Electroactivity of NiCr Catalysts for Urea Oxidation in Alkaline Electrolyte

Article abstract of DOI:10.1002/cctc.201700451

Inexpensive Ni-based catalysts have shown comparable urea oxidation activity to that of precious metals in an alkaline electrolyte. We investigated the urea oxidation on binary NiCr catalysts supported on a carbon matrix in 1 m KOH and 0.33 m urea. The amount of Cr was optimized in the catalyst layer, and it was found that the catalyst with 40 % Cr (Ni₆₀Cr₄₀/C) exhibits the highest urea oxidation activity of 2933 mA mg_Ni ^?1 at 0.55 V; with an onset potential of 0.32 V vs. Ag/AgCl. Chronoamperometry curves of NiCr catalysts show stability over 2000 s in oxidizing urea. A lower charge-transfer resistance of 3.3 Ω cm² at 0.40 V was calculated on Ni₆₀Cr₄₀/C compared to that on Ni/C (13 Ω cm²). The exceptionally high mass activity of Ni₆₀Cr₄₀/C is attributed to its improved charge-transfer kinetics, enhanced surface coverage of Ni^II/Ni^III redox center, better dispersion of Ni nuclei, and a low-Tafel slope.

Full text of DOI:10.1002/cctc.201700451

Accepted Article
Title: Electroactivity of Urea Oxidation on NiCr Catalysts in Alkaline
Electrolyte
Authors: Ramesh Kumar Singh and Alex Schechter
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Electroactivity of Urea Oxidation on NiCr Catalysts in Alkaline
Electrolyte
Ramesh Kumar Singh[a] and Alex Schechter*[b]
chemical step.[5] Thereafter, the NiOOH
Abstract: Inexpensive Ni-based catalysts have shown comparable
product in
a
urea oxidation activity to that of precious metals in an alkaline
electrolyte. We investigated urea oxidation on binary NiCr supported
on a carbon matrix in 1 M KOH and 0.33 M urea. The amount of Cr
is optimized in the catalyst layer and it was found that catalyst with
40% Cr (Ni60Cr40/C) exhibits the highest urea oxidation activity of
intermediate was probed by in-situ surface-enhanced Raman
(SER) spectroscopy [6–8] and in-situ X-ray diffraction (XRD).[9] To
further improve urea oxidation on Ni catalysts, a variety of binary
and ternary Ni-based catalysts with enhanced urea oxidation,
, , , , and
such as Ni-Rh[10] Ni-Co[11,12] Ni-Zn-Co[13] Ni/WC[14]
2933 mA mg-1 at 0.55 V; with an onset potential of 0.32 V vs.
Ni
Ni/Sn[15] were reported. Most of the catalysts used for urea
Ag/AgCl. Chronoamperometry curves of NiCr catalysts show stability
over 2000 s in oxidizing urea. A lower charge-transfer resistance of
3.3 Ω cm2 at 0.40 V was calculated on Ni60Cr40/C compared to that of
Ni/C (13 Ω cm2). The exceptionally high mass activity of Ni60Cr40/C is
attributed to improved charge-transfer kinetics, enhanced surface
coverage of Ni(II)/Ni(III) redox center, better dispersion of Ni nuclei
and a low-Tafel slope.
oxidation were prepared by an electro-deposition method, where
[17]
[18]
the yield is limited[4,6,12,16]
.
NiCo2O4
,
Ni1.5Mn1.5O4
, and
[19]
LaNiO3
with improved performance towards urea oxidation
chemical synthesis method were recently
prepared by
a
reported. The mechanism on these catalysts is similar to that of
Ni.[6,16] Xu et al. critically reviewed various aspects of urea
oxidation on
a
variety of Ni surfaces.[20] Electrochemical
impedance spectroscopy investigation of urea oxidation on Ni
suggests both a direct and an indirect mechanism path.[21] To
date, Ni-based catalysts remain the state-of-the-art for the urea
oxidation reaction in alkaline electrolyte.
