This thesis focuses on the synthesis, characterization and performance towards CO2 electroreduction of mono and bi-metallic particles based on p-block metals. With an industrial perspective in mind... Show moreThis thesis focuses on the synthesis, characterization and performance towards CO2 electroreduction of mono and bi-metallic particles based on p-block metals. With an industrial perspective in mind, we try to synthesize particulate, high surface area materials with clean, scalable synthesis methods where possible and test their performance in H-Cell and gas diffusion electrode flow cell configurations. With a combination of characterization techniques, we find possible explanations for the catalytic behaviors. Show less
This thesis has shed light on some of the ways in which the local electrolyte composition can differ from the bulk and how these changes in the local reaction environment can determine the activity... Show moreThis thesis has shed light on some of the ways in which the local electrolyte composition can differ from the bulk and how these changes in the local reaction environment can determine the activity and/or selectivity of two important electrocatalytic reactions, namely, electrochemical CO2 reduction reaction (CO2RR) and hydrogen evolution reaction (HER). Show less
Electrocatalysis allows for storing electricity or converting it into chemical bonds, producing chemical building blocks and fuels using renewable resources. Therefore, it plays an important role... Show moreElectrocatalysis allows for storing electricity or converting it into chemical bonds, producing chemical building blocks and fuels using renewable resources. Therefore, it plays an important role in the transition towards a more sustainable future for our society through electrification. Still, to bring electrochemical technologies to industrial scale and make them competitive, optimization of various aspects of electrocatalytic reactions are needed. Many fundamental studies focus on understanding the catalyst surface, however, different components of the electrolyte, as pH and cations, have also shown to significantly affect the reaction activity and selectivity. In view of that, in this thesis, various aspects of the electrode-electrolyte interface are studied at different scales, using Scanning Electrochemical Microscopy (SECM), stationary and rotating-disc electrode voltammetry techniques, and bulk electrolysis. Show less
This dissertation focused on computational methods based on first principles calculations using the Density Functional Theory (DFT) framework. Emphasis was laid on affordable methods that can... Show moreThis dissertation focused on computational methods based on first principles calculations using the Density Functional Theory (DFT) framework. Emphasis was laid on affordable methods that can provide a tradeoff between computational expense and accuracy. Specifically, we investigated solvation effects near the surface of the electrode, used thermodynamic cycles to compute solution-phase energies and also proposed a workflow to detect gas-phase errors on the free energies of target molecules. We used these simple methods to study complex adsorption processes at the PdMLPt(111) electrode surface. DFT and experimental studies (performed by Dr. Chen from our group) were crucial to guide the investigation forward. Show less
The electrochemical oxygen reduction reaction (ORR) is an essential half-reaction for the utilization of hydrogen as a sustainable fuel, via the conversion of hydrogen to electrons and protons... Show moreThe electrochemical oxygen reduction reaction (ORR) is an essential half-reaction for the utilization of hydrogen as a sustainable fuel, via the conversion of hydrogen to electrons and protons facilitated by the ORR. In the most common fuel cells, the ORR is requires high loadings of non-abundant platinum based catalysts. Inspired by Laccase, a multicopper oxidase able to perform the ORR at a low overpotential, copper complexes have become interesting targets as non-precious metal catalysts for the ORR.In this thesis, the ORR performance of molecular copper catalysts and the involved catalytic mechanisms have been investigated. The previously undetermined electrocatalytic mechanism for the ORR by the Cu(tmpa) copper complex was elucidated. Hydrogen peroxide was shown to play an important role in the catalytic cycle as a reaction intermediate. This has interesting implications for the sustainable electrochemical production of hydrogen peroxide. Furthermore, the reduction of hydrogen peroxide shows striking similarities with Fenton-like reactions observed in copper containing enzymes. Finally, the performance of several different copper electrocatalysts for the reduction of oxygen and hydrogen peroxide was investigated and compared. Show less
Molecular complexes can be used as electrocatalysts for oxygen reduction, water oxidation, and/or hydrogen peroxide production. However, in situ degradation of these catalyst is a major issue. This... Show moreMolecular complexes can be used as electrocatalysts for oxygen reduction, water oxidation, and/or hydrogen peroxide production. However, in situ degradation of these catalyst is a major issue. This dissertations describes the analysis of degradation processes as well as the performance of various molecular electrocatalysts. In addition, complexes have been structurally modified to perform structure-activity studies that could to mechanistic insight. In addition, it is described how molecular catalysts can be beneficial to heterogeneous electrocatalysis as well. Show less
