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Acid-functionalised periodic mesoporous benzenosilica proton conductors
Solid State Ionics 225 (2012) 308–311
Contents lists available at SciVerse ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Acid-functionalised periodic mesoporous benzenosilica proton conductors Eddy M. Domingues, Maria A. Salvador, Paula Ferreira, Filipe M. Figueiredo ⁎ Department of Materials & Ceramic Engineering, CICECO, University of Aveiro, 3810‐193 Aveiro, Portugal
a r t i c l e
i n f o
Article history: Received 27 September 2011 Received in revised form 1 June 2012 Accepted 6 June 2012 Available online 23 June 2012 Keywords: Mesoporous Hybrids Organosilica Proton conductivity Fuel cell
a b s t r a c t An investigation of proton conducting periodic mesoporous benzenosilica hybrids functionalised with sulfonic or phosphonic acids obtained by a co-condensation route is presented. Protonic conductivity data obtained under different temperatures and relative humidity conditions are reported in order to assess the combined effects of composition and concentration of the functional acid group, and microstructural features such as pore size, specific surface area and meso- and molecular-scale order. It is found that the conductivity increases significantly with increasing acid load, specific surface area and relative humidity. A proton conductivity of 0.3 Sm− 1 at 100 °C and 100% relative humidity is reported for a sulfonic acid functionalised PMO which is three orders of magnitude higher than the value reported for a similar material obtained by grafting. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Acid-functionalised periodic mesoporous organosilicas (PMO) [1] may display high levels of proton conductivity [2–4] and can thus be suitable fillers to enhance the properties of polymeric membranes such as Nafion® [5]. The presence of a bissilylated benzene bridge along with sulfonic or phosphonic acid functionality may contribute to water mediated proton structural diffusion via hierarchically ordered structures with meso- and molecular-scale periodicity [6]. These structures may in principle be tuned to enhance the proton conductivity. Here we explore a novel co-condensation synthesis route of acidfunctionalised benzene bridged PMOs in order to assess the potential of these materials for application in components for proton exchange membrane fuel cells. 2. Experimental procedures Different amounts of 4-(trimethoxysilyl) benzylphosphonate (PSiP) or (3-mercaptopropyl)trimethoxysilane (MPTMS) were cocondensed with 1,4-bis(triethoxysilyl)benzene (BTEB) precursor in the presence of octadecyltrimethylammonium bromide (C18TMABr, CH3(CH2)17N(Br)(CH3)3, average Mn = ~ 392.5) or a tri-block copolymer (Pluronic®P123, EO20PO70EO20, average Mn = ~ 5800) structure directing agents in order to obtain materials with different levels of functionalisation and porosity, as schematised in Fig. 1. Microwave radiation was used in some steps of the synthesis to improve reaction ⁎ Corresponding author at: University of Aveiro, CICECO - Centre for Research in Ceramics and Composite Materials, Complexo de Laboratórios Tecnológicos, Room 29.3.26, 3810‐193 Aveiro, Portugal. Tel.: + 351 234 401 464; fax: + 351 234 370 204. E-mail address: [email protected] (F.M. Figueiredo). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2012.06.010
kinetics [7]. The structure and microstructure of the samples were assessed by a range of techniques, including X-ray diffraction (XRD) with Cu–Kα X-radiation (Rigaku Geigerflex D/Max–C) and with Cu–Kβ X-radiation (Bruker D8 Discover), transmission electron microscopy (TEM) imaging (Hitachi H9000-NA) − 196 °C nitrogen adsorption/desorption isotherms (Micromeritics Gemini V2.00) and potentiometric titration. Also, although not directly presented here, Fourier transform infrared spectroscopy (FTIR) of KBr pellets (FTIR Mattson-7000) was used to confirm the surfactant removal, and 29 Si, 31P and 13C solid-state magic-angle spinning nuclear magnetic resonance (MAS-NMR) – in some cases with the cross-polarisation technique (Bruker Avance 400 and 500) – was used to confirm the molecular structure of the compounds. The water uptake capacity (W) of the materials was determined as W = (Ww − Wd)Wd− 1 × 100, where Wd and Ww are the weights of the sample measured in wet and dry states. The Wd was determined after exposing the sample to an atmosphere of 97.3% relative humidity in a closed desiccator with a saturated K2SO4 solution, at room temperature for ca. 48 h. The weight of the samples was monitored over time in order to ensure that an equilibrium value was attained. The difference Ww − Wd was obtained directly from the plateaus of thermogravimetric curves collected in air. A list of the prepared samples is given in Table 1, together with some of the most relevant microstructural features. The proton conductivity (σ) was assessed by impedance spectroscopy of pelletized samples at different temperatures and humidity conditions. Disc shaped samples with a density of about 1 g cm − 3 were obtained after pressing the powders in a uniaxial press at 1.53 MPa, and then isostatically at 300 MPa. Silver electrodes were applied on both sides of the pellets by painting a commercial paste (Agar Scientific) and firing at 160 °C for 30 min. The spectra were collected in a pseudo 4-electrode configuration with an Agilent 2980A
E.M. Domingues et al. / Solid State Ionics 225 (2012) 308–311
309
Fig. 1. Synthesis of (A) sulfonic acid- and (B) phosphonic acid-functionalised PMOs.
