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Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation
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Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation M. Taei∗, E. Havakeshian, F Hasanpour, F. Abedi, M. Movahedi Chemistry Department, Payame Noor University, 19395-4697 Tehran, Islamic Republic of Iran
a r t i c l e
i n f o
Article history: Received 19 March 2016 Revised 16 June 2016 Accepted 11 July 2016 Available online xxx Keywords: Mg–Ca–Fe layered double hydroxide Au nanoparticles Ethanol oxidation Alkaline fuel cell
a b s t r a c t In this work, the influence of Mg–Ca–Fe layered double hydroxide (LDH) on the electrocatalytic activity of gold nanoparticles (AuNPs) electrodeposited on a glassy carbon electrode for ethanol oxidation in alkaline medium was investigated. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM) and cyclic voltammetry (CV) were used to characterize the synthesized Mg–Ca–Fe LDH powder and to investigate the electrochemical behaviors of the prepared electrodes. The results showed that the AuNPs/GCE exhibits higher current density for ethanol oxidation (about 2.9 times) and higher active surface area (about 2.5 times), after the deposition of the Mg–Ca–Fe LDH on its surface. It was also found that the ethanol oxidation reaction at the LDH/AuNPs/GCE is an adsorption-controlled process, while at the AuNPs/GCE it is a diffusion-controlled process. The investigation of the long-term stability of the electrodes using CV and chronoamperometry methods indicated that the Mg–Ca–Fe LDH by providing OHads species at the surface of the catalyst improves the endurance of the AuNPs against poisoning effects of intermediate species. Accordingly, the Mg–Ca–Fe LDH can be used to improve the electrocatalytic performance of supported AuNPs in the anode of alkaline ethanol fuel cells. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Ethanol oxidation has been the subject of many researches [1–3], because ethanol is considered as an attractive liquid fuel with a high energy density (8 kWh kg−1 ) for use in direct alcohol fuel cells (DAFCs) [4]. Moreover, ethanol can be produced in a large scale by the fermentation of biomass and has less toxicity than methanol. Pt and Pt-based catalysts have been extensively investigated as the electrocatalysts for the ethanol oxidation in direct ethanol fuel cells. However, some major problems such as the high price and limited supply of Pt metal, slow kinetics and facile poisoning have been limited the development of these fuel cells [5]. Therefore, the use of Pt-free catalysts with higher performance has been investigated in recent years. Among Pt-free catalysts, Au is considered as a promising substitute catalyst in alkaline medium, because Au is less expensive and more abundant on the earth than Pd and Pt metals. Also, it has been found that poisoning species are easily oxidized and removed from Au based-catalysts [6]. Nevertheless, since Au has weak adsorption properties, it does not easily catalyze
∗
Corresponding author. E-mail address: [email protected] (M. Taei).
the bond breaking or bond making steps in the alcohol oxidation [7]. Therefore, Au is usually modified with suitable compounds that catalyze the alcohol oxidation or/and have synergistic effects on its catalytic behaviors. Layered double hydroxides (LDHs) are a class of twodimensional nanostructured anionic clays and known as hydrotalcite-like compounds. Generally, these compounds contain the positively charged layers of edge-shared metal M (II) and M (III) hydroxide octahedral that neutralized by anions located in the interlayer spacing [8]. Desirable properties of LDHs such as low cost, high thermal stability and good catalytic activity make them interesting catalysts for use in fuel cells. It has been shown that LDH-noble metal nanocomposites exhibit a superior catalytic behavior [9,10]. In the case of LDH-AuNPs nanocomposites, they have exhibited the enhanced electrocatalytic activity for the oxidation of CO [11], methanol [12–14], glucose [15] and nitrite [16]. In this work, the electrocatalytic performance of a Mg–Ca– Fe LDH–AuNPs nanocomposite for ethanol oxidation in alkaline medium was investigated. The LDH/AuNPs/GCE is fabricated by electrodeposition of AuNPs on a glassy carbon electrode, and followed by Mg–Ca–Fe LDH coating. After charactering the electrode, the electrocatalytic activity of the LDH/AuNPs/GCE is investigated and compared with the AuNPs/GCE for ethanol oxidation in
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Please cite this article as: M. Taei et al., Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.07.011
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alkaline medium using cyclic voltammetry and chronoamperometry methods. 2. Experimental
The morphology of the synthesized Mg–Ca–Fe LDH sample is investigated using FE-SEM. As shown in Fig. 1C, the Mg–Ca–Fe LDH is plate-shape at a range of 30 to 100 nm in size. EDX analysis in Fig. 1D reveals that the sample is composed of Mg, Ca and Fe elements.
