Sophisticated Construction of Hollow Au–Ag–Cu Nanoflowers as

Oct 12, 2017 - The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02491. Typical T...
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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10490-10498

Sophisticated Construction of Hollow Au−Ag−Cu Nanoflowers as Highly Efficient Electrocatalysts toward Ethylene Glycol Oxidation Hui Xu,† Bo Yan,† Ke Zhang,† Jin Wang,† Shumin Li,† Caiqin Wang,† Zhiping Xiong,† Yukihide Shiraishi,‡ Yukou Du,*,†,‡ and Ping Yang† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University 199 Renai Road, 215123 Suzhou, People’s Republic of China ‡ Tokyo University of Science Yamaguchi, Sanyo-Onoda-shi, Yamaguchi 756-0884, Japan S Supporting Information *

ABSTRACT: Direct ethylene glycol fuel cells represent promising advanced portable energy storage and conversion devices, which can be well applied to serve as advanced sustainable energy technology to overcome the formidable challenges of ecological crisis and environmental pollution. However, many drawbacks of the newly established catalysts such as poor corrosion resistance and antipoisoning make them exhibit limited electrocatalytic performances. For addressing these issues, we herein combined both morphological and component advantages to engineer an advanced Au−Ag−Cu trimetallic electrocatayst with fascinating hollow nanoflowers structures. By virtue of such unique structure, the as-obtained Au−Ag−Cu hollow nanoflowers displayed unprecedentedly excellent electrocatalytic activity toward ethylene glycol oxidation with the highest mass activity of 5493.4 mA mg−1 Au , 5.9-fold enhancement than that of pure Au. We postulate that the strategy and electrocatalysts we engineered in this work will greatly boost the commercial development of fuel cell technologies. KEYWORDS: AuAgCu electrocatalysts, Hollow nanoflowers, High-performance, Ethylene glycol electrooxidation, Long-term stability

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INTRODUCTION In the state of the climate warming, countries of the world are focusing on clean and green renewable sources for future sustainable development. Fuel cells are a new class of clean energy equipment that has attracted increasing research interests for addressing the energy crisis and environmental contamination.1−3 Among various fuel cells, direct ethylene glycol fuel cells (DEGFCs), with pre-eminent energy density, low toxicity and renewability from biomass, have drawn increasing notice for serving as ideal energy storage and conversion devices.4,5 More importantly, the EG can also be produced as a byproduct from biomass, which is environmental benign enough to be applied in DEGFCs. Regardless of these advantageous terms, the bottleneck in enhancing fuel cell technologies for the electrooxidation reaction of liquid fuel is the shortage of cost-efficient catalysts, where even the most efficient precious-metal catalysts require a large quantity of electrocatalysts to reach the desired catalytic activities.6 With regard to this, tremendous efforts have been paid to address © 2017 American Chemical Society

these issues, the rational design and development of Pt- or Pdbased nanomaterials has played significant roles in the research field of DEGFCs for its extremely outstanding electrocatalytic activity and durability over the past decade,7,8 while both of the scarce natural abundance9 and poor antipoisoning10 impose major limitations on its role in future electrocatalyst applications. Accordingly, designing and modifying the nanocatalysts without the dopant of noble metal of Pt or Pd but greatly enhanced electrocatalytic performances has changed into the hotspot in recent years.11,12 Recently, Au emerged as the ideal electrocatalyst due to the fact introduction of Au nanocatalysts could simultaneously boost the intermediate oxidation and accelerate the reaction process. In addition, Au can be well applied in a series of reactions, such as the hydrogen evolution reaction, biological sensor, the electrooxidation of Received: July 23, 2017 Revised: September 27, 2017 Published: October 12, 2017 10490

DOI: 10.1021/acssuschemeng.7b02491 ACS Sustainable Chem. Eng. 2017, 5, 10490−10498

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Figure 1. Typical TEM images of AuAg1Cu1 HNFs with different magnifications.

