Bulk and surface changes in two proton-exchange membranes (Nafion-112 and Nafion-117) as a result of the incorporation of the IL-cation n-dodecyltriethylammonium (or DTA+) by a proton/cation exchange mechanism after immersion in a DTA+ aqueous solution were analysed by impedance spectroscopy (IS), differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), and contact angle measurements performed with dry samples of the original Nafion and Nafion-DTA+-modified membranes. Only slight differences were obtained in the incorporation degree and surface chemical nature depending on the membrane thickness, and DTA+ incorporation modified both the hydrophobic character of the original Nafion membranes and their thermal stability. Electrical characterization of the dry Nafion-112 membrane was performed by impedance spectroscopy while different HCl solutions were used for membrane potential measurements. A study of time evolution of the impedance curves measured in the system “IL aqueous solution/Nafion-112 membrane/IL aqueous solution” was also performed. This study allows us monitoring the electrical changes associated to the IL-cation incorporation in both the membrane and the membrane/IL solution interface, and it provides supplementary information on the characteristic of the Nafion/DTA+ hybrid material. Moreover, the results also show the significant effect of water on the electrical resistance of the Nafion-112/IL-cation-modified membrane. 1. Introduction As it is well known, ionic liquids are room temperature molten salts (RTILs) with very low vapour pressure and composed only of ions. These salts are characterized by weak interactions owing to the combination of a large cation and a charge-delocalized anion. Ionic liquids exhibit very interesting properties such as the solubilization of a large range of organic molecules and transition metal complexes or the possibility of their repeated use due to reduced environmental loss, while their good thermal stability tolerates reactive processes at high temperatures favouring a faster kinetic [1–4]. Various kinds of salts can be used to design an ionic liquid with tailoring properties for a given application, but typical IL-cations are imidazolium and quaternary ammonium salts, while hexafluorophosphate is a common anion. Due to their particular characteristics, the use of ILs in many areas of technology and science has increased lately and they can be used as solvents for organic reactions and catalysis, CO2 capture devices, selective separation of chemical species, dissolution of natural
References
[1]
T. Welton, “Room-temperature ionic liquids. solvents for synthesis and catalysis,” Chemical Reviews, vol. 99, no. 8, pp. 2071–2083, 1999.
[2]
F. van Rantwijk and R. A. Sheldon, “Biocatalysis in ionic liquids,” Chemical Reviews, vol. 107, no. 6, pp. 2757–2785, 2007.
[3]
Y. Hu and C. Xu, “Effect of the structures of ionic liquids on their physical-chemical properties and the phase behavior of the mixtures involving ionic liquids,” Chemical Reviews, 2005.
[4]
J. Ranke, S. Stolte, R. St？rmann, J. Aming, and B. Jastorff, “Design of sustainable chemical products—the example of ionic liquids,” Chemical Reviews, vol. 107, no. 6, pp. 2183–2206, 2007.
[5]
M. Armand, F. Endres, D. R. MacFarlane, H. Ohno, and B. Scrosati, “Ionic-liquid materials for the electrochemical challenges of the future,” Nature Materials, vol. 8, no. 8, pp. 621–629, 2009.
[6]
L. J. Lozano, C. Godínez, A. P. de los Ríos, F. J. Hernández-Fernández, S. Sánchez-Segado, and F. J. Alguacil, “Recent advances in supported ionic liquid membrane technology,” Journal of Membrane Science, vol. 376, no. 1-2, pp. 1–14, 2011.
[7]
F. Endres, M. Armand, and D. R. MacFarlane, Eds., Electrodeposition from Ionic Liquids, Wiley, 2008.
[8]
S. Rivera-Rubero and S. Baldelli, “Influence of water on the surface of hydrophilic and hydrophobic room-temperature ionic liquids,” Journal of the American Chemical Society, vol. 126, no. 38, pp. 11788–11789, 2004.
[9]
K. A. Mauritz and R. B. Moore, “State of understanding of Nafion,” Chemical Reviews, vol. 104, no. 10, pp. 4535–4585, 2004.
[10]
J. S. Lee, N. D. Quan, J. M. Hwang et al., “Polymer electrolyte membranes for fuel cells,” Journal of Industrial and Engineering Chemistry, vol. 12, no. 2, pp. 175–183, 2006.
[11]
M. Saito, N. Arimura, K. Hayamizu, and T. Okada, “Mechanisms of ion and water transport in perfluorosulfonated ionomer membranes for fuel cells,” Journal of Physical Chemistry B, vol. 108, no. 41, pp. 16064–16070, 2004.
[12]
A. Siu, J. Schmeisser, and S. Holdcroft, “Effect of water on the low temperature conductivity of polymer electrolytes,” Journal of Physical Chemistry B, vol. 110, no. 12, pp. 6072–6080, 2006.
[13]
M. Doyle, S. K. Choi, and G. Proulx, “High-temperature proton conducting membranes based on perfluorinated ionomer membrane-ionic liquid composites,” Journal of the Electrochemical Society, vol. 147, no. 1, pp. 34–37, 2000.
[14]
C. Schmidt, T. Glück, and G. Schmidt-Naake, “Modification of Nafion membranes by impregnation with ionic liquids,” Chemical Engineering and Technology, vol. 31, no. 1, pp. 13–22, 2008.
[15]
L. A. Neves, J. Benavente, I. M. Coelhoso, and J. G. Crespo, “Design and characterisation of Nafion membranes with incorporated ionic liquids cations,” Journal of Membrane Science, vol. 347, no. 1-2, pp. 42–52, 2010.
[16]
C. Iojoiu, M. Martinez, M. Hanna et al., “PILs-based Nafion membranes: a route to high-temperature PEFMCs dedicated to electric and hybrid vehicles,” Polymers for Advanced Technologies, vol. 19, no. 10, pp. 1406–1414, 2008.