2.1. Materials and apparatus 3.2. Characterizations of the prepared electrodes All chemicals were of analytical grades and were purchased from Aldrich Chemicals (Milwaukee, USA). All solutions were prepared just prior to use with deionized and double distilled water. X-ray diffraction pattern of the sample was obtained by Holland Philips Xpert, X-ray diffractometer with Cu-Kα radiation. Fourier transformed infrared (FT-IR) spectrum of the sample in a pressed KBr matrix was recorded on a FT-IR, JASCO 4200 spectrometer in the range 40 0 0–40 0 cm−1 . A Metrohm instrument, Model 797 VA processor, or an Autolab potentiostat-galvanostat, Model PGSTAT302, were used to perform all electrochemical experiments. The conventional three-electrode cell containing a platinum wire counter electrode, Ag/AgCl (3.0 mol l–1 KCl) as a reference electrode and a modified or unmodified glassy carbon electrode (GCE) as a working electrode, was used for all the electrochemical experiments. The geometric surface area of the used GCE was 0.0314 cm2 . All the potentials reported in this work are vs. Ag/AgCl (3.0 moll–1 KCl). 2.2. Synthesis of Mg–Ca–Fe LDH The procedure used for the synthesis of the Mg–Ca–Fe LDH followed a co-precipitation method reported previously [17]. Briefly, 0.025 mol of CaCl2 . 2H2 O, 0.025 mol of MgCl2 and 0.02 mol of FeCl3 .10 H2 O were dissolved in 50 ml of deionized water. Then, the above solution was slowly added into the 100 ml of alkaline solution which containing 0.12 mol of NaOH under vigorous magnetic stirring. The suspension was aged under magnetic stirring for 18 h at 25 °C. The product was washed with deionized water several times and dried at 50 °C. 2.3. Electrode preparation After the polishing of a glassy carbon electrode (GCE) with 5 μm alumina slurry on a polishing cloth, the electrode was sonicated in a mixture of ethanol/distilled water solution (50% v/v) for 10 min. Afterwards, the AuNPs were electrodeposited on the GCE by applying 40 continuous cycles cyclic voltammetry in the potential range of −1.50 to 0 V with a scan rate of 50 mV s−1 . The used solution for the AuNPs electrodeposition was 4.0 × 10−3 mol l–1 HAuCl4 .3H2 O and 0.10 mol l–1 KNO3 . The synthesized Mg–Ca–Fe LDH powder was dispersed in distilled water (0.10 mg LDHs per 10 ml) using ultrasonic agitation until a relative stable suspension was obtained. Then, 5.0 μl of the LDH suspension was dropped on the electrode and allowed to dry in the air. 3. Result and discussion 3.1. Characterizations of the synthesized Mg–Ca–Fe LDH The Mg–Ca–Fe LDH sample was characterized by XRD pattern and FT-IR spectrum, as shown in Fig. 1A and B. The XRD pattern for the Mg–Ca–Fe LDH sample is in good agreement with that previously reported [17]. FT-IR spectra indicates that vibrations characteristic of the LDH structure, including the band of HOH bending in water molecule at 1550–1700 cm−1 , the vibrations of metal-O or metal-OH band at 500–750 cm−1 and vibration of M–O–M lattice at 428–433 cm−1 [18]. As shown in Fig. 1B, the broad band of OH vibration at 3431 cm−1 and the band vibrations at 729, 589 and 428 cm−1 were recorded by FT-IR spectrum of the Mg–Ca–Fe LDH.
Fig. 2A and B show the FE-SEM images obtained from the AuNPs/GCE and LDH/AuNPs/GCE surface, respectively. As clearly seen, AuNPs with and an average diameter of 50 nm cover nearly whole the GCE surface. After coating the surface with Mg–Ca–Fe LDH, many nanoparticles with a diameter of about 28 nm cover the surface (Fig. 2B). From the close examination of the Mg–Ca–Fe LDH sample in Fig. 1C, it is found that each LDH plate is composed of some connected nanoparticles with a size of about 30 nm. This LDH nanoparticles are separated from each other during dispersion in distilled water using ultrasonic agitation, and consequently, form a uniform layer of LDH nanoparticles on the AuNPs/GCE surface after deposition. The EDX data obtained from the surface of the AuNPs/GCE after coating the Mg–Ca–Fe LDH (Fig. 2C and D) present the existence of Mg, Ca, Fe, Au and oxygen on the surface with a (Mg + Ca): Al atomic ratio of 1.92 and an Au: LDH atomic ratio of 1.31. This demonstrates the successful deposition of the Mg–Ca–Fe LDH and AuNPs on the GCE surface. Moreover, the molecular composition of Mg–Ca–Fe LDH was verified by atomic absorption spectroscopy after acid digestion. The results showed that the ratio of Mg:Fe:Ca are close to unity. The electrochemical behaviors of the prepared electrodes in alkaline medium were investigated using cyclic voltammetry. As seen in Fig. 3, GCE does not exhibit no distinctive peak in the potential range of −0.40 to 0.80 V (curve a), while two anodic peaks at about 0.28 and 0.48 V and two cathodic peaks at about −0.10 and 0.10 V appear after the electrodeposition of AuNPs on the GCE (curve c). These anodic and cathodic peaks correspond to the formation and reduction of Au oxides and hydroxides, respectively. After the deposition of the Mg–Ca–Fe LDH onto the Au/GCE surface, a broad anodic peak from −0.10 to 0.25 V appears. This peak is due to the LDH oxidation, since the similar increase is also seen at the LDH/GCE (curve b in Fig. 3), as compared with the GCE. A cathodic peak at about 0.05 V is also observed in curve b, which is related to the conversion of Fe2+ /Fe3+ associated with OH− [19]. 3.3. Ethanol electrooxidation The electrocatalytic activity of the prepared electrodes towards ethanol oxidation was studied using cyclic voltammetry in 0.50 M KOH solution containing 0.50 M ethanol with a scan rate of 50 mV s−1 . As shown in Fig. 4 curve b, the LDH/GCE presents two anodic peaks at about 0.28 and 0.04 V in the forward and backward scans, respectively. The anodic peak in the forward scan is related to the oxidation of freshly chemisorbed ethanol species and the anodic peak in the backward scan is corresponding to the oxidative removal of CO and other carbonaceous species remianed on the surface in the forward scan [4,20]. The ethanol oxidation at the Au/GCE (curve c) starts at about −0.25 V and reaches to the maximum current density at the peak potential of 0.18 V. The peak observed at 0.47 V is related to the oxidation of the Au species, according to AuOHads + OH−→AuO + H2 O + e [21]. The comparison of voltammogarams b and c shows that although the ethanol oxidation at the LDH/GCE and Au/GCE occurs in the same potential region, the Au/GCE exhibits 0.2 V lower onset potential (Eonset ) and higher current density (about 180 times) in comparison with the LDH/GCE, indicating higher catalytic activity of the Au/GCE. As clearly seen in Fig. 4, the deposition of the Mg–Ca–Fe LDH onto the Au/GCE surface results in a decrease in Eonset and
Please cite this article as: M. Taei et al., Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.07.011
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B
C
D
Fig. 1. (A) XRD pattern, (B) FT-IR spectra, (C) FE-SEM image, and (D) EDX analysis of the Mg–Ca–Fe LDH sample.
