Preparation of Fe-Doped Ba_0.7Pr_0.3CoO_3−δ Perovskite Oxide for Electrocatalytic Hydrogen Evolution

Abstract

Developing efficient and stable electrocatalysts for the hydrogen evolution reaction (HER) is critical for advancing clean and sustainable energy technologies. Herein, a Fe-doped perovskite oxide Ba_0.7Pr_0.3Co_0.8Fe_0.2O_3−δ (BPCF_0.2) was successfully synthesized via the sol–gel method. By regulating the stoichiometric ratio of precursors and calcination temperature, a stable single-phase perovskite structure was achieved. X-ray photoelectron spectroscopy (XPS) analysis indicated that Fe incorporation increased the proportion of high-valent Co species and lattice oxygen content, which respectively reduced the charge transfer resistance of BPCF_0.2, thereby significantly enhancing catalytic performance. Electrochemical measurements revealed that BPCF_0.2 exhibited remarkable HER activity in 1.0 M KOH, achieving an overpotential of 172 mV at a current density of 10 mA cm⁻², with no significant decay during 200 h of continuous HER testing at 100 mA cm⁻². These results demonstrate that the Fe doping strategy can effectively optimize the electronic structure, providing valuable insights for the development of perovskite-based HER catalysts.

1. Introduction

Electrocatalytic water splitting has garnered widespread attention owing to its advantages of facile scalability and compatibility with industrialization [1,2,3,4]. However, the state-of-the-art electrocatalysts for water electrolysis currently available are predominantly reliant on precious metals [5,6,7]. The exorbitant cost and inadequate long-term stability of these materials hinder their large-scale practical application, thereby prompting the exploration of earth-abundant transition metals as alternatives to precious metals. Perovskite oxides have attracted considerable research interest due to their unique merits, including tunable crystal structures, facile and controllable synthetic routes, and versatile structural design strategies [8,9,10]. The canonical chemical formula of perovskite oxides is ABO₃, where the A-site is typically occupied by rare-earth or alkaline-earth metal cations with a coordination number of twelve, while the B-site usually accommodates transition metal cations with a coordination number of six [11,12]. In general, the electrocatalytic activity of perovskite oxides is primarily dominated by the B-site elements, whereas the A-site elements mainly modulate the structure of the material and the oxidation state of B-site cations [13,14]. Consequently, rational heteroatom doping or substitution at the A and B sites represents an effective approach to remarkably enhance the electrocatalytic performance of perovskite oxides.

Currently, BaCoO₃-based perovskites have been demonstrated to possess great potential in the field of water electrolysis, with numerous relevant research reports published [15,16,17]. Shao et al. designed and synthesized Sn and Fe doped perovskite BaCo_0.7Fe_0.2Sn_0.1O_3−δ for an electrocatalytic oxygen evolution reaction (OER) catalyst [18]. The introduction of heteroatoms comprehensively enhanced the hydroxyl (OH⁻) adsorption capacity, oxygen (O₂) desorption capability, charge transfer efficiency, and electrical conductivity of the material, accompanied by excellent long-term stability. Grenier et al. found that incorporating an appropriate amount of Ba²⁺ doping ions at the A-site of PrSrCo₂O_5.95 can increase the content of high-valence Co ions [19]. This adjustment is beneficial for optimizing the adsorption energy of the catalyst toward both the −OOH intermediate and O₂, thereby enhancing the oxygen evolution reaction (OER) activity. According to the currently available literature, a large number of perovskite catalysts have been developed for OER in alkaline electrolytes, whereas reports on their application in the hydrogen evolution reaction (HER) remain relatively scarce [20,21,22,23,24]. This is mainly attributed to the difficulty in regulating the adsorption capacity of perovskite materials toward hydrogen intermediates [25,26]. Therefore, the preparation of perovskite oxides with high HER activity remains challenges.

