Development and characterization of crosslinked ionomer membranes based upon sulfinated and sulfonated PSU crosslinked PSU blend membranes by disproportionation of sulfinic acid groups

journal of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 139 (1998) 211-225

Development and characterization of crosslinked ionomer membranes based upon sulfinated and sulfonated PSU Crosslinked PSU blend membranes by disproportionation of sulfinic acid groups Jochen Kerres*, Wei Cui, Ralf Disson, Wolfgang Neubrand University of Stuttgart, Institute for Chemical Engineering, B6blinger Str. 72, D-70199 Stuttgart, Germany

Received 19 March 1997; received in revised form 16 September 1997; accepted 1 October 1997

Abstract Crosslinked sulfonated ion-exchange blend membranes have been produced via a new crosslinking process. The blends have been obtained from mixing PSU-SO3H and PSU-SO2H in different sulfinic acid/sulfonic acid relations in N-methyl pyrrolidone. The crosslinking process consists of the disproportionation between SOzH groups which occurs during membrane formation. These membranes have been characterized in terms of ion-exchange capacity, ion-resistance, swelling, ionpermeability, and ion-permselectivity. Some of the membranes have been applied to electro-membrane processes, as electrodialysis, and PEM fuel cells (PEM=polymer electrolyte membrane). The advantages of the sulfinate disproportionation crosslinking process are: (i) the crosslinking process is easy to do; (ii) the ion-exchange capacities of crosslinked membranes and thus their ionic resistance, swelling and permselectivity can be varied in a broad range. The crosslinked blend membranes show good thermal stabilities and are suitable for application in electrodialysis. Although the property profile of the blend membranes still has to be improved further, it is demonstrated that they are in principle suitable for application in PEM fuel cells. © 1998 Elsevier Science B.V. Keywords: PSU Udel; Sulfinate; Sulfonate; Crosslinked sulfonated PSU blend membranes; Disproportionation; Resistance;

Permeability; Permselectivity; Swelling

1. I n t r o d u c t i o n Ion-exchange membranes suitable for electromembrane processes should have the following properties:

*Corresponding author. Tel.: +49 711 641 2244; fax: +49 711 641 2242. 0376-7388/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. Pll S0376-7388(97)00253-6

(i) a high ion-conductivity which is accomplished by a high ion-exchange group content (named in the following as ion-exchange capacity (IEC)); (ii) a high ion-permselectivity which is especially important for electrodialysis (ED) because only a high membrane ion-permselectivity leads to a high energyefficiency of the ED process; (iii) moderate swelling because too much swelling leads to a poor mechanical stability of the membrane

212

J. Kerres et al./Journal of Membrane Science 139 (1998) 211-225

which is disadvantageous for the application in electro-membrane processes like ED and polymer-electrolyte fuel cells (PEFC). Zschocke and Quellmalz [1] pointed out that there is the following relation between IEC, ion-conductivity, permselectivity and swelling: when the IEC is increased, both the ion-conductivity and the swelling are increased, while the ion-permselectivity decreases, The reason for the decrease in ion-permselectivity with increasing swelling is facilitation of diffusion of coions into the swollen polymer membrane network since the Donnan exclusion does no longer work efficiently. The decrease in ion-permselectivity and mechanical stability with increasing IEC can be avoided by crosslinking the membranes. This reduces the swelling. In the literature, different procedures for the production of crosslinked ion-exchange membranes are described. The most important are: (1) Copolymerization of styrene and divinylbenzene in thin layers, subsequent sulfonation of the crosslinked membrane [2,3]. Most of the commercial ion-exchange membranes are based on this procedure, (2) "/-irradiation of teflon/FEP foils (FEP=fluori nated ethylene polymer) with subsequent grafting of styrene/divinylbenzene onto the produced radical sites, followed by sulfonation of the grafted foils [4]. Disadvantage of procedures (1) and (2) is the oxidation instability of the polystyrene/divinylbenzene network because of the weakness of tertiary C - H bonds against oxidant attack [5]. Another disadvantage of these procedures is that the thickness of the membranes is fixed either by the copolymerization conditions or by the thickness of the teflon/FEP foils, (3) Activation of the ion-exchange groups of ionomers with subsequent reaction with di- or oligofunctional crosslinkers. One example of this type of crosslinking reactions is the activation of sulfonic acid groups via conversion to the sulfonic acid chloride/ bromide or to the sulfonic acid imidazolide with subsequent reaction with (aromatic/aliphatic) diamines [6,7]. However the sulfonamide crosslinking bridges formed by this reaction are not sufficiently hydrolysis-stable. This limits the applicability of such crosslinked membranes in electro-membrane processes, Thus there is a need for the development of new, easy to do, and reproducible crosslinking processes