electrocatalysts.29 In view of these, if we can integrate the favorable conditions of hollow nanoflower structure with composition advantages to fabricate the ideal Au−Ag−Cu hollow nanoflowers (Au−Ag−Cu HNFs) catalysts, it will be favorable for further achieving the commercial development of DEGFCs and beyond.30 In accordance with this guideline, we herein demonstrated a combined seed mediated and galvanic replacement method to successfully synthesize the high-yield Au−Ag−Cu HNFs catalysts with rough open surfaces. Impressively, the electrochemical measurements and controlled experiments revealed that such Au−Ag−Cu HNFs catalysts exhibited unprecedentedly high electrocatalytic activity toward EGOR with the mass activity of 5493.4 mA mg−1 Au , 5.9-fold higher than that of pure Au (929.1 mA mg−1 Au ), as well as the superior long-term stability after successive cyclic voltammetry (CV) of 500 cycles, which render them to be promising anode electrocatalysts for DEGFCs. It is postulated that the strategy and the Au−Ag− Cu HNFs catalysts we developed in this work will greatly boost the commercial development of DEGFCs and solve the energy crisis and environmental pollution to some extent.

alcohols and so on. More importantly, different from Pt, Au is also not prone to be poisoned by some toxic intermediates; all of these favorable terms have made it a desirable electrocatalyst for commercial application in DEGFCs. Therefore, Au and Aubased nanomaterials have been proposed to serve as highly efficient electrocatalysts for improving the catalytic performances for ethylene glycol oxidation reaction (EGOR) due to their superior long-term durability and capacity for reducing intermediates poisoning.13,14 In this regard, the rational fabrications and modifications of a novel category of Aubased nanocatalysts will be favorable for accelerating the commercialization of DEGFCs.15 The electrocatalytic performances of nanocatalysts have been demonstrated to rely on their shapes and structures.16 Therefore, some nanocatalysts with ideal patterns such as nanocubes,17 nanowires18 and nanoflowers19,20 until now have been proposed for maximizing the utilization efficiency and electrocatalytic performances. Among multitudinous configurations, the hollow nanoflower structure, with the characteristics of much higher surface-to-volume ratio but lower material density as compared to those of bulk materials, has become more popular in both scientific research and industrial production in the past decades.21−23 In addition to the morphology influence, the compositions of catalysts are also important for affecting final electrocatalytic performances. Alloying Au with other transition metals such as Ag24−26 and Cu27 has been demonstrated to be an efficient approach to assist in enhancing electrocatalytic performances through synergistic and electronic effects. Besides, as an ancillary ligand, Ag can also promote intermediate aldehyde oxidation and cleavage of C−C bonds of alcohol species. Specifically, apart from Ag, the addition of Cu can greatly improve the electrocatalytic performance of Au,28 which can assist in regenerating some new surface active sites of the

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EXPERIMENTAL SECTION

Preparation of Porosity Hollow Au−Ag−Cu Nanoflowers. In a typical synthesis, 300 mg of ascorbic acid (AA) and 30 mg of polyvinylpyrrolidone (PVP) were dissolved into 8 mL of EG in a 25 mL volumetric flask. After complete dissolution of PVP and AA, the volumetric flask was then heated to 85 °C in an oil bath. After that, 2 mL of solution containing 3.2 mg of CuSO4 was dropped into the homogeneous solution. After continuous reaction for 30 min, 2 mL of AgNO3 solution (10 mM) was dropped into the above solution with rapid stirring. After reacting for another 10 min, we then added 4 mL of KBr (250 mg) solution to the above solution and kept consecutive reaction in this condition for 1 h. Subsequently, 0.82 mL of HAuCl4 10491

DOI: 10.1021/acssuschemeng.7b02491 ACS Sustainable Chem. Eng. 2017, 5, 10490−10498

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Figure 2. HAADF-STEM images and typical EDS mapping images of hollow AuAg1Cu1 nanoflowers.

