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Vertically aligned carbon-coated titanium dioxide nanorod arrays on carbon paper with low platinum for proton exchange membrane fuel cells
Journal of Power Sources 276 (2015) 80e88
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Vertically aligned carbon-coated titanium dioxide nanorod arrays on carbon paper with low platinum for proton exchange membrane fuel cells Shangfeng Jiang a, b, Baolian Yi a, Changkun Zhang a, b, Sa Liu a, b, Hongmei Yu a, *, Zhigang Shao a, * a Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
h i g h l i g h t s Vertically aligned TiO2eC nanorod arrays were directly grown on carbon paper. PteTiO2eC electrode without PTFE and Nafion. Generating power 342.6 mW cm2 with ultra low loading (28.67 mg cm2). PteTiO2eC electrode exhibited high stability.
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Article history: Received 27 August 2014 Received in revised form 18 November 2014 Accepted 20 November 2014 Available online 21 November 2014
Carbon-coated titanium dioxide (TiO2eC) has received much attention as a catalyst support in proton exchange membrane fuel cells. In this study, TiO2 nanorod arrays (NRs) are hydrothermally grown on carbon paper and converted into TiO2eC NRs by heat treatment at 900 C under methane atmosphere. Then, platinum nanoparticles are sputtered onto the TiO2 NRs by physical vapor deposition to produce Pt eTiO2eC. The as-prepared PteTiO2eC exhibits high stability during accelerated durability tests. As compared with the commercial gas diffusion electrode (GDE, 34.4% decrease), a minor reduction in the electrochemically active surface area of the PteTiO2eC electrode after 1500 cycles (10.6% decrease) is observed. When the as-prepared electrode with ultra-low platinum content (Pt loading: 28.67 mg cm2) is employed as the cathode of a single cell, the electrode generates power that is 4.84 that of the commercial GDE (Pt loading: 400 mg cm2). An electrode that generates power of 11.9 kW g1 Pt (as the cathode) is proposed. The fabricated PteTiO2eC electrode can be used in proton exchange membrane fuel cells. © 2014 Elsevier B.V. All rights reserved.
Keywords: Proton exchange membrane fuel cells Carbon-coated titanium dioxide nanorod arrays Low platinum loading
1. Introduction Proton exchange membrane fuel cells (PEMFCs) have attracted considerable attention as one of the most promising energy conversion technologies for stationary and transportation applications because of the high energy conversion efficiency, low or zero emission, low operating temperature, and quick start-up of PEMFCs [1]. However, the commercialization of PEMFCs is limited by their
* Corresponding authors. E-mail addresses: [email protected] (H. Yu), [email protected] (Z. Shao). http://dx.doi.org/10.1016/j.jpowsour.2014.11.093 0378-7753/© 2014 Elsevier B.V. All rights reserved.
high costs and the limited reserves of platinum (Pt), which is widely used in PEMFCs [1,2]. Thus, reducing the amount of Pt in PEMFCs has been intensively examined. High-surface area and electronicconducting carbon materials, such as carbon black, mesoporous carbon [3], and carbon fibers [4], are applied to increase the dispersion of Pt-based electrocatalysts and consequently improve the utilization of Pt. Other new approaches for improving Pt utilization are based on coreeshell catalyst [5], dealloyed catalyst nanoparticles [6,7], and monolayer Pt [8]. However, the effective utilization of catalysts depends not only on the intrinsic catalytic activity but also on the structure of the catalyst layer [9].
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Fig. 1. Schematic illustration of the synthesis of the PteTiO2eC NRs on a carbon paper.
The catalyst layer is one of the key components of a membrane electrode assembly (MEA), in which electrochemical reactions occur. The traditional catalyst layer with a thickness of 10 mme100 mm (usually <50 mm) consists of carbon-supported electrocatalyst powders, proton-conducting ionomer (i.e., Nafion, DuPont, USA), and/or hydrophobic binders [i.e., polytetrafluoroethylene (PTFE)] [10,11]. If the carbon-supported Pt catalysts or Ptalloy nanoparticles are wrapped by PTFE (electronic insulator) or inaccessible to the Nafion ionomer, the electron conductivity would decrease and proton transport resistance would increase, respectively. This behavior results in low electrocatalyst utilization, which leads to the necessity of high Pt loading to maintain high fuel cell performance. The 3M Company designed a highly efficient catalyst layer with ordered whiskers as catalyst support (Pt loading <0.15 mg cm2), which exhibited high fuel cell performance [12]. In addition to the 3M Company that developed electrodes, Tian et al. [9] employed aligned carbon nanotube arrays as the catalyst support in PEMFCs with ultra-low Pt loading down to 35 mg cm2 (the cathode) and showed excellent performance, which was comparable to that of the commercial Pt/C with 400 mg cm2 loading (the
cathode). Therefore, advanced catalyst layer materials can help to improve Pt utilization and show high performance with low Pt loading. TiO2 nanoarrays (nanorods, nanowires, and nanotubes) have been used as fuel cell catalyst support in a single cell [13]. However, the wide application of TiO2 nanorod arrays (NRs) in fuel cells is hindered by the low electronic conductivity of these NRs. Several studies have focused on the carbon-coated TiO2 structure, which combines the electron conductivity of carbon with the chemical stability of TiO2 [14]. With the carbon shell as the electron conductor, Pt nanoparticles mainly act as catalyst; thus, the amount of Pt can be reduced further. In this paper, an electrode of TiO2eC NRs is proposed, in which TiO2 was grown directly on carbon paper. Aligned TiO2 NRs were synthesized in two steps according to a previous report [15]. After the TiO2 NRs were directly hydrothermally grown on a carbon paper, the as-prepared samples were heat-treated under methane atmosphere at 900 C for 2 h to obtain TiO2eC NRs [16]. The physical and electrochemical characteristics of the prepared
Fig. 2. FESEM images of the morphologies of the different TiO2 NRs. The various TiCl4 solution concentrations used were (a) 0.05, (b) 0.1, (c) 0.15, and (d) 0.2 M. The conditions of hydrothermal process were 150 C for 24 h with 1.1 mL TBT precursor. Scale bar: 100 nm.
