Electrolytes:New materials

Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova

Laurea Magistrale in Scienza dei MaterialiMateriali Inorganici Funzionali

Bibliography1. N.Q. Minh, T. Takahashi: Science and technology of ceramic fuel cells

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7438. N. Trofimenko et al Solid State Ionics 118 (1999) 215-2279. I.R. Evans et al Chem. Mater 17 (2005) 4074-407710. S.S. Pramana et al Acta Crystallographica B 63 (2007) 597-60211. M.C. Martin-Sedeno et al Chem. Mater 16 (2004) 4960-496812. E. Kendrick et al Solid State Ionics 179 (2008) 819-82213. P.J. Wilde et al Solid State Ionics 112 (1998) 173-18314. A.S. Nowick et al Solid State Ionics 125 (1999) 303-31115. A. Arulraj et al Chem. Mater. 14 (2002) 2492

Comparison of the oxide ionconductivity in Fe, Co, or Ni doped

LaGaO3 based oxide.

YSZ = Zr0.86Y0.14O2SDC = Ce0.8Sm0.2O2

LSGM = La0.8Sr0.2Ga0.8Mg0.2O3, LSGMF = La0.8Sr0.2Ga0.8Mg0.17Fe0.03O3,LSGMC = La0.8Sr0.2Ga0.8Mg0.115Co0.085O3, LSGMN = La0.8Sr0.2Ga0.8Mg0.13Ni0.07O3.

���� Specific conductivity and activation energy vsGoldschmidt tolerance factor and free volume of the unit

cell

Perovskites: ABO3

���� A = La; B = Ga

Ionic radii versus coordinationnumber for the perovskites. For Co , Fe , Mn the radii of the high spin

states were used.

(La0.9Sr0.1)MIIIO3-δδδδ

(LN0.9Ca0.1)Ga0.9Mg0.1O3

LN = Nd, La

(Nd1-xAKx)Ga0.9Mg0.1O3

AK = Sr, Ca

(Nd0.9AK0.1)GaO3

AK = Sr, Ca, Ba

(LN0.9Ca0.1)GaO3

LN = La, Sm, Pr, Nd

(Ba2.9A0.1)Ga2O6

A = Na, K, Rb

SammelsSammels etet al., 1990 al., 1990 –– CockCock etet al., 1991al., 1991

� the appearance of an optimum free volume, Vf, value(which exhibits the minumun activation energy for anion

conduction) = 30-35 Å3 range

Vf = difference between unit cell volume per chemical formula of compound and the volume occupied by constituent ions: the

“space” in the perovskite structure

No consideration on distortion

NomuraNomura e e TanaseTanase 19971997

pre-exponential factors, A have almost the same value (in the temperature range 730–980°C), except for the Sc compound which has a

low relative density. Under this condition, oxide ion conductivity is mainly determined by E .

Is expected to be negligible since the dopant cation (Sr2+ 1.44Å) has almost the same size as the host one, (La3+ = 1.36Å)

σT = A exp (Ea/kT)

Ea = ∆Hm + ∆Ha

Crystal structure of cubicperovskite (a) and orthorhombic

perovskite (b)

O2-

B-cations

A-cations

Free volume and tolerance factor as a function of the idealized cubic lattice

parameter, a°, at RT

Free volume dependence of total conductivityin air (black) and N2 (open symbols)

TM-doped La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM)(Cr, Mn, Fe, Co) y = 0.1-0.3

SrCO3, MgCO3, Ga2O3, La2O3, Cr2O3, Mn2O3, Fe2O3, Co3O4 mixed in a mortar, heated in air for 20 h at 1350°C. Crushed and ground for 2 h. The

powder was pressed into pellets and sintered in at 1450°C for 20 h.

LaGaO3 = orthorhombic+ Sr and Mg = cubic

High cobalt content = Hexagonal

y = 0.1

CoCo

y = 0.3

Splitting of the cubic (110) reflection intothe (110) and (104) reflections of the

hexagonal cell

With decreasing size of the doped cations the main peak of the perovskite-phase shifts tobigger angles according todecreasing unit cell volume

� Reduction (10-10 Pa 800°C) and Mg play an important role in the solubility of Co ions in the lattice = cubic structure

SammelsSammels 19901990

� high oxygen conductivity in mixed oxides characterized by a high free volume of the unit cell and a weak association between

mobile ions and the lattice bulkt-factor near to 1: no distorsion of the perovskite

Cation and doping cations almost equal radiir = (rA + rO)/√2(rB + rO)

