CF4 plasma-treated porous silicon nanowire arrays laminated with MnO2 nanoflakes for asymmetric pseudocapacitors

Abstract

High energy density is a necessity of the electrochemical energy storage (EES) systems for practical application in many fields, including implantable devices, backup systems, sensors, and portable electronics. In the present work, we developed a simple and up-front method to achieve high energy- and power-density asymmetric pseudocapacitors (APCs) from silicon nanowires (SiNW)-laminated intrinsic pseudocapacitive two-dimensional (2D) nanoflakes. Initially, the impermeable SiNWs are treated with carbon tetrafluoride (CF4) plasma to form porous silicon nanowires (pSiNWs). 2D intrinsic pseudocapacitive MnO2 (2D MnO2) nanoflakes were decorated on the pSiNWs to fabricate core-shell-structured 2D MnO2/pSiNWs for high-energy APCs. The nanostructuring over the one-dimensional (1D) SiNWs via the plasma process and the 2D nature of the MnO2 nanoflakes simultaneously provided large active sites for charge storage and transport processes. As a result, the optimized 2D MnO2/pSiNW electrode material exhibited a specific capacitance of 311.89 F/g at an applied current density of 2 A/g and 100% coulombic efficiency at all current densities, with excellent cycling durability. To evaluate the actual cell applicability of the prepared core-shell electrode, APCs were designed using the 2D MnO2/pSiNWs as the positive electrode and activated carbon (C) as the negative electrode. The assembled APC showed a maximum energy density of 93.31 mWh/cm2 and a power density of approximately 1.51 mW/cm2, with a large operational voltage window (2.2 V) and capacitive retention of 89.5% over 10,000 cycles. The strategic construction of the 2D MnO2/pSiNWs and the extracted output demonstrated that the fabricated electrodes and/or devices are promising candidates for next-generation high-performance pseudocapacitors.