4593
J. Phys. Chem. 1982, 86, 4593-4598
tiating the numerator and denominator with respect to C1. After simplifying and reusing the identity A-9 we obtain
AH(di1) = ([cmc(V,, [cmcV,P
+ Vd)R1 + CNf(n)A"] n
+ CNi(n)l?"] n
[ V,p(Ci
(A-11)
which by definition is equal to (rIJ1 evaluated at Ci. Substitution of this result into (A-7) gives eq 4 of the text.
Appendix B. Derivation of Equation 5: Enthalpy of Dilution for the Sphere-to-Rod Transition We consider an ampule of volume V, containing an initial concentration of detergent Ci (moles per unit volume) dissolved in solution, which is to be diluted into a calorimeter solution of volume Vd whose initial detergent concentration corresponds to the cmc. The final system (after dilution) has a volume V, + V,, and a concen+ Vcd). The tration Cf equal to (cmc + Ci)Vslnp/(VamP measured quantity AH(di1) is defined to be the enthalpy change per micellized molecule resulting from dilution and, in the present theory, is assumed to reflect only the changes in the micelle size distribution brought about by the dilution. Note that the design of the experiment ensures that the number of moles of micellized molecules remains equal to V,,(Ci - cmc) and thus demicellization does not contribute to AH(di1). Accordingly, AH(di1) can be related to the difference in enthalpy content of the entire system (ampule plus calorimeter solution) before and after dilution. Using the same notation for the partial molar enthalpy contributions of the monomer E' and micellar species l?" as in Appendix A, we can express AH(di1) as
- [cmcVdR1])/ - cm41 (B-1)
where Ni(n) and Nf(n) denote the number of moles of micelle n-mers in the initial and final states, respectively. Upon simplifying eq B-1 we obtain C[Nf(n)- Ni(n)Il?" AH(di1) = 03-2) V,,(Ci - cmc) which is a general result showing that AH(di1) only depends on the changes in the micelle size distribution. Again, we may use the model of the sphere-to-rod transition to relate Hn to n (eq A-6) which gives after simplifying -no(RR - P)C [Nf(n)- Ni(n)] nbno
AH(di1) =
V,,(Ci - cmc) where use of the identity C nNf(n) = C nNi(n) = V,,,(Ci
03-3)
-
cmc) (B-4)
n2no
nbno
has been made. Using this identity again in the denominator of eq B-3 leads to the final result C Nf(n) C Ni(n) ntno nbno AH(di1) = -no(RR- E%) C nNf(n) C nNi(n)
(
nbnO
n2no
)
03-5)
which is identical with eq 5 of the text.
Microemulsions as Photogalvanic Cell Fluids. The Surfactant Thlonine-Iron( II)System N. S. Dixit and R. A. Mackay' Depadment of Chemlstv, Drexel Unlverslty, Phlladelphie, Pennsylvanie 19 104 (Recelvd: December 3, 198 1; I n Final Form: JuW 26, 1982)
The current and voltage responses of the totally illuminated thin-layer photogalvanic cell have been investigated in microemulsions, micellar solution, and water. Results indicate that there is a significant enhancement of power output in anionic microemulsion compared to that in cationic microemulsion, micellar solution, and water. The power conversion efficiency of the photocell has been examined as a function of the various experimental parameters such as dye concentration, light intensity, pH, and distance between the electrodes. The highest percent solar engineering efficiency obtained in this study is 0.33 X The optical properties of the newly synthesized dye are also presented.
Introduction The feasibility of utilizing the thionine (ThH+)-FeZf reversible photoredox reaction in photogalvanic cells for converting solar photons into electrical energy has been extensively The fundamental photo(1) E. Rabinowitch, J . Chem. Phys., 8, 551 (1940). (2) A. E. Potter and L. H. Thaller, Solar Energy, 4, 1 (1959).
