Ultra-hydrophilic porous carbons and their supercapacitor performance using pure water as electrolyte

The growing need for renewable energies requires the development of efficient energy storage technologies such as supercapacitors. Using aqueous electrolytes, usually high ion concentrations are required to generate high capacitances and power densities. Herein, we report an anomalous electrochemical charge storage behavior of supercapacitors in a very diluted electrolyte and even pure water, enabled by a densely N- and O-doped ultra-hydrophilic carbon (DUT-108). Minimizing the electrolyte concentration from 1 down to 0.001 M, the capacitance of DUT-108 only decreases from 192 to 147 F/g; while a commercially available model carbon (ROX) decreases considerably in capacitances (i.e., 90 to 32 F/g). More interestingly, when using water, DUT-108 still reaches 137 F/g in contrast to hydrophobic ROX (27 F/g). Impedance analysis further confirms low resistance change by showing a significantly higher ion diffusion ability of charge carriers within DUT-108. We hypothesize that this phenomenon could be driven by proton hopping on the amphoteric N- and O-groups and the abrupt passing of protons to neighboring water molecules. These findings are quite interesting, since water is environmentally friendly, safe and biocompatible as compared with corrosive acid/base and organic electrolytes. Therefore, the proof-of-concept pure water electrolyte could open new avenues for applications in bioelectronics.