The transition from declining conventional fossil fuel energy to renewables is one of the most significant challenges facing humanity. Hydrogen is anticipated as the key future energy vector. This is sought to bring more value and utility to renewable energy resources, and eventually providing an energy storage medium to replace fossil fuels such as for automotive applications. This thesis contains an investigation of hydrogen production through renewable low-cost and low-carbon processes. Literature reviews of conventional and renewable H₂ production methods and storage (compression, liquefaction, adsorption materials and hydrides) are presented in detail (in appendices). Particular attention has been given to energy efficiency, cost and practicality of processes. Electrolysis of water is investigated in detail. Wind turbines and solar photovoltaics are reviewed and physically investigated as key renewable electrical energy sources for renewable H₂ production via electrolysis. Conventional and novel electrical power control is investigated and tested to support low-cost wind/solar-powered electrolysis. Biological H₂ production from mixed-acid fermentation of Escherichia coli is practically investigated from the energy-physics perspective and considered as one possible route to permit renewable H₂ production in the long-term. Photocatalytic materials are also investigated as additional future routes for renewable H₂ production; in this work they are investigated using nanoscale materials processing and surface analysis techniques. This thesis has an energy-focussed, applied and practical theme, achieving a broad investigation of the topics herein. Experimental investigations were chosen based upon relevance, practicality, concurrent research, availability of resources, and for application of novel nanoscale materials processing. Power control elements for wind-powered H₂ production have for example been optimised by complete investigation of supply/load characteristics rather than adopting the more conventional power electrical/electronic approach. Ultimately the work here aims to demonstrate (at small-scale) that renewable H₂ production can be achieved at relatively low cost, e.g. by wind-powered electrolysis, inferring that pathways can be established within existing means to produce much larger quantities of renewable H₂ economically.

Many communities around the world have no access to an electricity grid. To supply power to these people, stand-alone power systems are often used, the majority of which are based on diesel generators. Rising fuel costs and environmental concerns make the use of renewable energy in stand-alone systems increasingly attractive. The research reported in this thesis was to demonstrate a stand-alone power system based exclusively on renewable energy sources. To achieve this, a DC electrical backbone is used. Power electronic converters are used to interconnect the loads and generators and hydrogen is used as an inter-seasonal energy store. The design and control of the DC based stand-alone power system forms the primary focus of this research. A demonstration system has been implemented at West Beacon Farm in the UK. Substantial data has been collected that confirms the successful operation of the system.

Hydrogen storage in lightweight hydrides continues to attract significant interest as the lack of a safe and efficient storage of hydrogen remains the major technological barrier to the widespread use of hydrogen as a fuel. The metal borohydrides Ca(BH₄)₂ and LiBH₄ form the subject of this thesis; three aspects of considerable academic interest were investigated by density functional theory (DFT) and molecular dynamics (MD) modelling. (i) High-pressure crystal structures of Ca(BH₄)₂ were predicted from a structural analogy between metal borohydrides and isoelectronic metal oxides. The structural stability of hydrogen storage materials under high pressure is an important aspect, as high-pressure polymorphs may provide structures with better hydrogen desorption properties. The isoelectronic analogue of Ca(BH₄)₂ is TiO₂, and structural equivalents of Ca(BH₄)₂ in the baddeleyite, columbite and cotunnite structures of TiO₂ were found to be stable at elevated pressure. Thermodynamic stability was evaluated by computing the Gibbs energy with respect to pressure and temperature. The pressure-dependence of the Helmholtz energy was determined to described a third-order Birch-Murnaghan equation of state, and the harmonic approximation was used to compute the vibrational energy levels and the Helmholtz energy as a function of temperature. The proposed structures are consistent with reports of two hitherto unidentified high-pressure phases observed experimentally. (ii) The disordered structure of the high-temperature phase of LiBH4 was studied by ab initio molecular dynamics (MD) at temperatures ranging from 200-535 K. It was found that the model emerging from analysis of the MD simulations properly accounts for dynamical disorder and fundamentally differs from the published experimental and theoretical structures. The validity of the MD model was corroborated by comparison of calculated pair distribution functions, vibrational spectra and a crystallographic model with neutron diffraction data; good agreement was found. A reassignment of the space group from P63mc to P63/mmc is proposed based on evidence for additional symmetry from MD simulations. (iii) Finally, a new MD-based method was developed to simulate fast ionic diffusion in LiBH₄. The colour diffusion algorithm - a nonequilibrium molecular dynamics method originally developed for the study of model fluids - was adapted and applied to self-diffusion of atoms in a solid for the first time. Calculated diffusion coefficients agreed very well with published measurements, and diffusion pathways that include collective particle effects were determined directly from the simulation results, thereby opening up a promising and efficient new method for the study of phenomena such as superionic conduction.