Hydrogen fuel cells. Fuel cells: a glimpse into the future Fuel cell installations

They operate the spacecraft of the US National Aeronautics and Space Administration (NASA). They provide power to the computers of the First National Bank in Omaha. They are used on some public city buses in Chicago.

These are all fuel cells. Fuel cells are electrochemical devices that produce electricity without combustion - chemically, in much the same way as batteries. The only difference is that they use different chemicals, hydrogen and oxygen, and the product of the chemical reaction is water. Natural gas can also be used, but when using hydrocarbon fuels, of course, a certain level of carbon dioxide emissions is inevitable.

Because fuel cells can operate with high efficiency and no harmful emissions, they hold great promise as a sustainable energy source that will help reduce emissions of greenhouse gases and other pollutants. The main obstacle to widespread use of fuel cells is their high cost compared to other devices that generate electricity or propel vehicles.

History of development

The first fuel cells were demonstrated by Sir William Groves in 1839. Groves showed that the process of electrolysis - the splitting of water into hydrogen and oxygen under the influence of an electric current - is reversible. That is, hydrogen and oxygen can be combined chemically to form electricity.

After this was demonstrated, many scientists rushed to study fuel cells with zeal, but the invention of the internal combustion engine and the development of oil reserve infrastructure in the second half of the nineteenth century left the development of fuel cells far behind. The development of fuel cells was further hampered by their high cost.

A surge in the development of fuel cells occurred in the 50s, when NASA turned to them in connection with the need for a compact electric generator for space flights. The investment was made and the Apollo and Gemini flights were powered by fuel cells. Spacecraft also run on fuel cells.

Fuel cells are still largely an experimental technology, but several companies are already selling them on the commercial market. In the last nearly ten years alone, significant advances have been made in commercial fuel cell technology.

How does a fuel cell work?

Fuel cells are similar to batteries - they produce electricity through a chemical reaction. In contrast, internal combustion engines burn fuel and thus produce heat, which is then converted into mechanical energy. Unless the heat from the exhaust gases is used in some way (for example, for heating or air conditioning), then the efficiency of the internal combustion engine can be said to be quite low. For example, the efficiency of fuel cells when used in a vehicle, a project currently under development, is expected to be more than twice the efficiency of today's typical gasoline engines used in automobiles.

Although both batteries and fuel cells produce electricity chemically, they perform two very different functions. Batteries are stored energy devices: the electricity they produce is the result of a chemical reaction of a substance that is already inside them. Fuel cells do not store energy, but rather convert some of the energy from externally supplied fuel into electricity. In this respect, a fuel cell is more like a conventional power plant.

There are several different types of fuel cells. The simplest fuel cell consists of a special membrane known as an electrolyte. Powdered electrodes are applied on both sides of the membrane. This design - an electrolyte surrounded by two electrodes - is a separate element. Hydrogen goes to one side (anode), and oxygen (air) to the other (cathode). Different chemical reactions occur at each electrode.

At the anode, hydrogen breaks down into a mixture of protons and electrons. In some fuel cells, the electrodes are surrounded by a catalyst, usually made of platinum or other noble metals, which promotes the dissociation reaction:

2H2 ==> 4H+ + 4e-.

H2 = diatomic hydrogen molecule, form, in

in which hydrogen is present in the form of a gas;

H+ = ionized hydrogen, i.e. proton;

e- = electron.

The operation of a fuel cell is based on the fact that the electrolyte allows protons to pass through it (towards the cathode), but electrons do not. Electrons move to the cathode along an external conductive circuit. This movement of electrons is an electrical current that can be used to drive an external device connected to the fuel cell, such as an electric motor or light bulb. This device is usually called a "load".

At the cathode side of the fuel cell, protons (that have passed through the electrolyte) and electrons (that have passed through the external load) are “recombined” and react with the oxygen supplied to the cathode to form water, H2O:

4H+ + 4e- + O2 ==> 2H2O.

The total reaction in a fuel cell is written as follows:

2H2 + O2 ==> 2H2O.

In their work, fuel cells use hydrogen fuel and oxygen from the air. Hydrogen can be supplied directly or by separating it from an external fuel source such as natural gas, gasoline or methanol. In the case of an external source, it must be chemically converted to extract the hydrogen. This process is called "reforming". Hydrogen can also be produced from ammonia, alternative resources such as gas from city landfills and wastewater treatment plants, and through water electrolysis, which uses electricity to break water into hydrogen and oxygen. Currently, most fuel cell technologies used in transportation use methanol.

Various means have been developed to reform fuels to produce hydrogen for fuel cells. The US Department of Energy has developed a fuel unit inside a gasoline reformer to supply hydrogen to a self-contained fuel cell. Researchers from the Pacific Northwest National Laboratory in the US have demonstrated a compact fuel reformer one-tenth the size of a power supply. American utility Northwest Power Systems and Sandia National Laboratories have demonstrated a fuel reformer that converts diesel fuel into hydrogen for fuel cells.

Individually, the fuel cells produce about 0.7-1.0V each. To increase the voltage, the elements are assembled into a “cascade”, i.e. serial connection. To create more current, sets of cascaded elements are connected in parallel. If you combine fuel cell cascades with a fuel system, an air supply and cooling system, and a control system, you get a fuel cell engine. This engine can power a vehicle, a stationary power plant, or a portable electric generator6. Fuel cell engines come in different sizes depending on the application, the type of fuel cell and the fuel used. For example, each of the four separate 200 kW stationary power plants installed at a bank in Omaha is approximately the size of a truck trailer.

Applications

Fuel cells can be used in both stationary and mobile devices. In response to tightening emissions regulations in the United States, automakers including DaimlerChrysler, Toyota, Ford, General Motors, Volkswagen, Honda and Nissan have begun experimenting with and demonstrating fuel cell-powered vehicles. The first commercial fuel cell vehicles are expected to hit the road in 2004 or 2005.

A major milestone in the development of fuel cell technology was the June 1993 demonstration of Ballard Power System's experimental 32-foot city bus powered by a 90-kilowatt hydrogen fuel cell engine. Since then, many different types and different generations of fuel cell passenger vehicles have been developed and put into service, running on different types of fuel. Since late 1996, three hydrogen fuel cell golf carts have been in use in Palm Desert, California. On the roads of Chicago, Illinois; Vancouver, British Columbia; and Oslo, Norway, city buses powered by fuel cells are being tested. Taxis powered by alkaline fuel cells are being tested on the streets of London.

Stationary installations using fuel cell technology are also being demonstrated, but they are not yet widely used commercially. First National Bank of Omaha in Nebraska uses a fuel cell system to power its computers because the system is more reliable than the old system, which ran off the main grid with backup battery power. The world's largest commercial fuel cell system, rated at 1.2 MW, will soon be installed at a mail processing center in Alaska. Fuel cell-powered portable laptop computers, control systems used in wastewater treatment plants and vending machines are also being tested and demonstrated.

"Pros and cons"

Fuel cells have a number of advantages. While modern internal combustion engines are only 12-15% efficient, fuel cells are 50% efficient. The efficiency of fuel cells can remain quite high even when they are not used at full rated power, which is a serious advantage compared to gasoline engines.

