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Conductivity and selectivity of ion exchange membranes: structure-correlations
DESALINATION ELSEVIER
Desalination 147 (2002) 359-361
www.elsevier.com/Iocate/desal
Conductivity and selectivity of ion exchange membranes: structure-correlations G6rald Pourcelly lnstitut Europden des Membranes, CC 047, Universitd Montpellier 11, Place Eugbne Bataillon, 34095 Montpellier C~dex 5, France Tel. +33 (0) 467149110; Fax: +33 (0) 467149119; email: [email protected]
Received 30 January 2002; accepted 13 February 2002
Abstract
Apart from their stability in various operating conditions, the two main properties of ion-exchange membranes (IEMs) are the conductivity and selectivity, but both are difficult to obtain simultaneously at high values. This presentation deals with different ways of measurements, including specific characterization such as electrochemical impedance spectroscopy, Raman vibrational spectroscopy and radiotracers. Different transport models to reach fundamental transport parameters are also presented. Two cases of structure correlations will illustrate this topic: a cation-exchange membrane in strong basic solutions and a high-proton-conducting membrane based on sulfonated polyimide polymers. Keywords: Conductivity; Selectivity; Ion-exchange membranes; Membrane structure
The transport properties of ion-exchange membranes (IEMs) depend on their synthesis parameters such as nature o f the polymer matrix, degree o f crosslinking and nature and concentration o f the fixed ionic charges. The main properties of the so-obtained IEMs are the chemical-, thermal- and mechanical stability along with the swelling, conductivity and selectivity. The conductivity is the ability o f a membrane to conduct electrical current while the
selectivity is the ability o f a membrane to separate ions. There is no strict relationship between the synthesis parameters and the properties. Some o f them are even counteracting. Generally speaking, a high value o f conductivity does not favour a high selectivity, and a wide gap exits between microporous IEMs and sensors. The conductivity of IEMs can be measured using alternating- or direct-current methods. The former are the most suitable with either a fixed
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Toulouse, France, July 7-12, 2002.
0011-9164/02/$- See front matter © 2002 Elsevier Science B.V. All rights reserved PI1:S0011-9164(02)00609-4
360
G. Pourcelly / Desalination 147 (2002) 359-361
frequency (clip cell etc.) or applying transient techniques [electrochemical impedance spectroscopy, (EIS)]. In this case, information concerning the membrane structure [1] or the thickness of Donnan boundary layers can also be obtained [2]. Direct-current method can be applied through linear voltamperometry or chronopotentiometry [3]. The selectivity of IEMs can be measured from zero-current membrane potential (diffusional method) or from the conventional Hittorf' method based on the balance sheet between compartments. However, the most accurate method giving migrational transport numbers is from radiotracer measurements. This method can also provide self-diffusion coefficients. Coupling EIS and radiotracers gives diffusion coefficients whose values differ according to the method of measurements. At last, coupling Raman vibrational spectroscopy and radiotracers allow for a better knowledge of the ionic composition of the conductive phase of the IEM [4]. The conductivity of an IEM depends not only on the synthesis parameters but also on the synthesis protocol. Membranes grafted by irradiation (FEP-PSSA) do not exhibit specific ionic clusters. Thermoplastic membranes (Nation, Dow) show a clear phase separation between hydrophilic clusters and hydrophobic matrix with a strcure sensitive to hydration, temperature and nature of counterions. Block copolymer membranes (sulfonated polyimides) have a structure depending on the nature and length of hydrophylic and hydrophobic blocks. The selectivity of IEMs depends mainly on the synthesis parameters and significant increase of selectivity can be obtained by surface modification [5]. However, this technique is not efficient for the proton leakage reduction through anion-exchange membranes. An example of selectivity increase between monovalent and divalent cations is illustrated in Figs. 1 and 2. The first example of structure correlation concerns IEMs with different degrees of
Layer
A+
B"
j*"
,..
j I i
I L Fig. 1. Surfacemodificationofa CEM.
