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Three-dimensional assembly of building blocks for the fabrication of Pd aerogel as a high performance electrocatalyst toward ethanol oxidation
Accepted Manuscript Three-dimensional assembly of building blocks for the fabrication of Pd aerogel as a high performance electrocatalyst toward ethanol oxidation Abdollatif Shafaei Douk, Hamideh Saravani, Meissam Noroozifar PII:
S0013-4686(18)30817-X
DOI:
10.1016/j.electacta.2018.04.073
Reference:
EA 31640
To appear in:
Electrochimica Acta
Received Date: 16 December 2017 Revised Date:
24 March 2018
Accepted Date: 10 April 2018
Please cite this article as: A. Shafaei Douk, H. Saravani, M. Noroozifar, Three-dimensional assembly of building blocks for the fabrication of Pd aerogel as a high performance electrocatalyst toward ethanol oxidation, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.04.073. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Three-dimensional assembly of building blocks for the fabrication of Pd aerogel as a high
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performance electrocatalyst toward ethanol oxidation
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Abdollatif Shafaei Douk*, Hamideh Saravani*, Meissam Noroozifar
*
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Department of Chemistry, University of Sistan and Baluchestan, P.O. Box 98135-674, Zahedan, Iran
Corresponding author. Tel.: +98-54-3341-6464; Fax: +98-54-3341-6888.
E-mail: [email protected] (A.Shafaei Douk) E-mail: [email protected] (H. Saravani),
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Abstract Porous noble metal nanostructures with highly porous and extended surface areas are a long-
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desired target in both industry and academia. Among them, noble metal aerogels have emerged as state-of-the-art catalysts and these exceptional structures play crucial roles in various applications such as catalysis, sensors, etc. Until now, various methods have been published for
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the creation of noble metal aerogels, while their development and synthesis suffer from timeconsuming multistep procedures. In this paper, we present an efficient method for the creation of
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porous three-dimensional network of Pd aerogel by a self-assembly process. This method offers several advantages over other methods such as being one-pot synthesis, simplicity and fast. Pd aerogel is synthesized by reducing H2PdCl4 in the presence of sodium carbonate using glyoxylic acid monohydrate as a reductant agent in a short time followed by supercritical drying. The Pd
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aerogel serves as an anode catalyst for the electrooxidation reaction of ethanol and exhibits superior electrocatalytic activity and durability compared to Pd/C catalyst. We believe that the Pd aerogel synthesized via this method is a promising catalyst for direct ethanol fuel cells
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(DEFCs) and will also open large opportunities for other applications.
Keywords: Aerogel, Ethanol oxidation, Noble metal aerogels, Porous noble metal nanostructures, Self-assembly, Three-dimensional network
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1. Introduction Direct ethanol fuel cells (DEFCs) have received worldwide attention owing to their high energy
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density, security, low operating temperature, and low pollutant emission [1-3]. Nonetheless, the commercial application of DEFCs still faces three critical obstacles: relatively high costs, insufficient performance, and inadequate long-term durability of catalysts. To overcome these issues, various methods have been proposed by tailoring the design of the size, composition, and
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shape of the catalyst [4-10].
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Nowadays, new self-assembled architectures have received worldwide attention in nanoscience owing to their widespread applications in optics, sensors, catalysis, etc [11-14]. Self-assembly processes exhibit the most useful strategy to create complex materials with unique structures. During the past years, self-assembled architectures have been applied to fabricate porous
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materials, which can be branched out into three various categories in accordance with their pore sizes: microporous (<2 nm), mesoporous (2-50 nm), and macroporous (>50 nm) [15]. The rational design of porous nanomaterials with extended surfaces such as nanosponges, nanowires
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and nanosheets, has created enormous opportunities to improve catalytic activity and stability [16-19]. In this context, a very effective class of supportless catalyst nanomaterials are noble
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metal aerogels consisting of expanded metal backbone nanonetworks considered as state-of-theart catalysts.
Aerogels are unique solid materials with ultralow densities, high porosities, great surface areas, and profuse open interconnected pores [11-14]. They are a cross-linked system, usually a result of self-assembly of metal nanoparticles (NPs). Self-assembly is a usual bottom-up approach to obtain well-defined complex architectures. Metal NPs with logically designed sizes, morphologies, shapes and structures act primarily as the building blocks to make template-free
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two-dimensional (2D) or 3D structures [20]. These novel noble metal nanostructures not only combine the physical and chemical properties of nanomaterials with those of the macroscale, but also contain attractive properties in terms of their composition, different sizes, and
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morphological effects [11-14, 20]. These unique porous metallic 3D networks have two primary advantages: (1) the self-supporting property of these porous noble metal aerogels may hinder the loss of durability observed in metallic catalysts supported on carbon owing to corrosion; (2) the
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macroporous 3D network of the aerogels exhibits high active surface areas and allows reactants to readily access the active sites and enhance catalytic performance [12, 20]. Moreover, noble
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metal aerogels contain the unique properties of metals (such as excellent electrical and thermal conductivity, catalytic performance and ductility/malleability) as well as the unique properties of common aerogels (high surface area, extremely low density, and high porosity) [12]. Therefore, noble metal aerogels can be used as unsupported catalyst materials.
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It is worth noting that Kistler and co-workers pioneered the first aerogels synthesis (silica and alike) [21]. Since then, a lot of research has been carried out for the development of aerogels. Eychmüller and colleagues successfully realized the controlled destabilization of citrate-
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stabilized noble metal NPs in aqueous solution and carried out pioneering work to create mono-
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and bimetallic aerogels, opening a novel road to self-support porous 3D networks [11, 12, 15]. These exceptional assembled structures offer pore size distribution: microporous, mesoporous and macroporous. Micropore and mesopore properties create high specific surface areas while macrospore character guarantees easy access of molecules to the surface [15]. Therefore, novel exceptional materials have found fascinating applications such as polymer electrolyte fuel cells (PEFCs), oxygen reduction reaction (ORR), thermal insulators and components of electrochemical devices owing to their unique physical and chemical properties [15, 22-25].
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Nonetheless, the synthesis and development of noble metal aerogels compared to some inorganic aerogels such as metal oxide, carbon materials, and other hybrid systems are very limited despite the urgent need for their widespread applications. Therefore, a large number of studies have been
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devoted to new and suitable approaches for the creation of noble metal aerogels.
One of the key challenges in the development of noble metal aerogels is the formation of hydrogel. During the past years, various methods have been successfully developed to facilitate
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the gelation process. For example, Eychmüller and co-workers [22] showed that the formation of
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Pd hydrogels is controlled by using the Ca2+ concentration. Moreover, Lin and co-workers reported the one step in situ reduction of metals with sodium borohydride (NaBH4) in aqueous media at elevated temperature for the preparation of MCu (M = Pd, Pt, and Au) hydrogels. Likewise, Eychmüller and co-workers [26] demonstrated that the Pd hydrogels can be spontaneously formed with NaBH4 in the presence of α-, β-, or γ-cyclodextrins (PdCD) without
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additional treatment and next PdCD aerogels were obtained with supercritical CO2 drying. Bigall and co-workers reported that lightweight aerogels of Pt nanocubes and nanospheres can be formed by direct destabilization from nonpolar colloidal solution utilizing hydrazine
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monohydrate as gelation reagent [27]. The shape-controlled Au/Ag/Pd alloy NPs and their self-
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assembly into monolithic aerogels for ethanol oxidation reaction (EOR) were reported by Arachchige and co-workers [28]. However, despite numerous efforts made for the development of aerogels, the search to facilitate the gelation process is very important. In this paper, we demonstrate an efficient method for the creation of Pd aerogel. Herein, for the first time, glyoxylic acid monohydrate is utilized as reducing reagent for the formation of Pd aerogel. This method provides a number of advantages including being one-pot synthesis, surfactant free, simple and fast. The Pd aerogel was synthesized by reducing H2PdCl4 in the
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presence of sodium carbonate using glyoxylic acid monohydrate as a reductant agent in a short time followed by supercritical drying. These porous noble metal nanostructures were used for the electrooxidation reaction of ethanol, and showed superior electrocatalytic activity and durability
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compared to the Pd/C catalyst toward EOR in alkaline media. The schematic representation of
shown in Fig. 1.
