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large-scale clean energy solution (let us also not forget that oxygen gas, the other product of water splitting, is not very valuable, at least on Earth). Other value-added chemicals, such as oxidants (H2O2, HClO, H2S2O8)6,7,11 or biomass-derived chemicals,12 could also be produced using PEC devices, sometimes still together with the production of hydrogen gas, as illustrated in Figure 1e. These chemical oxidants and/or valuable organic chemicals have higher economical values due to current challenges in their production or transportation; therefore, using sunlight to produce them locally with PEC devices likely has more chance to succeed economically first than PEC water splitting. Enabling PEC generation of these chemicals with higher efficiency and lower cost will require somewhat different sets of materials, different design strategies, and overcoming new scientific challenges. This could be a field that is ripe for intense fundamental study. Another interesting new direction is to integrate PEC photoelectrodes with biological organisms, such as chemicalproducing bacteria, to enable generation of more complex organic molecules.13 Tapping into the biological processes or even genetically engineering the natural processes for new functions could enable the production of chemicals with even higher value and that are not possible to produce using simple inorganic-based PEC systems with an even higher energy conversion efficiency than that of the purely biological systems.14 The biological organisms need aqueous solutions in PEC systems to survive and function well, and the inorganic photoelectrodes could filter out the high-energy photons harmful to organisms, but there are still many challenging scientific questions to address on the interface between biological systems and inorganic systems (such as photoelectrodes and electrolytes).15 There are two basic types of PEC cells:16 photoelectrosynthetic cells (Figure 1f), which include PEC water splitting, and regenerative PEC cells (Figure 1g). The latter include the well-known dye-sensitized solar cells (DSSCs), in which redox couples such as I−/I3− collect the photogenerated charge carriers at the surface of semiconductors and perform the opposite redox reaction at a counter electrode to generate electricity (Figure 1d). DSSCs have found some practical applications due to their low cost, flexible form factor, and ease of fabrication, and the fundamental understanding gained in DSSCs has helped the development of photoelectrosynthetic cells.9 Generally, in PEC devices, (liquid) electrolytes are in contact with semiconductor electrode materials, which poses challenges in terms of protecting electrodes from corrosion but also presents opportunities to perform the functions of charge transfer and collection differently from PV solar cells. For
unlight is the most abundant renewable energy source available on Earth, but it is intermittent and diffuse. In addition to the photovoltaic (PV) solar cell technology that effectively converts solar energy directly to electrical energy, solar-driven photoelectrochemical (PEC) water splitting for production of hydrogen and oxygen gases and PEC reduction of CO2 to fuels have been studied as promising approaches to provide affordable clean energy, reduce our reliance on fossil fuels, and mitigate the impact of climate change.1 They provide a chemical solution to the intermittency challenge as a large amount of energy can be stored in the chemical bonds of hydrogen or hydrocarbon fuels that could be later released and utilized in fuel cells or internal combustion engines. Ever since the seminal work on PEC water splitting in the 1970s,2 significant research progress has been made in the development of semiconductors, catalysts, integrated photoelectrodes, and devices for PEC water splitting,3 yet many scientific and technological challenges remain and no successful technologies have emerged.4 The interest in PEC CO2 reduction is more recent and also significant because it could provide the additional benefit of mitigating the effect of a rising CO2 level. These contemporary research interests are also well reflected in the papers published in ACS Energy Letters.5−9 Considering the commercial success of conventional PV solar cell technologies, their accelerating rate of deployment, and continuing decrease in the cost of solar electricity, it would be useful to think more about the potential pathways to practical, and commercially viable, applications of PEC solar energy conversion.4 The promise of artificial photosynthesis based on solar PEC devices has not yet been realized commercially, despite the fact that the biological photosynthetic process is responsible for the modern agriculture that produces food and bioenergy and also for the deposited fossil fuels over long geological timeframes. Considering the strengths and weaknesses of various competing technologies,4 what may be the unique benefits of PEC devices that could enable the realization of some applications with significant advantages or solve problems that are otherwise very difficult to solve? After decades of research, some re-evaluation and fresh thinking about alternative, or maybe even completely new, approaches for utilizing PEC designs might be productive for future progress, especially in the context of the current status of other related energy and chemical technologies. One thing is very clear: we can do much more with solar PEC devices beyond just splitting water and reducing CO2 (Figure 1a). In fact, arguments could be made that using solar energy to make high-volume and low-value chemicals such as hydrogen gas and hydrocarbons is less likely to be economically viable10 despite the significant promise of a © 2018 American Chemical Society
Published: October 12, 2018 2610
DOI: 10.1021/acsenergylett.8b01800 ACS Energy Lett. 2018, 3, 2610−2612
Editorial
Cite This: ACS Energy Lett. 2018, 3, 2610−2612
ACS Energy Letters
Editorial
Figure 1. Basic types of photoelectrochemical (PEC) solar energy conversion systems (photoelectrosynthetic cells shown in (f) vs regenerative PEC cells shown in (g)) and different ways that they can be used: (a) produce hydrogen from splitting water or hydrocarbons by CO2 reduction, (b) integrated with biological organisms to produce chemicals, (c) integrated with a redox flow battery in a solar flow battery (SFB), (d) directly output electricity in dye-sensitized solar cells, (e) produce other value-added chemicals, and (h) other potentially new approaches using PEC solar energy conversion. A single n-type semiconductor photoelectrode is shown for most schemes for the sake of simplicity but the complete systems may also include a photocathode.
