Hydrogen evolution assisted electrodeposition of porous Cu-Ni alloy electrodes and their use for nitrate reduction in alkali

Article abstract of DOI:10.1016/j.electacta.2014.04.048

Porous Cu-Ni alloys with a wide range of compositions have been deposited from citrate baths containing the two metal ions, under application of large current densities (-3 A cm^-2) leading to vigorous hydrogen evolution. The deposited alloy layers show macroscopic porosity and a spongy material made of a network of tiny dendrites. Porous RDEs reveal a significant current increase - as compared to ordinary polished electrodes - in the fast ferrocyanide reduction under pure mass transport control. Voltammograms of nitrate reduction in alkali at porous RDEs show at all potentials larger currents in comparison with experiments at compact electrodes of same composition. Electrolyses at porous sheet electrodes show large stable currents, leading to fast and selective nitrate reduction to ammonia.

Full text of DOI:10.1016/j.electacta.2014.04.048

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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Hydrogen evolution assisted electrodeposition of porous Cu-Ni alloy
electrodes and their use for nitrate reduction in alkali
1
∗,1
1
Luca Mattarozzi , Sandro Cattarin , Nicola Comisso , Arianna Gambirasi,
1
1
1
Paolo Guerriero, Marco Musiani , Lourdes Vázquez-Gómez , Enrico Verlato
Istituto per l’Energetica e le Interfasi (IENI)-CNR, C.so Stati Uniti 4, 35127 Padova, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Porous Cu-Ni alloys with a wide range of compositions have been deposited from citrate baths containing
the two metal ions, under application of large current densities (-3 A cm ) leading to vigorous hydrogen
Received 27 November 2013
Received in revised form 7 April 2014
Accepted 7 April 2014
−2
evolution. The deposited alloy layers show macroscopic porosity and a spongy material made of a network
of tiny dendrites. Porous RDEs reveal a significant current increase – as compared to ordinary polished
electrodes – in the fast ferrocyanide reduction under pure mass transport control. Voltammograms of
nitrate reduction in alkali at porous RDEs show at all potentials larger currents in comparison with
experiments at compact electrodes of same composition. Electrolyses at porous sheet electrodes show
large stable currents, leading to fast and selective nitrate reduction to ammonia.
Available online xxx
Keywords:
Templated Electrodeposition
Bubble Template
Porous Alloy
©
2014 Published by Elsevier Ltd.
Electrocatalysis
Nitrate
1
. Introduction
An alternative simple approach to preparation of macroporous
layers over a large area is the deposition of metals from aqueous
solutions under high cathodic currents, resulting in vigorous hydro-
gen evolution: in this regime, indicated in literature as “hydrogen
bubble dynamic template deposition” [11], the gas bubbles act as
a faint template shaping the deposit morphology and disappearing
upon current interruption, so that no template removal is required
and preparation occurs in a single step. An additional important
advantage of this method is that, unlike those based on solid tem-
plates, it may permit formation of porous layers on substrates
of complex macroscopic morphology, including e.g. grids. Several
porous metals have been deposited with this approach, including
Cu [11–15], Sn [12], Ni [16–19], Co [20], Ag [21,22] and Au [23] but
very few alloys that are, to the best of our knowledge, all Cu-based
[24–27]. Among them, porous Cu-Ni has been deposited [26] in
order to prepare a nanoporous Ni/NiO foam by selective electrodis-
solution of Cu, but the alloy itself was not tested in electrochemical
applications.
Templated electrodeposition is a promising method for the
preparation of porous conductive structures [1], with potential
applications as electrode materials in fields like electroanalysis,
energy conversion and wastewater treatment. The method uses a
number of templates, with a built-in porosity like the membranes of
track-etched polycarbonate [2–4] and anodic alumina [5,6], or with
a porosity resulting from assembly of micro- and submicro-objects
like the layers of packed microspheres made of polystyrene [7,8]
or silica [9,10]. Electrodeposition of metals or other conductors in
the interstices provides an accurate inverse replica of the mould,
and after template removal (by burning or dissolution in suitable
media, in most cases hazardous) a spongy material is obtained, with
an inner porosity of defined shape, size and organization.
Despite the great modellistic value, the mentioned procedures
are complex, costly and time consuming, and may not be suitable
for the scaling up. Moreover, in all applications in which the current
is controlled by diffusion of a reacting species, only a macroporos-
ity with typical dimension in the range of tens of microns appears
useful to enhance transport, whereas a submicron porosity will not
improve significantly electrode performances [9].
