Catalytic activity of electrolytic palladium deposits on porous nickel substrates

Article abstract of DOI:10.1134/S107042721108009X

Catalytic activity of palladium layers in reduction of atmospheric oxygen and oxidation of methanol and ethanol in an alkaline medium was studied for electrolytic palladium deposits on porous nickel substrates by cyclic voltammetry on a rotating disk electrode. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S107042721108009X

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 8, pp. 1347–1353. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © T.G. Nikiforova, T.V. Savel’eva, O.A. Datskevich, 2011, published in Zhurnal Prikladnoi Khimii, 2011, Vol. 84, No. 8, pp. 1282–
1288.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Catalytic Activity of Electrolytic Palladium Deposits
on Porous Nickel Substrates
T. G. Nikiforova, T. V. Savel’eva, and O. A. Datskevich
St. Petersburg State University, St. Petersburg, Russia
Received November 11, 2010
Abstract—Catalytic activity of palladium layers in reduction of atmospheric oxygen and oxidation of methanol
and ethanol in an alkaline medium was studied for electrolytic palladium deposits on porous nickel substrates
by cyclic voltammetry on a rotating disk electrode.
DOI: 10.1134/S107042721108009X
It is known that the catalytic activity of palladium is
frequently comparable with that of platinum, which
makes palladium promising as an electrocatalyst for
fuel cells. A particular attention is paid in development
of new palladium catalysts to the nature and state of
substrates on which palladium is deposited and to the
method of deposition of the metal. Use of nickel as a
catalyst in fuel cells is promising because of its high
corrosion resistance and comparatively low cost. Of
particular interest in this case are deposits with large
roughness factor and good mechanical strength.
Co3O4, and Mn3O4 as substrates for palladium catalysts
improves their activity in reactions of ethanol
oxidation. The improved activity is due to their
increased resistance to poisoning by products of these
reactions, introduced into the catalysts.
Palladium catalysts deposited on various substrates,
as well as double and triple palladium-containing
catalysts, exhibit a sufficient activity in methanol and
ethanol oxidation in alkaline solutions [13–28]. Fabrica-
tion of palladium catalysts by electrochemical reduc-
tion of various complex compounds of palladium(II)
on electrically conducting substrates has been com-
paratively rarely reported in the literature, although it
has been shown [29] that this method has a number of
advantages in the case of deposition of small
controlled amounts of palladium.
The problem of application of pure electrolytic
deposits of nickel as electrocatalysts has been little
discussed in the literature. Use of electrochemically
obtained highly porous metallic nickel matrices with
large roughness factor and good mechanical resistance
as Raney electrodes was mentioned in [1]. The
possibility of using hollow nickel spheres constituted
by needle-like nickel particles as catalysts in methanol
and ethanol oxidation in an alkaline medium was noted
in [2]. In [3–6], particles of metallic nickel were
dispersed in conducting polymeric films when
fabricating active catalysts for methanol oxidation. It is
known that nickel favors an increase in catalytic
activity when used as a substrate for double (Pt–Ru/Ni)
[7] and triple (Pt–Ru–Ni/C) catalysts [8–11] and in the
form of oxides [12].
The goal of our study was to obtain porous nickel
deposits by procedures suggested in the literature and
to electrochemically deposit catalytic layers of metallic
palladium onto these substrates from solutions of
glycinate-chloride complexes of palladium(II). The
catalytic activity of the electrodes was evaluated by the
rates of reduction of atmospheric oxygen and oxidation
of alcohols (methanol and ethanol) in an alkaline
aqueous solution.
EXPERIMENTAL
It has been shown [13–16] that palladium catalysts
exhibit a higher activity and stability in ethanol
oxidation in alkaline solutions, compared with
platinum catalysts. Use of such oxides as NiO, CeO2,
We used the following reagents: NiSO4, CuSO4,
and NiCl2 of analytically pure grade; NH4Cl and NaCl
of chemically pure grade; H3BO3 of analytically pure
1347

1348
NIKIFOROVA et al.
