the thionine-coated electrode for photogalvanic cells

654 J. EIectrochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY March 1980

Lett., 28, 95 (1976). 11. T. C. Arnoldussen, Paper 199 presented at The

Electrochemical Society Meeting, Las Vegas, Nev- ada, October 17-22, 1976.

12. E. Gileadi, E. Kirowa-Eisner, and J. Penciner, "Interfacial Electrochemistry," p. 83, Addison- Wesley, Reading, Mass. (1975).

13. S. W. Feldberg, in "Electroanalytical Chemistry," Vol. 3, A. J. Bard, Editor, Chap. 4, Marcel Dek- ker, Inc., New York (1965).

14. S. W. Feldberg, in "Computers in Chemistry and Instrumentation," Vol. 2, J. S. Mattson, H. B.

Mark, Jr., and H. C. MacDonald, Jr., Editors, Chap. 7, Marcel Dekker, New York (1972).

15. A. J. Bard and L. R. Faulkner, "Electrochemical Methods," Appendix B, Wiley, New York (1980).

16. I. B. Goldberg, A. J. Bard, and S. W. Feldberg, J. Phys. Chem., 76, 2250 (1972).

17. R. S. Crandall, P. J. Wajtoneicz, and B. W. Faugh- nan, Solid State Commun., 18, 1409 (1976).

18. B. Reichman and A. J. Bard, This Journal, In press. 19. A. T. Hubbard and F. C. Anson, in "E]ectro-

analytical Chemistry," Vol. 4, A. J. Bard, Editor, Chap. 2, Marcel Dekker, Inc., New York (1966).

The Thionine-Coated Electrode for Photogalvanic Cells W. John Albery, Andrew W. Foulds, Keith J. Hall, and A. Robert Hillman Department of Chemistry, Imperial College, London SW7 2AY, England

ABSTRACT

The successful operation of a photogalvanic cell for solar energy conversion requires that the illuminated electrode should discriminate between the two redox couples in solution. In the case of the iron-thionine system the electrode must oxidize photogenerated leucothionine but not reduce the photogenerated Fe(III) . Modified electrodes with coatings of thionine of up to 20 monolayers can be prepared on Pt and SnO~. These electrodes have been investigated using ring disk, cyclic voltammetry, XPES, and spectroelectrochemical measurements. Results for the modified electrode kinetics are presented for the following systems: thionine, disulfonated thionine, Fe (I ~D, Fe (CN) ~4-, Ru (bpy) ~3 +, Ce(IV), quinone, and N,N,N',N'-tetramethyl-p-phenylenediamine. The results for the Fe (III) and thionine systems show that this modified electrode is suit- able for the iron-thionine photogalvanic cell.

A typical photogalvanic cell for solar energy conversion is shown in Fig. 1. The iron-thionine system for such a cell works according to the following reaction scheme (1-3)

hv T h > Th*

H+ Th* + Fe(II) > S' Jr Fe(III)

H + S'+S" ,)Th+L

Illuminated electrode L

Dark electrode

2Fe ( I I I ) + 2e

where Th is

�9 ) T h + 2 e + 3 H +

) 2Fe (II)

H 2 N ~ NI-I~. L is

H

I-I3N + ~ ~ NH3 +

and S' is the semithionine radical. In order to obtain power from the cell it is essential

that the illuminated electrode should discriminate between the photogenerated leucothionine (L) and Fe(III) (4). If the electrode does not so discriminate, then addition of the electrode reactions in the reaction scheme shows that the electrode merely catalyzes the back-reaction of photogenerated products into the original reactants

Key words: photogalvanic cells, modified electrodes, thionine.

L + 2Fe (III) -> Th + 2Fe(H)

The illuminated electrode must remove one of the photogenerated products, in this case L, and force the other, Fe(III) , to diffuse across the cell and react on

ILLUMINATED/4 ELECTRODE

I

I

m

m

E

m

__--___1 '~'NDARK ELECTRODE

t PHOTO GA LVA N IC SOLUTION

Fig. 1. Typical photogalvanic cell ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.130.252.222Downloaded on 2014-07-10 to IP

http://ecsdl.org/site/terms_use

Vol. 127, No. 3 PHOTOGALVANIC CELLSL

the dark electrode. It should be noted that for an effi- cient photogalvanic cell the electrode kinetics of the two electrodes must be different (5). If the Fe( I I I ) reaction is blocked on the dark electrode as well as the i l luminated electrode then the cell operates only as a concentrat ion cell and produces insufficient voltage of ~ R T / F (6). In this paper we describe how to modify the i l luminated electrode by coating it with thionine itself. The modified electrode then discriminates between the photogenerated L and Fe (III) . A photogalvanic cell with a modified and an unmodified SnO2 electrode will then possess the necessary differential electrode kinetics (7).

