Anodic overoxidation of poly-p-phenylene in aqueous electrolytes

157

J. Elec~roanol. Chem., 216 (1987) 157-168

Elsevier Sequoia S.A., Lausarme - Primed in The Netherlands

ANmIC OVEROXIDATION ELECTROLYTES

OF POLY-pPHENYLENE

IN AQUEOUS

F. BECK and A. PRUSS University-GH-

Duisbur~

FB 6 -

Elektrochemie,

Lothatwasse

I, D-4100 Duisburg I (F.R.G.)

(Received 22nd April 1986; in revised form 14th July 1986)

ABSTRACT Poly-p-phenylene (PPP), synthesized chemically by the Kovacil! method, was mixed with 7.5% carbon black and used as a pressed thin-layer electrode (100 pm) on Pt mesh. Its overoxidation as an anode in aqueous acid electrolytes (8-18 M HsSO,, 8-11.3 M HCIO,, 8 M HBF,) was investigated systematically. Slow cyclic voltammetty reveals that the reversible redox peak 1, leading to an insertion compound up to 1 F per mole (CsH,),, is followed by a large second peak 2 at more positive potentials, corresponding to about 15 F with respect to the unit mentioned above. Peak separation increases with increasing acid concentration due to a negative shift of the insertion potential (peak 1) by 60 mV mol-’ dm3, but normal Nernst behaviour is observed in the case of peak 2. The strong decay of the current efficiency for reversible cycling of PPP at concentrations below 10 M can be understood readily from these findings. The overoxidation starts with the formation of the polymer radical cation, which is oxidized further to the quinone, then to the product of ring opening and finally to the product of cleavage of the polymer chain. The final product seems to be a derivative of maleic acid. The p-quinone path is preferred over the oquinone path, quite contrary to the case with graphite. The practical consequences with respect to battery application are discussed in detail.

(1) INTRODUCTION

Organic polymers with conjugated double bonds are known to be organic semiconductors [l]. Partial oxidation of the molecular lattice, which is accompanied by the insertion of anions to compensate the space charge in the solid, leads to a great enhancement in electronic conductivity. The oxidation can be performed by chemical or electrochemical means. This process is known to be reuersibfe, e.g. in the cases of polyacetylene [2], poly-p-phenylene [3] or polypyrrole [4] as characteristic examples. Chemical or cathodic reduction of the insertion compound removes the anions from the host lattice, and returns it back to the semiconducting state. Based on this reversible switching between the semiconducting and metallic states, applications of these new materials as battery electrodes, electrocatalysts and electrochemical displays have been proposed.

158

In this connection the phenomenon of irreversible overoxidation of the polymer backbone at more positive potentials is of great interest. In the case of poly-p-phenylene (polybenzene, PPP), whose reversible oxidation in aqueous electrolytes has been investigated by us in detail [5-71, nothing is known with regard to overoxidation. We would like to report on our combined electrochemical and chemical/analytical measurements which yielded a mechanism for the anodic degradation of PPP. As before, comparison is made with the electrochemical behaviour of the related graphite electrode. (2) EXPERIMENTAL

The polymer was prepared according to the standard method described by KovaciE and co-workers [S]. The mole ratio was C,H,:AlCl,:H,O:CuCl, = 2:1:0.11:1 at a temperature of 70°C cf. ref. 5. PPP was obtained as a brown, insoluble and infusible powder. It was thoroughly mixed with carbon black Corax L@ (Degussa Company) * to yield a composite of 92.5% PPP and 7.5% carbon black. A pressed thin-layer electrode (PTL) was constructed by applying this mixture to base electrodes of Pt mesh (400 cm-*) at a pressure of 50 MPa at 20°C. The final thickness of this PPP electrode was about 100 pm with an area of 4-5 cm*. Contact was made with a brass holder outside the electrolyte. For electrochemical measurements, especially very slow (us = 0.02-0.05 mV s-l) cyclic voltammetry, a conventional three-electrode cell was used. The reference electrode was a Hg/Hg,SO_,/l M H,SO, electrode. The potentials against this electrode, which is itself 674 mV more positive than the SHE, are denoted in the following as U,. Electrolytes were made from distilled water and 96% H,SO, (p.a.), 70% HClO, (p.a.) and 50% HBF, (purum), respectively. The temperature was 20°C throughout. Special working electrodes and analytical procedures are described in connection with the results. (3) RESULTS