In this dissertation iron-based homogeneous catalysts were synthesized, characterized and investigated for water oxidation activity. The catalysts were studied under electrochemical conditions in... Show moreIn this dissertation iron-based homogeneous catalysts were synthesized, characterized and investigated for water oxidation activity. The catalysts were studied under electrochemical conditions in order to compare the electrochemical approach to the catalysis based on the use of sacrificial oxidants. The mechanisms under which these catalysts operate have been explored with particular attention to the O−O bond formation step. The combination of electrochemical techniques and in situ characterization techniques allowed for the identification of the active intermediates responsible for catalysis. The influence of the presence of water oxidation catalysts in solution on the evolution of carbon dioxide from the surface of a pyrolytic graphite working electrode was also investigated. Overall, the results of this work demonstrate that the combination of in operando and in situ (spectro)electrochemical techniques allows for a complete investigation of the catalytic mechanism of the water oxidation reaction. Show less
Advanced sensing techniques require graphene with high quality and well-controlled surface chemistry. The intrinsic high mobility, low electrical noises and uniform graphitic crystallinity are the... Show moreAdvanced sensing techniques require graphene with high quality and well-controlled surface chemistry. The intrinsic high mobility, low electrical noises and uniform graphitic crystallinity are the prerequisites for high-performance graphene electronics. More importantly, chemical functionalization contributes to unlock the sensing potential of the graphene basal plane. This thesis focuses on manipulating the surface chemistry of a graphene monolayer and explores the impacts on the electrical and electrochemical properties for sensing applications. Heteroatoms like hydrogen, nitrogen and oxygen were systematically introduced into the graphene lattice as defect sites to modify the surface chemistry, and consequently the electronic properties and sensing performance. In summary, a correlation between the in-plane electron transport and the electrochemical activity of hydrogenated graphene was studied by modulating the density of H-sp3 defects. Moreover, cleaning effect on the graphene surface caused by hydrogenation process and the corresponding mechanism were discussed. The electrocatalysis of oxygen reduction reaction on nitrogen doped monolayer graphene was conducted to pinpoint the catalytic active sites. The mechanics of a centimeter-scale graphene floating on water was characterized by biaxial compression. Finally, the chemically modified graphene was tested for field-effect sensing of gas molecules. Show less
Sustainable energy from wind and solar is most readily available near the sea. Seawater electrolysis would thus be a highly promising method for intermittently storing surplus electricity from... Show moreSustainable energy from wind and solar is most readily available near the sea. Seawater electrolysis would thus be a highly promising method for intermittently storing surplus electricity from these sources, in the form of hydrogen. Unfortunately, the direct use of seawater in electrolysers brings with it a selectivity problem, caused by the chloride salts in such water. Instead of forming oxygen at the anode, which is environmentally harmless and thus the desired product, the formation of toxic chlorine becomes possible in seawater, and this reaction has to be avoided. This thesis is focussed on how the anodic evolution of oxygen and chlorine compete, and how selectivity between these two reactions may be optimized for the benefit of seawater electrolysis, and electrocatalysis in general. Show less
Three pyridyl‐amide substituted (benz)imidazolium salts H2L1Cl, H2L2Cl and H2L3Cl were synthesized and successfully employed as ligand precursors for the syntheses of novel nickel(II) and cobalt... Show moreThree pyridyl‐amide substituted (benz)imidazolium salts H2L1Cl, H2L2Cl and H2L3Cl were synthesized and successfully employed as ligand precursors for the syntheses of novel nickel(II) and cobalt(III) complexes. The compounds H2L1Cl and H2L2Cl are precursors to tetradentate ligands and differ in the nature of the N‐heterocyclic carbene (NHC) functionality, being imidazole‐based and benzimidazole‐based, respectively. The ligand precursor H2L3Cl resembles H2L1Cl, but with one of the pyridyl groups replaced with a benzyl group, thus yielding a potential tridentate ligand. The nickel(II) compounds [Ni(L1)]Cl and [Ni(L2)]PF6 were obtained, bearing tetradentate ligands comprising an amidate and two pyridine nitrogen donor atoms and an (NHC) carbon donor. Single crystal X‐ray crystallography revealed that the nickel ions in both compounds are in slightly distorted square‐planar geometries. Reactions of cobalt salts with the ligands H2L1Cl and H2L3Cl resulted in the cobalt(III) compounds [Co(L1)2]Cl and [Co(L3)2]PF6; the cobalt ions in both complexes are in octahedral geometries, bound by two tridentate ligands in a meridional binding mode, with two dangling pyridine and benzyl groups, respectively. The four compounds show electrocatalytic activity in proton reduction in dimethylformamide solutions in presence of acetic acid; their activity is compared using cyclic voltammetry and quantified with gas chromatography. Show less
Cathodic corrosion is a relatively unknown phenomenon that can severely etch metallic electrodes at cathodic (negative) potentials. In spite of these remarkable changes that are caused by cathodic... Show moreCathodic corrosion is a relatively unknown phenomenon that can severely etch metallic electrodes at cathodic (negative) potentials. In spite of these remarkable changes that are caused by cathodic corrosion, the phenomenon is stil not fully understood. Cathodic corrosion is therefore the focus of this PhD thesis. The first three experimental chapters of the thesis focus on characterizing platinum, rhodium and gold electrodes before and after cathodic corrosion in a variety of working solutions. In doing so, these chapters establish surprisingly mild corrosion onset potentials and reveal an etching anistropy that depends on the cation in the working solution. Additional density functional theory calculations suggest a similarly significant role for adsorbed hydrogen. These result suggest the existence of ternary metal hydrides during cathodic corrosion. The role of hydrides is further studied in the fourth experimental chapter through X-ray absorption spectroscopy. These four fundamental chapters are followed by two more applied chapters. The first of these tailors the activity of a platinum single crystal towards oxygen reduction, by using cathodic corrosion. The second applied chapter uses cathodic corrosion to create and thoroughly characterize alloyed nanoparticles. Combined, these fundamental and applied chapters provide valuable new information towards understanding and applying cathodic corrosion. Show less
