Zr-Doped Ru-Based Catalyst for Highly Active and Durable Acidic Oxygen Evolution Reaction

The escalating environmental pollution caused by traditional energy sources (such as coal, oil, and natural gas) has accelerated the development and adoption of clean and renewable energy alternatives [1,2,3]. For proton exchange membrane water electrolysis—a technology capable of producing green hydrogen with high durability and high current density—the anodic oxygen evolution reaction plays a decisive role in determining the energy conversion efficiency [4,5,6]. However, most OER catalysts exhibit low OER kinetics and unsatisfactory stability in acidic environments [7,8,9,10]. Iridium-based materials are regarded as the state-of-the-art catalysts for the OER; nevertheless, their prohibitively high cost poses a significant barrier to commercial expansion [11,12]. In contrast, ruthenium-based catalysts have emerged as promising alternatives for PEM water electrolysis due to their lower cost and outstanding OER activity under acidic conditions [13,14,15]. However, pristine RuO₂ catalysts still suffer from instability in acidic electrolytes. Overoxidation issues and the participation of lattice oxygen in the reaction lead to the rupture and reformation of Ru–O bonds, ultimately resulting in the deactivation of RuO₂ catalysts [16,17]. Therefore, the development of highly active and stable ruthenium-based anodic catalysts is crucial for advancing acidic water electrolysis technology.

Based on existing research, the performance enhancement of Ru-based catalysts primarily focuses on two strategies: the first involves enhancing the intrinsic activity of catalytic materials through the introduction of defect engineering (such as oxygen vacancies), heterojunctions, and lattice distortion; the second strategy entails introducing metal elements with weaker electronegativity than Ru to lower the oxidation state of Ru via doping and inhibit its overoxidation during the OER, thereby enhancing the stability of the catalytic material [18,19,20]. Regarding the selection of dopant elements, this study comprehensively considered the electronic structure, lattice matching, and stability in acidic electrolytes. First, elements with lower electronegativity were screened, as they are favorable for donating electrons to Ru, thereby weakening Ru–O covalency. Furthermore, considering that the Ru-based active species in the catalyst ultimately form RuO₂ dominated by Ru⁴⁺, titanium-group elements (e.g., Ti, Hf, Zr) that exist in the +4 oxidation state were selected to preserve the structural integrity as much as possible. This isovalent substitution mitigates charge imbalance and lattice distortion caused by heterovalent doping. Second, M⁴⁺ ions possess an empty d-orbital configuration, endowing their corresponding oxides with excellent acid corrosion resistance and thermal stability—key characteristics required for an efficient acidic OER catalyst. Finally, to synergistically enhance both the catalytic activity and stability of the material, Zr was chosen as the dopant metal because it has a lower electronegativity than Ti while being less expensive than Hf [21,22]. The introduction of Zr ions can provide additional electrons to the Ru centers, stabilize their valence state, and prevent the overoxidation of Ru that leads to the formation of soluble high-valent ruthenium oxides. Additionally, the differing ionic radii between Zr and Ru ions may induce controllable lattice distortion, optimizing the electronic structure of Ru sites within the catalyst and their adsorption energy for oxygen-containing intermediates [23,24,25]. In this study, Ru and Zr precursors were loaded onto pretreated Ti mesh surfaces using a drop-coating method, followed by a one-step pyrolysis process under a nitrogen atmosphere to synthesize a Zr-doped RuO₂ nanosphere composite catalyst for the OER under acidic conditions (hereinafter referred to as RuZrO_x/Ti). Benefiting from the introduction of the new metal element Zr, the RuZrO_x/Ti nanosphere catalyst exhibits overpotentials of 199 mV, 382 mV, and 512 mV at current densities of 10 mA·cm⁻², 50 mA·cm⁻², and 100 mA·cm⁻², respectively. Furthermore, the RuZrO_x/Ti nanosphere catalyst can operate stably for 400 h at a current density of 100 mA·cm⁻². In addition, in situ Raman spectroscopy indicates that RuZrO_x/Ti does not undergo significant irreversible degradation or overoxidation during the OER process. Therefore, this study introduces a strategy through doping engineering to enhance catalyst activity and stability, providing an additional option for the synthesis of OER catalysts in acidic environments.

Ruthenium(III) chloride trihydrate (RuCl₃·3H₂O) and zirconium(IV) nitrate pentahydrate (Zr(NO₃)₄·5H₂O) were purchased from Aladdin Industrial Corporation (Shanghai, China). Ethanol (C₂H₆O) was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Titanium mesh (Ti mesh) was supplied by Kunshan Hengbo New Materials Co., Ltd. (Kunshan, China). All chemical reagents were of analytical grade and used as received without further purification.

