- Email: [email protected]
Effects of inorganic substances on water splitting in ion-exchange membranes
Journal of Colloid and Interface Science 273 (2004) 533–539 www.elsevier.com/locate/jcis
Effects of inorganic substances on water splitting in ion-exchange membranes II. Optimal contents of inorganic substances in preparing bipolar membranes Moon-Sung Kang, Yong-Jin Choi, and Seung-Hyeon Moon ∗ Department of Environmental Science and Engineering, Kwangju Institute of Science & Technology (K-JIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, South Korea Received 22 July 2003; accepted 23 January 2004
Abstract An approach to enhancing the water-splitting performance of bipolar membranes (BPMs) is introducing an inorganic substance at the bipolar (BP) junction. In this study, the immobilization of inorganic matters (i.e., iron hydroxides and silicon compounds) at the BP junction and the optimum concentration have been investigated. To immobilize these inorganic matters, novel methods (i.e., electrodeposition of the iron hydroxide and processing of the sol–gel to introduce silicon groups at the BP junction) were suggested. At optimal concentrations, the immobilized inorganic matters significantly enhanced the water-splitting fluxes, indicating that they provide alternative paths for water dissociation, but on the other hand possibly reduce the polarization of water molecules between the sulfonic acid and quaternary ammonium groups at high contents. Consequently, the amount of inorganic substances introduced should be optimized to obtain the maximum water splitting in the BPM. 2004 Elsevier Inc. All rights reserved. Keywords: Water-splitting performance; Bipolar membranes; Inorganic substance; Electrodeposition; Polarization of water molecules
1. Introduction Water-splitting electrodialysis (WSED) using bipolar membrane (BPM) is known to be an efficient technology for producing acid/base without the production of undesirable by-products and the use of additional chemicals. WSED technology is also considered to be a more cost-effective process, compared to conventional electrolysis for the production of acids and bases [1]. Since BPM is a key component in WSED, the development of high-performance BPM is essential to increase the process efficiency. It is known that the water-splitting property of a BPM can be greatly enhanced by introducing an inorganic substance, such as metal hydroxides, at the bipolar (BP) junction [1–3]. The major factors contributing to the enhancement in water splitting by inorganic substances may include (i) increase in the electric fields generated at the BP junction [4]; (ii) catalytic water* Corresponding author.
E-mail address: [email protected] (S.-H. Moon). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.01.051
splitting reactions between metal hydroxides (or metal ions) and water molecules [5,6]; (iii) increase in the water activity (i.e., enhancement of interface wetness) [2]; (iv) increase in the looseness of the bonds of water [3,7]. It seems that these factors contribute the water-splitting mechanism complexly. In a previous study, the electrochemical characteristics of ion-exchange membranes coated with iron hydroxide/oxide and silica sol have been investigated [4]. From the results, the bipolar structure immobilized on the membrane surface significantly increased water splitting due to the enhancement in water polarization with the help of strong electric fields. Moreover, it was revealed that metal oxides or ≡Si– OH groups, as well as metal hydroxides, are effective catalysts for water dissociation. In this study, iron hydroxides and silicon groups (i.e., ≡Si–OH) have been examined as possible catalysts to enhance the water-splitting capability of BPM. To immobilize the inorganic substances at a BP junction, novel methods (i.e., electrodeposition of iron hydroxide and in situ sol–gel processing to introduce the sili-
534
M.-S. Kang et al. / Journal of Colloid and Interface Science 273 (2004) 533–539
Fig. 2. pC–pH diagram of hydroxo iron(III) complexes.
Fe(H2 O)2 (OH)− 4 + H2 O → Fe(H2 O)3 (OH)3(S) + OH− .
(4)
The metal species may react with the quaternary ammonium groups of the AEL. The reaction mechanisms are [9]
Fig. 1. Schematic illustration of water-splitting mechanism by iron hydroxides at the BP junction.
con groups) were suggested and their optimum contents for a high-performance bipolar membrane were evaluated.
2. Water splitting mechanism by inorganic substances Fig. 1 shows a schematic illustration of the mechanism of water splitting by metal hydroxides at a BP junction. It was assumed that the pH in the reaction layer where water splitting occurs varies from about zero, at the boundary in the cation exchange layer, to about 14, at the boundary region in the anion exchange layer of a BPM [8]. Therefore, based on the pC–pH diagram for hydroxo iron(III) complexes (Fig. 2), the reactions occurring under acidic conditions near the boundary region of the cation exchange layer (CEL) are expected to be 2+ + H3 O+ , Fe(H2 O)3+ 6 + H2 O → Fe(H2 O)5 (OH)
Fe(H2 O)5 (OH)2+ + H2 O → Fe(H2 O)3+ 6
+ OH− .
(5)
→ M(H2 O)3 (OH)3(S) + B+ + OH− , B+
(6) N+ )
where is a quaternary ammonium group (–R3 in the anion-exchange layer. This reaction model indicates that the metal species are immobilized in the BPMs as a hydroxide and react with the water molecules and the quaternary ammonium groups reversibly in the presence of a strong local electric field. This reaction mechanism seems to be dominant in water-splitting reactions induced by metallic species. Likewise, other inorganic matter such as silicon hydroxide groups (≡SiOH) might also accelerate water splitting at a BP junction. In this case, water splitting may occur between ≡SiOH groups and quaternary ammonium groups, as well as between ≡SiOH and sulfonic acid groups, according to the following reactions [10]: under acidic conditions near CEL at the BP junction,
(1)
− ≡SiOH + H2 O ≡SiOH+ 2 + OH ,
(7)
(2)
+ ≡SiOH+ 2 + H2 O ≡SiOH + H3 O ;
(8)
In this case, however, positively charged metal ions may easily migrate out of the membrane under an electric field. The catalytic reactions occurring under basic conditions near the boundary region of the anion exchange layer (AEL) are as follows: Fe(H2 O)3 (OH)3(S) + H2 O + → Fe(H2 O)2 (OH)− 4 + H3 O ,
M(H2 O)3 (OH)3(S) + B+ + H2 O + → (B+ ) M(H2 O)2 (OH)− 4 + H3 O , (B+ ) M(H2 O)2 (OH)− 4 + H2 O
(3)
under basic conditions near AEL at the BP junction, ≡SiOH + H2 O ≡SiO− + H3 O+ ,
(9)
≡SiO− + H2 O ≡SiOH + OH− .
