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PPY-camphorsulfonic acid composite coating for 304SS bipolar plates in proton exchange membrane fuel cell
G Model JIEC 4726 No. of Pages 11
Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
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Enhanced anticorrosion performance of PPY-graphene oxide/PPYcamphorsulfonic acid composite coating for 304SS bipolar plates in proton exchange membrane fuel cell Li Jianga,b , Junaid Ali Syedc, Guoli Zhanga , Yujie Maa , Jun Mad, Hongbin Lua,** , Xiangkang Menga,* a Institute of Materials Engineering, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, PR China b School of Chemistry, Biology and Materials Engineering, Suzhou University of Science and Technology, Suzhou 215009, PR China c Department of Metallurgy and Materials Engineering, Mehran University of Engineering and Technology, Jamshoro, Pakistan d School of Mechatronics & Traffic Engineering, Nantong College of Science and Technology, Nantong 226007, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 8 April 2019 Received in revised form 8 July 2019 Accepted 12 August 2019 Available online xxx
The enhanced corrosion resistance with sustained conductivity are the prerequisites of stainless steel bipolar plates for practical application in proton exchange membrane fuel cell. Herein, we prepare a conductive polypyrrole-graphene oxide/polypyrrole-camphorsulfonic acid bilayer composite coating (PPY-GO/PPY-CSA) on 304 stainless steel bipolar plate by electrodeposition method. The electrochemical tests are conducted in the simulated bipolar plates working environment, the potentiostatic polarization results imply that the PPY-GO/PPY-CSA composite coating offers stable corrosion resistance with low potentiostatic corrosion current density in comparison with the PPY-GO coating. The corresponding electrochemical impedance spectroscopy measurements reveal that the PPY-GO/PPY-CSA composite coating exhibits satisfactory conductivity and displays sustained anodic protection effect with superior anticorrosion performance during the 696 h of immersion. The excellent corrosion protection ability of the PPY-GO/PPY-CSA composite coating owing to its good adhesion strength, compact structure, satisfactory conductivity as well as the synergetic interaction between the two layers of the coating. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Bipolar plates Corrosion protection Polypyrrole Graphene oxide Camphorsulfonic acid
Introduction The shortage of resources and increasing environmental pollution have aroused wide public concern for renewable and green energy systems [1–4]. Proton exchange membrane fuel cell (PEMFC) is one of the environmentally friendly energy generation system, has attracted significant attention due to the high efficiency, low emission and large power density [5,6]. Bipolar plates is an important component in PEMFC (occupies ca. 80% of the weight and ca. 45% of the cost of PEMFC stacks) [5], which plays a crucial role in the durability and performance of the fuel cell [7]. The aggressive acidic working environment determines that the
* Corresponding author. ** Corresponding author at: Institute of Materials Engineering, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Jiangsu, China. E-mail addresses: [email protected] (H. Lu), [email protected] (X. Meng).
bipolar plate material must have sturdy chemical stability, satisfactory corrosion resistance and good conductivity [6,8]. Economical metallic bipolar plates such as stainless steel (SS), aluminum and nickel plates have been broadly explored for the substitution of conventional graphite bipolar plates due to their good conductivity, high mechanical strength and low gas permeability [2,6]. Although the metallic bipolar plates have many advantages, however, the formation of oxide film at metal surface during the long time service in PEMFC environment weakens the conductivity of the bipolar plates [9,10]. Moreover, the metallic bipolar plates are vulnerable to the attack of corrosive species and the produced corrosion products would contaminate the internal environment of PEMFC, further reduces its lifespan [11]. Consequently, various types of corrosion inhibiting coatings with high electrical conductivity have been developed for the metallic bipolar plates [8,12]. The conductive polymer coatings based on polyaniline (PANI), polypyrrole (PPY) etc. are considered as suitable candidates due to their chemical stability, economical production as well as satisfactory conductivity [13–16]. Numerous
https://doi.org/10.1016/j.jiec.2019.08.032 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: L. Jiang, et al., Enhanced anticorrosion performance of PPY-graphene oxide/PPY-camphorsulfonic acid composite coating for 304SS bipolar plates in proton exchange membrane fuel cell, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j. jiec.2019.08.032
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reports show the convenience of electrodeposition method for the fabrication of conductive polymer coatings and the corrosion protection ability of the polymer coatings for metallic bipolar plates is wildly studied [9,10,17,18]. Specifically, the conductivity of the coating is affected by the doping anions in polymer matrix [19,20]. In general, the conductive polymers have two states that are the oxidized and reduced states, the polymer exhibits conductivity in the doped oxidized state and could passivate the underlying substrate through anodic protection effect, also the coating layer acts as a barrier that can physically insulate the substrate from the corrosive environment [8,21]. However, the electrochemically synthesized conductive polymer films usually have poor adhesion strength and exhibit microscopic defects that provide the diffusion pathway to the corrosive species, eventually result in the initiation of corrosion process beneath the coating [19,20]. Besides, the conductive polymer can be gradually reduced in the harsh PEMFC environment during long-term service, which affects the coating conductivity and deteriorates the anodic protection ability of the coating [9,11]. Therefore, modification of the coating defects and maintain the coatings conductivity are the crucial concern for the practical application of conductive polymer coatings, which could enhance the stability of the anticorrosion performance for bipolar plates in PEMFC environment. A most robust and feasible alternative way is to fabricate the bilayer conductive polymer coatings with organic and inorganic additives. Ren et al. [11,18] described that the electrodeposited bilayer PPY/PANI film significantly enhances the anticorrosion property of 304SS bipolar plates in PEMFC environment. Pan et al. [10] also envisaged that the bilayer conductive polymer film could provide superior protection ability to the copper bipolar plates and the coating exhibits relatively low contact resistance. The functional material graphene oxide (GO) has extensive research in various fields such as lithium batteries, supercapacitors and anticorrosion coatings due to its large surface area, high band gap and the excellent chemical stability [22–25]. It is reported that the introduction of GO in the anticorrosion coating significantly advances the coatings adhesion property, further improves the anticorrosion performance [26]. The functional groups e.g., carboxyl groups in GO could provide reactive sites that several studies have been focused on the combination of conductive polymers with GO. Ramezanzadeh et al. [22] described that the insertion of GO in PANI improves the conductivity of PANI and the PANI-GO composite enhances the corrosion resistance of zinc-rich epoxy coated SS in NaCl environment. GO and conductive polymer composites can also be synthesized through electrochemical method. Zhu et al. [27] described that the oxygen-containing groups in GO sheets could act as charge-balancing dopants within PPY that facilitates the co-electrodeposition of PPY-GO composites through potentiostatic method, and the prepared composites exhibit excellent energy storage property. Qi et al. [28] revealed that GO could directly introduced into PPY matrix without other dopants through pulse electrochemical incorporation. In the previous work, we have also prepared the high-performance anticorrosion coating consist of PPY and GO by in-situ electrodeposition method [29]. Previously, we demonstrated that PPY doped with the organic camphorsulfonic acid (CSA) which has spatial molecular structure could enhance the coatings conductivity and sustain the anodic protection for 304SS bipolar plates [9]. In order to further reduce the coating defects and enhance the long-term anticorrosion performance of 304SS bipolar plates in the chloride containing PEMFC environment, this study aims to develop a novel bilayer anticorrosion composite coating system for 304SS which contains a polypyrrole-graphene oxide (PPY-GO) layer and a polypyrrolecamphorsulfonic acid (PPY-CSA) layer by electrodeposition technique. We speculated that the incorporated GO in PPY matrix at
PPY-GO layer could enhance the coatings adhesion property and the PPY doped with CSA layer could improve the conductivity of the coating, hence the PPY-GO/PPY-CSA composite coating would offer superior corrosion protection to the bipolar plate in PEMFC. Experimental Materials The graphite powder (spectral pure) with ca. 45–48 mm size was obtained from Sinpharm Chemical Reagent Co., Ltd., while pyrrole monomer and other chemicals were purchased from Shanghai Aladdin Bio-Chem. 304SS coupons were offered by Senda Decoration Materials Co. Ltd., mechanically polished by SiC papers then ultrasonically washed by acetone and ethanol, respectively. GO was synthesized from graphite powder by using the modified Hummer’s method [30,31] and the homogeneous GO aqueous dispersion was used for the coating electrodeposition. Electrodeposition of coatings The electrodeposition process and electrochemical analyses of the coatings were all conducted with Metrohm PGSTAT 302N instrument at room temperature. The coatings were prepared through galvanostatical electrodeposition process in the three electrodes system that the saturated calomel electrode (SCE) as reference electrode and a platinum mesh counter electrode, whereas the working electrode was the treated 304SS coupon with 1 cm2 exposed area. The mixed solution which contains 0.5 mg mL1 GO and 0.4 M pyrrole was stirred to form a uniform supporting electrolyte for electrodeposition of PPY-GO layer and the electrodeposition process was galvanostatically performed for 10 min with a current density of 1 mA cm2. Then the PPY-CSA layer was electrodeposited on the top of PPY-GO layer for 15 min at 1 mA cm2 in the mixed solution that composed of 0.5 M CSA with 0.4 M pyrrole monomer. The thickness of the prepared PPY-GO/ PPY-CSA composite coating is ca. 4.2 mm. For ease of comparison, the single PPY-GO coating was also electrodeposited in the supporting electrolyte which contains 0.5 mg mL1 GO and 0.4 M pyrrole in the same current density for 25 min. Instrumentation The scanning electron microscope (SEM) images of the samples were acquired with the JOEL JSM-6510 SEM system and the FT-IR spectra of the two kinds of coating were recorded on a PerkinElmer Fourier transform infrared spectrometer. The Raman spectra of the samples were accomplished with LabRAM HR Evolution Raman microscope with 532 nm excitation laser wavelength. The X-ray photoelectron spectroscopy (XPS) examined the valence states of elements were conducted with Thermo Fisher Scientific K-Alpha XPS instrument. Atomic force microscopy (AFM) studies were accomplished by using Vecco Metrology atomic force microscope. The cross-cut technique along with tape test was used to evaluate the coatings adhesion strength and the adhesion degree was assessed from the lowest 0B grade to highest 5B grade based on ASTM D3359. The concentration of dissolved metal ions in the electrolyte after 10 h of potentiostatic polarization was measured by using the Perkin–Elmer Optima 5300DV Inductive Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). The electrochemical properties of the bare and coated 304SS bipolar plates were tested in 0.3 M HCl solution to simulate a definite aggressive PEMFC environment and all the experiments were initiated at a steady-state open circuit potential (OCP). The potentiostatic polarization tests were achieved in 0.6 VSCE, which approaches the PEMFC cathode operating potential [32]. The
