Strongly nonlinear dynamics of electrolytes in large ac voltages

We study the response of a model micro-electrochemical cell to a large ac voltage of frequency comparable to the inverse cell relaxation time. To bring out the basic physics, we consider the simplest possible model of a symmetric binary electrolyte confined between parallel-plate blocking electrodes, ignoring any transverse instability or fluid flow. We analyze the resulting one-dimensional problem by matched asymptotic expansions in the limit of thin double layers and extend previous work into the strongly nonlinear regime, which is characterized by two novel features - significant salt depletion in the electrolyte near the electrodes and, at very large voltage, the breakdown of the quasi-equilibrium structure of the double layers. The former leads to the prediction of "ac capacitive desalination", since there is a time-averaged transfer of salt from the bulk to the double layers, via oscillating diffusion layers. The latter is associated with transient diffusion limitation, which drives the formation and collapse of space-charge layers, even in the absence of any net Faradaic current through the cell. We also predict that steric effects of finite ion sizes (going beyond dilute solution theory) act to suppress the strongly nonlinear regime in the limit of concentrated electrolytes, ionic liquids and molten salts. Beyond the model problem, our reduced equations for thin double layers, based on uniformly valid matched asymptotic expansions, provide a useful mathematical framework to describe additional nonlinear responses to large ac voltages, such as Faradaic reactions, electro-osmotic instabilities, and induced-charge electrokinetic phenomena.Comment: 30 pages, 17 eps-figures, RevTe

Nonlinear electrochemical relaxation around conductors

We analyze the simplest problem of electrochemical relaxation in more than one dimension - the response of an uncharged, ideally polarizable metallic sphere (or cylinder) in a symmetric, binary electrolyte to a uniform electric field. In order to go beyond the circuit approximation for thin double layers, our analysis is based on the Poisson-Nernst-Planck (PNP) equations of dilute solution theory. Unlike most previous studies, however, we focus on the nonlinear regime, where the applied voltage across the conductor is larger than the thermal voltage. In such strong electric fields, the classical model predicts that the double layer adsorbs enough ions to produce bulk concentration gradients and surface conduction. Our analysis begins with a general derivation of surface conservation laws in the thin double-layer limit, which provide effective boundary conditions on the quasi-neutral bulk. We solve the resulting nonlinear partial differential equations numerically for strong fields and also perform a time-dependent asymptotic analysis for weaker fields, where bulk diffusion and surface conduction arise as first-order corrections. We also derive various dimensionless parameters comparing surface to bulk transport processes, which generalize the Bikerman-Dukhin number. Our results have basic relevance for double-layer charging dynamics and nonlinear electrokinetics in the ubiquitous PNP approximation.Comment: 25 pages, 17 figures, 4 table

Diffuse-Charge Dynamics in Electrochemical Systems

The response of a model micro-electrochemical system to a time-dependent applied voltage is analyzed. The article begins with a fresh historical review including electrochemistry, colloidal science, and microfluidics. The model problem consists of a symmetric binary electrolyte between parallel-plate, blocking electrodes which suddenly apply a voltage. Compact Stern layers on the electrodes are also taken into account. The Nernst-Planck-Poisson equations are first linearized and solved by Laplace transforms for small voltages, and numerical solutions are obtained for large voltages. The ``weakly nonlinear'' limit of thin double layers is then analyzed by matched asymptotic expansions in the small parameter $\epsilon = \lambda_D/L$, where $\lambda_D$ is the screening length and $L$ the electrode separation. At leading order, the system initially behaves like an RC circuit with a response time of $\lambda_D L / D$ (not $\lambda_D^2/D$), where $D$ is the ionic diffusivity, but nonlinearity violates this common picture and introduce multiple time scales. The charging process slows down, and neutral-salt adsorption by the diffuse part of the double layer couples to bulk diffusion at the time scale, $L^2/D$. In the ``strongly nonlinear'' regime (controlled by a dimensionless parameter resembling the Dukhin number), this effect produces bulk concentration gradients, and, at very large voltages, transient space charge. The article concludes with an overview of more general situations involving surface conduction, multi-component electrolytes, and Faradaic processes.Comment: 10 figs, 26 pages (double-column), 141 reference

Multiparameter Telemetry as a Sensitive Screening Method to Detect Vaccine Reactogenicity in Mice

