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Article

Strain Effect in PdCu Alloy Metallene for Enhanced Formic Acid Electrooxidation Reaction

by
Kaili Wang
1,
Zhen Cao
1,* and
Jia He
2,*
1
School of Chemistry & Chemical Engineering and Environmental Engineering, Weifang University, Weifang 261061, China
2
School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(10), 967; https://doi.org/10.3390/catal15100967
Submission received: 13 September 2025 / Revised: 29 September 2025 / Accepted: 9 October 2025 / Published: 10 October 2025
(This article belongs to the Special Issue Nanostructured Catalysts for Emerging Electrochemical Technologies)
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Abstract

Developing high-activity and high-durability Pd-based electrocatalysts is an important strategy to promote their commercial application. Herein, a smaller particle size and ultrathin sheet-like PdCu alloy metallene (PdCuene) were successfully prepared by using a one-pot wet chemistry method for FAOR. Experimental measurements indicated that the introduction Cu into Pd lattice induces a significant compressive strain effect through lattice mismatch between Pd and Cu, and the strain effect optimizes the electronic structure of Pd, as well as the high electrochemical surface area, increased exposure of active sites, and appropriate lattice strain have been demonstrated as factors that influence the enhancement of intrinsic activity and the acceleration of kinetics, thereby improving FAOR performance. Moreover, the stronger lattice strain of 0.85% would facilitate surface adsorption and dissociation of formic acid. Specifically, the optimized PdCuene exhibits enhanced mass activity and specific activity with current densities of 2.31 A mgPd−1 and 4.09 mA cm−2, respectively, which transcend the activities of Pd metallene (1.44 A mgPd−1 and 2.73 mA cm−2) and commercial Pd/C (0.6 A mgPd−1 and 1.53 mA cm−2). Meanwhile, PdCuene displayed obvious enhanced durability. The work provides an approach to modulate the lattice strain engineering, which represents a highly promising strategy for designing efficient FAOR electrocatalysts.