In this study, a facile synthesis strategy was employed to successfully fabricate RuZrO_x/Ti nanosphere composite catalysts via a drop-coating method followed by one-step pyrolysis on pretreated Ti mesh substrates. First, the Ti mesh was etched using a mixed solution of hydrofluoric acid, nitric acid, and deionized water in a volume ratio of 1:1:2 to remove the natural oxide layer and surface contaminants, thereby exposing the pristine Ti surface and increasing the surface roughness. This treatment optimized the interfacial bonding strength between the active material and the Ti mesh substrate, enhancing the stability of the active material loading process [26]. Subsequently, 0.3 mmol of RuCl₃·3H₂O and 0.008 mmol of Zr(NO₃)₄·5H₂O (As shown in Figure S1, the LSV curves of the samples at different molar ratios indicate that this molar ratio has been verified as the optimal one) were dissolved in 20 mL of a mixed solution consisting of anhydrous ethanol and deionized water in a volume ratio of 9:1. The thoroughly mixed solution was drop-coated onto the Ti mesh surface to obtain the precursor samples (The average loading mass of the active substance is between 0.2 mg and 0.3 mg), which were then placed in a vacuum drying oven for subsequent use. The precursor samples were subjected to pyrolysis treatment at 400 °C for 4 h in two different atmospheres: one in a quartz tube under a N₂ flow, and the other in a muffle furnace under ambient air. Finally, the catalyst obtained from pyrolysis under N₂ atmosphere was denoted as RuZrO_x/Ti-1, while the catalyst obtained from pyrolysis under air atmosphere was denoted as RuZrO_x/Ti-2.

The micromorphology and elemental distribution of the samples were characterized using scanning electron microscopy (SEM) with a Nova Nano SEM 450 instrument (FEI, Hillsboro, OR, USA). The chemical composition and valence states of the samples were analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. XRD patterns were obtained using a D/max 2200 diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu-Kα radiation, while XPS measurements were performed on a PHI 5500 spectrometer (ULVAC-PHI, Chigasaki, Japan). Raman spectroscopy was employed to monitor the evolution of chemical bonds during the OER process using a LabRam Odyssey Raman spectrometer (Horiba Scientific, Tokyo, Japan).

Electrochemical measurements were performed using a standard three-electrode configuration. In this system, the as-prepared catalyst samples served as the working electrode, a carbon rod was used as the counter electrode, and a saturated calomel electrode (SCE) functioned as the reference electrode. The electrolyte was 0.5 M H₂SO₄ solution (pH ≈ 0). All potentials measured against the SCE were converted to the reversible hydrogen electrode (RHE) scale according to the equation: E_RHE = E_SCE + 0.242 + 0.0591 pH.

Linear sweep voltammetry (LSV) curves were recorded at a scan rate of 5 mV·s⁻¹. The overpotential values reported in this study were obtained directly from the LSV curves without iR compensation. The stability of the oxygen evolution reaction (OER) was evaluated at a fixed current density of 10 mA·cm⁻². To determine the double-layer capacitance (C_dl), cyclic voltammetry (CV) curves were collected in the non-Faradaic potential region at scan rates of 20, 40, 60, 80, and 100 mV·s⁻¹. The C_dl value was calculated using the equation 1/2 × Δj = ν × C_dl, where Δj represents the current density difference obtained from the CV curves, and ν is the scan rate. The electrochemical surface area (ECSA) is proportional to the double-layer capacitance and was quantitatively determined using the formula ECSA = C_dl/C_s. In this study, the specific capacitance (C_s) was taken as 0.04 mF·cm⁻², a commonly reported value in the literature [27,28]. Electrochemical impedance spectroscopy (EIS) Nyquist plots were recorded over a frequency range of 100,000 Hz to 0.1 Hz.

The preparation process of the RuZrO_x/Ti catalyst is illustrated in Figure 1. The precursor solution was drop-coated onto the Ti mesh surface, followed by a one-step pyrolysis method to successfully synthesize a self-supporting acidic OER electrocatalytic electrode. To investigate the morphological characteristics of the catalyst material, SEM images of RuZrO_x/Ti-1 were collected (Figure 2a,b). Observation of the low-magnification SEM image (Figure 2a) reveals that the active material is uniformly adhered to the surface of the Ti mesh framework. At higher magnification (Figure 2b), it can be observed that RuZrO_x exhibits a nanosphere morphology (The particle size distribution graph is shown in Figure S2), and within this uniformly distributed and dense nanosphere arrangement, numerous pore structures are also present. This spherical and porous surface facilitates more sufficient contact between the active material and the electrolyte, thereby enhancing the OER performance. Furthermore, EDS mapping analysis was conducted on the RuZrO_x/Ti-1 material to verify the elemental distribution on the material surface. From Figure 2c–f, it can be observed that four elements—Ti, Ru, Zr, and O—are present within the catalytic material, and no element enrichment phenomenon was observed, indicating that RuZrO_x is uniformly loaded on the Ti mesh surface.

