Breaking the mold: Rethinking defects in Pb-free vacancy ordered perovskite for enhanced CO₂ reduction and supercapacitor functionality

The growth of hybrid halide perovskite single crystals has gathered significant attention due to their low trap density and fewer defects, which make them promising candidates for enhancing the performance of optoelectronic devices. However, in this work, we have explored the potential advantages of defects and vacancies in lead-free perovskites, specifically for applications in CO2 reduction and energy storage. We have synthesized vacancy-ordered lead-free perovskite single crystals, Cs3Bi2Br9 and Cs3Bi2Cl9, using a fast-cooling process before grinding them to prepare a nanocrystalline powder. This method deviating from the traditional slow cooling process, creates more defects and vacancies in these nanomaterials. Interestingly, these defects, often viewed as detrimental in most optoelectronic applications, have proven beneficial for energy storage in our study. During the fast-cooling process, C--O, C-O, and O-Bi-O bonds are formed in both halide perovskites indicating adsorption and formation of products. Therefore, these materials could be used in CO2 reduction without the use of a metal-organic framework. These bonds are found to be absent in defect-free perovskites produced by the traditional slow cooling process. Furthermore, the specific energy density of supercapacitors fabricated from these nanocrystalline materials is increased by 15-20 % compared to the traditional slow-cooling perovskite materials. This enhancement in energy density underscores the potential of these vacancy-rich perovskite materials in developing supercapacitors with better storage performance. Overall, this work shows how defects and vacancies engineering in lead-free halide perovskite's single crystal growth can be used to create new opportunities for their use in energy storage and CO2 reduction technologies.