In the Laboratory
Structure and Content of Some Primary Batteries
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Michael J. Smith* Departamento de Química, Universidade do Minho, Largo do Paço, 4700-320 Braga, Portugal Colin A. Vincent School of Chemistry, University of St. Andrews, St. Andrews, Fife, KY16 9ST, Scotland
Commercial batteries, now consumer products of major importance (1, 2), may be regarded as well-designed miniature chemical plants, fabricated for the conversion of chemical energy into electrical power. In recognition of their economic impact and their widespread everyday use, chemistry course curricula often include a study of these devices, although there are significant difficulties in providing students with appropriate practical experience. In previous papers (3, 4) we have described experiments that characterize the electrochemical processes in a commercial cell. In the experiment described here the structure and content of the most common AA-sized (LR6 or R6) consumer primary batteries is studied. The aim is to familiarize students with the components and construction of these devices, and, given the stoichiometry of the cell reaction, to allow them to calculate the anode and cathode capacities. These values may be compared with the practical capacity obtained when the cells are discharged through a resistor. Three types of cell are considered: the zinc–carbon or general purpose cell, the zinc chloride or (extra) heavy-duty cell, and the alkaline manganese or alkaline cell. All three are variants on the original Leclanché cell proposed in 1866 and use the same active reagents, zinc and manganese dioxide. While the reaction products are somewhat different, the electrochemistry of the three cells is basically the same: anode reaction:
Zn → Zn2+ + 2e
cathode reaction:
2Mn + 2e → 2Mn IV
(1) III
dioxide and synthetic carbon, which acts as the current collector. In this cell the anode consists of zinc powder suspended in a polymer-based gel. The polymer support contains alkaline electrolyte, and a synthetic fabric separator is used to enclose the anode composite mass. Experimental Procedure The first stage of this experiment involves opening the case of a commercial cell using a handsaw and removing the two active materials. Where the cell is of the zinc–carbon or heavy-duty class, the case itself forms the zinc anode. The composite cathode mixture is removed quantitatively from the open case and both electrodes are weighed using an analytical balance (Table 1). In the alkaline cell the cathode is disposed around the case wall and the anode mix is in the center of the cell within a separator envelope. In this case the anode composite must be extracted from the supporting gel before weighing. A sample of the composite cathode mix is analyzed by dissolving the manganese dioxide and filtering the carbon suspension through a preweighed filter paper. The weight gain of the filter paper provides a simple method for obtaining an accurate value for the carbon content. The manganese dioxide content of the composite cathode removed from the cell is analyzed by dissolving an accurately known mass of the mixture and carrying out a standard volumetric analysis.
(2)
The outer case of modern zinc–carbon cells (Fig. 1a) is manufactured using a zinc alloy. The electrolyte has evolved from the saturated ammonium chloride solution proposed by Leclanché, and manufacturers currently use ammonium chloride, zinc chloride, and water in a range of compositions. Inexpensive manganese dioxide produced simply by washing ores from suitable geological sources is still commonly used in this type of cell. Manganese dioxide is a poor electronic conductor, and therefore the cathode mix also contains finely divided carbon, to provide adequate electronic contact with the carbon rod collector. The cathode is separated from the zinc can by a microporous paper sheath saturated with electrolyte. The architecture of the zinc chloride cell is similar to that of the zinc–carbon cell, but the ammonium chloride in the electrolyte is almost completely replaced by zinc chloride. A sophisticated sealing system is used, since leakage of this acidic electrolyte would present a more serious problem, and oxygen ingress must be avoided. Purer synthetic manganese dioxide is generally used in zinc chloride cells. In cells based on alkaline electrolytes the configuration is reversed (compare Figs. 1a and 1b). The cell is enclosed in a thin steel case, which is mechanically robust and resistant to corrosion. The cathode mix is composed of purified manganese
Figure 1. Schematic of typical cell configurations: (a) zinc–carbon, (b) alkaline cell, (c) detail of alkaline cell.
