English

• 0.   Introduction

  With the rapid development of hydrogen energy technologies, proton exchange membrane fuel cells (PEMFCs) have emerged as promising energy conversion devices for transportation and stationary power applications owing to their high energy-conversion efficiency, low operating temperature, and near-zero pollutant emissions^[1-4]. However, their large-scale commercialization remains hindered by the sluggish oxygen reduction reaction (ORR) kinetics at the cathode^[2-4] and the substantial reliance on platinum (Pt)-based catalysts^{[1, 5-6]}. Although various low-cost alternatives, including carbon-based catalysts and transition-metal compounds, have been explored, their catalytic activity and long-term stability remain insufficient for practical PEMFC applications.

  Alloying Pt with 3d transition metals (e.g., Fe, Co, Ni, and Cu) has been widely recognized as an effective strategy to enhance Pt utilization efficiency and regulate its electronic structure^[7]. Pt-based alloys can optimize the adsorption energies of oxygen-containing intermediates (e.g., OH*) through ligand and strain effects, thereby facilitating the oxidative removal of poisoning species (e.g., CO*) and improving ORR activity and durability^[8-13]. Among these alloying elements, Cu has attracted particular interest owing to its abundance, low cost, and tunable electronic properties. The incorporation of Cu into Pt induces favorable electronic and geometric effects, while the similar atomic radii of Cu and Pt (127.8 and 138.7 pm, respectively) generate moderate lattice strain and optimize the d-band electronic structure of Pt. Furthermore, Cu can be selectively removed under acidic conditions, to form a Pt-enriched surface, improving structural stability and CO* tolerance while creating abundant surface vacancies that facilitate ORR kinetics^{[9-11, 14]}.

  Various PtCu nanostructures, including nanoparticles^{[2, 15]}, nanosheets^[16], nanowires^[17], nanoframes^{[5, 18]}, and atomically dispersed structures^{[12, 19]}, have been developed to maximize active-site exposure and enhance catalytic performance^{[18, 20-21]}. However, structural degradation caused by dissolution, aggregation, and collapse during long-term operation remains a major challenge. Porous architectures constructed through dealloying can provide high surface areas, interconnected frameworks, and abundant accessible active sites, providing an effective strategy to simultaneously improve both catalytic activity and stability^{[3, 9]}.

  Recently, 3D nanoporous conductive networks (3D NCNs) have attracted considerable attention as advanced Pt-based ORR catalysts due to their large surface area, continuous electron-transport pathways, abundant interfacial active sites, and robust structural integrity^[22-24]. However, current fabrication methods, including template-assisted synthesis and galvanic replacement reactions, often involve complicated procedures, surfactants, or organic solvents, which increase costs and hinder large-scale production^{[6, 10, 13, 18, 25-30]}.

  Therefore, developing a simple, environmentally friendly, and industrially scalable strategy for constructing Pt-based 3D NCN catalysts remains highly desirable. In this work, we report a one-pot, surfactant-free, inorganic-salt-mediated self-assembly approach for the rapid synthesis of PtCu 3D NCNs. The structural and compositional characteristics of the resulting materials were systematically investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Their electrocatalytic activity and durability toward the ORR were evaluated in 0.1 mol·L^-1 HClO₄ electrolyte. This work provides a facile and scalable route for the fabrication of high-performance Pt-based ORR catalysts and offers insights into the rational design of cost-effective electrocatalysts for advanced PEMFC applications.

  1.   Experimental

  1.1   Chemical reagents

  Potassium chloride (KCl), aluminum chloride hexahydrate (AlCl₃·6H₂O), chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O), copper chloride dihydrate (CuCl₂·2H₂O), urea, sodium hydroxide (NaOH), acetic acid (C₂H₄O₂, 60%), absolute ethanol, and isopropanol (99%) were purchased from Macklin Chemical Reagent Co., Ltd. A 5% Nafion solution was obtained from DuPont. All reagents were of analytical grade and used as received without further purification. Deionized (DI) water with a resistivity higher than 18.0 MΩ·cm was produced using an ultrapure water purification system and used throughout all experiments.

