English

• The electrochemical water splitting to generate hydrogen is recognized as the most promising substitutions of fossil fuels to mitigate the energy crisis associated with the environmental pollution. Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are involved in water splitting, demanding additional potential to drive because of the endothermic reactions [1–4]. To accelerate these reactions, platinum (Pt) and ruthenium/iridium oxides (RuO₂/IrO₂) has been adopted as HER and OER electrocatalyst, respectively [5–8]. However, water splitting in acid is retarded by the low catalytic activity and stability of the OER electrocatalyst, facing the high energy barrier for water dissociation and rapid surface reconstruction associated with the electrochemical dissolution [9,10]; as a result, the longevity is shortened by the deterioration of OER electrocatalyst. Compared to RuO₂, IrO₂ shows a lower OER performance stemming its higher binding strengthen with oxygen reaction intermediates; however, the stability of IrO₂ is better than RuO₂ ascribing to higher oxidization potential. Therefore, the promotions in stability and OER activity of RuO₂ and IrO₂ are vital for the industrialization of acidic water splitting.

  Taking catalytic activity for consideration, modulations in electronic structure and morphology are the most attractive strategies since the electrocatalysis strongly replies the electronic configuration and the number of active sites [11,12]. Construction of heterostructure and introduction of hetero-atom dopants have been tremendously investigated as effective methodologies to adjust the electronic structure of active site. Metal (Ni [13,14], Mn [15,16], Ta [17,18], etc.) and non-metal heteroatoms (B [19], S [20], etc.) have been invited to RuO₂ and IrO₂ to boost the OER performance and stability. Apart from the catalytic activity, the durability of the electrocatalyst relies on the structural stability, resistance towards the consistent surface reconstruction as well as the oxidation state of Ru/Ir atoms [21]. The invitation of hetero-dopant to RuO₂ or IrO₂ also could enhance the stability since more oxygen vacancies were created and the over oxidation of Ru/Ir has been restricted. Based on the above analysis, the introduction of metal dopants can efficiently solve the low catalytic activity and stability issues simultaneously. In order to make the formed electrocatalyst with bifunctionality, HER and OER performance, Pt atom is an attractive candidate as dopant for RuO₂ and IrO₂. Huang et al. have reported the formation of Pt doped RuO₂ with interstitial carbon atoms [22]; while, the effects caused by Pt dopants on stability and catalytic activity under OER condition have not been investigated. Besides, the insight for boosted acidic OER performance associated with stability after Pt doping has not been clearly reported yet, especially the stability caused by the overoxidation of Ru during OER at atomic level.

  Herein, in this work, we have synthesized Pt doped RuO₂ nanosheets with one step pyrolysis. The as-prepared Pt-RuO₂ showed its high bifunctionality towards HER and OER catalysis, demanding only 36 mV and 221 mV overpotential to attain 10 mA/cm², which was decreased by 8 mV and 23 mV with relative to the commercial Pt/C and IrO₂, respectively. The boosted OER activity originated to the increment in e_g orbital filling of Ru atom weakening the binding strengthen with oxygen intermediates. The similar HER performance to Pt/C achieved by Pt-RuO₂ stemmed from the similar Gibbs free energy to Pt (111). Only 1.5 V was required to reach 10 mA/cm² in acidic water splitting for Pt-RuO₂. This strategy has also been extended to IrO₂ electrocatalyst.

