Highly durable silica coated Pt/Cs with different surfactant types for proton exchange membrane fuel cell applications

Abstract

Platinum supported on carbon Vulcan XC-72 (Pt/Cs) for application as a cathode in proton exchange membrane fuel cells (PEMFCs) was coated with silica layers by a sol–gel method with three types of surfactants with different charging properties. The three various types of surfactants (1) cationic surfactant (cetyltrimethylammonium bromide (CTAB)), (2) anionic surfactant (sodium dodecylbezenesulfonate (SDBS)), and (3) non-ionic surfactant (Pluronic 123 (P123)) were applied to prevent agglomeration of the Pt nanoparticles and prevent detachment of the Pt nanoparticles from the carbon supports during operation. The degree of improvement depended on the type of surfactant applied in the sol–gel method. The formation of silica layers by SDBS and P123 significantly improved the durability of the Pt/Cs catalysts under acid conditions. Silica coated Pt/Cs formed using SDBS and P123 showed improved durability after 500 cycles in a cyclic voltammetry test in 0.5 M sulfuric acid (H₂SO₄) by 27.3% and 22.7%, respectively.

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1. Introduction

Proton exchange membrane fuel cells (PEMFCs) are promising alternative energy sources because of advantages such as low temperature operation, low emissions, and high energy efficiency.^1,2 Pt nanoparticles have been used as a catalyst for the oxygen reduction reaction (ORR) at the cathode in PEMFCs. The Pt nanoparticles could be seriously deactivated under cathode conditions.^3–6 Pt nanoparticles at the cathode might easily migrate on the carbon supports and subsequently agglomerate or detach from the carbon supports. Pt nanoparticles can also grow through Ostwald ripening, where the Pt atoms in small Pt nanoparticles dissolve to form cationic Pt species that are subsequently deposited onto large metal particles, which results in decreasing the total surface active area of the catalysts. The alloy formation between Pt and other metal species such as Au, Co, or Pd has been widely studied in order to improve the activity and durability of the catalysts under PEMFCs cathode conditions.^7–10

Electrocatalytic activity of Pt alloys with Ni, Co, and Fe, formed by sputtering, was investigated by Toda et al. Maximum activity was observed at ca. 30, 40, and 50% content of Ni, Co, and Fe, respectively, by which 10, 15, and 20 times larger kinetic current densities than that of pure Pt. B. Lim et al. investigated that the Pd–Pt nanodendrites were two and a half times more active on the basis of equivalent Pt mass for the ORR than the state-of-the-art Pt/Cs catalyst and five times more active than the first-generation supportless Pt-black catalyst. The alloy catalysts have higher activity and durability compared to that of pure Pt catalysts. However, the metal species added to the Pt catalysts also eventually dissolve under the cathode conditions, which cause a loss in activity during PEMFCs operation. The dissolved metal species are also deposited in the polymer electrolyte membrane, which results in decreasing of proton conductivity.^11–13 In the present study, commercial Pt/C catalysts were covered with silica layers to (1) avoid the agglomeration of the nanoparticles and (2) to prevent detachment from carbon supports. This study is expected to provide an alternative solution for high durability electrode material for PEMFCs.

2. Experimental

2.1 Catalyst preparation

In this study, silica coated Pt/Cs was synthesized by sol–gel method with three different types of surfactant. Commercial Pt/C catalysts were dispersed in 100 mL surfactant solutions (cetyltrimethylammonium bromide (CTAB), sodium dodecylbezenesulfonate (SDBS), and Pluronic 123 (P123)) by sonication for 1 hour. Then 0.1 M NaOH solutions were added to adjust pH to pH 10 and stirred for 5 minutes. Finally, 100 μL triethyl orthosilicate (TEOS) in 2 mL ethanol (EtOH) was injected and stirred for 2 hours. The sample was filtrated and washed several times by distilled water prior to drying at 80 °C for 6 hours.

2.2 Characterization of catalysts

The content of SiO₂ in silica coated Pt/Cs with three different types of surfactant was evaluated by X-ray fluorescence spectroscopy (XRF). The samples were ground into a fine powder and spread evenly on the polyethylene terephthalate film.

The morphology and particle size of Pt nanoparticles before and after silica coating in durability test were examined by transmission electron microscopy (TEM) with an acceleration voltage of 200 kV. The TEM images of the samples were recorded with a JEM-2500SE.

Electrochemical measurements were carried out using a three-compartment electrochemical cell with a Pt wire and saturated Ag/AgCl electrode serving as the counter and reference electrodes, respectively. For cyclic voltammograms (CVs) measurement, a glassy carbon disk electrode (3 mm diameter) was used as substrate for the catalysts and polished to a mirror finish. Catalyst ink was prepared by blending the catalyst, ethanol, and 5% Nafion by ultrasonic device. The ink was deposited on the glassy carbon disk and dried. The amount of Pt/Cs and silica coated Pt/Cs on the working electrode were adjusted to 200 μg for electrochemical analyses. The working electrode was immersed in a N₂-purged electrolyte solution of 0.5 M H₂SO₄ at room temperature. CVs analyses were measured at a scan rate of 50 mV s⁻¹ between −0.2 and 1.20 V in N₂-purged H₂SO₄ electrolyte. Prior to the measurement of the CVs, 30 cycles of potential cycling between −0.2 and 1.20 V were performed for the working electrode in N₂-purged H₂SO₄ electrolyte to clean the catalyst surfaces. The electrochemically active surface area (ECSA) and normalized electrochemically active surface area (N-ECSA) of Pt nanoparticles were determined from cyclic voltammogram acquired during desorption of hydrogen.