Please cite this article as: M. Taei et al., Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.07.011
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Fig. 2. FE-SEM images of the (A) AuNPs/GCE, (B) LDH/AuNPs/GCE, and (C, D) EDX analysis of the LDH/AuNPs/GCE surface.
an increase in the current density of the ethanol oxidation, indicating the LDH/AuNPs/GCE has a higher catalytic activity than the Au/GCE. Generally, the real surface area (Sreal ) of a catalyst influences its catalytic activity. Therefore, Sreal for the AuNPs and LDH/AuNPs modified-GCE was obtained by applying cyclic voltammetry at different scan rates (ν ) in 5.0 mM [Fe(CN)6 ]3-/4− solution and plotting Ip versus ν 1/2 , as shown in Fig. 5. Finally, Sreal was calculated using Randles–Sevcik equation, IP = 2.69 × 105 n3/2AD1/2 C ν 1/2, where n is the number of electrons transferred in the redox reaction (n = 1), A is the electrode surface area, D is the dif-
fusion coefficient, C is the reactant concentration, IP refers to the redox peak current and ν is the scan rate. Accordingly, Sreal (provided using both of the AuNPs and LDH) is obtained 0.032 cm2 for the AuNPs/GCE and 0.079 cm2 for the LDH/AuNPs/GCE. The comparison of the obtained values indicates that the Mg–Ca–Fe LDH increases the active surface area of the electrode. As shown in Fig. 4, ethanol electrooxidation is done at both LDH and AuNPs sites in the same potential ranges. Therefore, AuNPs and Mg–Ca– Fe LDH are electroactive material for ethanol oxidation. However, as seen in the Fig. 4, the current density of ethanol oxidation on LDH/GCE is much lower than that on AuNPs. Therefore, the LDH
Please cite this article as: M. Taei et al., Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.07.011
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Fig. 3. Cyclic voltammograms of (a) GCE, (b) LDH/GCE, (c) AuNPs/GCE, and (d) LDH/AuNPs/GCE in deaerated 0.50 M KOH at a scan rate of 50 mV s−1 .
Fig. 4. Cyclic voltammograms of (a) GCE, (b) LDH/GCE, (c) AuNPs/GCE, and (d) LDH/AuNPs/GCE in deaerated 0.50 M KOH and 0.50 M ethanol at a scan rate of 50 mV s−1 .
Fig. 5. Ip versus ν 1/2 for (A) AuNPs/GCE, and (B) LDH/AuNPs/GCE in 5.0 mM [Fe(CN)6 ]3 − /4− .
surface area can have a low effect. To omit the effect of Sreal on the catalytic activity, the current peak was normalized by dividing IPf to Sreal . The obtained values were 0.0157 and 0.0179 A cm−2 for the AuNPs/GCE and LDH/AuNPs/GCE, respectively. This indicates that the Mg–Ca–Fe LDH has synergistic effects. To find the mechanism of the ethanol oxidation at the electrodes, cyclic voltammetry was performed at different scan rates in 0.50 M ethanol solution. Generally, using the dependence of peak current Ip on the scan rate (υ ), the process is distinguished to be
weather under diffusion or adsorption controlled currents. when Ip changes linearly with υ , the process is under adsorption control, whereas if Ip changes linearly with υ 1/2 , the process is under diffusion control. For kinetic currents, Ip is independent of υ (i.e. Ip = f(υ )) [22]. Accordingly, the linear plot of the peak current density of ethanol oxidation versus υ 1/2 for the LDH/AuNPs/GCE (Fig. 6A) and versus υ for the AuNPs/GCE (Fig. 6B) indicates that the ethanol oxidation at the LDH/AuNPs/GCE is under adsorption control, while it is under diffusion control for the AuNPs/GCE. Therefore, the
Please cite this article as: M. Taei et al., Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.07.011
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Fig. 6. (A) Cyclic voltammograms at different scan rates and its related plot of the peak current density of ethanol oxidation versus the square root of the scan rate for the LDH/AuNPs/GCE, and (B) the plot of the peak current density of ethanol oxidation versus the scan rate for the AuNPs/GCE.
