Synthesis and Characterization of La0.75Sr0.25Cr0.5Mn0.5O3-δ Nanoparticles Using a Combustion Method for Solid Oxide Fuel Cells

doi:10.4236/njgc.2011.12010

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New Journal of Glass and Ceramics, 2011, 1, 58-62

doi:10.4236/njgc.2011 .12007 Published Online July 2011 (http://www.SciRP.org/journal/njgc)

Synthesis and Characterization of

La0.75Sr0.25Cr0.5Mn0.5O3-δ Nanoparticles

Using a Combustion Method f or Solid Oxide

Fuel Cells

V. S. Reddy Channu1, Ru d ol f Ho lz e1, E dwin H. Walker Jr.2, Rajamohan R. Kalluru3

1Institut für Chemie, AG Elektrochemie, Technische Universität Chemnitz, Chemnitz, German y; 2Department of Chemistry, South-

ern University and A&M College, Baton Rouge, USA; 3KITE College of Professional Engineering Sciences, Jawaharlal Nehru

Technological University of Hyderabad, Shabad, India.

Email: [email protected]

Received April 11th, 2011; revised May 9th, 2011; accepted May 16th, 2011.

ABSTRACT

La0.75Sr0.25Cr0.5Mn 0. 5O3-δ (LSCM) perovskite nanoparticles for use as anode material in intermediate temperature solid

oxide fu el cells (IT-SOFCs) w e re syn th esi zed usi ng 3,3’,3”-nitrilotripropio nic a cid (NTP), citric acid and oxalic acid as

carriers via a combustion method. The influence of the carrier on phase and morphology of the obtained pristine prod-

ucts was characterized using X-ray diffraction (XRD), thermal gravimetric analysis (TGA), and scanning electron mi-

croscopy (SEM). XRD results showed, that the LSCM had rhombohedral symmetry with R-3c space group; a single

phase LSCM perovskite formed after calcination of fired gel at 1200˚C for 7 h. Scanning electron microscopy analysis

of the pristine powders showed spherical shape and particle sizes in the range of 50 - 500 nm..

Keywords: Nanoparticles, Combustion method, Morphology, Carriers

1. Introduction

A fuel cell is an electrochemical device that converts

chemical energy of fuels (hydrogen, methane, butane or

even gasoline and diesel) into electrical energy by ex-

ploiting the natural tendency of oxygen and hydrogen to

react. Fuel cells are simple devices co nta ini ng no mo vin g

parts and only four functional components namely ca-

thode, electrolyte, anode and interconnect. Solid oxide

fuel cells (SOFCs) are considered to be among the most

versatile power production facilities. Their unique cha-

racteristics include extre me efficiency, sig nificant energy

conversion rate with a wide range of fuels and pollu-

tion-free operation. On the other hand high operating

temperature (about 1000˚C) of the SOFC results in pro b-

lems including difficult sealing between cells with flat

plate configurations and thermal expansion mismatches

between components. In addition, the high operating

temperature places rigorous constraints on materials se-

lection and results in difficult fab rication proc e sses [1].

Recently, perovskite based conducting oxides, such as

substituted lanthanum chromites or lanthanum manga-

nates and strontium titanates, received great attention as

alternative anode materials for solid oxide fuel cells [2].

Particularly, (La0.75 Sr 0.25)1-xCr0.5 Mn 0.5O3−δ perovskite phase

(LS C M) has been considered as a capable anode material

for SOFCs d ue to its electro catal ytic/catalytic acti vity for

oxidation of methane fuel in the absence of steam, re-

duced carbon deposition, and high durability a gainst sul-

fur poisoning and good electrical properties [3,4]. Pre-

dominantly, this composition of LSCM has shown good

stability in fuels and in air, and has good resistance to-

wards carbon deposition and low polarization resistance

when used with hydrocarbon fuels [5]. LSCM can also

be used as a cathode, thus facilitating fuel cells with a

symmetrical structure (LSCM/electrolyte/LSCM) [6,7].

Up till now, various chemical methods were reported

in the literature for the synthesis of LSCM powders, for

example, glycine nitrate method [3], EDTA (ethylenedia

minetetraacetic acid) chelating method [8,9], combustion

synthesis [7,10], solid-state reactio n [4,6,11-15], gel-casting

[16,17] and co-prec ipitation method [18 ]. The so lid-state

Synthesis and Characterization of La0.75Sr0.25Cr0.5Mn0.5O3-δ Nanoparticles

Using a combustion M ethod for Solid Oxide Fuel Cells

method has some disadvantag es suc h as high temperature

(above 1300˚C) and long duration of synthesis of LSCM

and non-homogeneity in pa rticle size and low purity [19].

However, to synthesize homogeneous fine particles of

pure phase LSCM powders requires low temperature and

short duration of synthesis. The solution combustion

method is suitable to synthesize nanosize LSCM particles

with good homogeneity [20]. The present work reports on

the synthesis of nanostructured La0.75Sr 0. 25 Cr0.5Mn0.5 O3−δ

(LSCM) perovskite by a combustion method using

3,3’,3”-nitrilotripr opionic ac id, oxalic acid and citric acid

as carriers.

