VULCAN-SUPPORTED Pt ELECTROCATALYSTS FOR PEMFCs PREPARED USING SUPERCRITICAL CARBON DIOXIDE DEPOSITION

Transcription

1 Chemical Engineering Communications ISSN: (Print) (Online) Journal homepage: VULCAN-SUPPORTED Pt ELECTROCATALYSTS FOR PEMFCs PREPARED USING SUPERCRITICAL CARBON DIOXIDE DEPOSITION Ayşe Bayrakçeken, Alevtina Smirnova, Usanee Kitkamthorn, Mark Aindow, Lemi Türker, İnc[idot] Eroğlu & Can Erkey To cite this article: Ayşe Bayrakçeken, Alevtina Smirnova, Usanee Kitkamthorn, Mark Aindow, Lemi Türker, İnc[idot] Eroğlu & Can Erkey (2008) VULCAN-SUPPORTED Pt ELECTROCATALYSTS FOR PEMFCs PREPARED USING SUPERCRITICAL CARBON DIOXIDE DEPOSITION, Chemical Engineering Communications, 196:1-2, , DOI: / To link to this article: Published online: 20 Oct Submit your article to this journal Article views: 576 View related articles Citing articles: 10 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 24 November 2017, At: 06:38

2 Chem. Eng. Comm., 196: , 2009 Copyright # Taylor & Francis Group, LLC ISSN: print/ online DOI: / Vulcan-Supported Pt Electrocatalysts for PEMFCs Prepared using Supercritical Carbon Dioxide Deposition AYŞE BAYRAKÇEKEN, 1 ALEVTINA SMIRNOVA, 2 USANEE KITKAMTHORN, 3 MARK AINDOW, 2 LEMI TÜRKER, 4 İNCİ EROĞLU, 1 AND CAN ERKEY 5 1 Department of Chemical Engineering, Middle East Technical University, Ankara 2 Department of Chemical, Materials and Biomolecular Engineering, and Institute of Materials Science, University of Connecticut, Storrs, Connecticut, USA 3 School of Metallurgical Engineering, Suranaree University of Technology, Muang, Nakorn Ratchasima, Thailand 4 Department of Chemistry, Middle East Technical University, Ankara, Turkey 5 Department of Chemical and Biological Engineering, Koc University, Istanbul, Turkey In this study, supercritical carbon dioxide (scco 2 ) deposition was used to prepare vulcan-supported Pt (Pt=Vulcan) electrocatalysts for proton exchange membrane fuel cells (PEMFCs), and the effects of process variables on the properties of the electrocatalysts were investigated. The two different methods used to reduce the organometallic precursor were thermal reduction in nitrogen at atmospheric pressure and thermal reduction in scco 2. In the former method, the maximum Pt loading achieved was 9%, and this was governed by the adsorption isotherm of the Pt precursor between the scco 2 phase and the Vulcan phase. By using the latter method, higher Pt loadings of 15% and 35% could be achieved. The Pt=Vulcan catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and cyclic voltammetry (CV). The average particle sizes for Pt=Vulcan 9%, 15%, and 35% catalysts were 1.2, 1.3, and 2 nm, respectively. Electrochemical surface areas obtained from CV data were found to vary with the Pt loading. Keywords Carbon black; Pt nanoparticles; Supercritical carbon dioxide deposition Introduction The heart of a proton exchange membrane fuel cell (PEMFC) is the membrane electrode assembly (MEA), which is comprised of a proton-conducting membrane sandwiched between the anode and cathode (Hoogers, 2003). These electrodes generally contain platinum-based metallic particles, which serve as very effective catalysts for Address correspondence to Can Erkey, Dept. of Chemical and Biological Engineering, Koc University, Sariyer, Istanbul, Turkey. cerkey@ku.edu.tr 194

