Chitosan with various degrees of carboxylation as hydrogel electrolyte for pseudo solid-state supercapacitors

Chitosan (CS), an eco-friendly amino polysaccharide with various degrees of carboxylation (DC) as hydrogel electrolyte for symmetric pseudo solid-state supercapacitors is investigated. The carboxylated reaction is carried out at pH of 6,8, and 10 for products denoted as CCS6, CCS8, and CCS10, respectively. The DC of CCS6, CCS8, and CCS10 is 100.9%, 20%, and 17.8% from the calculation of nuclear magnetic resonance spectra. Both hydrophilicity and water retention capability increase with the increase in DC. Thus, CCS6 hydrogel possesses the smallest contact angle and the largest fraction of heterogeneous melting peaks evidenced by contact angle measurement and differential scanning calorimetry. As for supercapacitor performances, Warburg region resistances of CS, CCS6, CCS8, and CCS10 are estimated as 0.5, 0.2, 0.1, and 0.45(Ω∙cm²). Therefore, interfacial ion diffusion resistance decreases with the increase in DC. Additionally, CCS6 with the highest DC results in the largest electrochemical window of 1.8 V. At current density of 1A∙g⁻¹, the specific capacitances of CS, CCS6, CCS8, and CCS10 supercapacitors are 35F∙g-1, 72.5F∙g⁻¹, 49.2F∙g⁻¹, and 40.5F∙g⁻¹. Among all hydrogel electrolytes, CCS6 possesses the highest DC, the smallest ion diffusion resistance, the largest electrochemical window, and the largest specific capacitance.