1. Introduction
Growing demands for energy and global warming concerns
inspire the development of renewable abundant energy sources.
Direct urea fuel cells (DUFCs) are regarded as a clean energy
conversion device. DUFCs have recently gained significant
attention because urea is used as an alternative fuel to
hydrogen, which mitigates problems associated with hydrogen
production, storage, and transportation.[1–3] However, the power
density of DUFCs is lower compared to polymer electrolyte
membrane fuel cells (PEMFC), due to the sluggish urea
oxidation kinetics at the anode. Urea oxidation on Ni-based
catalysts was first reported by G. G. Botte’s group in an alkaline
solution.[4] They proposed a direct mechanism of urea oxidation
on Ni, where a nickel oxyhydroxide (NiOOH) intermediate
initiates urea oxidation in a sequence of electrochemical steps.
Density functional theory calculation suggests an indirect path of
urea oxidation, where urea reacts with NiOOH to form its final
A NiCr catalyst was recently used in the anode of an alkaline
polymer electrolyte fuel cell (APEFC) to improve cell
performance.[22] It was suggested that Cr helps modify the
electronic structure of the d-band by weakening the Ni-O
interaction.[22] The NiCr/C catalyst also shows excellent
tolerance towards quaternary ammonium salts in APMFCs.[23]
The enhanced methanol oxidation[24] and hydrogen evolution[23]
activities of bimetallic NiCr catalysts in the alkaline medium were
reported.
In this article, urea oxidation on NiCr/C catalysts is reported for
the first time. The rigorous chemical and electrochemical
analysis of NiCr ratios were carried out in order to identify the
most active composition of this catalyst. The highest urea
oxidation activity of Ni60Cr40/C catalysts is correlated with the
charge-transfer kinetics, particle dispersion, and surface
coverage of Ni(II)/Ni(III) redox using a broad range of analytical
methods.
[a]
[b]
Dr. Ramesh Kumar Singh
Department of Chemical Sciences
Ariel University
2. Results and Discussion
Ariel-40700, Israel
The chemical composition of the NiCr catalysts was verified by
inductively coupled plasma-optical emission spectrometry (ICP-
OES) on the acid dissolved sample and is in good agreement
(less than 1%) with the nominal sample composition expected
from the reactant ratio (Table S1).
Prof. Alex Schechter
Department of Chemical Sciences
Ariel University
Ariel-40700, Israel
E-mail: salex@ariel.ac.il
1
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X-ray diffraction (XRD) patterns of NiCr/C powder are shown in
Figure 1. For comparison, Ni/C and Cr/C XRD patterns are also
included in the same figure. The peak at 25° is ascribed to the
Vulcan XC-72 carbon-support. The peak observed at 33.7° is
assigned to Ni(OH)2, the peak at 44° arises from Ni(101) which
indicates the formation of metallic Ni, and the peak at 60.2° is
attributed to the hexagonal structure of Ni.[11] With the
incorporation of Cr, the XRD peaks’ intensity decreases sharply
and almost disappear above 40 at.% Cr. Pure Cr depicts almost
no crystalline peaks. Moreover, Cr peaks are completely missing
from the XRD pattern. It thus appears that Cr increases the
amorphization of NiCr particles with no direct evidence of NiCr
alloy formation. It is therefore concluded that chromium controls
the Ni crystallite growth while promoting Ni(OH)2 formation up to
40 at.% Cr. Above 40 at.% Cr, the Ni(101) peak is apparent. The
crystallite size estimated from the peak broadening of Ni(101)
and shown in Table S2. The presence of Ni and Cr is verified
from the ICP-OES and scanning electron microscopy (SEM)
elemental mapping is shown in the next section.
A selected area SEM image is shown in Fig. 2(c) and the
corresponding elemental mapping distributions of Cr and Ni are
shown in Fig. 2(d) and (e), respectively, which could indicate the
formation of segregated phases of Ni and Cr. Atomic
percentages of Ni and Cr obtained from EDS are in good
agreement with ICP-OES results.
