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The proton exchange membrane fuel cell generates electricity from hydrogen and oxygen (from air) through electrocatalytic reactions in an electrochemical cell. The Pt/C catalyst, commonly used in PEM fuel cells, is very sensitive to impurities that can interact with the active catalyst sites and limit fuel cell performance. Unfortunately, most hydrogen is currently produced from fossil sources, and inevitably contains impurities. The subject of this thesis is the effect of hydrogen impurities on the operation of a PEM fuel cell using a Pt/C anode. The impurities studied are carbon monoxide (CO), carbon dioxide (CO2), and selected hydrocarbons. Particular focus is given to the interaction between the impurities studied and the anode catalyst. The main method used in the study involved performing cyclic voltammetry and mass spectrometry, simultaneously. Other electrochemical techniques are also employed. The results show that all the impurities studied adsorb to some extent on the Pt/C catalyst surface, and require potentials comparable to that of CO oxidation, i.e., about 0.6V, or higher to be removed by oxidation to CO2. For complete oxidation of propene, and toluene, potentials of above 0.8, and 1.0V, respectively, are required. The unsaturated hydrocarbons can be desorbed to some extent by reduction, but oxidation is required for complete removal. Adsorption of ethene, propene, and CO2 is dependent on the presence of adsorbed or gaseous hydrogen. Hydrogen inhibits ethene and propene adsorption, but facilitates CO2 adsorption. Adsorption of methane and propane is very limited and high concentrations of methane cause dilution effects only. The adlayer formed on the Pt/C anode catalyst in the presence of CO2, or moderate amounts of hydrocarbons, is found to be insffuciently complete to notably interfere with the hydrogen oxidation reaction. Higher concentrations of toluene do, however, limit the reaction. / Polymerelektrolytbränslecellen genererar elektricitet fran vätgas och syrgas (fran luft) genom elektrokatalytiska reaktioner i en elektrokemisk cell. Den platina-baserade katalysator som oftast används i dessa bränsleceller är känslig mot föroreningar, då dessa kan interagera med katalysatorns aktiva yta, och därmed begränsna bränslecellens prestanda. Tyvärr produceras dagens vätgas huvudsakligen fran fossila källor och innehåller därför oundvikligen föroreningar. Denna avhandling behandlar hur olika vätgasföroreningar påverkar katalysatorns aktivitet och bränslecellens drift. De föroreningar som studeras är kolmonoxid (CO) och koldioxid (CO2), samt ett antal mindre kolväten. Störst fokus ligger på hur dessa föroreningar interagerar med anodens Pt/C katalysator. Den metod som huvudsakligen används är cyklisk voltammetri kombinerat med masspektrometri, men flera elektrokemiska metoder har använts. Resultaten visar att alla undersökta föroreningar adsorberar på Pt/C katalysatorns yta i större eller mindre utstreckning. For att avlägsna det adsoberade skiktet genom oxidation till CO2 krävs potentialer jämförbara med CO oxidation, dvs ca 0,6V, eller högre. Fullständig oxidation av propen eller toluen kräver potentialer högre än 0,8V respektive 1,0V. De omättade kolvätena kan delvis avlägsnas genom reduktion, men fullständig avlägsning kräver oxidation. Närvaron av väte, i gasform eller adsorberat pa katalysatorn, hämmar adsorptionen av eten och propen, men främjar CO2 adsorption. Metan och propan adsorberar i mycket begränsad utstreckning på Pt/C katalysatorns yta. De prestandaförluster som uppstår av höga koncentrationer av metan förklaras av utspädning av vätgasen. Det adsorberade skiktet som bildas när Pt/C katalysatorn exponeras för CO2 eller måttliga koncentrationer av studerade kolväten, är inte tillräckligt heltäckande for att märkbart påverka vätgasreduktionen. Däremot kan höga koncentrationer av toluen begränsa reaktionen. / <p>QC 20161010</p>

Nos últimos anos, materiais baseados em grafeno têm atraído grande interesse na área eletroquímica devido às suas excelentes propriedades eletrônicas. Neste trabalho, apresentamos a obtenção de óxido de grafeno reduzido utilizando métodos eletroquímicos. A redução eletroquímica do óxido de grafeno (OG) foi realizada na superfície de um eletrodo de carbono vítreo (ECV) utilizando voltametria cíclica. O óxido de grafeno reduzido eletroquimicamente (OG-RE) foi caracterizado utilizando espectroscopia Raman, microscopia eletrônica de varredura (MEV), microscopia de força atômica (MFA), espectroscopia de fotoelétrons excitados por raios-X (XPS) e espectroscopia de impedância eletroquímica. A relação ID/IG obtida através dos espectros Raman do OG e do OG-RE foram de 0,98 e 1,15, respectivamente, indicando que o processo de redução resultou em uma maior desorganização estrutural. A espectroscopia de fotoelétrons de raios-X mostrou que a proporção entre C e O no OG foi de 45,7% e, que após a redução eletroquímica essa relação diminuiu para 38,5%. O eletrodo modificado com OG-RE foi empregado para quantificação de furosemida (FUR) em amostras farmacêuticas utilizando voltametria de pulso diferencial e análise por injeção em fluxo com detecção amperométrica. Os limites de detecção e quantificação calculados para o método proposto foram de 0,35 e 1,18 &#181;mol L-1, respectivamente. Visando à aplicação em amostras de interesse clínico, fabricou-se um sensor descartável e de baixo custo para a detecção de FUR utilizando papel sulfite pintado com lápis de desenho e ativado por laser de CO2. Os estudos voltamétricos utilizando o [Ru(NH3)6]Cl3 mostraram que o transporte de massa neste dispositivo foi governado por difusão e que o tratamento com laser diminui a resistência a transferência de carga, assim como resultou em um menor problema relacionado ao envenenamento da superfície do eletrodo / In recent years, graphene-based materials have attracted great interest in the area of electrochemistry due to its excellent electronic properties. In this work, we present the production of reduced graphene oxide using electrochemical methods. The electrochemical reduction of graphene oxide (GO) was carried out on the surface of a glassy carbon electrode (GCE) using the cyclic voltammetry technique. Electrochemically reduced graphene oxide (ER-GO) was characterized using Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), Xray excited photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). ID / IG ratio obtained through Raman spectra of GO and ER-GO were 0.98 and 1.15, respectively, indicating that the reduction process resulted in greater structural disorganization. XPS showed that the ratio between C and O in the GO was 45.7% and that after electrochemical reduction this ratio decreased to 38.5%. The ER-GO modified electrode was used as a sensor for furosemide (FUR) in pharmaceutical samples using the techniques of differential pulse voltammetry and flow injection analysis with amperometric detection. The limits of detection and quantification for the proposed method were 0.35 and 1.18 &#181;mol L-1, respectively. Aiming to the application in clinical samples a disposable, low-cost paper-based sensor for the detection of FUR was fabricated using office paper painted with drawing pencil and activated by CO2 laser. Voltammetric studies using [Ru(NH3)6]Cl3 have shown that mass transport in this device was controlled by diffusion and the laser decreases resistance to charge transfer, as well as, avoided the problem with electrode surface poisoning

