- Email: [email protected]
Study on ferrocene derivatives diffusion dynamics in polymer electrolyte by solid-state voltammetry
Electrochimico Acta. Vol. 41, No. IS, pp.2395-2398. 1996 CopyrightC 1996 ElsewerScienceLtd. PrIntedin Great Britain.All rightsreserved 0013 4686196 $lS.tXl+ 000
Pergamon PII:SOO13-4686(96)00019-9
STUDY ON FERROCENE DERIVATIVES DIFFUSION DYNAMICS IN POLYMER ELECTROLYTE BY SOLID-STATE VOLTAMMETRY HUAFANG
Laboratory
of Electroanalytical Chemistry, of Sciences, Changchun,
ZHOU
SHAOJUN DONG*
and
Changchun Institute of Applied Chemistry, Jilin 130022, People’s Republic of China
Chinese Academy
(Received 27 November 1995)
Abstract-The
diffusion rates of seven ferrocene derivatives have been estimated in polyelectrolyte PEG. LiCIO, by using non-steady-state chronoamperometry. The D,,, of ferrocene derivatives increases obeys the Arrhenius equation. The D,,, of with temperature, and the dependency of D,,, on temperature ferrocene derivatives decreases with increasing size of electroactive species. The AD,,, values of D,, T, increase with increasing size of diffusion species. The dependency of D,,, on the size of and D,,,, ferrocene at T < T, is larger than that at T > T, in the polyelectrolyte. On the other hand, the diffusion behaviour of ferrocene derivatives is qualitatively analyzed by using cyclic voltammetry. Copyright (“’ 1996 Elsevier Science Ltd Key words: ferrocene metry.
derivatives,
diffusion,
temperature
INTRODUCTION Polymer-based solid electrolytes have attracted increasing attention, because of their potential applications as solid polymer electrolytes with high ionic conductivities in high energy density batteries, electrochromic displays and other solid-state ionic devices[ l-61. These polymer electrolytes have the important property of dissolving many electroactive species[7], which diffuse in the polymer as in a fluid electrolyte solution, except more slowly. This property provides the basis for quantitative solid state voltammetric investigations of mass transport in the polymer electrolyte[8]. However, mass transport and electron-transfer dynamics in polymer solvents depend on many factors, including the nature and concentration of electroactive species and salts, temperature, plasticization and phase structure of the polymeric media. The aim of the present work is to investigate the dependence of diffusional behaviour on the nature of electroactive species in the polymer electrolyte PEG. LiClO, The apparent diffusion coefficients Dapp of seven ferrocenes have been estimated in PEG. LiClO, by using non-steady-state chronoamperometry[9-IO]. The results show that the Dapp decreases with increasing size of ferrocene. The temperature dependence of Dapp obeys the Arrhenius equation. EXPERIMENTAL Ferrocene derivatives such as ferrocene (Fc), ethyleneferrocene (EFc), cr-hydroxyethylferrocene (HFc), l,l’-bis-hydroxyethylferrocene (BHFc), acetoferro*Author to whom correspondence
should
be addressed. 2395
effects, polymer
electrolyte.
solid-state
voltam-
cene (AFc), l,l’-bis-acetoferrocene (BAFc), l,l’-biscarboxyferrocene (BCFc) and lithium perchlorate (analytical grade) were obtained from the Beijing Pure Chemical Co. (People’s Republic of China). Polyethylene glycol (PEG, average M = 1000) was purchased from Merck (Schuchardt, Germany). A composite microelectrode cell was made with a Pt working electrode (diameter: Sprn and 50pm), a Pt counterelectrode (diameter: 1 mm) and an Ag wire reference electrode (diameter: 0.5 mm). The working electrode was polished with metallographic abrasive paper W14(03”) and W10(04#) (Shanghai sandstone factory), and finally cleaned in an ultrasonic water bath. The radius of the electrode was determined voltammetrically from plots of the steady-state current versus concentration for a system with a known diffusion coefficient, 5 mM Fe(CN)i in 1 M KCl. All electrochemical experiments were carried out using a conventional single-compartment Pyrex glass cell with 50ml capacity, which was equipped with a jacket allowing circulation of water from a thermostat (Model CS 501, Chongqing test equipment factory). The temperature of experiments was monitored by a model 501 thermostat. The polymer solution PEG.1000 was prepared by mixing an amount of LiClO, electrolyte at melting ratio of the concentration temperature and oxygen : cation was 32 : 1 (0 : Li). An appropriate amount of ferrocene derivatives was dissolved in a known amount of PEG. LiClO, polymer electrolyte with heating and stirring over 10h. Cyclic voltammetric and chronoamperometric experiments were carried out with a CV-47 voltammograph (BAS, USA) and 902-PA (Wuhan University, China) in a Faraday cage. All potentials are reported relative to the Ag electrode.