(3) L. J. Miller, A Feasibility Study of a Thionine Photogalvanic Power Generation System, Final Report, Contract No. AF33 (616)-7911, Sunstrand Aviation ASTIA Document No. 282870, 1962. (4) W. D. K. Clark and J. A. Eckert, Solar Energy, 17, 147 (1975). (5)R. Gomer, Electrochim. Acta, 20, 13 (1975). (6)D. E. Hall, W. D. K. Clark, J. A. Eckert, N. N. Lichtin, and P. D. Wildes, Am. Ceram. SOC.Bull., 56, 408 (1977). (7) R. A. Hann, G. Read, D. R. Rosseinsky and P. Wassell, Nature (London),244, 126 (1973). 0022-365418212086-4593$01.25/0
chemical and electrochemical reactions for such a photogalvanic cell are
-
+ hv 'ThH+* 5 3ThH+* H+ 3ThH+*+ Fez+ [ThHz]+.+ Fe3+ ThH+
2[ThH2]+. e [ThH312++ ThH+
-+ -
anode reaction [ThH3l2+ ThH+ + 2H+ + 2e-
(1)
(2) (3) (4)
cathodic reaction 2Fe3+ 2e2Fe2+ (5) Although, some reportsgconsider that the electrode active (8) M.Z.Hoffman and N. N. Lichtin, "Solar Energy Chemical Conversion and Storage",R. R. Hautala, R. B. King, and C. Kutal,Ed., The Humane Press, New Jersey, pp 153-87, and the references cited therein.
0 1982 American Chemical Society
4594
The Journal of Physical Chemistty, Vol. 86, No. 23, 1982
species is the one-electron-reduced product (the semithionine), a recent reportlo shows that the two-electronreduced thionine (the leucothionine) is formed to the extent of about 95%, which is then reoxidized at the electrode-liquid junction. G ~ m e rand , ~ more recently Albery and Archer,11-14 have developed a general theoretical treatment for a photogalvanic cell, and optimum conditions for the maximum efficiency of the photocell are suggested. Based on the proposals made by these authors, attempts have been made to enhance the efficiency of the cell by varying the experimental conditions. First, by changing the configuration of photogalvanic cell from a convential design1to a so-called totally illuminated thin-layer (TITL) cell.486915-20 The latter cell employs a semitransparent semiconductingelectrode (e.g., n-SnOJ as the anode, which discriminates to some extent between the ThH+/ThH?+ and Fe2+/Fe3+redox couples. The cathode is usually a metal-coated glass plate, and the electrodes are separated by 25-100-pm inert spacers. Photocells with both transparent electrodes have also been developed.21*22Second, additional absorbers which transfer excitation energy to the principal sensitizer have also been covalently bonded to a polymer matrix.23 Third, the selectivity of the n-Sn02 electrode for reduced thionine has been increased by electrodeposition of the thionine on the electrode.24 The TITL cells have gained wider popularity in the field of construction of solar transducers utilizing chemical systems due to their ease of fabrication and better sunlight engineering efficiency (SEE, eq 6) compared to the classical SEE =
Dixit and Mackay
TABLE I : Microemulsion Compositions proportion, w t % -
component
scsa
CTAB~
surfactant a q u e o u s H,SO, mineral oil hexadecane 1-pentanol 1-butanol
12.4 59.6
17.8 60.0
8.8 4.0
19.2 18.2
SCS, s o d i u m cetyl sulfate (anionic). CTAB, cetyltrimethylammonium bromide (cationic). a
vents used in the previous studies; (ii) lack of a mechanism to separate the charge carriers formed in the photochemical reaction, which results in a net low quantum efficiency for the forward reaction; (iii) lack of selectivity of the electrodes to discriminate between the redox couples. Any improvement in either the photochemical (i-ii) or electrochemical determinant is expected to give a better SEE for a photocell. The efficiency of the photochemical determinants may be enhanced by carrying out the reactions in various organized media such as micellar solutions or microemulsions.26yn The electrochemicaldeterminant can be improved either by surface modification or by chemical derivatization of the selective electrode. In the present paper we report our studies on the photoelectrochemical responses of a TITL photogalvanic cell containing the newly synthesized surfactant thionine (CloThH+,I)and the Fe2+/Fe3+couple in microemulsion
