Focus

Solar fuels are substances that store solar energy in the form of usable chemical energy. For them to be appropriate substances have to meet various requirements, which include the ability for their efficient production, sufficient energy density, and flexible conversion into heat, electrical, or mechanical energy. An essential requisite is environmental friendliness in order to sustainably incorporate conversion products into the global circulation of matter of the biosphere. This can be fulfilled now only by H2 and to some extent by some carbohydrates directly or indirectly produced by a solar energy source. Direct conversion of solar energy into free chemical energy – either by hydrogen or by hydrocarbon production – requires the development of efficient catalysts for the oxidation of water. This represents a huge materials design challenge, because multiple requirements for catalyst materials must be addressed simultaneously. After an introduction concerning the materials requirements for solar fuel production, storage, transport, and consumption, this chapter focuses on the topic of water-oxidation catalysis: what we can we learn from evolution for the development of an artificial oxygen-evolution center?

Synopsis

Solar fuels are substances that store solar energy as usable chemical energy at rates that allow sustainable conversion into other forms of energy. Oxidation of water driven by solar light, in order to gain a proton source for either hydrogen or hydrocarbon formation, seems to be the key issue. There are three fundamental steps in the conversion of solar energy into chemical energy (the first two of which also apply to the conversion of solar energy into electrical energy in photovoltaic cells; see Chapter 18). The first is light capture – absorbing the sunlight and transforming it into chemical energy of excited electron–hole pairs. The second is electron and hole transfer – separating and transporting sunlight-excited electrons and holes from their original sites in order to use them. The third is catalysis – the efficient generation and breaking of chemical bonds using the electrons and holes produced in this process to reduce and oxidize compounds, respectively. Although much progress has been made in catalytic control of reaction paths for the production of hydrocarbon and ammonia compounds, detailed understanding of the atomic processes occurring at catalysts and materials design of highly active and specific catalysts are still in their early stages. Here, the outstanding problem is the control of multielectron-transfer reactions, such as the evolution of oxygen from water, which requires four electrons for the liberation of one molecule of oxygen. Thermodynamics promise an energetically most favorable possibility for the oxidation of water when all four electrons are extracted in a correlated way with a minimum identical energy input (1.23 eV). Multielectron transfer requires a well-balanced coordinated chemical reaction involving multiple electron transfers through a sufficiently complex reacting chemical catalyst.

Focus

Energy efficiency has been recognized as the most effective near-term means to meet the energy and environmental crisis we face today. In the USA, buildings are the largest energy-consumption sector of the economy. Residential and commercial buildings combined consume over 40% of the primary energy and over two-thirds of the total electricity [1]. Heating and cooling are the largest portions of this. Demonstration homes have shown that the heating consumption can be reduced by as much as 90% by the proper application of very thick thermal insulation in the walls, roof, and windows [2]. One challenge is the development of very thin economical insulation materials that provide the same performance.

Synopsis

Thermal insulations comprise a wide variety of materials whose primary function is the reduction of heat and mass transfer. These insulations are made from foams, fibers, and other fine-structured solids that encapsulate a gas or are held in vacuum. In buildings, insulation improves energy efficiency by reducing heat loss in winter and heat gain in summer. Even modern windows have been designed to act as insulators to improve building performance. Appliances such as refrigerators and ovens use insulation to maintain temperature and to be more energy-efficient. Insulations are also used in industrial operations such as furnaces for metal and glass manufacture as well as as a means to control silicon-chip formation. In space and on Earth, insulations are used for protection in harsh environments. The development of the next generation of insulations requires an understanding of the physics of heat transfer and of the role advanced materials play in limiting heat transfer by the mechanisms of conduction, radiation, and convection.

Focus

The Sun's radiation can be modeled as a 6,000-K blackbody radiator. Whereas photovoltaics (PV) can convert the part of the Sun's spectrum to electrical energy, over 40% of that spectrum, namely, the infrared (IR) range, is lost as heat. In solar thermoelectrics (TE), the thermal energy from the IR range is converted directly into electricity. Therefore, a solar PV–TE hybrid system would have access to the entire spectrum of the Sun.

