Dissertations / Theses on the topic 'Electrocatalytic CO2 reduction'

Thesis advisor: Matthias Waegele
Heterogeneous electrocatalytic reduction is an environmentally friendly method for the conversion of abundant feedstock molecules into valuable products. Examples include the reduction of carbon dioxide to hydrocarbons and the reduction of nitrate to ammonia. Heterogeneous electrocatalysis occurs at the interface between an electrode and an electrolyte. Interfacial properties, such as surface morphology, interfacial electric field, interfacial water structure, and local pH, can substantially influence the activity and selectivity of electrocatalytic reduction processes. However, a comprehensive, molecular-level understanding of how these interfacial properties control electrocatalysis is still largely lacking to date. To develop such an understanding, it is essential to probe the properties of the electrocatalytic interface under operating conditions. This great experimental challenge is further compounded by the fact that the interface often undergoes dynamic changes during catalysis. In this thesis, we took a multimodal approach to characterize the aqueous electrolyte/copper interface during CO2/CO reduction and hydrogen evolution. Copper is the only pure metal that promotes the reduction of CO2/CO to hydrocarbons at significant reaction rates. The hydrogen evolution reaction is the main competing reaction in aqueous electrolytes. It is therefore essential to understand how these reactions are controlled by the properties of the interface. In the first part of this thesis, we employed in-situ surface-enhanced infrared absorption spectroscopy (SEIRAS) and surface-enhanced Raman spectroscopy (SERS) to investigate dynamic changes of the copper electrode surface. We found that the polycrystalline copper electrode surface undergoes a reconstruction process upon adsorption of CO. The formation of nanoscale metal clusters on the electrode manifests itself by the appearance of a new CO stretch band, which arises from a CO sub-population bound to undercoordinated copper atoms. The formation of these clusters is reversible, that is, they disappear upon desorption of CO. This work demonstratesthat a reaction intermediate such as CO can induce dynamic and reversible changes in the surface morphology of a heterogeneous catalyst. Because the changes are reversible, they would escape ex situ measurements. Our findings highlight the need for probing catalytic surfaces under operating conditions. In the second part of this thesis, we focused on how the electrolyte influences electrocatalysis at the aqueous electrolyte/copper electrode interface. Specifically, we explored the mechanisms by which cations of the supporting electrolyte affect the reduction of CO and the hydrogen evolution reaction on copper. With differential electrochemical mass spectrometry (DEMS), we determined to what extent the reduction of CO to ethylene is affected by the identity of the cations of the supporting electrolyte. Ethylene is produced in the presence of methyl4N+ and ethyl4N+ cations, whereas this product is not synthesized in propyl4N+- and butyl4N+-containing electrolytes. With SEIRAS, we found that an intermolecular interaction between surface-adsorbed CO and interfacial water is disrupted in the presence of the two larger cations. This observation suggests that this interaction promotes the hydrogenation of surface-bound CO to ethylene. This work illustrates that weak intermolecular interactions can substantially influence electrocatalytic processes. In a related study, we examined the effect of alkali metal cations of the supporting electrolyte on the hydrogen evolution reaction. We found that, in alkaline conditions, changing the cation from Na+ to Cs+ has no measurable effect on the HER. Because it is well-established that Cs+ promotes the reduction of CO2/CO to hydrocarbons, the results illustrate the changing the alkali cation enables the selective promotion of this pathway under alkaline conditions. Further, we found that in 0.1 M solutions of NaOH and CsOH of the highest commercially available purity grades, trace impurities of iron deposit on the copper electrode during the hydrogen evolution reaction. Because iron is a better catalyst for the hyrogen evolution reaction than copper, the rate of the hydrogen evolution reaction is enhanced by up to a factor of 5. These findings demonstrate that trace impurities of this ubiquitous metal pose a great challenge for the development of selective catalytic processes for CO2/CO reduction. This thesis provides a critical study of how the interfacial properties change under the electrocatalytic reduction of CO2/CO and hydrogen evolution conditions. The properties of both Cu electrode and the electrolyte contribute to the control of the selectivity of these complex electrocatalytic processes
Thesis (PhD) — Boston College, 2021
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Chemistry

Thesis advisor: Matthias M. Waegele
Studying the adsorption and reaction kinetics of surface-bound chemical species, on different metal catalysts or electrodes, is of paramount importance in the development of inhomogeneous catalytic methodology. Our study of the oxidation of CO on platinum was accomplished by designing a thin layer flow cell in an external reflection configuration. A charge-injection circuit was successfully implemented which decreased the time required to charge the double layer in the electrochemical cell. We were able to obtain a signal via Stark shift spectrum, of the adsorbed CO, using the thin layer cell configuration. Additionally, electrochemical impedance spectroscopy was used as a diagnostic tool to assess the effect of electrode geometry, on the voltage response, in the thin layer cell. The coupling of visible light-driven photoexciation with transition metal catalytic plat- forms is emerging as a synthetic strategy to achieve unique reactivity that has previously been inaccessible. One such example is the iridium/nickel-dipyridyl system discovered recently. Characterizing the interactions between the iridium and nickel catalysts, under reaction conditions, is important to develop a better understanding of the system. In order to apply infrared spectroscopic measurement techniques, in-situ, we made modifications to the synthetic scheme by changing the solvent and by utilizing different iridium catalysts for the synthesis of the desired methyl 4-(benzoyloxy)benzoate product. Using our trans- mission infrared setup we effectively demonstrated in-situ product detection of the aryl- ester coupled product. Additionally, after constructing a transient infrared pump-probe setup, we collected preliminary results of the triplet state lifetime of the iridium dye. The surface morphology of copper has been shown to affect the electrochemical reduction of CO2. Using surface-enhanced Raman spectroscopies, the reversible formation of nanoscale metal clusters on a copper electrode was revealed at sufficiently cathodic potentials where we observed the appearance of a new band at 2080 cm-1 corresponding to C≡O adsorbed to undercoordinated copper defect sites. The formation of new undercoordinated sites additionally resulted in the surface enhancement of the Raman scattering which amplified the intensity of the other spectral bands
Thesis (MS) — Boston College, 2019
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Chemistry

