J . Chem. SOC., Faraday Trans. I , 1982, 78, 3417-3429 Electrochemistry of Pol yace t ylene, ( CH)z Characteristics of Polyacetylene Cathodes BY KEIICHI KANETO, MACRAE MAXFIELD, DAVID P. NAIRNS AND ALAN G. MACDIARMID* Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, U.S.A. AND ALAN J. HEEGER Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, U.S.A. Received 1st April, 1982 The relationship of cell potential to degree of oxidation, coulombic and energy efficiencies, constant-current discharge characteristics, energy density and maximum power density of a partly oxidized polyacetylene, [CH(ClO,),] 0, < 0.07), cathode in a cell of the type [CH(ClO,),],ILiClO,ILi are discussed. Coulombic efficiencies ranging from 100 to 86 % and energy efficiencies ranging from 8 1 to 68 %during a chargedischarge cycle are found at oxidation levels ranging from 1.54 to 6.0%.Energy densities of ca. 255 W h kg-’ (based on the weights of the electroactive materials involved in the discharge process) are obtained for 7.0% oxidized polyacetylene cathodes under constant-current discharge conditions. Maximum power densities of ca. 30 kW kg-’ are observed. We have shown previously that polyacetylene, (CH),, may be controllably oxidized or reduced electrochemically with the incorporation of a variety of counter anions or cations, and that as oxidation or reduction proceeds its conductivity increases through the semiconducting to the metallic regime to give ultimately a series of ‘organic metals’.l We have also demonstrated that ca.0.1 mm thick films of partly oxidized or reduced (CH), may be used as the cathode and anode materials, respectively, in lightweight, rechargeable storage b a t t e r i e ~ . ~ ? ~ Polyacetylene is the first example of a covalent polymer whose conductivity can be increased, by chemical or electrochemical oxidation or reduction, into the metallic regime. Increases in electrical conductivity of over twelve orders of magnitude have been 0bserved.l The partly oxidized material exists as a stabilized polycarbonium ion, (CHY+),, in combination with the corresponding number of monovalent counter anions, A-, such that the overall composition is [CHu+A;],. Similarly, the partly reduced material exists as a polycarbanion, (CHY-)$, in combination with the corresponding number of monovalent counter cations, M+, such that the overall composition is [M,+CHY-],.Extensive studies of the partly oxidized material have shown that a semiconductor-metal transition occurs at oxidation concentrations > 7 mol% with a highly conducting transitional regime extending from ca. 0.1 mol% up to 7 mol%. In the concentration range > 1 mol%, the conductivity increases upon further oxidation at a relatively slow rate up to a value of ca. lo3 Q-l cm-l at room temperature. We summarize here selected important electrochemical properties of the more extensively studied partly oxidized (CH),, such as voltage against degree of oxidation curves, coulombic and energy efficiencies, constant-current discharge characteristics, energy density and maximum power density, which demonstrate its potential for use as a cathode material in rechargeable batteries.34173418 ELECTROCHEMISTRY OF POLYACETYLENE EXPERIMENTAL All investigations were performed using cells constructed by sandwiching a separator between a (CH), film electrode and a lithium electrode, squeezing the assembly into rectangular glass tubing (3 mm x 10 mm) adding the electrolyte together with ca. 200 mg of Woelm B Super 1 basic alumina and sealing the tube under vacuum across the protruding electrode leads. Using this method of construction the minimum amount of electrolyte was employed, air was excluded and the electrodes were held firmly in place, ca. 0.5 mm apart from each other. The (CH), electrodes were constructed of a single sheet of cis-rich (CH), (ca.85% cis isomer; 1-2.5 cm2; 0.1 mm thick; 3-10 mg). On oxidation to the levels involved in the present study, spontaneous isomerization to the trans isomer occurs. Cis film was used in construction of the cells since it is more flexible and is therefore more easily folded into the current collectors, as described below. One current collector was constructed by spot welding a 10 mil* platinum wire lead to a piece of 52 mesh platinum gauze of similar area to the (CH), film. The other current collector was constructed by spot welding a 10 mil nickel wire lead to a piece of 189 mesh nickel grid (Delker Corp.) of similar area to the (CH), film. This nickel grid was then folded tightly around a piece of 0.13 mm lithium ribbon (Alfa Ventron Corp.) whose area was approximately half that of the nickel grid.This composite was then