AAIIIIII11--aaAAIIII

aaBBIIIIIIOO33--xxrA = (1-a)rAIII + arAII x = a/2

rB = (1-b)rBIII + brBII x = b/2AAIIIIIIBBIIIIII11--bbBBIIII

bbOO33--xx

� Additional doping with transition metal cations M on B-sitewhich have non-integer oxidation states depending on the

oxidation conditionsLa0.9Sr0.1(Ga0.9(FeIII1-zFeIVz)0.1)0.8Mg0.2O3-x+δ

RealReal t t factorsfactors

Oxygen ∆O exchanged from air-oxidized samples of

La0.9Sr0.1(Ga1-yCoy)0.8Mg0.2O3-x-δ

as a function of pO2 at 800°C.

Oxygen stoichiometry range, δ, mean ionic radii and t-factor

FreeFree lattice volume and t lattice volume and t factorsfactors

Lattice parameters of the cubic phase of

La0.9Sr0.1(Ga1-yCoy)0.8Mg0.2O3-x-δ vsthe mean ionic radii of the transition metal ions, M,

calculated on the basis of the experimentally determined

oxidation states

Free volumes and t-factorsfor the air-oxidized samples

as functions of dopant concentrations for

La0.9Sr0.1(Ga1-yMy)0.8Mg0.2O3-x-δ

(M=Co, Fe).

ConductivityConductivity

�Conductivity increases with T� For the highest dopant

concentrations a maximum is observed (800°C for Co, 700°C for Fe) = change in conductivity from

semiconductive to metallic

Co 0.0 Co 0.1

La0.85Co 0.1Co 0.2

Co 0.25

Co 0.3

Fe 0.1

Fe 0.2

La0.85Fe0.1

Fe 0.3

ConductivityConductivity

Activation Energy = Migration Enthalpy(small differences in the size of host and dopant =

negligible association phenomena)

Activation Energy decreases with doping

IonicIonic and and ElectronicElectronic ConductivityConductivity

In the range up to y = 0.1 Co and Fe no dependence of conductivity on PO2

= ionic domainY > 0.1 = p-conduction

Conductivity of La0.9Sr0.1(Ga1-yMy)0.8Mg0.2O3-x-

δ (M = Co, Fe) as a function of dopant concentration y at 800°C

Conductivity of La0.9Sr0.1(Ga1-yMy)0.8Mg0.2O3-x-δ (M = Co, Fe) as a function

of dopant concentration y and PO2

0.2650.4330.310

0.707Ea(Co)

0.16410.13240.0603δ (Co)0.96530.96420.9613t (Co)

12.68213.15614.307Vf(Co)

0.3110.3680.657Ea(Fe)

0.09630.06420.0344δ (Fe)0.96210.96120.9605t (Fe)

13.47213.86114.436Vf(Fe)

y = 0.3y = 0.2y = 0.1

0.707(0.694)

0.657(0.578)

0.8980.745

0.748Ea

0.06030.03440.02110.004δ

0.96130.96050.96080.9599t

14.307(16.102)

14.436(16.583)

14.36214.817Vf

CoFeMnCr

Conductivity estimations

La0.85Sr0.1(Ga0.9My)0.8Mg0.2O2x-δM = Mn, Cr, Co, Fe

(La0.85Sr0.1(Ga0.9M0.1)0.8Mg0.2O2x-δ M = Fe, Co)

� > TM doping > δ; in the row Cr<Mn<Fe<Co. �

� y > 0.1: conductivity increases due toadditional p-type conduction.

� Mn, Cr n-type conductivity

Cr

Mn

Co

CoFe

Perovskites and RP

La0.8Sr0.2Ga0.8Fe0.2O2x-δ

25x10-3

20

15

10

5

0

j O2

(ml*

cm-1

*min

-1)

800750700650600Temperature (°C)

Specific oxygen permeation rate obtained for:

La2Cu0.8Co0.2O4

La0.7Sr0.3CuO3

La1Cu0.3Co0.7O3

n = 1

General formulas:

An+mBnO3n+m

Or:

(AO)m[ABO3]n

n perovskite planes followed by m AO NaCl type planes

Conductivity estimations

� jt = total current density, je and jO2- = electronic and ionic partial currents in C·cm-2·s-1.

With: � F = Faraday constant (C·mol-1), R = universal

gas constant (J·mol-1·K-1), T = absolute temperature (K), σe and σi = electronic and ionic conductivity (S·cm-1), tm = membrane thickness (cm), Ph and Pl = oxygen partial pressure at the O2-rich and O2-deficient sides respectively (Pa).

