international journal of hydrogen energy 34 (2009) 453–458
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Optimized Cr-nitride film on 316L stainless steel as proton
exchange membrane fuel cell bipolar plate
Yu Fua,c,d, Guoqiang Linb, Ming Houc,*, Bo Wub, Hongkai Lib, Lixing Haoc,
Zhigang Shaoc, Baolian Yic
a
Dalian Sunrise Power Co., LTD, Dalian 116025, China
Dalian University of Technology, Dalian 116024, China
c
Fuel Cell Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
d
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
b
article info
abstract
Article history:
Pulsed bias arc ion plating was used to form Cr-nitride films on stainless steel as bipolar
Received 3 June 2008
plate of proton exchange membrane fuel cell. Surface micrograph, film thickness, film
Received in revised form
composition, corrosion resistance, interfacial conductivity and contact angle with water of
12 September 2008
the sample obtained at the optimal flow rate of N2 were investigated. The atomic ratio of Cr
Accepted 26 September 2008
to N was close to 2:1 and the CrN phase with crystal planes of (111), (200), (220) and (311)
Available online 21 November 2008
was found in the film. Potentiodynamic and potentiostatic tests showed that the corrosion
resistance of the bipolar plate sample was greatly enhanced. The contact resistance
Keywords:
between the bipolar plate sample and Toray carbon paper was about two orders of
Metal bipolar plate
magnitude lower than that of 316L stainless steel. The contact angle of the sample with
Pulsed bias arc ion plating
water was 95 , which is beneficial for water management in fuel cells.
Flow rate of N2
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
reserved.
1.
Introduction
Bipolar plate is an important component of proton exchange
membrane fuel cell (PEMFC), and it accounts for most of the total
weight. The bipolar plate serves as the functions of distributing
reactants uniformly over the active areas, removing heat from
the active areas, collecting currents from cells, preventing
leakage of reactants and coolant. An ideal bipolar plate material
should have the characteristics as follows: (1) high corrosion
resistance in PEMFC environment; (2) low interfacial contact
resistance; (3) big contact angle with water; (4) lightweight; (5)
high mechanical strength; and (6) cost-effective etc.
Forming a film with good corrosion resistance and high interfacial conductivity on stainless steel is one of the possible solutions. Metal nitrides are the promising surface modification
materials for metal bipolar plate owing to their good corrosion
resistance and high conductivity. Most of the researches have been
focused on Cr-nitrides in recent years [1–10]. In our previous work
[11], Cr-nitride gradient films were coated on 316L stainless steel by
pulsed bias arc ion plating (PBAIP) as bipolar plate for PEMFC. The
composition of film varied with the flow rates of N2. Films obtained
at N2 gas flow rates from 25 to 100 sccm exhibited high interfacial
conductivity, good corrosion resistance and big contact angle,
which showed great potential in the application of PEMFC.
The aim of the present work was to optimize the flow rate of
N2 and obtain bipolar plate with better performance. Surface
micrograph, film thickness, film composition, corrosion resistance, interfacial conductivity and contact angle with water of
the bipolar plate sample prepared at the optimal flow rate of N2
were investigated.
* Corresponding author. Tel.: þ86 411 84379051; fax: þ86 411 84379185.
E-mail address: [email protected] (M. Hou).
0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2008.09.104
454
international journal of hydrogen energy 34 (2009) 453–458
Table 1 – Different flow rates of N2 applied in this study.
Code
N2 flow rate (sccm)
2.
1
2
3
4
5
6
7
8
100
75
50
25
20
15
10
5
Experimental
The 316L stainless steel (bright annealed) with the size of
100 100 0.1 mm3 was chosen as the base metal of bipolar
plate. Details of forming the Cr-nitride films are described in
our previous paper [10]. By changing the flow rate of N2, film
with different composition could be obtained. However, the
total pressure in the chamber, which is a sum of the N2
pressure and the Ar pressure, was kept constant at
5.0 103 Pa. The flow rates of N2 conducted in this study are
listed in Table 1. In this study, duration of the treatment for all
the samples was 100 min. And the temperature of the samples
during the treatment was round about 200 C.
Surface topography of the bipolar plate sample was performed with VK-8550 Super Depth Surface Profile Measurement Microscope. Thickness of the film was also measured by
protecting some area of the substrate. X-ray energy dispersive
spectrometer was used to characterize the composition of the
film and the phase was identified by X-ray diffraction. The
corrosion characteristics of the bipolar plate samples were
analyzed by potentiodynamic and potentiostatic tests in 0.5 M
H2SO4 þ 5 ppm F solution. The tests were performed at 25 C
and 70 C. The corrosion solution was bubbled with either air
(simulating the PEMFC cathodic environment) or hydrogen
gas (simulating the PEMFC anodic environment) prior to and
during the electrochemical measurements. The initial contact
resistances between the bipolar plate sample and bare TGP-H060 Toray carbon paper were also measured. During the tests,
the compacting force was increased with a step of 5 N s1. In
addition, the contact angles of the samples with water were
measured by a JC2000A Contact Angle Measurement system
for investigating the surface energy.