From Figure 3, distinct lattice fringes can be observed in the RuZrO_x/Ti-1 sample. Figure 3g,h clearly show lattice fringes corresponding exclusively to the (110) crystal plane of RuO₂. Notably, due to lattice expansion induced by Zr doping, the interplanar spacing of the (110) plane increases from 0.318 nm to 0.319 nm. Furthermore, local lattice distortion can also be observed in Figure 3b. In the EDS mappings shown in Figure 3c–f, the presence of Zr element is detected in addition to Ru and O. All of the above observations indicate the successful doping of Zr. The XRD patterns of the RuZrO_x/Ti-1, RuZrO_x/Ti-2, and RuO_x/Ti catalyst samples are presented in Figure 4a. Due to the strong diffraction peaks of the Ti substrate and the significant overlap between the peak positions of Ti (PDF#44-1294) and RuO₂ (PDF#40-1290), it is extremely difficult to observe characteristic peaks attributable to ruthenium oxide in the XRD patterns of the RuZrO_x/Ti-1 and RuO_x/Ti samples calcined under a N₂ atmosphere [27]. The diffraction peak s located at 35°, 38°, 40°, 53°, 63°, 71°, 76°, 77°, 82°, 87°, and 93° can all be attributed to Ti, corresponding to its (100), (002), (101), (102), (110), (103), (112), (201), (004), (202), and (104) crystal planes, respectively. The diffraction peaks at 35°, 40°, 77°, and 93° are overlapping peaks of Ru₂ and Ti, assignable to the (101), (111), (212), and (330) crystal planes of RuO₂, respectively. Furthermore, by observing the RuZrO_x/Ti-2 sample calcined in an air atmosphere, characteristic peaks corresponding to the (110) and (211) crystal planes of RuO₂ can be discerned at 28° and 54°. This suggests that the catalyst sample calcined under a N₂ atmosphere was not fully oxidized, resulting in a relatively low RuO₂ content.

Due to the excessively strong diffraction peaks originating from the Ti substrate, the interpretability of the XRD patterns regarding the phase composition of the as-prepared material is compromised. To further verify the successful synthesis of the catalyst material, X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical composition and elemental valence states of the catalyst samples. The survey spectra shown in Figure 4b,c reveal the presence of Ru, Zr, and Ti elements in the RuZrO_x/Ti-1 sample, confirming the successful introduction of Zr into the catalyst. In contrast, the RuO_x/Ti sample contains only Ru and Ti. Figure 5a displays the high-resolution Ru 3d spectrum, which comprises three main peaks and one satellite peak [29]. Specifically, the peaks at binding energies of 280.8 eV, 282.3 eV, and 283.3 eV correspond to Ru⁴⁺, Ru³⁺, and a satellite peak, respectively. Due to the close proximity of the binding energy of the carbon peak (C 1s) to that of the Ru peaks in the Ru 3d region, a main C peak is also observable at 284.7 eV in the Ru 3d spectrum. The high-resolution Ru 3p spectrum is shown in Figure 5b. In the RuO_x/Ti sample, peaks for Ru⁴⁺ and Ru³⁺ are located at 462 eV and 464 eV, respectively, while a satellite peak is observed at 466.7 eV. Notably, compared to the Ru 3p spectrum of the RuO_x/Ti sample, the characteristic Ru peaks for the RuZrO_x/Ti-1 sample shift towards higher binding energies. The peaks for Ru⁴⁺ and Ru³⁺ are observed at 463 eV and 464.5 eV, respectively [30]. This phenomenon indicates a lower oxidation state of Ru in RuZrOx-1 compared to RuO_x/Ti, demonstrating that electron transfer from Zr to Ru occurs upon doping with the less electronegative Zr. This promotes the formation of lower-valence Ru and increases its proportion, which is beneficial for enhancing the catalytic activity and stability of the catalyst [11,27]. Additionally, due to the spectral overlap between the Ti 2p and Ru 3p regions, a peak attributable to Ti can be observed at 458.5 eV in the high-resolution Ru 3p spectrum [31]. The position of this Ti peak remains consistent and is unaffected by Zr doping.

