Layered Titanates Intercalating Organic Guest Spacers for Organic/Inorganic Proton Conductors

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1 / The Electrochemical Society Layered Titanates Intercalating Organic Guest Spacers for Organic/Inorganic Proton Conductors C. de Bonis a, A. D Epifanio a, B. Mecheri a, E. Traversa a,b, M. Miyayama c, and S. Licoccia a a Department of Chemical Science and Technology & NAST Center, University of Rome Tor Vergata, Via della Ricerca Scientifica, Rome 00133, ITALY b International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki , JAPAN c Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1, Meguro-ku, Tokyo , JAPAN Two layered organic/inorganic tetratitanate derivatives containing sulfanilic acid (SA) were synthesized from potassium tetratitanate (K 2 Ti 4 O 9 ) or its acid form (H 2 Ti 4 O 9 ). Scanning electron microscopy (SEM) observations were performed to investigate the microstructural features of the samples. The thermal stability of the compounds was tested by thermogravimetric analysis (TGA). The effect of the presence of organic guest molecules in the layered tetratitanates was investigated in terms of proton exchange capacity (PEC) and conductivity (σ), to evaluate the use of these compounds as fillers for nanocomposite proton exchange membranes for fuel cell applications. Introduction During the last decades, layered compounds (i.e. titanates, zirconium phosphates, silicates etc.) have attracted considerable attention for applications in absorbing materials, proton exchange membrane fuel cells (PEMFCs), batteries and microelectronic devices, due to their structural characteristics (1,2). The peculiar property of lamellar compounds is their ability to exfoliate by insertion of delamination solvents and agents between the layers. The exfoliated sheets can be considered as new classes of two-dimensional (2D) nanoscale materials, which may exhibit novel physical and chemical properties associated with their anisotropic structure, high aspect ratio, high specific surface area and thickness in the nanometer scale. Concerning PEMFC applications, numerous research efforts have been focused on the use of layered metal oxides as fillers to prepare nanocomposite polymeric membranes (3,4). In general, the higher the interface interaction between the polymer and the dispersed particles, the greater is the filler influence on the original characteristics of the polymer. Thus, the presence of 2D nanoparticles as filler can improve some important properties of the membranes, providing physical barriers to fuel crossover and a mechanical reinforcement (5). Starting from lamellar metal oxides, the nanosheets can be obtained moving the layers apart and weakening the forces which keep the planar macromolecules together in the crystal. Concerning layered titanates, a remarkable decrease of the interactions 2091

2 between adjacent layers is expected as a consequence of the intercalation of organic species, such as organoammonium cations, alkyldiamines, and pyridine (6,7). When the foreseen application is in the field of proton exchange membranes, particularly interesting is the introduction of organic guest molecules with functional proton-donor groups into the interlayer spaces that adds proton conductivity to the material, allowing to obtain organic/inorganic proton conductors. In this work, we report the preparation of layered tetratitanate derivatives containing sulfanilic acid (SA), starting from potassium tetratitanate (K 2 Ti 4 O 9 ) or its acid form (H 2 Ti 4 O 9 ). The products were characterized by means of field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX) microanalysis, thermogravimetric analysis (TGA) and electrochemical impedance spectroscopy (EIS). The attention was focused on the effect caused by the introduction of SA on the proton-conducting features of layered tetratitanate derivatives, to evaluate their use as filler in nanocomposite membranes for PEMFCs. Materials Experimental Synthesis of K 2 Ti 4 O 9. Starting materials were dry reagent grade. The reactant mixture of TiO 2 / K 2 CO 3 in molar ratio 4:1 was prepared by mixing and homogenizing the powders in an agate mortar. The mixture was heated in an alumina crucible, at a rate of 10 C/min up to 950 C (soaking time: 4 h), and cooling down to 20 C with a rate of 10 C/min. Synthesis of H 2 Ti 4 O 9. K 2 Ti 4 O 9 (2.0 g) was poured in 200 ml of 1 M HCl solution and kept under stirring for three days. HCl solution was renewed each day to ensure complete ion exchange. The resulting product was filtered off, washed with distilled water until no chloride ions were detected and dried under vacuum. Synthesis of (SAH) x Ti 4 O 9. K 2 Ti 4 O 9 (2 g) was poured in 100 ml of distilled water at 80 C and stirred for 1 h. SA (5.02 g) was poured in 50 ml of distilled water at 80 C and then 2.4 ml of HCl 37% were added. The two solutions were mixed and kept under stirring at 80 C for 4 days. The resulting product was filtered off, washed with distilled water until no chloride ions were detected and dried under vacuum. Synthesis of SA-H 2 Ti 4 O 9. H 2 Ti 4 O 9 (1 g) was poured in 100 ml of distilled water at 80 C and stirred for 1 h. SA (3.08 g) was poured in 50 ml of distilled water at 80 C. Then, the two solutions were mixed and kept under stirring at 80 C for 4 days. The resulting product was filtered off, washed with distilled water and dried under vacuum. Methods SEM images were acquired using a FE-SEM (LEO model Supra 35) instrument. EDX microanalysis was performed using an INCA Energy 300 spectrometer. TGA was carried out using a thermobalance (STA 409, Netzsch), in air flow (80 ml/min), with a heating rate of 5 C/min. Proton exchange capacity (PEC) was determined by acid-base titration. In a typical experiment 50 mg of sample were dried under vacuum at 60 C for 1h, then poured in an appropriate amount of 1 M NaCl solution, and kept under stirring overnight at room temperature, so that H + ions of the powder acid sites could be replaced by Na + ions. The solution was then titrated with 0.1 M NaOH (Aldrich, volumetric standard). PEC values were calculated by the following equation: 2092

