Design Principles and Developments of Integrated Solar Flow Batteries

CONSPECTUS: Due to the intermittent nature of sunlight, practical round-trip solar energy utilization systems require both efficient solar energy conversion and inexpensive large-scale energy storage. Conventional round-trip solar energy utilization systems typically rely on the combination of two or more separated devices to fulfill such requirements. Integrated solar flow batteries (SFBs) are a new type of device that integrates solar energy conversion and electrochemical storage. In SFBs, the solar energy absorbed by photoelectrodes is converted into chemical energy by charging up redox couples dissolved in electrolyte solutions in contact with the photoelectrodes. To deliver electricity on demand, the reverse redox reactions are carried out to release chemical energy stored in redox couples as one would do in the discharge of a normal redox flow battery (RFB). The integrated design of SFBs enables all the functions demanded by round trip solar energy utilization systems to be realized within a single device. Leveraging rapidly developing parallel technologies of photovoltaic solar cells and RFBs, significant progress in the field of SFBs has been made in the past few years. This Account aims to provide a general reference and tutorial for researchers who are interested in SFBs, and to describe the design principles and thus facilitate the development of this nascent field. The operation principle of SFBs is built on the working mechanism of RFBs and photoelectrochemical (PEC) cells, so we first describe the basic concept and important features of RFBs and redox couples with the emphasis on the quantitative understanding of RFB cell potentials. We also introduce different types of PEC cells and highlight two different photoelectrode designs that are commonly seen in SFB literature: simple semiconductor photoelectrodes and PV cell photoelectrodes. A set of experimental protocols for characterizing the redox couples, RFBs, photoelectrodes, and SFBs are presented to promote comparable assessment and discussion of important figures of merits of SFBs. Solar-to-output electricity efficiency (SOEE) defines the round trip energy efficiency of SFBs and has received substantial research attention. We introduce a quantitative simulation method to find the relationship between the SOEE and cell potential of SFBs and reveal the design principles for highly efficient SFBs. Several other important performance metrics of SFBs are also introduced. Then we review the historical development of SFBs and identify the state-of-the-art demonstrations at each development stage with more emphasis on our own research efforts in developing SFBs built with PV photoelectrodes. Finally, we preview some promising future directions and the challenges for advancing both the scientific understanding and practical applications of SFBs.