(10)
The reaction by which protons are added to or removed from the oxide (i.e., the acidity of the M–OH group (M = metal)) depends on the metal atom [10]. The pH at which the particle
M.-S. Kang et al. / Journal of Colloid and Interface Science 273 (2004) 533–539
is neutrally charged is called the point of zero charge (PZC). At pH < PZC, Eq. (7) predominates, and the particles are positively charged, whereas at pH > PZC, Eq. (9) gives the particle a negative charge. Note that the PZC of silicon oxides is near 2.5 [10]. The surface charge density of ≡Si–OH groups, σs , at the BP junction may be represented by [11] eN σs.b = 1 + 10(pHs−PZC) near the cation-exchange layer, (11) eN σs.a = − 1 + 10(PZC−pHs) near the anion-exchange layer, (12) where PZC is the point of zero charge of the Si–OH group, pHs the surface pH, N the total site charge density, and e the Coulombic charge. Therefore, it is expected that the water-splitting reactions of ≡Si–OH groups may occur at the BP interface near both the anion-exchange layer and the cation-exchange layer. The reversible water-splitting reactions can proceed according to Eqs. (7)–(10) since dissociated water ions (i.e., H+ and OH− ) are instantly removed from the reaction layer under the reverse bias condition.
3. Experimental 3.1. Materials Neosepta CM-1 (Tokuyama Corp., Japan) and Nafion 117 (DuPont, USA) were used as a cation-exchange layer (CEL) in the preparation of a bipolar membrane. A traditional paste method was employed to prepare the anion-exchange layer (i.e., poly(vinylbenzyl) chloride–divinylbenzene–styrene). Vinylbenzyl chloride (VBC, Aldrich, USA) was selected as the F-monomer to introduce the quaternary ammonium groups and styrene (Sty, Kanto Chem., Japan) was used for the co-polymerization. Divinylbenzene (DVB, Aldrich, USA) was used as the crosslinker and benzoyl peroxide (BPO, Fluka, Switzerland) as the initiator. VBC was purified to remove inhibitors using the inhibitor-removal column (Aldrich, USA). Sty and DVB were treated with 10 wt% aqueous sodium hydroxide and then were washed with distilled water several times. BPO was recrystallized in a methanol/water mixture at −5 ◦ C to purify it. Acrylonitrile butadiene rubber (NBR) was added to the monomer mixture to enhance the mechanical properties of the membrane. In the monomer mixture, the weight ratio VBC to Sty was varied in the range 0.5–3 and the contents of DVB were 3, 6, or 10 wt%. The weight fraction of NBR relative to the monomer mixture (VBC/Sty/DVB) was fixed at 1:6. To immobilize the inorganic substances at a BP junction, iron(III) chloride (Aldrich, USA) and tetraethoxylsilane (TEOS, Si(OC2 H5 )4 , Aldrich, USA) were employed as the precursors.
535
3.2. Bipolar membrane preparation To prepare the BPMs, initially, the surface of the cationexchange membrane (CM-1/Nafion 117) was roughened with a fine sandpaper to increase the contact area and, thereby, the strength of the bonding [1,3]. The monomer paste was cast onto the cation-exchange membrane and fixed on a glass plate, and then another glass plate was superimposed upon it. The glass plates were then sealed with a cloth tape to prevent leakage of the monomers during copolymerization. Polymerization was allowed to continue for 4 h at 80 ◦ C and then the prepared membranes were immersed in a 0.5 mol dm−3 trimethylamine (TMA, Aldrich, USA) aqueous solution to introduce quaternary ammonium groups. After being washed with distilled water, the prepared membranes were stored in 0.5 mol dm−3 NaCl. 3.3. Immobilization of inorganic substances To immobilize iron hydroxides (amorphous Fe(OH)3 ) at the BP junction, two methods were employed in this study. First, iron hydroxides were immobilized on membrane (CM-1/Nafion 117) surface by immersing the membrane successively in a 0.025 mol dm−3 FeCl3 solution and a 0.1 mol dm−3 NaOH solution for a certain time. After the immobilization of iron hydroxides, the BPMs were prepared according to previously mentioned procedure. The metal contents were quantitatively analyzed via an electron spectroscopy for chemical analysis (ESCA, ESCALAB MK-II, VG Scientific Ltd., England). The second method was the electrodeposition of iron hydroxides. Fig. 3 shows the schematic drawing of the electrodeposition of metal hydroxides at a BP junction. It was expected that the iron hydroxides would be immobilized near the AEL at the BP junction under a forward-bias condition (with a low current flow). The immobilization was performed using a
Fig. 3. Schematic illustration describing the electrodeposition of iron hydroxides at the BP junction.
536
M.-S. Kang et al. / Journal of Colloid and Interface Science 273 (2004) 533–539 Hydrolysis H+
Si(OR)4 + H2 O → (OR)3 Si–OH + ROH Condensation (RO)3 Si–OH + RO–Si(OR)3 → (RO)3 Si–O–Si(OR)3 + ROH Substitution Si–(OEt) + MeOH → Si(OMe) + EtOH (R = Et or H); TEOS, tetraethoxylsilane (Si(OC2 H5 )4 ) Fig. 5. Acid-catalyzed sol–gel reactions occurring within H2 O/TEOS mixture [12].
condensation of the SiOH groups. Fig. 5 exhibits the acidcatalyzed sol–gel reactions occurring within the H2 O/TEOS mixture. The silicon contents were also quantitatively evaluated via ESCA. After the sol–gel process, BPMs were prepared according to the preparation method described in the previous section. The BPMs prepared were then stored in 0.5 mol dm−3 NaCl for more than a day. 3.4. Membrane characterizations
Fig. 4. Procedures for preparing BPMs containing silicon groups at the BP junction.