HUAFANG ZHOUand SHAOJUN DONG
2396 RESULTS
AND DISCUSSION
Cyclic voltammetry
The shape of the voltammograms provides a qualitative indication of the magnitude of Dapg for the electroactive species in polyelectrolyte. Figure 1 shows typical cyclic voltammograms of a-hydroxyethylferrocene at different temperatures in PEG. LiClO, (0 : Li = 32 : 1) polymer electrolyte. A lo-fold change of peak current occurs over a 30°C temperature range, while the voltammograms change in shape as well as in magnitude. The voltammogram shows a peak shape at lower temperature (30-4o”C), and a sigmoidal shape at higher temperature (50-60°C). There are some parameters that control the voltammetric wave shapes, such as microdisk electrode radius r, potential sweep rate v, diffusion coefficient D of electroactive species and temperature. The effects of these factors can be accounted for through the dimensionless parameter p, which was employed by Aoki et a[.[ 111: p = (nFr*v/RTD)“*.
(1)
When p is large, linear diffusion predominates and the voltammogram is peak shaped. Conversely, when p is small, radial diffusion predominates and a sigmoidal voltammogram is obtained. When T and D are large and other factors are fixed, the value of p is small and sigmoidal voltammograms are obtained as shown in Fig. 1. On the other hand, when T and D are small, the value of p is large and a peak shaped voltammogram is obtained. Figure 2 shows some of the typical cyclic voltammograms of a-hydroxyethylferrocene at various scan rates in
be-
11.25nA
J
I
I
0. 0
0. 2
I 0. 4
I
0. 6
E/V vs. Ag Fig. 2. Cyclic voltammograms of 1OmM a-hydroxyethylferrocene in PEG. LiClO, (0 : Li = 32 : 1) at different scan rates at microdisk electrode (r = 24.9pm). Potential scan rates (mVs_‘): (a) 5, (b) 20, (c) 50, (d) 100, (e) 150. Temperature: 30°C.
PEG. LiCIOL . At 30°C when the scan rate is increased, the oxidation peak (E,,) is shifted toward more positive and the reduction peak (E,,) toward more negative potentials, resulting in a larger peak separation (AE,). However, the formal potential (E”) remains relatively constant at all scan rates. A large value of AE, may come from a large uncompensated resistance between the working and the reference electrodes[12]. Estimation of diffusion coefficient
d
I
0.0
I
I
0.4
I
1
0.8
ENvKAg Fig. 1. Cyclic voltammograms of 1OmM a-hydroxyethylferrocene at different temperatures in PEG. LiCIO, (0 : Li = 32 : 1) at microdisk electrode (r = 24.9pm). Potential scan rate: SmVs-‘. Temperature: (a) 30°C. (b) 4O”C,(c) WC, (d) 60°C.
Electrochemical determination of Dapp is important to understand the transfer kinetics and electrode reaction mechanisms of an electroactive solute in a solid-state polymer. In this paper, the non-steadystate chronoamperometry at a ultramicrodisk electrode[9, lo] has been used to estimate the transport properties in the solid-state polymer. The principle is that the diffusion limiting current under a potential step gives the following equation for a short time: i = z”‘nFD,‘it
C, r*/t’/* + 4nFD,,,
C, r
(2)
where r is the radius of the ultramicrodisk electrode, n is the number of electrons, t is the potential step time, Dapp and C, are the apparent diffusion coefft-
Ferrocene derivatives diffusion dynamics
2397
and concentration of electroactive species, respectively, and other symbols have their usual meaning. There is, at worst, a 6.8% error[15] associated with equation (2) in contrast to the exact expression[16]. From a plot of i vs. t-l/*, the slope k and the intercept b, the value of Dappcan be obtained as cient
Dapp= n(br/4k)‘.