Cu, Ni and some of their alloys with noble and non-noble metals
are materials of choice for electroreduction of nitrogen oxyanions
−
[28–38]. The good activity of Cu-Ni alloys in performing NO3 reduc-
tion has been ascribed [33] to some sort of synergistic mechanism
−
in which Cu sites perform an efficient adsorption of NO , and Ni
3
sites an efficient adsorption of H-atoms resulting from discharge
+
of H /H O. The reduction mechanism would proceed by transfer of
∗ Corresponding author.
2
Hads to the O-moieties of the oxyanion and elimination of hydroxyl
ions, with an overall activity enhancement in comparison with the
E-mail address: s.cattarin@ieni.cnr.it (S. Cattarin).
1
ISE Member.
http://dx.doi.org/10.1016/j.electacta.2014.04.048
0
013-4686/© 2014 Published by Elsevier Ltd.
Please cite this article in press as: L. Mattarozzi, et al., Hydrogen evolution assisted electrodeposition of porous Cu-Ni alloy electrodes
and their use for nitrate reduction in alkali, Electrochim. Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.04.048

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Table 1
Compositions of deposition baths and of the resulting porous Cu-Ni alloys (estimated by EDS).
Bath
Bath composition:0.3
M Na citrate, 1 M (NH4)2SO4
and
Composition in at%, alloy
deposited on Nb RDEs
Composition in at%, alloy
deposited on Nb sheets (stirred
solutions)
−
1
(2500 min
)
I
II
III
IV
0.05 M CuSO4, 0.25 M NiSO4
0.08 M CuSO4, 0.175 M NiSO4
0.125 M CuSO4, 0.125 M NiSO4
0.175 M CuSO4, 0.10 M NiSO4
Cu33Ni67
Cu52Ni48
Cu70Ni30
Cu84Ni16
Cu25Ni75
Cu44Ni56
Cu66Ni34
Cu82Ni18
single metals. Porous Cu-Ni electrodes may display even better per-
formances, as indicated by preliminary evidence [39]. In this paper,
we report thoroughly on deposition and characterisation of porous
Cu-Ni materials, and present comparative tests of nitrate reduction
in alkali at compact and porous electrodes, showing the superior
performances of the latter.
efficiency of 100% - actually verified only for compositions with
Cu ≥ 60 at% [38]. For comparative EIS investigations a few compact
electrodes were mirror polished with 0.3 ␮m alumina and washed
in a water ultrasonic bath; they will be referred to as “compact
polished”. Deposition of spongy electrodes, referred to as “porous”,
−
1
was performed either on RDEs rotated at 2500 min , or on sheet
electrodes while keeping the solutions agitated by a magnetic stir-
rer. Alloy layers of suitable morphology were obtained at a current
2
. Experimental
−
2
density of -3.0 A cm from baths I-IV (pH 4.1 - 4.5) described in
Table 1.
2.1. Materials and electrochemical equipment
Alloy compositions in at%, estimated by EDS analysis and indi-
cated as CuxNi100-x, depended on stirring conditions, and higher Cu
contents were obtained for deposition on RDEs than on sheet elec-
trodes. SEM images were obtained with a FEI Quanta 200 FEG ESEM
instrument, equipped with a field emission gun, operating at an
accelerating voltage variable in the range of 20-30 kV. EDS analyses
were performed on Nb substrates using an EDAX Genesis energy-
dispersive X-ray spectrometer at an accelerating voltage of 25 kV.
EDS data obtained from spongy samples are normally questionable,
but appear reliable in the present case considering that the mate-
rial consists only of Cu and Ni, two elements with peaks close in
energy. Electron backscatter diffraction (EBSD) investigations were
performed as reported elsewhere [4].
The cell used for depositions was a single compartment cell,
equipped with a Pt wire counter electrode wound as a spiral (total
2
estimated surface of ca. 15 cm ) and fixed to the inner cell wall.
The solutions were prepared from deionized water (by a Millipore
Elix3 system, resistivity ␳ > 15 Mꢀ cm) and high purity chemicals
Sigma-Aldrich, puriss. p.a. ACS reagents). RDE electrodes were
(
made of Cu rods (Goodfellow, 99.999% purity, diameter 0.635 cm,
2
section area 0.317 cm ) or Nb rods (Goodfellow, 99.9% purity, diam-
2
eter 0.56 cm, section area 0.247 cm ), inserted in a PTFE sheath
and mounted on a EDI 101 rotating unit (Radiometer). Nb elec-
trodes were used for samples devoted to EDS analyses (see below)
