Inorganic Compounds with Unusual PropertiesâII - American

24 Sunlight Engineering Efficiency of Thin-Layer

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Iron-Thiazine Photogalvanic Cells Evidence T h a t Surface-Induced Back Reaction Is a K e y Limiting Factor NORMAN N. LICHTIN , PETER D . WILDES, a n d TERRY L. O S I F Department of Chemistry, Boston University, Boston, MA 02215 1

DALE E. HALL E x x o n Research & Engineering C o . , L i n d e n , N J 07036 2

Absorption spectra and redox potentials in acid solution limit sunlight engineering efficiency (S.E.E.) of unsensitized iron -thiazine photogalvanic cells to ~ 2%. The highest S.E.E. value obtained with totally illuminated single thin-layer (TI-TL) iron-thionine cells with SnO anodes and Pt cathodes, .036%, corresponds to V ~ 35% of theoretical limit. Potentials at the selective anode are dominated by the dye-leucodye couple. Potentials at the poorly selective cathode are dominated by the iron couple. I varies linearly with photostationary concentration of leucothionine and, with electrode spacing ≤ 50μm, is not limited by solution lifetime of charge carriers. Inefficient electron transfer at the electrodes is believed to reduce S.E.E. by a factor of ~ 5, possibly because of surface-promoted back reaction on SnO . 2

power

point

sc

2

p h o t o g a l v a n i c c e l l is a system o f m a n y components t h a t m u s t b e c a r e f u l l y m a t c h e d to e a c h other a n d , f o r u s e as a solar t r a n s d u c e r , m u s t b e m a t c h e d to t h e i n s o l a t i o n s p e c t r u m . N o p r a c t i c a l p h o t o g a l v a n i c c e l l has y e t b e e n a c h i e v e d . R a t i o n a l o p t i m i z a t i o n of p h o t o g a l v a n i c c o n A

Senior author. Present address: Paul D. Merica Research Laboratory, The International Nickel Co., Suffern, NY 10901 1

1

0-8412-0429-2/79/33-173-296$05.00/0 © 1979 American Chemical Society

King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

24.

LICHTIN E T A L .

Sunlight Engineering

297

Efficiency

v e r s i o n requires u n d e r s t a n d i n g of a l l aspects of t h e system. T h e s e aspects i n c l u d e p h o t o c h e m i s t r y , e l e c t r o c h e m i s t r y , ground-state s o l u t i o n c h e m i s t r y , d e v i c e d e s i g n , a n d d e v i c e materials. I r o n - t h i a z i n e p h o t o g a l v a n i c cells use t h e p h o t o r e d o x reactions

of

F e ( I I ) w i t h t h i a z i n e dyes, r e p r e s e n t e d f o r t h i o n i n e b y R e a c t i o n s 1, 2, 3, 4, a n d 5, to c o n v e r t p h o t o n energy i n t o c h e m i c a l p o t e n t i a l . T h e s p o n taneous g r o u n d state reactions r e p r e s e n t e d b y R e a c t i o n s 6, 7, 8, a n d 9 also o c c u r i n h o m o g e n e o u s

solution during illumination.

Photogalvanic

a c t i o n results w h e n h o m o g e n e o u s R e a c t i o n s 7, 8, a n d 9 are r e p l a c e d b y a n o d i c o x i d a t i o n of T H Fe(III). TH

4

2 +

4

2 +

and T H 2

c o u p l e d w i t h c a t h o d i c r e d u c t i o n of

+

T h e f r e e energy c h a n g e

for the oxidation of leucothionine,

, to t h i o n i n e , T H , b y F e ( I I I ) i n aqueous s u l f u r i c a c i d at p H = 2 +

corresponds to E

o

r

= 0.28 V ( I ) .

Established Elementary Steps in the Iron-Thionine Reaction in Acid Solution

TW(So)

Photoredox

— -» T i m ) ~ 600 nm

(1)

TH*0Si)-»TH (So)

(2)

+

H

+

T H ( S i ) -> T H * ( T i ) - * T H +

W C T O

^TH (S ) +

2 +

2

2

2 T H •* + H * ? ± T H + T H 4

2

1

+

4

+

+ Fe(II)

2 +

* + Fe(III) T± "Complex"

"Complex" - » T H 2

+

+ 2 H + Fe(II) +

TH -* + Fe(n)->TH 2

+

+ H * + Fe(II)

Thionine A

m a x

(3) (4)

T H s ^ T i ) + F e ( I I ) -> T H -

TH

7

+ H*

0

2

(7 )

601 n m


(5) (6) (7) (8) (9)

298

INORGANIC COMPOUNDS W I T H UNUSUAL PROPERTIES

T h e p i o n e e r i n g w o r k of R a b i n o w i t c h ( 2 )

achieved

II

photogalvanic

t r a n s d u c t i o n b y means of a c e l l i n w h i c h a p l a t i n u m electrode i n contact w i t h i l l u m i n a t e d s o l u t i o n s e r v e d as the a n o d e w h i l e a s i m i l a r

electrode

i n c o n t a c t w i t h s o l u t i o n m a i n t a i n e d i n the d a r k s e r v e d as the

cathode.