AND DISCUSSION

(3. I) Slow cyclic voltammetty Figures 1 and 2 show the slow cyclic voltammetric curves for H,SO, and HClO, at three levels of acid concentration. The voltage scan rate was set at a very low value in order to ensure a complete conversion of the organic material within the organic layer. Prior to the curves shown in Figs. 1 and 2, three activation cycles were performed within the potential range of the first reversible peak. The potential, capacity and reversible behaviour of this primary peak correspond completely to the results for the reversible insertion and removal of anions [5].

* Denoted as “carbon black” in the following.

159

Fig. 1. Slow cyclic voltammetry of PTL electrode (m = mass of PPP in the mixture with 7.5% carbon black); (1) 12 M HzS04, m = 40.4 mg, us = 0.05 mV s-l y= 0.13, y2=1.65; (2) 14 M H2S04. m = 52.5 mg, us = 0.02 mV s-l, n = 0.15, yz =1.90; (3) 16 M HzSO.+, m = 33.5 mg, us = 0.05 mV s-l, JJ~= 0.11, y2 =1.60. A = 5 cm2, d = 0.1 mm, base electrode: Pt.

The second peak is much larger. While the insertion potential UT, found for the first peak, depends strongly on the acid concentration, the foot potktial U, or the peak potential UP of the large second peak are almost independent of it. The

0

0.4

0.8

1,2

“*IV

Fig. 2. Slow cyclic voltammetry, as in Fig. 1: (1) 8 M HClO,, m = 67 mg. us = 0.05 mV s-l. y1 = 0.09, y2-1.60; (2) 10 M HCIO,, m=74 mg, us-O.02 mV s-l, y, = 0.14. y2 -1.90; (3) 11.3 M HCIO,, m=68mg, os=0.05mVs-‘, y,= 0.09, y2 = 1.55. A = 4 cm2, d - 0.1 mm, base electrode: Pt.

160 TABLE 1 Reversible oxidation (Yl, U’,) and overoxidation ( Yz, C&F) of poly-p-phenylene in aqueous acids at 20°C. Averaged values for n parallel runs. Electrodes l-9 were of the PTL-type, cf. Sect. 2 No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Electrolyte M M M M M

n

“s /mV s-l

Yl

Y2

Y2/Y1

_AYl,max

H,SO., H,SO, H2S04 H2SOd H2S04

2 2 3 1 1

0.05 0.05 0.02 0.05 0.05

0.13 0.15 0.11 0.1

1.65 1.65 1.90 1.60 1.70

13 13 15 17

10 10 11 10 11

8 M HClO., 10 M HCIO, 11.3 M HClO.,

1 1 2

0.05 0.02 0.05

0.09 0.14 0.09

1.60 1.90 1.55

18 14 17

10 12 10

1

0.05

-

1.4

-

-

8 2 5 8 2

0.05 0.05 0.05 0.05 0.05

0.04 0.04 0.12 0.01 -

1.3 1.4 2-3 0.3 1.2

33 35 17-25 30 -

8 9 12-18 2 7

10 12 14 16 18

8 M HBF., 14 M H2S04 ’ 14MH2SOdb 14MH,SO,= 14 M H2SOdd 14 M H,SO,=

= &

us.1 P

0.39 0.28 0.17 -0.02

0.57 0.50 0.45 0.42 0.45

0.92 0.96 0.90 0.83 0.95

0.55 0.45 0.29

0.53 0.52 0.49

0.95 0.91 0.91

0.63

0.87

0.51 0.54 0.60 0.50 0.50

0.90 0.83 0.85

0.34 0.51 0.45 0.30 0.35?

a Pressed layer (on Ti) with 50% PE binder. b Free-standing thin foil with binder. ’ Very thin (3 pm) layer (dip coating) on Pt sheet. d Dispersion electrode with PPP alone. ’ Dispersion electrode with PPP, precipitated onto carbon black in the course of synthesis.

