La Incorporated into L1₀-PtFe Nanoalloys as a Highly Active and Durable Oxygen Reduction Catalyst

Abstract

Pt–transition metal intermetallic compounds have been recognized as promising catalysts for oxygen reduction reaction (ORR). However, further enhancing the activity and durability of this kind of catalyst is still necessary. Herein, we report a novel L1₀-type PtFe intermetallic nanoalloy with the partial substitution of Fe sites by La as a highly active and stable catalyst towards ORR. This new intermetallic nanoalloy retains an ordered structure after the incorporation of La confirmed by XRD, XPS and TEM results and the ordered PtFe_0.5La_0.5 nanoparticles are embedded in porous carbon (L1₀-PtFe_0.5La_0.5@C) in very uniform particle size of around 2 nm. This L1₀-PtFe_0.5La_0.5@C catalyst exhibits a half-wave potential of 933 mV, which is about 12 mV and 70 mV higher than those of L1₀-PtFe@C and commercial Pt/C catalysts, respectively. Moreover, it also achieves an enhanced mass activity of 0.79 A mg_Pt⁻¹ at 0.90 V, which outperforms the performance of commercial Pt/C (0.10 A mg_Pt⁻¹). In addition, it also shows excellent stability with only 3 mV negative shift in half-wave potential after 20k CV cycles of accelerated durability testing. This high activity and stability may be attributed to the incorporation of La in the PtFe lattice, which induces the formation of a compressively strained Pt overlayer in acidic media which not only tunes the surface strain of Pt sites but also possesses robust resistance to the dissolution of Fe and La. This work also provides a new direction for the development of Pt-based intermetallic catalysts for efficient catalysis applications.

1. Introduction

Proton exchange membrane fuel cells (PEMFCs) are expected to be a green and sustainable source of power in a future society due to their zero-emission operation [1,2]. However, the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode leads to a high overpotential, reducing PEMFC efficiency and necessitating highly active catalysts [3,4]. Furthermore, the harsh operating conditions (a strongly acidic and oxidative environment) at the cathode also require catalysts to possess high stability [5]. Therefore, developing highly durable and robust electrocatalysts remains a major obstacle for PEMFC application [6,7,8]. Platinum–transition metal alloy nanomaterials, particularly those alloying with Ni or Co, have been intensively researched for decades due to their promising ORR activity [4,9,10,11]. In Pt–transition metal alloys, the coupling between the 3d orbitals of the transition metals and the 5d orbitals of Pt modifies key parameters, including Pt’s 5d band vacancies, Pt-Pt atomic spacing, and Pt coordination number [12]. These modifications collectively cause a downward shift in the d-band center of Pt atoms away from the Fermi level [4,13]. Consequently, the electronic structure of Pt is altered, weakening the binding energy between Pt atoms and oxygen-containing species [10,14,15,16]. In contrast to the disordered atomic arrangement in solid–solution alloys, Pt-based intermetallic compounds with well-defined ordered structures, which generally include directed covalent bonds between ordered atoms, preventing the oxidation and dissolution of the transition metal under harsh acidic conditions and high potential, typically exhibit superior catalytic activity and long-term durability [7,17]. For example, Wang et al. [18] reported the ordered Pt₃Co catalyst exhibited an over 200% increase in mass activity towards ORR compared with the disordered Pt₃Co alloy and an excellent stability with minimal loss of activity after 5000 potential cycles.

In the past decade, a new kind of Pt-based alloy, Pt–rare earth metal alloys (Pt-RE alloys), has also been explored as an ORR catalyst [19,20,21,22]. For example, Hu et al. [23,24] prepared a series of Pt-RE nanoalloys (Pt₃Y, Pt₅La, Pt₅Ce, Pt₂Sm, Pt₂Gd, and Pt₃Tb) through the in situ-formed g-C₃N₄ network structure to stabilize rare earth metal atoms, and among them, Pt₂Gd alloy exhibited the best ORR catalytic activity with a specific activity of 3.9 mA cm⁻², which is 5.3 times higher than that of commercial Pt/C. Theoretical and experimental studies reveal that the ORR activity enhancement of Pt-RE alloys should originate from the formation of a compressively strained Pt overlayer [25]. Furthermore, given the exceptionally negative alloy formation energies and the formation of a protective Pt overlayer in acidic media, Pt-RE alloys present very stable alloy structures in acidic conditions [17,26,27]. Inspired by these developments, the introduction of rare earth metal atoms into intermetallic compounds might be an efficient strategy to delay the dissolution of transition metals and further tune the electronic structure of Pt atoms, thereby leading to improved activity and stability of intermetallic compounds in the acid environment of a fuel cell [28]. For example, Li et al. [29] incorporated La into the L1₂-Pt₃Co intermetallic compound with a quasi-covalent Pt-M interaction, enhancing ORR activity and durability. Therefore, further exploration of incorporating rare earth metals into other kinds of ordered Pt–transition metal nanoalloys is highly worthwhile.