W e have been concerned w i t h understanding and o p t i m i z i n g an area d e v i c e , the t o t a l l y i l l u m i n a t e d t h i n - l a y e r

(TI-TL)

first

I n this d e v i c e

described by Clark and Eckert

transduction

has

been

achieved

(3).

b y u s i n g one

photogalvanic

cell,

photogalvanic

transparent

electrode,

u s u a l l y n - t y p e S n 0 , w h i c h is m o r e r e v e r s i b l e to the d y e : r e d u c e d 2

dye

c o u p l e t h a n to the F e ( I I I ) : F e ( I I ) c o u p l e , together w i t h a s e c o n d r e l a t i v e l y nonselective electrode, u s u a l l y either p l a t i n u m o r i n d i u m t i n o x i d e (ITO).

I n s u c h a c e l l , the S n 0

electrode is the anode.

2

In principle, it

w o u l d b e d e s i r a b l e to r e p l a c e t h e nonselective electrode b y one w h i c h is m u c h m o r e r e v e r s i b l e to the F e ( I I I ) : F e ( I I )

c o u p l e t h a n t o the

dye

couple. P r a c t i c a l c o n v e r s i o n efficiency of a solar t r a n s d u c e r is m e a s u r e d t h e s u n l i g h t e n g i n e e r i n g efficiency

by

(S.E.E.):

100 X e l e c t r i c a l energy or power d e l i v e r e d to l o a d at the p o w e r p o i n t

g ^ g

i n c i d e n t s u n l i g h t energy or power T h e best S . E . E . values that w e h a v e o b t a i n e d w i t h the single l a y e r T I - T L iron-thionine cell w i t h S n 0

a n o d e a n d P t c a t h o d e h a v e b e e n i n the r a n g e

2

. 0 2 - . 0 3 6 % . T h e s e values h a v e b e e n o b t a i n e d u s i n g 50 v / v %

aqueous

C H C N as solvent a n d solutions c o n t a i n i n g ^ . 0 0 1 M d y e , . 0 1 M s u l f u r i c 3

acid, and F e ( I I ) , w i t h S 0

4

2

" a n d H S 0 " as the o n l y anions, 4

p r e s e n t i n i t i a l l y at its i m p u r i t y l e v e l

(~5XlO" M), 5

and

Fe(III)

electrodes

s p a c e d 80 /xm apart. T h e use of transparent nonselective electrodes, e.g., i n d i u m t i n o x i d e , makes p o s s i b l e m u l t i t h i n - l a y e r ( M T L ) cells i n w h i c h the layers are i n series o p t i c a l l y a n d i n p a r a l l e l e l e c t r i c a l l y . A s u n l i g h t efficiency of was obtained w i t h a M T L cell constructed

.063%

of f o u r 80 /mi layers

(4).

S u c h cells are c a p a b l e of a b s o r b i n g a l a r g e r p r o p o r t i o n of i n c i d e n t l i g h t w h i l e m a i n t a i n i n g the e l e c t r o c h e m i c a l properties of t h i n - l a y e r cells.

Processes in the

TI-TL

Iron—Thionine Photogalvanic

Cell

Photogalvanic transduction i n the T I - T L iron-thionine cell or similar cells u s i n g other thiazines processes

(5):

(1)

(4)

absorption

c a n b e a n a l y z e d i n terms of five b a s i c of

incident

light;

(2)

conversion

a b s o r b e d r a d i a n t energy i n t o c h e m i c a l p o t e n t i a l of c h a r g e carriers;


of (3)

24.

Sunlight

LICHTIN ET A L .

Engineering

d i f f u s i o n of c h a r g e carriers to electrode surfaces; ( 4 ) to

electrode

surfaces;

and

(5)

d e l i v e r y of

299

Efficiency

transfer of c h a r g e

electrical current

to

the

external c i r c u i t . T h e t h e o r e t i c a l m a x i m u m S . E . E . of a n i r o n - t h i o n i n e c e l l at p H = is ~ 2 % , b a s e d o n a b i l i t y to absorb u p to 1 5 %

of t h e i n s o l a t i o n

2

flux,

1 0 0 % q u a n t u m efficiency i n p h o t o r e d o x reactions, a n d c o n v e r s i o n of 2 V p h o t o n s to 0.28 V e l e c t r i c a l p o t e n t i a l . T h e best S . E . E . a c h i e v e d w i t h a s i n g l e element T I - T L c e l l at p H = 2 has b e e n less t h a n 1/50

of

the

t h e o r e t i c a l m a x i m u m . F u r t h e r m o r e , the t h e o r e t i c a l l i m i t is l o w c o m p a r e d w i t h S . E . E . values a l r e a d y a c h i e v e d w i t h m a n y o t h e r solar t r a n s d u c t i o n devices.

It is thus necessary to find means of b o t h i n c r e a s i n g t h e theo-

r e t i c a l l i m i t a n d c o m i n g close to a c t u a l l y r e a c h i n g this l i m i t i f p h o t o g a l v a n i c t r a n s d u c t i o n is to r e m a i n a significant o p t i o n f o r use of solar energy. W e h a v e p e r f o r m e d a v a r i e t y of studies of e a c h of the five basic steps of p h o t o g a l v a n i c t r a n s d u c t i o n so that reasons f o r losses of e n e r g y c o u l d b e u n d e r s t o o d a n d steps t a k e n to e l i m i n a t e or r e d u c e these losses. Absorption

of

Light

E f f i c i e n t a b s o r p t i o n of l i g h t i n a t h i n - l a y e r c e l l requires 1 0 " M thionine ( c 3

m a x

10"

to

2

~ 6 X 1 0 M " c m " ) or other t h i a z i n e d y e . A t s u c h 4

1

1

concentrations, association of these dyes to d i m e r s a n d h i g h e r o l i g o m e r s is v e r y extensive i n aqueous s o l u t i o n . S i n c e o n l y m o n o m e r i c d y e c o n tributes to t r a n s d u c t i o n of l i g h t to e l e c t r i c i t y a n d the a b s o r p t i o n s p e c t r u m of d i m e r i c d y e extensively overlaps t h a t of m o n o m e r , it is necessary to suppress association

(6).

