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
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
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
The PhD project was aimed to understand the role of the solvent in the hydrogen oxidation and evolution reactions on platinum and gold. This approach sheds light on the molecular origins... Show more The PhD project was aimed to understand the role of the solvent in the hydrogen oxidation and evolution reactions on platinum and gold. This approach sheds light on the molecular origins affecting the kinetics of the hydrogen evolution reaction, as a promising source of energy in the era of sustainable energy production and storage. Ultimately, this work demonstrates the importance of the solvent in the hydrogen electrocatalysis, specifically, water, by settling its role as a solvent, as a proton donor, and by preferential proton solvation, clarifying a long-existing debate regarding the pH dependence of the hydrogen evolution, and setting a path for future exploration of solvent-electrode interfaces for the tailoring of electrocatalytic reactions. Show less
This thesis discusses the parameters affecting the catalysis for the electrochemical conversion of water into oxygen. The slow kinetics for the oxygen evolution reaction (OER) is one of the major... Show moreThis thesis discusses the parameters affecting the catalysis for the electrochemical conversion of water into oxygen. The slow kinetics for the oxygen evolution reaction (OER) is one of the major bottlenecks in the solar energy-to-fuels conversion process, which reduces the efficiency for the photo-electrochemical fuels generation (artificial photosynthesis). The work shows that to enhance the kinetics for the oxygen evolution reaction, one should not only look at the catalysts but also consider the synergy between catalyst and electrolyte. A more general approach that considers the electrochemical interface as a whole (electrode + electrolyte) is therefore the most promising route towards optimal activity. Show less
Nitrate reduction on Sn-modified polycrystalline Pt has been investigated. NO is the main product at high Sn coverage, whereas N2O is dominant at low Sn coverage. The N2O reduction on Sn-modified... Show moreNitrate reduction on Sn-modified polycrystalline Pt has been investigated. NO is the main product at high Sn coverage, whereas N2O is dominant at low Sn coverage. The N2O reduction on Sn-modified Pt electrodes indicates electrochemical formation of N2 is related to pristine Pt sites. Moreover, homogeneous chemical reactions of intermediates products also contribute to N2O and N2 formation of in solution. The p-block metals have been studied: Cd, In and Sn show a promoting effect; Ga shows a limited enhancement; Tl shows a special promoting effect in sulfuric acid; Pb shows a weak formation of N2O. Density Functional Theory calculations show that Sn and In enhance nitrate adsorption compared with pristine Pt. Moreover, ammonia is found as the only product on Pt. After modification by Sn, hydroxylamine is specifically found with nitrite, which supports that nitrate reduction to nitrite is enhanced by Sn and Sn could steer the hydrogenation of NOads. However, solution pH is an important factor. On Pt, nitrate reduction is only observed in acidic solution. On Rh, a higher activity is observed in wide pH, which suggests a mechanism that HNO3 molecule is the active species. However, Rh additionally shows a special ability to reduce NO3- directly. Show less
This project has dealt with the mechanistic study of the electrocatalytic nitrite reduction, the selectivity-determining step of nitrate reduction. Nitrate is a polluting ion targeted by wastewater... Show moreThis project has dealt with the mechanistic study of the electrocatalytic nitrite reduction, the selectivity-determining step of nitrate reduction. Nitrate is a polluting ion targeted by wastewater remediation; electrochemistry strives to achieve selectivity to harmless products (N2). A multi-pronged approach has been followed, aimed at establishing the influence of several variables (electrocatalyst material, surface structure, pH and electrode potential) on the catalytic activity and the product distribution, which has been determined with in situ analytical techniques (mass spectrometry and infrared spectroscopy). The molecular underpinnings of nitrite reduction have thereby been unravelled for transition metals, showing that an optimal catalytic performance is achieved when metals intermediate affinities to reaction intermediates (Sabatier Principle). The all-important concept of structure sensitivity also applies to nitrite reduction at Pt electrodes, although only in alkaline media: a Pt(100) single-crystal is the sole Pt surface able to achieve the desired direct conversion of nitrite into 100% N2. Such selectivity is unparalleled for a simple monometallic surface and is an outstanding finding. Additionally, the nitrite-reducing performance of bio-inspired catalysts, (electroactive metalloporphyrins) was investigated. A further side-project of this PhD thesis has also been the electrochemical characterization of preferentially-oriented cuboid Pt nanoparticles synthesized with the innovative __cathodic corrosion__. Show less
