Electrode surface area

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 value of Eff is affected by many experimental conditions other than the electrolyte and anode materials. The experimental conditions include such factors as the cell configuration, electrode orientation, electrode surface area, working electrode substrate, charge-discharge currents, charge quantity, and amount of electrolyte. 

For the experiments shown in Fig. 5.30 the ratio Cdi2/Cdi] is on the average 2500, very close to the ratio NG/Ntpb ( 3570)54 where N0 is the gas-exposed electrode surface area and Ntpb is the surface area of the three phase boundaries. These quantities were measured via surface titration and via SEM and the techniques described in section 5.7.2, respectively. Thus once N0 has been measured, AC Impedance spectroscopy allows for an estimation of the three-phase-boundary (tpb) length via ... 

In summary AC impedance spectroscopy provides concrete evidence for the formation of an effective electrochemical double layer over the entire gas-exposed electrode surface. The capacitance of this metal/gas double layer is of the order of 100-300 pF/cm2, comparable to that corresponding to the metal/solid electrolyte double layer. Furthermore it permits estimation of the three-phase-boundary length via Eq. 5.62 once the gas exposed electrode surface area NG is known. 

In the case of electrochemically promoted (NEMCA) catalysts we concentrate on the adsorption on the gas-exposed electrode surface and not at the three-phase-boundaries (tpb). The surface area, Ntpb, of the three-phase-boundaries is usually at least a factor of 100 smaller than the gas-exposed catalyst-electrode surface area Nq. Adsorption at the tpb plays an important role in the electrocatalysis at the tpb, which can affect indirectly the NEMCA behaviour of the electrode. But it contributes little directly to the measured catalytic rate and thus can be neglected. Its effect is built in UWr and [Pg.306]

The micro structured platelets, hold in a non-conducting housing, were realized by etching of metal foils and laser cutting techniques [69]. Owing to the small Nemst diffusion layer thickness, fast mass transfer between the electrodes is achievable. The electrode surface area normalized by cell volume amounts to 40 000 m m". This value clearly exceeds the specific surface areas of conventional mono- and bipolar cells of 10-100 m m. ... 

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 basic defect of film capacitors is tfieir low value of specific electrostatic capacity. Therefore, such capacitors are practically useful only in the pico- and nanofarad range. For this reason, valiant attempts have been made in recent years to increase the specific capacity of capacitors per unit of mass, volume, and plate (electrode) surface area. 

Voltage, Electrode Surface Area, and Temperature Effects... 

In the present communication we report on the effects of voltage, temperature and electrode surface area on the transient and steady-state behavior of the system. The change in the rate of C2Hi 0 production can exceed the rate of 0 pumping by a factor of 400 and is proportional to the anodic overvoltage both at steady state and during transients. 

In the present communication we establish the effect of cell voltage, temperature and catalyst-electrode surface area on the... 

Figure 9. Effect of Ag catalyst-electrode surface area Q on the relative steady-state increase in the rates of epoxidation rt and deep oxidation r2 at constant imposed current i = 100 /jA, constant gas composition, 400°C, Po /Pet 7. Key O, r10/Ars and , ru/Ar%.

Figure 10. Effect of Ag catalyst-electrode surface area Q on the cell relaxation time constant. Conditions 400°C, PqJPet — 7.

The capaciatance C depends on the gas-electrode-electrolyte interline "area" but not on the total electrode surface area S. If the porosity of all the electrode catalysts used is the same, which is a reasonable assumption since they were all prepared by the same calcination procedure, it follows that the interline "area" is proportional to the flat electrolyte surface area A, i.e. the constant X equals X A, where X is another constant which does not depend on any macroscopic dimension. 

Q Reactive oxygen uptake, mols 02 proportional to catalyst-electrode surface area S. 

Here, i is the faradaic current, n is the number of electrons transferred per molecule, F is the Faraday constant, A is the electrode surface area, k is the rate constant, and Cr is the bulk concentration of the reactant in units of mol cm-3. In general, the rate constant depends on the applied potential, and an important parameter is ke, the standard rate constant (more typically designated as k°), which is the forward rate constant when the applied potential equals the formal potential. Since there is zero driving force at the formal potential, the standard rate constant is analogous to the self-exchange rate constant of a homogeneous electron-transfer reaction. 

---