A fuel cell consists of an anode, a cathode and an ion conductor. The electrodes are metal plates that work as a catalyst and are coated with platinum or palladium. Between the two electrodes is an electrolyte, which may be in the form of a membrane and provides for ion exchange. The electrolyte can be a dielectric, an acid, alkali or a polymeric membrane. Polymeric membranes( PEM) or proton membranes (PEM) can be penetrated by protons and can pass through the membrane from the anode to the cathode. For gases, the membranes are relatively impermeable.
Hydrogen (H2) or a hydrogen-containing compound such as methane or methanol flows around the anode of the fuel cell. Oxidation takes place at it, forming hydrogen ions (protons). The hydrogen ions migrate through the ion conductor and react with the oxygen (O2) at the cathode to form water (H2O). The reduction result is pure water. These hydrogen-oxygen fuel cells are the most advanced and can be used in electric vehicles.
The energy generation of a fuel cell starts as soon as hydrogen is supplied to the anode. The properties and structure of the ion conductor are of particular importance. Fuel cells are designated according to the membrane technology. For example, there is the PEM fuel cell, which uses a proton exchange membrane (PEM). It is also known as a Polymer Electrolyte Fuel Cell (PEFC). There are also the solid oxide fuel cell( SOFC), the alkaline fuel cell( AFC), the direct methanol fuel cell( DMFC), the phosphoric acid fuel cell( PAFC) and the molten carbonate fuel cell ( MCFC). In terms of working temperature, fuel cells can be divided into the groups of high-temperature fuel cells, medium-temperature fuel cells and low-temperature fuel cells.
Characteristics of fuel cells
The theoretical voltages generated by fuel cells are 1.23 V without load. In practice, cell voltages are between 0.5 V and 1 V. Higher voltages and power can be achieved by connecting several fuel cells in series. Such a fuel cell stack is called a stack. The total voltage of a stack is the sum of the fuel cells connected in series. The total voltage can be tapped at the two end plates of the stack.
Fuel cells are suitable for energy storage; at 100 Wh/kg to 1,000 Wh/kg, their energy density is many times higher than that of batteries. In contrast, the power density of 2 W/kg to 200 W/kg is lower than that of batteries or supercapacitors. The efficiency of fuel cells is up to 70 %, depending on the type of fuel cell.
Fuel cells are used in emergency power generators, in UPS systems and in fuel cell electric vehicles, for energy supply and electrical energy storage. In miniaturized size, micro fuel cells are also used as micro generators to power miniature systems and components.