Design Principles and Developments of Integrated Solar Flow Batteries

Due to the intermittent nature of sunlight,practical round-trip solar energy utilization systems require bothefficient solar energy conversion and inexpensive large-scale energystorage. Conventional round-trip solar energy utilization systemstypically rely on the combination of two or more separated devicesto fulfill such requirements. Integrated solarflow batteries (SFBs)are a new type of device that integrates solar energy conversion andelectrochemical storage. In SFBs, the solar energy absorbed byphotoelectrodes is converted into chemical energy by charging upredox couples dissolved in electrolyte solutions in contact with thephotoelectrodes. To deliver electricity on demand, the reverse redoxreactions are carried out to release chemical energy stored in redoxcouples as one would do in the discharge of a normal redoxflowbattery (RFB). The integrated design of SFBs enables all the functions demanded by round trip solar energy utilization systems to berealized within a single device. Leveraging rapidly developing parallel technologies of photovoltaic solar cells and RFBs, significantprogress in thefield of SFBs has been made in the past few years. This Account aims to provide a general reference and tutorial forresearchers who are interested in SFBs, and to describe the design principles and thus facilitate the development of this nascentfield.The operation principle of SFBs is built on the working mechanism of RFBs and photoelectrochemical (PEC) cells, so wefirstdescribe the basic concept and important features of RFBs and redox couples with the emphasis on the quantitative understanding ofRFB cell potentials. We also introduce different types of PEC cells and highlight two different photoelectrode designs that arecommonly seen in SFB literature: simple semiconductor photoelectrodes and PV cell photoelectrodes. A set of experimentalprotocols for characterizing the redox couples, RFBs, photoelectrodes, and SFBs are presented to promote comparable assessmentand discussion of importantfigures of merits of SFBs.Solar-to-output electricity efficiency (SOEE) defines the round trip energy efficiency of SFBs and has received substantial researchattention. We introduce a quantitative simulation method tofind the relationship between the SOEE and cell potential of SFBs andreveal the design principles for highly efficient SFBs. Several other important performance metrics of SFBs are also introduced. Thenwe review the historical development of SFBs and identify the state-of-the-art demonstrations at each development stage with moreemphasis on our own research efforts in developing SFBs built with PV photoelectrodes. Finally, we preview some promising futuredirections and the challenges for advancing both the scientific understanding and practical applications of SFBs.