Carbon-based electrodes such as for incorporation into ultracapacitors or other high power density energy storage devices, include activated carbon, carbon black, binder and at least one molecular sieve material. The molecular sieve component can adsorb and trap water, which can facilitate the use of the device at higher voltage, such as greater than 3V. The molecular sieve material may be incorporated into the carbon-based electrodes or formed as a layer over a carbon-based electrode surface.