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Article

Electrochemical Quartz Microbalance for Studying Electrodeposited Pt Catalysts for Methanol Oxidation Reaction

by
Bangfeng Zong
1,
Xiaojun Pan
2,
Bo Wei
2,
Lifang Zhang
2,
Xiangxiong Feng
2,
Chenggong Hou
2,
Hai Yan
2,
Wenju Xie
3,
Guicheng Liu
2,3,* and
Feng Ye
2,*
1
School of Mechanical and Electronic Engineering, Suzhou University, Suzhou 234000, China
2
Key Laboratory of Power Station Energy Transfer Conversion and System of MOE, School of Energy Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China
3
Fujian Provincial Key Laboratory of Eco-Industrial Green Technology, Wuyi University, Nanping 354300, China
*
Authors to whom correspondence should be addressed.
Inorganics 2025, 13(4), 111; https://doi.org/10.3390/inorganics13040111
Submission received: 18 February 2025 / Revised: 26 March 2025 / Accepted: 31 March 2025 / Published: 3 April 2025
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Abstract

:
Pt catalysts are investigated for methanol oxidation in direct methanol fuel cells, utilizing the electrochemical quartz microbalance method (EQCM) with exceptional resolution and sensitivity. Pt catalysts were deposited onto the gas-diffusion layer of carbon using stationary potential electrodeposition. Physical characterization and electrochemical tests were performed. SEM results showed that Pt presented dendrite crystals with nanoscale facets. Cyclic voltammetry (CV) demonstrated that the current density for the methanol oxidation reaction highly reached 1020 mA·cm−2 for the deposited Pt catalyst by EQCM. The dendrite crystal structures of deposited Pt provide much area for high catalytic activity. It found that the peak density of the Pt catalysts for the methanol oxidation reaction decreased after five cycles. Furthermore, the response frequency for the adsorption of the deposited Pt catalysts was investigated using EQCM and compared with commercial PtRu catalysts. The results showed that the response frequency of the Pt catalysts decreased more rapidly than that of the PtRu catalysts. It is possible for the adsorption of small organic molecules on Pt catalysts to occur during the methanol electro-oxidation with COad intermediates. The reaction mechanism is preliminarily discussed by the electrochemical measurement combined with EQCM.

