Bio-Inspired Constructs for Sustainable Energy Production and Use

Sustainable energy production for human use based upon solar powered bio-inspired constructs and renewable resources is achievable. However, several challenges must be overcome before this goal is realized.

The goal of research in artificial photosynthesis is to abstract the key photosynthetic processes using synthetic schemes, suggest hybrid systems and to engineer living cells to increase the efficiency of the synthesis of energy-rich fuel materials such as hydrogen and reduced carbon compounds.

Energy Conversion in Living Cells

Reprinted with permission from L'Actualilté Chimique

Energy transduction takes place in biological "fuel cells" known as mitochondria where electrons are collected by the enzymatic oxidation of reduced carbon compounds, a process that reduces NAD⁺ to NADH and ultimately reduces O2 to water. This is accomplished by a series of electron carriers (electron transport system) which use the drop in electrochemical potential to pump protons across the mitochondrial inner membrane. The resulting proton thermodynamic gradient or its thermodynamic equivalent, ATP, is used to do synthesis, mechanical and transport work, and information processing work. The mitochondrial energy-transducing membranes operate at near the thermodynamic limit of efficiency.In contrast the efficiency of hydrogen/air (O2) fuel cells is limited by the overpotential at the oxygen-reducing cathodic electrode.

Photosynthesis vs. Photovoltaics and Electrochemistry

Reprinted with permission from L'Actualilté Chimique

Photoinduced charge separation is common to both photovoltaic and photosynthetic processes. The red arrows delineate connections in function between the two systems. In photosynthesis, charge separation yields highly oxidizing and highly reducing chemical species that carry out water oxidation into O2, and carbon reduction into the energy-rich carbon compounds, the building blocks of living cells. Oxidation/reduction chemistry as carried out electrochemically in batteries and fuel cells and the energy output of photovoltaic devices produce electromotive force (emf), the current and voltage carried by the metallic conductors and electrodes, which couples the chemical oxidation and reduction half reactions together.

Light Harvesting, Energy Transfer and Charge Separation in Artificial Photosynthesis

Reprinted with permission from L'Actualilté Chimique

Artificial photosynthesis includes the design and synthesis of molecular constructs that mimic key functions found in natural photosynthetic membranes. An artificial construct that exhibits the primary photophysical functions of light absorption, energy transfer and charge separation found in natural photosynthesis is shown above.The central hexaphenylbenzene core that bears five light-harvesting 9,10-bis(phenylethynyl)-anthracene (BPEA) groups and a Zn porphyrin. The reaction center comprises the Zn-porphyrin as the primary electron donor and a C60 moiety as the acceptor. The BPEA units act as antennas to provide substantial absorption cross section in the blue-green region of the spectrum.

A Hybrid Photobiofuel Cell

Reprinted with permission from L'Actualilté Chimique

In this hybrid photobiofuel cell the electrons are obtained at the anode by oxidation of biomass such as glucose or short chain alcohols and a reductive chemical reaction at the cathode coupled with transport of charge between the anodic and cathodic solutions completes the circuit. In the anode solution, a soluble dehydrogenase enzyme is used to oxidize a biological substrate and reduce NAD⁺ to NADH. A porphyrin-sensitized high-surface area TiO2 photoanode is excited by visible photons, resulting in electron injection from the first excited singlet state of the porphyrin into the conduction band of the titanium (IV) oxide. These electrons are collected at the back contact of the photoanode and passed via the connecting wire to the cathode. The porphyrin radical cation resulting from photoinduced electron injection oxidizes the NADH product of the enzymatic reactions, regenerating NAD⁺ and poising the NAD⁺/NADH couple oxidizing. The photochemical step increases the negative potential of the electrons derived from biomass. Once injected into the TiO2 conduction band, the electrons are conducted to the cathode where the emf is sufficient to reduce protons to hydrogen gas. Under our typical operating conditions (pH 8.0) the H⁺/H2 couple is among the most reducing species encountered in biology, and hydrogen production is therefore considered a proxy for the synthesis of a variety of products. The ability of the biohybrid photoelectrochemical cell to evolve hydrogen indicates that there is sufficient driving force for other types of bio-inspired reductive synthesis to be carried out with appropriate catalysts.

Read the full article in L′Actualité Chimique, 2007, No. 308039, pp.50-56

For further information, please, contact Ana Moore (amoore@asu.edu), Devens Gust (Gust@asu.edu) and Tom Moore (tom.moore@asu.edu)