Modulating the electronic configuration of Co species in MOF/MXene nanosheet derived Co-based mixed spinel oxides for an efficient oxygen evolution reaction

Abstract

The electronic configuration of Co cations at octahedral (Oct) sites plays a crucial role in the catalytic activity of Co₃O₄-based spinel oxides toward the oxygen evolution reaction (OER). However, there are few reports on modulating the electronic configuration of partial Co_Oct³⁺ (t⁶_2ge⁰_g) to Co_Oct²⁺ (t⁶_2ge¹_g) for further promoting their OER performances. Herein, a metal–organic framework (MOF)/MXene composite material pyrolysis–reorganization strategy is developed to obtain heterogeneous Co-based mixed spinel oxides, Co₃O₄/Co₂TiO₄. By regulating the pyrolysis temperature, the mesoporous structures and the electronic configuration of Co_Oct cations of mixed spinel oxides can be simultaneously optimized, leading to exceptional OER performances with low overpotentials of 280 mV on a glassy carbon electrode and 260 mV on Ni foam at 10 mA cm⁻² as well as good stability in alkaline solution. The synergistic catalytic effect between Co_Oct³⁺ in Co₃O₄ and Co_Oct²⁺ in Co₂TiO₄ is shown to be crucial for improving the OER activity. This finding will provide a new pathway for promoting the OER activity of Co-based spinel oxides.

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Introduction

The oxygen-evolution reaction (OER) is an important half-reaction in electrolytic water splitting to hydrogen and metal–air batteries, while its slow kinetics and large reaction energy barriers greatly limit the overall efficiency of these clean energy systems.¹ Exploring an excellent electrocatalytic OER catalyst is a key point to solve these problems.^2–4 It is well known that noble metal catalysts such as IrO₂ and RuO₂ are considered to be the best OER electrocatalysts, but their high instability in acidic/alkaline media and high cost severely restrict their application.⁵ In virtue of low cost, environmental friendliness, and ready tunability, cobalt-based spinel oxides have been widely studied as substitutes for noble metal-based electrocatalysts for the OER.^6–9 They can be described as AB₂O₄, where oxygen anions are arranged in a cubic close-packed lattice with metal ions located in tetrahedral (Td) and octahedral (Oct) interstices.¹⁰ A general principle demonstrates that the redox activity of Co cations in Oct sites is crucial for the OER process. This could be attributed to the high-lying e_g orbital directly pointing to six adjacent O in octahedral coordination, thus creating a strong spatial overlap with an O 2p orbital.^10–13 Particularly, the moderate e_g occupancy (e_g ≈ 1) of the metal cation at the Oct sites can balance the competition between rate-limiting steps to realize drastically improved OER activity.^14–16 These findings inspire more in-depth research on Co-based spinel oxides, such as regulating the composition and valence state, to tune their e_g occupancy.

Co₃O₄, a typical Co-based spinel oxide, is highly attractive due to its competitive activity. The Td and Oct-occupied Co cations in Co₃O₄ would prefer the electronic configuration of Co_Td²⁺ (e⁴_gt³_2g) and Co_Oct³⁺ (t⁶_2ge⁰_g), respectively.¹⁷ The strong orbital overlap between the octahedral Co and O is responsible for the excellent OER activity.¹⁸ To date, various synthetic strategies have been explored to modulate the electronic configuration of Co species for advanced OER activity.^19,20 Among them, using metal–organic framework (MOF) materials as precursors to prepare Co₃O₄-based catalysts has great advantages in terms of both regulable architectures and chemical composition.²¹ For example, Lou's group employed Co-based zeolitic imidazolate framework-67 (ZIF-67) nanoplates as the precursors for the preparation of metal doped Co₃O₄ electrocatalysts with improved OER activity.²² They also synthesized Co₃O₄/Co–Fe oxide double-shelled nanoboxes by a novel MOF hybrid-assisted approach, achieving excellent OER performance.²³ Wang et al. reported the controllable pryolysis of bimetallic Mo–Co Prussian blue analogue nanoframes to generate interconnected Co₃O₄–Mo₂N heterostructures with superior OER activity.²⁴ Besides, Wang's group revealed that Co_Oct²⁺ sites exhibit better activity than Co_Td²⁺.²⁵ However, more attention is currently focused on tuning the ratio of Co_Td²⁺/Co_Oct³⁺ in Co₃O₄ or functionalizing it with other heteroatoms/metal compounds,^26–31 and the perspective of introducing partial Co_Oct²⁺ (t⁶_2ge¹_g) in Co₃O₄ for further promoting the OER performance has been long ignored. Mixed Co-based spinel oxides assembled from normal and inverse spinel-structures contain different coordination environments and oxidation states of transition metals, providing them tunable geometrical configurations to optimize OER activity. Therefore, one or more components are introduced into MOFs as co-precursors to derive mixed Co₃O₄-based spinel oxides containing Co_Oct²⁺ and Co_Oct³⁺ species which are of great significance in regulating OER performance.

