Binary mixtures of two protic ionic liquids (PILs), namely, diethylmethylammonium hydrogensulfate ([dema]HSO4) and diethylmethylammonium bis(trifluoromethanesulfonyl)amide ([dema][NTf2]), were prepared by mixing in various weight ratios for prospective use as fuel cell electrolytes. The binary mixtures showed significantly higher electrochemical activity compared with the constituent pure PILs, and the activity changed depending on the composition of the mixtures. Specifically, the open circuit potential (OCP) for a H2 | O2 cell using a binary electrolyte consisting of 56 wt % [dema][NTf2] was 1.03 V vs a reversible hydrogen electrode (RHE), whereas the values were 0.90 and 0.77 V for pure [dema]HSO4 and [dema][NTf2], respectively, under similar conditions. The electrochemical activity of the binary mixtures was interpreted by comparing their molecular characteristics inferred from Fourier transform infrared (FT-IR) and 1H NMR spectroscopy with those of the constituent PILs. The binary systems showed enha...