Ultrathin PtxSn1–x Nanowires for Methanol and Ethanol Oxidation

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Ultrathin PtxSn1−x Nanowires for Methanol and Ethanol Oxidation Reactions: Tuning Performance by Varying Chemical Composition Luyao Li,† Haiqing Liu,† Chao Qin,† Zhixiu Liang,† Alexis Scida,† Shiyu Yue,† Xiao Tong,‡ Radoslav R. Adzic,§ and Stanislaus S. Wong*,† †

Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, United States Center for Functional Nanomaterials, Brookhaven National Laboratory, Building 735, Upton, New York 11973, United States § Chemistry Department, Brookhaven National Laboratory, Building 555, Upton, New York 11973, United States ‡

S Supporting Information *

ABSTRACT: Pt-based alloys denote promising catalysts for the methanol oxidation reaction (MOR) and the ethanol oxidation reaction (EOR), due to their enhanced activity toward alcohol-oxidation reactions and reduced cost as compared with Pt alone. Among all of these binary systems, PtSn has been reported to exhibit superior methanol/ethanol oxidation activity. In this study, we deliberatively tailor chemical composition, reduce size, and optimize morphology of the catalyst in an effort to understand structure−property correlations that can be used to improve upon the electrocatalytic activity of these systems. Previous work performed by our group suggested that Pt-based catalysts, possessing an ultrathin one-dimensional (1D) structure, dramatically promote both cathodic and anodic reactions with respect to their zerodimensional (0D) counterparts. Herein, a novel set of ultrathin binary Pt−Sn 1D nanowire (NW) catalysts with rationally controlled chemical compositions, i.e., Pt9Sn1, Pt8Sn2, and Pt7Sn3, has been synthesized using a facile, roomtemperature, wet-solution-based method. The crystallinity and chemical composition of these as-prepared samples were initially characterized using XRD, XPS, and EDX. Results revealed that this synthetic protocol could successfully generate PtSn alloys with purposely tunable chemical compositions. TEM and HRTEM verified the structural integrity of our ultrathin 1D NW morphology for our Pt9Sn1, Pt8Sn2, and Pt7Sn3 samples. The effects of varying Sn content within these alloy samples toward the electro-oxidation reaction of methanol and ethanol were probed using cyclic voltammetry (CV) in acidic media. Within this series, we find that the optimized chemical composition for both the MOR and the EOR is Pt7Sn3. KEYWORDS: fuel cell, PtSn, ultrathin nanowires, methanol oxidation reaction, ethanol oxidation reaction, electrocatalysis, alloys

1. INTRODUCTION Precious noble metals, especially Pt, have been touted as one of the best classes of anodic electrocatalysts available, due to their high intrinsic catalytic activity and durability.1 Nonetheless, though direct methanol fuel cells (DMFCs) and direct ethanol fuel cells (DEFCs) represent a promising fuel cell option for future applications in the automotive, portable power, and electronics industries, much effort still needs to be expended in terms of overcoming the high production costs and the relatively low efficiencies of these systems, which can be traced in large measure to the fact that typically used elements such as Pt tend to be both scarce and expensive. Moreover, these Pt-based electrocatalysts possess poor reaction kinetics and preferentially form partially oxidized products, such as acetic acid and acetaldehyde, as opposed to CO2. In addition, typical electrocatalysts lack the stability and durability required for long-term fuel cell applications, due to CO poisoning, metal dissolution, and surface oxidation.2 To mitigate for all of these deficiencies, over the years, we have explored the idea of synergistically and simultaneously © XXXX American Chemical Society

combining the favorable attributes of (a) a one-dimensional (1D) morphology (versus zero-dimensional (0D) nanoparticles (NPs)), (b) an ultrathin size, and (c) alloy formation as independent physicochemical “knobs” in order to “tune” methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) activity metrics and kinetics.3 As precedence for the appropriateness of our approach on comparable systems, it is worth noting as an example that our as-prepared Pt ∼ Pd1−xCux ultrathin core-shell nanowires4 consistently outperformed not only commercial Pt NPs but also ultrathin Pt nanowires by several-fold factors of improvement for both the MOR and EOR reactions in alkaline media. Moreover, MOR and EOR performance with the chemical composition of our ultrathin Pt ∼ Pd1−xCux nanowires correlated with and appeared to rise with increasing Cu content. We proceed to systematically justify our Received: December 3, 2017 Accepted: February 9, 2018