Fig. 7. (A) The normalized forward anodic peak current of ethanol oxidation over 250 cycles, and (B) chronoamperograms of (a) AuNPs/GCE and (b) LDH/AuNPs/GCE at 0 V in 0.50 M KOH and 0.50 M ethanol solution.
Mg–Ca–Fe LDH by providing the higher active surface area increases the ethanol concentration at the AuNPs centers through the adsorption of ethanol molecules and thereby, enhances the catalytic activity of the LDH/AuNPs/GCE. To compare the electrochemical performance of the proposed electrode (LDH/AuNPs/GCE) and LDH composite-based electrodes studied for ethanol oxidation in other literature, the obtained parameters such as Eonset , EPf and JPf were listed in Table 1. As clearly seen, the LDH/AuNPs/GCE has a lower Eonset and higher JPf , and consequently, higher catalytic activity than the NiAl-LDH/NMCC. In comparison with MgFe-LDH/GCE, the LDH/AuNPs/GCE exhibits a lower JPf , but 0.70 V lower Eonset . The Pt/CNTs/ Nix Mg1− x Al2 O4 /C has a higher catalytic activity in the case of both JPf and Eonset . However, it is remarkable that the expensive Pt has been used in the preparation of this catalyst. Moreover, the way employed for the preparation of the LDH/AuNPs/GCE in this work is simpler than that for the preparation of the Pt/CNTs/ Nix Mg1− x Al2 O4 /C.
3.4. Stability studies To investigate the endurance of the LDH/AuNPs/GCE against poisoning effects of intermediate species that are formed during the ethanol oxidation reaction, 250 successive cyclic voltammetry for the LDH/AuNPs/GCE and AuNPs/GCE were recorded in the potential range of −0.40 to 0.80 V at a scan rate of 50 mV s−1 in deaerated 0.50 M ethanol solution. Fig. 7A shows that the forward anodic peak current for ethanol oxidation at the LDH/AuNPs/GCE decreases gradually and reaches at 250th cycle to about 73% of its initial value. Whereas, it is observed that the anodic peak current at the AuNPs/GCE decreases sharply after 90th cycle and finally, reaches to about 20% of its value in the first cycle. The increase in I/IFirst during the initial cycles may be due to the catalyst activation or penetration of electrolyte into the catalyst film. Fig. 7B shows the chronoamperograms obtained at the constant potential of 0 V for 10 0 0 s in 0.50 M ethanol solution. As
Please cite this article as: M. Taei et al., Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.07.011
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Table 1 Comparison of the electrochemical performance parameters of the LDH/AuNPs/GCE with other modified electrodes for ethanol oxidation reaction. Electrode composition
Electrolyte (M) + ethanol (M)
Eonset (V)
EPf c (V)
JPf d (mA cm−2 )
Ref.
AuNPs/GCE LDH/AuNPs/GCE NiAlLDH/NMCCa MgFe-LDH/GCE Pt/CNTsb / Nix Mg1− x Al2 O4 /C
KOH (0.50)+ 0.50 KOH (0.50)+ 0.50 NaOH (0.1)+ 0.05 KOH (1.0)+ 1.0 KOH (0.5)+ 0.5
−0.25 −0.30 0.40 ∼0.4 −0.55
0.18 0.24 0.70 0.45 −0.10
16 45 7.32 172.3 58
This work This work [8] [19] [24]
a b c d
NMCC: nanoparticle modified carbon ceramic. CNTs: carbon nanotubes. EPf : forward peak potential. JPf : forward peak current with respect to the geometric area.
clearly seen, the LDH/AuNPs/GCE has higher steady state current density than the AuNPs/GCE. All of the results indicate that by the deposition of the Mg–Ca–Fe LDH on the AuNPs/GCE, the long-term stability of the electrode against the poisoning species increases. As shown by FT-IR spectra in Section 3.1, there are OHads species on the structure of the Mg–Ca–Fe LDH. It is well known that these OHads can react with CO-like intermediate species to produce CO2 or water soluble products, and thereby the Au active sites become free for further electrochemical reaction [23]. 4. Conclusion In this work, the effect of the Mg–Ca–Fe layered double hydroxide on the catalytic activity of AuNPs-modified GCE towards the ethanol oxidation in alkaline medium was studied using cyclic voltammetry and chronoamperometry. The results indicated that the Mg–Ca–Fe LDH enhances the electrocatalytic activity of the AuNPs by providing a high active surface area and increasing the ethanol concentration at AuNPs sites through adsorption of ethanol molecules. Moreover, the long-term stability studies show that the endurance of the AuNPs against poisoning effects of intermediate species is improved by loading the Mg–Ca–Fe LDH on the surface of the AuNPs/GCE. The investigation of the other LDHs for the increase of the catalytic performance of Au nanostructures can be the subject of future works.
