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PdCu@Pd Nanocube with Pt-like Activity for Hydrogen Evolution Reaction Jing Li, Feng Li, Si-Xuan Guo, Jie Zhang, and Jiantai Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01241 • Publication Date (Web): 15 Feb 2017 Downloaded from http://pubs.acs.org on February 20, 2017

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ACS Applied Materials & Interfaces

PdCu@Pd Nanocube with Pt-like Activity for Hydrogen Evolution Reaction Jing Li, †§‡ Feng Li, †‡ Si-Xuan Guo, § Jie Zhang*§ and Jiantai Ma*†

†

State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of

Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China §

School of Chemistry and ARC Centre of Excellence for Electromaterials Science,

Monash University, Clayton, Vic. 3800, Australia

KEYWORDS: PdCu@Pd nanocubes, Anodization, Hydrogen evolution reaction,

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Pt-like activity, Electrocatalysis ABSTRACT

The electronic properties of metal surfaces can be modulated to weaken the binding energy of adsorbed H-intermediates on the catalyst surface thus enhance the catalytic activity for the hydrogen evolution reaction (HER). Here we firstly prepare PdCu alloy nanocubes (NCs) by co-reduction of Cu(acac)2 (acac =acetylacetonate) and Na2PdCl4 in the presence of oleylamine (OAm) and trioctylphosphine (TOP). The PdCu NC coated glassy carbon electrode is then anodized at a constant potential of 0.51 V vs. Ag/AgCl at room temperature in 0.5 M H2SO4 solution for 10 s, which converts PdCu NCs into core@shell PdCu@Pd NCs that show much enhanced Pt-like activity for the HER and much more robust durability. The improvements in surface property and HER activity are rationalized based on strain and ligand effects that enhance the activity of the edge-exposed Pd atoms on core@shell PdCu@Pd structure. This work opens up new perspective of simultaneously reducing metal Pd cost and achieving excellent performance towards the HER.

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1. INTRODUCTION Hydrogen is now widely considered as the fuel source of the future owing to its high energy efficiency and zero CO2 emission.1-3 Hydrogen can be produced by electrochemical reduction of protons in water using electricity from renewable sources, such as solar and wind. This hydrogen evolution reaction (HER) also provides an effective mechanism in storing energy from intermittent sources in the form of chemical energy.4, 5 To enhance the kinetics of HER without consuming extra energy, a catalyst must be present to initiate proton reduction with minimal overpotential (η) and to facilitate hydrogen production with high faradaic efficiency.6 At present, Pt-based metals are the most effective HER catalysts. However, they are unsuitable for large scale practical application in hydrogen production due to their scarcity and hence high cost.7-10 It is therefore essential to search for economical and effective Pt-free electrocatalysts. In this case, Pd could be considered as a good candidate for catalyzing the HER due to its similarly superior catalytic ability and comparatively abundant resource.11

In our previous study, we have demonstrated both experimentally and theoretically that Pd nanocubes (NCs) are more active electrocatalyts for the HER than Pd octahedrons.12 However, the cost of metallic Pd catalyst required was still relatively high. Thus, Pd-based bimetallic electrocatalysts, such as Pd-monolayer core-shell structure or alloys of Pd with cheaper transition metals, are attracting increasing attention in recent years due to their lower cost and higher activities.13-16 These bimetallic approaches can modify the surface atomic structure of the catalysts to 3



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further enhance the catalytic performance.17-18 Among these various Pd-based electrocatalysts, PdCu nanoparticles have been intensively studied in a variety of reactions such as formic acid oxidation,19 CO oxidation,20 and CO2 reduction.21 The reason why such alloy nanoparticles of expensive metal coupled with non-expensive metal are popular electrocatalysts is that they not only promote electrochemical performances but also reduce the material cost.22-24 Recently, Shao and co-workers have reported the synthesis of PdCu nanoparticles (NPs) supported on carbon (PdCu NPs/C), which displayed a high catalytic activity for the oxygen reduction reaction.25 However, the preparation of PdCu NCs with controllable morphologies for the application in the HER has not been explored.

