Electrochemistry Communications 13 (2011) 570–573

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Electrochemistry Communications

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Preparation and characterization of nano-tube and nano-rod structuredLa0.8Sr0.2MnO3-δ/Zr0.92Y0.08O2 composite cathodes for solid oxide fuel cells

Naiqing Zhang a,⁎, Juan Li b, Zhilong He b, Kening Sun a,⁎a Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, PR Chinab Department of Chemistry, Harbin Institute of Technology, Harbin 150001, PR China

⁎ Corresponding authors. Tel./fax: +86 451 8641 215E-mail addresses: znqmww@163.com (N. Zhang), ke

(K. Sun).

1388-2481/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.elecom.2011.03.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 January 2011Received in revised form 9 March 2011Accepted 9 March 2011Available online 17 March 2011

Keywords:Solid oxide fuel cellsComposite cathodesNano-tubeNano-rod

In this work, La0.8Sr0.2MnO3-δ/Zr0.92Y0.08O2 (LSM/YSZ) composite nano-tubes are co-synthesized by a porewetting technique as a cathodematerial for solid oxide fuel cells (SOFCs). A fast-firingmethod is introduced toimprove the contact between the composite cathode and the YSZ electrolyte, as well as to retain the originalnano-tube structure. The morphology and structure studies indicate that the LSM/YSZ composite cathodeswhich undergo a 1100 °C heat treatment present nano-tube and nano-rod structure (with heating and coolingrates of 200 and 100 °C min−1). Area specific resistance (ASR) of the composite cathodes is characterized byelectrochemical impedance spectroscopy. The as-prepared nanostructured composite cathode shows low ASRvalues, which is mainly due to small grain size, homogeneous particle distribution and fine pore structure ofthe material.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Research on novel electrode materials [1–3] and/or uniquemicrostructure [4–6] is one of the critical issues in the developmentof new generation solid oxide fuel cells (SOFCs). Nanomaterials,including nano-wires, nano-tubes, nano-rods and nano-belts, havegreat potential to increase the triple phase boundary (TPB, where thegas, electrode, and electrolyte are in contact) length and dramaticallyenhance the SOFC performance. Recently, Bellino et al. [5,7] reportedthe synthesis of cobaltite nano-tubes and a new architecture based onthese materials as cathodes for SOFCs, which show impressively lowpolarization resistance.

So far, cobaltite nano-tubes have been used to fabricate cathodesfor SOFCs. However, the fabrication of composite cathodes showingnano-tube structure has not yet been reported. It is well known thatthe addition of an ionically conductive phase to the electronic ormixed ionic and electronic conductive (MIEC) oxides significantlyreduces the interfacial resistance compared to that of single-phaseoxides [8–13]. Thus, it is of great significance to develop nano-tubestructured composite cathodes which will combine the abovementioned advantages. In this study, La0.8Sr0.2MnO3-δ/Zr0.92Y0.08O2

(LSM/YSZ) composite nano-tubes were co-synthesized by pore

wetting technique as cathode materials for SOFCs. Fast-firing methodwas used to attach the composite cathode to the electrolyte. Whenused as cathode in SOFCs, the as-prepared nanostructured compositesof LSM/YSZ show low area specific resistance (ASR) values.

2. Experimental

Commercial polycarbonate (PC) membranes (Whatman, UK) withpore size of 400 nmwereused as templates. Themixture (0.15 mol L−1)of La(NO3)3·6H2O, Sr(NO3)2, Mn(NO3)2·4H2O, Y(NO3)3·6H2O and ZrO(NO3)2·2H2O in de-ionized water was stirred for 20 min to make theprecursor solution of La0.8Sr0.2MnO3-δ and Zr0.92Y0.08O2 (weight ratio1:1). A few drops of the precursor solution were spread on a glass slideand covered with the PC membranes. The membranes would be filledwith the solution due to capillary force. After having been kept at 30 °Cin a vacuum for 12 h, thefilled PCmembraneswere calcined in a furnaceat 800 °C for 10 min with the heating rate of 2 °C min−1 and cooled toroom temperature naturally in the furnace. The resulting powder was acollection of LSM/YSZ composite nano-tubes.

The resulting samples were characterized by means of X-raydiffraction (XRD, Rigaku D/max-ПB) using Cu Kα radiation. Thecrystalline size was calculated based on Scherrer's equation. Themorphology and microstructure of the samples were examined byscanning electron microscope (SEM, FEI Quanta 200f, Holland) andtransmission electron microscope (TEM, FEI Tecnai G2, Holland)operating at 300 kV.

571N. Zhang et al. / Electrochemistry Communications 13 (2011) 570–573

A three-electrode cell was fabricated for electrochemical imped-ance spectroscopy (EIS) measurements, as described in our previouspaper [8]. Measurements were taken in air over a temperature rangeof 700–850 °C, at zero bias. A screen printed layer of LSM/YSZcomposites on the YSZ electrolyte was sintered through a fast-firingtechnique at 1100 °C for 1 minwithheating and cooling rates of 100 and200 °C min−1, respectively. The cathode area was 0.25 cm2. The Ni/YSZanode-supported YSZ electrolyte was prepared by tape-casting for fuelcell tests, as introduced in our previous paper [14].

