Synthesis and Characterization onf recycled ZnO from exhausted ZnMnO2 batteries, ans study of the potential application of the oxide as a Photocalizer and Pseudocapacitor
Name: PALOMA POSSATI PASOLINI
Publication date: 04/07/2025
Examining board:
Name |
Role |
|---|---|
| JOSIMAR RIBEIRO | Examinador Interno |
| LUIZ CARLOS PIMENTEL ALMEIDA | Examinador Externo |
| MARCOS BENEDITO JOSE GERALDO DE FREITAS | Presidente |
| PEDRO VITOR MORBACH DIXINI | Coorientador |
Summary: This work presents an integrated route for the recycling of ZnMnO alkaline batteries and the conversion of the zinc anode into zinc oxide (ZnO). Initially, the batteries were discharged ensuring the complete removal of residual energy and the safety of the process. Then, the metal casing was mechanically removed, and the zinc anode was separated, macerated and subjected to leaching in a 1 mol L¹ citric acid solution with the addition of hydrogen peroxide, at 80 °C, under constant stirring. This step promoted the complexation of the Zn² ion by the chelating action of citrate, keeping it in solution and avoiding the formation of zinc hydroxide precipitates. After filtration and washing with ultrapure water, a sol–gel gel was obtained, which was dried at 80 °C for 12 h to remove residual solvents. Calcination of the dry precursor at 700 °C for 5 h in a muffle furnace resulted in crystalline ZnO with porous morphology and a high degree of purity. The characterization of the synthesized ZnO used several techniques that, in a complementary manner, confirmed its composition, structure and morphology. X-ray diffraction showed that the material crystallized in the hexagonal phase characteristic of ZnO. Thermogravimetric analysis demonstrated the gradual removal of water and organic residues during heating, indicating the formation of pure oxide after the decomposition of the precursors. Raman spectroscopy highlighted the emergence of vibrational modes typical of ZnO only after heat treatment, reflecting an increase in crystallinity. SEM micrographs showed porous clusters of particles, whose elemental composition, assessed by EDS, confirmed the predominance of zinc and oxygen. Finally, TEM revealed the presence of well-crystalline nanoparticles, exhibiting crystallographic planes characteristic of the wurtzite structure and indicating point defects in the lattice. The pseudocapacitive performance of ZnO was quite limited, highlighting the need for additional modifications to make it truly efficient. When used as an electrode in a threephase cell, the material presented a capacitance of only 1.10 F, significantly lower than the typical values of pseudocapacitors based on doped transition oxides. The behavior during cycling was also unsatisfactory, demonstrating that ZnO without additional treatment does not sustain the Faradaic reactions necessary for charge storage. These results highlight that, in isolation, ZnO obtained by the sol-gel process does not have adequate pseudocapacitive characteristics and should be combined with carbonaceous conductors, heterostructures or doped with transition metals to improve its electrical conductivity, density of active sites and cycling stability before being considered for application in supercapacitors. In heterogeneous photocatalysis tests, ZnO was applied to the degradation of methyl orange (5 mg L¹) under UV irradiation at 265 nm and 365 nm. The irradiance of 265 nm, above the band gap of ZnO (~3.2 eV), allowed the generation of reactive species and resulted in more than 80% removal of the dye in 150 min. Under 365 nm, no significant degradation was observed, confirming that only photons with sufficient energy to excite ZnO are effective in the process.
