Wenjie Li1 Hui-Chun Fu2 Yuzhou Zhao1 Jr-Hau He2 Song Jin1

1, University of Wisconsin-Madison, Madison, Wisconsin, United States
2, Electrical and Mathematical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal, , Saudi Arabia

The practical utilization of solar energy demands not only efficient energy conversion but also inexpensive large scale energy storage. Building on mature regenerative solar cells and emerging electrochemical redox flow batteries (RFBs), more efficient, scalable, compact and cost-effective hybrid energy conversion and storage devices could be realized. Here we present an integrated solar energy conversion and electrochemical storage device by integrating regenerative solar cells in aqueous electrolytes with RFBs using the same pair of organic redox couples. In such an integrated solar flow battery (SFB) device, solar energy is absorbed by semiconductor electrodes and photoexcited caries are collected at the semiconductor-liquid electrolyte interface and used to convert the redox couples in the RFB to fully charge up the battery (i.e. store the solar energy into the redox couples). When electricity is needed, the charged up redox couples will be discharged on the surface of carbon felt electrodes as one would do in the discharge of a RFB to generate the electricity. We demonstrated that such an integrated SFB device can be charged under solar illumination without external electric bias and deliver a high discharge capacity comparable with state of the art RFBs over many cycles. This integrated device can utilize solar energy efficiently -- an overall direct solar-to-output electricity efficiency (SOEE) of 14% has been achieved with advanced performance optimization. With comprehensive mechanism study and deeper understanding of the operation principles of SFBs, we also propose a set of design principles for highly efficient integrated SFB devices.