Solar Fuels and Solar Chemicals Industry - American Chemical Society

Commentary pubs.acs.org/accounts

Solar Fuels and Solar Chemicals Industry Published as part of the Accounts of Chemical Research special issue “Holy Grails in Chemistry”. Daniel G. Nocera Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States ABSTRACT: Two decades of solar energy research, since the “Holy Grails” Account on Artificial Photosynthesis, has delivered astounding discovery that sets the stage for a paradigm shift from a fuels and chemicals industry powered by fossil fuels to one powered by the sun.

S

olar is a renewable energy source of sufficient scale to meet rising global energy demand over the next century.1 The precipitous drop in cost of Si solar cells in recent years, owing to manufacturing capabilities in China, has made solargenerated energy costs competitive with traditional energy generation in many markets2 and, in turn, has made energy storage the single most critical challenge to the widespread implementation of renewable energy.3 Owing to the diurnal variation in local insolation, widespread utilization of solar energy is impeded unless there are efficient and cost-effective methods for its storage. In this regard, the “Holy Grails” Account from two decades ago by Bard and Fox, entitled Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen,4 and preceding articles of over a decade before5 were prescient in identifying solar storage as the future research frontier of energy science. The conversion of water to H2 and O2 is one of the most practical carbon-neutral schemes to store solar energy on a global scale.1 Indeed natural photosynthesis powers the planet by converting the primary absorption of solar energy into hole and electron equivalents, which are delivered to Photosystems II and I, respectively, to produce oxygen and hydrogen in the form of NADPH.6 The NADPH reducing equivalents are combined with CO2 to furnish carbohydrates. On an electron equivalency basis, the production of the carbohydrate stores only 0.01 eV more energy than water splitting. Thus, the storage of solar energy in photosynthesis is achieved by water splitting; the carbohydrate is Nature’s method of storing the hydrogen released from the water splitting reaction (Figure 1). Accordingly, energy research of the past two decades has focused on the “holy grail” of water splitting. Of the two water-splitting half reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), the latter is more kinetically demanding because it © 2017 American Chemical Society

Figure 1. Photosynthesis stores solar photons in the four electron/ four proton reaction of water splitting to produce oxygen and hydrogen in the form of NADPH. The hydrogen is stored by combining it with carbon dioxide to produce carbohydrate in a thermoneutral reaction. Modified from ref 15.

requires the coupling of four electrons to four protons in the cleavage of four O−H bonds and formation of two O−O bonds.7,8 In this area of science, the discovery of an oxidic cobaltate OER catalyst that self-assembles from water in the presence of phosphate9 has spurred the development of other OER catalysts created from earth abundant elements of Mn, Co, and Ni.10−12 Oxidic Mn, Co, and Ni catalysts that are distinguished by their self-healing properties13 enable OER to be performed from water under benign conditions. By working under such conditions, a stable photoelectrochemical (PEC) cellthe artificial leafmay be constructed with facility by interfacing OER and HER catalysts simply as coatings on Received: December 8, 2016 Published: March 21, 2017 616

DOI: 10.1021/acs.accounts.6b00615 Acc. Chem. Res. 2017, 50, 616−619

Commentary

Accounts of Chemical Research silicon via conducting interfaces.14,15 The artificial leaf deviates from decades of work on liquid-junction PECs, which rely on a single material to perform light absorption, charge separation, and catalysis.16 Conversely, the buried junction configuration of the artificial leaf relaxes the constraints imposed by liquidjunction PECs because light absorption and one-electron charge separation may be independently optimized from the four-electron, four-proton chemistry that is needed for water splitting (Figure 2). For this reason, buried junction PECs

