Binary metal sulfides and polypyrrole on vertically aligned carbon nanotube arrays/carbon fiber paper as high-performance electrodes

Abstract

Pseudocapacitive materials, such as binary metal sulfides, show great promise as electrode candidates for energy storage devices due to their higher specific capacitance than that of mono-metal sulfides and binary metal oxides, but generally suffer from low energy densities when assembled in supercapacitor devices. To push the energy density limit of pseudocapacitive materials in devices, a new class of electrode materials with favorable architectures is strongly needed. Here, rationally designed and coaxially grown Ni–Co–S and polypyrrole on vertically aligned carbon nanotube (VA-CNT) arrays/3D carbon fiber paper (CFP) is presented as a novel freestanding electrode for energy storage devices. Our study has revealed that the catalyst preparation is the key step and the presence of an Al₂O₃ buffer layer is essential for the growth of VA-CNTs. The 3D hierarchical VA-CNTs/CFP allows high areal loading and high utilization efficiency of the active materials. The binder-free asymmetric energy storage devices with Ni–Co–S/VA-CNTs/CFP as the positive electrode and polypyrrole/VA-CNTs/CFP as the negative electrode, respectively, demonstrate a high energy density of 82 W h kg⁻¹ at 200 W kg⁻¹.