The Ti₃C₂T_x nanosheet material (T_x stands for the surface F, OH and O terminations), one of the most widely studied two-dimensional early transition metal carbides (labeled MXenes), with excellent metallic conductivity, ultrathin structure and rich surface negative groups, has been considered as a promising substrate. The strategy of combining Ti₃C₂T_x nanosheets with various active materials such as SnO₂, LDH and Co(OH)₂ to improve electrochemical performances was proposed.^32–34 More recently, we have also fabricated strongly coupled Ti₃C₂T_x/Co-MOF hybrid nanosheets by a simple solvothermal process.³⁵ Inspired by this finding, and considering Ti₃C₂T_x can be pyrolysed to TiO₂ under certain temperature and atmosphere, the Co₃O₄ derived from a Co-MOF might react with TiO₂ to generate the inverse spinel cobalt-orthotitanate Co₂TiO₄ (Co_Td²⁺[Ti_Oh⁴⁺Co_Oh²⁺]O₄). Therefore, the tightly coupled Co-MOF/Ti₃C₂T_x composite is expected to be a promising co-precursor to derive the mixed Co₃O₄/Co₂TiO₄ spinel oxide, which would be an ideal material to study the role of Co_Oct²⁺ (t⁶_2ge¹_g) in the OER activity of Co₃O₄.

Bearing these aspects in mind, herein, we for the first time report the synthesis of Co₃O₄/Co₂TiO₄ mixed spinel oxides via the pyrolysis–reorganization strategy of Co-MOF/Ti₃C₂T_x hybrid nanosheets. Detailed analyses revealed that the pore size and ratio of Co₃O₄ and Co₂TiO₄ can be effectively tuned by modulating the pyrolysis temperature, thereby realizing the effective regulation of OER activity through structural and composition aspects. From the viewpoint of composition, the tuning of temperature can lead to the partial conversion of Co_Oct³⁺ (t⁶_2ge⁰_g) to Co_Oct²⁺ (t⁶_2ge¹_g). Benefiting from the mesoporous structures and electronic configuration regulation, efficient OER activity was achieved for the mixed spinel oxides. This work shows the promising application of Co₃O₄/Co₂TiO₄ mixed spinel oxides in the OER and also provides a new avenue for the application of MOF/MXene composites.

Experimental section

Materials

Ti₃AlC₂ was purchased from Laizhou Kai Kai Ceramic Materials Co. Ltd. Cobalt chloride hexahydrate (CoCl₂·6H₂O), terephthalic acid (1,4-BDC), N,N-dimethylformamide (DMF), Nafion solution (5 wt%) and 40% hydrofluoric acid (HF) were purchased from Macklin Chemical Reagent. Ethanol (CH₃CH₂OH) and potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents were of analytical grade and were used without further purification.

Preparation of CoTiO_x-T catalysts

The Co–BDC/Ti₃C₂T_x composite was prepared according to the method recently reported by our group.³⁵ Then, the composite was treated in a tube furnace under a normal nitrogen atmosphere at 550, 650, 750, 800 and 900 °C, respectively. Each sample was treated for 3 h, with a heating rate of 5 °C min⁻¹, and naturally cooled to room temperature to obtain CoTiO_x-550, CoTiO_x-650, CoTiO_x-750, CoTiO_x-800 and CoTiO_x-900.

