Our overarching goals are to optimise the performance, durability, efficiency and compactness of the fuel cells in accordance with the requirements of the application in question. The polymer electrolyte membrane (PEM) fuel cell components that we have developed range from electrodes right through to bipolar plates.

Our work primarily involves characterisation, performance optimisation and the improvement of service life. This includes efforts to develop manufacturing methods, characterise the structure and surface of materials including estimating ageing processes, and error analyses.

In addition, we also perform experimental studies on new functional materials and carry out investigations aimed at developing new concepts for electrodes and membrane electrode assemblies, and determining the resistance of seals, gas diffusion systems and bipolar plates.

Contact

Dr. Joachim Scholta
+49 (0)731 95 30-206
MEAs

// Optimisation of membrane-electrode assemblies

Membrane-electrode assemblies (MEAs) are the core components of fuel cells. Their functioning is determined by the ion-conducting electrolyte membrane, the catalyst layers on the anode and cathode sides, and the gas diffusion layers (GDL). Optimal coordination of these functional layers is essential. In addition to this, the MEA also forms part of the seal design. The mechanical characteristics of the materials used are also important alongside their conductivity and chemical stability.

Optimal coordination of the properties of the electrolyte membrane, the catalyst layers and the gas diffusion layers is crucial to achieve high output densities. The use of new electrolyte membranes makes it possible to reduce the internal resistance or minimise reactant transfer between the anode and cathode. Using these membranes in an MEA often requires adjustments to the composition and microstructure of the catalyst layers and gas diffusion layers.

Read also the Catalysts chapter.

// Membrane-electrode assembly for PEM fuel cells

So-called polymer, perfluorinated sulfonic acid membranes (PFSA) have become established for hydrogen-consuming PEM fuel cells, with thicknesses now dropping to < 18 µm. For alcohol fuel cells such as direct methanol fuel cells (DMFC), new polyaryl membranes are also being studied. Currently, research is focusing on minimising the resistance and reactant transfer. The interface design between the electrolyte membrane and the electrode, the adjustment of the wetting characteristics between the catalyst layer and gas diffusion layer, the optimisation of the material transport characteristics and the adaptation of the operating conditions can all potentially increase the power density and service life of MEAs.


Catalysts

// Catalysts in PEM fuel cells

A basic requirement for the high activity and long service life of PEM fuel cells is the stability of the precious-metal and carrier particles. The standard for use in highly-active electrodes for PEM fuel cells are precious-metal nanoparticles deposited on porous, acid-resistant and electrically conductive support materials.

The highest activities for oxygen reduction are generally achieved with platinum alloys (e.g. Pt/Co) on a high-surface carbon black carrier. However, these materials are not stable in the long term under the conditions of use of the fuel cell. The use of graphite-based carrier materials coated with pure platinum nanoparticles increases the stability of the catalyst, but at the expense of activity. Oxidic carrier materials also offer stability, but have an entirely different surface chemistry and generally have lower electrical conductivity than carbon.

Comparison of the particle size achieved and the reproducibility of different preparation methods for the deposition of platinum nanoparticles on Vulcan XC-72 carbon powder.
Comparison of the particle size achieved and the reproducibility of different preparation methods for the deposition of platinum nanoparticles on Vulcan XC-72 carbon powder.

The aim of catalyst development is to preserve the high activity of carbon-black-based platinum catalysts on corrosion-resistant carrier materials. For this purpose, platinum or platinum alloy nanoparticles are deposited on carbon-based and carbon-free carrier materials. The surface chemistry and pore structure of the carrier material play a key role when selecting the synthesis method and the corresponding implementation procedure. Reductive processes with formaldehyde or polyols and production using precious-metal peroxides have proven themselves for the manufacturing of active catalysts on oxidation stable carriers.

Read also the Membrane-electrode assembly chapter.

Electrodes

// Development of electrodes for PEM fuel cells

High-performance electrodes for polymer electrolyte membrane fuel cells (PEMFC) consist of a combination of a catalyst layer with a gas diffusion layer. The catalyst layer can be applied to a membrane (CCM = catalyst coated membrane) or the gas diffusion layer (GDE = gas diffusion electrode).

ZSW offers appropriate methods for developing the catalyst layers. For example, spray coating is a flexible tool for new materials. Then, if the electrode and material are evaluated positively, the process can be transferred to printing processes (screen printing, inkjet printing) or other continuous coating processes.

SEM image of an electrode catalyst layer.
Layer development using spray coating. Picture: ZSW/Bosa
Layer development using spray coating. Picture: ZSW/Bosa

 

 

The key elements of electrode development include

  • Composition of electronic and ionic conductivity
  • Pore distribution
  • Wettability of the catalytic active layer
  • Coordination of the catalyst layer with the gas diffusion layer
Gas Diffusion Layers

// Characterisation of gas diffusion layers

For successful use in fuel cells, gas diffusion layers (GDLs) must perform many tasks: alongside electrical and thermal contacting and current dissipation, the GDL must feed the reactants – generally hydrogen and atmospheric oxygen – and remove the product water. For this purpose, the gas diffusion layer requires a high porosity with sufficient mechanical stability and suitable wettability properties for liquid water.

One example of a method of characterising gas diffusion electrodes and gas distribution structures is micro-computer tomography (µ-CT system). This method can be used to examine GDL structures, including their water content, even in a compressed state.

Various characterisation methods, selecting the right method and interpreting the results are key components of our expertise on fuel cells.

Micro-computer tomography (µ-CT) for GDL structural images
Micro-computer tomography (µ-CT) for GDL structural images. Source: ZSW
Cross-section (tomogram) of a GDL with integrated microporous layer (MPL) for identification of free volume between fibres and binder material.
Cross-section (tomogram) of a GDL with integrated microporous layer (MPL) for identification of free volume between fibres and binder material.

// Water management in gas diffusion layers

Water management is crucial when optimising gas diffusion layers for use in fuel cells. The properties of fuel cells in service can be studied with a high degree of reliability and with minimal effort using radiography methods.

In addition, the properties of the materials relevant to the water content can also be determined outside the cell (ex-situ). The methods of determining the contact angle, element distribution and the water penetration behaviour into a GDL (Leverett function) are used here. The procedures are explained in the PDFs in the download box.

 

For furhter information about our investigation methods, please check the PDFs in downloads as well as our services or selected research projects.

Bipolar Plates

// Bipolar plates: Design and characterisation

The geometry of bipolar plates, including the gas distribution structure on them, is of crucial importance for the design and construction of robust, high-performance fuel cells. As well as forming part of the process of designing fuel cells, which also includes designing the bipolar plate, ex-situ and in-situ characterisation of bipolar plates makes a major contribution to understanding the functioning and to further optimisation. Alongside the studies described below regarding the water balance in fuel cells, comprehensive investigations of the mechanical and electrical characterisations are also performed.

Real flow field and CAD design, with gas distributor structure.
Real flow field and CAD design, with gas distributor structure.
Contours of water saturation.
Contours of water saturation.

// Water balance in gas distributor channels

The condensate discharge properties of gas distributor channels in bipolar plates can generally be determined using individual experimental setups. However, these do not permit full replication of fuel cell operation, which is why studying neutron radiographic methods offers an excellent alternative to determine the condensate distribution within the gas distributor structure. This allows a direct connection to be made between the visible water movements and the type of condensate discharge, and conclusions can thus be drawn regarding the performance of the fuel cell.

In addition, the institute has comprehensive expertise in studying condensate discharge properties by modelling (discharge criteria, VOF modelling, interpretation).