SOLAR HYDROGEN “Utilising Nature’s Most Abundant Resources – SUNLIGHT AND WATER” Stephen Dennison and Fessehaye Zemichae

1. O H H SOLAR HYDROGEN “Utilising Nature’s Most Abundant Resources – SUNLIGHT AND WATER” Stephen Dennison and Fessehaye Zemichael Department of Chemical Engineering, Imperial College London SW7 2AZ, UK, WHY HYDROGEN? With the decline in reserves and increasing concern over fossil fuel use, there is a demand for a revolution of energy carrier. • The requirements of this new energy carrier are: • CLEAN • RENEWABLE • AVAILABLE • The challenges are: • AFFORDABLE • EFFICIENT • SUSTAINABLE Potentially HYDROGEN fulfils all of these criteria. It can be used as both an environmentally viable fuel for transportation, a chemical feedstock and for electric power generation using fuel cells. There is also particular interest in hydrogen for novel processes for CO2 reduction and homologation. Thus, Research and Development into the production of hydrogen gas efficiently from renewable sources is of paramount importance. The transition from a fossil fuel-based economy to a Hydrogen energy-based economy, however, is fraught with many technical challenges; from production of sufficient quantities of Hydrogen to its storage, transmission and distribution. The aim of this project is to generate carbon-free hydrogen using solar energy, utilising virtually unlimited resources – SUNLIGHT and WATER. It is a major multi-Department programme at Imperial College, funded to a value of £4.2M by EPSRC. It is unique in its multi-disciplinary approach, starting from studies at the molecular level in Biology and Chemistry, through to systems and reactor design in Chemical Engineering. • PHOTOCHEMICAL CHALLENGES: • Major efforts have been made in the search for materials for the efficient photoelectroysis of water. The key requirements are for a semiconductor with: • Band gap matched to the solar spectrum (Eg ~1.5 eV) for maximum • absorption of solar energy • Energy levels suitable to carry out both oxidation and reduction of water • Stability under the conditions of water electrolysis (especially oxygen • evolution) • Generally, these requirements are mutually exclusive, with most stable materials being oxides, with Eg > 2 eV. There has been success in the sensitisation of larger band gap materials with light absorbing dyes, although significant technical barriers remain in the development and application of devices based on this principle. • BIOPHOTOLYSIS CHALLENGES: • Bio-hydrogen systems are under intense investigation to find ways to improve both the rates of H2 production and the ultimate yield of H2. • Hydrogen production by direct photolysis using green algae is currently limited by three parameters:- • Solar conversion efficiency of the photosynthetic apparatus; • H2 synthesis processes (i.e. the need to separate the processes of H2O • oxidation from H2 synthesis); and • Bioreactor design and cost. • KEY ACTIVITIES: • A number of approaches to improve H2 production by green algae are currently under investigation:- • (A) Genetic engineering of metabolic pathways and light gathering antennae, • (B) Optimization of light input into photo-bioreactors, and • (C) Improvements to the two-phase H2 production systems used with green • algae (phase-1 growth and phase-2 H2 production). • (D) Nutrient optimisation • (E) Screening of Algal collections for naturally high H2 producing strains with a • view to future genetic manipulation • Design Considerations For Appropriate Photo-bioreactor: • Achieve a high surface to volume ratio (minimise light gradients) • Reactor costs ( material properties; strength, durability, spectral properties • and diffusion coefficient for hydrogen) • Hydrogen production via indirect photolysis using green algae can be improved by screening for high yielding wild-type strains. Genetic modification of strains to switch off competing fermentative pathways and increase starch accumulation may yield significant increases in H2 production. • Finally, optimization of cultivation conditions such as light intensity, pH, temperature, and nutrient content, as well as maintaining low partial pressures of H2 and CO2 will contribute to increased H2 production. Figure: showing Biophotolytic pathways for H2-production Figure: showing principles of operation of a simple photoelectrolysis device. • Key Activities: • Materials selection/development for best opto-electrochemical • performance (Eg, stability) • Development of fabrication methods, e.g. spray pyrolysis, • electrodeposition • Reactor design to: maximise illuminated area per unit volume • anode-cathode geometry to minimise resistive losses and achieve • efficient gas separation (H2 from O2) • Approaches: • Investigation of Fe2O3 (Eg = 2.1 eV): cheap; can be produced in • thin-film form by a range of different methods. • Alternative materials: band-gap engineered TiO2 and their properties • Reactor design: modelling will be used to determine reactor • configuration. CONCLUSION This project links catalytic efficiency at the molecular level to reactor design and engineering, for the generation of truly carbon-free hydrogen. It demonstrates power of an integrated, cross-disciplinary approach to address a problem of global significance.