Upgrading the toolbox for fermentation of crude syngas: Process characterization for complete carbon usage, cyanide adaption and production of C4 components

Synthesegas (Syngas), Industrieabgase und andere C1-Moleküle haben sich in den letzten Jahren als interessante Substrate für die biotechnologische Produktion von Kraftstoffen und Plattformchemikalien herauskristallisiert. Syngas ist in diesem Zusammenhang eine Mischung aus Wasserstoff, Kohlenmonoxid und Kohlendioxid, welche über Vergasung von Biomasse und organischen Abfällen (z. B. Kommunalabfälle oder Klärschlamm) hergestellt wird. Organismen, die in der Syngas-Fermentation eingesetzt werden können, gehören zur Klasse der acetogenen Bakterien. Diese nutzen einen einzigartigen Stoffwechselweg zur Kohlenstofffixierung in dem zwei Moleküle CO oder CO2 über sequenzielle Reaktionen zu einem Molekül Acetyl-CoA kondensiert werden. Dieser wird reduktiver Acetyl-CoA Weg oder Wood-Ljungdahl-Stoffwechselweg (WLS) genannt. Natürliche Produkte die aus dem WLS abgeleitet werden können sind Essigsäure, Ethanol, Buttersäure, n-Butanol oder 2,3‑Butanidol. Clostridium ljungdahlii ist einer der Modelorganismen für acetogene Bakterien. Bisher konzentrieren sich Forschungen der synthetischen Biologie, Gentechnik und Prozessentwicklung hauptsächlich darauf, die Produktion natürlicher C4-Moleküle eines Organismus zu erhöhen oder neue Stoffwechselwege für C4- und C6-Moleküle zu implementieren. Bioenergetische Limitierungen verhindern jedoch die Produktion dieser Zielprodukte mit hohen Ausbeuten. Dies liegt darin begründet, dass acetogene Bakterien bevorzugt Essigsäure oder Ethanol herstellen, da dies einen höheren Energiegewinn für die Zelle bedeutet, als die Produktion von C4- oder C6-Molekülen. Die Limitierung des Massentransfers von Kohlenmonoxid und Wasserstoff von der Gasphase in die Flüssigphase ist einer der Punkte, die häufig im Zusammenhang mit Limitierungen der Syngas-Fermentation in der Literatur genannt werden. Möglichkeiten den Massentransfer zu verbessern sind zum einen die Vergrößerung der Phasengrenzfläche Gas-Flüssig durch erhöhte Rührerdrehzahl und/oder Begasungsrate und zum andern, die Erhöhung der Sättigungskonzentration von Kohlenmonoxid und Wasserstoff durch das Anheben des Partialdruckes. Sollen allerdings Produkte mit geringer Wertschöpfung produziert werden, so hat sich gezeigt, dass die Verbesserung des Massentransfers über Erhöhung der Rührerdrehzahl nicht wirtschaftlich ist. Während die Verbesserung des Massentransfers über die Steigerung der Begasungsrate den Umsatz beeinträchtigen kann, führen höhere Gelöstkonzentrationen von Wasserstoff und Kohlenmonoxid zu Stoffwechselinhibierungen und dadurch zu einer Reduktion der Effizienz des Prozesses. Da Roh-Syngas neben den genannten Hauptkomponenten auch Verunreinigungen wie Stickoxide, Schwefelwasserstoff oder Blausäure enthält, arbeiten viele Forschergruppen mit gereinigten oder synthetischen Syngas-Mischungen. Jedoch ist die Reinigung von Gas ein kostenintensiver Schritt. Die Möglichkeit ungereinigtes oder nur Teilgereinigtes Syngas einsetzten zu können, würde die Wirtschaftlichkeit jedes Syngas-Fermentationsprozesses verbessern. Leider sind die meisten, der in Roh-Syngas enthaltenen Stoffe, bekannte Katalysatorgifte. Bakterien