Energy Conversion in Natural and Artificial Photosynthesis

134 Pages·2018·4.65 MB·English

by  Katharina Brinkert

Springer Series in Chemical Physics 117
Katharina Brinkert
Energy Conversion
in Natural
and Artificial
Photosynthesis
Springer Series in Chemical Physics
Volume 117
Series editors
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Jan Peter Toennies, Göttingen, Germany
Kaoru Yamanouchi, Tokyo, Japan
Wolfgang Zinth, München, Germany
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More information about this series at http://www.springer.com/series/676
Katharina Brinkert
Energy Conversion in Natural
fi
and Arti cial Photosynthesis
123
Katharina Brinkert
Division of Chemistry
andChemical Engineering
California Institute ofTechnology
Pasadena,CA
USA
and
European Space Agency
ESTEC
Noordwijk
TheNetherlands
ISSN 0172-6218
SpringerSeries inChemical Physics
ISBN978-3-319-77979-9 ISBN978-3-319-77980-5 (eBook)
https://doi.org/10.1007/978-3-319-77980-5
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Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Oxygenic Photosynthesis—A Brief Overview . . . . . . . . . . . . . . . . . 3
2.1 The Light Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 The Dark Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3 Architecture, Structure and Function of the Energy
Conversion Centers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1 Overview of the Evolution and Structure of the Photosynthetic
Reaction Centers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1 Antenna System Organization. . . . . . . . . . . . . . . . . . . . . 10
3.1.2 Reaction Center Structures and Cofactor
Arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2 Semiconductor Photochemistry and Principles
of Photoelectrocatalysis: Energetics, Charge Separation
and Excitation Transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.2.1 Photoelectrochemistry of Semiconductors . . . . . . . . . . . . 23
3.2.2 Photoelectrode Materials . . . . . . . . . . . . . . . . . . . . . . . . 25
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4 Electron Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.1 Electron Transfer, Bioenergetics and Redox Potentials
of Cofactors in Photosystem II . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.2 Proton-Coupled Electron Transfer in Photosystem II:
Kinetics and Thermodynamics. . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.2.1 The Tyrosyl Radicals in Photosystem II:
Why D and Why Z? . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.3 Back-Reactions, Short-Circuits and Leaks: Kinetics
and Pathways of Charge Recombination . . . . . . . . . . . . . . . . . . 41
v
vi Contents
4.4 Photoelectrochemistry in Semiconductors: Electron
Transfer and Recombination Reactions. . . . . . . . . . . . . . . . . . . . 43
4.4.1 Nanostructured Photoelectrodes . . . . . . . . . . . . . . . . . . . 50
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
5 Water Oxidation Catalysis and Hydrogen Evolution . . . . . . . . . . . 55
5.1 Structure and Function of Nature’s Oxygen Evolving
Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5.1.1 Channel Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5.1.2 Catalytic Cycle and Manganese Oxidation States. . . . . . . 58
5.2 Photoelectrocatalysis for Solar Water-Splitting . . . . . . . . . . . . . . 62
5.2.1 Electrocatalysts: Oxygen Evolution on Semiconductor
Photoelectrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.3 Molecular Catalysts for Water Oxidation . . . . . . . . . . . . . . . . . . 65
5.4 Photoelectrochemical Hydrogen Production . . . . . . . . . . . . . . . . 69
5.4.1 Catalyst Materials and Reaction Mechanisms. . . . . . . . . . 70
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6 Carbon Fixation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
6.1 RubisCO—Structure, Functionality and Catalytic
Efficiencies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
6.2 Photoelectrochemical CO Reduction: Materials,
2
Challenges and Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6.2.1 Sequential Versus Concerted Proton-Coupled
Electron Transfer Mechanisms . . . . . . . . . . . . . . . . . . . . 82
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
7 Protection Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
7.1 Photodamage in Photosystem II. . . . . . . . . . . . . . . . . . . . . . . . . 88
7.1.1 Protection Against Light Damage . . . . . . . . . . . . . . . . . . 89
7.2 Catalyst Stability and Photocorrosion in Photoelectrochemical
Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
7.2.1 Role and Function of Protection Layers . . . . . . . . . . . . . 92
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
8 Biomimetic Systems for Artificial Photosynthesis . . . . . . . . . . . . . . 97
8.1 Photosynthetic Model Systems: Concepts and Ideas
to Realize Artificial Photosynthesis . . . . . . . . . . . . . . . . . . . . . . 97
