Development of non-covalent ternary polymer-CNT composites as a novel supporting material for electrooxidation of glycerol

Article abstract of DOI:10.1039/c5ra17172e

The present study is focused on the development of a novel ternary polymer-CNT composite (ATCP-CP-BZD-CNT) by using three different monomers namely amine terminated cyclophosphazene (ATCP), hexachlorocyclotri-phosphazene (CP) and 2,2′-benzidinedisulfonic acid (BZD) as a novel catalyst support for biobased alcohol fuel cell applications. In this regard, Pt and Pt-Sn nanoparticles were deposited on the ATCP-CP-BZD-CNT composite material through a formaldehyde reduction method using hexachloroplatinic acid hexahydrate and stannous(ii) chloride as metal precursor materials. The platinum nanoparticles and platinum-tin loaded ATCP-CP-BZD-CNT composites were characterized by various analytic techniques. The activity and stability of the Pt/ATCP-CP-BZD-CNT and Pt-Sn/ATCP-CP-BZD-CNT catalysts were analyzed by cyclic voltammetry and chronoamperometry techniques in the presence of 0.5 M glycerol and 0.5 M KOH solution. From the results, it has been concluded that the Pt/ATCP-CP-BZD-CNT and Pt-Sn/ATCP-CP-BZD-CNT electrocatalysts exhibit significant higher anodic oxidation current and lower onset potential when compared to those of Pt loaded Vulcan carbon and CNT materials. The various experimental studies demonstrate that the ATCP-CP-BZD-CNT composite shows a huge impact on the physicochemical properties of Pt/ATCP-CP-BZD-CNT and Pt-Sn/ATCP-CP-BZD-CNT catalysts for the electrooxidation of glycerol. Furthermore, the cyclic voltammetry and chronoamperometry analysis of glycerol in alkaline medium concludes that the Pt/ATCP-CP-BZD-CNT and Pt-Sn/ATCP-CP-BZD-CNT catalysts are still electroactive even after the completion of 100 cycles of various scan potentials.

Full text of DOI:10.1039/c5ra17172e

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Page 1 of 36
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Development of non-covalent ternary polymer-CNT composite as a novel supporting
material for electrooxidation of glycerol
Dakshinamoorthy Prasanna and Vaithilingam Selvaraj*
Nanotech Research Lab, Department of Chemistry, University College of Engineering
Villupuram, (A Constituent College of Anna University, Chennai) Kakuppam, Villupuramꢀ
605 103, Tamilnadu, India.
Abstract
The present study is focused on the development of a novel ternary polymerꢀCNT
composite (ATCPꢀCPꢀBZDꢀCNT) by using three different monomers namely amine
terminated cyclophosphazene (ATCP), hexachlorocyclotriꢀ phosphazene (CP) and 2,2’ꢀ
benzidinedisulfonic acid (BZD) as a novel catalyst support for biobased alcohol fuel cell
applications. In this regard, Pt and PtꢀSn nanoparticles were deposited on ATCPꢀCPꢀBZDꢀ
CNT composite material through formaldehyde reduction method using hexachloroplatinic
acid hexahydrate and stannous (II) chloride as metal precursor materials. The platinum
nanoparticles and platinumꢀtin loaded ATCPꢀCPꢀBZDꢀCNT composites were characterized
by various analytic techniques. The activity and stability of Pt/ATCPꢀCPꢀBZDꢀCNT and Ptꢀ
Sn/ATCPꢀCPꢀBZDꢀCNT catalysts were analyzed by cyclic voltammetry and
chronoamperometry technique in the presence of 0.5 M glycerol and 0.5 M KOH solution.
From the result, it has been concluded that the Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀ
CPꢀBZDꢀCNT electrocatalysts exhibit significant higher anodic oxidation current and lower
onset potential when compared to that of Pt loaded vulcan carbon and CNT materials. The
various experimental studies demonstrate that the ATCPꢀCPꢀBZDꢀCNT composite shows a
huge impact on the physicochemical properties of Pt/ATCPꢀCPꢀBZDꢀCNT and Ptꢀ
1

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Sn/ATCPꢀCPꢀBZDꢀCNT catalyst for the electrooxidation oxidation of glycerol. Further, the
cyclic voltammetry and chronoamperometry analysis of glycerol in alkaline medium
concludes that Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT catalyst is still
electroactive even after completion of 100 cycles of various scan potential.
Keywords: Platinum, ternary polymerꢀCNT composite, carbon nanotubes, glycerol, alcohol
fuel cell, electrooxidation
*Corresponding Author: vaithilingamselvaraj@gmail.com, vselva@aucev.edu.in and
rajselva_77@yahoo.co.in (V.Selvaraj), Fax No. +91ꢀ4146 224500
Introduction
Alkaline direct alcohol fuel cells (ADAFCs) are very attractive, renewable and clean
energy devices for portable electronics and stationary applications1. For direct alcohol fuel
cells (DAFCs), glycerol has found to be most sustainable fuel than the methanol and
ethanol. Glycerol is glowing for its attractive properties such as low cost, simple purification
and storage, nonꢀvolatility and environment friendliness, nonꢀtoxicity, bioꢀrenewability, a
charge capacity of 5.13 Ah m/L and high volumetric energy density of 6.4 kWh/L. 1,2,3 The
bioꢀmethanol and bioꢀethanol are obtained by the fermentation process of biomass. But,
glycerol is frequently obtained as a bioꢀwaste byproduct in the production of biodiesel
through a transꢀesterification reaction.4 Glycerol derived from natural oil has found to be
low in market price when compared to other alcohols like methanol, ethanol, etc. So, the
present work is focused to utilize the biowaste as an alcohol fuel for the generation of
electrical energy.
