Chemical Bulletin of Politehnica University of Ti misoara, ROMANIA [623658]

Chemical Bulletin of “Politehnica” University of Ti misoara, ROMANIA
Series of Chemistry and Environmental Engineering
Chem. Bull. "POLITEHNICA" Univ. (Timisoara) Volume 61(75), 1, 2016

Ca Doped Y-114 Layered Cobalt Perovskite Electrodes with Catalytic
Effect for Methanol Electrooxidation in Alkaline So lutions.
I. Voltammetric studies

M.L. Dan, N. Vaszilcsin, D.A. Duca and V.D. Craia J oldes

University Politehnica Timisoara, Faculty of Indust rial Chemistry and Environmental Engineering, 30022 3, Parvan 6, Timisoara,
Romania, e-mail: [anonimizat]

Abstract: This paper investigates anodic oxidation of methano l on a calcium doped cobalt layered perovskite type 114
electrode in aqueous alkaline solution (1M KOH). El ectrocatalytic activity formethanol anodic oxidatio n becomes a
serious issue, especially due to the utilization of layered cobalt perovskite electrodes in fuel cells . In order to understand
the oxidation mechanism on the surface of these typ e of electrodes, comprehensive researches were nece ssary.
Electrochemical behavior has been studied by cyclic voltammetry and linear polarization.

Keywords: Y0.5 Ca 0.5 BaCo 4O7, cobalt layered perovskite, methanolelectrooxidati on.

1. Introduction

Direct alkaline alcohol fuel cells with methanol or
ethanol benefit of height interest over the past 60 years
because of their advantageous reaction mechanism an d
kinetics in alkaline media, higher energy densities
achievable and easy handling of the liquid fuels [1 , 2].
This cell was initially equipped with Pt and Pt all oys
catalysts. Due to high costs, Pt was gradually repl aced
with others materials, having lower prices and
approximately the same catalytic activity: Pd, Au a nd Ag.
Currently, non-precious metal catalysts, lanthanum,
strontium oxides and perovskite-type oxides are tes ted
[2].First studies of alcohol oxidation in fuel cell s were
conducted by Palve in 1954 who demonstrated the
possibility to use methanol as fuel in aqueous elec trolytes
[3]. In direct alcohol fuel cells, methanol is dire ctly
oxidised at the anode in acidic or alkaline support
electrolytes. Methanol use was favored because it i s the
simplest alcohol with only one carbon in its molecu le and
without C-C bonds [2].
It is known that for alcohols oxidation processes,
electrocatalysts perform better in alkaline electro lytes.
Justi and Winsel developed in 1955 the first direct
methanol fuel cells operating in alkaline media, eq uipped
with porous nickel anode and porous nickel-silver c athode
[3].
Electrochemical reactions occurring at electrodes a re:
on the anode surface, methanol is oxidised to carbo n
dioxide releasing six electrons (1) and simultaneou sly, on
the cathode side, molecular oxygen accepts electron s and
it is reduced to hydroxide ions (2). Hydroxide ions
migrate from the bulk of the solution to the anode [2].

CH 3OH + 6OH -→ CO 2 + 5H 2O + 6e – E° =̵ 0.81 V (1)

3/2O 2 + 3H 2O + 6e – →6OH – E° = +0.40 V (2)
The characteristic overall reaction of direct metha nol
fuel cell is (3):

CH 3OH + 3/2O 2→CO 2 + 2H 2O E° = +1.21 V (3)

The disadvantage of alkaline media is progressive
carbonation with CO 2when carbonate and dicarbonate are
produced in the cathodic compartment (2'); this wil l affect
the cell performance, lowering the concentration of the
electrolytes.