Theory The kinetics of the electrode processes were mea-

sured on the rotat ing disk electrode. For the reaction

0 + ne<-~-R

we can derive the general equat ion

l n { i O �9 )2/3 _ _ , +

e~o E• = y = In ~--~-o' /

a(E -- E')F [13

RT

where io and iR are the l imit ing currents for the reduction of O and oxidation of R, respectively, E' is the formal potential for the O, R couple in the par t icular medium, kD,o' is the mass t ransfer rate constant for O, and ko' is the s tandard electrochemical rate constant for the O, R couple at E _-- E'.

At a rotat ing disk electrode (11, 12)

kD,O' = 1.55Do~/3~-l/sW 1/~ [2]

io -- nAFkD,o'[O] [3]

v is the kinematic viscosity and W is the rotat ion speed in Hz~

For the i r reversible reduction of O the current voltage curve is observed when (E -- E') is fair ly negative and hence the exponent ial term in [1] may be neglected leading to the usual Tafel equation corrected for mass t ranspor t

In this work we observe l imiting currents that are not ent i re ly t ransport controlled. There is a two-step reaction at the electrode where the rate constant, k~', of one of the steps does not depend on potential. For such a scheme with two consecutive steps we re- placed kD,o' in [1] or [3] with"

kD,? ' - 1 : kD,O ' - 1 -~- k?'--I [~] where kD,?' will be determined by the smaller of the two rate constants kD,o' and k~.'. At reducing potentials we then obtain from [3] and [2] the usual Koutecky Levich equation (13) for the l imit ing cur ren t

nAF[O] 1 1 0.65Do-2/3u 1Is 1

- - k ' + k ? ' = W ~ / ~ b ~ [6] io D,O

Experimental All the electrochemical exper iments were carried

out at 25~ and all potentials reported with respect to the saturated calomel electrode. Rotating disk and r ing disk studies were performed using an Oxford Electrodes motor controller, rotat ing assembly, and control electronics. Ant imony-doped SnO2 electrodes were prepared by spraying a solution of 10 ml SnC14 and 0.6 ml SbC15 added to 20 ml ethyl acetate for 3 sec

655

onto the end of a quartz rod heated to 1000~ and rota t ing at 5 Hz. Spectra were recorded on a Unicam SP 600 recording spectrophotometer. XPES spectra were measured on an AEI ES 200 B photoelectron spectrometer using Mg K~ exciting radiation; the base pressure was ,-~ 10 -9 Torr. The conductance of the thionine coating was measured in the d-c mode after vapor deposition of an Au contact.

Results and Discussion Preparation and properties of modified electrodes.--

Both Pt and SnO2 electrodes can be coated with a stable layer of thionine by holding the electrode at 1.1-L5V for a period of several minutes in a solution containing thionine. Figure 2 shows a r ing disk exper iment which monitors the consumption of thionine dur ing the coating process. From the decrease in the t ranspor t l imited current for thionine at the r ing electrode we can calculate from N - l f ~ i R d t the amount of thionine con- sumed on the disk where N is the collection efficiency (8, 9). When the modified electrode is removed from the thionine solution, washed and placed in background electrolyte (e.g., 50 mmole H2SO4) containing no t~o- nine, cyclic vol tammograms as shown in Fig. 3 are

1.1 ED/V.4

0.1

iu/mA 0.0

11 -iR/)JA

10

0 I I 2 L

t/min Fig. 2. Ring disk study of coating process; ED, disk potential;

iD, disk current; iR ring current. Amount of thionine coated an disk found from N -1 f AiR dt.

0.25 1

i/mA /

I 0 - 1 0 0 ~

-0.25 _

. i / j J

EImV(SCE)

Fig. 3. Cyclic voltammogram of thionlne-coated Pt electrode in 0.05M H2S04 containing no thionine.