Fig. 1 – Passive current densities in 0.5 M H2SO4 D 5 ppm
FL solution at 25 8C and contact resistances with carbon
paper under 1.2 MPa of the samples prepared with
different flow rates of N2 by PBAIP.
Fig. 2 – The image of 1000X micrograph of the film obtained
at 20 sccm N2 flow on 316L stainless steel.
3.
Results and discussion
3.1.
Optimization of the flow rate of N2
The corrosion resistance and interfacial conductivity are the
two main properties of metal bipolar plate materials. The
parameters of passive current density (the current density
under passivation potential) and interfacial contact resistance
were used to evaluate theses properties. All the bipolar plate
samples were prepared at different flow rates of N2 by PBAIP.
Passive current density in 0.5 M H2SO4 þ 5 ppm F solution at
25 C and initial contact resistance with carbon paper under
1.2 MPa of all the samples are shown in Fig. 1. The corrosion
resistance of the samples increased with the flow rate of N2
and came to a head at 20–25 sccm. Then the corrosion resistance of the samples decreased with the flow rate of N2. As for
the interfacial contact resistance, it decreased with the
increment of N2 flow rate when the flow rates of N2 was lower
than 20 sccm; then it increased evidently to a high level at
Fig. 3 – The image of 1000X micrograph of the bare 316L
stainless steel.
international journal of hydrogen energy 34 (2009) 453–458
25 sccm; when the flow rates of N2 was over 50 sccm, the ICR
stayed a relative low level. As a whole, the sample obtained at
20 sccm N2 exhibited the best corrosion resistance and the
highest interfacial conductivity simultaneously. Other characteristics of the bipolar plate sample obtained at the optimal
flow rate were analyzed in detail as follows.
3.2.
Characterization of the film
Fig. 2 shows the micrograph of the sample prepared at
20 sccm nitrogen flow. The picture shows a uniform, homogeneous film with regular stripes and a few protuberances on
the 316L stainless steel base metal. These regular stripes were
induced by the base metal which could also be seen on the
surface of the untreated 316L stainless steel (Fig. 3). Although
455
the pulsed bias arc is helpful to reduce droplets on the surface,
there are still minor quantities of Cr droplets on the film.
During the process of forming the film, N2 can also react with
the Cr droplets. Thus the protuberances in the film are a kind
of defects but not pinholes, so the influence of protuberances
can be negligible.
Before preparing the film, some area of the substrate was
protected. After the film was formed, the thickness difference
between the coated area and the uncoated area could be
considered as the thickness of the film. Usually, the
‘‘Maximum depth’’ in Fig. 4 is regarded as thickness of the
film. Thickness of the films obtained at the optimal N2 flow
rate at three different points are 0.768 mm, 0.81 mm and
0.81 mm, respectively. So the average thickness of the film
obtained at 20 sccm N2 was 0.80 mm.
Fig. 4 – Thickness of the film obtained at 20 sccm N2 flow.
456
international journal of hydrogen energy 34 (2009) 453–458
Table 2 – Elementary composition of the film obtained at
20 sccm N2 on pure Fe plate.
Element
NK
Cr K
Wt%
At%
11.8
88.2
33.2
66.8
To eliminate the effect of the Cr in 316L stainless steel, pure
Fe was used as the base metal for composition analysis of the
film. X-ray energy dispersive spectrometer analysis of the film
obtained at 20 sccm N2 flow on pure Fe plate shows that the
final composition of the film was 11.8 wt.% N and 88.2 wt.% Cr,
and the atomic ratio of Cr to N is very close to 2:1 (Table 2).
Further analysis by X-ray diffraction (Fig. 5) indicates that
there was CrN phase with crystal planes of (111), (200), (220)
and (311) on the film. As we know, the coating material
obtained by PBAIP is of metastable state. So the film obtained
at 20 sccm N2 might be with CrN structure, which is a kind of
structure with high chemical stability; what is more, there is
almost one more Cr atom in the face-centered cubic CrN and
the additional Cr enhances the conductivity greatly.
3.3.