In this work, we demonstrate a novel L1₀-type intermetallic compound (L1₀-PtFe_0.5La_0.5@C) with the partial substitution of Fe sites with the rare earth metal of La. The as-prepared L1₀-PtFe_0.5La_0.5@C sample maintains ordered features after La incorporation. The obtained L1₀-PtFe_0.5La_0.5@C catalyst exhibits excellent ORR catalytic activity with a half-wave potential of 933 mV and a mass activity and specific activity of 1.28 and 1.32 times higher than those of the L1₀-PtFe@C catalyst, respectively. In addition, it also delivers a very stable performance in mass activity retention (0.70 A mg_Pt⁻¹) after 20k cycles of accelerated durability tests.

2. Results and Discussion

2.1. Synthesis and Characterization of L1₀-PtFe_0.5La_0.5@C Catalyst

As shown in Figure 1, a sonication assistant incipient wetness impregnation method including the thermal reduction process was employed to synthesize La-incorporated L1₀-PtFe nanoalloys embedded in porous carbon (L1₀-PtFe_0.5La_0.5@C) [30,31]. Typically, a certain amount of H₂PtCl₆·6H₂O, Fe(NO₃)₂·9H₂O, La(NO₃)₃·6H₂O and cyanamide was impregnated in Ketjenblack EC 600JD (KB600) and then annealed in a H₂-Ar atmosphere (5% H₂) with a program of 700 °C for 2 h and 900 °C for 2 h, and subsequently cooled down to 600 °C with a ramp rate of 1 °C min⁻¹ to obtain the L1₀-PtFe_0.5La_0.5@C sample. The surface pores with a pore size of 3–5 nm in KB600 act as the nanoreactor and can restrict the growth and aggregation of PtFe_0.5La_0.5@C nanoparticles under high temperature which is reported in our previous works [32,33]. In addition, the high-temperature annealing and slow-rate cooling to 600 °C processes are beneficial to the formation of ordered structure [34]. As comparisons, the PtFe@C and PtFe_0.5@C samples were also prepared by the same method without adding La(NO₃)₃·6H₂O or the different ratios of Pt and Fe precursors, respectively. The Pt weight percentages for the L1₀-PtFe_0.5La_0.5@C, PtFe@C and PtFe_0.5@C samples were 15.37 wt%, 15.19 wt% and 16.37 wt%, respectively, determined by inductively coupled plasma–optical emission spectroscopy (ICP-OES) (Tables S1–S3) and the molar ratios of Pt, Fe and La are consistent with their feeding ratios.

X-ray diffraction (XRD) analysis was firstly conducted to confirm the ordered structure of PtFe_0.5La_0.5 nanoparticles in the L1₀-PtFe_0.5La_0.5@C sample. Figure 2 shows the X-ray diffraction (XRD) patterns of L1₀-PtFe_0.5La_0.5@C sample with the L1₀-PtFe@C sample as a comparison. The superlattice-related peaks located at 24.1° and 33.0°, corresponding to the (001) and (100) facets of L1₀-PtFe (PDF card No. 26-1139), respectively, were observed in XRD patterns for the both L1₀-PtFe_0.5La_0.5@C and L1₀-PtFe@C samples, indicating the maintenance of an ordered structure after La substitution [35]. In addition, a broader peak around 24° can be assigned to the (002) plane of carbon which is overlapped with the superlattice peak. Moreover, it can be clearly observed that the peak around 41.1° corresponding to the (111) facet of L1₀-PtFe in the L1₀-PtFe_0.5La_0.5@C sample shifted to a lower angel compared to the pure L1₀-PtFe@C sample. That may be associated with the larger atomic radius of La compared to Fe and this result suggests the success of La substitution in Fe sites [36]. In addition, the broader diffraction peaks of the L1₀-PtFe_0.5La_0.5@C sample, such as the peak associated with the (111) facet, compared to the L1₀-PtFe@C sample, may reveal the smaller particle size or crystal distortion after La substitution [37]. The crystallite sizes for these samples calculated by the Scherrer equation based on the (111) facet-associated peaks were listed in Table S4, indicating the smaller particle size of the L1₀-PtFe_0.5La_0.5@C sample compared to the L1₀-PtFe@C and PtFe_0.5@C samples. In addition, as shown in Figure S1, the PtFe_0.5@C showed typical diffraction peaks associated with PtFe alloys with diffraction peaks shifting to a high degree compared to pure Pt [38], which further confirms the incorporation of La into the L1₀-PtFe lattice for the L1₀-PtFe_0.5La_0.5@C sample. In addition, although the L1₀-PtFe_0.5La_0.5@C sample still presented the characteristic features of an L1₀-type ordered structure after the incorporation of La, we need to point out that its ordering degree may be different to the L1₀-PtFe@C sample. Therefore, the tuning of ordering degree of this kind of rare earth metal-incorporated Pt–transition metal ordered nanoalloy needs to receive more attention in the future.

