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Cite this: DOI: 10.1039/c4cc09019e Received 14th November 2014, Accepted 9th January 2015
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Third-body effects of native surfactants on Pt nanoparticle electrocatalysts in proton exchange fuel cells† Young-Hoon Chung,a Soo Jin Kim,a Dong Young Chung,bc Hee Young Park,a Yung-Eun Sung,bc Sung Jong Yooa and Jong Hyun Jang*ad
DOI: 10.1039/c4cc09019e www.rsc.org/chemcomm
The residual surfactant organic molecules on electrocatalysts are expected to enhance the tolerance to specific anion adsorption, whereas the surfactants have been generally regarded as contaminants that block active surfaces. In this study, the Pt nanoparticles with adsorbed surfactants were prepared, and their electrochemical characteristics at various phosphoric acid concentrations were studied by the half-cell test. The third-body effect was experimentally confirmed by the single-cell test with a phosphoric acid-doped polybenzimidazole membrane.
For decades, proton exchange membrane fuel cells (PEMFCs) have attracted much attention as renewable energy sources.1 However, a number of technical issues still remain prior to commercialization, including the requirement for expensive electrocatalysts to promote the oxygen reduction reaction (ORR) at cathodes.2 Among various efforts to improve the cathodic ORR activity, the most common approach is to control the electronic structure of the platinum electrocatalyst by introducing transition metals,3 applying various support materials,4 or utilizing organic compounds.5 Recently, the high-temperature PEMFCs that utilize H3PO4-doped polybenzimidazole (PBI) membranes have been intensively researched, due to the enhanced kinetics and water/heat management at higher temperatures (B180 1C). However, there exists an additional technical issue of Pt poisoning by H3PO4.6 In another approach to enhancing the electrochemical activity, the Conway group reported the so-called ‘‘third-body (or ensemble)
a
Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea. E-mail: [email protected]; Fax: +82-2-958-5199; Tel: +82-2-958-5287 b Chemical and Biological Engineering, Seoul National University (SNU), Seoul 151-742, Republic of Korea c Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742, Republic of Korea d Green School, Korea University, Seoul 136-713, Republic of Korea † Electronic supplementary information (ESI) available: Experimental details, supplementary electrochemical data for cPt–OA, XRD spectra, XPS spectrum, TGA curves of sPt–OA, and current densities in a single-cell scale. See DOI: 10.1039/c4cc09019e
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effect’’, which refers to the blocking of poisonous spectator species on the electrocatalyst.7 It was reported that, for smooth platinum, an enhancement of the formic acid oxidation activity was achieved by introducing acetonitrile (CH3CN), which was explained by the blocking of the surface from the adsorption of inhibitor species. Genorio et al. reported that the ORR is selectively inhibited over the hydrogen oxidation reaction by calix[4]-arene molecules with truncated cone-like structures.8 This study revealed that, to effectively exploit the third-body effect for the ORR with respect to specific anion adsorption, the modified organic molecules should leave sufficient room for contact of the oxygen reactants with the catalyst. A similar effect was also reported for the ORR in the presence of phosphoric acid.7,8 Strmcnik et al. reported that poisoning by specific adsorption of anions (H2SO4 and H3PO4) could be dramatically decreased by the adsorption of cyanide (CN ) on the platinum surface, which was ascribed to the selective blocking of phosphate anions.9 Recently, Chung et al. reported that, when commercial Pt/C catalysts were modified by oleylamine (OA) with surface coverage up to 30%, the electronic structure of the Pt nanoparticles was altered, i.e., the d-band center was downshifted, to provide significantly enhanced ORR activity.5b In addition, when H3PO4 was added to the electrolyte solution, the oleylamine-adsorbed Pt/C showed enhanced ORR activity retention (up to 71%) compared to the untreated Pt/C (47%), which can be ascribed to the third-body effect of the adsorbed oleylamine molecules blocking phosphate anions. Therefore, the third-body effect, which has been studied at the halfcell level, is expected to be utilized to enhance the cell performances of practical PBI-based PEMFCs. To utilize the third-body effect in practical PEMFC systems, it would be highly desirable to develop a direct synthesis of OA-adsorbed Pt/C, where the OA adsorption step is eliminated. In the conventional colloidal-reduction preparation of platinum-based electrocatalysts,10 the particle size is generally controlled to a few nanometers by using organic species as surfactants, which entropically control growth during the preparation process.11 Then, because the strongly adsorbed surfactants cover a large portion of the surface active sites and proportionally reduce the ORR activity,3f
