Symmetry Evolution of La₂O₃ from P$\stackrel{\mathrm{-}}{3}$m1 to P6₃/mmc for Enhanced Electrocatalytic H₂O₂ Production

Abstract

Electrocatalytic H₂O₂ production via the two-electron oxygen reduction reaction (ORR) is a highly sustainable alternative to industrial methods. To further optimize non-noble catalysts, we report an interfacial engineering strategy to stabilize the metastable P6₃/mmc-La₂O₃ phase on SrTiO₃. This symmetry evolution from the low-symmetry $\mathrm{P}\stackrel{\mathrm{-}}{3}\mathrm{m}1$ (trigonal) to the high-symmetry P6₃/mmc (hexagonal) space group yields a composite with >95% H₂O₂ selectivity. Mechanistic studies demonstrate that the symmetry-regulated interface optimizes *OOH conversion and suppresses O–O bond cleavage. This work offers a robust design principle for high-performance, noble-metal-free H₂O₂ electrosynthesis.

1. Introduction

Hydrogen peroxide (H₂O₂) is a versatile and industrially important oxidant that has been widely used in fields such as textile bleaching, wastewater treatment, chemical synthesis, and biological disinfection [1,2,3,4]. To date, the anthraquinone process remains the dominant industrial route for large-scale H₂O₂ production. However, this method involves multistep hydrogenation/oxidation cycles and extensive solvent management, which translate into energy-intensive operation, a complex reaction flow, and non-negligible environmental burdens [5,6]. In contrast, electrocatalytic synthesis has emerged as a promising alternative owing to its low energy requirements, mild reaction conditions, and minimal environmental impact. Among these, the two-electron oxygen reduction reaction (ORR) provides a sustainable and environmentally friendly pathway to generate H₂O₂ under mild conditions [7,8,9,10].

One of the major challenges of the ORR lies in controlling the selectivity between two competing reaction pathways: the four-electron pathway to H₂O and the two-electron pathway to H₂O₂ [8,11,12]. An effective two-electron ORR catalyst requires intermediate oxygen binding, which is sufficient to enable O₂ activation and *OOH formation, while avoiding excessive adsorption that drives O–O bond scission and the four-electron pathway. Noble metals such as platinum (Pt) and palladium (Pd) typically exhibit high O–O bond dissociation ability, thus favoring the four-electron pathway and resulting in poor H₂O₂ selectivity [13,14,15,16]. Therefore, the development of cost-effective, stable, and highly selective non-noble metal catalysts for the two-electron ORR is of great significance [12,17,18].

Rare-earth oxides and perovskite-type oxides have been increasingly explored as electrocatalysts, because their electronic structures and defect chemistry can be engineered over a wide range, enabling tailored surface reactivity [19,20,21,22]. Rare earth oxides, especially La₂O₃, have been extensively studied in heterogeneous catalysis due to their tunable electronic structures [23]. However, the influence of crystal symmetry on their electrocatalytic behavior remains insufficiently explored. La₂O₃ can exist in various polymorphs with different space groups, such as $\mathrm{P}\stackrel{\mathrm{-}}{3}\mathrm{m}1$ and P6₃/mmc, which differ in atomic arrangement, coordination environment, and symmetry operation. This structural variation may affect the distribution of surface electronic states and the uniformity of active sites, thereby influencing the adsorption configuration of reaction intermediates. Nevertheless, the relationship between La₂O₃ symmetry evolution and the pathway selectivity of ORR, particularly for H₂O₂ formation, has not been systematically investigated. Perovskite oxides (e.g., SrTiO₃) have been extensively studied as electrocatalysts due to their structural stability and tunable electronic properties. SrTiO₃ can provide efficient electron transport channels and stable catalyst supports [24,25]. However, due to the imperfect binding of oxygen intermediates, the intrinsic activity of SrTiO₃ for the selective 2e⁻ ORR is generally limited. On the other hand, although La₂O₃ exhibits good adsorption properties for oxygen-containing species, its relatively low conductivity may hinder effective charge transfer in the electrocatalytic process. Constructing a composite system that integrates La₂O₃ and SrTiO₃ therefore presents a promising strategy to combine their complementary advantages. More importantly, the formation of the La₂O₃/SrTiO₃ heterointerface may induce electronic structure redistribution, thereby optimizing the adsorption strength of the *OOH intermediate and improving the selectivity for H₂O₂. Furthermore, the SrTiO₃ matrix may serve as a structural template to stabilize specific polymorphs of La₂O₃, enabling symmetry evolution that is otherwise difficult to achieve under conventional conditions. However, the interaction mechanisms between interfacial interactions, symmetry evolution, and catalytic selectivity in such composite systems remain unclear.

