Article pubs.acs.org/Langmuir
Highly Active Platinum Nanoparticles on Graphene Nanosheets with a Significant Improvement in Stability and CO Tolerance Daping He, Kun Cheng, Huaiguang Li, Tao Peng, Feng Xu, Shichun Mu,* and Mu Pan State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China ABSTRACT: Graphene nanosheets (GNS) supporting Pt nanoparticles (PNs) are prepared using perfluorosulfonic acid (PFSA) as a functionalization and anchoring agent. Transmission electron microscope (TEM) results indicate that the prepared Pt NPs are uniformly deposited on GNS with a narrow particle size ranging from 1 to 4 nm in diameter. A high catalytic activity of this novel catalyst is observed by both cyclic voltammetry and oxygen reduction reaction (ORR) measurements due to the increasing of proton (H+) transmission channels. Significantly, this novel PFSA-functionalized Pt/GNS (PFSA-Pt/GNS) catalyst reveals a better CO oxidation and lower loss rate of electrochemical active area in comparison with that of the plain Pt/GNS and conventional Pt/C catalysts, indicating our PFSA-Pt/GNS catalysts hold much higher stability and CO tolerance by virtue of introduction of PFSA.
1. INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) have attracted much attention in recent years as a result of their high efficiency in converting chemical energy into electric power and minimal environmental impact.1−3 However, a big challenge in the commercialization of PEMFCs is the performance loss during extended operation and repeated cycling of fuel cells. The degradation of Pt or Pt alloy based catalysts, caused mainly by support oxidation, is recognized as one of the most important reasons for the decline of fuel cell performance.4,5 The Pt nanoparticles (NPs) detach from conventional carbon black supports because of the heavy corrosion they suffered under extremely harsh working environments, part of them dissolving with cationic Pt (Pt2+, Pt4+) and some others agglomerating into larger particles. Moreover, a portion of the Pt NPs is trapped in the micropores of porous carbon support surfaces and cannot work due to the absence of the effective triple-phase boundary (TPB) which is essential for PEMFCs.5−8 Thus, considerable efforts have been devoted to the development of new alternative support materials to improve the stability of catalysts. Graphene nanosheets (GNS) exhibit a structure of 2D sheets composed of sp2-bonded carbon atoms with one or more atomic thickness and have unique physical properties such as high surface areas (theoretical specific surface area of 2600 m2 g−1), superior electric conductivities, and excellent mechanical strength and elasticity.9−11 Compared to the other carbon materials, the electron (e−) shows nearly ballistic transport speed in GNS, and the unique 2D structure of GNS12,13 with large area brings a continuous electron transport channel, which facilitates increasing of the TPB, thereby enhancing the Pt utilization and activity. Hence, it is expected that GNS may © 2012 American Chemical Society
offer a new carbon−metal nanocomposite material for the next generation catalyst in fuel cells. One of the most promising methods to obtain GNS is by chemical reduction of graphene oxide (GO) in solution.14,15 Most of the previous studies for the GNS-supported Pt catalysts have employed GO as the catalyst support, since there are abundant oxygen-containing functional groups such as hydroxyl, epoxide, and carboxyl groups on GO. The presence of such functional groups does make GO a good support for uniformly dispersing and depositing of Pt NPs with a small particle size; however, it leads to a drastic decrease of the conductivity and stability as a result of a loss in the conjugated sp2 network. Therefore, GO is needed to be reduced further to GNS, which have a great increase in the conductivity and stability by restoring the hybridized sp2 network.16−19 Recently, several studies have reported that using GNS directly as supports to prepare Pt/ GNS catalysts shows an improved activity, CO tolerance, and stability in fuel cell performance.20−22 However, challenges still exist to obtain the highly dispersed Pt NPs and higher Pt loading on the inert surface of GNS, especially to greatly improve the CO tolerance and stability of Pt/GNS. In our prior work, macromolecule polymers including polyaniline (PANI) and perfluorosulfonic acid (PFSA) were applied to carbon nanotubes (CNTs) supported Pt catalysts to improve the dispersion of Pt NPs on graphitic surfaces.23,24 In addition, Tian et al.25 and our previous studies26 showed that SO3− end groups of PFSA facilitate the reaction species transfer for oxygen reduction reaction (ORR) of Pt NPs and enhance Received: November 18, 2011 Revised: January 14, 2012 Published: January 25, 2012 3979
