Synthesis of strontium-incorporation SnO₂ for oxygen evolution reaction

Summary

Modulating the electronic structure of metal oxides is an effective strategy to enhance the electrocatalytic oxygen evolution reaction (OER). Shifting the conduction band (CB) to more negative potentials facilitates interfacial charge transfer, improving OER kinetics. Here, strontium (Sr) is incorporated into SnO₂ to shift the CB, induce lattice strain, and generate oxygen vacancies. Structural and electrochemical analyses reveal that intermediate Sr incorporation optimally balances electronic modulation and defect density, yielding the highest OER activity. This catalyst delivers an overpotential of 430 mV at 10 mA cm⁻², a Tafel slope of 68 mV dec⁻¹, and stable operation over 5.2 h in 1.0 M KOH. The enhanced performance is driven primarily by improved interfacial charge transfer kinetics rather than benchmark intrinsic activity. This work demonstrates how aliovalent doping tunes the band structure and defect chemistry of cost-effective oxides, providing a scalable strategy for designing non-precious metal water splitting catalysts.

Graphical abstract

Highlights

• •Sr-incorporation shifts SnO₂ conduction band negatively and enhances OER performance
• •Solid solutions with 10%–100% Sr were synthesized to determine optimal OER activity
• •Structural and electrochemical analyses confirm Sr-SnO₂ potential for water splitting

Catalysis; Materials science; Engineering materials

Introduction

Currently, the demand for renewable energy is rapidly increasing to overcome the depletion of fossil fuels. In this context, electrocatalytic water splitting to produce O₂ and H₂ is a promising pathway to store solar energy in the form of chemical energy. However, the sluggish kinetics of OER are due to the thermodynamic uphill battle and the complex four electron transfer process.^1,2,3,4 Although the most efficient electrocatalysts such as RuO₂ and IrO₂, require substantial overpotential to achieve the current density of 10 mA cm⁻².^5,6 These challenges have stimulated the search for non-precious metal oxides for OER.^7,8,9,10,11 Metal oxide semiconductors are a class of materials that gain influence in electrocatalysis owing to their adjustable energy band gap and remarkable chemical and mechanical stability.^12,13 Among the various metal oxides, tin (IV) oxide (SnO₂) is a promising electrode material for energy storage and conversion.¹⁴ This is due to its optical and electrical properties, low cost, non-toxic, a suitable band gap of 3.6 eV, a theoretical capacity of 1,378 mAh g⁻¹, and strong electrochemical activity.^15,16

The efficiency of the OER is predominantly influenced by the electrical and chemical properties of the electrocatalysts at the catalyst/electrolyte contact. Band energies of metal oxides dictate electron transfer and the covalency of the transition metal-oxygen (TM-O) bond.^17,18 They directly influence the Schottky barrier, which serves as an indicator for electron conduction at the oxide-electrolyte interface.^19,20,21 Therefore, designing electrocatalysts for efficient electron transfer is crucial and in demand.^21,22,23 In addition, the four-electron OER mechanism relies on intermediates containing oxygen, such as ∗OH, ∗O, and ∗OOH, and SnO₂ has a lower binding affinity for these compounds.^24,25 When compared to more reactive oxides such as RuO₂ or IrO₂, SnO₂ exhibits an inert nature, making it a poor choice for facilitating these processes.^26,27

A tuning of the electronic properties of the SnO₂ could be a practical way to address the efficient electron transfer process at the electrode-electrolyte interface.²⁸ To alter the electronic structure, the incorporation of aliovalent or isovalent cations into the SnO₂ lattice can be an effective strategy.²⁹ For example, transition metals such as Co, Ni, Fe, or Mn incorporation increases the amount of oxygen vacancies and improves conductivity by creating new electronic states close to the conduction band (CB) edge.³⁰ The impact of transition metals doping, including Ag, Zn, Ru, Co, and Mn, on the electrochemical characteristics of SnO₂ has been widely.^31,32 Whereas strontium (Sr²⁺) or another alkaline earth metal could improve OER performance by changing the band structure, increasing defect density, and facilitating charge carrier mobility.³³ Sr-doped SnO₂ has been widely explored for photocatalytic applications.^34,35 For example, Haya et al.³⁶ reported the Sr-doped SnO₂ for the degradation of methylene blue. Ahmed et al.³⁷ have reported that the Sr doping generates the oxygen vacancies in the Sr-doped SnO₂ which enhanced the methylene blue photodegradation. However, the Sr-incorporated SnO₂ could further induce defects in the SnO₂ lattices and could also shift the band positions. The defects in the SnO₂ lattice and the electronic band positions would be vital for improved catalytic activity of the SnO₂. Moreover, it is well known that the SnO₂ supported metal oxides electrocatalysts have been explored for the electrocatalytic OER and HER applications.^{38,39,40,41,42,43,44,45,46,47} However, the Sr-incorporated SnO₂ could yield interesting electrocatalytic outcomes due the larger defects and optimal band position for efficient electron transfer for OER.

In this work, we synthesized Sr-incorporated SnO₂ by precipitation method due to simplicity, scalability, and cost-effectiveness and studied the influence of Sr incorporation on the SnO₂ lattice for OER applications. To explore the effect of Sr incorporation, the Sr molar ratio was increased from 10% to 90% in SnO₂. The as-synthesized electrocatalysts were characterized by powder X-ray diffraction (pXRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) techniques. The optical analysis was performed by UV-Vis absorptions spectroscopy to determine the band gap and the valence band, whereas, the CB was determined by Mott-Schottky plot. The 50% Sr-incorporated SnO₂ shows the 430 mV overpotential to reach the 10 mA cm⁻² and Tafel slope of 68 mV dec⁻¹. The electrochemical analysis shows that the introduction of Sr²⁺ into the SnO₂ enhances the number of active sites, increases the surface area, and creates oxygen vacancies in the host lattice, and optimized band positions resulting in enhanced electrical conductivity. These results will offer insights into the design of new materials to enhance OER activities. This work demonstrates how aliovalent alkaline-earth (Sr) doping modulates the electronic structure and defect chemistry of SnO₂, a low-cost, stable oxide rarely optimized for OER. By shifting the CB and inducing lattice strain, we enhance charge transfer kinetics, offering a scalable, single-phase alternative to complex composite electrocatalysts for water splitting.

Results and discussion

Structure of the catalysts

Sr-incorporated SnO₂ with various molar ratios were synthesized by adding a calculated amount of strontium nitrate into the tin (II) chloride solution before NaOH addition. The chemical steps involved in synthesis are presented in Figure 1 for 10%–90% Sr incorporated tin oxide. The molar ratios were adjusted by increasing concentration of Sr and decreasing the concentration of Sn molar ratios. Moreover, for comparison pure SnO₂ and SrO were synthesized.