3 Supported Pt Electrocatalysts for PEMFCs 195 oxidation and reduction reactions (Costamagna and Srinivasan, 2001). Platinum is, however, expensive, and this causes the price of MEAs to be prohibitively high, inhibiting the commercialization and widespread usage of PEMFCs (Xiong and Manthiram, 2005). The catalytic activity of the catalyst and the utilization depend critically on the contact between the electrolyte and the Pt particles (Pan et al., 2005), as well as on the size of the platinum particles. Therefore, tremendous efforts have been devoted to reduce the sizes of Pt particles that can be produced on carbon supports. The catalyst preparation techniques used most commonly in the literature are impregnationreduction (Kawaguchi et al., 2005), microemulsion-based synthesis (Escudero et al., 2002), and ion exchange (Shao et al., 2006). The types of carbon supports used in these preparations include carbon blacks (Gloaguen et al., 1997), multiwall carbon nanotubes (MWCNT) (Liu et al., 2002; Li et al., 2003; Bayrakçeken et al., 2007), carbon aerogels (Marie et al., 2004), activated carbons (Maruyama and Abe, 2003), black pearl (BP2000) (Amine et al., 1998), and carbon cryogels (Babić et al., 2006). Carbon black is the most widely used of these supports because it has good electrical conductivity and high corrosion resistance and is inexpensive; Vulcan XC72R (Cabot) is a carbon black used commonly for fuel cell applications (Raghuveer and Manthiram, 2004). One of the most promising catalyst preparation techniques under development is the supercritical carbon dioxide (scco 2 ) deposition method (Zhang and Erkey, 2006), which results in metallic particles with small sizes and homogeneous dispersions (Zhang et al., 2005a,b). Furthermore, it is possible to control the metal loading thermodynamically using this method (Saquing et al., 2004; Bayrakçeken et al., 2007). The scco 2 method involves impregnating the support substrate with an organometallic precursor by dissolving the precursor in scco 2 and then exposing the substrate to this solution. Thermal treatment of the impregnated substrate results in supported nanoparticles of the metal. When reduction is carried out at atmospheric pressure, the maximum loading is governed by the adsorption isotherm of the precursor between the carbon black phase and the scco 2 phase, but higher loadings are possible when the reduction is carried out in scco 2. In this study, we investigated the effect of the method used to reduce the Pt precursor to its metal form on the properties of Pt=Vulcan. Specifically, we investigated two different reduction methods: thermal reduction in N 2 at atmospheric pressure and thermal reduction in scco 2. The resulting catalysts were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), and cyclic voltammetry (CV) measurements. Experimental Section Catalyst Preparation Carbon black, Vulcan XC-72R (Vulcan) (Cabot International), was impregnated with Pt using the scco 2 deposition method. The total, meso=macroporous, and micro surface areas of Vulcan were 235, 152.6, and 82.4 m 2 =g, respectively (Zhang et al., 2004). Prior to impregnation, carbon black was heat treated in a pyrolysis oven at 423 K for 4 h in N 2 (99.999%, Airgas) atmosphere. In this synthesis, dimethyl (cyclooctadiene) platinum (II) (PtMe 2 COD) (99.9%) (Strem) was used as the Pt precursor.

4 196 A. Bayrakçeken et al. Figure 1. Experimental setup for supercritical deposition. A schematic diagram for the supercritical deposition setup is given in Figure 1. The deposition apparatus consists of a 54 ml custom-manufactured stainless steel vessel equipped with two sapphire windows, 25 mm in diameter, and sealed with poly (ether ether ketone) O-rings. A T-type thermocouple assembly (Omega Engineering, PX KGV), a vent line and a rupture disk assembly (Autoclave Engineers) are also attached to the vessel. Thermal Reduction in Nitrogen at Atmospheric Pressure The heat-treated carbon black was placed into a pouch made of a filter paper, and the pouch was placed in the vessel together with a measured amount of PtMe 2 COD precursor and a stirring bar. For the desired metal loading, the amount of the PtMe 2- COD precursor was determined by using the adsorption isotherm of PtMe 2 COD onto the carbon black. A stainless steel screen was used to separate the stirring bar from the filter paper pouch. The vessel was sealed and heated to 343 K using a circulating heater=cooler apparatus (Fisher Scientific Isotemp Refrigerated Circulator Model 80) and then charged slowly with carbon dioxide (99.998%, Airgas) to a pressure of 24.2 MPa using a syringe pump (Isco, 260D). These conditions were maintained for a period of 6 h, which was enough for the system to reach equilibrium, and then the vessel was then depressurized. After allowing the vessel to cool, the pouch was removed and the impregnated carbon black was weighed to determine the amount of precursor adsorbed using an analytical balance (Adventure Model Ar 2140) accurate to 0.1 mg. Subsequently, the impregnated carbon black was placed in an alumina process tube in a tube furnace (Thermolyne Tube Furnace), and the precursor was reduced thermally under flowing N 2 (100 cm 3 min 1 ) for 4 h at 473 K. By using this reduction method Pt=Vulcan (9%) catalyst was prepared. Thermal Reduction in scco 2 The heat-treated carbon black was placed into a pouch made of a filter paper, and the pouch was placed in the vessel together with a measured amount of the required precursor. The vessel was heated to 343 K using a circulating heater=cooler apparatus and then charged slowly with carbon dioxide (99.998%, Airgas) to a pressure

