Morgan L. Thomas

Application of Ionic Liquids to Energy Storage and Conversion Materials and Devices

Ionic liquids (ILs) are liquids consisting entirely of ions and can be further defined as molten salts having melting points lower than 100 °C. One of the most important research areas for IL utilization is undoubtedly their energy application, especially for energy storage and conversion materials and devices, because there is a continuously increasing demand for clean and sustainable energy. In this article, various application of ILs are reviewed by focusing on their use as electrolyte materials for Li/Na ion batteries, Li-sulfur batteries, Li-oxygen batteries, and nonhumidified fuel cells and as carbon precursors for electrode catalysts of fuel cells and electrode materials for batteries and supercapacitors. Due to their characteristic properties such as nonvolatility, high thermal stability, and high ionic conductivity, ILs appear to meet the rigorous demands/criteria of these various applications. However, for further development, specific applications for which these characteristic properties become unique (i.e., not easily achieved by other materials) must be explored. Thus, through strong demands for research and consideration of ILs unique properties, we will be able to identify indispensable applications for ILs.

A structured three-dimensional polymer electrolyte with enlarged active reaction zone for Li-O2 batteries

The application of conventional solid polymer electrolyte (SPE) to lithium-oxygen (Li–O2) batteries has suffered from a limited active reaction zone due to thick SPE and subsequent lack of O2 gas diffusion route in the positive electrode. Here we present a new design for a three-dimensional (3-D) SPE structure, incorporating a carbon nanotube (CNT) electrode, adapted for a gas-based energy storage system. The void spaces in the porous CNT/SPE film allow an increased depth of diffusion of O2 gas, providing an enlarged active reaction zone where Li+ ions, O2 gas, and electrons can interact. Furthermore, the thin SPE layer along the CNT, forming the core/shell nanostructure, aids in the smooth electron transfer when O2 gas approaches the CNT surface. Therefore, the 3-D CNT/SPE electrode structure enhances the capacity in the SPE-based Li–O2 cell. However, intrinsic instability of poly(ethylene oxide) (PEO) of the SPE matrix to superoxide (O2·−) and high voltage gives rise to severe side reactions, convincing us of the need for development of a more stable electrolyte for use in this CNT/SPE design.

Selective aerobic oxidation of para-xylene in sub- and supercritical water. Part 2. The discovery of better catalysts

An extensive and systematic study has been carried out on the catalytic effect of more than 20 elements on the aerobic oxidation of p-xylene to terephthalic acid in super- and subcritical water. Reactions have been performed in a continuous reactor under catalyst unsaturated conditions. Reaction product, by-products and intermediates have been quantified as well as the burn (the amount of CO2 originating from total oxidation of p-xylene). CuBr2 has been found to be a superior catalyst to MnBr2, which has been widely used in the literature for this reaction in water at high temperatures. At catalyst unsaturated conditions (i.e. with low concentrations of catalyst), MnBr2 gives a terephthalic acid yield of 36.1% whereas CuBr2 enhances this value to 55.6%. A strong synergistic effect has been found between CuBr2 and other metals and sources of bromide. Indeed, we show that Cu/Co/Br, Cu/Co/NH4/Br and other mixtures give better results than CuBr2 reaching a terephthalic acid yield of 70.5% for the four component catalyst. The compositions of the catalyst as well as the reactor temperature have been optimized and their effects on the analyzed compounds are discussed. A substantial amount of additional data is included in the electronic supplementary information.

Stability of Glyme Solvate Ionic Liquid as an Electrolyte for Rechargeable Li−O2 Batteries

A solvate ionic liquid (SIL) was compared with a conventional organic solvent for the electrolyte of the Li-O2 battery. An equimolar mixture of triglyme (G3) and lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]), and a G3/Li[TFSA] mixture containing excess glyme were chosen as the SIL and the conventional electrolyte, respectively. Charge behavior and accompanying gas evolution of the two electrolytes was investigated by electrochemical mass spectrometry (ECMS). From the linear sweep voltammetry performed on an as-prepared cell, we demonstrate that the SIL has a higher oxidative stability than the conventional electrolyte and, furthermore, offers the advantage of lower volatility, which would benefit an open-type lithium-O2 cell design. Moreover, CO2 evolution during galvanostatic charge was less in the SIL, which implies less side reaction. However, O2 evolution during charge did not reach the theoretical value in either of the two electrolytes. Several mass spectral fragments were generated during the charge process, which provided evidence for side reactions of glyme-based electrolytes. We further relate the difference in observed discharge product morphology for these electrolytes to the solubility of the superoxide intermediate, determined by rotating ring disk electrode (RRDE) measurements.

Selective aerobic oxidation of para-xylene in sub- and supercritical water. Part 3: effects of geometry and mixing in laboratory scale continuous reactors

In this paper we report a strong dependence of the observed performance of the catalyst on the geometry and the configuration of laboratory scale reactors in the continuous aerobic oxidation of p-xylene in supercritical water. Small differences, such as the length of the feed pipes protruding into the reactor, have a very large effect on the observed yields and selectivities as well as on the reproducibility of the results. Different reactor designs also exert an influence on the perceived catalyst performance. We demonstrate that these effects are consistent with the relative efficiency of mixing of the reactant streams in the different reactors. The overall conclusion is that caution is required when comparing sets of data derived from studying such reactions even in apparently similar experimental arrangements.