  1.2   Preparation of catalyst

  As a representative example, Pt₃Cu₁ 3D NCNs (denoted as Pt₃Cu₁, where the precursor molar ratio of Pt to Cu was 3) were synthesized via a one-pot solvothermal self-assembly strategy. In a typical synthesis, 385 μL of KCl solution (0.02 mol·L^-1) and 1.53 mL of AlCl₃ solution (0.02 mol·L^-1) were sequentially added to 14 mL of isopropanol under stirring. The resulting mixture was ultrasonicated to ensure homogeneous dispersion. Subsequently, 3.0 mL of H₂PtCl₆ solution (0.038 6 mol·L^-1) and 185 μL of CuCl₂ solution (0.20 mol·L^-1) were added successively, followed by stirring and ultrasonication for 30 min. Next, 4.0 mL of urea solution (0.05 mol·L^-1) was introduced dropwise under continuous magnetic stirring. After an additional 30 min of ultrasonication, the pH of the suspension was adjusted to 12 using 0.5 mol·L^-1 NaOH solution. The resulting mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and maintained at 220 ℃ for 4 h. After naturally cooling to room temperature, XC-72 carbon black was added to the obtained product, and the suspension was stirred for 2 h to facilitate catalyst loading. The metal loading (mass fraction) of all catalysts was fixed at 20%. The solid product was then collected by centrifugation, thoroughly washed, and dried for subsequent treatment and the Pt₃Cu₁/C catalyst was obtained. To investigate the effect of alloy composition on catalytic performance, a series of Pt_xCu₁/C catalysts (x=1, 2, 4, 5; x is the precursor molar ratio of Pt to Cu) were prepared using the same procedure. If carbon loading was not carried out, the product (Pt_xCu₁) was obtained after filtration, washing, and drying the above suspension. The as-prepared Pt_xCu₁/C catalysts were further subjected to a chemical dealloying treatment. Specifically, the samples were refluxed in a 60% acetic acid solution at 60 ℃ for 6 h under a nitrogen atmosphere. After dealloying, the catalysts were washed repeatedly with ethanol, collected by centrifugation, and dried overnight at 50 ℃. Unless otherwise treated, all final catalysts mentioned in the following text underwent dealloying treatment.

  For comparison, monometallic Pt 3D NCNs (denoted as Pt) and Pt/C 3D NCNs (denoted as Pt/C) were synthesized following the same procedure in the absence of the Cu precursor. A commercial Pt/C catalyst (TKK, denoted as com-Pt/C) was also employed as a benchmark catalyst.

  1.3   Characterization and electrochemical measurements

  The crystal structure, morphology, elemental distribution, surface composition, and metal contents of the catalysts were characterized using various physicochemical techniques. Electrochemical measurements were performed to evaluate the ORR activity and durability of the catalysts. Detailed descriptions of the characterization methods and electrochemical testing procedures were provided in the Supporting information.

  2.   Results and discussion

  2.1   Formation mechanism of Pt₃Cu₁

  The role of Al³⁺ in mediating the formation of Pt₃Cu₁ was systematically investigated by varying the amount of AlCl₃ while keeping the KCl dosage constant. Previous studies have demonstrated that high-valent cations, such as Fe³⁺ and Al³⁺, can facilitate the face-to-face self-assembly of two-dimensional nanostructures, ultimately leading to the formation of one-dimensional superlattice nanowires^[31-32]. These cations exhibit a strong affinity for metal ions, forming stable complexes that promote the desired assembly behavior^[33]. Additionally, anions such as Cl^- can selectively adsorb onto the (100) crystal plane of the face-centered cubic (fcc) lattice, inhibiting growth along this direction and promoting anisotropic evolution of the nanostructures^{[2, 34]}.

  In our experiments, the molar ratio (y) of metal ions (Mⁿ⁺, defined as the total of Pt²⁺ and Cu²⁺ ions) to Al³⁺ was used as a controlling parameter. The resulting samples are denoted as Pt₃Cu₁-y (y=1, 3, 5). At a high ratio (y=5), nanoparticles formed via isopropanol reduction were well dispersed, with only a few short chains connecting the particles (Fig. 1a). Increasing the Al³⁺ content induced the formation of a 3D nanowire network, which became more defined as the ratio approached 3 (Fig. 1b). At a higher Al³⁺ concentration (y=1), significant nanoparticle aggregation occurred, which could not be effectively dispersed by ultrasonication (Fig. 1c). Therefore, the sample prepared with a metal ion to Al³⁺ ratio of 3 was selected as the optimized catalyst for further investigation in this work (Unless otherwise specified in the following text, Pt₃Cu₁ refers to this sample).