sind zwar generell robuster gegenüber Katalysatorgiften als chemische Katalysatoren, aber Studien mit Extrakten aus aufgeschlossenen Zellen oder gereinigten Enzymen zeigen, dass manche der Verunreinigungen die zentralen Enzyme des WLS inhibieren. Daten, die den Effekt von Syngas-Verunreinigungen auf ganze Zellen acetogener Bakterien zeigen, sind dagegen nur wenige in der Literatur zu finden. Im Zuge dieser Doktorarbeit wurden die folgenden vier Themen untersucht: • Der Einfluss erhöhten Massentransfers durch gesteigerte Begasungsrate • Verbesserung des Massentransfers durch erhöhen des Systemdrucks und Partialdrucks der Substrate • Der Einfluss von Cyanid auf Wachstum und Produktbildung von C. ljungdahlii • Prozesskopplung über sequentielle Mischkultur zur Produktion von Äpfelsäure Hinsichtlich der Massentransfereigenschaften wurden zwei Rührerandordnungen mit drei unterschiedlichen Begasungsraten getestet. Eine ermöglichte die Teilweise Rückführung von Gas aus dem Kopfraum des Reaktors. Es zeigte sich für alle Begasungsraten, dass das Maximum des volumenbezogenen Stoffübergangskoeffizienten (kla-Wert) bei einer Rührerdrehzahl von 800 min-1 liegt. Über Batch-Kultivierungen im 1,5 L Maßstab konnte gezeigt werden, dass trotz steigender kla-Werte mit steigender Begasungsrate, die Konversion von Substraten zu Produkten bei höheren Begasungsraten ineffizienter wird. Der Einfluss der Begasungsrate auf den kla-Wert ist klein, verglichen mit dem Einfluss der Begasungsrate. Zudem Verringern steigenden Begasungsraten die Verweilzeit der Gasblasen in der Flüssigkeit. Dadurch wird mit steigender Begasungsrate zunehmend ungenutztes Substrat aus dem Reaktor ausgetragen. In diesem Punkt zeigt sich die Rühreranordnung, die eine Teilrückführung von Gas aus dem Kopfraum des Reaktors gestattet, überlegen und erreicht bessere Konversionseffizienten obwohl der kla-Wert im Vergleich zur anderen Rühreranordnung kleiner ist. Aufgrund der möglichen Nachteile, die mit einer Steigerung des Massentransfers durch Erhöhung der Begasungsrate verbunden sind, sollen Experimente mit erhöhtem Druck zeigen, ob dies eine Alternative darstellt, um den Massentransfer zu verbessern. Zu diesem Zweck wurden Experimente aus dem 1,5 L Maßstab in den 2,5 L Maßstab hochskaliert und Versuche bei absoluten Systemdrücken von 1 bar, 4 bar und 7 bar, bei ansonsten konstantem volumenbezogenen Massenfluss der Gase und konstanter Gaszusammensetzung, durchgeführt. Um mögliche Inhibierungen durch erhöhte Gelöstkonzentrationen von Kohlenmonoxid zu vermeiden wurde hierfür ein Gasgemisch aus CO2, H2 und N2 verwendet. Für dieses Gasgemisch konnte erstmals gezeigt werden, dass eine direkte Steigerung der Substratpartialdrücke keine Verbesserungen bei Wachstum und Produktbildung zur Folge hat. Im Gegenteil, das Wachstum ging mit steigendem Druck zurück und die Produktbildung stagnierte. Darüber hinaus änderte sich die Produktzusammensetzung von 2,4 % Ameisensäure, 86,5 % Essigsäure und 11,1 % Ethanol bei atmosphärischem Druck zu 82,7 % Ameisensäure, 15,6 % Essigsäure und 1,7 % Ethanol bei einem Systemdruck von 7 bar. Da aber der Massenfluss an Substraten über alle Druckstufen konstant gehalten wurde, war die Produktausbeute und Konversionseffizienz bei 7 bar um den Faktor 7,5 höher, als bisher in der