1. Introduction

Direct formic acid fuel cells (DFAFCs), as highly promising portable electrochemical energy devices for clean and efficient renewable energy conversion technologies, have garnered widespread attention due to their advantages, including high energy density, low operating temperature, and safe, convenient fuel storage and transportation [1,2,3,4]. Formic acid electrooxidation reaction (FAOR) is the key in the anode of DFAFCs. However, the slow kinetics of anodic FAOR constrained the commercialization process of DFAFCs. The key to solving the problem lies in the efficient and low-cost electrocatalysts. Currently, Pt-based and Pd-based nanocatalysts are the primary electrocatalysts due to their unique electronic structure, which enables them to catalyze the dehydrogenation of formic acid molecules. Compared to Pt-based catalysts, Pd-based catalysts exhibit higher initial activity for FAOR and preferentially comply with the ideal “direct pathway” (HCOOH → CO2 + 2H+ + 2e−) rather than the “indirect pathway” (HCOOH → COads + H2O → CO2 + 2H+ + 2e−) [5,6,7]. However, Pd-based electrocatalysts have two significant disadvantages in practical applications: First, they inevitably generate trace amounts of CO species during the reaction, which occupy Pd active sites and cause rapid catalyst deactivation; Secondly, the scarcity and high cost of the precious metal Pd constrain its large-scale application [8,9,10]. To overcome the issues outlined above, many studies mainly focus on the following approaches: reducing the size and controlling the morphology of nanocatalysts enhances the abundance of active sites; it also regulates the electronic states of Pd to optimize the adsorption energy of reaction intermediates through electronic structure engineering strategies such as alloying and surface modification [11,12]. In the regulation of morphogenesis, two-dimensional (2D) ultrathin metallene has garnered considerable attention owing to its unique structural and chemical characteristics. Its atomic-scale thickness ensures exceptionally high atomic utilization and an unsaturated coordination atomic environment [13]. Meanwhile, their extensive lateral dimensions offer continuous and abundant active sites [14]. Moreover, their flexible 2D planar structure contributes to maintaining structural stability during electrocatalysis reactions [15].
Previous studies have shown that two-dimensional nanocatalysts [16], particularly those based on Pd metallene, exhibit superior activity compared to conventional nanoparticles in FAOR. Simply controlling the morphology cannot fundamentally address the issues of insufficient intrinsic activity of Pd and CO poisoning on Pd surface sites [17]. Therefore, combining the advantages of morphology adjusting with the electronic effects induced by alloying is regarded as a more promising approach. By alloying Pd with transition metals (e.g., Cu, Ni, Ag), synergistic effects arising from differing atomic radii and electronegativity can effectively modulate the d-band centers of Pd [18,19,20,21,22,23,24,25]. The resulting lattice strain effect (such as lattice compression or tension) serves as a powerful tool for controlling electronic structure. For example, when Cu atoms are introduced into the host Pd metal, they would induce compressive strain because the different lattice constant results in lattice mismatch. As shown in the d-band center theory, the compressive strain can shift downward the d-band center of Pd to generally weaken their adsorption strength toward reaction intermediates, such as the CO* intermediate, which is vital for FAOR to avoid strong intermediate adsorption. As an inexpensive and abundant metal, Cu has a smaller atomic radius of 128 pm than Pd, which is 137 pm, making it an ideal choice for introducing compressive strain into the Pd lattice. Theoretical calculations have demonstrated that compressive strain induced by alloying Cu into the Pd lattice effectively reduces the adsorption energy of CO* at Pd sites, thereby significantly improving the anti-poisoning performance of Pd-based electrocatalyst [26,27,28]. However, conventional alloy nanoparticles typically display spherical-like structures in which numerous internal atoms remain buried and are unable to participate in surface electrocatalysis reactions. Furthermore, the uneven strain distribution makes it difficult to maximize the surface strain effects.
Herein, the work is designed to synthesize a 2D ultrathin PdCuene nanocatalyst using a simple one-pot wet chemistry method. The as-prepared PdCuene nanocatalysts provide a large electrochemically active surface area and abundant surface unsaturated atoms; the alloying effect between Pd and Cu optimizes the electronic structure, as well as the intense in-plane compressive strain induced on the Pd atomic layer due to the lattice constant difference between Pd and Cu, all significantly enhance the adsorption behavior of reaction intermediates on surface active sites. The systematic electrochemical and structural characterizations of transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have shown that PdCuene displayed the improved FAOR activity, for example, the mass activity of PdCuene catalysts is 2.31 A mgPd−1, which is 1.6 and 3.9 folds higher than that of Pd metallene (1.44 A mgPd−1) and commercial Pd/C (0.60 A mgPd−1), respectively. Moreover, PdCuene displayed obvious enhanced durability after 3600s of long-term operation. In short, 2D PdCuene electrocatalysts have achieved breakthroughs in mass activity, specific activity, and stability, providing new insights and an experimental basis for designing the next generation of high-performance electrocatalysts for DFAFC devices.

