Cost effective and energy efficient catalytic support of Co and Ni in Pd matrix toward ethanol oxidation reaction: Product analysis and mechanistic interpretation

Article abstract of DOI:10.1016/j.apcata.2018.05.015

The present investigation deals with the comparative analysis of electro-catalytic behaviour of Pt, Pd and ternary combinations of Co and Ni with Pd NPs, supported on vulcan XC72 as the anode component in direct ethanol fuel cell (DEFC) operating in alkaline environment. Catalyst NPs were synthesized by ethylene glycol reduction method and their structure, composition and surface morphology were determined through XRD, EDAX and TEM techniques. The superb catalytic efficiency of PdCoNi/C toward ethanol oxidation reaction (EOR) is ascribed to the catalytic intervention of transition metal ad atoms and their surface oxides, culminating to enhanced electrochemical surface area, preferred OH^? adsorption on the surface and remarkable yield of oxidation products (CH₃CO₂^? and CO₃^{2 ?}) estimated by ion chromatography. The performance output parameters collectively substantiate not only to the catalytic superiority of the PdCoNi/C catalyst but also affordability to a considerable extent over both the Pt/C and Pd/C catalysts.

Full text of DOI:10.1016/j.apcata.2018.05.015

Accepted Manuscript
Title: Cost effective and energy efficient catalytic support of
Co and Ni in Pd matrix toward ethanol oxidation reaction:
product analysis and mechanistic interpretation
Authors: Achintya Mondal, Abhishek De, Jayati Datta
PII:
S0926-860X(18)30236-9
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.05.015
APCATA 16664
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
3-3-2018
11-5-2018
18-5-2018
Please cite this article as: Mondal A, De A, Datta J, Cost effective and energy
efficient catalytic support of Co and Ni in Pd matrix toward ethanol oxidation reaction:
product analysis and mechanistic interpretation, Applied Catalysis A, General (2018),
https://doi.org/10.1016/j.apcata.2018.05.015
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Cost effective and energy efficient catalytic support of Co and Ni in
Pd matrix toward ethanol oxidation reaction: product analysis and
mechanistic interpretation
Achintya Mondal, Abhishek De and Jayati Datta*‡
Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah-711103, India
*
E-mail: jayati_datta@rediffmail.com
‡
Present Addresses: Department of Chemistry, Renewable Energy Research Centre, Heritage Institute of
Technology, Kolkata-700107, West Bengal, India.
Graphical Abstract:
1

Highlights

Pd40Co38Ni22/C catalyst with closed packed structure bear particle size in the range 2.5-
.5 nm and demonstrate remarkable functional features for ethanol oxidation in DEFC.
3

Smart catalytic intervention of the transition metal oxides/ hydroxides in the ternary
matrix accelerate the EOR almost toward completion, as evidenced by high yield of
acetate and carbonate as oxidation products.

Power density derived from an in-house DEFC using Pd40Co38Ni22/C is 47.71% higher
than with Pd/C and the ternary composition proved to be extremely poison tolerant.
Abstract. The present investigation deals with the comparative analysis of electro-catalytic
behaviour of Pt, Pd and ternary combinations of Co and Ni with Pd NPs, supported on vulcan
XC72 as the anode component in direct ethanol fuel cell (DEFC) operating in alkaline
environment. Catalyst NPs were synthesized by ethylene glycol reduction method and their
structure, composition and surface morphology were determined through XRD, EDAX and TEM
techniques. The superb catalytic efficiency of PdCoNi/C toward ethanol oxidation reaction
(EOR) is ascribed to the catalytic intervention of transition metal ad atoms and their surface
−
oxides, culminating to enhanced electrochemical surface area, preferred OH adsorption on the
−
and CO
2 −) estimated by ion
surface and remarkable yield of oxidation products (CH
3
CO
2
3
chromatography. The performance output parameters collectively substantiate not only to the
catalytic superiority of the PdCoNi/C catalyst but also affordability to a considerable extent over
both the Pt/C and Pd/C catalysts.
2

Keywords. direct ethanol fuel cell, ethylene glycol reduction, Pd based catalyst, transition metal
oxide, ion chromatography.
1
. Introduction
Among the fuel cell systems, in recent times, direct ethanol fuel cell (DEFC) has been one of the
cleanest choice for energy conversion with high potential advantages that ethanol could offer as a
fuel, in respect of 12 electron release during complete oxidation, its abundance from biomass as
well as compatibility to the existing fuel infrastructure [1-3]. However, being C2 molecule,
electro-catalysis of ethanol is constrained with sluggish oxidation kinetics due to difficulty in C-
C bond cleavage and ultimate poor poison tolerance [4]. Efforts are taken to overcome the
situation by designing stringent catalysts within multimetallic framework that improve the
functional behaviour in terms of electronic as well as, bi-functional effects of the co-metals that
proactively take part in the electro-catalysis.
Particularly in search of non Pt catalyst for ethanol oxidation reaction (EOR), Pd has been
reported to be an efficient base metal in alkali environment [5, 36]. There are few reports which
explain the significant role of oxides and/ hydroxides toward electro-oxidation of alcohols on Pd
surfaces. The combinatorial approach towards developing energy efficient electro-catalyst
involves incorporation of noble/ non noble metals into the Pd matrix [6- 12].
Fengping Hu et. al. [13] reported on the promoting activity of oxides on modified Pd surfaces
with significant performance stability towards ethanol oxidation reactions in alkali medium.
Addition of Co to Pd can facilitate the removal of adsorbed ethoxy intermediates due to easy
adsorption of hydroxyl groups on Co and/ Co-oxide at lower potentials as reported by S. Han et
3