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Fig. 3. SEM images of different morphologies of the prepared TiO2 NRs around on a carbon paper with increasing reaction time (Reactions were performed at 0.05 M titanium (IV) chloride solution for the preparation of the TiO2 seeds). Scale bar: 1 mm (insert: the magnified images with scale bar 1 mm).
electrode were examined, and compared with a commercial GDE and a home-made GDE (GDE-1, without conducting ionomer, Nafion). 2. Experimental 2.1. Preparation of TiO2 NRs TiO2 NRs on carbon paper were synthesized with a seedeassisted hydrothermal method reported by Lu [15,17]. The
carbon papers (TGP-H-060, without further treatment) were ultrasonically cleaned for 30 min in a mixture of deionized water, ethanol, and acetone (1:1:1 in volume ratio). The cleaned carbon paper was impregnated in 0.05 Me0.2 M titanium (IV) chloride aqueous solution on one side for 10 min and then dried. To obtain TiO2 seeds, the dried carbon paper was further heated in a furnace in air at 350 C for 10 min. Then, 0.05 mLe1.1 mL tetrabutyl titanate (TBT) solution was dropped into 37 mL concentrated hydrochloric acid and 37 mL deionized water mixture under stirring. After stirring for 15 min, the mixture solution together with the carbon paper coated with TiO2 seeds was transferred into a 100 mL Teflon-lined stainless steel autoclave. Hydrothermal treatment was performed at 150 C with different durations, and then the autoclave was cooled down to room temperature. The resulting sample was cleaned with deionized water to remove the excessive ions. Finally, the sample was annealed in a furnace in air at 550 C for 1 h.
2.2. Preparation of PteTiO2eC electrode
Fig. 4. Reaction time and length of TiO2 nanorod.
To obtain TiO2eC NRs, the carbon papers with TiO2 NRs were heated under methane gas flow [16]. The TiO2 NR sample around the carbon paper was put into a quartz boat in a quartz tube system with a flow of methane atmosphere. The sample was heated from room temperature to 900 C under methane with the flow rate of 60 mL min1 and then maintained for 2 h. PteTiO2eC was prepared by sputtering Pt nanoparticles onto TiO2 NRs by physical vapor deposition (PVD). Deposition was performed in an argon atmosphere with a working pressure of 8.9 101 Pa. The deposition process was maintained for 5 min.
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Fig. 5. FESEM images of the (a) TiO2 NRs on a carbon paper, (b) TiO2eC NRs, (c) the PteTiO2eC NRs, and (d) a home-made GDE-1 (Scale bar: 1 mm).
2.3. Physical characterization of samples Field-emission scanning electron microscopy (FESEM, Zeiss supra 55; Germany) and transmission electron microscopy (TEM, JEM-2000EX at 120 KV) were employed to observe the morphology and structure of the samples. The crystal phase and the composition of the products were measured by X-ray diffraction (XRD, BrukerD8, Cu Ka radiation) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCA Lab250 Xi spectrometer, Al Ka in twin
anode), respectively. The hydrophobicity of the PteTiO2eC samples was measured by a drop shape analyzer (DSA100; Germany, KRUSS). The conductivity of the as-synthesized samples was measured by four-point probe measuring system (Suzhou Jingge Electronic Co., Ltd; ST2253 and ST2258). The Pt content of the electrodes was measured by inductively coupled plasma-optical emission spectroscopy (Leeman Plasma-Spec-I).
2.4. Electrochemical measurements The CV curves of the cathode of a single cell were measured in a humidified N2 instead of O2 at the cathode side. The cathode was the working electrode, and the anode was the counter electrode. The electrochemically active surface area (ECSA) was calculated by integrating the areas of hydrogen desorption after subtraction of the double-layer region. Then, the value of ECSA (m2 g1) was calculated using Equation (1):
ECSA ¼
Fig. 6. XRD spectra of (a) as-prepared TiO2 NRs coated on a carbon paper, (b) TiO2eC coated on a carbon paper, and (c) rutile TiO2.
QH 104 Lpt *Qf
(1)
where QH (mC cm2) is the coulombic charge of hydrogen desorption, LPt (g cm2) is the Pt loading on the working electrode, and Qf (0.21 mC cm2) represents the desorption charge of a monolayer of hydrogen atoms on a clean Pt surface [18]. The accelerated durability tests (ADT) was performed in a conventional three-electrode system with PteTiO2eC as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a Pt-foil as the counter electrode in N2-purged 0.5 M H2SO4 solution. The test was performed in the potential range of 0.241 V to 0.959 V (vs. SCE) for 1500 cycles with the scan rate of 50 mV s1 at room temperature.
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Fig. 7. TEM images of the as-prepared (a, b) TiO2eC NRs and (c) PteTiO2eC samples.
