Unexpected High Rate Capability Enhancement By MnO2 Incorporation in Cellulose-Derived Carbon Nanofiber Electrodes for Electrochemical Capacitors

Abstract

Electrochemical capacitors (ECs) can provide ultra-long cycle life and ultra-fast energy delivery, which are impossible to reach by most battery technologies. Therefore, ECs are irreplaceable in applications such as energy harvesting systems and grid power buffer [1, 2]. The compositing between carbon and manganese oxides (denoted here as carbon/MnO2) is a popular strategy in the materials research community for getting higher energy densities of EC electrodes [3, 4]. MnO2 is well acknowledged for its high pseudocapacitance due to surface-confined redox processes that take place during charging/discharging, but also known for its poor intrinsic electrical conductivity (0.1 to 1 mS/m) that restricts high power applications [5]. The combination of carbon and MnO2 circumvents the limitation of MnO2 by taking advantage of good rate capability endowed by superior electrical conductivity of carbon. Normally, the fast rate capability (in terms of a retention value of capacitance at high current density relative to that at low current density) of the composite is enhanced compared to pure MnO2, but still remains inferior to pure carbon [6]. However, a greatly enhanced rate capability (78.2%, the ratio of 15 A/g capacitance to 0.5 A/g capacitance) compared to the original carbon (10.2%) was observed after the incorporation of MnO2 into a cellulose-derived nitrogen-doped carbon nanofibers (NCNF) system presented here. In the present study, the composite material (NCNF/MnO2) was prepared via the carbonization of electrospun cellulose and subsequent reaction between carbon nanofibers and permanganate (MnO4 -) in a liquid phase under a hydrothermal condition. The above processes generate a physical blend of the supporting NCNF matrix and MnO2. The unexpected high rate capability enhancement in this system can be explained by a significant change in carbon surface functional groups, specifically, the increased fraction of phenolic groups and the decreased fraction of carboxylic groups. This study presents not only a high performance electrode material for ECs (over 110 F/g specific capacitance, 78.2% rate capability and 96.3% capacitance retention over 50 000 cycles), but also highlights the critical role of carbon surface modification that is induced by the compositing processes, especially in the process deployed in this work, i.e. the electroless deposition of MnO2 on carbon via reaction with MnO4 - in liquid media.