al. [14] Bimetallic catalysts are normally found to exhibit higher currents for ethanol oxidation
than their single counterparts; however, the oxidation product selectivity is shifted predominantly
toward the acetic acid pathway, leading to less efficient use of the fuel [15-17, 25- 26]. Recently,
PdAu bimetallic nanoparticles (NPs) with unalloyed nanostructures have been reported by our
group, to exhibit much improved electro-catalytic behaviour in EOR study [18]. Shen et. al. [19]
reported ternary PdIrNi nano catalyst with increased peak power density compared to the
monometallic Pd nano catalyst. Bambagioni et. al. synthesized Pd based ternary alloys in
alkaline medium and demonstrated promising EOR activities on Pd-(Ni-Zn)/C and Pd-(Ni-Zn-
P)/C catalysts [20, 27]. In several of our earlier reports we have discussed on the involvement of
Ni and its oxide/hydroxide species as well as Au NPs in the EOR sequence over PdNi and
PdNiAu NPs [21, 22]. Besides all these, the introduction of a second or third metal or metal
oxide promoter, particularly from the transition metal series, to a non Pt base matrix has been
rarely reported.
In course of searching cheap and efficient electro-catalyst for ethanol oxidation in fuel cell, in
this attempt, we have synthesized carbon supported nano-structured ternary PdCoNi catalyst for
the study of EOR and demonstrated the concerted effort of Co and Ni toward promoting the
catalytic performances of the ternary matrix. The single and ternary catalyst matrices were
developed through the reduction of the respective metal chlorides (Pt, Pd, Co and Ni) by
modified ethylene glycol reduction method [23-24, 28, 37]. Physical and electrochemical
characterizations were performed in order to determine particle size, morphology, identify
phases and derive electrode kinetic behaviour of the synthesized materials toward EOR in
alkaline electrolyte at room temperature. The working electrolytes were further subjected to ion
chromatographic analysis to identify and estimate the products formed during the course of EOR,
4

thereby elucidating the mechanistic pathway. Finally, the single cell testing is carried out using
the membrane electrode assembly (MEA) comprising of synthesized catalysts as anode, Pt/C
catalyst as cathode and the alkaline anion exchange membrane sandwiched in between.
2
. Experimental Section
2
.1. Synthesis of Catalysts
A 40:60 catalyst-support ratio was maintained during synthesizing the catalyst regime by taking
appropriate amount of Vulcan XC-72 carbon (Carbon India Ltd., Brunauer-Emmett- Teller
2
− 1
(
BET) surface area, 288 m g ) and the metal precursors H
2
PtCl
6
, 6H
2
O, PdCl
2
,xH
2
O,
NiCl (Arora Matthey Ltd.) and CoCl2,6H O (A.R, E Merck, Germany) in their respective
2
2
proportions. The mixture was subjected to chemical reduction using ethylene glycol (E-Merck)
0
solution at 160 C temperature and in alkaline pH to obtain the respective metallic deposits of Pt,
Pd, Ni, Co as well as PdCoNi (1:1:1). Similar synthetic procedure was followed for developing
PdCo/C and PdNi/C binary catalysts used only for surveying the electrochemical surface of the
catalysts and to figure out the additional contribution of the transitional metals in the ternary
matrix.
2
.2. Surface Morphology and Structural Characterization
The crystalline structures of the supported catalysts were revealed through the powder X-ray
diffraction (XRD) technique using a Rigaku (mini flex II) parallel beam X-ray diffractometer
operating with Cu Kα radiation (λ = 0.1540 nm) generated at 35 kV and 30mA. Scans were
−
1
recorded at 1° min for 2θ values between 20°and 90°. The XRD patterns were analyzed
following the JCPDS file, and Scherrer and Bragg’s formulae were used to calculate the mean
diameter and the lattice parameter of each of the catalyst. The chemical compositions of the
5