1. Introduction

Platinum (Pt) catalysts are widely used in direct methanol fuel cells (DMFCs) for methanol oxidation due to their high catalytic activity [1,2]. However, over time, Pt catalysts suffer from poisoning, which significantly reduces their efficiency [3,4]. Factors such as reaction temperature, overpotential, and the coverage of small organic molecules have been identified as contributors to Pt catalyst degradation [5,6]. Recent studies have highlighted the importance of bimetallic catalysts (e.g., PtRu, PtCo, and PtNi) in enhancing MOR activity and CO tolerance. For instance, Zhang et al. [7] demonstrated that PtRu/C catalysts exhibited superior CO oxidation kinetics due to the bifunctional mechanism and ligand effects. Similarly, Huang et al. [8] reported that PtCo/C catalysts showed enhanced catalytic activity through an optimized electronic structure and reduced adsorption energy of CO intermediates. Igarashi et al. [9] have explored the degradation mechanisms of Pt catalysts. They found that the increased adsorption of organic molecules, such as CO, significantly accelerated catalyst degradation. Matthijs et al. [10] used surface techniques such as the surface X-ray diffraction method to study the adsorption of small organic molecules on Pt catalysts. They found that the adsorption of small molecules led to a significant reduction in Pt catalytic activity, with a reaction barrier ranging from 1.45 to 2.18 eV below 550 K. Additionally, other advanced techniques, including nuclear magnetic resonance spectroscopy and online electrochemical mass spectrometry, have been applied to detect intermediates and monitor chemical reactions [11]. Monatomic catalyst poisoning, often due to feed impurities such as sulfur or CO, has been identified as a major factor in the decline of Pt catalyst activity [12]. Despite the availability of various evaluation techniques for Pt degradation, there remains a need for higher resolution and sensitivity in these assessments.
In recent years, the electrochemical quartz microbalance (EQCM) has gained significant attention due to its ability to assess catalytic mechanisms with high time resolution and sensitivity (up to 2.17 Hz cm2 ng−1) [13,14]. EQCM operates based on the piezoelectric properties of quartz crystals, where mass changes on the electrode surface induce measurable shifts in the crystal’s resonant frequency. This relationship is quantified by the Sauerbrey equation (Δf = −SΔm), where Δf is the frequency shift, Δm is the mass change per unit area, and S is the sensitivity constant of the crystal [15]. By correlating frequency shifts with electrochemical signals, EQCM enables real-time monitoring of nanoscale mass variations during reactions, making it uniquely suited for studying dynamic processes such as adsorption, desorption, and degradation. EQCM has been widely applied in various areas of electrochemistry, such as metal electrodeposition [16,17], dissolution [18], electrochemical adsorption [19], conductive polymer films [20], and electrode surface morphology [21]. For instance, Bruckenstein et al. [22] integrated quartz crystal microbalance with electrolyte solutions to study electrochemical processes. Ivan et al. [23] employed DFT-assisted EQCM to quantitatively analyze the phase transition of α-Ni(OH)2(H2O) after cycling in alkaline electrolytes. The results showed that ionic embedding displaced structural water, promoting electrochemical degradation and aging of the material, as indicated by changes in quartz crystal frequency. Moreover, EQCM allows the detection of mass changes (∆m) due to adsorption, deposition, desorption, or dissolution on the electrode surface, as reflected by changes in quartz crystal frequency (∆f) during electrochemical redox processes. Yi et al. [24] investigated mass changes in MnO2 electrodes electrodeposited in various electrolytes using EQCM. The results showed that the cation radius of MnO2 decreased, along with a reduction in the deviation ratio of mass and charge changes from the corresponding cation’s equivalent mass. These findings highlight that EQCM is a promising electrochemical research technique for studying the degradation mechanisms of Pt catalysts in DMFCs [25].
The electrodeposition method has proven effective for enhancing Pt utilization. Recent advances in electrodeposition techniques have focused on optimizing deposition parameters (e.g., potential, current density, and electrolyte composition) to control catalyst morphology and improve performance [26]. For example, Liu et al. [27] demonstrated that pulse electrodeposition could produce Pt catalysts with high surface area and improve MOR activity. Similarly, Shen et al. [28] reported that the addition of surfactants during electrodeposition could tailor the size and distribution of Pt nanoparticles, leading to enhanced catalytic performance. Using the electrodeposition method, Pt catalysts could be selectively deposited at the three-phase interface of the electrode. Thus, the electrodeposition method is considered a promising approach for improving Pt utilization [29]. Various electrodeposition techniques, employing current or potential signals, have been reported [30,31]. Mohammad et al. [32] prepared Pt-based Pt/NiCu/MDAB catalysts by electrodeposition and co-catalytic strategy, which were small in particle size and well dispersed on the polymer matrix. Jin et al. [33] used H2PtCl6 as a platinum source to precisely control the atomic structure of Pt via in situ electrodeposition. The synthesized Pt-Ni(OH)2 exhibited excellent intrinsic catalytic activity due to strong adsorption and rapid dissociation of water molecules. Additionally, the choice of substrate morphology, such as graphene, carbon nanotubes, or mesoporous carbon, has been shown to influence the nucleation and growth kinetics of Pt during electrodeposition, ultimately affecting the catalyst’s electrochemical performance [34]. For example, Wang et al. [35] demonstrated that three-dimensional graphene substrates promoted the formation of highly dispersed Pt nanoparticles with improved mass activity for methanol oxidation. Li et al. [36] showed that carbon-based substrates with high conductivity and surface area could significantly enhance the dispersion and activity of Pt nanoparticles. Moreover, the deposition conditions, including electrolyte composition, pH, and potential, play a critical role in controlling the size, shape, and crystallographic orientation of Pt deposits [37]. These findings underscore the importance of optimizing both substrate design and deposition parameters to achieve high-performance Pt catalysts for the methanol oxidation reaction.
In this work, Pt catalysts on XC-72 were successfully prepared using the electrodeposition method without the need for protective reagents or thermal treatments. SEM was used to characterize the morphology of the deposited Pt catalysts. The performance of the Pt catalysts was evaluated and compared with that of commercial PtRu catalysts for methanol oxidation using the CV method. The Pt catalysts exhibited a small dendritic morphology, and the current density increased significantly during methanol oxidation after several cycles. However, the stability of the Pt catalysts declined significantly. The catalytic mechanism of the Pt catalysts for methanol oxidation was further investigated using EQCM. The results showed that the current density of the methanol oxidation reaction for the EQCM-deposited Pt catalyst reached 1020 mA cm−2. The response frequency of the as-prepared Pt catalyst decreased more rapidly than that of the commercial PtRu catalyst after several voltammetric cycles in the voltage range of 0.591–0.991 V. This suggests that while the Pt catalyst exhibits high catalytic activity, prolonged CO adsorption on the catalyst surface leads to poisoning and degradation of its performance.

