30
31
32
33
34
35
M
Percentage o f nomlnal c a p c l r y
Flgure 5 Discharge-chargecycles of 5 % nominal capacily for 0 80.80-mAh secondary m c - s ver oxlae cells: (a) 4 mA. lo) 32 mA
of the system decreases during discharge ( A P= -34.2 JK-1 mol-I), and thismust therefore be compensated by anequivalent increase in the disorder of the surroundings brought about by the transfer of heat from the system. At 298K, the minimum heat that must be evolved in order for the discharge to proceed spontaneously is TASe or 10.2 kJ mol-I. Thus the maximum work that can he extracted is, as we have seen, (317.5 - 10.2) or 307.3 kJ mol-l. If the cell operates in an "irreversible" manner, i.e., far from equilibrium, then further energy is dissipated as heat in addition to the 10.2 kJ mol-l, and the amount of work done is less. I t is clear from Table 2 that the efficiency of the electrochemical conversion falls off rapidly as higher currentsare drawn from the cell. Note that the values of conversion efficiency are dependent not only on the type and design of the cell and the mode of discharge, but also on its previous history (i.e., on any previous chargeldischarge cycles), as may be readily demonstrated. Cycle Efficiency
Cvcle efficiencv measurements were carried out bv suhprimary A d secondary cells to shallow discharge/ charge cycles a t different rates. A "shallow" cycle is one that involves only a small fraction of the practical capacity of the cell. I t is safe to recharge primarv miniature cells based on aqueous systems a t low rates f i r a small fraction of the capacity. If high currents are used, however, gassing may occur and the cell may burst open, possibly violentl@. Further, i t must be stressed that experiments to recharge primarynonaqueous cells based on lithium may result in explosive hazards, and should never be attempted. I t is common prac-
jetting
Table 3. Cycle Efflciencles tor Shallow (5%) Cycllng In Prlmary and Secondary Mlnlature Zlnc-Sllver Oxlde Cells as a Functlon ol Rate Cwem (mA) Rimsry ceN (D 350) 5 10 20
Secondary cell (B 80) 2 4
8 16 32 48
686
Rate
Percentage efficiency of cycle
0.05C 0.1C 0.2C
0.025C 0.05C 0.1C 0.ZC 0.4C 0.6C
Journal of Chemical Education
94 91 86
Current
/ mA
Figure 6. Power levels as a function of cunem for a D 350, 100-mAh primary zlnc-silver oxide cell.
tice to standardize the values of current drawn in such experiments in terms of the nominal capacity of the cell. Thus H C-rate of unity corresponds to a current that would completely charge or discharge the cell in 1h. At a C-rate of 0.1 this process would take 10 h, and a t a C-rate of 10 it would take 0.1 h, etc. Two typical cycles, corresponding to low and hieh rates. for enerev conversion corresoondine to 5% of nominal capacity of: B 80 cell are shown in ~ i & e 5. The cvcle efficiencies. as defined in ea 9a ahove.. are eiven for a variety of rates for primary and secondary cells in Table 3. Again i t is clear that the cycle efficiency falls off a t higher rates where increased values of polarization leads t o a high dissipation of energy as heat.
-
Battery Power
As the current withdrawn from a cell is increased, the cell voltage falls, a t first slowly and then more rapidly. The power delivered therefore rises to a maximum and then falls as the cell voltage collapses. The maximum power point is easily found experimentally. Figure 6 shows the results of polarizing a D 350 primary cell a t progressively higher currents. In each case a constant current was drawn for 15 s before the voltage was recorded and the current incremented. A peak power of over 50 mW was recorded in this instance. Energy Density and Power Density
In order to compare different systems, it is useful to define energy density and power density as the energy and power, respectively, delivered per unit mass or unit volume of battery. For applications such as traction, mass-based energy and power densities are clearly very important. For miniature cells. the corresoondine volume-based characteristics are generilly most uskful. .' The D 350 and B 80 cells had volumes of 0.29 cm3 and 0.80 em3, respectively. A practical energy density of just over 0.4 Wh cm+ was therefore obtained for the primary cell discharge shown in Figure 3, where the cell is operating a t 100 FA and hence a t a continuous power level equivalent t o 5 X W ~ m - The ~ . same type of cell showed a peak power density of 0.17 W em-3. At 5 mA, the secondary cell delivered 0.13 Wh, thus showing a volumetric energy density of 0.16 Wh ~ m - ~This . cell, however, was able to work a t much hieher rates than the orimarv cell. Thus even with a drain of 2 5 m ~the , energy density was still over 0.13 Wh ~ m - but ~, the power densitv a t this rate was now eauivalent to 0.044 W
Linden, D. In Handof Batteries and Fuel Cells; Llnden, D., Ed.; McGraw-Hill: New York. 1984; pp 4-21.
