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Article Cite This: ACS Omega 2018, 3, 8724−8732
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Facile Synthesis of PtCu Alloy/Graphene Oxide Hybrids as Improved Electrocatalysts for Alkaline Fuel Cells Taiyang Liu,† Chaozhong Li,†,‡ and Qiang Yuan*,†,‡ †
College of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou 550025, P. R. China Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China
‡
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S Supporting Information *
ABSTRACT: Morphology-controllable preparation of Pt-based nanoalloys supporting on carbonaceous materials is a potential strategy to enhance the catalytic properties for oxygen reduction reaction (ORR) and ethanol oxidation reaction (EOR); they are recognized as irreplaceable electrode reactions in proton-exchange membrane ethanol fuel cells. Herein, we exhibit a facile, one-step synthesis method to directly prepare composition-tunable PtCu alloy/graphene oxide (GO) hybrids. The structure of the as-synthesized PtCu alloy/GO hybrids has been analyzed using transmission electron microscopy, high resolution transmission electron microscopy, energy-dispersive X-ray, X-ray diffraction, inductively coupled plasma, and X-ray photoelectron spectroscopy. In the PtCu alloy/ GO hybrids, the PtCu alloy nanoparticles well disperse on GO, and the size is below 5.0 nm. The catalysis for ORR and EOR of the as-synthesized PtCu/GO hybrids has been evaluated in alkaline solution. Compared to commercial Pt/C, the PtCu/GO hybrids exhibit much higher mass activity and stability. The mass activities toward ORR/EOR on Pt75.4Cu24.6/GO hybrids are 5.3/2.36 times higher than the commercial Pt/C. This study proves that the as-synthesized PtCu/GO hybrids can be used as improved catalysts for ORR and EOR in alkaline medium.
1. INTRODUCTION Fuel cells that transform chemical energy into electric energy have given rise to great interest due to their potential application for clean and sustainable systems of energy generation with many merits such as low pollutant emission, high energy density, and low operating temperature.1−10 So far, platinum (Pt) is still recognized to be the most active and commonly used catalyst in fuel cells among the pure metal catalysts. However, the activity, stability, and scarcity of Pt become the main limiting factors impeding the commercial application of fuel cells. To conquer these barriers, a variety of strategies have been exploited to enhance the mass activity and/or stability: (1) reducing the size of Pt catalyst, which can provide more active sites, thus increasing Pt utilization per mass;11−13 (2) alloying with earth-abundant transition metals (Cu, Zn, Ni, and Fe),14−20 which may boost the mass activity and stability of Pt-based catalysts due to the synergetic effects that result from the change of the electronic and chemical natures of nanoalloy surface;21−24 and (3) supporting the Ptbased catalysts on the cheap carriers, which can not only permit one to greatly reduce the loading of Pt but also increase the mass activity and stability.25−33 Therefore, the combination of the above-mentioned factors is crucial to real application of fuel cells. So far, various techniques, such as impregnation method,31,32 electrodeposition,34 microwave-assisted process,35 supercritical method36 and ultrasonic method,37 have been © 2018 American Chemical Society
employed to anchor the Pt-based nanoparticles on different substrates for minimizing the precious Pt. However, these methods, more or less, often have defects such as multistep, time-consuming process, the pretreatment of the support, or weak governing of the shape/dispersion of Pt-based nanocrystals on the support. For instance, the unpretreated carbon nanotubes (CNTs) are rather inert; to coat the Pt-based nanocatalysts on the CNTs, CNTs must be pretreated in a mixture of HNO 3 and H 2 SO 4 through refluxing. 38,39 Compared to CNTs, the graphene oxide (GO) has larger surface area and abundant functional groups on the surface, thus leading to easier anchoring of the Pt-based nanocatalysts on GO.27,40,41 However, the development of a facile method to fabricating GO−Pt-based nanocrystal hybrids with size, shape, and dispersion control is of significant challenge. Herein, we introduce a direct, one-pot synthesis method to prepare sub-5.0 nm PtCu alloy supported on GO based on our previous report.42 The obtained PtCu alloy nanoparticles have uniform size and are well dispersed on GO. The as-prepared PtCu/GO hybrids displayed much higher mass activity and stability than commercial Pt/C for oxygen reduction reaction (ORR) and ethanol oxidation reaction (EOR) in alkaline medium. Received: June 14, 2018 Accepted: July 24, 2018 Published: August 7, 2018 8724
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Figure 1. Representative TEM (a−c) and high resolution TEM (HRTEM) (d) images of the as-synthesized Pt75.4Cu24.6/GO hybrids.
