Thickness-Gated Water and Electron Transport in Near-Critical Ultrathin Polyethylene Membrane Electrodes.

Electrochemical systems are constrained by a materials paradox: chemically robust porous separators are typically hydrophobic and electronically insulating, forcing bulky, carbon-rich interfaces to couple mass transport and electrical contact. Here we show that pushing freestanding porous polyethylene into a near-critical ultrathin regime turns thickness into a geometric gate, enabling these functions to be switched without chemical modification. Near an inferred lower bound (∼10 nm), a 60 nm membrane already enters this regime: liquid-entry pressure collapses into a sub-bar window, enabling aqueous-electrolyte permeation, while the electrical threshold shifts to sub-volt soft dielectric breakdown, yielding persistent through-plane conductivity. The same scaffold enhances nanowire-matrix coupling via high conformability and real contact area, enabling a binder- and carbon-free ∼300 nm ultrathin membrane electrode. As a stringent liquid-phase electrochemical validation, this membrane electrode drives selective nitrate-to-ammonia electroreduction in flowing alkaline electrolytes with sustained performance. Near-critical ultrathinness thus emerges as a gateable design dimension for re-imagining commodity plastics as transmembrane-active platforms.