Heteropolyacid-Mediated Self-Assembly of Heteropolyacid-Modified

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Heteropolyacid-Mediated Self-Assembly of HeteropolyacidModified Pristine Graphene Supported Pd Nanoflowers for Superior Catalytic Performance toward Formic Acid Oxidation Xiuling Fan, Weiyong Yuan, Dao Hua Zhang, and Chang Ming Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00081 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Applied Energy Materials

Heteropolyacid-Mediated Self-Assembly of Heteropolyacid-Modified Pristine Graphene Supported Pd Nanoflowers for Superior Catalytic Performance toward Formic Acid Oxidation Xiuling Fan,†,‡ Weiyong Yuan,*,†,‡ Dao Hua Zhang,§ and Chang Ming Li†,‡,ǁ †

Institute for Clean energy & Advanced Materials, Faculty of Materials & Energy, Southwest

University, Chongqing 400715, China ‡

Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies,

Chongqing 400715, China §

School of Electrical and Electronic Engineering, Nanyang Technological University, 50

Nanyang Avenue, Singapore 639798, Singapore ǁ

Institute for Materials Science and Devices, Suzhou University of Science and Technology,

Suzhou 215011, China KEYWORDS: formic acid oxidation, Pd-based electrocatalysts, self-assembly, pH, heteropolyacid

ACS Paragon Plus Environment

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ABSTRACT: The in-situ growth of Pd nanoflowers on pristine graphene is achieved using phosphomolybdic acid (HPMo) to mediate self-assembly. The HPMo serves simultaneously as a linker, stabilizer, and structure-directing agent, and the nanoflowers are formed by kinetically controlled growth. When the resulting material, Pd nanoflowers on HPMo-modified graphene (HPMo-G) support, is used to catalyze formic acid oxidation reaction (FAOR), much higher catalytic activity and durability are found than with HPMo-G supported Pd nanospheres, graphene supported Pd nanoparticles, and commercial Pd/C catalysts. The catalytic activity for Pd nanoflowers on HPMo-G is also among the highest reported for Pd-based catalysts. The superior electrocatalytic performance is attributed to the unique nanoflower shape, a promotion by the HPMo mediator, and the excellent support properties of pristine graphene. The use of HPMo to mediate self-assembly of metals on graphene can be extended to fabricate other hybrid nanostructures promising broad applicability.

1. INTRODUCTION The direct formic acid fuel cell (DFAFC) is one of the most promising energy conversion devices for broad applications such as electric vehicles and portable electronics because of its low operating temperature, high theoretical open circuit voltage (1.48 V), easy fuel storage, and low fuel crossover through the proton exchange membrane.1-3 Pt is perhaps the most commonly used anode catalyst in DFAFCs, but it is very scarce and expensive, and its performance is severely restricted by low CO tolerance.1-4 On the other hand, Pd is much cheaper and more earth-abundant, and possesses better activity than Pt.1,2,4,5 Therefore, it is highly promising to develop new Pd-based electrocatalysts with high activity and durability to significantly accelerate the formic acid oxidation reaction (FAOR) for practical DFAFC applications.


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Graphene, a two-dimensional (2-D) π-π conjugated structure with superior properties such as ultrahigh surface area and excellent conductivity, could serve as a support of Pd to greatly improve its dispersion and inhibit aggregation as well as create interconnected ultrafast electron conducting paths, thus boosting its catalytic performance toward the FAOR.6-8 Nanostructuring of Pd in various shapes such as nanowires, nanosheets, and nanodendrites could also remarkably enhance its catalytic activity toward the FAOR due to its increased catalytically active sites and unique shape-dependent physicochemical properties;9-11 Adding co-catalysts could further significantly promote the catalytic activity and durability via changing its electronic structure or interacting with reaction intermediates.4,12,13 Therefore, it is highly desirable to fabricate graphene supported Pd nanostructures with co-catalysts incorporated for synergistically enhancing the catalytic performance. Nevertheless, large and aggregated Pd particles are easily formed on pristine graphene because of its inertness, and although graphene oxide or heavily oxidized graphene can be used, the performance of the catalysts is compromised even after sophisticated reduction under severe reducing conditions due to the irreversibly damaged π-π conjugated structure.6,14,15 It is even more difficult to directly and robustly grow dense and uniform Pd nanostructures with unique shapes on pristine graphene for high catalytic activity and durability.6,7,16 Furthermore, it is challenging to efficiently integrate co-catalysts into the nanohybrid to generate strong interaction with the catalysts for high promotion effect.17-19 Non-covalent functionalization of pristine graphene could facilitate growth of Pd nanostructures and introduce co-catalytic properties, while maintaining its intrinsic electronic structure.20-24 Heteropolyacids (HPAs) hold great promise for this purpose due to the following advantages: firstly, their large interaction with pristine graphene could guarantee robust immobilization of them on graphene with high surface density, and also promote dispersion of


