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Shape-controlled synthesis of Pd polyhedron supported on polyethyleneimine-reduced graphene oxide for enhancing the efficiency of hydrogen evolution reaction
Journal of Power Sources 302 (2016) 343e351
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Shape-controlled synthesis of Pd polyhedron supported on polyethyleneimine-reduced graphene oxide for enhancing the efficiency of hydrogen evolution reaction Jing Li a, b, Panpan Zhou a, Feng Li a, b, Jianxin Ma a, b, Yang Liu a, b, Xueyao Zhang a, b, Hongfei Huo a, b, Jun Jin a, b, **, Jiantai Ma a, b, * a
State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China Gansu Provincial Engineering Laboratory for Chemical Catalysis, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Polyhedral Pd nanoparticles with controlled sizes and shapes were prepared. Pd nanocubes have high current density in HER. The DFT shows electron density distribution of Pd {100} is better than Pd {111}. The novel catalyst exhibits excellent HER activity comparable with that of Pt/C. It opens new insights into design efficient Pd catalyst with low loading in HER.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 August 2015 Received in revised form 13 October 2015 Accepted 15 October 2015 Available online 11 November 2015
The catalytic activity of noble-metal nanoparticles (NPs) often has closely connection with their sizes and geometric shape. In the work, polyhedral NPs of palladium (Pd) with controlled sizes, shapes, and different proportions of {100} to {111} facets on the surface were prepared by a seed-mediated approach. Electrochemical experiment demonstrates that the catalytic performance of the Pd nanocubes (NCs) enclosed by {100} facets is more active than Pd octahedrons enclosed by {111} facets for the hydrogen evolution reaction (HER), which is consistent with density functional theory (DFT) calculation results. Meanwhile, with the assistance of a polyethyleneimineereduced graphene oxide (PEIerGO) support, the examined Pd cube/PEIerGO50:1 (10 wt. %) electrocatalyst presents outstanding HER activity comparable with that of commercial Pt/C catalyst. This correlation between the HER catalytic activity and surface structure will contribute to the reasonable design of Pd catalysts for HER with high efficiency and low metal loading. © 2015 Elsevier B.V. All rights reserved.
Keywords: Palladium Nanocubes Shapeedependence Hydrogen evolution reaction Density functional theory
* Corresponding author. State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China. ** Corresponding author. State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China. E-mail addresses: [email protected] (J. Jin), [email protected] (J. Ma). http://dx.doi.org/10.1016/j.jpowsour.2015.10.050 0378-7753/© 2015 Elsevier B.V. All rights reserved.
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1. Introduction With the decreasing availability of fossil fuels and increasing demand for clean energy, hydrogen, which has high specific energy storage density and perfect environmental friendliness, is increasingly considered as the fuel of the future [1,2]. Among current technologies to produce hydrogen, water splitting (H2O / H2 þ 1/2 O2) has been considered as the best method because the method is renewable and environmental friendly [3,4]. The water splitting reaction is composed of two half reactions: the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Catalysts are often used to reduce the overpotential and consequently promote the efficiency of this important HER [5,6]. In the HER, acidic electrolytes are more favorable than alkaline electrolytes because these units are more compact and could potentially run in reverse mode to produce electricity (i.e., in fuel cells) [7]. Noble metals are usually considered as the best candidates for the HER under acidic conditions due to their excellent catalytic activity. The most representative noble metal is platinum (Pt), which has lower overpotential and higher current density [8,9]. However, its expensive cost and scarce resource definitely prohibit its commercial application in the HER. Metal palladium (Pd) relying on its remarkable catalytic capabilities and relatively ample resource is the best substitute for Pt in many areas involving HER [10]. It is well established that the activity and selectivity of Pd nanoparticles (NPs) have strong correlation with the size and geometric shape such as the type of facet exposed on the surface [11]. Therefore, how to tailor these parameters to maximize the catalytic performance of a Pd catalyst is worth to be studied. The Pd catalysts at the moment reported in the oxidation or reduction reactions do not present a breakthrough mainly based on Pd NPs without a uniform and welledefined atomic structure on the surface. To improve the catalytic performance and minimize the loading, it is necessary to replace them with even NPs enclosed by a specific set of facets. Many studies have been performed with various noble metals as model systems applied in electrochemical reactions [12e15]. For example, Zhang et al. reported that {110}faceted Pd nanocrystals show higher catalytic activity than {111}faceted NCs and {111}-faceted octahedron in hydrazine oxidation reaction [12]. Wang et al. studied the shape-dependent catalytic activity of oxygen reduction reaction (ORR) on silver nanodecahedra and NCs [13]. In the case of PtePderGO stack structure reported by Bai, {100} facets are selected for Pd and Pt, as they have relatively high current density in HER [14]. The above reports on the shapeedependence of a catalytic reaction inspire us to explore how the morphology of Pd NPs enclosed by {100} facets or {111} facets on the surface influences the HER performance in this work. Reduced graphene oxide (rGO) is a twoedimensional (2D) nanomaterial of sp2ehybridized carbon containing many special properties like high surface area, enhanced mobility of charge carriers and good stability, which also make rGO ideal platform for anchoring functional nanomaterials [16,17]. Our previous studies have shown that PEI decorated graphene oxide (GO) is beneficial to improve the electronic transmission and synergistically promote the electrocatalytic activity of the HER [18,19]. It was reported that GO can be reduced to rGO in the presence of amineecontaining molecules [20]. It is therefore rational to believe that the PEIerGO hybrid can be a promising support material for HER. Herein we prepared the Pd NCs with different sizes by a facile and aqueous approach, then used the Pd NCs of 9.7 nm width as the seeds to further synthesize relatively uniform polyhedrons such as truncated cubes (named Pd cubetransform-1), cuboctahedrons (named Pd cubetransform-2), truncated octahedrons (named Pd cubetransform-3), and octahedrons respectively. Electrochemical
experiment demonstrates that the catalytic activity of HER increases when the area ratio of {111} to {100} facets on the surface of a Pd polyhedron decreases. The experimental observations are supported by the density functional theory (DFT) calculation results. After comparing these polyhedrons, Pd NCs not only have relatively high current density in HER, but offer a flat surface for contact with the substrates [21]. Furthermore, with the synergistic catalytic assistance of a PEIerGO support, the examined Pd cube/ PEIerGO50:1 (10 wt. %) electrocatalyst exhibits an outstanding HER performance with a Tafel slope of 34 mV dec1 and an exchange current density of 2.29 mA cm2, achieving high stability simultaneously. Thus, these findings confirm that the Pdebased electrocatalysts are extremely promising for their applications in the HER. Scheme 1. 2. Experimental 2.1. Materials Natural graphite powder (325 mesh) was obtained from Alfa Aesar, polyethyleneimine (PEI, Mn ¼ 100,000) was purchased from Aladdin Reagent Co. Ltd., Na2PdCl4 (98%), Leascorbic acid (AA), Poly (vinyl pyrrolidone) (PVP, MW z 55,000) were purchased from Aldrich. All the other commercial chemicals were used without further purification. 2.2. Synthesis of Pd cubes In a typical synthesis of Pd NCs, 10.0 mL of an aqueous solution containing 105 mg of PVP, 60 mg of AA, and different amounts of KBr or KCl were placed in a 25 mL vial, and preeheated at 80 C for 5 min under magnetic stirring. Then, 3.0 mL of an aqueous solution containing 57 mg of Na2PdCl4 was added. The reaction was allowed to proceed at 80 C for 3 h. The product was collected by centrifugation and washed with acetone and ethanol to remove excess PVP, the final product was reedispersed in water (1.0 mg mL1 in concentration). 2.3. Synthesis of Pd polyhedrons In a typical synthesis of Pd polyhedrons, 3 mL of aqueous Na2PdCl4 solution (32 mM) was introduced into 8 mL of an aqueous solution containing 105 mg of PVP, 100 mL of HCHO, and 0.3 mL of an aqueous suspension (1.8 mg mL1 in concentration) of Pd NCs seeds with 9.7 nm in edge length, which had been heated at 60 C for 5 min under magnetic stirring in a 25 mL vial. The addition of Na2PdCl4 in the following amounts gave Pd cubetransform-1 (5.8 mg), Pd cubetransform-2 (8.7 mg), Pd cubetransform-3 (17.4 mg), and octahedrons (29.0 mg), respectively. The concentration of Na2PdCl4 solution is maintained at 32 mM. Each reaction was allowed to proceed at 60 C for 3 h. After collection by centrifugation and washing with acetone and ethanol, the final product was redispersed in water. 2.4. Synthesis of the PEIerGO support GO was prepared by the modified Hummers method [22]. After being carefully washed with deionized water until pH > 6, GO was freeze dried for the preparation of PEIerGO. To synthesize the PEIerGO composite (e.g., weight ratio ¼ 50:1), 10 mg of GO was added to 100 mL of water, and sonicated for 30 min to form a homogeneous yellowish brown solution. Then, 10 mL aqueous suspension of GO (0.1 mg mL1) was mixed with 50 mL PEI (1% aqueous solution), followed by sonication for another 30 min at room temperature. After being kept at 60 C for 12 h, the mixture
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Scheme 1. Illustration of the formation of Pd cube/PEIerGO nanocomposite.