Electrochemical measurements

The electrochemical measurements were performed on an electrochemical workstation (CHI660E) at room temperature using a standard three-electrode in 1.0 M KOH electrolyte. Hg/HgO was used as the reference electrode. The as-prepared samples were coated on glassy carbon electrode (GC) and nickel foam (NF) substrates with different loading amounts as the working electrodes, respectively. For the GC electrode, a platinum wire was used as the counter electrode. 4 mg of sample and 30 μL Nafion solution (5 wt%) were dispersed in 1 mL of water/ethanol mixed solution (volume ratio of 3 : 1) by sonicating for 40 min to form a homogeneous ink. 5 μL ink was loaded at 0.285 mg cm⁻² onto the glassy carbon electrode (3 mm diameter). For nickel foam substrates, a platinum sheet was applied as the counter electrode. The sample, acetylene black and PTFE in a mass ratio of 85 : 10 : 5 were mixed to form a homogeneous slurry with the assistance of ethanol. Then the slurry was coated on the NF to obtain a working electrode, with a loading amount of 10 mg cm⁻². Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed at a scan rate of 30 mV s⁻¹. Electrochemically active surface area (ECSA) was determined from double-layer capacitance using CV in the potential range between 0.15 and 0.25 V (vs. Hg/HgO) with scan speeds of 20, 40, 60, 80 and 100 mV s⁻¹. By determining the ΔJ (J_a − J_c) at 0.2 V (vs. Hg/HgO) against the scan rate, the half of the linear slope was applied to determine the ECSA. Electrochemical impedance spectroscopy was performed in the frequency range from 0.1 to 100 kHz at 0.6 V (vs. Hg/HgO). The stability of the catalyst was determined using constant voltage–time current (I–T) data with a current density of 10 mA cm⁻². All potentials in OER tests were calibrated to the reversible hydrogen electrode (RHE) based on the formula E_RHE = E_Hg/HgO + 0.098 V + 0.0591pH (pH = 14). All potentials are given without IR correction.

Characterization

X-ray diffraction (XRD) was studied on an Ultima IV with Cu Kα radiation (λ = 1.5406 Å). The microstructure was analysed by transmission electron microscopy (TEM, JEM-2100Plus) and scanning electron microscopy (SEM, JSM-7500F). Thermogravimetric analysis was performed on a Synchronous thermal analyzer (STA 449 F5). The specific surface areas of the samples were measured using N₂ sorption–desorption isotherms, based on the Brunauer–Emmett–Teller model (BET; ASAP 2020). X-ray photoelectron spectra (XPS) were acquired on an ESCALAB250XI. Raman spectroscopy was performed on a Thermo Scientific DXR.

Results and discussion

The preparation process of the mixed Co-based spinel oxides is illustrated in Fig. S1.† First, the Co–BDC/Ti₃C₂T_x hybrid nanosheets were fabricated using a solvothermal strategy recently reported by our group.³⁵ The obtained precursor was then pyrolyzed into mixed Co-based spinel oxides under an ordinary nitrogen atmosphere. In order to understand the pyrolysis process of the Co–BDC/Ti₃C₂T_x precursor, the TG–DTG curve was first obtained. As shown in Fig. 1a, a slight weight loss is observed before 420 °C, corresponding to the escape of physically adsorbed water molecules. Next, a massive dissociation is found in the range of 420–540 °C, which could be ascribed to the decomposition and reorganization of Co–BDC and Ti₃C₂T_x, as shown by the corresponding DTG curve. Based on the result, we explored the composition and structure of the pyrolysis products at 550–900 °C. Moreover, all of the as-obtained products are named CoTiO_x-T for convenience, where T denotes the pyrolysis temperature in °C.

Fig. 1 (a) TG–DTG curves of the Co–BDC/Ti₃C₂T_x precursor. (b) XRD patterns of CoTiO_x at 550–900 °C. (c) XRD patterns of Ti₃C₂T_x annealed at 550 and 650 °C. (d) Raman spectroscopy of CoTiO_x at 550–900 °C.

