Radiolytic Preparation of Electrocatalysts with Pt-Co and Pt-Sn Nanoparticles for a Proton Exchange Membrane Fuel Cell

Kim, Sang Kyum; Park, Ji Yun; Hwang, Soon Choel; Lee, Do Kyun; Lee, Sang Heon; Han, Moon Hee; Rhee, Young Woo

doi:https://doi.org/10.1155/2014/960379

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Volume 2014 | Article ID 960379 | https://doi.org/10.1155/2014/960379

Radiolytic Preparation of Electrocatalysts with Pt-Co and Pt-Sn Nanoparticles for a Proton Exchange Membrane Fuel Cell

Sang Kyum Kim,¹Ji Yun Park,²Soon Choel Hwang,¹Do Kyun Lee,²Sang Heon Lee,²Moon Hee Han,¹and Young Woo Rhee¹

Academic Editor: Ajay Soni

Received30 Dec 2013

Accepted10 Apr 2014

Published04 May 2014

Abstract

Nanosized Pt-Sn/VC and Pt-Co/VC electrocatalysts were prepared by a one-step radiation-induced reduction (30 kGy) process using distilled water as the solvent and Vulcan XC72 as the supporting material. While the Pt-Co/VC electrodes were compared with Pt/VC (40 wt%, HiSpec 4000), in terms of their electrocatalytic activity towards the oxidation of H₂, the Pt-Co/VC electrodes were evaluated in terms of their activity towards the hydrogen oxidation reaction (HOR) and compared with Pt/VC (40 wt%, HiSpec 4000), Pt-Co/VC, and Pt-Sn/VC in a single cell. Additionally, the prepared electrocatalyst samples (Pt-Co/VC and Pt-Sn/VC) were characterized by transmission electron microscopy (TEM), scanning electron microscope (SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrochemical surface area (ECSA), and fuel cell polarization performance.

1. Introduction

Fuel cells have garnered global attention in recent decades due to their high efficiency and environmental compatibility [1, 2]. Among various types of fuel cells, proton exchange membrane fuel cells (PEMFC) have shown great promise as an alternative source of power generation for transportation applications due to their low operating temperature, fast startup, high power density, and low emission of pollutants [1–5]. However, to make PEMFCs commercially viable, some technical and economical challenges have to be overcome, including the poor kinetics of the anodic reaction, the complicated catalyst loading process, and the high cost of electrocatalysts [6–9].

In a standard H₂ fuel cell, the PEMFC anode catalyst facilitates the following hydrogen oxidation reaction (HOR):

Currently, the best electrocatalyst for the HOR is platinum (Pt). The extremely high activity of Pt is believed to be due to the nearly optimal bonding affinity that exists between Pt and hydrogen. The bonding is strong enough to promote facile absorption of H₂ from the gas phase onto a Pt surface and the subsequent electron transfer, but the bonding is weak enough to allow desorption of the resultant H⁺ ion into the electrolyte [10–14]. In contrast, the bond between H₂ and metals like W, Mo, Nb, and Ta is too strong, resulting in a stable hydride phase. On the other hand, the bond between H₂ and metals like Co, Pb, Sn, Zn, Ag, Cu, and Au is too weak, resulting in little or no absorption [15–21].

Although Pt is expensive, it proves to be an exceptionally effective catalyst for the HOR. Using the well-developed Pt/VC catalyst approach, whereby ultrasmall (2-3 nm) Pt particles are supported on a high-surface-area carbon powder, only an extremely small amount of Pt catalyst is required. Thus, typical Pt loadings in PEMFC anodes have been successfully reduced to around 0.05 mg Pt/cm². At these levels, the anode Pt catalyst expense is relatively modest compared to the expense associated with other components in the fuel cell. For example, a 50 kW automotive fuel cell stack, operating at a power density of 1.0 W/cm², would require about 2.5 grams of Pt for the anode catalyst. At a price of $42/g, this represents a Pt material cost of approximately $100 [1–3, 14, 16].

In this study, we prepared the Pt-Co/VC (Vulcan XC72) and Pt-Sn/VC electrocatalysts by one-step radiation-induced reduction. The obtained electrocatalysts have overcome the disadvantages of the PEMFC (e.g., high cost and a weak bond with H₂). Pt-Co/VC and Pt-Sn/VC electrocatalysts were then characterized by transmission electron microscopy (TEM), scanning electron microscope-energy dispersive spectroscopic (SEM-EDS), thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrochemical surface area (ECSA), and fuel cell polarization performance [17, 18].

