Fabrication of Pb-Containing PtAu Nanoflowers via Galvanic Replacement Method for Electrocatalytical Oxidation of Methanol

Abstract

A Pb-containing PtAu nanoflower electrocatalyst was deposited on the cathode via galvanic replacement reaction in a double-cabin galvanic cell (DCGC) with a Cu plate as the anode, a multiwalled carbon nanotube (MWCNT) modified glassy carbon electrode (GCE) as the cathode, 0.1 M HClO₄ aqueous solution as the anolyte, and Pb²⁺-containing Pt⁴⁺ salt and Au³⁺ salt mixed aqueous solution as the catholyte, respectively, and the electrocatalytic performance of the modified electrode toward methanol oxidation in the alkaline medium was investigated. Electrochemical studies reveal that the stripping of bulk Cu can induce underpotential deposition (UPD) of Pb on Pt during the galvanic replacement reaction, which affects the morphology and composition of Pb-containing PtAu nanoparticles. Under the optimal experimental conditions, a Pb-Pt₃Au₁/MWCNTs/GCE shows the highest activity and the best stability toward electrocatalytic oxidation of methanol in the alkaline medium, and the Pt active area-normalized specific electrocatalytic activity of Pb-Pt₃Au₁/MWCNTs/GCE is as high as 59.8 mA cm_Pt⁻². We believe that the method presented here of depositing highly active noble metal nanostructures by galvanic replacement reaction in a DCGC device is expected to be widely applied in the preparation of nanomaterials for their study in fuel cells and electrocatalysis.

1. Introduction

Direct methanol fuel cells (DMFCs) have attracted great interest as a renewable energy supply due to their advantages of low operating temperature, easy operation, and high energy density [1]. Currently, Pt, as a commonly used methanol electrocatalyst, is widely used in DMFC research, but Pt is expensive, scarce, and easily deactivated by CO_ads intermediates generated by methanol electrooxidation, which limits its commercial application in DMFCs [2]. Therefore, the key to the development of DMFCs is to develop efficient, low-cost, and anti-poisoning methanol electrocatalysts. Bi (or tri)-component Pt-based electrocatalysts formed by Pt with other transition metals or noble metals can effectively reduce the Pt content and improve the activity and anti-poisoning ability of Pt. So far, various Pt-based multi-component electrocatalysts have been studied, such as Pt-Mn [3], Pt-Fe [4], Pt-Co [5], Pt-Ru [6], Pt-Ni [7], Pt-Cu [8], Pt-Pd [9], Pt-Ag [10], Pt-Mo [11], and PtRuFe [12]. These multi-component Pt-based electrocatalysts can significantly improve the electrocatalytic activity and stability of Pt through electronic effects and bifunctional effects between components.