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DOI: 10.1021/acsanm.7b00289 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials emphasis on carefully tuning and tailoring parameters (a−c), as follows. Why a 1D morphology? Crystalline 1D nanostructures possess (i) high aspect ratios, (ii) fewer lattice boundaries, and (iii) a high number of low-coordination sites, all of which are desirable design attributes of fuel cell catalysts.5 Moreover, the surfaces of 1D morphologies can be tuned so as to preferentially display different crystal facets. Anisotropic nanostructures such as Pt nanowires (NWs) maintain a favorable downshift in the Pt dband, which has been noted both spectroscopically as well as by means of first-principles calculations. The downshift in the Pt dband contributes to a weaker d−π* interaction with the adsorbed CO, thereby improving Pt’s ability to oxidize adsorbed CO at potentials closer to the thermodynamic potential for MOR, for instance. Our group and several others have extensively studied the morphology-dependent performance in the context of Ptbased catalysts for MOR. As a relevant example,6 we found that 50 nm Pt NWs maintain a nearly 3-fold higher current density after onset as compared with Pt NP/C. In addition to enhanced MOR kinetics, anisotropic nanostructures have also been widely demonstrated to be more stable toward degradation under realistic fuel cell conditions. For instance, we have shown that ultrathin Pt∼Pd core∼shell NWs (i.e., diameter = 2 nm) undergo essentially no loss in mass activity over the course of an extended durability protocol of up to 30 000 electrochemical cycles (0.6− 1.0 V vs RHE) when employed as ORR electrocatalysts.7 By contrast, the corresponding activity of analogous Pt NP/C declined by as much as 40−50% over the same durability test duration. Why an ultrathin size? Ultrathin nanowires are expected to maintain slightly contracted surfaces, which can weaken the interaction with O2 and prevent passivation of the catalyst by O2. Moreover, ultrathin NWs may also be more chemically homogeneous and structurally monodisperse, and maintain fewer defect sites. Not surprisingly, we have found that ultrathin Pt NWs (i.e., diameters Pt8Sn2 NWs (0.58 mA/cm2) > Pt9Sn1 NWs (0.46 mA/cm2) > Pt NWs (0.06 mA/cm2). These collective results corroborate findings from Gharibi et al., who analyzed the MOR activity of analogous PtSn composites possessing different Pt/Sn atomic ratios and found that Pt−Sn with a 70:30 composition yielded a much better performance as compared with either 85:15 Pt−Sn, 65:35 Pt−Sn, or pure Pt, denoting a set of data which was ultimately explained by the particularly high CO tolerance of Pt70Sn30 nanoparticles.37 Moreover, another analogous study performed by Liu et al. also concurred with our general observations in that they reported that the magnitude of the observed peak current density of methanol oxidation within a series of PtxSn1−x/C catalysts increased as the amount of Sn increased, following the order of Pt65Sn35/C > Pt80Sn20/C > Pt/C > Pt50Sn50/C. This trend was primarily attributed to the key underlying influence of the bifunctional mechanism.10 In support of this mechanistic explanation, a theoretical study of MOR on Pt3Sn(111) using a combination of periodic density functional theory and microkinetic modeling not only indicated that the Sn site is likely to be the most favored adsorption site for methanol and analogous derivatives, thereby highlighting the compositional significance of incorporating Sn within the alloy, but also substantiated the notion of a higher CO resistance for the Pt3Sn alloy itself as compared with Pt alone.62 The corresponding EOR process was studied on Pt7Sn3 NWs, Pt8Sn2 NWs, Pt9Sn1 NWs, and Pt NWs by cyclic voltammetry in acidic electrolyte at room temperature. Collected currents were normalized by the ECSA. Typical profiles are shown in Figure 6A. We detected similar behavior to that of MOR in that the onset potential (Figure 6B) also perceptibly decreased to lower values with increasing Sn content. The observed trend implied that EOR likely becomes easier after introducing Sn atoms within the Pt-based lattice,27,63 corroborating the relevance and relatively greater importance of the “bifunctional mechanism” in explaining the electrochemical behavior of our ultrathin PtSn NWs. The mechanism of EOR on Pt and PtSn has been explored independently. On Pt alone,64 the initial steps of ethanol oxidation start at around 0.3 V (vs RHE). At higher potentials (i.e., > 0.8 V vs RHE),65,66 acetaldehyde and acetic acid can form, whereas CO2 is produced at lower potentials (i.e., < 0.4 V vs RHE)34,47 and a slew of unwanted oxygenated species tend to be created at intermediate potentials (i.e., 0.6 V < E < 0.8 V vs RHE).31 By contrast, within Pt−Sn alloys, ethanol oxidation is dramatically accelerated, because tin and the corresponding tin oxides are able to provide for oxygenated species at lower potentials, which can subsequently react with CO poisoning intermediates not only more easily but also more rapidly. As demonstrated by our own CO-stripping profiles, the “CO oxidation” process on the PtSn surface occurs over a much larger potential range as compared with Pt alone, and as such, helps to

