Brownian-snowball-mechanism-induced hierarchical cobalt sulfide for supercapacitors

Transition metal sulfides with hierarchical (ball in ball) micro/nanoporous structures have drawn wide spread interests for various applications, such as energy storage, catalysis, solar cells, owing to their unique features and intriguing properties. However, precise control of synthesizing process for hierarchical porous nanospheres of transition metal sulfides remains a big challenge. In addition, the charging and discharging process of transition metal sulfides in electrochemical storage is still a black box. Herein we design and precise control of synthesizing a transition metal sulfide with hierarchical porous nanosphere structure, namely cobalt sulfide hierarchical porous nanospheres (HPNs). Brownian snowball mechanism is put forward to explain the formation mechanism of CoS samples, which is supported by first-principles calculations. The proposed Brownian snowball mechanism helps us understand the formation process and facilitates comprehensive and precise manipulation, a key requirement for industrial scale-up. The CoS hierarchical porous nanospheres with a specific surface area of approximately 140 m2/g possess a total specific capacitance of 1310 F/g and 932 F/g at a current density of 5 A/g when used as electrode materials for electrochemical capacitors. The capacitance reflected hybrid electrical energy storage and is separated into double layer charging and the Faradaic contribution from the OH− ions reactions with surface atoms. It is demonstrated that Faradaic capacitance dominates in CoS hierarchical porous nanospheres exceeding 770 F/g by an analysis of the voltammetric sweep data.