Carbon-Supported PdCu Alloy as Extraordinary Electrocatalysts for Methanol Electrooxidation in Alkaline Direct Methanol Fuel Cells

Abstract

Palladium (Pd) nanostructures are highly active non-platinum anodic electrocatalysts in alkaline direct methanol fuel cells (DMFCs), and their electrocatalytic performance relies highly on their morphology and composition. This study reports the preparation, characterizations, and electrocatalytic properties of palladium-copper alloys loaded on the carbon support. XC-72 was used as a support, and hydrazine hydrate served as a reducing agent. Pd_xCu_y/XC-72 nanoalloy catalysts were prepared in a one-step chemical reduction process with different ratios of Pd and Cu. A range of analytical techniques was used to characterize the microstructure and electronic properties of the catalysts, including transmission electron microscopy (TEM), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma emission spectroscopy (ICP-OES). Benefiting from excellent electronic structure, Pd₃Cu₂/XC-72 achieves higher mass activity enhancement and improves durability for MOR. Considering the simple synthesis, excellent activity, and long-term stability, Pd_xCu_y/XC-72 anodic electrocatalysts will be highly promising in alkaline DMFCs.

1. Introduction

With the rapid development of human society, the demand for energy in all countries is increasing, and the accompanying environmental problems are becoming more serious. It has become an imperative development strategy to develop innovative, sustainable, and environmentally friendly energy conversion technologies and devices. Fortunately, Direct Methanol Fuel Cells (DMFCs), as a new energy conversion device with simple structure, high energy density, fast charging, and environmental advantages, can transform direct chemical energy stored in methanol into electricity, which has attracted much attention in recent years [1,2,3,4,5]. At the same time, methanol is a relatively abundant product of the chemical industry, and the development of DMFCs is also considered to be the key to achieving carbon neutrality [6,7,8,9,10]. However, the anodic catalyst with excellent performance and low cost is a challenge for the commercial use of DMFCs. Therefore, it is essential to design and prepare the anode catalyst of DMFCs scientifically [11].

Pt has been verified to be the best catalyst for DMFCs; however, the active site of Pt is easily poisoned by CO_ads produced in the process of methanol oxidation, and the methanol oxidation activity on the Pt electrode surface consequently decreases. In order to improve the anti-CO poisoning ability of catalysts, the research of DMFCs anode catalysts mainly focused on Pt and PtM (M = Pd [12], Ru [13], Fe [14], Co [15], Ni [16], Cu [17], Sn [18], and Ag [19]). Some studies have shown that there are two mechanisms of bifunctional mechanism and electronic effect in improving the anti-CO poisoning ability of Pt-based alloy catalysts. The bifunctional mechanism holds that methanol takes the adsorption–dehydrogenation processes on the surface of Pt. The second metal or metal oxide which is introduced promotes the dissociation of the electrolyte to form more OH_ads, which is conducive to faster oxidation removal of CO_ads adsorbed on Pt [20,21]. On the other hand, the second metal reduces the adsorption energy of CO_ads on the Pt surface by affecting the electronic properties of Pt, thus inhibiting the adsorption of CO_ads on the Pt surface. The Pd-based catalyst is more abundant, has higher electrocatalytic activity for small alcohol molecules in alkaline media, and is resistant to CO poisoning. Thus, it can be used as a substitute for Pt-based anode catalysts [22].

Therefore, in order to further improve the electrocatalytic activity and economical utilization of the Pd catalyst, the second metal is usually introduced to form the alloy PdM (M = Cu [23], Co [24], Ag [25], Fe [26], and Sn [27]). Comparing it with a single metal, the bimetallic catalyst exposed to more active sites had higher activity and stability, owing to the shift of the d-band center of Pd, which is caused by the electronic effects and geometric changes of the catalyst. In addition, the support with a large specific surface area can provide a channel for electron transfer, such as carbon black, carbon nanotubes, and graphene, thus maintaining the excellent conductivity and performance of the catalyst.

In recent years, palladium copper alloy nanoparticles have attracted much attention due to their excellent small molecule oxidation properties of alcohols and ability to resist CO_ads poisoning. The reason for this is that the d-band center of Pd shifts after the alloy formation between Cu and Pd, and the addition of copper reduces the binding energy of Pdⁿ⁺. In contrast, Pd increases the binding energy of Cuⁿ⁺, which weakens the adsorption of CO_ads on the catalyst surface.

