Browsing by Author "Gupta, Ram K."

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    Alloying normal and inverse spinel (Zn–Co ferrite) nanostructures via direct precursor pyrolysis for enhanced supercapacitance and water splitting
    (Elsevier, 2023) Malima, Nyemaga Masanje; Khan, Malik Dilshad; Choi, Jonghyun; Gupta, Ram K.; Revaprasadu, Neerish
    The nanocrystals comprising of multiple redox-active cations are generally proficient electrocatalysts for renewable and sustainable energy applications. Ferrites have rich redox chemistry and crystallographically, they can be classified into normal spinel structures or inverse spinel structures, depending on the type of occupancy at tetrahedral and octahedral sites. Herein, taking advantage of the simplicity and environmental benignity of the solventless method, a series of mixed inverse and normal spinel nanocatalysts (Co1-xZnxFe2O4 (0 = x ≤ 1)) have been successfully synthesized and tested for supercapacitance and overall water splitting. The p-XRD and EDX analyses confirmed the successful nucleation of CoFe2O4 and ZnFe2O4 to form monophasic Co1-xZnxFe2O4 solid solutions over the entire composition range. The solid solutions with composition Co0·4Zn0·6Fe2O4 demonstrated higher discharge time, indicating higher specific capacitance of the material than other electrode compositions. The Co0·8Zn0·2Fe2O4 showed relatively low overpotentials of 317 mV to afford the current density of 10 mA/cm2 for oxygen evolution reaction (OER), while Co0·6Zn0·4Fe2O4 exhibited an overpotential of 169 mV for hydrogen evolution reaction (HER), outperforming most of the electrocatalysts reported in the literature. More impressively, such solid solutions demonstrated negligible deviation between the first and the 1 k cycles, suggesting high durability for the electrolysis of water.
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    Eco-friendly mixed metal (Mg–Ni) ferrite nanosheets for efficient electrocatalytic water splitting
    (Springer Science and Business Media LLC, 2023) Malima, Nyemaga M.; Khan, Malik Dilshad; Masikane, Siphamandla C.; de Souza, Felipe M.; Choi, Jonghyun; Gupta, Ram K.; Revaprasadu, Neerish
    Eco-friendly and cost-effective catalysts with multiple active sites, large surface area, high stability and catalytic activity are highly desired for efficient water splitting as a sustainable green energy source. Within this line, a facile synthetic approach based on solventless thermolysis was employed for the simple and tunable synthesis of Ni1−xMgxFe2O4 (0 ≤ x ≤ 1) nanosheets. The characterization of nanosheets (via p-XRD, EDX, SEM, TEM, HRTEM, and SAED) revealed that the pristine ferrites (NiFe2O4 and MgFe2O4), and their solid solutions maintain the same cubic symmetry throughout the composition regulation. Elucidation of the electrochemical performance of the nanoferrite solid solutions showed that by tuning the local chemical environment of Ni in NiFe2O4 via Mg substitution, the intrinsic catalytic activity was enhanced. Evidently, the optimized Ni0.4Mg0.6Fe2O4 catalyst showed drastically enhanced HER activity with a much lower overpotential of 121 mV compared to the pristine NiFe2O4 catalyst. Moreover, Ni0.2Mg0.8Fe2O4 catalyst exhibited the best OER performance with a low overpotential of 284 mV at 10 mA/cm2 in 1 M KOH. This enhanced electrocatalytic activity could be due to improved electronic conductivity caused by the partial substitution of Ni2+ by Mg2+ in the NiFe2O4 matrix as well as the synergistic effect in the Mg-substituted NiFe2O4. Our results suggest a feasible route for developing earth-abundant metal oxide-based electrocatalysts for future water electrolysis applications.
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    Solventless synthesis of nanospinel Ni 1− x Co x Fe 2 O 4 (0≤ x≤ 1) solid solutions for efficient electrochemical water splitting and supercapacitance
    (The Royal Society of Chemistry, 2021) Malima, Nyemaga Masanje; Malik, Dilshad Khan; Choi, Jonghyun; Gupta, Ram K.; Mashazi, Philani; Nyokong, Tebello; Revaprasadu, Neerish
    The formation of solid solutions represents a robust strategy for modulating the electronic properties and improving the electrochemical performance of spinel ferrites. However, solid solutions have been predominantly prepared via wet chemical routes, which involve the use of harmful and/or expensive chemicals. In the present study, a facile, inexpensive and environmentally benign solventless route is employed for the composition-controlled synthesis of nanoscopic Ni1−xCoxFe2O4 (0 ≤ x ≤ 1) solid solutions. The physicochemical characterization of the samples was performed by p-XRD, SEM, EDX, XPS, TEM, HRTEM and UV-Vis techniques. A systematic investigation was also carried out to elucidate the electrochemical performance of the prepared nanospinels towards energy generation and storage. Based on the results of CV, GCD, and stability tests, the Ni0.4Co0.6Fe2O4 electrode showed the highest performance for the supercapacitor electrode exhibiting a specific capacitance of 237 F g−1, superior energy density of 10.3 W h kg−1 and a high power density with a peak value of 4208 W kg−1, and 100% of its charge storage capacity was retained after 4000 cycles with 97% coulombic efficiency. For HER, the Ni0.6Co0.4Fe2O4 and CoFe2O4 electrodes showed low overpotentials of 168 and 169 mV, respectively, indicating better catalytic activity. For OER, the Ni0.8Co0.2Fe2O4 electrode exhibited a lower overpotential of 320 mV at a current density of 10 mA cm−2, with a Tafel slope of 79 mV dec−1, demonstrating a fast and efficient process. These results indicated that nanospinel ferrite solid solutions could be employed as promising electrode materials for supercapacitor and water splitting applications.