2. Experimental

Nanostructured La0.75Sr0. 25Cr0.5Mn0.5O3−δ (LSCM) pero-

vskite anode material was synthesized by a combustion

technique. Three batches of LSCM solutions are pre-

pared with stoichiometric amounts of lanthanum nitrate

(La(NO3)3·6H2O), strontium nitrate (Sr(NO3)2), chro-

mium nitrate (Cr(NO3)3·9H2O) and manganese nitrate

(Mn(NO3)2) in distilled water under constant stirring.

First, a stoichiometric amount of citric acid (C6H8O7

H2O) and 15 ml ethyle ne glycol, second, oxalic acid and

15 ml ethylene glycol, and 3,3’,3”-nitrilotripro pionic acid

and 15 ml et h yle ne gl yc ol whi c h ar e che latin g a ge nt s a nd

fuel, were also dissolved in LSCM solution. The stoi-

chiometric ratio of carriers to nitrates was 2 [21]. A gel

was formed with continuous stirring and mild heating at

120˚C. The gel was dried at room temperature for over-

night a nd the n heated at 350˚C for 30 min. The resulting

powders were ground in an agate mortar and heated in air

at 1000˚C for 7 h. F inal l y, the ground product was heated

at 1200˚C for 7 h in air.

Crystallographic information of the samples was ob-

tained using an X-ray powder diffractometer (D8 Ad-

vanced Brucker) with graphite monochromatized Cu Kα

radiation (λ = 1.54187 Å). Diffraction data were col-

lected over the 2θ range of 15˚ to 80˚. The morphologies

of the resulting products were characterized using a

scanning electron microscope (SEM, JEOL JSM 6390).

For the TGA measurements a TA 600, operating in dy-

namic mode (heating rate = 10˚C/min) , was employed.

3. Results and Discussion

Phase purity and crystallographic information of the

synthesized La0.75Sr0.25Cr0.5Mn0.5O3-δperovskite nanopar-

ticles were characterized using powder X-ray dif fraction.

The XRD patterns of LSCM perovskite nanoparticles

are shown in Figure 1. All diffraction patterns of

La0.75Sr0. 25Cr0.5Mn0.5O3-δ perovskite nanoparticles show

characteristic peaks of the perovskite phase. The XRD

patterns are in good agreement with the standard data for

rhombohedral symmetry with a = 5.4736 Å, b = 5.4736

Å, c = 13.2898 Å and R-3c space group (JCPDS #

01-070-8673). No impurity phases were observed pre-

sumably because in the solid-state reaction the pure

phase of La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM ) perovskite

was formed at 1200˚C [21]. The crystallite size of the

LSCM nanoparticles was calculated using Debye-Scherrer

formula at (110) plane. The size of the LSCM-particles

prepared NTP-assisted is 57.65 nm, with citric acid and

oxalic acid the value is 68.63 nm.

In the synthesis of La0.75Sr 0. 25Cr0.5Mn0.5O3-δ (LSCM)

perovskite nanoparticles the carriers (NTP, oxalic acid

and c itri c aci d) he lp i n ob tai ning a ho moge neous mi xtur e

of the cations in a solution through forming metal com-

plexes, they also help in the reduction of nitrates in a

combustion process, releasing a large amount of heat.

This is sho wn by an exothermic peak at 377˚C on a DS C

curve (Figure 2). During the solid-state reduction

process, metal cations and oxygen anions stay in the

react and mixture during the formation of perovskite

phase in t his combustion process. The large heat released

during combustion might be of the help to overcome the

lattice energy, which is required for the formation of the

perovskite phase, and also the completion of the nu-

cleation by the rearrangement of atoms by short distance

diffusion. The fast combustion process might not be of

help for the diffusion of atoms far from each other and

hence the particle size of LSCM powder remained in the

nanometer range [22].

Thermogravimetric analysis and differential scanning

calorimetry (TGA-DSC) curves with NTP-assisted

LSCM dried as a gel at 120˚C are sho wn in Figure 2. The

weight losses happen in three steps. The total weight loss

20 30 40 50 60 7080

0.75

0.25

0.5

3-δ

(NTP +EG at 1200

2θ/°

57.65 nm

68.63 nm

0.75

0.25

0.5

3-δ

(OA+EG at 1200

68.63 nm

0.75

0.25

0.5

3-δ

(CA+EG at 1200

Intensity/s

-1

(214)

(024)

(006)

(202)

(113)

(104)

(110)

(JCPDS # 01-070-8673)

(012)

Figure 1. XRD patterns of La0.75Sr0.25Cr0.5Mn0.5O3-δ

(LSCM) perovskite phase nanopa rt icles.

