- Email: [email protected]
Facile fabrication of stable PdCu clusters uniformly decorated on graphene as an efficient electrocatalyst for formic acid oxidation
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Facile fabrication of stable PdCu clusters uniformly decorated on graphene as an efficient electrocatalyst for formic acid oxidation Zhe Zhang a, Yuyan Gong a, Diben Wu a, Zhi Li a, Qing Li a, Linwei Zheng a, Wei Chen b, Weiyong Yuan c, Lian Ying Zhang a,b,* a
Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, 266071, PR China b CAS Key Laboratory of Low-Coal Conversion Science & Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, PR China c Institute for Clean Energy & Advanced Materials, Southwest University, Chongqing 400715, PR China
article info
abstract
Article history:
Small-sized nanocrystals uniformly distributed on suitable supports are promising as
Received 26 September 2018
active electrocatalysts due to their unique electronic structure and high accessible surface
Received in revised form
area. Herein, we report the synthesis of graphene supported PdCu nanoclusters with a
7 November 2018
uniform size of 2.3 nm via a facile one-pot solvothermal method. This nanohybrid exhibits
Accepted 3 December 2018
much higher inherent catalytic activity and stability for formic acid oxidation in compar-
Available online 28 December 2018
ison to graphene supported Pd clusters and commercial Pd/C. The PdCu nanoclusters can maintain their original size and dispersity on graphene even after 200 CV cycles. This work
Keywords:
offers an efficient low Pd-loading catalyst for formic acid oxidation, and the synthetic
Palladium copper alloy
approach is of significance to preparing uniform metal-based clusters decorated on sup-
Cluster
ports with superior catalytic properties for fuel cells and sensors.
Electrocatalyst
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Formic acid oxidation Fuel cell
Introduction Controlling the shape and size of metal nanocrystals has attracted particular interest in recent years due to their tailorable properties and promising applications especially in electrocatalysis, batteries, and sensors [1e4]. The crystals with ultrafine size are beneficial for obtaining large surface area and high fraction of edge or corner atoms, which could greatly promote their catalytic activity [5e7]. Currently,
solution phase synthesis is one of the most commonly used approach due to its ability to precisely control nanocrystal shape and size [8]. Yang and Xia et al. reported that the formation of metal nanocrystals with the solution phase route containing two key steps of nucleation and growth [9,10]. Zero-valent metal atoms could be produced depending on decomposition or reduction of corresponding metal precursors, followed by aggregation into nuclei such as small clusters when the nuclei concentration achieves supersaturation [11]. However, once the concentration of metal atoms
* Corresponding author. E-mail address: [email protected] (L.Y. Zhang). https://doi.org/10.1016/j.ijhydene.2018.12.004 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
2732
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
drops below the level of the supersaturation, nucleation stops, and the newly formed metal atoms will be grown onto previously generated nuclei [12]. Therefore, the shape and size of nanocrystals could be controlled by thermodynamics and/or kinetics [13,14]. Up to date, small-sized nanocrystals below 3 nm have been synthesized by the typical solution phase synthesis, showing enhanced properties in electrocatalysis. Sun et al. synthesized 3.0 nm Pt polyhedral via solvothermal approach in the presence of trace amount of Fe(CO)5 [15]. Du et al. prepared unsupported PdBi nanodots (~2.5 nm) using wet-chemical method [16]. Tang et al. synthesized “raisin bun”-like Pd clusters (~1.7 nm) by a one-pot hydrothermal method [17]. Nevertheless, all of these synthetic approaches involve unsupported nanoparticles, which suffer from unfavorable aggregation and surface area degradation in harsh reaction conditions (for example, low pH and high potentials). Attachment of above-mentioned small-sized nanocrystals on a suitable support such as graphene has been demonstrated as a significant route to suppress nanocrystal aggregation or Ostwald ripening for large specific surface area, high catalytic properties [18,19]. Metal nanocrystals could be easily anchored and immobilized on graphene surface due to a strong metal-support interaction [20]. According to density functional theory and bond-order potential simulations, Fampiou et al. reported that the catalytic behaviors of graphene-supported Pt nanocrystals could be improved due to the promoted charge-transfer from the nanoparticles to graphene [21]. Nevertheless, dispersing small-sized nanoparticles below 3.0 nm uniformly on support especially graphene is desirable and also an open challenge. Formic acid oxidation (FOR) is the essential anode reaction for direct formic acid fuel cell, which is regarded as one of promising power sources for portable electronic device and hybrid vehicle because of its high theoretical open-circuit potential (1.45 V), low crossover effect and pollution [22e25]. Currently, Pd is the most commonly used catalysts for FOR, but it suffers from high costs and low abundance [26]. Alloying Pd with non-noble transition metals (M) is a promising strategy to decrease the cost of Pd while improve its catalytic activity [27]. Cu has lower d-band center (~2.67 eV) than Pd (~1.83 eV), and thus could be a desirable alloying component for Pd to tune down its d-band center in terms of electronic and strain effects [28,29]. The down shifted d-band center of Pd was demonstrated to enhance CO-poisoning tolerance and facilitate 2e- FOR on Pd crystal surfaces [30]. Therefore, bimetallic PdCu system is believed to be a promising and active anode catalyst toward FOR. Up to date, researchers mainly focus on shape and composition of PdCu-based nanostructures for the enhancement of catalytic properties in comparison with plain Pd toward FOR [31e35]. To the best of our knowledge, small-sized PdCu alloy nanoclusters below 3.0 nm uniformly on graphene have not been synthesized, and their catalytic behaviors and mechanism of FOR have not been investigated and explored. In this study, PdCu clusters with a narrow size of 2.3 nm uniformly distributed on graphene (PdCu@Graphene) were synthesized successfully by a universal solution phase route. Ascorbic acid served as a reducing agent to synthesize PdCu clusters by the reduction of Pd (acac)2 and Cu(ac)2 with the assistance of benzyl alcohol and polyvinylpyrrolidone (PVP). The size of clusters and
their dispersity on graphene could be well maintained even after the compositions tuning. Toward electrooxidation of formic acid, the tailored Pd70Cu30@Graphene exhibits much higher catalytic activity and stability in comparison to Pd@Graphene and commercial Pd/C. Additionally, this unique PdCu clusters are generated under a low-temperature and the synthetic approach does not involve high-poisoning reactants, suggesting its great potential applications in the fields such as fuel cells and sensors.
Experimental section Synthesis of graphene First of all, graphene oxide was synthesized from graphite with the modified Hummers approach [22,36]. The synthesized graphene oxide was dried in 65 C overnight, followed by transferred into a glass bottle which was further heated to 120 C for 3 h under vacuum. After that, the bottle was heated to 250 C quickly, and the collected highly loose black powder was named as graphene.
Preparation of graphene supported PdCu clusters 20.0 mg Pd (acac)2 and 4.0 mg Cu(ac)2 were added into 5.0 mL benzyl alcohol, then the solution was stirred for 30 min under room temperature. After that, 19.8 mg graphene, 50.0 mg ascorbic acid, 86.0 mg polyvinylpyrrolidone (PVP, MW ¼ 30,000) and 10.0 mL ethanol were added to the above solution under ultrasonication for 2 h. The mixed solution was heated to 95 C for 10 h, and the sample was collected by centrifugation, washed with ethanol/deionized water mixture, and dried at 65 C for 8 h. The obtained product was named as PdCu@Graphene. The atomic ratio of Pd to Cu was tuned for comparison.
Electrochemical and physicochemical measurements Field emission scanning electron microscope (JEOL-JSM7800F) and transmission electron microscope (JEOL-JEM2100) were selected to characterize the size and nanostructure of samples in this work. Crystal structure of prepared materials were obtained by XRD (Shimadzu XRD-7000) with Cu Ka radiation (l ¼ 0.1541 nm). The nitrogen isotherm of PdCu@Graphene and commercial Pd/C were tested on automated gas sorption system (QuantachromeAutosorb iQ3) under 77.3 K. The electrochemical tests were carried with a CHI-760 E electrochemical workstation. A three-electrode cell was used in this work. A glassy carbon disk electrode was selected as the working electrode. Saturated calomel electrode (SCE) was used as reference electrode, while a platinum foil was served as counter electrode. 6.0 mg of prepared PdCu@Graphene, 0.9 mL DI water, 1.0 mL ethanol as well as 0.1 mL Nafion solution (5 wt%) were mixed together to obtain catalyst ink. After ultrasonicated for 0.5 h, 6 mL ink was dropped on working electrode and dried in room temperature. Electrocatalytic properties of all the catalysts in this work were investigated in H2SO4 or H2SO4 þ HCOOH solution, respectively. Scan rate is 50 mV s1. Electrochemical impedance
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
spectroscopy (EIS) tests were performed at the open circuit potential, and corresponding frequency range is between 100 K Hz and 0.05 Hz.
Results and discussion Physical characterization of Pd-based materials The TEM image (Fig. 1a) shows typical morphology of commercial Pd/C, of which Pd nanocrystals are dispersed on carbon support with an average size of 6.1 nm. Obviously a macroporous structure of prepared graphene could be observed in Fig. 1b, and PdCu clusters are uniformly dispersed on graphene surface with a narrow size of 2.3 nm (Fig. 1cee).
2733
Fig. 1d displays the HRTEM image of one PdCu cluster. It has a high crystallinity and the calculated period of adjacent fringes is 0.218 nm, which is attributed to PdCu(111) lattice spacing of an FCC structure. Our previous work demonstrated that the lattice spacing of Pd (111) is 0.225 nm [22], thus the decreased lattice distance of the PdCu(111) could be due to a certain lattice contraction, suggesting that Cu is successfully introduced into Pd forming an alloy structure [37,38]. Furthermore, the size and dispersity of Pd and PdCu clusters on graphene could be well maintained even after the composition tuning (Fig. 2), suggesting that the introduced Cu did not affect the basic morphology of Pd clusters and their dispersity. This approach is expected to be a promising route to synthesize other uniform metal-based nanoclusters decorated on supports for broad applications such as fuel cells and sensors.
