High Temperature Ion Conducting Ceramics by T

High Temperature Ion Conducting Ceramics by T. A. Ramanarayanan, S. C. Singhal, and E. D. Wachsman

esearch in materials today has range in oxygen stoichiometry. hole pair formation and the concentra- progressed from materials exhibit- Depending on their defect chemistry and tion of aliovalent dopants (such as Y3+ 4+ ing structural functionality to environment, these materials can exhibit on a Zr site - YZr’), one can calculate the those exhibiting electronic func- exclusively ionic (electrolyte) or electron- concentration of all species as a function R tionality. The study of solid-state ic conduction, or mixed (both ionic and of the external oxygen partial pressure, ionics ushers in a new era of chemically electronic) conduction. PO2. This solution for the defect concen- functional materials. This chemical func- Ionic conduction involves hopping trations is shown graphically in a typical tionality arises out of the defect equilib- from an occupied to a vacant lattice (or electrolyte defect equilibrium diagram, ria of these materials, and results in the interstitial) site, as shown in Fig. 1a. At Fig. 1b. (For a review of defect equilibria, ability to transport chemical species and high temperatures, the ionic conductivi- the reader is referred to References 1-2 actively participate in chemical reactions ty of these solid-state materials is compa- and the numerous references therein.) at their surface. These chemically func- rable to that of aqueous solutions at This diagram shows three regions, low, ambient conditions. For example, at tional materials provide a promise for the intermediate, and high PO2, in which a future. They allow the harnessing of our 1000°C the ionic conductivity of yttria- pair of defects dominate the neutrality natural hydrocarbon energy resources stabilized zirconia (YSZ) is 0.1 S·cm-1, relationship due to their relative high through highly efficient and non-pollut- and in fact, there are numerous oxides concentration. Thus the same material ing methods. that have significantly higher conductiv- can exhibit respectively, n-type, ionic, Numerous solid-state materials con- ity at lower temperatures (albeit with a and p-type conductivity depending on + + stability trade off). The high conductivi- duct cations (e.g., Li and Na ) and PO2. Each of the mobile species is then anions (e.g., F- and O2-) and are therefore ty of these materials is attributable to the transported through the material in chemically functional. Among these, high concentration of mobile defects: response to an applied chemical (due to oxygen vacancies (V ••), oxygen intersti- ∆ activities in the ECS High Temperature o a PO2) or electrical potential. Materials Division tend to focus on oxide tials (Oi”), electrons (e’), and electron- Oxide materials such as YSZ, rare ion conducting ceramics. These materials holes (h•). Each of these defects can be earth doped ceria, rare earth doped bis- are used in, and are being investigated treated as a chemical species and their muth oxide, and doped lanthanum gal- for, a wide variety of technological appli- concentration determined from thermo- lates have been widely investigated as cations. Their required functionality goes dynamics. By evaluating the heteroge- oxide ion conductors. Of these materials, far beyond mechanical strength and neous equilibria between gaseous oxygen YSZ has been most successfully toughness, to include such properties as and the solid state, employed. The yttrium oxide dopant ionic and electronic conductivity, ther- O x = 1/2O + V ••+ 2e’ (1) serves dual roles: it stabilizes the high mochemical stability, and catalytic activi- o 2 o temperature cubic phase in zirconia and ty. These latter properties arise out of combined with the internal chemical also generates oxygen vacancies through their defect chemistry and resultant wide equilibria for ionic defect and electron- the following defect reaction:

(a) (b)

FIG. 1. (a) Mechanism of transport from Log (Concentration) an occupied (tetrahedrally coordinated) anion lattice site to a vacant anion site in a fluorite oxide; (b) Concentration of defects in yttria-stabilized zirconia Log (P ) as a function of PO2. o2

