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Feasibility study of an external manifold for planarintermediate-temperature solid oxide fuel cells stack

Dong Yan, Zhu Bin, Dawei Fang, Jun Luo, Xiaopeng Wang, Jian Pu*, Bo Chi, Li Jian,Yisheng Zhang

School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University

of Science & Technology, Wuhan, Hubei 430074, PR China

a r t i c l e i n f o

Article history:

Received 5 March 2012

Received in revised form

3 June 2012

Accepted 5 June 2012

Available online 3 August 2012

Keywords:

Solid oxide fuel cell

External manifold

Stack

Degradation

Computer simulation

* Corresponding author. Tel./fax: þ86 27 8755E-mail address: pujian@hust.edu.cn (J. Pu

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2012.06.0

a b s t r a c t

A 3-cell stack of anode supported planar solid oxide fuel cell was built to evaluate the

application of an external-manifold design in this research. This short stack was operated

with hydrogen as fuel and air as oxidant at 750 �C. The stack had an OCV of 3.36 V,

produced about 100 W in total power with a power density of 0.56 W/cm2. The stack also

underwent 51 h degradation test at the current density of 0.55 A/cm2. The test results have

demonstrated that this external-manifold stack had an excellent and steady performance

during the test. Computer simulation was employed to help optimizing the parameters of

the design and explaining the different performances between the cells. The simulation

results suggested that the external-manifold design could generate a uniform gas distri-

bution for a short stack, and the different performances of the individual cells were mainly

caused by the uneven temperatures distribution between the cells.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction operation. To overcome these technology difficulties and

The advantage of the combined electrical and waste heat

keeps the study of solid oxide fuel cell (SOFC) more attractive

than that of the low temperature fuel cell such as proton

exchange membrane fuel cell (PEMFC) and phosphoric acid

fuel cell (PAFC). The high power density will also make the

SOFC cheaper, more compact and feasible than the molten

carbonate fuel cells (MCFC). A planar SOFC operated in the

range of 650e750 �C is even more attractive than the high

temperature SOFC operated under 900e1100 �C due to the low

cost of themetallic interconnect design. Hence, planar SOFC is

considered as the next generation of renewable and environ-

mental friendly energy device. However, the commercial

application of SOFC requires the solutions of several draw-

backs including high cost and low reliability in long-term

8142.).2012, Hydrogen Energy P20

support the industry efforts, several researches have focused

on such fields [1e6].

In a typical planar SOFC, a single cell with 81 cm2 active

reaction areahas the capability ofmaximumOCV (open circuit

voltage) of 1.2 V and power of 80 W at the current density of

around 1 A/cm2 [1,7e10]. Cells are connected by interconnects

in series to obtain a higher power output for a stack. There are

different systems to feed the fuel and oxidant onto the two

sides of cells in the stack. The SOFC stack can be divided into

several types: external-manifold system [11], internal-mani-

fold system [12], disk type [13] and envelope type [14], etc. The

most commonly used type is the internal-manifold design that

involves the gas distribution channels within the stacked

interconnect plates. Comparatively, the external-manifold

design uses separated manifolds to distribute the inlet and

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

Fig. 1 e Schematic diagram of metallic corrugated current

collector in cathode side.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 6 6 0e6 6 6 661

outlet gases andhence could reduce the cost of building a stack

by greatly simplify the interconnect design and minimize the

gas leak by reducing the sealing length. The most important

advantage of the external-manifold design is that the mani-

folds are thermally decoupled from the stack, hence the

temperature difference between the inlet and outlet of the

stack can be minimized. However, not much work has been

done by using the external-manifold design in SOFC. Chung

et al. [11] has built an external stackwith only power density of

0.22 W/cm2 and a very limited lifetime. Ma et al. [15] also

studied the computer modeling of gas flow distribution and

heat transfer for the external-manifold stack.

In themeantime, thenumericalmethod is alsonecessary to

help designing the external SOFC stack and system.Models are

built in computers to investigate the electrical, mechanical,

thermal and fluid dynamic properties of the stack. Those

studies can optimize the design before the stack is actually

assembled. However, there are uncertain factors such as flat-

ness and thickness of the cell, and pinholes in the electrolyte,

gas leaking of the sealant etc. in a real stack, which will make

the computer simulation deviated from the virtual state of

stack. Thus, it is necessary tomodify the theoretical computer

calculation by combining with designed experiments.