Figure 3. Representative TEM images of (A and C) AuAg1Cu2 and (B and D) AuAg2Cu1 with different magnifications. (24.3 mM) solution was dropped into the homogeneous mixture to react for another 90 min under this condition. The as-prepared nanocatalyst was denoted as AuAg1Cu1. For making a detailed comparison, the other two types of AuAgCu nanocatalysts with different compositions were also prepared via adjusting the amount of CuSO4 and AgNO3. In addition, the AuAg and AuCu nanocrystals were also prepared for further comparison. Characterizations of Au−Ag−Cu HNFs. The morphological and structural features of the as-prepared products were analyzed via a TECNAI-G20 electron microscope. High-magnification transmission electron microscopy and EDS mapping were performed via TECNAIF20 electron microscope. For studying the structure and crystal properties of the products, the XRD analysis technique was also been employed. In addition, X-ray photoelectron spectroscopy (XPS) was conducted to investigate compositions and elemental valences. Electrochemical Measurements. In this work, all the electrochemical measurements were operated in the CHI 760E electrochemical workstation. For the electrochemical measurements, the

glassy carbon electrode (GCE) needed to be polished with alumina powder and then rinsed with double deionized water and ethanol. Subsequently, 10 μL of electrocatalysts solution was added to the surface of the prepolished GCE, with drying at room temperature; the mass of Au on the GCE was calculated to be 3.9 μg. In addition, 3 μL of Nafion (0.05%) was also attached to the surface of catalysts inks and then dried before measurements to avoid the loss of catalysts during the reaction. After that, the modified electrodes were used to perform the cyclic voltammetry (CV) for further evaluating their electrocatalytic performances.

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RESULTS AND DISCUSSION Physicochemical Characterizations of Au−Ag−Cu HNFs. As is mentioned, the typical Au−Ag−Cu HNFs were fabricated via a combined seed mediated and galvanic replacement method at milder conditions. The morphological features were first evaluated by TEM. As is mentioned above, 10492

DOI: 10.1021/acssuschemeng.7b02491 ACS Sustainable Chem. Eng. 2017, 5, 10490−10498

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ACS Sustainable Chemistry & Engineering the typical Au-Ag-Cu HNFs were fabricated via a combined seed mediated and galvanic replacement method at milder condition. The morphological features were firstly evaluated by TEM. Figure 1 displays the unique AuAg1Cu1 hollow nanoflower structure with a rich hump on the surface. After detailed observation, we find that the surface of HNFs assembled orderly with some regular granules, which were favorable for providing some nanochannels31 to greatly increase the specific surface active areas and leading to the great enhancements of electrocatalytic performances.32 For further revealing the structure features, the HAADFSTEM images are discretely collected in Figure 2, in which the hollow and flower-like structure could be clearly observed. In addition, EDS element mapping on single Au−Ag−Cu HNFs shown in Figure 2 indicated that Au, Ag and Cu atoms overlapped throughout the whole hollow nanoflower. More interestingly, from the overlapping element mapping of Au, Ag and Cu, both the Au and Ag atoms are well-dispersed on the wall of the nanoflower. For understanding the influences of atomic ratio on the morphology of such Au−Ag−Cu HNFs, the TEM images of AuAg1Cu2 and AuAg2Cu1 have also been recorded. As it can be vividly observed in Figure 3, same as Au−Ag−Cu HNFs, both the AuAg1Cu2 and AuAg2Cu1 nanocrystals displayed the typical hollow nanoflowers, confirming such an approach can be well applied to synthesize porous Au−Ag−Cu HNFs with high yield regardless of the variations of atomic ratio.33−35 Apart from these, we also made investigations of the TEM images of AuCu and AuAg nanocrystals synthesized under the same conditions but without the addition of AgNO3 and CuSO4, respectively. As can be observed in Figure S1A,B, the reaction without the addition of AgNO3 formed a large amount of irregular AuCu nanoparticles with various sizes. The reaction in the absence of CuSO4 established some hollow nanostructures with anomalistic shapes. By virtue of these analyses, both AgNO3 and CuSO4 played significant roles in controlling formation of hollow nanoflower structures in this work.36 In general, since the standard reduction potential of standard reduction potentials of Cu2+/Cu (0.3419 V vs SHE) is lower than those of Ag+/Ag (0.7991 V vs SHE) and AuCl4−/Au (0.99 V vs SHE), it is reasonable to postulate that the galvanic displacement reaction contributes to the formation of hollow nanoflower structure by serving the above Cu seeds as template.37,38 Therefore, in this work, the dominate reaction is the galvanic replacements among Cu seeds, AgNO3 and HAuCl4, which may account for the formation of such hollow Au−Ag−Cu nanoflowers.39 On the purpose of characterizing the phase structure of these unique Au−Ag−Cu HNFs, X-ray diffraction (XRD) has also been employed. From the XRD patterns in Figure 4, we can observe the typical face centered cubic (fcc) crystalline Au.40 More interestingly, after comparing with the pure Au, we found that the diffraction peak of Au−Ag−Cu HNFs shifted slightly to a higher degree. In addition, after a detailed observation, we found that the position of the Au−Ag−Cu HNFs diffraction peak located between the AuAg and AuCu, which can be ascribed to the formation of alloy phase in such AuAgCu HNFs.41 We have also employed the XPS analysis technique to further investigate the surface compositions and chemical valence states of Au, Ag and Cu as well as their electronic coupling effect in the AuAgCu HNFs. As illustrated in Figure 5A, the peaks located at the binding energies (BEs) around 85, 285, 370 and