was purified by centrifugation and dried using a vacuum freezing dryer to constant weight. The PEIerGO composites with weight ratios of 100: 1, 1: 1, 1: 3 were also prepared by the similar method. 2.5. Preparation of Pd cubes/PEIerGO50:1 (10 wt. %) composite materials The 10 mg of PEIerGO was suspended in 20 mL ethanol solution with magnetic stirring. After that, 1 mL of Pd NCs (1.0 mg mL1 in concentration) was added dropwise into the above PEIerGO suspension under sonication for 15 min. The mixed suspension was then stirred magnetically for 12 h. Finally, the product was collected by centrifugation and washed several times with H2O and ethanol. The final product was dried in a vacuum oven at 40 C overnight. 2.6. Electrochemical measurements Electrochemical measurements were carried out in a standard three-electrode cell with a CHI model 660E electrochemical workstation (Shanghai Chenhua Instrument Factory, China) at room temperature. A platinum wire was used as the counter electrode and an AgjAgCl (in saturated KCl) electrode as the reference. The working electrode was a glassy carbon electrode (GCE, diameter: 3.0 mm) polished with Al2O3 powders (Aldrich, 0.05 mm). The working electrodes were fabricated as follows: 5 mg of the sample was dispersed in a 5 mL mixed solution of deionized water, ethanol and Nafion solution (2:1:1) using ultrasonic treatment to produce a homogeneous suspension. Then, the well-dispersed suspension (10 mL) was drop-cast onto the GCE, which was then air-dried at room temperature. The total metal loading weights are kept constant for the assessment. All the potentials reported in our work were against reversible hydrogen electrode (RHE) through RHE calibration. For conversion of the obtained potential (vs Ag/AgCl) to RHE, Eq. (1) was used [23].
ERHE ¼ EAg=AgCl þ 0:059pH þ EoAg=AgCl EoAg=AgCl ¼ 0:209 V
(1)
2.7. Characterizations A FEI-TECNAI G2 transmission electron microscope operating at 200 kV (FEI company) was used to make the transmission electron microscopy (TEM) images and high resolution TEM (HRTEM) images. Elemental composition data were collected by Energy dispersive X-ray (EDX) performed using a TECNAI G2 microscope. Raman spectra were recorded on an in Via Reinishaw confocal spectroscope with 633 nm laser excitation. X-ray photoelectron spectroscopy (XPS) was recorded on a PHI-5702 instruments. Electrochemistry was performed with a model CHI Instrument 660E electrochemical workstation (Shanghai Chenhua Equipment, China). 3. Results and discussion Fig. 1(AeE) shows a typical TEM image of the Pd NCs with different size by employing different amounts of halide species such as Br and Cl. Other factors such as temperature may affect the growth of Pd NPs (Fig. 1F). The size distribution derived from the TEM image by counting ca. 100 cubes is plotted in the inset of Fig. 1. The details about controlled preparation of Pd NCs are listed in Table. S1. Samples (A-E) respectively correspond to six samples shown in TEM images. After comparing their catalytic performance of HER (Fig. S5), the Pd NCs of 9.7 nm width with purity approaching 100% shown in Fig. 1A are used as seeds for further preparing Pd polyhedrons. Fig. 2 shows TEM images and HR-TEM images of the Pd polyhedrons how continuous growth on the Pd {100} facets eventually leading to an octahedron enclosed by {111} facets. It can be seen that at a small amount of Na2PdCl4 (5.8 mg), the as-obtained polyhedrons are a slightly larger size cube (Fig. 2B) than the original cubic seeds (Fig. 2A). When the amount of Na2PdCl4 is increased to 8.7 mg and 17.4 mg, the polyhedrons evolves into cuboctahedron (Fig. 2C) and truncated octahedrons (Fig. 2D), respectively. Introducing an even larger amount of Na2PdCl4 (29.0 mg), the polyhedrons eventually become octahedrons with edge length of 18.7 nm as shown in Fig. 1E. Fig. 2aee are the HRTEM of Pd polyhedrons. In this process of derived from Pd cube to octahedron, the proportions of Pd {111} to {100} facets are gradually
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Fig. 1. TEM images of Pd nanocubes prepared in different reaction conditions. The inset is size distribution histograms of each sample.
increasing, which is beneficial for allowing us to study the relationship between the surface atomic structure and HER. Then, TEM is continuously used to investigate the morphologies and dispersion of the Pd cubes deposited on the PEIerGO50:1 support (Fig. 3A and B). The Pd cubes with an average width of 9.7 nm are adsorbed relatively uniformly and tightly on the PEI functionalized GO sheets by the interaction of palladium-nitride (PdeN). The EDX pattern in the inset of Fig. 3A reveals that Pd, C and N are the predominant species and the Cu element arising from the copper grid is also detected. The existence of the N element reveals that GO has been functionalized by PEI. Raman spectroscopy is a conventional tool to detect the ordered and disordered crystalline structure of graphene. The typical Raman spectra of GO, PEIerGO50:1 and Pd cube/PEIerGO50:1 nanocomposite are shown in Fig. S1. The level of chemical modification of the graphitic carbon sample is commonly quantified by the intensity ratio of the D band to G band (ID/IG), which is correlated with the average size of sp2 domains, that is, the smaller the size of sp2 domains, the higher the intensity ratio (ID/IG) [24,25]. The ID/IG ratios for GO, PEIerGO50:1 and Pd cube/PEIerGO50:1 are 0.96, 1.03 and 1.12, respectively. The increase of ID/IG ratios for PEIerGO50:1 and Pd cube/PEIerGO50:1 nanocomposites as compared with GO indicates that the new domains of conjugated carbon atoms are formed due to the removal of the oxygenous groups. Previous reports have shown the increase of the ID/IG ratio
after reduction of GO, which is probably due to the introduction of new defects after the reduction at relatively high temperature [25]. Thus, it is concluded that GO in PEIerGO50:1 and Pd cube/ PEIerGO50:1 nanocomposites have been well deoxygenated and reduced. X-ray photoelectron spectroscopy (XPS) is employed to study the composition of the composite of Pd cube/PEIerGO50: 1. According to the C 1s XPS spectra of the RGO as shown in Fig. S2B, the peaks at 284.6, 286.1, 287.7 and 288.6 eV corresponding to the oxygen and carbon atoms in the forms CeC, CeO, C]O, and COOH [26]. The XPS of the composite in the N 1s region is shown in Fig. S2C, the N 1s spectrum can be deconvoluted to several individual peaks which are assigned to ¼ Ne (398.8 eV), eNHe (399.2 eV), and eNH2 (399.9 eV), which indicates the GO sheets were successfully decorated by PEI [27]. The main doublet peak in Fig. S2D could be attributed to Pd 3d. The binding energy of the doublet peaks at 335.4 eV (assigned to Pd0 3d5/2) and 340.7 eV (assigned to Pd0 3d3/2) indicate that all Pd atoms are present in the Pd0 state [28]. Fig. S3 shows the FTIR spectra of GO, PEI, PEIerGO50:1, and Pd cube/PEIerGO50:1. As shown in Fig. S3c, a new band at 1460 cm1 (CeN stretching vibration) appears for PEIerGO50:1, indicating that some PEI is covalently linked to the PEIerGO surface by amide bonds. After decorating the Pd NCs on the PEIerGO50:1 sheet, the stretching vibrations of the main functional groups shifts to higher
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Fig. 2. (AeE) TEM images of the Pd polyhedrons obtained by controlling the amount of Na2PdCl4: (a) Pd cube, (b) Pd cubetransform-1, (c) Pd cubetransform-2, (d) Pd cubetransform-3, and (e) Pd octahedron. The inset of TEM images (E) is size distribution histograms; HR-TEM images of the Pd polyhedrons are given in the Fig. 2 (aee).