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.130.252.222Downloaded on 2014-07-10 to IP


656 J. EZectrochem. Soc.: ELECTROCHEMICAL SCIENCE AND T E C H N O L O G Y March 1980

obtained. The peak height of these vol tammograms varies l inear ly with sweep rate as expected for surface bound species. From the charge passed per cycle we can also calculate the amount of thionine in the layer. The coverages found from cyclic vol tammetry agree with those from the r ing disk exper iment but are usual ly about 20% lower. Coatings of up to 20 monolayers can be obtained and these coatings are stable in background electrolyte for at least several months. XPES studies, see Fig. 4, of a coated electrode show identical peaks to those of thionine and the Pt 4f peak is complely absent (10).

The cyclic vol tammograms before and after the XPES exper iment show that exposure to high vacuum for several hours only reduces the coverage by about 10%.

Cls

Compared to thionine in solution the visible spec- t rum of a coated SnO2 electrode on a quartz plate shows a broadened peak at much the same wavelength (~.max/nm "~ 600) (10). At reducing potentials this peak disappears. As shown in Fig. 5 the proportion of thionine, f, and leucothionine in the layer obeys the Nernst equation

n F -- c [7]

We find n = 2 and Ec'/mV = 180. This value is s imilar to that of thionine in our solution ( E ' / m V ---- 208). The resistance of 1 cm 2 of a layer coated at 1.1V containing 6 nmole cm -2 was found to be 6012. From these results we conclude firstly that the layer consists of thionine or leucothionine depending on the electrode potential. Secondly, the layer is remarkab ly stable and coherent; it cannot be washed off and the disappearance of the Pt signal in the XPE spectrum is par t icular ly note- worthy. Thirdly, for cur ren t densities less than 150 ~A cm -2 the voltage drop across the layer is less than 10 mV and can be neglected.

Thion ine . - -We now examine the electrode kinetics of various redox couples on the thionine-coated electrode star t ing with thionine itself. Figure 6 shows typical current voltage curves for the reduction of thionine on a clean and a thionine-coated Pt electrode. On the clean electrode the system is reversible. On the coated electrode the system is near ly reversible. Figure 7 shows the current voltage data plotted according to [1]. Kinetic parameters for different electrodes are collected in Table I. We conclude firstly that on the coated electrode there is little difference between Pt and SnO2 as the substrate surface. Secondly the electrochemical rate constant is decreased for electrodes coated at higher voltages and for longer times. In par - t icular there is a significant reduct ion in rate constant for electrodes coated at 1.4V or above for periods longer than one hour. It should be noted from Fig. 2 that after the ini t ial coating process lasting two ra in-

~ ls Nls Pt5d I Pt4f

I I I I I 600 1000

Fig. 4. Typical XPE spectra. 1he top trace shows a Pt electrode that has been exposed to thionine solution at 0.4V. The principal feature is the Pt 4f signal. The second and third traces show Pt electrodes coated at 1.0 and 1.1V, respectively. The Pt signal be- comes progressively suppressed; C, N, and S peaks corresponding to thionine are found, x-axis, KE/eV.

/o///o r I I

100 150 200 250

Fig. 5. Plot of Nernst Eq. [7] describing the bleaching of the thionine layer, x-axis, E/mV(SCE); y-axis, In(f/1 - - f).



Vo[ 127, No. 3

25

2o . . . . . . ; /

/I 16 / / /

/ / Th 10 / ' I / Fei'rrr

/ i , , .................. ioo

/ l/y,. / ....'" " /

600 500 400 300 200 100

Fig. 6. Typical current voltage curves for thionine (Th) and Fe(lll) on an uncoated ( - - - and . . . . ) and thionine-coated Pt electrode ( and . . . . ). x-axis, E/mV(SCE); left-hand y-axis, ITh/~A; right-hand y-axis, iFean)/#A.

PHOTOGALVANIC CELLS

OF

6 5 7

utes, the r ing electrode does not detect any fur ther consumption of thionine on the disk electrode. From cyclic vo l tammetry (see Fig. 3) the amount of adsorbed thionine does not change significantly after the ini t ia l coating process.

