Electrical Double Layer as a Voltage Divider: Suppressing Counterion Transport in Atomically Thin Nanopores

Surface charge in nanochannels is traditionally understood to enhance the ionic conductance. Particularly in the low-concentration regime, the electrical double layer (EDL) enriches counterions significantly above bulk levels, thereby sustaining high conductance. Experimental deviations from this behavior, such as anomalous conductance scaling, are typically attributed to surface physicochemical limitations or an insufficient ion supply. However, through finite element method simulations of ion transport in atomically thin nanopores, we reveal that the EDL itself acts as a potent voltage divider. This effect intensifies by up to an order of magnitude as the surface charge density increases or the bulk concentration decreases. Consequently, the local driving field within the pore is severely suppressed, rendering the enriched counterions ineffective for conduction despite their high local density. We derive a phenomenological model for this field attenuation and combine it with a refined, beyond-Donnan ion distribution profile to construct a conductance formulation that accurately reproduces the observed scaling behavior. Our findings unveil a dual nature of the EDL: while capable of recruiting conductive ions, the EDL can paradoxically function as a formidable barrier to transport via voltage division, challenging the classical view of surface-charge-governed conduction.