Structure Reinforced Birnessite in Extended Potential Window for Supercapacitors

Although δ-MnO2 with layered structure are promising electrode materials for supercapacitors, its poor electronic conductivity and structural instability limit its electrochemical performance. In this work, a structure reinforced birnessite with improved electron transfer is developed via Cr doping in δ-MnO2 using hydrothermal synthesis. The Cr-doped δ-MnO2 exhibits significantly improved specific capacitance and cycling stability in an extended potential window (0-1.2 V vs. Ag/AgCl) as compared to the pristine δ-MnO2. In specific, the 2 mol% Cr-doped δ-MnO2 (0.02CrMO) electrode exhibits large specific capacitances of 250 F g-1 at 0.2 A g-1 and 150 F g-1 at 10 A g-1 (~5 mg cm-2 mass loading) as well as outstanding capacitance retention of 82.6% after 30000 cycles. Both calculations and experimental results demonstrate that Cr doping in δ-MnO2 can narrow down the band gap, enhancing electron transfer, and strengthen the layered structure with suppressed Jahn-Teller distortion, resulting in reduced Mn dissolution and phase transition.