Sulfonated aromatic ionomers: Analysis of proton conductivity and proton mobility

Solid State Ionics 225 (2012) 255–259

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Sulfonated aromatic ionomers: Analysis of proton conductivity and proton mobility Philippe Knauth a,⁎, Maria Luisa Di Vona b a b

Aix-Marseille Université-CNRS, UMR 6264 : Laboratoire Chimie Provence, 13397 Marseille, France Università Roma Tor Vergata, Dip. Scienze e Tecnologie Chimiche, 00133 Roma, Italy

a r t i c l e

i n f o

Article history: Received 10 September 2011 Received in revised form 21 January 2012 Accepted 27 January 2012 Available online 25 February 2012 Keywords: Proton conductivity Polymer electrolytes Composites Fuel cells

a b s t r a c t Recent progress on proton-conducting Sulfonated Aromatic Polymers is reviewed, including composite and cross-linked ionomers. Proton conductivity and water uptake, studied as function of relative humidity, can be combined to calculate the effective proton mobility in ionomers. The plot of proton mobility versus proton concentration shows common features for various ionomers, which can be related to percolation and tortuosity of hydrated nanometric channels. Proton conductivity can even be enhanced in cross-linked ionomers, possibly due to a reduction of channel tortuosity. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Solid proton conductor membranes have many high technological applications, including hydrogen generation by water electrolysis [1,2], hydrogen and direct methanol fuel cells [3–5], and, more recently, electrochemical chlorine production [6]. At high temperature, proton-conducting solid oxides with perovskite structure have been identified as most promising membrane materials [7,8]; at low and intermediate temperature, various polymeric materials appear as the best choice, including so-called hydrated acidic polymers. In these, the hydrophobic polymer matrix contains a concentrated acidic solution inside hydrophilic nanometric domains, percolating through the membrane [9–11]. This complex nanocomposite structure must be preserved during operation, whatever the operating conditions (temperature, relative humidity…). The proton conductivity of such membranes is obviously an important criterion of choice, because it determines a part of the overpotential of the electrochemical cell, the so-called Ohmic drop [12,13], but it is not the only one. Actually, the membranes operate in an aggressive environment and under harmful experimental conditions, including highly oxidizing (and reducing) conditions, the presence of a corrosive concentrated acidic solution, and temperatures, which can be quite high for a complex polymer nanocomposite. Under these conditions, other properties such as chemical and thermal stability as well as mechanical properties become of paramount importance [12]. After unsuccessful initial experiments with polystyrene sulfonic acid [13], used in Gemini space flights in the 1960s, but which proved

⁎ Corresponding author. Fax: + 33 413 55 1808. E-mail address: [email protected] (P. Knauth). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2012.01.043

unstable under fuel cell operating conditions due to oxidation of C\H bonds, the most widely used polymers have a perfluorinated backbone and are made acidic by introduction of sulfonic acid groups in side chains of the backbone. Nafion is the most popular ionomer of this family [4,14–16], but short side-chain polymers [17,18] have also been developed more recently. The high stability of these polymers is related to the strength of the C\F bond, making however the preparation of side-chain substituted materials difficult and therefore very costly. Furthermore, polyfluorinated polymers tend to be unstable at temperatures above 80 °C [19,20]. Many researches have been developed over the years in order to find alternative and less expensive proton-conducting polymers. The most promising group of materials is probably that of Sulfonated Aromatic Polymers (SAPs) [21–24]. In this case, the polymer backbone is made of highly stable phenyl rings, which show high thermal and chemical stability due to very stable aromatic C\C and C\H bonds. The polymers can be sulfonated by a simple aromatic substitution reaction and the cost of SAPs is consequently significantly below that of perfluorinated ionomers [25,26]. However, the proton conduction mechanism is quite similar to that of Nafion that is protons are transported by a concentrated acidic solution inside nanometric percolating channels in a highly hydrophobic polymer matrix [27]. Some of the problems encountered in Nafiontype ionomers are thus found also in SAPs, including a tendency to degradation of properties at temperatures above 80 °C, especially at low relative humidity [28]. On the other hand, the polymer matrix can soften at high relative humidity, if the polymer is highly sulfonated; an excess of plasticizing water can lead to loss of mechanical resistance, with swelling of the polymer and finally dissolution in water [26,29,30]. There are two basic choices in order to further improve polymer membranes [31,32] for fuel cell and electrolyser applications:

