Spinel ferrite MFe2O4 (M = Ni, Co, or Cu) nanoparticles prepared by a proteic sol-gel route for oxygen evolution reaction

Enhanced Electrocatalytic Activity of Ecofriendly and Earth-Abundant (Zn,Cu)Fe2O4 + CuO Nanocomposites for Water Splitting

The projection of sustainable, low-cost, and environmentally friendly energy technologies demands innovation of electrocatalysts utilizing earth-abundant materials. The current study aims to improve the catalytic activity of spinel zinc ferrite (ZF), which is an earth-abundant and economically viable material, via a doping strategy. The spinel ZF shows a weak catalytic activity for water splitting, whereas the substitution of Cu ions at octahedral sites results in improving the catalytic performance in both acidic and basic electrolytes. Structural characterization using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction demonstrates that, depending on the Cu concentration, Cu ions either incorporate into spinel Zn-ferrite oxide as doping agents or form CuO nanocomposites, where Cu-induced construction of a composite containing ZCF nanoparticles and CuO nanophase coexists. Substituting Zn with Cu in the octahedral sites of the ZF crystal structure leads to a decrease in the unit cell lattice parameter, and the crystal symmetry is impacted, including the creation of strain and dislocation density. HRTEM analyses provide evidence that the ZF particles nucleate and grow randomly due to the asymmetric reaction dynamics of spinel oxide and the lack of surfactant, while the ZCF nanoparticles are elongated in preferential orientation, forming oriented nanoparticles with a greater surface-to-volume ratio. To attain the current density of 10 mA cm-2, the nanocomposite of the ZCF-50 electrode shows the lowest overpotential of 280 mV for oxygen evolution reaction (OER) among other electrodes. The Tafel slope also decreases significantly in which the nanocomposite of ZCF-50 shows the lowest value of 80 mV dec-1. The measured double-layer capacitance (Cdl) for the nanocomposite structure of ZCF-50 offers the highest value of 27 mF cm-2, which indicates that the nanocomposite contains the largest electrochemically active surface area (ECSA). The catalytic activity of Cu-doped spinel ZCF for hydrogen evolution reaction is also evaluated. The nanocomposite of ZCF-50 shows the lowest onset overpotential of 60 mV compared to 200 mV for the ZF electrode. The obtained Cdl over cathodic potentials for the ZCF-50 electrode shows the highest value of 11.3 mF cm-2 compared with other electrodes. These results confirm that ZCF-50 contains the largest ECSA and highest electrochemical activity. Electrochemical impedance spectroscopy studies also demonstrate that the ZCF-50 electrode shows the lowest charge-transfer resistance, indicating that the catalytic OER is improved significantly at its interfaces. We realize that Cu doping into the ferrite structure and the formation of the CuO semishells synergistically can improve interparticle and transparticle charge transfer.

Advances in Nickel-Based Catalysts for Alkaline Water Electrolysis: Comprehensive Review of Current Research Direction for HER and OER Applications

Nickel-based catalysts are among the most promising materials for electrocatalytic water splitting, particularly for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media. Their abundance, cost-effectiveness, and tunable electrochemical properties make them attractive alternatives to precious metal catalysts. This review provides a comprehensive analysis of the advancements in nickel-based catalysts, including pure nickel, alloys, oxides, hydroxides, and spinels, emphasizing their synthesis methods, structural properties, and electrocatalytic performance. Recent nanostructuring, doping, and hybridization innovations with conductive supports have significantly enhanced catalytic activity, stability, and efficiency. Despite notable progress, challenges remain in improving long-term durability, minimizing surface degradation, and scaling up production for industrial applications. Addressing these limitations through advanced catalyst design, in situ characterization, and integration with renewable energy sources will be crucial for widely adopting nickel-based catalysts in sustainable hydrogen production. This review highlights the key developments and future directions in the field, underscoring the role of nickel-based materials in enabling the hydrogen economy and global decarbonization efforts.