  The as-synthesized electrocatalysts have been tested by XRD to know the phase information. As shown in Fig. 1a, diffraction peak at 28.2°, 35.1° and 54.5° have been recorded for RuO₂-350, matched with PDF #43-1027, indicating the formation of crystal RuO₂, which was similar to the commercial RuO₂ (c-RuO₂). Moreover, diffraction peak at 28.2° was the dominant (110) crystal facet of RuO₂. Interestingly, with the Pt dopants, the XRD pattern of Pt-RuO₂ was similar to that of RuO₂-350 and c-RuO₂ suggesting the introduction of Pt dopants did not induce the phase separation. It was noticed that Pt-IrO₂ also exhibited similar XRD diffraction peak to IrO₂ shown in Fig. S1 (Supporting information). Moreover, the different electronegativities between Pt and Ru would also modulate the electronic configuration of Ru active sites. Therefore, the XPS tests have been carried out (Fig. S2 in Supporting information). As shown in Fig. 1b, the Ru 3p XPS spectra, consisted of Ru 3p_3/2 and Ru 3p_1/2 at 462.1 and 484.7 eV, fitted to metallic Ru and Ru⁴⁺ species. A higher content of metallic Ru was noticed for Pt-RuO₂ according to the quantitative analysis of XPS, demonstrating that the introduction of Pt dopant alleviated the oxidation state of Ru since Pt could donate its electrons to oxygen atoms [23]. Besides, the binding energy of Ru 3p for Pt-RuO₂ was lower than RuO₂-350 highlighting a lower oxidation state of Ru in Pt-RuO₂. As shown in Fig. 1c, the Pt 4f XPS spectrum was deconvoluted into Pt⁰ 4f_5/2, Pt²⁺ 4f_5/2, Pt⁰ 4f_7/2 and Pt²⁺ 4f_7/2 species at 76.9, 78.5, 73.6 and 75.2 eV [24]. Compared to commercial Pt/C, more Pt oxidized species were observed for Pt-RuO₂ because the Pt dopants were predominantly coordinated with oxygen atoms. As shown in Fig. 1d, the O 1s peaks were fitted to lattice oxygen from RuO₂, oxygen vacancy and adsorbed H₂O. Due to the different electronegativity, a higher percentage of oxygen vacancy was found for Pt-RuO₂ [25]. More Ru atoms with lower oxidized state (Ru^δ+, δ < 4) would enhance the acidic OER performance since a higher binding strength with oxygen intermediate was found for the crystal RuO₂ [26]; moreover, nucleophilic attack by H₂O will be accelerated on Ru atoms with low valance.

  Figure 1

  

  Figure 1.  (a) XRD patterns of c-RuO₂, RuO₂–350, Pt-RuO₂. (b) Ru 3p spectra of RuO₂–350 and Pt-RuO₂. (c) Pt 4f spectra of Pt-RuO₂ and Pt/C. (d) O 1s spectra of RuO₂–350 and Pt-RuO₂.

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  As shown in Fig. 2a, the SEM image indicated nanoparticle structure of the formed Pt-RuO₂, similar to the RuO₂–350 and c-RuO₂ (Fig. S3 in Supporting information). As shown in Fig. 2b, the TEM image also suggested the nanoparticle structure of Pt-RuO₂ with a diameter of 5 nm. The SEAD pattern (Fig. 2c) confirmed the presence of (110), (101), (211) and (301) facets from RuO₂ in Pt-RuO₂ and no crystal Pt was observed. The HR-TEM image in Fig. 2d clearly showed the lattice spacing of 0.31 nm assigned to the dominant (110) plane of RuO₂. As shown in Fig. 2e, the Pt, Ru and O elements were well distributed on the architecture of Pt-RuO₂. Differently, with the doping of Pt atoms, the morphology was different to c-IrO₂ shown in Fig. S4 (Supporting information). And the porous structure was favorable for the mass transfer at high current density.

  Figure 2

  

  Figure 2.  (a) SEM, (b) TEM, (c) SAED, (d) HR-TEM, (e) HAADF images and corresponding EDS mapping of Pt-RuO₂.

