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• W2893025345 abstract "•Monolithic integration of solar energy conversion, storage, and electricity delivery•Record solar-to-output electricity efficiency of 14.1%•General design principles for further development of highly efficient integrated device Because of the intermittent nature of sunlight, the design of practical round-trip solar energy utilization systems requires both efficient solar energy conversion and storage. Compared with separated solar energy conversion and storage devices, combining the functions of separated devices into a single device allows us to bypass the intermediate step of electricity generation, which represents a more efficient, compact, and cost-effective approach to utilizing solar energy. Here, we present a monolithically integrated solar flow battery device that builds on III-V solar cells and organic redox species. The excellent performance of this device and the general design principles proposed here promise a general approach for storing the intermittent solar energy electrochemically with high storage capacity and efficiency, which will accelerate the large-scale deployment of solar energy technologies, especially in remote locations, to enable practical off-grid electrification. Challenges posed by the intermittency of solar energy source necessitate the integration of solar energy conversion with scalable energy storage systems. The monolithic integration of photoelectrochemical solar energy conversion and electrochemical energy storage offers an efficient and compact approach toward practical solar energy utilization. Here, we present the design principles for and the demonstration of a highly efficient integrated solar flow battery (SFB) device with a record solar-to-output electricity efficiency of 14.1%. Such SFB devices can be configured to perform all the requisite functions from solar energy harvest to electricity redelivery without external bias. Capitalizing on high-efficiency and high-photovoltage tandem III-V photoelectrodes that are properly matched with high-cell-voltage redox flow batteries and carefully designed flow field architecture, we reveal the general design principles for efficient SFBs. These results will enable a highly efficient approach for practical off-grid solar utilization and electrification. Challenges posed by the intermittency of solar energy source necessitate the integration of solar energy conversion with scalable energy storage systems. The monolithic integration of photoelectrochemical solar energy conversion and electrochemical energy storage offers an efficient and compact approach toward practical solar energy utilization. Here, we present the design principles for and the demonstration of a highly efficient integrated solar flow battery (SFB) device with a record solar-to-output electricity efficiency of 14.1%. Such SFB devices can be configured to perform all the requisite functions from solar energy harvest to electricity redelivery without external bias. Capitalizing on high-efficiency and high-photovoltage tandem III-V photoelectrodes that are properly matched with high-cell-voltage redox flow batteries and carefully designed flow field architecture, we reveal the general design principles for efficient SFBs. These results will enable a highly efficient approach for practical off-grid solar utilization and electrification. The practical utilization of solar energy demands not only efficient energy conversion but also inexpensive large-scale energy storage to accommodate the intermittency of sunlight.1Lewis N.S. Research opportunities to advance solar energy utilization.Science. 2016; 351: aad1920Crossref PubMed Scopus (1122) Google Scholar Natural photosynthesis represents a promising approach to efficiently utilize solar energy by converting and storing solar energy in chemical bonds. Studies since the 1970s2Honda K. Fujishima A. Photolysis-decomposition of water at the surface of an irradiated semiconductor.Nature. 1972; 238: 37-38Crossref PubMed Scopus (26245) Google Scholar have shown that artificial photosynthesis can also be accomplished with semiconductors in direct contact with liquid electrolytes to perform photoelectrolysis.3Walter 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: 6446-6473Crossref PubMed Scopus (7542) Google Scholar, 4Nocera D.G. The artificial leaf.Acc. Chem. Res. 2012; 45: 767-776Crossref PubMed Scopus (1358) Google Scholar While much research effort has focused on storing solar energy in molecular fuels by irreversible photoelectrochemical (PEC) reactions, such as PEC water splitting3Walter 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: 6446-6473Crossref PubMed Scopus (7542) Google Scholar, 4Nocera D.G. The artificial leaf.Acc. Chem. Res. 2012; 45: 767-776Crossref PubMed Scopus (1358) Google Scholar and carbon dioxide reduction reaction,5Montoya J.H. Seitz L.C. Chakthranont P. Vojvodic A. Jaramillo T.F. Norskov J.K. Materials for solar fuels and chemicals.Nat. Mater. 