Superior Electrochemical Flow Supercapacitors Based on Ultrasmall Carbon Dots

Electrochemical flow capacitors (EFCs) exhibit microsecond-level response times in grid-scale energy storage systems and are capable of simultaneously decoupling power density from energy density. However, the carbon slurries typically employed in EFCs often suffer from poor fluidity, low specific capacity, and suboptimal rate performance, which have collectively impeded their large-scale application. In this work, we synthesized ultrasmall carbon nanodots (CDs) with a rich array of oxygen-containing functional groups on their edge structures through a simple mixed-acid hydrothermal method. Different from conventional EFC slurries based on micrometer-sized activated carbons, this work establishes a size-isolated materials platform in which three CDs share essentially identical surface chemistry but differ only in particle size (20.3, 10.0, and ~3.0 nm), so that the size-dependent interplay among interparticle interactions, collision dynamics, and pseudocapacitive charge storage can be decoupled and quantified by combining DLVO-based theoretical modeling with electrochemical measurements. These CDs were then combined with a KOH electrolyte to produce a carbon slurry that exhibits excellent suspension stability, fluidity, and dispersibility. Owing to the pseudocapacitance contribution from the edge functional groups, the EFC exhibited a high specific capacitance of 583 F/g and a low solution resistance of 0.352 Ω under static conditions. Under flowing conditions, the EFC maintained a high specific capacitance of 486 F/g, with a coulombic efficiency of 98%. These results demonstrate that ultrasmall CDs are a viable active material for EFC slurries.