Exploration of pH-Driven Morphology and Defect Evolution in Nickel Cobalt Carbonate Hydroxide Nanomaterial for...

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.