J. Phys. Chem. 1995, 99, 16676-16683
16676
Electron Transport and Frozen Concentration Gradients in a Mixed Valent Viologen Molten Salt Roger H. Terrill, Tsuyonobu HatazawaJ and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received: April 11, 1995@
This paper shows that electrolytically generated crossed concentration gradients of viologen (2+) and viologen (1+) in films of its mixed valent molten salt on interdigitated array electrodes can be thermally and stably frozen in place in the interelectrode gaps. A study is presented of properties of the gradient-containing films, as compared to films that are non-mixed valent (V2+)or that are mixed valent but lack concentration gradients. Comparisons of charge transport measurements show that the 2-5 pm wide concentration gradients are persistent in the molten salt at lowered temperatures, where ionic motions are quenched relative to electron hopping as shown by differences in conductivity between mixed valent and non-mixed valent films as the temperature is lowered from +50 to -70 "C. Differences in the magnitude and shape of high-field currentgradient-containing samples are interpreted voltage curves taken from mixed valent nongradient and V2+N+ with an electron-hopping model that includes a parameter for kinetic dispersity . Differences between the capacitance of the mixed valent and non-mixed valent phase of the viologen molten salt are consistent with the formation of an electronic space charge at the metallredox conductor interface.
Electron-hopping (or redox) conduction has been the subject of considerable experimental,',* and theoretical3 research, particularly in molecular and polymeric systems that contain electronically well-defined electron donor and acceptor sites. The preparation of chemically defined microstructures made from redox polymers has also been of interest for some time.'$4 Most of these microstructures are based on some spatially defined change in chemical composition; a few, such as chargetrapped bilayer electrodes? contain energy stored by means of multiple layers of different redox polymers. We are not aware, however, of redox polymer microstructures that contain, within a single film having the same molecular composition, a stored gradient of oxidation states with associated built-in electrical potential gradient. Such stored gradients, in more elaborated microstructures: may provide current-potential behavior formally analogous to that of pn junctions in doped semiconductors, although the redox materials are not themselves semiconductors. It seemed of interest, then, to demonstrate how "frozen concentration gradients" might be generated in a molecular redox material and to explore their electrical properties in relation to those of the redox material lacking such gradients. The material used to illustrate frozen concentration gradients is the 2+/1+ mixed valent form of the (2+) viologen compound.6
The donor and acceptor states of this molecule will be abbreviated as V2+ and V'. The BF4- salt of the dipositive form, V2+, is at room temperature a non-birefringent, highly viscous amber liquid; that is, it is a concentrated redox molten salt, approximately 2 M in V2+ sites. The V2+/+mixed valent form is an intensely blue colored material. No diluents are Present address: Applied Electronics Research Laboratories, Sekisui Chemical Co., Ltd., 32 Wadai Tsukuba-shi Ibaraki, 300-42 Japan. Abstract published in Advance ACS Absrrucrs, October 15, 1995. +
@
0022-365419512099- 16676$09.00/0
added; the V2+ and V2+/+viologen melts are employed as neat materials. A recent electrochemical characterization of the V2+ viologen melt6 has shown that it is ionically conductive at room temperature. Microelectrode voltammetry in the solvent-free redox liquid exhibits two quasi-reversible one-electron reductions, to the V+ and Vo states? similar to those observed in related redox melt systems' synthesized in our laboratory. The key to establishing frozen concentration gradients in a mixed valent polymer (or in a viscous molten salt like V2+'+) lies in understanding the relationship between its ionic and electron-hopping conductivities. We have shown previously that ionic transport in dry mixed-valent redox polymers at room temperature can be slow relative to electron-hopping-based charge transport, dry Nafion loaded with Os complexesla and polyvinylferrocene' being examples. In electropolymerized Os and Ru bipyridine complexes,Ic-J ionic transport is appreciable at room temperature in the dry mixed valent material, but is thoroughly quenched at lowered temperatures. The viologen molten salt used here resembles the latter material, having a significant ionic conductivity at room temperature that is quenched, relative to electron conductivity, at lowered temperatures. We will show that at temperatures above about -10 "C, both ionic motions and V2+/+ electron hopping contribute to charge flow and that at lower temperatures the conductivity is govemed solely by V2+/+electron hopping. The conductivity measurements are made on films of the viologen melt coated onto interdigitated array electrodes (IDAs) with interfinger gap dimensions of 2-5 pm. The bimolecular electron self-exchange mechanism expected for electron transport through a mixed valent viologen film on an IDA is shown schematically in Figure 1. Crossed concentration gradients of V2+ and V+ are readily prepared electrolytically at room temperature, by applying a potential bias to films of mixed valent V2+/' on IDAs. Then, still under the same bias, cooling to ionically nonconductive temperatures stores the concentration gradient in the film even when the gradient-formingpotential bias is removed. The V2+/+ donor-acceptor gradients are expected to be stable when ionic conductivity is too small to allow reorganization of the ionic 0 1995 American Chemical Society
Mixed Valent Viologen Molten Salt
J. Phys. Chem., Vol. 99, No. 45, 1995 16677
I D A Electrode: Top View.