(3)
According to equations (2) and (3), the values of ferrocene derivatives at different temperatures have been estimated in the polyelectrolyte PEG LiClO, and listed in Table 1. D app for
Dependence of Da+,,,on temperature and size of ferrocene derivatives
According to Table 1, the plots of log Dapp versus temperature T are shown in Figs 3-5. Figure 3 displays the temperature dependence of Dapp for Fc, EFc and BCFc in PEG. LiClO, . Figure 4 is the plot of Da,, vs. T for HFc and BHFc. Figure 5 is the plot of Dapp vs. T for AFc and BAFc. These three figures reflect the following phenomena. (i) The values of Dappincrease strongly with temperature, resulting in
3.0
3.2 IDXNl-
3.4
Fig. 4. Temperature dependences of diffusion coelfcient of 1OmM ferrocene derivatives in PEG LiCIO,. (a) HFc, (b) BHFc.
a lOtSfold change of D,,, for Fc and its derivatives over a 40°C temperature range. The temperature dependence of D,,, can be elucidated by the free volume model. The diffusion coefficient of a small
-7 -
-ii
-B-
%
.
B
_g_
3 -10 -
3.0
3.2
-111
3.4
I
3.0
1KKV-T
T(C) Fc EFc HFc BHFc AFc BAFc BCFc * 16°C +48°C.
1. The diffusion
18
coefftcients
20
0.1 0.62
(D,,,
25
0.38 0.004 0.01
0.012
Fig. 5. Temperature dependences of diffusion coefficient of 1OmM ferrocene derivatives in PEG LiCIO, (a) AFc, (b) BAFc.
x lOa, cm’s_‘) peratures
of ferrocene
derivatives
at different
tem-
30
35
40
45
50
55
60
0.85 3.2 1.2 0.85 2.1 0.008 0.016
2.6
0.30 0.14 0.28 0.50 0.007 0.008
3.0 6.4 4.0 2.2 3.5 0.34 1.1
4.6 1.2
8.0 8.7 5.3 4.0 4.9 2.4 4.9
19 11 1.2
0.12 0.08 0.03
0.002*
3.4
1CKD-f
Fig. 3. Temperature dependences of experimentally obtained diffusion coefficient (D,,,) of 1OmM ferrocene derivatives in PEG. LiCIO,. (a) EFc, (b) Fc, (c) BCFc. Table
I
3.2
3.8 1.5 2.5 0.43 0.83
3.5 4.5 0.99 2.6’
2.6
HUAFANGZHOUand SHAOJUN DONG
2398
molecule and the free volume are related as[13] D ~PP= BRT
exp( - V*/I$)
(4)
where B and V* are constants and V, is the free volume. According to equation (4), V, increases with temperature causing an increase of Dapp. (ii) The plots of DaPP vs. T display a sharp discontinuity leading to much slower diffusion below a certain temperature. The discontinuity in DnPPvalues occurs in the range of the melt transition temperature (Td of the polymer electrolyte, ie about 30-35°C. The discontinuity coincides with the formation or dissolution of ordered regions in the polymer[14], which impede molecular diffusion and ionic mobility. On the other hand, the Dppp values above T, (T > T,) are about 10 times larger than that below T, (T < T,). The values AD,,, (the difference between D,, T, and DT>T,) increase with increasing the size of ferrocene. The plots of DaPP vs. T are approximately of an Arrhenius type both above and below T,. (iii) According to Table 1, the values Dppp decrease (Fc > HFc > BHFc > AFc > BCFc > BAFc) with increasing size of electroactive species (Fc < BCFc, HFc c BHFc, AFc < BAFc). Figure 3 shows that the D.PP values fall in the following order: EFc > Fc > BCFc. Figure 4 shows that Dnpp for HFc is larger than that for BHFc owing to the larger size of BHFc. Figure 5 is analogous to Fig. 4. These phenomena indicate that the molecular size of the diffusion species influences the free volume, and thus the diffusion rates of electroactive species. In a polymer electrolyte, Dppp is mainly dependent on the size of the diffusion species and the polymer chain when other conditions are kept constant. In the above behavior, the dependence of Dapp on the size of diffusion species (or volume) is larger than that on the interactions. (iv) The dependence of Dppp on the size of ferrocene derivatives at T < T, is larger than that at T > T,. Figures 4 and 5 show that the values (AD,,, = DHF~-DBHF~ or AD,,, = of AD,,, are larger at T < T,, but smaller at DAFC-DBAFC) T > T,. On the other hand, AD,,, decreases as the temperature increases. These phenomena indicate that the dependence of Dnpp on the size of the species is larger at lower temperature owing to a smaller free volume. Conversely, the dependence of Dapp on the size of the species is smaller at higher temperature owing to a larger free volume.
