J . Chem. SOC., Faraday Trans. 1, 1986,82, 2385-2400 Polyaniline, a Novel Conducting Polymer Morphology and Chemistry of its Oxidation and Reduction in Aqueous Electrolytes Wu-Song Huang, Brian D. Humphrey and Alan G. MacDiarmid" Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19 104, U . S. A . The emeraldine salt forrn of polyaniline, conducting in the metallic regime, can be synthesized electrochemically as a film exhibiting a well defined fibrillar morphology closely resembling that of polyacetylene. Cyclic volt- ammograms of chemically synthesized and electrochemically synthesized polyaniline are essentially identical. Probable chemical changes which occur and the compounds which are formed when chemically synthesized poly- aniline is electrochemically oxidized and reduced between - 0.2 and 1 .O V us.SCE in aqueous HCl solutions at pH values ranging from -2.12 (6.0 mol dm-3) to 4.0 have been deduced from cyclic voltametric studies. These are shown to be consistent with previous chemical and conductivity studies of emeraldine base and emeraldine salt forms of polyaniline. It is proposed that the emeraldine salt form of polyaniline has a symmetrical conjugated structure having extensive charge delocalization resulting from a new type of doping of an organic polymer-salt formation rather than oxidation which occurs in the p-doping of all other conducting polymer systems. The emeraldine salt form of polyaniline, synthesized by the chemical or electrochemical oxidative polymerization of aniline in aqueous acid sol~tionl-~ is currently arousing considerable interest since it exhibits conductivity in the metallic regime (ca.5 W1 cm-l).l-14 [For a full description of the nomenclature used in describing the various forms of polyaniline see ref. (14).] Chemical synthesis results in a powder which can be compressed into pellets. Electrochemical synthesis gives cohesive films which have been reported to show a fairly smooth featureless topography by scanning electron micr0scopy.~9 The emeraldine salt is believed to have the composition + + {[-(C6H,)-N(H)-(C6H4)-N(H)* (C6H,)-N(H)=(C6H,)=N(H)~}~ A- A- where A- is an anion. It may also be synthesized by 'doping' the emeraldine base forrn of polyaniline [-(C6H4)-N(H>-(C6H4)-N(H) (C6H4)-N=(C6H4)=N-1 1, which consists of equal numbers of reduced [-(C6H4)-N(H)-(C6H4)-N(H)-], (IA), and oxidized [-(c6H4)-N=(c6H,)=N-], (2A), repeat units with aqueous 1.0 mol dm-3 HCl.149 l5 This results in an increase in conductivity of ca.lolo. The aqueous electrochemistry of polyaniline films polymerized on electrodes has been studied previously.6*9-13 A variety of effects have been observed depending on the conditions used in preparing the films. Cyclic voltammograms of films prepared by potential cycling between -0.2 and 0.8 V us. SCE or by potentiostatic oxidative polymerization at 0.9 V us. SCE exhibit redox processes associated with degradation products of the polymer.6 On the other hand cyclic voltammograms of films prepared 23852386 Electroactivity of Polyaniline at lower oxidation potentials, e.g.(0.65 V us. SCE) only exhibit degradation products when scanned to higher potentials ( > 0.7 V us. SCE).6, l2 The oxidation potential of the first redox process (I$ = 0.13 V us. SCE in 1 .O mol dm-3 HCl) does not change in the presence of different cations and varies little in the presence of different anions, but does change with the pH of the electr01yte.l~ In the range pH 1 to -2 the first redox peak shifts 59 mV per pH unit to higher potential as the pH is decreased; however, the first redox peak does not shift between pH 1 and pH 4. No study of the effect of pH on the second redox process occurring at a higher potential has been reported.6* 12* l3 The electroactivity of the film ceases at pH >4.13 No detailed interpretation of these observations in terms of the chemical transformations which occur during oxidation and reduction has been given.The present investigation was carried out for the purpose of (i) elucidating the chemical reactions which occur and the materials which are formed during the electrochemical oxidation and reduction of polyaniline in aqueous solutions of varying pH values and (ii) relating these observations to a proposed structure for the highly conducting emeraldine salt form of polyaniline. Experimental Reagents Reagent-grade C,H,NH,, NH,OH and concentrated HCl were purchased from MCB Manufacturing Chemists, Inc. A.C.S.