Effect of pore geometry on the electrocatalytic performance of nickel cobaltite/ carbon xerogel nanocomposite for methanol oxidation

Article abstract of DOI:10.1016/j.electacta.2017.10.152

A new nickel cobaltite/carbon xerogel nanocomposite (NiCo₂O₄/CX) is obtained by preparing carbon xerogel (CX) by using sol-gel technique followed by growing the NiCo₂O₄ nanoneedles over CX using hydrothermal method. The used characterization techniques are XRD, HRTEM, FESEM and surface area measurements. The physicochemical properties of NiCo₂O₄/CX show face centered cubic nanoneedles -like structures containing different lengths and widths. In addition, BET analysis clarify that NiCo₂O₄/CX nanocomposite is mesoporous with uniform cylindrical pore geometry with two modular systems compared to narrow necks ink-bottle pore geometry for pristine NiCo₂O₄. Electrocatalytic activity and stability of glassy carbon modified electrode with NiCo₂O₄/CX towards methanol electro-oxidation in basic medium are studied using cyclic voltammetry and chronoamperometry techniques. Data shows, the NiCo₂O₄/CX modified electrode gives a catalytic current density of 98 mA cm^?2 at 0.29 V with a good stability of 90.6% current retention after 100 repetitive cycles compared to 37.8 mA cm^?2 at 0.38 V for pristine NiCo₂O₄. The highly electrocatalytic activity of NiCo₂O₄/CX modified electrode is correlated to the synergistic effects between the pore geometry and accessible porosity.

Full text of DOI:10.1016/j.electacta.2017.10.152

Accepted Manuscript
Effect of pore geometry on the electrocatalytic performance of nickel cobaltite/ carbon
xerogel nanocomposite for methanol oxidation
Mohamed M. EL-Deeb, Waleed M.A. El Rouby, Abdalla Abdelwahab, Ahmed A.
Farghali
PII:
S0013-4686(17)32274-0
10.1016/j.electacta.2017.10.152
EA 30538
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 8 August 2017
Revised Date: 11 October 2017
Accepted Date: 23 October 2017
Please cite this article as: M.M. EL-Deeb, W.M.A. El Rouby, A. Abdelwahab, A.A. Farghali, Effect of
pore geometry on the electrocatalytic performance of nickel cobaltite/ carbon xerogel nanocomposite for
methanol oxidation, Electrochimica Acta (2017), doi: 10.1016/j.electacta.2017.10.152.
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ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Effect of pore geometry on the electrocatalytic performance of nickel cobaltite/ carbon
xerogel nanocomposite for methanol oxidation
Mohamed M. EL-Deeb1,2, Waleed M. A. El Rouby3, Abdalla Abdelwahab3 and Ahmed A.
Farghali3*
1. Chemistry Department, Faculty of Science, Beni-Suef University, 62511 Beni-Suef, Egypt.
2. Chemistry Department, Faculty of Science, Ha’il University, 81451 Hail, P.O. Box 2440,
KSA.
3. Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for
Advanced Science (PSAS), Beni-Suef University, 62511 Beni-Suef, Egypt.
* Corresponding Author: Ahmed A. Farghali
E-mail: d_farghali@yahoo.com
Tel/Fax: +2 082 2244135
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Abstract
A new nickel cobaltite/carbon xerogel nanocomposite (NiCo2O4/CX) is obtained by
preparing carbon xerogel (CX) by using sol-gel technique followed by growing the NiCo2O4
nanoneedles over CX using hydrothermal method. The used characterization techniques are
XRD, HRTEM, FESEM and surface area measurements. The physicochemical properties of
NiCo2O4/CX show face centered cubic nanoneedles -like structures containing different lengths
and widths. In addition, BET analysis clarify that NiCo2O4/CX nanocomposite is mesoporous
with uniform cylindrical pore geometry with two modular systems compared to narrow necks
ink-bottle pore geometry for pristine NiCo2O4. Electrocatalytic activity and stability of glassy
carbon modified electrode with NiCo2O4/CX towards methanol electro-oxidation in basic
medium are studied using cyclic voltammetry and chronoamperometry techniques. Data shows,
the NiCo2O4/CX modified electrode gives a catalytic current density of 98 mA.cm-2 at 0.29 V
with a good stability of 90.6 % current retention after 100 repetitive cycles compared to 37.8
mA.cm-2 at 0.38 V for pristine NiCo2O4. The highly electrocatalytic activity of NiCo2O4/CX
modified electrode is correlated to the synergistic effects between the pore geometry and
accessible porosity.
