R. Cava et al. Progress in Solid State Chemistry 30(2002)1-101 and learn about this kind of interdisciplinary effort. Research activities that promote interdisciplinary interactions and efforts in an academic setting will be a necessary first step. Although the discussion of research and education is often kept separate in these sorts of reports, research is education--particularly in new, interdisciplin- ary areas Naturally, there is also potential profit in re-examination of the traditional curricu- lum. Learning about interdisciplinary science, however, cannot occur through a com- pletely new curriculum-academic chemists and physicists will not accept a"weak ening of training"in their area. Refusal to abridge current courses leads to more courses, which the academic community also doesnt like. Thus, this problem is not simply solved. In redefining and refining an interdisciplinary curriculum, the follow ng two general guidelines are proposed 1. Develop educational concepts generating students fluent in languages of various 2. Define the central concepts in each field and develop a curriculum that teaches concepts and uses synergistic examples whenever possible to show the parallels Is there any model for promoting these types of interdisciplinary educational efforts? Bioengineering departments created over the past decade at many univer- sities provide either a starting point, or a model or a partner for this new kind of education. Such departments are generally relatively new, yet some have acquired enough age that they can provide an initial assessment of agendas that do and do not work. Overall, however, education(which is, in part, research) remains a funda- mental challenge 4. Energy storage and conversion Sossina Haile, Bruce Dunn, Debra rolison, Alan Jacobson, Arumugam Manthi ram,Arthur Nozik, Channing Ahn, Mercouri Kanatzidis 4. Introduction World oil reserves can be anticipated to meet world demand for perhaps another 70 years [50. While this timescale may not evoke an immediate call to action, atmospheric CO2 levels, deriving primarily from fossil fuel combustion [51], have risen dramatically since pre-industrial times, from 280 ppm to 370 ppm today, and are predicted to reach between 500 and 700 ppm by the year 2100 [52], and thus demand a response from the scientific community. Solid state chemists have much to offer by way of solutions. Indeed, viable, long-term solutions to meet our energy needs while maintaining the quality of our environment will increasingly depend on electrochemical processes within and at the surface of solids. Photovoltaics, fuel cells, thermoelectrics and batteries are all devices in which energy storage or conver
26 R.J. Cava et al. / Progress in Solid State Chemistry 30 (2002) 1–101 and learn about this kind of interdisciplinary effort. Research activities that promote interdisciplinary interactions and efforts in an academic setting will be a necessary first step. Although the discussion of research and education is often kept separate in these sorts of reports, research is education—particularly in new, interdisciplinary areas. Naturally, there is also potential profit in re-examination of the traditional curriculum. Learning about interdisciplinary science, however, cannot occur through a completely new curriculum—academic chemists and physicists will not accept a “weakening of training” in their area. Refusal to abridge current courses leads to more courses, which the academic community also doesn’t like. Thus, this problem is not simply solved. In redefining and refining an interdisciplinary curriculum, the following two general guidelines are proposed: 1. Develop educational concepts generating students fluent in languages of various communities. 2. Define the central concepts in each field and develop a curriculum that teaches concepts and uses synergistic examples whenever possible to show the parallels developing between the fields. Is there any model for promoting these types of interdisciplinary educational efforts? Bioengineering departments created over the past decade at many universities provide either a starting point, or a model or a partner for this new kind of education. Such departments are generally relatively new, yet some have acquired enough age that they can provide an initial assessment of agendas that do and do not work. Overall, however, education (which is, in part, research) remains a fundamental challenge. 4. Energy storage and conversion Sossina Haile, Bruce Dunn, Debra Rolison, Alan Jacobson, Arumugam Manthiram, Arthur Nozik, Channing Ahn, Mercouri Kanatzidis 4.1. Introduction World oil reserves can be anticipated to meet world demand for perhaps another 70 years [50]. While this timescale may not evoke an immediate call to action, atmospheric CO2 levels, deriving primarily from fossil fuel combustion [51], have risen dramatically since pre-industrial times, from 280 ppm to 370 ppm today, and are predicted to reach between 500 and 700 ppm by the year 2100 [52], and thus demand a response from the scientific community. Solid state chemists have much to offer by way of solutions. Indeed, viable, long-term solutions to meet our energy needs while maintaining the quality of our environment will increasingly depend on electrochemical processes within and at the surface of solids. Photovoltaics, fuel cells, thermoelectrics and batteries are all devices in which energy storage or conver-