Literatur beschrieben. Die Literatur betrachtet Cyanid als die kritischste Komponente in Roh-Syngas, die erst aus dem Gasstrom entfernt werden muss, bevor das Gas für die Fermentation eingesetzt werden kann. Die Ergebnisse in dieser Doktorarbeit zeigen jedoch zum ersten mal, dass C. ljungdahlii unter heterotrophen Bedingungen an bis zu 1,0 mM Cyanid adaptiert werden kann. Unter autotrophen Bedingungen ist eine Adaptation bis zu 0,1 mM Cyanid möglich. Nachdem die Zellen an Cyanid adaptiert wurden, werden die maximale Wachstumsrate und Biomasse nichtmehr von Cyanid beeinflusst, bis die oben genannten Grenzwerte erreicht sind. Die Ausbeutekoeffizienten aus Versuchen unter heterotrophen Bedingungen zeigen, dass bei einer Cyanidkonzentration von 1,0 mM die Aktivität der Kohlenmonoxiddehydrogenase (CODH) vollständig inhibiert und der WLS inaktiv ist. Die Ergebnisse unter autotrophen Bedingungen bestätigen dies, hier konnte bei einer Cyanidkonzentration von 1,0 mM weder Wachstum noch Produktbildung beobachtet werden. Im Zusammenhang mit der Inhibierung der CODH konnte zudem erstmals beobachtet werden, dass die Produktzusammensetzung sich mit steigender Cyanidkonzentration ändert und mehr Ethanol gebildet wird. Bei 0 mM Cyanid bestehen die Produkte zu 97 % aus Essigsäure und 3 % aus Ethanol während bei 1,0 mM Cyanid die Produktzusammensetzung 20 % Essigsäure und 80 % Ethanol ist (heterotrophe Kulturen). Unter autotrophen Bedingungen (Wachstum auf Syngas) setzen sich die Produkte in Gegenwart von 0,1 mM Cyanid zu 80 % aus Essigsäure und zu 20 % aus Ethanol zusammen. Um die Limitierungen bei der Produktion von gewünschten C4 oder C6 Molekülen zu umgehen, wurde zusammen mit Dr.-Ing. Stefan Dörsam ein Prozess zur Produktion von Äpfelsäure (2-Hydroxybernsteinsäure) mit syngas als initiale Kohlenstoffquelle entworfen (vgl. Kapitel vier der Doktorarbeit Evaluation of Renewable Resources as Carbon Sources for Organic Acid Production with Filamentous Fungi). Die Herstellung von Äpfelsäure mit filamentösen Pilzen, wie zum Beispiel Aspergillus oryzae, resultiert in hohen substratbezogenen Ausbeuten. Allerdings sind diese Verfahren bisher auf Zucker als Kohlenstoffquelle limitiert. In Kooperation mit Dr.-Ing. Stefan Dörsam wurde ein zweistufiger Prozess entworfen der es ermöglicht Äpfelsäure aus Syngas herzustellen. Bei dieser sequentiellen Mischkultur ist Essigsäure der Metabolit, der beide Prozessstufen miteinander verbindet. Da die A. oryzae-Stufe dieses Prozesses auf Stickstofflimitierung angewiesen ist, war es zunächst notwendig, die Konzentration an Ammoniumchlorid im Medium für die Syngasfermentation so weit zu reduzieren, dass am Ende der Syngas-Stufe kein Ammonium-Stickstoff mehr enthalten ist. Während der Syngasfermentation wurden Wasserstoff und Kohlenmonoxid zu Essigsäure (88,9 %) und Ethanol (11,1 %) umgewandelt. Die Ausbeute lag bei 0,86 g g-1. Aus dieser Mischung wurde während der A. oryzae-Stufe des Prozesses die Essigsäure in Äpfelsäure umgewandelt. Hierbei lag die Ausbeute bei 0,33 g g-1. Daraus ergab sich eine Gesamtausbeute von 0,28 Gramm Äpfelsäure pro Gramm eingesetztem Wasserstoff und Kohlenmonoxid. Damit konnte erstmalig gezeigt werden, dass über Verfahrenskopplung höhere Ausbeuten an hochwertigen C4-Molekülen erreicht werden können