3. Results and discussion

3.1 Proposed mechanism of silica coating of Pt/Cs with various surfactant

The mechanism of the silica coating on Pt/C is depended on the type of surfactant applied for the coating process. Three kinds of surfactant with different properties were chosen for this study (1) CTAB, a cationic surfactant which consists of positively charged head-group with hydrophilic nature and a hydrophobic tail. (2) SDBS, an anionic surfactant which consists of negatively charged head-group with hydrophilic nature and a hydrophobic tail, and (3) P123, a non-ionic surfactant which consists of polyethylene glycol with hydrophilic nature and polypropylene glycol with hydrophobic nature.

The mechanism of the growing of CTAB is suggested as follows (Fig. 1A): the surfactant was adsorbed on Pt/C surface due to the hydrophobic bonding and electrostatic bonding.^14,15 The surfactant induced nucleation of silica precursor and grew onto the silica by electrostatic interaction between positively charge of surfactant and negatively charge of silica.^16,17 In the case of SDBS (Fig. 1B), the surfactant was adsorbed on Pt/C surface by π–π bonding and hydrophobic bonding.^18,19 Since the surfactant and silica were both negative charge, a positive charged mediator (Na⁺) was required to establish the electrostatic interaction between silica and SDBS.^16,17 If the surfactant was changed to P123 (Fig. 1C), it behaved a different mechanism compared to that of CTAB and SDBS. P123 was firstly adsorbed on Pt/C surface by hydrophobic bonding,²⁰ followed by the nucleation of silica precursor. The silica and surfactant was then bonded by hydrogen bonding.^16,17 The formation of hydrogen bonding can be direct by bulk water between ether group of P123 and silanol (Si–OH) group of silica.

Fig. 1 Schematic drawing of possible silica coated Pt/Cs mechanism.

3.2 Morphology of silica-coated Pt particles

Fig. 2 shows TEM images of Pt/Cs and silica coated Pt/Cs prepared at different surfactant. The Pt particles size of Pt/Cs before silica coating was about 3.8 ± 1.3 nm and the Pt particles size after silica coating with CTAB, SDBS, and P123 were 4.0 ± 1.5 nm, 3.9 ± 1.5 nm, and 3.8 ± 1.7 nm, respectively. It means that silica coating was not effect to Pt particles size of Pt/Cs.

Fig. 2 TEM images of Pt/Cs before and after silica coating with different surfactant (A) Pt/Cs before silica coating, (B) silica coated Pt/Cs with CTAB, (C) silica coated Pt/Cs with SDBS, (D) silica coated Pt/Cs with P123.

Table 1 presents the SiO₂ contents in various silica coated Pt/Cs samples prepared with different types of surfactant. The amount of silica was contained in Pt/Cs depend on the charge of surfactant. The cationic surfactant could introduce silica on Pt/Cs higher than the anionic surfactant and non-ionic surfactant, respectively.

Table 1 Contents of SiO₂ in silica coated Pt/Cs with different types of surfactant

Sample	Content of SiO2 (wt%)
Silica coated Pt/Cs–CTAB	7.40
Silica coated Pt/Cs–SDBS	5.80
Silica coated Pt/Cs–P123	2.32

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3.3 Electrochemical analyses

In general, the durability of Pt/Cs is examined by potential cycling in acid media solution.^21–23 Fig. 3 shows CVs of Pt/Cs and silica coated Pt/C with different surfactant during the durability tests. The potential of these catalysts was repeatedly changed between −0.2 and 1.20 V in N₂-purged 0.5 M H₂SO₄. Two peak couples were found in the CV for fresh Pt/Cs. One peak couple at −0.2 to 0.1 V was assignable to the adsorption and desorption of hydrogen on Pt metal and the other couple to the oxidation and reduction of Pt metal.^24,25 The peak currents in the CVs for Pt/Cs were significantly decreased with the number of the potential cycling after 500 cycles. These results indicate that Pt metal particles in Pt/Cs were seriously agglomerated and detached from carbon supports during the potential cycling experiment in H₂SO₄ electrolyte. On the other hand, two peak couples were also observed in the CVs for all silica coated Pt/Cs with different surfactant. The peak currents in the CVs for silica coated with CTAB were similar to that of Pt/Cs but in the case of SBDS and P123, they were significantly higher than Pt/Cs. Therefore, the coverage with silica by using SDBS and P123 improves the durability of Pt/Cs. This result is in agreement with the finding of Takenaka et al.^24–26 in which durability of silica coated Pt/Cs was increased from that of non-coated Pt/Cs. In their case, hydrolysis of 3-aminopropyltriethoxysilane and tetraorthosilicate was used for the silica coating without any usage of surfactants. Takenaka et al. have examined the catalytic activity of silica coated Pt catalyst, the silica coated Pt catalysts exhibited similar activity for the oxygen reduction reaction to non-coated Pt catalyst.^24,27

