Holy Grails of Solar Photochemistry
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skites and the reasons behind observing relatively low efficiency in such materials.6 We expect to see new research advances to address the challenges of solar fuel production and emerging technologies for photovoltaics. Perovskite solar cells, organic photovoltaics, and solar fuels were the key topics discussed during the recent KAUST Research Conference 2016: Emerging Concepts and Materials in Solar Energy Conversion at King Abdullah University of Science & Technology, Saudi Arabia (Oct. 31−Nov. 2, 2016). The discussions focused on recent developments in each of these areas, and the presenters pointed out the need to further explore optical and electronic properties of advanced energy materials. On behalf of ACS Energy Letters, I presented the young scientist presentation award to Xiwen Gong from the University of Toronto. Prof. Kirk Schanze, EIC, ACS Applied Materials & Interfaces and Prof. Jillian Buriak, EIC, Chemistry of Materials presented two other awards sponsored by the ACS (see Figure 1). I also would like to take this opportunity to introduce our new Senior Editor, Prof. Lin Chen of Northwestern University and Argonne National Laboratory. Prof. Chen has been an active researcher in artificial photosynthesis and photoinduced charge transfer processes. Two major research areas that she is currently engaged in are (a) excited/transient state structure, dynamics, and function correlations in transition metal complexes for solar fuel and electricity generation and (b) electronic processes in organic materials for photovoltaic and optoelectronic applications. Fundamental energy conversion processes in the context of light−matter interactions from Xrays to terahertz radiation continue to be a major theme of her research. We welcome Prof. Chen to ACS Energy Letters’ team of editors.
hotocatalytic water splitting to produce hydrogen and CO2 reduction to produce C1 fuels (carbon monoxide, formic acid, methanol, methane) have been the holy grails of solar photochemistry in the new millennium. The thermodynamic and kinetic challenges to achieve multiple proton-coupled electron transfer are well-known, and pursuit continues to design suitable photocatalysts and electrocatalysts to address them. Despite 40 year long research efforts, we have yet to make a major breakthrough from the point of view of a practical device. Yet, every week, we see a media headline proclaiming the discovery of a new photocatalyst or the design of a nanostructure assembly as breakthrough research to split water into hydrogen or reduce CO2 into C1 fuels. Often, these coverages end up with an optimistic statement similar to “... the new discovery may lead to increased efficiency of energy conversion and meet the clean energy demand in the future.” How valid are such statements when we have yet to develop a prototype reactor to demonstrate the feasibility of such an approach? Although many laboratory-scale experiments have reported significant progress in terms of catalyst design, scaling up these processes for real-world application is a daunting task. Any new design used to mitigate the problem should be economical and more efficient than simple photovoltaic-driven electrolysis. Our authors should keep this in mind while claiming “high efficiency” or “breakthrough performance” of photocatalysts in their work. In the previous issue of ACS Energy Letters, we published three Viewpoints from leading experts who discussed issues related to CO2 reduction as fuels. Profs. Parkinson and Osterloh discussed the limitations of using CO2 as a feedstock for generating C1 fuels through photocatalysis, while, on the other hand, Prof. Robert made a case for the usefulness of electroactive catalysts that can assist in CO2 reduction.1−3 The important points discussed in these Viewpoints should be taken into consideration while making claims of CO2 mitigation through solar photochemistry. The Review article by Serpone and co-workers pointed out the reasons behind low hydrogen and oxygen yields in a photocatalytic water splitting process.4 The other holy grail of solar photochemistry is the development of advanced photovoltaic technologies that can compete with conventional silicon photovoltaics. On the basis of advances made in dye-sensitized solar cells (DSSCs), organic photovoltaics (OPVs), and quantum dot solar cells (QDSCs), we have now succeeded in making lead halide perovskite solar cells with efficiencies as high as 22%. The performance of these cells directly competes with that of other thin-film photovoltaics (e.g., CIGS, CdTe). Efforts are underway to tune the band gap of these materials through variation in the halide composition, to address stability issues through cation interplay, and to develop non-lead-based perovskite materials. In this issue is a Perspective by Slotcavage, Karunadasa, and McGehee, which addresses halide ion segregation in mixed halide perovskites and discusses the origin of light-induced migration of bromide and iodide ions.5 In their Perspective, Giustino and Snaith discuss opportunities to develop non-lead-based perov© 2016 American Chemical Society
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RELATED READINGS (1) Parkinson, B. Advantages of Solar Hydrogen Compared to Direct Carbon Dioxide Reduction for Solar Fuel Production. Published: December 9, 2016 1273
DOI: 10.1021/acsenergylett.6b00590 ACS Energy Lett. 2016, 1, 1273−1274
Editorial
http://pubs.acs.org/journal/aelccp
ACS Energy Letters
Editorial
Figure 1. Editors and KAUST Research Conference 2016 award winners. L−R: Kirk Schanze, Ibrahim Dursun (KAUST), Tatsuya Shinagawa (KAUST), Nicholas Davy (Princeton University), Xiwen Gong (University of Toronto), Jillian Buriak, and Prashant Kamat. (Photo courtesy of P. Kamat.)
ACS Energy Lett. 2016, 1, 1057−1059. DOI: 10.1021/ acsenergylett.6b00377 (2) Osterloh, F. E. The Low Concentration of CO2 in the Atmosphere is an Obstacle to a Sustainable Artificial Photosynthesis Fuel Cycle Based on Carbon. ACS Energy Lett. 2016, 1, 1060−1061. DOI: 10.1021/acsenergylett.6b00493. (3) Tatin, A.; Bonin, J.; Robert, M. A Case for Electrofuels. ACS Energy Lett. 2016, 1, 1062−1064. DOI: 10.1021/ acsenergylett.6b00510. (4) Serpone, N.; Emeline, A. V.; Ryabchuk, V. K.; Kuznetsov, V. N.; Artem’ev, Y. M.; Horikoshi, S. Why Do Hydrogen and Oxygen Yields from Semiconductor-Based Photocatalyzed Water Splitting Remain Disappointingly Low? Intrinsic and Extrinsic Factors Impacting Surface Redox Reactions. ACS Energy Lett. 2016, 1, 931−948. DOI: 10.1021/acsenergylett.6b00391. (5) Slotcavage, D. J.; Karunadasa, H. I.; McGehee, M. D. Light-Induced Phase Segregation in Halide−Perovskite Absorbers. ACS Energy Lett. 2016, 1, XXXX−XXXX. DOI: 10.1021/acsenergylett.6b00495. (6) Giustino, F.; Snaith, H. Toward Lead-Free Perovskite Solar Cells. ACS Energy Lett. 2016, 1, XXXX−XXXX. DOI: 10.1021/acsenergylett.6b00499.
Prashant V. Kamat, Editor-in-Chief, ACS Energy Letters
University of Notre Dame, Notre Dame, Indiana 46556, United States
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AUTHOR INFORMATION
Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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DOI: 10.1021/acsenergylett.6b00590 ACS Energy Lett. 2016, 1, 1273−1274