The effect of particle size on structural and catalysts for oxygen evolution reaction of (CoFeNiMnCr)3O4 prepared by controlled synthesis with polyvinylpyrrolidone (PVP)

In this study, high-entropy spinel oxides (CoNiMnFeCr)3O4 were synthesized using a PVP-assisted sol-gel method, marking the first report of this approach for producing high-entropy oxides. This method provides new insights into morphology customization through precise temperature control during calcination. Samples were calcined at 800, 900, and 1000 °C, and structural, optical, and electrochemical characterizations were performed to evaluate the impact of synthesis conditions on the oxygen evolution reaction (OER) performance. X-ray diffraction (XRD) confirmed the formation of a single-phase spinel structure with face-centered cubic symmetry. UV-Vis spectroscopy revealed a band gap shift associated with calcination temperature, indicating subtle changes in electronic structure that can influence catalytic behavior. The S-HEO 800 sample exhibited the highest catalytic activity, achieving an overpotential of 316 mV at 10 mA cm-2. Electrochemical tests showed excellent short-term durability, with the electrodes maintaining stable performance for 24 h at 10 mA cm-2. Field emission gun scanning electron microscopy (FEGSEM) analysis revealed that particle size increased with calcination temperature, ranging from 96 nm (S-HEO 800) to 475 nm (S-HEO 1000). X-ray photoelectron spectroscopy (XPS) showed a higher concentration of Cr6+, Cr4+, and Ni3+ ions on the surface of S-HEO 800, correlating with its superior OER performance. Additionally, Raman and FT-IR spectra confirmed the formation of the spinel phase and provided insights into metal-oxygen bonding. Electrochemical impedance spectroscopy (EIS) results indicated that S-HEO 800 exhibited the lowest charge transfer resistance (Rct), further supporting its enhanced catalytic behavior. These findings demonstrate the potential of the PVP-assisted sol-gel method to produce customized high-entropy oxides with tunable morphology, making them promising candidates for energy conversion applications, particularly in water electrolysis.

Enhancement of dielectric permittivity and Havriliak-Negami relaxation mechanism in MnFe2O4through Dy substitution

Pristine and Dy substituted MnFe2O4,MnFe2-xDyxO4(x= 0.00, 0.02, 0.04, 0.06, 0.08 & 0.10) were successfully synthesized by sol-gel method to investigate the dielectric properties of the system. MnFe2O4exhibits a high dielectric permittivity of order 104which is further augmented by 60% through Dy substitution. This is owing to the rise in interfacial polarization resulting from localized states, dipolar polarization arising from the multiple valence states of Fe and Mn ions, atomic polarization due to structural distortion induced by strain, and electronic polarization stemming from the concentration of free charge carriers. The enhancement of induced strain, mixed valence ratio of Fe2+/Fe3+and Mn4+/Mn2+, localized states, and free charge carrier concentration are confirmed from the XRD, XPS, and optical studies, respectively. The dielectric relaxation mechanism of MnFe2-xDyxO4follows a modified Havriliak-Negami relaxation model with conductivity contribution. Complex impedance analyses further validate the contribution of grain-grain boundary mechanisms to the dielectric properties confirmed through Nyquist plots. A comprehensive analysis of conductivity reveals the significant impact of Dy substitution on the electrical conductivity of MnFe2O4. This influence is strongly related to the variations in the concentration of free charge carriers within the MnFe2-xDyxO4system. The understanding of the underlying physics governing the dielectric properties of Dy-substituted MnFe2O4not only enhances the fundamental knowledge of material behavior but also opens new avenues for the design and optimization of advanced electronic and communication devices.

Silver nanoparticle-decorated NiFe2O4/CuWO4 heterostructure electrocatalyst for oxygen evolution reactions

In this work, Ag nanoparticles decorated with NiFe2O4/CuWO4 heterostructure were synthesized using the step-wise precipitation method. The influence of varying Ag loading on the NiFe2O4/CuWO4 heterostructure and its electrochemical OER performance was extensively studied in 1 M KOH electrolyte. The obtained LSV profile was analyzed to determine the overpotential, Tafel slope, and onset potential. The heterostructure with an optimal Ag loading of 5 wt% required the least overpotential (1.60 V vs. RHE) for generating a current density of 10 mA cm-2 with a lower Tafel slope of 44.5 mV dec-1, indicating its faster OER kinetics. Furthermore, the composite remained stable over a period of 24 hours with a minimum rise in the overpotential after the stability test. The enhanced OER performance of the as-prepared catalyst can be attributed to the presence of multiple metallic elements in the Ag-loaded NiFe2O4/CuWO4 composite, which created a diverse array of oxygen-vacant sites with varying reactivity, enhancing the charge-transfer kinetics; and thus contributing to the overall efficiency of OER. Therefore, optimizing the Ag concentration and engineering a microstructure represents an encouraging strategy for developing cost-effective catalysts for next-generation energy-conversion applications.