It is well-known that the mechanisms of the HER include the adsorption of hydrogen (*H) on the electrode surface. According to the Sabatier principle,26 in order to achieve the best catalytic performance, the binding energy of the reaction intermediates on the electrode surface should not be too weak or too strong. On this basis, the optimal binding energy of *H on the electrocatalytic sites for the HER should be ~ 0.09 eV, which is slightly lower than that on Pt, Pd or Rh. In principle, the electronic properties of these metal surface can be tuned via the formation of bulk alloys or core-shell (monolayer) structures to provide an optimal binding strength for *H to improve the catalytic activity of these metals.27, 28

In this work, in order to optimize the binding energy for *H and maximize the number of active sites on the surface of the catalyst, we design a novel core@shell

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PdCu@Pd NC catalyst for highly efficient hydrogen production using a procedure described in Scheme 1. First, we synthesize highly mondisperse PdCu NCs with an average size of 11.7 nm using a modified literature procedure.29 In the synthesis, Pd/Cu compositions are controlled by the molar ratio of Na2PdCl4 and [Cu(acac)2] (acac=acetylacetonate), and the surface Cu is etched in a controlled manner by anodization in an acid solution, resulting in the core@shell structured PdCu@Pd NCs. The as-synthesized PdCu@Pd NCs shows much enhanced HER activity in comparison with PdCu NCs, spherical PdCu NPs and Pd NCs reported in previous work,12 and is comparable to that of commercial Pt catalysts but with much enhanced durability.

Scheme 1. Illustration of the formation of PdCu@Pd nanocubes.

2. EXPERIMENTAL SECTION 2.1. Materials. Oleylamine (OAm, >70%), trioctylphosphine (TOP), ascorbic Acid (AA), poly (vinyl pyrrolidone) (PVP), Cu(acac)2 (acac = acetylacetonate) (95%), Na2PdCl4 (98%), Nafion (5%) were purchased from Sigma Aldrich. The commercial 5



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Pt/C catalyst (20 wt. % loading) was obtained from Dalian Trico Chemical Co. Ltd. The deionized water was obtained from a Millipore Autopure system. All reagents were analytical grade and used without further purification.

2.2. Synthesis of monodisperse PdCu nanocubes. A modified literature procedure was used in the synthesis of monodisperse PdCu nanocubes.29 In brief, 0.2 mmol Na2PdCl4 and 0.2 mmol [Cu(acac)2] were added to 10 mL OAm with magnetic stirring. The solution was heated at 100 ℃ for 30 min under a N2 atmosphere, and then 1.1 mL TOP was added dropwise to the mixture. The reaction temperature was then increased to 250 ℃ at a speed of 10 ℃ per minute and kept at 250 ℃ for 30 min until the solution color turned black. Finally, the product was separated from the solution by centrifugation and washed with ethanol. 2.3. Synthesis of monodisperse PdCu nanoparticles. 0.2 mmol Na2PdCl4 and 0.2 mmol [Cu(acac)2] were added to 10 mL OAm with magnetic stirring at a temperature of 100 ℃ for 30 min under a N2 atmosphere. Then the reaction temperature was increased to 200 ℃ at a speed of 10 ℃ per minute and kept at 200 ℃ for 1 h until the solution color turned black. Finally, the product was separated from the solution by centrifugation and washed with ethanol.

2.4. Synthesis of monodisperse Pd nanocubes. A literature procedure was used in the synthesis of monodisperse Pd nanocubes.12 In brief, an aqueous solution (20 mL) containing 210 mg PVP, 120 mg AA, and 600 mg KBr was prepared and heated up to 80 ℃ for 10 min under magnetic stirring. After that, 6.0 mL of an aqueous solution

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containing 114 mg Na2PdCl4 was added dropwise. The mixture was kept at 80 ℃ for 3 h. Finally, the product was separated from the solution by centrifugation and washed with ethanol.

2.5. Synthesis of monodisperse Pd nanoparticles. 0.2 mmol Na2PdCl4 was added to 10.0 mL OAm with magnetic stirring. The solution was heated up to 100 ℃ and kept for 30 min under a N2 atmosphere. 2.0 mL TOP was then added dropwise to the mixture, and the solution temperature was increased to 300 ℃ at a speed of 10 ℃ per minute and kept at 300 ℃ for 1h until the solution color became black. Finally, the product was separated from the solution by centrifugation and washed with ethanol.

