Solar vanadium redox-flow battery powered by thin-film silicon photovoltaics for efficient photoelectrochemical energy storage

Solar-powered vanadium redox-flow batteries (VRFB) have emerged as an attractive method for large-scale and efficient energy storage and conversion. However, due to the stringent charging voltage requirements of vanadium-based systems (1.4-1.7 V), common photobatteries, applying standard photovoltaics with nonoptimized photovoltages, cannot be completely charged bias-free, i.e. by only using bias-free solar energy, or if they can be, only at unpractical low current densities of just a few mA cm(-2). In response to this critical challenge, the present study aimed to design and test a compact device combining a high-photovoltage silicon multijunction solar cell with an all-vanadium continuous-flow battery. In particular, we applied a monolithic triple junction solar cell, which can provide photovoltage of up to 2.2 V. Additionally, we have introduced the concept of increased illumination intensity for the solar VRFB. As a first demonstration, a complete bias-free solar charging at 25 mA cm(-2) (300 mW cm(-2) illumination) is reported. Moreover, we investigated the influence of the operation parameters of the redox-flow battery itself: the membrane type and the vanadium concentration in the electrolyte (i.e. storage capacity). The presented results provide evidence that the low-cost thin-film silicon based solar VRFB can be considered as an outstanding alternative for practical energy storage and conversion usage. A maximum bias-free solar conversion efficiency of 12.3% was achieved during charging, combined with promising and competitive energy efficiencies for the complete charge-discharge process that can guarantee an overall solar-to-electricity conversion efficiency of >10%.