3. Results and discussion

The SEMmicrograph of LSM/YSZ composite nano-tubes fabricatedwith template-directed synthesis method is shown in Fig. 1a. Thenano-tubes were found to be 4–6 μm in length and 260±10 nm inouter diameter. Furthermore, the microstructures of the compositenano-tubes were investigated directly by taking the TEM image(Fig. 1b), indicating that the composites consist of nanometer sizedLSM and YSZ particles (10–15 nm). The XRD pattern of the LSM/YSZnano-tubes after calcination at 800 °C is presented in the inset ofFig. 1a. We can see that all peaks agree well with the fluorite YSZ andperovskite LSM phases. The mean crystalline size estimated withScherrer's equation is approximately 17.6 nm and 10.3 nm for LSMand YSZ, which agrees well with the TEM image shown in Fig. 1b.

It is well known that significant grain growth leads to severeagglomeration of nanomaterial at temperatures ≥1000 °C [5]. In thisreport, a fast-firing process with fast heating and cooling rates as wellas a short dwell time was carefully chosen, aiming to suppress the

Fig. 1. (a) SEM and (b) TEM micrographs of LSM/YSZ composite nano-tubes, the inset in (a)cathodes sintered at 1100 °C for 1 min with heating and cooling rates of (c) 200 °C min−1 an

grain growth and retain the nano-tube structure simultaneously. Theas-prepared LSM/YSZ composite nano-tubes were screen-printedonto solid YSZ electrolytes, followed by heating at 1100 °C for 1 minwithheating and cooling rates of 200 and100 °C min−1 (coded as L/Y200and L/Y100). Fig. 1c and d compared the micrographs of differentcathodes. It can be observed that the L/Y100 composite cathode turnsinto dense nano-rods while the hollow nano-tube structure is retainedfor L/Y200 cathode. The inset in Fig. 1c shows the cross-section SEMmicrograph of the L/Y200 sample. The 12 μmthick cathode film has goodcontact with the electrolyte membrane.

The fast firing profile recommended to maintain the nanostruc-tures will put thermal stresses on the SOFCs during preparation. Inorder to validate the application of fast firing in the state-of-the-artSOFC architecture, the power output test on a real SOFC device hasbeen conducted. The I–V curve of the fuel cell is shown in the inset ofFig. 2a. The OCVs which are very close to the cell theoretical voltage inthe testing temperature range from 650 to 800 °C certify the highquality of YSZ electrolyte film. Thus, the YSZ electrolyte thin film couldendure the thermal stresses.

To understand the influence of heating rate on the electrochemicalperformance of SOFC cathodes, the two types of cathodes (L/Y200 andL/Y100) were investigated by EIS measurements at 700–850 °C in airatmosphere. Fig. 2a displays the evolution of the Nyquist plots withtemperature for the L/Y200 cathode. The resistances were obtained byfitting EIS data with the equivalent circuit model LRel(QRH)(QRL). Theintercept of the semicircle on the real axis in the high frequencyrepresents the ohmic resistance (Rel), and (RHQ), (RLQ) representthe high frequency and low frequency arc, respectively. The radius

is the XRD pattern of the nano-tubes; Surface SEM micrographs of LSM/YSZ composited (d) 100 °C min−1, the inset in (c) is the corresponding cross-section SEMmicrograph.

Fig. 2. (a) Nyquist plots of the L/Y200 composite cathode at 700–850 °C, symbolscorrespond to experimental data, solid lines correspond to fitted results using theequivalent circuit LRel(QRH)(QRL), the numbers indicate the frequency (logarithm), andthe inset shows the I–V curve of the real SOFC device; (b) RH, RL and ASRs of L/Y200 andL/Y100 composite cathodes as a function of temperature; (c) Schematic illustration ofthe ORR on the nano-tube and nano-rod LSM/YSZ composite cathodes.

Table 1Comparative ASR values and experimental parameters of LSM/YSZ composite cathodes bas

Cathode Composition(LSM:YSZ)

Thermal treatment Thickness(μm)

Meansize

La0.8Sr0.2MnO3-δ-YSZ 50:50 1100 °C, 1 min (200 °C min−1) 12 80–12La0.7Sr0.3MnO3-YSZ 47–32–21 1100 °C, 4 h 25–30 30–50La0.8Sr0.2MnO3-δ-YSZ 80:20 1200 °C, 2 h 10 1–2 μmLa0.8Sr0.2MnO3-δ-YSZ 50:50 1175 °C, 1 h 20–24 200 nmLa0.8Sr0.2MnO3-δ-YSZ 50:50 1150 °C, 4 h 20±1 0.3–0.4La0.85Sr015MnO3-δ-YSZ 8:2 1000 °C, 3 h 35 0.24 μmLSM–YSZ(impregnation)

1:1 650–750 °C, 1 h 10–20 YSZ 0.5impregLSM 50

(La0.85Sr015)0.9MnO3-δ-Zr0.85Y015MnO1.92

6:4 1150 °C, 2 h 10 0.2–0.3

a Not reported.b Not reported or could not get precise values.

