Interplay between the Structure and Relaxations in Selemion AMV

Article pubs.acs.org/JPCC

Interplay between the Structure and Relaxations in Selemion AMV Hydroxide Conducting Membranes for AEMFC Applications Guinevere A. Giffin,† Sandra Lavina,† Giuseppe Pace,‡ and Vito Di Noto*,† †

Dipartimento di Scienze Chimiche, Università di Padova, Via Marzolo 1, I-35131, Padova (PD), Italy Dipartimento di Scienze Chimiche, Istituto di Scienze e Tecnologie Molecolari, ISTM-CNR and INSTM, Via Marzolo 1, I-35131 Padova (Pd), Italy

‡

ABSTRACT: Selemion AMV was studied to examine the relationship between the membrane chemical structure and its properties. The structure of AMV consists of two components: a functionalized polystyrene copolymer containing the ionexchange moieties and PVC which is likely blended with copolymer. The PVC is primarily responsible for the mechanical properties of the membrane but seems to undergo degradation during the anion-exchange process that leads to a reduction of the storage modulus. The functionalized polystyrene copolymer with the ion-exchange groups is primarily responsible for the electrical properties of the membrane. The AMV and AMVOH membranes exhibited conductivities of 2 and 7 mS cm−1 at 25 °C, respectively. The membrane exhibited Arrhenius behavior in all conditions that suggests the dynamics of the membrane are not significantly involved in the mechanism of long-range conduction. The AMVOH membrane has two predominant pathways of charge exchange through the membrane: one through the bulk of the hydrophilic domains that is associated with the electrode polarization and another along the interface between the hydrophobic and hydrophilic domains associated with the interfacial polarization. These two separate phenomena provide significant percolation pathways that merge into a single contribution to the long-range charge migration at high water content. However, conduction along the interface does not provide an important contribution to the long-range conductivity in the chloride form of the membrane.

1. INTRODUCTION Anion exchange membrane fuel cells (AEMFCs) have advantages over both proton exchange membrane fuel cells (PEMFCs) and traditional alkaline fuel cells (AFCs). As compared to PEMFCs, AEMFCs have more facile kinetics at the electrodes.1−3 Therefore, noble-metal catalysts that are the current standard in PEMFCs are not necessary.1,4 Instead, cheaper metals can be used such as silver- or nickel-based catalysts.1 In addition, AEMFCs tend to have a reduced fuel crossover as compared to PEMFCs due to the ion migration that opposes fuel migration from the anode to the cathode.1 The solid electrolyte membrane obviates the need of highly corrosive liquid electrolytes such as KOH required in AFCs. In addition, the use of polymeric membranes can reduce the susceptibility of solid carbonate formation within the electrolyte due to the immobility of the counterions in the ionomer membrane.2,3 There are several challenges that must be overcome before AEMFC technology can undergo widespread implementation. One important challenge is design of a polymeric membrane that is highly stable, has a low susceptibility to CO2, and has high hydroxide conductivity given the lower mobility of the hydroxide ions as compared to protons.1−3 In order to design novel membrane materials that can meet these requirements, a © 2012 American Chemical Society

better understanding of the current state-of-the-art membranes is required. Recent reviews have detailed the many synthetic avenues implemented to produce new membranes that meet the required stability and conductivity requirements. 2,3 However, only a few studies have been done to understand the fundamentals of hydroxide transport through anion exchange membranes.3−5 Anion exchange membranes have been primarily used for water purification applications.6−9 With this goal in mind, the properties of anion exchange membranes have been extensively studied in the presence of electrolyte solutions and in a halide anionic form.10,11 Unfortunately, the performance of membranes in the presence of an electrolyte solution is not likely comparable to their performance for the transport of hydroxide ions in the presence of water. In addition, it is likely that the vast amount of research that has been conducted on transport in proton exchange membranes is not completely transferable to AEMs due to the lower mobility of hydroxide ions and the different types of ion exchange moieties. Therefore, detailed fundamental studies on the correlation between the structure Received: September 24, 2012 Revised: October 26, 2012 Published: October 29, 2012 23965

