Galvanostatic Polarization Method

This way of determining the cation transference number involves some underlying assumptions, too binary electrolyte with the cation as active species, no convection, semi-infinite diffusion, and one-dimensional cell geometry. Furthermore, the method combines the results of three different measurements, which is very time-consuming. Nevertheless, the calculation of transference numbers does not assume ideality or diluted solutions, making it more appHcable for modelling transport parameters of hthium-ion batteries. 

Transference numbers in literature are not linear with concentration. 

---

Hafezi H, Newman J (2000) Verification and analysis of transference number measurements by the galvanostatic polarization method. J Electrochem Soc 147(8) 3036-3042... 

Side reactions can introduce error into the measurement of physical properties in three ways [73]. Current is consumed by the side reaction, introducing error into calculations of the amount of current that went into the main reaction. Bulk concentrations of salt or solvent may change if the side reaction is substantial, and soluble products of reaction may affect the activity of the electrolyte. Finally, the side reaction causes the potential of the electrode to be a mixed (corrosion) potential. It is commonly assumed that the lithium electrode is covered by the SEI layer. However, there is strong evidence that, in many situations, the protection is not complete and side reactions involving the solvent or anion continuously occur. Such reactions can increase the concentration of lithium ions adjacent to a lithium electrode, introducing error into measurements of the variation in potential with apparent electrolyte concentration, particularly at low electrolyte concentrations. Such concentration-ceU measurements are used to obtain activity coefficients and transference numbers via the galvanostatic polarization method. Simulations of the type described in this section can be used to analyze how much error is introduced by the side reaction [73]. It may be preferable to use a less reactive reference electrode, such as Li4Ti50,2 [74], to reduce this error. 

Polarization probe.s. Polarization methods other than LPR are also of use in process control and corrosion analysis, but only a few systems are offered commercially. These systems use such polarization techniques as galvanodynamic or potentiodynamic, potentiostatic or galvanostatic, potentiostaircase or galvanostaircase, or cyclic polarization methods. Some systems involving these techniques are, in fact, used regularly in processing plants. These methods are used in situ or... 

Figure 38. The voltage response on a galvanostatic polarization of a mixed conductor with ion-blocking electrodes ( eqc. (G3)3,15). Reprinted from J. Maier, Evaluation of Electrochemical Methods in Solid State Research and Their Generalization for Defects with Variable Charges , Z. Physik. Chemie N.F., 191-215, Copyright 1984 with permission from Oldenbourg Verlagsgruppe.

As mentioned earlier, electrolytes used in lithium batteries are usually concentrated, binary electrolytes that exhibit nonideal behavior. In addition, polymer and gel electrolytes are opaque, highly resistive, and sticky, and therefore their transference numbers are not easily measurable using traditional techniques such as the Hittorf or moving boundary methods. Recent theoretical studies have described the substantial error involved in measuring transference numbers with techniques that assume ideal behavior [14, 15], and have described how experimental data can be interpreted rigorously using concentrated-solution theory to obtain transference numbers. One method is the galvanostatic polarization technique [120,121,122] ... 

Anodes for boilers can be tested by such methods. Good-quality magnesium anodes have a mass loss rate per unit area < 30 g m d", corresponding to a current yield of >18% under galvanostatic anode loading of 50 /xA cm" in 10 M NaCl at 60°C. In 10 M NaCl at 60°C, the potential should not be more positive than t/jj = -0.9 V for the same polarization conditions [27],... 

Steady-state measurements can be made under both galvanostatic and potentiostatic conditions. It is irrelevant for the results of the measurements whether the current or the potential was set first. But in certain cases in which the polarization (/ vs. E) curve is nonmonotonic and includes a falling section (BC in Fig. 12.4), the potentiostatic method has important advantages, since it allows the potential to be set to any point along the curve and the corresponding current measured. But when the galvanostatic method is used, an increase in current beyond point B causes a jump in potential to point D (i.e., the potential changes discontinuously from the value Eg to the value Eg,) and the entire intermediate part of the curve is inaccessible. 

A version of the galvanostatic method is that where the current is turned off (or a current f = 0 is applied ) and the polarization decay curve is measured. Consider an electrode which up to the time t = 0, when the current was turned off, had the potentiaf F at the net current density When the current is turned off, the ohmic voftage drop in the electrolyte gap between the electrode and the tip of the Luggin capillary vanishes, so that the potential instantaneously shifts to the value F (Fig. 12.11). After that the electrode potential returns (falls) relatively slowly to its open-circuit value, for which a certain nonfaradaic charging current is required. Since ip + =... 

Although the CMT method was originally developed to measure the corrosion rate at the corrosion potential, it has been demonstrated that it can also be used, with some restrictions, to measure the dissolution rate of a polarized electrode. The device for polarization can be a galvanostat or a potentiostat, the operation of which must not interfere with the pH measurements. Most important, the counter electrode must be in the same cell compartment as the experimental electrode and its content well mixed. 

A cell with a capacity of 1 L was made of mild steel. An amorphous carbon rod (diameter 25 mm length 15 cm) was used as anode, the inside wall of the cell as cathode and a platinum wire was used as reference electrode. The anode compartment of the cell was separated from the cathode compartment by a skirt of steel welded to the cell cover. The anode gas was passed through a tube filled with tablets of NaF to absorb anhyd HF gas and then led to a gas sampler. Fluorine was detected with K.I soln. After the starting material was added into the molten KIIF2/HF salt, the electrolyte was pre-electrolyzed at a low current density until NF2 was detected, and then current efficiency of each product and polarization curves by galvanostatic or potential sweep method were determined (Table 1). At optimum conditions the current efficiency of NF3 was 55%. 

Fig. 12. Polarization curves of various metal oxides on Ti substrate electrodes in 02-saturated 4 M KOH obtained by pseudo-steady-state galvanostatic method (3 min./point). Curves recorded from low to high currents T = 22°C [243]. a, Fe A, Pr , Pd , Rh , Ir O, Ru.

The effect of ultrasound on the process of tellurium anodic dissolution in alkaline solutions was studied by the method of plotting polarization and galvanostatic curves [148]. Tests were made in NaOH solutions (concentrations of 0—20 g/L), subjected to the action of ultrasound at a frequency 17.5 kHz and using Te electrodeposited under ultrasound. The anodic polarization curves plotted without ultrasound and in its presence shifted with increased NaOH concentration towards negative values as a result of the increasing rate of Te anodic dissolution. The presence of ultrasound inhibited the process of Te anodic dissolution, probably due to the desorption of OFT anions from the anode surface. This sonoelectrodeposited Te thus showed greater corrosion resistance in alkaline solution than that deposited... 

This assertion is supported by the data in Table 1 concerning the behaviour of ARMCO iron in 1 m HCl solutions at various temperatures [40]. The direct evaluation of the corrosion current density. Id, was obtained from the determination of the concentration of ferrous ions, entered the solution, by the spectrophotometric technique. The galvanostatic pulse polarization curves were performed using the CORRCONTROL system [41] and the GALIMP program [42]. The corrosion current density, Ic, was computed using the NOLI method [34]. 

The determinations of R which was made using high-frequency alternating current, showed that its value was approximately 0.3 H. The polarization curves were performed using the galvanostatic pulse technique and their geometric shapes were analyzed by the NOLI method [34],... 

In the galvanostatic method, constant current steps are applied for a predetermined duration and to measure the overvoltage. The polarization resistance curve is constructed by plotting the values of the overvoltage as a function of applied current for each current step. The slope at the origin of the plot gives the polarization resistance. Because the metal... 

---