special ceremic materials

7/28/2019 Special Ceremic Materials

1/13

1st Meeting of Romanian Hydrogen and Fuel Cell Technology PlatformCALIMANESTI-CACIULATA, VALCEA, OCTOBER 13, 2005

Paper 18/1

SPECIAL CERAMIC MATERIALS FOR SOFC-IT TYPE FUEL CELLS

Mircea GHITULESCU, Georgeta VELCIU, Lelian CIOROIANU,

Gabriela CIOROIANU, Gela PROROCU

ENERGETIC RESEARCH AND MODERNIZING INSTITUTE ICEMENERG S.A.

Energeticienilor Bvd., no. 8, district 3, Bucharest, phone: +4021 3462772, fax 4021 3465310

e-mail: [email protected]

AbstractFuel cells are, in the time being, the cleanest electrical energy production technology. Solid electrolyte

fuel cells suffered, lately, an intensive development. The actual tendency is to produce solid electrolyte fuel cells,

which function at intermediate temperatures (650-8500C) SOFC-IT.

The present paper shows the stage and functionality of a SOFC-IT type fuel cell. Types of ceramic

materials and the best compositions used in construction of fuel cell elements are also shown, as well as the

corresponding obtaining technologies.

1. INTRODUCTION

The European Union policy and the Kyoto Protocol impose important demands to the

energetic systems: reduction of the pollutant emissions and the rational use of energetic

resources.

Reduction of pollutant emissions is an essential condition, in regards to the fact that

over half of the noxes emitted in the environment are the result on electrical and thermal

energy production processes in classical thermal plants (SO2, NO2, CO2, slag, ash and thermal

pollution). [1]

The energetic resources are very important, being influenced by the uncertainties

accounted in insuring the supply, by the exploitation and conservation of superior fuels

resources for their use as raw materials or fuels for transport.

In present, there are researched alternative routes for electrical energy production

through using some maximum yield technologies, increased reliability, and minimum

pollution. From this point of view, the fuel cells are considered the most clean technologies

for obtaining of electrical energy.

For the period 2000-2030, studies done in the European Union show an increase of the

global energy needs with 33% and a double of the CO2 emissions 9from 6.3 to 13 mil. tones).

[1] There are known numerous possibilities of saving fuels and energy, as well as of using

some clean energy sources, which allow the accomplishment of the Kyoto objectives in

regards to reducing the gas emissions.

In the collaboration in the European Research Area frame, the efficient coordination

and support of European Union organizations are the key factors for accelerating the activities

in the fuel cells field until the commercial stage.


2/13


Paper 18/2

2. ACTUAL STAGE OF DEVELOPMENT

IN THE SOFC-IT TYPE FUEL CELLS FIELD

Solid electrolyte fuel cells (SOFC) represent a potential alternative to energy

producing conventional technologies, offering the possibility of combined production of

electrical energy and heat with very high efficiency.

SOFC are energetic installations which produce energy through the direct conversion

of gas fuels, natural gas, coal gas, ethanol, methanol etc.), reducing the pollutant emissions of

NOx, SOx and gases with green house effect, CO2.

In the last period, the SOFC technology has encountered major progresses in regards

to the power density, as well as the life time.

Solving some technical, economical and ecological problems, allowed the fast

development of solid electrolyte fuel cells.

Fuel cells can be used in stationary decentralized power plants as energetic generators

having the possibility of cogeneration, capitalizing the heat from exhaust worm gases.

In compared to other types of fuel cells (MAFC, PAFC), the solid electrolyte fuel cells

(SOFC) have the following advantages:

clean conversion technology (the level of pollutant emissions NOx, SOx, CO2, under 10mg/m

3consumed gas)

net electrical yield of 55-60% low phonic pollution level (without noise, vibrations) different areas of applications (industry, domestic)

In present, there are in development three principal configurations (monolithic, tubular,

and planar). There are differences in opinions on the configuration to be adopted in order to

obtain the most advantageous solutions.

The planar type SOFC is still in the laboratory stage, while the tubular construction is

produced and tested.

Companies like Westinghouse E.C. Arfgone National Laboratories-USA, Dornier

GmbH-Germany have studied and developed the SOFC tubular configuration.

