pH-Regulated host-guest architecture in molybdenum Dioxide/Carbon sphere composites for flexible solid-state supercapacitors

Compositing molybdenum dioxide (MoO2) with carbon sphere (CS) can greatly enhance the conductivity and structural instability to achieve its high theoretical capacitances. The controllable regulation of local architecture and host–guest interaction in the two-phase composites is highly desirable, but commonly hindered by the significant difference in formation mechanism between MoO2 and CS. Herein, a pH-regulated pre-intervention strategy was developed to provide control over the MoO2/CS architecture by adjusting the interplay between the precipitation kinetics of MoO2 and the polymerization effect of hydrochar precursor. By varying the pre-synthesis pH from 10.0 to 0.1, the corresponding dominant formation process of precursor changes from inside-out Ostwald ripening to protonation-induced electrostatic interactions, resulting in the structural evolution of MoO2/CS from hollow CS-encapsulating MoO2 nanosheets and solid CS-supporting MoO2 nanoparticles. The electrochemical test proved the former architecture possesses faster electrode kinetics, lower charge/ion-transfer resistance, and better electrochemical stability. An extended oxidation-etching method was further introduced to prepare mesoporous CS with excellent supercapacitive properties. A flexible solid-state asymmetric supercapacitor composed of optimal MoO2/CS structure as the positive electrode and mesoporous CS as the negative electrode delivers a high energy density of 52.3 W h kg−1 and power density of 14500 W kg−1, and good cycling stability with 89.7 % retention of the initial capacitance after 8000 cycles.