to exclude interference from the substrate. Cathodes for prolonged
electrolyses were obtained depositing alloy layers on both faces
of Nb sheets (Goodfellow, 99.5% purity) or Cu sheets (Goodfellow
2.3. Electrochemical tests in cathodic reactions.
9
9.9% purity) over a surface of 10 mm x 15 mm (for a total surface of
2
about 3 cm ). No significant difference was observed between the
layers deposited on the two metals, presumably because the sub-
strates are rapidly covered in the initial stages of alloy deposition
and most of the layer growth occurs under essentially identical con-
ditions. Prior to deposition, electrodes were abraded with emery
paper 1000 grit, rinsed in a water ultrasound bath for 15 minutes
and dried in air. All data concerning current densities, deposition
charge and impedance measurements are referred to geometric
area.
Ferricyanide reduction was studied with polished Au and porous
Cu-Ni RDEs in deaerated solutions containing 20 mM K3Fe(CN)6,
20 mM K4Fe(CN)6 and 1 M KOH. Experiments of nitrate reduc-
tion were performed in a two-compartment deaerated cell, with
100 ml of 1 M NaOH and 0.1 M NaNO3 in the working compartment.
Compact and porous Cu-Ni RDEs were used for recording cyclic
voltammograms and chronoamperometries, compact and porous
sheet electrodes (about 3.0 cm2 of immersed area) for prolonged
electrolyses. Analyses of the products were performed by with-
drawing 0.1 mL samples from the cathodic compartment with a
precision pipette and assessing the products with a Metrohm model
850 Professional IC Ion Chromatograph [38].
◦
Electrodeposition experiments were performed at T = 22 C,
under potentiostatic or galvanostatic control, using an Autolab
PGSTAT 302 N, equipped with a booster providing currents as high
as 10 A. In potentiostatic experiments, a (Hg/HgO/1 M KOH) was
used as reference electrode and all potentials are referred accord-
ingly in text and figures. EIS measurements were taken with a
Solartron 1286 Electrochemical interface and a Solartron 1254 FRA,
both controlled by a ZPlot-ZView commercial software, covering
the frequency range 10 kHz to 0.01 Hz with 8 points per decade. The
electrode was polarized at the applied potential E = -0.70 V, where
the electrode is blocking, to avoid any significant faradaic reaction;
the potential modulation was 10 mV rms, checked to be low enough
to ensure linear response.
3. Results and discussion
3.1. Electrodeposition of porous Cu-Ni on RDE electrodes.
A series of preliminary electrodepositions is performed to iden-
tify good operating conditions. The baths used for these preliminary
trials are either 0.08 M CuSO , 0.175 M NiSO , 0.30 M citrate, or
4
4
a similar solution containing either 1 M (NH ) SO or 2 M NH Cl
4
2
4
4
(bath II in Table 1). A benign role of the ammonium ion is indeed
reported for deposition of the (porous) single metals, Cu [14] and
Ni [16].
2.2. Alloy deposition and characterization
With the ammonium-free bath the deposited layer is rather
powdery under all the explored deposition conditions. With the
ammonium sulphate bath, conversely, it is possible to obtain the
desired morphology by acting on the current density, as illus-
Ordinary deposits, referred to in the following as “compact”,
−2
were obtained potentiostatically (8 C cm ) from a citrate bath con-
taining 0.7 M NiSO , 20 mM CuSO and 0.26 M trisodium citrate, pH
4
4
−
2
6
.0, according to a procedure reported in detail elsewhere [38]. Film
thickness could be estimated around 2.8 ␮m assuming a current
trated in Fig. 1. The deposit obtained with jdep = -0.3 A cm
is
−
2
globular (Fig. 1a); that obtained with jdep = -1.0 A cm shows an
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Fig. 2. SEM images of deposits obtained on Nb RDE (2500 min 1), after galvanostatic
−
−
2
−2
polarization (deposition charge 40 C cm ) at jdep = -3.0 A cm , in: a) bath I, alloy
Cu33Ni67; b) bath III, alloy Cu70Ni30.
Fig. 2 reports the SEM images of deposits obtained on RDEs
−
1
−2
(
(
2500 min ), after galvanostatic polarization at j = -3.0 A cm
dep
−
2
deposition charge 40 C cm ) in the sulphate solutions I) and III)
of Table 1. The images show a rather homogeneous, macroporous
D structure, with features in the range of tens of microns. The Cu
3
content increases in the series I) to IV), but deposits with Cu con-
tent above 70 at% show a tendency to fracture, increasing with the
Cu content and rather marked for samples obtained from bath IV.
