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polyaniline coatings in simulated environments of a proton exchange membrane fuel cell
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Electrochemical corrosion characteristics of conducting polypyrrole/polyaniline coatings in simulated environments of a proton exchange membrane fuel cell Y.J. Ren a,*, J. Chen a, C.L. Zeng b, C. Li a, J.J. He a a
Department of Energy and Power Engineering, Changsha University of Science and Technology, Changsha 410076, China b State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China
article info
abstract
Article history:
A bilayer conducting polymer coating consisting of an inner layer of polypyrrole (Ppy) with
Received 10 November 2015
large dodecylsulfate ionic groups and an external polyaniline (Pani) layer with small SO2 4
Accepted 26 March 2016
groups was electrodeposited on type 304 stainless steel bipolar plates of a proton-exchange
Available online xxx
membrane fuel cell (PEMFC). The corrosion performance of conducting Ppy/Pani bilayer coatings on 304SS in simulated cathode and anode environments (0.1 M H2SO4 solution
Keywords:
bubbled with air or H2 at 80 C) of a PEMFC was investigated by electrochemical impedance
Proton exchange membrane fuel cell
spectroscopy, polarization and open circuit potential measurements. The experimental
Polypyrrole/polyaniline bilayer
results showed that the Ppy/Pani bilayer increased the free corrosion potential of the steel
coatings
by about 310 mV (SCE) and 270 mV (SCE) in simulated cathodic and anodic environments of
Stainless steel bipolar plates
the PEMFC, respectively. Long-term exposure studies showed that the bilayer was highly
Corrosion
stable and inhibited the corrosion of the steel effectively in simulated cathodic and anodic environments, which was attributed to its “self-healing” effect. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The proton-exchange membrane fuel cell (PEMFC) is a clean and highly efficient power generation technology that converts hydrogen and oxygen gases into electricity and consists of a catalyst, a membrane electrode and bipolar plates. Of these components, the bipolar plates are the bulkiest component and also one of the most expensive parts to
manufacture [1]. Until now, the main materials used for bipolar plates have been graphite and graphite composites. However, their low mechanical strength and high gas permeability inhibit their commercial application. In comparison, metallic bipolar plates possess excellent electrical conductivity, high strength, low gas permeability and can be manufactured at low cost and can also be processed into thin plates. Consequently, the volume and weight of a PEMFC are both decreased and the power density of the fuel cell stack is
* Corresponding author. E-mail address: [email protected] (Y.J. Ren). http://dx.doi.org/10.1016/j.ijhydene.2016.03.184 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ren YJ, et al., Electrochemical corrosion characteristics of conducting polypyrrole/polyaniline coatings in simulated environments of a proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.03.184
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increased. However, in the weak acidic environment of a PEMFC (pH 2e4), which is correlated with the release of sulfonate groups and hydrogen ions from the proton-exchange membrane, metallic bipolar plates are usually attacked, with the formation of passive films and the dissolution of metallic ions [2]. In recent years, coatings with high corrosion resistance and electrical conductivity have been applied to protect metallic bipolar plates [3e7]. Conducting polymers have been proposed as candidates for corrosion protection of metallic bipolar plates [8e11]. Joseph et al. investigated Pani and Ppy conducting polymers electrodeposited on stainless steel bipolar plates and suggested that both coatings could improve the corrosion resistance of stainless steel bipolar plates with low contact resistance [8]. However, the stability of the coatings for extended immersion times was not investigated. Similarly, Gonzalez-Rodriguez found an increased corrosion resistance of polypyrrole coatings on 304 stainless steel (304SS) bipolar plates [9]. However, the coating degraded rapidly with prolonged immersion time. Thus, it is essential to boost the reliability of conductive polymers as a protective layer for metallic bipolar plates. Deyab proposed that a polyaniline coating mixed with carbon nanotubes (CNTs) could improve the corrosion resistance of aluminum bipolar plates effectively [12]. Rajkumar confirmed that the composite coatings improved the corrosion resistance of the single coating in acidic solution [13]. It is acknowledged that a conductive polymer could reoxidize the metal that is attacked, with the formation of a passive film, at the same time as the polymer is reduced, accompanied by the release of doped ions [14]. Once the polymer is completely reduced, it acts merely as a physical barrier [15]. In this study, a Ppy/Pani bilayer with a self-healing capacity was electropolymerized on type 304SS. To evaluate the stability of the bilayer credibly, the corrosion performance of the Ppy/Pani bilayer was investigated in simulated anodic and cathodic environments of a PEMFC.
Experimental procedures Electrodeposition of Ppy/Pani coatings Sheet-shaped sections of 304SS with an exposed surface area of 1 cm2 were coated with epoxy resin, followed by grinding with 240# grit emery paper and cleaning with distilled water and acetone. Electrodeposition was carried out in a glass cell with a stainless steel plate as the counter electrode. For the preparation of the bilayer coatings, as described in Ref. [16], the inner Ppy layer doped with DS was galvanostatically electrodeposited on the stainless steel in aqueous 0.4 M pyrrole containing 0.15 M sodium dodecylsulfate (SDS) supporting electrolyte at 5 C under a nitrogen atmosphere at a current density of 3 mA cm2 for 12 min. The outer Pani layer with the small counter-ion SO2 4 was electrodeposited over the Ppy layer from aqueous solutions of 0.5 M aniline and 1 M H2SO4 at 5 C using a cyclic voltammetry technique. The thickness of the bilayer coatings was around 15 mm.
