Research Article pubs.acs.org/journal/ascecg
Site-Selective, Low-Loading, Au Nanoparticle−Polyaniline Hybrid Coatings with Enhanced Corrosion Resistance and Conductivity for Fuel Cells Kun Zhang and Surbhi Sharma* School of Chemical Engineering, University of Birmingham, Edgbaston, B15 2TT, United Kingdom
ACS Sustainable Chem. Eng. 2017.5:277-286. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 06/30/18. For personal use only.
S Supporting Information *
ABSTRACT: Ultralow loading of Au nanoparticles (0.038 mg cm−2) and a polyaniline hybrid coating (AuNP-PANI) were deposited on stainless steel (SS316L) coupons. The unique two-step approach utilizing electrochemical deposition via cyclic voltammetry enabled growth of AuNPs in the fibrous PANI micropores thereby achieving enhanced surface coverage of SS316L and minimizing substrate corrosion via blocking of pores. The hybrid coatings revealed significantly low interfacial contact resistance values of 16.6 mΩ cm2 (achieving the US DOE 2017 targets). Potentiodynamic tests revealed the excellent corrosion resistance of AuNP-PANI hybrid coatings with a corrosion potential of 0.61 VSHE, which is positively shifted by 790 and 390 mV as compared to bare SS316L and PANISS316L, respectively. The corresponding potentiostatic corrosion current density of AuNP-PANI was reduced to 0.63 μA/cm2 from 2.28 and 0.65 μA/cm2 for bare SS316L and PANI-SS316L samples, respectively, thereby providing excellent stability in the cathodic polymer electrolyte fuel cell (PEFC) environment. Extensive electron microscopy and X-ray diffraction studies showed uniformly placed AuNP bundles with an average cluster size of 16 nm with Au (200) as the most prominent crystallite phase. Thermogravimetric studies revealed that the AuNPs did not affect the thermal stability of PANI which remained stable up to 260 °C which is well above the operating temperature of conventional PEFCs, making them highly suitable for coatings on bipolar plates for enhanced conductivity and corrosion resistance. KEYWORDS: Polymer electrolyte fuel cells, Bipolar plates, Interfacial contact resistance, Corrosion resistance, Au-PANI, Metal−polymer coatings, Polyaniline
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INTRODUCTION Depleting resources and increasing environmental concerns have led to an enormous body of research into renewable and green energy systems. Polymer electrolyte fuel cells (PEFCs) are widely recognized as the most promising alternatives to existing internal combustion engines. Near zero emissions, low operating temperatures, quick start-up, and high power density make them ideal for automotive applications.1−5 However, the high component costs and durability issues still limit the competitiveness of these systems.6 Within PEFCs, the bipolar plates (BPPs) still account for the second most expensive component (second to catalyst), accounting for 37% of the component costs and are the heaviest components in the fuel cell stack.4,7−9 The traditionally used (expensive to machine and relatively brittle) graphite and graphite composite plates are fast losing their reputation to the metal (mostly stainless steel) based BPPs.10 These allow the use of thin (1 mm or less) BPPs reducing the bulk weight and size of the stacks while reducing the machining costs.11 However, the metal BPPs are more susceptible to corrosion as compared to graphite based plates and often form a passive chromium oxide layer which decreases its conductivity.12,13 Consequently, extensive work © 2016 American Chemical Society
using a variety of materials has been reported on protective and conductive coatings for stainless steel. The various coatings can be categorized according to the method used and indeed the coating material. According to the type of coating material, there are three main categories (i) metal based coatings (which include Au, Ti based, Cr based etc.),4,5,14−20 (ii) conductive polymer coatings (which include polyaniline and polypyrrole),13,21−23 and (iii) carbon coatings (graphite, graphene based).9,24−26 The coating methods can be broadly divided into two categories (i) physical vapor deposition (PVD)17,27,28 and (ii) electrochemical methods (which include electroplating, cyclic voltammetry etc.).10,13,18 Even though the metal nitride coatings like CrN have been thoroughly studied; the precious and expensive metals like Au and Ti based coatings continue to be the most promising metal-based coatings. Moreover, most metal-based coatings are prepared using expensive PVD techniques, which significantly add to the preparation and overall costs. Conducting polymers such as polyaniline (PANI) Received: July 5, 2016 Revised: October 5, 2016 Published: October 31, 2016 277
DOI: 10.1021/acssuschemeng.6b01504 ACS Sustainable Chem. Eng. 2017, 5, 277−286
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. (a) Cyclic voltammograms showing electrochemical deposition of AuNPs on PANI-SS316L at a scan rate of 25 mV/s. (inset) Close up of peak at 0.03 V. (b) ICR measurements as a function of number of CV cycles for AuNP-PANI-SS316L at a compressive force of 140 N/cm2 and the Au loading profile. Here, a red triangle shows the loading after the (70 + 30) cycles. (c) Potentiodynamic polarization curves of the three samples in 1 mM H2SO4 solution at 80 °C with inset showing the AuNP-PANI coated SS316L. (d) Potentiostatic polarization curves of the three samples held at 1 VSHE in 1 mM H2SO4 solution purged with O2, 80 °C.