With the blend of addressing issues of sustainable energy with the environmental worries regarding emission of greenhouse gases, there is a motivation to target the efficient chemical reduction of CO2. Re(I) integrated photosensitizers and catalysts, synthesized from commercially available ligands, are introduced with the selective CO2 reduction of formic acid, making for a unique class of Re(I) catalysts typically selective for CO as a reduction product. Furthermore, synthesized Zn(II) phosphino aminopyridine complexes are structurally and computationally characterized as well as observed to function as unprecedented electrocatalysts for the reduction of CO2 to formic acid and CO. Lastly, with the importance and popularity of N-heterocyclic carbenes (NHCs) as a class of ligands in the field of organometallic catalysis, six-membered perimidine based carbenes are further explored. Synthesis of a chelating pyridyl-perimidine NHC in addition to potential transition metal catalysts are also attempted.

Over the last decades, many steps have been taken in the search for an effective method to reduce carbon dioxide to small organic molecules that can be used as fuel or building blocks for the chemical industry. The object of this work is to prepare two bimetallic electrocatalysts utilizing Cu combined with Au or In for the efficient and selective reduction of CO2 to CO, HCOOH and small fuels molecules. The copper-gold electrode is prepared through the electrodeposition of Cu on the surface of Au, using the underpotential deposition (UPD) technique to obtain a copper monolayer. The prepared electrode shows a high current density compared to Au electrode. Bimetallic metal oxides of CuInO2 is used as the precursor to prepare Cu-In alloys electrodes for electrochemical reduction of CO2. The electrocatalyst preparation is carried out using a thermal reducing treatment able to form different catalytic surfaces with different Cu-In alloys or single-phase metals. The best sample shows a high faradaic efficiency toward CO (71%) at the low overpotential of −0.8 V vs RHE. This study shows two examples of scalable and inexpensive preparation methods of bimetallic surfaces, which may use as selective electrocatalysts for the aqueous reduction of CO2.

Las etapas de transferencia electrónica o transferencia de carga involucradas en reacciones electroquímicas juegan un papel muy importante en un gran número de procesos biológicos y bioquímicos. Hoy en día, el interés de la comunidad científica se centra en explorar y entender exhaustivamente la naturaleza biológica y química de los fenómenos bioelectroquímicos que ocurren en los seres vivos, con el objeto de mimetizarlos en el laboratorio. Los procesos bioelectrocatalíticos presentan un amplio abanico de aplicaciones dirigidas al: (i) desarrollo de biorreactores electroquímicos para la mitigación de las emisiones de gases de efecto invernadero, la eliminación de contaminantes presentes en aguas residuales y urbanas, o la síntesis de productos con alto valor añadido para la industria, (ii) el desarrollo de biopilas y biobaterías, y (iii) el desarrollo de (bio)sensores electroquímicos con fines analíticos. Sin embargo, la implantación en el mercado de dispositivos basados en procesos biocatalíticos aún se enfrenta a varios desafíos, como son la robustez, la estabilidad a largo plazo, la reproducibilidad y la rentabilidad de producción en términos de materiales y fabricación de los dispositivos electroquímicos. La motivación de esta tesis doctoral es la de enfrentarse a algunos de los desafíos con los que se encuentra hoy en día la bioelectrocatálisis, para ello esta tesis doctoral se centra, principalmente en el estudio de nuevos materiales y mejora de rutas y estrategias bioelectrocatalíticas, con la finalidad de desarrollar dispositivos electroquímicos con aplicaciones analíticas y en la obtención de productos de valor añadido. En primer lugar esta tesis doctoral recoge el estudio y desarrollo de (bio)sensores electroquímicos para la determinación de lactato, L-cisteína, peróxido de hidrógeno y pH en medios biológicos complejos, y en segundo lugar estudia la bioelectrosíntesis de ácido fórmico a través de la reducción bioelectroquímica de dióxido de carbono.

This dissertation relates to the application of density functional theory to the design of novel nanoelectrocatalysts for various electrochemical reduction reactions such as carbon dioxide reduction reactions, carbon monoxide reduction reactions and nitrogen reduction reactions. Many electrocatalysts with high activity, excellent selectivity and stability were designed and engineered using first principle calculations. These findings could potentially guide the experimentalists for creating clean and sustainable energy resources.