2.5. MEA preparation and performance test The MEA was fabricated using the as-prepared PteTiO2eC electrode (without proton conducting ionomer, Nafion) as one side of the MEA, the other side was home-made catalyst-coated membrane (CCM, membrane: Nafion 212; Pt loading: 0.2 mg cm2). The modified MEA was hot-pressed at 140 C and 0.2 MPa (gauge) for 2 min. The effective area of the single cell was 4 cm2. A commercial GDE electrode (Sunrise Power Co. China, GDE04a; Pt loading: 0.4 mg cm2) was employed as the reference. At the same time, the GDE-1 which consisted of a catalyst layer attached to a microporous layer, was also used as the reference. The catalyst ink was composed of carbon-based Pt, hydrophobic binder (PTFE), and without proton-conducting ionomer Nafion (Pt loading: 0.1274 mg cm2). The GDE-1 was used as the cathode and the home-made CCM (Pt loading: 0.2 mg cm2) as the anode. The MEA was hot-pressed at 140 C and 0.2 MPa (gauge) for 2 min. During the performance test, the cell operating temperature was 65 C. Then, H2 and O2 (99.99% pure) with corresponding flow rates of 50 and 200 mL min1 at 0.05 MPa (backpressure) were fed into the anode and the cathode, respectively. Both gases were humidified through a humidifier at the operating temperature of 65 C before feeding into a single cell. The KFM-2030 impedance meter (Kikusui, Japan) was used for i-V curve test. The electrochemical
impedance spectroscopy (EIS) was performed using PARSTAT-2273 frequency response analyzer within the frequency range from 10,000 Hz to 0.1 Hz. 3. Results and discussion The TiO2 NRs are successfully grown on carbon paper via a seedeassisted process that was reported by Lu et al. [15], but the conditions that affected the morphology of the TiO2 nanorods were not investigated in the paper of Lu et al. The fabrication process is schematically illustrated in Fig. 1. Without TiO2 seeds on the carbon paper, TiO2 NRs can barely grow as previously reported by Cai et al. [19]. Thus, before the hydrothermal process, TiO2 seeds were initially coated onto carbon paper, and different densities and morphologies of TiO2 seeds were obtained by changing the titanium (IV) chloride solution concentrations. A series of TiCl4 solutions with different concentrations were employed to investigate the influence of TiCl4 concentration on the morphology of TiO2 NRs. The morphologies of the synthesized TiO2 NRs with different titanium (IV) chloride solution concentrations indicate that higher concentration of titanium (IV) chloride solution leads to the disordering and aggregation of TiO2 NRs (Fig. 2). Higher concentration of titanium (IV) chloride produces thicker and denser TiO2 seeds layer, resulting in disordered TiO2 NRs. Relatively uniform TiO2 NRs
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Fig. 8. XPS spectra of the as-prepared TiO2eC sample on a carbon paper. (a) XPS survey, (b) Ti2p spectrum of TiO2eC on a carbon paper, (c) C1s XPS spectrum of the TiO2eC sample, and (d) O1s XPS spectrum of the TiO2eC sample.
are obtained with a 0.05 Me0.1 M TiCl4 solution (Fig. 2a and b). Thus, an appropriate concentration of TiCl4 solution is important for preparing ordered TiO2 NRs on carbon paper. Furthermore, with prolonged reaction time, the amount of TiO2 NRs that cover the surface of carbon fibers also increases, producing a denser and thicker TiO2 NR film. The morphologies of the films obtained under different reaction times are shown in Fig. 3, and the relationship of the TiO2 NR length and the reaction time is shown in Fig. 4. The amount of TBT solution is also important for the preparation of uniform TiO2 NRs. When 0.55 mL TBT solution is added, disordered TiO2 nanowires are acquired (Fig. 2a0 ec0 ), even at 0.2 M TiCl4 solution concentration (Fig. 2d and d0 ). Therefore, a comprehensive study on the seeding condition has shown that a suitable amount of TBT precursor and reaction time is needed to prepare uniform TiO2 NRs on carbon paper. These results confirm the findings of a previous study on ketone/HCl-based solvothermal system [19]. After the vertically aligned TiO2 NRs were grown on carbon paper, the as-prepared sample was heat-treated under methane atmosphere to form TiO2eC. FESEM images (Fig. 5a and b) reveal that the uniform morphology of TiO2 NRs is preserved after carbon coating. The crystal structure of TiO2 NRs remains in the rutile phase (according to JCPDS 021e1276) after carbon coating, which is confirmed by XRD patterns (Fig. 6). A 4 nm-thick carbon layer covers the surface of the TiO2 nanorods as shown in the TEM images (Fig. 7a and b). Pt nanoparticles are uniformly deposited onto TiO2eC NRs (Fig. 7c). X-ray photoelectron spectroscopy (XPS) was employed to examine the TiO2 NR component. The XPS survey spectra suggest that these NRs consist of Ti, O, and C. The peaks centered at 459.5 and 465.2 eV are attributed to Ti2p3/2 and Ti2p1/2
of rutile TiO2 [20]. The C1s spectrum is shown in Fig. 8c. The peak can be divided into three peaks with the value of 284.3, 284.7, and 285.9 eV, respectively, which are attributed to the CeC, CeOH, and CeO bonds, respectively [21]. No carbide-related peak is found, which indicates that the carbon is not doped but coated on the surface of the TiO2 NRs (Fig. 8c) [22]. Moreover, the electronic conductivity of the as-prepared samples was measured by a digital four-probe tester (Suzhou Jingge Electronic, Co., Ltd; ST2253 and ST2258A). The measured electronic conductivities of the TiO2eC and bare TiO2 samples are approximately 201 and 6.7 105 S cm1, respectively. The evident increase in electronic conductivity of as-synthesized TiO2eC sample is attributed to the coated carbon layer. Thus, the TiO2eC samples consist of carbon layer-coated rutile TiO2 nanorods. The PteTiO2eC sample was tested in a single cell as the cathode and compared with a commercial GDE and a GDE-1 to evaluate the electrochemical performance. When the PteTiO2eC sample is used as the cathode, the maximum power density is 342.6 mW cm2. Meanwhile, using a commercial GDE as the cathode generates a maximum power density of 983.1 mW cm2. When the GDE-1 was used as the cathode, the maximum power density is 260.7 mW cm2. The as-prepared electrode generates power of 11.9 KW g1 Pt , which is 4.84 of the commercial GDE. The i-V curves are shown in Fig. 9. Without conducting ionomer Nafion, both of the PteTiO2eC electrode and the GDE-1 as the cathode in a single cell can work effectively. The protons are probably transport by the water film on the surface of the catalyst. The proton conductivity of Pt surface as a function of RH and temperature has been reported by
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Fig. 9. i-V curves of MEAs with (a) a PteTiO2eC electrode and a home-made GDE as the cathode, (b) a commercial GDE as the cathode; another side of the single cell was home-made CCM.