V Fig. 2. I-V response: (a) before, (b) atter modification. crosslinking and grafting ratio, regarding their conductivity and selectivity properties towards sodium ions (for cation-) and nitrate ions (for anion-exchange membranes) [6]. The second example concerns the wide family of sulfonated polyimides mainly focusing on their proton conductivity in order to be used in lowtemperature fuel cells. These membranes are shown to be good candidates as model compounds in order to understand the structurecorrelation property relationships in ionomer membranes. The dimension and the distribution
G. Pourcelly / Desalination 147 (2002) 359-361
o f the ionic domains can be changed by modifying the blockness nature and the rigidity o f the polymer chain [7].
[4]
References [1] G. Pourcelly, A. Oikonomou, H.D. Hurwitz and C. Gavach, Influence of the water content on the kinetics of ion transport in Nation perfluorosulphonic membranes, J. Electroanal. Chem., 287 (1990) 43-59. [2] P. Sistat, Apport des techniques dectriques de relaxation ~t la compr6hension des ph6nom~nes de transport de mati&e dans un syst~me membrane ionique-solution, Thesis, University Montpellier 2, France, 1997. [3] P. Sistat and G. Pourcelly, Chronopotentiometric response of an ion-exchange membrane in the underlimiting current range. Transport phenomena within
[5]
[6]
[7]
361
the diffusion layers, J. Membr. Sci., 123 (1997) 121131. I. Tugas, G. Pourcelly, J.L. Bribes and C. Gavach, Identification of the ionic species in anion exchange membranes equilibrated with sulphuric acid solutions by means of Raman spectroscopy and radiotracers, J. Membr. Sci., 78 (1993) 25-33. P. Sistat, G. Pourcelly, M. Boucher and C. Gavach, Electrodialysis of acid effluents containing divalent salts. Recovery of acid with a action-exchange membrane modified in situ, J. Appl. Electrochem., 27 (1997) 65-77. S. Resbeut, Propri6t~ des membranes ¢Schangeuses d'anions en relation avec leur structure, Thesis, University Montpellier 2, France, 1999. N. Comet, Relation entre la structure et les propri6t6s de membranes en polyimide sulfon6 pour pile combustible H2/O2, Thesis, University of Lyon 1, France, 1999.
Desalination 147 (2002) 359-361
www.elsevier.com/Iocate/desal
Conductivity and selectivity of ion exchange membranes: structure-correlations G6rald Pourcelly lnstitut Europden des Membranes, CC 047, Universitd Montpellier 11, Place Eugbne Bataillon, 34095 Montpellier C~dex 5, France Tel. +33 (0) 467149110; Fax: +33 (0) 467149119; email: [email protected]
Received 30 January 2002; accepted 13 February 2002
Abstract
Apart from their stability in various operating conditions, the two main properties of ion-exchange membranes (IEMs) are the conductivity and selectivity, but both are difficult to obtain simultaneously at high values. This presentation deals with different ways of measurements, including specific characterization such as electrochemical impedance spectroscopy, Raman vibrational spectroscopy and radiotracers. Different transport models to reach fundamental transport parameters are also presented. Two cases of structure correlations will illustrate this topic: a cation-exchange membrane in strong basic solutions and a high-proton-conducting membrane based on sulfonated polyimide polymers. Keywords: Conductivity; Selectivity; Ion-exchange membranes; Membrane structure
The transport properties of ion-exchange membranes (IEMs) depend on their synthesis parameters such as nature o f the polymer matrix, degree o f crosslinking and nature and concentration o f the fixed ionic charges. The main properties of the so-obtained IEMs are the chemical-, thermal- and mechanical stability along with the swelling, conductivity and selectivity. The conductivity is the ability o f a membrane to conduct electrical current while the
selectivity is the ability o f a membrane to separate ions. There is no strict relationship between the synthesis parameters and the properties. Some o f them are even counteracting. Generally speaking, a high value o f conductivity does not favour a high selectivity, and a wide gap exits between microporous IEMs and sensors. The conductivity of IEMs can be measured using alternating- or direct-current methods. The former are the most suitable with either a fixed
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Toulouse, France, July 7-12, 2002.