dx.doi.org/10.1021/jp3094879 | J. Phys. Chem. C 2012, 116, 23965−23973

The Journal of Physical Chemistry C

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the dry samples, and CO2 absorption for the hydroxideexchanged samples. The geometrical cell constant was determined by measuring the electrode−electrolyte contact surface and the distance between electrodes with a micrometer. Corrections for thermal expansion of the cell were not used. The electrical spectra measured as a function of relative humidity were measured using pressure, temperature, dew point broadband electrical spectroscopy (PTD-BES). PTD-BES has been previously described in detail18 and is only summarized here. The AMVOH membrane was sandwiched between porous steel electrodes covered with platinum (GKT Sinter Metals, Germany) to facilitate water exchange into the sample. The sample holder was placed into a special homemade chamber which allowed the control of the desired parameters with a high precision.18 The humidity inside the cell was generated with a 5 SLPM 750/2200W DPH Dew Point Humidifier from Arbin Instruments (College Station, TX). The carrier gas was N2, and the sample was maintained under N2 while the sample was dried as the temperature was increased. The humidity, dew point (Tdp), and cell temperature (Tcell) were controlled and monitored using an optical system based on a laser beam reflected on a Peltier stainless steel mirror (Optidew Remote from Michell Instruments, Milan, Italy) and a Pt100 sensor. The complex impedance was measured with an Agilent 4294A impedance analyzer between frequencies of 40 and 107 Hz at Tcell = 30, 35, 50, and 70 °C and various Tdp ≤ Tcell.

and the mechanism of hydroxide transport through anion exchange membranes are necessary to devise new and more efficient hydroxide conducting membranes. In this work, Selemion AMV, a commercial reference AEM, which was obtained from Asahi Glass Company, is studied in both the chloride and hydroxide anion form. Selemion AMV has been extensively studied for electrodialysis applications,7,9−11 but it has never been examined for application in an AEMFC. AMV is a functionalized polystyrene-based copolymer containing quaternary ammonium groups. Several membranes containing functionalized benzyl moieties have been proposed for application in AEMFCs.12−17 Therefore, Selemion AMV is a particularly good starting point for the definition of the structure−property relationships that govern the physical and chemical properties which influence hydroxide transport and the conduction mechanism in such AEMs.

2. EXPERIMENTAL SECTION 2.1. Materials. The Selemion AMV ionomer membranes in the chloride form were provided by the Asahi Glass Co. (Japan). The membrane has a thickness of 0.11−0.15 mm. The anion exchange was carried out under a nitrogen atmosphere by soaking each 5 × 5 cm piece of membrane in ∼50 mL of 1 M KOH for 5, 12, and 3 h with a new solution for each time increment. The membranes were then immersed three times for 1 h in degassed double-distilled water to remove any excess KOH. After the anion exchange, the AMVOH membranes were maintained in a nitrogen atmosphere in double-distilled water to prevent the absorption of CO2. The “wet” samples were used as prepared. The “dry” samples were obtained by drying the polymers for 48 h under vacuum at room temperature. The “dry” samples were maintained under an argon atmosphere. 2.2. Instruments and Methods. Thermogravimetric analyses (TGA) were performed with a High Resolution TGA 2950 (TA Instruments) thermobalance working under a N2 flux of 100 cm3 min−1 and with a resolution of 1 μg. A 2920 differential scanning calorimeter (TA Instruments) equipped with a liquid nitrogen cooling system operating under a nitrogen flux of 30 cm3/min with a heating rate of 3 °C/min and a modulation of ±0.95 °C every 60 s in the temperature range from −150 to 350 °C was used to probe the thermal phase transitions. Samples of ∼4 mg were hermetically sealed in an aluminum pan. Dynamic mechanical analyses (DMA) were carried out with a TA Instruments DMA Q800 instrument equipped a film/fiber tension clamp. Data were collected between −10 and 200 °C with a heating rate of 4 °C min−1 by subjecting a rectangular film with dimensions of ca. 25 (height) × 6 (width) × 0.11 (thickness) mm to an oscillatory sinusoidal tensile deformation with an amplitude of 4 μm at 1 Hz and a 0.05 N preloading force. The wet and dry electrical spectra were measured with broadband electrical spectroscopy (BES) in the frequency range from 10 mHz to 10 MHz using a Novocontrol Alpha analyzer over the temperature range from −105 to 105 °C. The temperature was controlled using a homemade cryostat operating with an N2 gas jet heating and cooling system. Temperatures were measured with accuracy greater than ±0.4 °C. The membranes were sandwiched between two circular gold electrodes inside a closed cylindrical Teflon cell. A small amount of water was added to the cell for the wet membrane. The cell was closed to avoid water loss during the measurement of wet samples, water adsorption before the measurement of

