UNIVERSITY OF CRAIOVA

DOCTORAL SCHOOL OF SCIENCES

DOMAIN: CHEMISTRY

Abstract of the Ph.D. thesis

DEVELOPMENT OF NEW OXIDE

SYSTEMS FOR SOLID OXIDE FUEL CELLS

Ph.D. Student:

VOINEA ELENA-ADRIANA

Ph.D. Supervisor:

Prof. univ. dr. POPESCU ALEXANDRU

Craiova

2017

2 Abstract

Abstract

The technological evolution and the continuous growth of the planet's population have

led to an increase in energy demand. The prospect of exhaustion of fossil fuel resources as

well as environmental concerns have prompted intensified research to identify new, non-

polluting and efficient energy sources. In this context, fuel cells are an alternative to

conventional technologies.

Fuel cells are devices that convert chemical energy stored in a gaseous fuel directly

into electrical energy. The fuel oxidation and oxygen reduction (oxygen from the air)

reactions take place at the fuel cell anode and cathode, respectively, the two electrodes being

separated by a dense, ionic conductive membrane. The yield of this process is far superior to

the direct combustion process of the same fuel. There are several types of fuel cells, and a

classification criterion for them is the electrolyte used.

Solid oxide fuel cells (SOFCs) are devices with all components in solid state,

operating at high temperatures (800 – 1000C). Due to the high operating temperatures, the

requirements imposed on the components are drastic and the manufacturing costs are high.

One of the research directions in the field of solid oxide fuel cells is to reduce their

operating temperature down to 500 – 800C. This objective can be achieved by identification

of new materials for cell components and development of new processing technologies.

The main objective of this thesis is the synthesis, characterization and electrochemical

testing of new materials for SOFC electrolyte.

The thesis is structured in five chapters, the first two consisting in a bibliographic

study of the current state of the research regarding the subject being addressed, as well as a

description of the synthesis methods and the characterization techniques of the ceramic

materials. An analysis of the literature in the field in recent years was the subject of an article

published in the Annals of the University of Craiova - Chemistry Series. Other three main

chapters were added, which have been the subject of four scientific papers published or sent

for publication at: Journal of Alloys and Compounds, Ceramics International, Journal of

Electroceramics, Journal of Solid State Chemistry. In addition, the results were

communicated to seven international scientific events.

In the first chapter the principle of SOFC operation, as well as the main classes of

materials used in manufacturing of their components: electrodes (anode, cathode) and

electrolyte are presented.

3 Abstract

Chapter II presents the synthesis methods frequently involved in the production of

solid oxides for solid oxide fuel cells, as well as a brief description of the techniques used for

the characterization and testing of the synthesized materials.

In Chapter III, emphasis was placed on CeO2-based materials with fluorite structure,

highlighting the effect of rare earth (Dy or Yb) doping and strontium co-doping on the

structure, morphology and electrical conductivity of cerium oxide.

Chapter IV presents electrolyte materials with pyrochlore crystalline structure; their

doping possibilities are superior to the fluorites, the first one presenting two types of cationic

centers. Thus, the effect of the cation from the center A and the effect of the dopants on both

types of centers on the structure, morphology and electrical conductivity of rare earth

titanates was investigated.

In Chapter V the synthesis, the physico-chemical characterization and the testing of

materials based on lanthanum titanate with crystalline cuspidine structure are presented.

In the following, the scientific content of each chapter will be presented.

Chapter I: MATERIALS FOR SOLID OXIDE FUEL CELLS

Chapter I presents the principle of SOFC operation, as well as the main classes of

materials used in manufacturing of their components: electrodes (anode, cathode) and

electrolyte.

Fuel cells are devices that can produce energy conversion with greater efficiency

compared to the conventional technologies, such as coal-fired power plants and internal

combustion engine power generators. They allow the use of a wide range of fuels: hydrogen,

CO, hydrocarbons, they do not produce noise or vibration, have no moving parts and they

have low SOx and NOx emissions.

Solid oxide fuel cells (SOFCs) consist in a dense electrolyte layer sandwiched

between two porous electrodes (anode and cathode), all components being solid (Scheme 1).

The oxygen molecules supplied to the cathode react with the electrons coming from the

external circuit to form the oxygen ions, which migrate through the electrolyte to the anode.