Recently, Ye et al. [28] reported a PdCu catalyst with an unique core–shell structure. In an alkaline medium, the catalyst has excellent catalytic performance for methanol oxidation reaction (MOR), and the enhanced activity is rigorously ascribed to the adsorption strength of OH_ads, which is good for the removal of adsorbed COads and increases the ability of CO_ads to resist CO_ads poisoning. Shih et al. [23] studied porous PdCu NPs for electrocatalytic oxidation of methanol with excellent electrochemical activity, greater stability, and lower cost-effectiveness. The copper content of a catalyst is known to have a significant effect on its morphology and catalytic activity. Additionally, the copper content controls the morphology and affects the catalytic activity toward the MOR in alkaline media. This is an important discovery, because it can help to optimize the performance of catalysts for various applications. Saleem et al. [29] prepared an element segregation phenomenon in two-dimensional (2D) core–shell nanoplates, which showed superior electrocatalytic activity and stability towards the MOR compared to the commercial Pt/C catalyst. Unfortunately, the complex synthesis process impedes its practical application in DMFCs. Therefore, it is urgent to design and synthesize an efficient MOR catalyst using convenient, rapid, and eco-friendly synthetic methods.

Based on the above views, in this work, we synthesized PdCu nanoalloys on Vulcan XC-72 support by means of the liquid reduction method, which can synthesize many alloy catalysts in a short time under mild conditions, and meets the requirements of green chemistry. In addition, we synthesized a series of PdCu nanoalloys with different atomic ratios to study the influences of Cu addition on the electronic environment and the lattice strain of the catalysts.

2. Materials and Methods

2.1. Materials and Reagents

Palladium chloride PdCl₂ (>99.9%); ethylene glycol (>99.8%), hydrazine hydrate (>85%), XC-72, concentrated hydrochloric acid (36~38%), copper nitrate solution (>99%), and all other reagents were of analytical grade and purchased from MACKLIN (Shanghai, China) without further purification.

2.2. Preparation of Catalysts

The supported Pd_xCu_y/XC-72 (x + y = 5) nanoalloy catalysts with different compositions were prepared by one-step chemical reduction, with hydrazine hydrate as a reducing agent and controlling the total metal loading at 30%. Typically, taking Pd₁Cu₁/XC-72 as an example, 50.0 mg XC-72 was accurately weighed and dispersed in 200 mL ethylene glycol for 30 min by ultrasound, and 2 mL hydrazine hydrate was added to the mixture under stirring. Then, 11.41 mL of H₂ PdCl₄ (1 mg/mL) and 8.011 mg Cu(NO₃)₂·3H₂O mixed precursor solution was added to it by dropping, and the mixture was reduced by stirring for two hours, followed by filtration, washing, and drying at 70 °C overnight to obtain Pd₁Cu₁/XC-72.

2.3. Physical Property Characterization

In order to conduct an X-ray diffraction (XRD, Bruker D8 Advance, Germany) analysis, a Bruker D8 Advance, equipped with Cu target Kα radiation as the X-ray source (λ = 0.15404 nm), was employed. Transmission electron microscopy (TEM, FEI Talos F200x, USA) and high-resolution transmission electron microscopy (HRTEM, FEI Talos F200x, USA) were also performed on a JEOL 2100F TEM with an accelerating voltage of 200 kV. Inductively coupled plasma-atomic emission spectroscopy (ICP-OES, Agilent 5110(OES), USA) was a powerful analytical tool that could measure a wide range of elements in a sample. In this study, ICP-OES was used to measure the concentration of elements in the sample. This information was then used to understand the chemical composition of the sample. To this end, a certain amount of the sample was digested by microwave with aqua regia at 200 °C for 1 h. This process was repeated five times until it was clear and transparent, to ensure that the solids were completely digested. Then, the as-obtained solution was volume-constant at 25 mL and diluted 100 times. Moreover, in order to obtain the electronic structure information of the catalyst surface, the Pd_xCu_y/XC-72 was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, America). The as-synthesized materials were evaluated for electrochemical characterization using cyclic voltammetry (CV) and chronoamperometry (CA).

