Solid Electrolytes: Advances in Science and Technology
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Advances in Science and Technology Himanshu Jain, John O. Thomas, and M. Stanley Whittingham, Guest Editors
The interdisciplinary area of science and engineering dealing with solid electrolytes and mixed conductors is frequently known as solid-state ionics. It concerns materials that show rapid ionic motion with or without electronic conductivity, from basic science through application. Interest in solid-state ionic materials has continued for the past few decades due to several important, promising applications, such as fuel cells, batteries, sensors, and electrochemical pumps. The principle behind these applications is simply either the Nernst law (as exemplified by Equation 1 in the article by Singhal in this issue) or Faraday’s laws of electrochemistry (which connect current flow to mass flow), as applied to a cell consisting of an electrolyte and two electrodes. However, the technological issues are complex, and demands on materials can be very diverse, as illustrated by the five articles in this issue. These articles are based, in part, on the invited talks presented at a symposium in April 1999 on the same subject at Lehigh University to commemorate G.C. Farrington’s inauguration as its president. The first report of ionic motion in solids is attributed to Michael Faraday,1 who by 1839 had noted that lead fluoride and silver sulfide become highly conducting when heated. Little activity was reported in this field during the remainder of the 19th century. However, Haber and Tolloczko2 in 1904 recognized that galvanic cells could be constructed with solid electrolytes. Shortly thereafter, Katayama3 used such cells to measure thermodynamic properties, and Tubandt4 showed that Faraday’s laws applied to solid electrolytes, just as they do to aqueous solutions. One of the most studied solid electrolytes is doped zirconium oxide, which
MRS BULLETIN/MARCH 2000
is a major component of the solid-oxide fuel cells discussed in Singhal’s article. In addition to its use in solid-oxide fuel cells, it is presently used in commercial oxygen sensors—for example, to control combustion in automobiles and to measure oxygen concentration in molten metals—and also in steam electrolyzers. The first use of zirconia was in 1900 in the Nernst glower, a device Nernst5 produced to convert electricity into light by the passage of oxygen. He optimized on the composition 85wt%ZrO2-15wt%Y2O3, and this was termed the “Nernst mass.” In 1937, Baur and Preis6 used this electrolyte in high-temperature fuel cells. It was commercialized in fuel cells in the U.S. space program. Singhal describes the current status of fuel-cell technology for high-efficiency, pollution-free electric power generation approaching the 250kW level. All present commercial batteries use liquid electrolytes, either aqueous (in the case of lead-acid, MnO2 dry-cell, Ni/Cd, and Ni/MeH batteries) or organic (in the case of the Sony Li-ion cell). These cells must also contain a separator to prevent physical contact between the anode and the cathode. Thus a solid electrolyte s
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