  Figure 1

  

  Figure 1.  TEM images of Pt₃Cu₁-y: (a) y=5, (b) y=3, (c) y=1

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  Based on these experimental results and previously reported literature, we propose the following formation mechanism of Pt₃Cu₁, as illustrated in Fig. 2:

  Figure 2

  

  Figure 2.  Schematic diagram of the Pt₃Cu₁ formation process

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  (1) Nucleation: Al³⁺, as a trivalent cation with high charge density, forms stable ion pairs with negatively charged [PtCl₆]^2- (the Pt species in the reaction system exists as the anionic complex [PtCl₆]^2-) and [CuCl₄]^2- (the Cu²⁺ also mainly exists as [CuCl₄]^2- under high Cl^- concentration conditions) complexes via electrostatic interaction. This association reduces electrostatic repulsion between the metal complex anions, facilitating aggregation. Subsequent reduction by isopropanol produces nanocrystalline nuclei, which are further capped by amide groups generated from urea decomposition.

  (2) Directed assembly and growth: Al³⁺ mediates the ordered assembly of nanoparticles through electrostatic adsorption, promoting the formation of a continuous network structure^[33]. Simultaneously, Cl^- selectively adsorbs on the (100) plane of the fcc PtCu alloy, suppressing crystal growth along this direction and driving anisotropic growth toward the (111) orientation^[34]. High-resolution TEM (HRTEM) analysis confirmed that the (111) and (220) planes of PtCu alloy, with interplanar spacings of 0.219 and 0.139 nm, respectively, dominate the exposed surfaces, whereas the (200) plane (0.191 nm) was rarely observed, consistent with the selective adsorption of Cl^- on the (100) plane of the PtCu alloy. The moderate aspect ratio of the final products arises from the concurrent progress of crystal growth and Al³⁺-induced interparticle assembly. As nanoparticles evolve into short nanowires, Al³⁺ acts as a linker, cross-linking these units into a 3D network while preventing excessive elongation of individual nanowires. This synergistic effect results in an interconnected zigzag nanoporous network rather than discrete nanowires with high aspect ratios.

  2.2   Characterization of the catalysts

  2.2.1   Morphological and structural characteristics

  The morphology and microstructure of the assynthesized Pt₃Cu₁ were first investigated by TEM. As shown in Fig. 3a, the catalyst exhibited a typical 3D NCN composed of interconnected open channels and ultrathin nanowires. Higher-magnification TEM images (Fig. 3b and 3c) reveal a distinctive zigzag nanowire morphology, with an average diameter of approximately 9.6 nm.

  Figure 3

  

  Figure 3.  (a-c) TEM images, (d-f) HRTEM images, (g-i) elemental mappings, and (j) EDS spectrum of as-synthesized Pt₃Cu₁ catalyst

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  To further elucidate the crystalline structure, HRTEM analyses were performed at representative branch tips and network junctions (Fig. 3d-3f). Well-defined lattice fringes were clearly observed throughout the nanowire framework. In addition, numerous grain boundaries, twin boundaries, and vacancy-related defects could be identified. These structural defects are expected to generate local lattice distortions and modify the electronic environment of surface atoms, which may provide additional catalytically active sites. The zigzag configuration of the nanowire further introduces abundant step and corner sites while preserving the structural continuity of the conductive network. The measured lattice spacings of 0.219, 0.191, and 0.139 nm correspond to the (111), (200), and (220) planes of an fcc PtCu alloy phase, respectively, confirming successful alloy formation^{[13, 27]}. Such an interconnected porous architecture is beneficial for efficient electron transport, rapid mass diffusion, and enhanced structural robustness, which are advantageous for electrocatalytic applications^{[5, 31]}.

  Elemental distributions were further examined using HAADF-STEM coupled with EDS. As shown in Fig. 3g-3i, Pt and Cu were homogeneously distributed throughout the entire network without detectable elemental segregation, indicating the formation of a uniform alloy structure. Quantitative analysis from the EDS spectrum (Fig. 3j) gave an atomic ratio of Pt to Cu of 3.18, which was slightly higher than the nominal precursor ratio (3) and agreed well with the ICP-AES result (3.26). This slight increase in Pt content is attributed to the partial dissolution of Cu during the subsequent dealloying process.

  2.2.2   Crystal structure and phase composition

  The crystal structure of the Pt_xCu₁/C was further investigated by XRD. As shown in Fig. 4a, Pt exhibited characteristic diffraction peaks at 39.76°, 46.24°, 67.45°, and 81.28°, corresponding to the (111), (200), (220), and (311) planes of the fcc Pt (PDF No.04-0802), respectively^{[20, 22]}. After alloying with Cu, all Pt_xCu₁ retained the fcc structure, while their diffraction peaks gradually shifted toward higher 2θ values with increasing Cu content. This shift reflects lattice contraction arising from the incorporation of smaller Cu atoms into the Pt lattice.