multimetallic catalyst layers were determined by energy-dispersive X-ray spectroscopy (EDX)
using a Link ISIS EDX detector (Oxford Instruments, UK) coupled with the transmission
electron microscope. The catalysts suspended on a standard carbon-coated Cu grid were
subjected to high-resolution transmission electron microscopy (HRTEM) using JEOL JEM 2100
operating at an accelerating voltage of 200 kV. Specimens for TEM analysis were prepared by
suspending the particles in alcohol in an ultrasonic cleaner (Model AU-30C, 4 L capacity, 100
W, 28 KHz, 130 VA, timer 15 mins, Eyela, Tokyo, Rikakikai Co., LTD). A drop of the
suspension was deposited onto a standard carbon-coated Cu grid (GCU300, 300 mesh square,
ProSciTech, Australia) and allowed to dry before being inserted into the microscope. The
particle size distribution (histograms) was obtained by the observation of about 300 particles
from the four different locations of the corresponding TEM images.
2
.3. Electrochemical Studies
The fabrication of the electrodes included usual method of preparing catalyst ink by mixing 2-
propanol (GR, Merck) with an appropriate amount of the catalyst in 5 wt% Nafion ionomer
(Eloctrochem. Inc., USA) followed by sonication. A calculated amount of this slurry was
2
micropipette out onto the graphite plate (GLM grade, Graphite India Ltd.) of 0.65 cm surface
−
2
area maintaining a constant catalyst loading of 0.77 mg cm . Electrochemical measurements
were recorded by the help of a microprocessor controlled potentiostat/ galvanostat (PG stat 12
and FRA modules, Ecochemie BV, Netherlands) in a three electrode assembly cell containing 1
M ethanol (Merck, A.R) in 0.5 M NaOH solution and consisting the synthesized catalysts as the
working electrodes, bright Pt-foil (10 mm × 10 mm) counter electrode and mercury/ mercurous
oxide (Hg/HgO) reference electrode with respect to which the potentials in this article are
expressed.
6

2
.4. Estimation of Reaction Products
The ion chromatography system (Metrohm’s Advanced Modular Ion Chromatography)
consisting of a L-7100 pump (Metrohm Ltd.), a conductivity detector, and Metrosep A Supp 5-
2
50 and Metrosep A Supp 4-550 organic acid columns was employed to quantify the yields of
acetate and carbonate ions produced during the electrolysis by taking aliquots from the working
electrolyte after polarization at a constant potential of -300 mV for 1 hr at room temperature. The
entire work of electrochemical characterization and the chromatographic estimation were carried
out in inert atmosphere by purging N
2
(XL grade from BOC India Ltd.) into the solution before
atmosphere within the working cell set up.
experimentation and also maintaining N
2
2
.5. Performance Testing of DEFC
Membrane Electrode Assembly (MEAs) was prepared by hot pressing and arranged with
synthesized catalysts (as anode), alkaline anion exchange membrane, AEM (A-006, purchased
from Tokuyama Corp. Japan) and Pt/C (40% metal/C as cathode) catalyst. Metal loading is
-2
same (1.0 mg cm Pt loading) for both the anode and cathode catalysts and the exposed area of
2
the MEA is 1cm . Fuel cell testing was performed at 40 °C with the test station of Fuel Cell
Technologies Inc, USA. Flow of alkaline ethanol solution to the anode is maintained at 1.0 mL
−
1
3
−1
min , while oxygen is fed into the cathode chamber at a flow rate of 100 cm (STP) min .
3
. Results & Discussion
3
.1. Electron Microscopy and X-ray Diffraction Studies
7

Figure 1 shows the typical EDX spectrum of the as-prepared PdCoNi/C catalyst. The
compositional analysis is given in Table 1. The EDX analysis of the metallic phase demonstrates
the presence of each of the individual metals with proportionate existence of 40:38:22 in the
PdCoNi/C matrix. The EDX composition of the as-prepared PdCoNi/C catalyst more or less
agreed with the precursor bath compositions except that Ni was less deposited compared to the
other two metals.
Figure 2a shows the XRD patterns of Pd/C, Co/C, Ni/C and ternary catalyst Pd40Co38Ni22/C.
Each of the XRD patterns show characteristic peaks of face centered cubic (fcc) crystalline facets
of Pd, Co and Ni which exhibit diffraction of (111), (200), (220), (311), and (222) facets
corresponding to fcc crystal structure according to the respective JCPDS file 46-1043 for Pd, 15-
0
806 for Co and 04-0850 for Ni. The diffraction peaks of Pd/C like Pd (111), Pd (200), Pd (220),
0
0
0
0
and Pd (311) appear at 39.85 , 46.4 , 67.924 and 81.8 respectively, for Co/C the respective
0
0
0
facets are (111), (200) and (220) corresponding to 2θ = 45 , 52.02 , and 75.77 and for Ni/C the
0
0
0
(
111), (200) and (220) facets appear at 2θ = 43.79 , 51.02 and 74.74 respectively. The largest
peak shift is noticed for Pd40Co38Ni22/Ccatalyst for which Pd peaks (111), (200), (220), and (311)
0
are shifted 0.2 right, indicating possible alloy formation in the ternary matrix. For
0
Pd40Co38Ni22/Cthe shifted peak with respect to Pd/C corresponded to 2θ values at 40.05 for (111)
0
0
0
0
(
Figure 2b), 46.67 for (200), 68.3 for (220), 81.83 for (311) and 86.51 for (222) facets.
Similarly the peak shifting inPd40Co38Ni22/Cwith respect to Co/C are obtained at 2θ =
0
0
0
0
4
5
5.10 (111), 52.42 (200), 76.0 (220) and with respect to Ni/C, obtained at 2θ = 44.20 (111),
0
0
1.26 (200) and 78.01 (220).
Some of the critical oxide phases identified (Figure 2c) in the XRD pattern of the ternary matrix
are Ni
2
O
3
H (141, JCPDS file 40-1179), NiOOH (108, JCPDS file 06-0075), NiOOH (211 and
8

