Density Functional Theory (DFT) has been implemented, within an ab-initio pseudopotential framework, in simulations of materials involving weak intermolecular interactions.

Anthropogenic greenhouse gas emissions and rising oil prices call for a sustainable transportable energy source. Hydrogen is a promising replacement for carbon based fuels in vehicles. The HyS cycle, proposed by the Westinghouse Corporation, is a route to hydrogen production on a scale large enough to satisfy the requirements of the transport industry. The process is a hybrid thermochemical cycle based on the decomposition of sulphuric acid.

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Although the field of solar water splitting is now forty years old, in recent years there has been an upsurge of research in this area, with the aim of using sunlight to produce hydrogen cheaply and efficiently. Hematite (α-Fe2O3) is of particular interest as a photoanode material for solar water splitting, due to its optimum band gap (2.0-2.2 eV) and visible light absorption and stability. Various modifications – including nanostructuring and doping – have been investigated as routes to improved efficiencies, thought to be limited by long visible light absorption depths, low charge carrier mobilities and slow hole-transfer kinetics. Additionally, an anodic applied bias is required for water oxidation to occur on hematite. Improved understanding of the role of applied bias and the processes limiting the performance of hematite photoanodes will lead to more directed routes to photoanode architectures with increased efficiencies. This Thesis describes the results of transient absorption spectroscopy studies, in conjunction with photoelectrochemical measurements, of hematite photoanodes. Transient absorption spectroscopy on microsecond-second timescales allows direct monitoring of the recombination, trapping and reaction of photogenerated holes, both in isolated hematite films, and in photoanodes in a fully functional photoelectrochemical cell. Transient photocurrent measurements probe electron extraction from the photoanode on microsecond-millisecond timescales. The charge carrier dynamics are found to be strongly dependent on the electron density, which is controlled by applied electrical bias. The photocurrent generated is found to correlate with the population of long-lived holes, determined by the kinetics of electron-hole recombination. Generally, effects which lower electron density result in retarded electron-hole recombination kinetics, increasing the population of long-lived holes and hence increasing the photocurrent. Following an introduction and review of the literature, the first results chapter reports that the effect of a positive applied bias is to retard the otherwise dominant electron-hole recombination, increasing the lifetime of photogenerated holes such that water oxidation can occur. The relative timescales of recombination, electron extraction and water oxidation as a function of applied bias are discussed in the following chapter, in conjunction with the results of excitation density studies. The third results chapter compares the charge carrier dynamics in photoanodes with different nanomorphologies. The fourth results chapter discusses the effect of an energetic trap state on charge carrier dynamics, while the effects of surface treatment with cobalt, which is shown to retard recombination at low applied bias, is reported in the final results chapter. Overall conclusions are drawn and the implications of these for photoelectrode design are discussed.

Hydrogen has a great potential to become a wide-scale, carbon free, sustainable energy carrier of the future. However its implementation and final utilization especially in mobile applications is still limited because of several technological and socio-economical barriers, mainly to do with safe, efficient storage of hydrogen with high gravimetric and volumetric storage capacities. Physisorption into nanoporous materials is an attractive option as it benefits from rapid, fully reversible adsorption/desorption and can store significant amounts of hydrogen at more moderate temperature and pressures conditions than conventional liquefaction (20.3 K) or compression (350‒700 bar). Nevertheless, the critical challenge exists to define the experimental methods that allow accurate hydrogen sorption determination and reduce discrepancies in measurements between different laboratories. This thesis presents an investigation of the experimental methodology of hydrogen sorption in porous materials. A set of nanoporous samples and characterisation techniques have been tested rigorously to explore experimental uncertainty and provide universally reproducible procedures. The validity of the standard methods and some new approaches for experimental data collection and analysis are presented. High repeatability of gas sorption isotherms measured gravimetrically and volumetrically at 77 K on reference TE 7 III carbon beads sample has been demonstrated in-house. A study has been conducted between seven laboratories to evaluate the reproducibility of nitrogen/hydrogen isotherms at 77 K according to a defined test protocol. Statistical analysis yields very good agreement between nitrogen and hydrogen sorption results. The Brunauer-Emmett-Teller surface area of 777.8 ± 6.2 m2 g-1 and Dubinin-Radushkevich micropore volume of 0.3766 ± 0.0078 cm3 g-1, have been determined. The excess hydrogen sorption capacities are found to be 1.65 ± 0.04 wt% and 2.33 ± 0.007 wt% for 1 bar and 20 bar hydrogen pressure, respectively. This study concludes that the accuracy of hydrogen sorption measurements have been pushed forward and methodology proposed here could contribute to improvements in certification of future hydrogen sorption methods.