3. RESULTS AND DISCUSSION 3.1. AMV Structure and Hydroxide Exchange. The exact structure of Selemion AMV is not known. However, recent studies11,19 have suggested that AMV is likely a blend of poly(vinyl chloride) (PVC) and a copolymer synthesized from styrene, chloromethylstyrene, and divinylbenzene. The copolymer is then functionalized by the introduction of an ammonium-based ion exchange group. The possible functional groups contained in this copolymer are shown in Figure 1. The

Figure 1. Possible functional groups contained within Selemion AMV.

ion exchange capacity is ∼1.9 mequiv/g.10 SEM studies11,20 have shown that the membrane has a basket-weave macroscale morphology, where the fibers of the functionalized copolymer blended with PVC are interwoven. The anion exchange process clearly illustrates one of the fundamental problems with many AEMs currently existing.4 Within 1 h of the initial immersion of the membrane in the 1 M KOH solution, the membrane changed from a pale yellow transparent material to an opaque black color. It has been suggested that this color change is due to the dehydrochlorination of the PVC component which was blended with the membrane for mechanical stability.4 Therefore, it is expected that the degradation of the PVC will have a significant impact on the mechanical properties of the hydroxide-exchanged membrane. 3.2. Thermal Analysis. The thermal stability and thermal transitions were studied with thermogravimetric analysis (TGA) and modulated differential scanning calorimetry (mDSC), respectively. The TGA profiles, shown in Figure 2, show a gradual mass loss until the first major thermal 23966



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transition of the functionalized PS moieties in the membrane. This transition will be discussed again with respect to the electrical spectra. There is an endothermic peak at −18 °C, followed by an exothermic peak at 27 °C. These peaks are probably due to the melting and subsequent recrystallization of small domains of the copolymer.23 Finally, there is an exothermic peak at ∼100 °C that is only present in the hydroxide-exchanged membrane. This peak is likely due to the elimination of water produced from hydroxide anions. 3.3. Dynamic Mechanical Analysis. The mechanical properties of AMV are investigated with DMA analysis. The DMA results reveal that the mechanical properties of the membrane are primarily derived from the PVC that was blended with the functionalized polystyrene copolymer. The storage (E′) and loss (E″) modulus for both the hydroxide and the chloride forms of AMV are shown in Figure 4. The storage

Figure 2. TGA profile of Selemion AMV in the chloride and hydroxide forms. The derivative wt % is shown in the inset.

elimination around 200 °C. The slow mass decrease is most likely due to the loss of small amounts of ammonium functionalities. The loss of the ion exchange groups at relatively low temperatures limits the operative conditions where the AEM could be employed. The overall decomposition profile is consistent with those of quaternary-ammonium polystyrene polyelectrolytes that have been previously studied.21 The first major mass elimination at ca. 200 °C is likely associated with the loss of the rest of ammonium moieties. The temperature of this event is slightly higher in the hydroxide-containing membrane. The large mass elimination at 400 °C is assigned to the decomposition of the polystyrene component. The mDSC curves of the dry AMV membrane in both the chloride and the hydroxide form are shown in Figure 3. There are several thermal transitions which can be associated with the various components of the membrane and the copolymer. There are two secondary transitions, Tg and TfPS, evident in the mDSC profiles. The Tg at ca. 90 °C is consistent with the glass transition of both PVC and PS.22 TfPS, which occurs at approximately −50 °C, is likely associated with a thermal

Figure 4. DMA storage (E′) and loss (E″) modulus of the dry Selemion AMV membranes in the chloride and hydroxide forms.

modulus of AMV shows a steady decrease until about 0 °C, which corresponds to a peak in the loss modulus. This peak is consistent with the β relaxation of pristine PVC.24 The second drop in the storage modulus corresponds to a peak in E″ and occurs at the glass transition (α) of PVC at ∼90 °C. The same behavior has been reported for pristine PVC.24 The anion exchange process results in a dramatic decrease of E′ in the hydroxide-exchanged membrane that is probably due to the degradation of the PVC during the exchange process. It is likely that degradation also produces the color change described above in section 3.1. It should be noted that at high temperatures (above αPVC) E′ of AMV and AMVOH are the same. Unlike pristine PVC, the storage modulus does not drop to zero even after the αPVC transition. This residual contribution to the mechanical properties likely comes from the functionalized copolymer component. There is a small increase in E′ above 0 °C in AMVOH. This increase is most likely due to the absorption of CO2 that is only present in significant quantities inside the instrumental setup above 0 °C. Below 0 °C, there is constant flux of nitrogen gas from the cooling system that allows the system to reach low temperatures. The N2 flux is significantly less at temperatures above 0 °C, which allows air to enter the system. The attribution of this increase in the storage modulus for the AMVOH membrane is supported by an isothermal DMA experiment (data not shown) that showed an increase in E′ over time as the membrane was exposed to air.

