The strong polarization effect of lanthanide metals for efficient alkaline hydrogen evolution

Abstract

Alkaline water (H₂O) electrolysis represents a commercially viable route for green hydrogen (H₂) production, but its efficiency remains fundamentally limited by the sluggish hydrogen evolution reaction (HER) kinetics. Herein, we synthesized PtRuLaPrEu high-entropy alloy aerogels (HEAAs) via a freeze–thaw synthesis strategy as a high-performance alkaline HER catalyst. The PtRuLaPrEu HEAAs achieve an ultralow overpotential of 19.2 mV at 10 mA cm⁻², outperforming PtRu metal aerogels (MAs, 55.7 mV) and commercial Pt/C (93.1 mV), with a lower Tafel slope of 32 mV dec⁻¹. Mechanism studies reveal that the excellent alkaline HER performance of the PtRuLaPrEu HEAAs is attributed to the strong polarization effect of the lanthanide elements (La, Pr, and Eu), which induces interfacial electric fields that promote H₂O polarization, facilitating the dynamic reconstruction of the optimized hydrogen-bond networks at the catalyst–electrolyte interface, accelerating the proton transfer kinetics. This work establishes a new paradigm for synergistic electronic structure engineering and interfacial microenvironment control in advanced electrocatalyst design.

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New concepts

This work demonstrates a breakthrough in alkaline hydrogen evolution reaction (HER) catalysis through the synergistic integration of lanthanide metals (Ln: La, Pr, Eu) into PtRu-based high-entropy alloy aerogels (HEAAs). The novel PtRuLaPrEu HEAAs achieve an ultralow overpotential of 19.2 mV at 10 mA cm⁻² and a Tafel slope of 32 mV dec⁻¹. The key innovation lies in leveraging the strong polarization effect of lanthanides, which induces interfacial electric fields that dynamically reconstruct hydrogen-bond networks at the catalyst–electrolyte interface, thereby accelerating the proton transfer kinetics. This work uniquely exploits the 4f electron configuration, high charge density, and oxygen affinity of lanthanides to simultaneously modulate electronic structures and interfacial microenvironments. In situ spectroscopy reveals that Ln-induced polarization enhances the proportion of interfacial H₂O_gap species, optimizing the hydrogen-bond connectivity critical for the alkaline HER. This dual engineering strategy—combining electronic regulation with interfacial microenvironment control—provides a transformative paradigm for electrocatalyst design, emphasizing the interplay between atomic-scale coordination and mesoscale hydrogen-bond dynamics. It advances materials science by bridging the gap between intrinsic activity enhancement and mass/charge transport optimization, offering a holistic approach to tackling sluggish alkaline HER kinetics.

1. Introduction

The development of clean and renewable energy systems has become imperative to address the dual challenges of escalating global energy demands and environmental degradation caused by fossil fuel consumption.^1–3 Hydrogen energy has emerged as a particularly promising alternative, due to its high energy density and clean characteristics.^4–6 Water (H₂O) electrolysis technology stands out as a critical pathway for sustainable hydrogen production, enabling efficient energy conversion from renewable electricity to high-purity hydrogen (H₂).^7–9 Among various electrolysis approaches, alkaline H₂O electrolysis (AWE) demonstrates distinct advantages over proton exchange membrane systems, particularly in terms of catalyst stability and scalability, while avoiding the corrosion issues and precious metal dependency associated with acidic environments.^10–12 However, the slow hydrolysis kinetics in alkaline environments lead to substantially lower hydrogen evolution reaction (HER) performances than in acidic systems, presenting a fundamental challenge for advancing alkaline electrolyzer efficiency.^13–15

