Manganese and selenium co-doped CeO₂@Co₃O₄ porous core–shell nanospheres for enhanced oxygen evolution reactions

Abstract

As one of the semi-reactions of water splitting, electrocatalytic oxygen evolution reactions (OERs) are key process to generate sustainable energy. Co-based spinel oxides are deemed as promising OER electrocatalysts, but the low intrinsic activity limits their further practical applications. Herein, we report an effective strategy to synthesize CoCeMn coordination-driven self-assembled aggregates (CDSAAs) by a one-pot multi-step method. It is different from the common mixed addition method to synthesize trimetallic nanomaterials. Subsequently, CoCeMn-CDSAAs served as self-templates, and Mn and Se co-doped CeO₂@Co₃O₄ porous core–shell nanospheres (MnSe–CeO₂@Co₃O₄) were obtained after calcination and selenization treatments. It is worth mentioning that CeO₂ and the doping Mn/Se elements increase the oxygen vacancy content and the ratio of Co³⁺/Co²⁺ on the surface respectively and can effectively accelerate the four-electron transfer process of OERs. Moreover, the special porous core–shell structure exposes more active sites and is also useful for boosting OER performance. When explored as an anode electrocatalyst for OERs, MnSe–CeO₂@Co₃O₄ exhibits excellent OER performance (284 mV@10 mA cm⁻²) and a long-time stability of 40 h in an alkaline medium. This work provides a feasible idea for the construction of multi-component porous core–shell nanoelectrocatalysts with non-noble metals.

---

1. Introduction

With the rapid increase in energy demand and the intensification of global environmental problems, it is important to develop renewable and clean energy sources.^1,2 Electrolysis of water is a promising way to generate pure hydrogen. However, the slow four-electron transfer process of oxygen evolution reactions (OERs) at the anode has restricted the efficiency of water splitting.^3–5 Although noble metal catalysts such as RuO₂ exhibit excellent OER performance, the high price and low natural abundance hinder their large-scale applications.⁶ Therefore, it is urgent to develop efficient and stable non-noble metal electrocatalysts for OERs.

In recent years, spinel transition-metal oxides have received extensive attention from researchers around the world due to their low price, easy availability and abundant storage.^7,8 Furthermore, Co₃O₄, as a cobalt-based spinel oxide, has been regarded as a potential OER electrocatalyst because of its excellent performance in an alkaline medium.^9,10 However, the practical application of pure Co₃O₄ is limited because of its poor intrinsic conductivity and insufficient active sites.¹¹ Developing efficient OER electrocatalysts with competitive price, low overpotential and long-time stability is challenging, and researchers have made considerable efforts to boost the OER performance of Co₃O₄ through these years.

On the one hand, the intrinsic activity of the materials can be improved obviously by increasing oxygen vacancies or doping other elements.^12–14 CeO₂, as a rare earth oxide, has been considered as an effective synergist for OERs due to its excellent redox ability and abundant oxygen vacancies.^15,16 After introducing CeO₂, the valence state conversion between Ce³⁺ and Ce⁴⁺ can accelerate electron transfer and increase the oxygen vacancies of materials, thus promoting the catalytic activity for OERs.¹⁷ Qiu et al. constructed a special CeO₂/Co₃O₄ heterojunction interface with enhanced OER performance, which is attributed to the abundant oxygen vacancies and interfacial coupling effect between the components.¹⁸ Moreover, doping other elements such as manganese (Mn),¹⁹ iron (Fe),²⁰ selenium (Se),²¹ and sulfur (S)²² can regulate the electronic structure of catalysts, resulting in faster four-electron transfer progress and is regarded as a promising method to enhance their intrinsic activity. Moreover, a higher content of Co³⁺ in nanomaterials also contributes to better OER performance.^23,24 Qi et al. designed a Mn and S dual-doped Co₃O₄ electrode array for efficient OERs. The excellent OER performance is attributed to their improved electrical conductivity and increasing ratio of Co³⁺/Co²⁺ after doping with Mn and S elements.²⁵ On the other hand, appropriate morphology designs such as core–shell structure can expose more active sites and facilitate charge transfer in materials.^26,27 Currently, numerous strategies have been developed to construct core–shell structures such as the Kirkendall effect,²⁸ Ostwald ripening,²⁹ and template method.³⁰ Zhang et al. synthesized pure CeO₂ core–shell nanospheres by a simple self-template and calcination method.³¹ However, the preparation of multielement core–shell hybrid nanomaterials is still a great challenge and has rarely been reported.

Herein, we report a facile approach to fabricate CoCeMn coordination-driven self-assembled aggregates (CDSAAs) by a simple one-pot multi-step method, in which the Ce element is enriched at the core of the solid nanosphere. Subsequently, CoCeMn-CDSAAs as self-temples were calcined in air to form Mn-doped CeO₂@Co₃O₄ porous core–shell nanospheres (Mn–CeO₂@Co₃O₄). Finally, Mn–CeO₂@Co₃O₄ was selenized in an Ar atmosphere and Se/Mn co-doped CeO₂@Co₃O₄ porous core–shell nanospheres (MnSe–CeO₂@Co₃O₄) were obtained (Fig. S1, ESI†). CeO₂ can effectively increase the oxygen vacancy content on the surface, which is helpful for promoting the OER performance. After doping of Se and Mn elements, the alteration of Co electronic structure can not only increase the electrical conductivity, but also improve the ratio of Co³⁺/Co²⁺ of the nanomaterial, which both contribute to enhancing the OER kinetic properties. In an alkaline medium of 1.0 M KOH, MnSe–CeO₂@Co₃O₄ exhibits a low overpotential of 284 mV@10 mA cm⁻², which is much lower than that of Co-oxides (419 mV@10 mA cm⁻²). In addition, they also exhibit the lowest Tafel slope of 94 mV dec¹¹ and excellent stability of 40 h.