8.1.1 Electron Transfer Studies . . . . . . . . . . . . . . . . . . . . . . . . 99
8.1.2 Proton-Coupled Electron Transfer Studies . . . . . . . . . . . . 102
8.1.3 Synthetic Mimics of the Mn CaO Cluster . . . . . . . . . . . 103
4 5
8.2 The Photoelectrochemical Device: Utilizing Solar Energy
for Water Oxidation, Hydrogen Production and CO Fixation . . . 105
2
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
Contents vii
9 Efficiency of Photosynthesis and Photoelectrochemical Cells. . . . . . 111
9.1 The Photosynthetic Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 111
9.2 Optimizing Photosynthesis?. . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
9.3 Efficiencies of Solar Water Splitting Cells . . . . . . . . . . . . . . . . . 115
9.3.1 Calculation of Solar-to-Chemical Conversion
Efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
9.4 Comparison of Photosynthetic and Photoelectrochemical
Cell Efficiency? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
10 Conclusion: Towards the Realization of an Artificial Leaf . . . . . . . 123
Index .... .... .... .... .... ..... .... .... .... .... .... ..... .... 125
Chapter 1
Introduction
About2.3billionyearsago,naturedevelopeditsmostimportantenergyconversion
process: photosynthesis. Since decades, the crucial elements and principles of
converting water and carbon dioxide into oxygen and chemical energy using
sunlight have inspired scientists in the design of artificial biomimetic systems:
collectingandstoringsolarenergyinchemicalbondsisahighlydesirableapproach
to solving the energy challenge. Artificial systems for solar-driven water-splitting
face, however, similar challenges to biological photosynthesis: efficient mecha-
nisms for light absorption, charge separation and transfer, catalysis and protection
againstcorrosionarerequiredtoprovideastableandconstantenergyfluxfromthe
photoelectrolysis of water.
Thismonographaimsatdiscussingthebasicprinciplesandprocessesofnatural
photosynthesis and comparing them directly with recent developments and con-
cepts currently realized in artificial photosynthetic systems, capable of utilizing
sunlight and converting carbon dioxide and water into a chemical fuel. Here, the
main focus lies on photoelectrochemical cells, where semiconducting photoanodes
and -cathodes modified with (electro-) catalysts are used to oxidize water, produce
hydrogen and reduce carbon dioxide in a monolithic device. Fundamental photo-
chemical and photophysical processes are presented and discussed along with
protection mechanisms and efficiency calculations for both, natural and artificial
photosynthesis. Key parameters are identified which are crucial for the efficient
operation of natural photosynthesis and their validity and applicability for the
design of artificial solar-driven water-splitting systems are further on examined.
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https://doi.org/10.1007/978-3-319-77980-5_1
Chapter 2
—
Oxygenic Photosynthesis A Brief
Overview
Oxygenic photosynthesis is the process in which oxygen and carbohydrates are
produced from water and carbon dioxide utilizing sunlight. With the ability of
convertingsolarenergyintochemicalenergy,oxygenicphotosynthesisremainsone
ofthemostimportantbiologicalprocessesonearth,producingdirectlyorindirectly
thebuildingblocksofalllivingorganismsandalsoofoil,gasandcoal.Incontrast
tosomeanaerobicphotosyntheticorganismswhichuseH Sasareducingagentand
2
produce sulphur as a major product, oxygenic photosynthesis utilizes H O as the
2
reducing agent. It is accomplished by a series of reactions which occur in the
chloroplastofphotosyntheticeukaryotesandcyanobacteriaandischaracterizedby
two main sets of reactions: the ‘light’ and the ‘dark’ reactions. Early biochemical
studies demonstrated that chloroplast thylakoid membranes oxidize H O, reduce
2
NADP to NADPH and synthesize ATP during the so-called ‘light reactions’ [1].
These reactions were shown to be catalyzed by two photosystems, Photosystem I
(PSI) and Photosystem II (PSII), an ATP synthase, which produces the energy
source ATP through a proton gradient formed by light-driven electron transfer
reactions and the cytochrome(cyt) b f complex, mediating the electron transport
6
between PSII and PSI and partly converting the redox energy into the proton
gradient used for ATP formation [2]. The ‘dark’ reactions utilize the reducing
equivalents NAPDH and ATP generated by the light reactions to reduce atmo-
spheric CO into carbohydrates, which act as the respiratory source.
2
2.1 The Light Reactions
For the light-driven reactions in photosynthesis, four protein complexes are
requiredwhichresideinthethylakoids,amembranecontinuumofflattenedsacsin
the inner membrane part of the chloroplast [3]. They form a three-dimensional
network enclosing an aqueous space called the lumen and are differentiated into
cylindrical stacked structures, the grana, and interconnecting single membrane
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