Carbon nanotubes supported platinum–gold alloy nanocrystals composites with ultrahigh activity for the formic acid oxidation reaction

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Carbon nanotubes supported platinumegold alloy nanocrystals composites with ultrahigh activity for the formic acid oxidation reaction Shu-He Han a,1, Yi-Gang Ji b,1, Shi-Hui Xing a, Jiao-Jiao Hui a, Qi Guo b, Feng Shi a,*, Pei Chen a,**, Yu Chen a a

School of Materials Science and Engineering, Shaanxi Normal University, Xi'an 710062, PR China Jiangsu Key Laboratory of Biofuction Molecule, Department of Life Sciences and Chemistry, Jiangsu Second Normal University, Nanjing 210013, PR China

b

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abstract

Article history:

Improving the electrocatalytic activity, durability, and utilization of anode eletrocatalysts

Received 28 July 2016

are crucial for accelerating commercialization of direct formic acid fuel cells. In this work,

Received in revised form

the multiwall carbon nanotubes (MWCNTs) supported PtAu alloy nanocrystals (MWCNTs/

24 August 2016

PtAu) composites are synthesized by a one-pot wet-chemical method using poly-

Accepted 5 September 2016

ethyleneimine as the complexant and surfactant. The physicochemical properties of the

Available online xxx

as-prepared MWCNTs/PtAu composites are characterized detailedly by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron

Keywords:

spectroscopy, etc. These structural investigations display the PtAu alloy nanocrystals are

PtAu alloy

highly dispersed on the surface of the MWCNTs. Cyclic voltammetry and chro-

Carbon nanotubes

noamperometry measurements show the as-prepared MWCNTs/PtAu composites signifi-

Formic acid

cantly enhance the direct dehydrogenation pathway of the formic acid oxidation reaction,

Electrooxidation

resulting in the improved electocatalytic activity and durability for the formic acid

Reaction pathway

electrooxidation. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction In comparison with direct alcohol (methanol and ethanol) fuel cells, direct formic acid fuel cells (DFAFCs) have obvious advantages, including the bigger open-circuit potential, higher specific energy density, and lower fuel crossover, etc [1e6]. So far, Pd-based nanocrystals are the most efficient and active electrocatalysts for the formic acid oxidation reaction (FAOR)

due to the dominant direct dehydrogenation pathway on the Pd surface (eq. (1)) [7e10]. However, Pd-based nanocrystals suffer from the low durability due to the Pd dissolution in the strong acidic solution with strong corrosivity, resulting in the rapid degradation of the DFAFCs performance. Pathway1 : HCOOH/HCOOads þ Hþ þ e /CO2 þ 2Hþ þ 2e (1)

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Shi), [email protected] (P. Chen). 1 These two authors made an equal contribution to this work. http://dx.doi.org/10.1016/j.ijhydene.2016.09.026 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Han S-H, et al., Carbon nanotubes supported platinumegold alloy nanocrystals composites with ultrahigh activity for the formic acid oxidation reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.09.026

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Pathway2 : HCOOH/COads þ H2 O/CO2 þ 2Hþ þ 2e

(2)

