Precise diameter control derived intermetallic PtNiCo nanowires: a simple two-step synthesis and operando XAS insight

Abstract

Developing efficient and high-performance catalysts is imperative for the commercial deployment of proton exchange membrane fuel cells (PEMFCs). Ordered intermetallic Pt-based nanowires emerge as a promising candidate, offering a powerful combination of remarkable catalytic activity due to their well-defined atomic arrangements, which modulate the d-band center of Pt and enhance oxygen reduction reaction (ORR) kinetics. However, synthesizing ordered nanowire structures with superior ORR activity remains a formidable challenge as the high-temperature treatment required to induce atomic ordering often leads to the collapse or fragmentation of ultrathin wires, while thicker nanowires, though structurally intact, frequently exhibit suboptimal catalytic activity due to limited surface reactivity and mass transport constraints. Herein, for the first time, we report a facile hydrothermal synthesis followed by direct annealing to produce structurally robust, ordered PtNiCo intermetallic nanowires (I-PtNiCo-NW/C) with precisely controlled diameters, eliminating the need for protective coatings or harsh post-treatment to preserve nanowire morphology. By systematically tuning the nanowire diameters (<5 nm, ∼15 nm, and ∼30 nm), we demonstrate that intermediate-diameter ordered nanowires (∼15 nm) strike an optimal balance between structural integrity and catalytic performance, and display superior ORR performance after the ordering procedure, with a specific activity of 3594 µA cm⁻², which is 2.5-fold higher than that of their disordered counterpart, and a mass activity of 1431 A g_Pt⁻¹ at 0.9 V vs. RHE. Notably, besides activity, the catalyst maintains exceptional durability, showing only a 5 mV loss in half-wave potential after 10 000 cycles. Operando X-ray absorption spectroscopy (XAS) revealed that enhanced local structure ordering and shorter Pt–Pt bond distances effectively suppress Pt–O formation during the ORR, increasing the activity and stability of the ordered phase. Hence, this study demonstrates a simple and scalable strategy for fabricating robust intermetallic nanowire catalysts with controlled diameters and provides operando evidence linking structural ordering with high ORR activity and durability, offering key design principles for future PEMFC catalyst development.

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Introduction

Proton exchange membrane fuel cells (PEMFCs) are recognized for their high efficiency and environmental compatibility, making them attractive for sustainable energy applications.^1–3 However, the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode remains a key limitation, necessitating substantial platinum (Pt) loadings to achieve the desired power output. This reliance on high Pt content hinders both cost-effectiveness and commercial scalability.^4–6 Consequently, improving the intrinsic activity of Pt-based catalysts while reducing precious metal usage remains a central objective in advancing PEMFC technology.

Alloying Pt with 3d transition metals has proven effective in enhancing ORR activity and stability while reducing Pt loading.^7–10 In particular, multi-metallic Pt-based alloys incorporating a third elemental component have demonstrated superior performance compared to their binary counterparts.¹¹ The presence of a third metal introduces additional ligand and strain effects, which collectively modulate the electronic structure of Pt and alter the 5d band vacancy, Pt–Pt bond distances, and Pt coordination environment. These changes lead to a downward shift of the d-band center, ultimately optimizing the electronic properties of Pt and significantly enhancing its catalytic activity.^12–16 Moreover, the altered local electronic environment can increase the oxidation potential of adjacent metal atoms, effectively suppressing their dissolution under electrochemical conditions and improving the long-term stability of the catalyst.¹⁷ To further enhance catalytic activity, Pt-based alloy nanocrystals have been engineered into a diverse array of morphologies, such as nanowires, nanosheets, and nanocages, to optimize surface area and expose more active sites.^18–22 Among these, nanowires have attracted particular interest due to their unique one-dimensional architecture, which offers several intrinsic advantages: enhanced electron and mass transport, improved catalyst utilization, and a high density of accessible active sites.^23,24 These features are particularly beneficial when high-index facets rich in low-coordinated atomic sites are exposed, promoting catalytic activity.²⁵ However, it is quite evident from reported literature that under harsh electrochemical conditions, transition metals are prone to spontaneous leaching, resulting in structural degradation, compositional instability, and a progressive loss of catalytic activity over time.^26,27

In contrast to disordered alloys, Pt intermetallic compounds (IMCs) feature an ordered atomic framework and precise stoichiometry, which promote stronger bonding between Pt and transition metals.^28,29 These ordered structures have been shown to improve activity and enhance stability by suppressing the dissolution of less noble metals under operating conditions due to the stronger interaction with Pt.³⁰ For instance, Gao et al. and Zhao et al. demonstrated PtCo and PtNi alloy nanoparticles with a face-centered tetragonal (fct) structure, respectively.^31,32 The fct structure exhibits significantly enhanced ORR activity and stability compared to their disordered face-centered cubic (fcc) counterparts. Thus, the strategic integration of nanowire morphology with atomic-scale intermetallic ordering creates a highly synergistic catalyst architecture, delivering both outstanding ORR activity and robust long-term stability under fuel cell operating conditions. Despite such advantages, achieving ordered IMC structures in Pt-based alloy nanowires remains challenging, as the high-temperature annealing required to induce ordering frequently causes nanowire fragmentation or particle aggregation.^23,33–36 As a result, many prior studies have focused on disordered alloy nanowires, with only very few reports successfully achieving ordered nanowire architectures. To overcome thermal instability, earlier strategies introduced high-melting-point elements or protective shells to preserve the nanowire structure during annealing.²⁷ For instance, Li et al. succeeded in synthesizing ordered Pt-based IMC nanowires by incorporating molybdenum and coating with silica, which improved thermal stability but required complex procedures involving hydrofluoric acid.³⁷ Yang et al. added high-melting-point metal Ir into PtFe nanowires and coated them with SiO₂ to maintain the morphology at high temperatures, leading to excellent activity, but the hard template etching step was still necessary.²⁷ These multistep approaches, while effective, limit scalability and practical application. Moreover, optimizing the nanowire diameter is crucial; thicker nanowires offer greater durability but compromise Pt utilization, while ultrathin ones enhance activity but suffer from structural instability under operating conditions. Therefore, there is an urgent need for a streamlined, template-free synthesis strategy capable of producing highly ordered nanowires while precisely preserving their one-dimensional morphology.

