Chemical fuel cells. Fuel cells

Fuel cells Fuel cells are chemical power sources. They directly convert fuel energy into electricity, bypassing ineffective combustion processes that involve large losses. This electrochemical device directly produces electricity as a result of highly efficient “cold” combustion of fuel.

Biochemists have established that a biological hydrogen-oxygen fuel cell is “built-in” into every living cell (see Chapter 2).

The source of hydrogen in the body is food - fats, proteins and carbohydrates. In the stomach, intestines, and cells, it is ultimately decomposed into monomers, which, in turn, after a series of chemical transformations, produce hydrogen attached to the carrier molecule.

Oxygen from the air enters the blood through the lungs, combines with hemoglobin and is distributed to all tissues. The process of combining hydrogen with oxygen forms the basis of the body’s bioenergetics. Here, under mild conditions (room temperature, normal pressure, aquatic environment), chemical energy with high efficiency is converted into thermal, mechanical (muscle movement), electricity (electric stingray), light (insects emitting light).

Man has once again repeated the device for generating energy created by nature. At the same time, this fact indicates the prospects of the direction. All processes in nature are very rational, so steps towards the real use of fuel cells give hope for the energy future.

The discovery of the hydrogen-oxygen fuel cell in 1838 belongs to the English scientist W. Grove. While studying the decomposition of water into hydrogen and oxygen, he discovered a side effect - the electrolyzer produced electricity.

What burns in a fuel cell?
Fossil fuels (coal, gas and oil) are composed primarily of carbon. When burned, fuel atoms lose electrons, and air oxygen atoms gain them. Thus, in the process of oxidation, carbon and oxygen atoms combine to form combustion products - carbon dioxide molecules. This process proceeds energetically: atoms and molecules of substances involved in combustion acquire high speeds, and this leads to an increase in their temperature. They begin to emit light - a flame appears.

The chemical reaction of carbon combustion has the form:

C + O2 = CO2 + heat

During the combustion process, chemical energy is converted into thermal energy due to the exchange of electrons between the fuel and oxidizer atoms. This exchange occurs chaotically.

Combustion is the exchange of electrons between atoms, and electric current is the directed movement of electrons. If electrons are forced to do work during a chemical reaction, the temperature of the combustion process will decrease. In a fuel cell, electrons are taken from reactants on one electrode, give up their energy in the form of an electric current, and are added to the reactants on another.

The basis of any HIT is two electrodes connected by an electrolyte. The fuel cell consists of an anode, a cathode and an electrolyte (see Chapter 2). It oxidizes at the anode, i.e. gives up electrons, a reducing agent (fuel CO or H2), free electrons from the anode enter the external circuit, and positive ions are retained at the anode-electrolyte interface (CO+, H+). From the other end of the chain, electrons approach the cathode, where a reduction reaction takes place (the addition of electrons by the oxidizing agent O2–). The oxidizing ions are then transferred by the electrolyte to the cathode.

In TE, three phases of a physicochemical system are brought together:

gas (fuel, oxidizer);
electrolyte (conductor of ions);
metal electrode (conductor of electrons).
In the fuel cell, the energy of the redox reaction is converted into electrical energy, and the processes of oxidation and reduction are spatially separated by the electrolyte. The electrodes and electrolyte do not participate in the reaction, but in real structures they become contaminated with fuel impurities over time. Electrochemical combustion can occur at low temperatures and with virtually no losses. In Fig. p087 shows a situation in which a mixture of gases (CO and H2) enters the fuel cell, i.e. it can burn gaseous fuel (see Chapter 1). Thus, TE turns out to be “omnivorous”.

What complicates the use of fuel cells is that the fuel needs to be “cooked” for them. For fuel cells, hydrogen is produced by conversion of organic fuel or gasification of coal. Therefore, the block diagram of a fuel cell power plant, in addition to fuel cell batteries, a DC-to-AC converter (see Chapter 3) and auxiliary equipment, includes a hydrogen production unit.

Two directions of fuel cell development

There are two areas of application of fuel cells: autonomous and large-scale energy.

For autonomous use, the main factors are specific characteristics and ease of use. The cost of generated energy is not the main indicator.

For large-scale energy production, efficiency is a decisive factor. In addition, installations must be durable, not contain expensive materials and use natural fuel when minimum costs for preparation.