Figure 2. Histogram of size distribution of Pt75.4Cu24.6 nanoalloys (a), EDX spectra (b), and XRD spectra (c) of the as-synthesized Pt75.4Cu24.6/GO hybrids. (d) EDX line scanning curves of one Pt75.4Cu24.6 nanoparticle.
obtained products was first observed through transmission electron microscopy (TEM). Figure 1a,b are the typical TEM images of the as-synthesized products. As can been seen, many uniform particles dispersed on the GO. The size of particles range from 4.2 to 5.1 nm, and the mean dimension is around 4.68 ± 0.06 nm (Figure 2a). After careful observation, we found that these particles were polyhedral in shape with many corners and edges (Figure 1c,d). Besides, the result of inductively coupled plasma mass spectrometry (ICP-MS) displayed that the atomic percentage of Pt and Cu was
2. RESULTS AND DISCUSSION In a typical synthesis, copper(II) acetylacetonate (Cu(acac)2), platinum(II) acetylacetonate (Pt(acac)2), sodium iodide (NaI), and poly(vinyl pyrrolidone) (PVP-8000) were dissolved in N,N-dimethylformamide (DMF). Then, hydroxylamine hydrochloride and graphene oxide (GO) were introduced to the mixed solution and stirred for 12 h. The resulting mixture was solvothermally treated in a 10 mL Teflon-lined stainless steel autoclave at 130 °C for 8 h. The morphology of the 8725
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peaked at around 71.08 and 74.38 eV, respectively (Figure 3a). The 2p3/2 and 2p1/2 regions of metallic Cu peaked at around 932.0 and 952.0 eV, respectively (Figure 3b). The XPS analysis shows that the Pt 4f binding energy of Pt75.4Cu24.6/GO hybrids is slightly blue-shifted relative to standard Pt.42,46,47 On the other hand, the peaks of Cu 2p of Pt75.4Cu24.6/GO hybrids are also blue-shift compared to that of standard Cu. These results indicate Pt atoms and Cu atoms have a mutual interaction of electrons in the Pt75.4Cu24.6/GO hybrids and also imply PtCu alloy formation. Previous works on Pt-based alloys also recorded the downshifts of the d-band center and found such a downshift may lower the oxygen adsorption strength, improve oxygenated intermediates adsorption/desorption, and enhance the ORR activity.48−50 Furthermore, by changing the raw ratio of Cu and Pt precursors, the Pt68.2Cu31.8/GO hybrids (Figure 4; the component was decided using ICP-MS) can be also prepared in the current synthetic system. As shown in Figure 4a−c, the diameter of PtCu nanoparticles was also below 5.0 nm, and the results of XRD, EDX, and EDX line scanning showed that the PtCu nanoparticles were alloy structure. The electrocatalytic properties of Pt75.4Cu24.6/GO hybrids and Pt68.2Cu31.8/GO hybrids toward ORR were evaluated and compared to commercial Pt/C (Figure 5 shows TEM images of commercial of Pt/C, with many similar particles dispersed on the carbon; the size of these particle ranges from 2.7 to 3.6 nm). Figure 6a shows the cyclic voltammetry (CV) curves of catalysts recorded with a scan rate of 50 mV·s−1 in N2saturated 0.1 M KOH electrolyte. As can be seen, there are distinct adsorption/desorption regions on Pt75.4Cu24.6/GO hybrids, Pt68.2Cu31.8/GO hybrids, and Pt/C from −0.8 to −0.45 V, which means that the surface structures of Pt75.4Cu24.6/GO hybrids and Pt68.2Cu31.8/GO hybrids are different from that of Pt/C due to the Pt alloying with Cu.