Please cite this article in press as: L. Jiang, et al., Enhanced anticorrosion performance of PPY-graphene oxide/PPY-camphorsulfonic acid composite coating for 304SS bipolar plates in proton exchange membrane fuel cell, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j. jiec.2019.08.032
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electrochemical impedance (EIS) measurements were obtained at OCP value under the frequency range of 100 kHz–0.01 Hz at 10 mV amplitude. Results and discussion For the synthesis of PPY-GO layer, the mutual repulsion of electric charges of the negatively charged groups in GO drive GO species towards anode [33]. Meanwhile, the pyrrole monomer and GO could be in contact by p–p interactions and electrostatic force [34]. Under the applied current, the GO species with negatively charged oxygen-containing groups acts as anionic dopants, thus could be incorporated into the polymer matrix to balance the positively charged PPY [35]. As a result, PPY and GO can be simultaneously in-situ electrodeposited and the uniform PPY-GO layer is obtained. Notably, GO can be reduced to reduced graphene oxide (rGO) at proper cathodic potentials [36], however, it is hard to be reduced and change to rGO under the given elcetrosynthesis conditions, which is similar to the previous reports [37,38]. Characterization of electrodeposited coatings Fig. 1a shows the FT-IR spectra of GO and two different coatings in the wavenumber range of 800–4000 cm1. As we analyzed previously [29], the GO shows typical absorption peaks of oxide groups, the vibration of O–H groups is at 3396 cm1 [27], the peaks at 1736 and 1417 cm1 are belong to carboxyl (C¼O) and C–OH stretching, respectively [22,39]. For PPY-GO coating, the peak at 3432 cm1 is the characteristic of the stretching of N–H in PPY [40]. Signal at 1534 cm1 is due to the C¼C stretching and the characteristic peak at 1174 cm1 comes from the in-plane bending vibration of C–H binds [28,41]. The characteristic peaks of GO are located at 1733 and 1041 cm1, assigned to C¼O and C–O groups, respectively [42,43]. These characteristic peaks imply successful introduction of GO in PPY matrix [27]. However, the absorption peaks of C–OH and the vibration of O–H related to GO are not clearly represented in the FT-IR spectrum of PPY-GO coating, which is due to the overlapping of the main peaks of GO and PPY [42]. The characteristic peaks of PPY-GO/PPY-CSA bilayer composite coating show slight shift due to the interaction of the two layers [10]. The absorption peak at 3424 cm1 is associated with N–H stretching in PPY while the signal at 1531 and 1168 cm1 are ascribed to the C¼C stretching and C–H in-plane vibration, respectively. The characteristic peak at 1732 cm1 belongs to the stretching vibration of C¼O and the signal for S¼O groups of CSA dopants in the outer PPY film peak is observed at 1186 cm1 [44,45], while C–O of alkoxy in GO is located at 1038 cm1 in the FT-IR spectra [27]. The powerful nondestructive Raman spectroscopy is often used for the structural analysis of carbon materials [46]. The Raman spectra of GO, PPY-GO and PPY-GO/PPY-CSA coatings are shown in
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Fig. 1b. It can be seen that the Raman spectrum of GO exhibits two peaks at 1348 and 1585 cm1with high intensity, which correspond to the D band and G band, respectively [47]. The D band is related to the structural defects, while the G band corresponds to the sp2 hybridized carbon atoms [48,49]. For PPY-GO coating, the D band and G band are shift to 1344 and 1568 cm1, respectively, implying the p–p interaction between PPY and GO species as reported previously [48,50]. As for PPY-GO/PPY-CSA composite coating, its respective D and G band are shifted to 1341 and 1563 cm1, which may due to the enhanced interaction of GO, PPY and CSA [48]. Moreover, the Raman spectrum of PPY-GO/PPY-CSA composite coating also clearly shows a broad peak at 1048 cm1 and two peaks at 928 and 970 cm1, resulting from the PPY species [50]. However, GO contribution in the PPY-GO coating is obviously stronger, while the characteristic peaks of PPY are relatively low in the Raman spectrum, which is similar to the previous report [48]. Generally, the intensity ratio of D and G band (ID/IG) specifies the structural disorder or defects of carbon materials [46]. Fig. 1b indicates that the ID/IG values are declined in the sequence of GO, PPY-GO and PPY-GO/PPY-CSA, which illustrates the decrease of defect sites after GO combined with PPY [51]. The above Raman analyses are in agreement with FT-IR results. XPS is an effective tool for studying the surface constitution and valence of the materials [40]. The XPS measurements were performed to explore the composition of PPY-GO/PPY-CSA composite coating, the C 1s, N 1s, O 1s and S 2p XPS spectra are displayed in Fig. 2. C 1s XPS spectrum are fitted by four peaks as presented in Fig. 2a, the peaks at 284.5 and 285.5 eV belong to C–C/ C–H and C–N bonds at PPY matrix, respectively [52,53], while the peaks at 284.9 and 287.6 eV are due to the respective C–S and C¼O binds in CSA matrix [54–56]. The N 1s XPS spectrum of PPY-GO/ PPY-CSA composite coating is decomposed into two peaks (Fig. 2b), the fitted peaks at 399.7 and 401.1 eV can be ascribed to the neutral NH– binds and the nitrogen with positive charge (N+) at the backbone of doped PPY, respectively [14,57]. The doping level of PPY in PPY-CSA layer can be assessed by determining the proportion of N+ in the total nitrogen (Ntotal) content [53] and the estimated N+/Ntotal ratio is 0.48. The decomposed O 1s XPS spectrum of PPY-GO/PPY-CSA composite coating can be fitted by three peaks as displayed in Fig. 2c. The peaks at the binding energy of 531.1, 532.1 and 533.5 eV are attribute to the C¼O, S¼O and S– OH bonds in CSA molecules, respectively [52,58]. The S 2p XPS spectrum of the covalently bonded sulfate anion from the outer PPY-CSA film has been deconvoluted in two peaks at 167.7 eV and 168.8 eV (Fig. 2d), which ascribed to S 2p 3/2 and S 2p 1/2, respectively [56,59]. The above results demonstrate that the PPY film doped with CSA has been electrodeposited successfully, which is consistent with the FT-IR and Raman analysis. In the previous work, we have detected the morphology of the dry GO aqueous dispersion, which shows a wavy morphology with
Fig. 1. FT-IR (a) and Raman (b) spectra of GO, PPY-GO and PPY-GO/PPY-CSA coatings.
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Fig. 2. XPS spectra of C 1s (a), N 1s (b), O 1s (c) and S 2p (d) for PPY-GO/PPY-CSA composite coating.
Fig. 3. The SEM and 3D AFM images of PPY-GO (a, c) and PPY-GO/PPY-CSA (b, d) coatings.
Please cite this article in press as: L. Jiang, et al., Enhanced anticorrosion performance of PPY-graphene oxide/PPY-camphorsulfonic acid composite coating for 304SS bipolar plates in proton exchange membrane fuel cell, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j. jiec.2019.08.032
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wrinkle characteristics [29]. The morphology of GO is similar with the results of previous studies [60,61]. Fig. 3 represented the SEM and AFM images of the prepared coatings. Fig. 3a implies that the obtained PPY-GO coating exhibits a homogenous surface and maintained the wrinkle GO characteristics (magnified inset), and this morphology is consistent with the co-electrodeposited PPYGO composites reported previously [62]. Fig. 3b indicates that the morphology of PPY-GO/PPY-CSA composite coating surface exhibits a relatively compact surface, composed of comparatively small packed PPY particles (magnified inset of Fig. 3b) compared to the PPY-GO coating shown in Fig. 3a, which is due to the CSA doping in PPY film. The surface roughness of the coatings was confirmed by the AFM technique, and the corresponding 3D AFM images are shown in Fig. 3c and d, respectively. The PPY-GO/PPY-CSA composite coating shows a rougher surface compared to the PPY-GO coating and its average roughness (Ra) value is found to be 57.3 nm, whereas the Ra value of the PPY-GO coating is 24.8 nm. Similar to the SEM morphology in Fig. 3b and d presents obvious compact PPY particles with spherical grains on PPY-GO/PPY-CSA composite coating surface, resulting from the presence of the spatial molecular CSA doped PPY layer. The good adhesion property of conductive polymer coating is vital to its anticorrosion performance because the corrosion process through coating delamination or blistering can be restrained in the aggressive environment [63,64]. The adhesion strength of the coatings was assessed through the cross-cut method and the optical images of the tested samples before and after the application of pressure-sensitive tape is shown in Fig. 4, the adhesion degree of the coating was determined on the basis of the ASTM D3359. After the removal of the tape, it is observed that the edges of the cross cuts for both PPY-GO and PPY-GO/PPY-CSA
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coatings are intact and smooth without detachments (Fig. 4c and d), hence the adhesion degree of these two coatings can be classified as 5B. The above results imply that the prepared coatings all exhibit satisfactory adhesion strength with the underlying substrates. This is mainly due to the synergistic effect of GO with PPY backbone and the formation of network structure between PPY and GO species in the PPY-GO layer as reported previously [26], which could provide high adhesion strength for both the PPY-GO and PPY-GO/PPY-CSA coatings. The anticorrosion performance of the metallic bipolar plate in the harsh PEMFC environment determines the lifespan of fuel cell and stable corrosion resistance is an important criteria to evaluate the bipolar plate performance. Generally, the cathodic working environment of bipolar plates is more aggressive than the anodic working environment [8,65]. Thus, the potentiostatic polarization testing for bare and coated 304SS in the simulated cathodic environment of PEMFC were conducted to investigate the stability of the samples. The time dependence potentiostatic polarization plots (10 h) of the tested samples at the potential of 0.6 VSCE are presented in Fig. 5. A low and stable current density (j) is consistent with the better anticorrosion performance of the tested sample [64]. As shown in Fig. 5, initially the current densities for all the samples exhibit a rapid decline tendency, owing to the passivation of the 304SS [8,66]. Although the j value of PPY-GO coated 304SS is much lower than that of uncoated sample, the current density for both bare and PPY-GO coated 304SS exhibit an increasing tendency through the prolonged polarization process, indicating the rapid dissolution of the passive films on 304SS surface and these two samples suffer with localized corrosion [10]. In contrast, the current density of PPY-GO/PPY-CSA coated 304SS fluctuated during the first 2 h of polarization, then decreased and keeps almost
Fig. 4. Microscope images of PPY-GO and PPY-GO/PPY-CSA coated 304SS before (a, b) and after (c, d) adhesion tape test.
