B-site substitution in A₂BO₄ Ruddlesden–Popper perovskites for enhanced OER and HER in alkaline medium

Abstract

In this work, we systematically investigated the optimization of water-splitting performance in A₂BO₄-type (214-type) Ruddlesden–Popper (R–P) perovskite catalysts via B-site substitution. PrSrCoO₄, previously identified as a highly active bifunctional electrocatalyst, was selected as the parent material. A series of B-site-substituted perovskites, including PrSrCo_0.6Fe_0.4O₄ (Fe0.4), PrSrCo_0.6Fe_0.2Ni_0.2O₄ (Ni0.2), PrSrCo_0.6Fe_0.3Ir_0.1O₄ (Ir0.1), and PrSrCo_0.6Fe_0.3Ru_0.1O₄ (Ru0.1), were successfully synthesized. Structural and magnetic characterization confirmed that all catalysts belong to the tetragonal I4/mmm space group and exhibit paramagnetic behavior at room temperature. Among the series, Ir0.1 exhibited the most remarkable bifunctional electrocatalytic performance, achieving low overpotentials of 277 mV for the oxygen evolution reaction (OER) and 279 mV for the hydrogen evolution reaction (HER) at 10 mA cm⁻². This enhanced catalytic activity is attributed to Ir-induced synergistic effects, including optimized surface oxygen species, a favorable shift in the d-band center, and an increased proportion of high-spin (HS) Co³⁺. Overall, our experimental results broaden the electrochemical application potential of 214-type R–P perovskites and provided an ideal platform for deeper mechanistic understanding and rational catalyst design.

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1. Introduction

In recent years, ABO₃-type perovskite oxides have attracted significant attention for electrocatalysis due to their unique crystal structures, tunable electronic properties, and abundant active sites.^1–5 Compared to conventional noble-metal-based catalysts (IrO₂, RuO₂, and Pt/C), perovskite materials offer opportunities to overcome performance limitations through compositional engineering strategies such as A/B-site substitution and oxygen vacancy modulation.^6–9 For instance, Liu and Dai et al. demonstrated that the introduction of oxygen vacancies into SrTi_0.7Ru_0.3O₃ activates the intrinsic OER activity of Ti³⁺ sites, while the Ti³⁺–O–Ru⁵⁺ super-exchange interaction facilitates charge redistribution, thereby significantly enhancing HER performance.^10,11 Kim et al. revealed that Fe in La_0.2Sr_0.8Co_1−xFe_xO_3−δ and Ba_0.5Sr_0.5Co_1−xFe_xO_3−δ modulates the Co–O coordination environment, thus playing a crucial role in enhanced OER activity.¹² Moreover, Qian et al. confirmed that Ce³⁺ in Ce–LaCoO₃ tunes the spin state of Co³⁺, simultaneously optimizing both OER and ORR activities.¹³ These studies highlight that elemental substitution is a highly effective strategy for improving the catalytic performance of perovskite catalysts.

214-type R–P perovskite oxides also exhibit intriguing physical and chemical properties due to their distinctive structures, making them attractive candidates in functional materials research. Representative examples include La_2−xSr_xCuO₄,¹⁴ a well-known high-temperature superconductor, and NaRTiO₄ (R = rare earth elements),¹⁵ which shows notable piezoelectric behavior. While several 214-type R–P catalysts such as Sr₂RuO₄, Sr₂IrO₄, and La₂NiO₄ have been extensively studied for water splitting,^16–18 other 214-type perovskite oxides remain largely underexplored in this field. In our previous research, we investigated the effect of A-site substitution on the electrocatalytic performance of LnSrCoO₄ (Ln = La, Pr, Sm, Eu, and Ga).¹⁹ However, the role of B-site substitution in enhancing the electrocatalytic activity of 214-type R–P perovskite has received far less attention. Further research is needed to elucidate the underlying mechanisms and establish effective design strategies for improving their electrocatalytic performance.