1
21 JCPDS file 27-0956), Co
3
O
4
(511), Ni(OH)
2
(110), NiOOH (310), Co
3
O
4
(440) and Ni(OH)
2
0
0
0
(
220, JCPDS file 22-0444) corresponding to the peaks appearing at 44.36 , 51.58 , 54.61 ,
0
0
0
0
0
5
9.71 , 60.87 , 62.14 ,65.0 and 70.78 respectively. The addition of Ni and/or Co into the Pd,
eventually suppresses the growth of the ensemble of metallites and the particle size is found to
decrease in the multimetallic matrix, as calculated using Scherrer’s equation D=0.9λ/ B cosθ
considering the Pd (111) peak, where D is the crystallite size nm, λ is the wave length (0.154 nm
for Cu-Kα), B is the full width at half-maximum (FWHM) (in radians) and θ is the diffraction
angle. Depending on the XRD results, the interplanar distance dhkl with Miller Indices (hkl) for
the catalysts were estimated according to Bragg’s equation nλ=2dsinθ. It is discernible from
Table 1 that incorporation of transition metals into the Pd result in decreasing lattice parameter,
inter planer distance and average crystallite size indicating alloy formation and /or close pack
multimetallic matrix formulation.
Figure. 3a and 3b shows the TEM images and histogram (size distribution) as derived from
ensemble of 300 particles of the Pd/C and Pd40Co38Ni22/Ccatalysts. As observed, the metal
particles on both the catalysts exhibit spherical shape and are well dispersed on the meso porous
carbon powder. The size distribution range from 1 to 7 nm and the average particles size in the
catalyst matrix Pd40Co38Ni22/C is 3.6 nm which corroborates with the XRD data. The SADP
represented by the inset of Figure 3b indicates the formation of nano-phases of Pd40Co38Ni22/C
with good crystallinity and the rings are indexed for an fcc crystal structure. Figure 3c shows the
typical fringe pattern of the Pd/C nano-particles indicating (111) plane of Pd. Some well defined
fringe-finger prints (Figure 3d) of Pd40Co38Ni22/C were used to measure the inter-planar d-
spacing with the help of corresponding FFT pattern (Figure 3d inset ) where from the lattice
spacing are found to be around 0.227, 0.233 and 0.204 nm, corresponding to (111) planes of Pd,
9

Co and Ni respectively. Although XRD results indicate alloy formation, distinct existence of
trimetallic phases of Co, Ni & Pd are also evidenced from the fringe patterns. This signifies that
the catalyst is not entirely composed of perfectly alloyed particles throughout the matrix but
there is possibility of the simultaneous existence of unalloyed composite particles of Pd, Co, Ni
along with the alloyed phases. A distinct quenching of the particle size in the trimetallic
composite is worth noting, 58 % from XRD and 41% from TEM analysis, compared to the single
metal catalyst. This is attributed to the comparatively lower atomic radii of the transitional co-
metals, Co (1.25 Å) and Ni (1.25 Å) compared to Pd (1.37 Å).
3
.2. Voltammetric studies
Figure.4 shows the cyclic voltammograms (CV) for Pt/C, Pd/C, Co/C, Ni/C and Pd40Co38Ni22/C
-1
catalysts obtained in 0.5 M NaOH solution at a scan rate of 50 mVs within the potential
window -1000 mV to 800 mV. The hydrogen region is prominent for Pt/C, Pd/C and the ternary
systems while no significant contribution is made by the bare Co and Ni in this region. The
respective CV of Co/C and Ni/C however demonstrate the oxide formation and oxide reduction
region as their special features.
The voltammogram of Pd40Co38Ni22/C catalyst exhibit enormous current density in the anodic
and cathodic sweeps compared to the single counterparts. In the anodic sweep considerable
expansion is observed for the critical regions like hydrogen adsorption region (HAD) followed
by premature double layer charging and pronounced current output in the oxide formation
region, unusually aggravated with the simultaneous effect of transition metals Co, Ni and Pd
itself. The double layer region covers a considerable potential range (480 mV) from -270 mV to
2
10 mV. The colossal oxidation current beyond 0.270 V is one of the remarkable features of the
surface functional behavior of the ternary catalyst. The oxide formation within potential span
1
0