  Figure 4

  

  Figure 4.  (a) XRD patterns and (b) lattice constants and lattice strains of Pt_xCu₁/C and Pt/C

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  Notably, all diffraction peaks of Pt_xCu₁/C were located between those of pure Pt (PDF No.04-0802) and pure Cu (PDF No.04-0836), and no diffraction peaks associated with separate Pt or Cu phases were detected, further confirming the formation of a single-phase PtCu alloy^{[13, 27]}. Meanwhile, the progressive broadening of diffraction peaks with increasing Cu content indicates a reduction in crystallite size and/or an increase in lattice distortion.

  To quantitatively evaluate the alloy-induced structural changes, the lattice constants (α) were calculated from the (111) diffraction peak using Bragg′s law^[35]:

  $ \alpha=\frac{\sqrt{3} \lambda}{2 \sin \theta} $

  (1)

  where λ is the wavelength of Cu Kα radiation (0.154 059 nm), and θ is the diffraction angle of the (111) plane. The lattice strain (S_l) was estimated according to:

  $ S_1=\left(d_{\text {alloy }}-d_{\mathrm{P}_1}\right) / d_{\mathrm{P}_1} \times 100 \% $

  (2)

  where d_alloy and d_Pt represent the nearest-neighbor bond lengths in the alloy and pure Pt structures, respectively. A negative S_l value indicates that the alloy bond length is reduced relative to pure Pt, resulting in compressive strain within the Pt lattice.

  As summarized in Fig. 4b, the lattice constant and the lattice strain values of all Pt_xCu₁/C were between those of pure Pt (0.392 31 nm) and pure Cu (0.361 49 nm), and increased approximately linearly with increasing Pt content, consistent with Vegard′s law^[8]. Correspondingly, the compressive strain became progressively stronger as the Cu concentration increased. The calculated lattice contraction values for Pt₅Cu₁/C, Pt₄Cu₁/C, Pt₃Cu₁/C, Pt₂Cu₁/C, and Pt₁Cu₁/C were 1.04%, 1.21%, 1.90%, 2.04%, and 2.87%, respectively. Such compressive strain can regulate the Pt d-band center and optimize the adsorption behavior of oxygen-containing intermediates, thereby promoting ORR activity.

  2.2.3   Surface chemical states and electronic interactions

  XPS was employed to investigate the surface composition and chemical states of Pt₃Cu₁/C. The survey spectrum reveals a surface atomic ratio of Pt to Cu of 3.35, which was higher than both the nominal precursor ratio (3) and the bulk composition determined by ICP-AES (3.26). Considering the surface-sensitive nature of XPS (probing depth of 2-5 nm), this result provides direct evidence for the preferential dissolution of Cu during dealloying and the formation of a Ptenriched surface layer^{[13, 22, 27]}. Consistently, HRTEM images (Fig. 3e) showed discernible lattice distortions near the surface region, which can be attributed to the lattice mismatch between the Pt-rich shell and the PtCu alloy core.

  Fig. 5 compares the XPS spectra of Pt₃Cu₁/C and Pt/C. In the Pt4f region (Fig. 5a), the Pt4f_7/2 and Pt4f_5/2 peaks of Pt₃Cu₁/C were located at 70.9 and 74.3 eV, respectively, exhibiting a negative shift of approximately 0.65 eV relative to those of Pt/C (71.5/75.0 eV). High-resolution Pt4f spectra fitting (Fig. 5b and 5c) indicates that Pt/C exists predominantly in the metallic state (Pt⁰), accompanied by a small amount of oxidized Pt²⁺ species. For Pt₃Cu₁/C, the major peaks at 71.0 and 74.2 eV correspond to Pt⁰, while the weaker components at 71.8 and 75.9 eV are assigned to Pt²⁺. Similar species are observed for Pt/C, which display Pt⁰ peaks at 71.6 and 74.9 eV and Pt²⁺ peaks at 72.5 and 76.3 eV^[36].

  Figure 5

  

  Figure 5.  (a) Pt4f XPS spectra for Pt₃Cu₁/C and Pt/C; Pt4f XPS spectra for (b) Pt₃Cu₁/C and (c) Pt/C; (d) Cu2p XPS spectra for Pt₃Cu₁/C

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  The Cu2p spectrum of Pt₃Cu₁/C (Fig. 5d) can be deconvoluted into Cu⁰ and Cu²⁺ components. Peaks at 932.3 and 952.0 eV are attributed to metallic Cu⁰ (Cu2p_3/2 and Cu2p_1/2), whereas those at 935.0 and 953.5 eV correspond to Cu²⁺ (Cu2p_3/2 and Cu2p_1/2) species. In addition, a characteristic shake-up satellite peak appeared at approximately 941.8 eV, confirming the presence of surface Cu²⁺ generated by air oxidation.