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Capacity (IEC) were determined by titration, thermogravimetric analysis or 1H NMR spectroscopy, according to published procedures [40,41]. The repeat unit of SPEEK is shown in Scheme 1a. The synthesis procedure of SOSiPEEK has been published previously [42,43]. In short, the commercial PEEK polymer is dissolved in chlorosulfonic acid at 50 °C. This leads to a chlorosulfonated, partially crosslinked polymer, soluble in hot organic solvents such as tetrahydrofurane (THF), where it can then react with butyl-lithium (BuLi) and SiCl4 to the final product, partially cross-linked, sulfonated and silylated PEEK. The samples are called SOSiPEEK/N, where N is the molar percentage of silylated monomer units. The degree of sulfonation of the material (DS = 0.8) and its degree of cross-linking (XL = 0.2) were determined by NMR spectroscopy [36]. The repeat unit of SOSi-PEEK/50 is shown in Scheme 1b. Silylated PPSU (SiPPSU, Scheme 1c) was synthesized by metalation reaction of PPSU with BuLi, followed by electrophilic substitution by phenyl-trichlorosilane (PhSiCl3) and subsequent hydrolysis. The original polymer PPSU was added in nitrogen atmosphere to anhydrous tetrahydrofurane. The solution was kept at room temperature for 1 h then cooled to −60 °C. After 1.5 h, an excess of BuLi and tetramethylenediamine (TMEDA) were added and the solution was stirred for 2 h at − 60 °C. At this point, PhSiCl3 (Aldrich, 97%) was added and the solution was slowly warmed to room temperature and then kept at reflux for 2 h [34]. The procedure for membrane preparation was solution casting using solvents with a high boiling point, typically dimethylsulfoxide (DMSO) [42]. As a general procedure, for SOSiPEEK and XL-SPEEK, 250 mg of sample was dissolved in 30 mL of solvent. The resulting solution was stirred for 4 h, evaporated to 5 mL, cast onto a Petri dish and heated to dryness. After cooling to room temperature, the resulting membranes were peeled off and dried at 120 °C for 12 h (SPEEK) or 24 h (SOSiPEEK) for solvent removal. Further heat treatments were performed for crosslinking SPEEK (XL-SPEEK): in that case the temperature of the heat treatment must be above 160 °C for times between 3 h and 24 h, depending on the desired degree of cross-linking. For SPEEK–SiPPSU composites, around 250 mg sample (containing 93 wt.% SPEEK and 7 wt.% SiPPSU) was sonicated in 30 mL of solvent.

(i) search for new proton-conducting polymers, and (ii) modification and optimization of existing polymers. After intensive literature research and bibliography, we believe that the second approach is more promising in terms of cost and simplicity. We have therefore in recent years concentrated on improvement of existing SAPs, introducing Van der Waals bonds or covalent bonds (“cross-links”). Fig. 1 shows the basic approaches that have been developed. Van der Waals bonds are considered in composite materials, where a second phase is added and dispersed inside the polymer matrix [33]: we have studied as second phase hybrid organic–inorganic materials (such as Si-PPSU [34]) and inexpensive nanocrystalline metal oxides (such as TiO2 [35]). Covalent cross-linking bonds by SO2 bridges can be formed by clever chemical synthesis (giving hybrid ionomers called SOSiPEEK [36]) or by thermal treatment (solvothermal cross-linking, for example XL-SPEEK [37] or XL-PPSU [38]). In the following, we will present highlights of our recent research on this topic. Proton conductivity of ionomers will be particularly discussed and a common phenomenological description of effective proton mobility in hydrated acidic polymers will be presented. 2. Experimental 2.1. Membrane synthesis Poly (ether–ether–ketone) (PEEK, Victrex, 450P, MW = 38,300), poly (phenyl-sulfone) (PPSU, Solvay, MW = 46173), functionalized titanium dioxide (Titanoxide-Hydrate, Anatase, 350 m 2/g, Crenox Pigments GmbH, Germany) and all other reagent-grade chemicals were used as received. Sulfonated PEEK (SPEEK) was prepared by reaction of PEEK with concentrated sulphuric acid at 50 °C for different times, between 2 h and 5 days, depending on the desired degree of sulfonation [39]. After this time, the solution was poured, under continuous stirring, into a large excess of ice-cold water. After standing overnight, the white precipitate was filtered and washed several times with cold water to neutral pH. The sulfonated polymer (SPEEK) was dried over night at 80–85 °C. The degree of sulfonation (DS) and Ion Exchange

Bond Energy

O C

O C O

SO3H O

O

O

XL-SPEEK

SO2 O

CrossLinked Ionomers

O

C O

SO3H

O

SO3H

Si(OH)3

O

SO3H

C

O

O O

C

SOSiPEEK

O

0.4

O

SO2

O

C O

O

O

C

0.4 Si(OH)3

0.2

O

SO3H SO3H

O O

O

SO3H

O

O

C

O

O S

O

C O

O

O

O

O

Y

X

Composite Ionomers

SO3H

S

O 0.95

SO3H

SPEEK-SiPPSU SO3H

O O

O

O C O

C O

X

TiO2

O

SPEEK-TiO2

Y

Fig. 1. Schematic representation of chemical composition and bond energies of modified ionomers.

