Platinum electrodes capacitance

When the solution is not quite inert, ac techniques are widely used to investigate the capacitance and other surface properties of platinum electrodes as well as of various other electrodes. Their chief advantage is the possibility to apply them in the case of electrodes passing some faradaic current. It is shown in Section 12.5.1 that in this case the electrode s capacitance can be determined by extrapolating results obtained at different ac frequencies to the region of high frequencies. This extrapolation can be used for electrodes where electrode reactions occur that have standard rate constants, of up to 1 cm/s. 

Transient measnrements (relaxation measurements) are made before transitory processes have ended, hence the current in the system consists of faradaic and non-faradaic components. Such measurements are made to determine the kinetic parameters of fast electrochemical reactions (by measuring the kinetic currents under conditions when the contribution of concentration polarization still is small) and also to determine the properties of electrode surfaces, in particular the EDL capacitance (by measuring the nonfaradaic current). In 1940, A. N. Frumkin, B. V. Ershler, and P. I. Dolin were the first to use a relaxation method for the study of fast kinetics when they used impedance measurements to study the kinetics of the hydrogen discharge on a platinum electrode. 

The differential capacity can be measured primarily with a capacity bridge, as originally proposed by W. Wien (see Section 5.5.3). The first precise experiments with this method were carried out by M. Proskurnin and A. N. Frumkin. D. C. Grahame perfected the apparatus, which employed a dropping mercury electrode located inside a spherical screen of platinized platinum. This platinum electrode has a high capacitance compared to a mercury drop and thus does not affect the meaurement, as the two capacitances are in series. The capacity component is measured for this system. As the flow rate of mercury is known, then the surface of the electrode A (square centimetres) is known at each instant ... 

Hiratsuka et al102 used water-soluble tetrasulfonated Co and Ni phthalocyanines (M-TSP) as homogeneous catalysts for C02 reduction to formic acid at an amalgamated platinum electrode. The current-potential and capacitance-potential curves showed that the reduction potential of C02 was reduced by ca. 0.2 to 0.4 V at 1 mA/cm2 in Clark-Lubs buffer solutions in the presence of catalysts compared to catalyst-free solutions. The authors suggested that a two-step mechanism for C02 reduction in which a C02-M-TSP complex was formed at ca. —0.8 V versus SCE, the first reduction wave of M-TSP, and then the reduction of C02-M-TSP took place at ca. -1.2 V versus SCE, the second reduction wave. Recently, metal phthalocyanines deposited on carbon electrodes have been used127 for electroreduction of C02 in aqueous solutions. The catalytic activity of the catalysts depended on the central metal ions and the relative order Co2+ > Ni2+ Fe2+ = Cu2+ > Cr3+, Sn2+ was obtained. On electrolysis at a potential between -1.2 and -1.4V (versus SCE), formic acid was the product with a current efficiency of ca. 60% in solutions of pH greater than 5, while at lower pH... 

It is desirable to measure R (and thus G) at a high frequency in order to reduce Xc to a negligible value compared to R. Unfortunately, other complications arise if the frequency is increased above a few kilohertz, and therefore other means must be devised to decrease Xc. A commonly used remedy is to increase the surface area and thus the capacitance of the electrodes as much as possible. A 100-fold area increase is obtained by platinizing the electrodes, that is, electrodepositing a layer of platinum black onto the platinum electrodes, usually from a solution of chloroplatinic acid [8]. 

The time (t) dependences of the charging current (Ic) of a cell where the capacitance of the electrode investigated is much smaller than that of the reference electrode (e.g., saturated calomel electrode) and the counter electrode (e.g., high surface area of platinum electrode) as well as the geometrical capacitance of the cell are neglected, therefore the total capacitance of the cell is equal to the capacitance of the electrode (Q) under study, and - ohmic (solution) resistance is Rs> - are as follows. 

Such an equation can be applied for the determination of - pseudocapacitances (e.g., hydrogen capacitance of a platinum electrode) in the case of the so-called charging curve experiments. 

An early attempt to make a real electrochemical sensor based on a molecularly imprinted methacrylate polymer utilised conductometric measurements on a field-effect capacitor [76]. A thin film of phenylalanine anilide-imprinted MAA-EDMA copolymer was deposited on the surface of semiconducting p-type silicon and covered with a perforated platinum electrode. An AC potential was applied between this electrode and an aluminium electrode on the back side of the semiconductor and the capacitance measured as a function of the potential when the device was exposed to the analyte in ethanol. The print molecule could be distinguished from phenylalanine but not from tyrosine anilide and the results were very variable between devices, which was attributed to difficulties in the film production. The mechanism by which analyte bound to the polymer might influence the capacitance is again rather unclear. 

Conductivity. A 900-ml dilution cell was used. The platinum electrodes had a light platinization, and the cell design ensured wide separation of the electrical leads to minimize capacitance effects. 