-certified NaCl and (NH,),S,O, were purchased from Fisher Scientific Co. Perchloric acid (70 % ) was purchased from Baker Chemical Co. and HBF, (48%) was purchased from Aldrich Chemical Co.The C,H,NH, was further purified by distillation. All other chemicals were used as purchased. Chemical Synthesis of Polyaniline The method149 l5 employed for synthesizing the polyaniline used in this study was based, in part, on previously described procedure^.^? An aqueous solution of (NH,),S,O, was added slowly to a solution of aniline dissolved in 1 .O mol dmP3 aqueous HC1 at ca. 5 "C. After 1 h the precipitate which had formed was removed by filtration, washed repeatedly with 1 .O mol dm-3 HCl and dried under dynamic vacuum for ca. 48 h. The material thus obtained was identified as emeraldine hydrochloride as described below. The emeraldine hydrochloride was converted into the emeraldine base by stirring with a ca.0.1 mol dm-3 solution of NH,OH for several hours. The material was dried under dynamic vacuum for ca. 48 h. If it was considered necessary to remove the lower-molecular-weight species (ca. 20% by weight) the emeraldine base was extracted with CH,CN until the extract was colourless. Complete elemental analyses1, of both the emeraldine base and its hydrochloride salt were consistent with the compositions proposed above.15 The sum of the individual percentage compositions by weight of various samples fell in the range 99.57-100.88 % , 1 4 9 l5 Elemental analyses are not a reliable criteria for unequivocally establishing the presence or absence of hydrogen atoms in the emeraldine base or in the emeraldine salt. However, the infrared spectra of both types of materials showed the presence of the expected types of N-H stretching ~ibrati0ns.l~ More important still, the amount of charge required to reduce electrochemically an analytically pure sample of chemically synthesized emeraldine base to the colourless leucoemeraldine base, [-(C,H,)-N(H)-(C,H,)-N(H)-],,, in aqueous acid solution (ca.pH 4) was consis- tent with the proposed composition of the emeraldine base.17W-S. Huang, B. D. Humphrey and A . G. MacDiarmid 2387 Electrochemical Synthesis of Polyaniline Polyaniline was synthesized electrochemically in the form of a high-quality smooth cohesive film on a platinum-foil electrode (ca. 1 cm2) by cycling (ca. 45 times) the platinum electrode between -0.20 and +0.75 V vs. SCE at a rate of 50 mV s-l in a solution consisting of 1 cm3 of aniline and 20 cm3 of ca.1 rnol dm-3 HCl. This procedure produced a ca. 0.2 pm thick film and took ca. 30 min. The polymerization was completed at a potential of ca. 0.4 V, characteristic of the emeraldine oxidation state of polyaniline since in this form the polyaniline is not oxidized by air. The film was then washed in 1.0 mol dm-3 HCl for ca. 2 min to free it from traces of aniline. Cyclic Voltammetry Studies Cyclic voltammetry studies of chemically synthesized polyaniline were performed by one or the other of two methods. (1) The polymer was ground to a fine powder and ca. 1 mg was impregnated into glass filter paper (ca. 1 cm2) either by a spatula or by a finger (rubber glove required). The filter paper was then shaken to remove an excess of polyaniline powder and was then placed on platinum mesh which was folded so as to encase the filter paper on both sides.(2) Finely ground polyaniline powder was suspended in acetone or chloroform and a few drops of the suspension were poured on to a platinum foil (ca. 1 cm2) electrode and allowed to dry in the air. Both methods yielded identical cyclic voltammograms. Cyclic voltammetric studies of chemically prepared polyaniline requires a precondi- tioning in order to obtain reproducible cyclic voltammograms. This may involve complete permeation of the electrolyte into the polymer powder particles. All results reported below were obtained with polymer powder/Pt electrodes after they had been cycled 10-20 times in 1 .O mol dm-3 HCl between - 0.20 and 0.40 V us.SCE. During this preconditioning the intensity of the anodic peak at 0.21 V increased and then became constant. For HCl concentrations < 1 mol dm-3, NaCl solution was added to maintain a C1- concentration of ca. 1.0 mol dm-3. All cyclic voltammetry studies were performed using a PAR model 173 potentiostat in conjunction with a PAR model 175 universal programmer. A standard three-electrode configuration involving a saturated calomel electrode (SCE) was used. The pH of the electrolyte was varied by changing the HC1 concentrations. Morphology Studies A smooth green film of emeraldine