Photoelectrochemical Catalysis with Polymer ... - ACS Publications

29 Photoelectrochemical Catalysis with Polymer Electrodes

Downloaded by UNIV OF CINCINNATI on February 18, 2015 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch029

HOWARD D. M E T T E E Youngstown State University, Chemistry Department, Youngstown, OH 44555

Polymer electrodes which absorb light and catalytic ally convert this energy into stored chemical poten t i a l , possibly with electrochemical assistance, using endoergic reactions such as water splitting, are of increasing interest as practical means of solar energy storage are sought. Developments in this field may be interpreted in terms of photocatalytic and electrochemical reactions catalysed by polymers. Emphasis is placed upon the roles of polymer films in protecting n-type semiconductors from anodic dissolution and in helping to under stand electron transfer mechanisms. Requirements of free-standing polymeric photoelectrochemical (PEC) catalysts are outlined. In 1972 Fujishima and Honda (1) focussed attention on the possibility of using visible light to photolytically sensitize the splitting of water into hydrogen and oxygen. This light driven, energy storing reaction has obvious attractions in that it produces a clean burning, re-cyclable fuel and it ultimately depends upon an infinite energy source. However, to couple ab sorbed solar energy to this thermodynamically uphill reaction requires photoelectrochemical (PEC) catalysts which will simutaneously oxidize and reduce water. PEC + hv solar 2 H0 y

>

4 H 0 + 4e~

•

1

2

2

• PEC* • 0 PEC* •

2

+ 4H

+ 4e

• 2H

2

_ + 4 OH

These catalysts must not only cause the birth of reactive inter mediates, which in some cases have been identified as Η', e", and OH' etc., but also prevent their back reaction until the O2 and H2 have formed. Obviously they must maintain their own chemical 0097-6156/83/0220-0473$06.50/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


474

POLYMERS IN SOLAR ENERGY UTILIZATION

i n t e g r i t y i n the process. Thus the f u n c t i o n a l requirements of these PEC c a t a l y s t s are more demanding than those o f t h e i r phot o c a t a l y t i c c o u n t e r p a r t s , which serve only t o a c c e l e r a t e energet i c a l l y downhill reactions. In the i n t e r v e n i n g decade s i n c e Fujishima and Honda's paper a great deal o f chemical e f f o r t has gone i n t o the design and cons t r u c t i o n of these s p e c i a l PEC c a t a l y s t s . The r o l e o f polymers has been an important one a t a l l l e v e l s o f i n v e s t i g a t i o n whether the system be c o l l o i d a l m i c e l l e s , v e s i c l e s , microemulsions o r " m i c r o e l e c t r o d e s " , o r b u l k semiconductors. Without polymeric support f o r the c a t a l y s t s i n c o l l o i d a l systems f o r i n s t a n c e , important d i a g n o s t i c r e a c t i o n s could not be detected. More s i g n i f i cant f o r the polymer chemist, however, i s the i n c r e a s i n g l y cent r a l r o l e polymers a r e p l a y i n g i n the a c t u a l l i g h t a b s o r p t i o n , charge s e p a r a t i o n and p a r t i c l e flow dynamics that c h a r a c t e r i z e the intermediate chemistry p r i o r to H2 and O2 formation. A completely polymeric u n i t which accomplishes s o l a r induced water s p l i t t i n g has not y e t been devised, nor indeed has any system which can s u c c e s s f u l l y compete w i t h the e f f i c i e n c y and l o n g e v i t y o f p h o t o v o l t a i c a l l y - d r i v e n water e l e c t r o l y s i s (0 ^ 10%). Thus current e f f o r t s are addressed a t s e l e c t i n g a p p r o p r i a t e l i g h t absorbing agents, charge c r e a t i n g and s e p a r a t i n g media, and c a t a l y t i c environments which meet the necessary thermodynamic and k i n e t i c requirements. Of course the more p r a c t i c a l c o n s i d e r a t i o n s o f reasonable quantum y i e l d , d u r a b i l i t y and cost cannot be ignored e i t h e r . Broadly speaking, polymers have c o n t r i b u t e d both to the s t a b i l i t y o f PEC c a t a l y s t s and t o the understanding and c o n t r o l o f charge m i g r a t i o n and redox chemistry i n these systems. In the f u t u r e , polymers o f f e r an e x t r a degree o f s y n t h e t i c f r e e dom which may be e x p l o i t e d t o enhance the quantum y i e l d s , durab i l i t y and economic p r a c t i c a l i t y of PEC systems s t i l l f u r t h e r . A number of c l o s e l y r e l a t e d reviews have been p u b l i s h e d r e c e n t l y which h i g h l i g h t a number o f approaches t o s o l a r induced water s p l i t t i n g . Bard (2) has summarized the semiconductor des i g n c r i t e r i a f o r example. More d e t a i l e d reviews by Nozik (3) and Wrighton (4) consider the i n t e r p l a y o f the thermodynamic and k i n e t i c behavior o f these semiconductor u n i t s , and Wrighton has helped develop the concept o f surface m o d i f i e d , semiconductor e l e c t r o d e s . C a l v i n (5) and P o r t e r (6) have emphasized the c l o s e s i m i l a r i t y of PEC water s p l i t t i n g t o n a t u r a l p h o t o s y n t h e s i s , thereby l e a d i n g t o the study o f biomimetic systems a t the c o l l o i d a l l e v e l . Whitten (7) has considered photoinduced e l e c t r o n t r a n s f e r i n homogeneous s o l u t i o n s , very f r e q u e n t l y i n v o l v i n g chlorophyll-like sensitizers. P u b l i c a t i o n of the proceedings o f three recent symposia d e a l ing w i t h PEC processes i n the past two years i n d i c a t e s the h i g h l e v e l o f chemical i n t e r e s t i n these systems. Two i n the A.C.S. Symposium s e r i e s (8,9) consider the importance o f i n t e r f a c e s i n PEC systems, w h i l e a Faraday D i s c u s s i o n Volume (10) contains p r i n c i p a l l y semiconductor c o n t r i b u t i o n s . I t appears that the