CONCLUSION The DaPP of seven ferrocene derivatives in PEG. LiC104 electrolyte have been estimated by using non-steady-state chronoamperometry. The dependence of Dapp on temperature is of the Arrhenius type, and the plots of Dapp versus T produce a
discontinuity. These discontinuities occur close to the melt transition temperature of the polymer electrolyte. The Dnpp values decrease with increasing size of ferrocene derivatives. The AD,,, values (between T > T, and T < T,) increase with increasing size of ferrocene derivatives. The dependence of Dngp on the size of the species is larger at lower temperature and smaller at higher temperature. The results indicate that the molecular size of the diffusion species has a predominant influence on its diffusion rate in the polyelectrolyte. Acknowledgement-This project was supported National Natural Science Foundation of China.
by the
REFERENCES 1. M. B. Armand, Solid State lonics, 9/10,745 (1983). 2. F. M. Gray, Solid Polymer Electrolytes, Fundamentals and Technological Applications. VCH, New York (1991). 3. C. K. Chiang, Polym. Commun. 22, 1454 (1981). 4. P. Passiniemi and 0. Inganas, Solid State lonics, 34, 225 (1983). 5. S. Passerini, B. Scrosati, A. Gorenstein, A. M. Andersson and C. G. Granqvist, J. electrochem. Sot. 136, 3394
(1989). 6. S. Pantaloni, S. Passerini and B. Scrosati, J. electrothem. Sot. 134,753 (1987). 7. (a) R. A. Reed, L. Geng and R. W. Murray, J. electroanal. Chem. 208. 185 (1986). (b) L. Gena. R. A. Reed. M. Longmire and R. W.‘ Murray; 1. phys:Chem. 91, i908 (1987). (c) L. Geng, R. A. Reed, M-H. Kim, T. T. Wooster, B. N. Oliver, J. Egekeze, R. T. Kennedy, J. W. Jorgenson, J. F. Parcher and R. W. Murray, J. Am. them. Sot. 111, 1614 (1989). 8. (a) M. L. Longmire, M. Watanabe, H. Zhang, T. T. Wooster and R. W. Murray, Anal. Chem. 6.2, 747 (1990). (b) T. T. Wooster, M. L. Longmire, H. Zhang, M. Watanabe and R. W. Murray, Anal. Chem. 64, 1132 (1992). 9. G. Che and S. Dong, J. electroanal. Chem. 346, 239 (1993). 10. H. Zhou, G. Che and S. Dong, manuscript in preparation. 11. K. Aoki, K. Honda, K. Tokuda, H. Matsuda and J. Ostervouna. J. electroanal. Chem. 219. 17 (1984). 12. R. S. Nich&on, Anal. Chem. 37,667 (1965). 13. M. Watanabe, T. T. Wooster and R. W. Murray, J. phys. Chem. %,4573 (1991). 14. (a). H. Dahms, J. phys. Chem. 72, 362 (1968). (b) I. Ruff and V. J. Friedrich, J. phys. Chem. 75,3297 (1971). 15. M. H. Cohen and D. Turnbull. J. them. Phvs. _ 31. 1164 (1959). 16. M. A. Ratner and D. F. Shriver, Chem. Rev. 88, 109 (1988). 17. G. Denuault, M. V. Mirkin and A. J. Bard, J. electroanal. Chem. 30&27 (1991). 18. D. Shoup and A. Szabo, .I. electroanal. Chem. 140, 237 (1982).
