Electrokinetic energy conversion in nanofluidic channels: Addressing the loose ends in nanodevice efficiency (original) (raw)
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Electrokinetic Pumping And Energy Conversion At Nanoscales
2014
The integration of the coupling effects of intrinsic wettability and surface charge in a nanochannel can cause non-intuitive behavior in the electrokinetic energy conversion processes. We demonstrate that in a nanofluidic device the energy conversion efficiencies may get amplified with an increase in surface charge density, not perpetually, but only over a narrow regime of low surface charges, and may get significantly attenuated to reach a plateau beyond a threshold surface charging condition. This results from the complex interplay between fluid structuration and ionic transport within a charged interfacial layer. We explain the corresponding findings from our molecular dynamics simulations with the aid of a simple modified continuum based theory. We attribute our findings to the four-way integration of surface charge, interfacial slip, ionic transport, and the water molecule structuration. The consequent complex non-linear nature of the energy transfer characteristics may bear fa...
We devise a new approach for capturing complex interfacial interactions over reduced length scales, towards predicting electrokinetic energy conversion efficiencies of nanofluidic devices. By embedding several aspects of intermolecular interactions in continuum based formalism, we show that our simple theory becomes capable of representing complex interconnections between electro-mechanics and hydrodynamics over reduced length scales. The predictions from our model are supported by reported experimental data, and are in excellent quantitative agreement with molecular dynamics simulations. The present model, thus, may be employed to rationalize the discrepancies between low energy conversion efficiencies of nanofluidic channels that have been realized from experiments, and the impractically high energy conversion efficiencies that have been routinely predicted by the existing theories.
The present study utilizes a modified Poisson-Boltzmann (MPB) equation to take into account ion-size (steric) effect and then performs a series of simulations to investigate the ionic transport phenomenon within two nanofluidic devices, namely a nanoslit and a nanotube. The results show that at all electrolyte concentrations, the streaming conductance increases when the ion-size effect is taken into account, particularly under conditions of high surface charge density. The net charge density is amplified by the ion-size effect for both nanogeometries as well. The enhancement in the streaming conductance is particularly pronounced in the nanotube since the net charge density has higher value by the geometry effect of the nanotube. The maximum ratio of net charge density (ρ e steric =ρ e nonÀsteric ) is 1.55 near the surface of nanotube. In addition, it is shown that for both configurations, the contribution of electroosmotic flow to the electrical conductance increases when the finite ion size is taken into account.
Nano Energy, 2017
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.
Molecular Theory for Electrokinetic Transport in pH-Regulated Nanochannels
Ion transport through nanochannels depends on various external driving forces as well as the structural and hydrodynamic inhomogeneity of the confined fluid inside of the pore. Conventional models of electrokinetic transport neglect the discrete nature of ionic species and electrostatic correlations important at the boundary and often lead to inconsistent predictions of the surface potential and the surface charge density. Here, we demonstrate that the electrokinetic phenomena can be successfully described by the classical density functional theory in conjunction with the Navier−Stokes equation for the fluid flow. The new theoretical procedure predicts ion conductivity in various pH-regulated nanochannels under different driving forces, in excellent agreement with experimental data.
Energy Conversion Efficiency of Nanofluidic Batteries: Hydrodynamic Slip and Access Resistance
The Journal of Physical Chemistry C, 2013
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.