Synopsis

With respect to solar energy conversion, PV devices utilize the UV region, whereas TE devices utilize the IR region (which is waste heat with respect to the PV devices) to generate electricity. In a solar PV–TE hybrid system, a high-efficiency solar collector would turn the sunlight (from the IR spectrum) into heat that would then be transformed by TE devices into usable electricity. In addition, the solar thermal energy could be stored in a thermal bath, or TE devices could be used to charge batteries that could then provide electricity when the Sun was not shining. Such a TE system would need to operate at around 1,000 K (~700 °C), and the materials would need to exhibit high ZT values around this temperature.

Focus

Nuclear power holds the promise of a sustainable, affordable, carbon-friendly source of energy for the twenty-first century on a scale that can help meet the world's growing need for energy and slow the pace of global climate change. However, a global expansion of nuclear power also poses significant challenges. Nuclear power must be economically competitive, safe, and secure; its waste must be safely disposed of; and, most importantly, the expansion of nuclear power should not lead to further proliferation of nuclear weapons. This chapter provides an overview of the proliferation risks of nuclear power and how they could be managed through a combination of technical, political, and institutional measures.

Synopsis

The million-fold increase in energy density in nuclear power compared with other traditional energy sources, such as chemical combustion, makes nuclear energy very attractive for the generation of electricity; however, it is exactly this high energy density that can be used to create weapons of unprecedented power and lethality. The development of commercial nuclear power has, since its inception, had to cope with the prospect of potentially aiding the spread of nuclear weapons. Although commercial nuclear power plants have not directly led to weapon proliferation, the technologies of the nuclear fuel cycle, namely fabricating and enriching fuel, operating the reactors, and dealing with the spent fuel, provides a means for countries to come perilously close to obtaining the fissile materials, 235U and 239Pu, which are required for nuclear weapons. Several countries have developed most of the technical essentials for nuclear weapons under the guise of pursuing nuclear power or research.

Focus

The historic and current global energy portfolio is dominated by fossil fuels, an affordable and plentiful means of energizing the development of human civilization. The focus of this chapter is to give an overview introduction to coal and coal conversion processes, with a focus on the development of materials and strategies that promote the efficient use of coal in an environmentally friendly manner.

Synopsis

Coal is the altered remains of biomass, and can be considered a means of storing solar energy on very long time scales. Coal has been used for centuries as a means of an affordable and plentiful energy source, and in recent years has been exploited for these characteristics through two processes: the direct combustion of coal to produce heat and electricity though steam cycles; and the gasification process that produces a highly combustible gas or liquid fuels and chemicals through liquefaction processes.

Focus

Life-cycle assessment (LCA) evaluates the energy and material requirements and resulting environmental impacts of a product or process over its entire life cycle from raw-material extraction to disposal. This examination across the life cycle provides a systems perspective that can aid decision making for product optimization, product selection, and supply-chain management.

Synopsis

Life-cycle assessment evaluates the environmental impacts of a product or process over its entire life cycle. It can provide an environmental profile of a system or process through the evaluation of inputs, outputs, and potential environmental impacts. There are generally two approaches to conducting an LCA, namely, a process-oriented approach and an economic input–output approach. In a process-oriented assessment, the inputs and outputs are itemized for each step in the process. Specifically, the five steps considered are raw-material acquisition or extraction, material processing, product manufacturing, use, and recovery and retirement. An optional transportation stage can also be added. In contrast, the latter type of assessment considers the required materials and energy resources (inputs) of a process to estimate the resulting environmental emissions (outputs).

Likely only the very brave will reach this point after having studied all of the “materials” (no pun intended) presented in this book. Most students will have used this book a bit as one chooses courses, focusing on what is needed, interesting, and/or challenging. For the reader, we would now like to raise some crosscutting concepts that any student of this information should consider.

To a large extent this book presents sets of chapters with insight into specific areas. In many cases the chapters present technology-specific views and understanding of the current state of the art or state of affairs for the specific area. The more technology-specific chapters provide an idea of what is needed in order to advance specific technologies, and present a picture of how that technology or the situation brought about by the technology may evolve.

Focus

What is called in this chapter the “engineering” of natural photosynthesis has been performed by evolution over several billions of years. Its optimization, against thermodynamic and other selection criteria from the biological environment, has led to a remarkably limited set of molecular and supramolecular motifs. Photosynthesis starts with absorption of light by mutually interacting chlorophyll (Chl) and related molecules. These are embedded in protein matrices that promote rapid transport of energy by excitons and lower the energy of transition states for charge separation and multielectron catalysis. An understanding of engineered natural photosynthesis is the underpinning of the design of artificial solar-to-fuel devices.

