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8/7/2019 Mo-Fe cathode
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Electrochimica Acta 50 (2005) 55945601
Kinetics of the hydrogen evolution reaction on FeMo filmdeposited on mild steel support in alkaline solution
N.R. Elezovic a, V.D. Jovic a, N.V. Krstajic b,
a Center for Multidisciplinary Studies, University of Belgrade, 11030 Belgrade, Serbia and Montenegrob Faculty of Technology and Metallurgy, University of Belgrade, 11000 Belgrade, Serbia and Montenegro
Received 12 November 2004; received in revised form 21 January 2005; accepted 5 March 2005
Available online 17 May 2005
Abstract
The mechanism and kinetics of the hydrogen evolution reaction were studied in 1.0 mol dm3 NaOH solution on FeMo alloy elec-
trodes prepared by electrodeposition at constant current densities from a pyrophosphate bath. A series of electrode containing 3459 at.%
Mo was prepared. Electrodes displayed porous character, and electrochemical impedance spectroscopy was used to characterized real
surface area. It was found that within the whole potential region the mechanism of the HER is a consecutive combination of the
Volmer step, followed dominantly by a rate controlling Heyrovsky step, while the contribution of the parallel Tafel step is negligible.
The kinetic parameters of the HER were determined. With an increase in the molybdenum content, the electrodes become more ac-
tive, and an increase in the real surface area is observed. The main factor influencing the electrode activity seems to be the real surface
area.
2005 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen evolution; FeMo electrodes; Alkaline solution; Impedance; Mechanism
1. Introduction
The hydrogen evolution reaction (HER) is of technologi-
cal importance for such processes as water electrolysis, chlo-
rate and chlorine production. For these applications materials
with low overpotential are needed.
Miles [1] suggested that a combination of two metals from
the two brunches of volcano curve could result in enhanced
catalytic activity. Jaksic [2] showed that a combination of Ni
or Co with Mo could result in a substantial enhancement of
the HER.Numerous studies thoroughlyinvestigated the kinetics and
mechanism of the HER at NiMo [312] alloys of various
compositions, obtained by suitable baths. All these electrodes
are characterized by having a high roughness factor.
However, despite extensive studies, in the case of chlorate
cell process, mild steel is a traditional cathode material and its
Corresponding author.
E-mail address: [email protected] (N.V. Krstajic).
substitution by more active catalysts based on combination
of Co or Ni with Mo up to now is not successful. The main
reason is probably the fact that Co or Ni ions present in the
solution at very low concentration as corrosion product cat-
alytically degrade active chlorine (ClO, HClO) which lead
to substantial current losses [13].
In this study, we report the results regarding the HER on
some FeMo coatings prepared by the electrochemical de-
position from suitable pyrophosphate bath [14] on mild steel
substrate. The main purpose is to study the mechanism and
kinetics in order to determine surface coverage by adsorbedhydrogen and to understand the source of the electrode ac-
tivity.
2. Theory
Generally, the mechanism of the HER in aqueous acid
solutions is treated as a combination of three basic steps, two
electrochemical and one chemical:
0013-4686/$ see front matter 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2005.03.037
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N.R. Elezovic et al. / Electrochimica Acta 50 (2005) 55945601 5595
H3O+ +M+ e
k1k1
MHads +H2O (1)
followed by
MHads +H3O+ + e
k2k2
M+H2 +H2O (2)
and/or
2MHadsk3k3
2M+H2 (3)
The rates of corresponding steps are given by
v1 = k1(1 H)exp
1F
RT
k1H
exp
(1 1)
F
RT
= k1(1 H) k
1H (4a)
v2 = k2H exp
2F
RT k2(1H)
exp
(1 2)
F
RT
= k2H k
2(1H) (4b)
v3 = k32H k3(1 H)
2 (4c)
In these rate laws surface concentrations of the interme-
diate Hads and the corresponding free adsorption sites at
the electrode are given as the coverage (H), or surface
sites concentrations (1H). The chemical rate constants
ki (mol cm2 s1) have nominally been written to include
H3O+
ion concentration, in the 0.5mol dm3
HClO4 solu-tion.