The modular design of fuel cells means that the power of a fuel cell power plant can be increased simply by adding more stages. This ensures that capacity underutilization is minimized, allowing for better matching of supply and demand. Since the efficiency of a fuel cell stack is determined by the performance of the individual cells, small fuel cell power plants operate as efficiently as large ones. Additionally, waste heat from stationary fuel cell systems can be used for water and space heating, further increasing energy efficiency.

There are virtually no harmful emissions when using fuel cells. When an engine runs on pure hydrogen, only heat and pure water vapor are produced as by-products. So on spaceships, astronauts drink water, which is formed as a result of the operation of onboard fuel cells. The composition of emissions depends on the nature of the hydrogen source. Methanol produces zero nitrogen oxide and carbon monoxide emissions and only small hydrocarbon emissions. Emissions increase as you move from hydrogen to methanol and gasoline, although even with gasoline, emissions will remain fairly low. In any case, replacing today's traditional internal combustion engines with fuel cells would lead to an overall reduction in CO2 and nitrogen oxide emissions.

The use of fuel cells provides flexibility to the energy infrastructure, creating additional opportunities for decentralized electricity production. The multiplicity of decentralized energy sources makes it possible to reduce losses during electricity transmission and develop energy markets (which is especially important for remote and rural areas with no access to power lines). With the help of fuel cells, individual residents or neighborhoods can provide most of their own electricity and thus significantly increase energy efficiency.

Fuel cells offer high quality energy and increased reliability. They are durable, have no moving parts, and produce a constant amount of energy.

However, fuel cell technology needs to be further improved to improve performance, reduce costs, and thus make fuel cells competitive with other energy technologies. It should be noted that when the cost characteristics of energy technologies are considered, comparisons should be made based on all component technology characteristics, including capital operating costs, pollutant emissions, energy quality, durability, decommissioning and flexibility.

Although hydrogen gas is the best fuel, the infrastructure or transport base for it does not yet exist. In the near future, existing fossil fuel supply systems (gas stations, etc.) could be used to provide power plants with sources of hydrogen in the form of gasoline, methanol or natural gas. This would eliminate the need for dedicated hydrogen filling stations, but would require each vehicle to have a fossil fuel-to-hydrogen converter ("reformer") installed. The disadvantage of this approach is that it uses fossil fuels and thus results in carbon dioxide emissions. Methanol, the current leading candidate, produces fewer emissions than gasoline, but would require a larger container in the vehicle because it takes up twice the space for the same energy content.

Unlike fossil fuel supply systems, solar and wind systems (using electricity to create hydrogen and oxygen from water) and direct photoconversion systems (using semiconductor materials or enzymes to produce hydrogen) could provide hydrogen supply without a reforming step, and thus Thus, emissions of harmful substances that are observed when using methanol or gasoline fuel cells could be avoided. The hydrogen could be stored and converted into electricity in the fuel cell as needed. Looking ahead, pairing fuel cells with these kinds of renewable energy sources is likely to be an effective strategy for providing a productive, environmentally smart, and versatile source of energy.

IEER's recommendations are that local, federal, and state governments devote a portion of their transportation procurement budgets to fuel cell vehicles, as well as stationary fuel cell systems, to provide heat and power for some of their significant or new buildings. This will promote the development of vital technology and reduce greenhouse gas emissions.

FUEL CELL
electrochemical generator, a device that provides direct conversion of chemical energy into electrical energy. Although the same thing happens in electric batteries, fuel cells have two important differences: 1) they function as long as the fuel and oxidizer are supplied from an external source; 2) the chemical composition of the electrolyte does not change during operation, i.e. The fuel cell does not need to be recharged.
see also BATTERY SUPPLY .
Operating principle. The fuel cell (Fig. 1) consists of two electrodes separated by an electrolyte, and systems for supplying fuel to one electrode and oxidizer to the other, as well as a system for removing reaction products. In most cases, catalysts are used to speed up a chemical reaction. The fuel cell is connected by an external electrical circuit to a load that consumes electricity.

In the one shown in Fig. In a fuel cell with an acidic electrolyte, hydrogen is fed through a hollow anode and enters the electrolyte through very small pores in the electrode material. In this case, hydrogen molecules decompose into atoms, which, as a result of chemisorption, each giving up one electron, turn into positively charged ions. This process can be described by the following equations:

Hydrogen ions diffuse through the electrolyte to the positive side of the cell. Oxygen supplied to the cathode passes into the electrolyte and also reacts on the surface of the electrode with the participation of a catalyst. When it combines with hydrogen ions and electrons that come from the external circuit, water is formed:

Fuel cells with an alkaline electrolyte (usually concentrated sodium or potassium hydroxides) undergo similar chemical reactions. Hydrogen passes through the anode and reacts in the presence of a catalyst with hydroxyl ions (OH-) present in the electrolyte to form water and an electron:

At the cathode, oxygen reacts with water contained in the electrolyte and electrons from the external circuit. In successive stages of reactions, hydroxyl ions are formed (as well as perhydroxyl O2H-). The resulting reaction at the cathode can be written as:

The flow of electrons and ions maintains the balance of charge and matter in the electrolyte. The water formed as a result of the reaction partially dilutes the electrolyte. In any fuel cell, some of the energy from a chemical reaction is converted into heat. The flow of electrons in an external circuit is a direct current that is used to do work. Most reactions in fuel cells provide an emf of about 1 V. Opening the circuit or stopping the movement of ions stops the fuel cell from operating. The process occurring in a hydrogen-oxygen fuel cell is the reverse in nature of the well-known electrolysis process, in which water dissociates when electric current passes through the electrolyte. Indeed, in some types of fuel cells the process can be reversed - by applying a voltage to the electrodes, water can be decomposed into hydrogen and oxygen, which can be collected on the electrodes. If you stop charging the cell and connect a load to it, such a regenerative fuel cell will immediately begin to operate in its normal mode. Theoretically, the dimensions of a fuel cell can be as large as desired. However, in practice, several cells are combined into small modules or batteries, which are connected either in series or in parallel.
Types of fuel cells. There are different types of fuel cells. They can be classified, for example, by the fuel used, operating pressure and temperature, and the nature of the application.
Hydrogen fuel cells. In this typical cell described above, hydrogen and oxygen are transferred to the electrolyte through microporous carbon or metal electrodes. High current density is achieved in elements operating at elevated temperatures (about 250 ° C) and high pressure. Cells that use hydrogen fuel, produced by processing hydrocarbon fuels such as natural gas or petroleum products, are likely to see the most widespread commercial use. By combining a large number of elements, you can create powerful energy systems. In these installations, the direct current generated by the elements is converted into alternating current with standard parameters. A new type of elements capable of operating on hydrogen and oxygen at normal temperatures and pressures are elements with ion-exchange membranes (Fig. 2). In these cells, instead of a liquid electrolyte, a polymer membrane is located between the electrodes, through which ions pass freely. In such elements, air can be used along with oxygen. The water formed during operation of the cell does not dissolve the solid electrolyte and can be easily removed.

Collier's Encyclopedia. - Open Society. 2000 .