The high magnification SEM images (Fig. 2, bottom) show that
the material consists of a network of small dendrites with typi-
cal dimensions below 500 nm. The bimodal porosity, common to
all porous metals grown in a similar way [13,14,22,23], originates
from the peculiar growth mechanism. The pits with a diameter of
tens of microns result from the shaping action exerted by hydro-
gen bubbles sticking to the electrode, whereas the spongy structure
made of needle-like crystals forms under mass transfer conditions.
The latter favour limited dendritic growth: the frequent bubbles
detachment causes solution renewal, limits the formation of con-
centration gradients and prevents the formation of long fragile
dendrites, typical of metals growth under pure transport control.
The resulting metal network is a foam-like structure with a good
mechanical resistance. The morphology of the porous alloy deposits
obtained on sheet electrodes polarised in a stirred solution is, dis-
regarding a somewhat higher tendency to fracture (Fig. 3a), very
similar to those of deposits obtained on RDEs (Fig. 1-2), indicating
that the deposition process and the local hydrodynamics are essen-
tially controlled by growth and detachment of hydrogen bubbles.
According to literature ([23] and references therein), growth
of spongy-like structures made of dendrites requires specific
electrocrystallization properties, namely high exchange current
Fig. 1. SEM images of deposits obtained on Nb RDE (2500 min−1) from bath II
−
2
(
Table 1) by galvanostatic polarization (deposition charge 40 C cm ) at different
−2
current densities: a) jdep = -0.3 A cm (alloy composition Cu75Ni25); b) jdep = -1.0 A
cm (Cu56Ni44); c) jdep = -3.0 A cm (Cu52Ni48).
−
2
−2
irregular, fractured spongy aspect (Fig. 1b); the layer obtained with
−2
jdep =-3.0 A cm shows a regular spongy material, with a 3D pat-
tern of macropores having diameters in the order of tens of microns
(Fig. 1c). Using ammonium chloride, the obtained morphologies are
very similar but the copper content is larger and the tendency of
the deposit to fracture more pronounced. For this reason only the
sulphate bath is used in the following.
The obtained deposits show good adhesion to the substrate and
good mechanical resistance, as proved by the regular, essentially
flawless structure, despite generation in a rude regime of hydro-
gen evolution, and the unchanged aspect of the porous layer after
density j , low deposition overpotential ␩ and low melting point
0
−
1
prolonged (1 h) operation as RDE at 900 min in nitrate reduction
see below), verified by an accurate SEM inspection. Hence, all the
Tm–properties typical e.g. of Cu. Conversely, formation of the men-
tioned structures is not expected for metals like Ni, with low
(
−
2
subsequent depositions are performed at jdep = -3.0 A cm , vary-
ing only concentrations of species in the bath. The current density
is large but not extreme and under our operating conditions the
sort of passivation phenomena termed in the literature “electrode
effects” [40–42], involving electrode blanketing by a gas layer, are
never observed.
exchange current density for deposition j , fairly high deposition
0
overpotential ␩, high melting point Tm, and indeed “porous” Ni
deposits [16–19] show an inner cauliflower structure and an outer
globular morphology, more dense and compact than that of Fig. 2.
Apparently, the presence of Cu creates favourable conditions for
formation of spongy binary alloys, even when the second metal is
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Fig. 4. Impedance response recorded at E = -0.70 V for a porous Cu70Ni30 stationary
−
2
disk electrode (deposition charge 40 C cm ), and a compact polished electrode of
"
same composition. Bi-logarithmic plots of the dependence of −Z on frequency,
showing very similar slopes (close to -0.88) in the shadowed frequency range.
electrodeposition efficiency with film thickening must also depend
on a relative increase of hydrogen evolution, but the reasons are
−
2
less obvious. Spongy deposits of about 5 mg cm obtained with
−
2
4
1
0 C cm show a thickness of 35-40 ␮m, an apparent density of
∼
.4 and, considering the compact alloy density (= 9), a void volume
fraction of 84-85%.
Selected porous Cu-Ni layers deposited on Nb sheets, with EDS
composition rather regularly spaced (Table 1), were submitted to
XRD analysis. The X-ray diffractograms, reported in a previous
paper [39], show alloy peaks located between the correspond-
ing peaks of the single metals, with a linear dependence of the
calculated cell parameter on composition, in agreement with the
formation of a solid solution respecting Vegard’s law. Consider-
ing the peak width at half height and using the Scherrer’s formula
we estimate the typical dimension of crystallites at about 13 nm.
