Combining the Advantages of Hollow and One-Dimensional

Publication Date (Web): March 5, 2019. Copyright © 2019 American Chemical Society. *E-mail: [email protected]., *E-mail: Wang.Yuesheng@ireq ca...
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Combining the Advantages of Hollow and One-Dimensional Structures: Balanced Activity and Stability toward Methanol Oxidation Based on the Interface of PtCo Nanochains Yunwei Liu,† Zelin Chen,† Chang Liu,† Jinfeng Zhang,† Xiaopeng Han,† Cheng Zhong,† Dewei Rao,‡ Yuesheng Wang,*,§ Wenbin Hu,† and Yida Deng*,†

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†

Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300372, People’s Republic of China ‡ School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, People’s Republic of China § Center of Excellence in Transportation Electrification and Energy Storage, Hydro-Québec, 1806 boulevard Lionel-boulet, Varennes, Quebec J3X 1S1, Canada S Supporting Information *

ABSTRACT: Active and durable electrocatalysts for methanol oxidation reaction (MOR) are of great importance to the practical application of direct methanol fuel cell technology. Although tremendous efforts have been devoted to optimize the electrocatalysts, these electrocatalysts still fall far short of expectation and suffer from rapid activity loss. Herein, we report a new strategy for designing PtCo nanochains as a methanol oxidation electrocatalyst with high activity and excellent stability. The obtained PtCo nanochains consist of a string of hollow spheres that forms a one-dimensional chain structure. We find that having a defect-rich interface between adjacent hollow spheres is responsible for high catalytic performance and stability. With this special morphology, the PtCo nanochains exhibit exceptional activity (3.73 A·mg−1Pt) and stability (90.8% of initial current retained after 1500 cycles) for methanol oxidation. These findings will provide a new insight in designing and synthesizing high-performance electrocatalysts. KEYWORDS: methanol oxidation reaction, methanol fuel cell, electrocatalyst, hollow spheres, defect-rich interface

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the stability of the catalysts for MOR, one-dimensional (1D) structures have been proposed for having desirable properties such as having fewer lattice boundaries, higher rate of electron and mass transport, being less vulnerable to dissolution, Ostward ripening, and aggregation.13 Both theoretical and experimental works proved that the change of the crystal structure caused by the defects can undoubtedly break the electron−hole symmetry, leading to a different catalytic property.14,15 Previous literature works have reported that the defect makes a big difference of the electronic structure, resulting in superior catalytic performance.16−18 Inspired by those findings, we design a facile one-pot synthesis strategy to assemble PtCo hollow nanochains (PtCo-HNC) with rich interfaces. The obtained PtCo-HNC is assembled from a single PtCo hollow sphere (PtCo-HNS) with lots of interfaces between adjacent hollow spheres. Meanwhile, the

aving both high energy utilization efficiency and low pollutant emission,1−4 direct methanol fuel cells (DMFC) have a promising future for industrial applications. Despite tremendous efforts devoted to explore efficient catalysts for methanol oxidation, the DMFC technology is still far from being commercialized due to the lack of highly efficient and stable electrocatalysts. Pt is considered as the most promising electrocatalyst for DMFCs. However, problems due to its sluggish kinetics, high cost, instability, and easy poisoning during operation5 severely hinder the practical application of Pt electrocatalyst. Numerous efforts were made to improve the catalytic activity and stability to deal with these problems. One of the effective strategies is to alloy Pt with transition metals, for example, Fe,6 Co,7 Ni,8 Mo,9 and Cu,10 etc. It has been reported that the synergistic effect of alloys can efficiently weaken the binding of CO on Pt, 11 resulting in an improved catalytic performance. Controlling the catalyst morphology and structure is another approach to enhance catalytic performance, for example, hollow structures.12 Although the catalytic activities of these hollow structures are high, their stability is low. To improve © XXXX American Chemical Society

Received: December 7, 2018 Accepted: March 5, 2019 Published: March 5, 2019 A

DOI: 10.1021/acsaem.8b02137 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

