Zeolitic imidazolate framework-67 embedded TEMPO-oxidized cellulose nanofibers/polyethyleneimine gel as Cu²⁺ adsorbent and its further application in supercapacitor

Heavy metals pollution is a problem of concern, and their recycling from waste water is a challenging work. In this study, epichlorohydrin crosslinked TEMPO-oxidized cellulose nanofibers/polyethyleneimine (TOCNFs/PEI, CP) gel was used as a matrix to in-situ plant Zeolitic imidazolate framework-67 (ZIF-67), fabricating a nanocomposite gel (CPZ) with abundant active sites to adsorb Cu²⁺ ions by mechanism of spontaneous adsorption on heterogeneous surfaces. Then two-step successive calcinations in N₂ and subsequent air were exerted for a Cu²⁺-loading gel (CPZ/Cu) and a virgin CPZ as a control, obtaining corresponding N-doped carbon composites containing metal oxides, i.e., CPZ/Cu-N₂-Air and CPZ-N₂-Air; and their application as supercapacitor electrodes was further investigated. The former demonstrates denser porous structure as well as lower surface area and pore volume than the latter owing to the generation of copper oxides, but better electrochemical properties are endowed, reasoned by the enhanced pseudo-capacitance from the synergism of Cu/Co oxides. The sample CPZ/Cu-N₂-Air demonstrates a promising potential as an energy storage material. This study provides an effective recycling method of Cu²⁺ ions from polluted water and develops an efficient solution for further utilization of the used polymer adsorbents containing heavy metals. This strategy achieves a closed-loop recycle from waste resources to advanced energy materials in a “trash to treasure” way, in line with the concept of environmental sustainability.