This study synthesized and systematically compared PtNiCo nanowires with three distinct diameters (<5 nm, ∼15 nm, and ∼30 nm). Ordered intermetallic PtNiCo nanowires with well-defined diameters were successfully prepared via a facile, template-free two-step approach involving solvothermal synthesis followed by thermal annealing without exotic additives or hazardous reagents. To the best of our knowledge, this was the first successful demonstration of structurally stable L1₀-ordered PtNiCo intermetallic nanowires synthesized by a hydrothermal route followed by thermal annealing with precise control over the nanowire diameter, eliminating the use of a hard template. The appropriate diameter (∼15 nm) of these nanowires effectively prevents morphological collapse during annealing, overcoming the fragmentation challenges typically observed in ultrathin 1D structures (<5 nm). Incorporating Co and Ni into the Pt lattice modulates its electronic configuration, enhancing ORR kinetics while maintaining long-term stability in acidic media. The resulting I-PtNiCo-NW/C-15 nm catalyst achieved a remarkable specific activity of 3594 µA cm⁻² and a mass activity of 1431 A g_Pt⁻¹ at 0.9 V vs. RHE, values that surpassed many previously reported Pt–M systems, maintaining high stability. We employed operando XAS at the Pt L₃-edge to elucidate the structure-activity relationship under realistic operating conditions.³⁸ This real-time structural probe revealed enhanced local ordering, reduced Pt–O interaction, and greater resistance to electrochemical degradation in the ordered catalyst.³¹ Together, these findings demonstrated that combining simplified nanowire synthesis, atomic ordering, and operando insights makes I-PtNiCo-NW/C-15 nm a highly promising ORR catalyst platform for next-generation PEMFC technologies.

Experimental

Materials

Pt(II) acetylacetonate (Pt(acac)₂, 98%), Ni(II) acetylacetonate (Ni(acac)₂, 95%), and Co(III) acetylacetonate (Co(acac)₃, 98%) were obtained from Sigma-Aldrich. Ethanol (99.5%), 2-propanol (99.7%), oleylamine (OAm, 85%), cyclohexane (99.5%), glucose, and hexadecyltrimethylammonium chloride (CTAC, 99%) were sourced from Fujifilm. The electrolyte was prepared using ultrapure water (Milli-Q, 18.2 MΩ) and ultrapure perchloric acid (HClO₄) from Kanto Chemical Co. Inc. Vulcan XC-72 carbon was supplied by Cabot Corporation, and gases such as O₂ (99.995%), N₂ (99.999%), and H₂ (5% in N₂ balance) were purchased from Kyoto Teisan Co., Ltd. A commercial Pt/C catalyst (29.1 wt%; TEC10V30E) was obtained from Tanaka Kikinzoku Kogyo Co., Ltd, Japan.

Synthesis of PtNiCo nanowires with controlled diameters

PtNiCo nanowires with an average diameter of ∼15 nm were synthesized by dissolving 0.127 mmol Pt(acac)₂, 0.032 mmol Ni(acac)₂, and 0.095 mmol Co(acac)₃ with a molar ratio of 4 : 1 : 3 in 45 mL of oleylamine (OAm), with 350 mg glucose (reducing agent) and 200 mg CTAC (surfactant). After 1 hour of ultrasonication at room temperature, the mixture was gently heated in a 60 °C water bath until a transparent solution formed. This solution was transferred into a Teflon-lined autoclave and thermally treated in a muffle furnace preheated to 200 °C for 10 hours. The product was naturally cooled, washed with ethanol/cyclohexane, and dried at room temperature. The resulting black powder was redispersed in a cyclohexane/ethanol solution containing Vulcan XC-72 carbon, sonicated in an ice bath for 2 hours, and centrifuged to yield a supported nanowire catalyst (D-PtNiCo-NW/C-15 nm, 16.1 wt% Pt).

Smaller nanowires (<5 nm, PtNiCo-NW/C-5 nm) were synthesized using the same protocol, with the addition of 15 mg W(CO)₆ as a growth modulator, co-dissolved in OAm with the metal precursors.

Larger nanowires (∼30 nm, PtNiCo-NW/C-30 nm) were obtained by modifying the heating profile: instead of using a pre-heated furnace, the autoclave was placed in a muffle furnace at room temperature and ramped to 200 °C at 3 °C min⁻¹. The reaction time, washing, and carbon loading steps remained unchanged. PtCo nanowires (PtCo-NW/C) were synthesized using a similar procedure to that of the PtNiCo nanowires, except that Ni(acac)₂ was omitted. Specifically, 0.127 mmol Pt(acac)₂ and 0.127 mmol Co(acac)₃ were dissolved in 45 mL of oleylamine, maintaining a Pt : Co molar ratio of 1 : 1. The same amounts of glucose (350 mg) and CTAC (200 mg) were added as the reducing agent and surfactant, respectively.

To induce intermetallic ordering, all nanowire samples were annealed at 550 °C for 10 hours under a 5% H₂/95% N₂ atmosphere, yielding PtNiCo-NW/C-5 nm, I-PtNiCo-NW/C-15 nm, I-PtNiCo-NW/C-30 nm and I-PtCo-NW/C, respectively.

Structural characterization

X-ray diffraction (XRD) analyses were performed using a Rigaku Ultima IV with Cu Kα radiation (λ = 1.54056 Å) at a scanning rate of 2° per minute to characterize the crystalline structure of the samples. Ex situ Pt L3-edge, Ni and Co K-edge XAS was conducted in transmission mode using a Si (111) monochromator at BL01B1 and BL14B2 of SPring-8, Japan. Surface chemical compositions were analyzed using X-ray Photoelectron Spectroscopy (XPS) on a PHI5000 VersaProbe II, utilizing monochromatic Al Kα radiation for high precision. Morphological features were evaluated via TEM (JEM-2200FS, JEOL, 200 kV), and detailed High-Resolution Transmission Electron Microscopy (HR-TEM) imaging and elemental mapping were conducted on a JEOL-JEM-ARM200F microscope equipped with an EDS detector.