2017; 16: 70-81Crossref Scopus (927) Google Scholar the great versatility of semiconductor-based photoelectrolysis also permits reversible redox couples to be used as solar energy storage media.6Luttmer J.D. Konrad D. Trachtenberg I. Electrode materials for hydrobromic acid electrolysis in Texas-Instruments solar chemical converter.J. Electrochem. Soc. 1985; 132: 1054-1058Crossref Scopus (28) Google Scholar, 7Heller A. Miller B. Thiel F.A. 11.5% solar conversion efficiency in the photocathodically protected p-InP/V3+-V2+-HCl/C semiconductor liquid junction cell.Appl. Phys. Lett. 1981; 38: 282-284Crossref Scopus (110) Google Scholar Moreover, reversible electrochemical reactions are also exactly what happens during the energy storage process in rechargeable batteries.8Yang Z. Zhang J. Kintner-Meyer M.C.W. Lu X. Choi D. Lemmon J.P. Liu J. Electrochemical energy storage for green grid.Chem. Rev. 2011; 111: 3577-3613Crossref PubMed Scopus (3748) Google Scholar In this way, the PEC solar energy conversion process can be seamlessly connected with rechargeable batteries by the common reversible redox reactions they share to realize an integrated device that can be directly charged by solar light and discharged like normal batteries when needed. The concept of the “solar rechargeable battery” was perhaps first demonstrated in 1976 with a polycrystalline CdSe photoelectrode and silver-silver sulfide solid battery electrode.9Hodes G. Manassen J. Cahen D. Photoelectrochemical energy conversion and storage using polycrystalline chalcogenide electrodes.Nature. 1976; 261: 403-404Crossref Scopus (411) Google Scholar Since then, various approaches toward integrated solar energy conversion and storage have been developed.10Schmidt D. Hager M.D. Schubert U.S. Photo-rechargeable electric energy storage systems.Adv. Energy Mater. 2015; 6: 1500369Crossref Scopus (122) Google Scholar, 11Yu M. McCulloch W.D. Huang Z. Trang B.B. Lu J. Amine K. Wu Y. Solar-powered electrochemical energy storage: an alternative to solar fuels.J. Mater. Chem. A. 2016; 4: 2766-2782Crossref Google Scholar, 12Gurung A. Qiao Q. Solar charging batteries: advances, challenges, and opportunities.Joule. 2018; 2: 1-14Abstract Full Text Full Text PDF Scopus (170) Google Scholar For example, common rechargeable batteries such as lithium-ion batteries,13Paolella A. Faure C. Bertoni G. Marras S. Guerfi A. Darwiche A. Hovington P. Commarieu B. Wang Z. Prato M. et al.Light-assisted delithiation of lithium iron phosphate nanocrystals towards photo-rechargeable lithium ion batteries.Nat. Commun. 2017; 8: 14643Crossref PubMed Scopus (123) Google Scholar batteries based on other inorganic chemistry,14Licht S. Hodes G. Tenne R. Manassen J. A light-variation insensitive high efficiency solar cell.Nature. 1987; 326: 863-864Crossref Scopus (117) Google Scholar and redox flow batteries (RFBs)15Cheng Q. Fan W. He Y. Ma P. Vanka S. Fan S. Mi Z. Wang D. Photorechargeable high voltage redox battery enabled by Ta3N5 and GaN/Si dual-photoelectrode.Adv. Mater. 2017; 351: 1700312-1700318Crossref Scopus (50) Google Scholar, 16Yu M. McCulloch W.D. Beauchamp D.R. Huang Z. Ren X. Wu Y. Aqueous lithium-iodine solar flow battery for the simultaneous conversion and storage of solar energy.J. Am. Chem. Soc. 2015; 137: 8332-8335Crossref PubMed Scopus (122) Google Scholar, 17Liao S. Zong X. Seger B. Pedersen T. Yao T. Ding C. Shi J. Chen J. Li C. Integrating a dual-silicon photoelectrochemical cell into a redox flow battery for unassisted photocharging.Nat. Commun. 2016; 7: 11474-11478Crossref PubMed Scopus (103) Google Scholar, 18Wedege K. Azevedo J. Khataee A. Bentien A. Mendes A. Direct solar charging of an organic-inorganic, stable, and aqueous alkaline redox flow battery with a hematite photoanode.Angew. Chem. Int. Ed. 2016; 55: 7142-7147Crossref PubMed Scopus (84) Google Scholar, 19Liu P. Cao Y.l. Li G.R. Gao X.P. Ai X.P. Yang H.X. A solar rechargeable flow battery based on photoregeneration of two soluble redox couples.ChemSusChem. 2013; 6: 802-806Crossref PubMed Scopus (92) Google Scholar, 20Wedege K. Bae D. Dražević E. Mendes A. Vesborg P.C.K. Bentien A. Unbiased, complete solar charging of a neutral flow battery by a single Si photocathode.RSC Adv. 2018; 8: 6331-6340Crossref Google Scholar, 21Zhou Y. Zhang S. Ding Y. Zhang L. Zhang C. Zhang X. Zhao Y. Yu G. Efficient solar energy harvesting and storage through a robust photocatalyst driving reversible redox reactions.Adv. Mater. 2018; 103: 1802294-1802297Crossref Scopus (44) Google Scholar can be integrated with different types of solar cells. Among these, the integration of PEC cells with RFBs is particularly attractive because of the wide selection of redox couples22Ding Y. Li Y.F. Yu G.H. Exploring bio-inspired quinone-based organic redox flow batteries: a combined experimental and computational study.Chem. 2016; 1: 790-801Abstract Full Text Full Text PDF Scopus (169) Google Scholar, 23Ding Y. Yu G. A bio-inspired, heavy-metal-free, dual-electrolyte liquid battery towards sustainable energy storage.Angew. Chem. Int. Ed. 2016; 55: 4772-4776Crossref PubMed Scopus (112) Google Scholar, 24Huskinson B. Marshak M.P. Suh C. Er S. Gerhardt M.R. Galvin C.J. Chen X. Aspuru-Guzik A. Gordon R.G. Aziz M.J. A metal-free organic-inorganic aqueous flow battery.Nature. 