Reference electrode capacitance

Figure Bl.28.8. Equivalent circuit for a tliree-electrode electrochemical cell. WE, CE and RE represent the working, counter and reference electrodes is the solution resistance, the uncompensated resistance, R the charge-transfer resistance, R the resistance of the reference electrode, the double-layer capacitance and the parasitic loss to tire ground.

Accurate control of potential, stability, frequency response and uniform current distribution required the following low resistance of the cell and reference electrode small stray capacitances small working electrode area small solution resistance between specimen and point at which potential is measured and a symmetrical electrode arrangement. Their design appears to have eliminated the need for the usual Luggin capillary probe. 

The Hg/V-methylformamide (NMF) interface has been studied by the capacitance method as a function of temperature.108,294,303 The potential of Hg was measured with respect to the reference electrode Ag/0.05 M AgC104 + 0.05 M NaC104 in water. The specific adsorption of C104 was found to be negligible at a < 6 /iC cm"2. The experimental capacitance data have been discussed in terms of the four-state model,121,291,294 which assumes the presence of both monomers and clusters in the surface layer of the solvent. The model has been found to describe the experimental picture qualitatively but not quantitatively. This is related to the fact that NMF is a strongly associated solvent.108,109,294,303... 

Metal/molten salt interfaces have been studied mainly by electrocapillary833-838 and differential capacitance839-841 methods. Sometimes the estance method has been used.842 Electrocapillary and impedance measurements in molten salts are complicated by nonideal polarizability of metals, as well as wetting of the glass capillary by liquid metals. The capacitance data for liquid and solid electrodes in contact with molten salt show a well-defined minimum in C,E curves and usually have a symmetrical parabolic form.8 10,839-841 Sometimes inflections or steps associated with adsorption processes arise, whose nature, however, is unclear.8,10 A minimum in the C,E curve lies at potentials close to the electrocapillary maximum, but some difference is observed, which is associated with errors in comparing reference electrode (usually Pb/2.5% PbCl2 + LiCl + KC1)840 potential values used in different studies.8,10 It should be noted that any comparison of experimental data in aqueous electrolytes and in molten salts is somewhat questionable. 

Nonfaradaic components associated with the uncompensated resistance between reference electrodes (7 ) and the double layer capacitance (Qi) can be accurately determined by AC impedance measurements. In this technique, a small AC potential perturbation is superimposed to the DC bias, and the resulting AC current is measured as a function of the frequency of modulation. The simplest circuit considered for a polarizable... 

FIG. 7 Simplified equivalent circuit for charge-transfer processes at externally biased ITIES. The parallel arrangement of double layer capacitance (Cdi), impedance of base electrolyte transfer (Zj,) and electron-transfer impedance (Zf) is coupled in series with the uncompensated resistance (R ) between the reference electrodes. (Reprinted from Ref. 74 with permission from Elsevier Science.)... 

FIGURE 7.2 Schematic of a DNA sensor based on a capacitive EIS structure. For operation, a DC (direct current) polarization voltage (VG) is applied via the reference electrode (RE) to set the working point of the EIS sensor, and a small AC (alternating current) voltage (E ) is applied to the system in order to measure the capacitance of the sensor. ssDNA - single-stranded DNA, cDNA - complementary DNA, dsDNA - double-stranded DNA. 

This is just what happens in an electrochemical cell when a potential is applied between the working and the reference electrodes the double layer setting up at the working electrode/solution interface generates capacitive currents. 

As a result of the difference of potential AE applied between the working and reference electrodes, a capacitive current generates inside the cell which flows as a function of time, according to the relation ... 

The interface impedance for a case such as Ag/Ag4Rbl5 will consist of a capacitance (derived from the Helmholtz formula) in parallel with i et so that in the complex plane impedance a semi-circle will be found from which Qi and can be evaluated. Rq will cause this semicircle to be offset from the origin by a high frequency semicircle due to the bulk impedance between the interface and the reference electrode (Fig. 10.12). There can be no Warburg impedance (a line at 45° to the real axis generally due to diffusion effects) in this case. 

Figure 53. Idealized half-cell response of a thin solid electrolyte cell, (a) Cell geometry including working electrodes A and B and reference electrode (s). (b) Equivalent circuit model for the cell in a, where the electrolyte and two electrodes have area-specific resistances and capacitances as indicated, (c) Total cell and half-cell impedance responses, calculated assuming the reference electrode remains equipotential with a planar surface located somewhere in the middle of the active region, halfway between the two working electrodes, as shown in a.

Undoubtedly, the mercury/aqueous solution interface, was in the past, the most intensively studied interface, which was reflected in a large number of original and review papers devoted to its description, for example. Ref. 1, and in the more recent work by Trasatti and Lust [2] on the potentials of zero charge. It is noteworthy that in view of numerous measurements of the double-layer capacitance at mercury brought in contact with NaF and Na2S04 solutions, the classical theory of Grahame [3] stiU holds [2]. According to Trasatti [4], the most reliable PZC value for Hg/H20 interface in the absence of specific adsorption equals to —0.433 0.001 V versus saturated calomel electrode, (SCE) residual uncertainty arises mainly from the unknown liquid junction potential at the electrolyte solution/SCE reference electrode boundary. 