yielding chemically and thermally stable ionomeric membranes. This paper presents a new crosslinking procedure leading to thermally stable crosslinked ionomer membranes with good ionic conductivities and ionic permselectivities [8]. In [9], a new process for the sulfonation of arylene polymers via the metalation route shown at the exampie of poly(ethersulfone) PSU Udel T M has been presented. The process includes the following steps: (1) deprotonation of PSU ortho to the sulfone bridge in the diarylsulfone portion of the PSU monomeric repeating unit with n-butyllithium; (2)reaction of the metalated PSU with SO2 to yield sulfinated PSU; (3) oxidation of the sulfinated PSU with NaOC1, KMnO4, or H202 to yield the PSU sulfonate. In the literature dealing with low-molecular sulfinate group chemistry, reactions of sulfinate groups which are applicable for crosslinking reactions of polymers have been found. The first reaction, which is presented in this paper, is a disproportionation reaction of sulfinic acid groups shown in Fig. 1 [10,11]. Fig. 1 shows that 3 molecules of sulfinic acid disproportionate under formation of one -S-S(O)2bridge and one molecule of sulfonic acid. Interestingly, in the first step of the disproportionation reaction, one of the sulfinate groups (left sulfinate group in Fig. 1) reacts as electrophile, while the other sulfinate group (right sulfinate group in Fig. 1) reacts as nucleophile [11]. The problem with this reaction is the fact that free sulfinic acids, which are acids of medium acidicity, are thermally instable (apart from their tendency to disproportionate, they also can undergo splitting-off I and thus cannot be isolated as pure compounds. Because of their instability, the free sulfinic acids must be produced in situ before the disproportionation reaction can take place. This can be affected by first dissolving the sulfinate salt 2 in a suitable solvent, followed by addition of strongly acidic ion-exchange resin (e.g. Dowex in H-- form) which swells in the solvent and thus makes an ion-exchange with the sulfinate possible, yielding the free sulfinic acid. After 1Desulfination [12] at temperatures >80-120°C, dependent of the type of sulfinic acid - aromatic sulfmic acids show better stabilities than aliphatic sulfinic acids ZThe sulfinate salt is much more stable than the free sulfinic acid, and thus can be isolated as pure compound

J. Kerres et al./Journal of Membrane Science 139 (1998) 211-225

~ o

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Fig. 1. Disproportionationreaction of PSU sulfmic acid. this procedure, the disproportionation reaction can proceed, Crosslinked PSU sulfonate ionomer membranes can be produced by application of the disproportionation reaction of sulfinate groups via the following route [8]: (i) Mixing of PSU sulfinate with PSU sulfonate (in Na/K/Li form, dependent from the oxidant used for the oxidation of PSU sulfinate) in a suitable solvent, (ii) Ion-exchange in order to yield the free PSU sulfinic/sulfonic acids in solution, (iii) Evaporation of the solvent; during solvent evaporation the crosslinking via disproportionation of the sulfinic acid groups takes place, By this route, crosslinked blend membranes are obtained where only the PSU sulfinate macro-molecules are crosslinked inter- and intramolecularly. The PSU sulfonate chains are entangled in the network. PSU sulfinate and PSU sulfonate are chemically

compatible, and thus the blend formed between them is transparent to visible light. 2. Experimental 2.1. Production of PSU sulfinate and PSU sulfonate Sulfonation of PSU was performed via the metalation-sulfination-oxydation route [9]. In this paper, 1-fold sulfinated/sulfonated PSU has been used for membrane formation. 2.2. Preparation of the polymeric solution and formation of the crosslinked membrane The crosslinked ionomer membranes crosslinked via this procedure are synthesized as follows: At first PSU sulfinate (Li salt) and PSU sulfonate (Li + salt or Na + salt) are dissolved together in the

214

J. Kerreset aL/Journal of Membrane Science 139 (1998) 211-225

desired weight relation PSU sulfonate/PSU sulfinate in N-methyl pyrrolidone (NMP). Normally quantities of 5-15 g polymer and 15-45 g NMP have been used for production of polymeric solution ( ~ 25% polymer content). After dissolution of the polymers a strong acidic cation-exchange resin (Dowex 50WX8, supplied by Aldrich, 2-6 g) in the H + form is added (3-5 fold excess, referring to entire sulfinate and sulfonate content) in order to obtain the sulfinate and sulfonate groups of the polymers in the H + form. This procedure is required to obtain the PSU-SO2H groups in situ because sulfinic acids are thermally very instable and thus cannot be isolated in pure state without some desulfination and disproportionation. The polymeric solution is stirred with the ion-exchange resin for 24 h. After the polymers have been transformed to the acidic form, the resin is removed via filtration of the solution, and the polymeric solution is cast on a glass plate. The solvent is then evaporated under reduced pressure (starting with 700 mbar, subsequent reduction to 10 mbar) and at temperatures from 90120°C. In preliminary experiments, the optimum solvent evaporation/crosslinking conditions for the membranes have been investigated. When the film is made in the membrane-machine, the heating programme of the oven is performed as follows: 1 h 90°C, 0.5 h 100°C, 0.5 h 110°C, 1 h 120°C. When the membrane is produced in the vacuum oven, the evaporation procedure is as follows: 1 h 90°C/900 mbar, 0.5 h 100°C/600 mbar, 0.5 h 110°C/400 mbar, 13 h 120°C/25 mbar. With these solvent evaporation procedures, membranes have been produced which contain no residual solvent (elemental analysis: 0% N) and show a very good fit between calculated and experimental IEC when the PSU-SO3H content is higher as 60%. This indicates optimal crosslinking, In the membranes formed only the PSU sulfinic acid macromolecules are crosslinked. The PSU sulfonic acid macromolecules are entangled in the formed network. The membranes are transparent to visible light, due to mutual affinity of PSU sulfinic acid and PSU sulfonic acid. 2.3. Membrane post-treatment

The membranes are post-treated for 24 h in (i) 1 N HCI at 80°C to complete the crosslinking reaction and (ii) in pure water to wash out the HC1 at 80°C.

