Potential shifts at electrodes coated with ion-exchange polymeric films

Voltammetry at electrodes modified with ion-exchange polymers, named "ion exchange voltammetry", has been recently developed for characterizing and determining quantitatively ionic electroactive analytes preconcentrated at the electrode surface. Like for other voltammetric techniques, characterization is based on the position of the response on the potential scale, but an appreciable difference is frequently observed between the formal half-wave potential for redox couples incorporated within ion-exchange polymeric films and those for the same redox couples in solution as measured at bare electrodes. Such a difference has been rationalized here by a generalized equation, inferred from a suitable elaboration of the Nernst equation, whose validity has been tested by a thorough investigation performed at glassy carbon electrodes modified with either cationic (Nafion) or anionic (Tosflex) polymeric films. With this purpose, the effect of both charge and concentration of the analyte and of the loading counterion, this last introduced as the cation or anion of the supporting electrolyte, of the ion-exchange selectivity coefficients of the redox partners and of their stoichiometric coefficients, as well as of the number of electrons involved in the charge transfer has been evaluated. The results obtained agree quite well with theoretical expectations and indicate that the potential shifts found are mainly conditioned by both charge and concentration of the counterion from the supporting electrolyte and by the ratio of the ion-exchange equilibrium constants for the two redox partners involved. Other parameters considered have no influence on the potential shift or lead to negligible effects, provided that the quantities of the redox partners incorporated within the ion-exchange coating represents less than 5% of the film capacity. Again in agreement with theoretical expectations, positive shifts are found for increasing supporting electrolyte concentrations when cationic redox species incorporated within cationic films are involved, while the opposite effect is found for anionic redox species incorporated within anionic films.     

Explanation of Misleading Nernst Slope by Boltzmann Equation

The common misuse in the literature of the terms Nernst slope, Nernst factor, and Nernst potential should be corrected. The membrane electrode potential is explained by the Boltzmann distribution equation. The slope of 59 mV per ionic unit is not unique to the Nernst equation. Both the Nernst equation and the Boltzmann distribution equation give the same 59-mV slope based on different mechanisms. The slope indicates only the change of −ΔGregardless of mechanisms. To avoid any misleading inference to the wrong equation mechanism, a person's name should not be used to denote a common slope. The factors affecting the slope, the electrode potential and its measurements, the importance of potential mechanism, and the modified Boltzmann equation are presented.     

Beyond potentiometry: robust electrochemical ion sensor concepts in view of remote chemical sensing

For about 100 years, potentiometry with ion-selective electrodes has been one of the dominating electroanalytical techniques. While great advances in terms of selective chemistries and materials have been achieved in recent years, the basic manner in which ion-selective membranes are used has not fundamentally changed. The potential readings are directly co-dependent on the potential at the reference electrode, which requires maintenance and for which very few accepted alternatives have been proposed. Fouling or clogging of the exposed electrode surfaces will lead to changes in the observed potential. At the same time, the Nernst equation predicts quite small potential changes, on the order of millivolts for concentration changes on the order of a factor two, making frequent recalibration, accurate temperature control and electrode maintenance key requirements of routine analytical measurements. While the relatively advanced selective materials developed for ion-selective sensors would be highly attractive for low power remote sensing application, one should consider solutions beyond classical potentiometry to make this technology practically feasible. This paper evaluates some recent examples that may be attractive solutions to the stated problems that face potentiometric measurements. These include high-amplitude sensing approaches, with sensitivities that are an order of magnitude larger than predicted by the Nernst equation; backside calibration potentiometry, where knowledge of the magnitude of the potential is irrelevant and the system is evaluated from the backside of the membrane; controlled current coulometry with ion-selective membranes, an attractive technique for calibration-free reagent delivery without the need for standards or volumetry; localized electrochemical titrations at ion-selective membranes, making it possible to design sensors that directly monitor parameters such as total acidity for which volumetric techniques were traditionally used; and controlled potential coulometry, where all ions of interest are selectively transferred into the ion-selective organic phase, forming a calibration-free technique that would be exquisitely suitable for remote sensing applications.     