I
Viologen film
Pt I D A Fingers
1
I
I D A Electrode: Cross Section of Fingers.
-
4r
U
10.2
9
1
Distance (pm)
Figure 1. Schematic of an IDA electrode, showing cross section of open-face sandwich structure and electron self-exchangesbetween V2t and V+ sites in a mixed valent film in the interfinger gap.
V2+'+ and BF4- sites at the lowered temperatures used to study electron transport, within the duration of the experiment. Delineation between mixed ionic-electronic and solely electronic conductivity is possible by comparisons to the non-mixed valent form V2+, which conducts by ionic modes alone. In the course of applying voltage steps to V2+'+ mixed valent "frozen-gradient'' films at lowered temperatures, we have also observed a transient component due to a capacitance that may represent formation of an electronic space charge at the metalviologen interface.
Experimental Section Chemicals. The synthesis and characterization of the tetrafluoroborate salt of the trimer-ethylene oxide tailed viologen (vide supra) has been described elsewhere.6 Acetonitrile for electrochemical experiments (Burdick and Jackson HPLC grade) was distilled from CaH2. LiC104 electrolyte (Aldrich analytical reagent grade) was dried under vacuum at 70 OC for several hours before use. IDA Electrodes. Interdigitated array (IDA) electrodes consisting of 50 (nf) interlocking pairs of 5 or 10 p m wide, 2 mm long ( l ) , and 0.1 pm thick (h) platinum film fingers separated by 5 or 2 pm, respectively, generously donated by Nippon Telephone and Telegraph, were patterned on Si/Si02 substrates with an insulating Si3N4 overlayer surrounding the electron pattern. Electrodes were cleaned by sonication and rinsing in methanol and Nanopure water. Prior to film casting, both sets of IDA fingers were independently made the working electrodes in an electrochemical cell, and the chemical reversibility of the 3+/2+ couple of an aqueous [Ru(NH3)6I3+standard was analyzed by cyclic voltammetry to confirm the area and electrochemical activity of the platinum fingers. Voltammetric peak separations were within experimental error of the ideal value of 59 mV. The cross sectional area of the parallel plate polymer sandwich (A) is taken as the area of the IDA finger walls facing each other, A = (nf - 1)lh. Mixed Valent Film and Gradient Preparations. Mixed valent V2+/+ solutions were prepared by bulk electrolysis of ca. 25 mM solutions of [V2+][BF& in freshly distilled acetonitrile, using a platinum mesh working electrode at the Eo' of the V2+'+ couple (as ascertained by a preliminary microdisk electrode voltammogram). The solution contained no supporting electrolyte, in order to minimize the ionic content of the subsequent 1:l V2+/+mixed valent films. The electrolysis was done in a three-compartment electrochemical cell with fine frits separating the working compartment from a Ag/AgBFdacetonitrile nonaqueous reference electrode compartment and a salt bridge compartment containing 0.1 M LiC104 acetonitrile
Figure 2. (top) Concentration profiles of V2+, Vt, and [BFJ counterions in a 1: 1 mixed valent Vz+'+ film containing concentration gradients produced by electrolysis at a 0.3 V potential bias. (bottom) Profile of Nemstian potentials and conductivity within the crossed concentration gradient film. IDA finger width 5 pm and gap 5 pm.