2. Results and Discussions

2.1. Performance of Electrodeposited Pt Catalysts on Diffusion Layer

The Pt catalysts prepared by electrodeposition were examined for their electrocatalytic activities towards the room-temperature methanol oxidation reaction. Pt catalysts were electrodeposited on the carbon substrates in the 3% H2PtCl6 solution at a stationary potential of −0.109 V at room temperature on the gas-diffusion layer made of carbon. The cyclic voltammetry curve of the Pt catalysts for the methanol oxidation reaction is shown in Figure 1. As illustrated in Figure 1, the forward sweep oxidation peak of Pt catalysts surprisingly displayed a current density of 1020 mA·cm−2 for MOR. The results showed that the Pt catalysts exhibited high catalytic activity for MOR. Additionally, from Figure 1, it can be seen that the current density of 200 mA·cm−2 exists before the methanol oxidation reaction starts. The high capacitive current may result from the activated carbon support, which is highly conductive and provides a high catalytic surface for the Pt catalysts. The result is in accordance with the report on the high capacity on the surface of the Pt electrode [38].
Additionally, the oxidation process of methanol is typically slower at lower potentials (e.g., 0.341 V), while the reaction rate may increase at medium potentials (e.g., 0.541 V) in relation to the current crossing phenomenon. Currents cross due to changes in current density at different potentials, particularly when methanol is being oxidized, where several reaction steps may occur, including direct oxidation (CH3OH → CO2 + 6H + 6e−) and indirect oxidation (if partial oxidation produces an intermediate product like CO, the subsequent oxidation of CO occurs) [39]. Adsorbed substances (such as CO, CO2 and other intermediates) on the surface of Pt may affect the current density, especially at higher potentials (such as 0.841 V), and CO adsorbed on the surface of the catalyst may inhibit further oxidation of methanol [40]. This dynamic balance of adsorption and desorption causes the current to cross at different potentials because the adsorbed material is oxidized or desorbed at certain potentials, changing the activity of the catalytic surface.
Figure 2 exhibits the chronocurrent curve of Pt catalysts in 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution. From Figure 2, it can be seen that a current of methanol oxidation suddenly increases within the first several seconds. After that, the current of methanol oxidation gradually declines and keeps stable at about 175 mA·cm−2. Pt catalysts show good catalytic performance. In addition, for only a few seconds, the high current is attributable to the electric double-layer capacity of Pt catalysts. These findings suggest that the as-prepared Pt catalysts exhibit outstanding catalytic performance in methanol electro-oxidation.
The SEM images of Pt catalysts are shown in Figure 3. From Figure 3, it is clear that the morphologies of the Pt particles appear to be leaf-like crystals with small dendritic crystal aggregations by the electrodeposition method. The particle sizes of the leaf-like crystal aggregations are about 200 nm in width. Furthermore, the aggregated crystal is composed of lots of smaller steps with those substructures of about 20 nm in width (in Figure 3b). The morphologies of Pt catalysts indicate that the particle size and morphology of the deposited Pt can be obtained by the electrodeposition method. The smaller Pt crystallite size usually has a higher surface area, which can be beneficial for the catalytic reactions.
It is evident that it is equally vital to consider the chemical and electrochemical stability of Pt catalysts. Moreover, we investigate the stability of the Pt catalysts. In order to determine the stability of electrodes, tests of life cycle were performed by the MOR on the Pt electrode in a 0.5 M H2SO4 + 1.0 mol·L−1 CH3OH solution at room temperature. To make a fast evaluation with the stability tests, the MOR CV curves for the Pt catalysts were operated in a 0.5 M H2SO4+ 1.0 mol·L−1 CH3OH solution between 0.241 V and 1.141 V at 100 mV s−1 with a certain number of cycles. Figure 4 shows the CV curves for Pt catalysts after five cycles. The curves were recorded in the same electrolyte. Following five cycles, the current density of the Pt catalysts remained nearly unchanged at 1020 mA·cm−2 for MOR. It is interesting that no degradation of the electrode performance is observed throughout the stability test. On the other hand, it is found that at the peak negative sweep, the current density is 1090 mA·cm−2 in the fifth cycle, which is significantly higher than 860 mA·cm−2 in the first cycle. It is possible that the adsorption capacity of small organic molecules on Pt catalysts is accelerated. It also indicated that the small-molecule adsorption was observed after a cycling period, which maybe causes the Pt poisoning. Therefore, it is vital that the small-molecule adsorption on the Pt surface of Pt catalysts be further investigated.