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Twenty milliliters of H2PtCl6·H2O (1.5 mg mL−1), 100 μL of 5 wt % PFSA Nafion (Dupont Co.), and 200 mL of ethylene glycol were mixed and vigorously stirred for 20 min at 25 °C, and then the pH of the solution was adjusted to 8−10 by using 1 M NaOH solution. PFSA-stabilized colloidal Pt NPs were obtained after refluxed at 130 °C for 1−2 h until the solution color changed from yellowish to black. Then 100 mg of graphene nanosheets (GNS) previously suspended into 50 mL of ethylene glycol by ultrasonic stirring for 30 min was added to the Pt colloid coupled with agitation for 11 h. After filtration, washing with deionized water, and drying at 100 °C in a vacuum oven, the PFSA-functionalized 20 wt % Pt loaded GNS catalysts were prepared. The Pt/GNS catalyst as a benchmark was prepared using the same method except the PFSA addition. 2.2. Characterization of Catalysts. Morphologies of the prepared catalysts were analyzed with a JEOL 2010 high-resolution transmission electron microscope (HRTEM). The prepared catalysts were dispersed ultrasonically in ethanol, and then an aliquot of this solution was deposited on a grid dried at room temperature. Infrared spectra of the prepared PFSA-Pt/GNS catalysts were carried out to detect the presence of PFSA in Pt/GNS catalysts with a Fourier transform infrared spectroscope (Nicolet MAGNA-IR 560, FT-IR). The reference PFSA was prepared by the same method. Thermo gravimetric analysis (TGA7, PerkinElmer, Norwalk, CT) was used to determine the PFSA content of PFSA-Pt/GNS under air flowing at 5 mL min−1. The temperature ranged from room temperature to 1000 °C at a heating rate of 5 °C min−1. The samples were heated at 80 °C for 8 h in a vacuum oven before the test. Energy dispersive X-ray (EDX) analysis was carried out to determine the element contents in the samples. X-ray photoelectron spectroscopy (XPS) measurements were performed with a VG Scientific ESCALAB 210 electron spectrometer using Mg KR radiation under a vacuum of 2 × 10−8 Pa. The electrochemical performance of the catalysts was tested with a three-electrode system. A saturated calomel (SCE) was used as a reference electrode, and platinum wire was used as a counter electrode. Six milligrams of catalyst powders were mixed with 1000 μL of deionized water and 42 μL of 5 wt % Nafion solution, followed by coating on a mirror-polished glassy carbon disk electrode (d = 3.0 mm) as a working electrode. The measurements were carried out in 0.5 M H2SO4 at a scan rate of 50 mV s−1 in a potential range from 0 to 1.2 V vs RHE at room temperature. The H2SO4 solution was saturated with pure argon to expel oxygen out of the solution. The specific electrochemical active area (ECA) was calculated from the following equation31,32
the metal−support interaction. Thus, the PFSA, such as Nafion, can enhance the catalytic activity of Pt catalysts by improving the dispersion of Pt NPs on supports and increasing the proton (H+) conductivity of catalyst layers and the TPB on Pt particle surfaces.27−29 Thus, the chosen polymer, PFSA, is favorable to improve both lifetime and performance of graphitic carbon supported Pt catalysts. These studies strongly suggested that polymer stabilization to improve catalyst performance would be a major research focus in the near future. Enlightened by our previous work, we present a new strategy to improve the dispersion of Pt NPs on the inert graphitic surfaces of GNS using the PFSA-functionalized Pt NPs. Illustrated in Scheme 1a is a PFSA molecule as both a stabilizer Scheme 1. (a) Schematic of Facilitate Electrocatalytic Reaction with Increased Triple-Phase Boundary for PFSAFunctionalized Pt/GNS Catalyst, (b) Mechanism of PFSAPromoted Pt/GNS for Enhancing CO Electro-Oxidation