22 The Electrochemical Society Interface • Summer 2001 x fuel cells provide many advantages over other cell components are chosen to Y2O3 = 2YZr’ + 3 Oo + Vo•• (2) traditional energy conversion systems have thermal expansion near this value thus creating more of the conducting including high efficiency, reliability, to minimize thermal stresses. species (Vo••). modularity, fuel adaptability, and very Lanthanum manganite suitably The high oxide ion conductivity over low levels of NOx and SOx emissions. doped with alkaline and rare earth ele- wide ranges of temperature and oxygen Quiet, vibration-free operation of SOFCs ments is generally used for the cathode. pressure in these materials has led to also eliminates noise usually associated Lanthanum manganite is a p-type per- their use as electrolytes in a variety of with conventional power generation sys- ovskite oxide and shows reversible oxi- electrochemical applications. High tem- tems. Furthermore, because of their high dation-reduction behavior. The materi- perature solid oxide fuel cells (SOFCs) for temperature of operation (~800-1000°C), al can have oxygen excess or deficiency electric power generation are now near- natural gas fuel can be reformed within depending upon the ambient oxygen ing commercialization. Oxygen sensors the cell stack eliminating the need for an partial pressure and temperature. The are widely used in combustion control, expensive, external reformer system. electronic conductivity of lanthanum especially in automobiles, atmosphere An SOFC essentially consists of two manganite is due to hopping of an elec- control in furnaces, and as monitors of porous electrodes separated by a dense, tron hole between the +3 and +4 oxygen concentration in molten metals. oxide ion conducting electrolyte. The valence states of Mn. This conductivity Other applications include electrochem- operating principle of such a cell is illus- is enhanced by doping with a divalent ical pumps for control of oxygen poten- trated in Fig. 2. Oxygen supplied at the ion such as calcium or strontium. tial, steam and CO2 electrolyzers, and cathode (air electrode) reacts with The reducing conditions on the fuel high temperature reactors for chemicals incoming electrons from the external side of an SOFC permit the use of a production from hydrocarbons. Some of circuit to form oxide ions, which migrate metal such as nickel for the anode. these applications are discussed below. to the anode (fuel electrode) through the However, the thermal expansion of oxide ion conducting electrolyte. At the nickel, 13.3 x 10-6/°C, is considerably Ceramic Fuel Cells anode, oxide ions combine with H2 larger than that of YSZ. Nickel can also for Power Generation (and/or CO) in the fuel to form H2O sinter at the cell operating temperature (and/or CO2), liberating electrons. resulting in a decrease in the fuel elec- High temperature ceramic fuel cells, Electrons (electricity) flow from the trode porosity. These problems are cir- more commonly known as solid oxide anode through the external circuit to the cumvented by forming a skeleton of fuel cells or SOFCs, offer a clean, low- cathode. To keep the cell resistance low, YSZ around the nickel particles. The pollution technology to electrochemi- the electrolyte (typically 10 mol% YSZ) is YSZ skeleton prevents sintering of the cally generate electricity at high efficien- fabricated in the form of a thin film. The nickel particles, decreases the fuel elec- cies; their efficiencies are not limited by thermal expansion of 10 mol% YSZ is trode thermal expansion coefficient the Carnot cycle of a heat engine. These about 10 x 10-6/°C; materials for all bringing it closer to that of the electrolyte, and provides better adhesion of the fuel electrode with the electrolyte. For the interconnection (the bipolar material), generally doped lanthanum chromite is used. Lanthanum chromite is a p-type conductor; its conductivity is due to small polaron hopping from room temperature to 1400°C at oxygen pressures as low as 10-16 kPa. The conductivity is enhanced as lower valence ions (e.g., Ca, Mg, Sr, etc.) are substitut- ed on either the La3+ or the Cr3+ sites. FIG. 2. (right) Operating Recently, high temperature metallic principle of a solid oxide alloys have also been employed as the fuel cell. interconnection material, particularly for cells intended for operation at lower temperatures (~ 800°C). FIG. 3. (below) (a) Tubular SOFC design; (b) Planar SOFCs of two different basic geome- SOFC design. tries are being developed; tubular and planar;3-6 these are illustrated in Fig. 3. (a) (b) The materials for cell components in these two designs are very similar in nature. In the tubular design, pioneered by the Siemens Westinghouse Power Corporation, the cell components are deposited in the form of thin layers on a cathode tube.5 The YSZ electrolyte is fabricated in the form of a 40 µm thick dense layer by an electrochemical vapor deposition process (EVD).7 Deposition of the YSZ electrolyte film by a more cost-effective non-EVD technique, such as plasma spraying or colloidal/elec- trophoretic deposition followed by sin-

