Fuel Cells

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Rechargeable batteries are not clearly enough to power electric cars for a considerable period of time. Without reliable and capacious batteries, you lose the challenge of mass mobility. So the industry is working more actively to the problem of the sources of alternative energy to oil. And the direction of the most promising alternative to lithium batteries, are the fuel cells.

The basic principle of the functioning of these cells was discovered by British scientist Sir William Grove in 1839. It is known as the father of the fuel cell. William Grove It has generated electricity by changing the electrolysis of water to extract hydrogen and oxygen.

After disconnecting the battery from the electrolytic cell, Grove He was surprised to find that the electrodes began to absorb the released gas and generate power. The discovery of the cold combustion process “idrochimica” Hydrogen has become a significant event in the energy sector.

The term itself “Fuel cell” He appeared later was proposed in 1889 gives Ludwig Mond e Charles Langer, who they have tried to create a device for the generation of electricity from the air and from the coal gas.

During normal combustion, oxygen oxidizes fossil fuels and the fuel chemical energy is converted into thermal energy in an inefficient way. It has been discovered that it is possible to perform the oxidation reaction, for example, hydrogen with oxygen, in an electrolytic medium in the presence of electrodes for obtaining an electrical current. Eg, supplying hydrogen at an electrode located in an alkaline medium, we get electrons:

2H₂ + 4OH- → 4H₂O + 4e-

that, passing through the external circuit, ranging electrode opposite, to which oxygen enters and where the reaction takes place:

4e- + O₂ + 2H₂O → 4OH-

You can see that the resulting reaction 2H₂ + O₂ → H₂O It is the same as the normal combustion, but in a fuel cell, or in an electrochemical generator, an electric current is generated with great efficiency and partially heat.

Note that carbone, carbon monoxide, alcohols, hydrazine and other organic substances may also be used as a fuel in fuel cells and air, hydrogen peroxide, chlorine, bromo, nitric acid, etc. They can be used as oxidizing agents.

In 1965 fuel cells were tested in the United States on the ship Gemini 5 e, subsequently, the Apollo ships for flights to the moon and in the Shuttle program.

The next phase of the rapid development of fuel cells began in the '90s and continues now. It is caused by the need for new and efficient energy sources in relation, on one side, the global environmental problem of the increase of greenhouse gas emissions from the combustion of fossil fuels and, on the other, with the exhaustion of such fuels. Since water is the final product of hydrogen combustion in a fuel cell, They are considered the cleanest environmental impact point of view. The main problem is just finding a cost-effective way to produce hydrogen.

Billions of financial investments in the development of fuel cells and hydrogen generators should lead to a breakthrough technology and make their use in the life of every day a reality. Already automotive giants like Ballard, Honda, Daimler Chrysler and General Motors are already testing cars and buses with fuel cell 50 kW. Several companies have developed demonstration plants that use fuel cells with a solid oxide electrolyte with a capacity of up to 500 kW. Ma, despite a significant step forward in the improvement of fuel cell performance, you need to solve many more problems related to their cost, reliability and safety.

In a fuel cell, unlike batteries and accumulators, both fuel and the oxidant are supplied from the outside. The fuel cell is just a mediator in the reaction and in ideal conditions might work for almost always. The beauty of this technology is that, in fact, l’element burns fuel and directly converts the energy released into electricity. With the direct combustion of fuel, It is oxidized with oxygen and the heat generated in this process is used to perform useful work.

In a fuel cell, as in batteries, the oxidation reactions of the fuel and oxygen reduction are spatially separated and the process of “combustion” It proceeds only if the cell emits current to the load. It is like a diesel generator, only without a diesel engine, smokeless, noise, overheating and with much higher efficiency. The latter is explained by the fact that, in the first place, there are no intermediate mechanical devices and, secondly, the fuel cell is not a heat engine and therefore does not obey the law of Carnot (or, its efficiency is not determined by the temperature difference).

The oxygen is used as oxidizing agent in fuel cells. Furthermore, since the oxygen in the air is quite, you do not need to worry about the supply of an oxidizing agent. As for the fuel, it is hydrogen. Then, the reaction occurs in the fuel cell:

2H₂ + O₂ → H₂O + electricity + heat.

Ithe result is useful energy and water vapor. The simplest design is a fuel cell with a proton exchange membrane (see Figure 1). It works as follows: the hydrogen that enters the element decomposes under the action of the catalyst into hydrogen ions and electrons H + positively charged. Then comes in a special membrane, which acts as an electrolyte in a conventional battery. Because of its chemical composition, the protons pass through itself, but traps electrons. Therefore, the electrons accumulated on the anode creates a negative charge in excess and hydrogen ions create a positive charge on the cathode (the element voltage is approximately 1 V).

To create high-power, a fuel cell is constituted by a plurality of cells. If you include an element in the load, the electrons will flow therethrough to the cathode, creating a current and completing the process of oxidation of hydrogen by oxygen. As a catalyst in said fuel cells, They are typically used microparticles of platinum deposited on a carbon fiber. Thanks to its structure, such a catalyst passes well gas and electricity. The membrane is typically made of a polymer Nafion sulfur-containing. The thickness of the membrane is of tenths of a millimeter. During the reaction,, naturally, Also heat is released, but it is not so much, then the operating temperature is maintained in the range 40-80 ° C.