However, a certain peak broadening might result from distribution
of compositions, leading us to underestimate the real crystallite
dimensions. Hence, we looked for further information performing
EBSD investigations [4] (not shown) directly at the dendrite scale.
The signal is frequently absent or rather blurry, indicating that the
crystallite dimensions are typically below instrumental resolution
(20 nm). A certain number of spots provides the expected EBSD
patterns of a FCC structure, but positions separated by a distance
in the order of 50 nm correspond to different reciprocal crystal ori-
entations. On the basis of Scherrer’s analysis and EBSD results we
can conclude that the material is highly polycrystalline, with crys-
tallite dimensions d typically in the range 10 nm < d < 20 nm and
occasionally with d > 20 nm.
Fig. 3. a) SEM image of the deposit (40 C cm−2) obtained on Nb sheet electrodes
2
(
3 cm ) from bath III, alloy composition Cu66Ni34; b) efficiency of alloy deposi-
tion, based on estimation of weight increase for deposits obtained from baths I-IV
Table 1), as a function of deposition charge.
(
not prone to deposit according to this morphology (see e.g. prepa-
ration of porous Cu-Ru [27]).
3.2. Characterization of the deposited porous alloy layers.
The composition of some deposited alloys, estimated by EDS
over a large area, is summarized in Table 1 for both RDEs and sheet
electrodes.
The charge efficiency of alloy deposition has been estimated on
sheet electrodes as
ꢁ
Qdep
2F
ꢁQtot
ꢁWexp
XCuAWCu + XNiAWNi
Eff% =
× 100 =
×
× 100
(1)
ꢁ
Qtot
where: F is the Faraday; ꢂQtot is the total charge passed; ꢂQdep
is the charge effectively used for deposition, calculated from the
weight increase associated to alloy deposition, ꢂWexp, and the
atomic fractions XCu and XNi of the two metals, estimated by EDS,
each multiplied by its corresponding atomic weight AW. Loss of
material due to powder formation and/or detachment of small
pieces was never appreciable during our short electrolyses, and
became significant only for deposition of much thicker alloy layers.
The obtained efficiency values, reported in Fig. 3b, are in the range
EIS spectra were recorded on stationary disk electrodes in 1 M
NaOH, at E = -0.70 V, a potential at which the electrode is blocking
"
[38]. The log-log plots of −Z vs frequency for porous and “compact
polished” Cu70Ni30 electrodes (Fig. 4) show a CPE behaviour [43],
"
i.e. a linear dependence of −Z on frequency over at least 2 decades
(0.5 Hz to 50 Hz), with a common slope of -0.88. In such cases the
ꢀ
−
1
ꢀ
capacity cannot be evaluated as C = [−2ꢃf Z ] at any frequency f.
Formulas proposed for the calculation of capacity from CPE param-
eters [44,45] might allow a sound determination of C only if the
reasons for the CPE behaviour were clearly identified [46], which
is not the case in the present study. Considering that similar slopes
are observed for both alloy morphologies, the roughness factor fr
−2
3
8-47% for a deposition charge of 20 C cm and decrease with film
thickening, attaining values in the range 32-40% for a deposition
−2
charge of 60 C cm . Moreover, for any given deposition charge the
estimated efficiency decreases on increasing the Ni content of the
deposit. The latter observation is consistent with the higher activ-
ity of Ni in hydrogen evolution, as compared to Cu; the decrease in
ꢀ
ꢀ
ꢀꢀ
ꢀꢀ
∼
"
can be directly estimated as the ratio fr = Z /Z - where Zc and Z
c
p
p
represent the imaginary parts of the impedance of the compact and
porous electrode, respectively, measured at the same frequency. In
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Fig. 6. Voltammograms (20 mV s−1) recorded at a, a’) compact and b, b’) porous
−
1
Cu70Ni30 RDEs. (900 min ). Solid curves represent experiments in a solution of 1 M
NaOH + 0.1 M NaNO3, dashed curves experiments in 1 M NaOH.
for PRDEs is a complex function of operating parameters including
thickness, radius and permeability of the porous layer. However,
the limiting situations are conceptually simple: at low rotation
rates the current is controlled by reactant diffusion to the outer
electrode surface (Levich dependence), but as ω is increased above
a critical value, the reaction involves the porous inner volume (elec-
trode perfusion) and the current exceeds the Levich dependence.
The deviation from Levich law is smaller in our experiments than in
Fig. 5. a) comparison of voltammograms recorded at 10 mV s−1 in a solution of
0
.020 M K3Fe(CN)6 and 0.020 M K4Fe(CN)6 in 1.0 M KOH at a polished Au RDE and a
−
1
−1
porous Cu70Ni30 RDE for different rotation speeds: ␻1 =225 min ; ␻2 = 900 min ;
␻
−
1
3 = 3600 min ; b) Levich plot of plateau current densities; the dashed line con-
necting the points relevant to porous Cu70Ni30 is merely a guide for the eye.