75.4:24.6 (the sample was denoted Pt 75.4 Cu 24.6 /GO). Furthermore, the energy-dispersive X-ray (EDX) spectra (Figure 2b) verified that the products consisted of Pt and Cu and the atomic percentages of Pt and Cu were 73.2:26.8, which was very much in accord with the result of ICP-MS. The X-ray diffraction (XRD) spectra (Figure 2c) of the as-prepared Pt75.4Cu24.6/GO hybrids have four diffraction peaks referred to as (111), (200), (220), and (311) lattice planes of the facecentered cubic crystal structure of Pt (JCPDS-65-2868). Peaks of pure Pt or pure Cu cannot be observed. Compared to pure Pt, these peaks moved to a bigger angle, which is caused by the small Cu atoms entering the Pt lattice,43−45 and this implied the formation of PtCu alloy. Besides, the analysis of EDX line scanning further confirmed the PtCu alloy structure (Figure 2d). It is generally accepted that X-ray photoelectron spectroscopy (XPS) is used to analyze the sub-2.0 nanometer region of nanoparticles. Figure 3a,b shows the XPS spectra of Pt 4f and
Figure 3. XPS spectra of the as-prepared Pt75.4Cu24.6/GO hybrids. (a) Pt 4f region and (b) Cu 2p region.
Cu 2p for the as-synthesized Pt75.4Cu24.6/GO hybrids. The atomic ratio of Pt/Cu is 77.2:27.8, which is very close to the result of ICP-MS. The 4f7/2 and 4f5/2 regions of metallic Pt
Figure 4. Representative TEM (a) and HRTEM (b) images. (c) Histogram of size distribution of Pt68.2Cu31.8 nanoalloy. (d) XRD spectra and (e) EDX spectra of the as-synthesized Pt68.2Cu31.8/GO hybrids. (f) EDX line scanning curves of one Pt68.2Cu31.8 nanoparticle. 8726
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Figure 5. TEM images of the commercial Pt/C (a, b), and histogram of size distribution of Pt particles.
of PtCu alloy happens because some hollows appear in the particles (Figure S3b,c) during the durability test. These would lead to the activity decline of Pt75.4Cu24.6/GO. Generally, the ORR on metal nanoparticles is considered as a multielectron reaction.53−55 For the sake of determining the electron transfer number on Pt75.4Cu24.6/GO, the ORR polarization curves of Pt75.4Cu24.6/GO were investigated by rotating disk electrode (RDE) measurements at different rotation rates from 100 to 2500 rpm in 0.1 M KOH solution (Figure 7a), and the Koutecky−Levich (K−L) equation was employed to calculate the electron transfer number on catalysts.53−56 As a comparison, the ORR polarization curves of Pt/C are also tested. The Koutecky−Levich (K−L) plots for Pt75.4Cu24.6/GO calculated at different potentials of 0.6 and 0.8 V are shown in Figure 7b. According to the calculated results, the electron transfer number on Pt75.4Cu24.6/GO is determined to be around 4, which shows that the ORR on Pt75.4Cu24.6/GO hybrid is a 4-electron process. And this result is similar to the Pt/C (Figure 7c,d). The electrocatalytic properties of the as-synthesized Pt75.4Cu24.6/GO hybrid, Pt68.2Cu31.8/GO hybrids, and the commercial Pt/C catalyst have also been investigated toward EOR in alkaline electrolyte. The current density (J) was normalized by ECSA and Pt loading. Figure 8a,b shows the cyclic voltammograms (CVs) of Pt75.4Cu24.6/GO hybrid, Pt68.2Cu31.8/GO hybrids, and the commercial Pt/C catalyst in N2-saturated 0.5 M KOH + 0.5 M ethanol solution with a scan rate of 50 mV·s−1 at room temperature. The specific