The electrochemical oxidation of hydrogen at activated platinum electrodes in room temperature ionic liquids as solvents

The oxidation of hydrogen was studied at an activated platinum micro-electrode by cyclic voltammetry in the following ionic liquids: [C2mim][NTf2], [C4mim][NTf2], [N6,2,2,2][NTf2], [P14,6,6,6][NTf2], [C4mim][OTf], [C4mim][BF4], [C4mim][PF6], [C4mim][NO3], [C6mim]Cl and [C6mim][FAP] (where [ C n mim ] + = 1 -alkyl- 3 -methylimidazolium , [ N 6 , 2 , 2 , 2 ] + = n -hexyltriethylammonium , [ P 14 , 6 , 6 , 6 ] + = tris ( n -hexyltetradecyl ) phosphonium, [ NTf 2 ] - = bis(trifluoromethylsulfonyl)amide , [ OTf ] - = trifluoromethlysulfonate and [ FAP ] - = tris(perfluoroethyl)trifluorophosphate ). Activation of the Pt electrode was necessary to obtain reliable and reproducible voltammetry. After activation of the electrode, the H2 oxidation waves were nearly electrochemically and chemically reversible in [ C n mim ] [ NTf 2 ] ionic liquids, chemically irreversible in [C6mim]Cl and [C4mim][NO3], and showed intermediate characteristics in OTf−, [ BF 4 ] - , [ PF 6 ] - , [FAP]− and other [ NTf 2 ] - -based ionic liquids. These differences reflect the contrasting interactions of protons with the respective RTIL anions. The oxidation peaks are reported relative to the half-wave potential of the cobaltocenium/cobaltocene redox couple in all ionic liquids studied, giving an indication of the relative proton interactions of each ionic liquid. A preliminary temperature study (ca. 298–333 K) has also been carried out in some of the ionic liquids. Diffusion coefficients and solubilities of hydrogen at 298 K were obtained from potential-step chronoamperometry, and there was no relationship found between the diffusion coefficients and solvent viscosity. RTILs possessing [ NTf 2 ] - and [FAP]− anions showed the highest micro-electrode peak currents for the oxidation in H2 saturated solutions, with[C4mim][NTf2] being the most sensitive. The large number of available RTIL anion/cation pairs allows scope for the possible electrochemical detection of hydrogen gas for use in gas sensor technology.