The high-resolution Zr 3d spectrum is presented in Figure 5c. Two distinct peaks are observed at binding energies of 181.7 eV and 184.2 eV, corresponding to Zr 3d5_5/2 and Zr 3d3_3/2, respectively [32]. The detection of clear Zr characteristic peaks in the XPS spectrum provides direct evidence for the successful doping of Zr. Finally, the high-resolution O 1s spectrum is displayed in Figure 5d. Characteristic peaks corresponding to adsorbed water, oxygen vacancies, –OH species, and lattice oxygen are located at 533.3 eV, 531.5 eV, 530 eV, and 529.4 eV, respectively. The presence of oxygen vacancies is conducive to the improvement of the catalyst’s performance. First, oxygen vacancies alter the local coordination environment and electronic density of states around the Ru active sites. The characteristic peaks in the Ru 3p XPS spectrum shift toward higher binding energies, indicating a reduction in the oxidation state of Ru and effective suppression of Ru overoxidation and dissolution. On the other hand, oxygen vacancies are conducive to triggering the lattice oxygen oxidation mechanism (LOM), which directly bypasses the high-energy-barrier step of *OOH formation [33,34,35,36]. The detection of Ru, Zr, O, and Ti in the XPS analysis corroborates the results obtained from the energy-dispersive X-ray spectroscopy (EDS) mapping under scanning electron microscopy.

As shown in the LSV curves of different samples in Figure 6a, RuZrO_x/Ti-1 exhibits superior OER performance compared to RuZrO_x/Ti-2, RuO_x/Ti, and the bare Ti mesh. The overpotentials of the four samples at a current density of 10 mA·cm⁻² are presented in Figure 6b, which are 199 mV (RuZrO_x/Ti-1), 254 mV (RuZrO_x/Ti-2), 382 mV (RuO_x/Ti), and 742 mV (Ti), respectively. The sufficiently high overpotential is sufficient to prove that the Ti mesh exists merely as a substrate and has a negligible contribution to the catalyst performance (The comparison of overpotentials and Tafel slopes of the RuZrOx/Ti-1 catalyst with those of other similar catalysts reported in recent years is presented in Table S1) [37,38,39,40]. Among these, RuZrO_x/Ti-1 displays the lowest overpotential, demonstrating the optimal OER activity. This enhancement can be attributed to the dense nanosphere structure and abundant void spaces on the surface of the RuZrO_x/Ti-1 catalyst, which increase the number of accessible active sites. Previous studies have indicated that a larger double-layer capacitance (C_dl) value signifies greater exposure of active sites during the electrocatalytic process, leading to correspondingly higher catalytic activity. As shown in the CV curves of RuZrO_x/Ti-1 at different scan rates (Figure 6d) and the corresponding linear fit for C_dl determination (Figure 6e), the as-prepared RuZrO_x/Ti-1 catalyst exhibits a C_dl value of 19.7 mF·cm⁻². This relatively high double-layer capacitance is indicative of a larger electrochemically active surface area.

The differences in reaction kinetics among the various catalyst samples were compared using Tafel slopes and electrochemical impedance spectroscopy. Studies have demonstrated that a smaller Tafel slope corresponds to faster reaction kinetics. As observed in the Tafel slope comparison of different samples in Figure 6c, the RuZrO_x/Ti-1 catalyst exhibits the smallest Tafel slope of 155 mV·dec⁻¹. In contrast, the Tafel slopes for the RuZrO_x/Ti-2 catalyst, RuO_x/Ti catalyst, and bare Ti mesh are 220 mV·dec⁻¹, 574 mV·dec⁻¹, and 1580 mV·dec⁻¹, respectively, all higher than that of RuZrO_x/Ti-1. The electrochemical impedance spectroscopy (EIS) results in Figure 6f further corroborate the faster reaction kinetics of the RuZrO_x/Ti-1 catalyst, which displays a charge transfer resistance of 19 Ω, lower than that of RuZrO_x/Ti-2 (23 Ω) and RuO_x/Ti (245 Ω). This indicates that the RuZrO_x/Ti-1 catalyst possesses a faster electron transfer rate during the OER process, thereby contributing to its enhanced catalytic performance.

