Core-Shell Structured Cobalt Sulfide/Cobalt Aluminum Hydroxide Nanosheet Arrays for Pseudocapacitor Application

The direct synthesis of nanostructured electrode materials on three-dimensional substrates is important for their practical application in electrochemical cells without requiring the use of organic additives or binders. In this study, we present a simple two-step process to synthesize a stable core-shell structured cobalt sulfide/cobalt aluminum hydroxide nanosheet (LDH-S) for pseudocapacitor electrode application. The cobalt aluminum layered double hydroxide (CoAl-LDH) nanoplates were synthesized in basic aqueous solution with a kinetically-controlled thickness. Owing to the facile diffusion of electrolytes through the nanoplates, thin CoAl-LDH nanoplates have higher specific capacitance values than thick nanoplates. The as-grown CoAl-LDH nanoplates were transformed into core-shell structured LDH-S nanosheets by a surface modification process in Na₂ S aqueous solution. The chemically robust cobalt sulfide (CoS) shell increased the electrochemical stability compared to the sulfide-free CoAl-LDH electrodes. The LDH-S electrodes exhibited high electrochemical performance in terms of specific capacitance and rate capability with a galvanostatic discharge of 1503 F g^-1 at a current density of 2 A g^-1 and a specific capacitance of 91 % at 50 A g^-1 .