Pergamon PII:SOO13-4686(96)00019-9
STUDY ON FERROCENE DERIVATIVES DIFFUSION DYNAMICS IN POLYMER ELECTROLYTE BY SOLID-STATE VOLTAMMETRY HUAFANG
Laboratory
of Electroanalytical Chemistry, of Sciences, Changchun,
ZHOU
SHAOJUN DONG*
and
Changchun Institute of Applied Chemistry, Jilin 130022, People’s Republic of China
Chinese Academy
(Received 27 November 1995)
Abstract-The
diffusion rates of seven ferrocene derivatives have been estimated in polyelectrolyte PEG. LiCIO, by using non-steady-state chronoamperometry. The D,,, of ferrocene derivatives increases obeys the Arrhenius equation. The D,,, of with temperature, and the dependency of D,,, on temperature ferrocene derivatives decreases with increasing size of electroactive species. The AD,,, values of D,, T, increase with increasing size of diffusion species. The dependency of D,,, on the size of and D,,,, ferrocene at T < T, is larger than that at T > T, in the polyelectrolyte. On the other hand, the diffusion behaviour of ferrocene derivatives is qualitatively analyzed by using cyclic voltammetry. Copyright (“’ 1996 Elsevier Science Ltd Key words: ferrocene metry.
derivatives,
diffusion,
temperature
INTRODUCTION Polymer-based solid electrolytes have attracted increasing attention, because of their potential applications as solid polymer electrolytes with high ionic conductivities in high energy density batteries, electrochromic displays and other solid-state ionic devices[ l-61. These polymer electrolytes have the important property of dissolving many electroactive species[7], which diffuse in the polymer as in a fluid electrolyte solution, except more slowly. This property provides the basis for quantitative solid state voltammetric investigations of mass transport in the polymer electrolyte[8]. However, mass transport and electron-transfer dynamics in polymer solvents depend on many factors, including the nature and concentration of electroactive species and salts, temperature, plasticization and phase structure of the polymeric media. The aim of the present work is to investigate the dependence of diffusional behaviour on the nature of electroactive species in the polymer electrolyte PEG. LiClO, The apparent diffusion coefficients Dapp of seven ferrocenes have been estimated in PEG. LiClO, by using non-steady-state chronoamperometry[9-IO]. The results show that the Dapp decreases with increasing size of ferrocene. The temperature dependence of Dapp obeys the Arrhenius equation. EXPERIMENTAL Ferrocene derivatives such as ferrocene (Fc), ethyleneferrocene (EFc), cr-hydroxyethylferrocene (HFc), l,l’-bis-hydroxyethylferrocene (BHFc), acetoferro*Author to whom correspondence
should
be addressed. 2395
effects, polymer
electrolyte.
solid-state
voltam-
cene (AFc), l,l’-bis-acetoferrocene (BAFc), l,l’-biscarboxyferrocene (BCFc) and lithium perchlorate (analytical grade) were obtained from the Beijing Pure Chemical Co. (People’s Republic of China). Polyethylene glycol (PEG, average M = 1000) was purchased from Merck (Schuchardt, Germany). A composite microelectrode cell was made with a Pt working electrode (diameter: Sprn and 50pm), a Pt counterelectrode (diameter: 1 mm) and an Ag wire reference electrode (diameter: 0.5 mm). The working electrode was polished with metallographic abrasive paper W14(03”) and W10(04#) (Shanghai sandstone factory), and finally cleaned in an ultrasonic water bath. The radius of the electrode was determined voltammetrically from plots of the steady-state current versus concentration for a system with a known diffusion coefficient, 5 mM Fe(CN)i in 1 M KCl. All electrochemical experiments were carried out using a conventional single-compartment Pyrex glass cell with 50ml capacity, which was equipped with a jacket allowing circulation of water from a thermostat (Model CS 501, Chongqing test equipment factory). The temperature of experiments was monitored by a model 501 thermostat. The polymer solution PEG.1000 was prepared by mixing an amount of LiClO, electrolyte at melting ratio of the concentration temperature and oxygen : cation was 32 : 1 (0 : Li). An appropriate amount of ferrocene derivatives was dissolved in a known amount of PEG. LiClO, polymer electrolyte with heating and stirring over 10h. Cyclic voltammetric and chronoamperometric experiments were carried out with a CV-47 voltammograph (BAS, USA) and 902-PA (Wuhan University, China) in a Faraday cage. All potentials are reported relative to the Ag electrode.