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In the Laboratory
In the second stage of the experiment, cells are discharged through a resistor to evaluate their energy content. As this experiment may last for several hours, the use of automatic data acquisition is convenient. Personal computers equipped with AD-converter cards1 provide an economic alternative to expensive storage voltmeters.2
Table 1. Comparison of Cells Type of Cell
Component Zinc metal/g
MnO2/g
Zinc–carbon
4.5548
6.0423
Heavy-duty
3.1286
8.0658
Alkaline
3.8688
10.5297
Hazards Most of the components of the mercury-, cadmium-, and lead-free cells recommended for study present no hazard for students. Manganese dioxide is classified as a material that may be harmful by inhalation, ingestion, or skin contact (5). The risks associated with this material can be satisfactorily reduced by wearing gloves, safety spectacles, and a lab coat. Although the quantities involved are small, instructors may prefer to carry out manipulations of the dried powder in a fume cupboard. There are, however, two other aspects of the experiment that require special attention. During the removal of the case, electrolyte (concentrated potassium hydroxide in the alkaline cell) may be released from the cell. In most cases this is not a problem because this class of cell (designated as “dry” cells) does not contain much liquid electrolyte and the leakage is easily controlled. Similarly, when washing the anode mixture of the alkaline electrolyte cells with a volume of potassium hydroxide care is required to avoid spillage or projection. The application of the normal rules of good laboratory practice should be sufficient to eliminate any hazards that may exist. Results and Discussion The theoretical capacity of a galvanic cell may be calculated from the masses of the active components (i.e., the zinc and manganese dioxide) using Faraday’s law. Assuming that the stoichiometry of the cell reaction is given by eqs 1 and 2, the theoretical anode capacity is given by
(mass of zinc/g) × (96,489 C mol1) × 2 (relative atomic mass of zinc/g mol1) × (3600 s/h)
Table 2. Theoretical Capacity of Cells Type of Cell
Theoretical Capacity/kJ Anode
Cathode
Zinc–carbon
21.4 ± 0.4
10.7 ± 0.2
Heavy duty
14.7 ± 0.3
14.3 ± 0.3
Alkaline
18.2 ± 0.4
18.6 ± 0.4
Alkaline cells are based on a steel case that is not involved in the cell reaction, and are anode limited (2). If this were not the case the potential of the zinc in a cell with an exhausted cathode, maintained under load, might be sufficient to produce a dangerous pressure buildup within the cell. The other cells do not suffer from this problem to the same extent because the carbon rod current collector acts as a porous element that can provide a controlled release of internal pressure. The practical capacity of a cell is the actual quantity of charge delivered during its discharge life. It is very dependent on the way in which the discharge is carried out. In general, the slower the discharge (i.e., the lower the current), the nearer will the practical capacity approach the theoretical value. The way the cell recovers during the open-circuit intervals is greatly affected by the nature of the manganese dioxide used. Alkaline cells are generally capable of effective continuous discharge at reasonably high rates. In Figure 2 we show continuous discharge curves for the three cell types under a constant load of 4.92 Ω. A high-rate discharge of this nature provides a test too severe for the zinc– carbon and heavy-duty cells, which give practical capacities to a 0.8-V cutoff of only 820 and 2300 J, respectively. The
that is, by (0.8198 × mass of zinc/g)/(A h). Similarly, the theoretical cathode capacity is given by (mass of manganesedioxide/g) × (96,489 C mol1) × 1 (relative molar mass of manganesedioxide/g mol1) × (3600 s/h)
that is, by (0.3083 × mass of manganese dioxide/g)/(A h). Theoretical anode and cathode capacities can be converted to joules by multiplying the value in A h by 3600 and multiplying the result by the potential of the cell prior to discharge. The calculated values for the three cells characterized in Table 1 are given in Table 2. In a balanced cell the anode and cathode capacities are equal. Cells in which the zinc anode also serves as a structural element are cathode limited so that only a fraction of the zinc is oxidized during cell discharge and even at the end of the useful life of the cell the container does not leak. The zinc–carbon cell examined has a typical performance.
520
Figure 2. Discharge curves for three cells under constant load conditions (4.92-Ω resistor): (a) zinc–carbon, (b) heavy duty, (c) alkaline cell.
Journal of Chemical Education • Vol. 78 No. 4 April 2001 • JChemEd.chem.wisc.edu
In the Laboratory
alkaline cell, however, discharges through 4.92 Ω to give a value of about 6300 J, corresponding to 35% of its theoretical capacity. It is straightforward to devise interesting projects to study how the practical capacity of primary cells is affected by the rate of discharge (4 ). Reference to the results shown in Table 1 and Figure 2 confirms that the more expensive cell with alkaline-based electrolyte has a superior electrochemical performance. This is a consequence not only of its greater mass of active material, but also of the superior material specification used in this class of cell. The results obtained with the zinc–carbon and heavy-duty cells fall within the range of values observed with cells from 15 manufacturers characterized by our students. In these cells the electrochemical behavior depends on the manufacturer’s choice of both active and passive materials, which is largely determined by cost. Conclusions In many cases students’ interest and motivation can be greatly enhanced by relating the content of an experimental procedure to a real-life situation. This experiment demonstrates the use of commercial primary cells in an undergraduate or high school research project to compare the characteristics of the products of different manufacturers. It gives students a hands-on experience that improves understanding of a commercially important product. While we have focused on the electrochemical context, clearly the procedure described might also be adapted to provide a source of material that students could extract and analyze in introductory analytical chemistry practical classes.
Acknowledgments We are grateful to the Fundação para a Ciência e Tecnologia for providing financial support through the IMAT Centre of the University of Minho and contract BSAB/85/99. W
Supplemental Material
Student handouts and additional information on the experiment are available in this issue of JCE Online. Notes 1. An appropriate basis for this data logger is a PICO ADC-11 from RS Components; http://RSwww.com. 2. Suggested equipment is Tektronix TX1 with WSTRM software; http://www.tek.com.
Literature Cited 1. Vincent, C. A.; Scrosati, B. Modern Batteries, An Introduction to Electrochemical Power Sources, 2nd ed.; Arnold/Wiley: New York, 1997. 2. Linden, D. Handbook of Batteries and Fuel Cells, 2nd ed.; McGraw-Hill: New York, 1996. 3. Smith, M. J.; Vincent, C. A. J. Chem. Educ. 1989, 66, 529– 531. 4. Smith, M. J.; Vincent, C. A. J. Chem. Educ. 1989, 66, 683– 687. 5. The Sigma Aldrich Library of Safety Data, 2nd ed.; Lange, R., Ed.; Sigma-Aldrich Corporation: Milwaukee, WI, 1988.
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