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Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation M. Taei∗, E. Havakeshian, F Hasanpour, F. Abedi, M. Movahedi Chemistry Department, Payame Noor University, 19395-4697 Tehran, Islamic Republic of Iran
a r t i c l e
i n f o
Article history: Received 19 March 2016 Revised 16 June 2016 Accepted 11 July 2016 Available online xxx Keywords: Mg–Ca–Fe layered double hydroxide Au nanoparticles Ethanol oxidation Alkaline fuel cell
a b s t r a c t In this work, the influence of Mg–Ca–Fe layered double hydroxide (LDH) on the electrocatalytic activity of gold nanoparticles (AuNPs) electrodeposited on a glassy carbon electrode for ethanol oxidation in alkaline medium was investigated. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM) and cyclic voltammetry (CV) were used to characterize the synthesized Mg–Ca–Fe LDH powder and to investigate the electrochemical behaviors of the prepared electrodes. The results showed that the AuNPs/GCE exhibits higher current density for ethanol oxidation (about 2.9 times) and higher active surface area (about 2.5 times), after the deposition of the Mg–Ca–Fe LDH on its surface. It was also found that the ethanol oxidation reaction at the LDH/AuNPs/GCE is an adsorption-controlled process, while at the AuNPs/GCE it is a diffusion-controlled process. The investigation of the long-term stability of the electrodes using CV and chronoamperometry methods indicated that the Mg–Ca–Fe LDH by providing OHads species at the surface of the catalyst improves the endurance of the AuNPs against poisoning effects of intermediate species. Accordingly, the Mg–Ca–Fe LDH can be used to improve the electrocatalytic performance of supported AuNPs in the anode of alkaline ethanol fuel cells. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Ethanol oxidation has been the subject of many researches [1–3], because ethanol is considered as an attractive liquid fuel with a high energy density (8 kWh kg−1 ) for use in direct alcohol fuel cells (DAFCs) [4]. Moreover, ethanol can be produced in a large scale by the fermentation of biomass and has less toxicity than methanol. Pt and Pt-based catalysts have been extensively investigated as the electrocatalysts for the ethanol oxidation in direct ethanol fuel cells. However, some major problems such as the high price and limited supply of Pt metal, slow kinetics and facile poisoning have been limited the development of these fuel cells [5]. Therefore, the use of Pt-free catalysts with higher performance has been investigated in recent years. Among Pt-free catalysts, Au is considered as a promising substitute catalyst in alkaline medium, because Au is less expensive and more abundant on the earth than Pd and Pt metals. Also, it has been found that poisoning species are easily oxidized and removed from Au based-catalysts [6]. Nevertheless, since Au has weak adsorption properties, it does not easily catalyze
∗
Corresponding author. E-mail address: [email protected] (M. Taei).
the bond breaking or bond making steps in the alcohol oxidation [7]. Therefore, Au is usually modified with suitable compounds that catalyze the alcohol oxidation or/and have synergistic effects on its catalytic behaviors. Layered double hydroxides (LDHs) are a class of twodimensional nanostructured anionic clays and known as hydrotalcite-like compounds. Generally, these compounds contain the positively charged layers of edge-shared metal M (II) and M (III) hydroxide octahedral that neutralized by anions located in the interlayer spacing [8]. Desirable properties of LDHs such as low cost, high thermal stability and good catalytic activity make them interesting catalysts for use in fuel cells. It has been shown that LDH-noble metal nanocomposites exhibit a superior catalytic behavior [9,10]. In the case of LDH-AuNPs nanocomposites, they have exhibited the enhanced electrocatalytic activity for the oxidation of CO [11], methanol [12–14], glucose [15] and nitrite [16]. In this work, the electrocatalytic performance of a Mg–Ca– Fe LDH–AuNPs nanocomposite for ethanol oxidation in alkaline medium was investigated. The LDH/AuNPs/GCE is fabricated by electrodeposition of AuNPs on a glassy carbon electrode, and followed by Mg–Ca–Fe LDH coating. After charactering the electrode, the electrocatalytic activity of the LDH/AuNPs/GCE is investigated and compared with the AuNPs/GCE for ethanol oxidation in
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Please cite this article as: M. Taei et al., Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.07.011
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alkaline medium using cyclic voltammetry and chronoamperometry methods. 2. Experimental
The morphology of the synthesized Mg–Ca–Fe LDH sample is investigated using FE-SEM. As shown in Fig. 1C, the Mg–Ca–Fe LDH is plate-shape at a range of 30 to 100 nm in size. EDX analysis in Fig. 1D reveals that the sample is composed of Mg, Ca and Fe elements.
