Mo doping and Se vacancy engineering for boosting electrocatalytic water oxidation by regulating the electronic structure of self-supported Co₉Se₈@NiSe

Abstract

Oxygen evolution reactions (OERs) are regarded as the rate-determining step of electrocatalytic overall water splitting, which endow OER electrocatalysts with the advantages of high activity, low cost, good conductivity, and excellent stability. Herein, a facile H₂O₂-assisted etching method is proposed for the fabrication of Mo-doped ultrathin Co₉Se₈@NiSe/NF-X heterojunctions with rich Se vacancies to boost electrocatalytic water oxidation. After step-by-step electronic structure modulation by Mo doping and Se vacancy engineering, the self-standing Mo-Co₉Se₈@NiSe/NF-60 heterojunctions deliver a current density of 50 mA cm⁻² with an overpotential of 343 mV and a cell voltage of only 1.87 V at 50 mA cm⁻² for overall water splitting in 1.0 M KOH. Our study opens up the possibility of realizing step-by-step electronic structure modulation of nonprecious OER electrocatalysts via heteroatom doping and vacancy engineering.

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1. Introduction

Developing an effective green hydrogen production strategy for solving the enormous energy crisis and accompanied environmental pollution has become increasingly important.^1,2 Water electrolysis involving anodic oxygen evolution reactions (OERs) and cathodic hydrogen evolution reactions (HERs) is regarded as a promising technology for large-scale hydrogen production.^3,4 However, the efficiency of overall water splitting is seriously hampered by the anodic half-reaction, which involves a four-electron process.^5,6 Currently, the state-of-the-art OER electrocatalysts are Ru/Ir and their oxides, mainly due to their suitable electronic structure and good durability.^7,8 Unfortunately, their scarcity and high cost have seriously impeded their industrial applications.^9,10 Therefore, it is imperative to design and prepare OER electrocatalysts with high activity, low cost, good conductivity, and excellent stability.

Nowadays, several strategies including morphology engineering,^11,12 composition tuning,^13–15 metal/non-metal doping,^15,16 and surface/interface engineering^17,18 are applied for designing and preparing high-performance OER electrocatalysts. Defective engineering, a recently proposed strategy, is regarded as an effective method for improving the intrinsic OER activity of metal oxides, metal phosphides, metal sulfides, and selenides. Nanocatalysts with rich P vacancies, S vacancies, O vacancies are reported to achieve outstanding OER performance, as vacancies can significantly affect the electronic structure of electrocatalysts, thus providing an ideal binding energy for the intermediates, which is a crucial requirement for developing highly efficient OER electrocatalysts.^19,20 Among these anion vacancies, Se vacancies have been reported to be favourable for the modulation of the oxidation state of metal ions. In addition, Se vacancies can also optimize the Gibbs free energy and partial density of states, thereby contributing to the outstanding OER performance. However, due to the complexity of preparation, there are few reports on Se-defective electrocatalysts.²¹ Apart from defect engineering, doping is an established strategy for improving the electrocatalytic OER performance due to its versatile capability for simultaneously tailoring the electronic structure and introducing extra active sites.^22,23 In view of this, integrating a nonprecious electrocatalyst with rich Se vacancies and doped heteroatom will greatly enhance electrocatalytic water oxidation.

ZIF-67, a typical metal–organic framework material has been widely applied to synthesize Co-based selenides via high-temperature calcination and using Se powder as a precursor, which consumes enormous energy, and the shape is also hard to control.²⁴ In our previous work, we have successfully fabricated CoSe₂ using ZIF-67 as a precursor and SeO₂ as a Se source.²⁵ Supports such as nickel foams, carbon cloth, and graphenes were widely used in the synthesis process of catalysts to improve the conductivity and stability.^26,27 Given this, a self-standing nanomaterial with a modulated electronic structure and rich Se vacancies may be a potential outstanding anodic catalyst for electrocatalytic water splitting.

