Interfacial engineering of a nanofibrous Ru/Cr₂O₃ heterojunction for efficient alkaline/acid-universal hydrogen evolution at the ampere level

Abstract

Interfacial engineering of a heterostructured electrocatalyst is an efficient way to boost hydrogen production, yet it still remains a challenging task to achieve superior performance at ampere-grade current density. Herein, a nanofibrous Ru/Cr₂O₃ heterojunction is prepared for alkaline/acid-universal hydrogen evolution. Theoretical calculations reveal that the introduction of Cr₂O₃ modulates the electronic structure of Ru, which is beneficial for *H desorption, resulting in a superior HER performance at ampere-grade current density. Accordingly, the resultant Ru/Cr₂O₃ catalyst presents an ultra-low overpotential of only 88 mV and a long-term stability of 300 h at 1 A cm⁻² in 1 M KOH. Furthermore, it also exhibits a small overpotential of 112 mV and steadily operates for 300 h at 1 A cm⁻² in 0.5 M H₂SO₄. The catalyst outperforms not only the benchmark Pt/C catalyst but also most of the top-performing catalysts reported to date. This study offers a novel conceptual approach for designing highly efficient electrocatalysts that hold significant promise for industrial-scale water splitting applications.

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Introduction

Today, environmental pollution and the energy crisis represent the most significant challenges confronting humanity.¹ Hydrogen (H₂) has the potential to emerge as a key sustainable and green energy source, given its combustion without emitting greenhouse gases, renewable nature and high gravimetric energy density.^2,3 As a vital half-reaction in water electrolysis, the hydrogen evolution reaction (HER) is widely recognized as an efficient, viable and promising route for the production of green H₂ due to its economic, clean and environmentally friendly attributes.^4–6 However, the current limitations of low efficiency, high overpotential, and the excessive reliance on the precious metal platinum (Pt) lead to prohibitively high costs for H₂ production. Additionally, many of the high-efficiency non-Pt electrocatalysts developed to date are primarily suited for low-current-density applications (less than 0.1 A cm⁻²), rather than the high-current-density environments (1 A cm⁻² or greater) that are essential for industrial-scale H₂ production.^7–9 Consequently, it is both challenging and crucial to design and fabricate efficient and cost-effective non-Pt HER electrocatalysts that can meet the demands of industrial H₂ production.^10–12

As a highly promising direction in electrocatalyst research, ruthenium (Ru)-based catalysts have garnered substantial interest due to their cost-effectiveness compared to Pt and iridium (Ir), as well as their similar binding energies to Pt–H bonds.^13–15 The electronic structure of a catalyst is pivotal in determining the reaction energy barrier during catalytic processes. To modulate this structure, a variety of strategies such as vacancy engineering, doping, and interface engineering have been employed.^16–20 For instance, a hierarchical MoS₂/MoP nanorod heterojunction is constructed through a sulfuration and phosphorization process using MoO₃ nanorods as the sacrificial template for the HER.²¹ The distinct hierarchical architecture of the MoS₂/MoP heterojunction facilitates electron transfer from S to P, resulting in an optimized H adsorption energy to promote the HER properties. Furthermore, the hierarchical structure provides abundant active sites and facilitates mass transfer, enabling easier H₂ release. While the creation of heterostructures significantly boosts the HER performance, it remains a formidable challenge to attain a universally high-performance catalyst that operates efficiently under both alkaline and acidic conditions at industrial-scale current densities for water splitting applications.

In this study, we successfully delineate a reliable electrospinning-calcination-thermal reduction process for constructing robust ruthenium–chromium trioxide (Ru/Cr₂O₃) heterojunction nanofibers (HNFs) as a highly efficient HER electrocatalyst for ampere-grade-current-density water electrolysis in alkaline and acidic media. The choice of Cr₂O₃ is due to its ability to modulate the electronic structure of metals and desirable chemical stability.^22,23 In addition, the nanofibrous structure offers efficient mass transfer, which is an ideal structural system for electrocatalysis.^24–26 The resulting Ru/Cr₂O₃ HNF catalyst showcases exceptional HER activity with an ultra-low overpotential of only 88 mV and a long-term stability of 300 h at 1 A cm⁻² in 1 M KOH. In addition, it also exhibits extremely high HER efficiency and outstanding durability in 0.5 M H₂SO₄. The excellent alkaline/acid-universal HER performances surpass that of commercial Pt/C and most documented electrocatalysts. Density functional theory (DFT) calculations reveal that the interfacial engineering in Ru/Cr₂O₃ HNFs modulates the electronic structure of Ru sites, thereby optimizing *H desorption, which yields superior HER properties in both acidic and alkaline environments.