Electrochemical measurements Electrochemical measurements in simulated cathodic and anodic environments (0.1 M H2SO4 bubbled with air or H2 at 80 C) were conducted in a three-electrode setup where a platinum sheet was selected as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. All tests were conducted with a Princeton Applied Research PAR2273 Potentiostat/Galvanostat under computer control. During the potentiodynamic polarization tests, the potential scan rate was 20 mV min1 after 1 h immersion. In addition, potentiostatic polarization measurements were carried out at 600 mV (SCE) and 240 mV (SCE) to evaluate the stability of the polymer coating further in simulated anodic and cathodic environments, respectively. Electrochemical impedance measurements were carried out between 0.01 and 100 kHz at open circuit potential. The amplitude of the input sine-wave voltage was 5 mV.
Results and discussion Electrochemical polarization measurements Fig. 1 shows the potentiodynamic polarization curves for Ppy/ Pani-coated 304SS and the bare steel after immersion for 1 h in the simulated anodic and cathodic environments. In the cathodic environment, 304SS was in the passive state at Ecorr and the free corrosion potential and current density were 262 mV (SCE) and 0.197 mA cm2, respectively. For Ppy/Panicoated steel the corrosion potential was 114 mV (SCE), improving by almost 380 mV (SCE) compared with the bare steel, and its corrosion current density was 0.161 mA cm2. The corrosion current and the corrosion potential of 304SS in the anodic environment were 0.122 mA cm2 and 341 mV (SCE), respectively, while those for the bilayer coated steel were 0.206 mA cm2 and 64 mV (SCE), respectively. It can be seen that the corrosion current density for the Ppy/Panicoated steel in the simulated anodic and cathodic environments is close to that of the 304SS, which is related to the oxidation of the bare steel and the polymer or the ionexchange between the interface of the Pani/Ppy bilayer and the solution [17]. Fig. 2 shows the potentiostatic polarization at 600 mV (SCE) and 240 mV (SCE) for the Ppy/Pani coatings. The polarization current density of Ppy/Pani-coated steel decreased significantly to a steady value with increasing time at 600 mV (SCE). There was no obvious degradation after 4 h of polarization, indicating that the bilayer coating could prevent the inward diffusion of aggressive ions effectively. Similarly, the polarization current density of Ppy/Pani decreased significantly initially at 240 mV (SCE) and then remained stable with extended polarization time. After polarization for 4 h, it could be observed that the samples turned dark green, which was related to the reduction of the external polyaniline film.
Open circuit potential measurements Fig. 3 shows the open circuit potential versus time curves for 304SS and the bilayer coating in simulated cathodic and
Please cite this article in press as: Ren YJ, et al., Electrochemical corrosion characteristics of conducting polypyrrole/polyaniline coatings in simulated environments of a proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.03.184
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Fig. 2 e Polarization current vs. time curves for 304SS with Ppy/Pani coating in 0.1 M H2SO4 solution bubbled with (a) air at 600 mV (SCE) (b) H2 at ¡240 mV (SCE) at 80 C. Fig. 1 e Potentiodynamic polarization curves of 304SS and 304SS with Ppy/Pani coating after immersion in 0.1 M H2SO4 solution bubbled with (a) air (b) H2 at 80 C for 1 h.
anodic environments, respectively. It can be observed that for 304SS, the Eocp remains at around 80 mV (SCE) and 150 mV (SCE) in the simulated cathodic and anodic environments. In contrast, Eocp for the Ppy/Pani-coated steel in the simulated cathodic environment increased gradually and then remained at around 310 mV (SCE) during exposure for 500 h, whilst in the simulated anodic environment it remained at around 270 mV (SCE). These comparative results indicate that in both simulated environments of the PEMFC the bilayer could effectively inhibit the penetration of corrosive species.
depressed loops in the low frequency region are difficult to observe. However, the corresponding Bode plots exhibit two obvious time constants, particular after the steel was immersed for 2 h. The impedance behavior could be described by the equivalent circuit shown in Fig. 6(a), in which Rs represents the electrolyte resistance, Rf and Cf represent the resistance and capacitance, respectively, of the porous corrosion product layer on the steel, and Rt and Cdl represent the charge transfer resistance and the double layer capacitance, respectively. In the fitting procedure, the non-ideal capacitive response of the corrosion system is considered instead of pure capacitance. That is, the constant phase elements (CPE) Qf and Qdl substitute for both Cf and Cdl. The impedance of the CPE can be expressed as
Electrochemical impedance measurements
ZCPE ¼
Typical Nyquist and Bode plots for the corrosion of 304SS in 0.1 M H2SO4 bubbled with air or H2 for prolonged immersion times are shown in Figs. 4 and 5. In both environments the impedance spectra show similar characteristics. The Nyquist plots are comprised of two depressed capacitive loops, and the
where Y0 represents the admittance magnitude of the CPE and n is the exponential term. Pure capacitance behavior of the corrosion system is represented by n ¼ 1. However, the value of n usually varies from zero to 1 for a practical system. A smaller value of n is related to a rougher electrode surface. The
1 n ðjuÞ Y0
Please cite this article in press as: Ren YJ, et al., Electrochemical corrosion characteristics of conducting polypyrrole/polyaniline coatings in simulated environments of a proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.03.184
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Fig. 3 e Open circuit potentialetime curves for 304SS and Ppy/Pani-coated steel in 0.1 M H2SO4 solution bubbled with (a) air (b) H2 at 80 C.