Overlaid steady state cyclic voltammograms (CVs) of NiCr/C
catalysts are shown in Fig. 3. CVs of Ni/C and Cr/C are included
for comparison. The CVs show the typical redox feature of Ni as
reported in the literature.[4,6,16] A featureless voltammogram of
Cr/C suggests that it is electrochemically inactive. The anodic
peak at ~0.38 V is ascribed to oxidation of Ni(II)(OH)2/Ni(III)OOH
and the cathodic peak is attributed to the reversible reduction of
Ni(III)OOH to Ni(II)(OH)2. Electrochemical parameters from Fig.
3 are given in Table S3. NiCr/C catalysts show a shift in redox
peaks to the negative potentials.
Ni/C
Ni90Cr10/C
Ni60Cr40/C
Ni40Cr60/C
Ni20Cr80/C
Cr/C
10
5
C
0
(a)
(b)
(c)
(d)
(e)
-5
-10
10
20
30
40
50
60
70
80
90
0.0
0.1
0.2
0.3
0.4
0.5
2  (degree)
E/V vs. Ag/AgCl
Figure 1. X-ray diffraction patterns of Ni/C (a), Ni80Cr20/C (b), Ni60Cr40/C (c),
Ni40Cr60/C (d), and Cr/C (e).
Figure 3. Cyclic voltammograms of NiCr/C catalysts in N2-saturated 1 M KOH
solution at a scan rate of 10 mV s-1.
Figure 2 presents a typical scanning electron microscopy (SEM)
image of a carbon-supported Ni60Cr40 catalyst. The morphology
of the catalysts is shown in Fig. 2(a) and the corresponding
energy dispersive spectroscopic (EDS) analysis of the encircled
region of Fig. 2(a) is shown in Fig. 2(b).
This shift is assigned to the lower redox potential of Cr(II)/Cr(III)
couple compared to that of Ni(II)/Ni(III), which may shift the
overall redox potential of NiCr compared to that of bare Ni. An
increase in the anodic peak current is observed with the
increase in Cr content up to 40%. Above 40%, the peak current
decreases gradually with the further increase in Cr content to
80%. The redox peak separation potential decreases gradually
from 140 to 60 mV with the increase in Cr content (Table S3).
An observed peak separation of only 60 mV is seen in 80% Cr,
which suggests high reversibility of Ni(II)/Ni(III) redox. This redox
peak separation is close to the theoretical reversible mixed-
diffusion controlled process (59 mV for single electron transfer).
[25] The electrochemical active surface area (ECSA) is calculated
from integration of the charge under the redox peaks using a
constant of 514 µC cm-2 for Ni surfaces.[26] A higher ECSA of
285 m2 g-1Ni is observed in Ni60Cr40/C compared to 123 m2 g-1Ni of
Figure 2. SEM image of Ni60Cr40/C (a), EDS of encircled region (b), SEM
image of Ni60Cr40/C where mapping was performed (c), elemental mapping of
Cr (d) and Ni (e).
2
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Ni/C supports better dispersion of smaller Ni nuclei. Thus, the
sharp redox features and small peak separation observed on
NiCr electrodes point to better charge-transfer kinetics.
compared to that of Ni. The similar Tafel slope of 28 mV decade-
1
was reported on Ni film.[16] The addition of Cr thus decreases
the Tafel slope by a factor of 2 on Ni60Cr40/C.
The urea oxidation activity of NiCr/C catalysts was measured in
0.33 M urea and 1 M KOH (Fig. 4(a)) and the CVs of Ni/C and
Cr/C are included for comparison. In the presence of urea, the
anodic current sharply increases in accordance with urea
oxidation facilitated by Ni(III) formation (Fig. 3). The oxidation
onset potential exactly overlaps that of Ni in KOH solution,
suggesting that the reaction is governed by the Ni(II)/Ni(III)
redox (Fig. S1). The cathodic peak, assigned to the reduction of
NiOOH which was not chemically reduced by urea, serves as an
indication that the chemical step (discussed in the next section)
is slower than NiOOH formation. At a slow scan rate of 1 mV s-1,
the cathodic peak current is minute, resulting from an almost
complete urea reduction of NiOOH as well as competitive
adsorption of hydroxide and urea to the remaining Ni(III) sites.