hydrofluoroborate was electrochemically deposited for ca. 16 h on a platinum anode at ca. 0.7 V vs. SCE in an electrolyte consisting of 5 cm3 of 48% HBF, and 1 cm3 of aniline in 10 cm3 water as described previously.8 Another film was electrochemically deposited on indium oxide conducting glass (Practical Products Co.) at a constant applied current of 50 pA cm-2 for 90 min in an electrolyte consisting of 4 cm3 of HClO, (70%) and 2 cm3 of aniline in 20 cm3 of water.The films were coated with ca. 100 A of gold and were examined with a Philips PSEM 500 scanning electron microscope. Results Cyclic voltammograms of chemically and electrochemically synthesized polyaniline were recorded in aqueous HC1 solutions of different concentrations at a sweep rate of 50 mV s-l. Both the emeraldine base and its hydrochloride salt are insoluble in all the aqueous electrolytes employed in the study. The ‘pH values’ given for solutions more acidic than pH 1.0 are actually Hammett acidity functions, which reflect the proton2388 Electroactivity of Polyaniline mA J.V I 1 I I I I I -0.2 0.0 0.2 0 . 4 0.6 0.8 1.0 EIV us. SCE Fig. 1. (a) Cyclic voltammogram (50 mV s-l) of chemically synthesized emeraldine hydrochloride in 1 .O mol dm-3 aqueous HCl. (b) Cyclic voltammogram (50 mV s-l) of electrochemically synthe- sized emeraldine hydrochloride in 1 .O mol dmP3 aqueous HCl. donating ability of highly acidic solutions more accurately than pH.l8, l9 In dilute solutions the Hammett function and pH are identical. A cyclic voltammogram (Pt mesh method) of chemically synthesized emeraldine hydrochloride in a 1 .O mol dm-3 aqueous HC1 electrolyte (pH - 0.2) is given in fig. 1 (a). This, as expected, is identical to a cyclic voltammogram of chemically synthesized emeraldine base, since the base is converted into the hydrochloride salt when placed in the 1.0 mol dm-3 HCl.*? l4 The cyclic voltammogram is essentially identical to the cyclic voltammogram of electrochemically synthesized polyaniline given in fig.1 (b), except that it has a small peak at +0.85 V us. SCE which is absent in the cyclic voltammogram of the chemically synthesized polymer. The colour changes observed during the oxidation and reduction process were identical for both the chemically and electrochemically synthesized materials. Cyclic voltammograms between -0.20 and 1.0 V us. SCE of the above chemically synthesized polyaniline using the same polyaniline electrode were recorded in a number ofdifferent electrolytes having pH values in the range - 0.2 to 4.0.Typical voltammograms are given in fig. 2. The potential of the first anodic peak (and the corresponding cathodic peak) is almost independent of the pH of the electrolyte at pH 1 and 2 and changes only slightly at a pH - 0.2 (1 .O mol dm-3 HC1). The potentials of the second anodic peak (and the corresponding cathodic peak) are strongly dependent on pH in the range of -0.2W-S. Huang, B. D. Humphrey and A . G . MacDiarmid 2389 I 1 I 1 I I 1 I I -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 EIV vs. SCE Fig. 2. Cyclic voltammograms of chemically synthesized emeraldine hydrochloride in electrolytes of (a) pH 2.0, (b) pH 1.0 and (c) pH -0.2 (1.0 mol dm-3 HCl). 0.8 L 0.0 1 .o 2.0 3.0 4.0 PH Fig. 3. Relationship between E; of the second redox process between pH -0.20 (1.0 mol dm-3 HC1) and pH 4.0.Slope = - 120 mV per pH unit. to 4.0. The relationship between Ej of this second redox process and pH in this range is given in fig. 3. On cycling between -0.2 and 1.0 V us. SCE in the pH range -0.2 to 4.0 degradation is observable after a few cycles, especially in the more acidic electrolytes. Note that no significant degradation is observable after 5 x lo3 cycles between - 0.2 and 0.6 V at ca. pH -0.2.6 Cyclic voltammograms of chemically synthesized polyaniline (Pt mesh method) were also recorded at pH values of -2.12 (6.0 mol dm-3 HCl), - 1.05 (3.0 mol dm-3 HC1) and -0.20 (1.0 mol dm-3 HCl) between -0.20 and 0.50 V us. SCE. They are given in2390 Electroactivity of Polyaniline 1 -0.2 0.0 0.2 0.L 0.6 EIV vs. SCE Fig. 4. Cyclic voltammograms (50 mV s-l) of the first redox process of chemically synthesized emeraldine hydrochloride in electrolytes of (a) pH -2.12 (6.0 mol dm-3 HCl), (b) pH - 1.05 (3.0 mol dmP3 HCl) and (c) pH - 0.2 (1 .O mol dm-3 HCl). 