two-compartment cell (with a membrane effective area of 0.785 cm2 ) and the concentration of iron hydroxides immobilized at the BP junction was controlled by varying the applied current density (i.e., 38.22/63.69/95.54/127.39 A m−2 for 10 min). The current was applied by an Agilent 6613C power supply (Hewlett Packard Co. Ltd., USA) connected to two Ag/AgCl plate electrodes. The BPMs prepared were then stored in 0.5 mol dm−3 NaCl for more than a day to remove free Fe3+ ions from the CEL. The immobilization of the silicon groups was carried out according to Mauritz’s method [12]. The procedures for preparation of the silicon-containing BPMs are schematically drawn in Fig. 4. Initially, Nafion membranes were + transformed into an acid form (–SO− 3 H ) since the polymer− + bound SO3 H groups conveniently serve to catalyze the sol–gel reactions [12]. The initialized membranes were then swollen in a mixture of 5:1 (v/v) MeOH:H2 O at 25 ◦ C for 24 h to allow water and TEOS to penetrate into the membrane. One side of the membranes was brought into contact with a H2 O/TEOS mixture (H2 O:TEOS = 4:1 (mol/mol)) in a two-compartment cell (with an effective membrane area of 12.56 cm2 ) with stirring. Finally, the membranes were surface-blotted and dried at 100 ◦ C under a vacuum for 48 h to remove trapped volatiles and promote further
Morphological characterization for prepared BPMs was carried out using a field-emission scanning electron microscope (FE-SEM, S-4700, Hitachi, Japan). The influence of silicon deposition on the functional groups on a membrane surface was investigated using an attenuated total refractive Fourier transform infrared (ATR-FTIR) spectrophotometer (Jasco 460 Plus, Japan). The contact angles of the samples were measured using a contact-angle meter (Tantec, USA). To measure the contact angles, 10 µl of water was dropped onto the dried membrane surface and then the angles were measured using a magnifying glass before the water was absorbed onto the membrane surface. Water-splitting experiments were carried out in a cell with six compartments, separated by a Neosepta CMX cation-exchange membranes (Tokuyama Corp.). A sodium sulfate solution (Na2 SO4 , 0.25 mol dm−3 ) was used as the electrolyte solution and a constant current was supplied through the cell with a power supply (Agilent 6613C, Hewlett Packard Co. Ltd., USA).
4. Results and discussion 4.1. FE-SEM image of prepared BPM Fig. 6 shows the cross-sectional FE-SEM image of the prepared BPM (Nafion/AEL) and also shows a welldeveloped bipolar junction, i.e., one without any gap. The thickness of the AEL was in the range of 80–100 µm. 4.2. Immobilization of iron hydroxides at the BP junction The results of ESCA analysis are summarized in Table 1. Fe contents (mg Fe/cm2) on the membrane surface
M.-S. Kang et al. / Journal of Colloid and Interface Science 273 (2004) 533–539
537
Fig. 6. Cross-sectional FE-SEM image of prepared BPM (×300).
Table 1 ESCA analysis for estimating metal (iron) content on membrane surface Time (min)
Peak area (S)
Peak area (Fe)
Fe/S
Fe-content (mg cm−2 )
0.5 1.5 3.0 5.0 30.0 720.0
6479.1 5933.4 13258.0 12911.2 7110.6 10758.6
11512.4 14702.9 35489.9 43069.3 36645.1 77620.5
1.78 2.48 2.68 3.34 5.15 7.21
1.82 2.53 2.74 3.41 5.26 7.36
were estimated from the Fe/S ratios. As expected, the Fe content increased with an increase in the treatment time. Fig. 7 exhibits variation in the water-splitting fluxes of prepared BPMs (Method 1) according to the Fe contents at the BP junction layer. The water-splitting fluxes increased with the increasing metal content, in the range of 2.0–3.0 mg Fe/cm2 . This clearly shows that the iron hydroxides provide alternative reaction paths for water splitting by forming reactive complexes [2]. However, the water-splitting flux decreased and then leveled off at metal concentrations higher than 3.0 mg Fe/cm2 . The excessive metal hydroxides seem to interfere with the polarization of water molecules, occurring between the sulfonic acid and quaternary ammonium groups, indicating that an optimum metal concentration exists for water splitting. Based on the preliminary experimental results (Method 1), BPMs in which iron hydroxides were introduced via electrodeposition (Method 2) were successfully prepared and characterized. Fig. 8 shows the water-splitting flux and the increase rate (RWS ) according to the Fe content. As found in preliminary experiments, the water-splitting fluxes increased with increases in the metal concentration up to about 2.0 mg Fe/cm2 and decreased at higher metal contents. Therefore, the optimum metal content was determined to be about 2.0 mg Fe/cm2 . The optimal Fe content (∼2.0 mg Fe/cm2 ) of BPMs prepared by Method 2 (i.e., electrodeposition) is shown to be lower than that (∼3.0 mg Fe/cm2 ) of BPMs prepared by Method 1, demonstrating that in situ immobilization of metal hydroxides by the electrodeposition method is more effective. Since the location of the immobilized metal hydroxides is also an important factor determining the water-splitting perfor-
Fig. 7. Variation in water-splitting fluxes according to Fe/S ratios at the BP junction layer.
Fig. 8. Variations in water-splitting fluxes and increase rates according to iron contents at the BP junction (electrodeposition). RWS = ((JA − JB )/JB ) × 100, JB = water-splitting flux before immobilization, JA = water-splitting flux after immobilization.
mance, as mentioned previously, the catalytic water-splitting effects may be more effective when they are immobilized near the AEL at the junction. Note that, in the case of Method 1, the iron hydroxides were deposited on the surface of the CEL. Fig. 9 shows the increase in water splitting (RWS ) according to the compositions of the anion-exchange layer. The RWS s increased with the increase in the DVB and Sty contents in the AEL. Note that the increases in the DVB and Sty contents mean a decrease in the amount of anion-exchangeable groups (quaternary ammonium groups) at the BP junction. Since the metal hydroxides function as OH-affinity groups like quaternary ammonium groups (see Eqs. 3 and 4), the increase in the anion-exchangeable groups may diminish the apparent catalytic effects of the metal hydroxides. Therefore, it appears that the quantitative balance between the OH-affinity and H-affinity groups also affects the water-splitting characteristics at the BP junction.
538
M.-S. Kang et al. / Journal of Colloid and Interface Science 273 (2004) 533–539
Fig. 9. Variations in water-splitting flux increase rates according to the compositions of anion-exchange layer (electrodeposition).
Fig. 11. Changes in contact angles on the surface of silicon-deposited Nafion membranes. Table 2 Silicon content of bipolar junction layer (estimated from ESCA analysis) and water-splitting performance of the BPMs Time (min)
Si/S
Si content (mg cm−2 )
Water-splitting fluxa (×105 mol min−1 cm−2 )
RWS (%)
0 1.0 2.0 3.0 5.0 10.0
– 0.096 0.306 0.333 0.358 0.368
– 0.098 0.312 0.340 0.365 0.375
2.03 ± 0.09 2.13 ± 0.13 2.44 ± 0.12 2.89 ± 0.19 2.39 ± 0.14 2.32 ± 0.12
– 4.93 20.20 42.33 17.59 14.43
a CEL = Nafion 117; AEL: VBC/Sty = (3/1), DVB = 10 wt%; 0.25 M Na2 SO4 /25 ◦ C/8 V (CV operation).
Fig. 10. FT-IR/ATR spectra of silicon-deposited Nafion membranes.