2.2. EQCM Analysis of Pt Catalysts

In this work, EQCM was used to assess the small organic molecule adsorption and reaction processing on the electrode surface. The EQCM technique is particularly advantageous for in situ monitoring of mass changes associated with electrochemical reactions. Frequency response analysis, by integrating to EQCM, provides a direct correlation between the frequency shift (Δf) and mass changes on the electrode, based on the Sauerbrey equation [15] (Δf = −2.26 × 10−6αBf2ΔM = −SΔm). Here, the term “S” represents a constant specific to the crystal oscillator electrode, incorporating factors such as the shear modulus and density of the quartz crystal. In the present study, EQCM reveals that the value of S is 406,635.9 Hz/mg (S = 17,648 Hz/0.0434 mg), indicating that the frequency changes by 406,635.9 Hz for every 1 mg mass variation on the crystal. This precise frequency response provides valuable insights into the dynamics of molecular adsorption on electrodes. Before delving into the experimental data, it is essential to highlight the broad application of frequency response analysis in electrochemical studies. It allows for real-time detection of changes in mass and provides a detailed understanding of reaction intermediates, such as the interaction of methanol molecules with Pt catalysts in fuel cell applications. Thus, frequency response analysis is a powerful tool to monitor the behavior of catalysts, providing information on adsorption, desorption, and reaction kinetics.
In the experiment, the interaction between small molecules and as-prepared Pt catalysts in a 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution is investigated by using EQCM, and the results are compared with those obtained using commercial PtRu catalysts. In the experiment, the interaction between small molecules and as-prepared Pt catalysts in a 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution is investigated by using EQCM, and the results are compared with those obtained using commercial PtRu catalysts. In this case, Pt catalysts and commercial PtRu catalysts were compared based on the same total precious metal loading (Pt + Ru). Additionally, we compare the properties of Pt-based catalysts found in the existing literature with the performance of our Pt catalysts, as summarized in Table 1.
The electrochemically active surface area (ECSA) of the Pt and PtRu catalysts was determined by integrating the hydrogen desorption region, indicated by the shaded area in the current-potential (I-E) data, recorded in 0.5 mol·L−1 H2SO4 solution at a scan rate of 100 mV s−1. For the polycrystalline Pt electrode, the charge density corresponding to hydrogen adsorption was 210 μC cm−2 [46]. The ECSA values for the Pt and PtRu catalysts were determined by normalizing the electrochemically active surface area to the geometric area of the carbon-based substrate (0.2 cm2). The results indicated that the ECSA values for the Pt and PtRu catalysts were 22.8 m2·g−1 and 17.8 m2·g−1, respectively. These findings suggest that the ECSA value of Pt is higher than that of PtRu, indicating that Pt possesses more active sites.
Figure 5 shows the cyclic voltammetry curves for as-prepared Pt catalysts and commercial PtRu catalysts in 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution by EQCM. The current densities are normalized to the ECSA to evaluate intrinsic activity. As shown in Figure 5a, the forward-sweep oxidation peak reveals that the homemade Pt catalyst exhibits a higher ECSA-normalized peak current density, suggesting superior catalytic performance for methanol electrooxidation compared to the commercial PtRu catalyst. However, the reverse-sweep oxidation peak demonstrates that the Pt catalysts display a higher current density (7.67 μA·cm−2) compared to PtRu catalysts. The finding suggests that prepared Pt catalysts are susceptible to the methanol electro-oxidation intermediate products [47]. Notably, the lower onset potential of the PtRu catalyst (almost 200 mV more negative than pure Pt) indicates its superior activity for MOR, likely due to the bifunctional mechanism and the ability of Ru to facilitate CO oxidation [48]. However, the Pt catalyst exhibited higher current density, suggesting that the intrinsic activity of Pt is higher than that