Photosynthetic Protein-Based Photovoltaics

Swee Ching Tan

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351646550
• Edition: 1
• Topic: Medicine
• Subtopic: Biochemistry in Medicine

Ever since the discovery of the photoelectric effect, researchers have been trying to improve the efficiency of converting sunlight into electricity through photovoltaic devices. Photosynthetic organisms provide clues for harvesting sunlight and storing the energy in chemical forms. This book offers a concise overview of the fundamental concepts of photosynthesis and the emerging photovoltaic technologies, casting light on the symbiotic relation between these spheres of science.

Although there are many books about the fundamentals of photosynthesis and the various aspects of the photosynthetic processes, this is the first volume to focus on the prospects of studying the photosynthetic proteins, understanding and applying their properties to design prospective solar energy conversion devices that are sustainable and efficient. All in all, the book aims to bring together the present know-how on organic photovoltaics and dye-sensitized solar cells with that of the emerging bio-photovoltaics and the underlying physics of photosynthesis to foster a more eclectic research that would converge towards a sustainable energy technology for the future.

The book mainly serves as a bridge to connect biochemists, who study photosynthetic proteins, and physicists and engineers who design and develop photovoltaic devices. Scientists, engineers and students in the fields of photosynthetic research and solar energy research can use this book as a ready reference.

Key selling features:

• Covers both methods and bio-based materials needed to build bio-based photovoltaics
• Focuses on both techniques and applications
• Summarizes the advantages and limitations of various techniques
• Contributors from multiple disciplines integrate the knowledge of photosynthetic proteins and the physics/engineering of photovoltaic devices.
• Includes adaptive designs and techniques used in other types of solar cells to for the design of protein-based PVs

1	Learning from Nature to Improve Solar Energy Conversion Devices Di Sheng Lee, Yoke Keng Ngeow, and Swee Ching Tan

CONTENTS

1.1 Introduction

1.2 Learning from Photosynthetic Organisms

1.2.1 Reaction Centers (RCs)

1.2.2 Light-Harvesting (LH) Complexes

1.2.3 Photoprotection

1.3 From Natural to Biomimetic Photosynthetic Device

1.4 From Natural to Biomimetic Leaf

1.5 Overview

References

1.1 INTRODUCTION

Global energy consumption models envisage that human civilization would require 46 TW of energy in 2100, which is a few times greater than current global energy consumption.¹ Coupled with an increasing world population, the emerging economies of India and China will pose great challenges to global energy demand. Now, fossil fuels account for most of the global energy demand, and the consumption is even more for developed countries. What’s more, oil production is estimated to decline around 2007–2038,² but recent studies suggest that oil production level has already reached its apex.³ Even if oil reserves run out, coal and gas remain to be exploited.⁴ Therefore, the exigent issue is not the fossil fuel constraint but the repercussions of burning fossil fuel, which releases a staggering amount of CO₂ into the atmosphere. The CO₂ level would rise to that of primordial times if every drop of fossil fuel remaining on our planet is burned.⁵ In the last 400,000 years, atmospheric CO₂ levels were stable at ~250 ppm but have risen sharply to 400 ppm.^6,7 The current staggering atmospheric CO₂ level is the culprit behind global warming; if global warming is left unchecked, it will spell doom not only for human civilization but also for all lives on Earth. For example, at atmospheric CO₂ levels of 450 ppm, irreversible coral reef damage is likely to happen.⁸ A level of 550 ppm would cause the melting of the West Antarctic ice sheet, the rising of sea level for 4–6 m,⁸ and the extinction of a 35% of flora and fauna.⁹ At levels of 650 ppm, thermohaline circulation would be disrupted, and local climate would change considerably.⁹ Some global climate change models¹⁰ suggest that the potential effects could be more serious than previously anticipated;^9,11,12 as such, it is imperative to keep the CO₂ level below 450 ppm. Since CO₂ emission is unlikely to be cut down soon, and the fact that CO₂ is stable in the atmosphere for a long period of time, it is improbable that the CO₂ level could be kept below 450 ppm. Hoffert and coworkers calculated that in order to keep atmospheric CO₂ levels below 450 ppm, we would need to use 11 TW of CO₂-emission-free fuel by 2025.¹ In other words, we are running out of time as we urgently need a CO₂ emission-free technology capable of producing energy equivalent to the global annual energy consumption rate in 2000 (13 TW) within 10 years. However, we are still heavily dependent on fossil fuels for many years to come; thus, it is important to minimize carbon dioxide release into the atmosphere and to develop carbon dioxide sequestration technologies as soon as possible.¹³ Hand in hand with this is definitely the development of highly efficient clean energy technologies.