activity (J) on the as-synthesized Pt75.4Cu24.6/GO hybrid is 12.3 mA·cm−2, which is 1.07 times higher than that on Pt68.2Cu31.8/GO hybrids (11.5 mA·cm−2) and 4.96 times higher than that on the commercial Pt/C catalyst (2.48 mA· cm−2) (Figure 8a). The mass activity on the Pt75.4Cu24.6/GO hybrid is 2.93 A·mgPt−1, which is 1.27 times higher than that on Pt68.2Cu31.8/GO hybrids (2.30 A·mgPt−1) and 2.36 times higher than that on the commercial Pt/C catalyst (1.24 A·mgPt−1) (Figure 8b). Moreover, the onset potential on Pt75.4Cu24.6/GO hybrid and Pt68.2Cu31.8/GO hybrids is much lower than that on
The electrochemical surface areas (ECSAs) of Pt75.4Cu24.6/GO and Pt68.2Cu31.8/GO hybrids and Pt/C are, respectively, 23.9, 20, and 50.0 m2·g−1 obtained by assuming a value of 210 μC· cm−2 of the hydrogen adsorption/desorption region of CVs.51,52 The ORR polarization curves (Figure 6b) of Pt75.4Cu24.6/GO, Pt68.2Cu31.8/GO, and Pt/C were obtained using a rotation rate of 1600 rpm at room temperature; the electrolyte was O2-saturated 0.1 M KOH, and the scan rate was 10 mV·s−1. It shows that the half-wave potential of Pt75.4Cu24.6/ GO was 0.928 V, which obviously enhances 52 mV higher than a commercial Pt/C catalyst (0.876 V). The mass activity on Pt75.4Cu24.6/GO hybrids is 0.74 A·mgPt−1 at 0.9 V using a reversible hydrogen electrode (RHE) as the reference, and this value is 1.35 times higher than that of Pt68.2Cu31.8/GO hybrids (0.55 A·mgPt−1) and 5.3 times higher than that of Pt/C (0.14 A·mgPt−1) (Figure 6c), 2.1 times higher than our previously reported sub-5.0 nm Pt68Cu32 polyhedral alloys,42 3.5 times higher than PtCu nanoframes,49 and 6.1 times higher than sub5.0 nm Pt3Cu nanoparticles.50 Furthermore, the specific activities on Pt75.4Cu24.6/GO, Pt68.2Cu31.8/GO, and Pt/C are 3.0, 2.75, and 0.28 mA·cm−2, respectively, which implies that the specific activity on PtCu/GO hybrids is around 10 times higher than that on Pt/C (Figure S1) and is still superior to previously reported sub-5.0 nm Pt68Cu32 polyhedral alloys,42 PtCu nanoframes,49 and sub-5.0 nm Pt3Cu nanoparticles50 (Figure S2). The stability of Pt75.4Cu24.6/GO was studied through accelerated degradation testing and performed with a linear potential scanning method; the potential cycle was from 0.6 to 1.0 V at 100 mV·s−1 in an O2-saturated 0.1 M KOH electrolyte. The Pt75.4Cu24.6/GO also shows much better mass activity than Pt/C after going through 1000 cycles. The mass activity on Pt75.4Cu24.6/GO (0.41 A·mgPt−1) is 3.4 times higher than that of Pt/C (0.12 A·mgPt−1) (Figure 6d). The Tafel plots also show that the Pt75.4Cu24.6/GO hybrids display greatly higher activity than do the commercial Pt/C catalysts (Figure 6e,f). However, the TEM images (Figure S3a) of Pt75.4Cu24.6/ GO after the durability test show that the particles have aggregation tendency. Simultaneously, the dealloying process 8727
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Figure 6. (a) CV curves of Pt75.4Cu24.6/GO, Pt68.2Cu31.8/GO, and Pt/C in 0.1 M KOH. (b) ORR polarization curves. (c) Comparison of ORR mass activities at 0.9 V of Pt75.4Cu24.6/GO, Pt68.2Cu31.8/GO, and Pt/C. (d) ORR polarization curves (inset is comparison of mass activity) of Pt75.4Cu24.6/GO and Pt/C after 1000 cycles. Tafel plots of mass activities (e) and specific activities (f) of Pt75.4Cu24.6/GO and Pt/C.