Puneet K. Sinha et al. [23]. The three-phase boundary can be organized by the protons, the water molecules and the electrons. The ECSA of the cathode was determined according to Equation (1). The ECSA of the PteTiO2eC cathode is 85.01 m2 g1, while that of commercial GDE cathode is 59.95 m2 g1, which indicates that the PteTiO2eC cathode presents a higher Pt utilization than the commercial GDE cathode (Fig. 10).
The durability of the as-synthesized sample was examined via ADT in the potential range of 0.241 Ve0.959 V (vs. SCE) for 1500 cycles at a scan rate of 50 mV s1 (Fig. 11). After ADT, the remaining ECSA of the PteTiO2eC electrode is 89.4%, which is larger than that of the commercial GDE (65.6%). This result indicates that the PteTiO2eC electrode is more stable than the commercial GDE. Contact angle measurement was used to examine the hydrophobicity of the PteTiO2eC electrode (a dense PteTiO2eC NRs film is coated on the surface of the carbon paper). The water contact angles of the PteTiO2eC sample and original carbon paper are 139.8 and 128.4 , respectively, which indicates the hydrophobicity of the PteTiO2eC sample (Fig. 12). Moreover, the PteTiO2eC NRs, with varying spacing between two nanorods (approximately 50 nme100 nm; Fig. 2a), improves the mass transport of the reactant and by-product water. Therefore, contrary to commercial GDE, the PteTiO2eC NRs electrode without the incorporation of PTFE can be used as an electrode material in a single cell. The thickness of a fuel cell catalyst layer is also important for fuel cell performance. The 2.1 mm-thick PteTiO2eC catalyst layer (before hot-pressing, Fig. 5c) generates a thin film on the surface of a carbon paper and thinner than the GDE-1 (~78 mm, Fig. 5d). The catalyst layer in the GDE-1 is 37.1 thicker than that of PteTiO2eC electrode, which results in the inefficient catalytic utilization and poor fuel cell performance (Fig. 9). EIS is a powerful tool to measure the resistance of a single cell. The interrupt in high frequency equals to the ohmic resistance, and the semicircle in the low frequency range represents the electrochemical resistance [24]. The ohmic resistance or internal resistance comprise electronic, ionic, and contact resistances, and important for single cell performance. The region between 0.7 V and 0.4 V of the slope of i-V curve is related to ohmic loss [25]. The absolute slope of the PteTiO2eC cathode is higher than that of the commercial GDE (Fig. 9), which suggests that the electrode has a higher internal resistance. The low electron conductivity of TiO2 seeds is one factor of the high internal resistance. The EIS spectra of MEA were measured at different current densities from 100 KHz to 0.1 Hz (Fig. 13). The charge transfer resistance (Rct) of the assynthesized electrode at each current density is higher than that of commercial GDE electrode, which confirms that the low electron conductivity of TiO2 seeds hinders the electron transport from carbon paper to PteTiO2eC. 4. Conclusions In summary, TiO2 NRs were directly grown on carbon paper, which were subsequently heated under methane atmosphere to
Fig. 10. CV curves of (a) PteTiO2eC electrode as the cathode, and (b) commercial GDE as the cathode, with scan rate of 50 mV s1;anode: H2, cathode: N2.
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Fig. 11. Accelerated durability test of as-prepared PteTiO2eC sample in N2-purged 0.5 M H2SO4 with scan rate of 50 mV s1.
Fig. 12. Contact angle measurement for (a) original carbon paper and (b) PteTiO2eC on a carbon paper.
Fig. 13. Nyquist plots of (a) PteTiO2eC as the cathode and (b) a commercial GDE as the cathode at different current densities.
form TiO2eC NRs. Then, Pt nanoparticles were sputtered onto the TiO2eC NRs by PVD to generate a PteTiO2eC electrode. When the prepared PteTiO2eC was used as the cathode of a single cell, the PteTiO2eC with low Pt loadings (28.67 mg cm2) generates the maximum power density of 342.6 mW cm2. The PteTiO2eC electrode consists of a thin catalyst layer (less than 2.1 mm) with a hydrophobicity property and a high utilization of Pt, resulting in relatively high performance. The prepared electrode without the proton-conducting ionomer (Nafion) and the binder (PTFE) are directly grown on a carbon paper, which can further reduce the cost of MEA. Moreover, the ADT result shows that the PteTiO2eC sample is more stable than the commercial GDE. Thus, the prepared electrode with low platinum loading and relative high stability is a promising material for fuel cells.
