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Enhanced electrocatalytic activity of ethanol oxidation reaction on palladium-silver nanoparticles via removable surface ligands Hucheng Zhang, Yingying Shang, Jing Zhao, and Jianji Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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ACS Applied Materials & Interfaces

Enhanced Electrocatalytic Activity of Ethanol Oxidation Reaction on Palladium-Silver Nanoparticles via Removable Surface Ligands Hucheng Zhang,* Yingying Shang, Jing Zhao, and Jianji Wang* Collaborative Innovation Centre of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, PR China KEYWORDS: Pd-Ag nanoparticles, inorganic ligands, ionic liquids, electrocatalysis, electrooxidation reaction

ABSTRACT: This work developed a facile colloidal route to synthesize BH4--capped PdxAgy nanoparticles (NPs) in water using the reducing ionic liquids of [Cnmim]BH4, and the resulting NPs were prone to form the nanocomposites with [amim]+-modified reduced graphene (RG). The removal of the metal-free inorganic ions of BH4- can create the profoundly exposed interfaces on the PdxAgy NPs during the electrooxidation, and favor the ethanol oxidation reaction (EOR) in lowering energy barrier. The counterions of [Cnmim]+ can gather ethanol, OH- ions, and the reaction intermediates on catalysts, and synergistically interact with RG to facilitate the charge transfer in nanocomposites. The interface-modified RG nanosheets can

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effectively segregate the PdxAgy NPs from aggregation during the EOR. Along with the small size of 4.7 nm, the high alloying degree of 60.2%, the large electrochemical active surface area of 64.1 m2·g-1, and the great peak current density of 1501 mA·cm-2·mg-1, Pd1Ag2@[C2mim]BH4amimRG nanocomposite exhibits the low oxidation potentials, strong poison resistance, and stable catalytic activity for EOR in alkaline media, and hence can be employed as a promising anodic catalyst in ethanol fuel cells.

1. INTRODUCTION The physicochemical properties of metal nanoparticles (NPs) depend not only on the architecture, but more remarkably on the size, shape, and chemical composition. The tunable properties have fascinated scientists to develop a large number of synthetic strategies in the production of metal NPs with a wide variety of nanostructures.1-6 Among these synthetic protocols, the conventional colloidal method is a facile, inexpensive, efficient, versatile route in contrast to other synthetic techniques, and various colloidal metal NPs in the presence of ligands have been grown by adjusting reaction conditions and precursor concentrations within appropriate solvent. The ligands in reaction mixture are critically important to synthesize the colloidal metal NPs, and protect them from aggregation or flocculation. However, the usually employed surface ligands are polymers or surfactants with long hydrocarbon chains,7-12 and a bulky organic shell is certainly built around each metal NPs. When a catalytic reaction occurs on the surfaces of metal NPs, the ligand shells act as the insulating barriers that block the transport of charges, and create the large steric effect that chokes the access of reactants. Although these ligands can be potentially removed by the exchange procedure with hydrazine, N,N-dimethylformamide, pyridine, or 1,4-phenylenediamine, the treatment usually alters the surface properties and imparts


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instability to NPs resulting mainly from the volatility and oxidizability of the small molecules.1317