The storage of hydrogen is one of the main problems that needs to be solved before hydrogen can become a real alternative to oil in mobile applications. Physisorption of hydrogen in an adsorbate is one of the possible solutions to this problem. This thesis studies the adsorption of hydrogen in Single-Walled Carbon Nanotubes (SWNTs). Neutron scattering techniques are used to probe the possible adsorption sites and the interaction between the hydrogen and the nanotubes at those sites.

Hydrogen is considered a possible alternative to fossil fuels. Hydrogen can be produced through dark fermentation with 1 mol hexose yielding a maximum of 4 mol hydrogen in association with acetate production, and 2 mol hydrogen in association with butyrate production. However, an economically and technically feasible process is yet to be established. So far research into fermentative hydrogen production has focused on pure and soluble carbohydrates, particularly glucose. To reduce substrate costs, use of more complex low-cost co- and waste products of the food industry or biomass crops which have undergone minimum pre-treatment would be desirable. Also, whilst much research to date has focused on use of pure bacterial strains, an easily obtainable mixed microflora would be preferable to avoid costs of substrate sterilisation. Therefore this research project focused on fermentative hydrogen production from three abundant (in the UK) low cost substrates, namely a wheat starch co-product, sugarbeet and perennial ryegrass. Anaerobic digester sludge obtained from the local sewage works was used as inoculum in a continuously stirred tank reactor. Production of hydrogen and other fermentation products was measured to gain information about the main metabolic pathways used. To lower hydrogen partial pressure the reactor was sparged with nitrogen and the effect on hydrogen production observed. It was demonstrated that stable fermentative hydrogen production from the wheat starch co-product and sugarbeet water extract was possible in continuous operation. Hydrogen production from grass extract was demonstrated in batch mode. Sparging with nitrogen significantly increased hydrogen yields, by 46% for the wheat starch co-product, by 67% for sugarbeet water extract, and by 184% for ryegrass extract. Maximum yields achieved were 1.9 mol hydrogen per mol hexose converted for 16 days on starch, 1.7 mol per mol hexose converted for 5 days on sugarbeet water extract and 0.8 mol hydrogen per mol hexose converted in batch from grass extract. Therefore up to 48% of the maximum theoretical hydrogen yield was produced. Various factors were identified as preventing higher hydrogen yields. Hydrogen production was more closely related to butyrate than acetate concentration. Also, lactate, ethanol and propionate, which are products of carbohydrate fermenting metabolic pathways that do not produce hydrogen, were detected, as were signs of hydrogen consuming homoacetogenesis in continuous operation.

