Electrode / electrolyte interface measurement

In galvanic cells it is only possible to determine the potential difference as a voltage between two half-cells, but not the absolute potential of the single electrode. To measure the potential difference it has to be ensured that an electrochemical equilibrium exists at the phase boundaries, e.g., at the electrode/electrolyte interface. At the least it is required that there is no flux of current in the external and internal circuits. 

By tradition, electrochemistry has been considered a branch of physical chemistry devoted to macroscopic models and theories. We measure macroscopic currents, electrodic potentials, consumed charges, conductivities, admittance, etc. All of these take place on a macroscopic scale and are the result of multiple molecular, atomic, or ionic events taking place at the electrode/electrolyte interface. Great efforts are being made by electrochemists to show that in a century where the most brilliant star of physical chemistry has been quantum chemistry, electrodes can be studied at an atomic level and elemental electron transfers measured.1 The problem is that elemental electrochemical steps and their kinetics and structural consequences cannot be extrapolated to macroscopic and industrial events without including the structure of the surface electrode. 

Another technique consists of MC measurements during potential modulation. In this case the MC change is measured synchronously with the potential change at an electrode/electrolyte interface and recorded. To a first approximation this information is equivalent to a first derivative of the just-explained MC-potential curve. However, the signals obtained will depend on the frequency of modulation, since it will influence the charge carrier profiles in the space charge layer of the semiconductor. 

Up to now only qualitative data have been available on potential-dependent MC measurements of electrochemical interfaces. When metals or other highly conducting materials are used, or when liquids are in play, special care has to be taken to allow access of microwave power to the active electrode/electrolyte interface. 

Electrochemical impedance spectroscopy leads to information on surface states and representative circuits of electrode/electrolyte interfaces. Here, the measurement technique involves potential modulation and the detection of phase shifts with respect to the generated current. The driving force in a microwave measurement is the microwave power, which is proportional to E2 (E = electrical microwave field). Therefore, for a microwave impedance measurement, the microwave power P has to be modulated to observe a phase shift with respect to the flux, the transmitted or reflected microwave power APIP. Phase-sensitive microwave conductivity (impedance) measurements, again provided that a reliable theory is available for combining them with an electrochemical impedance measurement, should lead to information on the kinetics of surface states and defects and the polarizability of surface states, and may lead to more reliable information on real representative circuits of electrodes. We suspect that representative electrical circuits for electrode/electrolyte interfaces may become directly determinable by combining phase-sensitive electrical and microwave conductivity measurements. However, up to now, in this early stage of development of microwave electrochemistry, only comparatively simple measurements can be evaluated. 

It is worth emphasizing that although overpotentials are usually associated with electrode-electrolyte interfaces, in reality they refer to, and are measured as, deviations of the potential (

associated with an electrode and not with an electrode-electrolyte interface, although the nature of this interface will, in general, dictate the magnitude of the measured overpotential. 

Lyden et al. [92] used in situ electrical impedance measurements to investigate the role of disorder in polysulfide PEC with electrodeposited, polycrystalline CdSe photoanodes. Their results were consistent with disorder-dominated percolation conduction and independent of any CdS formed on the anode surface (as verified by measurements in sulfide-free electrolyte). The source of the observed frequency dispersion was located at the polycrystalline electrode/electrolyte interface. 

TR-SFG measurements at a Pt electrode/electrolyte interface covered with a CO monolayer excited by the irradiation of picosecond visible pulses showed that the... 

Thus, in an electrochemical cell the electrolyte has a small but finite resistance, Rt, resulting in a potential drop, Fd, between the working and reference electrodes. From Ohm s law, VA = /Rc, where / is the current flowing across the working electrode/electrolyte interface. As a result of this resistance, the measured potential Vm is related to the real potential, Vrt by ... 

The electrochemical detection of pH can be carried out by voltammetry (amper-ometry) or potentiometry. Voltammetry is the measurement of the current potential relationship in an electrochemical cell. In voltammetry, the potential is applied to the electrochemical cell to force electrochemical reactions at the electrode-electrolyte interface. In potentiometry, the potential is measured between a pH electrode and a reference electrode of an electrochemical cell in response to the activity of an electrolyte in a solution under the condition of zero current. Since no current passes through the cell while the potential is measured, potentiometry is an equilibrium method. 

For the investigation of charge tranfer processes, one has the whole arsenal of techniques commonly used at one s disposal. As long as transport limitations do not play a role, cyclic voltammetry or potentiodynamic sweeps can be used. Otherwise, impedance techniques or pulse measurements can be employed. For a mass transport limitation of the reacting species from the electrolyte, the diffusion is usually not uniform and does not follow the common assumptions made in the analysis of current or potential transients. Experimental results referring to charge distribution and charge transfer reactions at the electrode-electrolyte interface will be discussed later. 

The charge distribution at metal electrode-electrolyte interfaces for liquid and frozen electrolytes has been investigated through capacity measurements using the lock-in technique and impedance spectroscopy. Before we discuss some of the important results, let us briefly consider some properties of the electrolyte in its liquid and frozen state. 