4.3. Immobilization of silicon groups at the BP junction As another inorganic substance for catalytic water splitting, silicon groups (i.e., ≡Si–OH groups) were selected. The effects of silica colloids on water splitting in the anion-exchange membranes were revealed in the previous study [4]. Fig. 10 shows the ATR spectra of silicon-deposited Nafion membranes according to the treatment time. The intensities of functional peaks decreased with an increase in the silicon-treatment time, indicating the deposition of the silicon compounds on the membrane surface. Since the peaks assigned to silicon compounds mostly overlapped with those of the Nafion-virgin membrane, it is not easy to identify the chemical structures of the silicon compounds formed on the membrane surface. However, the absorption band assigned to Si–OH groups (Si–O) was seen near 844 cm−1 in Fig. 10. The area ratio (peak (844 cm−1 )/peak (1380–1100 cm−1 )) increased with an increase in the treatment time, indicating that the number of silicon groups deposited on the membrane surface gradually increased.
Fig. 11 also shows the change in the contact angles on the surface of silicon-deposited Nafion membranes (Na+ -form). The contact angles decreased with an increase in the silicontreatment time. This result also indicates the increase in hydrophilic functional groups on the surface. This increase in the hydrophilicity can be explained by the formation of numerous ≡SiOH groups to which water molecules can be hydrogen-bonded [12]. Nevertheless, a decrease in the number of sulfonic acid groups due to the immobilization of the silicon groups (i.e., screening effect) may also affect the surface hydrophilicity simultaneously. As a result, the contact angles leveled off after a silicon-treatment time of about 5–10 min. The Si contents and the water-splitting performances, according to silicon-treatment time, for the prepared BPMs are shown in Table 2. Like the iron hydroxide-treated BPMs, the water splitting varied with the immobilization time and the optimal Si content was determined to be ∼0.34 mg Si/cm2 . The decrease in the water-splitting capabilities over the optimal Si content can be explained by the decrease in the number of sulfonic acid groups due to the excess number of silicon groups. Namely, the polarization effects brought on by the sulfonic acid groups at the BP junction seem to be weakened. The maximum RWS was found to be about 42% at the optimal Si content. Fig. 12 shows the changes in the
M.-S. Kang et al. / Journal of Colloid and Interface Science 273 (2004) 533–539
539
5.58 × 10−5 mol min−1 cm−2 ) under the same operation conditions.
5. Conclusions
Fig. 12. Variations in water-splitting fluxes of Nafion/AEL membranes with and without Si compounds at the BP junction according to operation time.
Table 3 Water-splitting performances of Nafion/AEL BPMs with and without catalytic inorganic substances (Si and Fe compounds) Membranea Nafion/AEL Nafion/Si/AELc Nafion/Fe/AELd
Water-splitting fluxb (×105 mol min−1 cm−2 )
RWS (%)
3.84 ± 0.12 5.13 ± 0.14 5.24 ± 0.15
– 34.90 36.46
a AEL: VBC/Sty = 1/2 (w/w); DVB = 6 wt%; amination time = 30 min. b Measured under constant voltage condition (10 V); 0.25 M Na SO ; 2 4 25 ◦ C. c Si content = 0.34 mg Si/cm2 (sol–gel processing). d Fe content = 1.99 mg Fe/cm2 (electrodeposition).
water-splitting fluxes as a function of elapsed time during a constant-voltage operation. The water-splitting fluxes of the BPM containing the silicon groups were somewhat enhanced during the second run, indicating that the activation of the silicon groups at the BP junction proceeded during an operation under reverse bias conditions. Table 3 shows the water-splitting performances of Nafion/ AEL BPMs with/without catalytic inorganic substances (Si and Fe compounds of optimal content). The result clearly shows that water-splitting performance of BPMs is dramatically enhanced by introducing inorganic matter with optimal content and silicon groups as well as transitionmetal hydroxides can be utilized as an effective catalyst for enhancing BPM performance. Although the catalytic effects of Si and Fe compounds could not be directly compared due to the different immobilizing method, the watersplitting performances of the BPMs were comparable at their optimal contents. In the optimum preparation condition, the water-splitting flux of the BPM (i.e., Nafion/Fe/AEL, 5.24 × 10−5 mol min−1 cm−2 ) was comparable to that of the commercialized BPM (i.e., BP-1, Tokuyama Corp., Japan,
In this study, metal hydroxides and silicon groups were quantitatively evaluated as catalysts for enhancing the watersplitting capability of BPMs. To immobilize the inorganic substances at the BP junction, novel methods (in situ electrodeposition of iron hydroxide and sol–gel processing to introduce silicon groups) were suggested. The water-splitting fluxes were significantly enhanced by the immobilization of the inorganic catalysts at their optimum content. The results imply that the inorganic substance provides alternative paths for water dissociation, but on the other hand they may reduce the polarization effects between the sulfonic acid and quaternary ammonium groups when present at high concentrations. From the results, it was concluded that the concentration of the catalyst should be optimized to maximize the water-splitting properties of the bipolar membrane. Moreover, it was revealed that the silicon group (i.e., ≡Si–OH), as well as the metal hydroxides, could be utilized as an effective water-splitting catalyst.
Acknowledgment This work was supported by the National Research Laboratory (NRL) Program of Korea Institute of Science and Technology Evaluation and Planning (Project 2000-N-NL01-C-185).
References [1] A.J.B. Kemperman (Ed.), Handbook on Bipolar Membrane Technology, Twente Univ. Press, Enschede, 2000. [2] R. Simons, J. Membr. Sci. 78 (1993) 13. [3] F. Hanada, K. Hirayama, N. Ohmura, S. Tanaka, U.S. Patent 5,221,455, 1993. [4] M.-S. Kang, Y.-J. Choi, H.-J. Lee, S.-H. Moon, J. Colloid Interface Sci. 273 (2004) 523–532. [5] N.V. Shel’deshov, V.I. Zabolotskii, V.V. Ganych, Russian J. Electrochem. 30 (12) (1994) 1458. [6] R. Simons, Electrochim. Acta 30 (3) (1985) 275. [7] L. Jialin, W. Yazhen, Y. Changying, L. Guangdou, S. Hong, J. Membr. Sci. 147 (1998) 247. [8] R. Simons, Electrochim. Acta 31 (9) (1986) 1175. [9] M.-S. Kang, A. Tanioka, S.-H. Moon, Korean J. Chem. Eng. 19 (1) (2002) 99. [10] C.J. Brinker, G.W. Scherer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, San Diego, 1990. [11] F. Jimbo, M. Higa, N. Minoura, A. Tanioka, Macromolecules 31 (1998) 1277. [12] Q. Deng, R.B. Moore, K.A. Mauritz, J. Appl. Polym. Sci. 68 (1998) 747.