Under open circuit conditions all three reaction steps are
in equilibrium with net reaction rates equal to zero so that
v1 = v2 = v3 = 0 (5)
In this case and according to Eq. (5) only four out of six rate
constants are independent parameters, i.e. k1, k2, k1, and k3,
while k2 = (k1k2)/k1, and k3 = k21k3/k
21.
The steady-state kinetics of the HER at constant current
density are characterized by the conditions of the charge bal-
ance (with rates in mol cm2 s1):
r0 =jF= (v1 + v2) (6)
and the mass balance with respect to the intermediate Hads:
r1 =q1
F
dH
dt= v1 v2 2v3 (7)
q1 is the charge necessary for a monolayer coverage by ad-
sorbed hydrogen.
When a particular value of the mass balance, i.e. r1 = 0
is set, the steady-state coverage can be calculated, assuming
that the Langmuir adsorption isotherm is operable. Calcu-
lations showed that the rate constants k2 and k3 of the
backward reactions of steps 2 and 3 could be neglected be-
cause they have no significant influence on the reaction rates
of the second (Eq. (4b)) and third step (Eq. (4c)) at overpo-
tential 60 mV, and the steady state coverage (H), of
the intermediate Hads species is the following function of the
corresponding rate constants:
H = (k1 + k
1 + k
2) +
(k1 + k
1+k
2)
2+ 8k1k3
4k3(8)
and the pseudocapacitance, C is given by
C =q1dH
d(9)
The presence of the electrochemical rate constants in Eq. (8)
clearly indicates the rather complex dependence ofH on
the electrode potential.
Theoretically, six variables, i.e. four independent chemi-
cal rate constants of the three basic steps and two symmetry
factors of the electrochemical steps (1 and 2) can describe
the mechanism of the HER on the electrode in the corre-
sponding solution. However, it can be reasonably assumed
for elementary electrode reactions, that 1 =2 = 0.5. With
this assumption the problem is reduced to the determination
of four independent variables, i.e. k1, k1, k2, k3.
The Faradaic impedance of the HER is described by the
following equation [15]:
1
Zf=
1Rct
+1
1Rp+ jCp
1
(10)
The equation is written directly from Armstrongs equivalent
circuit [16].
The equivalent circuit elements: Rct, Rp, Cp are complex
functions of the kinetic parameters and are given [17] by
R1ct = F
v1
E
H
+
v2
E
H
=
F2
RT
[k1(1 H) k
1H + k
2H] (11a)
Generally, it is reasonable to assume that more than one
of the basic steps is involved in the mechanism of the HER,which means that the value of the experimental Tafel slope
is determined by the reciprocal of the Faradaic resistance
(i.e. the sum of charge transfer and pseudo resistances)
(Rct + Rp)1, where the pseudo resistance is presented by
Rp = R20
Rct +R0(11b)
and the parameter R0 is defined as follows:
R10 =F2p
q1
v1
H
E
+
v2
H
E
(11c)
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Fig. 1. CPE equivalent circuit used for the HER.
with the time constant p equal to
1p =F
q1
2
v3
H
E
+
v2
H
E
v1
H
E
(11d)
However, on solid electrodes, the double-layer capaci-
tance is substituted by a constant phase element (CPE) its
impedance is given as [18]:
ZCPE =1
T(j)(12)
where Tis capacitance parameter (in Fcm2 s1) and 1
is a parameter characterizing rotation of the complex plane
impedance plot. The double-layer capacitance may be esti-
mated from [18]:
T= Cdl(R1 +R
1ct )
1(13)
This is the so-called CPE model and it is presented in
Fig. 1. It predicts the formation of two depressed semicircles
on the complexplane plots.
3. Experimental
3.1. Cell and chemicals
A conventional three-compartment cell was used. The
working electrode (WE) compartment was separated by frit-
ted glass discs from the other two compartments. The WE
compartment was jacketed and thermostated during measure-
ments at 25.0 C using an ultrathermostat. All measurements
were performed in 1.0 mol dm3 solution of NaOH (Spec-
trograde, Merck), prepared in deionized water.