In this study, the metastable phase of hexagonal La₂O₃ with P6₃/mmc symmetry has been stabilized and grown on SrTiO₃ template powders via a conventional solid-state reaction method. By tuning La₂O₃ content, we observe a composition-dependent symmetry evolution of the hexagonal La₂O₃ phase, where the phase assignment shifts from $\mathrm{P}\stackrel{\mathrm{-}}{3}\mathrm{m}1$ to P6₃/mmc. This enhanced symmetry thus improved the uniformity of surface electronic distribution. Electrochemical measurements demonstrated that, compared with pure La₂O₃ with $\mathrm{P}\stackrel{\mathrm{-}}{3}\mathrm{m}1$ symmetry and SrTiO₃, the optimized La₂O₃/SrTiO₃ composite catalyst delivers significantly higher H₂O₂ yield and two-electron selectivity. These results highlight the pivotal role of crystallographic symmetry evolution in steering the ORR toward the two-electron pathway. This study provides new insights into the role of crystallographic symmetry and interfacial synergy in regulating ORR selectivity.

2. Materials and Methods

2.1. Catalyst Synthesis

The schematic representation of the synthesis of Catalyst Synthesis is given in Figure 1. In this study, La₂O₃-SrTiO₃ composite electrocatalysts with different La₂O₃ content were synthesized via a conventional high-temperature solid-state reaction route. Specifically, SrTiO₃ and La(NO₃)₃·6H₂O powders were weighed to match the desired molar ratios between La₂O₃ and SrTiO₃, followed by thorough mixing and grinding for 1 h. The resulting mixture was heated to 1200 °C in air at a heating rate of 10 °C/min and calcined for 12 h to ensure complete decomposition of the nitrate precursor and formation of crystalline La₂O₃. At the same time, the high-temperature treatment promoted the interfacial interaction between La₂O₃ and SrTiO₃, which helped La₂O₃ to partially transform from the thermodynamically stable $\mathrm{P}\stackrel{\mathrm{-}}{3}\mathrm{m}1$ phase to the metastable P6₃/mmc phase, thereby obtaining La₂O₃-SrTiO₃ electrocatalysts with different La₂O₃ contents. The obtained samples are denoted as x%La₂O₃-SrTiO₃ (x = 1, 3, 5, 10, 15, and 20) according to the nominal La₂O₃ content, where x represents the molar percentage of La₂O₃ relative to SrTiO₃. For example, the 10% La₂O₃–SrTiO₃ sample corresponds to a molar ratio of La₂O₃:SrTiO₃ = 1:9. For comparison, pure La₂O₃ powders were yielded by annealing La(NO₃)₃·6H₂O powder under the same conditions.

Figure 1. Schematic illustration of the synthesis procedure of La₂O₃-SrTiO₃ composite electrocatalysts.

2.2. Physical Characterization

The phase composition of the samples was analyzed by X-ray diffraction (XRD, SmartLab, Rigaku, Tokyo, Japan). The morphology of the catalysts was examined by cold-field emission scanning electron microscopy (SEM, SU8600, Hitachi, Tokyo, Japan) and transmission electron microscopy (TEM, JEM-2100, JEOL Co., Ltd., Tokyo, Japan). X-ray photoelectron spectroscopy (XPS, Al Kα, ESCALAB QXi, Thermo Fisher Inc., Waltham, MA, USA) was employed to analyze the surface chemical composition of the electrocatalysts. In situ Fourier transform infrared spectroscopy (FTIR, Nicolet iS50 FT-IR, Thermo Fisher Scientific Inc., Waltham, MA, USA) and high-resolution mass spectrometry (HRMS, Q Extractive plus, Thermo Fisher Scientific Inc., Waltham, MA, USA) were employed to monitor the formation and evolution of surface reaction intermediates during the oxygen reduction reaction (ORR) process, thereby elucidating the reaction pathway and the underlying mechanism for H₂O₂ generation.