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Please insert Fig.1
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the preparation process of Pd aerogel and its application in the ethanol oxidation reaction are
2. Experimental
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2.1 Chemical reagents
Palladium (II) chloride (PdCl2) was purchased from Sigma-Aldrich Co. Sodium carbonate (Na2CO3) and glyoxylic acid monohydrate (C2H2O3.H2O) were purchased from Merck Co.
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2.2 Physical catalyst characterization
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Transmission electron microscope (TEM) and Field Emission Scanning Electron Microscopy (FESEM) images were employed to investigate the morphology of the Pd aerogel sample. The FESEM and TEM analyses were taken by using MIRA3 TESCAN and Zeiss-EM10C-100KV, respectively. Energy-Dispersive X-ray Spectroscopy (EDS) technique was carried out to determine the chemical compositions of the as-synthesized Pd aerogel by using SAMX electron microscope. XRD (X-Ray powder diffraction) pattern of the as-synthesized Pd aerogel catalyst sample was done by a Bruker, D8 ADVANCE XRD diffraction spectrometer for an angle range
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of 2θ = 30-90°. XRD analysis was conducted on a Philips diffractometer of X’pert company with graphite monochromatic Cu Kα radiation (λ = 1.54056 Å). The surface area of the Pd aerogel was evaluated by the Brunauer–Emmett–Teller (BET) method from the recorded N2 isotherm
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with Micromeritics TriStar II Plus. The pore size distribution was investigated by the Barrett– Joyner–Halenda (BJH) model.
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2.3 Electrochemical measurements
All electrochemical measurements were carried out in a standard three-electrode cell system
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using an SAMA-500 Electroanalyser (SAMA Research Center, Iran). The modified electrodes with the Pd aerogel and Pd/C were fabricated as follows: a certain mass of samples (that is 5 mg in the present work), 1 ml of chitosan (1 wt.%) and 4 ml of distilled water were ultrasonically mixed. Then, given amounts of the homogeneously mixed ink of each sample (5 µL for Pd/C and 2µL for Pd aerogel) were placed on a pre-polished glassy carbon (GC) electrode with a 0.0314
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cm-2 surface area. These modified electrodes were used as the working electrodes. The counter electrode was a Pt foil, while serving an Hg/HgO (MMO) electrode as the reference electrode.
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Electrochemical impedance spectroscopy (EIS) measurements were performed with an Autolab PGSTAT 128N (EcoChemie, Netherlands) potentiostat/galvanostat controlled by NOVA 1.11
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software. Electrochemical impedance tests for Pd aerogel and Pd/C catalysts were carried out in 5 mM [Fe (CN) 6]3-/4- fabricated in 0.1 M KCl. Electrochemical impedance was done over a frequency area of 0.1 Hz to 100 kHz with 0.02 V amplitude (rms). 2.4 Synthesis of Pd hydrogel/aerogel To synthesize the Pd aerogel, the following procedure was used: In a typical synthesis, 2 ml of (H2PdCl4) with concentration of 1 mg/ml was added into a certain amount (see Table 1) of
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sodium carbonate (Na2CO3) and glyoxylic acid monohydrate dissolved in 8 ml of deionized water at 65℃ until the reduction reaction was performed. Afterwards, the glassware was moved into the oven at 45°C. Next, the resulting Pd hydrogel was washed several times using
supercritical CO2 and Pd aerogel was obtained. 3. Results and discussion 3.1 Optimum condition for the creation of Pd hydrogel
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deionized water, ethanol, and acetone. Thereafter, the Pd hydrogel was dried by using
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The monolith network of the Pd aerogel was synthesized by reducing H2PdCl4 in the presence of sodium carbonate using glyoxylic acid monohydrate as a reductant agent followed by supercritical drying. Fig. 2 presents the formation of Pd hydrogel with different ratios of the glyoxylic acid monohydrate and sodium carbonate, and the results are summarized in Table 1.
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The porous 3D network is created by the controlled coalescence of Pd nanoparticles in aqueous media, and a self-supporting network is formed owing to their interconnection and interpenetration. A film is uploaded from Pd hydrogel for the demonstration of hydrogel It can be seen that the ratios of sodium
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monolithic (Supporting Information, Video file).
carbonate and glyoxylic acid monohydrate played a vital role in the formation of hydrogel.
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However, the gelation kinetics for Pd were found to be closely related to the concentration of sodium carbonate, and increase in the concentration of sodium carbonate effectively promoted gelation kinetics while glyoxylic acid monohydrate concentration remained constant. The change of the ratio of sodium carbonate in the synthesis process is accompanied by the faster gelation kinetic as well as lower gelation time. As shown, the Pd hydrogel is not formed with the ratios 1:1 and 1:2 from glyoxylic acid monohydrate and sodium carbonate, respectively. In addition,
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the ratios of the 1:3 and 1:4 from glyoxylic acid monohydrate and sodium carbonate lead to the formation of the Pd hydrogel while the quality of the hydrogels remains very poor. Based on the obtained results, we understand that the optimum condition for the creation of Pd hydrogel is the
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ratio of 1:6 (glyoxylic acid monohydrate1:6 sodium carbonate) with 60 min of time. Table 1 presents the concentrations effect of sodium carbonate and glyoxylic acid monohydrate for the creation of Pd hydrogel. Next, the Pd hydrogel was dried by using supercritical CO2 and Pd
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aerogel was obtained (shown in Fig. 3 (a, b)). Moreover, the inset of Fig. 3a indicates Pd hydrogel onto spatula. Fig. 3 (c) depicts a photograph of Pd aerogel that is balanced on the spine
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of a dandelion plant. It well confirms the ultralow density of the Pd aerogel synthesized via this approach.
Please insert Fig. 2
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Please insert Table.1 Please insert Fig. 3
3.2.1. TEM images
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3.2. Morphological and structural characteristics
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TEM and FESEM were utilized to investigate the morphology of the Pd aerogel prepared by the synthetic approach shown in Fig. 1. The TEM images of the Pd aerogel are shown in Fig. 4. The TEM images at various magnification obviously show the coalescence of the initial spherical nanoparticles into chainlike structures. These images exhibit the extended nanochains interconnected network system of the Pd aerogel. Please insert Fig. 4
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3.2.2. FESEM images Fig. 5(A, B) illustrates the FESEM micrographs at different magnifications for the
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morphological characterization of the Pd metal aerogel. It can be observed that the Pd aerogel represents a three-dimensional porous network with extended nanochains. As illustrated, this porous microstructure contained a wide range of pores, both micron and nanoscales, which could be distinguished using the FESEM micrographs, and nitrogen isotherm, respectively. Moreover,
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the TEM and FESEM micrographs of the Pd aerogel well confirm the formation of porous 3D
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network. The existence of the abundant open pores for the Pd aerogel can create a highly specific surface area which is examined by nitrogen isotherm.
Please insert Fig. 5 3.2.3. XRD and EDS characterizations
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To examine the crystallinity of the as-synthesized Pd aerogel, powder X-ray diffraction (XRD) measurement was performed. Fig. 5(C) illustrates the XRD pattern of the Pd aerogel catalyst sample. In the XRD pattern of the Pd aerogel, the five major diffraction peaks exist at the 2
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values of about 40.1˚, 46.6˚, 68.1˚, 82.1˚, and 86.6˚ which are ascribed to the (111), (200), (220), (311), and (222) reflection planes (JCPDS# 46-1043), respectively. These peaks agree with a
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face centered cubic (FCC) crystal structure of Pd. To study the chemical compositions of the assynthesized Pd aerogel, the EDS technique was applied. Fig. 5(D) presents the EDS spectrum of the Pd aerogel, and the results are summarized in Table inset of Fig. 5(D). It can be observed that the EDS spectrum demonstrates the presence of Pd in the Pd aerogel sample with mass percentage equal to 98.88 wt.%. The TEM, FESEM, XRD, and EDS analyses confirm the
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formation of the metallic Pd aerogel with a large number of open pores during the synthesis process shown in Fig. 1.
