Solar energy is particularly useful for off-the-grid applications, e.g., in developing regions. Due to its fluctuating nature, the use of solar energy requires rechargeable battery systems. Usually, light is harvested by solar cells and the electricity that is generated by these cells can then be stored in a battery. This extra step causes energy losses. Photo-active battery electrodes that can be recharged directly by light could allow the construction of batteries that avoid this problem entirely.

Buddha Deka Boruah, Michael De Volder, and colleagues, University of Cambridge, UK, have developed an aqueous zinc-ion battery that can harvest sunlight to recharge without external solar cells. The photoactive cathode consists of vanadium pentoxide (V₂O₅) nanofibers, poly(3-hexylthiophene-2,5-diyl) (P3HT), and reduced graphene oxide (rGO). The team combined this with a Zn metal anode and an aqueous electrolyte.

The vanadium pentoxide nanofibers both harvest light and allow for charge conduction. The photoexcited electrons are then transported to a carbon felt (CF) current collector. The P3HT polymer component prevents the recombination of charge carriers and the rGO component promotes their transport. Using this cathode composition, the team built fully functioning, light-rechargeable batteries. According to the researchers, the batteries have the highest light conversion efficiencies (1.2 %) reported for photo-rechargeable batteries so far.

• Photo-rechargeable zinc-ion batteries,
  Buddha Deka Boruah, Angus Mathieson, Bo Wen, Sascha Feldmann, Wesley M. Dose, Michael De Volder,
  Energy Environ. Sci. 2020.
  https://doi.org/10.1039/d0ee01392g