However, the cathodic charge on Ni/C (1 mC) is significantly
lower in the presence of urea compared to the cathodic charge
in a urea free solution (5.5 mC) of the same electrode and
suggests that most of the generated NiOOH react with urea. As
seen from Fig. 4(a), Cr/C displays very low currents suggests
that it is inactive towards urea oxidation. Electrochemical
parameters derived from Fig. 4(a) are listed in Table S4. With
the increase in Cr content, a decrease in onset potential is
observed up to 40% Cr, and it increases gradually with the
increase in Cr content above 40% (Fig. 4(b) and Table S4). A
significantly low onset potential of 0.32 V is observed with 40%
Cr in NiCr catalysts (Fig. 4(b)). This value is among the lowest
Figure 4. Cyclic voltammograms of NiCr/C catalysts in 0.33 M urea and 1 M
KOH at a scan rate of 10 mV s-1 (a), onset potential (b), and current density at
potentials of 0.40, 0.45, 0.50 and 0.55 V (c) as a function of Cr atomic %.
Table 1. Tafel slopes of various NiCr/C catalysts calculated from the
voltammograms in 0.33 M urea/1M KOH solution at a scan rate of 10 mV s-1.
Catalysts
Tafel Slope
(mV decade-1)
Ni/C
30
17
16
15
14
Ni90Cr10/C
Ni60Cr40/C
Ni40Cr60/C
Ni20Cr80/C
reported compared with 0.29 V [18], 0.34 V [15], and 0.35 V
[21]
under similar conditions. Fig. 4(c) displays current densities at
selected potentials of 0.40, 0.45, 0.50 and 0.55 V as a function
of atomic Cr %, which shows a volcano shape behavior. A
similar trend in current density is seen at all potentials, with the
highest current at 0.55 V.
CVs of Ni60Cr40/C were recorded at various scan rates with and
without urea in 1 M KOH electrolyte (Fig. 5). In light of the high
activity of the Ni60Cr40/C electrode, it was selected for this
analysis. Fig. 5(a) shows CVs recorded at various scan rates in
1 M KOH solution. Although the Ni(II)/Ni(III) peak current
increases, the anodic and cathodic peak potentials do not
change much with the scan rate (ʋ), in line with a redox
reversible reaction. A plot of cathodic current density as a
function of scan rate (ʋ) shows linear behavior and indicates a
surface confined redox (inset to Fig. 5(a)). This behavior is in
line with that reported by R. Zhang et al. where the peak current
density (Ip) proportional to the scan rate below 0.1 V s-1 is
attributed to surface-confined redox and above 0.1 V s-1, it
becomes proportional to the square root of the scan rate of a
dominant diffusion process.[27] The surface coverage of
Ni(II)/Ni(III) redox is estimated from Eqn. (1).[27]
The highest mass activity of 2933 mA mg-1 at 0.55 V was
Ni
calculated for Ni60Cr40/C and is 3.6-fold larger than Ni (800 mA
mg-1Ni). Ni1.5Mn1.5O4[18] was recently reported to exhibit enhanced
activity towards urea oxidation, where the maximum current of 7
mA cm-2 at 0.50 V is observed compared to 65 mA cm-2 at 0.50
V in Ni60Cr40/C under identical conditions. A fivefold increase in
mass activity was observed with Ni60Cr40/C compared to
[19]
LaNiO3
(371 mA mgoxide-1) in 0.33 M urea and 1 M KOH
solution.