Reactor designs and configurations for biological and bioelectrochemical C1 gas conversion: a review

Microbial C1 gas conversion technologies have developed into a potentially promising technology for converting waste gases (CO2, CO) into chemicals, fuels, and other materials. However, the mass transfer constraint of these poorly soluble substrates to microorganisms is an important challenge to maximize the efficiencies of the processes. These technologies have attracted significant scientific interest in recent years, and many reactor designs have been explored. Syngas fermentation and hydrogenotrophic methanation use molecular hydrogen as an electron donor. Furthermore, the sequestration of CO2 and the generation of valuable chemicals through the application of a biocathode in bioelectrochemical cells have been evaluated for their great potential to contribute to sustainability. Through a process termed microbial chain elongation, the product portfolio from C1 gas conversion may be expanded further by carefully driving microorganisms to perform acetogenesis, solventogenesis, and reverse -oxidation. The purpose of this review is to provide an overview of the various kinds of bioreactors that are employed in these microbial C1 conversion processes.This study was conducted in collaboration with researchers from four different institu tions (Dokuz Eylul University, Turkey; University of Minho, Portugal; Izmir Democracy University, Turkey, and University of A Coruña, Spain), who were supported by the following funding bodies: A.A. [Dokuz Eylul University, Scientific Research Foundation (DEU-BAP) (#2011.KB.FEN.046) and TUBİTAK (#119R029)]; L.P. [Portuguese Foundation for Science and Technology (FCT) (UIDB/04469/2020), and FCT and European Social Fund (POPH-QREN) (POCI-01-0145-FEDER 031377)]; T.K. [TUBİTAK-CAYDAG (118Y305)]; and H.N.A. [Xunta de Galicia (ED431C 2021/55)].A.A. acknowledges the support by Dokuz Eylul University, Scientific Research Foundation (DEU-BAP), Turkey, for the award on (#2011.KB.FEN.046) “Direct Electricity Generation from Treatment Plant Sludges by using MFCs” research project. A.A. acknowledges TUBİTAK for the support on #119R029 “Sustainable Energy Recovery from Treatment plant sludge, green waste and olive pomace via gasification process: Investigation of beneficial usage alternatives of gasification by-products”. L.P. acknowledges the Portuguese Foundation for Science and Technol ogy (FCT) under the scope of the strategic funding of UIDB/04469/2020 unit. Also, the financial sup port from Portuguese Foundation for Science and Technology (FCT) and European Social Fund (POPH-QREN) through the project INNOVsyn - Innovative strategies for syngas fermentation (POCI-01-0145-FEDER-031377) are gratefully acknowledged. T.K. acknowledges the support from The Scientific and Technological Research Council of Turkey (TUBITAK-CAYDAG) (project no: 118Y305). H.N.A. thanks the Xunta de Galicia (Spain) for his postdoctoral fellowship (ED481D 2019/033). H.N.A., belonging to the BIOENGIN group of the UDC, also acknowledges Xunta de Galicia for recognizing the group as a competitive Reference Research Group (GRC) (ED431C 2021/55).info:eu-repo/semantics/publishedVersio

Understanding the activity transport nexus in water and CO2_{2} electrolysis: State of the art, challenges and perspectives

This article reviews the challenge of expanding the current research focus on water and CO2 electrolysis from catalyst-related insights towards achieving complete understanding of the activity transport nexus within full electrolysis cells. The challenge arises from the complex interaction of a multitude of phenomena taking place at different scales that span several orders of magnitude. An overview of current research on materials and components, experiments and simulations are provided. As well as obvious differences, there are similar principles and phenomena within water and CO$_{2}$ electrolysis technologies, which are extracted. Against this background, a perspective on required future research within the individual fields, and the need for a multidisciplinary research approach across natural, materials and engineering sciences to tackle the activity transport nexus is presented

Industrial Chemistry Reactions: Kinetics, Mass Transfer and Industrial Reactor Design

Nowadays, the impressive progress of commercially available computers allows us to solve complicated mathematical problems in many scientific and technical fields. This revolution has reinvigorated all aspects of chemical engineering science. More sophisticated approaches to catalysis, kinetics, reactor design, and simulation have been developed thanks to the powerful calculation methods that have recently become available. It is well known that many chemical reactions are of great interest for industrial processes and must be conducted on a large scale in order to obtain needed information in thermodynamics, kinetics, and transport phenomena related to mass, energy, and momentum. For a reliable industrial-scale reactor design, all of this information must be employed in appropriate equations and mathematical models that allow for accurate and reliable simulations for scaling up purposes. The aim of this proposed Special Issue was to collect worldwide contributions from experts in the field of industrial reactor design based on kinetic and mass transfer studies. The following areas/sections were covered by the call for original papers: Kinetic studies on complex reaction schemes (multiphase systems); Kinetics and mass transfer in multifunctional reactors; Reactions in mass transfer-dominated regimes (fluid–solid and intraparticle diffusive limitations); Kinetic and mass transfer modeling using alternative approaches (ex. stochastic modeling); Simulations in pilot plants and industrial-sized reactors and scale-up studies based on kinetic studies (lab-to-plant approach)