In this work, we study the optimization of water-splitting performance in 214-type R–P perovskite catalysts through B-site substitution. Using PrSrCoO₄ as the parent material, the effects of substituting the B-site with transition metals (Fe, Ni) and noble metals (Ir, Ru) on the crystal structure, magnetic behavior, and electrocatalytic activity were comprehensively explored. All catalysts belonged to the tetragonal I4/mmm space group and exhibited paramagnetic behavior at room temperature, with Ir0.1 demonstrating the most outstanding bifunctional catalytic activity. Specifically, Ir0.1 showed a significantly higher mass activity (MA) and turnover frequency (TOF) compared to the other catalysts. XPS and magnetic measurements reveal that the substitution of Ir⁴⁺ enhances the surface oxygen species ratio, induces an upward shift in the d-band center, and increases the proportion of HS Co³⁺.

2. Results and discussion

Fig. 1a presents the layered perovskite crystal structure of PrSrCo_1−yFe_yO₄, which belongs to the K₂NiF₄-type structure.²⁰ In this structure, Pr/Sr cations occupy the A-sites, forming 9-fold coordination with oxygen anions, while Co/Fe cations occupy the B-sites within CoO₆/FeO₆ octahedra, forming six-fold coordination. The CoO₆/FeO₆ octahedra form the perovskite-like (Pr/Sr)(Co/Fe)O₃ layers, which alternately stack with rock-salt-type (Pr/Sr)O layers along the c-axis, resulting in the typical 214-type R–P phase structure. As shown in Fig. 1b, we selected PrSrCoO₄ as the parent perovskite and synthesized a series of Fe-substitution catalysts (PrSrCo_1−yFe_yO₄). It was found that the catalyst with a Co/Fe molar ratio of 0.6 : 0.4 exhibited the best bifunctional OER/HER performance (Fig. S1). Based on this optimal ratio, we further explored transition-metal (Ni) and noble-metal (Ir, Ru) substitution strategies, designing PrSrCo_0.6Fe_0.4−zNi_zO₄ (Fig. 1c) and PrSrCo_0.6Fe_0.4−xM_xO₄ (M = Ir, Ru), respectively. XRD results revealed that co-substitution with Fe and Ni (z ≤ 0.4) did not cause lattice distortion or phase separation, and the R–P structure remains stable. In contrast, increasing Ir or Ru content beyond x = 0.1 resulted in the appearance of secondary phase peaks in the XRD patterns (Fig. S2), indicating structural instability and phase impurity at higher noble metal contents. Furthermore, the effects of both temperature and calcination time were systematically investigated. It was found that any deviation from 1150 °C within the 1050–1250 °C range increased the impurity peaks, indicating a higher secondary phase content (Fig. S3a–c). Similarly, changing the calcination time revealed that a shorter duration of 5 hours resulted in significant impurities (Fig. S3d), while a longer 20 hour calcination caused a marked decline in OER and HER performance (Fig. S3e and f). Therefore, the optimal synthesis conditions were determined to be 1150 °C for 10 hours. Based on these optimized conditions, the best-performing catalysts from each substitution series were selected for comprehensive comparison: Fe0.4, Ni0.2 (Fig. S4), and the noble-metal-substitution samples Ir0.1 and Ru0.1.

Fig. 1 (a) Schematic illustration of the layered perovskite crystal structure of PrSrCo_1−yFe_yO₄. XRD patterns of (b) PrSrCo_1−yFe_yO₄ and (c) PrSrCo_0.6Fe_0.4−zNi_zO₄.

Fig. 2a shows the XRD patterns of Fe0.4, Ni0.2, Ir0.1 and Ru0.1. The diffraction peaks of all samples match well with the standard powder diffraction data (JCPDS Card No. 01-074-4676), indicating identical crystal structures for these four catalysts. Fig. 2b–e shows the Rietveld refinement results of the four catalysts based on the I4/mmm space group. The refined patterns exhibit excellent consistency between the experimental and calculated profiles, confirming the high reliability of the fitting. The corresponding lattice parameters and reliability factors are summarized in Table S1. The refinement results clearly indicate that all catalysts exhibit a pure 214-type R–P phase without detectable impurity peaks.