frequency as well, which is supposed to originate from the interaction of special PdeN [18]. The electrochemical activity of the HER has been evaluated using polarization curves. The polarization curves for a series of PEIerGO support (PEIerGO100:1/GCE, PEIerGO50:1/GCE, PEIerGO1:1/GCE, PEIerGO1:3/GCE) measured in 0.5 M H2SO4 at a scan rate of 5 mV s1 at room temperature were firstly recorded. As
shown in Fig. S4A, the PEIerGO catalysts present better catalytic activities than pure GO, confirming that the PEIerGO material has a high effective surface area which is beneficial for electronic transmission. Particularly, on a PEIerGO50:1/GCE, it shows the best HER activity with a current density of 28.4 mA cm2 at overpotential of 600 mV. The Tafel plots of these catalysts derived from the polarization curves shown in Fig. S4B fit well with the Tafel equation at
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Fig. 3. Typical TEM (A and B) images of Pd cube/PEIerGO50:1. The inset of TEM image (A) is EDX images of Pd cube/PEIerGO50:1.
different overpotential ranges, and only the linear portions were selected to obtain a clear comparison. A smaller Tafel slope means a faster increase of the HER rate with increasing potential [29]. The PEIerGO/GCE compared with the GO/GCE in Fig. S4, we infer that GO functionalized with PEI is available for promoting synergistically the HER. The polarization curves of the Pd NCs samples with different sizes measured in the same system are shown in Fig. S5. Samples (A-E) respectively correspond to six samples shown in TEM images of Fig. 1. The sizes of Pd NPs can be tuned in the range of 4e14 nm. The polarization curves of the current density plotted against potential show that the activity of 9.7 nm width NCs is highest because of its most positive onset potential than other NCs. In the meanwhile, the Pd NCs in Fig. 1A are more uniform, which is suitable as seeds for preparing Pd polyhedrons. Then these Pd polyhedron samples are measured in the same system as well. As shown in Fig. 4A, the total metal loading weights are kept constant for this assessment and the polarization curves show that the activities of Pd NCs are highest with the most positive onset potential of 37 mV and a current density of 99.7 mA cm2 at 200 mV vs. RHE compared to any other Pd polyhedron. Significantly, the Pd octahedron exhibits the unsatisfactory HER performance among all samples in terms of both the current densities and Tafel slopes. Meanwhile, with the proportions of {111} to {100} facets of Pd NPs increases, the HER performance shows a deterioration trend, demonstrating that the catalytic activity of Pd cube enclosed by {100} facets is more active than Pd octahedron enclosed by {111} facets for the HER. Theoretical calculation of the mechanism is mentioned below. The electron densities on Pd {100} and Pd {111} facets are investigated using DFT method. The generalized gradient
approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional is employed for the calculations [30]. An energy convergence criterion of 105 eV and an energy cutoff of 400.0 eV are used, and the k-points are set to 2 2 1 for the Brillouin zone integration. The criterion for the HellmannFeynman forces is smaller than 0.01 eV Å1. All calculations are performed using the Dmol3 software package [31]. The experimental results show that Pd cube enclosed by {100} facets exhibits better performance than the Pd octahedron enclosed by {111} facets in the HER reaction. The reason is that the electron density distribution on the Pd {100} surface is better than that on the Pd {111} surface, as shown in Fig. 5. Due to the electron requirement of the HER (2Hþ þ 2ee/H2), the Pd {100} surface possessing more electron density is capable of providing more electrons, so the much better HER performance for the Pd {100} surface is observed. In addition, rGO as a carrier can also supply electrons while a negative voltage is employed to the reaction system, and Pd NCs offer a flat surface for contact with the substrates, which is benefit for accumulating electron on the Pd surface when the Pd NCs are supported on rGO (Figure S6) [21]. Additionally, the stabilization of the adsorbed intermediate might also be an important factor, the Pd cube enclosed by {100} facets may be easier to form a stable intermediate state than Pd octahedron enclosed by {111} facets [32]. Furthermore, we study the optimum loading amount of Pd NCs supported on the PEIerGO50:1 in HER as shown in Fig. 6. The content of Pd NCs on the PEIerGO50:1 is identified by atomic absorption spectroscopy (AAS) as 5 wt. %, 10 wt. %, 15 wt. %, and 20 wt. %. These samples with various metal loadings show the catalytic performance of volcanic type trending. With the increase of Pd NCs content, the HER activity increases firstly and reaches the
Fig. 4. (a) Polarization curves (after iR-correction) obtained with several catalysts as indicated and (b) corresponding Tafel plots recorded on glassy carbon electrodes with a catalyst loading of 0.14 mg cm2.
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Fig. 5. The optimized models for (a) Pd {100} surface, (b) Pd {111} surface, (c) The electron density on Pd {100} surface, and (d) The electron density on Pd {111} surface.
maximum value when the content of Pd NCs is arrived at 10 wt. %. Then it drops down sharply, which indicates that the synergistic effect is not desired when the loading amount of Pd NCs is low or high. Therefore, the Pd cube/PEIerGO50:1 (10 wt. %) displays the best activity with a current density of 108 mA cm2 at an overpotential of 100 mV and the smallest slope value of 34 mV dec1 almost comparable with the catalytic activity of commercial Pt/C catalyst (33 mV dec1). Three possible reaction steps have been suggested for the HER in acidic media [33]. First is a primary discharge step (Volmer reaction)
This step is followed by either an electrochemical desorption step (Heyrovsky reaction),
H3 Oþ þ Hads þ e /H2 þ H2 O b¼
2:3RT z40mV ð1 þ aÞF
(3)
Or a recombination step (Tafel reaction),
Hads þ Hads /H2
H3 Oþ þ e /Hads þ H2 O 2:3RT z120mV b¼ aF
Where R is the ideal gas constant, T is the absolute temperature,
a z 0.5 is the symmetry coefficient, and F is the Faraday constant.
(2)
b¼
2:3RT z30mV 2F
(4)
The Tafel slope is an inherent property of the catalyst that is
Fig. 6. (a) Polarization curves (after iR-correction) obtained with several catalysts as indicated and (b) corresponding Tafel plots recorded on glassy carbon electrodes with a catalyst loading of 0.14 mg cm2.