Keywords: mesoporous NiCo2O4/CX; methanol electro-oxidation; electrocatalyst;
physicochemical properties
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1. Introduction
Electrocatalytic oxidation of alcohols is considered as one of the most important reactions
in drugs, dyes and fine chemicals as well as the potential fuel reactions in fuel cells [1-3]. Direct
methanol fuel cells (DMFCs) have recent interest as most important environmentally friendly
power sources for both mobile and stationary electronic devices [4, 5]. Alkaline DMFCs are
considered as more interested than acidic ones due to these potential uses of lower cost non
platinum catalysis to realize higher methanol oxidation as well as the less corrosive alkaline
environment enhances these longevity [6, 7]. Transition bimetallic oxides are used in the
magnetism, electronics, catalysis, sensors, energy storage and conversion as a result of the
synergistic enhancement in the catalytic reactions for electrochemical energy storage and
conversation applications [8-13]. Stable, electrochemically active and highly electrical
conductive NiCo2O4 is considered as one of the most interesting mixed valence transition metal
oxide. It has the spinal structure in which nickel occupies the octahedral site and cobalt is
distributed over both octahedral and tetrahedral sites [13, 14]. It is also used as electrode material
for a wide range of applications for oxygen reduction, hydrogen peroxide reduction and
methanol oxidation [15-22].
Direct modification of metal oxides on inert electrodes usually leads to their aggregation,
which considerably reduce their electrocatalysis and electrochemical performance. Combination
of metal oxides with conductive substrate materials, which mainly contain carbon materials, can
effectively overcome this problem. Among these materials, grapheme (Gr) and carbon xerogel
(CX) are commonly used. CX has excellent property such as high surface area, high porosity,
controllable pore size, high density, and high conductivity. It can be prepared in different forms
as well as their structural properties can be controlled according to the synthesis conditions [23-
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25]. Therefore, CX can be used as support in a wide range of applications such as catalysis [26,
27], electrocatalysis [28], adsorption [29, 30], energy storage [31-33] and biosensor [34].
Li et. al. [34] studied the CX/ZnO composite as an electrochemical biosensor, and reported
the higher electrocatalysis toward catecol with high sensitivity and selectivity. Pd/Ag bimetallic
catalysts supported on CX with various pore size were studied as selective catalysts for
hydrodechlorination of 1,2-dichloroethane into ethylene [26]. The data showed, the effectiveness
of the catalyst supported on the CX with 10 nm pores decreased with the pellet size whereas no
decrease was observed for the macroporous support (60 - 70 nm).
NiCo2O4 show much better electrocatalytic behavior for both CH3OH electro-oxidation and
H2O2 electroreduction than those of NiO and Co3O4. The electrocatalytic activity of NiCo2O4
electrode is mainly attributed to its rich binary electroactive sites of Ni and Co species, high
intrinsic electron conductivity and superior porous nanostructures [35]. Modification of NiCo2O4
with reduced graphene as a supporting material was investigated in methanol electro-oxidation
[36]. The data exhibits higher methanol oxidation current density as well as good stability than
NiCo2O4 and Pt/C electrocatalysts. Yu et al. [37] demonstrated higher methanol electro-
oxidation activity and good catalytic stability of mesoporous NiCo2O4 nanoneedles grown on 3D
graphene-nickel foam by the chemical vapor deposition method. NiCo2O4- reduced graphene
oxide (rGO) shows good catalytic activity in both electrochemical methanol and water oxidations
as demonstrated from its highly oxidation peak currents and smaller onset potential than the
individual NiCo2O4 and rGO electrodes due to the synergistic effects between NiCo2O4 and rGO
[38].