solution and leading in turn by a fine (porous Vycor) frit to the silver wire counter electrode compartment. Intrusion of counter electrode electrolysis products (Ag+) into the working electrode compartment, with consequence Ago deposition, was not observed. Charge balance in the working compartment was maintained solely by loss of [BF4]- andlor uptake of Li+ from the salt bridge. The V+ state is oxygen sensitive, and all electrolyses were carried out in a vigorously nitrogen purged glovebag, simultaneously purging the working electrode compartment with N2. Following completion of the electrolysis, V2+'+ solutions were immediately transferred to a small round bottom flask and concentrated ca. 5-fold under vacuum. Films were cast from this solution onto IDA electrodes in the glovebag and then immediately placed in an evacuated chamber ( K 5 0 mTorr) on a temperature-controlled (f0.5 "C) stage (local construction) and thoroughly dried. The temperature-controlled stage was ohmically heated by 60 W Omega Kapton-strip heaters powered by the 110 V triac output of an Omega CN76000 two-channel temperature controller. Below-ambient temperature control was achieved by circulating liquid-nitrogencooled N2 through copper coils on the bottom of the stage and controlling the nitrogen flow with the second channel of the temperature controller. Non-mixed valent films were similarly cast onto IDAs from acetonitrile solutions, evacuated, and dried. The [V2+][BF& melt is quite hygroscopic, and care was taken to assure that films of it and of the mixed valent form were kept rigorously dry during the analysis by continuous active pumping. The ionic conductivity of polyether phases is known to be very sensitive to water contaminant,* and film conductivities were observed to drop over several hours and then stabilize when held in vacuo and warmed to 50 "C. Mixed valent films containing concentration gradients were generated at room temperature from thoroughly dried films of the 1:1 mixed valent V2+'+ melt prepared on an IDA as above. In the latter films, the [BF4]-, V2+, and V+ concentrations are uniform across the interfinger gaps, as is the conductivity (a) at any point in the film. Application of a 0.3 V interfinger potential bias causes interfacial electrolysis; V2+ accumulates at one electrode and V+ at the other, and in time steady state gradients of these states and an associated gradient of the counterion BF4- are formed (Figure 2, top). This is signalled by attainment of a steady state current, which occurs within 300 s at 50 "C for the largest (5 pm) gap. Mobility of the BF4counterion is required for this electrolysis to occur. Subsequent cooling of the IDA sample, using the controlled stage, to
Terrill et al.
16678 J. Phys. Chem., Vol. 99,No. 45, 1995 temperatures of negligible BF4- mobility can preserve its spatial distribution and that of the V2+ and V+ concentration gradients. The high surface tension (or poor wetting of the Si3N4 substrate) of the non-mixed valent [V2+][BF4]2made it necessary to cast thick (> 100 pm) films of this material onto the IDAs to prevent flow and beading during conductivity measurements at higher temperatures. The mixed valent films were thinner than the non-mixed valent ones, but both were very thick relative to IDA finger and gap geometries. Conductivity and Other Measurements. The conductivities of non-mixed valent and mixed valent V2+/+films not containing concentration gradients were measured under vacuum using high-frequency (5-5000 Hz) sweeps of f 3 0 mV ('0 "C) to f 3 0 0 mV ( 5 0 "C) potential bias between the IDA finger sets. Sweeps were provided by a Hewlett Packard Model 8116A function generator, converting current to voltage with a homebuilt current follower circuit and recording i-E traces with a Nicolet 310 digital oscilloscope or a Date1 412 digitizing board in an IBM PC compatible microcomputer using locally written software. The time scale of the potential sweeps was chosen to be sufficiently short that no hysteresis is observed in i-E curves, meaning that ionic polarization is being outrun, and the potential bias does not result in any significant interfacial