Please cite this article in press as: L. Jiang, et al., Enhanced anticorrosion performance of PPY-graphene oxide/PPY-camphorsulfonic acid composite coating for 304SS bipolar plates in proton exchange membrane fuel cell, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j. jiec.2019.08.032
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element CPEf and Rf are attributed to the capacitance and resistance of the oxide film, respectively, while the CPEdl and Rct attributed to the capacitance of double-layer and the charge transfer resistance, respectively. For PPY-GO coating, the high frequency region of the Nyquist plot shows a capacitive response and a diffusion line can be observed at the low frequency region as present in Fig. 6a, correspondingly, the Bode plot shown in Fig. 6b presents one time constant. The EIS data of the PPY-GO coating can be fitted by the equivalent circuit presented in Fig. 6d, the element Zd is the diffusion impendence element, implying the charge diffusion within the conductive PPY-GO coating [17]. As for the PPY-GO/PPY-CSA composite coating, a diffusion impedance can be found in the intermediate frequency region as shown in the magnified inset of Fig. 6a, however, the finite length diffusion can be blocked by the compact coating and the low frequency region of the Nyquist plot presents a capacitive response. This different EIS response of PPY-GO/PPY-CSA coating might due to the bilayer structure of the composite coating. The EIS data of PPY-GO/PPYCSA coated 304SS is modeled with a new equivalent circuit presented in Fig. 6e, the two embedded RC circuits signifies the response of PPY-GO/PPY-CSA composite coating and the interaction at coating/substrate interface, respectively. For coated 304SS, the elements CPEc and Rc in the equivalent electric circuits (Fig. 6d and e) stand for the coating capacitance and the resistance of the micro-pores in the coating, respectively. Fig. 6a show satisfactory fitting by using the suggested circuits and the impedance of CPE is expressed in Eq. (1) [68].
Fig. 5. The potentiostatic polarization curves of the bare and coated 304SS held at 0.6 VSCE.
constant at low values. As reported previously, the fall of the current density implies the metastable localized corrosion that caused during polarization are repassivated [67]. Therefore, the low current density indicates that PPY-GO/PPY-CSA composite coating could serve as a qualified barrier to prevent the further inward migration of aggressive species in the harsh chloride containing environment and exhibits advanced anticorrosion property in the simulated cathode condition of PEMFC. The dissolved metal ions evolved from bipolar plate due to corrosion have significant influence on the performance of the fuel cell because the degradation of the membrane can be caused by the contamination of metal ions. The amount of the released metal ions is depending on the anticorrosion performance of the bipolar plates and the aggressive environment. The concentration of the dissolved Fe, Cr, Ni, Mn and Mo ions in the corrosive solution are determined through ICP-OES tests after potentiostatic polarization, and the results are summarized in Table 1. It can be seen that the total concentration of metal ions for the uncoated 304SS is much higher than those of PPY-GO and PPY-GO/PPY-CSA coated 304SS, which is due to the severe corrosion of the uncoated substrate during polarization as confirmed by Fig. 5. However, low concentration of the released metal ions for PPY-GO/PPY-CSA coated 304SS is observed in comparison with the PPY-GO coated substrate, due to the better anticorrosion property of PPY-GO/PPYCSA composite coating. Combining the potentiostatic polarization and ICP-OES results, it can be concluded that the PPY-GO/PPY-CSA composite coating has satisfactory corrosion protection ability in the simulated operation potential of PEMFC, which can effectively inhibit the release of metal ions from the 304SS bipolar plate in the corrosive chloride containing solution. The EIS response with the corresponding equivalent electric circuit for bare and coated 304SS in 0.3 M HCl solution are present in Fig. 6. Fig. 6a indicates that the Nyquist plot of uncoated 304SS exhibits two depressed capacitive loops and its corresponding Bode plot shows two time constant (Fig. 6b). Fig. 6c is used as the equivalent electric circuit for the EIS plot of the bare 304SS. The element Rs signifies the electrolyte resistance, the constant phase
ZCPE ¼
1 Y0 ðjvÞn
ð1Þ
Where Y0 and j in Eq. (1) are represent the admittance magnitude of CPE and the imaginary unit, respectively, the exponential term n is varies from 0–1 and implies an ideal capacitance response when n = 1. Fig. 7 is the EIS plots of bare 304SS, PPY-GO and PPY-GO/PPYCSA coated 304SS in 0.3 M HCl solution at different immersion time. Fig. 7a indicates that the depressed loops of uncoated 304SS at low frequency region are unremarkable after 12 h of immersion, however, the corresponding Bode plots shown in Fig. 7a present two time constant throughout the immersion time. The impendence of the bare 304SS increased initially in 0–24 h of immersion followed by a declined tendency. The fitting results of the uncoated 304SS are represented in Table 2, it can be seen that the Rf values increased with time before 24 h of immersion, which is due to the passivation of 304SS in the acidic environment [69]. However, this passive film gradually devastated because of the inward penetration of corrosive species, further leads to a decreasing trend of Rf values after 24 h of immersion. In addition, the increasing trend of Rct values during 0–72 h of immersion is observed, attributed to the accumulated corrosion products on 304SS surface. The decrease in Rct value at 120 h of immersion indicates the 304SS suffers severe corrosion due to the attack of corrosive species. For PPY-GO coated 304SS, the EIS plots and the fitted EIS parameters during immersion are represented in Fig. 7b and Table 3, respectively. Fig. 7b shows that the Nyquist plot of PPY-GO coating presents a similar shape with a single capacitive loop and a
Table 1 The concentration of the dissolved metal ions for uncoated and coated 304SS after 10 h of potentiostatic polarization. Samples
304SS PPY-GO PPY-GO/PPY-CSA
Metal ions concentration (mg L1) Fe
Cr
Ni
Mn
Mo
Total
503 5.22 2.66 0.21 0.68 0.04
123 2.63 0.62 0.08 0.16 0.06
60.9 2.15 0.31 0.02 0.079 0.03
7.27 0.87 0.042 0.01 0.011 0.01
0.29 0.07 0.012 0.01 –
694.46 3.644 0.93
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Fig. 6. Nyquist and Bode plots of the bare and coated 304SS (a, b); the equivalent electric circuits for the EIS plots of bare 304SS (c), PPY-GO (d) and PPY-GO/PPY-CSA (e) coated 304SS.
semi-infinite diffusion process associated with the straight line during immersion [70]. This semi-infinite diffusion behavior with immersion time implies that the ion diffusion behavior is the dominant charge transfer process [71]. The Nyquist plots in Fig. 7b shown that the capacitive loop of PPY-GO coated 304SS continually expanded at 0–456 h of immersion and exhibit a decreasing trend afterwards. Accordingly, the fitted Rc values display a similar variation tendency during immersion (Table 3). It can be seen that the PPY-GO coating exhibit relatively low initial Rc value compared to the uncoated 304SS (Table 2). Rc is the micro-pore resistance in the coating and it is the sum of Re and Ri, where Re represents the electronic resistance due to electrons movement in polymer chain while Ri is the ionic resistance within the coating defects [18]. Hence, the low Rc value of coated 304SS is mainly due to the conductive property of the coating. The increase of Rc values in Table 3 indicates the appearance of reduced PPY with decreased coating conductivity during immersion, which increase both Re and Ri values [18]. The coating could provide satisfactory corrosion protection to the underlying substrate during this reduction process, however, after PPY is completely reduced in the harsh chloride containing environment, the PPY-GO coating merely serve as a barrier to inhibit the ion diffusion by compromising on its anticorrosion property. Thereby, the Rc values of PPY-GO coated 304SS gradually decreased after 456 h of immersion. Fig. 7c displays the time dependence Nyquist and Bode plots for PPY-GO/PPY-CSA coating and the fitting results are listed in Table 4. Fig. 7c indicates that both the Nyquist and Bode plots show a consistent shape during immersion. The Rc values of PPY-GO/PPYCSA composite coating are relatively low with immersion time up to 696 h, indicating the coating exhibits relatively low electrical resistance. Since, the PPY-GO/PPY-CSA composite coating has good conductivity, the coating could afford enhanced anodic protection to the underneath 304SS during immersion [18]. The CPEc value is an important parameter to evaluate the deterioration of the anticorrosion coatings, Table 4 indicates that the PPY-GO/PPY-CSA composite coating exhibits lower CPEc values during immersion than those of the PPY-GO coating, indicating the composite coating could reduce the water adsorption and inhibit the electrolyte
diffusion more effectively [53,72]. The variation in Rct values is related to the electrochemical process at the coating/304SS interface. It is found that the Rct values of PPY-GO/PPY-CSA coated 304SS show a little decrease during immersion, but the Rct value after 696 h of immersion is still much higher than that of the uncoated 304SS (Table 2), implies the highly enhanced anticorrosion performance of 304SS. The above EIS results indicate that during the immersion process, the PPY-GO/PPY-CSA composite coating could provide more effective corrosion protection to the 304SS than PPY-GO coating through effective barrier and anodic protection effect. After 696 h of immersion, the morphology features of the tested samples are studied by means of AFM and SEM technologies. Fig. 8a and b show the topographical morphologies of PPY-GO and PPY-GO/PPY-CSA coatings, the roughness displays in Fig. 8a indicates that the Ra value of PPY-GO coating ca. 42 nm, this value is higher than its roughness before immersion (Fig. 3d), which may due to the PPY redox in the coating structure during the long-term immersion. For the PPY-GO/PPY-CSA composite coating, the surface roughness is ca. 54 nm (Fig. 8b) after immersion and there is no clear change in the topographical morphology compared to its 3D AFM image before immersion (Fig. 3e). The surface morphology of the two coatings after immersion are presented in Fig. 8c and d, respectively. It can be seen that both the coatings have relatively compact surface, comparatively, the PPYGO/PPY-CSA composite coating shows no significant morphology change after immersion and exhibits no visible microscopic defects on the coating surface (Fig. 8d), which is consistent with the AFM results. The AFM and SEM images of PPY-GO/PPY-CSA coating imply that the coating structure was merely affected during the long-term immersion. The respective interfaces underlying PPYGO and PPY-GO/PPY-CSA coatings are presented in Fig. 8e and f. The presence of localized pitting and crevice corrosion can be observed beneath the PPY-GO coating (Fig. 8e), signifying the weakened anticorrosion performance of PPY-GO coating during the 696 h of immersion in harsh chloride containing environment. In contrast, Fig. 8f shows that the surface of 304SS underlying the PPY-GO/PPY-CSA composite coating only exhibits slight corrosion
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Fig. 7. EIS response for bare 304SS (a), PPY-GO (b) and PPY-GO/PPY-CSA (c) coated 304SS at different immersion time.