ECA =
QH mqH
(2)
where QH is the charge for Hupd (H+ + e− = Hupd) adsorption, m is the amount of metal loading, and qH (210 μC cm−2) is the charge required for monolayer adsorption of hydrogen on Pt surfaces. Electrochemical accelerated durability tests (ADT) were employed to evaluate the long-term performance of catalysts. ADT is an inexpensive and time-effective method for screening catalysts for high stability and good performance.33 Using the same system as in a CV test, ADT was conducted in the current study with CV curves between 0.6 and 1.20 V. The ORR performance of the as-prepared catalysts was evaluated by the rotating disk electrode (RDE) technique in 0.5 M H2SO4 electrolyte at a sweep rate of 10 mV s−1 and a speed of 1600 rpm at room temperature. For the CO stripping voltammetry, CO gas (99.9% purity) was introduced and bubbled through the electrolyte of 0.5 M H2SO4 solution for 10 min, while the potential was held at 100 mV versus RHE. Then a pure N2 stream was purged for 30 min to remove the CO dissolved in the H2SO4 solution, and the electrodes were then cycled between 0.05 and 1.2 V versus RHE for three cycles at 50 mV/s.
of Pt NPs and a physical cross-linker between Pt NPs and GNS. This architecture of the composite catalyst promotes the extension of TPB on Pt NPs due to the unique 2D structure of GNS with continuous electron transport channel. We expect that through polymer functionalization Pt NPs with small size can be well-dispersed on the GNS surfaces and the stability of this novel catalyst is greatly improved. Significantly, we for the first time try to investigate how the PFSA functionalization of Pt NPs affects the CO tolerance of Pt catalysts.
2. EXPERIMENTAL SECTION 2.1. Preparation of PFSA-Pt/GNS and Pt/GNS Catalysts. GNS were obtained via reducing GO using hydrazine hydrate, and the graphene oxide was synthesized from graphite powder by the procedure describe previously.30 3980
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Figure 1. (a) TEM image of graphene nanposheets (GNS) and (b) high-resolution energy X-ray photoemission spectra of the C 1s region in GNS. The inset of panel b is the Raman spectra of GNS.
3. RESULTS AND DISCUSSION Figure 1a shows the TEM image of GNS. It can be seen clearly that there are many wrinkles on the sheets that may be important for preventing aggregation of graphene due to van der Waals forces during drying and for maintaining high surface area.20,34 The XPS of GNS (Figure 1b) and shows a strong C− C bond at 284.5 eV. Moreover, as shown in the inset of Figure 1b, the Raman peaks at 1348 and 1585 cm−1 could be ascribed to the D and G bands of GNS, respectively. The D band corresponds to defects and staging disorder in the GNS, while the G band is related to the graphitic hexagon-pinch mode (C sp2 atoms), indicating a good sp2 conjugation.35,36 We show the high uniformity of the prepared PFSAfunctionalized Pt nanocolloids with an average size of 3 nm for Pt NPs in Figure 2a. The EDX study of metal nanoparticles (Figure 2b) reveals the presence of elements C, F, and S on the Pt nanocolloid derived from the PFSA molecular layers on the outer surface of the metal particles. Figure 3 displays the Pt NPs distribution of PFSA-Pt/GNS, Pt/GNS, and commercial Pt/C catalysts. Pt NPs are welldispersed on GNS in the PFSA-Pt/GNS with diameters in the 1−4 nm range and a very narrow particle size distribution (Figure 3a,b), indicating that the long-chain PFSA polyelectrolyte can effectively trap and stabilize Pt NPs on GNS, resulting in a high dispersion of Pt NPs. By contrast, much lower dispersion of Pt NPs on GNS occurs in the Pt/GNS catalysts without PFSA stabilizer due to the Pt NPs aggregating with each other (Figure 3c, d), leading to a dramatically decreased surface area.37 Hence, it is important for PFSA to
Figure 2. (a) TEM image of the PFSA-functionalized Pt nanocolloids and (b) energy dispersive X-ray (EDX) pattern of the as-prepared Pt nanocolloids.
