Tandem Solar Flow Batteries for Conversion, Storage, and Utilization of Solar Energy
2018; Elsevier BV; Volume: 4; Issue: 11 Linguagem: Inglês
10.1016/j.chempr.2018.10.008
ISSN2451-9308
Autores Tópico(s)Solar-Powered Water Purification Methods
ResumoIn this issue of Chem, Jin and coworkers present the design principles and demonstration of a highly efficient integrated solar flow battery (SFB) device that can be configured to perform all the requisite functions from solar energy harvest to electricity redelivery with a record solar-to-output electricity efficiency of 14.1%. In this issue of Chem, Jin and coworkers present the design principles and demonstration of a highly efficient integrated solar flow battery (SFB) device that can be configured to perform all the requisite functions from solar energy harvest to electricity redelivery with a record solar-to-output electricity efficiency of 14.1%. Worldwide economic growth is coupled with the increasing energy demand. Driven by the necessity to reduce the reliance on fossil fuels and carbon emissions, the development of renewable-energy sources, such as solar and wind power, has accelerated in recent years much faster than ever before. For example, the growth of electricity production from solar and wind energy is at an average rate of about 6.3% per year.1US Energy Information Administration (2016). International Energy Outlook 2016. DOE/EIA-0484(2016). https://www.eia.gov/outlooks/ieo/pdf/0484(2016).pdf.Google Scholar The momentum toward renewable energy is irreversible. The practical utilization of solar energy requires both efficient, low-cost energy conversion and large-scale energy storage techniques because of the dispersion and intermittency of solar energy sources. Solar cells have been widely studied and implemented in the market. Meanwhile, several energy storage devices, such as secondary batteries (e.g., lead-acid, Li-ion, and redox flow batteries), flywheels, and electrochemical super-capacitors, are available for electricity energy storage. Because of the technological merits of high power input and output, decoupled energy and power, scalability, safety features, and abundant redox-active candidate materials, redox flow batteries (RFBs) are well suited for renewable-energy storage and electricity grid balancing.2Wei X. Pan W. Duan W. Hollas A. Yang Z. Li B. Nie Z. Liu J. Reed D. Wang W. Sprenkle V. Materials and systems for organic redox flow batteries: Status and challenges.ACS Energy Lett. 2017; 2: 2187-2204Crossref Scopus (277) Google Scholar Simultaneous solar energy conversion and storage have received increasing interest for efficiently utilizing the abundant yet intermittent solar energy.3Cao L. Skyllas-Kazacos M. Wang D.-W. Solar redox flow batteries: Mechanism, design, and measurement.Adv. Sustainable Syst. 2018; 2: 1800031Crossref Scopus (18) Google Scholar Solar rechargeable batteries combine the advantages of photoelectrochemical (PEC) devices and batteries and have emerged as an attractive alternative to artificial photosynthesis for large-scale solar energy harvesting and storage.3Cao L. Skyllas-Kazacos M. Wang D.-W. Solar redox flow batteries: Mechanism, design, and measurement.Adv. Sustainable Syst. 2018; 2: 1800031Crossref Scopus (18) Google Scholar Under the irradiation of sunlight, the photo-generated electron-hole pair can drive the redox reactions, which makes the integration of a solar cell and RFB possible. In fact, recent studies have demonstrated the feasibility of integrating semiconductor solar cells and RFBs.4Li W. Fu H.-C. Li L. Cabán-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 (86) Google Scholar, 5Liao 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: 11474Crossref PubMed Scopus (102) Google Scholar In these solar flow batteries (SFBs), the anolyte or catholyte is directly reduced or oxidized by the photogenerated electron or hole on the surface of photoelectrodes. There are several apparent technological advantages to using SFBs for integrated solar energy conversion and storage. First, SFBs can directly utilize photo-generated current to charge a RFB while preserving the merits of RFBs, including decoupled energy and power storage, safety, and scalability. Second, integrated