The value of ~ ~ 1.0 is in teres t ing It suggests that the ra te -de te rmin ing step in the reduct ion of thionine involves a react ion of a semithionine species which is in equi l ibr ium with the thionine. On a clean P t electrode as the pH is increased the value of ko' decreases. P re l imina ry results are consistent with the following scheme of squares (14, 15) mechanism

T h + H* ~t

e HTh2+ ~ .HS .+

H + $ R . D . S . e

H28.2+ -~ H~L +

HaL~+

In this scheme we have included the added protons explicitly. We have estimated the pK's of the t ransi- t ion states f rom the pK's of the thionine, semithionine, and leucothionine species (16-20). In order to explain the var iat ion of ko' with coating conditions (see Table I) the thionine species must be adsorbed on the electrode.

Disul]onated thionine.--Photogalvanic cells have so far been extremely inefficient for the conversion of solar energy (21) because thionine is not sufficiently soluble to absorb the solar radiat ion close enough to the i l luminated electrode (3). We have recently syn- thesized disulfonated thionine. The solubil i ty is in - creased so that solutions ~ 10-2M can be prepared and fur thermore from the visible spectrum there is no evidence for dimer formation. The disulfonated thionine cannot be coated onto the electrode in the same way as thionine. However the electrode kinetics of disulfonated thionine are very similar to those of thionine. Results are given in Table II.

Table I. Kinetic parameters for the reduction of thionine

Coating Coating Elec- voltage, time, ko', trode V hr a cm ksec -1

P t C l e a n - - - - > 3 0 SnO~ C l e a n - - 0.8 9.0 P t 1.1 ~4 0.9 9.3 SnO~ 1.t ~/4 1.2 9.0 P t 1.4 V4 1.0 2.8 Sn02 1.4 V4 0.9 6.1 P t 1.5 V4 1,2 2.9 Sn02 1.5 1/, 0.9 5.1 SnOz 1.1 1 0.9 5.5 SnO~ 1.4 1 0.9 0.86 S n 0 2 1.5 1 0.9 0.66 SnO~ 1.1 2 1.0 3.6 SnOs 1.5 3 1.0 0.70

3[

-Z,

-51 , , , 100 150 200 250

Fig. 7. Tafel plot from Eq. [1] for reduction of thionine at a thionine-coated Pt electrode, x-axis, EE/mV(SCE); y-axis, y.

Quinone.--Results for the reduct ion of quinone a r e given in Table III. In these experiments a Pt electrode was used throughout and the coating t ime was //'4 hr. The reduct ion was carried out in 0.1M H 2 S O 4 . It c a n be seen that the thionine coating has practically n o effect on the electrode kinetics of the quinone reduction.

N,N,N ' ,N ' - t e t ramethy l - p - pheny lened iamine . - -The normal reaction scheme ~or the oxidation of N,N,N',N'- t e t r amethy l -p -pheny lened iamine is as follows

NMe2 NMe2 NMe2

NMe2 NMe~ NMe~

Wurster ' s Blue +

As shown in Fig. 8 at pH 7 on a clean electrode t w o separate waves corresponding to the two steps in t h e

Table II. Kinetic parameters for disulfonated thionine

Coating Coating Elec- voltage, time, ko', trode V hr a cm ksec-~

P t - - - - 0.8 6.2 P t 1.1 Y, 0.8 2.7 P t 1.5 V, 0.9 1.5



658 J. Electrochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY March 1980

Table Ill. Kinetic parameters for reduction of quinone

Coating v o l t a g e , V ~z ko', c m k s e c 4

Clean 0.6 0.18 1.1 0.6 0.24 1.4 0.6 0.20 1.5 0.7 0.15

1.5

reaction are observed. On an electrode coated with thionine at 1.1V for Ya hr while the rate constant for the first wave is somewhat reduced, the second elec- 1.0 tron transfer is completely blocked. At present we c a n

offer no explanat ion for this complete blocking. I t is t rue that Wurster 's blue is positively charged but then the positively charged thionine reacts rapidly as shown in Fig. 6.

