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 (<i>C</i>_dl) 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 <i>C</i>_dl 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.