PEMFC technology is a promising alternative to conventional energy storage systems, especially for mobile applications and in the automotive sector. In a PEMFC, chemical energy is converted directly into electrical energy. During the reaction of hydrogen, which serves as fuel, with oxygen, only water is produced as "waste gas". In PEMFCs, the operating temperature is typically around 80°C, which means that the water produced is present in the cell in both liquid and gaseous form. In order to be able to generate electrical energy from this chemical reaction, the reduction of the oxygen and the oxidation of the hydrogen must take place spatially separated. This is achieved by the polymer membrane, which is only conductive for protons, but not for electrons. The proton conductivity depends decisively on the water content in the membrane. The higher the water content, the better the proton conductivity. Too much water, however, leads to flooding of the cathode side of the cell, which impedes oxygen transport and thus oxygen reduction. Water management is therefore of paramount importance for the performance of the PEMFC.
One focus of the modelling activities in the field of PEMFC is therefore the development of detailed cell models, which enable the investigation of the transport processes and electrochemistry spatially resolved in 2D/3D. The resulting insight into the processes that take place allows the cell design and operating conditions to be optimised. In addition, ageing processes such as the chemical degradation of the membrane or the degradation of the catalyst will also be investigated. The aim of modelling and simulating such processes is to predict the service life of the fuel cell and to develop optimisation strategies to reduce degradation.
Another important aspect, especially for automotive applications, is the cold-start capability of the fuel cell. The product water generated during oxygen reduction can freeze in the cell at low ambient temperatures, which can bring the function of the fuel cell to a standstill. Suitable start-up procedures must therefore be developed to ensure that the cell heats up sufficiently quickly to prevent it from freezing in winter. With the help of the physical cell models developed at DLR, such a cold start can be simulated and optimised in detail.
In addition to modelling at the cell level, microstructure-resolved modelling of individual cell components based on the lattice Boltzmann model is also being advanced. These models enable a detailed investigation of the transport processes in the components depending on their material properties. On the one hand, this allows the optimisation of the component structures and, on the other hand, the derivation of material-dependent effective transport properties, which are required for modelling at the cell level.
DMFCs are also low-temperature fuel cells in which methanol is used as fuel instead of hydrogen. Otherwise, the design is comparable to that of the PEMFC. The advantage of using methanol is that it is available in liquid form and therefore no pressure tanks are needed, as is the case with hydrogen storage. However, the use of methanol also brings with it some disadvantages. For example, it can easily pass through the membrane to the cathode, which on the one hand causes fuel to be lost and on the other hand lowers the cell voltage. Furthermore, the oxidation of methanol is very complex and requires special catalysts (typically platinum-ruthenium).
The DMFC models developed at DLR provide a detailed insight into the processes taking place, with the aim of developing optimised operating strategies. One focus is on modelling both reversible and irreversible degradation mechanisms. Here, too, the aim is to predict the service life under given operating conditions and to develop improved operating strategies. The NEOPARD-X software (Numerical Environment for the Optimisation of Performance And Reduction of Degradation of X (= energy conversion device)) developed at DLR is used for modelling at the individual cell level.