  Importantly, the negative shift of the Pt4f peaks together with the slight positive shift of the Cu2p peaks suggests electron transfer from Cu to Pt within the alloy framework. Such electronic interaction modifies the electronic structure of Pt and is expected to downshift the Pt d-band center, thereby optimizing the adsorption strength of ORR intermediates and enhancing catalytic activity^{[13, 18, 22]}.

  2.3   Electrochemical activity evaluation

  To evaluate the role of Cu incorporation in catalyst formation and electrocatalytic performance, monometallic Pt 3D NCNs were synthesized under identical conditions without the addition of Cu precursors. A com-Pt/C catalyst was also employed as a benchmark.

  Fig. 6a presents the cyclic voltammetry (CV) curves of the catalysts recorded in an N₂-saturated 0.5 mol·L^-1 H₂SO₄ solution. The ECSA was determined from the hydrogen underpotential deposition (Hupd) region. As summarized in Fig. 6b and Table S1, the ECSA values of Pt₁Cu₁/C, Pt₃Cu₁/C, Pt₅Cu₁/C, Pt/C, and com-Pt/C were calculated to be 39.3, 47.4, 41.3, 34.4, and 60.4 m²·g_Pt^-1, respectively. Among the synthesized catalysts, Pt₃Cu₁/C exhibited the largest ECSA, which can be attributed to the interconnected nanoporous framework and defect-rich zigzag nanowires that provide abundant accessible active sites.

  Figure 6

  

  Figure 6.  (a) CV curves of catalysts in a 0.5 mol·L^-1 H₂SO₄ solution at room temperature at a scanning rate of 50 mV·s^-1 and (b) the corresponding ECSA; (c) ORR polarization curves in O₂-satured 0.1 mol·L^-1 HClO₄ electrolyte under room temperature at a scan rate of 10 mV·s^-1 and a rotation rate of 1 600 r·min^-1 and (d) the corresponding MA and SA at 0.9 V (vs RHE)

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  Notably, all Pt_xCu₁/C displayed a pronounced positive shift of the oxide reduction peak compared with Pt/C and com-Pt/C. Since the oxide reduction potential is closely related to the adsorption strength of oxygen-containing intermediates on Pt surfaces, a more positive reduction peak generally indicates weaker oxygen binding and more favorable ORR kinetics^[5]. This observation suggests that alloying Pt with Cu effectively modifies the surface electronic structure and optimizes the adsorption behavior of reaction intermediates.

  The ORR activities of the catalysts were further evaluated in O₂-saturated 0.1 mol·L^-1 HClO₄ electrolyte using a rotating disk electrode (RDE). As shown in Fig. 6c, the half-wave potentials (E_1/2) of Pt₁Cu₁/C, Pt₃Cu₁/C, Pt₅Cu₁/C, Pt/C, and com-Pt/C were 0.872, 0.875, 0.852, 0.844, and 0.839 V, respectively (Table S1). All Pt_xCu₁/C catalysts exhibited higher ORR activity than both Pt/C and com-Pt/C, with Pt₃Cu₁/C delivering the most positive half-wave potential.

  The kinetic current densities were further extracted using the Koutecký-Levich (K-L) equation, and the corresponding mass activity (MA) and specific activity (SA) at 0.90 V (vs RHE) are summarized in Fig. 6d and Table S1^{[13, 23, 37]}. Pt₃Cu₁/C achieved an MA of 0.651 A·mg_Pt^-1, which was 1.26, 1.46, 1.69, and 2.60 times higher than those of Pt₁Cu₁/C, Pt₅Cu₁/C, Pt/C, and com-Pt/C, respectively. The corresponding SA of Pt₃Cu₁/C reached 1.373 mA·cm^-2, which was also the highest among all catalysts investigated.

  It is noteworthy that although com-Pt/C possessed the largest ECSA, its ORR activity remained significantly lower than that of Pt₃Cu₁/C. This result indicates that the enhanced catalytic performance cannot be solely attributed to an increase in surface area. Instead, the superior activity of Pt₃Cu₁/C primarily originates from the synergistic effects of PtCu alloying, including compressive strain, electronic structure modulation, and the formation of a Pt-enriched surface layer, all of which optimize the adsorption energetics of ORR intermediates.

