A ternary MOF-based Ce–Ni–Cu oxide for energy storage applications: structural, optical, and impedance studies

The escalating need for energy on a global scale and the necessity for sustainable energy solutions have spurred the advancement of sophisticated energy storage devices. This work presents a novel ternary Metal–Organic Framework (MOF)-based oxide, i.e., Ce–Ni–Cu@MOF, synthesized using the solvothermal process and it is used to study the dielectric properties. The as-synthesized MOF sample is characterized using XRD, SEM, FTIR, HRTEM, UV–vis-DRS, and XPS analysis. XRD infers the formation of multiple phases on cubic fluorite structure of CeO₂ with crystal volume determined as 162 ų. The d-spacing (CeO₂ = 0.312 nm) obtained from HRTEM images are well corroborated with XRD spectra. SEM image clearly evidenced the formation of agglomerated nanoparticles with non-uniform distribution of particles are observed. XPS confirms the presence of multi-oxidation states such as Ce³⁺, Ce⁴⁺, Cu⁺, Cu²⁺, Ni²⁺, and Ni³⁺. Indeed, the ratios of Ce³⁺/Ce⁴⁺, Cu⁺/Cu²⁺, and Ni²⁺/Ni³⁺ are observed as 1.12, 0.63, and 0.4, respectively. The sample is more photo sensitive in visible and UV region, because of its electronic transitions captured in UV–vis-DRS. Moreover, the direct band gap energy of Ce–Ni–Cu@MOF is obtained as 3.13 eV (ν = 396 nm) and this nanocomposite makes the electrons to transport easier and improved its electrochemical performance. The dielectric properties of Ce–Ni–Cu@MOF are investigated as a function of temperature from 30 to 150 °C between 100 Hz and 5 MHz. The results showed evidence of conduction through the grains and grain boundaries, which are determined through complex impedance analysis using an equivalent circuit model (represented by a Nyquist plot). Additionally, the sample Ce–Ni–Cu@MOF exhibited non-Debye relaxation behavior, as indicated by wider peaks above room temperature compared to the ideal Debye curve. These exceptional dielectric and conductive properties make it a suitable material for use in energy harvesting devices.