Galvanostatic operation

The main difference is that in the case of O2 conductors galvanostatic operation leads to a steady state (Figs. 4.13 and 4.14) while in the case of Na+ conductors galvanostatic operation does not lead to a steady state, as the Na coverage on the catalyst surface is continuously increasing (Fig. 4.15 and 4.17). 

Figure 4.26. Transient response of the rate of CO2 formation and of the catalyst potential during NO reduction by CO on Pt/p"-Al2C>396 upon imposition of fixed current (galvanostatic operation) showing the corresponding (Eq. 4.24) Na coverage on the Pt surface and the maximum measured (Eq. 4.34) promotion index PINa value. T=348°C, inlet composition Pno = Pco = 0.75 kPa. Reprinted with permission from Academic Press.

FIGURE 11.3 Distributions of reactant concentration near the electrode after different times of galvanostatic operation (a) t < (b) > t. ... 

Constant current electrolysis is an easy way to operate an electrochemical cell. Usually, it is also applied in industrial scale electrolysis. For laboratory scale experiments, inexpensive power supplies for constant current operation are available (also a potentiostat normally can work in galvanostatic operation). The transferred charge can be calculated directly by multiplication of cell current and time (no integration is needed). 

The electrode potentials (exactly the overvoltages) are dependant on the current density. Thus, using the galvanostatic operation mode, optimal results are attained only if a well-defined current density can be chosen with a clear difference between desired and undesired reactions, as in Fig. 1. This precondition is favored... 

For batch operation (see Sect. 2.3.4), the limiting current density is going to zero for increasing degree of conversion (see reactant 1 in Fig. 1). Here, the galvanostatic operation may only be acceptable if exclusively unproblematic side reactions occur, such as water electrolysis as solvent decomposition. In all other cases, better results can be expected using the potentiostatic operation (see next section). 

In some problematic cases, there will be no obvious limits available for the choice of the current density in galvanostatic operation. Concurrent reactions take place, resulting in a poor selectivity. But here the potentiostatic operation also cannot demonstrate its advantages, and probably the simpler galvanostatic operation may be applied. To find relatively suitable operation conditions, an experimental optimization of the current density should be carried out, perhaps including parameters... 

A potentiostat is relatively expensive, especially if high power is needed. A cheaper method is to use the galvanostatic operation and to measure continuously the potential and to adapt the cell current manually (or using a computer data acquisition system) in order to adjust the... 

It is the objective of this chapter to study this effect by a model-based analysis. The spatially distributed model used here will be derived in the next section. In a first step, the steady-state behavior of the model will be investigated by a phase plane analysis for the case of potentiostatic operation of the cell. In the second step, numerical bifurcation analysis of the model will be used to study the technically more interesting case of galvanostatic operation. 

In contrast to classical chemical reactors, a fuel cell provides the possibility to control the reaction rate directly from outside by setting the cell current, because the local cell current density and the local reaction rate are related by a constant factor. This operation of a fuel cell at constant cell current is more important than the potentiostatic operation from a technical point of view, as fuel cells typically are characterized by current-voltage plots. Because the integral Eq. (15) has to be included in the analysis, the investigation of the galvanostatic operation is more difficult and requires numerical methods. In the following, numerical bifurcation... 

Fig. 3.6. Codimension-1 singularities of a finite length system for the case of galvanostatic operation (/ is the bifurcation parameter) (a)-(f) point to the parameter values ofy and yA used in the one-parameter continuation of Fig. 3.7.

Usually, that is except under open circuit conditions, the working electrode is embedded in an electric circuit, which imposes a constraint on WE(f) and thus defines its value with respect to some potential scale (in our case Eq. (14b)). Moreover, WE will in general evolve in time and thus the external constraint directly influences the dynamics of the system. The two most important operation modes of electrochemical systems are the potentiostatic and the galvanostatic operation. 

The galvanostatic operation mode gives rise to the following integral constraint for the total current... 

Figure 22 Typical chronopotentiometric profile of a graphite (4% PVDF) electrode and Li counter electrode loaded with methyl formate-LiAsF61 M solution under C02 (6 atm), galvanostatic operation C/33 h, 0.5 mA/cm2 [83]. (With copyright from The Electrochemical Society Inc.)... 

Figure 6.32. Nyquist plots of fuel cell impedance at different times under galvanostatic operation during progressive CO poisoning of the anode [33], (Reprinted from Journal of Power Sources, 127(1-2), Wagner N, Gulzow E. Change of electrochemical impedance spectra (EIS) with time during CO-poisoning of the Pt-anode in a membrane fuel cell, 341-7, 2004, with permission from Elsevier and the authors.)...

With galvanostatic instead of potentiostatic control, the potential would vary along a CER, which could compound the problem of selectivity variation caused by concentration changes. In both potentiostatic and galvanostatic operation, the nonuniform conditions may promote side reactions, including local electrode deactivation processes. 

Tor galvanostatic operation, a current density (i.e. current per unit electrode area) up to 10 mA cm (but generally lower) is convenient. Better quality films (certainly in terms of anisotropy) can be obtained by scanning the voltage between 0 and 1.0 V or better still by applying a square-wave potential. ... 

An AMEL 553 galvanostat-potentiostat was used in order to supply constant currents (galvanostatic operation) or potentials (potentiostatic operation) [8] between the working and counter, or the working and reference electrodes, respectively. Most experiments were carried... 

Figure 8. Time dependence of the cell voltages in an E-design short-stack during galvanostatic operation at 0.3 A/cm and 800 °C with H2, followed by two periods of 20 thermal cycles each insert shows the time-temperature (down to 220 °C and 75 °C, resp.)...

Electrochemical reactors can be operated under conditions of constant electrode potential, constant current, or constant cell voltage. The first two are referred to as potentiostatic and galvanostatic modes of operation, respectively. The potenti-static method is characterized by constant values of the kinetic parameters and hence enables integration of the dififerential equations describing the different reactors. On the other hand, galvanostatic operation is characterized by an inevitable change of electrode potential with time, leading to variations in the kinetic parameters. Hence we restrict our treatment to potentiostatic operation. 

Galvanostatic operation is commonly adopted, however, because potentiostats are not available for large-scale operations and are also much costlier. 

Operation with Constant Cell Current (Galvanostatic Operation). .. 35... 

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