The greatest benefits come from using fuel cells in a car. Here, as nowhere else, the compactness of the fuel cell will have an impact. When directly obtaining electricity from fuel, the savings will be about 50%.

The idea of ​​using fuel cells in large-scale energy was first formulated by the German scientist W. Oswald in 1894. Later, the idea of ​​creating efficient sources of autonomous energy based on a fuel cell was developed.

After this, repeated attempts were made to use coal as an active substance in fuel cells. In the 30s, German researcher E. Bauer created a laboratory prototype of a fuel cell with a solid electrolyte for direct anodic oxidation of coal. At the same time, oxygen-hydrogen fuel cells were studied.

In 1958 in England, F. Bacon created the first oxygen-hydrogen installation with a power of 5 kW. But it was cumbersome due to the use of high gas pressure (2...4 MPa).

Since 1955, in the USA, K. Kordesh has been developing low-temperature oxygen-hydrogen fuel cells. They used carbon electrodes with platinum catalysts. In Germany, E. Just worked on the creation of non-platinum catalysts.

After 1960, demonstration and advertising samples were created. The first practical application of fuel cells was found on the Apollo spacecraft. They were the main power plants for powering on-board equipment and provided the astronauts with water and heat.

The main areas of use for autonomous fuel cell installations have been military and naval applications. At the end of the 60s, the volume of research on FC decreased, and after the 80s it increased again in relation to large-scale energy.

VARTA has developed fuel cells using double-sided gas diffusion electrodes. Electrodes of this type are called “Janus”. Siemens has developed electrodes with a power density of up to 90 W/kg. In the USA, work on oxygen-hydrogen cells is carried out by United Technology Corp.

In the large-scale energy sector, the use of fuel cells for large-scale energy storage, for example, the production of hydrogen (see Chapter 1), is very promising. (sun and wind) are dispersed (see Chapter 4). Their serious use, which cannot be avoided in the future, is unthinkable without capacious batteries that store energy in one form or another.

The problem of accumulation is already relevant today: daily and weekly fluctuations in the load of power systems significantly reduce their efficiency and require so-called maneuverable capacities. One of the options for electrochemical energy storage is a fuel cell in combination with electrolyzers and gas holders*.

* Gas ​​holder [gas + eng. holder] – storage for large quantities of gas.

First generation of fuel cells

The greatest technological perfection has been achieved by medium-temperature fuel cells of the first generation, operating at a temperature of 200...230°C on liquid fuel, natural gas or technical hydrogen*. The electrolyte in them is phosphoric acid, which fills a porous carbon matrix. The electrodes are made of carbon, and the catalyst is platinum (platinum is used in quantities of the order of several grams per kilowatt of power).

* Technical hydrogen is a product of conversion of organic fuel containing minor impurities of carbon monoxide.

One such power plant was commissioned in the state of California in 1991. It consists of eighteen batteries weighing 18 tons each and is housed in a housing with a diameter of just over 2 m and a height of about 5 m. A procedure has been thought out for replacing the battery using a frame structure moving on rails.

Two US fuel power plants were supplied to Japan. The first of them was launched at the beginning of 1983. The station's operational indicators corresponded to the calculated ones. It worked with a load from 25 to 80% of the rated load. The efficiency reached 30...37% - this is close to modern large thermal power plants. Its start-up time from a cold state is from 4 hours to 10 minutes, and the duration of the power change from zero to full is only 15 seconds.

Currently, small heating plants with a capacity of 40 kW with a fuel efficiency of about 80% are being tested in different parts of the United States. They can heat water up to 130°C and are located in laundries, sports complexes, communication points, etc. About a hundred installations have already worked for a total of hundreds of thousands of hours. The environmental friendliness of FC power plants allows them to be located directly in cities.

The first fuel power plant in New York, with a capacity of 4.5 MW, occupied an area of ​​1.3 hectares. Now, for new stations with a capacity two and a half times greater, a site measuring 30x60 m is needed. Several demonstration power plants with a capacity of 11 MW each are being built. The construction time (7 months) and the area (30x60 m) occupied by the power plant are striking. The estimated service life of new power plants is 30 years.

