1 Solar Electricity: Lessons Gained from Photosynthesis JAMES R. BOLTON
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The University of Western Ontario, Department of Chemistry, Photochemistry Unit, London, Ontario N6A 5B7 Canada
Nature has developed a mechanistically very complex yet conceptually very simple process for the conversion and storage of solar energy. In this lecture I shall first examine the reaction and mechanism of photosynthesis deriving insights into how nature has achieved this remarkable process. I shall then go on to describe various attempts to mimic the primary steps of photosynthesis. Finally, I shall speculate on how these insights into the mechanism of photosynthesis might be used to design a new type of solar cell for the conversion of light to electricity. The Photosynthesis Reaction The reaction of photosynthesis is clearly the most important chemical reaction since life could not exist for long without it. The overall reaction is CO (g) + H O(l) 2
light
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C H O (s) + O (g) 6
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where C H O (s) is D-glucose, the major energy-storage product of photosynthesis from which all plant and animal biomass is derived. The thermodynamic parameters for the photosynthesis reaction at 298K (25°C) are ΔΗ=467 kJmol ; ΔG=496 kJmol and E°=1.24V (1). It is helpful to think of the photosynthesis reaction as the sum of an oxidation half reaction and a reduction half reaction as shown in Figure 1. In fact, nature does separate these half re actions, in that the reduction of CO to carbohydrates occurs in the stroma of the chloroplast, the organelle in the leaf where the photosynthesis reaction occurs, - whereas, the light-driven oxidation half reaction takes place on the thylakoid membranes which make up the grana stacks within the chloroplast. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) carries the reducing power and most of the energy to the stroma to drive the fixation of CO with the help of some additional energy provided 6
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0097-6156/83/0211 -0003$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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C0 (g)
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8 hv
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/ carbon \f 1 fixation Y V cycle Λ
2NADPH+2H* + 3 ATP ^/light Y driven y 1 electron A JV transportJ\ 2NADP thylakoid +3ADP membrane 3Pi +
Λ stroma
2H 0(I) 2
0 (g) ?
+
l/6C H 0 (s)+ H 0(l) 6
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Figure 1. The separation of the half reaction in the chloroplast of the photosynthetic plant cell. The dark reaction (left) and the light-driven reactions (right) are shown. Key: NADP\ oxidized form of nicotinamide adenine dinucleotide phosphate; ATP, adenosine triphosphate; and P inorganic phosphate. if
In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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by adenosine triphosphate (ATP) which i s a l s o generated on the t h y l a k o i d membrane. [Readers i n t e r e s t e d i n the s t r u c t u r e and mechanism of photosynthesis should r e f e r to references (2-4)·] Although the mechanism of the c a r b o n - f i x a t i o n c y c l e i s very i n t e r e s t i n g biochemistry, i t i s the mechanism of the o x i d a t i o n h a l f r e a c t i o n which w i l l concern us most, because i t i s here that the conversion of s u n l i g h t to chemical energy takes p l a c e . We s h a l l now explore some of the d e t a i l s of t h i s remarkable process.
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Mechanism of the Primary Photochemical Reaction of Photosynthesis The primary photochemistry of photosynthesis takes place w i t h i n very s p e c i a l i z e d r e a c t i o n - c e n t e r p r o t e i n s s i t u a t e d i n the t h y l a k o i d membrane (see Figure 2) of the c h l o r o p l a s t (2) . Most (>99%) of the c h l o r o p h y l l and other pigments act as an antenna system to gather l i g h t photons and channel them to one of the two r e a c t i o n centers. In essence the antenna c h l o r o p h y l l system acts as a photon concentrator, concentrating the photon f l u x by a f a c t o r of ^300 over what i t would be without an antenna system. In both Photosystems I and I I the photochemically a c t i v e component (P700 or P680) i s thought to be a c h l o r o p h y l l a species. P700 i s l i k e l y a dimer of c h l o r o p h y l l a. The primary photochemical step then i n v o l v e s the t r a n s f e r of an e l e c t r o n from the donor (P700 or P680) to an acceptor species. In Photosystem I I the acceptor Q i s thought to be a molecule of plastoquinone. In Photosystem I 1
0
0 the acceptors are not as w e l l c h a r a c t e r i z e d ; A may be a molecule of c h l o r o p h y l l a i n a s p e c i a l environment and A i s thought to be an i r o n - s u l f u r center. Hence, i t appears that nature has chosen the f a s t e s t and perhaps simplest of a l l photochemical r e a c t i o n s , namely, photochemical e l e c t r o n t r a n s f e r , to trap the energy of the e l u s i v e sunbeam. In e f f e c t , the r e a c t i o n - c e n t e r p r o t e i n s of photosynthesis are s o l a r c e l l s converting l i g h t to e l e c t r i c i t y which i s then used to d r i v e the r e l a t i v e l y slow biochemical r e a c t i o n s which lead u l t i m a t e l y to D-glucose. These r e a c t i o n - c e n t e r s o l a r c e l l s are very e f f e c t i v e - the y i e l d f o r e l e c t r o n t r a n s f e r i s almost u n i t y , and the o v e r a l l s o l a r energy conversion to e l e c t r i c a l energy i s VL6% (5). This i s as good as or g e n e r a l l y much b e t t e r than most commercially a v a i l a b l e s i l i c o n s o l a r c e l l s . Now l e t us examine more c l o s e l y the d e t a i l s of how the x
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In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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NADPH
Figure 2 .
Model of the thylakoid membrane showing the various components involved in electron transport from H 0 to NADP\ 2
The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key: Z, the watersplitting enzyme which contains Μη; P680 and Q the primary donor and acceptor species in the reaction-center protein of Photosystem II; Qi and Q , probably plastoquinone molecules; PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside; Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I; P700 and At, the primary donor and acceptor species of the Photosystem I reaction-center protein; At, Fe-SA. and Fe-S , membrane-bound secondary acceptors which are probably Fe-S centers; Fd, soluble ferredoxin Fe-S protein; and fp, is theflavoproteinthat functions as the enzyme that carries out the reduction of NADP+ to NADPH. h
t
B
In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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e l e c t r o n i s t r a n s f e r r e d from one s i d e of the t h y l a k o i d membrane to the other. Figure 3 i l l u s t r a t e s a view of our current knowledge of the composition and s t r u c t u r e of the r e a c t i o n - c e n t e r p r o t e i n from photosynthetic b a c t e r i a which performs a photochemical e l e c t r o n - t r a n s f e r r e a c t i o n analogous to the r e a c t i o n s i n the two photosystems of green-plant photosynthesis (2, 7_ 8). The b a c t e r i a l r e a c t i o n - c e n t e r p r o t e i n contains 4 b a c t e r i o c h l o r o p h y l l molecules, 2 bacteriopheophytins, one nonheme i r o n and a ubiquinone molecule. Two of the b a c t e r i o c h l o r o p h y l l molecules form a s p e c i a l p a i r c a l l e d P870 which acts as the photochemical e l e c t r o n donor. The other two b a c t e r i o c h l o r o p h y l l s absorb at 800 nm and may act as the l a s t l i n k to the antenna system. Within