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the graphene for uniform growth of Pd; 25-27 secondly, their strong binding with metals could not only enable them to be efficient stabilizers and structure-directing agents for synthesis of Pd nanoparticles with uniform size and shape, but also allow for in-situ self-assembly of the Pd nanostructures on the HPA-modified graphene with high density, good dispersion, and high stability;28,29 thirdly, their oxidizing ability and intimate contact with Pd nanostructures could facilitate the oxidation of CO, and thus enhance the resistance of Pd to CO poisoning for high catalytic activity and durability toward FAOR.12,30 However, no one has utilized HPAs to noncovalently functionalize pristine graphene for controlled growth of unique Pd nanostructures, and no one has studied synergistic effects of the building blocks for significantly enhancing the catalytic performance of Pd catalysts. Reported approaches to fabricating Pd nanostructures include addition of surfactants/polymers and exertion of electrochemical potential, high temperature, and/or high pressure.10,31-33 Nevertheless, the surfactants/polymers frequently block the catalytically active sites of Pd, and the harsh conditions significantly compromise the scalability of the synthesis process as well as increase energy consumption.34,35 Herein, for the first time we report in-situ growth of Pd nanoflowers on pristine graphene at room temperature and ambient pressure without adding surfactants/polymers or applying electricity via a phosphomolybdic acid (HPMo)-mediated selfassembly strategy. HPMo was employed since it is a representative HPA and widely used in catalysis.25,30 Possible self-assembly mechanism was investigated based on experimental data. The nanohybrids were further applied as electrocatalysts toward FAOR. 2. EXPERIMETNAL SECTION


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2.1. Materials. HPMo, H2PdCl4, L-ascorbic acid (L-AA), Nafion (5%), and commercial Pd/C catalyst (30%) were purchased from Sigma-Aldrich. High-purity graphene (>98.3%, oxygen content 6–8%) was obtained from Sinocarbon Materials Technology Co., Ltd. The deionized (DI) water (18.2 MΩ cm-1) used throughout all experiments was produced by a Milli-Q water purification system (Millipore). 2.2. Preparation of HPMo-modified pristine graphene. 20 mg graphene was dispersed in 50 mL of 10 mg mL-1 of HPMo solution via ultrasonication. The suspension was then filtered, washed with DI water, and dried at 70 ⁰C under vacuum overnight. The obtained product was denoted as HPMo-G. 2.3. Self-assembly of Pd nanoflowers on HPMo-G. 2 mg of HPMo-G was dispersed in 10 mL of DI water via ultrasonication. 1.9 mL of 2.966 mM H2PdCl4 solution was then added under magnetic stirring. After adjusting the pH of the mixed solution to 5.5 using a 0.1 M NaOH solution, 2 mL of 14 mg mL-1 L-AA solution was injected quickly and stirred for 3 h. The suspension was then centrifuged and washed with DI water for three times, and with ethanol for three times. Finally, it was dried at 70 ⁰C under vacuum overnight. The solid product was denoted as Pd NF/HPMo-G. 2.4. Self-assembly of Pd nanospheres on HPMo-G. 2.94 mg HPMo-G was dispersed in 50 mL of DI water via ultrasonication, and transferred to a three-necked flask. 1.26 mL of 5.636 mM H2PdCl4 was then added under magnetic stirring. Subsequently, the mixed solution was heated to 180 ⁰C under refluxing, and 0.15 g sodium citrate dissolved in 1 mL of water was quickly injected into the solution. The mixture was reacted for 6 h, and was then centrifuged and


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washed with DI water for three times, and with ethanol for three times. Finally, it was dried at 70 ⁰C under vacuum overnight. The obtained product was denoted as Pd NS/HPMo-G. 2.5. Self-assembly of Pd nanoparticles on pristine G. The procedure is the same as that used for the self-assembly of Pd nanoflowers on HPMo-G except that pristine graphene rather than HPMo-G was used as the support. The product was denoted as Pd NP/G. 2.6. Characterization. Field emission scanning electron microscopy (FESEM) imaging was carried out on a JEOL JSM-7800F microscope. Energy dispersive X-ray (EDX) analysis was performed using an EDX analysis unit attached on the same microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were taken on a JEOL JEM2100 microscope. X-ray powder diffraction (XRD) patterns were obtained using a Shimadzu XRD-7000 diffractometer with Cu Kα line. 2.7. Electrochemical measurements. All electrochemical measurements were performed with a three-electrode system at room temperature on a CHI660E electrochemical workstation (CH Instruments, Inc., USA). A saturated calomel electrode (SCE) and a platinum foil were used as the reference electrode and counter electrode, respectively. For the preparation of the working electrode, a 1 mg mL-1 sample suspension in ethanol with added Nafion (the mass ratio of sample to Nafion was 4:1) was ultrasonicated for 10 min, and then 20 µL of the suspension was pipetted onto a clean and polished glassy carbon electrode with an area of 0.19625 cm2 and dried under an infrared lamp. CO stripping experiments were carried out in 0.5 M H2SO4 solution. The solution was bubbled with CO gas for 10 min at a potential of 0.259 V vs. RHE, followed by bubbling with N2 for 15 min to remove the residual CO in solution. The CO stripping voltammograms were measured within the potential range between 0.059 V and 1.259 V vs.