Results and discussion

The synthetic pathway for the Ru/Cr₂O₃ HNFs is depicted in Fig. 1a, encompassing the calcination and reduction steps of the precursor nanofibers (NFs). Field emission scanning electron microscopy (FESEM) is employed to observe and scrutinize the morphological changes of NFs during the preparation process. The precursor NFs consisting of poly(vinylpyrrolidone) (PVP) and Cr³⁺/Ru³⁺ are initially fabricated via an electrospinning technique, exhibiting an average diameter of approximately 453.9 nm (Fig. S1, ESI†). Following sequential calcination and reduction treatments, the mean diameter of the obtained Ru/Cr₂O₃ HNFs is reduced to be around 143.9 nm, which is caused by the thermal decomposition of the polymer within the precursor NFs under high temperature conditions (Fig. 1b and c). Moreover, the FESEM image also presents a rough surface of Ru/Cr₂O₃ HNFs, a characteristic further corroborated by the transmission electron microscopy (TEM) image (Fig. 1d). Indeed, the Ru/Cr₂O₃ HNFs are composed of a multitude of nanoparticles. The nanofibrous architecture with a similar rough surface is also observed in Ru/Cr₂O₃-1, Ru/Cr₂O₃-2 and Ru/Cr₂O₃-4 HNFs (Fig. S2, ESI†). For comparison, single metal-component R-Cr₂O₃ NFs and Ru NFs, as well as RuCrO_x composite NFs, are also constructed, all demonstrating the nanofibrous morphologies (Fig. S3, ESI†). The high-resolution TEM (HRTEM) image in Fig. 1e clearly reveals the heterointerface between the (101) plane of Ru and the (110) plane of Cr₂O₃, corresponding to the lattice fringe spacings of 0.206 nm and 0.247 nm, respectively. The heterojunctions with Cr₂O₃ and Ru species may be the key factor in the highly efficient active HER, which will be further discussed in the following investigation. The selected area electron diffraction (SAED) pattern in Fig. 1f aligns with the presence of multiple polycrystalline rings, which arises from the (101), (110) and (112) planes of Ru, and the (110), (116) and (300) planes of Cr₂O₃. Additionally, the energy-dispersive X-ray (EDX) spectroscopy analysis confirms the presence of Ru, Cr and O elements within the Ru/Cr₂O₃ HNFs (Fig. 1g). It is noted that the elemental contaminants such as C and Cu originate from the carbon-coated copper grid for TEM sample preparation, while Si is attributed to the instrument itself. The elemental mappings of Ru/Cr₂O₃ HNFs illustrate a uniform distribution of Ru, Cr and O across the entire NF structure (Fig. 1h). Furthermore, N₂ adsorption–desorption isotherms are utilized to provide the specific surface area and pore size distribution of the Ru/Cr₂O₃ HNFs (Fig. S4, ESI†). Herein, the Brunauer–Emmett–Teller (BET) surface area of Ru/Cr₂O₃ HNFs is evaluated to be 21.26 m² g⁻¹, and the pore size distribution is mainly dominated by mesopores.

Fig. 1 (a) Schematic illustration for the synthesis of Ru/Cr₂O₃ HNFs. (b) SEM image and (c) the corresponding diameter distribution of Ru/Cr₂O₃ HNFs. (d) TEM image of a single Ru/Cr₂O₃ HNF, (e) HRTEM image of Ru/Cr₂O₃ HNFs, showing a distinct interface between Ru(101) and Cr₂O₃(110). (f) SAED pattern and (g) EDX spectrum of Ru/Cr₂O₃ HNFs. (h) HAADF-STEM image and the corresponding elemental mappings of Ru, Cr and O in Ru/Cr₂O₃ HNFs.

