Alkali-assisted hydrothermal preparation of artificial humic acid from litchi wood and electrochemical performance of its hydrochar

Abstract

Utilizing soluble alkalis such as KOH and NaOH, hydrothermal humification technology (HTH) facilitates the conversion of biomass into artificial humic acid-like substances. However, the impact of less soluble bases like Ca(OH)2 has been underexplored. In this study, we investigates the use of Ca(OH)2 as an alternative to KOH in converting waste litchi wood into artificial humic acid (AHA) under hydrothermal conditions, and examined the potential of residual hydrochar in the field of electrochemistry. Under optimal conditions (20 % Ca(OH)2 addition, 225 ℃, and 4 h), 16.77 ± 0.49 % AHA was produced, with a yield and structure comparable to those obtained with KOH. During the hydrothermal process, Ca(OH)2 promotes the formation of small molecules that contribute to AHA formation, which then complex with Ca2+ and incorporate into the solid phase. This process significantly reduces carbon gas emissions by decreasing the loss of soluble sugars and absorbing CO2 (5.22 % with Ca(OH)2vs. 18.24 % with KOH). Additionally, after simple pyrolysis, the hydrochar exhibited a specific capacitance of 88.15 F−1 at 0.5 A g−1 and maintained over 97 % of its capacitance at 10 A g−1 after 10,000 cycles. This study highlights the potential of Ca(OH)2 in the hydrothermal humification and offers a novel approach for the sustainable utilization and high-value application of litchi wood and other biomass.