T h i s c a n b e d o n e i n v a r i o u s w a y s , e.g.,

i n c o r p o r a t i o n of surfactants.

by

Greatest i m p r o v e m e n t i n c e l l p e r f o r m a n c e

has b e e n a c h i e v e d b y u s i n g o r g a n i c co-solvents to suppress association. T h e most satisfactory solvent y e t i d e n t i f i e d is 50 v / v % aqueous in

which

1 X 10" M 3

solutions

of

thionine

are

virtually

CH CN 3

completely

monomeric. O n e a p p r o a c h to i n c r e a s i n g l i m i t i n g t h e o r e t i c a l efficiency is to use m i x t u r e s of p h o t o r e d o x dyes w h i c h c a n a b s o r b a l a r g e r p o r t i o n of the i n s o l a t i o n flux t h a n c a n o n l y a single d y e . T h i o n i n e a n d m e t h y l e n e b l u e c a n together absorb about 2 5 % of t h e i n s o l a t i o n s p e c t r u m at A i r M a s s — 1. T h e t h e o r e t i c a l m a x i m u m S . E . E . f o r a n i r o n - t h i o n i n e - m e t h y l e n e b l u e c e l l at p H = 2 is ~ 4 % .

A d d i t i v i t y i n o u t p u t of T I - T L S n 0 / P t 2

cells

c o n t a i n i n g b o t h dyes at 1 0 ~ M c o n c e n t r a t i o n has b e e n a p p r o a c h e d b u t , 4

w i t h b o t h dyes 1 0 " M , c e l l o u t p u t w a s no m o r e t h a n that o b t a i n e d w i t h 3

m e t h y l e n e b l u e alone a n d o n l y a b o u t 8 0 %

of the o u t p u t w i t h 1 0 " M 3

t h i o n i n e . T h e reason f o r this unsatisfactory result s t i l l is n o t established. R o u g h l y a d d i t i v e o u t p u t has b e e n a c h i e v e d w i t h 1 m M t h i o n i n e a n d 4 m M m e t h y l e n e b l u e i n c o r p o r a t e d i n different elements of a t w o - l a y e r c e l l


(4).

300

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

II

S e n s i t i z a t i o n o f p h o t o g a l v a n i c a c t i o n b y dyes w h i c h are themselves n o t c a p a b l e of p h o t o r e d o x a c t i o n has b e e n d e m o n s t r a t e d

(7).

Action

spectra of solutions c o n t a i n i n g r h o d a m i n e 6 G a n d t w o c o u m a r i n dyes i n addition

to t h i o n i n e a n d m e t h y l e n e

blue

closely p a r a l l e l a b s o r p t i o n

spectra, c o r r e s p o n d i n g to t h e p o s s i b l e use of a b o u t 5 0 % o f t h e i n s o l a t i o n s p e c t r u m a n d a t h e o r e t i c a l m a x i m u m s u n l i g h t e n g i n e e r i n g efficiency of ~7%.

It has b e e n

increase

t h e p o w e r o u t p u t o f i r o n - t h i o n i n e ( o r other t h i a z i n e )

demonstrated

that r h o d a m i n e 6 G does, i n fact, cells

u n d e r w h i t e - l i g h t i l l u m i n a t i o n ; a n a p p r o x i m a t e l y 4 0 % increase has b e e n o b s e r v e d u n d e r i l l u m i n a t i o n w i t h 35 m W c m "

2

Formation

in Solution

and Decay of Charge

Previous workers have

Carriers

s h o w n that,

(8).

for thionine, competition of

R e a c t i o n 2 w i t h R e a c t i o n 3 is essentially n e g l i g i b l e , i.e., t h e q u a n t u m y i e l d f o r intersystem c r o s s i n g f r o m S i to T i is close t o u n i t y ( 9 ) . W e h a v e s t u d i e d t h e d e p e n d e n c e of the rates o f i n t r i n s i c d e c a y o f t r i p l e t t h i o n i n e to t h e g r o u n d state, R e a c t i o n 4, a n d of R e a c t i o n 5, t h e r e d u c t i o n of t h e t r i p l e t b y F e ( I I ) , u p o n p H a n d t h e nature of solvent a n d a n i o n s u s i n g flash p h o t o l y t i c t e c h n i q u e s (10).

T h e s e measurements h a v e s h o w n

t h a t r e d u c t i o n b y F e ( I I ) is g r e a t l y f a v o r e d b y use of 5 0 v / v %

aqueous

C H C N r a t h e r t h a n p u r e w a t e r a n d b y use of sulfate r a t h e r t h a n F3CSO3". 3

U n d e r the favored conditions, . 0 1 M F e ( I I )

reduced 9 7 % of triplet

t h i o n i n e at p H = 2. I n a r e l a t e d s t u d y , i t w a s f o u n d that t h e l o g a r i t h m of t h e specific rate of d i s p r o p o r t i o n a t i o n of s e m i t h i o n i n e , t h e f o r w a r d d i r e c t i o n of R e a c t i o n 6, varies l i n e a r l y w i t h t h e v a l u e o f K o s o w e r s Z f o r t h e solvent i n w h i c h t h e r e a c t i o n occurs

(11).