Considering these points, in this work, Mo-doped Co₉Se₈@NiSe/NF with rich Se vacancies and heterointerfaces was facilely prepared by a wet-chemical method followed by H₂O₂ etching. The as-prepared electrocatalysts, which possessed ultrathin nanosheets, strong interfacial interaction, and tuned electronic structure, exhibited enviable electrocatalytic performance. In particular, the optimized Mo-Co₉Se₈@NiSe/NF-60 heterojunction needed an overpotential of only 343 mV to drive a current density of 50 mA cm⁻², and the Tafel slope is only 61.86 mV dec⁻¹. More importantly, the two-electrode coupled Co₉Se₈@NiSe/NF-60 heterojunction with Pt/C achieved a current density of 50 mA cm⁻² for overall water electrolysis with a cell voltage of 1.87 V.

2. Experimental section

2.1 Reagents and chemicals

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), 2-methylimidazole (2-MIM), sodium molybdate dihydrate (Na₂MoO₄·2H₂O), selenium dioxide (SeO₂), hydrogen peroxide (H₂O₂, 30%), and ethanol (C₂H₅OH) were purchased from Aladdin (Shanghai, China).

2.2 Apparatus and measurements

Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and high-angle aberration-corrected dark-field scan transmission electron microscopy (HAADF-STEM) were performed using a JEM 2100F transmission electron microscope at a voltage of 200 kV. Field emission scanning electron microscopy (FESEM) studies were carried out using a JEOL S4800 scan electron microscopy operating at 15 kV for obtaining FESEM images and elemental mapping. X-ray photoelectron spectroscopy (XPS) characterizations were carried out using a VG Scientific ESCALAB 210 electron spectrometer equipped with two ultrahigh vacuum chambers. Powder X-ray diffraction (PXRD) patterns were recorded using a Rigaku diffractometer with Cu-Kα radiation (λ = 0.15418 nm).

2.3 Preparation of ZIF-67/NF

A nickel foam (NF, 2 cm × 3 cm) was sonicated in acetone and ethanol for 15 min. Then, 0.821 g of 2-MIM was dissolved in 20 mL of deionized water to obtain solution A. Solution B was obtained by dissolving 0.728 g of Co(NO₃)₂·2H₂O in 20 mL of water. Then, solution B was injected into solution A and sonicated for 5 minutes. The pre-treated NF was immersed in the solution. After 4 h, the samples were obtained by washing with DI water and dried at 60 °C.

2.4 Preparation of Mo-ZIF-67/NF

First, 100 mg of NaMo₂O₄·2H₂O was dissolved in 50 mL of DI water and sonicated for complete dissolution. The as-obtained ZIF-67/NF was immersed in the solution to react for 25 min. Subsequently, Mo-doped ZIF-67/NF was obtained by washing several times with deionized water and dried at 60 °C.

2.5 Preparation of Mo-Co₉Se₈@NiSe/NF

First, 10 mg of SeO₂ and 0.1 mL of N₂H₄·H₂O were dissolved in 20 mL DI water. After that, the obtained Mo-ZIF-67/NF (2 cm × 1.5 cm) was placed into this solution within a 100 mL Teflon-lined stainless autoclave for continuous reaction for 8 h at 160 °C. After cooling to room temperature, Mo-Co₉Se₈@NiSe/NF was washed several times with DI water and dried at 60 °C.

2.6 Preparation of Mo-Co₉Se₈@NiSe/NF

To obtain Mo-Co₉Se₈@NiSe/NF with different numbers of Se vacancies, Mo-Co₉Se₈@NiSe/NF was etched for 40 s, 60 s and 80 s with 5 mol L⁻¹ H₂O₂ at room temperature. Then, they were washed with DI water and dried at 60 °C. The samples were labeled as Mo-Co₉Se₈@NiSe/NF-40, Mo-Co₉Se₈@NiSe/NF-60, and Mo-Co₉Se₈@NiSe/NF-80, respectively.