HUAFANG ZHOUand SHAOJUN DONG
2396 RESULTS
AND DISCUSSION
Cyclic voltammetry
The shape of the voltammograms provides a qualitative indication of the magnitude of Dapg for the electroactive species in polyelectrolyte. Figure 1 shows typical cyclic voltammograms of a-hydroxyethylferrocene at different temperatures in PEG. LiClO, (0 : Li = 32 : 1) polymer electrolyte. A lo-fold change of peak current occurs over a 30°C temperature range, while the voltammograms change in shape as well as in magnitude. The voltammogram shows a peak shape at lower temperature (30-4o”C), and a sigmoidal shape at higher temperature (50-60°C). There are some parameters that control the voltammetric wave shapes, such as microdisk electrode radius r, potential sweep rate v, diffusion coefficient D of electroactive species and temperature. The effects of these factors can be accounted for through the dimensionless parameter p, which was employed by Aoki et a[.[ 111: p = (nFr*v/RTD)“*.
(1)
When p is large, linear diffusion predominates and the voltammogram is peak shaped. Conversely, when p is small, radial diffusion predominates and a sigmoidal voltammogram is obtained. When T and D are large and other factors are fixed, the value of p is small and sigmoidal voltammograms are obtained as shown in Fig. 1. On the other hand, when T and D are small, the value of p is large and a peak shaped voltammogram is obtained. Figure 2 shows some of the typical cyclic voltammograms of a-hydroxyethylferrocene at various scan rates in
be-
11.25nA
J
I
I
0. 0
0. 2
I 0. 4
I
0. 6
E/V vs. Ag Fig. 2. Cyclic voltammograms of 1OmM a-hydroxyethylferrocene in PEG. LiClO, (0 : Li = 32 : 1) at different scan rates at microdisk electrode (r = 24.9pm). Potential scan rates (mVs_‘): (a) 5, (b) 20, (c) 50, (d) 100, (e) 150. Temperature: 30°C.
PEG. LiCIOL . At 30°C when the scan rate is increased, the oxidation peak (E,,) is shifted toward more positive and the reduction peak (E,,) toward more negative potentials, resulting in a larger peak separation (AE,). However, the formal potential (E”) remains relatively constant at all scan rates. A large value of AE, may come from a large uncompensated resistance between the working and the reference electrodes[12]. Estimation of diffusion coefficient
d
I
0.0
I
I
0.4
I
1
0.8
ENvKAg Fig. 1. Cyclic voltammograms of 1OmM a-hydroxyethylferrocene at different temperatures in PEG. LiCIO, (0 : Li = 32 : 1) at microdisk electrode (r = 24.9pm). Potential scan rate: SmVs-‘. Temperature: (a) 30°C. (b) 4O”C,(c) WC, (d) 60°C.
Electrochemical determination of Dapp is important to understand the transfer kinetics and electrode reaction mechanisms of an electroactive solute in a solid-state polymer. In this paper, the non-steadystate chronoamperometry at a ultramicrodisk electrode[9, lo] has been used to estimate the transport properties in the solid-state polymer. The principle is that the diffusion limiting current under a potential step gives the following equation for a short time: i = z”‘nFD,‘it
C, r*/t’/* + 4nFD,,,
C, r
(2)
where r is the radius of the ultramicrodisk electrode, n is the number of electrons, t is the potential step time, Dapp and C, are the apparent diffusion coefft-
Ferrocene derivatives diffusion dynamics
2397
and concentration of electroactive species, respectively, and other symbols have their usual meaning. There is, at worst, a 6.8% error[15] associated with equation (2) in contrast to the exact expression[16]. From a plot of i vs. t-l/*, the slope k and the intercept b, the value of Dappcan be obtained as cient
Dapp= n(br/4k)‘.