Apar t from the complication of Wurster 's blue and the small variat ions in Table I, we find that for these organic species the thionine coating does not produce a drastic change in the electrode kinetics. This is to be contrasted with the inorganic systems that we now consider. 0.5

Fe (IID.--Typical current voltage curves for the re - duction of Fe ( i t I ) are shown in Fig. 6. It can be seen at once that the coating produces firstly a shift of some 200 mV in the hal f -wave potential and secondly that the l imit ing cur ren t of the shiZted wave is very much less than that on the unt rea ted electrode. The var ia t ion of the l imit ing currents of the shifted waves with rota- tion speed are plotted in Fig. 9 according to tim Kou- tecky Levich equat ion Eq. [6]. From the intercept, which corresponds to infinite rotat ion SliCed, we can find values of k~.:'. For the rising par t of the current voltage 0.0 curve we can use the simple i rreversible Eq. [4] with io being the observed l imit ing current. A typical plot is shown in Fig. 10. Results for the Fe ( I I I ) system are collected in Table IV. Instead of extrapolat ing the results for ko' for the coated electrodes a long way to the E' for the Fe ( I I I ) / F e (II) couple, we have reported the results at E' for tnionine.

The first point to note is that for this reaction a is close to 0.5. This suggests that the potent ial dependent rate constant is still describing an electron transfer, even though the rate is much slower on a coated electrode compared to a clean electrode. The conductivi ty of the thionine film means that there cannot be large potential differences wi th in the film and hence the

3 0 0 -

200

Elec- t r o d e ma te -

r ia l

/ P t / / SnO=

P t Sn02 SnO~ Sn02 P t

~ " ~ ~ " [ [ [ P t 200 400 600 sno,

Pt SnO~ SnO~

100 -

0 0

Fig. 8. Current voltage curves for oxidation of N,N,N' ,N' - te t ra- methyl-p-pheny[enediamine on a clean ( ) and coated (- - -) Pt electrode, x-axis, E/mV(SCE); y-axis, i/,~A.

A

/ /

J /

I I 0.0 0.5 1.0

Fig. 9. goutecky Levich p l o t , Eq. [6], for reduction of Fe(lll) on a coated Pt electrode, x-axis, (W/Hz)-I/2; y-axis, (iL/mA) -1.

main potential drop occurs at the th ion ine /water in- terface. The second point to note is that, like thionine, al though Fe( I I I ) has different electrode kinetics on clean Pt and clean SnO,~, there is not much difference once the electrodes are coated. Thirdly, the existence of k,.' means that there must be a second step in the mechanism, the rate constant of which is not great ly affected by potential. These results will be fur ther discussed below.

Fe (CN)64---Resul ts for the oxidation of Fe (CN) 64- in 0.4M K2SO4 at pH 7 are similar to those for Fe (III) . Typical results are shown in Fig. 11. On a clean Pt electrode as expected the couple is reversible but on a coated electrode the wave is i rreversible and again there is a second step. For an electrode coated at 1.1V for ~/4 hr we find ko'/Cm ksec -1 ~ 0.6 and k~'/cm ksec -1 ___ 2.2. It is very str iking how the thionine coat has blocked this normal ly rapid electron transfer.

Table IV. Kinetic parameters for the reduction of Fe(lll)

Coating Coating ko'(~), k~', vol tage , t ime , Eli=, c m c m

V h r V a ksec -1 k s e c -~

Clean - - 0.42 0.4 100 Clean ~ 0.28 0.5 15.2

1.1 1/4 0.23 0.4 3.5 9.4 1.1 1/4 0.23 0.5 5.0 14 1.1 1 0.20 0.6 1.4 3.6 1.1 2 0.19 0.6 0.79 2.1 1.3 V4 0.22 0.4 2.0 6.3 1.4 ~/4 0.22 0.4 2.1 3.1 1.4 V4 0.23 0.5 3.7 9.5 1.5 1/4 0.17 0.4 0.88 2.8 1.5 1~ 0.20 0.5 2.0 6.2 1.5 3 0.11 0.3 0.072 0.30

<a> Value of k ' a t E ' f o r th ion ine w h e r e E ' / V = 0208.



PHOTOGALVANIC CELLS

-I

-2

V o i 127, No. 3

4 / 659

I I I f 100 200 300 ~00 500

Fig. 10. Typical Tafel plot, Eq. [4 ] , for reduction of Fe(lll) on a coated Pt electrode. Label x-axis, E/rnV(SCE); y-axis, y.