O

Si(OH)2 0.05

P. Knauth, M.L. Di Vona / Solid State Ionics 225 (2012) 255–259

257

a SO3H

O O

O

O

C

C

O

O

O Y

X

b O

SO3H

O

Si(OH)3 SO3H

C

O

O O

C

O

0.4

O

SO2

O

C O

O

O

C

0.4 Si(OH)3

0.2

O

c Ph O

O Si(OH)2 O

O S

S O

O

O O

0.95

0.05

Scheme 1. Repeat units of (a) SPEEK (DS = X), (b) SOSiPEEK (DS = 0.8) and (c) SiPPSU.

The resulting mixture was stirred for 4 h, evaporated to 5 mL, cast onto a Petri dish and heated to dryness. After cooling to room temperature, the resulting membranes were peeled off and dried at 120 °C for 12 h for solvent removal. For SPEEK–TiO2 composites, TiO2 functionalized with 10% polymethylhydrosiloxane was used (F-TiO2). SPEEK-based composite membranes containing 5 wt.% of F-TiO2 were typically prepared by dissolving 250 mg of S-PEEK in 20 ml of solvent and 12.5 mg of FTiO2 was then added [35]. After cooling to room temperature, the resulting membranes were peeled off and dried under vacuum at 80 °C for 24 h and then further dried in the oven at 140 °C for 64 h. The thickness of the resulting membranes was in the range 60–100 μm.

M(H2O) is the molar mass of water and IEC the ion exchange capacity of the ionomer (in meq/g). Through-plane conductivity measurements were carried out on membranes, 8 mm in diameter, sandwiched between gas diffusion electrodes (ELAT containing 1 mg cm − 2 Pt loading), which were pressed on the membrane faces by means of porous stainless steel discs. The membrane conductivity was determined as a function of temperature and relative humidity by impedance spectroscopy with a Solartron Sl 1260 Impedance Analyser in the frequency range 10 Hz to 1 MHz at a signal amplitude ≤100 mV. All reported conductivity values had reached a constant value for at least 2 h. Relative humidity was controlled as described in Ref. [28]. 3. Results

2.2. Membrane characterization Many different experimental techniques have been employed to characterize structure, thermal and mechanical properties of the investigated ionomers. They can be found in the original publications. Here we concentrate on proton conductivity measurements, coupled with water uptake measurements to determine the effective proton mobility in the ionomers. Water sorption isotherms were determined after equilibration with water vapor at different temperatures. The water sorption isotherms were recorded using a TA5000 thermogravimetric analyzer. Prior to all experiments, the membranes were first dried in situ 3 h at 80 °C under 0% RH (dry air flux). RH was then modified in 10% steps and the water uptake recorded at each step during 2 or 3 h. The reversibility of water uptake was checked by systematic desorption experiments, reducing RH with same steps. The hydration number λ, which is the number of water molecules per number of sulfonic acid groups, can be calculated from Water Uptake (WU, in %) according to equation: λ¼

nðH2 OÞ WU ⋅10 ¼ nðSO3 HÞ M ðH2 OÞ ⋅ IEC

ð1Þ

Fig. 2 shows the proton conductivity, σ, at 100 °C between 50 and 90% RH of typical composite membranes SPEEK–F-TiO2 and SPEEK– SiPPSU, a thermally cross-linked SPEEK membrane (XL-SPEEK) and, for comparison, a pristine SPEEK membrane with comparable DS and without any cross-linking or secondary phase. The degrees of sulfonation of the ionomers are indicated in the caption. One notices that the introduction of a second phase in composites leads to a slight decrease of proton conductivity vs. pristine SPEEK, but the conductivity remains stable even at 90% RH, whereas the conductivity of pristine SPEEK decays rapidly due to membrane swelling and loss of contact with the electrodes. This is an important advantage of composite materials; further conductivity improvement might be possible by optimization of the second phase, for example by surface functionalization [44]. More physico-chemical properties of composite ionomers can be found in references [35] (SPEEK–F-TiO2) and [34,45] (SPEEK–SiPPSU). Thermally cross-linked membranes (XL-SPEEK) show rather high proton conductivity although their DS is quite low, originating from the loss of sulfonic acid groups by the cross-linking reaction, which is an electrophilic aromatic substitution involving sulfonic acid groups [37]. A possible explanation for this surprisingly high proton