2

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Carbon nanotubes (CNT) have been exposed much more in the fundamental,
theoretical and applied research perspectives because of their extraordinary electrical,
mechanical and structural properties.5,6 In recent years, many efforts have been emphasized
for CNT due to their high electrical conductivity, chemical stability and more importantly
CNT is working as a support for nobleꢀmetal nanoparticle catalysts for fuel cells
applications.7,8 Pristine CNT exhibits poor dispersibility in liquids and agglomerates due to
its high surface free energy. Further, the pristine CNT doesn’t have sufficient binding sites
for the deposition of metal nanoparticles due to their high curvature and chemical inertness.9
As a result, metal nanoparticles on CNT have poor dispersion and large particle size, which
diminishes the electrocatalytic activity. In order to overcome such difficulties, the surface of
the pristine CNT is usually modified by harsh acid for functionalization, which cause
structural defects and reduces the electrical property of the surface modified CNT.10,11 So,
the recent researches are focused on the development of nonꢀcovalent functionalization of
CNT using various polymers. The binding between the CNT and polymer is only depend on
πꢀπ stacking, hydrogen bonds and van der Waals force of attraction, which will not destruct
the structure and electrical properties of CNT.12,13 However, the reliability of CNT should
be maintained because the weak interaction between the polymer and the CNT typically
leads to instability of the composites. The instability of the composites is negotiated by
introducing polymers with high crossꢀlinked structure and numerous low surface energy
groups.14
With this view, the present work is proposed to prepare a novel ternary polymerꢀ
CNT (ATCPꢀCPꢀBZDꢀCNT) composite through polycondensation method using three
different monomers namely amine terminated cyclotriphosphazene (ATCP),
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Hexachlorocyclotriphosphazene (CP) and 2,2’ꢀbenzidinedisulfonic acid (BZD). The
polymerization process leads to a high cross linked ATCPꢀCPꢀBZD ternary polymer
formation around the CNT material (core shell structure). To the best of our knowledge, this
is the first report involved to synthesize ternary polymerꢀCNT (ATCPꢀCPꢀBZDꢀCNT)
composite using three different monomers like amine terminated cyclotriphosphazene
(ATCP), hexachlorocyclotriphosphazene (CP) and 2,2’ꢀbenzidinedisulfonic acid (BZD) for
biobased fuel cell applications. The whole functionalization is nonꢀcovalent, which rarely
damage the chemical structure of the carbon nanotubes. As a result, the CNT has been
maintained with good electroconductivity and mechanical strength. More importantly,
platinum (Pt) and platinumꢀtin (PtꢀSn) nanoparticles have been successfully deposited on
the surface of ATCPꢀCPꢀBZDꢀCNT composites by formaldehyde reduction method15,16. The
platinum and platinumꢀtin nanoparticles deposited ATCPꢀCPꢀBZDꢀCNT catalysts were
characterized by HRTEM and EDX analyses. The electrocatalytic activity and stability of
Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT electrocatalyst were investigated
by using glycerol as a fuel in alkaline medium. The electrocatalysts developed in the present
investigation show more enhanced catalytic activity, stability and more negative onset
potential, which can be used in alkaline fuel cell applications.
Experimental methods
Materials
Hexachlorocyclotriphosphazene (CP), 2,2’ꢀbenzidinedisulfonic acid (BZD), 4ꢀ
acetamido phenol and multiwalled carbon nanotubes were purchased from Sigma Aldrich,
India. Reagent grade formaldehyde (HCHO), triethylamine (TEA), dimethyl sulphoxide
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(DMSO) glycerol and electrocatalyst precursor salt (hexachloroplatinic acid hexahydrate
(H2PtCl6·6H2O) and stannous (II) chloride dihydrate (SnCl2.2H2O)) were purchased from
SRL, India.
Preparation of amine terminated cyclotriphosphazene (ATCP)
A mixture of 4ꢀacetamido phenol (15.65 g, 0.1035 mol) and K2CO3 (21.09 g,
0.15824 mol) in dry acetone (200 ml) were effectively agitated at room temperature for 30
minutes. To this mixture, hexachlorocyclotriphosphazene (CP) (5g, 0.0143 mol) was added
and refluxed at 60°C for 4 days. Then, the resulted product was cooled to room temperature
and filtered. The product (ATCP) was further purified by using hexane and dried in a
vacuum oven at 50 °C for 2 h17ꢀ19
.
