Phosphorus-rich microporous carbons (P-carbons) prepared by a simple H3PO4 activation of three different carbon precursors exhibit enhanced supercapacitive performance in 1 M H2SO4 when highly stable performance can be achieved at potentials larger than the theoretical decomposition potential of water. This ability of P-carbons greatly enhances the energy density of supercapacitors that are capable of delivering 16 Wh/kg compared to 5 Wh/kg for the commercial carbon. An intercept-free multiple linear regression model confirms the strongest influence of phosphorus on capacitance together with micropores 0.65-0.83 nm in width that are the most effective in forming the electric double layer.