The WE compartment was saturated with purified hydro-
gen at standard pressure during measurements.
3.2. Electrodes
Allsamples were depositedon mild steel substrates having
a surface area of 1 cm2. The steel substrates were first sand
blasted using 50m particles, degreased in NaOH-saturated
ethanol for 5 min, then etched in 25 wt.% HCl for 2 min. Af-
ter this procedure samples were washed with distilled water,
dried and weighted and then immersed in the solution for
FeMo alloy electrodeposition. FeMo film was deposited
on one side of the electrode. The inactive walls of the sup-
port were protected from the solution by the alkaline resistant
epoxy resin. After deposition samples were washed, driedand weighted again to determine the mass of the alloy. All
solutions were made using distilled-deionized water and an-
alytical grade chemicals.
FeMo alloys were deposited to a constant charge of
36Ccm2 at three different current densities: 100 mA cm2
(FeMo100), 50 mA cm2 (FeMo50) and 20 mA cm2
(FeMo20), from the plating bath with the following compo-
sition: 9 g dm3 of FeCl3, 40 g dm3 Na2MoO4, 75 g dm
3
NaHCO3 and 45gdm3 Na4P2O7 at 60
C. A Pt mesh,
placed parallel to the cathode, was used as a counter elec-
trode during electrodeposition and electrolyte was moder-
ately stirred with the magnetic stirrer.
The counter electrode was a platinum sheet of 5 cm2 geo-metric area.
The reference electrode was the Hg|HgO electrode in
1.0moldm3 NaOH, held at a constant temperature of 25 C.
All potentials are referred to the standard hydrogen electrode
scale (SHE). The calculated value of the equilibrium poten-
tialoftheHERin1.0moldm3 solution of NaOH (pH13.86)
at 25.0 C is 0.818 V.
3.3. Measurements
Tafel lines were recorded using potentiostatic steady-state
voltammetry, point by point at 60 s intervals, in the range ofpotential from 1.12 to 0.82 V, using a PAR 273 potentio-
stat, with good reproducibility of measurement.
Whenever the potential of the WE approached approxi-
mately 1.1 V (or when current densities were close to, or
above approximately 0.1 A cm2) it was found that the un-
compensated solution resistance was significant. Therefore,
the IR drop was systematically determined in all measure-
ments, using ac impedance methods. All data presented in
this article are corrected for the IR drop.
Simultaneously with the Tafel lines, electrochemical
impedance spectra of the WE at selected constant potentials
were determined, using a PAR 273 potentiostat, together with
a PAR 5301 lock-in-amplifier, controlled through a GPBI
PC2A interface. The fast Fourier transformation (FFT) tech-
nique was used in the impedance measurements in the fre-
quency region below 5 Hz. So, both impedance spectra in
the complex plane and corresponding Bode diagrams were
obtained in the frequency range from 50 mHz to 100 kHz,
at the constant potentials of the WE. In all measurements
above 5 Hz, ten frequency points per decade were taken. The
real and imaginary components of the impedance spectra in
the complex plane were analyzed using the nonlinear least
squares (NLSs) fitting program to determine the parameters
of the proposed equivalent circuit. The rate constants were
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calculated by minimizing the residual (S) of the sum of each
experimental datum (the dc polarization measurements and
ac impedance results).
The real electrode surface area was evaluated from in situ
measurements of double-layer capacitance, determined from
initial potentialdecay slopes (d/dt) following interruption
of significant, overpotential ()-dependent, Faradaic currentspassing across the electrode/electrolyte interface. Potential-
relaxation transients were recorded digitally by computer
over five to six decades of time using a very fast electronic
switcher.
Scanning electron microscopy (SEM-JOEL 840) was
used to characterize the as-deposited surfaces and en-
ergy dispersive X-ray spectroscopy (EDS) to determine
alloy composition. Selected deposits were mounted in
cross-section, polished and examined by optical microscopy.
4. Results and discussion
4.1. Composition and morphology of the FeMo alloys
The influence of the current density (potential) on current
efficiency and alloy composition is presented in Table 1. As
can be seen the content of Mo increases, while the content of
Fe decreases with increasing average current density.