As a common noble metal, Au exhibits good chemical stability and weak catalytic activity in its bulk state, but nanosized Au particles show excellent electrocatalytic performance for small molecules [13,14]. The bi-component PtAu electrocatalyst can effectively enhance the anti-poisoning capability and catalytic activity of Pt due to the synergistic effect between Pt and Au components [15]. Currently, people use various methods to prepare PtAu electrocatalysts with different morphologies and compositions. For example, Zhang et al. prepared nanoporous PtAu electrocatalysts via dealloying and found that they could efficiently electrocatalyze the oxidation of methanol under acidic conditions [16]. Luo et al. synthesized PtAu nanoparticles supported on carbon via colloidal synthesis and observed that the bifunctional effect of Au could enhance the electrocatalytic activity of Pt toward methanol under alkaline conditions [17]. In addition, doping the PtAu electrocatalyst with other base metals (such as Cu and Pb) can further enhance the catalytic performance of the PtAu-based tri-component electrocatalyst. For instance, Wang et al. synthesized Au-Cu-X (X = Pt, Pd, Ag) ternary metal nanorods using Au-Cu nanorods as a galvanic cell displacement template, and the synthesized nanorods were found to be highly effective in the reduction of 4-nitrophenol [18]. To date, extensive studies have been conducted on the preparation of PtAu electrocatalysts; previous synthesis methods also have disadvantages such as high cost, complex operations, and environmental pollution. The galvanic cell method, due to its low energy consumption, simplicity, and low cost, is a typical green preparation technology. For example, Yoshii et al. synthesized Pd-Co bimetallic nanocatalysts supported on C using Co nanoparticles supported on C as a displacement template and found that they had high catalytic activity and selectivity for the hydrogenation of styrene [19]. In the previous galvanic cell method for the synthesis of Pt-based electrocatalysts, the anode and cathode reactions of the galvanic cell occurred in the same solution. Both reactions at the anode and cathode of the galvanic cell may affect the composition of the deposited Pt-based electrocatalyst. In some cases, further purification of the product is inevitably required, which adds complexity to the process and increases production costs, both of which are undesirable. The use of a double-cabin galvanic cell (DCGC) with separate anode and cathode compartments allows for the one-step deposition of clean, multi-component Pt-based electrocatalysts at the cathode, simplifying the operational steps. To our knowledge, there are rarely reports on the direct preparation of high-activity PtAu-based tri-component nanoelectrocatalyst-modified electrodes at the cathode using the DCGC setup.

In this work, using a high-purity Cu plate as the anode and a glassy carbon electrode (GCE) modified with multiwalled carbon nanotubes (MWCNTs) as the cathode in the DCGC device, a Pb-containing PtAu nanoflower electrocatalyst was deposited at the cathode through galvanic replacement, and the electrocatalytic performance of the resulting modified electrode toward methanol electrooxidation in an alkaline environment was investigated. Electrochemical studies show that the dissolution of bulk Cu during the galvanic reaction can induce underpotential deposition (UPD) of Pb on the Pt surface, leading to the deposition of Pb-containing PtAu nanoflower electrocatalysts on MWCNTs. Pb-containing PtAu nanoflower electrocatalysts exhibit excellent activity toward methanol electrocatalytic oxidation and have a potential application value in DMFCs.

2. Results and Discussion

2.1. Fabrication and Characterization of Pb-Containing PtAu Nanoparticles

Figure 1 shows the discharging current (i_cell) and the potential of a MWCNT/GCE cathode (E_cathode) during the depositing of Pb-Pt₃Au₁ and Pt₃Au₁ on its surface. When the anode and cathode are not connected, the electrochemical potential (ECP) of the Cu plate is approximately 0.008 V in 0.1 M HClO₄ solution (anode part), while the ECP of the MWCNT/GCE surface in the mixed solution of noble metal salts is about 0.78 V (cathode part). Therefore, after connecting the anode and cathode of the DCGC device with a Cu wire, the ECP difference between the anode and cathode drives the galvanic replacement reaction, leading to the dissolution of bulk Cu (Cu_bulk) at the anode and the deposition of noble metal nanoparticles on the surface of the MWCNTs/GCE at the cathode. During the deposition of Pt₃Au₁ from 3.0 mM H₂PtCl₆ + 1.0 mM HAuCl₄ + 0.1 M HClO₄ catholyte (without the presence of Pb(ClO₄)₂; Figure 1 b), the steady-state potential and i_cell of MWCNTs/GCE are 0.097 V and 52 μA, respectively. However, during the deposition of Pb-Pt₃Au₁ from 3.0 mM H₂PtCl₆ + 1.0 mM HAuCl₄ + 5.0 mM Pb(ClO₄)₂ + 0.1 M HClO₄ (with the presence of Pb(ClO₄)₂; Figure 1 a), the steady-state potential and i_cell of MWCNTs/GCE increase to 0.121 V and 62 μA, respectively. The response for the deposition of Pb-Pt₃Au₁ is larger than that for the deposition of Pt₃Au₁, which perhaps suggests that the addition of Pb(ClO₄)₂ to the catholyte facilitates the deposition of Pt and Au.