underlying PANI coating provides strong corrosion resistance, the AuNPs enhance the conductivity to achieve the desired interfacial contact resistance and further boost the corrosion resistance. The method allows for economical, controlled and convenient deposition technique which minimizes the precious metal loading, thus, making the whole process cost-effective and suitable for commercialization. The coatings were stable and were found to have very high corrosion resistance along with excellent ICR values, which were well within the DOE 2017 targets.1 To the best of our knowledge, no studies so far have explored the two-step electrochemical deposition technique for development of AuNP-PANI coatings on SS316L and investigated their corrosion resistance and contact resistance properties for application in PEFCs.
and polypyrrole (Ppy) with high redox potential have also received considerable interest as protective layers for stainless steel due to their corrosion protective properties. However, despite the term “conductive polymer” the intrinsic conductivity of these is nearly 2 orders lower than that of metals.29 As such these coatings often lead to increased interfacial contact resistance (ICR). In fact most studies investigating PANI and/or Ppy coatings for SS316L just focus on their corrosion resistant behavior and completely overlook the ICR values. Thin polymer coatings are, therefore, preferred in order to minimize the loss of conductivity but the coated materials still offer higher ICR as compared to uncoated SS316L and also risk the presence of micropores that restrict their durability. Composite coatings using conductive polymers with carbon nanostructures or metal NP and conducting polymer composites have been extensively studied for chemical sensors and biosensor applications.30 However, very selective (few and far between) studies have looked into the use of polymer− metal coatings for use as protective coatings for stainless steel.21,23,31 Moreover, these did not explore the ICR or the performance of such coatings under fuel cell specific conditions for applications as bipolar plates. In the current work, Au nanoparticles (AuNPs) and PANI hybrid coatings on SS316 were investigated using a two-step electrochemical deposition technique for application in PEFC BPPs. A thin PANI coating was deposited in the first step followed by AuNP growth. Unlike the homogeneous PVD deposition technique, this process allows growth of AuNPs at selective sites on the PANI coatings ensuring there are no micropores or pinholes in the final hybrid coatings. While the
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EXPERIMENTAL SECTION
Material Preparation. A stainless steel 316L (SS316L) substrate with a thickness of 0.1 mm was used as the substrate. Prior to each coating process, the SS316L substrates were cut into specimens of 35 mm × 17.5 mm. The cut samples were cleaned by immersing in 50% H2SO4 solution for 15 min at room temperature. This was done in order to remove the passive layer commonly formed on the SS316L surface. The cleaned SS316L substrates were thoroughly rinsed using distilled water to remove any residual H2SO4, cleaned with ethanol and finally dried in air. The freshly cleaned substrates were further used for coating deposition. Fabrication of AuNP-PANI Hybrid Coating. Aniline, gold(III) chloride trihydrate (HAuCl4·3H2O), and sulfuric acid (H2SO4) were purchased from Sigma-Aldrich. The deposition process was carried out with an IviumStat Potentiostat using a four-electrode system for simultaneously coating both sides of SS316L. The SS316L coupon was connected as working electrode, two platinum mesh were connected 278
DOI: 10.1021/acssuschemeng.6b01504 ACS Sustainable Chem. Eng. 2017, 5, 277−286
Research Article
ACS Sustainable Chemistry & Engineering as counter electrodes and Ag/AgCl (3 M KCl) was connected as reference electrode to enable coating on both sides of the SS316L. Deposition of PANI and AuNPs were accomplished in two separate steps. For the first step of coating, the electrochemical polymerization and deposition of PANI on SS316L substrate was carried out using cyclic voltammetry (CV) with a potential window from −180 mV to 1000 mV at a scan rate of 50 mV/s. Coatings were deposited using a 0.1 M H2SO4 solution containing 0.1 M aniline monomer. After the coating, the PANI coated SS316L was thoroughly washed with distilled water to remove H2SO4 and any aniline residues. The sample was then dried under N2 stream. In the second step, an aqueous solution of 0.1 M H2SO4 containing 0.1 mM HAuCl4 was used to electrochemically deposit AuNPs on PANI coated SS316L using CV method (potential window: −431 mV to 669 mV, scan rate: 25 mV/ s). After the coating process, the AuNP-PANI coated SS316L was washed thoroughly with distilled water to remove any unreacted HAuCl4 and H2SO4 residues, and then dried in air. Different coating thickness for AuNPs coatings were obtained by varying the number of CV cycles from 10 to 100 cycles while the cycle number for PANI coating was fixed to 5 cycles (Figure S1 shows a five cycle PANI coating; further details can be seen in the Supporting Information). Material Characterization and Electrochemical Testing. In order to study the phase states and the crystalline structure of the AuNP-PANI hybrid coatings, detailed material characterization studies were performed using scanning electron microscopy (SEM) (Tabletop Microscope TM3030, Hitachi-Hitec) and transmission electron microscopy (TEM) (JEOL 2100 electron microscope) coupled with an energy dispersive X-ray spectrometer (EDX). X-ray diffraction (XRD) (D2 PHASER second generation, Bruker), thermogravimetric analysis (TGA) (TG 209 F1), contact angle measurements (EasyDrop Contact Angle Measuring System), and Fourier transform infrared (FT-IR) spectroscopy (PerkinElmer FT-IR Spectrum 100) were also carried out to study composition, Au loading, and hydrophopic/ hydrophilic behavior. The AuNP loading was also measured using a Cubis Micro Balance (MSA2.7S0TRDM). The interfacial contact resistance (ICR) and corrosion resistance measurements (using linear sweep voltammetry and potentiostatic tests) were carried out in fuel cell specific cathode conditions as these are known to be more sever compared to anode conditions. Further details of the procedures can be found in the Supporting Information.