2014; 505: 195-198Crossref PubMed Scopus (1027) Google Scholar, 25Kwabi D.G. Lin K. Ji Y. Kerr E.F. Goulet M.-A. De Porcellinis D. Tabor D.P. Pollack D.A. Aspuru-Guzik A. Gordon R.G. et al.Alkaline quinone flow battery with long lifetime at pH 12.Joule. 2018; 2https://doi.org/10.1016/j.joule.2018.07.005Abstract Full Text Full Text PDF Scopus (221) Google Scholar and ease of scaling up the energy storage capacity in RFBs.26Park M. Ryu J. Wang W. Cho J. Material design and engineering of next-generation flow-battery technologies.Nat. Rev. Mater. 2016; 2: 16080Crossref Scopus (418) Google Scholar, 27Weber A.Z. Mench M.M. Meyers J.P. Ross P.N. Gostick J.T. Liu Q. Redox flow batteries: a review.J. Appl. Electrochem. 2011; 41: 1137-1164Crossref Scopus (1397) Google Scholar, 28Ding Y. Zhang C. Zhang L. Zhou Y. Yu G. Molecular engineering of organic electroactive materials for redox flow batteries.Chem. Soc. Rev. 2018; 47: 69-103Crossref PubMed Google Scholar Recently, by integrating silicon solar cells and all-organic quinone-based RFBs, the proof of concept for a bias-free solar energy conversion and electrochemical storage was demonstrated in a solar flow battery (SFB).29Li W. Fu H.-C. Li L. 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: 13104-13108Crossref PubMed Scopus (87) Google Scholar However, despite the much higher solar conversion efficiency of the silicon solar cells employed, this prototype device could only achieve a modest solar-to-output electricity efficiency (SOEE) of 1.7%, which is not sufficient for practical applications. This and other examples make it clear that simply integrating high-performance solar cells and RFBs does not necessarily guarantee an SFB with a high SOEE. With comprehensive mechanism study and deeper understanding of the operation principles of SFBs, we now propose a set of design principles for highly efficient integrated SFB devices. Generally, the photoelectrode used for solar energy conversion devices can be categorized into two types, semiconductor-liquid junction cells30Sivula K. van de Krol R. Semiconducting materials for photoelectrochemical energy conversion.Nat. Rev. Mater. 2016; 1: 15010Crossref Scopus (965) Google Scholar, 31Cha H.G. Choi K.S. Combined biomass valorization and hydrogen production in a photoelectrochemical cell.Nat. Chem. 2015; 7: 328-333Crossref PubMed Scopus (411) Google Scholar, 32Kamat P.V. Tvrdy K. Baker D.R. Radich J.G. Beyond photovoltaics: semiconductor nanoarchitectures for liquid-junction solar cells.Chem. Rev. 2010; 110: 6664-6688Crossref PubMed Scopus (680) Google Scholar and photovoltaic (PV) cells.33Khaselev O. Turner J.A. A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting.Science. 1998; 280: 425-427Crossref PubMed Scopus (1907) Google Scholar For semiconductor-liquid junction cells, energy-level matching between semiconductors and redox species is critical as it determines the photovoltage of such cells.21Zhou Y. Zhang S. Ding Y. Zhang L. Zhang C. Zhang X. Zhao Y. Yu G. Efficient solar energy harvesting and storage through a robust photocatalyst driving reversible redox reactions.Adv. Mater. 2018; 103: 1802294-1802297Crossref Scopus (44) Google Scholar On the other hand, the photovoltage of PV cells is generated by their internal solid-state junctions, and are thus insensitive to the redox potential of the specific redox couple used. By utilizing PV cells the difficulties in the overall device design and voltage matching can be greatly reduced, which makes it a good choice for the purpose of proof-of-principle demonstration. Here, we present a high-efficiency, monolithically integrated SFB device with a record average SOEE of 14.1% and demonstrate that solar energy harvest, conversion, storage, and redelivery can be completed by such a single integrated device without any external electrical energy input. This highly efficient SFB is enabled by high-photovoltage and highly efficient III-V tandem solar cells, carefully matching them with high-voltage RFBs, and dedicatedly designed zero-gap flow field architecture. Building a highly efficient integrated SFB device starts from designing the general structure of the device, followed by developing and studying the individual components that fit well with the general structure. As illustrated in Figure 1A, we design a simple three-electrode device by incorporating a semiconductor photoelectrode into the conventional two-electrode device design that has been used for most RFBs.26Park M. Ryu J. Wang W. Cho J. Material design and engineering of next-generation flow-battery technologies.Nat. Rev. Mater. 