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.

To combat the escalating environmental challenges and alleviate the current energy crisis, CO2 conversion to fuels and chemical feedstocks provides a reliable approach to mitigate the devastating impact of greenhouse emissions on climate change. CO2 conversion/reduction could be carried out by several methods; however, the electrochemical CO2 reduction (CO2R) approach has coupled several advantages. For instance, CO2R occurs in near-ambient reaction conditions and could be driven through the employment of renewable energy resources (wind or solar) to generate electricity. However, this reaction has a large energy barrier which requires a catalyst to facilitate its pathway. In this context, various catalyst designs were developed and investigated during the last decades, such as heterogenous (metal and metal oxide) and homogenous (organic molecules) catalysts. A new class of materials – atomically dispersed metal nitrogen–doped carbon support (M–N–C)– has emerged recently and showed remarkable enhancement for CO2R compared to the state-of-the-art. In particular, Ni–N–C catalysts have demonstrated an improved selectivity toward CO production compared to precious metal catalysts. Researchers have postulated this superior performance to the high atomic utilization (theoretically 100%) of the metal sites under reaction conditions and the enhanced electronic properties. In addition, intermetallic carbides have been included as a promising class of catalysts for CO2R due to their unique physical and chemical characteristics. These catalysts could be synthesized using different precursors; among them, MOFs are currently one of the most promising platforms that generate several catalyst designs. It was demonstrated that MOF’s unique characteristics, such as high surface area and porosity, would be transitioned to the derived catalysts. In this thesis, two MOF architectures (ZIF-8 and MOF-74) were initially selected to be employed as precursors for deriving atomically dispersed Ni–N–C catalysts. Both MOF-derived catalysts were evaluated for CO2R using a customized electrochemical cell (E-cell) with a 3–electrode configuration. The derived Ni–N–C catalysts using ZIF-8 and MOF-74 have achieved enhanced CO selectivity with Faradaic efficiencies (FE) > 90% at less negative applied potentials, –0.68 and –0.76 V vs RHE, respectively. Further, various synthetic conditions were explored in these studies, such as the role of the Ni content and the pyrolysis temperature on the resulted catalyst structure, and the electrocatalytic performance during CO2 electrolysis. Subsequently, one of the MOF topologies – ZIF-8 – was further utilized to develop other designs of electrocatalysts by introducing different synthetic conditions. This has resulted in generating various moieties that are able to produce CO during CO2R. For example, one derived catalyst design consists of homogenously distributed atomically dispersed dual Ni–Zn–NX/C sites. Whereas the other design demonstrated a heterogenous structure of Ni3ZnC-based particles anchored on atomically dispersed dual Ni–Zn–NX/C sites. Both electrocatalyst designs were integrated into a gas diffusion electrode (GDE) and evaluated for CO2R using an MEA-based electrolyzer. Our findings revealed that the co-existence of Ni3ZnC particles and dual Ni–Zn–NX/C active sites in a heterogenous structure has boosted the electrocatalytic activity towards CO production, achieving near unity CO FE at 448 mA/cm2 at an overall cell voltage of 3.1 V. Aside from the electrocatalytic performance, the nature of active sites in the developed catalyst designs has been studied using in-situ and ex-situ X-ray absorption spectroscopy. Other analytical techniques such as transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), powder X-ray diffraction (PXRD), and X-ray photoelectron spectroscopy (XPS) have also been used to identify the catalysts’ composition and morphology. / Thesis / Doctor of Philosophy (PhD) / This PhD thesis aims to develop and implement a sustainable technology that tackles increased CO2 emissions in the atmosphere and mitigates the greenhouse effect on climate change. The approach of this thesis focuses on developing efficient catalyst designs for CO2 electroreduction (CO2R) to CO as a beneficial chemical feedstock, and then pursues the practical implementation of these catalysts in an industrially relative reactor design in the form of a membrane electrode assembly (MEA)-type electrolyzer. This study selected atomically dispersed metal-doped nitrogen-carbon (M–N–C) and intermetallic carbide electrocatalysts as promising materials for CO2R. Among different precursors, metal-organic frameworks (MOFs) have been employed to synthesize the desired electrocatalysts due to their unique geometric structure and high surface area. On a fundamental level, our findings demonstrated that all MOF-derived catalysts have exhibited high selectivity towards CO during CO2 R. However, the conversion rates were governed by the nature of the active sites and the implemented electrochemical systems.