The chemical state of L1₀-PtFe_0.5La_0.5 nanoparticles in the L1₀-PtFe_0.5La_0.5@C sample was further analyzed using X-ray photoelectron spectroscopy (XPS). As shown in Figure 3a and Figure S2, the XPS survey spectrum shows that the L1₀-PtFe_0.5La_0.5@C possessed the characteristic Pt, Fe, La and C peaks. Figure 3b and Table S5 show that the Pt⁰ 4f binding energies of L1₀-PtFe_0.5La_0.5@C (71.2/74.5 eV) have a negative shift compared to L1₀-PtFe@C (71.4/74.7 eV) (Figure S3) and PtFe_0.5@C (71.5/74.8 eV) (Figure S4), indicating that more electrons were transferred to Pt [39]. Considering that the electronegativity of La (1.1) [40] is smaller than that of Fe (1.83) [41] and Pt (2.28) [40], the extra electrons transferred to Pt may be contributed by La, and that also indicates that La was successfully embedded in the PtFe lattice. Moreover, the appearance of the La⁰ 3d signal also suggests the existence of La incorporation into the L1₀-PtFe_0.5La_0.5@C sample (Figure 3c). By the way, compared to L1₀-PtFe@C and PtFe_0.5@C samples, the higher portion of Pt⁰ observed in the L1₀-PtFe_0.5La_0.5@C sample may predict better ORR activity [42]. In addition, the existence of La³⁺ [43], Fe²⁺ and Fe³⁺ species [44] in L1₀-PtFe_0.5La_0.5@C, L1₀-PtFe@C and PtFe_0.5@C samples may be caused by the surface oxidation in the atmosphere [45]. In addition, the surface composition of L1₀-PtFe_0.5La_0.5@C, L1₀-PtFe@C and PtFe_0.5@C samples measured by XPS is shown as Table S6. The similar surface compositions observed compared to bulk compositions (ICP results) may be associated with the L1-type ordered structure.

Transmission electron microscopy (TEM) and high-angle annular dark field scanning electron microscopy (HAADF-STEM) techniques was then used to analyze the morphology of the L1₀-PtFe_0.5La_0.5@C sample (Figure 4). The TEM (Figure 4a,b) and HAADF-STEM (Figure 4c) images show that the as-synthesized L1₀-tFe_0.5La_0.5@C displayed uniformly distributed nanoparticles embedded in porous carbon with the average nanoparticle size of around 2–3 nm. In addition, L1₀-PtFe@C and PtFe_0.5@C samples showed similar morphologies (Figures S5–S8). Moreover, as shown in Figure 4b and Figure S9, the interplanar spacing of 0.223 nm was observed in the high-resolution TEM (HRTEM) image of the L1₀-PtFe_0.5La_0.5@C sample, which may be indexed to the (111) planes of L1₀-PtFe. It needs to be pointed out that this value is a little bigger than that of the L1₀-PtFe@C sample (Figures S7b and S8) which shows the standard value (0.220 nm for (111) facet). That result is in consistent with the XRD results which further confirms the La incorporation into the PtFe lattice [28]. Furthermore, elemental mapping based on the corresponding HAADF image (Figure 4d–g) demonstrates the homogeneous distribution of Pt, Fe, and La, indicating the uniform formation of La-incorporated PtFe nanoparticles into the L1₀-PtFe_0.5La_0.5@C catalyst. In addition, surface compositions of these four samples from EDS elemental mapping are summarized as Table S7 and are similar to the results obtained by XPS.