Synopsis

Light energy is abundant and evenly spread over the surface of the Earth, and virtually all organisms depend on the conversion and chemical storage of light energy by natural photosynthesis. The advent of natural photosynthesis opened up new energy conversion pathways for progression toward thermodynamic equilibrium between the very hot Sun and the much colder Earth. The first-principles thermodynamic and mechanistic analysis of how natural photosynthesis is engineered by biological evolution in this chapter shows that combining different functionalities of light harvesting, charge separation, and catalysis in small molecules or at a single narrow interface is incompatible with high solar-to-fuel conversion efficiency, and that possible solutions to the problem of accumulation of solar energy in chemicals require complex device topologies. This led biology to a modular design approach that is the basis of photosynthesis. It is the result of an intensive evolutionary effort of shaping complex structures, driven by the abundant solar energy spread over the planet.

Focus

The use of wind and solar electricity generation has grown tremendously during the last decade. This raises the important question of how these variable and uncertain resources can be effectively used while maintaining reliable electricity generation.

Synopsis

The large-scale deployment of wind and solar energy creates challenges for grid operators to maintain reliable service. Wind and solar output are variable, uncertain, and often not correlated with normal demand patterns for electricity. At low penetrations in an energy system (up to about 20% on an energy basis) these energy sources act to reduce the fuel use and emissions from conventional power plants used to meet normal variations in electricity demand. These sources can also add varying levels of “firm capacity” to the system, depending on technology and location. Studies have found that current utility systems can accommodate these levels of variable generation sources with a combination of changes in operational practices, but without massive deployment of “enabling” technologies such as energy storage. However, the variability and uncertainty impose modest cost penalties, since utilities require increased operating reserves in order to maintain reliable service. At higher penetrations (beyond 20%) new methods of integrating renewables into the grid are required, including transmitting power over long distances to take advantage of spatial diversity and new generation technologies that can ramp rapidly to respond to variations in demand. At these penetrations variable generation sources also begin to affect the operation of baseload power plants, which creates more challenges for system operators, and may lead to curtailed wind and solar generation. This will begin to decrease the environmental benefits of these renewable sources. At very high penetrations (beyond 30%) the simple coincidence of energy supply and demand limits the useful contributions of wind and solar energy, with wind potentially exceeding the demand for electricity on occasions. This will require deployment of a variety of enabling technologies, including greater use of long-distance transmission, shiftable load, new demands for electricity, such as electric vehicles, and energy storage.

Focus

The financial future of firms that depend on materials can be permanently compromised if the availability of those materials is constrained. This chapter examines how limited materials availability can affect a firm; how a firm can know whether it is using materials that are at risk of becoming of limited availability; and what can be done to mitigate that risk. One mitigation strategy is to foster an effective recycling system. The chapter concludes by exploring the benefits of expanded recycling and some of the remaining challenges to making that happen.

Synopsis

Resource scarcity is a topic that has challenged scientists, engineers, and economists for centuries. Current interest in this topic stems from the central role of natural resources in our economy, the inherently finite supply of those resources, and the unprecedented rate of resource consumption.

Focus

Motor vehicles consume about 19% of the world's total energy supplies, with 95% of this amount being petroleum, accounting for about 60% of the total world petroleum production [1]. In the USA about 80.5% of the motorized transportation energy is consumed by road vehicles [2]. The recent increase in petroleum prices, expanding world economic prosperity, the probable peaking of conventional petroleum production in the coming decades, regulations to increase fuel economy standards, concerns about global climate change, and the recent release of significant quantities of oil as a result of the failure of the deep-sea well in the Gulf of Mexico all suggest the need to focus efforts to increase the efficiency of the use of, and develop alternatives for, petroleum-based fuels used in road transportation. Efforts to increase the energy efficiency of a vehicle will require improvements in materials and processes for propulsion systems and structures, new advanced propulsion systems, batteries, and alternative fuels.

Synopsis

In many industrial countries, road transportation accounts for a significant portion of the country's energy consumption. In developing countries, the use of energy for transportation is on the rise. Most studies indicate that 70%–80% of the energy usage during the life cycle of a road transportation vehicle is in the use phase, including maintenance. The remainder is energy usage in the production of the vehicles, including the production of the materials, supply of the fuel, and disposing of the vehicles. Fuel economy and greenhouse-gas-emission regulations in North America, Japan, and Europe are forcing manufacturers to look into reducing fuel consumption in any cost-effective manner possible. Thus, advances in many materials and processes will be required in efforts to increase the energy efficiency of motorized vehicles for road transportation.