3 PEC = V NaOH *C NaOH / W sample [1] where PEC is the proton exchange capacity (meq/g), V NaOH is the added titrant volume at the equivalent point (ml), C NaOH is the molar concentration of the titrant, and W sample is the dry mass of the sample (g). Electrochemical impedance spectroscopy (EIS) measurements were carried out using a multichannel potentiostat VMP3 (Princeton Applied Research). An applied voltage of 25 mv and a frequency range of 1 MHz to 1 Hz were used. Sample pellets were sandwiched between commercial electrodes (E-Tek ELAT HT 140E-W with a platinum loading of 5 g/m 2 ). Conductivity was recorded as a function of temperature in the C range, at saturated water vapor pressure (100% RH). Results and Discussion Potassium tetratitanate (K 2 Ti 4 O 9 ) was synthesized by solid state reaction. Structurally, the units are consisting of four TiO 6 octahedra arranged in a line by means of edge sharing and connected with each other to form two-dimensional sheets, whilst the potassium ions are situated in the interlayer regions (7). The acid form of tetratitanate, H 2 Ti 4 O 9, was obtained by K + /H + ion exchange via treatment of K 2 Ti 4 O 9 with HCl aqueous solution. Two different synthetic routes were followed to prepare tetratitanate derivatives containing sulfanilic acid. (SAH) x Ti 4 O 9 and SA-H 2 Ti 4 O 9 were synthesized starting from potassium tetratitanate or its acid form, respectively, as shown in Scheme 1. Route (a) consisted in a single step reaction leading to (SAH) x Ti 4 O 9, whilst SA-H 2 Ti 4 O 9 was prepared in two steps following route (b): preparation of the acid form of tetratitanate and subsequent reaction with SA. Scheme 1. Synthetic methodologies for the preparation of (SAH) x Ti 4 O 9 and SA-H 2 Ti 4 O 9. The morphology of layer tetratitanates was investigated by FE-SEM. Figure 1 shows the micrograph of SA-H 2 Ti 4 O 9 showing the presence of elongated fibrous structure. 2093

4 Cation exchange and reaction with SA did not modify the morphology of the original tetratitanate (not shown). Figure 1. SEM micrograph of SA-H 2 Ti 4 O 9. EDX microanalysis was used to assess the efficiency of the K + /H + exchange reaction and the content of SA in the samples. The presence of residual potassium and the low content of sulfur in (SAH) x Ti 4 O 9, indicated the poor yield obtained with synthetic route (a). To study the thermal evolution of (SAH) x Ti 4 O 9 and SA-H 2 Ti 4 O 9 and to quantify their SA content, TGA was carried out. Figure 2 shows the thermograms of tetratitanate derivatives. Both (SAH) x Ti 4 O 9 and SA-H 2 Ti 4 O 9 display two degradation ranges. The first one between ºC can be attributed to dehydration processes, whilst the weight loss between 250 and 550 C is due to the decomposition of SA (8) (a) 90 Weight% (b) Temperature / C Figure 2. TG curves of (a) (SAH) x Ti 4 O 9 and (b) SA-H 2 Ti 4 O 9. At temperatures above 600 C, the residual weight percentage for (SAH) x Ti 4 O 9 is larger than that of SA-H 2 Ti 4 O 9 because of the larger content in inorganic components, confirming that the substitution of potassium ions in K 2 Ti 4 O 9 with SA molecules is rather inefficient, in agreement with EDX microanalysis. The amount of SA per gram of compound was estimated considering the weight losses in the C range, where the decomposition of the organic component 2094