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3.2.4. Nitrogen isotherm According to the structure observed with FESEM micrographs, a high surface area is expected for the Pd aerogel. The high porosity and surface area of the Pd aerogel sample have been
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investigated by nitrogen adsorption/desorption isotherm (shown in Fig. 6). The isotherm of the Pd aerogel represents a multilayer adsorption behavior [11-14]. The surface area of the Pd
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aerogel sample was determined via analysis of the isotherm by the Brunauer-Emmett-Teller (BET) approach, yielding a value of 50.1m2g-1 for Pd aerogel. The inset of Fig. 6 displays the corresponding pore size distribution obtained from the desorption branch of the nitrogen isotherm using the BJH (Barrett-Joyner-Halenda) theory. The results represent narrow pore size
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distribution for the Pd aerogel in the range of mesopores (2-50 nm) with the mean pore size of about 11.4 nm. This porous 3D network is an excellent advantage for faster mass transport as well as minimizing diffusion barriers in various applications such as sensor or catalytic since the
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diffusion rates through 10-50 nm pores approach those of molecules in open media [11, 12, 14]. Please insert Fig. 6
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3.3. Electrocatalytic performances The electrocatalytic activity of the as-synthesized Pd aerogel was evaluated by using cyclic voltammetry (CV) in the aqueous solution 1.0 M KOH at ambient condition with a sweep rate of 20 mVs-1 and compared to that of Pd/C catalyst (20. wt %). Fig. 7(A) shows the CV profiles of the Pd/C (red line) and Pd aerogel (black line) in the absence of ethanol in the potential window 1.0 to + 0.5 V (vs. Hg/HgO). In each CV profile, the regions of I, II, III and IV, which are
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attributed to the hydrogen desorption, formation of Pd (II) oxide, reduction of Pd (II) oxide, and hydrogen adsorption peaks, respectively, can be observed. It is proved that, the electrochemical active surface area (ECSA) of an electrode is a crucial index to evaluate the activity of a catalyst.
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The ECSA of a catalyst not only provides significant information regarding the number of electrochemically active sites of a catalyst, but also is a suitable index to compare different electrocatalytic supports [29]. Cyclic Voltammetry (CV) is a useful technique to measure the
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ECSA of a catalyst. The values of ECSA for Pd aerogel and the Pd/C samples were measured by
=
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using the reduction charge of Pd (II) oxide according to the following equation (1) [30]:
1
.
×
Where, QP denotes coulombic charge (Q per mCcm-2) for the reduction of Pd (II) oxide achieved by integrating the charges related to the reduction of Pd (II) oxide for both catalysts; Pdm is the
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mass amount of Pd loaded (mgcm-2) on the GC electrode surface and a constant (0.405 mCcm-2) charge is needed, which corresponds to the reduction of a Pd (II) oxide monolayer. The ECSA values of the as-synthesized Pd aerogel and Pd/C samples are estimated to be 133.27 m2g-1 and
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10.06 m2g-1, respectively. It can be seen that, the adsorption/desorption peak currents of hydrogen on the Pd aerogel are much higher in comparison to the Pd/C catalyst, providing
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further evidence for the greater ECSA. The obtained results might be attributed to the porous 3D network with numerous open pores as well as high specific surface area of the Pd aerogel. Please insert Fig. 7
We examined the electrocatalytic activity of the as-synthesized Pd aerogel toward the electrochemical oxidation of ethanol in 1.0 M KOH solution with that of the Pd/C catalyst. The electrocatalytic performances of the as-synthesized Pd aerogel and Pd/C toward EOR were
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conducted by CV measures in the mixture of aqueous solution 1.0 M KOH containing 0.5 M C2H5OH at RT with a sweep rate of 20 mVs-1 and in the swept potential from -0.6 to +0.5 V vs. Hg/HgO (shown in Fig. 7B). Cyclic voltammograms for electrodes decorated with the Pd aerogel
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and Pd/C catalysts show two well-defined peak current densities, which are created in the forward and backward scans, respectively. The peak current densities of the Pd aerogel and Pd/C catalysts in the forward sweeps are related to the electrooxidation reaction of ethanol.
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Nevertheless, other sharp oxidation peak currents were created in the backward sweeps which appeared primarily owing to the removal of the incompletely oxidized carbonaceous
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intermediates produced during the forward sweep [3, 5]. In general, the oxidation peak current density during the forward sweep was utilized to evaluate the catalytic activity of a catalyst. It can be seen that the Pd aerogel gives much higher current density in comparison to the Pd/C catalyst (4703 vs 1007.9 mAmg-1Pd). Besides, the onset oxidation potentials on the Pd aerogel
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and Pd/C catalysts are about -542 mV and - 460 mV, respectively. The 82 mV negative shift of the onset oxidation potential for Pd aerogel compared to the Pd/C demonstrates an improvement in the kinetics of the EOR on the Pd aerogel catalyst surface. Furthermore, to study the tolerance
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of the catalyst to agglomeration of carbonaceous intermediate species, the ratio of the forward peak current density (Jf) to the backward peak current density (Jb) is commonly utilized. A high
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Jf/Jb ratio represents the efficient oxidation reaction of ethanol during the forward sweep with little agglomeration of carbonaceous intermediate species residue on the catalyst surface [26]. The ratio of Jf/Jb for Pd aerogel was determined to be 1.59, which is much higher compared to the Pd/C (0.6) catalyst. This suggests that the carbonaceous intermediate species are removed more effectively from Pd aerogel surface in comparison to the Pd/C catalyst [3]. Based on the obtained results, the Pd aerogel sample exhibits better electrocatalytic activity toward EOR
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compared to the Pd/C catalyst, which can be attributed to the mesoporous and macroporous characters of the Pd aerogel. Mesoporous character provides a high surface area, which allows faster mass transport through the profuse open pores and the macroporous character guarantees
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easy access of molecules to the active sites. Likewise, Table 2 presents the mass activity of modified electrodes containing Pd and Pd-based aerogel for electrooxidation reaction of ethanol in alkaline media. As shown, the Pd aerogel synthesized by this method compared to the other
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aerogels offers considerable mass activity, despite the lower ethanol concentration as well as
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scan rate.
Fig. 7(C) indicates the effect of potential sweep rate on the EOR for the modified electrode with the Pd aerogel in the solution 1.0 M KOH containing 0.5 M ethanol at RT. It can be observed that, the oxidation peak currents of ethanol increases with the sweep rate. Meanwhile, the peak potential of EOR slightly shifts to more positive potential along with the increase in the sweep
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rate. Moreover, Fig. 7(D) shows an approximate linear relationship between the oxidation peak currents of ethanol and the square root of sweep rates (v1/2). Hence, we can logically assume that
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the EOR on the Pd aerogel catalyst surface is controlled by the diffusion process [31-33]. Fig. 8(A, B) displays linear sweep voltammetry (LSV) measures for the decorated electrodes
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with the Pd aerogel and Pd/C catalysts at various temperatures with a sweep rate of 20 mVs-1 in the aqueous solution 1.0 M KOH containing 0.5 M ethanol and in the potential window -0.6 to +0.5 V (vs. Hg/HgO). It can be observed that, the increase in temperature is accompanied by higher oxidation peak current of ethanol and the slight shift of the peak potential to more positive potential. In addition, Fig. 8(C, D) shows the reciprocal relationship of logarithm of oxidation current densities versus temperature for both catalysts at a potential - 0.1 V. For the Pd aerogel and Pd/C samples, the values of apparent activation energy (Ea) were measured by suitable linear
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fit the relationship ln I vs T-1, using the Arrhenius relationship [34, 35] according to the following equation (2): = /
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2
Where, I denotes oxidation current density at given potential, T is the absolute temperature (in K), while R and Ea are gas constant and apparent activation energy (Ea), respectively. By using
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the Arrhenius equation, the value of apparent activation energy for the Pd aerogel sample was determined to be 4.82 kjmol-1, which is smaller compared to the Pd/C (22.6 kjmol-1) sample.
