In the Laboratory
Why Do Some Batteries Last Longer Than Others?
W
Michael J. Smith* Departamento de Química, Universidade do Minho, Largo do Paço, 4700-320 Braga, Portugal;
[email protected] Colin A. Vincent School of Chemistry, University of St. Andrews, Fife, KY 16 9ST, Scotland
Although the group of aqueous electrolyte-based cells is one of the oldest classes of primary cells (1), it is also one of the commercially most important. As a result of continual innovation in the design and manufacture of the Leclanché cell (2), variants based on the zinc anode and manganese dioxide cathode represent one of the strongest sectors of the portable energy market. The favorable position of this cell is due to various factors including low cost of raw materials, ease of manufacture, and performance characteristics that satisfy a wide range of portable devices. The use of commercial cells based on this chemical system as resources for student experiments (3) offers several advantages. The cells are inexpensive, readily available, and familiar—superficially, at least— to most students. Their introduction as experimental materials provides a clear link between the everyday environment and the study of chemistry. In this experiment we describe how the energy content of samples of cathode material, removed from cells of different manufacturers, can be compared using an appropriate test cell. The objective is to demonstrate the diversity of cathode specification and to provide an explanation for the observed range of performance of energy-converting devices. From the early days of development of the Leclanché cell it was realized that samples of naturally occurring MnO2 from different sources produced very different discharge performances. Manufacturers of modern cells of the lowest specification still use ores from Ghana, Mexico, or Gabon, which are prepared simply by crushing and washing. Although the active materials of these cells are zinc and MnO2, the lowperformance variants are generally designated as “zinc-carbon” cells. In modern versions the anode is manufactured from zinc sheet and the electrolyte is normally ammonium chloride with an amount of zinc chloride that varies from one manufacturer to another. The use of higher quality MnO2 in cell manufacture leads to better electrochemical performance, though naturally with an increase in production cost. A further improvement in performance can be obtained by adding greater amounts of zinc chloride, or even completely substituting the ammonium chloride in the electrolyte with this salt. Cells with a zinc chloride electrolyte composition are generally designated as “heavy-duty” cells because they have better performance at high current discharge. Although higher specification material is used in the heavy-duty cell, the internal structure of the cell is essentially the same as that of the zinc–carbon cell, illustrated in the supplemental material.W The devices known as “alkaline” cells differ from the zinc– carbon and heavy-duty cells in the internal distribution of components. The anode of the alkaline cell is composed of zinc granules suspended in a gel and contained in a cylindrical separator tube. The cathode is located around the interior
Figure 1. Detail of test cell assembly.
wall of the cell, in contact with the steel case. To improve electrochemical performance the cathode material is usually chemically or electrochemically modified manganese dioxide mixed with carbon powder, which acts as an electronic conductor; this is compressed to form the dense, hollow cylinder that contains the anode. The electrolyte of this cell is typically a concentrated solution of potassium hydroxide with small amounts of zinc oxide. Experimental Procedure Samples of composite cathode material from various cell manufacturers can be readily obtained by sectioning commercial cells of AA size (also identified as R6 or LR6). In practice this operation is straightforward and can be conveniently carried out using a bench vise and a junior hacksaw. The cathode material obtained from the cell is ground into a powder, transferred to a Büchner funnel, and washed to remove electrolyte residues. The cathode material is then dried in an oven before use. The test cell assembly developed for this study of electrode materials is illustrated in Figure 1 (see also Fig. 2 of the supplemental material).W Both the carbon rod and the zinc sheet anode referred to in this figure can be removed from the same commercial zinc–carbon cells that provide samples of composite cathode. The mass of the zinc anode used in the test cell must of course be such that the cathode limits the cell capacity. The assembly of the test cell is initiated by transferring a known mass of composite into the separator, embedding the tip of the carbon rod current collector into the cathode material, and compressing the powder firmly to produce a compact electrode block. The complete cathode assembly is located in position in the Perspex (or plexiglass) cover, the zinc sheet anode is connected to the contact, and the assembly of the cell is completed by the addition of a
JChemEd.chem.wisc.edu • Vol. 79 No. 7 July 2002 • Journal of Chemical Education
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In the Laboratory