Figure 4. XRD patterns of AuCu, AuAg, AuAgCu and pure Au.

950 eV are assigned to Au 4f, C 1s, Ag 3d and Cu 2p, respectively.42,43 From Figure 5B,C, we find that both the elementary valences of Au and Ag are the predominate state in the AuAgCu HNFs products. Besides, compared with pure Au, the BEs of Au 4f in AuAgCu HNFs exhibited an obvious shift to a higher BE, indicating the charge transfer occurred among Au, Ag and Cu.29 From Figure 5D, we observe that the XPS of Cu 2p could also be well fitted into two sets of peaks. The one set of peaks at 932.92 and 952.85 eV could be assigned to spin orbit states of zerovalent Cu 2p3/2 and 2p1/2, respectively. And the other set of weak peaks appeared at 934.96 and 955.18 eV could be assigned to the BEs of Cu (II) 2p3/2 and Cu (II) 2p1/2, respectively.44 In addition, the XPS of shaking up of the satellites in Cu 2p region was associated with its oxides, indicating that Cu on the surface of catalyst possessed high activity and could be easily oxidized,45 which is probably favorable for enhancing the performance of catalyst. Electrochemical Measurement. Considering such nanostructures, the electronic effect and synergistic effect among these three metals, they are thus highly expected to display outstandingly high electrocatalytic performances toward liquid fuel oxidation. In this aspect, we herein selected the EGOR as the model reaction system to study the electroatalytic performances of Au−Ag−Cu HNFs and further compared with AuAg, AuCu and pure Au nanocatalysts due to their advantages of nontoxicity, environmental benignity and reproducible.46 The EGOR was conducted in the solution of 1 M KOH + 1 M EG at the potential ranging from −0.9 to +0.3 V with the scanning rate of 50 mV/s. Figure 6A shows the typical CV curves of EGOR; as seen, the AuAgCu HNFs display superior catalytic activity of 5493.4 mA mg−1 Au , 5.9, 2.2 and 1.5 times higher than that of pure Au (929.1 mA mg−1 Au ), −1 AuCu (2487.4 mA mg−1 ) and AuAg (3704.3 mA mg Au Au ), respectively. For further demonstrating its superior electrocatalytic activity, we also made a comparison with commercial Pt/C and Pd/C, as displayed in Figure S2; the as-obtained AuAgCu HNFs could also display 6.4- and 5.6-fold enhancements than those of commercial Pd/C (852.8 mA mg−1 Pd ) and Pt/C (978.4 mA mg−1 Pt ), respectively. In addition, we also investigated their long-term durability, for which is a significant parameter to evaluate the properties of the electrocatalysts. Figure 6B,C show the CA curves of these four types of electrocatalysts at the fixed potential of −0.2 V for 3600 s and successive CVs of 500 cycles, respectively. As shown, the Au− Ag−Cu HNFs exhibited the slowest current decay over time and highest retained mass activity, demonstrating its superior durability and confirming the excellent electrocatalytic perform10493