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  Firstly, the acidic OER performance was measured in 0.5 mol/L H₂SO₄ electrolyte. As shown in Fig. 3a, as the percentage of Pt dopants affects the OER performance, Pt-RuO₂ with different Pt contents have been synthesized and tested. Pt-RuO₂–2 exhibited the highest OER performance among all Pt-RuO₂ electrocatalysts ascribed to the optimized electronic configurations of Ru atom since a moderate adsorption-desorption capability is required for active sites. Due to the highest OER performance of Pt-RuO₂–2, it was selected for comparison and simplified as Pt-RuO₂. Pt-RuO₂ required 221 mV overpotential to reach 10 mA/cm², which was lowered by 24 and 21 mV with respect to commercial RuO₂ (c-RuO₂) and RuO₂–350. Due to the highest OER performance of Pt-RuO₂–2, it was selected for comparison and simplified as Pt-RuO₂. RuO₂–350 exhibited a comparable OER activity to c-RuO₂; while, Pt-RuO₂ showed a superior OER performance with overpotential of 280 mV at 50 mA/cm², which was decreased by 54 mV than c-RuO₂. As shown in Fig. S5 (Supporting information), the number of electrocatalytic active sites was calculated and Pt-RuO₂ showed a 1.9- and 1.7-fold higher double layer capacitance (C_dl) by comparison with c-RuO₂ (22.4 mF/cm²) and RuO₂–350 (24.2 mF/cm²) ascribing to more Ru atoms with low valance state [27]. Furthermore, the charge transfer resistance (R_ct) was decreased by 27.6% after the introduction of Pt dopants (Fig. S6 in Supporting information). As shown in Fig. 3b, Tafel slope for Pt-RuO₂, c-RuO₂ and RuO₂–350 was 59.2, 118.1 and 69.4 mV/dec, respectively. A lower Tafel slope Tafel slope value demonstrated a faster acidic OER kinetics [28]. In order to know the superb OER performance, the polarization curves were recorded under various temperatures (Fig. S7 in Supporting information); therefore, the activation energy was estimated as 11.8, 27.1 and 19.5 kJ/mol for Pt-RuO₂, c-RuO₂ and RuO₂–350 (Fig. 3c), respectively. The lowest activation energy indicated that the water molecule was more prone to split on Pt-RuO₂. As displayed in Table S1 (Supporting information), the OER performance of Pt-RuO₂ was superior to those of the recently reported Ru based electrocatalysts. As shown in Fig. 3d, the OER performance of Pt-RuO₂ only increased by 6 mV at 10 mA/cm² explicating a superior stability. It was found that C_dl (Fig. S8 in Supporting information) and associated with R_ct (Fig. S9 in Supporting information) showed a slight decrement after 1000 cycles. Compared to Pt-RuO₂, RuO₂–350 showed a higher deterioration in OER performance (Fig. S10 in Supporting information); moreover, as shown in Fig. S11 (Supporting information), the OER performance of c-RuO₂ was seriously decayed with overpotential at 10 mA/cm² decreased by 60 mV due to its low resistance towards electrochemical dissolution witnessed by the decrement in C_dl (Fig. S12 in Supporting information). As shown in Fig. 3e, the OER activity of Pt-RuO₂ was stable for 20 h; while, the potential was sharply increased for c-RuO₂ originating from its low stability. The electrochemical impedance spectroscopy (EIS) can record the dynamic change from electrocatalyst-electrolyte and inner electrocatalyst by mean of phase angle at high and low frequency region, respectively [29]. As shown in Fig. 3f, a peak located at low frequency region was found and its intensity was decreased with applied potential because of the occurrence of OER catalysis. The decrement in phase angle was larger than that of c-RuO₂ (Fig. S13 in Supporting information) and RuO₂–350 (Fig. S14 in Supporting information) underlining a better OER performance of Pt-RuO₂. Furthermore, the R_ct was also lower than c-RuO₂ and RuO₂–350 at different potential (Fig. S15 in Supporting information). Thus, it was concluded that Pt dopants in RuO₂ triggered a better OER performance as well as stability. To confirm the superior structural stability, TEM and XRD tests have been carried out after chronopotentiometry test. As shown in Fig. S16 (Supporting information), XRD pattern of Pt-RuO₂ after stability test was resemble to that of the as-prepared electrocatalyst depicting the well-maintained RuO₂ phase after stability; moreover, the TEM image also confirmed the nanoparticle structure and (110) facet of RuO₂ in the post-electrolysis Pt-RuO₂. To extend this strategy, the OER performance and stability of Pt-IrO₂ were also tested. As shown in Fig. S17 (Supporting information), Pt-IrO₂ exhibited a better OER performance than c-IrO₂, whose overpotential at 10 mA/cm² decreased by 28 mV. Moreover, the C_dl value was increased by 10.7-time associated with decrement in R_ct (Fig. S18 in Supporting information). Besides, the OER performance of Pt-IrO₂ was also stable than c-RuO₂ (Fig. S19 in Supporting information). As shown in Fig. S20 (Supporting information), the Pt-IrO₂ also displayed a superior stability within 100 h. Moreover, the XRD pattern of Pt-IrO₂ indicated the well-maintained structure after long-term stability, demonstrating the importance of Pt dopants for boosting the OER stability.

  Figure 3

  

  Figure 3.  (a) OER performance, (b) Tafel slope and (c) activation energy of c-RuO₂, RuO₂–350 and Pt-RuO₂. (d) Cyclic stability of Pt-RuO₂. (e) Chronopotentiometry test of c-RuO₂ and Pt-RuO₂. (f) Bode plots of Pt-RuO₂ at various potential from 1.40 V to 1.58 V vs. RHE.

