Ph.D Thesis


Ph.D StudentRoy Pinhassi
SubjectEngineering of a Green System for the Production of
Photocurrent and Hydrogen through Photosynthesis
DepartmentDepartment of Energy
Supervisors Full Professors Schuster Gadi
Professor Rothschild Avner
Full Professors Adir Noam
Full Thesis textFull thesis text - English Version


Abstract

The efficient and inexpensive conversion of solar energy into storable and transportable fuel is an important goal, which may be addressed by photoelectrochemical water-splitting devices. Light-induced water-oxidation naturally occurs at the thylakoid membranes in cyanobacteria, algae and plants. We investigate here the utilization of thylakoid preparetion from the cyanobacteria Synechocystis (Syn) or from plants for the production of photocurrent or hydrogen fuel in a Bio-PhotoElectroChemical (BPEC) cell. First we develop methodologies to abstract photosynthetically-derived electrons from the thylakoids. Cytochrome c (cyt c) is the first candidate electron mediator, which has been found to abstract electrons from the QA site in photosystem II (PSII) of Syn thylakoids, an effect was enhenced after substitution the lysine at position 238 in the protein D1 to glutamic acid. Based on these findings and the high degree of similarity between plant and cyanobacterial photosynthetic complexes, we test cyt c as a mean for abstracting of electrons from plant thylakoids. We show that cyt c is reduced by spinach thylakoids five times faster comperd to Syn. However, in this case the site of electron abstraction is not at PSII,  regardeless of the identity of the amino acid at position 238 of D1. We then addressed to use thylakoid preparetions for photocurrent production. We found that Syn thylakoids produced an unmediated photocurrent density of 6 µM•cm-2, which was increased to 15 µM•cm-2 in the presence of the herbicide DCMU. Contraditory to the situation in Syn, significant photocurrents from plant thylakoids could be detected only in the presence of a mediator, and only in the absence of DCMU. We concluded that unlike in Syn thylakoids, where electrons can be withdrawn from PSII to cyt c or to an electrode, this could not be obtained in thylakoids of spinach or tobacco. We found that small chemical species are advantegeous to proteins as electron mediators, and obtained a maximal photocurrent densitiy of 100 µM•cm-2 from spinach thylakoids using the mediator DCBQ and a graphite anode. Employing the mediator potssium ferricyanide and a fluorinitaed-tin-oxide anode we achieved a maximal photocurrent density of 500 µM•cm-2. Next, we aimed at using the photocurrent in order to reduce protons to hydrogen gas. We show that in the presence of spinach thylakoid an external bias of merely 0.8V suffice to evolve hydrogen at the cathode, and that the Faradaic efficiency of this process is 67%. Thus, the photocatalytic water oxidation activity of PSII significantly contributes to reduce the bias otherwise needed to achieve hydrogen production under these pH conditions (~2.0 V). Further, we present a stand-alone mode of operation where the BPEC cell is coupled in tandem to a Si photovoltaic cell, which provides the bias required to produce hydrogen. We envision the development of methodologies to further increase the energetic output of the system and to extend the operational time, for insance by conveying the thylakoid slurry through a porous and transparent anode, to allow continuoes replacemnt of damaged thyalkoids, and to increse the effective area for charge collection.