2.2. ORR Performance of L1₀-PtFe_0.5La_0.5@C Catalyst

To investigate the ORR performance of the L1₀-PtFe_0.5La_0.5@C catalyst, the cyclic voltammogram (CV) curve was firstly recorded in Ar-saturated 0.1 M HClO₄ solution with a scan rate of 20 mV s⁻¹. The CV curves of L1₀-PtFe@C, PtFe_0.5@C and commercial Pt/C catalysts were also tested as references. As shown in Figure 5a, a higher peak potential associated with the surface oxide reduction of Pt for L1₀-PtFe_0.5La_0.5@C was observed, relative to that of L1₀-PtFe@C, disordered PtFe_0.5@C and commercial Pt/C, demonstrating the weakened Pt-OH_ads interaction, which suggests an improved ORR kinetics over the L1₀-PtFe_0.5La_0.5 surface [33]. And then, the electrochemical surface area (ECSA) can be further calculated through the hydrogen desorption peak area in the CV curves, and the L1₀-PtFe_0.5La_0.5@C sample showed an ECSA of 48.3 m² g_Pt⁻¹, which is a little higher than those of L1₀-PtFe@C (42.5 m² g_Pt⁻¹) and PtFe_0.5@C (39.7 m² g_Pt⁻¹) catalysts (Table S8). In addition, these values are a little lower than that of Pt/C (65.0 m² g_Pt⁻¹) which may be associated with the alloying of transition metals which suppresses H adsorption [46]. Considering the H_upd-based ECSA is usually underestimated due to H adsorption suppression on multimetallic surfaces, the ECSAs of these samples were also determined by the CO stripping method (Figure S10) and the L1₀-PtFe_0.5La_0.5@C sample displayed a comparable value with Pt/C which may be related to the similar particle size (Table S4). In addition, the L1₀-PtFe_0.5La_0.5@C sample showed the lowest CO oxidation peak potential (826 mV) among these samples, suggesting its good capability for CO tolerance [47].

The ORR polarization curve of the L1₀-PtFe_0.5La_0.5@C catalyst was then measured in an O₂-saturated 0.1 M HClO₄ solution with a scan rate of 10 mV s⁻¹ under the electrode rotation of 1600 rpm. As shown in Figure 5b, the half-wave potential (E_1/2) of the L1₀-PtFe_0.5La_0.5@C catalyst was about 933 mV, which is much more positive than that of L1₀-PtFe@C (921 mV) and PtFe_0.5@C (913 mV) and also 73 mV higher than that of the commercial Pt/C catalyst under the same condition. Figure 5c shows the Tafel slopes of the L1₀-PtFe_0.5La_0.5@C catalyst at the high-potential region from 1.0 to 0.9 V with the comparisons of L1₀-PtFe@C, PtFe_0.5@C and commercial Pt/C catalysts. The Tafel slope of the L1₀-PtFe_0.5La_0.5@C catalyst was fitted as 60 mV dec⁻¹, very close to L1₀-PtFe@C (66 mV dec⁻¹), PtFe_0.5@C (65 mV dec⁻¹) and the commercial Pt/C catalyst (72 mV dec⁻¹), which reveals a similar ORR pathway and rate-determined step over their Pt sites [48]. Furthermore, a slightly lower Tafel slope for the L1₀-PtFe_0.5La_0.5@C sample, compared to the rest of the samples, may indicate its enhanced reaction kinetics towards ORR. The kinetic mass activity (MA) and specific activity (SA) were also calculated by normalizing the kinetic current densities of catalysts at 0.9 V vs. RHE to the Pt loading and ECSA, respectively, with the Koutecky–Levich equation. As shown in Figure 5d and Table S8, the L1₀-PtFe_0.5La_0.5@C catalyst demonstrated a mass activity of 0.79 A mg_Pt⁻¹, which outperforms the L1₀-PtFe@C (0.62 A mg_Pt⁻¹) and the disordered PtFe_0.5@C (0.45 A mg_Pt⁻¹) samples, and reveals nearly eight-fold improvement over the commercial Pt/C catalyst (0.10 A mg_Pt⁻¹). Moreover, the specific activity of PtFe_0.5La_0.5@C was 1.63 mA cm⁻², which is also higher than those for L1₀-PtFe@C (1.24 mA cm⁻²), disordered PtFe_0.5@C (1.15 mA cm⁻²) and commercial Pt/C (0.16 mA cm⁻²) catalysts. These findings reveal that the substitution of Fe sites by La atoms in PtFe may effectively induce the formation of a strained Pt overlayer, which adjusts the adsorption ability of the surface Pt site to oxygen-containing intermediates, thus enhancing the ORR performance of L1₀-PtFe_0.5La_0.5@C [20].

