Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are promising for applications as inexpensive "designer" solvents exhibiting a host of tunable physicochemical properties. A detailed review of the current literature reveals the lack of predictive understanding of the microscopic mechanisms that govern the structure−property relationships in this class of solvents. Complex hydrogen bonding is postulated as the root cause of their melting point depressions and physicochemical properties; to understand these hydrogen bonded networks, it is imperative to study these systems as dynamic entities using both simulations and experiments. This review emphasizes recent research efforts in order to elucidate the next steps needed to develop a fundamental framework needed for a deeper understanding of DESs. It covers recent developments in DES research, frames outstanding scientific questions, and identifies promising research thrusts aligned with the advancement of the field toward predictive models and fundamental understanding of these solvents.

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. To achieve high temperature operation of proton exchange membrane fuel cells (PEMFC), preferably under ambient pressure, acid-base polymer membranes represent an effective approach. The phosphoric acid-doped polybenzimidazole membrane seems so far the most successful system in the field. It has in recent years motivated extensive research activities with great progress. This treatise is devoted to updating the development, covering polymer synthesis, membrane casting, physicochemical characterizations and fuel cell technologies. To optimize the membrane properties, high molecular weight polymers with synthetically modified or N-substituted structures have been synthesized. Techniques for membrane casting from organic solutions and directly from acid solutions have been developed. Ionic and covalent cross-linking as well as inorganic-organic composites has been explored. Membrane characterizations have been made including spectroscopy, water uptake and acid doping, thermal and oxidative stability, conductivity, electro-osmotic water drag, methanol crossover, solubility and permeability of gases, and oxygen reduction kinetics. Related fuel cell technologies such as electrode and MEA fabrication have been developed and high temperature PEMFC has been successfully demonstrated at temperatures of up to 200• C under ambient pressure. No gas humidification is mandatory, which enables the elimination of the complicated humidification system, compared with Nafion cells. Other operating features of the PBI cell include easy control of air flow rate, cell temperature and cooling. The PBI cell operating at above 150• C can tolerate up to 1% CO and 10 ppm SO 2 in the fuel stream, allowing for simplification of the fuel processing system and possible integration of the fuel cell stack with fuel processing units. Long-term durability with a degradation rate of 5 V h −1 has been achieved under continuous operation with hydrogen and air at 150-160• C. With load or thermal cycling, a performance loss of 300 V per cycle or 40 V h −1 per operating hour was observed. Further improvement should be done by, e.g. optimizing the thermal and chemical stability of the polymer, acid-base interaction and acid management, activity and stability of catalyst and more importantly the catalyst support, as well as the integral interface between electrode and membrane.

Polybenzimidazole films doped with phosphoric acid are being investigated as potential polymer electrolytes for use in hydrogen/air and direct methanol fuel cells. In this paper, we present experimental findings on the proton conductivity, water content, and methanol vapor permeability of this material, as well as preliminary fuel cell results. The low methanol vapor permeability of these electrolytes significantly reduces the adverse effects of methanol crossover typically observed in direct methanol polymer electrolyte membrane fuel cells.

Polybenzimidazole ͑PBI͒ film, a candidate polymer electrolyte membrane ͑PEM͒ for high-temperature ͑120-200°C͒ fuel cells, was cast from PBI/trifluoacetyl/H 3 PO 4 solution with constant molecular weight PBI powder and various acid doping levels. Conductivity measurements on these membranes were performed using an ac method under controlled temperature and relative humidity ͑RH͒. A complete set of conductivity data for H 3 PO 4 acid-doped PBI is presented as a function of temperature ͑60-200°C͒, RH ͑5-30%͒, and acid doping level ͑300-600 mol %͒. A mechanism of conductivity is proposed for the proton migration in this PBI/acid system based on this and previous work. Proton transfer in this system appears to occur along different paths for different doping levels, RHs, and temperatures. Hydrogen bonds immobilize the anions and form a network for proton transfer by a Grotthuss mechanism. The rate of proton transfer involving H 2 O is faster, leading to higher conductivity at higher RH. The order of the rate of proton transfer between various species is H 3 PO 4 (H 2 PO 4 Ϫ

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