LCR meter in the frequency range 20–2 × 10 6 Hz and using test signals of variable amplitude (10 mV–1 V range) to test for possible nonlinear system responses and at 100 mV for the final conductivity measurements. The samples were mounted on an appropriate support and placed in a climatic chamber made in-house, where variable relative humidity (r.h.) conditions were achieved by forcing an air stream (50 mL min − 1) through a water bath under controlled temperature. The temperature and r.h. were constantly monitored close to the sample with a capacitive humidity sensor. The temperature of the humidifier necessary to obtain 100% r.h. was adjusted in order to be as close as possible to the sample temperature in order to minimise possible water condensation effects. The spectra were analysed with ZView (Version 2.6b, 1990–2002, Scribner Associates) to assess the ohmic resistance (R) of the pellet, which was then used to obtain the conductivity by σ = R − 1(LS − 1), where L is the sample thickness and S is the electrodes surface area.
of the pore walls (taken as a0 minus the pore diameter) is considerably larger for the phosphonic acid-based materials. This large difference suggests a much less dense pore wall in the case of the P-PMOs.
A
S-PMO-1 P-PMO-1
3. Results and discussion 3.1. Materials synthesis and characterisation Fig. 2(A) shows typical powder XRD patterns collected at low diffraction angles for one sulfonic acid – (S-PMO-1) and one phosphonic acid – (P-PMO-1) functionalised materials. Both can be indexed onto the two-dimensional hexagonal symmetry (P6mm), as expected from the envisaged mesopore structure [1]. The d100 spacing of 4.29 nm for S-PMO-1 and 8.28 nm for P-PMO-1 correspond to pore structure cell parameters a0 = 2d1003 − 1/2 of 4.95 and 9.56 nm for S-PMO-1 and PPMO-1, respectively. The larger mesostructure cell parameter of the latter materials can be explained primarily by the much larger size of the P123 surfactant molecules when compared to C18TMABr, thus directing the formation of larger, ordered structures. Other factors such as the nature of ligand (ionic C18, amphoteric P123) and the pH of the synthesis media (basic for C18, acid for P123) may also have an effect, although expectedly less important. Since the average pore diameter (Table 1) is similar for all materials, the thickness
B
C
Table 1 List of prepared samples and relevant properties. Sample
SBET (m2 g− 1)
BJH average pore diameter (nm)
Acid loading (meq [H+] g− 1)
W (%)
Activation energy (kJ mol− 1)
P-PMO-1 P-PMO-2 S-PMO-1 S-PMO-2
616 82 774 715
3.3 3.4 3.4 3.2
0.24 1.83 0.30 0.58
27 12 34 42
31 15 11 19
Fig. 2. (A) Powder XRD patterns and (B,C) TEM images of (B) S-PMO-1 and (C) P-PMO-1 materials. The inset in (B) shows a view of the hexagonal array of pores along the (001) direction.