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

29.

ΜΕΤΤΕΕ

Photoelectrochemical Catalysis

475

semiconductor approach i s a dominant one at the moment, and i t should not be a t a l l s u r p r i s i n g to f i n d out that polymers have c o n t r i b u t e d mainly to r e s e a r c h i n t h i s area. This i s not to sug gest that work l i k e that of Regen and co-workers (11a), who have photopolymerized v e s i c l e w a l l s and thereby extended the t y p i c a l v e s i c l e l i f e t i m e from 48 hours to more than two weeks, i s i n c i d e n t a l . Indeed no one can f o r e t e l l at t h i s moment what the f i n a l forms of s u c c e s s f u l PEC c e l l s w i l l be, and c o l l o i d a l s u r f a c t a n t systems may w e l l be among them. However, polymers have been most e x t e n s i v e l y a p p l i e d to b u l k photoelectrodes and i t i s t h i s sub j e c t that p r i n c i p a l l y occupies t h i s review. Downloaded by UNIV OF CINCINNATI on February 18, 2015 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch029

O p e r a t i o n a l Background Figure 1 d e p i c t s the p h y s i c a l arrangement of a t y p i c a l PEC c e l l w i t h a polymer coated photoanode (e~ pass from the e l e c t r o l y t e through the polymer l a y e r to the working e l e c t r o d e ) , or pho tocathode. The polymer l a y e r may be e i t h e r c o v a l e n t l y attached or p h y s i c a l l y adsorped to the s u b s t r a t e , and i t f r e q u e n t l y con t a i n s a redox couple to be conductive. The c o n d u c t i v i t y of the polymer l a y e r may be due to a m e t a l - l i k e wide energy band (eg. p o l y p y r r o l e ( l i b ) ) , or may occur through a narrow energy window r e s u l t i n g from s p e c i f i c e l e c t r o a c t i v e s i t e s w i t h i n the f i l m (eg. f e r r o c e n e ) . The s u b s t r a t e may be a noble metal or g r a p h i t e , i n which case the s e n s i t i z e r i s embedded i n the f i l m , or a semicon ductor which may then assume the r o l e of s e n s i t i z e r . M e t a l e l e c trodes are o f t e n used i n c o n t r o l experiments to d i s t i n g u i s h f i l m behavior from that of semiconductors. An i l l u s t r a t i o n of a PEC c e l l of t h i s type, which operates i n r e v e r s e , may be found i n the work of R u b i n s t e i n and Bard (12). The w e l l known duPont polymer N a f i o n was dip coated on a g r a p h i t e e l e c t r o d e f o r t h i s experiment, and then immersed i n a s o l u t i o n of Ru(bpy)3Cl2 (bpy = 2 , 2 ' - b i p y r i d i n e ) . C y c l i c voltammograms o f t h i s t r e a t e d e l e c t r o d e showed broad o x i d a t i v e and r e d u c t i v e waves c l o s e to 1.2V vs NHE of the p o t e n t i a l sweep, c h a r a c t e r i s t i c o f the presence of the Ru(bpy)3^+ complex sequestered i n the polymer f i l m . However, when o x a l a t e i o n was added to the s u p p o r t i n g e l e c t r o l y t e , which i o n i s only o x i d i z e d at h i g h e r p o t e n t i a l s by p l a i n N a f i o n , three e f f e c t s were noted; namely, an enhanced o x i d a t i v e current ( m o r e ^ R u ( b p y ) R u ( b p y ) 3 ) , a damped r e t u r n Z

