Materials for Sustainable Energy

George Crabtree

The Sustainable Energy Challenge

At every stage of human development, energy has shaped both our aspirations and our limitations. In agrarian times, we relied on energy from the sun to grow our food and energy from domestic animals to do our work. The industrial revolution brought a powerful new feature – harnessing the energy of coal in the steam engine and later oil in the internal combustion engine to do orders of magnitude more work than humans or animals could provide, on demand and without the maintenance costs of animals. The use of energy to do work, the hallmark of the industrial revolution, gave birth to other revolutions that use energy in subtler but equally significant ways, for observation, communication, information, and decision-making.

Energy is among the fastest growing commodities in the world. By 2050 we will need twice the energy we use today, driven by the rising expectations of the developing world. The question is, where will it come from? Our use of traditional sources of energy – coal, oil and gas – is reaching fundamental limits that we should not, and in some cases cannot, exceed. These sources of energy are not sustainable.

The first big problem is supply. Oil accounts for 40% of world energy use; its production in the US peaked in the 1970s, and world production outside the OPEC countries is peaking now. Eventually OPEC oil production will peak, most predictions say by mid-century or sooner. If we do not wean ourselves from oil, the transition through the OPEC production peak will be very difficult. But apart from world supply, the US faces a much more pressing problem. Last year we imported nearly 60% of our oil. Our consumption continues to rise, and our domestic production continues to fall - these trends are irreversible. The cost of imported oil is huge - $700B/yr at the peak prices of summer 2008, and about $300B/yr at today’s prices. That money is drained from our shores, going to foreign producers where it cannot stimulate our economy or promote our recovery from the present recession. Furthermore, foreign oil supplies are uncertain, subject to interruption from terrorist acts, weather disasters, and internal political decisions in supplier countries. The flow of oil is vital to our economy and lifestyle – try to imagine our society without driving cars and trucks, which consume two thirds of the oil we use.

The second big problem — many say the more important one — is greenhouse gas emissions. Some effects of global warming, such as the decline in Arctic sea ice, are occurring at rates faster than we predicted just a decade ago. The 4th Intergovernmental Panel on Climate Change describes some of the consequences: a loss of Arctic sea ice and snow cover in the northern hemisphere, rising sea level, and the pole-ward migration of animal and plant species to maintain their preferred habitats. Once emitted, carbon dioxide takes 400 to 1000 years to settle in the deep ocean. That means that today’s emissions will affect not only our children and grandchildren, but also many generations beyond. Left unchecked, the consequences of greenhouse gas emissions could be drastic: disruption of established agricultural patterns, of economic networks, and of population centers in coastal areas. The ultimate severity of global warming depends not only on still-unpredictable ocean-atmosphere dynamics, but also on human choices - the balance between pre-emission mitigation of greenhouse gas releases and post-emission adaptation to global warming. An early shift to low carbon energy technologies will have the greatest impact on reducing both the deleterious consequences of global warming and the degree of adaptation required to accommodate them.

To reduce our dependency on imported oil and our greenhouse gas emissions, we must find alternatives to fossil fuel that are more sustainable. The ultimate sustainable energy sources are solar (including solar electricity, solar fuel, and solar heat), wind, geothermal and other fully renewable resources. These technologies operate qualitatively differently from fossil fuels and most are not yet sufficiently cost competitive to replace them. Carbon emissions, one of the harmful effects of fossil fuels, can be reduced by capture and sequestration underground, and by greater use of nuclear energy, where a new generation of reactors could be twice as efficient (and therefore half as costly per unit of output) as the commercial reactors of the 1960s that we now rely on.

Materials and Chemical Change

Every alternative to conventional fossil energy, however, faces roadblocks that cannot be solved without basic research on materials and chemical change [1]. No one knows what will happen to enormous quantities of supercritical carbon dioxide placed deep underground as it reacts with porous rocks. Will the carbon dioxide migrate great distances, perhaps affecting water supplies? Will it react to form solid, stable compounds? Will it stay underground? If it leaks out at 1% per year, it will return to the atmosphere in a century, far too short a time to mitigate climate change. If it leaks, will the heavy carbon dioxide collect in low areas and displace the oxygen we need to breathe? These questions require significant basic research to predict the phenomena, effectiveness, and safety of carbon sequestration.