potentials and surface-specific capacities of both peaks are summarized in Table 1. The latter have been normalized as degree of insertion yr and as degree of overoxidation yz with reference to one monomer unit C,H,. The second peak ends at U, = 1 V, rather distant from the final current rise at U, = 1.6 V, corresponding to oxygen evolution, cf. Fig. 2. The second peak is totally irreversible; no cathodic process can be detected on the reversed scan down to U, = 0 V and the curve coincides with the abscissa. Table 1 shows that the ratio of both peaks, y/y,, lies between 13 and 18. As the y, values never reach the maximal figure of 0.16, as found in ref. 7, but a further activation is to be expected in the course of the drastic oxidation at peak 2, where the primary process should then occur automatically, a second column ~~0.16 is established in addition. These ratios are between 10 and 12. The separation of the peaks is poor at lower acid concentrations, as is shown in the figures. This corresponds to the poor cycling current efficiency ar at 4-8 M acid concentration found previously [5]. Therefore it is no wonder that this poor peak separation is confirmed in 8 M HBF,. No distinct anodic oxidation in the region up to U, = 1.0 V could be detected for electrolytes such as 1 M H,SO,, (cf. Fig. 3) or 31 wt.% H,SiF,, where no reversible insertion could be found previously [5,6].

161

Fig. 3. Slow cyclic voltammetry, as in Fig. 1. Electrolyte: 1 M H2S0,.

Figure 4 compiles the dependence of the potentials on the molar acid concentration of H,SO,. The strong linear potential shift of U, is confiied. On the other hand, U, exhibits only a slight negative trend with increasing acid concentration

10

12

14

16

18

Fig. 4. Insertion potentials, U..J, and foot potentials, f&, in lo-18 M H2S04. ITL electrode as in Fig. 1. averaged line from ref. 5; (x) Q. (- - -) Nemst line. LI,.l. ( -)

(=.o)

162 TABLE 2 Specific conductivity I( of pressed p&lets made from PPP and carbon black (Corax L@‘)at 20°C. Pressure applied during the measurement was 300 bar No.

PPP/carbon

black system

PPP alone PPP precipitated onto 18% carbon black during synthesis PPP premixed with 7.5% carbon black Carbon black only

T&S

cm-’

10-s-10-10 10-6 10-3 10

according to the Nemst equation. The values for 10 M H,SO, clarify that overoxidation sets in only at a potential equal or negative to the insertion potential *. Other electrode designs than the standard setup described in Section 2 have been checked for handling the powdery electrode material. The results are included in Table 1, Nos. 10-14. The addition of 50% polythene binder as a PTL on Ti or as a 1 mm thick free-standing foil (Nos. 10 and 11) led to very low yt values, indicating non-quantitative conversion of the material. However, y2 was nearly of the right order. On the other hand, very thin layers on smooth Pt with PVC binder, made by dipping the electrode into a PVC solution in THF into which appropriate PPP and carbon black were dispersed [7], led to much larger yz values. This may be due to an electrocatalytic activity of the Pt base electrode immediately adjacent to the organic material. Finally, the dispersion electrode design [9] previously used for the electrochemical measurement of dispersed metal chelates [lo], was checked with PPP alone (No. 13). The feeder electrode was a mesh of Au/Pt (90/10). Only poor conversions were realized, even after extended intensive stirring and in the presence of a perfluorinated surfactant. However, improved behaviour was found for dispersions consisting of PPP, precipitated onto 18% carbon black in the course of synthesis. For further experimental designs, cf. ref. 11. As the potentials are strongly distorted in all cases, none of these designs is superior to the PTL electrode used throughout this paper. An interesting result concerning the specific conductivity of pressed pellets with these materials should be mentioned. As Table 2 shows, the specific conductivity K of PPP with 7.5% carbon black is much higher than K for PPP precipitated onto carbon black during synthesis. Transversal chains of “naked” carbon-black particles seem to be realistic in the former case as stated previously [5], but not in the latter, where eoery particle seems to be covered with a PPP-layer. (3.2) Analytical evaluation Analytical evaluation of oxidized samples was undertaken in order to supplement the electrochemical findings. The results of elementary analysis are compiled in

l

With p =103

$I cm, cf. Table 2, the IR drop is 10 mV at the most.