2016; 2: 16080Crossref Scopus (418) Google Scholar Consequently, this device can be operated as a normal RFB with only two carbon felt-based inert electrodes to charge and discharge the redox active species in the liquid electrolytes (Figure 1C), which are constantly circulated between the device and external storage tanks by pumps. More importantly, the charging of this device can also be accomplished by illuminating the photoelectrode with solar light to allow the harvest of photogenerated carriers by redox active species at the semiconductor-liquid electrolyte interface (Figure 1D). We can also operate this device just as a PV solar cell by cycling the redox couples between the photoelectrode and the counter electrode to directly extract the electricity (Figure 1E), which is how regenerative PEC liquid junction solar cells work.7Heller A. Miller B. Thiel F.A. 11.5% solar conversion efficiency in the photocathodically protected p-InP/V3+-V2+-HCl/C semiconductor liquid junction cell.Appl. Phys. Lett. 1981; 38: 282-284Crossref Scopus (110) Google Scholar To improve the flow dynamics of electrolyte as well as minimize ionic and contact resistance between each component, we configure the new integrated device to allow the membrane, electrode, and current collector to come in direct contact (Figure S1), resembling the zero-gap flow field cell architecture of RFBs.34Aaron D.S. Liu Q. Tang Z. Grim G.M. Papandrew A.B. Turhan A. Zawodzinski T.A. Mench M.M. Dramatic performance gains in vanadium redox flow batteries through modified cell architecture.J. Power Sources. 2012; 206: 450-453Crossref Scopus (358) Google Scholar We chose low-cost organic redox couples 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-OH-TEMPO) and methyl viologen (MV) as the anolyte and the catholyte, respectively, for the integrated SFB device. By reason of their proper formal potential (E0) matching, the combination of 4-OH-TEMPO and MV enabled a demonstration of RFBs with an exceptionally high cell voltage of 1.25 V (Figure 2A), which is a significantly high value for aqueous organic RFBs.35Liu T. Wei X. Nie Z. Sprenkle V. Wang W. A total organic aqueous redox flow battery employing a low cost and sustainable methyl viologen anolyte and 4-HO-TEMPO catholyte.Adv. Energy Mater. 2015; 6: 1501449Crossref Scopus (419) Google Scholar Although the potential difference between the two redox couples already reach the limit of thermodynamic water-splitting potential (1.23 V), the large overpotentials of water oxidation and reduction reactions on carbon-based electrodes under neutral condition leave at least 400 mV on each side to practically operate the anodic and cathodic redox reactions without electrolysis of water. Given the large E0 difference between the two redox couples, we utilized a high-photovoltage triple-junction III-V tandem photoelectrode that consists of an InGaP top cell (Eg = 1.85 eV), a GaAs middle cell (Eg = 1.42 eV), and a Ge bottom cell (Eg = 0.67 eV) (Figure 1B; for more device details see Figure S2).36Huang C.-W. Liao C.-H. Wu C.-H. Wu J.C.S. Photocatalytic water splitting to produce hydrogen using multi-junction solar cell with different deposited thin films.Sol. Energy Mater. Sol. Cells. 2012; 107: 322-328Crossref Scopus (19) Google Scholar Such monolithic III-V tandem heterojunctions have been proved to be the best for high-efficiency solar cells and furthermore have been shown to be an excellent candidate for PEC water splitting because of their near-ideal band-gap energy and adsorption-spectrum match with solar irradiation.33Khaselev O. Turner J.A. A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting.Science. 1998; 280: 425-427Crossref PubMed Scopus (1907) Google Scholar, 37Verlage E. Hu S. Liu R. Jones R.J.R. Sun K. Xiang C. Lewis N.S. Atwater H.A. A monolithically integrated, intrinsically safe, 10% efficient, solar-driven water-splitting system based on active, stable earth-abundant electrocatalysts in conjunction with tandem III-V light absorbers protected by amorphous TiO2 films.Energy Environ. Sci. 2015; 8: 3166-3172Crossref Google Scholar Moreover, the III-V tandem cell can provide a high photovoltage (2.4 V) out of a single cell. This high photovoltage, although not specifically critical for PV cells as the panel voltage can be easily increased by series tandem, is a key beneficial feature in integrated SFB device design to enable efficient photocharging of the device without external bias and a simpler three-electrode SFB device design (as shown in Figure 1A). Compared with the four-electrode SFBs previously demonstrated,29Li W. Fu H.-C. Li L. 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: 13104-13108Crossref PubMed Scopus (87) Google Scholar the three-electrode SFBs are easier to fabricate and operate because only one photoelectrode is needed and illumination comes from one side. Another advantage of these 4-OH-TEMPO and MV redox couples is that they both have relatively large solubility (>0.5 M) in neutral solution; thus, using neutral rather than acidic or alkaline electrolyte greatly reduces the corrosiveness of the electrolyte, and more stable devices readily achieved. We then studied the redox couples, which serve as the bridge connecting photocharging and electrical discharging processes. The E0 for 4-OH-TEMPO and MV redox couples are 0.80 V and −0.45 V in 2 M NaCl solution, respectively (Figure 2A). Besides the proper E0, the electrochemical kinetics and reversibility of the redox couples are also important for the power capability of RFB27Weber A.Z. Mench M.M. Meyers J.P. Ross P.N. Gostick J.T. Liu Q. Redox flow batteries: a review.J. Appl. Electrochem. 