Dynamics Of Electric Double Layer Research Articles

Robust Electrocatalytic CO2 Reduction in Acid Enabled by "Molecularly Charged" Cobalt Phthalocyanine: A Profound Understanding from Electric Double Layer.

Electrocatalytic CO2 reduction (eCO2R) in acid holds promise in renewable electricity-powered CO2 utilization with high efficiency, but the hydrogen evolution reaction (HER) often prevails and results in a low eCO2R selectivity. Here, using cobalt phthalocyanine/Ketjen black (CoPc/KB) as the model catalysts, we systematically study the effect of active site density, operational current density, and hydrated cations on the acidic eCO2R selectivity and decipher it through the componential dynamics of electric double layer (EDL). The optimal CoPc-4/KB demonstrates a near-unity CO Faradaic efficiency from 50 to 400 mA cm-2 and superb operational stability (>120 h) at 100 mA cm-2. Aided by in situ Raman and infrared spectroscopies, we reveal that the proper cations establish an electrostatic shield for mitigating bulk H+ penetration and mediate the interfacial water structure for suppressing HER. This study should elicit further profound thinking on robust eCO2R system design from the perspective of multiphasic and dynamic EDL.

Modular theory for EIS response of electron transfer coupled with electric double layer on rough and heterogeneous electrode

The dynamic phenomenological coupling of charge transfer kinetics with electric double layer (EDL) dynamics on a rough and heterogeneous electrode is a theoretically challenging problem. Here, we solve this complex problem using a modular theoretical approach for the electrochemical impedance spectroscopic (EIS) technique, also transcends the limitations of equivalent circuit model. Theory based on modular approach allow us to analyse the EIS response for multi phenomena over seven decades of frequency. Qualitatively EIS response can be classified in three frequency regimes: (i) ω>ωH, largely controlled by ohmic and EDL dynamics, (ii) ωH≥ω≥ωCH, coupling between EDL and electron transfer, and extent of coupling depends on applied potential and (iii) ω<ωCH, usually controlled by solvent kinetics. The frequency ωH is characteristic EDL formation frequency or inverse time for the closest approach of an ion to the electrode surface, which is also a prerequisite for an electron transfer step. The characteristic (low) frequency ωCH in aqueous system represents the onset of sluggish water splitting kinetics. The intermediate frequency response is dynamically influenced by the morphological complexities of electrode surface which is usually characterized through the finite fractal roughness. Our work highlights that the enhanced viscosity of the medium results in (i) lowering of ωH caused by increase in the electrolyte resistance, (ii) lowering of diffusion coefficient. Similarly, the temperature has significant influence on the ωH. Finally, this modular approach captures the experimental responses obtained through variation in externally applied potential, viscosity of the medium, temperature and the electroactive area.

Hierarchical Porous Electrode Impedance Model Based on Diffusion Dynamics and the Electrode Morphology and Prediction of Electric Double-Layer Structures

The capacitance performance of a hierarchical porous carbon (HPC) electrode mainly depends on the dipole storage capacity and the rate of electric double-layer (EDL) reorganization. However, it is difficult to directly measure the effect of microscopic changes of pores and electrolytic characteristics on the internal ionic mechanism of the EDL. Therefore, a model is proposed to easily and accurately describe both the diffusion and EDL dynamics of the HPC electrode. In general, diffusion in the mesoporous channel can be characterized in the middle-frequency region (5–500 Hz) and in the micropore in the low-frequency region (0.05–5 Hz). What is more, we have proven that the diffusion layer thickness is inversely proportional to the electrolytic concentration and positively proportional to the mesoporous size, microporous depth, and surface roughness. In particular, the thicker diffusion layer makes it easier for ions diffusing into micropores and a better EDL recombination in mesoporous channels. However, the thinner diffusion layer means a better EDL recombination in micropores. The low-frequency region (0.01–0.05 Hz) characterized the compact layer dynamics, which shows the constant phase element behavior obviously. Moreover, the compact layer thickness is inversely proportional to the surface heterogeneity, thus determining the dipole storage capacity. The model offers a general framework for impedance analysis and EDL prediction of HPC.

Electrochemical surface plasmon resonance measurements of camel-shaped static capacitance and slow dynamics of electric double layer structure at the ionic liquid/electrode interface.

Electrochemical surface plasmon resonance (ESPR) is applied to evaluate the relative static differential capacitance at the interface between 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquid (IL) and a gold electrode, based on the relationship between the SPR angle and surface charge density on the electrode. Potential-step and potential-scan ESPR measurements are used to probe the dynamics of the electric double layer (EDL) structure that exhibit anomalously slow and asymmetrical characteristics depending on the direction of potential perturbation. EDL dynamics respond at least 30 times more slowly to changes of potential in the positive direction than in the negative direction. ESPR experiments with the positive-going potential scan are significantly affected by the slow dynamics even at a slow scan. The surface charge density that reflects the relative static capacitance is obtained from the negative-going potential scans. The evaluated quasi-static differential capacitance exhibits a camel-shaped potential dependence, thereby agreeing with the prediction of the mean-field lattice gas model of the EDL in ILs. ESPR is shown to be an effective experimental method for determining relative values of the static differential capacitance.