Effects of inorganic substances on water splitting in ion-exchange membranes II. Optimal contents of inorganic substances in preparing bipolar membranes Moon-Sung Kang, Yong-Jin Choi, and Seung-Hyeon Moon ∗ Department of Environmental Science and Engineering, Kwangju Institute of Science & Technology (K-JIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, South Korea Received 22 July 2003; accepted 23 January 2004
Abstract An approach to enhancing the water-splitting performance of bipolar membranes (BPMs) is introducing an inorganic substance at the bipolar (BP) junction. In this study, the immobilization of inorganic matters (i.e., iron hydroxides and silicon compounds) at the BP junction and the optimum concentration have been investigated. To immobilize these inorganic matters, novel methods (i.e., electrodeposition of the iron hydroxide and processing of the sol–gel to introduce silicon groups at the BP junction) were suggested. At optimal concentrations, the immobilized inorganic matters significantly enhanced the water-splitting fluxes, indicating that they provide alternative paths for water dissociation, but on the other hand possibly reduce the polarization of water molecules between the sulfonic acid and quaternary ammonium groups at high contents. Consequently, the amount of inorganic substances introduced should be optimized to obtain the maximum water splitting in the BPM. 2004 Elsevier Inc. All rights reserved. Keywords: Water-splitting performance; Bipolar membranes; Inorganic substance; Electrodeposition; Polarization of water molecules
1. Introduction Water-splitting electrodialysis (WSED) using bipolar membrane (BPM) is known to be an efficient technology for producing acid/base without the production of undesirable by-products and the use of additional chemicals. WSED technology is also considered to be a more cost-effective process, compared to conventional electrolysis for the production of acids and bases [1]. Since BPM is a key component in WSED, the development of high-performance BPM is essential to increase the process efficiency. It is known that the water-splitting property of a BPM can be greatly enhanced by introducing an inorganic substance, such as metal hydroxides, at the bipolar (BP) junction [1–3]. The major factors contributing to the enhancement in water splitting by inorganic substances may include (i) increase in the electric fields generated at the BP junction [4]; (ii) catalytic water* Corresponding author.
E-mail address: [email protected] (S.-H. Moon). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.01.051
splitting reactions between metal hydroxides (or metal ions) and water molecules [5,6]; (iii) increase in the water activity (i.e., enhancement of interface wetness) [2]; (iv) increase in the looseness of the bonds of water [3,7]. It seems that these factors contribute the water-splitting mechanism complexly. In a previous study, the electrochemical characteristics of ion-exchange membranes coated with iron hydroxide/oxide and silica sol have been investigated [4]. From the results, the bipolar structure immobilized on the membrane surface significantly increased water splitting due to the enhancement in water polarization with the help of strong electric fields. Moreover, it was revealed that metal oxides or ≡Si– OH groups, as well as metal hydroxides, are effective catalysts for water dissociation. In this study, iron hydroxides and silicon groups (i.e., ≡Si–OH) have been examined as possible catalysts to enhance the water-splitting capability of BPM. To immobilize the inorganic substances at a BP junction, novel methods (i.e., electrodeposition of iron hydroxide and in situ sol–gel processing to introduce the sili-
534
M.-S. Kang et al. / Journal of Colloid and Interface Science 273 (2004) 533–539
Fig. 2. pC–pH diagram of hydroxo iron(III) complexes.
Fe(H2 O)2 (OH)− 4 + H2 O → Fe(H2 O)3 (OH)3(S) + OH− .
(4)
The metal species may react with the quaternary ammonium groups of the AEL. The reaction mechanisms are [9]
Fig. 1. Schematic illustration of water-splitting mechanism by iron hydroxides at the BP junction.
con groups) were suggested and their optimum contents for a high-performance bipolar membrane were evaluated.
2. Water splitting mechanism by inorganic substances Fig. 1 shows a schematic illustration of the mechanism of water splitting by metal hydroxides at a BP junction. It was assumed that the pH in the reaction layer where water splitting occurs varies from about zero, at the boundary in the cation exchange layer, to about 14, at the boundary region in the anion exchange layer of a BPM [8]. Therefore, based on the pC–pH diagram for hydroxo iron(III) complexes (Fig. 2), the reactions occurring under acidic conditions near the boundary region of the cation exchange layer (CEL) are expected to be 2+ + H3 O+ , Fe(H2 O)3+ 6 + H2 O → Fe(H2 O)5 (OH)
Fe(H2 O)5 (OH)2+ + H2 O → Fe(H2 O)3+ 6
+ OH− .
(5)
→ M(H2 O)3 (OH)3(S) + B+ + OH− , B+
(6) N+ )
where is a quaternary ammonium group (–R3 in the anion-exchange layer. This reaction model indicates that the metal species are immobilized in the BPMs as a hydroxide and react with the water molecules and the quaternary ammonium groups reversibly in the presence of a strong local electric field. This reaction mechanism seems to be dominant in water-splitting reactions induced by metallic species. Likewise, other inorganic matter such as silicon hydroxide groups (≡SiOH) might also accelerate water splitting at a BP junction. In this case, water splitting may occur between ≡SiOH groups and quaternary ammonium groups, as well as between ≡SiOH and sulfonic acid groups, according to the following reactions [10]: under acidic conditions near CEL at the BP junction,
(1)
− ≡SiOH + H2 O ≡SiOH+ 2 + OH ,
(7)
(2)
+ ≡SiOH+ 2 + H2 O ≡SiOH + H3 O ;
(8)
In this case, however, positively charged metal ions may easily migrate out of the membrane under an electric field. The catalytic reactions occurring under basic conditions near the boundary region of the anion exchange layer (AEL) are as follows: Fe(H2 O)3 (OH)3(S) + H2 O + → Fe(H2 O)2 (OH)− 4 + H3 O ,
M(H2 O)3 (OH)3(S) + B+ + H2 O + → (B+ ) M(H2 O)2 (OH)− 4 + H3 O , (B+ ) M(H2 O)2 (OH)− 4 + H2 O
(3)
under basic conditions near AEL at the BP junction, ≡SiOH + H2 O ≡SiO− + H3 O+ ,
(9)
≡SiO− + H2 O ≡SiOH + OH− .
