Energy Conversion Efficiency of Nanofluidic Batteries: Hydrodynamic Slip and Access Resistance

With asymptotic and numerical analyses, we systematically study the influence of slip length and access Ohmic resistance (due to pore-end field focusing and concentration polarization) on the energy conversion efficiency of pressure-driven electrolyte flow through a charged nanopore. Hydrodynamic slip reduces the percent of energy dissipated by viscous dissipation but, through electro-osmotic convective current, can also reduce the electrical resistance of the nanopore. Since the nanopore resistance is in parallel to the load access serial resistance, the latter effect can actually reduce useful current through the load. These two opposing effects of slip produce specific and finite optimum values of surface charge density and ionic strength. The optimization offers explicit analytical estimates for the realistic parameters and suggests an upper bound of 50% conversion efficiency at the slip length of 90 nm and 35% for measured electro-osmotic flow slip lengths of about 30 nm for charged channels.