Experimental corroboration of general phenomenological theory for dynamics of EDL in viscous medium on rough heterogeneous electrode

The phenomenological theory for the anomalous electric double layer (EDL) impedance response (see J. Phys. Chem. C 118, 2014, 5122-5133) for rough heterogeneous electrodes is simplified for roughness features larger than the electrolyte screening length. The electrochemical impedance responses (EIS) of moderately rough polycrystalline Pt electrodes in NaNO3 electrolyte dissolved in aqueous and viscous media are analyzed and interpreted with the phenomenological theory. Experiments support predictions of theory for the EDL relaxation on rough and heterogeneous electrode, and also emphasize that the dynamics can be decomposed into compact layer and diffuse layer processes. Influence of viscosity is used to unravel the characteristic high frequency diffuse EDL dynamics through NaNO3 solution in 90% glycerol (by volume). These experiments also contrast the influence of concentration on EDL dynamics in aqueous and viscous media. EIS identifies four phenomenological regimes along with their characteristics frequencies. Finally, this phenomenological theory brings in significant physical insight and provides a useful method for interpreting experimental results.

Theory of Anomalous Dynamics of Electric Double Layer at Heterogeneous and Rough Electrodes

A generalized model for dynamics of the electric double layer (EDL) at a heterogeneous and rough electrode is developed using the Debye–Falkenhagen equation for the potential. The influence of surface heterogeneities which causes the distribution in relaxation time in the compact layer is included through the current balance boundary constraint at the outer Helmholtz layer. The results for the admittance response are obtained for deterministic and stochastic roughness. The response for the deterministic surface is expressed as a functional of an arbitrary surface profile and the stochastic roughness as a functional of an arbitrary power spectrum of roughness. The dynamics is understood in terms of phenomenological (viz., dynamic diffuse layer and polarization) lengths and various relaxation (viz., compact layer, diffuse layer, and mixed) frequencies resulting from the interaction of compact and diffuse double layer. A strong influence of heterogeneity, finite fractal roughness, electrolyte concentration, and their diffusion coefficient is found. Our model unravels anomalous roughness-dependent pseudo-Gerischer behavior at high frequency, classical Helmholtz behavior at intermediate frequencies, and emergence of CPE at low frequency due to heterogeneity of the surface. Comparison of the theory with experimental data shows good agreement.

Debye–Falkenhagen dynamics of electric double layer in presence of electrode heterogeneities

A phenomenological theory of electric double layer (EDL) polarization of an electrode (in the absence and presence of electroactive species) is obtained using the Debye–Falkenhagen equation for the potential. The influence of surface heterogeneities on the compact layer causes the distribution in relaxation time, resulting in constant phase element (CPE) response. This contribution of the compact layer is included through the current balance boundary constraint at the outer Helmholtz plane. The results for the impedance and the capacitance are obtained in terms Debye screening length and dynamic polarization length which is dependent on the surface heterogeneity parameter. At frequencies less than the characteristic compact layer relaxation frequency, the EDL is controlled by the compact layer dynamics and shows CPE response. The intermediate frequencies show the emergence of pseudo-Gerischer and pseudo-Warburg like behavior for systems with lower concentration and lower diffusion coefficients of ions. EDL behaves like a resistor at larger frequencies. Theoretical results capture various observations in the experimental capacitance dispersion data.

Solvent Effects on the Transient Characteristics of Liquid-Gate Field Effect Transistors with Silicon Substrate

The transient characteristics of electric double layer (EDL) gated field-effect transistors with Si as an active semiconductor were studied using various electrolyte solutions of LiBF4 by applying a step-function voltage to determine the optimum electrolyte for semiconductor circuits using EDLs. The tR, determined by EDL dynamics in the present experiment, was minimum as a function of the kind of solvent used owing to the competing effects of the EDL thickness and viscosity. The responses of the electrolyte solutions with various solvents at the same concentration were classified into three categories on the basis of tR: slow response of a complex-forming solvent, intermediate response of protic solvents, and fast response of nonprotic solvents. The best response time was 55 µs when a 1.0 M acetonitrile solution was used as the liquid-gate insulator.

Electric double layer dynamics by high-field electroluminescence in aqueous solution (Destriau effect)

Transient luminescence induced by high electric fields (>2.5 x 10(4) V/cm) was observed in aqueous solutions of 6,8-dihydroxypyrene-1,3-disulfonic acid. The light emission takes place both at the leading and the trailing edges of the perturbing electric square pulse. It occurs only during the variation of the applied field and disappears with a time constant of 0.5 musec at constant field. In solid state phosphors, where this phenomenon has been reported previously, it is known as the Destriau effect. The mechanisms of excitation and emission can be explained by the electron injection model, in which the time constant of the luminescence decay is associated with the rate of formation of the space charge. This space charge appears as a separation of positive and negative charges into a double layer stabilized by the applied potential.