Performance of PEMFC with new polyvinyl-ionic liquids based membranes as electrolytes
Introduction
Polymer electrolyte membranes Fuel Cells, PEMFC, are considered one of the most important technologies for transport in medium term due to their high efficiency and low environmental impact. PEMFCs generally use as electrolyte perfluorosulfonated acid (PFSA)-based polymers such as Nafion. These polymers are mechanically, thermally and chemically stable and have very high proton conductivity [1]. However, its conductivity drops at temperatures higher than 100 °C because of the evaporation of water which has an important role in proton conduction. Higher temperatures are desirable because catalyst poisoning by absorbed CO can be reduced and hydrogen with less purity can be used. Besides, increasing the temperature the kinetics of the chemical reactions is accelerated [2], [3]. During the past few years a big progress has been made in the development of fuel cells but there are still technical and economic obstacles for the commercialization of this technology. Particularly, many efforts were made in the development of PEM with high conductivity at low relative humidity with the aim of reducing the cost and complexity of the system. Various studies have been carried out employing hybrid inorganic-organic PEMs to overcome this issue [4]. Alternatively, sulfonated hydrocarbon polymers such as poly (ether ketone) (PEK), poly (ether ether ketone) (PEEK) and polyimide (PI), which are cheaper than the perfluorinated ionomers and can be employed over a wide temperature range, have been extensively studied as polymer electrolytes for PEMFCs. However, these polymers, as in the case of Nafion, require humidification to maintain high proton conductivity, since water molecules play the role of proton carriers in such polymer electrolytes [5]. Other promising membranes are acid-doped PBI which do not rely on hydration for conductivity and they have mechanical flexibility at temperatures up to 200 °C. However, the leaching out of the acid component from the membrane during the operation represents a problem that must be solved. Besides, due to the large uptake of liquid acid these membranes display significant anisotropic swelling and the mechanical properties of these membranes may become critically poor [6].
Several studies have arisen in literature integrating the use of conventional polymers and ionic liquids for their use as electrolytes in PEMFC under anhydrous conditions [7], [8], [9]. Ionic liquids are defined as organic salts with melting points below or equal to room temperature. These compounds have a number of properties that make them suitable for this application, such as high conductivity in anhydrous conditions, high thermal and chemical stability, negligible vapor pressure and a large electrochemical window [10]. In this way, various authors have studied the modification of PFSA membranes trough the incorporation of ionic liquids in their structure [11], [12]. Moreover, recent literature shows the increased use of polymerized ionic liquids in fields like polymer chemistry and materials science [13]. They combine the properties of ionic liquids with the properties of polymers, encouraging their use in many applications such as separation of CO2 from flue gases, or as solid ionic electrolytes in batteries, supercapacitors and solar cells. Protic ionic liquids, which could be easily obtained through the combination of a Brønsted acid and a Brønsted base, have the ability to transfer protons from the acid to the base, leading the presence of proton donor and acceptor sites, which can be used to build up a hydrogen-bonded network [14]. Moreover, in most protic ionic liquids the proton migration is due to a vehicular mechanism, in which protic ionic liquids with the highest conductivities are those with highest fluidities [9]. A benefit of using PILs is that cells can be operated at temperatures above 100 °C under anhydrous conditions.
Due to the potential improvements that derive from the use of ionic liquids in fuel cell membranes, this work studies the performance of different types of protic imidazolium ionic liquids based membranes with high features for this applications, i) Nafion membranes modified with two different protic ionic liquids, 1-methyl-3-(4-sulfobutyl)-imidazolium bis(trifluoromethylsulfonyl)-imide ([HSO3-BMIm][NTf2]) and 1-butyl-3-(4-sulfobutyl)-imidazolium trifluoromethanesulfonate ([HSO3-BBIm][OTf]), and, ii) specifically designed ionic liquid polymerized membranes, 1-(4-sulfobutyl)-3-vinylimidazolium trifluoromethanesulfonate ([HSO3-BVIm][OTf]), that are reported for the first time and show promising results. Electrical impedance spectroscopy (EIS) was used to determine parameters that are essential for a better understanding of the electrodes and the membrane behavior in the fuel cells [15]. Furthermore, the resistance exerted by the electrolyte under the operating conditions has been determined.
Section snippets
Materials
Divinylbenzene (Sigma–Aldrich, 80%), 2-hydroxy-2-methyl propiophenone (Sigma–Aldrich, 97%), 1-butyl-3-(4-sulfobutyl)-imidazolium trifluoromethanesulfonate ([HSO3-BBIm][OTf]) (Sigma–Aldrich, ≥97%), 1-methylimidazole (Sigma–Aldrich, 99%), 1-vinylimidazole (Sigma–Aldrich, ≥99%), 1,4-butane sultone (Acros Organics, ≥99%), trifluoromethanesulfonic acid (Acros Organics, 99%, extra pure), and bis(trifluoromethane)sulfonamide lithium salt (Sigma–Aldrich, 99%) were commercially available and used
Thermal stability of protic ionic liquids and polymerized [HSO3-BVIm][OTf]
Polymeric membranes based on [HSO3-BVIm][OTf] were prepared by photopolymerization. The resulting membrane was flexible, transparent and the average thickness was 250–300 microns. Fig. 3 shows a photograph of polymerized [HSO3-BVIm][OTf] membrane together with the thermogravimetric curves of protic ionic liquids and polymerized [HSO3-BVIm][OTf].
Degradation of ionic liquids took place in two steps. The first weight loss occurring at T < 150 °C is associated with traces of water embedded in the
Conclusions
Membranes based on protic ionic liquids were prepared for their use as electrolytes in PEMFCs under anhydrous conditions. Nafion membranes impregnated with [HSO3-BBIm][OTf] achieve current densities of 217 mA/cm2 and maximum power of 44 mW/cm2 under anhydrous conditions at 25 °C. Polymerization of [HSO3-BVIm][OTf] provides solid electrolytes that reach current densities of 154 mA/cm2 and maximum power of 33 mW/cm2 at 25 °C without humidification. The analysis of the influence of the thickness
Acknowledgments
This research was supported by the Ministry of Education and Science under the project CTQ2008-00690. M. Vilas and E. Tojo would like to acknowledge to the University of Vigo (11VIA18) and the Xunta de Galicia (CN2012/184) for financial support.
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2021, Renewable EnergyCitation Excerpt :In recent years, many researchers have focused on incorporating proton conduction components into the polymer matrixes to improve the proton conductivity of membranes. Ionic liquids (ILs) based on heterocyclic nitrogen groups (imidazolium, pyridium, or pyrrole) are widely used in PEMs of various fuel cells due to excellent electrochemical performance [15–19]. However, the ILs tend to leach out from the polymer matrix after long-term usage, resulting in the electrochemical properties of the blend membranes deteriorates [20–22].