2.7 Electrochemical measurement

Electrochemical measurements were done using an electrochemical workstation in a three-electrode system, where a glassy carbon electrode covered with catalysts, a Ag/AgCl electrode, and a graphene rod were employed as the working, reference and counter electrodes, respectively. Linear sweep voltammetry at a scan rate of 5 mV s⁻¹ was conducted at room temperature. For Tafel plots, the linear portion is fitted to the Tafel equation. All the potentials reported in the present study were converted to a reversible hydrogen electrode (RHE) scale. The long-term stability was also tested at a current density of 50 mA cm⁻². EIS was performed in the frequency range of 0.01 to 100 000 Hz and an amplitude of 10 mV.

3. Results and discussion

The scheme presents the wet-chemical synthesis of Mo-Co₉Se₈@NiSe/NF-60 (Fig. 1a). As shown in Fig. S1a,† the pretreated nickel foam has an obvious three-dimensional network structure.²⁸ After in situ growth of ZIF-67 on the surface of the nickel foam by a hydrothermal method, is clearly observed that intensive triangular column-like ZIF-67 is anchored on the surface of the nickel foam (Fig. 1b). When the obtained ZIF-67/NF was immersed in a sodium molybdate solution for 25 min, an ion-exchange process will happen, which will lead to the formation of Mo-ZIF-67/NF ultrathin nanosheets (Fig. 1c).²⁹ Mo-Co₉Se₈@NiSe/NF was obtained by selenizing Mo-ZIF-67/NF employing SeO₂ as a selenium source under a mild hydrothermal condition. Fig. 1d and e show that Mo-Co₉Se₈@NiSe/NF could well inherit the morphology of ultrafine Mo-ZIF-67/NF. After immersion of Mo-Co₉Se₈@NiSe/NF in 5 M H₂O₂ for 60 s, a small amount of selenium ions can be oxidized into selenium by H₂O₂ and fall off the compound to form vacancies, and the obtained product was noted as Mo-Co₉Se₈@NiSe/NF-60.

Fig. 1 (a) Schematic illustration of the preparation of Mo-Co₉Se₈@NiSe/NF-60. SEM images of (b) ZIF-67/NF, (c) Mo-ZIF-67/NF, (d and e) Mo-Co₉Se₈@NiSe/NF, and (f and g) Mo-Co₉Se₈@NiSe/NF-60.

As shown in Fig. 1f and g, the same morphology as that of Mo-Co₉Se₈@NiSe/NF was also observed, implying that H₂O₂ etching would not damage the ultrathin structure. For comparison, the samples with different immersion durations were also prepared, and both Mo-Co₉Se₈@NiSe/NF-40 and Mo-Co₉Se₈@NiSe/NF-80 possessed a similar morphology to that of Mo-Co₉Se₈@NiSe/NF-60 (Fig. S1b and c†), which suggested that this method had good versatility. Moreover, the samples without H₂O₂ etching and Mo doping were also prepared by a simple method, which also exhibited an analogous morphology (Fig. S2†). Fig. S3† presents the XRD patterns of samples at different stages and their locally enlarged images. As shown in Fig. S3,† ZIF-67/NF and Mo-ZIF-67/NF showed similar XRD patterns to that of pre-treated NF, but the locally enlarged images of ZIF-67/NF showed faint peaks around 7° and 10°, which were assigned to the characterized peaks of ZIF-67, implying that ZIF-67 successfully grew on the surface of the NF.^30,31 After Mo doping, the appearance of a broad and large peak located 24.3° indicated the amorphous structure of Mo-ZIF-67/NF, suggesting that Mo doping not only destroyed the original morphology, but also altered its crystal structure. After further selenization and H₂O₂ etching, the XRD patterns of Mo-Co₉Se₈@NiSe/NF with different etching durations have similar peaks. After careful observation, it was clearly found that the peaks at 28.4° and 29.6° were pointed to the (222) plane of Co₉Se₈ (PDF # 09-0233). Meanwhile, the peaks around 30.8°, 33.9°, 38.5°, 46.5°, and 56.2° were ascribed to the (300), (021), (211), (131), and (401) crystal planes of NiSe (PDF # 18-0887), respectively, implying that Mo-Co₉Se₈@NiSe/NF-X was successfully fabricated.³²