Fig. 2 (a) XRD patterns of Fe0.4, Ni0.2, Ir0.1 and Ru0.1 catalysts. The XRD data for Fe0.4 and Ni0.2 used here are reproduced from Fig. 1b and c, respectively. Rietveld refinement XRD results of (b) Fe0.4, (c) Ni0.2, (d) Ir0.1 and (e) Ru0.1.

As shown in Fig. S5, the SEM images revealed that the Fe0.4, Ni0.2, Ir0.1 and Ru0.1 catalysts exhibit irregular, micron-sized block-like morphologies. The N₂ adsorption–desorption isotherms of the four catalysts display typical type II behavior (Fig. S6a), indicating a predominantly nonporous structure. The values of BET surface area are 2.309, 1.463, 2.353 and 0.952 m² g⁻¹ for Fe0.4, Ni0.2, Ir0.1 and Ru0.1, respectively. Among these four catalysts, Ir0.1 exhibited the largest specific surface area. BJH pore size distribution analysis further indicated the absence of distinct pore structures in all four 214-type R–P perovskite oxides (Fig. S6b), consistent with their morphologies observed by SEM.

To further investigate the atomic-scale structural characteristics and elemental distribution, Ir0.1, which possesses the highest surface area, was analyzed by HRTEM and EDX elemental mapping. The TEM image shows that Ir0.1 retains the irregular micron-sized block morphology with slight aggregation (Fig. 3a). The HRTEM image in Fig. 3b clearly reveals well-defined lattice fringes with an interplanar spacing of 6.24 Å, which closely matches the theoretical (002) plane spacing of 6.23 Å obtained from the XRD refinement. The corresponding FFT pattern acquired along the [2−10] zone axis exhibits distinct diffraction spots corresponding to the (004), (121), and (125) planes (inset of Fig. 3b), confirming its high crystallinity. The EDX mapping confirms that the Pr, Sr, Co, Fe, Ir, and O elements were uniformly distributed throughout the whole measured region (Fig. 3c), and the elemental ratios of Pr, Sr, Co, Fe, and Ir in Ir0.1 closely match the designed stoichiometry (Fig. S7).

Fig. 3 (a) TEM image of Ir0.1. (b) HRTEM image of Ir0.1. Inset: enlarged view of the red-highlighted region (below panel) with the corresponding FFT pattern along the [2−10] direction (upper panel). (c) EDX elemental mapping of Ir0.1.

Fig. 4 presents the OER performance of Fe0.4, Ni0.2, Ir0.1, and Ru0.1. As shown in Fig. 4a, Ir0.1 achieves a current density of 10 mA cm⁻² at an overpotential of 277 mV, significantly outperforming Fe0.4 (355 mV), Ni0.2 (332 mV), Ru0.1 (359 mV) and commercial RuO₂ (330 mV), thereby demonstrating superior OER activity. Kinetic analysis reveals that Ir0.1 exhibits the lowest Tafel slope (51.9 mV dec⁻¹) (Fig. 4b), indicating more favorable four-electron transfer kinetics. The EIS spectra shown in Fig. 4c further confirm that Ir0.1 possesses the lowest charge transfer resistance (R_ct), signifying the highest charge transport efficiency among the four catalysts. The charge transport capability follows the order: Ir0.1 > Ni0.2 > Fe0.4 > Ru0.1, which correlates well with their respective overpotential values, highlighting the critical role of interfacial charge transfer in OER performance.²¹

Fig. 4 The OER performance of Fe0.4, Ni0.2, Ir0.1 and Ru0.1 catalysts in 1 M KOH. (a) LSV curves. (b) Tafel slopes. (c) EIS spectra at 1.57 V vs. RHE. Inset: equivalent circuit diagram. (d) Double-layer capacitance (C_dl). (e) Comparison of the OER overpotential at 10 mA cm⁻² for our catalyst and previously reported perovskite oxides. The corresponding references are listed in Table S2.