the nanochain in Figure S3 indicates that the Pt loading on the surface is much higher than that in the core. The elemental mapping and line scan for PtCo-HNS were also carried out. As shown in Figure S4, the distributions of Pt and Co together with the line-scan result for a single nanosphere indicates that both PtCo-HNC and PtCo-HNS have the same element distribution. X-ray diffraction (XRD) characterizations were employed to obtain the crystallographic information on the different catalysts (Figure S5). Four broad peaks emerge at approximately 40.21°, 46.80°, 68.49°, and 82.51° and can be assigned to the (111), (200), (220), and (311) of fcc Pt planes (JCPDS No. 04-0802). Notably, the diffraction peaks were slightly shifted to a higher 2θ direction compared with that of pure Pt, indicating a decrease in lattice parameter caused by the alloying effect of Pt atoms and smaller Co atoms, which is consistent with Mathe’s work.19 The composition of the asprepared PtCo samples was found to be close to Pt3Co (JCPDS No. 29-0499). This is consistent with the inductively coupled plasma−mass spectrometry (ICP-MS) results (2.4:1, 2.8:1, and 2.9:1 for the Pt:Co atomic ratios of the PtCo-HNC, PtCo-HNC + HNS, and PtCo-HNS catalysts, respectively). Surface compositions of the as-synthesized catalysts were investigated by X-ray photoelectron spectra (XPS). According to the XPS results, the surface Pt loading of PtCo-HNS, PtCoHNC + HNS, and PtCo-HNC samples were 94.69%, 94.13%, and 93.65%, respectively. As shown in Figure S6, the Pt 4f core-level binding for all catalysts shifted to higher binding energies, which also appeared in previous studies.6,19 Comparing with electronic core-level binding energy of the metallic Pt 4f7/2 peak at 71.20 eV, the upshift magnitudes of Pt 4f7/2 binding energy are 0.50, 0.35, and 0.17 eV for PtCoHNC, PtCo-HNC + HNC, and PtCo-HNS, respectively. The MOR activity of as-prepared samples and commercial Pt/C (20%) was evaluated in alkaline media. Figure 2a illustrates the typical cyclic voltammogram (CV) profiles of PtCo catalysts and commercial Pt/C. From its integrated hydrogen desorption charge in the positive-going potential scan (−0.96 to − 0.60 V vs saturated silver chloride electrode), the specific electrochemical surface areas (ECSAs) of the catalysts are calculated. The ECSA values of PtCo-HNC, PtCo-HNC + HNS, PtCo-HNS, and commercial Pt/C are 49.45, 48.04, 38.78, and 59.56 m2·g−1, respectively (Table 1). Among the obtained catalysts, PtCo-HNC has the largest ECSA value which is also one of the largest reported in literature for Pt based materials.20−22 Figure 2b shows typical CV profiles of methanol oxidation. The specific activity and mass activity of the catalysts are calculated by dividing the peak current with ECSA and Pt mass (Table 1). Among all the electrocatalysts examined, PtCoHNC sample exhibits the highest mass activity of 3.73 A· mg−1Pt, which is larger than that of PtCo-HNC + HNS (3.06 A·mg−1Pt), 1.6 times that of PtCo-HNS (2.30 A·mg−1Pt), and 3 times as much as that of commercial Pt/C (1.24 A·mg−1Pt). To our knowledge, 3.73 A·mg−1Pt is one of the largest values reported in literature for Pt-based materials.23−25 PtCo-HNS sample has the smallest ECSA and mass activity value among the three samples. Structurally, hollow spheres should possess a higher specific surface area. But according to experimental results, the PtCo-HNS did not exhibit the highest activity. Through various analyses including XRD, XPS, line scan, and EDS mapping, we excluded other factors (phase, surface composition, and element distribution) that can affect the

PtCo-HNC combines the advantages of having both hollow and 1D structure. This unique structure possesses high specific surface area and high utilization efficiency of Pt as well as the excellent long-term durability in MOR electrocatalysis. After excluding the factors that can affect the activity and stability, we attribute the enhancement performance of PtCo nanochains to the appearance of the rich interfaces. The formation of the PtCo nanochains electrocatalyst with defect-rich interface was primarily driven by a galvanic replacement reaction between Co nanospheres and H2PtCl6 in an aqueous solution. As a capping agent and stabilizer, the amount of citric acid was found to be responsible for morphology variation. By adjusting the amount of citrate acid solution, PtCo-HNC, PtCo-HNC + HNS, and PtCo-HNS catalysts were obtained. If the citrate acid was absent, products of net-like nanoparticles were obtained (see Supporting Information Figure S1). The morphologies were observed by transmission electron microscopic (TEM). Low-magnification TEM images (Figure 1a,c) indicate that the as-prepared PtCo-HNS and PtCo-HNC,

Figure 1. (a, c) Low magnification and (b, d) high-resolution TEM (HRTEM) images of the PtCo-HNC and PtCo-HNS, respectively. (e−h) High-angle annular dark-field scanning TEM (HAADFSTEM) image and EDS elemental mapping of the Pt−Co bimetallic hollow nanochains.

with a length of several micrometers, could be synthesized in high yield. Figure S2 showed that the PtCo-HNC + HNS sample is a mixture of nanospheres and nanochains. The highresolution TEM (HRTEM) images (Figure 1b,d) revealed that the lattice spacing corresponds to the face-centered cubic (fcc) (111) and (200) planes of Pt. Moreover, an evident lattice contraction (0.217 nm) was observed compared to that of pure Pt (0.226 nm, PDF card no. 04-0802), indicating an alloy effect between Pt and Co atoms.3,17 The PtCo-HNC was also visualized by high-angle annular dark-field scanning TEM (HAADF-STEM) (Figure 1e−h). The elemental mapping revealed that Co and Pt were distributed throughout the whole chain-like structure. The line scan across a single nanosphere in B

