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Oct 25, 2017 - Department of Chemistry and Institute of Basic Science, Chonnam National ... H2 production, nowadays, intensive research have been focused.

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 39303-39311

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Highly Efficient Dual Active Palladium Nanonetwork Electrocatalyst for Ethanol Oxidation and Hydrogen Evolution Halima Begum, Mohammad Shamsuddin Ahmed, and Seungwon Jeon* Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-757, Republic of Korea S Supporting Information *

ABSTRACT: Tunable palladium nanonetwork (PdNN) has been developed for catalyzing ethanol oxidation reaction (EOR) and hydrogen evolution reaction (HER) in alkaline electrolyte. 3D PdNN is regarded as a dual active electrocatalyst for both EOR and HER for energy conversion application. The PdNN has been synthesized by the simple chemical route with the assistance of zinc precursor and a surfactant (i.e., cetyltrimethylammonium bromide, CTAB). The thickness of the network can be tuned by simply adjusting the concentration of CTAB. Both EOR and HER have been performed in an alkaline electrolyte, and characterized by different voltammetric methods. The 3D PdNN has shown 2.2-fold higher electrochemical surface area than the commercially available Pt/C including other tested catalysts with minimal Pd loading. As a result, it provides a higher density of EOR and HER active sites and facilitated the electron transport. For example, it shows 2.6-fold higher mass activity with significantly lower CO2 production for EOR and the similar overpotential (110 mV @ 10 mA cm−2) for HER compared to Pt/C with better reaction kinetics for both reactions. Thus, the PdNN is proved as an efficient electrocatalyst with better electrocatalytic activity and stability than state-of-the-art Pt/C for both EOR and HER because of the crystalline, monodispersed, and support-free porous nanonetwork. KEYWORDS: dual active electrocatalyst, ethanol oxidation, hydrogen evolution, nanonetworks, palladium durability and typical poisoning.19,20 However, to solve the problem of Pt dependence fully, relatively more abundant and cheaper Pd-based catalysts are more likely to be put into practical use for DMFCs in alkaline media, owing to its good tolerance to the poisoning effects from carbonaceous intermediates21−23 and stability.24,25 However, the efficiency and activity of Pd-based catalysts are still the main challenges for DEFCs applications.26 Thus, many efforts, such as nanoparticles (NPs), nanonetwork (NN), nanoflowers, nanotubes, or nanodendrites, have been explored to develop more electroactive and efficient Pd-based nanocatalysts for FCs application.8,27−32 Among them, metallic (i.e., Pd)-NN can not only maximize the Pd utilization but also enhance the catalytic activity due to its structural effects such as, self-support, large surface area, low density, and special electrical properties, resulting in an increase in electrochemically active surface area (ECSA) and associated with the higher performance.33,34 It is worth mentioning that the electrocatalytic performances of nanocatalysts are strongly dependent on their size and shape.35 Recently, bifunctional (dual active)36 or even trifunctional37 catalysts system have been reported, although it is still difficult to develop a bifunctional catalysts system for several electrochemical reactions due to the variation of mechanism among

1. INTRODUCTION The search for sustainable and renewable clean energy sources has been expended because of the fast expanding demand and environmental impact of traditional energy resources.1,2 In this regard, renewable electrochemical energy conversion devices are regarded as clean energy sources.3,4 Ethanol oxidation reaction (EOR) and hydrogen evolution reaction (HER) are two important electrochemical processes that take place in the energy conversion devices, such as fuel cells (FCs) and water splitting cells.5,6 The electrocatalytic EOR is greatly highlighted because ethanol has less toxicity, more sustainability, and higher energy density among different alcohol fuels, and thus, it is regarded as the superior fuel for direct alcohol fuel cells (DEFCs).7,8 Particularly, DEFCs has superior merits such as nearly zero pollution, renewable, easy transportable, and relatively low operating temperature.9,10 At the same time, the electrolysis of water is a cost-effective and eco-friendly technique for hydrogen gas (H2) production for hydrogen fuel cells.11 For increasing H2 production, nowadays, intensive research have been focused on finding higher-active as well as low cost catalysts for electrochemical decomposition of water.12,13 However, the overall efficiency of these energy conversion devices are rigorously hindered by the sluggish kinetics of those electrocatalytic reactions.14−16 For boosting the sluggish kinetics, Pt-based materials are widely used for electrocatalysis of those reactions.17,18 Unfortunately, the usages of Pt-based catalysts are susceptible due to the scarcity, high cost, inferior © 2017 American Chemical Society

