Hierarchical Multiscale Engineered Fe ₃ O ₄ /Ni Electrodes with Ultrafast Supercapacitive Energy Storage for Alternate Current Line‐Filtering

Since the first demonstration of 3D bicontinuous porous architectures as a promising electrode for rechargeable batteries, many attempts have been made to extend this concept to other applications. Although some recent investigations have shown potential of bicontinuous structures as supercapacitor electrodes, there is a lack of capturing their capability for ultrafast charge/discharge as well as of manipulating the components in a more careful manner. Herein, novel bicontinuous porous Fe 3 O 4 /Ni supercapacitor electrodes fabricated by hierarchical multiscale engineering across three different length scales are reported. The electrodes comprise mesostructured epitaxial Ni scaffold (atomic‐scale), ultrathin pseudocapacitive Fe 3 O 4 nanosheets (nanoscale), and interconnected periodic pores (mesoscale). It is highlighted that the electrodes can be cycled as a capacitor at an ultrahigh scan rate up to 100 V s −1 and also exhibit excellent line‐filtering properties at 120 Hz including the areal capacitance (272 μF cm −2 ), phase angle (−76°), and time constant (0.3 ms). This is attributed to the synergistic effects of rapid electron conduction, efficient utilization of surface‐limited reaction, and facile ion diffusion, enabled by engineering the properties at different levels of length scales. As a result, the electrode‐based symmetric supercapacitors enable remarkable line‐filtering performance with the significantly suppressed voltage ripple.