Acknowledgements This work is financially supported by the National Basic Research Program of China (973 Program No. 2012CB215500), the National Natural Science Foundations of China (Nos. 91434106 and 21176234). References [1] V. Metha, J.S. Cooper, J. Power Sources 114 (2003) 32e53. [2] J. Tian, A. Morozan, M.T. Sougrati, M. Lefevre, R. Chenitz, J.P. Dodelet, D. Jones, F. Jaouen, Angew. Chem. Int. Ed. 52 (2013) 6867e6870. [3] J.R.C. Salgadoa, F. Alcaide, G. Alvarez, L. Calvillo, M.J. L azaro, E. Pastor, J. Power Sources 195 (2010) 4022e4029. n, M.J. La zaro, I. Suelves, R. Moliner, V. Baglio, A. Stassi, A.S. Arico , [4] D. Sebastia Int. J. Hydrog. Energ. 37 (2012) 6253e6260. [5] M. Shao, K. Sasaki, Nebojs, S. Marinkovic, L. Zhang, R.R. Adzic, Electrochem. Commun. 9 (2007) 2848e2853.
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Vertically aligned carbon-coated titanium dioxide nanorod arrays on carbon paper with low platinum for proton exchange membrane fuel cells Shangfeng Jiang a, b, Baolian Yi a, Changkun Zhang a, b, Sa Liu a, b, Hongmei Yu a, *, Zhigang Shao a, * a Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
h i g h l i g h t s Vertically aligned TiO2eC nanorod arrays were directly grown on carbon paper. PteTiO2eC electrode without PTFE and Nafion. Generating power 342.6 mW cm2 with ultra low loading (28.67 mg cm2). PteTiO2eC electrode exhibited high stability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 August 2014 Received in revised form 18 November 2014 Accepted 20 November 2014 Available online 21 November 2014
Carbon-coated titanium dioxide (TiO2eC) has received much attention as a catalyst support in proton exchange membrane fuel cells. In this study, TiO2 nanorod arrays (NRs) are hydrothermally grown on carbon paper and converted into TiO2eC NRs by heat treatment at 900 C under methane atmosphere. Then, platinum nanoparticles are sputtered onto the TiO2 NRs by physical vapor deposition to produce Pt eTiO2eC. The as-prepared PteTiO2eC exhibits high stability during accelerated durability tests. As compared with the commercial gas diffusion electrode (GDE, 34.4% decrease), a minor reduction in the electrochemically active surface area of the PteTiO2eC electrode after 1500 cycles (10.6% decrease) is observed. When the as-prepared electrode with ultra-low platinum content (Pt loading: 28.67 mg cm2) is employed as the cathode of a single cell, the electrode generates power that is 4.84 that of the commercial GDE (Pt loading: 400 mg cm2). An electrode that generates power of 11.9 kW g1 Pt (as the cathode) is proposed. The fabricated PteTiO2eC electrode can be used in proton exchange membrane fuel cells. © 2014 Elsevier B.V. All rights reserved.
Keywords: Proton exchange membrane fuel cells Carbon-coated titanium dioxide nanorod arrays Low platinum loading
1. Introduction Proton exchange membrane fuel cells (PEMFCs) have attracted considerable attention as one of the most promising energy conversion technologies for stationary and transportation applications because of the high energy conversion efficiency, low or zero emission, low operating temperature, and quick start-up of PEMFCs [1]. However, the commercialization of PEMFCs is limited by their
* Corresponding authors. E-mail addresses: [email protected] (H. Yu), [email protected] (Z. Shao). http://dx.doi.org/10.1016/j.jpowsour.2014.11.093 0378-7753/© 2014 Elsevier B.V. All rights reserved.
high costs and the limited reserves of platinum (Pt), which is widely used in PEMFCs [1,2]. Thus, reducing the amount of Pt in PEMFCs has been intensively examined. High-surface area and electronicconducting carbon materials, such as carbon black, mesoporous carbon [3], and carbon fibers [4], are applied to increase the dispersion of Pt-based electrocatalysts and consequently improve the utilization of Pt. Other new approaches for improving Pt utilization are based on coreeshell catalyst [5], dealloyed catalyst nanoparticles [6,7], and monolayer Pt [8]. However, the effective utilization of catalysts depends not only on the intrinsic catalytic activity but also on the structure of the catalyst layer [9].
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Fig. 1. Schematic illustration of the synthesis of the PteTiO2eC NRs on a carbon paper.
The catalyst layer is one of the key components of a membrane electrode assembly (MEA), in which electrochemical reactions occur. The traditional catalyst layer with a thickness of 10 mme100 mm (usually <50 mm) consists of carbon-supported electrocatalyst powders, proton-conducting ionomer (i.e., Nafion, DuPont, USA), and/or hydrophobic binders [i.e., polytetrafluoroethylene (PTFE)] [10,11]. If the carbon-supported Pt catalysts or Ptalloy nanoparticles are wrapped by PTFE (electronic insulator) or inaccessible to the Nafion ionomer, the electron conductivity would decrease and proton transport resistance would increase, respectively. This behavior results in low electrocatalyst utilization, which leads to the necessity of high Pt loading to maintain high fuel cell performance. The 3M Company designed a highly efficient catalyst layer with ordered whiskers as catalyst support (Pt loading <0.15 mg cm2), which exhibited high fuel cell performance [12]. In addition to the 3M Company that developed electrodes, Tian et al. [9] employed aligned carbon nanotube arrays as the catalyst support in PEMFCs with ultra-low Pt loading down to 35 mg cm2 (the cathode) and showed excellent performance, which was comparable to that of the commercial Pt/C with 400 mg cm2 loading (the
cathode). Therefore, advanced catalyst layer materials can help to improve Pt utilization and show high performance with low Pt loading. TiO2 nanoarrays (nanorods, nanowires, and nanotubes) have been used as fuel cell catalyst support in a single cell [13]. However, the wide application of TiO2 nanorod arrays (NRs) in fuel cells is hindered by the low electronic conductivity of these NRs. Several studies have focused on the carbon-coated TiO2 structure, which combines the electron conductivity of carbon with the chemical stability of TiO2 [14]. With the carbon shell as the electron conductor, Pt nanoparticles mainly act as catalyst; thus, the amount of Pt can be reduced further. In this paper, an electrode of TiO2eC NRs is proposed, in which TiO2 was grown directly on carbon paper. Aligned TiO2 NRs were synthesized in two steps according to a previous report [15]. After the TiO2 NRs were directly hydrothermally grown on a carbon paper, the as-prepared samples were heat-treated under methane atmosphere at 900 C for 2 h to obtain TiO2eC NRs [16]. The physical and electrochemical characteristics of the prepared
Fig. 2. FESEM images of the morphologies of the different TiO2 NRs. The various TiCl4 solution concentrations used were (a) 0.05, (b) 0.1, (c) 0.15, and (d) 0.2 M. The conditions of hydrothermal process were 150 C for 24 h with 1.1 mL TBT precursor. Scale bar: 100 nm.