Currently, the great progress in high carrier mobility between the semiconducting nanocrystals has been achieved using molecular metal chalcogenide complexes as surface ligands.18-21 Alternatively, the capping ligands were also reported from some of small inorganic ions, such as oxoanions, halogen ions, SCN-,S2-,HS-,Se2-, HSe-,Te2-, HTe-, TeS32-,OH-, and NH2-.22-25 All investigations point to the good colloidal stability and the remarkably improved conductivity. These novel ideas open a space for the design and fabrication in nanotechnology, and for their practical applications in field-effect transistors, solar cells, photodetectors, and thermoelectrics. However, the above-mentioned inorganic ions cannot be directly employed as the stabilizers in the synthesis steps, and usually go through the ligand exchange to harvest the inorganic ionfunctionalized nanocrystals. In such exchange procedure, obviously, it is difficult to completely remove the strongly-adsorbed primitive ligands, but easy to leave the defects behind on the surfaces of semiconducting nanocrystals. Moreover, there are few reports on the inorganic ioncapped metal NPs for their applications in chemical catalysis. Pd-based electro-catalysts are the promising alternative of Pt-based materials in direct alcohol fuel cells, because Pd metal provides the potential solution to meet the requirements of the acceptable cost, natural-reserves, catalytic activity, and stability.26-35 Among the Pd-based electrocatalysts for ethanol oxidation reaction (EOR), it is reported that the foreign Ag metal in PdxAgy NPs not only increases the surface area and poisoning-tolerant ability,35-39 but also promotes the shift-up of the d-band center of Pd metal,40-42 and hence effectively improves the catalytic activity and steady-state behavior of Pd metal. However, the Pd-Ag NPs that are grown by the colloidal route usually exhibit the evidence of severe block of electron-communication


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owing to the presence of surface ligands, therefore, the output of metal colloids with “clean” interfaces still remains the challenges in nanocatalysis topics.43, 44 Using the ionic liquids of 1-alkyl-3-methylimidazolium borohydride ([Cnmim]BH4, n=2, 4, 6) as both reducers and stabilizers, in this work, the BH4--capped PdxAgy NPs of varying chemical constituents were grown in aqueous solution at room temperature, while no ligand exchange procedures were required. As illustrated in Scheme 1, The as-prepared PdxAgy@[Cnmim]BH4 NPs are readily recombined with the reduced graphene (RG) that is modified by 1-aminopropyl3-methylimidazolium bromide ([amim]Br) to produce the PdxAgy@[C2mim]BH4-amimRG nanocomposites. The ligand shells on these alloy NPs consist of BH4− ions and [Cnmim]+ counterions, and it is the BH4− ions that can be removed by electrooxidation to produce the PdxAgy NPs with almost “clean” interfaces. Besides the high alloying degree (Malloy), the large electrochemical active surface area (EASA) and the presence of [C2mim]+counterions, it is shown that the almost naked interfaces of Pd1Ag2 NPs in the nanocomposite are responsible for lowering the energy barrier of EOR in alkaline media and significantly enhancing the catalytic activity and steady state behaviour.

Scheme 1. Schematic illustration of the production of “ligand-free” electrocatalyst for EOR.


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2. EXPERIMENTAL SECTION 2.1. Chemical reagents. 1-ethyl-3-methylimidazolium bromide ([C2mim]Br, 99%), 1-butyl-3methylimidazolium bromide ([C4mim]Br, 99%), and 1-hexyl-3-methylimidazolium bromide ([C6mim]Br, 99%) were obtained from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. NaBH4 (98%), AgNO3 (99.8%), and K2PdCl4 (99.9%) were purchased from Aladdin reagent com (Shanghai, China). Acetone (AR), absolute ethanol (AR), and diethyl ether (AR) were purchased from Tianjin reagent com. All reagents were used as received without further purification. amimRG was synthesized from the single layer reduced graphene oxide (RG) that was modified by 1-aminopropyl-3-methylimidazolium bromide ([amim]Br) as reported in our previous work.45 2.2. Synthesis of reducing ionic liquids. In a typical procedure, 0.2g of [C2mim]Br and 0.2g of NaBH4 (the molar ratio of [C2mim]Br and NaBH4 at 1:5 ) were added into a 50 mL roundbottom flask, and dissolved in acetone of 25 mL. Firstly, the reaction mixture was subjected to sonication in 40oC for 32 h using an ultrasonic cleaner (KQ3200D, Kunshan, China), the precipitate of NaBr was removed by centrifugation at 18000 rpm for 10 min, and acetone was evaporated using rotavapor at 40oC. Then, the resulting product was dispersed in diethyl ether of 25 mL, and centrifuged at 18000 rpm for 10 min to separate out the residual NaBr and the excess NaBH4. The purified process was repeated twice, and finally the product of [C2mim]BH4 was dried in vacuum for 48 h. Holding the molar ratio of NaBH4 and [C4mim]Br or [C6mim]Br constant at 1:5, [C4mim]BH4 and [C6mim]BH4 were respectively prepared following the same procedures. The content of Br- ions in each ionic liquid was determined by ion selective electrode when the saturated calomel electrode (SCE) was used as the reference electrode.