fitting results for steel were observed to be consistent with the experimental measurements. The fitted results are listed in Tables 1 and 2. According to Table 1, the values of ndl increased, indicating that the surface
roughness decreased due to the formation of a passive film on the steel. At the same time, the values of Rf increased from 568.5 to 601.2 U cm2 after immersion for 100 h in the simulated cathodic environment. Nevertheless, the Rf decreased gradually after immersion for 100 h since bubbled H2 in the simulated anodic environments is not favorable for the formation of a passive film. Differing from the impedance spectra of the bare steel, those for the Ppy/Pani coated steel in simulated cathodic and anodic environments of a PEMFC consist of a small, depressed capacitive loop in the high-frequency region and a line in the low-frequency region during exposure of up to 500 h (Figs. 7 and 8). The high-frequency loop denotes the impedance of the Ppy/Pani coating, while the doping/undoping processes controlled by diffusion in the bilayer contribute to the line in the low-frequency region. The loop in the high-frequency region in the simulated anodic environment expanded slightly with increasing immersion time, indicating an increase of conductive resistance for the polymer. The impedance diagrams can be fitted by the equivalent circuit shown in Fig. 6(b), where Zd represents the diffusion impedance. The fitting results for steel were in good agreement with the experimental values. Electrochemical parameters are given in Tables 3 and 4. The protection mechanism of conductive polymer arises from the physical barrier effect and the anodic protection effect. For an electrically synthesized single conductive polymer film, the unvoidable micropores in it allow the inward diffusion of aggressive species, hence causing peeling of the films and deterioration of the substrates. In this work, the electropolymerization of external Pani within the network of the internal Ppy layer produces a complex structure, which reduces the porosity of the inner layer and suppresses the penetration of electrolyte through the pores of the polymer. Moreover, the inner Ppy layer doped with larger dopant dodecylsulfate (DS) behaves as a cation exchanger, and the top Pani doped with small dopant SO2 4 anions behaves as an anion exchanger [18]. A polymer capable of cationic exchange is permeable only to cations, whilst an anion-exchange polymer is permeable only to anions. In this way, the combination of two different ion-selective films could also inhibit effectively the ingress of corrosive species. Sato et al. [19] found
Fig. 4 e Nyquist and Bode plots for 304SS in 0.1 M H2SO4 solution bubbled with air at 80 C with prolonged immersion time. Please cite this article in press as: Ren YJ, et al., Electrochemical corrosion characteristics of conducting polypyrrole/polyaniline coatings in simulated environments of a proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.03.184
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Fig. 5 e Nyquist and Bode plots for 304SS in 0.1 M H2SO4 solution bubbled with H2 at 80 C with prolonged immersion time.
Fig. 6 e Equivalent circuit for fitting impedance diagrams for (a) 304SS (b) Ppy/Pani-coated 304SS in 0.1 M H2SO4 solution bubbled with air or H2 at 80 C.
Table 1 e Fitting results of impedance spectra for the corrosion of 304SS in 0.1 M H2SO4 solution bubbled with air at 80 C. Time (h) 2 24 48 100
Rs (U cm2)
Yf (U1 cm2 Sn)
nf
Rf (U cm2)
Ydl (U cm2 Sn)
ndl
Rt (U cm2)
6.618 7.163 11.76 7.292
2.381E4 1.030E4 9.863E5 9.553E5
0.815 0.886 0.884 0.884
568.5 452.0 574.7 601.2
2.638E4 3.400E4 2.974E4 2.858E4
0.517 0.626 0.716 0.731
1.765E4 1.571E4 1.369E4 1.338E4
Table 2 e Fitting results of impedance spectra for the corrosion of 304SS in 0.1 M H2SO4 solution bubbled with H2 at 80 C. Time (h) 2 24 48 100
Rs (U cm2)
Yf (U1 cm2 Sn)
nf
Rf (U cm2)
Ydl (U1 cm2 Sn)
ndl
Rt (U cm2)
7.044 6.036 6.396 6.031
2.359E4 2.391E4 2.390E4 1.894E4
0.809 0.802 0.788 0.803
758.3 693.4 477.1 272.8
5.932E4 1.655E4 1.887E4 1.930E4
0.532 0.723 0.661 0.687
7.096E3 1.443E4 2.826E4 2.033E4
that the outer layer of metal-passive films exhibits cationic selectivity which inhibits the migration of Cl in the corrosive solution through passive films, while the inner layer exhibits anionic selectivity which can block the transfer of metal cations. Thus, the bipolar coating exhibited excellent corrosion resistance. In this work, the complex structure of electropolymerized Ppy/Pani bilayer coating and the combination of different ion selectivities of Ppy and Pani films contributed to decreasing the penetration of corrosive species. Thus, the bilayer coating showed an excellent barrier effect. In addition, the anodic protection effect of the polymers for alloys is related to their electrical conductivity [20]. That is, if a
conductive polymer is reduced to an insulating state, with the anions in the chain dedoped or the insertion of cations in the solution into the polymer, it acts merely as a barrier layer. In this work, due to the large immobile anion, Ppy doped with large DS is reduced by the insertion of cations into the polymer [21]. However, due to the anion permselectivity of the external Pani film, the reduction of internal Ppy would be suppressed. The potential of polyaniline (0.4e1.0 V vs NHE, pH 7) is higher than that of polypyrrole (0.1e0.3 V vs NHE, pH 7) [22]. Consequently, even if the inner Ppy was reduced for the dissolution of metals or the cations in solution, the external Pani could oxidize it in situ and the reduced Pani could react
Please cite this article in press as: Ren YJ, et al., Electrochemical corrosion characteristics of conducting polypyrrole/polyaniline coatings in simulated environments of a proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.03.184
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Fig. 7 e Nyquist and Bode plots for 304SS with Ppy/Pani coating after immersion in 0.1 M H2SO4 solution bubbled with air at 80 C for with prolonged immersion time.
