Performance of symmetric cells with PAA-coated N-doped graphene (N-graphene for short) and YP-50 electrodes. YP-50 electrode in (a–c), N-graphene-CCF-GP electrode in (d, e), with electrolyte (0.5 M H2SO4 labeled as H2SO4 and 2 M Li2SO4 at pH ~6 labeled as Li2SO4) and PAA coating indicated where applicable. (a, d) CV curves at 2 mV s-1. (b, e) Galvanostatic charging/discharging curves at 1 A g-1. (c) Retention of initial capacitance of symmetric cell with coated vs. uncoated electrodes in 2 M Li2SO4 (pH ~6). Inset: threshold voltage for water splitting determined by H2 accumulation (measured by gas chromatography) in a sealed symmetric cell under 24 h CV sweeping at 2 mV s-1. (f) Ragone plot of specific energy versus specific power for several standard devices vs. our coated YP-50 and N-graphene symmetric cells using 2 M Li2SO4 (pH ~6) electrolyte.