Platinum-copper doped poly(sulfonyldiphenol/cyclophosphazene/benzidine)-graphene oxide composite as an electrode material for single stack direct alcohol alkaline fuel cells

Article abstract of DOI:10.1039/c7ra04525e

The present work has attempted to prepare platinum-copper (Pt-Cu) and platinum (Pt) nanoparticle-deposited poly(sulfonyldiphenol/cyclophosphazene/benzidine)-graphene oxide catalysts for the electrochemical oxidation of alcohols and single stack direct alkaline alcohol fuel cells. Electrochemical performance of methanol and ethylene glycol is measured using Pt-Cu/poly(SDP/CP/BZ)-GO and Pt/poly(SDP/CP/BZ)-GO catalysts as the working electrode in KOH solution using cyclic voltammetry analysis. Platinum-copper and platinum nanoparticles embedded with the poly(SDP/CP/BZ)-GO composite show enhanced electrooxidation current, lower onset potential and good CO tolerance compared to that of Pt/GO and Pt/poly(SDP/CP/BZ) catalysts. Further, the Pt-Cu/poly(SDP/CP/BZ)-GO catalyst exhibits enhanced catalytic activity with respect to lower onset potential and higher oxidation current than that of the Pt/poly(SDP/CP/BZ) catalyst. Hence, single stack direct alcohol alkaline fuel cells are constructed using the Pt-Cu/poly(SDP/CP/BZ)-GO catalyst as an electrode material. The optimum power densities of 112.24 and 140.67 mW cm^-2 are observed for methanol and ethylene glycol in single direct alkaline alcohol fuel cells.

Full text of DOI:10.1039/c7ra04525e

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Platinum–copper doped poly(sulfonyldiphenol/
cyclophosphazene/benzidine)–graphene oxide
composite as an electrode material for single stack
direct alcohol alkaline fuel cells
Cite this: RSC Adv., 2017, 7, 34922
Prasanna Dakshinamoorthy and Selvaraj Vaithilingam
*
The present work has attempted to prepare platinum–copper (Pt–Cu) and platinum (Pt) nanoparticle-deposited
poly(sulfonyldiphenol/cyclophosphazene/benzidine)–graphene oxide catalysts for the electrochemical
oxidation of alcohols and single stack direct alkaline alcohol fuel cells. Electrochemical performance of
methanol and ethylene glycol is measured using Pt–Cu/poly(SDP/CP/BZ)–GO and Pt/poly(SDP/CP/BZ)–GO
catalysts as the working electrode in KOH solution using cyclic voltammetry analysis. Platinum–copper
and platinum nanoparticles embedded with the poly(SDP/CP/BZ)–GO composite show enhanced
electrooxidation current, lower onset potential and good CO tolerance compared to that of Pt/GO and Pt/
poly(SDP/CP/BZ) catalysts. Further, the Pt–Cu/poly(SDP/CP/BZ)–GO catalyst exhibits enhanced catalytic
activity with respect to lower onset potential and higher oxidation current than that of the Pt/poly(SDP/CP/
BZ) catalyst. Hence, single stack direct alcohol alkaline fuel cells are constructed using the Pt–Cu/poly(SDP/
Received 22nd April 2017
Accepted 4th July 2017
DOI: 10.1039/c7ra04525e
ꢀ2
CP/BZ)–GO catalyst as an electrode material. The optimum power densities of 112.24 and 140.67 mW cm
rsc.li/rsc-advances
are observed for methanol and ethylene glycol in single direct alkaline alcohol fuel cells.
The development of novel electrode materials operated at low
temperature is essential for the diverse applications. A potential
1
. Introduction
The direct alkaline alcohol fuel cell (DAAFC) is a potent and advantage of the direct alkaline alcohol fuel cell is that the alcohol
attractive electrochemical device, which generates electricity from in alkaline medium is less structure sensitive of the catalyst than in
liquid alcohol fuel due to its promising energy conversion, lower acid media. Hence, the structure of the catalyst will not be changed
corrosion, high energy density, high energy conversion efficiency, during the electrooxidation of alcohol in alkaline medium.
low operating temperature and environmental friendliness. The
Graphene oxide (GO) is a good catalyst support due to its high
signicant advantages of direct alcohol fuel cells (DAFC) such as, electron in-plane transport rate, simple preparation method and
easy to handle, store, transport and refuel, make the DAFC an low cost carbon in comparison with CNT. Graphene oxide based
attractive substitute to batteries and combustion engines. Recent nanocomposites are paying more attention as a new material
reports state that the alcohol oxidation in alkaline medium is because of their extraordinary physical and chemical properties for
1–5
kinetically faster than in acidic media. Moreover, the cost of various applications. Graphene oxide based nanocomposites were
proton conducting polymer membranes like Naon used in direct applied for the fabrication of electronic devices, batteries, super
6–9
acid alcohol fuel cells is higher than that of anion exchange capacitor, solar cells, fuel cells and electrocatalytic applications.
membranes used in direct alkaline alcohol fuel cell applications. Further, the recent reports state that the GO supported platinum
Further, the poisoning issue of the electrode material and alcohol nanoparticles composite exhibits superior electrocatalytic activities
crossover limits the performance of proton exchange membranes. and high poison tolerance for alcohol oxidation when compared to
10–13
In addition, the direction of the electro-osmotic drag is from the that of other carbon supported platinum nanoparticles.
anode to the cathode in acid medium, which reduces the cell
Furthermore, to enhance the stability, activity and dispersibility
performance. The reaction kinetics and catalytic activities of the of metal nanoparticles, the GO are coupled with conductive poly-
anodic oxidation of fuel and the cathodic reduction of O
in mers like poly(aniline), poly(pyrrole), poly(thiophene), etc., to
alkaline media are signicantly higher than those in acidic media. a form conductive polymer/GO composites.