Figure 3. mDSC profile of the dry Selemion AMV membranes in the chloride and hydroxide forms. 23967



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Figure 5. 3D imaginary permittivity surfaces of the dry Selemion AMV membranes in the chloride and hydroxide forms.

3.4. Broadband Electrical Spectroscopy. Broadband electrical spectroscopy is a very powerful tool to study the electrical response of ionomers and to determine the conductivities, dielectric relaxations, and characteristic relaxation times of a material. These phenomena can be used to elucidate the mechanism of conduction within the material. While the mechanical properties primarily originate in the PVC component of the membrane blend, the electrical properties are primarily attributed to the functionalized polystyrene components of AMV. The conduction phenomena associated with the ion exchange groups have much higher permittivities and therefore will effectively mask the electrical response of the PVC. The 3D imaginary permittivity surfaces are shown for the dry AMV and AMVOH membranes in Figure 5. Three molecular relaxations and two electrical polarizations are visible in both surfaces. All three molecular relaxations are attributed to β-relaxation modes which originate in the local fluctuations of the dipole of the associated function groups. The highest permittivity and lowest frequency mode, the β1 relaxation, is associated with the functionalized polystyrene ion exchange groups. β2 is assigned to fluctuations of the PS phenyl groups.24 β3, which is only visible at the highest frequency and lowest temperatures, is attributed to local fluctuations of the dipoles perpendicular to the PVC chain axis.24 The assignments of the molecular relaxations are discussed in more detail later. The higher frequency polarization event, associated with the electrode polarization σEP, is due to the accumulation of charge at the interface between the sample and the electrodes. The lower frequency event, σIP, is assigned as an interfacial polarization and occurs as a result of the accumulation of charge at the interface between domains with different permittivities.25 The presence of interfacial polarization events in ionomers is indicative of a separation of phases into hydrophobic and hydrophilic domains.25 In AMV, the hydrophobic domains contain the PVC and PS components which both have low permittivities (pristine PVC: ε′ = 3.39 and pristine PS: ε′ = 2.5−2.6 at 1 kHz).22 The hydrophilic domains are composed of the functionalized PS containing the quaternary ammonium ion exchange groups, which are expected to have significantly higher permittivity values. A comparison of the 3D surfaces in Figure 5 illustrates that the intensity of the interfacial polarization phenomenon in AMVOH is larger than that in AMV. An examination of the temperature tan δ spectra, shown in Figure 6, clearly illustrates the effect of the anion exchange on the electrical response of the AMV materials. Both the β1 and the σIP events shift to lower temperatures in AMVOH. This

Figure 6. Plot of tan δ as a function of temperature for the wet and dry Selemion AMV membranes in the chloride and hydroxide forms. The curves from top to bottom are determined at the following frequencies: 10 kHz, 1 kHz, 100 Hz, 10 Hz, and 1 Hz. The curves at frequencies less than 10 kHz are vertically shifted to lower values of tan δ for clarity.

shift is most noticeable for the interfacial polarization and is due to the higher mobility of the hydroxide ion as compared to the mobility of the chloride ion. This higher mobility is evident by comparing the conductivities of the ions at infinite dilution (λ0,Cl− = 76.3 Ω−1 cm2 mol−1 and λ0,OH− = 199.1 Ω−1 cm2 mol−1)26 and results in the shift of the polarization event to a higher frequency at a given temperature. A similar trend is seen in the case of the wet membranes. Although all of the relaxation and polarization events are shifted to significantly lower temperatures due to the presence of water in the system, it is clear that the interfacial polarization event occurs at temperatures and frequencies much closer to the electrode polarization in AMVOH than in AMV. In both conditions, the intensity of the interfacial polarization is larger in the OH−-containing membrane. The 3D real conductivity surfaces, shown in Figure 7, also illustrate the intensity increase of the interfacial polarization phenomenon in AMVOH. In the σ′ representation, polarization phenomena appear as a plateau that is followed by a drop in the σ′ values at low frequency due to the polarization effect.25 This plateau corresponds to a peak in the imaginary permittivity spectra, which is related to the real conductivity by the relationship σ*(ω) = iωε0ε*(ω). The σ′ spectra can be divided into two regions delimited by the melting point of water. Below 23968