Fig. 1b shows the XRD patterns of the products at different pyrolysis temperatures including 550, 650, 750, 800 and 900 °C. It can be seen that when the pyrolysis temperature is 550 °C, Co₃O₄ (JCPDS card no. 42-1467) and very weak diffraction peaks at 35.36° and 62.2° that belong to the (311) and (440) planes of Co₂TiO₄ (JCPDS card no. 39-1410) are detected in the resulting product. Interestingly, more and stronger Co₂TiO₄ diffraction peaks appeared as the pyrolysis temperature increased. Of note, the (311) plane of both phases is the strongest peak. Based on the quantitative equation reported in previous literature about Co₃O₄ and Co₂TiO₄,^36,37 the weight ratio of Co₂TiO₄/Co₃O₄ is calculated to be 5.4, 10.9, 16.5, 26.1 and 37.8% for CoTiO_x-T (550–900 °C). Additionally, when the pyrolysis temperatures of the composite are 800 and 900 °C, a new phase of CoO (JCPDS card no. 43-1004) appears in the XRD patterns, in which the peaks at 36.5, 42.4, 61.5 and 73.7° are ascribed to (111), (200), (220) and (311). This result could be ascribed to the partial decomposition of Co₃O₄ to CoO at higher temperature, which can be confirmed by the significant decrease in the intensity of the (311) plane of Co₃O₄. Besides, the formation process of Co₂TiO₄ was also explored by annealing Ti₃C₂T_x at 550 and 650 °C. As shown in Fig. 1c, the XRD pattern of Ti₃C₂T_x annealed at 550 °C displayed that except for the generation of mixed phase TiO₂, the characteristic diffraction peak located at small angle of MXene still exists, which indicates that Ti₃C₂T_x is not completely transformed into TiO₂ at this temperature. Since a small amount of Ti₃C₂T_x with weak crystallization (Fig. 1c) was added during the preparation process of the Co–BDC/Ti₃C₂T_x precursor, its diffraction peaks are not detected in CoTiO_x-550. However, when Ti₃C₂T_x was annealed at 650 °C, almost all Ti₃C₂T_x was decomposed into TiO₂. Co₃O₄ is the pyrolysis product of single Co–BDC annealed at 650 °C, as confirmed by XRD (Fig. S2†). Thus, the formation process of Co₂TiO₄ above 650 °C could be attributed to the chemical reaction of Co₃O₄ and TiO₂. The diffraction peaks of excess unreacted TiO₂ are not detected in the XRD pattern of CoTiO_x-T (650–900 °C), which is mainly caused by the small yield of TiO₂ and the strong diffraction peak of Co₃O₄.

The structure information of the products was further evidenced by Raman spectroscopy. As shown in Fig. 1d, all samples have the same five Raman peaks, which located at 170–700 cm⁻¹ corresponding to the F¹_2g, E_g, F²_2g, F³_2g and A_1g modes of Co-based spinel oxides, respectively.³⁶ Among them, the F¹_2g mode is attributed to the translational movement of the whole tetrahedron. The two vibrational F²_2g and F³_2g modes belong to the asymmetric stretching and bending of the oxygen anion in the octahedral group (BO₆). The E_g corresponds to symmetric bending of oxygen with respect to cations in a tetrahedral surrounding. The strongest A_1g mode relates to the symmetric stretching of oxygen in the tetrahedral group (AO₄). It can be seen that all the five Raman modes shift to higher wavenumber with increasing temperature which is caused by the substitution of more Ti ions in the octahedral sites, and it increases the bond length between the octahedral cation and the oxygen (B–O).³⁶ This phenomenon is in good agreement with the XRD result. Noted that no characteristic peaks at 1360 cm⁻¹ and 1580 cm⁻¹ of carbon were observed, which indicates that the synthesized products have no carbon material. This phenomenon may be due to the fact that ordinary nitrogen contains oxygen and oxygen is generated in Co–BDC/Ti₃C₂T_x cracking, which promotes the conversion of all C into gaseous substances.