This relationship w a s

o b s e r v e d over three orders of m a g n i t u d e i n rate constant a n d v a r i a t i o n of Z b y -

17.

T h e k i n e t i c s a n d m e c h a n i s m of t h e b u l k r e a c t i o n o f l e u c o t h i o n i n e , TH

4

2 +

(12).

, w i t h F e ( I I I ) , also has b e e n s t u d i e d b y flash p h o t o l y t i c t e c h n i q u e T h e s e experiments h a v e s h o w n that t h e r e a c t i o n proceeds v i a

r e v e r s i b l e f o r m a t i o n o f a 1:1 association c o m p l e x , R e a c t i o n 7, a n d h a v e explored dependence

of e q u i l i b r i u m a n d rate constants

on p H , ionic

strength, a n d n a t u r e of solvent a n d anions. T h e p r o d u c t o f t h e association constant a n d t h e e l e c t r o n transfer rate constant, K k , 7

8

corresponds to a

s e c o n d - o r d e r rate constant w h i c h is rather s m a l l , ^ 350 t o 1 8 0 0 M " sec" 1

u n d e r t h e range of c o n d i t i o n s u s e d .

1

T h e magnitudes of intracomplex

e l e c t r o n transfer rate constants, k , are of the o r d e r of 1 sec" . F o r m a t i o n 8

1

of r e l a t i v e l y l o n g - l i v e d complexes of l e u c o t h i o n i n e w i t h F e ( I I I ) i n b u l k s o l u t i o n is consistent w i t h a m e c h a n i s m f o r loss of c h a r g e carriers at t h e Sn0

2

electrode that is suggested b e l o w .


24.

LICHTIN E T A L . A

Sunlight

Engineering

301

Efficiency

great d e a l of u s e f u l i n f o r m a t i o n has

been

obtained by

direct

m e a s u r e m e n t of the c o m p o s i t i o n a n d k i n e t i c s of r e l a x a t i o n of the t h i o n i n e p h o t o s t a t i o n a r y state ( 1 3 ) .

iron-

T h e m e a s u r e d d e p e n d e n c e of p h o t o -

stationary state c o m p o s i t i o n o n the i n i t i a l solute c o n c e n t r a t i o n s the n a t u r e of solvent has b e e n f o u n d to agree to w i t h i n

and on

experimental

error w i t h values c a l c u l a t e d f r o m m e a s u r e d rate constants f o r the r e l e v a n t e l e m e n t a r y reactions i n the system. I n t e r e s t i n g l y , u n d e r i l l u m i n a t i o n w i t h 100 m W c m " of l i g h t f r o m a X e n o n l a m p (essentially "1 s u n " ) , a s o l u t i o n 2

i n 50 v / v aqueous C H C N i n i t i a l l y . 0 0 1 M i n t h i o n i n e , . 0 1 M i n a c i d a n d 3

F e ( I I ) , a n d . 0 0 0 0 6 M i n F e ( I I I ) , w i t h sulfate as a n i o n , is 4 8 %

bleached.

T h i s d e g r e e of b l e a c h i n g is n e a r l y o p t i m a l since it p r o v i d e s a h i g h c o n c e n t r a t i o n of b o t h a b s o r b i n g d y e a n d c h a r g e - c a r r y i n g r e d u c e d

species.

T h e absence of d e t e c t a b l e a b s o r p t i o n at 390 a n d 770 n m i n the p h o tostationary state shows that less t h a n 1 0 %

of the r e d u c e d d y e is pres-

ent as s e m i t h i o n i n e u n d e r c o n d i t i o n s of o b s e r v a t i o n i n v o l v i n g photobleaching

of

10" -10" M 5

thionine

3

Calculations

(14).

k i n e t i c d a t a s h o w that no m o r e t h a n ~ 0 . 3 %

35-95% based

on

of the p h o t o b l e a c h e d

dye

is present as s e m i t h i o n i n e i n a s o l u t i o n i n i t i a l l y 1 0 " M i n t h i o n i n e a n d w i t h 3

other aspects of i n i t i a l c o m p o s i t i o n a n d i l l u m i n a t i o n as g i v e n above. a c o m p a r i s o n of s u c h results w i t h m e a s u r e d

monochromatic

From

quantum

y i e l d s f o r c u r r e n t g e n e r a t i o n , e.g., ~ 7 % at 578, 589, a n d 620 n m i n 80 /xm cells u n d e r s i m i l a r c o n d i t i o n s (4),

i t c a n b e c o n c l u d e d that l e u c o t h i o n i n e

is the p r i n c i p a l c h a r g e c a r r i e r d e r i v e d f r o m the d y e u n d e r these c o n d i t i o n s . K i n e t i c analysis of the o b s e r v e d l i n e a r d e p e n d e n c e first-order rate of r e l a x a t i o n of the p h o t o s t a t i o n a r y concentrations

of

thionine

and

Fe(III)

of the

gives

(13,14)

pseudo

state u p o n a

initial

completely

i n d e p e n d e n t d e t e r m i n a t i o n of the a p p a r e n t s e c o n d - o r d e r rate constant f o r the r e a c t i o n of F e ( I I I ) w i t h l e u c o t h i o n i n e , K k . 7

T h e resulting values

8

are i n excellent a g r e e m e n t w i t h those d e t e r m i n e d b y flash p h o t o l y s i s

(12).

