Yttria Stabilized Zirconia Thin Film as Solid Oxide Fuel Cell Electrolyte: Temperature Dependent Structures and Morphology

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 2, Pages: 604-609

J. Environ. Treat. Tech.

ISSN: 2309-1185

Journal web link: http://www.jett.dormaj.com

Yttria Stabilized Zirconia Thin Film as Solid

Oxide Fuel Cell Electrolyte: Temperature

Dependent Structures and Morphology

N. F. M. Rahimi , Sathiabama T. Thirugnana , S. K. Ghoshal

Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310, Skudai, Johor, Malaysia

Razak Faculty of Technology and Informatics, Universiti Teknologi Malaysia, 54100, Kuala Lumpur, Malaysia

Department ofPhysics, FacultyofScience & Laser Centre,Universiti Teknologi Malaysia 81310, Skudai, Johor, Malaysia

Received: 23/10/2019

Accepted: 23/02/2020

Published: 20/05/2020

Abstract

Fuel Cell is an electrochemical cell that supports clean and alternative energy that is mushrooming nowadays. Being a device

of clean energy production, highly efficient solid oxide fuel cells (SOFCs) are increasing in demands. It converts the chemical

energy into electrical energy in an environmentally-friendly way following green technology route. The SOFCs are one type of

technology that has great promise to improve energy efficiency and to provide the society with clean and abundant energy. Yttria-

stabilized zirconia (YSZ) is used as theelectrolyte in SOFC wherein its synthesis with controlled properties is important to obtain

the highest energy efficiency. The overall characteristics of the YSZ thin-film electrolyte in the SOFC are determined by its

structures and morphologies. Based on these factors, a series of YSZ thin films were deposited on the sapphirewafer substrateby

the dip-coating method and sintered in the temperature range of 900 – 1500 ˚C. The temperature dependent structural and

morphological attributes of such thin films were determined and the prepared samples were characterized using XRD, AFM and

Raman spectroscopy. TheXRD patterns of the samples revealed the change in the crystallinity and phase, with an increase in the

sintering temperatures while a tetragonal structurewas observed at 1300 ˚C. Furthermore, the Raman spectral analyses supported

the XRD results. The AFM morphology analysis of the thin films showed an increase in the grain size from 132.25 to 995.2 nm.

The observed temperature-dependent changes in the structures and morphological attributes of these films may be useful for

achieving high ionic conductivity required for an efficient SOFC construction.

Keywords: SOFC, Green technology, YSZ electrolyte, Thin film, Dip-coating, Structures, Morphology

Introduction

In recent years, generating highly efficient, clean, and

environmentally-friendly sources of energy, energy carrier,

and many more have become one of the biggest challenges

for researchers, engineers, and prosumers. There are a lot of

green technologies such as biomass, fuel cell and solar cell

1–3). One of thegreen technologies that have gained overall

interest is fuel cell technology due to global warming that is

now well underway due to the emission of flow out gasses,

electrical efficiencies (5). Themost important part of SOFC

is electrolyteas thesubject itself uses solid ceramic materials

as the electrolyte (7). Zirconia is widely used in a corrosive

environment. Usually, it can be found in pipes, steel alloys,

bricks, ceramic, and artificial gemstones. Zirconia is also

utilised for catalytic converters. In other words, zirconia

materials are very universal and widely used. Pure zirconia

(

ZrO ) is chosen as the electrolyte material for SOFC

application as it has more advantages in terms of its

properties, such as mechanical, optic, hardness, strength, and

high ionic conductivity (8–10). At atmospheric pressure,

x x

such as CO , NO and so on (4). It is worrisome for future

nature outcomes if this continues happening unchecked (5).

Therefore, it is important to improve the properties of the

YSZ electrolyte films useful for SOFCs. A fuel cell converts

chemical energy to electrical energy in which no combustion

is required during the process (6). In this paper, one of the

fuel cell types called Solid Oxide Fuel Cell (SOFC) is

presented. SOFC has the greatest potential of any fuel

technology due to low-cost ceramic materials and high

ZrO

has three polymorphic structures between room

temperature until its melting point is reached at 2826 ˚C,

namely monoclinic, tetragonal, and cubic (3,11). At room

temperature, themonoclinic structureis thermodynamically

stable before turning tetragonal when it reaches 1170 ˚C.

After that, above 2370 ˚C (12), a stable cubic structure is

formed. However, the tetragonal and cubic structures can be

Corresponding author: N. F. M. Rahimi, Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310,

Skudai, Johor, Malaysia. E-mail: fathirahrahimi@gmail.com.

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Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 2, Pages: 604-609

stabilised at room temperature either by doping with cation

covalent/trivalent as stabilisers (e.g. Y O , CaO, and MgO)

2 3

recorded between 20 ˚ to 90 ˚ with the step size of 0.01˚.