2.4. Electrochemical and Physical Characterization

The electrochemical performance of electrocatalysts were evaluated by workstation (CHI 660E) of a three-electrode system. The platinum electrode was the counter electrode, the calomel saturated electrode (SEC) was the reference electrode, and the glassy carbon electrode (GCE, 4.0 mm in diameter, 0.1256 cm [2]) coated with catalyst was the working electrode. The working electrode was prepared as follows: 2.0 mg of catalyst was accurately weighed, and 450 μL absolute ethanol, 50 μL 5% Nafion membrane solution, and 500 μL ultrapure water were successively added to the catalyst. It was then dispersed by ultrasound for 0.5 h to form a uniform inked mixture. Next, 5 μL was removed and dropped onto the surface of the glassy carbon electrode, which was allowed to dry naturally before testing. Cyclic voltammetry (CV) was used to test the activity of electrocatalytic methanol oxidation and the electrochemical active area (ECSA) in an alkaline environment. The MOR activity test electrolyte was 1 M KOH + 1.0 M CH₃OH solution saturated with N₂, and the ECSA test was placed into 1 M KOH solution saturated with N₂. The scanning speed was 50 mV/s, and the potential range was −0.9–0.3 V (vs. SCE). In this work, a chronoamperometry experiment was used to evaluate the stability of the catalyst. The specific operating conditions were as follows: a chronoamperometry test was performed at −0.2 V constant potential, and rapid scanning was performed at 0.1 V/s scanning speed. All above tests were performed at room temperature.

3. Results

3.1. Physical Characterization of Pd_xCu_y/XC-72

Figure 1 indicates the XRD profile of the as-synthesized Pd_xCu_y/XC-72. The XRD profile reveals that the PdCu NPs are well crystallized, as observed from the three prominent diffraction peaks. The three main diffraction peaks appear at 2θ = 40.6°, 46.7°, and 68.2° index to the (111), (200), and (220) crystal planes of face-centered cubic (fcc) Pd NPs. The Pd (111) shift peak is shown in Figure 1. After Cu incorporation, the diffraction peak at 40.6°, corresponding to the Pd (111) plane, slightly shifted to the higher values (namely, 40.61°, 40.26°, 40.28°, 40.20°, 40.00°). This shift may be due to the Pd lattice contraction, owing to the smaller radius of Cu (0.128 nm) compared to that of Pd (0.137 nm), indicating the formation of successful PdCu alloy NPs [30].

The dispersion of several catalysts with different PdCu ratios is reflected in the TEM images of Pd_xCu_y/XC-72 in Figure 2. It can be seen that the catalysts formed with our synthesized are well-dispersed, with a uniform size distribution, and the particle size of the formed catalysts gradually rises with the increase in the Pd ratio.

Figure 3 shows the HRTEM profile of the Pd₃Cu₂/XC-72. It can be seen that the metal is dispersed more evenly on the carbon support, and in the Figure 3c, the lattice fringes of the Pd(111) plane, Pd (200) plane, and Cu (111) plane can be clearly seen, which further verifies the formation of its alloy [31].

Moreover, we used the microwave digestion technique combined with inductively coupled plasma optical emission spectrometry (ICP-OES) analysis.

Table 1. ICP-OES results for PdxCuy/XC-72 catalysts.

Sample	ICP Results (wt%)		Theory of Loads (wt%)		Theoretical Metal Molar Ratio (Pd: Cu)	Actual Metal Molar Ratio (Pd: Cu)
	Pd	Cu	Pd	Cu
Pd1Cu4/XC-72	7.6	15.4	8.8	21.2	0.40	0.47
Pd2Cu3/XC-72	12.4	11.1	15.8	14.2	1.07	1.07
PdCu/XC-72	18.2	10.2	18.7	11.3	1.60	1.72
Pd3Cu2/XC-72	19.7	6.9	21.4	8.6	2.40	2.74
Pd4Cu1/XC-72	25.8	3.5	26.1	3.9	6.44	7.08

shows that the Pd metal content of Pd₁Cu₄/XC-72, Pd₂Cu₃/XC-72, PdCu/XC-72, Pd₃Cu₂/XC-72, and Pd₄Cu₁/XC-72 is 7.6, 12.4, 18.2, 19.7, and 25.8 wt%, respectively. In addition, the Cu metal content of Pd₁Cu₄/XC-72, Pd₂Cu₃/XC-72, PdCu/XC-72, Pd₃Cu₂/XC-72, and Pd₄Cu₁/XC-72 is 15.4, 11.1, 10.2, 6.9, and 3.5 wt%, respectively. This generally agrees with the ratio of PdCu alloy in our experimental design.

The composites were further examined by XPS in order to learn more about the surface electronic state of Pd_xCu_y/XC-72 (Figure 4). To ascertain the valence state of Pd in the hybrids, the XPS spectra of Pd 3d were also gathered. Metallic Pd⁰ is responsible for the Pd 3d5/2 peak at 335.2 and the 3d3/2 peak at 340.5 eV, and also the Pd 3d5/2 peak at 342.4 eV and 3d5/2 peak at 337.2 eV assigned to Pd²⁺. It can be seen that the majority of the metal precursors are successfully reduced and loaded onto the support by comparing the quantities of Pd⁰ and Pd²⁺. According to the Cu 2p spectra, the Pd_xCu_y/XC-72 peaks at 952.56 and 932.7 eV, respectively, correspond to Cu⁰ 2p1/2 and 2p3/2; the peaks at 953.7 and 933.6 eV, respectively, correspond to Cu²⁺ 2p1/2 and 2p3/2. The peaks at 944.8 and 941.9 eV, correspond to Cu²⁺ 2p3/2 satellite peaks. The overall intensity of the Pd peaks decreases with the addition of Cu, indicating less Pd exposure in the bimetallic.

