Engineering surface oxygen functionalities on reduced graphene oxide cathode for high performance flexible zinc-ion hybrid capacitor

The oxygen functionalities of reduced graphene oxide (rGO) cathode are demonstrated to provide additional pseudocapacitance with electrochemical adsorption of electrolyte ions in zinc-ion hybrid capacitor (ZHSC). However, the excess oxygen functionalities would reduce the conductivity of rGO, deteriorating electrons transfer and limiting the further improvement of electrochemical performance. Here, a three-dimensional hierarchical rGO/hydroquinone (rGO/HQ) hybrid hydrogel as ZHSC cathode is firstly prepared by hydrothermal assembly, in which high oxygen-content HQ molecules are tightly adhered on the conductive rGO framework in parallel via non-covalent interaction without compromising the conductive and porous advantages of rGO matrix. The flexible rGO assembly mainly offers large surface area for the deposition of HQ and smooth ions/ electrons transportation channels for the efficient utilization of active sites. The incorporated HQ molecules primarily contribute to the pseudocapacitance caused by electrochemical co-adsorption of Zn2+/H+ with oxygen functionalities. The multiple faradic reaction mechanisms of rGO/HQ cathode are clearly demonstrated by electrochemical analysis and ex situ characterization for the first time. Benefiting from the synergy, the optimum rGO/HQ based ZHSC delivers nearly two times higher specific capacitance (323.5 F g- 1 , 0.5 A g- 1 ) than pristine rGO counterpart, and impressive rate performance (75.7 %, 20 A g- 1 ). Meanwhile, the device also presents high energy density (112.57 Wh kg- 1 at 377.42 W kg- 1 ) and power density, as well as long cycle stability and mechanical flexibility. Accordingly, the high-performance flexible rGO/HQ based ZHSC shows a great application prospect in wearable/portable electronics.