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Figure 5. XPS spectra of (A) survey scan, (B) Au 4f, (C) Ag 3d and (D) Cu 2p in the AuAgCu HNFs.

Figure 6. (A) CV curves and (B) CA curves of pure Au, AuCu, AuAg and AuAgCu operated in the solution containing 1 M KOH and 1 M EG with scanning rate of 50 mV/s. (C) Durability comparison of pure Au, AuCu, AuAg and AuAgCu toward EGOR for successive CV of 500 cycles. (D) Retained mass activity and normalized current densities of these catalysts after 500 cycles.

ances. The histogram in Figure 6D showed the mass activity and normalized current density (The ratio of current density to initial current) of different electrocatalysts toward EGOR after

successive CVs of 500 cycles. As shown, the Au−Ag−Cu HNFs displayed the best electrocatalytic performances, whose retained mass activity and normalized current densities after successive 10494

DOI: 10.1021/acssuschemeng.7b02491 ACS Sustainable Chem. Eng. 2017, 5, 10490−10498

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Figure 7. (A) CV curves and (B) CA curves of AuAg1Cu1, AuAg1Cu2 and AuAg2Cu1 electrocatalysts operated in the solution containing 1 M KOH and 1 M EG with the scanning rate of 50 mV s−1. (C) Durability comparison of AuAg1Cu1, AuAg1Cu2 and AuAg2Cu2 electrocatalysts toward EGOR for successive CVs of 500 cycles. (D) Retained mass activity and normalized current densities of these catalysts after 500 cycles.

Figure 8. (A) Nyquist plots of EGOR on pure Au, AuCu, AuAg and AuAgCu catalysts at −0.3 V, and (B) Nyquist plots of EGOR on AuAg1Cu1, AuAg2Cu1 and AuAg1Cu2 electrocatalysts operated at −0.3 V.

CVs of 500 cycles were 2041 mA mg−1 Au and 36.8%, respectively, showing greater enhancements than those of pure Au (89 mA −1 mg−1 Au , 7.7%), AuCu (381 mA mgAu , 15.1%) and AuAg (740 mA −1 mgAu , 21.2%). All of these results from the CV and CA measurements further demonstrated that such Au−Ag−Cu HNFs possessed both greatly improved catalytic activity and durability toward EGOR.47 For the sake of studying the influences Au/Ag/Cu atomic ratio on the electrocatalytic performances, the CV for the other two types of AuAg1Cu2 and AuAg2Cu1 electrocatalysts have

also been evaluated in the solution containing 1 M KOH and 1 M EG. From Figure 7A, the mass activity of AuAg1Cu1 was calculated to be 5493.4 mA mg−1 Au , which was much higher than those of AuAg1Cu2 (3770.1 mA mg−1 Au ) and AuAg2Cu1 (4456.2 mA mg−1 Au ), indicating that when set the atomic ratio of Au:Ag:Cu for 1:1:1 could obtain the Au−Ag−Cu HNFs with the best electrocatalytic activity.48 Apart from the electrocatalytic activity, we also investigated the influences of atomic ratio for long-term durability. In Figure 7B, we see that the maximum peak current density and the relative value of the 10495

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technology, helping relieve the energy crisis and environmental pollution.