0 0 -2.0 -1.0 0.0 1.0 2.0 PH Fig. 5. Relationship between Ei for the first redox process between pH -2.12 (6.0 mol dm-3 HCl) and pH 2.0. Slope = -58 mV per pH unit.W-S. Huang, B. D. Humphrey and A . G. MacDiarmid 239 1 Fig. 6. Proposed resonance forms for the emeraldine salt form of polyaniline consisting of equal numbers of reduced, f(C,H,)-N(H)-(C,H,)-N(H)+, and oxidized and protonated, f(C,H,)-N=(C,H,)=N+, repeat units. H H + + fig. 4. Continuous cycling in 1 .O and 6.0 mol dmP3 HCl electrolytes in this voltage range gives no observable degradation.The relationship between the potential and electrolyte pH between pH -2.12 and 2.0 is given in fig. 5. However, oxidation to potentials > ca. 0.7 V in these electrolytes causes significant irreversible degradation of the polymer, especially when the acid strength is > ca. 1 mol dmP3. Degradation was so rapid that it was not possible to determine accurately the relationship between potential and pH in solutions much more concentrated than ca. 1 mol dm-3 (pH -0.2). Discussion The extremely large increase of electronic conductivity brought about by treating the emeraldine base form of polyaniline with aqueous acid involves a new type of doping of a conducting polymer. It occurs by proton addition to the polymer rather than by partial oxidation of the polymer 7~ system, as is the case in the p-doping of other conducting The formation of a nitrogen base salt rather than a potentially highly reactive carbonium ion is believed to be responsible for the high chemical stability of the material in the environment.Unlike all other conducting polymers, the conductivity of polyaniline depends on two variables instead of one, viz. the degree of oxidation of2392 Electroactiuity of Polyaniline the polyaniline and the degree of protonation of the material. The proton addition results in partial depopulation of the n system. It is proposed that the emeraldine salt form of polyaniline shows high conductivity because of extensive 7c conjugation in the polymer chain as shown by the four identical resonance forms in fig.6.14 If this should be the case then all nitrogen atoms, all C-N bonds and all C,H, rings would be identical. Each nitrogen atom would bear a +0.5 charge, all nitrogen atoms would bc intermediate between a single and double bond and all C,H, rings would be intermediate between benzenoid and quinoid, viz. The resulting highly conjugated n system, in addition to contributing to the high conductivity, would also be expected to impart extra stability to this form of polyaniline. It would also be expected to increase the strength of the emeraldine base to a value greater than that found in amines containing phenyl-nitrogen bonds. Such amines are known to be very weak bases [pK, of (C,H,),NHl = 1.0].21 Since amine nitrogen atoms are stronger bases than imine nitrogen atoms it might be expected that treatment of emeraldine base with HCl would lead to preferential protonation of the amine nitrogen atoms to give a material such as + + {€(C6H,)-N(H)2-(C,H4)-N(H)2 (c6H,>-N=(c6H,)=N~),.C1- C1- It is proposed here that when there are equal numbers of reduced, f (C,H,)-N(H)-(C,H,)-N( H) t, and oxidized, +( C,H,)-N=( C,H 4)=N +, repeat units in the polymer that protonation of the imine nitrogen is preferred, resulting in the formation of the completely symmetrical, resonance stabilized polymer having one hydrogen atom and an identical positive charge on each nitrogen atom. This is consistent with the well known properties of guanidine,, H N II which, except for quaternary ammonium hydroxides, is the strongest organic base known (pK, of [C(NH,),]+ = 13.6). The high basicity is believed to result from the large resonance stabilization (resulting from preferential protonation of the imine nitrogen atom) of the quanidinium ion relative to free guanidine.A large resonance stabilization is to be expected because the contributing resonance structures t C C C H,N/ ‘NH, H,N/ ‘NH, H,N/ N N h 2 + are identical.J . Chem. SOC., Furuduy Trans. I , Vol. 82, part 8 Plute 1 Plate 1. Scanning electron micrograph of a smooth emeraldine hydrofluoroborate film grown electrochemically on a platinum anode at a constant potential of ca. 0.7 V us. SCE for 16 h. W-S. Huang, B. D. Humphrey and A. G. MacDiarmid (Fucing p . 2392)J. Chem. SOC., Faraday Trans. 1, Vol.82, part 8 Plate 2 Plate 2. Scanning electron micrograph of an emeraldine hydroperchlorate film grown electro- chemically on indium oxide conducting glass at a constant current of 50 pA cmP2 for 90 min. W-S. Huang, B. D. Humphrey and A. G. MacDiarmidW-S. Huang, B. D. Humphrey and A . G. MacDiarmid I 1 I 2393 1 1 T u U 0 C a .