UV irradiation treatment on graphene/Zn3(VO4)2 composite electrodes to enhance their capacitance and energy density

This study presents the electrochemical performance of flexible supercapacitors (SCs) fabricated with graphene (G) electrodes. To enhance their capacitance, Zn₃(VO₄)₂ (ZVO) compound with morphology of microplates was added to the SC electrodes. Firstly, a reference device made solely with graphene electrodes was fabricated and named as GPR-SC, which produced an energy-density/capacitance of 75 Wh kg⁻¹/370.6 F g⁻¹. Later, the ZVO redox compound was incorporated to the graphene electrodes to make the G/ZVO-SC device, and this one had higher energy-density/capacitance of 188 Wh kg⁻¹/940.4 F g⁻¹, which represents an improvement of ⁓150% in the capacitance in comparison with the GPR-SC device. Interestingly, the G/ZVO-SC device demonstrated an operating voltage of 0.40 V during 10 hours of continuous use. To increase capacitance even more, G/ZVO electrodes were treated with ultraviolet (UV) light (254 nm,) which formed defects on them. These additional defects were redox sites available for the charge storage. In consequence, the capacitance and energy density increased to the ultimate values of 1231.9 F g⁻¹ and 246.0 Wh kg⁻¹, respectively. UV-Vis, photoluminescence, Raman and XPS spectroscopies identified grain boundary defects, changes in hybridization from sp² to sp³, the presence of vanadium species (V⁺⁴ and V⁺ 3 ), oxygen vacancies, and the presence of OH groups on the electrode surface. All those species/defects acted as redox centers for charge storage. It is worthy to mention that SCs made with G/ZVO electrodes had the highest capacitance retention of 88.5% and 80.6% after 1000 cycles of charge-discharge or bending. Surprisingly, G/ZVO-SC device was immersed in hot oil at 75 o C and its capacitance slightly decreased by 1.5%, demonstrating its resistance in hard conditions. This research demonstrates the effectiveness of UV irradiation to enhance the capacitance of electrodes without the need for complex physical/chemical processes. Due to the high values of capacitance and output voltage (0.85 V) produced by SCs studied here, they could be an alternative to substitute lithium-batteries. • Adding Zn₃(VO₄)₂ (ZVO) to the supercapacitor (SC) enhanced their capacitance by 154% • SC electrodes treated with UV light (254 nm) had additional defects on their surface • SC constructed with UV-treated electrodes had an ultimate capacitance of 1231.9 F g⁻¹ • V 4+ /V 5+ , oxygen vacancies and OH groups were redox (active) centers to store charge