2
14–16
Such, composite
materials show strong interaction with metal nanoparticles, which
exhibits better electrocatalytic activity and stability. Hence, a new
type of conductive polymer is attempted to coat over the graphene
Nanotech Research Lab, Department of Chemistry, University College of Engineering
Villupuram, (A Constituent College of Anna University, Chennai), Kakuppam, oxide surface to get covalent bonded conductive polymer–gra-
Villupuram-605 103, Tamilnadu, India. E-mail: vaithilingamselvaraj@gmail.com;
phene oxide composite along with p–p stack interaction. The
vselva@aucev.edu.in; rajselva_77@yahoo.co.in; Fax: +91-4146-224500
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designing of new composite materials with
a
2 2
hexa- mixture was stirred for another 1 h. Finally, 15 mL of 30% H O
chlorocyclotriphosphazene group as a linker for multiple mono- solution was added to the above reaction mixture in order to
mers as well as with graphene oxide have attracted more attention terminate the reaction. The solid product was separated by
because of their special desired coupling to get a new type of centrifugation process and washed three times with 5% HCl
linkage between graphene oxide and monomers during the poly- solution. The resulting solid was re-dispersed in DI water and
merization process. The composite covalent bond along with non- dialyzed for three days to remove the residual salts and acids.
covalent bond formation such as p–p stack interaction, the van The solid suspension was ltered and dried in a vacuum oven at
ꢁ
der Waals force of attraction and hydrogen bonding are expected 45 C for 48 h to obtain graphite oxide for further use.
to show good thermal stability, catalytic properties, electrical
17
conductivity, and stable under aggressive condition.
2.3. Preparation of poly(sulfonyldiphenol/
With this view in mind, the present work is projected to cyclophosphazene/benzidine)–graphene oxide (poly(SDP/CP/
prepare poly(sulfonyldiphenol/cyclophosphazene/benzidine) BZ)–GO) composite
polymer–graphene oxide composite as a supporting material
for platinum (Pt) and platinum–copper (Pt–Cu) nanoparticles.
Pt–Cu/poly(SDP/CP/BZ)–GO catalyst is fabricated and subjected
For the preparation of poly(SDP/CP/BZ)–GO composite, 10 mg
of graphene oxide was dispersed in 80 mL of DMSO solution.
2
0 mg of hexachlorocyclotriphosphazene (CP), 29 mg of benzi-
to electrooxidation of alcohol oxidation in alkaline medium.
Further, the electrooxidation of alcohol oxidation on Pt/pol-
y(SDP/CP/BZ)–GO, Pt/poly(SDP/CP/BZ) and Pt/GO were also
investigated in order to conrm the efficiency of poly(SDP/CP/
BZ)–GO composite as supporting material. The electrochemical
performance investigation concludes that the poly(SDP/CP/BZ)–
GO composite is a good supporting material for the deposition
of metal nanoparticles with uniform distribution and shows
improved electrocatalytic oxidation of methanol and ethylene
glycol for fuel cell applications. Further, the best performed
catalyst Pt–Cu/poly(SDP/CP/BZ)–GO was used to fabricate single
stack direct alcohol alkaline fuel cell and their performance
were checked under different experimental condition to opti-
mize the single stack test fuel cell.
0
dine, 29 mg of 4,4 -sulfonyldiphenol (SDP) and 4 mL of trie-
thylamine (TEA) were added to the above reaction mixture. The
polycondensation reaction was carried out in an ultrasonic bath
(100 W, 40 kHz) at room temperature for 12 h. Aer completion
of the reaction, the poly(SDP/CP/BZ)–GO composite was ltered
and its impurities were removed by washing repeatedly with
ꢁ
distilled water and then dried at 60 C in an oven for 24 h.
2.4. Synthesis of platinum and platinum–copper
nanoparticles deposited poly(SDP/CP/BZ)–GO composite
For the Pt/poly(SDP/CP/BZ)–GO catalyst preparation, 30 mg of
poly(SDP/CP/BZ)–GO composite was suspended in 5 mL
distilled water and stirred under ultrasonic treatment for
2 6 2
20 min. 54 mg of H PtCl $6H O dissolved in 20 mL of distilled
water is added to the above reaction mixture and the pH of the
reaction mixture was adjusted to 11 with 2.5 M NaOH. 1.0 mL of
formaldehyde (37%) was added to the above solution at 85 C in
2
. Experimental methods
ꢁ
2.1. Materials
order to initiate the reduction process to deposit the platinum on
poly(SDP/CP/BZ)–GO composite. The reaction was continued for
another 5 h in order to complete the reduction of platinum salts to
Pt nanoparticles. The resultant solid was ltered, washed with
distilled water and then dried at 60 C for 8 hours. The Pt–Cu
nanoparticles were deposited on 60 mg of poly(SDP/CP/BZ)–GO
Hexachlorocyclotriphosphazene (HCCP) and benzidine (BZ)
were purchased from Sigma Aldrich, India. Reagent grade 4,4 -
0
sulfonyldiphenol, potassium permanganate (KMnO
trated sulfuric acid (H SO ), sodium nitrate (NaNO
dehyde (HCHO), graphite powder, ethylene glycol, methanol,
ethanol, hexachloroplatinic acid hexahydrate (H PtCl $6H O)
and copper chloride (CuCl ) were purchased from SRL, India.