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RHE at a scan rate of 20 mV s-1. The electrocatalytic activity was evaluated via cyclic voltammetry (CV), which was performed in 0.5 M HCOOH + 0.5 M H2SO4 in the potential range between 0.059 V and 1.059 V vs. RHE at a scan rate of 50 mV s-1. The durability was studied via chronoamperometry at 0.409 V vs. RHE and CV in the potential range between 0.059 V and 1.059 V vs. RHE at a scan rate of 100 mV s-1. The electrochemical measurements were carried out without iR compensation. All the potentials are reported against the reversible hydrogen electrode (RHE). In 0.5 M H2SO4, E(RHE)=E(SCE) + 0.259 V, based on the RHE calibration experiment.36 3. RESULTS AND DISCUSSION 3.1. Scheme for Synthesis of Pd NF/HPMo-G. The process for the synthesis of Pd NF/HPMo-G is schematically shown in Scheme 1. First, the pristine graphene was noncovalently modified with HPMo to obtain HPMo-G. Then, Pd nanoflowers were in-situ selfassembled on HPMo-G via reduction of H2PdCl4 with L-AA to obtain Pd NF/HPMo-G.

Scheme 1. Synthesis process of Pd NF/HPMo-G.


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3.2. HPMo-Mediated Self-Assembly and Characterization. The non-covalent modification of HPMo on pristine graphene was investigated via dispersion experiments. HPMo-G can be easily dispersed in DI water (5-10 min bath sonication is enough for dispersion), and it can maintain the dispersion state for at least 8 h (Fig. 1(a)). In contrast, it is very challenging to disperse the pristine graphene in DI water, and even after vigorous ultrasonication, graphene is much more easily aggregated and precipitated (Fig. 1(a)). This result indicates that HPMo can be used for the surface modification of pristine graphene. The successful non-covalent modification of pristine graphene by HPMo is further confirmed by the EDX analysis result, which shows that the Mo content in HPMo-G is ~13% (Fig. 1(b)). It needs to be noted that this loading does not change significantly when changing the concentration of HPMo from 1 mg mL-1 to 20 mg mL-1 (the solution volume is 50 mL), and therefore the nanostructures of Pd subsequently grown on HPMo-G and their catalytic performance do not depend on the concentration of HPMo in this range. This is because the amount of HPMo in the solution is much larger than that selfassembled on the pristine graphene (5.2 mg on 20 mg pristine graphene based on the EDX result).


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Figure 1. (a) Photographs of pristine G (left) and HPMo-G (right) dispersed in DI water after 8 h. (b) EDX spectrum of HPMo-G. The peak of Si in (b) arises from the Si substrate on which HPMo-G was deposited by drop casting. HPMo-G was subsequently used as a support for the growth of Pd nanostructures. The FESEM images of the obtained hybrid (Fig. 2(a) and (b)) show that a high density of nanoflowers have been uniformly assembled all over the graphene nanosheets and there are no aggregations between these nanoflowers. The low-magnification TEM image (Fig. 2(c)) further indicates the uniform distribution and good dispersion of nanoflowers over large area of the graphene nanosheets. The high-magnification TEM images (Fig. 2(d) and (e)) reveal that the average size of nanoflowers is 16.9±0.8 nm (see the size distribution in Fig. S1), which is among the smallest and most uniform branched Pd nanostructures reported,9,37,38 and the density on the surface of graphene is 4.79×1015 m-2, which is also among the highest value reported for the graphene/RGO supported Pd nanostructures.38,39 The HRTEM image (Fig. 2(f)) displays clear lattice fringes with spacings of 0.23 nm, which correspond to (111) planes of face-centered cubic (fcc) Pd.10,40 The XRD pattern (Fig. 2(g)) shows several characteristic peaks located at 40.1⁰, 46.7⁰, 68.2⁰, 82.2⁰, and 86.6⁰, which can be indexed to the (111), (200), (220), (311), and (222) planes of fcc Pd, consistent with the HRTEM result.10,40 The very slight peak at ~24.9⁰ is caused by the π–π stacking between graphene nanosheets, which is hardly avoidable for pristine graphene with an ultralow oxygen content used in this work.41,42