The morphology of the as-prepared products is observed from scanning electron microscopy (SEM) images. As displayed in Fig. 2a–c, 2D porous interconnected network structures composed of tiny nanoparticles were generated at 550–750 °C, which could be attributed to the nanosheet morphology of the Co–BDC/Ti₃C₂T_x precursor. Moreover, the size of nanoparticles increases with elevating temperature. When the temperature rises to 800 and 900 °C, the agglomeration of CoTiO_x nanoparticles is obvious and it directly leads to the collapse of the network structure (Fig. 2d and e), which can result in the degradation of electrochemical performance. Nitrogen adsorption/desorption measurement was performed to study the surface area and pore size distribution of the products. As shown in Fig. S3,† all samples display type-IV isotherms. The CoTiO_x-650 sample exhibits the largest Brunauer–Emmett–Teller (BET) specific surface area of 12.32 m² g⁻¹ among the five composites. The pore size distribution of the samples was analyzed by the Barrett–Joyner–Halenda (BJH) method. Obviously, mesoporous structures with pore diameter in the range of 10–30 nm are produced in CoTiO_x composites (Fig. 2f). Moreover, it can be seen that there is a decrease in pore volume at 20–30 nm with increasing temperature and this is mainly ascribed to the nanoparticles that gradually sinter and agglomerate together, agreeing well with the results of SEM images. Compared with other samples, CoTiO_x-650 has a larger specific surface area and more abundant mesoporous structure, which would be beneficial for exposing rich active sites and facilitating electron/ion transfer at the electrode/electrolyte interface.

Fig. 2 (a–e) SEM images of CoTiO_x-T (550, 650, 750, 800 and 900 °C); (f) pore size distributions calculated from N₂ adsorption isotherms for all samples. Further characterization for CoTiO_x-650; (g) low and (h) high magnification TEM images; (i) HRTEM image and (j and k) the corresponding interplanar spacings of the lattice fringes; (l) HAADF image and (m–o) element mapping images.

Furthermore, the low and high magnification transmission electron microscopy (TEM) images of CoTiO_x-650 also show its porous structure with a 2D interconnected network based on uniform nanoparticles (Fig. 2g and h). As can be seen, the direct pyrolysis–reorganization of Co–BDC/Ti₃C₂T_x nanosheets into CoTiO_x can avoid interparticle sintering and effectively separate the nanoparticles, resulting in the formation of monodisperse nanocrystals with uniform size. Moreover, the corresponding high-resolution TEM (HRTEM) image in Fig. 2i clearly reveals the existence of hetero-interfaces where atom arrangement is quite distinct along the two sides of interfaces. The magnified HRTEM images (Fig. 2j and k) give two different interplanar distances of 0.24 nm and 0.49 nm, which can be well indexed to the (311) and (111) planes of the Co₃O₄ and Co₂TiO₄ phases, respectively, further confirming the co-existence of the two phases in CoTiO_x-650. Of note, in contrast to the well-ordered lattice atoms of Co₂TiO₄(111) (Fig. 2j), the obvious lattice distortions occur in the Co₃O₄(311) plane (Fig. 2k). Thereby, CoTiO_x-650 possesses a heterostructure at the atomic level and lattice distortions, which are expected to provide more active sites with high electrochemical reactivity for oxygen-containing intermediates, leading to enhanced OER performance.^38,39 The selected area electron diffraction (SAED) image in other areas of CoTiO_x-650 (Fig. S4†) also shows two sets of diffraction spots, which originate from the characteristic faces of Co₃O₄(311) and Co₂TiO₄(311). Notably, the TiO₂(110) lattice plane was also observed. This is because excess TiO₂ is produced by the pyrolysis of Ti₃C₂T_x at 650 °C, which almost has no OER activity (Fig. S5†). Besides, the HAADF (Fig. 2l) and elemental mapping images (Fig. 2m–o) reveal that the Ti distribution is different from that of Co and O in the 2D porous structure, further confirming the formation of a heterojunction in CoTiO_x-650. In a word, the tightly coupled Co–BDC/Ti₃C₂T_x precursor pyrolysis–reorganization strategy is successfully developed to obtain heterogeneous Co-based mixed spinel oxides with a porous interconnected network structure.