Preparation of ternary polymer-CNT (ATCP-CP-BZD-CNT) composite
In the typical synthesis, 20 mg of hexachlorocyclotriphosphazene, 22.5 mg of 2,2’ꢀ
benzidinedisulfonic acid (BZD) and 22.5 mg amine terminated cyclophosphazene (ATCP)
were dissolved in 80 mL of dimethyl sulfoxide (DMSO). To the above reaction mixture, 5
mg of multiwalled carbon nanotubes was added and dispersed using ultrasonicator for 10
mins. The polymerization of monomers over carbon nanotubes begins with the addition of 2
mL of triethylamine (TEA) into the above dispersion and maintained the ultrasound for 12
hours (100 W, 40 KHz) at room temperature. After that, the resultant
precipitate was isolated by the centrifugation process at 3000 rpm for 15
mins, washed three times with DMSO and then distilled water. The resultant
ternary polymerꢀCNT (ATCPꢀCPꢀBZDꢀCNT) composite was dried at 100°C for 12
hours.
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Synthesis of platinum and platinum-tin nanoparticles deposited ATCP-CP-BZD-CNT
composites
Platinum and platinumꢀtin nanoparticles were deposited on ATCPꢀCPꢀBZDꢀCNT
composite by the formaldehyde reduction method according to the scheme 1.
Scheme 1 Schematic representations for the preparation of ATCPꢀCPꢀBZDꢀCNT composite
and platinum and platinumꢀtin nanoparticles deposited ATCPꢀCPꢀBZDꢀCNT composites.
In this process, 30 mg of ATCPꢀCPꢀBZDꢀCNT composite was dispersed in 10 mL of
distilled water using ultrasound (100 W, 40 KHz) for 20 mins. Then the platinum precursor
solution with a concentration of 54mg/mL was added in dropwise to the above solution and
2.5M NaOH solution was used to adjust the pH of the reaction mixture (pH=11). Finally,
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platinum nanoparticles were deposited by adding a formaldehyde solution at 85°C and the
reaction was refluxed at the same temperature for 5 hours in an inert
atmosphere. After the completion of reduction reaction, the solid Pt/ATCPꢀCPꢀBZDꢀCNT
was centrifuged,washed with distilled water and then dried at 60°C for 10 hours. By
using the above process, the Pt-Sn/ATCPꢀCPꢀBZDꢀCNT electrocatalyst was
synthesized. The prepared electrocatalysts have been used for the fabrication of electrode
towards glycerol fuel cell applications.
Fabrication of Pt/ATCP-CP-BZD-CNT and Pt-Sn/ATCP-CP-BZD-CNT electrodes
The working electrode has been fabricated on graphite electrode through drop
casting method. Typically, 5 mg of Pt/ATCPꢀCPꢀBZDꢀCNT electrocatalyst, 100 ꢁL of
Nafion solution (5 wt%, Aldrich) and isopropanol (1:1) were mixed using an ultrasonic
bath. 25 µL of this mixture was coated on a freshly polished graphite electrode. The
electrocatalyst coated graphite electrode was dried at room temperature for 6 hours to
remove the solvent like isopropanol. By using the same procedure, the PtꢀSn/ATCPꢀCPꢀ
BZDꢀCNT has been fabricated and utilized towards the electrooxidation glycerol.
Physicochemical and electrochemical characterizations
The physicochemical characterizations were carried out by using FTIR, HRTEM and
EDX analysis. The prepared ATCPꢀCPꢀBZDꢀCNT composite has been examined by fourier
transform infrared (FTIR, Thermo Nicolet Model: 6700) technique. The surface morphology
and size of metal nanoparticles deposited on ATCPꢀCPꢀBZDꢀCNT composite were analysed
by high resolution transmission electron microscope (HRꢀTEM, JEOL) with an accelerating
voltage of 120 kV. Energy dispersive Xꢀray analysis (EDX, INCA200) confirms the
presence of platinum and platinumꢀtin nanoparticles on ATCPꢀCPꢀBZDꢀCNT composite.
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The electrochemical characterizations were carried out using a multichannel
electrochemical workstation with inbuilt FRA system (Biologic SAS, Model VSP2). A
standard threeꢀelectrode glass cell equipped with novel modified graphite electrodes as
working, saturated calomel electrode as reference and a Pt wire as counter electrode were
employed. Further, the electrocatalytic activity of glycerol oxidation has been investigated
in 0.5 M KOH and 0.5 M glycerol solution with a potential range of ꢀ1.0 to 0.3 V at a scan
rate of 20, 50 and 100 mVsꢀ1.
The intermediate products formed during the glycerol electrooxidation were
determined by using highꢀperformance liquid chromatography (Dionex system P680
HPLC). It works with an isocratic elution and mainly includes an autosampler (ASI 100
Automated Sample Injector), a sample loop (20 ꢁL), and an ionꢀexclusion column (Aminex
HPXꢀ87H), which were operated at room temperature. The analytes were separated with
diluted sulfuric acid (3.3 mmol L−1 H2SO4, MERCK 96%) used as eluent with a 0.6 mL
min−1 flow rate. The chromatograph was equipped with an UV−Vis detector (λ = 210 nm)
followed by a refractive index detector (IOTA2).