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Figure 7. 3D real conductivity surfaces of the wet Selemion AMV membranes in the chloride and hydroxide forms.

the melting point of water at the highest frequencies, the molecular relaxation phenomena are characterized by the typical sharp increase in the σ′ values.25 In AMVOH, two clear plateaus represent two distinct and significant polarization phenomena. The lower frequency plateau is present in the chloride form of the AMV membrane at much lower frequencies and only at temperatures much closer to 0 °C. As was shown in the temperature tan δ spectra, the hydroxide exchange has resulted in a shift of this interfacial polarization event to higher frequencies. In AMVOH above 0 °C, the electrode and interfacial polarization events likely merge into a single contribution when liquid water is present within the hydrophilic domains. Additionally, there is a new low-frequency contribution that probably does not make a large contribution to the overall σ′ values. The conductivity values associated with the polarization events, the dielectric strengths, and the characteristic relaxations times attributed to the molecular relaxations are obtained by fitting the experimental profiles of ε*(ω) = ε′(ω) − iε″(ω) with eq 1.25 ⎛ σ ⎞ ε*(ω) = −i⎜ 0 ⎟ + ⎝ ωε0 ⎠ 3

+

∑ k=1

2

∑ j=1

Figure 8. Conductivity values associated with the polarization phenomena for the Selemion AMV membranes in the chloride and hydroxide forms.

between the conductivity values of the wet and the dry conditions for both AMV and AMVOH. Water in the membrane acts to solvate the ion exchange groups and reduces the electrostatic interactions between the anion and the cation. The increase in the intensity of the interfacial polarization phenomenon, which was evident in Figures 7 and 8, is also visible here as an increase in the values of the associated conductivity. The conductivity values of AMVOH are about half an order of magnitude higher than those of AMV. This increase in the conductivity is likely due to the higher mobility of the hydroxide ion as compared to the chloride ion.26 For AMVOH below the melting point of water and in dry conditions, the values of σIP are less than an order of magnitude lower than σEP. In these conditions, the conductivity values imply that there are two significant pathways for ion migration through the membrane that contribute to the overall bulk conductivity and σTOT = σEP + σIP. This trend does not occur under any conditions in the chloride form of AMV. In the case of AMV, the conductivity associated with the electrode polarization can be approximated as the total bulk conductivity. The conductivity σEP can be considered as conduction through the bulk of the hydrophilic domains, while σIP is associated with charge migration along the interface between the hydrophobic and hydrophilic domains. The difference between the contribution of σIP in the Cl− and OH− forms of AMV can likely be attributed to a stronger interaction between the OH− ion and the amine groups as compared to the Cl− ion. Therefore, it can be assumed that in AMVOH the interface between the hydrophobic and hydrophilic domains provides a good alternative percolation pathway for hydroxide ions in the AMV membrane.

σj(iωτj)γj iω[1 + (iωτj)γj ]

Δεk + ε∞ [1 + (iωτk)μk ]

(1)

The first term describes the conductivity that occurs at frequencies significantly below the experimentally measured range. The second term describes the polarization phenomena. The variables σj and τj are the conductivity and relaxation time associated with the jth event, while γj is a shape parameter that describes the broadness and asymmetry of the jth peak. The third term represents the molecular relaxations through a Cole−Cole type equation,25 where k corresponds to the events β1, β2, or β3. ω = 2πf is the angular frequency of the electric field, τk is the relaxation time of the kth event of intensity Δεk, and μk is a shape parameter bound to the distribution of the relaxation times associated with the kth event. The fourth term, ε∞, is the electronic contribution to the permittivity of the material. Equation 1 is used to simultaneously fit the experimental tan δ, ε′, ε″, σ′, and σ″ spectra which are determined from the permittivity spectra according to the relationship between the complex conductivity and complex permittivity: σ*(ω) = iωε0ε*(ω). The values of log σi are reported as a function of 1000/T in Figure 8. There is a difference of at least 4 orders of magnitude 23969