- d e c r e a s i n g acidity peak position m A o l e t 1 Fig. 7. Cyclic voltammogram (50 mV s-l) of chemically synthesized emeraldine hydrochloride powder in 1 .O mol dm-3 HC1 (pH - 0.20) with schematic representations of the change in potential of the peaks as a function of the pH of the electrolyte. Approximate colours of the polyaniline observed at different stages of oxidation are also included. A recent investigation’ has shown that minimum resistivity of polyaniline is attained when it is electrochemically converted in aqueous solution into an oxidation state approximately midway between the completely oxidized and the completely reduced forms.This is consistent with the formulation of the emeraldine salt given above. However, no studies have been reported which indicate the nature of this ‘half-oxidized’ form, the nature of the polyaniline species which are formed or the reactions which are involved when this half-oxidized form is oxidized or reduced. It has also been shown23 that polyaniline can be used as an anode or cathode in rechargeable batteries in aqueous electrolytes. The chemical changes which the polyaniline undergoes during the charge and discharge processes have not, however, been formulated.Morphology A scanning electron micrograph (plate 1) of a smooth emeraldine hydrofluoroborate film grown electrochemically on a platinum anode at a constant potential of ca. 0.7 V us. SCE for ca. 16 h showed a relatively even compact microspheroid surface morphology. This may be compared with the scanning electron micrograph (plate 2) of an emeraldine hydroperchlorate film grown electrochemically on indium oxide conducting glass at a constant current of 50 pA cm-2 for 90 min. It is interesting to observe clearly defined fibrils on a compact microspheroid underlayer of polymer. The fibrillar morphology of the polyaniline resembles that of (CH),23 very closely, but the fibril diameter (ca. 2000 A) is greater than that found in (CH), (ca. 200 A).The fibrillar morphology reported for p~lyparaphenylene~~ and poly(3-methylthi0phene)~~ is not as similar to that of (CH), as is that here observed for the emeraldine hydroperchlorate. Note that no fibrillar morphology has been reported for polypyrrole26 and p~lythiophene.~~. 27 It is not yet known if the difference in morphology of the two emeraldine salt samples is due to the2394 Electroactivity of Polyaniline different anions employed, the nature of the electrode or the difference in the electrochemical procedures used. The dependence of the morphology upon these variables is being investigated. Electrochemical Studies A cyclic voltammogram of chemically synthesized emeraldine hydrochloride powder in 1.0 mol dm-3 HC1 (pH -0.20) together with the approximate colours at various potentials, which are identical to those observed with the electrochemically polymerized films, is given in fig.7. The approximate composition corresponding to a given colour together with the classical names28 are listed below: leucoemeraldine base (1A repeat units only; fully reduced material) [-(C,H,)-N(H)-(C,H,)-N(H)-I,, (Pale Yellow) (protonated form : pale yellow) protoemeraldine base (1A and 2A repeat units) (protonated form: light green) emeraldine base (1A and 2A repeat units) [[-(C,H,)-N(H)-(C,H,)-N(H) $& (c,H,)-N=(c,H,)=N-],], (dark blue) (protonated form : green) nigraniline base (1A and 2A repeat units) [[-(C,H,)-N(H)-(C&,)-N(H) +E-(C6H4)-N=( C,H,)=N-],], (blue-black) (protonated form: blue) pernigraniline base (2A units only ; fully oxidized material) [-(c,H4)-N=(c,H4)=N-]4x (violet) (protonated form: violet).The above colours are those observed with thin films or layers of the material on a highly reflective metal surface such as Pt. They are presumably dominated largely by the absorption spectra of the material associated with light reflected from the underlying metal surface. Bulk powders of the polymers are various shades of black in their oxidized forms. There is actually a continuum of oxidation states ranging all the way from the completely reduced leucoemeraldine to the completely oxidized pernigraniline forms. The intermediate oxidation states given above are those which have been reported as being synthesized by chemical oxidizing or reducing agents having characteristic standard reduction potentials.28 Since the degree of protonation is not known for all these species in acids of varying strengths only the composition of the free unprotonated base is listed.All species can, however, be expected to be protonated to some extent depending on the pH of the solution and the