Unlike wind, geothermal, and hydroelectric energy, solar energy is one of the most copious (178,000 TW year⁻¹) and accessible sources of clean energy.¹⁵ Solar energy can be economically viable even in regions where sunlight is scarce. The bottleneck, however, is the development of affordable photovoltaic systems with high efficiency. In fact, an area of 634 × 634 (equivalent to 4.4% of the Sahara desert) is enough for today’s commercial solar cells to supply global annual energy consumption.¹⁶ Yet, the costs of constructing and installing such extended solar systems are still much higher compared to those of fossil fuels. Hence, we have two challenges ahead. The first is to develop more efficient solar energy systems. Existing solar capture technologies are improving on a daily basis.^17,18 In the foreseeable future, solar systems will be more affordable as biomimetic materials inspired by natural photosynthesis would improve the current solar systems.^19,20 The second is to implement a carbon tax as well as include the costs related to decommissioning nuclear power plants and radioactive waste in order to increase the competition in the energy market. The sun provides solar energy at a rate of 172,500 TW to our planet annually. Since incident sunlight is reflected and absorbed by the atmosphere, only ~65,000 TW of it reaches the hydrosphere and 15,600 TW of it falls on land (Figure 1.1).²¹ To better appreciate the power of the sun, consider that the energy from an hour of sunlight is enough to power the total energy humanity uses in a year.²² The sun spectrum is represented by air mass 0 (AM0), while the amount of sunlight that reaches the Earth has been reduced significantly by the atmosphere and hence is represented by air mass 1.5 (AM1.5); the air mass is the path length that light takes through the atmosphere (Figure 1.2). To fully make use of sunlight, solar-energy converters have to be sufficiently cheap, robust, and efficient; however, current solar technologies are a far cry from being the primary energy source for humanity.

FIGURE 1.1 Rates of solar energy reaching different parts of the Earth. The sun provides solar energy at a rate of 172,500 TW to our planet annually. Since incident sunlight is reflected and absorbed by the atmosphere, only ~65,000 TW of it reaches the hydrosphere and 15,600 TW of it falls on land. (With kind permission from Springer Science+Business Media: Photosynth. Res., 120, 2014, 59–70, Sherman, B.D. et al.)

Mother Nature’s photosynthesis might be the solution to our energy crisis as this elegant process has undergone billions of years of evolution to increase its efficiency to capture the sun’s energy.^24,25 Biological cells evolved highly efficient and sophisticated molecular machineries that can adapt to different environmental conditions. Balzini et al. wrote, “An intelligent approach toward the design of artificial systems for solar energy conversion is to take the natural solar energy conversion sequence (i.e., the light reactions of photosynthesis) as a model and see whether the natural devices can be replaced by artificial ones.”²⁶ As such, photochemists have long foreseen the potentials hidden within the natural solar systems, and recent progress in obtaining atomically resolved structures have shed light on the photosynthetic molecular machinery in terms of structure and function that so elegantly converts sunlight into useful chemical energy. These seminal discoveries were made on the purple bacteria, which is an anaerobic photosynthetic bacteria. Comparable to the role of the hydrogen atom in understanding basic physics and chemistry in earlier times of the scientific era, the photosynthetic machinery of purple bacteria with its multicomponent organization holds the key to unlock the secrets behind the functions and mechanisms of molecular machineries in photosynthetic organisms. Although purple bacteria have similar solar energy conversion as those of green plants, purple bacteria’s ones are simpler and better understood. As such, the photosynthetic machinery of purple bacteria is chosen as a model of biomimicry as a green plant’s two-photosystem photosynthetic machinery is quite complicated. In purple bacteria, the photosynthetic process occurs across a cell membrane and is facilitated by some membrane proteins. It makes use of both visible and near-infrared photons. Antenna systems are employed to collect sunlight and maximize the absorption of wavelengths obtainable from the surroundings. Within the antenna, electronic excitation transfers from one chromophore to another and eventually to a reaction center, where charge separation occurs across the bilayer and where the excitation energy is converted to chemical energy. A bacteriochlorophyll “special pair” near the outside of the cell membrane and a series of steps involving several donor-acceptor cofactors are involved. It is possible to build a more energy-efficient artificial reaction center that employs fewer redox centers.²⁷ Even though most wavelengths of the visible region can be efficiently trapped by photos...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• About the Editor
• Contributors
• Chapter 1 Learning from Nature to Improve Solar Energy Conversion Devices
• Chapter 2 Developments in Electrodes and Electrolytes of Dye-Sensitized Solar Cells
• Chapter 3 Organic Bulk Heterojunction Solar Cells
• Chapter 4 Reaction Centers as Nanoscale Photovoltaic Devices
• Chapter 5 Bio-photoelectrochemical Cells: Protein Immobilization Routes and Electron Transfer Modes
• Chapter 6 Electronics, Photonics, and Device Physics in Protein Biophotovoltaics
• Chapter 7 Challenges and Opportunities of Photosynthetic Protein-Based Solar Cell
• Index