4
), concen-
2
4
3
), formal-
ꢁ
2
6
2
composite by following similar experimental procedure with 54 mg
of H PtCl $6H O, 14 mg of CuCl and 2 mL of HCHO to get Pt–Cu/
2 6 2 2
2
poly(SDP/CP/BZ)–GO catalyst. For comparative studies, the plat-
inum nanoparticles were also deposited on graphene oxide and
poly(SDP/CP/BZ) by utilizing similar experimental conditions.
2
.2. Preparation of graphene oxide
Graphene oxide was prepared from the natural ake graphite
according to the modied Hummers method.
method, 2.0 g of graphite powder and 4.0 g of NaNO were
18,19
In this
2.5. Physicochemical and electrochemical characterizations
3
ꢁ
added to 80 mL of cold (0 C) concentrated H SO in an ice bath. The physicochemical characterizations were done by using
2
4
4
Then, KMnO (8.0 g) was added gradually under stirring and FTIR, HRTEM and EDX analysis. The prepared graphene oxide
care was taken to maintain the reaction temperature below and poly(SDP/CP/BZ) materials have been examined by Fourier
ꢁ
10
C. The reaction mixture was continuously stirred for transform infrared (FTIR, Thermo Nicolet Model: 6700) spec-
another 4 h during which the reaction temperature is main- troscopy. The Raman spectra of graphite powder and graphene
ꢁ
tained below 10 C. Next, the reaction mixture was stirred at oxide are recorded using 1064 nm line of Nd:YAG laser as the
ꢁ
ꢀ1
3
5 C for further 4 h and then diluted with 200 mL of deionized excitation wavelength in the region 500–2500 cm on a Thermo
(
DI) water. The addition of water in concentrated sulfuric acid Electron corporation model Nexus 670 spectrophotometer. X-
ꢁ
medium releases a large amount of heat and thus the addition ray diffraction patterns were measured at 25 C temperature
of water was performed under an ice bath environment to keep by examining the diffraction angle 2q from 0 to 10 as the
the temperature below 100 C. Aer adding DI water, the standard on a Rich Seifert (Model 3000) X-ray powder
ꢁ
ꢁ
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diffractometer. The surface morphology, size of platinum and were performed with a galvanostat (Hokuto Denko, HA-501G).
platinum–copper nanoparticles deposited on poly(SDP/CP/BZ)– Each measurement was started aer the fuel was poured in
GO composite are analyzed by high resolution transmission the cell for 30 min. The cell voltage was recorded aer setting up
electron microscope (HR-TEM, JEOL) with an accelerating the current for 3 min to stabilize the voltage. The single direct
voltage of 120 kV. Energy dispersive X-ray analysis (EDX, INCA200) alcohol alkaline fuel cell operated at different concentration
substantiates the presence of platinum and platinum–copper and temperature for the optimization process.
nanoparticles on poly(SDP/CP/BZ)–GO composite.
3
. Results and discussions
2.6. Electrochemical characterizations
3.1. FT-IR and FT-Raman spectroscopy
Multichannel electrochemical workstation with inbuilt FRA
system (Biologic SAS, Model VSP2) was used for electrochemical
characterizations with a standard three-electrode glass cell
equipped with novel modied graphite electrodes as working,
saturated calomel electrode as reference and a platinum wire as
counter electrodes. Further, the electrocatalytic activity of
alcohol (methanol and ethylene glycol) oxidation has been
investigated in 0.5 M KOH and 0.5 M alcohol solution with
a potential range of ꢀ1.0 to 0.3 V at a scan rate of 20, 50 and
Graphite and graphene oxide materials were characterized by
FTIR and Raman analyses to conrm their respective structures.
Fig. 1(a) displays the FTIR spectra of graphite and graphene
oxide materials. The peaks appeared at 3405 and 1403 cm are
corresponding to the O–H stretching and bending vibration
present in graphene oxide. The C]O stretching vibration of
ꢀ1
–
COOH groups present in the graphene oxide sheets is observed
ꢀ1
at 1727 cm (ref. 20) and the C]O stretching of hydrogen
ꢀ
1
bonded carboxylic group has appeared at 1624 cm . The
characteristic peaks corresponding to the functional groups of
graphene oxide were absent in the case of graphite powder,
which infers the conversion of graphite powder into graphene
oxide material.
ꢀ
1
1
00 mV s . Chronoamperometric analysis was also done in
.5 M KOH and 0.5 M alcohol solution at its maximum peak
0
current potential. The CO stripping testing was carried out by
holding the working electrode in the carbon monoxide satu-
rated KOH solution (0.5 M) for 500 s, and then the working
electrode is placed in the nitrogen gas purged 0.5 M KOH
solution to record the CO stripping proles. The intermediate
products formed upon electrooxidation of ethylene glycol were
determined by using high-performance liquid chromatography
Raman spectroscopy is an effective, non-destructive and well
know tool to nd the structural characteristics and quality of
graphene based materials. So, the Raman spectra were carried
out for graphite and graphene oxide materials (Fig. 1(b)). The
Raman spectrum of graphite powder shows a single sharp G
(Dionex system P680 HPLC). It works with an isocratic elution
ꢀ
1
band at 1630 cm , which is corresponding to the vibration of
and mainly includes an autosampler (ASI 100 Automated
Sample Injector), a sample loop (20 mL), and an ion-exclusion
column (Aminex HPX-87H), which are operated at room
temperature. The analytes were separated with diluted KOH
2
sp carbon. The graphene oxide shows two peaks at 1645 and
ꢀ1
1
380 cm are known as the G band (growth band) and D band
(
disordered band), respectively. The broad and intense D-band
ꢀ1
3
appeared at 1380 cm
is corresponding to sp hybridized
ꢀ
1
ꢀ1
(
3.3 mmol L , Merck 96%) used as a eluent with 0.6 mL min
carbon atoms, which are conrming the graphene oxide
conversion from graphite material. Further, the shi in the G
band (growth band) is observed for graphene oxide, which
run rate. The chromatograph was equipped with an UV-Vis
detector (l ¼ 210 nm) followed by a refractive index detector
(IOTA2).