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additional tedious processes such as heat treatment, plasma etching, and acid leaching are generally performed to remove the adsorbed organic species and maximize the amount of active surface.10c,12 In this study, we propose that platinum nanoparticles prepared using adsorbed organic surfactants, such as OA, can exert a positive role in the operation of practical high-temperature PEMFCs through the aid of the third-body effect. To accomplish this, the effects of high level OA adsorption (coverage (y) B 0.7) were first evaluated by analyzing OA-adsorbed commercial Pt/C catalysts (cPt–OA) in comparison to untreated Pt/C (cPt), focusing on the third-body effect to block the poisonous phosphate anion spectator species. For the preparation of cPt–OA, the excess amount of OA was added to the dispersion solution of commercial Pt/C (20 wt%, Johnson Matthey) in ethanol, and the mixture solution was stirred overnight. Detailed procedures are described in the ESI† (S1.1). Considering practical applications, we then tried to directly synthesize the OA-adsorbed Pt/C catalyst (sPt–OA) via colloidal reduction methods using oleylamine as a surfactant, where the conventional thermal/chemical treatment step to remove surfactant species was eliminated. For this, precursor solution was prepared by dispersing carbon black and oleylamine in ethanol, and adding platinum(IV) chloride. Then, sodium borohydride was introduced into the solution to reduce platinum precursors to form carbon-supported Pt nanoparticles with adsorbed OA (S1.2 in the ESI†). Then, the ORR activity of sPt–OA was evaluated in a single-cell test at 160 1C, utilizing PBI membranes with H3PO4-doping as the polymer electrolytes. Based on a previous report,5b oleylamine was used as the blocking species for the third-body effect, where its bulky aliphatic hydrocarbon chain was expected to selectively block the adsorption of large anions and allow the access of smaller oxygen molecules onto the Pt surface (Scheme 1). To confirm the modification of the platinum surface through oleylamine treatment, near-edge X-ray absorption fine structure (NEXAFS) and cyclic voltammetry (CV) studies were carried out (Fig. 1a and b). The NEXAFS spectra of the C K edge, which provide information about the surface bonding environment of carbon from the initial 1s electronic structure, were measured at the 4D beamline of the Pohang Light Source-II (PLS-II, 3 GeV) using soft X-rays.13 The experimental details are described in the ESI.† As shown in Fig. 1a, the oleylamine treatment clearly increases the intensity of the photon energy at 287–290 eV, which is reportedly associated with the s* electronic state of the C–H bond.14 Therefore, it can be concluded that a significant amount of OA is adsorbed on the Pt surface. Fig. 1b presents the CVs of cPt and cPt–OA in different electrolyte solutions: 0.1 M HClO4 or a mixture of 0.1 M HClO4 + 0.1 M H3PO4. In the hydrogen underpotential deposition region, the peak area for proton adsorption/ desorption (Had 2 H+ + e ) is significantly decreased for cPt–OA compared to the untreated cPt. From the integrated desorption charge, the electrochemically active surface area (ECSA) was calculated to be 33.5 (cPt) and 11.7 m2 gPt 1 (cPt–OA), based on the hydrogen desorption charge of 0.21 mC mPt 2. This ECSA decrease indicates that oleylamine was effectively adsorbed and blocked about 70% of the surface sites on the platinum nanoparticles. The third-body effect of cPt–OA in comparison with cPt was evaluated by electrochemical analysis using HClO4 electrolyte
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Scheme 1 Schematic illustration of third-body effects on a modified platinum surface for the oxygen reduction reaction in the presence of the specifically adsorbed anions.
Fig. 1 (a) Near-edge X-ray absorption fine structure (NEXAFS) spectra of the C K edge using the 4D beamline of the Pohang Light Source-II (PLS-II, 3 GeV). (b) Cyclic voltammograms (CVs). (c) Polarization curves of the oxygen reduction reaction (ORR) at a rotation speed of 1600 rpm. (d) Mass-normalized kinetic current densities at 0.9 V for cPt and cPt–OA in various electrolytes. All electrochemical measurements were conducted at room temperature and standardized against the reversible hydrogen electrode (RHE).