Figure 1.

Illustration of the synthesis of Sr-incorporated SnO₂

Figure 2A show the pXRD spectra of pure and Sr-incorporated SnO₂. Pure SnO₂ shows diffraction peaks at 2 theta angles of 25.89°, 33.13°, 37.12°, 50.83°, 54.04°, 56.76°, 61.02°, 63.85°, 65.09°, 70.71°, and 77.88°. These are indexed as (110), (101), (200), (220), (002), (102), (112), (301), (202), and (321) planes, which belong to the rutile-type SnO₂ crystal structure (space group P42/mm) as specified by JCPDS no. 41–1445.⁴⁸ SrO shows major diffraction peaks at 23.15°, 36.73°, 51.23°, 61.15°, and 73.21° from (110), (200), (220), (311), and (400) diffraction planes, respectively, which correspond to the cubic phase of SrO (JCPDS file no: 48–1477).⁴⁹ Sr-incorporated SnO₂ exhibits diffraction peaks at 25.57°, 33.04°, 36.84°, 50.66°, 53.75°, 56.43°, 60.51°, 62.21°, 64.40°, 70.34°, and 77.16° that are indexed as (110), (101), (200), (220), (002), (102), (112), (301), (202), and (321) planes, respectively. As shown in Figure 2B, the most intense peak exhibits a shift and broadening with increasing Sr concentration. Notably, the peak for the 10% Sr sample initially shifts to a slightly lower angle compared to pure SnO₂, which may be attributed to lattice stress relaxation. However, with further increases in Sr content (30%–50%), the peak shifts toward higher angles and broadens significantly. This trend at higher concentrations is indicative of lattice contraction and increased microstrain induced by the incorporation of Sr ions into the SnO₂ matrix. The peak shift is attributed to lattice expansion caused by the substitution of Sn⁴⁺ ions (0.71 Å) by larger Sr²⁺ ions (1.12 Å). Furthermore, the peak broadening is attributed to a reduction in crystallite size and increased microstrain induced by the lattice distortion upon Sr incorporation, as confirmed by Williamson-Hall analysis (Figure 2C; Table 1). Given the significant difference in ionic radius between Sn⁴⁺ (0.71 Å) and Sr²⁺ (1.12 Å), the incorporation mechanism is complex and likely involves a combination of substitutional and interstitial doping, rather than uniform lattice substitution. The large ionic radius of Sr²⁺ induces substantial lattice distortion and microstrain as shown in Figure 2C, suggesting that Sr²⁺ ions may occupy interstitial sites or distorted lattice positions. To maintain charge neutrality due to the valence mismatch (2+ vs. 4+), the incorporation of Sr²⁺ likely promotes the formation of oxygen vacancies. Furthermore, at higher Sr²⁺ concentrations (70%–90%), the solubility limit is exceeded, leading to surface segregation and the formation of secondary SrO rich phases, consistent with the observed agglomeration and performance decline.⁵⁰ Two separate techniques, the Debye-Scherrer and the Williamson-Hall (W-H) methods, were used to find the average crystallite size. The Debye-Scherrer and W-H formulas and given by Equations 1 and 2, respectively.

$$D=\frac{0·98\lambda }{\beta \cos \theta }$$ (Equation 1)

$$\frac{\mathrm{B}\cos \mathrm{\theta }}{\mathrm{\lambda }}=\frac{\mathrm{k}}{\mathrm{D}}+\frac{\mathrm{\eta }\sin \mathrm{ϴ}}{\mathrm{\lambda }}\text{.}$$ (Equation 2)

Where D is the crystalline size; η is the effective strain; θ is the glancing angle; k is the form factor; β is the full width at half maximum (FWHM). While the reciprocal of the intercept measures the crystallite size (D), the effective lattice strain (η) is determined from the slope of the linear fit of the W-H plot as shown in Figure 2C. It is observed that the crystalline size calculated by both the aforementioned methods decreases with the Sr-concentration increase in the SnO₂ lattice (Table 1). The 50% Sr-incorporated SnO₂ shows the lowest crystalline size calculated by both methods. The small crystalline size offers more surface area with large number of active sites. Likewise, the lattice strain in the SnO₂ lattice was induced by Sr incorporation, which restricts the grain evolution. The ionic radius of Sr²⁺ is 1.12 Å, and Sn⁴⁺ is 0.71 Å.⁴⁸ The large difference in ionic radius causes the large lattice strain. The maximum strain was observed in the 50% Sr-incorporated SnO₂. The large lattice strain indicates more lattice defects and lattice changes on the surface of materials, which alters the binding behaviors of adsorbents.^50,51 Moreover, Equation 3 was used to find out the lattice parameters (a = b ≠ c) for tetragonal constructions, as given in Table 1 a

$$\frac{1}{d_{hkl}^2}=\frac{h^2+k^2}{a^2}+\frac{l^2}{c^2}\text{,}$$ (Equation 3)

where d is the interplanar distance, (h k l) are the Miller indices of the crystallographic planes, and (a, b, and c) are the lattice parameters. The lattice parameters also increase upon increasing Sr incorporation in the SnO₂ lattice, which is also due to the larger ionic radius of Sr²⁺ than Sn⁴⁺. The systematic increase in lattice parameters and the corresponding rise in microstrain (Table 1) provide strong evidence for the incorporation of Sr ions into the SnO₂ lattice. At higher Sr concentrations (70% and 90%), the solubility limit of Sr in the SnO₂ lattice is likely exceeded due to the large ionic radius mismatch, leading to phase separation and the formation of SrO like phases. This structural change correlates with the observed decline in OER performance for these samples compared to the optimally doped 50% Sr-incorporated SnO₂.

Figure 2.

pXRD and Williamson-Hall plot of pure SnO₂ and Sr-incorporated SnO₂ and pure SrO

(A) pXRD patterns of pure SnO₂, Sr-incorporated SnO₂, and pure SrO.

(B) Magnified spectra of peak (100) plan of pure SnO₂, Sr-incorporated SnO₂, and pure SrO.

(C) W-H plot with linear fitting of of pure SnO₂, Sr-incorporated SnO₂, and pure SrO.

Table 1.