2.1. Materials and apparatus 3.2. Characterizations of the prepared electrodes All chemicals were of analytical grades and were purchased from Aldrich Chemicals (Milwaukee, USA). All solutions were prepared just prior to use with deionized and double distilled water. X-ray diffraction pattern of the sample was obtained by Holland Philips Xpert, X-ray diffractometer with Cu-Kα radiation. Fourier transformed infrared (FT-IR) spectrum of the sample in a pressed KBr matrix was recorded on a FT-IR, JASCO 4200 spectrometer in the range 40 0 0–40 0 cm−1 . A Metrohm instrument, Model 797 VA processor, or an Autolab potentiostat-galvanostat, Model PGSTAT302, were used to perform all electrochemical experiments. The conventional three-electrode cell containing a platinum wire counter electrode, Ag/AgCl (3.0 mol l–1 KCl) as a reference electrode and a modified or unmodified glassy carbon electrode (GCE) as a working electrode, was used for all the electrochemical experiments. The geometric surface area of the used GCE was 0.0314 cm2 . All the potentials reported in this work are vs. Ag/AgCl (3.0 moll–1 KCl). 2.2. Synthesis of Mg–Ca–Fe LDH The procedure used for the synthesis of the Mg–Ca–Fe LDH followed a co-precipitation method reported previously [17]. Briefly, 0.025 mol of CaCl2 . 2H2 O, 0.025 mol of MgCl2 and 0.02 mol of FeCl3 .10 H2 O were dissolved in 50 ml of deionized water. Then, the above solution was slowly added into the 100 ml of alkaline solution which containing 0.12 mol of NaOH under vigorous magnetic stirring. The suspension was aged under magnetic stirring for 18 h at 25 °C. The product was washed with deionized water several times and dried at 50 °C. 2.3. Electrode preparation After the polishing of a glassy carbon electrode (GCE) with 5 μm alumina slurry on a polishing cloth, the electrode was sonicated in a mixture of ethanol/distilled water solution (50% v/v) for 10 min. Afterwards, the AuNPs were electrodeposited on the GCE by applying 40 continuous cycles cyclic voltammetry in the potential range of −1.50 to 0 V with a scan rate of 50 mV s−1 . The used solution for the AuNPs electrodeposition was 4.0 × 10−3 mol l–1 HAuCl4 .3H2 O and 0.10 mol l–1 KNO3 . The synthesized Mg–Ca–Fe LDH powder was dispersed in distilled water (0.10 mg LDHs per 10 ml) using ultrasonic agitation until a relative stable suspension was obtained. Then, 5.0 μl of the LDH suspension was dropped on the electrode and allowed to dry in the air. 3. Result and discussion 3.1. Characterizations of the synthesized Mg–Ca–Fe LDH The Mg–Ca–Fe LDH sample was characterized by XRD pattern and FT-IR spectrum, as shown in Fig. 1A and B. The XRD pattern for the Mg–Ca–Fe LDH sample is in good agreement with that previously reported [17]. FT-IR spectra indicates that vibrations characteristic of the LDH structure, including the band of HOH bending in water molecule at 1550–1700 cm−1 , the vibrations of metal-O or metal-OH band at 500–750 cm−1 and vibration of M–O–M lattice at 428–433 cm−1 [18]. As shown in Fig. 1B, the broad band of OH vibration at 3431 cm−1 and the band vibrations at 729, 589 and 428 cm−1 were recorded by FT-IR spectrum of the Mg–Ca–Fe LDH.
Fig. 2A and B show the FE-SEM images obtained from the AuNPs/GCE and LDH/AuNPs/GCE surface, respectively. As clearly seen, AuNPs with and an average diameter of 50 nm cover nearly whole the GCE surface. After coating the surface with Mg–Ca–Fe LDH, many nanoparticles with a diameter of about 28 nm cover the surface (Fig. 2B). From the close examination of the Mg–Ca–Fe LDH sample in Fig. 1C, it is found that each LDH plate is composed of some connected nanoparticles with a size of about 30 nm. This LDH nanoparticles are separated from each other during dispersion in distilled water using ultrasonic agitation, and consequently, form a uniform layer of LDH nanoparticles on the AuNPs/GCE surface after deposition. The EDX data obtained from the surface of the AuNPs/GCE after coating the Mg–Ca–Fe LDH (Fig. 2C and D) present the existence of Mg, Ca, Fe, Au and oxygen on the surface with a (Mg + Ca): Al atomic ratio of 1.92 and an Au: LDH atomic ratio of 1.31. This demonstrates the successful deposition of the Mg–Ca–Fe LDH and AuNPs on the GCE surface. Moreover, the molecular composition of Mg–Ca–Fe LDH was verified by atomic absorption spectroscopy after acid digestion. The results showed that the ratio of Mg:Fe:Ca are close to unity. The electrochemical behaviors of the prepared electrodes in alkaline medium were investigated using cyclic voltammetry. As seen in Fig. 3, GCE does not exhibit no distinctive peak in the potential range of −0.40 to 0.80 V (curve a), while two anodic peaks at about 0.28 and 0.48 V and two cathodic peaks at about −0.10 and 0.10 V appear after the electrodeposition of AuNPs on the GCE (curve c). These anodic and cathodic peaks correspond to the formation and reduction of Au oxides and hydroxides, respectively. After the deposition of the Mg–Ca–Fe LDH onto the Au/GCE surface, a broad anodic peak from −0.10 to 0.25 V appears. This peak is due to the LDH oxidation, since the similar increase is also seen at the LDH/GCE (curve b in Fig. 3), as compared with the GCE. A cathodic peak at about 0.05 V is also observed in curve b, which is related to the conversion of Fe2+ /Fe3+ associated with OH− [19]. 3.3. Ethanol electrooxidation The electrocatalytic activity of the prepared electrodes towards ethanol oxidation was studied using cyclic voltammetry in 0.50 M KOH solution containing 0.50 M ethanol with a scan rate of 50 mV s−1 . As shown in Fig. 4 curve b, the LDH/GCE presents two anodic peaks at about 0.28 and 0.04 V in the forward and backward scans, respectively. The anodic peak in the forward scan is related to the oxidation of freshly chemisorbed ethanol species and the anodic peak in the backward scan is corresponding to the oxidative removal of CO and other carbonaceous species remianed on the surface in the forward scan [4,20]. The ethanol oxidation at the Au/GCE (curve c) starts at about −0.25 V and reaches to the maximum current density at the peak potential of 0.18 V. The peak observed at 0.47 V is related to the oxidation of the Au species, according to AuOHads + OH−→AuO + H2 O + e [21]. The comparison of voltammogarams b and c shows that although the ethanol oxidation at the LDH/GCE and Au/GCE occurs in the same potential region, the Au/GCE exhibits 0.2 V lower onset potential (Eonset ) and higher current density (about 180 times) in comparison with the LDH/GCE, indicating higher catalytic activity of the Au/GCE. As clearly seen in Fig. 4, the deposition of the Mg–Ca–Fe LDH onto the Au/GCE surface results in a decrease in Eonset and
Please cite this article as: M. Taei et al., Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.07.011
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A
B
C
D
Fig. 1. (A) XRD pattern, (B) FT-IR spectra, (C) FE-SEM image, and (D) EDX analysis of the Mg–Ca–Fe LDH sample.