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Apparent activation energy values for the Pd aerogel and Pd/C samples are demonstrative of a faster charge transfer process and better intrinsic activity of the Pd aerogel than those of Pd/C catalyst.
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It is proved that the durability of a catalyst sample is a crucial index for the commercialization of the catalyst. However, redox processes occurring on the surface of catalysts often severely decrease the catalytic activity and durability of the catalyst. The long-term stability of the as-
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synthesized Pd aerogel and Pd/C catalysts for the oxidation process of ethanol were also studied at a potential of -0.2 V (vs. Hg/HgO) for 7200 s with a scan rate of 20 mVs-1 in the mixture of an
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aqueous solution 1.0 M KOH containing 0.5 M ethanol at RT and the chronoamperometry (CA) curves are shown in Fig. 8(E). In each curve of the CA related to the Pd aerogel and Pd/C, at the initial step of the CA tests, both catalysts illustrated a high current due to the double layer charging process as well as abundant available active sites on the Pd aerogel and Pd/C surfaces. Owing to the formation of some intermediate species during the oxidation process of ethanol, both catalysts represented a dramatic decay at current densities before a steady current status was
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achieved. The intense drop in current densities was due to the formation and adsorption of intermediate species on the catalysts surface, which led to the blockage of active sites as well as poisoning of the catalysts [36-37]. After an early decay in activity, the currents were stabilized at
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1.3 mAcm-2 on Pd/C and 81.4 mAcm-2 on the Pd aerogel, respectively. The much higher elecrocatalytic activity and durability of the Pd aerogel sample compared to the Pd/C catalyst are ascribed to two crucial reasons: (1) the presence of profuse open pores as well as easy access to
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the surface of Pd aerogel and (2) the self-supporting character of the resultant aerogel sample
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which may hinder the loss of durability observed in Pd supported on carbon owing to corrosion. Electrochemical impedance spectroscopy (EIS) experiments were carried out to further evaluate the electrochemical behaviors of the as-synthesized Pd aerogel and Pd/C catalysts. Fig. 8(F) depicts the Nyquist plots of the as-synthesized Pd aerogel and Pd/C catalysts. The Nyquist plots show that the charge transfer process takes place on the surface of the Pd aerogel and Pd/C
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catalysts. In general, the increase in the diameter of the semicircle leads to an increase in the charge transfer resistance (RCT), while a lower RCT value demonstrates a faster charge transfer process. The RCT values of the as-synthesized Pd aerogel and Pd/C catalysts are estimated to be
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50 Ω and 354 Ω, respectively. The standard exchange current density (i0) is often utilized to
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evaluate the catalytic activity, which can be calculated according to the following relationship (3) [38, 39]:
i0 = / CT
3
Where, n equals 1 which denotes the number of electrons involved in the charge transfer process, while T, F, R, and RCT are the absolute temperature (that is 298 K), Faradic constant, gas constant, and charge transfer resistance in Ω, respectively. The i0 values of the as-synthesized Pd aerogel and Pd/C samples were measured 5.14×10-4 and 7.2×10-5 mA, respectively. This result
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indicates that the rate of electron-transfer is largely facilitated on the Pd aerogel which is in
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excellent agreement with the electrochemical results.
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4. Conclusions
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In summary, we demonstrated a one-pot, simple, and fast method to prepare Pd aerogel by reducing H2PdCl4 in the presence of sodium carbonate using glyoxylic acid monohydrate as a reductant agent followed by supercritical drying. This method offers several advantages over other methods such as being one-pot synthesis, simple and fast. The porous three-dimensional network of the Pd aerogel with high surface area and large open pores were confirmed by the
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TEM, FESEM, XRD, EDS and N2 isotherm techniques. The Pd aerogel was applied as an anode catalyst toward electrooxidation of ethanol, and showed superior electrocatalytic activity and durability compared to the Pd/C. The considerable electrocatalytic activity of the Pd aerogel
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compared to the Pd/C catalyst is attributed to the mesoporous and macroporous characters of the
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Pd aerogel. Mesoporous character creates a high surface area, which allows faster mass transport through the profuse pores and macroporous character guarantees easy access of molecules to the active sites. Moreover, the self-supporting property of the Pd aerogel may prevent the loss of stability observed in Pd catalyst supported on carbon due to corrosion. We believe that the porous exceptional 3D nanostructures synthesized by this method are an efficient catalyst for DEFCs as well as other applications such as sensors, catalysis, material engineering, etc.
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Acknowledgment The authors acknowledge Dr. Mahdi Shafiee Afarani for kind comments. Moreover, special
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thanks go to Mehdi Zareie Yazdan-abad and Ebrahim Mesgari for their technical helps.
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Ratio of GA:SC GA:SC GA:SC GA:SC GA:SC GA:SC GA:SC GAa:SCb 1:1 1:2 1:3 1:4 1:5 1:6 1:7 c Time/min NR NR 245 180 135 60 65 a
Glyoxylic acid monohydrate. Sodium carbonate. c No reaction.
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a
Pdα-CD Pdβ-CDb Pdγ-CDc Au/Ag/Pd Pd68Cu32 d Pd83Ni17HNS
Pd Pd
Electrolyte Concentration (M)
Ethanol Concentration (M)
Scan rate (mVs-1)
Mass activity (Amg-1)
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.5
50.0 50.0 50.0 50.0 50.0 50.0 50.0 20.0
7.388 7.830 4.119 20.55 3.472 3.63 3.70 4.703
a
Pd aerogel modified by α-cyclodextrins. Pd aerogel modified by β-cyclodextrins. c Pd aerogel modified by γ-cyclodextrins. d PdNi Hollow Nanospheres.
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Ref. [26] [26] [26] [28] [13] [40] [41]
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Aerogel
this work
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Figure captions Fig. 1. Schematic illustration of the preparation process of Pd aerogel and its application in ethanol oxidation.
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Fig. 2. Photographs showing the concentrations effect of sodium carbonate and glyoxylic acid monohydrate for the creation of the Pd aerogel. Fig. 3. (a, b) Photographs showing the preparation of Pd aerogel by the Pd hydrogel supercritical drying and inset of Fig.3a indicates Pd hydrogel onto spatula, (c) a photograph of Pd aerogel balanced on the spine of a dandelion plant.
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Fig. 4. TEM images of the Pd aerogel at various magnifications. The TEM images display the the coalescence of the Pd nanoparticles into chainlike network structures.
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Fig. 5. (A, B) FESEM images at different magnifications show porous 3D network with large open pores of the Pd aerogel, and C) X-Ray Diffraction (XRD) and D) Energy dispersive X-ray spectroscopy (EDS) of the Pd aerogel. Fig .6. Nitrogen isotherm shows surface area of 50.1 m2g-1 for Pd aerogel. The inset of Fig. 6 illustrates the corresponding pore size distribution and the presence of mesopores can be concluded from BJH theory
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Fig. 7. A) Cyclic voltammograms of the Pd aerogel and Pd/C catalysts in 1.0 M KOH solution and B) 1.0 M KOH + 0.5 M C2H5OH solution at sweep rate of 20 mV s-1, and C) CV curves of the EOR on the Pd aerogel at different scan rates in the 0.5 M C2H5OH +1.0 M KOH solution and D) plot of forward peak current density vs. the square root of the scan rate.
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Fig. 8. LSV curves in the mixture of a 1.0 M KOH and 0.5 M ethanol solution at different temperatures for A) Pd aerogel and B) Pd/C catalysts at scan rate of 20 mV s-1 and Arrhenius plots for C) Pd aerogel and D) Pd/C at -0.1 V, and Chronoamperometry curves of the Pd aerogel and Pd/C at potential of -0.2 V (vs. Hg/HgO) for 7200 s (E) and the Nyquist plots recorded at potential of 0 V for the Pd aerogel and Pd/C in the solution of 5 mM [Fe (CN) 6]3- / 4- prepared in 0.1 M KCl (F).
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One-pot and surfactant free method to fabricate the Pd aerogel. Fast strategy for the creation of Pd aerogel assembled by nanochaines. Pd aerogel shows high catalytic activity and durability toward ethanol oxidation. The porous 3D network of Pd aerogel is formed during the self-assembly process.
