Oxygen vacancy assisted Ru–Ni(OH)₂ for efficient ethylene glycol electrooxidation reaction

Abstract

Ethylene glycol oxidation (EGOR) is an important step in polyethylene terephthalate (PET) recycling, and it is a thermodynamically more favorable anode reaction in comparison with the oxygen evolution reaction (OER). A cross-network framework of Ru and S-doped nickel hydroxide with abundant oxygen vacancies (Ru,S–Ni(OH)₂–O_V) on nickel foam (NF) is synthesized by a corrosion method, which is used to produce formate by electrocatalysis of ethylene glycol. Benefiting from the introduction of the S element promoting the formation of oxygen vacancies, Ru,S–Ni(OH)₂–O_V has a strong OH* adsorption capacity when the EGOR occurs, which is demonstrated by the Bode phase diagram and hydroxyl adsorption capacitance (C_φ). With an operating potential of only 1.42 V, the electrode can drive a high current density of 500 mA cm⁻². Ru,S–Ni(OH)₂–O_V also shows excellent selectivity for formate over a wide potential window. The assembled two electrodes for an anion exchange membrane electrolytic cell (AEMWE) can achieve a current density of 500 mA cm⁻² at a voltage of only 1.74 V. This work provides new insights into the development of efficient EGOR electrocatalysts.

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1. Introduction

With the advantages of high energy density and zero emissions, H₂ is a promising alternative to fossil fuels that can alleviate global environmental problems and energy crises.^1–3 Although electrochemical water splitting technology has been applied commercially, it has become a bottleneck restricting the development of electrochemical water splitting technology due to the slow kinetics of anodes.^4–7 The anodic small molecule reaction is a strategy to replace the oxygen evolution reaction. Small molecule anodic oxidation reactions, especially electrochemical synthesis reactions (such as the oxidation of alcohols and aldehydes), have attracted much attention.^8,9 Alcohol oxidation has a lower thermodynamic barrier, which can effectively reduce the potential of the anode reaction, so the anodic alcohol oxidation has the potential to replace the oxygen evolution reaction.¹⁰ At the same time, electrocatalytic reforming of ethylene glycol, which is produced by the hydrolysis of polyethylene terephthalate (PET), is a feasible way to increase PET utilization.^11–14 Therefore, producing high value-added formate by electrocatalysis of ethylene glycol at the anode and producing hydrogen at the cathode effectively achieves two goals with one process.^15,16

Common effective catalysts for ethylene glycol oxidation are transition metal hydroxides. To improve the activity of ethylene glycol oxidation catalysts, many methods have been used to modify them.^17–20 Liu and his colleagues²¹ synthesized a simple Ir–CuPd monocrystalline mesoporous nano-tetrahedron as an efficient electrocatalyst to promote the oxidative decomposition of C–C bonds in the electrocatalysis of alkaline EGOR. Detailed mechanism studies show that the improved performance comes from the synergistic effect of the tetrahedral structure and composition, which kinetically accelerates electron/reactant transport within the penetrating mesopore active site and promotes the oxidative cleavage of the high energy barrier C–C bond in EG to obtain the desired C1 product. Qiu and his colleagues²² constructed a Pt/γ-NiOOH/NF electrocatalyst by a corrosion method and realized the selective oxidation of PET plastic to glycolic acid driven by a low potential (0.55 V vs. RHE) with a low noble metal loading (0.18 mg cm⁻²). This highly active and stable catalyst can effectively oxidize glycol to glycolate in the anode chamber in a wide reactant concentration range, with high productivity and faradaic efficiency. However, ethylene glycol oxidation electrocatalysts still have some problems such as insufficient activity and poor stability.^23–26 Defect engineering is an effective strategy to improve electrocatalytic activity by adjusting the electronic structure and surface properties.^27–29

In this work, a cross-network framework of Ru,S–Ni(OH)₂–O_V with highly accessible active sites on nickel foam (NF) is synthesized by a corrosion method. Its Bode phase diagram and hydroxyl adsorption capacitance (C_φ) show that the introduction of the S element and the continuous dissolution of the S element promote the generation of oxygen vacancies and enhance the adsorption of the active intermediate OH*. Nuclear magnetic resonance ¹H shows that Ru,S–Ni(OH)₂–O_V exhibits excellent selectivity for formate at an application potential. With an operating potential of only 1.42 V, the electrode can drive a high current density of 500 mA cm⁻². The assembled two electrodes for an AEMWE can drive a current density of 500 mA cm⁻² at the voltage of only 1.74 V. Finally, by successfully recycling terephthalic acid (TPA) and potassium dihydrogen (KDF) on PET through electrocatalysis, the advantages of Ru,S–Ni(OH)₂–O_V are further highlighted, achieving the dual effect of value-added product production and hydrogen production.