Corrosion resistance
Potentiodynamic
tests
were
conducted
in
0.5 M
H2SO4 þ 5 ppm F solution bubbled with air at 70 C to investigate the corrosion resistance of the bipolar plate sample. The
two potentiodynamic curves of the bipolar plate sample and
316L stainless steel are shown in Fig. 6. From the slopes of the
anodic curve and the cathodic curve, the corresponding
corrosion current can be determined. The corrosion current
density of the bipolar plate sample was about 106 A cm2. As
the applied potential increases from 0 to 0.75 V versus SCE, the
corrosion current increased from 106 to 105 A cm2 accordingly. As for the 316L stainless steel, the corrosion current
density was about 103.5 A cm2, over two orders of magnitude
higher than that of the coated sample. What is more, it could
be passivated spontaneously under the test conditions and
the passive current density of 316L stainless steel was
Fig. 5 – Displays the X-ray diffraction pattern of the film
obtained at 20 sccm N2 flow on Fe.
Fig. 6 – Potentiodynamic curves of the bipolar plate
samples in 0.5 M H2SO4 D 5 ppm FL with a scan rate of
2 mV sL1 at 70 8C bubbled with air.
104.5 A cm2. The potentiodynamic curves of the bipolar plate
sample and 316L stainless steel in 0.5 M H2SO4 þ 5 ppm F
solution at 70 C bubbled with H2 are shown in Fig. 7. The
bipolar plate sample was in passive state under test condition
with a passive current density of 105 A cm2, which shows an
obvious anticorrosion characteristic compared with 316L
stainless steel.
In order to study the corrosion behaviors of the bipolar
plate sample in the actual working conditions of PEMFC,
potentiostatic tests were conducted at 0.6 V versus SCE
bubbled with air to simulate the cathode working condition
and at 0.1 V versus SCE bubbled with H2 to simulate the
anode working condition. In the simulated cathode condition
(Fig. 8), the current density of the sample with Cr-nitride film
obtained at 20 sccm N2 stabilized within 3 min and stayed in
the range of 107.0–108.0 A cm2. The current density of the
Fig. 7 – Potentiodynamic curves of the bipolar plate
samples in 0.5 M H2SO4 D 5 ppm FL with a scan rate of
2 mV sL1 at 70 8C bubbled with H2.
international journal of hydrogen energy 34 (2009) 453–458
Fig. 8 – Potentiostatic curves of the bipolar plate samples at
0.6 V versus SCE in 0.5 M H2SO4 D 5 ppm FL at 70 8C
bubbled with air.
316L stainless steel stabilized after 15 min immerging and
varied in the range of 105.0–106.0 A cm2, which is near one
order of magnitude higher than that of the coated sample. As
for the tests in simulated anode condition, results are shown
in Fig. 9. The current densities of the coated sample and 316L
stainless steel were higher than that performed in the simulated cathode condition. But the current density of coated
sample was also about one order of magnitude lower than that
of 316L stainless steel.
As a whole, the bipolar plate with Cr-nitride film obtained
at 20 sccm N2 exhibited higher anticorrosion behavior
compared with the 316L stainless steel.
3.4.
Contact resistance
Initial interfacial contact resistance of the bipolar plate
sample performed at 20 sccm N2 with Toray carbon paper is
Fig. 9 – Potentiostatic curves of the bipolar plate samples
at L0.1 V versus SCE in 0.5 M H2SO4 D 5 ppm FL at 70 8C
bubbled with H2.
457
Fig. 10 – Interfacial contact resistance of the bipolar plate
sample obtained at 20 sccm N2 with Toray carbon paper.
shown in Fig. 10. The value of contact resistance decreased
with the increasing compact pressure. But the coated sample
exhibited excellent interfacial conductivity in all the range of
compact pressures. The contact resistance between the
coated bipolar plate sample and Toray carbon paper is about
two orders of magnitude lower than that of 316L stainless
steel. The interfacial contact resistance between the bipolar
plate sample and carbon paper is in the range of
8.4–11.8 mU cm2 under the common compacting pressure of
0.8–1.2 MPa in PEMFC. The higher output power could be
obtained with the lower interfacial contact resistance. So the
Cr-nitride film obtained at 20 sccm N2 is helpful to improve the
output power.
3.5.
Contact angle
The contact angles of the bipolar plate sample and untreated
316L stainless steel with water are shown in Fig. 11 and Fig. 12,
respectively. Obviously, the coated sample has a bigger
contact angle (95 ) than 316L stainless steel (73 ). As it is
Fig. 11 – Initial contact angle of the bipolar plate sample
obtained at 20 sccm N2 with water.
458
international journal of hydrogen energy 34 (2009) 453–458
the sample with water was 95 , which is beneficial for water
management in fuel cell.
Coated 316L stainless steel plate with Cr-nitride film at
20 sccm N2 shows good corrosion resistance, high interfacial
conductivity and well hydrophobic characteristic, so it
exhibits great promising in the application of PEMFC. Durability and performance of this kind of bipolar plate material in
actual PEMFC environment is to be studied in the future.
Acknowledgments
This work was financially supported by the National High
Technology Research and Development Program of China (863
Program, No. 2007AA03Z221).
