Using graphene nanosheets as a conductive additive to enhance the capacitive performance of α-MnO2

Graphene nanosheets (GNs) were directly used as a type of novel but powerful planar conductive additive in the α-MnO2-based electrode material for electrochemical capacitors (ECs), to improve the low electronic conductivity of α-MnO2. It was found that the specific capacitance (SC) and cycling performance of α-MnO2 were obviously enhanced when GNs co-existed with acetylene black (AB), a conventional carbon-based conductive agent, at an appropriate weight ratio in the electrode material (GNs and AB were 10 wt% and 5 wt% of total weight, respectively). The unusual phenomenon was attributed to the following two reasons: (i) the planar graphene conductive additive could bridge active α-MnO2 particles more effectively via a “plane-to-point” conducting mode and (ii) AB particles might serve as the fillings in the electrode material and connect the isolated α-MnO2 particles to GNs through a “filling effect”, thereby constructing a novel and more effective conducting network. In this way, the synergy effect between the “plane-to-point” mode (due to GNs) and the “filling” mode (due to AB) significantly decreased the charge-transfer resistance of the α-MnO2-based electrode. With much faster charge-transfer process, the capacitive performance of α-MnO2 was greatly enhanced. On the contrary, when GNs were excess, the effective conducting network was weakened by the agglomeration of GNs and the absence of AB, leading to a lower conductivity and non-enhanced capacitive performance of α-MnO2.