Concentration Polarization Process Research Articles

Super-resolved dynamics of isolated zinc formation during extremely fast electrochemical deposition/dissolution processes.

The development of zinc-air batteries with high-rate capability and long lifespan is critically important for their practical use, especially in smart grid and electric vehicle application. The formation of isolated zinc (i-Zn) on the zinc anode surface, however, could easily lead to deteriorated performance, such as rapid capacity decay. In particular, under the fast charging/discharging conditions, the electrochemical activities on the anode surface are complicated and severely suppressed. Thus, it is highly desirable to deeply understand the formation mechanism of i-Zn and its relationship with the electrochemical performance during extremely high-rate cycling. Herein, we employed a super-resolution dark-field microscope to in situ analyze the evolution dynamics of the electrolyte-Zn interface during the extremely fast electrochemical deposition/dissolution processes. The unique phenomenon of nanoscopic i-Zn generation under the condition is unveiled. We discovered that the rapid conversion of nanoscopic i-Zn fragments into passivated products could greatly exacerbate the concentration polarization process and increase the overpotential. In addition, the role of large-sized i-Zn fragments in reducing the coulombic efficiency is further elucidated. This information could aid the rational design of highly effective anodes for extremely high-rate zinc-based batteries and other battery systems.

An improved single particle model for lithium-ion batteries based on main stress factor compensation

Lithium-ion battery is a complex thermoelectric coupling system. In order to understand its internal mechanism, it is necessary to build an accurate electrochemical model, which can be used for state estimation, life prediction, fault diagnosis. However, many stress factors affect the electrochemical reaction of the battery. The key to establishing an accurate electrochemical model is to determine the main stress factors. Based on the traditional single particle model, this paper studies the main stress factors that affect the polarization process of the battery. The main stress factor is used to compensate and modify the critical parameters of polarization, and a more accurate single particle model is established. Firstly, the concentration polarization process is added to the traditional single particle model. Secondly, through the orthogonal stress experiment, the principal stresses in temperature, current, and SOC are discussed for each polarization process. Thirdly, the parameters of the model are identified by the impulse current test and least-squares fitting, and compensated by the main stress factors. Finally, the compensated model is verified under different experimental conditions. The experimental results show that the current and temperature are the main stress factors affecting the concentration polarization and activation polarization of the cell. Considering the non-main factor SOC not only does not significantly improve the model accuracy, but also increases the workload of model parameter identification. The compensated model has better accuracy and adaptability than the original model.

Characterization and polarization DRT analysis of direct ethanol solid oxide fuel cells using low fuel partial pressures

Commercial Ni-YSZ anodes of solid oxide fuel cells (SOFCs) are susceptible to carbon deposits using hydrocarbons as fuel. Herein, we explore a simple approach of low fuel partial pressure to investigate the relevant limiting factors for ethanol-fueled SOFCs, providing the evaluation of the electrochemical performance and the carbon tolerance. As the partial pressure of ethanol fuel increases from 30% to 60%, the best cell performance gradually increases from 710.2 mW cm−2 to 856.8 mW cm−2 at 800 °C, which still lower than those of hydrogen-fueled SOFCs (1061.4 mW cm−2 at 800 °C), and the corresponding improvement in concentration polarization can be clearly observed especially in high current discharge area. The corresponding impedance spectra and electrode polarization processes of ethanol-fueled SOFCs are further studied by the equivalent circuit fitting and the distribution function of relaxation time (DRT) analysis, finding that the key improvement of the speed control steps comes from the concentration polarization process with the increase of the fuel partial pressure. Small amount of carbon deposition is observed on the Ni-YSZ anode substrate by SEM-EDS images after short-term discharging testing, and the safety utilization boundaries of ethanol-fueled SOFCs with a certain steam concentration can be predicted by the thermodynamic equilibria calculation.

Propagating behavior comparison of analytes and background electrolytes in a concentration polarization process

Concentration polarization (CP) is an electrochemical phenomenon related to the concentration gradient (under a transversal electric field) near an ion selective membrane surface. During concentration polarization process, quantitatively investigate the background electrolyte (BGE) concentration distribution is a very challenging problem. In this study, we conducted transient simulations within 2D micro-nanochannel domain to capture the dynamic nature of concentration polarization process and compared the propagating behavior of analytes and BGE. The simulation results show that the depletion area of chloride ion and analyte are both increase with time at ohmic and limiting region. The normalized depletion area (NDA) differences increases with decreasing BGE concentration. For the high applied voltage (overlimiting region), chloride ion and analyte at the anodic side microchannel are depleted everywhere that the NDA tends to 1 after an increasing step because of the electroconvection effect. This means there is no depletion area (DA) differences between two species in this plateau region. These results tell us the fluorescent ion indicator can’t show the exact BGE concentration distribution during CP process and we need to find new technique to visualize the concentration field.