T h e same analysis leads to e v a l u a t i o n of the rate constant f o r s y n p r o p o r t i o n a t i o n of l e u c o t h i o n i n e to g i v e s e m i t h i o n i n e , k. .

K n o w l e d g e of

G

the

rate constants f o r b o t h the f o r w a r d a n d reverse d i r e c t i o n s of R e a c t i o n 6 makes p o s s i b l e e v a l u a t i o n o f the e q u i l i b r i u m constant f o r this r e a c t i o n , KG, u n d e r a v a r i e t y of c o n d i t i o n s (14). [Semi] /[Leuco][Thionine] 2

=

T h e r e s u l t i n g v a l u e s , e.g., K

Q

=

(.6 ± .2) X 10" i n 1 0 " M aqueous s u l f u r i c 6

2

a c i d , are less b y b e t w e e n f o u r a n d five orders of m a g n i t u d e t h a n a v a l u e estimated b y M i c h a e l i s (15),

w h i c h has l e d a n u m b e r of w o r k e r s to

c o n c l u d e that s e m i t h i o n i n e is the p r i n c i p a l r e d u c e d f o r m of the d y e i n i r o n - t h i o n i n e p h o t o g a l v a n i c cells. possible

e v a l u a t i o n of the

A k n o w l e d g e of K

potentials

e q u i l i b r i a of t h i o n i n e , e.g., E ° ' / s = T

for

.196 ±

the

two

6

also has

one-electron

.004 V a n d E

0 ,

s / L

=

made redox .570

±

.005 V vs. N H E i n 1 0 ~ M aqueous s u l f u r i c a c i d . I n 1 0 ~ M solutions of 2

2

s u l f u r i c a c i d i n 50 v / v % aqueous C H C N , E ° ' / s = 3

T

.176 ± .005 V a n d


302

INORGANIC COMPOUNDS W I T H UNUSUAL

PROPERTIES

E S / L = . 5 3 0 ± . 0 0 5 V v s . N H E . T h e s e a n d r e l a t e d values h a v e

been

O ,

u s e d i n c o n s t r u c t i n g a n energy d i a g r a m f o r t h e S n 0 " - e l e c t r o l y t e 2

II

interface

that is d e s c r i b e d b e l o w i n t h e section o n e l e c t r o n transfer at t h e elect r o d e - s o l u t i o n interface.

Diffusion

of Charge

Carriers

to the

Electrodes

S u b s t i t u t i o n of e x p e r i m e n t a l l y d e t e r m i n e d l i f e t i m e s f o r r e l a x a t i o n o f t h e p h o t o s t a t i o n a r y state ( 1 3 ) , 1 . 6 sec u n d e r t h e c o n d i t i o n s i n d i c a t e d above, i n t o t h e d i f f u s i o n e q u a t i o n gives 5 0 /mi as t h e average d i f f u s i o n l e n g t h f o r c h a r g e carriers i n s o l u t i o n u n d e r these c o n d i t i o n s . T h u s , loss of c h a r g e carriers b y r e c o m b i n a t i o n i n b u l k s o l u t i o n c a n n o t b e a m a j o r process w i t h the 2 5 a n d 8 0 /nn e l e c t r o d e separations u s e d i n most of o u r T I - T L cells.

Transfer

of Charge

to the

Electrodes

Short c i r c u i t c u r r e n t o f T I - T L cells varies l i n e a r l y w i t h c o n c e n t r a t i o n of p h o t o b l e a c h e d d y e , m o s t l y l e u c o t h i o n i n e (16).

C u r r e n t p e r u n i t area

of e l e c t r o d e p e r u n i t c o n c e n t r a t i o n of l e u c o t h i o n i n e w a s f o u n d to b e the same f o r T I - T L cells w i t h 2 5 a n d 8 0 /nn spacings of a g i v e n set o f electrodes aqueous

a n d increased CH CN 3

s l i g h t l y w i t h t h e p r o p o r t i o n of C H C N 3

solutions.

T h e best efficiencies

obtained w i t h

in

Sn0

2

anodes i n 5 0 v / v % aqueous C H C N w i t h sulfate as a n i o n c o r r e s p o n d e d 3

to a b o u t 1 3 0 / x A / m M - c m .

S u b s t i t u t i o n o f this v a l u e i n t h e N e r n s t d i f f u -

2

s i o n - l a y e r e q u a t i o n i n d i c a t e s a d i f f u s i o n - l a y e r thickness of 1 0 0 /mi. T h e f a c t that essentially i d e n t i c a l c u r r e n t densities w e r e o b t a i n e d w i t h 2 5 and

8 0 fim electrode separations i n d i c a t e s , h o w e v e r , that t h e d i f f u s i o n

l a y e r w a s n o greater

t h a n 2 5 /xm i n thickness a n d c o u l d h a v e

been

s i g n i f i c a n t l y less. T h i s suggests that at least 7 5 % of charge carriers that r e a c h e d those p a r t i c u l a r electrodes d i d n o t p r o d u c e c u r r e n t i n t h e e x t e r n a l c i r c u i t a n d w e r e w a s t e d . L i t t l e of this loss is i n c u r r e d w i t h i n t h e electrodes after electron transfer, as reverse b i a s i n g o f t h e c e l l increases t h e current b y only 2 5 - 5 0 % .