The Electrochemical Society Interface • Summer 2001 23 tering, is also being investigated to is installed at the National Fuel Cell gas separator contacting the anode and reduce cell cost. Research Center on the University of the cathode of adjoining cells. The dense The interconnection is deposited in California at Irvine campus, and is electrolyte and interconnection are fab- the form of an 85 µm thick, 9 mm wide presently undergoing proof-of-concept ricated by tape casting, powder sintering strip along the cathode tube length by testing. or CVD, whereas the porous electrodes plasma spraying. The anode, 100-150 Even though the tubular SOFCs have are applied by slurry methods, screen µm thick layer of nickel/YSZ, is deposit- progressed the most, their electrical resis- printing, or by plasma spraying. ed by first applying nickel powder slur- tance is high, specific power output Sizeable cost reductions are possible ry over the electrolyte and then grow- (W/cm2) and volumetric power density with the planar design through a concept ing YSZ around the nickel particles by (W/cm3) low,5 and manufacturing costs called “mass customization” that is being the same EVD process as used for high. The low power density (0.25 to pursued in the recently initiated U.S. depositing the electrolyte. Deposition 0.30 W/cm2) makes tubular SOFCs suit- Department of Energy’s Solid-State of a Ni-YSZ slurry over the electrolyte able only for stationary power genera- Energy Conversion Alliance (SECA). This followed by sintering has also yielded tion and not very attractive for mobile concept involves the development of a 3 anodes that are equivalent in perfor- applications. Planar, particularly anode- to 10 kW size core SOFC module, that can mance to those fabricated by the EVD supported, SOFCs, in contrast, are capa- be mass produced and then combined for process; use of this non-EVD process ble of achieving very high power densi- different size applications in stationary results in a substantial reduction in the ties of up to about 2 W/cm2.8 In the pla- power generation, transportation, and cost of manufacturing SOFCs. nar design, the cell components are con- military market sectors, thus eliminating A large number of tubular SOFCs has figured as thin, flat plates. The intercon- the need to produce custom-designed been electrically tested for up to about nection having ribs on both sides forms and inherently more expensive fuel cell 70,000 hours. These cells perform satis- gas flow channels and serves as a bipolar stacks to meet a specific power rating. factorily for extended periods of time under a variety of operating conditions with less than 0.1% per 1,000 hour performance degradation. To construct an electric generator, individual cells are connected both in parallel and in series to form a semi-rigid bundle that becomes the basic building block of the generator. A three-in-parallel by eight- in-series cell bundle is shown in Fig. 4.6 Siemens Westinghouse has designed, built, and operated successively larger SOFC power generation systems since 1984. A 100 kW atmospheric SOFC power generation system, shown in Fig. 5, operated very successfully in The Netherlands for a (contract) period of two years without any measurable performance degradation. The system provided 110 kW ac to the Dutch grid at an efficiency of 47%. In addition, it produced approximately 85 kW of hot water for the local district heating system. Such atmospheric SOFC systems FIG. 4. Photograph of a three-in-parallel and eight-in-series cell bundle. are ideally suited for distributed power generation and cogeneration. SOFC operation at elevated pressures yields a higher cell voltage at any current density due to increased Nernst potential and reduced cathode polariza- tion, and thereby permits higher stack efficiency and greater power output. With pressurized operation, SOFCs can be successfully used as replacements for combustors in gas turbines; such pressurized SOFC-gas turbine hybrid power systems are calculated to reach efficiencies approaching 70%. Siemens Westinghouse, under the sponsorship of Southern California Edison, recently fabricated a 250 kW class pressurized SOFC-gas turbine hybrid power system; this system combines a 200 kW SOFC module operating at 355 kPa pressure and a 50 kW microturbine. The system FIG. 5. Siemens Westinghouse 100 kW SOFC system installed in The Netherlands.