163 TABLE 3 Elementary analysis of samples Sample

Number of samples

Average composition

1 PPP after synthesis 2 PPP after total overoxidation in 14 M H,SO., (y2 =1.9)

6 2

C6H36.

3.800.1

C6H34,

3.6°0.9...10

3 PPP after overoxidation and NaOH extraction

2

C6H3

6OO

0.4

3...0.35

Table 3. Oxygen was determined directly. It is clear from these results that the oxygen content increases from an average of 0.3 per C, to 1.0 per C,. The original value can be restored by NaOH extraction. After anodic overoxidation the PPP/carbon-black mixture adheres only loosely to the Pt mesh electrode and can be removed mechanically. This material, together with the small shedded solids, was collected in a sintered-glass filter crucible and washed free of acid. Direct titration of the dispersed material with 0.1 M NaOH failed. However, digesting 65 mg (corrected as pure PPP) with 20 ml 0.1 M NaOH for 20 h, filtering and rinsing thoroughly, acidifying the filtrate with 25 ml 0.05 M H,SO,, and backtitrating with 0.1 M NaOH (after extended purging with Ar to remove dissolved CO*) leads (averaged over 5 independent runs) to a consumption of 6.00 ml 0.1 M NaOH *. Thus, 100 pmol acid groups are detected for 65 mg PPP after overoxidation by acid/base titration. No additional amounts could be detected as solute in the primary acid electrolyte filtrate, which had a brown colour, or in the remaining solid after NaOH extraction. From these results we calculate to 100 pmol carboxyl groups per 65 mg or 0.85 mm01 PPP units (C,H,) or 0.7 mol carboxyl groups per C,,H,,. As no step could be detected in the titration curve (in the mixture with excess dilute H2S04), carboxylic groups of the benzoic acid type @K= 4.2) can be ruled out in favour of aromatic dicarboxylic acids such as phthalic acid (pK, = 2.9) or maleic acid (derivatives) with a much larger first acidity constant. IR spectroscopy revealed aromatically bound -COOH groups to be present after overoxidation (bands at 1595 and 1670 cm-‘), cf. Fig. 5. The frequency shift to lower wavenumbers is indicative of a dicarboxylic acid. Furthermore, the new band at 1250 cm-’ is due to C-OH vibrations in the carboxylic group. As expected, in all cases aromatic C-H and C-C vibrations are observed. After alkaline extraction of the solid, the intensity of -COOH and C-OH in the non-dissolved residue decreased strongly.

This procedure was originally proposed by Boehm et al. [12-141 for the determination of acid surface groups on carbon black.

l

164

Fig. 5. Infrared spectroscopy of PPP samples (1% in pressed KBr pellets): (1) PPP with 7.5% carbon black; (2) PPP as (I), but after anodic overoxidation in 14 M H2S04; (3) PPP as (2), but after extraction with 0.1 M NaOH.

(3.3) Poly-(2,6-dimethyl-1,4-phenylenoxide),

DMPPO

Additional information on the potentialities of the electrochemical conversion of organic polymer/carbon black mixtures has been collected with the above-men-

i

j

/mAk2

1

Fig. 6. Slow cyclic voltammetry us = 2 mV s-l, first 4 cycles.

of DMPPO in 10 M H2S04.

pellet electrode with 7.5% carbon black,

165

tioned polyether, a product of Chemische Werke Htils (CWI-I) AG. The usual mixtures of the white polymer with 7.5 wt.% carbon black were pressed to mechanically stable pellet electrodes as described previously [5]. Figure 6 shows slow cyclic current-voltage curves in 10 M H2S04. Clearly, after oxidation at potentials positive of U, = 0.4 V, a reversible redox peak develops at U, = 0 V. Contrary to PPP, a C-O heterobond extends along the backbone of the polymer, which is subject to anodic cleavage according to eqn. (1):

(1)