Fig. 8 e Nyquist and Bode plots for 304SS with Ppy/Pani coating after immersion in 0.1 M H2SO4 solution bubbled with H2 at 80 C with prolonged immersion time.
Table 3 e Fitting results of impedance spectra for the corrosion of 304SS with Ppy/Pani coating in 0.1 M H2SO4 solution bubbled with air at 80 C. Time (h) 2 24 120 168 240 500
Rs (U cm2)
Yf (U1 cm2 Sn)
nf
Rf (U cm2)
Yd (U1 cm2 Sn)
nd
1.384 2.316 1.662 2.894 3.038 3.210
0.729 0.437 0.186 0.163 0.112 0.100
0.764 0.826 0.771 0.758 0.668 0.682
0.976 0.557 0.343 0.437 0.476 0.493
0.905 1.302 0.935 1.057 1.001 0.911
0.983 0.890 0.842 0.880 0.899 0.973
Table 4 e Fitting results of impedance spectra for the corrosion of 304SS with Ppy/Pani coating in 0.1 M H2SO4 solution bubbled with H2 at 80 C. Time (h) 2 24 72 192 312
Rs (U cm2)
Yf (U1 cm2 Sn)
nf
Rf (U cm2)
Yd (U1 cm2 Sn)
nd
3.049 3.654 3.513 3.917 4.105
0.268 0.126 0.119 9.471E4 4.225E3
0.724 0.901 0.890 0.717 0.518
0.739 0.598 0.640 1.276 1.574
0.354 0.755 0.741 0.885 0.680
0.981 0.894 0.890 0.922 0.886
Please cite this article in press as: Ren YJ, et al., Electrochemical corrosion characteristics of conducting polypyrrole/polyaniline coatings in simulated environments of a proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.03.184
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with dissolved oxygen in solution. The relevant reactions are as follows:
2þ
Ppy þ EM
þ
þ A / Ppy A þ LE
O2 þ LE þ H2O / EM2þ þ 2OH
(1)
7
Acknowledgments This paper was funded by the National Natural Science Foundation of China (Grant No. 51301026), Natural Science Foundation of Hunan Province (Grant No. 14JJ6019) and Educational Commission of Hunan Province (Grant No. 13B128).
(2)
EM2þ and LE represent the oxidized and reduced states of Pani, respectively. A is the anion in the corrosive solution. Namely, the top Pani serves as an anion reservoir which “heals” the reduced inner Ppy and keeps the bilayer conductive, and hence provides further anodic protection for the substrate. The bilayer exhibits self-healing capacities like an “intelligent” coating. According to Tables 3 and 4, Rf decreased from 0.976 U cm2 to 0.343 U cm2 and then remained stable for up to 500 h in the cathodic environment. In the anodic environment, the values of Rf also decreased for the initial 72 h, then increased to 2.731 U cm2 for 500 h, indicating that the electrical resistance increased. Rf is the sum of the electronic resistance Re and the ionic resistance Ri of electrolyte within the pores of the film [23]. The decreased Rf for the bilayer in the cathodic environment is related to the oxidation of Pani and the penetration of electrolyte within the pores of Pani for up to 500 h. It also demonstrates that the Ppy/Pani bilayer could provide effective protection for the steel in a cathodic environment with low conductivity resistance. In the simulated anodic environment, the decrease Rf after immersion for 72 h may relate to the decreased ionic resistance Ri of the electrolyte within the pores of the films. Due to lack of oxygen, the oxidation of Pani as reaction (2) is suppressed. Therefore, Rf increased slightly after immersion for up to 500 h.
Conclusions The corrosion performance of a Ppy (DS)/Pani (SO2 4 ) bilayer coating on 304SS bipolar plates of PEMFC was investigated in simulated cathodic and anodic environments (0.1 M H2SO4 bubbled with air or H2 at 80 C). In both the cathodic and anodic environments, the Ppy/Pani bilayer coatings were able to improve the corrosion potential of 304SS significantly. The composite coatings exhibited stability, with a corrosion potential of approximately 310 mV (SCE) and 270 mV (SCE) during exposure for up to 500 h in simulated cathodic and anodic environments. Following polarization at 600 mV (SCE) or 240 mV (SCE) for 4 h, the polarization current density of Ppy/Pani coated steel remained at a steady value. Electrochemical impedance spectra showed that in the cathodic environment of a PEMFC, the film resistance of the Ppy/Pani bilayer decreased during the initial stage and then remained stable after immersion for 500 h. In contrast, in the simulated anodic environment, the film resistance of the bilayer coating increased slightly. The bilayer could therefore significantly improve the corrosion resistance of steel in simulated cathodic and anodic environments.
references
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[16] Ren YJ, Zeng CL. Effect of conducting composite polypyrrole/ polyaniline coatings on the corrosion resistance of type 304 stainless steel for bipolar plates of proton-exchange membrane fuel cells. J Power Sources 2008;182:524e30. [17] Dong YH, Zhang QY, Su XM, Zhou Q. Preparation and investigation of the protective properties of bipolar coatings. Prog Org Coat 2013;76:662e9. [18] Ehrenbeck C, Juttner K. Development of an anion/cation permeable free-standing membrane based on electrochemical switching of polypyrrole. Electrochim Acta 1996;41:511e8. [19] Santos JR, Mattoso LHC, Motheo AJ. Investigation of corrosion protection of steel by polyaniline films. Electrochim Acta 1997;43:309e13.