R. Cava et al. Progress in Solid State Chemistry 30(2002)1-101 ion relies on a coupling of chemical, thermal and/or electrical phenomena within the solid state. The direct and elegant coupling of these phenomena, in turn, is responsible for high efficiencies in fuel cells and batteries, and for the ability to use energy sources that might otherwise have been lost, such as sunlight in photovoltaics and "waste" heat in thermoelectrics In discussing the broad topic of energy, it is necessary to distinguish between sources, both primary and secondary, and technologies that serve as either energy conversion devices or energy storage devices, Table 2. The vast majority of our planets energy resources are available in the form of fossil fuels, whereas our con Table 2 Energy sources and technologies Primary Fossil/chemical (oil, natural gas) Chemical(H2, methanol) Thermal (waste heat) Energy Technologi Conversion Combustion engine hemical→( thermal→) mechanical(→ electrical) sil→ chemical Fuel Cell hemical→ electrical Photoelectro-chemical cell Thermoelectric heat→ electrical Stirling engine heat→ mechanical(→ electrical) Technology Interconversion =二 Electrolyzers are considered energy storage rather than conversion devices because their input is electrical energy, which must be generated using some other conversion device
R.J. Cava et al. / Progress in Solid State Chemistry 30 (2002) 1–101 27 sion relies on a coupling of chemical, thermal and/or electrical phenomena within the solid state. The direct and elegant coupling of these phenomena, in turn, is responsible for high efficiencies in fuel cells and batteries, and for the ability to use energy sources that might otherwise have been lost, such as sunlight in photovoltaics and “waste” heat in thermoelectrics. In discussing the broad topic of energy, it is necessary to distinguish between sources, both primary and secondary, and technologies that serve as either energy conversion devices or energy storage devices, Table 2. The vast majority of our planet’s energy resources are available in the form of fossil fuels, whereas our conTable 2 Energy sources and technologies Energy Sources Primary Secondary Fossil/chemical (oil, natural gas) Chemical (H2, methanol) Hydroelectric Thermal (waste heat) Geothermal Wind Solar Energy Technologies Conversion Technology Conversion Combustion engine chemical → (thermal →) mechanical (→ electrical) Reactor fossil → chemical Fuel Cell chemical → electrical Photovoltaic solar → electrical Photoelectro-chemical cell solar → chemical Thermoelectric heat → electrical Stirling engine heat → mechanical (→ electrical) Storage Technology Interconversion Battery chemical ↔ electrical Capacitor chemical ↔ electrical Electrolyzera electrical ↔ chemical Flywheel electrical ↔ mechanical Hydrogen storage chemical a Electrolyzers are considered energy storage rather than conversion devices because their input is electrical energy, which must be generated using some other conversion device
R. Cava et al. /Progress in Solid State Chemistry 30(2002)1-101 sumption of energy is primarily in the form of electricity or mechanical work. As will be demonstrated below, solid state materials chemistry can play a critical role in many electricity generating technologies, particularly those which bypass mechan- ical work as an intermediate step. In addition, energy transmission technologies (involving superconducting electric cables)and fuel stream purification technologies (involving hydrogen separation membranes), also stand to benefit tremendously from advances in solid state materials chemistr 4.2. Materials and technologies 4.2.1. Membranes: reactors and separators As our fossil fuel-based energy economy relies to a greater and greater extent on natural gas rather than oil, it is clear that highly efficient means of converting gaseous methane to liquid chemicals, which can be more easily transported, are required Similarly, means for generating and separating oxygen and/or hydrogen are becoming increasingly important. Over the past decade oxygen ion transport membrane reactors have attracted considerable interest as cost-effective alternatives to conventional methane conversion processes and for oxygen separation. The membrane in such a reactor is comprised of a dense ceramic oxide that has both high electronic and high oxide ion conductivity at temperatures in the range 700C-1100oC. At the air-side of the membrane, the surface catalyzes the reduction of oxygen [1/202+2e] the other surface catalyzes either the oxygen recombination reaction when the mem- brane is used as an oxygen separator, or the methane partial oxidation reaction [CH4+O--CO+H2+2e]. The overall oxygen transport process is driven by the gradient in the oxygen chemical potential. Analogous reactors/separators can be envi- sioned based on mixed proton/electron conductors, although relatively few oxides with good conductivity of both protons and electrons have been reported to date Hydrogen separation membranes are particularly relevant to energy technologies: in PEM(proton exchange membrane) fuel cells that use hydrogen produced by on- board reforming of hydrocarbons, significant process simplification can be achieved by replacing portions of the Co clean-up hardware by a hydrogen separation mem- Considerable progress has been made over the past decade in the development of oxygen ion-transport membranes for use in methane partial oxidation. Most are based on defect perovskites(or related structures) containing a variable valence element