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Front.Chem. Res, 2019, Vol: 2, Issue: 1, pages: 38-41.
Research article
Frontiers in chemical research 2020-07-16 doi: 10.22034/fcr.2020.125028.1018
*Corresponding author: M. Roushani, Tel/fax: +98 843 2227022, Email: [email protected] and
Frontiers in Chemical Research
Electrocatalytic oxidation of ethanol on the copper, carbon paste and glassy
carbon electrode modified with Cu-BDC MOF
Mahmoud Roushani*, Behruz Zare Dizajdizi
Department of Chemistry, Faculty of Science, Ilam University, P.O. Box, 69315516, Ilam, Iran.
Received: 20 April 2020
Accepted: 29 April 2020
Published online: 16 July 2020
Abstract Fuel cells are promising alternatives in power generation. Direct
ethanol fuel cells (DEFCs) offer significant advantages due to the comparatively
safe handling, non-toxicity and renewability of ethanol as well as its high power
density. Development of the efficient catalysts for ethanol electroxidation has
attracted great attention and represents one of the major challenges in
electrocatalysis. This work investigates ethanol electrooxidation on Cu-BDC MOF catalyst-modified electrodes. The catalysts are characterized by X-ray
diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive
spectroscopy (EDS), cyclic voltammetry (CV) and chronoamperometry (CA)
techniques. The Cu-BDC MOF catalyst shows significantly improved catalytic
activity and high durability for ethanol electrooxidation.
Keywords: Ethanol; Cu-BDC MOF; Electroxidation; Chronoamperometry.
1. Introduction
Over the recent decades it has become evident that, on the one hand,
the fossil fuel reserves would finally be exhausted and, on the other,
by using fossil fuels the concentration of CO2 in the atmosphere has
been exceeded. Thus, during past decades, great attentions have
been paid to the development or improvement of new energy
sources. Direct alcohol fuel cells are promising alternatives for
portable power resources [1-3]. Among the various liquid fuels so
far being attempted for the use in direct oxidation fuel cells
(DOFCs), ethanol has been extensively studied because of its
relatively high abundance, comparatively safe handling and low
toxicity than other alcohols and also having higher energy density
[4-9].
The highly catalytic activity and low cost of the materials used to
fabricate the fuel cells are some of the several challenges which
must be overcome before the commercialization of fuel cells [10-
13]. Metal-organic frameworks (MOFs) have attracted intensive
interests because of their unique structural properties, such as high
surface areas, tunable pore sizes, and open metal sites, which enable
them to have potential applications in gas storage, catalysis,
sensors, magnetism, and luminescence [14–19]. Recently, few
studies have demonstrated the promised application of MOFs for
the electrochemical sensing [20-22]. By contrast, little effort has
been exerted towards the utilization of MOFs as electrocatalysts.
In this paper, we fabricated new modified electrodes based on
MOFs as the modifier. The constructed electrodes were applied for
electro-catalytic oxidation of ethanol in alkaline solution by cyclic
voltammetry and chronoamperometry. In addition, the catalytic
activity of the modified electrodes is discussed and compared to the
previously reported modified electrodes for electro-catalytic
oxidation of ethanol; the MOF modified electrodes possessed the
advantages of inexpensive reagents, simple operation, and
moderate linear concentration range.
2. Materials and methods
2.1. Materials
Copper nitrate trihydrate, terephthalic acid, ethanol and N,N
dimethylformamide were all purchased from Sigma‐Aldrich and
other regents were obtained from Merck Company. All the used
chemicals were of analytical grade and were used without further
purifications. Aqueous solutions were prepared with doubly
distilled water.
2.2 Instrumentation and electrochemical measurements Electrochemical measurements were carried out in a conventional
three electrode assembly cell incorporating the modified and
unmodified electrode as the working electrode, an Ag/AgCl
electrode as reference electrode and Pt wire as counter electrode.
An electrocatalytic activity of the catalyst was characterized by
cyclic voltammetry (CV) and chronoamperometry (CA)
techniques.
2.3 Preparation of MOF electrocatalysts
Cu-BDC MOF was synthesized according to the literature reported
method [23]. In short, equimolar quantities of copper nitrate
trihydrate (7.5 mmol) and terephthalic acid (7.5 mmol) in 150 mL
DMF were used. This solution was placed in a Teflon-lined
autoclave in an oven at 110 °C for 36 h. Then, Teflon-lined
autoclave cooled slowly to room temperature. It was observed that
the blue precipitated product was washed one time with DMF.
