New Oxide-Ion Electrolytes
- PDF / 643,899 Bytes
- 15 Pages / 420.48 x 639 pts Page_size
- 116 Downloads / 201 Views
NEW OXIDE-ION ELECTROLYTES J.B. Goodenough and Y.S. Zhen Center for Materials Science and Engineering, ETC 5.160 University of Texas at Austin, Austin, TX 78712-1084
ABSTRACT Strategic considerations for the design of new O 2 --ion solid electrolytes are presented, and preliminary measurements of the oxide-ion conductivity 0o = (BIT) exp (-EA/kT) and transport number to = ao0/a are reported for several oxides with perovsldte-related structures. Ba2In205 exhibits a first-order transition at Tt - 930"C from the brownmillerite structure to a cubic perovskite with disordered oxygen vacancies and fast 0 2 --ion conduction. Ba3In2MO8 (M = Ce, Zr, or Hf) contains oxygen vacancies ordered into every second B02 (001) plane. An octahedralsite preference of the disordered M(IV) cations makes incomplete the oxygen-vacancy ordering, so a high extrinsic oo with EA - 0.6 eV is found for T < 400 *C. The transport properties of Ba3In2ZrO8 at 400 °C are equivalent to those of the stabilized zirconias at 800 'C. Ba8In6017 consists of an intergrowth of (BaO)2 rocksalt and Ba31n3O7.5 oxygen-deficient perovskite layers. Although there is no evidence of long-range ordering of the oxygen vacancies, nevertheless an EA = 1.06 eV and transport properties equivalent to stabilized zirconia indicate retention of short-range correlations to at least 950 *C. Attempts to prepare Ba8Y60 17 yielded a mixture of hexagonal Ba 3Y 4O9 and unidentified phases. An EA = 0.45 eV above a Tt = 370 *C gives a remarkable 02-. ion conductivity at temperatures T > Tt in both the mixed-phase and pure Ba 3Y4O9 at temperatures T > 370 *C, but the samples are extremely hygroscopic. INTRODUCTION An electrochemical cell is designed to have a dc electronic current flowing in an external circuit that is balanced by a purely ionic current in the internal circuit. A solid electrolyte acts, ideally, not only as an internal electronic insulator separating the two electrodes of the cell, but also as a conductor of the working ion of the cell. It is particularly useful where the reactants at the two cell electrodes are either gaseous or liquid. An oxide-ion electrolyte is a conductor of 02- ions; it may be used in either an open-circuit or a power device. These two types of applications are illustrated schematically in Fig. 1. In each case, the oxide-ion electrolyte is in the form of a thin sheet of large surface area having porous electrodes on opposite faces. The open-circuit application of Fig. 1(a) represents an oxygen sensor or meter, a reference oxygen partial pressure p82 on one side is used to determine an unknown po 2 on the other via a
Mat. Res. Soc. Symp. Proc. Vol. 210. t1991 Materials Research Society
288
Mobile 202.Ion
P0
2
(a) Fig. 1 02
÷4e-
202.
V
Mobile o"2oIon
.v0 2 +4e" 202 (pump; V =vapp ) 2 H 2 +20 2( fuel cell;
An 0 2 --ion solid-electrolyte
separator used in (a) an oxygen sensor
and (b) an oxygen pump (V = Vapp) or
a hydrogen-air fuel cell (V = Vc - 11()).
2H 2 0 +4e' V = Voc l)
(b)
measurement of the open-circuit voltage Voc generate
Data Loading...