minescence due to Ru(bpy)3 "K*J. Thus i t appears that the power f u l Ru(bpy)3 oxidant c a t a l y t i c a l l y o x i d i z e s o x a l a t e w i t h s u f f i c i e n t excess energy to produce an e l e c t r o n i c a l l y e x c i t e d s t a t e . E l e c t r o c h e m i c a l Polymers. PEC experiments w i t h polymer i n t e r f a c e s a r i s e from e l e c t r o c h e m i c a l work by Anson (13a, b, c) Kuwana (14), M i l l e r (15) and others. This f i e l d has been review ed by Murray (16) and S n e l l and Keenan (17). A study by Anson and Oyama (13b) i l l u s t r a t e s s e v e r a l general p r i n c i p l e s of f i l m s .



476


CATALYST

t POLYMER

Î SUBSTRATE

Figure 1. A photoelectrochemical c e l l w i t h a p o l y m e r / e l e c t r o l y t e i n t e r f a c e c o n t a i n i n g a l i g h t absorbing s e n s i t i z e r (S) embedded i n the polymer. L i g h t absorption may enable a redox r e a c t i o n o f (R) d i s s o l v e d i n the e l e c t r o l y t e . When a semiconductor i s the s u b s t r a t e , i t i s a l s o o f t e n the s e n s i t i z e r . (WE and CE denote working and counter e l e c t r o d e s ) .


29.