electrical power output of the photocell (6) total incident light power
cells. The best SEE achieved so far for a single cell is 0.03% ? This figure is at least 100 times less than the SEE obtained for photovoltaic devices with either solid-solid or solid-liquid junctions.25 The low efficiency of the TITL devices is due to several factors: (i) low concentration of the primary absorber which is limited by its solubility in buffer or in H20:CH3CN(1:l by v/v), which are the sol(9) L. Tenger, "Solar Energy Photochemical Conversion and Storage", S. Claesson and L. Engstrom, Ed., NE Project 6562071, National Swedish Board for Energy Source Development, Stockholm,1977,Chapter 111, and the references cited therein. (10) T. L. Osif, N. N. Lichtin, and M. Z. Hoffman, J. Phys. Chem., 82, 1778 (1978). (11)W. J. Albery and M. D. Archer, Nature (London),270,399 (1977). (12) W. J. Albery and M. D. Archer, J.Electrochem. SOC.,124, 688 (1977). (13) W. J. Albery and M. D. Archer, J. Electroanal. Chem., 86, 1 (1978). (14) W. J. Albery and M. D. Archer, J. Electroanal. Chem., 86, 19 (1978). (15) P. D. Wildes, M. Z. Hoffman, and N. N. Lichtin, "Proceedings of the International Symposium of Solar Energy", J. Berkowitz and L. Lesk, Ed., The Electochemical Society, Princeton, NJ, 1976, p 128. (16) D. E. Hall, J.Electrochem. Soc., 124, 804 (1977). (17) P. D. Wildes, K. T. Brown, M. Z. Hoffman, and N. N. Lichtin, Solar Energy, 19, 579 (1977). (18) P. D. Wildes, D. R. Hobart, N. N. Lichtin, D. E. Hall, and J. A. Eckert, Solar Energy, 19, 657 (1977). (19) D. E. Hall, P. D. Wildes, and N. N. Lichtin, J,Electrochem. Soc., 125, 1365 (1978),. (20) P. D. Wildes and N. N. Lichtin, J. Am. Chem. Soc., 100, 6568 (1978). (21) D. E. Hall, J. A. Eckert, N. N. Lichtin, and P. D. Wildes, J. Electrochem. SOC.,123, 1705 (1976). (22) P. D. Wildes. K. T. Brown, M. Z. Hoffman. D. E. Hall. and N. N. Lichtin, Solar Energy, 19, 579 (1977). (23) K. Shigehara, M. Nishimura and E. Tsuchida, Bull. Chem. SOC. Jpn., 50, 3397 (1977). (24) W. J. Albery, W. R. Brown, F. S. fisher, A. W. Foulds, K. J. Hall, A. R. Hillman, R. G. Edgell, and A. F. Orchard, J.Electrochem. Soc., 107, 37 (1980). (25) A. Heller, B. Miller, and F. A. Thiel, Appl. Phys. Lett., 38, 285 (1980).
c=o I
solvent. The results have been compared with the ThH+/Fe2+/Fe3+system in both water and aqueous micellar solutions. Experimental Section
Thionine hydrochloride (Aldrich) was recrystallized twice from 50% ethanol before use and its purity was checked by visible spectrum (4 = 5.64 X lo4;lit.,285.76 X 104 M-I cm-I). Decanoyl chloride (Aldrich),FeS04.7H20, Fe2(S04)3(Fisher), and sodium dodecyl sulfate (Sigma) were used as received. The composition^^^ of the microemulsion systems used in this study are given in Table I. The phase diagrams for these systems are given in ref 30 and 31. Laboratory deionized distilled water was redistilled from alkaline KMn04 in a Pyrex still. The surfactant derivative of thionine was prepared at room temperature by monoacylation of thionine (0.76 mM) and decanoyl chloride (0.9 mM) in dry acetonitrile (500 mL). It was recrystallized from a H20:C2H50H(1:3 v/v) solvent mixture. Chemical analysis of the compound was (26) S. J. Gregoritch and J. K. Thomas, J. Phys. Chem., 84, 1491 (1980). (27) M. Gratzel, Ber. Bunsenges. Phys. Chem., 84, 981 (1980). (28) E. Rabinowitch and L. R. Epstein, J.Phys. Chem., 63,69 (1941). (29) C. E. Jones, L. E. Weaner, and R. A. Mackay, J. Phys. Chem., 84, 1495 (1980). (30) R. A. Mackay, K. Letts, and C. Jones in 'Micellization, Solubilization and Microemulsion", Vol2, K. L. Mittal Ed., Plenum Press, New York, 1977, pp 801-16. (31) R. A. Mackay and C. Hermansky, J.Phys. Chem., 85, 739 (1981).