2. Results and discussion

As described in Scheme 1, MnSe–CeO₂@Co₃O₄ was synthesized via a simple one-pot multi-step reaction, calcination and selenization strategy. First, Ce(OAc)₃, Mn(OAc)₂, and Co(OAc)₂ (molar ratio of 1 : 4 : 8) were added in turn to a methanolic solution of HL₁₀, and a CoCeMn-CDSAA precursor was successfully synthesized by coordination assembly of metal ions and HL₁₀. Subsequently, Mn–CeO₂@Co₃O₄ was obtained after calcination of CoCeMn-CDSAAs in an air atmosphere. On the one hand, the enrichment of Ce elements in the CoCeMn-CDSAAs contributes to the formation of core–shell structures, which can be confirmed by the later characterization. On the other hand, CO_x and H₂O derived from the decomposition of organic ligands facilitate the generation of porous structures in the process of calcination. Finally, MnSe–CeO₂@Co₃O₄ was obtained by selenization of Mn–CeO₂@Co₃O₄ in an Ar atmosphere. The MnSe–CeO₂@Co₃O₄ porous core shell nanospheres with high oxygen vacancy contents and a high ratio of Co³⁺/Co²⁺ ultimately exhibit excellent electrocatalytic performance.

Scheme 1 Schematic illustration of the synthesis procedure of MnSe–CeO₂@Co₃O₄.

From the perspective of morphology, CoCeMn-CDSAAs are smooth solid nanospheres with a diameter of about 1 μm (Fig. 1(a) and (b)). Moreover, the larger magnification TEM image reveals that there is a darker shadow inside the nanospheres (Fig. 1(c)). A series of CoCeMn-CDSAAs with different metal ratios can be easily obtained when adjusting the amount of different metal salts added in the one-pot multi-step method. Meanwhile, it can be illustrated by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) that the elemental ratios of Co and Ce roughly match the feeding ratios and the Mn element is mainly doped by small amounts (Table S1, ESI†). When adjusting metal ratios or synthesizing by a one-step method, the obtained nanospheres show different sizes or inhomogeneity (Fig. S2–S6, ESI†). When illustrated from a structural perspective, X-ray diffraction (XRD) and selected area electron diffraction (SAED) results both indicate that CoCeMn-CDSAA nanospheres are amorphous (Fig. 1(d) and (e)). Similarly, monometallic and bimetallic nanospheres can also be easily synthesized, and XRD patterns demonstrate that Co-CDSAAs, CoCe-CDSAAs and CoMn-CDSAAs are also amorphous (Fig. S7, ESI†). Moreover, the Fourier transform infrared (FT-IR) spectroscopy characterizations (Fig. S8, ESI†) reveal that Co-CDSAAs, CoMn-CDSAAs, CoCe-CDSAAs and CoCeMn-CDSAAs show the same IR absorption peaks, which suggest that they have the same coordination mode, and thus, they have similar morphological characteristics.^32,33 EDX elemental mapping results indicate that the Ce element enriches at the core of the sphere, while C, N, O, Co and Mn elements are uniformly distributed in the solid nanospheres (Fig. 1(f)), which is the basis for the formation of porous core–shell structures after calcination.

Fig. 1 (a) SEM and (b), (c) TEM images of CoCeMn-CDSAAs. (d) XRD spectrum and (e) SAED pattern of CoCeMn-CDSAAs. (f) HAADF-STEM image of CoCeMn-CDSAAs and corresponding EDX mapping of C, N, O, Co, Ce and Mn elements.

According to thermogravimetric (TGA) curves (Fig. S9, ESI†), CoCeMn-CDSAAs are able to form corresponding oxides after annealing at over 400 °C. Based on energy-dispersive X-ray spectroscopy (EDX) data (Table S2, ESI†), the metal ratios in the corresponding metal oxides remain consistent with the precursors. Interestingly, the high content of Ce elements can effectively strengthen their structural stability under high temperature conditions. The intensity of the diffraction peaks belonging to CeO₂ in the XRD pattern gradually decreases (Fig. S10, ESI†), which also indicates that the content of CeO₂ gradually decreases from CoCeMn-oxides-1 to CoCeMn-oxides-5. With the decrease in the content of Ce elements, the solid structure of the nanospheres starts to disappear and forms a porous core–shell structure (Fig. S11 and S12, ESI†). When the atomic ratio of Co/Ce is 8.54 in CoCeMn-CDSAAs, Mn-CeO₂@Co₃O₄ exhibits a complete porous core–shell structure after calcination (Fig. 2(a) and (b)). When the content of Ce elements continues to be reduced, the structure of CoCeMn-oxides-5 collapses (Fig. S11g and S12g, ESI†). Follow-up electrochemical active surface area (ESCA) results (Fig. S17d, ESI†) also reveal that Mn–CeO₂@Co₃O₄ possesses the most abundant electrochemical active sites among CoCeMn-oxides-x (x = 1–5), which may be due to their well-defined porous core–shell structure. Fig. 2 shows the successful formation of Mn–CeO₂@Co₃O₄ after calcination. Evidently, the diameter of Mn–CeO₂@Co₃O₄ shrinks to about 600 nm, and a porous core–shell structure is formed (Fig. 2(a)–(c)). The XRD pattern, high-resolution TEM (HRTEM) image and SAED pattern all show that there is a two-component heterogeneous structure in Mn–CeO₂@Co₃O₄ (Fig. 2(d)–(f)), which corresponds to CeO₂ (PDF#34-0394) and Co₃O₄ (PDF#43-1003), respectively. Compared with CoCe-oxides, there is a significant negative shift of Co₃O₄ (311) in Mn–CeO₂@Co₃O₄ (Fig. S13b, ESI†), which proves the successful doping of Mn elements in Co₃O₄.^34,35 In addition, the EDX elemental mapping (Fig. 2(g)) also reveals the formation of CeO₂ and Co₃O₄ heterostructures and uniform distribution of C, N, O, and Mn elements in Mn–CeO₂@Co₃O₄. When other factors are adjusted to synthesize the comparison material, the spherical morphology of CoCeMn-oxides-x (x = 6–10) are largely preserved though most of nanospheres showed unsatisfactory core–shell morphology (Fig. S4–S6, ESI†). However, the spherical morphology of Co-oxides, CoMn-oxides and CoCe-oxides was severely fragmented after calcination (Fig. S10 and S11, ESI†). All these results indicate that coexistence of Co, Ce and Mn elements is important for maintaining the spherical morphology. Moreover, a suitable metal ratio in precursors and a unique one-pot multi-step method both contribute to favorable porous core shell structures after calcination.