5 Supported Pt Electrocatalysts for PEMFCs 197 of 13.6 MPa using a syringe pump. These conditions were maintained overnight and then the temperature was increased to 413 K for 6 h for thermal reduction, which caused the pressure to increase to 31 MPa. After allowing the vessel to cool, the pouch was removed and the impregnated carbon black was weighed to determine the amount of Pt adsorbed. The increase of the weight of the sample was equal to the amount of Pt in the PtMe 2 COD placed in the vessel, indicating complete conversion of the precursor to Pt on the surface. This reduction method was used to produce Pt=Vulcan (15%) and (35%) catalysts. Further reduction of the Pt=Vulcan Vulcan catalysts taken from the vessel in a tube furnace under flowing N 2 did not cause any significant changes in the Pt loading. Catalyst Characterization Structural Characterization The prepared Pt=Vulcan 9%, 15%, and 35% catalysts were characterized by XRD or TEM in the as-synthesized condition with no subsequent cleaning or purification steps. XRD data were obtained by using a Cu Ka source in a powder diffractometer (Scintag XDS 2000). The diffractometer was operated in continuous scan mode at a scan rate of 0.6 deg min 1 in the range of 5 80 (2h). TEM samples were produced by dispersing the material ultrasonically in ethanol, transferring a drop of this suspension onto a copper mesh grid coated with a holey carbon film, and then allowing the ethanol to evaporate. The samples were examined in a TEM (UHR JEOL 2010 FasTEM) operating at an accelerating voltage of 200 kv. Electrochemical Characterization The electrochemical characterization of these catalysts was obtained by using CV scans. The CV measurements were carried out in a standard three-electrode electrochemical cell. An Ag=AgCl, Cl electrode was used as the reference, which was externally connected to the cell by a salt bridge filled with 0.1 M KCl solution and placed as close as possible to the working electrode to decrease the ohmic resistance. Pt wire and a glassy carbon (GC) electrode (5 mm in diameter) were used as the counter and working electrodes, respectively. Catalyst ink was prepared by mixing measured amounts of the prepared catalysts with 1 ml deionized water, 1 ml 1,2-propandiol, and 400 ml 5% Nafion solution (Ion Solutions Inc.). The suspension was homogenized for 1 h using a homogenizer (Ultra-Turrax 1 T25). Then, 5.8 ml of this solution was deposited onto the GC electrode and dried overnight. The amount of Pt=Vulcan placed in the solution was adjusted so that the platinum loading per unit area on the GC electrode was 28 mgpt cm 2. Cyclic voltammograms were recorded in a 0.1 M HClO 4 electrolyte that was saturated with hydrogen for 30 min to remove the oxygen. All the experiments were performed at room temperature. After 10 cycles between 0 and 0.8 V at a scan rate of 50 mv s 1 the stabilized CV curves were recorded. CV data were reported with respect to a normal hydrogen electrode (NHE). Results and Discussion The Pt loading can be controlled up to a certain level that depends on the adsorption isotherm if the reduction is carried out after depressurization at atmospheric pressure. The adsorption isotherm for PtMe 2 COD on Vulcan in scco 2 is given in

6 198 A. Bayrakçeken et al. Figure 2. Adsorption isotherm and Langmuir fit for Pt precursor (PtMe 2 COD) on Vulcan XC-72R in scco 2 at 343 K and 24.2 MPa. Table I. Langmuir equilibrium constants for PtMe 2 COD on Vulcan XC-72R in scco 2 Catalyst K 1 (mg=g Vulcan) Q 0 (g scco 2 =mg) K 1 Q 0 (g scco 2 =g Vulcan) R 2 Pt=Vulcan Figure 3. XRD patterns for Pt=Vulcan 9%, 15%, and 35% catalysts.