Although platinum (Pt)-based catalysts demonstrate optimal hydrogen adsorption (H*) strength approaching the theoretical volcano peak for the acidic HER,^16,17 their alkaline performance remains suboptimal due to the limited H₂O dissociation capability (Volmer step: H₂O + e⁻ + * → H* + OH⁻).^18–20 Recent strategies incorporating 3d transition metal additives (Ni(OH)₂,²¹ Co(OH)₂, Ru,^22,23etc.) have shown promise by synergistically enhancing the H₂O dissociation and reaction kinetics. Beyond electrocatalytic reactions, the alkaline HER also involves mass transport and charge transfer. For example, Li et al.²⁴ revealed pH-dependent electrical double layer (EDL) restructuring, where reduced hydrogen (H)-bond network connectivity under alkaline electrolytes impedes proton (H⁺) transfer compared to acidic electrolytes. Unlike the aforementioned 3d transition metals, lanthanide metals (Ln) present unique opportunities for catalytic interface engineering through their distinctive 4f electron configuration and oxygen affinity: (i) atomic-level tuning of active site coordination via lanthanide contraction effects; (ii) Lewis acid-mediated optimization of the interfacial reaction pathways through hydroxyl species adsorption; (iii) electric field-induced H₂O molecule polarization for H-bond network reconstruction.²⁵ Despite these advantages, the mechanistic understanding of Ln-element effects on both the electronic structure and interfacial microenvironment remains incomplete.

Herein, we construct a novel PtRuLaPrEu high entropy alloy aerogel (HEAA) system to elucidate Ln-mediated enhancement mechanisms in the alkaline HER. The freeze–thaw synthesized HEAAs demonstrate exceptional catalytic performance, achieving an ultralow overpotential of 19.2 mV at 10 mA cm⁻² (η₁₀), significantly surpassing PtRu metal aerogels (MAs, 55.7 mV) and commercial Pt/C (93.1 mV). Moreover, the remarkably low Tafel slope of 32 mV dec⁻¹ further confirms the accelerated reaction kinetics compared to controls (PtRu MAs, 69 mV dec⁻¹ and 20% Pt/C, 107 mV dec⁻¹). Furthermore, in situ infrared results demonstrate that compared with the PtRu MA surface, under the strong local electric field of Ln metal, H₂O induces polarization at the interface between the HEAA surface and KOH electrolyte, forming a dynamically stable H-bond network, improving the proton transfer efficiency, and accelerating the kinetics of the alkaline HER. This study provides a new paradigm for the synergistic regulation of the electronic structure interface microenvironment in the design of efficient alkaline HER catalysts.

2. Results and discussion

2.1. Synthesis and characterization of PtRuLaPrEu HEAAs and PtRu MAs

PtRuLaPrEu HEAAs, PtRu MAs and PtRuLn trimetal materials were prepared via the freeze–thaw synthesis process (Fig. 1a). Specifically, chloroplatinic (IV) acid hexahydrate (H₂PtCl₆·6H₂O), ruthenium (III) trichloride hydrate (RuCl₃·xH₂O), lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O), praseodymium chloride hydrate (PrCl₃·xH₂O), europium chloride (EuCl₃), and sodium citrate were dissolved in H₂O under stirring at 25 °C, then sodium borohydride (NaBH₄) was added under stirring for another 0.5 h, the H₂O was changed until the pH was neutral, and then the reaction solution was freeze dried for 10 h at −50 °C to obtain PtRuLaPrEu HEAAs. The scanning electron microscope (SEM, Fig. 1b) and transmission electron microscope (TEM, Fig. 1c) images of the PtRuLaPrEu HEAAs exhibit a honeycomb-like porous and three-dimensional network structure, confirming the successful preparation of HEAAs. The X-ray diffraction (XRD) pattern of the PtRuLaPrEu HEAAs in Fig. 1d shows that the diffraction peak at 40.8° belongs to the (111) crystal plane of the face centered cubic (fcc) metal. However, in the XRD pattern of the PtRu MAs in Fig. S1, the diffraction peaks belong to the hexagonal close packed (hcp) phase, and compared with Ru (Ru PDF # 06-0663), the PtRu MAs are shifted towards a lower angle, indicating lattice expansion. To definitively prove that the larger lanthanide atoms (La, Pr, and Eu) are also successfully incorporated despite this net contraction, we performed a critical control experiment. We synthesized a PtRuLa series (PtRu_2.75La_0.5, PtRu_2.75La₁, and PtRu_2.75La_1.5, Fig. S2) and analyzed their XRD patterns. As the content of the large La atom increased in the PtRuLa series, their XRD peaks progressively shift to lower angles, confirming lanthanide-induced lattice expansion. Most importantly, the diffraction peak for the PtRuLaPrEu HEAAs (40.9°, Fig. 1d) is located at a lower angle compared to PtRu_2.75La_0.5 (41.3°), PtRu_2.75La₁ (41.2°), and PtRu_2.75La_1.5 (41.0°). This unambiguous result demonstrates that the incorporation of additional lanthanides (Pr and Eu) in the HEAAs introduces an expansive strain that partially offsets the strong contraction from Ru.