  To further elucidate the ORR pathway, rotating ring-disk electrode (RRDE) measurements were conducted. Based on the disk and ring currents (Fig.S2a), the electron transfer number (n) and H₂O₂ yield (Y) were calculated according to Eq.3 and 4, where I_D is the disk current corresponding to the overall ORR process, I_R is the ring current derived from the oxidation of H₂O₂ species collected at the ring electrode, and N is the collection efficiency of the RRDE electrode. The results are shown in Fig.S2b. Throughout the investigated potential range, Pt₃Cu₁/C exhibited an average electron transfer number of approximately 3.99 and an H₂O₂ yield below 5%, indicating that oxygen reduction proceeds predominantly through a highly efficient four-electron pathway with negligible peroxide formation. These results are consistent with those reported for state-of-the-art Pt-based ORR catalysts^{[13, 23]}.

  $ Y=200 I_{\mathrm{R}} /\left(I_{\mathrm{D}}+I_{\mathrm{R}} / N\right)^2 $

  (3)

  $ n=4 I_{\mathrm{D}} /\left(I_{\mathrm{D}}+I_{\mathrm{R}} / N\right) $

  (4)

  The overall electrochemical performance of all catalysts is summarized in Table S1. Among the investigated samples, Pt₃Cu₁/C exhibited the most balanced combination of ECSA, half-wave potential, MA, and SA, demonstrating its superior intrinsic ORR activity. In addition, the methanol oxidation reaction (MOR) performance of Pt₃Cu₁/C was evaluated to explore its potential applicability beyond ORR, and the corresponding results are presented in Fig.S3.

  To investigate the influence of inorganic salt composition on catalyst formation and electrocatalytic behavior, catalysts prepared with different molar ratios of Mⁿ⁺ to Al³⁺ were also examined. Detailed structural and electrochemical characterization results are provided in Fig.S1. As shown in Fig.S1, the ECSA of the catalysts increased from 96.4 to 118.5 m²·g_Pt^-1 as the y value decreased from 5 to 3, while further decreasing y to 1 resulted in a reduced ECSA of 80.3 m²·g_Pt^-1, indicating that the Mⁿ⁺/Al³⁺ ratio plays a critical role in regulating the formation of surface-active sites. The ORR measurements further confirmed this trend, with Pt₃Cu₁/C when y=3 exhibiting the highest half-wave potential (0.873 V) and MA (0.651 A·mg_Pt^-1), significantly outperforming other catalysts with different Mⁿ⁺/Al³⁺ ratios and com-Pt/C. The superior ORR performance of Pt₃Cu₁/C when y=3 can be attributed to the defect-rich 3D porous architecture induced by an appropriate Al³⁺ content, which not only enhances the exposure of Pt active sites but also optimizes oxygen intermediate adsorption through electronic modulation of the PtCu alloy, thereby accelerating the ORR kinetic process.

  2.4   Electrochemical durability evaluation

  The long-term durability of electrocatalysts is a critical prerequisite for their practical application in PEMFCs. To evaluate catalyst stability under harsh operating conditions, accelerated durability tests (ADT) were performed for Pt₃Cu₁/C and com-Pt/C in an O₂-saturated 0.1 mol·L^-1 HClO₄ electrolyte^[38]. The potential was cycled between 0.60 and 1.00 V (vs RHE) at a scan rate of 100 mV·s^-1 for up to 30 000 cycles. The electrochemical properties before and after ADT were compared to assess the structural and catalytic stability of the catalysts (Fig. 7).

  Figure 7

  

  Figure 7.  ORR polarization curves of (a) Pt₃Cu₁/C and (b) com-Pt/C catalysts before and after ADT in O₂-saturated 0.1 mol·L^-1 HClO₄ electrolyte under room temperature at a rotation speed of 1 600 r·min^-1 and a scanning rate of 10 mV·s^-1 and the corresponding CV curves in a 0.5 mol·L^-1 H₂SO₄ solution at a scan rate of 50 mV·s^-1 under room temperature (inset); (c) ECSA and (d) MA at 0.9 V (vs RHE) of the catalysts before and after ADT

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  Fig. 7a and 7b present the ORR polarization curves of Pt₃Cu₁/C and com-Pt/C before and after ADT. The corresponding CV curves recorded in an N₂-saturated 0.5 mol·L^-1 H₂SO₄ are shown in the insets. The evolution of ECSA and MA during the durability test is summarized in Fig. 7c and 7d, respectively. After 30 000 cycles, the ECSA of com-Pt/C decreased from 60.4 to 35.9 m²·g_Pt^-1, corresponding to only 59% retention of its initial value. This substantial loss is primarily attributed to Ostwald ripening, nanoparticle migration, and particle aggregation during prolonged electrochemical cycling. In contrast, Pt₃Cu₁/C retained an ECSA of 35.5 m²·g_Pt^-1 after 30 000 cycles, corresponding to 75% retention of the initial value. The significantly improved ECSA retention demonstrates the superior structural robustness of the 3D nanoporous network.