477


METTEE

Their work i n c l u d e d c o a t i n g p y r o l y t i c graphite e l e c t r o d e s w i t h p o l y ( 4 - v i n y l p y r i d i n e ) and immersing them i n aqueous s o l u t i o n s o f R u ( e d t a ) 0 H 2 . A slow and steady growth of the redox waves of the R u / couple was observed as the complex entered what was otherwise an e l e c t r o c h e m i c a l l y quiet f i l m . C o n t r o l experiments showed that when i n the f i l m , the R u i o n c h e m i c a l l y coordinated w i t h p y r i d i n e by l i g a n d s u b s t i t u t i o n . This work showed that t h i c k durable polymer f i l m s could conveniently be made, and that t h e i r e l e c t r o c h e m i c a l a c t i v i t y could r e s u l t from the redox coup l e s sequestered from the ambient s o l u t i o n . The p r i n c i p l e of "mediated" e l e c t r o n t r a n s f e r , whereby e l e c trons are passed from the reduced form of a r e l a t i v e l y negative redox couple to the o x i d i z e d form of a r e l a t i v e l y p o s i t i v e couple, has been demonstrated to occur between two polymer l a y e r s o f s l i g h t l y d i f f e r e n t R u ( b p y ) 3 complex polymers by Murray and coworkers (18). This k i n d of stepwise, u n i d i r e c t i o n a l e l e c t r o n t r a n s f e r may be very s i g n i f i c a n t i n f u t u r e polymer coated PEC c e l l s which seek to separate charge, and of a d d i t i o n a l i n t e r e s t , Ru*I(bpy)3 complexes are f r e q u e n t l y used as c y c l i c PEC c a t a l y s t s i n water s p l i t t i n g experiments. Some d e t a i l s of t h i s experiment are thus i n f o r m a t i v e . To demonstrate u n i d i r e c t i o n a l charge flow v i a e l e c t r o n media t i o n , Murray's group e l e c t r o c h e m i c a l l y polymerized complexes [ R u ( b p y ) ( v p y ) 2 ] , A, and [ R u ( b p y ) ( v p y ) C l ] + , B, on Pt e l e c trodes i n CH3CN. (vpy i s 4 - v i n y l p y r i d i n e . ) The order of deposit i o n of t h e f i l m s i s c r u c i a l of course s i n c e [RU '2+(A)] = +1.23 and E [Ru ( B ) ] = +.76 V vs SSCE and the inner f i l m mediator (poly(A)) would not be expected to move e l e c t r o n s u p h i l l . The r e s u l t s are summarized i n Figure 2, where i t i s c l e a r that both redox waves a s s o c i a t e d w i t h the outer f i l m couple (poly (B)) are m i s s i n g i n the dual l a y e r system ( F i g 2 ( b ) ) . The (A + B) copolymerized s i n g l e f i l m e l e c t r o d e ( F i g 2 ( c ) ) shows the e l e c t r o n i c presence of both couples at the Pt/polymer i n t e r f a c e . Thus the inner polymer l a y e r has seemingly screened communication between the h o l e ( h ) a t the Pt s u r f a c e and the redox couple i n the outer f i l m . 3+/2+ E l e c t r o n i c mediation, the passage of e~ from one Ru couple to another, i s shown by c o n s i d e r i n g the d e t a i l s of F i g u r e 2(b). One important feature i s the o x i d a t i v e s p i k e , or prewave, i n the f i r s t o x i d a t i v e scan, and i t s absence from subsequent scans i f the r e t u r n p o t e n t i a l i s not swept negative past about - I V . A second i s the pronounced cathodic s p i k e on the l a r g e r e d u c t i o n wave i f t h i s negative scan i s c a r r i e d out subsequent to a p o s i t i v e one. In the f i r s t anodic scan, the Ru ions i n both polymer l a y e r s begin as R u . When the e l e c t r o d e p o t e n t i a l f i r s t passes +1.15 V, the outer l a y e r Ru +(B) -> R u ( B ) i s s i g n a l l e d as a sharp s p i k e , and the inner l a y e r R u ( A ) -* R u ( A ) a c t s as a mediator. When ~ +1.23 V i s passed, the remaining Ru (A) R u ( A ) occurs. However, on the r e t u r n scan, only 111

1 1 1

1 1


1 1 1

II

2+

2

2

3 +

F

o

3 + / 2 +

g u r f

+

2 +

2

3+

2+

2+

3+

3+



478


Figure 2. a) A double polymer l a y e r on a P t substrate w i t h energy l e v e l s used to i n t e r p r e t voltammograms (b) and ( c ) . Scan (c) i s that o f a s i n g l e l a y e r o f copolymerized complex A and B. Adapted from Murray et a l . ( 1 8 ) .


29.

479


METTEE

3+

2

the inner l a y e r r e d u c t i o n R u ( A ) -> Ru +(A) occurs s i n c e the R u (B) i s e n e r g e t i c a l l y and s p a t i a l l y i n a c c e s s i b l e (cf F i g . 21(a)). Unless e l e c t r o n s whose p o t e n t i a l i s more negative than +.76 V can be s u p p l i e d to the outer f i l m , the R u ( B ) s i t e s w i l l remain trapped there. Subsequent anodic scans do not r e v e a l the presence of Ru +(B) i n the outer l a y e r , as few i f any are present. The outer l a y e r h o l e s at +.76 V may be discharged, and the o x i d a t i v e prewave returned, however, i f e i t h e r the monomer o f Ru +(B) i s added to the e l e c t r o l y t e (E '[Ru +/ +(B)] = +.76 V) o r i f the p o t e n t i a l i s swept negative enough to reduce bpy° -»· bpy i n the inner l a y e r l i g a n d s . (See redox values i n Figure 2 ( a ) ) . In t h i s l a t t e r case a pronounced cathodic spike s i g n a l s the r e duction of R u ( B ) -> R u ( B ) i n the outer l a y e r , at a p o t e n t i a l n e a r l y 2 V more negative than necessary to e f f e c t the r e d u c t i o n ! One obvious c o n c l u s i o n to be drawn from these experiments i s that these polymer f i l m s are e s s e n t i a l l y i n s u l a t i n g , except i n the r a t h e r narrow energy range a s s o c i a t e d w i t h t h e i r included redox couple. Another i s that charge r e c t i f i c a t i o n may be sustained i n a laminate polymer system, thereby mimicking the r o l e played by the p o t e n t i a l gradient w i t h i n the d e p l e t i o n l a y e r of conventional p-n semiconductor j u n c t i o n s . F u r t h e r d i s c u s s i o n of the o x i d a t i v e and r e d u c t i v e prewaves may be found i n the work of Meyer (19). Examples of the propagation of a redox r e a c t i o n through polymers f i l m s may be found i n the work of M i l l e r (15a). J

