Uphill Anion Pumping through Triangular Nanofluidic Device of Reconstructed Layered Double Hydroxide

Abstract

Numerous research efforts have been devoted to replicate the functioning of biological ion pumps in nanofluidic devices. Unlike previous reports of cationic pumps, positively charged CoAl layered double hydroxide (CoAl LDH) is exploited here for uphill pumping of anionic species. Nanofluidic membranes prepared by self-assembling exfoliated layers of CoAl LDH exhibit excellent anionic transport characteristics. At the surface-charge-governed regime, the positively charged CoAl LDH membrane (p-LDHM) showed a remarkable OH– ion conductivity of ∼2 mS cm–1. The remarkable mobility of OH– ions (4 × 10–4 cm2 V–1 S–1) in the atomically thin channel of p-LDHM is attributed to the tiny activation energy (0.09 eV) required for Grotthuss-like hopping of the ions between the positive charges of densely packed CoAl LDH layers. The lamellar p-LDHM was also found to be suitable for energy harvesting via salinity gradient, and a power density up to 0.7 Wm–2 was achieved under a 1000-fold concentration gradient. The triangular p-LDHM displays a diode-like nonlinear I–V curve, attributed to a combination of unipolar conductivity of counterions inside the two-dimensional (2D) nanochannels and geometrical asymmetry. The triangular p-LDHM pumps anions against the concentration gradient (up to 1000-fold), under fluctuating external potentials with a mean value of zero.