Does Heat Play a Role in the Observed Behavior of Aqueous Photobatteries?

Abstract

Light-rechargeable photobatteries have emerged as an elegant solution to address the intermittency of solar irradiation by harvesting and storing solar energy directly through a battery electrode. Recently, a number of compact two-electrode photobatteries have been proposed, showing increases in capacity and open-circuit voltage upon illumination. Here, we analyze the thermal contributions to this increase in capacity under galvanostatic and photocharging conditions in two promising photoactive cathode materials, V₂O₅ and LiMn₂O₄. We propose an improved cell and experimental design and perform temperature-controlled photoelectrochemical measurements using these materials as photocathodes. We show that the photoenhanced capacities of these materials under 1 sun irradiation can be attributed mostly to thermal effects. Using <i>operando</i> reflection spectroscopy, we show that the spectral behavior of the photocathode changes as a function of the state of charge, resulting in changing optical absorption properties. Through this technique, we show that the band gap of V₂O₅ vanishes after continued zinc ion intercalation, making it unsuitable as a photocathode beyond a certain discharge voltage. These results and experimental techniques will enable the rational selection and testing of materials for next-generation photo-rechargeable systems.