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

Abstract

Solar-powered vanadium redox-flow batteries (VRFB) have emerged as an attractive alternative to 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 highphotovoltage 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%.

Multilayered Hematite Nanowires with Thin‐Film Silicon Photovoltaics in an All‐Earth‐Abundant Hybrid Tandem Device for Solar Water Splitting

“…Nevertheless, to reach the target hydrogen production required in industry or domestic usage, it is crucial to identify methods to increase the bias‐free photocurrent without expensive upscaling. The most efficient and straightforward method to achieve this involves increasing the available illumination intensity for the hybrid tandem device by light concentration . The effect of concentrated light illumination on the silicon photocathodes (solid curves) and on the photoanode (dashed curves) is also shown in Figure .…”

“…The value of the onset potential, besides catalysis, also depends on the internal structure of the photoabsorbing material, that is, on the recombination and charge‐transfer processes . Additionally, the photoelectrodes were heated up during concentrated illumination, which usually results in a slight decrease in the photovoltage . Recombination and temperature effects therefore might have hampered a more pronounced increase in the onset potential, as one would expect from semiconductor devices upon increased illumination intensity, that is, in the presence of more photogenerated charge carriers.…”

“…One critical point to consider is the heating up of the photocathode. In continuous operation with concentrated illumination, the temperature of the photocathode will inevitably increase, which, in general, will result in a slight decrease in the photovoltage owing to changes in the bandgap energies of the thin‐film silicon absorber layers . This effect might have shifted the operation point of the hybrid PEC‐WS device into a lower current‐density region.…”

“…The most efficient and straightforward method to achieve this involves increasing the available illumination intensity for the hybrid tandemd evice by light concentration. [14,15] Thee ffect of concentrated light illumination on the silicon photocathodes( solid curves) and on the photoanode (dashedcurves) is also shown in Figure 3. We tested concentration factors of 2a nd 3, giving illumination intensities (P in )o f 200 (purple curves) and 300 mW cm À2 (orange curves), respectively.P lease note that the light concentration was achieved by approachingt he hybrid tandemt ot he sun simulator,a nd thus, somes mall deviations from the exact concentration factors might be introduced.…”

“…[16] Additionally,t he photoelectrodes were heatedu pd uring concentrated illumination, which usually results in aslight decreaseinthe photovolt-age. [15] Recombination andt emperature effects therefore might have hampered am ore pronounced increase in the onset potential, as one would expect from semiconductor devices upon increased illumination intensity,t hat is, in the presence of more photogenerated charge carriers. The influence of highert emperatures on the j-V characteristics of the hematite photoanode is presentedi nF igure S5 (Supporting Information).…”