Although pure Pt meal has very excellent tolerance to acidic solution, the previous investigation have demonstrated the indirect dehydration pathway with COads intermediate (eq. (2)) preferentially occurs on the pure Pt surface, which results in very low reactivity for the FAOR [11,12]. Fortunately, the direct dehydrogenation pathway of the FAOR on the Pt surface (eq. (1)) can be enhanced remarkably by alloying Pt with second metal element due to the ensemble effect [13e33]. Thus, the high active and stable Pt-based bimetallic/trimetallic nanocrystals have been widely exploited as the efficient Pdalternative elelctrocatalysts for the FAOR [13e26,29e33]. Besides the reactivity and durability, improving the utilization of noble metal anode eletrocatalyst is also a key issue for accelerating commercialization of the DFAFCs. The numerous investigations have indicated that carbon nanomaterials can effectively serve as supporting materials to disperse Pt nanocrystals and improve their electrocatalytic efficiency [34e39]. At present, carbon nanotubes (CNTs), a relatively new carbon nanomaterial, have been widely used as supporting material due to their unique one-dimensional structure, high surface area, and fascinating electrical/mechanical properties, which improve the utilization of noble Pt metal [40e48]. Among various Pt-based bimetallic/trimetallic nanocrystals, PtAu bimetallic nanocrystals have been considered as highly competitive candidate for the FAOR due to their high activity and excellent durability [49e59]. At present, to our best knowledge, there are only two reports on decorating PtAu alloy nanocrystals on CNTs for the FAOR [60,61]. However, these reported PtAu/CNTs composites can't effectively enhance the direct dehydrogenation pathway of the FAOR, resulting in the low reactivity for the FAOR yet. In this work, we successfully synthesized highly dispersed multiwall carbon nanotubes (MWCNTs) supported PtAu alloy nanocrystals (MWCNTs/PtAu) composites with assistance of polyethyleneimine (PEI, Scheme S1), which effectively electrocatalyzed FAOR through the direct dehydrogenation pathway and displayed an increase in mass activity by a factor of 12.2 compared to commercial Pt/C electrocatalyst.

deionized water and stirred for 20 min. Then, 7.1 mg of MWCNTs was added into the mixture solution with ultrasonic treatment for 30 min. Subsequently, 5 mL of 0.15 M NaBH4 aqueous solution was added into the mixture solution and stirred for 60 min. After reaction, the obtained MWCNTs/PtAu composites were separated by centrifugation, washed with deionized water, and then dried in a vacuum at 40  C for 24 h. Using similar procedure, MWCNTs/Au, and MWCNTs/Pt composites were also synthesized.

Electrochemical measurements Electrochemical tests were performed on a CHI 660 D electrochemical analyzer at 30  C. Platinum wire, saturated calomel electrode (SCE), and catalyst modified glassy carbon electrode (GCE) were used as the counter electrode, reference electrode, and working electrode, respectively. Potentials in this work were reported with respect to the SCE. The working electrode was prepared according to the procedure reported previously [62,63]. The 6 mL of 2 mg mL1 catalyst ink was spread on the GCE with 3 mm diameter. Then, 3.5 mL Nafion (5 wt.%) was coated onto GCE surface. The electrochemically active surface area (ECSA) of electrocatalyst was measured in H2SO4 by integrating hydrogen adsorption charge and assuming a value of 210 mC cm2 for the monolayer Had atoms on the Pt surface [64].

Instruments The interactions between PEI and metal ions were investigated by Ultraviolet and visible spectroscopy (UVevis, Shimadzu UV-2600U). The chemical composition, surface charge, crystal structure, and morphology of the catalysts were characterized by inductively coupled plasma atomic emission spectrometry (ICP-AES, Leeman), energy dispersive X-ray spectroscopy (EDX, JSM-2010), X-ray photoelectron spectroscopy (XPS, AXIS ULTRA), zeta potential analyzer (Malvern Zetasizer Nano ZS90), X-ray diffraction (XRD, D/max-rC X-ray diffractometer), scanning electron microscopy (SEM, JSM2010), transmission electron microscopy (TEM, JEM-2100F), and high angle annular dark-field scanning TEM (HAADFSTEM, JEM-2100F). The binding energy was calibrated by means of the C 1s peak energy of 284.5 eV.

Experimental Reagents and chemicals

Results and discussion

The highly pure carboxylated MWCNTs (outer diameter: 30e50 nm; surface area: >30 m2 g1) were purchased from Chengdu Organic Chemicals Co. Ltd, Chinese Academy of Sciences. Polyethyleneimine (PEI, Scheme S1, Mw ¼ 600) was purchased from Aladdin Industrial Corporation. Commercial 40 wt. % Pt/C elelctrocatalyst was supplied from Johnson Matthey Corporation. Other reagents were of analytical reagent grade and used without further purification.

