FUEL CELLS

 

Hydrogen

Before its discovery, hydrogen had been mistaken for other gases. In 1766, the English chemist Henry Cavendish showed that hydrogen was formed by applying the sulphuric acid on metals and therefore, he was considered the discoverer of hydrogen. Later, he showed that water was the result of the reaction between hydrogen and oxygen. In 1781, Joseph Priestley called this gas „the inflammable air”. The French chemist Antoine Laurent Lavoisier named this gas hydrogenium (it forms water). Liquid hydrogen was first produced in 1898 by James Dewar.


Hydrogen is the most frequent element in the Universe. In space, it is present in three forms: ions (protons), atoms and diatomic molecules. On Earth, it appears only in molecules.
In combination with other elements, hydrogen is widespread, and its most frequent and important form is water (H2O). Water is the basis of life.

 


Figure 3.1


Hydrogen has the properties of a gas and rises, due to its density which is smaller than that of the air. Beaware when using hydrogen in close spaces, where hydrogen may combine with air and the resulting mixture explodes.


In figure 3.1, energy values are presented for different types of fuels.
In the diagram, one can notice the big difference between hydrogen and other types of fuels.

 


Fuel Cell Construction and Operation


The mode of operation of a fuel cell is presented in figure 3.2. In a fuel cell, hydrogen reacts with oxygen forming water. The two gases are separated by an electrolyte (for example, an artificial-plastic membrane) and exchange negative electrons through an electric conductor.

This flow of electrons transforms the cell into a source (stream) of energy. The resulting heat will also be used. At anode, hydrogen will be split into positive ions and negative electrons, by means of a catalyst. A distinction must be made between the actual sense of the electrons in the circuit and, implicitly, that of the electric current, and the conventional sense.

Figure 3.2

Each hydrogen atom will give up an electron, which will head for the cathode (the positive terminal) through a conductor. Thus, the electric current circulates, and by means of it, the hydrogen ions will combine with oxygen, resulting water.

Figure 3.3

According to the laws of electrochemistry, one fuel cell can supply a voltage of 1 volt. In order to get higher voltages more cells must be serially connected, and for higher currents more cells must be connected in parallel. Therefore, cell panels are obtained.


The chemical reactions are presented in figure 3.3.


In a fuel cell, the chemical energy accumulated in the fuel is converted to electrical energy and thermal energy.

Fuel energy = Electrical energy + Thermal energy

Figure 3.4 presents the „products” of the fuel cell.

Figure 3.4

When burning the fuel, its energy is released through the heat of the reaction. In a fuel cell, hydrogen reacts with oxygen, and on a mole of hydrogen, an amount of energy of 286kJ is given off. This value is the enthalpy of the H reaction or, at constant pressure, is called calorific value.


Fuel Cell Applications

Fuel cells are mostly used for the propulsion of the vehicles or for steamers. The claims on the fuel cell are tremendous, because of a small volume, they supply a power of over 50 kW and must guarantee a long operating time. They are generally supplied with high-pressure hydrogen. Hydrogen will be compressed to 700 bar and stored in safe tanks.


The concepts of propulsion with liquid hydrogen or the ad hoc production of the hydrogen have been accomplished successfully. PEFC is used as type of fuel cell.

Most automobile manufacturers developed fuel-cell automobiles, which are in the stage of tests. Fuel cells are used both as batteries in small devices and for portable generators of up to 1 kW. They are an efficient alternative of re-chargeable cells. In comparison with batteries, they offer a longer operating time and an unlimited number of recharging cycles.
The greatest advantages are the reduced weight and an increased power of recharging. The most common in use fuel cells are PEFC and DMFC.


The fields of use are communication devices, torches, mobile phones, laptops, and portable devices for producing electrical energy.


Types of Fuel Cells

Phosphoric Acid Fuel Cells (PAFC)

The operating temperature of 130-200oC creates ideal conditions to use the phosphoric fuel cell for the steady production of energy in small domestic central heating systems.
The first commercial plants are already on the market and serve to supply __blocks and small factories with heat and electric current. Phosphoric fuel cells are used together with hydrogen. Gas or methanol can also be used by means of a converter.

Characteristics:

  1. ions of hydrogen are exchanged, so the membrane must be permanently wet;
  2. the liquid phosphoric acid is used as electrolyte;
  3. plants are sensitive to carbon monoxide, which involves cleaning the gas from the process;
  4. the operating temperature: 130-200 oC;
  5. power: 50-500 W;
  6. efficiency: 48-60 %;
  7. they are used for domestic central heating systems.


Molten Carbonate Fuel Cell (MCFC)

The high operating temperature, 650oC, also facilitates, besides the production of electric current and heat, the production of steam. Due to the high temperature, inside the cell, the gas may be coverted internally into hydrogen and carbon dioxide. An external converter is not necessary. The high temperatures and the liquid salts of the electrolyte attack the material.

Characteristics:

  1. only combustion gases which contain carbon can be used, hydrogen cannot be used;
  2. a problem is represented by the slow dissolution of the cathode into the electrolyte;
  3. more resistant materials should be used;
  4. operating temperature: 650 oC;
  5. there are equipments which develop 250 kW, and others, of 2.2 MW, are being developed at present;
  6. efficiency: 48-60 %;
  7. they are used for domestic central heating systems.


Solid Oxide Fuel Cell (SOFC)

The solid oxide fuel cell operating temperature is the highest, 800 -1000oC, and it will also be used in industrial thermal power stations. Smaller domestic systems can also be developed. For the latter type, a tube-like form is also being studied.
The cell operates with hydrogen, which, due to high temperaturates can be obtained from natural gas, through an internal process.

Figure 3.5 presents the operating principle of a solid oxide fuel cell (SOFC).

Figure 3.5

 

Characteristics:

  1. oxygen ions are exchanged in a ceramic electrolyte of improved zirconium;
  2. it is necessary to find thinner electrolytes, which can operate at lower temperatures;
  3. power: 1-2,5 MW;
  4. they are particularly used for thermal power stations.


Alkaline Fuel Cell (AFC)

The alkaline fuel cells were used aboard the spaceship Apollo. They represent the classical fuel cells. Due to their electrical efficiency and the easily controllable operating temperature of 80 oC, they are still used for space flights at present.

Characteristics:

  1. the reaction is very sensitive due to the traces of carbon dioxide and oxygen;
  2. the operating temperature: 60-90 oC;
  3. plants with powers from 1 kW to 120 kW have been created;
  4. the efficiency is up to 60 %;
  5. they are used for space flights and yachts.


Polymer Electrolyte Fuel Cell (PEFC)

The name of this cell comes from a foil of polymer used as a partition between the anode and cathode. The prototypes are used as energy sources for vehicles, domestic central heating or batteries for laptop, mobile phones. The cell uses refined hydrogen. Natural gas and petrol can also be used with a converter.

Characteristics:

  1. positive hydrogen ions are exchanged, so the membrane must be permanently wet;
  2. plants are sensitive to carbon monoxide, which involves cleaning the gas used in the process;
  3. the operating temperature: 0-80 oC;
  4. power up to 250 kW;
  5. efficiency with hydrogen: 60%;
  6. efficiency with methane: 40%;
  7. they are used for electric current supply, in the field of automobiles, and for thermal power stations.

Direct Methanol Fuel Cell (DMFC)

Methanol is liquid, so it can be used easily. It operates without a converter.
The electrolyte is a membrane, and the operating temperature is 60-130 oC

Characteristics:

  1. positive hydrogen ions are exchanged;
  2. the operating temperature: 60-130 oC;
  3. they are used for small devices.