Nos últimos anos, materiais baseados em grafeno têm atraído grande interesse na área eletroquímica devido às suas excelentes propriedades eletrônicas. Neste trabalho, apresentamos a obtenção de óxido de grafeno reduzido utilizando métodos eletroquímicos. A redução eletroquímica do óxido de grafeno (OG) foi realizada na superfície de um eletrodo de carbono vítreo (ECV) utilizando voltametria cíclica. O óxido de grafeno reduzido eletroquimicamente (OG-RE) foi caracterizado utilizando espectroscopia Raman, microscopia eletrônica de varredura (MEV), microscopia de força atômica (MFA), espectroscopia de fotoelétrons excitados por raios-X (XPS) e espectroscopia de impedância eletroquímica. A relação ID/IG obtida através dos espectros Raman do OG e do OG-RE foram de 0,98 e 1,15, respectivamente, indicando que o processo de redução resultou em uma maior desorganização estrutural. A espectroscopia de fotoelétrons de raios-X mostrou que a proporção entre C e O no OG foi de 45,7% e, que após a redução eletroquímica essa relação diminuiu para 38,5%. O eletrodo modificado com OG-RE foi empregado para quantificação de furosemida (FUR) em amostras farmacêuticas utilizando voltametria de pulso diferencial e análise por injeção em fluxo com detecção amperométrica. Os limites de detecção e quantificação calculados para o método proposto foram de 0,35 e 1,18 &#181;mol L-1, respectivamente. Visando à aplicação em amostras de interesse clínico, fabricou-se um sensor descartável e de baixo custo para a detecção de FUR utilizando papel sulfite pintado com lápis de desenho e ativado por laser de CO2. Os estudos voltamétricos utilizando o [Ru(NH3)6]Cl3 mostraram que o transporte de massa neste dispositivo foi governado por difusão e que o tratamento com laser diminui a resistência a transferência de carga, assim como resultou em um menor problema relacionado ao envenenamento da superfície do eletrodo / In recent years, graphene-based materials have attracted great interest in the area of electrochemistry due to its excellent electronic properties. In this work, we present the production of reduced graphene oxide using electrochemical methods. The electrochemical reduction of graphene oxide (GO) was carried out on the surface of a glassy carbon electrode (GCE) using the cyclic voltammetry technique. Electrochemically reduced graphene oxide (ER-GO) was characterized using Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), Xray excited photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). ID / IG ratio obtained through Raman spectra of GO and ER-GO were 0.98 and 1.15, respectively, indicating that the reduction process resulted in greater structural disorganization. XPS showed that the ratio between C and O in the GO was 45.7% and that after electrochemical reduction this ratio decreased to 38.5%. The ER-GO modified electrode was used as a sensor for furosemide (FUR) in pharmaceutical samples using the techniques of differential pulse voltammetry and flow injection analysis with amperometric detection. The limits of detection and quantification for the proposed method were 0.35 and 1.18 &#181;mol L-1, respectively. Aiming to the application in clinical samples a disposable, low-cost paper-based sensor for the detection of FUR was fabricated using office paper painted with drawing pencil and activated by CO2 laser. Voltammetric studies using [Ru(NH3)6]Cl3 have shown that mass transport in this device was controlled by diffusion and the laser decreases resistance to charge transfer, as well as, avoided the problem with electrode surface poisoning

Dans cette thèse, des travaux de recherche ont été menés pour immobiliser différentes enzymes (oxydases et déshydrogénases) au sein d'une matrice de silice dans le but de construire un biocapteur ampérométrique. Des matériaux nanostructurés ont ensuite été introduit dans ce système afin d'améliorer les caractéristiques analytiques de ces biocapteurs. La méthode de dépôt sol-gel par assistance électrochimique a été choisie pour l'immobilisation des enzymes à la surface des électrodes et des nanomatériaux car elle donne la possibilité de contrôler finement l'épaisseur du film déposé afin de couvrir individuellement ces objets. La faisabilité de cette approche a été montrée par la modification de nanofibres de platine présentant une grande surface active pour l'oxydation de H2O2 produit en présence de biocomposite silice-glucose oxidase. Le dépôt sol-gel électrochimiquement assisté permet également la modification d'électrodes imprimées en or par un biocomposite silice-choline oxidase, ce qui donne la possibilité de construire rapidement un biocapteur à choline présentant de très bonnes caractéristiques analytiques. Les nanotubes de carbone ont également été choisis comme matrice pour l'immobilisation de déshydrogénases car ils permettent d'obtenir une grande aire spécifique et des propriétés catalytiques intéressantes pour l'oxydation du co-facteur enzymatique NADH. La méthode de dépôt électrophorétique a été utilisée pour créer des couches de nanotubes poreuses ayant une épaisseur contrôlée à la surface d'un support de carbone vitreux. Les électrodes ainsi préparées présentent de bonne performances électrochimiques, permettant notamment de déplacer le potentiel d'oxydation de NADH et d'augmenter la sensibilité de détection. Un biocomposite silice-sorbitol déshydrogénase a ensuite été déposé à la surface de la couche de nanotubes de carbone en utilisant la méthode de dépôt sol-gel assisté par électrochimie pour construire un biocapteur à sorbitol. La méthode de dépôt électrophorétique a enfin été appliquée pour la première fois à l'élaboration d'assemblages de nanotubes de carbone macroporeux. De tells assemblages ont été utilisés comme support pour co-immobiliser la sorbitol déshydrogénase et le co-facteur enzymatique au sein des macropores, ce qui a permis d'augmenter la sensibilité de la détection du sorbitol par comparaison avec un assemblage de nanotubes de carbone non macroporeux / In this thesis, the research work was focused on the immobilization of different enzymes (oxidases and dehydrogenases) into biocomposite silica matrix with the aim of amperometric biosensors construction. Then, the structured nanomaterials were introduced in the system