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Synthesis of Polymer Derived
Boron-Doped Rare Earth Stabilized
Bismuth Oxide Nanocomposites
for SOFC Applications

Prof.Dr. İbrahim USLU


Outline
• Fuel Cells and SOFCs
• Solid oxide fuel cell history
• Design and operation of SOFCs
• R&D On Fuel Cell
• RE Stabilized Bismuth Oxides
• Techniques used in our studies
– Electrospinning technique
– Polymer precursor technique
• Results of our studies


Fuel CELLs
• Fuel cells convert chemical energy of a fuel gas directly into
electrical work, and are efficient and environmentally clean,
since no combustion is required.
• Moreover, fuel cells have the potential for development to a
sufficient size for applications for commercial electricity
generation.


Solid oxide fuel cells
• SOFC are based on the concept of oxygen ion conducting
electrolyte through which the oxide ions (O2-) migrate from the
air electrode (cathode) side to the fuel electrode (anode) side
where they react with the fuel (H2, CH4, etc.) to generate an
electrical voltage.


Christan Friedrich Schönbein
• Schönbein discovered the principle of the fuel cell in 1838.
• It was his Welsh friend Sir William Robert Grove who
developed the first prototype using hydrogen and oxygen to
create electricity in 1845.


Grove's Fuel Cell
• Grove discovered that by arranging two platinum electrodes
with one end of each immersed in a container of sulfuric acid
and the other ends separately sealed in containers of oxygen
and hydrogen, a constant current would flow between the
electrodes.


Ludwig Mond
• Ludwig Mond (1839–1909) and
assistant Carl Langer described
their experiments with a
hydrogen–oxygen fuel cell that
attained 6 amps per square foot
(measuring the surface area of
the electrode) at 0.73 V.


Friedrich Wilhelm Ostwald
• Ostwald (1853–1932), a founder of the field of physical
chemistry, provided much of the theoretical understanding of
how fuel cells operate.


NASA spent tens of millions of dollars
• In connection with the space program Apollo in 1960, NASA
spent tens of millions of dollars in a successful program that
used hydrogen-based fuel cells to power the on-board
electrical systems on the Apollo journey to the moon.


Solid oxide fuel cell history
• The operation of the first ceramic fuel cell at 1000°C, by Baur
&Preis, was achieved in 1937.
• Researchers at Westinghouse, for example, experimented with
a cell using zirconium oxide (zirconia) and calcium oxide in
1962.


Solid oxide fuel cell history
• However, electrolytes based on zirconia have a relatively
low oxide ion conductivity at temperatures below 800 K and
require very high sintering temperatures (often higher than
2000 K).
• For example δ-phase-Bi2O3 is two orders of magnitude
higher conductivity than zirconia.
• Replacement of zirconia with δ-phase-Bi2O3 based ion
conductor, would give a signifcant reduction in the material
and fabrication problems together with an improvement in
the e•-ciency and longevity of the cell.


Fuel cell was built by Siemens
Westinghouse
• The fuel cell was built by Siemens
Westinghouse and the microturbine by
Northern Research and Engineering
Corporation.
• In a year of actual operating conditions,
the 220 kW SOFC, running on natural
gas is achieving an efficiency of 60%.
• Also, a world record for SOFC operation,
roughly eight years, still stands, and the
prototype cells have demonstrated two
critical successes:
– the ability to withstand more than 100
thermal cycles, and
– voltage degradation of less than 0.1% per
thousand h.


Historical review of fuel cells


TODAY Fuel Cells
• Today, fuel cells are common in spaceflight (Space Shuttle,
Skylab and Gemini spacecrafts), transportation and make
sense for use as portable power, home power generation
and large power generation.


Design and operation of SOFCs
• Cells are being constructed in two main configurations,
• Tubular cells or rolled tubes, such as those being
developed at Westinghouse Electric Corporation since the
late 1950s,
• Flat-plates configuration adopted more recently.


Tubular cells


Tubular Solid Oxide Fuel Cell


Flat-plates configuration


Components of the SOFCs
• A SOFC is mainly composed of two electrodes (the anode
and the cathode), and a solid electrolyte.