  A similar trend was observed for catalytic activity. As shown in Fig. 7d and Table S2, the MA of com-Pt/C at 0.90 V (vs RHE) decreased to 0.107 A·mg_Pt^-1 after 30 000 cycles, retaining only 43% of its initial activity. By comparison, Pt₃Cu₁/C maintained a MA of 0.374 A·mg_Pt^-1, corresponding to an activity retention of 57%. Even after extensive durability testing, the MA of Pt₃Cu₁/C remained substantially higher than that of com-Pt/C. A comparison with recently reported Pt-based ORR catalysts is provided in Table S3.

  To elucidate the origin of the enhanced durability, post-ADT structural characterization was performed (Fig.S5). Although the Pt-enriched surface generated by chemical dealloying cannot completely suppress Cu dissolution under prolonged cycling conditions, it effectively slows Cu leaching and mitigates catastrophic structural degradation. TEM images collected after ADT reveal partial nanowire coarsening and localized aggregation; however, the overall 3D interconnected framework remained largely intact. Such structural preservation contributes significantly to the superior ECSA retention observed for Pt₃Cu₁/C. Further evidence is provided by the post-ADT XPS spectra, where the Cu2p signal intensity decreased markedly, indicating substantial dissolution of surface Cu species during long-term electrochemical operation. Nevertheless, the remaining Pt-rich shell effectively stabilizes the alloy framework and prevents rapid collapse of the porous architecture. Consequently, Pt₃Cu₁/C exhibited considerably higher ECSA retention (75%) than its monometallic Pt counterparts (52%).

  The beneficial role of dealloying is further associated with the generation of Cu vacancies. Numerous studies have shown that vacancy sites formed after selective Cu removal can regulate the adsorption energies of oxygen-containing intermediates (HO* and O*), thereby facilitating ORR kinetics and enhancing catalytic activity^{[2-3, 5, 15, 22]}. The existence of such vacancy-rich structures is supported by several experimental observations. First, XPS analysis revealed a higher surface atomic ratio (3.35) of Pt to Cu than the bulk composition determined by ICP-AES (3.26), indicating preferential surface Cu dissolution. Second, ICP-AES measurements showed a measurable decrease in Cu content after dealloying. Third, HRTEM images (Fig. 3e) reveal lattice distortions and vacancy-type defects within the subsurface region. These observations collectively support the formation of a Ptenriched and vacancy-rich surface structure.

  Based on the experimental results and discussions above, we conclude that PtCu exhibits excellent ORR activities and stabilities in acidic media, primarily due to the following four key factors: (1) optimal Pt/Cu composition and electronic regulation. The introduction of Cu atoms effectively promotes the removal of toxic intermediates (e.g., CO*) adsorbed on adjacent Pt active sites, thereby freeing up Pt active sites^{[18, 20, 38-39]}. An appropriate Cu content optimizes the binding strength between Pt and reaction intermediates, accelerating the oxidation kinetics of these intermediates^[40]. However, excessive Cu content may inhibit overall activity due to its own low-potential oxidation inertness to alcohols^[41]. Therefore, an optimal ratio of Pt to Cu significantly enhances electrocatalytic performance by balancing the surface Pt and Cu sites. (2) Abundant defects and 3D porous structure. The presence of grain boundaries (line defects) and vacancies (point defects) in the catalyst provides abundant active catalytic sites^{[3, 7, 22, 42]}. Additionally, the 3D interconnected porous network structure not only enhances the accessibility of active sites but also facilitates the mass transfer of reactants and products, further improving catalytic efficiency. (3) Synergistic catalytic effect of Pt and Cu. The electronic and geometric strain effects induced by alloying create a significant synergistic interaction between Pt and Cu. This interaction optimizes the reaction pathways and energy barriers on the catalytic surface, thereby enhancing the intrinsic activity for both MOR^[43] (Fig.S3) and ORR^{[3, 17, 18, 22]}. (4) Pt-enriched surface layer. The formation of a Pt-enriched surface layer during the chemical dealloying process effectively prevents further Cu leaching and stabilizes the crystal structure. Combined with the Cu vacancies generated after dealloying, which have been shown to enhance the adsorption energy of HO* and O* intermediates^{[2, 5, 22]}, the catalyst exhibits high activity, stability, and anti-poisoning ability. This proves that interfacial engineering is an effective strategy for the rational design of PEMFC catalysts.

  These combined features make Pt₃Cu₁/C a highly effective and stable catalyst for electrocatalytic applications in PEMFCs.