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Table 1 Onset potentials, Tafel slopes, exchange current densities j0 and TOF value for various catalysts. catalyst
Oneset potential (mV)
Tafel slope b (mV dec1)
j0 (mA cm2)
TOF (s1)
Pd cube Pd octahedron Pd cube/PEI-rGO50:1 (5 wt. %) Pd cube/PEI-rGO50:1 (10 wt. %) Pd cube/PEI-rGO50:1 (15 wt. %) Pd cube/PEI-rGO50:1 (20 wt. %) Pt/C
37 120 8
62 98 35
0.351 0.064 0.184
0.73 0.134 0.383
3
34
1.10
2.29
4
48
0.576
1.197
10
60
0.223
0.465
3
33
1.46
3.05
determined by the rate-limiting step of the HER. The determination and interpretation of the Tafel slope are important for elucidation of the elementary steps involved. The observed Tafel slope of 34 mV dec1 in the current work suggesting that electrochemical desorption and recombination is the rate-limiting step. The most inherent measure of activity for the HER is the exchange current density, j0, which is determined by fitting j-E data to the Tafel equation [34]. The most active catalyst of Pd cube/ PEIerGO50:1 (10 wt. %) shows the largest exchange current density of 1.10 mA cm2. To obtain a direct site-to-site comparison, the rough estimation of TOFs follows Jaramillo's method [35]. The Pd cube/PEIerGO50:1 (10 wt. %) shows the highest TOF value of 2.29 s1, indicating an excellent intrinsic HER activity. More details are showed in Table 1. The HER kinetics at the electrode/electrolyte interface is further investigated by electrochemical impedance spectroscopy (EIS). Nyquist and bode plots of the Pd cube, PEIerGO50:1 and Pd cube/ PEIerGO50:1 by applying an AC voltage with 5 mV amplitude in a frequency range from 100 000 to 0.01 Hz and recorded at 0.25 V vs. RHE in 0.5 M H2SO4 solution is shown in Fig. 7 A, B. Equivalent electric circuit is used for fitting the EIS experimental results on Pd cube/PEIerGO50:1 to model the impedance of HER (inset of Fig. 7B) [36]. For the Pd cube/PEIerGO50:1 catalyst, the two-time constant model observed from Fig. 7B that predicts the appearance of two depressed semicircles in Fig. 7A, which means the electrode process is controlled by the electrochemical reaction process and diffusion process. The high frequency semicircle (CPEl - Rct) is related to the charge transfer kinetics, being essential to high catalytic activity of the material for HER (the impedance of this electrochemical reaction process is mainly affected by the charge transfer resistance (Rct) and double layer capacitance (CPE1)). In the
spectra, the diameter of arc is a measure of Rct related to the charge transfer reaction kinetics during the HER process. It is revealed that the Pd cube/PEIerGO50:1 hybrid and the Pd cube hybrid show a much smaller radius of the semicircle in the Nyquist plots compared with PEIerGO50:1 material, indicating the higher conductivity of the Pd cube/PEIerGO50:1 hybrid catalyst due to the presence of Pd and PEIerGO, which enables simple and effective electrical transfer that minimized parasitic ohmic losses [37]. The low frequency semicircle (Cp-Rp) is associated with diffusion process, which is mainly affected by the electrode surface roughness (CPE2) and H diffusion effects (Rp). The low-medium frequency semicircle observed in Fig. 7A for the Pd cube and PEIerGO50:1 catalysts means that the HER process is accompanied by H absorption, which may take place either by a two-step (indirect) or by a direct absorption mechanism [38,39]. Another important criterion for an excellent electrocatalyst is high durability. To assess this, we cycled the Pd cube/PEIerGO50:1 (10 wt. %) hybrid continuously for 1000 cycles. At the end of the cycling, the electrocatalyst exhibits similar ieV curves to those obtained in the initial test, indicating that the Pd cube/PEIerGO50:1 composite catalyst exhibits excellent HER activity with reasonable long-term stability (Fig. 8A). Meanwhile, the chronoamperometric measurement was used to further appraise the durability of catalysts (inset of Fig. 8A). The result shows no obvious performance degradation, suggesting that the Pd cube/PEIerGO50:1 (10 wt. %) electrocatalyst maintains its catalytic activity for at least 72 h in acid electrolysis. Fig. 8B shows the hydrogen amount produced in electrolysis experiment at 150 mV overpotential in 0.5 M H2SO4. The experimental line (red) overlays with a slight deviation with the calculated hydrogen production according to the cumulative charge, assuming a theoretical value of 100% Faraday's yield (black) that means all current is used to produce hydrogen gas [40]. Compared with the theoretical value, detected value of Faraday's efficiency is 95.6%. The reasons may be come from the following several aspects: (1) the formation of palladium hydride on the electrode will consume some current [32,41]. (2) the bubbles on the electrode surface are difficult to fall off during the process of growing up. (3) part of current is consumed on the electrode surface, for forming the electric double layer. This experiment also shows that the Pd cube/PEIerGO50:1 (10 wt. %) electrocatalyst is stable during the electrolysis. 4. Conclusion In summary, polyhedral NPs of Pd serving as model catalysts are used to uncover the correlation between the surface structure and the catalytic performance of HER. The DFT results support the
Fig. 7. Nyquist (a) and bode (b) plots of Pd cube/PEIerGO50:1, PEIerGO50:1 and Pd cube.
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Fig. 8. (a) A durability test of the Pd cube/PEIerGO50:1 electrode. The inset is Chronoamperometric measurement at 150 mV overpotential. (b) Current efficiency for hydrogen production catalyzed by the Pd cube/PEIerGO50:1 (10 wt. %) hybrid on a glassy carbon electrode with a catalyst loading of 0.14 mg cm2 in 0.5 M H2SO4 and at 150 mV overpotential.
experimental observations that the catalytic activity for HER successively becomes better with an increasing in the proportion of {100} facets. Immobilizing the Pd NCs onto PEIerGO support, the observed Pd cube/PEIerGO50:1 (10 wt. %) composite achieves an impressive HER performance with a current density up to 108 mA cm2 at an overpotential of 100 mV and the value of the Tafel slope (34 mV dec1) is almost comparable with the catalytic activity of commercial Pt/C catalyst (33 mV dec1). It offers the possibility of increasing the catalytic efficiency of the HER while reducing the weight of precious metals used. We envision that the combination of our experimental results and theoretical calculations would provide fresh insights into the rational design of efficient catalysts for the HER from a different perspective. Acknowledgments This research was supported from the National Science Foundation of China (NO. 21345003), the Natural Science Foundation of Gansu (No.145RJZA132 and 143GKDA013). the Key Laboratory of Catalytic engineering of Gansu Province and Resources Utilization, Gansu Province for financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.10.050. References [1] J.A. Turner, Science 285 (1999) 687e689. [2] F. Li, L. Zhang, J. Li, X. Lin, X. Li, Y. Fang, J. Huang, W. Li, M. Tian, J. Jin, R. Li, J. Power Sources 292 (2015) 15e22. [3] B.E. Conway, B.V. Tilak, Electrochim. Acta 47 (2002) 3571e3594. [4] B. Lassalle-Kaiser, D. Merki, H. Vrubel, S. Gul, V.K. Yachandra, X. Hu, J. Yano, J. Am. Chem. Soc. 137 (2015) 314e321. [5] W. Cui, Q. Liu, Z. Xing, A.M. Asiri, K.A. Alamry, X. Sun, Appl. Catal. B 164 (2015) 144e150. [6] T. Liao, Z. Sun, C. Sun, S.X. Dou, D.J. Searles, Sci. Rep. 4 (2014) 6256. [7] D. Kong, H. Wang, Z. Lu, Y. Cui, J. Am. Chem. Soc. 136 (2014) 4897e4900. [8] F. Raimondi, G.G. Scherer, R. Kotz, A. Wokaun, Angew. Chem. Int. Ed. 44 (2005) 2190e2209. €lle, A. Otto, Surf. Sci. 597 (2005) 110e118. [9] R. To [10] A.K. Vijh, Mater. Chem. 4 (1979) 51e66. [11] Y. Xia, X. Xia, H.C. Peng, J. Am. Chem. Soc. 137 (2015) 7947e7966.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Shape-controlled synthesis of Pd polyhedron supported on polyethyleneimine-reduced graphene oxide for enhancing the efficiency of hydrogen evolution reaction Jing Li a, b, Panpan Zhou a, Feng Li a, b, Jianxin Ma a, b, Yang Liu a, b, Xueyao Zhang a, b, Hongfei Huo a, b, Jun Jin a, b, **, Jiantai Ma a, b, * a
State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China Gansu Provincial Engineering Laboratory for Chemical Catalysis, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Polyhedral Pd nanoparticles with controlled sizes and shapes were prepared. Pd nanocubes have high current density in HER. The DFT shows electron density distribution of Pd {100} is better than Pd {111}. The novel catalyst exhibits excellent HER activity comparable with that of Pt/C. It opens new insights into design efficient Pd catalyst with low loading in HER.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 August 2015 Received in revised form 13 October 2015 Accepted 15 October 2015 Available online 11 November 2015
The catalytic activity of noble-metal nanoparticles (NPs) often has closely connection with their sizes and geometric shape. In the work, polyhedral NPs of palladium (Pd) with controlled sizes, shapes, and different proportions of {100} to {111} facets on the surface were prepared by a seed-mediated approach. Electrochemical experiment demonstrates that the catalytic performance of the Pd nanocubes (NCs) enclosed by {100} facets is more active than Pd octahedrons enclosed by {111} facets for the hydrogen evolution reaction (HER), which is consistent with density functional theory (DFT) calculation results. Meanwhile, with the assistance of a polyethyleneimineereduced graphene oxide (PEIerGO) support, the examined Pd cube/PEIerGO50:1 (10 wt. %) electrocatalyst presents outstanding HER activity comparable with that of commercial Pt/C catalyst. This correlation between the HER catalytic activity and surface structure will contribute to the reasonable design of Pd catalysts for HER with high efficiency and low metal loading. © 2015 Elsevier B.V. All rights reserved.