3+

2

2

3

2

m


3+

2+

P h o t o c a t a l y t i c Polymers. The e l e c t r o c h e m i c a l experiments c i t e d above were chosen from a vast body of recent polymer coated e l e c t r o d e work. L i k e w i s e the f i e l d of polymer supported photoredox c a t a l y s t s i s a l s o broad and has a more extended h i s t o r y . P o s s i b l y a common l i n k a g e between e l e c t r o c h e m i c a l and photochemic a l c a t a l y s e s , a s s i s t e d by polymers, can be t r a c e d to o x i d a t i o n of a s c o r b i c a c i d (AH2). In 1966 Davidov (20a) found that l i g h t exposed, p o l y a c r y l o n i t r i l e (PAN) c o n t a i n i n g s o l u t i o n s of AH2 consumed oxygen i n a measurably d i f f e r e n t manner than s i m i l a r s o l u t i o n s without AH2. (Simple photoabsorption of 02 a l s o o c c u r s ) . In 1977 Kuwana and coworkers (14) found that RF discharged vapors of b e n z i d i n e (BZ) l e d to the formation of a f i l m on graphite e l e c trodes (PBZ) which e l e c t r o c h e m i c a l l y c a t a l y s e d the same o x i d a t i o n . (Naked graphite e l e c t r o d e s functioned very p o o r l y ) . hv

e'




480

I t may be that t h i s coincidence i s nothing more than f o r t u i t o u s , but i t i s worth n o t i n g that whether a chemical o x i d i z i n g o r r e ducing p o t e n t i a l i s produced at the molecular l e v e l by the a c t i o n of l i g h t o r an e l e c t r i c a l f i e l d , the r e s u l t i s e s s e n t i a l l y the same. An important d i f f e r e n c e remains, however, between e l e c t r o and p h o t o c a t a l y t i c processes. I n the e l e c t r o c a t a l y t i c case, the c r e a t i o n o f the charge gradient i s the i n i t i a l a c t , the one l e a d ing to charge s e p a r a t i o n and thereby chemical p o t e n t i a l . On the other hand, p h o t o l y t i c a l l y e x c i t e d molecules may undergo numerous d i s s i p a t i n g processes such as fluorescence and i n t e r n a l convers i o n , not to mention i r r e v e r s i b l e photochemistry. Thus i t i s ess e n t i a l t o reduce these d i s s i p a t i v e processes t o a minimum, and to the extent that polymers can do t h i s and promote r a p i d charge s e p a r a t i o n , polymers w i l l continue t o f i n d a u s e f u l f u n c t i o n . I t i s u s e f u l to note here that nature has a l r e a d y accomp l i s h e d the goal o f l i g h t induced charge s e p a r a t i o n i n the react i o n centers of Rps..sphéroïdes,, a photosynthetic b a c t e r i a . These l a r g e macromolecular assemblies c o n t a i n organized u n i t s o f cytochrome C ( c y t C), b a c t e r i o c h l o r o p h y l l dimers ( ( B C h l ^ ) , bacteriopheophytin (BPh), a metal-ion f r e e c h l o r o p h y l l , an i r o n complexed quinone (FeQ) and a second quinone molecule (Q2). The l i g h t absorbing and charge s e p a r a t i n g sequence, c r e a t i n g i n the end both o x i d i z i n g and reducing power on opposite s i d e s of a membrane, i s c u r r e n t l y v i z u a l i z e d as f o l l o w s (20b), where the p r i mary donor and acceptor appear i n [ ] , and the time s c a l e i s given on the r i g h t . Species

Time (picoseconds)

cyt

C,[(BChl) ,BPh],QFe,Q

cyt

Ψ hv C,[(BChl)*,BPh],QFe,Q Ψ

2

2

0

2

0

+ cyt

C,[(BChl) ,BPh*],QFe,Q Ψ 2

Photoelectrochemical Catalysis with Polymer ... - ACS Publications

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