Capacitor. The potential difference between two conductors A and B at the same temperature connected by a circuit containing no source of e.m.f. is given by the difference in work function (pa — a according to Equation (2). The application of a potential Fo = < s — a between the two conductors brings them to the same potential, but for a value F 5 Fo there will be a net charge on one of the conductors of (F — Vo)/C, where C is the capacitance. If F is adjusted so that F = Fo, no current flows when the capacitance is varied and F = 4>a — A This is the basis of the capacitor method, where the surface under consideration and a reference electrode form the plates of a capacitor. The reference electrode must remain inert at normal temperatures when exposed to gas or vapor during the measure-... 

A few comments are in order on the probable validity of conclusions based on this equivalent circuit to real cells. Quite simply stated, real cells that are properly designed will have the same properties as dummy cells of the same values of Rs, Ru, and Cdl. Important design features of a cell are (1) equal resistance between all points on the surface of the working electrode and the auxiliary electrode (2) low-impedance reference electrode and (3) low stray capacitance between electrodes, between leads, and to shields. Spherical symmetry is a good, but somewhat inconvenient, method of meeting the first requirement a parallel arrangement also works with planar electrodes. At the very... 

An equivalent circuit of the three-electrode cell discussed in Chapters 6 and 7 is illustrated in Figure 9.1. In this simple model, Rr is the resistance of the reference electrode (including the resistance of a reference electrode probe, i.e., salt bridge), Rc is the resistance between the reference probe tip and the auxiliary electrode (which is compensated for by the potentiostat), Ru is the uncompensated resistance between the reference probe and the working-electrode interphase (Rt is the total cell resistance between the auxiliary and working electrodes and is equal to the sum of Rc and Ru), Cdl is the double-layer capacitance of the working-electrode interface, and Zf is the faradaic impedance of the electrode reaction. 

Figure 9.1 Equivalent circuit of an electrochemical cell. A, Auxiliary electrode R, reference electrode W, working electrode Rc, compensated resistance R , uncompensated resistance Rr, reference electrode impedance Zf, faradaic impedance Cdl, doublelayer capacitance.

A standard method to determine capacitance is cyclic voltammetry. One electrode, made of the material of interest (with know surface area A) and a counter electrode, are introduced into the electrolyte solution. A reference electrode can be used in addition. Then a triangular potential is applied and the electric current is measured. From the current, the capacitance can be calculated. 

Is it possible to obtain the information about the pH of the solution without a reference electrode using capacitive measurement ... 

To understand the electrical behaviour of the LAPS-based measurement, the LAPS set-up can be represented by an electrical equivalent circuit (see Fig. 5.2). Vbias represents the voltage source to apply the dc voltage to the LAPS structure. Re is a simple presentation of the reference electrode and the electrolyte resistance followed by a interface capacitance Cinterface (this complex capacitance can be further simulated by different proposed models as they are described, e.g., in Refs. [2,21,22]). In series to the interface capacitance, the insulator capacitance Cj will summarise the capacitances of all insulating layers of the LAPS device. The electrical current due to the photogeneration of electron-hole pairs can be modelled as current source Ip in parallel to the... 

Figure 6.4 Cell design for the study of fast electrode reactions. System provides low-resistance reference electrode and low stray capacitances (1) counter-electrode chamber (2) Kel-F top (3) Teflon bottom (4) working electrode (5) reference-electrode groove (6) reference-electrode connection (7) Viton O-ring (8) stainless-steel spacer (9) stainless-steel locating pin.

To prepare a working electrode, carbon powder (PFA-P7-H or PFA-AN8-H) was mixed with polytetrafluoroethylene (PTFE) (5 wt%) to form a pellet and it was sandwiched in Ni mesh as a current collector. The EDL capacitance properties were measured by CV and galvanostatic charge/ discharge in a three-electrode cell vs. Ag/AgCl reference electrode. 1 M-(C2H5)4NBF4 in PC was used as a nonaqueous electrolyte. 

The values of specific capacitance and cell resistance obtained with symmetric two-electrode capacitors built with a-Mn02/CNTs composite electrodes are presented in Table 8.3. The addition of nanotubes to a-Mn02 causes a drastic decrease of cell resistance and an increase of specific capacitance referred to the mass of a-Mn02 H20. However, when the specific capacitance is referred to the total mass of the composite electrode material, to be realistic, a CNTs loading higher than 10-15 wt% does not improve the electrodes performance. Therefore, 10-15 wt% of CNTs conducting additive seems to be an optimal amount both on the point of view of electrodes capacitance... 

Channel activity is best studied electrochemically as charged species cross a cell membrane or artificial lipid bilayer. There is a difference in electrical potential between the interior and exterior of a cell leading to the membrane itself having a resting potential between -50 and -100 mV. This can be determined by placing a microelectrode inside the cell and measuring the potential difference between it and a reference electrode placed in the extracellular solution. Subsequent changes in electrical current or capacitance are indicative of a transmembrane flux of ions. 

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