Anode and Cothode
• The anode, conducts the electrons that are freed from the
hydrogen molecules so that they can be used in an external
circuit.
• The cathode, distribute the oxygen to the surface of the
catalyst. It also conducts the electrons back from the
external circuit to the catalyst, where they can recombine
with the hydrogen ions and oxygen to form water.


Electrolyte and Catalyst
• The electrolyte is the proton exchange membrane only
conducts positively charged ions.
• The membrane blocks electrons.

• The catalyst facilitates the reaction of oxygen and hydrogen.
• It is usually made of platinum porous nanoparticles very
thinly coated onto carbon paper or cloth so that the
maximum surface area of the platinum can be exposed to
the hydrogen or oxygen.


The key requirement for the solid electrolyte
• it has good ionic conduction to minimize cell impedance, but
also;
• has little or no electronic conduction to minimize leakage
currents,


How Fuel Cell Works
• The pressurized H2 gas entering the fuel cell on the anode side. This gas is forced
through the catalyst by the pressure. When an H2 molecule comes in contact with the
platinum on the catalyst, it splits into two H+ ions and two electrons (e-). The
electrons are conducted through the anode, where they make their way through the
external circuit and return to the cathode side of the fuel cell.
• On the cathode side, O2 gas is being forced through the cathode, where it forms
two oxygen atoms. Each of these atoms has a strong negative charge and charge
attracts the two H+ ions through the membrane, where they combine with an oxygen
atom and two of the electrons from the external circuit to form H2O.


How Fueş Cell Works
SOFC Technology & Advantages
• SOFCs have a modular and do not present any moving
parts, thereby are quiet enough to be installed indoors.


SOFC Technology and Advantages
• SOFCs are the most efficient (fuel input to electricity output)
fuel cell electricity generators currently being developed world-
wide.
• SOFCs are flexible in the choice of fuel such as carbon-
based fuels, eg, natural gas.
• SOFCs do not have problems with electrolyte management
(liquid electrolytes, for example, which are corrosive and
difficult to handle).
• SOFCs have a potential long life expectancy of more than
40000–80000 h.


R&D On Fuel Cell
• US, Canada and Japan significantly increased their funding
for fuel cell R&D.
• The superior fuel flexibility is due primarily to the higher
operating temperature, which increases reaction rates in
the fuel, but also increases the rates of undesired reactions
and creates thermal stresses during thermal cycling.
• Thus, the development and fabrication of materials to meet
these requirements is a major challenge for the implementation
of cost effective SOFCs


Development & fabrication
Oxide ion conductivity
• At present electrolyte materials used in SOFC Technology are
based on doped zirconia systems.
• Yttria stabilised zirconia, (YSZ) is a typical electrolyte
material.
• However, electrolytes based on zirconia have a low ionic
conductivity compared to bismuth oxide (Bi2O3) based
electrolytes.


Disadvantages of YSZ
• Compared to Bi2O3 solid electrolytes based on ZrO2,
– have a relatively low oxide ion conductivity and
– require very high sintering temperatures (>2000 K).


Bi2O3
One of the best oxygen ion conductors

• Cubic Bismuth oxide is one of the best oxygen ion
conductors and is of considerable interest in SOFC and
oxygen sensors.


δ- Bi2O3
high concentration of oxygen vacancies

• The concentration of oxygen vacancies of Bi2O3 can be as high
as 25% of the total amount of anions,
• With such a high concentration of oxygen vacancies, the
conductivity of δ-Bi2O3 can be two orders of magnitude higher
than that of the common oxide conductor, YSZ.


Bi2O3 has high ionic conductivity
• Bi2O3 has high ionic
conductivity, but decomposes
at low oxygen partial
pressures, which prevents it
from being used in SOFC.
• The δ-phase is one of the four
known polymorphs of Bi2O3
solid, and it is stable only in the
temperature range from 730ºC-
825 ºC.


Bi2O3, cracking due to the volume changes
• δ- Bi2O3, has exceptionally high ionic conductivity, but is
subject to cracking due to the volume changes associated with
phase transformation during cyclic heating and cooling.
• The high-temperature form of Bi2O3 can be stabilized with
REs to eliminate the fracture problem, but the ionic conductivity
is reduced.