At the anode, the oxygen ions react with H2 or another fuel to form H2O (and/or CO2),

releasing electrons, which pass through the external circuit to the cathode, producing

electricity. Electrochemical reactions generate electricity as long as oxygen and fuel are fed

to the electrodes.

4 Abstract

Scheme 1. Schematic presentation of the processes taking place in a fuel cell.

The high operating temperatures of SOFCs can lead to multiple problems of the

materials used for their components, including electrostatic sintering, catalyst contamination,

diffusion at the interface between electrolyte and electrode materials, thermal instability and

mechanical or thermal stresses caused by differences in thermal expansion coefficients

(TECs) of fuel cell components.

The high operating temperatures of SOFCs exert numerous constraints in the material

selection process, appropriate to each SOFC component. As a result, for the selection of

SOFC materials must be taken into account their stability, the microstructural changes which

may occur during operation and the decrease of the operating temperature in the range of 500

– 800oC.

The SOFC anode must be a good catalyst for fuel oxidation, must be stable in a

reducing environment, good electrical conductor, and must be sufficiently porous to allow

access of the gaseous species to the electrolyte / anode interface where the fuel oxidation

reactions occur. Other requirements include: thermal expansion coefficient compatible with

electrolyte materials and interconnectors, chemical stability with electrolyte and

interconnectors, and applicability in the use of a wide range of fuels and impurities (such as

sulfur).

Nowadays, Ni-YSZ cermet is the most commonly used material for the anode

manufacture. The aim to develop new anode materials has led to the research and

development of various types of materials such as perovskite type anode materials, ceramic

composites, titanate based or doped titanate materials, spinel materials, pyrochlores, tungsten

bronzes and fluorites.

5 Abstract

The SOFCs cathode must have high electrical conductivity, chemical and dimensional

stability during the manufacture of the cell and during its operation, a thermal expansion

similar to other cell components, compatibility and minimal reactivity with the electrolyte

and the interconnectors and a high catalytic activity for the dissociation of oxygen molecules

and their reduction.

For the SOFC cathode, besides the noble metals, the perovskite oxide conductors are

used: LaMnO3, LSM, LSCF, BSCF, etc.

In a solid oxide fuel cell, the electrolyte must exhibit high ionic conductivity, dense

structure and chemical stability. Also, the thermal expansion coefficients corresponding to

the electrodes and the electrolyte must be compatible.

In addition to YSZ, which is the most commonly used material for electrolyte,

research in the field has led to the development of new materials such as: CeO2 and Bi2O3

based oxides with fluorite structure, LaGaO3 based perovskites, La2Mo2O9 and Bi2V2O11

based derivatives, Ba2In2O5 perovskite, pyrochlores and CeO2 doped materials.

Chapter II: SYNTHESIS AND CHARACTERIZATION OF THE CERAMIC

MATERIALS

Chapter II presents the synthesis methods frequently involved in the production of

solid oxides for SOFCs, as well as a brief description of the techniques used for the

characterization and testing of synthesized materials.

Among the methods of chemical synthesis, the sol-gel method is of special interest

due to its simplicity and versatility. This method involves the preparation of a mixture in the

solution, thus ensuring homogeneity at molecular level; the composition of the product can be

easily adapted by varying the ratio of precursor amount in solution. Due to its advantages,

this synthesis method is increasingly used for the manufacture of materials involved in the

fabrication of various components of solid oxide fuel cells, such as electrolyte and cathode

materials.

In general, the characterization of a solid material involves two main processes:

Structural analysis

Analysis of properties

Structural analysis is carried out using a variety of microscopic and spectroscopic

techniques, while the characterization of properties is quite varied and depends on individual

6 Abstract

applications. Various methods of characterization have been used over the years to

investigate the structure and properties of solid materials.

The characterization techniques used to investigate the materials studied in this Ph.D.

thesis have been briefly described.

As characterization techniques were used:

X-Ray Diffraction (XRD);

Scanning Electron Microscopy with Energy-dispersive X-ray spectroscopy

(SEM/EDXS);

Raman Spectroscopy;

Thermodilatometric analysis (TDA);

Thermogravimetric analysis (TGA);

X-ray photoelectron spectroscopy (XPS);

Electrochemical Impedance Spectroscopy (EIS).