Meanwhile, the Atomic Force Microscope(Nanowizard® 3

Atomic Force Microscope) was utilised to analyse the

morphology, surface roughness, and thickness of the thin

films. Theimage of the AFM was recorded with a scale of 3

µm . Raman Spectrometer (UNIDRON Micro Raman

Mapping System) was employed to measure the vibration

bond and crystal structure of the thin films.

or by lowering the crystallite size so that it can fix to more

effective applications in various fields (13). In the SOFC

system, a cubic structure is the most stable structure for

electrolytes and performs the best and highest ionic

conductivity functions due to its equal number number of

vacant oxygen sites in all crystalline lattice directions.

Among all dopants used to stabilise ZrO

, the most suitable

typeis Y because of high oxygen ion conductivity. Yttria

has the cubic form of rare earth oxide structure by itself

according to the ordering of oxygen vacancies in thefluorite

Results and Discussions

.1 X-Ray Diffraction crystal phase analysis

Figure 1 shows theXRD patterns of thesynthesised YSZ

2 2 3

structure, along with thedirections [111] (14). ZrO -Y O or

thin films with one coating layer sintered at different

temperatures. The samples sintered at 900, 1000, 1300,

Yttria-stabilised zirconia (YSZ) is the cubic solid solution

system, whereas tetragonal is also unexceptional because of

both structures’ desire to stabiliseat a high temperature (15).

When increasing the value of doping, the conductivity of

YSZ will rise and reach the maximum, thus yielding many

contributions in wide-ranging applications. It offers good

properties such as high mechanical strength, good chemical

stability, high level of oxygen-ion conductivity, and low

thermal conductivity (16,17). The amount of yttriumdoping

is one of theparameters that must becontrolled as it will later

influence thegrain size, martensitetemperature, and strength

properties. According to (11,18), 8mol% of doping yttrium

show the highest ionic conductivity performance.

400, and 1500 ˚C were named as YSZ9, YSZ10, YSZ13,

YSZ14, and YSZ15, respectively. The peak appeared when

thethin films weresintered at 1300 ˚C and above dueto grain

growth happening during the heating process. The XRD

patterns revealed thetetragonal structurefor YSZ13, YSZ14,

and YSZ15 with thehighest peak (101) corresponding to the

tetragonal structure. The peak (101) increased from YSZ13

to YSZ14 and decreased at YSZ15. The change of crystal

phase in the thin films is shown in Figure 2 by the shifting

peak at (101) and its increasing intensity. The shifting

showed that the stress and strain effect happened in the film

growth during the heating process. The highest intensity

indicated more crystallinity in the thin films accordingly (8).

The crystallitesize, strain and lattice parameter were shown

in Table 1. The crystallite size was calculated by using

Scherrer Formula where the full width at half maximum

(FWHM) of the most intense diffraction peak was chosen:

Materials and Experimentals

.1 Chemicals Materials

Chemicals acquired from Sigma Aldrich to deposit the

YSZ films were Yttria-stabilised zirconia (IV) oxide,

ethanol, and polyethylene glycol (PEG).

푘휆

퐷 =

.2 Instruments

(1)

훽 cos 휃

The dip-coater machine (PTL-MM01) was used for the

preparation of thin films. The hotplate(IKA C-MAG HS 7)

and ultrasonic bath (BRANSON 3510) were also utilised.

where D is Crystallite Size, k is shape factor constant, β is

FWHM, θ is diffraction angle (in radians) and λ is

wavelength of X-ray. The lattice strain, ε was calculated by

Williamson-Hall relation:

.3 YSZ thin films deposition

By mixing yttria-stabilised zirconia (IV) oxide (Sigma

Aldrich, particle size below 100 nm) with PEG as the binder

in a certain amount of ethanol, thesuspension for dip coating

was prepared. Themixture was stirred for 2 h on thehotplate

at room temperature before subjected to the ultrasonic

treatment for another 30 min. Sapphirewafer was used as the

substrate, with a dimension of 1 cm × 1 cm. The samples

were cleaned in the ultrasonic bath using ethanol followed

by acetonebefore they wererinsed in distilled water and kept

dry and clean before they could be used. The coating speed

was fixed at 150 mm/s as each layer of the coating was

prepared. The final process of sintering process was set for

훽 푐표푠 휃



(2)

In equation 2 ε is latticestrain. Last relation equation was

used to find the lattice parameter of tetragonal YSZ:

_푑2

ℎ²+ 푘²

푙²

_푐2

_푎2

(3)

where d is lattice planer sapcing, h, k, and l is the value of

miller indices for a specific Bragg refelction, while a and c

both is lattice parameter. A small difference was seen in the

value of a and c for YSZ13 and YSZ14 compared to YSZ15,

which was reflected by the change in bond distance during

the heating process (19). Besides, the lattice parameter was

calculated after confirming the tetragonal structure

according to the existence of peak at (101), (002) and (110).

Therefore, only YSZ13, YSZ14 and YSZ15 were recorded,

whereby YSZ14 has the highest crystallite size. Moreover, a

slightly shift peak towards the right for YSZ13 and YSZ15

00 ˚C, 1000 ˚C, 1300 ˚C, 1400 ˚C, and 1500 ˚C, resulting

in the samples labelled as YSZ9, YSZ10, YSZ13, YSZ14,

and YSZ15 respectively, based on the temperature used.