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-1

-2 λ = 10

λ=5

λ=3

-3

log[u(H+)/(cm2.V-1.s-1)]

-2

-3

-4 50

60

70

80

90

RH/%

-4 -5 -6 -7 -8

Fig. 2. Proton conductivity at 100 °C between 50 and 90% RH of pristine SPEEK (DS = 0.7, ), thermally cross-linked SPEEK (DS = 0.7, ●), SPEEK (DS = 0.9)–SiPPSU composite ▲, SPEEK (DS = 0.75)/F-TiO2 composite ○.

-9 0

0.5

1

1.5

2

log[c(H+)/(mol.L-1)] conductivity will be given later. The great potential of these inexpensive, thermally cross-linked polymers for application as durable proton-conducting membranes is evident. More data on mechanical and thermal properties can be found in reference [38]. 4. Discussion In order to better understand the influence of the polymer matrix on the conduction properties of the acidic solution inside nanometric channels, we have calculated the effective proton mobility u(H +) in the solution using the classical equation [46]:   þ u H ¼

σ F ⋅ cðHþ Þ

ð2Þ

F is Faraday's constant and c(H +) is the molar proton concentration (in mol/L), which can be calculated from the water uptake WU (in %) of ionomers, determined at the same temperature and relative humidity as the proton conductivity, according to the equation:   IEC d 100 þ ⋅ ⋅ c H ¼ WU

ð3Þ

IEC is the ion exchange capacity (in meq/g) of the ionomers and d is the density of the solution. Two assumptions are made for this calculation: 1. all the water sorbed by the membrane goes into the acidic solution, and 2. the solution density is taken as 1, which is an approximation. The effective proton mobility can then be plotted against the molar proton concentration (u(H +) = f(c(H +))) as shown in Fig. 3. In this double-logarithmic plot are represented SPEEK (DS = 0.9), SPPSU (DS = 2.0) and SOSiPEEK (DS = 0.8) at 25 °C, SPPSU at 50 °C and composite SPEEK (DS = 0.9)-SiPPSU at 100 °C. Furthermore, standard data for Nafion [47] are added for comparison and some values of hydration number are indicated. Data at different temperatures are shown in order to estimate the validity of our conclusions over a large temperature range, significant for applications. At low proton concentrations (up to about 10 mol/L), one can notice a very similar decrease of the effective proton mobility with increasing proton concentration, which can be well described by a power law u(H +) = A[c(H +)] − 3 (see also Table 1). As the negatively charged sulfonate groups are fixed on the channel surface, there is a spatial separation between them and the protons, which move inside the acidic solution. This separation leads to electrostatic forces, which cause a non-homogeneous arrangement of protons inside the solution. The decrease of effective proton mobility with increasing proton concentration is related to the preferred proton localization near sulfonate groups, limiting dissociation and leading to an effective proton trapping.

Fig. 3. Effective proton mobility in various proton-conducting ionomers versus square root of proton concentration: Nafion at 25 °C Δ; SPEEK (DS = 0.9) at 25 °C ●; SOSiPEEK (DS = 0.8) at 25 °C ■; SPPSU (DS = 2.0) at 25 °C ○ and at 50 °C □; SPEEK (DS = 0.9)– SiPPSU at 100 °C ▲. The solid lines represent least-squares fits using power laws. The best fit parameters are reported in Table 1.

The effective proton mobility u(H +) can furthermore be related, as shown for proton conductivity in reference [48], to the membrane porosity ε and tortuosity τ:   ε  B þ þ u H ¼ u H τ

