Fuel cells

Functionality, types and applications

Source: stock.adobe.com

DiLiCo engineering's measurement technology has so far been used most for fuel cells. But what actually is a fuel cell? How does this technology work, what are the different types of fuel cells and where are they used? Which applications are already ready for the market and which still require further research and development work? You will find answers to these questions on this page. In addition, current research topics in the fuel cell sector are discussed and it is presented in which research projects DiLiCo eingineering is contributing its expertise and actively promoting the development of this technology.

What are fuel cells?

The fuel cell is one of the hydrogen technologies and, like the redox flow battery or the electrolyser, is an electrochemical energy converter. Through an electrochemical reaction, chemical energy is converted into electrical energy. This happens in the fuel cell through a so-called cold combustion. In this process, a fuel reacts with another chemical substance (usually oxygen or carbon dioxide) and generates an electrical voltage. The difference to hot combustion is that in cold combustion, the energy generated by the reaction or combustion is not transferred to another medium to generate electrical or kinetic energy.

The functioning principle of the fuel cell was already discovered in 1838 by the German Christian Friedrich Schönbein and the Briton William Robert Grove in 1839. However, since around the same time it was also discovered how to generate electricity with a generator, the principle of the fuel cell did not become established at that time.

How is a fuel cell constructed?

Image: Structure of a fuel cell or fuel cell stack.

The structure of a fuel cell is similar for all types of fuel cells. A fuel cell always consists of two electrodes, an anode and a cathode. Depending on the type, the current flows in the form of negative electrons from the anode to the cathode or vice versa. Between the anode and the cathode is the electrolyte, which varies depending on the fuel cell type.

In addition to the electrodes and the electrolyte, a fuel cell also consists of two bipolar plates, which ensure that the fuel is evenly supplied to the cell and regulate the delivery of thermal and electrical energy. If several such fuel cells are connected in series, this is called a fuel cell stack or fuel cell stack. Connecting the individual cells in series serves to increase the power of a fuel cell stack. This possibility is a significant advantage of this technology, as it allows for a very flexible scalability of the power.

How does a fuel cell work?

The function of a fuel cell differs depending on the type and the fuel and electrolyte used. In the following, the functional principle is explained on the basis of the proton exchange membrane fuel cell (PEMFC).

The structure of a proton exchange membrane fuel cell consists of a bipolar plate, a gas diffusion layer (GDL for short, ensures the controlled entry of hydrogen or oxygen into the cell) followed by the anode. Then comes a thin proton-conducting membrane. On the other side of the membrane comes the cathode, another GDL and finally another bipolar plate.

On the anode side, hydrogen is supplied to the cell. This splits at the membrane so that the positive hydrogen ions can migrate through the membrane to the cathode side. The membrane does not allow the negative electrons from the hydrogen to pass through and they have to take a detour to the cathode, creating an electric voltage. On the cathode side, (air) oxygen is supplied and the positive hydrogen ions combine with the oxygen and the negative electrons. The emission of this reaction is pure water.

Image: Schematic representation of how a PEM fuel cell works.

The diversions that the electrodes have to take from the anode to the cathode produces direct current. This can be used for an electrical load such as an electric motor.

What types of fuel cells are there?

Fuel cells can be divided into two different categories, into low-temperature fuel cells which have an operating temperature of below 150 °C and into high-temperature fuel cells which have an operating temperature of between 150 and 1,000 °C. In each of these two categories there are three different types of fuel cells, which have different fuels, power classes and applications depending on the type. The following table provides an overview of the different fuel cell types. The three most commonly used fuel cell types worldwide are described in more detail below.

Low temperature (LT) fuel cells High temperature (HT) fuel cells
Electrolyte Proton conducting membrane Proton conducting membrane Kale lye Phosphoric acid Molten carbonate Oxide ceramic
Temperature range ˂ 150 °C ˂ 150 °C ˂ 150 °C 200 °C 650 °C 650 - 1.000 °C
Fuel Methanol Hydrogen, natural gas Hydrogen Hydrogen Natural gas Hydrogen, natural gas
Reading area W/kW W/kW W/kW kW kW/MW kW/MW

Table: The different types of fuel cells.

