Thermal dependence of nanofluidic energy conversion by reverse electrodialysis.

Junho Hwang,Wei-Lun Hsu,Tatsuki Sekimoto,Akira Endo,Hirofumi Daiguji,Sho Kataoka

doi:10.1039/c7nr04387b

Abstract

The thermal dependence of salinity-gradient-driven energy conversion by reverse electrodialysis using a mesoporous silica thin film with pores ca. 2-3 nm in diameter was studied in a temperature range of 293-333 K. As the temperature increases, the surface charge density of mesopores increases owing to an increase in the zeta potential of the pore walls, which in turn increases the concentration of counter-ions in the electrical double layer. The ion mobility also increases with increasing temperature owing to a decrease in the liquid viscosity. As a result, the temperature increase improves the ion conductance of mesopores both in the surface-charge-governed regime at low ion concentrations and in the bulk regime at high ion concentrations. However, further increases in temperature induce bubble nucleation. In particular, in highly concentrated salt solutions, hydrophobic patches appear on the pore surfaces because of the salting-out effect and mask the surface charge. The weakened polarity in mesopores allows more co-ions to enter them, decreasing the potential difference across the film, resulting in a serious deterioration of the energy conversion efficiency. The thermal dependence of the performance characteristics of mesoporous-silica-based nanofluidic devices was also evaluated.

Highlights

Functional nanostructured-material-based energy conversion systems can be exploited as reliable alternative energy sources for emerging miniaturized electrical devices.[1,2,3] In particular, the nanofluidic-based power generator using concentrationgradient-driven ion-selective transport, which is known as the reverse electrodialysis (RED) power generator (Fig. 1a), has recently attracted increasing attention and is intensively studied to improve its performance characteristics such as open-circuit voltage (Voc), short-circuit current (Isc), maximum power density, and maximum energy conversion efficiency
The fully packaged nanofluidic device for energy conversion studies was prepared by combining a rectangularly patterned mesoporous silica (MPS) thin film obtained by photolithography with a microstructured poly(dimethylsiloxane) (PDMS) chip embossed by the replica micromolding method
The increase in the conductance with the increasing temperature can be attributed to the increased surface charge density and the decreased viscosity of the aqueous solutions

Summary

Introduction

Functional nanostructured-material-based energy conversion systems can be exploited as reliable alternative energy sources for emerging miniaturized electrical devices.[1,2,3] In particular, the nanofluidic-based power generator using concentrationgradient-driven ion-selective transport, which is known as the reverse electrodialysis (RED) power generator (Fig. 1a), has recently attracted increasing attention and is intensively studied to improve its performance characteristics such as open-circuit voltage (Voc), short-circuit current (Isc), maximum power density ( pmax), and maximum energy conversion efficiency (ηmax). In silica–aqueous solution systems, a temperature increase can affect the equilibrium constants of the dissociation and protonation reactions of silanol groups, and the dissociation constant of water, the ion diffusivity, and the liquid viscosity. All of these changes can affect the conductance of the nanofluidic channels/pores.[11] As the temperature increases, the surface charge density of the silica mesopores increases,[12,13,14,15] and the concentration of the counter-ions in the electrical double layer (EDL) increases.

Objectives

Results

Conclusion