Electrochemistry of the Zinc-Silver Oxide System Part 2: Practical Measurements of Energy Conversion Using Commercial Miniature Cells Michael J. Smith University of Minho, Braga 4700, Portugal Colin A. Vincent' University of St. Andrews, St. Andrews. Fife KY 16 9ST. Scotland Following the development of microelectronic equipment, and in particular the introduction of the electric watch and pocket calculator, miniature electrochemical power sources known as "button cells", with diameters ranging from under 1cm to over 3 cm, and heights of 2-4 mm, have become an increasingly important consumer p ~ o d u c t Such . ~ cells may be regarded as well-designed chemical plant, fabricated for the conversion of the chemical energy of particular chemical reactions into electrical power. As such, they can also be used as an extremely valuable but widely available teaching resource both for the illustration of basic electrocbernical ideas, and for the practical study of concepts such as reacpower, enerw stortion rate. enerm -. conversion efficiency, . . .~ age cycles, etc. The emf of a reversible electrochemical cell is simply relat. ed to the Gibbs free energy of the cell reaction, which is a measure of the theoretical maximum "useful work" that can be ohtained from the cell. In an earlier paper3 we suggested experiments with commercial zinc-silver oxide miniature and AS+ cells that led to the determination of AGO. for the reaction
@.
Here we describe experiments in which such cells are discharged and charged under controlled conditions so that practical energy conversion capabilities and a numher of other parameters may be studied. We have again chosen the zinc-silver oxide system because the cell reaction can be assumed for our purposes to he a straightforward reversible process. First, however, we summarize the quantitative relationships pertaining to the operation of electrochemical cells and briefly discuss practical cells based on the above reaction.
where eo is the charge on an electron (1.601 X 10-l9 C ) and L is the Avogadro constant. The term (Leo) is known as Faraday's constant, F, and has a value of 96,490 C mol-', so that we have Rate of cell readion = iI2F rnol s-'
(2)
Note that the current gives a direct measure of the reaction rate; this is one of the most useful features of an electrochemical reaction. Cell Capacity, Energy, Electrochemical Efficiency, and Power From a practical point of view, a cell may be characterized in terms of the available capacity, the total useful energy, and the power that i t can deliver. For purposes of comparison, these factors are often normalized in terms of the overall weight or volume of the cell as will be discussed below. The theoretical capacity may be calculated as QT = x(nF)
(3)
where x is the numher of moles of reaction associated with complete discharge of the cell (and fixed by the quantity of reactants present) and n is the numher of electrons involved in the cell reaction (two in the case of eq 1).The practical capacity, Qp will be lower than QT if reactants can be consumed in ways that do not lead to charge transfer a t the electrodes, e.g., by corrosion processes or other forms of chemical attack. Qp may be readily measured by integrating the current that flows as the cell is completely discharged:
Q~ =
I
idt
(4)
0
Current and Reaction Rate By dividing the cell reaction 1into two "half-reactions" and Ag,O(s)
+ H,0(1) + 2e
-
20H-(aq) t 2Ag(s)
(lb)
i t is seen that each time a zinc atom is oxidized t o a zinc ion a t the anode, two electrons pass through the external circuit to the cathode where two silver ions are reduced:
In practice the limit of integration corresponds to the time a t which the current has fallen to zero following complete consumption of the reactants. We have seen3that, for areversible process, the maximum amount of energy available to do external work is given for 1 mol of reaction by -AG = nFE
(5)
where E is the emf of the cell, so that the theoretical auailable energy is
ST = x(nFE)
(6)
When current is drawn from a cell, the voltage a t its termi-
' To whom all correspondence should be addressed.