Τα τελευταία χρόνια έχει αυξηθεί ιδιαίτερα το ενδιαφέρον για το υδρογόνο, καθώς η αξιοποίηση του θα μπορούσε να οδηγήσει σε μείωση της ανησυχίας για την εξάντληση των αποθεμάτων των ορυκτών κυρίως καυσίμων και της επιβάρυνσης του περιβάλλοντος. Ο προσανατολισμός προς την κατεύθυνση του υδρογόνου οφείλεται κυρίως στο υψηλό ενεργειακό περιεχόμενο του έναντι άλλων συμβατικών καυσίμων καθώς επίσης και στο γεγονός ότι κάνει "καθαρή" καύση αφού όταν καίγεται με οξυγόνο παράγει μόνο νερό και θερμότητα με αποτέλεσμα να μην ρυπαίνει το περιβάλλον. Πιο συγκεκριμένα, στη παρούσα εργασία αναλύεται η τεχνολογία παραγωγής υδρογόνου με τη μέθοδο της ηλεκτρόλυσης. Η ηλεκτρόλυση λαμβάνει χώρα σε ηλεκτρολυτικό κελί τύπου Hofmann. Ως ηλεκτρόδιο ανόδου χρησιμοποιήθηκε ανοξείδωτος χάλυβας StS304, ενώ ως ηλεκτρόδιο καθόδου υπόστρωμα χαλκού πάνω στο οποίο εναποτέθηκαν με τη μέθοδο της ηλεκτροχημικής εναπόθεσης, λεπτά υμένια μεταβατικών μετάλλων με βάση το νικέλιο. Τα υμένια που παρασκευάστηκαν ήταν NiFe, NiFeZn, NiCoZn, NiMoFe, NiZn και NiMoZn. Κύριος στόχος της εργασίας ήταν η μελέτη των ηλεκτροκαταλυτικών ιδιοτήτων των παραπάνω υμενίων ως προς την παραγωγή υδρογόνου. Αρχικά μελετήθηκαν τα υμένια σε πραγματικές συνθήκες ηλεκτρόλυσης, μετρώντας την παραγωγή υδρογόνου για συγκεκριμένες πάντα συνθήκες. Στη συνέχεια έγιναν μετρήσεις υπερδυναμικού παραγωγής υδρογόνου και από τα δεδομένα αυτά και με τη βοήθεια των διαγραμμάτων Tafel υπολογίστηκαν οι ηλεκτροκαταλυτικοί συντελεστές, όπως η πυκνότητα ρεύματος ανταλλαγής και η κλίση Tafel. Τέλος, για μια πιο ολοκληρωμένη εικόνα για την επίδραση των συνθηκών της ηλεκτροχημικής εναπόθεσης στην μορφολογία της επιφάνειας των λεπτών υμενίων. πραγματοποιήθηκαν μετρήσεις σε ηλεκτρονικό μικροσκόπιο σάρωσης (SEM). / Recently the interest in hydrogen has increased as the utilization could lead to reduced concern about depletion of fossil fuels . The increased interest for hydrogen is mainly due to its high energy content compared to other fossil fuels, as well as the fact that it is a completely clean burning fuel; its only product is water. In the present study hydrogen production by electrolysis is analyzed. Electrolysis takes place in type Hofmann electrolytic cell. As anode electrode stainless steel StS304 was used. As cathode electrode copper substrate was used; in particular we have deposited films of the transition metal Ni on copper substrates. The alloys we fabricated were NiFe, NiFeZn, NiCoZn, NiMoFe, NiZn and NiMoZn. The main objective of this work was to study the electrocatalytic properties of these films for hydrogen production. At first we studied the electrocatalytic properties of the thin film alloys during the electrolytic production process using the same conditions every time. Subsequently, we measured the hydrogen production overvoltage, and using these data, we constructed Tafel diagrams and calculate Electrocatalytic coefficients sush as: exchange current density and Tafel slope. Finally SEM , gave us a complete picture of the effect of the electrochemical deposition conditions on the morphology of the surface of thin films.

Hydrogen is a potential future energy storage medium to supplement a variety of renewable energy sources. It can be regarded as an environmentally-friendly fuel, especially when it is extracted from water using electricity obtained from solar panels or wind turbines. The focus in this thesis is on solar energy, and the theoretical background (i.e., PSCAD computer simulation) and experimental work related to a water-splitting, hydrogen-production system are presented. The hydrogen production system was powered by a photovoltaic (PV) array using a proton exchange membrane (PEM) electrolyser. The PV array and PEM electrolyser display an inherently non-linear current-voltage relationship that requires optimal matching of maximum operating power. Optimal matching between the PV system and the electrolyser is essential to maximise the transfer of electrical energy and the rate of hydrogen production. A DC/DC converter is used for power matching by shifting the PEM electrolyser I-V curve as closely as possible toward the maximum power the PV can deliver. By taking advantage of the I-V characteristics of the electrolyser (i.e., the DC/DC converter output voltage is essentially constant whereas the current increases dramatically), we demonstrated experimentally and in simulations that the hydrogen production of the PV-electrolyser system can be optimised by adjusting the duty cycle generated by the pulse-width modulation (PWM) circuit. The strategy used was to fix the duty cycle at the ratio of the PV maximum power voltage to the electrolyser operating voltage. A stand-alone PV energy system, using hydrogen as the storage medium, was designed. The system would be suitable for providing power for a family's house located in a remote area in the Libyan Sahara.