2
might be due to the reduction in size and in-plane sp domains
due to extensive oxidation. This indicates the defects within the
2
.7. Procedure for the construction of single stack test fuel
cell
Direct alcohol alkaline fuel cell was prepared by utilizing anion graphene “honeycomb” structure of GO.
exchange membrane (AEM) (thickness: 28 mm, IEC: 1.9 mmol Fig. 2 shows the FTIR spectra of graphene oxide and pol-
, Tokuyama Co.). 5 mg of electrode catalyst was dispersed in y(SDP/CP/BZ)–GO composite. The FTIR spectrum of poly(SDP/
21,22
graphitic sheets and edges.
The G band arises due to the
hexagonal lattice vibrations of GO, which is the characteristic of
ꢀ
1
g
ꢀ
1
a mixture 1 : 1 (v/v) ratio of 2-ethoxyethanol and anion exchange CP/BZ)–GO has the characteristic bands at 3309 and 840 cm
ionomer (Tokuyama Co.), and then stirred to form homoge- for –N–H stretching and wagging vibrations, respectively. The
neous slurry. Catalyst (2.5 mg) was coated on carbon paper by peak corresponding to C]O stretching of carboxylic group is
ꢀ1
brush coating method. The sandwiched layer like structure of observed at 1768 cm
.
ꢀ
1
the anode and cathode on both the sides of anion exchange
The shi in the peak position from 1727 to 1768 cm for
ꢁ
membrane was obtained by hot-pressing (120 C, 2 MPa for 10 C]O stretching of carboxylic group as in the case of poly(SDP/
min) method. The prepared sandwich type of anode and CP/BZ)–GO composite when compared to GO material indicates
cathode layers were placed between two copper meshes as the chemical interaction between the polymer and GO material.
current collector. The alcohols (methanol and ethylene glycol) The stretching and bending vibrations of O–H in GO spectrum
were purged in the inlet at anode side. The fuel diffused into the at 3405 and 1403 cm were disappeared as in the case of pol-
anode catalyst layer from the tank through the anode current y(SDP/CP/BZ)–GO composite, which conrms the chemical
collectors, while oxygen transferred into the cathode catalyst bonding between the polymer and GO material. The O]S]O
ꢀ
1
ꢀ1
layer from the surrounding air through the cathode current stretching vibration was appeared at 1380 cm indicates the
collectors by natural convection. The I–V curve measurements presence of sulfonyldiphenol on poly(SDP/CP/BZ)–GO material.
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Fig. 1 (a) FT-IR and (b) FT-Raman spectra of graphite and graphene oxide materials.
nanoparticles on poly(SDP/CP/BZ)–GO composite surface might be
due to the presence of multiple functional groups such as –NH,
P]N–, –C]O and S]O on poly(SDP/CP/BZ)–GO surface, which
are acting as a good anchoring site for metal nanoparticles depo-
sition. In addition to that the poly(SDP/CP/BZ)–GO composite will
favor good dispersion of Pt and Pt–Cu nanoparticles with an
average particle size of 2.8 and 2.2 nm, respectively.
Further, the EDX spectra of Pt/poly(SDP/CP/BZ)–GO and Pt–
Cu/poly(SDP/CP/BZ)–GO catalysts conrm the presence of
platinum and platinum–copper nanoparticles on the poly(SDP/
CP/BZ)–GO composite (Fig. 3(c and d) and Table 1). The EDX
spectra result also conrms that the composition of Pt and Cu
are nearly 1 : 1 ratio, which is good agreement with the calcu-
lated theoretical values.
3.3. Cyclic voltammetry analysis
Fig. 2 FTIR spectrum of graphene oxide and poly(SDP/CP/BZ)–GO
composite.
3.3.1. Electrooxidation of alcohols. The electrocatalytic
performance of Pt/poly(SDP/CP/BZ)–GO and Pt–Cu/poly(SDP/
CP/BZ)–GO catalysts were checked by cyclic voltammetry (CV)
studies in alkaline solutions of methanol and ethylene glycol
fuels. Fig. 4 shows cyclic voltammograms of Pt/poly(SDP/CP/
ꢀ
1
The characteristic peaks observed at 1268 and 968 cm
conrm the presence of P–O–C and P–N–C bond in poly(SDP/ BZ)–GO and Pt–Cu/poly(SDP/CP/BZ)–GO electrodes in 0.5 M
CP/BZ)–GO material. Further, the aromatic –C–H stretching KOH, 0.5 M KOH + 0.5 M methanol and 0.5 M KOH + 0.5 M
ꢀ
1
ꢀ1
vibration was observed at 3043 cm . Thus, the above discus- ethylene glycol at scan rate 50 mV s . The oxidation current of
0
sions conrm that the poly(SDP/CP/BZ)–GO is composed of 4,4 - methanol and ethylene glycol on Pt/poly(SDP/CP/BZ)–GO and
sulfonyldiphenol (SDP), cyclophosphazene (CP), benzidine and Pt–Cu/poly(SDP/CP/BZ)–GO catalysts are tabulated in the Table
GO materials.