2. Synthesis of the electrocatalyst

For the synthesis of Ru,S–Ni(OH)₂–O_V, 50 mg of RuCl₃, 0.1 g of NaCl, and 20 mg of NaHS were added to deionized water and stirred for 10 minutes. After mixing evenly, a piece of pre-treated nickel foam (2 cm × 2 cm) was impregnated in the mixing solution and stirred for 6 hours at a low speed.

Synthesis of Ru–Ni(OH)₂: except without NaHS, all the experimental methods were consistent with the synthesis methods of Ru,S–Ni(OH)₂–O_V.

3. Results and discussion

3.1. Synthesis and characterization of Ru, S–Ni(OH)₂–O_V

As shown in Fig. 1a, Ru,S–Ni(OH)₂–O_V was synthesized by a corrosion method. Fig. S1† shows an optical photograph of the synthesized sample at each step. It can be clearly seen from the scanning electron microscopy image that the surface of the pre-treated nickel foam is very smooth (Fig. S2†). After the addition of RuCl₃ and NaHS, a cross-network framework is formed on the foam nickel substrate and the surface is uniformly grown. The highly distributed network structure is conducive to exposing more active sites (Fig. 1b). The Transmission Electron Microscopy (TEM) image in Fig. 1c clearly shows the edge array of nanosheets. The Ru,S–Ni(OH)₂–O_V nanosheet array growing vertically on this conductive substrate can provide good active sites and excellent mass transfer performance.¹¹ In order to further understand the composition of the catalyst, a High Resolution Transmission Electron Microscopy (HRTEM) image was used for analysis. It was found that the nanosheet array was dominated by Ni(OH)₂ with lattice spacings of 2.71 and 2.33 nm, corresponding to the (101) and (100) planes of Ni(OH)₂, respectively (Fig. 1e). The selected region electron diffraction of Ru,S–Ni(OH)₂–O_V confirms that the diffraction ring corresponds to the (101), (110) and (103) crystal faces of Ni(OH)₂. The EDS-mapping image analysis in Fig. 1f shows that Ni, Ru, and O elements are uniformly present in the nanosheet structure. As shown in Fig. 1g and S3,† the XRD patterns of Ru,S–Ni(OH)₂–O_V and Ru–Ni(OH)₂ were in good agreement with Ni(OH)₂ (JCPDS No. 14-0117) and Ni (JCPDS No. 04-0850), and the introduction of the S element does not change the original phase.

Fig. 1 (a) Schematic diagram of the synthesis, (b) SEM, (c) TEM, (d) HRTEM, (e) SAED pattern, (f) EDS-mapping, and (g) XRD pattern of Ru,S–Ni(OH)₂–O_V.

In order to understand the structure, chemical state and surface composition, X-ray photoelectron spectroscopy (XPS) was performed. From the XPS results of Ru,S–Ni(OH)₂–O_V, Ni, Ru, S, O and other elements can be found, which is consistent with the results of the EDS-mapping above (Fig. 2a). In the Ni 2p spectrum of Ru,S–Ni(OH)₂–O_V (Fig. 2b), the two prominent peaks at binding energies of 873.18 and 855.48 eV are attributed to Ni 2p_1/2 and Ni 2p_3/2 of Ni(II), respectively, and the two corresponding satellite peaks are located at 879.38 and 861.28 eV.³⁰ The chemical state of Ru in the catalyst was further studied (Fig. 2c). In Ru,S–Ni(OH)₂–O_V, the main peaks of 462.5 and 484.5 eV are observed, which can be attributed to Ru 3p_3/2 and Ru 3p_1/2, respectively, and the binding energy of the main peaks of Ru is positively shifted when the S element is added.³¹ The S 2p spectrum validates the prominent peak deconvolution to S 2p_3/2 (163.5 eV), S 2p_1/2 (163.9 eV), and S–O bonds (168.0 eV) (Fig. 2d).³² The O 1s spectrum consists of three peaks with binding energies of 532.3, 531.0 and 529.1 eV, which are related to lattice oxygen (O_L), oxygen vacancies (O_V) and chemical surface adsorbed oxygen (O_A), respectively (Fig. 2e).³³ Ru–Ni(OH)₂ and Ru,S–Ni(OH)₂–O_V were characterized by electron paramagnetic resonance (EPR). Compared with the original Ru–Ni(OH)₂ with a weaker signal, Ru,S–Ni(OH)₂–O_V shows a strong peak at a g value of 2.004, which shows that the introduction of the S element promotes the generation of oxygen vacancies.