Figure 2 shows the SEM images of the obtained Pt₃Au₁/MWCNTs/GCE and Pb-Pt₃Au₁/MWCNTs/GCE. For Pt₃Au₁/MWCNTs/GCE (Figure 2A,B), a large number of spherical nanoparticles with diameters of 40–240 nm are distributed on the surface of MWCNTs. However, in Pb-Pt₃Au₁/MWCNTs/GCE (Figure 2C,D), flower-like nanoparticles grow on the surface of MWCNTs, with diameters ranging from 110 to 450 nm, and the density of nanoparticles on the surface of Pb-Pt₃Au₁/MWCNTs/GCE is also lower than that of Pt₃Au₁/MWCNTs/GCE. EDS characterization (Figure 3) also proved that Pt and Au have been deposited on the surface of MWCNTs/GCE, and the atomic percentages of Pt and Au for Pt₃Au₁ nanoparticles were determined to be 53.9% and 46.1% (Figure 3A), respectively, which was unproportional to that of the catholyte solution (mole concentration ratio for c_H2PtCl6:c_HAuCl4 = 3:1). Due to the higher ECP of Au versus that of Pt, the reaction kinetics for reducing AuCl₄⁻ is anticipated to be significantly faster than that for reducing PtCl₆²⁻, thus leading to the deposition of more Au in Pt₃Au₁ nanoparticles even at a high Pt/Au mole ratio in the catholyte. For Pb-Pt₃Au₁/MWCNTs/GCE, there are a small number of Pb atoms present in the obtained PtAu nanoparticles (Figure 3B), and the atomic percentages of Pt, Au, and Pb were determined to be 50.0%, 42.9%, and 7.1%, respectively, which indicated the occurrence of cathodic deposition of Pb from the Pb(ClO₄)₂-containing catholyte during the GRR deposition of Pb-Pt₃Au₁ nanoparticles, despite the higher activity of bulk lead (Pb_bulk) compared to Cu_bulk (anode here). The above results indicate that when Pb(ClO₄)₂ is added to the catholyte, the co-deposition of Pb on the cathode can influence the morphology and composition of PtAu nanoparticles, leading to the formation of Pb-containing PtAu nanoflower particles. Indeed, previous studies have also reported that the addition of transition metal ions (such as Cu²⁺) can control the crystal structure and morphology for synthesizing noble metal nanostructures [20].

Moreover, Figure S1 shows the X-ray diffraction (XRD) patterns of the as-synthesized Pt₃Au₁ and Pb-Pt₃Au₁ thin film sample on MWCNTs. The peaks at 37.1° and 42.4° are assigned to Au(111) and Pt(111), respectively, according to JCPDS 04-0784 of pure Pt and JCPDS 04-0802 of pure Au, and the peaks at 73.3° and 76.4° may correspond to (311) and (222) planes. In our case, no XRD peaks of Pb are observed on Pb-Pt₃Au₁ that may be the Pb of UPD is too small to produce an X-ray diffraction signal. X-ray photoelectron spectroscopy (XPS) was applied to characterize the Pt₃Au₁ and Pb-Pt₃Au₁ thin film sample also (as shown in Figure S2). The two 4f binding states of zero valence, at 84.4 (4f_5/2) and 87.5 eV (4f_7/2) for Au⁰ and at 71.2 (4f_5/2) and 74.5 eV (4f_7/2) for Pt⁰, were observed on both Pt₃Au₁ and Pb-Pt₃Au₁, respectively. Moreover, Pb may exist as Pb²⁺ in Pb-Pt₃Au₁, with the peak of Pb²⁺ 4f_7/2 and 4f_5/2 appearing at 139.6 eV and 144.6 eV, respectively, which is a large shift compared to pure Pb⁰, indicating the electronic interaction between PtAu and Pb.

