Temperature Influence on the Synthesis of Pt/C Catalysts for Polymer Electrolyte Membrane Fuel Cells

Abstract

To reduce the manufacturing cost of polymer electrolyte membrane fuel cells (PEMFCs), tests targeting the decrease of reaction temperature and the amount of reducing agent in the polyol method for the synthesis of Pt/C catalysts were conducted. The reaction temperature in the polyol method was changed from 50 to 160 °C. Through XRD and TGA, it was determined that the reduction of platinum ions by the oxidation of ethylene glycol started at 70 °C. Below a 60 °C reaction temperature, Pt (1 1 1) peaks in XRD were barely visible, indicating that no deposition occurred. TEM revealed that Pt particles were well-dispersed above a 100 °C reaction temperature. For manufacturing platinum catalysts using the polyol method, it was found that 100 °C is the optimal synthesis temperature. Additionally, it was found that similar performance can be achieved by adding water to decrease the amount of ethylene glycol during synthesis. Finally, considering various analyses, it is evident that the dispersion, size, and crystallinity of platinum particles had the most significant impact on performance.

1. Introduction

Currently, we face various environmental challenges such as serious pollution caused by the Fourth Industrial Revolution and indiscriminate destruction of nature to obtain resources. This has led to issues including global warming and resource depletion [1,2]. Therefore, there is a focus on minimizing environmental pollution by reducing carbon emissions. To address these issues, research is underway in various renewable energy sources and related fields [3,4]. Among them, fuel cells are receiving significant attention and are categorized based on operating temperature and electrolyte type. Polymer electrolyte membrane fuel cells (PEMFCs), which utilize only hydrogen and oxygen to produce water without environmental pollution, are garnering attention. They operate at relatively low temperatures and provide high power density, making them suitable for applications such as automobiles, buses, and residential fuel cells [5,6]. The structure of a PEMFC consists of an anode plate, cathode plate, gas diffusion layer, catalyst layer, and PEM (polymer electrolyte membrane). The combination of these components, excluding the anode plate and cathode plate, is referred to as the membrane electrode assembly (MEA). The catalyst layer of the MEA primarily utilizes platinum catalysts to enhance the slow oxygen reduction reaction (ORR) [2,5,7,8]. Therefore, the particle size of platinum catalysts significantly influences the performance of PEMFC [9,10]. The methods for catalyst synthesis include various techniques such as electroless deposition (ED) [11], the polyol process, micro emulsion method [12], colloidal method [13] and hydrothermal method [14,15]. Among these methodologies, the polyol method was selected, considering its simple conditions, minimal material requirements for synthesis, and effectiveness in achieving nano-sized particles [7,15,16]. Polyol refers to alcohols that contain two or more hydroxyl groups (-OH) per molecule. The polyol method, a type of reduction process, involves heating the solution to an optimal temperature to reduce metal ions through the oxidation reaction of polyols [17,18]. Thus, polyols serve as reducing agents, playing the role of electron donors in the reduction process. The types of polyol solvents are diverse, and in this experiment, ethylene glycol (CH₂OHCH₂OH, EG) was used for the process. These two chemical equations describe the reaction: 2CH₂OH-CH₂OH → 2CH₃-CHO + 2H₂O and M²⁺ + 2CH₃CHO → 2M + CH₃-CO-CO-CH₃ + 2H⁺ [1,19]. Heating induces the dehydration process of EG, leading to its transformation into acetaldehyde. In this process, EG provides electrons to metal ions, and upon receiving electrons, the metal ions become metal particles [20]. The particle size can be controlled by adjusting the concentration of the glycolate anion generated during the oxidation process of ethylene glycol. The glycolate anion serves as a role in forming electrical repulsion between metal particles, preventing agglomeration between particles. Therefore, particle size control is achievable with just one reducing agent without the need for additional dispersants [7,8,10,21,22]. Using this method, the platinum catalyst produced was employed to facilitate the oxygen reduction reaction (ORR). However, platinum, being a precious metal, increases the manufacturing cost of fuel cells. Looking at the price composition of a fuel cell system, the catalyst, particularly platinum, constitutes the largest proportion. Therefore, platinum catalysts significantly impact the manufacturing cost of PEMFC [6,13,23].

