Carbon is widely used as the electrode material in supercapacitors but has not reached its maximum performance because of poor optimization of the electrode–electrolyte interaction. Taking inspiration from recent advances in “water-in-salt” electrolytes where the electrochemistry changes significantly because of the scarcity of free water, we propose a new electrode–electrolyte system comprising nitrogen-doped, reduced graphene oxide (N-RGO) electrodes and highly acidic electrolytes. We show that the tuning of the carbon electrode–electrolyte interaction (protonation) via control of electrolyte acidity through free water content (acid concentration) leads to an ∼100% increase in specific capacitance, excellent rate capability (238.7 F g–1 at 100 A g–1), and high-power performance. This high rate capability is enabled by protonation of N-RGO in concentrated sulfuric acid, which swells its restacked sheets, allowing sufficient access, yet fast transport of protons to the bulk electrode via the established hydrogen bonding network of acid molecules (Grotthuss mechanism). In situ Raman and electron paramagnetic resonance spectroscopies further reveal that this increased specific capacitance is due to both the enhanced double-layer capacitance and pseudocapacitance in highly acidic electrolytes. This approach has been adapted to conventional carbons and nonaqueous highly acidic electrolytes (e.g., 5 M sulfuric acid in trifluoroacetic acid), opening new opportunities in the supercapacitor design.