A Novel Cell-Array Design for SingleChamber SOFC MicrostackMingliang Liu1, Zhe Lü1*, Bo Wei1, Xiqiang Huang1, Kongfa Chen1, and Wenhui Su1,2

1 Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150080, China2 International Center for Material Physics, Academia, Shenyang 110015, China

Received January 18, 2009; accepted April 19, 2009

1 Introduction

Single chamber solid oxide fuel cell (SC-SOFC) is a type ofsolid oxide fuel cell operating in a diluted gas mixture of fueland oxidant. The operation principle of SC-SOFC was basedon the catalytic selectivity of the electrodes towards the fuel/oxidant gas mixture [1]. The anode must be selective to theelectrochemical oxidation reaction of fuel while the cathodemust be selective to the electrochemical reduction of the oxi-dant and inert towards the oxidation of the fuel. For example,in a methane–oxygen mixture, CH4 is partly oxidised to H2

and CO at the anode, which reacts with oxygen ions from theelectrolyte subsequently, producing CO2 and H2O. The oxida-tion reactions are

CH4 12

O2→CO 2H2 (1)

H2 O2→H2O 2e (2)

CO O2→CO2 2e (3)

At the other side of the electrolyte, the cathode has a high-er electrocatalytic activity for the reduction of oxygen

12

O2 2e→O2 (4)

Because of the consumption of oxygen at the anode side, agradient of the oxygen ion activity can be generated betweencathode and anode. As a result, an electromotive force (EMF)

is built up between anode and cathode; thus SC-SOFCs canwork and generate electric energy.

SC-SOFCs have several advantages over the conventionaldual-chamber design such as rapid startup, improved ther-mal and mechanical shock resistance and eliminated sealingprocesses. The prospect of small fuel cell stack for portableapplications is considered in terms of impressive high energydensity, technological feasibility, cost, safety and convenience[2]. For SC-SOFCs, high power density has been obtained[3–6]. In particular, SC-SOFCs are easily arranged into micro-stack by eliminating the sealing processes and the fabricationcost, mass and volume of stack are reduced.

For SC-SOFCs stack, Hibino et al. [7] demonstrated thatsome unit cells arranged on the same face of the electrolytecould be connected in series/in parallel with one another.Suzuki et al. [8] fabricated an Sm-doped ceria (SDC) electro-lyte supported double cell module, which has generated anopen circuit voltage (OCV) of 1.5 V and a maximum outputof about 17 mW at 550 °C. Buergler et al. [9] reported micro-single chamber SOFCs consisted of an array of 19 individualcells in parallel and fabricated by micromoulding in capil-laries of electrodes on Ce0.8Gd0.1O1.95 electrolytes which hasgenerated a maximum power density of 17 mW cm–2. Shao etal. [10] reported a thermally self-sustained, anode-facing-

–[*] Corresponding author, lvzhe@hit.edu.cn

AbstractA novel design of single chamber solid oxide fuel cell (SC-SOFC) microstack with cell-array arrangement is fabricatedand operated successfully in a methane–oxygen–nitrogenmixture. The small stack, consisting of five anode-supportedsingle cells connected in series, exhibits an open circuit vol-tage (OCV) of 4.74 V at the furnace temperature of 600 °Cand a maximum power output of 420 mW (total active elec-

trode area is 1.4 cm2) at the furnace temperature of 700 °C.A gas mixture of CH4/O2 = 1 leads to best performance andstability.

Keywords: Cell Array, Flow Rate, Methane–Oxygen–Nitro-gen Mixture, Microstack, Single Chamber Solid Oxide FuelCell

FUEL CELLS 09, 2009, No. 5, 717–721 © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 717

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DOI: 10.1002/fuce.200900010

Liu et al.: A Novel Cell-Array Design for Single Chamber SOFC Microstack

anode, microstack with two anode-supported cells, which hasgenerated an OCV of 1.44 V and a power output of about350 mW at 1.0 V. We fabricated an anode-facing-cathodestack using two single cells with an anode-supported config-uration, producing a maximum power output of 371 mW(1 cm2) [11]. Recently, Hibino and co-workers [12, 13] haveoperated an SC-SOFC stack in engine exhaust with a highoutput of 1 W (12 cells, 12 × 5 cm2). Wei et al. [14] proposed anovel star-shape design of SC-SOFC microstack, which wasoperated successfully in a methane–oxygen mixture. Theseresults demonstrated the possibility of SC-SOFC microstackfor portable applications.

In this study, we fabricate an anode-facing-cathode micro-stack with a novel cell-array arrangement. The stack, consist-ing of five single cells connected in series, is operated success-fully in a methane–oxygen–nitrogen mixture at a furnacetemperature ranging from 600 to 750 °C. Effects of methane/oxygen ratio, operating temperature and flow rate on cell per-formance are studied in order to optimise the operating con-ditions of the microstack.

