Photoelectrochemical, all-soluble iron redox-flow battery for the direct conversion of solar energy

A photoelectrochemical redox-flow battery (RFB) employing an all-soluble, aqueous coordination chemistry of the element iron is developed. The system is based on the ferro/ferricyanide redox couple as posolyte, the iron-triethanolamine (TEOA) complex as negolyte and a Ge/GaAs/GaInP triple-junction solar cell (TJSC) as power source. Combining a discharging flow-cell with the photoelectrochemical unit, we enable an operando monitoring of degradation phenomena at the working electrode similar to a rotating ring-disc electrode (RRDE). In this manner, an irreversible deactivation of an unprotected TJSC is observed after 15 h of operation. In a post-mortem SEM/EDX analysis, this performance loss is identified to result from a dissolution of the top junctions of the photoabsorber. Mitigating this corrosion is achieved by introducing a 150 nm thick protection layer of TiO2 via reactive sputtering. In this manner, a stable performance for more than 120 h is obtained. In view of the fact that the iron species selected for this study are affordable, abundant, water-based and non-toxic our device is a prototype of a green photoelectrochemical RFB at laboratory scale.