In this work, focusing on the BaCoO₃ parent perovskite, a dual-doped perovskite oxide Ba_0.7Pr_0.3Co_0.8Fe_0.2O_3−δ (denoted as BPCF_0.2) with Pr substitution at the A-site and Fe doping at the B-site was rationally designed and synthesized via a sol–gel method. By precisely regulating the stoichiometric ratio of precursors and optimizing the calcination temperature, the as-prepared doped material was endowed with a pure single-phase perovskite structure. BPCF_0.2 delivers remarkable HER activity, requiring an overpotential of merely 172 mV to achieve a current density of 10 mA cm⁻². Moreover, it maintains outstanding HER stability at 100 mA cm⁻² with negligible performance degradation over a 200-h durability test. Detailed characterizations reveal that Fe doping not only increases the proportion of high-valent Co sites in the perovskite lattice but also enhances the relative content of lattice oxygen. These changes concurrently increase the number of catalytically active sites, thereby synergistically enhancing the electrocatalytic performance of the material. Furthermore, the catalyst in situ generates BaCO₃ during the HER process to maintain charge balance, which is conducive to inhibiting perovskite corrosion and improving the structural and electrochemical stability of the catalyst. This work successfully designs a perovskite oxide with prominent HER activity through a synergistic A/B-site doping strategy, highlighting that this approach can be effectively applied to the development of high-efficiency non-precious metal hydrogen evolution catalysts.

2. Results and Discussion

The synthesis procedure Ba_0.7Pr_0.3Co_1−xFe_xO_3−δ (denoted as BPCF_x, x = 0, 0.1, 0.2, 0.3) is illustrated in Figure 1a. Among all BPCF_x samples with different compositional ratios and calcination temperatures, the BPCF_0.2 exhibited the most remarkable hydrogen evolution reaction (HER) activities. Consequently, it was selected as the representative sample in this work for in-depth investigation. X-ray diffraction (XRD) measurements were carried out to characterize the crystal structure of the as-prepared BPCF_x samples. As presented in Figure 1b, the BPC sample is mainly composed of a tetragonal perovskite phase (PDF#97-009-7117) with a minor amount of hexagonal perovskite phase (PDF#97-001-5257). After Fe doping, all BPCF_x perovskite changes to a single cubic perovskite phase (PDF#97-026-2123) with a space group of Pm-3 m. The diffraction peaks observed at 22.4°, 32.0°, 39.4°, 45.9°, 51.7°, 57.1°, and 66.9° correspond to the (100), (110), (111), (200), (210), (211), and (220) crystal planes, respectively [27]. With the increase in Fe doping content, the diffraction peaks of the perovskite phase exhibit a gradual shift toward lower 2θ angles. This phenomenon can be attributed to the smaller ionic radii of Co^3+/4+ (r = 0.61/0.53 Å) compared to those of Fe^3+/4+ (r = 0.65/0.59 Å), which induces a slight expansion of the perovskite lattice [28,29]. In addition, an analysis of the XRD patterns of BPCF_0.2 at different calcination temperatures (Figure 1c) reveals that a single cubic perovskite phase is formed exclusively within the temperature range of 900 °C to 1100 °C, with no other impurity phases detected. This result indicates that introducing Fe doping at the Co site of BPC (corrected from BCP) facilitates the formation of a single-phase perovskite structure at relatively lower calcination temperatures. Energy-dispersive X-ray spectroscopy (EDX) characterization results confirmed that the samples were mainly composed of Pr, Ba, Co, Fe, and O elements (Figure 1d and Figure S1). The actual content of each element was in good agreement with the preset feeding ratios and was consistent with the quantitative analysis results obtained by inductively coupled plasma-optical emission spectrometry (ICP-OES) (Table S1). This consistency further verifies the successful preparation of Ba_0.7Pr_0.3Co_1−xFe_xO_3−δ samples with the designed stoichiometric composition.

