information about the proteins that perform primary photosynthetic energy conversion is driving
the surge in new information on how this process works.
The second part of this survey discusses current advances in the field of photosynthesis and
explains how this knowledge may be exploited to develop bio-inspired systems for solar fuels
production.
The use of robust bio-inspired chemical systems to carry out the primary processes that lead to
the photoconversion and use of solar energy to produce fuels is detailed in the third section of
this survey. Prototype systems have demonstrated the fundamental steps necessary to complete
the photoconversion process, yet many challenges remain, including finding ways to integrate
subsystems for optimal performance and understanding the fundamental concepts of energy and
charge flow within complex integrated systems.
In the fourth section of the survey, we focus on the design of catalysts that can use the chemical
energy derived from sunlight to carry out the critical fuel formation step. Because this research
area is still in its infancy, many important challenges remain, such as developing ways to use
multiple solar-derived charges to catalyze CH 4 and H 2 production. Finally, we discuss some of
the major scientific challenges at the cutting edge of knowledge: integrating subsystems for
photo-driven solar fuels formation, optimizing their performance, and providing a versatile and
dependable means to ensure their long functional lifetime.
CURRENT STATUS
Biomass-derived Fuels
Biomass has been used as an energy source over the entire span of human existence. It has been,
and continues to be, a functioning resource for energy production that is being exploited on a
significant scale both in developing and industrialized countries. The overall energy efficiency of
biomass energy conversion systems is, however, quite low: less than 1% of the incident
insolation is stored as chemical fuels. As a consequence, it is important to address new ways to
increase the efficiencies of the many biological pathways leading from photosynthetic light
capture through the production of polysaccharides and their subsequent conversion into liquid
fuels. Improvements may be obtained by genetic modifications of the organisms responsible for
the production of biomass and by re-engineering enzyme catalysts that convert biomass into
liquid fuels. Current research directions focus largely on secondary energy production from
already-available biomass to produce liquid fuels by: (1) increasing cellulose-to-sugar
conversion for the production of ethanol; and (2) developing biomass gasification technologies
that produce synthesis-gas (syngas) — a mixture of CO and H 2 for use in fuel-forming reactions.
The structures of plants contain large amounts of cellulose that cannot be readily used as a
feedstock for producing liquid fuels (Aden et al. 2002). As a consequence, current research has
focused on methods for converting cellulose into its component sugars, which can subsequently
be converted into ethanol. Methods for treating cellulose to obtain sugars range from acid
treatment to using specific enzymes to catalyze this process. By using only the experimentally
achieved process parameters and a feedstock cost of $53/dry ton, the calculated minimum