Fig. 1 e (a) TEM image of commercial Pd/C. (b) SEM, (cee) TEM images of PdCu@Graphene. (f) HRTEM image of one PdCu cluster on graphene. Insets: corresponding size distribution of nanocrystals.
2734
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
Fig. 2 e TEM image of (a) Pd@Graphene, (b) Pd75Cu25@Graphene, (c) Pd65Cu35@Graphene and (d) Pd60Cu40@Graphene, respectively.
The nitrogen isotherms (Fig. 3a) show that prepared PdCu@Graphene materials deliver a much high specific surface area of 813.8 m2 g1, which is about four times larger than that of commercial Pd/C (206.4 m2 g1). Furthermore, PdCu@Graphene possesses much more mesopores (centered ~3.65 nm) in comparison to commercial Pd/C (Fig. 3b). Combining with the SEM image analysis in Fig. 1b, the prepared PdCu@Graphene in this work indeed owns rich macro- and mesopores, which can provide high electrocatalytic surface area and promote mass transport and electronic kinetics in electrocatalysis [22,39]. For further analysis of the crystallographic properties of PdCu clusters and commercial Pd/C, XRD was taken and shown in Fig. 3c. The typical diffraction peaks at 40.2 , 46.8 , 68.4 and 82.3 are ascribed to Pd (111), Pd (200), Pd (220) and Pd (311) planes, respectively. The diffraction peaks of PdCu@Graphene shift to higher positions in comparison to commercial Pd/C, demonstrating the formation of an alloy phase of PdCu because of the smaller atomic radius of Cu (0.128 nm) than that of Pd (0.137 nm) [40]. This result is in good accordance with the TEM characterization in Fig. 1f. Further, the diffraction peak of PdCu (111) exhibits a wider half-width in comparison with commercial Pd/C, indicating its smaller crystallite size of PdCu clusters according to the Debye-Scherrer formula [41]. The metal loading for PdCu@Graphene is 21.46%, and the atomic ratio between Pd and Cu for PdCu@Graphene is 2.40:1.01 (Fig. 3d), which agrees well with inductively coupled plasma atomic emission spectrophotometry analysis. Thus, the prepared PdCu clusters on graphene is named as Pd70Cu30@Graphene.
Electrocatalytic behaviours of Pd-based catalysts for formic acid oxidation Since the homogeneous PdCu clusters are decorated on graphene, they are expected to present a remarkable electrochemically active surface area (ECSA) and modified electronic structure for high catalytic activities [42]. In detail, small-sized clusters are believed to provide high accessible surface area, which stands for high catalytic active sites. Previous works demonstrated that Pd alloyed with Cu could change the electronic structure of Pd resulting in a low activation energy of formic acid and weak bond strength between PdCu crystal surface and immediate species [31,35]. All the catalysts (Fig. 4a) exhibit typical hydrogen adsorption/desorption over the range of 0.23 ~ 0.05 V and the formation of a OHad layer (2H2O ¼ OHad þ H3Oþ þ ee) beyond ~0.5 V, where OHad stands for the adsorbed hydroxyl species [43]. Interestingly, Pd70Cu30@Graphene exhibits around 60 mV more negative onset potential of adsorbed hydroxyl species than both Pd@Graphene and commercial Pd/C (Fig. 4b). Thus, the introduction of Cu element into Pd nanocrystals of Pd70Cu30@Graphene offers favourable oxygen-containing species such as OHad to oxidize poisoning species under a more negative potential over Pd@Graphene and commercial Pd/C, leading to a faster electrode kinetics for FOR [44]. The shifted onset potential of OHad for Pd70Cu30@Graphene could also be found in other Pd-based catalysts such as PdNi-NNs/RGO [45], Pd6Co/ 3DGraphene [46], and PdCu/C [47]. The peaks around 0.4 V are
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
2735
Fig. 3 e (a) The nitrogen isotherm of PdCu@Graphene and commercial Pd/C. (b) BJH pore size distribution, (c) XRD patterns of commercial Pd/C and PdCu@Graphene. (d) EDS spectrum of PdCu@Graphene.
associated with Pd oxide reduction, and Pd70Cu30@Graphene shows obviously larger peak area than that of Pd@Graphene and commercial Pd/C. ECSA is a significant parameter to evaluate the quantity of active sites for electrode, and can be calculated using the equation as follows [22]. ECSA ¼ Q= ð0:424 Pdm Þ
(1)
where Pdm is the weight of Pd loaded on working electrode, and Q is the reduction charge of Pd(II) oxide integrated from
corresponding reduction peak area. We supposed that 0.424 mC cm2 was needed for the reduction of a Pd(II) oxide monolayer. Pd70Cu30@Graphene presents much higher calculated ECSA (64.1 m2 g1) in comparison with Pd@Graphene (36.7 m2 g1) and commercial Pd/C (31.5 m2 g1), suggesting that prepared Pd70Cu30@Graphene could provide more catalytic activity sites, which is beneficial for promoting catalytic performance. The ECSA of Pd70Cu30@Graphene is also higher than most of previously reported Pd-based catalyst. (Table 1)
Fig. 4 e The CV curves of prepared Pd70Cu30@Graphene, Pd@Graphene and commercial Pd/C in 0.5 M H2SO4 solution.
2736
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
Table 1 e Formic acid oxidation behavior of different Pd-based catalysts. Electrocatalyst
ECSA
Peak potential (mVvs. SCE)
Peak current density (mA mg1Pd)
Ref
Pd6Co/3DG PdCu/C Pd nanowires porous Pd sheets PdeSn alloy PdCu alloy Pd NF/HPMo-G PdeFe/RGO PdCu/CNTs NPePd50Cu50 Pd70Cu30@Graphene
51.0 e 45.2 12.9 18.6 33.2 62.3 e 39.07 28.0 64.1
156 290 355 160 198 203 353 220 153 225 150
430.8 194.5 758.0 409.3 553.4 517.0 1020.0 1000.0 252.0 1069.0 1289.0
Ref [46] Ref [47] Ref [53] Ref [54] Ref [55] Ref [56] Ref [57] Ref [58] Ref [59] Ref [60] This work
Toward FOR, two oxidation peaks can be observed for all Pd-based catalysts in this study (Fig. 5a). The catalytic current density about 0.1e0.3 V (HCOOH / CO2 þ 2Hþ þ 2e) is much larger than that about 0.4e0.6 V (HCOOH / COads þ H2O / CO2 þ 2Hþ þ 2e), suggesting that the three catalysts mainly through a direct electron transfer pathway. However, the peak current density of catalyst Pd70Cu30@Graphene is 1289.0 mA mg1Pd, which is about 2.1 times higher than that of Pd@Graphene (627.2 mA mg1Pd) and even 2.5 times higher than that of commercial Pd/C (526.3 mA mg1Pd). Fundamen-
tally, oxidation peak potential is a significant parameter to estimate the catalytic activity of an electrode, and the more negative oxidation peak potential implies the lower overpotential and higher energy conversion efficiency. The oxidation peak potential of Pd70Cu30@Graphene catalyst is 149.8 mV, which is much more negative in comparison with both Pd@Graphene (172.1 mV) and commercial Pd/C (194.6 mV). While Pd70Cu30@Graphene catalyst also delivers more negative onset potential and half-wave potential for FOR in comparison to both Pd@Graphene and commercial Pd/C.
Fig. 5 e (a) CV curves on Pd70Cu30@Graphene, Pd@Graphene and commercial Pd/C and (b) corresponding catalytic activity comparison concerning active Pd at peak potentials.
Fig. 6 e (a) CV curves of Pd70Cu30@Graphene, Pd75Cu25@Graphene Pd65Cu350@Graphene and Pd60Cu40@Graphene catalysts toward FOR and corresponding (b) CV curves testing in 0.5 M H2SO4 electrolyte.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
Additionally, Pd70Cu30@Graphene shows much higher mass activity (normalized with the metal loading Pd) and specific activity (normalized with ECSA) over Pd/Graphene and commercial Pd/C (Fig. 5b). These results demonstrated that the Cu modification of Pd70Cu30@Graphene could indeed enhance electrode kinetics for FOR and owns the highest inherent electrocatalytic activity in comparison to Pd@Graphene and commercial Pd/C. To investigate the effects of Cu in PdCu@Graphene toward electrooxidation of formic acid, the stoichiometric ratio for Pd to Cu was tuned and corresponding electrochemical behaviors were assessed as in Fig. 6a. The peak current density increased with decreasing loading of Cu in PdCu@Graphene system and the optimized stoichiometric ratio of Pd and Cu is 70:30, which
2737
delivers the highest peak current density up to 1289.0 mA mg1Pd. The peak current density is reduced when further decreasing the Cu loading in PdCu@Graphene. Further, CV curves were measured to evaluate the ECSA of tuned composition of PdCu@Graphene systems (Fig. 6b), and Pd70Cu30@Graphene shows the highest peak area of Pd oxide redox and largest ECSA over other PdCu-based catalysts, suggesting its most available catalytic active sites. Thus, appropriate loading of Cu in PdCu bimetallic catalyst is believed to effectively modifying electronic and geometric structures of Pd surface affecting the ECSA and electrode kinetics of FOR. This finding concurs with the result reported in our previously work [48]. The electrochemical stability of Pd-based catalysts was evaluated by amperometric i-t tests as in Fig. 7a. Obviously,
Fig. 7 e (a) I-t curves of Pd70Cu30@Graphene, Pd@Graphene and commercial Pd/C. Fixed potential: 0.15 V (vs.SCE). The comparison of peak current density for commercial Pd/C (b), Pd@Graphene (c) and Pd70Cu30@Graphene (d) before and after 200 cycles. TEM images of commercial Pd/C (e) and Pd70Cu30@Graphene (f) after 200 cycles.