Doubling the efficiency of nuclear reactors and coal-fired power plants is a worthy sustainability goal. Higher efficiency requires operating at much higher temperatures, in turn requiring materials that withstand extreme environments, not only of temperature but also chemical corrosion and, for nuclear reactors, high radiation doses. Designing and fabricating these "extreme materials" is a significant basic research challenge.

Solar electricity needs higher efficiency, lower cost solar cells to compete with fossil electricity, and both wind and solar need long-distance electricity transmission to get power from renewable wind resources in the upper midwest or solar resources in the southwest to the population centers of the east and west coasts. Solar and wind are intermittent, requiring either companion conventional plants as backup or large-scale electrical energy storage to be effective. Long distance transmission lines require lower-loss cables, such as superconductors operating at DC and buried underground to minimize weather damage and the growing objection to unsightly infrastructure. Storing electricity at the utility scale is new territory - there is no conventional technology and all the potential options such as electrochemical flow batteries and fuel production are heavily dependent on new, complex functional materials and chemistry.

We would like to make chemical fuel from the sun, such as liquid fuel from the cellulose in the stalks and leaves of plants. Corn ethanol is now technologically ready for deployment, but its capacity is limited to a fraction of the gasoline needed for transportation, it displaces food, and at best produces only slightly more energy than it consumes. Cellulosic fuels are a potentially much bigger winner, but we lack the fundamental knowledge for the cost effective conversion of cellulose to fermentable sugars or directly to fuel. Fuel can also be made directly from carbon dioxide and water, without relying on plants or biological processes. An energy source is required for this uphill reaction, either heat from the sun to drive high temperature thermochemistry or photonic excitation of electrons at room temperature to drive photo-chemistry, an artificial version of biological photosynthesis. Both of these sustainable routes to fuel production require basic research in the materials and chemistry of splitting water and carbon dioxide and subsequent synthesis of fuels like methane, methanol, and hydrocarbon chains.

The roadblocks to deploying more sustainable next generation energy technologies are fundamental, not incremental. Refinement of existing technologies is not capable of delivering the alternative energy we need. Qualitatively new, more sustainable energy technologies are, however, within reach. There is no law of physics, chemistry, thermodynamics or economics that precludes their operation. We simply have to develop the materials and learn to control the physical and chemical phenomena that will enable them.

Interdisciplinary Science

Nearly every step in the energy chain involves the conversion of energy from one form to another: photons to electrons, heat to motion, chemical bonds to heat or to electrons. These conversions depend on physical and chemical phenomena, such as the photo-excitation of an electron in a semiconductor, the transfer of electrons and energy in chemical reactions, or the transmission of an electron without loss in a superconducting wire. The challenge in sustainable energy is to understand and control these physical phenomena to produce more efficient energy conversions. These phenomena are complex and often take place at nanometer or smaller length scales, and at pico- or femto-second time scales. We are beginning to probe these ultrasmall and ultrafast regimes, but to a large extent the atomic and molecular details of many energy conversion phenomena remain hidden. It takes the best science, including forefront experiments, theory, and computation, to understand how these conversion phenomena work.

Beyond complex phenomena, there is the equally difficult challenge of complex materials. The materials of sustainable energy applications are different from those of traditional energy usage based on combustion. The primary materials of combustion energy are the fuels, prized for their high energy content and the heat they release on burning. Fuels are commodities, used once and consumed in the combustion process. In contrast, sustainable energy materials direct the conversion of energy from one form to another and are not consumed in the process. They are expected to continue operating for many conversion cycles, with lifetimes as long as 30 years. These materials are much more complex than the commodity fuels of combustion. A solar cell, for example, must convert a photon to an excited electron and a hole, separate the electron and hole with a space charge at a p-n junction, and transport the electron and hole to external electrodes without allowing them to decay across the band gap. Each of these steps is a separate and specific function with stringent materials requirements, on the band gap for excitation, the impurity doping profile for charge separation, and the structural perfection for transport to the electrodes. The great progress in silicon solar cell efficiency, from 6% for the first prototypes in 1954 to over 20% in the best commercial cells today, is due to scientific advances in understanding the electronic structure and dynamics in semiconductors, and to enormous advances in perfecting silicon materials. Silicon is arguably the best-understood and most precisely controllable material in the world. A similar level of understanding of other sustainable energy conversion materials is needed to achieve the required technical performance and economic competitiveness.