suitable volume of 1 M potassium hydroxide as electrolyte. In this format the overall reaction that takes place may be written formally as Zn(s) + 2MnO2(s) + H2O(ᐉ) → ZnO(s) + 2MnO⭈OH(s) The assembled cell is connected to a digital voltmeter and the discharge is started by connecting a resistor of a known value to the cell. During the discharge readings of the cell potential are taken at convenient intervals until the “cutoff potential” of 0.2 V. When this value is reached the resistor is disconnected and the discharge process terminated. The calculation of the energy converted by a cell involves various steps. From the reading of cell potential and the value of the resistor used to discharge the cell, it is possible to calculate the instantaneous current that passes through the cell. The product of the potential and current at any given time during the discharge gives the power developed by the cell at the instant at which the potential reading was taken. The area under a graph of instantaneous power as a function of time during the discharge gives the energy converted by the cell under the specific conditions of discharge used in the experiment. Typical discharge curves are illustrated in Figure 2. The use of samples of material extracted from various sources confirms that commercial cells of different provenance have significantly different energy contents. The results shown in Table 1 are typical of commercially available cells. If this experiment is carried out using samples of known mass of composite electrode materials extracted from cells produced by different manufacturers, the gravimetric energy density of these materials can be compared directly. The price of a cell also allows an estimate of the amount of energy available per unit cost, permitting direct comparisons between products from different suppliers, as shown in Table 1. If a more quantitative analysis of the composite cathode mixture is of interest, the carbon and manganese dioxide contents of the composite cathode can be determined using published gravimetric and titrimetric analysis routines (3, 4). Typical results for the cells in this study are given in Table 2. The evaluation of the energy content demonstrates clearly that the cathode materials used by different manufacturers have very different capacities. The energy available from these samples is largely determined by the ability of certain manganese dioxide preparations to withstand high currents without undue polarization. The rate at which the test cell is discharged militates against high energy capacity being obtained from natural manganese ores. However, synthetic preparations of manganese dioxide are able to withstand these high discharge rates without excessive reduction in the cell operating potential and therefore convert energy more efficiently. While the specification of the cathode material influences the behavior of the cell, the total energetic content of a cell also depends on the mass of cathode material that is packed into the cell. In spite of having very similar external dimensions, the total mass of cells can differ very significantly, as confirmed by Table 2. The total masses of the cells studied by our students varied between about 16 and 25 g, and the mass of cathode composite varied between about 5 and 11 g, providing further justification for the range of cell performance observed. Clearly the quality of the cell depends on a careful selection of all the components, active and inactive. The gravimetric 852
Figure 2. Typical discharge curves for some commercial cells.
Table 1. Energy Content of Some Commercial Cells Cell Cost/Eurosa Energy Cost/(J/Euro) a
Cell
Energy Content/J
A
1,090
0.24
4,540
B
1,970
0.37
5,320
C
2,440
0.49
4,980
D
5,860
0.49
11,960
E
5,400
0.81
6,670
aThe
cell cost is expressed in Euros. One Euro is worth approximately 0.880 U. S. dollars.
Table 2. Quantitative Measurements of Some Commercial Cells Cathode Composite
Cell
Total Mass of Cell/g
A
15.59
4.90
31.5
21.3
222
B
18.79
5.21
27.7
19.7
378
C
18.67
5.12
27.4
19.6
477
D
24.34
9.75
40.0
3.7
601
E
24.26
10.45
43.0
6.5
517
Mass/g
% of Cell Carbon Mass Content (%)
Energy Density/ (J/g)
energy density, calculated from the mass of cathode material and total energy content of the cells (Table 2), demonstrates that the cell that provided more energy per unit cost also contains the electrode material with highest specification. It is interesting that in this case the material used in the most expensive cell does not have the best electrochemical characteristics nor does this cell represent the best choice from an economic point of view. Hazards We stress that only mercury-, cadmium-, and lead-free Leclanché cells are recommended for this experiment. With these cells the only significant chemical hazards involved in the experiment are associated with the electrolyte (concentrated potassium hydroxide in the alkaline cell). In most commercial “dry” cells the volume of liquid electrolyte is small and contact can easily be avoided by wearing gloves, safety spectacles, and a lab coat. The powdered cathode material is considered to
Journal of Chemical Education • Vol. 79 No. 7 July 2002 • JChemEd.chem.wisc.edu
In the Laboratory
be harmful. However, the application of the normal rules of good laboratory practice should be sufficient to control any ingestion, inhalation, or contact hazards that exist. Finally, care should be taken with the saw used to open the commercial cells, and instructors should remind students of this danger. The use of a suitable bench vise to support the cell will help to minimize this risk.
reflects the manufacturer’s options in a series of technical, safety, and perhaps environmental decisions. This material may be used to stimulate discussion of these options, emphasizing the link between chemistry and real-life situations. WSupplemental
Material
Instructions for students and notes for the instructor are available in this issue of JCE Online.
Conclusions These experiments can be readily modified to provide motivating practical classes for students at an upper secondary school or initial university level. With suitable adaptation they might also be used as a basis for a class project at a lower secondary school level. The main purpose of the practical work is to familiarize students with the structure of cells and batteries, essential components of a huge range of portable electronic devices. On the basis of experiments presented in this paper, students can also begin to appreciate that the choice of components of a cell, or indeed any commercial product,
Literature Cited 1. Heise, G. W.; Cahoon, N. C. The Primary Battery, Vol. 1; The Electrochemical Society: New York, 1970. 2. Vincent, C. A; Scrosati, B. Modern Batteries, An Introduction to Electrochemical Power Sources, 2nd ed.; Wiley: New York, 1997. 3. Smith, M. J.; Vincent, C. A. J. Chem. Educ. 2001, 78, 519–521. 4. Bassett, J; Denney, R. C.; Jeffrey, G. H.; Mendham, J. Vogel´s Textbook of Quantitative Inorganic Analysis, 4th ed.; Longman Group: London, 1978.
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