CO 2 + 2OH – → CO 32- + H 2O (2')

The new electrode materials family, oxide-ion
conducting perovskites, have appeared in the litera ture for
several years, but only recently compositions with high
enough conductivities to be usedin fuel cells have been
obtained [4]. Perovskiteelectrocatalysts as SrPdO 3,
SrRuO 3, La 0.8 Ce 0.2 CoO 3, La 0.8 Sr 0.2 CoO 3 have
demonstrated activity towards direct methanol oxida tion
during cyclic voltammetry measurements [5].
Lanthanum-based perovskite-type oxides, among
which La 2-xSr xNiO 4 (0 ≤ x ≤ 1) known as functional
materials with a wide range of applications, have b een
used as electrocatalysts for alkaline fuel cells an d as
environmental catalysts: hydrocarbon oxidation, CO
oxidation and NO x reduction [6, 7].
Of late years, a new class of transitional metals
mixed oxides, named layered cobalt perovskites type 114,
was most investigated due to their structural, magn etic
and electrochemical properties. Based on these pro perties
cobalt perovskites can be used as membranes with hi gh
oxygen permeability, oxygen sensors and also fuel c ells
electrodes. Part of this family is Y 0.5 Ca 0.5 BaCo 4O7, first
synthesized by M. Valldor by partial substitution o f Y 3+
with low valence Ca 2+ cationsinperovskite structure [8].
Experimental researches have shown there is a corre lation
between compound structure and his properties,
especially due to the variable cobalt ions valence and it

Chem. Bull. "POLITEHNICA" Univ. (Timisoara) Volume 61(75), 1, 2016

was found that oxygen adsorption properties can be
modified greatly by Y 3+ ions substitution in the original
structure of YBaCo 4O7 [9, 10].
In this work, new aspects of the electrocatalytic
oxidation of methanol on Y 0.5 Ca 0.5 BaCo 4O7 electrode
have been studied using cyclic and linear voltammet ry.
Also, a mechanism was proposed for the electrochemi cal
reaction. The results are expected to provide basic
information in understanding the methanol oxidation
reaction (MOR) process on 114 cobalt layered perovs kites
catalysts.

2. Experimental

Y0.5 Ca 0.5 BaCo 4O7perovskite was obtained using solid
state reaction, mixing the precursors Y 2O3 (Aldrich
99,99%), CaCO 3 (Aldrich 99,99%), BaCO 3 (Aldrich
99,99%) and CoO 1.38 (99,99% Normapur),according to
the stoichiometric cation ratio. After decarbonatio n at
600°C, the powder was reground, fired in air for 48 h at
1100°C and then removed rapidly from furnace and se t
ambient temperature. The mixture was reground again ,
pressed into discs (1 cm 2) and sintered at 1100°C for 24 h
in air[11].The structure of obtained Y 0.5 Ca 0.5 BaCo 4O7 pure
compound was checked by X-Ray powder diffraction
(RigakuUltima IV).
Electrochemical tests were performed at room
temperature using a SP-150 potentiostat/galvanostat (Bio-
Logic, SAS, France).A100 mL typical glass cell was
equipped with three electrodes: working electrodes
consisting of skeletal nickel samples, Ag/AgCl refe rence
electrode and two graphite rods used as counter
electrodes. For performed experiments, the exposed
surface of working electrode was 0.2 cm 2.
All potentials are given versus the reference elect rode
(Eref = 0.197 V vs NHE).
Cyclic voltammograms (CVs) were recorded at
different scan rate, between 5 and 500 mVs -1. Linear
polarization curves were registered potentiostatica lly with
1 mVs -1 scan rate. 1 mol L-1 KOH solution (prepared
using Merck KOH, p.a.) ensured the alkaline media u sed
in all experimental studies. Different concentratio ns of
methanol were added: 0.06, 0.12, 0.25, 0.5, 1 and 2 mol
L-1, all prepared from Sigma-Aldrich reagent p.a. min
99.8%.

3. Results and Discussion

3.1. Preliminary data

Electrochemical behaviour of studied perovskite in
alkaline solution (1M KOH) can be described by the
following reaction [11]:

Y0.5 Ca 0.5 BaCo 4O7+ 2δHO -↔Y0.5 Ca 0.5 BaCo 4O7+ δ + δH2O + 2 δe-
(4)

Simultaneously, anodic oxygen evolution (5) and
cathodic hydrogen evolution (6) occur:
4HO – → O 2 + 2H 2O + 4e – (5)

2H 2O + 2e – → H 2 + 2HO – (6)

Anodic oxidation process of Y 0.5 Ca 0.5 BaCo 4O7
perovskite consists in oxygen insertion in oxide
structure, assigned to Co (II) oxidation (7) [11]:

Co(II) → Co(III) + e – (7)