electrolysis even at the higher temperatures. Conductivities were generally very stable when observed over extended periods of time. Conductivities of mixed valent films containing and not containing frozen concentration gradient microstructures were also compared using potential sweeps of much higher amplitude and larger than the bias used to create the concentration gradients (0.3 V). Applications of large potential sweeps or steps to frozen gradient films were "one-shot" cycles, and the potential bias was returned to the original 0.3 V during rest cycles. In fitting eq 2. (vide infra) to experimental i-E curves, an initial estimate of the product p k ~ xwas made (by linear regression) from the current-potential response near zero potential (eq 3), and the p parameter was then adjusted to optimize the overall fit to eq 2. Fits to simulated data converge consistently on kEx and p values to within &I%. Comparisons were made between conductivity measurements made with ac impedence, linear potential sweep, and potentialstep methods. Alternating current impedance spectra (taken between lo5 and lo-' Hz with a Shlumberger Model 1255 frequency response analyzer and Shlumberger Model 1286 potentiostat operating under the control of a PC and using commercial acquisition software) displayed a well-formed RC semicircle. Conductivity measurements by these methods were in good agreement. Most data were acquired with the potential sweep technique owing to its convenience, the simplicity of data analysis, and the ease of adjusting current sensitivity under conditions of high resistance. Measurements of open circuit potential were made with a Keithley Model 197 digital multimeter (ZIN > loio S Z ) .
Results and Discussion Results for electron-hopping transport in 1:l mixed valent [V2+/f][BF4-]~.5films will be presented first, to establish a basis for how to preserve frozen gradients and to establish a basis for comparison between films with and without frozen gradients. These results amount to a restricted examination of the electron transport itself; an analysis of the effects of varied levels of mixed valency and of dilution, such as reported previously from this laboratory,'a-f remains to be carried out. Current-Vottage Response of Warm 1:l Mixed Valent [VZ+/+][BF4-]1.5Films. At room and elevated temperatures,
C
1
-0.1 0
0.1
Enias/Volts
A
-0.1 0
0.1
Figure 3. Cyclical potential sweeps on a 1:1 mixed valent V2+'+film on an IDA (finger width 5 pm and gap 5 pm) at 50 "C. Curves A-D have current sensitivities S = 3 x 3x 3 x and 3 x A, respectively. The cross represents zero current and potential.
application of slow sweeps ( < 1 VIS) of potential bias across the IDA finger sets (5 pm finger width, 5 pm gap) results in voltammograms (50 "C, under Nz) for a 1:l mixed valent [V2+/+][BF4-]i.5film as shown in Figure 3. The voltammetric wave shape indicates that interfacial electrolysis is occurring, which in turn means that the ionic conductivity of the [V2+/+][BF4-]l,5 melt phase is substantial. For the faster potential sweeps, the shapes of the voltammograms (Figure 3C,D) approach that characteristic of linear diffusion geometry around the electrode fingers. On the longer time scale of the slowest potential sweep (1 mV/s, Figure 3A), the diffusion profiles of V2+ and Vf states extend across the 5 pm interfinger gap, and sigmoidally shaped current-voltage curves with welldefined plateau currents are obtained, indicatingIO that steady state V2+ and V+ concentration profiles have been developed across the gap. The concentration gradients that are formed at the f 3 0 0 mV terminus of the potential sweep in Figure 3A are the same as those "frozen" into the film when the potential sweep is halted at f 3 0 0 mV and the temperature then lowered under this potential bias. For a given mixed valent film sample, steady state voltammograms like that in Figure 3A can be observed repeatedly over long periods of time. Limiting currents (I'LIM) of such voltammograms generally decay by less than ca. 5-10% upon repeated observation over a 24 h period; the decay is presumed to arise from residual air oxidation of V+. The apparent