Defect Induced in 3D-Rhombohedral MnCO<sub>3</sub> Microcrystals by Substitution of Transition Metals for Aqueous and Solid-State Hybrid Supercapacitors
M. Karuppaiah, P. Sakthivel, S. Asaithambi, L. Krishna Bharat, Goli Nagaraju, Balamurugan Karuppannan, R. Yuvakkumar, G. Ravi
Abstract
In this context, we synthesize pristine and transition metal (Fe, Co, Ni, and Cu) (TM)-doped uniform, 3D-rhombohedral manganese carbonate (MnCO3) microcrystals using a simple hydrothermal method. The substitution of different TMs notably induces structural defects in the 3D-rhombohedral microcrystals as evidenced by X-ray diffraction data and micro-Raman spectra. From the detailed investigation, the optimum Fe-MnCO3 sample exhibits a higher specific capacity of 55.83 mAh g–1 at 1 A g–1 with a remarkable rate capability of 64.39%. This is attributed to uniform morphology, abundant electroactive sites, homogeneous particle size, a higher amount of structural defects, and lower electronic/ionic resistance. Further, aqueous and solid-state hybrid supercapacitors (HSCs) were fabricated using 3D-rhombohedral Fe-MnCO3 as a positive electrode and activated carbon (AC) as a negative electrode in aqueous KOH and PVA/KOH polymer gel electrolytes, respectively. The aqueous HSC (AHSC) and solid-state HSC (SHSC) devices deliver maximum energy densities of 22.38 and 19.74 Wh kg–1 at power densities of 374 and 380 W kg–1 along with good initial capacitance retentions of 88.85% and 82.63% after 10,000 cycles, respectively. The practical applicability of fabricated devices is proved by successfully powering a pair of light-emitting diodes (LEDs) of different colors (red, green, and blue) using two SHSCs connected in series.