Fig. 2 (a) XPS of Ru,S–Ni(OH)₂–O_V, high-resolution XPS of (b) Ni 2p and (c) Ru 3p of Ru–Ni(OH)₂ and Ru,S–Ni(OH)₂–O_V, (d) S 2p and (e) O 1s of Ru,S–Ni(OH)₂–O_V and (f) EPR spectrum of Ru–Ni(OH)₂ and Ru,S–Ni(OH)₂–O_V.

3.2. Electrocatalytic performance for EGOR

To study the electrocatalytic activity for EGOR, Ru,S–Ni(OH)₂–O_V was tested in a three-electrode system. First, the effects of the introduction of oxygen vacancies on the oxidation performance of the EGOR are investigated. The LSV curves and corresponding Tafel slope in Fig. S4† show that when Ru is loaded to form Ru–Ni(OH)₂, the EGOR performance is improved, and the performance of the EGOR is further improved when NaHS is added to introduce O vacancies to form Ru,S–Ni(OH)₂–O_V. It is necessary to explore the optimal doping content of NaHS. As shown in Fig. S5a,† when the mass of NaHS is 20 mg, Ru,S–Ni(OH)₂–O_V has the best EGOR performance. Therefore, the EGOR activity of Ru,S–Ni(OH)₂–O_V was characterized by selecting the best S doping amount in the subsequent experiments. Fig. S5b and c† show the LSV curves of Ru,S–Ni(OH)₂–O_V in a series of alkaline glycol solutions with a concentration gradient. As the concentration of EG increases, the number of ethylene glycol molecules in the solution increases, and the catalytic performance of the EGOR improves. When the concentration of ethylene glycol is greater than 0.5 M, the reduction in active sites on the catalyst surface leads to decreased catalytic performance for the EGOR.³⁴ An anode potential of 1.70 V is required to achieve 500 mA cm⁻² in 1.0 M KOH. When 0.5 M EG is added, a potential of only 1.42 V is needed to achieve the same current density, a significant reduction compared to that of the OER (Fig. 3a). At a series of industrial current densities, the potential for EGOR reduces at least 210 mV relative to that of the OER (Fig. 3b). Ru,S–Ni(OH)₂–O_V has a low Tafel slope when the EGOR occurs, which means that Ru,S–Ni(OH)₂–O_V has better catalytic kinetics in 1.0 M KOH + 0.5 M EG (Fig. 3c). In order to investigate the superiority of the Ru,S–Ni(OH)₂–O_V catalyst, a water contact angle (CA) test was carried out. The hydrophilicity of pure nickel foam is low (the water CA is 93°), while the water CA of Ru,S–Ni(OH)₂–O_V is about 0°, indicating that Ru,S–Ni(OH)₂–O_V has super hydrophilicity. The low water contact angle indicates that water droplets rapidly diffuse upon contact with Ru,S–Ni(OH)₂–O_V. Moreover, the excellent hydrophilic properties are more conducive to the diffusion of active substances (Fig. S6†). Meanwhile, the Nyquist diagram shows that the R_ct of Ru,S–Ni(OH)₂–O_V (1.45 Ω) is lower than those of Ru–Ni(OH)₂ (2.04 Ω) and pure nickel foam (9.32 Ω), indicating that Ru,S–Ni(OH)₂–O_V has faster charge transfer kinetics during the EGOR process (Fig. 3d). The introduction of NaHS can effectively promote the generation of oxygen vacancies, and the generated oxygen vacancies can accelerate the EGOR process. To illustrate the intrinsic activity of the catalyst, the C_dl and ECSA were calculated from the integrated charge in the CV curve (Fig. 3e, S7a–c†). In contrast, the C_dl of Ru,S–Ni(OH)₂–O_V (19.2 mF cm⁻²) is much higher than