Synthesis of Pseudocapacitive Porous Metal Oxide Nanoclusters Anchored on Graphene for Aqueous Energy Storage Devices with High Energy Density and Long Cycling Stability along with Ultrafast Charging Capability

Realization of safe electrochemical energy storages with high energy density and long cycle life along with the high power density enabling fast charging is a major challenge. Here, a strategy to realize high‐performance aqueous energy storages using porous Mn3O4 (p‐MG) positive and porous Fe2O3 (p‐FG) negative electrodes, where granular nanoclusters composing nanoparticles are produced on graphene through lithiation‐induced conversion and the shortened ion diffusion lengths in p‐MG and p‐FG give fast charging rate and excellent cycle stability is reported. Furthermore, it is found from cyclic voltammetry curves and specific capacitances that porous metal oxide structures play mainly as redox reaction sites, while graphene structures provide electrical conductivity to active sites. Indeed, the full‐cell configuration of p‐MG and p‐FG in a hybrid capacitor exhibits a distinguished high energy density exceeding those of aqueous batteries, in addition to excellent capacity retention over 30 000 redox cycles and the energy density 2.5‐fold higher than that of its counterpart with pristine Mn3O4 and Fe2O3 nanocrystals. Additionally, this capacitor shows the high power density allowing ultrafast charging in that the full cells in series can be charged within several seconds by the rapid USB charger, thus outperforming those of typical aqueous batteries by about 100‐fold. Aqueous hybrid capacitors assembled in full cell configurations of pseudocapacitive porous metal oxide nanocluster electrodes with the energy density exceeding those of aqueous batteries, while achieving excellent capacity retention over long 30 000 charging/discharging cycles and the high power density exceeding those of aqueous batteries by about 100‐fold are reported.