2.6. Electrochemical characterization. The electrochemical experiments were undertaken at 20 ℃ in a typical three-electrode setup using an electrochemical workstation (CH 660E, CH Instrument, Texas, USA). A glassy carbon electrode (GCE, 3.0 mm diameter), Pt wire and Ag/AgCl (saturated KCl) were employed as the working, counter and reference electrodes, respectively. The coating solution used for the modification of electrodes was prepared by dispersing 5 mg of the catalyst in a mixture of 2.5 mL DI water, 1.25 mL Nafion solution and 1.25 mL ethanol (v:v:v=2:1:1), followed by sonication for 30 min. 10 µL of the dispersion was drop-casted onto the glassy carbon electrode (catalyst loading: ~ 0.14 mg cm-2), and dried in air at room temperature. The solution was purged with N2 for 10 min before measurements. IR-correction was done using the function available with the instrument.

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The measured potential (vs. EAg/AgCl) was converted to versus the reversible electrode (RHE) scale (ERHE) according to the following equation (Eq. 1) using the known potential of Ag/AgCl (saturated KCl) of 0.204 V vs NHE.30

ERHE = EAg/AgCl + 0.058pH + 0.204

(1)

2.7. Electrolysis and product analysis. Potentiostatic electrolysis was performed at an overpotential of 250 mV in an air-tight H-shaped electrolysis cell equipped with a PdCu@Pd NCs modified GC working electrode (catalyst loading 0.14 mg cm-2) together with aforementioned reference and counter electrodes. Prior to electrolysis, the solution was purged with N2 for 30 min. The generated H2 gas was sampled (50

µL) and analyzed offline with a gas chromatograph (Shanghai Ke Chuang GC9800) equipped with a molecular sieve 5A column and a thermal conductivity detector. N2 was used as the carrier gas.

2.8. Spectroscopic and microscopic characterization. The compositions of the nanomaterials were determined by ICP-AES (PerkinElmer Optima 4300 DV). A transmission electron microscope (TEM, FEI-TECNAI G2) was used to obtain the morphology of the prepared nanomaterials. A TECNAI G2 microscope equipped with an energy dispersive X-ray (EDX) spectrometer was used to collect the elemental composition data. All X-ray photoelectron spectroscopy (XPS) spectra obtained from a Perkin-Elmer PHI-5702 instrument are calibrated by the position of the C 1s peak. X-ray diffraction (XRD) measurements were undertaken on a Rigaku D/max-2400 diffractometer, using Cu-Kα radiation as the X-ray source in the 2θ range of 10 – 90°. 8


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3. RESULTS AND DISCUSSION 3.1. Synthesis and characterization. PdCu NCs of 1:1 molar ratio was prepared in 10.0 mL OAm with TOP as the stabilizer, which plays an important role in controlling the morphology of the PdCu NCs. The transmission electron microscopic (TEM) image in Figure 1a shows that small spherical nanoparticles are formed in the absence of TOP. Upon addition of TOP, cubic nanoparticles start to emerge. As the amount of TOP increases, the size of the nanocubes gradually becomes larger, ranging from 10 nm to 30 nm with a few spherical nanoparticles. Uniform nanocubes with an average size of 11.7 nm were obtained in the presence of 1.1 mL of TOP. The presence of TOP stabilizes the {100} facet to lower the total surface energy, which favors the formation of PdCu nanocubes.29 Further increase in TOP amount results in the formation of a mixture of spherical nanoparticles and larger polyhedral nanoparticles.

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Figure 1. TEM images of the products synthesized using 0.2 mmol Na2PdCl4 and (a) 0 mL; (b) 0.5 mL; (c) 0.9 mL; (d) 1.1 mL; (e) 1.5 mL; and (f) 2 mL TOP.

