Photoelectrochemical Hydrogen Production
(Using Solar Power to Directly Split Water)
Department of Mechanical
1
Engineering, Yuan Ze
Department of Mechanical Engineering, Yuan Ze University
University
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Biological Hydrogen Production
Department of Mechanical
2
Engineering, Yuan Ze
Department of Mechanical Engineering, Yuan Ze University
University
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Department of Mechanical
3
Engineering, Yuan Ze
Department of Mechanical Engineering, Yuan Ze University
University
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Materials Requirements for
Photobiological Hydrogen Production
• Biological H2 production is expected to be one of many processes that
contribute to the ultimate supply of H2.
• 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
H2 to displace all of the gasoline currently used in the U.S. (236 million
vehicles).
• The underlying assumptions are that H2 could be produced from water at
10 % efficiency (the maximum theoretical efficiency of an algal H2
production system is about 13 %) and that fuel cell-powered vehicles
could get 60 miles/kg of H2.
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•
DESCRIPTION OF THE PROCESS
 The photobiological production of H2 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 H2,
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 O2, protons, and electrons, deriving the requisite energy
from sunlight.
 However, instead of using the protons and electrons to reduce CO2 and storing the energy as
starch molecules (the normal function of photosynthesis), these organisms can recombine
the protons and electrons and evolve H2 gas under anaerobic conditions (figure 5.1) using a
reaction catalyzed by an induced hydrogenase enzyme.
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Department of Mechanical
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• The enzymes responsible for H2 production are
metallocatalysts that belong to the classes of [NiFe]- (in
cyanobacteria) or [NiFe]- (in green algae) hydrogenases.
• Although sustained and continuous photobiological H2
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:
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1.
The hydrogenase genes in green algae and in some cyanobacteria are not
expressed (the genes are not turned on) in the presence of O2, 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 O2; 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 O2.
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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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Engineering, Yuan Ze
University
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Department of Mechanical
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References
• Hydrogen Production: Overview of Technology Options,
FreedomCAR and Fuel Partnership, January 2009.
• Materials for the Hydrogen Economy, Jones, R. H. and Thomas,
G. J., ed., CRC Press, Boca Raton, 2008.
Department of Mechanical
11
Engineering, Yuan Ze
Department of Mechanical Engineering, Yuan Ze University
University
11