2.2. Electrochemical Study of the Mechanism of Pb Co-Deposition at the Cathode During the GRR Process in the DCGC Device

It is significant and intriguing to study the mechanism of Pb co-deposition at the cathode when depositing Pt and Au in the DCGC device with a Cu plate as the anode. Given that the ECP of Pb_bulk is lower than that of Cu_bulk, the dissolution of Cu_bulk, inducing the deposition of Pb_bulk during the GRR for depositing Pb-Pt₃Au₁ nanoparticles at the cathode, is thermodynamically impossible. Here, we studied the electrochemical deposition behavior of Pb on Pt or Au surfaces in cyclic voltammetry (CV) experiments. Figure 4 shows the CV curves for electroplated Pt-modified Au (Pt_pla/Au; as shown in Figure S3) and bare Au electrodes in a solution containing 3.0 mM Pb(ClO₄)₂ + 0.1 M HClO₄. For Pt_pla/Au, during the negative-going scan, the UPD of Pb on the Pt surface starts at 0.55 V [21]. The UPD of Pb on the Pt surface (Pb⁰_UPD) significantly impedes the adsorption/desorption behavior of H on the Pt surface between 0 and −0.28 V. The UPD of Pb terminates at −0.45 V, where the bulk deposition of Pb (Pb⁰_bulk) begins. For the bare Au electrode, during the negative-going scan, the UPD of Pb on Au starts at 0.06 V, and two distinct pairs of oxidation-reduction peaks appear at 0.01/−0.01 V and −0.23/−0.25 V, corresponding to the UPD behavior of Pb on the Au surface [22]. The UPD of Pb terminates at −0.45 V, where the bulk deposition of Pb occurs, and the dissolution peak of bulk Pb appears at −0.35 V. The CV experiments indicate that the deposition/dissolution potentials of Pb⁰_UPD are higher than those of Pb⁰_bulk, i.e., the ECP of Pb²⁺/Pb⁰_UPD is higher than that of Pb²⁺/Pb⁰_bulk, suggesting the thermodynamic possibility of the bulk dissolution of an active metal inducing the UPD of its ions (or other ions) on a relatively less active metal surface (such as Pt or Au).

In the DCGC device with a Cu plate as the anode, during the GRR for depositing Pb-Pt₃Au₁, the steady-state potential of the MWCNTs/GCE is 0.121 V, which falls within the potential range for the UPD of Pb on the Pt surface (0.55~−0.45 V) but is higher than the potential range for the UPD of Pb on the Au surface (0.06~−0.45 V). This indicates that during the deposition of Pb-Pt₃Au₁, Pb can undergo UPD on the Pt surface but not on the Au surface. Hence, the bulk dissolution of Cu at the anode can induce the UPD of Pb on Pt at the cathode in the DCGC device. The UPD effect of Pb on the Pt surface influences the growth process of PtAu nanoparticles, leading to the deposition of nanoflower structures on the MWCNT surface. Simultaneously, a small number of Pb⁰_UPD atoms are embedded within the PtAu nanoparticles, forming a Pb-containing PtAu nanoflower structure.

2.3. Electrocatalytical Oxidation of Methanol on Pb-Containing PtAu Nanoparticles in Alkaline Solution

Figure 5 shows the cyclic voltammograms (CVs) of Pb-Pt₃Au₁/MWCNTs/GCE, Pt₃Au₁/MWCNTs/GCE, and Pb-Pt/MWCNTs/GCE in a 1.0 M KOH (Figure 5A) and 1.0 M CH₃OH + 1.0 M KOH solution (Figure 5B,C). For Pb-Pt₃Au₁/MWCNTs/GCE (Figure 5A), the adsorption/desorption peaks of hydrogen on the Pt surface appear between −0.65 V and −1.0 V. The redox peaks at −0.13 V and −0.38 V correspond to the formation and reduction of Pt oxide [23]. Additionally, the redox peaks at 0.3 V and 0.03 V are attributed to the formation and reduction of Au oxide, which is consistent with the redox peaks of Au on the surface of Pb-Au/MWCNTs/GCE (inset of Figure 5A). The electrochemical characteristic peaks on the surfaces of Pt₃Au₁/MWCNTs/GCE and Pb-Pt/MWCNTs/GCE are similar to those of Pb-Pt₃Au₁/MWCNTs/GCE.