Microemulsions as Photogalvanic Cell Fluids
The Journal of Physical Chemistry, Vol. 86, No. 23, 1982 4595
a
I-
z 5.20
w
0 LL
4.16
0 0 \ I
a
*
0
"
b
3.12
I-
i>
Ex
2.08
W
I .04
b
500
600 Wavelength (nm)
700
Figure 2. Visible absorption spectra of CloThH+ (1.92 pM) in 60 % SCS microemulsion (solid line) and water (dotted line). hv
TABLE 11: Visible Absorption Maxima of Surfactant Thionine (1.92 MM)
Figure 1. a: (a) convex lens; (b) Corning cutoff filter below 400 nm; (c) bell jar; (d) SnO, anode; (e) Pt cathode; (f) teflon spacer; (9) photogalvanic solution. b: (h) and (i) Teflon and aluminum rectangular plates; (j) screw heads.
performed by Micro Analysis Inc., Wilmington, DE. Anal. Calcd €or C22H28N30SC1:C, 63.23; H, 6.70; N, 10.06; C1, 8.50. Found: C, 61.80; H, 6.28; N, 9.70; C1, 7.95. Electronic absorption spectra were recorded on a Perkin-Elmer 320 spectrophotometer. The photovoltage and photocurrent of the cell were monitored with a Keithley 1608 digital multimeter, coupled to a Fisher Recordall Series 5000 recorder. A standard resistor (General Radio Co.) was introduced in the circuit for the current measurement. For irradiation of the photocell a parallel beam of light from a Bausch and Lomb tensor lamp, filtered through a 400-nm Corning cutoff filter, was used. The lamp has a three-position switch to vary the intensity of the light, which was monitored by a laser power meter (Scientech,Inc.) having a flat response in the visible region. The photogalvanic action spectra were recorded by an assembly of a 450-W Xe lamp (Schoeffel Instruments Corp.) and Beckmann DU prism dispersion monochromator. The intensity of the monochromatic light was determined by an EG and G lite mike, Model 560B. A Surfass conductivity bridge (RCM 15B1) was used to measure the resistance of the electrode materials and the photocell. The TITL photocell used in the present investigation along with the experimental arrangement for the measurement of cell outputs are schematically shown in Figure 1, a and b. The cell essentially consisted of a n-Sn02 anode (PPG, with R = 22e240 $2 cm-I). The Pt electrodes were made in this laboratory by painting a well-cleaned microscope slide with platinum DNS solution (Johnson Matthey Inc.) and baking it in an electric oven around 300 "C until a Pt mirror is formed. A thin film of silver print
solvent br",anm water 567 (sh), 598 (3.54 X 60%SCS microemulsion 573 (sh), 604 (6.56 x e in brackets (cm' mol-').
lo4) lo4)
(G.C. electronics) was applied at the end of the electrodes for electrical contacts. The electrodes were separated by 50-, 80-, and 100-pm teflon spacers. M FeSThe photogalvanic solution consisted of o4.7H20,4.4 X lo4 M Fe2(S0J3, and ThH+ or CloThH+ of known concentration in the appropriate medium. The solution was previously deaerated by bubbling ultrapure helium (99.9% purity) for about 15 min. The deaerated solution (1-3 drops depending upon the spacer thickness) was placed at the center of the Pt electrode over which the working electrode, along with the spacer, were carefully placed. This was then sandwiched between the teflon and A1 rectangular plates as shown Figure lb. The entire cell assembly was placed in a bell jar through which helium was flushed continuously at the rate of 10 mL s-l. The surface area of the electrode exposed for the illumination was 0.7 cm2. The photoresponses were measured as described above. The total resistance of the cell in dark and under illumination was about 500 Q. The electrodes were cleaned successively with detergent solution, acetone, and distilled water. The electrodes were then dried in helium atmosphere and used immediately. No mechanical force was applied to the surface of the electrodes after drying.