Results and discussions
FTIR analysis of amine terminated cyclotriphosphazene (ATCP)
Figure 1 shows the FTꢀIR spectrum of amine terminated cyclotriphosphazene. The
two broad band’s observed at 3438 and 3338 cmꢀ1 were corresponding to ꢀNH2 stretching
vibrations, in fact they are also ascertain the presence of the amine group in the ATCP
material (Figure 1). The peaks observed at 3043 and 2925 cmꢀ1 were corresponding to ring
CꢀH stretching vibrations. The aromatic CꢀC stretching vibrations were noticed at 1507 and
1621 cmꢀ1. The characteristic stretching vibrations of PꢀOꢀPh, PꢀNꢀP and PꢀOꢀC were
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visualized at 1256, 1178 and 952 cmꢀ1 respectively. The aromatic CꢀH bending vibrations
,
were appeared at 832 and 695 cmꢀ1.
Figure 1 FTꢀIR spectrum of amine terminated cyclophosphazene (ATCP).
FTIR analysis of ATCP-CP-BZD and ATCP-CP-BZD-CNT materials
Figure 2 FTIR spectra of ATCPꢀCPꢀBZD and ATCPꢀCPꢀBZDꢀCNT materials
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Figure 2 shows the FTIR spectra of ATCPꢀCPꢀBZD and ATCPꢀCPꢀBZDꢀCNT
materials. From the FTIR spectrum of ATCPꢀCPꢀBZD material, the characteristic peaks of ꢀ
NꢀH, CꢀH, C=C, PꢀOꢀC, PꢀNꢀP, PꢀOꢀC and CꢀS were observed at 3370 cmꢀ1, 2920 cmꢀ1,
1620 cmꢀ1, 1361 cmꢀ1, 1190 cmꢀ1, 695 cmꢀ1 and 623 cmꢀ1 which indicates the existence of
ATCP and BZD in ATCPꢀCPꢀBZD material. Furthermore, importantly the P–NH–(Ph) peak
at 962 cmꢀ1 representing the link formation of the CP with ATCP and BZD. Thus the above
discussion can reasonably ascertain the polymerization between CP, ATCP and BZD
through the polycondensation process. FTIR spectrum of ATCPꢀCPꢀBZDꢀCNT composite
has all the characteristic stretching vibration that was observed as in the case of ATCPꢀCPꢀ
BZD materials, which confirm the existence of ATCPꢀCPꢀBZD material on the surface of
the CNT. The above analysis can make a conclusion that the ATCPꢀCPꢀBZDꢀCNT contain
the structural unit of amine terminated cyclotriphosphazene, 2,2’ꢀbenzidinedisulfonic acid
and hexachlorocyclotriphosphazene.
XRD analysis
The XRD analyses were carried out for CNT, ATCPꢀCPꢀBZD and ATCPꢀCPꢀBZDꢀ
CNT materials (Figure 3). The characteristic peaks of the CNT were observed at 25.53° and
41.01° for 002 and 100 diffraction plane of carbon in CNT. A broad peak was observed
between 5 and 15° in the case of ATCPꢀCPꢀBZD material, which confirms the amorphous
nature of the resultant ATCPꢀCPꢀBZD material. The above characteristic peaks of CNT and
ATCPꢀCPꢀBZD were appeared in the XRD spectrum of ATCPꢀCPꢀBZDꢀCNT composite,
which confirms the existence of ATCPꢀCPꢀBZD on the surface of CNT material.
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Figure 3 Xꢀray diffraction patterns of ATCPꢀCPꢀBZDꢀCNT, CNT and ATCPꢀCPꢀBZD
materials
Figure 4 Xꢀray diffraction patterns of Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀ
CNT electrocatalyst
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From the XRD patterns of Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT
(Figure 4), the diffraction pattern that is corresponding to the fcc structure of Pt
nanoparticles were observed at the Bragg angles (2θ) of 39.44°, 45.08° and 66.98° for the
diffraction planes (111), (200) and (220) along with the characteristic peak of ATCPꢀCPꢀ
BZD and CNT. Similar XRD patterns were also observed for PtꢀSn/ATCPꢀCPꢀBZD as in
the case of Pt/ATCPꢀCPꢀBZDꢀCNT and the peaks were located at higher 2θ value when
compared to that of the Pt/PCFAꢀCNT material, which indicates the formation of PtꢀSn
alloy on the surface of ATCPꢀCPꢀBZDꢀCNT material. However, the peak intensity
corresponding to ATCPꢀCPꢀBZD was decreased, which confirms the presence of Pt and
PtꢀSn nanoparticles on the ATCPꢀCPꢀBZD but not on CNT. Further, the average particle
size of Pt and PtꢀSn nanoparticels were calculated by using Scherrer equation20 are 2.4 and
2.6 nm, respectively.
Figure 5 Optical images of CNT (left) and ATCPꢀCPꢀBZDꢀCNT (right) materials dispersed
in water.