Keywords: Palladium Nanocubes Shapeedependence Hydrogen evolution reaction Density functional theory
* Corresponding author. State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China. ** Corresponding author. State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China. E-mail addresses: [email protected] (J. Jin), [email protected] (J. Ma). http://dx.doi.org/10.1016/j.jpowsour.2015.10.050 0378-7753/© 2015 Elsevier B.V. All rights reserved.
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1. Introduction With the decreasing availability of fossil fuels and increasing demand for clean energy, hydrogen, which has high specific energy storage density and perfect environmental friendliness, is increasingly considered as the fuel of the future [1,2]. Among current technologies to produce hydrogen, water splitting (H2O / H2 þ 1/2 O2) has been considered as the best method because the method is renewable and environmental friendly [3,4]. The water splitting reaction is composed of two half reactions: the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Catalysts are often used to reduce the overpotential and consequently promote the efficiency of this important HER [5,6]. In the HER, acidic electrolytes are more favorable than alkaline electrolytes because these units are more compact and could potentially run in reverse mode to produce electricity (i.e., in fuel cells) [7]. Noble metals are usually considered as the best candidates for the HER under acidic conditions due to their excellent catalytic activity. The most representative noble metal is platinum (Pt), which has lower overpotential and higher current density [8,9]. However, its expensive cost and scarce resource definitely prohibit its commercial application in the HER. Metal palladium (Pd) relying on its remarkable catalytic capabilities and relatively ample resource is the best substitute for Pt in many areas involving HER [10]. It is well established that the activity and selectivity of Pd nanoparticles (NPs) have strong correlation with the size and geometric shape such as the type of facet exposed on the surface [11]. Therefore, how to tailor these parameters to maximize the catalytic performance of a Pd catalyst is worth to be studied. The Pd catalysts at the moment reported in the oxidation or reduction reactions do not present a breakthrough mainly based on Pd NPs without a uniform and welledefined atomic structure on the surface. To improve the catalytic performance and minimize the loading, it is necessary to replace them with even NPs enclosed by a specific set of facets. Many studies have been performed with various noble metals as model systems applied in electrochemical reactions [12e15]. For example, Zhang et al. reported that {110}faceted Pd nanocrystals show higher catalytic activity than {111}faceted NCs and {111}-faceted octahedron in hydrazine oxidation reaction [12]. Wang et al. studied the shape-dependent catalytic activity of oxygen reduction reaction (ORR) on silver nanodecahedra and NCs [13]. In the case of PtePderGO stack structure reported by Bai, {100} facets are selected for Pd and Pt, as they have relatively high current density in HER [14]. The above reports on the shapeedependence of a catalytic reaction inspire us to explore how the morphology of Pd NPs enclosed by {100} facets or {111} facets on the surface influences the HER performance in this work. Reduced graphene oxide (rGO) is a twoedimensional (2D) nanomaterial of sp2ehybridized carbon containing many special properties like high surface area, enhanced mobility of charge carriers and good stability, which also make rGO ideal platform for anchoring functional nanomaterials [16,17]. Our previous studies have shown that PEI decorated graphene oxide (GO) is beneficial to improve the electronic transmission and synergistically promote the electrocatalytic activity of the HER [18,19]. It was reported that GO can be reduced to rGO in the presence of amineecontaining molecules [20]. It is therefore rational to believe that the PEIerGO hybrid can be a promising support material for HER. Herein we prepared the Pd NCs with different sizes by a facile and aqueous approach, then used the Pd NCs of 9.7 nm width as the seeds to further synthesize relatively uniform polyhedrons such as truncated cubes (named Pd cubetransform-1), cuboctahedrons (named Pd cubetransform-2), truncated octahedrons (named Pd cubetransform-3), and octahedrons respectively. Electrochemical
experiment demonstrates that the catalytic activity of HER increases when the area ratio of {111} to {100} facets on the surface of a Pd polyhedron decreases. The experimental observations are supported by the density functional theory (DFT) calculation results. After comparing these polyhedrons, Pd NCs not only have relatively high current density in HER, but offer a flat surface for contact with the substrates [21]. Furthermore, with the synergistic catalytic assistance of a PEIerGO support, the examined Pd cube/ PEIerGO50:1 (10 wt. %) electrocatalyst exhibits an outstanding HER performance with a Tafel slope of 34 mV dec1 and an exchange current density of 2.29 mA cm2, achieving high stability simultaneously. Thus, these findings confirm that the Pdebased electrocatalysts are extremely promising for their applications in the HER. Scheme 1. 2. Experimental 2.1. Materials Natural graphite powder (325 mesh) was obtained from Alfa Aesar, polyethyleneimine (PEI, Mn ¼ 100,000) was purchased from Aladdin Reagent Co. Ltd., Na2PdCl4 (98%), Leascorbic acid (AA), Poly (vinyl pyrrolidone) (PVP, MW z 55,000) were purchased from Aldrich. All the other commercial chemicals were used without further purification. 2.2. Synthesis of Pd cubes In a typical synthesis of Pd NCs, 10.0 mL of an aqueous solution containing 105 mg of PVP, 60 mg of AA, and different amounts of KBr or KCl were placed in a 25 mL vial, and preeheated at 80 C for 5 min under magnetic stirring. Then, 3.0 mL of an aqueous solution containing 57 mg of Na2PdCl4 was added. The reaction was allowed to proceed at 80 C for 3 h. The product was collected by centrifugation and washed with acetone and ethanol to remove excess PVP, the final product was reedispersed in water (1.0 mg mL1 in concentration). 2.3. Synthesis of Pd polyhedrons In a typical synthesis of Pd polyhedrons, 3 mL of aqueous Na2PdCl4 solution (32 mM) was introduced into 8 mL of an aqueous solution containing 105 mg of PVP, 100 mL of HCHO, and 0.3 mL of an aqueous suspension (1.8 mg mL1 in concentration) of Pd NCs seeds with 9.7 nm in edge length, which had been heated at 60 C for 5 min under magnetic stirring in a 25 mL vial. The addition of Na2PdCl4 in the following amounts gave Pd cubetransform-1 (5.8 mg), Pd cubetransform-2 (8.7 mg), Pd cubetransform-3 (17.4 mg), and octahedrons (29.0 mg), respectively. The concentration of Na2PdCl4 solution is maintained at 32 mM. Each reaction was allowed to proceed at 60 C for 3 h. After collection by centrifugation and washing with acetone and ethanol, the final product was redispersed in water. 2.4. Synthesis of the PEIerGO support GO was prepared by the modified Hummers method [22]. After being carefully washed with deionized water until pH > 6, GO was freeze dried for the preparation of PEIerGO. To synthesize the PEIerGO composite (e.g., weight ratio ¼ 50:1), 10 mg of GO was added to 100 mL of water, and sonicated for 30 min to form a homogeneous yellowish brown solution. Then, 10 mL aqueous suspension of GO (0.1 mg mL1) was mixed with 50 mL PEI (1% aqueous solution), followed by sonication for another 30 min at room temperature. After being kept at 60 C for 12 h, the mixture
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Scheme 1. Illustration of the formation of Pd cube/PEIerGO nanocomposite.