The orientation of water molecules at the interface is an important ingredient in understanding the properties of the surface region. A large body of data is available on the stmcture of water at metal surfaces measured under ultrahigh vacuum (UHV) conditions, but it is expected that the orientation of water molecules under the conditions that exist at the electrode/electrolyte interface is very different. As mentioned earlier, the fact that the minimum energy required to eject an electron from the surface (the work function) is lower when the metal is in contact with water... 

Figure 26. Predictions of the Adler model shown in Figure 25 assuming interfacial electrochemical kinetics are fast, (a) Predicted steady-state profile of the oxygen vacancy concentration ( ) in the mixed conductor as a function of distance from the electrode/electrolyte interface, (b) Predicted impedance, (c) Measured impedance of Lao.6Cao.4Feo.8-Coo.203-(5 electrodes on SDC at 700 °C in air, fit to the model shown in b using nonlinear complex least squares. Data are from ref 171.

Dedeloudis, C. Eransaer, J. Celts, J.-P. Surface Force Measurements at a Copper Electrode/ Electrolyte Interface./. Phys. Chem. B 2000, 104, 2060-2066. 

What Happens When One Tries to Measure the Potential Difference Across a Single Electrode/Electrolyte Interface ... 

The system created by the measuring procedure is in fact an electrochemical system, or cell, consisting of two electronic conductors (electrodes) immersed in an ionic conductor (electrolyte). All one can measure, in practice, is the potential difference across a system of interfaces, ora cell, not the potential difference across one electrode/electrolyte interface. 

In the particular cell (Fig. 6.28) generated by the measuring process, it will be only as a special case that the metal Mj (of the electrode/electrolyte interface under study) is identical with the metal M2 (the connecting wires of the measuring instrument). In general, M, and M2 will be different metals, say, platinum and copper. The meeting of the platinum and copper phases produces another double layer and an... 

The discussion so far can be summarized as follows The value of the potential difference across a single electrode/electrolyte interface cannot be measured with potential measuring instruments. The sum of the potential differences across at least... 

As explained in Section 6.3.11, the inner potential difference—A( )—seems to encompass all the sources of potential differences across an electrified interface—Ax and A jf—and therefore it can be considered as a total (or absolute ) potential across the electrode/electrolyte interface. However, is the inner potential apractical potential First, the inner potential cannot be experimentally measured (Section 6.3.11). Second, its zero point or reference state is an electron at rest at infinite separation from all charges (Sections 6.3.6 and 6.3.8), a reference state impossible to reach experimentally. Third, it involves the electrostatic potential within the interior of the phase relative to the uncharged infinity, but it does not include any term describing the interactions of the electron when it is inside the conducting electrode. Thus, going back to the question posed before, the inner potential can be considered as a kind of absolute potential, but it is not useful in practical experiments. Separation of its components, A% and A f, helped in understanding the nature of the potential drop across the metal/solution interface, but it failed when we tried to measure it and use it to predict, for example, the direction of reactions. Does this mean then that the electrochemist is defeated and unable to obtain absolute potentials of electrodes ... 

Consider two electrode/electrolyte interfaces (Fig. 7.173), M/S and M /S, which are assembled to form an electrochemical system or cell Recalling that potential differences are always measured between two metals of the same composition, a metal M" that is identical in composition to M is attached to M. Under these circumstances, the potential difference V across the whole system, or cell, has been shown [Eq. 6.53)] to be given by the inner potential of the electrode on the right minus the inner potential of a wire of the same composition connected to the electrode on the left ... 

In order to study electrochemical reactions at the electrode—electrolyte interface, one needs to consider that the observed current flowing through the cell is composed of partial formal anodic and cathodic currents associated with oxidation and reduction processes, respectively. However, these currents cannot be measured individually by experiment since the net electrical current flowing through the external circuit reflects the balance of both processes, i.e. / = /a — /c. 

The exchange current density introduced by Butler (33) is a measure of the exchange of O and R at the electrode—electrolyte interface at equilibrium and gives an indication of the activation barrier height when both reactant and product are at the same free energy level. 

Electroanalytical techniques, such as conductometry [174], potentiometry [22], voltammetry [6], chronoamperometry [25] and EIS [175], have been used extensively for transduction of the detection signal in the MIP-based chemosensors. The chemosensor response may be due to different interfacial phenomena occurring at the electrode-electrolyte interface [16], which will be discussed below in the respective sections. The electrochemical transduction scheme can be devised for accurate measurements tailored to the analytes exhibiting either faradic or non-faradic electrode behaviour. In many instances, the detection medium is an inert buffer solution [24]. In order to enhance the chemosensor response, some of the... 

Potentiometric methods in potentiometric methods, the equilibrium potential of the working electrode (see section2.2) is measured against the potential of a reference electrode. That potential results from an equilibrium established over the electrode-electrolyte interface and provides information about the analyte taking part in this equilibrium. 

Voltammetric methods in these methods, a potential is applied to the working electrode using a three-electrode setup (see section 1.6). The electrical current, resulting from charge transfer over the electrode-electrolyte interface, is measured and reveals information about the analyte that takes part in the charge transfer reaction. The potential applied can be constant (chronoamperometry, section2.5), varied linearly (cyclic voltammetry, section 2.3) or varied in other ways (Chapter 2). 

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