Department of Mechanical Engineering, Yuan Ze University 1 1 Photoelectrochemical Hydrogen Production (Using Solar Power to Directly Split Water)

Department of Mechanical Engineering, Yuan Ze University 2 2 Biological Hydrogen Production

Department of Mechanical Engineering, Yuan Ze University 3 3

4 Materials Requirements for Photobiological Hydrogen Production Biological H 2 production is expected to be one of many processes that contribute to the ultimate supply of H 2. But it is useful to consider that in an area of less than 5,000 square miles (about 0.12 % of the U.S. land area) of bioreactor footprint in the desert southwest, photobiological processes could in principle produce enough H 2 to displace all of the gasoline currently used in the U.S. (236 million vehicles). The underlying assumptions are that H 2 could be produced from water at 10 % efficiency (the maximum theoretical efficiency of an algal H 2 production system is about 13 %) and that fuel cell-powered vehicles could get 60 miles/kg of H 2.

Department of Mechanical Engineering, Yuan Ze University 5 DESCRIPTION OF THE PROCESS The photobiological production of H 2 is a property of only three classes of organisms photosynthetic bacteria, cyanobacteria, and green algae. These organisms use their photosynthetic apparatus to absorb sunlight and convert it into chemical energy. Water (green algae and cyanobacteria) or an organic/inorganic acid (photosynthetic bacteria) is the electron donor. This review will focus on oxygenic organisms such as green algae and some cyanobacteria, which produce hydrogen directly from water, without an intermediary biomass accumulation stage. This process is considered to have highest potential sunlight conversion efficiency to H 2, which as mentioned above, could be on the order of 10 to 13 %. To accomplish this, green algae and cyanobacteria can utilize the normal components of photosynthesis to split water into O 2, protons, and electrons, deriving the requisite energy from sunlight. However, instead of using the protons and electrons to reduce CO 2 and storing the energy as starch molecules (the normal function of photosynthesis), these organisms can recombine the protons and electrons and evolve H 2 gas under anaerobic conditions (figure 5.1) using a reaction catalyzed by an induced hydrogenase enzyme.

Department of Mechanical Engineering, Yuan Ze University 6

7 The enzymes responsible for H 2 production are metallocatalysts that belong to the classes of [NiFe]- (in cyanobacteria) or [NiFe]- (in green algae) hydrogenases. Although sustained and continuous photobiological H 2 production has been achieved with green algae, the establishment and maintenance of culture anaerobiosis is currently a sine qua non for the process. This requirement results from the following biological realities:

Department of Mechanical Engineering, Yuan Ze University 8 1.The hydrogenase genes in green algae and in some cyanobacteria are not expressed (the genes are not turned on) in the presence of O 2, and a variable period of anaerobiosis is required to induce gene transcription. 2.The expression and function of the genes that catalyze the assembly of the catalytic metallocluster of the algal [FeFe]-hydrogenases require anaerobiosis, while in most cyanobacteria, the [NiFe]-hydrogenase genes are expressed and the protein assembled in the presence of O 2 ; in this case, the proteins are assembled in an inactive form, but can be activated quickly when exposed to anaerobic conditions. 3.The activity of the catalytic metallocluster of the hydrogenases is inhibited reversibly (in cyanobacteria) or irreversibly (in green algae) by the presence of O 2.

Department of Mechanical Engineering, Yuan Ze University 9 Reactor Materials Engineering design of full-scale photobioreactors and the balance of the facility for photobiological hydrogen production has not been considered beyond very general concepts. Consequently, there has been little effort to identify construction material and establish boundaries for specifying materials performance and properties.

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Department of Mechanical Engineering, Yuan Ze University 11 Department of Mechanical Engineering, Yuan Ze University 11 References Hydrogen Production: Overview of Technology Options, FreedomCAR and Fuel Partnership, January Materials for the Hydrogen Economy, Jones, R. H. and Thomas, G. J., ed., CRC Press, Boca Raton, 2008.