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The conductivity values follow Arrhenius behavior in all conditions for both the AMV and AMVOH membranes, which indicates that ion migration occurs via hopping processes between coordination sites and is not significantly affected by the dynamics of the polymeric matrix. This behavior is different from what is typically exhibited by proton exchange membranes (PEMs). In PEMs above the alpha transition of the host matrix or in the presence of liquid water, the charge migration occurs via a membrane-mediated process.27−29 The lack of a correlation between the dynamics of the host polymer was also seen in a diblock functionalized PS/poly(methylbutylene) anion exchange membrane.30 It can be hypothesized that the bulky quaternary ammonium ion exchange moieties attached to large phenyl groups inhibit a membrane-mediated process. The activation energies associated with the Arrhenius fits of the conductivity values and their associated relaxation frequencies (data not shown) are given in Table 1. In the dry

The dielectric strengths and the characteristic relaxation frequencies associated with the molecular relaxation phenomena are shown in Figure 9. All the molecular relaxation

Table 1. Activation Energies Associated with the Conductivity Values and the Characteristic Relaxation Frequencies of the Conductivity and Molecular Relaxations for AMV and AMVOHa

Figure 9. Dielectric strength and characteristic relaxation frequencies for the molecular relaxations of AMV and AMVOH.

phenomena are associated with the local fluctuations of dipoles of the various functional groups of the membrane. β1 is associated with the functionalized polystyrene ion exchange groups, β2 is assigned to fluctuations of the PS phenyl groups,24 and β3 is attributed to the PVC blended with the functionalized PS copolymer.24 All the molecular relaxations follow Arrhenius behavior as is expected for β modes.25 The β1 relaxation can be divided in two temperature regions in the dry membranes delimited by a temperature equal to −45 °C. This temperature is consistent with the TfPS transition that was reported in the thermal analysis in section 3.2. The activation energies reported in Table 1 for the β1 relaxation are lower below −45 °C, while the dielectric strengths steadily increase below the transition temperature but remain relatively constant after −45 °C. Both of these trends are consistent with a thermal transition occurring within a material. The dielectric strength of β1 is significantly larger than either β2 or β3, which is consistent with the assignment of this relaxation to the functionalized polystyrene group which would have a larger dipole moment than PS. The activation energies for β1 are higher for AMVOH than for AMV, which supports a stronger interaction between the hydroxide ion and the ammonium groups than between the chloride and the ammonium groups as suggested by the conductivity activation energies. The frequencies associated with the β2 and β3 relaxations are consistent with those of pristine PS and PVC, respectively, which supports their assignments.24 In the wet membranes, all the relaxation modes shift to higher frequencies due to membrane plasticization by water. The dielectric strengths associated with the β1 relaxation are higher than those normally expected for molecular relaxations,25 but this is likely due to water solvating the ionic groups. A comparison of the activation energies associated with the molecular relaxations and those of the conduction phenomena can be used to deduce a correlation between the dynamics of the polymer matrix and the charge migration phenomena. Typically when the dynamics of the polymer mediate charge migration, the activation energies of molecular relaxations of the involved moieties are quite close to those of the conductivity values.29 In the AMV membranes, there is little correlation between these activation energies. In the dry conditions, the activation energies of the molecular relaxations

Ea (kJ/mol) dry AMV σEP > 0 °C σEP < 0 °C σIP > 0 °C σIP < 0 °C fσEP > 0 °C fσEP < 0 °C fσIP > 0 °C

wet AMVOH

82 ± 1

106 ± 2

85 ± 5

128 ± 3

64 ± 1

94 ± 1

AMV 23 46 9 105 13

± ± ± ± ±

1 1 2 11 2

47 ± 1

134 ± 4

fβ1 > −45 °C

64 ± 1

85 ± 2

31 ± 2

fβ1 < −45 °C

17 ± 2

10 ± 2

55 ± 1

fβ2

51 ± 1

51 ± 1

70 ± 2

fβ2 high T

12 38 28 38 7

± ± ± ± ±

1 1 2 1 1

43 ± 1 10 ± 3

39 ± 9

fσIP < 0 °C

AMVOH

64 ± 7

30 ± 1 70 ± 2

105 ± 15

a All of the activation energies were determined by fitting the experimental data with the Arrhenius equation and are expressed in kJ/mol.