24 The Electrochemical Society Interface • Summer 2001 Even though the polymer electrolyte fuels feasible. Also, no noble metal cata- development of low-cost oxidation- membrane (PEM) fuel cell is generally lysts are used, thus reducing the cost of resistant metallic alloys for use as inter- regarded as the fuel cell of choice for the cells. The initial application of connection, ability for rapid start up transportation applications, they require SOFCs in the transportation sector will and thermal cycling, and lower cost. pure H2, with no CO, as fuel to operate be for on-board auxiliary power units successfully. However, presently no H2 (APUs). Such APUs, operating on exist- Conducting Ceramics infrastructure exists, and on-board ing fuel base (gasoline, diesel), will sup- for Production of Chemicals reformer systems to produce H2 from the ply the ever-increasing electrical power existing fuel base (gasoline, diesel) are demands of luxury automobiles, recre- An important application of con- technically challenging, complex, and ational vehicles, and heavy-duty trucks. ducting ceramics is in the production of expensive. Furthermore, it is difficult to Delphi Automotive Systems recently high value chemicals using high tem- eliminate all the CO from the reformate developed and tested a 5 kW APU using perature processes. Two types of ceram- stream. In contrast, SOFCs can use CO anode-supported planar SOFCs. The ics are useful in this regard: mixed elec- along with H as fuel, and their higher challenge in commercializing planar 2 tron-ion conductors for permeation operating temperature and availability SOFCs offering high power densities membrane reactors, and purely ionic of water on the anode side makes on-cell requires successful development of seals conducting materials for solid elec- or in-stack reformation of hydrocarbon for isolating oxidant from the fuel, trolyte reactors. Ceramics in which oxide ions are the main ionically conducting species have received the Related High Temperature Materials Division Symposia widest interest. Mixed Conducting Membrane International Symposium on Ionic and Mixed Conducting Ceramics Reactors—The idea that significant lev- This continuing symposia series made its debut at the fall 1991 meeting of the Society els of electronic conduction can be in Phoenix, Arizona. During the past decade, it has provided a rich forum for the inter- deliberately introduced into predomi- national research community to share and discuss the forefront activities that are nantly ionically conducting systems ongoing in the exciting field of ionic and mixed conducting ceramics. Some of the spe- goes back to early work by Engell and cific themes include ionic transport in solid electrolytes, mixed conduction in ceram- Vygen9 on slags produced in steel mak- ics, thermo- and chemo-mechanical properties of mixed conductors, hydrocarbon con- ing operations where they observed the version by ceramic electrochemistry, electrocatalytic phenomena, electrode reactions involving ceramic cells, ceramics-based fuel cells and batteries, and thin film ceramic existence of Fe in two valence states (+2 membranes. and +3) depending on the oxygen par- The lead symposium organizer is T. A. Ramanarayanan, Exxon/Mobil Research and tial pressure, leading to electronic con- Engineering Company (e-mail: [email protected]). The next (fourth) symposium in duction in the slags. This idea has been this series will be held at the ECS/ISE Joint International Meeting in San Francisco, used in introducing electronic conduc- California, September 2-7, 2001. tion into the ZrO2-Y2O3 matrix by dis- International Symposium on Solid Oxide Fuel Cells (SOFCs) solving an oxide whose cation can exist