3
50-600 mV resembles the pattern of Co/C as well as Ni/C catalysts, however with much
amplified current production indicating the formation of an oxide reservoir in the multimetallic
catalyst matrix. This is distinctly reflected in the reverse scan where the usual Pd-oxide reduction
appears with enhanced current output in the potential range -200 mV to -650 mV and an
additional cathodic peak appears with much broader area and high current output indicating
reduction of co-metal oxides (Co and Ni) in the potential range 410 mV to -150 mV, henceforth
designated as Transition Metal Oxide Reduction (TMOR) Co,Ni region. The expanded area in the
cathodic reduction regions, collectively for TMOR as well as Pd oxide reduction become an
added advantage in course of promoting EOR kinetics as discussed later.
The electrochemical surface area (ECSA) of the catalysts were derived from the voltammgrams
by integrating the charge on hydrogen adsorption–desorption (HAD) and adsorbed oxide
reduction (AOR) regions and summarized in Table 2 [21, 29]. Although Pt surface exhibits
considerable ECSA values, it ultimately suffers from poor poison tolerance, whereas ECSA
values for both HAD and AOR regions were extraordinarily increased for Pd40Co38Ni22/C. This
may be a reflection of particle size diminution in the mixed metallic matrix as discussed earlier.
Moreover, the special feature of the huge oxide store discernible in the forward scan may also be
responsible for the enhanced cathodic current in the AOR region in the reverse sweep.
For better understanding of the synergic effect manifested in the voltammetric feature of the
ternary matrix, a magnified view of the voltammogram of the single metal and ternary catalyst
(
Pd40Co38Ni22/C) are presented in Figure S1 (supplementary material) along with the probable
surface reactions. It may be understood that the charges corresponding to the different regions of
the voltammogram reflects the relative electro-chemical activity of the catalyst surface. The
values obtained from the CV plots are summarized in Table 3. The most interesting phenomena
1
1

for the ternary catalyst is that it produces enormously large area in the (TMOR)Co,Ni region that
add testament to the superior surface electro-activity of this ternary composition.
At this stage, for precise screening of the electrochemical surface activity and to elucidate the
multi coupled effect of Co & Ni in the Pd matrix, voltammograms were also recorded with
binary PdCo/Cand PdNi/C catalysts in 0.5 M NaOH solution under similar conditions and
demonstrated in Figure S2 (supplementary material). The binary catalysts also show similar
characteristic feature in the TMOR region, however not as much enlarged as in the case of the
combined ternary matrix. It is seen from Table 3 that the charges exhibited by the ternary
catalyst, especially in TMOR region, is almost 4.6, 4.1, 8 and 6.5 times greater than those
obtained with the respective catalysts Co/C, Ni/C, PdCo/C and PdNi/C under similar
potentiodynamic condition. Hence, the individual contributions of Co and Ni toward formulating
a catalytically efficient ternary (PdCoNi) matrix, is an advantageous conjunction translating to
fabulous increase in the area [(TMOR)Co,Ni+ AORPd] which in effect becomes extremely useful
in promoting EOR kinetics as recorded by the outstanding electro-efficiency of this particular
combination.
In fact the concerted effort of each of the co-metals in this multimetallic ensemble creates
structurally and electronically favored ternary matrix (Pd40Co38Ni22/C) with ample
electrochemically active reaction centers. The smaller particle size and uniform size distribution
throughout the catalyst matrix obtained from XRD and TEM analysis are also in tandem with the
higher ECSA values for the Pd40Co38Ni22/C catalyst.
Infact the electronic effect in the combined matrix also plays an important role in propagating the
EOR as discussed in the next section. The charge distribution among the co-metals is depicted in
Scheme1. Due to higher ionization energy of Pd (8.34 eV) than Co (7.88 eV) and Ni (7.63 eV),
1
2

the electronic charge transfer may take place from Ni to Pd, Co to Pd and Ni to Co to Pd. This
induces extra positive charge on Ni and Co atoms and facilitates the formation of different
-
oxides/hydroxides of Co and Ni while δ charges on Pd sites facilitate easy anchoring of the
ethanol molecule undergoing the electro-oxidation.
The existence of NiOOH species on Ni containing noble metal catalyst surface is well known,
and this moiety actively takes part in hydrogen spillover and increases the charge in that region.
On the other hand, Co, having affinity toward forming multivalent oxides, provides substantial
oxide coverage throughout the catalyst matrix.
Figure 5 represents the characteristic voltammograms of EOR in the potential range -1 V to 0.8
V on Pt/C, Pd/C, Co/C, Ni/C and Pd40Co38Ni22/C. The voltamograms clearly demonstrates that
the combined formulation of the cheap transition metals and Pd can greatly enhance the
electrochemical activity for ethanol oxidation in alkaline medium. Form the inset of Figure 5,
the onset potential (Eonset) of EOR follow the order Pd40Co38Ni22/C< Pt/C <Pd/C. As expected,
the renewed oxidation (in the reverse sweep)on the ternary catalyst starts much earlier (less – ve
potential) compared to the single metal catalyst and the current density (Irenewed) and the onset
potential (Erenewed onset) for the renewed oxidation follow the positive potential shift
PdCoNi/C>>Pd/C> Pt/C as shown in Figure 6a and 6b. The respective forward and renewed
-
2
-2
oxidation peak current densities are 108 mA cm and 83 mA cm for Pd40Co38Ni22/C, 75.9 mA
-2
-2
-2
-2
cm and 32 mA cm for Pt/C while 65mA cm and 28 mA cm for Pd/C. Figure 7 shows that
the forward and renewed oxidation peak potential difference is much reduced in case of
Pd40Co38Ni22/C which is a typical behavior that translates to catalytic superiority of the ternary
composite over the others. All these electrochemical features like higher peak current and
reduced value of E
f
-E , the early onset of the renewed oxidation reflect the excellence of the
b
1
3