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Indeed, the XRD patterns of the sulfonic-based materials (with much thinner wall) display an additional reflection with 0.76 nm spacing (Fig. 2(A)), which is ascribed to the molecular scale periodicity in the pore wall, along the (001) direction [1]. This reflection is not apparent in the patterns of the phosphonic materials, which is indicative of an amorphous pore wall structure. This is to some extent typical of PMOs obtained with triblock copolymer surfactants [8] such as the P123 used here in the synthesis of P-PMO, probably coupled to a possible effect of the different S- and P-ligand structures. These microstructural features were confirmed by TEM. Figs. 1(B) and (C) clearly show the PMO channelled microstructure, as well as the hexagonal mesopore pattern (inset). The specific surface area values (SBET) were in the 600–700 m 2 g − 1 range for materials with acid loads up to 0.60 meq g − 1, but they can be decreased to less than 100 m 2 g − 1 for ~ 1.8 meq [H +] g − 1 (Table 1). The average pore size, determined from the N2 sorption isotherms, is slightly lower than 3.5 nm for all materials, further confirming their classification as of mesoporous type. The acid load in the materials can be varied significantly during the co-condensation by using different amounts of precursors (PSiP, MTPMS), or by controlling the reaction time. In general, the acid content has no effect on the pore structure (cell parameter and average pore diameter are nearly identical [7]). However, as mentioned above, the specific surface area (and consequently the pore volume) decreases with increasing acid concentration (Table 1). The effect, noticed previously [4], is more pronounced for the P-PMO materials, probably as a consequence of the phosphonic ligand being larger than the sulfonic, and of the different nature of the structure directing agents. The extremely low SBET value and the extraordinarily high acid loading observed for P-PMO-2 on comparison with the other samples suggest that the condensation of the excess phosphonic groups may occur in regions not accessible to the sorbate gas. Indeed, this sample, despite having the highest acid content, has the lowest water uptake capacity (Table 1). Table 1 also shows that for samples with similarly high SBET, the hydration level increases with increasing acid functionalisation (shown by S-PMO samples) and that the sulfonic acid based materials tend to display higher W values than P-PMO-1. While the levels of functionalisation of the selection of materials reported here are not extraordinarily high (values of the order of 1 meq [H +] g − 1 can be obtained [7]), their combination with other properties (SBET and W) allows the comparison of the proton conductivity of materials with comparable SBET values but different functional acidic group, and materials with different SBET and acid loading, but the same functional group. 3.2. Proton conductivity Impedance spectra collected at different temperatures and under different r.h. conditions displayed enormous differences, both in shape (relaxation frequency distribution) and magnitude (ohmic resistance). Fig. 3 shows Nyquist plots obtained under variable r.h. conditions, normalised to the maximum in Z' for the sake of comparison. The spectra at low r.h. are dominated by one large resistance (in the MΩ range). The increase in r.h. up to ca. 50% leads to a progressive abatement of the amplitude of the semicircle. The magnitude of the impedances continues to decrease by further increasing the humidity, but the shape of the spectra then changes almost abruptly. While this trend is observed for both series of materials, the type of change is of a different kind for S-PMO and P-PMO. In the latter, the spectra evolve displaying three typical features: a high frequency contribution with the lowest impedance; an intermediate contribution with higher amplitude; and, finally, a tail at low frequencies that indicates a third phenomenon with the highest resistance (Fig. 3A). In the same high humidity range, the spectra of the sulfonic materials are dominated by one single contribution that is ascribed to the electrode impedance,
Fig. 3. Evolution of the impedance spectra at 100 °C with humidity for (A) P-PMO-2 and (B) S-PMO-1 samples. The values are normalised to the maximum in Z'. Note that the impedance is decreased by orders of magnitude with increasing relative humidity.
which does not reach the expected intercept with the real axis at high frequency, even for nearly saturated atmospheres (Fig. 3B). For this reason, the ohmic resistance of S-PMO is assumed as the Z' value at the lowest phase angle, in the high frequency region, as in previous work on similar systems [3]. The three contributions in the spectra of the phosphonic acid functionalised materials display a more complex picture. On exploring the possible non-linear behaviour of the impedance in this system, it has been found that only the high frequency contribution is independent of the test signal amplitude (Fig. 4). This is a strong indication that the intermediate and low frequency phenomena are related to electrode reactions, whereas the high frequency contribution corresponds to the ohmic resistance of the material. Similar types of spectra were obtained for phosphonic acid functionalised amorphous silica [4]. The strongly overlapping contributions make it difficult to assess this resistance and hence to estimate the conductivity of the material. Considering the complexity and the absence of a physical model for this particular system, we have retained the simplest possible description of the spectra by an equivalent circuit consisting of a series association of three parallel RCcpe elements, where R is a resistance and Ccpe a constant phase element, necessary to account for some level of depression of the semicircles. In this case, the resistance corresponding to the high frequency contribution can be obtained by fitting the data to this equivalent circuit, and then used to estimate the proton conductivity. The obtained conductivity values are plotted in Arrhenius coordinates in Fig. 5. Here we can distinguish the effects of the specific surface
E.M. Domingues et al. / Solid State Ionics 225 (2012) 308–311
311
Fig. 4. Impedance spectra for P-PMO-1 collected with variable test signal amplitude at 82 °C and 100% r.h.