2.4. Membrane characterization 2.4.1. Thermogravimetry The thermal stability of the membranes was investigated by thermogravimetric analysis (TGA). The TGA analyses have been performed at the Institut fur Kunststoffpriffung (IKP) of the University of Stuttgart. The TGA conditions have been: heating rate 10°C/min, starting mass of sample 10o20 mg, N2-atmosphere. 2.4.2. Determination of the ion-exchange capacity The ion-exchange capacity of the sulfinated PSU is determined via 1H-NMR as described in [9]. The ionexchange capacity (IEC) of the sulfonated PSU and of the crosslinked membranes is determined via titration by following procedure: 0.5-1 g of the PSU sulfonate/ the crosslinked membrane in the SO3H form is stirred in 50 ml of saturated NaC1 solution. Due to the huge excess of Na + ions, the H-- ions of the polymer are released. Then the slurry is titrated with 0.1 N NaOH solution. 2.4.3. Determination of the swelling degree The SO3 H form membrane samples were swollen in water at 80°C for 2 days. At this temperature, no bleeding of the PSU-SO3H chains out of the membrane network into the hot water occurs. This could be shown experimentally by determination of IEC of the membranes. Then the water is cooled to ambient temperature, the membranes are removed, and surface-attached water is quickly removed with tissue paper. Subsequently, the wet weight mwet is determined. After drying in the oven at 80-90°C, their dry weight md~y is determined. The membranes still contain a small amount of hydrate water as could be confirmed by thermogravimetry and differential scanning calorimetry. However, the amount of residual water lies in the range between 1 and 2% and thus leads to a negligible change of the swelling value. For dense membranes, which are considered here, the swelling SW can be calculated via following formula: mwet - mdry SWx 1001%] (1) mdry The SO3Na form membranes are conditioned for 2 days in saturated NaC1 solution. Subsequently they are

J. Kerres et al./Journal of Membrane Science 139 (1998) 211-225

215

A U (voltage drop) (to computer)

[~~

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ill ~ teatmemN~e@~ /

calomelelectrode ~Haber-Luggin-capifilled lary,with 1NKClsolution(saltbridge,

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=,I I I N

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Fig. 2. Setup for the determination of the dc resistance of the ionomer membranes.

rinsed with water until the solution is Na+-free. The further handling follows the procedure indicated

is calculated as follows:

above.

2XUmeas PS -- AUthe~ x 1001%]

2.4.4. Determination o f the electric resistance

The theoretical voltage drop (concentration potential) between the two solutions is calculated via the Nernst equation:

The ionic membrane resistances have been measured in an electrolysis setup under electrolysis conditions (Fig. 2) and under current densities from 0 to 0.06 A]cm 2. As electrolytes 0.5 N HC1 and 0.5 N NaC1 at 25°C have been applied.

2.4.5. Determination ofpermselectivity and transport number

For the measurement of the ion-perrnselectivity of the membranes the static method was used. The potential difference (AUm~as.) is measured between two solutions of different concentrations (0.1/0.5 N NaC1 or HC1) separated by the membrane, at a ternperature of 25°C. Before the measurement, the membrahe is equilibrated in the 0.5 N electrolyte solution. After reaching an equilibrium, the potential difference is measured. The permselectivity PS of the membrane

AUtheo

--

R T x l n ~/C±1~71 ) \

(2)

(3)

F In Eq. (3) 71, "72 are the mean activity coefficients of the ions in the solutions with the concentrations c~ and 2 c+, F is the Faraday constant, R is the gas constant, and T the absolute temperature. The electrometric transport number t~n which describes the contribution of a certain cation or anion to the entire current transport in the membrane, is then calculated from the transport number t~ of the counterions in the solution and the permselectivity as follows: ~ = PS x (1 - t~) + ~ (4) The sum of all transport numbers of ions in solution and membrane is one.

J. Kerres et al./Journal of Membrane Science 139 (1998)211-225

216

2.4.6. Permeability For the permeability measurements, a cell consisting of two chambers has been used. One of the chambers is rinsed with the feed (a NaC1 solution or HC1 solution, volume=0.5 1), the other with the permeate (initially water, volume=0.25 1). Both the feed and the permeate are held at 25°C. During the experiment, ions permeate through the membrane into the permeate such that the conductivity of the permeate increases with time. The conductivity changes are measured with a conductivity cell, whose data are then recorded by a computer. The flux of the ions through the membrane is described by Fick's first law: dc rn N = -D × -dx

(5)

(D: diffusivity of the electrolyte, dcm/dx: change of concentration in membrane) With the distribution coefficient k for the sorption equilibrium, and the permeability P Eq. (5) transforms to:

0.2 mol/1 have been applied as electrolytes. The current efficiency was calculated via following equation:

QF __ F x Zj x Anj t=T f t=o Idt

r / J - Qtlaeo

(8)

For the energy consumption the following relation holds: E -- Qtheo X U(t) _ ft=o t=T I x U(t)dt Anj X Mj Anj X Mj

(9)

In Eqs. (8) and (9): r/j, current efficiency; F, Faraday number; zj, valence of ion sort j; An/, transported molar substance quantity; E, energy consumption; U(t), voltage drop per cell pair; Mj, molar mass of ion sort j.