RE Stabilized Bi2O3
• Stabilize Bi2O3 with REs the ion conductivities firstly increase
and then decrease, therefore a maximum value is obtained
• The conductivity increases along with oxygen vacancies.
• However, if the number of oxygen vacancies reaches a
certain value, the oxygen vacancies begin to become
ordering.
• This results in the weakening of oxygen ionic diffusion
because of the decrease of effective vacancies.


The ionic conductivity of Dy2O3
Stabilized Bi2O3

• The ionic conductivity of Dy2O3
stabilized Bi2O3 from x=0.25 to 0.60
• It is apparent that the highest
conductivity, for a stable fcc
structure, was reported for the
sample x=0.28, with a value of
– 7.1 10-3 Scm-1 at 500 ºC, and
– 0.14 Scm-1 at 700 ºC.
• Doping does not result in increase
in the oxygen ion vacancy
concentration and increasing
dopant amount just decreases the
ionic conductivity

The oxygen ion conductivity
values of Bi2O3
• Bi2O3 stabilized with Neodymia,
Lantania and Erbia and the
samples are also compared to
YSZ.


Conductivity values of RE oxide stabilized Bi2O3

• Er2O3 stabilized Bi2O3
has been shown to
have one of the
highest oxygen ion
conductivities systems
in air; ~0.4 S cm-1.


RE stabilized Bi2O3 Oxide İon Conductivity

• At high temperature,
the material has a
higher oxide ionic
conductivity.


Boron oxide used as dopant
• The addition of boron oxide to the composite materials as a
dopant is very beneficial.
• It is an effective sintering aid because of its low melting point
(460 °C), which could help during the sintering process.
• In this study, boric acid was chosen as the cheapest and
nontoxic source of boric oxide


Boric Acid-PVA cross-linking reaction
• White boric acid powder upon addition to water form tetrahedral [B(OH)4]-(aq) ions.
•
• B(OH)3(aq) + H2O(l) → [B(OH)4]-(aq) + H+(Aq) (1)

• Boric acid is a monoprotic acid and hydroxyl groups (OH) of [B(OH)4]-(aq) tetrahedral
ions.
• Boric acid react with PVA and weak cross-linking within the polymer resulting in
formation of the viscoelastic gel with tetrahedral [BO4]- ions .


In this study
• Five kinds of rare earth stabilized bismuth oxide ceramics,
(RE=Dy, Y, Ho, Er and La), were synthesized using
– A) the polymeric precursor technique and
– B) electrospinning technique.
• Calcining and sintering of
– A) the polymeric precursor technique or
– B) the electrospun nano-fibers at 850 ºC,
• Bi2O3 and RE oxide composite powder were obtained and
their characterization and electrical properties were
investigated.


A) Polymer precursor technique
• In this study boron and RE-Bismuth acetate containing PVA
polymer solution were used as a polymeric precursor.
• Polymeric precursor was calcined to remove the organics and
to obtain homogeneous crystalline ceramic powders.
• The main advantages of such polymer-derived ceramics are the
homogeneity of the precursors on a molecular level, the low
processing temperatures compared to conventional powder
mixing-milling and then sintering method.


B) Electrospinning technique

• In this study boron containing Bi2O3-La2O3 composite PVA
polymer solution was also electrospun to obtain
ultrahomogenus and nanosized fiber structures.


Electrospun nanofibers
• The polymer solution was poured in a syringe and connected to
a high-voltage supply (applied voltage 18 kV).
• The solution was delivered by a syringe pump with a flow rate
0.5 ml/h.
• Then the fibers were dried in vacuum for 12 h at 80 C.


Electrospun fiber diameter distributions
• The average fiber diameters for electropsun boron doped
and undoped nanofibers were in the range of 200 nm and
500 nm.


nano-grain size composites were obtained
from electrospinning technique


Advantages of nano-grains
• Nano-grains have greater ratios of surface area to volume,
which means a greater ratio of grain boundary to
dislocations.
• The more grain boundaries that exist, the higher the
strength becomes.
• Thus, an easy way to improve the strength of a material is
to make the grains as small as possible, increasing the
amount of grain boundary


Sintering after the calcination
• The grains of sintering samples increased with respect to
calcined samples, which can be explained by the fact that the
grains of Bi2O3 gradually grow larger after the second heat
treatment of the samples.