(10)
The reaction by which protons are added to or removed from the oxide (i.e., the acidity of the M–OH group (M = metal)) depends on the metal atom [10]. The pH at which the particle
M.-S. Kang et al. / Journal of Colloid and Interface Science 273 (2004) 533–539
is neutrally charged is called the point of zero charge (PZC). At pH < PZC, Eq. (7) predominates, and the particles are positively charged, whereas at pH > PZC, Eq. (9) gives the particle a negative charge. Note that the PZC of silicon oxides is near 2.5 [10]. The surface charge density of ≡Si–OH groups, σs , at the BP junction may be represented by [11] eN σs.b = 1 + 10(pHs−PZC) near the cation-exchange layer, (11) eN σs.a = − 1 + 10(PZC−pHs) near the anion-exchange layer, (12) where PZC is the point of zero charge of the Si–OH group, pHs the surface pH, N the total site charge density, and e the Coulombic charge. Therefore, it is expected that the water-splitting reactions of ≡Si–OH groups may occur at the BP interface near both the anion-exchange layer and the cation-exchange layer. The reversible water-splitting reactions can proceed according to Eqs. (7)–(10) since dissociated water ions (i.e., H+ and OH− ) are instantly removed from the reaction layer under the reverse bias condition.
3. Experimental 3.1. Materials Neosepta CM-1 (Tokuyama Corp., Japan) and Nafion 117 (DuPont, USA) were used as a cation-exchange layer (CEL) in the preparation of a bipolar membrane. A traditional paste method was employed to prepare the anion-exchange layer (i.e., poly(vinylbenzyl) chloride–divinylbenzene–styrene). Vinylbenzyl chloride (VBC, Aldrich, USA) was selected as the F-monomer to introduce the quaternary ammonium groups and styrene (Sty, Kanto Chem., Japan) was used for the co-polymerization. Divinylbenzene (DVB, Aldrich, USA) was used as the crosslinker and benzoyl peroxide (BPO, Fluka, Switzerland) as the initiator. VBC was purified to remove inhibitors using the inhibitor-removal column (Aldrich, USA). Sty and DVB were treated with 10 wt% aqueous sodium hydroxide and then were washed with distilled water several times. BPO was recrystallized in a methanol/water mixture at −5 ◦ C to purify it. Acrylonitrile butadiene rubber (NBR) was added to the monomer mixture to enhance the mechanical properties of the membrane. In the monomer mixture, the weight ratio VBC to Sty was varied in the range 0.5–3 and the contents of DVB were 3, 6, or 10 wt%. The weight fraction of NBR relative to the monomer mixture (VBC/Sty/DVB) was fixed at 1:6. To immobilize the inorganic substances at a BP junction, iron(III) chloride (Aldrich, USA) and tetraethoxylsilane (TEOS, Si(OC2 H5 )4 , Aldrich, USA) were employed as the precursors.
535
3.2. Bipolar membrane preparation To prepare the BPMs, initially, the surface of the cationexchange membrane (CM-1/Nafion 117) was roughened with a fine sandpaper to increase the contact area and, thereby, the strength of the bonding [1,3]. The monomer paste was cast onto the cation-exchange membrane and fixed on a glass plate, and then another glass plate was superimposed upon it. The glass plates were then sealed with a cloth tape to prevent leakage of the monomers during copolymerization. Polymerization was allowed to continue for 4 h at 80 ◦ C and then the prepared membranes were immersed in a 0.5 mol dm−3 trimethylamine (TMA, Aldrich, USA) aqueous solution to introduce quaternary ammonium groups. After being washed with distilled water, the prepared membranes were stored in 0.5 mol dm−3 NaCl. 3.3. Immobilization of inorganic substances To immobilize iron hydroxides (amorphous Fe(OH)3 ) at the BP junction, two methods were employed in this study. First, iron hydroxides were immobilized on membrane (CM-1/Nafion 117) surface by immersing the membrane successively in a 0.025 mol dm−3 FeCl3 solution and a 0.1 mol dm−3 NaOH solution for a certain time. After the immobilization of iron hydroxides, the BPMs were prepared according to previously mentioned procedure. The metal contents were quantitatively analyzed via an electron spectroscopy for chemical analysis (ESCA, ESCALAB MK-II, VG Scientific Ltd., England). The second method was the electrodeposition of iron hydroxides. Fig. 3 shows the schematic drawing of the electrodeposition of metal hydroxides at a BP junction. It was expected that the iron hydroxides would be immobilized near the AEL at the BP junction under a forward-bias condition (with a low current flow). The immobilization was performed using a
Fig. 3. Schematic illustration describing the electrodeposition of iron hydroxides at the BP junction.
536
M.-S. Kang et al. / Journal of Colloid and Interface Science 273 (2004) 533–539 Hydrolysis H+
Si(OR)4 + H2 O → (OR)3 Si–OH + ROH Condensation (RO)3 Si–OH + RO–Si(OR)3 → (RO)3 Si–O–Si(OR)3 + ROH Substitution Si–(OEt) + MeOH → Si(OMe) + EtOH (R = Et or H); TEOS, tetraethoxylsilane (Si(OC2 H5 )4 ) Fig. 5. Acid-catalyzed sol–gel reactions occurring within H2 O/TEOS mixture [12].
condensation of the SiOH groups. Fig. 5 exhibits the acidcatalyzed sol–gel reactions occurring within the H2 O/TEOS mixture. The silicon contents were also quantitatively evaluated via ESCA. After the sol–gel process, BPMs were prepared according to the preparation method described in the previous section. The BPMs prepared were then stored in 0.5 mol dm−3 NaCl for more than a day. 3.4. Membrane characterizations
Fig. 4. Procedures for preparing BPMs containing silicon groups at the BP junction.