Furthermore, the morphology of BPCF_0.2 was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The BPCF_0.2 exhibited an irregular block-like structure with pores of varying sizes. Such porous architecture effectively increases the accessible surface area between the catalyst and electrolyte, shortens the mass/charge transport pathways, and thus significantly boosts the electrocatalytic activity (Figure 1e and Figure S2). Meanwhile, SEM images of the undoped BPC and Fe-doped BPCF_x samples showed similar morphological features, indicating that the crystal phase transition did not alter the material morphology (Figure S3). To gain deeper insight into the composition and crystal structure of BPCF_0.2, high-resolution transmission electron microscopy (HRTEM) combined with fast Fourier-transform (FFT) characterization was performed along the [001] zone axis. Distinct lattice fringes were clearly visualized, with an interplanar spacing of 0.28 nm that matches the (110) crystal plane of the cubic perovskite phase (Figure 1f). The corresponding FFT pattern (Figure 1g) further corroborates the successful formation of the cubic perovskite structure, confirming the crystallinity and phase purity of the sample. In addition, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and corresponding elemental mapping (Figure 1h) were employed to comprehensively analyze the elemental distribution of BPCF_0.2. The results demonstrated that Ba, Pr, Co, Fe, and O elements were uniformly distributed throughout the sample, confirming the successful doping of Fe into the perovskite lattice.

The chemical composition and surface oxidation states of BPC and BPCF_0.2 were further investigated by X-ray photoelectron spectroscopy (XPS). As shown in the XPS survey spectra (Figure 2a), BPC is composed of Ba, Pr, Co and O, while BPCF_0.2 additionally contains Fe; these results are in good agreement with the EDX and ICP-OES analyses. In perovskite oxides, B-site metal elements typically serve as the catalytically active sites, whereas A-site elements mainly function to stabilize the crystal structure and modulate the oxidation states of B-site metal ions. The Pr 3d XPS spectrum (Figure 2b) reveals that the oxidation state of Pr in BPC and BPCF_0.2 remains almost unchanged after Fe doping. Correspondingly, the binding energy of Ba 3d shows no discernible shift, demonstrating that Fe doping exerts no influence on the oxidation states of A-site elements (Figure 2c). For the Co 2p spectrum (Figure 2c), the characteristic peaks of BPC at 780.0 and 795.2 eV are assigned to Co³⁺ species [30,31]. After Fe incorporation, the Co 2p binding energy exhibits a 0.2 eV negative shift (towards lower binding energy), indicative of an enhanced oxidation state of Co ions [32,33]. Given the close proximity of the Co 2p and Ba 3d characteristic peaks that may cause spectral interference, the Co 3p orbital was additionally characterized for verification. As presented in Figure S4, the Co 3p binding energy of BPCF_0.2 also shifts negatively relative to BPC, which further confirms that Fe doping leads to the oxidation of Co ions to higher valence states. In cubic perovskites, Co³⁺ ions generally adopt a low-spin electronic configuration (t_2g⁶e_g⁰). In comparison, enhancing the cobalt valence state effectively reduces the electron density around Co³⁺ sites, favoring the formation of Co⁴⁺ (t_2g⁴e_g¹) [34]. Relative to Co³⁺, Co⁴⁺ possesses more unpaired electrons, which significantly enhances the electronic conductivity of the catalyst. As is known, efficient electron transfer throughout the electrode is essential to achieve high current densities for electrocatalytic reactions [8,35]. Therefore, superior electrical conductivity represents a key prerequisite for high-performance electrocatalysts. The Fe 2p XPS spectrum further confirms the coexistence of Fe²⁺, Fe³⁺, and Fe⁴⁺ ions on the BPCF_0.2 surface [36,37,38]. Notably, in the cubic perovskite structure, Fe⁴⁺ ions possess an electronic configuration of t_2g³e_g¹, which also contains an increased number of unpaired electrons [39,40]. This distinctive electronic structure of BPCF_0.2 favors the enhancement of electronic conductivity in the perovskite oxide, further optimizing its electrocatalytic activity and synergistically complementing the beneficial effect of elevated Co valence states on the overall catalytic performance [2,41]. The O 1s XPS spectra of BPC and BPCF_0.2 are displayed in Figure 2d, with detailed peak deconvolution data summarized in Table S2. All O 1s spectra can be well resolved into four distinct characteristic peaks, which are assigned to lattice oxygen (O²⁻, 528.4 eV), highly oxidative oxygen species (O₂²⁻/O⁻, 529.3 eV), hydroxyl groups or adsorbed oxygen (–OH/O₂, 531.1 eV), and surface-adsorbed water (H₂O, 532.8 eV), respectively [42,43]. The surface O₂²⁻/O⁻ species are closely associated with oxygen vacancies in perovskite catalysts. The non-metal oxygen sites such as oxygen vacancies and lattice oxygen can serve as favorable active sites for the adsorption of hydrogen intermediates and enhance electronic conductivity, thus playing a critical role in boosting the HER kinetics [8,27]. Quantitative analysis indicates that the relative content of oxygen vacancies remains essentially unchanged, while the lattice oxygen content in BPCF_0.2 reaches 24.6%, which is higher than the 19.3% observed in BPC (Table S2). This difference in non-metal oxygen sites-related species content is consistent with the trend of HER activity observed for the two samples, further verifying the positive correlation between oxygen vacancies and catalytic performance.