[
47–49] due to the low thickness/radius ratio of the Cu-Ni porous
the frequency range f = 0.5-50 Hz, one finds a rather constant value
electrodes and the narrower explored ω range.
∼
fr = 360. Considering that the polished sample is not ideally smooth,
The Levich dependence implies a diffusion layer thickness ı0
=
we can take 360 as the minimum roughness factor of the porous
electrode. With the same approach, the roughness factor of a com-
1/3 −1/2
ꢄ1/6; ı0 values of 25.4 ␮m, 12.7 ␮m and 6.35 ␮m
1
.61DO ω
−1
−1
−1
may be calculated for ω = 225 min , 900 min , and 3600 min ,
respectively, using ꢄ = 0.01 cm
s
or lower than, the typical dimensions of pores mouth and so an
increased accessibility of the inner pore surface may enhance the
current even when the flow rate of the electrolyte through the
porous electrode is not very significant. Current enhancement due
to onset of turbulence is unlikely to occur [48].
∼
pact (not polished) Cu70Ni30 electrode was evaluated as fr =1.5:
2
−1
−5
cm2
s
[50] and DO =0.45·10
hence, a porous Cu70Ni30 electrode has a surface area that is about
−1
(relevant to 2 M NaOH, [51]). These values are comparable to,
2
40 times that of a compact one.
3.3. Use as cathode for a kinetically fast reaction. Reduction of
ferricyanide in alkali.
To evaluate the effect of layer morphology on transport, we
have performed comparative experiments on a model system,
i.e. ferricyanide reduction in alkali, at a smooth mirror-like Au
RDE providing a good reference flat surface, and at a porous
RDE of composition Cu70Ni30, providing very regular morphology
and macropores with open structure (Fig. 2). The voltammograms
3
.4. Voltammetric investigations of the reduction of nitrate ions
in alkali.
We have tested the Cu-Ni electrodes as cathodes for reduction
of nitrate in 1.0 M NaOH, an alkaline medium that may be consid-
ered representative of some industrial waste solutions [29,30], but
also of the treatment of polluted neutral wastewaters, since the
reduction processes
(
Fig. 5a) are recorded in a solution of 20 mM K Fe(CN)6 and
3
2
0 mM K Fe(CN) in 1.0 M KOH, at different rotation speeds
4 6
− −1 −1
1
(
ω = 225 min , 900 min , 3600 min ). The dependence of the
current measured at a porous Cu-Ni electrode on ω (Fig. 5b) resem-
bles to those reported in the literature for porous RDEs (PRDE)
based on carbon fibre layers [47–49]: in the limit of low rotation
rates the dependence tends to Levich relation [50]
NO + H O + 2e → NO2 + 2OH−
NO + 5H O + 6e → NH + 7OH
NO + 6H O + 8e → NH + 9OH
produce a local alkalinisation of the solution.
Fig. 6 shows the voltammograms recorded at Cu70Ni30 RDEs
−
−
−
(3)
(4)
(5)
3
2
−
−
−
2
2
3
−
−
−
3
2
3
jlim = 0.62nFAD /3ꢄ−1/6ω1/2CO
2
O
(2)
but on increasing ω the current exceeds markedly the values estab-
lished by that relation [48,49]. The quantitative jlim-ω dependence
−
1
(900 min ) in 1.0 M NaOH + 0.1 M NaNO3 (solid curves) and 1.0 M
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Fig. 8. Chronoamperometric curves recorded in 1 M NaOH + 0.1 M NaNO3, applied
potential E = -1.20 V vs Hg/HgO, at compact and porous Cu70Ni30 electrodes: RDEs
−
1
rotated at 900 min
dashed lines).
(solid lines); stationary sheet electrodes, stirred solution
(
being less negative by ca. 100 mV. Since the size of the peak is
controlled by various factors, including nitrate ion mass transport,
adsorption of reaction intermediates and H atoms, and possibly
pore filling by gaseous H , the peak current is not expected to scale
2
with the true surface area. One may note that the peak current
enhancement factor (ca. 2) is larger than that measured with the
diffusion controlled ferricyanide reduction at the same rotation rate
(ca. 1.35, compare Fig. 5).
−1) recorded at compact and porous RDEs
3.5. Constant potential electrolyses and analysis of reaction
products.