that of Ru–Ni(OH)₂ (6.0 mF cm⁻²), indicating that Ru,S–Ni(OH)₂–O_V has abundant active sites in the electrocatalytic reaction process and is suitable for use at high current. The ECSA was positively correlated with C_dl, and it is found that Ru,S–Ni(OH)₂–O_V has larger electrochemically active areas, indicating a higher reactivity. In addition, Ru,S–Ni(OH)₂–O_V also shows high TOF values, indicating that Ru,S–Ni(OH)₂–O_V has a strong intrinsic activity (Fig. S8a and b†). The TOFs of the catalysts were compared (Fig. S8c†) and Ru,S–Ni(OH)₂–O_V has a higher TOF than Ru–Ni(OH)₂. In summary, the introduction of oxygen vacancies increased the catalytic activity of Ru,S–Ni(OH)₂–O_V, including its potential, Tafel slope, TOF, C_dl and EIS (Fig. 3f). In addition, the EGOR performance of the electrocatalyst is also better than that of some recently reported catalysts (Table S1†). The hydroxyl and methyl groups in alcohol molecules also have important effects on the catalytic activity, so LSV tests were carried out in different alcohol solutions (Fig. S9a and b†). For ethylene glycol, the current density is 799 mA cm⁻² at a potential of 1.50 V. Meanwhile, for ethanol and methanol, the current densities are only 317 and 406 mA cm⁻², respectively. It can be seen from the above results that the alcohol oxidation activity follows the following trend: ethylene glycol > methanol > ethanol. It seems that alcohols with more adjacent hydroxyl groups tend to exhibit higher activity. However, the presence of methyl substituents inhibits the activity of adjacent hydroxyl groups, resulting in lower oxidation activity of ethanol than that of methanol. In addition, at the same concentration, the number of hydroxyl groups in monoalcohol is less than that in diol, resulting in lower activity of methanol than that of ethylene glycol. To test the faradaic efficiency of oxidizing products and resultant products of EG, an i–t test was carried out in an electrolytic cell. The anode cell contains 40 mL of 1.0 M KOH + 0.5 M EG solution. The composition of the electrolytic products of ethylene glycol alkaline solution was characterized and quantitatively analyzed by ¹H NMR spectroscopy (Fig. 3g, S10†). Formate is the only product of ethylene glycol oxidation and Ru,S–Ni(OH)₂–O_V shows excellent selectivity for formate at the application potential. In addition, the faradaic efficiency of formate can reach over 80%. The maximum faradaic efficiency of formate is 91.2% at 1.40 V. With prolonged current density output, the concentration of ethylene glycol decreases, so replacing the electrolyte at intervals of about 12 hours is necessary. As shown in Fig. 3h, the current density is effectively restored after electrolyte replacement, and Ru,S–Ni(OH)₂–O_V can maintain stable performance for 75 hours at 100 mA cm⁻², indicating that Ru,S–Ni(OH)₂–O_V has excellent durability. In addition, after a long-term stability test, the nanoarray structure on the surface can be seen basically unchanged in SEM images (Fig. S11a and b†), while a relatively complete nanoedge array can be seen in TEM images (Fig. S11c†). Ru,S–Ni(OH)₂–O_V has excellent durability and is suitable for high current operation.

