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The performance of supercapacitor electrode materials was greatly affected by the specific surface area. The urchin-like NiCo₂O₄ was transformed into porous NiCo₂O₄ (AA-NiCo₂O${}_4)$ using the acid–alkali treatment method. The specific surface area of AA-NiCo₂O₄ was 165.0660 m²/g, which was about three times larger than that of NiCo₂O₄. The specific capacitance of the AA-NiCo₂O₄ was enhanced significantly (1700 F/g at 1 A/g), and AA-NiCo₂O₄ possesses good rate capacitance (1277 F/g at 10 A/g). This is mainly attributed to the larger specific surface area, fast and convenient electron–ion transport and redox reaction. Therefore, AA-NiCo₂O₄ is a promising high-performance supercapacitor electrode material.

To find a solution to the global energy demand, efficient energy production and storage devices are utmost required. Taking advantage of the unique combination of hydrophilicity and conductivity of MXene, a bifunctional nonnoble metal-based electrode NiCo₂O₄/NiO/MXene (CNOT) is developed. Low conductivity and aggregation of transition metal oxides are compensated by making a hybrid of NiCo₂O₄/NiO with MXene. CNOT, as an anode catalyst in direct methanol fuel cell (DMFC), offers methanol oxidation reaction current density of 15A/g and low onset potential. Symmetric supercapacitor developed using CNOT in 3M KOH solution offers 0.9V potential window, and 32.66Fg${}^{-1}$ specific capacitance at 2.5A/g. Whereas, symmetric supercapacitor CNOT//CNOT in PVA/KOH hydrogel polymer electrolyte provides a broader window of 1.4V, with specific capacitance of 87.331Fg${}^{-1}$, and very high energy and power density of 23.77Wh/kg and 1808.87W/kg, respectively, at 2.5A/g. The hydrogel polymer electrolyte (PVA/KOH) outperforms aqueous 3M KOH by providing a larger window, higher capacitance, excellent energy and power density. Thus, the hybrid electrode provides synergistic effects of the electro-active NiCo₂O₄, NiO and MXene nanosheets and exhibits versatility in DMFC and symmetric supercapacitor.

The composite supercapacitor electrodes were rationally fabricated by facile electrochemical deposition of polypyrrole (PPy) on NiCo₂O₄ nanowire arrays which were grown radially on carbon fiber (CF). When used as electrodes in supercapacitors, the composite nanostructures demonstrated prominent electrochemical performances with a high areal capacitance (1.44F/cm² at a current density of 2mA/cm²), a good rate capability (80.5% when the current density increases from 2mA/cm² to 20mA/cm²), and a good cycling ability (85% of the initial specific capacitance remained after 5000 cycles at a high current density of 10mA/cm²). The excellent electrochemical performance of NiCo₂O₄@PPy nanostructures can be mainly ascribed to the good electrical conductivity of PPy, the enhanced adherent force between electrode materials and CF to hold the electrode fragments together by means of NiCo₂O₄ nanowires, the short ion diffusion pathway in ordered porous NiCo₂O₄ nanowires and the three-dimensional nanostructures.

Highly efficient, cost-effective and durable electrocatalysts for water splitting are crucial for energy conversion and storage. Transition-metal phosphides have been proven to be efficient catalysts for water splitting. In this paper, surface phosphation of 3D NiCo₂O₄ nanowires grown on Ni foam (P-NiCo₂O₄/NF) have been prepared to investigate the effect of surface phosphating on catalyst activity. XRD and XPS results demonstrate that P element has been decorated on the surface of the NiCo₂O₄ nanowires. The electrochemical results prove that P-NiCo₂O₄/NF shows better electrochemical performance than pure NiCo₂O₄/NF as an electrode for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of water splitting. It achieves a current density of 10mA cm² at an overpotential of 279mV and 164mV for OER and HER in 1.0M KOH electrolyte, respectively. In addition, the P-NiCo₂O₄/NF$\parallel$P-NiCo₂O₄/NF electrode is constructed by employing P-NiCo₂O₄/NF as both the anode and cathode, it only requires a low 1.68V of cell voltage to reach the current density of 10mA cm${}^{-2}$. Notably, P-NiCo₂O₄/NF$\parallel$P-NiCo₂O₄/NF also exhibits excellent stability for over 30h-long. These results indicate that surface phosphation is an effective approach to improve the electrochemical performance of NiCo₂O₄/NF electrode materials.

The morphology of nanomaterials plays an important role in the electrochemical sensing performance. Herein, the morphology-dependent electrochemical sensing properties of NiCo₂O₄ for glucose were studied. NiCo₂O₄ with one-dimensional (1D) rod structure or two-dimensional (2D) sheet structure was synthesized by just changing solvent composition. The morphology, structure and electrochemical sensing performance of NiCo₂O₄ were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and amperometric methods. The results of CV characterization show that the magnitude of the oxidation peak current increase obtained on rod-like NiCo₂O₄ is nearly two times higher than that of sheet-like NiCo₂O₄, which is due to the faster electron transfer rate of rod-like NiCo₂O₄. Rod-like NiCo₂O₄ exhibited higher electrocatalytic activity toward glucose oxidation with a wide linear range of 0.02–5.1mM, a low detection limit of 2.0$\mu$M and an ultrahigh sensitivity of 2040$\mu$A$\cdot$mM${}^{-1}$$\cdot$cm${}^{-2}$. Our findings offer a novel morphology-controllable synthesis strategy to understand the morphology impact on the electrochemical performances of NiCo₂O₄, and represent a facile design of electrocatalysts for sensors.