The HER performances of the as-prepared catalysts were evaluated in 1.0 M KOH electrolyte using a standard three-electrode system. As demonstrated in Figure 3a, among all Fe-doped BPCF_x samples, BPCF_0.2 displayed the superior catalytic activity, with an overpotential of 172 mV at 10 mA cm⁻², outperforming BPC (232 mV), BPCF_0.1 (195 mV), BPCF_0.3 (192 mV), and even approaching the commercial Pt/C at high current density. Furthermore, the BPCF_0.2 exhibits more favorable reaction kinetics, as evidenced by its lower Tafel slope of 58.0 mV dec⁻¹ compared to BPC (80 mV dec⁻¹), BPCF_0.1 (86.3 mV dec⁻¹), and BPCF_0.3 (85.1 mV dec⁻¹). Additionally, the HER activities of BPCF_0.2 calcined at different temperatures were examined (Figure S6). At a current density of 10 mA cm⁻², the overpotentials of samples calcined at 900 °C, 1000 °C, and 1100 °C were 190 mV, 172 mV, and 197 mV, with corresponding Tafel slopes of 68.1 mV dec⁻¹, 58.0 mV dec⁻¹, and 69.9 mV dec⁻¹, respectively. These findings demonstrate that 1000 °C is the optimal calcination temperature for BPCF_0.2 to achieve enhanced HER activity.

