Interfacial charge transfer dynamics in cerium metal-organic framework through non-covalent interactions with hydrogen-rich aqueous and nitrogen-rich ionic electrolytes for energy storage applications

Metal-organic frameworks (MOFs) hold significant potential for energy storage, but their performance is hindered by narrow operating potential windows (OPW) and low energy densities. This study focuses on the interfacial charge transfer dynamics of non-covalently bonded Cerium-Benzene-1,4-dicarboxylic acid (Ce-BDC MOF) in ethyldimethylpropylammonium bis(trifluoromethylsulfonyl)imide ionic liquids electrolyte (EDMPATFSI ILs), aiming to improve energy storage compared to H2SO4 aqueous electrolyte. The Ce-BDC MOF shows promising pseudocapacitive behavior in H2SO4, yet it is limited to a narrow 1 V operating window. In contrast, using EDMPA-TFSI ILs expands this window to 3 V, significantly enhancing energy density and specific capacity. The Ce-BDC MOF symmetric pseudocapacitor in EDMPA-TFSI ILs achieves a high specific capacity of 248 mAh g-1 at 1 A g-1 and an energy density of 154 Wh kg-1 at 442 W kg-1, surpassing aqueous solutions. However, longterm stability is a concern as prolonged cycling may degrade the ILs at higher voltages. The ILs is degraded to form sulfate ions found during the stability test, which is caused by the prolonged exposure of the sulfonyl groups in the TFSI anions with the trifluoromethyl group and the imide group to high voltages. The carboxylic acid groups of the BDC linker interact with the imide group of TFSI, leading to the replacement of the amine group in the sulfonyl group and the subsequent breakdown of the sulfonyl bond. To address the degradation of ILs bonding an electron-withdrawing group to the MOF can increase the acidity of the carboxylic groups, reducing their reactivity with the electrolyte. This approach could enhance the performance of MOF-based energy storage devices.