The preparation of thin gel electrolyte membranes with controllable thickness is important to explore the thickness-dependent electrochemical behaviors; this can further guide the fabrication of energy devices. Here we employ an in situ polymerization method to prepare a BSA–PDMAA–SiO2 cross-linked nanocomposite hydrogel on surfaces of eggshell membranes, which can be used as integrated separator and electrolyte in a supercapacitor after absorbing the electrolyte. The novel controlled thickness of the coated hydrogel therefore offers superior space utilization essential for all-solid-state devices. The composite gel can reach a high ionic conductivity of 8.8 mS cm−1 and a resulting Csp value of 161 F g−1 at the current density of 1 A g−1 when assembled in the supercapacitor, while the eggshell membrane based device has limited values of 2.7 mS cm−1 and 88 F g−1. A new insight into hybrid material preparation from low-cost natural life waste is presented in this work to obtain high performance gel electrolytes in energy devices.