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  As HER is the cathodic reaction for overall water splitting, the HER activity was also tested. As shown in Fig. 4a, overpotential at 10 mA/cm² was 36, 261 and 44 mV for Pt-RuO₂, RuO₂–350 and Pt/C, respectively. The HER performance of Pt-RuO₂ was significantly enhanced with relative to RuO₂–350 due to the presence of Pt dopants; moreover, Pt-RuO₂ showed a similar HER activity to commercial Pt/C. Furthermore, the C_dl estimated from cyclic voltammetry (CV) curve was 44.8 and 36.3 mF/cm² for Pt-RuO₂ and Pt/C (Fig. S21 in Supporting information), respectively. In addition, R_ct in HER catalysis was also comparable (Fig. S22 in Supporting information), indicating the substitution of Pt/C during HER catalysis. The Tafel slope of Pt-RuO₂ was 31.3 mV/dec, lower than Pt/C (33.6 mV/dec, Fig. 4b). The Tafel slope indicated that Pt-RuO₂ and Pt/C followed Volmer-Tafel mechanism under HER condition [30,31]. Compared to the reported electrocatalysts (Table S2 in Supporting information), Pt-RuO₂ was recognized as one of the most efficient HER electrocatalyst. As shown in Fig. 4c, the HER performance of Pt-RuO₂ was almost stable during 1000 cycles stemming from the robust structural stability evidenced by the similar C_dl (Fig. S23 in Supporting information) and R_ct (Fig. S24 in Supporting information). In contrast, Pt/C showed an unacceptable degradation after 1000 cycle (Fig. S25 in Supporting information) attributing to the low electronic interplay between Pt and carbon atoms. The long-term operation test revealed that Pt-RuO₂ sustained its HER performance without decay for 20 h (Fig. 4d). Therefore, Pt dopants triggered a similar HER performance to Pt/C; while, an enhanced stability was achieved. To prove the strategy, the HER performance of Pt-IrO₂ was also measured shown in Fig. S26 (Supporting information), in which a comparable HER performance to Pt/C was recorded for Pt-IrO₂; additionally, the stability was also superior to Pt/C (Fig. S27 in Supporting information).

  Figure 4

  

  Figure 4.  (a) HER performance and (b) Tafel slope of Pt-RuO₂ and Pt/C. (c) Cyclic stability and (d) chronopotentiometry test of Pt-RuO₂. (e) Overall water splitting performance and (f) stability of various electrocatalysts. (g) I-V polarization curves of membrane electrode assembly from Pt/C||RuO₂ and Pt-RuO₂||Pt-RuO₂ at 80 ℃.

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  The overall water splitting was tested and Pt/C||Pt-RuO₂ required 1.78 V to attain 100 mA/cm² (Fig. 4e), which was 130 mV lower than the commercial system (Pt/C||RuO₂) because of a higher OER performance of Pt-RuO₂. Besides, taking Pt-RuO₂ as bifunctional electrocatalyst, it demanded only 1.73 V to reach 100 mA/cm², slightly lower than Pt/C||RuO₂. As shown in Fig. 4f, the potential was stable at 1.6 V for Pt-RuO₂||Pt-RuO₂ within 30 h; however, the applied potential was sharply increased for Pt/C||RuO₂ indicating the exceptional stability of the formed Pt-RuO₂. The rapid increment in applied potential was due to the fast electrochemical dissolution of RuO₂. To know the practical application, the membrane electrode assembly (MEA) has been fabricated. It was found that the voltage was decreased by 0.16 V for Pt-RuO₂ with relative to Pt||c-RuO₂ (Fig. 4g) due to the robust OER performance of Pt-RuO₂.

  As shown in Fig. 5a, the charge density analysis indicated that the Pt atom was positively charged; while, the electrons were enriched for the nearby Ru atoms, in line with the XPS analysis. To deeply understand the electronic adjustment, density of state (DOS) has been carried out (Fig. 5b). The d band center of Ru in Pt-RuO₂ was −1.46 eV, downshifted with relative to pure RuO₂, revealing a weaker binding strength with oxygen moieties under OER condition since the strong binding strength between RuO₂ and oxygen intermediates hindered its OER performance. Consequently, the p band from oxygen atom in Pt-RuO₂ was upshifted by comparison with RuO₂ (Fig. 5c), creating more electron hole favorable for the assistance of deprotonation. The over-oxidation of Ru atom to RuO₄^2- has been recognized as the dominant reason for low stability; while, this process could be suppressed by accelerating the electron transfer. The Δε_d-p value was 1.76 and 2.3 eV for Pt-RuO₂ and RuO₂ (Fig. 5c), respectively. The lowered Δε_d-p value was a significant signal for enhanced electronic transfer during OER catalysis; therefore, the overoxidation of Ru to RuO₄^2- has been well suppressed and a robust structural stability was achieved. In order to know the details of electronic state in d band, the e_g orbital filling has been estimated. As shown in Fig. 5d, the e_g filling was 1.18 and 1.73 for RuO₂ and Pt-RuO₂. The increment in e_g filling significantly weakened the hybridization between electrons from outmost e_g and reaction intermediates, enhancing the OER performance [32]. Moreover, more electrons located in the exterior e_g orbital offered more active sites for OER catalysis. Furthermore, the adsorptions of oxygen intermediates have been evaluated (Fig. S28 in Supporting information). It has been found that the initial protonation of H₂O to generate OH* species was more difficult to occur on Pt-RuO₂, demanding 1.65 eV; however, the potential-limiting step, deprotonation of the secondary H₂O for O—O coupling, was decreased by 0.19 eV in comparison with RuO₂ (Fig. 5e). Therefore, a boosted OER performance was achieved. To know the binding strength between active site and OER intermediates, the results have been displayed in Fig. 5f. The generated OH* and O* species strongly adsorbed on the active site of RuO₂ resulting in difficulty in the conversion [33–35]. As shown in Fig. 5g, the Gibbs free energy for hydrogen adsorption of Pt site from Pt-RuO₂ and Pt (111) has been calculated (Fig. S29 in Supporting information). The result indicated that a similar ΔG_H* value was observed for two structures unveiling a comparable HER catalytic activity [36].