Westinghouse Electric Corp, Pittsburgh PA (USA) designed some pilot stations with

an over 40,000 hours functioning. The company has distributed two demonstrative units of 25

kW for two Japanese public services units and a Californian public service similar one.

The RISO research center, in Denmark, has the most recent results in the planar SOFC

field, succeeding in constructing and testing a 70 cells module, each with a 50 cm2

surface.

The maximum obtained power was 507 W for a functioning temperature of 10000C.

Promoting SOFC with intermediate temperature functioning (IT-SOFC)

The new generations of fuel cells have solid electrolyte and function at intermediate

temperatures, below 8000C (SOFC-IT).

The investment and exploitation costs for solid electrolyte fuel cells remain increased


3/13


Paper 18/3

because of the high temperature of the functioning (about 10000C). There are researched

superalloys and ceramic materials with electroconductive properties for these properties.

Also, the high functioning temperature leads to gas leaking, cracks and other functioning and

maintenance problems for the SOFC assembly. This is the reason why the elimination of

these setbacks is pursued through the reduction of the functioning temperature.

The optimum functioning temperature is between 6500C and 800

0C, a sufficient

temperature for fuel internal reforming, reducing the investment, functioning and maintenance

costs.

At the temperature 8000C or lower, there can be used classical stainless steel instead of

special materials used at high temperature and also, the lower temperature allows the increase

of the life time of the fuel cells.

The quantity of electricity produced and the temperature for which the efficiency is

maximum highly depend on the electrolyte potential to allow conduction through oxygen

ions.

In conventional SOFCs, the inherent problem is the functioning of yttrium stabilized

zirconia (YSZ) at the temperature of 10000C. At lower temperatures, the electrical resistance

of YSZ increases, lowering the performances at an economical level. To overcome these

difficulties, a new alternative of electrode or a new fuel cell model is necessary.

Different institutions and companies (Royal Institute of Technology, EPRI, Sulzer

HEXIS Ltd. etc.) directed the research to lower the functioning temperature of SOFC. An

approach of the study of some very thin electrolytes or the use of some alternative electrolytematerials.

The decrease of the electrolyte thickness is the most direct approach of to maintain the

SOFC performances at lower temperature. In this case, the oxygen ions have traveled a

smaller distance through the electrolyte, the total resistance of the cell being able to be

decreased, even if the resistance per electrolyte unit increases due to the decreasing of the

temperature. This approach allows the use of YSZ as electrolyte.

To maintain the performances, the thickness of the electrolyte must be smaller than 10

m. the obtianing of this thickness is difficult to accomplish because of the cracks that can

appear, not wanted in the electrolyte structure. To obtain thin layers of YSZ, chemical

methods were applied, as the layers in colloidal suspension technique, obtaining layers of 4-

10 m thickness.

In laboratory tests of a solid electrolyte fuel cell, in which the electrolyte is a thin

layer, with functioning at 700-8000C, there were obtained very good results.

The development of some ceramic oxides composite materials will allow the

intermediate temperature (


4/13


Paper 18/4

The increased performances of IT-SOFCs and the extremely high conductivity of

ceramic composite electrolytes imposes the development of the fuel cells field.

Through lowering the functioning temperature of the SOFC system, the fabrication

costs of components is expected to be reduced with 30-40%.

General functioning concept for solid electrolyte fuel cells SOFC-IT

A fuel cell is based on an electrochemical energy conversion process. The energy

conversion inside the fuel cell is clean and silent.

The solid electrolyte fuel cell, SOFC-IT, is comprised of two different compartments

for the fuel and oxidant continuous access. The compartments are separated by a component

of ceramic conductive material through oxygen ions that functions as electrolyte, which is

applied on each of the two component surfaces with electrode function: anode or fuel

electrode and cathode or oxygen electrode (fig. 1).