Tafel slopes of NiCr/C catalysts in the presence of urea are
given in Table 1. A Tafel slope of 16 mV decade-1 was observed
on Ni60Cr40/C, which is lower than that of Ni/C (30 mV decade-1),
suggesting a sharp increase in the current of NiCr catalysts
Ip = n2F2 ʋ AГ / (4RT)
(1)
3
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Where Ip: peak current, n: number of electrons transferred, F:
A similar increase in cathodic current with scan rate was
Faraday constant, ʋ: scan rate, A: geometrical electrode area, Г:
reported by Bryan K. Boggs et al. in the presence of urea.[4] The
surface coverage of Ni(II)/Ni(III) redox, R: universal gas constant, fact that the increase in the urea oxidation current is not
and T: temperature.
proportional to the scan rate, contrary to 1 M KOH, further
supports the claim that the urea oxidation reaction is not purely
electrochemical and that it proceeds via an electrochemical-
chemical (EC) mechanism.[6] It is important to note that no
change in the voltammogram was observed above 0.45 V,
indicating that no water oxidation takes place.
From the slope of the Ipc vs scan rate plot, the surface coverage
of the Ni(II)/Ni(III) redox sites was estimated to be 7.5 × 10-7
mole cm-2, which is higher than Ni/C (5.0 × 10-7 mole cm-2) and
in agreement with the activity and better dispersion of the Ni
active sites.
Chronoamperometric experiments of NiCr catalysts were carried
out to evaluate the stability of catalysts for urea oxidation (Fig. 6).
A stable current persists during a measurement time of 2000s in
oxidizing urea. The higher current observed with Ni60Cr40/C, with
respect to other NiCr and Ni electrodes, is in line with the
voltammetric results (Fig. 4(a)). The noise in the
chronoamperometric curves is due to the formation of N2 and
CO2 bubbles on the electrode surface with time.
Fig. 5(b) shows voltammograms of Ni60Cr40/C as a function of
scan rate in a urea solution. An increase in anodic current
density is observed with a scan rate at a potential above 0.45 V.
With the increase in scan rate from 1 to 10 mV s-1, the current at
0.55 V changes by 55%, from 55 to 85 mA cm-2. An increase in
the cathodic peak current with the scan rate is moderate. This is
attributed to the sluggish urea oxidation reaction rather than the
electro-oxidation of Ni(OH)2 to NiOOH (Eqn. 3).
30
Ni90Cr10/C
-1
y = - 0.784 - 0.7026 x
R-square = 0.985
(a) Ni60Cr40/C
-2
Ni60Cr40/C
Ni40Cr60/C
Ni/C
25
20
15
10
5
10
-3
-4
-5
-6
5
-7
-8
0
2
4
6
8
10
Scan rate (mV s-1
)
0
-5
0
0
250 500 750 1000 1250 1500 1750 2000
Time (s)
(b)
10 mV s-1
5 mV s-1
2 mV s-1
1 mV s-1
80
60
40
20
0
Figure 6. Chronoamperometry plot of NiCr/C catalysts in 0.33 M urea/1 M
KOH at 0.40 V vs. Ag/AgCl.
Electrochemical impedance spectroscopy was performed to
further probe the urea oxidation reaction on NiCr/C catalysts.
Typical Nyquist plots of NiCr/C catalysts at potentials of 350 and
400 mV are shown in Figs. 7(a) and (b), respectively. The
charge-transfer resistance proportional to the semi-circle
diameter decreases by one order of magnitude with the increase
in potential from 350 to 400 mV. The diameter of the semicircle
gives charge-transfer resistance (Rct). The Rct of NiCr catalysts
is given in Table S5. At 350 mV, the charge-transfer resistance
of Ni/C is 95 Ω-cm2 and decreases sharply to 13 Ω-cm2 at 400
mV. The lowest charge-transfer resistance of only 6 and 3.3 Ω-
cm2 at the potential of 350 and 400 mV, respectively, was
calculated on Ni60Cr40/C (Table S5). Thus, the Ni60Cr40/C catalyst
exhibits excellent charge-transfer kinetics towards urea oxidation.