Methanol anodic oxidation is a complex process,
involving 6-electron transfer and several intermedi ate
organic species are produced [12]. Oxidation mechan ism
of methanol in alkaline medium on metallic electrod e,
like Pt or Pt-alloys, was complete described by S. S.
Mahapatraand J. Datta in 2011 and proceeds through the
following steps [12]:

M + (CH 3OH) sol →M–(CH 3OH) ads (8)
M–(CH 3OH) ads + OH – → M–(CH 3O) ads + H 2O + e – (9)
M–(CH 3O) ads + OH – → M–(CH 2O) ads + H 2O + e – (10)
M–(CH 2O) ads + OH – → M–(CHO) ads + H 2O + e – (11)
M–(CHO) ads + OH – → M–(CO) ads + H 2O+e – (12)
M–(CO) ads + OH – → M–(COOH) ads + e – (13)
M–(COOH) ads + OH -→ M + CO 2 + H 2O+e – (14)

Reactions (12) and (13) can proceed directly to CO 2
production [12]:

M–(CHO) ads + 3OH – → M + CO 2 + 2H 2O + 3e – (12')
M–(CO) ads + 2OH – → M + CO 2 + H 2O + 2e – (13')

In Figure 1 linear voltammograms recorded with
1 mV s -1scan rateare presented, in absence and presenceof
methanol lowest concentrations usedin the alkaline
electrolyte.

Figure 1. Linear voltammograms recorded on Y 0.5 Ca 0.5 BaCo 4O7
electrode in 1M KOH solution,without and with 0.06 M and
0.12M MeOH, at 1 mV s-1

Analyzing voltammograms from Figure 1, it can be
observed the potential domain specific for methanol
oxidation, between +0.25 and +1.30 V, includes the
electrooxidation potential of Co(II) to Co(III).

Chem. Bull. "POLITEHNICA" Univ. (Timisoara) Volume 61(75), 1, 2016

This shows the potential region for Co(II)
electrooxidation inside of perovskitevis-`a-vis of
methanol anodic oxidation is the same. This behavio r is
somehow specific for layered perovskites, being
previously shown also for LaSrNiO 4 in alkaline media [7].
Methanol anodic oxidation process on
Y0.5 Ca 0.5 BaCo 4O7perovskiteoccurs with similar
mechanism ason La 2−xSr xNiO 4 in KOH solutions and
involves the following steps [7]:

Co(II) →Co(III) + e- (15)
Co(III) + OH -↔ Co(III)·OH + e- (16)
Co(III) + CH 3OH ↔ Co(III)·CH 3OH (17)
Co(III)·CH 3OH + Co(III)·OH + OH -→ Co(III)·CH 2O +
Co(II) + 2H 2O (18)
Co(III)·CH 2O + Co(III)·OH + OH -→products (19)

Methanol electrocatalytic oxidation seems to be
mediated by the Co(III)/Co(II) redox couple from in side
of perovskite. Also, methanol dehydrogenation is
catalyzed by the same couple. Reactions (15) and (2 0)
take place rapidly [7].
According to the processes at the interface
electrode/electrolyte, linear voltammograms from Fi gure
1 can be divided in three potential domains for bot h
perovskite oxidation and MOR: I – oxidation region, II –
limiting current domain and III – domain of oxygen
evolution reaction (OER) on electrode surface.
Table 1 presents the values of conductivity for
electrolyte solutions used in experimental data.

TABLE 1. Conductivity of different electrolytes used in
experimental studies

Electrolyte Conductivity [S m -1]
Distilled water 2.25·10 -6
MeOH p.a. min 99.8% 11.22·10 -6
KOH 1M (BS) 168.8·10 -3
BS + MeOH 0.06M 165.1·10 -3
BS + MeOH 0.12M 163.5·10 -3
BS + MeOH 0.25M 157.5·10 -3
BS + MeOH 0.5M 149.7·10 -3
BS + MeOH 1M 140.3·10 -3
BS + MeOH 2M 127.9·10 -3

Analysing data from Table 1, it can be noticed the
decrease of the conductivity along with the increas ing of
methanol concentration. In electrochemical
measurements, the effect of these values on MOR has
been observed.