Crystal size, microstrain, and lattice parameters of pure SnO₂ and 10%–50% Sr-incorporated SnO₂

Catalysts	Debye Scherrer crystal size (nm)	W-H crystal size (nm)	Strain (η) × 10−4	Lattice parameters
				a = b	c
Pure SnO2	9.83	12.31	6.54	4.752	3.174
10% Sr	8.73	11.87	7.21	4.757	3.179
30% Sr	7.61	10.83	8.24	4.761	3.201
50% Sr	6.39	9.24	10.3	4.766	3.204

Figure 3 shows the SEM images of the pure SnO₂ and Sr-incorporated SnO₂ and EDX elemental mapping. Most of the pure SnO₂ particles have flake-like morphology (Figure 3A). The EDX elemental mapping shows uniform distribution of Sn and O in the as-synthesized SnO₂ (Figures 3B and 3C). Likewise, pure SrO shows large particles of undefined morphology due to agglomeration (Figure 3D). EDX elemental mapping also confirms the presence of Sr and O in the pure SrO (Figures 3E and 3F). The 30% Sr-incorporated SnO₂ produces small particles with granular morphology as shown in Figure 3G. The representative elements are also observed in the 30% Sr-incorporated SnO₂ samples (Figures 3H–3J). Further increase in Sr concentration to 50% yields smaller spherical particles with the disappearance of large flakes (Figure 3K). The formation of small particles is well aligned with the XRD results. Furthermore, the uniform distribution of Sr, Sn, and O in the as-synthesized samples is observed in elemental mapping as shown in Figures 3L–3N. The further increase in Sr concentration to 70% in SnO₂ causes particle aggregation, causing the increase in size as shown in Figure 3O. The aggregation of particles is due to the reduced surface energy by Sr incorporation into SnO₂, which stabilizes particles by forming larger particles. EDX mapping also reveals the uniform distribution of elements in the samples (Figures 3P–3R).

Figure 3.

Morphological characterization and elemental distribution of as-synthesized SnO₂ compositions

(A) SEM image of pure SnO₂ at resolution of 50 μm.

(B) EDX elemental mapping showing the distribution of O in pure SnO₂ at resolution of 0.2 mm.

(C) EDX elemental mapping showing the distribution of Sn in pure SnO₂ at resolution of 0.2 mm.

(C) SEM image of pure SrO at resolution of 20 μm.

(E) EDX elemental mapping showing the distribution of O in SrO at resolution of 0.5 mm.

(F) EDX elemental mapping showing the distribution of Sr in SrO at resolution of 0.5 mm.

(G) SEM image of pure 30% Sr-incorporated SnO₂ at resolution of 50 μm.

(H) EDX elemental mapping showing the distribution of O in 30% Sr-incorporated SnO₂ at resolution of 0.5 mm.

(I) EDX elemental mapping showing the distribution of Sn in 30% Sr-incorporated SnO₂ at resolution of 0.5 mm.

(J) EDX elemental mapping showing the distribution of Sr in 30% Sr-incorporated SnO₂ at resolution of 0.5 mm.

(K) SEM image of pure 50% Sr-incorporated SnO₂ at resolution of 10 μm.

(L) EDX elemental mapping showing the distribution of O in 50% Sr incorporated SnO₂ at resolution of 0.2 mm.

(M) EDX elemental mapping showing the distribution of Sn in 50% Sr incorporated SnO₂ at resolution of 0.2 mm.

(N) EDX elemental mapping showing the distribution of Sr in 50% Sr incorporated SnO₂ at resolution of 0.2 mm.

(O) SEM image of 70% Sr-incorporated SnO₂ at resolution of 50 μm.

(P) EDX elemental mapping showing the distribution of O in 70% Sr-incorporated SnO₂ at resolution of 0.1 mm.

(Q) EDX elemental mapping showing the distribution of Sn in 70% Sr-incorporated SnO₂ at resolution of 0.1 mm.

(R) EDX elemental mapping showing the distribution of Sr in 70% Sr-incorporated SnO₂ at resolution of 0.1 mm.

Figures S1A–S1E (supplemental information) shows the EDX spectra of pure and Sr-incorporated SnO₂. Table 2 displays the calculated and observed atomic% of Sn, Sr, and O in as-synthesized samples. To better understand the impact of Sr incorporation on the O and Sn, the O/Sn, and Sr/Sn ratio were calculated from the EDX results. It is observed that all the as-synthesized samples are oxygen deficient and Sn rich. However, SrO was oxygen rich in nature. Therefore, the incorporation of Sr in the SnO₂ produces the oxygen deficient materials. However, the large discrepancies reflect differences between stoichiometric atomic fractions and semi-quantitative EDX results rather than calculation errors. EDX is a surface or near-surface, semi-quantitative technique with limited sensitivity to light elements and to elements present at very low concentration; it is also sensitive to acquisition parameters (accelerating voltage, detector window), matrix absorption, and peak overlaps. In our case, the most likely contributors are undercounting of oxygen and low Sr counts (below or near the detection limit) and sampling inhomogeneity (Sr may be present in a minor/segregated phase). The homogeneous distribution of Sr within the SnO₂ matrix was confirmed by SEM-EDX elemental mapping (Figure 3). The observed lattice expansion and increased microstrain (Table 1) confirm the presence of lattice distortions and defects induced by Sr incorporation.

Table 2.

Weight % and atomic % of synthesized materials from EDX spectra

Sample	Sn (Cal.)	Sn (Obs.)	% error	Sr (Cal.)	Sr (Obs.)	% error	O (Cal.)	O (obs.)	% error	O/Sn (cal.)	O/Sn (obs.)	% error	Sr/Sn (Cal.)	Sr/Sn (Obs.)	% error
SnO2	33.3	68.3	104.9	–	–	–	66.6	31.6	−52.4	2	0.46	−76.8	–	–	–
30% Sr	25.9	48.3	86.5	11.1	0.22	−98.0	62.9	26.9	−57.2	2.42	0.55	−77.0	0.42	0.004	−98.9
50% Sr	20	29.9	49.8	20	0.17	−99.5	60	30.9	−48.4	3	1.03	−65.5	1	0.005	−99.4
70% Sr	13.0	37.9	191.1	30.4	0.24	−99.2	56.5	31.3	−44.5	4.33	0.82	−80.9	2.33	0.006	−99.7
SrO	0	0	–	50	15.8	−68.2	50	84.1	68.2	–	–	–	–	–	–