(3)
According to equations (2) and (3), the values of ferrocene derivatives at different temperatures have been estimated in the polyelectrolyte PEG LiClO, and listed in Table 1. D app for
Dependence of Da+,,,on temperature and size of ferrocene derivatives
According to Table 1, the plots of log Dapp versus temperature T are shown in Figs 3-5. Figure 3 displays the temperature dependence of Dapp for Fc, EFc and BCFc in PEG. LiClO, . Figure 4 is the plot of Da,, vs. T for HFc and BHFc. Figure 5 is the plot of Dapp vs. T for AFc and BAFc. These three figures reflect the following phenomena. (i) The values of Dappincrease strongly with temperature, resulting in
3.0
3.2 IDXNl-
3.4
Fig. 4. Temperature dependences of diffusion coelfcient of 1OmM ferrocene derivatives in PEG LiCIO,. (a) HFc, (b) BHFc.
a lOtSfold change of D,,, for Fc and its derivatives over a 40°C temperature range. The temperature dependence of D,,, can be elucidated by the free volume model. The diffusion coefficient of a small
-7 -
-ii
-B-
%
.
B
_g_
3 -10 -
3.0
3.2
-111
3.4
I
3.0
1KKV-T
T(C) Fc EFc HFc BHFc AFc BAFc BCFc * 16°C +48°C.
1. The diffusion
18
coefftcients
20
0.1 0.62
(D,,,
25
0.38 0.004 0.01
0.012
Fig. 5. Temperature dependences of diffusion coefficient of 1OmM ferrocene derivatives in PEG LiCIO, (a) AFc, (b) BAFc.
x lOa, cm’s_‘) peratures
of ferrocene
derivatives
at different
tem-
30
35
40
45
50
55
60
0.85 3.2 1.2 0.85 2.1 0.008 0.016
2.6
0.30 0.14 0.28 0.50 0.007 0.008
3.0 6.4 4.0 2.2 3.5 0.34 1.1
4.6 1.2
8.0 8.7 5.3 4.0 4.9 2.4 4.9
19 11 1.2
0.12 0.08 0.03
0.002*
3.4
1CKD-f
Fig. 3. Temperature dependences of experimentally obtained diffusion coefficient (D,,,) of 1OmM ferrocene derivatives in PEG. LiCIO,. (a) EFc, (b) Fc, (c) BCFc. Table
I
3.2
3.8 1.5 2.5 0.43 0.83
3.5 4.5 0.99 2.6’
2.6
HUAFANGZHOUand SHAOJUN DONG
2398
molecule and the free volume are related as[13] D ~PP= BRT
exp( - V*/I$)
(4)
where B and V* are constants and V, is the free volume. According to equation (4), V, increases with temperature causing an increase of Dapp. (ii) The plots of DaPP vs. T display a sharp discontinuity leading to much slower diffusion below a certain temperature. The discontinuity in DnPPvalues occurs in the range of the melt transition temperature (Td of the polymer electrolyte, ie about 30-35°C. The discontinuity coincides with the formation or dissolution of ordered regions in the polymer[14], which impede molecular diffusion and ionic mobility. On the other hand, the Dppp values above T, (T > T,) are about 10 times larger than that below T, (T < T,). The values AD,,, (the difference between D,, T, and DT>T,) increase with increasing the size of ferrocene. The plots of DaPP vs. T are approximately of an Arrhenius type both above and below T,. (iii) According to Table 1, the values Dppp decrease (Fc > HFc > BHFc > AFc > BCFc > BAFc) with increasing size of electroactive species (Fc < BCFc, HFc c BHFc, AFc < BAFc). Figure 3 shows that the D.PP values fall in the following order: EFc > Fc > BCFc. Figure 4 shows that Dnpp for HFc is larger than that for BHFc owing to the larger size of BHFc. Figure 5 is analogous to Fig. 4. These phenomena indicate that the molecular size of the diffusion species influences the free volume, and thus the diffusion rates of electroactive species. In a polymer electrolyte, Dppp is mainly dependent on the size of the diffusion species and the polymer chain when other conditions are kept constant. In the above behavior, the dependence of Dapp on the size of diffusion species (or volume) is larger than that on the interactions. (iv) The dependence of Dppp on the size of ferrocene derivatives at T < T, is larger than that at T > T,. Figures 4 and 5 show that the values (AD,,, = DHF~-DBHF~ or AD,,, = of AD,,, are larger at T < T,, but smaller at DAFC-DBAFC) T > T,. On the other hand, AD,,, decreases as the temperature increases. These phenomena indicate that the dependence of Dnpp on the size of the species is larger at lower temperature owing to a smaller free volume. Conversely, the dependence of Dapp on the size of the species is smaller at higher temperature owing to a larger free volume.
