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Technical Communication

Micro-tubular, solid oxide fuel cell stack operated undersingle-chamber conditions

Naveed Akhtar*, Kevin Kendall

Department of Chemical Engineering, University of Birmingham, B15 2TT, UK

a r t i c l e i n f o

Article history:

Received 6 April 2011

Received in revised form

13 July 2011

Accepted 14 July 2011

Available online 19 August 2011

Keywords:

Solid oxide fuel cell

Single-chamber

Stack

Inert gas

Micro-tubular

* Corresponding author. Present address: DDen Dolech 2, Helix, STW 1.44, P.O. Box 513

E-mail addresses: N.Akhtar@tue.nl., navtURL: https://sites.google.com/site/navtar4

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.07.057

a b s t r a c t

A micro-tubular, solid oxide fuel cell stack has been developed and operated under single-

chamber conditions. The stack, made of three single-cells, arranged in triangular config-

uration, was operated between 500 and 700 �C with varying methane/air mixtures. The

results show that the operating conditions for the stack differ significantly than the single-

cell operation reported in our earlier study. The stack operated at 600 �C with methane/

oxygen mixture of 1.0 gives stable performance for up to 48 h, whereas for the single-cell,

this mixing ratio was not suitable. The increase in the inert gas flow rate improves the

stack performance up to a certain extent, beyond that; the power output by the stack

reduces due to extensive dilution of the reactants. It is concluded that both, the operating

conditions and the addition of inert gas, need to be tuned according to the number of cells

present within the stack.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction range of few milli-Watts which is not suitable for practical

Single-chamber solid oxide fuel cells (SC-SOFCs) operate on

a uniformmixture of fuel and oxidant by ensuring the selective

catalytic and electrochemical reactions on each electrode [1]. It

is preferred that the anode electrode should provide a syngas

with a higher hydrogen to carbon monoxide ratio, which will

undergo an electrochemical oxidation reaction at the reactive

region within the anode. On the other hand, the cathode elec-

trode should remain inert towards any fuel processing reac-

tions that consume oxygen parasitically; instead only electro-

chemical oxygen reduction is preferred at the cathode [2].

The power produced from a single-cell under single-

chamber conditions is reported to be too low, mainly in the

epartment of Chemical, 5600 MB Eindhoven, Thear433@yahoo.com. (N. Ak32/.2011, Hydrogen Energy P

applications [3,4]. Therefore, the development of stack design

in a single-chamber configuration is gaining recent interest

[5]. The single-chamber SOFC stack appears to be more

advantageous than its counterpart dual-chamber SOFC stack,

mainly due to: 1) its compact and simplified design can im-

prove thermal and shock resistance, 2) no need for channel-

ling, elimination of bipolar plates, thus reducedmanufacturing

cost, volume and mass, 3) in-situ exothermic reactions can

provide sufficient heat to maintain the cell temperature, thus

lowering the heat input, 4) the sealing process and associated

complex thermo-mechanical requirements are eliminated.

Although, there are numerous research studies on single-cell

level, the stack design in SC-SOFC remained less explored [3].

Engineering and Chemistry, Technical University Eindhoven,Netherlands. Tel.: þ31 62 2296029; fax: þ31 40 2446653.htar).

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 3 0 8 3e1 3 0 8 813084

Shao et al. [4] developed a thermally self-sustaining micro-

SC-SOFC stack, demonstrating higher power output and rapid

start-up using propane. Their results show an approximately

double voltage and more than double the power of a single-

cell, sufficient enough to power a 1.5 V MP3 player.

Hibino et al. [6] developed a 12-cell stack powered by the

exhaust gas from amotorcycle engine. They reported that the

optimal operating conditions for SC-SOFC stack were similar

to those at the engine exhaust, therefore, no additional

measures were required to control the gas composition,

temperature and flow rate. It was also demonstrated that the

stack exhibited high tolerance towards thermal cycling and

mechanical rupture. Based on their investigation, they

reported that the cathode material should be given more

attention in improving the stack performance.