immobilize the Pt NPs and avoid their aggregation on GNS. The commercial Pt/C catalyst as a benchmark has 2−5 nm Pt particle size. The HRTEM images (Figure 3b,d) show that the space of lattice fringes is 0.22 nm, corresponding with the (111) planes of crystalline Pt. Figure 4 shows the TG and DSC data of PFSA-Pt/GNS and Pt/GNS catalysts. For PFSA-Pt/GNS (Figure 4a), there is a sharp decrease before 400 °C, and a minor peak occurs at 394 °C, and this should be due to the decomposition of PFSA in the catalyst, which is consistent with the TG and DSC data of PFSA. As shown in the inset of Figure 4a, PFSA begins to lose its weight at about 300 °C and the DSC reaches a peak around 400 °C. However, in the case of Pt/GNS, there is almost no weight loss below 400 °C and it drops sharply above 400 °C (Figure 4b), which is attributed to the oxidation of GNS. FTIR spectra of the PFSA and PFSA-Pt/GNS catalysts are given in Figure 5. For the PFSA-Pt/GNS catalyst, the strong characteristic band at 1156 and 1225 cm−1 can be assigned to CF2 stretching vibration corresponding to the bands at 1154 and 1234 cm−1 in PFSA, respectively. For the other bands, 3981
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Figure 3. TEM images of the PFSA-Pt/GNS (a, b), Pt/GNS (c, d), and Pt/C (e) catalysts.
1060 cm−1 is associated with −SO3 stretching vibration and 980 cm−1 is assigned to the C−O−C symmetric stretching vibration in PFSA. The result combining the EDX study of Pt NPs (Figure 2b) well-verifies the presence of PFSA in the as-prepared Pt/GNS catalysts.
Shown in Figure 6a are the XPS spectra of PFSA-Pt/GNS and Pt/GNS. Besides the C(1s) signal at 284.2 eV and the O(1s) signal at 543.1 eV, the Pt(4f), Pt(5s), Pt(4p), and Pt(4d) signals appear in both samples. It is noteworthy that the S(2p) at 166.6 eV and F(1s) at 688.9 eV signal originating from PFSA 3982
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PFSA-Pt/CNT and Pt/GNS. The principle peaks are attributed to Pt0 at 71.4 eV (4f7/2) and 74.7 eV (4f5/2), while 72.8 and 76.1, and 75.0 and 78.2 eV are assigned to Pt in 2+ and 4+ states, respectively.38 The two catalysts show essentially identical ratios of metallic Pt to oxide species, and no significant differences are observed in the abundance ratio. However, a shift to higher energy is found in the binding energies of the PFSA-Pt/GNS. The slight shift of the Pt peak toward higher binding energies is due likely to the presence of PFSA and the effect of small particles in the PFSA-Pt/GNS catalysts. Figure 7 presents CV curves of PFSA-Pt/GNS, Pt/GNS, and Pt/C catalysts recorded at room temperature. The ECA is
Figure 4. (a) TGA data of the PFSA-Pt/GNS and PFSA (inset) and (b) TGA data of the Pt/GNS.