Chapter III: SOLID OXIDES WITH FLUORITE STRUCTURE

Chapter III presents CeO2-based materials with fluorite structure, highlighting the

effect of rare earth (Dy or Yb) doping and strontium co-doping on the structure, morphology

and electrical conductivity of cerium oxide.

Rare earth doped ceria materials (especially Gd or Sm doped ceria) proved to possess

high oxide ion conductivity in the intermediate temperature range (500-800°C), being an

interesting candidate for SOFC electrolyte.

Ceria doped with 15% mol Dy (Ce0.85Dy0.15O2-δ) or 15% mol Yb (Ce0.85Yb0.15O2-δ), as

well as powders with the composition Ce0.85Dy0.15-xSrxO2-δ (x=0.05, 0.075) and Ce0.85Yb0.15-

xSrxO2-δ (x=0.05, 0.075) were synthesized using a Pechini method.

For the electrochemical characterization, the calcinated powders were shaped into

pellets by uniaxially pressing in a 10 mm diameter stainless steel die. Thus obtained pellets

were sintered at 1300oC for10 h.

In the XRD patterns of the calcined powders and sintered pellets only the

characteristic peaks of fluorite structure (space group Fd-3m) were evidenced (Figure 1), the

evolution of the lattice parameter with average ionic radius of dopants being linear (Figure 2).

7 Abstract

20 40 60 80 100 120

x=0.075

x=0.05

Inte

nsi

ty (

a.u

.)

2

x=0

0.96 1.00 1.04 1.08 1.12 1.16

5.405

5.410

5.415

5.420

5.425

5.430Ce

0.85Dy

0.075Sr

0.075O

2-

Ce0.85

Yb0.075

Sr0.075

O2-

Ce0.85

Dy0.10

Sr0.05

O2-

Ce0.85

Yb0.10

Sr0.05

O2-

Ce0.85

Dy0.15

O2-

Lat

tice

par

amet

er (

An

gst

rom

)

Average dopant ionic radius (Angstrom)

Ce0.85

Yb0.15

O2-

R=0.98

Figure 1. XRD patterns of

Ce0.85Dy0.15-xSrxO2- calcined powders.

Figure 2. Evolution of lattice parameter with

dopant ionic radius for the investigated

samples.

The morphology of the sintered samples was evidenced using scanning electron

microscopy (Figure 3) and the distribution maps of the dopants were developed using EDS.

For the samples doped with Dy and Yb, the pellets have grains with submicrometric

dimensions, with a relatively uniform distribution of the dopant. From a morphological point

of view, the addition of strontium leads to an increase in the grain size of the sintered pellets

from the nanometer range to micrometer range. In addition, increasing the Sr content leads to

an increase in grain sizes.

The ionic conductivity in the air was measured using electrochemical impedance

spectroscopy. The impedance measurements were performed with a Zhaner IM6eX analyzer

in the 3 MHz – 0.10 Hz frequency range. Gold was used as electrode and the amplitude of the

AC perturbation signal was 50 mV. Measurements were carried out in the temperature range

200 – 800°C.

At temperatures between 200 – 500°C, only one arc was evidenced in the Nyquist plot

of EIS spectra, excepting the electrode response. At temperatures exceeding 500°C, only the

contribution of electrode was observed in Nyquist plots.

Doping with Dy provides better ionic conductivity than Yb. Partial substitution of Dy

or Yb with Sr leads to a significant increase in ionic conductivity only in the Ce0.85Yb0.15-

xSrxO2- system (Figure 3). The sample with the composition Ce0.85Yb0.10Sr0.05O2- showed the

highest ionic conductivity (8.6x10-4 S/cm at 500oC) and the lowest activation energy of

conduction (0.71 eV) among all investigated samples.

8 Abstract

0.99 1.02 1.05 1.08 1.11 1.14

0.0

3.0x10-4

6.0x10-4

9.0x10-4

1.2x10-3

1.5x10-3

=0.1125

=0.1

t (

-1 c

m-1

)

Average dopant radius (Angstrom)

400oC

500oC

600oC

=0.075

Figure 3. Variation of total ionic conductivity for doped ceria systems with average

ionic radius of the dopants and SEM images of the sintered pellets.

Chapter IV: SOLID OXIDES WITH PYROCHLORE STRUCTURE

Chapter IV presents electrolyte materials with pyrochlore crystalline structure, whose

doping possibilities are superior to the fluorites, the first one presenting two types of cationic

centers. Thus, the effect of the cation from the center A and the effect of the dopants on both

types of centers on the structure, morphology and electrical conductivity of rare earth

titanates was investigated.