Oxide-Encapsulated Electrocatalysts for Solar Fuels Production

As the cost of solar energy continues to drop, the major hurdle limiting the widespread use of intermittent renewable solar energy is the lack of efficient and cost-effective energy storage. Electrochemical technologies, such as electrolyzers, photoelectrochemical cells, and fuel cells, have the potential to compensate for solar energy intermittency on a large scale, by converting excess solar energy into storable solar fuels, such as hydrogen (H2), which can be converted back to electrical energy at a later time. However, improvements in the efficiency and lifetime of these technologies, in particular the electrocatalysts, are necessary for their commercialization. During operation, efficiency losses result from energetic penalties (overpotentials) associated with several processes occurring at or near the electrocatalyst/electrolyte (ohmic resistance, kinetic barriers, and mass transport limitations). These losses can be further exacerbated due to electrocatalyst durability issues such as dissolution, agglomeration, detachment, and poisoning. A major challenge in electrocatalysis field is developing methods to mitigate these losses without adversely affecting the electrocatalytic stability, selectivity, and/or activity. One promising solution is an oxide-encapsulated electrocatalyst architecture, which has been shown to improve electrocatalyst durability and provide mechanisms for controlling reaction pathways. Previous studies on oxide-encapsulated electrocatalysts, in which metal catalysts are fully or partially covered by ultrathin layers of permeable oxide films, have mostly focused on supported nanoparticles because of their high electrochemically active surface area per catalyst loading. However, these nanoparticle-based architectures tend to have poorly defined and/or non-uniform structures which make it difficult to understand and elucidate structure-property-relationships. This dissertation investigates well-defined oxide-coated electrocatalysts, which serve as model platforms for gaining a fundamental understanding of kinetic and transport phenomena that underlie their operation. This dissertation presents three studies which highlight the versatile functionalities of oxide-encapsulated electrocatalysts to improve the electrocatalyst stability, selectivity, and activity in different electrochemical systems. This dissertation demonstrates the ability of room temperature synthesized silicon oxide (SiOx)-encapsulated Pt electrocatalysts to: i) stabilize nanoparticles and improve electron transfer, ii) mitigate catalyst poisoning and control reaction pathways through selective transport, and iii) alter reaction energetics associated with catalysis at the buried interface. First, this dissertation establishes the ability of room temperature synthesized SiOx coatings to stabilize nanoparticle electrocatalysts by mitigating electrocatalyst migration, coalescence, and detachment on metal-insulator-semiconductor (MIS) photoelectrodes for solar-driven water splitting. Metallic Pt nanoparticles are inherently unstable on the insulating support due to poor physical adhesion and electronic coupling between Pt and SiO2. To overcome this issue, a room temperature UV ozone synthesis process was used to deposit 2-10 nm thick SiOx overlayers on top of electrodeposited Pt nanoparticles to stabilize Pt on the electrode surface. The photoelectrodes containing oxide-encapsulated electrocatalysts exhibit superior durability and electron transfer (ohmic) properties compared to the photoelectrode that lacked the SiOx encapsulation. While this study demonstrates that the oxide-encapsulated electrocatalyst architecture improves the stability of electrocatalytic nanoparticles deposited on insulating materials, it does not elucidate how reactants and products transport through the SiOx barrier to reach the Pt surface. In order to gain a better understanding of kinetic and transport phenomena that govern performance of oxide-encapsulated electrocatalysts, the following studies investigate model electrodes consisting of continuous SiOx overlayers of uniform thickness deposited onto smooth Pt thin films. This planar electrode geometry allows for simple and unambiguous characterization of structure-property relationships. The next study systematically evaluates the influence of SiOx thickness on the HER performance to understand species transport through SiOx. Through detailed characterization and electroanalytical tests, it is shown that proton and H2 transport occur primarily through the SiOx coating such that the HER occurs at the buried Pt|SiOx interface. Importantly, the SiOx nanomembranes were found to exhibit high selectivity for proton and H2 transport compared to Cu2+, a model HER poison. Leveraging this property, it is shown that SiOx–encapsulation can enable poison-resistant operation of Pt HER electrocatalysts. This oxide-encapsulated architecture offers a promising approach to enhancing electrocatalyst stability while incorporating advanced catalytic functionalities such as poison resistance or tunable reaction selectivity. The final study demonstrates ability of SiOx overlayers to alter reaction energetics associated with catalysis at the buried interface. Carbon monoxide (CO), methanol, and ethanol oxidation reactions are studied for their relevance in direct alcohol fuel cell applications. Oxide-supported catalysts have been shown to enhance alcohol oxidation by promoting CO oxidation at metal/oxide interfacial regions through the so-called bifunctional mechanism, in which hydroxyls on the oxide facilitate the removal of adsorbed CO−intermediates from active sites. A key advantage of the oxide-encapsulated electrocatalyst design compared to oxide–supported nanoparticles is that the former maximizes the density of metal/oxide interfacial sites. This study shows that the SiOx overlayer provides proximal hydroxyls, in the form of silanol groups, which can enhance CO and alcohol oxidation through unique interactions at the buried Pt|SiOx interface. Overall, this dissertation highlights the potential of using oxide-encapsulated electrocatalysts for stable, selective, and efficient electrochemical production and use of solar fuels

