Three-Dimensional Pd3Pb Nanosheet Assemblies: High-Performance

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Three-Dimensional Pd3Pb Nanosheet Assemblies: High-Performance Non-Pt Electrocatalysts for Bifunctional Fuel Cell Reactions Lingzheng Bu, Chongyang Tang, Qi Shao, Xing Zhu, and Xiaoqing Huang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00455 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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Three-Dimensional Pd3Pb Nanosheet Assemblies: HighPerformance Non-Pt Electrocatalysts for Bifunctional Fuel Cell Reactions Lingzheng Bu,1† Chongyang Tang,1,2† Qi Shao,1 Xing Zhu,3 and Xiaoqing Huang1* 1

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, China. 2

School of Physics and Technology, Wuhan University, Hubei 430072, China. 3

Testing & Analysis Center, Soochow University, Jiangsu 215123, China. †

These authors contributed equally to this work.

*

To whom correspondence should be addressed. E-mail: [email protected]

Abstract: Even though extensive efforts have been devoted to pursue the promising electrocatalysts, the design of high-efficiency and low-cost electrocatalysts continues to be a formidable challenge for commercializing the electrochemical energy technologies. Herein, we report the successful creation of a class of three-dimensional ordered Pd3Pb nanosheet assemblies (NSAs) via a wet-chemical approach. Such controlled nanostructures with ordered phase and highly open structure exhibit enhanced

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electrochemical activity and durability for both anodic methanol oxidation reaction (MOR) and cathodic oxygen reduction reaction (ORR). In particular, the ordered Pd3Pb NSAs show the ORR mass and specific activities of 0.697 A mgPd-1 and 0.989 mA cm-2 at 0.90 V versus reversible hydrogen electrode (RHE), which are 8.3 (10.9) and 7.5 (6.4) times higher than those of the commercial Pt/C (Pd/C), respectively, making them among the most efficient catalyst ever achieved in the Pd-based ORR catalysts to date. In addition, the ordered Pd3Pb NSAs can even sustain at least 20000 potential cycles with limited activity decay and structure change.

Keywords: Non-Platinum, Palladium, Ordered, Nanosheet Assembly, Fuel Cell Reaction

INTRODUCTION Direct methanol fuel cell (DMFC) is expected to be a promising candidate to meet inexorable demand of high-density energy, which requires prominently active and highly durable electrocatalysts for both the anodic methanol oxidation reaction (MOR) and cathodic oxygen reduction reaction (ORR).1-3 Although the precious and rare metal catalysts of platinum (Pt) and Pt-based alloys have been efficient for the fuel cell reactions,4-8 the progress of fuel cells largely necessitates the catalysts with superior performance yet largely reduced Pt usage.9 Non-precious catalysts, such as perovskite,10,11 transition metal oxide,12 transition metal phosphide,13 transition metal carbide,9 and heteroatom-doped nanocarbon,14 have been comprehensively studied for MOR and ORR, but the common drawbacks of limited activity and low durability have been generally faced. Although palladium (Pd) has emerged as the most potential candidate to replace Pt as the highly efficient MOR and ORR catalysts,15-18 the monometallic Pd has its trouble to engineer with excellent properties towards fuel cell reactions, because its electronic structure is not beneficial for the electron transfer during ORR process compared with


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Pt.19,20 To modulate the electronic structure of Pd, extensive works have been devoted to designing bimetallic Pd-based catalysts,15,21-23 but the majority reported catalysts are limited to the conventional disordered structures, which still suffer from low activity and durability. From the structural view, the ordered structures with precise atom arrangements can provide the predictable modulation over the geometry, surface and electronic structures, which can be hardly realized by the conventional disordered ones.24-27 In light of the high enthalpy of mixing, the ordered structures can also exhibit enhanced catalytic stability.28 Nevertheless, the reports on successfully controlling over the ordered Pd-based catalysts are still very limited so far. Most previous ordered Pdbased nanocrystals (NCs) are confined to zero-dimensional (0D) nanostructures with ill-defined morphology.28,29 2D structure is of great significance for enhancing electrochemical performance due to the largely improved atom utilization compared with 0D and 1D structures. Based on the above considerations, ordered Pd-based NCs with 2D structures are highly desirable for MOR and ORR. Herein, we present a new class of Pd-based 3D architectures assembled from 2D Pd-lead (Pb) nanosheets for the first time as efficient bifunctional fuel cell electrocatalysts. The 3D ordered Pd3Pb nanosheet assemblies (NSAs) not only improve surface area to effectively enhance the atom utilization for promoting the activity, but also contribute to the enhanced stability, where the Pd3Pb NSAs exhibit the largely enhanced performance for MOR, superior to the Pd3Pb NPs, the commercial Pd/C (SigmaAldrich, 10 wt% Pd) as well as the commercial Pt/C (JM, 20 wt% Pt). The Pd3Pb NSAs were further demonstrated as one of the most efficient ORR eletrocatalysts with high specific and mass activities, 7.5 and 8.3 times higher than those of the commercial Pt/C and 6.4 and 10.9 times greater than those of the commercial Pd/C. The Pd3Pd NSAs/C also showed the enhanced ORR durability with negligible activity decay, limited morphology/composition changes after 20000 potential sweeps.