e.g. (La, Sr)(Fe, Co)O3- 8 and SrFe CoosOr Relative to conventional partial oxidation routes, such membranes reactors offer very high selectivity, with methane conversion rates of up to 98% and CO selectivities of up to 90%[53. While these results demonstrate exceptional promise, several challenges remain to be addressed In parti- cular, the materials implemented to date require relatively high temperatures in order to attain acceptable fux(high diffusion rates, rapid surface kinetics and high elec tronic conductivity). Moreover, because the specific volume of the membrane material is strongly dependent on oxygen partial pressure, the chemical potential gradient across the membrane leads to severe internal stresses and often failure. The development of oxides with a lesser dependence of chemical bond distances on tran-
28 R.J. Cava et al. / Progress in Solid State Chemistry 30 (2002) 1–101 sumption of energy is primarily in the form of electricity or mechanical work. As will be demonstrated below, solid state materials chemistry can play a critical role in many electricity generating technologies, particularly those which bypass mechanical work as an intermediate step. In addition, energy transmission technologies (involving superconducting electric cables) and fuel stream purification technologies (involving hydrogen separation membranes), also stand to benefit tremendously from advances in solid state materials chemistry. 4.2. Materials and technologies 4.2.1. Membranes: reactors and separators As our fossil fuel-based energy economy relies to a greater and greater extent on natural gas rather than oil, it is clear that highly efficient means of converting gaseous methane to liquid chemicals, which can be more easily transported, are required. Similarly, means for generating and separating oxygen and/or hydrogen are becoming increasingly important. Over the past decade oxygen ion transport membrane reactors have attracted considerable interest as cost-effective alternatives to conventional methane conversion processes and for oxygen separation. The membrane in such a reactor is comprised of a dense ceramic oxide that has both high electronic and high oxide ion conductivity at temperatures in the range 700°C–1100°C. At the air-side of the membrane, the surface catalyzes the reduction of oxygen [1/2O2+2e→O= ]; the other surface catalyzes either the oxygen recombination reaction when the membrane is used as an oxygen separator, or the methane partial oxidation reaction [CH4+O= →CO+H2+2e]. The overall oxygen transport process is driven by the gradient in the oxygen chemical potential. Analogous reactors/separators can be envisioned based on mixed proton/electron conductors, although relatively few oxides with good conductivity of both protons and electrons have been reported to date. Hydrogen separation membranes are particularly relevant to energy technologies: in PEM (proton exchange membrane) fuel cells that use hydrogen produced by onboard reforming of hydrocarbons, significant process simplification can be achieved by replacing portions of the CO clean-up hardware by a hydrogen separation membrane. Considerable progress has been made over the past decade in the development of oxygen ion-transport membranes for use in methane partial oxidation. Most are based on defect perovskites (or related structures) containing a variable valence element, e.g. (La,Sr)(Fe,Co)O3-δ and SrFeCo0.5Ox. Relative to conventional partial oxidation routes, such membranes reactors offer very high selectivity, with methane conversion rates of up to 98% and CO selectivities of up to 90% [53]. While these results demonstrate exceptional promise, several challenges remain to be addressed. In particular, the materials implemented to date require relatively high temperatures in order to attain acceptable flux (high diffusion rates, rapid surface kinetics and high electronic conductivity). Moreover, because the specific volume of the membrane material is strongly dependent on oxygen partial pressure, the chemical potential gradient across the membrane leads to severe internal stresses and often failure. The development of oxides with a lesser dependence of chemical bond distances on tran-
R. Cava et al. /Progress in Solid State Chemistry 30(2002)1-101 ition metal valence is thus a significant challenge and opportunity to the solid state chemistry community Two main types of inorganic hydrogen membranes have been studied based either on microporous materials(porous silica, zeolites and related) and ion transport sys- tems. The microporous materials separate hydrogen as molecular hydrogen and rely on selective diffusion through pores($2 nm) in the solid phase. lon transport sys- tems, in contrast, require dissociative adsorption of hydrogen at the membrane sur- faces and coupled transport of H*+e. Silver-palladium membranes are the most studied of this type but oxides have also been considered. Many proton conducting pervoskites, such as doped barium cerate, exhibit primarily protonic conductivity at temperature below -500oC, but become mixed proton-electron conductors at elev ated temperatures, >700C, and are thus viable for high-temperature membrane reac- tor applications. Alternatively, dual phase composites of nickel with proton con- ducting perovskites can be implemented at lower temperatures, as can mixed electronic-protonic conducting transition metal oxides, originally developed for elec trochromic applications. At