Session R17: Electrokinetic Transport II

Conducting Polymer Jets in Electrostatically-Assisted Direct Ink Writing (eDIW)

The conducting polymer PEDOT:PSS has been in focus for its properties, suitable for the Direct Ink Writing (DIW) technique, such as high chemical stability and electrical conductivity. However, the process faces issues with jet stability. Modifying the solution with polyethylene oxide (PEO) can enhance the printability, along with an additional electric field (eDIW) feature. This work studies the PEDOT:PSS PEO mixture for eDIW, analyzing the jet flow subjected to the electric field (EF). The experimental results are compared to a novel theory also developed in this work to predict the electric field strength, the electric force and the resulting jet deflection. In the experiments a Phantom camera was used to record the jet at different EF strengths (0 to 10 kV), and the recorded jet configurations were compared to the theoretical predictions. The electrically-driven deflection for the 300 μm diameter jets became visible at 5 kV and intensified significantly at 9 kV. To measure the electric charge carried by the jet, a separate experiment was conducted: the jet was extruded onto a grounded plate and the electric current values were measured.   

Giant osmotic power generation in subnanometer-diameter single-wall carbon nanotubes

In nano- or angstrom-scale 1-D pores, surface charge significantly influences ion transport, and enables potential applications in desalination, ultrafiltration, and energy conversion. Ion transport through 1D nanopores with homogeneously distributed charge, such as boron nitride nanotubes, has been recently studied, with such nanotubes demonstrating the capacity to generate "giant" osmotic power under a salt gradient. However, the mechanisms of ion transport in inhomogeneously charged 1D nanopores remain intriguing. Here, we explore the use of 0.8-nm and 3-nm-diameter single-wall carbon-nanotubes (SWCNTs) with highly charged entrance regions and comparatively uncharged center regions. The SWCNTs reveal selective ion transport and exceptionally high osmotic-power densities comparable to and exceeding that of boron nitride nanotubes. To explain this phenomenon, we use analysis and finite-element modelling to compare diffusio-osmotic flow in homogeneously and inhomogeneously charged nanotubes. The evidence indicates that the highly charged entrance region dominates ion selectivity and generates electrical current induced by salinity gradients. Understanding the ion-transport mechanisms in inhomogeneously charged, small-diameter CNTs may enable ion-selective nanotube membranes optimized for different energy-conversion and separation processes, including desalination and osmotic-power harvesting.   

Net electroosmotic flow along an uncharged polarizable surface under DC electric field.

To induce a net electroosmotic flow (EOF) in the vicinity of an uncharged polarizable surface or around an immersed polarizable body by a comparatively weak DC electric field, a symmetry-breaking phenomenon needs to be invoked. Previous studies reveal that the symmetry can be broken either via the geometry of the surface/body or by modifying the surface properties. We find that even without such types of symmetry breaking, a substantial net EOF develops under a weak DC electric field in a charge-asymmetric electrolyte with different valences of anions and cations. We numerically solve the coupled set of Poisson-Nernst-Planck and Navier-Stokes equations around a spherical particle and in a nanochannel, especially for the case that the solid material is highly electrically polarizable. We also derive an analytical expression for the electrophoretic mobility of the particle in the limit of very high dielectric permittivity and thin electric double layer. The derived scaling relationships indicate that giant net flow velocities of the order of meters per second may occur in the nanochannel, which opens up new opportunities for future research.   

Influence of grafting properties on electrokinetic flow of polyelectrolyte solutions in brush-grafted microchannels

Brush-grafted channels have the advantage of a tunable and wide range of response to external stimuli, allowing us to use them in various applications. We modeled the electrokinetic flows of Newtonian as well as polyelectrolyte (PE) solutions in PE brush-grafted microchannels, on the basis of the continuum approach. In our model framework, the Poisson-Nernst-Planck equations are explicitly solved for the electrostatic field incorporated with the Alexander-de Gennes model for PE brush-layer and each ion concentration estimated by multi-species ion balance. Accounting for the Brinkman hydrodynamic friction inside the brush-layer, Bird-Carreau constitutive model is applied in the Cauchy momentum equation to describe the PE solution of anionic polyacrylic acid (PAA). This presentation reports in-detail the new results regarding the effects of grafting properties in terms of grafting density and Kuhn segment length. The electrostatic potential increases with increasing grafting density, whereas the surface potential decreases with increasing Kuhn length clearly unlike in the bulk. It is emphasized that the flow velocity decreases with either higher grafting density owing to enhanced PAA chain friction or larger Kuhn length according to higher flow retardation due to chain stiffness. The corresponding viscosity profile inside the channel is also examined with variations of pH and concentration of PAA dispersion.   

Charging dynamics of electrical double layers in a pore with an axially varying radius: Impact of pore shape and roughness

Electric double-layer capacitors (EDLCs) are devices that provide high power density and fast charging times, offering a unique advantage over batteries in high-power density applications. Previous studies have used direct numerical simulations to explore ionic transport inside porous electrodes. However, these simulations are computationally expensive and limited to single pores. To enhance EDLCs, it is essential to simulate ionic transport in pore networks containing thousands or millions of pores.

In our prior work, we linearized the Poisson-Nernst–Planck equations to develop an equivalent model of ionic transport in a network of straight cylindrical pores. In this study, we expand the model and derive effective equations for a pore where the radius varies along the axial direction. We investigate linearly converging or diverging pores and find that convergence allows for faster charge times. Additionally, we focus on a geometry with cyclic variation to model a "rough pore" and thoroughly examine the impact of phase variation of sinusoidal functions on charging times. Our work opens opportunities to study networks of shape-changing pores, which will more accurately capture the performance of EDLCs and aid in their future design.   