Fig. 1 The synthesis and structural characterization of the PtRuLaPrEu HEAAs. (a) A schematic illustration of the freeze–thaw synthesis process, (b) an SEM image, (c) a TEM image, (d) the XRD pattern, (e) a HRTEM image, (f) HAADF-STEM-EDS elemental mapping images, and (g) the N₂ adsorption/desorption isotherm and pore size distribution (inset).

Additionally, in high-resolution transmission electron microscopy (HRTEM, Fig. 1e), the interplanar spacings of the PtRuLaPrEu HEAAs (0.194 nm) are smaller than the (200) crystal plane of fcc Pt metal (0.196 nm), consistent with the cell shrinkage observed in XRD. Notably, many grain boundaries are observed in the PtRuLaPrEu HEAAs, providing conditions for the generation of unsaturated sites to enhance its performance. Moreover, the high angle annular dark field-scanning TEM-energy dispersive spectrum (HAADF-STEM-EDS) element mapping indicates that Pt, Ru, La, Pr, and Eu elements are uniformly distributed in the PtRuLaPrEu HEAAs (Fig. 1f). According to the nitrogen adsorption/desorption curve and Brunauer–Emmett–Teller theory, the specific surface area and pore volume of the PtRuLaPrEu HEAAs are 24.2 m² g⁻¹ and 0.138 cm³ g⁻¹, respectively (Fig. 1g).

The coordination environments of Pt in the PtRuLaPrEu HEAAs and PtRu MAs were examined using X-ray absorption near-edge structure spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS). The Pt L3-edge XANES spectra for both PtRuLaPrEu HEAAs and PtRu MAs exhibit a white-line intensity like Pt foil (Fig. 2a), confirming the presence of Pt⁰ species.²⁶ The Pt L3-edge EXAFS spectra (Fig. 2b) and wavelet transformed profiles (Fig. 2c) reveal that, like Pt foil, Pt–M bonds between Pt and coordinating metal atoms are observed in both the PtPtRuLaPrEu HEAAs and PtRu MAs, but no obvious Pt–O bonds are detected. In the PtPtRuLaPrEu HEAAs, the bond lengths for Pt–Pt, Pt–Ru, and Pt–Ln (Ln = La, Pr and Eu) are 2.67, 2.65, and 2.87 Å, respectively, with coordination numbers of 4.0, 2.6, and 0.7. In contrast, the PtRu MAs exhibit Pt–Pt and Pt–Ru bond lengths of 2.73 and 2.69 Å, with coordination numbers of 1.1 and 7.4, respectively (Table S1 and Fig. S3). Furthermore, the surface electronic configuration was systematically investigated through X-ray photoelectron spectroscopy (XPS). The Pt 4f XPS spectrum of the PtRuLaPrEu HEAAs (Fig. 2d) resolves into two distinct doublets corresponding to metallic Pt⁰ (4f_7/2 at 71.3 eV) and oxidized Pt²⁺ species,^27,28 with the dominant Pt⁰ 4f_7/2 peak exhibiting a 0.7 eV negative shift compared to PtRu MAs (72.0 eV). This coordinated electronic modulation is further evidenced in the Ru 3d spectrum (Fig. 2e), where the Ru⁰ 3d_5/2 peak position shifts downward by 1.1 eV to 279.8 eV relative to the PtRu MAs. The systematic binding energy reduction (for both Pt and Ru) strongly indicates electron enrichment at the Pt/Ru active sites through intermetallic charge transfer. This phenomenon originates from the substantial electronegativity gradient between the constituent elements: Pt (2.28) and Ru (2.20) acting as electron acceptors from the more electropositive lanthanides La (1.10), Pr (1.13), and Eu (1.20). Surface Ln (La, Pr, and Eu) species mainly exist in the form of oxidized states in the PtRuLaPrEu HEAAs (Fig. 2f).^29–33 The inductively coupled plasma (ICP) results indicate that the atomic ratio of Pt : Ru : La : Pr : Eu in the PtRuLaPrEu HEAAs is 1 : 2.25 : 0.75 : 1.36 : 1.35. According to the above formula, it can be calculated that the configurational entropy of the PtRuLaPrEu HEAAs is 1.54R, indicating that the PtRuLaPrEu HEAAs are high entropy alloy materials. To theoretically verify the feasibility of forming PtRuLaPrEu high entropy alloys, we have employed the Vienna Ab initio Simulation Package (VASP) to perform all density functional theory (DFT) calculations with the generalized gradient approximation (GGA) using the Hubbard U correction (DFT+U) method (Fig. S4). The calculated average atomic formation energy is −3.58 eV per atom. The negative formation energy clearly indicates that the five-element alloy phase is thermodynamically stable relative to its pure elemental separated state. This exothermic reaction (energy reduction) strongly drives the formation of the alloy, rather than phase separation. This theoretical calculation result is highly consistent with our experimental result of successfully synthesizing a uniform high entropy alloy with a single fcc solid solution structure, which confirms the rationality of the experiment from an energy perspective.

