Solvent Effects in Biomass-Derived Activated Carbons: New Insights for Their Doping/Functionalization toward Potential Hydrogen Storage Applications

Biomass-derived activated carbons play an important role in H2 storage applications since their structural and chemical properties can be modulated by adjusting the activating methods and experimental parameters as well as by functionalization with heteroatoms. However, unfavorable reaction conditions are usually required, which may compromise the carbonaceous framework, negatively impacting on the hydrogen storage performance. In this context, this work investigates the potential modification effects of different solvents on activated carbons (ACs) under mild conditions, with a focus on structural and textural rearrangements. ACs were treated, among others, with solvents such as toluene (TOL), tetrahydrofuran (THF), and isopropyl alcohol (IPA) at 353 K for a variable amount of time. Structural and textural analyses revealed that solvents might have a significant impact on the microporosity, chemical functionalization, and specific surface area (SBET) of ACs, thus potentially affecting their further chemical functionalization. TOL and IPA treatments demonstrated the solvent's role in framework reorganization, enhancing microporosity and storage capacity over reaction time. In contrast, THF exposure led to a decrease in textural properties and thermal stability, attributed to disruptive reaction conditions above the solvent’s boiling point. Furthermore, the presence of atmospheric oxygen was found to induce the formation of oxygenated functional groups in the graphitic carbon structure, which contributed to structural instability even if facilitating the framework reordering during prolonged treatments. Although treated samples exhibited reduced hydrogen uptake compared to the parent AC, selected treatments with toluene and IPA demonstrated promising improvements in adsorption efficiency (i.e., H2 uptake/SBET). This study opens the possibility of an effective biomass-derived AC modification, without the need to employ high-energy-consuming thermal treatments, thus maximizing the potential of greener processes.