facilitates the integration of separate solar energy conversion and storage functions into a single device, therefore bypassing the intermediate step of electricity generation. This may prove to be a more efficient, compact, and cost-effective approach to harvest, store, and utilize solar energy. Efficient SFBs can serve as distributed and standalone solar energy conversion and storage systems in remote locations and enable off-grid electrification. Recently, by building on high-performance tandem III−V solar cells and high-voltage organic RFBs, a monolithically integrated SFB with a high roundtrip solar-tooutput electricity efficiency (SOEE) above 14% has been demonstrated.19 This success illustrates the promise of SFBs by taking advantage of the strengths of PEC designs. Following the design principles that are rooted in fundamental photoelectrochemistry could lead to the realization of more efficient and less expensive SFBs using a broad range of mature or emerging PEC solar materials.20 It will always be difficult to “predict” technological breakthroughs (otherwise they probably would not be “breakthroughs”). To turn potentially promising technologies into practical successes, it helps to not be afraid of trying something new and to be open-minded about what the new approaches can and cannot do well and, importantly, to think about strategies carefully and learn from the iterative experience. Perhaps it is interesting to examine the case of liquid crystal displays (LCDs) that are ubiquitous nowadays. If we look at the early monochrome LCDs made in the 1980s, which were both extremely expensive and poor in resolution in comparison with the dominant cathode ray tube displays at the time, LCD’s dominance today would have been unthinkable. If it were not for LCDs being the only solution for laptop computer monitors, there would not have been the significant investment in research and development over the years that led to the dramatic improvement of LCD technology and
example, the kinetics of direct chemical conversion of many redox couples on the surface of PEC semiconductors can be very fast. However, much is still unknown about such fundamental processes at the semiconductor−electrolyte interface, such as the nature of surface states,17 which are unique to PEC devices. In contrast to DSSCs, the traditional liquid-junction PEC regenerative solar cells using inorganic semiconductors have not been able to compete with solid-state PV solar cells. However, regenerative PEC cells utilizing redox couples offer an interesting opportunity to address the grid-scale energy storage challenges due to the intermittent nature of sunlight. Over the last few decades, while PEC water splitting was studied in the laboratories, more efficient and less expensive PV solar cells have been deployed at an accelerating rate, such that further utilization of solar electricity (or other intermittent renewable energy) depends critically on inexpensive large-scale energy storage solutions to prevent the electric grid from becoming unstable. Among various energy storage technologies, redox flow batteries (RFBs) that utilize redox couples in liquid electrolytes are especially promising for large-scale grid-level energy storage due to their intrinsic scalability and lower cost.8 Recently, hybrid solar-charged storage devices that integrate regenerative PEC cells and RFBs that share the same pair of redox couples have been developed.18 In these integrated solar flow batteries (SFBs), solar energy is absorbed by semiconductors to directly charge up the redox couples at the photoelectrode surface (Figure 1c). When electricity is needed, the chemical energy stored in the redox couples can be discharged to generate the electricity, like in a RFB. Properly designed SFB devices can be charged under sunlight illumination without any external electric bias and deliver a high energy density comparable to that of state-ofthe-art RFBs over many cycles. The use of PEC systems 2611
DOI: 10.1021/acsenergylett.8b01800 ACS Energy Lett. 2018, 3, 2610−2612
ACS Energy Letters
Editorial
(13) Liu, C.; Gallagher, J. J.; Sakimoto, K. K.; Nichols, E. M.; Chang, C. J.; Chang, M. C. Y.; Yang, P. D. Nanowire-Bacteria Hybrids for Unassisted Solar Carbon Dioxide Fixation to Value-Added Chemicals. Nano Lett. 2015, 15, 3634−3639. (14) Liu, C.; Colon, 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. (15) Sakimoto, K. K.; Kornienko, N.; Cestellos-Blanco, S.; Lim, J.; Liu, C.; Yang, P. D. Physical Biology of the Materials-Microorganism Interface. J. Am. Chem. Soc. 2018, 140, 1978−1985. (16) Gratzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338−344. (17) Lin, F.; Boettcher, S. W. Adaptive Semiconductor/Electrocatalyst Junctions in Water-Splitting Photoanodes. Nat. Mater. 2014, 13, 81. (18) Li, W. J.; Fu, H. C.; Li, L. S.; Caban-Acevedo, M.; He, J. H.; Jin, S. Integrated Photoelectrochemical Solar Energy Conversion and Organic Redox Flow Battery Devices. Angew. Chem., Int. Ed. 2016, 55, 13104−13108. (19) Li, W.; Fu, H.-C.; Zhao, Y.; He, J.-H.; Jin, S. 14.1% Efficient Monolithically Integrated Solar Flow Battery. Chem. 2018, DOI: 10.1016/j.chempr.2018.08.023. (20) Sivula, K.; van de Krol, R. Semiconducting Materials for Photoelectrochemical Energy Conversion. Nat. Rev. Mater. 2016, 1, 15010.
