Fast-Galvanostatic Transients

The galvanostatic-transient behavior of the catalyst potential and that of the reaction rate dne to anodic cnrrent steps, was investigated as a function of the applied cnrrent. The reaction was the combnstion of ethylene at 375°C over Ir02A SZ catalyst at highly oxidative feed conditions (C2H4 O2 = 1 100). Transients under cmrent application (termed polarization) and after cnrrent intermption (relaxation) were recorded. In order to achieve well-established steady-states, long polarization of at least 100 min and relaxation of at least 200 min were applied. 

The reaction rate transient was faster under polarization than during relaxation, and experiments made evident that the kinetics of polarization and relaxation depended differently upon the applied current. On one hand, the higher the current, the faster the increase in reaction rate under polarization. On the other hand, the decrease in reaction rate during relaxation was rather independent of the current applied in the preceding polarization. 

The variation of the reaction rate between the two extremes, i.e., the open-circuit reaction rate, o, and the steady-state promoted reaction rate, A st, was fully reversible. A possible irreversible contribution to the promotion effect (permanent NEMCA) was avoided by limiting the polarizing current to 10 (tA. The current dependence of the steady-state promoted catalytic reaction rate was also investigated. The total increase in reaction rate, was a continuously increasing function 

The complexity of the reaction rate transients, which consist of one fast and one slow stage, is in agreement with the cyclic voltammetric evidence abont the existence of differently accessible regions for surface charging. The first rapid step (a) is believed to be dne to accumulation of promoting species over the gas-exposed catalyst surface by the mechanism of backspillover, while the second step (b) is due to current-assisted chemical surface modification. Since no correlation between potential transients and reaction rate transients was manifested, a dynamic approach is justified and the applied current —rather than the catalyst overpotential— may be an appropriate parameter to describe the transient behavior of ethylene combustion rate at electrochemically promoted Ir02AfSZ film catalysts. For the interpretation of the fast transient steps (a) and (c), a dynamic model of electrochemical promotion has been developed, as presented in detail in Section 11.3. 

It was concluded that a dynamic approach considering the applied current as key parameter is well adapted for the interpretation of galvanostatic electrochemical promotion of ethylene combustion over Ir02 catalysts. Both, the transient and the steady-state behavior of the system, were satisfactorily described by the proposed model, which assumes free surface site dependent formation, rapid spreading-out (backspillover), and first order rate consumption of the O promoter. 

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The use of galvanostatic transients enabled the measurement of the poten-tiodynamic behavior of Li electrodes in a nearly steady state condition of the Li/film/solution system [21,81], It appeared that Li electrodes behave potentio-dynamically, as predicted by Eqs. (5)—(12), Section III.C a linear, Tafel-like, log i versus T dependence was observed [Eq. (8)], and the Tafel slope [Eq. (10)] could be correlated to the thickness of the surface films (calculated from the overall surface film capacitance [21,81]). From measurements at low overpotentials, /o, and thus the average surface film resistivity, could be measured according to Eq. (11), Section m.C [21,81], Another useful approach is the fast measurement of open circuit potentials of Li electrodes prepared fresh in solution versus a normal Li/Li+ reference electrode [90,91,235], While lithium reference electrodes are usually denoted as Li/Li+, the potential of these electrodes at steady state depends on the metal/film and film/solution interfaces, as well as on the Li+ concentration in both film and solution phases [236], However, since Li electrodes in many solutions reach a steady state stability, their potential may be regarded as quite stable within reasonable time tables (hours —> days, depending on the system s surface chemistry and related aging processes). 

Due to the strong lateral repulsive interactions between the parallel-oriented surface dipoles, the migration of these dipoles on the catalyst surface is fast. The rate of migration is not limited by surface diffusion but rather by the rate, I/nF, of creation of the surface dipoles at the tpb metal-solid electrolyte gas. Consequently the time, T, required to form the "effective electrochemical double layer" during galvanostatic transients is of the order of ... 

Galvanostatic transient for Ni dissolution in saturated NiClj solution with potential increase after the induction time X and the ohmic drop in pores measured by superimposed fast short additional galvanostatic transients. (From Strehblow, H.-H. and Wenners, ]., Electrochim. Acta, 22,421,1977.)... 

The complex transient r vs t, or equivalently r vs 0Na or r vs Uwr behaviour of Fig. 4.15 parallels the steady-state rvs UWr behaviour shown in Fig. 4.16, where for each point UWr has been imposed potentiostatically, until the current I has vanished and the corresponding rate value, r, has been measured. This shows that the catalyst surface readjusts fairly fast to the galvanostatically imposed transient 0Na values (Fig. 4.15). The dashed and dotted line transients on the same figure were obtained with the same gaseous composition but with initial Uwr values of 0 and -0.3 V respectively. It is noteworthy that the three transients are practically identical which shows the reversibility of the system. 

Galvanostatic pulse excitation technique requires a fast E - i conversion device to switch from potentiostatic to galvanostatic conditions. The analysis of E(t) transients is rather complex since the nucleation and growth kinetics of the 3D Me bulk phase are changed continuously by the varying supersaturation. 

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