Additionally, the durability of the catalysts was also evaluated through an accelerated stability test (ADT) by the potential cycling between 0.6 and 1.0 V for up to 20k cycles in air-saturated 0.1 M HClO₄ solution. Figure 6a shows the CV curves of L1₀-PtFe_0.5La_0.5@C before and after 20k CV cycles of ADT and the change in the ECSA of the L1₀-PtFe_0.5La_0.5@C sample was only 6.4% after 20k potential cycles, which is significantly smaller than that of 36.1% for the commercial Pt/C catalyst (Figure 6b), suggesting the excellent retention of Pt active sites for the L1₀-PtFe_0.5La_0.5@C sample. After 20k cycles of ADT, the half-wave potential of L1₀-PtFe_0.5La_0.5@C was lost by only 3 mV (Figure 6c), while commercial Pt/C experienced 31 mV degradation (Figure 6d). Moreover, the MA of PtFe_0.5La_0.5@C is 88.6% maintained, demonstrating remarkable stability (Figure 6e). In comparison, the mass activity of the Pt/C was lost by about 50%. The structural stability of the L1₀-PtFe_0.5La_0.5@C catalyst after 20k of potential cycles was further examined by microscopic imaging. Figure 7 and Figure S11 present typical TEM and HAADF-STEM images of the L1₀-PtFe_0.5La_0.5@C catalyst at various magnifications after the ADT. The images show minimal change in particle size, revealing high resistance to agglomeration and sintering which may be related to the features of the embedded structure [33]. Furthermore, the negligible change in lattice spacing (Figure 7c) indicates excellent structural integrity of L1₀-PtFe_0.5La_0.5 nanoparticles, pointing to robust resistance to the dissolution of Fe and La in acidic environments, ensuring long-term stability, which is in agreement with its electrochemical results above. In addition, a slight decrease in Fe/La contents against Pt after ADT may be associated with the surface decomposition of Fe/La and the formation of Pt layers during this process. That superior durability may be due to the La occupation in the PtFe generating a strong bonding characteristic and a protective Pt overlayer which is similar to Pt-RE alloys [39].

3. Experimental Section

3.1. Chemicals

Lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O, 99.9%), iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O, 99.99%) and anhydrous ethanol were bought from Titan Technology Co., Ltd. (Shanghai, China). 2-Propanol, cyanamide, perchloric acid (70%) and chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O, 37.50% Pt basis) were purchased from Sigma-Aldrich (Saint Louis, MO, USA). Ketjenblack EC-600JD was purchased from Akzo Nobel (Amsterdam, The Netherlands). Commercial Pt/C (46.6 wt% Pt, Pt particle size: 2.6 nm) was purchased from Tanaka Kikinzoku International Inc. (Tokyo, Japan) All chemicals were used as received without further purification.

3.2. Synthesis of La-Incorporated L1₀-PtFe Intermetallic Compound (L1₀-PtFe_0.5La_0.5@C)

La-incorporated L1₀-PtFe intermetallic compound (L1₀-PtFe_0.5La_0.5@C) was synthesized by a method combining the wetness impregnation and thermal annealing processes. Typically, the anhydrous ethanol solution containing 50 umol H₂PtCl₆·6H₂O, 25 umol Fe(NO₃)₃·9H₂O, 25 umol La(NO₃)₃·6H₂O and 80 mg cyanamide was added into a vial with 50 mg Ketjenblack EC-600JD and further sonicated for 2 h. After drying at 70 °C for 12 h, 300 mg cyanamide was further added through the same method as above. And then, the dried powder was transferred into a tube furnace for further annealing under a 5% H₂/95% Ar atmosphere at 700 °C for 2 h and 900 °C for 2 h (ramp rate: 10 °C/min), and subsequently cooled down to 600 °C with a ramp rate of 1 °C min⁻¹, followed with cooling naturally to room temperature to obtain the L1₀-PtFe_0.5La_0.5@C catalyst. As comparisons, L1₀-PtFe@C and disordered PtFe_0.5@C were also prepared by the same method except for the absence of La(NO₃)₃·6H₂O or the different ratios of Pt and Fe precursors.