2. Experimental

2.1. Chemicals

In this experiment, the H₂PtCl₆·H₂O (37.5% Pt), CoCl₂·H₂O (47.4% Co), and SnCl₂·H₂O (52.0% Sn) were of analytical reagent grade (Sigma-Aldrich, USA) and were used without further purification. Carbon black (Vulcan XC72) was purchased from CABOT Co., Ltd. (USA). Nafion (perfluorinated ion-exchange resin, 5% (w/v) solution in a solution of 90% aliphatic alcohol/10% water mixture) was also purchased from Sigma-Aldrich (USA). Solutions for the experiments were prepared with water purified in an aquaMAX-Basic 360 series plus water purification system (YL Instruments Co., Ltd., Korea). The final resistance of the water was 18.2 MΩcm⁻¹ and the solutions were degassed prior to each measurement. The other chemicals used in the experiment were of reagent grade.

2.2. One-Step Preparation of Electrocatalysts by Radiation-Induced Reduction

First, VC (Vulcan XC72) was purified to remove the noncrystallized carbon impurities. VC was treated with a mixture of H₂SO₄/H₂O (2 : 8 vol%) and, in the process, VC was purified. The purified VC was used as the supporting material for deposition of the catalysts. The Pt-Co/VC catalyst was prepared as follows: H₂PtCl₆·H₂O (0.43 g) and CoCl₂·H₂O (0.37 g) were dissolved in a mixture of deionized water (188 mL) and 2-propanol (12.0 mL) was added as a radical scavenger. Next, 1.0 g of the purified VC support was added to the previously mentioned solution. The PH of the reaction solution was adjusted to 10.0 using NaOH. Nitrogen was bubbled through the solution for 30 min to remove oxygen and then the solution was irradiated under atmospheric pressure and ambient temperature. A total irradiation dose of 30 kGy (dose rate = 6.48 × 10⁵/h) was applied. Pt-Co nanoparticle-deposited VC catalysts were precipitated after γ-irradiation. The Pt-Sn/VC electrocatalysts were prepared as described above. The molar ratios of the input metal ions are Pt-Co/VC (Pt/Co = 0.6/0.4) and Pt-Sn/VC (Pt/Sn = 0.6/0.4).

2.3. Characterization of Electrocatalysts Surface

The particle size and morphology of the electrocatalysts were analyzed by SEM-EDS images obtained with a Hitachi S-4800 microscope operated at 30 kV. The HR-TEM images were obtained using a JEOL JEM-2100 microscope operated at 200 kV. The TGA of the PEM was made on the TA instruments of the TGA S-1000 model with a heating rate of 10°C/min, in a temperature range of 0–800°C. XRD was conducted using a Japanese Rigaku D/max γA XRD, equipped with graphite monochromatized Cu Kα radiation (λ = 0.15414 nm). The XPS analyses were carried out with a MultiLab 2000 (Thermo) XPS using a monochromated Al Ka source under a base pressure of 2.6 × 10⁻⁹ torr.

2.4. Characterization of Electrochemical Performance Measurement

Electrochemical measurements were carried out at room temperature in a three-electrode cell connected to an electrochemical analysis. A glassy carbon, coated with a catalyst, was used as the working electrode. An Ag/AgCl electrode and a Pt electrode were used as a reference electrode and as a counter electrode, respectively. Unless otherwise stated, all potentials were relative to the Ag/AgCl. HOR activity tests can be conducted by cyclic voltammogram (CV). With the CV, the HOR was measured in oxygen-saturated 0.5 M H₂SO₄ at 25°C with the scan rate of 0.1 V s⁻¹, in the potential ranging from −0.2 V to +0.3 V.