Characterization of MWCNTs/PtAu composites

Preparation of MWCNTs/PtAu composites 0.25 mL of 0.02 M H2PtCl6, 0.4 mL of 0.05 M HAuCl4, and 0.5 mL of 0.5 M PEI aqueous solutions were added into the 8.0 mL

The chemical composition of MWCNTs/PtAu composites was analyzed by ICP-AES. MWCNTs/PtAu composites contain about 7.4 wt.% Pt and 29.8 wt.% Au, which is close to its theoretical metal loading (40 wt.%). Fig. 1A shows the EDX spectrum of MWCNTs/PtAu composites. Pt/Au atomic ratio is measured to be 1:4.1, which is close to ICP-AES result (1:3.9). These experimental data demonstrate that the H2PtCl6 and HAuCl4 precursors are reduced completely by NaBH4. Fig. 1B shows XRD patterns of MWCNTs/Au, MWCNTs/PtAu, and MWCNTs/Pt composites. All composites show a characteristic C{002} diffraction peak at 25.9 , and a typical face-centered

Please cite this article in press as: Han S-H, et al., Carbon nanotubes supported platinumegold alloy nanocrystals composites with ultrahigh activity for the formic acid oxidation reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.09.026

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Fig. 1 e (A) EDX spectrum of MWCNTs/PtAu composites. (B) XRD patterns of (a) MWCNTs/Au, (b) MWCNTs/PtAu, and (c) MWCNTs/Pt composites.

cubic (fcc) structure. For MWCNTs/PtAu composites, the four characteristic fcc diffraction peak are located at the position between the corresponding Au diffraction peaks at MWCNTs/ Au composites and Pt diffraction peaks at MWCNTs/Pt composites, indicating the formation of PtAu alloy. The particle size of the deposited PtAu alloy nanoparticles was estimated to be 3.2 nm according to Scherrer's formula. In the absence of PEI, the weak diffraction peaks of Pt are observed at the obtained MWCNTs/PtAu composites (Fig. S1), indicating the low alloying degree. The control experiment clearly demonstrates the existence of PEI facilitates the formation of PtAu alloy. In order to explore the reason, UVevis and linear sweep voltammetry measurements were carried out. Upon the addition of PEI solution in H2PtCl6 or HAuCl4 solution, their UVevis spectra change obviously (Fig. S2), indicating the formation of PEIePtIV and PEIeAuIII complexes, respectively [65]. Linear sweep voltammetry measurements show the existence of PEI can effectively decrease the reduction potentials of both H2PtCl6 and HAuCl4 due to the coordination interaction, and the reduction potential of PEIePtIV complex is close to that of PEIeAuIII complex (Fig. S3). The similarity in reduction potentials allows PtIV and AuIII precursors to be reduced simultaneously by NaBH4, resulting in the generation of PtAu alloy. The morphology and structure of MWCNTs/PtAu composites were evaluated by TEM. Fig. 2A shows TEM images of MWCNTs/PtAu composites. As observed, the spherical PtAu alloy nanocrystals with the average size of 3.42 nm are highly dispersed on the MWCNTs surface. In a control experiment without PEI, the obtained MWCNTs/PteAu composites generate the obvious aggregation (Fig. S4). This fact suggests the PEI effectively acts as surfactant to inhibit the aggregation of PtAu alloy nanocrystals due to its bulky molecule volume and excellent hydrophilic property [64,66]. The carboxylated MWCNTs is negatively charged (zeta potential: e28.2 mV) whereas both PEIePtIV and PEIeAuIII complexes are positively charged due to the protonation of eNH2 groups [64,66]. Thus, the efficient anchorage of PEIePtIV and PEIeAuIII complexes on carboxylated MWCNTs via electrostatic interaction also contributes to the well dispersion of PtAu alloy nanocrystals on the surface of MWCNTs [64]. Fig. 2B shows the HAADF-STEM image and corresponding EDX element mapping patterns of MWCNTs/PtAu composites. The EDX element mapping patterns show Pt, Au, and C elements evenly distribute on the

