Electrodes with Electrodeposited Water-excluding Polymer Coating Enable High-Voltage Aqueous Supercapacitors

Aqueous supercapacitors are powerful energy sources,but they are limited by energy density that is much lower than lithium-ion batteries.Since raising the voltage beyond the thermodynamic potential for water splitting(1.23 V)can boost the energy density,there has been much effort on water-stabilizing salvation additives such as Li_(2)SO_(4) that can provide an aqueous electrolyte capable of withstanding~1.8 V.Guided by the first-principles calculations that reveal water can promote hydrogen and oxygen evolution reactions,here,we pursue a new strategy of covering the electrode with a dense electroplated polymerized polyacrylic acid,which is an electron insulator but a proton conductor and proton reservoir.The combined effect of salvation and coating expands the electrochemical window throughout pH 3 to pH 10 to 2.4V for both fast and slow proton-mediated redox reactions.This allows activated carbon to quadruple the energy density,a kilogram of nitrogen-doped graphene to provide 127 Watt-hour,and both to have improved endurance because of suppression of water-mediated corrosion.Therefore,aqueous supercapacitors can now achieve energy densities quite comparable to that of a lithium-ion battery,but at 100 times the charging/discharging speed and cycle durability.