A p p a r e n t l y , most of this loss results f r o m

inefficiency of e l e c t r o n transfer b e t w e e n

solution charge

carriers a n d

electrode(s). S i n g l e electrode potentials h a v e b e e n m e a s u r e d at S n 0

2

anodes a n d

I T O cathodes f o r solutions i n 5 0 v / v % aqueous C H C N , . 0 1 M i n H S 0 , 3

i n w h i c h the p h o t o s t a t i o n a r y ratio of T H to T H +

4

2 +

2

4

v a r i e d b y a f a c t o r of

m o r e t h a n 1 0 a n d t h e F e ( I I I ) : F e ( I I ) ratio v a r i e d b y a factor of a p p r o x i m a t e l y six ( I ) . T h e p o t e n t i a l at the S n O a n o d e p a r a l l e l e d t h e p o t e n t i a l 2

calculated for the T H : T H +

4

2 +

couple f r o m k n o w n compositions w i t h the

a i d of t h e N e r n s t p o t e n t i a l e q u a t i o n .

M e a s u r e d values w e r e , h o w e v e r ,


24.

Sunlight

LICHTIN E T A L .

Engineering

303

Efficiency

m o r e p o s i t i v e ( f o r t h e p a r t i c u l a r samples of S n 0

2

used) than calculated

values b y ~ 100 m V . M e a s u r e d potentials at t h e I T O c a t h o d e p a r a l l e l e d those c a l c u l a t e d f o r t h e F e ( I I I ) : F e ( I I ) c o u p l e b u t w e r e a b o u t 70 m V m o r e n e g a t i v e ( f o r t h e p a r t i c u l a r samples of I T O u s e d ) .

C e l l voltages

w e r e thus a b o u t 170 m V smaller t h a n w o u l d b e e x p e c t e d f r o m a c e l l w i t h a n a n o d e f u l l y selective f o r t h e T H : T H +

4

couple a n d a cathode fully

2 +

selective f o r the F e ( I I I ) : F e ( I I ) c o u p l e . E l e c t r o n transfer processes at t h e S n 0

2

electrode h a v e b e e n s t u d i e d

extensively b y c y c l i c v o l t a m m e t r y a n d other e l e c t r o c h e m i c a l

techniques

T h e t i n o x i d e electrodes w e r e c h a r a c t e r i z e d b y d e t e r m i n i n g

(4,6,17).

c h a r g e c a r r i e r densities a n d flat-band potentials b y means of S c h o t t k y M o t t plots of 1 / C vs. E.

T h e highly conductive

2

defect-structure S n 0 10

20

and

2

(30-100

ohms/sq)

h a d a c h a r g e c a r r i e r d e n s i t y i n t h e range ( 4 - 7 )

X

c m " . T h e flat b a n d p o t e n t i a l w a s + .05 to + .25 r e l a t i v e to N H E 1

essentially i n d e p e n d e n t of the solvent i n contact w i t h the

Thus, E ° ' p

H =

=

2

Sn0 . 2

of the t h i o n i n e - s e m i t h i o n i n e c o u p l e falls w i t h i n t h e flat

b a n d p o t e n t i a l range w i t h either . 0 1 M aqueous s u l f u r i c a c i d o r . 0 1 M s u l f u r i c a c i d i n 50 v / v % aqueous C H C N

as solvent.

3

E ' 0

p H

= 2 of t h e

s e m i t h i o n i n e - l e u c o t h i o n i n e c o u p l e is at least .28 V p o s i t i v e w i t h respect to the flat-band p o t e n t i a l i n aqueous C H C N a n d m o r e p o s i t i v e b y a b o u t 3

.04 V i n the neat aqueous solvent w h i l e E ' of t h e F e ( I I I ) : F e ( I I ) c o u p l e 0

is at least .4 V p o s i t i v e to the flat b a n d p o t e n t i a l i n b o t h m e d i a . F i g u r e 1, t a k e n f r o m R e f . 17, s u m m a r i z e s i n t e r f a c i a l energies. (18)

that the S n 0

2

T h e s e d a t a suggest

electrode s h o u l d b e r e l a t i v e l y p o o r l y selective since

l e u c o t h i o n i n e is, as s h o w n above, t h e p r i n c i p a l r e d u c e d f o r m of t h e d y e at the p h o t o s t a t i o n a r y state. T h e results also suggest that selectivity o f the S n 0

2

a n o d e w i t h respect to t h e l e u c o t h i o n i n e - s e m i t h i o n i n e c o u p l e

s h o u l d b e better w i t h 50 v / v % aqueous C H C N as solvent t h a n w i t h 3

neat w a t e r . Results of c y c l i c v o l t a m m e t r i c measurements at the S n 0

2

electrode

are s u m m a r i z e d b e l o w .