2. Results and Discussion

As illustrated in Figure 1a, the ultrathin PdCu nanosheet was fabricated through a one-pot wet chemical approach that a clean and homogeneous solution of metal acetylacetonate salts, such as palladium acetylacetonate (Pd(acac)2) and copper acetylacetonate (Cu(acac)2), along with acetic acid as reduction agent and W(CO)6 serving as capping agent in dimethyl formamide solution to form two-dimensional sheet structure, respectively. As displayed in Figure 1b,c, the representative scanning electron microscope (SEM) image displayed an open porous structure formed by the overlapping of two-dimensional (2D) ultrathin nanosheets, confirming a highly curled morphology [29]. This product primarily consists of 2D nanosheets with an average lateral dimension of approximately 150 nanometers. Its structure resembles graphene, so the PdCu nanosheet was denoted as PdCuene. Meanwhile, the SEM image of PdCuene presents a highly curved morphology with the features of flexibility and ultrathin. The energy-dispersive X-ray spectroscopy (EDX) spectrum revealed that the PdCuene had a Pt/Cu/W atomic ratio of 87.3/11.4/1.3, as shown in Figure 1d. In contrast, SEM images of Pdene catalysts, as shown in Figure S1, reveal that they exhibit relatively large two-dimensional sheet structures compared to PdCuene. Additionally, the atomic ratio of Pd/W was determined to be 95.1/4.9 by EDX analysis (Figures S2 and S3). It can be revealed that the doping of Cu atoms into the Pd lattice reduces the surface area of the nanosheets, with Cu atoms occupying W atomic sites to reduce the doping content of W atoms (Table S1). Preparation under the same reaction conditions as for PdCuene but in the absence of W(CO)6 resulted in the formation of non-uniform nanosheet products. As shown in Figure S1, the reaction solution without added W(CO)6 produced a green transparent solution after reacting for 1 h in a 110 °C oil bath. And no precipitate was produced through using centrifugation, suggesting that W(CO)6 not only serves as a two-dimensional nanostructure directing agent but also plays the essential reduction agent role of CO molecules produced by the slow decomposition of W(CO)6 in the synthesis of ultrathin 2D nanostructures. The transmission electron microscope (TEM) image presents a typical two-dimensional ultrathin nanosheet structure with abundant wrinkles and curves serving as electrochemically active sites for the obtained PdCuene (Figure 1e). The high-resolution TEM (HRTEM) image displays a lattice spacing of 2.22 Å that can be attributed to the (111) plane of the fcc Pd-based structure (Figure 1f). Furthermore, scanning transmission electron microscope (STEM) image (Figure 1g) and STEM energy dispersive spectroscopy (STEM-EDS) elemental mapping imaged (Figure 1h–k) reveals the uniform distribution of Pd and Cu elements throughout the two-dimension sheet of PdCuene, further revealing the successful doping of evenly distribution of Cu atoms, as shown in Figure 1h,i. Uniform Cu atom doping, combined with ultrathin and small lateral size of the PdCuene, offers increased accessible active sites to improve the atomic utilization of Pd for electrocatalytic applications.
X-ray diffraction (XRD) analysis was performed to reveal the crystal structure and phase of PdCuene and Pdene nanocatalysts [30,31]. The characteristic peaks at about 40.5°, 47.1°, 68.7°, 82.8° and 87.5° for PdCuene corresponding to a typical single-phase face-centered cubic (fcc) crystal structure of (111), (200), (220), (311) and (222), respectively, indicating a significant positive shift of 0.23° and 0.39° for (111) lattice compare to Pdene and the standard Pd (PDF# 46–1043), respectively, confirming the successful incorporation of Cu atoms into Pdene to induce comprehensive strain [32,33]. The compressive strain induced by the ultrathin sheet structure is confirmed by a positive shift in the XRD peaks in comparison with the standard Pd (PDF# 46–1043), which corresponds with previous studies [34,35]. Lattice parameters calculated based on the (111) peak position were presented in Table S2, and the lattice constants (a) of Pdene and PdCuene are 3.878 Å and 3.857 Å, with approximately 0.31% and 0.85% compressive strain compared to the standard Pd (PDF# 46–1043) with a lattice constant of 3.890 Å, respectively. Furthermore, the compressive strain of PdCuene was calculated to be about 0.85% based on the lattice spacing in the crystalline area of the PdCuene, which is about 2.224 Å relative to the standard Pd (PDF# 46–1043) with a lattice spacing of 2.244 Å. Macroscopic strain in PdCuene nanocatalysts typically results from multiple contributing factors, primarily including the following: (1) Lattice doping or mismatch can induce the intrinsic strain through doping the different atomic radius of Pd (0.137 nm) and Cu (0.128 nm), and thus the incorporation of Cu atoms into the Pd lattice does indeed induce compressive strain. (2) Surface effects can also induce the non-intrinsic strain. The PdCuene with ultrathin two-dimensional structure and microscopic curvature can induce shifts in diffraction peak positions due to surface relaxation or lattice bending.