[17]
N. P. Berezina, S. V. Timofeev, and N. A. Kononenko, “Effect of conditioning techniques of perfluorinated sulphocationic membranes on their hydrophylic and electrotransport properties,” Journal of Membrane Science, vol. 209, no. 2, pp. 509–518, 2002.
[18]
I. A. Stenina, P. Sistat, A. I. Rebrov, G. Pourcelly, and A. B. Yaroslavtsev, “Ion mobility in Nafion-117 membranes,” Desalination, vol. 170, no. 1, pp. 49–57, 2004.
[19]
V. S. Silva, B. Ruffmann, S. Vetter et al., “Mass transport of direct methanol fuel cell species in sulfonated poly(ether ether ketone) membranes,” Electrochimica Acta, vol. 51, no. 18, pp. 3699–3706, 2006.
[20]
M. Lavorgna, M. Gilbert, L. Mascia, G. Mensitieri, G. Scherillo, and G. Ercolano, “Hybridization of Nafion membranes with an acid functionalised polysiloxane: effect of morphology on water sorption and proton conductivity,” Journal of Membrane Science, vol. 330, no. 1-2, pp. 214–226, 2009.
[21]
E. Cho, J. S. Park, S. S. Sekhon et al., “A study on proton conductivity of composite membranes with various ionic liquids for high-temperature anhydrous fuel cells,” Journal of the Electrochemical Society, vol. 156, no. 2, pp. B197–B202, 2009.
[22]
J.-S. Park, M.-S. Shin, S. S. Sekhon, Y.-W. Choi, and T.-H. Yang, “Effect of annealing on Nafion recast membranes containing ionic liquids,” Journal of the Korean Electrochemical Society, vol. 14, pp. 9–15, 2011.
[23]
L. A. Neves, I. M. Coelhoso, and J. G. Crespo, “Methanol and gas crossover through modified Nafion membranes by incorporation of ionic liquid cations,” Journal of Membrane Science, vol. 360, no. 1-2, pp. 363–370, 2010.
[24]
R. Fortunato, C. A. M. Afonso, J. Benavente, E. Rodriguez-Castellón, and J. G. Crespo, “Stability of supported ionic liquid membranes as studied by X-ray photoelectron spectroscopy,” Journal of Membrane Science, vol. 256, no. 1-2, pp. 216–223, 2005.
[25]
F. J. Hernández-Fernández, A. P. de los Ríos, F. Tomás-Alonso, J. M. Palacios, and G. Víllora, “Preparation of supported ionic liquid membranes: influence of the ionic liquid immobilization method on their operational stability,” Journal of Membrane Science, vol. 341, no. 1-2, pp. 172–177, 2009.
[26]
R. Fortunato, L. C. Branco, C. A. M. Afonso, J. Benavente, and J. G. Crespo, “Electrical impedance spectroscopy characterisation of supported ionic liquid membranes,” Journal of Membrane Science, vol. 270, no. 1-2, pp. 42–49, 2006.
[27]
J. D. Ramos, C. Milano, V. Romero et al., “Water effect on physical-chemical and elastic parameters for a dense cellulose regenerated membrane: transport of different aqueous electrolyte solutions,” Journal of Membrane Science, vol. 352, no. 1-2, pp. 153–159, 2010.
[28]
Celula Laura Ceramics International and J. Benavente, “Electrical characterization of membranes,” in Monitoring and Visualizing Membrane-Based Process, C. Güell, M. Ferrando, and F. López, Eds., Wiley, 2009.
[29]
M. J. Ariza, J. Benavente, and E. Rodríguez-Castellón, Handbook of Membranes: Properties, Performance and Applications, Nova Science Publishers, New York, NY, USA, 2009.
[30]
National Institute of Standard and Technology (NIST), X-Ray Data Base, Gaithersburg, Md, USA, 1997.
[31]
V. Di Noto, E. Negro, J. Y. Sanchez, and C. Lojoiu, “Structure-relaxation interplay of a new nanostructured membrane based on tetraethylammonium trifluoromethanesulfonate ionic liquid and neutralized nation 117 for high-temperature fuel cells,” Journal of the American Chemical Society, vol. 132, no. 7, pp. 2183–2195, 2010.
[32]
J. R. Macdonalds, Impedance Spectroscopy, Wiley, New York, NY, USA, 1987.
[33]
J. Benavente, “Use of impedance spectroscopy for characterization of membranes and the effect of different modifications,” in Membrane Modification: Technology and Applications, N. Hilal, M. Khayet, and Ch. J. Wright, Eds., CRC Press, 2012.
[34]
J. Benavente, “Electrochemical impedance spectroscopy as a tool for characterization of membranes in contact with electrolyte solutions in Surface Electrical Phenomena,” in Membranes and Microchannels, A. Szymczyk, Ed., Transworld Reserach Network, 2008.
[35]
N. Lakshminarayanaiah, Transport Phenomena in Membranes, Academic Press, New York, NY, USA, 1969.
[36]
A. Ca？as, M. J. Ariza, and J. Benavente, “Characterization of active and porous sublayers of a composite reverse osmosis membrane by impedance spectroscopy, streaming and membrane potentials, salt diffusion and X-ray photoelectron spectroscopy measurements,” Journal of Membrane Science, vol. 183, no. 1, pp. 135–146, 2001.
[37]
C. Torras, X. Zhang, R. García-Valls, and J. Benavente, “Morphological, chemical surface and electrical characterizations of lignosulfonate-modified membranes,” Journal of Membrane Science, vol. 297, no. 1-2, pp. 130–140, 2007.