Fig. 2 Structure and surface properties. Pt L3-edge (a) XANES spectra, (b) EXAFS Fourier, and (c) wavelet transform profiles of PtRuLaPrEu HEAAs, PtRu MAs, Pt foil, and PtO₂. (d) Pt 4f and (e) Ru 3d XPS spectra of PtRuLaPrEu HEAAs and PtRu MAs. (f) Eu, Pr, and La 3d XPS spectra of the PtRuLaPrEu HEAAs.

2.2. The alkaline HER properties of the PtRuLaPrEu HEAAs and PtRu MAs

In the subsequent phase of the study, the pre-prepared HEAA and MA ink was applied to the working electrode. Prior to the electrochemical measurements, calibration of the Hg/HgO and Ag/AgCl reference electrodes was performed (Fig. S5). The electrochemical HER experiments were executed at ambient temperature within a conventional three-electrode configuration, utilizing commercially available 20% Pt/C (Johnson–Matthey) as a reference material. The alkaline HER performance of various catalysts was assessed following 50 cycles of cyclic voltammetry (CV) activation at a scan rate of 300 mV s⁻¹ in 0.5 M H₂SO₄ (Fig. S26) and 4 cycles of CV at a scan rate of 50 mV s⁻¹ in 1 M KOH (Fig. 3a). During the forward scanning, a distinct desorption peak is observed in the potential range of 0.05 V to 0.40 V_RHE, which is due to the underpotential deposition of hydrogen (H-UPD). The H-UPD peak provides a quantitative measure of the electrochemically active surface area (ECSA) relevant to the alkaline environment. The PtRuLaPrEu HEAAs exhibit a larger ECSA (48.5 m² g⁻¹) compared to the PtRu MAs (10.6 m² g⁻¹). The HER performance was analyzed through linear sweep voltammetry (LSV), with quantitative comparisons made based on the overpotential at 10 mA cm⁻² (η₁₀), the Tafel slope, and the turnover frequency (TOF). Notably, the PtRuLaPrEu HEAAs demonstrated superior alkaline HER activity compared to both the 20% Pt/C and other synthesized catalysts (as illustrated in Fig. 3b and Fig. S7–S14). Specifically, the PtRuLaPrEu HEAAs achieved an exceptionally low overpotential of 19.2 mV at 10 mA cm⁻² (η₁₀), significantly outperforming the PtRu MAs at 55.7 mV and commercial 20 wt% Pt/C at 93.1 mV. Furthermore, the Tafel slope for the PtRuLaPrEu HEAAs was measured at 32 mV dec⁻¹, which is considerably lower than that of the PtRu MAs (69 mV dec⁻¹) and 20% Pt/C (107 mV dec⁻¹), indicating enhanced kinetics for the alkaline HER (Fig. 3c). The Tafel reaction (2H* → H₂ + 2*) was identified as the rate-controlling step for the alkaline HER on the PtRuLaPrEu HEAAs, whereas the Volmer reaction (H₂O + e⁻ + * → H* + OH⁻) or the Heyrovsky step (H* + H₂O + e⁻ → OH⁻ + H₂ + *) was determined to be the rate-determining step for the PtRu MAs and 20 wt% Pt/C.³⁴ Additionally, the number of active sites was estimated using the CO-stripping method (Fig. S15), which facilitated the calculation of the TOF. The PtRuLaPrEu HEAAs exhibited a TOF of 4.65 s⁻¹ at an overpotential of 70 mV, which is 2.9 and 2.8 times greater than that of the PtRu MAs and 20% Pt/C, respectively (Fig. 3d). Subsequently, the catalytic activity for the alkaline HER of self-made samples, such as the PtRuLaPrEu HEAAs after CV activation in different electrolytes (Fig. S6), PtRux MAs with different Pt:Ru ratios (Fig. S7), PtRuLnx (Fig. S8–S12), PtRuLaPrEu NPs (Fig. S13), and RuLaPrEu MAs (Fig. S14), are all lower than that of the PtRuLaPrEu HEAAs.