Further the cross linking density and core shell formation was confirmed by the
simple dispersion method as per the previous method.14 Figure 5 shows the optical images
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of CNT and ATCPꢀCPꢀBZDꢀCNT dispersion in water. From the Figure 5, it was visualized
that the CNT displays poor dispersion in water compared to ATCPꢀCPꢀBZDꢀCNT
composites because of its high surface free energy. But, nonꢀcovalent functionalized ATCPꢀ
CPꢀBZDꢀCNT composite with highly crossꢀlinked structure, numerous low surface energy–
HSO3 and ꢀNH groups on its surface can greatly improve the dispersibility of CNT material.
Based on these results, it was concluded that the CNT was fully covered by ATCPꢀCPꢀBZD
through crossꢀlinked core structure (Figure 5) as shown in the Scheme 1.
X-ray photoelectron spectroscopy
Figure 6 (a) XPS full scan spectrum of PtꢀSn/ATCPꢀCPꢀBZDꢀCNT and XPS spectra of (b)
Pt 4f peak (c) Sn 3d peak in PtꢀSn/ATCPꢀCPꢀBZDꢀCNT catalyst.
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XPS analysis was carried out to determine the chemical state of individual metal
present in PtꢀSn/ATCPꢀCPꢀBZDꢀCNT catalyst. Figure 6 shows the XPS spectrum of Ptꢀ
Sn/ATCPꢀCPꢀBZDꢀCNT catalyst. The full scan XPS spectrum also confirms the presence of
Pt and Sn in the PtꢀSn/ATCPꢀCPꢀBZDꢀCNT catalyst. The images in the Figure 6 (b & c)
show the XPS spectra of Pt(4f) and Sn(3d) in PtꢀSn/ATCPꢀCPꢀBZDꢀCNT catalyst. The
binding energy values of Pt(4f7/2) and Pt(4f5/2) are 71.5 and 74.7 eV, respectively. These
observed binding energy values are good concords with the literature values of 71.2 and
74.4 eV, which authenticate that the Pt is in the zero valence state. 21, 22 The binding energy
value of Sn(3d5/2) is 485.4 eV, which is also confirming that the Sn is in the zero valence
state.23
High resolution transmission electron microscopy
Figure 7 TEM Images of ATCPꢀCPꢀBZD ꢀCNT composites
Figure 7 shows the TEM image of ATCPꢀCPꢀBZDꢀCNT composite material. The
coreꢀshell structure of ATCPꢀCPꢀBZDꢀCNT composite was clearly visualized from
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HRTEM image of ATCPꢀCPꢀBZDꢀCNT material. From the Figure 7, it could be seen that
the ATCPꢀCPꢀBZD was uniformly coated on CNT with the shell thickness in the range of 3ꢀ
5 nm. Further, it was also noticed that the surface of the CNT is almost covered by the
ATCPꢀCPꢀBZD material, which was in good accord with the previous report 14,24
.
Figure 8 HRTEM images of (a) Pt/ATCPꢀCPꢀBZDꢀCNT and (b) PtꢀSn/ATCPꢀCPꢀBZDꢀ
CNT electrocatalysts.
HRTEM images of Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT were
shown in the Figure 8 (a &b). From the Figure 8, the shape, size and dispersion of metal
nanoparticles on the surface of ATCPꢀCPꢀBZDꢀCNT composite was investigated. The TEM
picture clearly demonstrates that the platinum and platinumꢀtin nanoparticles are spherical
in shape and are uniformly deposited on the surface ATCPꢀCPꢀBZDꢀCNT composite. The
average particle size of platinum and platinumꢀtin nanoparticles were found to be ∼2.0 nm.
The small size and uniform distribution of Pt and PtꢀSn nanoparticles were obtained due to
the presence of three functional groups (i) ꢀSO3H groups (ii) ꢀNH groups and (iii) ꢀO groups
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that are available on the surface of ATCPꢀCPꢀBZDꢀCNT composite (scheme 1). Further by
comparing both the TEM images (Figures 7 and 8 (a&b) and XRD analysis, it was
concluded that the most of the metal (Pt and PtꢀSn) nanoparticles were present on the
surface of ATCPꢀCPꢀBZD but not on CNT.
Figure 9 HRTEM image showing a single particle of PtꢀSn/ATCPꢀCPꢀBZDꢀCNT catalyst.
Additionally, the existence of Pt and Sn and their contact information on Ptꢀ
Sn/ATCPꢀCPꢀBZDꢀCNT catalyst was shown in Figure 9. The interꢀplanar distance
(spacing) was measured for a particle present in the HRTEM picture and its value was 2.51
Å, which is corresponding to (111) plane of PtꢀSn bimetal present on ATCPꢀCPꢀBZDꢀCNT
catalyst. Further, the existence of Pt and Sn and their contact information were visualized
from the TEM picture (and insert picture) of PtꢀSn/ATCPꢀCPꢀBZDꢀCNT catalyst. Hence,
the HRTEM image of the single particle (Figure 9) confirms the formation of PtꢀSn bimetal
and their contact information on ATCPꢀCPꢀBZDꢀCNT surface during the formaldehyde
reduction procedure.
