Well-Ordered Oxygen-Deficient CoMoO₄ and Fe₂O₃ Nanoplate Arrays on 3D Graphene Foam: Toward Flexible Asymmetric...

In this work, we report the development of well-ordered hydrogenated CoMoO₄ (H-CoMoO₄) and hydrogenated Fe₂O₃ (H-Fe₂O₃) nanoplate arrays on 3D graphene foam (GF) and explore their practice application as binder-free electrodes in assembling flexible all-solid-state asymmetric supercapacitor (ASC) devices. Our results show that the monolithic 3D porous GF prepared by solution casting method using Ni foam template possesses large surface area, superior electrical conductivity, and sufficient surface functional groups, which not only facilitate in situ growth of CoMoO₄ and Fe₂O₃ nanoplates but also contribute the double-layer capacitance of the resultant supercapacitor. The well-ordered pseudocapacitive metal oxide nanoplate arrays standing up on 3D GF scaffold can provide efficient space and shorten the length for electrolyte diffusion from the outer to the inner region of the electrode material for Faradaic energy storage. Furthermore, one of our major findings is that the introduction of oxygen vacancies in CoMoO₄ and Fe₂O₃ nanoplates by hydrogenation treatment can increase their electronic conductivity as well as improve their donor density and surface properties, which gives rise to a substantially improved electrochemical performance. Benefiting from the synergistic contributions of different components in the nanohybrid electrode, the resultant flexible ASC device with GF/H-CoMoO₄ as the positive electrode and GF/H-Fe₂O₃ as the negative electrode achieves a wide operation voltage of 1.5 V and a maximum volumetric specific capacitance of 3.6 F cm^-3, which is two times larger than that of the Ni/GF/CoMoO₄//Ni/GF/Fe₂O₃ device (1.8 F cm^-3), and the rate capability is up to 70% as the current density increases from 2 to 200 mA cm^-3. Moreover, the Ni/GF/H-CoMoO₄//Ni/GF/H-Fe₂O₃ device also exhibits a high energy density of 1.13 mWh cm^-3 and a high power density of 150 mW cm^-3, good mechanical flexibility with the decrease in capacitance of less than 4% after being bent inward to different angles and inward to 90° 200 times, and good cycling stability of 93.1% capacitance retention after 5000 cycles.