  Figure 5

  

  Figure 5.  (a) Charge distribution of Pt-RuO₂. (b) Density of state, (c) band center, (d) e_g filling and OER energy barriers of RuO₂ and Pt-RuO₂ with (e) U = 0 V and (f) U = 1.23 V. (g) Gibbs free energy for hydrogen of Pt-RuO₂ and Pt (111).

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  In summary, we have incorporated Pt dopants to RuO₂ and IrO₂ as bifunctional electrocatalyst for OER and HER in acidic media. It has been found that the turnover frequency at 1.6 V vs. RHE was boosted by 3.5-fold for Pt-RuO₂ with relative to commercial RuO₂; in addition, the Pt dopants also triggered a better stability due to the strong electronic interplay between Pt and RuO₂ suppressing the overoxidation of Ru. Furthermore, the HER performance for Pt-RuO₂ was similar to the counterpart of commercial Pt/C stemming from the resemble Gibbs free energy for hydrogen. In the acidic water splitting, the applied potential was decreased by 180 mV to reach 100 mA/cm² for Pt-RuO₂||Pt-RuO₂ than Pt/C||RuO₂.

  Declaration of competing interest

  The authors declare no competing financial interest.

  CRediT authorship contribution statement

  Qing Li: Data curation. Bing Wang: Data curation. Qi Xu: Data curation. Ruiyou Liu: Data curation. Chen Li: Data curation. Fang Luo: Data curation. Yifei Li: Data curation. Yingjie Yu: Data curation. Zehui Yang: Writing – review & editing, Writing – original draft, Supervision.

  Acknowledgments

  This work is supported by the National Natural Science Foundation of China (No. 22209126) and the Opening Project of Key Laboratory of Optoelectronic Chemical Materials and Devices, Ministry of Education, Jianghan University (No. JDGD-202314). The authors also thank Dr. Liu from the Analytical and Testing Center of Wuhan Textile University for her assistance with XPS test and helpful analysis. The computation is completed in the HPC Platform of Wuhan Textile University.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111658.

  
  
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• 

  Figure 1  (a) XRD patterns of c-RuO₂, RuO₂–350, Pt-RuO₂. (b) Ru 3p spectra of RuO₂–350 and Pt-RuO₂. (c) Pt 4f spectra of Pt-RuO₂ and Pt/C. (d) O 1s spectra of RuO₂–350 and Pt-RuO₂.

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  Figure 2  (a) SEM, (b) TEM, (c) SAED, (d) HR-TEM, (e) HAADF images and corresponding EDS mapping of Pt-RuO₂.

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  Figure 3  (a) OER performance, (b) Tafel slope and (c) activation energy of c-RuO₂, RuO₂–350 and Pt-RuO₂. (d) Cyclic stability of Pt-RuO₂. (e) Chronopotentiometry test of c-RuO₂ and Pt-RuO₂. (f) Bode plots of Pt-RuO₂ at various potential from 1.40 V to 1.58 V vs. RHE.

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  Figure 4  (a) HER performance and (b) Tafel slope of Pt-RuO₂ and Pt/C. (c) Cyclic stability and (d) chronopotentiometry test of Pt-RuO₂. (e) Overall water splitting performance and (f) stability of various electrocatalysts. (g) I-V polarization curves of membrane electrode assembly from Pt/C||RuO₂ and Pt-RuO₂||Pt-RuO₂ at 80 ℃.

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  Figure 5  (a) Charge distribution of Pt-RuO₂. (b) Density of state, (c) band center, (d) e_g filling and OER energy barriers of RuO₂ and Pt-RuO₂ with (e) U = 0 V and (f) U = 1.23 V. (g) Gibbs free energy for hydrogen of Pt-RuO₂ and Pt (111).

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