Electrochemistry of the catalysts

The as-synthesized electrocatalysts exhibited distinct cyclic voltammetry (CV) behavior in 1.0 M KOH, as shown in Figures S2A–S2G (supplemental information). For more detailed explorations, we further increased the number of molar ratios that includes 10% Sr, 30% Sr, 50% Sr, 70% Sr, 90% Sr, along with pure SnO₂ and SrO and measured their electrochemical behavior under same conditions. In the cyclic voltammograms of the pure and Sr-incorporated SnO₂, distinct oxidation and reduction peaks are observed. These peaks are assigned to the quasi-reversible redox transformation between Sn²⁺ and Sn⁴⁺ states. In the alkaline electrolyte, this process is associated with the formation of Sn-OOH species on the electrode surface. The quasi-reversible nature of these peaks, rather than purely capacitive behavior, is attributed to the modified surface chemistry and increased density of oxygen vacancies induced by Sr incorporation, which enhances the surface reactivity toward the electrolyte ions. Increasing the scan rate leads to higher current densities in both oxidation and reduction peaks (Figures S2A–S2G supplemental information). This arises from quasi-reversible reactions occurring at the electrolyte-electrode junction.⁵² The anodic and cathodic peak currents versus the square root of the scan rate exhibit a linear relationship as shown in Figures 4A and 4B. The 50% Sr incorporated SnO₂ exhibits the highest slope, which is 4.029 V^1/2 (mV)^1/2 for anodic peak current and −2.836 V^1/2 (mV)^1/2 for cathodic peak current, respectively. These slopes are inductive of the OH⁻ ion diffusion on the surface of the electrode and suggest the enhanced formation of species like Sn-OOH. The formation of large number of Sn-OOH species, indicates the 50% Sr-incorporated SnO₂ will have enhanced electrocatalytic performance. At the electrode to electrolyte junction, a double layer is established, comprising a charge layer on the surface of the electrode and a countercharge layer within the electrolyte. The average double layer capacitance (Cdl) values for pure SnO₂, 10%, Sr, 30% Sr, 90% Sr, and SrO were found to be 0.02 mF cm⁻², 0.07 mF cm⁻², 0.11 mF cm⁻², 0.12 mF cm⁻², 0.10 mF cm⁻², 0.05 mF cm⁻², and 0.02 mF cm⁻², respectively (Figure 4C). The 50% Sr-incorporated SnO₂ exhibits the highest Cdl value of 0.12 mF cm⁻². Higher Cdl values suggest that the 50% Sr-incorporated SnO₂ has more active sites for electrocatalytic activity. Electrochemical surface area (ECSA) is an additional metric to correlate the catalytic efficiency of the materials. It is computed using the Cdl values. The ECSA of pure SnO₂, 10%–90% Sr-incorporated SnO₂ and pure SrO were 0.30 cm², 0.94 cm², 1.37 cm², 1.60 cm², 1.31 cm², 0.64 cm², and 0.32 cm², respectively, as shown in Figure 4D. As expected, the 50% Sr incorporation SnO₂ exhibits the highest ECSA value of 1.60 cm², which shows that this electrocatalyst has a large surface area and more active sites for electrocatalytic activity. Based on these findings, it is expected that 50% Sr-incorporated SnO₂ could show improved electrocatalytic performance compared to pure SnO₂, the other Sr-incorporated concentrations, and the SrO electrocatalysts.

Figure 4.

Electrochemical studies of as-synthesized samples to calculate Cdl, and ECSA

(A) Plot of the anodic current densities versus the square root of different scan rates.

(B) Plot of the cathodic current densities versus the square root of different scan rates.

(C) Double layer capacitances (Cdl) of as-synthesized samples.

(D) Bar graph illustrating the electrochemical surface area (ECSA) of as-synthesized SnO₂, Sr-incorporated SnO₂, and pure SrO in 1.0 M KOH electrolyte.

Linear sweep voltammetry (LSV) was conducted using a typical three-electrode setup in 1.0 M KOH medium within a potential range of 1.3–2.0 V (vs. RHE). The onset potential for pure SnO₂, 10%–90% Sr-incorporated SnO₂ and for pure SrO were observed at 1.65 V, 1.62 V, 1.58 V, 1.46 V, 1.51 V, 1.66 V, and 1.70 V (vs. RHE), respectively (Figure 5A). Whereas the overpotentials at 10 mA cm⁻² were 600 mV, 560 mV, 460 mV, 430 mV, 470 mV, 560 mV, and 580 mV for pure SnO₂, 10%–90% Sr-incorporated SnO₂, and SrO, respectively. The performance of the 50% Sr-incorporated SnO₂ indicates that it is a promising non-precious metal oxide catalyst for OER applications. The performance is comparable to many reported non-precious metal oxide electrocatalysts. While noble metal benchmarks like RuO₂ typically show lower overpotentials, the 50% Sr-incorporated SnO₂ serves as a cost-effective alternative with reasonable activity.⁵³ While this overpotential of Sr-incorporated SnO₂ is higher than that of noble metal benchmarks such as RuO₂ or IrO₂ (<300 mV), the Sr-incorporated SnO₂ catalyst offers significant economic advantages and stability, making it a competitive candidate for large scale applications where catalyst cost is a critical factor. Whereas, the bare Ni foam exhibits an overpotential of 817 mV at 10 mA cm⁻² (Figures 5A and S3A). As expected, the 50% Sr-incorporated catalyst shows the lowest overpotential among all synthesized electrocatalysts. The lower overpotential value of 50% Sr-incorporated SnO₂ electrocatalysts is well aligned with the Cdl and ECSA results. A lower overpotential indicates that the electrochemical process is facilitated with a reduced applied voltage. The dependence of OER performance on Sr content (Figure 5A) exhibits a volcano-type trend, peaking at 50% Sr incorporation. The enhancement from 10% to 50% Sr is attributed to the synergistic effects of increased lattice strain (inducing more defect active sites), optimized band alignment (negative CB shift), and enlarged ECSA. However, at higher concentrations (70% and 90%), the performance declines. This is likely due to the saturation of Sr in the SnO₂ lattice, leading to phase separation (formation of SrO clusters) and particle aggregation, which disrupt the conductive network and reduce the number of accessible active sites. To distinguish the intrinsic activity from the surface area effects, the specific activity was calculated by normalizing the geometric current density with the ECSA. The specific activity normalized by ECSA was determined at 400 mV overpotential. The pure SnO₂ shows the specific activity of 1.15 mA cm⁻², while 0.37 mA cm⁻², 2.69 mA cm⁻², 3.17 mA cm⁻², 2.91 mA cm⁻², 1.01 mA cm⁻², and 1.06 mA cm⁻² specific activity values were observed for 10%Sr, 30% Sr, 50% Sr, 70% Sr, 90% Sr, and Pure SrO, respectively. The 50% Sr-incorporated SnO₂ exhibited the highest specific activity, confirming that the enhancement in OER performance is driven by both the increased electrochemical surface area and the modified intrinsic electronic properties.

Figure 5.

Electrocatalytic performance characterization of SnO₂, 10%–90% Sr-incorporated SnO₂, and SrO

(A) iR corrected LSVs of pure SnO₂, 10%–90% Sr-incorporated SnO₂, and SrO were recorded at 5 mV s⁻¹ scan rate in 1.0 M KOH electrolyte.