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Fig. 3. SEM images of different morphologies of the prepared TiO2 NRs around on a carbon paper with increasing reaction time (Reactions were performed at 0.05 M titanium (IV) chloride solution for the preparation of the TiO2 seeds). Scale bar: 1 mm (insert: the magnified images with scale bar 1 mm).
electrode were examined, and compared with a commercial GDE and a home-made GDE (GDE-1, without conducting ionomer, Nafion). 2. Experimental 2.1. Preparation of TiO2 NRs TiO2 NRs on carbon paper were synthesized with a seedeassisted hydrothermal method reported by Lu [15,17]. The
carbon papers (TGP-H-060, without further treatment) were ultrasonically cleaned for 30 min in a mixture of deionized water, ethanol, and acetone (1:1:1 in volume ratio). The cleaned carbon paper was impregnated in 0.05 Me0.2 M titanium (IV) chloride aqueous solution on one side for 10 min and then dried. To obtain TiO2 seeds, the dried carbon paper was further heated in a furnace in air at 350 C for 10 min. Then, 0.05 mLe1.1 mL tetrabutyl titanate (TBT) solution was dropped into 37 mL concentrated hydrochloric acid and 37 mL deionized water mixture under stirring. After stirring for 15 min, the mixture solution together with the carbon paper coated with TiO2 seeds was transferred into a 100 mL Teflon-lined stainless steel autoclave. Hydrothermal treatment was performed at 150 C with different durations, and then the autoclave was cooled down to room temperature. The resulting sample was cleaned with deionized water to remove the excessive ions. Finally, the sample was annealed in a furnace in air at 550 C for 1 h.
2.2. Preparation of PteTiO2eC electrode
Fig. 4. Reaction time and length of TiO2 nanorod.
To obtain TiO2eC NRs, the carbon papers with TiO2 NRs were heated under methane gas flow [16]. The TiO2 NR sample around the carbon paper was put into a quartz boat in a quartz tube system with a flow of methane atmosphere. The sample was heated from room temperature to 900 C under methane with the flow rate of 60 mL min1 and then maintained for 2 h. PteTiO2eC was prepared by sputtering Pt nanoparticles onto TiO2 NRs by physical vapor deposition (PVD). Deposition was performed in an argon atmosphere with a working pressure of 8.9 101 Pa. The deposition process was maintained for 5 min.
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Fig. 5. FESEM images of the (a) TiO2 NRs on a carbon paper, (b) TiO2eC NRs, (c) the PteTiO2eC NRs, and (d) a home-made GDE-1 (Scale bar: 1 mm).
2.3. Physical characterization of samples Field-emission scanning electron microscopy (FESEM, Zeiss supra 55; Germany) and transmission electron microscopy (TEM, JEM-2000EX at 120 KV) were employed to observe the morphology and structure of the samples. The crystal phase and the composition of the products were measured by X-ray diffraction (XRD, BrukerD8, Cu Ka radiation) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCA Lab250 Xi spectrometer, Al Ka in twin
anode), respectively. The hydrophobicity of the PteTiO2eC samples was measured by a drop shape analyzer (DSA100; Germany, KRUSS). The conductivity of the as-synthesized samples was measured by four-point probe measuring system (Suzhou Jingge Electronic Co., Ltd; ST2253 and ST2258). The Pt content of the electrodes was measured by inductively coupled plasma-optical emission spectroscopy (Leeman Plasma-Spec-I).
2.4. Electrochemical measurements The CV curves of the cathode of a single cell were measured in a humidified N2 instead of O2 at the cathode side. The cathode was the working electrode, and the anode was the counter electrode. The electrochemically active surface area (ECSA) was calculated by integrating the areas of hydrogen desorption after subtraction of the double-layer region. Then, the value of ECSA (m2 g1) was calculated using Equation (1):
ECSA ¼
Fig. 6. XRD spectra of (a) as-prepared TiO2 NRs coated on a carbon paper, (b) TiO2eC coated on a carbon paper, and (c) rutile TiO2.
QH 104 Lpt *Qf
(1)
where QH (mC cm2) is the coulombic charge of hydrogen desorption, LPt (g cm2) is the Pt loading on the working electrode, and Qf (0.21 mC cm2) represents the desorption charge of a monolayer of hydrogen atoms on a clean Pt surface [18]. The accelerated durability tests (ADT) was performed in a conventional three-electrode system with PteTiO2eC as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a Pt-foil as the counter electrode in N2-purged 0.5 M H2SO4 solution. The test was performed in the potential range of 0.241 V to 0.959 V (vs. SCE) for 1500 cycles with the scan rate of 50 mV s1 at room temperature.
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Fig. 7. TEM images of the as-prepared (a, b) TiO2eC NRs and (c) PteTiO2eC samples.
