Na/K-containing birnessite-type MnO₂/chitosan-derived carbon composite for high-performance supercapacitor...

In this study, a multifunctional composite electrode material, Na/K-containing MnO₂/chitosan carbon composite, integrating a biopolymer-derived carbon matrix with multiple heteroatom dopants (N, S, Na, K) and pseudocapacitive MnOₓ species, was successfully synthesized via a hydrothermal method. Chitosan simultaneously functions as the structural scaffold, nitrogen source, and surface anchoring matrix for Na, K, S, and Mn species. Structural characterization was performed using XRD, XPS, FTIR, Raman spectroscopy, N₂ adsorption/desorption, and SEM/EDS analyses. The electrochemical charge storage properties were systematically evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) in 1 M KOH electrolyte, employing three-electrode and symmetric two-electrode configurations. XPS analyses confirmed the successful incorporation of N (3.42 at.%), Mn (3.86 at.%), K (0.57 at.%), Na (1.25 at.%), and S (0.42 at.%) into the composite structure. In the three-electrode configuration, an exceptionally high specific capacitance of 428.75 Fg^-1 was achieved at 0.7 Ag^-1. The low equivalent series resistance (ESR ≈ 1.8 Ω) from EIS analyses confirms significantly enhanced electronic conductivity. In the symmetric two-electrode configuration, a specific capacitance of 385.35 F g^-1, energy density of 23.8 Wh kg^-1, and power density of 707.07 W kg^-1 were obtained at 0.7 A g^-1. Long-term cycling tests over 10,000 GCD cycles demonstrated 82.26% capacitance retention, confirming excellent electrochemical durability. These results indicate that the synergistic interplay between the electric double-layer capacitive (EDLC) behavior of the porous chitosan-derived carbon framework and the pseudocapacitive contributions of nitrogen/sulfur heteroatom functionalities alongside manganese oxide species renders Na/K-containing MnO₂/Chit carbon composite a highly promising electrode candidate for next-generation supercapacitor applications.