grade; and KOH of chemically pure grade. The study
was performed in a hermetically sealed three-electrode
electrochemical cell with separated cathode and anode
spaces. Prior to carrying out an experiment, we kept
the cell in concentrated sulfuric acid and then washed
it with twice-distilled water. All the solutions used in
the study were prepared from twice-distilled water. A
glassy graphite rotating disk electrode with an area of
0.07 cm2 served as the cathode. A layer of porous
nickel was formed on this electrode by the
electrochemical methods described in [1, 30], and then
palladium was deposited. A platinum wire sealed in a
glass tube served as the auxiliary electrode. A calomel
electrode in a saturated KCl solution (sat.c.e) was used
as reference. All the potentials are given relative to this
electrode. The study was carried out at a temperature
of 18 ± 2°C in the atmosphere of argon.
and copper. In what follows, we designate the substrate
produced by this method as Ni–Cu.
To characterize the roughness of nickel substrates,
we estimated their capacitance, as it was done in [31].
For this purpose, we measured pulsed galvanostatic E,t
curves. The height of galvanostatic pulses was varied
in the range 1–15 μA, and values of the derivative
(dE/dt)t=0 were determined from the initial portions of
the E,t curves. Then, the capacitance C (F cm–2) was
calculated by the equation
I/S
,
C = −
(1)
(dE/dt)t=0
where I is the current (A), and S, size of the visible
electrode surface (S = 0.07 cm2).
The observed constancy of the electrode
capacitance in the galvanostatic pulse range we used
indicates that the values of the capacitance correspond
to charging of the electric double layer and are not
complicated by occurrence of Faraday processes.
Large values fluctuating in the range 2–4 mF cm–2
were obtained for the capacitance of nickel electrodes,
which confirms that highly porous nickel substrates are
obtained.
The measuring setup comprised a PI-50-1.1 poten-
tiostat, PR-8 programming unit, and X–Y recorder.
The study was performed in various modes: galvano-
static, potentiostatic, and potentiodynamic at various
potential sweep rates. The voltammetric curves re-
corded by the self-recorder were digitized with Windic
program and processed with OriginPro software.
Pulsed galvanostatic curves were recorded with a
Grafit-2 electronic self-recorder.
On porous Ni substrates, we measured cyclic
voltammetric (CVA) curves in a 1 M KOH solution.
Figure 1 shows voltammograms obtained on a nickel
electrode at various potential sweep rates. The arrows
show the directions of potential variation. It can be
seen in Fig. 1 that the CVA curves show at positive
potentials (>+0.4 V) peaks corresponding, according to
[4], to the process of nickel recharging at the electrode,
to which the following reaction may correspond:
Porous nickel was electrochemically deposited onto
a glassy graphite disk electrode in two ways. By the
method described in [1], nickel was deposited from a
solution of composition 0.2 M NiCl2 + 2 M NH4Cl in
the galvanostatic mode in the course of 100 s at a
current density of 0.1 A cm–2. The mass of the
deposited nickel was about 3 mg cm–2. Under these
conditions, highly porous metallic matrices with large
roughness factor and high mechanical strength are
formed [1]. In what follows, we designate nickel
produced by this method as Ni.
Ni(OH)2 + OH– = NiOOH + H2O + e.
The nickel recharging peaks obtained at various
electrode rotation speeds (from 350 to 1490 rpm) were
independent of the disk rotation speed. This means that
the process of nickel recharging is not controlled by
diffusion from solution. On plotting the dependence of
the logarithm of the peak current, ln Ip, on the
logarithm of the potential sweep rate, ln ν, and that of
the peak potential Ep on the logarithm of the potential
sweep rate, ln ν, we found that ln Ip linearly depends
on ln ν, with a slope ratio of approximately 0.6. There
also exists a linear relationship between Ep and ln ν.
This indicates that the process occurring at the
electrode is irreversible.
The procedure described in [30] was used to
deposit a Ni–Cu alloy from a solution of composition
1 M NiSO4 + 0.01 M CuSO4 + 0.5 M H3BO3 in the
potentiostatic mode at a potential E = –0.8 V by
passing about 0.05 C cm–2 of electricity. Further,
copper was anodically etched-out in the same solution
at a potential E = +0.5 V, and the amount of passed
electricity was used to calculate the mass of dissolved
copper. The Ni : Cu ratios in alloys obtained at
different deposition potentials, reported in [30], were
used to evaluate the composition of the alloy obtained
upon etching-out of copper. This alloy contained
approximately equal amounts (300 μg cm–2) of nickel
Small controlled amounts of metallic palladium
were deposited onto porous nickel substrates from a
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CATALYTIC ACTIVITY OF ELECTROLYTIC PALLADIUM DEPOSITS
1349
I, μA
I, μA
E, mV
Fig. 1. CVA curves obtained on a nickel electrode at
E, mV
various potential sweep rates. (I) Current and (E) potential;
the same for Figs. 2–6. Potential sweep rate (V s–1):
(1) 0.05, (2) 0.02, and (3) 0.01; the same for Fig. 5.