Rare-earth titanate pyrochlores have been investigated as potential materials for solid

oxide fuel cells (SOFCs).

M2Ti2O7 (where M = Y, Gd, Sm) powders were synthesized by a Pechini method

using Y(NO3)36H2O (Alfa Aesar, ≥ 99,9%), Gd(NO3)36H2O (Sigma-Aldrich, ≥ 99,9%),

Sm(NO3)36H2O (Sigma-Aldrich, ≥ 99,9%), and titanium (IV) isopropoxide (Sigma-Aldrich,

≥97%) as precursors. After excess solvent removal, the obtained gels were dried at 120oC

overnight. TG/DSC analysis of the resulted precursor resins was performed in order to

establish the optimal thermal treatment which can lead to solid oxides. According to TG/DSC

results, the precursor resins of Gd2Ti2O7 and Y2Ti2O7 were calcined in air at 850oC. Higher

temperature was necessary to get Sm2Ti2O7 single phase pyrochlore.

9 Abstract

The XRD patterns of the investigated powders with the composition Gd2Ti2O7 and

Y2Ti2O7, obtained after calcination at 850C, exhibited only the characteristic reflections of

pyrochlore structure. Sm2Ti2O7 powder calcined at 850C does not have a monophasic

crystalline structure. Higher temperature was necessary in order to obtain a pure pyrochlore

phase (1200oC) (Figure 4).

10 20 30 40 50 60 70 80

2

Gd2Ti

2O

7

Y2Ti

2O

7

Inte

nsi

ty (

u.a

.)

Sm2Ti

2O

7 850

oC

Sm2Ti

2O

7 1200

oC

Figure 4. XRD patterns of the calcined powders.

High-temperature X-ray diffraction (HT – XRD) in air and H2 (2%) – Ar mixture was

also performed on calcined powders. Diffraction data were collected at RT and in the

temperature range 50 – 800oC during heating/cooling cycles with a 50oC step and a

heating/cooling rate of 1oC /min. Before XRD data collection, the sample was kept at the

temperature of interest for 5 min.

HT-XRD measurements under oxidizing and highly reducing atmospheres on

M2Ti2O7 pyrochlores (M = Gd, Y, Sm) evidenced the decomposition of all investigated

samples into the corresponding single oxides under hydrogen containing atmosphere.

For the electrochemical characterization, the powders were shaped into pellets by

uniaxial pressing in a 10 mm diameter stainless steel die and further isostatically pressed

under 180 MPa to obtain green pellets. These pellets were sintered at 1200oC for 24 h using a

10 Abstract

heating rate of 2oC /min. Relative densities of all sintered pellets were estimated to be higher

than 90% of their theoretical density. Gold was used as electrode. The AC impedance

spectroscopy measurements were performed on sintered pellets in air and Ar/H2 (2 vol%)

mixture, in the temperature range 300 – 800oC, using a SI 1266 Solartron Impedance/Gain-

Phase Analyzer in the 30 MHz–0.10 Hz frequency range. The amplitude of the AC

perturbation signal was 50 mV.

Sm2Ti2O7 has the highest conductivity in the investigated temperature range and the

lowest activation energy of conduction under air, while Gd2Ti2O7 possess the lowest

conductivity and the highest activation energy.

Under highly reducing atmosphere, a strong increase in bulk conductivity (four orders

of magnitude) can be noticed for Gd2Ti2O7, while in the case of Y2Ti2O7, an increase in bulk

conductivity with three orders of magnitude after exposure to reducing atmosphere can be

observed (Figure 5).

1,02 1,04 1,06 1,08-12

-10

-8

-6

-4

-2 500oC

600oC

700oC

ln(

volT

/cm

-1K

)

rM

(Angstrom)

(a)

1,02 1,04 1,06 1,08-4

-3

-2

-1

0

1

(b)

ln(

volT

/cm

-1K

)

rM

(Angstrom)

500oC

600oC

700oC

Figure 5. Isotherms of bulk conductivities for samples investigated in:

(a) air and (b) 2%vol H2 in Ar.

The doping of RE and / or Ti sites may determine an increase in ionic conductivity of

the oxide.

Powders with composition Gd2-xSrxTi2O7- (x=0, 0.1, 0.2, 0.3, 0.4) were synthesized.