Table 2 The fitting results of EIS plots for uncoated 304SS at different immersion time in 0.3 M HCl solution. Time (h)
Rs (V cm2)
CPEf (mV1 cm2 sn)
nf
0 12 24 72 120
3.22 0.06 3.17 0.03 3.31 0.09 3.42 0.06 3.23 0.04
0.572 0.04 0.624 0.15 1.34 0.13 1.83 0.09 2.25 0.11
0.85 0.02 0.86 0.03 0.88 0.01 0.87 0.01 0.83 0.02
Rf (V cm2) 114.2 4.54 215.4 3.88 273.3 3.59 153.2 1.98 138.8 1.39
CPEdl (mV1 cm2 sn)
ndl
Rct (V cm2)
73.6 0.32 37.8 0.21 1.93 0.09 2.90 0.07 4.31 0.11
0.82 0.03 0.87 0.01 0.82 0.03 0.83 0.02 0.81 0.03
35.74 1.53 102.9 4.16 130.5 3.28 195.6 2.08 174.1 5.29
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Table 3 The fitting results of EIS plots for PPY-GO coated 304SS at different immersion time in 0.3 M HCl solution. Time (h)
Rs (V cm2)
CPEc (mV1 cm2 sn)
nc
Rc (V cm2)
Zd-R (V cm2)
0 48 168 264 360 456 552 696
11.64 0.73 11.85 0.42 11.06 0.47 12.13 0.28 16.39 0.93 13.13 0.32 13.04 0.33 13.74 0.51
71.4 0.91 68.9 0.79 61.6 0.99 60.5 0.51 60.4 0.55 61.7 0.62 61.9 0.75 58.2 1.39
0.86 0.01 0.88 0.02 0.88 0.01 0.86 0.03 0.85 0.01 0.85 0.03 0.85 0.02 0.86 0.02
22.98 0.93 173.82 2.71 610.34 4.83 854.91 7.92 1618.12 7.16 2113.22 10.26 1714.21 8.37 1642.46 15.69
13.09 1.08 128.65 3.56 268.24 8.58 349.28 7.93 496.24 10.28 558.93 9.69 469.35 12.58 432.48 11.23
Table 4 The fitting results of EIS plots for PPY-GO/PPY-CSA coated 304SS at different immersion time in 0.3 M HCl solution. Time (h) 0 48 168 264 360 456 552 696
Rs (V cm2) 11.55 0.18 11.66 0.62 11.28 0.15 11.4 0.26 11.75 0.35 13.43 0.29 14.18 0.51 11.76 0.21
CPEc (mV1 cm2 sn) 27.7 2.08 10.6 0.72 20.9 0.18 25.7 0.25 24.2 1.08 27.9 0.89 30.4 1.58 26.5 2.14
nc 0.73 0.03 0.85 0.02 0.86 0.02 0.85 0.01 0.85 0.02 0.86 0.03 0.85 0.02 0.86 0.01
Rc (V cm2)
Zd-R (V cm2)
CPEdl (mV1 cm2 sn)
5.79 0.09 7.07 0.06 16.61 0.12 18.25 0.31 20.23 0.26 28.67 0.72 32.05 0.45 36.31 1.07
3.51 0.36 2.38 0.52 8.75 0.84 11.26 1.23 15.68 0.98 24.26 1.02 36.71 1.38 46.02 1.43
46.8 1.96 44.9 0.68 38.6 0.72 37.2 0.67 34.7 0.98 33.3 1.48 33.9 3.66 33.7 1.53
ndl 0.99 0.01 0.98 0.01 0.97 0.02 0.98 0.01 0.99 0.01 0.99 0.01 0.98 0.02 0.99 0.01
Rct (V cm2) 16670 193 16689 233 15520 201 15862 185 15469 150 14623 98 14070 102 14180 75
Fig. 8. 3D AFM images of PPY-GO and PPY-GO/PPY-CSA coatings after immersion (a, b); the SEM images of PPY-GO and PPY-GO/PPY-CSA coatings (c, d) and the respective morphology at coating/substrate interface (e, f) after 696 h of immersion.
Fig. 9. Schematic diagram of the corrosion protection mechanism for the 304SS coated by PPY-GO/PPY-CSA composite coating.
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(rectangular area), which demonstrates that the coating could provide effective corrosion protection to 304SS during 696 h of immersion, these results are in agreement with the EIS analysis.
interaction of the two layers sustains the coatings conductivity and tailors the defects of the coating during the prolonged service time. As a result, the composite coating exhibits superior anticorrosion performance.
Mechanism The major culprits for corrosion are H2O, O2, Cl and other aggressive species, these corrosive species could migrate to coating/substrate interface by the ion-exchange and inward diffusion, hence the anticorrosion coatings must have compact structure and good adhesion property. During corrosion process, the dominate cathodic reaction of the conductive polymer coated 304SS takes place at coating/solution interface, i.e., the reduction process of the conductive polymer [20]. In this process, the doped ions (A) are released from the polymer matrix as shown in Eq. (2). Fe2+ ions are released from the substrate at the anodic zone, however, due to the instinct passivation property of the conductive polymers, the released Fe2+ ions could form a protective oxide layer which contains Fe2O3 at coating/substrate interface along with the polymers reduction (anodic protection effect). Moreover, the reduced PPY can be re-oxidized in the presence of oxygen and the cations, as shown in Eq. (3). The conductive polymer coating could act as an ionic membrane during the ionic transport process and this ionic transport is decided by the ion-exchange ability of the polymer film [19,20]. Thereby, the mobility of doped ions within the polymer matrix has critical influence on the anodic protection property of the coating. In our work, the combination of PPY-GO layer and PPY-CSA layer forms a network and the complex structure provides effective corrosion protection to the 304SS bipolar plate. The schematic corrosion protection mechanism diagram for PPY-GO/PPY-CSA composite coating is shown in Fig. 9. PPY+A + e → PPY0 + A
(2)
O2 + PPY0 + 4H+ → PPY+ + 2H2O
(3)
Initially, the CSA dopants of the PPY film improved the conductivity of the composite coating and the spatial structured of CSA makes the dedoping as well as the ion-exchange processes more difficult. Due to the cation permselectivity of the PPY-CSA film [9,19], the aggressive anions (Cl) in the environment are repelled while the cations (H+) required for the re-oxidization of PPY-GO layer (Eq. (3)) could be transported, hence the conductivity of the composite coating during immersion can be sustained. Later on, the PPY-GO layer strengthens the coatings adhesion property and the introduced functional groups from GO in PPY chain are hard to release or exchange with other ions due to their immobile property. Since GO has large and fully passivated surface, the existence of GO in PPY film extends the diffusion pathway of the partial diffused corrosive species and makes the pathway more circuitous as shown in Fig. 9a. Finally, the doped ions are stable in PPY matrix and both the layers serve as permselective barrier membranes to inhibit the diffusion of small ions from the external environment. Besides, the partly released sulfonate anions from the PPY-CSA layer could result in the formation of insoluble complex with the dissolved Fe2+ ions which migrate out through the coating defects, further tailor the defects of the PPY-GO layer and repassivate the substrate (Fig. 9b) [73]. Thereby, the aggressive ions can be trapped within the PPY-GO/PPY-CSA composite coating even if the ion penetration occurred during the prolonged immersion time and the long-term anticorrosion property of the coating can be reinforced. According to the above discussion, we deduce that the PPY-GO/ PPY-CSA composite coating exhibits cation permselectivity and presents hindrance towards ion- exchange. Meanwhile, the
Conclusion In this work, the conductive PPY-GO/PPY-CSA composite coating composed of a GO modified PPY layer and a spatial structured CSA doped PPY has been successfully electrodeposited on 304SS bipolar plate. It is found that the PPY-GO/PPY-CSA composite coating has a compact structure and exhibits better anticorrosion property than the PPY-GO coating in the simulated harsh chloride containing PEMFC environment. The potentiostatic polarization and EIS results indicate that the PPY-GO/PPY-CSA composite coating presents stable corrosion protection ability during the long-term immersion and exhibits satisfactory conductivity. The superior anticorrosion property of the composite coating results from the synergistic effect of PPY-GO and PPY-CSA layers. Because, the introduced GO in PPY layer strengthens the adhesion of the coatings and prolongs the diffusion path of the corrosive species, whereas the PPY-CSA layer impedes the ionexchange and maintains the coatings conductivity with enhanced anodic protection. Hence, this type of conductive PPY-GO/PPY-CSA anticorrosion coating shows an application potential on 304SS bipolar plates and has a promising prospect in PEMFC. Acknowledgements The authors would like to acknowledge the National Natural Science Foundation of China (No. 51501088, 51771090), the Science & Technology Support Plan of Jiangsu Province (No. BE2014039, BY2015080), the Qing Lan Project of Jiangsu Province, the Basic Research Priorities Program of Nantong (JC201803) and the Natural Science Foundation of Jiangsu Province (BK20161289) for their joint financial support. References [1] V.R. Stamenkovic, D. Strmcnik, P.P. Lopes, N.M. Markovic, Nat. Mater. 16 (2016) 57. [2] J. Jin, Z. Zhu, D. Zheng, Int. J. Hydrog. Energy 42 (2017) 11758. [3] A.E. Fetohi, R.M.A. Hameed, K.M. El-Khatib, J. Ind. Eng. Chem. 30 (2015) 239. [4] L. Zhang, X.-F. Zhang, X.-L. Chen, A.-J. Wang, D.-M. Han, Z.-G. Wang, J.-J. Feng, J. Colloid Interface Sci. 536 (2019) 556. [5] Y.J. Ren, M.R. Anisur, W. Qiu, J.J. He, S. Al-Saadi, R.K. Singh Raman, J. Power Sources 362 (2017) 366. [6] R. Wlodarczyk, D. Zasada, S. Morel, A. Kacprzak, Int. J. Hydrog. Energy 41 (2016) 17644. [7] H. Tsuchiya, O. Kobayashi, Int. J. Hydrog. Energy 29 (2004) 985. [8] K. Zhang, S. Sharma, ACS Sustain. Chem. Eng. 5 (2017) 277. [9] L. Jiang, J.A. Syed, Y. Gao, Q. Zhang, J. Zhao, H. Lu, X. Meng, Appl. Surf. Sci. 426 (2017) 87. [10] T.J. Pan, X.W. Zuo, T. Wang, J. Hu, Z.D. Chen, Y.J. Ren, J. Power Sources 302 (2016) 180. [11] Y.J. Ren, J. Chen, C.L. Zeng, C. Li, J.J. He, Int. J. Hydrog. Energy 41 (2016) 8542. [12] D. Zhang, L. Duan, L. Guo, Z. Wang, J. Zhao, W.H. Tuan, K. Niihara, Int. J. Hydrog. Energy 36 (2011) 9155. [13] Y.Z. Gao, J.A. Syed, H.B. Lu, X.K. Meng, Appl. Surf. Sci. 360 (2016) 389. [14] Y. Wang, Y. Qiu, Z. Chen, X. Guo, Corros. Sci. 118 (2017) 96. [15] C. Borsoi, A.J. Zattera, C.A. Ferreira, Appl. Surf. Sci. 364 (2016) 124. [16] Z. Chen, Y. Jin, W. Yang, B. Xu, Y. Chen, X. Yin, Y. Liu, J. Ind. Eng. Chem. 75 (2019) 178. [17] S. Joseph, J.C. McClure, R. Chianelli, P. Pich, P.J. Sebastian, Int. J. Hydrog. Energy 30 (2005) 1339. [18] Y.J. Ren, C.L. Zeng, J. Power Sources 182 (2008) 524. [19] D. Kowalski, M. Ueda, T. Ohtsuka, J. Mater. Chem. 20 (2010) 7630. [20] B. Zeybek, N.Ö. Pekmez, E. Kılıç, Electrochim. Acta 56 (2011) 9277. [21] L. Zhong, S. Xiao, J. Hu, H. Zhu, F. Gan, Corros. Sci. 48 (2006) 3960. [22] B. Ramezanzadeh, M.H. Mohamadzadeh Moghadam, N. Shohani, M. Mahdavian, Chem. Eng. J. 320 (2017) 363. [23] K. Catt, H. Li, C.X. Tracy, Acta Biomater. 48 (2017) 530.
