Porous AgPt@Pt Nanooctahedra as an Efficient Catalyst toward

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Porous AgPt@Pt Nanooctahedra as an Efficient Catalyst toward Formic Acid Oxidation with Predominant Dehydrogenation Pathway Xian Jiang, Xiaoxiao Yan, Wangyu Ren, Yufeng Jia, Jianian Chen, Dongmei Sun, Lin Xu, and Ya-wen Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11895 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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ACS Applied Materials & Interfaces

Porous AgPt@Pt Nanooctahedra as an Efficient Catalyst toward Formic Acid Oxidation with Predominant Dehydrogenation Pathway Xian Jiang,† Xiaoxiao Yan,† Wangyu Ren,† Yufeng Jia, † Jianian Chen, † Dongmei Sun,† Lin Xu,*,† and Yawen Tang,*,†

†

Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation

Center of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China.

* Corresponding Authors: [email protected] (L. Xu) [email protected] (Y. W. Tang)

ABSTRACT For direct formic acid fuel cells (DFAFCs), the dehydrogenation pathway is a desired reaction pathway, to boost the overall cell efficiency. Elaborate composition tuning and nanostructure engineering provide two promising strategies to design efficient electrocatalysts for DFAFCs. Herein, we present a facile synthesis of porous AgPt bimetallic nanooctahedra with enriched Pt surface (denoted as AgPt@Pt nanooctahedra) by a selective etching strategy. The smart integration of geometric and

ACS Paragon Plus Environment


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electronic effect confers a substantial enhancement of desired dehydrogenation pathway as well as electro-oxidation activity for the formic acid oxidation reaction (FAOR). We anticipate that the obtained nanocatalyst may hold great promises in fuel cell devices, and furthermore, the facile synthetic strategy demonstrated here can be extendable for the fabrication of other multi-component nanoalloys with desirable morphologies and enhanced electrocatalytic performances.

KEYWORDS: alloy, porous nanooctahedra, electrocatalysts, formic acid oxidation reaction, dehydrogenation pathway.

INTRODUCTION As a kind of promising clean energy sources, direct formic acid fuel cells (DFAFCs) have stimulated enormous scientific and technological attention due to their high energy density, low fuel crossover and environmentally benign products.1-12 Although Pt nanostructures have been demonstrated as the highly effective catalysts toward the formic acid oxidation reaction (FAOR), their extremely high cost, poor durability as well as susceptibility to CO poisoning greatly hinder the

practical commercialization of DFAFCs.13-14 It is

well-established that electrocatalysis of HCOOH to CO2 on Pt-based electrocatalysts proceeds via two pathways: (1) the direct pathway in which HCOOH is directly oxidized to CO2 (dehydrogenation pathway), and (2) the indirect pathway in which HCOOH undergoes dehydration to form


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intermediate CO species which may poison the catalyst surfaces (dehydration pathway).2,

5, 10

While for practical DFAFCs, the direct pathway, namely

dehydrogenation pathway, is the desired reaction pathway due to the following reasons: (1) the dehydrogenation pathway could lower the polarization overpotential of the anodic reaction, and thus provide higher working voltage in a practical DFAFC. (2) the dehydration pathway could generate intermediate CO, which will poison the catalyst and hence reduce the catalytic activity and stability. In this context, it is of great importance to develop effective strategies for preparing electrocatalysts with largely reduced consumption of Pt while still exhibiting superior performance and prominent dehydrogenation pathway. It has been well demonstrated that the activity and poisoning resistance of a nanocatalyst could be effectively improved through rational manipulation of the morphology, exposed facets, and composition of the nanostructures.15-17 Consequently, elaborate composition tuning and nanostructure engineering provide two promising strategies to design efficient Pt-based catalysts for FAORs. Generally, alloying Pt with a less-expensive metal could not only reduce the consumption of scarce Pt, but also simultaneously modify electronic structures of the principal metal of Pt, which often leads to extraordinary catalytic performances, in comparison with the monometallic counterpart.18-19 Among various metals, experimental and theoretical investigations have manifested that Ag is a promising candidate to form Ag-Pt alloy with improved