The materials of sustainable energy are highly interdisciplinary. This is a great challenge and a great opportunity. Semiconductor solar cells can be adapted to split water from sunlight, using the electrons and holes to do chemistry at the electrode surfaces instead of tapping them off to an external circuit. A catalyst is needed to promote the water splitting reaction near ambient temperatures, combined with favorable nanoscale architecture creating high surface area and active catalytic sites. The interdisciplinary science of solar water splitting requires physicists and chemists working together, exposing new directions that neither could have found alone. Biology is needed as well: green plants split water at room temperature by an entirely different mechanism that we are now probing at sub-nanometer length scales and are beginning to imitate in the laboratory.

Meeting the Energy Challenge

Creating sustainable energy technologies is a monumental scientific challenge for basic materials and chemical research. Not only must we observe and understand atomic and nanoscale energy conversion phenomena, we must learn how to control these phenomena at the nanoscale to produce targeted functional outcomes that operate with high efficiency. These challenges are within reach using the rapidly developing tools of nanoscience, materials simulation on high performance computers, and characterization of structure and dynamics by scanning probes, scattering of electrons, neutrons and x-rays. Cracking the grand challenges of sustainable energy phenomena, however, requires dream teams of the best scientists, using the best equipment, focused on the most important problems. Such teams do not typically exist at a single institution or within a single discipline. They must be created deliberately to be multi-institutional and multi-disciplinary, and given sufficient resources for sufficient time to solve the basic science challenges. The Energy Frontier Research Centers established by the Office of Basic Energy Sciences are premier examples of this style of high risk-high payoff targeted basic science; we must insure that this new research paradigm remains an enduring and vital force for the decade or more required to bring sustainable energy to technical and economic viability.

In addition to dream teams of established scientists, we must recruit and train a whole new generation of interdisciplinary sustainable energy scientists. The time scale of sustainable energy is long, because the research problems are diverse and challenging, and because even potentially disruptive technologies in the energy arena, such as plug-in hybrid cars, can take decades to overcome market inertia. The knowledge and wisdom developed by today's dream teams must be passed on to the next generation through education and collaborative mentoring. Aggressive programs of fellowship and research awards for graduate students, postdocs and early career scientists must be established. The best and the brightest must be attracted and embraced by the sustainable energy enterprise.

The sustainable energy challenge is clear. The series of ten Basic Research Needs workshops and reports on the Basic Energy Sciences website outlines the current status of sustainable energy technologies, the scientific roadblocks to competitive viability and the promising research directions to overcome the roadblocks [2]. With this foundation in place, it remains to pursue the promising research directions with the best scientific talent working in interdisciplinary teams until the problems are solved. The prize is not only within reach and well worth the race, it is necessary if we are to create a secure and sustainable energy future.

References

[1]New Science for a Secure and Sustainable Energy Future, Basic Energy Sciences Advisory Committee, December 2008
[2]The Basic Research Needs Workshops, Office of Basic Energy Sciences

George Crabtree is a senior Scientist and Distinguished Fellow in the Materials Science Division of Argonne National Laboratory. This article is based on a plenary presentation of the same title at the March APS Meeting, Pittsburgh, 2009, and on the Basic Energy Sciences Advisory Committee report, New Science for a Secure and Sustainable Energy Future (Ref. 1). e-mail: crabtree@anl.gov

This contribution has not been peer refereed. It represents solely the view(s) of the author(s) and not necessarily the views of APS.