2.4.8. PEM fuel cell application

(6)

One of the crosslinked membranes (PSU-M-65-3, I E C = I . 4 meq/g, thickness 120 ~tm) was tested in a polymer electrolyte fuel cell (PEFC) [13]. The conditions applied have been: Amembrane=25 cm2; electrodes: E-Tek, Pt loading 0.4 mg Pt]cm 2 (20% Pt onto Vulcan XC-72); mechanical cold-pressing of electrodes and membrane; cell temperature: 50°C;

(c: electrolyte concentration) From the concentration changes of the electrolyte solution the permeability coefficient is calculated by the following equation:

Pr~:=l.3 bar, po~=l.5 bar; gas humidification conditions: Thum~d(H2)=60°C, rhumid(O2)=24°C; gas ternperatures at cell inlet: Tin,rh=50°C, Tin,o~=30°C; PEFC experiment duration: 1 week.

dc N = -D × k x ~=

dm

P × t = - -h- × In

dc -P × ~

Ic°-ctp(l+b)+

cO _ cO

b×c°]

(7)

3. R e s u l t s a n d d i s c u s s i o n

3.1. Thermogravimetric characterization In Eq. (7) h : [A × (1 +b)]/Vp and b : Vp/Vf. c ° and co are the concentrations of the electrolyte in feed and permeate at t=0, Cp t is the concentration in the permeate at t, A is the membrane area, and dm is the thickness of membrane. The permeability coefficient can be determined as slope of the straight line P × t :f(t).

2.4. 7. Application in electrodialysis Two of the PSU membranes (one uncrosslinked PSU-SO3H membrane and one crosslinked membrane) have been investigated in an electrodialysis setup in desalination and concentration mode in order to test their suitability for this electromembrane process. HC1 solutions with initial concentrations of

In Fig. 3, the TGA curve of the blend membrane M-70 (made from 70 wt% PSU-SO3H and 30 wt% PSU-SO2H) is shown, in Fig. 4, the TGA curve of M-30 (made from 30 wt% PSU-SO3H and 70 wt% PSU-SO2H). The TGA traces indicate very good thermal stability of the crosslinked membranes. The weight loss up to 200°C is due to H20 evaporation, as could be proved by DSC measurements.

3.2. Obtained results In Table 1, the characterization results of the crosslinked membranes in the SO3H form and SO3Na form are presented.

J. Kerres et al./Journal of Membrane Science 139 (1998) 211-225 ZCtrr/l(errmn FN B4. KT. 142

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Fig. 3. TGA curve of M-70 (made from 70 wt% PSU-SO3H and 30 wt% PSU-SO2H).

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Fig. 4. TGA curve of M-30 (made from 30 wt% PSU-SO3H and 70 wt% PSU-SOzH).

3.3. Calculated and obtained ion-exchange capacity In Fig. 5, the obtained and expected IEC's of some of the membranes are shown. The expected IEC's have been calculated under the assumption that the crosslinking reaction proceeds with a yield of 100%. The

IEC of the membrane can be calculated via following formula: IECMem= Wpsuso3H × IECpsuso3n + WpSUSO2H IECpsuso2H × (10) 3

0 30 50 65 65 65 70 80 90 100 100

M-0 M-30 M-50 M-65 M-65-2 M-65-3 M-70 M-80 M-90 M-100-1 M-100-2

104 65 76 88 115 120 84 98 120 125

dM ~m]

116

30 59

-

37 67 67.2 112 193

13 15 15 30 57

SWNa+ [%]

-

11 20 19 34 57

SWH÷ [%]

0.41 0.51 0.63 1.25 1.49 1.40 1.35 1.45 1.59 1.6 1.57

IEC [meq/ga~y]

31535 534.2 119.7 36.6 18 23 30.8 18.8 67.2 5.7

H~ a Rsp [f~ cm]

.

8.97E-14 2.81E-13 6.95E-13 1.71E-12 . . . 2.47E-12 2.88E-11 . . 3.26E-10

[mE/s]

[f~ cm]

56524 3876 1185 459 120 . 393 109 165 46 75

PH+ b

gNa + a

5.92E-14 3.73E-13 8.01E-13 3.08E-12 . . . . 3.60E-12 4.40E-11 . . 2.76E-10 4.20E-12

[m2/s]

PNa+ b

72.4

. 93.2 89.2

88.7 91.7 92.9 90.7

[%]

PSH+ c

0.954

. 0.988 0.982

0.981 0.986 0.989 0.984

[%]

tH+ c

94.2 74.7 93.3 15.6 95.8

13.7 95 95 92.9 85.4

[%]

PSNa+ c

0.964 0.843 0.958 0.477 0,974

0.465 0.969 0.969 0.956 0.909

tNa+ c

dM: m e m b r a n e thickness; SW: swelling; IEC: i o n - e x c h a n g e capacity; R~p: specific resistance; RA: area resistance; P: permeability; PS: permselectivity; tH+ : transport number. In 0.5 N HC1 or NaC1; b 0.5 N HC1 or NaClllmembranellH20; ~ 0.5 N HC1 or NaClllmembranel]0.1 N HC1 or NaC1.