X-ray photoelectron spectroscopy (XPS) analysis was conducted to investigate the surface chemical states involving chemical valence state and localized electronic structure of Pdene and PdCuene electrocatalysts [31,36]. As displayed in Figure 2b, the survey XPS spectrum further confirms that PdCuene consists of Pd and Cu elements. High-resolution Pd 3d (Figure 2c) and Cu 2p (Figure 2d) spectra reveal that the majority of Pd and Cu are in the metallic state [37,38,39,40,41]. High-resolution Pd 3d spectra exhibited two pairs of core-level peaks: Low-intensity peaks at 336.0/341.26 eV could be assigned to metallic Pd (0), while high-intensity peaks located at 337.4/342.66 eV indicate the presence of oxidized Pd (II) species. Notably, the PdCuene demonstrated a 0.3 eV negative shift in Pd0 binding energy relative to Pdene, indicating that Cu donates electrons to enhance electron density at Pd by the alloying effect [42]. High-resolution W 4f of Pdene displayed an obvious two pairs of core-level peaks, but PdCuene has no signal of W 4f (Figure S4). According to these XPS results, the electronic structure of Pd can be regulated through alloying with Cu atoms via ligand effects owing to the different atomic diameters of Cu and Pd [43]. Meanwhile, the valence electrons of Cu (3d104s1) can transfer to Pd through the alloy interface due to the higher electronegativity of Pd, which is approximately 2.2, compared to that of Cu with about 1.9, thereby filling the d band and increasing the d electron density of Pd. In summary, the introduction of Cu atoms achieves the reduction of some Pd to the metallic state through electron transfer and structural regulation, while injecting electrons into Pd, thereby optimizing the electronic structure of the PdCuene electrocatalysts.
As a proof-of-concept application, the electrocatalysis of formic acid oxidation reaction (FAOR) over the PdCuene was carried out by assembling a three-electrode system [44,45]. The electrocatalytic performance of PdCuene towards FAOR was evaluated in comparison with commercial Pd/C and PdCuene, as shown in Figure 3. These three catalysts were activated in N2-saturated 0.1 M HClO4 solution to obtain a stable cyclic voltammetry (CV) curve, as shown in Figure 3a. The typical hydrogen adsorption/desorption region and PdO reduction region on Pd-based surface can be observed within the potential range from −0.24 V to 0 V and 0.3 V to 0.6 V vs. SCE, respectively, where the hydrogen adsorption/desorption region of Pdene and PdCuene showed a high current relative to commercial Pd/C owing to the ultrathin nature that exposes nearly every Pd atom at the surface, making it a potential active site. The specific surface area far exceeds that of spherical nanoparticles of equivalent mass. The electrochemical surface areas (ECSAs) of commercial Pd/C, Pdene, and PdCuene catalysts were calculated by the Pd oxidation peak charge integration using the standard 405 μC cm−2 PdO monolayer assumption, which were 39.2 m2 g−1, 52.7 m2 g−1, and 56.5 m2 g−1, respectively, and the higher ECSAs of metallene catalysts can be ascribed to the ultrathin ligament structure rather than the particle size of commercial Pd/C, which is consistent with the previous studies about ultrathin Pd-based metallene catalysts. As shown in Figure 3b, for the FAOR performance test, the MOR mass activity of PdCuene catalysts (2.31 A mgPd−1) outperformed Pd/C (0.60 A mgPd−1) and Pdene (1.44 A mgPd−1). In addition, the onset potential of the PdCuene catalyst is significantly lower than that of the two catalysts for comparison, revealing a stronger adsorption and activation ability to formic acid molecules. Meanwhile, as described in Figure 3c, the specific activity of the PdCuene catalyst demonstrates a peak current density of 4.09 mA cm−2, which is 2.67 and 1.50 times higher than that of commercial Pd/C (1.53 mA cm−2) and Pdene (2.73 mA cm−2), as shown in Figure 3c.