La combustió excessiva de combustibles fòssils té com a resultat l’emissió de diòxid de carboni (CO2), que està desencadenant problemes ambientals creixents, com ara l’escalfament global, l’augment del nivell del mar, el clima extrem i l’extinció d’espècies. Per tant, les tecnologies per a la conversió de CO2 en altres productes de valor estan jugant un paper vital per eliminar la concentració de CO2 a l’atmosfera. En aquest sentit, la conversió electroquímica de CO2, alimentat per energia renovable, en productes químics útils es considera una solució elegant per aconseguir el cicle del carboni. Tanmateix, a causa de la interioritat de les molècules de CO2 i de la reacció competitiva d’evolució d’hidrogen (HER), els principals reptes de CO2 RR són l’elevat requeriment de sobrepotencial associat a una termodinàmica desfavorable i una baixa eficiència faradaica (FE) per a un producte concret. Per tant, buscar un electrocatalitzador d’alta eficiència i econòmic és raonable i necessari per a aplicacions pràctiques. En les darreres dècades, els marcs metal·lorgànics (MOF) van absorbir les enormes consideracions en el camp de l’electrocatàlisi a causa de la seva gran superfície específica, una rica estructura de porus i llocs actius uniformement dispersos. Tot i que tenen un gran potencial en electrocàlisi, la majoria dels materials MOF encara pateixen una activitat insuficient, baixa conductivitat i poca estabilitat, cosa que dificultaria les seves aplicacions pràctiques. Especialment, en el camp del CO2 RR, s’han de tenir en compte molts paràmetres importants, inclosa una alta eficiència faradaica (FE), l’excessiu baix sobrepotencial, una gran densitat de corrent i una estabilitat robusta, entre d’altres. Per tant, el disseny racional dels MOF per complir els requisits anteriors tant com sigui possible és crucial per explotar el seu futur en aplicacions de CO2 RR. Per tant, en aquesta dissertació, vam fer molts esforços per desenvolupar catalitzadors basats en MOFs/derivats amb una eficiència, activitat i estabilitat superiors per augmentar el rendiment del CO2 RR. Aquesta dissertació es divideix en 5 capítols: El capítol 1 presenta les idees sobre els conceptes fonamentals sobre CO2 RR electroquímic, que inclou la cèl·lula fonamental de CO2 RR electroquímica, que revisa els productes de reducció comuns i les seves vies senzilles. En aquest capítol també es presenta la visió general de paràmetres importants que afecten el CO2 RR, inclosos diferents catalitzadors dels darrers anys i electròlits, i les mètriques rellevants que avaluen els electrocatalitzadors, així com les limitacions de la reducció electroquímica de CO2. El capítol 2 tracta de la fabricació de ZIF-8 modificat a la superfície com a elèctrode basat en MOFs per a un CO2 RR electroquímic per generar CO. En aquest treball, hem modificat la superfície del MOF ZIF-8 a partir d’introduir un petita proporció d’àcid 2,5-dihidroxyterephthalic (DOBDC), aconseguint una densitat de corrent de CO 2,5 vegades superior i una eficiència faradaica augmentada. Al capítol 3, s’utilitza una ruta fàcil per introduir grups que contenen O enllaçats axialment en un catalitzador Fe-N-C mitjançant piròlisi de marcs orgànics metàl·lics basats en Zn dopats amb Fe (IRMOF-3), formant àtoms individuals de Fe molt dispersos amb llocs actius de HO-FeN4. A causa de la modulació de l’entorn local induïda per aquests grups -OH, el catalitzador D-Fe-N-C presenta una activitat CO2 RR millorada, que inclou una alta selectivitat amb una eficiència faradaica de CO, i una estabilitat robusta , que és superior a la dels llocs FeN4 normals reportats sense grups -OH. Al capítol 4, vam proposar la introducció d’àtoms de Fe en catalitzadors de Ni-N-C per produir catalitzadors amb àtoms individualitzats (Ni/Fe-N-C) de doble metall (bimetàl·lics) de cara al CO2 RR per aconseguir una alta selectivitat i activitat simultàniament. Finally, Chapter 5 summarizes the general conclusions.
La combustión excesiva de combustibles fósiles da como resultado la emisión de dióxido de carbono (CO2), que desencadenó crecientes problemas ambientales, como el calentamiento global, el aumento del nivel del mar, el clima extremo y la extinción de especies. Por lo tanto, las tecnologías para la conversión de CO2 en otros productos de valor jugaron un papel vital para eliminar la concentración de CO2 en la atmósfera. En ese sentido, la conversión electroquímica de CO2 alimentado por energía renovable en productos químicos útiles se considera una solución elegante para lograr el ciclo del carbono. Sin embargo, debido a la interioridad de las moléculas de CO2 y la reacción competitiva de evolución de hidrógeno (HER), los principales desafíos en el campo CO2 RR son el alto requerimiento de sobrepotencial que representa la termodinámica desfavorable y la baja eficiencia faradaica (FE) para los productos objetivo. Por lo tanto, la búsqueda de un electrocatalizador económico y de alta eficiencia es sensato y necesario para aplicaciones prácticas. En las últimas décadas, las estructuras organometálicas (MOF) absorbieron las enormes consideraciones en el campo de la electrocatálisis debido a su gran área de superficie específica, rica estructura de poros y sitios activos uniformemente dispersos. Aunque con grandes potenciales en electrocatálisis, la mayoría de los materiales MOF todavía sufren de actividad insuficiente, baja conductividad y poca estabilidad, lo que dificultaría sus aplicaciones prácticas. Especialmente, en el campo de CO2 RR, se deben considerar muchos parámetros importantes, incluida la alta eficiencia faradaica (FE), bajo sobrepotencial, gran densidad de corriente y estabilidad robusta, etc. Por lo tanto, el diseño racional de MOF para cumplir con los requisitos anteriores tanto como sea posible es crucial para explotar sus futuras aplicaciones de CO2 RR. Por lo tanto, en esta disertación, hicimos muchos esfuerzos para desarrollar catalizadores basados en MOFs / derivados de MOF con eficiencia, actividad y estabilidad superiores para aumentar el rendimiento de CO2 RR. Esta disertación se divide en 5 capítulos: El capítulo 1 es la información sobre los conceptos fundamentales sobre la CO2 RR electroquímico, que incluye la celda fundamental de la CO2 RR electroquímico, revisa los productos de reducción comunes y sus vías simples. Mientras tanto, la descripción general de los parámetros importantes que afectan la CO2 RR, incluidos los diferentes catalizadores en los últimos años y el electrolito, y las métricas relevantes que evalúan los electrocatalizadores. El Capítulo 2 trata de la fabricación de ZIF-8 modificado en superficie como electrodo basado en MOF para CO2 RR electroquímico para generar CO. En este trabajo, se preparó un ZIF-8 modificado en superficie mediante la introducción de una proporción muy pequeña de ácido 2,5-dihidroxitereftálico (DOBDC) en ZIF-8, logrando una densidad de corriente de CO mayor. En el Capítulo 3, se utiliza una ruta fácil para introducir grupos que contienen O con enlaces axiales en un catalizador de Fe-N-C a través de la pirólisis de estructuras orgánicas metálicas a base de Zn dopado con Fe (IRMOF-3), formando átomos únicos de Fe altamente dispersos con sitios activos HO-FeN4. Debido a la modulación del ambiente local inducida por tales grupos -OH, el catalizador D-Fe-N-C exhibe una actividad CO2 RR mejorada, incluida una alta selectividad con alta eficiencia Faradaica de CO y una estabilidad sólida. En el capítulo 4, proponemos que la introducción de átomos de Fe en catalizadores de Ni-N-C fabrica catalizadores de un solo átomo de metal doble (Ni/Fe-N-C) hacia CO2 RR para lograr una alta selectividad y actividad simultáneamente. El catalizador de doble metal optimizado mostró excelentes rendimientos, obteniendo una alta selectividad con eficiencia faradaica CO a un bajo sobrepotencial, superior a las contrapartes de un solo metal. Finalmente, el Capítulo 5 resume las conclusiones generales.
The excessive combustion of fossil fuels results in the emission of carbon dioxide (CO2), which triggers increasing environmental problems, such as, global warming, rising sea levels, extreme weather, and species extinction. Therefore, the technologies for conversion of CO2 into other value products plays a vital role in order to eliminate the CO2 concentration in atmosphere. Thereinto, electrochemical conversion of CO2 powered by renewable energy to useful chemicals is considered as an elegant solution to achieve the carbon cycle. However, due to the innerness of CO2 molecules and competitive hydrogen evolution reaction (HER), the main challenges in the field CO2 RR are the high overpotential requirement that represents the unfavourable thermodynamics and low Faradaic efficiency (FE) for the target products. Therefore, searching for a high-efficient and cost-friendly electrocatalyst is sensible and necessary for practical applications. In the past decades, metal-organic frameworks (MOFs) engrossed the enormous considerations in the field of electrocatalysis because of their large specific surface area, rich pore structure, and uniformly dispersed active sites. Although they have a great potential in electrocatalysis, most MOFs materials still suffer from insufficient activity, low conductivity, and poor stability, which would hinder their practical applications. Especially, in the field of CO2 RR, many important parameters, including high FE, low overpotential, large current density and robust stability among others, should be considered. Thus, the rational design of MOFs to fulfil the above requirements as much as possible is crucial for exploiting their future in CO2 RR applications. Therefore, in this dissertation, we made many efforts to develop MOFs-based/derived catalysts with superior efficiency, activity, and stability for boosting the CO2 RR performance. This dissertation is divided into 5 chapters: Chapter 1 is the insights on the fundamental concepts about electrochemical CO2 RR, which includes the fundamental cell of electrochemical CO2 RR, reviews the common reduction products and their simple pathways. Meanwhile, the overview of important parameters affecting CO2 RR, including different catalysts over the past years, electrolyte, and the relevant metrics evaluating the electrocatalysts as well as limitations of electrochemical CO2 reduction are also presented in this chapter. In addition, this chapter summarizes the fundamental concepts about MOFs materials and their high-temperature pyrolysis derived materials as the electrocatalysts. Chapter 2 deals with the fabrication of surface modified ZIF-8 as MOFs-based electrode for electrochemical CO2 RR to generate CO. In this work, a surface modified ZIF-8 has been prepared through introducing a very small proportion 2,5-dihidroxyterephthalic acid (DOBDC) into ZIF-8, achieving a higher current density of CO and a boosted Faradaic efficiency. In Chapter 3, a facile route is used to introduce axial bonded O-containing groups into a Fe-N-C catalyst through pyrolysis of Fe-doped Zn-based metal organic frameworks (IRMOF-3), forming highly dispersed Fe single atoms with HO-FeN4 active sites. Due to the local environment modulation induced by such -OH groups, the D-Fe-N-C catalyst exhibits an enhanced CO2 RR activity, including a high selectivity with CO Faradaic efficiency, and a robust stability, which is higher than that of the reported normal FeN4 sites without -OH groups. In Chapter 4, we proposed that introducing Fe atoms into Ni-N-C catalysts fabricates double metal (bimetallic) single-atom catalysts (Ni/Fe-N-C) towards CO2 RR to achieve a high selectivity and activity simultaneously. The optimized double-metal Ni/Fe-N-C catalyst showed an excellent performance, obtaining a high selectivity with a high CO Faradaic efficiency at a low overpotential. The performance obtained is superior to both single metal counterparts and other state-of-the-art M-N-C catalysts, proving that regulating single active sites with a second metal site potentially breaks the single metal-based activity benchmark to obtain the high selectivity and activity in CO2 RR, simultaneously. Finally, Chapter 5 summarizes the general conclusions.
Universitat Autònoma de Barcelona. Programa de Doctorat en Ciència de Materials