placed on top of the separator [of similar size to the (CH), film] which was placed on top of the (CH), film which was itself placed on top of the platinum gauze. The whole assembly was then folded tightly in half so that the Li-Ni electrode was at the inside centre and the platinum grid was on the outside of the whole assembly. The whole assembly was then inserted into the rectangular glass tubing. Hydrophobic polypropylene (Celanese Celgard K-442) was used as the separator material in the cell potential and the energy density studies; freshly kiln dried (600 "C) glass filter-paper (Reeve Angel 934AH) was employed in the power density studies. The electrolyte solution was 1 .O mol dm-3 LiClO, (Alfa Ventron Corp.) in propylene carbonate.The solvent, electrolyte and lithium were purified and handled as described previ~usly.~ Charging of cells was performed with a Princeton Applied Research potentiostat/galvanostat model 173 by oxidizing the (CH), electrode either at constant current [ca. 0.1 mA per mg of (CH),], or at a series of stepped up constant potentials. Discharge studies were carried out in a similar manner, either at a constant-current discharge or at a series of stepped down constant applied potentials. Coulombs passed during charge or discharge cycles were recorded using a PAR coulometer, model 179. Voltages, resistances and currents were measured with a Keithley 177 microvolt digital multimeter.Voltage-charge, voltage-time and current-time graphs were recorded with a Houston Instruments Corp. Omnigraphics XY recorder. All voltages are given with respect to a lithium electrode. It must be stressed that the electrochemical characteristics of (CH), electrodes are extremely sensitive to the method of cell construction, presence of impurities (especially oxygen), relative ratio of electrolyte to (CH),, charging conditions etc. The results given in this report were obtained by following the described procedures exactly. RESULTS AND DISCUSSION CELL POTENTIAL (v) AND DEGREE OF OXIDATION (@ The positive and negative terminals of a d.c. power supply were attached to the (CH), and Li electrodes, respectively, immersed in a solution of 1 .O mol dm-3 LiClO, in propylene carbonate, and the cell was charged at a constant applied voltage [initial current ca.0.1 mA per mg of (CH),]. When the charging current fell to ca. 0.0 14 mA per mg of (CH),, the applied voltage was increased by steps of ca. 0.02 to 0.3 V. This step charging procedure was repeated at increasingly higher applied voltages until the desired final voltage was reached. The overall charging process in which the (CH), is oxidized and the Li+ is reduced is represented by (CH), + xyLi+(ClO,)- -+ [CHy+(ClO,);], + xyLi. (1) * 1 mil = 0.001 in = 2.54 x 10-5 m.KANETO, MAXFIELD, NAIRNS, MACDIARMID A N D HEEGER 3419 The charging process was interrupted at periodic intervals and V (o.c., immediate), the open-circuit voltage, was measured within 2-3 s of the interruption of the charging process.An open-circuit voltage is actually a voltage measured while the cell is being discharged at a very low rate, in these experiments at ca. 0.3 PA. Hence, during the few seconds needed to perform an open-circuit voltage measurement, negligible discharge of the cell occurs. The percentage oxidation of the polyacetylene electrode at each V(0.c.) measurement was calculated from the amount of charge passed and from the weight of (CH), (3.5 mg) employed. The total time taken to oxidize the (CH), to 6.00/,, i.e. to composition [CH(C104)o.06],, was ca. 3 h. The curve showing the relationship between V(o.c., immediate) and the percentage oxidation is given in fig. 1. 4.0 3.5 5 c - v L. 3.0 1 I I I I I A similar procedure was carried out with a different but similarly constructed cell [using 8.1 mg of (CH),., except that the cell was permitted to stand for 5 min after the charging process was interrupted before the open-circuit voltage, V (o.c., 5 min) was measured. The time taken for the whole experiment was 7 h.The curve showing the relationship between V(o.c., 5 min) and the percentage oxidation is given in fig. 1. In another study, using the same cell employed in the V(o.c., immediate) experiment described above, the charging process was interrupted at periodic intervals and the cell was permitted to stand for ca. 24 h to permit diffusion of the (C10,)- ions from the exterior to the interior of the ca. 200 I$ diameter (CH), fibrils. The open-circuit voltage, V (o.c., 24 h), was then measured. The cell was then discharged [reverse reaction to that given by eqn (I)] to 2.50 V, a voltage characteristic of parent, neutral (CH),, during 16 h using the method described