2.3. Electrochemical Measurements

Electrochemical measurements were carried out using an electrochemical workstation (CHI760E, CH Instruments, Shanghai, China) coupled with a rotating ring-disk electrode (RRDE) system (AFMSRCE type, Pine Instruments Corporation, Durham, NC, USA). Prior to the measurements, catalyst inks were prepared as follows: 5 mg of the as-synthesized catalyst powder was dispersed in 1 mL of anhydrous ethanol and mixed with 10 µL of Nafion solution (5 wt%, DuPont, Wilmington, DE, USA). The mixture was ultrasonicated for 30 min to form a homogeneous suspension.

When conducting tests using an electrochemical workstation, different doses of catalyst ink were taken and dropped onto an indium tin oxide (ITO) conductive glass fixed with a rubber sealing ring using a pipette. The ink was then dried under an infrared lamp and used as the working electrode. Ag/AgCl was used as the reference electrode, and a Pt mesh was used as the counter electrode. Based on this, the electrochemical behavior under different reaction conditions in 0.1 M KCl solution was studied, and the yield of H₂O₂ in the solution after each reaction was determined using the iodometric titration method.

In RRDE experiments, the geometric areas of the ring and disk electrodes were 0.2475 cm² and 0.1866 cm², respectively. A 10 µL aliquot of the catalyst ink was drop-cast onto the disk electrode and allowed to dry naturally, forming a uniform catalytic film. The Ag/AgCl electrode and a Pt mesh were used as the reference and counter electrodes, respectively. The reference electrode potential was calibrated to the reversible hydrogen electrode (RHE) using the following equation:

E (V vs. RHE) = E (V vs. Ag/AgCl) + E^Θ(Ag/AgCl) + 0.059 × pH

2.4. Theoretical Calculations

We used the DFT as implemented in the Vienna Ab initio simulation package (VASP) in all calculations. The exchange-correlation potential is described by using the generalized gradient approximation of Perdew–Burke–Ernzerhof (GGA-PBE). The projector augmented-wave (PAW) method is employed to treat interactions between ion cores and valence electrons. The plane-wave cutoff energy was fixed to 450 eV. Given structural models were relaxed until the Hellmann–Feynman forces were smaller than −0.02 eV/Å and the change in energy was smaller than 105 eV. Grimme’s DFT-D3 methodology was used to describe the dispersion interactions among all the atoms in adsorption models.

The Gibbs free energy change is defined as:

ΔG = ΔE + ΔZPE − TΔS

where ΔE is the electronic energy calculated with VASP, ΔZPE and ΔS are the zero-point energy difference and the entropy change between the products and reactants, respectively, and T is the temperature (298.15 K).