A crescente emissão do CO2 para a atmosfera, causada pela matriz energética dependente dos combustíveis fósseis tem gerado a necessidade de sistemas que o utilizem como matéria-prima para a produção ou armazenamento de energia. Em vista disso, este trabalho teve como objetivo o estudo do ciclo de estocagem de hidrogênio baseado em etapas eletrocatalíticas da eletro-redução e eletro-oxidação do par CO2/HCOO-. Para o processo de eletro-redução, foram utilizados eletrocatalisadores suportados em pó de carbono formados à base de estanho (Sn/C) e de estanho modificado com cobalto (Co-Sn/C), cobre (Cu-Sn/C) e paládio (Sn-Pd/C). Os materiais foram sintetizados pelo método de impregnação seguido por tratamento térmico e caracterizados fisicamente por Difratometria de Raios X (DRX) e Espectroscopia por energia Dispersiva de Raios X (EDX). Os testes eletroquímicos foram realizados via cronoamperometria (eletrólise) e a quantificação dos íons formato por Cromatografia Líquida de Alta Eficiência (CLAE) e voltametria cíclica (VC). Os resultados obtidos mostraram que os materiais nanoestruturados sintetizados apresentaram estruturas cristalinas, sendo que o estanho apresentou-se na forma de SnO2, mas sofrendo eletro-redução em condições in situ para SnO ou SnOH. Os resultados eletroquímicos mostraram que o Sn/C eletrocatalisa a redução do CO2 para HCOO-, sendo que a quantificação por VC utilizando eletrodos de paládio e platina indicaram correntes de pico crescentes até o potencial de eletrólise de -1,6 V vs. Ag/AgCl/Cl-. Ademais, experimentos de eletrólise evidenciaram o aumento linear da concentração de HCOO- após 6 horas de polarização, indicando alta estabilidade do eletrocatalisador de Sn/C. A atividade eletrocatalítica dos eletrocatalisadores à base de estanho frente a redução de CO2 para HCOO- foi atribuída a dois aspectos: (i) o estanho favorece a adsorção ou interação do CO2 através dos átomos de oxigênio, possibilitando a transferência de prótons e elétrons sem a quebra da ligação C-O e/ou; (ii) a presença de espécies SnOH na superfície, mesmo em baixos potenciais, permite a interação com o CO2 e leva à formação de intermediários adsorvidos reativos, que sofrem a adição de prótons e elétrons para a formação de HCOO-. A eficiência máxima de corrente faradaica para a formação de HCOO- foi de aproximadamente 7 % tendo a reação de desprendimento de hidrogênio (HER) como rota paralela. A investigação da influência da natureza do eletrocatalisador mostrou inatividade do material de Co-Sn/C, mas com aumento da atividade de Cu-Sn/C para a eletro-redução de CO2, quando comparado com Sn/C puro.
With the increase CO2 emissions into atmosphere caused mainly by the energy dependence on fossil fuels, systems for generation or storage of clean energy has been studied to couple CO2 as feedstock. This work proposed a hydrogen storage cycle based on electrocatalytic steps of pair CO2/HCOO-, such electroreduction and electrooxidation. For electroreduction process were used carbon-supported tin-based electrocatalysts (Sn/C) and tin modified with cobalt (Co-Sn/C), copper (Cu-Sn/C) and palladium (Sn-Pd/C). The materials were synthesized by impregnation method followed of thermal treatment, and X Ray Diffraction (XRD) and Energy Dispersive X-ray Spectroscopy (EDS) techniques were used for physical characterization. Electrochemical tests were performed via chronoamperometry (electrolysis) and the quantification of formate ions by High Performance Liquid Chromatography (HPLC) and cyclic voltammetry (CV). Results of synthesized nanostructured materials showed crystalline structures with tin as SnO2 species, but tin oxide suffering electroreduction to SnO or SnOH in situ conditions. Electrochemical results presented that the Sn/C catalyzes the CO2 reduction to HCOO-, with an increase peak current until electrolysis potential of -1.6 V vs. Ag/AgCl/Cl- quantified by CV on palladium and platinum electrodes. Moreover, electrolysis measurements demonstrated the linear increase of HCOO- concentration after polarization for 6 hours, which indicates the high stability of Sn/C electrocatalyst. The electrocatalytic activity of tin-based electrocatalysts for CO2 reduction into HCOO- was attributed to two aspects: (i) tin favors the adsorption or interaction of CO2 through oxygen atoms, which enables the proton and electron transfer without breaking C-O bond and/or; (ii) the presence on surface of SnOH species allows the interaction with CO2 even at low potential, and leads to the formation of reactive intermediates adsorbed that undergo addition of protons and electrons to form HCOO-. Maximum Faradaic efficiency for HCOO- formation was near 7% with Hydrogen Evolution Reaction (HER) as parallel route. Investigation of the influence of the electrocatalyst nature showed inactivity of CO-Sn/C material, but the activity of CO2 electroreduction increased on Cu-Sn/C material as compared to Sn/C pure.

