An electrochemical study of the oxidation of hydrogen at platinum electrodes in several room temperature ionic liquids

The electrochemical oxidation of dissolved hydrogen gas has been studied in a range of room-temperature ionic liquids (RTILs), namely [C ₂mim][NTf₂], [C₄mim][NTf₂], [N _6,2,2,2][NTf₂], [P_14,6,6,6][NTf₂], [C₄mpyrr]-[NTf₂], [C₄mim][BF₄], [C₄mim][PF₆], [C₄mim][OTf], and [C ₆mim]Cl on a platinum microdisk electrode of diameter 10 μm. In all cases, except [C₆mim]Cl, a broad quasi-electrochemically reversible oxidation peak between 0.3 to 1.3 V vs Ag was seen prior to electrode activation ([C₆mim]Cl showed an almost irreversible wave). When the electrode was pre-anodized ("activated") at 2.0 V vs Ag for 1 min, the peak separations became smaller, and the peak shape became more electrochemically reversible. It is thought that the electrogenerated protons chemically combine with the anions (A^-) of the RTIL. The appearance and position of the reverse (reduction) peak on the voltammograms is thought to depend on three factors: (1) the stability of the protonated anion, HA, (2) the position of equilibrium of the protonation reaction HA ⇌ H⁺ + A^-, and (3) any follow-up chemistry, e.g., dissociation or reaction of the protonated anion, HA. This is discussed for the five different anions studied. The reduction of HNTf₂ was also studied in two [NTf ₂]^--based RTILs and was compared to the oxidation waves from hydrogen. The results have implications for the defining of pK_a in RTIL media, for the development of suitable reference electrodes for use in RTILs, and in the possible amperometric sensing of H₂ gas.