Chem. Mater. 2010, 22, 585–586 585 DOI:10.1021/cm903734p
Preface to Chemistry of Materials Special Issue on The Materials Chemistry of Energy Conversion There is little question that sustainable energy is one of the most pressing issues facing society today. Access to plentiful, inexpensive, and environmentally benign energy resources would free nations to pursue their greatest human and economic potential. In contrast, continuation along the present energy-consumption path, in which more than 80% of our energy is derived from fossil fuels,1 can lead only to increasing geopolitical tensions and environmental catastrophe. Not only are fossil fuel resources finite, their consumption has contributed to the dramatic increase in atmospheric CO2 observed over the past 150 years, to levels not experienced by the planet for at least 15 million years.2 Expansion of renewable energy technologies and increases in energy efficiencies may alter “business-as-usual” projections of fossil-fuel usage. However, current renewable technologies are more expensive than those utilizing fossil fuels and often perform at significantly lower energy efficiencies. Accepting the thermodynamic premise that energy is a conserved quantity, all “renewable” and energy “generation” technologies are in fact concerned with the conversion of energy from one form to another, and central to this conversion process are materials. Materials largely define, for example, photochemical devices (fuels from light), photovoltaics (electricity from light) thermochemical devices (fuels from heat), fuel cells (electricity from fuels), and thermoelectrics (electricity from heat). Materials are also at the heart of advanced electricity storage (batteries) and transmission (superconductors) technologies. Accordingly, advances in energy conversion and related materials (whether in the form of new compounds, new processing routes, or new configurations) are certain to play a central role in addressing our pressing energy problems and thereby in ensuring economic vitality. This special issue highlights a number of new directions in materials chemistry relevant to energy conversion. The response to the call for papers has been outstanding and our special issue comprises more than 70 manuscripts. Approximately15% are reviews, with the remainder being communications or articles providing a snapshot of current areas of research and pointing toward new directions for potential future breakthroughs. Photochemical approaches to the conversion of solar energy into storable chemical fuels require materials that capture photons, generating electron-hole pairs, and (1) International Energy Outlook 2009. http://www.eia.doe.gov/oiaf/ ieo/world.html (accessed 11/29/09). (2) Tripati, A. K.; Roberts, C. D.; Eagle, R. A. Coupling of CO2 and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years. Science 2009, 326, 1394–1387. r 2010 American Chemical Society
catalysts that subsequently utilize the electrons and holes, respectively, for oxidation and reduction reactions. In the simplest case of water dissociation, both hydrogenand oxygen-generation materials are required, and both are presented here. Like photochemical cells, photovoltaics require materials that capture photons and generate electron-hole pairs, but these electrical species are directly extracted in the form of electricity. Improvements in photovoltaic efficiencies through new materials and chemical manipulation of nano- and mesoscopic materials, among other approaches, are described in this issue. Solid-oxide and polymer-electrolyte membrane fuel cells have the potential to reduce consumption of fossil fuels through increased energy-conversion efficiency. In a fuel cell, a chemical fuel is supplied to the anode, where the electrochemical oxidation of the fuel occurs, whereas oxygen is supplied to the cathode, where electrochemical reduction occurs. The ions generated and consumed in this process traverse the electrolyte, while the electrons travel through an exterior circuit, generating electricity. Fuel cells are attractive because this electrochemical utilization of chemical fuels is inherently more efficient than combustion. In addition, they are extremely compatible with hydrogen and methanol, which can ideally be obtained renewably through photochemical or thermochemical approaches. There are a number of materials challenges in fuel cells presented in this special issue along with two reviews on new chemical systems and new materials for solid-oxide fuel cells. Research in the area of thermoelectrics has undergone a remarkable resurgence over the past decade, primarily because of the infusion of new ideas and discovery of new materials with high values of the thermoelectric figure-of-merit, zT (directly related to high efficiency of heat conversion into electrical energy). With sufficiently high values of zT, it is possible to generate electricity from waste heat, a highly attractive prospect. In a thermoelectric, thermally-excited charge carriers (electrons or holes) travel down a temperature gradient, ideally transferring high electrical current with minimal heat transport. Several new materials and nanostructured materials that address the challenge of obtaining high electrical conductivity and thermopower while maintaining low thermal conductivity are presented, along with several reviews. Global interest in batteries, particularly those for electric vehicles, has grown dramatically in very recent history. As with fuel cells, batteries can enable vehicle operation using energy from renewable resources. This possibility of substantial impact on the transportation
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sector has encouraged new ideas and materials for advancing battery technology. In a further analogy to fuel cells, batteries are comprised of three critical materials components: an anode, a cathode, and an electrolyte. The materials challenges and opportunities are accordingly rather similar, involving enhancing ion and electron transport through chemical modification, and minimizing diffusion distances through architectural engineering. Lithium-storage capability of nanomaterials and challenges for rechargeable batteries are tackled in several reviews and articles. The papers in this issue give examples of multidisciplinary work and provide a balance between current reviews and research. The cover art consists of a collage of images selected from those contributed and highlight the topics mentioned above. We thank all the authors who provided
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manuscripts for this special issue, in particular those who wrote reviews. These insightful reviews synthesize ideas and recent trends in a manner that helps point toward future directions. In sum, this special issue provides an overview of topics of current research in the important area of materials chemistry for energy conversion. We hope that it will further stimulate research and results, accelerating materials discoveries and thereby improving performance through the development, synthesis, processing, and application of novel materials for energy conversion. Sossina Haile, Susan M. Kauzlarich, Peter Battle, and John Greedan Special Issue Editors Received December 12, 2009