Figure 7. CV curves of PFSA-Pt/GNS, Pt/GNS, and Pt/C catalysts.
calculated by measuring the charge collected in the hydrogen adsorption/desorption region after double-layer correction and assuming a value of 210 μC cm−2 for the adsorption of a hydrogen monolayer.31,32 The plain Pt/GNS (50.2 m2 g−1) has a comparable ECA with Pt/C catalysts (48.7 m2 g−1), as shown in the inset of Figure 7. By contrast, the PFSA-Pt/GNS presents a significantly high ECA (74.2 m2 g−1), which is about 1.5 times of that of Pt/GNS and Pt/C, indicating the PFSA-Pt/ GNS has a higher electrochemical activity than both Pt/GNS
Figure 5. FTIR spectra of PFSA (a) and PFSA-Pt/GNS (b) catalyst.
is observed only in the PFSA-Pt/CNT but not Pt/GNS samples. The Pt(4f) line in Figure 6b was obtained from
Figure 6. (a) XPS spectra of PFSA-Pt/GNS and Pt/GNS and (b) XPS spectra of Pt (4f) bonds. 3983
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and Pt/C catalysts. The improved catalytic activity can be ascribed to the increased TPB on PFSA-functionalized Pt surfaces and the well-dispersed Pt NPs on GNS. Polarization curves for the ORR on these catalysts are shown in Figure 8. It can be seen that the PFSA-Pt/GNS catalyst has a
Figure 9. CO-stripping voltammetry for PFSA-Pt/GNS, Pt/GNS, and Pt/C catalysts at 0.5 mol L−1 H2SO4 electrolyte, at room temperature. The sweep rate is 50 mV s−1.
−1
Figure 8. Current−potential curves for ORR in O2-satuated 0.5 mol L H2SO4 and comparisons of ORR mass activities and specific activity of catalysts.
795 mV. For Pt/GNS catalysts, it shows a higher CO tolerance with a lower peak potential (828 mV) and onset potential (745 mV) compared to Pt/C, which due to the different carbon support effect. When it comes to PFSA-modified Pt/GNS, the peak potential is only 792 mV, and the onset potential is reached at about 678 mV, which show a further negative shift of 36 mV in peak potential and 97 mV in onset potential compared to that of Pt/GNS. In addition, PFSA-Pt/GNS also shows a smaller peak current of CO oxidation peak than Pt/C and Pt/GNS, indicating low CO adsorption amount on the surface of the PFSA-Pt/GNS catalyst. Typically, the lower onset and peak potentials are considered to manifest better catalytic activity for CO oxidation, indicating a better CO tolerance of our new catalysts. We believe that the enhanced CO tolerance of the PFSApromoted Pt catalyst can be attributed to the adsorption effect of PFSA polymer. The elimination of adsorbed CO can be described by the following reactions:40,41
higher half-wave potential, 15 and 40 mV more than that of the Pt/GNS and Pt/C catalysts, respectively. The diffusion-limiting currents were obtained in the potential region below 0.6 V, whereas a mixed kinetic-diffusion control region occurs between 0.7 and 0.9 V. The kinetic current was calculated from the ORR polarization curve according to the Koutecky− Levich equation,32 which can be described as follows 1 1 1 = + i ik id
(3)
where i is the experimentally measured current, id is the diffusion-limiting current, and ik is the kinetic current. Then, the kinetic current was calculated based on the following equation iid ik = id − i (4) i mass activity = k m
Pt + H2O → PtOHads + H+ + e−
(5)
(1)
PtCOads + PtOHads → 2Pt + CO2 + H+ + e−
where m is the amount of Pt loading. Both the PFSA-Pt/G and Pt/GNS show a slight smaller limiting current in ORR than Pt/ C. It is believed that the diffusion-limiting currents are strongly affected by the structure of the catalyst supporting material.39 The calculated results are shown in the inset of Figure 8. The ORR activities were calculated at 0.9 V versus RHE. The Pt mass activity of PFSA-Pt/GNS (7.2 mA mg−1) is 1.2 times of that of Pt/GNS (6.4 mA mg−1) and 1.6 times of Pt/C (4.4 mA mg−1), indicating that the PFSA-Pt/GNS catalyst has a higher ORR activity compared to both Pt/GNS and commercial Pt/C. The enhanced catalytic activity can be due to the unique conductivity of GNS and an increased TPB on PFSA-stabilized Pt surfaces. This is also in accordance with the trend observed for CV results in Figure 7. The CO tolerance of catalysts was performed by COstripping voltammetry as described in the Experimental Section. The curves were recorded at 50 mV/s in 0.5 M H2SO4 at room temperature, as shown in Figure 9. For conventional Pt/C catalyst, the peak potential is 846 mV, and the onset potential is