X-ray photoelectron spectroscopy (XPS) was further conducted to investigate the composition and valence state of CoTiO_x-650. To better reveal the change of the phase composition, the XPS spectra of CoTiO_x-550 and CoTiO_x-750 were also studied for comparison. As shown in Fig. 3a, the survey spectra demonstrate the presence of Ti, O and Co in the three samples. The fine-scanned Co 2p XPS spectra with fitted peaks are presented in Fig. 3b, in which the characteristic peaks at 780.3 and 795.3 eV belong to Co³⁺ species, while the two peaks at 781.8 and 796.9 eV are ascribed to Co²⁺ species, along with satellite peaks at 787.9 and 804.1 eV.²⁶ The relative atomic ratio of Co²⁺/Co³⁺ on the surface of the samples could be obtained by comparing the area that the fitted curve covered. It could be clearly seen that the atomic ratio of Co²⁺/Co³⁺ (1.47) in CoTiO_x-750 is higher than that in CoTiO_x-650 (1.38) and CoTiO_x-550 (1.28), revealing that more Co²⁺ species are generated in CoTiO_x-750, which agrees well with the XRD result. Since the Co ions of Co₂TiO₄ are composed of 1/2 Co_Oh²⁺ and 1/2 Co_Td²⁺, the amount of Co_Oh²⁺ increases in CoTiO_x-750. The fine-scanned Ti 2p XPS spectra of these samples in Fig. 3c exhibit the same two peaks at 458.1 eV (2p_3/2) and 463.8 eV (2p_1/2) with a separation value of 5.7 eV, confirming the same chemical valence of the Ti⁴⁺ valence state.⁴⁰ As for O 1s XPS spectra (Fig. 3d), the samples display similar spectra. Specifically, the peak at 530.1 eV is typical for metal–oxygen bonds (M–O), and the peak located at 532.5 eV is assigned to the surface-adsorbed water molecules (H₂O_ads). The peak at 531.2 eV indicates the presence of a small amount of oxygen vacancies in samples, which is beneficial for the electrocatalytic OER process.⁹ Besides, the electronic structures of the CoTiO_x-650 and CoTiO_x-550 samples were also analyzed by soft X-ray adsorption near-edge structure (XANES). Furthermore, the XANES spectra of single Co₃O₄ derived from the Co-MOF at 650 °C were also studied for comparison. As displayed in Fig. 3e, the Co L-edge of Co₃O₄ shows the main peak for Co³⁺ both in L_III and L_II, and a shoulder peak with lower intensity for Co²⁺ in L_III.⁴¹ For CoTiO_x-650, the intensity ratio of Co²⁺/Co³⁺ is 0.85, which is larger than that of CoTiO_x-550 (0.82) and Co₃O₄ (0.67), further showing the existence of more Co_Oh²⁺ in CoTiO_x-650. As for the Ti L-edge, the peak positions of the two samples show the same feature and two bands (L_III and L_II) with 2p spin–orbit coupling. The two peaks are split into t_2g and e_g characteristic of the TiO₆ ligand field (Oh).⁴¹ The two samples exhibit similar intensity ratios of t_2g/e_g, implying almost no change in the surface Ti valence state. As is known, the occupancy of e_g orbitals in 3d transition-metal-based electrocatalysts determines the strength of the metal–oxygen bond, which strongly affects the OER overpotential.¹⁴ Electrocatalysts with too few or too much e_g filling (0 or 2) would make the oxygen binding too weak or too strong, respectively, resulting in poor OER performance. The OER activity can be optimized with moderate e_g filling (e_g ≈ 1) of the octahedral sites, which could balance the competition between rate-limiting steps.¹⁶ Compared with the rich Co_Oct³⁺ (t⁶_2ge⁰_g) electronic configuration in CoTiO_x-550, the presence of more Co_Oct²⁺ (t⁶_2ge¹_g) in CoTiO_x-650 would bring efficient OER activity.

Fig. 3 XPS spectra: (a) survey, (b) Co 2p, (c) Ti 2p and (d) O 1s of CoTiO_x-550–750; (e) Co L-edge XANES spectra of Co₃O₄, CoTiO_x-650 and CoTiO_x-550; (f) Ti L-edge XANES spectra of CoTiO_x-650 and CoTiO_x-550.