In this study, heterogeneous electron-transfer kinetics were measured for the following Se(VI)/Se(IV), As(V)/As(III), Fe(CN)5 /Fe(CN)5 ", and Fe(III)/Fe(II). All experiments were done at pH 6.0 with the exception of the iron couple, which was done at pH 3.0. Using electron-transfer kinetic constants, aqueous diffusion coefficients, aqueous concentrations, starting potentials, and a constant double-layer capacitance model, values for the change of EMF as a function of time for a platinum electrode were calculated numerically. The result of this simulation was then compared to the observed potentiometric response for a solution of the same concentration. 

The second technique involves using a four probe apparatus, similar to that described by Cahan and Wainright. The membrane sample is placed in an PTFE apparatus which is equipped with two platinum strips in contact with the film, as shown in Fig. 1.115. Two platinum electrodes in a fixed geometry (distance of 1.026 cm) were placed on the surface of the film to measure the membrane potential and capacitance. Conductivity measurements could be obtained by utilizing complex impedance plots, which employ a circuit diagram... 

Bond and coworkers [36] have probed the ability of microelectrodes to determine low concentrations of electroactive species using flow injection analysis. Ferrocene was chosen as a test system to avoid any complications associated with irreversible reactions. Measuring concentrations of the order of 10 nM proved challenging and required the use of a battery operated two-electrode potentiostat because of 50-Hz noise coming from the mains power supply. Bond has also shown that it may be easier to realize low limits of detection using macro- rather than microelectrodes [37]. For example, the electrochemical detection of As (111) at a platinum electrode in an HPLC system becomes less favorable as the electrode radius decreases. Thus, while 10 nM As(III) could be detected at a 50- xm-radius microelectrode, the limit of detection increased to 500 nM when a 2.5-pm-radius electrode was used. This falloff in performance appears to arise because of imperfect seals and high stray capacitance for the smaller electrodes. 

With no redox reactions (e.g., hy using a platinum electrode in saline), the small AC voltage will result in an AC current dependent on douhle-layer capacitance and other components. 

Take for example the oxidation of Fe " ions to Fe ions in a unidirectional system with a planar interface between a platinum electrode and an aqueous solution which contains both ferrous and ferric ions and a supporting electrolyte. If the capacitive current can be ignored, then the boundary conditions for the two electroactive species can be presented in the following way ... 

The small-signal impedances of the platinum and the tungsten electrodes are mainly capacitive. For the platinum electrodes the impedance is given by a capacitance of 10p,Fmm the average electrode has a surface area of 10 pm. The tungsten electrode has a capacitance of 0.4 pm mm the average electrode surface area is 30 pm. It is noticed that platinum-black increases the effective smface area compared to the tungsten electrode. 

It is usually believed that high frequency capacitance obtained from impedance spectroscopy can represent ionic double layer capacity. However in general this is not the case for platinum electrode, this is the reason why one arrow in Fig. 1 (in the left) is crossed. In contrast to Cf measured under equilibrium conditions (by means of isoelectric potential shifts), non-equilibrium impedance response can contain a contribution from Ah (and/or Ao, surface concentration of oxygen-containing species). These contributions are determined by Ah and Ao potential derivatives and their free electrode charge derivatives, and in general can be either positive, or negative. 

Equation (6.1.1.2) indicates that a semi-log plot of the capacitive current vs. time should be linear with slope 1/RC and intercept In AE/R thus allowing the cell resistance and electrode capacitance to be determined. For the experiment illustrated in Figure 6.1.1.2, a linear semi-log plot is observed indicating that there is a single response time characterizing the electrochemical cell. This behavior is consistent with a clean, unmodified electrode surface and a well-constructed electrode (Section 6.1.3). The measured resistance and capacitance for this 10 pm radius platinum microelectrode are 19,500 780 Q and 1.5 0.1 X 10 ° F, respectively. 

Figure 2.6. Polarization capacitance Cp as a function of frequency for disc electrode and opposing point (platinum electrodes). 10 mA current. (Disc electrode 6.25 cm. )...

In the experimental situation, the polarization capacitance usually does not vary directly as a power of frequency and Fricke s law holds only approximately, as 9 varies from 30° to 45° in the normal experimental situation. Variations in m range from 0.06 to 0.87 (Fricke, 1932) depending upon electrode material and the electrolyte used. For platinized platinum electrodes with physiological saline as the electrolyte. 

If we extrapolate the data to infinite frequency, where Cp = 0, then we see that the true sample capacitance Q = 0.001 fiF. If we now plot Cp versus frequency on a double logarithmic basis, we obtain the result shown in Figure 2.8c. The slope of this line is 0.85 and is the coefficient m which appears in the expression of Fricke s law. Normally m varies from 0.3 to 0.5 for platinum electrodes with a saline electrolyte. For the hypothetical sample selected, m = 0.85. 

It was observed that 2-methyl benzene thiol suppresses the hydrogen adsorption on platinum and iron. 1,2-Ethanedithiol and 2,2 -(ethylenedioxy)diethanethiol adsorb strongly on platinum electrodes, suppress the adsorption of hydrogen from water, and also resist oxidative degradation. However, iron electrodes only show a five to ten times increase in polarization resistance and five to ten times reduction in double-layer capacitance. Therefore, 1,2-eth-anedithiol and 2,2 -(ethylenedioxy)diethanethiol are considered as promising additives for improving the charge efficiency of the iron electrode (106). 

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