diffusion coefficient D ~ p for p the V2+'+ couple in its 1:l mixed valent film can be obtained from i ~ ofl voltammograms like Figure 3A, using relations by Aoki et al.Ioa that account for the contribution of transport in regions of V2+'+ atop the IDA fingers as well as that in the interfinger gaps. We have tested this relation previously.i0b The limiting current of Figure 3A (5 p m finger width, 5 pm gap IDA, at 50 "C) gives with the Aokiioaequation an apparent diffusion coefficient D ~ p p = 2.57 x cm2/s. Analysis of the 100 mV/s voltammogram (obtained in the same experiment, Figure 3C) as a linear diffusion result, using th Randles-Sevcik9 equation, returns a value of 2.62 x cm2/s, in good agreement. In a different experiment, using a 3 pm finger15 pm gap IDA at 30 "C, Aoki and Randles-Sevcik analyses of (nearly) steady state limiting (at 3 mV/s) and of peak currents (at 300 mV/s) give values of and 4.6 x 1O-Io cm2/s,respectively, again DAPP = 4.3 x showing that the results under different diffusion geometries are consistent. The average of three experiments performed in the 25-30 "C temperature range is D ~ p = p (3.6 f 1.2) x 1O-Io cm2/s, which is in good agreement with that obtained in room temperature experiments from microdisk electrode voltammetry6 of bulk samples of [V2+][BF4-]~,5melts, (1.3 f 0.5) x 1O-Io cm2/s. This latter agreement is significant by showing that our experimental protocol effectively conserves the air sensitive V+
~
J. Phys. Chem., Vol. 99, No. 45, 1995 16679
Mixed Valent Viologen Molten Salt
TABLE 1: Selected Rate Constants for 1:l Mixed Valent [V2+/+][BF&,sUniform Composition and Concentration-Gradient-ContainingFilms on IDAs frozen concentration gradient no concentration gradient T kEx % dev. 10'2 u kEx % dev. 1012 u ("C) (M-I s-l) p offit (C2-Icm-l) (M-I S - I ) p offit (Q-I cm-I) -20 -30
-50 -70
5.5 8.0 10.8 10.7
3.4 1.05 0.073 0.038
. . ~
9.5 7.4 23 24
120 57 5.8 3.4
0.92 0.41 0.082
0.018
Fit Current Measured
40 pA/cmz
100,000V/ctn
Figure 4. Response of uniform and concentration-gradient-containing 1: 1 mixed valent films on an IDA with finger width 5 pm and gap 5 pm, at -30 "C to a & l o 0 V linear potential sweep. Best kEx fits were 0.84 and 0.42 M-' s-', and best p fits were 6.5 and 9.6 for the uniform and gradient-containing samples, respectively. Responses were taken at sweep rates sufficiently large as to be sweep-rate independent, typically 0.1 - 1 VIS. The cross represents zero current and potential.
state during film casting on the IDA (a less than 1:l mixed valent composition would producelo a substantially smaller DAPP). The bimolecular electron self-exchange rate constant lc~xfor the V2+/+couple is related to D ~ p pby the relation1'
+
(1) DAPP= DPHYS kE,62C16 where Dp~ysis the physical diffusion coefficient of the viologen cation, C is the total V2+/+concentration, and 6 is the electron hop distance. If one assumes that Dp~ysis negligible relative to the rate of electron exchange between V2i and V+, taking 6 = [CNA]-"~ 0.94 nm, gives kEx = 1.5 X lo5 and 8.8 X lo5 M-' s-I from the 25-30 and 50 "C D~ppresults, respectively. Electron-Hopping Rates and Conductivity in Films with No Concentration Gradients. Summarized briefly here are previously describedla-c.f-hrelations that are used to interpret i-E curves for electron hopping driven by an electrical gradient. The electron hopping-based conductivity of a mixed valent redox material at large potential bias can be described with the following relation,
10.7 9.6 11.4 13.1
10 13
64 27
19
7.0
29
1.9
CJGRADIENT~ UUNIFORM
0.53 0.47 1.2 0.56 av = 0.69 k 0.26
PGRADIENTI
PUNIFORM 1.94 1.20 1.06 1.22 1.36
* 0.40