was purified by centrifugation and dried using a vacuum freezing dryer to constant weight. The PEIerGO composites with weight ratios of 100: 1, 1: 1, 1: 3 were also prepared by the similar method. 2.5. Preparation of Pd cubes/PEIerGO50:1 (10 wt. %) composite materials The 10 mg of PEIerGO was suspended in 20 mL ethanol solution with magnetic stirring. After that, 1 mL of Pd NCs (1.0 mg mL1 in concentration) was added dropwise into the above PEIerGO suspension under sonication for 15 min. The mixed suspension was then stirred magnetically for 12 h. Finally, the product was collected by centrifugation and washed several times with H2O and ethanol. The final product was dried in a vacuum oven at 40 C overnight. 2.6. Electrochemical measurements Electrochemical measurements were carried out in a standard three-electrode cell with a CHI model 660E electrochemical workstation (Shanghai Chenhua Instrument Factory, China) at room temperature. A platinum wire was used as the counter electrode and an AgjAgCl (in saturated KCl) electrode as the reference. The working electrode was a glassy carbon electrode (GCE, diameter: 3.0 mm) polished with Al2O3 powders (Aldrich, 0.05 mm). The working electrodes were fabricated as follows: 5 mg of the sample was dispersed in a 5 mL mixed solution of deionized water, ethanol and Nafion solution (2:1:1) using ultrasonic treatment to produce a homogeneous suspension. Then, the well-dispersed suspension (10 mL) was drop-cast onto the GCE, which was then air-dried at room temperature. The total metal loading weights are kept constant for the assessment. All the potentials reported in our work were against reversible hydrogen electrode (RHE) through RHE calibration. For conversion of the obtained potential (vs Ag/AgCl) to RHE, Eq. (1) was used [23].
ERHE ¼ EAg=AgCl þ 0:059pH þ EoAg=AgCl EoAg=AgCl ¼ 0:209 V
(1)
2.7. Characterizations A FEI-TECNAI G2 transmission electron microscope operating at 200 kV (FEI company) was used to make the transmission electron microscopy (TEM) images and high resolution TEM (HRTEM) images. Elemental composition data were collected by Energy dispersive X-ray (EDX) performed using a TECNAI G2 microscope. Raman spectra were recorded on an in Via Reinishaw confocal spectroscope with 633 nm laser excitation. X-ray photoelectron spectroscopy (XPS) was recorded on a PHI-5702 instruments. Electrochemistry was performed with a model CHI Instrument 660E electrochemical workstation (Shanghai Chenhua Equipment, China). 3. Results and discussion Fig. 1(AeE) shows a typical TEM image of the Pd NCs with different size by employing different amounts of halide species such as Br and Cl. Other factors such as temperature may affect the growth of Pd NPs (Fig. 1F). The size distribution derived from the TEM image by counting ca. 100 cubes is plotted in the inset of Fig. 1. The details about controlled preparation of Pd NCs are listed in Table. S1. Samples (A-E) respectively correspond to six samples shown in TEM images. After comparing their catalytic performance of HER (Fig. S5), the Pd NCs of 9.7 nm width with purity approaching 100% shown in Fig. 1A are used as seeds for further preparing Pd polyhedrons. Fig. 2 shows TEM images and HR-TEM images of the Pd polyhedrons how continuous growth on the Pd {100} facets eventually leading to an octahedron enclosed by {111} facets. It can be seen that at a small amount of Na2PdCl4 (5.8 mg), the as-obtained polyhedrons are a slightly larger size cube (Fig. 2B) than the original cubic seeds (Fig. 2A). When the amount of Na2PdCl4 is increased to 8.7 mg and 17.4 mg, the polyhedrons evolves into cuboctahedron (Fig. 2C) and truncated octahedrons (Fig. 2D), respectively. Introducing an even larger amount of Na2PdCl4 (29.0 mg), the polyhedrons eventually become octahedrons with edge length of 18.7 nm as shown in Fig. 1E. Fig. 2aee are the HRTEM of Pd polyhedrons. In this process of derived from Pd cube to octahedron, the proportions of Pd {111} to {100} facets are gradually
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Fig. 1. TEM images of Pd nanocubes prepared in different reaction conditions. The inset is size distribution histograms of each sample.
increasing, which is beneficial for allowing us to study the relationship between the surface atomic structure and HER. Then, TEM is continuously used to investigate the morphologies and dispersion of the Pd cubes deposited on the PEIerGO50:1 support (Fig. 3A and B). The Pd cubes with an average width of 9.7 nm are adsorbed relatively uniformly and tightly on the PEI functionalized GO sheets by the interaction of palladium-nitride (PdeN). The EDX pattern in the inset of Fig. 3A reveals that Pd, C and N are the predominant species and the Cu element arising from the copper grid is also detected. The existence of the N element reveals that GO has been functionalized by PEI. Raman spectroscopy is a conventional tool to detect the ordered and disordered crystalline structure of graphene. The typical Raman spectra of GO, PEIerGO50:1 and Pd cube/PEIerGO50:1 nanocomposite are shown in Fig. S1. The level of chemical modification of the graphitic carbon sample is commonly quantified by the intensity ratio of the D band to G band (ID/IG), which is correlated with the average size of sp2 domains, that is, the smaller the size of sp2 domains, the higher the intensity ratio (ID/IG) [24,25]. The ID/IG ratios for GO, PEIerGO50:1 and Pd cube/PEIerGO50:1 are 0.96, 1.03 and 1.12, respectively. The increase of ID/IG ratios for PEIerGO50:1 and Pd cube/PEIerGO50:1 nanocomposites as compared with GO indicates that the new domains of conjugated carbon atoms are formed due to the removal of the oxygenous groups. Previous reports have shown the increase of the ID/IG ratio
after reduction of GO, which is probably due to the introduction of new defects after the reduction at relatively high temperature [25]. Thus, it is concluded that GO in PEIerGO50:1 and Pd cube/ PEIerGO50:1 nanocomposites have been well deoxygenated and reduced. X-ray photoelectron spectroscopy (XPS) is employed to study the composition of the composite of Pd cube/PEIerGO50: 1. According to the C 1s XPS spectra of the RGO as shown in Fig. S2B, the peaks at 284.6, 286.1, 287.7 and 288.6 eV corresponding to the oxygen and carbon atoms in the forms CeC, CeO, C]O, and COOH [26]. The XPS of the composite in the N 1s region is shown in Fig. S2C, the N 1s spectrum can be deconvoluted to several individual peaks which are assigned to ¼ Ne (398.8 eV), eNHe (399.2 eV), and eNH2 (399.9 eV), which indicates the GO sheets were successfully decorated by PEI [27]. The main doublet peak in Fig. S2D could be attributed to Pd 3d. The binding energy of the doublet peaks at 335.4 eV (assigned to Pd0 3d5/2) and 340.7 eV (assigned to Pd0 3d3/2) indicate that all Pd atoms are present in the Pd0 state [28]. Fig. S3 shows the FTIR spectra of GO, PEI, PEIerGO50:1, and Pd cube/PEIerGO50:1. As shown in Fig. S3c, a new band at 1460 cm1 (CeN stretching vibration) appears for PEIerGO50:1, indicating that some PEI is covalently linked to the PEIerGO surface by amide bonds. After decorating the Pd NCs on the PEIerGO50:1 sheet, the stretching vibrations of the main functional groups shifts to higher
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Fig. 2. (AeE) TEM images of the Pd polyhedrons obtained by controlling the amount of Na2PdCl4: (a) Pd cube, (b) Pd cubetransform-1, (c) Pd cubetransform-2, (d) Pd cubetransform-3, and (e) Pd octahedron. The inset of TEM images (E) is size distribution histograms; HR-TEM images of the Pd polyhedrons are given in the Fig. 2 (aee).