.5 Samples Characterizations

The X-ray Diffractometer (Rigaku smartlab X-ray

Diffractometer) With Cu Kα radiation of wavelength, λ ≈

.154 nm) was used to analysethestructuralcharacterization

of thin films. The pattern of X-ray diffraction (XRD) was

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2020, Volume 8, Issue 2, Pages: 604-609

was observed where YSZ14 recorded the smallest stress

value. From this analysis, it can be concluded that YSZ14 or

1400 ˚C is the most optimizetemperaturefor preparing YSZ

thin film electrolyte.

Figure 1: XRD patterns of the deposited YSZ thin films

Figure 2: Magnified view corresponding to the high intensity peak (101) from XRD

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2020, Volume 8, Issue 2, Pages: 604-609

Figure 3: Raman spectra of the selected films

Figure 4: a) YSZ10, b) YSZ13, c) YSZ14 and d) YSZ15 are 3-Dimensional AFM

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2020, Volume 8, Issue 2, Pages: 604-609

Table 1: Crystallite size, lattice strain and lattice parameter

of the prepared YSZ thin films

Crystallite

Conclusions

A series of YSZ electrolyte thin films were deposited

onto thec-planeof thesapphiresubstrateat different sintered

temperatures and the samples were characterised

accordingly. The sintering temperature variation was found

to significantly affect the structures and morphologies of

these films. The XRD analyses showed the formation of

stable crystalline structures of YSZ sintered at 1400 ˚C. The

Raman spectra confirmed the occurrence of two prominent

crystalline YSZ peaks at 1400 ˚C, thereby supporting the

XRD observation. The morphology of the samples sintered

at higher temperatures revealed highly disturbed growths

with thegrains merging together, roughening of surface, and

size enlargements. It was worth the efforts to improve the

YSZ thin film structures fromtetragonal to cubic as thecubic

phase was more stable. Hence, the present findings may be

useful for the development of SOFCs to support the green

technology for industrial developments and humanity.

Sample

Code

Lattice

Size, D

nm)

Strain (nm) Parameter

(

a = 3.598

0.0054

YSZ13

YSZ14

YSZ15

24.06

34.55

31.02

c = 5.186

a = 3.601

c = 5.186

0.0038

a = 3.532

c = 5.068

0.004

.2 Raman spectra

Figure 3 presents the selected Raman spectra of YSZ

films sintered at 1000 ˚C, 1300 ˚C, 1400 ˚C and 1500 ˚C,

whereby thesamples are denoted as YSZ10, YSZ13, YSZ14

and YSZ15, respectively. The observed Raman peaks

showed three prominent peaks related to the E

and E (3) symmetric vibration modes of tetragonal YSZ

crystal(20). YSZ10 and YSZ13 revealed a single peak at 500

g g

(1), E (2)

Aknowledgments

Authors and researchers are sincerely grateful to

acknowledge the financial support fromUTM and Ministry

cm , depicting a tetragonal YSZ with coupled Zr-O bonding

and stretching mode. Thestretching mode O-Zr-O appeared

at tetragonal YSZ of YSZ14 thin film at its peak of 281.91

Education

through

GUP

Vote

number

cm . The three samples supported the XRD result in

identifying thestructureof thefilm, but YSZ15 did not show

any signal at all due to some defects occurring during high

temperature treatments.

Q.K1300000.2540.20H27 and FRGS 5F050.

Ethical issue

Authors are aware of, and comply with, best practice in

publication ethics specifically with regard to authorship

.3 Three-Dimensional (3D) AFM analysis

(avoidance of guest authorship), dual submission and

Figure 4 depicts the 3-Dimensional AFM images of the

manipulation of the figures, competing interests, and the

compliance with policies on research ethics. Authors adhere

to publication requirements that submitted work is original

and has not been published elsewhere in any language.

YSZ thin films. The YSZ thin films displayed homogeneous

surfaces for YSZ10, YSZ13, YSZ14, and YSZ15, whereas

YSZ9 was neglected due to the peak absence in the XRD

pattern prior. Grain growth of the YSZ thin films could be

seen in the images. By increasing the temperature, the grain

boundaries were distinguished and small grains stuck

together to form larger grains. The grain size of the film is

calculated and shown in Table 2. It could be observed that

the grain size increased with incremental sintering

temperatures. Similarly, the root mean square roughness

Competing interests

The authors declare that there is no conflict of interest

that would prejudice the impartiality of this scientific work.

Authors’ contribution

All authors of this study have a complete contribution

for data collection, data analyses and manuscript writing

(RMS) and average roughness (Ra) were important

parameters, whereby the roughness also increased as the

sintering temperature increased. Even though the roughness

for YSZ15 decreased from 181.1 nm to 129.4 nm, the grain

size was too big to be considered. This would be important

for subsequent testing and analysis, such as in terms of

electrical properties in order to achieve a high performance

of ionic conductivity.

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