dispersity) and eq 2 when fit to experimental current-potential curves yield the same average rate constants and are thus simply alternative models for representing disperse kinetics. (The parameter p is analogous to the dispersity factor E of the ScherMontroll-F'fister theory.I3) Previous workla-c~f~h has indicated that the product pkEx (rather than REX) is the best representation of the effective self-exchange rate constant in the kinetically disperse reaction; pkEx agrees with rate constants obtained by other approaches not involving consideration of the dispersity. The rhs terms in (2) represent downfield and upfield electron hops; accounting for both is necessary because, although the overall potential gradient AEld is large, the molecular, intersite one AEhld is small. For example, for 6 = 1 nm, and AEld = lo5 Vlcm, the intersite driving force is only 10 mV, or about 40% O f kT2g8. In the limit of low AE (Le., [pFGAE]/[2dRT] F/cm2 and bulk dielectric constant 250, respectively. The nongradient result may more reflect a bulk property; the result for the gradient-containing film (which has a greater nearelectrode resistance), an interfacial property. Up to this point we have not addressed the distribution of potential in the films with frozen gradients, which now becomes important in consideration of the transient results. The simplest case is that for which the potential applied to the frozen gradient is the same as the original or gradient-forming bias. Then, the potential or quasi-Fermi level3 of the concentration gradient is at any interfinger distance x across the gap simply given by the Nemst e q ~ a t i o n , ~
RT v2'(x> E(x) = E" = -lnnF v+(x)
(7)
The frozen V2+ and V+ gradients create a gradient in electrochemical potential in the interfinger gap. The gradient of the electrochemical potential described by eq 7 is not constant, especially when a bias as large as 300 mV has been applied, as was done in Figure 2, lower. The gradient is steeper near the electrode interfaces than in the bulk region. The temperature
16682 J. Phys. Chem., Vol. 99, No. 45, 1995
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Terrill et al.
Circuit
25" c
Oe41
References and Notes
.
I
-30"C j Re-Open Circuit
10
and Masao Morita of NTT for providing the Pt interdigited array electrodes used in this work.
20
30
40
50
time I sec Figure 8. Open circuit potential measurements of gradient-containing samples at 25, -10, and -30 "C.
dependence of the interfacial concentration would be accommodated by a tiny movement of charge as the gradient is frozen, a minor effect. Equation 7 is insufficient when a new potential, different from the original bias, is applied to a frozen-gradient-containing film, and we do not have ready an adequate theoretical statement for this somewhat complex case. Qualitatively, as long as ion mobility in the film's bulk remains negligible, the potential distribution is governed jointly by the pre-existing V2+ and V+ distribution in the film and the spatial distribution of film electron conductivity. The new potential will also be accommodated at the fildelectrode interfaces by space charge injection of holes or electrons to modify the interfacial V2+ and V+ potentials andor by some degree of reorganization of the interfacial counterions lying in the large interfacial electrical gradients. Figure 8 shows open circuit potential measurements on gradient-containing films near room temperature (top) and at -10 and -30 "C. At room temperature when the potential bias is removed, the film's open circuit potential rapidly decays, reflecting the rapid ionic and electronic transport rates in the film. The concentration gradient is not frozen and decays. At lower temperatures, on the other hand, removal of the applied bias yields an open circuit potential that on the time scale shown remains relatively constant, reflecting the much lower ionic and electronic conductivity of the "frozen" material. Also, the measured open circuit potential reflects the most recently applied bias (up to at least 2.2 V), not simply that used originally to prepare the concentration gradient. Eventually, presumably, the space charges that these potentials represent would become selfdischarged through the film's electronic conductivity. If the film is simply shorted for a few seconds and then returned to open circuit, the open circuit potential gradually recovers in the -10 OC case, but barely measurably in the -30 "C case. The capacitance of the electrometer and leads contribute 110-120 pF to the Coulombic capacity of the circuit, a quantity of charge which should decay through the film resistance (< 10" SZ at -10 "C,