To verify our speculation, the OER performances of the as-prepared CoTiO_x samples were evaluated in a three-electrode cell with an electrochemical workstation (CHI660E) at room temperature in 1.0 M KOH solution. The Pt electrode and HgO/Hg electrode were used as the counter electrode and reference electrode, respectively. First, the catalysts with 0.285 mg cm⁻² uniform loading on the glassy carbon electrode were used as working electrodes for electrochemical tests. Fig. 4a shows the polarization curves of five CoTiO_x samples and commercial IrO₂, in which CoTiO_x-650 obviously exhibits the best OER activity among the catalysts, and the relative OER performance of these catalysts is 650 > 750 > 550 > 800 > 900. Specifically, as displayed in Fig. 4b, CoTiO_x-650 gives a small onset overpotential of 250 mV at a current density of 1 mA cm⁻², revealing the good OER activity. It's well known that the overpotential requirement for achieving a current density of 10 mA cm⁻² is an important parameter to estimate OER activity. Herein, CoTiO_x-650 shows a remarkably low overpotential (η₁₀) of 280 mV, which is smaller than that of CoTiO_x-550 (346 mV), CoTiO_x-750 (287 mV), CoTiO_x-800 (377 mV), CoTiO_x-900 (387 mV) and IrO₂ (367 mV). Besides, its superior catalytic activity is further reflected by comparing the slopes of Tafel plots (Fig. 4c) for CoTiO_x-650 (66.4 mV dec⁻¹) with those of CoTiO_x-550 (97 mV dec⁻¹), CoTiO_x-750 (79.1 mV dec⁻¹), CoTiO_x-800 (67.4 mV dec⁻¹), CoTiO_x-900 (66.7 mV dec⁻¹) and IrO₂ (85 mV dec⁻¹), revealing that CoTiO_x-650 is more beneficial for practical application.

Fig. 4 OER performances of catalysts coated on glassy carbon electrodes: (a) polarization curves of the as-obtained CoTiO_x-T samples; (b) comparison diagram of these samples in terms of onset overpotential at 1 mA cm⁻² and overpotential at 10 mA cm⁻²; (c) the corresponding Tafel plots of the catalysts; (d) the differences in current density variation at an overpotential of 0.2 V plotted against scan rate, where the slope is twice C_dl; (e) Nyquist plots; (f) polarization curves of CoTiO_x-650 initially and after 4000 CV cycles.

To reveal the reason for the excellent OER performance of CoTiO_x-650, the electrochemical double-layer capacitance (C_dl), which is linearly proportional to the electrochemically effective surface area, was measured by the cyclic voltammetry (CV) method. As displayed in Fig. S6† and Fig. 4d, CoTiO_x-650 shows the largest C_dl of 1.9 mF cm⁻² among these CoTiO_x samples, indicating the highest electrochemically active surface area, which can expose more active sites, and it greatly contributes to the excellent OER performance. Moreover, the current density further normalized with the electrochemically active surface area was captured to estimate the intrinsic OER activity of the CoTiO_x samples. As shown in Fig. S7,† the normalized LSV curves indicate that the OER activity of CoTiO_x-750 is slightly higher than that of CoTiO_x-650, and both are greater than that of CoTiO_x-550. This result can be ascribed to the generation of more Co₂TiO₄ with higher Co_Oh²⁺ in CoTiO_x-750, revealing the important positive effect of Co_Oh²⁺ on the intrinsic OER activity. However, due to the higher specific surface area and much richer porous structure of CoTiO_x-650 than CoTiO_x-750, its overall OER activity is better. As for CoTiO_x-800 and CoTiO_x-900, despite more Co₂TiO₄ generation, their OER activity still drops sharply, indicating that the decomposition of Co₃O₄ to CoO is unfavorable for OER activity, meanwhile showing the important role of Co_Oh³⁺ in the OER process. In a word, the synergistic catalytic effect between Co_Oct³⁺ in Co₃O₄ and Co_Oct²⁺ in Co₂TiO₄ is shown to be crucial for improving the OER activity.

More insights into the structure–performance relationship of the as-obtained mixed spinel oxides were revealed by electrochemical impedance spectra (EIS). As displayed in Fig. 4e, the Nyquist plots reveal a significant decrease of the charge-transfer resistance (R_ct) from 250 Ω (CoTiO_x-900) to 18 Ω (CoTiO_x-650), indicating the fastest charge transport kinetics of CoTiO_x-650. This result could be ascribed to the highly porous structure and the synergistic catalytic effect between Co_Oct³⁺ in Co₃O₄ and Co_Oct²⁺ in Co₂TiO₄ of CoTiO_x-650. Specifically, the former is favourable for exposing rich active sites and facilitating electron/ion transfer at the electrode/electrolyte interface,⁴² while the latter can directly promote more electron transfer between surface octahedral cations (CoO₆) and adsorbed-OOH intermediates,²⁵ thereby leading to the fast charge transport process of CoTiO_x-650, which is responsible for the superior OER performance. Besides, stability is another important parameter for advanced electrocatalysts. Therefore, a long-term CV cycling test was conducted to measure the stability of CoTiO_x-650. As displayed in Fig. 4f, the polarization curves measured before and after 4000 CV cycles were compared, which shows negligible difference between the initial polarization curve and the final one, indicating the excellent durability of CoTiO_x-650.