Electrochemical testing

All electrochemical experiments were conducted in a three-electrode system at room temperature. A reversible hydrogen electrode (RHE, 5% H2) served as the reference electrode, a platinum mesh as the counter electrode, and a glassy carbon rotating disk electrode (RDE) (5 mm diameter) coated with the catalyst as the working electrode. The RDE was polished using alumina and rinsed thoroughly with water. Catalyst ink was prepared by dispersing 2 mg of catalyst in a 0.3 : 1.7 water/isopropanol mixture containing 20 µL of 5 wt% Nafion® solution, followed by 30 minutes of sonication. A controlled amount of ink was cast onto the RDE to achieve a Pt loading of 6.5 µg cm⁻² for the synthesized nanowire samples. For the commercial Pt/C reference, the same catalyst loading was used, the corresponding Pt mass on the electrode was 11.5 µg cm⁻².

Cleaning cyclic voltammetry (CV) was recorded in N₂-saturated 0.1 M HClO₄ over 50 cycles (0.05–1.2 V vs. RHE at 100 mV s⁻¹). The final CV profile was recorded at 50 mV s⁻¹ from 0.05 V–1.2 V vs. RHE. ORR activity was measured by linear sweep voltammetry (LSV) in O₂-saturated 0.1 M HClO₄ (0.2–1.2 V vs. RHE at 10 mV s⁻¹), with additional scans at varying RDE rotation rates (100–2500 rpm). Background correction was applied using N₂-saturated electrolyte. Koutechy–Levich analysis at 0.9 V vs. RHE was used for kinetic evaluation. Electrochemical active surface area (ECSA) was calculated from the hydrogen adsorption charge, assuming 210 µC cm⁻² for monolayer adsorption.

Durability was assessed by an accelerated degradation protocol involving 10 000 potential cycles between 0.60 V and 1.00 V vs. RHE, with 3-second holds at each potential. Post accelerated degradation test (ADT) LSVs were used to evaluate stability losses.

Operando X-ray absorption spectroscopy (XAS)

Operando XAS measurements of the Pt L₃-edge were carried out at the BL36XU beamline (SPring-8, Japan) in transmission mode using a Si (111) monochromator and a Ru mirror to achieve a monochromatic beam. For high-energy resolution fluorescence detection (HERFD)-XAS at the Co and Ni K-edge, measurements were conducted at the BL39XU beamline. X-ray emission spectroscopy (XES) was utilized to monochromatize the emitted fluorescent X-rays before data acquisition, with Si (220) monochromators and a 4.5 mrad total-reflection Rh mirror used to collimate and monochromate the beam. The operando cell was fabricated from PEEK and included an RHE reference (electrolyte-filled with 0.1 M HClO₄), a platinum mesh counter electrode, and a catalyst-coated glassy carbon RDE.³⁹ Gas introduction was managed via a PTFE inlet and check valve to maintain pressure. The electrode was preconditioned by 50 CV cycles under RDE-like conditions to establish a stable surface state before data collection. X-rays passed through a 25 µm Kapton® film window, and data analysis was performed using the ATHENA and ARTEMIS packages in the IFEFFIT suite.

Results and discussion

Morphology and crystallinity of nanowires

In a typical synthesis, PtNiCo alloy nanowires with three distinct diameters (<5 nm, ∼15 nm, and ∼30 nm) were successfully prepared using a hydrothermal method, which provided a stable temperature and pressure environment for the concurrent reduction of Pt(acac)₂, Ni(acac)₂, and Co(acac)₃ in oleylamine (OAm). Glucose served as the reducing agent, while the concentration of CTAC was carefully adjusted to control the nanowire length and diameter as shown in Fig. S1.⁴⁰ The growth of PtNiCo nanowires is proposed to occur via CTAC-directed anisotropic assembly in OAm, where Pt is reduced first and Co/Ni incorporation follows more slowly, as shown in the scheme in Fig. S2.^41,42 For the formation of sub-5 nm nanowires, carbonyl compounds act as structure-directing agents by releasing CO ligands that strongly adsorb on Pt atoms and promote anisotropic growth. This CO-mediated effect is the key factor for producing ultrathin nanowires, whereas the specific metal carbonyl (e.g., Mo or W) plays a secondary role. Additional analysis, such as XPS survey spectrum (Fig. S3a) and the EDS elemental mapping results (Fig. S3b and c), confirmed the absence of any W signal, indicating that W is not incorporated into the final product. In contrast, in this work, we focus on slightly thicker nanowires (∼15 nm), which are more thermally stable during the high-temperature ordering process.^43–45 Under optimized reaction conditions, including heating ramp and CTAC concentration (Fig. S4a and Table S1), uniform alloy nanowires with controlled diameters were obtained, as confirmed by Transmission Electron Microscopy (TEM) images in Fig. 1a–c, corresponding to <5 nm, ∼15 nm, and ∼30 nm nanowires, respectively. Inductively coupled plasma (ICP) analysis revealed nearly complete metal reduction, yielding final atomic ratios of Pt₅₄Co₃₆Ni₁₀, Pt₅₀Co₃₆Ni₁₄, and Pt₄₉Co₃₄Ni₁₇ for the <5 nm, ∼15 nm, and ∼30 nm nanowires, respectively. These results demonstrate the advantage of the hydrothermal method in translating the initial precursor ratios into the final alloy composition under optimized conditions, with only minor drift arising from kinetic factors during heating, especially when compared to conventional oil-bath synthesis methods.⁴⁰ Subsequent thermal annealing at 550 °C for 10 hours under a 5% H₂/95% N₂ atmosphere successfully drove atomic interdiffusion and lattice contraction and induced the formation of the ordered L1₀ intermetallic phase in the ∼15 nm and ∼30 nm nanowires.^41,46 As shown in Fig. 1d, thin nanowires (<5 nm) experienced significant morphological degradation, including fragmentation into nanoparticles or aggregation into thicker wires, due to their poor thermal stability. In contrast, the ∼15 nm and ∼30 nm nanowires largely preserved their morphology after annealing (Fig. 1e, f, S5a and b).²⁷ Fig. S6 presents the powder X-ray diffraction (PXRD) analysis used to examine the crystal structures of PtNiCo nanowires before and after thermal treatment. As shown in Fig. S6a, the as-synthesized D-PtNiCo-NW/C-15 nm exhibited broad diffraction peaks corresponding to a face-centered cubic (fcc) structure, with peak positions located between those of pure Pt and Co. After annealing at 550 °C for 10 hours, the I-PtNiCo-NW/C-15 nm sample displayed additional reflections indexed to the (001), (110), (002), (201), (112), (220), and (202) planes, along with a noticeable shift of the (111) peak toward higher angles.⁴⁶ These features confirmed the formation of the L1₀ ordered intermetallic phase, consistent with PDF card JCPDS no. 43-1358.^46,47 PXRD patterns of nanowires with different diameters after annealing are shown in Fig. S6b and d. Notably, the ∼5 nm PtNiCo nanowires did not exhibit superlattice reflections even after 10 or 20 hours of annealing at 550 °C, suggesting that such thin nanowires were unable to undergo ordering under these thermal conditions. Previous work by Yang et al. demonstrated the formation of ordered structures at a significantly higher temperature (670 °C) for similarly thin nanowires, highlighting the need for elevated annealing temperatures and protective shells to stabilize the morphology.²⁷ These results emphasized the importance of nanowire diameter in promoting the formation of L1₀ ordering through simple annealing at moderate temperatures without requiring protective coatings. Besides, the successful formation of L1₀ ordering in I-PtCo-NW/C was confirmed by the appearance of a distinct (110) superlattice peak at 2θ = 33.4°, as shown in Fig. S6c. Furthermore, the structural parameters summarized in Tables S2 and S3 indicate that incorporation of Ni induces lattice strain relative to the binary alloy, and that ordering amplifies this effect compared with the disordered counterpart. Such lattice strain modulates the surface electronic structure of Pt sites, thereby optimizing adsorption–desorption behaviour and contributing to the enhanced catalytic activity.^48,49

