Photochemical Water Splitting

Materials and Applications

Neelu Chouhan, Ru-Shi Liu, Jiujun Zhang

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781315279633
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Organic Chemistry

Cleavage of water to its constituents (i.e., hydrogen and oxygen) for production of hydrogen energy at an industrial scale is one of the "holy grails" of materials science. That can be done by utilizing the renewable energy resource i.e. sunlight and photocatalytic material. The sunlight and water are abundant and free of cost available at this planet. But the development of a stable, efficient and cost-effective photocatalytic material to split water is still a great challenge. To develop the effective materials for photocatalytic water splitting, various type of materials with different sizes and structures from nano to giant have been explored that includes metal oxides, metal chalcogenides, carbides, nitrides, phosphides, and so on. Fundamental concepts and state of art materials for the water splitting are also discussed to understand the phenomenon/mechanism behind the photoelectrochemical water splitting. This book gives a comprehensive overview and description of the manufacturing of photocatalytic materials and devices for water splitting by controlling the chemical composition, particle size, morphology, orientation and aspect ratios of the materials. The real technological breakthroughs in the development of the photoactive materials with considerable efficiency, are well conversed to bring out the practical aspects of the technique and its commercialization.

1	Introduction to Hydrogen as a Green Fuel

1.1 INTRODUCTION

Hydrogen gas can be seen as a future renewable energy (RE) carrier/fuel by virtue of the fact that it gives water as a combustion product without evolving the “greenhouse gases” such as CO₂. Hydrogen is considered the most clean and storable energy carrier of the future if it can be produced from a renewable energy source via a CO₂-neutral and efficient route. Solar water splitting is a renewable and sustainable fuel production method because it can utilize sunlight, the most abundant energy source on Earth, and water, the most abundant natural resource available on Earth. Water splitting can be carried out using coupled solar cell–water electrolysis systems, but efficiency loss between the two systems and high installation cost make them a less attractive option. As an alternative route, the direct photoelectrochemical (PEC) cell is potentially more economical because it combines the functions of a solar cell and an electrolyzer in a single device.

1.2 CURRENT ENERGY SCENARIO

Energy is a lever to trigger the speed of development in all segments of life (economical, social, and political) for safer, affordable, cleaner, and more habitable environmental conditions, that required for better standard of living. A secured, uninterrupted, affordable, and adequate energy supply is required to sustain global economic growth and stability. Worldwide our current energy storage contains 1047.7 billion barrels of oil, 5501.5 trillion standard cubic foot (scf) of natural gas (NG), and 984 billion tons of coal as conventional energy sources that might be sufficient to satisfy our energy needs for 40.2, 53.8, and 205 years, respectively. In addition, these energy sources are not proportionately distributed throughout the world. For example, the United States contains about 25% of world coal reserves while Middle East countries account for about 60% of oil reserves. This results in energy insecurity in countries that have inadequate energy assets/supply that are the most probable grounds for political disturbances. According to the estimated record, nearly one-quarter of world’s population (1.6 billion) still does not have electricity today. However, the continuous increase in energy requirements has been putting a lot of pressure on conventional energy sources. But the limited availability of the fossil fuels and corresponding environmental threats compel us to explore an uninterrupted supply of energy by utilizing alternative sources. All conventional sources of energy are carbon rich and so their combustion leads to CO₂ emission (main greenhouse gas) that adds to the extra burden on its naturally occurring amount. CO₂ as a greenhouse gas absorbs the infrared part of the sun’s radiation and reradiates it back to Earth’s surface, which traps the heat and keeps Earth 30° warmer than it would be otherwise—but without greenhouse gases, Earth would be too cold to live. But the additional CO₂ leads to an extra rise in temperature (van Ruijven et al. 2011). As a consequence, Earth’s average temperature increases, which will result in unpredictable changes in weather patterns in the form of floods, droughts, and submerging of low-lying areas due to melting of ice at the poles. The current concentration level of CO₂ in the atmosphere is around 390 ppm (in January 2011) and scientists suggest that this value should drop to 350 ppm, otherwise it should lead to irreversible catastrophic effects. CO₂ emission was found to be 27 gigatons in 2005 and is expected to boost up to 42 gigatons in 2030 and 62 gigatons in 2050. The countrywise contribution to CO₂ emission is shown in Figure 1.1 (U.S. Environmental Protection Agency 2013). Most of this emission comes from power, industrial, and transportation sectors.