(2)
A reasonable CO electro-oxidation mechanism on the PFSApromoted Pt/GNS catalyst is shown in Scheme 1b. First, the presence of sulfonic (−SO3−) groups in the PFSA promotes water absorption though a hydrogen bond, leading to more OHads species production by dissociating water onto the Pt sites in terms of reaction 1, and the formation of an active oxy compound Pt−OHads promotes CO oxidation to CO2 through reaction 2 and results in decreasing of CO poisoning toward Pt catalysts. Second, bound CO molecules might attach on the long double helix chains of PFSA, which decreases the catalyst deactivation through the PFSA protection. This can be also in accordance with the lowest peak current of PFSA-Pt/GNS catalyst in Figure 9. At last, the improved dispersion of Pt particles as well as the effective extension of the TPB also plays an important role in improving CO oxidation. The stability of catalyst over different electrochemical oxidation cycles is shown in Figure 10. The ECA loss of all 3984
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC) (50972112) and the Major State Basic Research Development Program of China (973 Program) (No.2012CB215504).
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Figure 10. Changes of ECA of catalysts with the increased potential cycles.
catalysts decreases with the number of cycles under the same electrochemical acceleration test. According to the correlations, the durability of the Pt/C is much lower than that of the Pt/ GNS in this work. After 4000 cycles, only 6.1% of the initial ESA of the Pt/C remained while 35.2% of the initial ECA for the Pt/GNS remained, but surprisingly, the PFSA-Pt/GNS retains 45.6% after the same cycles. It is evident that the PFSAPt/GNS has a higher stability than Pt/GNS and Pt/C. Such findings clearly demonstrate that our prepared PFSA-Pt/GNS has an electrochemical durability up to 7.5 times higher than that of the Pt/C and 1.5 times higher than that of the plain Pt/ GNS under the same ADT conditions. The greatly improved durability can be caused by the PFSA-functionalized Pt NPs on GNS, which can effectively prevent the Pt NPs from aggregating with each other.
4. CONCLUSION A simple approach to synthesize the well-dispersed Pt/GNS catalyst using PFSA functionalization was demonstrated, and well-dispersed Pt NPs with small and narrowly distributed particle size were obtained on graphene nanosheets (GNS). The introduction of PFSA (Nafion) offers an enhancement of the Pt−graphene interaction and the stability for the Pt/GNS catalysts. Also, it can provide more channels for H+ transmission toward Pt surfaces, which extends the effective TPB and introduces more active catalytic sites. Moreover, it facilitates CO water absorption on the catalyst and formation of an active oxycompound of Pt−OH, promoting CO oxidation to CO2. The electrochemical tests show that the PFSAfunctionalized Pt NPs supported on GNS have a higher catalytic activity: 1.5 times than that of both Pt/C and Pt/GNS catalysts in electrochemical surface area and 1.6 and 1.2 times than that of Pt/C and Pt/GNS in oxygen reduction activity, respectively. Moreover, a much higher stability can be achieved in comparison with both the pristine Pt/GNS and commercial Pt/C catalysts. This research significantly raises the possibility of using graphene nanosheets as catalyst supports in proton exchange membrane fuel cells. Furthermore, the polymer-modified Pt NPs method showed that a better CO tolerance can provide an effective way to explore new antipoisoning catalysts for PEMFC application.
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REFERENCES
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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on Jan 25, 2012, with errors in the Figure 2 caption. The corrected version was reposted on Feb 14, 2012.
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dx.doi.org/10.1021/la2045493 | Langmuir 2012, 28, 3979−3986