2011; 41: 1137-1164Crossref Scopus (1397) Google Scholar and even more important for the efficient charge transfer from the semiconductors to electrolytes in PEC devices.29Li W. Fu H.-C. Li L. 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: 13104-13108Crossref PubMed Scopus (87) Google Scholar Detailed cyclic voltammetry studies at various scan rates (Figure S3) revealed that both redox couples have remarkable electrochemical reversibility and rapid diffusion rate, similar to that of other commonly used fast organic redox couples, such as quinones.24Huskinson B. Marshak M.P. Suh C. Er S. Gerhardt M.R. Galvin C.J. Chen X. Aspuru-Guzik A. Gordon R.G. Aziz M.J. A metal-free organic-inorganic aqueous flow battery.Nature. 2014; 505: 195-198Crossref PubMed Scopus (1027) Google Scholar Building on the excellent electrochemical properties of the redox couples, we tested the RFB by using 0.1 M 4-OH-TEMPO as anolyte and 0.1 M MV as catholyte, both with 2 M NaCl as supporting electrolyte. The RFB charge-discharge cycling test was carried out in the device shown in Figure S1 at desired constant current density with cutoff voltages of 0.5 and 1.5 V. The representative cycling behavior at 20 mA cm−2 (Figure 2B) shows a stable voltage profile over at least ten cycles with an average open-circuit voltage of around 1.2 V. The rate performance study of the RFB at various current densities (Figure 2C) shows that the current efficiency (CE) stays at 99% for all the rates while the energy efficiency drops from 91.9% to 73.0% as the rate increases from 10 to 50 mA cm−2. The RFB cycling performance achieved by our SFB device is comparable with that of the previously demonstrated 4-OH-TEMPO/MV RFB,35Liu T. Wei X. Nie Z. Sprenkle V. Wang W. A total organic aqueous redox flow battery employing a low cost and sustainable methyl viologen anolyte and 4-HO-TEMPO catholyte.Adv. Energy Mater. 2015; 6: 1501449Crossref Scopus (419) Google Scholar indicating that, although not specifically optimized for RFB performance, the design of this integrated SFB device is competitive with the state-of-the-art RFB architecture. We then characterized the performance of the solid-state III-V tandem solar cell under 1 sun (100 mW cm−2) of AM 1.5G simulated solar illumination. The linear sweep voltammetry curve in Figure 3A shows that the solid-state tandem cell exhibits an open-circuit potential (Voc) of 2.41 V, a short-circuit current density (Jsc) of 12.72 mA cm−2, a fill factor (FF) of 85.0%, and a power conversion efficiency (PCE) of 26.1%. Previous efforts to use III-V semiconductors in PEC cells showed that III-V semiconductors are very prone to photocorrosion in aqueous electrolytes, especially under extreme pH conditions that are conducive to PEC water electrolysis.36Huang C.-W. Liao C.-H. Wu C.-H. Wu J.C.S. Photocatalytic water splitting to produce hydrogen using multi-junction solar cell with different deposited thin films.Sol. Energy Mater. Sol. Cells. 2012; 107: 322-328Crossref Scopus (19) Google Scholar Although the neutral electrolyte adopted in our SFB device partially alleviated the corrosive attack by H+ or OH− ions, a surface protection layer for the photoelectrode was still required to achieve stable operation. TiO2 has been widely used as the protection layer and has shown good stability and low charge-transfer resistance in aqueous electrolytes under various pH conditions.38Mei B. Pedersen T. Malacrida P. Bae D. Frydendal R. Hansen O. Vesborg P.C.K. Seger B. Chorkendorff I. Crystalline TiO2: a generic and effective electron-conducting protection layer for photoanodes and -cathodes.J. Phys. Chem. C. 2015; 119: 15019-15027Crossref Scopus (76) Google Scholar, 39Hu S. Shaner M.R. Beardslee J.A. Lichterman M. Brunschwig B.S. Lewis N.S. Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation.Science. 2014; 344: 1005-1009Crossref PubMed Scopus (1049) Google Scholar Therefore, we deposited a Ti/TiO2 (5/40 nm) thin film on the back side of the III-V cell (Ge bottom cell side) by using sputter coating and atomic layer deposition (ALD) to protect the photoelectrode. A 5 nm layer of Pt was then sputter coated on top of the TiO2 film to provide a stable ohmic contact between TiO2 and electrolyte (see Figure 1B). As illustrated in Figures 1D and 1E, we can configure the integrated SFB device to two different solar modes under illumination. Under solar cell mode (Figure 1E), the photoelectrode and anode are connected to allow photo-oxidation of 4-OH-TEMPO at the surface of photoelectrode and reduction of [4-OH-TEMPO]+ at the surface of anode, just like regenerative PEC liquid junction solar cells.7Heller A. Miller B. Thiel F.A. 11.5% solar conversion efficiency in the photocathodically protected p-InP/V3+-V2+-HCl/C semiconductor liquid junction cell.Appl. Phys. Lett. 1981; 38: 282-284Crossref Scopus (110) Google Scholar Thus, the solar energy input can be directly converted and delivered as electrical energy output to