3.2. Cyclic voltammetry studies

On CVs recorded at 100 mVs -1in alkaline solutions,
in the absence and presence of methanol, in a wide range
of the potential (+1.75 and -1.75 V), depicted in F igure 2,
there is observed a pronounced anodic peak associat ed
with all methanol oxidation steps described by reac tions
(15) – (20) and with Co(II) to Co(III) oxidation fo r blank
solution. When the electrode potential is over +1.0 0V,
OER on electrode surface can be noticed. On the backward scan, a low intensity reduction peak assoc iated
with adsorbed oxygen reduction was recorded. When t he
electrode potential becomes more negative than -0.7 5V,
hydrogen evolution reaction (HER) occurs.

Figure 2. Cyclic voltammograms recorded on Y 0.5 Ca 0.5 BaCo 4O7
electrode in 1MKOH solution, without and with diffe rent concentration
of MeOH, at 100 mVs -1

CVs recorded with low scan rate (10 mV s-1) in the
potential range from 0 to -1.50 V show a separation of the
reduction peaks (Figure 3). Starting from open circ uit
potential (OCP) towards anodic polarization (+1.60 V),
characteristic peaks which may be attributed to met hanol
oxidation reaction products are not distinguished.

Figure 3. Cyclic voltammograms recorded on Y 0.5 Ca 0.5 BaCo 4O7
electrode in 1M KOH solution, without and with
different concentration of MeOH, at 10 mVs -1

The oxidation-reduction peaks in the potential rang e
from +0.50 to -1.30 V are detailed in Figure 4. The se are
associated with formaldehyde/formate redox couple i n
alkaline medium.

Chem. Bull. "POLITEHNICA" Univ. (Timisoara) Volume 61(75), 1, 2016

Figure 4. Oxidation-reduction peaks for HCHO/HCOO – redox couple
from cyclic voltammograms

Formaldehyde (HCHO) to formate (HCOO -)
oxidationdepends on the methanol concentration in t he
electrolyte solution, increasing the peak intensity along
with the increasing of the concentration. As well, from the
curve shape it can be seenthat oxidation is a multi stage
process.
It should be emphasized that oxidation-reduction
peaks corresponding to HCOH/HCOO – couple are
recorded in the same potential range as for Co(II)/ Co(III)
couple when electrolyte solution without methanol
addition have been used, thus confirming the prelim inary
results.
Cyclic voltammetric studies have shown that it is
possible to separate the peaks associated with the
electrochemical processes occurring at the interfac e
Y0.5 Ca 0.5 BaCo 4O7 – 1M KOH + methanol only if the
potential scan rate is around 5 mVs -1. The CVs recorded
for 1M and 2M methanol added in alkaline electrolyt e are
depicted in Figure 5.
Starting from OCP towards anodic polarization,
methanol oxidation domain (1) is revealed, followed by
limiting current plateau (2) characteristic for thi s reaction.
At higher potentials, over +1.25 V, the current inc rease
(3) was associated with the OER.
On the backward scan of the CVs, when the electrode
potential shifts towards more negative values, a
pronounced cathodic peak (4), associated with forma te
ions reduction to formaldehyde appears around -1.00 V.
At more negative electrode potentials, the current increase
(5) is assigned to HER. When the potential is scann ed in
anodic direction, up to OCP, an oxidation peak (6) is
observed at approximately -0.50 V due to the
formaldehyde oxidation to formate ions or other pro ducts.

Figure 5. Cyclic voltammograms recorded on Y 0.5 Ca 0.5 BaCo 4O7
electrode in 1M KOH solution with 1M and 2M MeOH, a t 5 mV s-1

Comparing voltammograms drawn in the same
potentialrange, an increase of current density rela ted to
above processes is ascertained for electrolyte solu tion
containing higher concentration of methanol, as wel l as a
shift of OER to more negative potentials.

Figure 6. Cyclic voltammograms recorded on Y0.5Ca0. 5BaCo4O7
electrode in 1M KOH solution with 1M and 2M MeOH, a t 5 mV s-1

Oxidation and reduction processes at the interface
make possible the voltammograms partition in four
potential domains (Figure 6), characteristic for MO R on
Y0.5 Ca 0.5 BaCo 4O7 in alkaline media: I – HER domain, II –
formaldehyde/formate ions redox couple domain, III –
MOR on 114 layered cobalt perovskite surface domain
and IV – OER domain.
In Figure 7 CVs (5 cycles) plotted on
Y0.5 Ca 0.5 BaCo 4O7 in alkaline solutions with addition of
0.5 M, respectively 2.0 M methanol are presented.