X-ray diffraction (XRD) analysis was performed to elucidate the phase structure of the prepared Ru/Cr₂O₃ HNFs. Fig. 2a reveals that the RuO₂ NFs, characterized by the distinctive lattice planes of JCPDS No. 43-1027, are successfully reduced to form Ru NFs (JCPDS No. 06-0663) after the in situ H₂/argon thermal reduction process. Conversely, the Cr₂O₃ NFs retain their original crystal structure, displaying the characteristic lattice planes of Cr₂O₃ (JCPDS No. 38-1479) even after H₂/argon thermal treatment, which demonstrates no reduction occurring (Fig. 2b). In Fig. 2c, the XRD pattern of the as-synthesized RuCrO_x NF sample shows the characteristic peaks of both RuO₂ and Cr₂O₃. Subsequent in situ H₂/argon reduction yields the Ru/Cr₂O₃ HNFs, where the XRD pattern indicates the persistence of Cr₂O₃ diffraction peaks, while the peaks associated with RuO₂ have completely vanished (Fig. 2c). In their place, the distinctive peaks of metallic Ru (JCPDS No. 06-0663) emerge, aligning with the HRTEM image findings previously discussed (Fig. 1e). Consistent outcomes are observed in the other comparative HNFs, with all diffraction peaks aligning with those of Ru (JCPDS No. 06-0663) and Cr₂O₃ (JCPDS No. 38-1479) (Fig. S5, ESI†). X-ray photoelectron spectroscopy (XPS) is further conducted to probe the surface chemical characteristics and electronic configuration of Ru/Cr₂O₃ HNFs. The survey spectrum presented in Fig. S6, ESI† confirms the presence of Ru, Cr, and O in Ru/Cr₂O₃ HNFs. Fig. 2d shows the high-resolution Ru 3p spectrum of Ru/Cr₂O₃ HNFs, which reveals two distinct doublets: the peaks positioned at 462.7 and 484.9 eV are ascribed to Ru⁰ 3p_3/2 and Ru⁰ 3p_1/2, respectively, while the weak peaks at 465.6 and 487.8 eV correspond to Ru^δ+ 3p_3/2 and Ru^δ+ 3p_1/2.^27,28 The oxidized Ru^δ+ species may originate from exposure to air leading to surface oxidation and their interaction with the Cr₂O₃ component.^29,30 In contrast, the Ru 3p XPS spectrum of RuCrO_x NFs shows a pair of strong peaks located at 463.6 and 485.9 eV, attributable to Ru⁴⁺ (Fig. S7, ESI†).³¹ In the Cr 2p spectrum of Ru/Cr₂O₃ HNFs (Fig. 2e), the prominent characteristic peaks observed at binding energies of 576.9 and 587.0 eV are attributed to Cr³⁺ 2p_3/2 and Cr³⁺ 2p_1/2, respectively.^22,32,33 In addition, the narrow-scan O 1s spectrum reveals two distinct components with binding energies at 530.7 and 532.1 eV (Fig. 2f), which correspond to lattice oxygen and adsorbed oxygen species, respectively.^34–36 These findings collectively provide compelling evidence that Ru/Cr₂O₃ HNFs have been successfully synthesized.

Fig. 2 XRD patterns of (a) RuO₂ NFs and Ru NFs, (b) Cr₂O₃ NFs and R-Cr₂O₃ NFs, and (c) RuCrO_x NFs and Ru/Cr₂O₃ HNFs. Typical high-resolution (d) Ru 3p, (e) Cr 2p and (f) O 1s XPS spectra of Ru/Cr₂O₃ HNFs.