Direct methanol fuel cell (DMFC)

The DMFC (Direct Methanol Fuel Cell) is operated between 60 and 120 °C and thus belongs to the low-temperature fuel cells. The fuel is methanol together with water, which is fed to the anode and then oxidised. At the cathode, (air) oxygen is added and electrical energy is produced, as well as water as a waste product. Direct methanol fuel cells are used for small emergency power generators in camping or for remote measuring stations.

Proton exchange membrane fuel cell (PEMFC)

The PEMFC (Proton Exchange Membrane Fuel Cell) belongs to the low-temperature fuel cells at an operating temperature between 70 - 90 °C. However, there are also PEM fuel cells that are operated at an operating temperature of approx. 180 °C as high-temperature fuel cells.

Through a chemical reaction within the cell of hydrogen and oxygen, an electrical voltage is generated. Here, too, the waste product of the reaction is pure water. Platinum is often used as the catalyst in the membrane for the reaction. Due to the simple and light construction of this type of fuel cell, the PEMFC is the most produced and used fuel cell worldwide.

Solid oxide fuel cell (SOFC)

The SOFC (Solid Oxid Fuel Cell) is operated at a temperature between 650 and 1,000 °C and thus belongs to the high-temperature fuel cells. A solid ceramic material serves as the electrolyte. Hydrogen as well as natural gas or biogas can be used as fuel. Depending on the use of the fuel, either water or surplus fuel gas remains as waste product.

Due to the high operating temperature, the SOFC has a high electrical efficiency of up to 60%. This makes this type of fuel cell particularly well suited for use as a combined heat and power plant in the form of a cogeneration plant (in short: CHP). The SOFC is the second most produced and used fuel cell in the world after the PEMFC.

Applications of fuel cells

The advantage of the flexible power scaling of fuel cell technology makes it possible to apply the technology in many different places. Last but not least, the different types of fuel cells with their different operating characteristics and associated advantages and disadvantages also contribute to the technology having a wide range of applications. The areas of application are divided into portable, mobile and stationary applications.

Portable applications

Portable applications include fuel cells that operate in a small power range, mostly below 100 watts. In this power range, one can find applications in the camping sector, for example, as a small emergency power generator from the German company SFC Energy. In addition, there is a fuel cell trailer from the company Sunfire Fuel Cells for mobile power and heat supply for single-family households that do not have a gas connection.

But solutions for self-sufficient and off-grid power supply are also already on the market. In portable applications, the DMFC or SOFC types are often used, as both fuel cells can be operated with liquefied gas or methanol, which is easier to store than pure hydrogen.

Image: Portable fuel cell emergency power generator on methanol base.

Mobile applications

Image: Hydrogen train Coradia iLint. Image by Erich Westendarp from Pixabay

The mobile applications of fuel cells are particularly in the public focus due to the ambitious climate targets and the automotive location Germany. Fuel cells can be used as energy converters in cars and trucks and the electricity generated by the fuel cell can be used to power an electric motor. If the hydrogen used is green, i.e. produced from renewable energy sources with the help of electrolyzers, the cars and trucks run without CO2 emissions. In cars and trucks, the PEMFC with lightweight metallic bipolar plates are used due to their light weight.

In addition, the fuel cell offers further possibilities in mobile applications. For example, it can be used to power trains that run on non-electrified lines. The first hydrogen train from the French company Alstom has already been running in northern Germany since 2019. But ships can also be powered by fuel cells in the future. Various research projects with PEMFC or DMFC fuel cells are already underway in order to convert shipping to an environmentally friendly drive.

Stationary applications

Stationary applications of fuel cells are already very widespread in practical use in Japan. Europe has already supported the use of fuel cells through various funding and research projects. The best-known application of fuel cells in the stationary sector is cogeneration with fuel cell heating or fuel cell CHP.

Gas is used, mostly natural gas or biomethane, to produce hydrogen by steam reforming. The hydrogen is then used to operate a fuel cell that generates electricity and heat. Market-ready solutions already exist for single-family homes, but also for small businesses and multi-family homes. The electrical power range of the market-ready products is between 0.75 kW and 5 kW. In this application range, the PEMFC and the SOFC are most widely used.