Suppression of ion conductance by electro-osmotic flow in nano-channels with weakly overlapping electrical double layers

This theoretical study investigates the nonlinear ionic current-voltage characteristics of nano-channels that have weakly overlapping electrical double layers. Numerical simulations as well as a 1-D mathematical model are developed to reveal that the electro-osmotic flow (EOF) interplays with the concentration-polarization process and depletes the ion concentration inside the channels, thus significantly suppressing the channel conductance. The conductance may be restored at high electrical biases in the presence of recirculating vortices within the channels. As a result of the EOF-driven ion depletion, a limiting-conductance behavior is identified, which is intrinsically different from the classical limiting-current behavior.

Identification of dynamic model parameters for lithium-ion batteries used in hybrid electric vehicles

This paper presents an electrical equivalent circuit model for lithium-ion batteries used for hybrid electric vehicles (HEV). The model has two RC networks characterizing battery activation and concentration polarization process. The parameters of the model are identified using combined experimental and Extended Kalman Filter (EKF) recursive methods. The open-circuit voltage and ohmic resistance of the battery are directly measured and calculated from experimental measurements, respectively. The rest of the coupled dynamic parameters, i.e. the RC network parameters, are estimated using the EKF method. Experimental and simulation results are presented to demonstrate the efficacy of the proposed circuit model and parameter identification techniques for simulating battery dynamics.

Role of Surface Conductivity in the Dynamic Mobility of Concentrated Suspensions

In this article, a cell model is used for the evaluation of the alternating current (ac) mobility (dynamic mobility) of spherical particles in suspensions of arbitrary volume fractions of solids. The main subject is the consideration of the role of the electrical conductivity (SLC or K(sigmai)) of the stagnant layer (SL) on the mobility. It is assumed that the total surface conductivity (K(sigma)), resulting from both K(sigmai) and the diffuse layer conductivity (K(sigmad)), is constant in the cases considered and that it is the K(sigmai)-K(sigmad) balance that determines the SL effects. We first explore the effect of K(sigmai) on the frequency dependence of the dynamic mobility. It is found that the mobility decreases on average, for any frequency, when K(sigmai) increases. This is a consequence of stagnancy: ions in the SL, although contributing to the surface conductivity, do not drag liquid with them when they migrate and do not contribute to electro-osmotic flow or, equivalently, to electrophoresis. Three relaxations are observed in the mobility-frequency spectrum: inertial (the particle and liquid motions are hindered), Maxwell-Wagner-O'Konski (ions in the double layer cannot follow the field oscillations and can move only over a distance much smaller that the diffuse layer thickness), and the so-called alpha or concentration polarization process (the ions can rearrange around the particle, but they cannot form the electrolyte concentration field that appears at low frequency). Whereas the first two relaxations are little affected by K(sigmai), the alpha process undergoes significant changes. Thus, the mobility increases with frequency around the alpha relaxation region if K(sigmai) is negligible, but it decreases with frequency in the same interval if K(sigmai) is finite. With the aim of explaining this behavior, we calculate the capillary osmosis velocity field that is the fluid flow provoked by the concentration gradient around the particle. The calculations presented demonstrate that the velocity is reduced (for each frequency and position) when the SLC is raised. It is proposed that such a decrease adds to that due to the changes in the induced dipole moment of the particle, also favoring a decrease in the mobility. These tendencies are also present when the volume fraction of solids, phi, is modified, although higher phi values somewhat hide the effect of K(sigmai), as in fact observed with all features of electrokinetics associated with the phenomenon of concentration polarization.

Refractometric measurement of polarized layer structure: studies of hyaluronic acid ultrafiltration

In order to gain insight into the concentration polarization process, we have constructed a novel ultrafiltration cell and experimental system which allows direct, non-invasive, real time measurement of polarized layer structure. The device employs a differential refractometric method of polymer detection, and consists essentially of a laser beam passing through (in order): the polarized layer, an angled glass interface, and a reference solution. The deflection of the emerging laser beam is sensed and can be directly related to the polymer concentration at measurement locations upstream of the membrane. The apparatus can resolve polymer concentrations as low as 60–70 ppm; however, spatially resolving profiles within 200 μm of the membrane face in problematic. We have used this system, in addition to the more conventional measurements of total polarized layer resistance, to carry out preliminary studies of the ultrafiltration of the random coiling polyelectrolyte hyaluronic acid. Quasi-steady and transient concentration profiles are presented and compared with theory. Discrepancies between theory and experiment, possibly due to membrane fouling, were observed. We conclude that the differential refractometric approach is a potentially valuable tool for studying polymer ultrafiltration and the physicochemical behaviour of polymer solutions.

The concentration polarization effect in a multicomponent electrolyte solution—The human erythrocyte

A thermodynamic model describing the concentration polarization of solutes within cells during osmotic experiments is presented. The intracellular RBC solution is modeled as an ideal, hydrated and non-dilute salt-protein-water electrolyte solution in which the various solute species are allowed to diffuse independently. Application of this model to the case of human erythrocytes being cooled at subzero temperatures indicates that at the optimum cooling rate for the cryopreservation of RBCs a significant amount of solute polarization occurs within the intracellular solution, resulting in the temporary spatial separation of the salt and protein components. The concentration polarization process also significantly affects the transport of water out of RBCs during the cooling process. On the basis of these results, we believe that the concentration polarization phenomenon plays a significant role in determining the survival probability of cells at both high and low cooling rates.