Fig. 3 Alkaline HER properties. (a) CVs, (b) LSVs, (c) Tafel slopes, (d) TOFs, and (e) a summary of the major performance metrics of the PtRuLaPrEu HEAAs, PtRu MAs, and 20% Pt/C in 1 M KOH without iR compensation. A comparison of (f) η₁₀ and Tafel slopes, and (g) TOFs between the PtRuLaPrEu HEAAs and reported catalysts for the alkaline HER in 1 M KOH. (h) The long-term electrochemical durability of the PtRuLaPrEu HEAAs for the HER in 1 M KOH.

Subsequently, an i–t curve was employed to evaluate the durability at a current density of 10 mA cm⁻² on the glassy carbon electrode. After a duration of 10 hours (Fig. S16), the i on the PtRuLaPrEu HEAAs was maintained at 83%, which is markedly superior to the performance of PtRu MAs (66%) and Pt/C (43%), thereby indicating enhanced stability of the PtRuLaPrEu HEAAs for the alkaline HER compared to the PtRu MAs and Pt/C. Fig. 3e presents radar plots illustrating the primary performance evaluation parameters for the HER, including η₁₀, TOF, initial potential, Tafel slope, TOF and current density at 70 mV overpotential for the PtRuLaPrEu HEAAs, PtRu MAs, and 20% Pt/C. The data indicate that the PtRuLaPrEu HEAAs exhibit markedly superior performance in the alkaline HER compared to both the PtRu MA and the 20 wt% Pt/C catalysts across these parameters. Furthermore, in terms of η₁₀, Tafel slope, and TOF (Fig. 3f, g and Tables S2, S3), the PtRuLaPrEu HEAAs also surpass the majority of previously documented high-entropy alloys and noble metal-based catalysts. Additionally, the long-term stability of the PtRuLaPrEu HEAAs and PtRu MAs, when coated on carbon paper, was assessed through chronoamperometry (Fig. 3h and Fig. S17). The findings revealed that after 245 hours, the potential at η₁₀ shifted from −0.9517 V_Hg/HgO to −0.9611 V_Hg/HgO, demonstrating a minimal decay rate of only 3.8 × 10⁻⁵ V h⁻¹. The results from these revised experiments demonstrate that the overall stability of the PtRuLaPrEu HEAAs (Fig. 3h) is, in fact, comparable to that of the PtRu MAs (Fig. S17). To directly address the concern of Ln leaching during operation (Fig. S18), we performed ICP analysis on the electrolyte after a prolonged stability test (25 hours at 100 mA cm⁻²). The results indicate that the leaching amounts and leaching rates are 5.1 × 10⁻⁵ mg (0.043%) for La, 4.0 × 10⁻⁵ mg (0.019%) for Pr, and 4.7 × 10⁻⁵ mg (0.020%) for Eu. These exceedingly low levels confirm that the incorporation of lanthanide elements within the HEAA structure is highly stable, and significant leaching does not occur under high-current-density HER conditions. XRD and HRTEM were conducted on the PtRuLaPrEu HEAAs post-stability testing, revealing that its (111) crystal plane retained the fcc metal structure (Fig. S19).