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D
Fig. 2. FE-SEM images of the (A) AuNPs/GCE, (B) LDH/AuNPs/GCE, and (C, D) EDX analysis of the LDH/AuNPs/GCE surface.
an increase in the current density of the ethanol oxidation, indicating the LDH/AuNPs/GCE has a higher catalytic activity than the Au/GCE. Generally, the real surface area (Sreal ) of a catalyst influences its catalytic activity. Therefore, Sreal for the AuNPs and LDH/AuNPs modified-GCE was obtained by applying cyclic voltammetry at different scan rates (ν ) in 5.0 mM [Fe(CN)6 ]3-/4− solution and plotting Ip versus ν 1/2 , as shown in Fig. 5. Finally, Sreal was calculated using Randles–Sevcik equation, IP = 2.69 × 105 n3/2AD1/2 C ν 1/2, where n is the number of electrons transferred in the redox reaction (n = 1), A is the electrode surface area, D is the dif-
fusion coefficient, C is the reactant concentration, IP refers to the redox peak current and ν is the scan rate. Accordingly, Sreal (provided using both of the AuNPs and LDH) is obtained 0.032 cm2 for the AuNPs/GCE and 0.079 cm2 for the LDH/AuNPs/GCE. The comparison of the obtained values indicates that the Mg–Ca–Fe LDH increases the active surface area of the electrode. As shown in Fig. 4, ethanol electrooxidation is done at both LDH and AuNPs sites in the same potential ranges. Therefore, AuNPs and Mg–Ca– Fe LDH are electroactive material for ethanol oxidation. However, as seen in the Fig. 4, the current density of ethanol oxidation on LDH/GCE is much lower than that on AuNPs. Therefore, the LDH
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Fig. 3. Cyclic voltammograms of (a) GCE, (b) LDH/GCE, (c) AuNPs/GCE, and (d) LDH/AuNPs/GCE in deaerated 0.50 M KOH at a scan rate of 50 mV s−1 .
Fig. 4. Cyclic voltammograms of (a) GCE, (b) LDH/GCE, (c) AuNPs/GCE, and (d) LDH/AuNPs/GCE in deaerated 0.50 M KOH and 0.50 M ethanol at a scan rate of 50 mV s−1 .
Fig. 5. Ip versus ν 1/2 for (A) AuNPs/GCE, and (B) LDH/AuNPs/GCE in 5.0 mM [Fe(CN)6 ]3 − /4− .
surface area can have a low effect. To omit the effect of Sreal on the catalytic activity, the current peak was normalized by dividing IPf to Sreal . The obtained values were 0.0157 and 0.0179 A cm−2 for the AuNPs/GCE and LDH/AuNPs/GCE, respectively. This indicates that the Mg–Ca–Fe LDH has synergistic effects. To find the mechanism of the ethanol oxidation at the electrodes, cyclic voltammetry was performed at different scan rates in 0.50 M ethanol solution. Generally, using the dependence of peak current Ip on the scan rate (υ ), the process is distinguished to be
weather under diffusion or adsorption controlled currents. when Ip changes linearly with υ , the process is under adsorption control, whereas if Ip changes linearly with υ 1/2 , the process is under diffusion control. For kinetic currents, Ip is independent of υ (i.e. Ip = f(υ )) [22]. Accordingly, the linear plot of the peak current density of ethanol oxidation versus υ 1/2 for the LDH/AuNPs/GCE (Fig. 6A) and versus υ for the AuNPs/GCE (Fig. 6B) indicates that the ethanol oxidation at the LDH/AuNPs/GCE is under adsorption control, while it is under diffusion control for the AuNPs/GCE. Therefore, the
Please cite this article as: M. Taei et al., Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.07.011
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Fig. 6. (A) Cyclic voltammograms at different scan rates and its related plot of the peak current density of ethanol oxidation versus the square root of the scan rate for the LDH/AuNPs/GCE, and (B) the plot of the peak current density of ethanol oxidation versus the scan rate for the AuNPs/GCE.