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2.3. Preparation of PdxAgy@[Cnmim]BH4–amimRG nanocomposites. The PdxAgy NPs with different composition were synthesized by controlling the concentration of metal precursors and the type of ionic liquids in the reaction mixture. For the synthesis of Pd1Ag2 NPs capped by [C2mim]BH4 (Pd1Ag2@[C2mim]BH4), typically, the mixture of metal precursors was prepared by mixing 3mL K2PdCl4 aqueous solution of 3 mmol·L-1 with 6 mL AgNO3 aqueous solution of 3 mmol·L-1, and then added dropwise into 18 mL [C2mim]BH4 aqueous solution of 12 mmol·L-1 at 0 oC under intense stirring. The reaction mixture was continually stirred for another 30 min, and the colour evolved from light yellow to light brown. The resulting Pd1Ag2NPs were purified by the centrifugation at 18000 rpm for 10 min, and rinsed with deionized water. The appropriate amount of PdxAgy NPs was combined with 10mL amimRG aqueous dispersion of 0.5 g·L-1, and then sonicated for 4h at room temperature to prepare the PdxAgy@[Cnmim]BH4-amimRG nanocomposite with the loaded metals of 20wt%. The resulting nanocomposite was centrifuged and rinsed by deionized water repeatedly for three times. 2.4. Characterization methods. The sample solution was dropped onto copper grid and dried under vacuum, and the images were taken from JEM-2100 transmission electron microscope (TEM). Thermogravimetric analysis (TGA) was conducted on a Netzsch STA449C at the heating rate of 5◦C min−1 under N2. After samples were filled into the disposable plastic cuvettes, the data of zeta potential were collected using Zetasizer Nano-ZS90 (Malvern instruments) with a 633 nm He-Ne laser. The analysis of X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi spectrometer (Thermo Fisher Scientific Inc.) using Al Kα X-ray source (10mA, 15 kV). The pattern of X-ray diffraction (XRD) was measured on a Bruker D8 Advance using Cu Kα1 (1.54056 Å) radiation.


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2.5. Electrochemical measurements. The glassy carbon electrode (GCE) with a diameter of 3mm was respectively polished with 1.0, 0.3, and 0.05µm alumina suspension, and rinsed by deionized water. Then, 10mg of PdxAgy@[Cnmim]BH4-amimRG nanocomposite was dispersed in 2mL of deionized water with the help of the sonication for 30 min. the working electrode was prepared

by

coating

6.5µL

homogeneous

slurry

of

PdxAgy@[Cnmim]BH4-amimRG

nanocomposite on the GCE surface, and dried in ultrapure N2 at room temperature for use. The electrochemical measurements were conducted at room temperature on a CHI660D electrochemical workstation (Chenhua Instruments Co. Ltd.) installed a conventional threeelectrode cell. The nanocomposite-modified GCE was employed as the working electrode, a Pt foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Before measurements, 1.0 mol·L-1 KOH aqueous solution with and without 1.0 mol·L-1 ethanol was deaerated by bubbling ultrapure N2 for 30 min to remove potentially dissolved O2. All potentials are referred to the reversible hydrogen electrode (RHE) in the working electrode. The alternating current impedance spectra of the nanocomposites were measured respectively in the N2-saturated 1 mol·L-1 KOH solution containing 1 mol·L-1 ethanol and in 2.5 mmol·L-1 [(Fe(CN)6]3−/[(Fe(CN)6]4−(1:1) solution containing 0.5 mol·L-1 KCl over a frequency range from 0.1 to 105 Hz and with initiative potential of 0.21 V, amplitude of 0.005 V, and quiet time of 2 s. 3. RESULTS AND DISCUSSION [Cnmim]BH4 can be synthesized by the metathesis reaction of [Cnmim]Br with NaBH4 in acetone. The ion selective electrode analysis was employed to evaluate the reaction extent. Based on the work curve of the electrode potential with the concentration of [C2mim]Br, [C4mim]Br, or [C6mim]Br in water, the content of Br- ions in [Cnmim]BH4 was determined to be less than 0.044 wt.%, a quality index to meet the following synthesis requirements. In the presence of