In order to obtain detailed information of Mo-Co₉Se₈@NiSe/NF-60, the ultrathin nanosheet morphology was further established and explored. As shown in Fig. 2a, the folded ultrathin morphology of Mo-Co₉Se₈@NiSe/NF-60 is clearly observed. High-resolution TEM, which is assigned to the olive square area of Fig. 2b, shows two different lattice fringes at 0.291 nm and 0.240 nm, corresponding to the (222) lattice plane of Co₉Se₈ and the (211) lattice plane of NiSe, respectively.^33,34 The corresponding selected area electron diffraction (SAED) pattern (Fig. 2c, the purple circle area in Fig. 2a) obviously exhibited two sets of diffraction rings, which could be indexed to the (222) plane of Co₉Se₈ and the (211) plane of NiSe, which is consistent with the results of HRTEM. High-angle annular dark-field scanning TEM (HAADF-STEM; Fig. 2d and e) and EDS were also employed to explore the distribution of surface elements, which presented the uniform distribution of Co, Ni, Se, and Mo elements on the surface of Mo-Co₉Se₈@NiSe/NF-60. The line-scan profiles (corresponding the orange arrow line in Fig. 2d) and EDS results showed the coexistence of Ni, Co, Ni, Se, and Mo throughout Mo-Co₉Se₈@NiSe/NF-60, and the atomic number ratio of Co, Ni, Se, and Mo was measured to be about 68/148/73/1 (Fig. S4 and S5†). Fig. 2f presented the locally enlarged images and the Se vacancies can be clearly observed in the orange rectangle area, suggesting that H₂O₂ etching also generates Se vacancies. Electron paramagnetic resonance (EPR) was used to further study the relationship of etching time with numbers of vacancies (Fig. 2g). Mo-Co₉Se₈@NiSe/NF has a faint intensity, indicating that samples without H₂O₂ etching nearly have no Se vacancies. When Mo-Co₉Se₈@NiSe/NF was etched in a H₂O₂ solution for 40 s, the EPR intensity becomes stronger, implying the appearance of more defects and unpaired electrons.^35,36 The results also indicate that H₂O₂ etching is an effective method for generating Se vacancies.

Fig. 2 (a) TEM image, (b) HRTEM images (inset I-1 and II-2 correspond to the area of red square I and orange square II), (c) SAED patterns, (d) HAADF-STEM image, (e) EDX images, and (f) local HRTEM images of Mo-Co₉Se₈@NiSe/NF-60. (g) EPR spectra of etched Mo-Co₉Se₈@NiSe/NF with different etching durations compared to Mo-Co₉Se₈@NiSe/NF.

Considering its ultrathin nanosheet configuration, heterointerfaces, polycrystalline structure, and rich Se vacancies, the electrocatalytic OER performance of Mo-Co₉Se₈@NiSe/NF was confidently evaluated. Fig. 3a shows that optimized Mo-Co₉Se₈@NiSe/NF-60 exhibited outstanding OER activity, which outperformed the referenced RuO₂, Co₉Se₈@NiSe/NF, and Mo-Co₉Se₈@NiSe/NF. The large oxidation peaks in the range of 1.3 to 1.5 V were ascribed to the oxidation of Ni species.³⁷ Moreover, other samples also showed better OER activity than that of RuO₂, Co₉Se₈@NiSe/NF, and Mo-Co₉Se₈@NiSe/NF, indicating that the prepared Mo-Co₉Se₈@NiSe/NF was a series of promising OER electrocatalysts.³⁸ For intuitively comparing the OER activity of different electrocatalysts, the overpotentials of different samples at 50 mA cm⁻² are shown in Fig. 3b, where Mo-Co₉Se₈@NiSe/NF-60 needed an overpotential of only 343 mV to achieve 50 mA cm⁻², which was 31 mV, 7 mV, 61 mV, 56 mV and 111 mV lower than those of Mo-Co₉Se₈@NiSe/NF-40 (374 mV), Mo-Co₉Se₈@NiSe/NF-80 (350 mV), Mo-Co₉Se₈@NiSe/NF (404 mV), Co₉Se₈@NiSe/NF-60 (399) and RuO₂ (454 mV), respectively, implying that Mo doping and Se vacancies synergistically improve the electrocatalytic OER performance.