in multiple valence states; cerium This continuing symposia series provides an international forum for the presentation and discussion of developments related to solid oxide fuel cells based on zirconia or oxide, titanium oxide, and terbium other oxide electrolytes. Topics addressed include materials for cell components. (e.g. oxide have been investigated. Cales and 10 electrolyte, electrodes, and interconnection); fabrication methods for complete cells Baumard showed that in the ZrO2- and components; cell design, electrochemical performance and modeling; stacks and Y2O3-CeO2 system, a maximum in elec- systems for residential and automotive applications; and field tests of SOFC demon- tronic conduction occurred corre- stration systems. sponding to oxygen partial pressures at This symposia series started in 1989 at the Society’s fall meeting in Hollywood, 3+ 4+ Florida and has since become the leading symposium in the field of solid oxide fuel which Ce and Ce ions would exist cells. The symposium is held every two years, rotated among the USA, Europe, and in roughly equal amounts. This is Japan, and co-sponsored by the SOFC Society of Japan. because the probability of electron The lead symposium organizer is S. C. Singhal, Pacific Northwest National jump between these two valence states Laboratory (email: [email protected]). The next symposium in this series (SOFC-VIII) is proportional to the product of their rd will be held at the Society’s 203 meeting in Paris, France, April 27-May 2, 2003. concentrations, which is maximized International Symposium on Solid-State Ionic Devices when the two concentrations are equal. The above mixed conductors belong Solid-state electrochemical devices, such as batteries, fuel cells, membranes, and sen- to the fluorite structure. Additionally, sors, are becoming more and more important for our technologically driven lifestyles. The development of these devices involves common research themes such as ion trans- compositions belonging to the per- port, interfacial phenomena, and device design and performance. The intent of this ovskite, pyrochlore, and brownmillerite symposium is to bring together researchers in all fields of solid-state ionics. This con- classes have also been shown to exhibit tinuing symposia is a forum for current advances in solid-state ion conducting materi- mixed conduction. In addition to these als, regardless of the class of materials or whether the solid-state is amorphous or crys- single-phase mixed conducting sys- talline, and the design, fabrication, and performance of devices that utilize them. tems, dual phase materials have been Topics covered include modeling and characterization of defect equilibria, ionic and electronic transport, interfacial and electrocatalytic properties of ion conducting developed where one phase fulfils the ceramics, novel synthesis and processing of thin films, membranes, and nanostruc- ionic conduction requirements while tured materials or devices; the effect of nanostructures on ionic transport and catalyt- the other provides the electronic trans- ic activity; electrode kinetics, interfacial phenomena, and electrode microstructure per- port path. taining to chemical sensors, fuel cells, gas separation membranes and reactors, solid- Hazbun11 describes in U.S. Patent state battery and microbattery electrodes. 4,827,071 a process for hydrocarbon The lead symposium organizer is E. D. Wachsman, University of Florida (email: [email protected]). The next symposium in this series (SSID-III) will be held at conversion reactions involving a dou- the Society’s 202nd meeting in Salt Lake City, Utah, October 20-25, 2002. ble-layered ceramic reactor. One layer is a compact mixed conductor made