2738
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
Fig. 8 e (a) Tafel plots and (b) Nyquist plots of Pd70Cu30@Graphene, Pd@Graphene and commercial Pd/C.
Pd70Cu30@Graphene has much higher catalytic current density at the initial in comparison with Pd@Graphene and commercial Pd/C, while the former holds the highest catalytic current density even after 4000s testing, implying its best durability and more beneficial for practical application. The seriously decreased current densities of all Pd-based catalysts could be ascribed to the slow deactivation by adsorption of poisoning intermediates [42]. The dynamic durability of various Pd-based catalysts was also investigated in an accelerated stability test by recording 200 cycles (Fig. 7bed). After 200 CV cycles, commercial Pd/C could maintain 83.4% of its original peak current density, and corresponding value for Pd@Graphene is 85.1%. However, Pd70Cu30@Graphene could keep 95.6% of its original peak current density. This result confirms that Pd70Cu30@Graphene indeed has better durability in comparison to Pd@Graphene and commercial Pd/C. The crystal size and morphology of commercial Pd/C and Pd70Cu30@Graphene after 200 cycles testing were characterized as in Fig. 7e and f. Pd nanocrystals occur obvious aggregation in commercial Pd/C. While for Pd70Cu30@Graphene, PdCu nanoclusters could maintain their uniform distribution on graphene surface with an average size of 3.4 nm. Previously work demonstrated that the crystals aggregation mainly derived from Oswald ripening phenomenon [49,50]. Thus, we argue that the Pd nanoclusters modified by Cu element forming a PdCu alloy surface is favourable to suppress Oswald ripening for great durability, which could be contributed to the tuned electronic effect between Pd nanocrystals and introduced Cu.
transfer kinetics than that of both Pd@Graphene and commercial Pd/C, indicating a favourable reactants adsorption amount of Pd70Cu30@Graphene and confirming its highest inherent catalytic activity toward FOR. Furthermore, electrochemical impedance spectroscopy, a useful technique for evaluating the dynamics, was selected in this work to investigate the charge-transfer resistance for FOR in current work [51]. The semicircle over high and medium frequency region tells the charge-transfer impedance (Rct) between electrode and electrolyte interface, and the larger diameter of the semicircle corresponds to the larger Rct [52]. The calculated Rct of Pd70Cu30@Graphene catalyst is 128.3 U(Fig. 8b), which is lower than that of Pd@Graphene (155.9 U) and commercial Pd/ C (464.6 U). Standard exchange current density (i0) can be selected to estimate intrinsic catalytic activity of Pd-based catalysts in this work, which is usually studied with the equation as follows [22]. i0 ¼ R T=ðn F Rct Þ
(2)
where R and F are gas and faradic constant, respectively. T means absolute temperature. The calculated result reveals that Pd70Cu30@Graphene holds a highest value of i0 (8.08 104 A cm2) over Pd@Graphene (6.65 104 A cm2) and commercial Pd/C (2.23 104 A cm2), further confirming its highest inherent catalytic activity for FOR, which also concurs with the CV results in Fig. 5a and Tafel slopes analysis in Fig. 8a. Comparing with previously reported Pd-based catalysts in Table 1, Pd70Cu30@Graphene also presents the highest electrocatalytic activity for formic acid oxidation in terms of ECSA, oxidation peak potential and peak current density.
The insights of catalytic enhancement mechanism for Pd70Cu30@Graphene
Conclusions Tafel slope is a significant parameter to uncover the catalytic mechanism and the reactants adsorption amount in kineticcontrol region in catalytic process [26]. The calculated Tafel slope for commercial Pd/C and Pd@Graphene catalyst is 169.4 mV decade1 and 160.1 mV decade1, respectively (Fig. 8a). While for Pd70Cu30@Graphene, the calculated Tafel slope is only 133.5 mV decade1. The smaller Tafel slope value of Pd70Cu30@Graphene catalyst suggested its faster charge-
A new facile one-pot solvothermal method was demonstrated to synthesize stable PdCu nanoclusters uniformly decorated on graphene with an average size of 2.3 nm. After tuning composition of PdxCu clusters, graphene decorated with Pd70Cu30 clusters exhibited an oxidation peak current density up to 1289.0 mA mg1Pd toward FOR, which is 2.5 times higher than that of commercial Pd/C (526.3 mA mg1Pd). This is also
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
the highest value for PdCu-based catalysts toward FOR. The PdCu nanoclusters could maintain their original morphology and structure even after 200 CV cycles. This work offers a lowPd loading but highly active catalyst toward formic acid oxidation, and a one-pot solvothermal method, which opens new opportunities to synthesize uniform metal alloy-based nanoclusters decorated on supports for electrocatalysis and sensing devices. Zhe Zhang and Yuyan Gong contributed equally to this work. The authors declare no competing financial interest.
Acknowledgment We gratefully acknowledge to the financial support from the Thousand Talents Plan, the World-Class University and Discipline, the Taishan Scholar's Advantageous and Distinctive Discipline Program of Shandong Province, the world-Class Discipline Program of Shandong Province, CAS Key Laboratory of Low-Coal Conversion Science & Engineering (KLLCCSE201706, SARI, CAS), Key Research and Development Plan of Shandong Province (No. 2018GGX102019), Natural Science Foundation of Shandong Province (No. ZR2017BB022), China Postdoctoral Science Foundation (No. 2017M612201), Applied Basic Research Program of Qingdao (No. 18-2-2-5-jch) as well as scientific research fund from Qingdao university (No. 41117010098) are also acknowledged.
references
[1] Jin H, Guo C, Liu X, Liu J, Vasileff A, Jiao Y, Zheng Y, Qiao SZ. Emerging two-dimensional nanomaterials for electrocatalysis. Chem Rev 2018;118(13):6337e408. [2] Li DN, Wang AJ, Wei J, Zhang QL, Feng JJ. Facile synthesis of flower-like Au@AuPd nanocrystals with highly electrocatalytic activity for formic acid oxidation and hydrogen evolution reactions. Int J Hydrogen Energy 2017;42(31):19894e902. [3] Zhang LY, Wu D, Gong Y, Liu H, Chen Wei, Bi L. Carbon monoxide-templated synthesis of coral-like clean PtPd nanochains as efficient oxygen reduction catalyst. ChemElectroChem 2018;5:2403e8. [4] Rahm JM, Erhart P. Beyond magic numbers: atomic scale equilibrium nanoparticle shapes for any size. Nano Lett 2017;17(9):5775e81. [5] Zhu Q-L, Xu Q. Immobilization of ultrafine metal nanoparticles to high-surface-area materials and their catalytic applications. Inside Chem 2016;1(2):220e45. [6] Yuan W, Fan X, Cui ZM, Chen T, Dong Z, Li CM. Controllably self-assembled graphene-supported Au@Pt bimetallic nanodendrites as superior electrocatalysts for methanol oxidation in direct methanol fuel cells. J Mater Chem 2016;4(19):7352e64. [7] Lu S, Hu Y, Wan S, McCaffrey R, Jin Y, Gu H, Zhang W. Synthesis of ultrafine and highly dispersed metal nanoparticles confined in a thioether-containing covalent organic framework and their catalytic applications. J Am Chem Soc 2017;139(47):17082e8. [8] Ruditskiy A, Xia Y. The science and art of carving metal nanocrystals. ACS Nano 2017;11(1):23e7. [9] Tao AR, Habas S, Yang P. Shape control of colloidal metal nanocrystals. Small 2008;4(3):310e25.
2739
[10] Xia Y, Gilroy KD, Peng HC, Xia X. Seed-mediated growth of colloidal metal nanocrystals. Angew Chem Int Ed 2017;56(1):60e95. n LM. The relevance of light in the [11] Grzelczak M, Liz-Marza formation of colloidal metal nanoparticles. Chem Soc Rev 2014;43(7):2089e97. [12] Wang Y, Choi SI, Zhao X, Xie S, Peng HC, Chi M, Huang CZ, Xia Y. Polyol synthesis of ultrathin Pd nanowires via attachment-based growth and their enhanced activity towards formic acid oxidation. Adv Funct Mater 2013;24(1):131e9. [13] Xia Y, Xia X, Peng H-C. Shape-controlled synthesis of colloidal metal nanocrystals: thermodynamic versus kinetic products. J Am Chem Soc 2015;137(25):7947e66. [14] Wang Y, He J, Liu C, Chong WH, Chen H. Thermodynamics versus kinetics in nanosynthesis. Angew Chem Int Ed 2015;54(7):2022e51. [15] Wang C, Daimon H, Onodera T, Koda T, Sun S. A general approach to the size- and shape-controlled synthesis of platinum nanoparticles and their catalytic reduction of oxygen. Angew Chem Int Ed 2010;120(19):3644e7. [16] Xu H, Zhang K, Yan B, Wang J, Wang C, Li S, Gu Z, Du Y, Yang P. Ultra-uniform PdBi nanodots with high activity towards formic acid oxidation. J Power Sources 2017;356:27e35. [17] Wang X, Yang J, Yin H, Song R, Tang Z. “Raisin bun”-like nanocomposites of palladium clusters and porphyrin for superior formic acid oxidation. Adv Mater 2013;25(19):2728e32. [18] Liu M, Zhang R, Chen W. Graphene-supported nanoelectrocatalysts for fuel cells: synthesis, properties, and applications. Chem Rev 2014;114(10):5117e60. [19] Zhou X, Qiao J, Yang L, Zhang J. A review of graphene-based nanostructural materials for both catalyst supports and metal-free catalysts in PEM fuel cell oxygen reduction reactions. Adv Energy Mater 2014;4(8):1301523. [20] Tan C, Huang X, Zhang H. Synthesis and applications of graphene-based noble metal nanostructures. Mater Today 2013;16(1e2):29e36. [21] Fampiou I, Ramasubramaniam A. Binding of Pt nanoclusters to point defects in graphene: adsorption, morphology, and electronic structure. J Phys Chem C 2012;116(11):6543e55. [22] Zhang LY, Zhao ZL, Li CM. Formic acid-reduced ultrasmall Pd nanocrystals on graphene to provide superior electrocatalytic activity and stability toward formic acid oxidation. Nano Energy 2015;11:71e7. [23] Fan H, Cheng M, Wang L, Song Y, Cui Y, Wang R. Extraordinary electrocatalytic performance for formic acid oxidation by the synergistic effect of Pt and Au on carbon black. Nano Energy 2018;48:1e9. [24] Yan X, Hu X, Fu G, Xu L, Lee JM, Tang Y. Facile synthesis of porous Pd3Pt half-shells with rich “active sites” as efficient catalysts for formic acid oxidation. Small 2018;14(13):1703940. [25] Gong Y, Liu X, Gong Y, Wu D, Xu B, Bi L, Zhang LY, Zhao XS. Synthesis of defect-rich palladium-tin alloy nanochain networks for formic acid oxidation. J Colloid Interface Sci 2018;530:189e95. [26] Wang F, Xue H, Tian Z, Xing W, Feng L. Fe2P as a novel efficient catalyst promoter in Pd/C system for formic acid electro-oxidation in fuel cells reaction. J Power Sources 2018;375:37e42. [27] Liu Z, Fu G, Li J, Liu Z, Xu L, Sun D, Tang Y. Facile synthesis based on novel carbon-supported cyanogel of structurally ordered Pd 3 Fe/C as electrocatalyst for formic acid oxidation. Nano Res 2018;11(9):4686e96. [28] Zhang LY, Gong Y, Wu D, Wu G, Xu B, Bi L, Yuan W, Cui Z. Twisted palladium-copper nanochains toward efficient electrocatalytic oxidation of formic acid. J Colloid Interface Sci 2019;537:366e74.