See what "FUEL CELL" is in other dictionaries:

FUEL CELL, ELECTROCHEMICAL CELL for directly converting the oxidation energy of fuel into electrical energy. Suitably designed electrodes are immersed in the ELECTROLYTE and fuel (eg hydrogen) is supplied to one... Scientific and technical encyclopedic dictionary

A galvanic cell in which the redox reaction is maintained by a continuous supply of reagents (fuel, such as hydrogen, and oxidizing agent, such as oxygen) from special reservoirs. The most important component... ... Big Encyclopedic Dictionary

fuel cell- A primary element in which electrical energy is generated through electrochemical reactions between active substances continuously supplied to the electrodes from the outside. [GOST 15596 82] EN fuel cell cell that can change chemical energy from... ... Technical Translator's Guide

Direct methanol fuel cell Fuel cell is an electrochemical device similar to but different from a galvanic cell... Wikipedia

Fuel cell is an electrochemical device similar to a galvanic cell, but differs from it in that the substances for the electrochemical reaction are supplied to it from the outside - in contrast to the limited amount of energy stored in a galvanic cell or battery.

Rice. 1. Some fuel cells

Fuel cells convert the chemical energy of fuel into electricity, bypassing ineffective combustion processes that occur with large losses. They convert hydrogen and oxygen into electricity through a chemical reaction. As a result of this process, water is formed and a large amount of heat is released. A fuel cell is very similar to a battery that can be charged and then use the stored electrical energy. The inventor of the fuel cell is considered to be William R. Grove, who invented it back in 1839. This fuel cell used a sulfuric acid solution as an electrolyte and hydrogen as a fuel, which was combined with oxygen in an oxidizing agent. Until recently, fuel cells were used only in laboratories and on spacecraft.

Rice. 2.

Unlike other power generators, such as internal combustion engines or turbines powered by gas, coal, fuel oil, etc., fuel cells do not burn fuel. This means no noisy high-pressure rotors, no loud exhaust noise, no vibrations. Fuel cells produce electricity through a silent electrochemical reaction. Another feature of fuel cells is that they convert the chemical energy of the fuel directly into electricity, heat and water.

Fuel cells are highly efficient and do not produce large amounts of greenhouse gases such as carbon dioxide, methane and nitrous oxide. The only emissions from fuel cells are water in the form of steam and a small amount of carbon dioxide, which is not released at all if pure hydrogen is used as fuel. Fuel cells are assembled into assemblies and then into individual functional modules.

Fuel cells have no moving parts (at least not within the cell itself) and therefore do not obey Carnot's law. That is, they will have greater than 50% efficiency and are especially effective at low loads. Thus, fuel cell vehicles can become (and have already proven to be) more fuel efficient than conventional vehicles in real-world driving conditions.

The fuel cell produces a constant voltage electric current that can be used to drive the electric motor, lighting, and other electrical systems in the vehicle.

There are several types of fuel cells, differing in the chemical processes used. Fuel cells are usually classified by the type of electrolyte they use.

Some types of fuel cells are promising for power plant propulsion, while others are promising for portable devices or to drive cars.

1. Alkaline fuel cells (ALFC)

Alkaline fuel cell- This is one of the very first elements developed. Alkaline fuel cells (AFC) are one of the most studied technologies, used since the mid-60s of the twentieth century by NASA in the Apollo and Space Shuttle programs. On board these spacecraft, fuel cells produce electrical energy and potable water.

Rice. 3.

Alkaline fuel cells are one of the most efficient cells used to generate electricity, with power generation efficiency reaching up to 70%.

Alkaline fuel cells use an electrolyte, an aqueous solution of potassium hydroxide, contained in a porous, stabilized matrix. The potassium hydroxide concentration may vary depending on the operating temperature of the fuel cell, which ranges from 65°C to 220°C. The charge carrier in SHTE is the hydroxyl ion (OH-), moving from the cathode to the anode, where it reacts with hydrogen, producing water and electrons. The water produced at the anode moves back to the cathode, again generating hydroxyl ions there. As a result of this series of reactions taking place in the fuel cell, electricity and, as a by-product, heat are produced:

Reaction at the anode: 2H2 + 4OH- => 4H2O + 4e

Reaction at the cathode: O2 + 2H2O + 4e- => 4OH

General reaction of the system: 2H2 + O2 => 2H2O

The advantage of SHTE is that these fuel cells are the cheapest to produce, since the catalyst needed on the electrodes can be any of the substances that are cheaper than those used as catalysts for other fuel cells. In addition, SHTEs operate at relatively low temperatures and are among the most efficient.

One of the characteristic features of SHTE is its high sensitivity to CO2, which may be contained in fuel or air. CO2 reacts with the electrolyte, quickly poisons it, and greatly reduces the efficiency of the fuel cell. Therefore, the use of SHTE is limited to enclosed spaces, such as space and underwater vehicles; they operate on pure hydrogen and oxygen.

2. Molten carbonate fuel cells (MCFC)

Fuel cells with molten carbonate electrolyte are high temperature fuel cells. The high operating temperature allows the direct use of natural gas without a fuel processor and low calorific value fuel gas from industrial processes and other sources. This process was developed in the mid-60s of the twentieth century. Since then, production technology, performance and reliability have been improved.

Rice. 4.

The operation of RCFC differs from other fuel cells. These cells use an electrolyte made from a mixture of molten carbonate salts. Currently, two types of mixtures are used: lithium carbonate and potassium carbonate or lithium carbonate and sodium carbonate. To melt carbonate salts and achieve a high degree of ion mobility in the electrolyte, fuel cells with molten carbonate electrolyte operate at high temperatures (650°C). Efficiency varies between 60-80%.

When heated to a temperature of 650°C, the salts become a conductor for carbonate ions (CO32-). These ions pass from the cathode to the anode, where they combine with hydrogen to form water, carbon dioxide and free electrons. These electrons are sent through an external electrical circuit back to the cathode, generating electric current and heat as a by-product.

Reaction at the anode: CO32- + H2 => H2O + CO2 + 2e

Reaction at the cathode: CO2 + 1/2O2 + 2e- => CO32-

General reaction of the element: H2(g) + 1/2O2(g) + CO2(cathode) => H2O(g) + CO2(anode)

The high operating temperatures of molten carbonate electrolyte fuel cells have certain advantages. The advantage is the ability to use standard materials (stainless steel sheets and nickel catalyst on the electrodes). The waste heat can be used to produce high pressure steam. High reaction temperatures in the electrolyte also have their advantages. The use of high temperatures requires a long time to achieve optimal operating conditions, and the system responds more slowly to changes in energy consumption. These characteristics allow the use of fuel cell installations with molten carbonate electrolyte under constant power conditions. High temperatures prevent damage to the fuel cell by carbon monoxide, “poisoning,” etc.

Fuel cells with molten carbonate electrolyte are suitable for use in large stationary installations. Thermal power plants with an electrical output power of 2.8 MW are commercially produced. Installations with output power up to 100 MW are being developed.

3. Phosphoric acid fuel cells (PAFC)

Fuel cells based on phosphoric (orthophosphoric) acid became the first fuel cells for commercial use. This process was developed in the mid-60s of the twentieth century, tests have been carried out since the 70s of the twentieth century. The result was increased stability and performance and reduced cost.

Rice. 5.

Phosphoric (orthophosphoric) acid fuel cells use an electrolyte based on orthophosphoric acid (H3PO4) at concentrations up to 100%. The ionic conductivity of phosphoric acid is low at low temperatures, so these fuel cells are used at temperatures up to 150-220 °C.