Liu et al. [7] fabricated a micro-SC-SOFC stack of two cells

employing anode-facing-cathode configuration. The stack was

operated with nitrogen diluted methaneeoxygen mixtures.

They investigated the effect of gas flow rate, mixing ratio and

furnace temperature on the performance of a single-cell and

stack. According to them, the stack performance was not only

limited by the performance of each single-cell but also affected

by the stack arrangement, especially the distance between the

cells and the electrode face configuration. In order to findmore

about the effect of distance between the cells on the stack

performance, they reported another study, giving further

suggestions in this direction [8]. For an anode-facing-cathode

configuration, it was concluded that a cell distance of 2 mm

was very critical to the performance of the stack, whereas,

a distance of 4 mm gave negligible effect on the stack perfor-

mance. It was also reported that the mixing ratio was another

influencing parameter; especially the amount of oxygen could

have an impact on the individual cell performance, which

subsequently affected the stack performance.

Suzuki et al. [9] fabricated an electrolyte supported two-cell

SC-SOFC module with different electrode areas which were

operated with propaneeair mixtures. In order to get more

voltage out of the cell, the same electrode area was utilized in

two cells and the voltage (in series configuration) became

approximately doubled to that of a single-cell of same total

area. They further reported that the open circuit voltage of

the two-cell module decreases with increase in tempera-

ture, while that of a single-cell was relatively stable. They

attributed this effect to local temperature variations and gas

flow in the measuring system.

Recently, Wei et al. [10] proposed a novel, star shape micro-

SC-SOFC stack operated with methaneeoxygen mixture. The

stack consisted of four-anode supported cells, connected in

series to power a USB-fan. According to them, the symmetrical

configuration of the cells ensures identical operation of each

cell because ofnearlyuniformgasdistribution aroundeachcell.

A study made by Liu et al. [11] demonstrated a novel, array

arranged micro-SC-SOFC stack operated with methanee

oxygenenitrogen mixture. Five cells with anode-facing-

cathode configurationwere array arranged on a ceramic board

with square holes. Their results show that amethaneeoxygen

mixture of one leads to best performance and stability of the

stack.

In another study, Liu et al. [5] investigated the perfor-

mance of an annular SC-SOFC micro-stack array operated

with nitrogen diluted methaneeoxygen mixtures. The array

consists of four cells arranged in series. They observed that

the maximum power output of the stack decreases with

increasing methaneeoxygen ratio. The increase in methane

flow rate improved the stack performance; however,

increasing nitrogen flow rate resulted in decrease of power

output. In order to increase the fuel utilization and analyze

the surplus gas, they put an additional cell behind the stack in

the gas flow direction. Although, the output power of this

additional cell was lower than the average performance of

a single-cell in the stack array; it helped in improving the fuel

utilization.

As can be seen from above, all of the research studies

considered planar configuration for stack development. To

the authors’ knowledge there is no study available on

stack development for tubular SC-SOFC configuration, besides

their demonstrated advantages over planar and co-planar

designs [3]. The present authors are the first to report on the

development and operation of amicro-tubular SC-SOFC stack.

The objective of this short communication is to show that

SC-SOFC reactor can be built and its stacking and operation is

much simpler than a conventional SOFC stack. For example,

the three individual cells can be electrically connected with

just a silver wire wrapping, and a uniform air/fuel mixture

is introduced into the gas-chamber without any need of

individual channelling for directing air and fuel separately.

Furthermore, the manifold sealing is entirely eliminated in

our design. The detailed investigation of the stack properties,

such as impedance measurements, redox testing, and inno-

vative electrical connections must have to be determined for

successful applications of such devices.

2. Experimental

The cells were made of NieYSZ/YSZ/LSM, anode/electrolyte/

cathode materials and operated with methane/air mixture.

The anode and electrolyte co-extruded micro-tubes were

purchased from Adaptive Materials Inc. (USA). The cathode

ink (for preparation methodology see Ref. [12]) was made

in-house and has the following composition: La0.5Sr0.5MnO3.