below.The percentage oxidation of the polyacetylene cathode at each point was then calculated from the amount of charge (in coulombs) released during discharge. The curve showing the relationship between V(o.c., 24 h) and the percentage oxidation of the (CH), is given in fig. 1. The data for this experiment were collected over a period of three weeks. The empirical equation representing the V(o.c., 24 h) against percentage oxidation curve is given by the relationship (2) V(o.c., 24 h) = 3.43 + 0.14 In y3420 ELECTROCHEMISTRY OF POLYACETYLENE where y is the percentage oxidation. The excellent agreement between the curve (for values of y > 0.05%) given by this equation and the experimental points is shown in fig.1. In all the studies described above, significant oxidation occurred only at an applied potential > ca. 3.1 V. After the onset of oxidation, the V(0.c.) rises rapidly with increasing oxidation up to oxidation levels of ca. 1 % and then increases more slowly. Thus the electrochemical potential of a (CH), cathode can be varied over a relatively large range depending on the degree of oxidation. From fig. 1 it is apparent that for a given level of oxidation the observed open-circuit voltage decreases in the order V(o.c., immediate) > V(o.c., 5 min) > V(0.c. 24 h). This is interpreted as indicating that diffusion of the (C10,)- ions from the surface of the fibrils to the interior is relatively slow.Since the open-circuit voltage will be determined by that portion of the (CH), in contact with the electrolyte, the open-circuit voltages will decrease with time on standing as the extent of oxidation on the exterior of the fibrils decreases and the extent of oxidation in the interior increases by this diffusion process. The important role of diffusion is confirmed by the observation that the open-circuit voltage increases with time after rapid partial discharge. In this case, the (C10,)- ions and the associated positive charges on the polymer diffuse from the more highly oxidized interior of fibrils to the (ClO,)--depleted fibril exterior. Increases of almost 1 V were observed after cells were permitted to stand for 20 min after a relatively rapid discharge to 2.50 V (see below).Studies are in progress to determine the time taken to attain diffusion equilibrium within a fibril. For a 200 A (CH), fibril the diffusion time constant, 7,* is ca. 2 days;, for a 50 A fibril it is ca. 8 ha5 These values appear to vary significantly with the value of the applied voltage and extent of oxidation. The relationship between the open-circuit voltage and the degree of oxidation of the (CH), is believed to be given most accurately by the V(o.c., 24 h) curve because: (i) equilibration of C10, ions will be more complete than in the experiments from which the V(o.c., immediate) and V(o.c., 5 min) curves are generated and (ii) the amount of charge released during discharge is believed to give a more reliable measure of the extent of oxidation than the amount injected during charging, since at higher levels of oxidation some charge is lost during a charge-discharge cycle.Possible reasons for the loss of charge are discussed below. COULOMBIC EFFICIENCY A N D ENERGY EFFICIENCY A cell containing 3.5 mg of (CH), was charged at a constant applied potential of a magnitude such that the initial current was ca. 0.1 mA per mg of (CH),. When the charging current fell to ca. 10% of the initial value, the applied voltage was increased to give again a charging current of ca. 0.1 mA per mg of (CH),. When the charging current fell to ca. 10% of the initial value, the applied potential was again increased and the process was repeated as many times as necessary in a given experiment. This procedure results in a charging rate corresponding to ca.2% oxidation of the (CH), film per hour. The charging current was periodically discontinued and ~(o.c., immediate) was measured. The amount of charge involved in charging [Q(in)] to this value of V(o.c., immediate) was recorded and used to calculate the apparent extent of oxidation of the (CH)x at this value of V(o.c., immediate). The term ' apparent extent * 7 is related to the time taken to reach diffusion equilibrium in the following way: consider a solid cylinder surrounded by a constant concentration, C,,, of a substance, A, which is diffusing into the material of the cylinder. At any given time the average concentration of A in the cylinder is gven by c.At time = 2, c/Co = 0.74; at time = 27, C/Co = 0.91; at time = 32, c/Co = 0.97; at time = 45, C/Co = 0.99. Hence, diffusion equilrbrium is virtually established after time = 4 ~ . ~4.0 TABLE 1 .