Spinel ferrites are versatile, low-cost, and abundant metal oxides with remarkable electronic and magnetic properties, which find several applications. Among them, they have been considered part of the next generation of electrochemical energy storage materials due to their variable oxidation states, low environmental toxicity, and possible synthesis through simple green chemical processing. However, most traditional procedures lead to the formation of poorly controlled materials (in terms of size, shape, composition, and/or crystalline structure). Thus, we report herein a cellulose nanofibers-mediated green procedure to prepare controlled highly porous nanocorals comprised of spinel Zn-ferrites. Then, they presented remarkable applications as electrodes in supercapacitors, which were thoroughly and critically discussed. The spinel Zn-ferrites nanocorals supercapacitor showed a much higher maximum specific capacitance (2031.81 F g⁻¹ at a current density of 1 A g⁻¹) than Fe₂O₃ and ZnO counterparts prepared by a similar approach (189.74 and 24.39 F g⁻¹ at a current density of 1 A g⁻¹). Its cyclic stability was also scrutinized via galvanostatic charging/discharging and electrochemical impedance spectroscopy, indicating excellent long-term stability. In addition, we manufactured an asymmetric supercapacitor device, which offered a high energy density value of 18.1 Wh kg⁻¹ at a power density of 2609.2 W kg⁻¹ (at 1 A g⁻¹ in 2.0 mol L⁻¹ KOH electrolyte). Based on our findings, we believe that higher performances observed for spinel Zn-ferrites nanocorals could be explained by their unique crystal structure and electronic configuration based on crystal field stabilization energy, which provides an electrostatic repulsion between the d electrons and the p orbitals of the surrounding oxygen anions, creating a level of energy that determines their final supercapacitance then evidenced, which is a very interesting property that could be explored for the production of clean energy storage devices.

We report on the experimentally observed variation of the superconducting critical temperature (T _c ) of Ni/Bi systems as a function of the total deposited film thickness and on its explanation using a theoretical model. Two series of Ni/Bi systems have been analyzed which were obtained by depositions of Ni onto Bi in the proportions Ni3Bix (3 nm of Ni onto x nm of Bi) and NiyBi6y (y nm Ni onto 6 y nm of Bi). As shown recently, the formation of the superconducting compound NiBi₃ at Ni/Bi interfaces in the resulting NiBi₃-Bi films is thermodynamically favored by a volume contraction. Here we corroborate this result and estimate the thickness of the resulting NiBi₃ and of the remaining Bi layers for the Ni3Bix and NiyBi6y series using the laws of mass and conservation of number of atoms. We consider the resulting film as being made up of two homogeneous and uniform layers of NiBi₃ and Bi, respectively, and study this idealizing model using the Bogoliubov de Gennes (BdG) equations. It is assumed that superconductivity originates in the NiBi₃ layer and penetrates the Bi layers via a potential barrier. Our theoretical calculations predict the dependence of T _c with respect to the thicknesses of the NiBi₃ and Bi layers, and also with the strength of the potential barrier that blocks the migration of electrons from the NiBi₃ to the Bi layer. The calculations show that the superconducting gap also exists in Bi, although much weaker than in the NiBi₃ layer. We compare the predicted T _c values with the experimental data and find sufficient agreement to suggest that our model can explain the experimentally observed variation of T _c with thickness. We interpret this dependence as shape resonance oscillations which are derived from the BdG theory applied to thin superconducting films.

Hydrogen has been explored as a greener alternative for greenhouse gas emissions reduction. Sodium borohydride (NaBH₄) is a favorable hydrogen carrier due to its high hydrogen content, safe handling, and rapid hydrogen release. This work presents a novel synthesis of the catalyst CoFe₂O₄/Fe₂O₃ using nanocellulose fibers (TCNF) as reactive templates for metal adsorption and subsequent calcination. The resulting material was tested for H₂ production from basic NaBH₄ aqueous solutions (10–55 °C). The catalyst’s composition is 74.8 wt% CoFe₂O₄, 25 wt% Fe₂O₃, and 0.2 wt% Fe₂(SO₄)₃ with agglomerated spheroidal particles (15–20 nm) and homogeneous Fe and Co distribution. The catalyst produced 1785 mL of H₂ in 15 min at 25 °C (50 mg catalyst, 4.0% NaBH₄, and 2.5 wt% NaOH), close to the stoichiometric maximum (2086 mL). The maximum H₂ generation rate (HGR) reached 3.55 L min⁻¹ g_cat⁻¹ at 40 °C. Activation energies were determined using empirical (38.4 ± 5.3 kJ mol⁻¹) and Langmuir–Hinshelwood (L–H) models (42.2 ± 5.8 kJ mol⁻¹), consistent with values for other Co-ferrite catalysts. Kinetic data fitted better to the L–H model, suggesting that boron complex adsorption precedes H₂ evolution.