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2018 (4)

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Dissertation
Development of a scalable, sunlight-driven and water vapour-fed electrolyzer for hydrogen production

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In the future, hydrogen will be able to play an important role as a transportation fuel and storage medium for renewable sources. Current hydrogen production is still dominated by steam reforming of fossil fuels, but the share of more sustainable alternatives such as alkaline and proton exchange membrane (PEM) electrolysis is growing. However, these systems pose different issues, of which the high capital costs and high prices of the required noble metal catalysts are most prominent. A vapour-fed solar hydrogen generator implementing an anion exchange membrane was investigated as an interesting and cost-saving alternative. The objective of the thesis was to develop an electrolyzer, integrating different alternatives for three components: the electrolyzer frame, the current collector and the membrane electrode assembly (MEA). It was aimed to operate using water vapour from ambient air and solar energy captured by a solar panel as only resources. An electrolyzer frame was designed and 3D printed in a polymer material, VeroWhitePlus. Various alternatives were evaluated as current collectors of which the most promising configuration, with a negligible Ohmic resistance, was a combination of nickel foil as current collector and nickel foam as gas diffusion layer (GDL). The most essential component influencing electrolyzer performance is the MEA. A KOH-impregnated PVA membrane was used in all MEAs. Ionomer spraying, electrodeposition of NiFe (anode) and NiMo (cathode) on the Ni foam GDL and the catalyst coated membrane (CCM) method were evaluated as methods to increase activity and stability of the MEA. Spraying of the Ni foam with ionomer did not show any significant benefits. The only long term stable CCM, sprayed with a Ni nanoparticle and PVA ionomer ink (ratio 2:1), attained a current density of 10 mA/cm² at 2.59 V. The most promising combination was a MEA with the membrane sandwiched between two Ni foams electrodeposited with NiFe and NiMo and sprayed with FAA ionomer, with an active surface of 4 cm². Stable current densities of 10 mA/cm² were attained at a potential of 1.82 V. This MEA was scaled-up to an active surface of 25 cm² and tested with ambient air as water vapour supply. The electrolyzer was coupled to a solar panel with a DC/DC converter providing a constant potential of 1.808 V. The experiment proved successful, attaining current densities of 4 mA/cm² and an estimated solar-to-hydrogen efficiency of 1.38%. Further research is needed to investigate long term stability and to optimize all integrated components of the electrolyzer.

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Dissertation
Kinetische studie van de splitsing van waterdamp met alternatieve katalysators en anionenuitwisselingsmembranen.

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Samenvatting Het opslaan van hernieuwbare energie in waterstofgas als hernieuwbare brandstof heeft het potentieel een lacune te vullen in de huidige energietransitie. Het biedt door zijn hoge energiedensiteit opportuniteiten voor toepassingen waar elektrificatie (voorlopig) geen antwoord op heeft. Onderzoek richt zich daarom op het verhogen van de efficiëntie van de productie van waterstof. Een veelbelovende technologie daarvoor is foto-elektrolyse,- waarbij waterdamp uit omgevingslucht wordt aangevoerd. Om foto-elektrolyseapparaten te optimaliseren moeten alle componenten ervan zo efficiënt mogelijk en betaalbaar worden gemaakt. Deze masterproef richt zich op de katalysator, die nodig is om water te splitsen naar waterstof- en zuurstofgas. Het doel van dit onderzoek was om de kinetische eigenschappen van de splitsing van waterdamp op platina en andere katalysators te analyseren. Daarvoor ging de aandacht uit naar de invloed van een afnemende relatieve luchtvochtigheid op de kinetiek van watersplitsing. Het uiteindelijke doel van die analyse is een beter betaalbaar alternatief te vinden voor platina en andere edelmetalen, zonder bij lagere luchtvochtigheden te veel in te boeten op de snelheid van de reactie. In een eerste stap werden daarvoor experimenten uitgevoerd met platina interdigitated electrode arrays (IDE’s), die ohmse verliezen minimaliseren, met daarbovenop een Nafionfilm als elektrolyt. Tijdens gasfase-experimenten werd de relatieve luchtvochtigheid gevarieerd en de stroomdensiteit gemeten in functie van de aangelegde potentiaal. Door meting van de celweerstand kon de potentiaal voor ohmse verliezen gecorrigeerd worden. Uit de verschillende experimenten werd zo de schijnbare reactie-orde van water bepaald. Er bleek dat die beïnvloed werd door een complex samenspel van verschillende processen. De microkinetische reactie-orde was een eerste factor van belang. Uit experimenten bij hoge relatieve vochtigheid bleek een schijnbare 0e orde, maar die orde nam toe bij lagere luchtvochtigheden. Dat kon te wijten zijn aan diffusielimitaties van waterdamp. Een indicatie daarvoor volgde uit de waarneming dat de schijnbare orde toenam bij toenemende filmdikte van Nafion. Daarnaast werd waargenomen dat een grotere breedte van de IDE-vingers ook tot een toegenomen schijnbare reactie-orde leidde. Opnieuw was dit mogelijk te wijten aan diffusie van waterdamp. Er werd echter vermoed dat ook een heterogenere verdeling van de stroomdensiteit hier meespeelde, als reactie op toegenomen ohmse verliezen. In een volgende stap werden verschillende alkalische anionenuitwisselingsmembranen (AEM’s) gekarakteriseerd. Er werd een geschikte AEM en depositiemethode geïdentificeerd voor het testen van onedele oxidatiekatalysators. Een laatste stap omvatte het testen van alternatieve katalysators. Daarbij werd de oxidatiekatalysator nikkelijzer succesvol afgezet op platina IDE’s door middel van elektrodepositie, maar werden nog geen resultaten in de gasfase geboekt.