Fig. 7. Voltammograms (20 mV
(
s
900 min 1) of indicated compositions, in a solution of 1 M NaOH + 0.1 M NaNO3,
−
after correction for IR drop and subtraction of hydrogen evolution current (see text).
Chronoamperometric experiments have been recorded in 0.1 M
NaOH (dashed curves). The voltammogram of nitrate reduction at
compact electrodes (curve a) shows a large current peak that does
not attain a diffusion plateau, as already observed in a previous
publication for lower nitrate concentrations [38]. The experiment
recorded at the porous electrode (curve b) shows a shift of current
onset towards less negative potentials and a much larger current
maximum, which appears located at more negative potentials. It
must be considered that the two electrodes display a different abil-
ity to perform the hydrogen evolution reaction (curves a’, b’), and
that IR drop distorts the curves to different extents.
NaNO + 1 M NaOH at E = -1.20 V with compact and porous Cu70Ni30
3
RDEs, a composition offering a good compromise between catalytic
efficiency (larger at large Cu contents [38]) and mechanical stabil-
ity (lower for Cu contents exceeding 70 at%). Results reported in
Fig. 8 (solid lines) show that the initial current (t = 0) is about twice
larger at the porous electrode, in basic agreement with voltammet-
ric data. However, a major difference in time evolution is observed:
during the first hour of polarisation, the current decays substan-
tially at the compact electrode, and remains almost constant at
the porous electrode, with a minor decay basically reflecting the
decrease of concentration of the active species in solution. After
1 h of polarisation the current observed at the porous electrode is
ca. 30 times larger than that observed at compact electrodes. The
Hence, in Fig. 7 we compare also the plots of the net current of
nitrate reduction ꢂj, calculated by subtracting the current density
measured, at each (real) potential, in 1 M NaOH from that mea-
2
sured in 1.0 M NaOH + 0.1 M NaNO . We have first corrected the
experiments recorded at the two sheet electrodes (3 cm ), compact
3
curves for the IR drop, then subtracted the curves point-by-point at
and porous (Fig. 8, dashed lines), show a lower current density due
to the less efficient mass transport regime, but analogous j-time
dependencies.
Results of prolonged electrolyses are summarized in Table 2,
reporting the analytical data and some basic elaborations. In par-
(
corresponding) real electrode potentials. These corrected curves,
relevant to all the 4 compositions reported in Table 1 for RDEs, show
that at the porous electrodes the foot of the voltammetric j-E curve
is displaced by almost 200 mV towards less negative potentials,
with respect to the compact electrodes. However, if the current
densities measured at porous and compact electrodes are referred
to the estimated true surface areas (ca. 240 times larger for the
former, see section 3.2), the j-E curves tend to overlap in the poten-
tial range where the nitrate reduction reaction is expected to be
under pure kinetic control. This indicates that the shift in the cur-
rent onset is not due to real electrocatalytic effects but rather to
geometric factors, that may be nonetheless valuable in applications
ticular: a) the selectivity for NH production, defined as the ratio
3
−
−
−
[NH ]t/([NO ] –[NO ]t), where [NH ]t, [NO ]t indicate the con-
centrations at time t and [NO ] the initial concentration (0.1 M) at
t = 0; b) the current efficiency for production of NO2 and NH , esti-
3
3
in
3
3
3
−
3
in
−
3
mated as the ratio % between the charge used to generate the given
species and the total charge, and “other”, calculated as the comple-
ment to 100%. The last term includes production of H (according to
2
literature, the dominant side product) and possible minor amounts
[
52]. The curves in Fig. 7 also show that the peak currents mea-
of N . The total final concentration of nitrogen-containing species is
regularly below the initial NO3 concentration (100 mM), indicating
that a few % of the product is lost undetected. Considering that the
2
−
sured at porous electrodes are twice larger than those observed at a
compact electrode of very similar composition, the peak potentials
Please cite this article in press as: L. Mattarozzi, et al., Hydrogen evolution assisted electrodeposition of porous Cu-Ni alloy electrodes
and their use for nitrate reduction in alkali, Electrochim. Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.04.048

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7
Table 2
−
2
Products of electrolysis of 100 ml of alkaline solutions (1 M NaOH) containing 0.1 M NO3 . Alloy sheet electrodes, surface of 3 cm , applied potential E = -1.2 V.