As far as we know, the nickel cobaltite/carbon xerogel nanocomposite (NiCo2O4/CX)
have never been used as electrocatalyst in methanol electro-oxidation in basic medium.
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Therefore, the goal of our work is to prepare and characterize a new NiCo2O4/CX nano
composite as well as investigating its modification to glassy carbon electrode as an
electrocatalyst towards methanol electro-oxidation in basic medium using cyclic voltammetry
and chronoamperometric techniques. Also, the electrocatalytic activity of the modified electrode
will be correlated to the synergistic effects between the pore geometry and accessible porosity.
2. Materials and methods
2.1. Materials
All the purchased chemicals are of analytical grade and used without any further
treatment (Resorcinol, formaldehyde, cobalt acetate (Co(Ac)2.6H2O), Nickel acetate
(Ni(Ac)2.6H2O), urea, ethylene glycol, ethanol, N, N-dimethylformamide (DMF), Nafion
solution (5 wt.%), isopropanol and potassium hydroxide). Distilled water is used in preparing all
the solutions in this work.
2.2. Preparation of carbon xerogels (CX)
CX is prepared by the sol-gel method as reported previously [39]. Briefly, the resorcinol
(R) and formaldehyde (F) are dissolved in water (W) in presence of cobalt acetate as a catalyst
(C) instead of alkaline catalyst to improve the produced surface area. The weight of metal
catalyst is calculated to be 1% of the produced CX. The used molar ratios for R/F and R/W are
0.5 and 0.06, respectively. After preparation of initial mixture, the mixture is subjected into
o
o
stirring, then cast into glass molds. The curing temperature is 40 C for one day then, 80 C for
five days. The produced organic gel is then subjected to microwave drying to obtain their
corresponding organic xerogel. Finally, CX is obtained with carbonization of organic xerogel at
900 oC for 2 hours, with a heating rate of 1 oC/min in presence of N2 gas flow.
2.3. Preparation of nickel cobaltite (NiCo2O4)
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The metal precursors Ni(Ac)2.6H2O (0.250 gm, 1 mmol), Co(Ac)2.4H2O (0.5 gm, 2
mmol), and urea (0.72 gm, 12 mmol) are dissolved in 60 ml of H2O and ethylene glycol solution
[40]. The ratio between H2O and ethylene glycol is 1:2. The solution is then put in the ultrasonic
probe (SONICS, Probe diameter 5-8 mm, pulse on/off 9.9 sec and 3 sec) for 15 min., the solution
o
is transferred into 90 ml Teflon-lined stainless steel autoclave at 180 C for 12 h. The products
are collected from the autoclave by centrifugation, washed several times with distilled water and
ethanol, and then dried at 60 oC for 24 h. The dried powder is calcinated at 370 oC for 3 h in the
presence of air to produce the NiCo2O4 powder.
2.4. Preparation of nickel cobaltite/xerogels composite (NiCo2O4/CX)
40 mg of CX is dispersed into 40 ml of N, N-dimethylformamide (DMF) solvent by using
an ultrasonic probe for 15 minutes. Then, a solution containing Ni(Ac)2.6H2O (0.125 gm, 0.5
mmol), Co(Ac)2.4H2O (0.250 gm, 1 mmol), and urea (0.360 gm, 6 mmol) in 30 ml of H2O and
ethylene glycol solution is added and ultrasonicated again for 15 minutes. Put in autoclave to
o
o
perform the hydrothermal process at 180 C for 12 h, dried at 60 C for 24 h, and calcinated at
370 oC for 3 h.
