Development and characterization of coconut shell-derived carboxymethyl cellulose and nanocellulose as potential proton-conducting polymer electrolytes

The development of sustainable solid polymer electrolytes from agricultural waste offers an environmentally friendly alternative to conventional synthetic polymers for energy storage applications. In this study, cellulose and nanocellulose were extracted from coconut shell powder and chemically modified into carboxymethyl cellulose (CMC) and carboxymethyl nanocellulose (CMNC), followed by complexation with NH ₄ NO ₃ through the solvent-casting method. Successful carboxymethylation was confirmed by ¹ H and ¹³ C NMR, where three carboxyl carbon signals at 177.43, 179.61, and 180.96 ppm verified the incorporation of carboxymethyl groups without polymer degradation. ATR-FTIR analysis confirmed the removal of lignin and hemicellulose and the formation of carboxylate groups. FESEM analysis revealed the morphology of coconut shell powder and cellulose. TEM analysis confirmed the nanoscale morphology of nanocellulose, with 54.05% of the particles exhibiting a size distribution within the range of 0–100 nm. Thermogravimetric analysis showed that CMNC exhibited lower thermal stability and degraded at 190°C due to the formation of sulfated amorphous regions during acid hydrolysis. X-ray diffraction analysis demonstrated that carboxymethylation reduced the crystallinity index from 46.40% to 64.27% for cellulose and nanocellulose to 3.54% and 20.45% for CMC and CMNC, respectively. Electrochemical impedance spectroscopy revealed that the CMC electrolyte achieved the highest ionic conductivity of 2.87 × 10 ^-1 S cm ^-1 at 30 wt% NH ₄ NO ₃ , whereas CMNC (2.07 × 10 ^-3 S cm ^-1 ) highlighted its potential as a sustainable proton-conducting electrolyte for proton battery applications.