Fig. 3 CVs for Pt/Cs and silica coated Pt/Cs with different surfactant in N₂-purged 0.5 M H₂SO₄ during the durability tests (A) Pt/Cs, (B) silica coated with CTAB, (C) silica coated with SDBS, (D) silica coated with P123.

The TEM images of Pt/Cs and silica coated Pt/Cs with different surfactant after the durability tests shown in Fig. 4. For Pt/Cs without silica coating (Fig. 4A), we observed severe nanoparticle detachment from carbon supports after 500 cycles. In contrast, silica coated Pt/Cs with different surfactant significantly has successfully secured the Pt nanoparticles on carbon supports. The mechanism which was proposed in Fig. 1 seems realistic. However, the images demonstrated certain degree of agglomeration of Pt nanoparticles. The agglomeration of Pt particles of silica coated Pt/Cs with CTAB was obviously higher than that of silica coated Pt/Cs with SDBS and silica coated Pt/Cs with P123. Therefore, silica coating indicates clearly prevent the Pt particles detachment from carbon supports. In Section 3.1, we have discussed the mechanism of silica coating of Pt/Cs with various types of surfactant. The silica coating was either bonded to the Pt/C surface by electrostatic interaction for charged surfactant (CTAB and SDBS) or hydrogen bonding (P123). This bonding is relatively strong and it might secure the position of small Pt NPs on carbon during extreme condition such as acidic environment. As a result, agglomeration or detachment can be successfully avoided.

Fig. 4 TEM images of (A) Pt/Cs, (B) silica coated Pt/Cs with CTAB, (C) silica coated Pt/Cs with SDBS, (D) silica coated Pt/Cs with P123 after the durability tests 500 cycles.

Fig. 5 shows the change of ECSA of each Pt catalyst during the durability tests. The ECSA of Pt/Cs was about 66 m² g_Pt⁻¹ during the durability tests. The fresh silica coated Pt/Cs–CTAB, silica coated Pt/Cs–SDBS and silica coated Pt/Cs–P123 catalyst had a smaller ECSA (57, 62 and 63 m² g_Pt⁻¹, respectively) than the fresh Pt/CS catalyst. After 125 cycles, All silica coated Pt/Cs had a larger ECSA than Pt/Cs. After 500 cycles, silica coated Pt/Cs–SDBS and silica coated Pt/Cs–P123 catalyst still had a larger ECSA than Pt/Cs. The Normalized electrochemically active surface area (N-ECSA) of Pt metal particles for each Pt catalyst during the durability tests was evaluated from each CV shown in Fig. 3. The results are summarized in Fig. 6.

Fig. 5 The electrochemically active surface area for Pt/Cs and silica coated Pt/Cs with different surfactant during the durability tests.

Fig. 6 Normalized electrochemically active surface area for Pt/Cs and silica coated Pt/Cs with different surfactant during the durability tests.

The N-ECSA for fresh Pt/Cs decreased sharply as the number of potential cycles increased. The N-ECSA for Pt/Cs was reduced to 0.44 after 500 cycles. The N-ECSA for fresh silica coated Pt/Cs with CTAB, SDBS, and P123 were reduced to 0.45, 0.56, and 0.54 after 500 cycles, respectively. Fig. 7 shows the zeta potential of Pt/Cs in different pH, when pH changes from basic to acid condition, the zeta potential has a tendency from negative to positive. As a result, there is the repulsion force between positively charge of surfactant, positively charge of Pt/Cs (pH < 1), and positively charge of silica in acid condition (pH < 2)²⁸ during the durability test which affect to no significantly durable enhancement in case of CTAB. However, the silica coated Pt/Cs with SDBS and P123 thus maintains the active surface area of Pt which is higher than Pt/Cs about 27.3% and 22.7%, respectively.

Fig. 7 Zeta potential of Pt/Cs dispersed in water with different pH.

4. Conclusions

Silica coated Pt/Cs catalysts by sol–gel method for PEMFCs application has shown improvement of the catalysts durability under acidic condition. The degree of improvement depended on the type of surfactant applied in the coating method. Silica coated Pt/Cs by SDBS and P123 improved the durability by 27.3% and 22.7%, respectively, compared to that of non-coated Pt/Cs after 500 cycles in CV test under 0.5 M H₂SO₄ electrolytes. The N-ECSA of silica coated Pt/Cs by using SDBS and P123 was higher than that of non-coated Pt/Cs by preventing agglomeration and detachment of Pt nanoparticles from carbon support.

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