In-depth study of electrochemical capacitor performance of MnO2 during phase transition from Ramsdellite-MnO2 to Birnessite-MnO2

In this work, we report a birnessite-MnO₂/ramsdellite-MnO₂/F_SnO₂ (FTO) composite planar electrode with 3D peony-like micro-structure for micro-supercapacitors (m-SCs), which is prepared via an original series resistor electrochemical deposition (SRED) of MnO₂ and subsequent secondary-growth. The composite electrode achieves an excellent specific capacitance of 1261 F g^-1 at 10 A g⁻¹ and extraordinary cycling stability with a significant 304% capacity retention after 10000 GCD cycles. Different from conventional cases, even though it is Mn⁺⁴, the MnO₂ prepared by the SRED would undergo a secondary-growth with phase transition process during the GCD cycling, which could dramatically improve the pseudocapacitance performance by surface/inner-surface and diffusion-controlled insertion capacity aspects. The mechanism is in-depth discovered by deconvolution of double-layer and Faradaic insertion capacity at different stages of the phase transition, and first principle calculation based on density functional theory (DFT). The secondary-growth process also offers a fabulous self-repairing feature of the SC electrode materials, which caters the maintenance-free and long-life requirement of the m-SCs application. Scan rate dependency of the bulk and the surface/near-surface pseudocapacitance is confirmed via de-convoluted the total capacities and the redox peaks of CV curves with different scan rates.