The conventional polymer-electrolyte membranes used in low-temperature fuel cells are limited to operating temperatures below about 120 ºC, as they must be fully hydrated to facilitate proton transport. Protic ionic liquids (PILs) are ionic liquids formed by transferring protons from Brønsted acids to Brønsted bases, and it has recently been shown that some ammonium-based PILs inherently exhibit high proton conductivities. Consequently, PILs have been proposed for use as electrolytes in non-humidified fuel cells that can operate above 120 ºC (at intermediate temperatures). However, while they nominally consist entirely of ions, PILs can often contain a significant quantity of neutral species (either molecules or ion clusters) that can affect the physicochemical properties of the liquids.

In this contribution, we first describe an electroanalytical method for detecting and quantifying residual Brønsted acids in a series of ammonium-based PILs. Ultramicroelectrode voltammetry reveals that some of the accepted methods for synthesizing PILs can readily result in the formation of nonstoichiometric PILs containing up to 230 mmol dm⁻³ residual acid. We will then show that residual acid in PILs can have a drastic effect on the electrocatalytic oxygen reduction reaction (ORR; O₂ + 4e⁻+ 4H⁺→ 2H₂O) in the PILs. For example, the potential at which the ORR occurs at Pt in the PIL diethylmethylammonium trifluoromethanesulfonate, [dema][TfO], decreases linearly as the strength of the proton donor in the liquid decreases. In “pure” [dema][TfO], in which the proton donors during the ORR are the [dema]⁺ cations of the PIL (pK_a = 10), the onset potential of the ORR is the same as that of the hydrogen oxidation reaction (HOR) in the PIL. These observations have significant implications for the use of PILs as electrolytes in fuel cells and indicate that the best PILs are highly "acidic" liquids that can support oxygen reduction at high potentials.