In addition to applications for electricity and heat generation for buildings, fuel cells are also used in the stationary sector for off-grid emergency power supply or grid backup systems. Due to the easy maintenance of the systems and the easier storage of hydrogen compared to diesel, fuel cells are very well suited to safeguarding radio masts, emergency power systems of substations or other critical infrastructures. PEMFCs and DMFCs are mostly used here.

Image: Stationary application: The fuel cell CHP inhouse5000+.

Research in the field of fuel cells

Although the functional principle has been known since the 19th century, the safe and efficient application of this technology still requires a lot of research. Due to the high complexity of the fuel cell in construction and operation, many problems still need to be solved. However, much progress has been made in recent years. The fact that hydrogen technologies and thus also the fuel cell have moved considerably into the public focus since 2019 means that a significant push in the further development of this technology can be expected in the next few years.

A particularly interesting field of research is the combination of electrolysis and fuel cells in one and the same system. This is possible with so-called reversible fuel cells. They can be operated bidirectionally and have a fuel cell mode in which you generate electricity from hydrogen and a electrolysis mode in which you generate hydrogen from electricity and water. Otto-von-Guericke University Magdeburg, together with a consortium from industry and research, is researching such a reversible fuel cell based on PEM technology in the research project RE-FLEX (Unitary REversible PEM fuel cells for FLEXible energy storage).

In addition to basic scientific research, such as in the RE-FLEX project, a lot of work is also being done in the economy on the practical use of fuel cells and hydrogen technologies. DiLiCo engineering was able to plan and build the Energy Pavilion in Bitterfeld-Wolfen on the test site of MITNETZ GAS GmbH as part of the HYPOS projects H2-Netz and H2-Home with its many years of experience in the field of hydrogen as service provider. In this exhibition pavilion, the project participants will show how the supply of buildings with heat and electricity from hydrogen can look in the future. The focus is also on the integration of hydrogen into the natural gas network or the transport of pure hydrogen via a gas network.

Lifetime of fuel cells

An important research focus in the field of fuel cells is the improvement of the service life of fuel cells. Particularly in stationary applications, such as fuel cell-based heating, the service life is a very important factor for the economic success of the technology. While mobile applications require relatively few operating hours over the entire product lifetime, in the stationary sector fuel cell heating systems must enable up to 80,000 operating hours and more.

Measure fuel cell voltages

To achieve these operating hours, the individual cells of a fuel cell stack must be permanently monitored during operation. Because if just one cell fails, the entire fuel cell fails. The DiLiCo cell voltage measuring system is designed precisely for this application. It monitors the individual cells of a stack and can send a signal to the system's controller in case of deviations to change the operation and avoid failure of the fuel cell. This is a particular advantage for the customer, as his system does not fail.

Series production of fuel cells

A current challenge in fuel cell technology is that the production of the stacks is currently still manufactured, making them very expensive. Companies are working to automate the production process and thus reduce production costs while increasing quantities. There are some problems that need to be solved here, as some components of a fuel cell are very complex to manufacture.

An example of a complex component is the membrane in the PEMFC. These have to be coated with a catalyst in a costly process and cut to fit the cell. Automating this process is a major challenge. The assembly of the individual components into a cell and, in the next step, into a stack, is also a complex automation process that in part requires completely new production facilities. DiLiCo engineering develops products for the series production of fuel cells. As part of a research project in the Central Innovation Programme for SMEs (ZIM) funding programm,

Image: The measurement device DiLiCo cell voltage for single cell voltage measurement.

DiLiCo engineering and its partner ZBT - Zentrum für BrennstoffzellenTechnik GmbH in Duisburg are developing a solution for simple and fast contacting of fuel cells for cell voltage measurement. This solution is an extension of DiLiCo cell voltage and is intended to be a cost effective solution for the integration of a cell voltage measurement in the series production of fuel cells. This solution optimises the reliability of fuel cells that are manufactured in series. This brings significant advantages for the manufacturer in terms of the production and follow-up costs of his systems.