Recent progress in the studies of electrochemical interfaces by surface plasmon resonance spectroscopy and microscopy

Surface plasmon resonance (SPR) is a label-free spectroscopic technique that is highly sensitive to various surface reactions. Incorporating SPR into electrochemical measurements has emerged as a powerful method to study both faradaic and non-faradaic processes. SPR microscopy (SPRM) integrates an optical microscope into SPR detection, which further offers high throughput detection and spatially resolved information at an electrode surface and thus, has attracted attention especially in single entity electrochemical studies. In this review, the progress in the studies of electrochemical interfaces by SPR and SPRM during the past two years will be discussed.     

Predicting pH dependencies of electrode surface reactions in electrocatalysis

It is shown how to adopt the Nernst equation to electrode potential-dependent Gibbs energies, calculated for reactants and products from density functional theory, to make predictions of reversible potentials for redox reactions on electrode surfaces in electrolytes of any pH. The theory is general because any spectator species may be included in electrochemical interface. We demonstrate its application to H and OH deposition on Pt(111).     

Faradaic double layer depolarization in electrokinetics: Onsager relations and substrate limitations

More often than not, the measurement of interfacial potentials by means of electrokinetic techniques is affected by interfering processes that may relax or even annihilate their primary response function. Among these processes are faradaic ones, provided that the substrate is sufficiently conducting and a redox function is available, and non-faradaic ones, if geometrical constraints are in effect. Ample experimental evidence is available, e.g., in the collapse of streaming potentials generated by metal/electrolyte solution interfaces, the bipolar microelectrodic redox processes in fluidized beds of metallic particles, and the "superfast" electrophoresis of dispersed ion exchanger particles and electron-conducting particles. Common feature of these apparently disparate phenomena is that the lateral electric field is affected by coupling with transversal depolarization fields, or by conductance gradients due to Donnan effects. Recent work has rigorously analyzed the deformation of the lateral electric field in a (streaming potential) slit cell by electron transfer reactions at the interface, taking into account both convective diffusion of the electroactive species and kinetics of the interfacial electron transfer reaction. Here a common, generic basis for faradaic and non-faradaic double layer depolarization is formulated along the lines set by Onsager, and methodologies for retrieving the underlying electrokinetic parameters from experimental data are evaluated. Particular attention is paid to the limitations of double layer polarization, as posed by the substrate.     

Sensing mechanism of non-equilibrium solid-electrolyte-based chemical sensors

Solid electrolytes can be used in several different types of chemical sensors. A common approach is to use the equilibrium potential generated across a solid electrolyte given by the Nernst equation as the sensing signal. However, in some cases, stable electrode materials are not available to establish equilibrium potentials, so non-equilibrium approaches are necessary. The sensing signal generated by such sensors is often described by the mixed potential theory, in which a pair of electrochemical reactions establishes a steady state at the electrode, such that the electrons produced by an oxidation reaction are consumed by a reduction reaction. The rates of both reactions depend on several factors, such as electron exchange, active area, and gas phase diffusion, so establishment of the steady-state potential is complex and alternative explanations have been proposed. This paper will review and discuss the mechanisms proposed to explain the sensor response of non-equilibrium-based electrochemical sensors.     

The electrochemistry of particles, droplets, and vesicles �C the present situation and future tasks

Presently, a plethora of techniques is available to study the electrochemical properties of solid inorganic and organic micro- and nano-particles immobilized on electrode surfaces, provided that they possess a faradaic electroactivity. Similarily, immobilized droplets of liquids and solutions, which are immiscible with the electrolyte solution, give access to the three-phase electrochemistry of redox centers in the droplets, allowing determinations of free energies of ion transfer between the immiscible liquid phases. Possible and necessary future activities in the field of immobilized particles and droplets will be discussed here. The electrochemistry of suspended micro- and nano-particles possessing faradaic electroactivity is much more complex and needs special attention in future research. Finally, the electrochemistry of liposomes and biological vesicles, which do not possess faradaic activity, but the ability to produce capacitive signals upon attachment to electrodes, will be discussed focusing on possible future developments.     