2. Pt–Cu/poly(SDP/CP/BZ)–GO catalyst shows higher oxidation
current for methanol and ethylene glycol when compared to
that of Pt/poly(SDP/CP/BZ)–GO catalyst. For comparative
studies, electrooxidation of methanol were also carried out on
the Pt/GO and Pt/poly(SDP/CP/BZ) catalysts and their results
3.2. High resolution transmission electron microscope and
energy dispersive X-ray analysis
The size distribution, shape and surface morphology of the plat- conclude that the Pt/poly(SDP/CP/BZ)–GO catalyst shows
inum and platinum–copper nanoparticles present on poly(SDP/ enhanced catalytic activity current for methanol compared to
CP/BZ)–GO composite was analyzed by HRTEM measurements. Pt/GO and Pt/poly(SDP/CP/BZ) catalysts.
Fig. 3(a and b) shows the TEM photos of Pt/poly(SDP/CP/BZ)–GO
and Pt–Cu/poly(SDP/CP/BZ)–GO catalysts. The TEM photos CP/BZ)–GO catalyst might be due to (i) the introduction of
Fig. 3(a and b)) conrm the uniform deposited of platinum and poly(SDP/CP/BZ) polymer on graphene oxide with multi func-
The enhanced electrocatalytic performance of Pt/poly(SDP/
(
platinum–copper nanoparticles on the surface of the poly(SDP/CP/ tional groups such as –OH, –NH and S]O groups, which are
BZ)–GO. The uniform size and homogeneous distribution of metal helping to get the uniform size distribution of nanoparticles
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Fig. 3 HRTEM pictures of (a) Pt (b) Pt–Cu decorated poly(SDP/CP/BZ)–GO and EDX spectra of (c) Pt (d) Pt–Cu decorated poly(SDP/CP/BZ)–GO
catalysts.
with reduced aggregations. Their results generate more active interaction between metal nanoparticles and poly(SDP/CP/BZ)–
sites for adsorption of alcohols (methanol and ethylene glycol) GO composite.
towards electrochemical oxidation, (ii) poly(SDP/CP/BZ) present
Further, to reduce the cost of the electrode material without
in Pt/poly(SDP/CP/BZ)–GO can prevent the agglomeration of compromising the catalytic activity, the non precious metal was
graphene oxide as well as metal nanoparticles, which bring doped with platinum metal. This is not only reducing the cost,
homogeneous coexistence between the nanoparticles and (iii) but also enhances the catalytic activity. For this case, the copper
the electron transfer efficiency through an interfacial (Cu) was doped along with platinum on poly(SDP/CP/BZ)–GO
composite as supporting through in situ reduction method to
form Pt–Cu/poly(SDP/CP/BZ)–GO bimetallic system. The resul-
Table 1 Atomic and weight composition of Pt/poly(SDP/CP/BZ)–GO tant Pt–Cu/poly(SDP/CP/BZ)–GO catalyst was subjected to
and Pt–Cu/poly(SDP/CP/BZ)–GO catalysts
methanol and ethylene glycol oxidation in alkaline medium.
The cyclic voltammograms of Pt–Cu/poly(SDP/CP/BZ)–GO cata-
Catalysts
Weight composition%
Atomic composition%
lyst for methanol and ethylene glycol in 0.5 M KOH solution at
ꢀ
1
a scan rate of 50 mV s were shown in Fig. 4.
Pt100
100
100
Pt50–Cu50
51 : 49
50 : 50
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Fig. 4 Cyclic voltammograms of Pt and Pt–Cu decorated poly(SDP/CP/BZ)–GO electrode in (a) 0.5 M KOH (b) 0.5 M KOH + 0.5 M methanol and
ꢀ1
(
c) 0.5 M KOH + 0.5 M ethylene glycol at scan rate 50 mV s .
Pt–Cu/poly(SDP/CP/BZ)–GO catalyst shows signicantly distribution of electrons in platinum. This effect is making more
ꢀ
1
enhance oxidation current of 45.03 and 49.60 mA mg
for capable and possibilities for the adsorption of alcohols such as
methanol and ethylene glycol oxidation, respectively. From the methanol and ethylene glycol on platinum surfaces. Hence, this
results (Table 2), it was concluded that Pt–Cu/poly(SDP/CP/BZ)–GO phenomenon stimulates to increases the adsorption and the
catalyst shows considerably greater oxidation peak current than electrooxidation rate of methanol and ethylene glycol, etc.
that of Pt/poly(SDP/CP/BZ)–GO catalyst. The enhanced electro-
Further, the enhanced electrooxidation of methanol and
catalytic activity of Pt–Cu/poly(SDP/CP/BZ)–GO may be stimulated ethylene glycol can be explained based on their intermediate
due to the changes in the geometric and electronic structure of the formation during the oxidation process as given below.