(B) Tafel plot to determine the kinetic parameters of as-synthesized pure SnO₂, 10%–90% Sr-incorporated SnO₂, and SrO electrocatalyst.

(C) EIS Nyquist plot of pure SnO₂, 10%–90% Sr-incorporated SnO₂, and SrO electrocatalysts. Inset: circuit diagram of constant phase element (CPE) fitting model.

(D) Chronopotentiometry of 50% Sr-incorporated SnO₂ electrocatalysis over 19,000 s.

Electrocatalysis performance is determined by considering kinetics as an additional parameter. The Tafel plot is a visual depiction employed to examine the kinetics of electrochemical processes. Figure 5B shows the Tafel slopes for pure SnO₂, 10%–90% Sr-incorporated SnO₂ and pure SrO, which were 202 mV dec⁻¹, 118 mV dec⁻¹, 107 mV dec⁻¹, 68 mV dec⁻¹, 139 mV dec⁻¹, 147 mV dec⁻¹, and 266 mV dec⁻¹, respectively. The lower Tafel slope of 50% Sr-incorporated SnO₂ suggests an accelerated reaction kinetics and an increased efficiency of the catalyst.

Extensive comparison based on the overpotential and Tafel slope of different SnO₂-based electrocatalysts and as-synthesized Sr-incorporated SnO₂ is given in Table 3. The Tafel slope is a crucial parameter in electrocatalysis, reflecting the kinetics of the catalytic reaction. Lower Tafel slopes indicate improved kinetics and increased catalytic efficiency, making them highly desirable for applications like the OER. Various heterojunctions or composites of SnO₂ have shown lower overpotential and fast kinetics due to more active sites and favorable charge generation compared to Sr-incorporated SnO₂.²¹ However, Sr-incorporated SnO₂ shows significantly good overpotential and Tafel slope as electrocatalysts. As shown in Table 3, SnO₂-based heterojunctions or composites often exhibit lower overpotentials than the Sr-incorporated SnO₂ reported here. However, these superior metrics are typically attributed to synergistic effects at the heterointerfaces rather than the intrinsic modification of the SnO₂ lattice. In the context of single-phase SnO₂ catalysts, our 50% Sr-incorporated SnO₂ demonstrates competitive performance. Its enhancement stems from the effective modulation of the electronic structure, offering a scalable and cost effective strategy that avoids the complexity of composite synthesis.⁴³ These electrocatalysts offers wide range of factors that improve the performance; however, in the present study, the effect of band position and oxygen vacancies has been explored.^{17,38,40,43,46,54,55,56,57,58,59,60}

Table 3.

Comparison of the performance of SnO₂-based electrocatalysts

Electrocatalysts	Overpotential (mV)@10 mA cm−2	Tafel slope (mV dec−1)	Reference
SnO2/MnTe	181	33	Zahra et al.38
IrO2@SnO2	240	65	Bhanja et al.40
NiO-SnO2	320	149	Naresh et al.43
SnO2@CrS2	142	52	Aslam et al.54
SnO2/MnO2	360	38.9	Alwadai et al.55
SnO2QDs@CoFe2O4	290	85	Sreekanth et al.56
CuO-SnO2	139	245	Kumar et al.46
Co-doped SnO2@CoS	200	99	Nath et al.17
CdSe/SnO2	233	89	Nisa et al.57
Mn3O4@SnO2/Co3O4	420	70.1	Song et al.58
SnO2@g-C3N4@SnS2	475	253	Zhang et al.59
SnO2@NiFe-LDH	209	47.7	Yin et al.60
Sr-incorporated SnO2	430	68	Present work

Electrochemical impedance spectroscopy (EIS) was conducted in a 1.0 M KOH solution under open-circuit voltage, and the results are fitted with an equivalent constant phase element (CPE) circuit, as shown in Figure 5C. The CPE circuit components, Rsol is solution resistance represents the ohmic resistance of the electrolyte, separator, and cell hardware. It is the high-frequency intercept in the Nyquist plot and reflects the ionic conductivity of the electrolyte and the geometry of the cell. Rct (charge-transfer resistance, Ω) parameter corresponds to the resistance to electron transfer across the electrode/electrolyte interface. It is associated with the semicircle diameter in the Nyquist plot and reflects the kinetics of the electrochemical reaction occurring at the electrode surface. In the CPE, Y₀ is the magnitude of the admittance. It quantifies the capacitive response of the interface. A higher Y₀ indicates greater interfacial capacitance, which is influenced by double-layer formation, electrode surface area, and porosity. In a CPE circuit diagram, the parameter α (alpha) represents the phase angle factor that describes how far the system deviates from an ideal capacitor. In electrocatalysis, α is crucial because it reflects surface heterogeneity, roughness, and non-ideal charge distribution at the electrode-electrolyte interface, directly influencing catalytic efficiency.⁶¹

Pure SnO₂, 10%–90% Sr-incorporated SnO₂, and SrO have solution resistances (Rsol) of 4.4 Ω, 2.96 Ω, 2.08 Ω, 1.93 Ω, 3.42 Ω, 3.44 Ω, and 3.43 Ω, respectively (Table 4). The Ni foam shows the Rsol value of 2.53 Ω (Table S1, supplemental information; Figure S3B, supporting information). A low solution resistance indicates a high conductivity of the electrolyte. It implies that the electrolyte facilitates a smooth and effective passage of the electric current, hence reducing the ohmic drop across the solution. While Rsol (solution resistance) is anticipated to remain constant under the same electrolyte conditions, slight variations in the observed values can be attributed to physical differences in the catalyst layer, its interaction with the Ni foam, and its porosity, all of which affect ionic transport. The modifications have a negligible effect on the assessment of charge transfer dynamics, which is mainly regulated by Rct.⁶² The Rct values of pure SnO₂, 10%–90% Sr-incorporated SnO₂, SrO, and RuO₂ were 18.45 Ω, 11.91 Ω, 3.86 Ω, 1.12 Ω, 11.84 Ω, 8.97 Ω, 14.74 Ω and 6.36 Ω, respectively in Table 4. The lowest Rct is obtained for the 50% Sr-incorporated SnO₂, suggesting a faster electron transfer relative to the other catalysts. Y₀ denotes the amplitude of the impedance associated with the CPEs fitting model. However, due to variable reaction kinetics, electrode inhomogeneity, surface roughness, and porosity, real surfaces behave differently from ideal ones. Effective capacitance, as well as the electrode surface’s heterogeneity and roughness, are all provided by the CPE fitting model.⁶³ A dimensionless parameter that ranges from 0 to 1, the CPE exponent (α) indicates the degree of divergence from an ideal capacitor when using a CPE in circuit analysis.⁶⁴ A more uniform charge distribution and increased charge storage capacity are indicated by a larger value of alpha, 50% Sr-incorporated SnO₂ exhibits a smoother and more uniform electrode surface.