Content PSU-SO3H [%]

Membrane

Table 1 Characterization results o f the m e m b r a n e s in the H + f o r m and in the N a + f o r m

t,o t.~

',o

e~

~

~,~

to

J. Kerres et al./Journal of Membrane Science 139 (1998) 211-225

1,6 ~

~ 1,42~expected 1 g 0,8 E 0,6 0,4 - 0,2 0

| |

0

i I = i 10 20 30 40

~ lEG found i i i t i 50 60 70 80 90 100

ContentPSU-SO3H [wl%] Fig. 5. Calculated and experimentally obtained IEC of the crosslinked membranes,

All the membranes shown in Fig. 5 have been produced by applying the membrane-pulling machine which guarantees similar membrane-forming and evaporation conditions. From Fig. 5 it can be seen that for membranes with low IEC and thus high initial PSUSO2H content (up to ~ 50-60% PSU-SO2H) there is a significant gap between theoretical and experimental IEC (M-0: 0% PSU-SO3H, IECoveran,calc=0.67 meq/g, IECpst3_so3a,calc=0.0meq/g, IECfou,d=0.41 meq/g; M-30: 30% PSU-SO3H, IEC ..... U,calc=0.96 meq/g, IECpstJ_so3n,calc=0.48 meq/g, IECfouna=0.51 meq/g; M-50: 50% PSU-SO3H, IECoverall,ea]c=l.16 meq/g, IECpst3_SO3H,~al~=0.8 meq/g, IECfou,~=0.63 meq/g), Apart from their low IEC, the aforementioned membranes are crosslinked as could be proved by dissolution experiments in NMP. Insoluble material remained even when the NMP temperature was raised to 80°C. In addition, by FTIR it could be proved that no residual sulfinate groups are present in the crosslinked membranes (the symmetrical S = O stretching vibration (u s°) of the sulfinate group which occurs at aromatic sulfinates in the range between 960 and 980 cm -1 [14] was not found in the FFIR spectra of the crosslinked membranes), Reasons for the low measured IEC of the aforementioned membranes could be: (i) A part of the sulfinic acid groups could be split off during membrane formation - the splitting-off of sulfinic acid groups is a competing reaction to the crosslinking reaction [12]. (ii) The membranes with 0, 30 and 50% PSU-SO3H content have low IEC's lying below the percolation threshold of ionic conductivity. This means that the

219

ion-exchange groups are isolated in the membrane matrix, and thus they are partially notlow accessible to ion-exchange. This would explain the measured IEC which is in the case of M-50 (50% PSU-SO3H) even lower than the calculated IEC of the PSU-SOaH blend component alone. When comparing the IEC of the membrane containing 50 wt% PSU-SO3H with the IEC of the membrane containing 65 wt% PSU-SO3H, a drastical change of IEC is observed: Membranes containing >65 wt% PSU-SO3H show a very good fit between calculated and experimental IEC. This indicates an optimal crosslinking reaction of the sulfinic acid groups. Possible explanations for the drastical IEC change between 50 and 65 wt% PSU-SO3H content of the membranes are: (i) The percolation threshold of the ionic conductivity is crossed between 50 and 65 wt% PSU-SO3H content in the blend membrane - above the percolation threshold, ion-exchange groups are connected, leading to an ionic conductivity over the whole membrane. (ii) Taking into account, that the blend membranes could show (partial) microphase-separation between the PSU-SO3H and the PSU-SO2H component, between 50 and 65 wt% PSU-SO3H content, a phase-transition could occur from continuous PSUSO2H phase/discontinuous PSU-SO3H phase to continuous PSU-SO3H phase/discontinuous PSU-SO2H phase. When PSU-SO2H is the discontinuous polymer microphase, it might be better protected against splitring-off of the sulfinic acid group than if it is the continuous microphase - when PSU-SO2H is discontinuous, it is surrounded by PSU-SO3H groups which are capable to form hydrogen bridges to the SO2H groups. These hydrogen bridges could catalyze the disproportionation crosslinking reaction so that the crosslinking reaction is favored, compared with the splitting-off reaction. This phase transition from continuous PSU-SO2H phase/discontinuous PSU-SO3H phase to discontinuous PSU-SO2H phase/continuous PSU-SO3H phase also leads to percolation of ionic conductivity of the membrane. Summarizing it can be said, that it is very important to investigate the phase structure of the crosslinked PSU blend membranes with suitable methods in future research. Thus one can conclude that at membranes with high PSU-SO3H content >65 wt%, which are the only

220

J. Kerres et al./Journal of Membrane Science 139 (1998) 211-225 200-

[]

150-

I

•

H + form

o

Na + form

o ~100[]

O

50[]

o 0

I

0

0,5

I

1

I

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1,5

2

IEC [meq/g] Fig. 6. Swelling of the crosslinked PSU-sulfonate blend membranes in dependence of the IEC for the PSU-SO3Na and the PSU-SO3H form.

membranes finding practical interest because of their high ionic conductivities, the splitting-off of the sulfinic acid group is only a minor side reaction, compared with the crosslinking reaction.

