Achieving the "carbon peaking and carbon neutrality" targets requires efficient energy storage technologies. Supercapacitors with the high power density, rapid charge-discharge, and long cycle life show a great potential in renewable energy, electric vehicles, and smart grids. However, their low energy density limits wider applications. Transition metal oxides such as RuO₂ and MnO₂ are promising electrode materials due to their reversible faradaic reactions and high theoretical capacitance. Single-component oxides suffer from insufficient active sites and structural degradation, impairing rate capability and stability. Multi-metal oxide strategies leverage synergistic effects to optimize lattice, charge distribution, morphology, and performance. Cobalt-based oxides provide abundant Co²⁺/Co³⁺ redox pairs, manganese adds multiple valence states and structural buffering, while zinc enhances conductivity and stability. Nevertheless, low conductivity and poor kinetics persist. Oxygen vacancies improve conductivity, tune band structure, and create active sites via reducing bandgap and accelerating kinetics. In this work, oxygen-vacancy-rich Ov-Co₃Zn₁Mn_0.5O₅ nanoflowers were synthesized via hydrothermal-annealing-NaBH₄ reduction and multi-metal synergy and defect engineering to optimize morphology and electronic structure. This approach could validate the synergistic benefits and offer a strategy for high-performance transition metal oxide supercapacitor electrodes.

All chemicals were of analytical grade and used as received. Nickel foam substrates (2 cm × 3 cm) were pretreated in HCl solution of 3 mol/L under ultrasonication for 5 min to remove surface oxides and thoroughly rinsed with deionized water. For the synthesis of Co₃Zn₁Mn_0.5O₅, stoichiometric amounts of Co(CH₃COO)₂·6H₂O (3 mmol), Zn(CH₃COO)₂·H₂O (1 mmol), MnC₄H₆O₄·4H₂O (0.5 mmol), and urea (0.2 g) were dissolved in deionized water of 60 mL to form a homogeneous precursor solution. The solution together with the pretreated nickel foam was sealed in a 100 mL Teflon-lined autoclave and maintained at 120 ℃ for 6 h. After natural cooling, the nickel foam-supported precursor was washed, dried, and subsequently annealed in air at 400 ℃ for 2 h, yielding Co₃Zn₁Mn_0.5O₅ with a mass loading of 1.12 mg·cm^-2. For comparison, binary and other ternary oxides (i.e., Co₃Zn₁O₄, Co₃Mn_0.5O₄, Zn₁Mn_0.5O₂, Co₂Zn₁Mn_0.5O₄, and Co₄Zn₁Mn_0.5O₆) were prepared under the same conditions above. Oxygen vacancies were introduced via immersing the annealed Co₃Zn₁Mn_0.5O₅ in NaBH₄ solution for 12 min, followed by vacuum drying at 80 ℃ for 2 h to obtain Ov-Co₃Zn₁Mn_0.5O₅ (a mass loading of 1.02 mg·cm^-2). Comparative samples Ov-Co₂Zn₁Mn_0.5O₄ and Ov-Co₄Zn₁Mn_0.5O₆ were also prepared.

The morphology was determined by field-emission scanning electron microscopy (S-4800, Hitachi Co., Japan) and transmission electron microscopy (JEM-2100F, JEOL Co., Japan). The rystal structure was identified by X-ray diffraction (D/MAX-2500, Rigaku Co., Japan, Cu K_α, λ = 0.154156 nm). The surface chemical states and valence distributions were analyzed by X-ray photoelectron spectroscopy (K-Alpha, Thermo Co., USA). The oxygen vacancy concentration was quantified by electron paramagnetic resonance (EMXplus, Bruker Co., Germany) and thermogravimetric analysis (TG 209 F3 Netzsch Co., Germany). The electrochemical measurements were conducted in a three-electrode configuration with 6 mol/L KOH electrolyte using a CHI760e workstation. The cyclic voltammetry and galvanostatic charge-discharge within 0-0.5 V at different scan rates of 10-100 mV·s^-1 and various current densities were detremined. The electrochemical impedance spectroscopy was performed on a BioLogic SP-150 workstation. The density functional theory calculations were carried out by a software named Materials Studio package using the projector-augmented wave method.