7 Supported Pt Electrocatalysts for PEMFCs 199 Figure 2. This isotherm could be fitted to Langmuir-type model as given in Equation (1): q ¼ K 1Q 0 c ð1þ 1 þ K 1 c where K 1 (mg=g Vulcan) is the Langmuir adsorption constant, Q 0 (g scco 2 =mg) is the adsorption capacity, c (mg=g scco 2 ) is the solution concentration, and q (mg=g Vulcan) is the uptake by the adsorbent. The Langmuir equilibrium constants obtained by regression for this system are given in Table I. The high K 1 Q 0 value of g(scco 2 )=g(vulcan) indicates that PtMe 2 COD has a higher affinity for Vulcan than for scco 2. These data show that there is a limiting value for q of mg=g(vulcan), which corresponds to a Pt loading of 9%. The thermal reduction in scco 2 method was used to increase the Pt loadings. In this method, an amount of precursor corresponding to the required Pt loading was put in the vessel. By increasing the temperature in the supercritical carbon dioxide Figure 4. Bright field and high resolution TEM images for (a), (b) Pt=Vulcan 9% and (c), (d) Pt=Vulcan 15%.

8 200 A. Bayrakçeken et al. phase, all of the precursor placed into the vessel could be converted to Pt. Catalysts with loadings of 15% and 35% were produced in this manner. The XRD spectra for the Pt=Vulcan (9%), Pt=Vulcan (15%), and Pt=Vulcan (35%) catalysts are given in Figure 3. For all catalysts the characteristic {111}, {200}, {220} peaks were obtained for face-centered cubic (fcc) Pt. Because the {111} and {200} Pt peaks overlapped with the carbon peaks, the Pt particle sizes were calculated from the Scherrer equation using the full width at half maximum of the {220} peaks. In this manner, the Pt particle sizes obtained for Pt=Vulcan 9%, 15%, and 35% catalysts were 1.2, 1.3, and 2 nm, respectively. The surface area (SA) of the metal phase (Pt) was calculated from these particle sizes by assuming that the particles are monodisperse and spherical (Pozio et al., 2002). Figure 4 contains examples of the bright field and high-resolution TEM images obtained from the Pt=Vulcan 9% (Figure 4(a), (b)) and Pt=Vulcan 15% (Figure 4(c), (d)) catalysts. Such micrographs confirm that the Pt particles produced by the scco 2 deposition method are extremely fine with a narrow particle size distribution and a uniform spatial distribution. The mean particle sizes measured from such images for Pt=Vulcan 9% and Pt=Vulcan 15% catalysts are in very good agreement with the ones obtained from the XRD spectra. This indicates that the individual Pt particles were single crystals, as shown previously for deposition from scco 2 onto various carbons (Zhang et al., 2005b). For catalysts prepared by thermal reduction in scco 2, it was observed that when the platinum loading on Vulcan was increased, the particle size also increased. This behavior indicates that the mechanism for deposition in this method is the following: 1. The Vulcan is impregnated with the precursor up to the maximum adsorption capacity depending on the adsorption isotherm for the experimental conditions used. 2. When the temperature is increased, there is decomposition of the adsorbed precursor molecules and growth of the particles. 3. The precursor molecules in the scco 2 phase then are adsorbed on the carbon sites vacated by the decomposed precursor molecules. These adsorbed molecules are in Figure 5. Cyclic voltammograms of Pt=Vulcan 15% and Pt=Vulcan 35% catalysts in 0.1 M HClO 4 electrolyte in H 2 atmosphere at a scan rate of 50 mv s 1.