A current of i A (i.e., i Cs-1) therefore corresponds to a reaction rate for the cell process (eq 1) of
Vincent, C. A.; Bonino, F.; Lazzari, M.: Scrosati, B. Modem Baneries; Arnold: London, 1984. Smith, M. J.; Vincent C. A. J. Chem. ~duc.1988, 86, 529. Volume 66 Number 8 August 1969
683
nals falls due to the internal resistance of the electrolyte and other cell comoonents and to resistance associated with the electrode reactions, which increases with current. Hence the practical available energy, Sp, depends on the way the cell is constructed, its history, and, in particular, on the way i t is discharged. I t may be measured using the relationship
where E and i are the measured cell voltage and current, respectively, and the integration limit is as defined for eq 4. Most cell tests are based on discharge (continuous or intermittent) throueh a constant load. Under these circumstances;both t& cell current and voltage vary with time and each must be monitored in order to evaluate the inteeral. In this paper we suggest exploiting the benefits of c c k t a n t current discharge for which &,=iIoa~dt
(7.4
and for which i t is necessary to record only the cell voltage as a function of time durine discharee. The efficiency of a primary cell as an energy converter is given simply by Efficiency =
(8)
Cycle Energy Efficiency For secondary cells, one of the most important characteristics is the ratio of electrical energy obtained during discharee t o that exoended on chareine: ", this is known as the cycleefficiency. ~ blarge-scale r practical applications of batteries such as in load levelline or electric traction. this ratio is always likely t o he the critical factor in assessing the commercial viability of a system. The cycle efficiency is given by S Edi.idi.dtl.f E&,& (9) where Edia and E,h are the instantaneous cell voltages on discharge and charge, respectively, and id;, and i,h are the corresponding instantaneous currents. I t is assumed here that an equal amount of charge is passed in each part of the cycle. The practical study of cycle energy efficiency is again simplified by using constant currents: for such experiments, and where idis = ich, the ratio is given by S EdkdtlSEchdt (94 This parameter is dependent on the magnitude of the currents drawn and the oercentaee of the theoretical caoacitv . . involved in the cycle. Battery Powet Power is the rate of doing work, and the instantaneous power being delivered by working cell is given by the product of voltage and current in units of J s-1 or W.
a
The Zlnc-Sllver Oxlde Cell System The zinc-silver oxide cell has been known for over a century: indeed the first recorded battery was the "silver-zinc oile" r e ~ o r t e dbv Volta in 1800. Modern commercial orima&cells iange from miniature cells of the type studied'in this oaoer . . with total enereies of under 1 Wh (3600 J) to reserve batteries used in military equipment-e.g., for supplying up to 300 kW for several minutes to drive a toruedo motor or for providing energy for missile hydraulics and guidance systems where the assembly must be able t o withstand shock levels of up t o 2000 G. Practical secondary cells based on this system have been available for almost 50 years: because of their high energy-to-weight ratio, they are now widely used in portable e q u i p m e n t T V cameras, medical instrumentation, night vision devices, and in aerospace applications such 684
Journal of Chemical Education
Figure 1.Crws &ion of a D 357. 100-mAh rinc-silver oxide primary cell. 1: outer top; 2: i n n s top: 3: top subassembly; 4: sealing gasket: 5: anode; 6: absorbem: 7: separator membrane; 8: calhode: 9: can: 10: can/eathode subassembly; 11: sleeve. (By courtesy of Duracell UK.) as the life-support plant used by Apollo astronauts. Very large submarine batteries have also been constructed. Miniature Cyllndrlcal Cells The engineering design of modern button cells is highly soohisticated. and comoonents are manufactured to verv close toleranc&. A sectiin through a typical 100 mAh cell (D 350. Duracell UK) is shown in Fieure 1. The cathode is composed of Ag?O (mixed with graphite to improve the electronic conductivitv), which is comuressed into a pellet and inserted into the nickel-plated steel "can" or case. Arnalgamated zinc powder of high surface area, with a gelling agent such as sodium polyacrylate, forms the anode, and is located in the laminated copper/stainless steel cap. The electrolyte is a concentrated aqueous solution of KOH or NaOH, saturated with Zn(OH)aZ-. The success of this type of power source is, however, also very dependent on the design and materials of the final two components, namely the separator and the sealing gasket. Modern separators are commonly based on a polymer membrane that is permeable to hydroxide ions but that retards the diffusion of soluble silver species toward the anode, together with a fibrous electrolyte absorbent. For serondar; cells the membrane must be protected on the one side hy a "poiitive interseparator" that minimizes oxidative attack by silver oxide a n d b n the other by a "negative interseparator" that prevents zinc dendrite penetration and internal short circuits during recharge. T o seal the cell, a nylon gasket is inserted between the cell case and cap; the case is then pressed in such a way as to place the gasket under compression, so effecting a radial seal. A secondarv seal is then formed bv c r i m ~ i n the e edee of the case over tLe gasket. The selection of a n s o n polymer with suitable mechanical characteristics is critical4. Experlmedal We have studied energy conversion processes using two types of zinc-silver oxide button cells: (1)primary cells of 100-mAhnominal capacity (D 350, Duracell UK), and (2) secondary cells of 80-mAh nominal capacity (B 80, Medicharge). Cell holders that gave reliable electrical connections were of the type described previously3. Cell voltages were generally recorded using a standard digital voltmeter; a potentiometrie chart recorder might also be used. The key procedure both for simplifying the treatment of results and, more importantly, for helping student understanding was the use of a constant current mode in cell discharge and cycle experiments. The circuit diagram, far a simple galvanostat capable of delivering constant currents of 10-100 mA into low-impedance Loads is shorn in Figure 2. The device is based on the inexpensive 759 operational amplifier and may be quickly assembled by an
'
Crompton. T . R. SmaN Batteries: Primary Cells: Macrnillan: London, 1982; p 111.