The C_dl curves, as presented in Fig. 4d and S8, indicate that Ir0.1 has the highest C_dl value of 5.60 mF cm⁻², representing 3.4-fold, 1.2-fold, and 1.8-fold enhancements compared to Fe0.4, Ni0.2, and Ru0.1, respectively. This suggests that Ir0.1 possesses a larger electrochemical active surface area, providing more active sites for catalytic reactions and thereby enhancing OER performance.^22,23 Furthermore, Ir0.1 exhibits the highest MA and TOF within the potential range of 1.5–1.65 V (Fig. S9a and b), indicating that the substitution of Ir improves both active site density and single-site efficiency. Long-term stability tests under a constant current density of 10 mA cm⁻² show that Ir0.1 maintains optimal durability with a minimal voltage increase of 10 mV over 85 hours (Fig. S9c), markedly surpassing Fe0.4/Ni0.2 (which show a 19 mV increase over 66 hours) and Ru0.1 (which deteriorate rapidly within 5 hours). In addition, a comparative analysis with state-of-the-art perovskite materials reveals that Ir0.1 exhibits superior OER performance, exceeding even that of the pure Ir-based perovskite, SrIrO₃ (Fig. 4e).

The HER performance of Fe0.4, Ni0.2, Ir0.1, Ru0.1 and 20 wt% Pt/C is presented in Fig. 5. At 10 mA cm⁻², the corresponding overpotentials in Fig. 5a are 346, 355, 279, 384 and 51 mV, respectively. Among the four 214-type R–P perovskite catalysts, Ir0.1 exhibits the most favorable HER activity with an overpotential of 279 mV, significantly outperforming the other catalysts. In contrast, Ru0.1 showed the lowest catalytic performance with the highest overpotential of 384 mV. In terms of reaction kinetics, all catalysts exhibit Tafel slopes exceeding 120 mV dec⁻¹ (Fig. 5b), specifically 205.5, 187.2, 195.8, and 246.4 mV dec⁻¹ for Fe0.4, Ni0.2, Ir0.1, and Ru0.1, respectively. This indicates that the Volmer step is the rate-determining step in the HER process for all catalysts.^24,25 Notably, although Ir0.1 does not exhibit the lowest Tafel slope, it shows the smallest overpotential, highlighting that electrocatalytic performance arises from the synergistic effects of multiple factors.^26–28

Fig. 5 The HER performance of Fe0.4, Ni0.2, Ir0.1 and Ru0.1 catalysts in 1 M KOH. (a) LSV curves. (b) Tafel slopes. (c) EIS spectra at −0.32 V vs. RHE. Inset: equivalent circuit diagram. (d) C_dl. (e) Comparison of the HER overpotential at 10 mA cm⁻² for our catalyst and previously reported perovskite oxides. The corresponding references are listed in Table S3.

As shown in Fig. 5c and d and S10, EIS and C_dl measurements further confirm the superior electrocatalytic performance of Ir0.1. It displays the lowest R_ct, indicating enhanced charge transport capability, and the highest C_dl value of 10.30 mF cm⁻², suggesting a larger electrochemical active surface area and more active sites for the HER. Furthermore, Ir0.1 showed remarkable advantages in both MA and TOF within the potential range of −0.3 to −0.45 V (Fig. S11a and b), indicating that the substitution of Ir enhances both the density of active sites and the intrinsic activity of each site. Long-term stability tests in Fig. S11c further revealed that Ir0.1 maintained superior stability compared to other samples, with only a minimal voltage increase of 9 mV during continuous operation over 86 hours. After comparison with other reported perovskite catalysts, it is found that the synthesized Ir0.1 demonstrates competitive catalytic performance (Fig. 5e).