2 Experimental

Anode-supported fuel cells composed of YSZ membranes,Ni/YSZ anodes and modified La0.7Sr0.3MnO3 (LSM) cathodeswere fabricated (Figure 1a). YSZ (TZ-8Y, Tosoh Corp.), NiOand flour were mixed at a weight ratio of 5:5:2 as the greenanode powder. The powder was pressed into pellets of10 mm diameter, followed by pre-calcination at 1000 °C for2 h, which served as the anode substrates. Dense YSZ mem-branes were fabricated on the anode substrates using a slurryspin coating method [15]. The bi-layer of YSZ and anode sub-strate was co-sintered at 1400 °C for 4 h. LSM paste wasprinted on the electrolyte membranes and subsequently sin-tered at 1100 °C for 2 h. Then the cathodes were impregnatedby 3 mol L–1 Sm0.2Ce0.8(NO3)x solution and heat-treated at850 °C for 1 h to improve the cell performance [16]. Theactive area of the cathode was 0.28 cm2. The anode substrateswere reduced in hydrogen at 700 °C before assembly. The

reduction of anode was performed by using the conventionalconfiguration for dual-chamber SOFCs, i.e., the anode wasfed hydrogen while the cathode was exposed to air in orderto avoid the reduction of cathodes. The surfaces of electrodeswere covered with a thin layer of Ag paste as a current collec-tor and the cells were then connected by a silver sheet or sil-ver wire.

Figure 1 illustrates the schematic diagram of the cell con-figuration, ceramic board with square holes and the stack insingle chamber. The series-wound stack using anode-facing-cathode configuration consisted of five single cells. A piece oflightweight dichroite board (Figure 1b) with many squareholes (side length ∼1 mm) was used as a supporter to assem-ble the cell array. In order to fix the cells, the surface of cera-mic board was fluted. The distance of the cells which wereconnected by silver sheet is about 2 mm.

The performance of the stack was tested in a quartz tube(Figure 1c, the inner diameter and outer diameter were ∼1.75and 2.07 cm, respectively). The mixed gas was composed ofmethane, oxygen and nitrogen. The ratio of methane to oxy-gen ranging from 1 to 2 was controlled by mass flow control-lers (MFCs, D08-4D/2M, Seven-Star Huachuang, China). Theflow rate of nitrogen was also controlled by MFC. The perfor-mance of the stack was measured at a furnace temperatureranging from 600 to 750 °C. Two K-type thermocouples wereused to monitor both the furnace temperature (Tf) and theactual stack temperature (Ts). I–V performance and AC impe-dance spectra of the stack were measured by a Solartron SI1287 electrochemical interface and a Solartron SI 1260 impe-dance gain/phase analyser. The frequency was changed from0.1 Hz to 91 kHz for the impedance spectra measurements.

3 Results and Discussion

The gas composition has a significant influence on stackperformance. Figure 2 shows I–V and I–P curves of the stackfor various CH4/O2 ratios (m) at 600 °C. The total flow rate ofCH4 and O2 was fixed at 200 sccm. The OCV is 4.74 V atm = 1 and it remains stable at different CH4/O2 ratios. The

average OCV for individual cell is about 0.95 V. Thepower output decreased as the CH4/O2 ratioincreased, which is consistent with our previouswork [11]. In our previous 2-cell stack, the cathodefaces the narrow gas path between itself and theanode of the next cell was influenced by the oxygenpartial pressure [11]. It is also the reason that theI–V and I–P curves of the stack for m = 1.5 and 2were not as steady as those for m = 1, as shown inFigure 2. So an optimal fuel-to-oxygen ratio isimportant for SC-SOFC stack.

Figure 3 displays I–V and I–P curves of the stackoperated from 600 to 700 °C (furnace temperature)at the flow rates of CH4 = 100 sccm, O2 = 100 sccmand N2 = 400 sccm. Maximum power outputs of175, 244 and 275 mW are achieved at 600, 650 and

Cathode ~15μm

Electrolyte ~15μm

Anode ~0.6mm

Silver wire

CH4+O2+N2

Silver sheet

Ceramic boardSilver wire

Square hole

Quartz tube

Ceramic board

Square holes

(a) (b) (c) Fig. 1 The schematic diagram of the cell configuration, ceramic board and the stackin single chamber (a) single cell, (b) ceramic board with square holes, (c) stack in sin-gle gas chamber.

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Liu et al.: A Novel Cell-Array Design for Single Chamber SOFC Microstack

700 °C, respectively. The OCV of the stack decreases withincrease in temperature from 4.74 V at 600 °C to 4.53 V at700 °C. Figure 4 shows the impedance spectra of the micro-stack at various furnace temperatures. The ohmic resistanceincluding electrolyte and interconnect contributionsdecreases with the increase in temperature, and it accountsfor less than 9% of the total microstack resistance. Thus, thestack performance is primarily limited by the electrode polar-isation resistance.

