Influence of NH4Cl additive in a VO2+/VO2+ - AQDS/AQDS2− solar redox flow battery

Abstract

Solar redox flow batteries are a relatively new type of redox flow battery technology that uses solar energy to directly store chemical energy. Here we present a solar redox flow battery that uses a MoS2@TiO2 thin film with a Nafion protection layer supported on FTO glass substrate as photoanode, employing VO2+/VO2+ and AQDS/AQDS2− as redox active species. When the solar radiation strikes the photoelectrode, the photogenerated holes oxidize VO2+ to VO2+, while the photogenerated electrons reduce AQDS to AQDS2− at the counter electrode. The oxidized form of V5+ and reduced form of AQDS2− thus retain the chemical energy and can release the stored charged via the reverse electrochemical reaction. The addition of NH4Cl to the electrolyte was found to have a positive impact on the electrochemical performance of the redox flow cell. This effect was more evident for the VOSO4 electrolyte, leading to an enhancement of the voltaic and energy efficiencies of more than 17.5%. The results suggest that NH4Cl promotes both mass transport of the vanadium redox species and charge transfer of the AQDS in the electrolyte. The solar-to-output energy conversion efficiency (SOEE) of the solar redox flow battery using 1.6 g L−1 NH4Cl in both anolyte and catholyte reached 9.73%, and an energy density of 87.45% after 10 consecutive one-hour photocharging cycles. Additionally, the use of Nafion to protect the MoS2@TiO2 photoanode from photocorrosion was explored. The Nafion layer ensured an increased stability of MoS2@TiO2 against the strong acidic environment while maintaining effective light response, which translated into enhanced photon and mass transport. An energy storage capacity of ∼60 mAh L−1 after 1-hour photocharging was observed.