power external load. The PEC performance of the tandem photoelectrode in 0.1 M 4-OH-TEMPO aqueous solution under 1-sun simulated illumination by an EKE-type lamp (solid blue curve in Figure 3B) was very close to the J-V performance of the solid-state PV cell (Figure 3A), especially for the Voc and Jsc. Note that Figure 3B is displayed in current, not current density, as the areas of photoelectrode (∼0.4 cm2) and the carbon felt RFB electrode (4 cm2) are different. The lower FF of the photoelectrode (60.3%) in comparison with that of the solid-state cell may be attributed to the mass transport losses of the 4-OH-TEMPO redox couple and the electrolyte ohmic losses between the photoelectrode and anode, which is commonly observed in many PEC cells.3Walter 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: 6446-6473Crossref PubMed Scopus (7542) Google Scholar In contrast, under solar recharge mode (Figure 1D), the photoelectrode and cathode are connected to drive the photo-oxidation of 4-OH-TEMPO at photoelectrode and simultaneous reduction of MV2+ at carbon felt cathode. Solar energy can be harvested by the photoelectrode and stored as chemical energy by the redox reactions under solar recharge mode and released under RFB mode (Figure 1C) as electrical energy when needed. The dashed blue curve in Figure 3B shows the PEC performance of the photoelectrode under solar recharge mode, which can be well matched by cathodically offsetting the solid blue curve. The potential offset between the two PEC I-V curves (∼1.0 V) comes from the equilibrium potential (Eeq) difference between 4-OH-TEMPO and MV redox couples at the specific state of charge (SOC) where the measurements were performed, which agrees well with the open-circuit voltage of the RFB tested at the same SOC (red line in Figure 3B). By overlaying the polarization curve of the RFB and the I-V curve of the photoelectrode under solar cell mode, the operation point of the integrated SFB can be found as the intersection of the two curves. From the overlaid I-V curves shown in Figure 3B, we can estimate a bias-free solar recharging current of 5.56 mA for the integrated SFB device. Moreover, we use a specific figure of merit, SOEE, to evaluate the overall efficiency of the SFB device, which is defined asSOEE(%)=EdischargingEillumination=∫IoutVoutdt∫SAdt,(Equation 1) where Edischarging is the usable electrical energy delivered by the integrated SFB device and Eillumination is the total solar energy input.29Li W. Fu H.-C. Li L. 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: 13104-13108Crossref PubMed Scopus (87) Google Scholar If the RFB polarization curve intersects with the plateau part of the photoelectrode J-V curve (see an example in Figure 3B), the SOEE of the integrated device can be estimated with the following equation:estimatedSOEE=Jsc(photo)×Voc(RFB)×CE×VES,(Equation 2) where Jsc (photo) is the short-circuit current density of the photoelectrode, Voc(RFB) is the open-circuit voltage of the RFB, and CE and VE are the estimated current efficiency and voltage efficiency of the SFB. From the data shown in Figure 3B, we can estimate an SOEE of 13.3% for the SFB device (see calculation details in the Experimental Procedures). In light of the excellent and reproducible performance of the RFB as well as the good performance from the tandem III-V photoelectrode, we built the integrated SFB devices by using the same RFB and PEC components (Figure S1). The cycling behavior of the SFB was characterized with two potentiostats configured to solar recharge mode and RFB mode to monitor the photocurrent delivered by the photoelectrode and the cell potential of the integrated SFB device, respectively. The blue curve in Figure 4A shows that the photocurrent density under 1-sun illumination during the unassisted photocharging process stays at 14.5 mA cm−2 over ten photocharging cycles with a fluctuation of ±1 mA cm−2 that is likely due to the instability of the light source. After each photocharging cycle, we discharged the device by applying a discharging current of −10 mA until the cell potential reached 0.5 V. The CE and VE of the SFB can be calculated by the same methods used for normal RFBs. Figure S4 shows that the integrated device features both high CE and VE with average efficiencies of 96.2% and 96.6% over ten cycles, respectively. On the basis of the cycling data, we can calculate the actual SOEE for the SFB to directly evaluate its overall efficiency. The integrated SFB device achieved a stable SOEE over ten cycles with an average of 14.1%, which is over 8-fold higher than the prototype device demonstrated previously29Li W. Fu H.-C. Li L. 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: 13104-13108Crossref PubMed Scopus (87) Google Scholar and the highest published so far among all integrated solar rechargeable battery devices.10Schmidt D. Hager M.D. Schubert U.S. Photo-rechargeable electric energy storage systems.Adv. Energy Mater. 