Mechanism of cathodic reduction of cobalt pyrophosphate complex

Article abstract of DOI:10.1134/S1070427212050096

Potentiostatic and potentiodynamic methods were employed to study the kinetics of electrochemical reduction of the pyrophosphate cobalt complex to the metal from electrolytes of various compositions in the atmosphere of hydrogen. The reaction orders of cobalt reduction with respect to pyrophosphate anions and hydroxyl ions were found. A stage-by-stage mechanism of the cathodic reduction of cobalt was suggested.

Full text of DOI:10.1134/S1070427212050096

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2012, Vol. 85, No. 5, pp. 736−741. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © A.V. Krasikov, V.L. Krasikov, 2012, published in Zhurnal Prikladnoi Khimii, 2012, Vol. 85, No. 5, pp. 735−740.
PHYSICOCHEMICAL STUDIES
OF SYSTEMS AND PROCESSES
Mechanism of Cathodic Reduction
of Cobalt Pyrophosphate Complex
A. V. Krasikova and V. L. Krasikovb
aSt. Petersburg State Technological Institute, St. Petersburg, Russia
bRussian Institute of Radionavigation and Time RIRT, Joint-Stock Company, St. Petersburg, Russia
e-mail: kras257@km.ru
Received January 23, 2012
Abstract—Potentiostatic and potentiodynamic methods were employed to study the kinetics of electrochemical
reduction of the pyrophosphate cobalt complex to the metal from electrolytes of various compositions in the at-
mosphere of hydrogen. The reaction orders of cobalt reduction with respect to pyrophosphate anions and hydroxy
ions were found. A stage-by-stage mechanism of the cathodic reduction of cobalt was suggested.
DOI: 10.1134/S1070427212050096
In our previous study [1], we examined the
fundamental aspects of the cathodic reduction of the
pyrophosphate cobalt complex; however, results of
experiments performed in an electrolyte contacting
with air were distorted by the presence of oxygen. In
studies of the electrodeposition kinetics of metals and
alloys that exhibit a tendency toward oxidation and have
a low hydrogen evolution overvoltage, it is advisable to
remove dissolved oxygen. Commonly this is done by
bubbling the electrolyte with an inert gas, but continuous
variation of the partial pressure of hydrogen near the
cathode can strongly affect experimental results [2, 3].
were bubbled with hydrogen for 1 h under permanent
stirring with a magnetic rabble. The cell was placed in
an air thermostat whose temperature was maintained at
22±1°C.
The working electrodes were prepared as it was
done in [1], with a 3-μm-thick cobalt layer deposited
onto an isotropic special-purity graphite electrode
with an area of 0.70 cm2. Then the electrode was
thoroughly washed, rapidly dried, when necessary, in a
flow of purified nitrogen, and weighed on an analytical
balance. After that the electrode was again washed
with twice-distilled water and transferred to a cell with
electrolyte preliminarily bubbled with hydrogen. After
the electrode was submerged in the electrolyte, its zero-
current potential stabilized in 2–3 min to be nearly equal
to the potential of a reversible hydrogen electrode in the
same solution and independent of agitation. Thus, cobalt
in the atmosphere of hydrogen acted as a reversible
hydrogen electrode whose potential depended only
on the solution pH. This is due the fact that hydrogen
exchange current density on cobalt is rather high [4, 5]
and substantially exceeds the cobalt exchange current
density [6–8]. Adsorbed atomic hydrogen hindered
adsorption of oxygen and oxidation of the metal. The
potential of the working electrode was measured relative
to a silver chloride reference electrode (E0 = +202 mV);
In this study, we examined the kinetics of cobalt
electrodeposition from a hydrogen-saturated electrolyte,
which made it possible to eliminate interfering factors.
EXPERIMENTAL
The experimental procedure described in most
detail in [1] has some important distinctions in the
present study. Electrolytes of required composition
were prepared not earlier than 24 h before being used.
Immediately before experiments, the electrolytes were
filtered through a fluoroplastic membrane with a pore
diameter of 0.45 μm, poured into a thoroughly washed
three-electrode working cell with separated spaces, and
736

MECHANISM OF CATHODIC REDUCTION OF COBALT PYROPHOSPHATE COMPLEX
737
all the potentials in the figures and text are given on the
E, mV
standard hydrogen scale.