of PtRu. Figure 5b shows the relation Δf with E for as-prepared Pt catalysts and the commercial PtRu catalysts in the potential from 0.241 V to 0.991 V. The frequency change (Δf) observed in EQCM measurements was normalized to the ECSA to quantify the mass change per unit active area. The frequency response of the as-prepared Pt catalysts, as measured by EQCM, decreases significantly between 0.591 V and 0.991 V. This indicated a higher mass loading on the electrode surface compared to the commercial PtRu catalysts. The mass change is consistent with the adsorption of small organic molecules, such as CO, which is a well-documented intermediate in methanol oxidation on Pt catalysts. The larger frequency diminution observed for the as-prepared Pt catalysts suggests a stronger adsorption capacity for methanol oxidation intermediates, which may contribute to their higher catalytic activity. In contrast, the reduced frequency of PtRu catalysts can be attributed to the reduced Pt content and the presence of Ru, which promotes the oxidation of adsorbed species, including CO and other intermediates, thereby reducing their accumulation on the catalyst surface during the MOR process [49].
Chronoamperometry curves for as-prepared Pt catalysts at the potential of 0.841 V in a 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution can be observed in Figure 6a. Initially, the current density increases rapidly within the first few seconds, likely due to double-layer charging. Over time, the catalytic activity of the Pt catalysts surpasses that of the commercial PtRu catalysts for the first 600 s. However, from 600 to 1300 s, the current density of Pt catalysts for MOR begins to decline, approaching that of the PtRu catalysts. This suggests that while Pt catalysts exhibit excellent onset catalytic activity, the reaction rate is hindered by the irreversible adsorption of CO, a common intermediate in methanol oxidation. These observations are consistent with the EQCM data, where the frequency shift increases with time, indicating the accumulation of adsorbed species, such as CO. To further assess the effects of poisoning during methanol oxidation, EQCM measurements of both as-prepared Pt and commercial PtRu catalysts in 0.5 M H2SO4 + 1.0 M CH3OH solution were conducted, as shown in Figure 6b. The frequency change for methanol oxidation on the as-prepared Pt catalysts is much larger than that for PtRu catalysts, suggesting that small organic molecules are more readily adsorbed onto the Pt surface. This result aligns with the chronoamperometry findings and reinforces the superior onset catalytic performance of Pt catalysts in MOR.
It is known that the electrochemical oxidation of methanol involves two steps: (1) adsorption on the electrocatalyst surface and deprotonation to form carbonaceous intermediates; (2) dissociation of water to produce oxygenated species, which interact with carbon intermediates and release CO2. For as-prepared Pt catalysts and the commercial PtRu in acidic media, PtRu electrocatalysts exhibit superior stability and activity in both electrochemical oxidation and methanol adsorption [50]. The procedure for electrooxidizing methanol on the surface of platinum or platinum alloy catalysts involves the following steps [51]:
CHOH3 + Pt(s) → Pt-CH2OH + H+ + e−
Pt-CH2OH + Pt(s) → Pt2-CHOH + H+ + e−
Pt2-CHOH + Pt(s) → Pt3-CHOH + H+ + e−
Pt3-CHOH → Pt-CO + 2Pt(s) + H+ + e−
M(s) + H2O → M-OH + H+ + e−
Pt-CO + M-OH → PtM + CO2 + H+ + e−
In addition, at low reaction temperatures, methanol undergoes an incomplete oxidation reaction to produce formaldehyde or formic acid:
Pt-CH2OH → Pt(s) + HCHO + H+ + e−
Pt2-CHOH + M-OH → Pt2M(s) + HCOOH + H+ + e−
Based on this series of reactions, the small organic molecules generated from CH3OH are further enhanced by the presence of a Pt-based catalyst in the MOR. As a result, the Pt electrode surface presents significantly roughened and larger frequency changes, as shown in Figure 3 and Figure 6, leading to the significant mass change for the adsorption of COad. In addition, such an elementary step of the COad oxidation reaction at pure Pt with COad intermediate products is quite distinct from that described for the Pt alloys with Ru atoms on the top surface by the electrochemical measurements combined with EQCM.