in order to improve the characteristics of biosensors. The method of electrochemically-assisted deposition was chosen for the immobilization of enzymes on the surface of nanomaterials as it provides possibility of fine tuning of film thickness allowing covering each individual nanoobject. The feasibility of this was shown while modifying the platinum nanofibers, which demonstrate high electroactive surface and H2O2 oxidation rate, with silica-glucose oxidase biocomposite. The electrochemically-assisted deposition also allows the express modification of gold screen-printed electrodes with silica-choline oxidase biocomposite making possible the quick fabrication of cheap choline biosensors with high analytical characteristics. The carbon nanotubes was chosen as matrix for the immobilization of dehydrogenases as they have high surface area and electrocatalytic properties towards the oxidation of enzymatic co-factor NADH. The method of electrophoretic deposition was used for the creation of porous CNT-layer with controllable thickness on the surface of glassy carbon electrode. Created thereby matrix demonstrated good electrochemical performance significantly shifting the potential and increasing sensitivity of NADH oxidation. Then the biocomposite silica-sorbitol dehydrogenase film was deposited on the CNT-layer by means of electrochemically-assisted deposition for the construction of high-performance sorbitol biosensor. The method of electrophoretic deposition was also applied for the first time for the construction of thick macroporous CNT-assemblies with uniform pore size using template approach. Such macroporous CNT-electrode was used for the co-immobilization of sorbitol dehydrogenase and enzymatic co-factor inside the pores demonstrating higher sensitivity of sorbitol detection in comparison with nonporous CNT-electrode

Le travail de thèse est consacré au développement d'électrodes composites à base d'argiles naturelles et à leur caractérisation électrochimique à des fins analytiques, notamment pour le dosage de cations métalliques (Cu2+) et de quelques substances à visée thérapeutique. Le premier chapitre expérimental est consacré à une étude comparée des caractéristiques chimiques, structurales et physico-chimiques de deux argiles naturelles extraites sur deux sites en Roumanie. Il apparaît que l'argile de Valea Chioarului, de type montmorillonite, présente les propriétés les plus intéressantes avec une surface spécifique importante (190 m2/g). Le deuxième chapitre fait état d'une analyse prospective des deux argiles comme matériaux d'électrodes en utilisant un film composite argile/poly-électrolyte. Trois molécules d'intérêt pharmaceutique ont été choisies pour cible, l'acide ascorbique, la riboflavine et l'acétaminophène. Cette dernière molécule peut être détectée soit par oxydation directe, soit via une réaction électro-enzymatique avec la peroxydase de raifort. Les troisième et quatrième chapitres sont consacrés à l'utilisation d'argiles échangées par des cations ammonium pour la détection du cuivre. Ainsi, l'argile de Valea Chioarului a été échangée partiellement avec du tétrabutylammumium. La perméabilité des films d'organo-argile recouverts d'un film de cellulose a été étudiée en utilisant différentes sondes électro-actives de charge différentes, Ru(NH3)63+, [Fe(CN)6]3-, Fc(MeOH)2. Les conditions expérimentales ont été ensuite optimisées pour la détection du Cu(II) et Cd(II) avec ces électrodes modifiées en voltammétrie à vague carrée. Dans le dernier chapitre, une configuration originale d'électrode modifiée a été mise au point. Il s'agit d'électrodes composites argile-silice mésoporeuse préparée par électrodéposition en présence ou non d'un tensioactif, le cétyltriméthylammonium. La perméabilité des films a également été étudiée avec les mêmes molécules que précédemment. Il apparait que l'extraction du tensio-actif conduit à des films homogènes très poreux et stables qui présentent des propriétés d'accumulation du Cu(II) / The thesis is devoted to the development of composite electrodes based on natural clays and their electrochemical characterization for analytical purposes, in particular for the determination of metal cations (Cu2+) and some therapeutic substances. The first experimental chapter is devoted to a comparative study of the chemical, structural and physico-chemical properties of two natural clays extracted from two sites in Romania. It appears that Valea Chioarului clay, of montmorillonite type, has the most interesting properties with a high specific surface area (190 m2/g). The second chapter presents a prospective analysis of the two clays as electrode materials using a clay/polyelectrolyte composite film. Three target molecules of pharmaceutical interest have been selected: ascorbic acid, riboflavin, and acetaminophen. This last molecule can be detected either by direct oxidation or through an electro-enzymatic reaction with horseradish peroxidase. The third and fourth chapters are devoted to the use of clays exchanged by ammonium cations for copper detection. Thus, Valea Chioarului clay was partially exchanged with tétrabutylammumium. The permeability of the organoclay films coated with a cellulose membrane was studied by using different electroactive probes of different charging, Ru(NH3)63+, [Fe(CN)6]3-, Fc(MeOH)2. The experimental conditions were then optimized for the detection of Cu (II) and Cd (II) with the modified electrodes by square wave voltammetry. In the last chapter, an original modified electrode configuration was developed. This implies composite mesoporous silica-clay electrodes prepared by electrodeposition in the presence or absence of a surfactant, like cetyltrimethylammonium. The permeability of the films was also investigated with the above mentioned molecules. It appears that the extraction of the surfactant leads to very stable and homogeneous porous films that show Cu (II) accumulation properties

