Exploration of pH‐Driven Morphology and Defect Evolution in Nickel Cobalt Carbonate Hydroxide Nanomaterial for Supercapacitor Applications

The pH-optimized NCCH-2 mL (at pH = 8.25) sample exhibits a hierarchical nanowire-on-nanosheet morphology, maximum Brunauer–Emmett–Teller surface area (101.14 m² g⁻¹), and a defect-rich structure with mixed Ni^{2 +}/Ni^{3 +} and Co^{2 +}/Co^{3 +} oxidation -states, delivering a high specific capacitance of 3061.1 F g⁻¹ at 0.5 A g⁻¹ with a high stability of ≈99% over 5000 cycles.

Nickel cobalt carbonate hydroxide (NCCH) nanostructures with tunable morphologies, crystallite sizes, and defect structures are synthesized using a pH-modulated hydrothermal approach to explore the correlation between structural properties and electrochemical performance. Significant variations in crystallinity, surface area, chemical structure, and morphology are observed, as confirmed by synchrotron X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller analyses. Among the samples, the one synthesized at pH = 8.25 exhibits the most optimized physicochemical characteristics, including the highest surface area, smallest crystallite size, and a unique dual-phase nanowire-on-nanosheet morphology. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy analyses reveal the presence of abundant transition metal vacancies and/or oxygen interstitials on the surface. These defect-engineered features result in exceptional electrochemical performance, delivering a high specific capacitance of 3061.1 F g⁻¹ at 0.5 A g⁻¹, 2620.0 F g⁻¹ at 1 A g⁻¹, and 1533.3 F g⁻¹ at 10 A g⁻¹, and ≈99% capacitance retention over 5000 cycles at current density of 5 A g⁻¹. This study underscores the effectiveness of pH modulation in designing defect-rich NCCH nanostructures for high-performance supercapacitor applications, establishing a clear link between structure, defects, and electrochemical behavior.

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