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Figure 1. (A) TEM images of Pd1Ag2@[C2mim]BH4 NPs; the inset is the high-resolution image of a single nanoparticle. (B) Chemical element content of Pd1Ag2@[emim]BH4 NPs before (red) and after (blue) the electro-oxidation. [Cnmim]BH4, the PdxAgy@[Cnmim]BH4 NPs were synthesized by chemical co-reduction of the metal precursors in aqueous solution without the other ligands. Figure 1A shows the TEM images of the as-prepared Pd1Ag2@ [C2mim]BH4 NPs, and represents the good uniformity in size with an average diameter of 4.7 ± 0.1 nm. The high-resolution TEM image indicates the good crystallinity in Pd-Ag alloy, in which the clearly resolved lattice spacing of 0.235 nm over a single nanoparticle corresponds to the (111) plane of the metal NPs. As observed from the TGA data for Pd1Ag2@[C2mim]BH4 NPs (Figure S1), a significant mass loss between 253 and 324oC is ascribed to the pyrolysis of the ionic liquid, and suggests the ligand shells are built by [C2mim]BH4 with the content of 13.3 wt.% in the whole metal NPs. Furthermore, the zeta-potential of -39.2 mV (Figure S2) implies the negatively charged surfaces


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of Pd1Ag2@[C2mim]BH4 NPs should originate from the adherence of BH4- to metal NPs, and the strong electrostatic repulsions among the NPs hold the high colloidal stability. In this portrait of ligand shells, it is that the Pd1Ag2 core is coated by dense BH4- ions, and in turn the BH4- ions are surrounded by the [C2mim]+ counterions. The composition of ligand shells can be further confirmed from the XPS analysis of Pd1Ag2@[C2mim]BH4 NPs. The XPS survey spectrum reveals that the NPs are consisted of Pd, Ag, B, C, N, and O elements with high B content up to 17.34 at.% (Figure 1B and Figure S3). The B 1s peak at 187.6 eV in the high-resolution XPS spectrum is very significant, and designates the typical chemical state of BH4- (Figure S3). After the Pd1Ag2@[C2mim]BH4 NPs were used as anodic material in the deaerated KOH aqueous solution of 1 mol·L-1, however, it was noticed that the B element in these NPs was hardly detected by XPS measurement because BH4- ions were turned into B2H6 gas during the electro-oxidation,46 whereas the contents of C and N elements decreased a little. Evidently, the electro-oxidized treatment can remove the capped anions, and lead to the profoundly exposed interfaces of the metal NPs. The almost “clean” interfaces would improve the electrocatalytic activity of the metal NPs in EOR as mentioned later. BH4- ions are the most commonly used reducing agent for the synthesis of metal NPs, and the stable PdxAgy@[C2mim]BH4 NPs, including the monometallic Pd@[C2mim]BH4 and Ag@[C2mim]BH4 NPs, could been produced by the one-pot coreduction of [C2mim]BH4 in aqueous solutions. The X-ray diffraction (XRD) patterns for the series of PdxAgy@[C2mim]BH4 NPs are shown in Figure 2A. In the 2θ range from 35 to 50o, the face-centered cubic lattice of Pd exhibits two diffraction peaks at 40o and 46.8o that are respectively attributed to (111) and (200) crystalline plane. With increasing the silver content, the shift of Pd diffraction peaks toward low