3.4. Swelling in dependence of ion-exchange capacity In Fig. 6, the swelling of crosslinked PSU-SO3Na and PSU-SO3H blend membranes in dependence of 1EC is shown, The characteristics of the swelling/IEC curve is the same for both the PSU-SO3Na and PSU-SO3H form. However, the absolute swelling values are higher at the PSU-SO3H membranes. This can be explained with the larger hydratation shell of the H + ions, compared with the hydratation shell of the Na + ions [15]. The drastical increase in swelling for IEC's >1.4 meq/g can again be explained by taking into account the assumed phase structure of the blends: at higher IEC's, the crosslinked polymer microphase is discontinuous which hinders an efficient swelling reduction of the membrane,

3.5. Swelling in dependence of temperature and electrolyte The membrane M-90 was selected for determination of swelling in dependence of temperature and electrolyte, because this membrane has the lowest

crosslinking density of all membranes investigated. This membrane has been produced from a polymeric solution containing 90% PSU-SO3H and 10% PSUSO2H. The results of the swelling experiments are shown in Fig. 7. From Fig. 7 it can be seen that the swelling behaviour in the three liquids is similar until reaching ~75°C. For higher temperatures the swelling in water increases strongly, compared with swelling in NaaSO4 and HC1 solution. These findings can be explained with the osmotic pressure difference between membrahe and outer solutions which is higher for swelling in water, compared to swelling in the two electrolytes. The osmotic pressure difference between membrane and outer solution increases with temperature for the swelling in water, because the osmotic pressure difference is temperature-dependent.

3.6. Resistance in dependence of capacity The DC (direct current) ionic resistances of the ionexchange membranes have been measured in the Na + and H + form with 0.5 M NaC1 and HC1 solutions. At this concentration the resistances of membranes and electrolytes are sufficiently different from each other to end up in a low measurement error. In Fig. 8, the Na + and H + resistances of the membranes in dependence of the IEC are shown. It can be seen that at low IEC's the resistances are very high. This can be explained in the following way:

J. Kerres et aL /Journal of Membrane Science 139 (1998) 211-225

221

H20 70

~

0.5NNa2S

41

60 A

10 0

I

I

40

20

I

60

80

100

T[°C] Fig. 7. Swelling in dependence of temperature for M-90 in water, 1 N HC1 and 0.5 N Na~SO4.

100000 10000 oE 1000 .~

I~ ~

I

I • H+ form

+

form

100 10

1

I 0

0,5

I 1 EC[meq/g]

I

I

1,5

2

Fig. 8. Specific resistances of crosslinked PSU-SO3H membranes in the Na÷ and the H+ form in dependence of the IEC at 25°C.

at low IEC the ion-exchange groups are isolated from each other in the membrane matrix so that the counterions cannot migrate through the whole membrane. From a IEC of 0.6-0.8 meq SO3HJg on the resistance decreases strongly. This can be explained with the beginning formation of ion-conducting paths through the membrane in this IEC range. Between capacities of 1.3-1.5 meq SO3H/g the resistance of the membranes is strongly reduced further. This is due to the strongly increasing swelling of the membranes in this capacity range - the more the membrane-polymer network is swollen, the less is the hindrance of the ion-transport through the membrane. The resistances o f membranes in SO3 H form are by a factor 5-15 lower than the resistances of membranes in SO3Na form. This is due to the higher mobility of H +, compared to Na +.

3.7. Permselectivity and transport numbers in dependence o f capacity The permselectivity of the crosslinked blend membranes has been determined by applying a 0.5 N electrolyte solution (NaC1 or HC1, respectively) on the one side of the membrane, and a 0.1 N electrolyte solution on the other side o f the membrane. From the permselectivity, the transport numbers for the respective counterions (Na + or H +, respectively) have been calculated via Eq. (3). In Fig. 9, the Na + and H + transport numbers (tNa+ and tn+ ) are shown in dependence of the IEC of the membranes. From Fig. 9, the following can be concluded: At the membrane with the lowest IEC (0.41 meq SO3Na/g Polymer) the transport number for the

,

1 0,9 0,8 0,7 0,6

A I¢ tt

0,5

x

~ ~ A A

X

0,4 0,3 0,2 0,1 0 0

i 0,5

] Xt,,..

[

i 1 lEe [meq/gl

x

I 1,5

i 2

Fig. 9. t~a÷ and tH+ in dependence of the 1EC of the crosslinked blend membranes.

222

J. Kerres et al./Journal of Membrane Science 139 (1998) 211-225

1,00E-09'

I

x 1,00E-10'

Na+form I

~1,

~, H + form I

x

1,00E-11' Q. 1,00E-12' 1,00E- 13'

~,

1,00E-14 0

I

I

I

I

0,5

1

1,5

2

IEC [meq/g]

Fig. 10. Diffusional permeability of the membranes as a function of IEC (electrolyte: 0.5 N NaC1 or HCI).

