Graphene/Sr,Bi-based mixed oxide composite for highly efficient supercapacitors and its use for the degradation of dyes/herbicides

This work reports the electrochemical and photocatalytic performance of a graphene/Sr,Bi-based mixed oxides (BiSr) composite. The BiSr powder was synthesized by the combustion method, and it is mainly composed by Sr 2 Bi 2 O 5 , Sr 0.75 Bi 1.36 O 0.25 and SrBiO 2 Cl phases according to X-ray diffraction analysis. Firstly, a reference supercapacitor (SC) was made with graphene electrodes and without BiSr powder. This device produced a capacitance of 555.8 F g −1 and an energy density of 77.2 W·h kg −1 . After the addition of BiSr to the graphene electrodes, the capacitance and energy density increased to 803.6 F g −1 and 111.6 W·h kg −1 , respectively. This means an increment of capacitance by 44.6%, which occurred because SC electrodes made with BiSr contained additional surface defects and Bi 2+ /Bi 3+ redox centers for charge storage. Later, the Graphene/BiSr (G/BiSr) composite was deposited onto recycled black polyester fabric (F) to form a photocatalytic composite floater (FG/BiSr), which was used for the removal of methylene blue (MB) and diuron contaminants under sunlight. Compared with BiSr powder, which removed 97.2% of MB and 47.7% of diuron herbicide in 150 min, the FG/BiSr composite produced higher degradation efficiencies (99.0% for MB and 98.6% for diuron). This composite floater induced higher degradation rates for contaminants because it has higher content of oxygen vacancy defects compared to the BiSr powder as confirmed by UV–vis and Raman measurements. Such defects delayed the recombination of photogenerated electron-hole pairs, increasing the availability of electrons for the photocatalytic degradation of contaminants. Scavenger experiments indicated that •OH radicals were the main reactive species responsible for pollutant degradation. Moreover, total organic carbon (TOC) measurements indicated that the FG/BiSr composite produced the highest mineralization (degradation of by-products) of 90% and 86% for MB and diuron, respectively. Overall, the combined effect of graphene (material for fast transport of charge), redox-active bismuth species, and oxygen vacancies leads to improved electrochemical performance and to the enhancement of photocatalytic degradation of contaminants. Hence, graphene/BiSr composite enhanced the charge storage and accelerated the degradation of dye/herbicide contaminants, which is of great interest in water treatment plants.