Fig. 2. CVA curves obtained in a 1 M KOH solution.
(1) Glassy graphite, (2) Ni–Cu, and (3) Pd/Ni–Cu.
solution containing 5 × 10–4 M of glycinate-chloride
complexes of palladium(II), Pd(Hgly)2Cl2. As shown
in [32], palladium is deposited from these solutions
with a 100% current efficiency. Palladium was
deposited onto a rotating electrode in the potentio-
dynamic mode, with the potential varied from +0.15 to
–0.25 V at a potential sweep rate of 0.005 V s–1. At
different numbers of potential scanning cycles, we
obtained electrodes with different contents of metallic
palladium (from 6 to 390 μg cm–2).
done because, as shown in [33], a monolayer coverage
of the palladium surface by adsorbed oxygen atoms is
obtained at a potential of 1.3 V relative to the
equilibrium hydrogen electrode in the solution under
study. The method in which the true surface area of
palladium is found from removal peaks of a monolayer
of adsorbed oxygen atoms is frequently employed in
studies performed on palladium electrodes [34, 35].
Figure 2 shows CVA curves obtained at a potential
sweep rate of 0.05 V s–1 on a glassy graphite electrode
(curve 1), glassy graphite electrode coated with porous
nickel (Ni–Cu) (curve 2), and palladium electrode with
a porous nickel substrate (Pd/Ni–Cu) at a palladium
content of 216 μg cm–2 (curve 3). It can be seen that no
electrochemical process occurs on the glassy graphite
electrode in a wide range of potentials. On the Ni–Cu
electrode, charging of the capacitance of the double
electric layer on well developed surface is observed,
and on the palladium Pd/Ni–Cu electrode, clearly
pronounced peaks of adsorption–desorption of atomic
hydrogen and oxygen appear at potentials in the range
under study.
On electrodes with various catalytic layers, we
measured cyclic voltammograms in a 1 M KOH solu-
tion and studied the catalytic activity of these elec-
trodes in reduction of atmospheric oxygen and oxida-
tion of methanol and ethanol in an alkaline medium.
Prior to measuring cyclic voltammograms on all
palladium electrodes, we preliminarily cleaned the
surface of palladium by cycling the electrode potential
for 20 min in the range from hydrogen evolution
potentials to oxygen evolution potentials, as described
in [29]. Then we measured CVA curves at various
potential sweep rates (0.05, 0.02, and 0.01 V s–1) in the
range 0–1.3 V relative to a reversible hydrogen
electrode in the same solution. The CVA curves
measured on a palladium electrode in this range of
potentials make it possible to evaluate the true
palladium surface area by the quantity of electricity
consumed for removal of a monolayer of atomic
oxygen adsorbed on the palladium surface. This can be
The adsorption–desorption peaks of hydrogen and
oxygen become more pronounced with increasing
content of palladium in a catalytic layer and on passing
from the Pd/Ni–Cu electrode to Pd/Ni at the same
content of palladium, which indicates that the active
surface areas of palladium are different on these
electrodes.
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NIKIFOROVA et al.