area and of the acid composition and load. It can be seen that samples with high SBET have higher conductivity, confirming the key role of the surface on the conductivity of these materials. This is clearly noticed in the case of the P-PMO. The difficult access of water to the functional phosphonic sites in P-PMO-2 severely limits the proton conductivity, even for a material with such a high acid load (1.83 meq[H+] g− 1). When a large surface area is available (as in P-PMO-1) the material can reach a conductivity of ~0.01 Sm− 1 at 106 °C, which is an interesting value if taking into account the relatively low acid content (0.24 meq [H+]g− 1) and the extremely low density of these samples. The activation energy for the less conductive P-PMO-2 sample (~15 kJ mol− 1) is close to the values reported for Nafion (10 kJ mol− 1) [9]. The conductivity of the best sulfonic acid-based sample (S-PMO-2, 0.3 Sm− 1) is about one order of magnitude higher than P-PMO-1, whereas the activation energy is noticeably smaller (19 vs. 31 kJ mol− 1). This may be the result of the higher water retention capacity measured for the materials with the sulfonic acid group, also in agreement with [10]. In a similar manner, the comparison of the two S-PMO samples with similar SBET values shows the expected increase of the conductivity with increasing acid load, in par with an increasing water uptake capacity (Table 1). The conductivity of a similar material obtained by grafting of the silanol groups of pristine benzene–PMO with 1.14 meq [H +] g − 1 (SBET = ~ 840 m 2 g − 1, also prepared using C18TMABr) was recently reported (3 × 10 − 4 Sm − 1 at 100 °C) [11] and it is about three orders of magnitude lower than for the S-PMO-2 samples prepared in this study. This interesting performance of S-PMO-2 reinforces the potential of the co-condensation approach proposed in this work. It should be noticed that, while a conductivity of 0.3 Sm − 1 is still relatively low when compared to Nafion (~10 Sm − 1 at 100 °C [9]), the values reported here are obtained on powder compacts, which have a significant interparticle non-conducting volume. Therefore, the true bulk conductivity of such materials is certainly higher. 4. Conclusions Proton conducting periodic mesoporous organosilicas can be functionalised with different acid functional groups by a co-condensation route. The sulfonic acid-based materials can be synthesised in basic media with a cationic structure directing agent (C18TMABr), displaying
Fig. 5. Arrhenius plots for the proton conductivity obtained at ~ 100% r.h.
simultaneously high levels of acid functionalisation and surface areas. The functionalisation with phosphonic acid groups could only be achieved in acidic media with an amphoteric surfactant (co-polymer P123). In this case, the mesoporosity significantly decreases with increasing acid load. The proton conductivity is strongly enhanced for materials with high surface area, high acid concentration and for increasing temperature and humidity of the surrounding atmosphere. The highest conductivity obtained for a sulfonic acid functionalised PMO (0.3 Sm − 1 at 100 °C and 100% relative humidity) is three orders of magnitude higher than the value reported for a similar material obtained by grafting, thus demonstrating the potential of the co-condensation route for the synthesis of acid-functionalised PMOs with high proton conductivity. Acknowledgements This work was supported by the FCT under projects HyPEM - FEDER/ QREN - COMPETE PTDC/CTM– CER/109843/2009 and Pest-C/CTM/ LA0011/2011. E. Domingues and M.A. Salvador are particularly indebted to FCT (SFRH/BD/48042/2008 and SFRH/BD/60903/2009). References [1] S. Inagaki, S. Guan, T. Ohsuna, O. Terasaki, Nature 416 (2002) 304–307. [2] S. Mikhailenko, D. Desplantier-Giscard, C. Danumah, S. Kaliaguine, Microporous Mesoporous Mater. 52 (2002) 29–37. [3] R. Marschall, J. Rathousky, M. Wark, Chem. Mater. 19 (2007) 6401–6407. [4] Y.G. Jin, S.Z. Qiao, Z.P. Xu, Z. Yan, Y. Huang, J.C. Diniz da Costa, G.Q. Lu, J. Mater. Chem. 19 (2009) 2363–2372. [5] F. Pereira, K. Vallé, P. Belleville, A. Morin, S. Lambert, C. Sanchez, Chem. Mater. 20 (2008) 1710–1718. [6] Q.H. Yang, M.P. Kapoor, S. Inagaki, J. Am. Chem. Soc. 124 (2002) 9694–9695. [7] E.M. Domingues, N. Bion, P. Ferreira, F.M. Figueiredo, Ciênc. Tecnol. Mater. 23 (1/2) (2011) 20–24. [8] Y. Goto, K. Okamoto, S. Inagaki, Bull. Chem. Soc. Jpn. 78 (2005) 932–936. [9] G. Alberti, M. Casciola, L. Massinelli, B. Bauer, J. Membr. Sci. 185 (2001) 73–81. [10] M. Schuster, T. Rager, A. Noda, K.D. Kreuer, J. Maier, Fuel Cells 5 (2005) 355–365. [11] M. Sharifi, C. Köhler, P. Tölle, T. Frauenheim, M. Wark, Small 7 (2011) 1086–1097.