Fig. 3 (a) LSV polarization curves of Mo-Co₉Se₈@NiSe/NF-40, Mo-Co₉Se₈@NiSe/NF-60, Mo-Co₉Se₈@NiSe/NF-80, Mo-Co₉Se₈@NiSe/NF, Co₉Se₈@NiSe/NF-60 and RuO₂. (b) Corresponding histograms of overpotentials of different products at 50 mA cm⁻². (c) Tafel plots of various OER electrocatalysts. (d) Current density as a function of the scan rate for different electrodes. (e) Nyquist plots of different products at 0.4 V. (f) Prolonged CP of Mo-Co₉Se₈@NiSe/NF-60 at a current density of 50 mA cm⁻².

Subsequently, the Tafel plots were employed to evaluate the OER kinetics of different products. Fig. 3c shows that Mo-Co₉Se₈@NiSe/NF-60 possessed the smallest Tafel slope of 61.86 mV dec⁻¹. Compared with Mo-Co₉Se₈@NiSe/NF-40 (67.66 mV dec⁻¹), Mo-Co₉Se₈@NiSe/NF-80 (70.62 mV dec⁻¹), Mo-Co₉Se₈@NiSe/NF (68.05 mV dec⁻¹), and Co₉Se₈@NiSe/NF-40 (93.88 mV dec⁻¹), it was clearly found that Mo-Co₉Se₈@NiSe/NF-60 owned the optimal OER kinetics in all the samples. Fig. 3d represents the C_dl values of various samples, and Co₉Se₈@NiSe/NF-60 could obtain a value of 6.52 mF cm⁻², which was much higher than those of Mo-Co₉Se₈@NiSe/NF-40 (3.69 mF cm⁻²), Mo-Co₉Se₈@NiSe/NF-80 (5.94 mF cm⁻²), Mo-Co₉Se₈@NiSe/NF (2.49 mF cm⁻²), and Co₉Se₈@NiSe/NF-40 (2.60 mF cm⁻²), manifesting that the appropriate etching time was crucial for improving the electrochemical active surface area (ECSA). The Nyquist plots displayed in Fig. 3e show that all the samples had similar charge transfer resistance (R_ct) values, implying that the introduction of supports can effectively improve the conductivity of the materials. The long-term durability evaluated by performing chronopotentiometry is another important criterion for evaluating potential applications. Fig. 3f shows that the electrode modified by the Mo-Co₉Se₈@NiSe/NF-60 electrode can maintain an unchanged voltage for undergoing at least 88-hour CP tests at a current density of 50 mA cm⁻², strongly implying that Mo-Co₉Se₈@NiSe/NF-60 has excellent stability.

