An Optimized Angular Total Internal Reflection Sensor With High Resolution in Vanadium Flow Batteries

An optimized angular total internal reflection (TIR) sensor system without sensing films is proposed to detect the state of charge (SOC) of the vanadium redox flow battery (VFB) in real time. In order to balance the pixel sensitivity and the system noise considering the pixel discreteness of the charge-coupled device (CCD), the distance between the sensor chip and the CCD is adjusted to obtain the optimal refractive index (RI) resolution. The optimal RI resolution is <inline-formula> <tex-math notation="LaTeX">$4.18\times 10^{-6}$ </tex-math></inline-formula> refractive index unit (RIU) at a distance of 257 mm with a dynamic range of 0.074 RIU. An algorithm of N-point average is utilized to further reduce the system noise, and therefore the RI resolution even reaches <inline-formula> <tex-math notation="LaTeX">$8.13 \times 10^{-7}$ </tex-math></inline-formula> RIU at the optimal distance. The whole discharge process of the VFB detected by the optimal TIR sensor demonstrates that during three stages (VO²⁺ to VO²⁺, VO²⁺ to V³⁺, and V³⁺ to V²⁺) of the valence state variation, the RI of the electrolyte is linear with the valence state in each stage. The valence state resolutions of three stages are, respectively, <inline-formula> <tex-math notation="LaTeX">$6.56 \times 10^{-5}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$10.04 \times 10^{-5}$ </tex-math></inline-formula>, and <inline-formula> <tex-math notation="LaTeX">$5.35 \times 10^{-5}$ </tex-math></inline-formula>. This experiment verifies that the TIR sensor with high RI resolution can detect the SOC variation rate of the VFB in real time, providing a research tool for studying the kinetics of electrochemical reactions.