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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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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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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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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.
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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.
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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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