Graphene intermediated synthesis of thin-packing stacking-free MnO2 nanoflakes for high power output at high mass loading

Abstract

Numerous advanced pseudocapacitive MnO2 with much-improved power output have been reported in recent years. However, most of their performance is verified in thin film form, with a mass density of less than 3 mg cm−2, which falls short of the commercial electrode demand of 10 mg cm−2. This high mass loading will inevitably diminish its power output, the crucial superiority of capacitive materials, to a considerable extent. To address this issue, their kinetic analysis for seeking out the rate-determining step (RDS) is a prerequisite. Herein, a comprehensive kinetic analysis protocol has been developed combining the interpretation of capacitance fitting and impedance spectroscopy. The results have demonstrated the RDS is dependent on the mass loading status, and it switches to resistance-controlled under high mass loading due to the high resistance from both thick-packing and laminar-stacking of MnO2 flakes. To break such restriction, 3-D graphene foam intermediated MnO2 hybrid structures have been synthesized via a strategy of high-voltage graphite exfoliation paired with simultaneous Mn2+oxidation. More than constructing an efficient electron migration network, the underlying graphene skeleton also tunes the MnO2 nanostructure on its surface, leading to a thin-packing and stacking-free structure configuration. As a result, the optimized sample achieves an energy density of 8 mWh cm−2 at a power density of 3,012 mW cm−2 in a symmetrical capacitor, which is much higher than 0.1 mWh cm−2 at 1,700 mW cm−2 for the control sample. In summary, this study highlights the "mass effect" of energy storage kinetics for pseudocapacitive MnO2 and proposes a graphene-intermediated protocol to improve its power output at high mass loading, which could be valuable in guiding its commercialization.