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S0013-4686(18)30817-X
DOI:
10.1016/j.electacta.2018.04.073
Reference:
EA 31640
To appear in:
Electrochimica Acta
Received Date: 16 December 2017 Revised Date:
24 March 2018
Accepted Date: 10 April 2018
Please cite this article as: A. Shafaei Douk, H. Saravani, M. Noroozifar, Three-dimensional assembly of building blocks for the fabrication of Pd aerogel as a high performance electrocatalyst toward ethanol oxidation, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.04.073. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Three-dimensional assembly of building blocks for the fabrication of Pd aerogel as a high
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performance electrocatalyst toward ethanol oxidation
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Abdollatif Shafaei Douk*, Hamideh Saravani*, Meissam Noroozifar
*
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Department of Chemistry, University of Sistan and Baluchestan, P.O. Box 98135-674, Zahedan, Iran
Corresponding author. Tel.: +98-54-3341-6464; Fax: +98-54-3341-6888.
E-mail: [email protected] (A.Shafaei Douk) E-mail: [email protected] (H. Saravani),
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Abstract Porous noble metal nanostructures with highly porous and extended surface areas are a long-
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desired target in both industry and academia. Among them, noble metal aerogels have emerged as state-of-the-art catalysts and these exceptional structures play crucial roles in various applications such as catalysis, sensors, etc. Until now, various methods have been published for
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the creation of noble metal aerogels, while their development and synthesis suffer from timeconsuming multistep procedures. In this paper, we present an efficient method for the creation of
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porous three-dimensional network of Pd aerogel by a self-assembly process. This method offers several advantages over other methods such as being one-pot synthesis, simplicity and fast. Pd aerogel is synthesized by reducing H2PdCl4 in the presence of sodium carbonate using glyoxylic acid monohydrate as a reductant agent in a short time followed by supercritical drying. The Pd
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aerogel serves as an anode catalyst for the electrooxidation reaction of ethanol and exhibits superior electrocatalytic activity and durability compared to Pd/C catalyst. We believe that the Pd aerogel synthesized via this method is a promising catalyst for direct ethanol fuel cells
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(DEFCs) and will also open large opportunities for other applications.
Keywords: Aerogel, Ethanol oxidation, Noble metal aerogels, Porous noble metal nanostructures, Self-assembly, Three-dimensional network
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1. Introduction Direct ethanol fuel cells (DEFCs) have received worldwide attention owing to their high energy
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density, security, low operating temperature, and low pollutant emission [1-3]. Nonetheless, the commercial application of DEFCs still faces three critical obstacles: relatively high costs, insufficient performance, and inadequate long-term durability of catalysts. To overcome these issues, various methods have been proposed by tailoring the design of the size, composition, and
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shape of the catalyst [4-10].
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Nowadays, new self-assembled architectures have received worldwide attention in nanoscience owing to their widespread applications in optics, sensors, catalysis, etc [11-14]. Self-assembly processes exhibit the most useful strategy to create complex materials with unique structures. During the past years, self-assembled architectures have been applied to fabricate porous
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materials, which can be branched out into three various categories in accordance with their pore sizes: microporous (<2 nm), mesoporous (2-50 nm), and macroporous (>50 nm) [15]. The rational design of porous nanomaterials with extended surfaces such as nanosponges, nanowires
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and nanosheets, has created enormous opportunities to improve catalytic activity and stability [16-19]. In this context, a very effective class of supportless catalyst nanomaterials are noble
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metal aerogels consisting of expanded metal backbone nanonetworks considered as state-of-theart catalysts.
Aerogels are unique solid materials with ultralow densities, high porosities, great surface areas, and profuse open interconnected pores [11-14]. They are a cross-linked system, usually a result of self-assembly of metal nanoparticles (NPs). Self-assembly is a usual bottom-up approach to obtain well-defined complex architectures. Metal NPs with logically designed sizes, morphologies, shapes and structures act primarily as the building blocks to make template-free
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two-dimensional (2D) or 3D structures [20]. These novel noble metal nanostructures not only combine the physical and chemical properties of nanomaterials with those of the macroscale, but also contain attractive properties in terms of their composition, different sizes, and
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morphological effects [11-14, 20]. These unique porous metallic 3D networks have two primary advantages: (1) the self-supporting property of these porous noble metal aerogels may hinder the loss of durability observed in metallic catalysts supported on carbon owing to corrosion; (2) the
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macroporous 3D network of the aerogels exhibits high active surface areas and allows reactants to readily access the active sites and enhance catalytic performance [12, 20]. Moreover, noble
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metal aerogels contain the unique properties of metals (such as excellent electrical and thermal conductivity, catalytic performance and ductility/malleability) as well as the unique properties of common aerogels (high surface area, extremely low density, and high porosity) [12]. Therefore, noble metal aerogels can be used as unsupported catalyst materials.
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It is worth noting that Kistler and co-workers pioneered the first aerogels synthesis (silica and alike) [21]. Since then, a lot of research has been carried out for the development of aerogels. Eychmüller and colleagues successfully realized the controlled destabilization of citrate-
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stabilized noble metal NPs in aqueous solution and carried out pioneering work to create mono-
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and bimetallic aerogels, opening a novel road to self-support porous 3D networks [11, 12, 15]. These exceptional assembled structures offer pore size distribution: microporous, mesoporous and macroporous. Micropore and mesopore properties create high specific surface areas while macrospore character guarantees easy access of molecules to the surface [15]. Therefore, novel exceptional materials have found fascinating applications such as polymer electrolyte fuel cells (PEFCs), oxygen reduction reaction (ORR), thermal insulators and components of electrochemical devices owing to their unique physical and chemical properties [15, 22-25].
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Nonetheless, the synthesis and development of noble metal aerogels compared to some inorganic aerogels such as metal oxide, carbon materials, and other hybrid systems are very limited despite the urgent need for their widespread applications. Therefore, a large number of studies have been
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devoted to new and suitable approaches for the creation of noble metal aerogels.
One of the key challenges in the development of noble metal aerogels is the formation of hydrogel. During the past years, various methods have been successfully developed to facilitate
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the gelation process. For example, Eychmüller and co-workers [22] showed that the formation of
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Pd hydrogels is controlled by using the Ca2+ concentration. Moreover, Lin and co-workers reported the one step in situ reduction of metals with sodium borohydride (NaBH4) in aqueous media at elevated temperature for the preparation of MCu (M = Pd, Pt, and Au) hydrogels. Likewise, Eychmüller and co-workers [26] demonstrated that the Pd hydrogels can be spontaneously formed with NaBH4 in the presence of α-, β-, or γ-cyclodextrins (PdCD) without
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additional treatment and next PdCD aerogels were obtained with supercritical CO2 drying. Bigall and co-workers reported that lightweight aerogels of Pt nanocubes and nanospheres can be formed by direct destabilization from nonpolar colloidal solution utilizing hydrazine
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monohydrate as gelation reagent [27]. The shape-controlled Au/Ag/Pd alloy NPs and their self-
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assembly into monolithic aerogels for ethanol oxidation reaction (EOR) were reported by Arachchige and co-workers [28]. However, despite numerous efforts made for the development of aerogels, the search to facilitate the gelation process is very important. In this paper, we demonstrate an efficient method for the creation of Pd aerogel. Herein, for the first time, glyoxylic acid monohydrate is utilized as reducing reagent for the formation of Pd aerogel. This method provides a number of advantages including being one-pot synthesis, surfactant free, simple and fast. The Pd aerogel was synthesized by reducing H2PdCl4 in the
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presence of sodium carbonate using glyoxylic acid monohydrate as a reductant agent in a short time followed by supercritical drying. These porous noble metal nanostructures were used for the electrooxidation reaction of ethanol, and showed superior electrocatalytic activity and durability
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compared to the Pd/C catalyst toward EOR in alkaline media. The schematic representation of
shown in Fig. 1.