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Dissertation
Evaluation of layered double hydroxides in anion exchange composite membranes for water vapor electrolysis

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In the search for a highly flexible energy conversion system for production of hydrogen gas from renewable energy sources, a water vapor-fed electrolyser is developed. The use of water vapor instead of liquid water establishes significant flexibility gains, but also invokes challenges for all components that make up the electrolysissystem. Assolidelectrolyte,ananionexchangemembraneisneededthat maintains good ionic conductivity at a varying relative humidity. For this purpose, composite membranes with poly(vinyl alcohol) (PVA), impregnated with 4 M KOH, and layered double hydroxides (LDHs) were investigated in this master thesis. In a first step, various LDHs were screened for ionic conductivity and water uptake, while more information on the mobility of the charge-balancing anion and the hydroxide ions in the LDH interlayer was gathered. Electrochemical impedance spectroscopy showed clear differences in ionic conductivity, with the valence to dehydrated ion radius ratio as a promising predictor. According to literature, only the hydroxide ions contribute to this conductivity, while the charge-balancing anion is assumed immobile. However, in-depth electrochemical characterization of the LDHs,inwhichthewatersplittingreactionwasusedtolooksolelyatthehydroxide ions, showed that gradients in hydroxide concentration can form within the LDHs. This concentration polarization effect, which is reported here for the first time, can only happen when the charge-balancing anion is mobile as well. A PVA membrane and two composite membranes with 10 wt% of either nitrateorcarbonate-intercalatedLDH,i.e. “PVAMgAlNO3”and“PVAMgAlCO3”respectively, were investigated thoroughly. The water uptake of the PVA membrane was shown to be enhanced by LDH addition. The total water uptake of the composite membrane even surpassed the expected uptake, based on the sum of the contributions of both constituents. This points towards interaction effects between the LDHs and the PVA matrix. In the electrochemical tests, the PVA MgAl CO3 showed slightly higher in-plane ionic conductivity at 60, 80 and 95 % relative humidity than the bare PVA membrane. The PVA MgAl NO3 membrane showed poor homogeneityduetoissuesinthesynthesismethod,andthereforeshowedpoorelectrochemical properties and performance. In cyclic voltammetric and chronopotentiometric experiments in an electrolysis cell at different relative humidities, the PVA MgAl CO3 membrane was outperformed by the PVA membrane. The differences were mainly ascribed to stronger diffusion limitation of water. Apart from the issues in the synthesis procedure, strong indications were found that slight differences in the procedure of impregnating the membrane with KOH can also cause large changes in performance, obscuring the actual effect of the LDH addition. Therefore, it is suggested to perform future testing with homogeneous anion exchange membranes, that have fixed cationic head-groups for ionic conductivity.

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Dissertation
Synthesis and characterization of supported platinum model catalysts

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Improvements to obtain the most efficient industrial catalysts are often obtained by a trial-and-error approach instead of rational design based on the fundamental understanding of catalyst structure and performance. The non-uniformity intrinsic to heterogeneous catalysts makes isolating and investigating the different parameters and their implications on catalytic behavior hard. Therefore we aim to develop a model catalyst with reduced complexity which allows to discern and study the different effects influencing catalytic behavior. In this thesis, model catalysts consisting of platinum nanoparticles loaded onto oxide supports were synthesized, characterized and investigated for their catalytic activity. The goal of this thesis is to synthesize a model Pt catalyst system with appropriate size and shape of the support and with good dispersion of the Pt nanoparticles. In search for a suitable support material, an overview of different materials and their properties was made based on literature data. A material was considered suitable when it was spherical, non-porous and several magnitudes (> 100 nm) larger than the active platinum particles, with a narrow particle size distribution. Alumina and silica particles were selected as support and subsequently synthesized. In a next step, platinum was deposited on the surface using conventional chemical methods. The samples were characterized through SEM, TEM, XRD and N2-physisorption. The morphology of the alumina samples could be described as a combination of spherical and plate-like structures, while the silica particles were uniform and spherical. Alumina calcined at 1100°C and silica both had low porosity. On the alumina support, the strong electrostatic adsorption method resulted in small platinum particles with a narrow particle size and uniform spatial distribution. On the silica support, platinum cluster formation and non-uniform particle size and spatial distribution was observed for Pt deposition with Pt(NH3)4Cl2.H2O while Pt(NH3)4(OH)2.xH2O resulted in small uniform Pt particles. Catalytic performance was determined through hydroisomerization/hydrocracking of n-decane which requires a bifunctional catalyst with both metal and acid sites. The synthesized Pt model systems were therefore mixed with a NaY zeolite. Parameters influencing the activity and selectivity are dispersion of the metal particles and the proximity between metal and acid sites. Different support materials, chemical synthesis methods, platinum precursors and combinations with different zeolites were compared. On silica the impregnation method with Pt(NH3)4Cl2.H2O in combination with CBV 712 (NaY zeolite) showed highest activity and selectivity towards isomerized products. Platinum deposited on alumina using a wet impregnation method with SEA in combination with CBV 712 gave even better results, however when platinum was deposited on CBV 712 the best catalytic results are obtained.

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