Received: July 7, 2017 Accepted: October 25, 2017 Published: October 25, 2017 39303

DOI: 10.1021/acsami.7b09855 ACS Appl. Mater. Interfaces 2017, 9, 39303−39311

Research Article

ACS Applied Materials & Interfaces

magnetic stirring continued to ensure adequate mixing. Under continuous stirring, 1 mL of 100 mM NaBH4 was added to the solution dropwise. Afterward, the above solution kept in an oven for steady-state heating at 60 °C for 12 h. After completion of the reaction, the solution was centrifuged thoroughly with excess ethanol for two times. Subsequently, for removing Zn by etching treatment, centrifuged with 100 mM HCl and finally followed by the centrifuging with ethanol for another two times. The as-obtained black solid, PdNN, was dried at 40 °C for 12 h. For comparison, PdNPs and PdZnNN catalysts were prepared with same protocol but no addition of CTAB solution and no wash by 100 mM HCl, respectively. Also, carbon-based 20% Pt−Ru (atomic ratio of 1:1) catalyst was prepared with the same protocol described earlier.18 The physical and electrochemical characterizations were described in the Supporting Information.

reactions. However, there is no single Pd-catalyst that can perform as a bifunctional catalyst for both EOR and HER. Therefore, new strategies to develop a cost-effective and highperformance EOR and HER bifunctional electrocatalysts as the replacement of Pt catalysts are still anticipated. On the basis of the above discussions, we have developed a simple strategy to synthesize self-supported PdNN with interconnected porous nanoarchitecture. For the first time, the PdNN has been employed successfully as dual active Pdbased electrocatalyst for EOR and HER. The as-prepared PdNN showed better electrocatalytic activities for both EOR and HER with superior stability than the commercially available Pt/C. The porous PdNN ensured a high density of active sites with outstanding mass and charge transport for EOR and HER. Therefore, the PdNN is demonstrated to be a promising lowcost and efficient alternative for Pt catalysts because of the ∼40% less amount of Pd had been used than Pt during catalysis in this study (Table 1).

3. RESULT AND DISCUSSIONS 3.1. Catalysts Characterization. The surface morphology of as synthesized PdNN, PdNPs, and PdZnNN materials was first characterized by the TEM technique (Figure 2). The TEM image confirms the formation of 3D porous nanonetworks-like structure of PdNN sample (Figure 2a). According to the corresponding SAED pattern in Figure 2a inset, PdNN sample displays a good crystalline nature. The magnified image of PdNN nanocatalyst (Figure 2b) clearly presenting that the PdNN appeared monodispersed and interconnected with 1D net-like structure which finally appear as 3D network. The average diameter of 1D net is 4.9 nm. From high resolution TEM (HRTEM) image (Figure 2b inset), it was found that the lattice planes with d-spacing of 0.222 and 0.195 nm, which attributed to the face centered cubic (fcc) structure of Pd (111) and Pd (200) planes, respectively.38,39 The lattice fringes were coherently extended over the entire nanonetwork, signifying highly crystalline PdNN. TEM image of PdNPs shows an aggregated NPs morphology through the whole sample (Figure 2c), where CTAB was not been used. Although the size distribution was not uniformed in this sample, an average diameter size was calculated as ∼8 nm. The HRTEM image (Figure 2c inset) of single PdNP shows the calculated lattice dspacing are to be 0.222 and 0.195 nm, in accordance with their own interplanar distance of (111) and (200) lattice planes of Pd which is same with PdNN. However, the TEM image of PdZnNN also shows the formation of porous networks-like structure of PdZnNN sample (Figure 2d). The HRTEM image

Table 1. Electrochemical Performances Comparison among All Tested Catalysts name −2

metal loading (μgPd cm ) ECSA (m2 g−1 Pd ) MA (A mg−1 Pd ) @ jf Eonset (V) EOR HER jf/jb ηF (%) @ −0.2 V η10 for HER (mV) stability ECSA (%) EOR (%) HER (%) a

PdNN

PdNPs

PdZnNN

Pt/Ca

16.9 85.6 2.04 −0.75 −1.1 1.8 22.5 110 81.8 43.8 92.3

22.6 21.3 0.29 −0.65 −1.1 0.8 61 170 36.5 22.1

25.5 13.7 0.18 −0.63 −1.11 1.5 37.5

28.3 38.6 0.58 −0.71 −1.1 0.74 79 110 43.5 9.1 86.4

20.5 7.8

Normalized to the Pt mass.