3. Results and Discussion

As illustrated by the X-ray diffraction (XRD, SmartLab, Rigaku) patterns in Figure 2a,b, annealing the mixture of La(NO₃)₃·6H₂O and SrTiO₃ effectively induces the formation of a La₂O₃/SrTiO₃ heterostructure. The diffraction peaks of La₂O₃ can be indexed to two phases with different symmetries. For the $\mathrm{P}\stackrel{\mathrm{-}}{3}\mathrm{m}1$ phase, the main characteristic peaks are observed at 2θ ≈ 26.08°, 29.08°, 29.92°, 39.46°, 46.02°, 52.1°, 55.38° and 55.9°, corresponding to the (100), (002), (011), (012), (110), (103), (112) and (201) planes. The appearance of diffraction peaks at 2θ ≈ 27.42° and 28.11° can be assigned to the (002) and (101) planes of the P6₃/mmc phase. With increasing La₂O₃ fraction, the relative contribution of the P6₃/mmc phase exhibits a non-monotonic dependence, reaching its peak at x = 10% and decreasing at higher La₂O₃ loading. It is worth noting that the $\mathrm{P}\stackrel{\mathrm{-}}{3}\mathrm{m}1$ is considered the most thermodynamically favored polymorph of La₂O₃ under ambient conditions, whereas formation of the metastable P6₃/mmc phase typically occurs only at temperatures exceeding 2000 °C [26]. Moreover, annealing pure La(NO₃)₃·6H₂O powder exclusively yields the $\mathrm{P}\stackrel{\mathrm{-}}{3}\mathrm{m}1$ phase. Consequently, the appearance of the P6₃/mmc phase in the composites can be attributed to the SrTiO₃ matrix acting as a structural template. Such stabilization may arise from the epitaxial strain originating from the SrTiO₃ lattice, which could lower the energetic penalty for forming the metastable P6₃/mmc phase. To further elucidate the phase distribution and interfacial structure between the La₂O₃ and SrTiO₃ phases, transmission electron microscopy (TEM, JEM-2100, JEOL Co., Ltd.) analysis was performed on the representative 10%La₂O₃-SrTiO₃ sample. As shown in Figure 2c, it shows lattice stripes of 0.166 nm, 0.225 nm and 0.321 nm, which belong to the (201) crystal plane of La₂O₃ ($\mathrm{P}\stackrel{\mathrm{-}}{3}\mathrm{m}1$), the (111) crystal plane of SrTiO₃ and the (002) crystal plane of La₂O₃ (P6₃/mmc), respectively. By combining the EDS plot (Figure S1 and Table S1), we can see that the elements are uniformly distributed. It is inferred that La₂O₃ domains (in both $\mathrm{P}\stackrel{\mathrm{-}}{3}\mathrm{m}1$ and P6₃/mmc phases) with the particle size of 10~30 nm are well dispersed on the surface of the SrTiO₃ matrix. It is hypothesized that during the initial stage of annealing, the decomposition of La(NO₃)₃·6H₂O releases acidic gaseous species, which may mildly etch the SrTiO₃ surface and promote subsequent La₂O₃ nucleation [27,28]. This process could create additional active sites, enabling more uniform dispersion of La₂O₃ upon high-temperature conversion of the precursor [29]. As a result, a uniform distribution of La₂O₃ is achieved, promoting efficient contact between the active species and the catalyst during electrocatalysis. X-ray photoelectron spectroscopy (XPS) results showed that the La 3d spectrum exhibited characteristic peaks corresponding to La³⁺, confirming the presence of La–O bonds in La₂O₃. The Ti 2p spectrum showed two main peaks belonging to Ti⁴⁺, indicating the presence of Ti–O bonds in the SrTiO₃ lattice. While high concentrations of oxygen vacancies and La doping would induce Ti³⁺ formation [25,30], no Ti³⁺ was observed in this study, suggesting a negligible donor-doping effect from either La³⁺ or oxygen vacancy. This further indicates that the compositing between La₂O₃ and SrTiO₃ did not introduce a donor doping from La³⁺ to the Ti 3d orbital, where only Ti 2p peaks attributed to Ti⁴⁺ can be observed (Figure S2). Therefore, these observations support the formation of a surface-decorated La₂O₃/SrTiO₃ composite, where La₂O₃ is primarily located or near the SrTiO₃ surface rather than being incorporated into the SrTiO₃ lattice.

Figure 2. Structural characterization of La₂O₃-SrTiO₃ composite powders. (a) XRD patterns of La₂O₃-SrTiO₃ composite powders with different La₂O₃ fractions. (b) Magnified XRD patterns of the dashed box in (a). (c) TEM images of 10% La₂O₃-SrTiO₃ sample.

When comparing the catalytic performance of catalysts, differences in electrochemically active surface area (ECSA) must be considered to more accurately assess the surface effect on ORR activity. Bilayer capacitance (C_dl) is used to estimate the corresponding ECSA differences. Bilayer capacitance is determined by CV curves using the following equation:

Δj = (j_a − j_c)/2 = C_dl × v

(1)

In the charging current density plot, the scan rate (v) is plotted on the x-axis and Δj on the y-axis, where j_a and j_c represent the anode and cathode current densities at ΔE = 0 V (potential window was 0.927 V to 1.027 V vs. RHE), respectively, and v represents the scan rate in mV s⁻¹. The slope of the plot represents the electric double-layer capacitance (C_dl). The non-Faradic current density based on the ECSA was then estimated using the following equation:

