Within solid oxide fuel cells SOFCs, for instance, nanostructured ionic and electronic con-ducting materials can increase the electrochemical per-formance of the cathode and thus could p
Trang 1Solid: Mixed Ionic-Electronic Conductors
E Ivers-Tiffe´ e, Universita¨t Karlsruhe (TH), Karlsruhe, Germany
& 2009 Elsevier B.V All rights reserved.
Introduction
Mixed ionic–electronic conductors (MIECs) have been
and continue to be of interest for strategic applications
related to energy conversion and environmental
moni-toring including batteries, fuel cells, permeation
mem-branes, and sensors Within solid oxide fuel cells (SOFCs),
for instance, nanostructured ionic and electronic
con-ducting materials can increase the electrochemical
per-formance of the cathode and thus could potentially
facilitate lower-temperature operation and thereby
pro-vide faster start-up times, improved stability, and less
complicated thermal management
Mixed Conduction
The electrical conductivity s of any given material is the
sum of contributions from all electrically charged mobile
species, i.e., electronic parts (se,sh) as well as
contri-butions from ionic charge carriers (sion):
s¼ seþ shþ sion¼ e0ðnmnþ pmpÞ þX
i
zie0Nimi
with n, p, and Nithe concentrations of electrons e, holes
h, and ions (several mobile species i are generally
con-sidered), respectively, and mn, mp, and mitheir respective
mobilities (e0is the elementary charge and zithe valence
of the ion with index i )
Mostly, one type of carrier dominates charge
trans-port, so the contributions from the so-called minority
carriers can usually be neglected In many materials,
electronic conduction prevails (sEse or sEsh),
classi-fying them as electronic conductors; in some materials
ionic conduction dominates (sEsion) under certain
conditions (e.g., solid oxide electrolytes where the
transport of oxygen ions prevails, cf.Electrolytes: Solid:
Oxygen Ions), classifying them as ionic conductors, and a
certain class of materials is described as MIEC: here,
depending on experimental conditions, both ionic and
electronic transport must be taken into account
The fraction of the total conductivity caused by the
individual charge carriers (e.g., ion with index i ) is usually
described by the so-called transference number ti:
tiEsi
s
For electronic conductors, the sum of electron and hole
transference numbers, teþ th, is unity Yet, in principle, tiis
never truly zero, thus making mixed conduction the normal case For practical reasons, however, the term
‘mixed conduction’ should only be applied when both ions and electronic charge carriers significantly contribute to the overall conductivity
Electronic conductivity is determined by the elec-tronic bandgap, depending on the properties of the ions the material is composed of, whereas ionic conductivity
is related to its crystal structure Oxygen ion conduction
in oxides can occur via transport of oxygen vacancies or interstitial oxygen ions, depending on the crystal struc-ture Both are considered as defects with regard to the ideal crystal structure In a pure compound, intrinsic defects are formed as a function of temperature, in ac-cordance with thermodynamic considerations The presence of aliovalent ions (dopants) leads to the for-mation of extrinsic defects
In many oxides, the oxygen ion transport takes place
by means of a hopping mechanism via vacant lattice sites, resulting in a thermal activation behavior of the con-ductivity sion:
sion¼s0
Texp EA
kT
where T is the absolute temperature, s0a constant, and
EAthe activation energy
In some metal oxide compounds, oxide ions can ex-hibit high values of mobility (By way of comparison, the mobility of the cations is usually far lower.) Then, the ambient conditions (temperature T, oxygen partial pres-sure pO2) imposed on the material can result in a quick electrochemical equilibration Consider an oxide where oxygen exchange with the ambient gas phase takes place
at sufficiently high temperatures by means of oxygen vacancies in the anionic sublattice This is expressed in Kro¨ger–Vink notation by the reaction
Ox
O" V
O þ 2e0þ1
2O2
Thereby, the concentration of oxygen vacancies Vdd
O
changes and can be determined from the corresponding mass action law Because this reaction also involves electronic charge carriers (e0), their concentration n takes
on a new value, too As in any semiconductor, n and p are coupled (nppexpðEg=kT Þ, where Eg is the bandgap energy), thus influencing p as well
In the presence of further charge carriers (e.g., dop-ants), more defect-chemical equations have to be
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