confirm the peak position for Au NP deposition, AuNPs were electrodeposited on a bare SS316L surface as shown in Supporting Information (Figure S2) using the same potential cycle and deposition conditions where a single reduction peak (0.01 VAg/AgCl) corresponding to Au(III) reduction to Au(0) was observed. This was found to be in good agreement with Gotti et al’s work.38 It also confirmed that the reduction of Au(III) to Au(0) undergoes one-step electron transfer. Following on from this, the peak at 0.03 VAg/AgCl in the reverse cycle (as seen in Figure 1) was also deconvoluted into its two component peaks associated with Au deposition and PANI oxidation (Figure S3), respectively. The overall reaction mechanism and AuNP nucleation process can be expressed using the following equation:35,36 [AuCl4]− + 3e− → Au + 4Cl−
(1)
It was observed (Figure 1a) that the peak at 0.2 VAg/AgCl, in the forward scan, slowly shifted to a more positive potential as the number of cycles progressed from the 10th cycle to the 100th. This is attributed to growth of AuNPs occurring on existing nucleation centers on PANI-SS316L during previous scans. This is consistent with previous thermodynamic and experimental studies on electrochemical deposition, which reported that electrodeposition preferentially takes place on nanoparticles formed during previous sweep cycles (rather than nucleation on new sites on the substrate). This is due to the lower activation/overpotential (less energy) required for the electrodeposition of AuNPs on Au than that on the substrate.35,38,39 It is also speculated that during prolonged deposition time, more Au is loaded on the PANI-SS316L surface which may have resulted in aggregate formation,40 more detail can be found in the SEM discussion. Interestingly, however, a slow but noticeable decrease in the current intensity of the Au(III) to Au reduction peak (0.03 VAg/AgCl) was observed as the CV scans proceeded further from 10 cycles to 100 cycles (see inset in Figure 1a). In fact, by the 100th cycle, the peak disappeared completely (inset Figure 1a). This observation was mainly attributed to the consumption of AuCl4− complex ion in the diffusion layer, leading to a continuous decrease of HAuCl4 concentration in the electrolyte. 35 Consequently, this would result in a residual concentration (of AuCl4− complex ion) lower than the minimal concentration required for the deposition reaction to take place. The effect of this was also seen in the ICR response of the various AuNP-PANI coatings and is discussed later in this study. Interfacial Contact Resistance and Corrosion Studies. The ICR properties of different AuNP-PANI hybrid coated SS316L (AuNP-PANI-SS316L) prepared using varying CV cycles were investigated as a function of Au loading, as shown in Figure 1b. The ICR of uncoated cleaned SS316L used in this work was recorded as 65.5 ± 5.4 mΩ cm2, and although PANI films were deposited with very low thicknesses in this work (5 CV cycles), the ICR still remained high for the PANI coated samples (Figure 1b), as compared to the required DOE standards.1 With the increase in Au loading from 0.0038 (10 cycles) to 0.0228 mg/cm2 (70 cycles), the contact resistance of AuNP-PANI-SS316L decreased from 160.2 ± 24.1 to 39.5 ± 9.3 mΩ cm2. This is accredited to the presence of AuNPs on PANI structure, which facilitates an easier pathway for the transport of electrons via PANI channels resulting in improved electronic conductivity. This is consistent with previous reports investigating Au-PANI for solar cells.41 It should be noted that
