Development and Investigation of an Efficient Electrolysis Process for The
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Development and Investigation of an Efficient Electrolysis Process for the Conversion of Carbon Dioxide to Formate Von der Fakultät Chemie der Universität Stuttgart zur Erlangung der Würde eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigte Abhandlung Vorgelegt von Armin Löwe aus Kirchheim unter Teck Hauptberichter: Prof. Dr.-Ing. Elias Klemm Mitberichter: Prof. Dr. rer. nat. Andreas Friedrich Prüfungsvorsitzender: Prof. Dr. rer. nat. Bernhard Hauer Tag der mündlichen Prüfung: 01.03.2021 Institut für Technische Chemie der Universität Stuttgart 2021 Die Verantwortung für den Inhalt dieser Veröffentlichung liegt beim Autor. The author is responsible for the content of this publication. Erklärung über die Eigenständigkeit der Dissertation Ich versichere, dass ich die vorliegende Arbeit mit dem Titel „Development and Investigation of an Efficient Electrolysis Process for the Conversion of Carbon Dioxide to Formate“ selbständig verfasst und keine anderen als die angegebenen Quellen und Hilfsmittel benutzt habe; aus fremden Quellen entnommene Passagen und Gedanken sind als solche kenntlich gemacht. Declaration of Authorship I hereby certify that the dissertation entitled „Development and Investigation of an Efficient Electrolysis Process for the Conversion of Carbon Dioxide to Formate“ is entirely my own work except where otherwise indicated. Passages and ideas from other sources have been clearly marked. Stuttgart, September 11th, 2020 Place, date Signature - Armin Löwe Acknowledgment My greatest gratitude goes to Prof. Dr.-Ing. Elias Klemm for providing me the opportunity to work on this exceptional interesting topic that I hope will accompany my future career. Special thanks also for allowing me to pursue own ideas, providing a great working environment, imparting knowledge and for the detailed corrections on the present work. I want to extend my gratitude also to Dennis Kopljar who introduced me to this, at that time, com- pletely unknown topic. Also, to Fabian Bienen. Thanks to the both of you for the large number of fruitful discussions, helpful tips, for assisting with preparations and, Dennis, your great work I could start from. Thanks a lot to the whole institute, but most importantly to the electrochemical reaction engineer- ing group, for the great teamwork, wonderful discussions no matter day or night, awesome working environment and a lot of fun. Furthermore I want to thank all colleagues for the conducted measurements. Heike Fingerle for trying so hard to measure our unwilling samples. Barbara Gehring, Dennis Beierlein and Jan Florenski for measuring all those TGA samples and Efi Hadjixenophontos for taking the TEM images. To all colleagues, i’d like to thank you for the wonderful time at this institute! You live your life by a code, an ethos. Every man does. It’s your shoreline. It’s what guides you home. And trust me, you’re always trying to get home. - Act of Valor, 2012 Dedicated to my parents. Contents 1 Abstract 1 2 Zusammenfassung5 3 Introduction 9 4 Theoretical Background 13 4.1 Electrochemical basics................................... 13 4.1.1 The electrical double layer............................. 14 4.1.2 Cell voltage and electrode potentials........................ 16 4.1.3 Kinetic aspects................................... 18 4.1.4 Quantitative relations................................ 24 4.1.5 Important aspects of electrochemical cells..................... 25 4.2 Carbon dioxide reduction reaction............................. 27 4.2.1 Fundamentals.................................... 27 4.2.2 Equilibrium potentials............................... 29 4.2.3 Electrocatalytic conversion of carbon dioxide................... 31 4.2.4 Influence of operating conditions.......................... 37 4.3 Gas diffusion electrodes................................... 43 4.3.1 Wetting behavior.................................. 46 4.3.2 Mass transport in gas diffusion electrodes..................... 49 4.3.3 Equivalent circuit model of porous electrodes................... 50 4.4 Process concepts for carbon dioxide electrolysis...................... 53 4.4.1 Aspects to consider for alkaline reaction conditions................ 53 4.4.2 Coupling with downstream bipolar membrane electrodialysis........... 54 Contents 4.4.3 Alternative anode reactions............................. 56 5 Motivation and Objectives 59 6 Experimental Section 63 6.1 Preparation methods.................................... 63 6.1.1 Homogeneous precipitation of supported metal oxides.............. 63 6.1.2 Oxidative pretreatment of the Vulcan XC 72 support............... 65 6.1.3 Synthesis of supported palladium nanoparticles.................. 