2740
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
[29] Wang C, Chen DP, Sang X, Unocic RR, Skrabalak SE. Sizedependent disordereorder transformation in the synthesis of monodisperse intermetallic PdCu nanocatalysts. ACS Nano 2016;10(6):6345e53. [30] Chen Y, Yang Y, Fu G, Xu L, Sun D, Lee J-M, Tang Y. Coreeshell CuPd@ Pd tetrahedra with concave structures and Pd-enriched surface boost formic acid oxidation. J Mater Chem 2018;6:10632e8. [31] Yang N, Zhang Z, Chen B, Huang Y, Chen J, Lai Z, Chen Y, Sindoro M, Wang AL, Cheng H. Synthesis of ultrathin PdCu alloy nanosheets used as a highly efficient electrocatalyst for formic acid oxidation. Adv Mater 2017;29(29). [32] Ye W, Chen S, Ye M, Ren C, Ma J, Long R, Wang C, Yang J, Song L, Xiong Y. Pt4PdCu0.4 alloy nanoframes as highly efficient and robust bifunctional electrocatalysts for oxygen reduction reaction and formic acid oxidation. Nano Energy 2017;39:532e8. [33] Dong Q, Zeng W, Wan H, Yu S, Guo C, Huang M. Highly effective PdCu electrocatalysts supported on polyelectrolyte functionalized titanium dioxide for direct formic acid fuel cells. Int J Electrochem Sci 2017;12(2):1389e400. [34] Chen Y, Yang Y, Fu G, Xu L, Sun D, Lee JM, Tang Y. Coreeshell CuPd@Pd tetrahedra with concave structures and Pdenriched surface boost formic acid oxidation. J Mater Chem 2018;6(23):10632e8. [35] Jiang B, Li C, Malgras V, Bando Y, Yamauchi Y. Threedimensional hyperbranched PdCu nanostructures with high electrocatalytic activity. Chem Commun 2016;52(6):1186e9. [36] Hummers Jr WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80(6). 1339-1339. [37] Zhu C, Shi Q, Fu S, Song J, Xia H, Du Dan, Lin Y. Efficient synthesis of MCu (M¼Pd, Pt, and Au) aerogels with accelerated gelation kinetics and their high electrocatalytic activity. Adv Mater 2016;28:8779e83. [38] Zhu C, Wen D, Oschatz M, Holzschuh M, Liu W, Herrmann AK, Simon F, Kaskel S, Eychmu¨ller A. Kinetically controlled synthesis of PdNi bimetallic porous nanostructures with enhanced electrocatalytic activity. Small 2015;11(12):1430e4. [39] Zhang LY, Zhang W, Zhou Z, Li CM. g-Fe2O3 nanocrystalsanchored macro/meso-porous graphene as a highly efficient adsorbent toward removal of methylene blue. J Colloid Interface Sci 2016;476:200e5. [40] Mandal K, Bhattacharjee D, Roy PS, Bhattacharya SK, Dasgupta S. Room temperature synthesis of PdeCu nanoalloy catalyst with enhanced electrocatalytic activity for the methanol oxidation reaction. Appl Catal Gen 2015;492:100e6. [41] Wang Z, Fan H, Liang H, Ma J, Li S, Song Y, Wang R. Microfluidic synthesis and characterization of FePtSn/C catalysts with enhanced electro-catalytic performance for direct methanol fuel cells. Electrochim Acta 2017;230:245e54. [42] Ma Y, Yin L, Cao G, Huang Q, He M, Wei W, Zhao H, Zhang D, Wang M, Yang T. Pt-Pd bimetal popcorn nanocrystals: enhancing the catalytic performance by combination effect of stable multipetals nanostructure and highly accessible active sites. Small 2018;14(14):1703613. [43] Lim B, Jiang M, Camargo PHC, Cho EC, Tao J, Lu X, Zhu Y, Xia Y. Pd-Pt bimetallic nanodendrites with high activity for oxygen reduction. Science 2009;324(5932):1302e5. [44] Yuan T, Chen H-Y, Ma X, Feng J-J, Yuan P-X, Wang A-J. Simple synthesis of self-supported hierarchical AuPd alloyed nanowire networks for boosting electrocatalytic activity toward formic acid oxidation. J Colloid Interface Sci 2018;513:324e30.
[45] Bin D, Yang B, Ren F, Zhang K, Yang P, Du Y. Facile synthesis of PdNi nanowire networks supported on reduced graphene oxide with enhanced catalytic performance for formic acid oxidation. J Mater Chem 2015;3(26):14001e6. [46] Zhang LY, Zhao ZL, Yuan W, Li CM. Facile one-pot surfactant-free synthesis of uniform Pd6Co nanocrystals on 3D graphene as an efficient electrocatalyst toward formic acid oxidation. Nanoscale 2016;8(4):1905e9. [47] Wang L, Zhai JJ, Jiang K, Wang JQ, Cai WB. PdeCu/C electrocatalysts synthesized by one-pot polyol reduction toward formic acid oxidation: structural characterization and electrocatalytic performance. Int J Hydrogen Energy 2015;40(4):1726e34. [48] Zhang LY, Gong Y, Wu D, Li Z, Li Q, Zheng L, Chen W. Palladium-cobalt nanodots anchored on graphene: In-situ synthesis, and application as an anode catalyst for direct formic acid fuel cells. Appl Surf Sci 2019;469:305e11. [49] Zhang T, Wang P, Chen H, Pei P. A review of automotive proton exchange membrane fuel cell degradation under start-stop operating condition. Appl Energy 2018;223:249e62. [50] Zhang LY, Gong Y, Liu H, Yuan W, Liu Z. Ultrasmall and uniform Pt3Au clusters strongly suppress Ostwald ripening for efficient ethanol oxidation. Electrochem Commun 2017;84:1e5. [51] Guo CX, Zhang LY, Miao J, Zhang J, Li CM. DNAFunctionalized graphene to guide growth of highly active Pd nanocrystals as efficient electrocatalyst for direct formic acid fuel cells. Adv Energy Mater 2013;3(2):167e71. [52] Zhang M, Yu X, Ma H, Du W, Qu L, Li C, Shi G. Robust graphene composite films for multifunctional electrochemical capacitors with an ultrawide range of areal mass loading toward high-rate frequency response and ultrahigh specific capacitance. Energy Environ Sci 2018;11(3):559e65. [53] Xu D, Liu X, Han M, Bao J. Facile synthesis of ultrathin singlecrystalline palladium nanowires with enhanced electrocatalytic activities. Chem Commun 2016;52(88):12996e9. [54] Qiu X, Zhang H, Wu P, Zhang F, Wei S, Sun D, Xu L, Tang Y. One-pot synthesis of freestanding porous palladium nanosheets as highly efficient electrocatalysts for formic acid oxidation. Adv Funct Mater 2017;27(1):1603852. [55] Sun D, Si L, Fu G, Liu C, Sun D, Chen Y, Tang Y, Lu T. Nanobranched porous palladiumetin intermetallics: onestep synthesis and their superior electrocatalysis towards formic acid oxidation. J Power Sources 2015;280(7):141e6. [56] Yang F, Zhang Y, Liu P-F, Cui Y, Ge X-R, Jing Q-S. Pd-Cu alloy with hierarchical network structure as enhanced electrocatalysts for formic acid oxidation. Int J Hydrogen Energy 2016;41(16):6773e80. [57] Fan X, Yuan W, Zhang DH, Li CM. Heteropolyacid-mediated self-assembly of heteropolyacid-modified pristine graphene supported Pd nanoflowers for superior catalytic performance toward formic acid oxidation. ACS Appl Energy Mater 2018;1(2):411e20. [58] Feng A, Bai J, Shao W, Hong W, Tian ZQ, Xiao Z. Surfactantfree PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation. Int J Hydrogen Energy 2017;42(22):15196e202. [59] Zhu F, Ma G, Bai Z, Hang R, Tang B, Zhang Z, Wang X. High activity of carbon nanotubes supported binary and ternary Pd-based catalysts for methanol, ethanol and formic acid electro-oxidation. J Power Sources 2013;242:610e20. [60] Xu C, Liu A, Qiu H, Liu Y. Electrochem Commun 2011;13:766e9.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Facile fabrication of stable PdCu clusters uniformly decorated on graphene as an efficient electrocatalyst for formic acid oxidation Zhe Zhang a, Yuyan Gong a, Diben Wu a, Zhi Li a, Qing Li a, Linwei Zheng a, Wei Chen b, Weiyong Yuan c, Lian Ying Zhang a,b,* a
Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, 266071, PR China b CAS Key Laboratory of Low-Coal Conversion Science & Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, PR China c Institute for Clean Energy & Advanced Materials, Southwest University, Chongqing 400715, PR China
article info
abstract
Article history:
Small-sized nanocrystals uniformly distributed on suitable supports are promising as
Received 26 September 2018
active electrocatalysts due to their unique electronic structure and high accessible surface
Received in revised form
area. Herein, we report the synthesis of graphene supported PdCu nanoclusters with a
7 November 2018
uniform size of 2.3 nm via a facile one-pot solvothermal method. This nanohybrid exhibits
Accepted 3 December 2018
much higher inherent catalytic activity and stability for formic acid oxidation in compar-
Available online 28 December 2018
ison to graphene supported Pd clusters and commercial Pd/C. The PdCu nanoclusters can maintain their original size and dispersity on graphene even after 200 CV cycles. This work
Keywords:
offers an efficient low Pd-loading catalyst for formic acid oxidation, and the synthetic
Palladium copper alloy
approach is of significance to preparing uniform metal-based clusters decorated on sup-
Cluster
ports with superior catalytic properties for fuel cells and sensors.
