Charging constant-current charge

A second approach to coulometry is to use a constant current in place of a constant potential (Figure 11.23). Controlled-current coulometry, also known as amperostatic coulometry or coulometric titrimetry, has two advantages over controlled-potential coulometry. First, using a constant current makes for a more rapid analysis since the current does not decrease over time. Thus, a typical analysis time for controlled-current coulometry is less than 10 min, as opposed to approximately 30-60 min for controlled-potential coulometry. Second, with a constant current the total charge is simply the product of current and time (equation 11.24). A method for integrating the current-time curve, therefore, is not necessary. 

Fig. 9. Discharge and charging curves for a sintered iron electrode at a constant current of 0.2 A where the apparent geometrical surface area is 36 cm and porosity is 65%. A and B represent the discharging and charging regions, respectively. Overall electrode reactions, midpoint potentials, and, in parentheses, theoretical potentials at pH 15 ate Al, n-Fe + 2 OH Fe(OH)2 + 2, 0.88 V (1.03 V) B, Fe(OH)2 FeOOH + H+ +, 0.63 V (0.72 V) C,...

Fig. 11. Constant-current charge curve for a high rate Ag—Zn ceU at room temperature. Charging carried out at the 10-h rate.

For the amplifier pulse to be recognized in the ADC, it must exceed the lower level set by a discriminator, which is used to prevent noise pulses from jamming the converter. Once the pulse is accepted it is used to charge a capacitor that is discharged through a constant current source attached to an address clock typically... 

Freshly assembled lithium/carbon coin cells typically have voltages between 2.8 and 3.4 volts. The cells are in their fully charged state which means that no lithium is inserted in the carbon anode. Then the coin cells are tested with computer-controlled constant-current cyclers having currents stable to 1%. The cells are placed in thermostats at a particular set temperature v/hich is stable to 0.1°C during the test. Most of our cells were tested at 30°C. 

Figure 8. Charge characteristics of an Ni-Cd battery at a constant current (cell type 1200SC temperature 20 °C).

Figure 7. First- and second- cycle constant-current charge/discharge curves of graphite Timrex KS44 in LiN(S02CF3)2/ethylene carbonate/dimethyl carbonate as the electrolyte (CilT irreversible specific charge Crev =reversible specific charge) [2J.

Figure 15. First- cycle constant-current charge/discharge curve of hard carbon ("Carbotron P"). The figure has been reproduced with kind permission of Kureha Chemical Industry Co., Ltd. [2381.

Polymerization at constant current is most convenient for controlling the thickness of the deposited film. Charges of ca. 0.3, 0.2, and 0.08 C cm-2 are required to produce 1 fim of polypyrrole,59 poly(3-methylthio-phene)60 (no data are available for polythiophene), and polyaniline 43 respectively. Although these values can reasonably be used to estimate the thicknesses of most electrochemically formed conducting polymer films, it should be noted that they have considerable (ca. 30%) uncertainties. For each polymer, the relationship between charge and film thickness can... 

A particular constant current density is applied to the electrode, and the potential variation is followed as a function of time. When there is no electrode reaction, the entire current is a nonfaradaic charging current According to Eq. (12.13), the slope of the E vs. t curve (Fig. 12.10, curve 1) is determined by the EDL capacitance. 

When the nonfaradaic current is not small enough, the appropriate correction must be included when constructing the curves. At constant current, the charge consumed... 

In coulometry, one must define exactly the amount of charge that was consumed at the electrode up to the moment when the endpoint signal appeared. In galvanosta-tic experiments (at constant current), the charge is defined as the product of current and the exactly measured time. However, in experiments with currents changing continuously in time, it is more convenient to use special coulometers, which are counters for the quantity of charge passed. Electrochemical coulometers are based on the laws of Faraday with them the volume of gas or mercury liberated, which is proportional to charge, is measured. Electromechanical coulometers are also available. 

Generally similar multiple step constant current with overcharge control recommended for fast charging NiMH... 

Figure 7. Constant current charge/discharge cycling ofhighly graphitic carbon fiber P100 (Amoco) in 1.8 MLiAlCl4 in SOCl2 electrolyte, i = 300 pA mg 1, cut-off 0-2V vs. Li/Li+ [4],...

Figure 8. Constant current charge/discharge cycling (1.-3. cycles) of graphite (Lonza KS44 synthetic graphite) in 1 MLiCl04 in y-hutyrolactone as electrolyte without and with C02 (saturated in electrolyte) as electrolyte additive, i lOpA mg 1, cut-off 0-1.5V vs. Li/Li+...

Figure 11. First cycle constant current charge/discharge curves of synthetic graphite TIMREX SFG 44 using 1 MLiCl04 in PC PS (propylene sulfite) (95 5 by volume) as electrolyte, i = +20 mA g1, cut-off = 1.8/0.025 Vvs. Li/Li+.

Figure 12. F cycle constant current charge curves of synthetic graphite LONZA KS 44 (i) using 1 MLiN(S02CF3)2 in EC.DME (dimethoxy ethane, CH3OCH2CH2OCH3) (3 2) as electrolyte. The measurement was stopped when the graphite was exfoliated, (ii) using 1 M LiN(S02CF3)2 in EC F-DME (partially fluorinated dimethoxy ethane, CH3OCF2CF2OCH3) (3 2) as electrolyte, i = 20 mA g1, cut-off = 0.0 V vs. Li/Li+ (adaptedfrom [12]).

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