Residential and transportation accounts for almost half of the world’s total energy consumption and thus for a large portion of CO₂ emissions. Sunlight is probably the most convenient renewable energy source for producing in-loco thermal and electrical energy for buildings. PV panels produce already roof cheaper than grid electricity in several EU countries such as as Denmark, Germany, Italy, Portugal and Spain. However, given the intermittent nature of solar energy, energy storage is required.

Photoelectrochemical (PEC) cells provide an alternative platform for converting sunlight into storable fuels easily convertible into electricity. While originally the research on PEC cells targeted the solar hydrogen production, scientists still struggling to develop a viable PEC system mostly because of the fixed energy levels imposed by the water splitting reaction which often do not match the ones of the semiconductors used. [1] More recently, a new concept was demonstrated to surpass these obstacles, combining the advantages of the PEC cells and of the redox flow batteries (RFB) in a device initially named solar redox flow cell (SRFC). This technology uses the full potential of a PEC cell to charge electrochemical redox pairs in the liquid phase – Fig. 1. The stored chemical energy can be easily and efficiently converted into electricity at a RFB and the harvested thermal energy serves for thermal comfort and for sanitary waters.

The concept of a SRFC was described for the first time in the 80s [2], staying dormant until 2012; however, in only seven years, the SRFC efficiency equals the PEC water splitting with extensive years of research. Still, a high energy density and stable SRFC solely charged using sunlight, combining low cost, stable and efficient redox pairs, electrolytes and photoelectrode materials is elusive. The present work aims at studying the use of a stable hematite (α-Fe₂O₃) photoelectrode coupled with fast redox couples, such as ferrocyanide [3] or iodine [4] on the positive side, and organic anthraquinone-2,7-disulfonate disodium (AQDS) on the negative side. Hematite surface treatments employing an annealing treatment and using a FeNiOx co-catalyst allowed to achieve a photocurrent > 2.5 mA∙cm^-2 and a photovoltage of 0.8 V. Several cell arrangements were also studied for the design and construction of an innovative SRFC device. The optimized hematite-based SRFC coupled with ferrocyanide/AQDS redox pairs demonstrated to be stable over a long-term period of 1000 h. The hematite also yielded a stable performance in contact with iodide/iodine solution with pH 5.5. Finally, a tandem system coupling a hematite photoelectrode in series with a dye-sensitized solar cell (DSSC) was demonstrated providing 1.6 V of photovoltage that is sufficient to fully charge the AQDS/iodide SRFC. [4]