2.5. Characterization of the prepared materials
The Microstructures of the prepared materials are examined using a Transmission Electron
Microscope (JEOL JEM 2100, Japan) with an acceleration voltage of 200 KV and the surface
morphologies are checked using Scanning Electron Microscope (FEI- Quanta FEG-250 SEM,
Switzerland). Crystallinity and the phase identification are analyzed using X-ray diffractometer
(PANalytical Empyrean, Netherlands) using CuKα radiation (wavelength λ=1.54045Å) at an
accelerating voltage 40 KV, current 35 mA, scan range 10 - 80° and scan step 0:05°. BET
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surface area, pore volume and pore size distribution are determined using a surface area analyzer
(TriStar II 3020, Micromeritics, USA) at 77 K.
2.6. Modification of glassy carbon electrode
The glassy carbon electrode is modified by mixing 5.0 mg of the CX, NiCo2O4 and
NiCo2O4/CX with 30 µL of Nafion solution (5 wt.%) and 400 µL of isopropanol. The slurry is
sonicated for 30 min at room temperature, then 10 µL of the sonicated suspension is withdrawn
by micropipette and spread on the active area of the glassy carbon electrode with 3 mm of
diameter and 0.0706 cm2 of surface electrode area. The electrode was subjected to drying at 80
oC for 10 min.
2.7. Electrochemical measurements
Electrochemical measurements are performed using the Potentiostat/ Galvanostat
(AUTOLAB PGSTAT 302N) in standard three-compartment glass cell with the glassy carbon
(GC) working electrode, Pt counter electrode and Ag/AgCl reference electrode at room
temperature. Prior to each experiment, the working electrode is polished to mirror surface with
alumina, rinsed with acetone and finally washed and dried. NOVA 1.11 software is used for
recording and fitting the obtained data.
3. Results and Discussion
3.1. Physicochemical properties
The phase composition and the crystal structure of the as-prepared CX, NiCo2O4 and
NiCo2O4/CX were investigated using X-Ray Diffraction analysis and the XRD patterns are
shown in Figure 1a. From the Figure, the broad diffraction lines in case of both NiCo2O4, and
NiCo2O4/CX composite illustrate the crystalline nanostructures. The amorphous CX structure is
observed with two main peaks at 31o and 36o for the metallic cobalt that be used as a catalyst
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during its preparation. Also the patterns for NiCo2O4 and NiCo2O4/CX show diffraction peaks at
2θ related to the (220), (311), (400), (511) and (440) planes of face centered cubic crystalline
structure of NiCo2O4 [14, 41]. In additions, the purity of NiCo2O4 phase is confirmed by the
absence of any other impurity peaks in the XRD patterns and in case of NiCo2O4/CX there is a
peak at 2θ = 51.8364 which belong to (4 2 2) plane of NiCo2O4.
Surface morphology of the as-prepared samples is studied using FESEM (Figures 1b-d).
Figure 1b shows porous, spherical particles connected to each other for CX network which is in
aggrement with previously published resorcinol-formaldehyde carbon gels [39]. Homogeneous
growth of porous, nano particles NiCo2O4 on CX surface (Figure 1d) are observed as
nanoneedles-like structures. The homogeneous growth and anchoring of the NiCo2O4
nanoneedles over the CX surface are also confirmed using HRTEM image as shown in Figure
2C. The presence of cobalt that be used as a catalyst during the CX preparations is embedded
inside the carbon matrix with different sizes in nanometric scale (Figure 2a). Also, the formation
of NiCo2O4 nanoneedles is clearly seen in Figure 2b.
