CNT/turanite/FeNdCo-alloy electrodes to enhance the capacitance of waterproof/eco-friendly supercapacitors

Abstract

The electrochemical performance of flexible CNT-based supercapacitors (SCs) was enhanced by adding Cu₅(VO₄)₂(OH)₄ (CuVO, turanite), and FeNdCo alloy (FNC) on their electrodes. Firstly, we fabricated SCs with an acidic electrolyte (PVA/H₃PO₄) and electrodes of CNT/CuVO, CNT/FNC or CNT/CuVO/FNC. Those devices exhibited capacitances/energy densities in the range of 530–710 F g⁻¹/106–142 W h kg⁻¹. Later, eco-friendly devices were fabricated with seawater electrolyte instead of acidic electrolyte and the capacitances/energy-densities increased to the range of 552–869 F g⁻¹/110–151 W h kg⁻¹. Thus, using the seawater electrolyte enhanced the capacitance/energy-density by 4–22%. To demonstrate that such eco-friendly devices are waterproof, we evaluated their electrochemical performance underwater and found an increment of the capacitance from 902.7 to 1093.7 F g⁻¹ after increasing the temperature of the water from 28 to 95 °C (boiling point). We also reached an outstanding energy density of 197.6 W h kg⁻¹ underwater. When the SCs are exposed to high temperatures underwater, the CNT-fibers in the SC-electrodes are more relaxed/open (verified by SEM images), allowing a better charge-storage. XPS/Raman/UV-VIS spectra demonstrated a higher presence of Fe³⁺/oxygen-vacancies in the SC-electrodes of the devices made with seawater, and additional redox species of V³⁺/Cu⁰/Co²⁺. Therefore, they had higher capacitance than the devices made with acidic electrolyte.