Effect of Li+, Mg2+, and Al3+ Substitution on the Performance of Nickel Ferrite-Based Hydroelectric Cells

Abstract

Defect engineering in nickel ferrite has been performed to enhance the power output of the hydroelectric cell (HEC). Oxygen vacancies can be tuned in the ferrite using different valence element substitutions to enhance water dissociation by hydroelectric cells. In this work, Li+, Mg2+, and Al3+ substituted at the nickel site in nickel ferrite (NiFe2O4) are synthesized and studied for hydroelectric cell devices. Introducing these Li+, Mg2+, and Al3+ substituents (of a different atomic radius and valence charge state) in nickel ferrite leads to the generation of oxygen vacancy differently, which is discussed thoroughly in this work. In comparison to Mg and Al, lithium substitution lowers the overall cationic charge in nickel ferrite, leading to formation of a high number of oxygen vacancies for overall ionic charge compensation. Lithium-substituted nickel ferrite (NLFO) has a high number of defects (oxygen vacancies, etc.) compared to Mg-substituted (NMFO) and Al-substituted (NAFO) nickel ferrite, confirmed by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. The highest peak area of oxygen vacancies in the O 1s core-level spectra has been obtained for the case of NLFO among NLFO, NMFO, and NAFO. These vacancy sites provide a large number of adsorption sites for water molecule adsorption and dissociation. A high lattice strain (9.83 × 10–4) is induced in NLFO calculated by the Williamson–Hall plot. The porous nature of all samples has been confirmed from FESEM and BET analysis. The charge transfer processes in dry and wet HECs have been analyzed by fitting an equivalent RC circuit in the Nyquist spectra. The lithium-substituted nickel ferrite HEC with rich oxygen vacancies generates a high short-circuit current of ∼58.7 mA, which is ∼2 times higher than that of NMFO HEC and ∼3 times higher than that of NAFO HEC.