Cu are highly selective for the CDR of CO2 to CO in water at low overpotential.18 The ability of oxide derived nanoparticles of Cu to further reduce CO to ethanol, acetate, and n-propanol allows for the construction of a two-step CDR conversion of CO2 to C2+ liquid fuels.19 Whereas most CDR processes to date involve an electrochemical energy input for catalysis, a new approach to CDR is emerging that is based on a thermal chemistry using alkali carbonate molten salts as catalysts.20 Molten salts containing alkali cations furnish CO32−, which is able to deprotonate unactivated C−H bonds of simple organic substrates, generating carbanion intermediates that undergo C−C bond-forming reactions with CO2 to make carboxylates. The “carbonate catalysis” approach has been further elaborated by using the CO32− anion to deprotonate H2 to furnish formate, which subsequently undergoes carboxylation and reduction reactions to produce C2+ carboxylates. Because “carbonate catalysis” may be driven by a solar thermal input as opposed to a solar electricity input, intrinsically higher solar-to-fuels efficiencies may be achieved inasmuch as a solar-thermal powered process is limited by a Carnot efficiency (>50%) as opposed to the efficiencies offered by conventional solar photovoltaics (20%). Bioorganisms may promote CDR to complex C2+ products with impressive selectivity, but they suffer from low photosynthetic efficiencies.21 This limitation may be overcome by interfacing the bioorganisms to solar-driven inorganic systems. The metabolic pathways of bioorganisms to perform CDR may be powered by electrons directly from inorganic semiconducting materials, either from nanowires or nanoparticles to ensure a high surface area for the electron supply.22,23 Alternatively, the metabolic pathways of a bioorganism may be powered by hydrogen via a natural hydrogenase24 or via an inorganic catalyst that functions as an artificial hydrogenase.25 By interfacing bioorganisms such as Ralstonia eutropha with biocompatible water-splitting catalysts, a microbial/inorganic system has been developed to convert carbon dioxide, along with the hydrogen produced from water splitting, into biomass in excess of 10% energy efficiency, greatly exceeding the 1% yield of stored energy by natural photosynthesis. Extending this approach, R. eutropha may be engineered to directly synthesize C2+ liquid fuels at 5−7% energy efficiency along a bifurcated biomass-fuels metabolic pathway.26 A ledger of science advances since the “Holy Grails” Account on Artificial Photosynthesis shows that we can generate solar energy in a cost-effective manner with silicon, store solar energy in the form of the chemical fuels of hydrogen, biomass, and liquid fuels and do so at efficiencies well beyond that of Nature. These discoveries put society on a path to restructuring its fuels industry, currently based on carbon, to one based on solar energy. But this new “holy grail” is the tip of the iceberg. The buried junction allows any catalytic oxidation−reduction process to be solar powered, depending on the catalysts that are used to coat the semiconductor. Thus, many processes in the chemical industry may be transitioned from carbon-based processing to solar-based processing. And the powerful tool of synthetic biology allows the hybrid inorganic/biological approach to be generalized to a renewable chemical synthesis platform, depending on the biomachinery to which watersplitting is coupled (Figure 3). For a carbon-to-solar industrial transformation to be realized, high throughput will be needed to meet the demands of manufacturing scale processes. Thus, catalysts with higher turnover will be needed that operate at rates commensurate with solar concentration. Similarly, the

Figure 2. In a liquid junction photoelectrochemical (PEC) cell, the semiconductor (SC) performs light absorption, charge separation, and catalysis; this was the design highlighted in the original water-splitting Holy Grail Account4 and is the design that has occupied researchers for the last four decades. However, a good semiconductor has minimal defects and separates a single charge. A good water splitting catalyst creates defects (bond breaking and protonation of oxygen) and manages four protons and four electrons. This incompatibility between a good semiconductor and good catalyst engenders significant challenges in designing solar water-splitting devices based on liquid junction PECs. In a buried junction PEC, the SC and catalyst are separate materials, and thus they may be optimized independently, and consequently, the field of water splitting has advanced rapidly with their implementation.