2.5. MEA preparation and performance test The MEA was fabricated using the as-prepared PteTiO2eC electrode (without proton conducting ionomer, Nafion) as one side of the MEA, the other side was home-made catalyst-coated membrane (CCM, membrane: Nafion 212; Pt loading: 0.2 mg cm2). The modified MEA was hot-pressed at 140 C and 0.2 MPa (gauge) for 2 min. The effective area of the single cell was 4 cm2. A commercial GDE electrode (Sunrise Power Co. China, GDE04a; Pt loading: 0.4 mg cm2) was employed as the reference. At the same time, the GDE-1 which consisted of a catalyst layer attached to a microporous layer, was also used as the reference. The catalyst ink was composed of carbon-based Pt, hydrophobic binder (PTFE), and without proton-conducting ionomer Nafion (Pt loading: 0.1274 mg cm2). The GDE-1 was used as the cathode and the home-made CCM (Pt loading: 0.2 mg cm2) as the anode. The MEA was hot-pressed at 140 C and 0.2 MPa (gauge) for 2 min. During the performance test, the cell operating temperature was 65 C. Then, H2 and O2 (99.99% pure) with corresponding flow rates of 50 and 200 mL min1 at 0.05 MPa (backpressure) were fed into the anode and the cathode, respectively. Both gases were humidified through a humidifier at the operating temperature of 65 C before feeding into a single cell. The KFM-2030 impedance meter (Kikusui, Japan) was used for i-V curve test. The electrochemical
impedance spectroscopy (EIS) was performed using PARSTAT-2273 frequency response analyzer within the frequency range from 10,000 Hz to 0.1 Hz. 3. Results and discussion The TiO2 NRs are successfully grown on carbon paper via a seedeassisted process that was reported by Lu et al. [15], but the conditions that affected the morphology of the TiO2 nanorods were not investigated in the paper of Lu et al. The fabrication process is schematically illustrated in Fig. 1. Without TiO2 seeds on the carbon paper, TiO2 NRs can barely grow as previously reported by Cai et al. [19]. Thus, before the hydrothermal process, TiO2 seeds were initially coated onto carbon paper, and different densities and morphologies of TiO2 seeds were obtained by changing the titanium (IV) chloride solution concentrations. A series of TiCl4 solutions with different concentrations were employed to investigate the influence of TiCl4 concentration on the morphology of TiO2 NRs. The morphologies of the synthesized TiO2 NRs with different titanium (IV) chloride solution concentrations indicate that higher concentration of titanium (IV) chloride solution leads to the disordering and aggregation of TiO2 NRs (Fig. 2). Higher concentration of titanium (IV) chloride produces thicker and denser TiO2 seeds layer, resulting in disordered TiO2 NRs. Relatively uniform TiO2 NRs
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Fig. 8. XPS spectra of the as-prepared TiO2eC sample on a carbon paper. (a) XPS survey, (b) Ti2p spectrum of TiO2eC on a carbon paper, (c) C1s XPS spectrum of the TiO2eC sample, and (d) O1s XPS spectrum of the TiO2eC sample.
are obtained with a 0.05 Me0.1 M TiCl4 solution (Fig. 2a and b). Thus, an appropriate concentration of TiCl4 solution is important for preparing ordered TiO2 NRs on carbon paper. Furthermore, with prolonged reaction time, the amount of TiO2 NRs that cover the surface of carbon fibers also increases, producing a denser and thicker TiO2 NR film. The morphologies of the films obtained under different reaction times are shown in Fig. 3, and the relationship of the TiO2 NR length and the reaction time is shown in Fig. 4. The amount of TBT solution is also important for the preparation of uniform TiO2 NRs. When 0.55 mL TBT solution is added, disordered TiO2 nanowires are acquired (Fig. 2a0 ec0 ), even at 0.2 M TiCl4 solution concentration (Fig. 2d and d0 ). Therefore, a comprehensive study on the seeding condition has shown that a suitable amount of TBT precursor and reaction time is needed to prepare uniform TiO2 NRs on carbon paper. These results confirm the findings of a previous study on ketone/HCl-based solvothermal system [19]. After the vertically aligned TiO2 NRs were grown on carbon paper, the as-prepared sample was heat-treated under methane atmosphere to form TiO2eC. FESEM images (Fig. 5a and b) reveal that the uniform morphology of TiO2 NRs is preserved after carbon coating. The crystal structure of TiO2 NRs remains in the rutile phase (according to JCPDS 021e1276) after carbon coating, which is confirmed by XRD patterns (Fig. 6). A 4 nm-thick carbon layer covers the surface of the TiO2 nanorods as shown in the TEM images (Fig. 7a and b). Pt nanoparticles are uniformly deposited onto TiO2eC NRs (Fig. 7c). X-ray photoelectron spectroscopy (XPS) was employed to examine the TiO2 NR component. The XPS survey spectra suggest that these NRs consist of Ti, O, and C. The peaks centered at 459.5 and 465.2 eV are attributed to Ti2p3/2 and Ti2p1/2
of rutile TiO2 [20]. The C1s spectrum is shown in Fig. 8c. The peak can be divided into three peaks with the value of 284.3, 284.7, and 285.9 eV, respectively, which are attributed to the CeC, CeOH, and CeO bonds, respectively [21]. No carbide-related peak is found, which indicates that the carbon is not doped but coated on the surface of the TiO2 NRs (Fig. 8c) [22]. Moreover, the electronic conductivity of the as-prepared samples was measured by a digital four-probe tester (Suzhou Jingge Electronic, Co., Ltd; ST2253 and ST2258A). The measured electronic conductivities of the TiO2eC and bare TiO2 samples are approximately 201 and 6.7 105 S cm1, respectively. The evident increase in electronic conductivity of as-synthesized TiO2eC sample is attributed to the coated carbon layer. Thus, the TiO2eC samples consist of carbon layer-coated rutile TiO2 nanorods. The PteTiO2eC sample was tested in a single cell as the cathode and compared with a commercial GDE and a GDE-1 to evaluate the electrochemical performance. When the PteTiO2eC sample is used as the cathode, the maximum power density is 342.6 mW cm2. Meanwhile, using a commercial GDE as the cathode generates a maximum power density of 983.1 mW cm2. When the GDE-1 was used as the cathode, the maximum power density is 260.7 mW cm2. The as-prepared electrode generates power of 11.9 KW g1 Pt , which is 4.84 of the commercial GDE. The i-V curves are shown in Fig. 9. Without conducting ionomer Nafion, both of the PteTiO2eC electrode and the GDE-1 as the cathode in a single cell can work effectively. The protons are probably transport by the water film on the surface of the catalyst. The proton conductivity of Pt surface as a function of RH and temperature has been reported by
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Fig. 9. i-V curves of MEAs with (a) a PteTiO2eC electrode and a home-made GDE as the cathode, (b) a commercial GDE as the cathode; another side of the single cell was home-made CCM.