Contents lists available at SciVerse ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Acid-functionalised periodic mesoporous benzenosilica proton conductors Eddy M. Domingues, Maria A. Salvador, Paula Ferreira, Filipe M. Figueiredo ⁎ Department of Materials & Ceramic Engineering, CICECO, University of Aveiro, 3810‐193 Aveiro, Portugal
a r t i c l e
i n f o
Article history: Received 27 September 2011 Received in revised form 1 June 2012 Accepted 6 June 2012 Available online 23 June 2012 Keywords: Mesoporous Hybrids Organosilica Proton conductivity Fuel cell
a b s t r a c t An investigation of proton conducting periodic mesoporous benzenosilica hybrids functionalised with sulfonic or phosphonic acids obtained by a co-condensation route is presented. Protonic conductivity data obtained under different temperatures and relative humidity conditions are reported in order to assess the combined effects of composition and concentration of the functional acid group, and microstructural features such as pore size, specific surface area and meso- and molecular-scale order. It is found that the conductivity increases significantly with increasing acid load, specific surface area and relative humidity. A proton conductivity of 0.3 Sm− 1 at 100 °C and 100% relative humidity is reported for a sulfonic acid functionalised PMO which is three orders of magnitude higher than the value reported for a similar material obtained by grafting. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Acid-functionalised periodic mesoporous organosilicas (PMO) [1] may display high levels of proton conductivity [2–4] and can thus be suitable fillers to enhance the properties of polymeric membranes such as Nafion® [5]. The presence of a bissilylated benzene bridge along with sulfonic or phosphonic acid functionality may contribute to water mediated proton structural diffusion via hierarchically ordered structures with meso- and molecular-scale periodicity [6]. These structures may in principle be tuned to enhance the proton conductivity. Here we explore a novel co-condensation synthesis route of acidfunctionalised benzene bridged PMOs in order to assess the potential of these materials for application in components for proton exchange membrane fuel cells. 2. Experimental procedures Different amounts of 4-(trimethoxysilyl) benzylphosphonate (PSiP) or (3-mercaptopropyl)trimethoxysilane (MPTMS) were cocondensed with 1,4-bis(triethoxysilyl)benzene (BTEB) precursor in the presence of octadecyltrimethylammonium bromide (C18TMABr, CH3(CH2)17N(Br)(CH3)3, average Mn = ~ 392.5) or a tri-block copolymer (Pluronic®P123, EO20PO70EO20, average Mn = ~ 5800) structure directing agents in order to obtain materials with different levels of functionalisation and porosity, as schematised in Fig. 1. Microwave radiation was used in some steps of the synthesis to improve reaction ⁎ Corresponding author at: University of Aveiro, CICECO - Centre for Research in Ceramics and Composite Materials, Complexo de Laboratórios Tecnológicos, Room 29.3.26, 3810‐193 Aveiro, Portugal. Tel.: + 351 234 401 464; fax: + 351 234 370 204. E-mail address: [email protected] (F.M. Figueiredo). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2012.06.010
kinetics [7]. The structure and microstructure of the samples were assessed by a range of techniques, including X-ray diffraction (XRD) with Cu–Kα X-radiation (Rigaku Geigerflex D/Max–C) and with Cu–Kβ X-radiation (Bruker D8 Discover), transmission electron microscopy (TEM) imaging (Hitachi H9000-NA) − 196 °C nitrogen adsorption/desorption isotherms (Micromeritics Gemini V2.00) and potentiometric titration. Also, although not directly presented here, Fourier transform infrared spectroscopy (FTIR) of KBr pellets (FTIR Mattson-7000) was used to confirm the surfactant removal, and 29 Si, 31P and 13C solid-state magic-angle spinning nuclear magnetic resonance (MAS-NMR) – in some cases with the cross-polarisation technique (Bruker Avance 400 and 500) – was used to confirm the molecular structure of the compounds. The water uptake capacity (W) of the materials was determined as W = (Ww − Wd)Wd− 1 × 100, where Wd and Ww are the weights of the sample measured in wet and dry states. The Wd was determined after exposing the sample to an atmosphere of 97.3% relative humidity in a closed desiccator with a saturated K2SO4 solution, at room temperature for ca. 48 h. The weight of the samples was monitored over time in order to ensure that an equilibrium value was attained. The difference Ww − Wd was obtained directly from the plateaus of thermogravimetric curves collected in air. A list of the prepared samples is given in Table 1, together with some of the most relevant microstructural features. The proton conductivity (σ) was assessed by impedance spectroscopy of pelletized samples at different temperatures and humidity conditions. Disc shaped samples with a density of about 1 g cm − 3 were obtained after pressing the powders in a uniaxial press at 1.53 MPa, and then isostatically at 300 MPa. Silver electrodes were applied on both sides of the pellets by painting a commercial paste (Agar Scientific) and firing at 160 °C for 30 min. The spectra were collected in a pseudo 4-electrode configuration with an Agilent 2980A
E.M. Domingues et al. / Solid State Ionics 225 (2012) 308–311
309
Fig. 1. Synthesis of (A) sulfonic acid- and (B) phosphonic acid-functionalised PMOs.