Table 4.

EIS parameters obtained from the EIS data fitting with CPE model

Catalyst	Rct Ω	Rsol Ω	Yo	Alpha
SnO2	18.45	4.4	0.006	0.71
10% Sr	11.91	2.96	0.026	0.69
30% Sr	3.86	2.08	0.115	0.41
50% Sr	1.12	1.93	0.114	0.66
70% Sr	11.84	3.42	0.035	0.81
90% Sr	8.97	3.44	0.032	0.82
Pure SrO	14.74	3.43	0.044	0.95
RuO2	6.36	2.19	0.028	0.72

Based on the aforementioned results, the 50% Sr-incorporated SnO₂ electrocatalyst shows an efficient OER performance with low overpotential, fast kinetics, and low resistance. Therefore, the short terms stability of the synthesized 50% Sr-incorporated SnO₂ was assessed at a steady voltage of 1.4 V, with current measurements taken over 19,000 s (5.2 h). As illustrated in Figure 5D, the 50% Sr catalyst displays a well-defined plateau with minimal distortion, suggesting that the electrode retains stable electrochemical performance under the applied current. No notable changes or degradation were observed at the electrode surface that could influence the peak shape or position indicating the stability of the electrocatalysts in the tested duration.²⁵

Correlation between OER performance and electronic structure of Sr-incorporated SnO₂

The electronic structures of Sr-incorporated SnO₂ were studied by combining Mott-Schottky plots with UV-Vis absorption spectroscopy. Table 5 gives the calculated band gaps, CB, and valence band of as-synthesized samples. To determine the band gap, the UV-Vis absorption spectroscopy was used. Figure S4A (supplemental information) shows the UV-Vis absorption spectra of pure SnO₂ and Sr-incorporated SnO₂ and pure SrO. It can be observed that the absorption band edges shifted to longer wavelengths with an increase in the concentration of Sr in SnO₂. Therefore, the incorporation of Sr into the SnO₂ lattice causes the optical band gap to decrease. The band gaps of pure SnO₂, 10%–90% Sr, and pure SrO were 3.55 eV, 3.37 eV, 3.15 eV, 2.38 eV, 3.09 eV, 4.74 eV, and 4.71 eV, respectively, as shown in Figures S3B–S3H (supplemental information). The reduced band gap is likely attributed to the formation of metal interstitials, oxygen vacancies, and sp-d hybridization interactions. The significant shift in the CB potential (Table 5) and the corresponding improvement in charge transfer kinetics (Figure 5) provide strong indirect evidence for the modification of the electronic structure and defect landscape. Notably, the band gap increases significantly for the 90% Sr sample (4.74 eV), deviating from the narrowing trend observed at lower concentrations. This anomaly is attributed to the phase separation and surface segregation that occurs at high Sr loadings. The formation of insulating SrO rich secondary phases and severe particle agglomeration disrupts the continuous electronic structure of SnO₂, leading to an apparent widening of the optical band gap.

Table 5.

Band gaps, conduction, and valence bands of pure SnO₂, 10%–90% Sr incorporated SnO_2, and pure SrO

Catalysts	Band gap (Eg, eV)	Conduction band (ECB, eV)	Valence band (EVB, eV)
SnO2	3.55	0.04	3.59
10% Sr	3.22	−0.02	3.2
30% Sr	3.12	−0.14	2.98
50% Sr	2.38	−0.52	1.86
70% Sr	3.08	−0.47	2.61
90% Sr	4.74	−0.42	4.32
Pure SrO	4.72	−0.33	4.39

The Mott-Schottky plot was used to determine the CB energy of as-synthesized samples, as given in Figure 6A. Pure SnO₂ has a CB potential of 0.04 V. The incorporation of Sr into SnO₂ shifted the CB potential to a more negative value. The incorporation of 10%–90% Sr into SnO₂ shows the CB potentials of −0.02, −0.14, −0.52, −0.47, and −0.42 V, while it is −0.33 V in pure SrO. A more negative CB potential signifies a reduction in the energy levels of the CB relative to the reference electrode.²⁰ Since SnO₂ is an n-type semiconductor, the positive slope in the Mott-Schottky plot confirms the presence of donor states. The flat-band potential determined from the intercept corresponds to the bottom of the CB (ECB).

Figure 6.

Mott-Schottky plot and illustration showing the shifting of band potentials

(A) Mott-Schottky plot of pure SnO₂, 10%–90% Sr-incorporated SnO_2, and pure SrO with their respective CB potentials.

(B) Illustration of the CB shifting to a more negative potential and valence band to a positive potential, and introduction of hole states below the CB maximum by Sr incorporation of SnO₂.

An increase in the concentration of Sr in SnO₂ tunes the electronic structure of SnO₂, leading to an improvement in electron transfer during the electrocatalysis. Sr²⁺ incorporation in the SnO₂ shifts the CB toward a more negative potential (vs. RHE) away from the Fermi level, whereas the valence band potential shifts upward closer to the Fermi level. In this case, the Fermi level moves closer to the valence band maximum (VBM) of SnO₂ and induces the formation of hole states below the CB minimum (CBM), close to the Fermi level (Figure 6B).⁶⁵ Moreover, Sr incorporation also increases the filling of electrons in the VBM which is a key parameter to improve the orbital overlap with OER intermediates, that favor the formation of these intermediates.⁶⁶ Therefore, 50% Sr-incorporated SnO₂ having the highest CB potential (−0.52 eV) and lowest VB potential (1.86 eV vs. RHE) with electron-rich VBM and electron-deficient CBM has improved OER performance. The shift of the CB toward a more negative potential is relevant to OER kinetics as it facilitates interfacial electron transfer. In n-type semiconductors like SnO₂, a more negative CB position (closer to the vacuum level) reduces the energy barrier for charge transport, which is experimentally confirmed by the reduced Rct in EIS. Furthermore, the increase in lattice strain (η) and the narrowing of the band gap (Eg) provide strong evidence for defect formation (e.g., oxygen vacancies) induced by Sr incorporation. At higher concentrations (70%–90%), the electrochemical performance and peak broadening suggest the onset of phase coexistence, where excess Sr segregates to form insulating phases, disrupting the conductive SnO₂ network.