extent of oxidation of the polymer, the extent of protonation decreasing with increasing extent of oxidation. For the completely oxidized pernigraniline it is highly probable that there is very little or essentially no protonation in solutions having pH > 1. For a given oxidation state, protonation will increase with increasing acidity of the electrolyte. The smallest number of -(C6H4)-N(H)- and/or =(C6H4)=N- repeat units which can be used which will permit interconversion between the above five compositions is eight.The formulae given above for the three partly oxidized forms of polyaniline basesW-S. Huang, B. D. Humphrey and A . G . MacDiarmid 2395 do not necessarily represent the relative arrangements of 1A and 2A repeat units14 in a given polymer chain; indeed, it might be expected that different repeat units will be distributed uniformly throughout a polymer chain. It is clearly apparent that the protoemeraldine, emeraldine and nigraniline bases intermediate between the fully reduced leucoemeraldine base and the fully oxidized pernigraniline base represent only a small number of the possible combinations of 1A and 2A repeat units which could be used to represent polymers having increasing degrees of oxidation ranging all the way from the fully reduced leucoemeraldine to the fully oxidized pernigraniline polymers.For the reaction the Nernst equation is A -+ An+ +ne- (1) Eqn (2) shows that the reduction potential for the reaction is independent of pH. However, for the reaction the Nernst equation is AH, +A+nH++ne- (3) RT [A][H+], RT [A] RT n F [AH,] n F 0.059 Ered = E;ed +- In n F [AH,] = ged+- In -+- ln[H+]n [*I +0.059 log [H+]. = g e d + y l o g p n [AH,] (4) It shows that the reduction potential is clearly dependent on pH. If the pH is varied in a system consisting of a given fixed ratio of a compound, AH, and A, then a plot of E us. pH will give a straight line of slope 0.059 V per pH unit if the numbers of protons and electrons involved in the oxidation reaction are equal.However, in a reaction such as AH:-+A+2H++e- (7) where the number of protons liberated is not equal to the number of electrons, e.g. where the number of protons liberated is twice the number of electrons, an analogous plot would have a slope of (2 x 0.059) = 0.1 18 V per pH unit. These simple concepts can be used conveniently to determine the chemical reactions which occur during the electrochemical oxidation and reduction of polyaniline. It will be shown that the oxidation processes occurring for peaks 1 and 2 (fig. 7) in the cyclic voltammetry studies of polyaniline represent oxidation processes which differ in the number of protons which are lost per electron transferred. Oxidation Reactions associated with Peak 1 in the Cyclic Voltammetric Studies Oxidation Reactions between ca.pH I and 4 (Peak 1) In this pH range the potential of the initial oxidation reaction which occurs (first portion of peak 1, fig. 2 and 7) is found experimentally to be essentially independent of pH ; hence no protons are involved in the oxidation reaction. It is believed to involve the con- version of pale yellow leucoemeraldine base to the light green protonated form of pro toemeraldine :2396 Electroactivity of PolyaniIine The second oxidation reaction occurs during the second half of peak 1 to give the dark green protonated form of emeraldine : + + €(C6H,)-N(H)-(C6H4)-N(H)~ (C6H4)-N(H)=(C6H4)= N(H)+x + 2x c1- 1 c1- c1- 1 L L The electrochemical reduction reactions are the reverse of those given above. These reactions occur during the time taken for a single cyclic voltammetric scan (ca.24 s for the oxidation step). This study does not indicate whether the protonated units15 would spontaneously undergo deprotonation with the loss of HCI and be converted to f- (c6H,)-N=(c,H4)=r;(H)~ (2s’) or [-(C6H4)-N=(C,H4)=N-] (2A) c1- units if the polymer were permitted to stand in the electrolyte for an extended period in the absence of an applied potential. However, our previous investigationsA* l4 based on elemental analyses of material dried under dynamic vacuum for ca. 48 h show clearly that equilibration of emeraldine base with aqueous HC1 for 55 h results in protonation of ca. 25% of the nitrogen atoms at a pH of 1 and ca. 50% of the nitrogen atoms at a pH of - 0.2 (1 .O mol dm-3).At pH values > 4 there is essentially no protonation. Hence spontaneous deprotonation cannot be extensive in the more acidic solutions, even on standing for an extended period in the absence of an applied potential. Note