LCR meter in the frequency range 20–2 × 10 6 Hz and using test signals of variable amplitude (10 mV–1 V range) to test for possible nonlinear system responses and at 100 mV for the final conductivity measurements. The samples were mounted on an appropriate support and placed in a climatic chamber made in-house, where variable relative humidity (r.h.) conditions were achieved by forcing an air stream (50 mL min − 1) through a water bath under controlled temperature. The temperature and r.h. were constantly monitored close to the sample with a capacitive humidity sensor. The temperature of the humidifier necessary to obtain 100% r.h. was adjusted in order to be as close as possible to the sample temperature in order to minimise possible water condensation effects. The spectra were analysed with ZView (Version 2.6b, 1990–2002, Scribner Associates) to assess the ohmic resistance (R) of the pellet, which was then used to obtain the conductivity by σ = R − 1(LS − 1), where L is the sample thickness and S is the electrodes surface area.
of the pore walls (taken as a0 minus the pore diameter) is considerably larger for the phosphonic acid-based materials. This large difference suggests a much less dense pore wall in the case of the P-PMOs.
A
S-PMO-1 P-PMO-1
3. Results and discussion 3.1. Materials synthesis and characterisation Fig. 2(A) shows typical powder XRD patterns collected at low diffraction angles for one sulfonic acid – (S-PMO-1) and one phosphonic acid – (P-PMO-1) functionalised materials. Both can be indexed onto the two-dimensional hexagonal symmetry (P6mm), as expected from the envisaged mesopore structure [1]. The d100 spacing of 4.29 nm for S-PMO-1 and 8.28 nm for P-PMO-1 correspond to pore structure cell parameters a0 = 2d1003 − 1/2 of 4.95 and 9.56 nm for S-PMO-1 and PPMO-1, respectively. The larger mesostructure cell parameter of the latter materials can be explained primarily by the much larger size of the P123 surfactant molecules when compared to C18TMABr, thus directing the formation of larger, ordered structures. Other factors such as the nature of ligand (ionic C18, amphoteric P123) and the pH of the synthesis media (basic for C18, acid for P123) may also have an effect, although expectedly less important. Since the average pore diameter (Table 1) is similar for all materials, the thickness
B
C
Table 1 List of prepared samples and relevant properties. Sample
SBET (m2 g− 1)
BJH average pore diameter (nm)
Acid loading (meq [H+] g− 1)
W (%)
Activation energy (kJ mol− 1)
P-PMO-1 P-PMO-2 S-PMO-1 S-PMO-2
616 82 774 715
3.3 3.4 3.4 3.2
0.24 1.83 0.30 0.58
27 12 34 42
31 15 11 19
Fig. 2. (A) Powder XRD patterns and (B,C) TEM images of (B) S-PMO-1 and (C) P-PMO-1 materials. The inset in (B) shows a view of the hexagonal array of pores along the (001) direction.