• that the grains of Bi2O3 gradually grow larger as the sintering
temperature becomes higher.


Boron doped & undoped nano-composites
using electrospinning technique
• Boron undoped & the sample with very little boron addition consist of spheroidal
shaped grains. Average grain diameters of boron doped and undoped
nanocrystalline calcined powders were measured as 170 nm &120 nm respectively.
• “Given that the radius of B3+ (0.023 nm) is much smaller than that of Bi3+ (0.117 nm),
it is difficult for B3+ to replace the Bi3+ site. Boron ions may enter the interstitial site of
Bi2O3 crystal structure and lead to the swell of the crystallite size.


Further increase in boron doping
• Further increase of the boron may cause a decrease in
crystallite size and transition to the amorphous glassy
structure which is consisted with the literature.


FT-IR Spectrum
• It is concluded from the FTIR spectrum that no water vapor can adsorb
on the surface of the ceramic powder and all the carbons were removed
after sintering process of the samples.


Boron Doped and Sintered Composites


Crystallite sizes
• Crystallite sizes of the samples were evaluated using Scherrer’s equation.
• This result shows that holmia stabilized bismuth oxide nanoceramic powders
consisted of crystallites with a diameter of 37 nm.
• In addition, the crystallite size the calculated microstrain (ε) and dislocation
density (δ) values are also given in the Table.


BET Analysis

• The BET results show that boron undoped and doped Bi2O3-
La2O3 nanocrystalline powder ceramic structures sintered at
800 oC have surface area of 20.44 m2/g and 12.93 m2/g,
respectively.
• The surface area of undoped powder is thus about 1.6 times
larger than that of the doped one.
• The smaller grain size, as observed by SEM analysis given
above, also supports high surface area of the undoped
powders.


XRD Results of calcined &sintered Powders

• The decrease in the
width of the X-ray
peaks of sintered
samples with respect
to calcined ones
corresponds to a
reduction in the
crystallite size.


Electrical Conductivity values of Er
stabilized Bi2O3

• Measurements of
temperature
dependence electrical
conductivity (sdc) for
polymer-derived erbia
stabilized bismuth oxide
ceramic were carried
out in the wide
temperature range of
648-1175 K.
• Electrical conductivity
was measured by
raising and decreasing
temperature.

stabilized Bi2O3

• There are two distinct
region of the
conductivity curve
which is indicate a
transition occur at
650 C.
• This transition can be
attributed to the
order-disorder
transition which is a
higher-order or critical
point transition


stabilized Bi2O3
• At high temperature region (>650 C), the order-disorder
transition may be attributed to a charge transfer between the
oxide ions and vacant orbital of Er3+ cations in the distorted
crystal structure.
• The oxygen lattice points of the Er2O3 doped -phase Bi2O3 are
not completely occupied with oxygen ions.
• Thorough the solid state reactions of Bi2O3 doped with Er2O3,
Er3+ cations preferentially substitute at the fcc sites in the
crystal structure.
• This was an indication that Er2O3 dissolves in -type Bi2O3
matrix.
• Some of the oxygen lattice points located around fcc sites may
be vacant forming an oxygen vacancy.
• Prof.Dr. İbrahim USLU

stabilized Bi2O3
• In the references, the fcc crystal structure has an oxygen
deficient fluorite structure with two units, and two oxygen ion
vacant sites per unit cell and bismuth ions in the structure
are located on fcc sites, and differ only in the location of the
oxygen ions [K.R. Kendall, C. Navas, J.K. Thomas, H.C. zurLoye, Chem. Mater. 8 (1996) 642-649].
• Therefore, oxygen vacancies are partly responsible for the
order-disorder transition.


sol–gel prepared holmium doped Bi2O3


Boron Doped holmium Stabilized
Bi2O3 via Electrospun


Gadolinia Stabilized Bismuth Oxide with
Boron via Electrospun


Sol-Gel Gadolinia doped Bismuth Oxide


Boron Doped Bismuth Oxide-Erbium
Oxide Powder


Boron doped Bi2O3-La2O3 via electrospun


PVA (Bi2O3 doped)


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