inexperienced person at a cost of no mare than a 50-mL Quickfit flask. A 15-V single-ended dc supply is required. Six-millimeter pitch striphoard was used to simplify construction; details are available from the authors. The magnitude of the current through the external circuit is controlled hy the 20-turn variable resistance, PI, and measured either by an ammeter in series with the cell under test or by applying a voltmeter to the 20-0 resistor, R1.A list of components is given in Table 1. Results and Discussion
~~~
Energy Conversion Efficiency for Cell Discharge The discharge curve for a primary D350 button cell at a constant current of 100 *A is shown in Figure 3; this is a plot of cell voltage against time. The practical capacity is approximately (0.1 m.4 X 768 h) or 77 mAh, in comparison with the rated value of 100 mAh. This cell is designed to supply low currents, so that discharge a t higher rates leads to very low
Table 1. Component Llst lor Gatvanostat RI RP R3
R, R* Pt
iC 1
20 O f 10% 10kfl*1%
5kO*1% loon+ 1%
loon*
1%
soon, 20 turn cermet himer (% in.) type 759 (PA 759 UiC)
conversion efficiency. Such primary cells are not therefore ~ r a c t i c afor l student exoeriments on complete discharge (1) because of the long time scale of the experiment, and (2) expense, since each cell can only be used once. Secondary B 80 cells were discharged using constant currents in the range 1-50 mA. In between discharges, cells were recharged using a standard procedure of an 8-mA constant current for 10 h, followed by application of a constant potential of 1.60 V for P5 h. Results of some of these exoeriments are shown in Figure 4, which alsc~includes an example of a ronstnnt load disrharee. Fur discharges at rurrenrs 5 25mA. the capacity, given b; the product of the current and discharge time to 0.5 V, was always within the range 81 f 3 mAh. For higher currents, the practical capacity fell off quite rapidly. Hence, assuming a theoretical capacity of 81 mAh, and an emf for the cell reaction of 1.591 V a t 25 O C , the theoretical energy available is 0.129 Wh or 465 J, corresponding to 0.00146 mol of reaction. The practical energy available, and the energy efficiency (based on this capacity) for the various rates ofdischarge are given in Table 2. If a mole of zinc and a mole of silver oxide were able to react directly under isothermal conditions a t 25 "C, 317.5 kJ of energy would be given out to the surroundings, corresponding to AHe for the reaction. By carrying out the same reaction in an electrochemical cell, part of this energy may be harnessed as useful work. The lower the rate of discharge, the more nearly will the cell operate in a reversible manner, and the more energy can be extracted as work. Whatever way the discharge is uaertaken, the total energy change of the system is always 317.5 kJ, i.e., from the first law of thermodynamics the sum of the heat evolved and the work done is constant. If the cell is simply short-circuited, no work is done and all the energy is given out to the surroundings as heat. In contrast, because of the operation of the second law, it is not nossible to harness all the enerev as useful work. For a reaction to occur spontaneously, the entropy of the universe must increase. In the reaction we are studying, the disorder
1W
0.5 W 0.5 W 0.5 w 0.5 w
~
~~
~
--
Figure 2. Gaivanostat (conslantcurrent supply)clrcuit diagram
I
100
300
200 Tlne
400
5M)
/ nlnuroP
Figure 4. Discharge curves for a B 80. 80mAh zinc-sllvar oxide secondary cell. (a)35 mA: (b) 25 mA: (c) 15 mA: (d) 10 mA. Dashed line: constant load discharge into 56.2 0. Table 2. Energy Conversion Elflclency for the Complete Discharge of a B 80, 80-mAh Secondary Zlnc-Sllver Oxide Cell
Current
Practical capacity
Practical available energy
ImAI
ImAH
ImWH
Energy conversion
efficiency i%)
Tlme / daye Figure 3. Discharge ourve for a D 350, 10hAh zinc-silver oxide primary cell at a constant current of 100 pA.
Volume 66
Number 8
August 1989
685
Conclusions
commercial miniature cells provide a valuable resource for the practical study of the principles of electrochemical energy conversion. A variety of experiments may be devised to illustrate concepts such as energy storage cycles, energy and power density, chemical-to-electrical conversion efficiency, etc. The use of a simple galvanostat (constant current source), as described, greatly simplifies both practical
and conceptual difficulties. The zinc-silver oxide system is particularly recommended because of its straightforward
We wish to thank DuraceU UK, a division of Duracell Batteries, Ltd., for providing the scale drawing reproduced in Figure 1.
Volume 66
Number 8
August 1989
687