Electrocatalyst
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Formic acid oxidation Fuel cell
Introduction Controlling the shape and size of metal nanocrystals has attracted particular interest in recent years due to their tailorable properties and promising applications especially in electrocatalysis, batteries, and sensors [1e4]. The crystals with ultrafine size are beneficial for obtaining large surface area and high fraction of edge or corner atoms, which could greatly promote their catalytic activity [5e7]. Currently,
solution phase synthesis is one of the most commonly used approach due to its ability to precisely control nanocrystal shape and size [8]. Yang and Xia et al. reported that the formation of metal nanocrystals with the solution phase route containing two key steps of nucleation and growth [9,10]. Zero-valent metal atoms could be produced depending on decomposition or reduction of corresponding metal precursors, followed by aggregation into nuclei such as small clusters when the nuclei concentration achieves supersaturation [11]. However, once the concentration of metal atoms
* Corresponding author. E-mail address: [email protected] (L.Y. Zhang). https://doi.org/10.1016/j.ijhydene.2018.12.004 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
2732
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
drops below the level of the supersaturation, nucleation stops, and the newly formed metal atoms will be grown onto previously generated nuclei [12]. Therefore, the shape and size of nanocrystals could be controlled by thermodynamics and/or kinetics [13,14]. Up to date, small-sized nanocrystals below 3 nm have been synthesized by the typical solution phase synthesis, showing enhanced properties in electrocatalysis. Sun et al. synthesized 3.0 nm Pt polyhedral via solvothermal approach in the presence of trace amount of Fe(CO)5 [15]. Du et al. prepared unsupported PdBi nanodots (~2.5 nm) using wet-chemical method [16]. Tang et al. synthesized “raisin bun”-like Pd clusters (~1.7 nm) by a one-pot hydrothermal method [17]. Nevertheless, all of these synthetic approaches involve unsupported nanoparticles, which suffer from unfavorable aggregation and surface area degradation in harsh reaction conditions (for example, low pH and high potentials). Attachment of above-mentioned small-sized nanocrystals on a suitable support such as graphene has been demonstrated as a significant route to suppress nanocrystal aggregation or Ostwald ripening for large specific surface area, high catalytic properties [18,19]. Metal nanocrystals could be easily anchored and immobilized on graphene surface due to a strong metal-support interaction [20]. According to density functional theory and bond-order potential simulations, Fampiou et al. reported that the catalytic behaviors of graphene-supported Pt nanocrystals could be improved due to the promoted charge-transfer from the nanoparticles to graphene [21]. Nevertheless, dispersing small-sized nanoparticles below 3.0 nm uniformly on support especially graphene is desirable and also an open challenge. Formic acid oxidation (FOR) is the essential anode reaction for direct formic acid fuel cell, which is regarded as one of promising power sources for portable electronic device and hybrid vehicle because of its high theoretical open-circuit potential (1.45 V), low crossover effect and pollution [22e25]. Currently, Pd is the most commonly used catalysts for FOR, but it suffers from high costs and low abundance [26]. Alloying Pd with non-noble transition metals (M) is a promising strategy to decrease the cost of Pd while improve its catalytic activity [27]. Cu has lower d-band center (~2.67 eV) than Pd (~1.83 eV), and thus could be a desirable alloying component for Pd to tune down its d-band center in terms of electronic and strain effects [28,29]. The down shifted d-band center of Pd was demonstrated to enhance CO-poisoning tolerance and facilitate 2e- FOR on Pd crystal surfaces [30]. Therefore, bimetallic PdCu system is believed to be a promising and active anode catalyst toward FOR. Up to date, researchers mainly focus on shape and composition of PdCu-based nanostructures for the enhancement of catalytic properties in comparison with plain Pd toward FOR [31e35]. To the best of our knowledge, small-sized PdCu alloy nanoclusters below 3.0 nm uniformly on graphene have not been synthesized, and their catalytic behaviors and mechanism of FOR have not been investigated and explored. In this study, PdCu clusters with a narrow size of 2.3 nm uniformly distributed on graphene (PdCu@Graphene) were synthesized successfully by a universal solution phase route. Ascorbic acid served as a reducing agent to synthesize PdCu clusters by the reduction of Pd (acac)2 and Cu(ac)2 with the assistance of benzyl alcohol and polyvinylpyrrolidone (PVP). The size of clusters and
their dispersity on graphene could be well maintained even after the compositions tuning. Toward electrooxidation of formic acid, the tailored Pd70Cu30@Graphene exhibits much higher catalytic activity and stability in comparison to Pd@Graphene and commercial Pd/C. Additionally, this unique PdCu clusters are generated under a low-temperature and the synthetic approach does not involve high-poisoning reactants, suggesting its great potential applications in the fields such as fuel cells and sensors.
Experimental section Synthesis of graphene First of all, graphene oxide was synthesized from graphite with the modified Hummers approach [22,36]. The synthesized graphene oxide was dried in 65 C overnight, followed by transferred into a glass bottle which was further heated to 120 C for 3 h under vacuum. After that, the bottle was heated to 250 C quickly, and the collected highly loose black powder was named as graphene.
Preparation of graphene supported PdCu clusters 20.0 mg Pd (acac)2 and 4.0 mg Cu(ac)2 were added into 5.0 mL benzyl alcohol, then the solution was stirred for 30 min under room temperature. After that, 19.8 mg graphene, 50.0 mg ascorbic acid, 86.0 mg polyvinylpyrrolidone (PVP, MW ¼ 30,000) and 10.0 mL ethanol were added to the above solution under ultrasonication for 2 h. The mixed solution was heated to 95 C for 10 h, and the sample was collected by centrifugation, washed with ethanol/deionized water mixture, and dried at 65 C for 8 h. The obtained product was named as PdCu@Graphene. The atomic ratio of Pd to Cu was tuned for comparison.
Electrochemical and physicochemical measurements Field emission scanning electron microscope (JEOL-JSM7800F) and transmission electron microscope (JEOL-JEM2100) were selected to characterize the size and nanostructure of samples in this work. Crystal structure of prepared materials were obtained by XRD (Shimadzu XRD-7000) with Cu Ka radiation (l ¼ 0.1541 nm). The nitrogen isotherm of PdCu@Graphene and commercial Pd/C were tested on automated gas sorption system (QuantachromeAutosorb iQ3) under 77.3 K. The electrochemical tests were carried with a CHI-760 E electrochemical workstation. A three-electrode cell was used in this work. A glassy carbon disk electrode was selected as the working electrode. Saturated calomel electrode (SCE) was used as reference electrode, while a platinum foil was served as counter electrode. 6.0 mg of prepared PdCu@Graphene, 0.9 mL DI water, 1.0 mL ethanol as well as 0.1 mL Nafion solution (5 wt%) were mixed together to obtain catalyst ink. After ultrasonicated for 0.5 h, 6 mL ink was dropped on working electrode and dried in room temperature. Electrocatalytic properties of all the catalysts in this work were investigated in H2SO4 or H2SO4 þ HCOOH solution, respectively. Scan rate is 50 mV s1. Electrochemical impedance
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
spectroscopy (EIS) tests were performed at the open circuit potential, and corresponding frequency range is between 100 K Hz and 0.05 Hz.
Results and discussion Physical characterization of Pd-based materials The TEM image (Fig. 1a) shows typical morphology of commercial Pd/C, of which Pd nanocrystals are dispersed on carbon support with an average size of 6.1 nm. Obviously a macroporous structure of prepared graphene could be observed in Fig. 1b, and PdCu clusters are uniformly dispersed on graphene surface with a narrow size of 2.3 nm (Fig. 1cee).
2733
Fig. 1d displays the HRTEM image of one PdCu cluster. It has a high crystallinity and the calculated period of adjacent fringes is 0.218 nm, which is attributed to PdCu(111) lattice spacing of an FCC structure. Our previous work demonstrated that the lattice spacing of Pd (111) is 0.225 nm [22], thus the decreased lattice distance of the PdCu(111) could be due to a certain lattice contraction, suggesting that Cu is successfully introduced into Pd forming an alloy structure [37,38]. Furthermore, the size and dispersity of Pd and PdCu clusters on graphene could be well maintained even after the composition tuning (Fig. 2), suggesting that the introduced Cu did not affect the basic morphology of Pd clusters and their dispersity. This approach is expected to be a promising route to synthesize other uniform metal-based nanoclusters decorated on supports for broad applications such as fuel cells and sensors.