This thesis delivers an in-depth investigation into electrochemical carbon dioxide reduction (CO2R), a process with the potential to convert CO2 gas into value-added chemicals and fuels. However, the efficiency and operational durability of current CO2 reduction processes are limited by catalytic performance. To address this, the thesis focuses on gaining a deep understanding of the transformations that CO2R electrocatalysts undergo under realistic conditions, such as morphological, phase structure, and compositional changes. These insights inform the design of next-generation materials by identifying performance descriptors and degradation patterns. A key aspect of this thesis is the development and application of in-situ liquid phase transmission electron microscopy (LP-TEM), an advanced platform that directly correlates nanoscale changes in catalyst materials under the influence of electrode potentials in CO2R reactive environments. Despite its potential, the use of in-situ LP-TEM presents a range of challenges, which this thesis addresses alongside exploring potential advancements for enhancing its accuracy and applicability. With the evolution of nanofabricated liquid cells, dynamic nanoparticle tracking, and high-resolution imaging in a liquid medium, this technology can more accurately mimic realistic exposure conditions. Cumulatively, this thesis systematically navigates the technical hurdles, advancements, and future prospects of in-situ LP-TEM in the context of electrochemical CO2R. The findings not only advance our understanding of the in-situ LP-TEM technical process but also guide new researchers in the field of in-situ TEM of electrocatalyst materials, aiding in the optimization of catalyst design, and paving the way for more sustainable and economically competitive CO2R technologies. The application of in-situ LP-TEM extends to the examination of two specific catalysts: Palladium (Pd) and a bi-metallic alloy of Copper (Cu) and Silver (Ag). By employing in-situ LP-TEM and selected area diffraction (SAD) measurements, we trace the morphological and phase structure transformations of the Pd catalyst under CO2R conditions. Interestingly, our findings indicate that alterations in reaction energetics, rather than morphological or phase structure changes, chiefly govern catalyst selectivity. This provides invaluable insights for designing more efficient catalysts. Further, we observe the morphological transformation of a metallic copper catalyst structure into a Cu-Ag bimetallic alloy during a galvanic replacement method. We then investigate the stability of both catalyst structures under operational CO2R conditions. Our results reveal that the metallic Cu structure undergoes significant morphological deformation during CO2R, leading to migration, detachment, and recrystallization of the catalyst surface. Contrarily, the Cu-Ag bimetallic alloy demonstrates notable thermodynamic stability under a similar applied potential. / Thesis / Candidate in Philosophy / This PhD thesis focuses on the development and implementation of cutting-edge technologies to address the climate change implications of CO2 emissions - a potent greenhouse gas. CO2 molecules could be electrochemically converted into various chemical feedstock and fuels. This process involves the development of efficient catalyst designs that can reduce CO2 gas at high conversion rates. Acquiring mechanistic insights on the behavior of the developed catalysts under reaction conditions would significantly assist on producing performance descriptors for catalyst design in CO2 conversion approach. Among a range of different advanced techniques, in-situ liquid phase transmission electron microscopy (LP-TEM) technology is selected for this study. This technique is capable of correlating dynamic nanoscale compositional and morphological changes with the electrochemical response of the catalysts. The primary focus of the thesis is on developing and implementing in-situ LP-TEM techniques to achieve electrochemical CO2 conditions while tracking particle morphology and phase structures as functions of electrochemical potential and time. Furthermore, the thesis investigates the performance of different catalyst designs under CO2 reduction (CO2R) operational conditions, which includes palladium (Pd) nanoparticles and copper–silver (Cu–Ag) bimetallic alloys. On a fundamental level, these studies provide a detailed understanding of the phase transformation and structural changes of these catalysts during CO2R that contributes valuable knowledge to the field and can be used to design next-generation CO2R catalysts.