3. Materials and Methods

3.1. Reagent and Instrumentation

The Nafion solution (5 wt%) and Vulcan XC-72© required for preparing the gas diffusion layer were obtained from Aldrich (St. Louis, MO, USA) and Cabot (Boston, MA, USA), respectively. Chloroplatinic acid (H2PtCl6·6H2O, AR) for electrodeposition of Pt nanoparticles was purchased from the Shenyang Nonferrous Metals Research Institute (Shenyang, China). Additionally, PtRu Black (85%) was acquired from Johnson Matthey (London, UK) for comparison with the Pt catalysts prepared for the EQCM test. All solutions were prepared using the deionized water.
In the first step, the electrode slurry solution was prepared by mixing 30 mg carbon black (Vulcan XC-72), 120 µL Nafion solution (5 wt%), and 6 mL ethanol thoroughly in a supersonic bath. Then, 100 µL of the prepared slurry was pipetted on a graphite electrode (diameter 8 mm) that had been polished to a mirror-like state and cleaned with ethanol and ultrapure water previously. After that, the electrode was dried at 60 °C for 5 h in a vacuum oven to vaporize the ethanol. The resulting carbon graphite electrode served as the substrate for depositing the Pt catalyst. The total charge density and the deposition current density were normalized to the geometric area of the carbon-based substrate (area 0.2 cm2). The processing treatment for Vulcan XC-72 (30 mg carbon black) is described in detail, as seen in reference [52]. In the second step, the Pt electrodes were fabricated by the stationary potential method. All electrodes were rinsed thoroughly with deionized water and dried at 60 °C in a vacuum oven immediately.
The electrochemical measurements were conducted using an electrochemical quartz microbalance and an electrochemical workstation of type QCA922 and PAR2273, respectively. Cell disruption was performed using a JY92-TM ultrasonic cell crusher from Ningbo Xinzhi Biotechnology Co., Ltd. (Ningbo, China). Subsequently. The cells were dried in a ZD78-A vacuum drying oven from Beijing Xingzheng Instrument of Equipment Factory (Beijing, China). A constant temperature was maintained with a DHT stirring constant temperature from Shandong Juancheng Hualu Electric Instrument Co., Ltd. (Heze, China). The deionized water in the experiments was prepared with the SYZ-A Quartz Subcritical Boiling High-Purity Water Distiller from China Jintan Danyang Gate Quartz Glass Factory (Jintan, China). Sample weight was measured using electronic scales (BS124S, Saiduo Lisi Instrument System Co., Ltd., Beijing, China). A saturated calomel electrode was purchased from Russell Technology Co., Ltd. (Shanghai, China). Precise sample transfer was carried out using a micropipettor (obtained from Excelsior Biochemical Reagent Instrument Co., Ltd., Shanghai, China).

3.2. Materials Synthesis

A specific quantity of Nafion solution (5 wt%), carbon black (Vulcan XC-72), and aqueous isopropanol was mixed and ultrasounded in an ice bath for 15 min to ensure homogeneity. The homogeneous mixture was then sprayed onto carbon paper (geometric surface area: 1 cm2) under heated conditions. After spraying, the coated carbon paper was dried at room temperature to ensure proper drying and bonding, resulting in the desired gas diffusion layer (GDL). In a 3% H2PtCl6 solution, Pt catalysts were prepared by electrodeposition onto the diffusion layers at a stationary potential of −0.35 V for 5 min, with a deposition loading of 0.20 mg cm−2. Additionally, the Pt catalysts were electrodeposited onto platinum crystal electrodes (geometric surface area: 0.20 cm2) under the same solution and potential conditions (−0.109 V for 5 min), with the same deposition loading of 0.20 mg cm−2, prior to quartz microbalance testing. The main difference between the two Pt catalysts is their deposition locations: one is on the diffusion layer, while the other is on the Pt crystal electrode. For comparison, commercial PtRu catalysts were prepared by drop-coating onto the platinum crystal electrode with the same loading of 0.20 mg cm−2. The PtRu catalyst was ultrasonically dispersed in the isopropanol–water solution to ensure uniform loading.

3.3. Physical and Electrochemical Measurement

The size and morphology of the Pt catalysts on the electrode surface were analyzed using a SUPRATM55 scanning electron microscope (SEM) manufactured by ZEISS (Oberkochen, Germany). Electrochemical testing of the electrodeposited Pt catalyst was performed in a standard three-electrode cell using a VMP2 multichannel potentiostat electrochemical workstation from Princeton Applied Research (Oak Ridge, TN, USA). A platinum electrode served as the counter electrode, while a saturated calomel electrode (SCE) was used for the reference electrode. The working electrode was the electrodeposited electrode prepared in this work. All potentials were converted to the reversible hydrogen electrode (RHE) scale using the following equation: ERHE = ESCE + 0.241 + 0.059 × pH, where ESCE is the potential measured versus SCE, and the pH of 0.5 M H2SO4 is assumed to be 0. Thus, the conversion formula used in this study is ERHE = ESCE + 0.241.
The electrochemically active surface areas of the Pt catalysts and commercial PtRu catalysts were carried out by CV in a 0.5 mol·L−1 H2SO4 solution between −0.1 and 1.5 V with a scan rate of 100 mV s−1. Meanwhile, the methanol oxidation reaction (MOR) activity of the Pt catalysts and PtRu catalysts was also determined by a CV technique in the 0.5 mol L−1 H2SO4 +1.0 mol L−1 CH3OH solution scanning between 0.241 to 1.141 V at 100 mV s−1. Prior to conducting the CV tests, 20 min of Ar gas were bubbled into the electrolyte solution to remove oxygen. Additionally, transient current measurements were conducted by holding the potential at 0.841 V for 60 s in the same electrolyte to assess the reaction kinetics. The electrochemical performance of electrodes prepared under different conditions was compared by cyclic voltammetry at room temperature.

