Self-Decoration of PtNi Alloy Nanoparticles on Multiwalled Carbon Nanotubes for Highly Efficient Methanol Electro-Oxidation
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A simple one-pot method was developed to prepare PtNi alloy nanoparticles, which can be self-decorated on multiwalled carbon nanotubes in [BMIm][BF4] ionic liquid. The nanohybrids are targeting stable nanocatalysts for fuel cell applications. The sizes of the supported PtNi nanoparticles are uniform and as small as 1–2 nm. Pt-to-Ni ratio was controllable by simply selecting a PtNi alloy target. The alloy nanoparticles with Pt-to-Ni ratio of 1:1 show high catalytic activity and stability for methanol electro-oxidation. The performance is much higher compared with those of both Pt-only nanoparticles and commercial Pt/C catalyst. The electronic structure characterization on the PtNi nanoparticles demonstrates that the electrons are transferred from Ni to Pt, which can suppress the CO poisoning effect.
KeywordsPtNi nanoparticles Multiwalled carbon nanotubes Methanol electro-oxidation
To satisfy increasing demands in energy and overcome issues of environment pollution, researches on sustainable and renewable energy which may replace traditional fossil fuels have become hot topics. Direct methanol fuel cells (DMFCs), a green power sources in vehicles and portable devices, have gained wide attentions [1, 2, 3]. Methanol as fuel has numerous advantages: simplicity in handling, storage, and transport; low cost and renewability because it can be easily obtained from fermentation of agricultural products [4, 5, 6]. The efficiency of methanol electro-oxidation is limited by poor kinetics and methanol crossover [6, 7], and thus the development of anode catalysts with high catalytic performance is in an urgent need to achieve efficient DMFCs. Pt is the most widely used catalyst for electrochemical oxidation of methanol. However, during the electro-oxidation process, CO molecules are produced as the intermediate, which could be adsorbed on the Pt surface and hardly be removed. As a consequence, the surface poisoning of Pt by CO will suppress the catalytic activity, and this issue must be addressed for the realization of efficient and stable Pt catalysts [7, 8].
The methanol electro-oxidation on Pt is considered to follow a three-step process. The first is the adsorption of methanol. Subsequently, Pt breaks the C–H bonds of methanol and then CO molecules will adsorb on the Pt surface, which is regarded as the dehydrogenation step. Eventually, CO is oxidized with the assistance of oxygen-containing species (e.g., –OH) formed on Pt [9, 10]. However, the oxygen-containing species are often formed on the Pt surface at high potentials [>0.7 V vs reversible hydrogen electrode (RHE)] [7, 10, 11]. Therefore, at the state of low potentials, the adsorbed CO molecules are very difficult to be removed, which results in poor activity of Pt catalysts.
Introducing a second or third metal, such as Ni, Au, Ru, Sn, Co, or Cu [12, 13, 14, 15, 16, 17, 18, 19], is an effective way to liberate the Pt surface from the CO adsorption. In this type of bimetallic systems, Pt plays a key role for the adsorption of methanol and dehydrogenation. On the other hand, the second metal could supply oxygen-containing species as the promoter for the CO oxidation at low potentials . The second metal could also modify the electronic structure of Pt by transferring electrons from the second metal to Pt and thus weakening the Pt–CO bonding energy [21, 22]. The binary and ternary Pd-based catalysts were also along the same path [23, 24]. Furthermore, the addition of a second metal will reduce the consumption of Pt, which is in favor of minimizing the catalyst cost.
Among bimetallic catalysts, PtNi nanoparticles (NPs) in particular are promising materials for methanol electro-oxidation since Ni is more economic than other metals [14, 22, 25]. PtNi catalysts with enhanced activity have been prepared by the microwave-assisted polyol reduction, and the enhancement in catalytic activity is attributed to their electronic structure modification . The X-ray photoelectron spectroscopy (XPS) results show that the Pt 4f peak for PtNi is shifted to the lower binding energy, demonstrating the electron transfer from Ni to Pt . The density functional theory (DFT) studies further indicate that incorporating Ni will induce an upshift in the d-band center of Pt, leading to a weakening of the interaction between Pt and CO [14, 26].
The reduction of metal ions in solution is a typical approach to synthesize metal and alloy NPs [25, 27], which often involves chemical additives and/or byproducts, such as surfactants or polymers used to stabilize metal NPs. However, the introduction of the organic materials would block the active surface of metal and alloy NPs to some extent. To realize an additive/byproduct free approach, we develop a room-temperature ionic liquid (RTIL)-assisted sputtering method to prepare supported metal and alloy NPs. RTILs herein play an important role for uniform dispersion of nanosupports and stabilization of small-sized metal NPs. In this method, metal is directly sputtered onto a suspension mixing RTILs and nanosupports [28, 29, 30, 31], where metal NPs may nucleate on the RTIL surface, and then diffuse into the suspension and finally self-decorate on the nanosupports [24, 32, 33, 34]. This method has been extended to prepare supported bimetallic NPs, such as AgPd and AgAu NPs decorated on graphene, and PdAu NPs decorated on carbon or TiO2 nanosupports [31, 35, 36, 37].