that the highly conducting form of polyaniline, i.e. the emeraldine salt, can be formed in two different ways as depicted below: f N(H)-(C6H4>-N(H>-(C6H4)-N(H)-(c6H4)-N(H)-(c6H*) 3 2 2 leucoemeraldine base (insulator) oxidation (no protonation or deprotonation) I-42 e - T x A-I (10) + f- N(H)-(C6H4)-N(H)=(C6H4)=N(H)-(c6H4)-N(H)-(c6H4) 3 2 2 A- A- emeraldine salt (conductivity in metallic regime) protonation (no oxidation) T [+4x H+A-] f N(H)-(C6H4)-N=(C6H4)=N-(c6H4)-N(H)-(c6H4) t225 emeraldine base (insulator). In reaction (10) the emeraldine salt is formed by oxidation involving no change in the number of hydrogen atoms attached to nitrogen atoms.In reaction (1 1) it is formed by protonation with no accompanying change in the formal oxidation state of the polymer.W-S. Huang, B. D. Humphrey and A . G. MacDiarmid 2397 Oxidation Reactions between ca. pH -0.2 and -2.12 (Peak 1) In this pH range (1.0-6.0 mol dm-3 HCl) the potential of peak 1 (fig. 4) and also its corresponding reduction peak, l’, move to lower values at a rate of ca. 58 mV per pH unit (see fig. 4, 5 and 7) as the pH is increased, indicating that equal numbers of protons and electrons are lost in the oxidation process. In these more acidic solutions it is believed that the amine nitrogen atoms in the [-(C6H4)-N(H)-(C6H4)-N(H)-] (1A) units in leucoemeraldine will be completely or partly protonated to give + P l - [-(C6H4)-N(H>2-(C6H4>-N(H)-1 ( ”>? or units,I4 No degradation occurred on recycling in this range.Since an iminium nitrogen atom can have only one attached proton, deprotonation must occur when an ammonium nitrogen atom is oxidized to an iminium nitrogen atom, uiz. + + This is then followed by further oxidation and deprotonation during the second part of peak 1, to give a protonated form of emeraldine: + + + + -+ + + +2xH++2xe-. (13) The product of reaction (13) apparently loses HCl from the protonated amine nitrogen atoms on drying under a dynamic vacuum for ca. 48 h, since elemental analysis of the resulting material corresponds to that of the emeraldine hydrochloride given by product of reaction (9).The observation that the polyaniline is oxidized more easily in less acidic solutions, i.e. Ered becomes less positive as the pH increases, is consistent with the above reactions. The reactions associated with the reduction process (peak 1’) are the reverse of the above. Oxidation Reactions between ca. pH -0.2 and 1 (Peak 1) As described above, the potential of peak 1 is essentially independent of pH in the pH range 1-4 and decreases at the rate of ca. 58 mV per pH unit with increasing pH in the pH range ca. -0.2 (1.0 mol dm-3 HCl) to ca. -2.12 (6.0 mol dm-3 HCl). In the intermediate pH range, - 0.2 to 1, where reactions (8), (9), (12) and (1 3) are all occurring simultaneously to varying extents, the slope of the curve relating potential to pH changes from 0 mV per pH unit (for pH values 1-4) to 58 mV per pH unit (for pH values -0.2 to - 2.12) as shown in fig.5.2398 Electroactivity of Polyanilitw Note that no irreversible degradation of the redox process associated with peak 1 (see fig. 7) occurred on cycling between - 0.2 and 0.5 V us. SCE at any pH in the range - 2.12 (6.0 mol dm-3) to 4. Oxidation Reactions associated with Peak 2 in the Cyclic Voltammetric Studies Oxidation Reactions between ca. pH -0.2 and 4 (Peak 2) In this pH range the potential of peak 2 (see fig. 2, 3 and 7 ) and also its corresponding rer~uction peak, 2’, move to lower values at a rate of ca. 120 mV per pH unit as the pH is ncreased from ca. -0.2 to 4. This is consistent with a reaction of the type given by reaction (7), where the number of protons involved is twice the number of electrons.The chief reaction associated with the first part of peak 2 is therefore believed to be of the tY Pe + + [-(C6H4)-N(H)-(C6H4)-N(H) (C6H4)-N(H)=(C6H4)=N(H) (C6H4) c1- c1- -N=(C6H4)=N-],, + 4X H+ + 2~ e- + 2x c1- (14) to give a partly protonated nigraniline polymer. The reaction associated with the second part of peak 2 is believed to involve the final oxidation and deprotonation of the product given by reaction (14), viz. [-(C,H4>-N(H)-(C6H4)-N(H) kd (c6H4)-~(H)=(C6H4)=~(H) (C6H4) c1- c1- -N=(C6H4)=N-],, - [-(c6H4)-N=(c6H4)=N-]4z + 4x H t + 2x e- + 2x c1- (1 5 ) to give the fully oxidized pernigraniline. The observation that the polyaniline is oxidized more easily in less acidic solutions, i.e.Ered becoming less positive as the pH increases, is consistent with the above reactions. Some irreversible degradation