The surface area and textural characteristics of all prepared samples are analyzed by N2
gas adsorption. The adsorption/desorption isotherms are obtained after degassing the samples at
o
130 C for 2 hours under He gas flow. The obtained texture characteristics for the samples are
compiled in Table 1. The N2 sorption isotherms and pore size distribution of the prepared
compounds are displayed in Figure 3a and b. The isotherms are typical IV type [42] correspond
to mesopore materials with a different shape in the hysteresis loops that depend on the
characteristic pore geometry. Both CX and NiCo2O4/CX exhibit H1 hysteresis loop while H2
hysteresis loop is observed for NiCo2O4 [43]. The H1 hysteresis loop is characterized by
agglomerates particles arranged in a fairly uniform way with cylindrical pore geometry indicates
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the relatively high pore size uniformity and facile pore connectivity. The H2 hysteresis loop
shows pore network connectivity and a pore blocking effect for ink-bottle pore with narrow
necks [43, 44]. The pore geometry is schematically represented in Figure 3c. In addition, the
pore size distribution of these compounds reveals that the porosity of NiCo2O4 is one mesopore
system, while the distribution of NiCo2O4/CX shows two modular systems.
3.2. Electrochemical activity of modified electrodes
The electrochemical activity of CX, NiCo2O4 and NiCo2O4/CX modified glassy carbon
(GC) electrodes is studied in 1.0 M KOH solution using cyclic voltammetry measurements in the
potential range between 0.0 and 0.6 V (vs. Ag/AgCl) at room temperature, and the data are
graphically represented in Figure 4a and b. Data illustrates that, the voltammograms of both GC
and GC/CX does not show any characteristic electrochemical response within the applied
potentials (Figure 4a), while the broad and strong characteristic redox peaks of the charge
transfer process of Ni2+/Ni+3 and Co2+/Co3+ are appearing in the main oxidation state for both
NiCo2O4 and NiCo2O4/CX (Figure 4b) modified electrodes [20, 22, 35, 45]. Simultaneous
contributions from both redox couples for Ni2+/Ni+3 and Co2+/Co3+ offer rich redox reactions
with two electroactive center and support the uses of these electrodes for electrochemical energy
storage and conversion applications [12, 13]. The possible redox reactions (Eq. 1) for the
electroactive centers on the modified electrodes in alkaline medium are derived based on the
previous reports [13, 22, 45, 46].
NiCo2O4 + OH- + H2O
NiOOH + 2CoOOH + e-
(1)
Also, from Figure 4b, the highly electroactive response of NiCo2O4/CX compared to NiCo2O4 is
concluded from its high anodic and cathodic current densities. This improvement in the
electrocatalytic activity of the modified electrode is observed although the specific surface area
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and the cumulative pore volume of NiCo2O4 are higher than that of NiCo2O4/CX as we note from
Table 1. These results are in contrary to be expected from previously reported for some modified
NiCo2O4 electrodes [13, 37, 38], that explained the highly electrocatalytic activity of the
modified NiCo2O4, as a result of increasing the specific surface area which enhances the
interfacial electroactive sites and increases the electrode/electrolyte interface. This explanation is
acceptable in case of compounds with same pore geometry. This observation is confirmed from
N2 sorption isotherm for both NiCo2O4/rGO and NiCo2O4 which has the same shape in the
hysteresis loops [38]. So we explain the improvement in the electrocatalytic activity of our
modified NiCo2O4/CX to the synergistic effect between NiCo2O4 and CX during the preparation
of NiCo2O4/CX nanocomposite. This effect leads to changing the pore geometry from the ink-
bottle pore with narrow necks in case of NiCo2O4 to cylindrical pore geometry in NiCo2O4/CX
as we inferred from their N2 adsorption/desorption isotherms. This conclusion is agreed well
with the important relation between the pore geometry and the activity of the catalyst [47].
According to our conclusion, the highly electrochemical activity of NiCo2O4/CX compared to
NiCo2O4 is correlated to its cylindrical pore geometry that allowing a good accessibility of the
electrolyte to the inner active sites of the electrode/electrolyte interfaces inside the pore as well
as, to the pore block effect for narrow necks ink-bottle pores of NiCo2O4. Besides, the higher
electrocatalytic activity NiCo2O4/CX mesopore can be attributed to its easily accessible porosity
that is formed as a result of the presence of two modular systems [48] which can be considered
as another factor that affects the electrochemical activity.
