Hydrogen Redox in Protic Ionic Liquids and a Direct Measurement of Proton Thermodynamics

Room temperature ionic liquids have attracted a great deal of interest in recent years due to their remarkable physicochemical properties including high thermal stability, wide electrochemical window, and low vapor pressure. A subclass of ionic liquids, protic ionic liquids (PILs), are formed by proton transfer from a Brønsted acid to a Brønsted base, and are good candidates as electrolytes in several applications, including fuel cells, because they integrate high ionicity and proton exchange kinetics with low vapor pressure. Here we present hydrogen redox results for a number of hydrogen-saturated PILs. Specifically we study the systems diethylmethylammonium bistrifluoromethanesulfonimide, diethylmethylammonium chloroaluminate, triethy-lammonium triflate, diethylmethylammonium triflate, dimethylethylammonium triflate, ethylammonium nitrate, pyridinium acetate, triethylammonium methane sulfonate, diethylmethylammonium methane sulfonate, and R-picolinium triflate. We observe a significant potential gap between the potential at which proton reduction occurs and the potential at which facile hydrogen oxidation occurs (with the gap ranging from ca. 0 to 800 mV). We show that this observation correlates with differences in the energetics for proton extraction from the anion (acid with the form HA) and from the cation (acid with the form BH+), which is defined by the differences in proton free energy between the Brønsted couples HA/H- and BH+/B. This energy gap and the associated equivalence point in the titration curve fix the proton activity in these systems and determine the electrochemical potential needed to activate a proton when no lower energy sites are available in the vicinity of the electrode. 1