Fig. 2 (a) SEM and (b), (c) TEM images of Mn–CeO₂@Co₃O₄. (d) XRD spectrum, (e) HRTEM image and (f) SAED pattern of Mn–CeO₂@Co₃O₄. (g) HAADF-STEM image of Mn–CeO₂@Co₃O₄ and the corresponding EDX mapping of C, N, O, Co, Ce and Mn elements.

Modulating the electronic structure of metal atoms by heteroatom doping can effectively optimize the kinetic process of OERs.³⁶ Therefore, Mn–CeO₂@Co₃O₄ is selenized to further enhance its OER performance. The SEM and TEM images of the obtained MnSe–CeO₂@Co₃O₄ are shown in Fig. 3, and the porous core–shell structure of Mn–CeO₂@Co₃O₄ is preserved. As exhibited in Fig. S14 (ESI†), the surface area is about 298.67 m² g⁻¹ and the corresponding pore size is about 4.26 nm for MnSe–CeO₂@Co₃O₄. Fig. 3(a)–(c) show that the morphology of MnSe–CeO₂@Co₃O₄ is almost unchanged after selenization, whose diameter is about 500 nm. The XRD, HRTEM and SAED results (Fig. 3(d)–(f)) all correspond to CeO₂ (PDF#34-0394) and Co₃O₄ (PDF#43-1003), revealing the presence of CeO₂ and Co₃O₄ heterostructures. In addition, there is no new diffraction peak of CoSe₂, which proves the successful doping of Se elements in Co₃O₄. The magnified XRD spectra show that there is almost no obvious movement for the diffraction peak belonging to Co₃O₄ (311) in MnSe–CeO₂@Co₃O₄ compared with Mn–CeO₂@Co₃O₄ (Fig. S13b, ESI†), implying that suitable elemental doping of Se elements can hardly alter the crystal structure of Mn–CeO₂@Co₃O₄.³⁷ EDX elemental mapping results (Fig. 3(g)) also show the presence of CeO₂ and Co₃O₄ heterostructures and the uniform distribution of C, N, O, Mn, Se elements in MnSe–CeO₂@Co₃O₄.

Fig. 3 (a) SEM and (b), (c) TEM images of MnSe–CeO₂@Co₃O₄. (d) XRD spectrum, (e) HRTEM image and (f) SAED pattern of MnSe–CeO₂@Co₃O₄. (g) HAADF-STEM image of MnSe–CeO₂@Co₃O₄ and the corresponding EDX mapping of C, N, O, Co, Ce, Mn and Se elements.

To further investigate the role of CeO₂ as well as the doping of Mn/Se elements in MnSe–CeO₂@Co₃O₄, the elements and the corresponding chemical statuses of samples were investigated by X-ray photoelectron spectroscopy (XPS). The presence of C, N, O, Co, Ce, Mn and Se elements is observed in both EDX (Fig. 4(a)) and survey spectra (Fig. 4(b)) of MnSe–CeO₂@Co₃O₄. The low content of Se and Mn elements in EDX spectrum can further prove their doping in MnSe–CeO₂@Co₃O₄. The orbital peak of Ce 3d splits into a series of Ce³⁺ and Ce⁴⁺ peaks, indicating that Ce³⁺ and Ce⁴⁺ coexist in MnSe–CeO₂@Co₃O₄ (Fig. 4(c)). The peaks at 885.2 and 903.9 eV are attributed to Ce³⁺, while the peaks at 882.4, 887.8, 898.3, 901.3, 907.4, and 916.7 eV are attributed to Ce⁴⁺.^38,39 The XPS spectrum of O 1s can be divided into three peaks (donated as O1, O2, and O3 in Fig. 4(d)). The O1 peak at 530.0 eV is attributed to the metal–oxygen bond in the material, the O2 peak at 531.2 eV is attributed to the oxygen vacancy and the O3 peak at 532.7 eV is attributed to the hydroxyl group adsorbed on the surface.⁴⁰ It is noteworthy that high oxygen vacancy contents can enhance the intrinsic activity of the material and optimize the kinetic properties of the OER progress.^41,42 The introduction of CeO₂ effectively increases the content of oxygen vacancies (Table S3, ESI†), which is useful for improving the intrinsic activity of nanomaterials. For Co 2p (Fig. 4(e)), the peaks at 796.7 and 781.5 eV correspond to Co 2p_1/2 and Co 2p_3/2 of Co²⁺, while the peaks at 795.0 and 779.9 eV are attributed to Co 2p_1/2 and Co 2p_3/2 of Co³⁺ with the presence of two vibrational satellite peaks (787.6 and 803.6 eV).^43,44 Compared with Co-oxides, there are positive shifts in the binding energy of Co 2p_3/2 after introduction of CeO₂ and Mn/Se elements (Fig. 4(e)), which implies the electron transfer among different components. This phenomenon may be due to the rearrangement of electron distribution resulting from the interaction of Co with CeO₂ or dopants. The strong interaction in MnSe–CeO₂@Co₃O₄ may modulate the frontier orbital energy of the catalysts, which is useful for improving the kinetic process of OERs.^45,46 Moreover, it is worth mentioning that the doping of Mn/Se elements can significantly increase the content of Co³⁺ (Table S3, ESI†), and high activity of Co³⁺ can effectively accelerate the kinetic process of OERs.^47,48 It is obvious that a high ratio of Co³⁺/Co²⁺ and a high oxygen vacancy content in MnSe–CeO₂@Co₃O₄ are important to enhance their electrochemical performance.