In addition, the intrinsic electrocatalytic activities were further assessed using ECSA-normalized LSV curves for both the OER and HER (Fig. S12). For the OER (Fig. S12b), Ir0.1 exhibits the highest intrinsic activity at low potentials (E < 1.6 V), while Fe0.4 shows the highest normalized current density at higher potentials due to its smallest ECSA. For the HER (Fig. S12d), Ir0.1 delivers the largest current density between 0 and −0.4 V, indicating superior intrinsic activity, but its ECSA-normalized performance decreases at lower potentials, becoming inferior to Fe0.4 and Ru0.1, due to the largest ECSA of Ir0.1. These results indicate that the factors governing catalytic performance change with increasing potential. In the low-potential region (OER < 1.6 V; HER > −0.4 V), where reaction rates are sluggish, the catalytic performance is mainly determined by the intrinsic activity of active sites. In this regime, Ir0.1 exhibits the best performance owing to its optimized electronic configuration (discussed in detail below). At higher potentials, mass-transport effects dominate. The largest ECSA of Ir0.1 enhances reactant adsorption and product desorption, improving its geometric-area-normalized activity (Fig. S12a and c). However, when normalized by ECSA to exclude the contribution of surface area, this mass-transfer limitation leads to relatively lower intrinsic activity for Ir0.1 compared to Fe0.4 (OER) and Fe0.4/Ru0.1 (HER).

The elemental composition and chemical states of the Fe0.4, Ni0.2, Ir0.1, and Ru0.1 catalysts were analyzed using XPS. As shown in Fig. S13a, the full spectra of all catalysts exhibit characteristic peaks corresponding to Pr, Sr, Co, Fe, and O. In addition, the distinct signals for Ni, Ir, and Ru are only observed in Ni0.2, Ir0.1, and Ru0.1, respectively, confirming the successful substitution of the corresponding elements. In Fig. S13b, the Fe 2p spectra of the four catalysts exhibit nearly identical peak positions and line shapes, with Fe³⁺ 2p_3/2 and 2p_1/2 peaks located at 711.5 eV and 723.8 eV, respectively.^29,30 This similarity indicates that the crystal field environment of Fe sites remains largely unchanged by Ni, Ir, or Ru substitution, suggesting a stable electronic structure for Fe sites. Furthermore, the valence states of substituting elements were investigated. For Ni0.2 (Fig. S13c), the Ni 2p_3/2 and 2p_1/2 peaks appear at 855.6 eV and 873.5 eV, respectively,^31,32 suggesting that Ni predominantly exists in the Ni³⁺. In Ir0.1, the Ir 4f spectrum shows 4f_7/2 and 4f_5/2 peaks at 63.1 eV and 66.1 eV, respectively (Fig. S13d),³³ which are consistent with the binding energies of Ir⁴⁺. Similarly, for Ru0.1, the Ru 3p_3/2 and 3p_1/2 peaks are located at 464.4 eV and 486.6 eV, respectively (Fig. S13e),³⁴ confirming the presence of Ru⁴⁺.

In the Co 2p spectra (Fig. 6a and S14), Fe0.4, Ni0.2, Ir0.1, and Ru0.1 exhibit highly similar peak features at 780.4 eV (Co 2p_3/2) and 795.6 eV (Co 2p_1/2), suggesting the comparable valence state of cobalt ions. To further investigate the oxidation states of Co ions, detailed peak deconvolution of the Co 2p spectra was performed in Fig. 6a. The results show that the Co³⁺ to Co²⁺ ratios are nearly identical across the four catalysts (Table S4), with an average Co oxidation state of 2.73. These results confirm that Co valence states remain unchanged upon substituting with Ni, Ir, or Ru. Therefore, the difference in catalytic performance cannot be attributed to changes in the Co oxidation state.