Electrokinetics in nanochannels
Journal of Colloid and Interface Science, 2008
In this paper a new model is described for calculating the electric potential field in a long, thin nanochannel with overlapped electric double layers. Electrolyte concentration in the nanochannel is predicted self-consistently via equilibrium between ionic solution in the wells and within the nanochannel. Differently than published models that require detailed iterative numerical solutions of coupled differential equations, the framework presented here is self-consistent and predictions are obtained solving a simple one-dimensional integral. The derivation clearly shows that the electric potential field depends on three new parameters: the ratio of ion density in the channel to ion density in the wells; the ratio of free-charge density to bulk ion density within the channel; and a modified Debye-Hückel thickness, which is the relevant scale for shielding of surface net charge. For completeness, three wall-surface boundary conditions are analyzed: specified zeta-potential; specified surface net charge density; and charge regulation. Predictions of experimentally observable quantities based on the model proposed here, such as depth-averaged electroosmotic flow and net ionic current, are significantly different than results from previous overlapped electric double layer models. In this first paper of a series of two, predictions are presented where channel depth is varied at constant well concentration. Results show that under conditions of electric double layer overlap, electroosmosis contributes only a small fraction of the net ionic current, and that most of the measurable current is due to ionic conduction in conditions of increased counterion density in the nanochannel. In the second of this two-paper series, predictions are presented where well-concentration is varied and the channel depth is held constant, and the model described here is employed to study the dependence of ion mobility on ionic strength, and compare predictions to measurements of ionic current as a function of channel depth and ion density.
Ion-Specific Anomalous Electrokinetic Effects in Hydrophobic Nanochannels
Physical Review Letters, 2007
We demonstrate with computer simulations that anomalous electrokinetic effects, such as ion specificity and non-zero zeta potentials for uncharged surfaces, are generic features of electro-osmotic flow in hydrophobic channels. This behavior is due to the stronger attraction of larger ions to the "vapour-liquid-like" interface induced by a hydrophobic surface. An analytical model involving a modified Poisson-Boltzmann description for the ion density distributions is proposed, which allows the anomalous flow profiles to be predicted quantitatively. This description incorporates as a crucial component an ion-size-dependent hydrophobic solvation energy. These results provide an effective framework for predicting specific ion effects, with important implications for the modeling of biological problems.
Role of Surface Chemistry in Nanoscale Electrokinetic Transport
his dissertation work presents the efforts to study the electrofluidics phenomena, with a focus on surface charge properties in nanoscale systems with the potential applications in imaging, energy conversion, ultrafiltration, DNA analysis/sequencing, DNA and protein transport, drug delivery, biological/chemical agent detection and micro/nano chip sensors. Since the ion or molecular or particle transport and also liquid confinement in nano- structures are strongly dominated by the surface charge properties, in regards of the fundamental understanding of electrofluidics at nanoscale, we have used surface charge chemistry properties based on 2-pK charging mechanism. Using this mechanism, we theoretically and analytically showed the surface charge properties of silica nanoparticles as a function of their size, pH level and salt ionic strength of aqueous solution. For a fixed particle size, the magnitude of the surface charge typically increases with an increase in pH or background salt concentration. Furthermore, we investigated the surface charge properties of a charged dielectric nanoparticle and flat wall in electrostatic interactions. According to the theoretical results strong interactions cause a non-uniform surface charge density on the nanoparticle and the plate as a result of the enhancement of proton concentration in the gap between the particle and the plate. This effect increases with decreased separation distance (κh). We moreover investigated the ion confinement inside the nanospaces and using a continuum model, we showed the proton enhancement in extended nanochannels. The proton enrichment at the center of the nanochannel is significant when the bulk pH is medium high and the salt concentration is relatively low. The results gathered are informative for the development of biomimetic nanofluidic apparatuses and the interpretation of relevant experimental data. Later, we have developed an analytical model for electroosmotic ion transport inside pH-regulated nanoslits and compared the results with the numerical study. We showed the influences of background salt concentration, pH level and the length of nanoslit on EOF velocity. The predictions show that the EOF velocity increases first and then decrease with background salt concentration increasing and the EOF velocity increases with pH level of aqueous solution.
Surface-Charge-Governed Ion Transport in Nanofluidic Channels
Physical Review Letters, 2004
A study of ion transport in aqueous-filled silica channels as thin as 70 nm reveals a remarkable degree of conduction at low salt concentrations that departs strongly from bulk behavior: In the dilute limit, the electrical conductances of channels saturate at a value that is independent of both the salt concentration and the channel height. Our data are well described by an electrokinetic model parametrized only by the surface-charge density. Using chemical surface modifications, we further demonstrate that at low salt concentrations, ion transport in nanochannels is governed by the surface charge.