Linear sweep voltammetry (LSV) curves have indicated that the oxidation current of PdCuene catalysts is higher than that of Pd/C and Pdene catalysts in the whole potential range, displaying the synergistic enhancement capability and fast reaction kinetics on FAOR [30,46,47], as presented in Figure 3d. The Tafel slopes of commercial Pd/C, Pdene, and PdCuene catalysts are 97.2 mV dec−1, 96.6 mV dec−1, and 80.8 mV dec−1, respectively (Figure 3e), and the smallest Tafel slope of PdCuene indicates significantly improved FAOR kinetics [48]. Electrochemical impedance spectroscopy (EIS) was conducted at 0 V vs. SCE voltages to elucidate the interfacial electrochemical behaviors of commercial Pd/C, Pdene, and PdCuene catalysts during the FAOR process. By fitting Nyquist plots (Figure 3f) [49,50], the charge transfer resistance (Rct) of the PdCuene catalyst decreases significantly, indicating faster reaction kinetics with rapid charge transfer and enhancing the surface adsorption of formic acid and its intermediates during the FAOR process [51].
The durability of electrocatalysts, as an important issue, restricts their practical application in DFAFC devices. As exhibited in Figure 4a, this enhanced durability stems from the unique structural configuration of PdCuene catalysts, which provides numerous catalytically active sites for formic acid activation while maintaining structural integrity. Shortly after testing commenced, the electrode’s current density exhibited a rapid decline during the initial phase due to the decrease in the methanol concentration gradient around the electrode. After the 3600 s tests at 0 V vs. SCE, the PdCuene catalysts demonstrated a sustained current density relative to commercial Pd/C and Pdene catalysts, further confirming the fact that the PdCuene catalysts showed high tolerance to CO poisoning for FAOR. Post-stability characterization revealed that the two-dimensional ultrathin sheet architecture was preserved without obvious particle enlargement, as shown in Figure S5. The corresponding SEM-EDX mapping and SEM-EDX spectrum (Figure S7) of PdCuene electrocatalysts after durability test evidence that Cu content shows a slight decrease from 11.4% to 8.8%, and the uniform redistribution of Pd and Cu elements throughout the two-dimensional sheet of PdCuene, indicating a stable PdCu-enriched ultrathin sheet structure. These results indicate that PdCuene catalysts possess ultrahigh durability, benefiting from the structural stability and high CO oxidation rate.
To study the reaction kinetics of the samples, the scan-rate-dependent MOR activities were tested, as presented in Figure S6. The kinetic index κ is a dimensionless parameter used to determine whether an electrochemical reaction is controlled by surface adsorption processes or diffusion processes. The slope (κ) is calculated by the resulting linear function of the peak mass activity (A mgPd−1) extracted from CV curves with different scan rates (ν) versus the corresponding ν converted to the extraction of square root values ν1/2 ((mV s−1)1/2). The kinetic reaction rate of PdCuene catalysts is calculated to be 0.28, according to the slope of current density for specific activity versus ν1/2, which is 13.2- and 6.0-fold higher than that of commercial Pd/C (0.06) and Pdene (0.10), respectively (Figure 4b), in good accordance with the high mass activity and specific activity of PdCuene catalysts. Moreover, the PdCuene catalysts almost represent the best FAOR electrocatalysts in comparison with other state-of-the-art representative Pd noble metal-based electrocatalysts as summarized in Figure 4c and Table S3 [6,52,53,54,55,56,57,58,59,60,61,62,63,64]. It was accepted that the CO poisoning intermediate on Pd active sites is inevitable during the FAOR process. Figure S9 displays the CO-stripping voltammogram curves for the pre-adsorbed CO oxidation of commercial Pd/C, Pdene, and PdCuene catalysts. The PdCuene nanocatalyst shows a lower oxidation potential of 647 mV vs. SCE than Pdene (712 mV vs. SCE) and commercial Pd/C (719 mV vs. SCE), indicating that the alloying of Cu into the Pd lattice induces compressive strain to promote CO oxidation at a low potential. As illustrated in Figure 4d, PdCuene catalysts would alter the reaction pathway through the alloying effect of Cu compared to pure Pd. The key advantage lies in significantly suppressing the formation of the poisoning intermediate CO* through the dehydration pathway during FAOR. As a species, CO* can strongly adsorb onto Pd active sites and lead to the deactivation of the catalytic surface. The introduction of Cu atoms into the Pd lattice can weaken the adsorption strength toward CO* on the Pd surface through both electronic and compressive effects. At the same time, it is more inclined to promote the direct conversion of formic acid into the final product CO2 via the dehydrogenation pathway (HCOOH → CO2 + 2H+ + 2e−), thereby reducing the poisoning of the catalyst and enhancing catalytic activity and stability [58].