of peak 2 is observed after several cycles, especially in more acidic solutions. This is believed to be associated with the hydrolysis of the imine nitrogen-carbon bond in -N=(C,H,)= groups, which become more abundant at higher levels of oxidation of the polymer, viz. -N=(c6H4)=+ H 2 0 --+ -NH, + O=(C6H4)=. (16) Oxidation Reactions between pH -0.2 and -2.12 (Peak 2) At pH values below - 0.2 the chief reaction associated with the second peak is probably of the type + + + + [-(C6H4>-N(H)2-(C6H4)-N(H), (C6H4)-N(H)=(C6H4)=N(H)-12~ - c1- c1- c1- c1- [-(C,H,)-k(H)=(C,H,)=N(H)-],, +4xH++4xe- (1 7) c1- c1- since protonation of both amine and imine nitrogen atoms is expected to be more extensive in these highly acidic solutions.On cycling between - 0.2 and 1 .O V in this pH range the intensities of peaks 2 and 2’ decreased rapidly, indicating irreversible decomposition, probably hydrolysis of the more highly oxidized material. Because of theW-S. Huang, B. D. Humphrey and A . G . MacDiarmid 2399 very great irreversible decomposition which occurred, which was significant even at a pH of ca. -0.2 for ca. 20 s it was not possible to obtain an accurate value for the dependence of potential on pH down to pH values of ca. -2.12 (6.0 mol dm-3 HCl). Hence the reactions given by reaction (17) should be regarded as only tentative. Effect of pH on the Electroactivity of Polyaniline If it is assumed that all the base forms of polyaniline, regardless of their extent of oxidation are insulators, then at pH values sufficiently high that no significant protonation can occur, all such materials would be electrochemically inactive.If however, the electrolyte were sufficiently acidic to permit some protonation of a partly oxidized form of polyaniline so that it exhibited some conductivity, then that part of the polymer in contact with the platinum electrode surface could undergo electrochemical redox reactions and would serve as a conducting medium for electron transport to the remainder of the polyaniline. Indeed, recent studies7 have shown that on electrochemically oxidizing polyaniline in 0.5 mol dm-3 NaHSO, solution (ca. pH 1) that the resistance falls to a minimum when the polymer is approximately half oxidized (emeraldine salt form using the present nomenclature) and then rises as oxidation proceeds further with presumably extensive deprotonation.These results are in agreement with those obtained in the present study. For example, the oxidation and reduction process described in the previous section did not occur readily at pH > 4, and it was necessary to increase markedly the current gain in order to obtain a meaningful reading. This is believed to be due to the fact that essentially no protonation of the polymer occurred in these very slightly acidic electrolytes and hence very little conductivity could be imparted to the polymer film. This was substantiated by the observation that at pH 6 for example, the polyaniline was essentially electrochemically inactive, but regained its electrochemical activity in a completely reversible manner when placed in a more acidic electrolyte.Conclusions The present study presents evidence to show that the term ‘polyaniline’ describes a class of compounds composed of species which differ in their degree of oxidation (ratio of imine nitrogen atoms to amine nitrogen atoms). It is proposed that the polymer base f(C,H,)-N(H)-(C,H,)-N(H) (C6H,)-N=(C6H4)=Njb can exist in principle in a continuum of oxidation states ranging from the completely reduced polymer where b = 0 to the completely oxidized polymer where a = 0. Each oxidation state, defined by a given fixed ratio of imine to amine nitrogen atoms, can exist in forms which differ from each other in their extent of protonation which depends on the experimental conditions to which the polymer base has been subjected.The electrochemical oxidation and reduction of polyaniline in aqueous electrolytes of varying pH values shows two classes of redox processes occurring at different potentials which differ from each other by the extent to which the processes are accompanied by deprotonation (during oxidation) or protonation (during reduction). The emeraldine salt of polyaniline can be synthesized in a form having a fibrillar morphology similar to that of polyacetylene. Its conductivity lies in the metallic regime and is qualitatively consistent with a proposed symmetrical conjugated structure having extensive charge delocalization. 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