The three-step hydrothermal-annealing-NaBH₄ reduction process yields uniform flower-like Ov-Co₃Zn₁Mn_0.5O₅ nanoflowers directly grown on nickel foam. The SEM images reveal that the binary oxides display irregular nanosheets or nanowires, whereas the ternary Co₃Zn₁Mn_0.5O₅ forms large-sized nanosheets assembled into a loose porous flower-like morphology that provides abundant redox active sites. After NaBH₄ treatment, the overall morphology is preserved, while porosity is markedly increased. The TEM images confirm the porous nanosheet structure in the both samples, with Co₃Zn₁Mn_0.5O₅ exhibiting more mesopores and atomic-level defects evidenced by disordered lattice fringes. The HRTEM images display lattice spacings of 0.202 nm and 0.156 nm, corresponding to the plane (400) of MnCo₂O_4.5 and the plane (511) of ZnCo₂O₄. The EDS mapping shows a homogeneous distribution of elements Co, Zn, Mn and O, and Mn content is lower than the nominal ratio. The XRD patterns confirm the coexistence of cubic spinel MnCo₂O_4.5 and ZnCo₂O₄ phases in all the samples, and Ov-Co₃Zn₁Mn_0.5O₅ shows a higher crystallinity. The TGA and EPR unambiguously verify abundant oxygen vacancies. The XPS analysis reveals positive shifts in Co 2p, Mn 2p, Zn 2p, and O 1s binding energies after reduction, accompanied by increased proportions of low-valence Co²⁺, Mn³⁺, and a larger O_V peak area, confirming mixed-valence states and defect formation. The results of electrochemical tests demonstrate clear synergistic effects. The CV curves at 10 mV·s^-1 exhibit the largest integrated area for the ternary composition. The oxygen-vacancy introduction further enlarges the area due to more active sites and lowered OH^- intercalation barrier. The b-values of 0.70 and 0.77 indicate combined diffusion and pseudocapacitive contributions. The GCD measurements deliver an ultrahigh specific capacitance of 1509.8 F·g^-1 at 2 A·g^-1 and retain 809.1 F·g^-1 at 20 A·g^-1, demonstrating an excellent rate capability. The assembled Ov-Co₃Zn₁Mn_0.5O₅//AC asymmetric supercapacitor maintains stable CV shapes up to 100 mV·s^-1 within a 1.6 V window, achieves a high energy density of 53.8 W·h·kg^-1 at 750 W·kg^-1, and retains 76.4% capacitance after 5000 cycles with the Coulombic efficiency of 99%. The DFT calculations reveal that oxygen vacancies eliminate the original 0.098 eV bandgap, introduce mid-gap defect states, increase carrier concentration and facilitate electron hopping between low-valence Co²⁺and Mn³⁺ sites, thereby significantly enhancing conductivity and redox kinetics.

The flower-like Ov-Co₃Zn₁Mn_0.5O₅ with rich oxygen vacancies was successfully synthesized by a facile hydrothermal-annealing-NaBH₄ reduction method. The unique nanosheets-assembled nanoflower structure could expose abundant active sites and enhance redox kinetics. The introduction of oxygen vacancies regulated the electronic structure, improved conductivity, and induced low-valence Co²⁺ and intermediate Mn³⁺ to promote redox reactions. The Ov-Co₃Zn₁Mn_0.5O₅ electrode delivered a high specific capacitance of 1509.8 F·g^-1 at 2 A·g^-1 with the excellent rate performance. The assembled asymmetric supercapacitor achieved 53.8 W·h·kg^-1 energy density at 750 W·kg^-1 and retained 76.4% of its initial capacitance after 5000 cycles with the Coulombic efficiency of 99%. This work could demonstrate the synergistic optimization of multi-metal composition and oxygen vacancy engineering for advanced transition metal oxide electrodes in supercapacitors.