Fig. 1 TEM images of the PtNiCo-NW/C catalyst before heat treatment with diameters of (a) <5 nm, (b) ∼15 nm and (c) ∼30 nm. TEM images of the PtNiCo-NW/C catalyst after heat treatment with diameters of (d) <5 nm, (e) ∼15 nm and (f) ∼30 nm.

The morphology of PtNiCo nanowires before and after intermetallic ordering (D-PtNiCo-NW/C-15 nm and I-PtNiCo-NW/C-15 nm) was further thoroughly characterized using HR-TEM and High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM). As shown in Fig. S7, the disordered D-PtNiCo-NW/C-15 nm sample exhibited an average diameter of ∼15 nm with a relatively irregular surface texture. Post-annealing, I-PtNiCo-NW/C-15 nm retained both its average diameter and the stepped surface features (Fig. S8); however, a significant decrease in nanowire length was observed, from approximately 200 nm to 50 nm (Fig. S7b and S8b), which was attributed to fragmentation induced by thermal stress during the ordering process. Importantly, nanowire length is not the dominant factor governing ORR performance. Instead, the presence and exposure of catalytically active (111) facets are more critical for enhancing activity.⁴⁰ STEM-based diameter distribution profiles (Fig. S7c and S8c) confirmed a uniform and narrow size distribution centered at around 15 nm for both pre- and post-annealed samples, indicating consistent synthesis quality. The nanowire surfaces, characterized by protrusions, step edges, and grain-boundary irregularities seen in both TEM and STEM images, may serve as additional active sites. These features could locally disrupt the interfacial water structure and facilitate desorption of oxygen intermediates, thereby promoting enhanced ORR kinetics.^50,51 A common challenge in nanowire-based ORR catalysts is maintaining morphological integrity during high-temperature annealing, which is essential for ordered phase formation. The I-PtNiCo-NW/C-15 nm sample demonstrates that increasing the nanowire diameter significantly improves thermal robustness. In contrast, thinner nanowires tend to fragment or aggregate at temperatures exceeding 400 °C (Fig. 2a–d).⁵² Further HAADF-STEM (Fig. S9a) and STEM-EDS analyses (Fig. S9b and c) reveal that the sub-5 nm PtNiCo nanowires retain a uniform fcc lattice without L1₀ superlattice reflections even after 20 hours of heat treatment, which can be attributed to insufficient atomic mobility, local compositional imbalance, and the absence of protective stabilization during annealing.^27,53–56 The 15 nm nanowires retained their structure and ordering without the need for protective shell coatings, highlighting an intrinsic advantage for practical catalyst design. Most D-PtNiCo-NW/C-15 nm nanowires exhibited a lattice spacing of 0.223 nm, corresponding to the (111) plane of the typical face-centered cubic (fcc) PtNiCo alloy structure. Following thermal annealing, a reduced lattice spacing of 0.213 nm and an additional region with a spacing of 0.266 nm were observed in high-resolution TEM images (Fig. 2a and b), corresponding to the (111) and (110) planes of the face-centered tetragonal (fct) L1₀-ordered PtNiCo phase, respectively. These results confirmed the formation of the L1₀ intermetallic structure, which is in agreement with PXRD findings (Fig. S6a). Using calibration mentioned in our previous work, the ordering degrees are approximately 18.2% for I-PtNiCo-NW/C-15 nm and 27.1% for I-PtNiCo-NW/C-30 nm (Fig. S10).⁵⁷ In addition, this also further revealed the exposure of catalytically active facets, along with steps and grain boundaries, indicating that the surface morphology was largely preserved after annealing. Atomic-scale imaging confirmed the crystal symmetry of the two phases: D-PtNiCo-NW/C-15 nm followed a cubic Fm3̄m [0 −1 1] orientation, while I-PtNiCo-NW/C-15 nm exhibited a tetragonal P4/mmm [0 1 0] structure (Fig. S11a and S12). STEM-EELS imaging revealed a disordered atomic distribution in D-PtNiCo-NW/C-15 nm, whereas a well-ordered atomic arrangement characteristic of the L1₀ phase was observed in I-PtNiCo-NW/C-15 nm (Fig. 2c, d, S11b and c). Notably, surface segregation of Ni was evident in the disordered sample, likely caused by thermodynamically driven surface segregation during alloy formation.¹⁷ EDS mapping supported this observation, showing a non-uniform surface composition in D-PtNiCo-NW/C-15 nm, with higher concentrations of Pt and Co and reduced Ni content (Fig. S13). After annealing, the elemental distribution in I-PtNiCo-NW/C-15 nm became more homogeneous. Pt atoms were found to alternate with Co/Ni atoms along specific crystallographic directions, further confirming the successful formation of the L1₀ ordered phase (Fig. S14).