2. EXPERIMENTAL SECTION 2.1. Synthesis of PdNN. The synthesis of all catalysts has been simplified by the schematic diagram in Figure 1. As can be explained, the 1 mL of 10 mM K2PdCl4 and 1 mL of 10 mM ZnCl2 were mixed in a 20 mL vial, followed by the addition of 1 mL of 80 mM cetyltrimethylammonium bromide (CTAB) into the vial under

Figure 1. Schematic synthesis of PdNN, PdZnNN, and PdNPs. 39304

DOI: 10.1021/acsami.7b09855 ACS Appl. Mater. Interfaces 2017, 9, 39303−39311

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was formed upon no addition of CTAB as observed in Figure 2c earlier, indicating low PdNN formation with thick in diameter size (8.8 ± 0.1 nm). At 80 mM concentration of CTAB, the PdNN was formed uniformly with aggregation-free distribution and the average diameter size was 4.9 ± 0.1 nm. At 120 mM concentration of CTAB, the PdNN was also formed uniformly with lower diameter size (4.5 ± 0.1 nm). The corresponding diameter size histograms for all samples can be seen in Figure S2. The N2 absorption/desorption isotherm is presented for better understanding the porous structure of PdNN, PdNPs and PdZnNN catalysts in Figure 4a. The presented isotherms

Figure 2. TEM images of PdNN (a and b), PdNPs (c), and PdZnNN (d). Insets: the SAED (a) and HRTEM image (b) of PdNN and the HRTEM images of PdNPs (c) and PdZnNN (d).

(Figure 2d inset) shows the lattice d-spacing which are calculated as 0.222 and 0.28 nm, which are corresponding to their own interplanar distance of (111) lattice plane of Pd and (100) lattice plane of ZnO, respectively.40 Therefore, it has confirmed that the CTAB has a great influence to prepare 3D nanonetwork-like structure. The numerical analysis of Pd and Zn elements in all prepared catalysts was also observed by EDX patterns (Figure S1). EDX patterns show Pd and Zn peaks, along with the Cu and O peaks. Cu peak is due to the TEM grid, which is made of copper, and O represents the state of Zn as ZnO. The amount of Pd and Zn (as wt %) is listed on the corresponding EDX patterns and Pd amount is 1.2, 1.6, and 1.8 wt % for PdNN, PdNPs, and PdZnNN, respectively. However, the tunable thickness of 1D net of PdNN was also observed by TEM analysis. The Figure 3 is displaying TEM images of three different PdNN samples, which were synthesized with 40 (a), 80 (b), and 120 mM (c) CTAB concentrations during the synthesis procedure. The uniformly distributed PdNN were formed upon CTAB usage. Upon low concentration of CTAB applied, the thickness was much higher and seems the coexistence of PdNPs and PdNN; while PdNPs

Figure 4. Nitrogen adsorption/desorption isotherms (a), the XRD patterns (b), core level of Pd 3d (c), and Zn 2p XPS spectra (d) of PdNN, PdNPs, and PdZnNN. Inset: Corresponding pore-size distribution of those catalysts (a).

are of type IV (according to the Brunauer−Deming−Deming− Teller classification), signifying the presence of nano- and/or meso-pores in those samples.41,42 An important parameter, specific surface area (SSA), can be determined from these isotherms profile.42 The SSA of PdNN, PdZnNN and PdNPs samples was determined as 454.1, 226.9, and 103.2 m2 g−1, respectively, by Brunauer−Emmett−Teller (BET) method. The SSA of PdNN was 4.4 magnitudes higher than that of PdNPs. Pore size distribution curves of all samples are

Figure 3. TEM images of PdNN prepared with CTAB concentrations of 40 (a), 80 (b), and 120 mM (c). 39305