Anodic Nanocatalysts for Formic Acid Fuel Cells: An Electrochemical Study

Direct formic acid fuel cells (DFAFCs) have been reported as a prominent source of alternative green energy and solution to imminent energy crisis for the last two decades. The challenge to commercialize DFAFCs is primarily the utilization of cost effective, high performance and durable anodic catalyst for formic acid oxidation (FAO). Consequently, this dissertation addresses the extensive electrochemical study of a number of nanomaterials towards the potential use as electrocatalysts for FAO. Morphology and elemental analyses of the prepared nanomaterials were obtained using electron microscopy techniques. After a general introduction and overall review of this dissertation (Chapter I), studies of the influence of chloride ions as contaminant on 20 wt% Pd/C were presented in Chapter II. The correlation between FAO peak current at glassy carbon electrode (GCE) coated with 20 wt% Pd/C (commercial), and the amount of chloride ions either added or leached from the frit of Ag/AgCl (3.0 M KCl) reference electrode were established. This study provides a guideline on how to choose a suitable reference electrode in fuel cell research. Chapter III reports the comparative study of three different carbon based support materials and the catalytic activities towards FAO using Pd-based mono and ternary composite nanocatalysts with commercial 20 wt% Pd/C (activated carbon). The nanocatalysts were synthesized using Pd2+, Ni2+ and Co2+ precursors on Vulcan XC-72, Ketjen Black EC600, and graphite nanoparticles support materials. Vulcan XC-72 supported catalysts showed the highest FAO activities, whereas Ketjen Black support showed the best performance in terms of long-term durability. All PdNiCo-ternary composites displayed superior catalytic efficiencies towards FAO. In Chapter IV, polyhedral oligomeric silsesquioxane (POSS) molecules were utilized as template to prepare Bi nanorods and Pd nanoparticles. Specifically, Bi nanorods were studied to evaluate the so-called third-body effect mechanism of FAO. Finally, in Chapter V, nine transition metal complexes, prepared using POSS ligand and procured, were blended individually with 20 wt% Pd/C and explored towards FAO activity and durability. These hybrid catalysts were then investigated and ranked in terms of catalytic activity and stability for FAO using electrochemical techniques. Potential composite nanomaterials were also evaluated and proposed for further study

Small-scale power generation using paper based fuel cells

Paper based microfluidics has emerged as a promising technology for the development of low cost energy devices such as, paper based fuel cells for powering point-of-care (POC) diagnostic devices. These devices include glucometers, pregnancy detection kits etc. The diagnostic devices that are used in the environmental, health monitoring, micro/nano electromechanical systems (MEMS), wireless electronics etc. require power for a short duration and in milli-nano watt range for the analysis and display of the results/diagnosis. These are referred to as micro-nano devices (MNS). These devices are low power consuming and require the electrical input for a few seconds to a few minutes and are usually disposed after one-time use. Currently Li-ion batteries are used as the source of power in these devices. Apart from batteries other potential powering sources are solar cells and microbial fuel cells, however these have certain limitations associated with them. For instance, functioning of solar cells is dependent on the availability of sunlight, microbial fuel cells deliver power in the microwatt ranges and require strict environment control for their operation. The use of batteries in single-use, disposable devices is not recommended as it leads to under utilization of stored energy and resourceful materials. Moreover, keeping in view of the growing energy demand which may surpass the available energy sources, finding cost-e�ective powering sources for low-power requiring devices is necessary. Micro-fuel cells can be thought of as an alternative to the aforementioned power sources. However, in general fuel cells require metering devices for the supply of fuel and oxidant, which makes the overall system cumbersome. Paper can transport liquid fuels due to its remarkable intrinsic properties such as, porosity and capillarity. Furthermore, paper is cheap, easily available, easy to shape and dispose (biodegradable), without posing any environmental threats. Due to these benefits paper can easily provide the platform for the functioning of energy devices which are based on liquid fuel, oxidants and electrolytes. Due its intrinsic transport properties, paper based fuel cells don’t need any external pumping mechanism or ancillary parts to pump the fluids through the system. This not only reduces the fabrication and operational complexity to a great extent, but also minimizes the overall cost. The development and fabrication of various paper-based fuel cells in di�erent architectures,has been extensively studied by researchers recently. However, some of these either use expensive noble metal catalyst (Pt, Au, Pd) to attain high power densities or micro-organisms (bacteria) for their operation (as in microbial fuel cell), which limits their application in MNSs. The primary objective of this thesis is to develop, fabricate and characterize low-cost, easy to use and disposable paper based fuel cells for powering micro-nano devices at room temperature