Single cells were constructed to evaluate the fuel cell polarization performance of the Pt-Co and Pt-Sn catalyst electrodes. Membrane electrode assembly (MEA), with an area of 10 cm², was used to construct a single cell, which had been fabricated by hot-pressing a pretreated Nafion 112 membrane (DuPont) between the anode and the cathode. The catalyst loadings were 0.2 mg of Pt at the cathode and 0.4 mg of Pt-Co, Pt-Sn at the anode. For all the tests, Pt (40 wt%)/VC (HiSpec 4000) was used as the anode catalyst. The catalyst inks were prepared in a mixture solution composed of an appropriate amount of DI water and the required amount of 5 wt% Nafion ionomer solution (Aldrich). The Nafion ionomer content was 20 wt% in the anode catalyst layer and 25 wt% in the cathode catalyst layer. The appropriate amount of catalyst inks was painted uniformly on Teflon-coated carbon paper (TGPH-090) and dried at 80°C overnight. Fuel cell polarization performance tests were conducted at 60°C with a WFCTS fuel cell test station.

3. Results and Discussion

Pt-M/VC catalysts were successfully prepared by a one-step radiation-induced reduction (RIR) process. Diverse papers have reported that the mechanism of RIR could be generally proven by the following equation [22–28]:

Among these, free radical and solvated electrons () were used as strong reducing agents to reduce the metal ions up to the zero-valence state, as shown in

Similarly, multivalent ions, such as Pt⁴⁺, Co²⁺, and Sn²⁺, are reduced by a multistep reaction. On the other hand, the hydroxyl radical has a high oxidizing capacity and metal atoms and ions can be oxidized by hydroxyl radical. In order to protect the oxidizing agent , 2-propanol was added to the reaction solution. The radical reacted with 2-propanol, as shown in (5). As a result, the metal ion was then reduced to a zero-valence metal atom by 2-propanol radical, as shown in (6):

Similar to the findings of Choi et al. [24, 26, 28], various metal and vinyl monomers were grafted onto the surface of MWNT via trapped radicals. The results showed that RIR was an effective metal deposition method on the surface of the VC-supported electrocatalysts.

Figure 1 shows the HR-TEM images of the VC-supported electrocatalysts prepared by one-step radiation reaction. They are (a) purified VC, (b) Pt-Co/VC, and (c) Pt-Sn/VC catalysts, respectively. As shown in Figure 1(a), there are no metallic nanoparticles on the surface of the VC. Figures 1(b) and 1(c) show that the HR-TEM images of the Pt-M/VC (M = Co, Sn) catalyst clearly provide evidence for more deposition of Pt-M alloy nanoparticles on the surface of VC composites compared to the surface of pristine VC. The mean particle size of the metallic nanoparticles was in 10 nm under.

(a)

(b)

(c)

Figure 2 shows SEM images of the VC-supported electrocatalysts prepared by one-step radiation reaction. They are (a) Pt-Co/VC and (b) Pt-Sn/VC catalysts, respectively. As shown in Figures 2(a) and 2(b), the SEM images of the Pt-M/VC catalyst clearly provide evidence for more deposition of Pt-M alloy nanoparticles on the surface of the VC composites compared to the surface of the pristine VC. The mean particle size of the metallic nanoparticles were 10 nm under. This also indicates that the RIR is capable of preparing a stoichiometric Pt-M/VC electrocatalyst.

(a)

(b)

Although various strategies were used for synthesis of the Pt-Co/VC and Pt-Sn/VC catalysts, the TGA data shown in Figure 3 reveals that the actual metal loadings in all the Pt-Co/VC and Pt-Sn/VC electrocatalysts are very close to the normal value of 57~62 wt%. This suggests that Pt-Co/VC and Pt-Sn/VC of the synthesis strategies are efficient for preparation of high metal loading Pt-M catalysts [29].

Figure 4 shows the XRD patterns for the Pt-Co/VC and Pt-Sn/VC electrocatalysts prepared by one-step radiation-induced reduction. The Pt-Co/VC and Pt-Sn/VC electrocatalysts exhibit characteristics of a Pt face-centered cubic (fcc) structure [29, 30]. The average particle sizes were calculated using a Debye-Scherrer equation from the broadening of the Pt (220) reflection [27, 28] as ca. 2.8 nm and 3.7 nm for Pt-Co/VC and Pt-Sn/VC, respectively. The particle sizes calculated from the XRD data for Pt-Co nanoparticles are generally in good agreement with the randomly measured particle sizes from the HR-TEM and HR-SEM images shown in Figures 1 and 2.

