Designing high-performance electrochemical energy-storage nanoarchitectures to balance rate and capacity.

The impressive specific capacitance and high-rate performance reported for many nanometric charge-storing films on planar substrates cannot impact a technology space beyond microdevices unless such performance translates into a macroscale form factor. In this report, we explore how the nanoscale-to-macroscale properties of the electrode architecture (pore size/distribution, void volume, thickness) define energy and power performance when scaled to technologically relevant dimensions. Our test bed is a device-ready electrode architecture in which scalable, manufacturable carbon nanofoam papers with tunable pore sizes (5-200 nm) and thickness (100-300 μm) are painted with ~10 nm coatings of manganese oxide (MnOx). The quantity of capacitance and the rate at which it is delivered for four different MnOx-C variants was assessed by fabricating symmetric electrochemical capacitors using a concentrated aqueous electrolyte. Carbon nanofoam papers containing primarily 10-20 nm mesopores support high MnOx loadings (60 wt%) and device-level capacitance (30 F g(-1)), but the small mesoporous network hinders electrolyte transport and the low void volume restricts the quantity of charge-compensating ions within the electrode, making the full capacitance only accessible at slow rates (5 mV s(-1)). Carbon nanofoam papers with macropores (100-200 nm) facilitate high rate operation (50 mV s(-1)), but deliver significantly lower device capacitance (13 F g(-1)) as a result of lower MnOx loadings (41 wt%). Devices comprising MnOx-carbon nanofoams with interconnecting networks of meso- and macropores balance capacitance and rate performance, delivering 33 F g(-1) at 5 mV s(-1) and 23 F g(-1) at 50 mV s(-1). The use of carbon nanofoam papers with size-tunable pore structures and thickness provides the opportunity to engineer the electrode architecture to deliver scalable quantities of capacitance (F cm(-2)) in tens of seconds with a single device.