Figure 1. Ceramic solid electrolyte fuel cell structure

At the fuel cell function base is the possibility of production of some electrochemical

reactions at high temperatures. The process starts from the cathode electrode where a

reduction reaction of the oxygen form air takes place with the formation of oxygen ions,

followed by their diffusion through the electrolyte structure towards the anode and the fuel

oxidation reaction (H2 and/or CO). As a result, the cathode is positively charged and the

anode negatively, between the two electrodes setting an electrical tension, E1 (fig. 2). [2, 3, 4]

Current

direction

Interconnect

Anode

Electrolyte

Cathode

Air

Interconnect

Fuel

Repeating

cell unit


5/13


Paper 18/5

Figure 2. Solid electrolyte fuel cell functioning principle

3. COMPOSITE MATERIALS AND TECHNOLOGIES

USED IN THE SOFC-IT OBTAINING

Fuel cell components

A fuel cell has three component parts: solid electrolyte, electrodes (cathode and

anode), and interconnect.

From the electrolyte configuration point of view, the solid electrolyte fuel cells are:

tubular (fig.3) and planar (fig.4).

Figure 3. Tubular SOFC fuel cell

Ni-ZrO2 - Anode

YSZ - electrolyte

LaMnO3 - Cathode

Fuel: CO, H2Combustion

Products: CO2, H2O

Usablepower

Anode

Interconnector

Electrolyte

Cathode

Air


6/13


Paper 18/6

Figure 4. Planar SOFC fuel cell

Materials and compositions used for their obtianing

In the low temperature solid electrolyte fuel cells domain, the materials used for

obtaining the cell components play an important role.

Electrolyte

The solid electrolyte must posses a very good ionic conductivity and, as much as

possible, the electronic conductivity to be missing.

The temperature dependence of the electric conductivity of a ceramic material

constitutes the criteria on which their use as electrolytes is based upon.

In a ceramic oxidic compound, MO, comprising cations M2+

and anions O2-

, the

electrical charge carriers are represented by the ionic and electronic crystalline lattice defects

formed at structural level. The defects quantity at the crystalline lattice level is created by

substitution processes of the cation M2+

with another cation having a close atomic radius and

a different valance, in general, smaller than of the replaced cation. Under the influence of

temperature, the possibility of migration of defects in the material structure determines the

electrical conductivity of the electrolyte material.

The obtaining of a ceramic material requires a certain technology to obtain the

structure necessary for the electrolyte function. Therefore, because must be defined

temperature and oxygen partial pressure intervals specific for the working media in which the

electrolyte can function.

For the high temperature cell electrolyte, the use of stabilized zirconia ceramic

materials (electric conductivity in the order 2.5 10-2

S/cm at 10000C) is recommended and

for low temperature cell electrolyte, 700-8000C, there are recommended materials included in

a greater number of ceramic oxides groups as:

cerium based materials in binary systems with alkaline metals (CaO, MgO, BaO) or rareearth oxides (La2O3, Y2O3, Gd2O3, Sm2O3, Nd2O3)

Cathode

Electrolyte

Anode

Fuel

Interconnector

Air

Interconnector


7/13


Paper 18/7

perovskite-type materials (lanthanum oxide based materials in bianry systems with CaO,BaO, SrO oxides, strontium or barium cerates or with rare earth doping etc.)

composite type materials (bismuth oxide based materials with different rare earth dopants:Y, Er, Nd)

Cerium oxide based oxidic compounds group represented an interest as electrolytesbecause of the fluorite type structure resembling stabilized zirconia, having oxygen ions with

a high degree of mobility in network and with the possibility to increase the structural defects

through doping processes.

The ceria oxide is instable in the higher temperature area of 10000C due to the cerium

ion valance state change from Ce4+

to Ce3+

, process that leads to the increase of stability,

especially in the presence of reducing atmosphere.

The CeO2 forming process in solid solution systems through rare earth oxides doping

(La2O3, Y2O3, Gd2O3) shown the possibility to increase the structural stability and obtaining

of some good quality electrolytes.

The conductivity studies on cerium oxide binary systems materials with 15 mol %

La2O3 have shown a behaviour of electric conductors comparable with the one exhibited by

the zirconia stabilized with CaO and values even greater in the lower temperature domain

(under 8000C).

