Scalable electrode materials with nanoporous current collector shells for supercapacitors with ultrahigh areal and volumetric capacitances

Supercapacitors are characterized by high power density, but a bottleneck exists regarding their limited energy density due to the conflict between achieving high mass loading and high specific capacitance with the current structural design. Although there have been concerted efforts to develop supercapacitors with high specific capacitance using nanostructured materials, the total mass loading of active materials is confined by their nano-scale thickness. Here we report an effective strategy to integrate active materials and current collectors through binding active materials and current collectors together in a 3D style. Contrary to the conventional configuration, hollow active materials were confined inside nano-scale current collectors in this work. The resulting nano-scale electrode materials could be compressed together in a scalable style to form nanoporous Fe₃O₄@Ni freestanding electrodes at the desired scale. Consequently, symmetric supercapacitors were fabricated by Fe₃O₄@Ni electrodes, and showed a capacitance of 11.2 F cm⁻², corresponding to an energy density of 94.4 mW h cm⁻³. The supercapacitor also exhibited an excellent cyclic capability with a capacitance retention of 107% after 10 000 cycles. The scalability of the electrode materials could be further enhanced by the addition of multiwalled carbon nanotubes (MCNTs) during the synthesis process, and the resulting 0.665-thick Fe₃O₄-MCNTs@Ni electrode with an Fe₃O₄content of 79.4 mg cm⁻²exhibited a superhigh areal capacitance of 82.6 F cm⁻²as well as a high volumetric capacitance of 1242 F cm⁻³. This work demonstrates that large mass loading, large specific capacitance, high energy density, and good cyclability of supercapacitors can be achieved simultaneously through the effective structural design of electrodes at the multi-scale.