Hydrogen was produced with a an electrochemical
generatorwithanoutputcapacityofupto12lh–1 andaset
of glass wash bottles and buffer vessels. The installation
for hydrogen production and purification enabled
accumulation of up to 2 l of purified and humidified
hydrogen and its delivery during several seconds into
the cell with the electrolyte, which markedly accelerated
a changeover of the atmosphere from air to hydrogen
and precluded oxidation of cobalt.
The kinetics of cobalt electrodeposition was studied
in the potentiostatic or potentiodynamic mode, with the
potential swept from the zero-current potential in the
cathodic direction and back at a scanning rate of 2 mV
s–1. An IPC Pro M potentiostat was used (Volta Limited-
Liability Company, St. Petersburg).
log i
Fig. 1. Partial PCvs of cobalt reduction from K6[Co(P2O7)2] +
K4P2O7 solutions with constant free K4P2O7 concentration
of 0.4 M (pH 10.0). (E) Potential and (i) current density; the
same for Figs. 2–4. Concentration of the cobalt pyrophosphate
complex (M): (1) 0.1, (2) 0.2, and (3) 0.3.
Partial polarization curves (PCs) of cobalt deposition
and hydrogen evolution were plotted using results of
a potentiostatic deposition of cobalt onto cobalt-plated
graphite during a fixed interval of time. The total
amount of electricity was automatically calculated by
a computer that controlled the potentiostat, and the
amount of electricity consumed for the cobalt deposition
was determined from the gain in the mass of the working
electrode. The partial curves were plotted using only that
range of potentials in which the linear Tafel dependence
was observed and pure cobalt was deposited without
trace amounts of a cobalt hydroxide.
E, mV
Figure 1a shows partial potentiostatic PCs for
cobalt reduction from electrolytes with various cobalt
concentrations, butconstantpH10.0andafreepotassium
pyrophosphate concentration of 0.4 M.1 It can be seen
in Fig. 1 that, at potentials in the range from –800 to
–1000 mV, the PCs are linear and have a Tafel slope of
129 ± 3 mV and a transfer coefficient α = 0.45 (22°C).
The dependence of the logarithm of the cobalt reduction
rate on the logarithm of the cobalt concentration in
solution at various potentials demonstrated that the
reaction order with respect to cobalt is 0.99 ± 0.06. This
gives a reason to believe that cobalt is discharged in
stages with a hindered first-electron transfer.
log i
Fig. 2. Partial PCvs of hydrogen evolution. (1) Joint reduction
of cobalt and hydrogen in a 0.2 M K6[Co(P2O7)2] + 0.4 M
K4P2O7 solution at pH 10, (2) on electrolytic** cobalt in
a 0.8 M K4P2O7 + 0.2 M K2SO4 + H2SO4 solution at pH 10,
(3) on cobalt in a 0.2 M Na3PO4 + H2SO4 solution at pH 10,
and (4) on cobalt in a 1.2 M H2SO4 solution.
The partial PCs of hydrogen evolution with
simultaneous deposition of cobalt have an unusually
large slope ratio of 160–170 mV (Fig. 2, curve 1).
This may result from hindrance to discharge of water
molecules because of a strong specific adsorption of
pyrophosphate anions. The conclusion is confirmed by
the fact that common PCs of hydrogen evolution on
electrolytic cobalt in a pyrophosphate solution (0.8 M
K4P2O7 + 0.2 M K2SO4, pH 10) also have a large slope
ratio (Fig. 2, curve 2), whereas the slope ratio of PCs
––––––––––––––––
1
For the reaction orders with respect to solution components to be
correctly determined, it is necessary to maintain a constant ionic
force by introducing an indifferent electrolyte. However, this was
impossible for the system under study because the solubility limits
of the salts will be thus exceeded.