two-compartment cell (with a membrane effective area of 0.785 cm2 ) and the concentration of iron hydroxides immobilized at the BP junction was controlled by varying the applied current density (i.e., 38.22/63.69/95.54/127.39 A m−2 for 10 min). The current was applied by an Agilent 6613C power supply (Hewlett Packard Co. Ltd., USA) connected to two Ag/AgCl plate electrodes. The BPMs prepared were then stored in 0.5 mol dm−3 NaCl for more than a day to remove free Fe3+ ions from the CEL. The immobilization of the silicon groups was carried out according to Mauritz’s method [12]. The procedures for preparation of the silicon-containing BPMs are schematically drawn in Fig. 4. Initially, Nafion membranes were + transformed into an acid form (–SO− 3 H ) since the polymer− + bound SO3 H groups conveniently serve to catalyze the sol–gel reactions [12]. The initialized membranes were then swollen in a mixture of 5:1 (v/v) MeOH:H2 O at 25 ◦ C for 24 h to allow water and TEOS to penetrate into the membrane. One side of the membranes was brought into contact with a H2 O/TEOS mixture (H2 O:TEOS = 4:1 (mol/mol)) in a two-compartment cell (with an effective membrane area of 12.56 cm2 ) with stirring. Finally, the membranes were surface-blotted and dried at 100 ◦ C under a vacuum for 48 h to remove trapped volatiles and promote further
Morphological characterization for prepared BPMs was carried out using a field-emission scanning electron microscope (FE-SEM, S-4700, Hitachi, Japan). The influence of silicon deposition on the functional groups on a membrane surface was investigated using an attenuated total refractive Fourier transform infrared (ATR-FTIR) spectrophotometer (Jasco 460 Plus, Japan). The contact angles of the samples were measured using a contact-angle meter (Tantec, USA). To measure the contact angles, 10 µl of water was dropped onto the dried membrane surface and then the angles were measured using a magnifying glass before the water was absorbed onto the membrane surface. Water-splitting experiments were carried out in a cell with six compartments, separated by a Neosepta CMX cation-exchange membranes (Tokuyama Corp.). A sodium sulfate solution (Na2 SO4 , 0.25 mol dm−3 ) was used as the electrolyte solution and a constant current was supplied through the cell with a power supply (Agilent 6613C, Hewlett Packard Co. Ltd., USA).
4. Results and discussion 4.1. FE-SEM image of prepared BPM Fig. 6 shows the cross-sectional FE-SEM image of the prepared BPM (Nafion/AEL) and also shows a welldeveloped bipolar junction, i.e., one without any gap. The thickness of the AEL was in the range of 80–100 µm. 4.2. Immobilization of iron hydroxides at the BP junction The results of ESCA analysis are summarized in Table 1. Fe contents (mg Fe/cm2) on the membrane surface
M.-S. Kang et al. / Journal of Colloid and Interface Science 273 (2004) 533–539
537
Fig. 6. Cross-sectional FE-SEM image of prepared BPM (×300).
Table 1 ESCA analysis for estimating metal (iron) content on membrane surface Time (min)
Peak area (S)
Peak area (Fe)
Fe/S
Fe-content (mg cm−2 )
0.5 1.5 3.0 5.0 30.0 720.0
6479.1 5933.4 13258.0 12911.2 7110.6 10758.6
11512.4 14702.9 35489.9 43069.3 36645.1 77620.5
1.78 2.48 2.68 3.34 5.15 7.21
1.82 2.53 2.74 3.41 5.26 7.36
were estimated from the Fe/S ratios. As expected, the Fe content increased with an increase in the treatment time. Fig. 7 exhibits variation in the water-splitting fluxes of prepared BPMs (Method 1) according to the Fe contents at the BP junction layer. The water-splitting fluxes increased with the increasing metal content, in the range of 2.0–3.0 mg Fe/cm2 . This clearly shows that the iron hydroxides provide alternative reaction paths for water splitting by forming reactive complexes [2]. However, the water-splitting flux decreased and then leveled off at metal concentrations higher than 3.0 mg Fe/cm2 . The excessive metal hydroxides seem to interfere with the polarization of water molecules, occurring between the sulfonic acid and quaternary ammonium groups, indicating that an optimum metal concentration exists for water splitting. Based on the preliminary experimental results (Method 1), BPMs in which iron hydroxides were introduced via electrodeposition (Method 2) were successfully prepared and characterized. Fig. 8 shows the water-splitting flux and the increase rate (RWS ) according to the Fe content. As found in preliminary experiments, the water-splitting fluxes increased with increases in the metal concentration up to about 2.0 mg Fe/cm2 and decreased at higher metal contents. Therefore, the optimum metal content was determined to be about 2.0 mg Fe/cm2 . The optimal Fe content (∼2.0 mg Fe/cm2 ) of BPMs prepared by Method 2 (i.e., electrodeposition) is shown to be lower than that (∼3.0 mg Fe/cm2 ) of BPMs prepared by Method 1, demonstrating that in situ immobilization of metal hydroxides by the electrodeposition method is more effective. Since the location of the immobilized metal hydroxides is also an important factor determining the water-splitting perfor-
Fig. 7. Variation in water-splitting fluxes according to Fe/S ratios at the BP junction layer.
Fig. 8. Variations in water-splitting fluxes and increase rates according to iron contents at the BP junction (electrodeposition). RWS = ((JA − JB )/JB ) × 100, JB = water-splitting flux before immobilization, JA = water-splitting flux after immobilization.
mance, as mentioned previously, the catalytic water-splitting effects may be more effective when they are immobilized near the AEL at the junction. Note that, in the case of Method 1, the iron hydroxides were deposited on the surface of the CEL. Fig. 9 shows the increase in water splitting (RWS ) according to the compositions of the anion-exchange layer. The RWS s increased with the increase in the DVB and Sty contents in the AEL. Note that the increases in the DVB and Sty contents mean a decrease in the amount of anion-exchangeable groups (quaternary ammonium groups) at the BP junction. Since the metal hydroxides function as OH-affinity groups like quaternary ammonium groups (see Eqs. 3 and 4), the increase in the anion-exchangeable groups may diminish the apparent catalytic effects of the metal hydroxides. Therefore, it appears that the quantitative balance between the OH-affinity and H-affinity groups also affects the water-splitting characteristics at the BP junction.
538
M.-S. Kang et al. / Journal of Colloid and Interface Science 273 (2004) 533–539
Fig. 9. Variations in water-splitting flux increase rates according to the compositions of anion-exchange layer (electrodeposition).
Fig. 11. Changes in contact angles on the surface of silicon-deposited Nafion membranes. Table 2 Silicon content of bipolar junction layer (estimated from ESCA analysis) and water-splitting performance of the BPMs Time (min)
Si/S
Si content (mg cm−2 )
Water-splitting fluxa (×105 mol min−1 cm−2 )
RWS (%)
0 1.0 2.0 3.0 5.0 10.0
– 0.096 0.306 0.333 0.358 0.368
– 0.098 0.312 0.340 0.365 0.375
2.03 ± 0.09 2.13 ± 0.13 2.44 ± 0.12 2.89 ± 0.19 2.39 ± 0.14 2.32 ± 0.12
– 4.93 20.20 42.33 17.59 14.43
a CEL = Nafion 117; AEL: VBC/Sty = (3/1), DVB = 10 wt%; 0.25 M Na2 SO4 /25 ◦ C/8 V (CV operation).
Fig. 10. FT-IR/ATR spectra of silicon-deposited Nafion membranes.