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Enhanced anticorrosion performance of PPY-graphene oxide/PPYcamphorsulfonic acid composite coating for 304SS bipolar plates in proton exchange membrane fuel cell Li Jianga,b , Junaid Ali Syedc, Guoli Zhanga , Yujie Maa , Jun Mad, Hongbin Lua,** , Xiangkang Menga,* a Institute of Materials Engineering, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, PR China b School of Chemistry, Biology and Materials Engineering, Suzhou University of Science and Technology, Suzhou 215009, PR China c Department of Metallurgy and Materials Engineering, Mehran University of Engineering and Technology, Jamshoro, Pakistan d School of Mechatronics & Traffic Engineering, Nantong College of Science and Technology, Nantong 226007, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 8 April 2019 Received in revised form 8 July 2019 Accepted 12 August 2019 Available online xxx
The enhanced corrosion resistance with sustained conductivity are the prerequisites of stainless steel bipolar plates for practical application in proton exchange membrane fuel cell. Herein, we prepare a conductive polypyrrole-graphene oxide/polypyrrole-camphorsulfonic acid bilayer composite coating (PPY-GO/PPY-CSA) on 304 stainless steel bipolar plate by electrodeposition method. The electrochemical tests are conducted in the simulated bipolar plates working environment, the potentiostatic polarization results imply that the PPY-GO/PPY-CSA composite coating offers stable corrosion resistance with low potentiostatic corrosion current density in comparison with the PPY-GO coating. The corresponding electrochemical impedance spectroscopy measurements reveal that the PPY-GO/PPY-CSA composite coating exhibits satisfactory conductivity and displays sustained anodic protection effect with superior anticorrosion performance during the 696 h of immersion. The excellent corrosion protection ability of the PPY-GO/PPY-CSA composite coating owing to its good adhesion strength, compact structure, satisfactory conductivity as well as the synergetic interaction between the two layers of the coating. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Bipolar plates Corrosion protection Polypyrrole Graphene oxide Camphorsulfonic acid
Introduction The shortage of resources and increasing environmental pollution have aroused wide public concern for renewable and green energy systems [1–4]. Proton exchange membrane fuel cell (PEMFC) is one of the environmentally friendly energy generation system, has attracted significant attention due to the high efficiency, low emission and large power density [5,6]. Bipolar plates is an important component in PEMFC (occupies ca. 80% of the weight and ca. 45% of the cost of PEMFC stacks) [5], which plays a crucial role in the durability and performance of the fuel cell [7]. The aggressive acidic working environment determines that the
* Corresponding author. ** Corresponding author at: Institute of Materials Engineering, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Jiangsu, China. E-mail addresses: [email protected] (H. Lu), [email protected] (X. Meng).
bipolar plate material must have sturdy chemical stability, satisfactory corrosion resistance and good conductivity [6,8]. Economical metallic bipolar plates such as stainless steel (SS), aluminum and nickel plates have been broadly explored for the substitution of conventional graphite bipolar plates due to their good conductivity, high mechanical strength and low gas permeability [2,6]. Although the metallic bipolar plates have many advantages, however, the formation of oxide film at metal surface during the long time service in PEMFC environment weakens the conductivity of the bipolar plates [9,10]. Moreover, the metallic bipolar plates are vulnerable to the attack of corrosive species and the produced corrosion products would contaminate the internal environment of PEMFC, further reduces its lifespan [11]. Consequently, various types of corrosion inhibiting coatings with high electrical conductivity have been developed for the metallic bipolar plates [8,12]. The conductive polymer coatings based on polyaniline (PANI), polypyrrole (PPY) etc. are considered as suitable candidates due to their chemical stability, economical production as well as satisfactory conductivity [13–16]. Numerous
https://doi.org/10.1016/j.jiec.2019.08.032 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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reports show the convenience of electrodeposition method for the fabrication of conductive polymer coatings and the corrosion protection ability of the polymer coatings for metallic bipolar plates is wildly studied [9,10,17,18]. Specifically, the conductivity of the coating is affected by the doping anions in polymer matrix [19,20]. In general, the conductive polymers have two states that are the oxidized and reduced states, the polymer exhibits conductivity in the doped oxidized state and could passivate the underlying substrate through anodic protection effect, also the coating layer acts as a barrier that can physically insulate the substrate from the corrosive environment [8,21]. However, the electrochemically synthesized conductive polymer films usually have poor adhesion strength and exhibit microscopic defects that provide the diffusion pathway to the corrosive species, eventually result in the initiation of corrosion process beneath the coating [19,20]. Besides, the conductive polymer can be gradually reduced in the harsh PEMFC environment during long-term service, which affects the coating conductivity and deteriorates the anodic protection ability of the coating [9,11]. Therefore, modification of the coating defects and maintain the coatings conductivity are the crucial concern for the practical application of conductive polymer coatings, which could enhance the stability of the anticorrosion performance for bipolar plates in PEMFC environment. A most robust and feasible alternative way is to fabricate the bilayer conductive polymer coatings with organic and inorganic additives. Ren et al. [11,18] described that the electrodeposited bilayer PPY/PANI film significantly enhances the anticorrosion property of 304SS bipolar plates in PEMFC environment. Pan et al. [10] also envisaged that the bilayer conductive polymer film could provide superior protection ability to the copper bipolar plates and the coating exhibits relatively low contact resistance. The functional material graphene oxide (GO) has extensive research in various fields such as lithium batteries, supercapacitors and anticorrosion coatings due to its large surface area, high band gap and the excellent chemical stability [22–25]. It is reported that the introduction of GO in the anticorrosion coating significantly advances the coatings adhesion property, further improves the anticorrosion performance [26]. The functional groups e.g., carboxyl groups in GO could provide reactive sites that several studies have been focused on the combination of conductive polymers with GO. Ramezanzadeh et al. [22] described that the insertion of GO in PANI improves the conductivity of PANI and the PANI-GO composite enhances the corrosion resistance of zinc-rich epoxy coated SS in NaCl environment. GO and conductive polymer composites can also be synthesized through electrochemical method. Zhu et al. [27] described that the oxygen-containing groups in GO sheets could act as charge-balancing dopants within PPY that facilitates the co-electrodeposition of PPY-GO composites through potentiostatic method, and the prepared composites exhibit excellent energy storage property. Qi et al. [28] revealed that GO could directly introduced into PPY matrix without other dopants through pulse electrochemical incorporation. In the previous work, we have also prepared the high-performance anticorrosion coating consist of PPY and GO by in-situ electrodeposition method [29]. Previously, we demonstrated that PPY doped with the organic camphorsulfonic acid (CSA) which has spatial molecular structure could enhance the coatings conductivity and sustain the anodic protection for 304SS bipolar plates [9]. In order to further reduce the coating defects and enhance the long-term anticorrosion performance of 304SS bipolar plates in the chloride containing PEMFC environment, this study aims to develop a novel bilayer anticorrosion composite coating system for 304SS which contains a polypyrrole-graphene oxide (PPY-GO) layer and a polypyrrolecamphorsulfonic acid (PPY-CSA) layer by electrodeposition technique. We speculated that the incorporated GO in PPY matrix at
PPY-GO layer could enhance the coatings adhesion property and the PPY doped with CSA layer could improve the conductivity of the coating, hence the PPY-GO/PPY-CSA composite coating would offer superior corrosion protection to the bipolar plate in PEMFC. Experimental Materials The graphite powder (spectral pure) with ca. 45–48 mm size was obtained from Sinpharm Chemical Reagent Co., Ltd., while pyrrole monomer and other chemicals were purchased from Shanghai Aladdin Bio-Chem. 304SS coupons were offered by Senda Decoration Materials Co. Ltd., mechanically polished by SiC papers then ultrasonically washed by acetone and ethanol, respectively. GO was synthesized from graphite powder by using the modified Hummer’s method [30,31] and the homogeneous GO aqueous dispersion was used for the coating electrodeposition. Electrodeposition of coatings The electrodeposition process and electrochemical analyses of the coatings were all conducted with Metrohm PGSTAT 302N instrument at room temperature. The coatings were prepared through galvanostatical electrodeposition process in the three electrodes system that the saturated calomel electrode (SCE) as reference electrode and a platinum mesh counter electrode, whereas the working electrode was the treated 304SS coupon with 1 cm2 exposed area. The mixed solution which contains 0.5 mg mL1 GO and 0.4 M pyrrole was stirred to form a uniform supporting electrolyte for electrodeposition of PPY-GO layer and the electrodeposition process was galvanostatically performed for 10 min with a current density of 1 mA cm2. Then the PPY-CSA layer was electrodeposited on the top of PPY-GO layer for 15 min at 1 mA cm2 in the mixed solution that composed of 0.5 M CSA with 0.4 M pyrrole monomer. The thickness of the prepared PPY-GO/ PPY-CSA composite coating is ca. 4.2 mm. For ease of comparison, the single PPY-GO coating was also electrodeposited in the supporting electrolyte which contains 0.5 mg mL1 GO and 0.4 M pyrrole in the same current density for 25 min. Instrumentation The scanning electron microscope (SEM) images of the samples were acquired with the JOEL JSM-6510 SEM system and the FT-IR spectra of the two kinds of coating were recorded on a PerkinElmer Fourier transform infrared spectrometer. The Raman spectra of the samples were accomplished with LabRAM HR Evolution Raman microscope with 532 nm excitation laser wavelength. The X-ray photoelectron spectroscopy (XPS) examined the valence states of elements were conducted with Thermo Fisher Scientific K-Alpha XPS instrument. Atomic force microscopy (AFM) studies were accomplished by using Vecco Metrology atomic force microscope. The cross-cut technique along with tape test was used to evaluate the coatings adhesion strength and the adhesion degree was assessed from the lowest 0B grade to highest 5B grade based on ASTM D3359. The concentration of dissolved metal ions in the electrolyte after 10 h of potentiostatic polarization was measured by using the Perkin–Elmer Optima 5300DV Inductive Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). The electrochemical properties of the bare and coated 304SS bipolar plates were tested in 0.3 M HCl solution to simulate a definite aggressive PEMFC environment and all the experiments were initiated at a steady-state open circuit potential (OCP). The potentiostatic polarization tests were achieved in 0.6 VSCE, which approaches the PEMFC cathode operating potential [32]. The
