No abstract

The differential capacitance of the electrical double layer at glassy carbon, platinum and gold electrodes immersed in various ionic liquids was measured using impedance spectroscopy. We discuss the influence of temperature, the composition of the ionic liquids and the electrode material on the differential capacitance/potential curves. For different systems these curves have various overall shapes, but all include several extremes and a common minimum near the open circuit potential. We attribute this minimum to the potential of zero charge (PZC). Significantly, the differential capacitance generally decreases if the applied potential is large and moving away from the PZC. This is attributed to lattice saturation [A. A. Kornyshev, J. Phys. Chem. B, 2007, 111, 5545] effects which result in a thicker double layer. The differential capacitance of the double layer grows and specific adsorption diminishes with increasing temperature. Specific adsorption of both cations and anions influences the shapes of curves close to the PZC. The general shape of differential capacitance/potential does not depend strongly on the identity of the electrode material.

The electrocatalytic reduction of CO in a 30 % (w/w) monoethanolamine (MEA) aqueous solution was undertaken at In, Sn, Bi, Pb, Pd, Ag, Cu and Zn metal electrodes. Upon the dissolution of CO , the non-conducting MEA solution is transformed into a conducting one, as is required for the electrochemical reduction of CO . Both an increase in the electrode surface porosity and the addition of the surfactant cetyltrimethylammonium bromide (CTAB) suppress the competing hydrogen evolution reaction; the latter has a significantly stronger impact. The combination of a porous metal electrode and the addition of 0.1 % (w/w) CTAB results in the reduction of molecular CO to CO and formate ions, and the product distribution is highly dependent on the identity of the metal electrode used. At a potential of -0.8 V versus the reversible hydrogen electrode (RHE) with an indium electrode with a coralline-like structure, the faradaic efficiencies for the generation of CO and [HCOO] ions are 22.8 and 54.5 %, respectively compared to efficiencies of 2.9 and 60.8 % with a porous lead electrode and 38.2 and 2.4 % with a porous silver electrode. Extensive data for the other five electrodes are also provided. The optimal conditions for CO reduction are identified, and mechanistic details for the reaction pathways are proposed in this proof-of-concept electrochemical study in a CO capture medium. The conditions and features needed to achieve industrially and commercially viable CO reduction in an amine-based capture medium are considered.

There has been considerable recent interest in the use of the imidazolium cation as a promoter in the heterogeneous and homogeneous electrocatalysis of CO2 reduction. However, despite its widespread use for this purpose, the mechanism by which imidazolium operates is not yet fully established. The present work reveals that enhanced catalytic activity is achieved by addition of many cations other than imidazolium. Under cyclic voltammetric conditions at a Ag electrode in acetonitrile solutions (0.1 M n-Bu4NPF6), 2.0 mM concentrations of imidazolium, pyrrolidium, ammonium, phosphonium, and (trimethylamine)-(dimethylethylamine)-dihydroborate cations can all enhance the kinetics of catalytic CO2 reduction with imidazolium and pyrrolidium being the most active. Analysis of the voltammetric data suggests that imidazolium cations achieve their impact by directly acting as cocatalysts with Ag whereas the other cations affect the reaction rate by modifying the electrochemical double layer. The results also confirm that the active form of the cocatalyst is the reduced imidazolium radical which forms a complex with CO2 before being further reduced to CO or other products at Ag and not an imidazolium carboxylate formed between an imidazolium carbene and CO2. In fact, imidazolium is deactivated during CO2 reduction by the latter reaction. Addition of water inhibits this deactivation pathway allowing the imidazolium cation to remain active in a long-term for CO2 reduction. In contrast, the pyrrolidium cation, where enhanced catalysis is attributed to an electrochemical double layer effect, retains its catalytic activity for very long periods of time regardless of the presence or absence of water.

The structure of the liquid-vacuum interface in room temperature ionic liquids (ILs) is investigated using angle-resolved X-ray photoelectron spectroscopy (ARXPS) and synchrotron X-ray photoelectron spectroscopy (SXPS). By varying the polar angle and comparing the results for the chosen ionic liquids, we identify the presence of a surface layer that is chemically different to the bulk. In particular, this layer: (i) is enriched by aliphatic carbon atoms from the saturated carbon chains of the anions and cations, and (ii) contains an unequal distribution of cations and anions in a direction normal to the surface. This unequal distribution creates a potential gradient which extends from the surface into the liquid. We show unequivocally that this layer is not due to the presence of impurities.