On the interpretation of the potentiometric response of the inert solid electrodes in the monitoring of the oscillatory processes involving hydrogen peroxide

Solid inert electrodes are frequently used in potentiometry. However, potentiometric responses may significantly depend on the inert electrode material, a fact which may manifest itself particularly distinctly for the dynamical chemical systems—oscillating processes. We found that for the homogeneous oscillators involving hydrogen peroxide and either thiocyanates or thiosulfates, the periodic variations of the platinum and palladium indicator electrode potential are both not in phase with the variations of the potential of the gold and glassy carbon electrodes, the latter two exhibiting in turn concordant, in-phase responses. Potentiometric responses were compared with the impedance characteristics of the electrodes during the oscillations. In spite of high mechanistic complexity of the studied homogeneous oscillatory systems, we explain different responses of inert electrodes in terms of the concept of the mixed electrode potential, i.e., determined by more than one redox couple of different kinetic characteristics (exchange current densities). In our model explanation, two coupled Ox₁/Red₁ and Ox₂/Red₂ redox systems are considered. It is suggested that for Au or glassy carbon electrodes, the mixed potential is largely determined by the Ox₁/Red₁ couple. For Pt or Pd electrodes, due to the catalytic effect of their surfaces on the Ox₂/Red₂ couple, its exchange current largely controls the measured mixed potential. Our concept is supported by numerical calculations involving the classical Brusselator as the model generator of chemical oscillations. The proper interpretation of potentiometric kinetic data is crucial for the diagnosis of the correct reaction mechanism.     

Dynamic diffuse double-layer model for the electrochemistry of nanometer-sized electrodes

A dynamic diffuse double-layer model is developed for describing the electrode/electrolyte interface bearing a redox reaction. It overcomes the dilemma of the traditional voltammetric theories based on the depletion layer and Frumkin's model for double-layer effects in predicating the voltammetric behavior of nanometer-sized electrodes. Starting from the Nernst-Planck equation, a dynamic interfacial concentration distribution is derived, which has a similar form to the Boltzmann distribution equation but contains the influence of current density. Incorporation of the dynamic concentration distribution into the Poisson and Butler-Volmer equations, respectively, produces a dynamic potential distribution equation containing the influence of current and a voltammetric equation containing the double-layer effects. Computation based on these two equations gives both the interfacial structure (potential and concentration profiles) and voltammetric behavior. The results show that the electrochemical interface at electrodes of nanometer scales is more like an electric-double-layer, whereas the interface at electrodes larger than 100 nm can be treated as a concentration depletion layer. The double-layer nature of the electrode/electrolyte interface of nanometer scale causes the voltammetric responses to vary with electrode size, reactant charge, the value of formal redox potential, and the dielectric properties of the compact double-layer. These voltammetric features are novel in comparison to the traditional voltammetric theory based on the transport of redox molecules in the depletion layer.     

On-line FFT faradaic admittance measurements application to A.C. cyclic voltammetry

A measurement approach is described and data are presented which demonstrate the ability to effect a.c. cyclic voltammetric measurements with the on-line digital FFT approach to faradaic admittance data acquisition. The equipment utilized enables complete faradaic admittance spectra to be obtained at an effective spectrum acquisition rate of 10 s^?1, so that the d.c. potential range encompassed by a typical cyclic wave can be encompassed with adequate resolution in the E_dc dimension in ≥6 s, approximately. The instrument features dynamic, computerized measurement and compensation of the non-faradaic ohmic resistance and double-layer capacitance contributions to the acquired total cell admittance. Measurements with quasi-reversible systems yield the expected faradaic admittance and phase angle responses over a quite generous bandwidth. Applications to mercury and platinum electrodes are illustrated.     

Advantage of Fractional Calculus Based Hybrid‐Theoretical‐Computational‐Experimental Approach for Alternating Current Voltammetry

The dynamic electrochemical behavior of electroactive species is believed to be represented better by the fractional calculus, because it can consider the history of mass‐transfers of that species near the electrode surface. The elucidation of mathematical fundamentals of fractional calculus has been recently introduced for batteries, supercapacitors and a few voltammetry studies. The working equations for faradaic fundamental and second‐harmonic (SHac) components of alternating current (ac) for ac voltammetry of an electrochemically reversible redox reaction on an electrode of macroscopic diameter have been derived here by using generalized formulae of the fractional calculus. A computation code is written in Python language with a matrix based algorithm developed based on latest, accurate, efficient and stable Grunwald‐Letnikov‐Improved fractional‐order differentiation equation. That computational code is used to find the concealed faradaic fundamental, SHac components of the total current and other double‐layer parameters of experimentally recorded voltammograms of ruthenium(III/II) redox reaction on gold‐disc electrode by a common electrochemical workstation without having inbuilt Fourier transformation features. The amplitude of the computed faradaic current concealed in the experimental data gets enhanced through this hybrid theoretical‐computational‐experimental approach and thus it keeps scope of application and further improvement in electroanalysis.     