For the electrocatalytic oxidation of methanol (Figure 5B), during the forward scan, the initial oxidation potential of methanol on the surface of Pb-Pt₃Au₁/MWCNTs/GCE is −0.64 V, which is shifted negatively by 30 mV and 130 mV compared to Pt₃Au₁/MWCNTs/GCE (−0.61 V) and Pb-Pt/MWCNTs/GCE (−0.51 V), respectively. The peak current for the oxidation of methanol on the surface of Pb-Pt₃Au₁/MWCNTs/GCE is 270 mA cm⁻², which is 2.62 times and 5.0 times higher than that of Pt₃Au₁/MWCNTs/GCE (103 mA cm⁻²) and Pb-Pt/MWCNTs/GCE (54 mA cm⁻²), respectively. Furthermore, we compared the electrochemical active area specific activity of Pt (mA cm_Pt⁻²), as shown in Figure 5C. The peak current for the oxidation of methanol on the surface of Pb-Pt₃Au₁/MWCNTs/GCE is 59.8 mA cm_Pt⁻², which is 2.48 times and 7.29 times higher than that of Pt₃Au₁/MWCNTs/GCE (24.1 mA cm_Pt⁻²) and Pb-Pt/MWCNTs/GCE (8.2 mA cm_Pt⁻²), respectively.

The above results indicate that Pb-Pt₃Au₁/MWCNTs/GCE exhibits excellent electrocatalytic performance toward methanol, primarily due to the following reasons: (1) compared to the spherical structure of Pt₃Au₁ nanoparticles, the Pb-Pt₃Au₁ nanoflower particles have more surface defect sites and a higher number of exposed unsaturated Pt atoms, which enhance their catalytic activity [24]; (2) both the bifunctional effect between Pt and Au atoms [17] and the electronic effect between Pt and Pb atoms [25] contribute to enhancing the catalytic performance of the Pb-Pt₃Au₁ ternary metal catalyst.

Since the catalytic performance of noble metal electrocatalysts is related to their morphology, particle size, and composition [26], the modified electrodes with different Pt and Au contents were prepared by changing the concentration ratio of H₂PtCl₆ and HAuCl₄ in the catholyte, and the electrocatalytic performance of the obtained modified electrodes for methanol was studied, as shown in Figure 6. Increasing the concentration of HAuCl₄ in the catholyte results in an increase in the Au content on the electrode surface, accompanied by a decrease in Pt content, as evidenced by the increased oxidation-reduction peak of Au and the decreased oxidation-reduction peak of Pt (Figure 6A). The peak current density of methanol electrocatalytic oxidation also increases with the increased concentration of HAuCl₄, reaching a maximum value of 270 mA cm⁻² at c_H2PtCl6: c_HAuCl4 = 3:1 (Pb-Pt₃Au₁/MWCNTs/GCE; Figure 6B), and then decreases with the increased HAuCl₄ concentration. The electrochemical active area specific activity of Pt (Figure 6C) also reaches a maximum value of 59.8 mA cm_Pt⁻² on Pb-Pt₃Au₁/MWCNTs/GCE. As the Au content on the electrode surface increases, the synergistic effect between Pt and Au atoms gradually intensifies. The activity of Pt atoms reaches its maximum at a ratio of c_H2PtCl6: c_HAuCl4 = 3:1. As the Au content on the electrode surface continues to increase, the Pt content gradually decreases, as evidenced by the reduced oxidation-reduction peak of Pt observed in the above electrochemical characterization. The probability of Pt atoms being surrounded by Au atoms increases, and some Pt atoms may be separated by Au atoms, reducing the number of adjacent Pt atoms, which may hinder the adsorption and dissociation of methanol on the Pt surface and lead to a decrease in the activity of Pt atoms [27]. It has also been reported that methanol can only undergo electrooxidation on a surface with at least three adjacent Pt atoms [28].