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measurements, all the catalysts were placed on GCE with same mass loading and investigated by CVs in Ar-purged 1 M KOH electrolyte which are displayed in Figure 5a. Figure 5a

presented in Figure 4a inset. All curves are showing a prominent pore size around 4 nm, which could be due to the nanopores in between nanoparticles. Particularly, PdNN showed another two large peaks at 16 and 50 nm, which could be ascribed to the empty spaces occurred from etching treatment. Therefore, confirming the hierarchical porous structure of PdNN, PdZnNN than PdNPs. This finding corresponded with the morphological analysis. The XRD patterns of all catalysts are shown in Figure 4b which characterizes their crystalline nature. The XRD patterns of PdNN and PdNPs reveal their fcc structures with five common peaks. The typical peaks around the 2θ values of 40.1°, 46.1°, 68.1°, 81.7°, and 85.9° are allocated to the crystalline Pd(111), Pd(200), Pd(220), Pd(311), and Pd(222) facets, respectively.43,44 All peaks in the XRD pattern are assigned to fcc crystalline Pd (JCPDS 05-0681). On the contrary, for the PdZnNN catalyst, there are many additional peaks for ZnO which indicating Zn existence in PdZnNN catalyst along with above-mentioned Pd indication peaks.45,46 The diffraction peaks at 2θ of 31.3°, 33.9°, 37.0°, 44.3°, 53.4°, 57.6°, 62.6°, and 28.8° were ascribed to the diffractions from the following ZnO planes: (100), (002), (101), (102), (110), (103), (200), and (201), respectively.4 However, no significant peak shift [i.e., Pd(220)] was observed from any catalyst compared to the fcc Pd (JCPDS 05-0681) (Figure S3), which suggests that Pd and Zn are in composite form rather than alloy form47,48 with this preparation method. On the basis of the XRD result, however, the alloy formation with novel metals may be hampered due to poor metallic nature of Zn. Other group also observed that there was no any alloy formation between Au and ZnO in their nanoarchitectures preparation.48 It should be mentioned that cometals were found to be well alloyed with Pd during the synthesis of networks catalysts (PdAg, PdPbPt) via previously reported method,27,49 probably due to the good metallic nature of those cometals (Ag and Pt). The details of the structural features with oxidation state of metals in PdNN, PdNPs, and PdZnNN samples were elucidated through the XPS analysis. Surface composition of all samples is analyzed, and the Pd/Zn ratios (wt %) are found as 1:0, 1:0.1 and 1:0.9 for PdNN, PdNPs, and PdZnNN, respectively, which is the consistent of EDX analysis. The core level of the Pd 3d and Zn 2p spectra are shown in Figure 4c and 4d, respectively. In Figure 4c, the Pd 3d spectra present a doublet consisting with two bands at 340.1 and 334.9 eV which are assigned to the (Pd 3d3/2) and (Pd 3d5/2), respectively, for all catalysts. The metallic states of Pd in those samples are investigated as in Figure S4. After close observation, the Pd peak positions for PdZnNN and PdNPs are as same as the peak position of PdNN. Such phenomenon is suggesting that no change transfer (or very weak) in the electronic structure of Pd from ZnO,47 which is consistent with the XRD analyses. The Zn 2p spectra show two symmetric peaks in Figure 4d, the peaks at 1021.85 and 1045.05 eV are assigned to the Zn 2p3/2 and Zn 2p1/2, respectively, which indicating that the Zn presence in the oxide form.48 No difference was observed in the peak position between PdNPs and PdZnNN Zn 2p spectra even though higher amount of ZnO existence in PdZnNN sample, indicating no charge transfer between ZnO and Pd. Similar behavior was also observed in core level of Pd 3d spectrum. 3.2. Electrochemical Measurements. We evaluated the catalytic activities of PdNPs, PdNN, and PdZnNN including Pt/C toward EOR and HER. Prior to electrochemical

Figure 5. CV curves recorded in Ar-saturated 1 M KOH electrolyte at a scan rate of 50 mV s−1 with scale bar of 3 mA cm−2 (a), the plot of ECSA value from durability test for 3000 CV cycles (b), the CV curves for EOR in the presence of 1 M EtOH with current density normalized to electrode area with scale bar of 10 mA cm−2 (c), and forward scan of same CV curves are shown with current density normalized to Pd or Pt mass for PdNN, PdNPs, PdZnNN, and Pt/C electrodes (d). Inset: Plot of jf/jb (d).