To further analyze the reasons for the performance improvement, XPS was performed for Mo-Co₉Se₈@NiSe/NF-60, Mo-Co₉Se₈@NiSe/NF, and Co₉Se₈@NiSe/NF-60. Fig. S6† manifests the coexistence of Co, Ni, Se, and Mo elements on the surface of Mo-Co₉Se₈@NiSe/NF-60, indicating that the final products were successfully prepared. As shown in Fig. 4a, the Co 2p spectrum of Mo-Co₉Se₈@NiSe/NF-60 can be split into six characteristic peaks. The two peaks located at 781.70 and 775.92 eV could be assigned to Co³⁺ 2p_3/2 and Co²⁺ 2p_3/2, respectively.³⁹ Meanwhile, the peaks at 797.70 and 803.43 eV could be ascribed to Co³⁺ 2p_1/2 and Co²⁺ 2p_1/2, respectively.⁴⁰ Moreover, two accompanying satellite peaks were approximately located at 787.00 and 808.02 eV, respectively.⁴¹ For the Mo 3d spectrum of Mo-Co₉Se₈@NiSe/NF-60 (Fig. 4b), the two main peaks at binding energies of 229.24 and 233.73 eV were assigned to Mo 3d_5/2 and Mo 3d_3/2, respectively.⁴² In the high-resolution spectrum of Ni 2p (Fig. 4c), the two peaks at 852.90 and 856.22 eV were attributed to Ni³⁺ 2p_3/2 and Ni²⁺ 2p_3/2, respectively,⁴³ the peaks located at 873.96 eV were ascribed to Ni 2p_1/2,⁴⁴ while the two accompanying peaks at 861.89 and 879.86 eV belonged to satellite peaks.⁴⁵ As for the Se 3d spectra displayed in Fig. 4d, the peaks at binding energies of 54.11 and 59.09 eV were ascribed to Mo 3d_5/2 and Mo 3d_3/2, respectively.⁴⁶ As for Mo-Co₉Se₈@NiSe/NF-60, the intensity of Se 3d was obviously reduced, indicating that H₂O₂ etching can effectively induce Se defects in Mo-Co₉Se₈@NiSe/NF-60.⁴⁷ Notably, the binding energies of Co 2p for Mo-Co₉Se₈@NiSe/NF-60 showed a step-by-step positive shift in comparison with Mo-Co₉Se₈@NiSe/NF without H₂O₂ etching and Co₉Se₈@NiSe/NF-60 without Mo doping. Meanwhile, Ni 2p also showed a similar change when compared with Co 2p. To further establish the stepwise electronic structure regulation, the surface valence band spectra and the calculated d-band centers of different electrocatalysts were obtained (Fig. S7†). Compared with Co₉Se₈@NiSe/NF-60, the calculated E_d (energy of the d-band center) of Mo-Co₉Se₈@NiSe/NF-60 was found to be closer to the Fermi level (0 eV). According to the reports in the literature, this phenomenon indicated stronger adsorption between electrochemically active sites and OER intermediates (OH*, OOH*, and O*). In addition, numerous experiments have witnessed that a higher E_d value was positively associated with the OER activity. Meanwhile, from the XPS results, the incorporation of Mo has changed the electronic structure of Mo-Co₉Se₈@NiSe/NF-60, leading to a higher binding energy. More interestingly, compared with Co₉Se₈@NiSe/NF, the calculated E_d value of Co₉Se₈@NiSe/NF-60 was also closer to the Fermi level, indicating that Se vacancies can elevate the value of E_d by regulating the electronic structure, thereby contributing to the substantial improvement in electrocatalytic OER activity. These results indicated that Mo doping and Se vacancies can regulate the electronic structure of Co₉Se₈@NiSe/NF step by step, which optimized the adsorption energy for oxygenated intermediates and thus promoted the catalytic activity.^48–52

Fig. 4 High-resolution XPS spectra of (a) Co 2p, (b) Mo 3d, (c) Ni 2p, and (d) Se 3d in Mo-Co₉Se₈@NiSe/NF-60, Mo-Co₉Se₈@NiSe/NF, and Co₉Se₈@NiSe/NF-60.