-COULOMBIC AND ENERGY EFFICIENCIES OF A [CH(ClO,),],lLiClO,(P.C.)ILi CELL* - - 2 coulombic efficiency energy efficiency oxidation [Q(out, total)/Q(in, total)] x 100 [E(out, total)/E(in, total)] x 100 (%> (%I (%) 4.5 I I I I 1 I -,B -0 -0-0-0-4 B - A0’ 3.5 .+’ I - v a 0 - 1 I I 1 I I 2.0 * 1.54 2.01U 2.17 2.51 4.0 6.0 100.0 99.2 100.1 95.8 89.5 85.7 80.8 (88.7)c 79.7b (89.5) 81.5 (89.2) 78.2 (87.7) 72.8 (78.8) 68.2 (74.8) a The discharge curves for 2.01,2.17 and 2.5 1 % oxidation were similar; for ease in examining the curves in fig. 2, the discharge curves for 2.01 and 2.51% oxidation are, therefore, not shown. Slower rates of charging and discharging results in higher energy efficiencies, e.g.85.8% at 2.0% oxidation. Values in parentheses are obtained from the area under V(0.c. immediate) curves during charging and discharging. * Nofe added inproqf: Studies performed since this manuscript was submitted for publication show that higher coulombic efficiencies at higher levels of oxidation are obtained consistently if the (CH), is oxidized at the rate of 1 % per hour at constant current charging conditions and is then discharged first at a constant applied current of ca. 0.2 mA mg-I to 2.50 V and then at a constant applied voltage of 2.50 V for 16 h. Coulombic efficiencies in excess of 92% have been obtained for oxidation levels of 8.0%, and of 87% for oxidation levels of 10.0%. These high coulombic efficiencies have also been confirmed in an independent laboratory (D.J. Frydrych and G. C. Farrington, personal communication; J . Electrochem. SOC. 1982, to be published).3422 ELECTROCHEMISTRY OF POLYACETYLENE of oxidation’ is employed since the system is not at diffusion equilibrium and also because the results of this study show that at higher levels of oxidation, complete charge recovery is not obtained. Further points were obtained by restarting the charging process with a higher applied voltage and repeating the above operations until the maximum value of ~ ( o . c . , immediate) desired in a given cycle was obtained. The total number of coulombs [Q (in total)] used in the charging operation in this cycle was then recorded and the degree of oxidation of the (CH), was calculated.An example of the stepped-up charging potential (solid-line steps) and the corresponding ~ ( o . c . , immediate) curve is shown in fig. 2 for the 6.0% oxidation experiment. An identical procedure was used in the studies at lower levels of oxidation shown in fig. 2 and summarized in table 1. The constant applied charging potentials used in these studies were significantly greater than the near diffusion equilibrium potentials, ~(o.c., 24 h), for a given degree of oxidation. For example, the final constant applied potential step for the 6% oxidized film given in fig. 2 was 3.95 V, implying a local surface oxidation level considerably in excess of 6%. This 3.95 V value may be compared with the V(o.c., 24 h) value [calculated from eqn (2)] of 3.68 V for 6.0% oxidized film.A similar procedure was used when discharging. The cell was discharged during 1 h to ca. 3 V in a series of constant applied potential steps, as illustrated for the 6.0% oxidation cycle in fig. 2. It was then finally discharged at a fixed potential of 2.50 V during ca. 16 h and the total amount of charge (in coulombs) liberated, Q(out, total) was recorded. The extent of apparent oxidation of the (CH), at any selected ~(o.c., immediate) value in the discharge cycle was calculated from Q(in, total) - Q(out), where Q(out) was the number of coulombs released on discharging to a given V(o.c., immediate) value in a given discharge cycle. From fig. 2 it can be seen that hysteresis is present in the charge-discharge curves.From the data obtained in these experiments, the coulombic efficiency of a charge-discharge cycle, Q(out, total)/Q(in, total), may be calculated, where Q(out, total) is the total number of coulombs released in the discharge process to 2.50 V in a given charge-discharge cycle. The energy efficiency of a given charge-discharge cycle, E(out, total)/&, total), may also be calculated. The total energy (amount of charge x voltage) expended in a given charge cycle, E(in, total), is given by the area under the applied stepped-up voltage curve (shown in fig. 2 for the 6.0% oxidized cycle). Similarly, the total energy released, E(out, total), in the discharge portion of a cycle is given by the area under the corresponding applied stepped-down voltage curve.Energy efficiencies calculated from the areas under the respective stepped-up charge and stepped-down discharge curves using the applied V(charge) and V(dis- charge) potentials at each step are given in table I for a number of different (CH), oxidation levels. The energy efficiencies listed will be dependent on the arbitrarily chosen values of the stepped-up charge and stepped-down discharge voltages used in a given experiment. Energy efficiencies obtained under different charge and discharge conditions will, therefore, be higher or lower than the values listed in table 1. If the areas under the V(o.c., immediate) charge and discharge curves are used, then the energy efficiencies given in parentheses in table I are obtained.These refer to the percentage of the stored energy in a given charge cycle which could be released if the cell were discharged so that the discharge potential followed the V(o.c., immediate) discharge curve all the way down to 2.50 V. These data suggest that cells containing (CH), electrodes may have excellent potential for use in storage batteries. Slower rates of charging and discharging will result in higher energy efficiencies due to reduction in 12R loss. As shown above, higher charging rates result in a greater degree of oxidation of the (CH), on the exterior of the fibrils since the dissipationKANETO, MAXFIELD, NAIRNS, MACDIARMID A N D HEEGER 3423 of charge to the interior of a fibril is limited by diffusion. Thus at higher charging rates a higher voltage is required to produce a given average level of oxidation.Conversely, at higher discharge rates the discharge potential will be lower because of depletion of charge on the exterior of the fibrils. Hence, the energy efficiencies under near equilibrium conditions should be greater than the values given in table 1. For example, charging to 2.0%, [V(charge) = 3.69 V], at an average current of ca. 0.015 mA per mg of (CH), and discharging in a series of decreasing constant potential steps to 2.50 V, in each of which the discharge current was similar to the above, gave an energy efficiency of 85.8%. The value corresponding to that given in parentheses in table 1 is 89.7%. The average charging and discharging currents in this experiment were ca.3-4 times smaller than the average currents used in obtaining the data in table 1 . Studies are in progress to determine whether the loss of charge over a complete charge-discharge cycle is real or only apparent. The loss of charge could be caused by chemical reaction of the electrolyte or by reaction of impurities in the electrolyte with [CHV+(ClO,);],. Preliminary studies4 suggest that the charging potential, which will determine the local extent of oxidation on the fibril surface, should not exceed ca. 3.75 V if chemical reaction of the film with the propylene carbonate/LiClO, electrolyte is to be avoided. Higher values may be possible with other electrolyte systems. The 100% coulombic efficiency studies given in table 1 were carried out at charging potentials < 3.75 V.Films consisting of smaller diameter fibrils, having smaller z values, can be oxidized more rapidly without exceeding this critical charging potential. However, at least a significant portion of the apparent loss of coulombs may be due simply to the fact that an insufficient amount of time had been allowed for complete diffusion of (C104)- ions from the interior of the fibrils to the surface during discharge. Factors which increase the amount of time elapsing between the initiation of a charge cycle and termination of a charge cycle will favour more complete diffusion of (C10,)- ions into the interior of a fibril. Such factors would therefore imply that a correspondingly longer time would be needed for (ClO,)--ion diffusion to the exterior of a fibril during complete discharge.If the coulombic loss is real and is caused by reaction of the (CHY+), species with the electrolyte or with traces of impurities, then the real percentage oxidation values in fig. 2 will be slightly less than shown, to the extent indicated by the coulombic efficiencies at various oxidation levels as given in table I . In situ optical studies5 during the oxidation-reduction cycle have recently been carried out. The optical cell utilized a thin (CH), film (ca. 2000 A thick by 2 cm2 in area; fibril diameter ca. 50 A) in 2 cm3 of electrolyte so that the electrolyte to polymer ratio was ca. lo5. Under these conditions, even trace impurities in the electrolyte might lead to harmful side reactions. Nevertheless, these studies show that the (CH), is not degraded either chemically or electrochemically during cycling at quasi-equilibrium diffusion conditions to a maximum charging voltage of 3.73 V (7.8% oxidation).Hence, the (CH), electrodes can be stable in the electrochemical environment under the experimental conditions employed in these optical studies. Other related investigations