SFBs have a more compact design and smaller footprint than decoupled solar cells and batteries. Third, SFBs have a simpler design and are more practical than PEC solar fuel production and utilization, which requires a solar electrolyzer, a fuel storage system (e.g., a pressurized tank is required for hydrogen storage), and a second fuel cell to complete the whole process of solar energy conversion, storage, and utilization. Fourth and finally, SFBs can be used as standalone off-grid power systems for commercial and residential buildings, as well as remote areas. To maximize the utilization efficiency of solar energy in SFBs, matching the energy level between the electrochemical potentials of the redox active materials and the band structures of photoelectrodes is critical.3Cao L. Skyllas-Kazacos M. Wang D.-W. Solar redox flow batteries: Mechanism, design, and measurement.Adv. Sustainable Syst. 2018; 2: 1800031Crossref Scopus (18) Google Scholar, 6Li W. Fu H.-C. Zhao Y. He J.-H. Jin S. 14.1% efficient monolithically integrated solar flow battery.Chem. 2018; 4: 2644-2657Abstract Full Text Full Text PDF Scopus (57) Google Scholar However, given the fixed band structures of the semiconductors, it is hard to find redox couples that simultaneously close to the band edge of a photoelectrode and have a large potential gap. A mismatch in energy levels can lead to a waste of solar energy harvested by the photoelectrode and thus yield a low voltage output and a low solar-to-output electricity efficiency (SOEE). To date, low cell voltages and low SOEE remain two big issues for solar rechargeable RFBs.4Li W. Fu H.-C. Li L. Cabán-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 (86) Google Scholar, 5Liao 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: 11474Crossref PubMed Scopus (102) Google Scholar High-voltage and stable aqueous organic RFBs (AORFBs) have been recently reported and are good options for incorporated into SFBs.7Janoschka T. Martin N. Martin U. Friebe C. Morgenstern S. Hiller H. Hager M.D. Schubert U.S. An aqueous, polymer-based redox-flow battery using non-corrosive, safe, and low-cost materials.Nature. 2015; 527: 78-81Crossref PubMed Scopus (605) Google Scholar, 8Liu 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. 2016; 6: 1501449Crossref Scopus (419) Google Scholar, 9Lin K. Chen Q. Gerhardt M.R. Tong L. Kim S.B. Eisenach L. Valle A.W. Hardee D. Gordon R.G. Aziz M.J. Marshak M.P. Alkaline quinone flow battery.Science. 2015; 349: 1529-1532Crossref PubMed Scopus (675) Google Scholar, 10DeBruler 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: 1-18Abstract Full Text Full Text PDF Scopus (194) Google Scholar Meanwhile, because the PEC reactions in SFBs directly happen on the surface of photoelectrodes, it is challenging to maintain the stability of photoelectrodes. In this issue of Chem, Jin and coworkers have designed a new SFB that is different from traditional integration.6Li W. Fu H.-C. Zhao Y. He J.-H. Jin S. 14.1% efficient monolithically integrated solar flow battery.Chem. 2018; 4: 2644-2657Abstract Full Text Full Text PDF Scopus (57) Google Scholar As shown in Figure 1A, the photoelectrode is protected from the electrolyte by a conductive Ti|TiO2|Pt layer to allow direct photovoltage harvest at the semiconductor-current collector interface to charge the RFB. There are two advantages of this SFB design. On the one hand, it could overcome the requirement of an energy-level match because of the existence of an internal p/n junction in the photoelectrode. Theoretically, any redox couples with a redox-potential gap smaller than the photovoltage of the photoelectrode can be used in this SFB system. It greatly expands the choice of redox couples and photoelectrodes. On the other hand, this new setup simplifies the overall device design. Because of the tandem heterojunction structure of the III-V tandem solar cell utilized here, a 2.4 V photovoltage was obtained, which is high enough to drive any aqueous RFBs. Moreover, the band-gap energy of the III-V tandem cell matches well with the solar irradiation, which makes it highly efficient for solar energy conversion. On the basis of these advantages, Jin and coworkers demonstrated a SFB by coupling a III-V tandem solar cell with a 1.25 V 4-OH-TEMPO/MV aqueous RFB, where 4-OH-TEMO and MV stand for 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl and methyl viologen, respectively. It should be noted that the 4-OH-TEMPO/MV aqueous RFB was previously reported by Liu et al. as one of the first pH-neutral AORFBs.8Liu 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. 