Fig. 7. (A) The normalized forward anodic peak current of ethanol oxidation over 250 cycles, and (B) chronoamperograms of (a) AuNPs/GCE and (b) LDH/AuNPs/GCE at 0 V in 0.50 M KOH and 0.50 M ethanol solution.
Mg–Ca–Fe LDH by providing the higher active surface area increases the ethanol concentration at the AuNPs centers through the adsorption of ethanol molecules and thereby, enhances the catalytic activity of the LDH/AuNPs/GCE. To compare the electrochemical performance of the proposed electrode (LDH/AuNPs/GCE) and LDH composite-based electrodes studied for ethanol oxidation in other literature, the obtained parameters such as Eonset , EPf and JPf were listed in Table 1. As clearly seen, the LDH/AuNPs/GCE has a lower Eonset and higher JPf , and consequently, higher catalytic activity than the NiAl-LDH/NMCC. In comparison with MgFe-LDH/GCE, the LDH/AuNPs/GCE exhibits a lower JPf , but 0.70 V lower Eonset . The Pt/CNTs/ Nix Mg1− x Al2 O4 /C has a higher catalytic activity in the case of both JPf and Eonset . However, it is remarkable that the expensive Pt has been used in the preparation of this catalyst. Moreover, the way employed for the preparation of the LDH/AuNPs/GCE in this work is simpler than that for the preparation of the Pt/CNTs/ Nix Mg1− x Al2 O4 /C.
3.4. Stability studies To investigate the endurance of the LDH/AuNPs/GCE against poisoning effects of intermediate species that are formed during the ethanol oxidation reaction, 250 successive cyclic voltammetry for the LDH/AuNPs/GCE and AuNPs/GCE were recorded in the potential range of −0.40 to 0.80 V at a scan rate of 50 mV s−1 in deaerated 0.50 M ethanol solution. Fig. 7A shows that the forward anodic peak current for ethanol oxidation at the LDH/AuNPs/GCE decreases gradually and reaches at 250th cycle to about 73% of its initial value. Whereas, it is observed that the anodic peak current at the AuNPs/GCE decreases sharply after 90th cycle and finally, reaches to about 20% of its value in the first cycle. The increase in I/IFirst during the initial cycles may be due to the catalyst activation or penetration of electrolyte into the catalyst film. Fig. 7B shows the chronoamperograms obtained at the constant potential of 0 V for 10 0 0 s in 0.50 M ethanol solution. As
Please cite this article as: M. Taei et al., Mg–Ca–Fe layered double hydroxide–gold nanoparticles as an efficient electrocatalyst for ethanol oxidation, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.07.011
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Table 1 Comparison of the electrochemical performance parameters of the LDH/AuNPs/GCE with other modified electrodes for ethanol oxidation reaction. Electrode composition
Electrolyte (M) + ethanol (M)
Eonset (V)
EPf c (V)
JPf d (mA cm−2 )
Ref.
AuNPs/GCE LDH/AuNPs/GCE NiAlLDH/NMCCa MgFe-LDH/GCE Pt/CNTsb / Nix Mg1− x Al2 O4 /C
KOH (0.50)+ 0.50 KOH (0.50)+ 0.50 NaOH (0.1)+ 0.05 KOH (1.0)+ 1.0 KOH (0.5)+ 0.5
−0.25 −0.30 0.40 ∼0.4 −0.55
0.18 0.24 0.70 0.45 −0.10
16 45 7.32 172.3 58
This work This work [8] [19] [24]
a b c d
NMCC: nanoparticle modified carbon ceramic. CNTs: carbon nanotubes. EPf : forward peak potential. JPf : forward peak current with respect to the geometric area.
clearly seen, the LDH/AuNPs/GCE has higher steady state current density than the AuNPs/GCE. All of the results indicate that by the deposition of the Mg–Ca–Fe LDH on the AuNPs/GCE, the long-term stability of the electrode against the poisoning species increases. As shown by FT-IR spectra in Section 3.1, there are OHads species on the structure of the Mg–Ca–Fe LDH. It is well known that these OHads can react with CO-like intermediate species to produce CO2 or water soluble products, and thereby the Au active sites become free for further electrochemical reaction [23]. 4. Conclusion In this work, the effect of the Mg–Ca–Fe layered double hydroxide on the catalytic activity of AuNPs-modified GCE towards the ethanol oxidation in alkaline medium was studied using cyclic voltammetry and chronoamperometry. The results indicated that the Mg–Ca–Fe LDH enhances the electrocatalytic activity of the AuNPs by providing a high active surface area and increasing the ethanol concentration at AuNPs sites through adsorption of ethanol molecules. Moreover, the long-term stability studies show that the endurance of the AuNPs against poisoning effects of intermediate species is improved by loading the Mg–Ca–Fe LDH on the surface of the AuNPs/GCE. The investigation of the other LDHs for the increase of the catalytic performance of Au nanostructures can be the subject of future works.
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