In the recent works, individual cell performance in the

stack has been investigated. Cell voltages and temperatures

were measured by probes and different computer models

were built to predict the temperature and gas distribution

inside the stack [16,17]. Those papers are mainly discussing

the non-homogeneous state in a short external-manifold

stack. This numerical study of evaluating the gas and

temperature distribution is based on our previous works of

having successfully demonstrated the planar anode sup-

ported SOFC single cell tests.

Fig. 2 e Schematic view of the external-manifold stack

design.

2. Design for external-manifold stack

2.1. Stack concept with major components

In this study, an integrated interconnect plate made of

stainless steel (SUS 430) is developed for the stack assembly of

which the schematic diagram for the corrugated current

collector for the cathode side is shown as in Fig. 1. There are

two strips at the counter edges of the square plate form the

flow channel. The sealants we used in the stack are Al2O3

based ceramic tapes developed in our previouswork [18]. They

are cut to form the same shape like the strips and placed onto

the two edges of the interconnect plate. The gas channel at

cathode side is formed by the corrugation plate as shown in

Fig. 1. On the other hand, the plate also serves as the current

collectors. The thickness of the sealing tape is carefully

calculated to ensure a precise match with the corrugation

plate. A cross-flow configuration is used in this external-

manifold stack design, in which the flow channel of the anode

side is perpendicular to that of the cathode side. Nickel foam is

used as gas distributor and current collector in the anode side.

Schematic diagram of the external-manifold SOFC stack is

shown in Fig. 2. Each repeated unit of the stack includes

a metallic interconnect, sealing tapes, nickel foam, cell and

the corrugation plate. The top and end plates are thicker than

the interconnect plate to provide a reliable pressure for the

stack. In order to obtain the individual voltage of each cell in

the stack, stainless steel wires were welded to the intercon-

nect plates, and they were led out through the holes drilled on

the air outlet manifold.

The cellsweused in the stack areYSZ-NiOanode supported

planarSOFCwith squaredshape. Theanode function layer, the

10 mm thick YSZ electrolyte and the cathode with 5 mm thick

were screen printed onto the tape-casted anode support

sequentially and co-fired to complete the formation of cell.

Anode supported cells with 10 � 10 cm square shape were

chosen for assembling the stack which has been described in

detail in previous works [8]. Those cells all performed steady

outputs of over 1.1 V OCV and over 0.7W/cm2 power density at

operation temperature of 750 �C at single cell tests.

2.2. Mathematical model of stack and basicassumptions

The flow distribution is crucial to the stack performance, and

it is also very difficult to measure in actual stack test.

Computer simulation will be helpful to analyze and help the

flow distribution in the stack design. Therefore a stack model

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for computer simulation was built. To simplify the calcula-

tion, some assumptions are made as follows:

1) The gas flow is in turbulence mode;

2) The system has reached a steady state;

3) The temperature is at constant of 750 �C;4) The gas is uncompressible.

The boundary conditions of the flow are listed as follows.

1) The flow speed at the entrance of the flow field V ¼ Q/S,

while Q is the quality of the flow, and S is the cross-

sectional area of the entrance.

2) The pressure at the outlet equals to standard atmosphere

because it exits to open air environment.

3) The flow speed on the surface of the walls is zero.

The control equations including

1) Mass conservation equation:

vr

vtþ vðruÞ

vxþ vðrvÞ

vyþ vðrwÞ

vz¼ 0 (1)

where r is the density of the fluid, u, v, w are the speed

components on x, y, z directions respectively.

2) Momentum conservation equation:

vðruÞvt

þ divðruuÞ ¼ �vpvx

þ vsxxvx

þ vsyxvy

þ vszxvz

þ Fx (2)

vðrvÞvt

þ divðrvuÞ ¼ �vp

vyþ vsxy

vxþ vsyy

vyþ vszy

vzþ Fy (3)

vðrwÞvt

þ divðrwuÞ ¼ �vpvx

þ vsxzvx

þ vsyzvy

þ vszzvz

þ Fz (4)

P is the pressure on the fluid element, s is the viscous stress

components on the surface of the element, Fx, Fy, Fz are the

force on the element.