present, microporous systems are limited by their low separation factors at reasonable fluxes, silver-palladium systems are limited for many applications by poisoning, and none of the other systems have adequate fluxes In order for the chemical and energy industries to be able to take full advantage of the high selectivities and yield-enhancements offered by membrane reactors in a broad range of reactions, significantly improved membrane materials are required Strategies are needed for the discovery of new materials that meet the complex per- formance criteria: fast diffusion rates, rapid surface kinetics, high electronic conduc try, with its historical strength in new materials discovery, has a significant role to play in advancing the field. Experimental synthetic efforts to identify improved mem- brane materials must be guided by computational efforts, that may be able to explain the properties of synthesized compounds, and uncover promising new directions Particularly challenging will be to develop a complete theoretical framework for understanding and enhancing surface catalysis by multi-component oxides Of the often competing materials properties required for membrane materials, dif- fusion, conductivity and surface kinetics establish the overall rate at which a desired reaction will occur, whereas chemical and mechanical stability establish the lifetime of the reactor Reaction rates can often be enhanced by architectural control, and as such do not rely solely on improvements in material properties. For example, thin membranes can bypass limitations in conductivity and diffusion rates, and can even reduce internal stresses, whereas roughened, high-area surfaces can bypass limi- tations in surface reaction kinetics. Taking advantage of this approach will require novel processing routes to yield supported thin films with controlled surface mor hology. The selectivity of zeolitic and other porous materials will also benefit from recent solid state chemistry advances in the fabrication of nanostructured materials with highly controlled pore sizes and pore size distribution
R.J. Cava et al. / Progress in Solid State Chemistry 30 (2002) 1–101 29 sition metal valence is thus a significant challenge and opportunity to the solid state chemistry community. Two main types of inorganic hydrogen membranes have been studied based either on microporous materials (porous silica, zeolites and related) and ion transport systems. The microporous materials separate hydrogen as molecular hydrogen and rely on selective diffusion through pores (2 nm) in the solid phase. Ion transport systems, in contrast, require dissociative adsorption of hydrogen at the membrane surfaces and coupled transport of H+ +e. Silver–palladium membranes are the most studied of this type but oxides have also been considered. Many proton conducting pervoskites, such as doped barium cerate, exhibit primarily protonic conductivity at temperature below ~500°C, but become mixed proton–electron conductors at elevated temperatures, 700°C, and are thus viable for high-temperature membrane reactor applications. Alternatively, dual phase composites of nickel with proton conducting perovskites can be implemented at lower temperatures, as can mixed electronic–protonic conducting transition metal oxides, originally developed for electrochromic applications. At present, microporous systems are limited by their low separation factors at reasonable fluxes, silver–palladium systems are limited for many applications by poisoning, and none of the other systems have adequate fluxes. In order for the chemical and energy industries to be able to take full advantage of the high selectivities and yield-enhancements offered by membrane reactors in a broad range of reactions, significantly improved membrane materials are required. Strategies are needed for the discovery of new materials that meet the complex performance criteria: fast diffusion rates, rapid surface kinetics, high electronic conductivity, and good chemical and mechanical stability. Accordingly, solid state chemistry, with its historical strength in new materials discovery, has a significant role to play in advancing the field. Experimental synthetic efforts to identify improved membrane materials must be guided by computational efforts, that may be able to explain the properties of synthesized compounds, and uncover promising new directions. Particularly challenging will be to develop a complete theoretical framework for understanding and enhancing surface catalysis by multi-component oxides. Of the often competing materials properties required for membrane materials, diffusion, conductivity and surface kinetics establish the overall rate at which a desired reaction will occur, whereas chemical and mechanical stability establish the lifetime of the reactor. Reaction rates can often be enhanced by architectural control, and as such do not rely solely on improvements in material properties. For example, thin membranes can bypass limitations in conductivity and diffusion rates, and can even reduce internal stresses, whereas roughened, high-area surfaces can bypass limitations in surface reaction kinetics. Taking advantage of this approach will require novel processing routes to yield supported thin films with controlled surface morphology. The selectivity of zeolitic and other porous materials will also benefit from recent solid state chemistry advances in the fabrication of nanostructured materials with highly controlled pore sizes and pore size distribution