composite in promoting the EOR kinetics by oxidative removal of the carbonaceous residues on
the multimetallic matrix, designed with non Pt noble and non noble transition metals.
3
.3.Chronoamperometic studies and estimation of oxidation products
The intermediates produced during the oxidation of ethanol get adsorbed on electro-catalysts and
poison their catalytic activity and in effect destroy their stability. Hence to investigate upon the
deactivation of the catalytic surface, chronoamperometric studies were carried out with Pd/C and
PdCoNi/C catalysts in the working electrolyte at a fixed potential of −0.3 V for 3600 s (Figure
8
a).
The resulting curves indicate that the PdCoNi/C electrode produce much higher EOR current
density after 3600 s while the poisoning rate (δ) is reduced to half (0.00479 % per second) the
1
/2
value obtained for Pd/C(0.01 % per second). The derivative Anson’s plot (Q Vs t , Figure 8b)
allows the determination of the amount of charge involved in the process for single and ternary
catalysts whereby faster electrolysis is demonstrated in case of the multimetallic matrix.
Figure 9a & 9b shows the ion chromatograms for the product analysis during EOR carried out
on Pd/C and Pd40Co38Ni22/C electrodes for a period of 1 h at room temperature. The respective
products, acetate and carbonate formed during the oxidation of ethanol were estimated and
summarized in Table 4.
On the basis of the remarkably high yield of acetate and carbonate (6.1 and 6.7 times than on
Pd/C and 4.8 and 5.7 times than on Pt/C) on the ternary catalyst, we have tried to outline a
mechanistic pathway for the oxidation of ethanol, assisted by the conjugate effort of the co-
metals in the trimetalic ensemble of NPs (Pd, Co and Ni) each contributing to the kinetic input
1
4

for the electron extraction process, where 60% of the catalyst loading is comprised of transition
metals.
3
.4. Mechanistic overview of EOR
-
Pd NPs have intrinsic ability to absorb OH at lower potentials in alkaline electrolyte while Co
and Ni both are oxophilic and enrich the catalyst surface with oxy / hydroxy species [Co
CoO, NiOOH, Ni H, Ni(OH) etc] that play vital role in activating the surface for favourable
electrochemical oxidation of ethanol molecule.
3
4
O ,
2
O
3
2
The fair possibilities of EOR to proceed along parallel mechanistic pathways are: (i) Faster
removal of intermediate species (CH
3
CO)ads/(CH
3
O)ads on the OH studded electrode surface at
-
low potentials leading to significant amount of acetate production as well as CO
3
formation to
some extent (ii) catalytic intervention of Ni through Ni(OH)
2
species and (iii) assistance of Co
oxide rich network by forming methoxy linkage that enable to reach the ultimate CO
production. The possible reaction sequences are highlighted.
2
(i) Water activation at low potential assisting EOR (rigorous involvement of OH ads)
(
퐶퐻 퐶퐻 푂퐻)
→ (퐶퐻 퐶퐻 푂퐻)
(3)
(4)
(5)
(6)
(7)
3
2
푠표푙
푎푑푠
푎푑푠
3
2
푎푑푠
−
+ 푒−
푂퐻 → (푂퐻)
−
+ ꢀ푒−
푎푑푠
(퐶퐻 퐶퐻 푂퐻)
+ ꢀ푂퐻 → (퐶퐻 퐶푂)
3
2
3
(퐶퐻 퐶푂)
+ (푂퐻)푎푑푠 → 퐶퐻 퐶푂푂퐻
3
3
푎푑푠
−
−
퐶퐻 퐶푂푂퐻 + 푂퐻 → 퐶퐻 퐶푂푂 + 퐻 푂
3
3
2
1
5