To further confirm the excellent OER performance of the CoTiO_x-650 catalyst, the electrocatalytic performances of the obtained samples coated on nickel (Ni) foam with a high loading amount of 10 mg cm⁻² were also estimated, as they are beneficial for industrial application. Meanwhile, the electrochemical performance of individual Co₃O₄ obtained by pyrolysis of the Co-MOF at 650 °C was also tested for comparison. As displayed in Fig. 5a–c, the trends of CoTiO_x-T coated on NF are similar to that in the case of using a GCE as the substrate, proving that CoTiO_x-650 has excellent substrate-independent OER activity.

Fig. 5 (a) Polarization curves of CoTiO_x-T samples and Co₃O₄ coated on Ni foam samples; (b) the corresponding Tafel plots of these catalysts; (c) Nyquist plots; (d) chronoamperometry of CoTiO_x-650 at a potential of 1.49 V vs. RHE.

The deposition of CoTiO_x-650 onto Ni foam results in a slightly enhanced OER performance with an η₁₀ overpotential of 260 mV, which could be attributed to the excellent conductivity of Ni foam. It is worth noting that the performance of CoTiO_x-650 is far superior to that of Co₃O₄, strongly confirming the crucial role of Co₂TiO₄ with abundant Co_Oh²⁺ in the OER activity of CoTiO_x-650. Besides, to test the stability of the modified Ni foam electrode, the chronoamperometry curve of CoTiO_x-650 at a potential of 1.49 V vs. RHE was obtained. As shown in Fig. 5d, CoTiO_x-650 exhibits a current density of 10 mA cm⁻² with a negligible loss even after the OER process for 25 h, further showing its superior stability for the OER. The above analyses demonstrate that CoTiO_x-650 has excellent OER activity, which is also remarkable compared with previously reported values for advanced catalysts (Table S1†). Finally, the XRD and XPS of CoTiO_x-650 after the stability test were investigated, as shown in Fig. S8.† It can be seen that except for some crystalline CoTiO_x-650 being oxidized to form amorphous CoOOH, the overall phase remains as Co₃O₄ and Co₂TiO₄ after the long-term OER test. In a word, CoTiO_x-650 is an efficient and stable OER catalyst.

Conclusions

In summary, we have successfully synthesized mixed Co-based spinel oxides as efficient catalysts for the OER via a simple MOF/MXene pyrolysis–reorganization strategy. Detailed electrochemical measurements demonstrate that their OER performances can be remarkably tuned by modulating the electronic configuration of Co species in the mixed spinel oxides. By changing the pyrolysis temperature, Co_Oct³⁺ (t⁶_2ge⁰_g) is partially converted into Co_Oct²⁺ (t⁶_2ge¹_g), which is beneficial to promote the OER activity. Meanwhile, a controllable mesoporous structure is also achieved in the catalyst, providing the 2D porous interconnected network morphology with maximum exposure of active sites. As a result, the optimal Co₃O₄/Co₂TiO₄ catalyst at 650 °C exhibits superior OER activity and excellent stability. Our study will provide valuable guidelines for designing advanced electrocatalysts from the pyrolysis–reorganization strategy of MOF/MXene-based composites.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the NSF of China (no. 21971143, 21805165), the 111 Project (D20015), the Major Research and Development Project of Hubei Three Gorges Laboratory (2022-3), and ITOYMR in the Higher Education Institutions of Hubei Province (T201904). The authors also thank Prof. W. S. Yan (National Synchrotron Radiation Laboratory, University of Science and Technology of China) for soft X-ray absorption spectrometry testing.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2qi02098j

‡ These authors contributed equally.

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