Fig. 6 XPS spectra of (a) Co 2p, (b) O 1s and (c) valence-band spectra for Fe0.4, Ni0.2, Ir0.1 and Ru0.1 catalysts.

The O 1s spectra of Fe0.4, Ni0.2, Ir0.1, and Ru0.1 as shown in Fig. 6b were analyzed to elucidate the intrinsic relationship between surface oxygen species and electrocatalytic activity. The spectra of all catalysts can be deconvoluted into four oxygen species: lattice oxygen species (O²⁻ at 529.1 eV), highly oxidative oxygen species (O₂²⁻/O⁻ at 530.5 eV), surface hydroxyl/adsorbed oxygen (OH⁻/O₂ at 531.5 eV), and adsorbed water (H₂O at 533.1 eV). Among the four oxygen species, the proportion of O₂²⁻/O⁻ is positively correlated with the concentration of oxygen vacancy. Oxygen vacancies not only enhance the adsorption/desorption kinetics of reaction intermediates but also modulate the coordination environment of Co–O bonds, thereby inducing local electronic structure rearrangements.^35,36 Previous studies have demonstrated that an increased ratio of O₂²⁻/O⁻ species is generally associated with improved catalytic activity.^37–39 As shown in Fig. 6b and summarized in Table S5, Ir0.1 exhibits the highest proportion of O₂²⁻/O⁻, which corresponds well with its superior OER and HER performance. Moreover, Ru0.1 exhibits the highest content of O²⁻ and OH⁻/O₂ species in the four catalysts, yet it shows the lowest catalytic performance, suggesting that these two types of surface oxygen species do not play a beneficial role in 214-type R–P perovskites. In addition, although Fe0.4 contains the highest fraction of H₂O species, its electrocatalytic activity is significantly lower than that of Ir0.1, indicating that the adsorbed water content is not the primary determinant of catalytic performance.

The valence-band spectra as shown in Fig. S15 for Fe0.4, Ni0.2, Ir0.1, and Ru0.1 exhibit similar spectral features, indicating broadly consistent valence band structures. Further analysis of the d-band center (ε_d) through valence-band spectra integration (Fig. 6c) shows that the ε_d of Ir0.1 (−3.81 eV) is shifted closer to the Fermi level compared to those of Fe0.4 (−3.99 eV), Ni0.2 (−3.95 eV), and Ru0.1 (−3.91 eV). According to the d-band center theory, an ε_d closer to the Fermi level reflects an increase in the energy of antibonding orbitals, resulting in a lower filling of the antibonding orbitals.^40,41 This modified electronic configuration leads to enhanced bonding stability and stronger adsorption between the catalytic active sites and the reaction intermediates, thereby improving electrocatalytic performance.^29,42 Correlating the d-band center with electrocatalytic performance (Fig. S16), a negative relationship is observed between ε_d and the overpotential at 10 mA cm⁻² (η₁₀) for both the OER and HER. Specifically, a higher ε_d (closer to the Fermi level) corresponds to a lower η₁₀ value. Obviously, Ir0.1, with the ε_d value closest to the Fermi level, achieves the best electrocatalytic performance. These findings highlight the unique advantage of Ir substitution, which induces an upward shift of the d-band center. This electronic modulation optimizes the adsorption strength between the active sites and reaction intermediates, effectively lowering the energy barriers for both the OER and HER. In addition, the Co–O bond length was also correlated with catalytic activities toward both reactions (Fig. S17). For the OER (Fig. S17a), the bond length follows the order Ir0.1 > Ni0.2 > Ru0.1 > Fe0.4, whereas the overpotential exhibits the opposite trend (Ir0.1 < Ni0.2 < Fe0.4 ≤ Ru0.1), revealing a clear negative correlation in which a longer Co–O bond generally promotes superior OER performance. A similar relationship is observed for the HER (Fig. S17b), where the overpotential sequence (Ir0.1 < Fe0.4 ≤ Ni0.2 < Ru0.1) inversely correlates with the bond-length trend. Although Fe0.4 slightly deviates from this relationship, the overall trend remains consistent, suggesting that a longer Co–O bond length tends to favor improved HER activity as well.

