Tuning Co-Ni Electrochemical Active Sites via Iron Incorporation in Carbonate Hydroxide Frameworks for High-Performance Supercapacitors

Abstract

Transition metal incorporation is widely recognised as an efficient approach to enhance charge storage capability and ion transport behaviour in electrochemically active materials. In the present work, Fe-doped CoNi carbonate hydroxide (CoNi-CH) electrode materials were successfully synthesized using a hydrothermal technique, with controlled variation in host metal composition. The influence of Fe substitution on the structural, morphological, and electrochemical characteristics of CoNi-CH was systematically examined by introducing Fe into the Co and Ni lattice sites, yielding FexCo(1−x)Ni-CH and FexNi(1−x)Co-CH systems, respectively. Structural analyses using X-ray diffraction, Raman spectroscopy, and FTIR confirmed that Fe substitution at the Co site leads to the emergence of a secondary FeOOH phase in the FexCo(1−x)Ni-CH samples. In contrast, Fe incorporation at the Ni site resulted in single-phase CoNi-CH formation without detectable impurities. Morphological investigations revealed that Fe doping significantly alters the nanorod architecture, causing noticeable changes in diameter, surface texture, and overall morphology. The introduction of Fe not only modifies the defect structure and surface characteristics of CoNi-CH but also substantially improves its electrochemical performance. The optimized Fe0.10Co0.90Ni-CH electrode delivered a specific capacitance of 1080 F/g at a current density of 1 A/g, outperforming the undoped material due to the synergistic contribution of the FeOOH phase. More importantly, the optimized Fe0.05Ni0.95Co-CH electrode exhibited an exceptionally high specific capacitance of 2898 F/g at 1 A/g, demonstrating that Fe substitution at the Ni site is more effective in enhancing the charge storage performance of CoNi-CH materials. Overall, this work highlights the significance of site-selective transition-metal doping as an effective strategy for engineering high-performance supercapacitor electrode materials and offers valuable insights for future energy storage material design.