Fuel Cell Technologies

Summary

The devices known as fuel cells convert the chemical energy stored in fuel materials directly into electrical energy, bypassing the thermal-energy stage. Among the many technologies used to convert chemical energy to electrical energy, fuel cells are favored for their high efficiency and low emissions. Because of their high efficiency, fuel cells have found applications in spacecraft and show great potential as energy sources in generating stations. Modern research includes the development of microbial fuel cells (MFCs).

Definition and Basic Principles

Fuel cells provide a clean and versatile means to convert chemical energy to electricity. The reaction between a fuel and an oxidizer is what generates electricity. The reactants flow into the cell, and the products of that reaction flow out of it, leaving the electrolyte behind. They can operate continuously as long as the necessary reactant and oxidant flows are maintained. Fuel cells differ from electrochemical cell batteries because they use reactants from an external source that must be replenished. This is known as a thermodynamically open system. Batteries store electrical energy chemically and are considered a thermodynamically closed system. In general, fuel cells consist of three components—the anode, where oxidation of the fuel occurs; the electrolyte, which allows ions but not electrons to pass through; and the cathode, which consumes electrons from the anode.

A fuel cell does not produce heat as a primary energy conversion mode and is not considered a heat engine. Consequently, fuel cell efficiencies are not limited by the Carnot efficiency. They convert chemical energy to electrical energy essentially in an isothermal manner.

Fuel cells can be distinguished byreactant type (hydrogen, methane, carbon monoxide, methanol for a fuel and oxygen, air, or chlorine for an oxidizer); electrolyte type (liquid or solid); and working temperature (low temperature, below 120 degrees Celsius; intermediate temperature, 120 degrees to 300 degrees Celsius; or high temperature, more than 600 degrees Celsius).

Background and History

The first fuel cell was developed by the Welsh physicist and judge Sir William Robert Grove in 1839, but fuel cells did not receive serious attention until the early 1960s, when they were used to produce water and electricity for the Gemini and Apollo space programs. These were the first practical fuel cell applications developed by Pratt & Whitney. In 1989, Canadian geophysicist Geoffrey Ballard's Ballard Power Systems and Perry Oceanographics developed a submarine powered by a polymer electrolyte membrane or proton exchange membrane fuel cell (PEMFC). In 1993, Ballard developed a fuel-cell-powered bus and later a PEMFC-powered passenger car. Also in the late twentieth century, United Technologies (UTC) manufactured a large stationary fuel cell system for the cogeneration power plant while continuously developing the fuel cells for the US space program. UTC (later bought by ClearEdge Power) also worked on developing fuel cells for automobiles. Siemens Westinghouse successfully operated a 100-kilowatt (kW) cogeneration solid oxide fuel cell (SOFC) system, and in the 2020s greater than 4 megawatts (MW) systems have been developed.

How It Works

Polymer Electrolyte Membranes or Proton Exchange Membrane Fuel Cells (PEMFCs). PEMFCs use a proton-conductive polymer membrane as an electrolyte. At the anode, the hydrogen separates into protons and electrons, and only the protons pass through the proton exchange membrane. The excess of electrons on the anode creates a voltage difference that can work across an exterior load. At the cathode, electrons and protons are consumed and water is formed.

For PEMFC, the water management is critical to the fuel cell performance. Excess water at the positive electrode leads to flooding of the membrane. Dehydration of the membrane leads to the increase of ohmic resistance. In addition, the catalyst of the membrane is sensitive to carbon monoxide poisoning. In practice, pure hydrogen gas is not economical to mass produce. Thus, hydrogen gas is typically produced by steam reforming of hydrocarbons, which contains carbon monoxide.

Research into microbial fuel cells is exploring the potential of electrons resulting from biochemical reactions. One area in development is bioenergy generation via wastewater treatment processes.

Direct Methanol Fuel Cells (DMFCs). Like PEMFCs, DMFCs also use a proton exchange membrane. The main advantage of DMFCs is the use of liquid methanol, which is more convenient and less dangerous than gaseous hydrogen. The efficiency has been low for DMFCs, so they are used where the energy and power density are more important than efficiency, such as in portable electronic devices.