  3.   Conclusions

  A facile, surfactant-free, and scalable strategy was developed for the synthesis of PtCu 3D NCNs with abundant surface defects and tunable compositions. This proposed method does not require surfactants, hard templates, or long reaction times, making it highly suitable for large-scale commercial applications. Structural characterization revealed that the synthesized PtCu NCNs possess a highly interconnected 3D porous architecture composed of defect-rich zigzag nanowires.

  The cooperative effects of Al³⁺ and Cl^- ions play a crucial role in directing the formation of the network structure, where Al³⁺ promotes nanoparticle assembly and framework formation, while Cl^- facilitates anisotropic growth. Furthermore, the dealloying process generates a Pt-enriched surface layer and abundant Cuvacancy defects, leading to optimized electronic structures and enhanced accessibility of catalytic active sites. By tuning the Pt/Cu precursor ratio, the catalytic performance of the PtCu NCNs could be effectively regulated. Among the investigated catalysts, Pt₃Cu₁/C 3D NCNs exhibited the best electrocatalytic performance, delivering an MA that was 2.60 times higher for ORR than that of the com-Pt/C catalyst. In addition, Pt₃Cu₁/C 3D NCNs demonstrated excellent durability, retaining 75% of their initial ECSA and 57% of their initial MA after 30 000 ADT cycles.

  The outstanding electrocatalytic performance of PtCu 3D NCNs is primarily attributed to their unique, highly interconnected, and intertwined 3D network structure, high specific surface area, high porosity, and abundant edge/corner atoms, which act as active sites. The synergistic effect of the PtCu alloy significantly enhances the catalyst′s performance, with Cu playing a key role in improving the catalyst′s resistance to poisoning, inducing lattice strain, and tuning the electronic structure. Therefore, Pt₃Cu₁/C 3D NCNs, with their simple and effective synthesis process and high catalytic activity, are promising candidates to replace com-Pt/C catalysts and offer attractive electrocatalytic materials for various energy-related applications.

  
  Acknowledgments: This work was supported by the Shanxi Province Science Foundation for Youth (Grant No.202403021222354), and the Scientific and Technological Innovation Program of Higher Education Institutions in Shanxi Province (Grant No.2023L429). Supporting information is available at http://www.wjhxxb.cn
  Author contributions: Yuexia Li (First Author) performed the preparation, evaluation, and characterization and wrote the manuscript. Guodi Chen and Jiani He performed data curation and formal analysis. Caiting Ji and Yan Sun supervised the work, managed the project, and acquired funding. Zhiyi Zhang (Corresponding Author) revised the manuscript. Jianhuang Zeng (Corresponding Author) conceived the project, designed the experiments, and took part in the discussions.
  Data and code availability: The relevant data can be obtained in the submitted supporting documents.
  Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
  Declarations
  Ethical approval: Not applicable.
  
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• 

  Figure 1  TEM images of Pt₃Cu₁-y: (a) y=5, (b) y=3, (c) y=1

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  Figure 2  Schematic diagram of the Pt₃Cu₁ formation process

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  Figure 3  (a-c) TEM images, (d-f) HRTEM images, (g-i) elemental mappings, and (j) EDS spectrum of as-synthesized Pt₃Cu₁ catalyst

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  Figure 4  (a) XRD patterns and (b) lattice constants and lattice strains of Pt_xCu₁/C and Pt/C

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  Figure 5  (a) Pt4f XPS spectra for Pt₃Cu₁/C and Pt/C; Pt4f XPS spectra for (b) Pt₃Cu₁/C and (c) Pt/C; (d) Cu2p XPS spectra for Pt₃Cu₁/C

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  Figure 6  (a) CV curves of catalysts in a 0.5 mol·L^-1 H₂SO₄ solution at room temperature at a scanning rate of 50 mV·s^-1 and (b) the corresponding ECSA; (c) ORR polarization curves in O₂-satured 0.1 mol·L^-1 HClO₄ electrolyte under room temperature at a scan rate of 10 mV·s^-1 and a rotation rate of 1 600 r·min^-1 and (d) the corresponding MA and SA at 0.9 V (vs RHE)

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  Figure 7  ORR polarization curves of (a) Pt₃Cu₁/C and (b) com-Pt/C catalysts before and after ADT in O₂-saturated 0.1 mol·L^-1 HClO₄ electrolyte under room temperature at a rotation speed of 1 600 r·min^-1 and a scanning rate of 10 mV·s^-1 and the corresponding CV curves in a 0.5 mol·L^-1 H₂SO₄ solution at a scan rate of 50 mV·s^-1 under room temperature (inset); (c) ECSA and (d) MA at 0.9 V (vs RHE) of the catalysts before and after ADT

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