Figure 5 represents the cyclic voltammograms of (a) NiCo2O4/CX and (b) NiCo2O4
modified electrodes in 1.0 M KOH with different scan rate from 5 to 60 mV/s at room
temperature. The linear dependence of the anodic and cathodic current densities of the redox
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peaks related to Ni2+/Ni+3 and Co2+/Co3+ with the square root of the scan rate (5 to 60 mV/s) as
shown in Figure 5 (c) NiCo2O4/CX and (d) NiCo2O4 indicates that there is an increase in the
redox reactions with changing the scan rate and the electrochemical process is controlled by the
diffusion of OH- into the mesopore cavities [49]. Moreover, the diffusion rate of OH- ions
through the cylindrical pore geometry of CX/NiCo2O4 to the inner wall of the active sites of the
pores is higher than that of the narrow necks ink-bottle pore geometry of NiCo2O4 which can be
inferred from variation of the anodic and cathodic current density values with the scan rate in
both compounds.
3.3. Electrocatalytic oxidation of methanol
The electrocatalytic activity of the prepared mesoporous NiCo2O4/CX modified electrode
towards methanol electro-oxidation in basic medium is investigated using cyclic voltammetry
technique in the potential range between 0.0 to 0.6 V (vs. Ag/AgCl). Figure 6a represents the
cyclic voltammograms of GC, NiCo2O4 and NiCo2O4/CX electrodes in 1.0 M KOH + 0.5M
CH3OH solution with a scan rate of 50 mV/s at room temperature. Data shows that the redox
peaks for Ni2+/Ni+3 and Co2+/Co3+ are disappeared and only one methanol oxidation peak appears
at 0.41, 0.38 and 0.29 V (vs.Ag/AgCl) for GC, NiCo2O4 and CX/NiCo2O4 modified electrodes
respectively. The negative shift for the methanol electro-oxidation potential at NiCo2O4/CX
comparing with GC and NiCo2O4 electrodes reveals the higher electrocatalytic activity of
NiCo2O4/CX towards methanol electro-oxidation.
Catalytic current density is considered as another factor to investigate the electrocatalytic
efficiency for the electro-oxidation process, a higher anodic oxidation current density for
methanol electro-oxidation reaches to 96.2 mA.cm-2 at NiCo2O4/CX electrode compared to 37.8
and 0.2 mA.cm-2 at NiCo2O4 and GC electrodes respectively indicating higher electrocatalytic
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response of NiCo2O4/CX modified electrode for methanol electro-oxidation. It’s interesting that,
the electrocatalytic activity of our mesoporous NiCo2O4/CX modified electrode is higher than
that of previously reported for NiCo2O4 and its modifications towards the electro-oxidation of
methanol [20, 37, 45, 50, 51] as presented in Table 2. Although our new mesopore NiCo2O4/CX
nanocomposite has a small specific surface area compared to the other reported modified
NiCo2O4 electrode, but they didn't report any correlations with the pore geometry but only
explain the effect of surface area. The highly electrocatalytic activity of NiCo2O4/CX is
explained to the synergistic effect between the positive effect of its cylindrical pore geometry on
the diffusion rate of both OH- and CH3OH into the electroactive centers from the solution to the
inner wall of the pore and the presence of two modular systems as illustrated before.