With the energy transition toward a renewable energy supply and a CO2-neutral economy, electrification of the energy system is rising in importance, which leads to the challenge of long-term... Show moreWith the energy transition toward a renewable energy supply and a CO2-neutral economy, electrification of the energy system is rising in importance, which leads to the challenge of long-term storage of renewable electricity. A promising option is the electrochemical conversion of biomass or carbon dioxide in chemicals as energy carrier. In this research, catalysis of the electrochemical CO2 reduction was studied to obtain liquid fuels. In this fundamental study we discovered that so-called disproportionation reactions may occur simultaneously with the CO2 reduction reaction influencing the product spectrum. Moreover, we focused on metalloprotoporphyrins immobilized on a graphite surface. We found that the selectivity can be steered toward formic acid with rhodium, tin or indium metal centers. Apart from intrinsic catalyst parameters, we studied the influence of parameters related to the immobilization and the composition of the electrolyte. We showed that the substrate and its pretreatment as well as encapsulation of the catalyst in polymers can have a signifcant influence on the electrocatalysis of CO2 reduction. The results obtained in this thesis provide insight in the energy efficiency, reaction rate and selectivity of the CO2 reduction reaction, and play an important role for the development of an industrially viable process. Show less
The electrochemical oxidation of ammonia to dinitrogen is a model reaction for the electrocatalysis of the nitrogen cycle, as it can contribute to the understanding of the making/breaking of NN, NO... Show moreThe electrochemical oxidation of ammonia to dinitrogen is a model reaction for the electrocatalysis of the nitrogen cycle, as it can contribute to the understanding of the making/breaking of NN, NO, or NH bonds. Moreover, it can be used as the anode reaction in ammonia electrolyzers for H2 production or in ammonia fuel cells. We study here the reaction on the N2-forming Pt(1 0 0) electrode using a combination of electrochemical methods, product characterization and computational methods, and suggest a mechanism that is compatible with the experimental and theoretical findings. We propose that N2 is formed via an ∗NH + ∗NH coupling step, in accordance with the Gerischer-Mauerer mechanism. Other NN bond-forming steps are considered less likely based on either their unfavourable energetics or the low coverage of the necessary monomers. The NN coupling is inhibited by strongly adsorbed ∗N and ∗NO species, which are formed by further oxidation of ∗NH. Show less
In this dissertation, the synthesis and characterization of a series of iron complexes based on different ligand platforms are described. The complexes are subsequently studied for their... Show moreIn this dissertation, the synthesis and characterization of a series of iron complexes based on different ligand platforms are described. The complexes are subsequently studied for their activity in catalytic water oxidation with the help of a variety of electroanalytical techniques. The results show that the catalytic activity of structurally related iron complexes correlates strongly with the electronics of the iron centre. Another potentially very important aspect in the field of homogeneous electrocatalysis which has so far received only very little attention in published literature is the influence of the nature of the electrode material on the resulting electrochemistry. The results discussed in thesis show that interactions between the working electrode and the catalyst in solution can exhibit a strong influence on the resulting electrochemistry. Overall, the results of this work demonstrate that iron-based complexes can indeed be made to work as electrocatalysts for the water oxidation reaction. Furthermore, the results show that the electronic structure of the iron centre is a promising target for the design of new and improved catalysts. Finally, the results also highlight the importance of trying out different electrode materials as part of routine tests of new potential electrocatalysts. Show less
Gezer, G.; Durán Jiménez, D.; Siegler, M.A.; Bouwman, E. 2017
The growing demand of energy indicates that global energy resources in the form of fossil fuels will not be sufficient in the future. In order to solve potential future energy problems development... Show moreThe growing demand of energy indicates that global energy resources in the form of fossil fuels will not be sufficient in the future. In order to solve potential future energy problems development of a sustainable hydrogen economy is highly desirable. Researchers are looking for new and cleaner ways for the production of dihydrogen gas. The structure and function of hydrogenases have raised the attention of synthetic chemists in the past decades, since new catalysts for proton reduction may be developed by using biomimetic, functional models of hydrogenases. Three types of hydrogenases are known, being the [FeFe], [Fe] and [NiFe] hydrogenases.A significant amount of data has been gathered over the years concerning the enzyme redox states and the reaction mechanism for the reversible heterolytic splitting of dihydrogen at the [NiFe] hydrogenase active site. The [NiFeSe] hydrogenases form a subclass of the [NiFe] hydrogenases, in which one of the cysteines (Cys) in the active site of the enzyme is replaced by selenocysteine (Sec). This thesis deals with the synthesis and characterization of new structural and functional models of the nickel-containing enzymes [NiFe] and [NiFeSe] hydrogenases for electrocatalytic hydrogen evolution. Show less
