Halide-mediated electrochemical functionalization of lignin for high-performance flexible humidity sensors

Lignin is an abundant, renewable aromatic biopolymer that remains largely underutilized despite its rich chemical potential. In this work, lignin is valorized through a mild electrochemical functionalization process using chloride ions as a redox mediator on a boron-doped diamond electrode. This halide-mediated oxidation introduces oxygen-containing functional groups, while simultaneously inducing particle fragmentation and increasing surface accessibility. These structural modifications significantly enhance the hydrophilicity and interfacial interaction of lignin with gold electrodes. The electrochemically oxidized lignin is combined with polyvinylpyrrolidone (PVP) and subsequently integrated as the active sensing layer in a resistive humidity sensor fabricated on flexible Au/Cr interdigitated electrodes. The resulting sensor achieves a high humidity response of up to 2.0 × 10⁷% over a broad relative humidity range (33-90% RH). The device exhibits an optimal dynamic operating range between 40% and 90% RH, featuring fast response and recovery times of 0.2  s. Kinetic limitations are observed at the lower humidity (ca. 200  s at 33% RH) as the isolated water monolayers lack continuous hydrogen-bonding required to facilitate rapid Grotthuss proton transport. Through this approach, we demonstrate an effective pathway for advanced biomass valorization in functional, moisture-responsive flexible electronics.