Le réchauffement climatique est dû principalement à l'émission anthropique du dioxyde de carbone (CO2) dans l'atmosphère. Une réduction électrocatalytique et sélective de cette molécule a été proposée au cours de ce projet comme une solution prometteuse pour synthétiser des produits à valeur ajoutée. Une telle réaction requiert l'utilisation de matériaux efficaces et bas coût. Pour ce faire, les travaux de cette thèse ont porté sur la préparation de catalyseurs à base de cuivre dispersés sur différents substrats carbonés tels que le Vulcan XC-72R, les carbones mésoporeux CMK-3 et FDU-15, et des tanins à base d'IS2M pour réduire le CO2 en milieu aqueux. Les matériaux d'électrode ont été préparés à l'aide de la méthode polyol assistée par micro-ondes. Leurs caractérisations physiques et l'analyse élémentaire confirment des compositions atomiques et des taux de charge métallique proches de celles théoriquement envisagées. L'acide formique et le monoxyde de carbone sont les deux produits carbonés issus de la réduction du CO2 (2 bar) réalisée par chronoampérométrie en milieu NaHCO3. La détection et l'identification des produits de réaction ont été effectuées par des méthodes chromatographiques (µ-GC et HPLC), spectrométrique (DEMS) et spectroscopique (RMN). Une sélectivité de la réaction vis-à-vis de HCOOH (62 %) a été obtenue sur une cathode de Cu50Pd50/C. Cette conversion sélective du CO2 en HCOOH s'explique par une conjugaison d'effets électroniques et géométriques dans la structure de surface du catalyseur bimétallique et aussi celui de la texture du substrat carboné
The anthropogenic emissions of carbon dioxide (CO2) are the major cause of global warming. The selective CO2 reduction reaction (CO2RR) of has been proposed as a promising, convenient and efficient method for sustainable energy conversion systems. The reduction of CO2 to energetically valuable products requires the use of an appropriate electrode material. This study focuses on the preparation of Cu-based electrocatalysts supported on different types of carbon materials such as Vulcan XC-72R, mesoporous carbon CMK-3, mesoporous carbon FDU-15 and tannin based mesoporous carbon IS2M for the CO2RR under mild conditions. Besides, Vulcan XC-72R carbon supported bimetallic copper/palladium alloy materials were prepared for increasing the Faradaic yield. These copper-based catalysts were electrochemically characterized and preparative electrolyses set at constant potential were carried out in order to investigate the reduction products distribution and Faradaic yields as a function of the applied potential and catalyst loading. Chemicals such as HCOOH, CO and H2 issued from the CO2RR, were determined with in-situ and ex-situ complementary (electro)analytical and spectroscopic techniques. The significant difference in the product distribution is probably due to the ensemble (geometry and ligand) effects in the bimetallic CuPd materials, and textural structure of the supporting substrates. Selective CO2 to-HCOOH conversion has been successfully undertaken on Cu50Pd50/C with 62 % Faradaic efficiency