At the anode, methanol oxidation on a catalyst layer forms carbon dioxide. Protons pass through the proton exchange membrane to the cathode. Water is produced by the reaction between protons and oxygen at the cathode and is consumed at the anode. Electrons are transported through an external circuit from anode to cathode, providing power to connected devices.

Solid Oxide Fuel Cells (SOFCs). Unlike PEMFCs, SOFCs can use hydrocarbon fuels directly and do not require fuel preprocessing to generate hydrogen prior to utilization. Rather, hydrogen and carbon monoxide are generated in situ, either by partial oxidation or, more typically, by steam reforming of the hydrocarbon fuel in the anode chamber of the fuel cell. SOFCs are all-solid electrochemical devices. There is no liquid electrolyte with its attendant material corrosion and electrolyte management problems. The high operating temperature (typically 500–1,000 degrees Celsius) allows internal re-forming, promotes rapid kinetics with nonprecious materials, and yields high-quality byproduct heat for cogeneration. The total efficiency of a cogeneration system can be as high as 90 percent—far beyond the conventional power-production system—according to the European Commission.

The function of the fuel cell with oxides is based on the activity of oxide ions passing from the cathode region to the anode region, where they combine with hydrogen or hydrocarbons. The freed electrons flow through the external circuit. The ideal performance of an SOFC depends on the electrochemical reaction that occurs with different fuels and oxygen.

Molten Carbonate Fuel Cells (MCFCs). MCFCs use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic matrix. Like SOFCs, MCFCs do not require an external reformer to convert fuels to hydrogen. Because of the high operating temperatures, these fuels are converted to hydrogen within the fuel cell itself by an internal re-forming process.

MCFCs are also able to use carbon oxides as fuel. They are not poisoned by carbon monoxide or carbon dioxide, thus MCFCs are advanced to use gases from coal so that they can be integrated with coal gasification.

Applications and Products

Hydrogen Fuel Cell Vehicles. In the early twenty-first century, the automobile and energy industries gained interest in fuel cell-powered vehicles as an alternative to internal combustion engine vehicles fueled by petroleum-based liquid. Several automobile manufacturers, including Hyundai, Honda, and Toyota, began developing hydrogen fuel cell vehicles. Energy industries have also installed hydrogen fueling stations in some large cities. In the United States, most fueling stations are in California, where most hydrogen fuel cell vehicles are sold.

Public buses provide better demonstrations of hydrogen fuel cell vehicles than passenger vehicles since public buses are operated and maintained by professionals and have more volume for hydrogen fuel storage than passenger vehicles. Several bus manufacturers, such as Toyota, Man, and Daimler, developed hydrogen fuel cell buses. They have been in service in Palm Springs, California; Nagoya, Japan; Vancouver, Canada; and Stockholm, Sweden. Some cities have reversed course on acquiring hydrogen fuel cell buses. For example, Montpellier, France, canceled an order for more than fifty buses in 2022 after the city determined that operating electric buses would be far less expensive.

In the early 2010s, hydrogen-powered vehicles were appealing compared with electric vehicles because of their quick refueling time and long range. However, as electric vehicle technology evolved, this advantage deteriorated in the early 2020s. Hydrogen fuel cell vehicles face substantial challenges, such as high costs of novel metal catalysts, safety of hydrogen fuel, effective storage of hydrogen onboard, and infrastructure needed for public refueling stations. Additionally, the majority of hydrogen used is grey hydrogen, a major pollutant.

Stationary Power Plants and Hybrid Power Systems. Siemens Westinghouse and UTC produced several power plant units in the range of about 100 kW by using SOFCs, MCFCs, and phosphoric acid fuel cells (PAFCs). Approximately half of the power plants were MCFC-based plants. They showed that these fuel cell systems had exceeded the research-and-discovery level and already produced an economic benefit. These systems generate power with less fossil fuel and lower emissions of greenhouse gases and other harmful products. Just a small number of PEMFC-based power plants were built as the cost of fuel cell materials was prohibitive. In many cases, fuel-cell-based stationary power plants are used for heat supply and power production, enabling so-called combined heat and power systems. Such systems increase the total efficiency of the power plants and offer an economic benefit.

Many efforts to develop hybrid power plants combining fuel cells and gas turbines have been made. While the high-temperature fuel cells, such as SOFCs and MCFCs, produce electrical power, the gas turbines produce additional electrical power from the heat produced by the fuel cells' operation. At the same time, the gas turbines compress the air fed into the fuel cells. The expected overall efficiency for the direct conversion of chemical energy to electrical energy is up to 80 percent.