Fig. 2 (a–c) HAADF-STEM images and (d) STEM-EELS mappings for I-PtNiCo-NW/C-15 nm.

Electrochemical activity

To assess the catalytic performance of the nanowire catalysts under ORR conditions, an initial electrochemical 50 cleaning CV was recorded in a N₂-saturated 0.1 M HClO₄ solution to eliminate residual surfactants and surface-bound organic species. This cleaning process facilitates the formation of a dynamically stable Pt-rich shell, enhancing surface accessibility. As shown in Fig. S15, the D-PtNiCo-NW/C-15 nm sample exhibited broader and more intense hydrogen underpotential deposition (H_upd) features compared to I-PtNiCo-NW/C-15 nm, indicating a larger number of electrochemically accessible Pt sites. The I-PtNiCo-NW/C-15 nm sample, in contrast, displayed narrower and more defined redox peaks, consistent with the formation of ordered atomic structures and exposure of well-defined low-index facets such as (111) and (110). The ECSA, derived from the H_upd region, decreased with increasing calcination temperature. D-PtNiCo-NW/C-15 nm exhibited a relatively high ECSA of 57 m² g⁻¹, whereas I-PtNiCo-NW/C-15 nm showed a lower value of 40 m² g⁻¹. This reduction in ECSA could be attributed to the coarsening of the nanowire network and partial sintering observed after high-temperature annealing, which was consistent with the shortening of nanowire length and diameter stability discussed in Fig. S7–S8. After 50 CV cycles, both samples showed good electrochemical stability, with minimal degradation in I-PtNiCo-NW/C-15 nm. Fig. S16 compares CV profiles of PtNiCo nanowires after heat treatment with varying diameters. The 5 nm nanowire sample showed the highest current response in the H_upd region due to a similar reason to D-PtNiCo-NW/C-15 nm, which did not form the ordered L1₀ structure after annealing; thus, more defects could be active sites. Meanwhile, the 30 nm sample exhibits significantly lower current and flatter features, suggesting reduced surface area and limited Pt site accessibility. These CV trends correlated well with the morphological evolution and facet development described earlier.

Furthermore, to examine the ORR activity of the catalysts, LSV was performed in O₂-saturated 0.1 M HClO₄ at 1600 rpm (Fig. 3a and S17), and the kinetic current was obtained after applying mass transport correction. The Koutecky–Levich plot confirmed a four-electron transfer pathway for the ORR, in agreement with previous reports.⁵⁸ Notably, I-PtNiCo-NW/C-15 nm exhibited a slight positive shift in half-wave potential compared to its disordered counterpart, indicating enhanced intrinsic ORR activity. As highlighted in Fig. 3b, I-PtNiCo-NW/C-15 nm achieved a specific activity of 3594 µA cm_Pt⁻², approximately 2.5 and 8.8 times higher than that of D-PtNiCo-NW/C-15 nm and commercial Pt/C 30%, respectively, along with an impressive mass activity of 1431 A g_Pt⁻¹. Despite the reduced ECSA, the specific activity (SA) at 0.9 V vs. RHE increased significantly with thermal treatment. These enhancements are strongly correlated with the formation of the L1₀ intermetallic structure, as shown by XRD in Fig. S6, and lead to strengthened Pt–Co/Ni bonding, facilitating ORR intermediate desorption.^59,60 The reproducibility of I-PtNiCo-NW/C-15 nm was also checked by synthesizing three difference batches and obtaining the XRD pattern, TEM images and electrochemical results, respectively (Fig. S18). Fig. 3c compares the ORR polarization curves of I-PtNiCo-NW/C-15 nm catalysts with varying nanowire diameters (5, 15, and 30 nm), while Fig. S19 provides the corresponding Koutecky–Levich analyses. The 15 nm nanowires exhibit the most positive half-wave potential and the highest limiting current density, indicating superior catalytic performance. As summarized in Fig. 3d, both mass and specific activities reach their maximum at this diameter, outperforming the smaller 5 nm and thicker 30 nm nanowires. The enhancement in the ORR activity of 15 nm nanowires could be due to well-defined ordering and a change in the electronic state of Pt due to the strain effect (discussed in detail in a later section). In contrast, the 5 nm sample, despite its high surface area, may suffer from increased disorder and facet loss, whereas the 30 nm sample exhibits a decreased surface-to-volume ratio and weaker ORR kinetics, as confirmed by the lower slope (Fig. S19e). These observations suggest that the 15 nm diameter struck an optimal balance between structural order, accessible active sites, and electronic structure, resulting in the best overall ORR performance. As shown in Fig. S20, I-PtNiCo-NW/C-15 nm exhibited slightly higher current density and a more positive onset potential than I-PtCo-NW/C, while achieving superior mass activity and specific activity, confirming that Ni incorporation enhances the intrinsic ORR activity without compromising electrochemical surface area.

