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

Abstract

The oxidation of hydrogen was studied at an activated platinum micro-electrode by cyclic voltammetry in the following ionic liquids: [C 2mim][NTf 2], [C 4mim][NTf 2], [N 6,2,2,2][NTf 2], [P 14,6,6,6][NTf 2], [C 4mim][OTf], [C 4mim][BF 4], [C 4mim][PF 6], [C 4mim][NO 3], [C 6mim]Cl and [C 6mim][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 H 2 oxidation waves were nearly electrochemically and chemically reversible in [ C n mim ] [ NTf 2 ] ionic liquids, chemically irreversible in [C 6mim]Cl and [C 4mim][NO 3], 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 H 2 saturated solutions, with[C 4mim][NTf 2] 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.