� Schematic diagram showing the relationship between the ABO3perovskite (a) and A2B2O5 brownmillerite (b) structures.

The former is converted to the latter by replacement of half the BO6octahedra by BO4 tetrahedra.

Conductivity in Brownmillerite

(a) ideal cubic perovskite parent structure type(b) orthorhombic brownmillerite structure

(c) tetragonal ‘‘defect perovskite’’ superstructure phaseThe BO6 octahedra are shown dark while the oxygen-deficient BO4

and BO6-2y polyhedra of (b) and (c) are shown in grey.The A cations are represented by the large grey balls.

� Variation of the ionic conductivity of Ba2In2O5 with temperature showing the abrupt increase between 1140 and 1230

K.

� Evolution of part of the powder neutron diffraction pattern of Ba2In2O5 with

temperature, illustrating the structural change from

brownmillerite to disordered perovskite at 1170 K.

Variation of the lattice parameters of the brownmillerite and perovskite phases of

Ba2In2O5 with temperature

brownmillerite

perovskite

Defect, protons and conductivity in brownmillerite-structured Ba2In2O5

� Structure = derivative of the perovskite with tetravalent M cations completely substituted by cations one less in valency: to maintain charge

balance, one-sixth of anions are removed

� The brownmillerite structure responds to the high concentration of vacancies by

ordering them in parallel rows (= alternating sequence of octahedral and tetrahedral

layers)� This ordering traps the vacancies at fixed positions: modest ion conductivity at low T

�These vacancies are normally un-occupied = interstitial sites

� At 925°C = dramatic increase in electrical conductivity as the trapped oxygen vacancies

become partially disordered� Further heating = vacancies become

completely disordered = structure reverts to a highly defective cubic perovskite.

The unit cell of Ba2In2O5 showing interstitial sites (open squares) and the layers of

crystallographically distinct oxygen sites. Indium ions sit at the entres of oxygen

octahedra while the larger barium cations occupy the open spaces between O(2)

ions.

1. Born model of polar solids with Coulombic potentials – Mott-Littleton approximation (ions in an inner region surrounding thedefect are treated explicitly while the remainder of the crystal

is treated by quasi-continuum methods)2. The considered defects: intrinsic, or thermal, defects such as vacancies and interstitials which combine as Schottky or

Frenkel pairs 3. Estimating oxygen migration paths and energies by placing single oxide ions in ‘transition’ states between normal anion

positions. The transition state was assumed to be the saddle-point of the energy surface between initial

4. Various charged defects on different crystallographic sites were treated to calculate the energies of possible redox

reactions5. Protons associated with each of the oxygen sites were

considered

Conductivity in BrownmilleriteOxygen ion conductivity

� Oxygen Frenkel pairs are the most energetically favourableintrinsic defects and are responsible for oxide ion conductivity in the

orthorhombic structure (i.e. at low temperature)

� The low Frenkelenergy suggests that

the defectconcentration at intermediate

temperatures in the brownmilleritestructure will be

significant

� > T = increment of Frenkel defects� O(1) sites more favourable

� = the coordination environments of O(1) and O(3) become similar until a critical

concentration of defects

� O(1) and O(3) indistinguishable: the ordered vacancies released can diffuse

rapidly through the material

� jumps between the shortest distance in the octahedral layers have the lowestactivation energy but Eatt in the other

directions are not much greater

� a slight anisotropy in oxide ion diffusionis therefore expected in [001] direction

Octahedral or tetrahedral jump?Which is the path?

Conductivity in BrownmilleriteElectronic conductivity

Oxidation most likely occurs via incorporation of oxygen at interstitial site in the tetrahedral layer

� The oxidation energy of 2.22 eV is sufficiently low that p-typeconductivity is expected to occur in mildly oxidizing environments

�The higher reduction energy shows that Ba2In2O5 is more resistant toreduction than oxidation.