Z.O0

300

200

lOO

0 I 0 500

///

/ /

~ / I I I 190 200 300 400

Fig. 11. Typlcal current voltage curves for oxidation of Fe(CN)84- on a clean ( ) and a coated (- - -) Pt electrode. Label x-axis, E/mV(SCE); y-axis, i/#A.

Ru(bpy)33+.~Even more dramatic are the results for the reduct ion of Ru(bpy)~ 3+ in 0.5M H2SO4 where bpy is 2,2' bipyridine. Results on a coated electrode are shown in Fig. 12. On a clean electrode Ru(bpy)z 8+ is reduced at potentials less than 1.0V, in a reversible wave. The thermodynamics for the reduct ion are fav- orable and the kinetics are rapid. However on the coated electrode the t ranspor t l imited cur ren t is not

200 400 600 800 1000 0 i I ~ i i /

/ I

! !

20 / /

! /

/ hO --- I/

Fig. 12. Current voltage curves for reductloa of Ru(bpy)3 3+ on a clean ( .... ) and a coated (0) Pt electrode. The solid line is the calculated curve for the two parallel processes discussed in the text. Label x-axis, E/mV(SCE); y-axis, iliA.

reached unt i l 0.30V. Impect ion of the current voltage curve shows that there are two paral le l processes. For potentials greater than 0.40V we find a small cur ren t which increases gradual ly as the potential decreases. Figure 13 shows a Tafel plot of this cur ren t according to Eq. [4]. A reasonable straight l ine is obtained wi th a low value of a ---- 0.05. At 0.4V a second process takes over and the current rises rapidly to its t ranspor t l imited value. To allow for the two paral lel processes we use a modified form of [4]

( i o - - / ~ o - - ~ ' l ) l n k v , ~ in y = in ~ -- i, 7o = k~'

(E - - ERu')F + = [8]

RT

where ii is the current due to the process plotted in Fig. 13 at E __~ 0.4V. When ii -- 0, [8] reduced to [4]. The data for the steep part of the wave are plotted in Fig. 14. A reasonable straight l ine is found with a close to 2. This high value of a cannot be caused by driving the electron t ransfer for the reduct ion of Ru( I I I ) . For that reaction we would expect [as was found for Fe ( I I I ) ] that = _~ 1~, and in any case ~ mus t be less than unity. Re turn ing to the data in Fig. 5 where n = 2 we propose that the second process is the reaction of Ru ( t I I ) wi th L formed on the outside of the layer

3II-~+Th+2e~ L f l--f

L + Ru(bpy)3 ~+ "-> S" 4- Ru(bpy)82+ -l" 2H + 1 - - f

The very small fraction of L(,-, 2 X 10 -3) present when the second process takes over can be estimated from the Nernst Eq. [7] and the value of Ec' for 0.5M H2SO4 where Ec'/V -- 0.270. Combining [7] and [8] we obta in

k2'exp [ - - 2 ( E - - E R u ' ) F ] ~E = k'(1 - ~)

[ --2(E--Eo')F ] _~ k' exp RT

1.5

10

05

j D / j _ o /

/ / 6 t

/ o / /

co

0 i I i os 0~6 017

Fig. 13. Tafel plot [4] for points in Fig. 12 for E/mV > 400. Label x-axis, E/V(SCE); y:axis, y.



660 J. Electrochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY March 1980

3

[3

I I ~ q \

0.32 0.3Z, 0.3 5 0.38 0./,0

Fig. 14. Plot of Eq. [8] for steep part of current voltage curve for reduction of Ru(bpy)33+ shown in Fig. 12. Label x-axis, E~ V(SCE); y-axis, y.

From the data for k2' in Fig. 13 we find

k ' /cm sec -1 = 9 [9]

It is interest ing that this fast rate constant can be measured in the steady state by the di lut ion of the active species on the surface of the electrode; Fig. 5 shows that the oxidation state of the thionine layer obeys the Nernst equation.

We have also measured the homogeneous rate constant, k2, for the reaction between Ru(bpy)33+ and leucothionine in 0.5M H2804 using stopped flow. We find

k2/M -1 sec -1 ---- 1 X l0 s

This rate constant and the heterogeneous rate constant in [9] are each some two to three orders of magni tude below the respective encounter rates (22) for neu t ra l species. This may be because of the coulombic in te r - action between Ru(bpy)a 8+ and the doubly charged leucothionine, or there may be a small free energy barr ie r of much the same size in both processes.