−
−
−
(
퐶퐻 퐶푂푂 )
+ ꢁ푂퐻 → (퐶퐻 푂)
+ 퐶푂 + 퐻 푂 + ꢀ푒
(8)
(9)
3
푎푑푠
3
푎푑푠
2
2
−
−
(
퐶퐻 퐶푂푂퐻)
+ ꢀ푂퐻 → (퐶퐻 푂)
+ 퐶푂 + ꢀ퐻 푂 + ꢀ푒
3
푎푑푠
3
푎푑푠 2 2
-
The adsorbed carbonaceous species (CH
poisoning by (CO)ads
3
O)ads readily interact with further OH leading to surface
.
−
−
(
퐶퐻 푂)
+ ꢀ푂퐻 → (퐶푂)
+ ꢀ퐻 푂 + ꢀ푒
(10)
3
푎푑푠
푎푑푠
2
-
However at suitable pH condition and with adequate supply of OH , (CO)ads is likely to get
oxidized, producing the ultimate species CO 2-.
3
−
2−
−
(
퐶푂)푎푑푠 + (푂퐻)푎푑푠 + ꢀ푂퐻 → 퐶푂 + ꢁ퐻 푂 + 푒
(11)
3
2
(ii) Role of Ni and its oxides
The contribution of Ni oxy/ hydroxyl species towards assisting EOR has been reported in our
earlier work with PdNiAu [8, 21, 30]. There are few reports that indicate Ni(OH) and NiOOH
2
bear high electron and proton conductivity. In alkaline medium, Ni(OH)
2
can produce NiOOH
+
-
+
species by releasing H which combines with OH to produce water while absorption of H by
NiOOH may also take place to produce Ni(OH)
2
. Thus a rapid equilibrium between the Ni(OH) /
2
NiOOH species is attained through the spillover of hydrogen, which eventually gears up the
oxidation of ethanol on the multimetallic matrix [31-32].
The catalytic intervention of NiOOH [33-34] toward ethanol oxidation is demonstrated in the
following reaction sequences and also schematically represented (Scheme 2):
−
−
푁푖(푂퐻) + 푂퐻 ↔ 푁푖푂푂퐻 + 퐻 푂 + 푒
(12)
(13)
2
2
−
푁푖푂푂퐻 + 퐶퐻 퐶퐻 푂퐻 → 퐶퐻 퐶퐻 푂 + 푁푖(푂퐻) + 푒
3
2
3
2
2
1
6

−
푁푖푂푂퐻 + 퐶퐻 퐶퐻 푂 → 퐶퐻 퐶퐻푂 + 푁푖(푂퐻) + 푒
(14)
(15)
(16)
3
2
3
2
−
푁푖푂푂퐻 + 퐶퐻 퐶퐻푂 → 퐶퐻 퐶푂 + 푁푖(푂퐻) + 푒
3
3
2
−
푁푖푂푂퐻 + 퐶퐻 퐶푂 → 퐶퐻 퐶푂푂퐻 + 푁푖(푂퐻) + 푒
3
3
2
Furthermore, Ni(OH)
2
is known to generate NiO that exists in the dimeric form (NiO)
2
[
푁푖(푂퐻) → 푁푖푂 + 퐻 푂 &ꢁ푁푖푂 → (푁푖푂) ], assisting the ethanol oxidation process as has
2
2
2
been elaborately discussed in our earlier study on EOR over PdNi/C catalyst [22].
moiety drags the reaction to the ultimate CO 2
-
formation in alkaline
It was suggested that (NiO)
2
3
medium by direct or indirect pathway (i) conversion of ethanol to acetaldehyde (ii) conversion
of acetaldehyede to acetic acid and (iii) conversion of acetaldehyede to carbonate.
(iii) Role of Co and its oxides
In case of Co/Co oxide existing in the matrix there is possible formation of [–O–Co–O–Co–O–]
type network which effectively can take part in the ethanol oxidation process [35]. In one of our
previous articles with PtPd/MoO
3
-Polypyrrole matrix, such kind of linkage in MoO has been
3
reported to be instrumental in the electro-catalytic oxidation of ethanol.
In the present case, we suggest that the intermediate (–CH
metal oxide network to produce the methoxy linkage (equation17). The latter interacts with
OH)ads on the metal matrix (M-OH) whereby electron extraction is facilitated and reaction
proceeds toward the final release of CO (equation 18-19) as described in the following reaction
3
CO)ads species gets associated with the
(
2
sequences and also represented schematically. (Scheme 3)
푀(퐶퐻 퐶푂) + 푂 ꢂ 퐶ꢃ ꢂ 푂 ꢂ 퐶ꢃ → 퐶퐻 푂 ꢂ 퐶ꢃ ꢂ 푂 ꢂ 퐶ꢃ + 푀 ꢂ 퐶푂
(17)
3
3
1
7