Simple combination of a protic salt and an iron halide: precursor for a Fe, N and S co-doped catalyst for the oxygen reduction reaction in alkaline and acidic media

The major bottleneck for widespread realization of fuel cells has been the usage of precious-metal-based electrocatalysts, such as Pt/C or Pt-alloy/C, at the cathode. Owing to the high cost and limited natural resources of platinum, nonprecious metal catalysts, such as iron-doped carbons, have emerged as promising substitute catalyst materials. Protic salts and protic ionic liquids can provide a simple, quick, and cost-effective approach for fabricating iron-doped carbons without any additives or high-surface-area carbon supports. We demonstrate here the fabrication of an efficient electrocatalyst based on a protic salt, 1,10-phenanthrolinium dihydrogen sulfate ([Phen][2HSO4]), in combination with iron(III) chloride, FeCl3, via nanocasting. A predominantly mesoporous architecture with a narrow pore size distribution was achieved owing to the suitability of the precursors and silica template. The material also exhibited in situ formation of Fe–N bonds in the nitrogen and sulfur co-doped carbons. We found that maximizing the content of this motif was beneficial towards efficient conversion of oxygen into water in alkaline media. The prototype catalyst after post treatment (acid leaching and 2nd carbonization) also exhibited efficient reduction of oxygen in acidic media. The contribution of Fe–N bonds to the observed activity in acidic solution was less than in alkaline solution. The simplicity of the synthesis of the protic salt and the versatility of the ionic liquid platform make this molecular-level carbon precursor a unique candidate for future development of iron doped catalysts for fuel cells.

A dramatic switch in selectivity in the catalytic dehydrogenation of 4-vinylcyclohexene in high pressure steam; a cautionary lesson for continuous flow reactions

We report an investigation of the continuous oxidative dehydrogenation of 4-vinylcyclohexene (VCH) in high pressure steam. Oxidative dehydrogenation reactions such as this are often limited by unselective or total oxidation. We find that these side-reactions not only occur in this case, but can also have a major influence on the selectivity; a small increase in flow rate results in a complete switch in selectivity of the reaction. Our results suggest that styrene (ST) is formed as the initial product but that unless the H2 is sequestered, ST may then be hydrogenated to yield ethylbenzene (EB). The observation of periodic temperature spikes near the surface of the catalyst bed indicate cycles of propagating flames occur in a relatively small volume of the reactor, leading to total oxidation of some VCH and removal of O2, which would otherwise sequester the H2. These flames give rise to the large variations in the observed product selectivity. We suggest that these observations may not be restricted to this reaction system and reactor configuration, and may occur in other situations where small-scale continuous flow oxidation reactors are used.

Protic ionic liquids with primary alkylamine-derived cations: the dominance of hydrogen bonding on observed physicochemical properties

Novel protic ionic liquids (PILs) were synthesized by neutralization of primary alkylamines with bis(trifluoromethanesulfonyl)amide acid. An extensive hydrogen bonding network in these PILs was observed via lower thermal stability, temperature dependent inversion from non-Newtonian to Newtonian fluidic behavior, and lower ionicity compared to their secondary and tertiary analogues.

Enhanced Electrochemical Stability of Molten Li Salt Hydrate Electrolytes by the Addition of Divalent Cations

Water can be an attractive solvent for Li-ion battery electrolytes owing to numerous advantages such as high polarity, nonflammability, environmental benignity, and abundance, provided that its narrow electrochemical potential window can be enhanced to a similar level to that of typical nonaqueous electrolytes. In recent years, significant improvements in the electrochemical stability of aqueous electrolytes have been achieved with molten salt hydrate electrolytes containing extremely high concentrations of Li salt. In this study, we investigated the effect of divalent salt additives (magnesium and calcium bis(trifluoromethanesulfonyl)amides) in a molten salt hydrate electrolyte (21 mol kg–1 lithium bis(trifluoromethanesulfonyl)amide) on the electrochemical stability and aqueous lithium secondary battery performance. We found that the electrochemical stability was further enhanced by the addition of the divalent salt. In particular, the reductive stability was increased by more than 1 V on the Al electrode in the presence of either of the divalent cations. Surface characterization with X-ray photoelectron spectroscopy suggests that a passivation layer formed on the Al electrode consists of inorganic salts (most notably fluorides) of the divalent cations and the less-soluble solid electrolyte interphase mitigated the reductive decomposition of water effectively. The enhanced electrochemical stability in the presence of the divalent salts resulted in a more-stable charge–discharge cycling of LiCoO2 and Li4Ti5O12 electrodes.

Ionic liquids at interfaces: general discussion

Andrew Abbott, Matthew Addicoat, Leigh Aldous, Radha Gobinda Bhuin, Natalia Borisenko, José Nuno Canongia Lopes, Ryan Clark, Samuel Coles, Margarida Costa Gomes, Benjamin Cross, Jeffrey Everts, Millicent Firestone, Ramesh Gardas, Matthieu Gras, Simon Halstead, Christopher Hardacre, John Holbrey, Toshiyuki Itoh, Vladislav Ivaništšev, Johan Jacquemin, Philip Jessop, Robert Jones, Barbara Kirchner, Sichao Li, Ruth Lynden-Bell, Doug MacFarlane, Florian Maier, Markus Mezger, Aǵılio Pádua, Octavian D. Pavel, Susan Perkin, Simon Purcell, Mark Rutland, John M. Slattery, Sefik Suzer, Kazuhisa Tamura, Morgan L. Thomas, Shraeddha Tiwari, Seiji Tsuzuki, Betul Uralcan, William Wallace, Masayoshi Watanabe and James Wishart