Buffer anions can enormously enhance the electrokinetic energy conversion in nanofluidics with highly overlapped double layers

Abstract

Electrokinetic energy, where the pressure-driven transport of ions through nanofluidics yields streaming potential/current, is one of the next generation, sustainable, clean energies by converting hydraulic to electrical power. In this study, we develop an analytical model taking into account many practical effects, such as the Stern layer, buffer anions (e.g., HEPES, ACES, and lactic acid), electric double layers (EDLs) overlap, and surface equilibrium reactions, to investigate the buffer effect on the electrokinetic energy conversion in a long, pH-regulated nanochannel. Taking the nanochannel made of silica as an example, we for the first time show that introducing buffer anions into the working fluid can significantly enhance not only the maximum electrokinetic power output but also the corresponding conversion efficiency in a nanochannel under the condition of highly overlapped EDLs (e.g., low background salt concentration and/or small channel height). With buffer anions, the performance of electrokinetic energy, depending on the salt concentration, pH, and nanoscale channel height, can be enhanced at a degree as high as 1.5–26 times, as compared to the case without buffer anions. This work provides a useful receipt for estimating the electrokinetic energy in the nanochannel in the presence of buffer anions and the finding is of crucial importance for renewable energy applications.