The electrochemical stability of electrocatalysts is an important indicator for practical applications. We also used a potentiostatic method to evaluate the stability of different modified electrodes in catalyzing the oxidation of methanol in 1.0 M CH₃OH + 1.0 M KOH solution at −0.25 V, as shown in Figure 7. The Pb-Pt₃Au₁/MWCNTs/GCE exhibited the slowest rate of catalytic current decay, with a catalytic current of 8.19 mA cm_Pt⁻² at 1800 s, which is significantly higher than that of Pb-Pt/MWCNTs/GCE (0.10 mA cm_Pt⁻²), Pb-Pt₃Au₃/MWCNTs/GCE (0.83 mA cm_Pt⁻²), Pb-Pt₃Au₅/MWCNTs/GCE (0.23 mA cm_Pt⁻²), and Pb-Pt₃Au₇/MWCNTs/GCE (0.11 mA cm_Pt⁻²). This indicates that the Pb-Pt₃Au₁ nanoelectrocatalyst can effectively remove toxic products such as CO_ads that are generated during the electrooxidation of methanol, maintaining the catalytic activity and electrochemical stability.

3. Materials and Methods

3.1. Instrumentation and Reagents

All electrochemical experiments were conducted on an Autolab PGSTAT 30 electrochemical workstation (Eco Chemie BV, Utrecht, The Netherlands) using a conventional three-electrode system. The working electrodes were either a glassy carbon electrode (GCE) with a diameter of 3 mm or a gold electrode (0.29 cm²). The reference electrode was a saturated calomel electrode (SCE) equipped with a salt bridge, and the counter electrode was a carbon rod. All potentials were referenced to the SCE. The JEOL JSM-1230 high-resolution field-emission scanning electron microscope (SEM, Akishima, Japan) and the accompanying energy-dispersive X-ray spectroscopy (EDS) were used for morphological characterization and elemental composition analysis, respectively. X-ray diffraction (XRD) pattern was collected on a D8 Discover X-ray diffractometer (Bruker Co., Billerica, MA, USA). X-ray photoelectron spectroscopy (XPS) spectra were taken using an ESCA Escalab 220i XL (Thermo Fisher Scientific, Waltham, MA, USA) with a monochromated Al Kα X-ray source (1486.6 eV).

For the experiments conducted in the DCGC, when no external load was applied, the discharge current (i_cell) of the galvanic cell was monitored using the ECN module of the Autolab PGSTAT 30 electrochemical workstation [29] (Metrohm, Herisau, Switzerland). The accuracy of this setup was verified using a calibrated 1.5 V dry cell (error within 1 mV) under a 100 kΩ load; the dry cell voltage measured by the ECN setup was 1.501 V, with an i_cell of 15.01 µA, strictly adhering to Ohm’s Law. The potentials of the anode and cathode in the galvanic cell setup were dynamically monitored using a high-impedance (R ≥ 10¹² Ω) voltmeter, which was modified from a pH meter (Leici, Shanghai, China). All experiments were conducted at room temperature (26 ± 2 °C).

N,N-Dimethylformamide (DMF), HAuCl₄, H₂PtCl₆, and methanol were purchased from Tianjin Chemical Reagent Station (Tianjin, China). Pb(ClO₄)₂ and high-purity Cu plate (99.999%) were purchased from Alfa Aesar company (Tianjin, China). Multiwalled carbon nanotubes (MWCNTs; 60 nm outer diameter and 40 nm inner diameter on average) were purchased from Chengdu Organic Chemicals Co. Ltd. (Chengdu, China) and purified in concentrated acids before use. Other reagents were of analytical grade. All solutions were prepared using Milli-Q ultrapure water (Millipore, Burlington, MA, USA, >18 M cm).