Synthesis and Characterization of La0.75Sr0.25Cr0.5Mn0.5O3-δ Nanopar ticles

Using a Combustion Method for Solid Oxide Fuel Cells

0200 400 600 800100012001400

LaSrCrMnO

+NTP

0.5mg

0.33mg

TGA

DSC

Temperatur e/

W eight loss/mg

1.25 mg

-300

-200

-100

100

200

Heat flow (mW )

Figure 2. TGA-DSC curves of La0.75Sr0.25Cr0.5Mn0.5O3-δ

(LSCM) perovskite g el precursor dried at 120˚C.

was 2.1 mg from room temperature to 1300˚C. The first

weight loss is 0.33 mg in the temperature range 22˚C -

236˚C; this can be attributed to evaporation of water

from the layers of LSCM. The second weight loss is 1.25

mg in the tempera ture ra nge 236˚C - 383˚C and the third

weight loss is 0.5 mg in the temperature range 383˚C -

837˚C. The second and third weight losses are attributed

to decomposition of the carrier (NTP) and evaporation of

structurally bounded water. The peaks located at about

295˚C and 377˚C are exothermic on the DSC curve due

to decomposition of nitrates and organic matter and cor-

respond to the second weight loss of 1.25 mg from about

236˚C to 400˚C as observed in the TGA curve.

To observe thermal effects above 350˚C, different

carrier-assisted LSCM samples were fired at 350˚C for 1

h. F igure 3 shows T GA-DT A cur ves of t he di ffere nt c ar-

rier’s assisted LSCM samples. The total weight loss was

0.97 mg in NTP assisted LSCM, 0.39 mg in oxalic acid

assisted LSCM, and 0.77 mg in citric acid assisted

LSCM from room temperature to 1300˚C. The first

weight loss is 0.32 mg in NTP assisted LSCM in the

temperature range 30˚C - 490˚C, whereas the first weight

loss is 0.2 mg in the oxalic acid used LSCM in range

30˚C - 338˚C and the first wei ght loss is 0.2 5 mg in citric

acid used LSCM in range 30˚C - 334˚C. The second

weight loss is 0. 9 mg in NT P used LS CM in the t emper-

ature range 490˚C - 822˚C, whereas the second weight

loss is 0.42 mg in citric ac id used LSCM in range 334˚C -

688˚C. The second weight loss was attributed to the reac-

tion between the residual nitrate and carriers after the

decomposition of the precursor and subsequent combus-

tion of organic components.

The morphologies of La0.75 Sr0.2 5 Cr0.5Mn0. 5O3 -δ pe-

rovskite nan-oparticles synthesized using different car-

02004006008001000 1200 1400

3.5

4.0

4.5

5.0

5.5

6.0

0.42mg

0.25mg

0.2mg 0.9mg

LaSrCrMnO

+NTP

LaSrCrMnO

+ C it iric Ac id

LaSrCrMnO

+ Ox a lic Acid

W eight loss/mg

Temperature/

0.32mg

Figure 3. TGA curves of La0.75 Sr0.25Cr0. 5Mn0.5O3-δ (LSCM)

perovskite gel precursor dried at 120ºC and then fired at

350˚C for 1h.

(a)

(b)

(c)

Figure 4. SEM images of La0.75Sr0.25Cr0.5Mn0.5O3-δ perovs-

kite phase nanoparticles (a) NTP-assisted, (b) oxalic acid

assisted and (c) citric acid assisted.

1μm

Synthesis and Characterization of La0.75Sr0.25Cr0.5Mn0.5O3-δ Nanoparticles

Using a combustion M ethod for Solid Oxide Fuel Cells

riers by combustion method as examined with scanning

electron microscopy are show n in Fig ure 4. NTP-assisted

LSCM shows many nanosized particles of sphe ricalshape

in the range 100 - 500 nm (Figure 4(a)). The LSCM

powders synthesized using oxalic acid and citric acid as

carriers consist of uniform nanoparticles shwoing less

agglomeration. The average particle size of the

La0.75Sr0. 25Cr0.5Mn0.5O3-δ perovskite powders calcinated

at 1200˚C is about 50 - 500 nm (Figures 4(a) and (b)).

The small size La0.75Sr0.25Cr0.5Mn0.5O3-δ perovskite nano-

particles are very active, small size is also beneficial for

decreasing the fabrication temperature of the anode film

and enhancing the catalytic proper ties.

4. Conclusions

La0.75Sr0. 25Cr0.5Mn0.5O3-δ (LSCM) perovskite phase na-

noparticles were successfully synthesized by solution

combustion method using different carriers (NTP, oxalic

acid, and citric acid) after calcination of fired gel at

1200˚C for 7 h. Scanning electron microscopy of the

as-synthesized powders showed spherical particle shapes

and sizes in the range of 50 - 500 nm. An exothermic

reaction between carriers and nitrates initiates the com-

bustion process. TGA and DSC analysis confirmed the

decomposition process of nitrates and the organic matter.

The combustion reactions took place in the temperature

range 200 ˚C to 400˚C.

5. Acknowledgements

One of the authors (V S Reddy Channu) thanks the Al-

exander von Humboldt Foundation for a fellowship.

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