The fuel cell
Reading time approx. 7 minutes – This first of two articles on the fuel cell looks at how a fuel cell works and the different types of fuel cell. If you have any questions or comments about this article, please feel free to contact us personally.
The social and political developments of recent years have brought the fuel cell, like the battery, back into the focus of research and the media. Just as the functioning of batteries has been known for some time, the fuel cell can also look back on an eventful history. As early as 1838, Christian Friedrich Schönbein discovered the effect of the so-called cold combustion of oxyhydrogen gas. However, due to the technically complex functioning of the fuel cell, inventions such as the steam engine and the petrol engine were able to prevail in the past, with a few exceptions. Only the awareness of the impending dangers of further global warming and the resulting regulation of pollutant emissions could enable the fuel cell to achieve a breakthrough in the future. As interest in this technology is growing, this article looks at how it works and the possibilities of this development.
How the fuel cell works
Unlike the battery, the fuel cell is not an energy storage device but solely an energy converter. The energy must be constantly supplied in chemically bound form for operation. Similar to the battery, the structure consists of two electrodes coated with a catalyst – the anode and the cathode – which are separated by an ionic conductor, the electrolyte. The energy is supplied by a chemical reaction between a fuel and an oxidising agent, both of which are continuously fed in via the electrodes.
One of the best-known fuel cells is the PEMFC (Proton Exchange Membrane Fuel Cell), often just called PEM. In it, molecular hydrogen with the molecular formula H2 is oxidised to protons or cations 2H+ at the anode with the help of a catalyst, releasing electrons. While the protons are fed to the oxidising agent through the electrolyte, the electrons flow to the cathode via an electrical consumer outside the fuel cell. There the oxidising agent, in this case atmospheric oxygen, is reduced to anions by absorbing electrons and reacts with the hydrogen ions to form water.
Regardless of the individual reaction equations, which depend on the pH value of the electrolyte, the following reaction equation results:
2H2 + O2 -> 2H2O + electrical and thermal energy
Even if the electrical voltage that occurs during the operation of fuel cells differs for the different variants, values above 1 VDC are rarely reached in practice. However, as these voltages cannot be utilised in a technically sensible way, several cells are connected in series to form a so-called stack, similar to a battery. Depending on the application, these stacks can supply voltages far in excess of 1000 VDC.
Variants and areas of application of a fuel cell
While the basic structure and mode of operation remain the same for all fuel cells, different variants can be distinguished by varying the electrolyte. Typical electrolytes consist of liquids, plastics or ceramics. The resulting operating parameters such as usable fuel, operating temperature and efficiency predestine them for specific applications.
– Alkaline fuel cell (AFC)
In the alkaline fuel cell, the oldest variant, potassium hydroxide solution is used as the electrolyte. As caustic potash reacts with CO2, only high-purity fuels can be used here. Until now, they have generally been used in space travel.
– Membrane fuel cell PEMFC or PEM
To operate the membrane fuel cell, the electrodes must be coated with a catalyst, e.g. platinum, which has a negative impact on manufacturing costs. As they can be operated with atmospheric oxygen at approx. 10-100°C and are relatively easy to handle, they are often used in mobile applications.
– Direct methanol fuel cell DMFC
The DMFC is a further development of the PEM and also works with a plastic membrane as the electrolyte. However, it can work with methanol instead of hydrogen at 60°-130°C, which offers several advantages when storing the fuel. This fuel cell could therefore be used in portable electronic devices as well as in vehicles in the future.
– Phosphoric acid fuel cell PAFC
This type of fuel cell, developed for continuous use, converts hydrogen-rich gas, such as natural gas and atmospheric oxygen, using an electrolyte of phosphoric acid bound in a plastic fleece. The reaction that occurs at approx. 200°C is particularly suitable for small and combined heat and power plants.
– Molten carbonate fuel cell MCFC
The MCFC, which operates at around 650°C, converts natural gas and atmospheric oxygen into electrical and thermal energy. This enables efficient use in power stations with sensible utilisation of waste heat.
– Solid oxide fuel cell SOFC
This cell, which consists exclusively of solids, requires a temperature between 500° and 1000°C and can be operated with hydrogen, natural gas, diesel or petrol without expensive catalysts. It is generally used in large power stations.