Second and third generation of fuel cells

The best characteristics already being designed modular installations with a capacity of 5 MW with second-generation medium-temperature fuel cells have. They operate at temperatures of 650...700°C. Their anodes are made from sintered particles of nickel and chromium, cathodes are made from sintered and oxidized aluminum, and the electrolyte is a molten mixture of lithium and potassium carbonates. Elevated temperature helps solve two major electrochemical problems:

reduce the “poisoning” of the catalyst by carbon monoxide;
increase the efficiency of the oxidizer reduction process at the cathode.
Third-generation high-temperature fuel cells with an electrolyte made of solid oxides (mainly zirconium dioxide) will be even more efficient. Their operating temperature is up to 1000°C. The efficiency of power plants with such fuel cells is close to 50%. Here, gasification products of solid coal with a significant content of carbon monoxide are also suitable as fuel. Equally important, the waste heat from high-temperature plants can be used to produce steam that drives the turbines of electric generators.

Vestingaus has been working on solid oxide fuel cells since 1958. It is developing power plants with a capacity of 25...200 kW, which can use gaseous fuel from coal. Experimental installations with a capacity of several megawatts are being prepared for testing. Another American company Engelgurd is designing 50 kW fuel cells running on methanol with phosphoric acid as an electrolyte.

More and more firms around the world are becoming involved in the creation of fuel technologies. The American United Technology and the Japanese Toshiba formed the International Fuel Cells Corporation. In Europe, fuel cells are being developed by the Belgian-Dutch consortium Elenko, the West German company Siemens, the Italian Fiat, and the English Jonson Metju.

Victor LAVRUS.

Fuel cell- what it is? When and how did he appear? Why is it needed and why do they talk about them so often nowadays? What are its applications, characteristics and properties? Unstoppable progress requires answers to all these questions!

What is a fuel cell?

Fuel cell- is a chemical current source or electrochemical generator; it is a device for converting chemical energy into electrical energy. In modern life, chemical power sources are used everywhere and are batteries for mobile phones, laptops, PDAs, as well as batteries in cars, uninterruptible power supplies, etc. The next stage in the development of this area will be the widespread distribution of fuel cells and this is an irrefutable fact.

History of fuel cells

The history of fuel cells is another story about how the properties of matter, once discovered on Earth, found wide application far in space, and at the turn of the millennium returned from heaven to Earth.

It all started in 1839, when the German chemist Christian Schönbein published the principles of the fuel cell in the Philosophical Journal. In the same year, an Englishman and Oxford graduate, William Robert Grove, designed a galvanic cell, later called the Grove galvanic cell, which is also recognized as the first fuel cell. The name “fuel cell” was given to the invention in the year of its anniversary - in 1889. Ludwig Mond and Karl Langer are the authors of the term.

A little earlier, in 1874, Jules Verne, in his novel The Mysterious Island, predicted the current energy situation, writing that “Water will one day be used as fuel, the hydrogen and oxygen of which it is composed will be used.”

Meanwhile, new technology power supply was gradually improved, and starting from the 50s of the 20th century, not even a year passed without announcements of the latest inventions in this area. In 1958, the first tractor powered by fuel cells appeared in the United States, in 1959. a 5kW power supply for a welding machine was released, etc. In the 70s, hydrogen technology took off into space: airplanes and rocket engines powered by hydrogen appeared. In the 60s, RSC Energia developed fuel cells for the Soviet lunar program. The Buran program also could not do without them: alkaline 10 kW fuel cells were developed. And towards the end of the century, fuel cells crossed zero altitude above sea level - based on them, power supply German submarine. Returning to Earth, the first locomotive was put into operation in the United States in 2009. Naturally, on fuel cells.

In all the wonderful history of fuel cells, the interesting thing is that the wheel still remains an invention of mankind that has no analogues in nature. The fact is that in their design and principle of operation, fuel cells are similar to a biological cell, which, in essence, is a miniature hydrogen-oxygen fuel cell. As a result, man once again invented something that nature has been using for millions of years.

