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: mahmoudroushani@yahoo.com and

m.roushani@ilam.ac.ir .

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

Frontiers in chemical research 2020-07-16

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.

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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