membrane, the activation energies associated with AMVOH are higher than those of AMV. This increase in activation energy is likely due to strong electrostatic interactions between the hydroxide ion and the quaternary ammonium groups. In contrast, the activation energies of the wet AMVOH are lower than those of AMV that, as expected, suggests a more facile charge migration in the hydroxide form of the membrane due to hydroxyl/water exchange effects. This conclusion is in agreement with the higher conductivity values associated with the AMVOH. It should be noted that in the wet membrane below the melting point of water, the activation energy associated with the interfacial conductivity of AMVOH is almost an order of magnitude smaller than that of AMV. It can be inferred from the increase in the conductivity that the water in the membrane begins to melt around −5 °C. Although there is not much difference between the activation energies of σIP above the melting point of water, recall from the discussion of Figure 7 that this σIP is the result of a new low-frequency component that contributes little to the overall conductivity. 23970



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are lower than those of the conduction phenomena, while in the wet conditions they are higher. In both cases, the lack of a correlation supports the conclusion that the dynamics of the polymer are not significantly involved in the charge migration process. The PTD-BES technique allows the electrical response of the AMVOH membrane to be determined as a function of relative humidity.18 The electrical response of membranes when water vapor is equilibrated within the membrane is different than when the membrane is immersed in liquid water.18 The real part of the conductivity is shown in Figure 10 at several cell

Figure 11. Conductivity values associated with the interfacial and electrode polarization phenomena plotted as a function of water activity.

similar to that of silica, which implies that dynamics of the AMV polymeric matrix have little effect on the anion exchange processes. A second interesting conclusion can be drawn by examining the conductivities associated with the interfacial and electrode polarization phenomena in Figure 11. At low water activities, these two polarization phenomena result in two separate contributions that are separated by several orders of magnitude. However as the water content within the membrane increases, the conductivity values converge. This behavior is consistent with the idea that the two separate phenomena merge into a single contribution in the presence of liquid water.

Figure 10. Real part of the conductivity at Tcell = 30, 35, 50, and 75 °C and various dew points for the AMVOH membrane.

temperatures and various dew points. Increasing Tdp, which implies an increase in the relative humidity, results in an increase in both the intensity and the frequency of the interfacial polarization phenomenon. Therefore, as the amount of water vapor equilibrated within the membrane rises, the conductivity along the interface plays an increasingly important role in the overall conduction mechanism. It should be noted that the decrease in the σ′ values at high frequencies ( f > ca. 106) is due to inductive effects within the cell. It is interesting to compare the behavior at 30 and 70 °C. At the highest dew point at 30 °C, there are still two separate contributions. However, in the same conditions at 70 °C, the interfacial and electrode polarization phenomena are no longer distinguishable. At lower dew points at 70 °C, there are two clear contributions as with the lower cell temperatures. This behavior supports the conclusion that the two polarization phenomena merge into a single contribution, and both contribute to the overall conductivity. The conductivity values associated with the electrode and interfacial phenomena were determined by fitting the data with eq 1. These conductivity values, which are plotted as a function of the water activity in Figure 11, show little dependence on the temperature of the sample. Instead, these values fall onto a master curve and seem to only depend on the water content within the membrane. The formation of a master curve has some important implications about the role of the membrane in the conduction mechanism. The conductivity values of hydrophilic mesoporous silica showed a similar behavior as a function of water activity in the sample.31 In a system such as silica, the dynamics of the silica cannot contribute to the longrange conductivity.31 Therefore, it can be inferred that a master curve is likely formed when the dynamics of the host matrix do not significantly contribute to long-range mechanism of conduction. Interestingly, AMVOH shows a behavior that is

4. CONCLUSIONS The study of the AMV membrane in both the chloride and hydroxide results in important conclusions that can be drawn about the properties of this anion exchange membrane. The structure of AMV consists of two components: the functionalized polystyrene copolymer containing the ion-exchange moieties and PVC which is likely blended with the copolymer. These two components play separate but important roles within the membrane. The PVC is primarily responsible for the mechanical properties of the membrane. However, PVC does not seem to highly stable in the basic environment encountered during the anion exchange process as was evident from the reduction of the mechanical properties in AMVOH. The functionalized polystyrene copolymer with the ion-exchange groups is primarily responsible for the electrical properties of the membrane. The electrical properties determined here are in good agreement with previously published results for AMV (7 mS cm−1 as calculated from the area specific resistance and the membrane thickness reported).3 The AMV and AMVOH membrane exhibited conductivities of 2 and 7 mS cm−1 at 25 °C, respectively. In all conditions, the conductivity of the membrane followed Arrhenius behavior which suggested that the dynamics of the host polymeric matrix are not significantly involved in the mechanism of long-range conduction. This conclusion was supported by the results of the PTD-BES experiments, which produced a master curve as a function of water activity. The formation of a master curve implies that the dynamics of the host matrix are not involved in the charge exchange processes. The AMVOH membrane showed two predominant pathways of long-range charge exchange events through the membrane: one through the bulk of the hydrophilic domains that is associated with the electrode 23971



