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sunlight utilization. Sluggish kinetics of hydrogen and oxygen evolution reactions limits the efficiency of converting and storing solar energy by photocatalytic water splitting. In comparison, the kinetics of appropriate redox couples in photocharge- able RFBs can be several orders of magnitudes faster. On the one hand, connecting well-developed photovoltaic devices and secondary batteries can realize solar en- ergy conversion and storage with a solar-to-output energy efficiency up to about 15%.85 On the other hand, photochargeable RFBs, converting and storing solar en- ergy simultaneously via the photoelectrochemical reactions, represent a more cost- effective technique for efficient solar utilization. The first study of photochargeable RFBs was proposed by Hodes et al., who used a polycrystalline CdSe as the photo- electrode to drive S/S2 redox reaction and Ag2S/Ag as the anode.86 In this work, low storage efficiency was achieved since membranes were not used to suppress self-discharge. Besides, the insertion and extraction of ions in solid materials is slug- gish for storing photogenerated charges, and Gao and coworkers first explored two soluble redox species as the catholyte and the anolyte in order to improve the reac- tion kinetics.87 In the designed photochargeable RFB, a dye-sensitized TiO2 film was employed as the photoelectrode, and the [Fe(C10H15)2]+/[Fe(C10H15)2]-based nega- tive redox species and the I/I3-based positive redox species were separated by a ceramic Li-ion-conducting membrane. This photochargeable RFB presented an overall photo-to-electricity efficiency of 0.15% and a good cyclability. As for the cell architecture of photochargeable RFBs, there are mainly three configura- tions with different working principles so far. The classic architecture, as presented in Figure 9A, uses one photoelectrode to produce holes and electrons for oxidation of the redox couples in the catholyte and reduction of the redox couples in the anolyte. This configuration has the simplest form, which involves only one photoelectrode, offer- ing appealing advantages of high scalability and low-cost cell design for large-scale ap- plications.87 Photoelectrodes with narrow band gaps can harvest more photons of sun- light, but two photoelectrodes in series are required to produce large enough photovoltage for charging RFBs. As shown in Figure 9B, Wang et al. reported a high- voltage photochargeable RFB (1.2 V) by combining the anthraquinone anolyte and ferrocyanide catholyte, which was powered by Ta3N5 and GaN/Si dual photoelectro- des.88 Likewise, dual-silicon photoelectrodes can be integrated with a quinone/bromine RFB and achieved a solar-to-output electricity efficiency (SOEE) of $3.2% by Li.89 Jin and coworkers combined dual-silicon photoelectrodes with an all-quinone RFB, which showed a high SOEE of 1.7%.90 In addition, when the voltage of RFB is larger than the photovoltage produced by the photoelectrode, external voltage bias should be applied to compensate the potential difference to drive the redox reaction. As shown in Figures 9C and 9D, Zhou et al. reported a photoassisted rechargeable sodium poly- sulfide/iodine battery employing TiO2 as the photoelectrode. Under solar irradiation, the charging voltage of the photoassisted cell was decreased to 0.08 V, which is even lower than the discharging voltage of 0.83 V; therefore, this system saved $90% input electric energy.91 Moreover, Wu et al. used a dye-sensitized TiO2 photoelectrode to assist the charging process of a Li-I RFB, which reduced the charging voltage from 3.3 to 2.9 V as well.92 The future development of photochargeable RFBs is focused on high SOEE, long cycling performance, and low cost and mainly depends on the progress of photovol- taic and photocatalytic materials and advanced RFBs. Currently, a number of low- cost, environmentally friendly, and robust photocatalysts with outstanding solar en- ergy conversion efficiency in artificial photosynthesis are available and applicable for new SPRC technologies. Yu and coworkers applied the promising visible photocata- lyst BiVO4 as a photoelectrode (Figure 9A) with aqueous Br3/Br and I3/I redox Chem 5, 1964–1987, August 8, 2019 1981PDF Image | Development of Redox Flow Batteries Based on New Chemistries
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