected to the main electricity grid. Another similar system was built in Hempstead, New York, in 2011.

Fuel cells can use low-quality gas from landfills or wastewater treatment plants to generate electricity and reduce methane emissions. A 2.8-megawatt fuel cell plant in California is the largest of its kind. Small fuel cells are also being developed for use in homes without grid access.

Combined heat and power (CHP) systems, including microCHP systems, generate both electricity and heat for homes, offices, and factories. These systems produce electricity and use waste heat to generate hot air and water. This helps save energy because waste heat is often wasted in traditional systems. A typical home fuel cell system can produce 1–3 kilowatts of electricity and 4–8 kilowatts of heat. Some CHP systems use waste heat for cooling in summer or heating in winter. The University of Minnesota holds a patent for this type of system.

CHP systems can reach 85% efficiency, with 40–60% of energy used for electricity and the rest for heat. Phosphoric-acid fuel cells (PAFC) are the most common type of CHP system globally and can achieve nearly 90% combined efficiency. Molten carbonate (MCFC) and solid-oxide fuel cells (SOFC) also generate electricity and heat, with about 60% electrical efficiency. However, CHP systems have disadvantages, such as slow start and stop times, high costs, short lifetimes, and the need for hot water storage tanks, which take up space in homes.

In 2012, fuel cell microCHP systems outsold traditional systems globally. Japan’s ENE FARM project reported installing 34,213 proton-exchange membrane fuel cells (PEMFC) and 2,224 solid-oxide fuel cells (SOFC) between 2012 and 2014, along with 30,000 units using liquefied natural gas (LNG) and 6,000 using liquefied petroleum gas (LPG).

Four fuel cell electric vehicles are available for sale or lease: the Honda Clarity, Toyota Mirai, Hyundai ix35 FCEV, and Hyundai Nexo. By the end of 2019, about 18,000 of these vehicles had been sold or leased worldwide. These vehicles can travel about 505 kilometers (314 miles) between refuels and can be refueled in about 5 minutes. Fuel cell electric vehicles are 53–59% efficient at one-quarter power and 42–53% efficient at full power. They can last over 120,000 kilometers (75,000 miles) with less than 10% performance loss. A 2017 study found that fuel cell vehicles using hydrogen from natural gas could use 40% less energy and emit 45% fewer greenhouse gases than traditional gasoline cars.

Toyota introduced its first fuel cell vehicle, the Mirai, in 2015 at a cost of $57,000. Hyundai launched a limited number of ix35 FCEVs under a lease agreement. Honda began leasing the Clarity Fuel Cell in 2016. Hyundai replaced the ix35 FCEV with the Nexo in 2018. Toyota released an improved version of the Mirai in 2020.

In 2024, Mirai owners in California filed a lawsuit against Toyota, claiming the company hid the lack of hydrogen fuel availability and violated advertising laws. The same year, Hyundai recalled 1,600 Nexo vehicles sold in the U.S. due to a risk of fuel leaks and fires from a faulty pressure relief device.

Some experts believe hydrogen fuel cell vehicles will not become economically viable or may take decades to succeed. Elon Musk, CEO of Tesla, stated in 2015 that fuel cells for cars are not commercially practical due to the inefficiency of hydrogen production, transportation, and storage, as well as the gas’s flammability. A 2012 report by Lux Research said the "hydrogen economy" was not closer to reality and predicted limited adoption by 2030. Other analyses point to the lack of hydrogen infrastructure in the U.S. as a major challenge for fuel cell vehicle use.

In 2014, Joseph Romm, author of The Hype About Hydrogen, noted that fuel cell vehicles had not solved issues like high fueling costs, lack of infrastructure, and pollution from hydrogen production. He said overcoming these challenges would require "several miracles" over the next few decades. He also stated that renewable energy cannot be used economically in fuel cell systems.

Markets and economics

In 2012, fuel cell industry revenues surpassed $1 billion worldwide. Asian Pacific countries shipped more than three-fourths of all fuel cell systems globally. In 2010, 140,000 fuel cell stacks were shipped worldwide, an increase from 11,000 shipments in 2007. From 2011 to 2012, worldwide fuel cell shipments grew by 85% annually. In 2011, Tanaka Kikinzoku expanded its manufacturing facilities. Approximately 50% of fuel cell shipments in 2010 were stationary fuel cells, up from about one-third in 2009. The four leading producers in the fuel cell industry were the United States, Germany, Japan, and South Korea. According to the Department of Energy Solid State Energy Conversion Alliance, stationary fuel cells generated power at approximately $724 to $775 per kilowatt installed as of January 2011. In 2011, Bloom Energy, a major fuel cell supplier, reported that its fuel cells produced power at 9–11 cents per kilowatt-hour, including fuel, maintenance, and hardware costs. In 2016, Samsung decided to stop fuel cell-related business projects because the market outlook was not favorable.

Research and development

• 2013: A British company called ACAL Energy created a fuel cell that it claimed could operate for 10,000 hours under conditions that mimic real driving. It stated that the cost to build such fuel cells could be reduced to $40 per kilowatt (about $9,000 for 300 horsepower).
• 2014: Scientists at Imperial College London discovered a new way to restore the performance of Proton Exchange Fuel Cells (PEFCs) that had been damaged by hydrogen sulfide. They successfully recovered 95–100% of the original performance of these cells. They also restored PEFCs affected by sulfur dioxide. This method works for multiple fuel cell stacks.
• 2019: Researchers at the U.S. Army Research Laboratory designed a fuel cell with two parts. One part produces hydrogen, and the other generates electricity using an internal hydrogen/air power system.
• 2022: Scientists from the University of Delaware developed a hydrogen-powered fuel cell that is expected to cost less to produce. This design operates at roughly $1.4 per kilowatt and removes carbon dioxide from the air that enters hydroxide exchange membrane fuel cells.
• 2024: Researchers at KAIST created a method to make protonic fuel cells by layering slurries using a process called tape casting. The layers were then heated to bond them together. Three different slurries were mixed using a technique called Resonant Acoustic Mixing (RAM) before being applied.