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Figure 2. (a) Low-magnification FESEM image of Pd NF/HPMo-G. (b) High-magnification FESEM image of Pd NF/HPMo-G. (c) Low-magnification TEM image of Pd NF/HPMo-G. (d


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and e) High-magnification TEM images of HPMo-G. (f) HRTEM image of the area marked with the blue rectangle in (e). (g) XRD pattern of Pd NF/HPMo-G. 3.3. Self-assembly Mechanism of Pd NF/HPMo-G. To study the mechanism for in-situ growth of Pd nanoflowers on HPMo-G, the pristine graphene was used as a support instead of HMPo-G. The resultant Pd nanoparticles mainly exhibit irregular shapes (Fig. 3(a) and (b)). Their dispersion is much less uniform than that of Pd nanoflowers supported on HPMo-G and there are serious aggregations between them. Their size distribution is also much broader, with some of them even larger than 100 nm. This result indicates that HPMo serves not only as a structure-directing agent and stabilizer to induce the formation of nanoflowers with narrow size distribution, but also as a linker for their high-density and uniform self-assembly. In contrast, it is rather difficult for Pd nuclei to be immobilized and well-dispersed on the inert pristine graphene, leading to their random attachment and growth to form non-uniform and severely aggregated Pd particles.

Figure 3. FESEM (a) and TEM (b) images of Pd nanoparticles supported on pristine graphene.


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Since solution pH can usually affect the reducing ability of a reductant,43-45 it was tuned to further investigate the in-situ growth mechanism. At a low pH of 3.2, only nanospheres are observed (Fig. 4(a)), and the size of Pd particles is 32.3±7.6 nm (see the size distribution in Fig. S2). When the solution pH is increased to 4.8, some nanoflowers appear, although most of them are still nanospheres (Fig. 4(b)). Their size decreases to 22.4±5.0 nm (see the size distribution in Fig. S3), and their density significantly increases (Fig. 4(b)). Interestingly, with further increasing the pH to 5.5, all the nanoparticles exhibit the flower shape (Fig. 4(c)). Their size further decreases to 16.9±0.8 nm (see the size distribution in Fig. S1), and the density becomes even higher (Fig. 4(c)). At higher pH, the reducing ability of L-AA is stronger, and therefore the nucleation process will be faster, leading to smaller size, narrower size distribution, and higher density.44-47 The higher content of nanoflowers formed at higher pH implies that the formation of nanoflowers is a kinetically controlled process.9,34 When the growth rate is low, the atom addition is slower than adatom diffusion. This allows for migration of adatoms on the nanocrystal surface to minimize the total surface energy, leading to the formation of nanospheres.9,46 However, when increasing the growth rate to a sufficiently high value by increasing the pH, a faster rate of atomic addition than that of adatom diffusion can make highenergy facets grow more quickly than that of low-energy ones, thus creating high-surface-energy nanostructures such as nanoflowers.9,34 It is noteworthy that no nanoflowers can be obtained at a higher pH of ~6.5 or above, which will severely damage the structure of HPMo (see Fig. S4 for the FESEM image of the Pd nanostructures formed at pH 6.9).26,48 This result further suggests that HPMo plays a critical role in the in-situ synthesis of Pd nanoflowers.


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Figure 4. FESEM images of HPMo-G supported Pd nanostructures obtained by reducing at pH 3.2 (a), 4.8 (b), and 5.5 (c). The above experimental results show that the HPMo-G supported Pd nanoflowers is formed via a HPMo-mediated self-assembly process in a kinetically controlled growth mode. HPMo can be non-covalently immobilized on pristine graphene to greatly improve its dispersion while serving as a linker, stabilizer, and structure-directing agent for the in-situ growth of high-density and well-dispersed Pd nanostructures with uniform size and shape.25-29 At appropriate pH, L-AA has a sufficiently strong reducing ability to promote the anisotropic overgrowth of Pd, while still maintaining the function of HPMo. It is very likely that the combination of the kinetically controlled process and HPMo-mediated growth synergistically contributes to the formation of nanoflower structure. The possible mechanism for the HPMo-mediated self-assembly of Pd NF/HPMo-G is illustrated in Scheme 2.