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Electrochemical corrosion characteristics of conducting polypyrrole/polyaniline coatings in simulated environments of a proton exchange membrane fuel cell Y.J. Ren a,*, J. Chen a, C.L. Zeng b, C. Li a, J.J. He a a
Department of Energy and Power Engineering, Changsha University of Science and Technology, Changsha 410076, China b State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China
article info
abstract
Article history:
A bilayer conducting polymer coating consisting of an inner layer of polypyrrole (Ppy) with
Received 10 November 2015
large dodecylsulfate ionic groups and an external polyaniline (Pani) layer with small SO2 4
Accepted 26 March 2016
groups was electrodeposited on type 304 stainless steel bipolar plates of a proton-exchange
Available online xxx
membrane fuel cell (PEMFC). The corrosion performance of conducting Ppy/Pani bilayer coatings on 304SS in simulated cathode and anode environments (0.1 M H2SO4 solution
Keywords:
bubbled with air or H2 at 80 C) of a PEMFC was investigated by electrochemical impedance
Proton exchange membrane fuel cell
spectroscopy, polarization and open circuit potential measurements. The experimental
Polypyrrole/polyaniline bilayer
results showed that the Ppy/Pani bilayer increased the free corrosion potential of the steel
coatings
by about 310 mV (SCE) and 270 mV (SCE) in simulated cathodic and anodic environments of
Stainless steel bipolar plates
the PEMFC, respectively. Long-term exposure studies showed that the bilayer was highly
Corrosion
stable and inhibited the corrosion of the steel effectively in simulated cathodic and anodic environments, which was attributed to its “self-healing” effect. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The proton-exchange membrane fuel cell (PEMFC) is a clean and highly efficient power generation technology that converts hydrogen and oxygen gases into electricity and consists of a catalyst, a membrane electrode and bipolar plates. Of these components, the bipolar plates are the bulkiest component and also one of the most expensive parts to
manufacture [1]. Until now, the main materials used for bipolar plates have been graphite and graphite composites. However, their low mechanical strength and high gas permeability inhibit their commercial application. In comparison, metallic bipolar plates possess excellent electrical conductivity, high strength, low gas permeability and can be manufactured at low cost and can also be processed into thin plates. Consequently, the volume and weight of a PEMFC are both decreased and the power density of the fuel cell stack is
* Corresponding author. E-mail address: [email protected] (Y.J. Ren). http://dx.doi.org/10.1016/j.ijhydene.2016.03.184 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ren YJ, et al., Electrochemical corrosion characteristics of conducting polypyrrole/polyaniline coatings in simulated environments of a proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.03.184
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increased. However, in the weak acidic environment of a PEMFC (pH 2e4), which is correlated with the release of sulfonate groups and hydrogen ions from the proton-exchange membrane, metallic bipolar plates are usually attacked, with the formation of passive films and the dissolution of metallic ions [2]. In recent years, coatings with high corrosion resistance and electrical conductivity have been applied to protect metallic bipolar plates [3e7]. Conducting polymers have been proposed as candidates for corrosion protection of metallic bipolar plates [8e11]. Joseph et al. investigated Pani and Ppy conducting polymers electrodeposited on stainless steel bipolar plates and suggested that both coatings could improve the corrosion resistance of stainless steel bipolar plates with low contact resistance [8]. However, the stability of the coatings for extended immersion times was not investigated. Similarly, Gonzalez-Rodriguez found an increased corrosion resistance of polypyrrole coatings on 304 stainless steel (304SS) bipolar plates [9]. However, the coating degraded rapidly with prolonged immersion time. Thus, it is essential to boost the reliability of conductive polymers as a protective layer for metallic bipolar plates. Deyab proposed that a polyaniline coating mixed with carbon nanotubes (CNTs) could improve the corrosion resistance of aluminum bipolar plates effectively [12]. Rajkumar confirmed that the composite coatings improved the corrosion resistance of the single coating in acidic solution [13]. It is acknowledged that a conductive polymer could reoxidize the metal that is attacked, with the formation of a passive film, at the same time as the polymer is reduced, accompanied by the release of doped ions [14]. Once the polymer is completely reduced, it acts merely as a physical barrier [15]. In this study, a Ppy/Pani bilayer with a self-healing capacity was electropolymerized on type 304SS. To evaluate the stability of the bilayer credibly, the corrosion performance of the Ppy/Pani bilayer was investigated in simulated anodic and cathodic environments of a PEMFC.
