Electrochemistry: Edison Cell (Iron-Nickel-Battery)

Computer-Interfaced Experiments - Voltage Measurement

Electrochemistry
Edison Cell (Iron-Nickel-Battery) - Model

Objectives: Charging and Discharging of the Battery, Determination of the Cell Capacity

Peter Keusch

Datalogging and data analysis using the Program CHEMEX and the Analog-Digital-Converter CHEMBOX
IBK electronic + informatic

German version

Chemicals:
20 % potassium hydroxide solution

Apparatus and glass wares:
600 mL beaker
iron sheet metal 5 cm × 10 cm
nickel plate 5 cm × 10 cm
iron wire net
nickel wire net
DC voltage source
ammeter
voltmeter
switch
slide rheostat 100 W
path cords
clamps

Hazards and safety precautions:

Potassium hydroxide solution is corrosive! Contact with skin can cause irritation or severe burns and scarring with greater exposures. Swallowing may cause severe burns of mouth, throat, and stomach.

Safety glasses and protective gloves required.

Theoretical background:

The secondary cell devised by Thomas Edison (1847-1931) uses an iron anode and a nickel(III) oxide-hydroxide cathode both immersed in an electrolyte of potassium hydroxide.

The reaction on discharge and charge is:

Preparation:

The iron sheet metal is wrapped by an iron wire net, firmly fastened with wires to the iron plate. In the same manner a nickel plate is enclosed by a nickel wire net. The two electrodes are hung in a beaker filled with 20 % potassium hydroxide solution. The surface area of the electrodes immersed in the electrolyte is approx. 75 cm². The ends of the wire nets are wedged into slotted rubber stoppers. Using clamps and clamp holders the rubber stoppers are attached to a stand. The metal plates ensure the accurate vertical placement of the metal nets in the potassium hydroxide solution.

The iron net is connected to the negative terminal, the nickel net to the positive terminal of DC voltage source. A slide rheostat is adjusted such that a constant current of 200 to 300 mA flows through the cell (Fig. 1). The battery is charged for 30 minutes.

Fig. 1: Charging of the cell

At the two electrodes a gassing occurs. The voltage of the cell decreases within a day below a value of 0.5 V (Fig. 3).

Next day the cell is charged for 10 minutes (as above described). The voltage rises to 1.7 to 1.9 V.

Fig. 2: Potential gradient while battery charging
(data logging with Cassy)
After power-off of the voltage source, one connects the nickel electrode via the ammeter, the slide rheostat and a switch to the iron electrode. The slide rheostat is adjusted such so that the reading of the ammeter is 20 mA (Fig. 3). The voltage decreases within 3 minutes to 0.2 V. After interruption of the external electric circuit the voltage rises again slowly to 1 V.

Fig. 3: Battery discharge

Experimental procedure:

Matching of the program CHEMEX
The positive terminal of the voltage source is connected via the slide rheostat and the ammeter to the nickel electrode. The iron electrode is connected to the negative terminal.The voltmeter displays a voltage of 1.5 V. The voltage produced by the electrochemical cell is used for the calibration. The program CHEMEX is switched to 'Options / Calibration /Sensor1'. One sets the first point of reference on 0 V. Afterwards the voltmeter is replaced by the CHEMBOX: the Ni-electrode is connected to the positive terminal, the Fe-electrode to the negative terminal of the 'Input Sensor1' of the CHEMBOX. As a second point of reference is taken the voltage value of the cell. In order to check the matching of the program one switches to the analog / digital display for voltage 1. If the appropriate voltage value is not displayed, the calibration is to be repeated.

Measurements:
After the cell is charged at a constant current of 100 mA, it is discharged at 20 mA. Next the re-charged battery is discharged at 30 mA and 40 mA, respectively. The change in voltage is recorded at a 2 second interval.

The Edison cell exhibits a high self discharge. The voltage decreases to 1.0 V within approx. 250 seconds. Not till then the voltage curve levels off as time goes on. Therefore the measurements are started at the time where a voltage of approximately 1.2 V has been reached.

Fig. 4: Self-discharge of the Edison cell

screen

Fig. 5: Real-time plot discharge with a current of 20 mA

Data analysis using Excel:

The pairs of measured values logged using the CHEMBOX/CHEMEX System are analyzed using the spread sheet program Microsoft Excel.

Fig. 6: Voltage changes during the discharge of the cell
discharge currents: 20 mA (1) 30 mA (2) 40 mA (3)

The voltage of the uncharged cell is approx. 1.2 V.

At the beginning of the discharge process, the voltage decreases abruptly. Then it remains rather constant for some time until it drops within a couple of seconds. At a voltage of 0.6 V the battery is to be regarded as discharged. A linear relationship is evident between amperage and discharge time (Fig. 7).

discharging amperage / discharge time

Fig. 7: Plot of amperage versus the discharge time

In order to discharge the cell initial currents I₀ are adjusted by selecting the appropriate resistance. The results are initial voltages U₀ corresponding to the currents I₀.

	I ₀ [ mA ]	U ₀ [ V ]
Measurement 1	20	1.162
Measurement 2	30	1.137
Measurement 3	40	1.107

Tab. 1: Initial currents and initial voltages when discharging

The time-dependent currents I (t) during the discharge process can be calculated with the following equation

I (t) = U · I ₀ / U ₀

Determination of the Battery capacity

The values for I (t) are determined according to the above mentioned formula (Tab. 2) and a plot of I versus t is permitted (Fig. 8).

Tab. 2: Spread sheet

Fig. 8: Plot of amperage I versus time t

Battery capacity is determined by the amount of electrical energy the battery can deliver over a certain period of time and is normally measured in Ampere hours (Ah)

As a result of integration of the current I (t) over the discharge time the battery capacity Q can be determined.

Q = ò I dt

Q is found by calculating the appropriate area below the curves in Fig. 9.

Tab. 3: Spread sheet Calculation of the battery capacity

A bar graph is selected (Fig. 9).

Fig. 9: Plot of I (t) versus t battery capacity Q = ò I dt

	I ₀ [ mA ]	Q [ mA · s ]
Measurement 1	20	3510
Measurement 2	30	3462
Measurement 3	40	2359

Tab. 4: Battery capacity Q

References:
A sealed, starved-electrolyte nickel–iron battery
Computer-Interfaced Experiments Electrochemistry: Lead Acid Battery (Model)

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