Enhanced voltage generation through electrolyte flow on liquid-filled surfaces

B Fan,A Bhattacharya,P R Bandaru

doi:10.1038/s41467-018-06297-9

B Fan, A Bhattacharya + Show 1 more

Open Access

https://doi.org/10.1038/s41467-018-06297-9

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Abstract

The generation of electrical voltage through the flow of an electrolyte over a charged surface may be used for energy transduction. Here, we show that enhanced electrical potential differences (i.e., streaming potential) may be obtained through the flow of salt water on liquid-filled surfaces that are infiltrated with a lower dielectric constant liquid, such as oil, to harness electrolyte slip and associated surface charge. A record-high figure of merit, in terms of the voltage generated per unit applied pressure, of 0.043 mV Pa−1 is obtained through the use of the liquid-filled surfaces. In comparison with air-filled surfaces, the figure of merit associated with the liquid-filled surface increases by a factor of 1.4. These results lay the basis for innovative surface charge engineering methodology for the study of electrokinetic phenomena at the microscale, with possible application in new electrical power sources.

Highlights

The generation of electrical voltage through the flow of an electrolyte over a charged surface may be used for energy transduction
On application of a pressure difference (ΔP) across the two ends of the channel, say, through a mechanical pump, Fig. 1a–c, the electrolyte flow would be mainly constituted by the mobile counterions in the diffuse layer with the consequent charge separation yielding a streaming potential (Vs) that is proportional to the ε (= εοεr- with εο = 8.854. 10−12 C2 N−1 m−2 as the free-space permittivity, and εr as the relative permittivity of electrolyte e.g., ~ 80 for 0.1 mM L−1 NaCl solution) and the zeta potential (ζ) at the edge of the shear plane where the mobile ion motion occurs and varies inversely with the dynamic viscosity of the electrolyte (η) and bulk electrolyte conductivity (κ), as described through the Helmholtz–Smoluchowski model[7,11]: Vs
The lower Vs of Air-filled surface (AFS) may be rationalized as due to the absence of a surface charge in the air regions[13], which overwhelms the contribution of the slip

Summary

Introduction

The generation of electrical voltage through the flow of an electrolyte over a charged surface may be used for energy transduction. Most work on harnessing the Vs through electrokinetic effects, to date, has been concerned with fluid flow over smooth surfaces (where the scale of roughness is smaller than λD), and very small potential differences of the order of 18 mV may be predicted and obtained[11], in correspondence to Eq (1), i.e., for 0.1 mM L−1 NaCl, with εr ~ 80, η ~ 10−3 Pa·s, ζ ~ 25 mV, κ ~ 10−3 S m−1, with ΔP ~ 1000 Pa. With the objective of obtaining significantly larger Vs, we indicate briefly the principles of our approach, which first involved the modulation of the effective ζ through introducing fluid slip via groove-patterned surfaces (in air-/liquid-filled surfaces) and modifying surface charges at the substrate surface.

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