Stirring Without Stirrers: Polymer Fouling-Driven Mass Transport Unlocks Order-of-Magnitude Gain in Electrochemiluminescence.

Electrode reactions are central to analytical chemistry and a green approach to chemical synthesis. Here, it is demonstrated that sacrificing electrode-electrolyte contact to microscale polymeric blocks creates fouled electrodes that outperform unobstructed ones. By tuning the dielectric's geometry, surface chemistry, and charge - and controlling electrode alignment relative to gravity - a paradigm shift in electrode design, from "clean" to "fouled," as a strategy to enhance reaction rates is proposed. Electrochemiluminescence (ECL) microscopy reveals that strategic electrode fouling enhances mass transport, primarily through electrochemically actuated lateral density gradients. Engineered fouling induces flow velocities up to 0.4 cm s^-1 in otherwise quiescent systems. Sub-millimeter plastic features boost local rates by up to 290%, while micrometer-scale arrays yield a 30% net electrolysis gain. Through electrolyte engineering, it is shown that beyond expected hydrophobic reactant enrichment, the chemistry of the insulator influences reaction rates via electroosmotic flow and Marangoni-driven convection at the insulator-electrode-electrolyte boundary. This work establishes engineered fouling as a powerful strategy for enhancing electrochemical processes and provides a framework for designing advanced electrode architectures for ECL and electrosynthetic applications.