The charge carrier in fuel cells of this type is hydrogen (H+, proton). A similar process occurs in proton exchange membrane fuel cells (PEMFCs), in which hydrogen supplied to the anode is split into protons and electrons. Protons travel through the electrolyte and combine with oxygen from the air at the cathode to form water. The electrons are sent through an external electrical circuit, thereby generating an electric current. Below are reactions that generate electric current and heat.

Reaction at the anode: 2H2 => 4H+ + 4e

Reaction at the cathode: O2(g) + 4H+ + 4e- => 2H2O

General reaction of the element: 2H2 + O2 => 2H2O

The efficiency of fuel cells based on phosphoric (orthophosphoric) acid is more than 40% when generating electrical energy. With combined production of heat and electricity, the overall efficiency is about 85%. In addition, given operating temperatures, waste heat can be used to heat water and generate atmospheric pressure steam.

The high performance of thermal power plants using fuel cells based on phosphoric (orthophosphoric) acid in the combined production of thermal and electrical energy is one of the advantages of this type of fuel cells. The units use carbon monoxide with a concentration of about 1.5%, which significantly expands the choice of fuel. Simple design, low degree of electrolyte volatility and increased stability are also advantages of such fuel cells.

Thermal power plants with electrical output power of up to 400 kW are commercially produced. Installations with a capacity of 11 MW have passed appropriate tests. Installations with output power up to 100 MW are being developed.

4. Proton exchange membrane fuel cells (PEMFC)

Proton exchange membrane fuel cells are considered the best type of fuel cells for generating power for vehicles, which can replace gasoline and diesel internal combustion engines. These fuel cells were first used by NASA for the Gemini program. Installations based on MOPFC with power from 1 W to 2 kW have been developed and demonstrated.

Rice. 6.

The electrolyte in these fuel cells is a solid polymer membrane (a thin film of plastic). When saturated with water, this polymer allows protons to pass through but does not conduct electrons.

The fuel is hydrogen, and the charge carrier is a hydrogen ion (proton). At the anode, the hydrogen molecule is split into a hydrogen ion (proton) and electrons. Hydrogen ions pass through the electrolyte to the cathode, and electrons move around the outer circle and produce electrical energy. Oxygen, which is taken from the air, is supplied to the cathode and combines with electrons and hydrogen ions to form water. The following reactions occur at the electrodes: Reaction at the anode: 2H2 + 4OH- => 4H2O + 4eReaction at the cathode: O2 + 2H2O + 4e- => 4OH Overall cell reaction: 2H2 + O2 => 2H2O Compared to other types of fuel cells, fuel cells with a proton exchange membrane produce more energy for a given volume or weight of the fuel cell. This feature allows them to be compact and lightweight. In addition, the operating temperature is less than 100°C, which allows you to quickly start operation. These characteristics, as well as the ability to quickly change energy output, are just a few that make these fuel cells a prime candidate for use in vehicles.

Another advantage is that the electrolyte is a solid rather than a liquid. It is easier to retain gases at the cathode and anode using a solid electrolyte, so such fuel cells are cheaper to produce. With a solid electrolyte, there are no orientation issues and fewer corrosion problems, increasing the longevity of the cell and its components.

Rice. 7.

5. Solid oxide fuel cells (SOFC)

Solid oxide fuel cells are the highest operating temperature fuel cells. The operating temperature can vary from 600°C to 1000°C, allowing the use of different types of fuel without special pre-treatment. To handle such high temperatures, the electrolyte used is a thin solid metal oxide on a ceramic base, often an alloy of yttrium and zirconium, which is a conductor of oxygen ions (O2-). The technology of using solid oxide fuel cells has been developing since the late 50s of the twentieth century and has two configurations: planar and tubular.

The solid electrolyte provides a sealed transition of gas from one electrode to another, while liquid electrolytes are located in a porous substrate. The charge carrier in fuel cells of this type is the oxygen ion (O2-). At the cathode, oxygen molecules from the air are separated into an oxygen ion and four electrons. Oxygen ions pass through the electrolyte and combine with hydrogen, creating four free electrons. The electrons are sent through an external electrical circuit, generating electric current and waste heat.

Rice. 8.

Reaction at the anode: 2H2 + 2O2- => 2H2O + 4e

Reaction at the cathode: O2 + 4e- => 2O2-

General reaction of the element: 2H2 + O2 => 2H2O

The efficiency of electrical energy production is the highest of all fuel cells - about 60%. In addition, high operating temperatures allow for the combined production of thermal and electrical energy to generate high-pressure steam. Combining a high-temperature fuel cell with a turbine makes it possible to create a hybrid fuel cell to increase the efficiency of generating electrical energy by up to 70%.

Solid oxide fuel cells operate at very high temperatures (600°C-1000°C), resulting in significant time required to reach optimal operating conditions and a slower system response to changes in energy consumption. At such high operating temperatures, no converter is required to recover hydrogen from the fuel, allowing the thermal power plant to operate with relatively impure fuels resulting from gasification of coal or waste gases, etc. The fuel cell is also excellent for high power applications, including industrial and large central power plants. Modules with an electrical output power of 100 kW are commercially produced.

6. Direct methanol oxidation fuel cells (DOMFC)

Direct methanol oxidation fuel cells They are successfully used in the field of powering mobile phones, laptops, as well as to create portable power sources, which is what the future use of such elements is aimed at.

The design of fuel cells with direct oxidation of methanol is similar to the design of fuel cells with a proton exchange membrane (MEPFC), i.e. A polymer is used as an electrolyte, and a hydrogen ion (proton) is used as a charge carrier. But liquid methanol (CH3OH) oxidizes in the presence of water at the anode, releasing CO2, hydrogen ions and electrons, which are sent through an external electrical circuit, thereby generating an electric current. Hydrogen ions pass through the electrolyte and react with oxygen from the air and electrons from the external circuit to form water at the anode.

Reaction at the anode: CH3OH + H2O => CO2 + 6H+ + 6eReaction at the cathode: 3/2O2 + 6H+ + 6e- => 3H2O General reaction of the element: CH3OH + 3/2O2 => CO2 + 2H2O The development of such fuel cells has been carried out since the beginning of the 90s s of the twentieth century and their specific power and efficiency were increased to 40%.

These elements were tested in the temperature range of 50-120°C. Because of their low operating temperatures and the absence of the need for a converter, such fuel cells are a prime candidate for use in mobile phones and other consumer products, as well as in car engines. Their advantage is also their small size.

7. Polymer electrolyte fuel cells (PEFC)

In the case of polymer electrolyte fuel cells, the polymer membrane consists of polymer fibers with water regions in which conduction water ions H2O+ (proton, red) attaches to a water molecule. Water molecules pose a problem due to slow ion exchange. Therefore, a high concentration of water is required both in the fuel and at the outlet electrodes, which limits the operating temperature to 100°C.