ꢀ
Pt nanoparticles with the introduction of second metal like copper,
(i) Methanol. The formation of HCOO ion and ‘CO’ species
chemical interaction between poly(SDP/CP/BZ) and GO present in formed during the electrooxidation of methanol,
poly(SDP/CP/BZ)–GO composite and the synergy effect between the
ꢀ
ꢀ
Pt + CH
3
OH + 4OH / Pt–CO + 4H
2
O + 4e
(1)
(2)
(3)
(4)
metal nanoparticles and poly(SDP/CP/BZ)–GO composite. In
addition to that the copper is more electronegative metal than the
platinum, so the interaction between platinum and copper present
in the Pt–Cu/poly(SDP/CP/BZ)–GO catalyst may change the
+
2
Pt–CO + Pt–OH / CO + 2Pt + H
ꢀ
ꢀ
Pt + CH OH + 5OH / Pt–OOCH + 4H O + 4e
3
2
ꢀ
Pt–OOCH + Pt–OH / 2Pt + CO
2
+ H
2
O + 2e
Table 2 Oxidation peak currents of methanol and ethylene glycol on
Pt/poly(SDP/CP/BZ)–GO and Pt–Cu/poly(SDP/CP/BZ)–GO catalysts
Hence, still there is possible to ‘CO’ poisonous species which
is unavoidable in the presence of platinum metal, though
enhanced oxidation current is observed as in the case of Pt/
poly(SDP/CP/BZ)–GO, when compared to Pt/GO and Pt/
poly(SDP/CP/BZ) catalysts.
ꢀ1
Current (mA mg
Methanol
)
Catalyst
Ethylene glycol
Pt/GO
10.50
13.21
27.24
45.03
12.83
16.49
37.99
49.60
According to the above proposed mechanism, major inter-
Pt/poly(SDP/CP/BZ)
Pt/poly(SDP/CP/BZ)–GO
Pt–Cu/poly(SDP/CP/BZ)–GO
ꢀ
mediate formed is HCOO and ‘CO’ poison during electro-
oxidation of methanol. Copper in Pt–Cu/poly(SDP/CP/BZ)–GO
ꢀ
catalyst will assist the catalyst to undergo through the ‘HCOO ’
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ꢀ
ion formation and it also assist to remove the HCOO formed BZ)–GO catalyst could enhance the regeneration of poisoned
during electrooxidation of methanol. In general, the electro- platinum, which is more reactive towards intermediate
24–27
oxidation of small organic compounds on metal electrodes products.
occur in the presence of reactive M(OH)ads species formed by Hence, the copper particles can increase the availability of
hydroxyl adsorption on the electrode surfaces. Hence, the easily more numbers of active Pt sites for the adsorption and elec-
formed Cu–OHads (at lower potential when compared to Pt) can trooxidation of ethylene glycol. The rate of oxidation of the
readily oxidize the Pt–(HCOO)ads to CO
2
, which can increase the intermediate product proceeding through eqn (8) is always
availability of active Pt sites for the adsorption and electro- higher than that of the oxidation proceeding through the eqn
oxidation of methanol (as given below).
(7), which might be due to the easy formation of Cu–OH species
at lower potential when compared to Pt–OH species.
PtꢀðHCOOÞ
ꢀ
ꢀ
ads
Cu þ OH /CuꢀOHads þ e ƒƒƒƒƒƒƒƒƒ!
PtꢀHCOOꢀ
ꢀ
þ 2eꢀ (5)
Pt þ OH/PtꢀOHads þ e ƒƒƒƒƒƒƒƒ!
Pt þ Cu þ H
2
O þ CO
2
2
Pt þ CO
2
þ H
2
O þ 2eꢀ
(7)
(8)
PtꢀCOads
ꢀ
Cu þ OH /CuꢀOHads þ eads ƒƒƒƒƒƒ ƒƒ !
PtꢀHCOOꢀ
ꢀ
þ 2eꢀ
Cu þ OH/CuꢀOHads þ e ƒƒƒƒƒƒƒƒ!
Pt þ Cu þ CO
2
(6)
Cu þ Pt þ CO
2
2
þ H O
The rate of reactions in accordance with the eqn (5) and (6)
are higher than that of the reaction occurring in accordance
with the eqn (2), which results enhanced catalytic activity in the
presence of ‘Cu’ nanoparticles on Pt/poly(SDP/CP/BZ)–GO
catalyst.
The above results conclude that the Pt–Cu/poly(SDP/CP/BZ)–
GO catalyst shows better catalytic activity through bi-functional
effect than that of Pt/poly(SDP/CP/BZ)–GO catalyst. In addition
to that Pt/poly(SDP/CP/BZ)–GO and Pt–Cu/poly(SDP/CP/BZ)–GO
catalysts show better catalytic activity than that of the Pt/GO and
(ii) Ethylene glycol oxidation. Ethylene glycol produces formic
Pt/poly(SDP/CP/BZ)
composite is a potent and good supporting material with
enhanced activities for fuel cell applications.
catalyst.
So,
poly(SDP/CP/BZ)–GO
acid as a major intermediate along with other byproducts such
as glyoxal, glycolic acid, glyoxylic acid and oxalic acid during the
electrooxidation process.