Initially, the HER properties of the synthesized Ru/Cr₂O₃ HNFs are examined in 1 M KOH. Furthermore, to provide a benchmark, the HER activities of commercial 20% Pt/C and other nanofibrous catalysts prepared using varied experimental parameters are assessed for comparison. Fig. 3a and S8, ESI† present the linear sweep voltammetry (LSV) curves along with the corresponding overpotentials required to reach current densities of 0.1 and 1 A cm⁻² for Ru/Cr₂O₃ HNFs, R-Cr₂O₃ NFs, RuCrO_x NFs, Ru NFs and commercial 20% Pt/C. Ru/Cr₂O₃ HNFs exhibit an overpotential of merely 41 mV at 0.1 A cm⁻², significantly outperforming the R-Cr₂O₃ NFs with negligible HER activity, as well as RuCrO_x NFs (135 mV), Ru NFs (88 mV) and 20% Pt/C catalyst (87 mV), respectively. Notably, the prepared Ru-based catalysts exhibit more favorable HER performances compared to the R-Cr₂O₃ NF sample, which illustrates the key role of Ru species in the heterojunctions. Afterwards, the Ru/Cr₂O₃ HNF catalyst is optimized by controlling the molar ratio between Ru and Cr in the spinning solution. As illustrated in Fig. S9, ESI,† the optimized Ru/Cr₂O₃ HNFs with the Ru : Cr molar ratio of 3 : 1 demonstrate superior HER activity compared to the other comparative HNFs. Moreover, the mass activity (MA_Ru) values are calculated to evaluate the practical viability and cost-effectiveness of the catalysts. As anticipated, Ru/Cr₂O₃ HNFs boast the highest MA_Ru value of 1536.11 A g_Ru⁻¹ at the overpotential of 100 mV, much better than other control catalysts, including Ru/Cr₂O₃-1 HNFs (898.62 A g_Ru⁻¹), Ru/Cr₂O₃-2 HNFs (1220.54 A g_Ru⁻¹), Ru/Cr₂O₃-4 HNFs (812.48 A g_Ru⁻¹), Ru NFs (129.89 A g_Ru⁻¹), and RuCrO_x NFs (48.73 A g_Ru⁻¹) (Fig. S10a, ESI†). After that, turnover frequency (TOF) values are calculated to delve into the intrinsic HER activity of catalysts. The highest TOF value of 0.604 s⁻¹ at the overpotential of 100 mV is achieved by Ru/Cr₂O₃ HNFs, which is also markedly superior to other control catalysts (Fig. S10b, ESI†). It is worth noting that all the prepared HNF samples possess much higher MA_Ru and TOF values than single component Ru NFs and unreduced RuCrO_x NF catalysts, further revealing the highly efficient active sites in the heterojunctions of Ru/Cr₂O₃ during the alkaline HER.

Fig. 3 HER performance of Ru/Cr₂O₃ HNFs and other catalysts in 1 M KOH. (a) LSV curves of different catalysts including Ru/Cr₂O₃ HNFs, R-Cr₂O₃ NFs, RuCrO_x NFs, Ru NFs and 20% Pt/C. (b) Tafel plots of Ru/Cr₂O₃ HNFs, RuCrO_x NFs, Ru NFs and 20% Pt/C. (c) Comparison of overpotentials at 1 A cm⁻² with Tafel slopes in diverse HER electrocatalysts. (d) Nyquist plots and fitting curves at −1.0 V versus the Hg/HgO electrode for the HER. (e) The relationship between half of the capacitive current density and the scan rates to yield C_dl values. (f) Comparison of the overpotential at 0.1 A cm⁻², Tafel slope, R_ct, C_dl, MA and TOF of Ru/Cr₂O₃ HNFs, Ru NFs, and RuCrO_x NFs. (g) The i–t curves of Ru/Cr₂O₃ HNFs, RuCrO_x NFs and commercial Pt/C catalyst at a current density of ≈1 A cm⁻².