shows CVs recorded from the four different catalysts in the potential range of −1.2 to 0.2 V (vs Ag/AgCl) at 50 mV s−1. The metal oxide (PdO or PtO) reduction peak located around −0.4 V in the backward CV scan is appeared in all curves. The ECSA of all Pd-catalysts was derived from the Coulombic charge (Q) for the reduction of PdO8,50 and the Q associated with hydrogen adsorption for Pt/C.2,50 For comparison, the PdO or PtO peaks at around −0.4 V and corresponding charge variation can be seen in Figure S5. The ECSA of all catalysts was obtained from eq 1 by normalizing against the Pd (or Pt) 2 mass and the values were 85.6, 23.9, 12.2 m2 g−1 Pd , and 38.6 m g−1 for the PdNN, PdNPs, PdZnNN, and Pt/C catalysts, Pt respectively (Table 1). The obtained ECSA for PdNN was 2.2 and 3.6 magnitudes higher than that of Pt/C and PdNPs, respectively. The Pd/Pt loading and the corresponding ECSA can be compared in Figure S6a.

ECSA =

Qo Qr × m

(1)

where Qo is the charge for oxide-reduction, Qr is the charge required for the monolayer adsorption of oxygen on Pd surface (405 μC cm−2)50 or hydrogen on Pt surface (210 μC cm−2),2 and m is the metal loading (in μg cm−2) onto the GCE. The durability of electrocatalysts is an important issue for widespread and commercial application of FCs. The stability experiment was performed for evaluating long-term electrochemical application and the ECSA was calculated for all catalysts up to 3000 CV cycles (Figure 5b). As shown in Figure 39306

DOI: 10.1021/acsami.7b09855 ACS Appl. Mater. Interfaces 2017, 9, 39303−39311

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ACS Applied Materials & Interfaces 5b, the ECSA of Pt/C dropped almost 53% of its initial ECSA after 1000th cycle, and the final ECSA after 3000 cycles dropped to 43%. The PdNN displays a great ECSA stability up to 87.6% after the 1000th cycle and remaining 81.7% after completing the durability test. On the contrary, the ECSA of PdNPs and PdZnNN catalysts dropped at 36.5% and 20.5%, respectively, of their initial value after finishing the test. Thus, the durability test demonstrating that PdNN has significantly higher stability than the traditional carbon supported Pt/C and other tested catalysts. It is worth to mention that the ECSA value is an indicator of the precious metals utilization and this result suggests a better utilization of Pd active sits in PdNN catalyst probably due to the higher SSA, porous structure and support free catalysis. For PdNPs and PdZnNN catalysts, however, under practical operating conditions, PdNPs are prone to migration and agglomeration,51 and Pd active site blocked by the Zn due to coexistence as less conductive oxide form without alloy formation,8,47 respectively, resulting in a decrease in ECSA and associated loss of performance on PdNPs and PdZnNN catalysts. 3.3. Electrochemical EOR. The electrocatalytic EOR on PdNPs, PdNN, PdZnNN, and Pt/C in Ar-saturated 1 M KOH electrolyte with containing 1 M ethanol was performed by CVs at a scan rate of 50 mV s−1. Two well-defined peaks are observed in each CV curve, which denoted as f-peak in the forward scan and b-peak in the backward scan, respectively, in Figure 5c. This feature are signifying the EOR at all tested catalysts, that are similar to other reports.27,28,30,47 The magnitude of f-peak is as a result of the oxidation of chemisorbed species that come from ethanol and b-peak is due to the carbonaceous species that not completely oxidized during forward scan.52 The onset potential (Eonset) was −0.75, −0.71, −0.65, and −0.63 V, and the area normalized current density at f-peak (jf) was 34.8, 16.4, 7.4, and 4.1 mA cm−2, for the PdNN, Pt/C, PdNPs, and PdZnNN catalysts, respectively. The highest jf was found at PdNN, which is 2.1, and 4.7 magnitudes superior to that of Pt/C and PdNPs, respectively. Therefore, the PdNN catalyst shows the highest electrocatalytic activity toward EOR in terms of higher Eonset and jf among all tested catalysts. The metal mass normalized forward scan of corresponding CVs curves are also shown in Figure 5d. Among all catalysts, PdNN delivers the best catalytic performance toward EOR with the highest mass activity (MA, 2.04 A mg−1 Pd ) at jf, which is 3.5, 6.2, and 12.7 magnitudes higher than that on −1 Pt/C (0.58 A mg−1 Pt ), PdNPs (0.33 A mgPd ), and PdZnNN −1 (0.16 A mgPd ), respectively. For better understanding, the comparison of specific activity (SA, ECSA normalized jf) and MA at jf for all catalysts are presented in Figure S6b. However, the MA calculated from PdNN is much higher even than that of other reported nanonetwork-like catalysts.27−29 Better comparison can be seen in Table 2. As mentioned earlier, the b-peak current density (jb) is associated with the oxidation of carbonaceous intermediates of ethanol dissociative adsorption. The magnitude of jb is caused by the accumulation of these intermediates and which is an indicator of catalyst poisoning.7,8,27,53 Typically, the ratio of both peaks (jf/jb) is used to evaluate the catalyst poisoning exposure or tolerance to accumulation of carbonaceous intermediates. The Figure 5d inset shows that the jf/jb values of the PdNN (1.8) is larger than that of all tested catalysts including Pt/C (0.74) (Table 1), indicating the better tolerance to carbonaceous intermediates accumulation of as-prepared porous PdNN. Therefore, PdNN shows best electrochemical