Figure 6. (A) Cyclic voltammograms for EOR, obtained in an Arsaturated 0.1 M HClO4 + 0.5 M CH3CH2OH solution, at a scan rate of 20 mV/s. (B) Magnification of the EOR onset region. The current has been normalized to the ECSA for Pt NWs (black), Pt9Sn1 NWs (orange), Pt8Sn2 NWs (blue), and Pt7Sn3 NWs (red). (C) Using the same color scheme, bar graph plots highlight EOR activity at 0.60 V (vs RHE) for Pt NWs, Pt9Sn1 NWs, Pt8Sn2 NWs, and Pt7Sn3 NWs, respectively. (D) Chronoamperometry measurements of ultrathin Pt NWs (black) and Pt7Sn3 NWs (red) in an Ar-saturated 0.1 M HClO4 + 0.5 M CH3CH2OH solution, obtained at a potential of 0.60 V (vs RHE) for a period of 60 min.

free up Pt sites for activity.12,64 Previous work performed by Massong et al.67 suggests that the desirable promoting effect induced by Sn within Pt-based alloys can be attributed to the possible formation of bridge-bonded CO (COB) on neighboring Pt active sites as opposed to linearly bonded CO (COL) alone. Not surprisingly, the Pt−Sn alloy yielded improved activity toward EOR with respect to pure Pt in the region of interest for fuel cell applications (i.e., 0.45−0.7 V vs RHE).68 Bar graphs highlighted in Figure 6C summarize the current density values of all of the samples at 0.60 V (vs RHE) and reveal the clear enhancement of the EOR catalytic activity upon Sn addition. Within our data, the EOR activity trend of all of the nanowires follows the order of Pt7Sn3 NWs (0.33 mA/cm2) > Pt8Sn2 NWs (0.24 mA/cm2) > Pt9Sn1 NWs (0.18 mA/cm2) > Pt NWs (0.08 mA/cm 2 ). This pattern is in an agreement with the corresponding trend in the onset potential. Specifically, we noted that our ultrathin Pt7Sn3 NWs possessed not only the lowest onset potential but also the highest current density of the entire series of samples fabricated. Jiang et al. recently published a report that correlated Pt/Sn ratios with the resulting activity of alloys toward EOR and found that the optimized Pt/Sn ratio was also 7:3, denoting data consistent with our own results.69 Within our system, upon increasing Sn content from 0% to 30%, we find that both of the MOR and EOR activities of our ultrathin NWs follow a similar trend. In particular, these results relate to the improved CO oxidation ability and CO tolerance of our Pt−Sn alloys, as supported by our acquired CV and CO stripping profiles. The enhanced MOR and EOR activities of the PtSn can mainly be explained by a synergistic interplay of favorable and complementary phenomena, allowing for a more efficient and effective alcohol oxidation process, induced not only by the improved CO tolerance of the alloy emanating from the presence of Sn (i.e., the bifunctional effect) but also by the lower I

DOI: 10.1021/acsanm.7b00289 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

existence of tin oxides which can promote alcohol oxidation most likely through a bifunctional effect. The Pt 4f XPS spectra also were consistent with an alteration of Pt’s intrinsic electronic structure. CO-stripping profiles were obtained on our samples, prior to proceeding with our MOR/EOR tests. In effect, our ultrathin alloy nanowires yielded a much lower onset potential as compared with a pure Pt sample alone, thereby highlighting the higher CO tolerance of our alloy samples in general. Both the MOR and EOR activities were measured within an acidic solution, and the catalytic activities of as-prepared ultrathin PtxSn1−x (0 ≤ x ≤ 0.3) nanowires were found to have been substantially enhanced with increasing Sn content. Coupled with complementary chronoamperometry data, Pt7Sn3 nanowires were found to have yielded the best activity and stability in terms of both MOR and EOR, especially as compared with control samples consisting of pure Pt NWs and a PtRu NP/C sample, with all of these observations likely attributable to a combination of both electronic and bifunctional effects. Given the inherent difficulty with oxidizing alcohols at lower temperatures, the use of higher temperatures may reveal even more attractive features of these catalysts, since these conditions can promote C−C bond scission and thereby potentially not only increase observed current and power densities but also enhance electrode kinetics.54 Overall, our as-prepared motif incorporating an ultrathin, one-dimensional (1D) morphology with controlled chemical composition represents a potentially generalizable architectural paradigm toward the creation of either Pt-based or Pt-free electrocatalysts, that are not only inexpensive and effective but also practical for incorporation within fuel cell configurations that transcend DMFCs and DEFCs.