Refined vaccines and adjuvants are urgently needed to advance immunization against global infectious challenges such as HIV, hepatitis C, tuberculosis and malaria. Large-scale screening efforts are ongoing to identify adjuvants with improved efficacy profiles. Reactogenicity often represents a major hurdle to the clinical use of new substances. Yet, irrespective of its importance, this parameter has remained difficult to screen for, owing to a lack of sensitive small animal models with a capacity for high throughput testing. Here we report that continuous telemetric measurements of heart rate, heart rate variability, body core temperature and locomotor activity in laboratory mice readily unmasked systemic side-effects of vaccination, which went undetected by conventional observational assessment and clinical scoring. Even minor aberrations in homeostasis were readily detected, ranging from sympathetic activation over transient pyrogenic effects to reduced physical activity and apathy. Results in real-time combined with the potential of scalability and partial automation in the industrial context suggest multiparameter telemetry in laboratory mice as a first-line screen for vaccine reactogenicity. This may accelerate vaccine discovery in general and may further the success of vaccines in combating infectious disease and cancer

Open-Circuit Photovoltage and Charge Recombination at Semiconductor/Liquid Interfaces

The open-circuit photovoltage (Voc) of semiconductor/ liquid junction solar cells is a critical parameter in determining the energy conversion efficiency. The fundamental process controlling Voc is the recombination of photoexcited electrons and holes. 1' 2 The lower the recombination rate, the larger the Voc. The predominant energy-loss mechanism is determined by competition among the following processes: majority-carrier thermionic emission over the surface barrier, ~ majority-carrier charge transfer across the semiconductor/liquid interface, ~' 4 minority-carrier diffusion/recombination in the bulk of the semiconductor, ~' 8 space-charge recombination, 7 and surface recombination mediated by recombination centers. 8-13 The extent to which each of these processes is understood differs considerably. For example, expressions describing the minority-carrier diffusion/recombination in the bulk semiconductor contacting a redox electrolyte is obtained by direct analogy to formulas developed for solid-state p-n junction devices. ~ When bulk diffusion/recombination is the dominant recombination process, the dependence of Vo~ on the semiconductor bandgap, doping level, and minority-carrier diffusion length can be expressed in simple analytic forms? In contrast, surface recombination has generally been treated in a more complex fashion by numerical simulation. TM In cases in which Voe is limited by surface recombination, no simple analytic expression exists for relating Vo~ and the surface recombination velocity (Sr)-Several groups TM have considered theoretically the effect of surface recombination on the performance of photoelectrochemical (PEC) cells. Although each treatment has achieved some success in describing a certain aspect of the effect of surface recombination, these treatments are generally considered qualitative3 ~ For the most part, it has been difficult to extract quantitative information on surface recombination from * Electrochemical Society Active Member. Visiting professor, on leave of absence from Korea University, Seoul, Korea. experimental results because of the number of adjustable (and often arbitrary) parameters involved in numerical analyses. Up to now, only one study 11 has dealt directly with the dependence of Voc on the surface recombination current; however, because bias-independent surface recombination currents in arbitrary units were used in the numerical calculation, it is difficult to apply the model of this study for interpreting quantitatively experimental measurements. Other studies 8-I~ have focused mainly on the general shape of the photocurrent-voltage (J-V) curves, without addressing the dependence of Voo on Sr. b The absence of a theoretical framework relating Sr to Vo~ impedes the understanding of such processes at the solid/liquid interface. In this article, we derive a simple quantitative expression, based on semiconductor solid-state theory, that directly relates Sr to Voc. The applicability of the expression to account for the PEC behavior of n-St/acetone with FeCp~ j~ (ferrocenium ion/ferrocene) is then investigated. Based on J-Vdata and the dependence of Voe on both the temperature and the concentration of FeCp~, we are able to exclude other possible recombination channels and identify surface recombination as the dominant recombination process in determining Voc. The surface recombination velocity deduced from experimental results compares favorably with reported values. The application of the analytic expression to other PEC systems reported in the literature is also discussed. b The effect of surface recombination is generally discussed in terms of the photoeurrent onset potential. However, unlike the concept of the "open-circuit photovoltage," the "photoeurrent onset potential" is an empirical quantity that cannot be precisely defined. The photocurrent onset potential depends on both Voc and the fill factor. The latter two parameters are more definable quantities and are more relevant in calculating the PEC conversion efficiency