The Tafel slope of Ru/Cr₂O₃ HNFs (5.2 mV dec⁻¹) is also notably lower than those of RuCrO_x NFs (44.5 mV dec⁻¹), Ru NFs (11.1 mV dec⁻¹) and the commercial 20% Pt/C catalyst (15.5 mV dec⁻¹), suggesting the Volmer–Tafel mechanism as the HER pathway^37,38 and signifying the exceptional catalytic kinetics of Ru/Cr₂O₃ NFs (Fig. 3b). The ability to achieve high catalytic activity at industrial-grade current density is essential for the practical application of electrocatalysts.^39,40 Remarkably, the Ru/Cr₂O₃ HNFs require an overpotential of only 88 mV to attain a current density of 1 A cm⁻² in 1 M KOH, which is also significantly better than the other comparative catalysts (Fig. S8 and S9, ESI†). Owing to the ultra-low overpotential at ampere-grade current density and Tafel slope, Ru/Cr₂O₃ HNFs exhibit the most outstanding HER activity among the recently reported high-current-density HER catalysts in alkaline environments (Fig. 3c and Table S1, ESI†).^{6,12,14,41–49} Subsequently, to elucidate the origin of the high catalytic activity of Ru/Cr₂O₃ HNFs in alkaline environments, electrochemical impedance spectroscopy (EIS) analysis is conducted to assess the interfacial charge transfer capability. The Nyquist plots obtained at the potential of −1.0 V vs. the Hg/HgO electrode, as depicted in Fig. 3d, reveal that the charge transfer resistance (R_ct) value of Ru/Cr₂O₃ HNFs (0.67 Ω) is much lower than those of commercial Pt/C (6.77 Ω), RuCrO_x NFs (32.56 Ω), Ru NFs (4.64 Ω), and R-Cr₂O₃ NFs (10 230 Ω), indicating a substantial enhancement in charge transfer capacity for the heterostructure, which contributes to the enhanced HER efficiency. The equivalent circuit model used for fitting the EIS data is presented in Fig. S11, ESI.†

The double-layer capacitance (C_dl) values for different catalysts are determined from cyclic voltammetry (CV) curves obtained in the non-Faraday potential region, which correlates positively with the electrochemical active surface area (ECSA) (Fig. S12 and S13, ESI†).⁵⁰ The ECSA values reflect the number of active sites available for the electrocatalytic process.^51,52 The C_dl value of the Ru/Cr₂O₃ HNFs is 88.17 mF cm⁻², which is approximately 61.23, 2.98, and 2.93 times as high as that of R-Cr₂O₃ NFs, RuCrO_x NFs and Ru NFs, respectively (Fig. 3e). Evidently, the Ru/Cr₂O₃ HNFs possess a higher estimated ECSA and roughness factor (RF) values of 198.38 cm² and 2204, respectively, compared to the other comparative catalysts (Table S2, ESI†). Based on these ECSA values, the ECSA-normalized LSVs are employed to gauge their intrinsic activity (Fig. S14a, ESI†). And as expected, Ru/Cr₂O₃ HNFs show the lowest overpotential of 91 mV at a current density of 0.5 mA cm_ECSA⁻² compared to that of RuCrO_x NFs and Ru NFs (Fig. S14b, ESI†). This demonstrates that the heterostructure not only effectively exposes a great number of HER active sites, but also significantly increases the activity of each site. Fig. 3f shows the six pivotal factors for HER catalysts. It is readily apparent that the Ru/Cr₂O₃ HNFs encompass the largest area in each category compared to the RuCrO_x NFs and Ru NFs, signifying their exceptional electrocatalytic performance in the HER process.

Beyond the exploration into electrocatalytic activity, the stability test is also a crucial factor in evaluating the viability of a catalyst for real-world application. Remarkably, the Ru/Cr₂O₃ HNFs work stably at a high current density of 1 A cm⁻², retaining over 90% of the initial current density even after 300 h of continuous operation in an alkaline medium, as evidenced by the i–t test (Fig. 3g). Obviously, the stability of Ru/Cr₂O₃ HNFs is considerably better than that of RuCrO_x NFs (45% after 15 h) and commercial Pt/C catalyst (30% after 20 h). To shed light on the structural stability of Ru/Cr₂O₃ HNFs throughout the HER process, a comprehensive suite of characterization studies was conducted after the i–t testing. Fig. S15, ESI† shows the SEM image of the Ru/Cr₂O₃ HNF catalyst post HER, illustrating its morphological and structural robustness. Moreover, the XRD spectra of the catalyst exhibit no discernible phase transformation, thereby realizing the stability of the crystal structure of the Ru/Cr₂O₃ HNFs (Fig. S16a, ESI†). The surface chemical valence states of the catalyst following the HER process are further probed in greater detail using XPS spectroscopy (Fig. S16b–d, ESI†). A comparison of the Ru 3p, Cr 2p and O 1s spectra of the Ru/Cr₂O₃ HNFs before and after the HER test indicates that there are no substantial alterations, underscoring the stability of the catalyst. Furthermore, the electrolyte for Ru/Cr₂O₃ HNFs after 300 h of continuous HER stability test in 1 M KOH was examined by inductively coupled plasma-optical emission spectrometry (ICP-OES) to estimate the dissolved amount of Ru. The results show that only a very small amount (1.7 wt%) of Ru is dissolved in the electrolyte during the HER stability test, indicating the excellent stability of Ru/Cr₂O₃ HNFs under alkaline conditions.

