Using anodic oxidation for boosting supercapacitive performance of thermally-oxidized cobalt coatings with various morphologies

Abstract

Metal oxide electrodes for supercapacitors are commonly synthesized through the hydrothermal method. Despite the simplicity and cost-effectiveness of electrodeposition for producing electrode materials, there is a scarcity of literature on electrodeposited electrodes. Previous studies have utilized nitrate-based solutions and subsequent heat-treatment to fabricate these materials without additional treatments to enhance their performance. This research explores the creation of cobalt coatings with varying morphologies (pyramidal, ridge-shaped, powdery, and rough nodular) using electrodeposition from sulfate-based solutions by adjusting deposition parameters. The cobalt coatings were then heat-treated at 550˚C for 3 h to convert them to cobalt oxide, a promising material for supercapacitor electrodes. The study investigates the impact of initial morphology on the structure, phases formed, and supercapacitive properties of the heat-treated coatings. Analysis of XRD and FTIR data indicated that the primary phase post-heat treatment was Co3O4, alongside some CoO. Cyclic voltammetry (CV) measurements in 1 M KOH solution revealed that the powdery coating exhibited the highest specific capacitance of 796±21 mF cm−2 (∼110 F g−1) at a scanning rate of 2 mV s−1, surpassing the heat-treated pyramidal coating by approximately tenfold. These CV findings were corroborated by galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) analyses. Furthermore, the heat-treated coatings underwent anodic oxidation in a 3 M KCl solution for 5 minutes to assess the impact of this lesser-known process on their capacitive behavior. Following anodic oxidation, the ridge-shaped and pyramidal coatings displayed significantly enhanced specific capacitance due to the formation of fine pores and surface modifications. Specifically, the specific capacitance of the ridge-shaped coating surged from 103±4 mF cm−2 (∼11 F g−1) to 1832±64 mF cm−2 (∼201 F g−1) at a scanning rate of 2 mV s−1 after anodic oxidation. Although the rough nodular coating also experienced an increase in specific capacitance, the morphology and electrochemical characteristics of the powdery coating remained relatively stable throughout the study.