0.0
0.1
0.2
0.3
0.4
0.5
E/V vs. Ag/AgCl
Figure 5. CVs of Ni60Cr40/C in N2-saturated 1 M KOH (a) and 0.33 M urea/1 M
KOH at various scan rates; inset to Fig. 5(a) shows a plot of cathodic peak
current density (Ipc) as a function of scan rate.
The fact that the cathodic current is low compared to the Ni
redox current is an indication of a slower chemical step (Eqn. 3)
than NiOOH formation (Eqn. 2).
E: 6Ni(OH)2(s) + 6OH- ↔ 6NiOOH (s) + 6H2O(l) + 6e-
(2)
C: CO(NH2)2(aq.) + 6NiOOH (s) + H2O(l) →N2(g) + 6Ni(OH)2(l) +
CO2(g) (3)
Where E: electrochemical step and C: chemical step, as
reported in the literature.[6] The oxidation products of urea are N2
and CO2.
4
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evidence for the tuning of the electronic property of Ni.[22] We
believe that the electronic effect of Cr on Ni surfaces is seen
from the volcano shape of the onset potential plot (Fig. 4(b)).
(b) 400 mV
80
60
40
20
0
(b) 400 mV
Ni/C
Ni90Cr10/C
12
9
120
Ni60Cr40/C
Ni40Cr60/C
Ni20Cr80/C
16
6
14
12
3
100
10
8
0
0
3
6
9
12
80
6
Zreal ( cm2)
4
10 20 30 40 50 60
60
Cr atmoic %
(a) 350 mV
40
20
80
60
40
20
0
0
350 mV
400 mV
-20
0
20
40
60
80
Cr atmoic %
Rct
Figure 8. The plot of charge-transfer resistance as a function of Cr at. % at
applied potentials of 350 and 400 mV. The inset shows the magnified view of
encircled region.
0
20
40
60
80
Zreal ( cm2)
Figure 7. Nyquist plots of NiCr/C catalysts at 350 mV (a) and 400 mV (b) in
0.33 M urea and 1 M KOH solution from 10 kHz to 50 mHz frequency range.
Inset to Fig. 7(b) shows the magnified view. The solid lines show the fitted
data.
3. Conclusions
Various compositions of NiCr/C were synthesized by direct
borohydride reduction at room temperature. The composition of
NiCr catalysts was studied and it was found that 40% Cr, 60% Ni
exhibits the highest urea oxidation activity of 2933 mA mg-1Ni at a
potential of 0.55 V with an onset potential of 0.32 V vs. Ag/AgCl.
This urea oxidation activity value is 3.6 times higher than for
Ni/C. However, bare Cr is electrochemically inactive towards
urea oxidation. With the increase in Cr content, the amorphous
phase of the catalyst was observed from the XRD patterns and
the reversibility of Ni(II)/Ni(III) redox increases. Cr seems to have
the dispersion effect to Ni nuclei. The lowest charge-transfer
resistance of Ni60Cr40/C reflects improved urea charge-transfer
kinetics. The exceptionally high mass activity of Ni60Cr40/C is
attributed to the improved charge-transfer kinetics, better
dispersion of Ni nuclei, high surface coverage of Ni(II)/Ni(III)
redox center and low Tafel slope.