Fig. 3 (a) ORR polarization curves and (b) comparison of mass (blue) and specific (red) activity of I-PtNiCo-NW/C-15 nm and D-PtNiCo-NW/C-15 nm compared with Pt/C 30% as the reference. (c) ORR polarization curves and (d) comparison of mass (blue) and specific (red) activity of PtNiCo-NW/C with different nanowire diameters (5 nm, 15 nm, and 30 nm) after annealing at 550 °C for 10 h. (e) ORR polarization curves of I-PtNiCo-NW/C-15 nm and D-PtNiCo-NW/C-15 nm before and after the ADT. (f) Comparison of the specific activity with recent studies.^{27,52,63,64,79–82}

Catalyst durability was further evaluated via an ADT, cycling between 0.60 V and 1.00 V vs. RHE. As shown in Fig. 3e and S21a, I-PtCo-NW/C and D-PtNiCo-NW/C-15 nm experienced a 26 mV and 24 mV negative shift in half-wave potential after 10 000 cycles, respectively, whereas I-PtNiCo-NW/C-15 nm exhibited only a ∼5 mV shift, demonstrating superior stability. Fig. S22a presents the ECSA retention of three nanowire catalysts (D-PtNiCo-NW/C-15 nm, I-PtNiCo-NW/C-15 nm, and I-PtCo-NW/C) calculated by using the H_upd region in CV curves in Fig. S21b–d, during the ADT cycling process. The I-PtNiCo-NW/C-15 nm catalyst and the PtCo-based nanowires exhibited a slight increase in ECSA possibly due to the gradual exposure of the fresh Pt surface during the beginning of the ADT.⁶¹ The ordered I-PtNiCo-NW/C-15 nm catalyst demonstrated superior retention compared to both the disordered counterpart and the PtCo-based nanowires. After 10 000 cycles, I-PtNiCo-NW/C-15 nm maintained unchanged its initial ECSA, while that of ECSA D-PtNiCo-NW/C-15 nm and I-PtCo-NW/C reduced more significantly, indicating enhanced electrochemical durability for the ternary intermetallic system. Furthermore, as evident from Fig. S22b, the I-PtNiCo-NW/C-15 nm catalyst exhibited the smallest decline (12%) in mass activity after the ADT test, followed by I-PtCo-NW/C (26%) and D-PtNiCo-NW/C-15 nm (29%). Even under extended cycling (Fig. S23), the catalyst showed exceptional stability, with merely a ∼20 mV half-wave potential shift after 30 000 cycles and nearly unchanged CV profiles. The slight early change is ascribed to Pt-skin formation that stabilizes the surface structure,⁶² while quantitative analysis (Fig. S23c) confirms 95–100% ECSA and 60–65% mass-activity retention. These results suggest that both atomic ordering and compositional tuning in the I-PtNiCo nanowires contribute to improved stability and retention of ORR performance under accelerated stress conditions. TEM images after ADT (Fig. S24) revealed the formation of hollow nanowire structures in D-PtNiCo-NW/C-15 nm, showing the instability of a sample without an ordered structure. In contrast, the ordered I-PtNiCo-NW/C-15 nm and I-PtCo-NW/C sample retained a more consistent morphology after ADT. Although partial structural degradation was observed, the nanowires remained largely intact, with surfaces showing step-edge enrichment. This behavior aligns with the Pt-skin formation mechanism, in which the dissolution and re-deposition of Pt form a protective layer that stabilizes surface active sites, confirmed by the STEM EDS line mapping and EELS mapping results of I-PtNiCo-NW/C-15 nm after ADT(Fig. S25), a phenomenon also reported by Zhuang et al.⁶³ To contextualize the performance of I-PtNiCo-NW/C-15 nm within the broader literature, Fig. 3f and Table S4 present a comparison with the reported Pt-based nanowire ORR catalysts. Notably, our catalyst surpassed many previously reported Pt–M nanowire systems in both specific and mass activity, while maintaining structural integrity over prolonged cycling, highlighting its potential for practical PEMFC applications. Owing to the superior ORR activity exhibited by PtNiCo-NW/C-15 nm, as evidenced by electrochemical characterization, subsequent investigations were centered on D-PtNiCo-NW/C-15 nm and I-PtNiCo-NW/C-15 nm. Furthermore, to investigate the higher ORR activity of the ordered phase and elucidate the effects of strain and modifications in the local electronic and geometric environment of Pt, ex situ and operando XAS analyses were performed.

Oxidation state, electronic and coordination study by XPS and ex situ and operando XAS

To assess the surface composition and oxidation states, X-ray photoelectron spectroscopy (XPS) was conducted on D-PtNiCo-NW/C-15 nm and I-PtNiCo-NW/C-15 nm (Fig. S26–S28 and Tables S5–S7). The Pt 4f spectra of I-PtNiCo-NW/C-15 nm displayed a higher ratio of metallic Pt⁰ relative to Pt²⁺ compared to D-PtNiCo-NW/C-15 nm, indicating a more metallic surface after annealing (Fig. S26). Compared with Pt 4f of D-PtNiCo-NW/C-15 nm, I-PtNiCo-NW-15 nm had a decreased binding energy, indicating a higher electron accumulation around the Pt in I-PtNiCo-NW/C-15 nm.⁶⁴ Similarly, Co 2p and Ni 2p spectra showed a notable decrease in oxidized Co³⁺ and Ni²⁺ species in I-PtNiCo-NW/C-15 nm (Fig. S27 and S28), reflecting the improved surface reduction state and alloying degree. Quantitative analysis (Table S5–S7) confirmed this trend, with the Pt⁰ fraction increasing by 4.5% and the Co⁰/Ni⁰ peaks becoming more prominent. These surface changes were consistent with a thermally induced phase transition that enhances the electronic conductivity and chemical stability of the catalyst. The enriched metallic character of I-PtNiCo-NW/C-15 nm supported its superior catalytic activity and durability under acidic ORR conditions.⁶⁵