comprising earth-abundant materials that operate in water at near-neutral conditions are now being adopted by many as a popular approach for solar-driven water-splitting17 with solarto-hydrogen (STH) efficiencies of 10% achieved and 15% STH efficiencies on the horizon. The solar energy stored by water splitting may be released in the useful form of electricity by a fuel cell. Nevertheless, an infrastructure does not currently exist to accommodate a water splitting/fuel cell energy system. The discovery of new materials to store hydrogen at high energy density will provide the impetus to greatly accelerate the development of such an infrastructure. In the absence of a hydrogen infrastructure, efforts have turned to fixing hydrogen with carbon dioxide to furnish fuels, chemicals, and biomass. As depicted in Figure 1, this target represents a complete artificial photosynthesis. There are many emerging carbon dioxide reduction (CDR) processes that furnish C1 products either by C−O bond cleavage to CO or hydrogenation of CO2 to C1 oxygenates (e.g., formate, methanol) or to methane. However, the most high-value CDR process is to produce carbon and hydrogen rich C2+ products (e.g., long-chain hydrocarbons, alcohols, and carboxylates). In accomplishing this chemistry, there are the challenges of effecting (i) extensive carbon−carbon bond formation (ii) with selectivity and (iii) at high solar energy efficiency. Abiotic and hybrid inorganic/biological systems have been pursued to accomplish carbon-rich CDR processes, and recent advances presage an extremely promising future. CDR to C2+ products has been achieved with admirable selectivity and energy efficiency using oxide-derived nanoparticles as electrocatalysts. Such nanoparticles of Au, Sn, and 617

DOI: 10.1021/acs.accounts.6b00615 Acc. Chem. Res. 2017, 50, 616−619

Commentary

Accounts of Chemical Research

Reach Baseload Electricity Costs. Energy Environ. Sci. 2012, 5, 5874− 5883. (3) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474−6502. (4) Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28, 141−145. (5) Graetzel, M. Artificial Photosynthesis: Water Cleavage into Hydrogen and Oxygen by Visible Light. Acc. Chem. Res. 1981, 14, 376−384. (6) Barber, J. Photosynthesic Energy Conversion: Natural and Artificial. Chem. Soc. Rev. 2009, 38, 185−196. (7) Cukier, R. I.; Nocera, D. G. Proton−Coupled Electron Transfer. Annu. Rev. Phys. Chem. 1998, 49, 337−369. (8) Huynh, M. H. V.; Meyer, T. J. Proton-Coupled Electron Transfer. Chem. Rev. 2007, 107, 5004−5064. (9) Kanan, M. W.; Nocera, D. G. In Situ Formation of an OxygenEvolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072−1075. (10) Bediako, D. K.; Lassalle-Kaiser, B.; Surendranath, Y.; Yano, J.; Yachandra, V. K.; Nocera, D. G. Structure−Activity Correlations in a Nickel-Borate Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2012, 134, 6801−6809. (11) Huynh, M.; Bediako, D. K.; Nocera, D. G. A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid. J. Am. Chem. Soc. 2014, 136, 6002−6010. (12) See also work on Co by H. Frei and T. D. Tilley, on Mn by H. Dau and G. C. Dismukes, and on Ni by C. P. Berlinguette and S. W. Boettcher. (13) Bediako, D. K.; Ullman, A. M.; Nocera, D. G. Oxygen Evolution Catalysis by Cobalt Oxido Thin Films. Top. Curr. Chem. 2015, 371, 173−214. (14) Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J. H.; Nocera, D. G. Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts. Science 2011, 334, 645−648. (15) Nocera, D. G. The Artificial Leaf. Acc. Chem. Res. 2012, 45, 767−776. (16) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446−6473. (17) Sun, K.; Liu, R.; Chen, Y.; Verlage, E.; Lewis, N. S.; Xiang, C. A Stabilized, Intrinsically Safe, 10% Efficient, Solar-Driven WaterSplitting Cell Incorporating Earth-Abundant Electrocatalysts with Steady-State pH Gradients and Product Separation Enabled by a Bipolar Membrane. Adv. Energy Mater. 2016, 6, 1600379. (18) Li, C. W.; Kanan, M. W. CO2 Reduction at Low Overpotential on Cu Electrodes Resulting from the Reduction of Thick Cu2O Films. J. Am. Chem. Soc. 2012, 134, 7231−7234. (19) Li, C. W.; Ciston, J.; Kanan, M. W. Electroreduction of Carbon Monoxide to Liquid Fuel on Oxide-Derived Nanocrystalline Copper. Nature 2014, 508, 504−507. (20) Banerjee, A.; Dick, G. R.; Yoshino, T.; Kanan, M. W. Carbon Dioxide Utilization via Carbonate-Promoted C−H Carboxylation. Nature 2016, 531, 215−219. (21) Blankenship, R. E.; Tiede, D. M.; Barber, J.; Brudvig, G. W.; Fleming, G.; Ghirardi, M.; Gunner, M. R.; Junge, W.; Kramer, D. M.; Melis, A.; Moore, T. A.; Moser, C. C.; Nocera, D. G.; Nozik, A. J.; Ort, D. R.; Parson, W. W.; Prince, R. C.; Sayre, R. T. Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement? Science 2011, 332, 805−809. (22) Liu, C.; Gallagher, J. J.; Sakimoto, K. K.; Nichols, E. M.; Chang, C. J.; Chang, M. C. Y.; Yang, P. Nanowire−Bacteria Hybrids for Unassisted Solar Carbon Dioxide Fixation to Value-Added Chemicals. Nano Lett. 2015, 15, 3634−3639. (23) Sakimoto, K. K.; Wong, A. B.; Yang, P. Self-Photosensitization of Nonphotosynthetic Bacteria for Solar-to-Chemical Production. Science 2016, 351, 74−77.