ECSA = C_dl/C_s

(2)

where C_s represented the specific capacitance of the electrode. The corresponding CV curves and the difference between the charging current density Δj and the scan rate (v) are shown in Figure S3. The C_dl values of different catalysts are shown in Table S2. Compared to SrTiO₃, the C_dl values of all catalysts increased, with the 10% La₂O₃-SrTiO₃ composite exhibiting the highest C_dl value, indicating that it possesses the largest electrochemically active surface area and a higher density of active sites. These results demonstrate that the introduction of La₂O₃ can effectively increase the exposure of catalytic sites and improve the electrochemical interface.

Furthermore, we conducted stability tests using chronoamperometry in an oxygen-rich KCl electrolyte at a reaction potential of 0 V (vs. RHE) to evaluate the catalyst’s durability. As shown in Figure S4, the current density of the 10% La₂O₃-SrTiO₃ electrocatalyst remained relatively stable during the test, indicating that the composite catalyst has good electrochemical stability.

The H₂O₂ electrosynthesis performance of La₂O₃-SrTiO₃ catalysts with different La₂O₃ content was evaluated at 0 V vs. reversible hydrogen electrode (RHE) using a standard three-electrode configuration, and the results are summarized in Figure 3. Figure 3a shows that with a fixed catalyst loading of 0.5 mg and electrolysis time of 1 h, the produced concentration of H₂O₂ follows a clear volcano-type trend with increasing La₂O₃ content. A maximum H₂O₂ yield of 228.70 μmol L⁻¹ was obtained in the 10%La₂O₃-SrTiO₃ sample. Notably, this volcano-like tendency is well consistent with the evolution of the P6₃/mmc La₂O₃ phase (Figure 2c), suggesting that the presence of the P6₃/mmc polymorph and its interface with SrTiO₃ contributes positively to H₂O₂ formation. Meanwhile, the pure La₂O₃ with $\mathrm{P}\stackrel{\mathrm{-}}{3}\mathrm{m}1$ symmetry displays a lower H₂O₂ yield of 138.64 μmol L⁻¹, suggesting that the enhanced performance of the composite cannot be explained by La₂O₃ alone and is instead linked to the symmetry-modulated La₂O₃ phase and interfacial effect. In the case of the 10%La₂O₃-SrTiO₃ sample, we further tested the time-dependent electrocatalytic performance. As shown in Figure 3b, prolonging the electrolysis to 5 h leads to a constant increase in H₂O₂ production without any decay of the reaction rate, indicating a robust catalytic activity over the tested duration. The generation of H₂O₂ usually involves two pathways, which are water oxidation reaction (WOR) and ORR. In order to clarify the reaction pathway in this study, we compared the electrocatalytic performance under different atmospheres (N₂, air, and O₂). As shown in Figure 3c, H₂O₂ formation is essentially suppressed under N₂, whereas it is significantly enhanced under O₂, with air giving an intermediate level. Therefore, these atmosphere-resolved results indicate that dissolved O2 is required for H₂O₂ formation, supporting ORR as the dominant pathway under the applied conditions. In addition, further evidence is provided by the ¹⁸O₂ isotope labeling experiment using 4-hydroxybenzoic acid (4-HPBA) as a molecular probe to trace the oxygen source in the generated H₂O₂. The emergence of a distinct signal at m/z = 139.028 in the high-resolution mass spectrum confirms incorporation of oxygen from O₂ into the generated H₂O₂ (Figure 3d) [31]. To further probe the evolution of reaction intermediates and the reaction mechanism of the La₂O₃-SrTiO₃ electrocatalyst during the ORR process, in situ Fourier-transform infrared (FTIR) spectroscopy (Nicolet iS50 FT-IR, Thermo Fisher Scientific Inc.) was performed on the 10%La₂O₃-SrTiO₃ sample in 0.1 M KCl solution. Figure 3e presents the FTIR spectra recorded at various applied potentials (vs. RHE). As the applied potential was gradually shifted to more negative values, an absorption band emerged and gradually intensified at approximately 1230 cm⁻¹ from 0.7 V, which can be assigned to the O–O stretching vibration of the hydroperoxyl (*OOH) intermediate during the reaction. In addition, the signals observed in the range of 1400–1450 cm⁻¹ are attributed to the vibrational features of H₂O₂.