show, quite conclusively, that traces of air and unknown impurities in the system cause the voltage of a cell to fall sharply on standing. The stability is increased after several charge-discharge cycles, possibly because initially the (CHy+), species acts as a scavenger for impurities. The V(0.c.) of a carefully prepared, previously cycled cell after 1 day was 3.67 V. This fell to 3.61 V in one week and during the next two weeks fell at a rate of 0.003 V per day to a value of 3.56 V.Detailed long-term stability studies are in progress.3424 ELECTROCHEMISTRY OF POLYACETYLENE ENERGY DENSITY The change in voltage of a cell as it is discharged at a constant current is an important characteristic of the cell. Ideally, it should show very little change in voltage until it is almost completely discharged when the voltage would then drop sharply. Fig. 3 shows the excellent characteristics of a cell constructed from 3.3 mg of (CH), oxidized to 7.0% when discharged at constant currents of 0.1, 0.55 and 1.0 mA to 2.50 V immediately after charging. Greater weights of film will naturally provide a longer 'plateau' region for any given discharge current. These discharge rates in A per kg of [CH(ClO,),~,,], are 19.5, 107 and 195, respectively. The total energy released upon discharge (charge x voltage) is given by the area under each curve.To obtain the energy density (W h kg-l) the mass of the electroactive material involved was calculated using the weight of [CH(ClO,),~,,], employed and the weight of Li consumed in the discharge reaction [the reverse reaction to that given in eqn (l)]. The energy density values obtained are: 0.1 mA, 258 W h kg-l; 0.55 mA, 255 W h kg-'; 1.0 mA, 254 W h kg-l when the cell is discharged to 2.50 V, the voltage characteristic of parent, neutral (CH),. The corresponding energy density values obtained from the curves in fig. 3 on discharge to 3.0 V, the approximate potential'at which the discharge voltage begins to drop rapidly, are 2 19,2 18 and 21 6 W h kg-l, respectively. These values may be compared with the previously reported energy density of 176 W h kg-l obtained in a 3 min short-circuit discharge of a cell using 6.0% oxidized (CH),.3 Empirical rules may be applied to obtain a rough estimate of the expected energy density of a packaged battery, including the weight of electrolyte, solvent and casing from the experimental energy density of 218 W h kg-l.A reduction factor of seven gives a reasonably conservative estimate and results in a value of 31 W h kg-' for the completely packaged battery. This is approximately the same value as that found for t/min FIG. 3.-Relationship between the discharge voltage, Y, and discharge time, t, during a constant current discharge at 0.1 mA ( x ), 0.55 mA (O), and 1.0 mA (0) of a (CH),Il mol dmP3 LiClO, in propylene carbonatelLi cell.KANETO, MAXFIELD, NAIRNS, MACDIARMID A N D HEEGER 3425 the average lead/acid automobile battery.It should be noted that the above energy- density value is for 7% oxidized (CH),. Higher levels of oxidation will result in larger energy densities. The coulombic efficiencies for the 0.1, 0.55 and 1 .O mA discharges shown in fig. 3 are given in table 2. After completion of a discharge given in fig. 3, the cell was permitted to rest for 20 min, during which time diffusion of (C10,)- from the interior to the exterior of a fibril continued; an increase in V(0.c.) was observed. An additional constant potential discharge at 2.90 V for 16 h resulted in the release of further charge, as shown in table 2.Note that the 86.7% coulombic efficiency value in table 2 for a 7.0% oxidized film is in complete agreement with the 85.7% value in table 1 for 6.0% oxidized film obtained in a different, although related, type of experiment. The release of additional charge is caused by diffusion of (C10,)- ions within a fibril, not to diffusion of Li+ ions in the electrolyte to the surface of the fibrils. This is obviously the case since if the total quantity of charge released were dictated by the rate of diffusion of Li+ ions within the electrolyte to the fibril surface, then more charge should have been liberated in the slower 0.1 mA discharge than in the faster 1.0 mA discharge. As shown in table 2, the reverse is the case for this 7.07; oxidized film, more charge being released in the faster discharge.The mechanism for this unexpected observation is presently being studied. It is believed to be related to the charging times used in each of the three discharge studies. These were 12, 7 and 5.5 h for the 0.1, 0.55 and 1.0 mA studies, respectively, suggesting that the smaller the time allowed for diffusion of (C10,)- ions into the (CH), during charging the greater is the coulombic recovery at these relatively high discharge rates. TABLE 2.