In addition to catalytic activity, the stability of a catalyst is another crucial parameter for evaluating its overall performance. The chronoamperometry curve for the RuZrO_x/Ti-1 catalyst was recorded at a fixed current density of 10 mA·cm⁻². The results, shown in Figure 6g, demonstrate that the sample remains stable over 400 h without any significant decay. After the stability test, the potential increased by only 6.9%, indicating that the RuZrO_x/Ti-1 catalyst exhibits excellent long-term stability. To verify the structural integrity of the RuZrO_x/Ti-1 catalyst after the stability test, a fresh XPS analysis was performed. The results are shown in Figure S3, Figure S4 and Figure S5. It can be observed that the XPS spectrum of the Ru 3d region displays the same features as those of the sample before the stability test, indicating that no significant dissolution or loss of Ru occurred during the stability test. Due to the low amount of Zr as a dopant and the prolonged stability test, the XPS signals in the Zr 3d region are relatively weak; nevertheless, the characteristic peaks corresponding to Zr 3d_3/2 and Zr 3d_5/2 can still be discerned. The O 1s XPS spectrum reveals that the Ru–OH peak of RuZrO_x/Ti-1 becomes stronger, which is a result of surface hydroxylation, indicating the formation of a stable active layer on the sample surface.

Moreover, in-situ Raman spectroscopy was employed to monitor the changes occurring in the catalysts during the OER. The in-situ Raman spectra of the RuZrO_x/Ti-1 catalyst and the RuO_x/Ti catalyst are shown in Figure 7a,b, respectively. In the Raman spectra of both samples at open circuit potential, four characteristic peaks attributable to RuO₂ can be observed. Specifically, the characteristic peaks located at 419 cm⁻¹ and 516 cm⁻¹ are assigned to the Eg vibration modes of RuO₂, while the peaks at 604 cm⁻¹ and 696 cm⁻¹ correspond to the A1g and B2g vibration modes of RuO₂, respectively [11,41,42,43]. It is worth noting that as the potential increases, the configuration of the Ru–O bonds in RuZrO_x/Ti-1 remains stable, exhibiting neither a red shift nor a blue shift with increasing voltage, which serves as evidence for its excellent stability. In contrast, for the RuO_x/Ti catalyst, as the potential increases, a significant overall red shift of the characteristic peaks at 419 cm⁻¹ and 604 cm⁻¹ can be clearly observed (from 419 cm⁻¹ to 410 cm⁻¹; from 604 cm⁻¹ to 599 cm⁻¹). This indicates that the Ru–O bond length begins to contract during the OER process, which is detrimental to maintaining the stability of the catalyst.

In this study, a promising RuZrO_x/Ti composite catalyst for acidic oxygen evolution was successfully fabricated via a drop-coating and one-step pyrolysis process. The catalyst surface exhibits a densely packed nanosphere morphology accompanied by abundant pore structures. The introduction of the metallic element Zr effectively modulated the molecular structure within the catalyst, optimizing the adsorption behavior of reaction intermediates during the OER in acidic media. This modulation not only lowers the reaction energy barrier but also inhibits the overoxidation of Ru, effectively suppressing the irreversible dissolution of Ru and the loss of lattice oxygen. The RuZrO_x/Ti catalyst achieves a low overpotential of only 199 mV at a current density of 10 mA·cm⁻² for the OER. Furthermore, it demonstrates exceptional long-term durability, operating stably for 400 h at a fixed current density of 10 mA·cm⁻². In summary, this work provides a viable strategy for designing high-performance acidic OER catalysts, thereby paving the way for the advancement of electrodes for water oxidation.

The following supporting information can be found at: https://www.sciepublish.com/article/pii/973, Figure S1: LSV curves at different molar ratios. Figure S2: Particle size distribution graph. Figure S3: XPS spectra of the Ru 3d orbitals after the stability test. Figure S4: XPS spectra of the Zr 3d orbitals after the stability test. Figure S5: XPS spectra of the O 1s orbitals after the stability test. Table S1: Specific capacitance and corresponding current density of various materials.

During the preparation of this manuscript, the author(s) used [Deepseek] in order to [make the sentence more fluent and standardized]. After using this tool/service, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the published article.

The authors appreciate the financial support from the National Natural Science Foundation of (China No. 52574436), the Industrial Innovation Talent Special Project of Yunnan Province’s Xingdian Talent Support Plan (No. yfgrc202417).

Investigation, J.Z. and X.W.; Resources, R.X.; Data Curation, J.Z.; Writing—Original Draft Preparation, J.Z.; Writing—Review & Editing, X.W.; Visualization, J.Z.; Supervision, X.W.; Project Administration, R.X.; Funding Acquisition, R.X.; Funding Acquisition, J.W.

Not applicable.

Not applicable.

The data that support the findings of this study are available from the corresponding author, [Prof. Xu, rdxupaper@aliyun.com], upon reasonable request.

The authors appreciate the financial support from National Natural Science Foundation of China (No. 52574436), The Industrial Innovation Talent Special Project of Yunnan Province’s Xingdian Talent Support Plan (No. yfgrc202417), and the Yunnan Fundamental Research Projects (Grant No. 202301AT070399).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.