2015; 6: 1500369Crossref Scopus (122) Google Scholar, 11Yu M. McCulloch W.D. Huang Z. Trang B.B. Lu J. Amine K. Wu Y. Solar-powered electrochemical energy storage: an alternative to solar fuels.J. Mater. Chem. A. 2016; 4: 2766-2782Crossref Google Scholar For a broader comparison, the SOEE and other key performance metrics of reported representative SFBs and other solar rechargeable batteries are summarized in Table S1. The record SOEE achieved by the integrated SFB device demonstrated here is enabled by the following set of design principles. First, just like the RFBs,27Weber A.Z. Mench M.M. Meyers J.P. Ross P.N. Gostick J.T. Liu Q. Redox flow batteries: a review.J. Appl. Electrochem. 2011; 41: 1137-1164Crossref Scopus (1397) Google Scholar for the integrated SFB devices, even with the same photoelectrode and electrolyte, different flow cell structures could result in significantly different device performance and characteristics, especially for the liquid junction photoelectrodes that are more sensitive to the mass transfer rate of redox active species.40Gibbons J.F. Cogan G.W. Gronet C.M. Lewis N.S. A 14% efficient nonaqueous semiconductor/liquid junction solar cell.Appl. Phys. Lett. 1984; 45: 1095Crossref Scopus (81) Google Scholar To accommodate all the components and functions yet maintain a high performance, the SFB device should be dedicatedly designed and optimized. The zero-gap structure of the SFB device employed here only allows a very thin liquid layer (∼2 mm) contacting with the photoelectrode, thus ensuring effective diffusion and convection of redox couples at moderate flow rate. More importantly, comparison of the efficiency of the RFB component and solar component clearly shows that here the SOEE is mainly limited by the solar conversion efficiency of the photoelectrode. The photoelectrode used here was fabricated with a triple-junction III-V solar cell that can absorb most of the solar irradiation across the whole solar spectrum to provide a high PCE and, more importantly, a high photovoltage. Lastly, the E0 difference between the redox couples used in anolyte and catholyte determines the cell voltage of the SFB, which can significantly affect the SOEE (as suggested by Equation 2) as well as the energy and power capacity of the device. The highly efficient SFB device demonstrated here illustrates the general principles for designing a highly efficient SFB device with the available high-performance solar cells and RFBs: the RFB cell voltage should be matched as closely as possible with the maximum power point of the photoelectrode (Figure 5). In the specific case at hand, the 4-OH-TEMPO/MV redox couple combination boasts one of the highest cell voltages (1.25 V) among the aqueous organic RFBs demonstrated so far, which is a great boost for the SOEE. To drive the unassisted photocharging of SFBs with such a high cell voltage, the Voc produced by the photoelectrode needs to be at least 1.4 V to compensate for the inevitable voltage losses. Therefore, Voc higher than 1.8 V is generally not useful for driving the 4-OH-TEMPO/MV redox reactions. This means that an excess photovoltage of around 0.6 V produced herein by the tandem III-V photoelectrode was not contributing to the SOEE. This voltage mismatch is the most significant reason for the efficiency loss from the PCE of the solar cells to the final SOEE. As illustrated in Figure 5, if an RFB with an even higher cell voltage can be employed to shift the solid red RFB polarization curve to the hypothetical dashed red curve, it can not only improve the SOEE but also raise the energy density of the SFB. Therefore, there is still much untapped potential in the tandem III-V photoelectrode to further increase the SOEE of SFBs. With many new and emerging redox couples that are being developed for RFBs,22Ding Y. Li Y.F. Yu G.H. Exploring bio-inspired quinone-based organic redox flow batteries: a combined experimental and computational study.Chem. 2016; 1: 790-801Abstract Full Text Full Text PDF Scopus (169) Google Scholar, 23Ding Y. Yu G. A bio-inspired, heavy-metal-free, dual-electrolyte liquid battery towards sustainable energy storage.Angew. Chem. Int. Ed. 2016; 55: 4772-4776Crossref PubMed Scopus (112) Google Scholar, 24Huskinson B. Marshak M.P. Suh C. Er S. Gerhardt M.R. Galvin C.J. Chen X. Aspuru-Guzik A. Gordon R.G. Aziz M.J. A metal-free organic-inorganic aqueous flow battery.Nature. 2014; 505: 195-198Crossref PubMed Scopus (1027) Google Scholar, 25Kwabi D.G. Lin K. Ji Y. Kerr E.F. Goulet M.-A. De Porcellinis D. Tabor D.P. Pollack D.A. Aspuru-Guzik A. Gordon R.G. et al.Alkaline quinone flow battery with long lifetime at pH 12.Joule. 2018; 2https://doi.org/10.1016/j.joule.2018.07.005Abstract Full Text Full Text PDF Scopus (221) Google Scholar, 26Park M. Ryu J. Wang W. Cho J. Material design and engineering of next-generation flow-battery technologies.Nat. Rev. Mater. 2016; 2: 16080Crossref Scopus (418) Google Scholar, 27Weber A.Z. Mench M.M. Meyers J.P. Ross P.N. Gostick J.T. Liu Q. Redox flow batteries: a review.J. Appl. Electrochem. 