solutions that contained H3PO4 at various concentrations (0, 0.01, and 0.1 M). Upon positive charging, the specifically adsorbed anions, such as the conjugate bases of H3PO4,15 are strongly adsorbed on the metal electrodes including the platinum surfaces and severely affect the activity of the electrochemical reactions.6a,b,d,16 In contrast, solvated ClO4 is only weakly adsorbed on the electrodes, due to its high hydration strength.17 In the case of cPt, the redox currents at around 0.8 V are decreased with H3PO4 addition, indicating that the OH adsorption/ desorption processes, i.e., Pt + H2O 2 Pt–OHad + H+ + e , are
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hindered by the adsorbed phosphate species that block the active sites.6a,b,16 In contrast, cPt–OA demonstrates nearly identical CV curves in both the presence and absence of H3PO4, although the OH redox currents are smaller than that of cPt. These features reveal that the OA-modified electrode is tolerant to the specifically adsorbed anions. For cPt and cPt–OA, the effect of H3PO4 concentration on the ORR activity was analyzed by polarization tests in oxygen-saturated electrolytes. A shown in Fig. 1c, with an increase in H3PO4 concentration, the polarization curves of cPt are gradually shifted in the direction of lower electrode potential, indicating that the ORR activity is significantly decreased by H3PO4 adsorption. This significant activity decay (ca. 71% at 0.1 M H3PO4) can be explained by the observation that phosphate anions were on three-fold sites and a decrease in the amount of available Pt surface.9,16 In contrast, cPt–OA shows a high retention of ORR activity with H3PO4 addition, which suggests that the OA molecules adsorbed on the surface of the platinum nanoparticles effectively block the poisonous spectator species through steric hindrance (third-body effect). When the H3PO4 amount is changed from 0 M to 0.1 M, the mass activity decay was significant for cPt ( 71%), whereas the performance decay for cPt–OA was much smaller (Fig. 1d). To confirm and practically utilize the third-body effect in fuel cells with PBI/PA membranes, OA-adsorbed Pt/C (sPt–OA) was directly synthesized and its ORR activity was evaluated. As oleylamine is the surfactant that is widely used to control the particle size of novel metals,10 the conventional colloidal reduction method with oleylamine surfactants was modified to eliminate the OA removal step (conventionally, it has been generally accepted that a removal process is required to obtain the clean surface after the preparation process).10c–f Fig. 2a shows the TEM images of the OAmodified carbon-supported platinum nanoparticles (sPt–OA) with diameters of 2–3 nm. A typical face centered cubic (fcc) structure was confirmed by the X-ray diffraction (XRD, Fig. S2, ESI†) pattern and the reduced fast Fourier transform (FFT) patterns. Also, the high resolution-transmission electron microscopy (HR-TEM) image exhibited a typical lattice distance of platinum (111). The Pt content of sPt–OA was determined to be 17.5 wt% from the residual weight after thermogravimetric analysis (TGA, Fig. S4, ESI†). The weight decrease in the temperature range between 150 and 400 1C, where adsorbed oleylamine molecules are thermally decomposed, is about 2.5 wt%.10b–d The X-ray absorption near edge structure (XANES) spectra of the Pt L3 edge demonstrate that the d-band vacancy of sPt–OA, which is proportional to the intensity of the white line of the L3 edge, was smaller than that of cPt. This indicates that the oleylamine molecules are adsorbed on the as-prepared sPt–OA nanoparticles, and thereby modify their electronic structures.5b The effect of PA adsorption on sPt–OA was evaluated by a halfcell test (Fig. 2c). It can also be noted that, compared to cPt–OA, sPt–OA has higher ORR activity in 0.1 M HClO4 solution and larger ORR decay with H3PO4 addition, which can be ascribed to the different OA coverage. These results suggest that sPt–OA can provide higher PEMFC performances with PBI/PA electrolytes. The sPt–OA catalyst was incorporated into a membrane electrode assembly (MEA) using a homemade H3PO4-doped
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Fig. 2 (a) Transmission electron microscopy (TEM) images of sPt–OA (left) as well as high resolution images, reduced fast Fourier transform (FFT) patterns, and inversed FFT images with masked Pt(111) facets (right). (b) X-ray absorption near-edge structure (XANES) of the Pt L3 edge using the 8 C beamline of the PLS-II. (c) CVs and polarization curves of the ORR at a rotation speed of 1600 rpm. (d) Mass-normalized kinetic current densities at 0.9 V for sPt–OA.
para-polybenzimidazole (p-PBI) membrane, and a single-cell test was carried out (Pt loading: 0.26 mgPt cm 2). As a reference sample, an MEA with a cPt catalyst was also evaluated (Pt loading: 0.30 mgPt cm 2). In order to fix the effect of MEA compression, a similar catalyst layer thickness was contrived by controlling the Pt loading. In high-temperature PEMFCs with PBI/PA membranes, it is well known that the electrocatalyst is severely poisoned by specific anion adsorption with H3PO4.6c,18,19 As shown in Fig. 3a, sPt–OA shows higher cell voltages in the low-current density region, which is consistent with the half-cell results (Fig. S5, ESI†). Note that single-cell performance in the low-current region is mainly dependent on the ORR activity of the cathode, whereas mass transport characteristics become dominant under high-current operation. The higher ORR activity of sPt–OA can be more clearly observed in the Tafel plot (Fig. 3b). By interpolation of the experimental data points, the kinetic current densities of cPt and sPt–OA were determined to be 0.024 and 0.038 A cm 2. Also note that the kinetic current density of sPt–OA is about 1.6 times higher than that of cPt, even though the Pt loading in sPt–OA is smaller by 13%. Therefore, it can be calculated that, in the single-cell test, the mass activity of sPt–OA is 1.9 times higher than that of cPt. The MEAs constructed with either cPt or sPt–OA as the electrocatalyst provided stable cell voltages over 150 h during constant current operation at 0.2 A cm 2. The higher cell performance could be confirmed over the entire range of the durability test. In summary, we determined that the third-body effect of oleylamine, a widely used surfactant, could enhance the ORR activity of Pt nanoparticles in the presence of specifically adsorbed anions despite the quite small ECSA. Electrochemical characterization showed that, for the synthesized Pt nanoparticles with adsorbed OA, the poisonous spectator anions were blocked and ORR activity was enhanced. Further, this concept could be extended to a practical PEMFC system with severe anion adsorption conditions, and enhanced cell performances were experimentally confirmed. This work was supported by Korean Government through the New & Renewable Energy Core Technology Program of the
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Fig. 3 Electrochemical measurements of cPt and sPt–OA in a membrane electrode assembly (MEA) using a H3PO4-doped polybenzimidazole (p-PBI) membrane. (a) Polarization curves. (b) Tafel plots with Ohmic resistance compensation. (c) Kinetic current density at 0.7 V. (d) Durability tests with a constant current density (0.2 A cm 2) over 150 h. All measurements were carried out with non-humidified H2/air feeds at 160 1C.
