The dream of solar-to-fuel synthesis lies in first replicating the ability of natural photosynthesis to fix CO2 into multi-carbon compounds through solar energy, and eventually surpass it through the rational design of advanced light absorbers and catalysts. While photovoltaic and photoelectrochemical systems often surpass nature in terms of light harvesting efficiency, the transduction of these photoelectrons into carbon-carbon bonds remains challenging for inorganic catalysts. In contrast, biological routes to CO2 fixation routinely produce a wide range of complex organic compounds with high selectivity. Capitalizing on millions of years of natural selection, plants and bacteria employ a host of catalytic enzymes assembled from earth abundant elements that demonstrate self-replication and self-repair for a low cost, highly stable biocatalytic system. However, the poor light harvesting of photosynthetic organisms limit their potential for solar-to-fuel synthesis, calling for a hybrid system that combines the best of both worlds: the light harvesting of inorganic semiconductors with the catalytic power of biological organisms. To demonstrate this concept, we have devised two inorganic-biological hybrid systems. The first combines the electrotrophic, CO2 fixing bacterium, Sporomusa ovata, with a biocompatible Si/TiO2 nanowire tandem array for unassisted production of acetic acid and O2 from CO2 and sunlight. This system exhibited low overpotential, high Faradaic efficiency, and long-term stability. We have also demonstrated the first self-photosensitization of a non-photosynthetic organism: Moorella thermoacetica with self-bioprecipitated CdS nanoparticles. The hybrid organism M. thermoacetica/CdS photosynthetically produced acetic acid, offering a completely self-replicating, low-cost, low temperature, solution-processed route to efficient solar CO2 fixation. The acetic acid was then passed on to a genetically engineered strain of Escherichia coli, which upgraded and diversified the single feedstock into butanol, the biopolymer polyhydroxybutyrate, and pharmaceutical precursors. This hybrid approach opens up a new parameter space to explore in the design of advanced solar-to-fuel technologies.

Inorganic-biological hybrid systems for solar-to-chemical production

251st National Meeting (2016)

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