The electrocatalytic mechanism for the methanol electro-oxidation is summarized as
previously explained [21, 22, 37], as follows:
NiCo2O4 + OH- + H2O
NiOOH + 2CoOOH + e-
Ni(OH)2 + CO2 + 1.5 H2O
2Co(OH)2 + 2CO2 + 3H2O
(2)
(3)
(4)
NiOOH + CH3OH + 1.25 O2
2CoOOH + 2 CH3OH + 2.5 O2
The direct proportionality of the catalytic current densities with the square root of the
potential sweep rate in case of NiCo2O4 and NiCo2O4/CX and modified electrodes is shown in
the inset of Figure 6b and c that refers to the methanol electro-oxidation is kinetically and
diffusion controlled process [52, 53]. The diffusion rate in case of CX/NiCo2O4 is higher than
that of NiCo2O4 as a result of the difference in the pore geometry between the two studied
compounds. These data are in a good agreement with other data obtained in the absence of
methanol.
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The electrochemical stability is an important factor to evaluate the electrocatalytic
performance of the mesoporous NiCo2O4 and NiCo2O4/CX modified electrodes. Figure 7 shows
the repetitive 100 cycling test for the modified electrodes at a scan rate of 50 mV/s in 1.0 M
KOH + 0.5 M CH3OH solution at room temperature. The catalytic current density performs
90.6% retention after 100 cycle for mesoporous NiCo2O4/CX modified electrode (Figure 7a)
comparing to 94.2% in case of NiCo2O4 modified electrode (Figure 7b). The decrease in the
electro-catalytic current density can be explained as a result of the decrease in methanol
concentration resulting from the continuous electro-oxidation of methanol on highly
electroactive NiCo2O4/CX modified electrode that support its highly electrocatalytic activity
[53].
In addition, chronoamperometry is used as a useful technique to investigate the
electrochemical stability of the prepared modified electrodes in 1.0 M KOH + 0.5M CH3OH
solution at room temperature. Figure 8 represents the chronoamperometry curves for CX,
NiCo2O4 and NiCo2O4/CX modified electrodes at 0.6 V (vs. Ag/AgCl) for 3600 s. It clearly
shows that, the higher methanol electro-oxidation current density is observed at NiCo2O4/CX
compared to NiCo2O4, while CX does not has any oxidation current density. This order is in a
good agreement with the electrocatalytic activity behavior studied using cyclic voltammetry
measurements. Additionally, the constant current plateau for both modified electrodes indicates
the stability of the NiCo2O4/CX and NiCo2O4 towards the electro-oxidation of methanol under
this condition [54].
4. Conclusion:
A new nanoneedle-like structures of mesoporous NiCo2O4/CX is prepared using sol-gel
method followed by hydrothermal method. N2-adsorption/desorption isotherms clarify the
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uniform cylindrical pore geometry of NiCo2O4/CX nanocomposite compared to narrow necks
ink-bottle pore geometry for pristine NiCo2O4. The pore size distribution of NiCo2O4/CX shows
two modular systems. NiCo2O4/CX modified electrode acts as an efficient electrocatalyst for
methanol electro-oxidation in basic medium with catalytic current density of 98 mA.cm-2 at
0.29V with a good stability of 90.6 % current retention after 100 repetitive cycles compared to
37.8 mA.cm-2 at 0.38 V for pristine NiCo2O4. Chronoamperometry measurements show the
stability of the NiCo2O4/CX modified electrodes, which is observed from its constant current
plateau over 3600 s. The electrocatalytic activity of NiCo2O4/CX modified electrode is correlated
to the synergistic effects between the pore geometry and accessible porosity.
Acknowledgment
The authors are gratefully acknowledge the support from Beni-Suef University, PSAS
and the financial support from STDF funds with IDs no 12711 and 14970.
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Table 1: Surface area analysis of the prepared materials
Pore
SBET
Pore Size
Volume
cm3/g
0.163
Sample
m2/g
701.9
80.4
nm
4.33
5.68
CX
NiCo2O4
0.148
NiCo2O4/CX
64.0
0.07
5.97

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Table 2: Electrocatalytic Activity of NiCo2O4/CX electrocatalyst towards methanol electro-oxidation compared with others reported
electrocatalysts.
