Dr. Saskia Heumann (geb. Buller) - Carbon Synthesis and Applications
|Diplom||Christian-Albrechts-Universität zu Kiel, Dipl. Chem. (2003-2008)|
|Diplom thesis||IFM-GEOMAR, Leibnitz Institut, Maritime Chemistry (2009)|
|Dr. rer. nat.||Christian-Albrechts-Universität zu Kiel, Inorganic Chemistry (Prof. Dr. W. Bensch) (2012)|
|Gruppenleiterin||MPI CEC (seit 2012)|
- S. Buller and J. Strunk, Nanostructure in energy conversion. J. Energy Chemistry, 25, 171-190 (2016), DOI: 10.1016/j.jechem.2016.01.025
- S. Buller, M. Heise-Podleska, N. Pfänder, M. Willinger and R. Schlögl, Carbon nanotubes as conducting support for potential Mn-oxide electrocatalysts: Influence of pre-treatment procedures. J. Energy Chemistry, 25, 265-271 (2016), DOI: 10.1016/j.jechem.2016.01.022
Carbon Synthesis and Application
The aim of our research is the knowledge-based development of advanced electrode materials consisting of a structured carbon backbone with specific functional groups that have the ability to anchor the catalytically active metal component and stabilize even small metal clusters as known from literature 1. The work is focused on electrode materials for OER in water electrolysis (Figure 1). Furthermore, we develop concepts of functional carbon materials for battery applications and gas phase catalysis (e. g. ODH).
Functionalized Carbon Materials - Synthesis and Characterization
The carbon synthesis strategy of the group is based on the use of molecular precursors and controllable condensation reactions in liquid phase (Figure 2). The model precursor of this bottom-up approach is glucose. By hydrothermal treatment the glucose converts into carbonaceous materials highly functionalized by oxygen functional groups.
The distribution of the oxygen functional groups, as well as the morphology of the carbonaceous product, is controlled by process parameters, in particular the pH.1 For lower initial synthesis pH, i. e. pH 0, extended carbonaceous structures were confirmed by Raman spectroscopy, whereas for pH > 3 furanic structural entities from the 5-hydroxymethyl furfural intermediate remained the dominant structural motive of the carbon (Figure 3). The high number of functional groups leads to intrinsic binding properties that allow the preparation of functional disc electrodes by pressing and thermal annealing to 900°C. Hence, the general used concept of drop coated glassy carbon discs for electrochemical testing can be replaces by carbon based disc electrodes of controllable structure and functionalization. In the absence of Nafion, material-only properties can be studied for further fundamental understanding of electrochemical processes. The macroscopic dimension of the bulk electrode allows quantitative analytical investigations after electrochemical testing.
For further variation of the functionalization i. a. nitrogen containing precursors are applied. Post-functionalization techniques such as plasma treatment, electrochemical oxidation and (hydro)thermal treatments in active gases/solvents complete the methodical variety for the introduction of desired surface termination for the stabilization of catalysts. In order to distinguish differently functionalized carbon materials by Raman spectroscopy, a fitting procedure was developed based on MWCNT.2 The theory derived fit resulted in accurate ratios of the ideal graphitic lattice vibrations (G-Band) and lattice vibrations induces by defects/functional groups (D- and D’-Band). Supported by further characterization techniques such as microscopy, XPS and thermal analysis the type and quantity of functionalization of graphitic carbon materials can be derived (Figure 4).
We further studied options of anchoring metals on structured carbon materials. We assume that for oxygen activation in the process of water splitting small cluster rather than single atoms are relevant for the active site. A close contact of catalytic active species to electric conductive carbon support is essential for high activity and stability of the materials. Therefore we applied special methods to achieve different complexities of the deposited central atom. Single atoms, atom ensembles, cluster or solids correspond to a gradual increase of the complexity of the system. Techniques like atomic layer deposition (ALD) (Figure 5), wet impregnation and adsorption of colloids (Figure 6) were applied. We focused on deposition of manganese oxides to draw back on the expertise and methodical assembly developed for photosystem II (Department Lubitz, MPI CEC). We synthesized different pre-treated multi-walled carbon nanotubes and studied the influence of functional groups and defects on the surface on the stability of the impregnated manganese oxides. In comparison to ALD deposition, it turned out that for wet impregnation rather defects in the structure have a stabilizing effect than the functional groups itself.3
Dr. Sylvia Becker
Dr. Marina Prenzel
Dr. Youngmi Yi
1. S. Buller and J. Strunk, Nanostructure in energy conversion. J. Energy Chemistry, 25, 171-190 (2016), DOI: 10.1016/j.jechem.2016.01.025
2. S. Reiche, N. Kowalew, R. Schlögl, Influence of Synthesis pH and Oxidative Strength of the Catalyzing Acid on the Morphology and Chemical Structure of Hydrothermal Carbon, ChemPhysChem. 16, 579 (2015)
3. P. Düngen, M. Prenzel, Casey Van Stappen, Norbert Pfänder, Saskia Heumann, and Robert Schlögl, Investigation of different pre-treated multi-walled carbon nanotubes by Raman spectroscopy, accepted Materials Sciences and Applications (MSA) special issue „Carbon Materials“, 88
4. S. Buller, M. Heise-Podleska, N. Pfänder, M. Willinger and R. Schlögl, Carbon nanotubes as conducting support for potential Mn-oxide electrocatalysts: Influence of pre-treatment procedures. J. Energy Chemistry, 25, 265-271 (2016), DOI: 10.1016/j.jechem.2016.01.022