d-band energy of Pt atoms responsible for weakening of the adsorption of CO intermediate species (i.e., the electronic effect).50 Furthermore, the electrochemical behavior of our nanocomposites was strongly affected by parameters such as the precise synthesis method involved, the degree of alloying, as well as the nature of the underlying substrate. Moreover, in addition to the bifunctional and electronic effects described above, the ultrathin size coupled with the 1-D nature of our samples also contributed to the enhanced performance of our samples as compared with standard Pt controls. Previous work performed by our group26 reported that PtRu NPs/C (electrochemically processed using exactly the same reaction parameters and conditions) possess a much lower (0.051 mA/cm2) MOR activity with respect to our ultrathin PtSn nanowires. In fact, this set of the PtRu NP/C controls achieved comparable current densities with respect to the commercial Pt NP/C sample itself.70 Stability tests were only performed on Pt7Sn3 NWs, since activity measurements had revealed that this particular chemical composition yielded the best MOR and EOR activity performance. Hence, with respect to MOR stability, the test was carried out at a lower potential reading of 0.5 V (vs RHE), which is more commonly accepted by the community. From Figure 5D, we deduced that the ultrathin Pt7Sn3 alloy nanowires achieved much higher steady state current densities at a fixed potential of 0.5 V (vs RHE) over the time range of 60 min, as compared with pure Pt ultrathin nanowires used as controls. Nonetheless, it should be noted that we had assessed MOR stability at the higher potential of 0.65 V (vs RHE), and we found these results (Figure S7) to be consistent with our findings at 0.5 V (vs RHE). Again, our data corroborate the idea that the Pt7Sn3 alloy NW catalyst maintains both greater MOR activity and stability as compared with Pt alone. By analogy with the MOR stability test, the corresponding EOR stability test was also performed on the highest performing Pt7Sn3 NWs. We have evaluated the stability test results of our Pt7Sn3 NWs and Pt NWs in Figure 6D. We have found that the ultrathin Pt7Sn3 alloy nanowires maintained higher steady state current densities at a fixed potential of 0.6 V (vs RHE) over the entire time range of 60 min, as compared with pure ultrathin Pt nanowires, designated as the control sample. Combining these data with our previous catalytic activity results, it is reasonable to claim that the PtSn alloy catalyst possesses not only higher EOR activity but also better stability as compared with Pt itself.

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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/acsanm.7b00289. XRD, EDX, electrochemical, and XPS data as well as representative TEM images on our samples in addition to a summary table of Pt6Sn4 and Pt5Sn5 nanoparticles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. ORCID

4. CONCLUSIONS A series of ultrathin Ptx-Sn1−x nanowires with various controlled chemical compositions has been synthesized using a facile, ambient, room-temperature method. The chemical composition and structural morphology of our samples have been examined using both XRD and TEM. To a large extent, the stoichiometry of these NWs could be directly controlled by modifying the corresponding stoichiometric ratio of the metallic precursors within the precursor solution. The final, as-obtained Pt/Sn atomic ratios were verified by using EDX analysis. However, as with both our Pd−Ni and Pd−Cu systems, we noted that the higher the quantity (>30%) of more oxophilic atoms (i.e., Sn) present within our reaction environment, the more difficult the resulting nanowire formation happened to be. Hence, ultrathin nanowires can only be obtained when the Sn content is within the range from 0% to 30%, since Pt6Sn4 and Pt5Sn5 yielded mainly nanoparticles. Although no obvious metallic tin or tin oxide peak was observed from the XRD spectra, the corresponding XPS spectra of Sn 3d, however, confirmed the

Luyao Li: 0000-0002-2922-0686 Stanislaus S. Wong: 0000-0001-7351-0739 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research for all authors was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. Experiments for this manuscript were performed in part at the Center for Functional Nanomaterials located at Brookhaven National Laboratory, which is supported by the U.S. Department of Energy under Contract No. DE-SC-00112704.

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DOI: 10.1021/acsanm.7b00289 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsanm.7b00289 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX