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The relationship between the production of energy and water causes some major issues. Because a shift is expected towards a more energy intensive water production and a water dependent energy production, there is a need for sustainable solutions. In this study, capturing water vapor from air with hygroscopic materials is examined, either to be used to produce water or to convert water into hydrogen. Hygroscopic materials are able to adsorb or absorb water vapor, depending on temperature, relative humidity and structural properties. Two main parts are examined: i) the water uptake capacity and ii) the water uptake rate with a non-isothermal kinetic model to study which parameters are important. An extensive literature study denoted that both materials based on water adsorption and absorption emerge as possible candidates for these applications. Therefore, mesoporous silica gels with pore sizes between 6 and 10 nm were examined and the water uptake curves were determined over a wide range of relative humidities and temperatures. This confirmed that silica gels are able to adsorb water up to one time their own weight, as was observed in literature. The adsorption isotherms of mesoporous silica gels show an increase in amount adsorbed at high relative humidities because of capillary condensation. A temperature rise results in a monotonically decreasing amount adsorbed at constant RH due to the principle of Le Châtelier. The hygroscopic salts on the other hand, absorb several times their own weight because of deliquescence and dissolve themselves in the absorbed water. They are thus able to capture much more water than silica gels but are more difficult to handle. A linkage between the solubility in water and the deliquescence relative humidity was observed. It followed that colligative effects are important because the vapor pressure over the solution is decreased. Furthermore, the adsorption isotherms of PVA sponges and desiccant clay were determined. These materials only adsorb up to 0.3 g/g . Polymers on the other hand, capture almost two times their own weight in water due to absorption. Especially silica gels and hygroscopic salts are considered as interesting options. Subsequently, the kinetics of water vapor adsorption and desorption on silica gels have been examined. A non-isothermal linear driving force model allowed to evaluate the adsorption rate and to determine the mass transfer coefficient k. Temperature effects were taken into account by developing a BET equation that describes the isotherms between 20°C and 50°C. The parameters of this BET equation were observed to vary linearly with temperature. Heat transfer was found to be important in the region of capillary condensation for large particles with a relatively small specific surface area. It was observed that k depends on the amount adsorbed. In the region of low water uptake, the mass transfer coefficient increases because the energy of the bond between the surface and the water molecules decreases towards a complete surface coverage. k then declines, probably because the bridging between the water clusters and the pore walls occurs, impeding diffusion in the gas phase. Eventually, k increases again at high RH, which may be because of a contribution of viscous flow. The diffusivity coefficients are influenced by bed effects. It was observed that the depth of the sample bed is crucial for the adsorption rate and probably causes a larger resistance than heat transfer.

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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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Water is an essential good that fulfills multiple roles in society. Water is used for drinking, agricultural irrigation, and it acts as a solvent, reactant, or cooling medium in industrial processes. Enormous amounts of fresh water are needed to maintain our standard of living. Water itself is plentiful, but most water on Earth is either saline, found deep underground, or is present as snow, ice, or water vapor and therefore unavailable for immediate use. A dynamic and uneven water distribution combined with high fresh water demand is a recipe for water scarcity. Reports show that one billion people already live in water-stressed areas; this number is likely to increase due to a growing world population, deterioration in water quality, declining groundwater levels, and the intensification of the water-energy nexus. Providing people with water at any time and any place presents the ultimate solution to water scarcity. Taking advantage of the currently untapped but ubiquitous water vapor from atmospheric air paves the way for solving local water scarcity.Water-from-air extraction processes for fresh water production are currently not considered in the water sector, although the technology has been around for some time. This observation necessitates a thorough evaluation of the bottlenecks hindering atmospheric water vapor from becoming a viable fresh water source. Water-from-air technologies are classified and sorted according to their underlying working principles to gain a thorough understanding of the water-from-air field in its current state. Conventional categories of water-from air technologies include direct air cooling and desiccant-based water harvesting. In the air cooling approach, the air cools down below the dew-point to allow for water condensation. Operating in subzero dew-point temperatures should be avoided and call for a different strategy: desiccant-based water harvesting. In this approach, water vapor is captured by a desiccant material. The water evaporates from this material in the next step by thermal heating, facilitating the final condensation step by raising the dew-point. Presented concepts