Electrode Surface Area Determination

PEC electrode performance must be normalized to the planar projected electrode surface area, which makes accurate determination of the electrode surface area very important. Several techniques can be used to determine the planar illuminated area. Two common approaches are presented here. 

2 Cell Setup and Connections for Three- and Two-Electrode Configurations 

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The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

The potential of the electrode surface is determined by the Nernst equation introduced in Sec. 1.3.3. In an equilibrium, the currents in anodic and cathodic directions are equal. If they are related to an electrode area, they are called exchange-current densities, j0 ... 

The Nernst equation is of limited use at low absolute concentrations of the ions. At concentrations of 10 to 10 mol/L and the customary ratios between electrode surface area and electrolyte volume (SIV 10 cm ), the number of ions present in the electric double layer is comparable with that in the bulk electrolyte. Hence, EDL formation is associated with a change in bulk concentration, and the potential will no longer be the equilibrium potential with respect to the original concentration. Moreover, at these concentrations the exchange current densities are greatly reduced, and the potential is readily altered under the influence of extraneous effects. An absolute concentration of the potential-determining substances of 10 to 10 mol/L can be regarded as the limit of application of the Nernst equation. Such a limitation does not exist for low-equilibrium concentrations. 

The arrows above and the symbols below the interfaces indicate the transfer of the charge at each interface when the concentration of NaF in the sample is abruptly increased. It is possible to estimate the actual number of ions that are required to establish the potential difference at the interfaces. A typical value for the doublelayer capacitor is 10 5 F cm 2. If a potential difference of n = 100 mV is established at this interface, the double-layer capacitor must be charged by the charge Q = nCdi = 10 6 coulombs. From Faraday s law (6.3), we see that it corresponds to approximately 10 11 mol cm 2 or 1012 ions cm 2 of the electrode surface area. Thus, a finite amount of the potential determining ions is removed from the sample but this charge is replenished through the liquid junction, in order to maintain electroneutrality. 

In the case of solid electrodes, knowledge of the real surface area is a prerequisite for the proper evaluation of the activity with respect to other samples from the same laboratory, or for the comparison of results from different laboratories. It is very difficult to determine the real surface area because there are no unique techniques applicable to all materials. When the surface area determination is lacking, an evaluation in terms of synergetic effects can only be ambiguous. 

Electrode surface area, geometric — The interfacial area Ageom determined based on the assumption that the interface is truly flat (2-dimensional), mostly calculated using the geometric data of the involved parts (i.e., surfaces are of a metal sheet, disk, or the surface of a metal drop assuming a certain drop shape) ... 

In electrocatalysis, the activity of different electrocatalysts is usually expressed via the exchange current I0, and the specific activity, via the exchange current density, iQ (A cm-2), still often computed on the basis of the superficial electrode surface area. Only when the current is normalized using the true surface area of the electrode-electrolyte interface, the comparison between different electrocatalysts is truly meaningful. The determination of the true surface area of porous electrodes is discussed in Sect. 2.3.5. 

Fig. 3.4 Digital picture of three PEC electrodes. The ruler is used to set the scale for surface area determination...

Theoretical concepts of specific (per unit true electrode surface area) EDL capacitance are based on the known classical theories of EDL developed by Helmholtz, Stern, Gouy-Chapman, Grahame, and so on. One of the directions of modem smdies of EDL is elucidation of ratios between different surface layer characteristics. These include specific capacitances in zero charge points, electronic work functions of metals, their liophilicity, zero charge potentials. Correlations are established between many of these characteristics in a number of metals and solvents. At the same time, there are significant deviations from main trends. Zero charge points were first determined for different carbon materials in the works of Frumkin et al. 

Three main factors determine how much electrical energy a capacitor can store the electrode surface area, the electrode... 

Film preparation plays a crucial role in determining the photoelectrochemical properties of phthalocyanine electrodes. Since the coupling of individual chromophores strongly depends on their relative orientation, the position of the absorption maximum and its width shows a clear dependence on the structure of thin films. Also the charge transport within phthalocyanine films, a fundamental necessity for the films to work as electrodes, depends upon the overlap of the frontier orbital wave functions. Beyond the microscopic structure of films also the morphology of films plays an important role. In the case of crystalline films, the orientation of crystallites relative to the electrode surface will be relevant because of anisotropies in optical absorption and charge transport. The size of the observed photocurrent directly depends on the real electrode surface area accessible by the electrolyte and this leads to a strong dependence on the porosity of the films. 

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