Table 2. Electrochemical Performances of PdNN on EOR Compared with Other Catalysts Including Nanonetwork-like Catalysts name

electrolyte

ECSA (m2 g−1 Pd )

MA (A mg−1 Pd )

PdNN [this work] Pd1Ni1/C14 Pd50Ag5027a Au1Pt128a Pt53Cu4729a Pd1Pb149a Pd40Ni43P1753 Pd1Au3 NSFs59 Pd73Cu2767a Pd1Au168a FePd−Fe2O3/MWNTs69 PdCu2-270

1 M KOH 1 M NaOH 1 M KOH 0.5 M H2SO4 0.5 M H2SO4 1 M KOH 1 M NaOH 0.5 M KOH 1 M KOH alkaline 1 M KOH 1 M KOH

85.6 67.3 32.81 101.14b 61.3b 15.43 63.22 4.89 58.3 18 120.4 59.27

2.04 2.37 1.97 0.567b 0.171b 6.936 4.42 0.317 ∼3 10.8 1.191 0.163

a Network-like catalyst. bECSA and MA were normalized to the Pt mass.

performance toward EOR in respect to the more negative Eonset, higher jf, better utilization of Pd mass and poisoning tolerance ability. 3.4. Kinetics of EOR. To understand the kinetics and final products of electrochemical EOR, however, the faradaic efficiency (ηF) can be evaluated and derived from the stoichiometry (average number of electrons transferred per ethanol molecule, nav). As in eq 2, the CO2 can be the major product through C−C bond breaking and 12 electron (e−) involved EOR which must shows higher ηF. On the contrary, in practice low yields of CO2 have generally been reported, with the major products being acetic acid (eq 3) and acetaldehyde (eq 4) through 4e− and 2e− involved EOR, respectively, which shows lower ηF.18,54 CH3CH 2OH + 3H 2O → 2CO2 + 12e− + 12H+

(2)

CH3CH 2OH + H 2O → CH3COOH + 4e− + 4H+ (3)

CH3CH 2OH → CH3CHO + 2e + 2H −

+

(4)

The ηF was calculated from the stoichiometry-based equation 554 and plotted as a function of potential for the PdNN, PdNPs, PdZnNN, and Pt/C catalysts in Figure 6a. As can be seen in Figure 6a, the ηF was much lower at the PdNN catalyst than at all catalysts. In contrast, the maximum ηF was obtained for Pt/ C and PdNPs. Thus, indicating the higher yield of CH3−CHO or CH3−COOH at the PdNN catalyst and the higher yields of CO2 that have generally been reported for ethanol oxidation at Pt/C.18

nif nav =∑ i (5) 12 12 where ni is the number of electrons transferred to form product i and f i is the fraction of ethanol converted to product i. We have confirmed the final product of EOR at the PdNN using the single potential alteration infrared reflectance spectroscopy (SPAIRS) technique. The SPAIRS spectrum showed several peaks which indicate the final product was mainly CH3−CHO or CH3−COOH at the PdNN catalyst in Figure S7. Briefly, the peak at 915 cm−1 is assigned to the C− C−O bond.55 The peak at 1250 cm−1 is due to the C−O ηF =

