Tuning Conductivity of Lead-Free Cs₂AgBiBr₆ Double Perovskite Ternary Composite with PEDOT:PSS and Carbon Black for Supercapacitor Application

Lead-based halide perovskites have been widely used as efficient energy materials due to their superior optoelectronic properties and mixed electronic-ionic conductivity. However, lead toxicity has been one of the key challenges for commercialization. Recently Cs₂AgBiBr₆, a lead-free double perovskite, has garnered significant interest due to its exceptional stability, nontoxic nature, and promising optoelectronic properties. But because of the low electronic and ionic conductivity of bismuth-based double perovskites, there is a challenge for their use in energy storage applications. To resolve this issue, we have incorporated carbon black and a conducting polymer poly(2,3-dihydrothieno-1,4-dioxin)-poly (styrene sulfonate) (PEDOT:PSS) as electronic and ionic conductivity agents respectively into the Cs₂AgBiBr₆ porous electrode. This ternary composite exhibits over 40% enhancement in specific capacitance as well as specific energy density compared with a binary composite of carbon black with a perovskite electrode. There is no significant change in the power density. However, only PEDOT:PSS as charge transporting materials in perovskite matrix results in lower energy density and power density despite lower charge transfer resistance (R_ct) at the electrode/electrolyte interface and higher dc ionic conductivity compared to perovskite/carbon composite electrodes. From the electrochemical impedance spectroscopy analysis, it is evident that balanced ionic and electronic conductivities are necessary to achieve optimal performance in lead-free perovskite-based supercapacitors. We also fabricated a solid-state symmetric supercapacitor using a quasi-solid-state gel electrolyte.