Bi0.84Sm0.16FeO3/graphene composite for the enhancement of capacitance of flexible supercapacitors and its use for the photocatalytic removal of methylene blue dye from tap water under sunlight

The specific capacitance and energy density of supercapacitors (SCs) were enhanced by depositing Sm-doped BiFeO 3 (Bi 0.84 Sm 0.16 FeO 3 ) nanoparticles on their electrodes. These nanoparticles were synthesized using the Pechini method, which produced Sm-doped BiFeO 3 (BiSm) with a rhombohedral phase. The SC electrodes were fabricated by depositing graphene (G) ink on recycled polypropylene (PP), followed by the addition of BiSm nanoparticles (PG/BiSm electrodes). The reference SC fabricated without BiSm demonstrated a maximum capacitance of 544.4 F g −1 and an energy density of 75.6 W h kg −1 . Upon incorporating BiSm nanoparticles into the SC electrodes, these values exhibited a significant increase, reaching 773.2 F g −1 and 107.4 W h kg −1 , representing a 42 % enhancement. UV–Vis, Raman, and X-ray photoelectron spectroscopies corroborated the existence of redox-active species (Bi 3+ /Bi 0 , Fe 3+ /Fe 2+ , and Sm 3+ /Sm 2+ ) and surface oxygen vacancies, which acted as redox centers for the charge storage in the SCs. Subsequently, the PG/BiSm electrode was extracted from “spent SC” and was re-utilized for the photocatalytic degradation of methylene blue (MB, 20 ppm) dye under sunlight. PG/BiSm electrode produced 100 % degradation of MB after 180 min, whereas the BiSm nanoparticles (powder) produced only 77.7 % degradation of MB under the same conditions. The enhanced photocatalytic activity of the PG/BiSm electrode was attributed to increased visible light absorption, which occurred due to band gap narrowing (2.15 eV) and to surface oxygen vacancies in BiSm nanoparticles, which were induced after introducing the Sm dopant in the BiFeO 3 host. In addition, a decrease in the recombination rate of photogenerated electron-hole pairs was observed, this was attributed to the transfer of electrons from the BiSm conduction band to surface oxygen vacancy sites and graphene (trap centers). This transfer increased the availability of electrons, thereby enhancing the photocatalytic degradation of MB. According to scavenger experiments, superoxide radicals (•O 2 − ) are the primary reactive species in the degradation of MB. Thus, the findings of this study demonstrated that Sm-doped BiFeO 3 nanoparticles combined with graphene represent a highly promising multifunctional material for applications in SCs and environmental remediation. • Graphene + Bi 0.84 Sm 0.16 FeO 3 (PG/BiSm) electrodes were used for supercapacitor (SC) and photocatalytic applications. • SC made with PG/BiSm electrodes exhibited a capacitance/energy-density of 773.2 F g −1 /107.4 W h kg −1 . • PG/BiSm electrode recycled from “spent SC” produced 100 % photocatalytic degradation of MB under sunlight. • Oxygen vacancies in PG/BiSm electrodes enhanced the capacitance or degradation for dyes.