3.3. Physical Characterizations

X-ray diffraction (XRD) measurement was conducted on an X-ray diffractometer (Rigaku, SmartLab-9 kW) (Tokyo, Japan,) under Cu Kα radiation (λ = 0.15406 nm) at a scan rate of 3° min⁻¹. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field scanning transmission electron microscopy (HADDF-STEM) were performed using the Thermo Fisher Talos F200X G2 (Waltham, MA, USA). The chemical composition of catalysts was determined by inductively coupled plasma atomic emission spectroscopy (710-ES, Varian, ICP-OES) (Palo Alto, CA, USA). X-ray photoelectron spectroscopy (XPS) investigations were carried out using a Thermo Scientific K-Alpha XPS spectrometer (Wilmington, DE, USA).

3.4. Electrochemical Measurements

The electrochemical measurements were conducted on a CHI 760E electrochemical workstation (Shanghai CH Instruments) (Shanghai, China) using a three-electrode test cell. The working electrode is a glassy carbon electrode (GCE) (d = 5 mm, the geometric electrode area is 0.196 cm²) coating with catalyst ink, while Pt foil and Ag/AgCl electrode (with saturated KCl solution) served as the counter electrode and reference electrode, respectively. For the electrolyte, 0.1 M HClO₄ solution was used. The test potentials (vs. Ag/AgCl) were all converted to the reversible hydrogen electrode potential scale according to the equation (E_RHE = E_Ag/AgCl + E_calibration). The Ag/AgCl electrode was calibrated by testing the open circuit potential against a reversible hydrogen electrode (Figure S12) for every use. The catalyst ink was prepared by dissolving a certain amount of catalyst into the mixture of ultrapure water/iso-propanol (v/v = 3:1) containing 5 wt% Nafion (the solvent and Nafion volume ratio was controlled as 97:3) and sonicating for 30 min in an ice bath. The working electrode was obtained by dropping 20 μL catalyst ink onto the GCE and drying at room temperature. The catalyst loading was controlled at 20 μg_Pt cm⁻². Cyclic voltammetry (CV) measurements were firstly conducted in Ar-saturated 0.1 M HClO₄ solution between 0.05 V and 1.05 V with a scan rate of 50 mV s⁻¹ until a stable CV curve was obtained. After the catalyst was activated, the CV curve was measured from 0.05 V to 1.05 V with a scan rate of 20 mV s⁻¹ in Ar-saturated 0.1 M HClO₄ solution. The ECSA was determined by integrating the hydrogen underpotential deposition area (H_upd) in the CV curve by taking a value of 210 μC cm⁻² for the adsorption of one monolayer of hydrogen. In addition, the ECSA of the catalyst was also determined by conducting CO stripping voltammetry. The working electrode was held at 0.05 V while purging CO through the electrolyte followed by saturating with Ar. The ECSA was calculated from the CO oxidation charge recorded at a scan rate of 50 mV s⁻¹ using a conversion coefficient of 390 μC cm⁻². The ORR polarization curve was collected in an O₂-saturated 0.1 M HClO₄ solution with a scan rate of 10 mV s⁻¹ with the electrode rotation rate of 1600 rpm. The kinetic current (I_k) can be calculated using the Koutecky–Levich equation which is expressed by ${\displaystyle \frac{1}{I}}={\displaystyle \frac{1}{I_k}}+{\displaystyle \frac{1}{I_d}}$, where I is the measured current and I_d is the diffusion limited current. The stability of catalyst was investigated in air-saturated 0.1 M HClO₄ solution by accelerated durability testing (ADT) with CV cycling between 0.6 V and 1.0 V with a scan rate of 50 mV s⁻¹ up to 20k cycles.

4. Conclusions

In summary, a rare earth metal, La, was introduced to partially substitute the Fe sites of L1₀ PtFe ordered nanoalloy and formed a novel L1₀-type intermetallic nanoalloy (L1₀-PtFe_0.5La_0.5@C) keeping the ordered structure features after La incorporation. The L1₀-PtFe_0.5La_0.5@C sample with a particle size around 2–3 nm embedded in porous carbon showed an excellent ORR performance with the half-wave potential of 933 mV, an intrinsic specific activity of 1.63 mA cm⁻² and a mass activity of 0.79 A mg_Pt⁻¹ at 0.90 V in acidic media (0.1 M HClO₄ electrolyte). All these key metrics exceeded those of the L1₀-PtFe@C and commercial Pt/C catalysts. Moreover, it also exhibited very stable ORR activity after 20k CV cycles ADT. The strong bonding character and the formation of a strained, protective Pt overlayer after La incorporation may be the reasons for its enhanced activity and durability. In addition, this study also expands the scope of intermetallic compounds and provides a new direction to develop Pt-based intermetallic catalysts.