According to estimates of the United Nations Environment Programme (UNEP), more than 1.8 billion people will be living in countries or regions with absolute water scarcity by 2025. The pressure on water resources is increased not only in arid and semiarid regions, but also in fast growing megacities around the world as a result of, amongst other factors, the changing nutritional and consumer behavior (rising living standards). Over 90 % of the annual water consumption of the newly industrializing and developing countries in the arid and semiarid climate zone is used for agricultural irrigation to ensure the nutrition of the population. Thus, since the beginning of the 20th century, the planned/controlled reuse of wastewater has developed into a central task of the sustainable water resources management. Wastewater represents a valuable resource in view of its composition (e. g. nutrients P, N for soil fertilizing) and its reliable, weather-independent availability in every household. The establishment of a closed-loop water management can enhance the efficiency of water usage. Therefore, activities in research and development are currently focused on decentralized and semi-centralized concepts, since their structures offer better conditions for the establishment of closed-loop systems and innovations in wastewater technology can be implemented more easily. In general, the hygienic quality requirements for wastewater reuse are predominantly oriented towards the planned usage. These are, in turn, regulated by thresholds and guidance values, e. g. for faecal indicator bacteria (e. g. faecal coliforms: E. coli), in widely differing norms and legal provisions specific to the respective countries. In Germany since 2005, small wastewater treatment plants can obtain the discharge class +H by the German Institute for Civil Engineering (DIBt: Deutsches Institut für Bautechnik) if secondary effluents contain less than 100 faecal coliforms (E. coli) per 100 mL. This ensures a safe effluent seepage in karst and water protection areas. Due to the infectious risk caused by a multitude of pathogens (bacteria, viruses, worm eggs, protozoa) which are still contained in wastewater after mechanical-biological treatment, specific disinfection methods are indispensable for their satisfactory reduction. Demands on disinfection methods for wastewater reclamation are quite complex. They should be characterized by a high and constant disinfection efficiency at low or moderate formation of disinfection by-products. The reclaimed wastewater should be able to be stored safely. Moreover, the disinfection method should be technically simple, scaleable, space-saving, subjected to low maintenance and realized at moderate investment and operating costs without applying external toxic chemicals. Established methods in decentralized wastewater disinfection are mainly based on membrane and UV technologies. However, these methods are currently working under high operating costs (high maintenance and cleaning efforts). Furthermore, the high investment costs of the membrane filtration are disadvantageous. In addition, both methods do not provide a disinfection residual. Thus, further research is required for the development and testing of alternative disinfection technologies. Against this background, the applicability of the electrolysis and UV/electrolysis hybrid technology for the decentralized wastewater reclamation was investigated and assessed in this dissertation. Results have shown that the electrochemical disinfection of biologically treated wastewater represents an efficient method at temperatures of > 6 °C, pH values of < 8.5 and DOC con-centrations of < 22 mg L-1. Under these conditions, an E. coli reduction of four log levels was achieved at a concentration of free chlorine ranging from 0.4 mg L-1 to 0.6 mg L-1 and at an after-reaction time of 15...20 min. However, it becomes simultaneously apparent that low temperatures, high pH values and high DOC concentrations are limiting parameters for this disinfection method to reclaim biologically treated wastewater. A high energy consumption of the electrolysis cell equipped with boron-doped diamond (BDD) electrodes (2...2.6 kWh m-3) represents a further unfavourable effect. Moreover, the undesired formation of chlorate (c = 1.3 mg L-1) and perchlorate (c = 18 mg L-1) at BDD electrodes can be considered as critical, since these disinfection by-products are, amongst others, human-toxicologically relevant. The concentration of adsorbable organically bound halogens (AOX) and trihalomethanes (THMs) proved to be marginal to moderate. Due to the synergistic effect of the combined application of UV irradiation (primary disinfection method) and electrolysis, the disadvantages of the single methods can be compensated. Decisive drawbacks of UV irradiation are photo and dark repair mechanisms of reversibly damaged bacteria. It was observed that the reactivation of reversibly UV-damaged E. coli even occurs at low temperatures (T = 10 °C) and strongly differing pH values (pH = 5.7...8.1) as well as at low light intensities and in darkness to an extent excluding a safe usage and storage of the reclaimed wastewater. The reactivation processes might be lowered by increased UV fluences. However, this is limited by high concentrations of total suspended solids (TSS). In spite of high UV fluences of > 400 J m-1, no complete removal of E. coli bacteria can be achieved at TSS concentrations of > 17 mg L-1. Therefore, it is indispensable to prevent bacterial reactivation caused by photo and dark repair processes. This topic was studied in the current work by electrochemically produced oxidants using an electrolysis cell positioned downstream of the UV unit. Results have shown that photo and dark reactivation were completely prevented by oxidants in a total concentration of 0.5...0.6 mg L-1 at a TSS concentration of 8...11 mg L-1, at pH values ranging from 5.7 to 8.1 and at temperatures ranging from 10 °C to 30 °C (t = 24....72 h). Even at a high TSS concentration of 75 mg L-1, the reactivation of E. coli (ctotal oxidants = 1.8 mg L-1) and, up to a TSS concentration of 32 mg L-1, the reactivation of total coliforms (except E. coli, ctotal oxidants = 1.0 mg L-1) can be prevented at a high initial germ concentration of 2…3 105 per 100 mL. The lowest energy consumption could be observed when mixed oxide electrodes (MOX electrodes) were applied. This result and the fact that no chlorate and perchlorate were observed at MOX electrodes argue for the application of these electrodes in practice. All in all, the UV/electrolysis hybrid technology represents an energy-efficient method for reclamation of biologically treated wastewater with TSS concentrations ranging from < 11 to 32 mg L-1 (E = 0.17…0.24 kWh m-3, MOX electrodes). Thereby, the reclaimed wastewater meet the hygienic quality requirements for a multitude of reuse categories starting from agricultural irrigation to urban and recreational reuse. Moreover, the requirements of the discharge class +H (100 faecal coliforms (E. coli) per 100 mL) are complied with reliably. The operational stability of the UV/electrolysis hybrid technology should also be ensured within the required maintenance intervals (t > 6 months). The undesired formation of coverings caused by biofouling processes on quartz glass surfaces could be prevented by electrochemically produced oxidants in a total concentration of 1 mg L-1 within an experimental duration of 5.5 months. However, the application of the UV/electrolysis hybrid technology is limited by increased particle concentrations and faecal loadings (initial E. coli concentration). The resulting enhanced demand of electrochemically produced oxidants for the prevention of bacterial reactivation results in a considerable increase of the electric charge input and energy consumption.