8. Solid acid fuel cells (SFC)

In solid acid fuel cells, the electrolyte (CsHSO4) does not contain water. The operating temperature is therefore 100-300°C. The rotation of the SO42 oxyanions allows the protons (red) to move as shown in the figure. Typically, a solid acid fuel cell is a sandwich in which a very thin layer of solid acid compound is sandwiched between two electrodes that are tightly pressed together to ensure good contact. When heated, the organic component evaporates, exiting through the pores in the electrodes, maintaining the ability of multiple contacts between the fuel (or oxygen at the other end of the element), the electrolyte and the electrodes.

Rice. 9.

9. Comparison of the most important characteristics of fuel cells

Rice. 10.

10. Use of fuel cells in cars

Rice. eleven.

Rice. 12.

A fuel cell is a device that efficiently produces heat and direct current through an electrochemical reaction and uses a hydrogen-rich fuel. Its operating principle is similar to that of a battery. Structurally, the fuel cell is represented by an electrolyte. What's so special about it? Unlike batteries, hydrogen fuel cells do not store electrical energy, do not require electricity to recharge, and do not discharge. The cells continue to produce electricity as long as they have a supply of air and fuel.

Peculiarities

The difference between fuel cells and other electricity generators is that they do not burn fuel during operation. Due to this feature, they do not require high-pressure rotors and do not emit loud noise or vibration. Electricity in fuel cells is generated through a silent electrochemical reaction. The chemical energy of the fuel in such devices is converted directly into water, heat and electricity.

Fuel cells are highly efficient and do not produce large amounts of greenhouse gases. The emission product during cell operation is a small amount of water in the form of steam and carbon dioxide, which is not released if pure hydrogen is used as fuel.

History of appearance

In the 1950s and 1960s, NASA's emerging need for energy sources for long-term space missions provoked one of the most critical challenges for fuel cells that existed at that time. Alkaline cells use oxygen and hydrogen as fuel, which are converted through an electrochemical reaction into byproducts useful during space flight - electricity, water and heat.

Fuel cells were first discovered at the beginning of the 19th century - in 1838. At the same time, the first information about their effectiveness appeared.

Work on fuel cells using alkaline electrolytes began in the late 1930s. Cells with nickel-plated electrodes under high pressure were not invented until 1939. During World War II, fuel cells consisting of alkaline cells with a diameter of about 25 centimeters were developed for British submarines.

Interest in them increased in the 1950-80s, characterized by a shortage of petroleum fuel. Countries around the world have begun to address air and environmental pollution issues in an effort to develop environmentally friendly fuel cell production technology is currently undergoing active development.

Principle of operation

Heat and electricity are generated by fuel cells as a result of an electrochemical reaction involving a cathode, anode and an electrolyte.

The cathode and anode are separated by a proton-conducting electrolyte. After oxygen enters the cathode and hydrogen enters the anode, a chemical reaction is started, resulting in heat, current and water.

Dissociates on the anode catalyst, which leads to the loss of electrons. Hydrogen ions enter the cathode through the electrolyte, while electrons pass through the external electrical network and create a direct current, which is used to power the equipment. An oxygen molecule on the cathode catalyst combines with an electron and an incoming proton, ultimately forming water, which is the only product of the reaction.

Types

The choice of a specific type of fuel cell depends on its application. All fuel cells are divided into two main categories - high temperature and low temperature. The latter use pure hydrogen as fuel. Such devices typically require processing of primary fuel into pure hydrogen. The process is carried out using special equipment.

High temperature fuel cells don't need this because they convert fuel at elevated temperatures, eliminating the need for hydrogen infrastructure.

The operating principle of hydrogen fuel cells is based on the conversion of chemical energy into electrical energy without ineffective combustion processes and the transformation of thermal energy into mechanical energy.

General concepts

Hydrogen fuel cells are electrochemical devices that produce electricity through highly efficient "cold" combustion of fuel. There are several types of such devices. The most promising technology is considered to be hydrogen-air fuel cells equipped with a proton exchange membrane PEMFC.

The proton-conducting polymer membrane is designed to separate two electrodes - the cathode and the anode. Each of them is represented by a carbon matrix with a catalyst deposited on it. dissociates on the anode catalyst, donating electrons. Cations are conducted to the cathode through the membrane, but electrons are transferred to the external circuit because the membrane is not designed to transfer electrons.

An oxygen molecule on the cathode catalyst combines with an electron from the electrical circuit and an incoming proton, ultimately forming water, which is the only product of the reaction.

Hydrogen fuel cells are used to manufacture membrane-electrode units, which act as the main generating elements of the energy system.

Advantages of Hydrogen Fuel Cells

Among them are:

• Increased specific heat capacity.
• Wide operating temperature range.
• No vibration, noise or heat stain.
• Cold start reliability.
• No self-discharge, which ensures long-term energy storage.
• Unlimited autonomy thanks to the ability to adjust energy intensity by changing the number of fuel cartridges.
• Providing virtually any energy intensity by changing the hydrogen storage capacity.
• Long service life.
• Quiet and environmentally friendly operation.
• High level of energy intensity.
• Tolerance to foreign impurities in hydrogen.

Application area

Due to their high efficiency, hydrogen fuel cells are used in various fields:

• Portable chargers.
• Power supply systems for UAVs.
• Uninterruptible power supplies.
• Other devices and equipment.

Prospects for hydrogen energy

The widespread use of hydrogen peroxide fuel cells will only be possible after the creation of an effective method for producing hydrogen. New ideas are required to bring the technology into active use, with high hopes placed on the concept of biofuel cells and nanotechnology. Some companies have relatively recently released effective catalysts based on various metals, at the same time information has appeared about the creation of fuel cells without membranes, which has made it possible to significantly reduce the cost of production and simplify the design of such devices. The advantages and characteristics of hydrogen fuel cells do not outweigh their main disadvantage - high cost, especially in comparison with hydrocarbon devices. The creation of one hydrogen power plant requires a minimum of 500 thousand dollars.

How to assemble a hydrogen fuel cell?

You can create a low-power fuel cell yourself in a regular home or school laboratory. The materials used are an old gas mask, pieces of plexiglass, an aqueous solution of ethyl alcohol and alkali.

The body of a hydrogen fuel cell is created with your own hands from plexiglass with a thickness of at least five millimeters. The partitions between the compartments can be thinner - about 3 millimeters. Plexiglas is glued together with a special glue made from chloroform or dichloroethane and plexiglass shavings. All work is carried out only with the hood running.

A hole with a diameter of 5-6 centimeters is drilled in the outer wall of the housing, into which a rubber stopper and a glass drain tube are inserted. Activated carbon from the gas mask is poured into the second and fourth compartments of the fuel cell housing - it will be used as an electrode.

Fuel will circulate in the first chamber, while the fifth is filled with air, from which oxygen will be supplied. The electrolyte, poured between the electrodes, is impregnated with a solution of paraffin and gasoline to prevent it from entering the air chamber. Copper plates with wires soldered to them are placed on the layer of coal, through which the current will be drained.

The assembled hydrogen fuel cell is charged with vodka diluted with water in a 1:1 ratio. Caustic potassium is carefully added to the resulting mixture: 70 grams of potassium dissolve in 200 grams of water.

Before testing a hydrogen fuel cell, fuel is poured into the first chamber and electrolyte into the third. The reading of a voltmeter connected to the electrodes should vary from 0.7 to 0.9 volts. To ensure continuous operation of the element, spent fuel must be removed, and new fuel must be poured through a rubber tube. By squeezing the tube, the fuel supply rate is adjusted. Such hydrogen fuel cells, assembled at home, have little power.

