Role of LiOH in Aqueous Electrocatalytic Defluorination of Perfluorooctanoic Sulfonate: Efficient Li-F Ion Pairing Prevents Anode Fouling by Produced Fluoride.

Per- and polyfluoroalkyl substances (PFAS) pose a significant environmental and health threat due to their high toxicity, widespread use, and persistence in the environment. Electrochemical methods have emerged as promising approaches for PFAS destruction, offering cost-effective and energy-efficient solutions. We established recently that electrocatalysis with nonprecious materials enabled the complete defluorination of perfluorooctanesulfonate (PFOS) in aqueous 8.0 M LiOH. Here, we reveal the mechanistic role of LiOH in the efficient aqueous electrocatalytic PFOS defluorination. Our results demonstrate that synergistic effects of high lithium and high hydroxide ion concentrations are essential for complete PFOS defluorination. Two-dimensional NMR data of electrolytes post pulsed electrolysis provide experimental evidence for Li-F ion pairing, which plays a crucial role in preventing anode fouling by produced fluoride, thus enabling sustained C-F bond cleavage. This Li-F ion pairing was increased at high pH, and elevated temperatures enhanced diffusion of Li-F ion pairs into the bulk electrolyte. High hydroxide ion concentrations additionally removed fluoride from the anode surface by competitive adsorption, corroborated by XPS data. Our findings provide quantitative mechanistic insights into the electrocatalytic defluorination process and offer a general route of enhancing the efficiency of anodic PFAS defluorination.