What would oxygen turn into another element?

The fuel cell converts chemical energy into electricity




Scheme of a fuel cell




Fuel cells generate electricity from hydrogen and oxygen, with varying degrees of heat being produced and water being produced as a "waste product". Like batteries and accumulators, they belong to the group of electrochemical energy converters, also known as "galvanic elements". In contrast to batteries, however, the energy supply from which they generate electricity is not stored in the cell, but is continuously supplied from the outside. You can therefore generate electricity as long as the supply of hydrogen and oxygen continues.

The first practically usable fuel cells were developed for use in submarines and spacecraft. During the moon landings as part of the Apollo program, fuel cells covered the spacecraft's electricity needs and at the same time supplied the astronauts with drinking water. Today we are working on using fuel cells as energy suppliers for electric cars, as power plants for the power supply and for many other purposes.

Electrodes and electrolyte

In principle, all fuel cells consist of two porous surfaces (electrodes), which are separated from each other by a third surface (electrolyte). Hydrogen is supplied to one electrode and oxygen to the other. Without the electrolyte, the two gases would mix: At best, normal combustion or an oxyhydrogen reaction would then occur. However, instead of normal combustion, the electrolyte causes an electrochemical reaction: positively charged hydrogen ions form on one electrode (anode) and negatively charged oxygen ions form on the other electrode (cathode). This creates an electrical voltage between the two electrodes, like between the poles of a battery. And as with this one, this voltage can be used practically if the electrodes are connected to one another via an external circuit.

Each cell has a voltage of around 1 volt

Like all galvanic elements, fuel cells generate direct current. The voltage per cell is around one volt, depending on the type. However, this "no-load" voltage drops more or less sharply as soon as a significant current flows. A compromise must therefore be found between current strength and voltage. As a rule, the cells are operated at a voltage of 0.7 volts.

In order to achieve higher voltages and outputs, the individual cells are put together in series or parallel connection to form stacks. These cell stacks are also referred to as "stacks" using an English expression. When "the" fuel cell is used as an electricity supplier, one always means a large number of cells of the same type, which have been combined in the form of "stacks" to form the desired power unit. The grid feed or the supply of AC devices takes place via an inverter.

In addition to electricity, there is always heat

The ideal fuel cell converts the chemical energy of the fuel gases one hundred percent into electricity. One speaks therefore of a "cold combustion" to emphasize the difference to normal combustion, in which the chemical reaction of the fuel with the oxygen in the air only produces heat as a comparatively inferior form of energy. In technical practice, however, things are not quite as ideal: Depending on the type, the fuel cell needs an operating temperature of 60 ° C to over 1000 ° C for electrochemical energy conversion. As a rule of thumb can apply. All fuel cells therefore not only generate electricity, but also heat. Naturally, this has a negative effect on their electrical efficiency. The energetic balance is improved, however, if the fuel cell is operated, for example, as a combined heat and power plant, which simultaneously supplies electricity and usable heat. In the case of high-temperature fuel cells, the waste heat can also be used to operate a downstream gas and steam turbine power plant, thus increasing the electrical efficiency.

The basic structure of a fuel cell is illustrated by this demonstration model of the PEMFC type, which was developed for teaching purposes and can therefore be dismantled: The electrodes are attached to the two sieve-shaped sheets (carbon felt with platinum as a catalyst). The electrolyte is located between the electrode sheets in the form of a solid plastic membrane. The demo cell has a voltage of 0.9 volts when idling and can set a corresponding tiny motor in rotation with 20 milliwatts. - Normal fuel cells are made up of a large number of such individual elements, which are combined by series or parallel connection to the desired performance level.