Breaking the Limits of Ionic Liquid-Based Supercapacitors: Mesoporous Carbon Electrodes Functionalized with Manganese Oxide Nanosplotches for Dense, Stable, and Wide-Temperature Energy Storage

Manganese oxide (MnO2) nanosplotches (NSs) are deposited on N- and S-doped ordered mesoporous carbon (N,S-CMK-3) essentially blocking microporosity. The obtained N,S-CMK-3/MnO2 composite materials are assembled into ionic liquid (IL)-based symmetric supercapacitors, which exhibit a high specific capacitance of 200 F g(-1) (0-3.5 V) at a scan rate of 2 mV s(-1), and good rate stability with 55.5% capacitance retention at a scan rate of 100 mV s(-1). The device can operate in a wide temperature range (-20 to 60 degrees C), and high cycling stability of N,S-CMK-3/MnO2 composite electrode is demonstrated. Lower energy of -3.56 eV can be achieved for the adsorption of 1-ethyl-3-methylimidazolium(+) (EMIM+) cation on the edge between MnO2 NSs and N,S-CMK-3 than on the plane of MnO2 NS (-3.04 eV), both being more preferred than the surface of pristine N,S-CMK-3 (-1.52 eV). This strengthening of the ion adsorption at the three-phase boundary between N,S-CMK-3, MnO2, and IL leads to enhancement of the specific capacity as compared to nondoped or MnO2-free reference materials. Supercapacitors based on such composite electrodes show significantly enhanced areal capacity pointing to energy storage in the mesopores rather than in the electrochemical surface layer, demonstrating a new energy storage mechanism in ILs.