2011; 41: 1137-1164Crossref Scopus (1397) Google Scholar, 28Ding Y. Zhang C. Zhang L. Zhou Y. Yu G. Molecular engineering of organic electroactive materials for redox flow batteries.Chem. Soc. Rev. 2018; 47: 69-103Crossref PubMed Google Scholar such as the one reported recently,41DeBruler C. Hu B. Moss J. Liu X. Luo J. Sun Y. Liu T.L. Designer two-electron storage viologen anolyte materials for neutral aqueous organic redox flow batteries.Chem. 2017; 3: 961-978Abstract Full Text Full Text PDF Scopus (194) Google Scholar this strategy promises a clear pathway for future developments. Moreover, not only is high (photo)voltage highly desirable for both the RFBs and the photoelectrodes used in SFBs, but properly matching their voltages is also the critical factor for taking advantage of such high-efficiency solar cells to enable the highest SOEE out of the integrated SFBs. An alternative strategy for improving voltage matching could be boosting the Jsc of the photoelectrode, which is usually accompanied by some sacrifice in the photovoltage (from solid blue to dashed blue curve in Figure 5); however, as long as the photovoltage is still higher than the RFB cell voltage, we can still achieve intersection at maximum power point. Such a design would involve tuning the band structures of the tandem III-V photoelectrodes or integrating other materials into tandem double junctions,42Vaisman M. Fan S.Z. Yaung K.N. Perl E. Martin-Martin D. Yu Z.S.J. Leilaeioun M. Holman Z.C. Lee M.L. 15.3%-Efficient GaAsP solar cells on GaP/Si templates.ACS Energy Lett. 2017; 2: 1911-1918Crossref Scopus (38) Google Scholar therefore making it more complicated. Because of the expensive III-V substrates and heteroepitaxial growth, the manufacturing cost for tandem III-V photoelectrodes may be too high to be employed for practical applications. At this early stage of the development of SFBs, we are trying to demonstrate the design principles and push the boundaries to show what could be possibly achieved, with some sacrifice regarding cost effectiveness. However, the cost of III-V cells may be reduced in the future by designing simpler tandem cells with a sufficiently high photovoltage42Vaisman M. Fan S.Z. Yaung K.N. Perl E. Martin-Martin D. Yu Z.S.J. Leilaeioun M. Holman Z.C. Lee M.L. 15.3%-Efficient GaAsP solar cells on GaP/Si templates.ACS Energy Lett. 2017; 2: 1911-1918Crossref Scopus (38) Google Scholar or adopting new fabrication methods, such as epitaxial liftoff.43Ward J.S. Remo T. Horowitz K. Woodhouse M. Sopori B. VanSant K. Basore P. Techno-economic analysis of three different substrate removal and reuse strategies for III-V solar cells.Prog. Photovolt. 2016; 24: 1284-1292Crossref Scopus (92) Google Scholar With further developments and proper device design following the design principles laid out herein, we believe that the capital cost for monolithically integrated SFB devices will not be higher than individually operated PV devices plus RFBs. Furthermore, developing new semiconductor materials30Sivula K. van de Krol R. Semiconducting materials for photoelectrochemical energy conversion.Nat. Rev. Mater. 2016; 1: 15010Crossref Scopus (965) Google Scholar and incorporating them into more efficient liquid junction cells7Heller A. Miller B. Thiel F.A. 11.5% solar conversion efficiency in the photocathodically protected p-InP/V3+-V2+-HCl/C semiconductor liquid junction cell.Appl. Phys. Lett. 1981; 38: 282-284Crossref Scopus (110) Google Scholar, 31Cha H.G. Choi K.S. Combined biomass valorization and hydrogen production in a photoelectrochemical cell.Nat. Chem. 2015; 7: 328-333Crossref PubMed Scopus (411) Google Scholar, 32Kamat P.V. Tvrdy K. Baker D.R. Radich J.G. Beyond photovoltaics: semiconductor nanoarchitectures for liquid-junction solar cells.Chem. Rev. 2010; 110: 6664-6688Crossref PubMed Scopus (680) Google Scholar could further simplify the SFB photoelectrode fabrication process and lower the cost. In conclusion, building on novel device design and a set of rational design principles, we demonstrated a high-performance monolithic solar energy conversion and storage device by using highly efficient and high-photovoltage tandem III-V solar cells and high-voltage 4-OH-TEMPO/MV RFBs. The integrated SFB device can be easily configured to three different operation modes to fit specific application requirements. Enabled by a high-efficiency photoelectrode, properly matched redox couples, and carefully designed flow field design, a record SOEE of 14.1% has been achieved for the SFB. Following the design rules proposed herein, the efficiency of such SFB devices in general could be further boosted by better voltage matching of the RFBs and solar cells either by enlarging the RFB cell potential with better redox couple choices or by tuning the band structure of solar cells to improve its Jsc. This work paves the way for a practical new approach to harvesting, storing, and utilizing the intermittent solar energy with unprecedented high energy conversion efficiency and energy storage density. These integrated SFBs will be especially suitable as distributed and stand-alone solar energy conversion and storage systems in remote locations and will enable practical off-grid electrification." @default.
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