Figure 3. Electrocatalytic H₂O₂ production performance and mechanistic evidence for La₂O₃-SrTiO₃ composite. (a) La₂O₃ content-dependent H₂O₂ yield of La₂O₃-SrTiO₃ composite powders under the O₂ saturation condition. (b) Time-dependent H₂O₂ yield over an optimized 10%La₂O₃-SrTiO₃ sample under the O₂ saturation condition. (c) H₂O₂ yields over a 10%La₂O₃-SrTiO₃ sample under different atmospheres. (d) LC-MS results of TECatal reaction by ¹⁸O isotope labeling. (e) In situ FTIR spectroscopy of a 10% La2O3-SrTiO3 sample at different applied voltages.

Based on the results above, a plausible reaction pathway involving a two-electron reduction mechanism has been deduced [11]:

O₂ + H⁺ + e⁻ → *OOH

     *OOH + H⁺ + e⁻ → H₂O₂

To further reveal the interfacial charge transport behavior of the La₂O₃-SrTiO₃ composite material, electrochemical impedance spectroscopy (EIS) was performed on different La₂O₃-SrTiO₃ composite material samples (Figure S5). The charge transfer resistance (R₁) was simulated using the equivalent circuit model shown in Figure S6, and the results are presented in Table S3. The composite catalyst with 10% La₂O₃ content exhibited the lowest R₁ values, indicating that this sample not only has the lowest bulk resistance but also the most rapid interfacial charge transfer process. This suggests that the introduction of an appropriate amount of La₂O₃ helps to construct a more efficient electron transport network and significantly reduces the energy barrier during the electrode reaction process.

To gain an in-depth understanding of the two-electron ORR pathway and catalytic mechanism of the La₂O₃-SrTiO₃ composite catalyst, density functional theory (DFT) calculations were further performed to analyze the electronic structure and the energetics of key reaction steps. Figure 4a presents the calculated electronic density of states (DOS) for SrTiO₃ and La₂O₃-SrTiO₃ composite model. The composite shows an increased density of electronic states near the Fermi level relative to pristine SrTiO₃, resulting from interfacial electronic reconstruction. Such a feature is expected to improve the electrical conductivity of the catalyst and thereby facilitate charge transfer during the electrocatalytic process. Moreover, the computed free energy profiles shown in Figure 4b reveal that pristine SrTiO₃ suffers from a large uphill free energy change for the rate-determining step (*OOH + H⁺ + e⁻ → H₂O₂), with a ΔG value of approximately 2.95 eV, indicating its unfavorable energetics to further activate and convert the *OOH intermediate. In contrast, this uphill free-energy requirement is significantly reduced in the La₂O₃-SrTiO₃ composite structure, with a ΔG value of approximately 1.87 eV, demonstrating that the composite interface effectively lowers the energetic barrier associated with H₂O₂ formation. These DFT results are in good agreement with the experimentally observed enhancement in H₂O₂ generation, supporting that interfacial coupling in La₂O₃-SrTiO₃ composite electrocatalyst can tune the adsorption energetics of oxygenated intermediates and lower the thermodynamic penalty for H₂O₂-forming steps in the two-electron ORR pathway.

Figure 4. Theoretical investigation of the two-electron ORR mechanism. (a) Total and partial electronic density of states (DOS) for the catalysts. (b) Gibbs free energy profiles for the two-electron ORR pathway on pristine SrTiO₃ and La₂O₃-SrTiO₃ composite surfaces. The insets display the optimized adsorption configurations of reaction intermediates.