Chem. Bull. "POLITEHNICA" Univ. (Timisoara) Volume 61(75), 1, 2016

Figure 7. Cyclic voltammograms (5 cycles) recorded on
Y0.5 Ca 0.5 BaCo 4O7 electrode in 1M KOH with 0.5M and 2M MeOH, at
10 mV s-1

Analysing the above CVs, it can be stated the peak
intensity increases with number of plotted cycles d ue to
the increasing amounts of methanol oxidation produc ts
formed, which then participate in electrode surface
reactions.
When a low concentration of methanol (0.5 M) is
used, on cathodic domainthe low intensity peak
corresponding to the reduction of adsorbed oxygen o n
electrode surface and from inside its poresis regis tered at
about +0.40 V. When a higher concentration of meth anol
is used(over 1 M), this peak disappears because the entire
quantity of cathodic reduced oxygen is immediately
consumed for the oxidation of methanol and/or its
intermediate products formed in the anodic process,
available in height concentration near the electrod e
surface.
Widening the polarization range in both anodic and
cathodic domains significantly influences the elect rode
process efficiency, noticeably more when 1 M methan ol is used in alkaline media, as can be seen in Figure 8 a and
b.
Thereby, using 1M methanol addition in electrolyte
solution, in +1.50 V up to -1.50 V potential range (Figure
8b),the limiting current density value ( ilim ) is
approximately 325 A m -2, and the current density of
formaldehyde/formate oxidation-reduction couple hav e
values: iox = 90 A m-2, respectively │ired │= 225 A m-2.
When the potential was scanned between +1.75 and -1 .75
V, the current density values change to: ilim = 575 A m-2,
iox = 110 A m-2 and │ired │= 325 A m-2,as can be
observed.
If methanol concentration in electrolyte solution i n
lower than 1 M, 0.125 M for instance (Figure 8 a), the
changes of corresponding formaldehyde/formate coupl e
are insignificant, only methanol oxidation current density
increase from 575 to 700 A m -2 being observed
In the same time, a slight catalytic effect on OER and
HER is observed by enlarging the potential scan ran ge,
regardless of methanol concentration.

3.3. Linear voltammetry studies

Linear voltammograms (LVs)plotted at low scan rate
(1 mV s -1) confirms the above presented results. The
curves obtained in alkaline media with different
concentrations of methanol are shown in Figure 9.
Analysis of linear voltammograms shape indicates
only one oxidation process on perovskite electrode
surface which validates the mechanism reaction desc ribed
by (15) – (20) equations.
Methanol oxidation limiting current density decreas e
along with the increasing of methanol concentration in
KOH electrolyte solutions, the same principle being
observed for conductivity values presented in Table 1.
Based on LVs discussed above, it can be stated the
optimum concentration domain of methanol in alkalin e
media for maximum efficiency electrooxidation range s
from0.006 to 0.125 M.

a) b)
Figure 8. Cyclic voltammograms recorded on Y 0.5 Ca 0.5 BaCo 4O7 electrode in 1M KOH with 0.125M (a) and 1M (b) MeO H, at 10 mV s-1, in different
potential ranges

Chem. Bull. "POLITEHNICA" Univ. (Timisoara) Volume 61(75), 1, 2016

Figure 9. Linear voltammograms recorded on Y 0.5 Ca 0.5 BaCo 4O7
electrode in 1M KOH solution with different MeOH co ncentration,
at 1 mV s-1

4. Conclusions

Experimental data presented have confirmed the
possibility to oxidize methanol in alkaline media o n an
electrode made of cobalt layer perovskite type 114.
From the perspective of using cobalt layered
perovskite as potential electrode materials in alka line
direct methanol fuel cells, the possibility to oxid ize
methanol directly on their surface provides a good reason
for continuing studies presented in this paper. A
deepening of the research is necessary both for com plete
characterization of processes occurring at the inte rface by
electrochemical impedance spectroscopy, and in or der to determine optimum characteristic parameters, from w hich
the most important is the efficiency of the oxidati on
process.

ACKNOWLEDGEMENT

This work was partially supported by University
Politehnica Timisoara in the frame of PhD studies.

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Received: 11 May 2016
Accepted: 13 June 2016