Efficient solar-to-fuels production from a hybrid microbial–water-splitting catalyst system

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 112; no. 8; pp. 2337 - 2342
• Main Authors: Torella, Joseph P., Gagliardi, Christopher J., Chen, Janice S., Bediako, D. Kwabena, Colón, Brendan, Way, Jeffery C., Silver, Pamela A., Nocera, Daniel G.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 24.02.2015; National Acad Sciences
• Subjects: Alcohols; Bacteria; Bioelectric Energy Sources - microbiology; Biofuels - microbiology; Biological Sciences; Biomass; Bioreactors - microbiology; Catalase - pharmacology; Catalysis; Catalysts; Cupriavidus necator - cytology; Cupriavidus necator - drug effects; Cupriavidus necator - growth & development; Cupriavidus necator - physiology; Electrodes; Genetic Engineering; Metals; Microbial Viability - drug effects; Photovoltaic cells; Physical Sciences; Ralstonia eutropha; Reactive Oxygen Species - metabolism; Solar Energy; Water; isopropanol; renewable energy; bioelectrochemistry; biofuel; Ralstonia
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1424872112

Photovoltaic cells have considerable potential to satisfy future renewable-energy needs, but efficient and scalable methods of storing the intermittent electricity they produce are required for the large-scale implementation of solar energy. Current solar-to-fuels storage cycles based on water splitting produce hydrogen and oxygen, which are attractive fuels in principle but confront practical limitations from the current energy infrastructure that is based on liquid fuels. In this work, we report the development of a scalable, integrated bioelectrochemical system in which the bacterium Ralstonia eutropha is used to efficiently convert CO ₂, along with H ₂ and O ₂ produced from water splitting, into biomass and fusel alcohols. Water-splitting catalysis was performed using catalysts that are made of earth-abundant metals and enable low overpotential water splitting. In this integrated setup, equivalent solar-to-biomass yields of up to 3.2% of the thermodynamic maximum exceed that of most terrestrial plants. Moreover, engineering of R. eutropha enabled production of the fusel alcohol isopropanol at up to 216 mg/L, the highest bioelectrochemical fuel yield yet reported by >300%. This work demonstrates that catalysts of biotic and abiotic origin can be interfaced to achieve challenging chemical energy-to-fuels transformations. Significance Renewable-fuels generation has emphasized water splitting to produce hydrogen and oxygen. For accelerated technology adoption, bridging hydrogen to liquid fuels is critical to the translation of solar-driven water splitting to current energy infrastructures. One approach to establishing this connection is to use the hydrogen from water splitting to reduce carbon dioxide to generate liquid fuels via a biocatalyst. We describe the integration of water-splitting catalysts comprised of earth-abundant components to wild-type and engineered Ralstonia eutropha to generate biomass and isopropyl alcohol, respectively. We establish the parameters for bacterial growth conditions at low overpotentials and consequently achieve overall efficiencies that are comparable to or exceed natural systems.