KETEP funded by MOTIE (No. 20133030011320), the Korea CCS R&D Center (KCRC) grant funded by MSIP (No. 2013M1A8A1038315), the National Research Foundation of Korea grant funded by MSIP (2014, University-Institute cooperation program), and the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation, MSIP (No. 2012M3A6A7054283). This work was also financially supported by KIST through the Institutional Program, COE program and K-GRL program.
Notes and references 1 M. K. Debe, Nature, 2012, 486, 43. 2 J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, ´nsson, J. Phys. Chem. B, 2004, 108, 17886. T. Bligaard and H. Jo 3 (a) Y.-H. Chung, S. J. Kim, D. Y. Chung, M. J. Lee, J. H. Jang and Y.-E. Sung, Phys. Chem. Chem. Phys., 2014, 16, 13726; (b) P. Strasser, S. Koh, T. Anniyev, J. Greeley, K. More, C. Yu, Z. Liu, S. Kaya, D. Nordlund, H. Ogasawara, M. F. Toney and A. Nilsson, Nat. Chem., 2010, 2, 454; (c) M. Escudero-Escribano, A. Verdaguer-Casadevall, P. Malacrida, U. Grønbjerg, B. P. Knudsen, A. K. Jepsen, J. Rossmeisl, I. E. L. Stephens and I. Chorkendorff, J. Am. Chem. Soc., 2012, 134, 16476; (d) J. Greeley, I. E. L. Stephens, A. S. Bondarenko, T. P. Johansson, H. A. Hansen, T. F. Jaramillo, J. Rossmeisl, I. Chorkendorff and J. K. Nørskov, Nat. Chem., 2009, 1, 552; (e) S. J. Hwang, S.-K. Kim, J.-G. Lee, S.-C. Lee, J. H. Jang, P. Kim, T.-H. Lim, Y.-E. Sung and S. J. Yoo, J. Am. Chem. Soc., 2012, 134, 19508; ( f ) V. R. Stamenkovic, B. S. Mun, M. Arenz, K. J. J. Mayrhofer, C. A. Lucas, G. Wang, P. N. Ross and N. M. Markovic, Nat. Mater., 2007, 6, 241.
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ChemComm 4 (a) E. Toyoda, R. Jinnouchi, T. Ohsuna, T. Hatanaka, T. Aizawa, S. Otani, Y. Kido and Y. Morimoto, Angew. Chem., Int. Ed., 2013, 52, 4137; (b) R. Kou, Y. Shao, D. Mei, Z. Nie, D. Wang, C. Wang, V. V. Viswanathan, S. Park, I. A. Aksay, Y. Lin, Y. Wang and J. Liu, J. Am. Chem. Soc., 2011, 133, 2541. 5 (a) J. Snyder, T. Fujita, M. W. Chen and J. Erlebacher, Nat. Mater., 2010, 9, 904; (b) Y.-H. Chung, D. Y. Chung, N. Jung and Y.-E. Sung, J. Phys. Chem. Lett., 2013, 4, 1304. 6 (a) Q. He, B. Shyam, M. Nishijima, D. Ramaker and S. Mukerjee, J. Phys. Chem. C, 2013, 117, 4877; (b) R. Gisbert, G. Garcı´a and M. T. M. Koper, Electrochim. Acta, 2010, 55, 7961; (c) Y. Zhai, H. Zhang, D. Xing and Z. G. Shao, J. Power Sources, 2007, 164, 126; (d) P. N. Ross and P. C. Andricacos, J. Electroanal. Chem. Interfacial Electrochem., 1983, 154, 205. 7 H. Angerstein-Kozlowska, B. MacDougall and B. E. Conway, J. Electrochem. Soc., 1973, 120, 756. 8 B. Genorio, D. Strmcnik, R. Subbaraman, D. Tripkovic, G. Karapetrov, ´, Nat. Mater., 2010, V. R. Stamenkovic, S. Pejovnik and N. M. Markovic 9, 998. 9 D. Strmcnik, M. Escudero-Escribano, K. Kodama, V. R. Stamenkovic, A. Cuesta and N. M. Markovic, Nat. Chem., 2010, 2, 880. 10 (a) J. Zhang, H. Yang, K. Yang, J. Fang, S. Zou, Z. Luo, H. Wang, I.