Electroactive fiber optics for real-time, multiscale probing of ion transport and interfacial chemistry

Abstract Understanding charge dynamics at electrochemical interfaces is essential for establishing the basic mechanisms that underpin the functionality of energy storage devices and ultimately improve their performance, safety and longevity. However, direct, time-resolved access to such information remains challenging due to the complex and convoluted conditions necessary for charge transfer between materials and phases. Here, we report an electroactive fiber-optic device that integrates plasmonic sensing with electrochemical functionality, enabling real-time, multi-scale tracking of interfacial physicochemical changes. Gold-coated tilted fiber Bragg gratings were employed as dual-purpose sensors, simultaneously acting as working electrodes and optical probes of the resulting dynamics. By quantifying spectral response we demonstrate the ability to discriminate charge transfer phenomena across multiple regimes: double-layer formation, ionic and molecular adsorption, faradaic reactions involving both soluble redox species and immobilized thin solid films. By integrating a novel ‘optical-mass’ sensitivity concept, this approach unlocks access to comprehensive operando, real-time monitoring in both aqueous and organic electrolytes. As a first step toward practical deployment, we further demonstrate the feasibility of embedding a surface plasmon resonance tilted fiber Bragg grating sensor within a sealed 18650 Na-ion cell, enabling simultaneous electrochemical and optical interrogation of parasitic redox processes during formation. These results provide a new foundation for non-invasive, mechanistic studies of critical importance to energy storage, electrocatalysis and advanced materials technologies.