Oxide powders were investigated by X-ray diffraction. In the case of a low degree of

substitution (x=0.1, 0.2) X-ray patterns have revealed only the peaks characteristic to the

pyrochlore structure. For higher dopant content (x = 0.3 and 0.4), the characteristic peak of

SrTiO3 is observed.

11 Abstract

Pyrochlore materials with the composition: Gd2Ti2O7; Gd1,9Sr0,1Ti2O7± (GS0,1T);

Gd1,8Sr0,2Ti2O7± (GS0,2T), were investigated by dilatometry, XRD and EIS.

The XRD patterns of the pellets sintered at 1200°C for 12h indicated the presence of a

single phase - the pyrochlore type - the position of the peaks being shifted according to lattice

parameters variation determined by the ionic radius of the dopant.

EIS measurements were carried out in the 30 MHz - 0,1 Hz frequency range, in open

circuit, using an amplitude of the AC perturbation signal of 50 mV; measurements were

performed in air and in reducing atmosphere (Ar – H2 2 % vol mixture).

GS0,1T sample presents the highest total electrical conductivity value in the air over

the entire investigated temperature range; doubling the dopant content (Sr) leads to a drastic

reduction in conductivity. However, in reducing atmosphere, Gd2Ti2O7 exhibits the highest

values of the total electrical conductivity (Figure 6).

0.8 1.0 1.2 1.4 1.6 1.8-12

-10

-8

-6

-4

-2

0

Gd2Ti

2O

7

GS0.1T

GS0.2T

ln (

T/

cm

-1)

1000/T (K-1)

(a)

0.8 1.0 1.2 1.4 1.6 1.8 2.0

-12

-9

-6

-3

0

3

Gd2Ti

2O

7

GS0.1T

GS0.2T

ln (

T/

cm

-1)

1000/T (K-1)

(b)

Figure 6. Arrhenius plots of total electrical conductivity for the measurements under air (a)

and reducing atmosphere (b).

The highest increase in total electrical conductivity when passing from oxidizing

atmosphere (air) to reducing atmosphere (the Ar – 2% H2 mixture) was obtained for

Gd2Ti2O7 sample, followed closely by the sample GS0,2T. Instead, the GS0.1T sample

exhibits a high degree of stability in the electrical conduction behavior.

The powders with Sm2Ti2-xAlxO7- (x=0, 0.10, 0.20) stoichiometry were synthesized

using a modified Pechini method.

X-ray diffraction of solid oxide powders obtained after calcination of the samples at

900oC revealed that the powders were crystalline and the diffraction peaks were indexed to

several crystalline phases, mainly Sm2Ti2O7 and Sm2TiO5.

12 Abstract

Raman spectra were recorded at room temperature using an inVia confocal Raman

microscope (Renishaw) with a diode DPSS visible laser source (532 nm) and a Peltier cooled

CCD detector. The single beam power of the laser was 25mW and the 50x objective of the

microscope was used.

Raman spectroscopy also revealed the presence of orthorhombic and cubic pyrochlore

phases.

The SEM images of powder calcined at 900oC evidenced an increase in the amount of

aluminum in the sample leads to an increase in the size of the aggregate.

Conventional sintering experiments were performed on 5 mm diameter and 4 – 7 mm

thick pellets, obtained by uniaxial pressing of the calcined powders. The measurements were

performed in static air with a 10°C/min heating rate using a L75 PT Linseis horizontal

dilatometer

The undoped sample exhibited the highest shrinkage rate and the largest linear

shrinkage (about 20% at 1450oC). The partial substitution of titanium with aluminum leads to

the decrease in linear shrinkage (about 10% at 1450oC) and shrinkage rate (0.47%/min for

Sm2Ti1.9Al0.1O7- at 1250°C and 0.42%/min for Sm2Ti1.8Al0.2O7- at 1200°C).

XRD patterns of samples with composition Sm2Ti2O7 and Sm2Ti1.9Al0.1O7- exhibited

only the characteristic peaks of cubic pyrochlore crystalline structure after sintering at

1300°C for 5 h. For Sm2Ti1.8Al0.2O7- sample, the XRD pattern also evidenced reflections

characteristic to Al2TiO5 secondary phase. However, the Raman spectra of the sintered pellets

exhibited only the characteristic peaks of the pyrochlore structure.