-T. Bae and D. Y. Jung, Adv. Funct. Mater., 2010, 20, 3727; (b) J. C. Bauer, D. Mullins, M. Li, Z. Wu, E. A. Payzant, S. H. Overbury and S. Dai, Phys. Chem. Chem. Phys., 2011, 13, 2571; (c) J. Luo, L. Wang, D. Mott, P. N. Njoki, N. Kariuki, C.-J. Zhong and T. He, J. Mater. Chem., 2006, 16, 1665; (d) C. Kim, S. S. Kim, S. Yang, J. W. Han and H. Lee, Chem. Commun., 2012, 48, 6396; (e) Z. Peng, H. You and H. Yang, ACS Nano, 2010, 4, 1501; ( f ) J. Zhang and J. Fang, J. Am. Chem. Soc., 2009, 131, 18543. 11 J. Park, J. Joo, G. K. Soon, Y. Jang and T. Hyeon, Angew. Chem., Int. Ed., 2007, 46, 4630. 12 (a) D. Li, C. Wang, D. Tripkovic, S. Sun, N. M. Markovic and V. R. Stamenkovic, ACS Catal., 2012, 2, 1358; (b) F. J. Vidal-Iglesias, ´n, E. Herrero, V. Montiel, A. Aldaz and J. M. Feliu, J. Solla-Gullo Electrochem. Commun., 2011, 13, 502. ¨hner, Chem. Soc. Rev., 2006, 35, 1244. 13 G. Ha 14 (a) J. Wang, J. Zhou, Y. Hu and T. Regier, Energy Environ. Sci., 2013, 6, 926; (b) G. Tzvetkov, P. Fernandes, S. Wenzel, A. Fery, G. Paradossi and R. H. Fink, Phys. Chem. Chem. Phys., 2009, 11, 1098; (c) C. Albonetti, S. Casalini, F. Borgatti, L. Floreano and F. Biscarini, Chem. Commun., 2011, 47, 8823; (d) M. E. Drew, A. R. Konicek, P. Jaroenapibal, R. W. Carpick and Y. Yamakoshi, J. Mater. Chem., 2012, 22, 12682. 15 A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals and Applications, Wiley, 2001. 16 Q. He, X. Yang, W. Chen, S. Mukerjee, B. Koel and S. Chen, Phys. Chem. Chem. Phys., 2010, 12, 12544. 17 (a) D. V. Tripkovic, D. Strmcnik, D. Van Der Vliet, V. Stamenkovic and N. M. Markovic, Faraday Discuss., 2008, 140, 25; (b) T. J. Schmidt, U. A. Paulus, H. A. Gasteiger and R. J. Behm, J. Electroanal. Chem., 2001, 508, 41. 18 G. Liu, H. Zhang, J. Hu, Y. Zhai, D. Xu and Z.-g. Shao, J. Power Sources, 2006, 162, 547. 19 S. J. Kim, H.-Y. Park, S. H. Ahn, B.-s. Lee, H.-J. Kim, E. Cho, D. Henkensmeier, S. W. Nam, S. H. Kim, S. J. Yoo and J. H. Jang, Appl. Catal., B, 2014, 158–159, 348.
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Cite this: DOI: 10.1039/c4cc09019e Received 14th November 2014, Accepted 9th January 2015
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Third-body effects of native surfactants on Pt nanoparticle electrocatalysts in proton exchange fuel cells† Young-Hoon Chung,a Soo Jin Kim,a Dong Young Chung,bc Hee Young Park,a Yung-Eun Sung,bc Sung Jong Yooa and Jong Hyun Jang*ad
DOI: 10.1039/c4cc09019e www.rsc.org/chemcomm
The residual surfactant organic molecules on electrocatalysts are expected to enhance the tolerance to specific anion adsorption, whereas the surfactants have been generally regarded as contaminants that block active surfaces. In this study, the Pt nanoparticles with adsorbed surfactants were prepared, and their electrochemical characteristics at various phosphoric acid concentrations were studied by the half-cell test. The third-body effect was experimentally confirmed by the single-cell test with a phosphoric acid-doped polybenzimidazole membrane.