R. Cava et al. /Progress in Solid State Chemistry 30(2002)1-101 4.2. 2. Fuel cells and regenerative fuel cells as electrolyzers Fuel cell technology, for both stationary and mobile applications, has the potential to improve energy efficiency and distribution and to reduce the environmental impact of the use of fossil fuels. Among the attractive features of fuel cell technology are the conversion of chemical energy into electrical energy with high efficiency; the distributed and modular nature; and low environmental impact(no NOx, low SOx no noise, no transmission lines). They will also serve a critical role should the gener ation of hydrogen from non-fossil sources and a truly closed energy cycle become a reality The central component of a fuel cell, shown schematically in Fig. 9, is the ion conducting electrolyte. In the simplest example, a fuel such as hydrogen is brought into the anode compartment and oxygen into the cathode compartment. Hydrogen is oxidized to form protons and electrons at the anode, protons travel through the electrolyte, and then react at the cathode with oxygen and electrons. The flow of ionic charge through the electrolyte is balanced by the flow of electronic charge through the device to which electrical power is provided. Solids in which protons, hydronium ions, hydroxide ions, oxide ions, ammonium ions, and even carbonate ions are mobile are all known, but for fuel cell applications, oxide ions offer perhaps the simplest overall system. This is because direct electrochemical oxidation of hydrocarbon fuel is possible when oxide ions are transported to the fuel compartment or anode. In contrast, in the case of a proton conducting electrolyte, the fuel is stripped of hydrogen, and the proton is transported to the air compartment or cath ode. If hydrocarbon fuels rather than hydrogen are to be utilized as the energy source at best, excess water is required in the fuel supply in order to convert this fuel to CO nd hydrogen, the latter of which is used in the electrochemical process. Hydronium hydroxide and carbonate ion conductors introduce further complications in fuel cell operation because an otherwise inert species, H2o in the case of hydronium and hydroxide conductors or CO2 in the case of carbonate conductors, must be continu- ously recycled through the system. State-of-the art fuel cell electrolytes are listed in Table 3, along with the fuels typically utilized, the recycled species, and temperatures Critical to the function of a fuel cell are the electrodes/electrocatalysts. within Electrolyte oxidant H2+2H+2e O2+2H++2e+H2O by-products by-products Fig 9. Schematic of a fuel cell. The overall chemical reaction is H2+1/202-H20. Anode and cathode
30 R.J. Cava et al. / Progress in Solid State Chemistry 30 (2002) 1–101 4.2.2. Fuel cells and regenerative fuel cells as electrolyzers Fuel cell technology, for both stationary and mobile applications, has the potential to improve energy efficiency and distribution and to reduce the environmental impact of the use of fossil fuels. Among the attractive features of fuel cell technology are: the conversion of chemical energy into electrical energy with high efficiency; the distributed and modular nature; and low environmental impact (no NOx, low SOx, no noise, no transmission lines). They will also serve a critical role should the generation of hydrogen from non-fossil sources and a truly closed energy cycle become a reality. The central component of a fuel cell, shown schematically in Fig. 9, is the ion conducting electrolyte. In the simplest example, a fuel such as hydrogen is brought into the anode compartment and oxygen into the cathode compartment. Hydrogen is oxidized to form protons and electrons at the anode, protons travel through the electrolyte, and then react at the cathode with oxygen and electrons. The flow of ionic charge through the electrolyte is balanced by the flow of electronic charge through the device to which electrical power is provided. Solids in which protons, hydronium ions, hydroxide ions, oxide ions, ammonium ions, and even carbonate ions are mobile are all known, but for fuel cell applications, oxide ions offer perhaps the simplest overall system. This is because direct electrochemical oxidation of a hydrocarbon fuel is possible when oxide ions are transported to the fuel compartment or anode. In contrast, in the case of a proton conducting electrolyte, the fuel is ‘stripped’ of hydrogen, and the proton is transported to the air compartment or cathode. If hydrocarbon fuels rather than hydrogen are to be utilized as the energy source, at best, excess water is required in the fuel supply in order to convert this fuel to CO2 and hydrogen, the latter of which is used in the electrochemical process. Hydronium, hydroxide and carbonate ion conductors introduce further complications in fuel cell operation because an otherwise inert species, H2O in the case of hydronium and hydroxide conductors or CO2 in the case of carbonate conductors, must be continuously recycled through the system. State-of-the art fuel cell electrolytes are listed in Table 3, along with the fuels typically utilized, the recycled species, and temperatures of operation. Critical to the function of a fuel cell are the electrodes/electrocatalysts. Within Fig. 9. Schematic of a fuel cell. The overall chemical reaction is H2+1/2O2→H2O. Anode and cathode reactions written assume a proton conducting electrolyte