A plot of charge-transfer resistance as a function of Cr atomic %
at potentials of 350 and 400 mV is shown in Fig. 8. The enlarged
region of the plot is shown in the inset to reflect differences in
charge-transfer resistance. The choice of these potentials
reflects the sharpest change in charge-transfer resistance,
above these potentials the differences among the catalyst
resistance are minute. At 350 mV, the resistance decreased
drastically from 95 to 12 Ω cm2 with an increase in Cr content
from 0 to 10%, where the lowest resistance of 6 Ω cm2 was
observed with 40% Cr. Above 40% Cr, an increase in charge-
transfer resistance was observed at both potentials. At 400 mV,
the resistance does not change sharply with Cr content, and the
lowest resistance of 3.3 Ω cm2 was observed at 40% Cr. The
charge-transfer corresponds to the current recorded in the
voltammetric results (Fig. 4(a)). The metallic chromium itself has
high charge-transfer resistance of 7890 Ω cm2 at 400 mV, which
means that it is inactive in urea oxidation. Ni(OH)2/NiOOH is
thus the active part in Ni:Cr. We, therefore, believe that the
mechanism of urea oxidation with NiCr/C catalysts is similar to
that reported by Botte's group, where NiOOH mediates the
preferential oxidation of urea to its products.[5,6,16] In addition,
adsorption of urea on Ni surface is facilitated through urea
carbon center based on the theoretical calculation by Daramola
et al.[5] As reported by Lu et al. the modification of Cr oxide on
Ni(111) surface decreases the density of state of the d-band by
the weakening of the Ni-O bond which gives the convincing
Experimental Section
Materials
Chromium (III) chloride hexahydrate (CrCl36H2O, 98%) and sodium
borohydride (NaBH4, 98%) from Acros Organics; urea (NH2CONH2:
99.0–100.5%) from Alfa Aesar; nickel (II) nitrate hexahydrate
(Ni(NO3)26H2O: 99.99%) from Strem Chemicals; Nafion (5 wt% solution
in aliphatic alcohol) from ion power; potassium hydroxide (KOH), and
isopropyl alcohol from Frutarom Ltd., Israel were used as received. High
purity (18.2 MΩ) de-ionized water (DIW) was used for the synthesis.
5
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Synthesis of Various Compositions of NiCr/C
References
The experimental conditions for catalyst synthesis are presented in Table
S6. For the typical catalyst synthesis, salts of Ni (Ni(NO3)26H2O) and Cr
(CrCl36H2O) were mixed in the required atomic ratio in an argon-purged
solution. The required amount (160 mg) of Vulcan XC-72R carbon was
added to maintain a 20 wt% loading of the catalyst on the carbon support
and the solution was stirred for another 1 h. The resultant solution was
reduced by drop-wise addition of freshly prepared 4 wt% NaBH4 while
stirring. After reduction, the black solution was centrifuged at 3500 rpm
for 10 min to collect the solid product. The black powder was washed 3
times with DIW, re-dispersed in isopropyl alcohol and dried at 70°C
overnight. Bare Ni/C was synthesized by a similar procedure, without the
addition of chromium salt.
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Electrode Fabrication
The glassy carbon disk electrodes were coated by catalyst ink. For the
typical preparation of catalyst ink: 25 mg catalyst, 2 mL isopropyl alcohol
(IPA), 1 mL water and 100 µL Nafion were sonicated for 30 min to form a
homogeneous suspension of the catalysts ink. Then, 6 µL of catalyst
suspension was drop-cast on the glassy carbon disk electrode to
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complex non-linear least square (CNLS) fit using ZsimpWin software
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We acknowledge the support of the Israeli Fuel Choices
Initiative and the Israel Science Foundation (ISF) for partially
supporting the research work through the Israel National
Research Center for Electrochemical Propulsion (INREP) and I-
CORE Program (number 2797/11).
Keywords: NiCr/C • urea oxidation • surface redox •
electrochemical impedance spectroscopy • borohydride
reduction
6
This article is protected by copyright. All rights reserved.

10.1002/cctc.201700451
ChemCatChem
FULL PAPER
FULL PAPER
Enhanced urea oxidation
activity of Ni60Cr40/C is
attributed to improved charge-
transfer kinetics, better
dispersion of Ni nuclei, high
surface coverage of Ni(II)/Ni(III)
redox center and low Tafel
slope.
Ramesh Kumar Singh, Alex
Schechter*
Electroactivity of Urea
Oxidation on NiCr Catalysts in
Alkaline Electrolyte
7
This article is protected by copyright. All rights reserved.