3. Materials and Methods

3.1. Chemicals and Materials

Palladium acetylacetonate (Pd(acac)2, 99%), copper acetylacetonate (Cu(acac)2, 98%), and tungsten hexacarbonyl (W(CO)6, 99.9%) were purchased from Aladdin (Shanghai, China). Perchloric acid (HClO4, 70.0~72.0%) was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Acetic acid (CH3COOH, GC, ≥99.5%), formic acid (HCOOH, 99.5%), and N, N-dimethylformamide (DMF, 99.9%) were obtained from Energy Chemical Reagent Co., Ltd. (Shanghai, China). Nafion solution (5 wt%) was purchased from Dupont. Commercial Pd/C (10 wt% Pd) was obtained from Hesen Electric Co., Ltd. (Shanghai, China). All chemicals were used as received without further purification.

3.2. Synthesis of PdCuene Nanocatalysts

The preparation of PdCuene nanocatalysts uses a simple one-pot wet chemistry method. The specific steps are as follows: 20 mg Pd(acac)2, 6 mg Cu(acac)2, and 40 mg W(CO)6 were dispersed directly into a 50-milliliter glass vial containing 16 mL of a DMF solution. The above solution will be sonicated for 10 min until a transparent solution is formed. Then, the obtained transparent solution was mixed with 4 mL of acetic acid, and meanwhile, magnetic stirring ensured uniform mixing. Next, the above solution was heated in an oil bath from room temperature to 110 °C and maintained at 110 °C for 1 h. The resulting black product was isolated by centrifugation, washed repeatedly with ethanol 3 times, and finally vacuum-dried at 45 °C for 12 h to obtain the PdCuene nanocatalyst for further use. Pdene catalysts can be obtained by the same method described above, except for adding Cu(acac)2 reagent.

3.3. Characterization

Scanning electron microscopy (SEM) images of Pdene and PdCuene nanomaterials were obtained by using a SIGMA HV microscope (ZEISS, Oberkochen, Germany). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of PdCuene nanomaterial were obtained by using a JEM 2100plus microscope (JEOL; Akishima, Japan) operated at 200 kV, and the corresponding high-angle annular dark field (HAADF) images and energy-dispersive spectroscopy (EDS) elemental mapping images of Pd, Cu, and W elements were performed. The X-ray diffraction (XRD) spectra of Pdene and PdCuene nanomaterials were recorded through a Rigaku SmartLab SE X-ray diffractometer with a Cu Kα source (Akishima, Japan). X-ray photoelectron spectra (XPS) were obtained on a Thermo Scientific Nexsa X-ray photoelectron spectrometer using Al-Kα X-ray as the excitation source (Waltham, MA, USA).

3.4. Electrochemical Measurements

Electrochemical analyses were conducted using a CS310M (Wuhan Corrtest Instruments Corp., Ltd., Wuhan, China) electrochemical workstation under ambient conditions, employing a conventional three-electrode configuration: the saturated SCE, a carbon rod (6 mm in diameter), and a catalyst-coated glassy carbon (GC) electrode were used as the reference electrode, the counter electrode, and the working electrode, respectively. The catalyst inks were prepared by ultrasonicating a mixture of the prepared Pd/C, Pdene, or PdCuene catalysts and an ethanol solution (the ethanol/ultrapure water volume ratio was 1/1). A certain amount of ink was coated onto a GC electrode (4 mm in diameter) to obtain catalyst loadings of 40 mgPd/cm2, 25 mgPd/cm2, and 25 mgPt/cm2, respectively. And then, 4 μL of Nafion solution (0.05 wt%) was sequentially coated on the GC electrode as the working electrode. The electrochemical surface areas (ECSAs) of commercial Pd/C, Pdene, and PdCuene catalysts were calculated by the Pd oxidation peak charge integration using the standard 405 μC/cm2 PdO monolayer assumption, and cyclic voltammetric (CV) measurements in a N2-saturated 0.1 M HClO4 electrolyte with 50 mV/s scan rate between −0.24 and 0.9 V versus SCE reference. For formic acid oxidation reaction (FAOR) evaluations, deaerated electrolytes containing 0.1 M HClO4 and 0.5 M formic acid were prepared through 30 min nitrogen bubbling, CV, and linear sweep voltammetry (LSV) measurements in a N2-saturated 0.1 M HClO4 electrolyte with 50 mV/s and 5 mV/s scan rate between −0.24 and 0.9 V versus SCE reference, respectively. Chronoamperometric (CA) stability tests were performed at 0 V vs. SCE for 1 h in the reaction solution. Electrochemical impedance spectroscopy (EIS) tests were conducted over a frequency range of 0.01 Hz to 100 kHz, applying a 5 mV alternating voltage signal to assess the system’s response. All the electrochemical data presented in this work were iR-compensated by 85% using the positive feedback technique.