310
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Indeed, the XRD patterns of the sulfonic-based materials (with much thinner wall) display an additional reflection with 0.76 nm spacing (Fig. 2(A)), which is ascribed to the molecular scale periodicity in the pore wall, along the (001) direction [1]. This reflection is not apparent in the patterns of the phosphonic materials, which is indicative of an amorphous pore wall structure. This is to some extent typical of PMOs obtained with triblock copolymer surfactants [8] such as the P123 used here in the synthesis of P-PMO, probably coupled to a possible effect of the different S- and P-ligand structures. These microstructural features were confirmed by TEM. Figs. 1(B) and (C) clearly show the PMO channelled microstructure, as well as the hexagonal mesopore pattern (inset). The specific surface area values (SBET) were in the 600–700 m 2 g − 1 range for materials with acid loads up to 0.60 meq g − 1, but they can be decreased to less than 100 m 2 g − 1 for ~ 1.8 meq [H +] g − 1 (Table 1). The average pore size, determined from the N2 sorption isotherms, is slightly lower than 3.5 nm for all materials, further confirming their classification as of mesoporous type. The acid load in the materials can be varied significantly during the co-condensation by using different amounts of precursors (PSiP, MTPMS), or by controlling the reaction time. In general, the acid content has no effect on the pore structure (cell parameter and average pore diameter are nearly identical [7]). However, as mentioned above, the specific surface area (and consequently the pore volume) decreases with increasing acid concentration (Table 1). The effect, noticed previously [4], is more pronounced for the P-PMO materials, probably as a consequence of the phosphonic ligand being larger than the sulfonic, and of the different nature of the structure directing agents. The extremely low SBET value and the extraordinarily high acid loading observed for P-PMO-2 on comparison with the other samples suggest that the condensation of the excess phosphonic groups may occur in regions not accessible to the sorbate gas. Indeed, this sample, despite having the highest acid content, has the lowest water uptake capacity (Table 1). Table 1 also shows that for samples with similarly high SBET, the hydration level increases with increasing acid functionalisation (shown by S-PMO samples) and that the sulfonic acid based materials tend to display higher W values than P-PMO-1. While the levels of functionalisation of the selection of materials reported here are not extraordinarily high (values of the order of 1 meq [H +] g − 1 can be obtained [7]), their combination with other properties (SBET and W) allows the comparison of the proton conductivity of materials with comparable SBET values but different functional acidic group, and materials with different SBET and acid loading, but the same functional group. 3.2. Proton conductivity Impedance spectra collected at different temperatures and under different r.h. conditions displayed enormous differences, both in shape (relaxation frequency distribution) and magnitude (ohmic resistance). Fig. 3 shows Nyquist plots obtained under variable r.h. conditions, normalised to the maximum in Z' for the sake of comparison. The spectra at low r.h. are dominated by one large resistance (in the MΩ range). The increase in r.h. up to ca. 50% leads to a progressive abatement of the amplitude of the semicircle. The magnitude of the impedances continues to decrease by further increasing the humidity, but the shape of the spectra then changes almost abruptly. While this trend is observed for both series of materials, the type of change is of a different kind for S-PMO and P-PMO. In the latter, the spectra evolve displaying three typical features: a high frequency contribution with the lowest impedance; an intermediate contribution with higher amplitude; and, finally, a tail at low frequencies that indicates a third phenomenon with the highest resistance (Fig. 3A). In the same high humidity range, the spectra of the sulfonic materials are dominated by one single contribution that is ascribed to the electrode impedance,
Fig. 3. Evolution of the impedance spectra at 100 °C with humidity for (A) P-PMO-2 and (B) S-PMO-1 samples. The values are normalised to the maximum in Z'. Note that the impedance is decreased by orders of magnitude with increasing relative humidity.
which does not reach the expected intercept with the real axis at high frequency, even for nearly saturated atmospheres (Fig. 3B). For this reason, the ohmic resistance of S-PMO is assumed as the Z' value at the lowest phase angle, in the high frequency region, as in previous work on similar systems [3]. The three contributions in the spectra of the phosphonic acid functionalised materials display a more complex picture. On exploring the possible non-linear behaviour of the impedance in this system, it has been found that only the high frequency contribution is independent of the test signal amplitude (Fig. 4). This is a strong indication that the intermediate and low frequency phenomena are related to electrode reactions, whereas the high frequency contribution corresponds to the ohmic resistance of the material. Similar types of spectra were obtained for phosphonic acid functionalised amorphous silica [4]. The strongly overlapping contributions make it difficult to assess this resistance and hence to estimate the conductivity of the material. Considering the complexity and the absence of a physical model for this particular system, we have retained the simplest possible description of the spectra by an equivalent circuit consisting of a series association of three parallel RCcpe elements, where R is a resistance and Ccpe a constant phase element, necessary to account for some level of depression of the semicircles. In this case, the resistance corresponding to the high frequency contribution can be obtained by fitting the data to this equivalent circuit, and then used to estimate the proton conductivity. The obtained conductivity values are plotted in Arrhenius coordinates in Fig. 5. Here we can distinguish the effects of the specific surface
E.M. Domingues et al. / Solid State Ionics 225 (2012) 308–311
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Fig. 4. Impedance spectra for P-PMO-1 collected with variable test signal amplitude at 82 °C and 100% r.h.