To further investigate the local electronic states and atomic structures of ordered and disordered PtNiCo nanowires with a diameter of 15 nm, XAS was performed at the Pt L₃-edge, Ni K-edge, and Co K-edge (Fig. 4). The normalized Pt L₃-edge X-ray absorption near edge structure (XANES) spectra (Fig. 4a) showed that both D-PtNiCo-NW/C-15 nm and I-PtNiCo-NW/C-15 nm exhibited white-line intensities close to that of Pt foil, indicating the metallic state of Pt species.⁶⁶ Notably, I-PtNiCo-NW/C-15 nm displayed a slightly reduced white-line intensity, implying a more metallic character and stronger interaction with other metals after thermal ordering.⁶⁷ The Ni K-edge (Fig. 4b) and Co K-edge (Fig. 4c) XANES spectra also showed minor edge shifts toward lower energies in I-PtNiCo-NW/C-15 nm, suggesting reduced oxidation states of Ni and Co, further supporting improved alloying and electronic delocalization in the ordered structure. The results of XANES spectra were consistent with XPS results, further proving the homogenous elemental distribution. In addition, subtle peak splitting features which appeared in the Ni and Co XANES spectra after annealing were attributed to increased hybridization and electronic transitions from 1s to unoccupied 3d–4p states, reflecting the formation of an ordered intermetallic phase.⁶⁸ Further Fourier-transformed EXAFS spectra further elucidated the local coordination environments. The Pt L₃-edge FT-EXAFS (Fig. 4d) spectra showed that I-PtNiCo-NW/C-15 nm had a more intense and sharper first-shell peak compared to its disordered counterpart and a more negative shift in the Pt–Pt signal, indicating enhanced local ordering and shorter Pt–M (M = Pt, Co, Ni) bond distances, which is 0.07 Å and 0.01 Å shorter than Pt/C and D-PtNiCo-NW/C-15 nm, respectively. These results were consistent with the quantitative EXAFS fitting results shown in Fig. S29 and Table S8, and confirmed the improved structural ordering in the L1₀ intermetallic phase. Similar trends were observed for the Ni and Co K-edges (Fig. 4e and f), where the Ni–Ni signal and Co–Co signal had a positive shift in I-PtNiCo-NW/C-15 nm catalysts, implying stronger and more ordered metal–metal coordination around Ni and Co, proving the existence of lattice contraction.^52,69Fig. 4g–i present the wavelet transform (WT) analysis of the Pt L₃-edge EXAFS data. The WT maps clearly differentiate the backscattering contributions of neighboring atoms based on both radial distance and k-space resolution. Compared to D-PtNiCo-NW/C-15 nm, I-PtNiCo-NW/C-15 nm showed a more intense, concentrated and well-resolved WT maximum, reflecting a more defined Pt–Co/Ni coordination environment. The WT maxima also appeared at slightly higher k-values in the ordered sample, consistent with shorter and stronger metal–metal interactions, which should result in higher ORR activities.^70,71

Fig. 4 (a) Pt L₃-edge, (b) Ni K-edge and (c) Co K-edge XANES spectra of D-PtNiCo-NW/C-15 nm and I-PtNiCo-NW/C-15 nm, compared with metal foil and metal oxide as the reference. (d) Pt L3-edge, (e) Ni K-edge and (f) Co K-edge FT-EXAFS spectra of D-PtNiCo-NW/C-15 nm and I-PtNiCo-NW/C-15 nm, compared with metal foil and metal oxide as the reference. Wavelet transforms of Pt L₃-edge EXAFS spectra of (g) Pt foil, (h) D-PtNiCo-NW/C-15 nm and (i) I-PtNiCo-NW/C-15 nm.

To gain real-time insights into the evolution of Pt active sites during the ORR, operando XAS was conducted at the Pt L₃-edge. As shown in Fig. 5a and b, the white-line intensities of both D-PtNiCo-NW/C-15 nm and I-PtNiCo-NW/C-15 nm increased with increasing electrode potential, reflecting partial oxidation of Pt at higher anodic potentials.²³ This behavior corresponds to the dynamic redox cycling typical under ORR-relevant conditions. Notably, the white-line intensity in I-PtNiCo-NW/C-15 nm remained consistently lower than in D-PtNiCo-NW/C-15 nm across the full potential range, indicating a more metallic character and greater electron occupancy in Pt 5d orbitals.⁷² This trend aligned with earlier XANES and XPS findings, which demonstrated that intermetallic ordering via thermal annealing enhances electron delocalization and metallic conductivity due to the formation of the L1₀ phase.⁶⁵

Fig. 5 Pt L₃-edge XANES spectra of (a) D-PtNiCo-NW/C-15 nm and (b) I-PtNiCo-NW/C-15 nm. (c) Δµ of the white line peak heights for D-PtNiCo-NW/C-15 nm and I-PtNiCo-NW/C-15 nm. FT-EXAFS spectra of (d) D-PtNiCo-NW/C-15 nm and (e) I-PtNiCo-NW/C-15 nm. (f) Pt–Pt bond length from the corresponding fitting results for D-PtNiCo-NW/C-15 nm and I-PtNiCo-NW/C-15 nm.

As shown in Fig. 5c, quantitative analysis of the Pt white-line intensity revealed a more gradual increase in oxide content for I-PtNiCo-NW/C-15 nm relative to the disordered counterpart. This slower oxidation progression underscores the stabilizing effect of L1₀ intermetallic ordering, which weakened Pt–O bond strength and suppressed the accumulation of oxygenated species at elevated potentials. Supporting literature has shown that transition metals such as Co can adsorb hydroxyl intermediates (OH_ad) even under moderate conditions, accelerating the oxidation of both Co/Ni and nearby Pt atoms.^31,73 In contrast, I-PtNiCo-NW/C-15 nm benefited from its ordered framework, where minor Co/Ni dissolution during early cycling triggers the formation of a Pt-enriched shell. This self-passivating surface layer effectively shielded the internal alloy from further oxidative degradation.⁷⁴