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RESULTS AND DISCUSSION Varying thicknesses of AuNP coatings (10−100 cycles) were deposited on PANI coated SS316L (PANI-SS316L from here on) samples. All PANI-SS316L samples consisted of 5 CV cycle PANI coatings in order to minimize the porosity, thickness, and the resulting resistance of the PANI coatings. This was optimized based on previously reported studies, which show that thicker PANI coatings result in a porous fibrous structure.22,32 Moreover, the intrinsic conductivity of PANI is nearly 2 orders lower than that of metals29 and hence thicker coatings result in higher interfacial contact resistance. The CV curves of PANI coating showed the typical PANI polymerization peaks as can be seen in the Supporting Information (Figure S1). Figure 1a shows the consecutive cyclic voltammograms for 100 cycles recorded during the deposition of Au NPs on PANI-SS316. Three sets of redox peaks (namely 0.2, 0.45. and 0.55 V) were observed in these CV scans. These peak positions correspond to the transition of PANI phases as reported in literature and seen in Figure S1.33,34 However, in the reverse scan (from 0.669 to 0.431 V), the peak at around 0.03 VAg/AgCl demonstrates the typical reduction of Au(III) to Au(0),35−37 resulting in the electrochemical deposition of AuNPs on to the PANI-SS316L substrate. This peak (at 0.03 VAg/AgCl) also relates to the formation of leucoemeraldine and is slowly shifted to more positive value (around 0.2 VAg/AgCl) as the number of cycles progress to the 100th cycle. To further 279
DOI: 10.1021/acssuschemeng.6b01504 ACS Sustainable Chem. Eng. 2017, 5, 277−286
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Coating Conditions and Properties Listed in Comparison to Previous Literature Studiesa SS316L coating method coating thickness ICR (mΩ cm2)
PANI-SS316L
AuNP-PANI-SS316L
65.5 ± 5.4
CV (50 mV/s) 5 cy 393.3 ± 18
CV (25 mV/s) 20 nm 16.6 ± 0.13
test condition
1 mM H2SO4 (pH = 2.75) 80 °C
1 mM H2SO4 (pH = 2.75) 80 °C
1 mM H2SO4 (pH = 2.75) 80 °C
Icorr (μA/cm2)
2.28
0.65
0.63
Vcorr (VSHE)
−0.18
0.22
0.61
polarization resistance (kΩ cm2)
7.3
61.1
68.3
contact angle (deg)
39.9
74.5
81.1
ref
this study
this study
this study
a
Au-SS316L electroplating 2, 10, 1000 nm 2 nm: 80 10 nm: 4 1 μm: 5 H2SO4 (pH = 2) 80 °C 2 nm: 3 10 nm: 0.8 1 μm: 0.17 2 nm: −0.06 10 nm:0.26 1 μm: 0.24
Yoon et al. 200818
Au-SS316L classified 20 nm 0.9
PANI-SS316L CV (20 mV/s) 2, 3, 4 cy
0.05 mM 1 M H2SO4 + 2 ppm of HF (pH = 0.5) 70 °C H2SO4 80 °C 0.33
0.49/NHE
Kumar et al. 201042
2 cy: 1.29 3 cy: 1.08 4 cy: 0.44 2 cy: 0.45 3 cy: 0.49 4 cy: 0.66 2 cy: 28.5 3 cy: 32.7 4 cy: 80.5 2 cy: 61.40 3 cy: 62.87 4 cy: 63.34 Le et al. 200922
CV−cyclic voltammetry; cy−cycles; Icorr−corrosion current density, Vcorr−corrosion potential.
loading of this new sample was determined to be 0.0389 mg/ cm2. These AuNP-PANI-SS316L samples, which demonstrated suitable ICR for use in fuel cell applications, were further thoroughly characterized using corrosion resistance studies as well as various materials studies. Electrochemical Corrosion Tests. Figure 1c presents the potentiodynamic polarization curves for uncoated SS316L, PANI-SS316L, and AuNP-PANI-SS316L in the simulated PEFC cathodic environment. These were conducted in 1 mM H2SO4 solution bubbled with N2 at 80 °C. The corrosion current and corrosion potential of both bare and coated substrates were calculated from the intercept of the anodic and cathodic Tafel slopes of the potentiodynamic polarization curves (Table 1). As can be seen in Figure 1c and Table 1, the polarization curve for bare SS316L showed a typical active− passive behavior with corrosion potential around −0.18 VSHE and a corrosion current density of 2.28 μA/cm2. On the introduction of PANI, the PANI-SS316L corrosion potential shifted to a much more noble value of 0.22 VSHE making it more corrosion resistant. Furthermore, the corrosion current density was also observed to decrease to 0.65 μA/cm2, which easily meets the DOE requirement (