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Scheme 2. Possible mechanism for the HPMo-mediated self-assembly of Pd NF/HPMo-G. 3.4. Electrocatalytic Performance of Pd NF/HPMo-G. Pd NF/HPMo-G was then used as an electrocatalyst for FAOR. Pd NS/HPMo-G, in which the size of Pd nanospheres is slightly smaller than that of Pd nanoflowers in Pd NF/HPMo-G (see the synthesis process in the experimental part, the FESEM and TEM images in Fig. S5, and the size and distribution in Fig. S6) was used for comparison to investigate the shape effect. These Pd nanospheres are much smaller than those obtained by directly regulating the pH (Fig. 4(a)) and their size distribution is also much narrower, leading to higher performance (see Fig. S7). The reason for not using the Pd nanospheres synthesized via pH control as a comparison is that they are much larger than the Pd nanoflowers and thus it is difficult to exclude the size effect. Pd NP/G was also studied to reveal the critical role of HPMo in the electrocatalytic performance of Pd NF/HPMo-G. In addition, commercial Pd/C was adopted as a benchmark catalyst to demonstrate the great potential of Pd NF/HPMo-G. Fig. 5 shows CO stripping voltammograms of Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G, and commercial Pd/C. The hydrogen desorption peaks are suppressed in the first scan


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since the catalyst surface is covered with CO. Distinct CO oxidation peaks are observed during the first CV scan, but disappear during the second scan, indicating the removal of the adsorbed monolayer of CO species.6,49 The electrochemically active surface area (ECSA) can be estimated from the area of CO oxidation peaks, assuming that 420 µC/cm2 is needed for the oxidation of CO monolayer,6,50 and the result shows that Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G, and commercial Pd/C have ECSAs of 62.3, 23.7, 30.5, and 41.2 m2/g, respectively. It is noteworthy that the ECSA of Pd NF/HPMo-G is much larger than that of Pd NS/HPMo-G despite the smaller size of the Pd nanosphere than that of the Pd nanoflower (Fig. 2(d) and (e), Fig. S5, and Fig. S6). This result shows unique advantages of the nanoflower shape, which allows much easier diffusion in-and-out of reactants and products for fast electrocatalytic reaction than conventional nanospheres. CO tolerance of the catalysts can be evaluated via the onset potential of the CO oxidation peak, since it reflects the extra potential required mainly for overcoming activation energy of the reaction and a smaller one means an easier reaction.6,49,51,52 The onset potential of Pd NF/HPMo-G is 0.812 V, which is 8 mV lower than that of Pd NS/HPMo-G (0.820 V), 45 mV lower than that of Pd NP/G (0.857 V), and 32 mV lower than that of commercial Pd/C (0.844 V). The similar onset potentials of Pd NF/HPMo-G and Pd NS/HPMoG indicate that there is very little effect of the shape on the resistance of Pd to CO in our system. Nevertheless, the much lower onset potential of Pd NF/HPMo-G than that of Pd NP/G demonstrates that HPMo can significantly increase the resistance of Pd to CO poisoning. The CO tolerance of Pd NF/HPMo-G is even higher than that of commercial Pd/C, revealing its great potential as a catalyst for FAOR. It needs to be noted that it is difficult to evaluate the reaction based on the peak position because of the large dependence of the surface diffusion of CO molecules on CO surface concentration and the morphology of the catalyst, and the different


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mechanisms for CO oxidation on Pd NF/HPMo-G (oxidation by HPMo) and commercial Pd/C (oxidation by adsorbed OH).12,30

Figure 5. CO stripping voltammograms of Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G and commercial Pd/C in 0.5 M H2SO4 solution with a scan rate of 20 mV s-1. Fig. 6(a) and (b) display mass and ECSA-normalized CV curves of Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G, and commercial Pd/C in 0.5 M HCOOH + 0.5 M H2SO4 solution recorded at a scan rate of 20 mV s−1, respectively. The forward scan is characterized by a strong current peak at ~0.36 V and a shoulder at ~0.81 V (Fig. 6(a) and (b)). The peak at ~0.36 V can be attributed to the direct oxidation of formic acid to form CO2 (dehydrogenation path) while the shoulder at ~0.81 V is related to the oxidation of the formic acid with the formation of intermediate CO generated from the dissociative adsorption step (dehydration path).1,2 The peak potential for FAOR on Pd NF/HPMo-G is 0.353 V, which is 9 mV lower than 0.362 V on Pd