Investigation of Electrical Double Layers of Ionic Liquids in Graphene-Capped Liquid Cells by Multimodal Synchrotron Infrared Nanospectroscopy for Advanced Energy Storage Devices

Electrical double layer capacitors (EDLCs) are prominent energy devices owing to their rapid charge/discharge processes and extremely long cycle life. However, the generally low energy density of EDLCs hinders their broad application. A major strategy to improve the energy density of EDLCs involves employing electrolytes capable of high operation voltages. Ionic liquids (ILs) are a novel class of electrolytes typically composed of asymmetric organic cations and weakly coordinated anions. They appear to be ideal candidates to replace the diluted aqueous and organic electrolytes in EDLCs for their wide electrochemical windows, which enable operation voltages much higher than conventional EDLCs and thus significantly improve the energy density. Moreover, the unique EDL structure of ILs may present great possibilities for superior performance to traditional electrolytes. The implementation of ILs as electrolytes in EDLCs for enhanced performance and functionalities is contingent upon understanding the behaviors of IL EDLs, a knowledge gap that exists thus far. Here, we incorporate ILs in custom-made graphene-capped liquid cells, which allow for synchrotron infrared nanospectroscopy (SINS) investigation of IL EDLs, in combination with multimodal atomic force microscopy characterizations. This approach effectively correlates the chemical and vibrational bond information of IL EDLs with their nanoscale ion ordering and charge distribution, leading to a comprehensive understanding of IL EDLs that was previously inaccessible.   

Massively non-linear increase in electroosmotic transport in nanochannels grafted with cationic polyelectrolyte brushes

In this study, we employ all-atom molecular dynamic (MD) simulations for probing the electroosmotic (EOS) transport in nanochannels grafted with cationic [poly(2-

(methacryloyloxy)ethyl) trimethylammonium chloride] (PMETAC) polyelectrolyte (PE) brushes. Our central finding is that there is a massively non-linear increase in the EOS flow strength with the electric field: with an increase in the electric field strength by two-fold, the maximum of the EOS velocity increases much more than two-fold. To explain this phenomenon, we first study the behavior of the brushes and the brush-supported counterions in presence of the applied electric field. At larger electric fields, the chloride counterions inside the brush layer moves very fast: this is possible due to the significantly large mobility of the chloride ions inside the PMETAC PE brush layer. The brush chains that interact strongly with these ions, as a result, experience significant bending and bucking in the direction of the counterion motion. Such changes in the brush configuration ensure an accumulation of a large number of counterions at the brush-layer-bulk interface. The water molecules hydrating these interface-localized counterions experience much smaller drag force from the brush layer, thereby demonstrating very high velocity (i.e., EOS velocity becomes very large) at larger electric fields. This leads to the highly intriguing significantly large non-linear increase in the EOS velocity in PEMTAC-brush-grafted nanochannels.    

Theoretical investigation of ion transport dynamics in current-time inside nanochannel and microchannel

In this paper, we study the nanochannel material membrane property to model the ion current-time dynamics. Here we consider a symmetric monovalent electrolyte (z^{+ = -}z- = z)  like KCl with bulk concentration, c₀ = 0.1 mM, and assume that the concentration of H⁺ and OH^- is much lower compared to the bulk concentration of the ionic species. The nanochannel length  (L_nanochannel)  is 5μm and height is 30nm per unit width. The applied voltage is 1V. We obtain the surface charge density of the nanochannel material is -1 mC/m² , the counter-ion concentration is c_K⁺= 0.74 mM and co-ion concentration c_Cl^-= 0.06 mM for surface potential = 52 mV from the ion-ion and ion-wall interactions [1]. The mass and charge (m_K⁺, q_K⁺)  for the counter-ion and co-ion (m_Cl^-, q_Cl^-) are obtained.  The electrokinetic velocity (velocity), acceleration (a) of the ionic species (velocity/time) is calculated. The ionic species position is tracked using a well known velocity-verlet algorithm [2]. The counter-ion current-time model is I_K⁺(t) = ((q_K⁺velocity)/L_nanochannel) + ((m_K⁺a velocity)/Voltage​​​​​​) , co-ion is I_Cl^-(t) = ((q_Cl^-velocity)/L_nanochannel) + ((m_Cl^-a velocity)/Voltage​​​​​​) and the total ionic current-time through the nanochannel is I(t) = I_K⁺(t) + I_Cl^- (t). The obtained current is validated with Ref. [1] for time step 100 ns. We study for microchannel of length = 1 μm and height of the microchannel = 1 μm per unit width. The nanochannel material membrane current-time is GUI applications in nanofluidic calculators and app  software.    

Reduced-Order Model of Multicomponent Electrolyte Transport in Bipolar Membranes

Recent advancements in water-dissociation catalysis and affordable renewable electricity have led to a growing interest in the application of bipolar membranes (BPMs) in electrodialysis, electrochemical carbon dioxide reduction, and other electrochemical processes. Modeling tools for BPMs commonly use direct-numerical simulations (DNS) to predict performance, which are computationally expensive and make insights into the key physics of the cells difficult to discern. As a result, a mechanistic understanding of BPMs remains unclear, which is a requisite to optimize their performance and durability for their independent technology applications. In this work, we present a reduced-order, steady-state model of a general multicomponent-electrolyte-BPM system. We systematically simplify Poisson-Nernst-Planck equations for a BPM system that consists of two bulk regions, a cation-exchange membrane, an anion-exchange membrane, and a BPM junction. DNS is used to validate the results from our model. We examine the effects of membrane-length asymmetry and pH gradients on the characteristic polarization curve for various fixed-charged groups. These findings are obtained with better numerical stability and faster computational speed than DNS without compromising the essential physics of the system.