Our energy consumption history (1990–2011) and futuristic energy projections (2011–2035) are shown in Figure 1.1b, which will be increased at a rate of 1.4% per year till 2035. This energy consumption profile reflected that the world average capacity of energy utilization rates have continued to rise over time, from about 65% in 1990 to about 80% today, with some increases still anticipated in the future. This increasing demand for energy, imposed challenges in front of us as the threat of disruptive climate and huge capital investments in energy segment. Both are becoming problematic issues for developed and developing countries. According to a report, there is an irrational ratio of the population and energy consumption rate found among the developed (20% and 60%) and developing countries (80% and 40%) (Nezhad 2009). Meeting this demand without further damage to the environment is a great challenge. There are two main approaches to achieve a long-range energy scenario: the first scenario involves the replacement of the long-term development process with advanced energy-producing technologies and/or implementation of hybrid processes instead of conventional fuels to reduce fuel consumption and to reduce the climate change effect via new advanced technologies used for gas conversion and, so, more fixations of the gases to value-added products will be achieved. The second scenario includes the development of alternative energy resources. The six available renewable resources in nature, biomass, hydropower, wind, solar, geothermal, and biofuels, are economically, socially, and environmentally sustainable. But no single approach is able to achieve the goal. Therefore, a number of energy scenarios are given by different agencies, including the Energy Information Administration (EIA), World Energy Council (WEC), International Energy Agency (IEA), and many more, using different proportions of both approaches. The most comprehensive and authentic analysis on the world energy scenario, based on the world’s facts and perceptions, was given by the IEA, which has constituted a committee of 5000 experts from 39 countries on Energy Research and Technology to develop a strategy for the world energy scenario for 2050. In their report they concluded that world energy consumption will be doubled by 2050 and carbon emission rate will increase by a factor of 2.5. Their recommendations focused on alternative resources of energy. To shape the world energy future, IEA projected strategic energy scenario planning for 2050, which is represented in Figure 1.2 (Nezhad 2007). This includes a step-by-step process to achieve the desired scenario. The process is initiated by identifying the scope of the scenario and then defining the main driving forces behind it. For developing the scenario model, system dynamics of the future energy market will be analyzed by identification of the interrelationship among the driving forces. Continuation of the status quo is not sustainable due to the rising demand for energy, particularly for fossil fuel, and unacceptable level of the CO₂ emission. Therefore, by employing the existing technologies or those that are under development, a path toward clean, competitive energy will be established by utilizing sustainable energy solutions. Subsequently, possible future energy scenario (a few models suggested by the IEA are the baseline for the accelerated technologies and blue map) models would be developed. They assume the future energy demand and level of CO₂ emissions in light of the above-defined driving forces and accordingly set periodical goals to meet the most probable energy assessment and reduced CO₂ emission level for 2050 by utilizing decision support software. Finally, strategies will be developed to accomplish the goal. No single strategy is enough to reach the desired level of energy production, consumption, and CO₂ reduction. Therefore, a fusion of the following strategies might be used for achieving the desired goal:

FIGURE 1.1 (a) Countrywise per capita CO₂ emission in percentage. (b) World energy consumption history (1990–2011) and futuristic energy projections (2011–2035); 1 Quadrillion BTU = 1015 BTU = 1015 x 1054 Joules. (From U.S.Environmental Protection Agency, Light-Duty Automotive Technology, Carbon Dioxide Emissions, and Fuel Economy Trends: 1975 Through 2012, March 2013.)

1. Research development, demonstration, and deployment of new technology

2. Investm...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Series Preface
• Preface
• Introduction
• Authors
• Chapter 1 Introduction to Hydrogen as a Green Fuel
• Chapter 2 Concepts in Photochemical Water Splitting
• Chapter 3 Water-Splitting Technologies for Hydrogen Generation
• Chapter 4 Electrochemical Water Splitting
• Chapter 5 Oxide Semiconductors (ZnO, TiO2, Fe2O3, WO3, etc.) as Photocatalysts for Water Splitting
• Chapter 6 Fundamental Understanding of the Photocatalytic Mechanisms
• Chapter 7 Nanostructured Semiconducting Materials for Water Splitting
• Index