The structural stability of Ir0.1 after long-term OER and HER testing was systematically evaluated. The XRD and Raman results confirm that the Ir0.1 catalyst retains its original crystalline structure without detectable phase transition or degradation (Fig. S18). HRTEM reveals the formation of a thin amorphous surface layer (yellow dashed lines), while the bulk remains well-crystallized, indicating excellent structural integrity (Fig. S19). Furthermore, the XPS spectra of Co 2p, Fe 2p, and Ir 4f show negligible changes before and after reaction (Fig. S20a–c), indicating stable oxidation states and chemical environments. In contrast, the O 1s spectra exhibit a significant decrease in the lattice oxygen, particularly after the OER (Fig. S20d), which is consistent with the surface amorphization revealed by HRTEM.

The magnetic behavior of Fe0.4, Ni0.2, Ir0.1, and Ru0.1 was systematically investigated through magnetic measurements. As shown in Fig. 7a, the zero-field-cooled (ZFC) χ(T) curve of Fe0.4 displays a clear shoulder at approximately 75 K, while this feature is significantly weakened in the field-cooled (FC) curve. Upon further cooling to around 50 K, a clear bifurcation appears between the ZFC and FC curves, suggesting the formation of a spin-glass or cluster-glass state in this system. Indeed, such magnetic states are commonly reported in Co-based perovskite oxides, including typical single perovskites such as La_1−xSr_xCoO₃ and double perovskites like Sr₂FeCoO₆ and La_1.5Ca_0.5CoMnO₆.^43–45 For Ni0.2 (Fig. 7b), a rapid increase in magnetic susceptibility is observed in both ZFC and FC curves around 180 K, which can be attributed to a ferromagnetic/ferrimagnetic phase transition. In the noble-metal-substitution systems, the magnetic behavior of both Ir0.1 and Ru0.1 is similar to that of LaSrCoO₄ (Fig. 7c and d),¹⁹ exhibiting spin freezing in these systems. Further, the M(H) curves reveal that the four catalysts exhibit weak ferromagnetic/ferrimagnetic behavior at 10 K, while displaying paramagnetism at 300 K (insets of Fig. 7a–d).

Fig. 7 ZFC and FC magnetic susceptibility χ(T) curves of (a) Fe0.4, (b) Ni0.2, (c) Ir0.1, and (d) Ru0.1. Inset: isothermal M(H) curves within the magnetic field range of −7 T to 7 T at 10 K and 300 K, respectively. Temperature dependence of the ZFC 1/χ of (e) Fe0.4, (f) Ni0.2, (g) Ir0.1, and (h) Ru0.1 in 0.1 T. The solid lines represent the CW law fitting.

The ZFC 1/χ(T) curves of Fe0.4, Ni0.2, Ir0.1, and Ru0.1 were fitted using the Curie–Weiss (CW) law to obtain the Weiss temperature (θ) and the effective magnetic moment (μ_eff), as summarized in Table. S6. For the Fe0.4 system, μ_eff (Co) was calculated using the formula: μ_eff = [μ_eff (Pr³⁺)² + 0.6μ_eff (Co)² + 0.4μ_eff (Fe³⁺)²]^1/2. Given that μ_eff (Pr³⁺) is 3.60 μ_B and μ_eff (Fe³⁺) in the octahedral coordination is 4.04 μ_B,^46–49 the calculated μ_eff (Co) in Fe0.4 is 3.15 μ_B. This value represents a significant deviation from the parent PrSrCoO₄. PrSrCoO₄ exhibits a μ_eff (Co) of 2.80 μ_B, which is consistent with the theoretical value for Co³⁺ in the intermediate-spin (IS) state (S = 1, μ_eff ≈ 2.83 μ_B), as discussed in our previous work.¹⁹ The increased μ_eff (Co) in Fe0.4 suggests a partial transition of Co³⁺ from an IS to a HS (S = 2, μ_eff = 4.90 μ_B), indicating that the substitution of Fe³⁺ induces a partial spin-state transition of Co³⁺, resulting in a mixed IS/HS configuration. For Ni0.2, a more pronounced spin-state transition is observed. Upon substituting with Ni³⁺ (μ_eff = 1.73 μ_B), the μ_eff (Co) increases to 3.66 μ_B, implying a higher proportion of HS Co³⁺. The enhanced OER performance of Ni0.2 compared to Fe0.4 may be partly attributed to the increased HS Co³⁺ content, while its slightly reduced HER activity suggests that the spin state of Co³⁺ plays distinct roles in the OER and HER processes.