Besides electronic structure optimization, the Sr-incorporated SnO₂ also show highly oxygen-deficient structures due to the ionic radii differences and structural distortions.⁶⁷ Both factors increase the active surface area of 50% Sr-incorporated SnO₂ and the electrocatalytic active sites on the electrode surface. The Sr-incorporated SnO₂ catalyst (50% Sr) exhibits an overpotential of 430 mV at 10 mA cm⁻², surpassing certain noble-metal benchmarks such as IrO₂ and NiFe-LDH. Nonetheless, it serves as an effective and economical option functioning proficiently as a catalyst. Considering the trade-off among cost, stability, and scalability, its performance is highly promising. The systematic Sr-incorporation approach aids in elucidating the modulation of activity in SnO₂-based electrocatalysts. The superior OER performance observed at the intermediate doping level (50% Sr) can be systematically explained by the interplay of three key parameters: lattice strain, defect concentration, and charge transfer resistance. First, the Williamson-Hall analysis indicates that lattice strain increases progressively up to 50% Sr. This strain distorts the lattice, tuning the adsorption energies of OER intermediates. Second, the defect concentration, reflected by the band gap narrowing and ECSA, peaks at 50%, maximizing the number of active sites. Third, the Rct reaches its minimum (1.12 Ω) at this composition, attributed to the optimal alignment of the CB (Table 5). At 10% Sr, the strain and electronic modulation are insufficient. Conversely, at 70%–90% Sr, excessive loading leads to phase separation and agglomeration, which disrupt the conductive pathways and increase Rct, thereby lowering the catalytic activity. Thus, the 50% Sr-doped SnO₂ represents the optimal balance between structural activation and kinetic efficiency.

In summary, we developed Sr-incorporated SnO₂ electrocatalysts as a cost effective alternative to precious metal-based catalysts. The Sr-incorporated SnO₂ exhibits enhanced OER performance by improved charge transfer due to optimized band potentials and oxygen-deficient structures. The 50% Sr-incorporated SnO₂ achieved an overpotential of 430 mV at 10 mA cm⁻². While further optimization is required to match noble metal performance, this work highlights the potential of band structure tuning in designing economical electrocatalysts for water splitting. The 50% Sr-incorporated SnO₂ has shown 430 mV overpotential at 10 mA cm⁻², presenting a significant 170 mV reduction compared to pure SnO₂ under the identical testing conditions. Whereas the same electrocatalyst has shown a drastically improved Tafel slope of 68 mV dec⁻¹ compared to 202 mV dec⁻¹ for pure SnO₂, indicating much faster reaction kinetics. Our findings have verified that the position of CB and VB and oxygen deficiencies play a vital role in determining the performance of electrocatalysts for OER. This study presents valuable insights for designing efficient and cost-effective electrocatalysts through adjusting the energy bands.

Limitations of the study

While this study demonstrates that Sr incorporation effectively modulates the electronic structure of SnO₂ to enhance OER performance, several limitations should be noted. First, regarding structural characterization, the lack of advanced surface analysis techniques such as X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) limits our ability to provide direct quantification of oxygen vacancy concentrations and atomic-scale visualization of lattice distortions. The conclusions regarding defect formation and phase separation are therefore inferred from pXRD lattice strain analysis and SEM-EDX mapping, rather than direct spectroscopic observation. Second, regarding the stability assessment, the chronopotentiometry test was conducted for 5.2 h at a current density of 10 mA cm⁻². While the catalyst exhibited stable potential retention during this period, this duration is insufficient to predict long term industrial durability (e.g., >1,000 h). Furthermore, post-stability characterizations (XRD/SEM of the used electrode) were not performed to verify structural integrity or elemental leaching (e.g., Sr dissolution) over time. Third, regarding the theoretical mechanism, this study relies on experimental Mott-Schottky and UV-Vis data to infer electronic structure modulation. Density functional theory (DFT) calculations were not performed to determine the specific adsorption energies of OER intermediates (∗OH, ∗O, and ∗OOH) on the Sr-incorporated surface. Consequently, the microscopic mechanism remains at the level of experimental correlation rather than atomic-scale theoretical validation. Finally, regarding performance, the overpotential of the optimal catalyst (430 mV at 10 mA cm⁻²) is higher than that of state-of-the-art noble metal benchmarks (RuO₂ and IrO₂). Therefore, the Sr-incorporated SnO₂ system presented here is best positioned as a cost effective, non-precious alternative for applications where catalyst cost is a primary constraint, rather than a direct replacement for noble metals in high-performance applications.

Resource availability

Lead contact

Requests for further information and resources should be directed to and will be fulfilled by the lead contact, Muhammad Aamir (amir.chem@must.edu.pk).

Materials availability

All data reported in this article will be shared by the lead contact upon request.

Data and code availability

• •All data reported in this article will be shared by the lead contact upon request.
• •This article does not report original code.
• •Any additional information required to reanalyze the data reported in this article is available from the lead contact upon request.

Acknowledgments

The authors extend their appreciation to the Researchers Supporting Project number (ORF-2025-579)), King Saud University, Riyadh, Saudi Arabia. Research at Queen’s was also supported by the Natural Sciences and Engineering Research Council of Canada (RGPIN-2020-07016). This research work is supported by the Ministry of Higher Education (MOHE) under the 2023 Translational Research Program for the Energy Sustainability Focus Area (project ID: MMUE/240001), the 2024 ASEAN IVO (project ID: 2024-02), and Multimedia University, Malaysia.

Author contributions

S.A., investigation, data curation, and writing – original draft.; M. Aamir, conceptualization, supervision, writing – review and editing, and project administration; I.E.L., validation, resources, writing – review and editing, and funding acquisition;: Q.W., investigation and formal analysis; J.A., visualization and software; M.E.K., methodology and software; H.A., resources and validation; H.F.A., resources and funding acquisition; M. Akhtaruzzaman, supervision and writing – review and editing; M.S., formal analysis and visualization; J.-M.N., supervision, writing – review and editing, and funding acquisition.

Declaration of interests

The authors declare no competing interests.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Stannous chloride hexahydrate (SnCl2·6H2O)	Sigma Aldrich	\
strontium nitrate (Sr(NO3)2)	Sigma Aldrich	\
sodium hydroxide (NaOH)	Sigma Aldrich	\
Potassium hydroxide (KOH)	Sigma Aldrich	\

Experimental model and study participant details

This study does not involve human subjects or experimental models.