The HER performance of these samples in acidic media (0.5 M H₂SO₄) is also evaluated using a standard three-electrode system. As depicted in Fig. 4a and S17, ESI,† the Ru/Cr₂O₃ HNFs still retain high catalytic activity, requiring a mere overpotential of 35 mV to reach a current density of 0.1 A cm⁻². Notably, this overpotential is not only significantly lower than those of RuCrO_x NFs (179 mV) and Ru NFs (176 mV), but also smaller than that of commercial Pt/C (41 mV). Similarly, the Ru/Cr₂O₃ HNFs also demonstrate the best HER activity in acidic media when compared to the samples obtained under other experimental conditions (Fig. S18, ESI†). In addition, in 0.5 M H₂SO₄, the Ru/Cr₂O₃ HNFs achieve the highest MA_Ru value of 900.28 A g_Ru⁻¹ at an overpotential of 100 mV, far surpassing the performance of the other control catalysts (Fig. S19a, ESI†). The Ru/Cr₂O₃ HNFs also attain the highest TOF value of 0.354 s⁻¹ at an overpotential of 100 mV in 0.5 M H₂SO₄, which is significantly superior to that of other control catalysts (Fig. S19b, ESI†). Additionally, the Ru/Cr₂O₃ HNFs achieve a remarkable current density of 1 A cm⁻² with a reduced overpotential of just 112 mV, outperforming the other control catalysts and highlighting their exceptional HER activity for practical application (Fig. S17 and S18b, ESI†). The corresponding Tafel slopes of Ru/Cr₂O₃ HNFs, commercial Pt/C, RuCrO_x NFs, and Ru NFs are 33.4, 38.3, 65.9, and 83.5 mV dec⁻¹, respectively (Fig. 4b). These findings also suggest that the heterostructure of Ru and Cr₂O₃ can markedly promote the HER reaction kinetics in acidic environments. Fortunately, the robust catalytic activity of Ru/Cr₂O₃ HNFs in 0.5 M H₂SO₄ is on par with or even exceeds the performance of recently reported top-tier electrocatalysts (Fig. 4c and Table S3, ESI†).^44,53–64 Furthermore, EIS measurement reveals that the Ru/Cr₂O₃ HNFs exhibit an exceptionally low R_ct of merely 2.0 Ω, which is far below than that of the other catalysts (Fig. 4d and S18c, ESI†). This low R_ct value suggests an enhanced electrical conductivity and efficient electron transfer capability within the Ru/Cr₂O₃ HNFs, beneficial for promoting the HER activity.^9,65 The ECSA values of these electrocatalysts are estimated by assessing the directly related C_dl values (Fig. S20 and S21, ESI†). As illustrated in Fig. 4e, the calculated C_dl value for Ru/Cr₂O₃ HNFs is 45.19 mF cm⁻², which exceeds that of RuCrO_x NFs (34.6 mF cm⁻²) and is substantially greater than those of the Ru NFs (3.13 mF cm⁻²) and R-Cr₂O₃ NFs (0.05 mF cm⁻²). Clearly, the Ru/Cr₂O₃ HNFs also possess a higher estimated ECSA and RF values of 101.68 cm² and 1130, respectively, compared to other catalysts in acidic environments (Table S4, ESI†). In addition, ECSA normalization is also performed to evaluate the intrinsic activity of synthesized catalysts under acidic conditions (Fig. S22, ESI†). The results show that Ru/Cr₂O₃ HNFs exhibit a low overpotential of only 92 mV at 0.5 mA cm_ECSA⁻², which is much better than that of RuCrO_x NFs (249 mV) and Ru NFs (137 mV), indicating that Ru/Cr₂O₃ HNFs possess both a larger ECSA and higher intrinsic activity.