3.2. Deposition of Pb-Containing PtAu Nanoparticles on MWCNTs/GCE in the DCGC Device

The GCE was sequentially polished with 1.0 μm and 0.05 μm alumina slurry on a chamois leather until a mirror-like finish was achieved. After each polishing step, the surface contaminants were washed off, and the electrode was cleaned in an ultrasonic water bath for 3 min each time; this process was repeated three times. Finally, the electrode was ultrasonically cleaned sequentially with 1:1 ethanol, 1:1 HNO₃, and distilled water. It was then subjected to cyclic voltammetry (CV) scans in 0.2 M HClO₄ solution (0 to 1.5 V, 30 mV s⁻¹) until reproducible voltammetric responses were observed. The resulting GCE was further subjected to CV scans in 2.0 mM K₄Fe(CN)₆ + 0.1 M Na₂SO₄ solution (−0.1 to 0.5 V, 50 mV s⁻¹). A peak-to-peak potential width of ca. 70 mV indicated that the electrode surface was clean. A 1 mg mL⁻¹ dispersion of MWCNTs in DMF was prepared and sonicated for 15 min. Then, 5 μL of the MWCNT dispersion was cast onto the GCE and air-dried to form the MWCNTs/GCE.

For the deposition of Pb-containing PtAu nanoparticles on MWCNTs/GCE in a DCGC device based on the GRR principle, a Cu plate served as the anode, the anolyte was 0.1 M HClO₄ aqueous solution, MWCNTs/GCE served as the cathode, and the catholyte was 3.0 mM H₂PtCl₆ + 1.0 mM HAuCl₄ + 5.0 mM Pb(ClO₄)₂ + 0.1 M HClO₄ aqueous solution. The catholyte and anolyte were connected using a homemade U-shaped glass tube salt bridge filled with saturated KCl solution. Scheme 1 shows the schematic diagram of the total experimental setup, similar to that of a previous report [30]. After switching on the DCGC, the ECN and voltmeter were used to monitor the i_cell and the potential of MWCNTs/GCE (vs SCE), respectively. The total discharging time in the DCGC was 240 s. The resulting modified electrode was denoted as Pb-Pt₃Au₁/MWCNTs/GCE. Varying the concentration ratio of H₂PtCl₆ and HAuCl₄ in the catholyte can obtain modified electrodes with different Pt and Au content.

For comparative studies, we also prepared other modified electrodes in a similar procedure, and the abbreviation of the modified electrodes and the corresponding composition of catholytes are listed in

Table 1. Abbreviation and the corresponding metal salt composition of catholyte for preparing the modified electrodes.

Abbreviation of Modified Electrodes	The Metal Salt Composition of Catholyte
Pb-Pt/MWCNTs/GCE	3.0 mM H2PtCl6 + 5.0 mM Pb(ClO4)2 + 0.1 M HClO4
Pb-Pt3Au0.5/MWCNTs/GCE	5.0 mM Pb(ClO4)2 + 3.0 mM H2PtCl6 + 0.5 mM HAuCl4 + 0.1 M HClO4
Pb-Pt3Au1/MWCNTs/GCE	5.0 mM Pb(ClO4)2 + 3.0 mM H2PtCl6 + 1.0 mM HAuCl4 + 0.1 M HClO4
Pb-Pt3Au3/MWCNTs/GCE	5.0 mM Pb(ClO4)2 + 3.0 mM H2PtCl6 + 3.0 mM HAuCl4 + 0.1 M HClO4
Pb-Pt3Au5/MWCNTs/GCE	5.0 mM Pb(ClO4)2 + 3.0 mM H2PtCl6 + 5.0 mM HAuCl4 + 0.1 M HClO4
Pt3Au1/MWCNTs/GCE	3.0 mM H2PtCl6 + 1.0 mM HAuCl4 + 0.1 M HClO4
Pb-Au/MWCNTs/GCE	3.0 mM HAuCl4 + 5.0 mM Pb(ClO4)2 + 0.1 M HClO4

.