23
According to the above proposed mechanism, the major
Overall, the oxidation current density of ethylene glycol is
product formed during ethylene glycol oxidation is HCOOH, found to be higher than that of the oxidation current density of
which can be easily removed by the adding copper as second methanol, which may be the nature of byproduct formed during
metal to Pt/poly(SDP/CP/BZ)–GO catalyst. The addition of the electrooxidation of methanol. But, the ethylene glycol
copper could increase the catalytic activity by adsorbing the oxidation produces various intermediates such as glyoxal, gly-
ꢀ
OH ions in the electrolyte solution (KOH) at lower potential colic acid, glyoxylic acid, oxalic acid and formic acid in alkaline
comparable to that of platinum. Hence, copper as co-catalyst solution (Fig. 7). Further, most of the C–C bond cleavage in
will help to remove the intermediate product like formic acid ethylene glycol takes place at higher potential, which will
or CO or some other intermediates formed during electro- prevent the CO poisoning of platinum. However, the methanol
oxidation of ethylene glycol.
cleavage mostly takes place at lower potential and hence the
In addition to that the electrooxidation small organic oxidation of ‘CO’ poison may not possible at a lower potential,
molecule/intermediates formed during the electrooxidation of which will decrease the active surface area that are essential for
alcohols on metal electrodes occurs in the presence of reactive the adsorption and oxidation of methanol. Hence, the prepared
M(OH)ads species formed through hydroxyl group adsorption on catalysts show higher oxidation current density for ethylene
the electrode surfaces. Hence, the formed Cu–OHads can readily glycol when compared to methanol.
oxidize the Pt–(HCOO)ads to CO
2
compared to that of Pt–OHads
.
3.3.2. CO stripping and chronoamperometric analysis. The
In this regard, the copper nanoparticles in Pt–Cu/poly(SDP/CP/ CO tolerance of Pt/poly(SDP/CP/BZ)–GO and Pt–Cu/poly(SDP/
34928 | RSC Adv., 2017, 7, 34922–34932
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Fig. 5 CO stripping voltammograms for (a) Pt (b) Pt–Cu decorated poly(SDP/CP/BZ)–GO catalysts in 0.5 M KOH solution at a scan rate 50 mV
ꢀ
1
s
and chronoamperograms of Pt and Pt–Cu decorated poly(SDP/CP/BZ)–GO catalysts in (c) 0.5 M KOH + 0.5 M methanol and (d) 0.5 M KOH +
.5 M ethylene glycol.
0
CP/BZ)–GO catalysts were determined by COads striping analysis conrms the relative stability, reproducibility and higher cata-
Fig. 5(a and b)). Pt–Cu/poly(SDP/CP/BZ)–GO catalyst shows lytic activity of Pt–Cu/poly(SDP/CP/BZ)–GO catalyst.
lower onset potential and peak potentials for carbon monoxide From the overall electrooxidation analysis, it was observed
(
oxidation than that of the Pt/poly(SDP/CP/BZ)–GO catalyst. The that the Pt–Cu/poly(SDP/CP/BZ)–GO catalyst exhibits higher
onset electrode potential is negatively shied by 23 mV, which catalytic activity when compared to Pt–Cu/poly(SDP/CP/BZ)–GO.
indicating that the doping of copper as co-metal on Pt/poly(SDP/ So, it has been nominated as the electrode material for the
CP/BZ)–GO catalyst can signicantly improve the CO resistance construction of single stack test fuel cell for its power genera-
or oxidation. Therefore, the CO stripping results authenticate tion performance.
the stability and reduced poisonous effect of Pt–Cu/poly(SDP/
CP/BZ)–GO catalyst.
3.3.3. HPLC analysis. High performance liquid chroma-
tography (HPLC) analyses were carried out to predict the
The stability with respect to time and reproducibility related intermediate products obtained during the ethylene glycol and
to activities of Pt/poly(SDP/CP/BZ)–GO and Pt–Cu/poly(SDP/CP/ glycerol oxidation on Pt, and Pt–Cu deposited poly(SDP/CP/BZ)–
BZ)–GO catalysts in methanol and ethylene glycol oxidation GO composite. The HPLC analysis conrms that Pt and Pt–Cu
were determined by chronoamperometry analysis. The chro- nanoparticles present on poly(SDP/CP/BZ)–GO are able to
noamperometry analysis results of Pt/poly(SDP/CP/BZ)–GO and oxidize the ethylene glycol into single carbon fragments at
Pt–Cu/poly(SDP/CP/BZ)–GO catalysts in 0.5 M KOH + 0.5 M a high percentage under mild experimental conditions. The
methanol and 0.5 M KOH + 0.5 M ethylene glycol solutions were chromatograms obtained from HPLC analysis during the elec-
shown in Fig. 5(c and d). The current density decreases rapidly trooxidation of ethylene glycol on Pt and Pt–Cu deposited pol-
for both the catalysts at the initial stage, which may be due to y(SDP/CP/BZ)–GO composite was shown in Fig. 6.
the formation of the intermediate carbonaceous species during
The Fig. 6 shows ve intermediate peaks, which are corre-
28
methanol and ethylene glycol oxidation reaction. The steady sponding to glyoxal, oxalic acid, glyoxylic acid, glycolic acid and
state current observed for the as prepared catalysts clearly formic acid formed during ethylene glycol oxidation. At rst, the
ethylene glycol oxidized to give ve intermediates such as
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Fig. 6 (a) Chromatograms obtained from HPLC analysis and (b) comparative bar graph of intermediates produced during the electrooxidation of
ethylene glycol on Pt/poly(SDP/CP/BZ)–GO and Pt–Cu/poly(SDP/CP/BZ)–GO catalysts.
glyoxal, oxalic acid, glyoxylic acid, glycolic acid and formic acid.