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activity and better CO-poisoning tolerance because of its relatively low cost (less than 1% of that of Pt) and unique synergy between Pt and Ag.20-22 On the other hand, the catalytic properties of nanocatalysts are sensitively correlated with their exposed facets and overall morphologies.23-24 Especially for FAOR, the formation of poisoning intermediate of CO is less favorable on Pt (111) facet, thus facilitating the elimination of undesired indirect oxidation pathway.17, 25-26 Furthermore, a careful engineering of nanocatalysts from solid to porous nanostructures could provide large exposed surface, reduced mass diffusion paths and high molecular accessibility, and thus giving rise to enhanced catalytic performances. More significantly, the inner surface could also be effectively utilized and the interior catalytic sites are not easy to be deactivated caused by particle agglomeration during the catalytic reaction as compared with exterior catalytic sites27-30. Taken together, porous Ag-Pt alloy nanostructures terminated with (111) facets are considered as an ideal candidate for efficient electrocatalyst in DFAFCs. With this information, we sought to synthesize and evaluate porous AgPt bimetallic nanooctahedra with enriched Pt surface (denoted as AgPt@Pt nanooctahedra) as catalysts for FAOR. These structures were identified as a promising catalytic platform due to the unique octahedral porous structure and alloy synergy. To achieve these structures, a selective etching strategy was employed to remove partial Ag segment from preformed octahedral AgPt nanoalloy (Figure 1). Upon a facile acid treatment, the composition, surface


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chemistry and porosity of the AgPt nanoalloy alter greatly, which is beneficial for improving the electrocatalysis performances. 2. EXPERIMENTAL Synthesis of porous AgPt@Pt nanooctahedra. Firstly, AgPt nanooctahedra were prepared through a PAH-assisted hydrothermal approach according to our previous protocols (see the Supporting Information for details).31 Subsequently, 20 mg of the preformed AgPt nanooctahedra were subjected to an excess amount of concentrated HNO3 followed by continuously sonicating for 12 h at room temperature. The product was finally collected by centrifugation, washed deionized water for three cycles, and then followed by vacuum-drying. For comparison, pure Pt nanooctahedra were synthesized according to the literature protocols.32 Characterization. Transmission electron microscopy (TEM) measurements were conducted on a JEOL JEM-2100F microscope at an accelerating voltage of 200 kV. Energy dispersive X-ray (EDX) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) as well as elemental mapping measurements were carried out on FEI Tecnai G2 F20 microscope built as an accessory on the JEOL JEM-2100F. X-ray photoelectron spectroscopy (XPS) measurements were investigated on Thermo VG Scientific ESCALAB 250 spectrometer. X-ray diffraction (XRD) patterns were acquired on a Model D/max-rC X-ray diffractometer using Cu Kα radiation source (λ =



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1.5406 Å). The Brunauer-Emmett-Teller (BET) specific surface areas of the catalysts were evaluated at 77 K using a Micromeritics ASAP 2050 system. Electrochemical

measurement.

All

electrochemical

experiments

were

performed in a three-electrode system on a CHI 660 C electrochemical analyzer (CH Instruments, Shanghai, Chenhua Co.). The AgPt@Pt nanooctahedron modified glassy carbon electrode (3 mm in diameter) served as a working electrode. To immobilize the nanocatalysts, 6 µL homogeneous suspension of AgPt@Pt nanooctahedra (Ccatalyst = 2 mg mL-1) was dropped onto the glassy carbon electrode. After drying under ambient condition, 3 µL of Nafion solution (5 wt. %) was added to the AgPt@Pt nanooctahedron modified electrode and followed by drying again. A Pt wire and a saturated calomel electrode (SCE) were used as auxiliary and reference electrode, respectively. All electrochemical tests were conducted at 30 oC. The electrochemical measurements for FAOR were carried out in a N2-saturated solution containing 0.5 M H2SO4 and 0.5 M HCOOH at a scan rate of 50 mV s-1.

3. RESULTS AND DISCUSSION For the synthesis of AgPt@Pt nanooctahedra, the starting material of uniform AgPt alloy nanooctahedra were initially synthesized by a hydrothermal method with the assistance of polyallylamine hydrochloride (PAH). EDX analysis result (Figure S1) of the initial AgPt alloy nanooctahedra implies that the obtained nanocrystals consist of Ag and Pt with a Ag/Pt atomic ratio of 50.1: 49.9, which is very similar to the feeding ratio (1: 1) in the precursor solution.