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Please insert Fig.1
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the preparation process of Pd aerogel and its application in the ethanol oxidation reaction are
2. Experimental
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2.1 Chemical reagents
Palladium (II) chloride (PdCl2) was purchased from Sigma-Aldrich Co. Sodium carbonate (Na2CO3) and glyoxylic acid monohydrate (C2H2O3.H2O) were purchased from Merck Co.
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2.2 Physical catalyst characterization
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Transmission electron microscope (TEM) and Field Emission Scanning Electron Microscopy (FESEM) images were employed to investigate the morphology of the Pd aerogel sample. The FESEM and TEM analyses were taken by using MIRA3 TESCAN and Zeiss-EM10C-100KV, respectively. Energy-Dispersive X-ray Spectroscopy (EDS) technique was carried out to determine the chemical compositions of the as-synthesized Pd aerogel by using SAMX electron microscope. XRD (X-Ray powder diffraction) pattern of the as-synthesized Pd aerogel catalyst sample was done by a Bruker, D8 ADVANCE XRD diffraction spectrometer for an angle range
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of 2θ = 30-90°. XRD analysis was conducted on a Philips diffractometer of X’pert company with graphite monochromatic Cu Kα radiation (λ = 1.54056 Å). The surface area of the Pd aerogel was evaluated by the Brunauer–Emmett–Teller (BET) method from the recorded N2 isotherm
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with Micromeritics TriStar II Plus. The pore size distribution was investigated by the Barrett– Joyner–Halenda (BJH) model.
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2.3 Electrochemical measurements
All electrochemical measurements were carried out in a standard three-electrode cell system
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using an SAMA-500 Electroanalyser (SAMA Research Center, Iran). The modified electrodes with the Pd aerogel and Pd/C were fabricated as follows: a certain mass of samples (that is 5 mg in the present work), 1 ml of chitosan (1 wt.%) and 4 ml of distilled water were ultrasonically mixed. Then, given amounts of the homogeneously mixed ink of each sample (5 µL for Pd/C and 2µL for Pd aerogel) were placed on a pre-polished glassy carbon (GC) electrode with a 0.0314
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cm-2 surface area. These modified electrodes were used as the working electrodes. The counter electrode was a Pt foil, while serving an Hg/HgO (MMO) electrode as the reference electrode.
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Electrochemical impedance spectroscopy (EIS) measurements were performed with an Autolab PGSTAT 128N (EcoChemie, Netherlands) potentiostat/galvanostat controlled by NOVA 1.11
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software. Electrochemical impedance tests for Pd aerogel and Pd/C catalysts were carried out in 5 mM [Fe (CN) 6]3-/4- fabricated in 0.1 M KCl. Electrochemical impedance was done over a frequency area of 0.1 Hz to 100 kHz with 0.02 V amplitude (rms). 2.4 Synthesis of Pd hydrogel/aerogel To synthesize the Pd aerogel, the following procedure was used: In a typical synthesis, 2 ml of (H2PdCl4) with concentration of 1 mg/ml was added into a certain amount (see Table 1) of
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sodium carbonate (Na2CO3) and glyoxylic acid monohydrate dissolved in 8 ml of deionized water at 65℃ until the reduction reaction was performed. Afterwards, the glassware was moved into the oven at 45°C. Next, the resulting Pd hydrogel was washed several times using
supercritical CO2 and Pd aerogel was obtained. 3. Results and discussion 3.1 Optimum condition for the creation of Pd hydrogel
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deionized water, ethanol, and acetone. Thereafter, the Pd hydrogel was dried by using
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The monolith network of the Pd aerogel was synthesized by reducing H2PdCl4 in the presence of sodium carbonate using glyoxylic acid monohydrate as a reductant agent followed by supercritical drying. Fig. 2 presents the formation of Pd hydrogel with different ratios of the glyoxylic acid monohydrate and sodium carbonate, and the results are summarized in Table 1.
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The porous 3D network is created by the controlled coalescence of Pd nanoparticles in aqueous media, and a self-supporting network is formed owing to their interconnection and interpenetration. A film is uploaded from Pd hydrogel for the demonstration of hydrogel It can be seen that the ratios of sodium
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monolithic (Supporting Information, Video file).
carbonate and glyoxylic acid monohydrate played a vital role in the formation of hydrogel.
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However, the gelation kinetics for Pd were found to be closely related to the concentration of sodium carbonate, and increase in the concentration of sodium carbonate effectively promoted gelation kinetics while glyoxylic acid monohydrate concentration remained constant. The change of the ratio of sodium carbonate in the synthesis process is accompanied by the faster gelation kinetic as well as lower gelation time. As shown, the Pd hydrogel is not formed with the ratios 1:1 and 1:2 from glyoxylic acid monohydrate and sodium carbonate, respectively. In addition,
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the ratios of the 1:3 and 1:4 from glyoxylic acid monohydrate and sodium carbonate lead to the formation of the Pd hydrogel while the quality of the hydrogels remains very poor. Based on the obtained results, we understand that the optimum condition for the creation of Pd hydrogel is the
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ratio of 1:6 (glyoxylic acid monohydrate1:6 sodium carbonate) with 60 min of time. Table 1 presents the concentrations effect of sodium carbonate and glyoxylic acid monohydrate for the creation of Pd hydrogel. Next, the Pd hydrogel was dried by using supercritical CO2 and Pd
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aerogel was obtained (shown in Fig. 3 (a, b)). Moreover, the inset of Fig. 3a indicates Pd hydrogel onto spatula. Fig. 3 (c) depicts a photograph of Pd aerogel that is balanced on the spine
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of a dandelion plant. It well confirms the ultralow density of the Pd aerogel synthesized via this approach.
Please insert Fig. 2
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Please insert Table.1 Please insert Fig. 3
3.2.1. TEM images
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3.2. Morphological and structural characteristics
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TEM and FESEM were utilized to investigate the morphology of the Pd aerogel prepared by the synthetic approach shown in Fig. 1. The TEM images of the Pd aerogel are shown in Fig. 4. The TEM images at various magnification obviously show the coalescence of the initial spherical nanoparticles into chainlike structures. These images exhibit the extended nanochains interconnected network system of the Pd aerogel. Please insert Fig. 4
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3.2.2. FESEM images Fig. 5(A, B) illustrates the FESEM micrographs at different magnifications for the
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morphological characterization of the Pd metal aerogel. It can be observed that the Pd aerogel represents a three-dimensional porous network with extended nanochains. As illustrated, this porous microstructure contained a wide range of pores, both micron and nanoscales, which could be distinguished using the FESEM micrographs, and nitrogen isotherm, respectively. Moreover,
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the TEM and FESEM micrographs of the Pd aerogel well confirm the formation of porous 3D
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network. The existence of the abundant open pores for the Pd aerogel can create a highly specific surface area which is examined by nitrogen isotherm.
Please insert Fig. 5 3.2.3. XRD and EDS characterizations
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To examine the crystallinity of the as-synthesized Pd aerogel, powder X-ray diffraction (XRD) measurement was performed. Fig. 5(C) illustrates the XRD pattern of the Pd aerogel catalyst sample. In the XRD pattern of the Pd aerogel, the five major diffraction peaks exist at the 2
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values of about 40.1˚, 46.6˚, 68.1˚, 82.1˚, and 86.6˚ which are ascribed to the (111), (200), (220), (311), and (222) reflection planes (JCPDS# 46-1043), respectively. These peaks agree with a
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face centered cubic (FCC) crystal structure of Pd. To study the chemical compositions of the assynthesized Pd aerogel, the EDS technique was applied. Fig. 5(D) presents the EDS spectrum of the Pd aerogel, and the results are summarized in Table inset of Fig. 5(D). It can be observed that the EDS spectrum demonstrates the presence of Pd in the Pd aerogel sample with mass percentage equal to 98.88 wt.%. The TEM, FESEM, XRD, and EDS analyses confirm the
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formation of the metallic Pd aerogel with a large number of open pores during the synthesis process shown in Fig. 1.