2016; 6: 1501449Crossref Scopus (419) Google Scholar Because of the large cell voltage of the 4-OH-TEMPO/MV AORFB and high efficiency of the solar cell, the newly designed SFB delivered a record 14.1% SOEE. In this work, Jin and coworkers demonstrate a three-electrode SFB device by incorporating an III-V tandem photoelectrode into conventional two-electrode RFBs. As shown in Figure 1B, this three-electrode design is highly flexible. It can be operated as a normal RFB for electricity energy storage with a cathode and anode for battery charge and discharge (Figure 1B, top). Under the RFB mode, the 4-OH-TEMPO/MV AORFB delivered 99% current efficiency and 91.9%–73.0% energy efficiency under an operating current density of 10–50 mA cm−2. The cycling stability was comparable with that in the previous report.8Liu 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. 2016; 6: 1501449Crossref Scopus (419) Google Scholar Combining the photoelectrode, anode, and anolyte, the device can be operated as a solar cell (Figure 1B, middle) to directly convert solar energy into electrical energy to drive external load. In the third mode, the photoelectrode and the cathode can be connected to yield a solar rechargeable flow battery (Figure 1B, bottom). Under this mode, the anolyte and catholyte are simultaneously oxidized and reduced by the photoelectrode, which realizes the solar energy harvesting and storage simultaneously. The stored energy can be then discharged to deliver electricity through the RFB mode. Under 1 sun illumination, this team obtained a photocurrent density of 14.5 mA cm−2, 96.2% current efficiency, and 96.6% voltage efficiency. It is exciting that a record SOEE of 14.1% was achieved, which is 4.7 times higher than the previous leader (3.2%).5Liao 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: 11474Crossref PubMed Scopus (102) Google Scholar As presented in this work, besides the solar energy conversion efficiency of the photoelectrode, the match between the photovoltage of the photoelectrode and the potentials of redox-active materials can significantly affect the SOEE of the SFBs. To maximize the utilization of the harvested solar energy, the RFB cell voltage should be close to the effective photovoltage of the photoelectrode. Either increasing the RFB voltage or reducing the photovoltage (thus boosting the photocurrent density) could lead to higher efficiency. In light of this, it is highly possible to further improve the SOEE by using well-orchestrated redox couples and meticulously designed photoelectrodes. For example, it is expected that combining this 2.4 V III-V tandem solar cell with a recently reported 1.72 V viologen/TEMPO RFB10DeBruler 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: 1-18Abstract Full Text Full Text PDF Scopus (194) Google Scholar could lead to a SFB with a higher SOEE and a higher energy density. In addition, the long-term cycling stability of SFBs remains to be demonstrated. In summary, along with the development of advanced AORFBs and advanced photoelectrode materials, the emerging SFB technology will be further advanced and is promising for practical applications in integrated solar energy conversion and storage. 14.1% Efficient Monolithically Integrated Solar Flow BatteryLi et al.ChemSeptember 27, 2018In BriefThe monolithic integration of photoelectrochemical solar energy conversion and electrochemical energy storage offers an efficient and compact approach toward practical solar energy utilization. This work presents the design principles for and the demonstration of a highly efficient integrated solar flow battery device with a record solar-to-output electricity efficiency. These results will enable a highly efficient approach for practical off-grid solar utilization and electrification. Full-Text PDF Open Archive
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