Η ηλεκτροχημική ενίσχυση της κατάλυσης (EPOC ή αλλιώς μη-φαρανταϊκή τροποποίηση της καταλυτικής ενεργότητας, φαινόμενο NEMCA) είναι ένα φαινόμενο όπου εφαρμογή μικρών ρευμάτων ή δυναμικών (±2V) σε ένα καταλύτη που είναι υποστηριγμένος σε ένα ηλεκτρολύτη, ιοντικό ή μικτό ιοντικό-ηλεκτρονικό αγωγό, μπορεί να επιφέρει τροποποιήσεις στην καταλυτική ενεργότητα αλλά και εκλεκτικότητα, με τρόπο ελεγχόμενο, αντιστρεπτό και έως ένα βαθμό προβλέψιμο. Η ηλεκτροχημική ενίσχυση έχει βρεθεί, με χρήση διαφόρων τεχνικών, ότι πηγάζει από την ηλεκτροχημικά ελεγχόμενη παροχή ενισχυτικών ιοντικών ειδών από το φορέα-ηλεκτρολύτη στα καταλυτικά σωματίδια. Στο πρώτο κεφάλαιο της παρούσας διατριβής γίνεται μια εκτεταμένη αναφορά στους στερεούς ηλεκτρολύτες, στις ιδιότητες τους και τους τομείς στους οποίους χρησιμοποιούνται με ιδιαίτερη σημασία στη σταθεροποιημένη με οξείδιο του υττρίου ζιρκονία (YSZ), που αποτελεί ένα πολύ συχνά χρησιμοποιούμενο αγωγό ιόντων οξυγόνου. Επίσης, εισάγονται οι έννοιες της μετανάστευσης (spillover) και της αντίστροφης μετανάστευσης (backspillover), οι οποίες χρησιμοποιούνται στην ερμηνεία και την κατανόηση του φαινομένου της ηλεκτροχημικής ενίσχυσης και των αλληλεπιδράσεων μετάλλου-φορέα (MSI). Στο δεύτερο κεφάλαιο γίνεται εισαγωγή στις γενικές αρχές του φαινομένου της ηλεκτροχημικής ενίσχυσης όπου παρουσιάζονται μερικά παραδείγματα εφαρμογής του και γίνεται ανασκόπηση όλων των εργασιών που έχουν εμφανιστεί στη βιβλιογραφία και αφορούν στο συγκεκριμένο φαινόμενο. Συζητείται, επίσης, η μελέτη του φαινομένου με χρήση διαφόρων πειραματικών τεχνικών, όπως ηλεκτροκινητικών πειραμάτων δυναμικής απόκρισης, μετρήσεων έργου εξόδου, κυκλικής βολταμμετρίας, XPS, TPD και STM, καθώς και θεωρητικών μελετών, με σκοπό την κατανόηση της αρχής του φαινομένου σε ατομικό επίπεδο καθώς και την επίλυση σημαντικών προβλημάτων που αφορούν στην ετερογενή κατάλυση. Με βάση τα αποτελέσματα από τις ανωτέρω μελέτες, παρουσιάζεται το μαθηματικό μοντέλο που έχει αναπτυχθεί και εξηγεί τα παρατηρούμενα φαινόμενα σε μοριακό επίπεδο καθώς και οι πρόσφατα εδραιωμένοι κανόνες που το διέπουν. Στο τρίτο κεφάλαιο παρουσιάζεται η αξιοποίηση του φαινομένου της ηλεκτροχημικής ενίσχυσης στην αντιμετώπιση ενός εκ των δυσκολότερων και προκλητικότερων προβλημάτων της ετερογενούς κατάλυσης που είναι η αναγωγή του μονοξειδίου του αζώτου (ΝΟ) από αιθυλένιο παρουσία υψηλής περίσσειας (10%) οξυγόνου. Στην μελέτη χρησιμοποιήθηκε ένας πρόσφατα ανεπτυγμένος και βελτιωμένος για την παρούσα διατριβή, μονολιθικός ηλεκτροχημικά ενισχυόμενος αντιδραστήρας (monolithic electrochemically promoted reactor, MEPR) εξοπλισμένος με 22 ηλεκτροχημικά καταλυτικά στοιχεία του τύπου Rh/YSZ/Pt με μικρό πάχος ηλεκτροδίων (~40 nm). Βρέθηκε, δε, ότι η βέλτιστη λειτουργία επιτυγχάνεται σε χαμηλές θερμοκρασίες (220-240οC) με σημαντική ηλεκτροχημική ενίσχυση ακόμα και κάτω από τις ανωτέρω ισχυρά οξειδωτικές συνθήκες (λόγος αέρα-καυσίμου=16.7, περίσσεια οξυγόνου=9.43). Σε αυτό το στενό θερμοκρασιακό εύρος η εκλεκτικότητα προς Ν2 που επετεύχθη από τα Rh/YSZ/Pt ηλεκτροκαταλυτικά στοιχεία, είναι περίπου 100% ενώ η παραγωγή των ανεπιθύμητων CO, ΝΟ2, Ν2Ο ήταν σχεδόν μη-ανιχνεύσιμη. Στο τέταρτο κεφάλαιο μελετάται η χρήση λεπτών καταλυτικών ηλεκτροδίων Pt σκελετικής δομής (Pt-skeletal/YSZ/Au) στην έκταση του φαινομένου της ηλεκτροχημικής ενίσχυσης, χρησιμοποιώντας την πρότυπη αντίδραση οξείδωσης αιθυλενίου, στον μονολιθικό ηλεκτροχημικά ενισχυόμενο αντιδραστήρα. Βρέθηκε ότι και τέτοιου τύπου ηλεκτρόδια – καταλυτικά υμένια είναι ιδιαίτερα καταλυτικά ενεργά και είναι δυνατό να ενισχυθούν ηλεκτροχημικά σε μεγάλο βαθμό. Επίσης, ο αντιδραστήρας λειτούργησε επιτυχώς και παρατηρήθηκε ηλεκτροχημική ενίσχυση, υπό υψηλές ογκομετρικές παροχές (25 l/min), με ταχύτητες χώρου αντιδραστήρα που είναι κοντά σε αυτές που λειτουργούν οι βιομηχανικοί αντιδραστήρες (12000 h-1). Στο πέμπτο κεφάλαιο παρουσιάζεται η ηλεκτροχημική ενίσχυση της αντίδρασης υδρογόνωσης του CO2 με στόχο την παραγωγή μεθανίου χρησιμοποιώντας ηλεκτροχημικά στοιχεία του τύπου Rh/YSZ/Pt. Βρέθηκε ότι η αντίδραση μπορεί να ενισχυθεί σε μεγάλο βαθμό και επιπλέον να τροποποιηθεί και η εκλεκτικότητά της σε CΗ4 που είναι και το επιθυμητό προϊόν. Στο έκτο κεφάλαιο παρουσιάζεται η μελέτη της αντίδραση οξείδωσης του SO2 προς SO3, μιας πολύ σημαντικής αντίδρασης από βιομηχανική (παραγωγή H2SO4) αλλά περιβαλλοντική άποψη, με χρήση του φαινομένου της ηλεκτροχημικής ενίσχυσης, σε λεπτά (~40 nm) ηλεκτροχημικά στοιχεία του τύπου Pt/YSZ/Au. Βρέθηκε πως ηλεκτροχημική ενίσχυση μπορεί να επιτευχθεί ακόμα και σε πολύ υψηλές ογκομετρικές παροχές (30 l/min), όπου αντιστοιχούν σε ταχύτητες χώρου αντιδραστήρα (14000 h-1) πολύ κοντά σε αυτές που λειτουργούν οι βιομηχανικοί αντιδραστήρες και να επιτευχθούν σχετικά υψηλές μετατροπές SO2. / Electrochemical Promotion of Catalysis (EPOC or Non-Faradaic Electrochemical Modification of Catalytic Activity, NEMCA effect) is a phenomenon where the application of small currents or potentials (±2V) alters the activity and selectivity of catalysts supported on ionic or mixed ionic-electronic conductors and modifies the catalytic activity and selectivity, in a controllable, reversible and to some extend predictable manner. As shown by numerous surface science techniques, including STM, electrochemical promotion is due to electrochemically controlled migration (backspillover) of promoting or poisoning ionic species (Oδ- in the case of YSZ) between the ionic or mixed ionic-electronic conductor and the gas exposed catalytic surface. The utilization of electrochemical promotion of catalysis, in order to tackle one of the most difficult and challenging problems of heterogeneous catalysis, which is the NO reduction under high excess oxygen (10%), has been performed. This gas mixture is a typical mixture in a lean-burn engine (or Diesel engine) exhaust. In this study, a recently developed and improved monolithic electrochemically promoted reactor (MEPR) has been used, equipped with 22 thin (~40 nm catalyst-electrode thickness) electrochemical catalytic elements Rh/YSZ/Pt type. It was found that there is an optimum operation temperature of the reactor, in the range from 220oC to 240oC, where the maximum electropromotion effect occurs, even under these extremely oxidizing conditions (air/fuel ratio = 16.7 and oxygen excess = 9.43%). In this narrow temperature window the selectivity to N2 was almost 100% since the production of the undesired N2O and NO2 was almost undetectable. Also, the use of thin catalytic Pt electrodes with skeletal structure (Ptskeletal/ YSZ/Au) was examined in the MEPR for the model reaction of C2H4 oxidation. It was found that such skeletal structure electrodes are catalytically active and can be electropromoted even under high gas flow rates (25 l/min) or high space velocity (HSV~12000 h-1), close to those that the industrial reactors operate. The electrochemical promotion of the CO2 hydrogenation reaction was also examined, towards methane production using Rh/YSZ/Pt type electrochemical catalytic elements. It was found that the reaction rates can be enhanced and similarly the selectivity to CH4 which found to increase upon polarization. Finally, the effect of electrochemical promotion was examined in the study of the SO2 oxidation to SO3 reaction, which is a very important reaction by industrial (H2SO4 production) and environmental point of interest. The monolithic reactor was equipped with 5 or 22 thin (~40 nm) Pt/YSZ/Au type electrocatalytic elements. It was found that electrochemical promotion can be obtained by positive polarization even under high hourly space velocities (14000 h-1), close to those that the industrial reactors operate, with relatively high SO2 conversions.

Η Ηλεκτροχημική Ενίσχυση της Κατάλυσης (ή φαινόμενο NEMCA) είναι ένα φαινόμενο όπου εφαρμογή μικρών ρευμάτων ή δυναμικών (±2 V) μπορεί να τροποποιήσει την ενεργότητα καταλυτών υποστηριγμένων σε ιοντικούς ή μικτούς ιοντικούς-ηλεκτρονικούς αγωγούς, να επηρεάσει την εκλεκτικότητα σε επιθυμητή κατεύθυνση και να μεταβάλλει τις ηλεκτρονικές και συνεπώς τις καταλυτικές ιδιότητες με τρόπο ελεγχόμενο, αντιστρεπτό και σε κάποιο βαθμό προβλέψιμο. Στην παρούσα διατριβή μελετήθηκε η επίδραση του πάχους του καταλυτικού υμενίου στο μέγεθος της ηλεκτροχημικής ενίσχυσης, χρησιμοποιώντας την αντίδραση της οξείδωσης του C2H4 σε πορώδη υμένια Pt πάχους μεταξύ 0.2 και 1.4 μm, εναποτεθειμένα με τη μέθοδο επάλειψης οργανομεταλλικής πάστας, σε στερεό ηλεκτρολύτη YSZ, έναν αγωγό ιόντων Ο2-. Βρέθηκε πως η αύξηση του πάχους των υμενίων που χρησιμοποιούνται στις μελέτες ηλεκτροχημικής ενίσχυσης, προκαλεί μείωση στο λόγο προσαύξησης του ρυθμού, ρ, συμπεριφορά που βρίσκεται σε καλή συμφωνία με τις αναλυτικές προβλέψεις του μαθηματικού μοντέλου που περιγράφει την επιφανειακή διάχυση-αντίδραση των προωθητικών ειδών. Με βάση τις επιτυχείς μελέτες ηλεκτροχημικής ενίσχυσης που έχουν πραγματοποιηθεί σε λεπτά (40 nm), εναποτεθειμένα με τη μέθοδο της ιοντοβολής (sputtering) καταλυτικά υμένια, έγινε επέκταση της μελέτης της επίδρασης του πάχους σε τόσο λεπτά υμένια. Συγκεκριμένα, εξετάσθηκε η καταλυτική και η ηλεκτροχημικά ενισχυμένη συμπεριφορά πολύ λεπτών (30-90 nm) καταλυτικών υμενίων εναποτεθειμένων με τη μέθοδο του sputtering, τη μέθοδο Pulsed Laser Deposition και την τεχνική εναπόθεσης με ατμό (vapor deposition). Τιμές του λόγου προσαύξησης του ρυθμού, ρ, έως και 440 και τιμές φαρανταϊκής απόδοσης, Λ, έως και 1000 παρατηρήθηκαν για τα υμένια που εναποτέθηκαν με τη μέθοδο του sputtering. Η διασπορά μετάλλου στα υμένια αυτά είναι έως και 20%, συγκρίσιμη δηλαδή με αυτή των εμπορικών υποστηριγμένων