Fig. 4 (a) EDX and (b) Survey spectra of MnSe–CeO₂@Co₃O₄. High-resolution XPS spectra of (c) Ce 3d and (f) Se 3d in MnSe–CeO₂@Co₃O₄. High-resolution XPS spectra of (d) O 1s and (e) Co 2p comparison among Co-oxides, CoCe-oxides, CoMn-oxides, Mn-CeO₂@Co₃O₄ and MnSe–CeO₂@Co₃O₄.

At room temperature, the OER performance of the prepared catalysts is explored in an alkaline medium of 1 M KOH. It should be noticed that MnSe–CeO₂@Co₃O₄ is chosen as the main discussion object in the following based on that Mn–CeO₂@Co₃O₄, as the precursor of MnSe–CeO₂@Co₃O₄, is demonstrated to have the most promising performance among CoCeMn-oxides-x (x = 1–10), as observed from Fig. S17 and S19 (ESI†). As shown in Fig. 5(a), the anodic linear scanning voltammetry (LSV) polarization curves of different catalysts exhibit significant differences at a scan rate of 10 mV s⁻¹. MnSe–CeO₂@Co₃O₄ exhibits the optimal overpotential of 284 mV among these comparison materials, which is significantly lower than that of Co-oxides (419 mV), CoMn-oxides (390 mV), CoCe-oxides (365 mV) and Mn–CeO₂@Co₃O₄ (336 mV). The OER performance of trimetallic oxides is better than that of monometallic or bimetallic oxides, indicating that the synergistic effect of multiple components and the special porous core–shell structure contribute to their excellent performance. In addition, the corresponding oxides showed similar OER results after selenization (Fig. S19b, ESI†), which are attributed to the synergy of multiple components and porous core shell structure. Moreover, the OER performance of MnSe–CeO₂@Co₃O₄ is superior to that of commercial RuO₂ (362 mV) (Fig. S18, ESI†) and most recently reported Co₃O₄-based or CeO₂-based catalysts (Fig. 5(f) and Table S4, ESI†). The Tafel slope can further explain the OER properties from a kinetic point of view. The smaller value of Tafel slope means the faster kinetic process of OERs.⁴⁹ Among these tested catalysts, MnSe–CeO₂@Co₃O₄ exhibits the smallest Tafel slope of 94 mV dec⁻¹, revealing their boosting OER kinetic process (Fig. 5(b)). The ESCAs are collected to probe the actual number of active sites involved in the electrochemical reaction. The ESCAs of the catalysts exhibit a positive correlation with double layer capacitance (C_dl) and the C_dl value can be obtained in the non-faradaic region with different scan rates (Fig. S20, ESI†).⁵⁰ As shown in Fig. 5(c), the C_dl value of MnSe–CeO₂@Co₃O₄ (25.3 mF cm⁻²) is much higher than that of other catalysts, which suggests that MnSe–CeO₂@Co₃O₄ has more active sites under the same loading condition. Electrochemical impedance spectroscopy (EIS) value can effectively reflect the electron transfer rate and resistance value between the catalyst and the electrolyte.⁵¹ The smaller radius of the impedance arc represents its smaller charge transfer resistance (R_ct), which results in the improved OER activity of the catalysts. The Nyquist curves (Fig. 5(e)) show that MnSe–CeO₂@Co₃O₄ has the smallest semi-circular diameter among catalysts, which means the smallest charge transfer resistance (R_ct) (only 34 Ω). This phenomenon indicates that there is a significant enhancement of the electrical conductivity in materials after the doping of Se elements. A series of OER tests have shown that the doping of CeO₂, Mn and Se elements can contribute significantly to the improvement of the OER performance. Compared with Co-oxides and CoMn-oxides, CoCe-oxides and CoCe–Mn-oxides clearly possess a lower overpotential, faster OER kinetic performance, larger electrochemically active surface area and smaller charge transfer resistance, respectively. This is attributed to the increased oxygen vacancy content caused by the introduction of CeO₂. Similarly, a series of performance tests can also show that Mn and Se elements provide higher levels of Co³⁺ to the catalyst in favor of the enhanced OER performance of the catalyst.

Fig. 5 (a) LSV polarization curves of Co-oxides, CoMn-oxides, CoCe-oxides, Mn–CeO₂@Co₃O₄ and MnSe–CeO₂@Co₃O₄ in 1 M KOH aqueous solution for OERs. (b) Corresponding Tafel slopes. (c) Corresponding overpotentials and Tafel slopes at 10 mA cm⁻². (d) Double-layer capacitance (Cdl) obtained by linear fitting of the capacitive currents. (e) EIS Nyquist plots of Co-oxides, CoMn-oxides, CoCe-oxides, Mn–CeO₂@Co₃O₄ and MnSe–CeO₂@Co₃O₄. Inset is the equivalent circuit. (f) Comparison of overpotentials at 10 mA cm⁻² for the MnSe–CeO₂@Co₃O₄ catalyst with the recently reported Co-based or Ce-based OER electrocatalysts.

The stability is an important parameter for evaluating their electrochemical performance, which contributes to the further practical application of the catalysts.^52,53 Cyclic voltammetry (CV) and chronopotentiometric (CP) methods are used to test the stability of MnSe–CeO₂@Co₃O₄. The OER polarization curve of MnSe–CeO₂@Co₃O₄ remains almost unchanged after 1000 CV cycles, reflecting their excellent stability (Fig. S21, ESI†). To reduce the effect of sample shedding, MnSe–CeO₂@Co₃O₄ dropped on an inactive carbon paper substrate for testing their OER performance. Reaching the current density of 10 mA cm⁻², the overpotential of MnSe–CeO₂@Co₃O₄ on the carbon paper is 310 mV in 1 M KOH solution (Fig. 6(a)) and the OER activity is almost unchanged after the CP test of 40 h (Fig. 6(b)). The TEM image (Fig. 6(c)) shows that the porous core–shell morphology of MnSe–CeO₂@Co₃O₄ is still preserved after the long-time CP test, revealing the superior structural stability of porous core–shell structures. Moreover, the main components of MnSe–CeO₂@Co₃O₄ are still Co₃O₄ and CeO₂ after stability testing (Fig. 6(d), Fig. S22 and S23, ESI†), indicating the stable composition of catalysts. The XPS spectra are employed to further understand the elemental valence changes of MnSe–CeO₂@Co₃O₄ after stability testing. It is worth noting that MnSe–CeO₂@Co₃O₄ still maintains a high ratio of Co³⁺/Co²⁺ (Fig. 6(e)) and a high oxygen vacancy content (Fig. 6(f)) after the stability test, which may be the reason for their excellent stability (Table S3, ESI†).