The prepared ink was painted on 40 mm length of the

as-received micro-tubes and left to dry overnight. The cells

were then sintered in a furnace up to 1150 �C to ensure well

adherence of the prepared cathode layer with the anode,

electrolyte layers. The anode electrode was exposed by

carefully removing 10 mm thin electrolyte layer. Thereafter,

silver paste was applied to the exposed anode for establishing

anode current collection and three small segments (3 mm

each) of the silver paste were applied to the cathode layer for

enhancing the current collection from the cathode side. The

silver wires were wrapped around the cells for current

collection in parallel. The active area of the single-cell was

calculated to be 2.51 cm2 based on an active cathode length of

40 mm and a micro-tube outer diameter of 2 mm.

For the preparation of stack, three single-cells were

arranged in a configuration as shown in Fig. 1. The current

collecting wire was first wrapped around the outer periphery

of joint cell structure and then an additional wrapping was

carried out through the inner pitch area as shown in Fig. 2(a).

Fig. 1 e Developed MT-SC-SOFC stacks.

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This was done in order to reduce the ohmic resistance for the

current collection through the inner surface area of the cells

that is not fully exposed to the gaseous mixture. Fig. 2(b)

shows the inner spacing between the cells. This free spacing

between the cells is important in order to ensure sufficient gas

supply in the sub-surface areas.

Fig. 2 e (a) SEM photograph showing cells arrangement

within the stack; (b) magnified view of Fig. 2(a).

The cells were characterized by measuring the I � V

curves by the following instrumentation: a glass tube

(13 mm diameter, 285 mm length) was used to function as

a gas-chamber inwhich the stackwas placed axisymmetrically

with the help of a holder. A brick furnace operated by

Eurotherm� 2402 controller was used to heat-up the gas-

chamber and maintain the required operating temperature. A

K-type thermocouple was used to measure the furnace

temperature and two Unit Instruments 7300 mass flow

controllers were used for measuring the methane/air flow

rates. The voltage and current data was measured by a poten-

tiostat which was computer programmed to manipulate the

I � V characteristic curves. The performance was measured by

fixing the current (load) and recording the cell potential. With

a step increase in current from the open circuit potential to the

short circuit current, the whole I � V curve was scanned. A

waiting time ofw5minwas realized between each consecutive

step increase in current in order to ensure equilibrium.

The gases used were methane (CP-grade), air (21% oxygen,

79% nitrogen) and pure nitrogen supplied by British Oxygen

Company (BOC). The total (methane/air) flow rate used was

85 ml min�1, unless stated otherwise.

3. Results and discussion

The three cells were first individually tested and their power

density falls within 4% (�). Such deviation of the power

density may be attributed to a slight variation in the cathode

thickness, as it was paint brushed and very hard to control the

thickness to the exact level. The stack power density was

slightly lower than the measured power density range of the

individual cells. We attribute this effect due to current

collection mechanism of the stack bundle that has an

additional wrapping of the silver wire (as discussed above),

which might reduce the pitch between the cells, and hence

insufficient oxygen supply within the pitch area (as shown

in Fig. 2(b)).

The stack was operated with methane/air mixture having

a total flow rate of 85 ml min�1. Fig. 3(aec) shows the stack

performance at three different operating furnace temperatures

of 500, 600 and 700 �C, respectively. As can be seen, for all

of the operated temperatures, the best performance was

observed at methaneeoxygen mixing ratio (Rmix) of 1.0. The

performance of the stack decays with increase in Rmix value,

and the poor performancewas observed atRmix¼ 2.0. The trend

of performance decay is in agreement with our earlier study

on single-cells; however, the stability of the stack needs to be

examined, as the single-cell performance was not stable at

Rmix¼ 1.0 [3]. The poor performance of SC-SOFCs at higher Rmix

values is reported to be due to less oxygen in the mixture. The

higher amount of oxygen in the mixture promotes methane

combustion, which raises the cell temperature and con-

sequently the cell performance is enhanced. Additionally, at

higher Rmix values, the performance loss in case of a stack (with

cathode located outside the tube) could be greater than

a single-cell performance drop. This is due to large cumulative

cathode area of the stack exposed to the open gaseous mixture

which depletes the surrounding oxidant more than the local

fuel depletion inside the individual tube.