Method details

Synthesis of Sr-incorporated tin oxide

Sr-incorporated tin oxide was synthesized by using the chemical precipitation method. A clear solution was prepared by dissolving 0.5 g of SnCl₂.6H₂O in 100 mL of distilled water under vigorous stirring. Afterwards 1.0 M NaOH solution was added drop-wise until the white precipitates were formed under stirring. The as-obtained precipitates were then filtered and washed with distilled water and ethanol to remove solution impurities and then dried at 120°C for 12 hours before being calcinated at 450°C for 2 hours to get a SnO₂. The same procedure was repeated with the mixture of a calculated amount of strontium nitrate (Sr(NO₃)₂) and SnCl₂.6H₂O in nominal molar ratios (10%-90%) to synthesize the Sr incorporated SnO₂. These values represent the precursor ratios used during the synthesis, which systematically drive the structural and electronic trends observed in this study. The as-obtained Sr incorporated SnO₂ were dried and stored for further characterizations.

Characterization techniques

PXRD diffraction patterns were obtained with the Bruker D8 Discover system, under Cu Kα X-ray irradiation with a wavelength of λ = 1.5406 Å. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) were conducted with the FESEM NOVA IT100LA instrument.

Electrochemical OER measurements

The electrochemical studies were conducted using an electrochemical workstation (Gamry Reference 3000) equipped with a three electrode system in 1.0 M KOH electrolyte solution. The as-synthesized electrocatalysts loaded on nickel foam was used as a working electrode, platinum as a counter electrode, and Ag/AgCl as a reference electrode. The three-dimensional porous architecture of nickel foam facilitates increased catalyst loading and enhanced charge mobility.⁶⁸ To guarantee consistent and repeatable electrochemical measurements, all working electrodes (Ni foam) were cut to a geometric area of 1 x 1 cm². The identical area of electrolyte was used in all the electrochemical tests. We did not alter the prepared electrodes in any way before using them for electrochemical analysis. Control tests indicate that the enhanced electrocatalytic activity is mostly attributable to the Sr-SnO₂ catalyst, even though Ni foam possesses some electrochemical activity in alkaline environments due to surface nickel oxide/hydroxide layers. Each electrochemical measurement was performed thrice to insure the reproducibility. Prior to catalyst loading, the Nickel foam (1 cm × 1 cm) was pretreated to remove the native oxide layer and surface impurities. The Ni foam was sonicated sequentially in 3 M HCl solution for 15 minutes, absolute ethanol for 10 minutes, and deionized water for 10 minutes. The cleaned foam was then dried in a vacuum oven at 60°C for 2 hours. The catalyst ink was prepared by dispersing 5 mg of the catalyst in a mixture of 450 μL of isopropanol, 530 μL of deionized water, and 20 μL of Nafion solution (5 wt%). The ink was drop cast onto pre-cleaned Ni foam and dried at 60°C. For direct benchmarking, commercial RuO₂ (Sigma-Aldrich, 99.9%) were loaded onto Ni foam with an identical mass loading and tested under the same electrochemical conditions as the Sr-incorporated SnO₂ catalysts.

Electrochemical methods including cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and Mott-Schottky plots were used to assess the performance of electrocatalysts in OER. An extra potential is required to overcome the intrinsic kinetic barrier in electrochemical water splitting, relevant to both the hydrogen evolution reaction and the OER, termed overpotential and represented by η. The reversible thermodynamic potential for the OER is 1.23 V relative to the overpotential at the specified current density without iR compensation, as described by Equation 4.⁶⁹

$${\mathrm{\eta }}_{\text{OER}}={\mathrm{E}}_{\text{RHE}}-1.23\mathrm{V}$$ (Equation 4)

The typical benchmark for current density is 10 mA cm⁻² for assessing OER electrocatalysts in solutions. The iR compensation is designed to address the voltage loss (i.e., iR drop) that occurs due to the electrolyte solution present between the working electrode and the reference electrode, where R denotes the resistance of the electrolyte solution. The iR compensation was applied via manual post-test correction. The solution resistance (Rs) was determined from the high-frequency intercept of the Nyquist plot obtained from EIS measurements. The potential was corrected by 85% of the solution resistance as Equation 5.⁷⁰

$${\mathrm{E}}_{\text{iRcorrected}}=\mathrm{E}-\text{iR}$$ (Equation 5)

LSV measurements were conducted in the potential window of 1.3 V to 2.0 V vs. RHE at a scan rate of 5 mV s^-1. Stability was assessed using chronopotentiometry at a constant current density of 10 mA cm^-2. EIS measurements were performed at open circuit potential over a frequency range of 0.1 Hz to 100 kHz with a 10 mV amplitude. The kinetic parameters derived from the Tafel plot are the Tafel slope. The Tafel plot is derived by plotting the logarithm of current density (J) versus the overpotential, using the reconfigured polarization curves, namely linear sweep voltammetry (LSV). It is defined as the dependence of current density on the iR-corrected overpotential. In Equation 6,⁷¹ the Tafel slope is obtained by calculating the slope of the fitted linear segment of the Tafel plot.

$$\mathrm{\eta }=b\log \left[\mathrm{J}\right]+\mathrm{a}$$ (Equation 6)

Where b is the Tafel slope in mV dec^-1, J is the current density in mA cm^-2, and η is the overpotential in V.

The double-layer capacitance (Cdl) was calculated from the slope of the linear lines derived from plotting the difference in double-layer charging current between anodic and cathodic sweeps within a completely non-Faradaic region of the potential window, against the various scan rates. The electrochemically active surface area (ECSA) is calculated by Equation 7.⁷²

$$\text{ECSA}=\frac{\mathrm{C}d\mathrm{l}}{{\mathrm{C}}_{\mathrm{s}}}$$ (Equation 7)

Where C_s is the material's specific capacitance. The Mott-Schottky study was carried out by using 0.1 M NaCl solution as the electrolyte, with an onset potential specified between -0.6 to 0.6 V to calculate the flat conduction band potential of the catalysts. This electrolyte was chosen because it is an inert electrolyte with negligible specific adsorption on the oxide surface, which allows for the accurate determination of the flat band potential and donor density without interference from faradaic reactions or complex ion adsorption that occur in strong alkaline solutions like KOH. The valence band (VB) potential is calculated by Equation 8.⁷³ To determine the potential of the conduction band (E_CB) and valence band (E_VB), we employed an empirical technique that relies on the electronegativity of the semiconductor. Band edge potentials are necessary for comprehending the charge transport mechanism of the OER.

$${\mathrm{E}}_{\text{VB}}={\mathrm{E}}_{\text{CB}}+{\mathrm{E}}_{\mathrm{g}}$$ (Equation 8)

Where E_VB is the energy level at the top of the valence band, E_g is the band gap and E_CB is the energy level near to lower extremity of the conduction band (CB).

Quantification and statistical analysis

We did not perform any statistical analysis to filter out relevant data.

Published: June 30, 2026