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Further, the EDX analysis confirms the existence of platinum and platinumꢀtin
metal nanoparticles on ATCPꢀCPꢀBZDꢀCNT surface (Figure 10 (a & b)). The nominal and
actual loading of mono and biꢀmetallic nanoparticles present on ATCPꢀCPꢀBZDꢀCNT
composite was shown in the Table 1. Further, it affords supporting evidences for the
existence of metal nanoparticles on ATCPꢀCPꢀBZDꢀCNT composite.
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Figure 10 Energy dispersive Xꢀray analysis pattern of (a) Pt/ATCPꢀCPꢀBZDꢀCNT and (b)
PtꢀSn/ATCPꢀCPꢀBZDꢀCNT composites
Table 1 Nominal and actual composition of the catalyst of metal nanoparticles obtained
from EDX analysis
Nominal composition (%) Actual composition (%)
Sl.No.
Catalyst
Pt
100
Sn
0
Pt
100
Sn
0
1
2
Pt/ATCPꢀCPꢀBZDꢀCNT
PtꢀSn/ATCPꢀCPꢀBZDꢀCNT
50
50
48.85
51.15
Electrooxidation of glycerol
Figure 11 Cyclic voltammograms of Pt/C, Pt/CNT and Pt/ATCPꢀCPꢀBZDꢀCNT catalysts in
0.5 M KOH and 0.5 M glycerol solution at 50 mVs−1.
Before the electrooxidation of glycerol on bimetallic system, it is necessary to study
the effect of the support materials for electrocatalysis process. For this, the electrooxidations
of glycerol were carried out on Pt/C and Pt/CNT along with Pt/ATCPꢀCPꢀBZDꢀCNT
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catalysts in alkali medium. The cyclic voltammograms of Pt/C, Pt/CNT and Pt/ATCPꢀCPꢀ
BZDꢀCNT catalyst in 0.5 M KOH and 0.5 M glycerol solution are shown in Figure 11.
From the Figure 11, it can be clearly seen that the glycerol oxidation peak was observed at ꢀ
0.05 V vs. SCE for Pt/ATCPꢀCPꢀBZDꢀCNT electrode with the oxidation current of 26.59
mA/mg and it was considerably greater than that of the Pt/CNT and Pt/C, which might be
due to the combined synergy effect of ATCPꢀCPꢀBZD and CNT in ATCPꢀCPꢀBZDꢀCNT
material.
Further, in the present work three potent functional groups namely –NH, ꢀSO3H and
–O were present on ATCPꢀCPꢀBZDꢀCNT novel support, which will favors more enhanced
effect than that of any other doped material. Hence, the functionalized CNT has a positive
influence on the nucleation growth and binding properties with metal nanoparticles, which
will also result smaller particle size, better dispersion and a uniform size distribution. The
good dispersion and distribution of the metal nanoparticle on support is one of the most
basic features for an efficient catalyst , which lead to increase in the surface area of catalyst.
Especially nitrogen doped CNT owns high surface nucleation sites, which allow the
anchorage and high dispersion of the metal nanoparticles on the surface of supporting
material that results in high interaction between nitrogenꢀdoped CNT support and the
catalytic metals25ꢀ27. Besides, nitrogen doped CNT can improve the durability of the CNT
supported catalyst due to the strong interaction between the nitrogen doped CNT and metal
nanoparticles, which is due to the electron donor behavior of nitrogen. So, nitrogen doped
CNT can be employed as a highꢀefficiency electrocatalyst for fuel cell application.26
Generally, in the negative scan the oxidation of glycerol does not occur until the
surface oxide are reduced, which results the reactivating of electrode surface for further
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glycerol adsorption and electrooxidation28. The quasiꢀsuperimposition of the curves during
the positive and the negative potential sweeps suggests the poisoning effects. Further, this
will limit the rate of glycerol adsorption on Pt surface and hence electroxidation current
decreases. From this literature observation, the present results conclude that there no such
positive curves were observed during the negative sweep, which demonstrates the reduced
poison effect on Pt/ATCPꢀCPꢀBZDꢀCNT anode catalyst.
From the literature reports28,29 it has been noticed that the oxidation of glycerol on
platinum surface yields intermediates such as glyceraldehyde, glyceric acid, glycolic acid
and formic acid. Though other products are formed, formic acid plays an important role as it
paves a way for the formation of poisonous intermediate CO(ads).28ꢀ30 Hence, based on the
present investigation results (HPLC results), the possible proposed mechanism was
hypothesized for glycerol oxidation on Pt/ATCPꢀCPꢀBZDꢀCNT electrocatalyst (Scheme 2).
Scheme 2 Proposed mechanism based on the HPLC result for glycerol oxidation on
Pt/ATCPꢀCPꢀBZDꢀCNT surface
During the oxidation of glycerol an evolution “CO2” were noticed, which further
support the mechanism25. From these studies, it could be understood that the formation of
CO is vanished or not found on the surface of Pt/ATCPꢀCPꢀBZDꢀCNT electrode at lower
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negative onset potential. So, the absence of CO can enhance the electrocatalytic activity of
Pt/ATCPꢀCPꢀBZDꢀCNT electrode towards glycerol oxidation.