In addition to TOP, the molar ratio of Pd and Cu is also an important factor that affects the morphology of the Pd/Cu nanomaterials (Figure 2). When the molar ratio of Pd and Cu is changed to 2:3 or 3:2 (Figure 2a, b), a few small spherical nanoparticles appear, whereas when the molar ratio of Pd/Cu is tuned to 1:2 or 2:1 (Figure 2c, d), bigger and more irregular shaped nanocubes are obtained. Furthermore, the cubic shape of the nanoparticles cannot be maintained if the ratio is either 1:3 or 3:1 (Figure 2e, f). Therefore, an optimal molar ratio of 1:1 is used to balance the morphology and Pd content of the PdCu NCs used in all studies described below unless otherwise stated. It should be noted that the above molar ratios refer to the 10


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ratios of Pd and Cu precursors used in the synthesis. The exact molar ratios of Pd to Cu in the final products are determined by ICP-AES and shown in Table S1. However, for convenience of presentation, the ratios of Pd and Cu precursors are used in the following nomenclatures to represent the binary composition of a nanomaterial. For example, PdCu1:1 represents the nanomaterial synthesized with 1:1 molar ratio of Pd and Cu precursors.

Figure 2. TEM images of PdCu NCs or NPs synthesized with different molar ratios of Pd:Cu: (a) 2:3; (b) 3:2; (c) 1:2; (d) 2:1; (e) 1:3 and (f) 3:1.

Representative TEM images of the as-synthesized PdCu NCs and PdCu NPs show that they are both monodisperse (Figures 3a and b). The high-resolution TEM

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(HR-TEM) images (Figures 3c and d) show two different lattice fringes with spacings of 0.297 nm and 0.215 nm, which correspond to the {100} and {111} planes on the PdCu NCs and PdCu NPs, respectively. The size distribution plots (Figures 3e and f) indicate that the average width or diameter of PdCu NCs and PdCu NPs are 12 ± 3 nm and 3.5 ± 0.5 nm, respectively. The X-ray diffraction (XRD) patterns of the as-synthesized PdCu NCs illustrated in Figure 3g show peaks at 2θ = 30.0 o, 42.9 o, 53.3 o, 62.4 o, 70.8o and 78.7 o, which are assigned to the (100), (110), (111), (200), (210) and (211) crystal planes, respectively.29 Based on the calculation from the full width at half height for PdCu NCs (100) using the Scherrer equation,31 the average size of PdCu NCs is 12.7 nm, which is in good agreement with the statistic results obtained from TEM images. Meanwhile, diffraction peaks for PdCu NPs at 2θ = 41.3 o

, 48.0 o, 70.0 o, 83.3 o can be ascribed to the (111), (200), (220), and (311) crystal

planes, respectively, indicating the face-centered cubic (fcc) structure of the nanoparticles.21 Moreover, only elements C, Pd and Cu are detected in the energy-dispersive X-ray (EDX) spectrum showed in Figure 3h. The X-ray photoelectron spectroscopic (XPS) data are also obtained to study the chemical properties of PdCu NCs, and the main peaks (Figure S1) could be assigned to Pd 3d and Cu 2p.

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Figure 3. (a-b) TEM images of the PdCu1:1 nanocubes (a) and nanoparticles (b) obtained by controlling the reaction temperature; HR-TEM images of the PdCu1:1 nanocubes (c) and nanoparticles (d); size distribution histograms of PdCu1:1 nanocubes (e) and nanoparticles (f) ; (g) XRD patterns of PdCu1:1 nanucubes and nanoparticles; (h) EDX spectrum of PdCu1:1 nanocubes.

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To investigate the distributions of different elements of the PdCu NCs, a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image together with EDX line scan are obtained (Figure 4). The high-resolution elemental mapping of the 11 nm size PdCu NCs and the EDX line scan across the PdCu NCs (Figures 4a−f) confirm that both Pd and Cu distribute evenly in each nanocube.

To obtain the PdCu@Cu NCs, the PdCu NCs were drop casted on a GCE electrode, and the modified electrode was then placed in contact with a 0.5 M H2SO4 solution and a constant potential of 0.51 V vs. Ag/AgCl was applied for 10 s. Figures 5a−f show the elemental mapping and EDX line scan on Pd and Cu distribution obtained after this electrochemical pretreatment. The STEM image reveals that the cubic structure retains. It can also be seen that Pd still distributes across the whole nanoparticles, but Cu locates only in the center region of the nanocubes, indicating that after anodization, the Cu on the surface of PdCu NCs is oxidatively etched away and the core@shell PdCu@Pd NCs is formed.

Figure

4.

HAADF-STEM

image

(a)

of

a

pile

of

PdCu

nanocubes.

