Abstract: To address the remaining limitations of gel electrolytes in terms of wide-voltage operation and thermal stability over a broad temperature range, urea was used as a hydrogen-bond donor and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄) as a hydrogen-bond acceptor to prepare a eutectic ionic mixture, which was then impregnated into a bacterial cellulose (BC) aerogel to obtain a eutectic ionic gel electrolyte (BUE-x). The microstructure, composition, and thermal behavior were characterized by SEM, FT-IR, XRD, DSC, and TGA, and activated-carbon symmetric supercapacitors were assembled to evaluate the electrochemical performance. The results show that when the molar ratio of urea to EMIMBF4 is 3∶7, the resulting gel electrolyte (BUE-3) is more uniformly distributed within the three-dimensional BC pore network, forming continuous ion-transport pathways while effectively suppressing local aggregation and crystallization. The room-temperature ionic conductivity reaches 0.59 mS·cm⁻¹, and the electrochemical stability window is 3.0 V. The assembled supercapacitor delivers a specific capacitance of 123 F·g⁻¹ at 1 A·g⁻¹, with an energy density of 38.4 Wh·kg⁻¹ at a power density of 749.2 W·kg⁻¹, while maintaining favorable electrochemical performance over 0-80℃. These results demonstrate that confining eutectic ionic mixtures within BC aerogels is a viable strategy for developing electrolytes that combine wide-voltage and wide-temperature performance, and provide a new perspective for multifunctional cellulose-based applications.