This thesis focuses on the synthesis, characterization and reactivity of group VII transition metal complexes. It begins with exploring a new pincer geometry of Re(I) compounds and then examining both Re(I) and Mn(I) compound as homogenous catalysts for photocatalytic and electrocatalytic reduction of CO2. In the first chapter, I focus on some recently reported approaches to photocatalytic and electrocatalytic reduction of CO2 using homogenous catalysts of transition metal. The second chapter presents efforts to capture Re(I) in a neutral N,N,N pincer scaffold and the resulting enhanced absorption of visible light. Most of these results have appeared in a publication. In this thesis, I only present my work on rhenium compounds that are supported by the bis(imino)pyridine ligand and an examination of the differences in properties between the bidentate and tridentate ligand geometries. Later I examine both tridentate and bidentate complexes for the photocatalytic and electrocatalytic reduction of CO2 to CO. The failure of tridentate Re1 bis(imino)pyridine compounds to reduce CO2 to CO prompted a change in direction to rhenium compounds that are supported with diimine ligands. Thus, I choose 4,5-diazafluoren-9-one as supporting ligand for rhenium and manganese. This chapter explained the reasons behind choosing these particular ligand and metal combinations. ReI and Mn1 compounds of 4,5-diazafluoren-9-one have shown activity for the photocatalytic and electrocatalytic reduction of CO2 to CO. In the fourth chapter, as rhenium and manganese compounds of 4,5-diazafluoren-9-one have shown the great ability of CO2 reduction to CO, the focus here was to modify the ligand by attaching a photosensitizer to the ligand in order to prepare supramolecular complexes that may increase the efficiency and yield of reduction products. In this chapter, I examined two types of the photosensitizer; tris(bipyridine)ruthenium(II)chloride and osmium dichloro bis(4,​4'-​dimethyl-​2,​2'-​bipyridine).

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Thesis advisor: Dunwei Wang / Electrochemical CO2 reduction reaction (CRR) provides a solution to both the increasing global demand of energy by forming valuable chemical products for fuel production, and global warming by reducing the amount of CO2 in the environment. To efficiently reduce CO2, we sought to understand the reaction mechanism using a polycrystalline Au electrode and the super concentrated LiTFSI solution (WiS) as the electrolyte. By varying both the electrolytic potential and the concentration of WiS, we investigated the factors determining product selectivity and found that reaction kinetics and mass transport together direct the selectivity towards CO. We probed the rate limiting step (RLS) of CO2 reduction by observing the variation of product distribution with water availability in solution, and discovered that the RLS was likely to involve only a single electron transfer to form COO*–. Lastly, we proposed that in WiS, H2O were the dominant proton sources for both CO2 reduction and H2 evolution reactions. In 21m WiS, the competing hydrogen evolution reaction was kinetically inhibited, so CO production was favored with a selectivity of 90% at a potential as early as -0.4V vs RHE. This study demonstrated the great potential of WiS as a platform for studying multi-proton, multi-electron transfer reactions. / Thesis (BS) — Boston College, 2018. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Scholar of the College. / Discipline: Chemistry.

Le couplage d’un traitement électrochimique avec un procédé biologique est une alternative prometteuse pour la dégradation de composés organo-halogénés biorécalcitrants dans l’environnement. Les procédés d’électroréduction, connus pour couper sélectivement la liaison carbone-halogène, ont été mis en oeuvre afin de réduire la toxicité des molécules cibles et augmenter leur biodégradabilité avant une minéralisation totale des polluants par un traitement biologique. Pour améliorer le rendement de déchloration, la cathode préalablement nickelée a été modifiée par des nanoparticules d’argent car l’argent est considéré comme l’un des meilleurs catalyseurs pour couper sélectivement la liaison carbonehalogène. Le feutre de graphite a été choisi comme support d’électrode pour sa grande surface spécifique. Le principal produit de déchloration de l’alachlor s’est révélé être biorécalcitrant. Pour surmonter ce problème, un traitement par procédé électro-Fenton a été mis en oeuvre pour dégrader les polluants cibles. Une amélioration significative de la biodégradabilité de la solution d’alachlor a pu être observée après le traitement électro- Fenton, et qui est renforcée quand l’atome de chlore a été préalablement éliminé de la structure de l’alachlor par électroréduction. Le bismuth a été également utilisé comme support d’électrode du fait de sa grande surtension visà- vis de la réduction de l’eau. Une grande sélectivité a pu être obtenue sur cathode de bismuth lors de la réduction d’herbicides du type chloracétamide. La réduction électrochimique du dioxyde de carbone a également été réalisée sur électrode de bismuth modifiée par des nanoparticules d’argent comme autre application de cette nouvelle électrode. / Electrochemical process coupling with a biological treatment is a promising alternative for the degradation of biorecalcitrant organo-halogenated compounds in the environment. The electroreduction treatment, known to cut selectively carbon-halogen bonds, was first implemented to decrease the toxicity of the target molecules and increase their biodegradability before a complete mineralization of the pollutants by a biological treatment. To improve the dechlorination efficiency, the cathode was modified by silver nanoparticles after a previous nickelisation, since silver is considered as one of the best electrocatalysts to selectively cleave the carbonhalogen bond. The graphite felt was chosen as the electrode support due to its high specific surface area. For alachlor herbicide, deschloroalachlor, the main by-product after dechlorination, was still biorecalcitrant. To overcome this issue, electro-Fenton treatment, in which hydroxyl radicals were generated to degrade the target pollutants, was implemented. Significant improvement of biodegradability of the alachlor solution was observed after electro-Fenton treatment, which was further improved when the chlorine atom was beforehand removed from the alachlor structure by the electroreduction process. Bismuth was also used as electrode support due to its high overpotential for hydrogen evolution. A high selectivity of chloroacetamide herbicides reduction was observed on the bismuth based cathode. As an extended application of the bismuth based cathode, the electrochemical reduction of carbon dioxide was performed on Bi electrode modified by silver nanoparticles.

