Graphene/Bi12TiO20:Li-MoS2/Na2Ti3O7 composite electrodes to enhance the capacitance/energy-density of supercapacitors operated under harsh environments

Li-doped Bi 12 TiO 20 (Bi 12 TiO 20 :Li) and MoS₂/Na₂Ti₃O₇ composites were utilized as redox powders to enhance the capacitance of graphene supercapacitors (SCaps), whose electrodes were made by printing graphene on recycled polyethylene. The SCaps made with graphene-based electrodes as well as the redox powders of Bi₁₂TiO₂₀:Li and MoS₂/Na₂Ti₃O₇ showed specific capacitance/energy-density values ( C t / E d ) of 420.6 F g −1 /84.2 Wh kg −1 and 345.8 F g −1 /69.2 Wh kg −1 , respectively. Two other SCaps were subsequently fabricated with the same graphene-based electrodes containing a mixture of Bi₁₂TiO₂₀:Li/(MoS₂/Na₂Ti₃O₇) redox powders with ratios of 50/50 or 25/75. These devices were tested in air at room temperature, producing specific capacitances/energy-densities of 407.6 F g −1 /81.5 Wh kg −1 and 686.4 F g –1 /137.5 Wh kg −1 , respectively. The best SCap made with a mixture of 25/75 of redox powders was subjected to 1000 charging/discharging cycles, which displayed at the end of the test a capacitance retention of 95.5 %. Furthermore, the supercapacitor made with a mixture of 25/75 of redox powders was electrochemically tested under harsh conditions: The first condition was underwater, in this case, the values of ( C t / E d ) were 976.5 F g −1 /177.2 Wh kg −1 . The second test was carried out in air, at a temperature of 60 °C, and the resulting ( C t / E d ) values were 734.9 F g −1 /146.8 Wh kg −1 . Those values reached at 60 °C surpassed these obtained in air at room temperature. Thus, the specific capacitance increased by 6.6 % due to heating and by 29.7 % when the device is underwater. On the other hand, Raman and XPS analysis revealed that the ion species of Mo 6+ /Mo 5+ , Bi 3+ /Bi 0 , Li 0 /Na 0 , Ti 3+ /Ti 2+ as well as oxygen-vacancy defects (OV x ) were the main redox centers that served to store energy in the supercapacitors. • Bi 12 TiO 20 :Li (BTL) and MoS₂/Na₂Ti₃O₇of (MNT) were used as redox powders in supercapacitors (Scaps). • The mixture BTL/MNT enhanced the capacitance 4.8 times when deposited on SCaps electrodes • High Capacitance of 976.5 F g −1 was obtained in devices made with BTL/MNT and tested underwater. • Maximum capacitance of 734.9 F g −1 was obtained in Scaps heated at 60 °C • Mo 6+ /Mo 5+ , Bi 3+ /Bi 0 , Li 0 , Na 0 , and Ti 3+ /Ti 2+ species were the redox centers to store charge.