DOI: 10.1021/acsaem.8b02137 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 2. (a) CV curves of the catalysts in 1.0 M N2-saturated KOH and (b) CV curves of the catalysts in 1 M methanol + 1 M KOH. (c) Electrochemical impedance spectroscopy of methanol oxidation in a solution containing 1.0 M methanol and 1.0 M KOH with PtCo-HNS, PtCoHNC + HNS, PtCo-HNC, and commercial Pt/C at a potential of −1.20 V vs Ag/AgCl. (d) CV curves of the catalysts before (solid) and after (dotted) cycles. (e) Histogram of the peek current before (red) and after (black) cycles and (f) variation of the electro-oxidation peak current of MOR in the forward scan with the CV numbers for all catalysts between −0.8 and 0.24 V in 1 M methanol + 1 M KOH solution at a scan rate of 50 mV s −1.

Therefore, one of the main aims of our study is to improve the durability of the PtCo-HNC catalyst. The durability of the prepared catalysts and commercial Pt/C toward methanol oxidation reaction was evaluated by multiple cyclic voltammetry scans between −0.8 and 0.24 V at a scan rate of 50 mV·s−1, 1500 cycles for PtCo-HNC and 1000 cycles for PtCo-HNC + HNS, PtCo-HNS, and commercial Pt/C. In general, the PtCo catalysts have a higher stability than commercial Pt/C (Figure 2d). After 1500 cycles, PtCo-HNC still retained 90.8% of its initial current (Figure 2e,f). Compared with other Pt based materials reported in literature,27−29 our catalysts exhibit outstanding stabilities. PtCo-HNC + HNS and PtCo-HNS retained 51.7% and 29.2% of their initial currents after 1000 cycles, respectively, and commercial Pt/C showed the worst stability as only 11.3% of its initial current was retained. The morphology and XPS date of all catalysts after electrochemical cycles were also examined (Figure S7, Figure S9). As shown, the structure of PtCo-HNC catalyst remained unchanged. In contrast, aggregation was observed for the PtCo-HNS catalyst. XPS results also indicated that PtCo-HNC is the most stable. For commercial Pt/C, as shown in Figure S8, the Pt particles were totally peeled off from carbon support after 1000 cycles. These findings suggest that the hollow nanochain structure is important for high catalytic stability. Figure 3a shows the CO-stripping voltammetry curves. For the commercial 20% Pt/C benchmark catalyst, one obvious oxidation peaks was observed in the first anodic scan at −0.3 V vs saturated sliver chloride and the onset potential of CO electroxidation is about −0.53 V. Alloying Pt with Co, as expected, promotes CO electroxidation, resulting in reduction of the onset potential. As shown in Figure 3a, all three PtCo samples showed a lower onset potential than commercial Pt/C at about −0.65, −0.62, and −0.60 V, respectively, for PtCoHNC, PtCo-HNC + HNS, and PtCo-HNS. Among the three

Table 1. Summary of Results for the Electrooxidation of Methanol Catalyzed by the PtCo Catalysts and the Commercial Pt/C Catalyst catalyst

ECSA (m2·g−1)

specific activity (A·m−2)

mass activity (A·mg−1Pt]

PtCo-HNS PtCo-HNC+HNS PtCo-HNC commercial Pt/C

38.78 48.04 49.45 59.56

57.46 62.68 73.21 20.88

2.30 3.06 3.73 1.24

activity and stability. Thus, we ascribed the enhancement performance due to structural difference at the interfaces. Electrochemical impedance spectroscopy (EIS) was carried out to investigate the interfacial processes and kinetics of electrode reactions. Figure 2c presents the Nyquist plots for MOR at −1.20 V (vs saturated sliver chloride) with catalysts in a solution containing 1.0 M KOH and 1.0 M methanol). The impedance data were analyzed by using IviumSoft with the equivalent circuits shown in the Figure 2c, where Rs stands for the series resistance, CPE is the double-layer capacitance, and Rct is the charge transfer resistance associated with MOR. The Rct value of PtCo-HNC is 17.08 Ω, much lower than that of commercial Pt/C (111.23 Ω) and other PtCo catalysts (22.87 Ω for PtCo-HNC + HNS; 43.45 Ω for PtCo-HNS). This result combining with previous research by others19,26 could also account for the high catalytic activity of PtCo-HNC in comparison to that of the other PtCo catalysts and commercial Pt/C. One problem plaguing the development of MOR electrocatalysts is poor long-term stability.23−25 Irrespective of initial activities, nearly all previous electrocatalysts suffer from quick activity loss and become largely inactive within several hundreds of seconds (Supporting Information Table 1). C