frequency as well, which is supposed to originate from the interaction of special PdeN [18]. The electrochemical activity of the HER has been evaluated using polarization curves. The polarization curves for a series of PEIerGO support (PEIerGO100:1/GCE, PEIerGO50:1/GCE, PEIerGO1:1/GCE, PEIerGO1:3/GCE) measured in 0.5 M H2SO4 at a scan rate of 5 mV s1 at room temperature were firstly recorded. As
shown in Fig. S4A, the PEIerGO catalysts present better catalytic activities than pure GO, confirming that the PEIerGO material has a high effective surface area which is beneficial for electronic transmission. Particularly, on a PEIerGO50:1/GCE, it shows the best HER activity with a current density of 28.4 mA cm2 at overpotential of 600 mV. The Tafel plots of these catalysts derived from the polarization curves shown in Fig. S4B fit well with the Tafel equation at
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Fig. 3. Typical TEM (A and B) images of Pd cube/PEIerGO50:1. The inset of TEM image (A) is EDX images of Pd cube/PEIerGO50:1.
different overpotential ranges, and only the linear portions were selected to obtain a clear comparison. A smaller Tafel slope means a faster increase of the HER rate with increasing potential [29]. The PEIerGO/GCE compared with the GO/GCE in Fig. S4, we infer that GO functionalized with PEI is available for promoting synergistically the HER. The polarization curves of the Pd NCs samples with different sizes measured in the same system are shown in Fig. S5. Samples (A-E) respectively correspond to six samples shown in TEM images of Fig. 1. The sizes of Pd NPs can be tuned in the range of 4e14 nm. The polarization curves of the current density plotted against potential show that the activity of 9.7 nm width NCs is highest because of its most positive onset potential than other NCs. In the meanwhile, the Pd NCs in Fig. 1A are more uniform, which is suitable as seeds for preparing Pd polyhedrons. Then these Pd polyhedron samples are measured in the same system as well. As shown in Fig. 4A, the total metal loading weights are kept constant for this assessment and the polarization curves show that the activities of Pd NCs are highest with the most positive onset potential of 37 mV and a current density of 99.7 mA cm2 at 200 mV vs. RHE compared to any other Pd polyhedron. Significantly, the Pd octahedron exhibits the unsatisfactory HER performance among all samples in terms of both the current densities and Tafel slopes. Meanwhile, with the proportions of {111} to {100} facets of Pd NPs increases, the HER performance shows a deterioration trend, demonstrating that the catalytic activity of Pd cube enclosed by {100} facets is more active than Pd octahedron enclosed by {111} facets for the HER. Theoretical calculation of the mechanism is mentioned below. The electron densities on Pd {100} and Pd {111} facets are investigated using DFT method. The generalized gradient
approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional is employed for the calculations [30]. An energy convergence criterion of 105 eV and an energy cutoff of 400.0 eV are used, and the k-points are set to 2 2 1 for the Brillouin zone integration. The criterion for the HellmannFeynman forces is smaller than 0.01 eV Å1. All calculations are performed using the Dmol3 software package [31]. The experimental results show that Pd cube enclosed by {100} facets exhibits better performance than the Pd octahedron enclosed by {111} facets in the HER reaction. The reason is that the electron density distribution on the Pd {100} surface is better than that on the Pd {111} surface, as shown in Fig. 5. Due to the electron requirement of the HER (2Hþ þ 2ee/H2), the Pd {100} surface possessing more electron density is capable of providing more electrons, so the much better HER performance for the Pd {100} surface is observed. In addition, rGO as a carrier can also supply electrons while a negative voltage is employed to the reaction system, and Pd NCs offer a flat surface for contact with the substrates, which is benefit for accumulating electron on the Pd surface when the Pd NCs are supported on rGO (Figure S6) [21]. Additionally, the stabilization of the adsorbed intermediate might also be an important factor, the Pd cube enclosed by {100} facets may be easier to form a stable intermediate state than Pd octahedron enclosed by {111} facets [32]. Furthermore, we study the optimum loading amount of Pd NCs supported on the PEIerGO50:1 in HER as shown in Fig. 6. The content of Pd NCs on the PEIerGO50:1 is identified by atomic absorption spectroscopy (AAS) as 5 wt. %, 10 wt. %, 15 wt. %, and 20 wt. %. These samples with various metal loadings show the catalytic performance of volcanic type trending. With the increase of Pd NCs content, the HER activity increases firstly and reaches the
Fig. 4. (a) Polarization curves (after iR-correction) obtained with several catalysts as indicated and (b) corresponding Tafel plots recorded on glassy carbon electrodes with a catalyst loading of 0.14 mg cm2.
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Fig. 5. The optimized models for (a) Pd {100} surface, (b) Pd {111} surface, (c) The electron density on Pd {100} surface, and (d) The electron density on Pd {111} surface.
maximum value when the content of Pd NCs is arrived at 10 wt. %. Then it drops down sharply, which indicates that the synergistic effect is not desired when the loading amount of Pd NCs is low or high. Therefore, the Pd cube/PEIerGO50:1 (10 wt. %) displays the best activity with a current density of 108 mA cm2 at an overpotential of 100 mV and the smallest slope value of 34 mV dec1 almost comparable with the catalytic activity of commercial Pt/C catalyst (33 mV dec1). Three possible reaction steps have been suggested for the HER in acidic media [33]. First is a primary discharge step (Volmer reaction)
This step is followed by either an electrochemical desorption step (Heyrovsky reaction),
H3 Oþ þ Hads þ e /H2 þ H2 O b¼
2:3RT z40mV ð1 þ aÞF
(3)
Or a recombination step (Tafel reaction),
Hads þ Hads /H2
H3 Oþ þ e /Hads þ H2 O 2:3RT z120mV b¼ aF
Where R is the ideal gas constant, T is the absolute temperature,
a z 0.5 is the symmetry coefficient, and F is the Faraday constant.
(2)
b¼
2:3RT z30mV 2F
(4)
The Tafel slope is an inherent property of the catalyst that is
Fig. 6. (a) Polarization curves (after iR-correction) obtained with several catalysts as indicated and (b) corresponding Tafel plots recorded on glassy carbon electrodes with a catalyst loading of 0.14 mg cm2.