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2. (A) XRD patterns of Pd@[C2mim]BH4 (a), Pd2Ag1@[C2mim]BH4 (b),

Pd1Ag1@[C2mim]BH4

(c),

Pd1Ag2@[C2mim]BH4

(d),

Pd1Ag3@[C2mim]BH4

(e),

Ag@[C2mim]BH4 (f). (B) Dependences of Pd (111) lattice constant (○) and alloying degree (■) on Ag mole fraction in the two metals from PdxAgy@[C2mim]BH4 NPs. diffraction angles indicates the formation of Pd-Ag alloy. In addition, the new peaks centered at 38o and 44o are ascribed to the separated Ag phase, suggesting the incomplete mergence of Ag into Pd lattice in high silver content. Based on the lattice constants (a) of Pd (111) crystalline plane respectively in pure Pd (aPd) and alloy phase (aalloy), the Pd-Ag Malloy (%) was estimated from the radii of Ag (rAg= 0.134nm) and Pd (rPd= 0.128nm) by the equation of Malloy(1-rAg / rPd)=1-aalloy / aPd (Figure 2B).47 Clearly, the Malloy of these bimetallic NPs depends on the Ag content in PdxAgy@ [C2mim]BH4 NPs. Pd1Ag2@ [C2mim]BH4 NPs represent the optimal intersolubility with Malloy of 60.2% at the Pd-Ag molar ratio of 1:2, and the relatively high Ag content implies the great dispersion of Pd in Ag and poisoning tolerance.36-39


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The amimRG sheets provide many scaffolds to land PdxAgy@[C2mim]BH4 NPs by the electrostatic interactions, π-π stacking, and hydrogen bonding of imidazolium cations with the BH4—capped NPs,45 and the nanocomposites of PdxAgy @[C2mim]BH4-amimRG are readily prepared by mixing one component with the other. As observed from the TEM image of Pd1Ag2@[C2mim]BH4-amimRG nanocomposite (Figure S4), the bimetallic NPs could be well spread over the amimRG sheets. This array of the NPs on the amimRG sheets could circumvent the catalyst agglomeration in EOR. The almost “naked” surfaces, the high dispersity of Pd in Ag, and the well-distributed NPs on amimRG sheets make the bimetal NPs to be expected as an efficient electro-catalyst. After the PdxAgy@[C2mim]BH4 NPs were respectively recombined with amimRG, their electrocatalytic activity was evaluated by the EASA. The cyclic voltammograms (CVs) of the PdxAgy@[C2mim]BH4-amimRG nanocomposites on GCE were recorded at a scan rate of 50 mV·s-1 in N2-saturated KOH aqueous solution of 1 mol·L-1 at room temperature (Figure 3A), and the current (i) was normalized by the loaded Pd mass on the electrode and the geometric area of GCE. It is detected that the peaks of hydrogen desorption on the catalysts shift from 0.44 V for Pd@[C2mim]BH4-amimRG to 0.39 V for Pd1Ag3@[C2mim]BH4-amimRG during the positive scan, and correspondingly the peaks of the reduction of PdO that is formed during the positive scan shift from 0.66 V to 0.61 V during the negative scan. The coulombic charge (Qs) can be determined by integrating the peak of PdO reduction. Given the conversion factor of Qc as 424 µC·cm-2,36 the EASAs could be calculated on the basis of the loaded Pd mass (m) on the electrode by the equation of EASA = Qs/(Qc×m) (Figure 3B). It is shown that Pd1Ag2@[C2mim]BH4 NPs in the nanocomposite possess the maximal EASA that is 3.65 times larger than the EASA of Pd@[C2mim]BH4-amimRG, furthermore, no reduction peak of Ag


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Figure 3. (A) CVs at a scan rate of 50 mV·s-1 in N2-saturated 1 mol·L-1 KOH aqueous solution for the electrocatalysts of Pd@[C2mim]BH4-amimRG (a), Pd2Ag1@[C2mim]BH4-amimRG (b), Pd1Ag1@[C2mim]BH4-amimRG