(4,6,17)

( 1 ) T h e m e c h a n i s m of r e d u c t i o n of t h i o n i n e is E E , i.e., n o d i s c e r n i b l e c h e m i c a l process takes p l a c e b e t w e e n the t w o electron-transfer steps. ( 2 ) R e a c t i o n s of t h e t h i o n i n e - l e u c o t h i o n i n e c o u p l e are k i n e t i c a l l y c o n t r o l l e d . R e c t i f i c a t i o n s i g n i f i c a n t l y reduces efficiency o f o x i d a t i o n of l e u c o t h i o n i n e at the S n 0 electrode. ( 3 ) C o n t r a r y to w h a t m i g h t b e e x p e c t e d b y c o n s i d e r i n g o n l y t h e i n t e r f a c i a l energy d i a g r a m , r e v e r s i b i l i t y is greater f o r t h e d y e c o u p l e w i t h neat H 0 as solvent t h a n w i t h 50 v / v % aqueous C H C N . ( 4 ) L e u c o t h i o n i n e is s t r o n g l y a d s o r b e d o n SnOo w h i l e t h i o n i n e is w e a k l y a d s o r b e d . A d s o r p t i o n is greater f r o m solutions i n p u r e w a t e r t h a n f r o m 50 v / v % aqueous C H C N . ( 5 ) T h e F e ( I I I ) : F e ( I I ) c o u p l e is s o m e w h a t m o r e r e v e r s i b l e at S n 0 w i t h 50 v / v % aqueous C H C N as solvent t h a n w i t h p u r e w a t e r , b u t w i t h either solvent i t is m u c h less r e v e r s i b l e at S n 0 t h a n t h e d y e c o u p l e i s . 2

2

3

3

2

3

2


304

INORGANIC

COMPOUNDS WITH UNUSUAL

PROPERTIES

II

E,°'

--

0.2

t

2

-pss

0.4

t E

F e

2+

/ F e

3+

F 2 /Fe3+ e

+

0.6

E.V.VS.SCE Journal of the Electrochemical Society

Figure 1. Interfacial energy diagram for the Sn0 anode in contact with the components of the ironthionine photogalvanic cell at pH = 2. Solid lines: aqueous sulfuric acid. Dashed lines: sulfuric acid in 50 v/v % aqueous CH CN. Taken from Ref. 17. 2

3

Summary The

and fifty-fold

Speculation to o n e h u n d r e d - f o l d f a c t o r b y w h i c h t h e best o b s e r v e d

values of S . E . E . f o r the i r o n - t h i o n i n e T I - T L c e l l w i t h a S n 0

2

anode a n d

a P t o r I T O c a t h o d e f a l l short o f t h e t h e o r e t i c a l m a x i m u m c a n b e a s c r i b e d , i n p a r t , to t w o r e l a t i v e l y s i m p l e factors. O n e o f these is inefficient a b s o r p t i o n of l i g h t . A t t h e p h o t o s t a t i o n a r y state, t h e c o n c e n t r a t i o n of u n b l e a c h e d d y e i n o p t i m i z e d cells w a s ~ 5 X 1 0 " M , so t h a t o n l y ~ 2 5 % o f t h e i n c i 4

dent light i n the wavelength region of the thionine b a n d was absorbed i n the 80 /xm-thick c e l l . If a b s o r p t i o n b y S n 0

2

a n d I T O i n the four-element

M T L c e l l is i g n o r e d , a p p r o x i m a t e l y t w o - t h i r d s of i n c i d e n t l i g h t i n t h e t h i o n i n e b a n d is a b s o r b e d b y t h e d y e . T h e difference i s , i n fact, s i m i l a r to t h e difference

i n S . E . E . o f single-element

T I - T L a n d four-element


24.

LICHTIN ET A L .

M T L cells.

Sunlight

Engineering

305

Efficiency

T h e other s i m p l e f a c t o r is the r e l a t i v e l y l o w c e l l p o t e n t i a l .

C e l l potentials at the p o w e r p o i n t w e r e ^ 100 m V , i.e., a b o u t 3 5 %

of

E ° ' P H = 2 of the c e l l . M o s t of this d e f i c i e n c y c a n b e a s c r i b e d to s e l e c t i v i t y of electrodes b e i n g o n l y p a r t i a l . T h e t w o loss-factors together f o r r e d u c t i o n of S . E . E . b y a f a c t o r of ~

account

12.

T a k e n together, five lines of e v i d e n c e suggest that t h e r e m a i n i n g f o u r - f o l d to e i g h t - f o l d loss factor c a n be a s c r i b e d to inefficient transfer of c h a r g e b e t w e e n s o l u t i o n c h a r g e carriers a n d S n 0

2

anodes.

One perti-

nent p i e c e of e v i d e n c e is the r e l a t i v e l y l o n g d i f f u s i o n l e n g t h c a l c u l a t e d f o r c h a r g e carriers i n s o l u t i o n . T h i s l e n g t h is sufficient to a l l o w most c h a r g e carriers to r e a c h t h e electrodes.

S e c o n d is t h e contrast b e t w e e n

w h i c h suggest that the thickness of t h e N e r n s t d i f f u s i o n l a y e r is 100 a n d e v i d e n c e that i t c a n n o t be m o r e t h a n 25 /xm i n thickness.

data fim

T h i r d is

the c y c l i c - v o l t a m m e t r i c e v i d e n c e that e l e c t r o n transfer at t h e S n 0 - s o l u 2

t i o n interface is c o n t r o l l e d k i n e t i c a l l y . F o u r t h is the c y c l i c - v o l t a m m e t r i c e v i d e n c e that l e u c o t h i o n i n e is strongly a d s o r b e d o n S n 0 . 2

F i f t h is t h e

e v i d e n c e that t h e r e a c t i o n of l e u c o t h i o n i n e w i t h F e ( I I I ) i n b u l k s o l u t i o n p r o c e e d s v i a a r e l a t i v e l y l o n g - l i v e d ( T ^ 1 sec)

complex.