Figure 5 shows I–V and I–P curves of the stack for variousCH4 flow rates with m = 1 and an N2 flow rate of 400 sccm at700 °C. The OCV of the stack remained constant at variousCH4 flow rates at 4.5 V. The peak power output increasedwith increase in gas flow rates. The maximum power outputs

are 237, 275, 300 and 327 mW at CH4 flow rates of80, 100, 120 and 140 sccm, respectively. The resi-dence time which is a very important parameter inSC-SOFC [17] is shortened with increase in gas flowrates. The fuel utilisations also decreased with thedecrease in the residence time. Therefore, a moder-ate CH4 flow rate should be used in this stack toobtain both high power output and acceptable fuelutilisation.

Figure 6 shows I–V and I–P curves of the stack atvarious N2 flow rates at 700 °C. The flow rates ofboth CH4 and O2 were fixed at 140 sccm. The OCVof the stack increased with increase in N2 flow rate.The maximum power output increased as the N2

flow rates decreased, while a slight drop offappeared at the lowest flow rate. The highest poweroutput is about 420 mW at an N2 flow rate of100 sccm, and the average power density of thisstack is about 300 mW cm–2. N2 served as a dilutedgas is important for the diffusion and flow of fuel/oxygen as well as the reactant absorption andproduction desorption process on electrodes. How-ever, higher flow rate of N2 would carry more heataway from the cell, reducing the actual temperatureof the cell. The overheat temperatures (Ts–Tf) are 8,12, 15, 18, 25, 33 and 48 °C, at N2 flow rates of 600,500, 400, 300, 200, 100 and 0 sccm, respectively. Infact, the temperature monitored by the thermocou-ple at the centre of stack in gas chamber is an aver-age temperature of the stack instead of one cell sur-face temperature. The cell surface temperatureshould be higher than that monitored in the gaschamber. At the same time, the higher N2 gas veloc-ity reduced the residence time and fuel utilisation.The highest power output was obtained at an N2

flow rate of 100 sccm because of the high actualtemperature of cell and long residence time. It alsoshows that the flow geometry of the stack and theconfiguration of the cell array are reasonable. So theCH4 and O2 can be easily transmitted to each cell.At the same time, the I–V and I–P curves of thestack for the lower flow rate of N2, shown in Fig-

ure 6, were not steady. So an appropriate N2 flow rate isnecessary for the successful operation of stack in single cham-ber conditions. The N2 flow rate range of 200–400 sccm is rea-sonable for the stack in this study.

The cell-array design consists of two stages: the first threeSOFC elements stage and the second two SOFC elementsstage. The addition of the second stage can increase the fuelutilisation [17]. The fuel utilisation can be estimated by thecurrent efficiency e IIF where I and IF are the actual cur-rent and the current calculated for 100% fuel conversion,respectively. IF can be calculated by combining Eqs. (1)–(3),the fuel flow rate and Faraday’s law. The highest current effi-ciency of the stack was estimated to be ∼2.4%. The fuel utilisa-tion is still low. However, this result is higher than that

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.140

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2

3

4

5

0.00

0.04

0.08

0.12

0.16

0.20

Pow

er o

utpu

t / W

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tage

/ V

Current / A

m=1m=1.5m= 2

600 ºCN2= 400 sccm

Fig. 2 I–V and I–P curves of the stack for various m values at 600 °C, N2 flow rate of400 sccm.

0.00 0.05 0.10 0.15 0.200

1

2

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5

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0.05

0.10

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600ºC 650ºC 700ºC

CH4-O

2-N

2=100sccm-100sccm-400sccm

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Fig. 3 I–V and I–P curves of the stack from 600 to 700 °C at CH4 flow rate of100 sccm, O2 flow rate of 100 sccm and N2 flow rate of 400 sccm (m = 1).

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obtained under the same conditions in our previous work[11]. In order to increase the fuel utilisation, several otherstages can be added to the stack. The fundamental methodis to improve the selectivity of the electrodes [17].

The cell-array design can offer several advantages:firstly, improved thermal and mechanical shock resistancessince single cells are settled in the ceramic board (load-bearing), secondly, flexible output capability and modular-isation since the number and the shape size are flexibleaccording to requirement. A unit stack of cell-array designis constructed with several single chamber fuel cells fixedon a ceramic board with square holes as a supporter, thenunit stacks can be assembled together to form a biggerstack module in series/parallel.

4 Conclusions

A novel cell-array stack design for SC-SOFC was fabri-cated and operated successfully in a methane–oxygen–nitrogen mixture. This stack consisted of five single cellsusing anode-supported YSZ electrolyte membrane con-nected in series. A CH4/O2 = 1 gas mixture leads to bestperformance and stability. An appropriate N2 flow rate isnecessary for the successful operation of stack in singlechamber conditions. The stack generated an OCV over4.5 V and a maximum power output of 420 mW (totalactive electrode area: 1.4 cm2) at the furnace temperatureof 700 °C. Our result demonstrates that the design is verypromising for micropower sources.

Acknowledgement

This research was supported by the Ministry of Scienceand Technology of China (2007AA05Z139).

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-Z'' /

Ω

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0.1

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Pow

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utpu

t / W

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