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A.V. KRASIKOV, V.L. KRASIKOV
in a sodium pyrophosphate solution (0.2 M Na3PO4 +
H2SO4) acidified to pH 10 (curve 3) and in a sulfuric
acid solution (curve 4) is 117–120 mV. The possibility
of a firm adsorption of pyrophosphate anions on the
cathode was confirmed by an electrochemical quartz
crystal microbalance in [9, 10]. It is noteworthy that,
irrespective of whether the hydrogen evolution in
a pyrophosphate electrolyte is the only cathodic process
or occurs in parallel with the deposition of cobalt, the
partial PCs nearly coincide (Fig. 2, curves 1 and 2).
The hydrogen exchange current density on cobalt in an
electrolyte composed of 0.2 M K6[Co(P2O7)2] + 0.4 M
K4P2O7 (pH 10.0), found by extrapolation of curve 1
to the potential of the reversible hydrogen electrode, is
18 μA cm–2, and the cobalt exchange current density in
an electrolyte bubbled with nitrogen by the procedure
described in [6] is substantially lower, 4.6 μA cm–2. For
comparison, the cobalt exchange current in a CoSO4
solution with pH 6 is 4.8 μA cm–2 [6].
of the electrode potential.
Consequently, the rate-determining electrochemical
stage of the first-electron transfer to an electroactive
species is preceded by detachment of one pyrophosphate
anion from the dipyrophosphate cobalt complex
[Co(P2O7)2]6– predominant in solution to give
a monopyrophosphate complex [Co(P2O7)]2–, which
also occurs in reduction of the pyrophosphate lead
complex on a dropping mercury electrode [11].
The effect of the solution pH was studied in a small
interval, 9.7–10.2, which is due to the narrow pH
range of stable existence of the pyrophosphate cobalt
complex [1]. The curves in Fig. 4 show that the rate of
cobalt reduction grows with increasing pH value. The
order of the cobalt reduction reaction with respect to
hydroxy ions depends on the electrode potential and
is 0.37–0.53, i.e., alkalization of the electrolyte leads
to a faster discharge of complex cobalt ions, but this
acceleration is substantially weaker than that expected
if OH– ions would be involved in the rate-determining
stage. However, it is difficult to make, in a narrow pH
range, a substantiated conclusion about how the activity
of hydroxy ions affects the discharge kinetics of cobalt
ions. The reasons why the reaction order is fractional are
considered below.
The influence exerted by the concentration of free
potassium pyrophosphate on the cobalt reduction
kinetics was studied in electrolytes with constant cobalt
complex concentration of 2 M. The partial PCs are
straight lines whose average slope is 129 mV (Fig. 3).
It was found from dependences of the logarithm of
the cobalt reduction rate on the logarithm of free
pyrophosphate ions concentration in solution at various
potentials that the reaction order with respect to the free
pyrophosphate anion is –1.00 ± 0.03 and is independent
Our study demonstrated that the pyrophosphate
cobalt complex is discharged in stages. The overall
process rate is limited by the electrochemical stage of
the first-electron transfer to an electroactive species.
E, mV
E, mV
log i
log i
Fig. 4. Effect of the pH value on the cobalt reduction rate
from a 0.2 M K6[Co(P2O7)2] + 0.4 M K4P2O7 electrolyte
solution. The solution pH: (1) 9.7; (2) 10.0; (3) 10.2.
Fig. 3. Effect of the free K4P2O7 concentration on the cobalt
reduction rate in electrolytes with pH 10 and cobalt complex
concentration of 0.2 M. Free K4P2O7 concentration (M): (1)
0.4, (2) 0.6, and (3) 0.8.
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MECHANISM OF CATHODIC REDUCTION OF COBALT PYROPHOSPHATE COMPLEX
739
However, the role of this species is not played by the
pyrophosphate cobalt complex predominant in solution:
the electron transfer is preceded by a fast chemical
reaction of detachment of a pyrophosphate anion.
Probably, the OH– ion is also involved in the preceding
chemical reaction.
KClsolutionwiththesamemolarconcentration. Because
the situation is the opposite [20], we can conclude that a
major part of alkali metal ions in solutions of potassium
pyrophosphate and pyrophosphate cobalt complex are
bound into an associate that is the stronger, the smaller
the radius of an unhydrated ion [11, 21]. The formation
of associates of pyrophosphate anions with alkali
metal cations has even been observed in fairly dilute
pyrophosphate solutions (5 × 10–3 to 7 × 10–2 M) [11,
21], moreover, the stability of the associates increases
in the order Cs+ < K+ < Na+ < Li+. This suggests that
a hydrate shell between alkali metal ions and oxygen
of the pyrophosphate is absent or weakened, i.e., the
interaction between the ions is very strong. It is known
that, in solutions of alkali metal halides, the radius
of a hydrated ion grows as its true radius decreases
[22]. This results in that the electrical conductivity of
salt solutions decreases in the order Cs+ > K+ > Na+
> Li+. The instability constants of some associates of
pyrophosphates with alkali metal cations were reported
in [18].