(c),

Pd1Ag3@[C2mim]BH4-amimRG

(d),

and

Pd1Ag2@[C2mim]BH4-amimRG (e). (B) Dependence of the corresponding EASAs on Ag mole fraction in the two metals from PdxAgy@[C2mim]BH4-amimRG nanocomposites. oxide is detected from the series of PdxAgy@[C2mim]BH4-amimRG nanocomposites during the negative scan. These experimental data indicate that the alloy of Pd with Ag not only expands the EASA of Pd metal, but also improves the electrochemical stability of Ag metal. The CVs of EOR on PdxAgy@[C2mim]BH4-amimRG modified GCE at room temperature were measured in the N2-saturated aqueous solution containing 1 mol·L-1 KOH and 1 mol·L-1 ethanol at a scan rate of 50 mV·s-1 (Figure 4A). There are two typical oxidation peaks in each CV: 48-51 one occurs during the positive scan and results from the ethanol oxidation, and the other


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occurs during the negative scan and results from the oxidation of carbonaceous intermediate species which are derived from the incompletely-oxidized ethanol during the positive scan.36-39 The ratio of I+ to I- was determined by the peak intensity of EOR respectively in the positive and negative scan, and the Tafel slope by the dependence of the potential (E) on log i (Figure S5). As shown in Figure 4B, Pd1Ag2@[C2mim]BH4-amimRG with the largest I+/I- but the smallest Tafel slope would be the most efficient electrocatalyst among these nanocomposites.37 Pd1Ag2@[C2mim]BH4-amimRG in the EOR represents the appealing characters of the low onset potentials (Eonset) and peak potentials (Epeak), as well as the greater peak current density (ipeak) which is 6.3 times higher than that of Pd@[C2mim]BH4-amimRG during the positive scan (Figure 4C). The electrocatalytic performance of Pd1Ag2@[C2mim]BH4-amimRG outperforms that of the Pd@[C2mim]BH4-amimRG, and the related Pd-based electrocatalysts to a certain extent as reported in literature (Table S1). Intrinsically, the nano-sized particles have the larger specific surface area and create more catalytic active sites on their interfaces, and the catalytic efficiency of EOR benefits from alloying Pd with Ag. In depth, it is distinguishingly crucial that the Pd1Ag2@ [C2mim]BH4 NPs are attached to the amimRG sheets, and subsequently the capped BH4- ions are cleared by the initializing electrooxidation on the electrode. In this situation, the interfaces of Pd1Ag2@[C2mim]BH4 NPs are fully exposed and almost “cleaned”. Piecing these advantages together, as a result, it is believed that the energy barrier of EOR is lowered significantly which in turn improves the catalytic activity on Pd1Ag2@[C2mim]BH4amimRG/GCE. Moreover, It is noticed that PdxAgy@[C2mim]BH4-amimRG catalysts represent the relatively high I- in the EOR besides the large I+. In order to confirm what is working, the electrical


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Figure 4. Performance of electrocatalysts in N2-saturated 1 mol·L-1 KOH aqueous solution containing 1 mol·L-1 ethanol. (A) CVs at a scan rate of 50 mV·s-1. (B) and (C) Dependences of the I+/I- (■), Tafel slope (○), ipeak (●), Eonset (▲), and Epeak (▼) on Ag mole fraction in the two metals of the nanocomposites. (D) Nyquist diagrams and (E) Rct of the as-prepared electrocatalysts (■) and of the electrocatalysts (▲) after 50 times of CV cycle, the inset is the equivalent circuit. (F) Electrocatalysts:

Chronoamperograms at constant potential of 0.67 V vs. RHE.