4. Conclusions

In this work, we successfully constructed two-dimensional ultrathin PdCuene catalysts via a one-pot wet-chemical method. The two-dimensional and ultrathin structure assembles into a three-dimensional porous structure, maximizing the exposure of active sites and facilitating the rapid transport of reactants. More importantly, the random occupation of Cu atoms into the Pd lattice greatly produces compressive strain of 0.85%, thus weakening the adsorption energy of CO* intermediates. Subsequently, the electrocatalytic performance of PdCuene catalysts for FAOR was systematically investigated. Its mass activity (2.31 A mgPt−1) and specific activity (4.09 mA cm−2) are significantly superior to those of commercial Pd/C and Pdene catalysts. The introduction of Cu atoms facilitated the electrocatalytic performance of alloy PdCuene through the synergistic mechanism: (i) The electronic effect coupled with Cu atoms and Pd atoms would regulate the d-band center of Pd to weaken CO* adsorption energy; (ii) The two-dimensional ultrathin structure and three-dimensional porous structure optimize the exposure of active sites and the mass transfer process. In addition, the PdCuene electrocatalyst has demonstrated structural stability in long-term tests. This research offers vital guidance for the design of two-dimensional, ultrathin, and alloy-structured Pd-based electrocatalysts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15100967/s1. Figure S1: Photographs of (a) before reaction and (b) after reaction for 110 °C with 1 h under the same reaction conditions as for PdCu bimetallene but in the absence of W(CO)6; Figure S2: SEM image of Pdene catalyst; Figure S3: EDX analysis of Pdene catalysts; Figure S4: Atomic ratio of Pd/W obtained from EDX analysis of Pdene catalysts; Figure S5: XPS spectra of W 4f for Pdene and PdCuene nanocatalysts; Figure S6: SEM image of PdCuene after the durability test; Figure S7: (a) SEM-EDX mapping and (b) SEM-EDS spectrum analysis of PdCuene catalysts after durability test; Figure S8: The scan-rate-dependent FAOR activities for (a) Pd/C, (b) Pdene and (c) PdCuene electrocatalysts; Figure S9: CO stripping curves of commercial Pd/C, Pdene and PdCuene nanocatalysts. Table S1: Atomic ratio for Pdene and PdCuene electrocatalysts; Table S2: Lattice parameters of Pdene and PdCuene obtained from HRTEM and XRD presented in Figure 1f and Figure 2a, respectively; Table S3: The ECSA, specific activity, and mass activity of PdCuene in comparison with the reported state-of-the-art values.

Author Contributions

Conceptualization, Z.C. and J.H.; methodology, J.H.; software, K.W.; validation, Z.C. and J.H.; formal analysis, K.W.; investigation, K.W.; resources, J.H.; data curation, K.W.; writing—original draft preparation, K.W.; writing—review and editing, Z.C. and J.H.; visualization, Z.C.; supervision, Z.C.; project administration, Z.C.; funding acquisition, K.W. and J.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 52073214), the Natural Science Foundation of Shandong Province (No. ZR2025QC566), the Science Foundation of Weifang University (No. 2023BS11), and the Science and Technology Development Plan Foundation of Weifang (No. 2024GX020).