Fig. 3 e IL V and IL P curves for the stack for different Rmix

values at (a) 500 �C, (b) 600 �C and (c) 700 �C.

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Based on our earlier study on cell temperature measure-

ments [13], we found a steep temperature gradient along the

cell length, and in full size cathode configuration with

furnace temperature of 700 �C, the measured temperature

gradient was w150 �C along the 55 mm cell length. This

shows that the cell performance is highly sensitive and

varying with respect to local cell temperature, so indeed the

cell performance is also varying along the cell length. The

variation in current density along the cell length has also

been studied via modelling [14]. The following conclusion

can be drawn:

� Either the cells should be made shorter than 55 mm length

to avoid major variation in the performance.

� Small addition of water vapour could induce steam

reforming, which could reduce these severe temperature

gradients by means of endothermic reactions.

� Highly thermal conductive materials must be employed to

take away excess heat generated from the cell andminimize

thermal overshoot.

In the present study, the highest power density of the

stack (w44 mW cm�2) was observed at an operating furnace

temperature of 600 �C, whilst the maximum open circuit

voltage (1.002 V)was obtained at 500 �C. It should be noted that

with increase in temperature, the open circuit potential

decreases due to the following reasons: 1) Gibbs free energy

decreases with increase in temperature and OCV is a ratio of

Gibbs free energy to the heating value of the fuel, 2) with

increase in operating temperature, methane parasitic

combustion on the cathode side also increases. Since the OCV

is defined as the ratio of partial pressure of oxygen at the

cathode to that at the anode, OCV is therefore decreased due

to reduced oxygen partial pressure on the cathode side.

Furthermore, as the operating temperature increases, ionic

conductivity of the electrolyte improves which enhances the

stack performance.

In case of single-cell performance (in Ref. [3]), the optimum

operating furnace temperature was found to be 750 �C,whereas, in case of three cell stack, its value was reduced to

600 �C. This decrement in temperature is due to sufficient

thermal heat produced by the stack that helped in lowering

the electrical input by the furnace. It should be noted that

the temperature increase and consequently the output

performance depend upon the total flow rate used, mixing

ratio, furnace temperature, cathode active area etc [3,13]. In the

present study, we havemeasured an increase in the single-cell

temperature of w50 �C (based on furnace temperature of

600 �C). The cumulative temperature rise in case of three cells

stack could be scaled up and falls in the range ofw120e150 �C.Therefore, with a three cells stack with an operating furnace

temperature of 600 �C, the estimated stack temperature is

w720e750 �C. It is concluded that with additional cells, the

stack might become thermally self-sustained and no electrical

input would be required. This gives an excellent opportunity to

develop a micro-tubular, SC-SOFC reactor for combined heat

and power (CHP) applications.

Although, the stack power density in the present study is

found to be lower than the other reported studies. The varia-

tion in power density among many groups is inherent due to

different manufacturing techniques (electrode/electrolyte

microstructure, thickness, sintering temperature), operating

temperature, flow rates, mixing ratio etc. It should be noted

that SC-SOFCs operated with higher flow rates could give

better performance but the efficiency and fuel utilization of

such cells will be extremely poor. It is not the only factor to

have the higher cell performance, but the fuel utilization and

cell efficiency are also of prime importance in practical

application of such devices. For detailed study on the above

subject, the reader is referred to Table 5 of Ref. [3]. It has been

found that the flow rate, operating temperature andmethane/

oxygen ratio do have a major influence in performance

Fig. 5 e Effect of inert gas flow rate on performance curves

at Rmix [ 1.0 and 600 �C.

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boost-up, but all these factors contribute to cell degradation to

some extent. For example, CH4/O2 ¼ 1.0 could result in nickel

oxidation due to higher amount of oxygen in the mixture.