For decades, proton exchange membrane fuel cells (PEMFCs) have attracted much attention as renewable energy sources.1 However, a number of technical issues still remain prior to commercialization, including the requirement for expensive electrocatalysts to promote the oxygen reduction reaction (ORR) at cathodes.2 Among various efforts to improve the cathodic ORR activity, the most common approach is to control the electronic structure of the platinum electrocatalyst by introducing transition metals,3 applying various support materials,4 or utilizing organic compounds.5 Recently, the high-temperature PEMFCs that utilize H3PO4-doped polybenzimidazole (PBI) membranes have been intensively researched, due to the enhanced kinetics and water/heat management at higher temperatures (B180 1C). However, there exists an additional technical issue of Pt poisoning by H3PO4.6 In another approach to enhancing the electrochemical activity, the Conway group reported the so-called ‘‘third-body (or ensemble)
a
Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea. E-mail: [email protected]; Fax: +82-2-958-5199; Tel: +82-2-958-5287 b Chemical and Biological Engineering, Seoul National University (SNU), Seoul 151-742, Republic of Korea c Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742, Republic of Korea d Green School, Korea University, Seoul 136-713, Republic of Korea † Electronic supplementary information (ESI) available: Experimental details, supplementary electrochemical data for cPt–OA, XRD spectra, XPS spectrum, TGA curves of sPt–OA, and current densities in a single-cell scale. See DOI: 10.1039/c4cc09019e
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effect’’, which refers to the blocking of poisonous spectator species on the electrocatalyst.7 It was reported that, for smooth platinum, an enhancement of the formic acid oxidation activity was achieved by introducing acetonitrile (CH3CN), which was explained by the blocking of the surface from the adsorption of inhibitor species. Genorio et al. reported that the ORR is selectively inhibited over the hydrogen oxidation reaction by calix[4]-arene molecules with truncated cone-like structures.8 This study revealed that, to effectively exploit the third-body effect for the ORR with respect to specific anion adsorption, the modified organic molecules should leave sufficient room for contact of the oxygen reactants with the catalyst. A similar effect was also reported for the ORR in the presence of phosphoric acid.7,8 Strmcnik et al. reported that poisoning by specific adsorption of anions (H2SO4 and H3PO4) could be dramatically decreased by the adsorption of cyanide (CN ) on the platinum surface, which was ascribed to the selective blocking of phosphate anions.9 Recently, Chung et al. reported that, when commercial Pt/C catalysts were modified by oleylamine (OA) with surface coverage up to 30%, the electronic structure of the Pt nanoparticles was altered, i.e., the d-band center was downshifted, to provide significantly enhanced ORR activity.5b In addition, when H3PO4 was added to the electrolyte solution, the oleylamine-adsorbed Pt/C showed enhanced ORR activity retention (up to 71%) compared to the untreated Pt/C (47%), which can be ascribed to the third-body effect of the adsorbed oleylamine molecules blocking phosphate anions. Therefore, the third-body effect, which has been studied at the halfcell level, is expected to be utilized to enhance the cell performances of practical PBI-based PEMFCs. To utilize the third-body effect in practical PEMFC systems, it would be highly desirable to develop a direct synthesis of OA-adsorbed Pt/C, where the OA adsorption step is eliminated. In the conventional colloidal-reduction preparation of platinum-based electrocatalysts,10 the particle size is generally controlled to a few nanometers by using organic species as surfactants, which entropically control growth during the preparation process.11 Then, because the strongly adsorbed surfactants cover a large portion of the surface active sites and proportionally reduce the ORR activity,3f
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additional tedious processes such as heat treatment, plasma etching, and acid leaching are generally performed to remove the adsorbed organic species and maximize the amount of active surface.10c,12 In this study, we propose that platinum nanoparticles prepared using adsorbed organic surfactants, such as OA, can exert a positive role in the operation of practical high-temperature PEMFCs through the aid of the third-body effect. To accomplish this, the effects of high level OA adsorption (coverage (y) B 0.7) were first evaluated by analyzing OA-adsorbed commercial Pt/C catalysts (cPt–OA) in comparison to untreated Pt/C (cPt), focusing on the third-body effect to block the poisonous phosphate anion spectator species. For the preparation of cPt–OA, the excess amount of OA was added to the dispersion solution of commercial Pt/C (20 wt%, Johnson Matthey) in ethanol, and the mixture solution was stirred overnight. Detailed procedures are described in the ESI† (S1.1). Considering practical applications, we then tried to directly synthesize the OA-adsorbed Pt/C catalyst (sPt–OA) via colloidal reduction methods using oleylamine as a surfactant, where the conventional thermal/chemical treatment step to remove surfactant species was eliminated. For this, precursor solution was prepared by dispersing carbon black