GO is better than that of Pt/C. Thus, this study provides a new type of catalyst with enhanced mass activity and stability toward ORR and EOR in alkaline medium.
commercial Pt/C (Figure 8c), which implies the much faster reaction kinetics toward ethanol oxidation on PtCu/GO hybrid. The stability of the prepared PtCu/GO hybrid was investigated through fixed potential amperometry performed at −0.1 V for 1 h (Figure 8d). As shown, the current density of PtCu/GO hybrids is bigger than that of Pt/C over the entire time, indicating their enhanced catalytic stability.
4. EXPERIMENTAL SECTION 4.1. Preparation of Pt75.4Cu24.6 Alloy/GO Hybrids. First, 12.0 mg of Pt(acac)2, 15.8 mg of Cu(acac)2, 99.2 mg of NaI, and 30.0 mg of poly(vinyl pyrrolidone) (PVP-8000) were added into 8 mL of DMF solution and stirred for 15 min. Then, 70.0 mg of hydroxylamine hydrochloride and 38.7 mg of graphene oxide were added to the mixed solution and stirred for 12 h. The homogeneous solution was poured into a 10 mL Teflon-lined stainless steel autoclave. Finally, the sealed vessel was placed for 8 h at 130 °C. The products were collected by centrifugation and washing cycles at 10 000 rpm for 30 min with ethanol for three times. The obtained product was redispersed in ethanol. By changing the amounts of Pt(acac)2 (12.0 mg) and Cu(acac) 2 (23.7 mg) precursors, the Pt68.2Cu31.8/GO hybrids can also be prepared in the current system. 4.2. Electrocatalytic Test. CHI 760E electrochemical analyzer (CHI Instruments, Shanghai, Chenghua Co., Ltd.) was used to record electrocatalytic tests performed in a
3. CONCLUSIONS In conclusion, this study introduces a one-step method to prepare composition-tunable PtCu alloys/GO hybrids. The catalytic performances of the as-prepared PtCu/GO hybrids toward ORR and EOR have been evaluated in alkaline medium. The as-synthesized Pt75.4Cu24.6/GO hybrids exhibit a mass activity of 0.74 A·mgPt−1 at 0.9 V versus RHE. This value is, respectively, 5.3 and 2.1 times higher than Pt/C (0.14 A· mgPt−1) and our previously reported, unsupported sub-5.0 nm Pt68Cu32 polyhedral alloys (0.34 A·mgPt−1). Compared to commercial Pt/C, the as-synthesized Pt75.4Cu24.6/GO hybrids also display improved electrocatalytic properties toward EOR. The mass activity on Pt75.4Cu24.6/GO hybrid (2.93 A·mgPt−1) is 2.36 times higher than that on Pt/C (1.24 A·mgPt−1). The current−time curves indicate that the stability of Pt75.4Cu24.6/ 8728
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Figure 7. (a) ORR polarization curves of Pt75.4Cu24.6/GO hybrids at different rotation rates. (b) Koutecky−Levich (K−L) plots of the ORR of Pt75.4Cu24.6/GO hybrids at 0.6 and 0.8 V. (c) ORR polarization curves of Pt/C at different rotation rates. (d) Koutecky−Levich (K−L) plots of the ORR of Pt/C at 0.6 and 0.8 V.