Conductivity in BrownmilleriteProtonic conductivity

Protonic conductors

� During processing at high T oxygen-ion vacancies react with oxygen to produce electron holes (oxygen-ion and electron hole conduction)

½ O2 + V¨O = OxO + 2h˙

� In hydrogen atmosphere: ½ H2 + h˙ = H+

� In wet atmosphere: H2O + 2h˙ = ½ O2 + 2H+

H2O + V¨O = OxO + 2H+

H2O + OXO + V¨O = 2 (OH)˙O

� Protons are considered to be interstitials

� Hydroxide ions are considered to migrate between sites adjacent to oxygen ions or via vacancies

� Hydroxide-ion conduction is considered to occur when water is present

� Conduction can be hypothesized by activation energy measurements

(1)

(2)

(3)

(4)(5)

Energies of proton incorporationOxygen vacancies filled by OH

groups

Accomodation of a OH group at a structural vacancy (interstitial site) with the proton associating with aninterstitial oxygen present as a

Frenkel defect:

Water incorporated as protonsand oxygen interstitials:Involving a Frenkel pair:OH groups occupy both anoxygen vacancy and an

interstitial site

The more favourable mechanism: simultaneous occupancy of interstitial and lattice oxygen sites by protons from a

single water molecule

Water incorporation involves the creation of a mixture of OH’O and OH’idefects (OH groups on both lattice and interstitial sites

Proton exchange between lattice and interstitial oxygen ions

Doping in Brownmillerite

Variation of the unit cell volume of Ba(InxZr1-x)O3-x/2 samples versus x.

� Introducing a high valence ion, Zr(IV), induces anionic disorder that transforms the structure to a defect perovskite

structure as the high-temperature phase

� (rIn3+=0.80Å, rZr4+=0.72Å) the cell volume decreases with increasing Zr content

� The increase observed at x = 0.6-0.7 is due to the increased hygroscopic nature of the

material

� > Zr(IV) > oxygen (i) introduced to maintain the charge balance > conductivity

Doping and Conductivity BrownmilleriteEffect of the size

Schematic illustration of Ba2(In1-xGax)2O5

Temperature dependence of electrical conductivity of Ba2(In1-xGax)2O5 (x = 0.00, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45)

Lattice constant decreases with increasingGa (Ga(III) = 0.620 Å, In(III) = 0.790 Å

Lattice parameter (a) versus Ga content.

XRD Pattern of Ba2(In1-xGax)2O5

Cubic

Doped Brownmillerite� The lower limit of dopant concentration to stabilize

perovskite at RT 20%� In doped Ba2In2O5 the amount of oxide ions introduced by

doping, lattice parameters and unit cell volume are important indetermining conduction properties

� Reported highest conductivity = 0.3 S/cm at 1220 K for (Ba0.3Sr0.2La0.5)2In2O5.2

Doping and Conductivity BrownmilleriteEffect of size and charge

Doping with W6+

� high amount of oxide ions introduced into the lattice:

Ba2(In1-xWx)2O5+3xDOPING WITH SMALL AMOUNT OF W MAY STABILIZE

THE PEROVSKITE-TYPE STRUCTURE AT LOW-TEMPERATURE

� The small ionic radius of W6+ may be advantageous for the stabilization of the structure

BaCO3 + In2O3 + WO3 mixed in agate mortar with ethanol then dried and calcined at 1473 K. The powder was pressed into a

pellet and sintered (1673 and 1723 K 1h)

W-doped Ba2In2O5

� W(VI) ionic radius = 600 pm; In(III) ionic

radius = 800 pm� large amount of oxide

ions with doping� Doping with W

stabilizes the cubic perovskite-type

structure down to RT� Lattice parameters

and cell volume decrease with increasing W

XRD of(a) Ba2In2O5

(b) Ba2(In0.95W0.05)2O5.15(c) Ba2(In0.90W0.10)2O5.30(d) Ba2(In0.85W0.15)2O5.45

Dependencies of lattice parameters and cell volume on

x in Ba2(In1-xWx)2O5+3x.

Conductivity of the W-doped Ba2In2O5

� W doping increases the conductivity.

� No anomalies of conductivity = no phase transition

Total electrical conductivity of Ba2(In0.95W0.05)2O5.15

and Ba2(In0.90W0.10)2O5.30Dashed line indicates the conductivity

of Ba2In2O5

Conductivity of the W-doped Ba2In2O5

EMF of the oxygenconcentration cell using Ba2(In0.95W0.05)2O5.15

and Ba2(In0.90W0.10)2O5.30as an electrolyte

Dry air, Pt | Ba2(In1-xWx)2O5+3x | Pt, dry air Wet air, Pt | Ba2(In1.9W0.1)2O5.15 | Pt, Wet air

EMF of the steamconcentration cell using Ba2(In0.95W0.05)2O5.15as an electrolyte

The The contribution contribution of proton to of proton to conduction conduction cannot be cannot be neglected, neglected, especially at especially at

lower lower temperatures temperatures

Doping with differenthigh valence state ions:

V5+, Mo6+, and W6+

BaCO3, In2O3, V2O5, MoO3, WO3 in alumina crucibles. Calcinedat 1000, 1200, and 1300°C for 12 h with intermediate

grindings. The powder was pressed into a pellet and sintered (1300°C)

XRD of Ba2In2-xVxO5+δ solid solution as a function of the substitution level x

(* = Ba3V2O8)

Evolution of the unit cell parameters of Ba2In2-xMexO5+δ solid solution as a function of the substitution level x

Ba2In2-xMexO5+δ solid solution at RT

BaBa22InIn22--xxMeMexxOO5+5+δδ (Me = V, Mo, W): RT(Me = V, Mo, W): RT

Orthorhombic to tetragonal: 925°C

BaBa22InIn22--xxMeMexxOO5+5+δδ (Me = V, Mo, W): High Temperature(Me = V, Mo, W): High Temperature

x = 0x = 0

Tetragonal to cubic: 1050°C

Formation of hydrated phase

between 200-325°C

� Uptake of water starting at 100°C followed by a dehydration at 300°C

� The release of water is characterised by two endothermic peaks: a two step mechanism

� On cooling the uptake is reversed

BaBa22InIn22OO55� Orthorhombic to tetragonal 925°C; tetragonal to cubic 1050°C

� hydrated phases between 200-325°C

BaBa22InIn1.91.9WW0.10.1OO5.155.15

BaBa22InIn1.91.9MoMo0.10.1OO5.155.15� Orthorhombic to tetragonal 600°C; tetragonal to cubic 925°C

� hydrated phases between 200-325°C

� Orthorhombic to tetragonal 700°C; tetragonal to cubic 925°C� hydrated phases between 200-325°C

� For the orthorhombic polymorph a decrease of the order-disorder transition temperature with the increase of substitution

level was observed� hydrated phases = proton conductivity at low temperature

BaBa22InIn1.91.9VV0.10.1OO5.055.05� Orthorhombic to tetragonal 650°C; tetragonal to cubic 925°C

� hydrated phases between 200-325°C

Complex plane diagrams of impedance, measured at 700°C, on Ba2In2-xMoxO5+δ,x =0, 0.1, 0.5.

Arrhenius plots for Ba2In2-xMoxO5+δ

x =0 (□), 0.1 () and 0.5 (∆).

Arrhenius plots for Ba2In2-xVxO5+x/2

x = 0, 0.1 (), 0.2 (∆) and 0.4 (□).

Orthorhombic to tetragonal

Orthorhombic to tetragonal

Conductivity and tetragonal structure

Doping with Ti4+

BaCO3, In2O3, TiO2, ground in acetone. Calcined at 1200 for 24 h. The powder was pressed into a pellet and sintered (1350°C

24 h)

� At RT, when x increases, the progressive filling of oxygen vacancies, concomitant with the

substitution of Ti for In, induces

� 0.075 < x < 0.15, the symmetry becomes tetragonal

� 0.15 < x < 1: disordered cubic perovskite structure

BaBa22InIn2(12(1--x)x)TiTi2x2xOO5+x5+x□□11--xx

x = 0.2x = 0.2

� for 0 < x < 0.075: disorder in the plane of oxygen vacancies observed in Ba2In2O5.5

OrthorhombicOrthorhombic

TetragonalTetragonal

from brownmillerite to tetragonal structure (x: 0.075 to 0.1) = drastic decrease of the Eatt: this structural evolution introduces

much more facile diffusion pathways

BaBa22InIn2(12(1--x)x)TiTi2x2xOO5+x5+x□□11--xx

The highest oxide-ion conductivity is observed for 0.1<x<0.33:σ = 0.5 x 10-2 S cm-1 at 700°C.

from tetragonal to cubic structure (x: 0.18) = cell volume remains constant

Progressive decrease of the perovskite cell volume together with the filling of oxygen vacancies tend to impede the oxide ion

diffusion

Doping in A: (Ba1–xLax)2In2O5+x

� 0.0 ≤ x ≤ 0.3 orthorhombic phase

� 0.3 < x ≤ 0.5tetragonal phase

�x > 0.5 cubic phase

Tilting of oxygen octahedra with La ion doping

� Conductivity independenton oxygen partial pressureat all the temperatures:Oxygen ion conductor

� Arrhenius plot for(Ba1–xLax)2In2O5+x system

� x = ● 0.0, ■ 0.1, ▲ 0.3, ♦ 0.5 and ▼ 0.7

Carrier increment

Association, Phase change

No carrier or mechanism change

Doping in A: (Ba1–xLax)2In2O5+x