Membrane electrodes sensitive to doubly charged surfactants. Application to a cationic gemini surfactant

The response of a membrane electrode is not always identical to that of a redox electrode. Indeed, in the case of membrane electrode the response is not due to a redox equilibrium but to a cross-membrane potential. So, the membrane electrode's response depends mainly on the carrier system and the nature of the membrane. The properties of the membrane can favour several reactions giving rise to different ionic species diffusing in the membrane. The expression of the cross-membrane potential thus depends on the number and quantities of these ionic species. To illustrate this, we established the equations for the case of a two-charge cation detected by a univalent charged carrier. We show that the Nernstian response is not applicable to membrane electrodes. This approach allowed us to interpret results obtained with a cationic gemini surfactant-selective electrode prepared in the laboratory. To prove the well working of this electrode, we determined the critical micelle concentration in water and several NaBr solutions (0.004, 0.006, 0.01 and 0.02 M) from which the counterion binding has been determined.     

Ion-transfer- and photo-electrochemistry at liquid|liquid|solid electrode triple phase boundary junctions: perspectives

Ion transfer at liquid|liquid junctions is one of the most fundamental processes in nature. It occurs coupled to simultaneous electron transfer at the line junction (or triple phase boundary) formed by the two liquids in contact to an electrode surface. The triple phase boundary can be assembled from a redox active microdroplet deposit of a water-immiscible liquid on a suitable electrode surface immersed into aqueous electrolyte. Ion transfer voltammetry measurements at this type of electrode allow both thermodynamic and kinetic parameters for coupled ion and electron transfer processes to be obtained. This overview summarises some recent advances in understanding and application of triple phase boundary redox processes at organic liquid|aqueous electrolyte|working electrode junctions. The design of novel types of electrodes is considered based on (i) extended triple phase boundaries, (ii) porous membrane processes, (iii) hydrodynamic effects, and (iv) generator-collector triple phase boundary systems. Novel facilitated ion transfer processes and photo-electrochemical processes at triple phase boundary electrodes are proposed. Potential future applications of triple phase boundary redox systems in electrosynthesis, sensing, and light energy harvesting are indicated.     

Recent advances in the synthesis of non-carbon two-dimensional electrode materials for the aqueous electrolyte-based supercapacitors

Supercapacitors (SCs) with high power density and long cycling span life are demanding energy storage devices that will be an attractive power solution to modern electronic and electrical applications. Numerous theoretical and experimental works have been devoted to exploring various possibilities to increase the functionality and the specific capacitance of electrodes for SCs. Non-carbon two-dimensional (2D) materials have been considered as encouraging electrode candidates for their chemical and physical advantages such as tunable surface chemistry, high electronic conductivity, large mechanical strength, more active sites, and dual non-faradaic and faradaic electrochemical performances. Besides, these 2D materials also play particular roles in constructing highway channels for fast ion diffusion. This concise review summarizes cutting-edge progress of some representative 2D non-carbon materials for the aqueous electrolyte-based SCs, including transition metal oxides (TMOs), transition metal hydroxides (TMHs), transition metal chalcogenides (TMCs), MXenes, metal-organic frameworks (MOFs) and some emerging materials. Different synthetic methods, effective structural designs and corresponding electrochemical performances are reviewed in detail. And we finally present a detailed discussion of the current intractable challenges and technical bottlenecks, and highlight future directions and opportunities for the development of next-generation high-performance energy storage devices.     