3) Energy conservation equation:

vðrTÞvt

þ divðruTÞ ¼ div

�kcp

gradT

�þ Sr (5)

cp is specific heat capacity, T is the temperature, k is the

coefficient of heat transfer of the fluid, Sr is the viscous

dissipation.

A simplified model is used to calculate the temperature

distribution of individual cells of the 3-cell stack used in the

present study. The module is based on a full size 3-cell stack

core without manifolds. The current collectors are simplified

to be machined rib design with straight gas channels. Heat

removed by the fuel and air are not considered in this case.

Some assumptions are also made as boundary conditions:

1) The cells are considered as a homogeneous heat source,

and the heat generation S is calculated by the function of

S ¼ i

�DHnF

� Ucell

�(6)

i is the current density,DH is the enthalpy changes of the SOFC

reaction, Ucell is the voltage of a single cell.

The stack is considered to be operated at the current

density of 0.25 A/cm2 and the voltage of each cell is assumed

to be 0.9 V.

2) The environment temperature is set to be a constant of

750 �C, the stack heat removed by the environment goes

only through the end plates of the stack core by heat

convection, and the convection does not happen on the

side faces of the stack.

The control equations including

v2Tvx2

þ v2Tvy2

þ v2Tvy2

þ qv

l¼ 0 (7)

1) The heat conduction equation

where qv is the strength of internal heat source and l is the

coefficient of thermal conductivity.

2) The convective equation

F ¼ hA�Tw � Tf

�(8)

The convective heat transfer on the end surface of the stack

can be described by the simple equation of Newton’s law of

cooling. The h is the coefficient of convective heat transfer,

and the A is the heat transfer area.

3. Experimental

The stack was heated to 750 �C in the furnace. During the

start-up heating, the stack was first fed with mixed gases of

nitrogen and 4 vol.% of hydrogen. When the temperature

reached 750 �C, the pure hydrogen was fed into the anode

channel. The cathode side was fed with air during the testing.

The current and voltage of the stack were controlled and

measured by the SOFC testing station (developed by Lisun

Corp. and SOFC R & D in HUST). The stack was tested by

measuring the power curve under current passage ranging

from 0 to 65 A. Then the current was kept at a steady state to

evaluate the degradation performance of the stack with lapse

time. After the stack durability test, another IeV curve testing

was performed to observe the variation that was caused by

constant current. A resistance instrument was applied to

measure the ohmic resistance of the stack.

4. Results and discussion

4.1. Performance and durability 3-cell stacks

In our previous work, the performance and durability of the

single cell used in the stack has been evaluated at different

temperatures [8]. Fig. 3 shows that the test result of the stack

is similar with that of the single cell. During the test, the open

circuit voltage (OCV) of the stack has reached its highest peak

Fig. 3 e Current densityevoltageepower profiles of 3-cell

stack (8 3 8 cm2 active area per cell) at 750 �C before and

after 51 h aging.Fig. 4 e Degradation of 3-cell stack under 550 mA/cm2

constant load at 750 �C.

Fig. 5 e SEM micrograph of the cross sectioned single cell

used in 3-cell stack.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 6 6 0e6 6 6 663

of 3.36 V and the average cell voltage is quite close to the value

of 1.15 V in the single cell testing [8]. According to Nernst

equation, the OCV only depends on the gas concentration of

hydrogen and oxygen in the electrode sides at a certain

temperature, so it can be concluded that the sealing materials

applied on the stack has successfully prevented gas leakage

inside the stack. Hence, it is reasonable to say that the

external manifold with ceramic-based sealant can achieve

the gas tight requirement of a SOFC stack. It is also noted that

the OCV of stack is slightly increased by 0.04 V after 51 h test,

which indicated that the operating load had not damage the

integrality of the thin dense electrolyte layer of cell. Moreover,

after being exposed in the reducing atmosphere for a long

time, the NiO of the anode layer had been further reduced into

metallic Ni in a slowly process. The reduction process

increased the porosity of the anode layer, and improved the

hydrogen diffusion rate. The anode functional layer tookmore

time to complete the NiO reduction than the anode support

layer due to the microstructure difference between them.