Results and Discussion Optical properties of CloThH+. The visible absorption spectrum of the dye is shown in Figure 2 and the band positions along with the molar extinction coefficients (E) are listed in Table 11. Clearly, the monomeric (T T * ) as well as dimeric and/or the first vibronic bands are red shifted in microemulsions compared to that in water. b o , the E values are greater in microemulsions. The band positions are not affected by the presence of Fe2+or Fe3+ suggestingno ground-stateinteraction between the dye and
-
4596
Dixit and Mackay
The Journal of Physical Chemisty, Vol. 86, No. 23, 1982
TABLE IV : Effect of t h e Microemulsion p H o n SEE o f t h e n-SnO,/C,,ThH+, F e 2 + ,F e 3 + / P tPhotocella
TABLE 111: Power O u t p u t of t h e n-SnO,/Pt TITL Photocella Containing 1 0 m M F e z + ,0.44 mM Fe3+, and Dye in Various Media
dY e
mediumb
C,,,ThH+
H,O 6 0 % SCS p E SDS ( 8 4 m M ) 60%CTABpE
ThH
+
H2O 6 0 % SCS M E
-I,, pHb
dye concn, mM
P,c 0.69 3.27
1.40 0.08 0.02
0.10 0.10 0.11
1.62
*
1 = 80 pm. pE denotes microemulstion. Prepared in pH 3.0 aqueous H,SO,. P = V, (in V) x I , ( A ) ; measured a t I , = 4 1 m W / c m 2 .
redox couple. Since CloThH+is amphiphillic, its aggregation behavior in water and in microemulsions were examined. A plot of €598 and 6567 vs. concentration in water show that the former decreases and the latter increases as the concentration of the dye increases. This kind of spectral behavior is characteristic of dyes undergoing aggregation in water.28 Surprisingly, in 60% SCS microemulsions both and c573 remain practically constant up to 0.1 mM. This is a strong indication that the dye molecule is completely solubilized in the microemulsion droplet, presumeably with the polar aromatic head group being located in the vicinity of the Stern layer. However, at higher concentration (greater than ca. 1.2 mM), aggregation of CloThH+to some extent is evident even in microemulsions. Photoelectrochemical Responses. The experimental arrangement for the measurement of open circuit photovoltage (V,) and photocurrent (I,) of the Sn02/Pt TITL photocell used in this work, along with the photoassisted electron-transfer cyclic reaction, is shown in Figure 1, a and b. In all experiments the galvanic solution contained 0.01 M Fez+,0.44 mM Fe3+,and a known dye concentration. The current was measured with an external load resistance of 2 ki?, so that the total resistance was 2.5 ki?. The power developed by the TITL cell containing Fez+, Fe3+,and dye in different media is listed in Table 111. A cell solution with either iron ions or the dye alone produced no photocurrent. Both Vp and I, were negative indicating the direction of flow of current is n-Sn02 (anode) Pt (cathode). Two points are immediately evident from the data in Table 111. First, the surfactant thionine is more effective than thionine. Second, anionic aggregates are more effective than cationic aggregates. For the anionic system, a microemulsion is more effective than normal micelles. It is possible that the greater effectiveness of CloThH+is due to an increase in Sn02electrode selectivity by adsorption of the dye. Two Sn02plates were immersed in ThH+ and CloThH+solution for 15 min, washed with water, and then extracted with acetone. A small amount of CloThH+, but no ThH+, was detected in the extract. The higher efficiency in anionic systems may also be due to the fact that Fez+is brought into closer proximity to the dye, and vice-versa in the cationic system. However, the reason why the back-reaction is apparently not also facilitated is not clear. Since the efficiency was higher in 60% SCS microemulsions, the photogalvanic responses were examined systematically in this medium to determine the effect of the operating parameters on the cell efficiency. Effect of p H . Although the redox potentials of CloThH+ have not been determined, they are expected to be similar to those of ThH+, which are pH dependent. Further, pH may influence significantly the nature of the electrodeactive species, their kinetics, and intrinsic lifetimes.
-
% SEEC
V,, m V
pA/cm2
x 10'
31 48 27 20 19 20
1.70 2.80 1.42 1.15 1.06 1.06
0.12
2.0 3.0 4.0 5.0 6.0 7.0
nW/cm2
0.13 0.11 0.12
-
0.33 0.09 0.05 0.04 0.04
a I = 80 p m ; I , = 4 1 m W / c m 2 ; [ C , , T h H + ] is ca. 2 mM in all the experiments. Refers t o t h e p H of t h e aqueous HISO,. T h e actual p H of the microemulsion may be different from this value. % S E E = [ VJ,,/I,] x 100. For t h e true SEE, t h e short circuit current should be used in place of I,.