decay for the AuAg1Cu1 catalyst were much lower than that of AuAg2Cu1 and AuAg1Cu2 catalysts, indicating the less decay and more stable properties when operated at the applied constant potentials on the AuAg1Cu1 catalyst modified electrode.49−51 Figure 7C,D show the successive CVs of 500 cycles and their retained mass activities of AuAgCu nanocatalysts with different compositions. Same as the CA results, the AuAg1Cu1 catalyst also displayed superior mass activity and normalized current density of 2041 mA mg−1 Au and 36.8%, respectively, much higher than those of AuAg1Cu2 (1287 mA −1 mg−1 Au , 28.1%) and AuAg2Cu1 (880 mA mgAu , 23.2%). Based upon above evaluations, AuAg1Cu1 HNFs possessed the superior electrocatalytic performances toward EGOR as compared to the other catalysts, which could serve as highperformance electrocatalysts for DEGFCs.52,53 For making a detailed understanding on the reaction processes, we herein conduct the electrochemical impedance spectroscopy (EIS) for all the catalysts.54,55 Remarkably, the diameter impedance arc (DIA) in Figure 8A obeyed the following order: Au > AuCu > AuAg > AuAg 1 Cu 1 , demonstrating that this Au−Ag−Cu HNFs possessed the superior electrical conductivity, further confirming their outstanding electrocatalytic performances. Figure 8B displays the Nyquist plots of EGOR on Au−Ag−Cu HNFs modified electrodes with different compositions. Apparently, as shown with Figure 8A, the AuAg1Cu1 also exhibited the smallest DIA, further indicating its better electrical conductivity. All of these results forcefully demonstrated the as-prepared Au−Ag−Cu HNFs could serve as the ideal electrocatalysts toward EGOR and beyond. In general, the as-prepared porous AuAgCu HNFs exhibit unprecedentedly excellent catalytic activity and durability toward EGOR, which can be ascribed to (1) such hollow nanoflower structure with much higher surface-to-volume ratio, but lower material density makes porous AuAgCu HNFs a desirable electrocatalysts for the fuel cells and beyond.56 (2) The introductions of Ag and Cu are conductive to activating the surface active sites of Au. (3) The alloy and electronic role among these three metals are benefiting for greatly improving the catalytic performances.57

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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/acssuschemeng.7b02491. Typical TEM images of AuCu and AuAg nanocrystals with different magnifications, CV of commercial Pd/C and Pt/C operated in 1 M KOH + 1 M EG solution, calculated mass activities of commercial Pd/C and Pt/C toward EGOR (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-512-65880089, Fax: 86-512-65880089; E-mail: duyk@ suda.edu.cn (Y. Du). ORCID

Yukou Du: 0000-0002-9161-1821 Ping Yang: 0000-0003-2420-7658 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 (Grant No. 51373111), the Suzhou Industry (SYG201636), the project of scientific and technologic infrastructure of Suzhou (SZS201708), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials.

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CONCLUSIONS To summarize, a combined seed mediated and galvanic replacement method has been developed to successfully prepare the ternary Au−Ag−Cu nanoparticles with fascinating hollow nanoflowers structure. Owing to the abundant surface active areas, high porosity and self-supportability features of the hollow nanoflower structure, the resultant ternary Au−Ag−Cu HNFs are highly expected to supply more surface active sites and efficient charge mass transfer and thus exhibit enormous potential in the application of electrocatalysis. A battery of electrochemical analyses have also revealed that the captivating Au−Ag−Cu HNFs possessed unprecedentedly high catalytic activity toward EGOR with mass activity of 5493.4 mA mg−1 Au , 5.9 times higher than that of Au. Additionally, the stability measurements have also demonstrated amazing durability with the highest normalized current density of 36.8% after successive CVs of 500 cycles. All of these favorable terms have indicated that the resulted Au−Ag−Cu HNFs possessed the promising prospect for the application in DEGFCs, and the synthesized strategy and the nanomaterials developed in this work would assist in boosting the development of sustainable energy 10496

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DOI: 10.1021/acssuschemeng.7b02491 ACS Sustainable Chem. Eng. 2017, 5, 10490−10498