Having been a hot topic for some time, the interest in recycling carbon dioxide to renewable liquid fuels or other valuable chemicals has rocketed since the adoption of the Paris Agreement on Climate Change. This is due to the EU ruling that from 2020, a considerable fraction of renewable fuel of non-biological origin has to be added to gasoline and the commitment of large air carriers like UA to go 50% carbon neutral by 2050. The primary novelty of this thesis was the development and conversion of the thermal catalyst indium oxide to an electrocatalyst that could do the conversion of formic acid and CO2 at ambient conditions with water as the only hydrogen source and the second starting compound. Here, the synthesis of indium oxide (In2O3) and supported iridium oxide (IrO2) electrocatalysts were done in-house. The crystallinities and average particle size characterizations were examined via powder X-ray diffraction. Scanning electron microscopy was used to study the surface morphologies of both electrocatalysts. Three different anodic electrocatalysts including 60:40 wt% IrO2:TaC, 70:30 wt% IrO2:TaC and 100:00 wt% IrO2:TaC were fabricated and employed for water electrolysis, with 70:30 wt% IrO2:TaC demonstrated to be of superior electrochemical activity and further employed for subsequent studies. Currently, In2O3 is the best thermal catalyst for methanol formation from CO2 . Here, the as-synthesized thermal catalyst was converted to a cathodic electrocatalyst by firstly making electroconductive material in a nanosize form with very small crystallite grains, which contain numerous defects near the surface; thus, making it more conductive. This was used to prepare high-performance membrane electrode assemblies (MEAs). The reaction cell containing the MEA that was set up by spray-coating the respective catalyst inks onto either Nafion® or a carbon gas diffusion cloth. The PEM electrolysis cell configuration with Nafion® as the polymer electrolyte was used. It minimizes Ohmic losses, and a standard TaC-supported IrO2 water-splitting catalyst served as the anode, titanium mesh served as anodic gas diffusion layer, and the experiments were conducted at ambient temperature. The cathode consisted of In2O3 spray-coated on carbon paper which acts as a gas diffusion layer and titanium mesh current collector. The cathode electrocatalyst was enhanced by the addition of a small amount of polytetrafluoroethylene to the nanosized In2O3 to facilitate diffusion of FA and CO2. The electrochemical characteristics were examined via cyclic voltammetry, linear sweep voltammetry and chronoamperometric methods. The infrared spectroelectrochemical cell was also used because it permits in-situ analysis of the change of reactant concentrations and ideally the identification of intermediates Addition of PTFE to the In2O3 electrocatalyst layer for FARR has led to significant improvement in current density from 1.94 mA/cm2 (without PTFE) to 66.0 mA/cm2 (with 0.15 wt% PTFE) and 70.3 mA/cm2 (with 0.30 wt% PTFE) which is a factor of ca. 34 and ca. 36 respectively at 2.4 V cell voltage. This further reduces the onset potential of the electroreduction by 0.4 V and notably, the cell Ohmic resistance was reduced by a factor of 15, implying that the activation energy of the electrode and the transport resistance in the porous structure are significantly reduced. This is due to the increase in the hydrophobicity in the porous electrocatalyst layer. The Tafel slope was also used to investigate the electrochemical reaction of water splitting, co-electrolysis of 4.30 M formic acid and water on In2O3 and PTFE-In2O3 cathodes. Tafel values of all the samples over the respective number of LSV cycles were consistent with each other. Tafel analysis of the PTFE-In2O3 electrode improves significantly with the lower Tafel slope in comparison with the PTFE-free In2O3 electrode. The steady-state current density experiment in the absence of any flow showed excellent stability over the investigation period. A current density observed to be limited to ca. 26 mA/cm2 in the absence of any flow over 24 h from the initial current density of 