The RTIL-assisted sputtering is utilized to synthesize PtNi alloy NPs decorated on multiwalled carbon nanotubes (MWNTs), which possess large surface area, high conductivity, and good chemical stability. In particular, the PtNi NPs were prepared straightforwardly by sputtering a PtNi alloy target, and thus the atomic ratio of Pt to Ni can be easily controlled by changing the alloy target. The self-decoration on MWNTs is demonstrated to be an effective way to stabilize the PtNi alloy NPs, which have small sizes of only a few nanometers and uniform distribution on the nanosupport. Upon the employment of an appropriate Pt-to-Ni ratio, the supported PtNi NPs showed high catalytic activity and high stability for methanol electro-oxidation. The catalytic performance of the alloy NPs is superior to those of the Pt-only NPs and the commercial Pt/C catalyst.
2.1 Preparation of Pt/PtNi-NP-MWNT Hybrids
RTIL, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]), purity >99 %), was purchased from Shanghai Cheng-Jie Chemical and dried in vacuum for 24 h before using. MWNTs with diameters ranging from 30 to 50 nm were purchased from Nanjing XFNANO Materials Tech. The commercial Pt/C catalyst was purchased from Alfa Aesar.
Firstly, 10 mg MWNTs was fully dispersed into 1.5 mL [BMIm][BF4], with ultrasonication for 30 min to produce the [BMIm][BF4]-MWNT suspension. A stainless steel pot containing the suspension was placed into the sputtering chamber. A Pt or PtNi alloy target was used to sputter Pt or PtNi, respectively, onto the suspension for 15 min in a desktop direct-current sputtering system (Quorum Technologies). Two PtNi alloy targets were selected: one is Pt rich and the other is relatively Ni rich. The actual Pt-to-Ni ratio of the PtNi NPs was estimated by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Vista-MPX), and two Pt-to-Ni atomic ratios of 1:1 and 1:3.5 were obtained. During the sputtering, the Ar working pressure and deposition rate were kept at ~0.01 mbar and 0.2 Å s−1, respectively. Eventually, the Pt/PtNi-NP-MWNT hybrids were separated from [BMIm][BF4] by centrifugation, and the supernatant liquid was decanted after the centrifugation. The hybrids were then washed by acetone several times to completely remove residual [BMIm][BF4]. To liberate the catalysts from possible surface contamination, the hybrids were annealed at ~300 °C for 1 h in H2-Ar environment (~2.5 mbar, H2:Ar = 1:9).
2.2 Characterization of Pt/PtNi-NP-MWNT Hybrids
Metal composition of different catalysts measured by ICP-AES
Metal loading [wt%]
Atomic ratio of Pt:Ni
2.3 Electrochemical Measurements
The electrochemical measurements were carried out using CHI660E with three-electrode configuration. A carbon electrode was used as the counter electrode, and an Ag/AgCl electrode was used as the reference electrode with respect to which potentials were measured. On the other hand, a catalyst-coated glassy carbon (GC) electrode (3 mm in diameter) was employed as the working electrode. Before the preparation of the working electrode, the GC electrode was polished using an alumina slurry of 0.05 μm. Subsequently, the GC electrode was ultrasonically washed in a mixed solution of ethanol and deionized water of 1:1 in volume, and then dried at room temperature.
The catalyst suspensions were obtained by fully dispersing 3 mg catalyst in a mixed solution of 1 mL ethanol and 7 μL Nafion solution (5 wt% purchased from Aldrich) with ultrasonication for 1 h. The working electrode was prepared by dropping 10 μL catalyst ink onto the GC electrode and then drying it in air, yielding a working electrode with a catalyst loading of 0.42 mg cm−2. The actual mass of Pt in the catalyst ink were 5.96, 1.75, 1.39, and 0.85 μg for Pt/C, Pt-only, PtNi (1:1) and PtNi (1:3.5), respectively. Cyclic voltammogram (CV) was carried out with typical parameters, at a scan rate of 50 mV s−1 in a solution of 1 M CH3OH + 0.5 M H2SO4, to evaluate the catalytic activity for methanol electro-oxidation.