Parameters characterizing the active surface area of catalytic palladium layers on various porous nickel substrates
Pd content,
QO, μC
nPd
Strue, cm2
D, %
K
d, μm
μg cm–2
Pd/Ni–Cu electrode
216
124
90
470
600
230
1.4×1015
1.8×1015
0.7×1015
1.12
1.5
3.6
1.7
16
20
8
1.0
1.42
0.46
0.88
0.54
Pd/Ni electrode
3.97
389
175
49
1900
1080
400
5.9×1015
3.2×1015
1.2×1015
0.3×1015
3.7
5.1
5.8
9.8
43
36
10
3
0.51
0.35
0.29
0.13
2.58
0.87
6
100
0.24
For all of the palladium electrodes obtained, we
determined from the area under the removal peak of a
monolayer of adsorbed oxygen (shaded region in
Fig. 2) in cyclic voltammograms measured in 1 M
KOH at various potential sweep rates the quantity of
electricity QO consumed for removal of a monolayer of
atomic oxygen adsorbed on the surface of palladium,
number nPd of surface palladium atoms, true surface
area Strue of palladium, roughness factor K of the
electrode, variance D (%) of the size of palladium
particles, and average size d of palladium particles in
the catalytic layer. This was done as described in [29].
The table lists values of these parameters, found for the
palladium electrodes under study.
varied from +0.10 to –0.60 V at a constant potential
sweep rate (0.01 V s–1).
Figure 3 shows voltammetric curves for reduction
of molecular oxygen on Pd/Ni electrodes with various
palladium contents at an electrode rotating speed of
1100 rpm. The same figure presents a similar curve
obtained on a porous NI substrate (curve 1). It can be
seen that the porous nickel electrode does not catalyze
the reaction of reduction of molecular oxygen, whereas
on Pd/Ni electrodes, oxygen reduction currents are
observed at potentials in the range under study. The
catalytic activity on Pd/Ni electrodes grows as the
content of palladium increases and, simultaneously, so
does the true surface area of palladium in the catalytic
layer (see table).
It can be seen in the table that, as the content of
palladium in a catalytic layer increases, the roughness
coefficient of the palladium electrode markedly grows
and the variance of the size of palladium particles
slightly decreases. The largest variance (D = 9.8%) is
observed for the electrode with the lowest content of
palladium (6 μg cm–2). The average diameter of
palladium particles in the catalytic layers varies from
0.1 to 1 μm, depending on the palladium content of the
electrode and on a substrate. Finer palladium particles
(d = 0.13 μm) were obtained on a porous Ni substrate
at the lowest content of palladium (6 μg cm–2).
The reduction currents of molecular oxygen on Pd/
Ni electrodes increase with the rotation speed of the
disk electrode, and the corresponding dependences of
the reduction currents on the disk rotation speed are
linear in the Levich–Koutecky coordinates, which
indicates that the process under study occurs under
diffusion-kinetic control [29].
We studied the catalytic activity of the palladium
catalysts we obtained in the reactions of methanol and
ethanol oxidation in an alkaline medium. We measured
CVA curves in a 1 M KOH solution with addition of
1 M of methanol or ethanol at different potential sweep
rates (0.05, 0.02, and 0.01 V s–1) in a wide range of
variation of the potential on the glassy graphite
electrode, porous nickel Ni–Cu and NI substrates, and
also on Pd/Ni–Cu and Pd/Ni palladium electrodes. No
The reaction in which atmospheric oxygen is
reduced on palladium catalytic layers was studied in a
nondearated 1 M solution of KOH on a rotating elec-
trode at various rotation speeds (from 730 to 3500 rpm).
Voltammetric curves were measured with the potential
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CATALYTIC ACTIVITY OF ELECTROLYTIC PALLADIUM DEPOSITS
1351
I, μA
I, μA
E, mV
E, mV
Fig. 3. Voltammetric curves for reduction of atmospheric
oxygen on Pd/Ni electrodes with various palladium
contents. Palladium content (μg cm–2): (1) 0, (2) 6, (3) 49,
(4) 175, and (5) 389.
Fig. 4. CVA curves obtained on a nickel electrode in solu-
tions of different compositions. Potential sweep rate
0.05 V s–1. (1) 1 M KOH and (2) 1 M KOH + 1 M C2H5OH.
I, μA
I, μA
E, mV
E, mV
Fig. 6. CVA curves obtained on Pd/Ni electrodes at a
potential sweep rate of 0.05 V s–1. Palladium content
(μg cm–2): (1) 44 and (2) 49. Solution composition: (1) 1 M
KOH + 1 M CH3OH and (2) 1 M KOH + 1 M C2H5OH.