are carried out in a passive operation mode in which the process is carried out based on natural phenomena or an active mode in which an auxiliary energy input is required to carry out the process. The underlying principles, advantages and disadvantages, a thermodynamic analysis, and the achieved efficiencies of devices are presented in this work. The climate-dependent specific water yield (L/kWh), an energy intensity measure, is introduced to evaluate the impact on the water-energy nexus. The low intrinsic specific water yields of water-from-air technologies create a major bottleneck for large-scale implementation. Water consumption in thermoelectric power plants drastically reduces the overall process efficiency in the scope of the water-energy nexus. In addition, current water-from-air extraction devices reach the thermodynamic boundaries, limiting further improvements. Alternative low energy-intensive atmospheric water harvesting concepts are needed to turn water vapor into a viable water resource.Conventional water-from-air harvesting processes are characterized by low maximal specific water yields. The phase transitions of desorption and condensation are energy-intensive process steps. Two alternative concepts are proposed which avoid these transitions and drastically reduce the energy demand. Thermo-responsive polymers are capable of immediately releasing trapped water as a liquid upon heating due to a hydrophilic-hydrophobic configuration change of the polymer chains. Another alternative is deliquescent salt reverse osmosis: the transfer of the seawater desalination concept to land. Deliquescent salts become liquid solutions above a certain relative humidity, making it possible to separate the solution into fresh water and a concentrated brine in a pressure-driven reverse osmosis unit. The concentrated brine can be reused to capture water vapor and close the process cycle. The process fundamentals, technological restrictions, and an energy analysis of these alternative concepts are provided. A world map indicating the optimal water-from-air extraction process is constructed based on regional climate conditions, maximum specific water yields, and practical considerations. The selection of water technologies used to evaluate the water-energy nexus has been updated with the air-based fresh water production technologies covered in this work.Most water-from-air extraction processes operate in two consecutive steps: water vapor capturing with a desiccant followed by a water separation step. The rate of water uptake is crucial for maximizing water production, but research focuses mainly on water uptake capacity and the separation step. A complete non-isothermal kinetic model of a desiccant bed of particles has been developed to elucidate the prevailing kinetic bottlenecks during water vapor adsorption. The mass transfer effects of particle diffusion and bed diffusion are investigated in conjunction with the heat transfer phenomena of bed thermal conductivity and air convection. The validity of the commonly held assumptions of neglecting thermal radiation and convective water vapor supply has been checked. In addition, the use of a complete multiphysics model allows for large step sizes of the boundary conditions, making it possible to determine the effect of the nonlinear adsorption isotherm shape on the mentioned mass and heat transfer phenomena. In general, limited improvements in water uptake rate are expected by enhancing heat and mass transfer. Practical solutions are given to improve the slow water vapor uptake of the passive desiccant-based technology.

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Valorisation of carbon dioxide by electrochemical reduction can play a role in the mitigation of climate change and simultaneously provide useful chemicals. Throughout the years formic acid has emerged as one of the most promising products, since it can be applied as a hydrogen carrier and thereby serve as a storage medium for energy produced by intermittent renewable energy sources. Besides, it can directly produce energy in a direct formic acid fuel cell (DFAFC) and has several other applications in the (bio)chemical industry. To develop towards large-scale CO2 reduction systems, a catalyst displaying a combination of high selectivity, high current density and long stability is crucial. With these requirements in mind, a benchmarking of catalysts was performed. Different support materials, metallic catalysts and synthesis methods were searched. A copper foam support was found beneficial for achieving high current densities (j). Through combining the support with several catalytic metals, such as palladium, bismuth and tin, well-performing electrodes were created. Especially the combination of the copper foam support and tin showed a high selectivity towards formic acid. The straightforward synthesis methods used allow for easy implementation and upscaling. Electrodeposition of tin on copper foam gave a faradaic efficiency (FE) of 66.2 % with a current density of 106.6 mA/cm2 at -2 V vs Ag/AgCl (-0.95 V vs RHE). Tin nanoparticles deposited on copper foam attained 71.2 % FE at a current density of 35.1 mA/cm2 at -1.6 V vs Ag/AgCl (-0.79 V vs RHE). High production rates of respectively 1.31 mmol/cm2h and 0.465 mmol/cm2h were achieved. Although the selectivities are not exceptional compared to the state-of-the-art, current densities and production rates are among the highest values reported in the liquid phase. Attention should now be pointed towards creating reaction conditions that overcome mass transfer limitations by CO2 diffusion, so that the catalysts that were created can live up to their potential. This could be done by, for example, changing the liquid electrolyte conditions or making the step to a zero-gap electrochemical reactor cell.