39307

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1 M ethanol solution, as displayed in Figure 6c. Initially, all curves are showing a rapid decay in current density, this is caused by the formation of poisonous intermediate species during EOR process in alkaline electrolyte. The slow decrease in the current density and reached at pseudo-steady-state was observed under the subsequent measurements.7,27,58 As expected, however, the PdNN shows higher current density (normalization to metal mass) at the start and the end of the test, and the stability remaining at 43.8%, which is 4.8 magnitude greater than the Pt/C (9.1%) indicating its better catalytic activity and stability against the poisoning.54,59,60 The surface morphology was also investigated after the EOR stability test. The PdNN (Figure 6c inset, (i) was much stable and nonaggregated in nature than that of PdNPs (Figure 6c inset, (ii) and PdZnNN (Figure 6c inset, (iii) catalysts. Furthermore, the PdNN catalyst showed a good reproducibility for EOR which investigated by five different PdNN-modified electrodes (Figure S10). 3.5. Electrochemical HER. To demonstrate the dual active electrocatalytic performance, we also tested HER activities for the PdNN, PdNPs, PdZnNN, and Pt/C using RDE measurements in 1 M KOH (Figure 7a). As can be seen, the PdNN is

Figure 6. Calculated faradaic efficiency (a), Tafel plots (b), and chronoamperometric curves (i vs t) recorded in 1 M KOH electrolyte in the presence of 1 M ethanol on PdNN, PdNPs, PdZnNN, and Pt/ C-modified electrodes (c). Insets: TEM images of PdNN (i), PdNPs (ii), and PdZnNN (iii) after i vs t test (c).

stretching. The peak at 1350 cm−1 is for − CH3, and at 1412 cm−1 is for O−C−O bond. The peak at 1710 cm−1 can be assigned to the CO bond.55 Therefore, it has been confirmed that the CH3−CHO or CH3−COOH was the main product at the PdNN catalyst. For comparing kinetic activities, the EOR CVs derived Tafel polarization plots of PdNN, PdNPs, PdZnNN, and Pt/C catalysts toward EOR are plotted in Figure 6b. The Tafel slope was derived by plotting the overpotential vs the log of j and the obtained slope values were 122, 128, 132, and 139 mV dec−1 for PdNN, PdNPs, PdZnNN and Pt/C, respectively. It is noteworthy that at lower Tafel slopes, the EOR charge-transfer kinetics on a catalyst can be faster than higher one which facile to the EOR process.8 Also, the potentiodynamic measurements of EOR at PdNN, PdNPs, and PdZnNN catalysts were studied by CVs upon addition of various concentrations of ethanol (Figure S8). In this experiment, however, nearly constant jf/jb was observed at PdNN compared to the PdNPs, and PdZnNN catalysts as the function of ethanol concentration which indicating stable and sensibly same amounts of intermediates were formed and the peak ratio was concentration independent at the PdNN site (Figure S9a). Also, nearly same f-peak potential can be observed at PdNN compared to PdNPs, and PdZnNN catalysts (Figure S9b), indicating uncompensated resistance may not be occurred in the test system.7 Moreover, the relationship between jf/jb and square root of scan rate is linearly proportional (Figure S9c), which suggests the EOR was a diffusion controlled process on PdNN catalyst.56,57 Furthermore, the f-peak position was shifted negatively with increasing ln of scan rates (Figure S9d), followed by the correlation coefficient of 0.999, suggesting the EOR was an irreversible electrode process on PdNN catalyst.8,56 The durability of the all tested catalysts toward EOR was monitored by the chronoamperometry technique. The i vs t curves were recorded on all tested catalysts at an applied potential of −0.3 V for 30 000 s in Ar-saturated 1 M KOH with

Figure 7. LSV curves for HER recorded in Ar-saturated 1 M KOH at a rotation speed of 1600 rpm and scan rate of 10 mV s−1 (a), the corresponding Tafel plots of PdNN, PdNPs, PdZnNN, and Pt/C (b), the LSV curves for HER before and after stability test (c), and the i vs t curves on PdNN, Pt/C and homemade Pt−Ru/C electrodes at an applied potential of 110 mV (d).

highly efficient for the HER with an Eonset of −1.1 V, beyond which the cathodic current density increases promptly. For comparing the electrocatalytic HER from different catalysts, the overpotential (η10) required to drive a current density of 10 mA cm−2 (j10) for the PdNN is 110 mV, which is considerably better than the PdNPs (170 mV), and similar to the Pt/C. The turnover frequencies (TOF) of HER were calculated using BET surface area.11 The PdNN catalyzed HER in the KOH electrolyte was with TOF of 3.1 s−1 at 110 mV (at j10) which is better than recently reported Pd-based HER catalysts.13,61 The calculated TOF of HER at PdNPs catalyst was 1.9 s−1 at 170 mV at j10, and the HER at PdZnNN was at below level (at 39308

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ACS Applied Materials & Interfaces j10). Thus, the remarkable HER activity at PdNN catalyst was higher than those of other tested catalysts. 3.6. Kinetic of HER. Tafel slope analysis of above-described LSV curves for HER on PdNN, PdNPs, PdZnNN, and Pt/C electrodes is given in Figure 7b. The Tafel slope values can be used to determine the rate determining step (RDS) of HER process.62 It is also noteworthy that in alkaline medium, the Tafel slopes for the Volmer, Heyrovsky, and Tafel steps as RDS are 120, 40, and 30 mV dec−1, respectively.63 However, at higher overpotentials, the slope values were 121, 133, 142, and 119 mV dec−1 for PdNN, PdNPs, PdZnNN and Pt/C catalysts, respectively. The Tafel slopes obtained in this work suggest that for all electrodes the RDS was the Volmer step as in equation 6.63 The lowest Tafel slope was found at PdNN among all Pdbased catalysts which indicating a fastest RDS and the best reaction kinetics than that on other tested catalysts in the alkaline medium.64 The similar values to those previously reported for other Pd-based materials.63,65 H 2O + e + Pd s ↔ PdHad + OH −





AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Tel.: +82 62 530 0064. Fax: +82 62 530 3389. ORCID

Seungwon Jeon: 0000-0002-4588-1361 Notes

(6)

The authors declare no competing financial interest.



where Had is adsorbed hydrogen and Pds is the hydrogen adsorption Pd-site. In this step, the reduction of water molecules with the formation of Had as an intermediate. For practical application, it is also equally important for PdNN catalyst to work stably for HER without any undesirable side-reactions. Figure 7c shows that the PdNN catalyst has good stability in alkaline electrolyte with only a slight negative shift overpotential (13 mV) after 1000 CV cycles, whereas, the overpotential required to achieve at j10 on the Pt/C catalyst increased by 55 mV after the same CV cycles, which signifies the superior durability of PdNN for HER.66 Furthermore, the PdNN (Figure 7d) showed an about 92.3% current retention after 15 000s investigated by chronoamperometry at an applied potential of 110 mV, which was much better than that of the homemade Pt−Ru/C (89%) and Pt/C (86.4%) electrodes. The poor stability was observed at Pt/C due to the typical poisoning during long run operation. However, this experiment is clearly indicating the remarkable long-term operational stability of PdNN. Therefore, the outstanding features of high activity, favorable kinetics, and strong durability indicate that PdNN is a suitable catalyst for HER.

ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (NRF2015R1D1A1A09059344).



REFERENCES

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4. CONCLUSION In this study, we have prepared a tunable Pd nanonetwork with the assistance of CTAB and Zn precursor, which employed as an electrocatalyst for both EOR and HER. The presence of hierarchical structure ensures a high surface area with abundant interfacial active sites for electrochemical reactions with minimal Pd usage. Owing to its crystalline nanonetwork porous structure and properties such as large surface area, and rich active sites, the newly developed PdNN was demonstrated to be an effective dual active electrocatalyst with excellent activities and stability. Therefore, our study on the facile synthesized PdNN has provided new application for dual active catalysts particularly for EOR and HER, which could disclose new possibilities for the development of sustainable energy conversion technology based on scalable, durable, and superior electrocatalysis.



Physical parameters and electrochemical characterization; EDX spectra; the diameter size histograms; enlarged XRD patterns; the core level of Pd 3d XPS spectra; backward scan of CVs and their corresponding charge density variation; plots of metal loading with corresponding ECSA and mass activity with specific activity; SPAIRS spectrum; CVs for EOR in various concentration of ethanol; plots of jf/jb and jf variation vs ethanol concentration, the plot of jf/jb versus square root of scan rate, and jf shift versus ln of scan rates (PDF)

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DOI: 10.1021/acsami.7b09855 ACS Appl. Mater. Interfaces 2017, 9, 39303−39311

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