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electrochemical impedance (EIS) measurements were obtained at OCP value under the frequency range of 100 kHz–0.01 Hz at 10 mV amplitude. Results and discussion For the synthesis of PPY-GO layer, the mutual repulsion of electric charges of the negatively charged groups in GO drive GO species towards anode [33]. Meanwhile, the pyrrole monomer and GO could be in contact by p–p interactions and electrostatic force [34]. Under the applied current, the GO species with negatively charged oxygen-containing groups acts as anionic dopants, thus could be incorporated into the polymer matrix to balance the positively charged PPY [35]. As a result, PPY and GO can be simultaneously in-situ electrodeposited and the uniform PPY-GO layer is obtained. Notably, GO can be reduced to reduced graphene oxide (rGO) at proper cathodic potentials [36], however, it is hard to be reduced and change to rGO under the given elcetrosynthesis conditions, which is similar to the previous reports [37,38]. Characterization of electrodeposited coatings Fig. 1a shows the FT-IR spectra of GO and two different coatings in the wavenumber range of 800–4000 cm1. As we analyzed previously [29], the GO shows typical absorption peaks of oxide groups, the vibration of O–H groups is at 3396 cm1 [27], the peaks at 1736 and 1417 cm1 are belong to carboxyl (C¼O) and C–OH stretching, respectively [22,39]. For PPY-GO coating, the peak at 3432 cm1 is the characteristic of the stretching of N–H in PPY [40]. Signal at 1534 cm1 is due to the C¼C stretching and the characteristic peak at 1174 cm1 comes from the in-plane bending vibration of C–H binds [28,41]. The characteristic peaks of GO are located at 1733 and 1041 cm1, assigned to C¼O and C–O groups, respectively [42,43]. These characteristic peaks imply successful introduction of GO in PPY matrix [27]. However, the absorption peaks of C–OH and the vibration of O–H related to GO are not clearly represented in the FT-IR spectrum of PPY-GO coating, which is due to the overlapping of the main peaks of GO and PPY [42]. The characteristic peaks of PPY-GO/PPY-CSA bilayer composite coating show slight shift due to the interaction of the two layers [10]. The absorption peak at 3424 cm1 is associated with N–H stretching in PPY while the signal at 1531 and 1168 cm1 are ascribed to the C¼C stretching and C–H in-plane vibration, respectively. The characteristic peak at 1732 cm1 belongs to the stretching vibration of C¼O and the signal for S¼O groups of CSA dopants in the outer PPY film peak is observed at 1186 cm1 [44,45], while C–O of alkoxy in GO is located at 1038 cm1 in the FT-IR spectra [27]. The powerful nondestructive Raman spectroscopy is often used for the structural analysis of carbon materials [46]. The Raman spectra of GO, PPY-GO and PPY-GO/PPY-CSA coatings are shown in
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Fig. 1b. It can be seen that the Raman spectrum of GO exhibits two peaks at 1348 and 1585 cm1with high intensity, which correspond to the D band and G band, respectively [47]. The D band is related to the structural defects, while the G band corresponds to the sp2 hybridized carbon atoms [48,49]. For PPY-GO coating, the D band and G band are shift to 1344 and 1568 cm1, respectively, implying the p–p interaction between PPY and GO species as reported previously [48,50]. As for PPY-GO/PPY-CSA composite coating, its respective D and G band are shifted to 1341 and 1563 cm1, which may due to the enhanced interaction of GO, PPY and CSA [48]. Moreover, the Raman spectrum of PPY-GO/PPY-CSA composite coating also clearly shows a broad peak at 1048 cm1 and two peaks at 928 and 970 cm1, resulting from the PPY species [50]. However, GO contribution in the PPY-GO coating is obviously stronger, while the characteristic peaks of PPY are relatively low in the Raman spectrum, which is similar to the previous report [48]. Generally, the intensity ratio of D and G band (ID/IG) specifies the structural disorder or defects of carbon materials [46]. Fig. 1b indicates that the ID/IG values are declined in the sequence of GO, PPY-GO and PPY-GO/PPY-CSA, which illustrates the decrease of defect sites after GO combined with PPY [51]. The above Raman analyses are in agreement with FT-IR results. XPS is an effective tool for studying the surface constitution and valence of the materials [40]. The XPS measurements were performed to explore the composition of PPY-GO/PPY-CSA composite coating, the C 1s, N 1s, O 1s and S 2p XPS spectra are displayed in Fig. 2. C 1s XPS spectrum are fitted by four peaks as presented in Fig. 2a, the peaks at 284.5 and 285.5 eV belong to C–C/ C–H and C–N bonds at PPY matrix, respectively [52,53], while the peaks at 284.9 and 287.6 eV are due to the respective C–S and C¼O binds in CSA matrix [54–56]. The N 1s XPS spectrum of PPY-GO/ PPY-CSA composite coating is decomposed into two peaks (Fig. 2b), the fitted peaks at 399.7 and 401.1 eV can be ascribed to the neutral NH– binds and the nitrogen with positive charge (N+) at the backbone of doped PPY, respectively [14,57]. The doping level of PPY in PPY-CSA layer can be assessed by determining the proportion of N+ in the total nitrogen (Ntotal) content [53] and the estimated N+/Ntotal ratio is 0.48. The decomposed O 1s XPS spectrum of PPY-GO/PPY-CSA composite coating can be fitted by three peaks as displayed in Fig. 2c. The peaks at the binding energy of 531.1, 532.1 and 533.5 eV are attribute to the C¼O, S¼O and S– OH bonds in CSA molecules, respectively [52,58]. The S 2p XPS spectrum of the covalently bonded sulfate anion from the outer PPY-CSA film has been deconvoluted in two peaks at 167.7 eV and 168.8 eV (Fig. 2d), which ascribed to S 2p 3/2 and S 2p 1/2, respectively [56,59]. The above results demonstrate that the PPY film doped with CSA has been electrodeposited successfully, which is consistent with the FT-IR and Raman analysis. In the previous work, we have detected the morphology of the dry GO aqueous dispersion, which shows a wavy morphology with
Fig. 1. FT-IR (a) and Raman (b) spectra of GO, PPY-GO and PPY-GO/PPY-CSA coatings.
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Fig. 2. XPS spectra of C 1s (a), N 1s (b), O 1s (c) and S 2p (d) for PPY-GO/PPY-CSA composite coating.
Fig. 3. The SEM and 3D AFM images of PPY-GO (a, c) and PPY-GO/PPY-CSA (b, d) coatings.
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wrinkle characteristics [29]. The morphology of GO is similar with the results of previous studies [60,61]. Fig. 3 represented the SEM and AFM images of the prepared coatings. Fig. 3a implies that the obtained PPY-GO coating exhibits a homogenous surface and maintained the wrinkle GO characteristics (magnified inset), and this morphology is consistent with the co-electrodeposited PPYGO composites reported previously [62]. Fig. 3b indicates that the morphology of PPY-GO/PPY-CSA composite coating surface exhibits a relatively compact surface, composed of comparatively small packed PPY particles (magnified inset of Fig. 3b) compared to the PPY-GO coating shown in Fig. 3a, which is due to the CSA doping in PPY film. The surface roughness of the coatings was confirmed by the AFM technique, and the corresponding 3D AFM images are shown in Fig. 3c and d, respectively. The PPY-GO/PPY-CSA composite coating shows a rougher surface compared to the PPY-GO coating and its average roughness (Ra) value is found to be 57.3 nm, whereas the Ra value of the PPY-GO coating is 24.8 nm. Similar to the SEM morphology in Fig. 3b and d presents obvious compact PPY particles with spherical grains on PPY-GO/PPY-CSA composite coating surface, resulting from the presence of the spatial molecular CSA doped PPY layer. The good adhesion property of conductive polymer coating is vital to its anticorrosion performance because the corrosion process through coating delamination or blistering can be restrained in the aggressive environment [63,64]. The adhesion strength of the coatings was assessed through the cross-cut method and the optical images of the tested samples before and after the application of pressure-sensitive tape is shown in Fig. 4, the adhesion degree of the coating was determined on the basis of the ASTM D3359. After the removal of the tape, it is observed that the edges of the cross cuts for both PPY-GO and PPY-GO/PPY-CSA
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coatings are intact and smooth without detachments (Fig. 4c and d), hence the adhesion degree of these two coatings can be classified as 5B. The above results imply that the prepared coatings all exhibit satisfactory adhesion strength with the underlying substrates. This is mainly due to the synergistic effect of GO with PPY backbone and the formation of network structure between PPY and GO species in the PPY-GO layer as reported previously [26], which could provide high adhesion strength for both the PPY-GO and PPY-GO/PPY-CSA coatings. The anticorrosion performance of the metallic bipolar plate in the harsh PEMFC environment determines the lifespan of fuel cell and stable corrosion resistance is an important criteria to evaluate the bipolar plate performance. Generally, the cathodic working environment of bipolar plates is more aggressive than the anodic working environment [8,65]. Thus, the potentiostatic polarization testing for bare and coated 304SS in the simulated cathodic environment of PEMFC were conducted to investigate the stability of the samples. The time dependence potentiostatic polarization plots (10 h) of the tested samples at the potential of 0.6 VSCE are presented in Fig. 5. A low and stable current density (j) is consistent with the better anticorrosion performance of the tested sample [64]. As shown in Fig. 5, initially the current densities for all the samples exhibit a rapid decline tendency, owing to the passivation of the 304SS [8,66]. Although the j value of PPY-GO coated 304SS is much lower than that of uncoated sample, the current density for both bare and PPY-GO coated 304SS exhibit an increasing tendency through the prolonged polarization process, indicating the rapid dissolution of the passive films on 304SS surface and these two samples suffer with localized corrosion [10]. In contrast, the current density of PPY-GO/PPY-CSA coated 304SS fluctuated during the first 2 h of polarization, then decreased and keeps almost
Fig. 4. Microscope images of PPY-GO and PPY-GO/PPY-CSA coated 304SS before (a, b) and after (c, d) adhesion tape test.
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element CPEf and Rf are attributed to the capacitance and resistance of the oxide film, respectively, while the CPEdl and Rct attributed to the capacitance of double-layer and the charge transfer resistance, respectively. For PPY-GO coating, the high frequency region of the Nyquist plot shows a capacitive response and a diffusion line can be observed at the low frequency region as present in Fig. 6a, correspondingly, the Bode plot shown in Fig. 6b presents one time constant. The EIS data of the PPY-GO coating can be fitted by the equivalent circuit presented in Fig. 6d, the element Zd is the diffusion impendence element, implying the charge diffusion within the conductive PPY-GO coating [17]. As for the PPY-GO/PPY-CSA composite coating, a diffusion impedance can be found in the intermediate frequency region as shown in the magnified inset of Fig. 6a, however, the finite length diffusion can be blocked by the compact coating and the low frequency region of the Nyquist plot presents a capacitive response. This different EIS response of PPY-GO/PPY-CSA coating might due to the bilayer structure of the composite coating. The EIS data of PPY-GO/PPYCSA coated 304SS is modeled with a new equivalent circuit presented in Fig. 6e, the two embedded RC circuits signifies the response of PPY-GO/PPY-CSA composite coating and the interaction at coating/substrate interface, respectively. For coated 304SS, the elements CPEc and Rc in the equivalent electric circuits (Fig. 6d and e) stand for the coating capacitance and the resistance of the micro-pores in the coating, respectively. Fig. 6a show satisfactory fitting by using the suggested circuits and the impedance of CPE is expressed in Eq. (1) [68].
