Sir William Grove was perceived as the discoverer of fuel cells in 1839, however, the initial developments prompting an operational fuel cell can be traced back to the early 1800’s. Throughout the rest of the century, scientists endeavoured to continue developing the fuel cells utilising various fuels and electrolytes. Further work that was carried out in the primary portion of the twentieth century provided the framework for systems eventually used in the Gemini and Apollo space flights. In 1959 Francis T.Bacon successfully demonstrated the first completely operational fuel cell.
Proton exchange membrane fuel cells were first used by NASA in the 1960’s as a major aspect of the Gemini space program, and were utilised on seven missions. Those fuel cells used pure oxygen and hydrogen as the reactant gases and were small-scale, expensive and not commercially viable. NASA’s interest and energy crisis in 1973 pushed further development of fuel cells. Since then, fuel cell research has continued unabated and fuel cells have been used successfully in a wide variety of applications.
Basic Principle
A device that converts chemical energy stored in gaseous molecules of a fuel and oxidant into electrical energy in a constant temperature process is known as a fuel cell. Unlike storage cells, fuel cells can be continuously fed with a fuel hence the electrical power output is sustained indefinitely. When hydrogen is used as the fuel, the only by-products formed are pure water and heat, i.e. the overall process is the reverse of water electrolysis. In electrolysis, an electric current is applied to water to give hydrogen and oxygen; in this process, hydrogen and oxygen are combined to produce electricity and water with addition of small amount of heat.
The single cell structure known as the membrane electrode assembly consists of an electrolyte which in contact with two electrodes (anode and cathode) on its both sides. Anode is a negative electrode and cathode is positive. Fuel is continuously fed to anode and oxidant is continuously fed to cathode.
Reactants of fuel cell are categorised on the basis of their electron donor and acceptor properties as fuels and oxidants. Fuels include pure hydrogen and hydrogen-containing gases, for example: methanol, ethanol, natural gas, biogas, diesel, etc. Oxidants mainly include pure oxygen and oxygen-containing gases, such as: air or halogens.
Most common fuel cell is the hydrogen driven fuel cell. In this type of cells hydrogen is converted into water through combustion and this reaction is spilt into two electrochemical reactions occurring at the anode and cathode, this is called the two half-cell reactions:
Hydrogen oxidation reaction at the anode: H2 = 2H+ + 2e–
Oxygen reduction reaction at the cathode: ½ O2 + 2H+ + 2e– = H2O
Combination of the two half-cell reactions gives the overall combustion reaction:
H2 + ½O2 ®H2O
The role of electrolyte in any fuel cell configuration is very important as it must insulate the two half-cell reactions electrically, while allowing ionic channel of protons to be produced from anode to cathode side where they combine to form a molecule of water. Therefore, electrolytes are conductors of protons and insulators of electricity.
Electrolytes also need to be impermeable to gaseous molecules to keep the anodic and cathodic compartments separate, and hence prevent any parasitic reactions resulting from crossover of gases. Furthermore, electrolytes need to be chemically resistant to any reactant or product during the process.
Electrons are hindered through the electrolyte, forcing them to flow another way. To this purpose, electrodes are connected to an external electrical circuit, which acts as a pathway for electrons. This arrangement allows direct collection of electricity. High surface area is a common and preferred feature of fuel cell electrodes as it helps maximise each half-cell reaction zone, therefore the electrode materials required are made out of relatively porous compounds.
Every type of fuel cell is characterized by its own particular geometry, dimensions, and materials; yet, the core of the device remains the same: it consists of an electrolyte, two electrodes, and two gas backing layers and most often, bipolar plates separating unit cells.
For the gas backings five main requirements must be fulfilled:
1.Good electronic conductivity to transport the electrons from the electrochemical oxidation of hydrogen most efficiently;
2.High gas permeability to allow easy access of the gas reactants from the feeding source to the reaction zone;
3.High porosity to optimize product water management in the system;
4.Good resistance strength to give a mechanical support to the MEA;
5.High corrosion resistance to the acidic environment in the fuel cell.
The bipolar plates are the interconnecting components that collect the electrons and drive them to the external circuit. They are grooved with channels for gas flow input and output and must manage water as well as possible. The design and the geometric dimensions of the channels (in the order of 1 mm) are crucial for obtaining a homogeneous transport of gases on the whole surface of electrodes, evacuate liquid water droplets formed by the fuel cell reaction, thus achieving stable continuous operation. As every component in a fuel cell, they must be corrosion-resistant; but unlike gas backings, the bipolar plates must be gastight.
A fuel cell can be seen with profit as a “chemical factory” that continuously transforms fuel energy into electricity as long as fuel is supplied. However, unlike internal combustion engines that can be regarded as factories as well, fuel cells rely on an electrochemical reaction involving the fuel, and not on its combustion.
During combustion, molecular hydrogen and oxygen bonds are broken and electrons reconfigure into molecular water bonds at a picosecond length scale. There is no possible way to “catch up” these free electrons and the net energy difference between molecular bonds in products vs. reactants can only be recovered in the most degraded form of energy, i.e. heat. A Carnot cycle involving the transformation of heat into mechanical and electrical energy is then involved in conventional methods for generating electricity: these successive steps of transformation of energy severely limit the overall efficiency of the process (which is by definition the product of the efficiency of the different steps).
In a fuel cell, the direct conversion of the chemical energy of covalent bonds into electrical energy is made possible through the spatial separation of the hydrogen and oxygen reactants by the electrolyte also called the “separator”. The electron transfer necessary to complete the bonding reconfiguration into water molecules occurs over a much longer length scale. This allows direct collection of electrons as a current in fuel cells and leads to fuel efficiencies two to three times higher than in internal combustion engines (depending on the fuel cell technology).
Unlike batteries, there is no chemical transformation of any component of the fuel cell device during operation and it can generate power without recharging, as long as it is being fed with fuel.