9 Supported Pt Electrocatalysts for PEMFCs 201 Table II. Electrochemical and the total surface area of the Pt=Vulcan catalysts Catalyst d (nm) SA Pt (m 2 =g) ESA Pt (m 2 =g) % Pt utilization (ESA=SA 100) Pt=Vulcan 9% Pt=Vulcan 15% Pt=Vulcan 35% From XRD data. turn reduced and the process continues until all the precursor molecules in the system are converted to the metal. Therefore, a higher precursor concentration in the fluid phase leads to a larger particle size on the surface. The cyclic voltammograms for the catalysts prepared with different Pt loadings are given in Figure 5. Although the highest ESA was obtained with Pt=Vulcan 9%, which was produced by ex situ reduction of the precursor at atmospheric pressure, the Pt utilization for this sample was just 74%, indicating that some of the metal s active sites for the electro-oxidation and -reduction reactions are not accessible to the electrolyte. Since the surface area of the micropores is a substantial fraction of the total surface area, this behavior could be due to some of the small Pt particles being deposited into the micropores of the carbon support, thereby hindering accessibility. For the samples produced by reduction in scco 2, the activity of the Pt=Vulcan 15% for electro-oxidation and reduction of the hydrogen is superior to that of Pt=Vulcan 35%. The electrochemical surface areas (ESAs) of the catalysts were calculated by using Equation (2), taking into account the hydrogen reduction peak: ESA ¼ A ð2þ K:L:S where A is the area under the hydrogen reduction part of the curve, excluding the double layer capacitance up to the second inflection point, K ¼ 0.21 mc cm 2 Pt 1, S is the scan rate (50 mv s 1 ), and L is the Pt loading on the electrode (Smirnova et al., 2005). The electrochemical and total surface areas of the Pt=Vulcan catalysts estimated from the XRD, TEM, and CV data are shown in Table II. Although the electrochemical surface area for Pt=Vulcan 15% was higher than that for Pt=Vulcan 35%, the Pt utilization was lower. This difference is, however, quite small (58% versus 62%) and may not be significant given the uncertainty in both the calculated and the measured average Pt particle sizes. We note that most of the difference between the electrochemical surface areas of the Pt=Vulcan 15% and Pt=Vulcan 35% catalysts can be attributed to the differences in the Pt particle sizes, since both the d and ESA Pt values differ by a factor of 1.5. Conclusion Supercritical carbon dioxide deposition was used to prepare platinum nanoparticles on Vulcan XC-72R carbon black. In this approach the carbon support is first impregnated with PtMe 2 COD from a solution in scco 2, and then the precursor is

10 202 A. Bayrakçeken et al. reduced thermally to metallic Pt by heating either ex situ in N 2 or in situ in scco 2. Both reduction techniques result in small (d ¼ 2 nm or less) Pt nanoparticles, as revealed by XRD and TEM data. Moreover, both the particle size and the spatial distribution of the particles are very uniform. The maximum Pt loading achieved with thermal reduction in N 2 was 9%, as expected from the adsorption isotherm. Using thermal reduction in scco 2, higher loadings can be achieved due to concurrent precursor reduction in and adsorption from the scco 2. Samples with Pt loadings of 15% and 35% were produced in this study. The electrochemical surface areas of the Pt=Vulcan 9%,15%, and 35% catalysts were investigated by using CV. The highest ESA and Pt utilization values were obtained with Pt=Vulcan 9%. The ESA of the Pt=Vulcan 15% catalyst was higher than that for Pt=Vulcan 35%, at similar Pt utilization values, and this can be ascribed to the differences in the particle sizes for these two samples. Acknowledgment This study is supported by Middle East Technical University Found of Scientific Research Projects BAP DPT.2002 K and BAP DPT. 2005K References Amine, K., Yasuda, K., and Takenaka, H. (1998). New process for loading highly active platinum on carbon black surface for application in polymer electrolyte fuel cell, Ann. Chim. Sci. Mater., 23, Babić, B. M., Vračar, Lj. M., Radmilović, V., and Krstajić, N. V. (2006). Carbon cryogel as support of platinum nano-sized electrocatalyst for the hydrogen oxidation reaction, Electrochim. Acta, 51, Bayrakceken, A., Kitkamthorn, U., Aindow, M., and Erkey, C. (2007). Decoration of multiwall carbon nanotubes with platinum nanoparticles using supercritical deposition with thermodynamic control of metal loading, Scr. Mater., 56, Costamagna, P. and Srinivasan, S. (2001). Quantum jumps in the PEMFC science and technology from the 1960s to the year Part 1. Fundamental scientific aspects, J. Power Sources, 102, Escudero, M. J., Honta~non, E., Schwartz, S., Boutonnet, B., and Daza, L. (2002). Development and performance characterisation of new electrocatalysts for PEMFC, J. Power Sources, 106, Gloaguen, F., Leger, J. M., and Lamy, C. (1997). Electrocatalytic oxidation of methanol on platinum nanoparticles electrodeposited onto porous carbon substrates, J. Appl. Electrochem., 27, Hoogers, G., ed. (2003). Fuel Cell Technology Handbook, CRC Press, Boca Raton, Fla. Kawaguchi, T., Sugimoto, W., Murakami, Y., and Takasu, Y. (2005). Particle growth behavior of carbon-supported Pt, Ru, PtRu catalysts prepared by an impregnation reductivepyrolysis method for direct methanol fuel cell anodes, J. Catal., 229, Li, W., Liang, C., Zhou, W., Qiu, J., Zhou, Z., Sun, G., and Xin, Q. (2003). Preparation and characterizaiton of multiwalled carbon nanotube-supported platinum for cathode catalysts of direct methanol fuel cells, J. Phys. Chem B, 107, Liu, Z., Lin, X., Lee, J. Y., Zhang, W., Han, M., and Gan, L. M. (2002). Preparation and characterization of platinum-based electrocatalysts on multiwalled carbon nanotubes for proton exchange membrane fuel cells, Langmuir, 18,