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Figure 1. Morphological and structural characterizations of 3D Pd3Pb NSAs. (a, b) HAADF-STEM images of Pd3Pb NSAs. The inset in (a) is the 3D model of an individual Pd3Pb NSA. (c) HAADF-STEM image and HAADFSTEM-EDS elemental mappings of individual Pd3Pb NSA (Pd mapping in green, Pb mapping in red, and integrated mappings of Pd and Pb are yellow). (d) HAADF-STEM images of an individual Pd3Pb NSA at different viewing angles from -50o to +50o. The compositional ratio of Pd/Pb is 74.8/25.2, as confirmed by ICP-AES.

RESULTS AND DISCUSSION The 3D ordered Pd3Pb NSAs were prepared by simultaneous reduction of Pd(acac)2 and Pb(acac)2 via a wet-chemical approach, in which phloroglucinol was chosen as the reducing agent, oleylamine


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(OAm) and oleic acid (OA) as the mixed solvents and surfactants, and FeCl3·6H2O as the structuredirecting agent. The detailed synthesis conditions, such as the precursors, the reducing agents, the solvents, and the species, have been explored in details (Supporting Information, Figures S1-S8). The results show that the controlled synthesis of 3D ordered Pd3Pb NSAs highly depends on the selective use of phloroglucinol and FeCl3·6H2O, the combined use of OAm and OA as well as the suitable volume ratio. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and TEM images of the Pd3Pb NSAs (Figure 1a, b and Figure S9a-d) indicated that the highly dispersive Pd3Pb NSAs were constructed with many 2D ultrathin nanosheet building blocks with the yield approaching 100%. They are uniform in size with the average edge length of 122 nm and thickness of 3.25 nm for the nanosheet building block (Figure S9e, f), as vividly presented by the 3D model of individual Pd3Pb NSA (the inset of Figure 1a). As revealed by ICP-AES, the ratio of Pd to Pb in these Pd3Pb NSAs is 74.8/25.2 (Pd/Pb), being consistent with the TEM energy-dispersive X-ray spectroscopy (TEM-EDS) (Figure S9g). The elemental distributions of Pd and Pb were revealed by the HAADFSTEM-EDS mapping (Figure 1c), where the Pd (green), Pb (red), and combined (yellow) images showed that all elements were distributed evenly throughout the whole Pd3Pb NSAs. The HAADFSTEM images of individual Pd3Pb NSA tilted from -50° to +50° revealed that each nanosheet spread along different directions but they were parallel or vertical for each other (Figure 1d). The large gaps between neighbor nanosheets can be clearly observed (Figure 1b and Figure S9b, d), ensuring the highly open structure of Pd3Pb NSAs. When Pd3Pb NSA is projected to a plane at vertical direction, the unique fence-shaped structure can be clearly observed. To the best of our knowledge, it is the first time that such unconventional 3D structure has been successfully obtained via a facile chemical approach. Due to the highly open 3D nanostructure and the thin nanosheet building block, the Pd utilization can be greatly enhanced, which can be beneficial for the catalytic optimization.


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Figure 2. Structure Analysis of 3D Pd3Pb NSAs. (a) TEM image of Pd3Pb NSA. (b) HRTEM image from the selected area in (a). (c) Crystal structure of ordered Pd3Pb. (d) PXRD pattern of the Pd3Pb NSAs.