Catalytic
Electrocatalyst
Medium
SSA (m2/g)
Pore Geometry*
Current density
Reference
(mA.cm-2)
NiCo2O4
1.0M KOH / 0.5M CH3OH
1.0M KOH / 0.5M CH3OH
1.0M KOH / 0.5M CH3OH
0M KOH / 0.5M CH3OH
1.0M KOH / 0.5M CH3OH
1.0M KOH / 0.5M CH3OH
1.0M KOH / 0.5M CH3OH
1.0M KOH / 0.5M CH3OH
1.0M KOH / 0.5M CH3OH
80.4
64.0
88.9
45.2
66.9
190
146.5
47
Type ( IV, H2)
Type (IV, H1)
Type (IV, H1)
Type ( IV, H3)
Type (IV, H3)
Type (IV, H1)
Type (IV, H4)
Type ( IV, H1)
Type (IV, H1)
--
37.8
98
Our work
Our work
[7]
NiCo2O4/CX
NiCo2O4
93
NiCO – NS
NiCO - NC
NiCo2O4
111
134
98
[16]
[16]
[21]
NiCo2O4/NF
NiCo2O4
40.9
12
[22]
[38]
NiCo2O4/rGo
85
17
[38]
NiCo2O4- 120 cal 1.0M KOH / 0.5M CH3OH
NiCo2O4- 200 cal 1.0M KOH / 0.5M CH3OH
*Concluded from N2 adsorption isotherm
88
8.85
7.31
[45]
77
--
[45]

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Figures caption
Figure 1: (a) XRD patterns of the as-prepared CX, NiCo2O4, and NiCo2O4/CX, SEM
images of the as-prepared (b) CX, (c) NiCo2O4, and (d) NiCo2O4/CX
Figure 2: HRTEM images of the as-prepared (a) CX , (b) NiCo2O4 and (c)
NiCo2O4/CX.
Figure 3: (a) N2-adsorption/desorption isotherms plots for CX, NiCo2O4 and
NiCo2O4/CX, (b) Pore size distribution plots for NiCo2O4 and NiCo2O4/CX, and (c)
Schematic diagram for the pore geometry.
Figure 4: Cyclic voltammograms of (a) GC and GC/CX, (b) NiCo2O4 and
NiCo2O4/CX modified electrodes in 1.0 M KOH solution with scan rate of 50 mV/s at
room temperature.
Figure 5: Cyclic voltammograms of (a) NiCo2O4/CX and (b) NiCo2O4 in 1.0 M KOH
at room temperature with different scan rates. The relationship between anodic and
cathodic current densities with the square root of the scan rate for (c) NiCo2O4/CX
and (d) NiCo2O4 .
Figure 6: Cyclic voltammograms of (a) GC, NiCo2O4 and NiCo2O4/CX electrodes in
1.0 M KOH + 0.5 M CH3OH solution with scan rate of 50 mV/s at room temperature,
(b) NiCo2O4 and (c) NiCo2O4/CX modified electrodes in 1.0 M KOH + 0.5 M
CH3OH solution at room temperature with different scan rate. The insets represent the
relationship between anodic current densities with the square root of the scan rate.
Figure 7: Repetitive 100th cycling test of NiCo2O4/CX (a) and NiCo2O4 (b) modified
electrodes in 1.0 M KOH + 0.5M CH3OH solution with scan rate of 50 mV/s at room
temperature.
Figure 8: Chronoamperometry curves for CX, NiCo2O4 and NiCo2O4/CX modified
electrodes in in 1.0 M KOH + 0.5 M CH3OH solution at room temperature at 0.6 V
(vs. Ag/AsCl)
(d)
1

(a)
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(c)
(c)
(f)
(b)
(c)
(d)
2

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(b)
(a)
Figure 1
(c)
Figure 2
3

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(a)
(b)
(b)
(Ǻ)
(c)
Figure 3
4

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(b)
(a)
Figure 4
5