11 Supported Pt Electrocatalysts for PEMFCs 203 Marie, J., Berthon-Fabry, S., Achard, P., Chatenet, M., Pradourat, A., and Chainet, E. (2004). Highly dispersed platinum on carbon aerogels as supported catalysts for PEM fuel cell electrodes: Comparison of two different synthesis pathways, J. Non-Cryst. Solids, 350, Maruyama, J. and Abe, I. (2003). Application of conventional activated carbon loaded with dispersed Pt to PEFC catalyst layer, Electrochim. Acta, 48, Pan, M., Tang, H. L., Jiang, S. P., and Liu, Z. (2005). Self-assembled membrane-electrodeassembly of polymer electrolyte fuel cells, Electrochem. Commun., 7, Pozio, A., Francesco, M. D. E., Cemi, A., Cardellini, F., and Giorgi, L. (2002). Comparison of high surface Pt=C catalysts by cyclic voltammetry, J. Power Sources, 105, Raghuveer, V. and Manthiram, A. (2004). Mesoporous carbon with larger pore diameter as an electrocatalyst support for methanol oxidation, Electrochem. Solid State Lett., 7, A336 A339. Saquing, C. D., Cheng, T. T., Aindow, M., and Erkey, C. (2004). Preparation of carbonaerogel-supported platinum electrocatalysts by supercritical deposition method, J. Phys. Chem. B, 108, Saquing, C. D., Kang, D., Aindow, M., and Erkey, C. (2005). Investigation of supercritical deposition of platinum nanoparticles into carbon aerogels, Microporous Mesoporous Mater., 80, Shao, Y., Yin, G., Wang, J., Gao, Y., and Shi, P. (2006). Multi-walled carbon nanotubes based Pt electrodes prepared with in situ ion exchange method for oxygen reduction, J. Power Sources, 161, Smirnova, A., Dong, X., Hara, H., Vasiliev, A., and Sammes, N. (2005). Novel carbon aerogel-supported catalysts for PEM fuel cell application, Int. J. Hydrogen Energy, 30, Xiong, L. and Manthiram, A. (2005). High performance membrane-electrode assemblies with ultra-low Pt loading for proton exchange membrane fuel cells, Electrochim. Acta, 50, Zhang, Y. and Erkey, C. (2006). Preparation of supported metallic nanoparticles using supercritical fluids: A review, J. Supercrit. Fluids, 38, Zhang, Y., Kang, D., Aindow, M., and Erkey, C. (2005a). Ruthenium-carbon aerogel nanocomposites via a supercritical fluid route, J. Phys. Chem., 109, Zhang, Y., Kang, D., Saquing, C., Aindow, M., and Erkey, C. (2005b). Supported platinum nanoparticles by supercritical deposition, Ind. Eng. Chem. Res., 44, Zhang, J., Wang, X., Wu, C., Wang, H., Yi, B., and Zhang, H. (2004). Preparation and characterization of Pt=C catalysts for PEMFC cathode: Effect of different reduction methods, React. Kinet. Catal. Lett., 83(2),