area and of the acid composition and load. It can be seen that samples with high SBET have higher conductivity, confirming the key role of the surface on the conductivity of these materials. This is clearly noticed in the case of the P-PMO. The difficult access of water to the functional phosphonic sites in P-PMO-2 severely limits the proton conductivity, even for a material with such a high acid load (1.83 meq[H+] g− 1). When a large surface area is available (as in P-PMO-1) the material can reach a conductivity of ~0.01 Sm− 1 at 106 °C, which is an interesting value if taking into account the relatively low acid content (0.24 meq [H+]g− 1) and the extremely low density of these samples. The activation energy for the less conductive P-PMO-2 sample (~15 kJ mol− 1) is close to the values reported for Nafion (10 kJ mol− 1) [9]. The conductivity of the best sulfonic acid-based sample (S-PMO-2, 0.3 Sm− 1) is about one order of magnitude higher than P-PMO-1, whereas the activation energy is noticeably smaller (19 vs. 31 kJ mol− 1). This may be the result of the higher water retention capacity measured for the materials with the sulfonic acid group, also in agreement with [10]. In a similar manner, the comparison of the two S-PMO samples with similar SBET values shows the expected increase of the conductivity with increasing acid load, in par with an increasing water uptake capacity (Table 1). The conductivity of a similar material obtained by grafting of the silanol groups of pristine benzene–PMO with 1.14 meq [H +] g − 1 (SBET = ~ 840 m 2 g − 1, also prepared using C18TMABr) was recently reported (3 × 10 − 4 Sm − 1 at 100 °C) [11] and it is about three orders of magnitude lower than for the S-PMO-2 samples prepared in this study. This interesting performance of S-PMO-2 reinforces the potential of the co-condensation approach proposed in this work. It should be noticed that, while a conductivity of 0.3 Sm − 1 is still relatively low when compared to Nafion (~10 Sm − 1 at 100 °C [9]), the values reported here are obtained on powder compacts, which have a significant interparticle non-conducting volume. Therefore, the true bulk conductivity of such materials is certainly higher. 4. Conclusions Proton conducting periodic mesoporous organosilicas can be functionalised with different acid functional groups by a co-condensation route. The sulfonic acid-based materials can be synthesised in basic media with a cationic structure directing agent (C18TMABr), displaying
Fig. 5. Arrhenius plots for the proton conductivity obtained at ~ 100% r.h.
simultaneously high levels of acid functionalisation and surface areas. The functionalisation with phosphonic acid groups could only be achieved in acidic media with an amphoteric surfactant (co-polymer P123). In this case, the mesoporosity significantly decreases with increasing acid load. The proton conductivity is strongly enhanced for materials with high surface area, high acid concentration and for increasing temperature and humidity of the surrounding atmosphere. The highest conductivity obtained for a sulfonic acid functionalised PMO (0.3 Sm − 1 at 100 °C and 100% relative humidity) is three orders of magnitude higher than the value reported for a similar material obtained by grafting, thus demonstrating the potential of the co-condensation route for the synthesis of acid-functionalised PMOs with high proton conductivity. Acknowledgements This work was supported by the FCT under projects HyPEM - FEDER/ QREN - COMPETE PTDC/CTM– CER/109843/2009 and Pest-C/CTM/ LA0011/2011. E. Domingues and M.A. Salvador are particularly indebted to FCT (SFRH/BD/48042/2008 and SFRH/BD/60903/2009). References [1] S. Inagaki, S. Guan, T. Ohsuna, O. Terasaki, Nature 416 (2002) 304–307. [2] S. Mikhailenko, D. Desplantier-Giscard, C. Danumah, S. Kaliaguine, Microporous Mesoporous Mater. 52 (2002) 29–37. [3] R. Marschall, J. Rathousky, M. Wark, Chem. Mater. 19 (2007) 6401–6407. [4] Y.G. Jin, S.Z. Qiao, Z.P. Xu, Z. Yan, Y. Huang, J.C. Diniz da Costa, G.Q. Lu, J. Mater. Chem. 19 (2009) 2363–2372. [5] F. Pereira, K. Vallé, P. Belleville, A. Morin, S. Lambert, C. Sanchez, Chem. Mater. 20 (2008) 1710–1718. [6] Q.H. Yang, M.P. Kapoor, S. Inagaki, J. Am. Chem. Soc. 124 (2002) 9694–9695. [7] E.M. Domingues, N. Bion, P. Ferreira, F.M. Figueiredo, Ciênc. Tecnol. Mater. 23 (1/2) (2011) 20–24. [8] Y. Goto, K. Okamoto, S. Inagaki, Bull. Chem. Soc. Jpn. 78 (2005) 932–936. [9] G. Alberti, M. Casciola, L. Massinelli, B. Bauer, J. Membr. Sci. 185 (2001) 73–81. [10] M. Schuster, T. Rager, A. Noda, K.D. Kreuer, J. Maier, Fuel Cells 5 (2005) 355–365. [11] M. Sharifi, C. Köhler, P. Tölle, T. Frauenheim, M. Wark, Small 7 (2011) 1086–1097.