In the conventional polyol process, the synthesis temperature is set at 160 °C for the experiments. Raising the temperature to the desired level takes more time and energy than raising a lower temperature. Additionally, the cooling process also requires relatively more time, necessitating the use of more nitrogen and cooling water. Moreover, a large amount of EG for the reduction of Pt ion leads to an increase in production costs, which exceeds 1000 times for Pt ion [17,18]. Therefore, in this study, we tried to reduce the synthesis temperature of the polyol method and decrease the amount of the reducing agent, EG, required for the synthesis. If changing the synthesis temperature, a critical factor in the catalyst manufacturing process, still yields the same performance, it can lead to cost savings.

2. Results and Discussion

Figure 1a depicts the pH change of the solution containing the Pt precursor and EG depending on the reaction temperature. As the temperature increases, there is a rising trend in pH at the end of the reduction reaction. Before synthesis, all components, including the precursor, support, ethylene glycol, and NaOH, had values close to 11 due to adding NaOH. However, after synthesis, it was changed as shown in Figure 1a. This is because EG was oxidized to glycolic and oxalic acids during the synthesis process, resulting in an acidic pH in the solution [24]. Therefore, as the pH approaches 1, it indicates that more platinum has been reduced. At lower temperatures, there is relatively more reduction, and at 100 °C, the pH is observed to be the lowest. After the addition of sulfuric acid, a similar pattern is observed, where the pH increases with higher temperatures.

Figure 2 shows TEM images of catalysts synthesized at 80, 100, 120, 140, and 160 °C, magnified by 50 nm. The light gray color represents carbon black, the support material for platinum catalysts, and the black dots represent platinum particles. All catalysts show that Pt particles were well dispersed on carbon black. Lower temperatures lead to reduced dispersion due to differences in solubility, resulting in increased particle size through decreased dispersion and increased aggregation [3]. At high temperatures, the rapid generation of platinum nuclei due to high energy causes them to agglomerate, leading to an increase in particle size [19].

The particle size distributions in Figure 3 were obtained from the TEM images in Figure 2, taken at a smaller magnification of 20 nm. The average particle size was found to become smaller with the increase of temperature. The high reduction rate at high temperatures enables immediate multiple nucleation when the precursor solution was dripped, and the particles produced under this condition have a relatively wide size distribution due to non-uniform particle growth and primary particle solidification. At low temperature, the reaction medium is maintained and ions are not reduced to atoms [25].

Figure 4 shows the XRD conducted to confirm the crystallinity and crystalline size of the synthesized platinum. XRD analysis is conducted to confirm the crystal structure of the platinum catalyst [16]. Additionally, both qualitative and quantitative analyses can be performed by measuring the diffraction angles of X-rays scattered from atoms in the sample [8,24]. The unique peaks of platinum, (1 1 1), (2 0 0), (2 2 0), and (3 1 1), appear at approximately 2θ = 40°, 46°, 68°, and 82°. These peaks exhibit distinctive characteristics depending on the material. The characteristic peak of carbon, (0 0 2), appears at approximately 2θ = 24° [17]. The platinum particles exhibit a face-centered cubic (FCC) crystal structure, as indicated by these peaks [5,26,27]. As the intensity of the peaks decreases, it indicates a smaller particle size or less crystalinity [3]. For comparison, XRD of commercial Pt/C product was added in Figure 4a. Catalysts synthesized at various temperatures show well-defined peaks corresponding to platinum’s unique peaks (1 1 1), (2 0 0), (2 2 0), and (3 1 1), indicating that the synthesized platinum catalysts have been successfully prepared without any impurities. Even in XRD, the presence of platinum particles at 60 °C was not found, and it was confirmed that the catalysts synthesized at temperatures above 70 °C exhibit peaks of almost the same height. Figure 4b provides the crystalline size of platinum catalysts calculated from XRD by the Scherrer equation. For temperatures excluding 50 and 60 °C, the crystalline sizes are maintained within the range of 3–4 nm. As the temperature increases, there is an observable increase in crystalline size. Smaller crystalline sizes contribute to higher dispersion, enhancing catalyst activity and overall performance. However, when the crystalline size is too small, as observed in the cases of 50 and 60 degrees, the catalyst may become embedded in the pores of the support, hindering effective reactions from taking place [10]. Furthermore, excessive reduction in crystalline size can lead to reduced crystallinity of platinum, resulting in a decrease in catalyst activity [13]. However, the size distribution in Figure 4b are slightly different from those in Figure 3. These differences may come from using different analytical techniques.

