State of Charge in a Vanadium Redox Flow Battery Measured via ¹H MR Relaxation with Low Field Portable Magnets

Vanadium redox flow batteries offer a promising solution for medium- to large-scale energy storage applications. Accurately monitoring the state of charge (SOC) of these batteries is crucial for optimizing long-term performance and ensuring effective battery control. Electrolyte crossover and side reactions can degrade battery performance, but traditional electrochemical techniques are often inadequate for diagnosing these issues. This study introduces a novel ¹H magnetic resonance approach to estimate the SOC by analyzing the bulk relaxation times, <i>T</i>₁ and <i>T</i>₂, in the electrolyte. The basis of the measurement is the paramagnetic relaxation enhancement effect of vanadium ions on the bulk solution. The four different vanadium oxidation states in the redox flow battery have very different effects on the bulk relaxation times. The different relaxivities of these four species permit the determination of concentration. The measurement employs two MR devices, one measuring the cathode electrolyte and one measuring the anode electrolyte. The magnets are small inexpensive, permanent magnets, Proteus magnets, with a ¹H resonance frequency of 20 MHz. To the best of our knowledge, this is the first analytical MR measurement employing two discrete MR magnets in close proximity. The prospect exists for simultaneous measurement with the two MR devices, although measurements are sequential in this study. We demonstrate how 2D maps correlating <i>T</i>₁ with <i>T</i>₁, <i>T</i>₂ with <i>T</i>₂, and <i>T</i>₁ with <i>T</i>₂, measured from both sides of the battery, can effectively "map" the SOC during operation.