Puneet K. Sinha et al. [23]. The three-phase boundary can be organized by the protons, the water molecules and the electrons. The ECSA of the cathode was determined according to Equation (1). The ECSA of the PteTiO2eC cathode is 85.01 m2 g1, while that of commercial GDE cathode is 59.95 m2 g1, which indicates that the PteTiO2eC cathode presents a higher Pt utilization than the commercial GDE cathode (Fig. 10).
The durability of the as-synthesized sample was examined via ADT in the potential range of 0.241 Ve0.959 V (vs. SCE) for 1500 cycles at a scan rate of 50 mV s1 (Fig. 11). After ADT, the remaining ECSA of the PteTiO2eC electrode is 89.4%, which is larger than that of the commercial GDE (65.6%). This result indicates that the PteTiO2eC electrode is more stable than the commercial GDE. Contact angle measurement was used to examine the hydrophobicity of the PteTiO2eC electrode (a dense PteTiO2eC NRs film is coated on the surface of the carbon paper). The water contact angles of the PteTiO2eC sample and original carbon paper are 139.8 and 128.4 , respectively, which indicates the hydrophobicity of the PteTiO2eC sample (Fig. 12). Moreover, the PteTiO2eC NRs, with varying spacing between two nanorods (approximately 50 nme100 nm; Fig. 2a), improves the mass transport of the reactant and by-product water. Therefore, contrary to commercial GDE, the PteTiO2eC NRs electrode without the incorporation of PTFE can be used as an electrode material in a single cell. The thickness of a fuel cell catalyst layer is also important for fuel cell performance. The 2.1 mm-thick PteTiO2eC catalyst layer (before hot-pressing, Fig. 5c) generates a thin film on the surface of a carbon paper and thinner than the GDE-1 (~78 mm, Fig. 5d). The catalyst layer in the GDE-1 is 37.1 thicker than that of PteTiO2eC electrode, which results in the inefficient catalytic utilization and poor fuel cell performance (Fig. 9). EIS is a powerful tool to measure the resistance of a single cell. The interrupt in high frequency equals to the ohmic resistance, and the semicircle in the low frequency range represents the electrochemical resistance [24]. The ohmic resistance or internal resistance comprise electronic, ionic, and contact resistances, and important for single cell performance. The region between 0.7 V and 0.4 V of the slope of i-V curve is related to ohmic loss [25]. The absolute slope of the PteTiO2eC cathode is higher than that of the commercial GDE (Fig. 9), which suggests that the electrode has a higher internal resistance. The low electron conductivity of TiO2 seeds is one factor of the high internal resistance. The EIS spectra of MEA were measured at different current densities from 100 KHz to 0.1 Hz (Fig. 13). The charge transfer resistance (Rct) of the assynthesized electrode at each current density is higher than that of commercial GDE electrode, which confirms that the low electron conductivity of TiO2 seeds hinders the electron transport from carbon paper to PteTiO2eC. 4. Conclusions In summary, TiO2 NRs were directly grown on carbon paper, which were subsequently heated under methane atmosphere to
Fig. 10. CV curves of (a) PteTiO2eC electrode as the cathode, and (b) commercial GDE as the cathode, with scan rate of 50 mV s1;anode: H2, cathode: N2.
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Fig. 11. Accelerated durability test of as-prepared PteTiO2eC sample in N2-purged 0.5 M H2SO4 with scan rate of 50 mV s1.
Fig. 12. Contact angle measurement for (a) original carbon paper and (b) PteTiO2eC on a carbon paper.
Fig. 13. Nyquist plots of (a) PteTiO2eC as the cathode and (b) a commercial GDE as the cathode at different current densities.
form TiO2eC NRs. Then, Pt nanoparticles were sputtered onto the TiO2eC NRs by PVD to generate a PteTiO2eC electrode. When the prepared PteTiO2eC was used as the cathode of a single cell, the PteTiO2eC with low Pt loadings (28.67 mg cm2) generates the maximum power density of 342.6 mW cm2. The PteTiO2eC electrode consists of a thin catalyst layer (less than 2.1 mm) with a hydrophobicity property and a high utilization of Pt, resulting in relatively high performance. The prepared electrode without the proton-conducting ionomer (Nafion) and the binder (PTFE) are directly grown on a carbon paper, which can further reduce the cost of MEA. Moreover, the ADT result shows that the PteTiO2eC sample is more stable than the commercial GDE. Thus, the prepared electrode with low platinum loading and relative high stability is a promising material for fuel cells.
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