Fig. 1 e (a) TEM image of commercial Pd/C. (b) SEM, (cee) TEM images of PdCu@Graphene. (f) HRTEM image of one PdCu cluster on graphene. Insets: corresponding size distribution of nanocrystals.
2734
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
Fig. 2 e TEM image of (a) Pd@Graphene, (b) Pd75Cu25@Graphene, (c) Pd65Cu35@Graphene and (d) Pd60Cu40@Graphene, respectively.
The nitrogen isotherms (Fig. 3a) show that prepared PdCu@Graphene materials deliver a much high specific surface area of 813.8 m2 g1, which is about four times larger than that of commercial Pd/C (206.4 m2 g1). Furthermore, PdCu@Graphene possesses much more mesopores (centered ~3.65 nm) in comparison to commercial Pd/C (Fig. 3b). Combining with the SEM image analysis in Fig. 1b, the prepared PdCu@Graphene in this work indeed owns rich macro- and mesopores, which can provide high electrocatalytic surface area and promote mass transport and electronic kinetics in electrocatalysis [22,39]. For further analysis of the crystallographic properties of PdCu clusters and commercial Pd/C, XRD was taken and shown in Fig. 3c. The typical diffraction peaks at 40.2 , 46.8 , 68.4 and 82.3 are ascribed to Pd (111), Pd (200), Pd (220) and Pd (311) planes, respectively. The diffraction peaks of PdCu@Graphene shift to higher positions in comparison to commercial Pd/C, demonstrating the formation of an alloy phase of PdCu because of the smaller atomic radius of Cu (0.128 nm) than that of Pd (0.137 nm) [40]. This result is in good accordance with the TEM characterization in Fig. 1f. Further, the diffraction peak of PdCu (111) exhibits a wider half-width in comparison with commercial Pd/C, indicating its smaller crystallite size of PdCu clusters according to the Debye-Scherrer formula [41]. The metal loading for PdCu@Graphene is 21.46%, and the atomic ratio between Pd and Cu for PdCu@Graphene is 2.40:1.01 (Fig. 3d), which agrees well with inductively coupled plasma atomic emission spectrophotometry analysis. Thus, the prepared PdCu clusters on graphene is named as Pd70Cu30@Graphene.
Electrocatalytic behaviours of Pd-based catalysts for formic acid oxidation Since the homogeneous PdCu clusters are decorated on graphene, they are expected to present a remarkable electrochemically active surface area (ECSA) and modified electronic structure for high catalytic activities [42]. In detail, small-sized clusters are believed to provide high accessible surface area, which stands for high catalytic active sites. Previous works demonstrated that Pd alloyed with Cu could change the electronic structure of Pd resulting in a low activation energy of formic acid and weak bond strength between PdCu crystal surface and immediate species [31,35]. All the catalysts (Fig. 4a) exhibit typical hydrogen adsorption/desorption over the range of 0.23 ~ 0.05 V and the formation of a OHad layer (2H2O ¼ OHad þ H3Oþ þ ee) beyond ~0.5 V, where OHad stands for the adsorbed hydroxyl species [43]. Interestingly, Pd70Cu30@Graphene exhibits around 60 mV more negative onset potential of adsorbed hydroxyl species than both Pd@Graphene and commercial Pd/C (Fig. 4b). Thus, the introduction of Cu element into Pd nanocrystals of Pd70Cu30@Graphene offers favourable oxygen-containing species such as OHad to oxidize poisoning species under a more negative potential over Pd@Graphene and commercial Pd/C, leading to a faster electrode kinetics for FOR [44]. The shifted onset potential of OHad for Pd70Cu30@Graphene could also be found in other Pd-based catalysts such as PdNi-NNs/RGO [45], Pd6Co/ 3DGraphene [46], and PdCu/C [47]. The peaks around 0.4 V are
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
2735
Fig. 3 e (a) The nitrogen isotherm of PdCu@Graphene and commercial Pd/C. (b) BJH pore size distribution, (c) XRD patterns of commercial Pd/C and PdCu@Graphene. (d) EDS spectrum of PdCu@Graphene.
associated with Pd oxide reduction, and Pd70Cu30@Graphene shows obviously larger peak area than that of Pd@Graphene and commercial Pd/C. ECSA is a significant parameter to evaluate the quantity of active sites for electrode, and can be calculated using the equation as follows [22]. ECSA ¼ Q= ð0:424 Pdm Þ
(1)
where Pdm is the weight of Pd loaded on working electrode, and Q is the reduction charge of Pd(II) oxide integrated from
corresponding reduction peak area. We supposed that 0.424 mC cm2 was needed for the reduction of a Pd(II) oxide monolayer. Pd70Cu30@Graphene presents much higher calculated ECSA (64.1 m2 g1) in comparison with Pd@Graphene (36.7 m2 g1) and commercial Pd/C (31.5 m2 g1), suggesting that prepared Pd70Cu30@Graphene could provide more catalytic activity sites, which is beneficial for promoting catalytic performance. The ECSA of Pd70Cu30@Graphene is also higher than most of previously reported Pd-based catalyst. (Table 1)
Fig. 4 e The CV curves of prepared Pd70Cu30@Graphene, Pd@Graphene and commercial Pd/C in 0.5 M H2SO4 solution.
2736
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
Table 1 e Formic acid oxidation behavior of different Pd-based catalysts. Electrocatalyst
ECSA
Peak potential (mVvs. SCE)
Peak current density (mA mg1Pd)
Ref
Pd6Co/3DG PdCu/C Pd nanowires porous Pd sheets PdeSn alloy PdCu alloy Pd NF/HPMo-G PdeFe/RGO PdCu/CNTs NPePd50Cu50 Pd70Cu30@Graphene
51.0 e 45.2 12.9 18.6 33.2 62.3 e 39.07 28.0 64.1
156 290 355 160 198 203 353 220 153 225 150
430.8 194.5 758.0 409.3 553.4 517.0 1020.0 1000.0 252.0 1069.0 1289.0
Ref [46] Ref [47] Ref [53] Ref [54] Ref [55] Ref [56] Ref [57] Ref [58] Ref [59] Ref [60] This work
Toward FOR, two oxidation peaks can be observed for all Pd-based catalysts in this study (Fig. 5a). The catalytic current density about 0.1e0.3 V (HCOOH / CO2 þ 2Hþ þ 2e) is much larger than that about 0.4e0.6 V (HCOOH / COads þ H2O / CO2 þ 2Hþ þ 2e), suggesting that the three catalysts mainly through a direct electron transfer pathway. However, the peak current density of catalyst Pd70Cu30@Graphene is 1289.0 mA mg1Pd, which is about 2.1 times higher than that of Pd@Graphene (627.2 mA mg1Pd) and even 2.5 times higher than that of commercial Pd/C (526.3 mA mg1Pd). Fundamen-
tally, oxidation peak potential is a significant parameter to estimate the catalytic activity of an electrode, and the more negative oxidation peak potential implies the lower overpotential and higher energy conversion efficiency. The oxidation peak potential of Pd70Cu30@Graphene catalyst is 149.8 mV, which is much more negative in comparison with both Pd@Graphene (172.1 mV) and commercial Pd/C (194.6 mV). While Pd70Cu30@Graphene catalyst also delivers more negative onset potential and half-wave potential for FOR in comparison to both Pd@Graphene and commercial Pd/C.
Fig. 5 e (a) CV curves on Pd70Cu30@Graphene, Pd@Graphene and commercial Pd/C and (b) corresponding catalytic activity comparison concerning active Pd at peak potentials.
Fig. 6 e (a) CV curves of Pd70Cu30@Graphene, Pd75Cu25@Graphene Pd65Cu350@Graphene and Pd60Cu40@Graphene catalysts toward FOR and corresponding (b) CV curves testing in 0.5 M H2SO4 electrolyte.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
Additionally, Pd70Cu30@Graphene shows much higher mass activity (normalized with the metal loading Pd) and specific activity (normalized with ECSA) over Pd/Graphene and commercial Pd/C (Fig. 5b). These results demonstrated that the Cu modification of Pd70Cu30@Graphene could indeed enhance electrode kinetics for FOR and owns the highest inherent electrocatalytic activity in comparison to Pd@Graphene and commercial Pd/C. To investigate the effects of Cu in PdCu@Graphene toward electrooxidation of formic acid, the stoichiometric ratio for Pd to Cu was tuned and corresponding electrochemical behaviors were assessed as in Fig. 6a. The peak current density increased with decreasing loading of Cu in PdCu@Graphene system and the optimized stoichiometric ratio of Pd and Cu is 70:30, which
2737
delivers the highest peak current density up to 1289.0 mA mg1Pd. The peak current density is reduced when further decreasing the Cu loading in PdCu@Graphene. Further, CV curves were measured to evaluate the ECSA of tuned composition of PdCu@Graphene systems (Fig. 6b), and Pd70Cu30@Graphene shows the highest peak area of Pd oxide redox and largest ECSA over other PdCu-based catalysts, suggesting its most available catalytic active sites. Thus, appropriate loading of Cu in PdCu bimetallic catalyst is believed to effectively modifying electronic and geometric structures of Pd surface affecting the ECSA and electrode kinetics of FOR. This finding concurs with the result reported in our previously work [48]. The electrochemical stability of Pd-based catalysts was evaluated by amperometric i-t tests as in Fig. 7a. Obviously,
Fig. 7 e (a) I-t curves of Pd70Cu30@Graphene, Pd@Graphene and commercial Pd/C. Fixed potential: 0.15 V (vs.SCE). The comparison of peak current density for commercial Pd/C (b), Pd@Graphene (c) and Pd70Cu30@Graphene (d) before and after 200 cycles. TEM images of commercial Pd/C (e) and Pd70Cu30@Graphene (f) after 200 cycles.
2738
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
Fig. 8 e (a) Tafel plots and (b) Nyquist plots of Pd70Cu30@Graphene, Pd@Graphene and commercial Pd/C.