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3.2.4. Nitrogen isotherm According to the structure observed with FESEM micrographs, a high surface area is expected for the Pd aerogel. The high porosity and surface area of the Pd aerogel sample have been
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investigated by nitrogen adsorption/desorption isotherm (shown in Fig. 6). The isotherm of the Pd aerogel represents a multilayer adsorption behavior [11-14]. The surface area of the Pd
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aerogel sample was determined via analysis of the isotherm by the Brunauer-Emmett-Teller (BET) approach, yielding a value of 50.1m2g-1 for Pd aerogel. The inset of Fig. 6 displays the corresponding pore size distribution obtained from the desorption branch of the nitrogen isotherm using the BJH (Barrett-Joyner-Halenda) theory. The results represent narrow pore size
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distribution for the Pd aerogel in the range of mesopores (2-50 nm) with the mean pore size of about 11.4 nm. This porous 3D network is an excellent advantage for faster mass transport as well as minimizing diffusion barriers in various applications such as sensor or catalytic since the
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diffusion rates through 10-50 nm pores approach those of molecules in open media [11, 12, 14]. Please insert Fig. 6
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3.3. Electrocatalytic performances The electrocatalytic activity of the as-synthesized Pd aerogel was evaluated by using cyclic voltammetry (CV) in the aqueous solution 1.0 M KOH at ambient condition with a sweep rate of 20 mVs-1 and compared to that of Pd/C catalyst (20. wt %). Fig. 7(A) shows the CV profiles of the Pd/C (red line) and Pd aerogel (black line) in the absence of ethanol in the potential window 1.0 to + 0.5 V (vs. Hg/HgO). In each CV profile, the regions of I, II, III and IV, which are
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attributed to the hydrogen desorption, formation of Pd (II) oxide, reduction of Pd (II) oxide, and hydrogen adsorption peaks, respectively, can be observed. It is proved that, the electrochemical active surface area (ECSA) of an electrode is a crucial index to evaluate the activity of a catalyst.
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The ECSA of a catalyst not only provides significant information regarding the number of electrochemically active sites of a catalyst, but also is a suitable index to compare different electrocatalytic supports [29]. Cyclic Voltammetry (CV) is a useful technique to measure the
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ECSA of a catalyst. The values of ECSA for Pd aerogel and the Pd/C samples were measured by
=
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using the reduction charge of Pd (II) oxide according to the following equation (1) [30]:
1
.
×
Where, QP denotes coulombic charge (Q per mCcm-2) for the reduction of Pd (II) oxide achieved by integrating the charges related to the reduction of Pd (II) oxide for both catalysts; Pdm is the
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mass amount of Pd loaded (mgcm-2) on the GC electrode surface and a constant (0.405 mCcm-2) charge is needed, which corresponds to the reduction of a Pd (II) oxide monolayer. The ECSA values of the as-synthesized Pd aerogel and Pd/C samples are estimated to be 133.27 m2g-1 and
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10.06 m2g-1, respectively. It can be seen that, the adsorption/desorption peak currents of hydrogen on the Pd aerogel are much higher in comparison to the Pd/C catalyst, providing
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further evidence for the greater ECSA. The obtained results might be attributed to the porous 3D network with numerous open pores as well as high specific surface area of the Pd aerogel. Please insert Fig. 7
We examined the electrocatalytic activity of the as-synthesized Pd aerogel toward the electrochemical oxidation of ethanol in 1.0 M KOH solution with that of the Pd/C catalyst. The electrocatalytic performances of the as-synthesized Pd aerogel and Pd/C toward EOR were
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conducted by CV measures in the mixture of aqueous solution 1.0 M KOH containing 0.5 M C2H5OH at RT with a sweep rate of 20 mVs-1 and in the swept potential from -0.6 to +0.5 V vs. Hg/HgO (shown in Fig. 7B). Cyclic voltammograms for electrodes decorated with the Pd aerogel
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and Pd/C catalysts show two well-defined peak current densities, which are created in the forward and backward scans, respectively. The peak current densities of the Pd aerogel and Pd/C catalysts in the forward sweeps are related to the electrooxidation reaction of ethanol.
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Nevertheless, other sharp oxidation peak currents were created in the backward sweeps which appeared primarily owing to the removal of the incompletely oxidized carbonaceous
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intermediates produced during the forward sweep [3, 5]. In general, the oxidation peak current density during the forward sweep was utilized to evaluate the catalytic activity of a catalyst. It can be seen that the Pd aerogel gives much higher current density in comparison to the Pd/C catalyst (4703 vs 1007.9 mAmg-1Pd). Besides, the onset oxidation potentials on the Pd aerogel
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and Pd/C catalysts are about -542 mV and - 460 mV, respectively. The 82 mV negative shift of the onset oxidation potential for Pd aerogel compared to the Pd/C demonstrates an improvement in the kinetics of the EOR on the Pd aerogel catalyst surface. Furthermore, to study the tolerance
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of the catalyst to agglomeration of carbonaceous intermediate species, the ratio of the forward peak current density (Jf) to the backward peak current density (Jb) is commonly utilized. A high
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Jf/Jb ratio represents the efficient oxidation reaction of ethanol during the forward sweep with little agglomeration of carbonaceous intermediate species residue on the catalyst surface [26]. The ratio of Jf/Jb for Pd aerogel was determined to be 1.59, which is much higher compared to the Pd/C (0.6) catalyst. This suggests that the carbonaceous intermediate species are removed more effectively from Pd aerogel surface in comparison to the Pd/C catalyst [3]. Based on the obtained results, the Pd aerogel sample exhibits better electrocatalytic activity toward EOR
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compared to the Pd/C catalyst, which can be attributed to the mesoporous and macroporous characters of the Pd aerogel. Mesoporous character provides a high surface area, which allows faster mass transport through the profuse open pores and the macroporous character guarantees
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easy access of molecules to the active sites. Likewise, Table 2 presents the mass activity of modified electrodes containing Pd and Pd-based aerogel for electrooxidation reaction of ethanol in alkaline media. As shown, the Pd aerogel synthesized by this method compared to the other
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aerogels offers considerable mass activity, despite the lower ethanol concentration as well as
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scan rate.
Fig. 7(C) indicates the effect of potential sweep rate on the EOR for the modified electrode with the Pd aerogel in the solution 1.0 M KOH containing 0.5 M ethanol at RT. It can be observed that, the oxidation peak currents of ethanol increases with the sweep rate. Meanwhile, the peak potential of EOR slightly shifts to more positive potential along with the increase in the sweep
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rate. Moreover, Fig. 7(D) shows an approximate linear relationship between the oxidation peak currents of ethanol and the square root of sweep rates (v1/2). Hence, we can logically assume that
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the EOR on the Pd aerogel catalyst surface is controlled by the diffusion process [31-33]. Fig. 8(A, B) displays linear sweep voltammetry (LSV) measures for the decorated electrodes
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with the Pd aerogel and Pd/C catalysts at various temperatures with a sweep rate of 20 mVs-1 in the aqueous solution 1.0 M KOH containing 0.5 M ethanol and in the potential window -0.6 to +0.5 V (vs. Hg/HgO). It can be observed that, the increase in temperature is accompanied by higher oxidation peak current of ethanol and the slight shift of the peak potential to more positive potential. In addition, Fig. 8(C, D) shows the reciprocal relationship of logarithm of oxidation current densities versus temperature for both catalysts at a potential - 0.1 V. For the Pd aerogel and Pd/C samples, the values of apparent activation energy (Ea) were measured by suitable linear
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fit the relationship ln I vs T-1, using the Arrhenius relationship [34, 35] according to the following equation (2): = /
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2
Where, I denotes oxidation current density at given potential, T is the absolute temperature (in K), while R and Ea are gas constant and apparent activation energy (Ea), respectively. By using
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the Arrhenius equation, the value of apparent activation energy for the Pd aerogel sample was determined to be 4.82 kjmol-1, which is smaller compared to the Pd/C (22.6 kjmol-1) sample.
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Apparent activation energy values for the Pd aerogel and Pd/C samples are demonstrative of a faster charge transfer process and better intrinsic activity of the Pd aerogel than those of Pd/C catalyst.