reduction of cost. Being the uniquely enabling technology for mobile computing has enabled LCD technology to grow from an expensive niche product to eventually replacing massproduced large TVs, where economies of scale have helped as well. As an optimist, I feel confident that there will be a unique “killer application” for PEC solar energy conversion that can be commercially viable. Maybe we are just not imaginative enough (yet).
Song Jin,* Senior Editor, ACS Energy Letters
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Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Song Jin: 0000-0001-8693-7010 Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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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) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (3) 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. (4) Ardo, S.; Fernandez Rivas, D.; Modestino, M. A.; Schulze Greiving, V.; Abdi, F. F.; Alarcon Llado, E.; Artero, V.; Ayers, K.; Battaglia, C.; Becker, J.-P.; et al. Pathways to electrochemical solarhydrogen technologies. Energy Environ. Sci. 2018, 103, 15729. (5) Cheng, W.-H.; Richter, M. H.; May, M. M.; Ohlmann, J.; Lackner, D.; Dimroth, F.; Hannappel, T.; Atwater, H. A.; Lewerenz, H.-J. Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. ACS Energy Lett. 2018, 3, 1795−1800. (6) Mase, K.; Yoneda, M.; Yamada, Y.; Fukuzumi, S. Efficient Photocatalytic Production of Hydrogen Peroxide from Water and Dioxygen with Bismuth Vanadate and a Cobalt(II) Chlorin Complex. ACS Energy Lett. 2016, 1, 913−919. (7) Sayama, K. Production of High-Value-Added Chemicals on Oxide Semiconductor Photoanodes under Visible Light for Solar Chemical-Conversion Processes. ACS Energy Lett. 2018, 3, 1093− 1101. (8) Wei, X. L.; Pan, W. X.; Duan, W. T.; Hollas, A.; Yang, Z.; Li, B.; Nie, Z. M.; Liu, J.; Reed, D.; Wang, W.; Sprenkle, V. Materials and Systems for Organic Redox Flow Batteries: Status and Challenges. ACS Energy Lett. 2017, 2, 2187−2204. (9) Eom, Y. K.; Nhon, L.; Leem, G.; Sherman, B. D.; Wang, D.; Troian-Gautier, L.; Kim, S.; Kim, J.; Meyer, T. J.; Reynolds, J. R.; Schanze, K. S. Visible-Light-Driven Photocatalytic Water Oxidation by a π-Conjugated Donor−Acceptor−Donor Chromophore/Catalyst Assembly. ACS Energy Letters 2018, 3, 2114−2119. (10) Palmer, C.; Saadi, F.; McFarland, E. W. Technoeconomics of Commodity Chemical Production Using Sunlight. ACS Sustainable Chem. Eng. 2018, 6, 7003−7009. (11) Shi, X. J.; Zhang, Y. R.; Siahrostami, S.; Zheng, X. L. LightDriven BiVO4-C Fuel Cell with Simultaneous Production of H2O2. Adv. Energy Mater. 2018, 8, 1801158. (12) Cha, H. G.; Choi, K.-S. Combined Biomass Valorization and Hydrogen Production in a Photoelectrochemical Cell. Nat. Chem. 2015, 7, 328−333. 2612
DOI: 10.1021/acsenergylett.8b01800 ACS Energy Lett. 2018, 3, 2610−2612