Fig. 4 HER performance of Ru/Cr₂O₃ HNFs and other catalysts in 0.5 M H₂SO₄. (a) LSV curves of different catalysts including Ru/Cr₂O₃ HNFs, R-Cr₂O₃ NFs, RuCrO_x NFs, Ru NFs and 20% Pt/C. (b) Tafel plots of Ru/Cr₂O₃ HNFs, RuCrO_x NFs, Ru NFs and 20% Pt/C. (c) Comparison of overpotentials at 1 A cm⁻² with Tafel slopes in diverse HER electrocatalysts. (d) Nyquist plots and fitting curves at −0.3 V versus the Ag/AgCl electrode for the HER. (e) The relationship between half of the difference in current density and the scan rates to yield C_dl values. (f) Comparison of the overpotential at 0.1 A cm⁻², Tafel slope, R_ct, C_dl, MA and TOF of Ru/Cr₂O₃ HNFs, Ru NFs, and RuCrO_x NFs. (g) The i–t curves of Ru/Cr₂O₃ HNFs, RuCrO_x NFs and commercial Pt/C catalyst at a current density of ≈1 A cm⁻².

Further, the radargram presented in Fig. 4f demonstrates that Ru/Cr₂O₃ HNFs exhibit an outstanding electrocatalytic performance compared to RuCrO_x NFs and Ru NFs for the acidic HER, suggesting the exceptional advantage of the heterojunction construction of metallic Ru and Cr₂O₃ in the acidic HER process. Additionally, Ru/Cr₂O₃ HNFs also showcase exceptional durability in catalyzing acidic HER. In a continuous i–t test conducted in 0.5 M H₂SO₄ with a constant applied potential, Ru/Cr₂O₃ HNFs sustain up to 300 h of HER progression at a high current density of 1 A cm⁻², retaining a stable current density (Fig. 4g). In contrast, both RuCrO_x NFs and the commercial Pt/C catalyst decay rapidly within 25 h, further confirming the superior long-term stability of the Ru/Cr₂O₃ HNFs. Finally, we have examined the surface structure and chemical compositions of the Ru/Cr₂O₃ NFs after the acidic HER stability test. The SEM image (Fig. S23, ESI†) and XRD pattern (Fig. S24a, ESI†) exhibit no significant alterations in either morphology or crystal structure. The surface chemical valence states of the catalyst after the acidic HER process is further investigated using XPS spectroscopy (Fig. S24b–d, ESI†). Compared with the Ru 3p, Cr 2p and O 1s spectra of Ru/Cr₂O₃ NFs before the HER test, no substantial differences are observed, demonstrating the compositional stability of the catalyst during the acidic HER process.

To delve into the origin of the high activity on Ru/Cr₂O₃, periodic density functional theory (DFT) calculations are conducted. The structural models are constructed including Ru(101), RuCrO_x and Ru/Cr₂O₃ (Fig. S25, ESI†). Fig. 5a displays the schematic diagram of the adsorption of *H₂O and *H on three different models.^66–68 The kinetic energy for H₂O adsorption (ΔG (*H₂O)), hydrogen adsorption (ΔG (*H)), and final H₂ release is calculated based on the optimized models for Ru/Cr₂O₃, Ru(101) and RuCrO_x. The adsorption energies of ΔG (*H₂O) and ΔG (*H) have been widely accepted as descriptors to evaluate the HER activity of catalysts. In Fig. 5b, the adsorption of H₂O occurs almost spontaneously on all three models since the ΔG (*H₂O) is less than 0. The value of ΔG (*H) close to 0 eV is considered to be ideal for the HER.^7,69 It can be found that the value of ΔG (*H) for Ru/Cr₂O₃ is |0.25 eV|, which is much closer to 0 eV than the corresponding values for Ru(101) (|−0.64 eV|) and RuCrO_x (|−0.84 eV|). This indicates that the interface of the Ru site in proximity to the Cr₂O₃ possesses the most favorable *H adsorption kinetics. Moreover, the hydrogen evolution process is exergonic on the surface of Ru/Cr₂O₃ while it is thermodynamically uphill on the surface of the other two models, indicating that the construction of the Ru/Cr₂O₃ heterojunction interface facilitates *H desorption and thus accelerates the release of H₂. The possible reason for this phenomenon is that the Cr₂O₃ surface effectively modulates the electronic properties of Ru, thereby optimizing the adsorption and desorption of *H (Fig. 5c). This leads to superior HER performance in both alkaline and acidic environments, which can be further substantiated by examining the projected density of states (PDOS). As depicted in Fig. 5d, the d-band centers of Ru in the Ru/Cr₂O₃, Ru(101) and RuCrO_x models relative to the Fermi level are calculated to be −0.98, −0.84 and +0.02 eV, respectively, illustrating that the d-band center is far away from the Fermi level at the heterojunction interface between Ru and Cr₂O₃. This downshift suggests a weakened interaction between *H and the Ru active site at the heterojunction interface,⁶⁷ which facilitates the H₂ release from the heterojunction interface of Ru/Cr₂O₃. Furthermore, the differential charge density maps visualize the interaction between *H, *H₂O and the surface of catalyst models (Fig. 5a). The robust interactions between *H and Ru in RuCrO_x lead to a strong adsorption of *H on RuCrO_x, resulting in the higher desorption energy of *H on RuCrO_x. Conversely, the weak interactions between *H and Ru in Ru/Cr₂O₃ facilitate *H desorption, thereby lowering the energy barrier for the HER. The above analysis underscores that the synergistic interaction between Ru and Cr₂O₃ enhances the electrocatalytic performance of Ru/Cr₂O₃ HNFs for the HER.

Fig. 5 (a) The charge differential density maps of the adsorption of *H and *H₂O on Ru(101), RuCrO_x and Ru/Cr₂O₃ models, where blue and yellow represent electron density decrease and increase, respectively. The green, cyan and red spheres represent Ru, Cr and O atoms, respectively. (b) Calculated adsorption energies of *H₂O and *H on the surface for Ru(101), RuCrO_x and Ru/Cr₂O₃ models. (c) Schematic illustration of the proposed HER catalytic mechanism on Ru/Cr₂O₃ HNFs. (d) The PDOS of Ru atoms for Ru(101), RuCrO_x and Ru/Cr₂O₃ models.

Conclusion

In summary, we have successfully fabricated a nanofibrous Ru/Cr₂O₃ heterojunction through an electrospinning-calcination-thermal reduction procedure for efficient alkaline/acid-universal electrocatalytic properties at industrial-grade current density. The obtained Ru/Cr₂O₃ HNF catalyst achieves an exceptional HER activity and long-term stability at a high current density of 1 A cm⁻², significantly outperforming commercial Pt/C and many recently reported top-tier electrocatalysts. DFT calculations reveal that the construction of the heterojunction of Ru/Cr₂O₃ optimizes the *H desorption capability during the HER process. In addition, the strong interactions between Ru and Cr₂O₃ enable more charge carriers participating in the electrocatalytic reactions, resulting in a promoted HER performance. This approach of significantly enhancing the alkaline/acid-universal HER properties at the ampere level can be broadened as a novel design paradigm for other catalyst systems, thereby advancing the evolution and practical utilization for high-efficiency H₂ production.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

X. Lu, M. Zhong and M. Gao supervised this project. X. Yu performed the experiments and characterized the catalysts. X. Yu, M. Xia, R. Qi, Y. Wang and M. Zhong analyzed the data. M. Gao carried out the theoretical calculation. X. Yu and M. Gao wrote the manuscript. X. Lu and M. Zhong revised the manuscript. All authors have approved the final version of the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (52273056 and 22208337) and the Jilin Province Science and Technology Development Program (YDZJ202501ZYTS305).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5sc00248f

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