A t least three paths b y w h i c h l e u c o t h i o n i n e c a n b e w a s t e d at t h e a n o d e suggest themselves. ( 1 ) A d s o r b e d l e u c o t h i o n i n e is o x i d i z e d at the interface b y F e ( I I I ) .

T h e r e s u l t i n g s e m i t h i o n i n e w o u l d b e e x p e c t e d to

r a p i d l y transfer a n e l e c t r o n to S n 0 . 2

loss-factor

of o n l y t w o .

F e ( I I I ) at the interface.

(2)

T h u s , this p a t h c o u l d i n t r o d u c e a

A d s o r b e d l e u c o t h i o n i n e complexes

with

T h e r e s u l t i n g c o m p l e x t h e n reacts i n t h e a d -

s o r b e d state. T h i s p a t h also w o u l d b e e x p e c t e d to i n t r o d u c e a loss-factor of o n l y t w o . ( 3 ) A d s o r b e d l e u c o t h i o n i n e complexes w i t h F e ( I I I ) at the interface.

T h e r e s u l t i n g c o m p l e x desorbs a n d diffuses b a c k i n t o b u l k

s o l u t i o n before u n d e r g o i n g i n t r a c o m p l e x e l e c t r o n transfer. T h i s p a t h c a n l e a d to c o m p l e t e wastage of c h a r g e carriers. If the analysis p r e s e n t e d i n this c h a p t e r is correct, p a t h ( 3 ) p l a y s a n i m p o r t a n t r o l e i n r e d u c i n g t h e efficiency of charge transfer at t h e S n 0

2

electrode.

Acknowledgment T h i s w o r k , a joint project of the D e p a r t m e n t of C h e m i s t r y of B o s t o n U n i v e r s i t y a n d t h e Solar E n e r g y C o n v e r s i o n U n i t of E x x o n R e s e a r c h a n d E n g i n e e r i n g C o . , w a s s u p p o r t e d b y the N a t i o n a l S c i e n c e

Foundation

R e s e a r c h A p p l i e d to N a t i o n a l N e e d s P r o g r a m u n d e r G r a n t N o . S E / A E R / 72-03579.

S o m e of the research o n s o l u t i o n k i n e t i c s w a s s u p p o r t e d b y

E n e r g y Research and Development Administration Contract N o . E Y - 7 6 S-02-2889.

M a n y v a l u a b l e discussions of v a r i o u s aspects of this w o r k

have b e e n h e l d w i t h M o r t o n Z . H o f f m a n .


306

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES

II

Literature Cited 1. Wildes, P. D., Brown, K. T., Lichtin, N. N., J. Am. Chem. Soc. (1978) 100, "Abstracts of Papers," National Meeting, ACS, Aug. 28-Sept. 2, 1971, PHYS 117. 2. Rabinowitch, E., J. Chem. Phys. (1940) 8, 560. 3. Clark, W. D. K., Eckert, J.A.,Sol. Energy (1975) 17, 147. 4. Hall, D. E., Eckert, J. A., Lichtin, N. N., Wildes, P. D.,J.Electrochem. Soc. (1976) 123, 1705. 5. Lichtin, N. N., "Photogalvanic Processes," in "Solar Power and Fuels," J. Bolton, Ed., pp. 119-142, Academic, New York, 1977. 6. Hall, D. E., Clark, W. D. K., Eckert, J. A., Lichtin, N. N., Wildes, P. D., Am. Ceram.Soc.,Bull. (1977) 56, 408. 7. Wildes, P. D., Hobart, D. R., Lichtin, N. N., Hall, D. E., Eckert, J. A., Sol. Energy (1977) 19, 567. 8. Lichtin, N. N., Wildes, P. D., U.S. Patent 4,052,536, "Electrolytes Which Are Useful in Solar Energy Conversion," 1977. 9. Havemann, R., Reimer, K.G.,Z. Phys. Chem. (Leipzig) (1961) 216, 334, and earlier papers. 10. Wildes, P. D., Lichtin, N. N., Hoffman, M. Z., Andrews, L., Linschitz, H., Photochem. Photobiol. (1977) 25, 21. 11. Wildes, P. D., Lichtin, N. N., Hoffman, M. Z., J. Am. Chem. Soc. (1975) 97, 2288. 12. Osif, T. L., Lichtin, N. N., Hoffman, M. Z., "Abstracts of Papers," National Meeting, 114th, ACS, Aug. 28-Sept. 2, 1977, PHYS 17. 13. Wildes, P. D., Lichtin, N. N., Hoffman, M. Z., "Application of Solution and Photo Dynamics to the Optimization of Output of Iron-Thiazine Photogalvanic Cells," in "Solar Energy," J. B. Berkowitz, I. A. Lesk, Eds., p. 128-138, The Electrochemical Society, 1976. 14. Wildes, P. D., Lichtin, N. N.,J.Phys. Chem. (1978) 82, 981. 15. Michaelis, L., Schubert, M. P., Granick, S., J. Am. Chem. Soc. (1940) 62, 204. 16. Wildes, P. D., Brown, K. T., Hoffman, M. Z., Lichtin, N. N., Hall, D. E., Sol. Energy (1977) 19, 579. 17. Hall, D. E., Wildes, P. D., Lichtin, N. N., J. Electrochem. Soc. (1978) 125, in press. 18. Gerischer, H., in "Semiconductor Electrochemistry in Physical Chemistry," L. Eyring, D. Henderson, W. Jost, Eds., pp. 463-542, Academic, New York, 1970. RECEIVED February 22, 1978.


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