Fig. 1 The principle of operation of the fuel cell

There are other types of fuel cells, that differ primarily in the type of electrolyte used. Almost all they require hydrogen as a fuel, so is a logical question: where to find it. Naturally, it would be possible to use compressed hydrogen in special containers, but then immediately arise problems associated with the transport and storage of this highly flammable gas at high pressure. Naturally, it is possible to use hydrogen in bound form as in the batteries to metal hydrides. But still remains the task of its extraction and transportation, because there is no infrastructure of hydrogen filling stations.

However, There is also a solution: the liquid fuel based on hydrocarbons can be used as a source of hydrogen. Eg, ethyl or methyl alcohol. It's true, here you are already need a special additional device: a fuel converter, which converts the alcohols in a mixture of gaseous H2 and CO2 at high temperature (for methanol will be somewhere around 240 ° C).

Fig.2 The principle of operation of a direct methanol fuel cell

The fundamental difference between the filling elements of hydrogen and methanol is the catalyst used. The catalyst in the direct methanol fuel cell removes the protons directly from a molecule of alcohol. Therefore, the fuel problem is solved: methyl alcohol is produced in series for the chemical industry, it is easy to store and transport and to load a fuel cell in methanol, it is sufficient to simply replace the cartridge with fuel. It's true, there is a downside: methanol is toxic. Furthermore, the efficiency of a direct methanol fuel cell is significantly lower than that of a hydrogen.

Fig.3. A fuel cell in methanol

The most attractive option is to use ethanol as fuel, since the production and distribution of alcoholic beverages of any composition and strength is well established around the world. However, the efficiency of fuel cells based on ethanol, unluckily, It is even lower than that of methanol.

As already noted in the field of fuel cells in many years of development, various types of fuel cells have been constructed. The fuel cells are classified by electrolyte and fuel type.

1. Electrolyte hydrogen-oxygen solid polymer.

2. Fuel Cells solid polymeric methanol.

3. Elements of alkaline electrolyte.

4. Fuel Cells phosphoric acid.

5. Fuel Cells of carbonates melted.

6. Solid Oxide Fuel Cells.

Ideally, the efficiency of fuel cells is very high, but in real terms there are losses associated with non-equilibrium processes, as ohmic losses due to the conductivity of the electrolyte and the electrodes, activation polarization and concentration and diffusion losses. Consequently, part of the energy generated in the fuel cells is converted into heat. The efforts of specialists aim to reduce these losses.

The main source is the ohmic losses, and the reason for the high price of fuel cells, are the membranes for the exchange of ions. They are now looking for alternative proton conducting polymers cheaper. Since the conductivity of these membranes (solid electrolytes) It reaches an acceptable value (10 Ohm / cm) only in the presence of water, the gases supplied to the fuel cell need to be further moistened in a special device, which also makes the system more expensive. In the catalytic gas diffusion electrodes, Some are used mainly platinum and other noble metals, and so far, no replacement was found equally efficient. Although the platinum content in fuel cells is of different mg / cm2, for large batteries its quantity reaches tens of grams.

During the design of fuel cells, It is paid much attention to the heat removal system, since high current density (up to 1A / cm2) the system overheats alone. For cooling, is used the water that circulates in the fuel cell through special channels and, low capacity, is used ventilation.

Then, in addition to the fuel battery cells in the same, a modern system of an electrochemical generator “grow up” with many auxiliary devices, come: pump, a compressor for the air supply, a hydrogen inlet, a gas humidifier, a cooling unit, a gas leakage control system, a DC converter to AC and a control processor and other, all this leads to the fact that the cost of the fuel cell system during the period 2004-2005 was of 2-3 thousand $ / kW. According to experts, fuel cells will be available for use in transport and stationary electric power plants at the price of 50-100 $ / kW.

For the introduction of fuel cells in everyday life, along with cheaper components, We must wait for new ideas and original approaches. In particular, high hopes are associated with the use of nanomaterials and nanotechnologies. Eg, several companies have recently announced the creation of super efficient catalysts, in particular, for an oxygen electrode based on nanoparticles of various metals groups.

There have been reports of fuel cells projects devoid of membrane in which the fuel cell is supplied liquid fuel (such as methanol) along with an oxidizing agent. An interesting concept is the concept developed elements of biofuels that operate in polluted water and consume dissolved oxygen as an oxidizing agent and organic impurities as a fuel.

First of all, it is necessary to solve the problem of miniaturization of fuel cells. After all, smaller is the fuel cell, less will be the power that will be capable of delivering, then they are constantly being developed new catalysts and electrodes that allow, at small sizes, to maximize the work surface.

The latest developments in the field of nanotechnology and nanomaterials (for example nanotubes) they are useful.

The second important issue that needs to be addressed is the price. In fact, as a catalyst in the majority of fuel cells, It is used very expensive platinum. Once again, some manufacturers are trying to make the most of the silicon technology already well developed.

Researchers have demonstrated the possibility of producing hydrogen from water and store it right between the carbon atoms of the graphene. To achieve this “mini factory” hydrogen, They have used a nickel-coated surface of graphene. “Combining the properties of the two materials”, affirm, “the device works as a catalyst that breaks down the molecules of water and divides them into hydrogen atoms and hydroxyl groups (OH)”. The hydrogen produced is then absorbed on the same graphene, which then it provides a natural system of storage at room temperature. To release the hydrogen, just heat the graphene at temperatures of about 130 ° C.

News of the first of December of the year just ended, Bosch batteries will develop fuel cell in the city of Wuxi, in eastern China, in order to supply the local emerging market for fuel cell vehicles.

We should not have to go abroad to find this technology, in Piedmont are experimenting powered by fuel cells trains.

As you can see the ultimate solution is just around the corner but many are scrambling to find, This is what happens with all the not yet mature technologies.