Afterwards, in order to remove the organic species trapped within
the pores, the sample was washed with excess hot methanol (70 oC)
for 4 h, and then it was filtered and dried at 60 oC overnight. The
Cu-BDC MOF was then characterized using a variety of different
techniques.
2.4 Preparation of the modified electrodes
In the case of glassy carbon (GC) and copper electrode (CE), to
perform the electrochemical measurements, the surface of the
electrodes was polished to a mirror finish using 0.05 μm Al2O3
suspensions before each measurement. The catalyst ink was
prepared by mixing 1.0 mg of the MOF powder with 2.0 ml
deionized water followed by an ultrasonic treatment for 10 min. In
the following, 10 µl of the homogeneous mixture was drop-coated
on the surface of the freshly polished electrode and, then, allowed
to dry at room temperature. To prepare carbon paste modified
electrode, 80.0 mg graphite powder, 50.0 µl paraffin oil and 3.0 mg
MOF powder were mixed. Afterwards, a plastic tube with 3.0 cm
depth and a given surface area was filled with the prepared paste.
The filled tube was connected to the copper wire for electrical
connection. Before each measurement, the surface of the modified
electrode was smoothed on a paper sheet to produce a smooth layer.
2.2 Physical characterizations
2.2.1 X-ray diffraction (XRD)
Roushani et al. 39
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Powder X-ray diffraction (XRD) analysis was performed with the
D/max-RB diffractometer (made in Japan) using a Cu Ka X-ray
source operating at 45 kV and 100 mA, scanning between 10 ̊and
90 ̊ at a rate of 40̊ min-1.
The morphology and microstructure of the synthesized MOF was
investigated by a SEM (Jeol JEM-1230, 100 kV) equipped with an
energy-dispersive spectroscopy (EDS).
3. Results and discussion
3.1. Physicochemical characterization
3.1.1. SEM/EDX
The morphologies of the synthesized Cu-BDC were examined by
scanning electron microscopy (SEM). A SEM image (Fig. 1A)
reveals that the resulting MOF takes a cubic morphology. The
composition of the prepared Cu-BDC MOF was investigated by
EDX analysis in Fig. 1B. An EDX result showed elemental peak
for copper at ~8.0 keV, which proved the existence of the copper in
the synthesized MOF.
Figure 1. (A) SEM and (B) EDX image of Cu-BDC MOF.
3.1.2. X-ray diffraction (XRD)
To further verify the structure of the synthesized Cu-BDC MOF,
the powder XRD pattern of the as-prepared MOF is presented in
Fig. 2B. The peaks in the powder diffraction scans were compared
against the predicted peaks from the literature. The main diffraction
peaks of the as-synthesized Cu-BDC were consistent with those
previously reported in the literature [25].
In order to confirm the formation of Cu-BDC MOF, the FT–IR
experiment was carried out. Fig. 2A shows the FT–IR spectra of the
synthesized MOF powder. The absorption peaks at about 1617,
1508 and 1395 cm−1 could be assigned to the characteristic
vibrations of C=O, and the peaks at about 674 cm−1 belonged to the
stretching vibrations of the Cu–O [24]. In addition, the broad peak
at around 3435 cm−1 in the FT–IR spectrum of Cu-BDC MOF could
be assigned to the O–H vibration of the intercalated water [26].
Figure 2. (A) FT-IR spectra of the Cu-BDC MOF. (B) XRD
pattern of as-synthesized Cu-BDC MOF.
3.2. Cyclic voltammetry
The electro-catalytic performance of the Cu-BDC MOF modified
electrodes for the oxidation of ethanol was investigated by cyclic
voltammetry. Fig. 3A, B and C illustrate the CVs of 1.0 mM ethanol
at the bare (a) and modified (b) glassy carbon, copper and carbon
paste electrodes, respectively, in 0.1 M NaOH solution at 50 mVs−1.
