Ion-exchange synthesis of microporous Co₃S₄ for enhanced electrochemical energy storage

Replacing oxygen in an oxide-based material with sulfur can produce improved flexibility and more efficient electron transport in its structure leading to enhanced electrical performance. Herein, facile template-free growth of free-standing cobalt (II, III) oxide (Co₃O₄) on Ni foam via a mild hydrothermal technique followed by its transformation to cobalt (II, III) sulfide (Co₃S₄) via an ion-exchange is reported. The microstructural morphology, phase, and porosity of the prepared Co₃O₄ and Co₃S₄ are characterized by FESEM, XRD, Raman, XPS, TEM, and BET analyses. The electrochemical performance of the Co₃S₄ film with a microporous morphology is considerably superior to that of Co₃O₄, exhibiting a high specific capacitance of 1604 F g⁻¹ (905 F g⁻¹ for Co₃O₄), the excellent restorative ability of ~99% at 1 A g⁻¹ (~96% for Co₃O₄), and good retention of 98% at 10 A g⁻¹ (~70% for Co₃O₄). The Co₃S₄ electrode shows excellent capacitance endurance even after 10 000 charge/discharge cycles and a high energy density of 128.32 Wh kg⁻¹ at 0.72 kW kg⁻¹. A fabricated symmetric Co₃S₄ supercapacitor device also reveals superior charge/discharge, restorative, and retention performances compared to a Co₃O₄ one. The excellent supercapacitive performance of phase-transformed Co₃S₄ electrode is due its large electrochemically active surface area along with synergetic effect of small charge transfer resistance and high relative diffusion coefficient.