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NS/HPMo-G, and 49 mV lower than 0.402 V on Pd NP/G (Fig. 6(a) and (b)). In addition, the mass-normalized CV curves (Fig. 6(a)) show a forward peak current density for the FAOR on Pd NF/HPMo-G is 1.02 A mg-1, which is 4.43 times higher than 0.23 A mg-1 on Pd NS/HPMo-G and 7.85 times higher than 0.13 A mg-1 on Pd NP/G, and the ECSA-normalized ones (Fig. 6(b)) indicate that the forward peak current on Pd NF/HPMo-G is 16.37 A m-2, which is 1.69 times higher than 9.70 A m-2 on Pd NS/HPMo-G and 3.84 times higher than 4.26 A m-2 on Pd NP/G. These results clearly show that the shape of Pd has a great effect on its catalytic activity, with the nanoflower more active than the nanosphere, and HPMo serves as a highly effective promoter to significantly enhance the catalytic activity of Pd catalysts. The catalytic activity of Pd NF/HPMo was further compared with that of the commercial Pd/C catalyst. Pd NF/HPMo-G shows 11 mV lower peak potential than commercial Pd/C (0.364 V) (Fig. 6(a) and (b)). In addition, its mass and ECSA-normalized forward peak current densities are 1.96 and 1.30 times, respectively higher than those of the commercial one (0.52 A mg-1 and 12.62 A m-2) (Fig. 6(a) and (b)). The catalytic activity of Pd NF/HPMo was also compared with other reported Pd-based catalysts. Since ECSA can vary substantially depending on the approaches and measurement conditions adopted, only the mass activities were collected for the comparison.53,54 The forward peak current density is among the highest reported and the peak potential the lowest for Pd-based FAOR catalysts (Table 1).


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Figure 6. Mass (a) and ECSA (b) normalized CV curves of Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G and commercial Pd/C in 0.5 M HCOOH + 0.5 M H2SO4 solution recorded at a scan rate of 20 mV s−1.


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Table 1. Detailed comparison of the performance of Pd NF/HPMo-G with those of representative Pd-based FAOR catalysts. Catalyst Pd/HPMo-PDDAMWCNTs 2D porous Pd nanosheets (100)-faceted Pd SP@rGO Pd nanowires Hierarchically porous Pd nanospheres 3D Graphene Hollow Nanospheres@Pallad ium-Networks Pd30La70/rGO Pd arrow-headed tripods after being electrochemically cleaned Three-dimensional highly branched palladium tetrapods Pd nanodendrite assemblies Pd hollow nanospheres Hollow Pd nanospheres with porous shells supported on graphene Pd network nanostructures Multipod Pd Nanocrystals Cu-templated Pd nanoparticles supported on graphene nanosheets Three-dimensionally ordered mesoporous Pd networks

Surface area (m2 g-1 Pd)

Forward peak current density (A mg-1 Pd)a

N.A.

0.945

12.9

0.4093

25.43

~0.61

45.2

0.758

51.6

0.305

39.3

0.9724

~0.41 (vs. RHE)

0.5

0.5 M H2SO4

41

57.80

0.9864

0.66 (vs. Ag/AgCl)

1.0

0.25 M H2SO4

2

20

0.493

~0.24 (vs. SCE)

0.5

0.5 M H2SO4

50

N.A.

0.3659

~0.11 (vs. SCE)

0.5

0.5 M H2SO4

40

23.9

0.4516

N.A.

1.011

84.14

0.5296

21.2

0.2806

21.05

0.17

72.72

0.4463

0.14 (vs. SCE)

0.5

0.5 M H2SO4

62

N.A.

0.2574

0.23 (vs. SCE)

0.5

0.5 M H2SO4

63

Forward peak potential (V) 0.17 (vs. Ag/AgCl) 0.16 (vs. SCE) 0.393 (vs. RHE) ~0.3 (vs. Ag/AgCl) 0.073 (vs. SCE)

~0.14 (vs. SCE) 0.22 (vs. SCE) 0.23 (vs. SCE) ~0.10 (vs. SCE) 0.17 (vs. SCE)

Formic acid concentration (M) 0.5 0.5 0.5 0.5 0.l

0.5 0.5

0.5

0.5 0.5

Electrolyte 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.1 M H2SO4

0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.1 M HClO4

0.5 M H2SO4 a “A mg-1 Pd” is the activity per mg of Pd-metal itself. It does not include the mass of supports. Pd NF/HPMo-G

62.3

1.02

0.353

0.5

Reference 4 10 55 11 56

57 58

59

60 61

This work


19


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Fig. 7(a) and (b) show the mass and ECSA-normalized chronoamperometric curves of Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G, and commercial Pd/C, respectively. For all these catalysts, the current density shows an initial rapid decay and remains almost unchanged after 3600 s (Fig. 7(a) and (b)). The current densities of Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G, and commercial Pd/C decrease to 21.4%, 10.6%, 1.85%, and 10.7% of their initial values (Fig. 7(a) and (b)). It is worth noting that Pd NF/HPMo-G loses much less current than Pd NS/HPMoG although their CO tolerance is similar. This result suggests that Pd nanoflowers are more able to withstand harsh electrochemical conditions than Pd nanospheres. This is further supported by its higher CV cycling stability (will be discussed later). It is very likely that Pd nanoflowers have much stronger interaction with HPMo-G than Pd nanospheres due to the much higher interface area between Pd nanoflowers and HPMo-G than between Pd nanospheres and HPMo-G, thus being more able to withstand harsh electrochemical conditions. The steady-state massnormalized current density of Pd NF/HPMo-G is 0.198 A mg-1, which is 8.25, 86.09, and 4.4 times higher than that of Pd NS/HPMo-G (0.024 A mg-1), Pd NP/G (0.0023 A mg-1), and commercial Pd/C (0.045 A mg-1), respectively (Fig. 7(a)), and the steady-state ECSA-normalized current density of Pd NF/HPMo-G is 3.18 A m-2, which is 3.15, 42.4, and 2.92 times higher than that of Pd NS/HPMo-G (1.01 A m-2), Pd NP/G (0.075 A m-2), and commercial Pd/C (1.09 A m2