From the results, it has been noticed that the Pt/ATCPꢀCPꢀBZDꢀCNT catalyst
exhibits higher electrocatalytic activity than that of the Pt/CNT and Pt/C catalysts in terms
of onset potential and oxidation current. This indicates that the supporting material (ATCPꢀ
CPꢀBZDꢀCNT) plays an important in the electrooxidation of glycerol. Further, to enhance
the catalytic activity and to reduce the cost of the electrode materials, platinum is alloyed
with tin on ATCPꢀCPꢀBZDꢀCNT composite to form PtꢀSn/ATCPꢀCPꢀBZDꢀCNT
electrocatalyst.
Figure 12 Cyclic voltammograms of Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀ
CNT electrodes in 0.5 M KOH solution at a scan rate of 50 mVsꢀ1.
To compare the catalytic activity and stability of Pt/ATCPꢀCPꢀBZDꢀCNT with Ptꢀ
Sn/ATCPꢀCPꢀBZDꢀCNT as an anode catalyst, the electrooxidation of glycerol in 0.5 M
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KOH solution was carried out by cyclic voltammetry and chronoamperometric techniques.
Figure 12 shows the cyclic voltammograms of Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀ
CPꢀBZDꢀCNT electrodes in 0.5 M KOH at a scan rate of 50 mVsꢀ1.
Figure 13 Cyclic voltammograms of PtꢀSn/ATCPꢀCPꢀBZDꢀCNT electrode in 0.5 M KOH +
0.5 M glycerol solution at a scan rate 50 mVsꢀ1.
From the Figure 12, it can be seen clearly that the oxidation peak was not observed
for Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT electrodes in 0.5 M KOH
solution. Figure 13 depicts the cyclic voltammetric responses of PtꢀSn/ATCPꢀCPꢀBZDꢀ
CNT electrodes in 0.5 M KOH + 0.5 M glycerol solution at 50mVsꢀ1.
From the Figure 13, it was noticed that the presence of tin as second metal has
shown remarkable effects towards glycerol oxidation. The enhanced glycerol oxidation peak
current of PtꢀSn/ATCPꢀCPꢀBZDꢀCNT is 38.89 mA, which was higher than that of
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Pt/ATCPꢀCPꢀBZDꢀCNT electrocatalyst (26.59 mA). Further, the reduced onset potential of
PtꢀSn/ATCPꢀCPꢀBZDꢀCNT is ꢀ0.74 V, which was more negative than that of Pt/ATCPꢀCPꢀ
BZDꢀCNT electrocatalyst (ꢀ0.64 V). From the results, it has been concluded that the
electrocataltytic activity of glycerol oxidation in the present study shows higher oxidation
and lower onset potential than those of Pt/CNT and Pt/C electrocatalyst (Figure 13). This
enhanced catalytic activity and reduced poison effect may be due to the synergy effect
among the metal (Pt and PtꢀSn) nanoparticles, ATCPꢀCPꢀBZD and CNT materials. From the
results, it has been concluded that the prepared Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀ
CPꢀBZDꢀCNT are good and potent electrocatalyst for glycerol oxidation.
Chromatographic analysis of intermediate product formed during glycerol oxidation
The high performance liquid chromatography (HPLC) measurements were carried
out to find the intermediate products formed during the electrooxidation of glycerol on
Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT catalysts. The HPLC analysis
evidences that both Pt and PtꢀSn nanoparticles present on ATCPꢀCPꢀBZDꢀCNT support are
able to dissociate glycerol into fragments containing a single carbon at a high percentage
under mild experimental conditions.
Figure 14 shows the chromatograms obtained from HPLC analysis during the
electrooxidation of glycerol on Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT
catalysts. The intermediate product formed during the electrooxidation of glycerol on
Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT catalysts were obtained by HPLC
analysis.
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Figure 14 Chromatograms obtained from HPLC analysis during the electrooxidation of
glycerol on Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT catalysts
Figure 15 shows the comparative bar graph of intermediates obtained during the
electrooxidation of glycerol on Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT
catalysts. The Pt/ATCPꢀCPꢀBZDꢀCNT catalyst exhibits 26.59 mA current for 4 hours and
yielding 66% of formic acid, 20% of glyceraldehyde and 14% of glycolic acid. Similarly,
PtꢀSn/ATCPꢀCPꢀBZDꢀCNT catalyst exhibits 38.89 mA current for the same 4 hours and
yielding 70% of formic acid, 17% of glyceraldehyde and 13% of glycolic acid. Glycerol is
first oxidized to form three intermediates namely glyceraldehyde, glycolic acid and formic
acid. The formed glyceraldehyde and glycolic acid are oxidized and they undergo cleavage
of C–C bond to produce CO2 as a byproduct. Similarly, the formic acid gets oxidized to give
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CO2. Unfortunately, the HPLC is unable to identify the formation of CO2 or carbonates.32
However, the cyclic voltammograms of Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀ
BZDꢀCNT catalysts (Figures 11 & 13) evidenced the absence CO poison on anode
catalyst.28 Hence, the above discussion gives clear information regarding the intermediate
products formed during the glycerol oxidation process except CO2 generations.