The Electrochemical Society Interface • Summer 2001 25 up of ZrO2-Y2O3 with dissolved CeO2 reacting mixture consisting of ethylene oxygen gas stream from ambient air. or TiO2 while the other layer is porous at a partial pressure of 0.36 kPa and oxy- Compact ceramic oxygen generators are ZrO2-Y2O3 containing an appropriate gen at a partial pressure of 4.6 kPa. With being developed to provide a continuous catalyst. Oxygen from air is ionized to no current passing through the electro- supply of oxygen-enriched air for people O2- at the tube exterior, oxide ions chemical cell, the reaction rate at the with breathing disorders. Similarly, these pass through the mixed conductor and working electrode is 1.5 x 10-8 mol devices can enrich the breathing oxygen appropriate oxidative reactions occur O/sec. Upon passing 1 µA through the concentration for high altitude aircrafts. at the interior tube surface. Hazbun cell so that oxide ions are transported to More recently this technology has describes the conversion of methane the working electrode (Pt), the reaction sparked the interest of NASA for space to ethane and ethylene in the temper- rate gradually increases, plateauing at 40 exploration. Because of the distance ature range 700-900°C, ethylene to x 10-8 mol O/sec. The rate enhancement involved, if we are to travel to other ethylene oxide between 270 and factor, namely the ratio of the maximum planets in the future, we need to utilize 350°C, and propylene to propylene rate with the applied current to the rate their resources (in-situ resource utiliza- oxide between 300 and 400°C. British without any current flow is 26. The dif- tion, or ISRU) for life support and pro- Petroleum (BP) has developed a syngas ference between the maximum reaction pellant production. As early as 20 years µ -8 (CO+H2) production technology rate at I=1 A and at I=0 is 38.5 x 10 from now, NASA plans on sending a called Electropox (Electrochemical mol O/s. If the increase in reaction rate is manned mission to Mars. A conceptual Partial Oxidation). The heart of the solely due to oxidation by the oxygen picture of the Mars Ascent Vehicle is process is a mixed conducting ceramic transported via the applied current, the shown in Fig. 7. In order for this to be membrane that separates oxygen from increase would be I/2 F where F is the feasible, this mission will require the uti- air by oxygen ionization, and the Faraday constant. The ratio of the actual lization of the Mars CO2 atmosphere to oxide ions transport to the opposite rate increase to that expected from produce the propellant for the return surface of the membrane to trigger the Faraday’s law is 74000. This ratio is trip and oxygen, both for breathing and partial oxidation reaction. Membrane called the NEMCA factor; its large value the fuel oxidant. The technology envi- materials of sufficient strength, chem- clearly indicates that the reaction rate sioned to make this possible is based on ical stability and performance have enhancement is non-Faradaic. When the a ceramic oxygen generator converting been developed. Based on bench-scale applied current is cut off, the reaction CO2 to O2 and CO. simulations, considerable savings are rate gradually decays to the original projected for this process when com- value before the application of current. High Temperature Sensors pared with conventional steam The understanding of this phenomenon reforming or combined reforming. BP is in its infancy but could be far reaching The most prevalent application of ion also has developed a cheap and reli- in its impact. conducting ceramics is the automotive able fabrication process that can be oxygen sensor. Every automobile sold in used to produce commercially viable Ceramic Oxygen Generators the U.S. since about 1975 (and in most membrane modules.12,13 for Life Support of the industrialized nations sometime Air Products & Chemicals and thereafter) has had its air/fuel ratio, and Praxair are similarly developing mixed thus its gas mileage and emissions, con- Electrochemical cells based on zircon- conducting membrane technology to trolled by this simple zirconia-based ia or ceria electrolytes are being devel- produce pure O (and N ) from air at a Nernstian device. Advances to this tech- 2 2 oped for a variety of life support systems smaller size scale where cryogenic nology have focused on expanding the because they can readily produce a pure plants are not considered economical. sensor range (using an amperometric Solid Electrolyte Reactors— Exclusive ionic conduction in solid electrolyte ceramics has led to their use in driving specific chemical reactions. Examples include high temperature steam electrolysis, upgrading of hydrocarbons, and pollution control (e.g., NOx reduction). Of particular interest in production of chemicals is a phenomenon termed NEMCA (Non- Faradaic Electrochemical Modification of Catalytic Activity) investigated by Vayenas and co-workers.14 Enhancement of catalytic activity due to the formation of heterojunctions between dissimilar materials or the application of an applied potential can influence the mechanistic steps involved in heterogeneous chemical reactions. The effect can be best described by referring to the graph in Fig. 6, which shows in the inset the electrochemical cell arrangement. The reaction under investigation is the oxi- FIG. 6. NEMCA effect in ethylene oxidation. The designations, SE, RE, CE, and WE refer to the solid electrolyte, refer- dation of ethylene on a Pt catalyst, the ence electrode, counter electrode and working electrode, respectively.

