The aim of our research is the knowledge-based development of advanced electrode materials consisting of a structured carbon backbone. The work is focused on electrode materials for the oxygen evolution reaction (OER) in water electrolysis. The general idea is to use glucose as model precursor for biomass (a), which can be used as cheap and abundant carbon source. The initial liquid solutions convert into modified liquid and solid carbonaceous materials during the autoclave process (b) under high pressure and temperature. The liquid products can be further converted in biorefinery facilities to produce fuels or other value-added chemicals. The solid products can be pressed into mechanically stable pellets by an annealing step under inert gas atmosphere. The high number of functional groups within the hydrothermal carbon (HTC) lead to intrinsic binding properties during the pellet formation (c). No additional binder, like for instance Nafion, is needed and the specific material properties can be investigated.
The carbon based electrode materials are applied in electrochemical cells (d) to investigate their electrochemical activity as well as stability. The gaseous products of hydrogen and oxygen are produced during the water splitting process. In this way, the electrical energy from renewable sources like solar or wind energy is converted into the chemical energy carrier hydrogen, which can be further converted to methanol or ammonia by using exhaust gases from industry (e).
Interests & Challenges:
The carbon synthesis strategy of the group is based on the use of molecular precursors and controllable condensation reactions in liquid phase. Glucose is used for this bottom-up approach to obtain solid hydrothermal carbons. The incorporation of oxygen or nitrogen functional groups can be adjusted by varying the initial synthesis pH and by the addition of nitrogen containing precursor like for example urotropine. 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. 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. 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. 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.
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 established characterization methods were modified to obtain deeper knowledge about the synthesized carbon materials and their modification under electrochemical conditions. For Raman spectroscopy, a single-phonon resonance (SPR) fitting procedure was developed for graphitic materials based on multi-walled carbon nanotubes (MWCNT). 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). Furthermore, the direct speciation separation of functional groups could be achieved with a non-linear heating procedure by the thermogravimetric mass spectrometry setup. Supported by further characterization techniques such as microscopy and XPS the type and quantity of functionalization of graphitic carbon materials can be derived.
We further studied options of anchoring metals on structured carbon materials. A strong interaction between catalytic active species and electric conductive carbon support is essential for high activity and stability of the catalyst materials. Techniques like atomic layer deposition (ALD), wet impregnation, adsorption of colloids or the usage of polymerized ionic liquids were applied to achieve intense synergy effects. We applied different pre-treated multi-walled carbon nanotubes as support materials and were able to generate atomically dispersed metal oxides on their surface. The materials showed remarkably high activities as well as stabilities.
1. 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. https://doi.org/10.1002/cphc.201402834
2. J. W. Straten, P. Schlecker, M. Krasowska, E. Veroutis, J. Granwehr, A.A. Auer, W. Hateba, S. Becker, R. Schlögl, S. Heumann, N-Funtionalized Hydrothermal Carbon Materials using Urotropine as N-Precursor, Chemistry - A European Journal, https://doi.org/10.1002/chem.201800341
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, Materials Sciences and Applications https://doi.org/10.4236/msa.2017.88044
4. P. Düngen, R. Schlögl, S. Heumann, Non-linear thermogravimetric mass spectrometry of carbon materials providing direct speciation separation of oxygen functional groups, Carbon https://doi.org/10.1016/j.carbon.2018.01.047
5. Y. Yi, G. Weinberg, M. Prenzel, M. Greiner, S. Heumann, S. Becker, R. Schlögl, Electrochemical corrosion of a glassy carbon electrode, Catalysis Today https://doi.org/10.1016/j.cattod.2017.07.013
6. P. Düngen, M. Greiner, K.-H. Boehm, I. Spanos, X. Huang, A.A. Auer, R. Schloegl, S. Heumann, Atomically dispersed vanadium oxides on multiwalled carbon nanotubes via atomic layer deposition: A multiparameter optimization, Journal of Vacuum Science & Technology A https://doi.org/10.1116/1.5006783
7. Y. Ding, A. Klyushin, X. Huang, T. Jones, D. Teschner, F. Girgsdies, T. Rodenas, R. Schlögl, S. Heumann, Cobalt Bridged with Ionic Liquid Polymer on Carbon Nanotube for Enhanced Oxygen Evolution Reaction Activity, Angewandte Chemie Int. Ed, doi.org/10.1002/anie.201711688, https://doi.org/10.1002/ange.201711688
|Dr. Sylvia Becker||Dr. Jan Willem Straten|
|Dr. Qingqing Gu||Dr. Shuchang Wu|
|Dr. Marina Prenzel||Dr. Youngmi Yi|
|Dr. Tania Rodenas|