Polydopamine-coated graphene for supercapacitors with improved electrochemical performances and reduced self-discharge

Graphene-based supercapacitors have attracted significant attention owning to the high specific surface area and electrical conductivity of graphene with the potential to deliver superior energy and power performances than other supercapacitor materials. However, graphene-based supercapacitors inevitably have self-discharge behavior like any other supercapacitors, resulting in rapid voltage drop and limiting their applications for long-term energy storage. Reported methods for mitigating the self-discharge of graphene-based supercapacitors are often associated with deteriorated performances such as decreased specific capacitances or rate performances. Here we report that self-polymerized dopamine on the surface of graphene can reduce the self-discharge of supercapacitors while achieving improved specific capacitance and rate performance. Specifically, a 40% increase in specific capacitance (150.8 vs. 108.0 F g–1 at 0.1 A g–1), much improved capacitance retention at high currents (80% vs. 55% at 10 A g–1), and a 37% reduction in open circuit voltage (OCV) decay rate (0.33 V vs. 0.52 V in 12 h) were attained simultaneously after PDA-coating on graphene. Analysis of the open circuit potential attenuation of both positive and negative electrodes of the supercapacitors suggests that the coating of PDA suppressed the self-discharge caused by diffusion-controlled faradaic reaction process. This can be attributed to the catechol and amine functional groups of PDA which not only offered additional pseudocapacitance when the electrodes were charged but also served as trapping sites for dissolved impurity ions and oxygen molecules so that the reactions of these species on graphene surface were impeded during open circuit test, leading to both improved energy storage performances and reduced self-discharge rate.