same region, further confirming the uniform distribution of PtAu alloy nanocrystals on the surface of MWCNTs. Meanwhile, the N element is also detected, demonstrating the existence of PEI. The appearance of the characteristic N1s XPS peak further confirms the existence of PEI at MWCNTs/PtAu composites (Fig. S5), which originates from the anchorage of PEI on both MWCNTs and PtAu nanocrystals due to electrostatic interaction and N-metal bond interaction, respectively [64]. Fig. 2C shows SEM image of MWCNTs/PtAu composites. As observed, MWCNTs/PtAu composites have threedimensionally and highly porous architecture. Such interconnected structure not only provides effectively continuous transport pathway of electron but also accelerates mass transport of reactant [40e48]. Fig. 2D shows high-resolution TEM (HRTEM) image of MWCNTs/PtAu composites. HRTEM image displays the clear lattice fingers. Fig. 2E shows the magnified HRTEM images. The magnified HRTEM images show the interval distance between adjacent lattice fringes is 0.230 nm. This value is larger than the {111} lattice spacing (0.226 nm) of the fcc Pt crystal but is smaller than that (0.235 nm) of the fcc Au crystal, further confirming the formation of PtAu alloy. The corresponding fast Fourier transform (FFT) patterns show the spot patterns with 6-fold rotational symmetry, implying the PtAu alloy nanocrystals at MWCNTs/PtAu composites are presented by {111} facets. The surface composition and electronic structure of MWCNTs/PtAu composites were analyzed by XPS. Fig. 3 shows Pt 4f and Au 4f XPS spectra of MWCNTs/PtAu composites. XPS data display Pt/Au atomic ratio is 1:3.4, which is close to ICPAES and EDX data. These experimental results demonstrate the similar chemical composition exists at bulk and surface, which is indicative of PtAu alloy formation. Meanwhile, it is observed that both Pt 4f and Au 4f binding energies of MWCNTs/PtAu composites exhibit 0.2 and 0.1 eV negative shift compared to the standard values of bulk Pt and Au [67], respectively. Obviously, the interaction between Pt and Au elements can't explain the phenomenon. Therefore, the strong electron donation must come from other places at MWCNTs/PtAu composites. The XPS and EDX mapping measurements have demonstrated the existence of PEI in MWCNTs/PtAu composites. The lone pair electrons of eNH2 groups at PEI can efficiently donate electrons to Pt and Au atoms due to strong NePt and NeAu bonds [64e66], which

Please cite this article in press as: Han S-H, et al., Carbon nanotubes supported platinumegold alloy nanocrystals composites with ultrahigh activity for the formic acid oxidation reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.09.026

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Fig. 2 e (A) Typical TEM images of MWCNTs/PtAu composites. Insert: the corresponding size distribution histogram. (B) HAADF-STEM image and corresponding EDX element mapping patterns of MWCNTs/PtAu composites. (C) Representative SEM image of MWCNTs/PtAu composites. (D) Typical HRTEM image of MWCNTs/PtAu composites. (E) The magnified HRTEM images recorded from region marked by blue and green squares in D and the corresponding FFT patterns. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3 e (A) Pt 4f and (B) Au 4f XPS spectra of MWCNTs/PtAu composites. The vertical black dotted lines in Fig. 3A and B represent the standard values of Pt 4f7/2 and Au 4f7/2, respectively.

Please cite this article in press as: Han S-H, et al., Carbon nanotubes supported platinumegold alloy nanocrystals composites with ultrahigh activity for the formic acid oxidation reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.09.026

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leads to the negative shift in both Pt 4f and Au 4f binding energies.