CONCLUSION The DaPP of seven ferrocene derivatives in PEG. LiC104 electrolyte have been estimated by using non-steady-state chronoamperometry. The dependence of Dapp on temperature is of the Arrhenius type, and the plots of Dapp versus T produce a
discontinuity. These discontinuities occur close to the melt transition temperature of the polymer electrolyte. The Dnpp values decrease with increasing size of ferrocene derivatives. The AD,,, values (between T > T, and T < T,) increase with increasing size of ferrocene derivatives. The dependence of Dngp on the size of the species is larger at lower temperature and smaller at higher temperature. The results indicate that the molecular size of the diffusion species has a predominant influence on its diffusion rate in the polyelectrolyte. Acknowledgement-This project was supported National Natural Science Foundation of China.
by the
REFERENCES 1. M. B. Armand, Solid State lonics, 9/10,745 (1983). 2. F. M. Gray, Solid Polymer Electrolytes, Fundamentals and Technological Applications. VCH, New York (1991). 3. C. K. Chiang, Polym. Commun. 22, 1454 (1981). 4. P. Passiniemi and 0. Inganas, Solid State lonics, 34, 225 (1983). 5. S. Passerini, B. Scrosati, A. Gorenstein, A. M. Andersson and C. G. Granqvist, J. electrochem. Sot. 136, 3394
(1989). 6. S. Pantaloni, S. Passerini and B. Scrosati, J. electrothem. Sot. 134,753 (1987). 7. (a) R. A. Reed, L. Geng and R. W. Murray, J. electroanal. Chem. 208. 185 (1986). (b) L. Gena. R. A. Reed. M. Longmire and R. W.‘ Murray; 1. phys:Chem. 91, i908 (1987). (c) L. Geng, R. A. Reed, M-H. Kim, T. T. Wooster, B. N. Oliver, J. Egekeze, R. T. Kennedy, J. W. Jorgenson, J. F. Parcher and R. W. Murray, J. Am. them. Sot. 111, 1614 (1989). 8. (a) M. L. Longmire, M. Watanabe, H. Zhang, T. T. Wooster and R. W. Murray, Anal. Chem. 6.2, 747 (1990). (b) T. T. Wooster, M. L. Longmire, H. Zhang, M. Watanabe and R. W. Murray, Anal. Chem. 64, 1132 (1992). 9. G. Che and S. Dong, J. electroanal. Chem. 346, 239 (1993). 10. H. Zhou, G. Che and S. Dong, manuscript in preparation. 11. K. Aoki, K. Honda, K. Tokuda, H. Matsuda and J. Ostervouna. J. electroanal. Chem. 219. 17 (1984). 12. R. S. Nich&on, Anal. Chem. 37,667 (1965). 13. M. Watanabe, T. T. Wooster and R. W. Murray, J. phys. Chem. %,4573 (1991). 14. (a). H. Dahms, J. phys. Chem. 72, 362 (1968). (b) I. Ruff and V. J. Friedrich, J. phys. Chem. 75,3297 (1971). 15. M. H. Cohen and D. Turnbull. J. them. Phvs. _ 31. 1164 (1959). 16. M. A. Ratner and D. F. Shriver, Chem. Rev. 88, 109 (1988). 17. G. Denuault, M. V. Mirkin and A. J. Bard, J. electroanal. Chem. 30&27 (1991). 18. D. Shoup and A. Szabo, .I. electroanal. Chem. 140, 237 (1982).