Fig. 3 (a) LSV curves of Ru,S–Ni(OH)₂–O_V, (b) potential comparison of Ru,S–Ni(OH)₂–O_V in both solutions, (c) Tafel plots, (d) C_dl, (e) Nyquist plot, and (f) comprehensive comparisons of the EGOR performance of Ru–Ni(OH)₂ and Ru,S–Ni(OH)₂–O_V, (g) faradaic efficiency for the formate product of Ru,S–Ni(OH)₂–O_V and the yield of formate, and (h) stability tests of Ru,S–Ni(OH)₂–O_V.

3.3. Mechanism for the enhanced EGOR

In order to further understand the real active site of Ru,S–Ni(OH)₂–O_V, the chemical surface composition and valence state of the catalyst were analyzed by high-resolution XPS before and after the EGOR test. The Ni 2p peak is positively shifted (Fig. S12a†), and Ni(III) is generated after the reaction, indicating that Ni is oxidized during the EGOR process, and the valence state increased. After the EGOR, the S 2p spectrum signal becomes weaker, indicating that S is leached on the catalyst surface (Fig. S12b†). From the O 1s spectrum of Fig. S12c,† the number of oxygen vacancies can be significantly observed to increase. In the process of testing, the S element will continue to dissolve, the number of oxygen vacancies will increase, and the doping of S will promote the continuous formation of oxygen vacancies, thus accelerating the EGOR. It is generally considered that the adsorption of ethylene glycol and OH* is the first step in electrocatalytic EG conversion in alkaline electrolytes. Therefore, this work measures the open circuit potential (OCP) of the absorbers inside the Helmholtz layer to evaluate the adsorption behavior of EG on different catalysts (Fig. S13†). Ru–Ni(OH)₂ and Ru,S–Ni(OH)₂–O_V were tested by a chronoamperometric method in 1.0 M KOH. It is observed that the OER performance of Ru,S–Ni(OH)₂–O_V is better than that of Ru–Ni(OH)₂. Then about 1.10 mL EG is injected and the open circuit potential of Ru,S–Ni(OH)₂–O_V (Δ ≈ 0.28 V) decreases significantly compared with Ru–Ni(OH)₂ (Δ ≈ 0.25 V), indicating that Ru,S–Ni(OH)₂–O_V has better selectivity for EGOR.³⁵ In order to explore the effect of oxygen vacancies on the OH* adsorption capacity of the catalyst, an in situ impedance test was performed in 1.0 M KOH + 0.5 M EG. As shown in the Nyquist diagram, the semi-circle of Ru,S–Ni(OH)₂–O_V is smaller than that of the Ru–Ni(OH)₂ electrode under a series of gradient potentials (Fig. 4a and b). The corresponding circuit diagram is presented in Fig. S14.† ³⁶ The Bode diagram also reflects the dynamic evolution process of the electrocatalyst, showing the change of the phase angle with frequency. The low frequency and high frequency phase angular peaks are respectively the surface charge conduction and electron transfer of the inner layer of the catalyst.^21,37,38 The Bode diagrams in Fig. 4c and d show that Ru,S–Ni(OH)₂–O_V exhibits additional peaks in the initial low frequency region, corresponding to the adsorption of OH*. Due to the high interfacial charge transfer resistance, the EGOR occurs very slowly. The adsorption peak of OH* shifted significantly at the potential of 1.30 V, indicating that Ru,S–Ni(OH)₂–O_V performed the EGOR quickly. The EGOR occurs about 50 mV earlier in Ru,S–Ni(OH)₂–O_V than in the Ru–Ni(OH)₂ electrode. The pseudo-capacitance variance generated by OH* (C_φ) and charge transfer impedance (R_ct) were analyzed at applied potentials. Ru,S–Ni(OH)₂–O_V is superior to Ru–Ni(OH)₂ under gradient potential, indicating that the introduction of oxygen vacancies greatly improves the EGOR activity and effectively improves the hydroxyl adsorption capacity and reduces the interfacial charge transfer resistance, thus accelerating chemical reaction kinetics (Fig. 4e and f). The introduction of oxygen vacancies reduces the charge-transfer resistance and interfacial impedance of the catalyst and increases the oxidation rate. In addition, Fig. S15a† shows the in situ impedance of Ru,S–Ni(OH)₂–O_V in 1.0 M KOH. The semicircle of Ru,S–Ni(OH)₂–O_V in 1.0 M KOH + 0.5 M EG is smaller than that in 1.0 M KOH at the corresponding potential, indicating that Ru,S–Ni(OH)₂–O_V has good selectivity to the EGOR. As can be seen from the Bode phase diagram in Fig. S15b,† with the increase of potential, OH* is rapidly adsorbed and the coverage rate increases, which is basically consistent with the LSV curve of Ru,S–Ni(OH)₂–O_V. When the potential of Ru,S–Ni(OH)₂–O_V reaches 1.40 V in 1.0 M KOH, an obvious peak is found in the low frequency region, and the peak moved to the high frequency region with the increase of potential, and the peak became higher and wider, corresponding to the OER. At the same time, Ru,S–Ni(OH)₂–O_V has larger C_φ and R_ct than the OER when the EGOR occurs at the same potential (Fig. S15c and d†). The changes in surface reaction and impedance with potential polarization are directly observed by in situ EIS. At low potential, the electrode surface involves the adsorption of Ru to OH*, and the oxidation of the Ni–O substance gradually occurs.³⁹ Around 1.30–1.40 V, ethylene glycol oxidation occurs on the electrode surface. After 1.40 V, the potential is large enough to drive the OER. Both charge transfer resistance and interface impedance decrease sharply with increasing potential (Fig. 4g).^40–42

Fig. 4 Operando electrochemical impedance spectra of (a) Ru,S–Ni(OH)₂–O_V and (b) Ru–Ni(OH)₂ in 1.0 M KOH + 0.5 M EG, the corresponding Bode phase plots of (c) Ru,S–Ni(OH)₂–O_V and (d) Ru–Ni(OH)₂ during the EGOR, and plots of (e) CPE_OHversus applied potential and (f) charge transfer resistance versus applied potential, and (g) the scheme of the anode reactions at varied potentials.