Fig. 6 (a) OER polarization curve of MnSe–CeO₂@Co₃O₄ loaded on the carbon paper and bare carbon paper. (b) CP curves of MnSe–CeO₂@Co₃O₄ loaded on the carbon paper at 10 mA cm⁻². (c) TEM image and (d) XRD pattern of MnSe–CeO₂@Co₃O₄ after the CP test. High-resolution XPS spectra of (e) Co 2p and (f) O 1s in MnSe–CeO₂@Co₃O₄ after the CP test.

The desirable electrochemical property of MnSe–CeO₂@Co₃O₄ may be attributed to the following reasons. First, BET and ESCA results both reveal that the porous core–shell structure possesses a larger specific surface area and more active sites for OERs. The large area of interfaces in contact with the electrolyte can accelerate the active intermediate transport and diffusion on the surface of catalysts, thus optimizing their catalytic capacity.^54,55 Second, high oxygen vacancy contents can strengthen the intrinsic activity of catalysts, which facilitates the reduction of the OER reaction energy barrier and optimizes their OER activity.⁵⁶ The XPS spectra illustrate that CeO₂ can effectively increase the content of oxygen vacancies among the above-mentioned nanomaterials (Table S3, ESI†), which is considered as one of the reasons for their favorable OER activity. Third, element doping can lead to a shift of the metal elements from a lower valence to a more active higher valence state (for instance, Co²⁺ species are oxidized to Co³⁺ species) (Table S3, ESI†). The high oxidation state of Co³⁺ can facilitate the adsorption of OER intermediates and the four-electron transfer process of OERs.^57,58 Moreover, the smallest Tafel slope of MnSe–CeO₂@Co₃O₄ also means a faster four-electron transfer process of OERs (Fig. 5(b)). Finally, the EIS value of MnSe–CeO₂@Co₃O₄ is significantly lower than that of other comparison catalysts (Fig. 5(e)). This is mainly because the introduced substances affect the coordination environment and electronic structure of the adjacent Co atoms, leading to lattice distortions and defects, which really enhance the electrical conductivity of the nanomaterials.⁵⁹ Therefore, MnSe–CeO₂@Co₃O₄ with the unique characteristics of morphology, structure and composition ultimately possess enhanced electrochemical properties.

3. Conclusions

In a word, a multicomponent doping strategy was proposed to fabricate MnSe–CeO₂@Co₃O₄ porous core–shell nanospheres with large surface area, high oxygen vacancy content, outstanding kinetic performance and excellent electrical conductivity. At a current density of 10 mA cm⁻², the overpotential of MnSe–CeO₂@Co₃O₄ is only 284 mV, which is significantly lower than that of Co-oxides (419 mV). Moreover, MnSe–CeO₂@Co₃O₄ exhibits a long-time stability of 40 h in 1 M KOH solution. Introduction of CeO₂ and doping of Mn/Se elements play an important role in elevating the oxygen vacancy level and the ratio of Co³⁺/Co²⁺ on the surface respectively, which are beneficial to optimize the four-electron transfer process. Besides, the enhanced electrical conductivity and porous core–shell structure improve the intrinsic activity of the materials, which both ultimately enhance their OER performance. This work not only supplies an effective catalyst with non-noble metals for OERs but also provides a meaningful reference for designing multi-component porous core–shell nanomaterials as more advanced electrocatalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

H. G. and X. C. acknowledge financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the project of Scientific and Technologic Infrastructure of Suzhou (SZS201708), and the Research Fund Program of Key Laboratory of Rare Mineral, MNR (No. KLRM-KF202004). We express our sincere thanks to Prof. Qi Shao (College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, Jiangsu, China) for assistance with CHI660E electrochemical workstation and valuable suggestions.