Small Power Generation for the Portable Electronic Devices. At the end of the twentieth century, the demand for electricity continued to increase in many applications, including portable electronics. Batteries have seen significant advances, but their power density is still far inferior to combustion devices. Typically, hydrocarbon fuels have 50 to 100 times more energy storage density than commercially available batteries. Even with low conversion efficiencies, fuel-driven generators will still have superior energy density. There has been considerable interest in miniaturizing thermochemical systems for electrical power generation for remote sensors, micro-robots, unmanned vehicles (UMVs), unmanned aerial vehicles (UAVs), and even portable electronic devices such as laptop computers and cell phones.

Much work on such systems has been developed by the military. The Defense Advanced Research Projects Agency (DARPA) has initiated and developed many portable power concepts using fuel cells. Industries such as Samsung, Sony, NEC, Toshiba, and Fujitsu have developed fuel cell–based portable power generation. Most devices were based on PEMFCs or DMFCs, which require lower operating temperatures than SOFCs. However, the development of SOFC-based portable power generation under the DARPA Microsystems Technology Office showed the feasibility of employing high-temperature fuel cells with appropriate thermal management.

The Military. In addition to the portable power generation for foot soldiers, the military market has been interested in developing medium-size power plants (a few hundred watts) for recharging various types of storage batteries and high stationary power plants (more than a few kW) for the auxiliary power units.

Military programs, in particular, have been interested in the direct use of logistic fuel (for example, Jet Propellant 8) for fuel cells because of the complexities and difficulties of the re-forming processes. While the new and improved reforming processes of logistic fuel were being developed to feed hydrogen into the fuel cells, direct jet-fuel SOFCs were also demonstrated by developing new anode materials with high resistance to coking and sulfur poisoning.

Careers and Course Work

Courses in chemistry, physics, electrochemistry, materials science, chemical engineering, and mechanical engineering make up the foundational requirements for students interested in pursuing careers in fuel cell research. Earning a bachelor of science degree in any of these fields would be appropriate preparation for graduate work in a similar area. In most circumstances, either a master's or doctorate is necessary for the most advanced career opportunities in both academia and industry.

Careers in the fuel cell field can take several different shapes. Fuel cell industries are the biggest employers of fuel cell engineers, who focus on developing and manufacturing new fuel cell units as well as maintaining or repairing fuel cell units. Fuel cell engineers often find work in other industries, including aviation, automotive, electronics, telecommunications, and education.

Many fuel cell engineers prefer employment within national laboratories and government agencies such as the Pacific Northwest National Laboratory, the National Renewable Energy Laboratory, the Argonne National Laboratory, the National Aeronautics and Space Administration (NASA), DOE, and DARPA. Others find work in academia. Such professionals divide their time between teaching university classes on fuel cells and conducting their own research.

Social Context and Future Prospects

In the future, it is unlikely that sustainable transportation will involve the use of conventional petroleum. Transportation energy technologies should be developed to provide an alternative to petroleum-based internal combustion engine vehicles. People evaluate vehicles not only based on fuel economy but also on performance. Vehicles using an alternative energy source should be designed with these parameters in mind.

One of the most promising energy sources for the future may be hydrogen. However, hydrogen fuel cell vehicles face cost and technical challenges, especially the fuel cell stack and onboard hydrogen storage. For fuel cell power plants, the economic and lifetime-related issues hinder the acceptance of fuel cell technologies. Such problems were not associated with fuel cells but with auxiliary fuel cell units such as thermal management, reactant storage, and water management. Therefore, the auxiliary units of fuel cell systems should be further developed to address these issues.

To improve performance, the fundamental problems of fuel cells related to electrocatalysis must be addressed as highly selective catalysts will provide better electrochemical reactions. Once new fuel cell technologies are successfully developed and meet safety requirements, the infrastructure to distribute and recycle fuel cells will also be necessary.

Other uses of hydrogen fuel cell technology in the twenty-first century include powering generative artificial intelligence technologies, boats and submarines, and portable and continuous power supplies. The manufacturing and distribution industries most heavily utilize fuel cell technology.

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