Operating principle of fuel cells

The principle of operation of fuel cells is obvious even from school curriculum in chemistry and it was precisely this that was laid down in the experiments of William Grove in 1839. The thing is that the process of water electrolysis (water dissociation) is reversible. Just as it is true that when an electric current is passed through water, the latter is split into hydrogen and oxygen, so the reverse is also true: hydrogen and oxygen can be combined to produce water and electricity. In Grove's experiment, two electrodes were placed in a chamber into which limited portions of pure hydrogen and oxygen were supplied under pressure. Due to the small volumes of gas, as well as due to the chemical properties of the carbon electrodes, a slow reaction occurred in the chamber with the release of heat, water and, most importantly, the formation of a potential difference between the electrodes.

The simplest fuel cell consists of a special membrane used as an electrolyte, on both sides of which powdered electrodes are applied. Hydrogen goes to one side (anode), and oxygen (air) goes 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, that promotes the dissociation reaction:

2H 2 → 4H + + 4e -

where H 2 is a diatomic hydrogen molecule (the form in which hydrogen is present as a gas); H + - ionized hydrogen (proton); e - - electron.

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

4H + + 4e - + O 2 → 2H 2 O

Total reaction in a fuel cell it is written like this:

2H 2 + O 2 → 2H 2 O

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 (a load, such as a light bulb):

Fuel cells use hydrogen fuel and oxygen to operate. The easiest way is with oxygen - it is taken from the air. Hydrogen can be supplied directly from a certain container or by isolating it from an external fuel source (natural gas, gasoline or methyl alcohol - methanol). In the case of an external source, it must be chemically converted to extract the hydrogen. Currently, most fuel cell technologies being developed for portable devices use methanol.

Characteristics of fuel cells

Fuel cells are analogous to existing batteries in the sense that in both cases electrical energy is obtained from chemical energy. But there are also fundamental differences:

• they only work as long as the fuel and oxidizer are supplied from an external source (i.e. they cannot store electrical energy),

  the chemical composition of the electrolyte does not change during operation (the fuel cell does not need to be recharged),

  they are completely independent of electricity (while conventional batteries store energy from the mains).

Each fuel cell creates voltage 1V.

Higher voltage is achieved by connecting them in series. An increase in power (current) is realized through a parallel connection of cascades of series-connected fuel cells. In fuel cells

there is no strict limitation on efficiency achieved through the direct conversion of fuel energy into electricity. When diesel generator sets burn fuel first, the resulting steam or gas rotates a turbine or shaft of an internal combustion engine, which in turn rotates an electric generator. The result is an efficiency of a maximum of 42%, but more often it is about 35-38%.,

Moreover, due to the many links, as well as due to thermodynamic limitations on the maximum efficiency of heat engines, the existing efficiency is unlikely to be raised higher. For existing fuel cells Efficiency is 60-80%,

Efficiency almost does not depend on load factor

Capacity is several times higher than in existing batteries, Complete no environmentally harmful emissions.

Only pure water vapor and thermal energy are released (unlike diesel generators, which have polluting

environment exhausts and requiring their removal). Types of fuel cells

Fuel cells

classified

according to the following characteristics:

according to the fuel used, by operating pressure and temperature,:

according to the nature of the application.

In general, the following are distinguished:

fuel cell types

Solid-oxide fuel cells (SOFC);

Fuel cell with a proton-exchange membrane fuel cell (PEMFC);

Reversible Fuel Cell (RFC);

Direct-methanol fuel cell (DMFC);

Molten-carbonate fuel cells (MCFC);

Phosphoric-acid fuel cells (PAFC);

The main problem of fuel cells is related to the need to have “packaged” hydrogen, which could be freely purchased. Obviously, the problem should be solved over time, but for now the situation raises a slight smile: what comes first - the chicken or the egg? Fuel cells are not yet developed enough to build hydrogen factories, but their progress is unthinkable without these factories. Here we note the problem of the hydrogen source.

Currently, hydrogen is produced from natural gas, but rising energy costs will also increase the price of hydrogen. At the same time, in hydrogen from natural gas, the presence of CO and H 2 S (hydrogen sulfide) is inevitable, which poison the catalyst.

Common platinum catalysts use a very expensive and irreplaceable metal - platinum. However, this problem is planned to be solved by using catalysts based on enzymes, which are a cheap and easily produced substance. The heat generated is also a problem. Efficiency will increase sharply if the generated heat is directed into a useful channel - to produce thermal energy for the heating system, to use it as waste heat in absorption

refrigeration machines

and so on.