Experimental procedures Electrodeposition of Ppy/Pani coatings Sheet-shaped sections of 304SS with an exposed surface area of 1 cm2 were coated with epoxy resin, followed by grinding with 240# grit emery paper and cleaning with distilled water and acetone. Electrodeposition was carried out in a glass cell with a stainless steel plate as the counter electrode. For the preparation of the bilayer coatings, as described in Ref. [16], the inner Ppy layer doped with DS was galvanostatically electrodeposited on the stainless steel in aqueous 0.4 M pyrrole containing 0.15 M sodium dodecylsulfate (SDS) supporting electrolyte at 5 C under a nitrogen atmosphere at a current density of 3 mA cm2 for 12 min. The outer Pani layer with the small counter-ion SO2 4 was electrodeposited over the Ppy layer from aqueous solutions of 0.5 M aniline and 1 M H2SO4 at 5 C using a cyclic voltammetry technique. The thickness of the bilayer coatings was around 15 mm.
Electrochemical measurements Electrochemical measurements in simulated cathodic and anodic environments (0.1 M H2SO4 bubbled with air or H2 at 80 C) were conducted in a three-electrode setup where a platinum sheet was selected as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. All tests were conducted with a Princeton Applied Research PAR2273 Potentiostat/Galvanostat under computer control. During the potentiodynamic polarization tests, the potential scan rate was 20 mV min1 after 1 h immersion. In addition, potentiostatic polarization measurements were carried out at 600 mV (SCE) and 240 mV (SCE) to evaluate the stability of the polymer coating further in simulated anodic and cathodic environments, respectively. Electrochemical impedance measurements were carried out between 0.01 and 100 kHz at open circuit potential. The amplitude of the input sine-wave voltage was 5 mV.
Results and discussion Electrochemical polarization measurements Fig. 1 shows the potentiodynamic polarization curves for Ppy/ Pani-coated 304SS and the bare steel after immersion for 1 h in the simulated anodic and cathodic environments. In the cathodic environment, 304SS was in the passive state at Ecorr and the free corrosion potential and current density were 262 mV (SCE) and 0.197 mA cm2, respectively. For Ppy/Panicoated steel the corrosion potential was 114 mV (SCE), improving by almost 380 mV (SCE) compared with the bare steel, and its corrosion current density was 0.161 mA cm2. The corrosion current and the corrosion potential of 304SS in the anodic environment were 0.122 mA cm2 and 341 mV (SCE), respectively, while those for the bilayer coated steel were 0.206 mA cm2 and 64 mV (SCE), respectively. It can be seen that the corrosion current density for the Ppy/Panicoated steel in the simulated anodic and cathodic environments is close to that of the 304SS, which is related to the oxidation of the bare steel and the polymer or the ionexchange between the interface of the Pani/Ppy bilayer and the solution [17]. Fig. 2 shows the potentiostatic polarization at 600 mV (SCE) and 240 mV (SCE) for the Ppy/Pani coatings. The polarization current density of Ppy/Pani-coated steel decreased significantly to a steady value with increasing time at 600 mV (SCE). There was no obvious degradation after 4 h of polarization, indicating that the bilayer coating could prevent the inward diffusion of aggressive ions effectively. Similarly, the polarization current density of Ppy/Pani decreased significantly initially at 240 mV (SCE) and then remained stable with extended polarization time. After polarization for 4 h, it could be observed that the samples turned dark green, which was related to the reduction of the external polyaniline film.
Open circuit potential measurements Fig. 3 shows the open circuit potential versus time curves for 304SS and the bilayer coating in simulated cathodic and
Please cite this article in press as: Ren YJ, et al., Electrochemical corrosion characteristics of conducting polypyrrole/polyaniline coatings in simulated environments of a proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.03.184
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Fig. 2 e Polarization current vs. time curves for 304SS with Ppy/Pani coating in 0.1 M H2SO4 solution bubbled with (a) air at 600 mV (SCE) (b) H2 at ¡240 mV (SCE) at 80 C. Fig. 1 e Potentiodynamic polarization curves of 304SS and 304SS with Ppy/Pani coating after immersion in 0.1 M H2SO4 solution bubbled with (a) air (b) H2 at 80 C for 1 h.
anodic environments, respectively. It can be observed that for 304SS, the Eocp remains at around 80 mV (SCE) and 150 mV (SCE) in the simulated cathodic and anodic environments. In contrast, Eocp for the Ppy/Pani-coated steel in the simulated cathodic environment increased gradually and then remained at around 310 mV (SCE) during exposure for 500 h, whilst in the simulated anodic environment it remained at around 270 mV (SCE). These comparative results indicate that in both simulated environments of the PEMFC the bilayer could effectively inhibit the penetration of corrosive species.
depressed loops in the low frequency region are difficult to observe. However, the corresponding Bode plots exhibit two obvious time constants, particular after the steel was immersed for 2 h. The impedance behavior could be described by the equivalent circuit shown in Fig. 6(a), in which Rs represents the electrolyte resistance, Rf and Cf represent the resistance and capacitance, respectively, of the porous corrosion product layer on the steel, and Rt and Cdl represent the charge transfer resistance and the double layer capacitance, respectively. In the fitting procedure, the non-ideal capacitive response of the corrosion system is considered instead of pure capacitance. That is, the constant phase elements (CPE) Qf and Qdl substitute for both Cf and Cdl. The impedance of the CPE can be expressed as
Electrochemical impedance measurements
ZCPE ¼
Typical Nyquist and Bode plots for the corrosion of 304SS in 0.1 M H2SO4 bubbled with air or H2 for prolonged immersion times are shown in Figs. 4 and 5. In both environments the impedance spectra show similar characteristics. The Nyquist plots are comprised of two depressed capacitive loops, and the
where Y0 represents the admittance magnitude of the CPE and n is the exponential term. Pure capacitance behavior of the corrosion system is represented by n ¼ 1. However, the value of n usually varies from zero to 1 for a practical system. A smaller value of n is related to a rougher electrode surface. The
1 n ðjuÞ Y0
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Fig. 3 e Open circuit potentialetime curves for 304SS and Ppy/Pani-coated steel in 0.1 M H2SO4 solution bubbled with (a) air (b) H2 at 80 C.