and oleylamine in ethanol, and adding platinum(IV) chloride. Then, sodium borohydride was introduced into the solution to reduce platinum precursors to form carbon-supported Pt nanoparticles with adsorbed OA (S1.2 in the ESI†). Then, the ORR activity of sPt–OA was evaluated in a single-cell test at 160 1C, utilizing PBI membranes with H3PO4-doping as the polymer electrolytes. Based on a previous report,5b oleylamine was used as the blocking species for the third-body effect, where its bulky aliphatic hydrocarbon chain was expected to selectively block the adsorption of large anions and allow the access of smaller oxygen molecules onto the Pt surface (Scheme 1). To confirm the modification of the platinum surface through oleylamine treatment, near-edge X-ray absorption fine structure (NEXAFS) and cyclic voltammetry (CV) studies were carried out (Fig. 1a and b). The NEXAFS spectra of the C K edge, which provide information about the surface bonding environment of carbon from the initial 1s electronic structure, were measured at the 4D beamline of the Pohang Light Source-II (PLS-II, 3 GeV) using soft X-rays.13 The experimental details are described in the ESI.† As shown in Fig. 1a, the oleylamine treatment clearly increases the intensity of the photon energy at 287–290 eV, which is reportedly associated with the s* electronic state of the C–H bond.14 Therefore, it can be concluded that a significant amount of OA is adsorbed on the Pt surface. Fig. 1b presents the CVs of cPt and cPt–OA in different electrolyte solutions: 0.1 M HClO4 or a mixture of 0.1 M HClO4 + 0.1 M H3PO4. In the hydrogen underpotential deposition region, the peak area for proton adsorption/ desorption (Had 2 H+ + e ) is significantly decreased for cPt–OA compared to the untreated cPt. From the integrated desorption charge, the electrochemically active surface area (ECSA) was calculated to be 33.5 (cPt) and 11.7 m2 gPt 1 (cPt–OA), based on the hydrogen desorption charge of 0.21 mC mPt 2. This ECSA decrease indicates that oleylamine was effectively adsorbed and blocked about 70% of the surface sites on the platinum nanoparticles. The third-body effect of cPt–OA in comparison with cPt was evaluated by electrochemical analysis using HClO4 electrolyte
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Scheme 1 Schematic illustration of third-body effects on a modified platinum surface for the oxygen reduction reaction in the presence of the specifically adsorbed anions.
Fig. 1 (a) Near-edge X-ray absorption fine structure (NEXAFS) spectra of the C K edge using the 4D beamline of the Pohang Light Source-II (PLS-II, 3 GeV). (b) Cyclic voltammograms (CVs). (c) Polarization curves of the oxygen reduction reaction (ORR) at a rotation speed of 1600 rpm. (d) Mass-normalized kinetic current densities at 0.9 V for cPt and cPt–OA in various electrolytes. All electrochemical measurements were conducted at room temperature and standardized against the reversible hydrogen electrode (RHE).
solutions that contained H3PO4 at various concentrations (0, 0.01, and 0.1 M). Upon positive charging, the specifically adsorbed anions, such as the conjugate bases of H3PO4,15 are strongly adsorbed on the metal electrodes including the platinum surfaces and severely affect the activity of the electrochemical reactions.6a,b,d,16 In contrast, solvated ClO4 is only weakly adsorbed on the electrodes, due to its high hydration strength.17 In the case of cPt, the redox currents at around 0.8 V are decreased with H3PO4 addition, indicating that the OH adsorption/ desorption processes, i.e., Pt + H2O 2 Pt–OHad + H+ + e , are
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hindered by the adsorbed phosphate species that block the active sites.6a,b,16 In contrast, cPt–OA demonstrates nearly identical CV curves in both the presence and absence of H3PO4, although the OH redox currents are smaller than that of cPt. These features reveal that the OA-modified electrode is tolerant to the specifically adsorbed anions. For cPt and cPt–OA, the effect of H3PO4 concentration on the ORR activity was analyzed by polarization tests in oxygen-saturated electrolytes. A shown in Fig. 1c, with an increase in H3PO4 concentration, the polarization curves of cPt are gradually shifted in the direction of lower electrode potential, indicating that the ORR activity is significantly decreased by H3PO4 adsorption. This significant activity decay (ca. 71% at 0.1 M H3PO4) can be explained by the observation that phosphate anions were on three-fold sites and a decrease in the amount of available Pt surface.9,16 In contrast, cPt–OA shows a high retention of ORR activity with H3PO4 addition, which suggests that the OA molecules adsorbed on the surface of the platinum nanoparticles effectively block the poisonous spectator species through steric hindrance (third-body effect). When the H3PO4 amount is changed from 0 M to 0.1 M, the mass activity decay was significant for cPt ( 71%), whereas the performance decay for cPt–OA was much smaller (Fig. 1d). To confirm and practically utilize the third-body effect in fuel cells with PBI/PA membranes, OA-adsorbed Pt/C (sPt–OA) was directly synthesized and its ORR activity was evaluated. As oleylamine is the surfactant that is widely used to control the particle size of novel metals,10 the conventional colloidal reduction method with oleylamine surfactants was modified to eliminate the OA removal step (conventionally, it has been