Mayara Osorio García

The use of biomass waste as a precursor to biochar represents a sustainable avenue for the development of materials for energy storage. In this research, added value is given to the residual biomass of the Cassia fistula tree pod to manufacture anodes in sodium-ion batteries. The biochar was prepared using impregnation agents (NaCl and H₃PO₄) at different temperatures. Samples were characterized by X-ray diffraction, Raman spectroscopy, nitrogen physisorption, impedance spectroscopy, cyclic voltammetry, and galvanostatic cycling. This allowed identification of structural changes, the degree of disorder and the porosity of the resulting material, in addition to evaluating the electrochemical behavior as an anode in a semi-cell. The yield of the pyrolysis process with NaCl was 25.97–30.46%, and with H₃PO₄, the yield was 35.07–49.30%. According to the X-ray technique, biochar made with NaCl impregnation has interlayer spacings (d) in the range of 0.38–0.40 nm, higher than the spacing in the structure of crystalline graphite (0.34 nm). In the case of H₃PO₄ impregnation, the interlayer spacing range was 0.37–0.38 nm. The results of Raman spectroscopy indicate that the synthesized samples correspond to hard carbons with short-range graphitic domains, characterized by a sp²/sp³ hybrid structure, with a high density of defects and amorphous regions. This type of microstructure is especially favorable for energy storage applications, as defects, edges, and disordered regions often act as active sites for ion adsorption and storage processes. Initial galvanostatic cycling tests showed stabilized capacities from desodiation of ~140 mAh/g for NaCl-impregnated anodes and ~60 mAh/g for H₃PO₄-impregnated anodes; these results correspond to anodes based on biochar derived from Cassia fistula pods pyrolyzed at 900 °C.

Biochars derived from post-consumer coffee residues were synthesized using NaCl and NaHCO₃ as impregnation agents, which were pyrolyzed at 500 and 1000 °C. Structural characterization revealed that NaHCO₃ treatment at 1000 °C generated a highly interconnected porous network, with a surface area of 1353.22 m² g⁻¹, pore volume of 0.83 cm³ g⁻¹, and average pore size of 2.6 nm. These features, confirmed by nitrogen physisorption and SEM, favor Na⁺ accessibility and insertion. XRD and Raman analyses indicated a predominantly amorphous carbon, with graphitic domains and an interplanar distance of ≈0.34 nm, providing both adsorption capacity and electrical conductivity. Electrochemical evaluation showed that BC_{NaHCO3-1000°C} achieved an initial capacity of 34 mAh g⁻¹, stable for more than 15 cycles, outperforming NaCl-treated biochars. However, despite the favorable morphology, the high surface area may also promote side reactions and irreversible capacity loss, limiting overall efficiency. These findings demonstrate the feasibility of valorizing coffee waste into carbonaceous materials for sodium-ion battery anodes, while highlighting the need for further optimization of porosity, graphitization, and compositional modifications to enhance energy storage performance.

Hydrogen storage in Mg/MgH₂ materials is still an active research topic. In this work, a mixture of Mg-15wt.% VCl₃ was produced by cryogenic ball milling and tested for hydrogen storage. Short milling time (1 h), liquid N₂ cooling, and the use of VCl₃ as an additive produced micro-flaked particles approximately 2.5–5.0 µm thick. The Mg-15wt.% VCl₃ mixture demonstrated hydrogen uptake even at near room-temperature (50 °C). Mg-15wt.% VCl₃ achieved ~5 wt.% hydrogen in 1 min at 300 °C/26 bar. The fast hydriding kinetics is attributed to a reduction of the activation energy of the hydriding reaction (E_{a hydriding} = 63.8 ± 5.6 kJ/mol). The dehydriding reaction occurred at high temperatures (300–350 °C) and 0.8–1 bar hydrogen pressure. The activation energy of the dehydriding reaction is 123.11 ± 0.6 kJ/mol. Cryomilling and VCl₃ drastically improved the hydriding/dehydriding of Mg/MgH₂.