To further uncover the morphological and structural features of such unique nanostructure, a series of techniques, such as high resolution TEM (HRTEM), selected-area electron diffraction (SAED), and powder X-ray diffraction (PXRD), were carried out (Figure 2 and Figure S10). The TEM image of an individual Pd3Pb NSA clearly demonstrates that the Pd3Pb NSA is comprised of ultrathin nanosheet building blocks with highly open 3D structure (Figure 2a and Figure S10a). The Pd3Pb NSAs are highly crystalline, as confirmed by the clear SAED pattern (Figure S10b). As shown in Figure 2b, the lattice fringe of 0.232 nm corresponding to the (111) plane of cubic Pd3Pb phase is clearly observed. The (111) facet with interplanar spacing of 0.232 nm and (200) facet with interplanar spacing of 0.202 nm for


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typical Pd3Pb NSAs are consistent well with the (111) plane (0.232 nm) and (200) plane (0.201 nm) of cubic Pd3Pb, respectively (Figure 2b and Figure S10c, d). The crystal structure model of the ordered Pd3Pb with a primitive cubic structure is shown in Figure 2c, which is in accordance with the space group Pm-3m. The PXRD pattern of Pd3Pb NSAs displays typical peaks that can be indexed as (100), (110), (111), (200), (210), (211), (220), (221), (310), (311), (222), and (320) reflections of the ordered Pd3Pb phase (JCPDS No. 65-3266) (Figure 2d). Theoretically, the intensity ratio of the (110) peak to the (111) peak can be used to roughly determine the ordering degree of the Pd3Pb crystal.29 As expected, the ratio of (110) peak to the (111) peak is calculated to 0.062 for Pd3Pb NSAs, which is very close to the theoretical value of 0.063, confirming that the Pd3Pb NSAs are fully ordered. Unlike the disordered catalysts, the ordered phases can exactly control over the optimization of catalysts in surface, geometry, and electronic structures, for which the highly ordered Pd3Pb NSAs are expected to exhibit superior performance.


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Figure 3. MOR performances of 3D Pd3Pb NSAs/C, Pd3Pb NPs/C, commercial Pt/C, and commercial Pd/C in alkaline condition. (a) CO stripping curves of different electrocatalysts. (b) Bar graph of ECSAs of different electrocatalysts. (c) CVs of different electrocatalysts for MOR at a scan rate of 50 mV s-1 in 1 M KOH + 1 M methanol. The inset in (c) shows the onset potentials of different electrocatalysts for MOR. (d) Bar graph of mass and specific activities of different electrocatalysts for MOR.

To have an intuitive insight of how advantageous the fully ordered and highly open structure is, the fuel cell reactions of MOR and ORR were chose to evaluate the performance of ordered Pd3Pb NSAs. For comparison, the Pd3Pb NPs with averaged size of 10.2 nm (Figure S11), the commercial Pt/C and the commercial Pd/C were used as references for comparisons. Before the electrochemical investigations, Pd3Pb NSAs and Pd3Pb NPs were loaded on carbon black (Vulcan XC72R carbon, C) by sonication and washed with the mixture of acetic acid/ethanol (Figures S12-S13). Figure 3a shows the CO-stripping curves of different catalysts in 0.1 M HClO4 solution at a rate of 20 mV s-1, by which their electrochemical active surface areas (ECSAs) were calculated. The Pd3Pb NSAs/C show the ECSA of 67.5 m2 g-1, which is higher than those of the Pd3Pb NPs/C (35.4 m2 g-1), the commercial Pd/C (41.5 m2 g-1) and the commercial Pt/C (63.8 m2 g-1) (Figure 3b). The Pd3Pb NSAs/C and Pd3Pb NPs/C as anodic MOR electrocatalysts were first investigated (Figure 3c, d, Figure S14 and Table S1). Figure 3c depicts the cyclic voltammograms (CVs) of different catalysts in 1 M KOH solution containing with 1 M methanol at room temperature. A wider potential window and lower onset potential for the Pd3Pb NSAs/C were observed than other catalysts (Figure 3c), showing better anodic electrooxidation activity.30 The mass and specific activities by normalizing peak current in the forward potential scan with regard to the loading amount of Pt or Pd and ECSA are shown in Figure 3d. As expected, all the Pd3Pb NCs/C presents excellent MOR activity, in which the Pd3Pb NSAs/C shows the mass activity of 2.98 A mgPd-1, 3.0 and 2.3 times higher than those