A special electrolyte behaviour was still observed at cerium oxide compositions doped

with gadolinium oxide (4 and 8 mol %). Conductivity studies done through impedance

spectroscopy have shown conductivity values of the order 10

-2

10

-1

S/cm at 800

0

C, samevalues being presented for stabilized zirconia at 1000

0C. It was observed, also, an important

influence of the working atmosphere for the composition with 8 mol %, the ionic conduction

domain being dependent on the decrease of oxygen partial pressure and increase of

temperature, an electronic contribution on the conduction process in the air atmosphere being

expected only above 10000C, exemplified in figure 5. [5]


8/13


Paper 18/8

Figure 5. Binary system CeO-Gd (8 mol % Gd2O3) electric conductivity

Neodymium oxide doping (8 and 16 mol %) at 7000C also showed a predominant

ionic conduction in the air atmosphere (figure 6).

Figure 6. Binary system CeO-NdO electric conductivity

The obtaining of some materials with cerium oxide in binary and ternary systems with

ZrO2 and Y2O3 was experimented and observed the fact that only compositions with

tetragonal monophase compositions have stable electrolyte properties under 10000

C for amaximum content of 12 mol % CeO2. In compositions with a high content of CeO2 more than


9/13


Paper 18/9

25 mol %, the electronic conductivity becomes predominant due to the chemical instability of

the cerium ion through valance change from 4+ to 3+, with the release of electrons.

Another conductor material group with electrolyte characteristics is comprised by thematerial group based on lanthanum oxide in binary systems with CaO, SrO and MgO type

oxides. The process of forming as solid solutions resembles the stabilized zirconia systems,

the La3+

ions being substituted by Ca2+

ions with the formation of anionic vacancies or

interstitial cations.

Some older experiments indicated an ionic conductivity for a doping with 15 mol %

CaO between 400-11000C and a conductivity of the order 10

-2S/cm at 900

0C and low oxygen

partial pressures. The importance of the ion radius that substitutes the lanthanum ion

determined the best qualities as ionic conductors in these binary systems. Therefore, for

temperatures between 500-8000C, doping with 1 mol % SrO or BaO, has lead to increased

conductivities of the order 10-2 10-3 S/cm at 8000C and, also, a majoritary ionic type

conductivity with similar values and low oxygen partial pressures, below 10-7

atm, only a

maximum 15 mol % content was obtained.

Another class of used materials to function as electrolyte are ceramic composite materialsbased on bismuth oxide.

This type of ceramic electrolyte is a composite having an exterior layer rich in zirconia

and yttrium with a minor quantity of bismuth oxideand another layer rich in bismuth oxide

and poorer in zirconia and yittria, presenting high electric conductivities at the operating

temperatures of 700-800

0

C.These can be used to function as eelctrolyte and the materials from the system Bi2O3-

Re (where Re is the rare earth oxide). Bi2O3 doped with Al2O3 (Bi2Al4O9) at 8000C has a

conductivity of the order 7 9 10-2

S/cm.

Materials with perovskite-type structure (AxByO3-) presents a mixed conductivity (ionicas well as protonic) where A ca be one of the elements Al, Zr, Nb, Bi and B can be Y, La, Nd,

Sm, Gd, and Yb. The perovskite-type structure of these materials determines their electrolyte

quality through the possibility of maintaining, in the crystalline lattice, a considerable quantity

of ions which can contribute to the formation of structural defects as charge carriers of anionic

nature (O2-

) as well as protonic (H+) or electronic (electron holes) nature. [6, 7]

Electrodes (cathodes, anodes)

Besides the electrolyte, SOFC cell performances are also influenced by the electrodes.

The electrodes actually represent the support and activators for the electrochemical reactions.

They insure also the charge carriers transport, totally for electrons and partially for ions.

Electrodes form, together with the electrolyte, a triple phase gas electronic solid

ionic solid boundary, at which level, the conduction mechanism is suddenly changed from

ionic to electronic (figure 7).


10/13


Paper 18/10

TPB

4e-O2

2O2-

YSZ

4e-

O2

2O2-

YSZ

O2

4e-

2O2-

Figure 7. Triple phase boundary formed at the solid electrolyte electrode interphase

The use of ceramic materials as SOFC-IT cell electrodes represented the most

advantageous solution currently accepted by these energetic systems constructors. These

oxidic compounds meant a material which, at the temperature of functioning of the cell (800-

10000C) have electrical conduction properties of the electronic preponderant type. As in the

case of electrolytic materials, the electric conductivity characteristic is generated at the

network structure of defects called electron holes.

