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Peter Holzapfel
Innovation Award for Peter Holzapfel
Peter Holzapfel from the Department of Electrocatalytic Interface Engineering won the Innovation Award of the German Hydrogen and Fuel Cell Association for his Master Thesis, which was supervised by Prof. Dr. Simon Thiele.
Mehr: Innovation Award for Peter Holzapfel …

Research

To approach the question above, there are three important subtopics which can be approached by more specific research questions:

  • What is the structure of a given electrocatalytic system? - Tomography
    The first question is how to get a structure at all, which for a multi-phase, nanoporous or even hierarchically structured layer system in electrocatalytic devices can be a challenge per se. In the past, we developed methods for nanotomographic reconstruction of nanoporous electrode materials by focused ion beam / scanning electron micrcoscopy tomography (FIB-SEMt). This method is based on infiltrating the pore space by atomic layer deposition. Furthermore, we investigate methods on how to assess hierarchical materials systems by the use of multiple tomographic approaches (e.g. FIB-SEMt and X-ray tomography) at the same time.
  • What is the structure-macroscopic property relationship in electrocatalytic systems? – Modeling
    In this area we combine nano- and microtomographic imaging with transport parameter simulation. The idea here is that by assessing the real structrure of e.g. the electrodes in a fuel cell by tomography and successive pore-scale modeling, an understanding of e.g. mass transport limiting factors or structural degradation during aging can be gained. Successivley this knowledge can be exploited to improve the structure in a specific system.
  • How to create the best suitable structure for an electrocatalytic system? – Additive Manufacturing
    To improve structure, additive manufacturing techniques that provide a spatial control are used. The idea here is to implement the improved structures that were previously found by modeling or by more heuristic approaches. For this purpose, different deposition techniques, such as spray coating or inkjet techniques or electrospinning are applied. In conjunction, these methods can be applied to receive composite membranes or structured membrane-electrode assemblies.


Additionally, we perform full cell tests using the various test benches available in our research unit. Further, we perform different synthesis approaches, particularly for the creation of nanoparticles.

Facilities

Advanced fabrication methods for membrane electrode assemblies

Analogous to fuel cells, Power-to-X technologies like water electrolysis and CO2-reduction in their heart consist of membrane electrode assemblies (MEA). It is our goal to improve such systems with regard to their structure on the micro- and nanometer scale. For this purpose, we use state of the art manufacturing techniques like spray-coating, electrospinning and ink-jet printing.
Our group has shown that it is possible to reduce the effective membrane thickness increasing the MEA performance particularly for fuel cells or water electrolysers. This is made possible by new fabrication techniques such as the direct membrane deposition (DMD) technique. The DMD process is as industrially scalable as e.g. the catalyst coated membrane (CCM). The thin layer engineering in our group is focused on, but not limited to, catalyst layers, membranes, and additives.
A composite membrane is what can be created by combining the ion-conducting polymer with additives (e. g. CeO2 nano particles) or other polymer structures that improve the membrane properties towards a desired direction. We investigate composite membrane approaches that use electrospun fiber mats together with additives to achieve high durability under arbitrary conditions.

Advanced fabrication methods for membrane elctrode assembliesCopyright: Dr. M. Klingele/IMTEK Freiburg

Equipment

  • Spray coaters (Biofluidix BioSpot Benchtop Workstation BT750, SonoTek ExactaCoat)
  • Electrospinner (KatoTech)
  • ZetaSizer Nano SP (Malvern Panalytical)
  • Turbiscan Tower (Formulaction)

Key publications

State of the art electrochemical performance tests

The in-house fabricated MEAs are analyzed using sophisticated test setups from Scribner Associates. The setups allow not only the acquisition of polarization curves, but also advanced electrochemical impedance spectroscopy (EIS) measurements. This enables us to achieve an in-depth understanding of the processes happening during operation of our model systems (typically 4-25 cm2 active area). Moreover, our setups allow product analysis with dedicated sensors.

State of the art electrochemical performance testsCopyright: Scribner Associates

Key publications

Advanced tomographic methods

Advanced tomographic methodsCopyright: Georg Pöhlein

Raman tomography of Nafion XLRaman tomography of Nafion XL. The membrane has a thickness of approx. 30 µm and is reinforced with a central layer of microporous PTFE (red), whereas the outer layers of the membrane consist mainly of Nafion (green). Since the membrane was imaged in water-immersion, water below and above the membrane is depicted in blue.
Copyright: HI ERN

Understanding the intricate interplay between nano-meter-sized structures in catalyst layers or membranes and electrochemical performance requires accurate imaging methods with high resolution. Tomographic methods using focused ion beam scanning electron microscopy (FIB-SEM) give us access to the structure of functional materials with sub micrometer resolution (resolution up to 10 nm in cutting direction of the ion beam). Thus, it is possible to visualize the structure of porous catalyst layers and other structures. The tomographic dataset then forms the basis for the determination of structure and transport parameters.

Due to the multitude of relevant size scales in energy conversion devices it is often necessary to combine several tomographic techniques. The sub nanometer structure of catalyst nanoparticles can only be resolved using TEM tomography. On the other hand, the structures of gas diffusion electrodes may require a broader overview. This is provided by x-ray tomography. Therefore, a multi-scale analysis may augment the insights gained from FIB-SEM tomography of catalyst layers. However, for materials that are not electrically conductive such as the membranes in fuel cells or electrolyzers the FIB-SEM technique fails.
Tomographic investigations of the latter are made possible with confocal Raman imaging. Confocal Raman microscopy allows both chemical characterization and high-resolution imaging up to a spatial resolution of < 1 µm. This tool can for example be used to analyze ionomer membranes in pristine and cycled status in order to evaluate degradation. Also, membrane reinforcements that can hardly be imaged by e.g. electron microscopy or X-ray tomography, often show remarkably different Raman spectra and can therefore be investigated with confocal Raman microscopy even in 3D if the samples are transparent.

Equipment

  • Zeiss Gemini II CrossBeam 540 FIB-SEM
  • WITec alpha300RA atomic force and confocal Raman microscope

Key publications

Selected publications

Electrocatalysis for fuel cells

Manufacturing of fuel cells

Imaging and virtual design of fuel cells, electrolysers and batteries