F ina l ly we r e tu rn to Fig. 12 and calculate the current voltage curve using the parameters derived from the analysis in Fig. 13 and 14. A good fit is found. The fact that in this t rea tment we use a = 2 confirms the conclusion from the conductance measurements the IR losses in the coat are negligible.

While our model for the second process fits the data very well, we do not as yet have a detailed model for the slow process occurring at potentials greater than 0.4V. The very low value of a means that the process i s not a s traightforward electron transfer. It may be that this process is similar to the k~,step in the reduction of Fe( I I I ) . The potential range over which we c a n observe the k~'process for Fe( I I I ) is too restricted to be certain if k,.' depends slighly on potential or not.

10[

150

200 &O0 600 800 1000 1200

/ I f I I I ! I l l l 1 l I I I

/

Fig. 15. Current voltage curves for reduction of Ce(IV) on a clean ( ) and a coated ( . . . . ) Pt electrode. Label x-axis, E/rnV(SCE); y-axis, i/#A.

Ce(IV).--The reduction of Ce(IV) resembles that of Ru(bpy)3 s+. A typical current voltage curve is shown in Fig. 15. Again two processes are evident. Following the same analysis as for Ru (bpy)38+ we find that the steep par t gives a somewhat curved TafeI plot with ~ varying between 1.4 and 2.0. The fact that these values are greater than uni ty suggests that we must again be seeing a reaction between L in the coat and Ce (IV).

EfIects of coating voltages and coat~ng t imes.--Be- cause of our interest in photogalvanic cells we have investigated the thionine and Fe( I I I ) systems in more detail. For these systems we find that there is some reduction in the kinetic parameters the higher the coating voltage and the longer the coating time. This effect can be seen in ko' in Table I and ko' and k~.' in Table IV. In Fig. 16 and ]7 we show that there is a correlation between the different sets of rate constants; there is a par t icular ly good correlation with a gradient of un i ty between ko' and k~' for the Fe( I I I ) system. Since these rate constants describe different processes we assume that the correlations arise through a general effect of the higher voltage or coating t ime on the coat itself. This affects all three processes, the reduction of thionine, the electrochemical step, and the kT' step in the reduct ion of F e ( I I l ) , to much the same ex- tent. The higher coating voltages and coating times may cause this, for instance, by blocking off active sites on the surface.

Inorganic electron transfers.--While as discussed above the coat seems to have no drastic effect on the reduct ion of thionines or of quinone, single electron transfers to inorganic ions which normal ly proceed by outer sphere mechanisms are in all cases blocked by the coat. This is probably because the thickness of the

Iog(k~feimllcm kg 1 )

/

/

/ o" i 2

Iog(k~,Fecm}lcm ks q)

Fig. 16. CorreLation between ko' and k~' from Table IV for reduction of Fe (111) at coated electrodes: Pt, O ; Sn02 i-].



VoL 12"/, No. 3 PHOTOGALVANIC CELLS 661

Iog(k'o.Th / cm ks-l}

-0.5 J

1.[) o

0,5 1.0

log (k %, Fe(~]/crn ks -1 )

0.5

Fig. 17. Correlation between ko' for thionine from Table I and k~.' for Fe(lll) from Table IV at coated electrodes: Pt, � 9 SnO2 [~.

coat prevents the direct tunnelling of electrons from the metal or SnO2 to the solvated inorganic ion. For strong oxidizing agents like Ru (III) and Ce (IV) it appears that the main path for reaction is the oxidation of leucothionine on the outside of the coat. For Fe (III), because there is not such a strong thermodynamic driving force, the reaction between Fe(IlI) and L is 7 orders of magnitude slower than diffusion control. Hence it appears that we have a two-step mechanism of which one step is an electron transfer and we sug- gest that the other is the formation of a "complex" between Fe(III) and leucothionine at the surface. Such complexes have been found in homogeneous reactions between Fe(III) and L (23). It is interesting that the half-wave potential for the reduction of Fe(III) is close to that of E' for thionine suggesting that the conversion of thionine to leucothionine in the coat may well be necessary for the reaction.