), respectively (Fig. 7(b)). This result shows that the shape of Pd also significantly affects its

durability, with the nanoflower much more durable than the nanosphere, and HPMo greatly enhances the durability of Pd. The high durability of Pd NF/HPMo-G is further supported by the negligible morphological change after the chronoamperometry test (Fig. S8(a)). Fig. 7(c) shows peak current retention versus cycle number curves of Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G, and commercial Pd/C. It is very interesting that the forward peak current


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density of the FAOR on Pd NF/HPMo-G increases by 1.9% after 100 cycles of CV measurements, indicating its excellent CV stability. This slight increase could be due to activation of the catalyst by the CV cycling.64,65 However, those of Pd NS/HPMo-G, Pd NP/G, and commercial Pd/C have decreased by 28.7%, 15.8%, and 72.8%, respectively, after the same number of CV cycles. The much higher CV cycling stability of Pd NF/HPMo-G than those of Pd NS/HPMo-G and Pd NP/G further demonstrates that Pd nanoflowers have much higher durability than Pd nanosphere, and HPMo remarkably promotes the durability of Pd catalysts. It is noteworthy that no significant change in morphology can be observed for Pd NF/HPMo-G after the CV cycling (Fig. S8(b)), further indicating its high stability.


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Figure 7. Mass (a) and ECSA (b) normalized chronoamperometric curves of Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G and commercial Pd/C in 0.5 M HCOOH + 0.5 M H2SO4 at 0.409 V, and the remaining percentage of peak current for Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G and commercial Pd/C after different numbers of CV cycles (c).


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The above electrochemical data unambiguously demonstrate superior catalytic activity and durability of Pd NF/HPMo-G. This remarkably enhanced catalytic performance can be ascribed to the great synergistic effects arising from the unique nanoflower shape, large promotion of catalytic property by HPMo, and excellent properties of pristine graphene: firstly, the small, high-density, and uniformly dispersed Pd nanoflowers attached on large-surface-area and highly conductive graphene provide large ECSA for FAOR; secondly, the unique structure of Pd nanoflowers could provide numerous defects such as steps and kinks, thus enhancing its intrinsic catalytic activity;9,34 thirdly, HPMo with strong CO oxidizing ability significantly enhances the CO tolerance of Pd, thus promoting the catalytic activity and durability;12,30 lastly, the strong binding between HPMo and Pd and the large interface area between HPMo-G and Pd nanoflowers further improves the stability under harsh electrochemical conditions.28,29 4. CONCLUSION For the first time, we have realized in-situ growth of small, uniform, high-density, and welldispersed Pd nanoflowers on pristine graphene via HPMo-mediated self-assembly, in which HPMo serves simultaneously as a linker, stabilizer, and structure-directing agent in this process, and the nanoflowers are formed under a kinetically controlled growth condition. The obtained HPMo-G supported Pd nanoflowers show a greatly enhanced electrocatalytic activity and durability compared to HPMo-G supported Pd nanospheres, graphene supported Pd nanoparticles, and commercial Pd/C toward FAOR in DFAFCs. The catalytic activity is also one of the highest among those of reported Pd-based catalysts. The superior electrocatalytic performance is synergistically caused by the unique nanoflower shape, large promotion effect of HPMo, and excellent properties of pristine graphene. This work not only develops a mild, facile, economical, and surfactant-free self-assembly strategy for the fabrication of pristine graphene


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supported Pd nanostructures with cocatalyst incorporated to achieve high catalytic performance toward FAOR, but also provides scientific insight into the possible mechanism of the selfassembly process, which can be further extended to fabricate other hybrid nanostructures for broad applications. Studies to develop new nanohybrids such as pristine graphene/CNTs supported and cocatalyst-integrated Pd nanoflowers with ultrasmall size (e.g., smaller than 10 nm) to further improve the catalytic performance is ongoing in our lab and very promising preliminary results have been accomplished. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Size distribution of Pd nanoflowers supported on HPMo-G, size distribution of Pd nanoparticles supported on HPMo-G assembled at pH 3.2, size distribution of Pd nanoparticles supported on HPMo-G assembled at pH 4.8, FESEM image of HPMo-G supported Pd nanostructures obtained by reducing at pH 6.9, FESEM and TEM images of Pd NS/HPMo-G, size distribution of Pd nanospheres in Pd NS/HPMo-G, mass normalized CV curves of Pd NS/HPMo-G synthesized via citrate reduction and via pH control, and FESEM images of Pd NF/HPMo-G after after the chronoamperometry test and after the CV cycling (PDF) AUTHOR INFORMATION Corresponding Author *Email: [email protected].


