Protic ionic liquids (ILs) have been recently studied as a potential approach to enhance oxygen reduction reaction (ORR) activity of carbon supported platinum catalysts (Pt/C) for application in polymer electrolyte membrane fuel cells. The high oxygen solubility in the ILs was suggested as one of the main reasons for the accelerated reaction rates. Because the nature of the anion of the IL has been associated with increased oxygen solubility, in this work we survey a number of ionic liquids with various anions to study this effect. While the search for direct correlation between the ORR activities and the oxygen solubilities does not produce any conclusive results, by contrast, the specific activity showed dependence on the availability of oxygenated species free Pt sites. This finding indicates that the inhibition of Pt oxidation and less adsorption of non-reactive species may also play an important role in the enhanced ORR activity. Moreover, the degree of IL coverage on the Pt surface was estimated using (bi)sulfate ions as an indicator. The surface coverage not only affected the ORR activity, but also the Pt dissolution process. This suggests that an optimal balance between activity and stability can be achieved on a partially covered Pt surface. The oxygen reduction reaction (ORR) that occurs on the cathode poses a major hurdle for the efficient utilization of polymer electrolyte fuel cells (PEMFC). Extensive studies have focused on developing novel catalysts to improve the efficiency of this reaction.1,2 The ORR involves multiple steps, among which O 2 + H + + e − ↔ OOH ads and OH ads + H + + e − ↔ H 2 O act as the potential rate determining steps. Simultaneously optimizing the Gibbs free energies for both steps to completely eliminate the overpotential is very challenging, and a ∼350 mV overpotenial is commonly observed, which is independent of the identity of the catalyst.3,4 The large overpotential has been generally attributed to the sluggish kinetics of the ORR and adsorption of oxygenated species (e.g. OH ad ) or other anions. 5 The coverage of the adsorbed oxygenated species (θ OH ad ) is unfavorable for the reaction, and the availability of the free metal surface (as expressed by the (1-θ OH ad ) term) is one of the governing factors for the ORR activity.6,7 As a result, much effort has been dedicated to weakening the bonding of OH to the catalyst surface either by shifting the d-band center of Pt 8,9 or by lateral repulsion from the supports (e.g. metal oxides). 10,11Anion adsorption on Pt also affects the ORR activity, and it is agreed that the occupation of active sites deactivates the sites and reduces the activity.12 In real-world membrane-electrode assembly (MEA), the Nafion membrane and ionomer, constituted by a Teflon-like backbone and an anionic cluster of sulfonic groups, 13 are widely employed as indispensable components. The interface between the Nafion and the metal has received great attention due to the strong irreversible sulfonate anions adsorption on the Pt. 12,[14][15][16] The de...

We synthesized ionic liquids (ILs) comprising an alkylphosphonium cation paired with phenolate, 4-nitrophenolate, and 4-methoxyphenolate anions that span a wide range of predicted reaction enthalpies with CO2. Each phenolate-based IL was characterized by spectroscopic techniques, and their physical properties (viscosity, conductivity, and CO2 solubility) were determined. We use the computational quantum chemical approach paired with experimental results to reveal the reaction mechanism of CO2 with phenolate ILs. Model chemistry shows that the oxygen atom of phenolate associates strongly with phosphonium cations and is able to deprotonate the cation to form an ylide with an affordable activation barrier. The ATR-FTIR and (31)P NMR spectra indicate that the phosphonium ylide formation and its reaction with CO2 are predominantly responsible for the observed CO2 uptake rather than direct anion-CO2 interaction.