DOI: 10.1021/acsaem.8b02137 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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for MOR pathway on Pt3Co(111) is CH2OH* → CHOH*, for which the difference Gibbs free energy is 0.489 eV, which implies a theoretical overpotential (η) of 0.489 V. This value is much lower than the theoretical η of MOR on Pt(111) (0.925 V), which explains the improved electrochemical performance due to Co alloying. To determine the structure−activity relationship of PtCoHNC, we further carried out an in-depth study at the interface of the nanochain structure using HRTEM imaging (Figure 4). HRTEM revealed that the interface is rich in defects. Furthermore, comparing with the lattice spacing 0.226 nm (111) in the core, an interplanar spacing varying from 0.205 to 0.211 nm was found at the interface, indicating the structural difference between the interface and core. Figure S11 shows the longitudinal line scan throughout PtCo-HNC and a Pt/Co ratio curve. Based on the line-scan results Pt/Co ratio was smaller at the interface than the core for PtCo-HNC. The higher Co concentration accounts for the presence of the lattice disorder and defects at the interface. Taking the electrochemical results and structural findings together, the high catalytic activity and stability of the PtCo-HNC catalyst are due to the presence of defect-rich sites, formed by the high Co content, at the interface. In summary, we report a facile one-step synthesis to prepare various PtCo catalysts with different morphologies. Among these catalysts, PtCo-HNC has a chain-like 1D structure consisting of interconnected hollow spheres. This special structure gives birth to a lot of interfaces. Further research indicates that there are abundant lattice distortions and rich defects. Due to lattice distortion and rich defects, the PtCoHNC catalyst exhibits the highest catalytic activity and stability for methanol electro-oxidation as compared to the other synthesized PtCo catalysts and commercial Pt/C catalyst. The subsequent CO-stripping experiment results and density functional theory simulations also indicate that PtCo-HNC has superior performance. The unprecedented performances of PtCo-HNC for MOR eletrocatalysis represent an important step forward in their commercial applications in DMFCs.

Figure 3. (a) Cycle voltammograms of CO stripping on the PtCoHNS, PtCo-HNC + HNS, PtCo-HNS, and commercial Pt/C in 1 M KOH solution at a scan of 50 mV s−1. (b) DFT of methanol decomposition to CO and CO oxidation to form intermediate COOH and its decomposition on Pt(111) and Pt3Co(111).

PtCo catalysts, the PtCo-HNS sample showed an obvious CO oxidation peak in the second cycle which illustrates there was residual CO remaining unoxidized during the first COstripping cycle, In the CO stripping of all three PtCo samples, instead of one obvious oxidation peak, two new broad peaks emerged at a potential of −0.6 to approximately −0.35 V. We attribute those former peaks to CO electroxidation at Pt sites in the periphery of Co, while the peaks at about −0.35 V to CO electroxidation at the Pt site far away from Co.25 With the help of bifunctional interaction between Pt and Co, the CO can be oxidized and removed efficiently at potentials relevant to MOR (>−0.5 V). As a consequence, the catalysts are incredibly CO resistant. Methanol decomposition on PtCo catalysts were also studied through density functional theory simulations. As displayed in Figure 3b, the methanol oxidation reaction pathway on Pt(111) and Pt3Co(111) are calculated (Figure S10 shows the structure models of the main catalytic phase). Clearly, from the reaction pathway on Pt (111) (black line), it can be found that the step of CO* → COOH* is the ratedetermining step for its high barrier of 0.925 eV, which means that the CO is difficult to remove, consistent with experimental result (Figure 3a) and other published works.30 Interestingly, the Co allying can significantly decrease CO adsorption, showing as a red line in Figure 3b, which should greatly suppress CO poisoning in MOR. Moreover, the highest barrier

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b02137.

Figure 4. (a, b) Representative high-resolution images at the interface of PtCo-HNC (the regions for panels a and b indexed below the TEM image correspond to the marked areas by numbers 1−6). D

DOI: 10.1021/acsaem.8b02137 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Experimental section, chemicals materials characterization, electrochemical measurements, TEM images, EDS mapping and line scans, XRD, XPS spectra, and DFT atomic structural models (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: Wang.Yuesheng@ireq ca. ORCID

Xiaopeng Han: 0000-0002-7557-7133 Cheng Zhong: 0000-0003-1852-5860 Dewei Rao: 0000-0003-0410-5259 Yida Deng: 0000-0002-8890-552X 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 Nos. 51571151, 51701139, 51671143, and U1601216). We are also grateful for the support of the National Supercomputer Center in Lv Liang of China. The calculations were performed on TianHe-2.

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DOI: 10.1021/acsaem.8b02137 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX