Enhancement of capacitance in CNT based supercapacitors by incorporating a Mg3(PO4)2/CuSO4 porous composite on their electrodes

Abstract

Carbon nanotube (CNT) based supercapacitors (SCs) were fabricated using magnesium phosphate (MgPO) as a redox material and recycled polypropylene (PP) as the mechanical support for the SC electrodes. Microscopy analysis indicated that the MgPO powder is composed of grains with sizes of 100–420 nm. Firstly, SCs were made with bare CNT electrodes, and these devices generated an energy density of 44.8 W h kg⁻¹ and a maximum capacitance of 243.9 F g⁻¹ (at 1 A g⁻¹). Next, MgPO powder was added to the supercapacitor electrodes resulting in an increase of energy density and capacitance to 88.7 W h kg⁻¹ and 443.8 F g⁻¹, respectively. The CP increased even more (44%) after adding a mixture of MgPO/CuSO₄ to the SC electrodes. Interestingly, the device made with MgPO/CuSO₄ had the best electrochemical stability, with a capacitance retention of 92% after 500 cycles of charge–discharge. Spectroscopy analysis (XPS, UV-vis, and Raman) of the supercapacitor electrodes demonstrated the existence of the following redox species, which are useful to store charge: Mg²⁺/Mg⁰, oxygen vacancies, and P³⁺/P⁵⁺. The best device made with a mixture of redox powder MgPO/CuSO₄ presented the highest capacitance (638.6 F g⁻¹) because extra redox centers of Cu⁺/Cu²⁺ are found on its electrodes. Thus, combining magnesium phosphate materials and copper sulfate is a useful strategy not only to improve the electrochemical stability of carbon nanotube-based supercapacitors, but also to increase their capacitance/energy density, which are of interest for application in flexible electronics.