In the electrocatalytic H₂O₂ production process, driving ORR toward a two-electron pathway is essential for efficient H₂O₂ generation. However, the competing four-electron reduction to H₂O often occurs on many oxide surfaces, making selectivity control a key criterion for catalyst design [32,33]. To quantify the two-electron ORR selectivity, rotating ring–disk electrode (RRDE) measurements were conducted in a conventional three-electrode system using the 10%La₂O₃-SrTiO₃ and pure SrTiO₃ as catalysts. All tests of the ring-disk electrode were performed in 0.1 M KOH electrolyte. As shown in Figure 5a, cyclic voltammetry (CV) curves were recorded in N₂ and O₂-saturated electrolytes to confirm ORR activity. Both catalysts exhibit negligible reduction peaks in N₂-saturated solution, whereas distinct reduction peaks appear under O₂-saturated conditions. Compared to SrTiO₃, the 10%La₂O₃-SrTiO₃ sample displays a higher reduction current and a more positive onset potential, indicating its superior ORR activity. Further evaluation was performed using linear sweep voltammetry (LSV) under different rotation rates for the 10%La₂O₃-SrTiO₃ sample, where the limiting diffusion current increases progressively with increasing rotation speed due to the enhanced mass transport and the reduced thickness of the diffusion layer (Figure S7) [34]. At a rotation rate of 1600 rpm, both disk and ring currents of 10%La₂O₃-SrTiO₃ and SrTiO₃ samples were recorded, as shown in Figure 5b. The 10%La₂O₃-SrTiO₃ sample delivers higher disk and ring currents than SrTiO₃, suggesting accelerated ORR kinetics and improved H₂O₂ formation. Based on the RRDE data, the H₂O₂ selectivity and electron transfer number (n) were calculated according to the following equations [35,36]:

$$n =4 · {\displaystyle \frac{I_{\mathrm{Disk}}}{I_{\mathrm{Disk}}+I_{\mathrm{Ring}}/N}}$$

(3)

$${\mathrm{H}}_2{\mathrm{O}}_2(\%) =200 · {\displaystyle \frac{I_{\mathrm{Ring}}/N}{I_{\mathrm{Disk}}+I_{\mathrm{Ring}}/N}}$$

(4)

where I_Disk is the disk current, I_Ring is the ring current, and N is the collection efficiency, which is 0.37 in this system. As illustrated in Figure 5c, the n value for the 10%La₂O₃-SrTiO₃ is close to 2, indicating a predominant two-electron ORR pathway for H₂O₂ production. Correspondingly, the H₂O₂ selectivity of 10%La₂O₃-SrTiO₃ exceeds 95% across the tested potentials (Figure 5d), whereas SrTiO₃ remains below 70%. As a result, compositing La₂O₃ with P6₃/mmc symmetry into SrTiO₃ brings a significant enhancement in the two-electron pathway, which improves the H₂O₂ production rate.

Figure 5. Selectivity of ORR in H₂O₂ production by La₂O₃-SrTiO₃ composite powders. (a) Cyclic voltammogram (CV) curves, (b) disk and ring currents at 1600 rpm, (c) electron transfer number, and (d) H₂O₂ selectivity calculated from rotating ring–disk electrode (RRDE) responses for pure SrTiO₃ and 10%La₂O₃-SrTiO₃ powders.

4. Conclusions

In this study, a series of La₂O₃-SrTiO₃ composite electrocatalysts with different La₂O₃ content were successfully synthesized via a solid-state method. The interaction between La₂O₃ and SrTiO₃ leads to discernible electronic modulation, which in turn governs the surface reactivity toward oxygen reduction. Specifically, a metastable hexagonal La₂O₃ phase with P6₃/mmc symmetry can be stabilized in the presence of SrTiO₃, which is typically inaccessible under conventional conditions. The evolution from a low-symmetry hexagonal phase (space group $\mathrm{P}\stackrel{\mathrm{-}}{3}\mathrm{m}1$) to a high-symmetry hexagonal phase (space group P6₃/mmc) correlates with improved H₂O₂ formation activity and selectivity, which we attribute to more favorable interfacial electronic structures and optimized adsorption energetics of *OOH intermediates. Furthermore, RRDE measurements confirm that the P6₃/mmc symmetry La₂O₃ significantly enhances the selectivity toward the two-electron ORR pathway, resulting in a high H₂O₂ selectivity of over 95%. These findings not only highlight the importance of structural symmetry in modulating oxygen reduction pathways but also demonstrate a robust “template-induction” strategy to stabilize metastable rare-earth oxides. This work provides a new paradigm for the rational design of high-performance, cost-effective non-noble metal catalysts for decentralized and efficient H₂O₂ electrosynthesis.