70.3 mA/cm2, the limitation is a result of FA transport across the diffusion layers in the electrocatalyst surface. This behavior was further investigated using the Cottrell equation and this was observed to qualitatively reproduce the experimental behavior, thereby confirming a diffusion layer that builds up, resulting in a reactant depletion near the electrode surface. For a long time, it was thought that formic acid is a dead-end that does not lead to larger product molecules. For the first time, the co-electrolysis of water and aqueous 4.30 M formic acid, the first stable intermediate of CO2 electroreduction, results in a mixture of methanol, ethanol and isopropanol with a maximum combined Faraday efficiency of 82.6% at 3.5 V and a space-time-yield of 0.431 g(alcohol)/h/g(cat) that compares well with results from heterogeneous catalysis. It was further discovered here that high Faraday efficiency of the alcohols and current density can be achieved under a relatively low overpotential by tuning the amount of PTFE used. FTIR spectroelectrochemistry was used to monitor the disappearance of FA and the formation or disappearance of CO2 reaction intermediates as a function of time and potentials. The consumption of FA propelled significant decreasing of absorption of up to 6 vibrational modes in the observation window including bands: at 3670 cm-1 belonging to the O-H stretching vibration, 3037 cm-1 assigned to the C-H stretching mode, 2120 cm-1 attributed to the C=O stretching mode, a double band near 1667/1589 cm-1, assigned to the vibrational modes with major FA C-O stretching character, and finally one at 1225 cm-1 which are somewhat higher than corresponding literature values, suggesting interactions with the catalyst and the presence of the aqueous environment. In the experiment performed with CO2 catholyte on PTFE-In2O3 (in the absence of FA), the CO2 band disappears as expected with no FA build-up, suggesting that formic acid is bypassed as an intermediate. An additional convincing difference was that while R/R0 is >1 dominated by the FA disappearance and CO2 formation in FA catholyte, it is <1 in CO2 catholyte, and the spectra revealed the CO2 disappearance with the formation of intermediates and products; seen as a broad structured background. The CO2 band changes in the positive direction, demonstrating that CO2 is used up with the applied potential going more negative. The CV experiments further established a cross-over oxidation peak which indicates multiple redox species or a multi-step parallel or consecutive mechanism with the PTFE-In2O3 cathode. This was due to the slow formation of redox-active intermediates and slow follow-up reactions occurring in the diffusion layer on the surface of the electrode. This further indicates that the appropriate amount of PTFE in the In2O3 catalyst layer would enhance the adhesion properties of the In2O3 catalyst layer on the carbon paper and create the hydrophobic channels in the catalytic layers. Finally, in agreement with the cyclic voltammetry, spectroelectrochemistry and electrolysis experiments, a plausible reaction mechanism for FA reduction to methanol on In2O3 cathode was proposed while the higher alcohols (i.e. C2 and C3 alcohols) may be formed through the same stepwise reduction pattern involving the different intermediate species formed. Therefore, this study established that the In2O3 electrocatalyst could do the conversion of formic acid (HCOOH) and CO2 at room temperature and with water in place of hydrogen as the second starting material in contrast to the known methods which were achieved at elevated temperatures. Importantly, the addition of PTFE facilitated FA and CO2 diffusion and enhanced the electrochemical performance of the In2O3 electrocatalyst. / Thesis (PhD (Chemistry))--University of Pretoria, 2020. / National Research Foundation and the World Academy of Sciences (NRF-TWAS): (UID: 105453 & Reference: SFH160618172220) National Research Foundation (NRF) S&F - Extended Support for Scholarships and Fellowships: (Reference No: MND190603441389, Unique Grant No: 121108) / Chemistry / PhD (Chemistry) / Unrestricted