5 occurs (8), obtaining 0.6 mmol/g for (SAH) x Ti 4 O 9 and 1.8 mmol/g for SA-H 2 Ti 4 O 9, respectively. Thus, the most appropriated reaction conditions are those where the acidic crystalline lamellar compound H 2 Ti 4 O 9 is used as host, despite two synthetic steps are necessary to obtain the final product. To investigate the effect of the introduction of SA in the lamellar tetratitanate, PEC values of SA-H 2 Ti 4 O 9 and its precursor H 2 Ti 4 O 9 were calculated using Eq. [1] and shown in Table I. The increased PEC value for the sample containing SA, indicated the increment of the amount of proton-donor groups due to the organic guest molecules. TABLE I. PEC values of the acid compounds. Sample PEC (meq/g) H 2 Ti 4 O SA-H 2 Ti 4 O The proton conductivity of H 2 Ti 4 O 9 and SA-H 2 Ti 4 O 9 was measured as a function of temperature. Figure 3 shows the relative Arrhenius plots T / C H 2 Ti 4 O 9 SA-H 2 Ti 4 O 9 log (σ / S cm -1 ) T -1 / K -1 Figure 3. Arrhenius plot of H 2 Ti 4 O 9 and SA-H 2 Ti 4 O 9, at RH = 100%. The product containing SA showed much larger conductivity values with respect to those of H 2 Ti 4 O 9 in the whole investigated temperature range, in agreement with its higher PEC value. In detail, at 140 C, SA-H 2 Ti 4 O 9 showed a proton conductivity of S/cm, which is one order of magnitude larger than that of H 2 Ti 4 O 9 ( S/cm). Conclusions Two layered tetratitanate derivatives containing SA were prepared starting from K 2 Ti 4 O 9 and H 2 Ti 4 O 9, following two different synthetic routes. In both cases, the elongated fibrous structure of the inorganic matrix was preserved upon the ion exchange and the reaction with sulfanilic acid. The most efficient preparative procedure required 2095

6 the conversion of the tetratitanate potassium salt into the acidic lamellar compound H 2 Ti 4 O 9 prior to reaction with the SA guest species. A two step synthetic process was then necessary to obtain the final product, SA-H 2 Ti 4 O 9. This organic/inorganic compound showed high PEC value and adequate proton conductivity up to 140 C, suitable as proton-conducting filler for nanocomposite membranes for fuel cell applications. Acknowledgments Thanks are due to Ms. Cadia D Ottavi for her valuable technical assistance. This work has been partially sponsored by the University of Tokyo, under the frame of the joint lab Development of fuel cells for intermediate-temperature operation using organicinorganic composite electrolyte membranes and by the Ministry for Foreign Affairs of Italy under the framework of the Italy-Japan Joint Lab on Materials Nanoarchitectonics for Sustainable Development. References 1. M. Osada, Y. Ebina, H. Funakubo, S. Yokoyama, T. Kiguchi, K. Takada and T. Sasaki, Adv. Mater., 18, 1023 (2006). 2. W. H. J. Hogarth, J. C. Diniz da Costa and G. Q. (Max) Lu, J. Power Sources, 142, 223 (2005). 3. G. Alberti, M. Casciola, M. Pica, T. Tarpanelli and M. Sganappa, Fuel Cells, (2005). 4. D. H. Jung, S. U. Peck, D. R. Shin and J. S. Kim, J. Power Sources, 118, 205 (2003). 5. J.-M. Thomassin, C. Pagnoulle, D. Bizzari, G. Caldarella, A. Germain and R. Jérôme, Solid State Ionics, 177, 1137 (2006). 6. D. Marani, A. D Epifanio, E. Traversa, M. Miyayama and S. Licoccia, Chem. Mater., 22, 1126 (2010). 7. L. M. Nunes, V. de A. Cardoso and C. Airoldi, Mater. Res. Bull., 41, 1089 (2006). 8. T. Fu, Z. Cui, S. Zhong, Y. Shi, C. Zhao, G. Zhang, K. Shao, H. Na and W. Xing, J. Power Sources, 185, 32 (2008). 2096