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Figure 12. Comparison of ion exchange in PEMs and AEMs. In general, in PEMs, the dynamics of the matrix facilitate the conduction process. In comparison, the matrix dynamics make no significant contribution in AEMs.

the PTD-BES measurements. G. Giffin thanks Regione del Veneto (SMUPR n. 4148, Polo di ricerca del settore fotovoltaico) for financial support.

polarization and another along the interface between the hydrophobic and hydrophilic domains associated with the interfacial polarization. These pathways are present as two different contributions in dry conditions, in the presence of solid water below 0 °C and when water vapor is equilibrated in the membrane. Only one contribution is visible when there is liquid water in the membrane. The relative humidity experiments show that these two pathways converge as the water content within the membrane increases. Therefore, it can be concluded that the single contribution that occurs in the presence of liquid water is really the sum of the two separate contributions that provide significant percolation pathways for long-range charge migration. In addition, it is important to note that the conduction component along the interface did not make as large a contribution to the total conductivity in the chloride form of the membrane. Therefore, it can be inferred that the stronger interactions between the mobile OH− ion and the ion exchange groups cannot be overlooked during a study of the charge conduction mechanism in anion exchange membranes. This study of the commercial Selemion AMV membrane has provided some important insights into the properties of anion exchange membranes and has highlighted some significant differences between anion and proton exchange membranes, particularly in terms of the contribution of the polymeric matrix to the charge migration process. These differences are highlighted in Figure 12. In general, in PEMs, the dynamics of the matrix facilitate the conduction process. In comparison, the matrix dynamics make no significant contribution in AEMs. More in-depth studies on the electrical properties of other types of anion exchange materials are necessary to determine if the results presented here can be generalized to AEMs and to achieve a good understanding of the general factors that influence the charge migration processes in AEMs.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +39 049 8275229. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Asahi Glass Company for providing the Selemion AMV membranes and M. Piccolo for assistance with 23972



Article

(24) McCrum, N. G.; Read, B. E.; Williams, G. Anelastic and Dielectric Effects in Polymeric solids; Dover Publications: New York, 1967. (25) Di Noto, V.; Giffin, G. A.; Vezzù, K.; Piga, M.; Lavina, S. Broadband Dielectric Spectroscopy: A Powerful Tool for the Determination of Charge Transfer Mechanisms in Ion Conductors. In Solid State Proton Conductors: Properties and Applications in Fuel Cells; Knauth, P., Di Vona, M. L., Eds.; Wiley: Chichester, 2012. (26) Atkins, P.; de Paula, J. Physical Chemistry, 8th ed.; Oxford University Press: Oxford, 2006. (27) Di Noto, V.; Boaretto, N.; Negro, E.; Stallworth, P. E.; Lavina, S.; Giffin, G. A.; Greenbaum, S. G. Int. J. Hydrogen Energy 2012, 37, 6215−6227. (28) Di Noto, V.; Piga, M.; Giffin, G. A.; Pace, G. J. Membr. Sci. 2012, 390−391, 58−67. (29) Giffin, G. A.; Piga, M.; Lavina, S.; Navarra, M. A.; D’Epifanio, A.; Scrosati, B.; Di Noto, V. J. Power Sources 2011, 198, 66−75. (30) Tsai, T.-H.; Maes, A. M.; Vandiver, M. A.; Versek, C.; Thorn, M.; Giffin, G. A.; Di Noto, V.; Seifert, S.; Tuominen, M.; Herring, A. M.; Coughlin, E. B. Manuscript to be submitted. (31) Di Noto, V.; Lavina, S.; Wintersgill, M. C.; Fontanella, J. J. Phys. Chem. Chem. Phys. 2010, 12, 5993−5997.

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Interplay between the Structure and Relaxations in Selemion AMV

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