From Fig. 3, it can be seen that at the modified electrodes, the
anodic current of ethanol oxidation which has been greatly
enhanced indicate that the oxidation of ethanol could be catalyzed
at Cu-BDC MOF modified electrodes. This proves that the Cu-BDC
MOF layer which is formed on the surface of the electrode bears
the main role in the electro-catalytic oxidation of ethanol. The
excellent performance of Cu-BDC MOF modified electrode might
be contributed to the following reason. The three-dimensional
network could enlarge the electro-active surface area, which
enhance the electrochemical responses through adsorbing more
ethanol on the modified electrode surface. As reported in the
previous works [27], the following mechanism is proposed for the
oxidation of ethanol on the surface of the modified electrodes: An
ethanol molecule is first adsorbed on active sites and, then, reacts
with (OH)ads species. Besides, the resultant (CH3CO)ads is likely
to further react with (OH)ads to produce CH3COOH, which exits in
the form of CH3COO− ions in alkaline solution.
3.3. Effect of the amount of Cu-BDC-MOF
40 Roushani et al.
2020-07-16 Frontiers in chemical research
The amount of Cu-BDC MOF plays an important role in the
fabrication of the modified electrodes. In order to examine this
parameter, different amounts of the catalyst were used for the
modification of electrodes.
Figure 3. (A) CV curves of the Cu-BDC/CE, (B) Cu-BDC/ CPE
and (C) Cu-BDC/GCE electrodes in alkaline media (0.1 M NaOH)
in the absence (a) and presence (b) of 1.0 mM EtOH at scan rate:
50 mVs-1.
CuII-BDC + CH3CH2OHbulk → CuII-BDC-(H3CH2OH)ads (1)
CuII-BDC-(H3CH2OH)ads → CuIII-BDC-(H3CH2OH)ads + e- (2)
CuIII-BDC-(H3CH2OH)ads + OH- → CuII-BDC-(H3CH2O)ads + H2O (3)
CuII-BDC-(H3CH2O)ads → CuIII-BDC-(H3CH2O)ads + e- (4)
CuIII-BDC-(H3CH2O)ads + OH- → CuII-BDC-(H3CHO)ads + H2O (5)
CuII-BDC-(H3CHO)ads → CuIII-BDC-(H3CHO)ads + e (6)
CuIII-BDC-(H3CHO)ads + OH- → CuII-BDC-(H3CO)ads + H2O (7)
CuII-BDC-(H3CO)ads → CuIII-BDC-(H3CO)ads + e- (8)
CuIII-BDC-(H3CO)ads + OH- → CuII-BDC-(H3CO)ads + H2O (9)
CuII-BDC-(H3CO)ads + OH- → CuIII-BDC-(H3COOH)ads + e- (10)
CuIII-BDC-(H3COOH)ads + OH- → CuII-BDC + H3COO- + H2O (11)
The dependency relationship between the amount of the catalyst
and the peak current of ethanol showed that the peak-current
achieved a maximum value when the amount of catalyst was 1.0
mg. Afterwards, the peak-current decreased with the increase in the
catalyst amount, which could be attributed to the fact that the
modification film was so thick which performed higher electrical
resistance for the electrons transfer. In other words, a too thick or
too thin film was adverse for the electrochemical response.
Therefore, 1.0 mg was chosen as the appropriate amount of catalyst
to achieve the highest sensitivity.
3.4. Chronoamperometry
The chronoamperometric i-t curves of Cu-BDC MOF were
obtained at a constant potential of 0.65 V for 1000 s in an ethanol
alkaline medium (0.1M NaOH, 0.01 M EtOH). The obtained results
of the chronoamperometric test indicate that the modification of
electrodes by the synthesized catalyst not only enhances the
catalytic activity, but also improves the stability of the modified
electrodes. In Fig. 4, copper modified electrode because of the fact
that the strong interaction between electrode and catalyst shows the
highest current density at 1000 s in the chronoamperometry, carbon
paste modified electrode takes the second place and glassy carbon
modified electrode shows the worst. Initial rapid decreases in
current density were observed for all of the modified electrodes,
which might be due to the formation of intermediate species, during
ethanol electrooxidation reaction.
Figure 4. Chronoamperometry curves of Cu-BDC/CE (a), Cu-
BDC/CPE (b) and Cu-BDC/GCE (c) electrodes in 1.0 mM EtOH
in the presence of 0.1 M NaOH at +0.7 V.
Roushani et al. 41
Frontiers in chemical research 2020-07-16
4. Conclusions
In summary, the amount of the Cu-BDC MOF can influence the
electrochemical properties of catalysts for EOR in the alkaline
solution. The synthesized Cu-BDC MOF enhances the catalytic
activity as well as the stability of the modified electrodes for
electroxidation of ethanol.