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of the commercial Pd/C (1.01 A mgPd-1) and commercial Pt/C (1.31 A mgPt-1), and even superior to the Pd3Pb NPs/C (1.44 A mgPd-1). It also exhibits the highest specific activity of 4.41 mA cm-2. By CV cycling in 1 M KOH + 1 M methanol solution for 1000 cycles, the Pd3Pb NSAs/C, Pd3Pb NPs/C, commercial Pt/C and commercial Pd/C can maintain 12.3%, 12.6%, 0.62% and 14.2% initial activities, showing the enhanced durability of Pd3Pb NSAs/C relative to Pt/C (Figure S14).

Figure 4. ORR activities of 3D Pd3Pb NSAs/C, Pd3Pb NPs/C, commercial Pt/C, and commercial Pd/C in alkaline condition. (a) ORR polarization curves and (b) the corresponding Tafel plots of different electrocatalysts. (c) Bar graph of mass and specific activities of different electrocatalysts. The activities and standard deviations were calculated based on five parallel measurements. (d) Improvement factors of different electrocatalysts versus the commercial Pt/C. The current density in (a) was normalized to the geometric area of a RDE (0.196 cm2).

We then evaluated the ORR performances of different catalysts in O2-saturated 0.1 M KOH solutions at room temperature with a sweep rate of 10 mV s-1 by using a rotating disk electrode (RDE)


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with a rotation rate of 1600 rpm. As shown in Figure 4a, we can see that the half-wave potential and onset potential of Pd3Pb NSAs/C is shifted considerably to highest potentials compared to those of the Pd3Pb NPs/C, commercial Pt/C, and commercial Pd/C, indicating that the Pd3Pb NSAs/C has the highest ORR activity among these investigated catalysts. As shown in Figure 4b, the Pd3Pb NSAs/C presents the smallest Tafel slope of 76.1 mV dec-1 in contrast to Pd3Pb NPs/C (89.2 mV dec-1), commercial Pt/C (97.5 mV dec-1), and commercial Pd/C (100.7 mV dec-1) (Table S2). We can see that the Tafel slopes of all catalysts at low overpotential were close to each other, demonstrating the similar ORR mechanism occurred in the Pd3Pb NCs/C and the commercial Pt/C: the main rate-determining step is the protonation of O2 on the active sites of catalysts.31 The Koutecky-Levich equation was used to determine the kinetic currents from polarization curves. For each catalyst, the kinetic current was normalized to loading amount and ECSA of Pd or Pt in order to generate mass and specific activities, respectively. As shown in Figure 4c, d and Table S2, the Pd3Pb NSAs/C exhibits the highest mass activity of 0.697 A mgPd-1 at 0.90 V versus a reversible hydrogen electrode (RHE), 8.3 and 10.9 times greater than those of the Pt/C (0.084 A mgPt-1) and the Pd/C (0.064 A mgPd-1), respectively. The specific activity of Pd3Pb NSAs/C (0.989 mA cm-2) is 7.5 and 6.4 times larger than those of the commercial Pt/C (0.132 mA cm-2) and the commercial Pd/C (0.154 mA cm-2) (Figure 4c, d and Table S2). The mass and specific activities of Pd3Pb NSAs/C are also higher than those of the Pd3Pb NPs/C (Figure 4c, d and Table S2), confirming that highly open 3D nanostructure is beneficial for up-grading the electrocatalysis. It should be emphasized that the mass activity of the Pd3Pb NSAs/C is quite promising, which is among the highest ORR activity of all the Pd-based catalysts reported to date in alkaline condition (Table S3), even better than many Pt-based ORR electrocatalysts (Table S4).


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Figure 5. ORR durability of 3D Pd3Pb NSAs/C, Pd3Pb NPs/C, commercial Pt/C, and commercial Pd/C in alkaline condition. (a) ORR polarization curves of Pd3Pb NSAs/C before and after various potential cycles. (b) ORR polarization curves of Pd3Pb NPs/C before and after various potential cycles. (c) ORR polarization curves of commercial Pt/C before and after various potential cycles. (d) ORR polarization curves of commercial Pd/C before and after various potential cycles. (e) The normalized mass activity changes of Pd3Pb NSAs/C, Pd3Pb NPs/C, commercial Pt/C and commercial Pd/C before and after various potential cycles. The current densities in (a), (b), (c) and (d) were normalized in reference to the geometric area of a RDE (0.196 cm2).