HAADFSTEM-EDX elemental mapping images of Cu–K (b), Cu–L (c), Pd–K (d) and

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Pd–L (e). (f) Cross-sectional compositional line profiles of Cu and Pd in PdCu nanocubes recorded along the line shown in the HAADF-STEM image (inset).

Figure 5. HAADF-STEM image (a) of a pile of PdCu@Pd nanocubes. HAADFSTEM-EDX elemental mapping images of Cu–K (b), Cu–L (c), Pd–K (d) and Pd–L (e). (f) Cross-sectional compositional line profiles of Cu and Pd in PdCu@Pd nanocubes recorded along the line shown in the HAADF-STEM image (inset).

3.2. Electrocatalytic performance. The electrocatalytic performances of PdCu nanocubes with different Pd/Cu molar ratios for the HER are studied in a 0.5 M H2SO4 solution and the HER polarization curves are illustrated in Figure 6a. These results show that PdCu1:1 NCs has the most positive onset potential and thus the smallest overpotential compared to other nanocubes with different compositions. Studies were also carried out with PdCu@Pd NCs, Pd NCs, PdCu1:1 NPs, Pd NPs and Cu NPs under the same conditions. The results show that PdCu@Pd NCs only require an overpotential of 10 mV to reach a current density of 68 mA cm-2, and have the most positive onset potential of -5 mV (Figure 6b). This performance is far superior to PdCu1:1, Pd and Cu NPs and is also considerably better than Pd and PdCu1:1 NCs. For comparison, the result obtained with the commercial Pt nanocatalysts is also included. 15



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The results suggest that the electrocatalytic performance of PdCu@Pd NCs is close to that of Pt nanocatalysts. This significant HER activity associated with PdCu@Pd NCs could be attributed to the strain and ligand effects on the d band structure of surface metal (i.e. Pd in this case) induced by the substrate metal (i.e. Cu in this case), which subsequently affect the kinetics of the HER that involves adsorbed hydrogen species as the reaction intermediates.27 Furthermore, since Cu atoms are removed from the surface during the anodization process, structural defects can be expected, in particular, vacancies would be formed on the alloy surface. As a result, the Pd atoms adjacent to the defects having lower coordination numbers can contribute to the enhanced HER activities as compared to the perfect surface.32 Therefore, better activity can be expected for PdCu@Pd NCs than PdCu NCs.

In order to further evaluate the catalytic activities of the catalysts, the Tafel slopes should be compared. The Tafel plots of PdCu@Pd NCs and commercial Pt catalysts are derived from the polarization curves obtained (Figure 6c). The linear sections of the Tafel plots are fitted to the Tafel equation, and the slope values are obtained. A small Tafel slope is preferred since the lower the Tafel slope, the faster the increase in HER rate when the driving force increases.33 As shown in Figure 6c, the smallest Tafel slope of 35 mV dec-1 is obtained for PdCu@Pd NCs, which is close to the value of 31 mV dec-1 for Pt/C catalyst. Figure 6d compared the η10

mA cm-2

values

(overpotential at a current density of 10 mA cm-2) and the onset potentials for different catalysts. The results show that the lowest HER onset potential (-5 mV) and

η10

mA cm-2

(68 mV) values obtained at PdCu@Pd NCs are close to those for the 16


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commercial Pt catalyst, and are much lower than those for other catalysts with different morphologies, indicating the highest catalytic activity of PdCu@Pd NCs. A detailed comparison of the catalytic activities of the catalysts reported herein and other high performing HER catalysts reported in the literature is shown in Table S2. It can therefore be concluded that PdCu@Pd NCs has the highest catalytic activity for the HER among all the catalysts studied and compared.

Figure 6. (a) and (b) Polarization curves (after iR-correction) obtained with several catalysts as indicated and (c) the corresponding Tafel plots recorded on glassy carbon electrodes with a catalyst loading of 0.14 mg cm-2. (d) comparison of the overpotential at a current density of 10 mA cm-2 (left) and the onset potential (right) of each catalyst.

