A composite electrode of graphene/Bi2Te3 for the fabrication of flexible supercapacitors/thermoelectric devices with high output voltage

In this research, supercapacitors (SCs) were made by using a novel H-type architecture and recycled plastics (used as mechanical support for the fabrication of the SC electrodes). The electrochemical tests indicated that SCs made with pure graphene electrodes produced capacitance/energy-density of 222.8 F g-1/31.1 Wh kg-1. After this, one of the graphene electrodes was substituted by a Bi2Te3 (BiTe) electrode, and the capacitance/energy-density increased up to 410.5 F g-1/67.3 Wh kg-1. Thus, the capacitance was enhanced by 84%. XPS and Raman spectroscopies demonstrated the presence of oxygen vacancies defects as well as Bi0/Bi3+/Bi5+ and Te0/Te2+ species on the SC electrodes, which were responsible for the charge storage by redox reactions. Subsequently, the cellulose separator in the SC made with BiTe was substituted by a copper sheet to form a thermoelectric device. This one produced a maximum output voltage of 116.4 mV and total power of 6.81 μW/cm2 when a temperature gradient of ΔT=100 K is applied between its electrodes. Such output voltage is one of the highest reported so far for a single device (normally that voltage is obtained with 5-6 devices connected in series). Additionally, the figure of merit (ZT) was calculated for the BiTe electrode and obtained values 2.84-2.92, which are among the highest reported so far for a thermoelectric material. This work demonstrated that flexible SCs/thermoelectric devices can be fabricated on recycled plastics, which reduces their cost. This is important to push their commercialization for wearable applications.