fitting results for steel were observed to be consistent with the experimental measurements. The fitted results are listed in Tables 1 and 2. According to Table 1, the values of ndl increased, indicating that the surface
roughness decreased due to the formation of a passive film on the steel. At the same time, the values of Rf increased from 568.5 to 601.2 U cm2 after immersion for 100 h in the simulated cathodic environment. Nevertheless, the Rf decreased gradually after immersion for 100 h since bubbled H2 in the simulated anodic environments is not favorable for the formation of a passive film. Differing from the impedance spectra of the bare steel, those for the Ppy/Pani coated steel in simulated cathodic and anodic environments of a PEMFC consist of a small, depressed capacitive loop in the high-frequency region and a line in the low-frequency region during exposure of up to 500 h (Figs. 7 and 8). The high-frequency loop denotes the impedance of the Ppy/Pani coating, while the doping/undoping processes controlled by diffusion in the bilayer contribute to the line in the low-frequency region. The loop in the high-frequency region in the simulated anodic environment expanded slightly with increasing immersion time, indicating an increase of conductive resistance for the polymer. The impedance diagrams can be fitted by the equivalent circuit shown in Fig. 6(b), where Zd represents the diffusion impedance. The fitting results for steel were in good agreement with the experimental values. Electrochemical parameters are given in Tables 3 and 4. The protection mechanism of conductive polymer arises from the physical barrier effect and the anodic protection effect. For an electrically synthesized single conductive polymer film, the unvoidable micropores in it allow the inward diffusion of aggressive species, hence causing peeling of the films and deterioration of the substrates. In this work, the electropolymerization of external Pani within the network of the internal Ppy layer produces a complex structure, which reduces the porosity of the inner layer and suppresses the penetration of electrolyte through the pores of the polymer. Moreover, the inner Ppy layer doped with larger dopant dodecylsulfate (DS) behaves as a cation exchanger, and the top Pani doped with small dopant SO2 4 anions behaves as an anion exchanger [18]. A polymer capable of cationic exchange is permeable only to cations, whilst an anion-exchange polymer is permeable only to anions. In this way, the combination of two different ion-selective films could also inhibit effectively the ingress of corrosive species. Sato et al. [19] found
Fig. 4 e Nyquist and Bode plots for 304SS in 0.1 M H2SO4 solution bubbled with air at 80 C with prolonged immersion time. Please cite this article in press as: Ren YJ, et al., Electrochemical corrosion characteristics of conducting polypyrrole/polyaniline coatings in simulated environments of a proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.03.184
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Fig. 5 e Nyquist and Bode plots for 304SS in 0.1 M H2SO4 solution bubbled with H2 at 80 C with prolonged immersion time.
Fig. 6 e Equivalent circuit for fitting impedance diagrams for (a) 304SS (b) Ppy/Pani-coated 304SS in 0.1 M H2SO4 solution bubbled with air or H2 at 80 C.
Table 1 e Fitting results of impedance spectra for the corrosion of 304SS in 0.1 M H2SO4 solution bubbled with air at 80 C. Time (h) 2 24 48 100
Rs (U cm2)
Yf (U1 cm2 Sn)
nf
Rf (U cm2)
Ydl (U cm2 Sn)
ndl
Rt (U cm2)
6.618 7.163 11.76 7.292
2.381E4 1.030E4 9.863E5 9.553E5
0.815 0.886 0.884 0.884
568.5 452.0 574.7 601.2
2.638E4 3.400E4 2.974E4 2.858E4
0.517 0.626 0.716 0.731
1.765E4 1.571E4 1.369E4 1.338E4
Table 2 e Fitting results of impedance spectra for the corrosion of 304SS in 0.1 M H2SO4 solution bubbled with H2 at 80 C. Time (h) 2 24 48 100
Rs (U cm2)
Yf (U1 cm2 Sn)
nf
Rf (U cm2)
Ydl (U1 cm2 Sn)
ndl
Rt (U cm2)
7.044 6.036 6.396 6.031
2.359E4 2.391E4 2.390E4 1.894E4
0.809 0.802 0.788 0.803
758.3 693.4 477.1 272.8
5.932E4 1.655E4 1.887E4 1.930E4
0.532 0.723 0.661 0.687
7.096E3 1.443E4 2.826E4 2.033E4
that the outer layer of metal-passive films exhibits cationic selectivity which inhibits the migration of Cl in the corrosive solution through passive films, while the inner layer exhibits anionic selectivity which can block the transfer of metal cations. Thus, the bipolar coating exhibited excellent corrosion resistance. In this work, the complex structure of electropolymerized Ppy/Pani bilayer coating and the combination of different ion selectivities of Ppy and Pani films contributed to decreasing the penetration of corrosive species. Thus, the bilayer coating showed an excellent barrier effect. In addition, the anodic protection effect of the polymers for alloys is related to their electrical conductivity [20]. That is, if a
conductive polymer is reduced to an insulating state, with the anions in the chain dedoped or the insertion of cations in the solution into the polymer, it acts merely as a barrier layer. In this work, due to the large immobile anion, Ppy doped with large DS is reduced by the insertion of cations into the polymer [21]. However, due to the anion permselectivity of the external Pani film, the reduction of internal Ppy would be suppressed. The potential of polyaniline (0.4e1.0 V vs NHE, pH 7) is higher than that of polypyrrole (0.1e0.3 V vs NHE, pH 7) [22]. Consequently, even if the inner Ppy was reduced for the dissolution of metals or the cations in solution, the external Pani could oxidize it in situ and the reduced Pani could react
Please cite this article in press as: Ren YJ, et al., Electrochemical corrosion characteristics of conducting polypyrrole/polyaniline coatings in simulated environments of a proton exchange membrane fuel cell, International Journal of Hydrogen Energy (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.03.184
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Fig. 7 e Nyquist and Bode plots for 304SS with Ppy/Pani coating after immersion in 0.1 M H2SO4 solution bubbled with air at 80 C for with prolonged immersion time.