2.3. In situ SEIRAS spectra study for the alkaline HER of PtRuLaPrEu HEAAs and PtRu MAs

To elucidate the mechanism by which lanthanides (Ln) enhance the catalytic performance of the PtRu catalyst for the alkaline HER, the electrochemical in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) technique was employed to capture the vibrational responses of interfacial H₂O molecules during the HER process. Fig. 4a and b display the in situ SEIRAS spectra of the PtRuLaPrEu HEAAs and PtRu MAs at an alkaline electrolyte (0.1 M KOH) interface, across a potential range of −0.70 V to −1.05 V_Hg/HgO. These spectra are depicted as differential spectra, with the open circuit voltage spectra serving as the background reference. As illustrated in Fig. 4c and Fig. S20, S21, the O–H stretching peak was deconvoluted into three distinct components through Gaussian fitting, denoted by blue, orange, and green shading, respectively.²⁴ The O–H stretching in the blue region corresponds to the first layer of H₂O molecules adjacent to the electrode surface, which not only coordinates with abundant K⁺ but also interacts with the catalyst's surface atoms (represented as H₂O_K⁺). The O–H stretching in the orange region pertains to interfacial H₂O molecules that are rich in hydrogen bonds and exhibit a relatively complete hydrogen bond structure akin to that of bulk H₂O (represented as H₂O_ag). The green region represents hydrogen bonds or interfacial water molecules within the gap region, characterized by suspended OH groups (represented as H₂O_gap) due to the disruption of the hydrogen bond network caused by K⁺. Among these three OH groups, H₂O_gap in alkaline electrolytes is critical as it influences the connectivity of the hydrogen bonding networks and the hydrogen transfer capability in interfacial regions, thus playing a significant role in the performance of the alkaline HER.³⁵ Notably, the O–H stretching band of H₂O_gap on the surface of the PtRuLaPrEu HEAAs (3397 cm⁻¹, Fig. 4d) exhibited a redshift of approximately 196 cm⁻¹ compared to that of the PtRu MAs (3593 cm⁻¹), indicating a weakening of the O–H bond stretching vibration in H₂O_gap. This phenomenon can be attributed to the presence of La, Pr, and Eu species, which possess large atomic radii, high charge densities, and significant polarization effects^25–27 on the surface of the PtRuLaPrEu HEAAs, leading to the polarization of adjacent H₂O molecules and the formation of additional hydrogen bonds in H₂O_gap. Furthermore, the proportion of H₂O_gap on the surface of the PtRuLaPrEu HEAAs (9.7–33.7%) is considerably higher than that on the PtRu MAs (3.2–5.2%).

Fig. 4 In situ SEIRAS spectra at potentials from −0.70 to −1.05 V_Hg/HgO for the (a) PtRuLaPrEu HEAAs and (b) PtRu MAs. (c) Deconvolution of the O–H stretching bands for the PtRuLaPrEu HEAAs and PtRu MAs at −1.05 V_Hg/HgO. (d) Wavenumbers (k) and proportions of H₂O_gap derived from the SEIRAS. (e) A schematic diagram of the electrochemical interface of the PtRuLaPrEu HEAAs and PtRu MAs during the HER in KOH solution.