Further insight into coordination and local structural changes was obtained from operando FT-EXAFS spectra, shown in Fig. 5d and e. The main Pt–M (M = Pt, Co, Ni) peak near 2.3 Å in I-PtNiCo-NW/C-15 nm showed a slight shift to shorter bond lengths and increased amplitude compared to that in D-PtNiCo-NW/C-15 nm. This observation was consistent with lattice compression and enhanced ordering in the L1₀ phase, as supported by complementary XRD and EXAFS fitting (Fig. S6, S29 and Table S8).⁷⁵ These structural refinements were known to increase the overlap between Pt 5d and transition metal orbitals, thereby enhancing electronic conductivity and catalytic turnover.⁷⁶ As the applied potential increased, both catalysts exhibited an emerging FT-EXAFS peak at around 1.6 Å, attributed to Pt–O interactions. However, the Pt–O signal was more prominent in D-PtNiCo-NW/C-15 nm, indicating greater susceptibility to oxidation. To quantify these potential-dependent changes of the Pt–Pt bond and Pt–M bond, EXAFS fitting was performed using fcc and fct structural models (Fig. S30, S31, Tables S9 and S10). From Fig. 5f, D-PtNiCo-NW/C-15 nm showed a decrease in Pt–Pt coordination number and bond length at around 0.5–0.6 V, which can be attributed to the selective leaching of Co/Ni and subsequent Pt surface reconstruction. In contrast, I-PtNiCo-NW/C-15 nm exhibited relatively stable Pt–Pt bond distances up to 1.1 V, highlighting its improved resistance to oxidation and lattice distortion. At 1.2 V, however, both catalysts exhibited increased bond lengths, particularly I-PtNiCo-NW/C-15 nm, reflecting the onset of oxidative instability in the Co/Ni sublattice.⁷⁷ What's more, post-ADT operando Pt L₃-edge XAS demonstrates structural stability of I-PtNiCo-NW/C-15 nm. Fig. S32a and b show that FT-EXAFS and XANES profiles before and after 10 k and 30 k cycles are essentially unchanged, indicating preserved Pt coordination and oxidation states. Operando EXAFS (Fig. S32c) retains a strong, potential-dependent Pt–M peak (∼2.3 Å) and only a weak Pt–O feature (∼1.6 Å) across 0.5–1.4 V, consistent with suppressed surface oxidation and a maintained ordered framework (also see Fig. 5d and e). Similarly, operando XANES (Fig. S32d) exhibits only a slight white-line increase without a positive edge shift, confirming limited Pt-oxide formation and preserved metallic bonding after cycling. Consistently, operando HERFD-XAS spectra at the Co and Ni K-edges for I-PtNiCo-NW/C-15 nm (Fig. S33) displayed negligible shifts in the pre-edge and main peak positions with increasing potential, indicating that Co and Ni remain largely unoxidized during the ORR. I-PtNiCo nanowires therefore catalyze the ORR via the four-electron pathway on Pt surfaces, with enhanced activity arising from a Pt-skin/ordered-core mechanism. The ordered Ni/Co sublattice shortens Pt–M bonds and downshifts the Pt d-band center, weakening *O/*OH binding toward the Sabatier optimum, while partial Ni/Co leaching exposes a Pt-rich surface with highly active (111)-like facets. This mechanistic picture is consistent with our prior PtNi nanowire study and with intermetallic Pt–M nanoparticles, where anisotropic strain optimizes Pt adsorption energetics and underpins both activity and durability.^52,78

Conclusions

In this study, we controlled the diameter of the synthesized PtNiCo intermetallic nanowires using a two-step hydrothermal method followed by a high temperature annealing process that avoids the need for protective coating or harsh chemicals. This synthesis yields nanowires with preserved one-dimensional morphology and well-defined L1₀ ordering, addressing a longstanding challenge in intermetallic nanostructure fabrication. TEM, XRD, and XAS confirmed the formation of an ordered nanowire phase, and XAS validated the significant changes in the local environment of Pt after ordering. The electrochemical results confirm that the intermediate diameter nanowire (I-PtNiCo-NW/C-15 nm) catalyst exhibits outstanding ORR activity and durability, achieving 3594 µA cm⁻² specific activity and 1431 A g_Pt⁻¹ mass activity, with minimal degradation (5 mV) after 10 000 cycles. Crucially, this work integrates state-of-the-art operando Pt L₃-edge XAS to directly correlate the catalyst structure with performance. The analysis revealed shortened Pt–M bond distances, greater metallic character, and a stable coordination environment even at applied electrochemical potential, demonstrating the vital role of intermetallic ordering in enhancing both catalytic activity and longevity. These insights reinforce the potential of engineered Pt-based intermetallic nanowires as next-generation PEMFC cathode catalysts and establish a pathway for future mechanistically guided material design.

Author contributions

Kuowei Liao: material synthesis, XAS measurement, collected the data, and carried out laboratory research. Weijie Cao: performed XAS measurement and material synthesis. Mukesh Kumar: performed analysis, validated the data, supervised the project, and wrote and revised the original draft. Neha Thakur: performed analysis and revised the original draft. Mitsuhiro Matsumoto: performed analysis and revised the original draft. Toshiki Watanabe: performed XAS measurement. Masashi Matsumoto: high resolution TEM measurements. Hideto Imai: high resolution TEM measurements. Tomoya Uruga: XAS measurements. Takuma Kaneko: XAS measurement. Ryota Sato: TEM and ICP measurements. Toshiharu Teranishi: TEM and ICP measurements. Yoshiharu Uchimoto: validated the data, supervised the project, provided resources, and revised the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary Information is available. See DOI: https://doi.org/10.1039/d5ta05902j.

Acknowledgements

This work was supported by the PEFC Project (20001199-0) and NEDO FC-Platform Project (20001310-0 and 25100665-0) commissioned by the New Energy and Industrial Technology Development Organization (NEDO). The synchrotron radiation experiments were performed at the beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal 2021B1010, 2022A1020, 2021A1665, 2021B1047, 2021B1048, 2022B1444, 2022B1031, 2022B1441, 2022B3751, 2023A1013, 2023A1044, and 2023A3751). The analysis using SPring-8 and high-resolution TEM measurements was performed under the NEDO FC Platform.

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