The product, 2,ddimethyl quinone, which is nearly insoluble, leads to the reversible redox peaks. In 1 M H,SG, (pH 0) we found as the redox potential U,:a = - 60 mV, and at pH 7 (phosphate buffer) U, = -450 mV, which is compatible with the normal pH-dependency of - 58 mV/pH for a quinone/hydroquinone equilibrium. The standard potential (PH 0) compares well with the value for 2,5-dimethyl-1,4benzoquinone, U,, = - 80 mV [15,16]. Under aprotic conditions (0.1 it4 NEt,ClO, in CH,CN), both 2,5dimethylquinone and 2,6dimethylquinone exhibit exactly the same standard potential, U&, = - 1090 mV. In this way it seems to be justified to identify the observed Us,-,= - 60 mV with the redox system 2,bdimethylquinone/ 2,6dimethylhydroquinone. A qualitative evaluation reveals that every second C-O bond of the polymer has been cleaved arbitrarily after four cycles. This comparative study shows that the anodic cleavage of aromatic C-O ether bond, studied repeatedly with monomers under aprotic conditions [17], is possible even with polymers in aqueous electrolytes. (3.4) Mechanism

of overoxidation

Our results show clearly that irreversible anodic overoxidation of PPP occurs through the reversible formation of a polymer radical cation salt (an anion insertion compound) at relatively negative potentials. In electrolytes where this primary step is not possible, the further oxidation is highly hindered, cf. Fig. 3. Overoxidation starts at the site of the radical cation. Its further oxidation to the dication generates a very reactive intermediate which is attacked instantanously by water as a nucleophile to yield a phenolic compound. This intermediate is more easily oxidized than the starting material in the o- or p-position producing a benzcatequine- or hydroquinone-type compound, which is immediately further oxidized to an o- or p-benzoquinonetype compound. These two routes are presented in Schemes 1 and 2. However, oxidation goes beyond the quinone state, finally leading to the cleavage of C-C bonds, the liberation of CO, and the formation of aromatic carboxylic acids. Our experimental findings are in good

-2 co, -2eS-2H’

-w

-2 e;-&H’

I H,O >

-C

-86,-8H’

-C

2 Ii*0

l-

9

+Q

O+-

‘coo* + 4Oc

Scheme 1. o-Benzoquinone route.

+ 2H20 -Se: -6H+

+7H20 -124$12H+

m

m

0.

+

kOOH

> - 2el-2H’

Ho0 c? -

-2

co, -Cf)

+ y----

167

agreement with the p-benzoquinone route (Scheme 2) for the following reasons: (1) The theoretical ratio y,: y2 = 1: 17 compares well with the experimental value 1: 13, while 1: 23 for Scheme 1 is too high. (2) According to the overall reaction C&H, + 9 H,O + C,,H,O, + 2 CO, + 18 H+ + 18 e- for Scheme 2, a mass gain of 12.2% of the solid is to be expected. We have found corrected l values of lo-15% i.e. of the same order. On the other hand, Scheme 1 is represented by the overall reaction C&H, + 12 H,O + C32H2404 + 4 CO, + 4 CO, + 24 H+ + 24 e- which corresponds to a mass gain of only 3.5%. (3) The increase of the oxygen content after overoxidation was found to be 0.7 0 per C,, which is compatible with 0.75 0 per C, (Scheme 1) as well as with 0.74 0 per C, (Scheme 2). (4) The results of acid-base titration lead to 0.7 mol carboxyl groups per Cs6HM, while both schemes postulate 2 mol carboxyl groups per C,,H,. However, some further degradation of these groups under CO, emission seems feasible. (5) Concerning the type of COOH groups, strongly acid groups (such as dicarboxylic acids) which were derived from the titration curve, agree with Scheme 2 only. (6) The IR spectrum is interpreted in terms of activated carboxylic groups. Our data clearly show that the p-quinone route is preferred over the o-quinone route. This follows additionally from steric considerations, because the oxygen-containing groups should occupy those sites with most space available. (3.5) Comparison

with other overoxidations

Overoxidation of graphite giving graphite oxide with a charge consumption up to 1 mol electrons/mol C (COH), has been known for a long time. The primary step is the formation of graphite intercalation compounds of the first stage in concentrated acids and of the second stage in more diluted acids [lo]. Graphite oxidation at the rims of carbon planes follows the o-quinone route [18,19]. This is also true for electrochemical oxidation [20,21]. Conducting polymers with negative insertion potentials are subject to easy overoxidation. This has been demonstrated for polyacetylene in aqueous [22] and non-aqueous [23] systems and for polypyrrole [24], and rationalizes their poor stability in the positive direction. (4) CONCLUSIONS

PPP is overoxidized through the insertion compound and the p-quinone state of every 6th benzene ring. The final product is a derivative of maleic acid. The preceding steps are ring opening and cleavage of the PPP chain. The potential/acid

* Correction was necessary due to some mass loss after treatment of unoxidized PPP samples with 14 M H2S0.,.