Determiningthenatureofthespeciesthatisdischarged
in the rate-determining stage is fairly important for
elucidating the mechanism by which complex ions are
discharged. However, the rate-determining stage is only
one of several successive stages in which the cobalt
pyrophosphate complex is discharged. To understand
the reduction mechanism as a whole, it is necessary to
first briefly consider ionic equilibria in solution. The
following facts should be taken into account.
The composition of the pyrophosphate cobalt
complex is described by the empirical formula
[Co(P2O7)2]6– [12, 13]. From the standpoint of
coordination chemistry, it is the most probable that the
cobalt atom in the pyrophosphate complex is in the
sp3d2 hybridization state and is bonded to four oxygen
atoms of two pyrophosphate ions and to two oxygen
atoms of two water molecules [Co(H2O)2(P2O7)2]6–.
The equilibria in solutions and the structure of some
crystalline pyrophosphates were reported in [14–16].
In the general case, the composition of associates
in a potassium pyrophosphate solution can be
represented by the formula [KxP2O7](4–x)–. The
dihydropyrophosphates M2H2P2O7 have the minimum
electrical conductivity [20] and solubility [1] and
crystallization of pyrophosphate solutions yields only
two types of pyrophosphoric acid salts, M2H2P2O7
and M4P2O7 [20], which are the most stable and have
geometrically and electrically symmetric molecules
[23]. Taking this circumstance into account, we can
assume that the associate mostly contains two alkali
metal cations, [K2P2O7]2–, in concentrated solutions.
The solubility product of freshly deposited cobalt
hydroxide (6.3 × 10–15) [17], decomposition onset
pH of the dipyrophosphate complex (11.6) [1], and
instability constants of the monopyrophosphate complex
[Co(P2O7)]2– (1 × 10–4) [18] can be used to calculate the
overall instability constant of the pyrophosphate cobalt
complex to be 2.5 × 10–10, and the first-stage instability
constant of the complex [Co(P2O7)2]6–, 2.5 × 10–6. This
suggeststhatcobaltpredominantlyexistsintheelectrolyte
as the dipyrophosphate complex [Co(P2O7)2]6–, and only
its minor part, as monopyrophosphate [Co(P2O7)]2–. In
addition, a certain amount of cobalt ions are present in
solution as hydroxypyrophosphate [Co(OH)P2O7]3–,
that was experimentally confirmed in [19], but the share
of these ions is small.
Finally, numerous researchers believe that the
cathodic reduction and anodic oxidation of iron group
metals in both acid and alkaline solutions occurs via the
stage of formation of an intermediate neutral species
MOHads [24–32]. With this circumstance and our data
on the effect of the activity of OH• ions taken into
account, it is rather probable that the pyrophosphate
cobalt complex is also reduced via formation of the
CoOHads species.
In addition to the processes of formation and
dissociation of complex ions in concentrated solutions,
it is also necessary to take into account ion–ion
interactions. Itishighlydoubtfulthatspecieswithsohigh
negative charges, P2O74– and [Co(P2O7)2]6–, can exist in
an aqueous solution. In the case of a full dissociation of
potassium pyrophosphate, the electrolyte would have a
substantially higher electrical conductivity than, e.g., a
The most probable discharge mechanism of the
pyrophosphate cobalt complex can be represented as
follows. Stable associates [KxP2O7](4–x)–, predominantly
K2P2O2–, are formed in a potassium pyrophosphate
solutio7n. Experiencing the electrostatic field of the
pyrophosphate anion, hydrated alkali metal cations
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740
A.V. KRASIKOV, V.L. KRASIKOV
cannot detach themselves from this anion and move
away into the solution bulk, being supposedly situated at
a distance comparable with the length of the potassium–
oxygen ionic bond in the crystal. Accordingly, the
dipyrophosphate cobalt complex exists in solution as an
associate anion with predominantly four potassium ions,
[K4Co(H2O)2(P2O7)2]2–. In solution, the associate anion
partly dissociates (water molecules are not shown for
simplifying the equations):
electron transfer in stage III, and is again regenerated
in stage IV. The hydroxy ion may be involved in the
process many times, and, therefore, the reaction order
with respect to OH– ions markedly exceeds unity.