Ce memoire est consacre a l'etude des proprietes electrochimiques, photochimiques et aux applications dans le domaine de la photoimagerie et de la reduction electrocatalytique du dioxyde de carbone d'une serie de complexes monobipyridine carbonyle de ruthenium en phase homogene ou immobilises a la surface d'une electrode sous forme de films polymeriques. Nous avons tout d'abord demontre que le complexe considere dans la litterature comme etant cisru(bpy)(co)#2cl#2 (bpy=2,2-bipyridine) est en realite constitue d'un melange de ru(bpy)(co)#2cl#2 et de ru(bpy)(co)cl#3, ce dernier a pu etre isole sous sa forme reduite par une methode electrochimique originale. L'etude electrochimique detaillee de ces complexes a ete realisee et il a ete montre que l'electroreduction de certains de ces complexes permet de preparer un nouveau type d'electrodes modifiees par des polymeres inorganiques (appeles film bleu: fb). Ce travail s'est poursuivi par la mise en evidence de phenomenes de substitution de ligands par des molecules de solvant lorsque ces complexes sont irradies par la lumiere visible. Lorsque ces complexes sont immobilises a la surface d'une electrode par l'intermediaire de films polypyrroliques fonctionnalises, leurs proprietes photochimiques sont conservees et ces phenomenes ont permis l'utilisation de ces films en photoimagerie. Enfin, ces films (fb et polypyrroliques) se sont averes etre d'excellentes cathodes moleculaires pour la reduction electrocatalytique du co#2, avec la formation quasi quantitative de co meme dans des electrolytes purement aqueux

Ce memoire est consacre a l'etude de l'electrocatalytique de la reduction du co#2 par des complexes mono et bis bipyridine bis carbonyle de ru(ii), en phase homogene ou supportee (electrodes modifiees). Une partie de ce travail traite des proprietes electrochimiques des complexes trans-(cl)-cis-(co)-ru(l)(co)#2cl#2 (l = 2,2'-bipyridine substituee ou non ou 1,10-phenanthroline) et cis-(bpy)-cis-(co)-ru(bpy)#2(co)#2#2#+, precurseurs de polymeres du type ru(l)(co)#2#n, espece electrocatalytique clees. Le mecanisme d'electrogeneration de ces polymeres organometallique a pu etre determine, grace en particulier a l'etude des complexes stereoisomeres cis-(cl)-cis-(co)-ru(l)(co)#2xy#n#+(x = cl#- ; y = cl#- ou c(o)och#3#-). Ceux-ci ont un comportement electrochimique different des complexes equivalents trans-(cl)-cis-(co) et conduisent exclusivement a des dimeres. Tous ces complexes se sont averes etre d'excellents catalyseurs pour l'electroreduction du co#2 en milieu hydro-organique en phase homogene ou supportee. Par ailleurs, le probleme de l'instabilite a l'oxygene des electrodes modifiees par les films de ru(l)(co)#2#n a ete contourne grace a l'utilisation d'un autre type de cathodes moleculaires. Ces dernieres ont ete realisees par immobilisation, a la surface d'electrodes, des complexes precurseurs, dans des films de polypyrrole fonctionnalises. Une etude detaillee des differents parametres influencant l'orientation de la reaction de reduction du co#2 vers la production d'ions formiate (en particulier la substitution du ligand bipyridine) dans un electrolyte purement aqueux, a permis d'etablir les conditions optimales pour l'obtention de rendements electriques quasi quantitatifs.

