We report a novel method for the large-scale fabrication of porous bulk silver thin sheets (PSTS) built from three-dimensionally interconnected nanoparticles (NPs). The synthesis starts with synthesizing silver sponges via an in situ growth of NPs which assemble into networks. The sponges are pressed into thin sheets before etching in acid. The resulting porosity is nearly homogeneous throughout the whole volume. The dependence on acid concentration was investigated and the average pore diameter can be controlled in a range of 83–145 nm by etching time. Growing metal oxides results in PSTS/Co₃O₄ composites which can be used directly as binder-free supercapacitor electrodes. The Co₃O₄ growth is optimized and the optimized composite electrode provides a much higher specific capacitance (1276 F g⁻¹ at 1 A g⁻¹) than previously reported for pure Co₃O₄ nanostructures with different shapes or those for Ag–Co₃O₄ composite nanowire array electrodes. The optimal electrode has a superior rate capability (still 986 F g⁻¹ at 10 A g⁻¹). The improvements are attributed to the continuous open porosity of PSTS and a direct contact between Co₃O₄ and Ag ligaments. The method can be extended to many other metals or alloys, promising wide application.