In Fig. 2 are shown typical top view of FeMo alloy (a)
and a cross section of the deposit of a thickness of about
20m (b). As can be seen morphology of FeMo alloys is
characterized by the presence of micro-cracks, with some
of them being up to 2m wide (Fig. 2(b)). It is important
to note that the surface of the samples deposited at lower
Table 1
The influence of the current density for deposition of FeMo coatings, jdep,
on current efficiency, i, and alloy composition
Electrode jdep(mAcm2)
Mo (at.%) Fe (at.%) Current efficiency,
i (%)
FeMo20 20 34.0 66.0 36.8
FeMo50 50 41.8 58.2 32.0
FeMo100 100 59.3 40.7 29.8
Table 2
Kinetic parameters for the HER obtained from the polarization curves in
1.0moldm3 NaOH at 25 C
Electrode FeMo20 FeMo50 FeMo100
b1 (mV) 36 35 37
b2 (mV) 127 124 125
j0 (106 A cm2) 1.8 4.2 20.4
j (103 A cm2) (=0.2 V) 14.6 32.7 59.3
current densities (examined with the naked eye) seemed to
be less rough.
4.2. Polarization measurements
The polarization curves obtained on different FeMo
electrodes are shown in Fig. 3, and Table 2 presents
the kinetics parameters j0 and Tafel slopes, b, for those
electrodes. Polarization curves were recorded after holding
the electrodes at a constant cathodic current density of
100mAcm2 for about 1 h. The electrode containing
59.3 at.% molybdenum (FeMo100) is found to have the
lowest hydrogen overpotential, and the highest exchange
current density, j0 2 105
A cm2
. The polarization
Fig. 2. (a) Scanning electron micrographs of the FeMo electrode surface (1) FeMo100 (59.3% Mo); 2) FeMo50 (41.8% Mo); (3) FeMo20 (34% Mo).
Magnification 1000. (b) Cross-section of the FeMo100 deposit of a thickness of about 20m.
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Fig. 3. Tafel polarization curves for the HER on FeMo electrodes in
1.0moldm3 NaOH at 25 C.
curves are characterized by two Tafel slopes at all electrodes,
40 mV at lower overpotential region and 120 mV at
high overpotential region.
4.3. ac Impedance
Typical complex plane plots for an electrode containing
59.3 at.% Mo (FeMo100) are shown in Fig. 4 and Bode
plots in Fig. 5. Two overlapped semicircles was found for
all FeMo electrodes at all overpotentials. Data presented in
Figs. 4 and 5 are interpreted by NLS fitting procedure [19]
to determine the elements of the equivalent circuit, given in
Fig. 1. The values of all parameters obtained by this proce-
dure, which are necessary to calculate therate constantsin the
mechanism of the HER are presented in Table 3. The ohmic
resistance of the solution was R =0.69cm2.
4.4. Surface roughness
Using the NLS fit of the experimental impedance data to
the CPE model parameters characterizing the double-layer
may be obtained. From the parameters Tand it is possible
to determine an average double-layer capacitance. However,
the value of the parameter , which is close to 0.5 is too
Fig. 4. Impedance spectra in the complex plane for the HER on FeMo100
electrode at four constant overpotential within the Tafel region of the po-
larization curve. Circled points are experimental data and solid lines are
calculated using the NLS fitting procedure.
Fig. 5. Bode plot for the HER on FeMo electrodes in 1.0mol dm3 NaOH
at 25 C. Solid lines are calculated using the NLS fitting procedure using the
corresponding values for q1, assigned in figure.
low and Eq. (13) cannot be used to estimate the average Cdl.
