A Fixed‐Charge Interphase Synchronizes Ion Transport to Suppress Space‐Charge‐Driven Inefficiency Under Nanoliter Confinement

Ion transport at electrified interfaces is conventionally described by the redistribution of mobile ions to preserve local electroneutrality. Under extreme electrolyte confinement, however, this assumption fails as the characteristic transport length approaches the Debye screening length, giving rise to space-charge accumulation and slow electrostatic relaxation that dominate interfacial kinetics. Here, we introduce a fixed-charge-selective interphase in which immobile anionic charges replace mobile electrolyte anions as the primary charge-compensating species, thereby establishing a chemically encoded electrostatic boundary condition. Using a glucose-derived network as a model system, we show that localized fixed charge enables cation-selective transport and suppresses extended space-charge layers (ESCLs) by eliminating the slow relaxation pathways. Spatiotemporal transport analysis reveals that this interphase collapses multi-timescale interfacial relaxation into a unified kinetic regime. When applied to nanoliter-confined electrochemical systems (45 nL), rest-induced Coulombic efficiency (CE) collapse is reduced from 40% to 5%, demonstrating stabilization of electrostatic relaxation during idle periods, which is a failure mode intrinsic to microscale devices operating under duty cycles. The concept is further validated under pH-coupled and oxidative-stress conditions, sustaining stable operation with strong rate capability. These results define a general chemical strategy for regulating interfacial ion transport under confinement by replacing mobile charge compensation with molecularly fixed charges.