Pd70Cu30@Graphene has much higher catalytic current density at the initial in comparison with Pd@Graphene and commercial Pd/C, while the former holds the highest catalytic current density even after 4000s testing, implying its best durability and more beneficial for practical application. The seriously decreased current densities of all Pd-based catalysts could be ascribed to the slow deactivation by adsorption of poisoning intermediates [42]. The dynamic durability of various Pd-based catalysts was also investigated in an accelerated stability test by recording 200 cycles (Fig. 7bed). After 200 CV cycles, commercial Pd/C could maintain 83.4% of its original peak current density, and corresponding value for Pd@Graphene is 85.1%. However, Pd70Cu30@Graphene could keep 95.6% of its original peak current density. This result confirms that Pd70Cu30@Graphene indeed has better durability in comparison to Pd@Graphene and commercial Pd/C. The crystal size and morphology of commercial Pd/C and Pd70Cu30@Graphene after 200 cycles testing were characterized as in Fig. 7e and f. Pd nanocrystals occur obvious aggregation in commercial Pd/C. While for Pd70Cu30@Graphene, PdCu nanoclusters could maintain their uniform distribution on graphene surface with an average size of 3.4 nm. Previously work demonstrated that the crystals aggregation mainly derived from Oswald ripening phenomenon [49,50]. Thus, we argue that the Pd nanoclusters modified by Cu element forming a PdCu alloy surface is favourable to suppress Oswald ripening for great durability, which could be contributed to the tuned electronic effect between Pd nanocrystals and introduced Cu.
transfer kinetics than that of both Pd@Graphene and commercial Pd/C, indicating a favourable reactants adsorption amount of Pd70Cu30@Graphene and confirming its highest inherent catalytic activity toward FOR. Furthermore, electrochemical impedance spectroscopy, a useful technique for evaluating the dynamics, was selected in this work to investigate the charge-transfer resistance for FOR in current work [51]. The semicircle over high and medium frequency region tells the charge-transfer impedance (Rct) between electrode and electrolyte interface, and the larger diameter of the semicircle corresponds to the larger Rct [52]. The calculated Rct of Pd70Cu30@Graphene catalyst is 128.3 U(Fig. 8b), which is lower than that of Pd@Graphene (155.9 U) and commercial Pd/ C (464.6 U). Standard exchange current density (i0) can be selected to estimate intrinsic catalytic activity of Pd-based catalysts in this work, which is usually studied with the equation as follows [22]. i0 ¼ R T=ðn F Rct Þ
(2)
where R and F are gas and faradic constant, respectively. T means absolute temperature. The calculated result reveals that Pd70Cu30@Graphene holds a highest value of i0 (8.08 104 A cm2) over Pd@Graphene (6.65 104 A cm2) and commercial Pd/C (2.23 104 A cm2), further confirming its highest inherent catalytic activity for FOR, which also concurs with the CV results in Fig. 5a and Tafel slopes analysis in Fig. 8a. Comparing with previously reported Pd-based catalysts in Table 1, Pd70Cu30@Graphene also presents the highest electrocatalytic activity for formic acid oxidation in terms of ECSA, oxidation peak potential and peak current density.
The insights of catalytic enhancement mechanism for Pd70Cu30@Graphene
Conclusions Tafel slope is a significant parameter to uncover the catalytic mechanism and the reactants adsorption amount in kineticcontrol region in catalytic process [26]. The calculated Tafel slope for commercial Pd/C and Pd@Graphene catalyst is 169.4 mV decade1 and 160.1 mV decade1, respectively (Fig. 8a). While for Pd70Cu30@Graphene, the calculated Tafel slope is only 133.5 mV decade1. The smaller Tafel slope value of Pd70Cu30@Graphene catalyst suggested its faster charge-
A new facile one-pot solvothermal method was demonstrated to synthesize stable PdCu nanoclusters uniformly decorated on graphene with an average size of 2.3 nm. After tuning composition of PdxCu clusters, graphene decorated with Pd70Cu30 clusters exhibited an oxidation peak current density up to 1289.0 mA mg1Pd toward FOR, which is 2.5 times higher than that of commercial Pd/C (526.3 mA mg1Pd). This is also
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
the highest value for PdCu-based catalysts toward FOR. The PdCu nanoclusters could maintain their original morphology and structure even after 200 CV cycles. This work offers a lowPd loading but highly active catalyst toward formic acid oxidation, and a one-pot solvothermal method, which opens new opportunities to synthesize uniform metal alloy-based nanoclusters decorated on supports for electrocatalysis and sensing devices. Zhe Zhang and Yuyan Gong contributed equally to this work. The authors declare no competing financial interest.
Acknowledgment We gratefully acknowledge to the financial support from the Thousand Talents Plan, the World-Class University and Discipline, the Taishan Scholar's Advantageous and Distinctive Discipline Program of Shandong Province, the world-Class Discipline Program of Shandong Province, CAS Key Laboratory of Low-Coal Conversion Science & Engineering (KLLCCSE201706, SARI, CAS), Key Research and Development Plan of Shandong Province (No. 2018GGX102019), Natural Science Foundation of Shandong Province (No. ZR2017BB022), China Postdoctoral Science Foundation (No. 2017M612201), Applied Basic Research Program of Qingdao (No. 18-2-2-5-jch) as well as scientific research fund from Qingdao university (No. 41117010098) are also acknowledged.
references
[1] Jin H, Guo C, Liu X, Liu J, Vasileff A, Jiao Y, Zheng Y, Qiao SZ. Emerging two-dimensional nanomaterials for electrocatalysis. Chem Rev 2018;118(13):6337e408. [2] Li DN, Wang AJ, Wei J, Zhang QL, Feng JJ. Facile synthesis of flower-like Au@AuPd nanocrystals with highly electrocatalytic activity for formic acid oxidation and hydrogen evolution reactions. Int J Hydrogen Energy 2017;42(31):19894e902. [3] Zhang LY, Wu D, Gong Y, Liu H, Chen Wei, Bi L. Carbon monoxide-templated synthesis of coral-like clean PtPd nanochains as efficient oxygen reduction catalyst. ChemElectroChem 2018;5:2403e8. [4] Rahm JM, Erhart P. Beyond magic numbers: atomic scale equilibrium nanoparticle shapes for any size. Nano Lett 2017;17(9):5775e81. [5] Zhu Q-L, Xu Q. Immobilization of ultrafine metal nanoparticles to high-surface-area materials and their catalytic applications. Inside Chem 2016;1(2):220e45. [6] Yuan W, Fan X, Cui ZM, Chen T, Dong Z, Li CM. Controllably self-assembled graphene-supported Au@Pt bimetallic nanodendrites as superior electrocatalysts for methanol oxidation in direct methanol fuel cells. J Mater Chem 2016;4(19):7352e64. [7] Lu S, Hu Y, Wan S, McCaffrey R, Jin Y, Gu H, Zhang W. Synthesis of ultrafine and highly dispersed metal nanoparticles confined in a thioether-containing covalent organic framework and their catalytic applications. J Am Chem Soc 2017;139(47):17082e8. [8] Ruditskiy A, Xia Y. The science and art of carving metal nanocrystals. ACS Nano 2017;11(1):23e7. [9] Tao AR, Habas S, Yang P. Shape control of colloidal metal nanocrystals. Small 2008;4(3):310e25.
2739
[10] Xia Y, Gilroy KD, Peng HC, Xia X. Seed-mediated growth of colloidal metal nanocrystals. Angew Chem Int Ed 2017;56(1):60e95. n LM. The relevance of light in the [11] Grzelczak M, Liz-Marza formation of colloidal metal nanoparticles. Chem Soc Rev 2014;43(7):2089e97. [12] Wang Y, Choi SI, Zhao X, Xie S, Peng HC, Chi M, Huang CZ, Xia Y. Polyol synthesis of ultrathin Pd nanowires via attachment-based growth and their enhanced activity towards formic acid oxidation. Adv Funct Mater 2013;24(1):131e9. [13] Xia Y, Xia X, Peng H-C. Shape-controlled synthesis of colloidal metal nanocrystals: thermodynamic versus kinetic products. J Am Chem Soc 2015;137(25):7947e66. [14] Wang Y, He J, Liu C, Chong WH, Chen H. Thermodynamics versus kinetics in nanosynthesis. Angew Chem Int Ed 2015;54(7):2022e51. [15] Wang C, Daimon H, Onodera T, Koda T, Sun S. A general approach to the size- and shape-controlled synthesis of platinum nanoparticles and their catalytic reduction of oxygen. Angew Chem Int Ed 2010;120(19):3644e7. [16] Xu H, Zhang K, Yan B, Wang J, Wang C, Li S, Gu Z, Du Y, Yang P. Ultra-uniform PdBi nanodots with high activity towards formic acid oxidation. J Power Sources 2017;356:27e35. [17] Wang X, Yang J, Yin H, Song R, Tang Z. “Raisin bun”-like nanocomposites of palladium clusters and porphyrin for superior formic acid oxidation. Adv Mater 2013;25(19):2728e32. [18] Liu M, Zhang R, Chen W. Graphene-supported nanoelectrocatalysts for fuel cells: synthesis, properties, and applications. Chem Rev 2014;114(10):5117e60. [19] Zhou X, Qiao J, Yang L, Zhang J. A review of graphene-based nanostructural materials for both catalyst supports and metal-free catalysts in PEM fuel cell oxygen reduction reactions. Adv Energy Mater 2014;4(8):1301523. [20] Tan C, Huang X, Zhang H. Synthesis and applications of graphene-based noble metal nanostructures. Mater Today 2013;16(1e2):29e36. [21] Fampiou I, Ramasubramaniam A. Binding of Pt nanoclusters to point defects in graphene: adsorption, morphology, and electronic structure. J Phys Chem C 2012;116(11):6543e55. [22] Zhang LY, Zhao ZL, Li CM. Formic acid-reduced ultrasmall Pd nanocrystals on graphene to provide superior electrocatalytic activity and stability toward formic acid oxidation. Nano Energy 2015;11:71e7. [23] Fan H, Cheng M, Wang L, Song Y, Cui Y, Wang R. Extraordinary electrocatalytic performance for formic acid oxidation by the synergistic effect of Pt and Au on carbon black. Nano Energy 2018;48:1e9. [24] Yan X, Hu X, Fu G, Xu L, Lee JM, Tang Y. Facile synthesis of porous Pd3Pt half-shells with rich “active sites” as efficient catalysts for formic acid oxidation. Small 2018;14(13):1703940. [25] Gong Y, Liu X, Gong Y, Wu D, Xu B, Bi L, Zhang LY, Zhao XS. Synthesis of defect-rich palladium-tin alloy nanochain networks for formic acid oxidation. J Colloid Interface Sci 2018;530:189e95. [26] Wang F, Xue H, Tian Z, Xing W, Feng L. Fe2P as a novel efficient catalyst promoter in Pd/C system for formic acid electro-oxidation in fuel cells reaction. J Power Sources 2018;375:37e42. [27] Liu Z, Fu G, Li J, Liu Z, Xu L, Sun D, Tang Y. Facile synthesis based on novel carbon-supported cyanogel of structurally ordered Pd 3 Fe/C as electrocatalyst for formic acid oxidation. Nano Res 2018;11(9):4686e96. [28] Zhang LY, Gong Y, Wu D, Wu G, Xu B, Bi L, Yuan W, Cui Z. Twisted palladium-copper nanochains toward efficient electrocatalytic oxidation of formic acid. J Colloid Interface Sci 2019;537:366e74.