-cOULOMBIC EFFICIENCIES, [Q(out, total)/Q(in, total)] X 100, FOR SELECTED CONSTANT CURRENT DISCHARGES OF A [CH(C104),~,,],~LiC104(P.C.)(Li CELL coulombic efficiency increase of total coulombic after discharge cycle V(0.c.) during efficiency after a 16 h I(discharge) shown in fig.3 20 min rest constant voltage discharge /mA to 2 . 5 0 V (%) period to 2.90 V (%) 0.1 0.55 1 .o 74.0 2.50 + 3.42 80.3 79.3 2.50 + 3.40 84.4 86.7 2.50 + 3.36 87.2 POWER DENSITY The power delivered by a cell is given by the product of the voltage and current during a discharge. Power is dependent on a number of factors related to the nature of the electroactive materials, the packaging of the electrodes and the internal resistance of the cell. In the simplest case, when the resistance of the external load, R,, through which the cell is discharged is equal to the internal resistance, Ri, the power delivered by the cell is at its maximum. The maximum power, PmaX, was measured in three ways by one or the other of the following relationships:w P h, rn TABLE 3.-TYPICAL MAXIMUM POWER DENSITIES OF [CH(ClO,),],lLiClO,lLi CELLS M M r CI el 0 0 weight 56 is 28f5 eqn (4) ; 1 4.2 f 0.1 4.9 & 0.2 2.1 3.52 22.4 22.5 f 0.2 1.8 f 0.2 - 2 4.2 4.9 2.1 3.54 21.3 22.0 86f 1 33+2 eqn ( 5 ) 4 37f7 eqn (4) g 3 7.0 8.2 2.0 3.52 13.0 11.0 1.83 - 4 7.0 8.4 2.5 3.55 11.0 10.5 1.80 36+7 eqn (4) 5 12.0 17.4 & 0.2 5.5 3.66 8.0 10.0 2.08 f 0.04 208 f 5 252 l b eqn (3) g of electro- maximuma weight of active oxidation ~(o.c., 15 h) power density, experiment (CH),/mg material/mg (%) /v R i m RlIQ V m P Im/mA Pm,,/kW kg-l method m - - 4 a The calculated error in the P,,, values arises from the uncertainties in measuring the mass of the (CH),, the discharge current and the discharge voltage, and in adjusting the variable resistor.The errors associated with each measurement are given for experiment 1. They are of similar magnitude m in the other experiments. The small error in experiment 5 reflects the greater accuracy in reading the discharge voltage, which fell less rapidly due to the greater extent of oxidation of the (CH),. r MKANETO, MAXFIELD, NAIRNS, MACDIARMID A N D HEEGER 3427 where V, and I , are the voltage and current, respectively, measured at the very beginning of a discharge cycle under matched load conditions. The maximum power density was calculated in kW kg-l. The internal resistance of the cell was first measured by charging the cell as described above at a rate of ca. 2% oxidation of the (CH), per hour. Charging was terminated at a level of oxidation required for a given experiment listed in table 3.After standing for ca. 5 h to permit partial equilibration of (C10,)- ions within the fibrils, V(o.c., 5 h) was measured and the cell was discharged for ca. 1 s through an ammeter and coulometer in series whose total resistance, R,, (ca. 2 a), had been determined previously by an ohmmeter. The discharge current, I, was measured and was used to calculate Ri, values of which are listed in table 3, by means of the relationship: The coulombs lost in this measurement were also recorded. The cell was recharged using the same number of coulombs lost in the above operation. To measure P,,,, the cell was permitted to stand for ca. 15 h and V(o.c., 15 h) was recorded. It was then discharged through one of the three circuits shown in fig.4 in which the total external load, R,, was made equal to Ri. The adjustable external load includes the resistance of any other instruments such as coulometers etc. V(O.C., 5 h) = (Ri + &,)I. ( 6 ) R I 1 3 R Q R P which might have been used in a circuit in certain instances. The circuits are based on the use of eqn (3), (4) and (5), respectively. Values of V, were measured ca. 2 s after a discharge was commenced. During the initial 2 s period, the voltage fell very rapidly from the V(0.c.) value and then decreased more slowly. Values of I, were taken to be the peak discharge current which rises from zero to a maximum within 1-2 s then declines steadily. The results of five typical experiments using different cells containing differing amounts of (CH), doped to different extents are given in table 3.With the exception of experiment 5, it can be seen that V, z V(o.c., 15 h)/2, indicating that maximum power discharge conditions are operative. In experiment 5 , the Ri and R, values are not exactly matched and hence V , is greater than V(o.c., 15 h)/2. In this case, the product of I , and R,, 2.08 V, is identical to the experimentally determined value of VIn. The results of a number of experiments show that the