Electrocatalytic performance measurements The electrochemical property of MWCNTs/PtAu composites was investigated and compared to commercial 40 wt.% Pt/C electrocatalyst by cyclic voltammetry (CV). Fig. 4 shows the CV curves of MWCNTs/PtAu composites and commercial Pt/C electrocatalyst. The ECSA of MWCNTs/PtAu composites and commercial Pt/C electrocatalyst are calculated to be 56.8 and 43.6 m2 g1 Pt . Although the mean particle size of MWCNTs/PtAu composites (ca. 3.5 nm) is bigger than that of commercial Pt/C electrocatalyst (ca. 2.4 nm, Fig. S6), the ECSA of MWCNTs/PtAu composites is comparable to that of commercial Pt/C electrocatalyst. This fact demonstrates that three-dimensionally and porous architecture of MWCNTs/PtAu composites facilitates the mass transfer of reactant during the electrochemical reaction. The electrocatalytic activities of MWCNTs/PtAu composites and Pt/C electrocatalyst were evaluated by CV. Fig. 5A shows the CV curves of MWCNTs/PtAu composites and commercial Pt/C electrocatalyst during the FAOR. In general, the FAOR on Pt-based nanocrystals obeys a dual-path mechanism that involves a dehydrogenation pathway (direct pathway) without COads poisoning intermediate and a dehydration

Fig. 4 e CV curves of (a) MWCNTs/PtAu composites and (b) commercial Pt/C electrocatalyst in N2-saturated 0.5 M H2SO4 solution at 50 mV s¡1.

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pathway (indirect pathway) with COads poisoning intermediate (eqs. (1) and (2)) [13e26,68e70]. The oxidation peaks of the FAOR at both electrocatalysts appear at 0.30 and 0.70 V in the forward and backward sweeps, corresponding to the direct oxidation and indirect oxidation of formic acid, respectively. The reaction pathway of the FAOR is determined by atomic arrangement of Pt-based nanocrystals. Compared to MWCNTs/Pt composites and MWCNTs/Au composites, MWCNTs/PtAu composites show improved FAOR activity (Fig. S7), indicating the single isolated Pt atom can effectively enhance the direct pathway of the FAOR [13e26,68e70]. For MWCNTs/PtAu composites, the oxidation peak of the FAOR via direct pathway at 0.3 V is enhanced significantly, in comparison with commercial Pt/C electrocatalyst. ECSAnormalized CV curves also reveal that specific activity of MWCNTs/PtAu composites is much better than that of commercial Pt/C electrocatalyst (Fig. S8). These facts indicate the FAOR on MWCNTs/PtAu composites proceeds dominantly through direct pathway due to the formation of PtAu alloy. At 0.3 V potential, Pt-mass activities of MWCNTs/PtAu composites and commercial Pt/C electrocatalyst for the FAOR are 1345 1 A g1 Pt and 110 A gPt , showing the improved reactivity of MWCNTs/PtAu composites. Meanwhile, the Pt-mass activity (1345 A g1 Pt ) of MWCNTs/PtAu composites for the FAOR at 0.3 V is also much higher than reported values of various Pt-based nanocrystals, such as PtPb nanoflowers (310 A g1 Pt ) [21], Pt/ graphene hybrids (280 A g1 Pt ) [64], PtAgCu@PtCu concave nanooctahedrons (320 A g1 Pt ) [22], Pt/Ni(OH)2eNiOOH/Pd multi-walled hollow nanorod arrays (500 A g1 Pt ) [71], and Pt1Au3 nanowires (1180 A gPt1) [72], Pt1Au8/graphene (570 A gPt1) [73], Pt10Au1 nanodendrites (170 A g1 Pt ) [57], which further confirm the high FAOR activity and high Pt utilization of MWCNTs/PtAu composites. Considering that Au is an expensive metal, metal-mass normalized CV curves were also investigated (Fig. S9). As observed, the metal-mass activity of MWCNTs/PtAu composites is still 3.03 times bigger than that of commercial Pt/C electrocatalyst at 0.3 V potential, indicating the promising practical application. It is well known that the electrocatalytic activity of electrocatalyst highly relates to its chemical composition [74,75]. Thus, the electrocatalytic activities of MWCNTs/PtAu composites with different PtAu atomic ratio for the FAOR were investigated (Fig. S10). As observed, Pt-mass activity of MWCNTs/PtAu