References

1. Z. Seh, J. Kibsgaard, C. Dickens, I. Chorkendorff, J. Norskov and T. Jaramillo, Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design, Science, 2017, 355, eaad4998.
2. B. Guan, L. Yu, J. Li and X. Lou, TiO₂ Nanoshells with Enhanced Lithium Storage Properties, Sci. Adv., 2016, 2, e1501554.
3. T. Tian, M. Zheng, J. Lin, X. Meng and Y. Ding, Amorphous Ni–Fe Double Hydroxide Hollow Nanocubes Enriched with Oxygen Vacancies as Efficient Electrocatalytic Water Oxidation Catalysts, Chem. Commun., 2019, 55, 1044–1047.
4. Y. Jiao, Y. Zheng, M. Jaroniec and S. Qiao, Design of Electrocatalysts for Oxygen- and Hydrogen-Involving Energy Conversion Reactions, Chem. Soc. Rev., 2015, 44, 2060–2086.
5. X. Xu, F. Song, X. Hu and A. Nickel, Iron Diselenide-Derived Efficient Oxygen-Evolution Catalyst, Nat. Commun., 2016, 7, 12324.
6. Y. Hou, M. Qiu, T. Zhang, J. Ma, S. Liu, X. Zhuang, C. Yuan and X. Feng, Ternary Porous Cobalt Phosphoselenide Nanosheets: An Efficient Electrocatalyst for Electrocatalytic and Photoelectrochemical Water Splitting, Adv. Mater., 2017, 29, 1604480.
7. Y. Zhou, S. Sun, J. Song, S. Xi, B. Chen, Y. Du, A. Fisher, F. Cheng, X. Wang, H. Zhang and Z. Xu, Enlarged Co–O Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction, Adv. Mater., 2018, 30, 1802912.
8. Y. Duan, S. Sun, Y. Sun, S. Xi, X. Chi, Q. Zhang, X. Ren, J. Wang, S. Ong, Y. Du, L. Gu, A. Grimaud and Z. Xu, Mastering Surface Reconstruction of Metastable Spinel Oxides for Better Water Oxidation, Adv. Mater., 2019, 31, 1807898.
9. T. Ma, S. Dai, M. Jaroniec and S. Qiao, Metal–Organic Framework Derived Hybrid Co₃O₄–Carbon Porous Nanowire Arrays as Reversible Oxygen Evolution Electrodes, J. Am. Chem. Soc., 2014, 136, 13925–13931.
10. T. Quast, S. Varhade, S. Saddeler, Y. Chen, C. Andronescu, S. Schulz and W. Schuhmann, Single Particle Nanoelectrochemistry Reveals the Catalytic Oxygen Evolution Reaction Activity of Co₃O₄ Nanocubes, Angew. Chem., Int. Ed., 2021, 60, 23444–23450.
11. S. Hung, Y. Hsu, C. Chang, C. Hsu, N. Suen, T. Chan and H. Chen, Unraveling Geometrical Site Confinement in Highly Efficient Iron-Doped Electrocatalysts toward Oxygen Evolution Reaction, Adv. Energy Mater., 2018, 8, 1701686.
12. H. Xu, J. Cao, C. Shan, B. Wang, P. Xi, W. Liu and Y. Tang, MOF-Derived Hollow CoS Decorated with CeO_x Nanoparticles for Boosting Oxygen Evolution Reaction Electrocatalysis, Angew. Chem., Int. Ed., 2018, 130, 8790–8794.
13. J. Zhou, H. Zheng, Q. Luan, X. Huang, Y. Li, Z. Xi, G. Lu, L. Xing and Y. Li, Improving the Oxygen Evolution Activity of Co₃O₄ by Introducing Ce Species Derived from Ce-substituted ZIF-67, Sustainable Energy Fuels, 2019, 3, 3201–3207.
14. X. Du, Y. Ding and X. Zhang, Selectively Se-Doped Co₃O₄@CeO₂ Nanoparticle-Dotted Nanoneedle Arrays for High-Efficiency Overall Water Splitting, Appl. Surf. Sci., 2021, 562, 150227.
15. W. Li, L. Zhao, C. Wang, X. Lu and W. Chen, Interface Engineering of Heterogeneous CeO₂–CoO Nanofibers with Rich Oxygen Vacancies for Enhanced Electrocatalytic Oxygen Evolution Performance, ACS Appl. Mater. Interfaces, 2021, 13, 46998–47009.
16. J. Huang, H. Sheng, R. Ross, J. Han, X. Wang, B. Song and S. Jin, Modifying Redox Properties and Local Bonding of Co₃O₄ by CeO₂ Enhances Oxygen Evolution Catalysis in Acid, Nat. Commun., 2021, 12, 3036.
17. J. Xia, H. Zhao, B. Huang, L. Xu, M. Luo, J. Wang, F. Luo, Y. Du and C. Yan, Efficient Optimization of Electron/Oxygen Pathway by Constructing Ceria/Hydroxide Interface for Highly Active Oxygen Evolution Reaction, Adv. Funct. Mater., 2020, 30, 1908367.
18. B. Qiu, C. Wang, N. Zhang, L. Cai, Y. Xiong and Y. Chai, CeO₂-Induced Interfacial Co²⁺ Octahedral Sites and Oxygen Vacancies for Water Oxidation, ACS Catal., 2019, 9, 6484–6490.
19. S. Chen, H. Huang, P. Jiang, K. Yang, J. Diao, S. Gong, S. Liu, M. Huang, H. Wang and Q. Chen, Mn-Doped RuO₂ Nanocrystals as Highly Active Electrocatalysts for Enhanced Oxygen Evolution in Acidic Media, ACS Catal., 2019, 10, 1152–1160.
20. K. Chang, D. Tran, J. Wang, N. Kim and J. Lee, A 3D Hierarchical Network Derived from 2D Fe-Doped NiSe Nanosheets/Carbon Nanotubes with Enhanced OER Performance for Overall Water Splitting, J. Mater. Chem. A, 2022, 10, 3102–3111.
21. Y. Huang, J. Wang, Y. Zou, L. Jiang, X. Liu, W. Jiang, H. Liu and J. Hu, Selective Se Doping of NiFe₂O₄ on an Active NiOOH Scaffold for Efficient and Robust Water Oxidation, Chin. J. Catal., 2021, 42, 1395–1403.