TGA was conducted to measure Pt weight in carbon black under air. TGA measures the weight changes of a sample over a set period as the temperature increases at a constant rate. After heating, the remaining sample’s weight ratio can be determined [28]. The experiment was conducted by supplying air and heating at a rate of 5 °C per minute up to 800 °C. This condition aims to remove carbon through air injection, leaving only platinum to verify whether the deposition onto the support material occurred as desired [28]. In Figure 5a, it can be observed that the commercially available sample and those synthesized above 100 °C achieved values similar to the target of 40 wt%. In the cases of 60 °C, the values deviated significantly from the target 40 wt%, measuring approximately 22.7 wt%. As shown in Figure 5b, the supported amounts for each temperature are clearly visible. Above 70 °C, the Pt loading percentages reached the desired value and were not changed depending on temperature. We concluded that the pH changes with synthesis temperature in Figure 1 have little correlation with Pt particle size (Figure 4) and loading amount (Figure 5).

The cell performances of the obtained catalysts were measured in PEMFC as shown in Figure 6a. The cell performances were changed by Pt size, which was induced by reaction temperature. This shows similar performances for Pt synthesized at 100 and 120 °C in comparison with the commercial catalyst. This can be inferred from the earlier TEM images, indicating that the highest dispersion, resulting in smaller particle sizes and a relatively higher active surface area, led to the best performance. Since other analyses showed almost similar values, the dispersion likely had the most significant impact. The bar graph on the right shows the performance differences at 0.7 V, 0.5 V, and 0.3 V for each temperature, emphasizing the distinct performance at temperatures of 100–120 °C when synthesized using the polyol method.

Figure 6c,d show the change of current densities at 0.5 and 0.3 V according to the crystalline size of Pt calculated from XRD. Both graphs show the current densities increased with the increasing crystalline size of Pt regardless of cell voltage. In this work, the range of crystalline size was 2.8~3.6 nm, depending on synthesis temperature. The cell with Pt ~3.5 nm in size showed the highest performance, which is similar to the results of Zhuang et al. and Shao et al. [10,29]. When the Pt particle size increases, the fraction of face atoms and defects are reversely changed, and oxygen binding energy of particle surface has maximum value around 3 nm size. Thus, in this study, the platinum particles were well crystallized at around 100 °C of synthesis temperature and had a size of ~3.5 nm.

To explore cost-saving by reducing the amount of ethylene glycol (EG) used as a reducing agent during synthesis, the reduction of Pt ions was carried out by adding water in EG solution at 100 °C. The original process was conducted with a Pt:EG ratio of ca. 1:1000 in solution. Since the excess amount of EG was significantly higher than that of Pt, an experiment was designed to reduce the amount of EG to confirm if the performance remained consistent even with a reduced EG quantity. Adding water decreased the Pt:EG ratio to 1:250 and the cell performance using Pt catalyst synthesized in solution with different ratios of Pt:EG was not changed until reaching to ratio of 1:250, as shown in Figure 7. This suggests that the presence of EG mixed with the platinum precursor is sufficient for achieving the same performance, allowing for the substitution of water instead of EG. This similarity is evident when observing the bar graph of Figure 7b, where both cases show nearly identical values.

3. Materials and Methods

H₂PtCl₆·6H₂O (Sigma-Aldrich, St. Louis, MO, USA), carbon black (Vulcan XC72, Cabot Corporation, Boston, MA, USA), ethylene glycol, sodium hydroxide bead, sulfuric acid (99.5%, Samchun Pure Chemical Co., Ltd., Pyeongtaek, Gyeonggi-do, Republic of Korea), and a Nafion 211 membrane (Dupont, Wilmington, DE, USA) were used as received. Deionized water was supplied from our laboratory.

H₂PtCl₆·6H₂O was dissolved in EG and the platinum solution was stored in a bottle wrapped with foil to prevent exposure to light, or in a brown reagent bottle. EG was added to H₂PtCl₆·6H₂O using a pipette and poured into a brown reagent bottle. Finally, H₂PtCl₆·6H₂O/EG solution was made with a clear orange color, as shown in Figure 8a. The polyol process was conducted as follows. First, 0.1 g of carbon black and 50 mL of EG were added to the beaker. After stirring the solution for 10 min, the solution was ultrasonically treated for 10 min. Then, 17.7 g of H₂PtCl₆·6H₂O/EG was added to this solution before 4 mL of 0.5 M NaOH was added to form Pt hydroxide sol. The reactor temperature was set to the desired temperature, and the synthesis was carried out for 3 h under a nitrogen atmosphere. After 3 h, it was cooled down to room temperature. Then, 0.1 M H₂SO₄ was added for the increase of platinum loading. The synthesized catalyst was washed using distilled water.

