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 operando 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.

Figure 1

Optical considerations while conducting experiments on light-rechargeable photobatteries. (a) Band structures of two hypothetical cathode materials. When the photocathode is illuminated, the populations of electrons and holes are represented by their respective quasi-Fermi levels. Depending on the relative position of these levels and the anode/cathode plating and de-intercalation potentials, photocharging may (Cathode Material 1) or may not (Cathode Material 2) be possible. (b) Schematic highlighting that intercalation can shift the positions of the electron and hole quasi-Fermi levels, affecting the material’s ability to be photocharged.

Figure 2

Thermal considerations while conducting experiments on light-rechargeable photobatteries. (a) Effects of heat and light on the capacity of a V₂O₅–Zn battery; both heat and light increase the capacity of the cell and accelerate its activation process. (b) Planar cell structure that enables simultaneous cooling of photobatteries during photoelectrochemical measurements, allowing for the thermal contributions to capacity enhancement to be subtracted.

Figure 3

Operando optical microscopy images of a cell (a) discharged to 0.2 V and (b) charged to 1.6 V. A change in color from yellowish to gray is seen at a discharge voltage of 0.8 V; this color is retained until the cell is charged to 1.25 V, at which point the yellowish color returns. This indicates that the optical and electronic properties of the system are changed as the zinc ions are intercalated. However, this change is at least partly reversible, as indicated by the return of color in the photocathode as ions are de-intercalated. All scale bars represent 400 μm.

Figure 4

Changes in the reflectance of V₂O₅ when cycled against a zinc anode. (a) Operando reflection spectra of V₂O₅ photocathodes cycled against a Zn metal anode for different discharge voltages. The discharge protocol is provided in Figure S8. A clear reflection edge at about 520 nm (indicating the onset of the band gap) is seen before the cell is cycled and after it is charged to 1.4 V. (b) Enlarged reflection spectra for the discharged photocathode seen in (a) at 0.75, 0.5, and 0.25 V. The magnitude of reflection is significantly diminished and the reflection edge has shifted to about 400 nm, indicating a significant change in optical properties due to intercalation. (c) Reflectance at 526 nm measured while the cell is galvanostatically discharged. The reflectance decreases continuously as ions are intercalated, before flatlining at 0.38 V. The purple line tracks the reflectance of the cell while it is resting under OCV conditions. (d) Reflectance spectra of cells after 1, 5, and 18 cycles, showcasing a continued decrease in the magnitude of reflectance when cycled between 0.2 and 1.6 V.

Figure 5

CV curves under dark and illuminated conditions for V₂O₅–Zn and LiMn₂O₄–Zn cells. (a) The active material in the cell is facing the illumination source, and (b) the active material is shielded from the light source by a layer of carbon. A similar experiment is performed for LiMn₂O₄–Zn cells, where (c) the active material is illuminated and (d) the active material is obscured by a layer of carbon. For both V₂O₅ (dark cycle 20 and light cycle 21 in (e) and (f)) and LiMn₂O₄ (dark cycle 15 and light cycle 16 in (g) and (h)), the area under the CV curves shows a very similar increase, regardless of whether the active material is facing or shielded from the light (22% vs 26% for V₂O₅ and 9% vs 7% for LiMn₂O₄).

Figure 6

Thermal controls on LiMn₂O₄–Zn cells. (a) A planar LiMn₂O₄–Zn cell was initially charged (Step 1) and discharged (Step 2) in the dark (no heating or illumination) to establish a baseline capacity. Next, the cell was charged under 1 sun illumination (Step 3) and discharged in the dark (Step 4). The temperature probe records a temperature of 32 °C. Then, the cell was charged while maintaining a probe temperature of 32 °C (Step 5) and discharged (Step 6). (b) The cell was charged and discharged in the dark (Steps 7 and 8) and then charged under 1 sun illumination while cooling to maintain the initial temperature of the cell (Step 9), followed by discharge under ambient conditions (Step 10). After subtraction of thermal effects, the discharge capacity of the cell is very similar to its original baseline. Panels (c) and (d) are enlarged versions of (a) and (b), respectively, in the voltage range of 1.7–2.1 V.

Figure 7

Decoupling thermal and light effects under OCV charging for a V₂O₅–Zn cell. (a) Voltage vs time plots are shown for a cell that was initially discharged to a voltage of 0.2 V and displayed a discharge capacity of 0.29 mAh. The cell was allowed to relax in the dark for 32 h and then discharged again. A discharge capacity of 0.06 mAh was recorded. (b) The cell was subsequently allowed to relax under 1 sun illumination for 26 h. A rise in OCV to 1.056 V is measured. The cell is then discharged to 0.2 V, and a discharge capacity of 0.1 mAh is observed. (c) The cell is then allowed to relax in an incubator for 27 h. The OCV of the cell rises to 1.045 V. The cell is then discharged to obtain a discharge capacity of 0.1 mAh.