Electrode material
Time (h)
Charge (C)
[NO3−]t ·10−3
M
[NO2−]t ·10−3
M
[NH3]t·10−3
M
a) Selectivity to NH3 (%)
b) Current efficiency
(
%) for production of
−
NO2
NH3
Other
Cu70Ni30 compact
0
2
4
6
8
0
1
2
3
4
6
8
0
100.0
94.2
92.2
90.2
88.9
100.0
72.8
53.1
38.7
28.4
17.4
13.2
0
0
3.4
4.6
5.7
7.0
0
23.2
41.9
56.6
67.2
78.1
83.2
-
-
-
-
329.5
452.2
562.6
672
2.4
3.2
3.9
4.2
0
2.4
3.7
3.8
3.4
2.5
1.5
58.3
58.9
58.2
62.7
-
85.4
89.4
92.3
93.8
94.6
95.9
14.1
13.7
13.5
12.1
-
2.5
2.1
1.6
1.2
0.8
0.4
79.2
78.7
78.5
79.8
-
97.2
96.9
97.6
97.8
95.8
94.1
6.6
7.5
8.0
8.1
-
0.3
1.0
0.8
0.9
3.5
5.5
Cu70Ni30 porous
0
1846
3340
4475
5305
6296
6826
a) Selectivity: moles of ammonia produced over moles of original oxyanion disappeared (see text)
b) Current Efficiency: charge used to generate the given product as a % of total charge
deficiency of total nitrogen correlates with the presence of ammo-
nia in solution, at least part of it may be attributed to ammonia
losses during electrolysis (from a non-tight cell) and sampling oper-
ations. However, some production of (undetected) nitrogen cannot
be ruled out. It must be noticed that the rate of disappearance of
nitrate (Table 2) is larger by factors of 7-9 at the porous electrode
in comparison with the compact one: this marked improvement
of performances results from the lower deactivation of the porous
electrode that is shown by the chronoamperometric curves in Fig. 8.
Hence, in comparison with the compact alloy the porous mate-
rial of equivalent composition offers better performances in terms
action of gas bubbles evolving during current application, and a
spongy structure appearing as a network of tiny polycristalline
dendrites.
Use of the spongy alloys as cathodes shows a significant cur-
rent increase for reactions under pure mass transport control, in
agreement with literature studies on porous rotating disc elec-
trodes [47–49]. The porous alloys show better activity and stability
of performance in the reduction of nitrate, in comparison with the
compact alloys, especially at moderately negative potentials (E = -
1.20 V vs Hg/HgO). Both improvements must be ascribed mainly
to their porous structure and morphology: (i) the current density,
referred to the geometric area, is enhanced by surface roughness and
electrode perfusion; (ii) the rate of electrode inactivation, ascribed
to poisoning intermediates, is lower due to the much larger real
surface area. The porous electrodes show also better selectivity,
−
−
of: i) faster elimination of NO ; ii) limited production of NO ; iii)
3
2
high selectivity towards formation of a main product (NH ), with
3
possible minor production of gaseous N . The good stability of elec-
2
trode performances (Fig. 8) allows fast and drastic reduction of the
nitrate concentration in a single treatment (Table 2).
as they produce almost exclusively a single species, NH , which
3
To clarify the reasons of the superior current stability at the
porous electrodes we have performed CVs (not reported) in 1 M
NaOH at compact and porous electrodes i) freshly deposited and
ii) submitted to 1 h polarization. The basic voltammetric peaks are
similar at the fresh and aged electrodes, and no remarkable addi-
tional peak is detected, indicating that the current decay observed
at the compact electrode during nitrate reduction isn’t primarily
due to deposition of impurities from the alkaline solution. Some
observed modifications of the pattern (relative peak heights of the
systems relevant to Cu and Ni) provide little information. In the
absence of firm evidence, the “passivation phenomena” observed
during nitrate reduction on compact electrodes may be tenta-
tively attributed to inactivation of catalytic surface sites, due e.g.
to adsorption of elusive, N-containing reaction intermediates [35].
Since the ratio of true surface area of the porous Cu70Ni30 elec-
trode to that of the compact is about 240, and the ratio of the
corresponding reduction currents varies from 2 to 25-30 during 1-
hour electrolyses, the observed difference in deactivation rates may
be explained assuming that the production of “poisoning” species
is proportional to current, and that a much smaller surface frac-
tion would be inactivated at the large area porous alloy than at the
compact material.
may be oxidized to harmless N in further steps of the remediation
treatment.
2
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and their use for nitrate reduction in alkali, Electrochim. Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.04.048

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Please cite this article in press as: L. Mattarozzi, et al., Hydrogen evolution assisted electrodeposition of porous Cu-Ni alloy electrodes
and their use for nitrate reduction in alkali, Electrochim. Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.04.048