The area specific resistance (ASR) measured in the stack

remain unchanged at 0.45 U cm2 after 51 h test. In general, the

electrical resistance of stack is considered to have originated

from cell conductivity, interconnect oxidation and the

interconnect-cell contact. The ASR per cell in the stack was

higher than that of the single cell as we mentioned in our

previous work [8]. And the stack with cells of 8 cm � 8 cm

active reaction area obtained a peak power density of over

560 mW/cm2 and power output of 107W at 750 �C. Comparing

with the peak power density of 770 mW/cm2 of single cell, the

stack only reached 70% capability of the single cell perfor-

mance. The increase of the ASR can be mainly attributed to

the uneven contact between cell and the adjacent compo-

nents in stack, in which the corrugation plate is used as the

cathode current collector rather than the machined rigid jig

used in the single cell test. Similar observations are also re-

ported by other research groups [19].

Fig. 4 shows the result of 51 h operation test of 3-cell stack.

Output voltage was continuously measured at current density

of 0.55 A/cm2. Stack voltage showed an initial improvement

from 2.28 V to 2.40 V during the first 19 h, then slowly decrease

to 2.31 V at the end of the test. One possible reason of this

small degradation can be the impact of voltage losses

according to Equation (9)

V ¼ E� h� RJ (9)

where V is the output voltage, E is equilibrium voltage such as

OCV, h is the polarization loss in the respective electrodes, R is

the total resistance from the cell, interconnect and contact

and so on, J is the current density. Fig. 5 shows the cell

microstructures of cross section from the stackwith the dense

YSZ layer, the porous cathode and the anode layer. It can be

observed that the thin YSZ layer is dense, without pinhole and

well bonded to the porous Ni-YSZ substrate. This observation

confirmed that a good electrochemical reaction was per-

formed in the stack test. In Fig. 3, after the 51 h test, the stack

Table 1 e ASR comparison of individual cells in the 3-cellstack shows that the middle cell had a lower resistance.

Top cell Middle cell Bottom cell

0.484 U cm2 0.420 U cm2 0.490 U cm2

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 8 ( 2 0 1 3 ) 6 6 0e6 6 6664

IeV curve is slightly higher than that of before the 51 h test.

Such phenomenon indicates that there are unchanged ohmic

resistance and activation polarization of the electrodes during

the stack test. Therefore, the stack sealing is the major factor

that if it will have degradation in the stack. In general, the

stack degradation is the combined effect of the reduction of

the anode and the sealant leakage. However, more work will

be done in the future to determine the actual reason of the

degradation and improve the prototype stack.

The voltages and powers of each individual cell in the 3-cell

stack are plotted in Fig. 6. The overlapped curves of the top

and the bottom cell show that the performances are quite

similar with those two cells. The voltage of the middle cell is

higher than that the other two. The different ASRs of the three

cells calculated from the curves are listed in Table 1.

It is quite clear that the middle cell has much lower ASR

than the other two cells. There might be several reasons for

the ASR differences: 1) the difference of cell structure. A

thicker electrolyte or cathode layer could increase the ohmic

resistance, or the different microstructures of electrode will

result different polarization and concentration resistance [20];

2) the difference of operating environments. A higher

temperature could reduce the ohmic resistance by improving

the conductivity of electrolyte and reduce the polarization

resistance by increasing the activity of the chemical reactions

[21]. Moreover, higher of partial pressure of oxygen can

increase the electromotive force and results in relatively

higher voltage [7,8]. In this study, as the fabrication processes

of the cells are consistent and stable, so there is no major

microstructure variation between different batches of the

cells. Hence, it ismore reasonable to attribute the difference of

cell voltages to the gases composition, temperature and

contact resistance in the stack.

However, it is difficult to measure the temperature and

partial pressure of each cell because the stack core is compact

andwith corrugated gas flow channel. It is quite a challenge in

the future to mount sensors onto the cell surfaces. In order to

demonstrate the relationship between some inconvenient

measure parameters and cell performance, computer simu-

lation was applied to conjecture gas concentration and

temperature distribution in stack.

Fig. 6 e The separated voltage of each cell in the 3-cell

stack.