Figure 15 The comparative bar graph of intermediates produced during the electrooxidation
of glycerol on Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT catalysts.
Effect of scan rates
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Figure 16 Effect of scan rate on (a) Pt/ATCPꢀCPꢀBZDꢀCNT (b) PtꢀSn/ATCPꢀCPꢀBZDꢀ
CNT nanocatalyst in 0.5 M KOH + 0.5 M glycerol (scan rate = 20, 50 and 100 mVsꢀ1). The
inset image shows the dependence of the oxidation currents on the square root of scan rates.
The high performance of Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT
modified electrodes make us to know the nature of mass transport behaviour of the alcohol.
The forward cycle of cyclic voltammograms has been recorded at three different scan rates
(20, 50 and 100 mVs−1) in 0.5 M KOH + 0.5 M glycerol solution (Figure 16 (a&b)).
Figure 16 confirms that the oxidation current increases with an increase in the scan rates.
The insert figure shows the plot between the oxidation current versus the square root of the
scan rates that furnishes the linear relations with linear regression equation correlation
coefficient values of 0.9995 and 0.9978 for Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀ
BZDꢀCNT respectively, which are confirming the transport properties of glycerol. The
above results concluded that the glycerol oxidation reaction is a diffusionꢀcontrolled
reaction processes.27
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Figure 17 Cyclic voltammograms of (a) Pt/ATCPꢀCPꢀBZDꢀCNT (b) PtꢀSn/ATCPꢀCPꢀ
BZDꢀCNT electrodes in 0.5M KOH solution at a scan rate of 50 mVsꢀ1 after
electrooxidation of glycerol.
Furthermore, the adsorption and diffusion of the glycerol have been observed
practically by carrying out the electrochemical measurement in 0.5 M KOH solution after
carrying out the glycerol oxidation reaction. From the Figure 17 (a & b), the oxidation peak
current is observed for first and second cycle of the voltammograms but in the third cycle no
oxidation peak current has been observed. The results confirm the diffusion and adsorption
of glycerol molecules on the surface of the Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀ
BZDꢀCNT materials. Hence, this is also practically confirming that the electrooxidation of
glycerol is diffusionꢀcontrol process.
Effect of concentration of glycerol
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Figure 18 Effect of glycerol concentration in 0.5 M KOH solution on Pt/ATCPꢀCPꢀBZDꢀ
CNT catalyst
The effect of concentrations of glycerol (0.25, 0.50, 0.75, 1.00 and 1.25 M) on the
surface of Pt/ATCPꢀCPꢀBZDꢀCNT electrode was carried out in 0.5 M KOH solution at a
scan rate of 50 mV/sꢀ1 (Figure 18). From the cyclic voltammogram, the oxidation peak
current increases with the increase of glycerol concentration up to 1.00 M and decrease
further increase in the concentration of glycerol. This can be attributed due to the saturation
of active sites on Pt/ATCPꢀCPꢀBZDꢀCNT electrocatalyst with glycerol that hinders the OH
adsorption on Pt sites, which is essential for the electrooxidation of glycerol. Hence, the
decrease of oxidation peak current has been observed in the case of 1.25M glycerol
solution.31
Chronoamperometric analysis
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Figure 19 Chronoamperograms of Pt/ATCPꢀCPꢀBZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀ
CNT electrocatalysts in 0.5 M KOH and 0.5 M glycerol solution at a potential of 50mVsꢀ1.
Figure 19 shows the chronoamperograms of Pt/ATCPꢀCPꢀBZDꢀCNT and Ptꢀ
Sn/ATCPꢀCPꢀBZDꢀCNT modified graphite electrodes in 0.5 M KOH + 0.5 M glycerol.
From the Figure 19, it was noticed that the steady state current decrease initially, but after a
few seconds the constant steady state current has been observed for Pt/ATCPꢀCPꢀBZDꢀCNT
and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT electrocatalysts. The decrease in the steady state current
initially is due to the formation of doubleꢀlayer charge and the carbonaceous intermediates
on the surface of the electrode during the electrooxidation of glycerol.33 The steady state
current observed from chronoamperometric analysis is in good accord with the current
observed from the cyclic voltammetry analysis.
From the chronoamperometric analysis, it was concluded that the Pt/ATCPꢀCPꢀ
BZDꢀCNT and PtꢀSn/ATCPꢀCPꢀBZDꢀCNT are good electrocatalysts with long term
stability for the glycerol oxidation, which are due to small particle size, uniform distribution
of metal nanoparticles on ATCPꢀCPꢀBZDꢀCNT composites and synergic effect among
metal nanoparticls (Pt & PtꢀSn), ATCPꢀCPꢀBZD and CNT in Pt/ATCPꢀCPꢀBZDꢀCNT and
PtꢀSn/ATCPꢀCPꢀBZDꢀCNT composites.
29