3.4. Test of Electrochemical Quartz Microbalance

The electrochemical quartz microbalance (EQCM) measurements were conducted in a standard three-electrode setup, as shown in Figure 7. The working electrode consisted of a platinum (Pt) crystal electrode supported by a quartz crystal with a base frequency of 9 MHz and an AT-cut type, measuring 0.20 cm2, as shown in Figure 8a. The electrode was immersed in the 0.5 mol L−1 H2SO4 + 1.0 mol L−1 CH3OH solution for testing. In order to facilitate comparison with the prepared Pt catalyst, the commercial PtRu catalyst (0.20 mg·cm−2) was drop-coated onto the working electrode and mixed by ultrasonication. The platinum crystal oscillator electrode is shown in Figure 8b. Furthermore, the counter electrode was a Pt sheet electrode, and the reference electrode was a saturated calomel electrode.
The EQCM technique was employed to investigate the adsorption of CO on the surface of the Pt crystal electrode during catalytic methanol electro-oxidation, thereby providing insights into the poisoning mechanism of the catalyst. In this experiment, both CV method and stationary potential method were employed. The CV method had a voltage range of 0.241 V to 0.991 V, with a scan rate of 5 mV s−1. The curves and corresponding response data of the input voltage were recorded. The potential in the stationary potential method was maintained at 0.841 V for 1300 s, and the curves and their corresponding voltage response data were recorded. Subsequently, the corresponding voltage input curve was converted into the frequency response curve.

4. Conclusions

The Pt catalysts with nanocrystals were successfully stationary potential electrodeposited on carbon electrodes, exhibiting leaf-like dendritic morphologies that enhance electrocatalytic activity. The Pt catalysts showed a high current density up to 1020 mA cm−2 for methanol oxidation, with the peak current increasing from 860 mA cm−2 to 1090 mA cm−2 after several cycles, suggesting that an activation process was present. The electrochemical active surface area (ECSA) of the Pt catalysts is higher than that of PtRu, indicating a larger number of active sites on the Pt surface. The CV curve results show that the PtRu catalysts have lower onset potentials, which is attributed to the bifunctional mechanism and the ability of Ru to promote CO oxidation. However, the Pt catalysts exhibited higher current densities, suggesting higher intrinsic activity. Additionally, EQCM results revealed a rapid frequency decrease for Pt catalysts compared to commercial PtRu, suggesting substantial adsorption of methanol oxidation intermediates, particularly COad. This adsorption led to catalyst poisoning and performance decline. This research provides a pathway toward developing high-performance and long-lasting anode catalysts for DMFCs.