26 The Electrochemical Society Interface • Summer 2001 Atmosphere Filter Intake Duct Methane Condenser Methane Pump Methane Hoses

H2 Intake Sabatier Reactors Water Electrolyzer

O2 Pump O2 Condenser CO2 Condenser CO2 Compressor

About the Authors

FIG. 7. Mars Ascent Vehicle based on ISRU propellants. T. A. (Ram) Ramanarayanan is Senior Scientific Associate at design) and enhancing the selectivity to References the Corporate Strategic Research specific gaseous species; the latter is crit- Laboratories of ExxonMobil Re- search and Engineering Company. ical for combustion control, in order to 1. H. Rickert, Electrochemistry of Solids, During his 22 year career at meet future environmental regulations. Springer-Verlag (1982). The issue is that ppm levels of pollutants 2. P. J. Gellings and H. J. Bouwmeester, eds., Exxon/ExxonMobil, his research Solid State Electrochemistry, CRC Press (1997). interests have been primarily in the such as NOx and CO have to be moni- tored and controlled in concentrations 3. N. Q. Minh and T. Takahashi, Science and fields of corrosion chemistry and Technology of Ceramic Fuel Cells, Elsevier electrochemistry in high temperature of O2, CO2, and H2O that are not only (1995). systems. orders of magnitude higher than the 4. Solid Oxide Fuel Cells VI, S. C. Singhal and species to be detected, but that also fluc- M. Dokiya, Editors, PV 99-19, The Subhash C. Singhal is Battelle tuate dramatically under typical driving Electrochemical Society Proceedings Series, Fellow and Director, Fuel Cells at Pennington, NJ (1999). conditions. Modifications of the elec- the Pacific Northwest National trode materials and microstructures are 5. S. C. Singhal, Solid State Ionics, 135, 305 (2000). Laboratory (operated by Battelle for being actively investigated to obtain the 6. S. C. Singhal, MRS Bulletin, 25, 16 (2000). the U.S. Department of Energy), needed selective response. 7. U. B. Pal and S. C. Singhal, J. Electrochem. Richland, Washington. Prior to join- Soc., 137, 2397 (1990). ing his present position in April Summary 8. J-W. Kim, A. Virkar, K-Z. Fung, K. Mehta, 2000, he was manager of fuel cell and S. C. Singhal, J. Electrochem. Soc., 146, technology at the Siemens Tremendous progress has been made 69 (1999). 9. H. J. Engell and P. Vygen, Ber. Bunsenges. Westinghouse Power Corporation in in understanding the defect chemistry Phys. Chem., 72, 5 (1968). Pittsburgh, PA for over 28 years. His and electronic and ionic conduction in 10. B. Cales and J. S. Baumard, J. Electrochem. research interests are in high temper- many high temperature ceramics over Soc., 131, 2407 (1984). ature materials and advanced ener- the last four decades. Such ceramics are 11. E. A. Hazbun, U.S. Pat. 4,827,071 (1989). gy conversion systems, particularly 12. T. L. Cable, J. G. Frye, Jr., and T. J. Mazanec, now attracting increasing attention for a solid oxide fuel cells. variety of applications, notable among Eur. Pat. 399833A1 (1990). 13. T. J. Mazanec, Solid State Ionics, 70-71, 11 these being fuel cells for power genera- (1994). Eric D. Wachsman is an Associate tion, production of chemicals, oxygen 14. C. G. Vayenas, M. M. Jaksic, S. I. Bebelis, Professor in the Department of generation for life support systems, and and S. G. Neophytides, in Modern Aspects of Materials Science and Engineering high temperature electrochemical sen- Electrochemistry, No. 29, J. O’M. Bockris, B. at the University of Florida. Prior to sors. Many of these applications promise E. Conway, and R. E. White, Editors., p. 57, joining his present position in to become wide-spread commercial suc- Plenum Press, New York (1996). January 1997, he was a Senior cesses in the coming years as the cost is Scientist at SRI International. Dr. brought down through further technical Wachsman’s research interests are advances and mass production. in ionic and electronic conducting ceramics, the development of moder- ate temperature solid oxide fuel cells (SOFC), gas separation membranes, and solid-state sensors.

The Electrochemical Society Interface • Summer 2001 27