Function and maintenance of fuel cells

Fuel cells are regarded as one of the drive options for cars and trucks. Commercial vehicle manufacturers in particular rely on this technology. How do fuel cells work and what is important when it comes to maintenance?

Fuel cells: And who invented it?

The principle of fuel cells was discovered in 1838 by the German-Swiss physicist Christian Friedrich Schönbein. He surrounded two platinum wires in sulphuric acid (electrolyte) with hydrogen and oxygen and was able to detect an electrical voltage between the wires. The British physicist Sir William Grove was also on board. Even back then, numerous scientists postulated that water could be the coal of the future. But of course, electricity was needed to produce hydrogen even at that time. Ideally we talk today about green hydrogen produced with electricity generated from renewable energies.

The structure of fuel cells

Fuel cells are energy converters. The chemical reaction energy of hydrogen and oxygen is converted into electrical and thermal energy. The oxidation of the hydrogen and the reduction of the oxygen is achieved by spatial separation using an electrolyte. The violent reaction of the well-known "oxyhydrogen test", or the energy released when hydrogen and oxygen react to form water, can be utilised.
There are a variety of fuel cell types that use natural gas or methanol as fuel, for example, as well as other electrolytes or oxidising agents. However, the most common type for use in cars/trucks is the hydrogen-oxygen fuel cell, for example the low-temperature proton exchange membrane fuel cell (PEMFC).

PEM fuel cells

The centrepiece of PEM fuel cells is the stack. The stacks contain a large number of proton exchange membranes arranged in series, which are permeable to protons, while the transport of hydrogen and oxygen is prevented. The individual membrane (solid electrolyte) consists of a central foil to which the electrodes (anode (-) / cathode (+), including a catalyst, are attached on each side. The PEM is surrounded by a gas-permeable diffusion layer.

"Bipolar plates" are arranged around the electrode-membrane units in the stack. "Bipolar" therefore refers on the one hand to the hydrogen-bearing anode plate and on the other hand to an oxygen-bearing cathode plate. The bipolar plates are used for the homogeneous distribution of hydrogen and oxygen, for sealing to the outside, for cooling the fuel cells and for the electrical connection of the cells. They are characterised by complex channels and are usually made of graphite, metal or composite materials.

The current flow

In the centre of the PEM fuel cells is the membrane in a rectangular plate shape, wrapped in the bipolar plates. The anode is supplied with hydrogen, the cathode with oxygen. The two electrodes are connected to one another. The precious metal catalyst splits the gas molecules. The hydrogen molecules (H2) are split into two protons. Each hydrogen atom releases its electron. Now the protons migrate through the semi-permeable membrane to the opposite, positively charged cathode, while the electrons take the route via the connecting line between the anode and cathode. When the protons and electrons of the hydrogen reach the cathode side, they react with the oxygen there to produce water. The by-product of this chemical reaction is heat and electrical energy. A voltage can be tapped at the end plates or at the junction of the anode and cathode.

Ancillary systems ensure optimum functionality

In addition to the fuel cell stack, various ancillary systems such as special hydrogen tanks, compressors, DC/DC converters, the recirculation section (for example with a blower) and a cooling system ensure that a fuel cell functions optimally. The filtration of the intake air and the regulation of humidity in the system play a decisive role here. The fuel cells are controlled and monitored by an electronic unit.

Type IV hydrogen tanks have recently been delivered from Forvia's new series production facility in Allenjoie, France. This plant, which is the first of its kind in Europe and North America, is expected to produce 100,000 tanks per year. Photo: Forvia

What influence does moisture have on fuel cells?

If the air is too dry, the membrane in the fuel cell stack dries out. This can reduce the mechanical strength of the membrane, which is responsible for proton transport. The reaction product water can be transferred from the exhaust air, the humid side, to the supply air, the dry side of the fuel cells, via a humidifier.

However, if the humidity of the air is too high, it can condense into water droplets. These can block the fine structures of the gas diffusion layer or the microporous layer. Water droplets that strike the turbine side of the electric compressor also have a negative effect on the durability of the fuel cells. For this reason, additional water separators are used.

Undesirable noise

Acoustics in fuel cells is an interesting topic, as we are not dealing with mechanical components such as in a combustion engine. In fact, disturbing noises can occur in the electric compressor or as flow noises in the pipes. Appropriate resonators attenuate the undesirable noise.

The maintenance of fuel cells

The fuel cell stack itself (theoretically) requires no servicing and is designed as an assembly for a high number of operating hours. However, the filter elements must be replaced at regular intervals. The granulate in the ion exchanger, which ensures that the conductivity of the coolant is in the correct range, is also system-specific. This must also be changed at defined intervals. The same applies to the cathode air filter. It must also be changed regularly. Then there is the cooling circuit. Other service-relevant components are located here: the ion exchanger filter and the coolant particle filter. This means that car and truck workshops should not run out of work, even when servicing vehicles with fuel cells.

The overall balance

Fuel cell vehicles, especially trucks, show their strengths where long ranges (such as long distances) and short refuelling times are required. They can be refuelled in just a few minutes. If the "Well2Wheel" approach is taken into account (from the production of hydrogen to the drive energy), experts estimate that fuel cells achieve an efficiency of 30 to 40 per cent. Synthetic fuels (also based on green hydrogen) have an efficiency in the range of around 20 to 40 per cent. At 60 to 70 per cent, battery electric vehicles show significantly higher values here, but are currently still subject to limitations in terms of charging time and range. In principle, however, the values depend heavily on the respective technology, the site conditions, the energy source and other factors. Rapid technological advances may continue to have a major impact on the efficiency of each of the three technologies in the future. However, hydrogen as a fuel represents a highly interesting opportunity to decouple the production of green hydrogen and its use both in terms of location and time and thus to be able to offer efficient and environmentally friendly drive systems. Fuel cells are therefore a highly interesting option in the drive mix of the future.