22. S. Li, L. Wang, H. Su, A. Hong, Y. Wang, H. Yang, L. Ge, W. Song, J. Liu, T. Ma, X. Bu and P. Feng, Electron Redistributed S-Doped Nickel Iron Phosphides Derived from One-Step Phosphatization of MOFs for Significantly Boosting Electrochemical Water Splitting, Adv. Funct. Mater., 2022, 32, 202200733.
23. X. Zhang, B. Li, M. Lan, S. Yang, Q. Xie, J. Xiao, F. Xiao and S. Wang, Cation Modulation of Cobalt Sulfide Supported by Mesopore-Rich Hydrangea-Like Carbon Nanoflower for Oxygen Electrocatalysis, ACS Appl. Mater. Interfaces, 2021, 13, 18683–18692.
24. C. Jiang, J. Yang, T. Zhao, L. Xiong, Z. Guo, Y. Ren, H. Qi, A. Wang and J. Tang, Co³⁺–O–V⁴⁺ Cluster in CoVO_x Nanorods for Efficient and Stable Electrochemical Oxygen Evolution, Appl. Catal., B, 2021, 282, 119571.
25. J. Qi, H. Wang, J. Lina, C. Li, X. Si, J. Cao, Z. Zhong and J. Feng, Mn and S Dual-Doping of MOF-Derived Co₃O₄ Electrode Array Increases the Efficiency of Electrocatalytic Generation of Oxygen, J. Colloid Interface Sci., 2019, 557, 28–33.
26. Z. Cai, X. Bu, P. Wang, W. Su, R. Wei, J. Ho, J. Yang and X. Wang, Simple and Cost Effective Fabrication of 3D Porous Core–Shell Ni Nanochains@NiFe Layered Double Hydroxide Nanosheet Bifunctional Electrocatalysts for Overall Water Splitting, J. Mater. Chem. A, 2019, 7, 21722–21729.
27. Y. Fu, W. Wang, J. Wang, X. Li, R. Shi, O. Peng, B. Chandrashekar, K. Liu, A. Amini and C. Cheng, MOFs-Derived ZnCo–Fe Core–Shell Nanocages with Remarkable Oxygen Evolution Reaction Performance, J. Mater. Chem. A, 2019, 7, 17299–17305.
28. X. Yang, Y. Zhang, Y. Wang, C. Xin, P. Zhang, D. Liu, B. Mamba, K. Kefeni, A. Kuvarega and J. Gui, Hollow β-Bi₂O₃@CeO₂ Heterostructure Microsphere with Controllable Crystal Phase for Efficient Photocatalysis, Chem. Eng. J., 2020, 387, 124100.
29. D. An, J. Wang, J. Zhang, X. Zhai, Z. Kang, W. Fan, J. Yan, Y. Liu, L. Lu, C. Jia, M. Wuttig, O. Cojocaru-Miredin, S. Chen, W. Wang, G. Snyder and Y. Yu, Retarding Ostwald Ripening through Gibbs Adsorption and Interfacial Complexions Leads to High-Performance SnTe Thermoelectrics, Energy Environ. Sci., 2021, 14, 5469–5479.
30. J. Liu, Z. Wang, Y. Chai, M. Kurmoo, Q. Zhao, X. Wang, C. Tung and D. Sun, Core Modulation of 70-Nuclei Core–Shell Silver Nanoclusters, Angew. Chem., Int. Ed., 2019, 58, 6276–6279.
31. L. Zhang, L. Zhang, G. Xu, C. Zhang, X. Li, Z. Sun and D. Jia, Low-Temperature CO Oxidation over CeO₂ and CeO₂@Co₃O₄ Core–Shell Microspheres, New J. Chem., 2017, 41, 13418–13424.
32. Y. Zhao, J. Zhang, J. Song, J. Li, J. Liu, T. Wu, P. Zhang and B. Han, Ru Nanoparticles Immobilized on Metal–Organic Framework Nanorods by Supercritical CO₂–Methanol Solution: Highly Efficient Catalyst, Green Chem., 2011, 13, 2078–2082.
33. X. Huang, C. Zhou, H. Liu, L. Zeng, X. Zhang, X. Han, F. Zhu, Y. Lu, X. Cao and H. Gu, In Situ Simultaneous Cavitation-Doping Approach for Constructing Bimetallic Metal–Organic Framework Hollow Nanospheres with Enhanced Electrocatalytic Hydrogen Production, Inorg. Chem., 2022, 61, 5977–5981.
34. G. Li, M. Chen, Y. Ouyang, D. Yao, L. Lu, L. Wang, X. Xia, W. Lei, S. Chen, D. Mandler and Q. Hao, Manganese Doped Co₃O₄ Mesoporous Nanoneedle Array for Long Cycle-stable Supercapacitors, Appl. Surf. Sci., 2019, 469, 941–950.
35. C. Huang, Y. Zhang, X. Li, H. Cao, Y. Guo and C. Zhang, Mn-Incorporated Co₃O₄ Bifunctional Electrocatalysts for Zinc–Air Battery Application: An Experimental and DFT Study, Appl. Catal., B, 2022, 319, 121909.
36. Y. Huang, L. Jiang, B. Shi, K. Ryan and J. Wang, Highly Efficient Oxygen Evolution Reaction Enabled by Phosphorus Doping of the Fe Electronic Structure in Iron–Nickel Selenide Nanosheets, Adv. Sci., 2021, 8, e2101775.
37. R. Li, Y. Guo, H. Chen, K. Wang, R. Tan, B. Long, Y. Tong, P. Tsiakaras, S. Song and Y. Wang, Anion–Cation Double Doped Co₃O₄ Microtube Architecture to Promote High-Valence Co Species Formation for Enhanced Oxygen Evolution Reaction, ACS Sustainable Chem. Eng., 2019, 7, 11901–11910.
38. L. Lv, D. Zha, Y. Ruan, Z. Li, X. Ao, J. Zheng, J. Jiang, H. Chen, W. Chiang, J. Chen and C. Wang, A Universal Method to Engineer Metal Oxide–Metal–Carbon Interface for Highly Efficient Oxygen Reduction, ACS Nano, 2018, 12, 3042–3051.
39. S. Putla., M. Amin., B. Reddy., A. Nafady., K. Al Farhan. and S. Bhargava, MnO_X Nanoparticle-Dispersed CeO₂ Nanocubes: A Remarkable Heteronanostructured System with Unusual Structural Characteristics and Superior Catalytic Performance, ACS Appl. Mater. Interfaces, 2015, 7, 16525–16535.
