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...

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.

Phosphonium-based ionic liquids (ILs) paired with aprotic heterocyclic anions (AHAs) are found to react with CO 2 to form both a carbamate product and a carboxyl product. The carboxyl product is formed primarily at elevated temperatures through the formation of a phosphonium ylide intermediate. The formation of the carboxyl product leads to the formation of the neutral azole, which can lead to an irreversible process if the neutral azole is sufficiently volatile. To understand how the ILs would behave in a CO 2 capture process operated at elevated temperatures, it was necessary to quantify the two reaction products. CO 2 was reacted with seven different AHA ILs to determine the equilibrium amounts of carbamate and carboxyl, the equilibrium constants for both reactions, and the rate of CO 2 absorption by each reaction path. The reactions were tracked and quantified in situ by using ATR-FTIR spectroscopy, while NMR spectroscopy was used after equilibrium was reached to determine the extent of each reaction at multiple temperatures and pressures. It was found that both the basicity and molecular size of the anion play key roles in the formation of the phosphonium ylide. In the extreme case of only half of the reacted product was the desired carbamate at 60 °C. Although there is a significant amount of the carboxyl product formed, the carbamate is kinetically favored.

The solubility of O 2 was measured gravimetrically in 16 ionic liquids (ILs) containing piperidinium, pyrrolidinium, ammonium, tetrahydrothiophenium, methyltriazabicyclodecene, or guanidinium cations paired with bis(trifluoromethanesulfonyl)imide, bis(perfluoroethylsulfonyl)imide, or perfluoropropanoyl(trifluoromethylsulfonyl)imide anions. The temperature dependence of the solubility was determined for eight of the ILs. Henry' law constants were determined from the solubility data as well as standard enthalpies and entropies of absorption for systems with temperature-dependent data. Knowledge of these thermodynamic properties is essential in guiding the structural design of ionic liquids for different applications involving O 2 . The O 2 solubility did not follow any obvious trend with regard to IL structure or molar volume; instead, the dipole−quadrupole interactions are the primary driving forces for O 2 dissolution in ionic liquids, as demonstrated by negative standard enthalpies of absorption for all of the ILs. In addition, diffusion coefficients of O 2 in the ILs as a function of temperature (293, 313, and 333 K) and applied O 2 pressure (1, 3, and 5 MPa) were obtained by fitting the time-dependent absorption data to a widely used mass diffusion model. The diffusion coefficients of O 2 in ionic liquids at 293 K and 1 MPa roughly correlate inversely with the pure IL viscosity, likely due to the relatively low and similar O 2 solubilities under these conditions. As expected, O 2 diffusivities increase with increasing temperature. However, they also increase with increasing pressure, likely due to decreasing solution viscosity with increasing gas solubility.

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