Fig. 5. CVA curves for oxidation of ethanol on a Pd/Ni
electrode at various potential sweep rates. Solution
composition: 1 M KOH + 1 M C2H5OH; palladium content
49 μg cm–2).
oxidation of methanol in the alkaline solution was
observed on the glassy graphite electrode and porous
nickel substrates in the whole potential range under
study. The Ni–Cu and Ni electrodes catalyze the
process of ethanol oxidation in the alkaline medium at
high positive potentials (>+0.5 V).
process of ethanol oxidation occurs on the oxidized
surface of nickel, in agreement with the data of [3, 4,
6]. In a reverse (cathodic) potential sweep, the
voltammetric curve obtained in a solution with
addition of ethanol shows a cathodic peak of nickel
recharging.
Figure 4 shows that, in ethanol oxidation on a
nickel substrate in an alkaline medium, there occurs
superposition of two processes: oxidation of ethanol
and recharging of nickel. It can be seen that the
In an alkaline solution with addition of ethanol, we
measured CVA curves on a Ni electrode at various
potential sweep rates. It was noted that the currents of
ethanol oxidation peaks grow with increasing potential
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1352
NIKIFOROVA et al.
sweep rate, with the potentials of forward sweep peaks
shifted to positive values. This is indicative of the
irreversible course of the oxidation process. Voltam-
metric curves measured at various rotation speeds of
the disk electrode demonstrated that the currents of the
ethanol oxidation peaks are independent of the rotation
speed and, consequently, the process of ethanol
oxidation on the nickel electrode is not controlled by
the diffusion from solution.
content of palladium. On palladium electrodes with the
same nickel substrates and close contents of palladium
in the catalytic layer, the currents of methanol and
ethanol oxidation are markedly different. The catalytic
activity of palladium in oxidation of methanol in an
alkaline medium substantially exceeds its catalytic
activity in oxidation of methanol, as it can be seen in
Fig. 6. Thus, catalytic layers produced by electro-
chemical deposition of palladium onto a porous nickel
substrate can be recommended for use as catalysts for
ethanol oxidation in an alkaline medium.
A catalytic process of methanol and ethanol
oxidation in an alkaline solution was observed on all of
the palladium electrodes we obtained. Comparison of
the data in Figs. 4 and 5 demonstrates that catalytic
oxidation of ethanol on palladium layers occurs at
substantially more negative potentials, compared with
the process on porous nickel substrates under the same
conditions. Figure 5 shows CVA curves for ethanol
oxidation, measured at different potential sweep rates
on the Pd/Ni electrode. It can be seen that the curves
obtained in an alkaline ethanolic solution on the
palladium electrode exhibit a single peak of ethanol
oxidation in a forward (anodic) potential sweep (peak
at potentials of about –0.20 V) and a single peak of
ethanol oxidation in a reverse (cathodic) potential
sweep (peak at potentials of about –0.40 V). The
currents of the peaks increase with the potential sweep
rate, and the potentials of the peaks observed in a
reverse (cathodic) potential sweep are more negative
than those obtained in an anodic potential sweep.
Similar voltammetric curves for ethanol oxidation in
alkaline solutions on palladium catalysts supported by
various substrates were reported, e.g., in [21, 26, 29].
CONCLUSIONS
(1) Various methods for obtaining porous nickel
deposits with large roughness coefficients were
developed.
(2) It was shown that palladium with particle sizes
of 0.1 to 1.0 μm is formed in electrodeposition of
metallic palladium onto porous nickel substrates from
solutions of glycinate-chloride complexes of palla-
dium(II).
(3) The catalytic activity of palladium electrodes in
the reaction of reduction of molecular oxygen was
determined.
(4) It was demonstrated that catalytic layers
produced by electrochemical deposition of palladium
onto porous nickel substrates can be recommended as
catalysts for ethanol oxidation in an alkaline medium.
They can be used as anode catalysts for direct ethanol
fuel cells.
The results we obtained can be explained as
follows. The process of ethanol oxidation occurs in
several stages to give several intermediate products.
Ethanol is oxidized in alkaline solutions on a
palladium electrode [37] via dissociative adsorption of
ethanol at low potentials (the peak in the anodic
potential sweep), which yields species of the inter-
mediate oxidation product, acetaldehyde, strongly ads-
orbed on palladium. In the reverse (cathodic) potential
sweep, the intermediate products of ethanol oxidation
are converted into the final product, acetate ions.
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