SEM analysis evidenced the retardation effect of aluminum doping on grain growth.

The gain sizes in undoped sintered sample are in the micrometer range, while for sample with

5 mol % Al the grains are in the nanometer range. However, the increase in Al content leads

to crack formation.

Chapter V: SOLID OXIDES WITH CUSPIDINE STRUCTURE

Chapter V presents the synthesis, physico-chemical characterization and testing of

materials based on lanthanum titanate with cuspidine crystalline structure.

Development of Solid Oxide Fuel Cells (SOFCs) depends, among others, on the

identification of new oxide ion conductors with high ionic conductivity in the intermediate

temperature range (500 – 700oC). In this context, a new class of oxide ion conductors with

13 Abstract

cuspidine structure with general formula M4(Si2O7)X2 (M – cation bivalent and X = OH, F,

O) recently emerged.

In this chapter, the possibility of aliovalent doping of lanthanum and titanium sites

was investigated, with the aim to enhance the conductivity of lanthanum titanate.

Powders with composition La4-xCaxTi2O10 (x=0, 0.1, 0.2) were synthesized using a

Pechini method. La(NO3)36H2O, Ca(NO3)24H2O and Ti(OPr)4 were used as precursors.

Solid resins obtained after removal of excess solvent were then calcined in air at 900oC for 5

h. A small quantity from the synthesized powders were heat treated in 2% H2/Ar atmosphere

at 900oC for 1 h. Powders calcined in air were further processed into pellets by uniaxial

pressing in a 10 mm diameter stainless steel die. The as-obtained pellets were sintered at

1200oC for 12 h.

XRD patterns reveal the formation of single-phase solid solutions for all investigated

samples. All characteristic peaks could be indexed based on orthorhombic cuspidine structure.

Pattern matching performed on XRD data led to the lattice parameters of the investigated

crystalline samples. Changes in lattice sizes with dopant concentration were evidenced. The unit

cell volume decreases with the increase in dopant concentration. The sintered undoped pellet

exhibit a higher unit cell volume compared to powders, while for the doped sample lower unit

cell volumes were obtained, indicating a better dissolution of dopant cation into the host lattice.

Raman spectra were recorded at room temperature using an inVia confocal Raman

microscope (Renishaw) with a diode DPSS visible laser source (532 nm) and a Peltier cooled

CCD detector. The single beam power of the laser was 25mW and the 50x objective of the

microscope was used.

An increase in the intensity of all Raman active modes with the increase in dopant

concentration was evidenced for the samples thermally treated in air. When treated under

reducing atmosphere, the intensity of Raman active modes decrease with increase in dopant

concentration. An increase in Raman active modes was evidenced for sintered pellets

compared to the corresponding calcined powders.

The sintered pellets exhibit relative densities higher than 90%. Their microstructure

was evidenced by HR – SEM (Figure 7). The undoped sample exhibits a broad range of grain

size distribution. The addition of calcium ions led to an increase in grain size. The grain size

distribution became narrower with the addition of calcium and with the increase in dopant

content.

14 Abstract

Figure 7. SEM micrographs of sintered pellets: (a) La4(Ti2O8)O2; (b) La3.9Ca0.1Ti2O10-δ;

(c) La3.8Ca0.2Ti2O10-δ.

Electrochemical impedance spectroscopy (EIS) was used for the evaluation of

electrical conductivity of polycrystalline solid oxides.

Doping with 2.5% and 5% mol Ca on La site led to an increase in electrical

conductivity (Figure 8). In addition, the conductivity increased with the increase in the

calcium content, with a simultaneous decrease in the activation energy of the electric

conduction. Exposure to reducing atmospheres caused a strong increase in the conductivity of

the doped samples, accompanied by an increase in activation energy.

0.90 0.95 1.00 1.05 1.10 1.15 1.20-14

-12

-10

-8

-6 x=0

x=0.1

x=0.2

Ea=1.45 eV

Ea=1.11 eV

Ln

(T

/-1

cm-1

K)

1000/T (K-1)

Ea=1.26 eV

Figure 8. Arrhenius plots of ionic conductivity for La4-xCaxTi2O10-δ under air.