The performance of an ionic liquid with an aprotic heterocyclic anion (AHA-IL), trihexyl(tetradecyl)phosphonium 2-cyanopyrrolide ([P][2-CNPyr]), for CO capture has been evaluated considering both the thermodynamics and the kinetics of the phenomena. Absorption gravimetric measurements of the gas-liquid equilibrium isotherms of CO-AHA-IL systems were carried out from 298 to 333 K and at pressures up to 15 bar, analyzing the role of both chemical and physical absorption phenomena in the overall CO solubility in the AHA-IL, as has been done previously. In addition, the kinetics of the CO chemical absorption process was evaluated by in situ Fourier transform infrared spectroscopy-attenuated total reflection, following the characteristic vibrational signals of the reactants and products over the reaction time. A chemical absorption model was used to describe the time-dependent concentration of species involved in the reactive absorption, obtaining kinetic parameters (such as chemical reaction kinetic constants and diffusion coefficients) as a function of temperatures and pressures. As expected, the results demonstrate that the CO absorption rate is mass-transfer-controlled because of the relatively high viscosity of AHA-IL. The AHA-IL was encapsulated in a porous carbon sphere (Encapsulated Ionic Liquid, ENIL) to improve the kinetic performance of the AHA-IL for CO capture. The newly synthesized AHA-ENIL material was evaluated as a CO sorbent with gravimetric absorption measurements. AHA-ENIL systems preserve the good CO absorption capacity of the AHA-IL but drastically enhance the CO absorption rate because of the increased gas-liquid surface contact area achieved by solvent encapsulation.

A series of room temperature ionic liquids (RTILs) based on 1-ethyl-3-methylimidazolium ([emim](+)) with different aprotic heterocyclic anions (AHAs) were synthesized and characterized as potential electrolyte candidates for lithium ion batteries. The density and transport properties of these ILs were measured over the temperature range between 283.15 and 343.15 K at ambient pressure. The temperature dependence of the transport properties (viscosity, ionic conductivity, self-diffusion coefficient, and molar conductivity) is fit well by the Vogel-Fulcher-Tamman (VFT) equation. The best-fit VFT parameters, as well as linear fits to the density, are reported. The ionicity of these ILs was quantified by the ratio of the molar conductivity obtained from the ionic conductivity and molar concentration to that calculated from the self-diffusion coefficients using the Nernst-Einstein equation. The results of this study, which is based on ILs composed of both a planar cation and planar anions, show that many of the [emim][AHA] ILs exhibit very good conductivity for their viscosities and provide insight into the design of ILs with enhanced dynamics that may be suitable for electrolyte applications.

Ionic liquids (ILs) with aprotic heterocyclic anions (AHAs) are promising candidates for post-combustion carbon capture technologies since they react with CO2 stoichiometrically and reversibly. CO2 solubilities in two AHA-ILs, triethyl­(octyl)­phosphonium 2-cyanopyrrolide ([P2228]­[2CNPyr]) and triethyl­(octyl)­phosphonium benzimidazolide ([P2228]­[BnIm]), are reported for multiple temperatures and a third, triethyl­(octyl)­phosphonium 6-bromobenzimidazolide ([P2228]­[6-BrBnIm]), at one temperature. Ionic liquid [P2228]­[2CNPyr] and phase-change ionic liquid (PCIL) tetraethylphosphonium benzimidazolide ([P2222]­[BnIm]) were encapsulated in a chemically compatible and CO2-permeable polydimethylsiloxane (PDMS) polymer shell in order to enhance absorption and desorption kinetics. Both the free and encapsulated [P2228]­[2CNPyr] and [P2222]­[BnIm] were subjected to thermodynamic testing. The CO2 solubilities in the encapsulated IL and PCIL were in good agreement with the free IL and PCIL, meaning that the encapsulation of IL and PCIL greatly enhanced the kinetics of CO2 absorption while maintaining the high CO2 capture capacity. Recyclability testing was also performed on both the free and encapsulated [P2228]­[2CNPyr] and [P2222]­[BnIm]. The IL and PCIL materials, as well as the capsules, were stable upon cycling, with the CO2 capacities for each cycle remaining unchanged. The IL and the PCIL showed no sign of degradation after cycling, which demonstrated excellent performance.