maximum power density values at oxidation levels > 2% do not vary greatly either with the percentage3428 ELECTROCHEMISTRY OF POLYACETYLENE oxidation or with the total mass of film employed. Maximum power densities in the range of 30 kW kg-l are obtained (see table 3). These are based on the weight of [CH(ClO,),], employed and the weight of lithium which would be oxidized in the discharge reaction if it went to completion. Application of the empirical conversion factor of seven discussed previously gives maximum power densities of ca.4-5 kW kg-l. This may be compared with that of ca. 0.2 kW kg-l for a typical lead/acid automobile battery . The extremely high maximum power density values are probably related primarily to the fact that the effective surface area of ca. 4 mg of (CH), film (1 cm2 x 0.01 cm) is ca. 2.5 x lo3 cm2. Since the fibrils composing the film are only ca. 200 A in diameter, no portion of the (CH), can ever be more than ca. 100 from the electrolyte. This, coupled with the large surface area of the film, permits good accessibility of the (CH), to the (ClO,)- ions with consequent extremely high current densities during discharge (and charge) processes.The high power density is also related to the fact that because of the high electronic conductivity of partly oxidized (CH),, the removal of positive charge can readily occur and is limited primarily by the rate at which the C10, ion can diffuse from the interior to the exterior of a fibril during discharge. Any partial removal of positive charge (i.e. addition of electronic charge) from the interior of a [CHY+(ClO,)-], fibril will greatly increase the electrostatic repulsion between the remaining (ClO,)- ions and will hence increase their rate of diffusion to the fibril exterior. Recent preliminary studies4 show that the diffusion constant for the diffusion of charge into (CH), fibrils depends on the potential applied to the (CH), electrode.This may, in part, be the origin of the higher coulombic efficiencies at higher rates of discharge discussed above. In general, thinner films result in larger power densities. This is probably related to the observation that in thin (CH), films, formed during shorter polymerization times, the fibril diameter is smaller than that found for fibrils in films produced during longer polymerization times.' The larger surface area per unit weight of the thin fibrils contributes greatly to the production of larger discharge currents and correspondingly larger power densities. Since the power delivered at any given time during discharge is the product of the discharge voltage and discharge current, the power against time relationships for constant discharge currents of 0.1,0.55 and 1 .O mA may be obtained simply from the data given in fig. 3. The power against time curves obviously have exactly the same shape as the voltage against time curves. The average power densities are 70, 354 and 591 W kg-l for the 0.1, 0.55 and 1.0 mA discharges, respectively. The present results suggest that electrochemical studies not only of (CH), but also of other conducting organic polymers represent an extensive area for further research, not only of fundamental scientific interest but of possible potential technological value. We thank Dr Shahab Etemad for suggesting the logarithmic relationship between the potential of partly oxidized (CH), and its degree of oxidation and Mr James H. Kaufman for many helpful discussions. This study was supported by the U.S. Department of Energy, Contract no. DE-AC02-8 1 -ERlOS32. A. G. MacDiarmid and A. J. Heeger, Synth. Met., 1979/80, 1, 101 ; A. J. Heeger and A. G. MacDiarmid, in The Physics and Chemistry of Low Dimensional Solids, ed. L. Alcacer (D. Reidel, Dordrecht, Holland, 1979), p. 353. P. J. Nigrey, A. G. MacDiarmid and A. J. Heeger, J . Chem. Soc., Chem. Commun., 1979, 594; P . J. Nigrey, D. Macinnes Jr, D. P. Nairns, A. G. MacDiarmid and A. J . Heeger, in Conductive Polymers, ed. R. B. Seymour (Plenum Press, New York, 1981), p. 227.KANETO, MAXFIELD, N A I R N S , MACDIARMID A N D HEEGER 3429 P. J. Nigrey, D. MacInnes Jr, D. P. Nairns, A. G. MacDiarmid and A. J. Heeger, J. Electrochem. Soc,, 1981, 128, 1651. K. Kaneto, A. G. MacDiarmid and A. J. Heeger, to be published. A. Feldblum, J. H. Kaufman, S. Etemad, A. J. Heeger, T-C. Chung and A. G. MacDiarmid, Phys. Rev. B, 1982, in press. W. Jost, Dzflusion (Academic Press, New York, 1960), p. 45. ' M. Aldissi, Ph.D. Thesis (Universite des Sciences et Techniques du Languedoc, Montpellier, 1981), pp. 48-60; M. Aldissi and F. Schue (U.S.T.L., Place E. Bataillon, 34060 Montpellier Cedex, France), unpublished observations. (PAPER 2/558)