Author Contributions

Conceptualization, X.P. and F.Y.; Methodology, B.Z., B.W., X.F. and G.L.; Software, X.P. and L.Z.; Validation, B.W. and X.F.; Formal analysis, X.P., B.W., L.Z., X.F. and H.Y.; Investigation, B.Z., X.P., B.W., L.Z., C.H. and F.Y.; Resources, X.P. and F.Y.; Writing—original draft, X.P. and F.Y.; Writing—review & editing, W.X. and G.L.; Visualization, C.H., H.Y., G.L. and F.Y.; Supervision, G.L.; Funding acquisition, G.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work is funded by The Central Guidance on Local Science and Technology Development Fund of Hebei Province (Grant No. 246Z4412G), the Natural Science Foundation of Beijing (2252024), the Open Project Program of Provincial Key Laboratory of Eco-Industrial Green Technology, Wuyi University (WYKF-GCT2024-2), and the key scientific research project of Suzhou university (No: 2022xhx251, 2024yzd18).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 13 00111 g001
Figure 1. Cyclic voltammetry curves of Pt catalysts electrodeposited on the prepared diffusion layer in 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution.
Figure 1. Cyclic voltammetry curves of Pt catalysts electrodeposited on the prepared diffusion layer in 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution.
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Figure 2. Chronocurrent curve of Pt catalysts electrodeposited on the prepared diffusion layer prepared in 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution tested at a potential of 0.841 V.
Figure 2. Chronocurrent curve of Pt catalysts electrodeposited on the prepared diffusion layer prepared in 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution tested at a potential of 0.841 V.
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Figure 3. SEM of Pt catalysts electrodeposited on the prepared diffusion layer at different magnification. (a) 10 μm, (b) 200 nm.
Figure 3. SEM of Pt catalysts electrodeposited on the prepared diffusion layer at different magnification. (a) 10 μm, (b) 200 nm.
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Figure 4. Voltammetry curves of Pt catalysts electrodeposited on the prepared diffusion layer in 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution after 5 cycles.
Figure 4. Voltammetry curves of Pt catalysts electrodeposited on the prepared diffusion layer in 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution after 5 cycles.
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Figure 5. (a) Cyclic voltammetry curves of as-prepared Pt catalysts and commercial PtRu catalysts in 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution, (b) Frequency response curve corresponding to the positive sweep voltammetry curve.
Figure 5. (a) Cyclic voltammetry curves of as-prepared Pt catalysts and commercial PtRu catalysts in 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution, (b) Frequency response curve corresponding to the positive sweep voltammetry curve.
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Figure 6. (a) chronoamperometry curves of prepared Pt catalysts and commercial PtRu catalysts at the potential of 0.841 V in 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution, (b) Frequency response curve corresponding to the reaction time.
Figure 6. (a) chronoamperometry curves of prepared Pt catalysts and commercial PtRu catalysts at the potential of 0.841 V in 0.5 mol·L−1 H2SO4 + 1.0 mol·L−1 CH3OH solution, (b) Frequency response curve corresponding to the reaction time.
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Figure 7. Electrolytic cell used in quartz microbalance.
Figure 7. Electrolytic cell used in quartz microbalance.
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Figure 8. Platinum crystal oscillator electrode in quartz microbalance experiment (a) before and (b) after dropping the catalyst.
Figure 8. Platinum crystal oscillator electrode in quartz microbalance experiment (a) before and (b) after dropping the catalyst.
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Table 1. Comparison of electrochemical performance for Pt-based catalysts.
Table 1. Comparison of electrochemical performance for Pt-based catalysts.
ElectrodePeak Current Density
(mA cm−2)		Onset Potential
(mV)
This Work1020757
Pt/C [41]3.85-
PtRu/C [42]20.1-
PtRu/C [43]25.18550
PtRu/NCNT [44]290.6-
PtRu/NCNT [45]30.95-
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Zong, B.; Pan, X.; Wei, B.; Zhang, L.; Feng, X.; Hou, C.; Yan, H.; Xie, W.; Liu, G.; Ye, F. Electrochemical Quartz Microbalance for Studying Electrodeposited Pt Catalysts for Methanol Oxidation Reaction. Inorganics 2025, 13, 111. https://doi.org/10.3390/inorganics13040111

AMA Style

Zong B, Pan X, Wei B, Zhang L, Feng X, Hou C, Yan H, Xie W, Liu G, Ye F. Electrochemical Quartz Microbalance for Studying Electrodeposited Pt Catalysts for Methanol Oxidation Reaction. Inorganics. 2025; 13(4):111. https://doi.org/10.3390/inorganics13040111

Chicago/Turabian Style

Zong, Bangfeng, Xiaojun Pan, Bo Wei, Lifang Zhang, Xiangxiong Feng, Chenggong Hou, Hai Yan, Wenju Xie, Guicheng Liu, and Feng Ye. 2025. "Electrochemical Quartz Microbalance for Studying Electrodeposited Pt Catalysts for Methanol Oxidation Reaction" Inorganics 13, no. 4: 111. https://doi.org/10.3390/inorganics13040111

APA Style

Zong, B., Pan, X., Wei, B., Zhang, L., Feng, X., Hou, C., Yan, H., Xie, W., Liu, G., & Ye, F. (2025). Electrochemical Quartz Microbalance for Studying Electrodeposited Pt Catalysts for Methanol Oxidation Reaction. Inorganics, 13(4), 111. https://doi.org/10.3390/inorganics13040111

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