572 N. Zhang et al. / Electrochemistry Communications 13 (2011) 570–573

difference of the arc between the intercepts in the high- and low-frequency region on the real axis corresponds to the ASR of thecathode. Fig. 2b shows the resistances of L/Y200 and L/Y100 compositecathodes as a function of temperature. The L/Y200 composite cathodewas found to have ASR values of 0.17, 0.25, 0.39 and 0.52 Ω cm2 at850, 800, 750 and 700 °C, respectively,which are lower than that of theL/Y100 composite cathode, reaching correspondingly 0.37, 0.49, 0.73and 1.18 Ω cm2.

Since the cathode composition and the test conditions are identical,the decrease in the ASR values can be ascribed to the microstructuredifference of the cathodes. It is well known that the oxygen reductionreaction (ORR) on a SOFC cathode takes place at the TPB region. Incomposite cathodes, it is generally recognized that the TPB regionexpands from the limits of the two-dimensional interface betweencathode and electrolyte to the entire cathode. A schematic diagramwas used to explain the ORR on the nano-tube and nano-rod LSM/YSZcomposite cathodes, as shown in Fig. 2c. Due to the hollow structure ofL/Y200 composite cathodes, the ORR takes place at both inner andouter surfaces of the nano-tubes, while only on the outer surface forthe nano-rod structure in L/Y100 cathodes. Moreover, pores of sub-micron and nanometer scale were found in L/Y200 composite cath-odes. Such architecture facilitates the gas-phase diffusion processes[15], which contributes a low ASR value.

Up to now, various approaches have been suggested to fabricateLSM/YSZ composite cathodes for SOFCs. These reported ASR values ofcathodes are summarized in Table 1. It is worth noting that the ASRsreported in this study are substantially lower than those reported inthe literature [4,8–13]. Especially, the as-prepared composite cathodeis favorable for a relatively low temperature, e.g. 700 °C. Severalgroups have pointed out that electrode microstructure (i.e., particlesize, pore size, and porosity) has a strong influence on the value of ASR[4–6]. Thus, the promising performance of the nanostructured L/Y200composite cathodes is attributed to the optimized microstructure, i.e.,small grain size, uniform size distribution, fine pore structure, andgood inter-connectivity between LSM and YSZ particles.

Although the L/Y200 composite cathode was found to have ASRvalue of 0.17Ω cm2 at 850 °C, grain growth of the nanoparticles at suchhigh temperature is inevitable for extended periods of time. However,LSM nanoparticles proved to be stable at 650 °C [16], indicating that thedurability of the nanostructures at a relatively low temperature isacceptable. Based on these considerations, the resultant microstructureproposed herein is more applicable for intermediate temperaturesSOFCs. The durability of the resultant microstructure at intermediatetemperatures will be tested further.

4. Conclusions

Nanostructures of LSM/YSZ composite have been co-synthesized byusing the pore wetting technique. SEM micrographs suggest that the

ed on YSZ electrolytes.

particle Electrolyte ASR (800 °C)(Ω cm2)

ASR (750 °C)(Ω cm2)

ASR (700 °C)(Ω cm2)

References

0 nm 8YSZ, 0.7 mm 0.25 0.39 0.52 Present worknm 8YSZ, 0.5 mm 0.445 b 3.0 Song et al. [4]

8YSZ, 0.6 mm 0.25 0.52 b Piao et al. [8]8YSZ , a 0.50 b b Wilson et al. [9]

μm 8YSZ, 1.2 mm b 1.31 2.49 Murray et al. [10]8YSZ, 0.5 mm 0.58 1.63 b Song et al. [11]

μmnatednm

8YSZ, 10 μm b 0.74 b Liang et al. [12]

μm 8YSZ, 0.2 mm b 0.9 b Kim et al. [13]

573N. Zhang et al. / Electrochemistry Communications 13 (2011) 570–573

nano-tube structure feature of LSM/YSZ is retained even after a 1100 °Cthermal treatment. The cathode with nano-tube structure shows ASRvalues of 0.17, 0.25, 0.39 and 0.52Ω cm−2 at 850, 800, 750 and 700 °C,respectively, much lower than those reported values of materialsexhibiting conventional microstructures. The high specific surface areaand multi-scale porosity of the nano-tube structure composite cathodeare the main reasons for the promising performance.

Acknowledgment

Thiswork is funded byNational Natural Science Foundation of China(No. 20906015) and Natural Scientific Research Innovation Foundationof Harbin Institute of Technology (No. HIT.NSRIF.2008.23).

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