Methanol Fuel Cells (DMFC): Real Applications

The greatest practical interest today is direct fuel cells based on methanol (Direct Methanol Fuel Cell, DMFC). The Portege M100 laptop running on a DMFC fuel cell looks like this:

The main task is to find options for using a methanol solution with its highest concentration. The problem is that methanol is a fairly strong poison, lethal in doses of several tens of grams. But the concentration of methanol directly affects the duration of operation. If previously a 3-10% methanol solution was used, then mobile phones and PDAs using a 50% solution have already appeared, and in 2008, in laboratory conditions, specialists from MTI MicroFuel Cells and, a little later, Toshiba obtained fuel cells operating on pure methanol.

Fuel cells are the future!

Finally, the great future of fuel cells is evident from the fact that international organization The IEC (International Electrotechnical Commission), which sets industry standards for electronic devices, has already announced the creation of a working group to develop an international standard for miniature fuel cells.

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 heat and power supply (or only power supply) were used Various types fuel cells.

Introduction

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

Originally used only in space industry, currently fuel cells are increasingly used in a variety of areas - as stationary power plants, autonomous sources of heat and power supply to buildings, engines Vehicle, 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 for production electrical energy

uses fuel supplied from an external source (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

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. Emissions of pollutants into the atmosphere from fuel cell operation are so low that in some areas of the United States their operation does not require special permission from government agencies

, controlling air quality.

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

fuel cells, they are continuously improved, and their cost is reduced. 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. 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 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).

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

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 us to completely avoid air pollution. It is planned to create a similar system, for example, in training center Adam Joseph Lewis at 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.

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 the external circuit is D.C., which 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 takes place 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).

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.

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 cells is used as power sources for a wide range of different devices, incl. 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 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 use. 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 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.

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.

Just as there are different types of internal combustion engines, there are different types of fuel cells - choosing the right type of fuel cell depends on its application. Fuel cells are divided into high temperature and low temperature. Low temperature fuel cells require relatively pure hydrogen as fuel. This often means that fuel processing is required to convert the primary fuel (such as natural gas) into pure hydrogen. This process consumes additional energy and requires special equipment. High Temperature Fuel Cells do not need this additional procedure, since they can carry out “internal transformation” of the fuel at elevated temperatures

, which means there is no need to invest money in hydrogen infrastructure.

Molten carbonate electrolyte fuel cells are high temperature fuel cells. High operating temperature allows direct use of natural gas without a fuel processor and fuel gas with low fuel calorific value production processes and from other sources. This process was developed in the mid-1960s. Since then, production technology, performance and reliability have been improved.

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 (CO 3 2-). 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: CO 3 2- + H 2 => H 2 O + CO 2 + 2e -
Reaction at the cathode: CO 2 + 1/2 O 2 + 2e - => CO 3 2-
General reaction of the element: H 2 (g) + 1/2 O 2 (g) + CO 2 (cathode) => H 2 O (g) + CO 2 (anode)

The high operating temperatures of molten carbonate electrolyte fuel cells have certain advantages. At high temperatures, natural gas is internally reformed, eliminating the need for a fuel processor. In addition, advantages include the ability to use standard construction materials such as stainless steel sheets and nickel catalyst on the electrodes. The waste heat can be used to generate high pressure steam for a variety of industrial and commercial purposes.

High reaction temperatures in the electrolyte also have their advantages. The use of high temperatures requires significant 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.

Phosphoric acid fuel cells (PAFC)

Phosphoric (orthophosphoric) acid fuel cells were the first fuel cells for commercial use. The process was developed in the mid-1960s and has been tested since the 1970s. Since then, stability and performance have been increased and cost has been reduced.

Phosphoric (orthophosphoric) acid fuel cells use an electrolyte based on orthophosphoric acid (H 3 PO 4) with a concentration of up to 100%. The ionic conductivity of phosphoric acid is low at low temperatures, for this reason 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: 2H 2 => 4H + + 4e -
Reaction at the cathode: O 2 (g) + 4H + + 4e - => 2H 2 O
General reaction of the element: 2H 2 + O 2 => 2H 2 O

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. In addition, CO 2 does not affect the electrolyte and the operation of the fuel cell; this type of cell works with reformed natural fuel. Simple design, low degree of electrolyte volatility and increased stability are also advantages of this type of fuel cell.