40. Q. Zhou, Y. Chen, G. Zhao, Y. Lin, Z. Yu, X. Xu, X. Wang, H. Liu, W. Sun and S. Dou, Active-Site-Enriched Iron-Doped Nickel/Cobalt Hydroxide Nanosheets for Enhanced Oxygen Evolution Reaction, ACS Catal., 2018, 8, 5382–5390.
41. S. Guo, J. Wang, Y. Sun, L. Peng and C. Li, Interface Engineering of Co₃O₄/CeO₂ Heterostructure in situ Embedded in Co/N-Doped Carbon Nanofibers Integrating Oxygen Vacancies as Effective Oxygen Cathode Catalyst for Li–O₂ Battery, Chem. Eng. J., 2022, 452, 39317.
42. D. Ghosh, M. Manikanta Kumar, C. Raj and D. Pradhan, Bifunctional Catalytic Activity of Solvothermally Synthesized CeO₂ Nanosphere/NiO Nanoflake Nanocomposites, ACS Appl. Energy Mater., 2022, 5, 5666–5679.
43. A. Saad, D. Liu, Y. Wu, Z. Song, Y. Li, T. Najam, K. Zong, P. Tsiakaras and X. Cai, Ag Nanoparticles Modified Crumpled Borophene Supported Co₃O₄ Catalyst Showing Superior Oxygen Evolution Reaction (OER) performance, Appl. Catal., B, 2021, 298, 120529.
44. X. Yang, J. Chen, Y. Chen, P. Feng, H. Lai, J. Li and X. Luo, Novel Co₃O₄ Nanoparticles/Nitrogen-Doped Carbon Composites with Extraordinary Catalytic Activity for Oxygen Evolution Reaction (OER), Nano-Micro Lett., 2018, 10, 1–11.
45. M. Gong, Y. Li, H. Wang, Y. Liang, J. Wu, J. Zhou, J. Wang, T. Regier, F. Wei and H. Dai, An Advanced Ni–Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation, J. Am. Chem. Soc., 2013, 135, 8452–8455.
46. Y. Guo, J. Tang, J. Henzie, B. Jiang, W. Xia, T. Chen, Y. Bando, Y. Kang, M. Hossain, Y. Sugahara and Y. Yamauchi, Mesoporous Iron-Doped MoS₂/CoMo₂S₄ Heterostructures through Organic Metal Cooperative Interactions on Spherical Micelles for Electrochemical Water Splitting, ACS Nano, 2020, 14, 4141–4152.
47. Y. Bi, Z. Cai, D. Zhou, Y. Tian, Q. Zhang, Q. Zhang, Y. Kuang, Y. Li, X. Sun and X. Duan, Understanding the Incorporating Effect of Co²⁺/Co³⁺ in NiFe-layered Double Hydroxide for Electrocatalytic Oxygen Evolution Reaction, J. Catal., 2018, 358, 100–107.
48. Z. Hao, P. Wei, Y. Yang, J. Sun, Y. Song, D. Guo and L. Liu, Self-Assembled CuCo₂S₄ Nanosheets with Rich Surface Co³⁺ as Efficient Electrocatalysts for Oxygen Evolution Reaction, Appl. Surf. Sci., 2021, 536, 147826.
49. L. Xu, Q. Jiang, Z. Xiao, X. Li, J. Huo, S. Wang and L. Dai, Plasma-Engraved Co₃O₄ Nanosheets with Oxygen Vacancies and High Surface Area for The Oxygen Evolution Reaction, Angew. Chem., Int. Ed., 2016, 128, 5363–5367.
50. Y. Zhao, C. Mavrokefalos, P. Zhang, R. Erni, J. Li, C. Triana and G. Patzke, Self-Templating Strategies for Transition Metal Sulfide Nanoboxes as Robust Bifunctional Electrocatalysts, Chem. Mater., 2020, 32, 1371–1383.
51. Y. Lu, D. Fan, Z. Chen, W. Xiao, C. Cao and X. Yang, Anchoring Co₃O₄ Nanoparticles on MXene for Efficient Electrocatalytic Oxygen Evolution, Sci. Bull., 2020, 65, 460–466.
52. J. Kang, X. Qiu, Q. Hu, J. Zhong, X. Gao, R. Huang, C. Wan, L. Liu, X. Duan and L. Guo, Valence Oscillation and Dynamic Active Sites in Monolayer NiCo Hydroxides for Water Oxidation, Nat. Catal., 2021, 4, 1050–1058.
53. F. Tang, S. Guo, Y. Sun, X. Lin, J. Qiu and A. Cao, Facile Synthesis of Fe-Doped CoO Nanotubes as High-Efficient Electrocatalysts for Oxygen Evolution Reaction, Small Struct., 2022, 3, 2100211.
54. Q. Qin, J. Hao and W. Zheng, Ni/Ni₃C Core/Shell Hierarchical Nanospheres with Enhanced Electrocatalytic Activity for Water Oxidation, ACS Appl. Mater. Interfaces, 2018, 10, 17827–17834.
55. Q. Hu, Z. Wang, X. Huang, Y. Qin, H. Yang, X. Ren, Q. Zhang, J. Liu, M. Shao and C. He, Integrating Well-Controlled Core–Shell Structures into “Superaerophobic” Electrodes for Water Oxidation at Large Current Densities, Appl. Catal., B, 2021, 286, 119920.
56. B. Zhang, J. Shan, X. Wang, Y. Hu and Y. Li, Ru/Rh Cation Doping and Oxygen-Vacancy Engineering of FeOOH Nanoarrays@Ti₃C₂T_x MXene Heterojunction for Highly Efficient and Stable Electrocatalytic Oxygen Evolution, Small, 2022, 18, e2200173.
57. D. Shao, P. Li, R. Zhang, C. Zhao, D. Wang and C. Zhao, One-Step Preparation of Fe-Doped Ni₃S₂/rGO@NF Electrode and Its Superior OER Performances, Int. J. Hydrogen Energy, 2019, 44, 2664–2674.
58. S. Li, S. Peng, L. Huang, X. Cui, A. Al-Enizi and G. Zheng, Carbon-Coated Co³⁺-Rich Cobalt Selenide Derived from ZIF-67 for Efficient Electrochemical Water Oxidation, ACS Appl. Mater. Interfaces, 2016, 8, 20534–20539.
59. S. Balaghi, C. Triana and G. Patzke, Molybdenum-Doped Manganese Oxide as a Highly Efficient and Economical Water Oxidation Catalyst, ACS Catal., 2020, 10, 2074–2087.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ya00315e

‡ These authors contributed equally.

---