There are three elementary reactions steps involved in the HER, which are the 17



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Volmer (electrochemical hydrogen adsorption), the Heyrovsky (electrochemical desorption) and the Tafel (chemical desorption) reactions.34, 35 Tafel slope, a crucial indicator of the HER electrocatalyst, have often been used to evaluate the catalysts and identify the rate determining step. Tafel slopes of about 120, 40 or 30 mV per decade are predicted when the rate-determining step is the Volmer, Heyrovsky or Tafel reaction, respectively. A Tafel slope of 35 mV dec-1 for PdCu@Pd NCs suggests that the desorption of *H is the rate-limiting step.

The exchange current density, j0, the measure of activity for the HER, is obtained from the Tafel equation at η = 0.36 The exchange current density of the PdCu@Pd NCs catalyst is calculated to be 0.74 mA cm-2. Under this condition, a turnover frequency (TOF) value of 1.53 s-1 is estimated for PdCu@Pd NCs, which is close to the value of 1.73 s-1 obtained with the commercial Pt.37 These values further confirm the outstanding HER activity of PdCu@Pd NCs. The comparison of onset potentials, Tafel slopes, exchange current densities j0 and TOF values for various catalysts are summarized in Table 1.

Table 1. Comparison of onset potentials, Tafel slopes, exchange current densities j0 and TOF values for various catalysts.

Catalyst

Onset potential (mV)

Tafel slope (mV dec-1)

j0 (mA cm-2)

TOF (s-1)

Commercial Pt PdCu@Pd NCs PdCu NCs Pd NCs PdCu NPs Pd NPs

-3 -5 -8 -37 -53 -125

31 35 47 62 160 152

0.83 0.74 0.27 0.35 0.24 0.059

1.73 1.53 0.57 0.73 0.51 0.13

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Cu NPs

-270

244

0.023

0.053

Electrochemical impedance spectroscopy (EIS) is a powerful and informative technique for the investigation of the properties of the modified electrodes and electrode kinetics in the HER. Nyquist and Bode plots for PdCu@Pd NCs obtained in 0.5 M H2SO4 solution at -0.25 V vs. RHE using an AC frequency in a range from 0.01 to 100000 Hz with an amplitude of 5 mV are shown in Figures 7 a and b. The Bode plots of PdCu NCs and PdCu NPs are shown in Figures S2 a and b for comparison. In the phase angle-frequency Bode plots (Figures 7b and S2a), two distinguishable peaks (two-time constant model) of the PdCu@Pd NCs and PdCu NCs catalysts correspond to two semicircles in Nyquist plots in Figure 7a. The high frequency semicircle (CPEl-Rct) is associated with the charge transfer (thus electrocatalytic) kinetics, and the lower the radius value the faster the reaction rate. The low frequency one (CPE2-Rp-W) is related to the diffusion process.38 It can be seen from Figure 7 that PdCu@Pd NCs show a much smaller radius of the semicircle in the Nyquist plot in comparison with PdCu NCs and PdCu NPs, indicating higher reaction rate of PdCu@Pd NCs due to its higher intrinsic activity and higher active area. The low-medium frequency semicircle observed in the inset of Figure 7a for the PdCu NPs catalyst implies that the HER process is accompanied by H adsorption via either an indirect two-step or a direct one-step pathways.38

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Figure 7. Nyquist (a) and Bode (b) plots of PdCu@Pd NCs. The brown line in (a) shows the fitted Nyquist plots using ZsimpWin program. Inset in (a): Nyquist plots for the designated catalysts.

The EIS data of PdCu@Pd NCs and PdCu NCs obtained at an overpotential of 250 mV are fitted using the electrical equivalent circuit diagrams given in the inset of Figure 7b with the fitting parameters listed in Table 2, where Rs is the sum of the electrolytic and electrical leads resistances, and ω* is the characteristic frequency, which is used to determine the value of double-layer capacitance, Cdl.39, 40 The value n, the phase shift, is a measure of the surface inhomogeneity, which results from surface roughness, grain boundaries, and distribution of the active sites and formation of porous layers.41, 42 The resistance values were calculated based on the known GCE geometric area (0.07 cm2).