Part 1

This article examines in more detail the principle of operation of fuel cells, their design, classification, advantages and disadvantages, scope of application, efficiency, history of creation and modern prospects for use. In the second part of the article, which will be published in the next issue of the ABOK magazine, provides examples of facilities where various types of fuel cells were used as sources of heat and power supply (or only power supply).

Introduction

Fuel cells are a very efficient, reliable, durable and environmentally friendly way to generate energy.

Initially used only in the space industry, fuel cells are now increasingly used in a variety of areas - as stationary power plants, autonomous sources of heat and electricity for buildings, vehicle engines, power supplies for laptops and mobile phones. Some of these devices are laboratory prototypes, some are undergoing pre-production testing or are used for demonstration purposes, but many models are mass-produced and used in commercial projects.

A fuel cell (electrochemical generator) is a device that directly converts the chemical energy of fuel (hydrogen) into electrical energy through an electrochemical reaction, in contrast to traditional technologies that use the combustion of solid, liquid and gaseous fuels. Direct electrochemical conversion of fuel is very effective and attractive from an environmental point of view, since the operation process produces a minimal amount of pollutants and there is no strong noise or vibration.

From a practical point of view, a fuel cell resembles a conventional voltaic battery. The difference is that the battery is initially charged, i.e. filled with “fuel”. During operation, “fuel” is consumed and the battery is discharged. Unlike a battery, a fuel cell uses fuel supplied from an external source to produce electrical energy (Fig. 1).

To produce electrical energy, not only pure hydrogen can be used, but also other hydrogen-containing raw materials, for example, natural gas, ammonia, methanol or gasoline. Ordinary air is used as a source of oxygen, also necessary for the reaction.

When using pure hydrogen as a fuel, the reaction products, in addition to electrical energy, are heat and water (or water vapor), i.e., gases that cause air pollution or cause the greenhouse effect are not emitted into the atmosphere. If a hydrogen-containing feedstock, such as natural gas, is used as a fuel, other gases such as carbon and nitrogen oxides will be a by-product of the reaction, but the amount is much lower than when burning the same amount of natural gas.

The process of chemically converting fuel to produce hydrogen is called reforming, and the corresponding device is called a reformer.

Advantages and disadvantages of fuel cells

Fuel cells are more energy efficient than internal combustion engines because there is no thermodynamic energy efficiency limitation for fuel cells. The efficiency of fuel cells is 50%, while the efficiency of internal combustion engines is 12-15%, and the efficiency of steam turbine power plants does not exceed 40%. By using heat and water, the efficiency of fuel cells is further increased.

Unlike, for example, internal combustion engines, the efficiency of fuel cells remains very high even when they are not operating at full power. In addition, the power of fuel cells can be increased by simply adding individual units, while the efficiency does not change, i.e. large installations are just as efficient as small ones. These circumstances make it possible to very flexibly select the composition of equipment in accordance with the wishes of the customer and ultimately lead to a reduction in equipment costs.

An important advantage of fuel cells is their environmental friendliness. Fuel cell emissions are so low that in some areas of the United States, their operation does not require special approval from government air quality regulators.

Fuel cells can be placed directly in a building, reducing losses during energy transportation, and the heat generated as a result of the reaction can be used to supply heat or hot water to the building. Autonomous sources of heat and electricity can be very beneficial in remote areas and in regions characterized by a shortage of electricity and its high cost, but at the same time there are reserves of hydrogen-containing raw materials (oil, natural gas).

The advantages of fuel cells are also the availability of fuel, reliability (there are no moving parts in a fuel cell), durability and ease of operation.

One of the main disadvantages of fuel cells today is their relatively high cost, but this disadvantage can soon be overcome - more and more companies are producing commercial samples of fuel cells, they are constantly being improved, and their cost is decreasing.

The most effective way is to use pure hydrogen as a fuel, but this will require the creation of a special infrastructure for its production and transportation. Currently, all commercial designs use natural gas and similar fuels. Motor vehicles can use regular gasoline, which will allow maintaining the existing developed network of gas stations. However, the use of such fuel leads to harmful emissions into the atmosphere (albeit very low) and complicates (and therefore increases the cost of) the fuel cell. In the future, the possibility of using environmentally friendly renewable energy sources (for example, solar or wind energy) to decompose water into hydrogen and oxygen using electrolysis, and then converting the resulting fuel in a fuel cell, is being considered. Such combined plants, operating in a closed cycle, can represent a completely environmentally friendly, reliable, durable and efficient source of energy.

Another feature of fuel cells is that they are most efficient when using both electrical and thermal energy simultaneously. However, not every facility has the opportunity to use thermal energy. If fuel cells are used only to generate electrical energy, their efficiency decreases, although it exceeds the efficiency of “traditional” installations.

History and modern use of fuel cells

The principle of operation of fuel cells was discovered in 1839. The English scientist William Robert Grove (1811-1896) discovered that the process of electrolysis - the decomposition of water into hydrogen and oxygen through electric current - is reversible, i.e. hydrogen and oxygen can be combined into water molecules without combustion, but with the release of heat and electric current. Grove called the device in which such a reaction was possible a “gas battery,” which was the first fuel cell.

The active development of technologies for the use of fuel cells began after the Second World War, and it is associated with the aerospace industry. At this time, a search was underway for an effective and reliable, but at the same time quite compact, source of energy. In the 1960s, NASA (National Aeronautics and Space Administration, NASA) specialists chose fuel cells as a power source for the spacecraft of the Apollo (manned flights to the Moon), Apollo-Soyuz, Gemini and Skylab programs. . The Apollo spacecraft used three 1.5 kW (2.2 kW peak) plants using cryogenic hydrogen and oxygen to produce electricity, heat and water. The mass of each installation was 113 kg. These three cells operated in parallel, but the energy generated by one unit was sufficient for a safe return. Over the course of 18 flights, the fuel cells operated for a total of 10,000 hours without any failures. Currently, fuel cells are used in the Space Shuttle, which uses three 12 W units to generate all the electrical energy on board the spacecraft (Fig. 2). The water obtained as a result of the electrochemical reaction is used for drinking water and also for cooling equipment.

In our country, work was also carried out on the creation of fuel cells for use in astronautics. For example, fuel cells were used to power the Soviet Buran reusable spacecraft.

Development of methods for the commercial use of fuel cells began in the mid-1960s. These developments were partially funded by government organizations.

Currently, the development of technologies for the use of fuel cells is proceeding in several directions. This is the creation of stationary power plants on fuel cells (both for centralized and decentralized energy supply), power plants for vehicles (samples of cars and buses on fuel cells have been created, including in our country) (Fig. 3), and also power supplies for various mobile devices (laptop computers, mobile phones, etc.) (Fig. 4).

Examples of the use of fuel cells in various fields are given in Table. 1.