2740
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 7 3 1 e2 7 4 0
[29] Wang C, Chen DP, Sang X, Unocic RR, Skrabalak SE. Sizedependent disordereorder transformation in the synthesis of monodisperse intermetallic PdCu nanocatalysts. ACS Nano 2016;10(6):6345e53. [30] Chen Y, Yang Y, Fu G, Xu L, Sun D, Lee J-M, Tang Y. Coreeshell CuPd@ Pd tetrahedra with concave structures and Pd-enriched surface boost formic acid oxidation. J Mater Chem 2018;6:10632e8. [31] Yang N, Zhang Z, Chen B, Huang Y, Chen J, Lai Z, Chen Y, Sindoro M, Wang AL, Cheng H. Synthesis of ultrathin PdCu alloy nanosheets used as a highly efficient electrocatalyst for formic acid oxidation. Adv Mater 2017;29(29). [32] Ye W, Chen S, Ye M, Ren C, Ma J, Long R, Wang C, Yang J, Song L, Xiong Y. Pt4PdCu0.4 alloy nanoframes as highly efficient and robust bifunctional electrocatalysts for oxygen reduction reaction and formic acid oxidation. Nano Energy 2017;39:532e8. [33] Dong Q, Zeng W, Wan H, Yu S, Guo C, Huang M. Highly effective PdCu electrocatalysts supported on polyelectrolyte functionalized titanium dioxide for direct formic acid fuel cells. Int J Electrochem Sci 2017;12(2):1389e400. [34] Chen Y, Yang Y, Fu G, Xu L, Sun D, Lee JM, Tang Y. Coreeshell CuPd@Pd tetrahedra with concave structures and Pdenriched surface boost formic acid oxidation. J Mater Chem 2018;6(23):10632e8. [35] Jiang B, Li C, Malgras V, Bando Y, Yamauchi Y. Threedimensional hyperbranched PdCu nanostructures with high electrocatalytic activity. Chem Commun 2016;52(6):1186e9. [36] Hummers Jr WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80(6). 1339-1339. [37] Zhu C, Shi Q, Fu S, Song J, Xia H, Du Dan, Lin Y. Efficient synthesis of MCu (M¼Pd, Pt, and Au) aerogels with accelerated gelation kinetics and their high electrocatalytic activity. Adv Mater 2016;28:8779e83. [38] Zhu C, Wen D, Oschatz M, Holzschuh M, Liu W, Herrmann AK, Simon F, Kaskel S, Eychmu¨ller A. Kinetically controlled synthesis of PdNi bimetallic porous nanostructures with enhanced electrocatalytic activity. Small 2015;11(12):1430e4. [39] Zhang LY, Zhang W, Zhou Z, Li CM. g-Fe2O3 nanocrystalsanchored macro/meso-porous graphene as a highly efficient adsorbent toward removal of methylene blue. J Colloid Interface Sci 2016;476:200e5. [40] Mandal K, Bhattacharjee D, Roy PS, Bhattacharya SK, Dasgupta S. Room temperature synthesis of PdeCu nanoalloy catalyst with enhanced electrocatalytic activity for the methanol oxidation reaction. Appl Catal Gen 2015;492:100e6. [41] Wang Z, Fan H, Liang H, Ma J, Li S, Song Y, Wang R. Microfluidic synthesis and characterization of FePtSn/C catalysts with enhanced electro-catalytic performance for direct methanol fuel cells. Electrochim Acta 2017;230:245e54. [42] Ma Y, Yin L, Cao G, Huang Q, He M, Wei W, Zhao H, Zhang D, Wang M, Yang T. Pt-Pd bimetal popcorn nanocrystals: enhancing the catalytic performance by combination effect of stable multipetals nanostructure and highly accessible active sites. Small 2018;14(14):1703613. [43] Lim B, Jiang M, Camargo PHC, Cho EC, Tao J, Lu X, Zhu Y, Xia Y. Pd-Pt bimetallic nanodendrites with high activity for oxygen reduction. Science 2009;324(5932):1302e5. [44] Yuan T, Chen H-Y, Ma X, Feng J-J, Yuan P-X, Wang A-J. Simple synthesis of self-supported hierarchical AuPd alloyed nanowire networks for boosting electrocatalytic activity toward formic acid oxidation. J Colloid Interface Sci 2018;513:324e30.
[45] Bin D, Yang B, Ren F, Zhang K, Yang P, Du Y. Facile synthesis of PdNi nanowire networks supported on reduced graphene oxide with enhanced catalytic performance for formic acid oxidation. J Mater Chem 2015;3(26):14001e6. [46] Zhang LY, Zhao ZL, Yuan W, Li CM. Facile one-pot surfactant-free synthesis of uniform Pd6Co nanocrystals on 3D graphene as an efficient electrocatalyst toward formic acid oxidation. Nanoscale 2016;8(4):1905e9. [47] Wang L, Zhai JJ, Jiang K, Wang JQ, Cai WB. PdeCu/C electrocatalysts synthesized by one-pot polyol reduction toward formic acid oxidation: structural characterization and electrocatalytic performance. Int J Hydrogen Energy 2015;40(4):1726e34. [48] Zhang LY, Gong Y, Wu D, Li Z, Li Q, Zheng L, Chen W. Palladium-cobalt nanodots anchored on graphene: In-situ synthesis, and application as an anode catalyst for direct formic acid fuel cells. Appl Surf Sci 2019;469:305e11. [49] Zhang T, Wang P, Chen H, Pei P. A review of automotive proton exchange membrane fuel cell degradation under start-stop operating condition. Appl Energy 2018;223:249e62. [50] Zhang LY, Gong Y, Liu H, Yuan W, Liu Z. Ultrasmall and uniform Pt3Au clusters strongly suppress Ostwald ripening for efficient ethanol oxidation. Electrochem Commun 2017;84:1e5. [51] Guo CX, Zhang LY, Miao J, Zhang J, Li CM. DNAFunctionalized graphene to guide growth of highly active Pd nanocrystals as efficient electrocatalyst for direct formic acid fuel cells. Adv Energy Mater 2013;3(2):167e71. [52] Zhang M, Yu X, Ma H, Du W, Qu L, Li C, Shi G. Robust graphene composite films for multifunctional electrochemical capacitors with an ultrawide range of areal mass loading toward high-rate frequency response and ultrahigh specific capacitance. Energy Environ Sci 2018;11(3):559e65. [53] Xu D, Liu X, Han M, Bao J. Facile synthesis of ultrathin singlecrystalline palladium nanowires with enhanced electrocatalytic activities. Chem Commun 2016;52(88):12996e9. [54] Qiu X, Zhang H, Wu P, Zhang F, Wei S, Sun D, Xu L, Tang Y. One-pot synthesis of freestanding porous palladium nanosheets as highly efficient electrocatalysts for formic acid oxidation. Adv Funct Mater 2017;27(1):1603852. [55] Sun D, Si L, Fu G, Liu C, Sun D, Chen Y, Tang Y, Lu T. Nanobranched porous palladiumetin intermetallics: onestep synthesis and their superior electrocatalysis towards formic acid oxidation. J Power Sources 2015;280(7):141e6. [56] Yang F, Zhang Y, Liu P-F, Cui Y, Ge X-R, Jing Q-S. Pd-Cu alloy with hierarchical network structure as enhanced electrocatalysts for formic acid oxidation. Int J Hydrogen Energy 2016;41(16):6773e80. [57] Fan X, Yuan W, Zhang DH, Li CM. Heteropolyacid-mediated self-assembly of heteropolyacid-modified pristine graphene supported Pd nanoflowers for superior catalytic performance toward formic acid oxidation. ACS Appl Energy Mater 2018;1(2):411e20. [58] Feng A, Bai J, Shao W, Hong W, Tian ZQ, Xiao Z. Surfactantfree PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation. Int J Hydrogen Energy 2017;42(22):15196e202. [59] Zhu F, Ma G, Bai Z, Hang R, Tang B, Zhang Z, Wang X. High activity of carbon nanotubes supported binary and ternary Pd-based catalysts for methanol, ethanol and formic acid electro-oxidation. J Power Sources 2013;242:610e20. [60] Xu C, Liu A, Qiu H, Liu Y. Electrochem Commun 2011;13:766e9.