Considering the promising OER performance of Mo-Co₉Se₈@NiSe/NF-60, its overall water splitting (OWS) performance was further explored. The equipment was assembled by coupling Mo-Co₉Se₈@NiSe/NF-60 with Pt/C to achieve overall water splitting (Fig. 5a). Fig. 5b shows that the Mo-Co₉Se₈@NiSe/NF-60||Pt/C couple exhibited good catalytic activity towards overall water splitting, which required a cell voltage of only 1.87 V to achieve 50 mA cm⁻², remarkably surpassing the state-of-the-art RuO₂||Pt/C coupling. Moreover, LSV showed that the Mo-Co₉Se₈@NiSe/NF-60||Pt/C couple retained the initial voltage even after 1000 potential cycles, implying that the assembled electrode couple possessed excellent durability for overall water splitting. Furthermore, a chronoamperometry (CP) test was also employed to further establish its durability. Fig. 5c shows the ignorable voltage degradation maintaining 50 mA cm⁻² for at least 25 h continuously. In order to analyze the structural stability of Mo-Co₉Se₈@NiSe/NF-60, the electronic structure, phase, and morphology of spent catalysts were characterized. As shown in Fig. S8,† the XPS spectra of the catalyst before and after CP tests were obtained to depict the electronic structure. Co 2p and Ni 2p showed a significant negative shift compared with the initial one, which suggested that the electronic structure was noticeably altered. As displayed in Fig. S9,† the spent catalysts showed a similar pattern with the initial one (Fig. S9a†), and the locally enlarged area also showed the (222) plane of Co₉Se₈ (PDF # 09-0233) and the (300), (021), (211), (131) and (401) planes of NiSe (PDF # 18-0887) (Fig. S9b†). Fig. S9c and d† further prove the unchanged ultrathin morphology after 25-hour CP tests.

Fig. 5 (a) Schematic illustration of the electrocatalytic overall water splitting based on a Mo-Co₉Se₈@NiSe/NF-60∥Pt/C couple. (b) LSV polarization curves of RuO₂∥Pt/C, initial Mo-Co₉Se₈@NiSe/NF-60∥Pt/C, and Mo-Co₉Se₈@NiSe/NF-60∥Pt/C after 1000 circles toward water electrolysis. (c) Long-term CP curve of the Mo-Co₉Se₈@NiSe/NF-60∥Pt/C couple at 50 mA cm⁻².

Combining the results of electrochemical tests and characterizations, we can reach the conclusion that the excellent electrocatalytic OER performance of Mo-Co₉Se₈@NiSe/NF-60 is primarily generated from the electronic structure modification originating from Mo doping and Se vacancies (Fig. 6). For one thing, Mo doping can modify the electronic configuration and optimize the adsorption energy of reaction intermediates, resulting in enhanced activities. For another, Se vacancies can also effectively modify the electronic or surface structures to boost the numbers and intrinsic activities of electroactive sites, which further promotes the catalytic OER activity. Moreover, the ultrathin and self-supported structure also contributes to the substantial improvement in its OER electrocatalytic activity and stability.

Fig. 6 Schematic illustration of the mechanism of OER activity based on Mo-Co₉Se₈@NiSe/NF-60.

4. Conclusions

In summary, a dual modulation strategy was presented for tuning the electronic structure of Co₉Se₈@NiSe/NF ultrathin nanosheet arrays with Mo doping and Se vacancies. It is demonstrated that a trace amount of Mo doping and H₂O₂ etching can modulate the electronic structure of Co₉Se₈@NiSe/NF step by step, which can optimize the binding strength with oxygenated intermediates. Moreover, the self-supported ultrathin structure can also expose more accessible active sites for intermediates. Benefitting from the modulated electronic structure and self-supported ultrathin architecture, such Mo-Co₉Se₈@NiSe/NF-60 can exhibit superb electrocatalytic OER performance with an overpotential of 343 mV at 50 mA cm⁻², and a Tafel slope of 61.86 mV dec⁻¹ in 1.0 M KOH. This work demonstrated the power of step by step electronic structure modification in designing outstanding OER electrocatalysts by doping and defect engineering. It is highly anticipated that more works on the electronic structure and morphology engineering of other potential materials may be stimulated.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financed by the National Natural Science Foundation of China (22202169).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nr05410h

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