Please insert Fig. 8
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It is proved that the durability of a catalyst sample is a crucial index for the commercialization of the catalyst. However, redox processes occurring on the surface of catalysts often severely decrease the catalytic activity and durability of the catalyst. The long-term stability of the as-
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synthesized Pd aerogel and Pd/C catalysts for the oxidation process of ethanol were also studied at a potential of -0.2 V (vs. Hg/HgO) for 7200 s with a scan rate of 20 mVs-1 in the mixture of an
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aqueous solution 1.0 M KOH containing 0.5 M ethanol at RT and the chronoamperometry (CA) curves are shown in Fig. 8(E). In each curve of the CA related to the Pd aerogel and Pd/C, at the initial step of the CA tests, both catalysts illustrated a high current due to the double layer charging process as well as abundant available active sites on the Pd aerogel and Pd/C surfaces. Owing to the formation of some intermediate species during the oxidation process of ethanol, both catalysts represented a dramatic decay at current densities before a steady current status was
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achieved. The intense drop in current densities was due to the formation and adsorption of intermediate species on the catalysts surface, which led to the blockage of active sites as well as poisoning of the catalysts [36-37]. After an early decay in activity, the currents were stabilized at
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1.3 mAcm-2 on Pd/C and 81.4 mAcm-2 on the Pd aerogel, respectively. The much higher elecrocatalytic activity and durability of the Pd aerogel sample compared to the Pd/C catalyst are ascribed to two crucial reasons: (1) the presence of profuse open pores as well as easy access to
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the surface of Pd aerogel and (2) the self-supporting character of the resultant aerogel sample
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which may hinder the loss of durability observed in Pd supported on carbon owing to corrosion. Electrochemical impedance spectroscopy (EIS) experiments were carried out to further evaluate the electrochemical behaviors of the as-synthesized Pd aerogel and Pd/C catalysts. Fig. 8(F) depicts the Nyquist plots of the as-synthesized Pd aerogel and Pd/C catalysts. The Nyquist plots show that the charge transfer process takes place on the surface of the Pd aerogel and Pd/C
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catalysts. In general, the increase in the diameter of the semicircle leads to an increase in the charge transfer resistance (RCT), while a lower RCT value demonstrates a faster charge transfer process. The RCT values of the as-synthesized Pd aerogel and Pd/C catalysts are estimated to be
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50 Ω and 354 Ω, respectively. The standard exchange current density (i0) is often utilized to
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evaluate the catalytic activity, which can be calculated according to the following relationship (3) [38, 39]:
i0 = / CT
3
Where, n equals 1 which denotes the number of electrons involved in the charge transfer process, while T, F, R, and RCT are the absolute temperature (that is 298 K), Faradic constant, gas constant, and charge transfer resistance in Ω, respectively. The i0 values of the as-synthesized Pd aerogel and Pd/C samples were measured 5.14×10-4 and 7.2×10-5 mA, respectively. This result
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indicates that the rate of electron-transfer is largely facilitated on the Pd aerogel which is in
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excellent agreement with the electrochemical results.
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4. Conclusions
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In summary, we demonstrated a one-pot, simple, and fast method to prepare Pd aerogel by reducing H2PdCl4 in the presence of sodium carbonate using glyoxylic acid monohydrate as a reductant agent followed by supercritical drying. This method offers several advantages over other methods such as being one-pot synthesis, simple and fast. The porous three-dimensional network of the Pd aerogel with high surface area and large open pores were confirmed by the
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TEM, FESEM, XRD, EDS and N2 isotherm techniques. The Pd aerogel was applied as an anode catalyst toward electrooxidation of ethanol, and showed superior electrocatalytic activity and durability compared to the Pd/C. The considerable electrocatalytic activity of the Pd aerogel
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compared to the Pd/C catalyst is attributed to the mesoporous and macroporous characters of the
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Pd aerogel. Mesoporous character creates a high surface area, which allows faster mass transport through the profuse pores and macroporous character guarantees easy access of molecules to the active sites. Moreover, the self-supporting property of the Pd aerogel may prevent the loss of stability observed in Pd catalyst supported on carbon due to corrosion. We believe that the porous exceptional 3D nanostructures synthesized by this method are an efficient catalyst for DEFCs as well as other applications such as sensors, catalysis, material engineering, etc.
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Acknowledgment The authors acknowledge Dr. Mahdi Shafiee Afarani for kind comments. Moreover, special
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thanks go to Mehdi Zareie Yazdan-abad and Ebrahim Mesgari for their technical helps.
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Ratio of GA:SC GA:SC GA:SC GA:SC GA:SC GA:SC GA:SC GAa:SCb 1:1 1:2 1:3 1:4 1:5 1:6 1:7 c Time/min NR NR 245 180 135 60 65 a
Glyoxylic acid monohydrate. Sodium carbonate. c No reaction.
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b
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a
Pdα-CD Pdβ-CDb Pdγ-CDc Au/Ag/Pd Pd68Cu32 d Pd83Ni17HNS
Pd Pd
Electrolyte Concentration (M)
Ethanol Concentration (M)
Scan rate (mVs-1)
Mass activity (Amg-1)
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.5
50.0 50.0 50.0 50.0 50.0 50.0 50.0 20.0
7.388 7.830 4.119 20.55 3.472 3.63 3.70 4.703
a
Pd aerogel modified by α-cyclodextrins. Pd aerogel modified by β-cyclodextrins. c Pd aerogel modified by γ-cyclodextrins. d PdNi Hollow Nanospheres.
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b
Ref. [26] [26] [26] [28] [13] [40] [41]
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Aerogel
this work
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Figure captions Fig. 1. Schematic illustration of the preparation process of Pd aerogel and its application in ethanol oxidation.
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Fig. 2. Photographs showing the concentrations effect of sodium carbonate and glyoxylic acid monohydrate for the creation of the Pd aerogel. Fig. 3. (a, b) Photographs showing the preparation of Pd aerogel by the Pd hydrogel supercritical drying and inset of Fig.3a indicates Pd hydrogel onto spatula, (c) a photograph of Pd aerogel balanced on the spine of a dandelion plant.
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Fig. 4. TEM images of the Pd aerogel at various magnifications. The TEM images display the the coalescence of the Pd nanoparticles into chainlike network structures.
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Fig. 5. (A, B) FESEM images at different magnifications show porous 3D network with large open pores of the Pd aerogel, and C) X-Ray Diffraction (XRD) and D) Energy dispersive X-ray spectroscopy (EDS) of the Pd aerogel. Fig .6. Nitrogen isotherm shows surface area of 50.1 m2g-1 for Pd aerogel. The inset of Fig. 6 illustrates the corresponding pore size distribution and the presence of mesopores can be concluded from BJH theory
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Fig. 7. A) Cyclic voltammograms of the Pd aerogel and Pd/C catalysts in 1.0 M KOH solution and B) 1.0 M KOH + 0.5 M C2H5OH solution at sweep rate of 20 mV s-1, and C) CV curves of the EOR on the Pd aerogel at different scan rates in the 0.5 M C2H5OH +1.0 M KOH solution and D) plot of forward peak current density vs. the square root of the scan rate.
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Fig. 8. LSV curves in the mixture of a 1.0 M KOH and 0.5 M ethanol solution at different temperatures for A) Pd aerogel and B) Pd/C catalysts at scan rate of 20 mV s-1 and Arrhenius plots for C) Pd aerogel and D) Pd/C at -0.1 V, and Chronoamperometry curves of the Pd aerogel and Pd/C at potential of -0.2 V (vs. Hg/HgO) for 7200 s (E) and the Nyquist plots recorded at potential of 0 V for the Pd aerogel and Pd/C in the solution of 5 mM [Fe (CN) 6]3- / 4- prepared in 0.1 M KCl (F).
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Figure 1
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Figure 2
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Figure 3
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Figure 7
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Figure 8
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One-pot and surfactant free method to fabricate the Pd aerogel. Fast strategy for the creation of Pd aerogel assembled by nanochaines. Pd aerogel shows high catalytic activity and durability toward ethanol oxidation. The porous 3D network of Pd aerogel is formed during the self-assembly process.
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