퐶퐻 푂 ꢂ 퐶ꢃ ꢂ 푂 ꢂ 퐶ꢃ + 푀 ꢂ 푂퐻 → 퐶퐻 푂 ꢂ 퐶ꢃ ꢂ 푂 ꢂ 퐶ꢃ ꢂ 푂퐻 + 푀
(18)
(19)
3
3
−
4ꢄ
ꢉ
퐶퐻 푂 ꢂ 퐶ꢃ ꢂ 푂 ꢂ 퐶ꢃ ꢂ 푂퐻 ꢅ ꢆꢇ 퐶푂 + 퐶ꢃ ꢂ 푂 ꢂ 퐶ꢃ + ꢈ퐻
3
2
The Scheme 4 represents an overall model for EOR phenomena on the multi-metallic catalyst
matrix. The addition of transition metals to Pd NPs remarkably improves the kinetic efficacy of
EOR by virtue of their excellent co-catalytic activity in terms of rapid elimination of
carbonaceous residues from the surface and propagating the reaction toward completion.
For the further evidences in support of the electro-catalytic activities, typical Nyquist plots are
derived from impedance spectra of Pd/C and Pd40Co38Ni22/C at the anodic potential of -300 mV.
Detailed discussions related to impedance study have been included in the supplementary
material.
3
.5. Electrical performance of DE(AEM)FC
Finally the performances of the catalysts (Figure 10) are tested in the single cell of the fuel cell
test station. Open circuit voltage (OCV) and maximum current density for Pd/C catalyst are
-2
found to be 0.76 V and 213mAcm while for the ternary catalyst the values are 0.83 V and 263
-2
mA cm respectively. Even when the Pd loading is 60 % reduced to in the ternary catalyst, the
performance characteristics (OCV and current density) are far beyond those for the individual
metal catalyst. Obviously the intervention of Co, Ni towards electro-catalysis is further reflected
-2
in the significant power density ~39.44 mW cm ) obtained for the ternary catalyst compared to
-2
2
6.70 mW cm for bare Pd catalyst.
4
. Conclusion
1
8

In this investigation, carbon supported ternary PdCoNi nano-catalysts were synthesized by the
simultaneous ethylene glycol reduction method and the determination of morphology, structure
and composition of the NPs reveal that the trimetallic particles composed of alloyed and
unalloyed phases bear average size in the range 2.5-3.5 nm and indicate ample existence of
transition metal oxides as vital component of the catalyst materials. A set of electrochemical
techniques like cyclic voltammetry, potentio-dynamic polarization, chronoamperometry and the
performance screening in an in house fabricated DE(AEM)FC demonstrates the outstanding
functional behaviour of the mixed metallic matrix particularly with the incorporation of Ni and
Co to Pd and the latter being reduced to 40% in the catalyst matrix. The transition metal/ metal
oxides take proactive role in the electro-catalysis and provide multifunctional activities in
tandem with Pd in the matrix. The ethylene glycol reduction proved to be an efficient route for
fabricating the catalyst NPs. A series of multiple reactions occur in coordination with the
reaction centers created at the self-designed multimetallic matrix which enable EOR to progress
toward completion. Overall, the synergy of the bifunctional effect and the electronic charge
distribution enable the Pd40Co38Ni22/C catalyst composition to promote ethanol oxidation at a
lower energy and faster rate than the single and binary catalyst counterparts.
Acknowledgements
Authors gratefully acknowledge the financial support by Council of Scientific and Industrial
Research (CSIR) [01(2847)/16/EMR-II, 12.05.2016], New Delhi, India and Department of
Science and Technology (DST) (SR/S1/PC-63/ 2012, 08.07.2013), New Delhi. A. De wishes to
acknowledge the Senior Research Fellowship from the Council of Scientific and Industrial
Research (CSIR), New Delhi, India.
1
9

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2
4

Figure.1. Typical EDX spectrum of Pd40Co38Ni22/C catalysts.
2
5

(b)
(a)
(
C)
Figure.2. (a) X-ray diffraction patterns of Pd/C, Co/C, Ni/C & Pd40Co38Ni22/C catalysts.
0
0
0
0
Magnified view of the diffraction peaks at 2θ (b) 35 to 45 and (c) 44 to 74 .
2
6

1
1
1
1
1
8
6
4
2
0
8
6
4
2
0
(a)
(
b)
2
1
1
0
5
0
5
0
3-4
4-5
5-6
6-7
7-8
Particle size/ nm
1-2 2-3 3-4 4-5 5-6 6-7
Particle size/ nm
(
d)
(c)
Figure. 3 TEM image with histogram (a) Pd/C (b) Pd40Co38Ni22/C, Inset: SADP, (c) Fringe
pattern of Pd/C, (d) Fringe pattern of a single particle having three different d-spacing for
Pd40Co38Ni22/C, inset: FFT pattern of the corresponding fringe pattern.
2
7

Figure.4. Cyclic voltammograms of Pt/C, Pd/C, Co/C, Ni/C and Pd40Co38Ni22/C in 0.5 M NaOH
−1
recorded at scan rate 0.05 Vs .
2
8

–
1
Figure.5. Cyclic voltammograms of EOR on single and ternary catalysts. Sweep rate: 50mVs .
-2
Inset: Bar diagram of the onset potential recorded at current outputs 4.00 mA cm for Pt/C, Pd/C
and Pd40Co38Ni22/C catalysts.
(a)
(b)
2
9