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

Proton exchange membrane fuel cells (PEMFCs)

Proton exchange membrane fuel cells are considered the best type of fuel cell for generating vehicle power, which can replace gasoline and diesel internal combustion engines. These fuel cells were first used by NASA for the Gemini program. Today, MOPFC installations with power from 1 W to 2 kW are being developed and demonstrated.

These fuel cells use a solid polymer membrane (a thin film of plastic) as the electrolyte. 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: 2H 2 + 4OH - => 4H 2 O + 4e -
Reaction at the cathode: O 2 + 2H 2 O + 4e - => 4OH -
General reaction of the element: 2H 2 + O 2 => 2H 2 O

Compared to other types of fuel cells, proton exchange membrane fuel cells produce more energy for a given fuel cell volume or weight. 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 operating. These characteristics, as well as the ability to quickly change energy output, are just some of the features 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, and therefore such fuel cells are cheaper to produce. Compared to other electrolytes, solid electrolytes do not pose any orientation issues, fewer corrosion problems, resulting in greater longevity of the cell and its components.

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 (O 2 -). Solid oxide fuel cell technology has been developing since the late 1950s. and has two configurations: flat 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 (O 2 -). 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.

Reaction at the anode: 2H 2 + 2O 2 - => 2H 2 O + 4e -
Reaction at the cathode: O 2 + 4e - => 2O 2 -
General reaction of the element: 2H 2 + O 2 => 2H 2 O

The efficiency of the produced electrical energy 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 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.

Direct methanol oxidation fuel cells (DOMFC)

The technology of using fuel cells with direct methanol oxidation is undergoing a period of active development. It has successfully proven itself in the field of powering mobile phones, laptops, as well as for creating portable power sources. This is what the future use of these elements is aimed at.

The design of fuel cells with direct oxidation of methanol is similar to 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. However, liquid methanol (CH 3 OH) oxidizes in the presence of water at the anode, releasing CO 2, 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: CH 3 OH + H 2 O => CO 2 + 6H + + 6e -
Reaction at the cathode: 3 / 2 O 2 + 6H + + 6e - => 3H 2 O
General reaction of the element: CH 3 OH + 3/2 O 2 => CO 2 + 2H 2 O

The development of these fuel cells began in the early 1990s. With the development of improved catalysts and other recent innovations, power density and efficiency have been increased to 40%.

These elements were tested in the temperature range of 50-120°C. Due to low operating temperatures and no need for a converter, direct methanol oxidation fuel cells are the best candidates for both mobile phones and other consumer goods, as well as in car engines. The advantage of this type of fuel cells is their small size, due to the use of liquid fuel, and the absence of the need to use a converter.

Alkaline fuel cells (ALFC)

Alkaline fuel cells (AFC) are one of the most studied technologies, used since the mid-1960s. by NASA in the Apollo and Space Shuttle programs. On board these spacecraft, fuel cells produce electrical energy and drinking water. 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: 2H 2 + 4OH - => 4H 2 O + 4e -
Reaction at the cathode: O 2 + 2H 2 O + 4e - => 4OH -
General reaction of the system: 2H 2 + O 2 => 2H 2 O

The advantage of SHTE is that these fuel cells are the cheapest to produce, since the catalyst required on the electrodes can be any of the substances that are cheaper than those used as catalysts for other fuel cells. In addition, SFCs operate at relatively low temperatures and are among the most efficient fuel cells - such characteristics can consequently contribute to faster power generation and high fuel efficiency.

One of the characteristic features of SHTE is its high sensitivity to CO 2, which may be contained in fuel or air. CO 2 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 must run on pure hydrogen and oxygen. Moreover, molecules such as CO, H 2 O and CH 4, which are safe for other fuel cells, and for some of them even act as fuel, are harmful to SHFC.

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.

Solid acid fuel cells (SFC)

In solid acid fuel cells, the electrolyte (C s HSO 4) does not contain water. The operating temperature is therefore 100-300°C. The rotation of the oxy anions SO 4 2- 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.

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.

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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 quantity 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.

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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.

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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 (sheet stainless steel 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.

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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.

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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.

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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.

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9. Comparison of the most important characteristics of fuel cells

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10. Use of fuel cells in cars

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