Current step, electrical double layer

For conducting and completely polarizable surfaces, the capacitance can be measured directly with great precision. In the most simple capacitance measurement, called chronoamperome-try, a potential step AU is applied to an electrode (Fig. 5.10). A current flows due to charging of the diffuse electrical double layer. The current flows until the capacitance is fully charged. By measuring the current as a function of time and integrating the curve with respect to time we get the charge Q. The total capacitance C is easily obtained from C = CA A = Q/AU. 

Currents measured in the potential step or potential sweep techniques consists not only of faradaic current, Ip, due to electrolysis, but also of charging current (non-faradaic current), Ic, which is required to charge the electric double layer. Thus total current is given by ... 

An electrochemical analyzer system (model 608B, CH Instruments) was applied to perform the measurements. Using chronoamperomefry (CA), the 10 sec potential step, in which the -0.5 volts potential of the working electrode is stepped against the reference electrode, the transient current from the polarizaion current of the electric double layer occurring at the interface was measured with IM Hz... 

The second expression for the current I f) must reveal the growth mechanism and, as an illustration, we consider the case of direct attachment of ions from the bulk of the electrolyte to the periphery of the two-dimensional cluster. The rate-determining step is supposed to be the ions transfer across the electric double layer. Then the current /i(0 is given by the Buttler-Volmer equation ... 

The first step in developing an equivalent electrical circuit for an electrochemical system is to analyze the nature of the overall current and potential. For example, in the simple case of the uniformly accessible electrode shovm in Figure 9.1(a), the overall potential is the sum of the interfacial potential V plus the Ohmic drop Rgi. Accordingly, the overall impedance is the sum of the interfacial impedance Zo plus the electrolyte resistance Re- At the interface itself, shown in Figure 9.1(b), the overall current is the sum of the Faradaic current if plus the charging current I c through the double layer capacitor C. Thus, the interfacial impedance results from the double-layer capacity in parallel with the Faradaic impedance Zf. 

As explained earlier, in transient electrochemical methods an electrical perturbation (potential, current, charge, and so on) is imposed at the working electrode during a time period 0 (usually less than 10 s) short enough for the diffusion layer 8 (2D0) to be smaller than the convection layer (S onv imposed by natural convection. Thus the electrochemical response of the system investigated depends on the exact perturbation as well as on the elapsed time. This duality is apparent when one considers a double-pulse potentiostatic perturbation applied to the electrode as in the double-step chronoampero-metric method. 

The complete process for the fabrication of the proton-conducting membranes, previously reported in [69,73], can be described as follows. A 4-inch 520 /itm thick n+ (100)-oriented double-side polished silicon wafer is first thermally oxidized in an oven at 1000 °C under O2 and water steam flows to obtain a 2 nm thick Si02 layer on both sides of the substrate. These layers will allow the electrical insulation between the two parts of the future fuel cell. Then these previous layers are covered with sputtered Cr-Au layers on both sides. The Cr layers are used as adherence layers for the Au layers and are relatively thin (30 nm). The Au layers are 800 nm thick and will serve as current collector layers for the fuel cells. They are also useful as masking layers for the next different etchings since Au is not etched neither by KOH solution nor by HF solution, the two wet etchants used in the next steps. 

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