[Paper Review] Chapter: Energy conversion at water-solid interfaces using electrokinetic effects

Cecilia Herrero, Aymeric Allemand+3 more|arXiv (Cornell University)|Apr 28, 2022

Nanopore and Nanochannel Transport Studies4 citations

TL;DR

This paper presents a theoretical framework for energy conversion at water-solid interfaces using electrokinetic effects in nanofluidic systems, leveraging linear irreversible thermodynamics to link thermodynamic gradients (e.g., salinity, temperature, electrostatic potential) to coupled fluxes (hydrodynamic, electrical, thermal). The key contribution is identifying optimal design parameters—such as low salt concentration, small channel size, and low thermal/electrical conductivity—for maximizing efficiency in harvesting osmotic and waste heat energy.

ABSTRACT

Our Society is in high need of alternatives to fossil fuels. Nanoporous systems filled with aqueous electrolytes show great promises for harvesting the osmotic energy of sea water or waste heat. At the core of energy conversion in such nanofluidic systems lie the so-called electrokinetic effects, coupling thermodynamic gradients and fluxes of different types (hydrodynamical, electrical, chemical, thermal) at electrified water-solid interfaces. This chapter starts by introducing the framework of linear irreversible thermodynamics, and how the latter can be used to describe the direct and coupled responses of a fluidic system, providing general relations between the different response coefficients. The chapter then focuses on the so-called osmotic flows, generated by non-hydrodynamic actuation at liquid-solid interfaces, and illustrate how the induced fluxes can be related to the microscopic properties of the water-solid interface. Finally, the chapter moves to electricity production from non-electric actuation, and discusses in particular the performance of nanofluidic systems for the harvesting of osmotic energy and waste heat.

Motivation & Objective

• To establish a theoretical foundation for energy conversion at water-solid interfaces using electrokinetic effects.
• To identify the key physical parameters governing efficiency in nanofluidic systems for osmotic and thermal energy harvesting.
• To bridge microscopic interfacial properties (e.g., electrical double layer structure) with macroscopic transport responses.
• To optimize system performance by balancing electrokinetic response against electrical conductance in nanochannels.

Proposed method

• Applies linear irreversible thermodynamics to model coupled fluxes (hydrodynamic, electrical, thermal) driven by thermodynamic gradients.
• Uses the Gibbs equation and generalized forces (affinities) to define thermodynamic driving potentials for energy, volume, particle, and charge transport.
• Derives response coefficients (e.g., electro-osmotic mobility, streaming current, Seebeck coefficient) from microscopic interfacial properties.
• Analyzes osmotic flows induced by non-hydrodynamic forces such as electrostatic or thermal gradients at electrified interfaces.
• Evaluates thermoelectric performance via the dimensionless figure of merit $ZT = M_{\rm te}^2 / (\sigma k T)$, linking it to interfacial transport coefficients.
• Considers the breakdown of continuum models at molecular-scale confinement, advocating for statistical physics and quantum-level descriptions.

Experimental results

Research questions

• RQ1 How can electrokinetic effects be systematically described using linear irreversible thermodynamics in nanofluidic systems?
• RQ2 What are the dominant microscopic factors that enhance osmotic flows at water-solid interfaces?
• RQ3 How does the overlap of electrical double layers (EDLs) in narrow channels affect energy conversion efficiency?
• RQ4 What are the optimal material and geometric parameters to maximize $ZT$ in nanofluidic thermoelectric systems?
• RQ5 What new phenomena emerge at the molecular scale that challenge classical continuum descriptions of interfacial transport?

Key findings

• Electrokinetic effects in nanofluidic systems arise from coupling between thermodynamic gradients and fluxes at electrified water-solid interfaces, mediated by the electrical double layer (EDL).
• Osmotic flows can be generated by non-hydrodynamic actuation (e.g., electrostatic or thermal gradients), with magnitude dependent on EDL structure and surface slip.
• The Seebeck coefficient $S_e = M_{\rm te}/(\sigma T)$ links thermoelectric response to the thermoelectric coefficient $M_{\rm te}$, which depends on interfacial properties.
• Maximizing $ZT$ requires minimizing both electrical and thermal conductivities of the system, especially in low-salt, sub-10 nm channels.
• EDL overlap in narrow channels invalidates thin EDL approximations, necessitating extended analytical models for accurate prediction of transport coefficients.
• At molecular-scale confinement, continuum models fail, and quantum and statistical physics approaches are required to capture emergent interfacial phenomena.