Fig. 5. The potentiostatic polarization curves of the bare and coated 304SS held at 0.6 VSCE.
constant at low values. As reported previously, the fall of the current density implies the metastable localized corrosion that caused during polarization are repassivated [67]. Therefore, the low current density indicates that PPY-GO/PPY-CSA composite coating could serve as a qualified barrier to prevent the further inward migration of aggressive species in the harsh chloride containing environment and exhibits advanced anticorrosion property in the simulated cathode condition of PEMFC. The dissolved metal ions evolved from bipolar plate due to corrosion have significant influence on the performance of the fuel cell because the degradation of the membrane can be caused by the contamination of metal ions. The amount of the released metal ions is depending on the anticorrosion performance of the bipolar plates and the aggressive environment. The concentration of the dissolved Fe, Cr, Ni, Mn and Mo ions in the corrosive solution are determined through ICP-OES tests after potentiostatic polarization, and the results are summarized in Table 1. It can be seen that the total concentration of metal ions for the uncoated 304SS is much higher than those of PPY-GO and PPY-GO/PPY-CSA coated 304SS, which is due to the severe corrosion of the uncoated substrate during polarization as confirmed by Fig. 5. However, low concentration of the released metal ions for PPY-GO/PPY-CSA coated 304SS is observed in comparison with the PPY-GO coated substrate, due to the better anticorrosion property of PPY-GO/PPYCSA composite coating. Combining the potentiostatic polarization and ICP-OES results, it can be concluded that the PPY-GO/PPY-CSA composite coating has satisfactory corrosion protection ability in the simulated operation potential of PEMFC, which can effectively inhibit the release of metal ions from the 304SS bipolar plate in the corrosive chloride containing solution. The EIS response with the corresponding equivalent electric circuit for bare and coated 304SS in 0.3 M HCl solution are present in Fig. 6. Fig. 6a indicates that the Nyquist plot of uncoated 304SS exhibits two depressed capacitive loops and its corresponding Bode plot shows two time constant (Fig. 6b). Fig. 6c is used as the equivalent electric circuit for the EIS plot of the bare 304SS. The element Rs signifies the electrolyte resistance, the constant phase
ZCPE ¼
1 Y0 ðjvÞn
ð1Þ
Where Y0 and j in Eq. (1) are represent the admittance magnitude of CPE and the imaginary unit, respectively, the exponential term n is varies from 0–1 and implies an ideal capacitance response when n = 1. Fig. 7 is the EIS plots of bare 304SS, PPY-GO and PPY-GO/PPYCSA coated 304SS in 0.3 M HCl solution at different immersion time. Fig. 7a indicates that the depressed loops of uncoated 304SS at low frequency region are unremarkable after 12 h of immersion, however, the corresponding Bode plots shown in Fig. 7a present two time constant throughout the immersion time. The impendence of the bare 304SS increased initially in 0–24 h of immersion followed by a declined tendency. The fitting results of the uncoated 304SS are represented in Table 2, it can be seen that the Rf values increased with time before 24 h of immersion, which is due to the passivation of 304SS in the acidic environment [69]. However, this passive film gradually devastated because of the inward penetration of corrosive species, further leads to a decreasing trend of Rf values after 24 h of immersion. In addition, the increasing trend of Rct values during 0–72 h of immersion is observed, attributed to the accumulated corrosion products on 304SS surface. The decrease in Rct value at 120 h of immersion indicates the 304SS suffers severe corrosion due to the attack of corrosive species. For PPY-GO coated 304SS, the EIS plots and the fitted EIS parameters during immersion are represented in Fig. 7b and Table 3, respectively. Fig. 7b shows that the Nyquist plot of PPY-GO coating presents a similar shape with a single capacitive loop and a
Table 1 The concentration of the dissolved metal ions for uncoated and coated 304SS after 10 h of potentiostatic polarization. Samples
304SS PPY-GO PPY-GO/PPY-CSA
Metal ions concentration (mg L1) Fe
Cr
Ni
Mn
Mo
Total
503 5.22 2.66 0.21 0.68 0.04
123 2.63 0.62 0.08 0.16 0.06
60.9 2.15 0.31 0.02 0.079 0.03
7.27 0.87 0.042 0.01 0.011 0.01
0.29 0.07 0.012 0.01 –
694.46 3.644 0.93
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Fig. 6. Nyquist and Bode plots of the bare and coated 304SS (a, b); the equivalent electric circuits for the EIS plots of bare 304SS (c), PPY-GO (d) and PPY-GO/PPY-CSA (e) coated 304SS.
semi-infinite diffusion process associated with the straight line during immersion [70]. This semi-infinite diffusion behavior with immersion time implies that the ion diffusion behavior is the dominant charge transfer process [71]. The Nyquist plots in Fig. 7b shown that the capacitive loop of PPY-GO coated 304SS continually expanded at 0–456 h of immersion and exhibit a decreasing trend afterwards. Accordingly, the fitted Rc values display a similar variation tendency during immersion (Table 3). It can be seen that the PPY-GO coating exhibit relatively low initial Rc value compared to the uncoated 304SS (Table 2). Rc is the micro-pore resistance in the coating and it is the sum of Re and Ri, where Re represents the electronic resistance due to electrons movement in polymer chain while Ri is the ionic resistance within the coating defects [18]. Hence, the low Rc value of coated 304SS is mainly due to the conductive property of the coating. The increase of Rc values in Table 3 indicates the appearance of reduced PPY with decreased coating conductivity during immersion, which increase both Re and Ri values [18]. The coating could provide satisfactory corrosion protection to the underlying substrate during this reduction process, however, after PPY is completely reduced in the harsh chloride containing environment, the PPY-GO coating merely serve as a barrier to inhibit the ion diffusion by compromising on its anticorrosion property. Thereby, the Rc values of PPY-GO coated 304SS gradually decreased after 456 h of immersion. Fig. 7c displays the time dependence Nyquist and Bode plots for PPY-GO/PPY-CSA coating and the fitting results are listed in Table 4. Fig. 7c indicates that both the Nyquist and Bode plots show a consistent shape during immersion. The Rc values of PPY-GO/PPYCSA composite coating are relatively low with immersion time up to 696 h, indicating the coating exhibits relatively low electrical resistance. Since, the PPY-GO/PPY-CSA composite coating has good conductivity, the coating could afford enhanced anodic protection to the underneath 304SS during immersion [18]. The CPEc value is an important parameter to evaluate the deterioration of the anticorrosion coatings, Table 4 indicates that the PPY-GO/PPY-CSA composite coating exhibits lower CPEc values during immersion than those of the PPY-GO coating, indicating the composite coating could reduce the water adsorption and inhibit the electrolyte
diffusion more effectively [53,72]. The variation in Rct values is related to the electrochemical process at the coating/304SS interface. It is found that the Rct values of PPY-GO/PPY-CSA coated 304SS show a little decrease during immersion, but the Rct value after 696 h of immersion is still much higher than that of the uncoated 304SS (Table 2), implies the highly enhanced anticorrosion performance of 304SS. The above EIS results indicate that during the immersion process, the PPY-GO/PPY-CSA composite coating could provide more effective corrosion protection to the 304SS than PPY-GO coating through effective barrier and anodic protection effect. After 696 h of immersion, the morphology features of the tested samples are studied by means of AFM and SEM technologies. Fig. 8a and b show the topographical morphologies of PPY-GO and PPY-GO/PPY-CSA coatings, the roughness displays in Fig. 8a indicates that the Ra value of PPY-GO coating ca. 42 nm, this value is higher than its roughness before immersion (Fig. 3d), which may due to the PPY redox in the coating structure during the long-term immersion. For the PPY-GO/PPY-CSA composite coating, the surface roughness is ca. 54 nm (Fig. 8b) after immersion and there is no clear change in the topographical morphology compared to its 3D AFM image before immersion (Fig. 3e). The surface morphology of the two coatings after immersion are presented in Fig. 8c and d, respectively. It can be seen that both the coatings have relatively compact surface, comparatively, the PPYGO/PPY-CSA composite coating shows no significant morphology change after immersion and exhibits no visible microscopic defects on the coating surface (Fig. 8d), which is consistent with the AFM results. The AFM and SEM images of PPY-GO/PPY-CSA coating imply that the coating structure was merely affected during the long-term immersion. The respective interfaces underlying PPYGO and PPY-GO/PPY-CSA coatings are presented in Fig. 8e and f. The presence of localized pitting and crevice corrosion can be observed beneath the PPY-GO coating (Fig. 8e), signifying the weakened anticorrosion performance of PPY-GO coating during the 696 h of immersion in harsh chloride containing environment. In contrast, Fig. 8f shows that the surface of 304SS underlying the PPY-GO/PPY-CSA composite coating only exhibits slight corrosion
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Fig. 7. EIS response for bare 304SS (a), PPY-GO (b) and PPY-GO/PPY-CSA (c) coated 304SS at different immersion time.