One of the first commercial fuel cell models designed for autonomous heat and power supply to buildings was the PC25 Model A manufactured by ONSI Corporation (now United Technologies, Inc.). This fuel cell with a rated power of 200 kW is a type of cell with an electrolyte based on phosphoric acid (Phosphoric Acid Fuel Cells, PAFC). The number “25” in the model name means the serial number of the design. Most previous models were experimental or test units, such as the 12.5 kW "PC11" model introduced in the 1970s. The new models increased the power extracted from an individual fuel cell, and also reduced the cost per kilowatt of energy produced. Currently, one of the most efficient commercial models is the PC25 Model C fuel cell. Like Model A, this is a fully automatic 200 kW PAFC fuel cell designed for on-site installation as a self-contained source of heat and power. Such a fuel cell can be installed outside a building. Externally, it is a parallelepiped 5.5 m long, 3 m wide and high, weighing 18,140 kg. The difference from previous models is an improved reformer and a higher current density.

In some types of fuel cells, the chemical process can be reversed: by applying a potential difference to the electrodes, water can be broken down into hydrogen and oxygen, which collect on the porous electrodes. When a load is connected, such a regenerative fuel cell will begin to produce electrical energy.

A promising direction for the use of fuel cells is their use in conjunction with renewable energy sources, for example, photovoltaic panels or wind power plants. This technology allows you to completely avoid air pollution. A similar system is planned to be created, for example, at the Adam Joseph Lewis Training Center in Oberlin (see ABOK, 2002, No. 5, p. 10). Currently, solar panels are used as one of the energy sources in this building. Together with NASA specialists, a project has been developed for using photovoltaic panels to produce hydrogen and oxygen from water by electrolysis. The hydrogen is then used in fuel cells to produce electricity and hot water. This will allow the building to maintain the functionality of all systems during cloudy days and at night.

Operating principle of fuel cells

Let's consider the principle of operation of a fuel cell using the example of a simple element with a proton exchange membrane (Proton Exchange Membrane, PEM). Such a cell consists of a polymer membrane placed between an anode (positive electrode) and a cathode (negative electrode) along with anode and cathode catalysts. The polymer membrane is used as an electrolyte. The diagram of the PEM element is shown in Fig. 5.

A proton exchange membrane (PEM) is a thin (about 2-7 sheets of paper thick) solid organic compound. This membrane functions as an electrolyte: it separates a substance into positively and negatively charged ions in the presence of water.

An oxidation process occurs at the anode, and a reduction process occurs at the cathode. The anode and cathode in a PEM cell are made of a porous material, which is a mixture of carbon and platinum particles. Platinum acts as a catalyst that promotes the dissociation reaction. The anode and cathode are made porous for the free passage of hydrogen and oxygen through them, respectively.

The anode and cathode are placed between two metal plates, which supply hydrogen and oxygen to the anode and cathode, and remove heat and water, as well as electrical energy.

Hydrogen molecules pass through channels in the plate to the anode, where the molecules are decomposed into individual atoms (Fig. 6).

Then, as a result of chemisorption in the presence of a catalyst, hydrogen atoms, each giving up one electron e –, are converted into positively charged hydrogen ions H +, i.e. protons (Fig. 7).

Positively charged hydrogen ions (protons) diffuse through the membrane to the cathode, and the flow of electrons is directed to the cathode through an external electrical circuit to which the load (consumer of electrical energy) is connected (Fig. 8).

Oxygen supplied to the cathode, in the presence of a catalyst, enters into a chemical reaction with hydrogen ions (protons) from the proton exchange membrane and electrons from the external electrical circuit (Fig. 9). As a result of a chemical reaction, water is formed.

The chemical reaction in other types of fuel cells (for example, with an acid electrolyte, which uses a solution of orthophosphoric acid H 3 PO 4) is absolutely identical to the chemical reaction in a fuel cell with a proton exchange membrane.

In any fuel cell, some of the energy from a chemical reaction is released as heat.

The flow of electrons in an external circuit is a direct current that is used to do work. Opening the external circuit or stopping the movement of hydrogen ions stops the chemical reaction.

The amount of electrical energy produced by a fuel cell depends on the type of fuel cell, geometric dimensions, temperature, gas pressure. A separate fuel cell provides an EMF of less than 1.16 V. The size of fuel cells can be increased, but in practice several elements connected into batteries are used (Fig. 10).

Fuel cell design

Let's look at the design of a fuel cell using the PC25 Model C as an example. The fuel cell diagram is shown in Fig. eleven.

The PC25 Model C fuel cell consists of three main parts: the fuel processor, the actual power generation section and the voltage converter.

The main part of the fuel cell - the power generation section - is a battery made up of 256 individual fuel cells. The fuel cell electrodes contain a platinum catalyst. These cells produce a constant electrical current of 1,400 amperes at 155 volts. The battery dimensions are approximately 2.9 m in length and 0.9 m in width and height.

Since the electrochemical process occurs at a temperature of 177 °C, it is necessary to heat the battery at the time of start-up and remove heat from it during operation. To achieve this, the fuel cell includes a separate water circuit, and the battery is equipped with special cooling plates.

The fuel processor converts natural gas into hydrogen needed for an electrochemical reaction. This process is called reforming. The main element of the fuel processor is the reformer. In the reformer, natural gas (or other hydrogen-containing fuel) reacts with water vapor at high temperature (900 °C) and high pressure in the presence of a nickel catalyst. In this case, the following chemical reactions occur:

CH 4 (methane) + H 2 O 3H 2 + CO

(the reaction is endothermic, with heat absorption);

CO + H 2 O H 2 + CO 2

(the reaction is exothermic, releasing heat).

The overall reaction is expressed by the equation:

CH 4 (methane) + 2H 2 O 4H 2 + CO 2

(the reaction is endothermic, with heat absorption).

To provide the high temperature required to convert natural gas, a portion of the spent fuel from the fuel cell stack is directed to a burner, which maintains the required reformer temperature.

The steam required for reforming is generated from condensate generated during operation of the fuel cell. This uses the heat removed from the battery of fuel cells (Fig. 12).

The fuel cell stack produces an intermittent direct current that is low voltage and high current. A voltage converter is used to convert it to industrial standard AC current. In addition, the voltage converter unit includes various control devices and safety interlock circuits that allow the fuel cell to be turned off in the event of various failures.

In such a fuel cell, approximately 40% of the fuel energy can be converted into electrical energy. Approximately the same amount, about 40% of the fuel energy, can be converted into thermal energy, which is then used as a heat source for heating, hot water supply and similar purposes. Thus, the total efficiency of such an installation can reach 80%.

An important advantage of such a source of heat and electricity is the possibility of its automatic operation. For maintenance, the owners of the facility where the fuel cell is installed do not need to maintain specially trained personnel - periodic maintenance can be carried out by employees of the operating organization.

Types of fuel cells

Currently, several types of fuel cells are known, differing in the composition of the electrolyte used. The following four types are most widespread (Table 2):

1. Fuel cells with a proton exchange membrane (Proton Exchange Membrane Fuel Cells, PEMFC).

2. Fuel cells based on orthophosphoric acid (Phosphoric Acid Fuel Cells, PAFC).

3. Fuel cells based on molten carbonate (Molten Carbonate Fuel Cells, MCFC).

4. Solid Oxide Fuel Cells (SOFC). Currently, the largest fleet of fuel cells is based on PAFC technology.

One of the key characteristics of different types of fuel cells is operating temperature. In many ways, it is the temperature that determines the area of ​​application of fuel cells. For example, high temperatures are critical for laptops, so proton exchange membrane fuel cells with low operating temperatures are being developed for this market segment.