For determination of double-layer capacity the open circuit
potential decay method was used. In this technique, poten-
tial decay is monitoring after the opening of the circuit. The
overpotentiallogarithm of time curve for FeMo100 elec-
trode is presentedin Fig.6. It is evident that theelectrode does
not relax to the equilibrium potential even after 10 seconds
indicating the presence of a large pseudocapacitance. Open
Table 3
Values of the approximated elements of the CPE equivalent circuit for the HER at FeMo100 in 1.0 moldm3 NaOH at 25.0 C, taken at the constant
overpotentials
(V) Rct ( cm2) Error (%) Rp ( cm
2) Error (%) Cp (Fcm2) Error (%) T(Fcm2 s1) Error (%) Error (%)
0.055 2.04 5.2 56.3 3.9 0.046 8.1 0.13 8.5 0.54 3.5
0.064 2.03 4.8 35.9 3.5 0.048 6.6 0.14 8.1 0.54 3.2
0.074 1.33 4.6 12.8 4.9 0.070 6.6 0.19 6.7 0.49 2.1
0.087 1.31 5.5 7.67 4.8 0.080 6.2 0.16 5.9 0.49 1.9
0.120 1.05 5.3 2.64 5.6 0.12 6.1 0.16 4.9 0.53 1.7
0.128 0.99 6.5 2.14 5.7 0.13 5.9 0.16 4.6 0.53 1.6
0.150 0.86 6.1 1.32 7.1 0.14 5.7 0.16 4.4 0.52 1.5
0.164 0.78 6.5 0.95 7.0 0.16 5.6 0.15 4.3 0.43 1.4
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Fig. 6. Overpotential, , against log time, tplot for the HER at FeMo100
electrode recordered after interruption of polarization at overpotential
=0.44 V in 1.0 mol dm3 NaOH at 25 C.
circuit potential decay can be can be presented by the follow-ing equation [20]:
C + Cdl =j
ddt
(14)
where C is an pseudocapacitance and j is the measured
steady-state current density at the overvoltage . In order to
analyze this equation the derivative d/dtmust be determined
from the experimental t curves. Corresponding d/dt
curves obtained after numerical differentiation is shown in
Fig.7. At more negative overpotentialregion, where the pseu-
docapacitance is negligible, the double-layer capacitance can
be determined. The corresponding C curves for FeMoelectrodes obtained using Eq. (14) are presented in Fig. 8.
The double-layer capacities Cdl, increase with an increase in
the molybdenum content (Table 4).
From the double-layer capacitances the real electrode sur-
face area andthe surface roughness(R)canbeestimated,ifthe
double-layer capacitance for a smooth surface is known. Tak-
ing into account the value of ca. 20 F cm2 used earlier in
the literature [21,22], the values of the surface roughness ob-
Table 4
Double-layer capacitance values, surface roughness, and intrinsic activities
for FeMo electrodes in 1.0 moldm3 NaOH at 25 C
Electrode Cdl (mFcm2) R j0R1 (108 A cm2)
FeMo20 2.5 125 1.4
FeMo50 7 350 1.2
FeMo100 11 550 3.7
Fig.7. against (d/dt) plots forthe HERat FeMo100electrode obtained
by the numerical differentiation of initial part oftcurve presented in this
figure.
Fig.8. The capacitance, C, against overpotential plots forthe HERat FeMo
electrodes in 1.0 moldm3 NaOH at 25 C.
tained are included in Table 4. They indicate that the surface
roughness increases with increase of molybdenum content in
the alloy.
However, one should keep in mind that if the high fre-
quency data produced a distorted semicircle in complex plane
impedance plot with the parameter 0.5, the short time po-
tential decay curves (presented in Fig. 8) are also affected by
this problem, because there is an equivalence between time
and frequency data, which means that it is not possible ac-
curately to determine the double-layer capacitance of these
electrodes even with the open circuit potential decay method.
Table 5
Values of the rate constants (in mol cm2 s1) for individual steps of the HER on FeMo electrodes in 1.0 mol dm3 NaOH at 25 C
Electrode k1 k1 k2 k2a k3 k3
a
FeMo20 (34 at.% Mo) 1.5108 1.0106 2.1109 3 1011 1108 2.31012
FeMo50 (41.8 at.% Mo) 3 108 2 106 3 109 4.5 1011 3108 6.81012
FeMo100 (59.2 a t.% Mo) 6 108 5 106 6 109 5 1011 6108 4.41012
a k2 = (k1k2)/k1; k3 = k21k3/k
21.