The cathode materials are, generally, MO or MO2 oxides and ABO3 oxidic compounds

having a perovskite-type phase structure. The formation of defects takes place through a

doping process with cations having different valance, resulting a valance state transition for

the cation being replaced through doping. [8] For instance, in the NiO compounds case,

through Li doping, it takes place the valence transition from Ni2+ to Ni3+ with the liberation of

one electron. By increasing the temperature, these electrons receive a sufficient energy to pass

from the valance band in the conduction band. The electric conductivity characteristic of the

material in function of the corresponding temperature for a conduction mechanism through

electron holes is expressed by the following relation:

)/(exp kTEg

T

A =

where: A constant dependent on material structure and charge carrier concentrationEg activation energy

Pursuing researches for new types of electronic conductor oxidic compounds at high

temperatures has lead to the obtaining of perovskite material groups based on Ca, Sr, Zn

doped lanthanum chromites and manganites. For these compounds, the partial substitution

process of La with Ca or Sr leads to the increase of the electronic defects quantity and, in the

same time, also has as result an increase of the chemical stability of the material.

The most known obtained composition are of the La1-xSrxMnO3, La1-xSrxCrO3, La1-

xSrxCoO3, La1-xCaCrO3 type.

With the same electronic predominant conductor quantity, there were obtained


11/13


Paper 18/11

materials and from solid solutions in binary and ternary systems of the cerium oxide with rare

earth oxides (Nd2O3, La2O3, Yb2O3, Gd2O3) or with Ta2O5 and Pr6O11, the conductive

phenomena being caused by the Ce4+

to Ce3+

transition.

Selecting the materials for the SOFC-IT anodes has imposed less problems in regards

of the criteria for electronic conductors with chemical stability in reducing conditions of the

working environment for high temperatures and moderate costs.

The studies of the anodic material recommended SOFC-IT cells indicated the

following groups of materials:

cermets based on cerium oxide ceramic composite materials cerium oxide based materialsThe cermets compositions, Ni-YSZ, seem to be the best anodic materials for

electrochemical installations of the SOFC type.

To accomplish an efficient electrode, an optimum mixture of the two phases and a

good physical contact between the both materials are necessary in order to attain the high

electrochemical performances and, in the same time, a high chemical stability and durability.

[9, 10]

Other compositions for anodes of the composite type are also the ones obtained from

NiO as electrolyte material of the gadolinium type doped with ceria (GDC) and samarium

doped with ceria (SDC). These anodic NiO/SDC and NiO/GSC materials have the best

electric conductivity when the eelctrolyte is based on cerium oxide.There are, also, other materials for anodes such as in the cerium oxide based CeO2

doped form or Ce0.6La0.4O1.8 and Ce0.6Y0.4O1.8 compounds. Their electronic type conduction is

predominant for a content greater than 50 mol % CeO2 and is due to the Ce4+

to Ce3+

transition.

Recent studies indicated the accomplishment of other compositions in the structure of

some solid solutions with rare earth oxides, SmO1.5, GdO1.5, NbO2.5 or in binary compositions

with ZrO2 and Y2O3 at a content between 50 90 mol % CeO2. [11]

In order to diminish the polarisation penomena at anodes and to improve the decreased

conversion yield of the cell, problems remain unsolved in the case of regular Ni/ZrO2 cermite

type anode materials, new materials as cermites Ru/ZrO2 or Ru/Al2O3 were developed, which

have an increased activity in all the SOFC-IT cell processes, even in the case of methane gas

fuel use.

Recent informations recommend the successful use of some oxidic compounds with

ZrO2 stabilized Y2O3 and admixture of TiO2, materials with a mixed electric, ionic and

electronic, conductivity and with a very good compatibility towards the cell electrolyte.

Technological ceramic materials processing solutions

To obtain the ceramic materials used in the components of a fuel cell, classical


12/13


Paper 18/12

ceramic technologies, as well as special unconventional technologies can be applied.

Each of these technologies allows the first stage to be preparing the material under the

ceramic powder shape having the stoechiometric composition specific for the oxidic

compound with electrolyte or electrodes qualities at temperatures below 8000C.

