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Structural Characterisation of Biominerals

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Structural Characterisation of Biominerals Carbon-based Electrode Materials for Application in the Electrochemical Water Splitting Process Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften – Dr. rer. nat. – vorgelegt von Jan Willem Straten geboren am 14.09.1987 Fakultät für Chemie der Universität Duisburg-Essen 2018 For Ethel and my family Die vorliegende Arbeit wurde im Zeitraum von Dezember 2013 bis Dezember 2017 im Arbeitskreis von Prof. Dr. Robert Schlögl am Max-Planck-Institut für Chemische Energiekonversion durchgeführt. Tag der Disputation: 6. September 2018 Gutachter: Prof. Dr. Robert Schlögl Prof. Dr. Malte Behrens Vorsitzender: Prof. Dr. Stefan Rumann Table of Contents Motivation _________________________________________________________________ 6 Abstract ___________________________________________________________________ 8 Kurzzusammenfassung _____________________________________________________ 11 1. INTRODUCTION _______________________________________________ 15 1.1. Brief Historical Review of Hydrothermal Carbon _________________________ 15 1.2. Hydrothermal Synthesis ______________________________________________ 18 1.3. Hydrothermal Carbonization (HTC) – Synthesis, Properties and Applications of HTC Chars ___________________________________________________________________ 21 2. THEORETICAL FUNDAMENTALS _______________________________ 39 2.1. Postulated Reaction Mechanisms during Precursor Decomposition and Spherical Particle Formation _____________________________________________________________ 39 2.2. Chemical Energy Conversion using N-Functionalized/Doped Carbon-based Electrode Materials – A “Black” Prospect For a Bright Future ________________________ 53 3. APPLIED CHARACTERIZATION METHODS ______________________ 67 4. RESULTS AND DISCUSSION ____________________________________ 72 4.1. Hydrothermal Synthesis of N-Functionalized HTC Chars ___________________ 72 4.2. UV/VIS Spectroscopy _________________________________________________ 76 4.3. High-Performance Liquid Chromatography (HPLC) ______________________ 79 4.4. Macroscopic and Microscopic Morphology of N-HTC Materials _____________ 82 4.5. Elemental Composition and Yields ______________________________________ 88 4.6. Structural Composition of N-HTC ______________________________________ 91 4.7. Computed FTIR Spectra ______________________________________________ 92 4.8. Computed and Experimental Gas Phase Infrared Spectra of Furan __________ 93 4.9. Vibrational Spectra of α-Oligofurans and N-Free HTC (Pure HTC Originating from Glucose) _________________________________________________________________ 95 4.10. Comparison of the Vibrational Spectrum of N-Free HTC and Additional Structures 98 4.11. Thermal Analysis of HTC ____________________________________________ 100 4.12. Surface Analysis ____________________________________________________ 105 4.13. Raman Spectroscopy ________________________________________________ 106 4.14. Electron Energy Loss Spectroscopy (EELS) _____________________________ 109 4.15. Structural Comparison of Different HTCs via Solid-State-NMR Spectroscopy 111 4.16. Structural Models ___________________________________________________ 119 4.17. Concluding Remarks ________________________________________________ 122 4.18. Pyrolyzed HTC Chars: Disc Electrode Preparation and Characterization ____ 126 4.19. Electrochemical Approach: Composition of Commercial Pellet Holder, Nail Polish and Standardization Protocol ___________________________________________________ 135 4.20. Electrochemical Characterization in Alkaline Medium ____________________ 138 4.21. Carbon Corrosion and Activation as well as Deactivation of the Surface Area _ 153 4.22. Post-Mortem Characterization: Analysis of Colored Electrolyte and Isolation of Residual of Colored Electrolyte after Electrochemical Stability Tests __________________ 155 4.23. Concluding Remarks ________________________________________________ 163 5. CONCLUSIONS _______________________________________________ 165 6. OUTLOOK ____________________________________________________ 166 7. REFERENCES ________________________________________________ 167 8. APPENDIX ___________________________________________________ 187 8.1. List of Abbreviations ________________________________________________ 187 8.2. List of Publications __________________________________________________ 190 8.3. Lebenslauf (Curriculum Vitae) ________________________________________ 194 8.4. Erklärung (Statement) _______________________________________________ 195 8.5. Acknowledgments ___________________________________________________ 196 Motivation Hydrothermal carbonization (HTC) to produce “green” carbon materials is a promising, sustainable technique to get rid of