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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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Since the start of the industrial revolution, access to large amounts of energy has driven human development. Decades of energy exploration have led to the use of coal, petroleum, natural gas and uranium. Today, climate change and environmental concerns are steering scientific research in the direction of renewable energy sources. More than half of the newly installed power capacity worldwide consists of hydroelectric, solar, wind and geothermal power. In this work a strategy is investigated to complement renewable power with renewable fuels. This strategy is solar water splitting.In solar water splitting, solar energy is used to split the water molecule into its constituents, hydrogen and oxygen. Hydrogen may be used as a reducing agent to obtain other products or can be directly used as a fuel to once again generate water. In this work, a two-compartment reactor is built with an ion exchange membrane separator. On either side of the membrane, a catalyst-coated carbon electrode is pressed. One electrode is coated with titanium dioxide semiconductor and faces a window to receive sunlight. Photons which exceed the band gap energy of the semiconductor excite electrons to the conduction band, leaving a hole in the valence band. The holes are at a sufficiently positive potential to oxidise water and produce molecular oxygen and protons. The oxygen gas is evacuated and the protons are transported through the ion exchange membrane to the second electrode. The conduction band electrons are transmitted to this electrode through an external circuit. The second electrode is coated with platinum catalyst, which performs the hydrogen evolution reaction using the proton and electron products from the titanium dioxide electrode. The hydrogen gas is thus produced in a separate compartment. Crossover of hydrogen and oxygen is prevented by the membrane separator.Such a photoelectrochemical reactor simultaneously manages the transport and reaction of photons, electrons, ions and molecules. There are many possible reactor designs which all follow similar principles. In Chapter 1, an account is given of the current stage of development of photoelectrochemical reactors. Their operation, recent progress and forthcoming directions are discussed. Over the last decade, increasing attention was given to integrated, autonomous cells. Their design has evolved from typical two- or three-electrode laboratory setups towards two-compartment sealed reactors with well-defined dimensions that require no external bias.In this work the monolithic, porous assembly of membrane and electrodes allows for free choice of operating conditions. An unavoidable requirement of electrochemical cells is the presence of a conducting pathway between anode and cathode. In practice a liquid electrolyte with high ionic strength is commonly used. In the monolithic assembly, the ion exchange membrane serves as solid electrolyte with charged sulfonic groups that are fixed to the polymer backbone. A flow of pure liquid water can be used, which contains no dissolved salts. Gas phase operation is possible, without requiring any liquid at all. Depending on the conditions used, the current response of the cell under illumination changes. In Chapter 2, the chronoamperometric cell signature is analysed by varying the operating conditions. Observed effects are attributed to capacitive charging of the semiconductor, build-up and side reactions of hydrogen and oxygen products, membrane dehydration and a proton concentration gradient.From this investigation it is clear that photoelectrochemical cells are complex systems that require a fine-tuned interplay of all the active components. In Chapter 3, layer-by-layer deposition is explored as a strategy to assemble these components. Layer-by-layer deposition is a versatile technique with a high degree of control over the type and amount of deposited material. Titanium dioxide nanoparticles are embedded in a polymer matrix to fabricate electrodes with low mass loading and high specific activity. The same type of film is also used for photocatalytic degradation of pollutants.In Chapter 4, the original reactor is revisited to explore a new concept: gas phase operation using outside air as the source of water. The reactor is installed on a rooftop to give the first ever demonstration of hydrogen production using solely natural sunlight and outside air. For this demonstration electrodes are fabricated using atomic layer deposition on carbon nanotubes to obtain thin film architectures which are inert towards molecular oxygen present in air. The operation in air opens completely new possibilities for solar hydrogen production. Contrary to liquid-based devices, gas bubbles, frost, membrane poisoning nor corrosion cause problems. Moreover, only limited peripherals are needed and no source of clean water. In Chapter 5, the state of the art of photoelectrochemical water splitting and photovoltaic-driven electrolysis is discussed, along with the potential of air-based water splitting. A model is built which shows that air contains about ten times more water than is required to run such devices.In Chapter 6 of this thesis, the potential role of solar fuels in a global energy landscape is investigated. More than half of our energy consumption requires fuels and a large portion of this could be supplied by air-based water splitting. Solar fuel production is usually envisioned as large-scale plants in deserts, using high-maintenance equipment and transporting the fuel to end users by pipelines or trucks. In this work, a second type of solar fuel production is added to the portfolio: smaller scale autonomous devices which readily produce hydrogen from the sun and the air that surround us.

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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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ln an electrocatalytic process, CO2 can be reduced to simple organic compounds in the presence of water and driven by electrical energy. When renewable energy is used and the CO2 does not originate from the combustion of fossil fuels, this allows the production of hydrocarbons in a sustainable manner. This research will focus initially on attempting to produce formic acid from water vapour and CO2. The research makes use of a gas phase electrocatalytic reactor, consisting of a separating membrane and electrocatalysts, that is coupled directly to a photovoltaic panel.

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One of the most important challenges of this century is the rebalancing of the carbon cycle on earth. To that purpose, Carbon Capture and Utilisation (CCU) technologies are being developed. This work focuses on the development and benchmarking of Membrane Electrode Assembly (MEA)- type reactors that electrochemically reduce CO2 to formic acid. With the emerging carbon trading schemes and taxes, a lot of attention has been going towards all kinds of CCU technologies, including electrochemical CO2 reduction. This resulted in impressive results in literature. Liquid-phase electrolysers have accomplished high selectivities and energy efficiencies. However, because of the low solubility of CO2 in liquid catholytes, rather low partial current densities (PCD) were reached. Therefore, in the most recent publications, often gas-phase (or solid-state) cathodes are used. In this master’s thesis, 2 promising reactor designs using gas-phase cathodes are evaluated and rationalised, namely, a 2-compartment concept using KOH anolyte and a 3-compartment concept. Then, based on the rationalisation of the findings in those setups, new all-solid-state reactor designs were proposed. The use of ion conducting polymers, like ionomer solutions and ion exchange beads proved to be very helpful in recreating the favourable reaction conditions, identified in the 2 reproduced designs, in an all-solid-state environment. Because the use of these ion exchange beads in electrolyser applications is rather new, data on their conductivity in gaseous environment is scarce. Thus, in this work, the conductivities of a selection of these beads were measured, as well as the dependence of the conductivity of the commonly used ion exchanger beads, Amberlyst-15, on the humidity of the gas flow. Successful synthesis of formic acid vapour in an all-solid-state reactor was demonstrated, which was only reported before by two other research groups (Lee et al. and Xia et al.) to this date [1–2]. Furthermore, it was concluded that when high product concentrations/purities are desirable, it becomes interesting to use the 2-compartment all-solid-state reactors proposed in this work, which have a substantially smaller resistance than gas-phase 3-compartment reactors. However, when the concentration/purity is not critical, the proven selectivity and stability of the 3-compartment design using liquid DI-water in the middle compartment, proposed in literature, is superior at the moment.