碩士
中原大學
化學研究所
104
For the past few years, untilization of fossil fuel as main energy source caused the serious greenhouse effect. Scientists attempted to develop an alternative energy source to solve the problem and reduce the CO2 emission. In this work, we successfully synthesized dinuclear DNICs containing pyrazolate bridging ligands [(NO)2Fe(μ-RPyr)2Fe(NO)2] (Pyr = pyrazolate, R = Me, NH2). These complexes show two electron reversible redox interconversion between {Fe(NO)2}9-{Fe(NO)2}9, {Fe(NO)2}9-{Fe(NO)2}10 and {Fe(NO)2}10-{Fe(NO)2}10. We try as whether catalytic reduction of water into hydrogen and electrocatalytic reduction with CO2. In the aqueous phase, our complexes have lower onset potential of electrocatalytic reduction of water -1.40 V(vs. SCE) and good stability in the prolonged electrolysis, and the Faraday efficiency of hydrogen generation reaches 100%. However, 1-Me and 1-NH2 can electrochemical react with CO2 in methanol, although about 80% of the electrons used in generating hydrogen when the process of electrolysis, we succeed reduce CO2 to formic, and the low conversion efficiency is the problem to be overcome in the future.

碩士
國立成功大學
化學工程學系
104
In this study, the electrochemical reduction of CO2 using the electrocatalytic systems, which consists of four types of surface-modified fluorine-doped tin oxide coated glass electrodes (FTO) and three kinds of transition metal (Co2+, Ni2+, Fe2+) nitroso–R complexes was investigated. By reducing the CO2 into methanol, it can not only eliminate the excess emission of CO2 in atmosphere but also generate an alternative energy to solve both severe greenhouse effect and the energy crisis. The four types of surface-modified FTO electrodes include bare FTO, Prussian blue modified FTO (FTO|PB), Platinum (Pt) modified FTO (FTO|Pt), and Prussian blue modified FTO|Pt (FTO|Pt|PB) electrodes. The PB thin film, prepared by electrodeposition, acts as the electron transfer mediator in this work, whereas the transition metal nitroso-R complexes was used as the electrocatalysts electrocatalyzing the electrochemical reduction of CO2. It was found that the electrocatalytic system containing cobalt nitroso-R complex showed best electrocatalytic activity towards the reduction of CO2; methanol production was noticed for all the four types of electrodes, and the maximal Faradaic efficiency of 52.83 ± 17.10% can be achieved using FTO|Pt|PB. The electrocatalyst system containing ferrous nitroso-R complex exhibited second best activity; the maximal Faradaic efficiency of 11.50 ± 12.14% can be achieved using FTO|Pt|PB. The electrocatalyst system containing nickel nitroso-R complex didn’t exhibit any activity towards the electrochemical reduction of CO2 into methanol. These results is quite different from the previous work done by Ogura group in 1980s, in which they employ Pt|PB as the working electrode and ferrous nitroso-R complex showed the best activity to methanol production with a Faradaic efficiency of 83%. The lower or ignorable activity of electrocatalytic systems containing ferrous nitroso-R complex and nickel nitroso-R complex could be attributed to the fact that the instability of these two complexes under cathodic conditions or other byproducts, which cannot be detected and quantified using gas chromatography, formed during the electrochemical reduction of CO2.

The continuous strong growth of CO2 emissions and the intensification of environmental impacts caused by this gas have aroused an increasing interest in the development of strategies to transform CO2. Formate dehydrogenases (FDH) are enzymes that perform the reversible interconversion of formate to CO2, hence these biocatalysts can transform CO2 into a compound that can be used either as a biofuel or as chemical precursor for sustainable chemical synthesis. Reports on direct electrochemical approaches, avoiding kinetic limitations of the mediating molecules and additional steps of cofactors regeneration, have been scarce until recently. In this Thesis, the electrochemical characterisation of the molybdenum-containing FDH from Desulfovibrio desulfuricans (DdFDH) was accomplished through non-mediated methods, in the absence of added substrates (non-turnover conditions), for the enzyme physically adsorbed onto a pyrolytic graphite electrode, at pH 6.5. A redox process with formal potential of -124 ± 11 mV vs NHE was assigned to the redox pair Mo (VI/IV) of the active centre. The heterogeneous electron transfer rate constant increased with the scan rate, which is indicative of a good communication between the enzyme and the electrode. The DdFDH catalytic response towards CO2 reduction was attained without mediators as well, upon the addition of saturated CO2 solution and sodium carbonate solution for the DdFDH adsorbed onto a stationary pyrolytic graphite electrode. The electrocatalysis towards CO2 reduction was also attained for DdFDH physically adsorbed on glassy carbon and graphite, under the hydrodynamic regime, and for the DdFDH encapsulated on felt carbon, a gas diffusion electrode.