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As revealed by typical TEM images (Figure 2a-b), the obtained nanocrystals take on a uniform octahedral morphology and have an average size of 22 nm in edge length. A HRTEM image performed on an individual octahedron demonstrates the well crystalline feature of the nanooctehedron (Figure 2c). The well-resolved lattice fringes along two adjacent edges of the octahedron are measured to be 0.229 nm, corresponding to the (111) lattice spacings of face-centered cubic (fcc) AgPt. Furthermore, the angle measured between the two adjacent edges is identified as 70.6 o, which matches closely with the theoretical value (70.5 o) in fcc-structured nanooctahedra. These results strongly suggest that the AgPt octahedra are bounded by eight (111) facets. Elemental mapping analyses (Figure 2d) obtained by HAADF-STEM demonstrate that Ag and Pt are homogeneously distributed throughout the as-prepared AgPt nanaooctahedra, verifying the alloy composition of the starting material. Cross-sectional compositional line profiles (Figure S2) of the preformed AgPt nanooctahedra indicate that the distribution ranges of Ag and Pt are almost overlapped,

corroborating

the

alloy

feature

of

the

preformed

AgPt

nanooctahedra. Such an alloy phase is further supported by XRD pattern (Figure 3) since all diffraction peaks are positioned between the standard peaks of pure fcc Ag and Pt, and neither a Ag nor Pt single component peak can be detected. The above results clearly imply the successful formation of AgPt nanoalloys bound by {111} facets.



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The obtained {111}-terminated AgPt nanooctahedron were then subjected to concentrated nitric acid solution to engineer its nanostructure. As we know, Pt is less vulnerable to HNO3 solution while most of the Ag atoms could be selectively removed from the AgPt alloy by HNO3 solution. It is notable that Ag atoms in the Ag-Pt alloy phase are less susceptible to chemical etching than pure Ag atoms. Thereby, partial Ag atoms are still remaining in the Ag-Pt alloy phase after etching process. From XRD pattern in Figure 3, it is evident that all diffraction peaks of the final product move to higher angle compared with the original alloyed AgPt nanooctahedra, yet still in an alloy phase due to the dealloying of Ag. As indicated by TEM images in Figure 4a, the octahedral structure could be essentially preserved during the etching process, and the average size is almost the same as the starting AgPt nanooctahedra. Impressively, a close inspection (Figure 4b) reveals that the resultant octahedral nanocrystals possess relative rough surface and obvious contrast from area to area can be observed throughout the nanooctahedra, indicating the porous feature of the final product. A HRTEM image (Figure 4c) taken from an individual porous nanooctahedron exhibits clear lattice fringes, indicating that they are still highly crystalline even after a severe chemical etching. The observed d-spacing of 0.230 nm can be ascribed to (111) lattice spacing of fcc AgPt. Notably, the surface becomes rougher and generates many atomic steps/corners, which can act as highly active sites for electrocatalysis as marked by red arrows in Figure 4c.33-34 The


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elemental mapping results (Figure 4d) suggest that the porous AgPt nanooctahedra are made of Ag and Pt. The EDX line scanning profiles displayed in Figure 4e-f manifest that the Pt signals are evidently higher than those of Ag in the exterior regions, demonstrating the Pt-enriched surface of the obtained product. Such customized Pt-enriched surface could substantially enhance the catalysis performance because of the modified electronic structure of surface Pt atoms caused by the underneath alloy effect and increased Pt utilization efficiency.35-39 N2 adsorption-desorption measurement results (Figure S3) imply that the Brunauer-Emmett-Teller (BET) surface area of the porous AgPt@Pt nanooctahedra is 30.0 m2 g-1, which is almost 2.2 times higher than that of solid counterpart (13.8 m2 g-1). As evidenced by the EDX spectrum in Figure 5a, the content of Ag in the final product is ~20%. While XPS measurement (Figure 5b) indicates the surface atomic ratio of Pt: Ag is 91.4: 8.6, which is apparently higher than the bulk composition identified by EDX (79.9: 20.1). Since XPS technique is a surface sensitive technique with a penetration depth of ca. 2 nm, this result unambiguously demonstrates a surface enrichment of the Pt component. Furthermore, the chemical states of Ag and Pt in the porous nanooctahedra exist predominantly as metallic states (Figure 5c-d). Taken together, all the data fully confirm the successful formation of porous alloyed AgPt nanooctahedra with enriched Pt surface. The

