Highly Efficient K0.15MnO₂ Birnessite Nanosheets for Stable Pseudocapacitive Cathodes

In this paper, we reported a facile synthesis of Birnessite K 0.15 MnO 2 ·0.43H 2 O nanosheets in a solution phase. The structural and electrochemical properties of the K 0.15 MnO 2 nanosheets for supercapacitor (SC) reactions were studied, and a gravimetric capacitance of 303 F/g was obtained at a charge/discharge current of 0.2 A/g. Electrochemical kinetics showed that a non-Faradaic (electrical double layer) current existed throughout the charging potential range, while a dominant Faradaic (pseudocapacitive) current was observed at high and low potentials during anodic and cathodic scans, respectively. Asymmetric pseudocapacitive full-cells were constructed with both anodic and cathodic K 0.15 MnO 2 composite materials and subjected to long-term galvanostatic charge/discharge analyses. A specific capacitance of 67.8 F/g was obtained for the cathodic K 0.15 MnO 2 full-cells after 1000 cycles, with a capacitive retention of 87.8% and Coulombic and energy efficiencies of ∼100 and ∼90%, respectively. In situ X-ray absorption near edge spectroscopy further corroborated the potential-dependent Faradaic reactions, suggesting a predominant change in valence state of K 0.15 MnO 2 to occur between 0.3 and 0.6 V (vs Ag/AgCl). The present study not only underscores the structure–function relationship of MnO 2 -based electrode materials for SC reactions but also provides a new approach in fabricating advanced pseudocapacitors by utilizing cost-effective transition metal oxide materials.