Figure 3. Hydrogenases, schematically represented by a nose, allow bioorganisms to “breathe in” hydrogen from water splitting and convert it to ATP to power the cell. Introduction of genes allows for the incorporation of specific “cellular factories” to manufacture desired chemicals and materials, depending on the biomachinery introduced into the organism.

metabolic pathways of bioorganisms will have to be accelerated. Attendant to all of these approaches will be the need for ingenious reactor designs that enable high mass fluxes of reactants and products and at the same time efficiently integrate solar power to catalyst systems, whether they be abiotic or hybrid biotic. Embodied in the catalysis and reactor designs are many challenges including the need to furnish products selectively, isolate those products, and capture CO2 at low energy intensities. Ultimately, any system must be stable and cost-effective over long-term operation. On this point of cost, over the last century, a multitrillion dollar infrastructure based on fossil fuels was put in place, and it has been paid off. Thus, it is unlikely that any discovery can be cost-competitive against this established fossil fuel infrastructure, unless policies such as those that price carbon are enacted. To this end, scientists will need to begin working in concert with policy makers to allow renewable energy markets to be established against the largess of the existing fossil fuel infrastructure. With a robust policy framework to accept renewable energy technology, the discovery emanating from energy science research will set a course for historic change to a global chemical and energy infrastructure powered directly by the sun.

■

AUTHOR INFORMATION

ORCID

Daniel G. Nocera: 0000-0001-5055-320X Notes

The author declares no competing financial interest.

■

ACKNOWLEDGMENTS The author thanks Timothy Cook for preparing the art in this article, and he acknowledges support of the U.S. Department of Energy, National Science Foundation, Air Force Office of Scientific Research. Funds to establish the First 100 W Program at Harvard University are also gratefully acknowledged.

■

REFERENCES

(1) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729−15735. (2) Powell, D. M.; Winkler, M. T.; Choi, H. J.; Simmons, C. B.; Needleman, D. B.; Buonassisi, T. Crystalline Silicon Photovoltaics: A Cost Analysis Framework for Determining Technology Pathways to 618

DOI: 10.1021/acs.accounts.6b00615 Acc. Chem. Res. 2017, 50, 616−619

Commentary

Accounts of Chemical Research (24) Torella, J. P.; Gagliardi, C. J.; Chen, J. S.; Bediako, D. K.; Colón, B.; Way, J. C.; Silver, P. A.; Nocera, D. G. Efficient Solar-to-Fuels Production from a Hybrid Microbial | Water-Splitting Catalyst System. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 2337−2342. (25) Nichols, E. M.; Gallagher, J. J.; Liu, C.; Su, Y.; Resascoe, J.; Yu, Y.; Sun, Y.; Yang, P.; Chang, M. C. Y.; Chang, C. J. Hybrid Bioinorganic Approach to Solar-to-Chemical Conversion. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 11461−11466. (26) Liu, C.; Colón, B. C.; Ziesack, M.; Silver, P. A.; Nocera, D. G. Water Splitting | Biosynthetic System with CO2 Reduction Efficiencies Exceeding Photosynthesis. Science 2016, 352, 1210−1213.

619

DOI: 10.1021/acs.accounts.6b00615 Acc. Chem. Res. 2017, 50, 616−619