Fig. 5 e (A) Pt-mass normalized CV curves of (a) MWCNTs/PtAu composites and (b) commercial Pt/C electrocatalyst in N2saturated 0.5 M H2SO4 þ 0.5 M HCOOH solution at 50 mV s¡1. (B) CA curves of (a) MWCNTs/PtAu composites and (b) commercial Pt/C electrocatalyst in N2-saturated 0.5 M H2SO4 þ 0.5 M HCOOH solution for 6000 s at 0.30 V potential. Please cite this article in press as: Han S-H, et al., Carbon nanotubes supported platinumegold alloy nanocrystals composites with ultrahigh activity for the formic acid oxidation reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.09.026

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composites at 0.3 V potential gradually increases with increasing Au amount. When AuPt atomic ratio reaches to 4:1, the Pt-mass activity of the MWCNTs/PtAu composites reaches a maximum and then retain constant after further increasing AuPt atomic ratio to 5:1. The durability of electrocatalysts is very critical for fuel cells application. Fig. 5B shows the chronoamperometry (CA) curves of MWCNTs/PtAu composites and commercial Pt/C electrocatalyst for the FAOR. As observed, the currents of the FAOR at both MWCNTs/PtAu composites and commercial Pt/C electrocatalyst decrease gradually. However, the current at MWCNTs/PtAu composites decreases more slowly as compared with commercial Pt/C electrocatalyst. For example, at 6000 s, the current of the FAOR at MWCNTs/PtAu composites remains 30.5% of their initial value at 20 s [7,8,76], which is much better than that (20.6%) at Pt/C electrocatalyst. Meanwhile, MWCNTs/PtAu composites exhibit higher current than commercial Pt/C electrocatalyst over the entire reaction time, also confirming the improved durability of MWCNTs/PtAu composites. On the one hand, the formation of PtAu alloy interrupts the contiguous Pt atoms, resulting in ensemble effect that enhances direct pathway of the FAOR. On the other hand, the negative shift of Pt binding energies also facilitate the direct pathway of the FAOR due to the electronic effect [13,21]. The enhanced direct pathway effectively decreases the COads poisoning effect, which contributes to the improved durability of MWCNTs/PtAu composites for the FAOR.

Conclusions We have developed a facile one-pot wet-chemical method for the preparation of high-quality MWCNTs/PtAu composites with the assistance of PEI. Due to the particular coordination interactions, PEI efficiently decreases the reduction potential difference between H2PtCl6/Pt and HAuCl4/Au redox pairs, resulting in the formation of PtAu alloy. Meanwhile, PEI effectively serves as surfactant to restrain the aggregation of PtAu alloy nanocrystals, resulting in small particle size and excellent dispersion of PtAu alloy nanocrystals on the MWCNTs surface. Based on the ensemble effect and electronic effect, MWCNTs/PtAu composites significantly enhance the direct pathway of the FAOR, resulting in the superior electocatalytic mass activity for the FAOR. In particular, MWCNTs/ PtAu composites also show the excellent durability for the FAOR due to the enhanced direct pathway. With high mass activity and good durability, the as-prepared MWCNTs/PtAu composites may have potential application in the DFAFCs.

Acknowledgments This research was sponsored by Natural Science Foundation of Shaanxi Province (2015JM2043), Doctoral Program of Higher Education of China (20130202120010), Key Science and Technology Program of Shaanxi Province (2014K1006), and Fundamental Research Funds for the Central Universities (GK201602002 and GK201503036).

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.09.026.

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