The interface electric field strength (E_field) is the ratio of surface charge density (σ) to dielectric constant (ε, E_field = σ/ε). According to the Gouy–Chapman–Stern model, σ is calculated by multiplying the double-layer capacitance (C_dl) by the electrode potential (σ = C_dl (E − E_pzc)). Therefore, the interface electric field strength of the PtRuLaPrEu HEAAs and PtRu MAs was estimated using electrochemical impedance spectroscopy (EIS, Fig. S22) and differential capacitance curves (Fig. S23) to quantify the polarization effects of Ln. After calculation, the interface E_field of the PtRuLaPrEu HEAAs is 1.33 times that of the PtRu MAs. Consequently, in comparison to the PtRu MA surface, the strong local electric field generated by the Ln metals facilitates the polarization of H₂O at the interface between the HEAA surface and the KOH electrolyte, resulting in more H₂O_gap and a stable hydrogen bond network that enhances the proton transfer efficiency and accelerates the kinetics of the alkaline HER (Fig. 4e). Furthermore, the cation effect has little effect on the catalytic performance of the PtRuLaPrEu HEAAs (Fig. S24) for the alkaline HER, indicating that the promoting effect of Ln on the hydrogen bond network offsets the damage of the cations to the hydrogen bond network under alkaline conditions. The D₂O isotope labelling experiment (Fig. S25) further demonstrates that the HER on the PtRuLaPrEu HEAAs exhibits a smaller change in Tafel slope (Δb, 16 vs. 28) and a lower kinetic isotope effect (KIE, Fig. S25c) than the PtRu MAs. These results provide direct evidence that the strong polarizing electric field generated by the Ln elements not only alters the static structure of interfacial water, but also directly optimizes the proton transfer pathway by lowering its energy barrier.

3. Conclusions

In summary, we have successfully synthesized PtRuLaPrEu HEAAs that demonstrate remarkable efficiency in the alkaline HER. The PtRuLaPrEu HEAAs exhibited outstanding catalytic activity, achieving an exceptionally low overpotential of 19.2 mV at a current density of 10 mA cm⁻², along with a Tafel slope of 32 mV dec⁻¹, significantly surpassing those of PtRu MAs (55.7 mV and 69 mV dec⁻¹) and commercial Pt/C (93.1 mV and 107 mV dec⁻¹). Furthermore, the PtRuLaPrEu HEAAs displayed impressive stability, with a potential decay rate of merely 3.8 × 10⁻⁵ V h⁻¹ over a duration of 245 hours, which can be attributed to their robust fcc structure and optimized electronic configuration. Structural characterization indicated that the incorporation of lanthanides (La, Pr, Eu) induced a phase transition from hcp to fcc, resulting in lattice contraction and an abundance of grain boundaries that enhanced the exposure of active sites. Mechanistic investigations utilizing in situ ATR-SEIRAS revealed that the lanthanides generate strong interfacial electric fields, which polarize water molecules and facilitate the reconstruction of hydrogen-bond networks. This phenomenon increases the proportion of interfacial H₂O_gap species, thereby promoting proton transfer, which is identified as the rate-limiting step. This study establishes a dual-modulation strategy that integrates electronic structure engineering with interfacial microenvironment control, providing a comprehensive framework for the design of high-performance alkaline HER catalysts.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: Experimental details and additional characterizations including additional XRD, HRTEM, FT-EXAFS fitting process diagram and theoretical simulation structures, CV, LSV, EIS, CP, I-t, and H−D exchange. See DOI: https://doi.org/10.1039/d5mh01022e.

Acknowledgements

We are especially grateful to the National Natural Science Foundation of China (22575132 and 22572098), the Natural Science Foundation of Shandong Province (ZR2023YQ014), Taishan Scholar Youth Expert Program in Shandong Province (tsqn202211122), the financial support by the Natural Science Foundation of Hebei Province (B2023201065), the financial support by Science Research Project of Hebei Education Department (BJK2024103), and the Venture Fund of Qingdao University. We thank Specreation Instruments Co., Ltd. for XAFS measurements and related analysis and beamline BL14W1 of the Shanghai Synchrotron Radiation Facility for the XAFS measurements.

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Footnote

† These authors contributed equally.

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