obtained

porous

AgPt@Pt

nanooctahedra

were

used

to

electro-catalyze the FAORs, and the electrocatalysis results were benchmarked



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against the solid octahedral AgPt alloy, and commercial Pt black catalyst. Figure S4 shows the cyclic voltammogram (CV) curves of the as-prepared porous AgPt@Pt nanooctahedra, solid octahedral AgPt alloy and commercial Pt black recorded in a N2-purged 0.5 M H2SO4 solution. According to the Coulombic charge for hydrogen adsorption,40 the electrochemically active surface area (ECSA) of the preformed solid octahedral AgPt alloy (11.9 m2 g-1) is obviously smaller than that of the commercial Pt black (17.5 m2 g-1). Such phenomena can be rationally ascribed to the following two reasons: (1) In comparison with the commercial Pt black (d = 8.5 nm),41 the obtained octahedral AgPt alloy possesses a larger average size (d = 22 nm, Figure 2a). (2) The doping of Ag would decrease the Pt content on the surface of octahedral AgPt alloy. Upon the acid etching, the solid octahedral AgPt alloy transforms to porous AgPt@Pt nanooctahedra with enriched Pt surface by dealloying of Ag. Therefore, the ECSA of the resultant AgPt@Pt nanooctahedra increases to 22.3 m2 g-1. This result agrees well with the aforementioned EDX-Mapping and XPS analysis. Figure 6a displays the Pt mass-normalized CV curves of the above catalysts toward the FAOR. Obviously, the porous AgPt@Pt nanooctahedra demonstrate an obvious negative onset potential and a remarkably enhanced activity in comparison with their solid counterparts, pure Pt nanooctahedra (Figure S5) and commercial Pt black catalyst. The mass-normalized current density of peak I on porous PtAg@Pt nanooctahedra reaches 282.6 mA mg-1,


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which is ca. 1.7-fold, 3.6-fold and 10.8-fold as large as that on solid PtAg nanooctahedra (162.8 mA mg-1), pure Pt nanooctahedra (77.9 mA mg-1) and Pt black (26.1 mA mg-1). For FAOR, it is established that the oxidation peak I located between 0.40 and 0.60 V and peak II located between 0.80 and 1.00 V (vs. RHE) are associated with the direct oxidation of formic acid via the dehydrogenation pathway (HCOOH → CO2 + 2H+ + 2e-) and dehydration pathway (HCOOH→COads + H2O→CO2 + 2H+ + 2e-), respectively.2, 5, 10 The ratio (R) of peak area I to peak area II can reflect the FAOR pathway of the electrocatalyst. A higher R indicates a favorable direct reaction pathway.42 In the present study, as illustrated in Figure S6 and Figure 6b, the R of the porous octahedral AgPt@Pt is about 2.77, which is much higher than the value of 1.22 for the solid PtAg nanooctahedra and commercial Pt black (0.31), indicating the much improved desirable direct reaction pathway on the porous AgPt@Pt nanooctahedra. The turnover frequency (TOF) number (Figure S7) of the porous AgPt@Pt nanooctahedra is calculated to be 3.15 atom-1 s-1, which is higher than that of AgPt nanooctahedra (2.98) or Pt black (0.31). This result further verifies the high electrocatalytic efficiency of the porous AgPt@Pt nanooctahedra. From the Tafel plots of the three catalysts displayed in Figure 6c, it is observed that all plots exhibit good linear relationship in lower current region. Moreover, the porous AgPt@Pt nanooctahedra show higher output current under the same potential in comparison with the two reference materials. In addition, the polarization over-potential at the porous AgPt@Pt



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nanooctahedra appears at a relative higher output current density. These results suggest that the FAOR occurred at the porous AgPt@Pt nanooctahedra has a faster kinetic rate. To further evaluate the durability of these catalysts, chronoamperometry (CA) measurements were performed in 0.5 M HCOOH + 0.5 M H2SO4 solutions at 0.35 V for 3000 s. The i-t curves in Figure 6d indicate that the oxidation current density of the FAOR on the porous AgPt@Pt nanooctahedra is significantly higher than that of solid AgPt nanooctahedra and commercial Pt black catalyst over the entire time course, further verifying that the porous AgPt@Pt nanooctahedra exhibit better electrocatalytic performance toward the FAOR. Furthermore, the porous octahedral skeleton could be still essentially

preserved

without a

remarkable

aggregation,

whereas the

commercial Pt black catalyst suffered a severe agglomeration after the stability test (Figure S8). As onset potential, peak current, and overall reaction efficiency are considered as the most important parameters in the assessment of electrocatalytic performance of an electrocatalyst, the relative lower onset potential, larger oxidation current, and predominant direct oxidation pathway obtained from the porous AgPt@Pt nanooctahedra strongly manifest their better catalytic performance toward the FAOR than their solid counterparts and commercial Pt black. The superior catalytic performance of the porous AgPt@Pt nanooctahedra could be rationally ascribed to the porous octahedral