Na + ions is not much higher than in a liquid electrolyte (t~a+=0,465, t~a+=0,38). This means that with Na + as counterion the Donnan exclusion does not work properly for the coion C1- because of the low IEC. At the (uncrosslinked) membrane with the highest IEC (1.57 meq SO3H/g) the permselectivity and thus the tNa+ transport number is again very low. This can be explained with the high swelling of this membrane which makes codiffusion of counter- and coions through the membrane possible [1].

1E-09o o• ~ 1E-11 "a." 1E-12

q::l

H+form

o

Na* form

O0

1E-13"

,

0

In Fig. 10 the Na+/H + permeability of the crosslinked blend membranes in dependence of the IEC is shown. In the diagrams (Fig. 10), 3 different parts of the P/IEC curve can be observed: (1) Up to a IEC of ~ 0.6--0.7 meq/g, a strong increase of the permeability is observed. This might be due to the percolation of ion-conductive channels in the membrane matrix, (2) Between 0.7 and 1.3 meq/g, the permeability increases only slightly. This can be explained with an only small increase of the swelling of the membrane matrix in this IEC range, (3) At IEC's of >1.3 meq/g the swelling of the membrane strongly increases. This leads to a strong increase of the diffusivity of the ions in the membrane and thus to an increase of the permeability over two decades. The strong dependence of permeabilty on the swelling is documented in Fig. 11.

[] On

1E-14

3.8. Permeability in dependence of capacity

[]

1E-IO-

50

= 1 O0

,

,

150

200

SW [%] Fig. 11. Diffusional permeability of the membranes as a function of swelling (electrolyte: 0.5 N NaCI or HC1).

3.9. Application in electrodialysis In Fig. 12(a), the ED "desalination" curve of M65-2 starting with a 0.2 mole/1 HC1 solution is shown, in Fig. 12(b), the ED concentration curve of M-65-2 also starting with a 0.2 mole/1 HC1 solution is shown. In Table 2, the results • f E D experiments are shown for the membranes M-65-2 (crosslinked PSU) and M100-1 (homogeneous PSU). For comparison, also the results of ED experiments for the commercial cationexchange membranes CMX and Nation are listed in Table 2. From Table 2 it can be concluded that the sulfonated PSU membranes are suitable for the application in ED because they show a similar performance as the commercial membranes CMX and Nation.

223

J. Kerres et al./Journal of Membrane Science 139 (1998) 211-225

,-.

(a)

0,2

(b)

0,4

o,ls

~.,,...,,~-:

-

~_~ o,s

8S_o~

g~-~o2

o,o

o,1

=

,-,

"~

0

0 0

200 400 time [min]

600

0

500 1000 time [min]

1500

Fig. 12. Electrolyte concentration in dependence of time for ED "desalination" (a) and for ED concentration Co) of 0.2 mole/1 HCI solution using the membrane M-65-2. Table 2 ED results of membranes (current efficiency, energy consumption) Membrane

?~HC1[%]

M-65-2 M- 100-1 CMX Nation

EHO [kWh/kg]

Desalination

Concentration

Desalination

Concentration

82.0 80.0 86.6 86.3

47.1 47.1 53.8 44.3

0.0237 0.0255 0.0247 0.0230

0.0424 0.0448 0.0394 0.0453

It is important to note that the relatively low current efficiency of HC1 concentration experiment is caused by proton leakage of A M X membrane used in the ED cell - no acid-blocker anion-exchange membrane has been used in the ED experiment, 3.10. Application in P E M f u e l cells

One of the crosslinked membranes (PSU-M-65-3, I E C = I . 4 meq/g) was tested for 1 week in a H2/O2

PEFC [ 13]. During this time, no performance decrease of the PEFC could be observed. In Fig. 13, some I / U curves of the membrane in the PEFC, recorded in succession (run-1 to run-6), are shown. The results indicate that the formation of the three phase electrode/gas/membrane interlayer required for a good PEFC performance is not sufficient by simply coldpressing together the components. This can be concluded from the limited voltage of the electrodemembrane-electrode (EME) unit, compared with

100( 75~

p [bar] Thmidiam

~ Nation ®

~=

50O"

Tin [°C] T,,,, l°C]

02 1.4it 24

H2 1.28 60 50

3

50

[] U[mV]-mnl ~

250-

~

O V [mV] -mn 2 [ ] U [mV]- run 3 /~ U [mY]- mn4

OO II~

0

I~ UImV]-run5 0

510

100

150'

200

I [mA/em2] Fig. 13. U/I curves of PSU-M-65-3 in a PEFC.

~]~ U [mV]- mn 6

224

Jr. Kerres et al./Journal of Membrane Science 139 (1998) 211-225

the results for application of the perfluorinated ionexchange membrane Nation in PEFC (Nation curve in Fig. 13). The PEFC performance of the PSU membranes might be improved by following measures: (i) reduction of the membrane thickness, (ii) optimization of the PEFC water management because the PSU membranes are very sensitive against drying-out which leads to a strong decrease of ionic membrane conductivity, (iii) surface treatment of the membrane to increase the number of SO3H groups in the surface, (iv) modification of electrode-membrane-electrode (EME) production process to improve the contact between electrodes and membrane. 3.11. A p p l i c a t i o n in o t h e r m e m b r a n e p r o c e s s e s

The crosslinked blend membranes have been successfully applied in the perstractive separation of pentene from pentene/pentane mixtures via facilitated transport of pentene through the blend membranes in the A g + form by specific interaction of the pentene double bond with the A g + ion [16].

4. Conclusions A new crosslinking process for cation-exchange polymer membranes has been developed. The membranes have been characterized in terms of ionexchange capacity, swelling, ionic conductivity (resistance), permselectivity/transport number, and permeability. Dependences between these properties have been determined and discussed. The advantages of the sulfinate disproportionation crosslinking process are: (i) The crosslinking process is easy to perform. (ii) Ion-exchange capacities of crosslinked membranes and thus their ionic resistance, swelling and permselectivity can be varied in a broad range. (iii) The membranes show a good thermal stability, (iv) The membranes are suitable for application in electrodialysis. It could be shown in preliminary experiments that the crosslinking process is also applicable to blends of poly(etheretherketone sulfonic a c i d ) a n d poly(sulfone sulfinic acid) yielding crosslinked, transparent membranes with good electrochemical properties. A paper presenting the properties of this crosslinked mem-

brane system is in preparation. It can be expected that other polymeric sulfonic acids can also be crosslinked with polymeric sulfinic acids via the presented crosslinking procedure. In future work it is also intended to investigate the topology of the developed crosslinked blend membranes in detail in order to obtain relationships between blend structures and blend properties. The knowledge of these relationships is the basis for the development of blend membranes with tailormade properties.

References [1] P. Zschocke, D. Quellmalz, Novel ion exchange membranes based on an aromatic polysulfone, J. Membr. Sci. 22 (1985) 325-332. [2] K. Kusomoto, T. Sata, Y. Mizutani, Modification of anionexchange membranes with polystyrene sulfonic acid, Polym. J. 8 (1976)225-226. [3] W.A. McRae, S.S. Alexander, Sulfonation agent and its use in preparing cation-exchange membranes, U.S. Patent 2,962,454, 1960. [4] G.G. Scherer, Polymer membranes for fuel cells, Ber. Bunsenges. Phys. Chem. 94 (1990) 1008-1014. [5] R.A. Assink, C. Arnold, Jr. Roger, P. Hollandsworth, Preparation of oxidatively stable cation-exchangemembranes by the elimination of tertiary hydrogens, J. Membr. Sci. 56 (1991) 143-151. [6] E Helmer-Metzmann, E Osan, A. Schneller, H. Ritter, K. Ledjeff, R. Nolte, R. Thorwirth, Polymerelektrolyt-Membran und Verfahren zu ihrer Herstellung, EP 0 574 791, 1993. [7] R. Nolte, K. Ledjeff, M. Bauer, R. Mtilhaupt, Partially sulfonated poly(arylene ether sulfone) - A versatile proton conducting material for modern energy conversion technologies, J. Memb. Sci. 83 (1993) 211-220. [8] J. Kerres, W. Cui, W. Schnurnberger, Vernetzung von modifizierten Engineering Thermoplasten, German Patent Application 19622337.7 from June 4th, 1996. [91 J. Kerres, W. Cui, S. Reichle, New sulfonated engineering polymers via the metalation route. 1. Sulfonated poly(ethersulfone) PSU Udel® via metalation-sulfination-oxidation, J. Polym. Sci.: Part A: Polym. Chem. 34 (1996) 2421-2438. [101 J.L. Kice, N.E. Pawlowski, The mechanism of the disproportionation of sulfinic acids. The thermal decomposition of p-toluenesulfinyl p-tolyl sulfone and its reaction with p-toluenesulfinic acid, J. Org. Chem. 28 (1963) 11621163. 111] T. Okuyama, Sulfinate ions as nucleophiles, in: S. Patai (Ed.), The Chemistry of Sulphinic Acids, Esters and their Derivatives, Wiley, New York, 1990, pp. 639~64. [12] C.J.M. Stirring, Sulphinic acids and carboxylic acids - A comparison, in: S. Patai (Ed.), The Chemistry of Sulphinic Acids, Esters and their Derivatives, Wiley, New York, 1990, pp. 1-7.

J. Kerres et al./Journal of Membrane Science 139 (1998) 211-225

[13] J. Kerres, W. Cui, W. Neubrand, S. Springer, S. Reichle, B. Striegel, G. Eigenberger, W. Schnurnberger, D. Bevers, N. Wagner, K. Bolwin, Synthesis of new sulfonated polymers and their application in electro-membrane processes, Lecture, Euromembrane '95 Congress, Bath (UK), 18 to 20th Sept 1995, Proceedings, 1995, pp. 1-284-1-289. [14] B.J. Lindberg, Studies on sulfinic acids. V. Correlation of the IR-frequencies of the sulfur-oxygen bonds in substituted

225

aromatic sulfinates and sulfonates with Hammet substituent constants, Acta Chem. Scand. 21 (1967) 841-842. [15] E Helfferich, Ion Exchangers, Vol. 1, McGraw-Hill, New York, 1962. [16] A.J. van Zyl, J. Kerres, W. Cui, Application of new sulfonated ionomer membranes in the separation of pentene and pentane by facilitated transport, J. Membr. Sci. 137 (1997) 173.