0011-9164/02/$- See front matter © 2002 Elsevier Science B.V. All rights reserved PI1:S0011-9164(02)00609-4
360
G. Pourcelly / Desalination 147 (2002) 359-361
frequency (clip cell etc.) or applying transient techniques [electrochemical impedance spectroscopy, (EIS)]. In this case, information concerning the membrane structure [1] or the thickness of Donnan boundary layers can also be obtained [2]. Direct-current method can be applied through linear voltamperometry or chronopotentiometry [3]. The selectivity of IEMs can be measured from zero-current membrane potential (diffusional method) or from the conventional Hittorf' method based on the balance sheet between compartments. However, the most accurate method giving migrational transport numbers is from radiotracer measurements. This method can also provide self-diffusion coefficients. Coupling EIS and radiotracers gives diffusion coefficients whose values differ according to the method of measurements. At last, coupling Raman vibrational spectroscopy and radiotracers allow for a better knowledge of the ionic composition of the conductive phase of the IEM [4]. The conductivity of an IEM depends not only on the synthesis parameters but also on the synthesis protocol. Membranes grafted by irradiation (FEP-PSSA) do not exhibit specific ionic clusters. Thermoplastic membranes (Nation, Dow) show a clear phase separation between hydrophilic clusters and hydrophobic matrix with a strcure sensitive to hydration, temperature and nature of counterions. Block copolymer membranes (sulfonated polyimides) have a structure depending on the nature and length of hydrophylic and hydrophobic blocks. The selectivity of IEMs depends mainly on the synthesis parameters and significant increase of selectivity can be obtained by surface modification [5]. However, this technique is not efficient for the proton leakage reduction through anion-exchange membranes. An example of selectivity increase between monovalent and divalent cations is illustrated in Figs. 1 and 2. The first example of structure correlation concerns IEMs with different degrees of
Layer
A+
B"
j*"
,..
j I i
I L Fig. 1. Surfacemodificationofa CEM.
V Fig. 2. I-V response: (a) before, (b) atter modification. crosslinking and grafting ratio, regarding their conductivity and selectivity properties towards sodium ions (for cation-) and nitrate ions (for anion-exchange membranes) [6]. The second example concerns the wide family of sulfonated polyimides mainly focusing on their proton conductivity in order to be used in lowtemperature fuel cells. These membranes are shown to be good candidates as model compounds in order to understand the structurecorrelation property relationships in ionomer membranes. The dimension and the distribution
G. Pourcelly / Desalination 147 (2002) 359-361
o f the ionic domains can be changed by modifying the blockness nature and the rigidity o f the polymer chain [7].
[4]
References [1] G. Pourcelly, A. Oikonomou, H.D. Hurwitz and C. Gavach, Influence of the water content on the kinetics of ion transport in Nation perfluorosulphonic membranes, J. Electroanal. Chem., 287 (1990) 43-59. [2] P. Sistat, Apport des techniques dectriques de relaxation ~t la compr6hension des ph6nom~nes de transport de mati&e dans un syst~me membrane ionique-solution, Thesis, University Montpellier 2, France, 1997. [3] P. Sistat and G. Pourcelly, Chronopotentiometric response of an ion-exchange membrane in the underlimiting current range. Transport phenomena within
[5]
[6]
[7]
361
the diffusion layers, J. Membr. Sci., 123 (1997) 121131. I. Tugas, G. Pourcelly, J.L. Bribes and C. Gavach, Identification of the ionic species in anion exchange membranes equilibrated with sulphuric acid solutions by means of Raman spectroscopy and radiotracers, J. Membr. Sci., 78 (1993) 25-33. P. Sistat, G. Pourcelly, M. Boucher and C. Gavach, Electrodialysis of acid effluents containing divalent salts. Recovery of acid with a action-exchange membrane modified in situ, J. Appl. Electrochem., 27 (1997) 65-77. S. Resbeut, Propri6t~ des membranes ¢Schangeuses d'anions en relation avec leur structure, Thesis, University Montpellier 2, France, 1999. N. Comet, Relation entre la structure et les propri6t6s de membranes en polyimide sulfon6 pour pile combustible H2/O2, Thesis, University of Lyon 1, France, 1999.