generally accepted that a removal process is required to obtain the clean surface after the preparation process).10c–f Fig. 2a shows the TEM images of the OAmodified carbon-supported platinum nanoparticles (sPt–OA) with diameters of 2–3 nm. A typical face centered cubic (fcc) structure was confirmed by the X-ray diffraction (XRD, Fig. S2, ESI†) pattern and the reduced fast Fourier transform (FFT) patterns. Also, the high resolution-transmission electron microscopy (HR-TEM) image exhibited a typical lattice distance of platinum (111). The Pt content of sPt–OA was determined to be 17.5 wt% from the residual weight after thermogravimetric analysis (TGA, Fig. S4, ESI†). The weight decrease in the temperature range between 150 and 400 1C, where adsorbed oleylamine molecules are thermally decomposed, is about 2.5 wt%.10b–d The X-ray absorption near edge structure (XANES) spectra of the Pt L3 edge demonstrate that the d-band vacancy of sPt–OA, which is proportional to the intensity of the white line of the L3 edge, was smaller than that of cPt. This indicates that the oleylamine molecules are adsorbed on the as-prepared sPt–OA nanoparticles, and thereby modify their electronic structures.5b The effect of PA adsorption on sPt–OA was evaluated by a halfcell test (Fig. 2c). It can also be noted that, compared to cPt–OA, sPt–OA has higher ORR activity in 0.1 M HClO4 solution and larger ORR decay with H3PO4 addition, which can be ascribed to the different OA coverage. These results suggest that sPt–OA can provide higher PEMFC performances with PBI/PA electrolytes. The sPt–OA catalyst was incorporated into a membrane electrode assembly (MEA) using a homemade H3PO4-doped
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Fig. 2 (a) Transmission electron microscopy (TEM) images of sPt–OA (left) as well as high resolution images, reduced fast Fourier transform (FFT) patterns, and inversed FFT images with masked Pt(111) facets (right). (b) X-ray absorption near-edge structure (XANES) of the Pt L3 edge using the 8 C beamline of the PLS-II. (c) CVs and polarization curves of the ORR at a rotation speed of 1600 rpm. (d) Mass-normalized kinetic current densities at 0.9 V for sPt–OA.
para-polybenzimidazole (p-PBI) membrane, and a single-cell test was carried out (Pt loading: 0.26 mgPt cm 2). As a reference sample, an MEA with a cPt catalyst was also evaluated (Pt loading: 0.30 mgPt cm 2). In order to fix the effect of MEA compression, a similar catalyst layer thickness was contrived by controlling the Pt loading. In high-temperature PEMFCs with PBI/PA membranes, it is well known that the electrocatalyst is severely poisoned by specific anion adsorption with H3PO4.6c,18,19 As shown in Fig. 3a, sPt–OA shows higher cell voltages in the low-current density region, which is consistent with the half-cell results (Fig. S5, ESI†). Note that single-cell performance in the low-current region is mainly dependent on the ORR activity of the cathode, whereas mass transport characteristics become dominant under high-current operation. The higher ORR activity of sPt–OA can be more clearly observed in the Tafel plot (Fig. 3b). By interpolation of the experimental data points, the kinetic current densities of cPt and sPt–OA were determined to be 0.024 and 0.038 A cm 2. Also note that the kinetic current density of sPt–OA is about 1.6 times higher than that of cPt, even though the Pt loading in sPt–OA is smaller by 13%. Therefore, it can be calculated that, in the single-cell test, the mass activity of sPt–OA is 1.9 times higher than that of cPt. The MEAs constructed with either cPt or sPt–OA as the electrocatalyst provided stable cell voltages over 150 h during constant current operation at 0.2 A cm 2. The higher cell performance could be confirmed over the entire range of the durability test. In summary, we determined that the third-body effect of oleylamine, a widely used surfactant, could enhance the ORR activity of Pt nanoparticles in the presence of specifically adsorbed anions despite the quite small ECSA. Electrochemical characterization showed that, for the synthesized Pt nanoparticles with adsorbed OA, the poisonous spectator anions were blocked and ORR activity was enhanced. Further, this concept could be extended to a practical PEMFC system with severe anion adsorption conditions, and enhanced cell performances were experimentally confirmed. This work was supported by Korean Government through the New & Renewable Energy Core Technology Program of the
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Fig. 3 Electrochemical measurements of cPt and sPt–OA in a membrane electrode assembly (MEA) using a H3PO4-doped polybenzimidazole (p-PBI) membrane. (a) Polarization curves. (b) Tafel plots with Ohmic resistance compensation. (c) Kinetic current density at 0.7 V. (d) Durability tests with a constant current density (0.2 A cm 2) over 150 h. All measurements were carried out with non-humidified H2/air feeds at 160 1C.
KETEP funded by MOTIE (No. 20133030011320), the Korea CCS R&D Center (KCRC) grant funded by MSIP (No. 2013M1A8A1038315), the National Research Foundation of Korea grant funded by MSIP (2014, University-Institute cooperation program), and the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation, MSIP (No. 2012M3A6A7054283). This work was also financially supported by KIST through the Institutional Program, COE program and K-GRL program.
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