Classical processing technology

In this case, the starting point is the highly pure chemical reactives (oxides and

carbonates). The technological flux stages are: wet homogenizing, drying, chemical synthesis

treatment (roasting), grounding after which is obtained the ceramic powder with structural

characteristics specific for the oxidic compound and the particle sizes required for shaping

under the form of necessary ceramic piece.

Coprecipitation processing technology

The coprecipitation method is a chemical processing method. In this case, there are

used salt solutions of the components which coprecipitate at a certain pH value, after

coprecipitate filtration, taking place the synthesis through roasting of the compound as a

ceramic powder with submicronic particles.

Applying one of these methods implies a series of technological or economical factors,

which can influence the selection of the most advantageous methods.

The obtaining, under the form of components of electrolyte and electrode with a

certain configuration, requires the use of classical shaping ceramic technologies (pressing,

aqueous suspension pouring or pressure pouring from termoplastic slip, extrusion, lamination)or unconventional technologies of deposition in thin layer through serigraphy, spraying in

plasma, electrochemical deposition in vapour state (EDV) etc.

Serigraphy technology

The serigraphy technology is an easy to perform technology, which leads to the

obtaining of thin layers. Using this technology, an optimum contact between electrode,

support and electrolyte is accomplished, and controlled sizes can be obtained. The optimum

thickness of the deposed layers was not established, but is indicated to be in the range 50

100 m.

The obtaining stages of the thin layers, through the serigraphy technology, of the

ceramic components are:

obtaining of electrolyte, ceramic electrodes, interconnect, under the form of aserigraphic ink

serigraphy drying of the serigraphic layers sintering or cosintering the thin layer components

The serographic ink is obtained according to the stages:

dosing of the electrolyte, electrode or interconnect ceramic powders


13/13


Paper 18/13

dosing the charges: binders plastifiers solvents dispersing agents thixotropic agents thickening agents

In order to be easily obtained, the serigraphic ink must have a good homogeneity and

adherence to the alumina support in the case of cathode and electrolyte, and the anode and

interconnect to adhere well to the electrolyte. Also, the serigraphic ink must have good

rheological properties to allow passing through the serigraphic sieve.

The cost, reproductibility, installations and even obtaining criteria of some

technological parameters have limited the use of some of these technologies, the most

economically convenient technologies being preferred in order to accomplish the solid

electrolyte fuel cell construction.

4. CONCLUSION

Through the development strategy in the field of Hydrogen and Fuel Cells at the

national level, the evolving of the main activity areas will be followed, in accordance with the

European strategy for hydrogen and combustion cells. The main research directions in the

field of solid electrolyte fuel cells are connected to the study of materials, in regards to: reduce the operating temperature increase the energetic efficiency improve the cells fiability internal reforming and direct use of methane use of some sealing materials resistant to high temperatures

5. BIBLIOGRAPHY

1. Philippe Busquin, Prospects for Fuek Cells in A European Resarch Area, Tervuren-Brussels, Belgium29

th

and 30th

of May 20002. L. Oniciu, Pile de combustie, Ed. stiintifica, Bucuresti, 19713. L. Oniciu, E. M. Rus, Surse electrochimice de putere", Ed. Dacia Cluj-Napoca, 19874. S.C. Singhal, Proceedings 2nd Internaional Symposium on SOFC, 2-5 July, 1991, pg. 31-355. P.Duran, J.R. Jurado s.a., Microstructural and electrical characterization of some ceria-gadolinia solid

electrolityes, High Tech Ceramics, 1987,pg.1943-1953

6. T.Ivers Tiffe and H.J. Oel, Electronic conductivity of ceria, High Tech Ceramics, 1987,pg.1933-19417. B.C.H. Steele, s.a., Solide State Ionics 40/41, 1990, pg. 3388. H.Iwahara, s.a., J. Electrochem. Soc., 135, 526, 19889. Kuo, J.H., s.a., Journal Solide State Chem., nr.83,199210. Marjan Marinsek, Andrej Degen and Jordan Macek, Key Engineering Materials, vol. 132-136,199611. Soren Prindahl, Bent F. Sorensen & Mogens Mogensen, Journal of the American Ceramics Society, vol.

3, nr.3, 2000

12. Suzuki. M., s.a., Proceedings 2nd Internaional Symposium on SOFC, 2-5 Yuly, 1991, Athens Greecee

special ceremic materials

Documents