For autonomous power supply of buildings, fuel cells of high installed power are required, and at the same time there is the possibility of using thermal energy, so other types of fuel cells can be used for these purposes.

Proton exchange membrane fuel cells (PEMFC)

These fuel cells operate at relatively low operating temperatures (60-160 °C). They have a high power density, allow you to quickly adjust the output power, and can be turned on quickly. The disadvantage of this type of element is the high requirements for fuel quality, since contaminated fuel can damage the membrane. The rated power of this type of fuel cells is 1-100 kW.

Proton exchange membrane fuel cells were originally developed by General Electric in the 1960s for NASA. This type of fuel cell uses a solid-state polymer electrolyte called a Proton Exchange Membrane (PEM). Protons can move through the proton exchange membrane, but electrons cannot pass through it, resulting in a potential difference between the cathode and anode. Because of their simplicity and reliability, such fuel cells were used as a power source on the manned Gemini spacecraft.

This type of fuel cell is used as a power source for a wide range of different devices, including prototypes and prototypes, from mobile phones to buses and stationary power systems. The low operating temperature allows such cells to be used to power various types of complex electronic devices. Their use is less effective as a source of heat and electricity supply to public and industrial buildings, where large volumes of thermal energy are required. At the same time, such elements are promising as an autonomous source of power supply for small residential buildings such as cottages built in regions with a hot climate.

Phosphoric Acid Fuel Cells (PAFC)

Tests of fuel cells of this type were carried out already in the early 1970s. Operating temperature range - 150-200 °C. The main area of ​​application is autonomous sources of heat and electricity supply of medium power (about 200 kW).

These fuel cells use a phosphoric acid solution as the electrolyte. The electrodes are made of paper coated with carbon in which a platinum catalyst is dispersed.

The electrical efficiency of PAFC fuel cells is 37-42%. However, since these fuel cells operate at a fairly high temperature, it is possible to use the steam generated as a result of operation. In this case, the overall efficiency can reach 80%.

To produce energy, hydrogen-containing feedstock must be converted into pure hydrogen through a reforming process. For example, if gasoline is used as fuel, it is necessary to remove sulfur-containing compounds, since sulfur can damage the platinum catalyst.

PAFC fuel cells were the first commercial fuel cells to be used economically. The most common model was the 200 kW PC25 fuel cell manufactured by ONSI Corporation (now United Technologies, Inc.) (Fig. 13). For example, these elements are used as a source of thermal and electrical energy in the police station in Central Park in New York or as an additional source of energy in the Conde Nast Building & Four Times Square. The largest installation of this type is being tested as an 11 MW power plant located in Japan.

Phosphoric acid fuel cells are also used as an energy source in vehicles. For example, in 1994, H-Power Corp., Georgetown University and the US Department of Energy equipped a bus with a 50 kW power plant.

Molten Carbonate Fuel Cells (MCFC)

Fuel cells of this type operate at very high temperatures - 600-700 °C. These operating temperatures allow the fuel to be used directly in the cell itself, without the use of a separate reformer. This process was called “internal reform”. It makes it possible to significantly simplify the design of the fuel cell.

Fuel cells based on molten carbonate require a significant start-up time and do not allow for prompt adjustment of output power, so their main area of ​​application is large stationary sources of thermal and electrical energy. However, they are characterized by high fuel conversion efficiency - 60% electrical efficiency and up to 85% overall efficiency.

In this type of fuel cell, the electrolyte consists of potassium carbonate and lithium carbonate salts heated to approximately 650 °C. Under these conditions, the salts are in a molten state, forming an electrolyte. At the anode, hydrogen reacts with CO 3 ions, forming water, carbon dioxide and releasing electrons, which are sent to the external circuit, and at the cathode, oxygen interacts with carbon dioxide and electrons from the external circuit, again forming CO 3 ions.

Laboratory samples of fuel cells of this type were created in the late 1950s by Dutch scientists G. H. J. Broers and J. A. A. Ketelaar. In the 1960s, engineer Francis T. Bacon, a descendant of the famous English writer and scientist of the 17th century, worked with these cells, which is why MCFC fuel cells are sometimes called Bacon cells. In the NASA Apollo, Apollo-Soyuz, and Scylab programs, these fuel cells were used as a source of energy supply (Fig. 14). During these same years, the US military department tested several samples of MCFC fuel cells produced by Texas Instruments, which used military grade gasoline as fuel. In the mid-1970s, the US Department of Energy began research to create a stationary fuel cell based on molten carbonate suitable for practical applications. In the 1990s, a number of commercial installations with rated power up to 250 kW were introduced, for example at the US Naval Air Station Miramar in California. In 1996, FuelCell Energy, Inc. launched a pre-production 2 MW plant in Santa Clara, California.

Solid-state oxide fuel cells (SOFC)

Solid-state oxide fuel cells are simple in design and operate at very high temperatures - 700-1,000 °C. Such high temperatures allow the use of relatively “dirty”, unrefined fuel. The same features as those of fuel cells based on molten carbonate determine a similar field of application - large stationary sources of thermal and electrical energy.

Solid oxide fuel cells are structurally different from fuel cells based on PAFC and MCFC technologies. The anode, cathode and electrolyte are made of special grades of ceramics. The most commonly used electrolyte is a mixture of zirconium oxide and calcium oxide, but other oxides can also be used. The electrolyte forms a crystal lattice coated on both sides with porous electrode material. Structurally, such elements are made in the form of tubes or flat circuit boards, which makes it possible to use technologies widely used in the electronics industry in their production. As a result, solid-state oxide fuel cells can operate at very high temperatures, making them advantageous for producing both electrical and thermal energy.

At high operating temperatures, oxygen ions are formed at the cathode, which migrate through the crystal lattice to the anode, where they interact with hydrogen ions, forming water and releasing free electrons. In this case, hydrogen is separated from natural gas directly in the cell, i.e. there is no need for a separate reformer.

The theoretical foundations for the creation of solid-state oxide fuel cells were laid in the late 1930s, when Swiss scientists Emil Bauer and H. Preis experimented with zirconium, yttrium, cerium, lanthanum and tungsten, using them as electrolytes.

The first prototypes of such fuel cells were created in the late 1950s by a number of American and Dutch companies. Most of these companies soon abandoned further research due to technological difficulties, but one of them, Westinghouse Electric Corp. (now Siemens Westinghouse Power Corporation), continued work. The company is currently accepting pre-orders for a commercial model of a tubular solid-state oxide fuel cell, expected to be available this year (Figure 15). The market segment of such elements is stationary installations for the production of thermal and electrical energy with a capacity of 250 kW to 5 MW.

SOFC fuel cells have demonstrated very high reliability. For example, a prototype fuel cell manufactured by Siemens Westinghouse has achieved 16,600 hours of operation and continues to operate, making it the longest continuous fuel cell life in the world.

The high-temperature, high-pressure operating mode of SOFC fuel cells allows for the creation of hybrid plants in which fuel cell emissions drive gas turbines used to generate electrical power. The first such hybrid installation is operating in Irvine, California. The rated power of this installation is 220 kW, of which 200 kW from the fuel cell and 20 kW from the microturbine generator.