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skeleton nanostructure with enriched Pt surface and synergistic effect between Pt and Ag. The synthesized alloyed porous AgPt@Pt nanooctahedra smartly fulfill the design rationales for efficient nanocatalysts for FAOR and will be of special interest for fuel cell applications due to the following fascinating features: (1) The creation of porous structure could not only provide a higher percentage of exterior and interior surface atoms, more reagent accessibility and thus enhanced activity in comparison with their solid counterparts, but also be beneficial to decrease the usage of Pt.43-44 (2) Formation of Ag-Pt would be beneficial to improve catalytic performance, and also economically desirable due to better Pt utilization efficiency, modified electronic structure and relatively low cost of Ag. (3) The enriched Pt surface could provide abundant catalytic sites and thus further increase Pt utilization efficiency.45-46 (4) The discontinuous Pt atoms are favourable for the dehydrogenation pathway due to the presence of the neighbored Ag atoms.47-48 (5) The octahedral nanostructures could exhibit a better anti-poisoning capability since they are mainly bound by (111) facets.17,

25-26

All of these unique attributes induced by judicious integration of

geometric and electronic effects make the obtained porous AgPt@Pt nanooctahedra attractive for electrocatalysis.

4. CONCLUSIONS In summary, we have presented a feasible and versatile synthesis of porous AgPt



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bimetallic nanooctahedra with enriched Pt surface by a selective etching strategy. Thanks to the unique octahedral porous structure and alloy synergy, the as-prepared porous

AgPt@Pt

nanooctahedra

exhibit

a

significant

enhanced

desirable

dehydrogenation pathway as well as electrocatalytic activity toward the FAOR, in comparison with their solid counterparts and commercial Pt black. It is anticipated that the present versatile strategy could open the avenue to rational design of other porous multi-component nanoalloys with interesting morphologies and enhanced electrocatalytic activities.

ASSOCIATED CONTENT Supporting Information Additional experimental data (Figure S1−8). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Tel: +86-25-85891651; Fax: +86-25-83243286. E-mail address: [email protected] (L. Xu) [email protected] (Y. W. Tang) Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS The authors acknowledge the financially support from NSFC (Nos. 21576139, 21273116, 21376122, and 21503111), University Postgraduate Research and Innovation Project in Jiangsu Province (KYZZ15_0213). The authors are also grateful for the help from National and Local Joint Engineering Research Center of Biomedical Functional Materials, and a project sponsored by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Figure 1. Schematic illustration of the preparation of porous AgPt@Pt nanooctahedra.



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Figure 2. Morphological characterizations of the starting AgPt alloy nanooctahedra. (a)-(b) Typical TEM images. (c) HRTEM image of a nanooctahedron and (d) EDX mapping images.

Figure 3. XRD patterns of the alloyed AgPt octahedral nanocrystals before and after chemical etching.


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Figure 4. Detailed morphological characterization of the as-prepared porous AgPt@Pt nanooctahedra. (a)-(b) Representative TEM images. (c) HRTEM image of an individual nanooctahedron. (d) Elemental mapping images. (e)-(f) HADDF-STEM image and cross-sectional compositional line profiles.



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Figure 5. (a) EDX analysis, (b) XPS spectrum, (c) Ag 3d region and (d) Pt 4f region of the obtained porous AgPt@Pt nanooctahedra.

Figure 6. Comparison of the FAOR electrocatalytic performance of the prepared porous AgPt@Pt nanooctahedra, solid AgPt nanooctahedra, Pt nanooctahedra and commercial Pt black catalyst. (a) CV curves of FAOR. (b) Ratios of Peak area I to Peak area II calculated from the deconvolution of FAOR CV curves. (c) Tafel plots and (d) i-t curves.


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Bao,

J.;

Dou,

M.;

Liu,

H.;

Wang,

F.;

Liu,


J.;

Li,

Z.; Ji,

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Porous AgPt@Pt Nanooctahedra as an Efficient Catalyst toward

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