The double-layer capacitance (C_dl) of the catalysts was determined via cyclic voltammetry (CV) measurements conducted in the non-Faradaic potential region (Figure S7), with the calculated results summarized in Figure 3c. The C_dl values of BPC, BPCF_0.1, BPCF_0.2, and BPCF_0.3 were 2.21 mF cm⁻², 5.81 mF cm⁻², 6.18 mF cm⁻², and 2.52 mF cm⁻², respectively, with BPCF_0.2 exhibiting the largest C_dl. Since C_dl is correlated with the electrochemically active surface area (ECSA), this result indicates that BPCF_0.2 possesses a larger ECSA, which can expose more catalytically active sites during the HER process. Electrochemical impedance spectroscopy (EIS) measurements were carried out to assess the catalytic reaction kinetics. From the Nyquist plots (Figure 3d) and corresponding fitting data (Table S3), the charge transfer resistances (R_ct) of BPC, BPCF_0.1, BPCF_0.2, and BPCF_0.3 were calculated as 30.6 Ω, 28.3 Ω, 12.4 Ω, and 18.7 Ω, respectively. This remarkable reduction in R_ct for Fe-doped samples, particularly BPCF_0.2, confirms that Fe doping optimizes the electronic structure of the perovskite, accelerates charge transport kinetics, and thereby enhances catalytic performance. The intrinsic catalytic activity of the catalysts was further compared by calculating the turnover frequency (TOF) values. As illustrated in Figure 3e, BPCF_0.2 exhibited the highest TOF among all tested samples, with values of 0.018 s⁻¹, 0.035 s⁻¹, and 0.079 s⁻¹ at overpotentials of 100 mV, 150 mV, and 200 mV, respectively. This indicates that appropriate Fe doping remarkably boosts the intrinsic catalytic activity of the BPCF_0.2, which can be attributed to the increased proportion of high-valent Co species and the enhanced number of active sites induced by Fe doping. The electrochemical stability of BPCF_0.2 was evaluated via chronopotentiometry (CP) and CV cycling tests. Compared with commercial Pt/C, BPCF_0.2 showed negligible overpotential decay after 200 h of continuous operation at a current density of 100 mA cm⁻², demonstrating superior long-term stability (Figure 3f). Furthermore, after 5000 CV cycles, the polarization curve of BPCF_0.2 was almost identical to the initial one, confirming the significant electrochemical durability of the catalyst (Figure 3g). These stability results confirm the exceptional catalytic stability of BPCF_0.2, highlighting its potential for practical large-scale hydrogen production. To verify the Faraday efficiency (FE) of BPCF_0.2 for HER, a two-electrode overall water splitting system was assembled using BPCF_0.2 as the cathode and commercial RuO₂ as the anode. Hydrogen bubbles generated at the cathode were collected, and the hydrogen production was quantified via the water displacement method (Figure 3h). At a current density of 100 mA cm⁻², the actual volume of collected hydrogen was nearly consistent with the theoretically calculated volume, yielding a Faraday efficiency of approximately 99% for BPCF_0.2 (Figure 3i). This result confirms that BPCF_0.2 exhibits high HER catalytic efficiency with minimal side reactions. Meanwhile, the overpotential at 10 mA cm⁻² and Tafel slope of BPCF_0.2 are superior to those of most recently reported high-performance perovskite-based electrocatalysts for HER (Table S4). Evidently, the BPCF_0.2 catalyst displays remarkable catalytic activity under alkaline conditions.

To unveil the origin of the HER catalytic activity of BPCF_0.2, systematic post-reaction characterizations were performed after the catalyst underwent an accelerated durability test (ADT) of 5000 CV cycles. XRD analysis of the post-ADT sample (Figure 4a) demonstrated that BPCF_0.2 predominantly retained its intrinsic perovskite structure. However, new diffraction peaks corresponding to the BaCO₃ phase were detected, which were absent in the fresh catalyst. The formation mechanism of BaCO₃ can be elaborated as follows: under the alkaline electrochemical test environment, large-radius Ba²⁺ ions in the perovskite lattice tend to leach out. Simultaneously, CO₂ from the ambient atmosphere dissolves in the alkaline electrolyte to form CO₃²⁻ ions, which then react with the leached Ba²⁺ ions and precipitate as BaCO₃ on the catalyst surface. Notably, this in situ formed BaCO₃ plays a crucial role in inhibiting the further dissolution of active metal components (Co, Fe, Pr) and preventing lattice destruction, thereby effectively enhancing the long-term electrochemical stability of BPCF_0.2 during the HER process [44,45].

Subsequently, the SEM image (Figure 4b) demonstrated that the BPCF_0.2 surface became rougher and more porous after HER testing, with the formation of a typical BaCO₃ nanorod structure. Raman spectra (Figure 4c) showed no distinct characteristic peaks before HER testing, whereas characteristic peaks at 134 cm⁻¹, 686 cm⁻¹, and 1057 cm⁻¹ appeared after the HER test, which match the standard Raman peaks of BaCO₃ [46,47]. This further corroborates the formation of the BaCO₃ after the HER test. Additionally, EDX analysis confirmed a reduction in Ba species, which is consistent with the formation of insoluble BaCO₃ that immobilizes Ba²⁺ ions (Figure S8). The XPS results further confirm the leaching of Ba²⁺ ions. As can be seen from Figure 4d, after undergoing HER, the Ba 2d shoulder signal in the Co2p & Ba2d XPS spectrum is significantly weakened, while the binding energy of Co 2p is basically consistent with that before the HER test, indicating that the valence state of Co has not changed. In addition, it can also be seen from Figure 4e that after undergoing HER, the valence state of Fe remains basically the same as before the reaction, indicating that the perovskite structure has not changed. Deconvolution analysis of the O 1s XPS spectrum (Figure 4f and Table S4) revealed a reduction in the relative content of O₂²⁻/O⁻ and O²⁻ species following the HER process, which provides direct evidence for the participation of oxygen vacancies and lattice oxygen in the catalytic reaction. Meanwhile, a substantial increase in the proportion of –OH/O₂ and H₂O_ads species was detected, indicating that the catalyst surface undergoes hydroxylation reconstruction during the HER process. This evolution may further modulate the surface active sites and promote catalytic kinetics.

3. Experimental Section

3.1. Chemicals

Praseodymium (III) nitrate hexahydrate (Pr(NO₃)₃·6H₂O), barium (II) nitrate (Ba(NO₃)₂), cobalt (II) nitrate hexahydrate (Co(NO₃)₂·6H₂O), Nafion solution (5 wt.%), and ethylenediaminetetraacetic acid (EDTA) were purchased from Aladdin (Shanghai, China). Iron (III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O), ethanol, potassium hydroxide (KOH, ≥85%), and aqueous ammonia (NH₃·H₂O, 25–28%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Commercial Pt/C (20 wt.%) was purchased from Alfa Aesar (China) Chemical Co., LTD. (Beijing, China). All chemicals were used as received without additional purification.

3.2. Synthesis of Ba_0.7Pr_0.3Co_1−xFe_xO_3−δ

Perovskite-type Ba_0.7Pr_0.3Co_1−xFe_xO_3−δ (denoted as BPCF_x, x = 0.1, 0.2, 0.3) was synthesized via a standard sol–gel route. Briefly, stoichiometric quantities of Ba(NO₃)₂, Pr(NO₃)₃·6H₂O, Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, and CA were weighed and dissolved in deionized water, followed by continuous stirring to achieve a homogeneous solution. Subsequently, a mixed solution consisting of EDTA and aqueous ammonia was then added to the above solution to adjust the pH to 7~8. The molar ratio of metal ions CA: EDTA was maintained at 1: 1.5: 1. Then, the resulting transparent solution was evaporated at 90 °C under continuous stirring until a gel was formed. After that, the gel was pretreated at 180 °C for 10 h to yield a black solid precursor. Finally, the precursor was calcined in air at 1000 °C for 4 h to obtain the target BPCF_x powders. For comparison, Ba_0.7Pr_0.3CoO_3−δ (BPC) was synthesized following the same procedure without the addition of Fe(NO₃)₃·9H₂O.

3.3. Characterizations

X-ray diffraction (XRD) measurements were conducted on a Bruker D8 Advance diffractometer (Karlsruhe, Germany) operating at 40 kV and 40 mA, which was equipped with Cu Kα radiation (λ = 1.5418 Å). The morphological features of the as-prepared samples were characterized using a field-emission scanning electron microscope (FESEM, Hitachi S-4800, Tokyo, Japan) and a high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F, Tokyo, Japan). Elemental mapping images were acquired via energy-dispersive X-ray spectroscopy (EDX) coupled with the JEM-2100F HRTEM. X-ray photoelectron spectroscopy (XPS) analyses were performed on a Thermo Fisher Scientific ESCALAB 250Xi spectrometer (Waltham, MA, USA), employing monochromatic Al Kα radiation (1486.6 eV) as the excitation source; the binding energy values were calibrated with reference to the C 1s peak at 248.8 eV. Inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 7900, Singapore) was utilized for the quantitative determination of Co and Fe contents in the samples. Raman spectra were recorded with a LabRAM HR Evolution Raman microscope (Palaiseau, France).

3.4. Electrochemical Measurements

All tests related to electrochemistry were conducted using a standard three-electrode system constructed on a CHI 760E workstation, with 1.0 M KOH solution as the electrolyte. A Hg/HgO electrode was used as the reference electrode, and a carbon rod electrode served as the counter electrode. The working electrode was prepared by first dispersing 2.5 mg of catalyst and 2 mg of conductive carbon black (Vulcan XC-72) in a mixed solvent consisting of 480 μL of ethanol and 20 μL of Nafion solution. After ultrasonicating the mixture for 40 min, 10 μL of the uniformly dispersed ink was dropped onto a glassy carbon (GC) electrode with a diameter of 5 mm, which was used for subsequent tests after drying. The Pt/C electrode was prepared under identical conditions for comparison. Before performing electrochemical tests, the working electrode was activated within the potential range of −0.8 to −1.4 V (vs. Hg/HgO) until the cyclic voltammetry (CV) curves stabilized, with a scan rate of 100 mV s⁻¹. All potentials were calibrated to the reversible hydrogen electrode (RHE) using the following equations:

E (vs. RHE) = E (vs. Hg/HgO) + 0.059 pH + 0.098 V

Linear sweep voltammetry (LSV) curves recorded at a scan rate of 5 mV s⁻¹ with 100% iR-compensation. The calculation data of double-layer capacitance (C_dl) was obtained through CV tests, which were performed in the non-Faradaic potential region (−0.6 to −0.7 V vs. Hg/HgO) with scan rates increasing from 20 mV s⁻¹ to 120 mV s⁻¹. The stability of the materials was tested by chronopotentiometry at a current density of 100 mA cm⁻². To determine the Faradaic efficiency (FE), the volume of H₂ produced was measured by the drainage method. The formula is as follows:

$$\mathrm{F}\mathrm{E}={\displaystyle \frac{\mathrm{T}\mathrm{h}\mathrm{e} \mathrm{a}\mathrm{c}\mathrm{t}\mathrm{u}\mathrm{a}\mathrm{l} \mathrm{a}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{g}\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d} \mathrm{g}\mathrm{a}\mathrm{s}}{\mathrm{T}\mathrm{h}\mathrm{e} \mathrm{t}\mathrm{h}\mathrm{e}\mathrm{o}\mathrm{r}\mathrm{e}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l} \mathrm{a}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{g}\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d} \mathrm{g}\mathrm{a}\mathrm{s}}}$$

The TOF (s⁻¹) was calculated using the following equation:

$$\mathrm{T}\mathrm{O}\mathrm{F}={\displaystyle \frac{I}{2\mathrm{n}F}}$$

where I is the current (A) at the corresponding overpotential from LSV measurements, the factor 2 is based on the assumption that two electrons are required to form one hydrogen molecule, n (mol) is the number of active sites calculated from Ru content loaded on the work electrode, and F is the Faraday constant (96,485.3 C mol⁻¹). In this work, Co atoms are assigned as the primary active sites for the HER, and the contents of Co in the as-prepared samples were determined by ICP-OES.

4. Conclusions

In conclusion, a Fe-doped Ba_0.7Pr_0.3Co_0.8Fe_0.2O_3−δ perovskite oxide with exceptional electrocatalytic HER performance was successfully synthesized. Fe doping at the Co-site in the perovskite lattice enhances the HER activity. Specifically, BPCF_0.2 delivered overpotentials of 172 mV at a current density of 10 mA cm⁻². Moreover, the BPCF_0.2 catalyst exhibited prominent long-term electrochemical stability, showing no noticeable performance degradation even after 200 h of continuous operation under a current density of 100 mA cm⁻². Mechanistically, Fe doping effectively regulated the oxidation states of Co sites in the perovskite, facilitating the formation of high-valence Co ions and increasing the concentration of lattice oxygen in the BPCF_0.2. Both of these factors synergistically contributed to a significant increment in the number of catalytically active sites available for HER. This work confirms that rational B-site doping is a feasible and efficient strategy for fabricating high-performance perovskite electrocatalysts, and this doping approach can be further adopted and explored in the future design and development of perovskite-based materials with superior HER activity.