Implications $or photogalvanic cells.--Finally we re- turn to the question of the selectivity of these electrodes for use in photogalvanic cells. Inspection of Fig. 6 shows that the thionine-coated electrode does have the required selectivity. The coating leaves the electrode kinetics of the thionine couple sufficiently rapid to handle the photogenerated thionine (3) but it blocks the reduction of Fe(III) . Thus an illuminated electrode working at a potential where it is oxidizing leucothionine will only have a small current caused by the simultaneous reduction of Fe(III) . Hence we conclude that the thionine coated electrode is an ideal illuminated electrode for the iron-thionine photogalvanic cell. The fact that it can be made so easily and that it is relatively stable are added advantages.

Acknowledgments We thank Mr. A. F. Orchard and Dr. R. G. Egdell

for making the XPES measurements reported in this

paper. We thank Dr. R. Whiteside for making the disulfonated thionine. We thank the SRC for equipment grants and for studentships for ARH and AWF. This is a contribution from the Oxford Imperial Energy Group.

Manuscript received Aug. 13, 1979. This was Paper 331 presented at the Boston, Massachusetts, Meeting of the Society, May 6-11, 1979.

Any discussion of this paper will appear in a Dis- cussion Section to be published in the December 1980 JOURNAL. All discussions for the December 1980 Dis- cussion Section should be submitted by Aug. 1, 1980.

REFERENCES

1. N. N. Lichtt~n, "Solar Power and Fuels," J. R. Bolton, Editor, p. 119 et seq., Academic Press, New York (1977).

2. M. I. C. Ferreira and A. Harriman, J. Chem. Soc. Faraday Trans. 1, 73, 1085 (1977).

3. W. J. Albery and A. W. Foulds, J. Photochem., 10, 41 (1979).

4. W. J. Albery and M. D. Archer, This Journal, 124, 688 (1977).

5. W. J. Albery and M. D. Archer, Nature, 270, 399 (1977).

6. W. J. Albery and M. D. Archer, J. ElectroanaL Chem. Interracial Electrochem., 86, 1 (1978).

7. W. J. Albery and M. D. Archer, ibid., 86, 19 (1978). 8. W. J. Albery and S. Bruckenstein, Trans. Faraday

Soc., 62, 1920 (1966). 9. W. J. Albery, ibid., 67, 153 (1971).

10. W. J. Albery, W. R. Bowen, F. S. Fisher, A. W. Foulds, K. J. Hall, A. R. Hillman, R. G. Egdell, and A. F. Orchard, J. ElectroanaL Chem. Inter- 5acial EIectrochem., To be published.

11. W. J. Albery, "Electrode Kinetics," p. 58, Clarendon Press, Oxford (1975).

12. V. G. Levich, "Physicochemical Hydrodynamics," p. 68, Prentice Hall, Englewood Cliffs, N.J. (1962).

13. J. Koutecky and V. G. Levich, Zh. Fiz. Khim., 32, 1565 (1956).

14. J. Jacq, Electrochim. Acta, 12, 1345 (1967). 15. W. J. Albery and M. L. Hitchman, "Ring Disc

Electrodes," p. 38 et seq, Clarendon Press, Ox- ford (1971).

16. W. M. Clark, H. D. Gibbs, and B. Cohen, Public Health Rep., 40, 1160 (1935).

17. S. Granick, L. Michealis, and M. P. Schubert, J. Am. Chem. Soc., 62, 1802 (1940).

18. J. Faure, R. Bonneau, and J. Joussot-Dubien, J. Chim. Phys., 65, 369 (1968).

19. L. F. Epstein, F. Karush, and E. Rabinowitch, J. Opt. Soc. Am., 31, 77 (1941).

20. I. Ferreira, Ph.D. Thesis, London University (1978). 21. D. E. Hall, W. D. K. Clark, J. A. Eckert, N. N.

Lichtin, and P. D. Wildes, Ceram. Bull., 56, 408 (1977).

22. R. A. Marcus, J. Phys. Chem., 67, 853 (1963). 23. T. L. Osif, N. N. Lichtin, and M. Z. Hoffman,, ibid.,

82, 1778 (1978).



the thionine-coated electrode for photogalvanic cells

Documents