4.2. Computer simulation for flow field of stack

Gas flow in the stack has been investigated by computer

simulation. Fig. 7 shows the flow speed distribution in the

corrugated flow field on the surface of electrodes. The gas is

considered to go through the field along the channels of the

corrugation. The drawing reveals that when the cross-

sectional area narrows at the interface of two rows of stag-

gered arranged walls, the gas will be sped up, and that is

marked with red color in the figure. A detailed plotting about

the speed vector in this area is shown in Fig. 8. According to

the boundary condition, the gas speed at the surface of the

barrier is quite slow. Behind the barrier, there is gas detained

Fig. 7 e Flow speed distribution in the cathode side of

metallic interconnect. (a) The air flows perpendicular to the

interconnect channel; (b) the air flows along the

interconnect channel.

Fig. 8 e Speed vector distribution in the cathode side of

metallic interconnect. Fig. 10 e Temperature distribution in the stack with

corrugation used as interconnect components.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 6 6 0e6 6 6 665

area where the flow speed is nearly zero. However, because

there is a high-speed area nearby, the gas in the detained area

is soon sped up by the fluid friction force. At the edge of the

two sides of the flow field, the flow speed is much lower and

marked as green and blue. According to some spatially

resolved studies of fuel cell with large active area, the scale-up

size of fuel cell could lead to the mal-distribution of the fuel

gas and electromotive force [8,22e25]. In general, CFD simu-

lation of the corrugated flow field shows that the flow can

maintain a relative uniform and highly turbulent flow

throughout the flow field, as shown in Fig. 7.

For the same considering, the gas distribution efficiency of

the manifold had been investigated by CFD. A full size model

of inlet-manifold chamber of a 5-cell stack with corrugated

distributor was built. Flow velocity map was shown in Fig. 9.

Although the flow has quite a high speed at the only entrance

in the manifold, the chamber of the manifold is large enough

to act as a buffer and slows down the flowwhen it convolutes.

At the entrances to the cell surface, the velocity has been

reduced by an order of magnitude comparing to that of the

inlet, and the velocity differences between layers are not

significant enough to influent the cell performance.

The temperature distribution in the whole stack core is

shown in Fig. 10. Assuming the electrochemical reaction as

Fig. 9 e Flow velocity drawing of the manifold and the

cathode side of interconnect channel.

the only heat source in the core, the cell may be heated to

nearly 800 �C. The temperature dropped gradually to the

environment temperature of about 750 �C at the end surfaces

of the stack. The temperature of the cell in themiddle is about

10 �C higher than the other two.

This is a rough calculation that did not consider the heat

being taken out by the fuel and air flowing through the

corrugated channels. But as the previous CFD calculation

shows that the gas distribution between the cells is even on

thewhole, we can speculate that equal heat is taken out by the

flows. Thus, we can assume the gas distribution would not

change the temperature differences between the cells. Such

a result is been reported by another research group that

developed on the internal-manifold stack [26]. Another

research also revealed that non-uniformity in gas distribution

does not have significant influence on the total stack voltage

[27]. Therefore, the voltage differences of cell in stack may be

reasonably explained from the actual operation temperature

of cell.

5. Conclusions

An external-manifold SOFC stack using a simple intercon-

nect with corrugation plate was designed and fabricated. A 3-

cell stack was assembled and its performance was evaluated

at operating temperature of 750 �C. The stack generated an

OCV of 3.36 V and power of 107 W at current density of

0.94 A/cm2 and voltages of individual cell were measured in

the stack. The successful run of stack with high performance

and low degradation demonstrates the feasible route of the

future SOFC stack with external-manifold design. Computer

modeling and calculation method was introduced to inves-

tigate the stack influenced by gas and temperature distribu-

tion. The calculation result indicates that external-manifold

SOFC stack can evenly distribute gases on the whole for

a short stack, but uneven temperature distribution in the

stack may cause the performance difference of individual

cell. The performance differences of individual cells were

caused by un-uniform distributed temperatures among

them.

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 8 ( 2 0 1 3 ) 6 6 0e6 6 6666

Acknowledgments

This research was financially supported by the National

Science Foundation of China under the project contract

U1134001, the “863” high-tech project under contract

2011AA050702. SEM and XRD analysis were assisted by the

Analytical and Testing Center of Huazhong University of

Science and Technology, and Dr Fan Jianzhong for helpful

discussions on stack design and test results analysis.

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