Probing electrical double layer via triboelectric charge transfer.

The nanoscale electrical double layer (EDL) governs macroscopic phenomena such as ion adsorption and reaction kinetics, serving as a fundamental determinant in diverse applications ranging from sensing, and catalysis, to energy storage. While classical EDL models primarily describe conductive interfaces, most naturally occurring EDLs form at non-conductive surfaces in liquid environment, where characterization remains fundamentally challenging due to the constraints of conventional techniques. Here, we present a triboelectric nanogenerator (TENG)-based triboelectric charge transfer probe that utilizes the intrinsic solid-liquid contact electrification (CE) process to operando monitor the formation and evolution of the EDL at non-conductive interfaces. This bias-free and electrode-independent approach enables direct probing of interfacial charge dynamics fundamentally inaccessible to conventional electrochemical approaches constrained by conductive substrate dependencies and external potential requirements. This method also reveals distinct EDL behaviors, particularly in electrolytes with asymmetric ion sizes at concentrations exceeding 10-1 M and at non-conductive interfaces. Its fundamental mechanism and measurement precision were rigorously validated via atomic force microscopy, Kelvin probe force microscopy, surface-enhanced Raman spectroscopy, and molecular dynamics simulations, establishing a robust analytical platform and theoretical basis for EDL studies. This work introduces a CE-based methodology for direct triboelectric charge characterization on dielectric surfaces, overcoming conventional conductive substrate limitations. By integrating classical EDL theory with triboelectric frameworks, we establish models resolving interfacial charge dynamics across diverse solid-liquid interfaces, including high ionic strength regimes. It confirms material-agnostic applicability. This paradigm simultaneously advances fundamental EDL mechanisms and enables programmable charge manipulation for next-generation iontronic power, sensing, and neuromorphic devices.