Enhancing the Capacitance of Battery-Type Hybrid Capacitors by Encapsulating MgO Nanoparticles in Porous Carbon as Reservoirs for OH- Ions from Electrolytes.

Abstract

A novel design of no-loading and bifunctional positive electrode, serving as an active material and current collector simultaneously, has been constructed by grass-like nickel foam which shows a battery-type performance and excellent areal specific capacity at 0.540 mA h·cm-2 (over 4500 mF·cm-2). To obtain a high-performance hybrid capacitor, layered porous carbonaceous composites C/MgO negative electrodes were fabricated, in which MgO nanoparticles serve as "reservoirs" for OH- ions from the electrolyte. Compared with other carbon materials, such as carbon fibers, hollow nanospheres, and nanotubes, the three-dimensional (3D) hierarchical heterostructures of the C/MgO electrode exhibit a higher storage performance of 424.1 mF·cm-2. Assembled by these two working electrodes, a hybrid capacitor with uncommon galvanostatic charge/discharge cycling curve has been well-investigated in an alkaline aqueous electrolyte system. This as-coupled hybrid capacitor exhibits an engaging activation process during multiple cycling tests and leads to a drastically improved energy density of 60% (from 80.4 to 128.8 μW h·cm-2), which can be attributed to a "match behavior" between its positive and negative electrodes.