Improved Capacitive Behavior of Birnessite Type Mn Oxide Coated on Activated Carbon Fibers

Birnessite type Mn oxide (potassium birnessite hydrate) powder (as- δ -MnO 2 ) with a layered microstructure was prepared via a hydrothermal process. To improve its capacitive performance, the microstructure was thermally modified (annealed) at 400 °C (400- δ -MnO 2 ) in a N 2 reducing environment. By removing the hydrated cation (K + ) layers inserted between the main layers of birnessite, intercalation/deintercalation of the electrolyte species (Li +1 ) became more effective. Characterization of the as- δ -MnO 2 and 400- δ -MnO 2 samples revealed that no phase transformation occurred during the annealing process. The microstructure became less crystalline and the total pore volume increased from 0.20 cm 3 g −1 to 0.43 cm 3 g −1 , while the oxidation state of Mn remained 4 + after annealing the as- δ -MnO 2 sample at 400 °C. The 400- δ -MnO 2 sample was then coated on asphaltene derived activated carbon fibers (ACF-400- δ -MnO 2 ) to improve the performance by making use of the high electrical conductivity and capacitive behavior of ACF. Coating the 400- δ -MnO 2 sample led to a significant increase in the capacitance (328 F g −1 and 195 F g −1 for ACF-400- δ -MnO 2 and 400- δ -MnO 2 at 0.4 A g −1 , respectively), improved energy and power values (∼7 kW kg −1 at ∼4.2 Wh kg −1 for ACF-400- δ -MnO 2 and 240 W kg −1 at 2.4 Wh kg −1 for 400- δ -MnO 2 ) and improved cycling behavior.