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The durability is another important criterion for the catalyst evaluation, a catalyst with better durability can exhibit a much longer life, which plays a crucial role for practical applications.32 The electrochemical durability tests were measured in N2-saturated 0.1 M KOH solutions at the potential between 0.6 V and 1.1 V versus RHE by using accelerated durability tests (ADTs) at a sweep rate of 100 mV s-1. Figure 5a shows that there is no obvious change of ORR polarization curves before and after 20000 sweeping cycles, and only 17.2% mass activity loss for the Pd3Pb NSAs/C. However, the Pd3Pb NPs/C showed an obvious shift in ORR polarization curve and 46.0% mass activity loss (Figure 5b). The commercial Pt/C and Pd/C showed relatively apparent changes in ORR polarization curves with 48.7% and 51.6% losses in mass activities (Figure 5c, d). The ORR durability of different catalysts are summarized in Figure 5e, clearly showing the best ORR stability of Pd3Pb NSAs/C among these four electrocatalysts studied. Moreover, the structure and composition of different electrocatalysts after ORR durability tests were also investigated in detail (Figures S12, S13, S15, S16). There was no obvious morphology change and negligible composition change (from 74.6/25.4 to 77.4/22.6) and elements distribution change for Pd3Pb NSAs/C after 20000 sweeps (Figure S12). However, the Pd3Pb NPs/C, the commercial Pt/C and the commercial Pd/C exhibited a large number of aggregation and size change after 20000 cycles (Figures S13, S15, S16), further demonstrating the enhanced ORR stability of Pd3Pb NSAs/C. We ascribe the enhanced electrochemical performance of Pd3Pb NSAs/C to the highly open nanostructure, the alloy effect as well as the highly ordered structure. The nanostructures with desirable assembling manner show enhanced performance as a result of their collective structure, being distinctly different from their corresponding individual building blocks.33 Due to the ultrathin nanosheet blocks of the Pd3Pb NSAs, the Pd utilization of the Pd3Pb NSAs is enhanced compared to that of the Pd3Pb NPs,


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which is in favour of enhancing the electrochemical activities. In addition, the introduction of metal to Pd can vary the Pd-Pd bond, leading to d-band center down-shift and electronic structure change of Pd.34 Additionally, the ordered structure tend to result in the high enthalpy of mixing, making the ordered Pd3Pb NSAs stable.28,35

CONCLUSIONS To summarize, we have demonstrated an effective wet-chemical strategy to successfully produce a unique class of ordered Pd3Pb NSAs with high-yield and high-quality for the first time. Due to the highly open structure, well-defined ultrathin building blocks, and the intermetallic Pd3Pb phase, the ordered Pd3Pb NSAs exhibit excellent activity and superior durability for bifunctional fuel cell reactions of MOR and ORR in alkaline media. Particularly, the Pd3Pb NSAs show the ORR mass activity of 0.697 A mgPd1

and the specific activity of 0.989 mA cm-2 at 0.90 V versus RHE, which are much higher than those of

the commercial Pd/C, the commercial Pt/C and the Pd3Pb NPs/C, and also superior ORR durability, making them among the most active and stable non-Pt ORR electrocatalysts to date. They also exhibit excellent performance for MOR with the highest activity and best durability among all the counterparts investigated. We expect our study will inspire the rational design of highly ordered non-Pt catalysts with precise control, and can significantly impact the fuel cell reactions and beyond.

ASSOCIATED CONTENT Supporting Information. Figure S1-16&Table S1-4. This material is available free of charge via the Internet at http://pubs.acs.org.


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Corresponding Author [email protected] ACKNOWLEDGMENT This work was financially supported by the Ministry of Science and Technology (2016YFA0204100, 2017YFA0208200), the National Natural Science Foundation of China (21571135), Young Thousand Talented Program, the start-up supports from Soochow University, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Three-Dimensional Pd3Pb Nanosheet Assemblies: High-Performance

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