Pd@[C2mim]BH4-amimRG


(c),

(a),



(d),

(b), and

Pd1Ag2@[C2mim]BH4-amimRG (e). impedance spectra of the nanocomposite-modified electrodes are measured in the N2-saturated 1 mol·L-1 KOH solution containing 1 mol·L-1 ethanol respectively before and after 50 CV cycles (Figure 4D). The charge-transfer resistances (Rct) of the EOR on these catalysts are determined


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by fitting the Nyquist plots in Figure 4D. As shown in Figure 4E, The Rct of Pd1Ag2@[C2mim]BH4-amimRG nanocomposite presents the minimum value, and almost remains unchanged after multiple CV cycles. These experimental facts imply the high catalytic activity and strong poisoning-resistance of the catalyst, and are in agreement with the following chronoamperometric analysis. Presumably, the desorption of the carbonaceous intermediates from the catalyst is difficult because of the strong interactions of [C2mim]+ counterions. This promotes the more complete oxidation of reaction intermediates during the negative scan, and causes the large I- in the EOR. The chronoamperometric measurements were carried out to evaluate the stability of the PdxAgy@[C2mim]BH4-amimRG catalysts in the N2-saturated aqueous solution containing 1 mol·L-1 KOH and 1 mol·L-1 ethanol at the constant potential of 0.67V (vs. RHE) over a period of 3000s (Figure 4F). It is observed that the current densities decay rapidly in the first 600 s and then become relatively stable. This sharp decrease results from the double-layer charging, and the oxidation of the BH4- ligand and adsorbed ethanol. Pd1Ag2@[C2mim]BH4-amimRG nanocomposite displays the highest steady-state current density, and reflects the well scavenging ability to liberate the catalytic active sites, indicating the most active EOR and the best poisoning-tolerant feature on this catalyst. To address the concern of the role played by [C2mim]+ counterions in EOR further, Pd1Ag2@[C4mim]BH4-amimRG/GCE

and

Pd1Ag2@[C6mim]BH4-amimRG/GCE

were

respectively prepared according to the same procedures, and their CVs and alternating current impedance spectra were recorded along with those of Pd1Ag2@[C2mim]BH4-amimRG/GCE for comparison (Figure 5A and 5B). These experimental data reveal that the longer alkyl side chain in the counterions represents the lower peak current density (Figure 5C), suggesting the


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-2000

300 (A)

(B) a

-1600

250

a

b c

200 b -Z'' / Ω

Pd

-1

-1200

c

-2

i / mA cm mg

-800

d 150

100 -400 C RL

50 0

Rct

W

0 0.2

0.4

0.6

0.8

0

1.0

100

200

E / V vs. RHE

300

400

500

600

Z' / Ω

1600 (C) 300

Pd

-1

200 1200

Rct / Ω

-1

1400

iprak / mA cm mg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

100

800 0 a

b

c

d

Figure 5. (A) CVs at a scan rate of 50 mV·s-1 for the electrocatalysts in N2-saturated 1 mol·L-1 KOH aqueous solution containing 1 mol·L-1 ethanol. (B) Nyquist diagrams of nanocompositemodified electrodes in 2.5 mmol·L−1 [(Fe(CN)6]3−/[(Fe(CN)6]4− (1:1) solution containing 0.1


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mol·L−1 KCl, the inset is the equivalent circuit. (C) Effects of the alkyl side chain in the counterions on the ipeak (red) and the Rct (blue) of the nanocomposites. Electrocatalysts: Pd1Ag2@

[C2mim]BH4-amimRG

(a),


(b),

Pd1Ag2@

[C6mim]BH4-amimRG (c), and amimRG (d). imidazolium rings exert the effects on EOR. The charge-transfer resistances (Rct) in these catalysts are evaluated by fitting the electrical impedance spectra using the equivalent circuit (Figure 5B, 5C and S6), and follow the order: Rct(Pd1Ag2@[C2mim]BH4-amimRG) < (Pd1Ag2@[C4mim]BH4-amimRG)

Enhanced Electrocatalytic Activity of Ethanol ... - ACS Publications

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