Transferring Electrochemical CO2 Reduction from Semi-Batch into Continuous Operation Mode Using Gas Diffusion Electrodes

The electrochemical reduction of C02 is a promising method for its conversion which still suffers from important challenges that have to be solved before indus­ trial realization becomes attractive. The optimization of gas diffusion electrodes is described with respect to catalyst dispersion and mass transport limitations allowing solubility issues to be circumvented and current densities to be increased to industrially relevant values. The transfer of the promising results from semi-batch experiments into continuous mode of operation is demonstrated, and it is indi­cated how the energetic efficiency can be significantly improved by the choice of electrolyte, in terms of concentration and type. Thereby ohmic losses can be decreased and the intrinsic activity be improved

CARBON CAPTURE AND UTILIZATION

As the world moves towards clean energy initiative, carbon capture and utilization technologies are key to achieving net zero emissions. CO2 capture with amines has many disadvantages and cannot be applied to commercial power plants. The current manuscript will address this issue as well as a solution that involves the use of low-cost alkali absorbent CO2 capture solutions, combined with an electrochemical regeneration method that uses the least amount of energy available for capture and regeneration. This research will also further address the issue of how to deal with the captured CO2. Several viable storage and utilization methods have been explored, as well as their technological readiness level. The first chapter will introduce the subject and present various CO2 utilization ideas. The second chapter will cover a novel topic: adding surfactants to improve the absorption performance of a low-cost sodium carbonate solution. The third chapter will focus on capturing NOx, SOx, and CO2 using a single absorption column. In Chapter 4, we will look at how to reduce the reagent regeneration energy from 4MJ/Kg to 1.18MJ/Kg by switching from thermal regeneration to electrolysis. Chapter 5 will discuss an electrochemical approach for converting the capture CO2 to Oxalic acid. Finally, in Chapter 5, we will present pilot scale experimental studies of CO2 capture using our absorption columns at the MTU steam plant