65 6.1.4 Preparation of gas diffusion electrodes....................... 65 6.2 Experimental setup..................................... 67 6.2.1 Details for the semi-batch mode of operation................... 68 6.2.2 Details for the continuous mode of operation................... 68 6.2.3 Used and developed electrochemical cells..................... 70 6.3 Analytical methods..................................... 75 6.3.1 Quantification of gaseous products......................... 75 6.3.2 Quantification of dissolved products........................ 75 6.3.3 Catalyst and electrode characterization...................... 76 6.4 Electroanalytical methods.................................. 79 6.4.1 Determination of electrode activity and wetting.................. 79 6.4.2 Determination of faradaic and energetic efficiencies................ 82 6.4.3 Estimations on the catalyst’s oxidation state.................... 84 6.5 Data evaluation....................................... 85 6.5.1 Analysis of semi-batch operation.......................... 85 6.5.2 Analysis of continuous operation.......................... 87 6.5.3 Double layer capacitance and activity evaluation................. 88 6.6 Fundamental investigations on new process concepts................... 89 6.6.1 Bipolar membrane electrodialysis for downstream processing.......... 89 6.6.2 Selective alcohol oxidation as alternative anode reaction............. 93 7 Results and Discussion 97 7.1 Reproducibility....................................... 99 7.1.1 Reproducibility of galvanostatic experiments................... 99 7.1.2 Reproducibility of polarization curves and double layer capacitance....... 102 II Contents 7.2 Investigations on the catalyst................................ 105 7.2.1 Type of catalyst................................... 105 7.2.2 Synthesis parameters for tin oxide based catalysts................. 110 7.2.3 Tin oxide loading.................................. 111 7.3 Investigations on process conditions............................ 117 7.3.1 The effect of temperature.............................. 117 7.3.2 Effects of the electrolyte.............................. 125 7.3.3 Requirements for reactant and electrolyte purity.................. 137 7.4 Further optimization of the electrode’s matrix....................... 143 7.4.1 Influence of preparation parameters........................ 143 7.4.2 Fine tuning the electrode’s hydrophobicity..................... 143 7.5 Merging single parameter optimizations.......................... 147 7.6 Temporal behavior..................................... 149 7.6.1 Long-term stability................................. 149 7.6.2 Transient behavior................................. 154 7.7 Scale-up........................................... 161 7.7.1 Scale-up of the catalyst synthesis......................... 161 7.7.2 Scale-up of the electrode preparation....................... 161 7.7.3 Scale-up of the electrochemical cell........................ 163 7.8 Process concepts...................................... 171 7.8.1 Coupling carbon dioxide electrolysis with bipolar membrane electrodialysis... 171 7.8.2 Coupling carbon dioxide reduction with selective alcohol oxidation....... 179 8 Conclusion and Outlook 183 8.1 Conclusion......................................... 183 8.2 Outlook........................................... 187 9 Appendix 211 9.1 Carbon dioxide reduction reaction - Standard potential calculation............ 213 9.2 Preparation methods - Additional derivations....................... 215 9.2.1 Preparation of the supported catalyst........................ 215 9.2.2 Preparation of gas diffusion electrodes....................... 216 9.3 Electroanalytical methods - Supporting explanations................... 217 9.4 Investigations on the catalyst................................ 220 III Contents 9.4.1 Type of catalyst................................... 220 9.4.2 Tin oxide loading - Estimations on the quantity of metallic tin.......... 222 9.5 Investigations on process conditions............................ 223 9.5.1 The effect of temperature.............................. 223 9.5.2 Concentration effects................................ 224 9.5.3 Electrolyte purity.................................. 225 9.6 Temporal behavior - Long-term stability.......................... 227 9.7 Process concepts...................................... 228 9.7.1 Operating carbon dioxide electrolysis in the recycled product mixture...... 228 9.7.2 Coupling carbon dioxide reduction with selective alcohol oxidation....... 229 IV Contents Listings Table 1: List of abbreviations. Abbreviation Description AB acetylene black ABS acrylonitrile butadiene styrene ACS American Chemical Society AEM anion exchange membrane AOR alcohol oxidation reaction AsB angle selective backscattered electron ATR-IR attenuated total reflection infrared