Tailoring hierarchical BiVO4 sub-micron particles for enhanced cyclability in asymmetric supercapacitor

Abstract

This work demonstrates the feasibility of sub-micron size metal oxides as a sustainable charge storage material in supercapacitors fabrication and addressing the cycle instability issues of pseudocapacitors. The sub-micron bismuth vanadate (BiVO4) particles were synthesized by two different routes, co-precipitation (BVO-N) and sonochemical (BVO-S) methods. The morphological investigation of BVO-S showed hierarchical microspheres with diameter ranges of 1–6 μm, which is more prominent in size compared to BVO-N particles (100 nm- 1-μm in diameter). The nitric acid plays a crucial role in stabilizing the BiVO4 particles in the co-precipitation process, whereas ultrasonic waves predominantly control the spherical particle formation in the sonochemical route. The electrochemical performance of BVO-N and BVO-S was tested in a potassium hydroxide (KOH) electrolyte. The charge and discharge cycle experiments showed BVO-S microspheres are more highly stable than that BVO-N. The BVO-N starts to degrade with its initial capacitance beyond 1000 cycles. The poor stability of BVO-N may be due to the breakdown of surface-adsorbed charged ionic species. As a result, BVO-S performs with a higher specific capacitance value of 214 F/g compared to BVO-N (124 F/g). Trasatti analysis revealed a balanced, synergistic behaviour of pseudocapacitance (61 %) and electric double layer capacitance (39 %) at the BVO-S is responsible for their high specific capacitance compared to BVO-N. The BVO-S has low intrinsic resistance due to the highly denser micro-spherical structure, allowing electrolyte ions to access the inner and outer surfaces of the BVO-S. Interestingly, charge transfer resistance was decreased after the cyclic stability test due to electrochemical activation and facilitates fast ion transport increasing the surface contact area of active material at the electrode-electrolyte interface. We fabricate the asymmetric cell with BVO-S microspheres (anode), activated carbon particles (cathode) and Poly (ethylene oxide) (PEO) /Polyethene glycol dimethyl ether (PEGDME)/KOH gel-based electrolyte. This asymmetric supercapacitor performs with a specific capacitance value of 153 F/g at 0.3 A/g under the cell voltage of 1.2 V. Also, it delivers 30.6 Whkg−1 of energy density and 1983 W kg−1 of power density. The cyclic stability of 98 % over 5000 cycles was achieved in this configuration. This performance is appreciable compared to the previous work on BiVO4-based asymmetry supercapacitors, particularly a capacitance retention (%) and potential window. Overall, an acid-free sonochemical processing route reported in this work is highly environmentally friendly. Likewise, the sub-micron metal oxide particle significantly improves their electrochemical stability without the coalesced together with any carbonaceous material. It can be transferred to synthesizing a broader choice of metal oxide based for enhanced cyclability with effective utilization of their charge storage behaviour that will perform high-power storage, which powers short-distance electric transportation.