I O
cc
I
w
E
08
a
' Lz
06
n w
N
2 z
04
K
0
z
02
480
520
560
600
640
Wavelength (nm)
Figure 3. Normalized spectra (1) absorption spectrum of &ThH+ (1.92 pM) In water. (2) Absorption spectrum of the same dye (2 mM) in 60% SCS microemulsion. (3) Current action spectrum at pH 3.0. (4) Current quantum yield spectrum at pH 3.0.
Therefore, the pH effects on SEE were examined. Table IV lists V , and Ipand SEE as a function of pH. The efficiency is a maximum at pH 3.0 and becomes constant in the pH range 5-7. Both V , and I , behave similarly. A similar study of ThH+ exhibited a maximum at pH 3.5.8 If we assume that [CloThH2+]is the main anodically active species, then the Nernst equation for the anode reaction is given by eq 7. This equation predicts that V , should
increase with increasing pH. This is observed in the present case only below pH 3, but V , remains constant in the pH range 5-7. This may be due in part to the shifting of the flat-band potential of n-Sn02, since it is pH dependent.32 The pH may also affect some of the kinetic parameters. A photocurrent action spectrum, normalized current quantum yield spectrum, and absorption spectra at two concentrations are shown in Figure 3. It is clear that the current action spectrum matches the absorption spectrum of the dye in the monomeric form. The broadened action spectrum is due to the use of a large bandpass to increase (32) F. Mollers, and R. Memming, Ber. Bunsenges. Phys. Chem., 76, 469 (1972). (33) W. J. Albery, P. N. Bartlett, J. P. Davies, A. W. Foulds, A. R. Hillman, and F. S. Bachiller, Faraday Discuss., 70, 341 (1980).
The Journal of Physical Chemistry, Vol. 86, No.
Microemulsions as Photogalvanic Cell Fluids
TABLE V : Dependence of V and I , on the Distance between the Electrodes ( I ) of t%en-SnOJPt Photocell" 1, Mm -VD, m V --ID,MA/cm2
50 80 100 a
18
0.28
48 24
2.80 0.42
23, 1982 4597
3
50
I o = 4 1 m W / c m 2 ;[ C , , T h H + ]= 1.97 mM.
40
2
,-..
N
the photocurrent. this suggests that the photoelectrochemical outputs of the cell are essentially due to the absorption of the photons by the monomeric dye form only. The highest current quantum yield (electrons/ photon) is estimated to be about 2.7% at 600 nm. The V, action spectrum is similar to the current action spectrum. Effect of Light Intensity and Dye Concentration. The effect of light intensity (Io)on I , and V at a fixed dye concentration is presented in Figure 4. T!ie photocurrent of the cell varies linearly with Io, but V, varies as log I,. Our observed dependence of I, and V , on Io is predicted by the theoretical treatment of Albery and Archer14for the operating conditions employed here, which is the "BeerLambert" case with an absorbance on the order of unity and a life space for the reduced thionine of about 10% of the cell spacing. In the notation of Albery and Archer, 0 1 and K > 10. If leucothionine is the only currentproducing species at the anode, and neglecting intermediate steps, a slope of 30 mV is expected for the plot of V , vs. log I,. The observed slope is 41 mV. Both I, and V , vary linearly with dye concentration as shown in Figure 5. The above treatment also predicts that I, is directly proportional to concentration, but that V , is independent of concentration under these conditions. Also, over the range of cell path lengths 50 < 1 < 100 pm, little variation in output is expected. However, a significant variation is observed (Table V), with a maximum SEE at about 80 pm. Finally, we would like to compare the experimental results of this investigation with those obtained by other workers employing Sn02/Pt TITL photocells. While attempting to make such a comparison sufficient care should be exercised since the experimental conditions vary from group to group. Further, owing to the complexity of the system it is extremely difficult to transform all the results to a common scale. However, in Table VI we have listed two other studies for which we could compute the SEE from the published data. The SEE values for our cells are less than those of Clark and Eckert6 and are comparable to those of Shigehara et al.23 It is significant to note that the current density of the CloThH+-containingphotocell is 7-40 times higher than the photocell using polymerized thionine.
E
\
-