4.3. Immobilization of silicon groups at the BP junction As another inorganic substance for catalytic water splitting, silicon groups (i.e., ≡Si–OH groups) were selected. The effects of silica colloids on water splitting in the anion-exchange membranes were revealed in the previous study [4]. Fig. 10 shows the ATR spectra of silicon-deposited Nafion membranes according to the treatment time. The intensities of functional peaks decreased with an increase in the silicon-treatment time, indicating the deposition of the silicon compounds on the membrane surface. Since the peaks assigned to silicon compounds mostly overlapped with those of the Nafion-virgin membrane, it is not easy to identify the chemical structures of the silicon compounds formed on the membrane surface. However, the absorption band assigned to Si–OH groups (Si–O) was seen near 844 cm−1 in Fig. 10. The area ratio (peak (844 cm−1 )/peak (1380–1100 cm−1 )) increased with an increase in the treatment time, indicating that the number of silicon groups deposited on the membrane surface gradually increased.
Fig. 11 also shows the change in the contact angles on the surface of silicon-deposited Nafion membranes (Na+ -form). The contact angles decreased with an increase in the silicontreatment time. This result also indicates the increase in hydrophilic functional groups on the surface. This increase in the hydrophilicity can be explained by the formation of numerous ≡SiOH groups to which water molecules can be hydrogen-bonded [12]. Nevertheless, a decrease in the number of sulfonic acid groups due to the immobilization of the silicon groups (i.e., screening effect) may also affect the surface hydrophilicity simultaneously. As a result, the contact angles leveled off after a silicon-treatment time of about 5–10 min. The Si contents and the water-splitting performances, according to silicon-treatment time, for the prepared BPMs are shown in Table 2. Like the iron hydroxide-treated BPMs, the water splitting varied with the immobilization time and the optimal Si content was determined to be ∼0.34 mg Si/cm2 . The decrease in the water-splitting capabilities over the optimal Si content can be explained by the decrease in the number of sulfonic acid groups due to the excess number of silicon groups. Namely, the polarization effects brought on by the sulfonic acid groups at the BP junction seem to be weakened. The maximum RWS was found to be about 42% at the optimal Si content. Fig. 12 shows the changes in the
M.-S. Kang et al. / Journal of Colloid and Interface Science 273 (2004) 533–539
539
5.58 × 10−5 mol min−1 cm−2 ) under the same operation conditions.
5. Conclusions
Fig. 12. Variations in water-splitting fluxes of Nafion/AEL membranes with and without Si compounds at the BP junction according to operation time.
Table 3 Water-splitting performances of Nafion/AEL BPMs with and without catalytic inorganic substances (Si and Fe compounds) Membranea Nafion/AEL Nafion/Si/AELc Nafion/Fe/AELd
Water-splitting fluxb (×105 mol min−1 cm−2 )
RWS (%)
3.84 ± 0.12 5.13 ± 0.14 5.24 ± 0.15
– 34.90 36.46
a AEL: VBC/Sty = 1/2 (w/w); DVB = 6 wt%; amination time = 30 min. b Measured under constant voltage condition (10 V); 0.25 M Na SO ; 2 4 25 ◦ C. c Si content = 0.34 mg Si/cm2 (sol–gel processing). d Fe content = 1.99 mg Fe/cm2 (electrodeposition).
water-splitting fluxes as a function of elapsed time during a constant-voltage operation. The water-splitting fluxes of the BPM containing the silicon groups were somewhat enhanced during the second run, indicating that the activation of the silicon groups at the BP junction proceeded during an operation under reverse bias conditions. Table 3 shows the water-splitting performances of Nafion/ AEL BPMs with/without catalytic inorganic substances (Si and Fe compounds of optimal content). The result clearly shows that water-splitting performance of BPMs is dramatically enhanced by introducing inorganic matter with optimal content and silicon groups as well as transitionmetal hydroxides can be utilized as an effective catalyst for enhancing BPM performance. Although the catalytic effects of Si and Fe compounds could not be directly compared due to the different immobilizing method, the watersplitting performances of the BPMs were comparable at their optimal contents. In the optimum preparation condition, the water-splitting flux of the BPM (i.e., Nafion/Fe/AEL, 5.24 × 10−5 mol min−1 cm−2 ) was comparable to that of the commercialized BPM (i.e., BP-1, Tokuyama Corp., Japan,
In this study, metal hydroxides and silicon groups were quantitatively evaluated as catalysts for enhancing the watersplitting capability of BPMs. To immobilize the inorganic substances at the BP junction, novel methods (in situ electrodeposition of iron hydroxide and sol–gel processing to introduce silicon groups) were suggested. The water-splitting fluxes were significantly enhanced by the immobilization of the inorganic catalysts at their optimum content. The results imply that the inorganic substance provides alternative paths for water dissociation, but on the other hand they may reduce the polarization effects between the sulfonic acid and quaternary ammonium groups when present at high concentrations. From the results, it was concluded that the concentration of the catalyst should be optimized to maximize the water-splitting properties of the bipolar membrane. Moreover, it was revealed that the silicon group (i.e., ≡Si–OH), as well as the metal hydroxides, could be utilized as an effective water-splitting catalyst.
Acknowledgment This work was supported by the National Research Laboratory (NRL) Program of Korea Institute of Science and Technology Evaluation and Planning (Project 2000-N-NL01-C-185).
References [1] A.J.B. Kemperman (Ed.), Handbook on Bipolar Membrane Technology, Twente Univ. Press, Enschede, 2000. [2] R. Simons, J. Membr. Sci. 78 (1993) 13. [3] F. Hanada, K. Hirayama, N. Ohmura, S. Tanaka, U.S. Patent 5,221,455, 1993. [4] M.-S. Kang, Y.-J. Choi, H.-J. Lee, S.-H. Moon, J. Colloid Interface Sci. 273 (2004) 523–532. [5] N.V. Shel’deshov, V.I. Zabolotskii, V.V. Ganych, Russian J. Electrochem. 30 (12) (1994) 1458. [6] R. Simons, Electrochim. Acta 30 (3) (1985) 275. [7] L. Jialin, W. Yazhen, Y. Changying, L. Guangdou, S. Hong, J. Membr. Sci. 147 (1998) 247. [8] R. Simons, Electrochim. Acta 31 (9) (1986) 1175. [9] M.-S. Kang, A. Tanioka, S.-H. Moon, Korean J. Chem. Eng. 19 (1) (2002) 99. [10] C.J. Brinker, G.W. Scherer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, San Diego, 1990. [11] F. Jimbo, M. Higa, N. Minoura, A. Tanioka, Macromolecules 31 (1998) 1277. [12] Q. Deng, R.B. Moore, K.A. Mauritz, J. Appl. Polym. Sci. 68 (1998) 747.