季铵盐结构与高氯酸根离子选择电极性能关系的研究

本文报道了由七种长链季铵盐制得的高氯酸根离子选择电极,测定了这些电极的Nernst响应范围和检测下限,并研究了各种阴离子的干扰。通过大量实验测得,随着季铵盐分子中取代烷基碳链的增长,电极电势响应范围增宽、检测下限降低、抗外部阴离子干扰能力增强。若取代烷基碳链过长,则季铵盐纯化困难,在膜相中溶解度降低,不适于电极的研制。根据用上述七种选择电极在相同水相条件下测得的数据表明,阴离子的干扰次序为:PA~->ClO_4~->BF_4~->SCN~->I~->ClO_3~->N_3~->N_3~->CN~-～Br~->H_2PO_4~->F~->Cl~-。这些数据对阴离子选择电极研制与相转移催化反应研究均有现实意义。     

Specific features of the behavior of an electrochemical system in the case of the Hopf instability for a spherical electrode

Model approximations are developed that allow establishing a quantitative relationship between the geometrical parameters of a spherical electrode, the faradaic impedance, and instabilities of the electrochemical system for an electrode reaction under potentiostatic conditions for the adsorption of species preceding their discharge. It is shown that the control parameter ω_H in the Hopf bifurcation point depends on the electrode size. The effect of the Nernst diffusion layer is observed at low frequencies in the range of negative faradaic impedance values.     

Wilhelm Ostwald’s role in the genesis and evolution of the Nernst equation

The historical origin of the Nernst equation can be traced back to Helmholtz’ treatment of the thermodynamics of galvanic cells and to Gibbs’ masterwork “On the Equilibrium of Heterogeneous Substances”. However, Nernst himself used a model of the metal/solution interface based on Arrhenius’ dissociation theory, together with some aspects of van’t Hoff’s osmotic pressure theory. Bancroft performed some initial studies of redox chains (cells) in Ostwald’s laboratory. Peters has advanced these studies and published an equation correctly describing the potential of an inert electrode in a solution containing a dissolved reversible redox pair. Riesenfeld has treated interfaces of immiscible electrolyte solutions and the partition equilibria of ions. Luther has shown how standard potentials of elements possessing several redox states are related. Fredenhagen was the first to understand that the series of standard potentials are solvent dependent. Nernst, Bancroft, Peters, Luther and Fredenhagen were pupils of Ostwald; Riesenfeld and Fredenhagen were students of Nernst. Indeed, the presiding genius of the whole endeavour was clearly Friedrich Wilhelm Ostwald. This new survey of the genesis and evolution of what we now call Nernst equation reveals the influence of Ostwald’s ideas on the theorizing process, and it is concluded that his share in the development of the modern theory deserves greater recognitions.     

Calibration of the Hg chalcogenide glass membrane ion-selective electrode in seawater media

It is shown that a chalcogenide glass mercury(II) ion-selective electrode (ISE) can be calibrated in chloride-free unbuffered and saline buffered standards, displaying near-Nernstian response over 19 orders of magnitude (i.e. 10(-20) to 10(-1) M Hg(2+)). Extended ageing of the ISE in seawater induced a memory effect, causing the electrode to respond in a sub-Nernstian fashion. Electrochemical impedance spectroscopy (EIS) demonstrated that the response of the Hg(II) ISE is underpinned by a charge transfer process, and seawater matrix effects are due to electrode passivation. It is shown that standard addition ISE potentiometry may compensate for interferences caused by the seawater matrix.     

Electrochemical Capacitors

The current literature sources on the electrochemical capacitors, which are divided into the film (dielectric), electrolytic, and supercapacitors, are reviewed. The supercapacitors are in turn subdivided into the double-layer capacitors, which use the EDL recharge on a highly-developed interfacial surface of electrodes; pseudocapacitors, where the charge is stored in a faradaic pseudocapacitance of sufficiently reversible redox reactions and the EDL capacitance; and hybrid capacitors, which employ a variety of electrodes. A macrokinetic theory of operation of double-layer capacitors is considered. Effect of various factors on the properties of electrodes utilized in supercapacitors is analyzed. A novel type of hybrid capacitor, which has a negative electrode of activated carbon cloth and a PbSO₄/PbO₂ positive electrode, is proposed. A theory of capillary equilibrium in hermetically sealed electrochemical capacitors is considered. Specific features of the application of voltammetric and impedance methods to studying electrochemical processes in supercapacitors are revealed. Characteristics of electrochemical capacitors and batteries are compared.