καταλυτών. Τέλος, παρουσιάζεται η λειτουργία ενός πρόσφατα ανεπτυγμένου μονολιθικού ηλεκτροχημικά ενισχυόμενου αντιδραστήρα (MEPR), χρησιμοποιώντας την περιβαλλοντικού ενδιαφέροντος αντίδραση της αναγωγής του ΝΟ από αιθυλένιο παρουσία Ο2. Χρησιμοποιώντας καταλυτικά στοιχεία τύπου Pt-Rh(1:1)/YSZ/Au, παρουσία 10% Ο2 και σε ογκομετρικές παροχές έως και 1000 cc/min, ο αντιδραστήρας λειτούργησε επιδεικνύοντας τιμές φαρανταϊκής απόδοσης που ξεπερνούν τη μονάδα και επιτυγχάνοντας 50% και 44% προσαύξηση στους ρυθμούς μετατροπής του καυσίμου και του ΝΟ αντίστοιχα. Αυτή η μελέτη είναι η πρώτη που επιδεικνύει ηλεκτροχημική ενίσχυση της αντίδρασης αναγωγής του NO σε τόσο υψηλές τιμές μερικής πίεσης οξυγόνου (10% O2), που είναι αντιπροσωπευτικές για εξατμίσεις μηχανών πτωχού καυσίμου και μηχανών Diesel. Ο MEPR αποδεσμεύει το φαινόμενο NEMCA από την έως σήμερα χρήση του στην καθαρά εργαστηριακή κλίμακα και δείχνει πολλά υποσχόμενος για την πρακτική εφαρμογή του φαινομένου. / The effect of Electrochemical Promotion of Catalysis (EPOC or NEMCA effect) is a phenomenon where application of small currents or potentials (±2 V) alters the activity and selectivity of catalysts supported on ionic or mixed ionic-electronic conductors and modifies the electronic and thus catalytic properties in a controllable, reversible and to some extent predictable manner. The effect of catalyst film thickness on the magnitude of electrochemical promotion (ρ and Λ values) has not been studied experimentally so far but a mathematical model has been developed, accounting for surface diffusion and reaction of the promoting species, which predicts a strong variation of ρ and Λ with catalyst film thickness L. In the present thesis is examined for the first time experimentally the effect of catalyst film thickness on the magnitude of the EPOC, using porous Pt catalyst-electrodes prepared from Engelhard Pt paste with thicknesses in the range 0.2 to 1.4 μm. It was found that increasing the thickness of porous catalyst films used in electrochemical promotion studies causes a decrease in the rate enhancement ratio, ρ, due to the gradual axial decrease from the three-phase-boundaries to the top of the film of the surface concentration of the promoting backspillover O2- species which diffuse and react on the porous catalyst surface. Increasing film thickness causes a moderate increase in the Faradaic efficiency, Λ, which can be predicted by the parameter 2Fro/I0. The ρ and Λ behaviour is in good agreement with the analytical model prediction and provides additional support for the O2- promoter reaction-diffusion model and for the sacrificial promoter mechanism of electrochemical promotion. Most electrochemical promotion studies have been carried out so far with thick (0.1 μm to 5 μm) porous metal catalyst films with a roughness factor of the order of 500 and small (typically less than 0.1%) metal dispersion, deposited on solid electrolytes using a variety of deposition techniques. Very recently, electropromotion studies have been extended to thin (40 nm) sputter coated porous metal catalysts with metal dispersion of the order of 10 to 30%. The effect of thickness with such thin (30 to 90 nm) sputtered Pt catalyst-electrodes on the magnitude of electrochemical promotion is discussed, as well as the effect of the catalyst deposition method (Sputtering, Pulsed Laser Deposition and Vapor Deposition) using the model reaction of ethylene oxidation. Rate enhancement ratio, ρ, values up to 440 and Λ values up to 1000 where obtained for the sputtered films, in agreement with the sacrificial promoter and diffusion-reaction models of EPOC which predict increase in ρ value with thinner films. An environmental interest reaction, the reduction of NO by ethylene in the presence of excess oxygen, was investigated in a recently developed MEPR. In this novel dismantlable monolithic-type electrochemically promoted catalytic reactor, thin (~40 nm) porous catalyst films are sputter-deposited on thin (0.25 mm) parallel solid electrolyte plates supported in the grooves of a ceramic monolithic holder and serve as electropromoted catalyst elements. Using Pt-Rh(1:1)/YSZ/Au-type catalyst elements, the 8-plate reactor operated with apparent Faradaic efficiency exceeding unity achieving significant and reversible enhancement in the rates of C2H4 and NO consumption in presence of up to 10% O2 in the feed at gas flow rates up to 1000 cc/min. The Pt-Rh co-sputtered films exhibited very good performance in terms of stability and selectivity for N2 formation, i.e. practically 100% under all reaction conditions. The reactor, which is a hybrid between a monolithic catalytic reactor and a flat-plate solid oxide fuel cell, permits easy practical utilization of the electrochemical promotion of catalysis.

