An Electrochemical Study of the Oxidation of Hydrogen at Platinum Electrodes in Several Room Temperature Ionic Liquids ² Debbie S. Silvester, ‡ Leigh Aldous, § Christopher Hardacre, § and Richard G. Compton* ,‡ Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, United Kingdom, School of Chemistry and Chemical Engineering/QUILL, Queen’s UniVersity Belfast, Belfast, Northern Ireland BT9 5AG, United Kingdom ReceiVed: NoVember 2, 2006; In Final Form: January 7, 2007 The electrochemical oxidation of dissolved hydrogen gas has been studied in a range of room-temperature ionic liquids (RTILs), namely [C 2 mim][NTf 2 ], [C 4 mim][NTf 2 ], [N 6,2,2,2 ][NTf 2 ], [P 14,6,6,6 ][NTf 2 ], [C 4 mpyrr]- [NTf 2 ], [C 4 mim][BF 4 ], [C 4 mim][PF 6 ], [C 4 mim][OTf], and [C 6 mim]Cl on a platinum microdisk electrode of diameter 10 μm. In all cases, except [C 6 mim]Cl, a broad quasi-electrochemically reversible oxidation peak between 0.3 to 1.3 V vs Ag was seen prior to electrode activation ([C 6 mim]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 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 HNTf 2 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 H 2 gas. 1. Introduction Room-temperature ionic liquids (RTILs) have attracted much attention recently in many areas of science, particularly in green synthesis, 1,2 mainly due to their nonvolatility and interesting physical characteristics. They have been used in many electro- chemical applications such as solar cells, 3 electrochemical sensors, 4 fuel cells, 5 capacitors, 6 and in lithium batteries 7 and are employed as solvents in electrochemical experiments 8,9 because they are intrinsically conductive (no need for supporting electrolyte). Also, they have wide electrochemical windows, allowing for electrodeposition 10 of metals and semiconductors normally out of the range of traditional solvents. They can also be employed as supporting electrolytes in other solvents such as acetonitrile (MeCN). 11 As part of a general understanding of electrochemistry in RTILs, we have previously looked at the mechanisms of various organic reactions, 12-16 with a view to identify any similarities or differences to the reactions in RTILs compared to conventional aprotic solvents. In general, the studies reveal that the mechanisms often appear to be similar in both media, with the main differences due to the higher viscosity of RTILs giving slower diffusion. In addition, the electrode kinetics of oxidation and reduction are often slowed in RTILs. 17-19 We have, however, shown that RTILs can offer some advantages over traditional solvents, for example, in studying the electro- chemistry of the inorganic compounds PCl 3 and POCl 3 . 20 These compounds were found to be “stable” in some RTILs 21 (but would otherwise hydrolyze in traditional molecular solvents), and the electrochemistry was studied in detail for the first time. One of the most important applications of RTILs in electro- chemistry to date is their use in gas sensors. 4,22,23 Their nonvolatility means that the sensor does not “dry out” (as with conventional aqueous solution-based Clark cells), and their high thermal stability allows gas sensing at high temperatures. A range of gases, including oxygen (O 2 ), 24 ammonia (NH 3 ), 25 and carbon dioxide (CO 2 , from O 2 and CO 2 simultaneously) 26 have been studied previously in RTILs, and these solvents have shown promise for use as electrolytes in a range of industrial gas sensors. It is of general interest to explore the oxidation of hydrogen (H 2 ) due to its major importance in fuel cells 27 among other applications. The oxidation of gaseous hydrogen is (at least in principle) believed to be conceptually one of the simplest processes, involving a simple one-electron transfer per proton. The electrochemical oxidation of hydrogen has been studied on platinum and palladium surfaces in protic solvents 28-31 such as H 2 O, but relatively little work has been done in aprotic solvents. Barrette and Sawyer 32 showed reversible oxidation waves for dissolved H 2 in dimethyl sulfoxide (DMSO), pyridine, N,N-dimethylformamide (DMF), and acetonitrile (MeCN) and observed broad, diffuse anodic peaks whose peak currents were irreproducible and not proportional to the partial pressure of H 2 . However, when the electrodes were activated electrochemi- cally (pre-anodized) at potentials more positive than the initial peaks, the H 2 oxidation peak changes from a shape characteristic of an electrochemically quasi-reversible process to one for a nearly reversible one-electron transfer. In all solvents except ² Part of the special issue “Physical Chemistry of Ionic Liquids”. * Author to whom all correspondence should be addressed. E-mail: richard.compton@chemistry.oxford.ac.uk. Telephone: +44 (0) 1865 275 413. Fax: +44 (0) 1865 275 410. ‡ Physical and Theoretical Chemistry Laboratory. § School of Chemistry and Chemical Engineering/QUILL. 5000 J. Phys. Chem. B 2007, 111, 5000-5007 10.1021/jp067236v CCC: $37.00 © 2007 American Chemical Society Published on Web 02/07/2007