갈바닉 치환 반응을 이용한 조성 및 구조가 개질된 팔라듐-구리 촉매의 합성 및 전기화학적 응용

학위논문(박사)--서울대학교 대학원 :공과대학 화학생물공학부,2019. 8. 김재정.Electronic structure and electrochemically active surface area of catalysts are the important factors to determine the catalytic activity of electrochemical reactions. The catalyst synthetic method using galvanic displacement reaction can provide highly active electro-catalysts by alloying and changing their geometric structure. For example, when immersing Cu substrate in a solution containing Pd2+ ions, Pd deposition occurs spontaneously by galvanic displacement reaction originated from the difference of standard reduction potentials between Pd and Cu. During Pd-Cu galvanic displacement reaction, Cu atoms in the substrate diffuse into Pd deposit forming Pd-Cu alloy by Kirkendall effect. The catalytic activity of Pd can be enhanced by alloying with Cu which can change the electronic structure of Pd by ligand effect. The structure of Pd-Cu catalyst can be controlled from planar to whisker shapes by manipulating the reaction kinetics of galvanic displacement. The addition of Cl- ions in the reaction bath accelerates the galvanic displacement reaction and develops a steep concentration gradient of Pd2+ ions near the surface of Cu substrate. This induced the deposition of Pd-Cu in the vertical direction to form a whisker instead of horizontal extension. To verify the advantage of whisker-structured catalyst, the catalytic activity of Pd-Cu whisker toward electrochemical ethanol oxidation reaction was investigated. Pd-Cu whisker showed 21 times higher electrocatalytic performance than planar Pd due to the large surface area of whisker structure which could provide more reactive sites and the modified electronic structure of catalyst by alloying Pd with Cu. The prepared Pd-Cu whisker catalyst was also applied for N2O reduction. To enhance the N2O approach to rough-surfaced catalyst and N2O dissolution, electrochemical system combined with Couette-Taylor flow (CTF) mixer was adopted. When Ta number exceeds the critical Ta number by increasing rotating speed of inner cylinder of CTF mixer, Taylor vortices evolve in the solution, resulting in the rapid N2O dissolution and enhancing N2O solubility. The effectiveness of enhanced N2O dissolution on the N2O reduction was investigated by applying electrical potential on the CTF mixer. Pd-Cu whisker catalyst was loaded on outer cylinder of CTF mixer which played a role of cathode for N2O reduction. N2O conversion of 99.99% was obtained with introduction of Taylor vortices in the solution using a CTF mixer. Therefore, it can be suggested that the synthetic method to control geometric and electronic structure of catalysts using galvanic displacement reaction and the CTF mixer/electrolysis reactor hybrid system for efficient reactant dissolution can significantly enhance the electrochemical performance. The electronic structure of Pd-Cu catalysts can be modulated by the addition of citric acid in the galvanic displacement bath. The atomic ratio of Pd to Cu in Pd-Cu catalysts, which determines the electronic structure, was controlled by varying citric acid concentration during the displacement. Citric acid was incorporated into the Pd-Cu deposit during galvanic displacement and restrained the diffusion of Cu from Cu substrate to deposit leading to decrease in Cu content in the Pd-Cu catalysts. The electrocatalytic activity for N2O reduction was strongly dependent on Pd/Cu composition in Pd-Cu catalysts. The optimum composition of Pd-Cu catalysts with the highest N2O reduction activity was Pd66Cu34. Moreover, the Pd66Cu34 catalyst showed remarkably enhanced mass activity for N2O reduction compared with a commercial Pd/C. The density functional theory (DFT) calculations revealed that the highest N2O reduction activity of Pd66Cu34 was attributed to the moderate bonding energy to reaction intermediates which resulted from interatomic charge transfer between Pd and Cu.Abstract i List of Tables vii List of Figures viii Chapter I. Introduction 1 1.1. Bimetallic catalyst for electrochemical reaction 1 1.2. Fabrication of bimetallic catalyst by galvanic displacement reaction 11 1.3. Couette-Taylor flow mixer in a two-phase gas-liquid system 21 1.4. Combination of electrochemical reactor and Couette-Taylor flow mixer 30 1.5. Purpose of this study 31 Chapter II. Experimental 33 2.1. Fabrication of Pd-Cu catalysts 33 2.1.1. Synthesis of Pd-Cu whisker catalyst with enlarged surface area 33 2.1.2. Synthesis of Pd-Cu film catalyst with controllable composition 34 2.2. Characterization of catalysts 35 2.3. Electrochemical analysis 36 2.4. Catalytic performance test 40 2.4.1. Electrochemical ethanol oxidation 40 2.4.2. Electrochemical N2O reduction 41 2.5. DFT calculation 44 2.6. Couette-Taylor flow mixer/electrolysis reactor hybrid system 46 Chapter III. Results and Discussion 51 3.1. Morphology-controlled synthesis of Pd-Cu catalyst via galvanic displacement deposition 51 3.1.1. Synthesis of Pd-Cu whisker catalyst with enlarged surface area 51 3.1.2. Mechanism of whisker formation 64 3.1.3. Catalytic performance toward electrochemical ethanol oxidation 74 3.1.4. Application of Pd-Cu whisker catalyst in Couette-Taylor flow mixer/electrolysis reactor hybrid system 80 3.2. Composition-controlled synthesis of Pd-Cu catalyst via galvanic displacement deposition 95 3.2.1. Citric acid-assisted synthesis of Pd-Cu catalyst with controllable surface composition 95 3.2.2. Catalytic performance toward electrochemical N2O reduction 107 Chapter IV. Conclusion 118 References 121 국문 초록 134Docto