Fig. 12 – Contact angle of the bare 316L stainless steel with
water.
known, the process in fuel cell is always accompanied with
water. To prevent the proton exchange membrane from
dehydration, the inlet gases need usually to be humidified. In
addition, water generated due to the reduction reaction of
oxygen in the fuel cell stack also accounts for the hydration of
the membrane. So the bipolar plates are often contacted with
the mixture of reactant gas and water. Water would block the
reactant gases from accessing to the electrode if the liquid
water could not be removed in time. The accumulated water
induces the electrode flooding phenomenon. Furthermore,
the water adhering on the surface of bipolar plate could
accelerate up the corrosion of metal bipolar plate. In addition,
the bipolar plate with well hydrophobic characteristic is
helpful for enhancing heat exchange between coolant and the
cell. By considering these reasons, the coated bipolar plate
with higher hydrophobic characteristic is helpful for water
removal in the stack and beneficial to the water management.
4.
Conclusions
Different compositions of Cr-nitride films could be obtained
by changing the flow rate of N2 during PBAIP process. The
bipolar plate sample produced at 20 sccm N2 exhibited the
best corrosion resistance and the highest interfacial conductivity simultaneously.
In the film obtained at the optimal flow rate of N2, the
atomic ratio of Cr to N was very close to 2:1 and CrN phase
with crystal planes of (111), (200), (220) and (311) was found.
Potentiodynamic and potentiostatic tests performed at
70 C bubbled with either hydrogen gas or air showed that the
corrosion resistance of bipolar plate sample was greatly
enhanced compared with 316L stainless steel. The initial
contact resistance between the bipolar plate sample and
Toray carbon paper was 8.4–11.8 mU cm2 under 0.8–1.2 MPa,
which is about two orders of magnitude lower than that of the
untreated 316L stainless steel. In addition, the contact angle of
references
[1] Bertrand G, Mahdjoub H, Meunier C. A study of the corrosion
behaviour and protective quality of sputtered chromium
nitride coatings. Surf Coat Technol 2000;126(2–3):199–209.
[2] Brady MP, Weisbrod K, Paulauskas I, Buchanan RA, More KL,
Wang H, et al. Preferential thermal nitridation to form pinhole free Cr-nitrides to protect proton exchange membrane
fuel cell metallic bipolar plates. Scripta Mater 2004;50(7):
1017–22.
[3] Paulauskas IE, Brady MP, Meyer III HM, Buchanan RA,
Walker LR. Corrosion behavior of CrN, Cr2N and p phase
surfaces on nitrided Ni-50Cr for proton exchange membrane
fuel cell bipolar plates. Corros Sci 2006;48(10):3157–71.
[4] Wang H, Brady MP, More KL, Meyer III HM, Turner JA.
Thermally nitrided stainless steels for polymer electrolyte
membrane fuel cell bipolar plates: part 2. Beneficial
modification of passive layer on AISI446. J Power Sources
2004;138(1–2):79–85.
[5] Wang H, Brady MP, Teeter G, Turner JA. Thermally nitrided
stainless steels for polymer electrolyte membrane fuel cell
bipolar plates: part 1: model Ni-50Cr and austenitic 349
alloys. J Power Sources 2004;138(1–2):86–93.
[6] Brady MP, Wang H, Yang B, Turner JA, Bordignon M,
Molins R, et al. Growth of Cr-nitrides on commercial Ni-Cr
and Fe-Cr base alloys to protect PEMFC bipolar plates. Int J
Hydrogen Energy 2007;32(16):3778–88.
[7] Tian RJ, Sun JC, Wang L. Effect of plasma nitriding on
behavior of austenitic stainless steel 304L bipolar plate in
proton exchange membrane fuel cell. J Power Sources 2007;
163(2):719–24.
[8] Tian RJ, Sun JC, Wang L. Plasma-nitrided austenitic stainless
steel 316L as bipolar plate for PEMFC. Int J Hydrogen Energy
2006;31(13):1874–8.
[9] Nam DG, Lee HC. Thermal nitridation of chromium
electroplated AISI316L stainless steel for polymer electrolyte
membrane fuel cell bipolar plate. J Power Sources 2007;
170(2):268–74.
[10] Lee HY, Lee SH, Kim JH, Kim MC, Wee DM. Thermally
nitrided Cu-5.3Cr alloy for application as metallic separators
in PEMFCs. Int J Hydrogen Energy 2008;22(15):4171–7.
[11] Fu Y, Hou M, Lin GQ, Hou JB, Shao ZG, Yi BL. Coated 316L
stainless steel with CrxN film as bipolar plate for PEMFC
prepared by pulsed bias arc ion plating. J Power Sources 2008;
176(1):282–6.