In order to study the doping potential of the Ti site from La4Ti2O10, cations with ionic

radii close to that of the host cation (Ti4+) were selected. The synthesis method used was also

the Pechini method. La(NO3)36H2O, Ti(OPr)4, (NH4)6Mo7O246H2O, Ga(NO3)3H2O,

Al(NO3)39H2O and Fe(NO3)39H2O were used as precursors. Solid resins obtained after

removal of excess solvent were then calcined in air at 400C, in a tube furnace, to remove the

organic components. X-ray diffraction analysis revealed the predominantly amorphous nature

of the powders thus obtained.

(a) (b) (c)

15 Abstract

Powders were processed into pellets by uniaxial pressing, followed by sintering at

1200C for 12 h. The phases present in the heat-treated pellets were identified by assigning

the peaks from the X-ray diffractograms. The doping attempts of the titanium site led to pure

orthorhombic crystalline phase of interest only in the case of gallium. When Mo, Fe and Al

were used as dopants, a biphasic mixture was initially obtained.

The increase of the temperature and the time of the thermal treatment (1400C, 48 h)

determined the diminishing of the secondary phase quantity.

General conclusions

The PhD thesis approaches a complex and actual topic, aiming to improve the

performance of SOFCs by developing new materials for their electrolyte. The studies

conducted led to the following conclusions:

Have been synthesized, characterized and tested solid oxides with the following

crystalline structures: cubic (fluorite and pyroclor type) and orthorhombic (cuspidine

type).

The method used for the synthesis of solid oxides was the Pechini method, modified in

some cases to ensure that cations are maintained in solution.

By the addition of aliovalent cations it was intended to increase the electrical conductivity

of these materials and to improve their sinterisablity.

CeO2 doping with rare earth cations increases its conductivity. Doping with Dy provides

better ionic conductivity than Yb.

Partial substitution of Dy or Yb with Sr leads to a significant increase in ionic

conductivity only in Ce0.85Yb0.15-xSrxO2- system.

The sample with the composition Ce0.85Yb0.10Sr0.05O2- showed the highest ionic

conductivity (8.6x10-4 S/cm la 500oC) and the lowest activation energy of the conduction

(0.71 eV) of all investigated samples with fluorite structure.

The synthesis process of rare earth titanates with pyrochlore crystalline structure was adapted

by adjusting the pH to 5 to avoid precipitation of the precursor cations.

The HT-XRD measurements carried out in the oxidizing and reducing atmosphere on the

pyrochlors with the general formula M2Ti2O7 (M = Gd, Y, Sm) revealed the

decomposition of all investigated samples in the corresponding simple oxides in the

presence of hydrogen.

Sm2Ti2O7 sample has been shown to have the best stability in reducing atmosphere.

16 Abstract

The variation of the conductivity of M2Ti2O7 pyrochlors in air with M3+ cations was

correlated with their structural disorder.

The doping of Gd2Ti2O7 with 5% mol Sr leads to an increase in conductivity in air, also

causing the behavior stabilization at different oxygen partial pressure. Doubling the Sr

content causes a decrease in electrical conductivity in the air.

Increasing the Sr content over 10% mol determines the appearance of the secondary

phase (SrTiO3).

By doping the Ti site in Sm2Ti2O7 with Al, the sintering temperature decreased, while the

nanometric dimensions of the granules in the sintered samples was maintained.

La4Ti2O10 cuspidine orthorhombic crystalline structure offers opportunities for doping on

La and Ti sites.

Doping the La site with 2.5% and 5% mol Ca has determined an increase in electrical

conductivity. In addition, the conductivity increased as the calcium content increased,

simultaneously recording a decrease in activation energy of the electrical conduction.

Exposure to hydrogen reducing atmospheres determined a strong increase in the

conductivity of the doped samples, accompanied by an increase in activation energy.

The doping attempts of the titanium site led to pure orthorhombic crystalline phase of

interest only in the case of gallium. When Mo, Fe and Al were used as dopants, a biphasic

mixture was initially obtained.

The increase of the temperature and the duration of the thermal treatment determined the

diminishing of the secondary phase quantity.

Experimental results have highlighted numerous possibilities for doping, especially for

solid oxides with pyrochlore and cuspidine crystalline structures.

Future studies will focus on the synthesis, characterization and testing of new crystalline

cuspidine structure materials, highlighting the effect of dopants on structural stability,

sinterability and electrical properties. New electrochemical testing techniques will be used to

determine the contribution of electrons and ions to total electrical conductivity.