Table 2 The fitting results of EIS plots for uncoated 304SS at different immersion time in 0.3 M HCl solution. Time (h)
Rs (V cm2)
CPEf (mV1 cm2 sn)
nf
0 12 24 72 120
3.22 0.06 3.17 0.03 3.31 0.09 3.42 0.06 3.23 0.04
0.572 0.04 0.624 0.15 1.34 0.13 1.83 0.09 2.25 0.11
0.85 0.02 0.86 0.03 0.88 0.01 0.87 0.01 0.83 0.02
Rf (V cm2) 114.2 4.54 215.4 3.88 273.3 3.59 153.2 1.98 138.8 1.39
CPEdl (mV1 cm2 sn)
ndl
Rct (V cm2)
73.6 0.32 37.8 0.21 1.93 0.09 2.90 0.07 4.31 0.11
0.82 0.03 0.87 0.01 0.82 0.03 0.83 0.02 0.81 0.03
35.74 1.53 102.9 4.16 130.5 3.28 195.6 2.08 174.1 5.29
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Table 3 The fitting results of EIS plots for PPY-GO coated 304SS at different immersion time in 0.3 M HCl solution. Time (h)
Rs (V cm2)
CPEc (mV1 cm2 sn)
nc
Rc (V cm2)
Zd-R (V cm2)
0 48 168 264 360 456 552 696
11.64 0.73 11.85 0.42 11.06 0.47 12.13 0.28 16.39 0.93 13.13 0.32 13.04 0.33 13.74 0.51
71.4 0.91 68.9 0.79 61.6 0.99 60.5 0.51 60.4 0.55 61.7 0.62 61.9 0.75 58.2 1.39
0.86 0.01 0.88 0.02 0.88 0.01 0.86 0.03 0.85 0.01 0.85 0.03 0.85 0.02 0.86 0.02
22.98 0.93 173.82 2.71 610.34 4.83 854.91 7.92 1618.12 7.16 2113.22 10.26 1714.21 8.37 1642.46 15.69
13.09 1.08 128.65 3.56 268.24 8.58 349.28 7.93 496.24 10.28 558.93 9.69 469.35 12.58 432.48 11.23
Table 4 The fitting results of EIS plots for PPY-GO/PPY-CSA coated 304SS at different immersion time in 0.3 M HCl solution. Time (h) 0 48 168 264 360 456 552 696
Rs (V cm2) 11.55 0.18 11.66 0.62 11.28 0.15 11.4 0.26 11.75 0.35 13.43 0.29 14.18 0.51 11.76 0.21
CPEc (mV1 cm2 sn) 27.7 2.08 10.6 0.72 20.9 0.18 25.7 0.25 24.2 1.08 27.9 0.89 30.4 1.58 26.5 2.14
nc 0.73 0.03 0.85 0.02 0.86 0.02 0.85 0.01 0.85 0.02 0.86 0.03 0.85 0.02 0.86 0.01
Rc (V cm2)
Zd-R (V cm2)
CPEdl (mV1 cm2 sn)
5.79 0.09 7.07 0.06 16.61 0.12 18.25 0.31 20.23 0.26 28.67 0.72 32.05 0.45 36.31 1.07
3.51 0.36 2.38 0.52 8.75 0.84 11.26 1.23 15.68 0.98 24.26 1.02 36.71 1.38 46.02 1.43
46.8 1.96 44.9 0.68 38.6 0.72 37.2 0.67 34.7 0.98 33.3 1.48 33.9 3.66 33.7 1.53
ndl 0.99 0.01 0.98 0.01 0.97 0.02 0.98 0.01 0.99 0.01 0.99 0.01 0.98 0.02 0.99 0.01
Rct (V cm2) 16670 193 16689 233 15520 201 15862 185 15469 150 14623 98 14070 102 14180 75
Fig. 8. 3D AFM images of PPY-GO and PPY-GO/PPY-CSA coatings after immersion (a, b); the SEM images of PPY-GO and PPY-GO/PPY-CSA coatings (c, d) and the respective morphology at coating/substrate interface (e, f) after 696 h of immersion.
Fig. 9. Schematic diagram of the corrosion protection mechanism for the 304SS coated by PPY-GO/PPY-CSA composite coating.
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(rectangular area), which demonstrates that the coating could provide effective corrosion protection to 304SS during 696 h of immersion, these results are in agreement with the EIS analysis.
interaction of the two layers sustains the coatings conductivity and tailors the defects of the coating during the prolonged service time. As a result, the composite coating exhibits superior anticorrosion performance.
Mechanism The major culprits for corrosion are H2O, O2, Cl and other aggressive species, these corrosive species could migrate to coating/substrate interface by the ion-exchange and inward diffusion, hence the anticorrosion coatings must have compact structure and good adhesion property. During corrosion process, the dominate cathodic reaction of the conductive polymer coated 304SS takes place at coating/solution interface, i.e., the reduction process of the conductive polymer [20]. In this process, the doped ions (A) are released from the polymer matrix as shown in Eq. (2). Fe2+ ions are released from the substrate at the anodic zone, however, due to the instinct passivation property of the conductive polymers, the released Fe2+ ions could form a protective oxide layer which contains Fe2O3 at coating/substrate interface along with the polymers reduction (anodic protection effect). Moreover, the reduced PPY can be re-oxidized in the presence of oxygen and the cations, as shown in Eq. (3). The conductive polymer coating could act as an ionic membrane during the ionic transport process and this ionic transport is decided by the ion-exchange ability of the polymer film [19,20]. Thereby, the mobility of doped ions within the polymer matrix has critical influence on the anodic protection property of the coating. In our work, the combination of PPY-GO layer and PPY-CSA layer forms a network and the complex structure provides effective corrosion protection to the 304SS bipolar plate. The schematic corrosion protection mechanism diagram for PPY-GO/PPY-CSA composite coating is shown in Fig. 9. PPY+A + e → PPY0 + A
(2)
O2 + PPY0 + 4H+ → PPY+ + 2H2O
(3)
Initially, the CSA dopants of the PPY film improved the conductivity of the composite coating and the spatial structured of CSA makes the dedoping as well as the ion-exchange processes more difficult. Due to the cation permselectivity of the PPY-CSA film [9,19], the aggressive anions (Cl) in the environment are repelled while the cations (H+) required for the re-oxidization of PPY-GO layer (Eq. (3)) could be transported, hence the conductivity of the composite coating during immersion can be sustained. Later on, the PPY-GO layer strengthens the coatings adhesion property and the introduced functional groups from GO in PPY chain are hard to release or exchange with other ions due to their immobile property. Since GO has large and fully passivated surface, the existence of GO in PPY film extends the diffusion pathway of the partial diffused corrosive species and makes the pathway more circuitous as shown in Fig. 9a. Finally, the doped ions are stable in PPY matrix and both the layers serve as permselective barrier membranes to inhibit the diffusion of small ions from the external environment. Besides, the partly released sulfonate anions from the PPY-CSA layer could result in the formation of insoluble complex with the dissolved Fe2+ ions which migrate out through the coating defects, further tailor the defects of the PPY-GO layer and repassivate the substrate (Fig. 9b) [73]. Thereby, the aggressive ions can be trapped within the PPY-GO/PPY-CSA composite coating even if the ion penetration occurred during the prolonged immersion time and the long-term anticorrosion property of the coating can be reinforced. According to the above discussion, we deduce that the PPY-GO/ PPY-CSA composite coating exhibits cation permselectivity and presents hindrance towards ion- exchange. Meanwhile, the
Conclusion In this work, the conductive PPY-GO/PPY-CSA composite coating composed of a GO modified PPY layer and a spatial structured CSA doped PPY has been successfully electrodeposited on 304SS bipolar plate. It is found that the PPY-GO/PPY-CSA composite coating has a compact structure and exhibits better anticorrosion property than the PPY-GO coating in the simulated harsh chloride containing PEMFC environment. The potentiostatic polarization and EIS results indicate that the PPY-GO/PPY-CSA composite coating presents stable corrosion protection ability during the long-term immersion and exhibits satisfactory conductivity. The superior anticorrosion property of the composite coating results from the synergistic effect of PPY-GO and PPY-CSA layers. Because, the introduced GO in PPY layer strengthens the adhesion of the coatings and prolongs the diffusion path of the corrosive species, whereas the PPY-CSA layer impedes the ionexchange and maintains the coatings conductivity with enhanced anodic protection. Hence, this type of conductive PPY-GO/PPY-CSA anticorrosion coating shows an application potential on 304SS bipolar plates and has a promising prospect in PEMFC. Acknowledgements The authors would like to acknowledge the National Natural Science Foundation of China (No. 51501088, 51771090), the Science & Technology Support Plan of Jiangsu Province (No. BE2014039, BY2015080), the Qing Lan Project of Jiangsu Province, the Basic Research Priorities Program of Nantong (JC201803) and the Natural Science Foundation of Jiangsu Province (BK20161289) for their joint financial support. References [1] V.R. Stamenkovic, D. Strmcnik, P.P. Lopes, N.M. Markovic, Nat. Mater. 16 (2016) 57. [2] J. Jin, Z. Zhu, D. Zheng, Int. J. Hydrog. Energy 42 (2017) 11758. [3] A.E. Fetohi, R.M.A. Hameed, K.M. El-Khatib, J. Ind. Eng. Chem. 30 (2015) 239. [4] L. Zhang, X.-F. Zhang, X.-L. Chen, A.-J. Wang, D.-M. Han, Z.-G. Wang, J.-J. Feng, J. Colloid Interface Sci. 536 (2019) 556. [5] Y.J. Ren, M.R. Anisur, W. Qiu, J.J. He, S. Al-Saadi, R.K. Singh Raman, J. Power Sources 362 (2017) 366. [6] R. Wlodarczyk, D. Zasada, S. Morel, A. Kacprzak, Int. J. Hydrog. Energy 41 (2016) 17644. [7] H. Tsuchiya, O. Kobayashi, Int. J. Hydrog. Energy 29 (2004) 985. [8] K. Zhang, S. Sharma, ACS Sustain. Chem. Eng. 5 (2017) 277. [9] L. Jiang, J.A. Syed, Y. Gao, Q. Zhang, J. Zhao, H. Lu, X. Meng, Appl. Surf. Sci. 426 (2017) 87. [10] T.J. Pan, X.W. Zuo, T. Wang, J. Hu, Z.D. Chen, Y.J. Ren, J. Power Sources 302 (2016) 180. [11] Y.J. Ren, J. Chen, C.L. Zeng, C. Li, J.J. He, Int. J. Hydrog. Energy 41 (2016) 8542. [12] D. Zhang, L. Duan, L. Guo, Z. Wang, J. Zhao, W.H. Tuan, K. Niihara, Int. J. Hydrog. Energy 36 (2011) 9155. [13] Y.Z. Gao, J.A. Syed, H.B. Lu, X.K. Meng, Appl. Surf. Sci. 360 (2016) 389. [14] Y. Wang, Y. Qiu, Z. Chen, X. Guo, Corros. Sci. 118 (2017) 96. [15] C. Borsoi, A.J. Zattera, C.A. Ferreira, Appl. Surf. Sci. 364 (2016) 124. [16] Z. Chen, Y. Jin, W. Yang, B. Xu, Y. Chen, X. Yin, Y. Liu, J. Ind. Eng. Chem. 75 (2019) 178. [17] S. Joseph, J.C. McClure, R. Chianelli, P. Pich, P.J. Sebastian, Int. J. Hydrog. Energy 30 (2005) 1339. [18] Y.J. Ren, C.L. Zeng, J. Power Sources 182 (2008) 524. [19] D. Kowalski, M. Ueda, T. Ohtsuka, J. Mater. Chem. 20 (2010) 7630. [20] B. Zeybek, N.Ö. Pekmez, E. Kılıç, Electrochim. Acta 56 (2011) 9277. [21] L. Zhong, S. Xiao, J. Hu, H. Zhu, F. Gan, Corros. Sci. 48 (2006) 3960. [22] B. Ramezanzadeh, M.H. Mohamadzadeh Moghadam, N. Shohani, M. Mahdavian, Chem. Eng. J. 320 (2017) 363. [23] K. Catt, H. Li, C.X. Tracy, Acta Biomater. 48 (2017) 530.
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