Area ratios for various Pd_xCu_y/XC-72 catalysts are shown in Figure 5. Since Pdⁿ⁺ to Pd⁰ and Cuⁿ⁺ to Cu⁰ are the alloys formed during reduction, other metals will change to create a relatively stable state during the alloy formation process. At this point, low-valence Cu 2p electrons will be transferred to Pd, reducing the positive state of Pdⁿ⁺ while increasing the valence state of low-valence Cu, resulting in a gradual rise in the concentration of Cuⁿ⁺.

However, Pd₃Cu₂/XC-72 has the highest value of Pd⁰ and Cuⁿ⁺ content. There is also a change in the binding energy of a fraction of the Pd, represented by the broadening of the peaks. The electronic structure of the surface Pd atoms is modified by alloying with the Cu atoms as the Cu content increases and the binding energy shifts to a higher value. These results further confirm that the alloy of Pd_xCu_y nanoparticles supported on XC-72 has been successfully synthesized by the chemical reduction method. In addition, the presence of Cu²⁺ may be caused by the surface oxidation or chemisorption of environmental oxygen during the synthesis process, as indicated by the shake-up satellite (sat.) peaks at the high binding energy side of the Cu 2p3/2 and Cu 2p1/2. The XPS spectra of the other catalysts are shown in Figure 4. The characteristic peaks for each catalyst are the same as the Pd_xCu_y/XC-72 XPS results.

3.2. Electrochemical Tests for MOR (Three-Electrode Cell)

As is summarized in Figure 6, the electrochemical performance of the five electrocatalysts is shown. Cyclic voltammetry (CV) was performed on each of the Pd₁Cu₄/XC-72, Pd₂Cu₃/XC-72, PdCu/XC-72, Pd₃Cu₂/XC-72, and Pd₄Cu₁/XC-72 electrodes in N₂-saturated 1.0 M KOH and 1.0 M CH₃OH. It can be seen that there are obvious differences in electrocatalytic activities with different amounts of Cu. The results depict that Pd₃Cu₂/XC-72, the maximum value, was obtained (1719 mA·mg⁻¹ Pd). Therefore, combined with XPS (Figure 4) and XRD (Figure 1) data, we can conclude that Pd₃Cu₂/XC-72 exhibits excellent catalytic performance due to its excellent electronic structure. In addition, it was found that the performance of the catalyst synthesized in this work was much higher than that of the catalysts synthesized in other works (

Table 2. Performance comparison of different palladium-based catalysts.

Catalyst	Test Condition	Scan Rate (mV/s)	Mass Activity (mA/mg)	Refs.
Pd3Cu2/XC-72	1 M KOH + 1 M Methanol	50 mV/s	1719 mA/mg	This Work
Pd/C	1 M KOH + 1 M methanol	50 mV/s	550 mA/mg	2019 [32]
Solid carbon sphere-supported Pd-CeO2 nanoparticles	1 M KOH + 1 M methanol	50 mV/s	900 mA/mg	2019 [32]
Pd59Cu33 Ru8 NSs	1 M KOH + 1 M methanol	50 mV/s	1660.8 mA/mg	2019 [33]
Pd-PdO PNTs-260	1 M KOH + 1 M methanol	50 mV/s	1111.3 mA/mg	2020 [34]
Pd59Fe27Pt14 NMs	1 M KOH + 1 M methanol	50 mV/s	1610 mA/mg	2020 [35]
Pd3Sn2 IMC	1 M KOH + 1 M methanol	50 mV/s	1300 mA/mg	2022 [36]
Pd/Se−Ti3C2	1 M KOH + 1 M methanol	50 mV/s	1046.2 mA/mg	2022 [27]
Pd72Cu14Co14/rGO	1 M KOH + 1 M methanol	50 mV/s	1062 mA/mg	2019 [31]

).

Generally, the ratio between the peak current in the forward (I_f) and the backward scan (I_b) is the measure for the tolerance of the catalyst against CO poisoning, with a higher I_f:I_b indicating higher tolerance. In our experiment, Pd₃Cu₂/XC-72 displays a higher I_f:I_b, which reflects its higher CO tolerance (

Table 3. Comparison of CO resistance of different catalysts.