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Fig. 9. Experimental (circled points) and simulated of: (1) Tafel plot; (2)
vs. log[1/(Rct +Rp)]; (3) vs. log (1/Rp); (4) vs. log (1/Rct) for the HER
at FeMo100 in 1.0mol dm3 NaOH solution.
Fig. 10. Simulated curves, numerically calculated for the set of rate
constants for the HER at FeMo electrodes in 1.0 mol dm3 NaOH solution
at 25 C.
Fig. 11. Overpotential dependence of the theoretically calculated individual rates for the Volmer, Heyrovsky and Tafel steps occurring simultaneously with the
HER on FeMo electrodes in 1.0mol dm3 NaOH solution at 25 C. Theoretical polarization curve, obtained by summing individual curves is represented by
full lines. Experimental data are presented by circled points.
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4.5. Rate constants
The rates constants of the individual steps for the HER
was determined from polarization measurements and fitted
parameters values of CPEequivalent circuit using NLSfitting
procedure [19].
The estimated values of the rate constants are presentedin the Table 5, for the apparent surface area of 1 cm2.
The complexplane impedance diagrams calculated from
the evaluated k1 values are presented by the solid lines in
Figs. 4 and 5.
Conway and coworkers [17] have shown that at suffi-
ciently high when H is almost constant and for Lang-
muiran adsorption, the theoretical separation between j
and the simulated log(Rct +Rp)1 plots (curves 1 and 2
in Fig. 9) isequalto log (F/RT). With = 0.5 and T=298K,
log(F/RT) = 1.29, which is close to the observed separation
between the two lines of 1.31. This analysis proves the Lang-
muiran adsorption of H for the HER at FeMo electrodes.
Using the values of the rate constants from Table 5, andEq. (8) it is possible to calculate the dependence ofH on the
overpotential of the FeMo electrodes, which are presented
in Fig. 10. It is interesting to note, that this dependence is
almost the same for all FeMo electrode.
The calculated values of the maximum charge of the WE,
q1, obtained by fitting of Bode plots (Fig. 5) are close to
15mCcm2 for FeMo100, 5 mC cm2 for FeMo50 and
3mCcm2 for FeMo20 electrodes.
The reaction rates of the Volmer, Heyrovsky and Tafel
steps in the mechanism of the HER and the resulting total
current value areall independently shown in Fig. 11, as calcu-
latedfromthecorresponding ki values, in a wideoverpotentialregion. In this figure, all the rate values, vi (mol cm
2 s1)
are shown in terms of current density, Fvi (Acm2).
From Fig. 11, one can conclude that the reaction
mechanism is a consecutive combination of Volmer and
Heyrovsky step and that Heyrovsky step prevails over Tafel
step in low overpotential region at all investigated FeMo
electrodes. Reaction rate is controlled by the Heyrovsky
reaction because of the much smaller k2 value as compared
with the k1 and k3 values.
The intrinsic activity, that is the rate constant of the
limiting step per real surface area or the exchange current
density per real surface are, k2/R, or j0/R, was estimated
(see Table 4). Intrinsic activities of FeMo20 (34 at.% Mo)
and FeMo50 (41.8 at.% Mo) are similar and smaller than
FeMo100 (59.3 at.% Mo).
5. Conclusions
Polarization measurements and the ac impedance mea-
surements were used to determine the mechanism and the
kinetics of the electrodeposited FeMo alloy electrodes. The
surface roughness estimated using the double-layer capac-
itance ratio shows that real surface area increases with in-
crease in the molybdenum content. This factor is principally
responsible for the increased activity.
The rate constants of the forward and backward re-
actions of Volmer, Heyrovsky and Tafel steps were esti-mated by a non-linear fitting method of the experimen-
tal results obtained from polarization and impedance mea-
surements. The HER proceeds through VolmerHeyrovsky
mechanism and reaction rate is controlled by Heyrovsky
step.
Acknowledgement
This paper has been supported by the Ministry of Sci-
ence and Environmental Protection, Republic of Serbia, un-
der Contract No. 1825.
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