Asymmetric and zinc-ion hybrid supercapacitors based on iron oxide and carbon dots

There is an urgent need to design and develop materials for high-performance electrochemical energy storage systems. This paper presents practical synthesis and novel combination of alpha-Fe2O3, carbon dots (CDs), and BiOCl for high performance asymmetric supercapacitors (ASCs) and zinc-ion hybrid supercapacitors (ZHSCs). The materials prepared by wet-chemical methods were probed with different analytical techniques. X-Ray Diffraction, Fourier Transform Infrared Spectroscopy) and Raman analyzes indicate presence of alkyl and aryl rich surface functional groups of CDs which contribute to conducive interface for the interplay between electrolyte and electrodes. The alpha-Fe2O3@CDs enabled -20 fold increase in the electrochemical performance; 3686 F g-1 at a current density of 1 A g -1 in 2 M KOH electrolyte. However, the maximum specific capacitance of pristine alpha-Fe2O3 was just 184 F g-1 at the same current density. ASCs and ZHSCs were then fabricated using alpha-Fe2O3@CDs as the cathode along with anodes composed of BiOCl and Zn foil, respectively. The assembled ASC device achieved the potential window of 1.8 V with a high specific capacitance of 251 F g-1 at a current density of 1 A g-1. The highest energy, and power densities were 28 Wh kg- 1 and 14.8 kW kg- 1, respectively. The formed ZHSC exhibited the maximum specific capacitance of 449 F g-1 at a current density of 1 A g-1 with 78 % retention after 15,000 cycles. ZHSC outperformed ASC in terms of the energy and power density by -5 (140 Wh kg-1) and - 2 (30 kW kg- 1) times, respectively. These results position alpha-Fe2O3@CDs as an extremely promising material for ASCs and ZHSCs.