3.4. Measurement of electrochemical properties in two-electrode systems

On this basis, the proof-of-concept of reforming PET plastics with Ru,S–Ni(OH)₂–O_V to produce commercial chemicals is further verified. An anion exchange membrane water electrolyzer was constructed with Ru,S–Ni(OH)₂–O_V as the anode and cathode and used to degrade the authentic PET hydrolysate. Fig. 5a shows the solution after PET hydrolysis. After electrolysis, a mixed solution containing terephthalate and formate is produced. The ¹H NMR spectrum confirms the product of the PET hydrolysate and electrolyte. The LSV curve in Fig. 5b shows that only 1.74 V battery voltage is required to achieve 500 mA cm⁻², which is about 260 mV lower than that of the overall water splitting. As shown in Fig. 5c, the potential required for EGOR∥HER electrodes to reach the industrial current densities of 100, 200, 300, 400, 500, and 600 mA cm⁻² is reduced by at least 210 mV relative to that of the OER∥HER. The Ru,S–Ni(OH)₂–O_V two-electrode system has a significant advantage in the EGOR (Fig. 5d). Two Ru,S–Ni(OH)₂–O_V electrodes are applied to an anion exchange membrane water electrolyzer (Fig. 5e). The output voltage of the glycol electrolyzer can be maintained stably for 30 hours at 100 mA cm⁻² (Fig. 5f). After PET electrolysis at 100 mA cm⁻², TPA is acidified and the filtrate is further condensed and crystallized to obtain KDF. The successful recovery of TPA and KDF using PET electrocatalytic cycling further highlights the important structural engineering of Ru,S–Ni(OH)₂–O_V in multifunctional electrochemical applications (Fig. 5g).

Fig. 5 (a) ¹H NMR spectrum, (b) LSV curves, (c) comparison of current density at different potentials, (d) advantages of the EGOR, (e) schematic representation of a two-electrode anion exchange membrane electrolytic cell used for PET electroreforming, (f) stability test for iR compensation of the EGOR∥HER at 100 mA cm⁻², and (g) schematic illustration of the process of PET electro-reforming and product separation of Ru,S–Ni(OH)₂–O_V.

4. Conclusion

In conclusion, a cross-network framework of Ru,S–Ni(OH)₂–O_V grown on nickel foam (NF) with highly open active sites was synthesized by a corrosion method, and ethylene glycol was catalyzed to produce industrial formic acid. Its Bode phase diagram and hydroxyl adsorption capacitance C_φ show that the introduction of the S element promotes the generation of oxygen vacancies and enhances the adsorption of Ru,S–Ni(OH)₂–O_V to the active intermediate OH*. With a working potential of only 1.42 V, the electrode can drive a high current density of 500 mA cm⁻², which is about 280 mV lower than that of the OER. Ru,S–Ni(OH)₂–O_V shows excellent selectivity for formate over the application potential range of 1.35 and 1.50 V. The Ru,S–Ni(OH)₂–O_V||Ru,S–Ni(OH)₂–O_V couple for an AEMWE can drive a current density of 500 mA cm⁻² at the voltage of only 1.74 V. This study provides new insights for the enhancement and exploration of EGOR electrocatalyst activity.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

Financial support from the National Natural Science Foundation of China (51772162, 21971132, and 52072197), the Outstanding Youth Foundation of Shandong Province, China (ZR2019JQ14), the Youth Innovation and Technology Foundation of Shandong Higher Education Institutions, China (2019KJC004), the Major Scientific and Technological Innovation Project (2019JZZY020405), the Major Basic Research Program of Natural Science Foundation of Shandong Province (Grant No. ZR2020ZD09), the 111 Project of China (Grant No. D20017), the Taishan Scholar Young Talent Program (tsqn201909114), the University Youth Innovation Team of Shandong Province (202201010318), and the Shandong Province “Double-Hundred Talent Plan” (WST2020003) is acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01372g

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