168

concentration behaviour, cf. Fig. 4, explains the relative instability of PPP with inserted anions at acid concentrations below 10 M. With y, : yz = 17: 1 for PPP, its sensitivity for overoxidation is much greater than that of graphite, where this ratio is 48 : 1 in aqueous acids. Our results present an interesting example of electrochemistry in the solid state. One of the early reports in this field described the anodic oxidation of naphthalene to naphthoquinone in the solid mixture with carbon black [25]. ACKNOWLEDGEMENT

Financial support of this work by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. The authors wish to thank Degussa AG, Frankfurt, for providing the carbon black and Dr. Brandt, Chemische Werke Hbls AG, for the DMPPO samples. REFERENCES 1 H. Meier, Organic Semiconductors, Verlag Chemie, Weinheim, 1974. 2 H. Sbirakawa, E.J. Louis, A.G. McDiarmid, C.K. Chiang and A.J. Heeger, J. Chem. Sot. Chem. Commun., (1977) 578. 3 D.M. Ivory, G.G. Miller, J.M. Sowa, L.W. Shacklette, R.R. Chance and R.H. Baugbmao, J. Chem. Phys., 71 (1979) 1506. 4 A.F. Diaz, K.K_ Kanazawa and G.P. Gardini, J. Chem. Sot. Chem. Commun., (1979) 635. 5 F. Beck and A. Pruss, EJectrochim. Acta, 28 (1983) 1847. 6 A. Pruss and F. Beck, J. Electroanal. Chem., 172 (1984) 281. 7 A. Pruss and F. Beck, J. Power Sources, 16 (1985) 179. 8 P. Kovacie and A. Kyriakis, J. Am. Chem. Sot., 85 (1%3) 454; Tetrahedron Lett., 467 (1962); P. Kovaciz and J. Oziomek, J. Org. Chem., 29 (1964) 11. 9 J. Held and H. Gexiscber, Ber. Bunsenges. Phys. Chem., 67 (1963) 921. 10 F. Beck, W. Dammert, J..Heiss, H. Hiller and R. Polster, Z. Naturforsch. Teil A, 28 (1973) 1009. 11 A. Pruss, Ph. D. Thesis, University Duisburg, 1986. 12 H.P. Boehm and F. Diehl, Z. Elektrochem., 66 (1962) 642. 13 H.P. Boehm, F. Diehl, W. Heck and R. Sappock, Angew. Chem., 76 (1964) 742. 14 H.P. Boehm, Farbe und Lack, 79 (1973) 419. 15 J.B. Conant and L.F. Fieser, J. Am. Chem. Sot., 45 (1923) 2194. 16 A.S. Lindsey, M.E. Peover and N.G. Savill, J. Chem. Sot., (1%2) 4558. 17 L.L. Miller, J.F. Wolf and E.A. Mageda, J. Am. Cbem. Sot., 93 (1971) 3306. 18 H.P. Bochm, M. Eckel and W. Scholl, Z. Anorg. Allg. Chem., 353 (1%7) 236. 19 D.W. van Krevelen, Fuel, 29 (1950) 269. 20 D.W. van Krevelen, Coal Typology, Elsevier, Amsterdam, l%l. 21 P.M. Dhooge and Sa-Moon Park, J. Electrochem. Sot., 130 (1983) 1539. 22 K.H. Dietz and F. Beck, J. Appl. Electrochem., 15 (1985) 159. 23 D. N&gele and R. Bittihn, DECHEMA-Monogr., 97 (1984) 377. 24 R. Qian, J. Qiu and B. Yan, Synth. Met., 14 (1986) 81. 25 E.G. White and A. Lowry, Trans. Electrochem. Sot., 6i(1932) 223.