In the noble-metal-substitution systems, the μ_eff (Co) values in Ir0.1 and Ru0.1 are 3.93 μ_B and 3.78 μ_B, respectively, where the μ_eff values for Ir⁴⁺ and Ru⁴⁺ are 0.10 μ_B and 2.00 μ_B.^50–52 Although charge compensation due to Ir⁴⁺ or Ru⁴⁺ substitution might induce a minor reduction of Co³⁺ to Co²⁺, the low substitution levels ensure that Co³⁺ remains the dominant species. Therefore, the analysis primarily focuses on changes in the spin state of Co³⁺ in Ir0.1 and Ru0.1. Among the four catalysts, Ir_0.1 exhibits the highest μ_eff (Co), indicating the largest proportion of HS Co³⁺, consistent with previous reports that associate HS Co³⁺ with enhanced water-splitting activity.^53–55 This provides a reasonable explanation for the superior bifunctional catalytic performance of Ir0.1. In contrast, although Ru0.1 shows the second-highest HS Co³⁺ content, it displays the lowest OER and HER activities in these four catalysts, which may be attributed to its lowest specific surface area and oxygen vacancy concentration. The results suggest that electrocatalytic performance arises from a synergistic interplay among intrinsic activity, surface active site density, and charge transport dynamics. While metal substitution can effectively tune the spin state of Co³⁺ to enhance intrinsic catalytic activity, the overall performance is constrained by a complex set of multiscale factors.

3. Conclusions

In summary, a series of 214-type R–P perovskite catalysts (Fe0.4, Ni0.2, Ir0.1, and Ru0.1) were successfully synthesized via a modified Pechini method. Among all the catalysts, Ir0.1 exhibits the most promising bifunctional electrocatalytic performance, with low overpotentials of 277 mV for the OER and 279 mV for the HER at 10 mA cm⁻², respectively. The substitution of Ir not only optimizes the surface oxygen species distribution (with an increased O₂²⁻/O⁻ ratio) and tunes the electronic structure (upward shift of the d-band center), but also significantly increases the proportion of HS Co³⁺. Moreover, Ir0.1 achieved significantly higher MA and TOF compared to the other catalysts. In short, our research elucidates the performance enhancement mechanism of transition metal and noble metal substitution in 214-type R–P perovskites from the perspectives of electronic structure modulation and spin-state tuning of Co³⁺. The experimental results provide valuable design strategies for developing high-performance 214-type R–P perovskite catalysts in water splitting.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available within the article and its supplementary information (SI). Additional raw data are available from the corresponding author, Kaibin Tang, upon reasonable request.

Supplementary information: includes SEM, EDS mapping, BET, XPS, XRD, HRTEM and electrocatalytic performance. See DOI: https://doi.org/10.1039/d5ce00782h.

Acknowledgements

We deeply appreciate Xuguang Liu for the technique assistance on this work. We are thankful for the technical support of magnetic measurement by MPMS3 at Instruments Center for Physical Science of University of Science and Technology of China. This work is supported by the National Natural Science Foundation of China (Grant No. 12204452) and the National Key R&D Program of China (2021YFB4001401).

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