Fig. 8 e Nyquist and Bode plots for 304SS with Ppy/Pani coating after immersion in 0.1 M H2SO4 solution bubbled with H2 at 80 C with prolonged immersion time.
Table 3 e Fitting results of impedance spectra for the corrosion of 304SS with Ppy/Pani coating in 0.1 M H2SO4 solution bubbled with air at 80 C. Time (h) 2 24 120 168 240 500
Rs (U cm2)
Yf (U1 cm2 Sn)
nf
Rf (U cm2)
Yd (U1 cm2 Sn)
nd
1.384 2.316 1.662 2.894 3.038 3.210
0.729 0.437 0.186 0.163 0.112 0.100
0.764 0.826 0.771 0.758 0.668 0.682
0.976 0.557 0.343 0.437 0.476 0.493
0.905 1.302 0.935 1.057 1.001 0.911
0.983 0.890 0.842 0.880 0.899 0.973
Table 4 e Fitting results of impedance spectra for the corrosion of 304SS with Ppy/Pani coating in 0.1 M H2SO4 solution bubbled with H2 at 80 C. Time (h) 2 24 72 192 312
Rs (U cm2)
Yf (U1 cm2 Sn)
nf
Rf (U cm2)
Yd (U1 cm2 Sn)
nd
3.049 3.654 3.513 3.917 4.105
0.268 0.126 0.119 9.471E4 4.225E3
0.724 0.901 0.890 0.717 0.518
0.739 0.598 0.640 1.276 1.574
0.354 0.755 0.741 0.885 0.680
0.981 0.894 0.890 0.922 0.886
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with dissolved oxygen in solution. The relevant reactions are as follows:
2þ
Ppy þ EM
þ
þ A / Ppy A þ LE
O2 þ LE þ H2O / EM2þ þ 2OH
(1)
7
Acknowledgments This paper was funded by the National Natural Science Foundation of China (Grant No. 51301026), Natural Science Foundation of Hunan Province (Grant No. 14JJ6019) and Educational Commission of Hunan Province (Grant No. 13B128).
(2)
EM2þ and LE represent the oxidized and reduced states of Pani, respectively. A is the anion in the corrosive solution. Namely, the top Pani serves as an anion reservoir which “heals” the reduced inner Ppy and keeps the bilayer conductive, and hence provides further anodic protection for the substrate. The bilayer exhibits self-healing capacities like an “intelligent” coating. According to Tables 3 and 4, Rf decreased from 0.976 U cm2 to 0.343 U cm2 and then remained stable for up to 500 h in the cathodic environment. In the anodic environment, the values of Rf also decreased for the initial 72 h, then increased to 2.731 U cm2 for 500 h, indicating that the electrical resistance increased. Rf is the sum of the electronic resistance Re and the ionic resistance Ri of electrolyte within the pores of the film [23]. The decreased Rf for the bilayer in the cathodic environment is related to the oxidation of Pani and the penetration of electrolyte within the pores of Pani for up to 500 h. It also demonstrates that the Ppy/Pani bilayer could provide effective protection for the steel in a cathodic environment with low conductivity resistance. In the simulated anodic environment, the decrease Rf after immersion for 72 h may relate to the decreased ionic resistance Ri of the electrolyte within the pores of the films. Due to lack of oxygen, the oxidation of Pani as reaction (2) is suppressed. Therefore, Rf increased slightly after immersion for up to 500 h.
Conclusions The corrosion performance of a Ppy (DS)/Pani (SO2 4 ) bilayer coating on 304SS bipolar plates of PEMFC was investigated in simulated cathodic and anodic environments (0.1 M H2SO4 bubbled with air or H2 at 80 C). In both the cathodic and anodic environments, the Ppy/Pani bilayer coatings were able to improve the corrosion potential of 304SS significantly. The composite coatings exhibited stability, with a corrosion potential of approximately 310 mV (SCE) and 270 mV (SCE) during exposure for up to 500 h in simulated cathodic and anodic environments. Following polarization at 600 mV (SCE) or 240 mV (SCE) for 4 h, the polarization current density of Ppy/Pani coated steel remained at a steady value. Electrochemical impedance spectra showed that in the cathodic environment of a PEMFC, the film resistance of the Ppy/Pani bilayer decreased during the initial stage and then remained stable after immersion for 500 h. In contrast, in the simulated anodic environment, the film resistance of the bilayer coating increased slightly. The bilayer could therefore significantly improve the corrosion resistance of steel in simulated cathodic and anodic environments.
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