Controlled and selective electrochemical CO2 reduction to hydrocarbons and oxygenates utilizing energy from renewables such as solar energy is a promising alternative approach to store energy in chemical bonds while simultaneously close the anthropogenic carbon cycle, thus to address the twin problems of fossil fuels depletion and environmental challenges. Copper-based electrocatalysts have been demonstrated promising performance for CO2 reduction. However, Cu usually converts CO2 into a mixture, where more than 16 different species have been identified, and the selective yield of any product is limited by the competing reactions. Other major bottlenecks of Cu electrochemical catalyzed CO2 reduction reaction include the competition of hydrogen evolution reaction (HER), high overpotentials needed towards desired product, and lack of high-value products. In this dissertation, we addressed these three issues via surface modification, sulfurization, and coupling cathodic/anodic reactions, respectively. Specifically, (1) we developed a benzimidazole (BIMH)-modified copper foil catalyst, where the formed Cu(BIM)x complexes on Cu surfaces can enhance the Faradaic efficiency (FE) of C2/C3 products. The overall FE for CO2 reduction reaches 92.1% and the undesired hydrogen evolution reaction (HER) is lowered to 7% at -1.07 VRHE. (2) We demonstrated that Cu2S nanoarrays enable the selective CO2 reduction to formate starting at a very low overpotential (~ 120 mV), with high current density (over -20 mA/cm2 at -0.89 VRHE), and good Faradaic efficiency (>75%) over a broad potential window (-0.7 VRHE to -0.9 VRHE). Further- more, Cu2S catalysts show excellent durability without deactivation following more than 15 cycles (1h per cycle) of operation. The notable reactivity toward CO2 reduction to formate achieved by Cu2S nanoarrays may be ascribed to their ability to facilitate CO2 activation by stabilizing the CO2•− intermediate more effectively than pristine Cu foil. (3) We reported that direct electrochemical conversion of CO2 to 2-bromoethanol, a valuable pharmaceutical intermediate, is enabled by coupling the anodic and cathodic reactions with the presence of potassium bromide electrolyte in a membraneless electrochemical cell. The maximum Faradaic Efficiency of converting CO2 to 2-bromoethanol that we achieved is 40 % at -1.01 VRHE with its partial current density of -19 mA cm-2.

The electrochemical CO2 reduction reaction (CRR) can combine carbon cycling with renewable energy to convert CO2 into high-value carbonaceous feedstocks. However, this process su ers from kinetically sluggish because of the complicated electron transfer and high energy barriers involved. Well-designed transition metal materials as promising electrocatalysts show remarkable catalytic activities for the CRR. Therefore, this Thesis is to study the catalytic activity and selectivity on these transition metal catalysts, and a fundamental understanding of the catalytic mechanism is given through a series of experimental and computational results using advanced synthesis methods, electrochemical measurements, material characterization including microscopy and spectroscopy, synchrotron-based X- ray spectroscopy, in situ spectroscopy, and density functional theory (DFT) calculations. The scope of this Thesis is narrowed to nanoscale and sub-nanoscale engineered 3d-block transition metal (mainly, Fe, Co, Ni, Cu) catalysts for the CRR process. In this Thesis, the rst section introduces research progress including catalytic performance and mechanisms on sub-nanoscale 3d-block transition metal catalysts for the CRR. The second section consists of published and submitted works: (1) The rst project starts with the investigation of the CRR on Ni catalysts. We engineered and alloyed Ni with Cu to obtain ultrasmall graphene-encapsulated Ni-Cu bimetallic catalysts. The Cu-lean catalyst exhibited signi cant activity and selectivity, and the highest Faradaic e ciency (FE) toward CO was 90% at -1.0 V vs. RHE. By coupling synchrotron-based X-ray absorption and in situ Raman spectroscopy studies, we found that there is a negative correlation with the Cu content in Ni-Cu catalyst and CO selectivity due to redistribution of the 3d electrons from Ni and Cu. (2) Because of the high catalytic activity was received on ultrasmall Ni-Cu particles, the second project aims to fabricate sub-nanoscale transition metal catalysts for the CRR. We synthesized atomically dispersed Fe immobilized within N-doped carbon nanosheets. The optimal Fe catalyst achieved FE of 90% toward CO at -0.58 V vs. RHE. A series of controlled tests revealed that there is a synergistic e ect between the Fe sites and the pyrrolic-N-framework which promotes the catalytic activity of CO evolution. (3) The third work is based on the previous Fe catalyst and investigates the unique single-atom Cu catalyst (Cu-N4-NG). The chemical structure and coordination environment of Cu-N4-NG were identi ed using synchrotron-based characterization. Compared to a traditional bulk Cu catalyst, Cu-N4-NG performed a FE of 80.6% towards CO at -1.0 V vs. RHE. The experimental results revealed that the presence of Cu-N4 moieties largely promotes CO2 activation and water dissociation, showing CO2 reduction is kinetically preferred on Cu-N4-NG. Also, the computational investigation suggested a thermodynamic explanation that CO2 reduction is less hindered on Cu-N4-NG compared to hydrogen evolution. (4) Although high FEs were obtained on single-atom transition metal catalyst shown in the previous two works, the two catalysts were not strictly single-atom catalysts with a uniform structure of M-N4, some coordination defects existed. Thus, graphene- supported metal phthalocyanine catalysts with M-N4 structure were reported in the fourth work, which achieved almost 100% CO2 conversion to CO on graphene- supported cobalt phthalocyanine. Further experimental studies showed that the phthalocyanines with graphene were signi cantly activated than the pure ones. A series of control tests uncovered that the graphene substrate facilitates electron transfer between the catalyst and CO2 molecules, which increased CO selectivity.
Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering and Advanced Materials, 2020