	Pd1Cu4/XC-72	Pd2Cu3/XC-72	PdCu/XC-72	Pd3Cu2/XC-72	Pd4Cu1/XC-72
Mass activity	311 mA/mgPd	1130 mA/mgPd	1189 mA/mgPd	1719 mA/mgPd	1409 mA/mgPd
If/Ib	3.41287	4.01877	1.999	5.307	4.785

). In other words, I_f:I_b is positively correlated with the area ratio of Pd⁰ and Cuⁿ⁺, which proves that higher Pd⁰ and Cuⁿ⁺ in the catalyst results in higher anti-CO performance.

Figure 7 summarizes the electrochemical performance of five electrocatalysts. Cyclic voltammetry(CV) is performed on each of the Pd₁Cu₄/XC-72, Pd₂Cu₃/XC-72, PdCu/XC-72, Pd₃Cu₂/XC-72 and Pd₄Cu₁/XC-72 electrode in N₂-saturated 1.0 M KOH. The PdO reduction peaks of the catalysts containing Cu occur at earlier potentials, as seen in Figure 6. This means less energy was needed to reduce PdO in the presence of Cu. This is probably the attraction of Pd to metal oxides like CuO, which provides more electronics to the Pd and PdO active sites. In other words, it is more difficult to reduce PdO oxides as CuO withdraws electrons from Pd.

The electrochemical active surface area (ECSA) is also an important index for evaluating the activity of electrocatalysts. In this work, the ESCA of different ratios of Pd_xCu_y/XC-72 is estimated by integrating the Pd reduction peaks in order to find the Coulombic charge of each catalyst, and then following the proposed method of Rand and Wood (Equation (1))

ECSA = Q/SL

(1)

where Q is the Coulombic charge (0.0536, 0.1316, 0.1173, 0.216562, and 0.1444 mC for Pd₁Cu₄/XC-72, Pd₂Cu₃/XC-72, PdCu/XC-72, Pd₃Cu₂/XC-72, and Pd₄Cu₁/XC-72, respectively), S is the Coulombic constant for a monolayer of Pd (0.424 mC cm⁻²), and L is the electrocatalyst loading (∼x μg (Pd)). The calculated ECSA Pd values decrease as follows: Pd₃Cu₂/XC-72 (2.384 m² g_Pd⁻¹) > Pd₂Cu₃/XC-72 (1.961 m² g_Pd⁻¹) > PdCu/XC-72 (1.47 m² g_Pd⁻¹) > Pd₁Cu₄/XC-72 (1.428 m² g_Pd⁻¹) > Pd₄Cu₁/XC-72 (1.305 m² g_Pd⁻¹). These results reveal that Cu acts as a promoter to boost the electrochemical activity of Pd.

Chronoamperometry (CA) curves are recorded in 1.0 M CH₃OH + 1.0 M KOH solution under a constant potential of −0.2 V, as shown in Figure 8. The high initial current density is caused by double-layer charging and abundant active sites for methanol activation. The current density dropped quickly within the first dozens of seconds due to the adsorption of CO produced from the MOR on the catalytic surface.

After 500 s of testing, Pd₃Cu₂/XC-72 still had the highest ultimate current density (122.3 mA mg⁻¹ Pd), further confirming that the Pd₃Cu₂/XC-72 nanocatalyst has the best electrocatalytic activity and long-term electrochemical durability for MOR. The synthesized Pd₃Cu₂/XC-72 nanoparticles have the advantages of large specific surface area, high alloying, strong electronic interactions between Pd and Cu atoms, a clear surface, and many active sites, which can significantly improve the electrocatalytic performance of MOR.

4. Conclusions

In summary, we studied a highly efficient palladium copper alloy catalyst with a coordination structure which was successfully synthesized by the liquid phase reduction method. During the synthesis, the ratios between Pd and Cu determined the alloying degree. The particular electronic characteristics endowed Pd₃Cu₂/XC-72 with large ECSA, fast mass transfer, and high self-stability, which contributed to the high electrocatalytic activity and excellent stability of Pd₃Cu₂/XC-72 for MOR in alkaline media. Additionally, Pd₃Cu₂/XC-72 exhibited enhanced MOR activity compared to Pd/C, indicating that the PdCu interface improved the MOR activity of Pd nanostructures. Experimental results demonstrated that the synergistic effect between Pd and Cu accelerated the oxidation of CO_ads, which are also responsible for MOR activity/stability enhancement of Pd₃Cu₂/XC-72. This study provides new ideas for the design of highly active Pd electrocatalysts, and demonstrates the great potential of Pd₃Cu₂/XC-72 as a DMFCs anode electrocatalyst.