depleting primary energy sources, in particular fossil fuels (oil, coal, natural gas). These carbonaceous materials are traced to inexhaustible, renewable energy sources (biomass) and thus preventing the evolution of greenhouse gases (e.g. CO2) or other toxic, polluting emissions contributing to anthropogenic climate change and the related global warming which our planet is confronting with. In nature, carbon exists in various allotropes, that is to say diamond, graphite or amorphous carbon. As carbon is a natural product of coalification of every sort of biomass, mankind has been using it for millions of years exploiting it to a great extent. With modern technology the demand of fossil fuels is increasing to an alarming scale. Therefore, a sustainable feedstock to overcome the dependence on fossil fuel- derived energy is “green” HTC material. The HTCs are used for the application in the electrochemical water splitting process which is known to be a cheap and clean route to decompose water into its elements oxygen and hydrogen. With its highest energy density and its environmentally benign combustion in fuel cells among all fuels, hydrogen gas is considered to be the fuel of the future. The electrochemical water oxidation implies this opportunity towards an alternative and profitable way to produce hydrogen Since the reserves of fossil fuels are finite and known to be exploited within the next couple of decades, major efforts need to be made in the context of renewable and alternative environmentally friendly resources. One of the concepts to replace primary energy with renewable energy sources is the solar refinery aiming a complete independence from primary energy carriers. Water splitting into the components hydrogen and oxygen is crucial for a sustainable and renewable energy conversion technology. The electrochemical water splitting reaction as a simple, clean and economical process plays an important role towards the production of hydrogen gas as chemical energy carrier. To date, several electrocatalysts have been investigated. Still, the leading anode materials are RuO2 or IrO2. The electrochemical reaction mechanism of RuO2 or IrO2 in the process of water electrolysis is not fully understood. In addition, RuO2 or IrO2 are expensive precious metal oxide catalysts. The oxygen evolution reaction at the anode is kinetically the most demanding and complicated step in the overall electrochemical water oxidation reaction compared to the hydrogen evolution at the cathode. Understanding the electrochemical water splitting process is therefore a key factor towards green production technologies and in particular the hydrogen-based energy economy. Hydrothermal carbon as “green” carbon material is used in the electrochemical water oxidation. Synthetically, hydrothermal carbonization is one way to transform waste biomass into promising carbon materials. The simultaneous introduction of nitrogen is supposed to enhance the material properties and makes it an appealing object for electrochemical applications. The purpose of this thesis has been to synthesize and to characterize N- functionalized hydrothermal carbon materials. It also has been to significant importance to understand and to study the synergetic effect of carbon in conjunction with nitrogen through spectroscopic and electrochemical investigations, more specifically, the electrochemical water splitting process as a key application to obtain a better structure-property correlation. Abstract The synthesis and characterization of N-HTC materials for the application in the water electrolysis are the main focus of this work. N-incorporation is of fundamental importance throughout this work since, first of all, N is an n-type dopant by acting as electron-donor. The technique of N-doping tunes the physicochemical material properties. It is cheap, non-toxic and has the advantage to tailor the desired electrical, mechanical, optical, magnetic, structural, morphological or chemical properties. N-doping may improve conductivity, surface wettability, catalytic or storage characteristics. Besides, N- doping creates active sites and thus might enhance electrocatalytic activity as well as long-term electrochemical stability. The hydrothermal synthetic route has been applied based on the precursors glucose and urotropine. During hydrothermal synthesis, urotropine together with glucose undergoes a plethora of complex reaction pathways. Ammonia, in particular, the decomposition product of urotropine, contributes to a wide range of reaction mechanisms. The molar ratio of glucose and urotropine has been modified in order to achieve a steady increase of the N-content. Urotropine was proved to be a highly effective N-precursor. With raising the mass fraction of urotropine towards glucose, maximal N-proportion of 19 wt% can be achieved.
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