Higher furnace temperature could result in thermally driven

ruptures of cathode and current collection materials. Higher

flow rates can lower the fuel utilization and electrical effi-

ciency of the cell.

Fig. 4 shows a short-term stability test of the stack at three

different mixing ratios. The current density was chosen cor-

responding to the peak power density at 600 �C for each case.

The results show that the mixing ratio has a greater influence

on the stability of the stack. For a stack of three cells, Rmix¼ 1.0

was sufficient in providing stable performance for the chosen

period. For Rmix ¼ 1.5, the stack current density was relatively

stable up to 20 h, thereafter, it starts to decrease and fluctu-

ations were observed. However, when Rmix was changed to

2.0, the performance of the stack became poor and the current

density reaches zero after only 8 h of continuous operation.

This result is in contrast with our previous study [3] on single-

cell’s stability test, where Rmix ¼ 1.0 was giving fluctuations in

current density, which is not the case here. The stack degra-

dation behaviour for different Rmix values is ascribed to the

following reasons: 1) when a lower amount of oxygen

(Rmix ¼ 2.0) was used, the cathode was reduced by the excess

methane in the mixture and the performance loss was

observed, 2) when a relatively higher amount of oxygen

(Rmix ¼ 1.5) was used, the initial degradation was slow until

oxygen deficiency appears due to sustained thermal heating,

3) with a further increase in the amount of oxygen (Rmix ¼ 1.0),

the total cathode surface area of the stack well matches with

the available amount of oxygen in the mixture and the stack

performed stably. It should be noted that each cell has its

own reactant feed inside the micro-tube supplied to the

anode, therefore, each cell’s anode functions independently.

However, the cathode feed is supplied in the gas-chamber

enclosure, the oxygen consumption on the cathode side

scales up with the total cathodic surface area. Therefore, the

mixing ratio is quite sensitive to the stable operation of

the stack, because if there is less oxygen in the mixture, the

oxygen starvation could occur on the cathode side and vice

versa. It is therefore concluded that the stability of the single-

Fig. 4 e A short-term stability test for stack current density

vs. time in hours.

cell and stack is sensitively dependent upon the chosen

mixing ratio and one should find an optimum mixing ratio

with respect to the number of cells (i.e. proportional to the

total electrode area under consideration) in operation.

Fig. 5 shows the effect of inert gas flow rate (nitrogen) on

the stack performance. The bench mark flow rate used was

85 ml min�1 (Rmix ¼ 1.0) with standard air and methane

composition. In order to examine the effect of flow rate,

nitrogen gas was added in themixture at a rate of 100, 200 and

300 ml min�1, making the total flow rate as 185, 285 and

385 ml min�1. The results show an optimum for inert gas

addition giving the best performance. The best performance

was obtained, when the total gas flow rate was increased from

85 ml min�1 to 185 ml min�1. With a further increase in the

total gas flow rate, the performance of the stack became poor.

This result suggests that the addition of inert gas helps in

improving the convective transport to a certain extent,

beyond that, the mixture becomes diluted and the perfor-

mance drops.

4. Conclusions

A micro-tubular, single-chamber solid oxide fuel cell stack

has been developed and operated in a methaneeair mixture.

A stack consisting of three cells, arranged in a triangular

configuration, was operated between 500 and 700 �C with

different mixing ratios of methane/oxygen. The results show

that the operating conditions for a stack and single-cells

differ significantly, mainly due to varying amount of oxygen

in the mixture. The best performance (w44 mW cm�2) for

a three cells stack was observed at 600 �C with methane/

oxygen ratio of 1.0. A short-term durability test shows that

the stack performance was stable at this operating condition,

while the performance was dropped to zero in about 8 h of

operation at Rmix ¼ 2.0. The inert gas flow rate is beneficial in

improving the stack performance up to a certain extent,

after which, the dilution of the reactants reduces the

performance.

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Acknowledgements

The authors would like to thank E.ON-UK and EPSRC-UK for

funding Mr. Naveed Akhtar through Dorothy Hodgkin Post-

graduate Award (DHPA) scheme.

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