References
[1] V.M. Barragn, A. Heinzel, J. Power Sources, 104 (2002) 66.
[2] H. Tang, S. Wang, M. Pan, S.P. Jiang, Y. Ruan, Electrochim. Acta, 52 (2007)
3714.
[3] F. Kadirgan, S. Beyhan, T. Atilan, Int. J. Hydrogen Energy, 34 (2009) 4312.
[4] R. Yue, H. Wang, D. Bin, J. Xu, Y. Du, W. Lu, J. Guo, J. Mater. Chem. A, 3
(2015) 1077.
[5] S.Y. Shen, T.S. Zhao, J.B. Xu, Int. J. Hydrogen Energy, 35 (2010) 1291.
[6] S. Abdullah, S.K. Kamarudin, U.A. Hasran, M.S. Masdar, W.R.W. Daud, J.
Power Sources, 262 (2014) 401.
[7] E. Antolini, J Power Sources, 170 (2007) 1.
[8] W. Wang, Y. Yang, Y. Liu, Z. Zhang, W. Dong, Z. Lei, J. Power Sources, 273
(2015) 631.
[9] M. SoledadUreta-Zañartu, C. Mascayano, C. Gutiérrez, Electrochim. Acta,
165 (2015) 232.
[10] C. Xu, P.K. Shen, X. Ji, R. Zeng, Y. Liu, Electrochem. Commun., 7 (2005)
1305.
[11] C. Lamy, S. Rousseau, E. Belgsir, C. Coutanceau, J.M. Léger, Electrochim.
Acta, 49 (2004) 3901.
[12] C. Xu, R. Zeng, P.K. Shen, Z. Wei, Electrochim. Acta, 51 (2005) 1031.
[13] X. Zhang, H. Zhu, Z. Guo, Y. Wei, F. Wang, Int. J. hydrogen energy, 35
(2010) 8841.
[14] Y. Fu, D. Sun, Y. Chen, R. Huang, Z. Ding, X. Fu, Z. Li, Angew. Chem.,
124 (2012) 3420.
[15] L.T.L. Nguyen, T.T. Nguyen, K.D. Nguyen, N.T.S. Phan, Appl. Catal. A,
425–426 (2012) 44.
[16] M. Jahan, Q. Bao, K.P. Loh, JACS, 134 (2012) 6707.
[17] L.L. Wen, F. Wang, J. Feng, K.L. Lv, C.G. Wang, D.F. Li, Cryst. Growth &
Des., 9 (2009) 3581.
[18] F. Luo, Y.X. Che, J.M. Zheng, Cryst. Growth & Des., 9 (2009) 1066.
[19] B. Zheng, R. Yun, J. Bai, Z. Lu, L. Du, Y. Li, Inorg. Chem., 52 (2013) 2823.
[20] B.V. Harbuzaru, A. Corma, F. Rey, J. Jord, D. Ananias, L. Carlos, J. Rocha,
Angew. Chem., 48 (2009) 6476.
[21] Y. Wang; Y. Wu; J. Xi, H. Ge; X. Hu, Analyst, 138 (2013) 5113.
[22] J. Zhao; C. Wei; H. Pang, Part Part Syst Charact, 32 (2014) 429.
[23] P. Puthiaraj, P. Suresh, K. Pitchumani. Green Chem., 16 (2014) 2865.
[24] K. Nakamoto, A.E. Martell, J. Chem. Physic, 32 (1960) 588.
[25] C.G. Carson, K. Hardcastle, J. Schwartz, X. Liu, C. Hoffmann, R.A.
Gerhardt, R. Tannenbaum. Eur. J. Inorg. Chem., 2009 (2009) 233843.
[26] F. Wang, H. Guo, Y. Chai, Y. Li, C. Liu, Microporous and Mesoporous
Mater., 173 (2013) 181.
[27] G. Karim-Nezhad, B. Zare-Dizajdizi, P. Seye-Dorraji, Catal. Commun., 12
(2011) 906.
How to cite this manuscript: Mahmoud Roushani*, Behruz Zare Dizajdizi. Electrocatalytic
oxidation of ethanol on the copper, carbon paste and glassy carbon electrode modified with Cu-
BDC MOF. Frontiers in Chemical Research, 2020, 1, 38-41. doi: 10.22034/fcr.2020.125028.1018