Figure 8 shows pictures of solution without carbon black (CB) taken during experiments. After reaching the desired temperature and waiting for approximately 2–30 min, the solution was collected using a pipette and photographed in a vial. This experiment involved observing the change in color with increasing temperature. The aim was to identify the point at which the platinum precursor undergoes reduction due to the oxidation reaction of EG. After the addition of NaOH, the solution exhibited yellow color. As the heating progressed, the color gradually changed, and nearing 100 °C, it turned into a deep black shade. This occurred as platinum ions were reduced to nano-sized platinum particles, absorbing light and resulting in the solution exhibiting a black color. Above 100 °C, the solution maintained a black color without further color changes. Even after the synthesis was completed and sulfuric acid was added with stirring, the solution retained the same color.

X-ray diffraction (XRD, EMPYREAN, Panalytical, Overijssel Almelo, the Netherlands) was used to check the particle size of the platinum catalyst synthesized at various temperatures and observe only the platinum peak without the presence of other metals. The XRD analysis was conducted in the range of 20–90 degrees with Cu Kα (λ = 1.5406 Å). The distribution and surface morphology of platinum on carbon were investigated using transmission electron microscopy (TEM, JEOL, JEM-ARM200F, Tokyo, Japan). Thermogravimetric analysis (TGA, Q50, Ta Instrument, Newcastle, DE, USA) was utilized to determine the loading amount of the synthesized platinum catalysts and to verify the absence of other substances.

Pt/C was added to a vial containing a magnetic stir bar. Water and IPA were added in a 3:2 ratio, with 6 mL and 4 mL added, respectively. The mixture was then sonicated for 10 min. The 5 wt% Nafion solution was added, and the mixture was stirred for 10 min. The catalyst ink was sprayed onto the GDL or membrane until the platinum loading reached 0.3 mg/cm². The target Pt loading value was achieved, and left to dry for one day. The Nafion 211 and GDL were positioned with the side coated with the catalyst and hot pressing was conducted to make the MEA.

A single cell was assembled with MEA, gaskets, a Au-coated copper plate current collector and two graphite plates with ribbed channels for the distribution of reactant gas behind the porous GDE. The MEA was located between two graphite plates like a sandwich. A copper plate acting as a current collector was located behind the graphite plate. The active size of the graphite plate was 25 cm². The test station for the single cell performance measurement was composed of a temperature controller, humidification chamber, flowmeter, back pressure regulator, and electronic loader. Humidified hydrogen (H₂) and air gas were supplied to the anode and cathode, respectively. The utilization of reactant gases was 50%. These gases were humidified at 100% relative humidity (RH) by bubbling through a humidification chamber filled with water. For the electrochemical performance measurement, cells were maintained at 80 °C. H₂ and air gases were supplied at 90 and 70 °C, respectively. The pressure of all supplied gases was 1 atm. To enhance accuracy, experiments were conducted two or more times for each synthesis temperature.

4. Conclusions

In the synthesis of Pt/C as catalysts for PEMFC using the polyol process, this study aimed to decrease the synthesis temperature and minimize the amount of reducing agent to reduce production cost. Through XRD analysis, it was confirmed that when the synthesis temperature was above 70 °C, the Pt was crystallized, exhibiting its own diffraction peaks. TGA results also confirmed that the platinum catalyst was synthesized at the target value of 40 wt% above 70 °C. It was confirmed from XRD that the particle size of Pt changed with synthesis temperature. We concluded that the pH changes with synthesis temperature have little correlation with Pt particle size and loading amount measured from XRD and TGA. In the cell tests, Pt/C synthesized at 100 °C showed the highest cell performance among catalysts synthesized from 80 to 160 °C, even though it had a relatively larger Pt particle size compared to the other catalysts. This suggests that the crystallinity of the Pt particle has a greater effect than particle size, in the case of particles under 4 nm in size. Additionally, it was observed that the cell performance remained consistent after decreasing the ratio of ethylene glycol (EG) as the reducing agent to the platinum precursor by adding water to the solution. In other words, it is possible to proceed with less reducing agent in the polyol method if the synthesis temperature is decreased to 100 °C.