Figure 8. (a) Specific activities and (b) mass activities on as-prepared Pt75.4Cu24.6/GO, Pt68.2Cu31.8/GO, and Pt/C with a scan rate of 50 mV·s−1 in 0.5 M ethanol + 0.5 M KOH electrolyte. (c) Linear voltammetry curves on Pt75.4Cu24.6/GO, Pt68.2Cu31.8/GO, and Pt/C. (d) Current−time curves of the Pt75.4Cu24.6/GO, Pt68.2Cu31.8/GO, and Pt/C obtained at −0.1 V for 1 h.
mm) electrode. The ink of catalyst was prepared by taking a certain amount of PtCu alloy/GO hybrids and Pt/C powder to disperse in superpure water under an ultrasonic bath. The
standard glass three-electrode cell at room temperature. Before the electrocatalytic test, alumina powder with sizes of 1, 0.5, and 0.05 μm was used to polish the glassy carbon (GC, Φ = 5 8729
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working electrode was prepared by spreading the ink on the GC electrode, dried under infrared lamp, and, finally, 4 μL of Nafion diluents (1 wt % Nafion solution) was spread onto the electrode surface. The CV curves were recorded in N2-saturated 0.1 M KOH electrolyte with a scan rate of 50 mV·s−1 ranging from −0.8 to 0.2 V (Ag/AgCl). To obtain a stable CV curve, the scan was cycled several times. CV tests toward ethanol oxidation were performed from −0.9 to 0.4 V (Ag/AgCl) with a scan rate of 50 mV·s−1 in 0.5 M ethanol + 0.5 M KOH electrolyte. ORR test was recorded with a CHI 760E electrochemical analyzer (CHI Instruments, Shanghai, Chenghua Co., Ltd.) and a GC rotating disk electrode (RDE). A reversible hydrogen electrode (RHE) and Pt mesh (1 cm × 1 cm) were utilized as a reference and counter electrode, respectively. The working electrode was prepared by dropping catalyst ink obtained by dispersing samples in a mixture of ethanol and water (volume ratio is 1:1) onto the GC RDE (geometric area of GC is 0.196 cm2 (Φ = 5 mm)). After the evaporation of solvent under an infrared lamp, 5 μL of Nafion diluents (0.1 wt % Nafion solution) was transferred onto the electrode surface before test. The ORR was carried out from 0.15 to 1.05 V (vs RHE) at a scan rate of 10 mV·s−1 and a rotation rate of 1600 rpm in 0.1 M KOH at room temperature after inletting O2 for half an hour until saturated. The ORR curves were corrected for capacitive currents in N2 saturation. The kinetic current (ik) was calculated from the ORR curves based on the Koutecky− Levich equation: (1/i = 1/ik + 1/iL) at 0.90 V and ECSA of each catalyst was utilized to normalize the current density. In the equation, iL and i, respectively, stand for the diffusion limiting current and measured current (0.90 V) at the kineticdiffusion control region.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01347. Histogram of specific activity of Pt75.4Cu24.6/GO, Pt68.2Cu31.8/GO; TEM images and HRTEM images of
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Pt75.4Cu24.6/GO (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Qiang Yuan: 0000-0003-3022-9925 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 (21571038 and 21361005), the Open Fund of the Key Lab of Organic Optoelectronics & Molecular Engineering (Tsinghua University), and the special fund for natural science of Guizhou University (201801). We also thank Professor Xun Wang (Tsinghua University) for the help of HRTEM and XPS experiments. 8730
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DOI: 10.1021/acsomega.8b01347 ACS Omega 2018, 3, 8724−8732
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DOI: 10.1021/acsomega.8b01347 ACS Omega 2018, 3, 8724−8732
