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Site –selective, Low-loading, Au Nanoparticle-Polyaniline Hybrid Coatings with Enhanced Corrosion Resistance and Conductivity for Fuel Cells Kun Zhang, and Surbhi Sharma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01504 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 2, 2016

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ACS Sustainable Chemistry & Engineering

Site –selective, Low-loading, Au Nanoparticle-Polyaniline Hybrid Coatings with Enhanced Corrosion Resistance and Conductivity for Fuel Cells Kun Zhang†, Surbhi Sharma†* †

School of Chemical Engineering, University of Birmingham, Edgbaston, B15 2TT, United Kingdom

*Contact for Correspondence: Dr Surbhi Sharma, School of Chemical Engineering, University of Birmingham, Edgbaston B15 2TT, United Kingdom Email: [email protected]; surbhi.1204@gmail

ACS Paragon Plus Environment

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ABSTRACT Ultra-low loading of Au nanoparticle (0.038 mg cm-2) and polyaniline hybrid coating (AuNPPANI) was deposited on stainless steel (SS316L) coupons. The unique two-step approach utilising electrochemical deposition via cyclic voltammetry enabled growth of AuNPs in the fibrous PANI micro-pores thereby achieving enhanced surface coverage of SS316L and minimising 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 mV and 390 mV as compared to bare SS316L and PANI-SS316L, 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 particle size of 16 nm with Au (200) as the most prominent crystallite phases. 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 recognised 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 using a variety of materials has been reported on protective and conductive coatings for stainless steel. The various coatings can be categorised 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 vapour 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) and polypyrrole (Ppy) with high redox potential have also received considerable interest as protective layers for stainless steel due to their corrosion protective properties.


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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 behaviour and completely overlook the ICR values. Thin polymer coatings are, therefore, preferred in order to minimise the loss of conductivity but the coated materials still offer higher ICR as compared to un-coated SS316L and also risk the presence of micro-pores 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 micro-pores or pinholes in the final hybrid coatings. While the 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 minimises the precious metal loading, thus, making the whole process cost effective and suitable for commercialisation. 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.


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EXPERIMENTAL Material Preparation Stainless steel 316L (SS316L) substrate with the 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 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 fourelectrode system for simultaneously coating both sides of SS316L. The SS316L coupon was connected as working electrode, two platinum mesh were connected 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 polymerisation 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


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cycles from 10 to 100 cycles while the cycle number for PANI coating was fixed to 5 cycles (Figure S1 shows 5 cycle PANI coating, further details can be seen in the Supporting Information).

Material Characterisation and Electrochemical Testing In order to study the phase states and the crystalline structure of the AuNP-PANI hybrid coatings, detailed material characterisation studies were performed using Scanning electron microscopy (SEM) (Tabletop Microscope TM3030, Hitachi-Hitec) and Transmission Electron microscopy (TEM) (JEOL 2100 electron microscope) coupled with energy dispersive X-ray spectrometer (EDX). X-ray diffraction (XRD) (D2 PHASER 2nd generation, Bruker), Thermogravimetirc analysis (TGA) (TG 209 F1), contact angle measurements (EasyDrop Contact Angle Measuring System) and Fourier transform Infra-red (FT-IR) Spectroscopy (PerkinElmer FT-IR Spectrum 100) were also carried out to study composition, Au loading and hydrophopic/hydrophilic behaviour. 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.

RESULTS AND DISCUSSION Varying thicknesses of AuNP coatings (10 to 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 minimise the porosity, thickness and the resulting resistance of the PANI coatings. This was optimised 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 metals [29] and hence thicker coatings result in higher interfacial contact resistance. The CV curves of PANI coating showed the typical PANI polymerisation peaks as can be seen in the Supporting Information (Figure S1). Figure 1(a) 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 V, 0.45 V and 0.55 V) were observed in these CV scans. These peak


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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 V 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 confirm the peak position for Au NP deposition, AuNPs were electrodeposited on 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 V

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]: [AuCl ] + 3e → Au + 4Cl

Equation (1)

It was observed (Figure 1(a)) 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 10th cycle to 100th cycle. This is attributed to growth of AuNPs occurring on existing nucleation centres 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


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may have resulted in aggregate formation [40], more detail can be found in 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 1(a)). In fact, by the 100th cycle, the peak disappeared completely (inset Figure 1(a)). 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 SS (AuNP-PANI-SS316L) prepared using varying CV cycles were investigated as a function of Au loading, as shown in Figure 2(a). 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 1(b)), as compared to the required DOE standards [1]. With the increase in Au loading from 0.0038 mg cm-2 (10 cycles) to 0.0228 mg cm-2 (70 cycles), the contact resistance of AuNP-PANI-SS316L decreased from 160.2 ± 24.1 mΩ cm2 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 the successive reduction in contact resistance became relatively moderate for the AuNP-PANI-SS316L subjected to 50 and 70 CV cycles. In fact, surprisingly, the sample with 100 CV cycle coating of AuNPs exhibited higher ICR value than the one exposed to 70 CV cycle for AuNPs deposition. This could be ascribed to the continuous decrease of HAuCl4 concentration in the electrolyte caused by consumption of AuCl4- (as mentioned above) resulted in a drastic decrease in the deposition rate of


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AuNPs beyond 50 cycles. To identify the precise reason for this behaviour another sample was coated for 70 cycles and again subjected to additional 30 cycles in a fresh solution of 0.1 M H2SO4 containing 0.1 mM HAuCl4. Figure 1(b) shows the cumulative Au loading in the Au-PANI-SS316L samples as the number of cycles increased, measured using a microbalance. This new coating prepared using (70+30) CV cycles demonstrated an ICR of 16.6 mΩ cm2 with an Au loading of 0.038 mg cm-2. The increased loading after the (70+30) cycles is shown as the red triangle in Figure 1(b). Looking at the Au loading profile, the relatively slow increase in the Au loading between 10-30 CV cycles is attributed to the formation of new Au nucleation sites. Beyond the 30 CV cycles the nearly linear increase in loading (up to 100 CV cycles) suggests that there is more growth and ripening of the existing Au particles than the formation of new nucleation sites [35, 38-40]. Note the slight curvature to the loading profile between 30 to 100 CV cycles suggesting decrease in deposition rate with increase in CV cycles (see Figures 1(b) and S4). However, the distinctively higher loading after (70+30) cycles clearly suggests an increased loading rate in the 30 CV cycles (due to higher AuCl4concentration in the fresh solution) post the initial 70 CV cycles resulting in a noticeably higher loading after (70+30) CV cycles as opposed to the loading obtained via an equivalent number of continuous 100 CV cycles. This validated the assumption about decreased rate of deposition due to continuous consumption of AuCl4- ions. The achieved ICR value of 16.6 mΩ cm2 is well within the U.S. department of energy (DOE) 2017 target of

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