As shown in Figure 5, the XPS data was employed to analyze the valence state and the surface composition of the metal catalyst nanoparticles. Figures 5(b) shows the C1s, Pt4f, and Sn3d peaks of the commercial Pt-Sn/VC catalysts prepared by RIR methods, respectively. The C1s peaks appear at 285 eV. The Pt4f peaks and Sn3d of the Pt-Sn/VC appear at 74, 71 eV, and 496 eV respectively. Figures 5(c) shows the C1s, Pt4f, and Co2P peaks of the commercial Pt-Sn/VC catalysts. The Pt4f peaks and the Co2p of the Pt-Co/VC appear at 74, 71 eV, and 785 eV, respectively [31–33]. As a result, the Pt-Co and Pt-Sn were successfully deposited onto the VC-supported electrocatalyst by RIR.

(a)

(b)

(c)

Catalyst utilization efficiency is a very crucial factor which reflects the properties of catalysts, which is generally determined by dividing the electrochemical active surface area (ECSA) by the chemical surface area (CSA). Electrochemical surface area (ECSA) represents intrinsic electrocatalytic activity of electrocatalysts. The ECSA can be estimated from the integrated charge (after subtraction of capacitance contribution) in the hydrogen absorption region of the steady-state CV in 0.5 mol H₂SO₄ under N₂ atmosphere, based on a monolayer hydrogen adsorption charge of 0.2 mC/cm² on crystalline Pt, Pt-Co, and Pt-Sn. Well-defined hydrogen adsorption/desorption characteristics were observed for Pt-M/VC electrocatalysts.

Figure 6 shows the cyclic voltammograms (CV) curves of the Pt/C (HiSpec 4000), Pt-Co/VC, and Pt-Sn/VC electrocatalysts in 0.5 M H₂SO₄ at room temperature under N₂ atmosphere for the ECSA. The measured ECSA for the Pt-Co/VC was found to be 92 m²g⁻¹, which is much higher than the measured ECSA of Pt-Sn/VC and Pt/C (HiSpec 4000), which were 67 m²g⁻¹ and 72 m²g⁻¹, respectively. Compared with Pt/VC and Pt-Sn/VC catalysts, higher ECSA for the Pt-Co/VC catalysts suggests better utilization efficiency due to smaller Pt-Co nanoparticles and better particle distribution [34–36].

(a)

(b)

Figure 7 shows the polarization performance and power density curves of PEMFCs at 60°C using Pt-Co/VC and Pt-Sn/VC electrocatalysts. The H₂-fueled fuel cell polarization at low current density is electrochemical-activation controlled and mainly attributed to the sluggish HOR at the anode surface. The lowest loss in polarization voltage was observed for the Pt-Co/VC catalyst compared to the Pt-Sn/VC catalyst, indicating that the highest catalytic activity was toward the HOR. The maximum power density is 345 mW cm⁻² for the Pt-Co/VC catalyst, which is much higher than the maximum power density observed for the Pt-Sn/VC (275 mW cm⁻²) and Pt/VC (252 mW cm⁻²) catalysts. Once more, this illustrates that the Pt-Co/VC catalyst exhibits better fuel cell performance compared to those other catalysts (HiSpec 4000, Pt-Sn/VC), which is in full agreement with electron microscopy (HR-TEM, HR-SEM) data and the XRD data. The Pt-Co/VC catalysts exhibited a power density that was similar to the HiSpec 4000 Pt (40 wt%)/VC catalysts [34, 35], respectively. Surprisingly, the Pt-Co catalysts outperformed the state-of-the-art commercial Pt/VC (HiSpec 4000), having half as much Pt loading. Higher HOR electrocatalytic activity and better fuel cell performance are attributable to its larger ECSA, which is closely related to the smaller nanoparticles and the greater uniform particle dispersion of the Pt-Co catalyst (one-step radiation-induced reaction) on the VC-supported electrocatalyst [35–37].

4. Conclusions

In this study, VC-supported electrocatalysts were synthesized by a one-step radiation-induced reaction process in aqueous solution at room temperature without reducing agents. The efficiency of the prepared electrocatalysts was evaluated. The following conclusions were drawn based on the experimental results. In spite of the low metallic contents of Pt-Co/VC electrocatalysts, there efficiencies for hydrogen oxidation reaction and power density were higher than those of the Pt/VC and Pt-Sn/VC electrocatalysts. The resulting Pt-Co/VC electrocatalyst has demonstrated its possibility as a promising electrocatalyst in PEMFC.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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Copyright © 2014 Sang Kyum Kim et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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