Focus

Materials advances could help to reduce the energy and environmental impacts of buildings. Globally, buildings consume 30%–40% of primary energy and account for 25%–33% of CO2 emissions. Building energy consumption emanates from a variety of sources, some of which are related to the building envelope or fabric, some to the equipment in the building, and some to both. Opportunities for reducing energy use in buildings through the application of innovative materials are therefore numerous, but there is no one system, component, or material whose improvement can alone solve the building energy problem. Many of the loads in a building are interactive, and this complicates cost–benefit analysis for new materials, components, and systems. Moreover, components and materials for buildings must meet stringent durability and cost–performance criteria to last the long service lifetimes of buildings and compete successfully in the marketplace.

Synopsis

The world's buildings account for about one-third of greenhouse-gas emissions. The authors of a number of studies have concluded that one of the most cost-effective ways to reduce carbon emissions is to increase the energy efficiency of the existing building stock via energy retrofits, and to require a high degree of efficiency in new buildings. Several nations and states have even set goals requiring new buildings to have “zero net energy” consumption by established dates in the future. Some of the increases in efficiency can be achieved with currently available technology by using advanced building energy simulation and optimization techniques to establish technology packages that deliver maximum energy savings for minimal cost. The technology packages will vary by building type and climate type, and will also need to differ according to the level of development in various parts of the world. To realize further efficiency gains, new and better materials and technologies will be needed. This presents a unique opportunity and challenge for materials scientists. Success will depend not only on mastery of the usual disciplines associated with materials science, but also on a holistic understanding of the comfort, construction, and energy systems in buildings. This chapter explores some foundational concepts about how energy is used in buildings, and also highlights areas where materials-science advances would be most beneficial.

Focus

Over the last two decades the problem of limited oil and gas supply versus new emerging-nation demands has created an immediate need for new technologies to alleviate global dependence on a hydrocarbon-based energy policy. This requires significant changes in the way global energy-system policy is managed and the rapid adoption/introduction of an array of new technologies that produce and use energy more efficiently and more cleanly than in the past. Specifically, these energy policy changes have directly led to more focus on commercial applications of alternative, sustainable energy policy. This chapter is centered on the establishment of a hydrogen-based economy, specifically as it relates to fuel cells. The net result is the concept of hydrogen and fuel cells as a practical foundation for implementing public policies responding to growing uncertainties about the security and long-term price of oil and environmental concerns. With the expectation that fuel cells and hydrogen can play a significant role in the global energy economy, governments are committing funds for research, development, and demonstration of hydrogen and fuel cells as they strive to create programs for viable infrastructure to support their use.

Synopsis

Fuel cells generate energy from controlled, spontaneous oxidation–reduction (redox) reactions. A fuel cell is a multi-component device with two electrodes, separated by an ionic conductive membrane, the positive anode where the oxidation reaction (loss of electrons LEO) occurs and the negative cathode where the reduction reaction (gain of electrons GER) occurs (LEO goes GER, lose electrons oxidation, gain electrons reduction). As with battery systems, there are several kinds of fuel cells, and each operates a bit differently, but in general the fuel cell uses hydrogen (or hydrogen-rich fuel) at the anode and oxygen (air) at the cathode, to create electricity. As mentioned, the typical fuel cell consists of two electrodes: the negative electrode (or anode) and a positive electrode (or cathode), separated by an ion (charge)-conducting electrolyte. In a model system, the hydrogen is fed to the anode, and oxygen is fed to the cathode. Through the utilization of the catalyst, the activation energy barrier for the separation of hydrogen atoms into protons (H+) and electrons (e−) is decreased substantially, making it kinetically viable at <80 °C, i.e., the catalyst lowers the activation barrier for the chemical reactions and increases the rate at which the reactions occur. The electrons generated are forced to go through an external circuit, which creates creating a flow of electrons (electricity). In order to complete the balance of charge required in redox reactions, the protons migrate through the electrolyte to the cathode, where they react with oxygen and the electrons to produce water and heat. So, as long as fuel (hydrogen) and air are supplied, the fuel cell will generate electricity. A single fuel cell generates a small amount of electricity so, in practice, similarly to what is done every day with loading multiple batteries to operate an electronic device, fuel cells are usually assembled into a stack of multiple cells to meet the specific power and energy requirements of the particular application.