Data Availability Statement

The data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Schematic illustration of the preparation process of PdCuene nanocatalysts. (b,c) SEM image of PdCuene. (d) SEM-EDX analysis of the atomic ratio Pd/Cu/W. (e) TEM image of PdCuene. (f) The HRTEM image and inset rectangles represent a lattice spacing of 2.22 Å for Pd (111). (g) STEM image and the corresponding STEM-EDS mapping of (h) Pd element, (i) Cu element, (j) W element, and (k) PdCuW mix elements for PdCuene.
Figure 1. (a) Schematic illustration of the preparation process of PdCuene nanocatalysts. (b,c) SEM image of PdCuene. (d) SEM-EDX analysis of the atomic ratio Pd/Cu/W. (e) TEM image of PdCuene. (f) The HRTEM image and inset rectangles represent a lattice spacing of 2.22 Å for Pd (111). (g) STEM image and the corresponding STEM-EDS mapping of (h) Pd element, (i) Cu element, (j) W element, and (k) PdCuW mix elements for PdCuene.
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Figure 2. (a) XRD patterns for Pdene and PdCuene electrocatalysts (the blue dotted line shows the positive shift of (111) peak for PdCuene compare to Pdene). (b) XPS full spectrum and (c) high-resolution Pd 3d for Pdene and PdCuene electrocatalysts. (d) High-resolution Cu 2p for PdCuene electrocatalyst.
Figure 2. (a) XRD patterns for Pdene and PdCuene electrocatalysts (the blue dotted line shows the positive shift of (111) peak for PdCuene compare to Pdene). (b) XPS full spectrum and (c) high-resolution Pd 3d for Pdene and PdCuene electrocatalysts. (d) High-resolution Cu 2p for PdCuene electrocatalyst.
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Figure 3. Electrocatalytic FAOR performances on PdCuene, Pdene, and commercial Pd/C catalysts. (a) CVs of the three catalysts in N2-saturated 0.1 M HClO4 aqueous solution with a scan rate of 50 mV s−1. (b) CVs recorded in N2-saturated 0.1 M HClO4 containing 0.5 M formic acid solution with a scan rate of 50 mV s−1 (the arrows indicate the direction of the forward oxidate peak). (c) The mass activity and specific activity of the three catalysts. (d) Linear sweep voltammetry (LSV) curve obtained in N2-saturated 0.1 M HClO4 containing 0.5 M formic acid solution with a scan rate of 5 mV s−1. (e) Linear regions of the Tafel plots. (f) Electrochemical impedance spectroscopy (EIS) curves of the three catalysts.
Figure 3. Electrocatalytic FAOR performances on PdCuene, Pdene, and commercial Pd/C catalysts. (a) CVs of the three catalysts in N2-saturated 0.1 M HClO4 aqueous solution with a scan rate of 50 mV s−1. (b) CVs recorded in N2-saturated 0.1 M HClO4 containing 0.5 M formic acid solution with a scan rate of 50 mV s−1 (the arrows indicate the direction of the forward oxidate peak). (c) The mass activity and specific activity of the three catalysts. (d) Linear sweep voltammetry (LSV) curve obtained in N2-saturated 0.1 M HClO4 containing 0.5 M formic acid solution with a scan rate of 5 mV s−1. (e) Linear regions of the Tafel plots. (f) Electrochemical impedance spectroscopy (EIS) curves of the three catalysts.
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Figure 4. (a) CA tests performed at 0 V vs. SCE in N2-saturated 0.1 M HClO4 containing 0.5 M formic acid solution. (b) Current density as a function of the square root of the scan rate (v1/2) of the samples. (c) Plot of ECSA (m2 g−1) vs. mass activity peak potential with literature results listed in Table S3. (d) Mechanism schematic diagram of the weakly adsorbed CO* and strongly adsorbed HCOOH* on the PdCuene surface for FAOR (the green and yellow balls represent Pd and Cu atoms, respectively. “*” represents the adsorbed species).
Figure 4. (a) CA tests performed at 0 V vs. SCE in N2-saturated 0.1 M HClO4 containing 0.5 M formic acid solution. (b) Current density as a function of the square root of the scan rate (v1/2) of the samples. (c) Plot of ECSA (m2 g−1) vs. mass activity peak potential with literature results listed in Table S3. (d) Mechanism schematic diagram of the weakly adsorbed CO* and strongly adsorbed HCOOH* on the PdCuene surface for FAOR (the green and yellow balls represent Pd and Cu atoms, respectively. “*” represents the adsorbed species).
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Wang, K.; Cao, Z.; He, J. Strain Effect in PdCu Alloy Metallene for Enhanced Formic Acid Electrooxidation Reaction. Catalysts 2025, 15, 967. https://doi.org/10.3390/catal15100967

AMA Style

Wang K, Cao Z, He J. Strain Effect in PdCu Alloy Metallene for Enhanced Formic Acid Electrooxidation Reaction. Catalysts. 2025; 15(10):967. https://doi.org/10.3390/catal15100967

Chicago/Turabian Style

Wang, Kaili, Zhen Cao, and Jia He. 2025. "Strain Effect in PdCu Alloy Metallene for Enhanced Formic Acid Electrooxidation Reaction" Catalysts 15, no. 10: 967. https://doi.org/10.3390/catal15100967

APA Style

Wang, K., Cao, Z., & He, J. (2025). Strain Effect in PdCu Alloy Metallene for Enhanced Formic Acid Electrooxidation Reaction. Catalysts, 15(10), 967. https://doi.org/10.3390/catal15100967

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