According to estimates of the United Nations Environment Programme (UNEP), more than 1.8 billion people will be living in countries or regions with absolute water scarcity by 2025. The pressure on water resources is increased not only in arid and semiarid regions, but also in fast growing megacities around the world as a result of, amongst other factors, the changing nutritional and consumer behavior (rising living standards). Over 90 % of the annual water consumption of the newly industrializing and developing countries in the arid and semiarid climate zone is used for agricultural irrigation to ensure the nutrition of the population. Thus, since the beginning of the 20th century, the planned/controlled reuse of wastewater has developed into a central task of the sustainable water resources management. Wastewater represents a valuable resource in view of its composition (e. g. nutrients P, N for soil fertilizing) and its reliable, weather-independent availability in every household. The establishment of a closed-loop water management can enhance the efficiency of water usage. Therefore, activities in research and development are currently focused on decentralized and semi-centralized concepts, since their structures offer better conditions for the establishment of closed-loop systems and innovations in wastewater technology can be implemented more easily. In general, the hygienic quality requirements for wastewater reuse are predominantly oriented towards the planned usage. These are, in turn, regulated by thresholds and guidance values, e. g. for faecal indicator bacteria (e. g. faecal coliforms: E. coli), in widely differing norms and legal provisions specific to the respective countries. In Germany since 2005, small wastewater treatment plants can obtain the discharge class +H by the German Institute for Civil Engineering (DIBt: Deutsches Institut für Bautechnik) if secondary effluents contain less than 100 faecal coliforms (E. coli) per 100 mL. This ensures a safe effluent seepage in karst and water protection areas. Due to the infectious risk caused by a multitude of pathogens (bacteria, viruses, worm eggs, protozoa) which are still contained in wastewater after mechanical-biological treatment, specific disinfection methods are indispensable for their satisfactory reduction. Demands on disinfection methods for wastewater reclamation are quite complex. They should be characterized by a high and constant disinfection efficiency at low or moderate formation of disinfection by-products. The reclaimed wastewater should be able to be stored safely. Moreover, the disinfection method should be technically simple, scaleable, space-saving, subjected to low maintenance and realized at moderate investment and operating costs without applying external toxic chemicals. Established methods in decentralized wastewater disinfection are mainly based on membrane and UV technologies. However, these methods are currently working under high operating costs (high maintenance and cleaning efforts). Furthermore, the high investment costs of the membrane filtration are disadvantageous. In addition, both methods do not provide a disinfection residual. Thus, further research is required for the development and testing of alternative disinfection technologies. Against this background, the applicability of the electrolysis and UV/electrolysis hybrid technology for the decentralized wastewater reclamation was investigated and assessed in this dissertation. Results have shown that the electrochemical disinfection of biologically treated wastewater represents an efficient method at temperatures of > 6 °C, pH values of < 8.5 and DOC con-centrations of < 22 mg L-1. Under these conditions, an E. coli reduction of four log levels was achieved at a concentration of free chlorine ranging from 0.4 mg L-1 to 0.6 mg L-1 and at an after-reaction time of 15...20 min. However, it becomes simultaneously apparent that low temperatures, high pH values and high DOC concentrations are limiting parameters for this disinfection method to reclaim biologically treated wastewater. A high energy consumption of the electrolysis cell equipped with boron-doped diamond (BDD) electrodes (2...2.6 kWh m-3) represents a further unfavourable effect. Moreover, the undesired formation of chlorate (c = 1.3 mg L-1) and perchlorate (c = 18 mg L-1) at BDD electrodes can be considered as critical, since these disinfection by-products are, amongst others, human-toxicologically relevant. The concentration of adsorbable organically bound halogens (AOX) and trihalomethanes (THMs) proved to be marginal to moderate. Due to the synergistic effect of the combined application of UV irradiation (primary disinfection method) and electrolysis, the disadvantages of the single methods can be compensated. Decisive drawbacks of UV irradiation are photo and dark repair mechanisms of reversibly damaged bacteria. It was observed that the reactivation of reversibly UV-damaged E. coli even occurs at low temperatures (T = 10 °C) and strongly differing pH values (pH = 5.7...8.1) as well as at low light intensities and in darkness to an extent excluding a safe usage and storage of the reclaimed wastewater. The reactivation processes might be lowered by increased UV fluences. However, this is limited by high concentrations of total suspended solids (TSS). In spite of high UV fluences of > 400 J m-1, no complete removal of E. coli bacteria can be achieved at TSS concentrations of > 17 mg L-1. Therefore, it is indispensable to prevent bacterial reactivation caused by photo and dark repair processes. This topic was studied in the current work by electrochemically produced oxidants using an electrolysis cell positioned downstream of the UV unit. Results have shown that photo and dark reactivation were completely prevented by oxidants in a total concentration of 0.5...0.6 mg L-1 at a TSS concentration of 8...11 mg L-1, at pH values ranging from 5.7 to 8.1 and at temperatures ranging from 10 °C to 30 °C (t = 24....72 h). Even at a high TSS concentration of 75 mg L-1, the reactivation of E. coli (ctotal oxidants = 1.8 mg L-1) and, up to a TSS concentration of 32 mg L-1, the reactivation of total coliforms (except E. coli, ctotal oxidants = 1.0 mg L-1) can be prevented at a high initial germ concentration of 2…3 105 per 100 mL. The lowest energy consumption could be observed when mixed oxide electrodes (MOX electrodes) were applied. This result and the fact that no chlorate and perchlorate were observed at MOX electrodes argue for the application of these electrodes in practice. All in all, the UV/electrolysis hybrid technology represents an energy-efficient method for reclamation of biologically treated wastewater with TSS concentrations ranging from < 11 to 32 mg L-1 (E = 0.17…0.24 kWh m-3, MOX electrodes). Thereby, the reclaimed wastewater meet the hygienic quality requirements for a multitude of reuse categories starting from agricultural irrigation to urban and recreational reuse. Moreover, the requirements of the discharge class +H (100 faecal coliforms (E. coli) per 100 mL) are complied with reliably. The operational stability of the UV/electrolysis hybrid technology should also be ensured within the required maintenance intervals (t > 6 months). The undesired formation of coverings caused by biofouling processes on quartz glass surfaces could be prevented by electrochemically produced oxidants in a total concentration of 1 mg L-1 within an experimental duration of 5.5 months. However, the application of the UV/electrolysis hybrid technology is limited by increased particle concentrations and faecal loadings (initial E. coli concentration). The resulting enhanced demand of electrochemically produced oxidants for the prevention of bacterial reactivation results in a considerable increase of the electric charge input and energy consumption.