350
J. Li et al. / Journal of Power Sources 302 (2016) 343e351
Table 1 Onset potentials, Tafel slopes, exchange current densities j0 and TOF value for various catalysts. catalyst
Oneset potential (mV)
Tafel slope b (mV dec1)
j0 (mA cm2)
TOF (s1)
Pd cube Pd octahedron Pd cube/PEI-rGO50:1 (5 wt. %) Pd cube/PEI-rGO50:1 (10 wt. %) Pd cube/PEI-rGO50:1 (15 wt. %) Pd cube/PEI-rGO50:1 (20 wt. %) Pt/C
37 120 8
62 98 35
0.351 0.064 0.184
0.73 0.134 0.383
3
34
1.10
2.29
4
48
0.576
1.197
10
60
0.223
0.465
3
33
1.46
3.05
determined by the rate-limiting step of the HER. The determination and interpretation of the Tafel slope are important for elucidation of the elementary steps involved. The observed Tafel slope of 34 mV dec1 in the current work suggesting that electrochemical desorption and recombination is the rate-limiting step. The most inherent measure of activity for the HER is the exchange current density, j0, which is determined by fitting j-E data to the Tafel equation [34]. The most active catalyst of Pd cube/ PEIerGO50:1 (10 wt. %) shows the largest exchange current density of 1.10 mA cm2. To obtain a direct site-to-site comparison, the rough estimation of TOFs follows Jaramillo's method [35]. The Pd cube/PEIerGO50:1 (10 wt. %) shows the highest TOF value of 2.29 s1, indicating an excellent intrinsic HER activity. More details are showed in Table 1. The HER kinetics at the electrode/electrolyte interface is further investigated by electrochemical impedance spectroscopy (EIS). Nyquist and bode plots of the Pd cube, PEIerGO50:1 and Pd cube/ PEIerGO50:1 by applying an AC voltage with 5 mV amplitude in a frequency range from 100 000 to 0.01 Hz and recorded at 0.25 V vs. RHE in 0.5 M H2SO4 solution is shown in Fig. 7 A, B. Equivalent electric circuit is used for fitting the EIS experimental results on Pd cube/PEIerGO50:1 to model the impedance of HER (inset of Fig. 7B) [36]. For the Pd cube/PEIerGO50:1 catalyst, the two-time constant model observed from Fig. 7B that predicts the appearance of two depressed semicircles in Fig. 7A, which means the electrode process is controlled by the electrochemical reaction process and diffusion process. The high frequency semicircle (CPEl - Rct) is related to the charge transfer kinetics, being essential to high catalytic activity of the material for HER (the impedance of this electrochemical reaction process is mainly affected by the charge transfer resistance (Rct) and double layer capacitance (CPE1)). In the
spectra, the diameter of arc is a measure of Rct related to the charge transfer reaction kinetics during the HER process. It is revealed that the Pd cube/PEIerGO50:1 hybrid and the Pd cube hybrid show a much smaller radius of the semicircle in the Nyquist plots compared with PEIerGO50:1 material, indicating the higher conductivity of the Pd cube/PEIerGO50:1 hybrid catalyst due to the presence of Pd and PEIerGO, which enables simple and effective electrical transfer that minimized parasitic ohmic losses [37]. The low frequency semicircle (Cp-Rp) is associated with diffusion process, which is mainly affected by the electrode surface roughness (CPE2) and H diffusion effects (Rp). The low-medium frequency semicircle observed in Fig. 7A for the Pd cube and PEIerGO50:1 catalysts means that the HER process is accompanied by H absorption, which may take place either by a two-step (indirect) or by a direct absorption mechanism [38,39]. Another important criterion for an excellent electrocatalyst is high durability. To assess this, we cycled the Pd cube/PEIerGO50:1 (10 wt. %) hybrid continuously for 1000 cycles. At the end of the cycling, the electrocatalyst exhibits similar ieV curves to those obtained in the initial test, indicating that the Pd cube/PEIerGO50:1 composite catalyst exhibits excellent HER activity with reasonable long-term stability (Fig. 8A). Meanwhile, the chronoamperometric measurement was used to further appraise the durability of catalysts (inset of Fig. 8A). The result shows no obvious performance degradation, suggesting that the Pd cube/PEIerGO50:1 (10 wt. %) electrocatalyst maintains its catalytic activity for at least 72 h in acid electrolysis. Fig. 8B shows the hydrogen amount produced in electrolysis experiment at 150 mV overpotential in 0.5 M H2SO4. The experimental line (red) overlays with a slight deviation with the calculated hydrogen production according to the cumulative charge, assuming a theoretical value of 100% Faraday's yield (black) that means all current is used to produce hydrogen gas [40]. Compared with the theoretical value, detected value of Faraday's efficiency is 95.6%. The reasons may be come from the following several aspects: (1) the formation of palladium hydride on the electrode will consume some current [32,41]. (2) the bubbles on the electrode surface are difficult to fall off during the process of growing up. (3) part of current is consumed on the electrode surface, for forming the electric double layer. This experiment also shows that the Pd cube/PEIerGO50:1 (10 wt. %) electrocatalyst is stable during the electrolysis. 4. Conclusion In summary, polyhedral NPs of Pd serving as model catalysts are used to uncover the correlation between the surface structure and the catalytic performance of HER. The DFT results support the
Fig. 7. Nyquist (a) and bode (b) plots of Pd cube/PEIerGO50:1, PEIerGO50:1 and Pd cube.
J. Li et al. / Journal of Power Sources 302 (2016) 343e351
351
Fig. 8. (a) A durability test of the Pd cube/PEIerGO50:1 electrode. The inset is Chronoamperometric measurement at 150 mV overpotential. (b) Current efficiency for hydrogen production catalyzed by the Pd cube/PEIerGO50:1 (10 wt. %) hybrid on a glassy carbon electrode with a catalyst loading of 0.14 mg cm2 in 0.5 M H2SO4 and at 150 mV overpotential.
experimental observations that the catalytic activity for HER successively becomes better with an increasing in the proportion of {100} facets. Immobilizing the Pd NCs onto PEIerGO support, the observed Pd cube/PEIerGO50:1 (10 wt. %) composite achieves an impressive HER performance with a current density up to 108 mA cm2 at an overpotential of 100 mV and the value of the Tafel slope (34 mV dec1) is almost comparable with the catalytic activity of commercial Pt/C catalyst (33 mV dec1). It offers the possibility of increasing the catalytic efficiency of the HER while reducing the weight of precious metals used. We envision that the combination of our experimental results and theoretical calculations would provide fresh insights into the rational design of efficient catalysts for the HER from a different perspective. Acknowledgments This research was supported from the National Science Foundation of China (NO. 21345003), the Natural Science Foundation of Gansu (No.145RJZA132 and 143GKDA013). the Key Laboratory of Catalytic engineering of Gansu Province and Resources Utilization, Gansu Province for financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.10.050. References [1] J.A. Turner, Science 285 (1999) 687e689. [2] F. Li, L. Zhang, J. Li, X. Lin, X. Li, Y. Fang, J. Huang, W. Li, M. Tian, J. Jin, R. Li, J. Power Sources 292 (2015) 15e22. [3] B.E. Conway, B.V. Tilak, Electrochim. Acta 47 (2002) 3571e3594. [4] B. Lassalle-Kaiser, D. Merki, H. Vrubel, S. Gul, V.K. Yachandra, X. Hu, J. Yano, J. Am. Chem. Soc. 137 (2015) 314e321. [5] W. Cui, Q. Liu, Z. Xing, A.M. Asiri, K.A. Alamry, X. Sun, Appl. Catal. B 164 (2015) 144e150. [6] T. Liao, Z. Sun, C. Sun, S.X. Dou, D.J. Searles, Sci. Rep. 4 (2014) 6256. [7] D. Kong, H. Wang, Z. Lu, Y. Cui, J. Am. Chem. Soc. 136 (2014) 4897e4900. [8] F. Raimondi, G.G. Scherer, R. Kotz, A. Wokaun, Angew. Chem. Int. Ed. 44 (2005) 2190e2209. €lle, A. Otto, Surf. Sci. 597 (2005) 110e118. [9] R. To [10] A.K. Vijh, Mater. Chem. 4 (1979) 51e66. [11] Y. Xia, X. Xia, H.C. Peng, J. Am. Chem. Soc. 137 (2015) 7947e7966.
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