3.3. CV Experiment to Study the Electrochemical Deposition Behavior of Pb on Pt and Au Surfaces

The pretreatment process of the gold electrode was as follows. To remove contaminants from the electrode surface, the gold electrode was first cleaned with H₂SO₄ + H₂O₂ (v/v, 3:1) solution for 3 min and then subjected to CV scanning in 0.2 M HClO₄ solution (0~1.5 V, 30 mV s⁻¹) until a reproducible voltammetric response was achieved. The resulting gold electrode was then subjected to CV scans in 2.0 mM K₄Fe(CN)₆ + 0.1 M Na₂SO₄ solution (−0.1 to 0.5 V, 50 mV s⁻¹). A peak-to-peak potential width of ca. 70 mV indicated that the electrode had been properly cleaned.

The gold electrode was placed in a 3.0 mM H₂PtCl₆ + 0.1 M HClO₄ solution and electroplated at a constant potential of 0 V for 300 s to obtain a Pt-modified Au electrode (Pt_pla/Au). The obtained Pt_pla/Au electrode was placed in 0.1 M H₂SO₄ for CV characterization (−0.3–1.5 V, 50 mV s⁻¹). After Pt plating, a reduction peak of Pt oxide appeared at 0.35 V, and the adsorption/desorption peak of H on the Pt surface appeared at 0–0.28 V. The reduction peak of Au oxide at 0.85 V nearly disappeared, indicating that the Au electrode surface was fully covered with Pt (Figure S3). Then, using Pt_pla/Au as the working electrode, the electrochemical deposition behavior of Pb on the Pt_pla/Au surface was investigated by the CV method in 3.0 mM Pb(ClO₄)₂ + 0.1 M HClO₄ solution. Similarly, a bare gold electrode was used as the working electrode to study the electrochemical deposition behavior of Pb on the gold electrode surface.

3.4. Electrochemical Characterization of the Modified Electrodes and Electrocatalytic Oxidation of Methanol

The modified electrode was subjected to CV characterization in 1.0 M KOH (−1.0 to 0.5 V, 50 mV s⁻¹). The electrocatalytic oxidation of methanol was conducted in 1.0 M CH₃OH + 1.0 M KOH solution (−1.0 to 0.5 V, 50 mV s⁻¹). The stability of the modified electrode toward methanol electrocatalytic oxidation was evaluated using a potentiostatic method in 1.0 M CH₃OH + 1.0 M KOH solution at a potential of −0.25 V for 1800 s. The electrochemical active area of Pt was calculated from the hydrogen adsorption peak in 1.0 M KOH solution, using a conversion factor of 210 μC cm⁻² [21]. All solutions were deoxygenated with high-purity nitrogen for at least 10 min.

4. Conclusions

In summary, a Pb-containing PtAu nanoflower electrocatalyst was deposited on the MWCNT/GCE cathode in a DCGC device. The electrochemical experimental results demonstrate that the bulk dissolution of Cu at the anode can induce the UPD of Pb on Pt at the cathode during the GRR process in the DCGC device. The UPD effect of Pb influenced the growth process of PtAu nanoparticles, leading to the formation of Pb-containing PtAu nanoflower structures. Under the optimal experimental conditions, the prepared Pb-Pt3Au1/MWCNTs/GCE showed the highest activity and stability for the electrocatalytic oxidation of methanol in an alkaline environment. The method of depositing high-activity electrocatalysts via GRR in the DCGC device is expected to have broad application value in electrocatalysis and fuel cell research.