Table 2. The electrochemical data determined from Nyquist plots of the catalysts

Catalyst

Overpotential (mV)

Rs Rct 2 (Ω cm ） (Ω cm2）

Cdl ω*=2πf -2 (µF cm ）（s-1）

PdCu@Pd NCs

50

0.48

20.4

1.68

20


2.92 × 104

n

0.74

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PdCu NCs

50

0.47

3.57

17.1

1.64 × 104

0.76

The values of Cdl and n are employed to provide additional information about the structures of PdCu@Pd NCs and PdCu NCs. It is well-known that Cdl is directly related to the active surface area, hence, higher Cdl value for PdCu@Pd NCs compared to that for PdCu NCs suggests the increase in electrochemically active surface area resulted from the proliferation of the active sites. Furthermore, the n values of PdCu@Pd NCs and PdCu NCs are 0.74 and 0.76, respectively. A smaller value indicates a rougher surface. Thus, anodization of PdCu NC has successfully formed a slightly rougher core@shell structure of PdCu@Pd NCs.40

In order to study the stability of PdCu@Pd NCs under the harsh catalytic turnover conditions, long-term potential cycling was carried out by taking 5000 potential sweeps between −0.5 and 0.5 V vs RHE, and the results are compared to those obtained at the commercial Pt catalyst. Substantial activity decrease was observed at the commercial Pt catalyst, but the activity of the PdCu@Pd NCs catalyst was maintained throughout the testing period (Figure 8a), indicating that the core@shell PdCu@Pd NCs has much superior durability over Pt in catalyzing the HER under acidic conditions. The durability of the catalysts is further evaluated by electrolysis in 0.5 M H2SO4 at an overpotential of 250 mV (in the inset of Figure 8a), and the results suggest that PdCu@Pd NCs are stable for at least 48 h in acid solution without obvious current decay. In the first 20 hours of electrolysis, H2 gas product was monitored every 5 hours for one hour by sampling every 10 min for gas chromatographic analysis. Figure 8b shows the volume of H2 produced as a function 21



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of time. The black line represents the theoretical values obtained by assuming a 100% faradaic efficiency, and the red line represents the experimental values measured by gas chromatography. The detected faradaic efficiency is 96.4%. The excellent agreement between theory and experiment implies that nearly all the current is due to H2 evolution, while the rest of the electricity consumed is mainly used to form palladium hydride on the electrode as suggested by the literature.43, 44 These results confirm that PdCu@Pd NCs is stable during the electrolysis process.

Figure 8. (a) Comparison of linear sweep voltammograms obtained at PdCu@Pd NCs modified GC and commercial Pt electrode at 1st (―) and 5000th cycle (----), the inset is the result obtained from a potentiostatic electrolysis study undertaken in 0.5 M H2SO4 at an overpotential of 250 mV for 48 hours; (b) The theoretically calculated (black line) and experimentally measured (red line) amount of evolved hydrogen as a function of time.

4. CONCLUSIONS In summary, we have synthesized monodisperse PdCu NCs by a facile way, that is, co-reduction of Cu(acac)2 and Na2PdCl4 at high temperature of 250 °C. The PdCu 22


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NCs are more active for the HER than either Pd NCs or PdCu NPs. Further anodization converts PdCu NCs to core@shell PdCu@Pd NCs, which shows significantly increased activity for the HER as evidenced by its onset potential of -5 mV vs RHE and Tafel slope of 35 mV dec-1 which are close to that obtained at the commercial Pt (-3 mV vs RHE and 31 mV dec-1, respectively). The core@shell PdCu@Pd NCs have a Pt-like activity and is even more stable than Pt catalyst, which is the most efficient non-Pt catalyst for the HER in acidic media. This novel method to produce PdCu@Pd NCs with outstanding performance could be widely applied for the preparation of other core-shell structured materials.

ASSOSIATED CONTENT Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Additional XPS spectra of Pd 3d and Cu 2p, Bode plots of PdCu nanocubes and PdCu nanoparticles, the composition of the PdCu catalysts determined by ICP-AES and the comparison of the HER performance of PdCu@Pd Nanocubes with some state-of-the-art nanocatalysts reported recently in the literature.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]

*E-mail: [email protected]

Author Contributions 23



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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research was supported by the National Science Foundation of China (NO. 21345003), the Fundamental Research Funds for the Central Universities (Grant NO. lzujbky-2016-k08), the Natural Science Foundation of Gansu (145RJZA132), the Key Laboratory of Catalytic Engineering of Gansu Province and Resources Utilization, Gansu Province for ﬁnancial support. JL is specifically thankful to the China Scholarship Council for a scholarship.

24


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