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ACKNOWLEDGMENT This work was financially supported by Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education of China, Chongqing Natural Science Foundation (Grant No. cstc2015jcyjA50029), the Fundamental Research Funds for the Central Universities (Grant No. XDJK2017B057), Start-up grant from Southwest University, China (Grant No. SWU114090), Program for Innovation Team Building at Institutions of Higher Education in Chongqing (Grant No. CXTDX201601011), and Chongqing Engineering Research Center for Micro-Nano Biomedical Materials and Devices. REFERENCES (1) Yu, X.; Pickup, P. G. Recent Advances in Direct Formic Acid Fuel Cells (DFAFC). J. Power Sources 2008, 182, 124-132. (2) Ali, H.; Kanodarwala, F. K.; Majeed, I.; Stride, J. A.; Nadeem, M. A. La2O3 Promoted Pd/rGO Electro-catalysts for Formic Acid Oxidation. ACS Appl. Mater. Interfaces 2016, 8, 32581−32590. (3) El-Nagar, G. A.; Mohammad, A. M.; El-Deab, M. S.; El-Anadouli, B. E. Propitious Dendritic Cu2O−Pt Nanostructured Anodes for Direct Formic Acid Fuel Cells. ACS Appl. Mater. Interfaces 2017, 9, 19766−19772. (4) Cui, Z.; Kulesza, P. J.; Li, C. M.; Xing, W.; Jiang, S. P. Pd Nanoparticles Supported on HPMo-PDDA-MWCNT and Their Activity for Formic Acid Oxidation Reaction of Fuel Cells. Int. J. Hydrogen Energy 2011, 36, 8508-8517.


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Scheme 1. Synthesis process of Pd NF/HPMo-G. 38x28mm (300 x 300 DPI)



Figure 1. (a) Photographs of pristine G (left) and HPMo-G (right) dispersed in DI water after 8 h. (b) EDX spectrum of HPMo-G. The peak of Si in (b) arises from the Si substrate on which HPMo-G was deposited by drop casting. 19x7mm (300 x 300 DPI)


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Figure 2. (a) Low-magnification FESEM image of Pd NF/HPMo-G. (b) High-magnification FESEM image of Pd NF/HPMo-G. (c) Low-magnification TEM image of Pd NF/HPMo-G. (d and e) High-magnification TEM images of HPMo-G. (f) HRTEM image of the area marked with the blue rectangle in (e). (g) XRD pattern of Pd NF/HPMo-G. 81x130mm (300 x 300 DPI)



Figure 3. FESEM (a) and TEM (b) images of Pd nanoparticles supported on pristine graphene. 23x10mm (300 x 300 DPI)


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Figure 4. FESEM images of HPMo-G supported Pd nanostructures obtained by reducing at pH 3.2 (a), 4.8 (b), and 5.5 (c). 12x3mm (300 x 300 DPI)



Scheme 2. Possible mechanism for the HPMo-mediated self-assembly of Pd NF/HPMo-G. 28x15mm (300 x 300 DPI)


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Figure 5. CO stripping voltammograms of Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G and commercial Pd/C in 0.5 M H2SO4 solution with a scan rate of 20 mV s-1. 38x29mm (300 x 300 DPI)



Figure 6. Mass (a) and ECSA (b) normalized CV curves of Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G and commercial Pd/C in 0.5 M HCOOH + 0.5 M H2SO4 solution recorded at a scan rate of 20 mV s−1. 76x115mm (300 x 300 DPI)


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Figure 7. Mass (a) and ECSA (b) normalized chronoamperometric curves of Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G and commercial Pd/C in 0.5 M HCOOH + 0.5 M H2SO4 at 0.15 V vs. SCE, and the remaining percentage of peak current for Pd NF/HPMo-G, Pd NS/HPMo-G, Pd NP/G and commercial Pd/C after different numbers of CV cycles (c). 110x241mm (300 x 300 DPI)



Table of Contents 35x15mm (600 x 600 DPI)


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Heteropolyacid-Mediated Self-Assembly of Heteropolyacid-Modified

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