Integrated design of hematite and dye-sensitized solar cell for unbiased solar charging of an organic-inorganic redox flow battery

Abstract

Integration of the photoelectrochemical cells and redox flow batteries (RFBs) is conceptualized as solar redox flow cells (SRFCs). The SRFC opens an opportunity for simultaneous electrochemical conversion and storage of solar energy in a single device. This work proposes a SRFC which integrates stable hematite (alpha-Fe2O3) photoanode with a near-neutral organic-inorganic RFB using organic and low-cost redox pair anthraquinone-2,7-disulfonate disodium (AQDS) on the negative side and iodide/iodine (I-/I-2) redox couple on the positive side. The AQDS/iodide RFB exhibited a stable battery cycling with a cell voltage of similar to 0.8 V, a high coulombic efficiency of 99.2% and a peak power density of 0.22 W.cm(-2). The hematite yielded a stable performance in contact with iodide/iodine solution with pH 5.5, despite known stability issues for pH < 7. Electrochemical impedance spectroscopy was used to elucidate the main semiconductor/electrolyte charge transfer mechanisms and rates and energy level alignment of hematite with redox pair voltages. Finally, to photocharge the AQDS/iodide SRFC unbiasedly, a tandem system was used which includes hematite photoelectrode connected in series with a dye-sensitized solar cell (DSSC). The tandem photoelectrode provided 1.6 V of photovoltage that is sufficient to fully charge the RFB.