3.3.4. Fabrication of single stack test cell using Pt–Cu/
The intermediates like glyoxal, oxalic acid, glyoxylic acid and poly(SDP/CP/BZ)–GO catalyst as electrode material. Initially, the
glycolic acid are further oxidized and they undergo cleavage of single stack direct alcohol alkaline fuel cell performances were
C–C bond to produce CO . In the same way, the formic acid checked with respect to different temperature and concentra-
2
oxidized to give CO . Hence, the above discussion gives clear tion of the fuels using Pt–Cu/poly(SDP/CP/BZ)–GO as electrode
2
information regarding the intermediate products formed material. For this, the Pt–Cu/poly(SDP/CP/BZ)–GO catalyst
ꢀ2
during the ethylene glycol oxidation.
(2 mg cm ) was used as electrode material with an active
Fig. 7 Current density with respect to different (a) concentration of methanol and (b) temperature (c) concentration of ethylene glycol and (d)
temperature on Pt–Cu/poly(SDP/CP/BZ)–GO electrode in single stack test cell.
34930 | RSC Adv., 2017, 7, 34922–34932
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2
surface area of 2.5 ꢂ 2.5 cm . The methanol ethylene glycol Pt–Cu/poly(SDP/CP/BZ)–GO electrode for ethylene glycol (2.0 M)
ꢀ
1
fuels were pumped with a ow rate of 2.0 mL min into the in 1.0 M KOH solution.
anode side and the pure oxygen gas was passed on the cathode Hence, the direct alkaline alcohol test fuel cell is compared
3
ꢀ1
side at a ow rate of 100 cm min . Fig. 7(a–d) show the cell using the concentration of 2.0 M alcohol fuels in 1.0 M KOH
ꢁ
polarization curves of methanol and ethylene glycol obtained by solution at a constant temperature of 70 C on Pt–Cu/poly(SDP/
plotting the current densities against the potential for Pt–Cu/ CP/BZ)–GO catalyst as electrode material. The performance of
poly(SDP/CP/BZ)–GO electrode in single direct alkaline alcohol Pt–Cu/poly(SDP/CP/BZ)–GO electrode in single stack fuel cell
fuel cells at different concentrations and temperatures. From with respect to the current density, power density and voltage
ꢁ
the single stack fuel cell test results, it was clearly noticed that relationship under optimum temperature (70 C) and alcohol
the current densities of the cell were increased with increasing concentration (2.0 M) were shown in Fig. 8. From the Fig. 8, the
ꢀ2
the concentration of the fuels and the temperature of the cell. maximum power densities of 112.24 and 140.67 mW cm were
Further, it was observed that the current density and open noticed for methanol and ethylene glycol fuels, respectively.
circuit potential were increased with respect to increases in the Further, the observed power density of the Pt–Cu/poly(SDP/CP/
temperature.
BZ)–GO catalysts are found to be higher than that of the liter-
29–53
For methanol fuel, the current density increases with ature data.
From the overall studies, it was concluded that
increasing the methanol concentration up to 4.5 M methanol the prepared Pt–Cu/poly(SDP/CP/BZ)–GO catalyst is a potent
solution in 1 M KOH solutions and further increase in the electrode for direct alkaline alcohol fuel cells.
concentration of fuel shows no signicant changes in the current
densities. So, the maximum current density noticed for methanol
ꢀ
2
4. Conclusion
solution was 97 mA cm (Fig. 7(a)). The large increase in the
current densities was noticed up to 2.0 M methanol solution,
which may be taken as an optimum concentration of methanol.
Hence, the effect of temperature in 2.0 M methanol + 1.0 M KOH
In the present investigation, poly(SDP/CP/BZ)–GO composite as
supporting material was prepared for the electrooxidation of
methanol and ethylene glycol in favor of fuel cell applications.
For this application, platinum and platinum–copper nano-
particles were loaded on the novel poly(SDP/CP/BZ)–GO
composite material. The resulted catalysts were subjected to
electrooxidation of alcohols and CO tolerance studies to
conrm their performance for fuel cell applications. The
bimetallic Pt–Cu nanoparticles loaded poly(SDP/CP/BZ)–GO
exhibits good CO tolerance and higher electrocatalytic activities
of alcohols than that of platinum loaded poly(SDP/CP/BZ)–GO
composite. Hence, the better performed novel Pt–Cu/poly(SDP/
CP/BZ)–GO material was used for fuel cell construction. The
single stack direct alkaline alcohol test fuel cell with an active
solution is carried out at different temperature ranging from 20,
ꢁ
3
0, 40, 50, 60, 70 and 80 C (Fig. 7(b)). The effect of temperature
results concludes that the maximum current density obtained is
ꢀ
2
ꢁ
1
37 mA cm at 70 C.
For ethylene glycol fuel, the current density increases up to
.5 M ethylene glycol in 1.0 M KOH solution. However, the
2
signicant increase in current density was noticed upto 2.0 M
though an increase in current density was observed upto 2.5 M
ethylene glycol. So, the optimum concentration of the ethylene
glycol is taken as 2.0 M in 1.0 M KOH solution since the uniform
or signicant increase in the current density is observed up to
ꢁ
ꢁ
2
.0 M (Fig. 7(c)). The current density was increased up to 80 C,
2
surface area of 6.25 cm shows the maximum power density of
though the current density is increased dramatically at 70 C
ꢀ2
112.24 and 140.67 mW cm for methanol and ethylene glycol
ꢁ
(
Fig. 7(d)). So, the optimum temperature is taken as 70 C for
fuels, respectively.
Acknowledgements
The authors like to thank DST/Nanomission, New Delhi, India
for the nancial support to carry out this work and the estab-
lishment of Nanotech Research Lab through the grant No. SR/
NM/NS-05/2011(G).
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34932 | RSC Adv., 2017, 7, 34922–34932
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