Ph.D Thesis


Ph.D StudentGadiel Saper
SubjectHarnessing Photosynthesis for H2 Production
DepartmentDepartment of Energy
Supervisors Full Professors Adir Noam
Professor Rothschild Avner
Full Professors Schuster Gadi


Abstract

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In the last 100 years, humans have been using fossil fuels as the main resource of energy. Not only is the supply of these fuels dwindling, but the use of these fuels emits carbon dioxide, which has been proposed to be a major factor in the elevation of global temperatures. In recent years, much research has been conducted in the field of alternative renewable energy, with an emphasis on the production of hydrogen as a solar fuel. Photosynthesis, nature’s “solar energy conversion machine”, is an evolutionary conserved process from bacteria to plants responsible for maintaining an oxygen rich atmosphere and for the production of complex carbohydrates. Using photosynthesis for the production of solar fuels is an up and coming field in academia with many studies focusing on various ways to extract electrons from the photosynthetic systems. Here we, describe the construction of two bio-photo-electrochemical cells (BPEC) based on either cyanobacterial cells or spinach photosynthetic membranes to produce hydrogen fuel. In the first system, we used live cyanobacteria cells that were gently treated, thus, allowing the extraction of electrons with an endogenous mediator to an external graphite electrode. This cell generates photocurrent of about 30 µA/cm2 at a potential of 150 mV vs. Ag/AgCl/3M NaCl for several hours. The highest photocurrent are obtained at 700 nm, suggesting that photosystem I is the photo active complex and that electron are extracted from photosystem I. Moreover, the photocurrent can be attained in a mutant lacking the entire photosystem II complex, indicating the involvement of another electron source coupled to photosystem I. The photocurrent is prolonged in the presence of glucose and inhibited in the presence of iodoacetate that blocks electron transfer in the respiratory system, proving that the electron source is the respiratory system. For the second BPEC we use spinach thylakoids. Here we extract the electron from photosystem II with exogenous ferricyanide to a fluorinated-tin-oxide electrode. The spinach membrane fragments can generate photocurrent of 500 µA/cm2 at a potential of 500 mV vs. Ag/AgCl/3M NaCl for about 10 min. 3-(3,4-dichlorophenyl)-1,1-dimethylurea, which inhibits electron transfer from QA to QB in photosystem II, decreases the photocurrent proving that the electron source is water oxidation by the activity of the photosystem II oxygen evolving complex. Moreover, we show that ferricyanide extracts the electrons mainly from the plastoquinone pool and partially also from other sources including but not limited to QB, Cytochrome b6f and PSI. For both systems the energy is used on the anode to produce hydrogen gas. The spinach BPEC produces high currents originating from water that decays relatively fast, but the active material can easily be replaced to maintain high rates of electron transfer. The cyanobacterial-cell based BPEC generates lower currents, yet, has a significantly longer lifetime. By studying and understanding both systems we lay the foundation for future research to combine the advantages of both systems.