3 Results and Discussion
3.1 Microscopic Structures
The metal composition and loading of the as-prepared nanocatalysts and commercial Pt/C catalyst were measured by ICP-AES, and the results are summarized in Table 1. The sputtering of a Pt-rich alloy target produces the PtNi NPs with measured Pt-to-Ni ratio of about 1:1. Ni appears to be easier to decorate onto MWNTs in [BMIm][BF4] compared with Pt [38, 39], presumably due to the fact that Ni is more active than Pt. On the other hand, employing a relatively Ni-rich alloy target leads to the PtNi NPs with measured Pt-to-Ni ratio of about 1:3.5. The two groups of PtNi samples with different Pt-to-Ni ratios and the pure Pt hybrid as the control sample were denoted as the PtNi (1:1), PtNi (1:3.5), and Pt-only NPs, respectively.
3.2 Electronic Structures
On the other hand, the XPS Ni 2p spectra of the hybrids are shown in Fig. 3b for comparison. In all the spectra, the Ni oxidized state (e.g., at binding energy of about 856 eV) together with a broad satellite peak at higher binding energy account for the vast majority. The Ni metallic state at binding energy of about 853 eV is present as well, whereas it is gradually disappeared with increasing the Pt proportion. The XPS Ni features are consistent with the Pt ones, indicating an electron loss of Ni and an electron gaining of Pt in the PtNi NPs.
3.3 Electrochemical Performance
Electrochemical parameters of different catalysts calculated from cyclic voltammetry curve
ECSA [m2 (gPt)−1]
Onset potential [V]
I f [mA (mgPt)−1]
I b [mA (mgPt)−1]
I f/I b
On the other hand, the onset potential for methanol electro-oxidation is another important parameter to evaluate the catalytic performance . In forward scan, the onset potentials are 0.38, 0.32, and 0.29 V for the Pt-only, PtNi (1:1), and PtNi (1:3.5) NPs, respectively, all of which are lower than the one of 0.40 V for the commercial Pt/C catalyst. Significantly, the addition of Ni can effectively lower the onset potential, indicating the superiority of the PtNi alloy NPs for methanol electro-oxidation. As shown in Table 2, the PtNi (1:1) and PtNi (1:3.5) NPs show much larger I f of 390 and 274 mA (mgPt)−1, respectively, in comparison to that of Pt/C (198 mA (mgPt)−1). The trend in I f is well consistent with the ECSA results shown in Fig. 5.
According to the XPS and XANES results, the electronic structure of Pt was modified by the electron transfer from Ni, which will reduce the density of unoccupied states in the Pt d-band and hence weaken the Pt–CO bonding. In addition, the oxygen-containing species, which may be produced from Ni(OH)2, can react with adsorbed CO on the Pt surface . This process will promote CO oxidation and then liberate the Pt surface from CO adsorption. Therefore, the PtNi alloy NPs show much higher catalytic activity than the Pt monometallic counterpart. However, excess Ni would cover the surface of Pt active sites and thus decrease the catalytic activity, such as in the case of the PtNi (1:3.5) NPs compared with the PtNi (1:1) NPs. As a consequence, an appropriate Pt-to-Ni ratio is needed to reach the highest catalytic performance. For instance, the PtNi (1:1) NPs are considered as having sufficient active sites and strong resistance against the CO poisoning at the same time.
Figure 6b shows a comparison in I f of the studied catalysts in multiple CV cycles. After over 900 CV cycles, I f of the PtNi (1:1) and PtNi (1:3.5) NPs maintain 52.1 and 50.5 % of their initial ones, respectively. In contrast, I f of the commercial Pt/C catalyst maintains only 36.6 % of its initial one. The results demonstrate the long-term stability of the PtNi-NP-MWNT hybrids. As for the Pt-only NPs, I f is increased first and then decreased with raising the cycle number. The origin of the I f increase in the beginning is unclear so far, whereas it is worth noting that I f for the Pt-only NPs are always smaller than those for the PtNi alloy NPs in all the cycles.
The PtNi alloy NPs decorated on MWNTs were successfully prepared by the RTIL-assisted sputtering method with a PtNi alloy target. The PtNi alloy NPs exhibit small size and uniform distribution, and significantly Pt-to-Ni ratio can be controlled by selecting an appropriate alloy target. The PtNi-NP-MWNT hybrids are demonstrated to have high catalytic activity and long-term stability for methanol electro-oxidation, which are far superior to both the Pt-only monometallic counterpart and the commercial Pt/C catalyst. The XPS and XANES results indicate the electron transfer from Ni to Pt in the PtNi NPs, and it is beneficial to reducing the CO poisoning on the Pt surface. The present approach is promising for simple preparation of alloy-based nanocatalysts used in high-performance DMFCs.
The authors thank the Taiwan Light Source (TLS) for providing XANES beam time and thank Dr. Jeng-Lung Chen and Dr. Jyh-Fu Lee for technical support at TLS. This work was supported by the National Natural Science Foundation of China (No. 61274019), the Soochow University-Western University Joint Centre for Synchrotron Radiation Research, the Collaborative Innovation Center of Suzhou Nano Science & Technology, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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