The final stage of the process is diffusion of a cobalt
adatom over the surface of the growing metal and its
incorporation into the crystal lattice:
Co0ads → Cocryst
(stage V)
[K4Co(P2O7)2]2– → [K2Co(P2O7)]0ads + [K2P2O7]2–
The last process has been insufficiently analyzed in
the literature and is rather complicated for description
even when cobalt clusters are formed from adatoms in
a vacuum [38].
(stage I)
In the strong electric field of the double layer, the
process is markedly enhanced, compared with the
solution bulk. The [K2Co(P2O7)]0 species is adsorbed
on the cathode surface, being appropriately oriented.
Alkali metal cations serve as cation bridges [33–36] and
enhance the adsorption of the associate [K2Co(P2O7)]0
just as this occurs in cathodic reduction of persulfate
Thus, in the scheme presented above, the electrically
active species is that formed on the electrode surface
from the dissociation product of the complex of cobalt
and hydroxy ion, predominant in solution. The scheme
we suggested most fully accounts for the experimental
data obtained.
anions [37]. The species [K2Co(P2O7)]0
adsorbed
ads
on the electrode surface interacts with the hydroxy ion
(stage II) to give an intermediate electrically active
CONCLUSIONS
species [K2Co(OH)(P2O7)]– in which bonds between
ads
the cobalt atom and oxygen atoms of the pyrophosphate
ion are weakened:
(1) A study of the reduction kinetics of the
pyrophosphate cobalt complex under conditions
precluding oxidation of cobalt demonstrated that the
discharge of the pyrophosphate cobalt complex is
described by a Tafel equation with a slope ratio of 129 ±
3 mV. The transfer coefficient is 0.45.
[K2Co(P2O7)]0ads + OH– → [K2Co(OH)(P2O7)]–ads
(stage I)
Further follows the first-electron transfer to the
electrically active species:
(2) It was found the order of the cobalt reduction
reaction is 1 with respect to cobalt ions and –1 with
respect to pyrophosphate ions and is independent of the
electrode potential.
[K2Co(OH)(P2O7)]–ads + e → CoOH.ads + [K2P2O7]2–
(stage III)
(3) It was shown that, depending on the electrode
potential, the reaction order with respect to hydroxy ions
is 0.37–0.53.
As a result, an adsorbed species CoOHads is formed
and the pyrophosphate anion [K2P2O7]2– is detached.
This stage is rate-determining. The subsequent fast
second-electron transfer leads to formation of a cobalt
adatom and detachment of the hydroxy anion:
(4) Ionic equilibria in a solution of the pyrophosphate
cobalt complex were considered. It was demonstrated
that associates with alkali metal ions are the most
probable form of existence of the pyrophosphate ion and
pyrophosphate cobalt complex.
CoOH.ads + e → Co0ads + OH–
(5) A mechanism of the cathodic reduction of cobalt
was suggested. The mechanism includes a preceding fast
chemical stage and a delayed electrochemical stage of the
first-electron transfer. The mechanism accounts for the
lowered reaction order with respect to the hydroxy ion.
(stage IV)
In this scheme, the OH– ion serves as a catalyst
that weakens the bond between the cobalt atom and
the pyrophosphate ion in stage II, facilitates the first-
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MECHANISM OF CATHODIC REDUCTION OF COBALT PYROPHOSPHATE COMPLEX
741
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ACKNOWLEDGMENTS
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The authors are grateful to Prof. V.I. Kravtsov
(St. Petersburg State University) for valuable remarks in
preparation of the manuscript for publication.
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skaya, K.K., Pirofosfatnye elektrolity (Pyrophosphate
Electrolytes), Kiev: Tekhnika, 1965.
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