ð4Þ

u(H +)° is the proton mobility in an aqueous acidic solution not contained in a polymer matrix. Porosity and tortuosity are phenomenological parameters used to describe the microstructure of the polymeric material. In our case, porosity corresponds to the amount of polymer matrix occupied by the acidic solution (ε = Vsol/Vtotal). The limiting cases ε = 0 (no acidic solution) and ε = 1 (no polymer) are obviously meaningless for our problem. Tortuosity is defined as the distance that a proton has to travel to cross the membrane divided by the membrane thickness. By definition, tortuosity τ > 1; τ = 1 corresponds to an ideal case of channels, which are ideally straight and perpendicular to the polymer surface. Porosity and tortuosity allow a mean-field description of the proton-conducting membrane: it is evident that the effective proton mobility increases with porosity and decreases with tortuosity. For ε = τ = 1, one can extrapolate for infinite dilution the proton mobility in pure water. The difference of effective proton mobility between Nafion and SPEEK observed in the plot indicates about an order of magnitude higher ratio ε/τ for Nafion, in accordance with the larger hydrophobic/ hydrophilic domain separation, leading especially to lower tortuosity. SPPSU shows also distinctive larger effective mobility than SPEEK, indicating a reduced tortuosity, related to the more rigid polymer backbone leading to a better nanophase separation. The surprisingly high proton conductivity of thermally cross-linked ionomers (Fig. 2) might also be related to a reduction of pore tortuosity between cross-linked macromolecular chains. For high proton concentrations, the curves for SPEEK, SOSiPEEK and SPPSU at 25 °C show a discontinuity, which Nafion does not

Table 1 Least-squares fit parameters above the percolation threshold for equation: log[u(H+)]= a log[c(H+)] + b (cf. Fig. 3, r: correlation coefficient). Ionomer

a

b

r

Nafion SPEEK SPPSU SPEEK–SiPPSU

− 2.9 − 2.6 − 2.9 − 3.2

− 2.0 − 2.9 − 2.1 − 1.2

0.998 0.980 0.998 0.997

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present. Composite SPEEK–SiPPSU shows a more continuous change of slope. The sudden change of slope for SPEEK and SOSiPEEK at high proton concentration (and thus low amount of water, around λ = 5, corresponding to a volume fraction of water above 0.2) can be related to the percolation threshold of hydrated channels through the polymer matrix. In fact, analytical calculations [48] and numerical simulations [49] performed in recent years have consistently established a percolation threshold of about λ = 5 for SPEEK, whereas no such threshold was found for Nafion, again due to the better nanophase separation in the latter. These two results are in very good agreement with our plot. Above the percolation threshold ϕc, the conductivity should increase as a power law: α

σ ∼ðϕ−ϕc Þ

ð5Þ

ϕ is the volume fraction of hydrophilic domains. The exponent α is a universal constant, depending on the dimensionality of the network: α ≈ 2 for a three-dimensional network [50]. Inserting Eq. (5) into Eq. (2) and recording that ϕ is inversely proportional to the proton concentration c(H +), one can now also understand the proportionality between u(H +) and c(H +) − 3 observed above the percolation threshold in Fig. 3. For randomly dispersed spherical hydrated clusters in the 3D polymer matrix, the percolation threshold for conduction, ϕc, is at a volume fraction below 0.3. In other words, sulfonic acid groups aggregate in spherical domains in SPEEK and must absorb sufficient water to achieve a hydrophilic volume fraction near 0.3 for conduction paths to traverse the polymer electrolyte [51]. The lamella structure suggested for Nafion [51] presents instead plate-like hydrophilic domains. The plates percolate through the hydrophobic matrix at a much lower volume fraction [52]. The percolation threshold is also lower for SPPSU than for SPEEK, consistent with a better nanophase separation, as explained above. In the composite SPEEK–SiPPSU, the percolation threshold is less evident and a more subtle change of slope is seen at high proton concentration: this might indicate the existence of interfacial conduction paths between the two polymers. Altogether, the presented mobility plot gives thus many interesting information on the investigated hydrated acidic polymers [53]. 5. Conclusion This article reviews recent progress in the synthesis of protonconducting sulfonated aromatic polymers. The studied polymer modifications include the introduction of Van der Waals bonds in composite polymers and covalent bonds in cross-linked polymers. The proton conductivity of these materials is stable at 100 °C under high relative humidity, which is an important asset for application in fuel cells. The decrease of effective proton mobility with increasing proton concentration can be described by a power law; it is related to the proton localization near sulfonate groups and space charge effects. Furthermore, the effective proton mobility can be related to percolation thresholds and channel tortuosity as phenomenological parameters. When designing new polymer electrolytes to replace Nafion having high proton conductivity at low relative humidity, the shape of hydrophilic domains formed in the polymer matrix is an important point to consider. Acknowledgments The EU-FP7 (FCH-JU) project “LoLiPEM-Long-life PEM-FCH &CHP systems at temperatures higher than 100 °C” (GA 245339) is gratefully acknowledged for co-funding this work.

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