Response-Surface-Guided Extraction and Copper Modification of Nanocellulose from Corn Cob and Hemp Hurds: Source- and Loading-Dependent Structural and Electrochemical Properties

Agricultural residues offer a source of nanocellulose for developing functional composites. This study examined how biomass source, extraction conditions, and copper loading affect the structural and electrochemical properties of nanocellulose derived from hemp hurds and corn cob. A citric-acid-assisted chemical–mechanical extraction route was investigated using response surface methodology, with extraction cycle number and sonication time as factors and FTIR-derived total crystallinity index and SEM-derived apparent fiber size as responses. The selected conditions were three cycles and 120 min for hemp hurds and three cycles and 75 min for corn cob, yielding measured mean apparent fiber sizes of 83.3 and 100 nm, respectively. Nanocellulose was modified with nominal Cu loadings of 15, 25, 50, and 75 wt%. At 0.1 Hz, impedance magnitude decreased from 274.15 and 247.67 kΩ for the respective bare references to below 10 kΩ for all Cu-modified groups. Cu incorporation also increased cycle-5 passed charge, with the corn-cob series showing its highest mean response at 50 wt%. FTIR, SEM, TGA, and XRD supported cellulose enrichment, nanoscale fibrillation, source-dependent thermal behavior, and crystalline metallic Cu formation. These findings support further evaluation of Cu-modified nanocellulose for electrochemical applications.