Influence of electrode assembly on catalytic activation and deactivation of a Pt film immobilized H+ conducting solid electrolyte in electrocatalytic reduction reactions

Article abstract of DOI:10.1039/c4ra15950k

Symmetric (Cu-Pt|Nafion|Pt-Cu) and asymmetric (Pt|Nafion|Pt-Cu) assemblies were fabricated to study the nitrate reduction processes at the cathode. The electrocatalytic nitrate reduction reactions were performed in these assemblies in order to investigate the prerequisite for the enhanced catalytic activity, electrochemical cell durability as well as preferable product selectivity resulting from the reduction of nitrate at the cathode. It has been observed for the symmetric assembly that Cu particles were oxidized on the anode surface under an applied potential and the resulting copper ions migrated to the cathode surface through the Nafion membrane, which deposited as copper oxide on the cathode surface. The formation of this copper oxide covering layer on the Pt-Cu cathode surface is attributed as the reason for the deactivation of the cathode that governed the reduced nitrate reduction along with increasing nitrite selectivity. These problems were addressed and resolved with the asymmetric design of the electrocatalytic reactor, where enhanced hydrogen evolution activates the surface by eroding the CuO over layer as well as speeding up the slow rate determining hydrogenation reactions.

Full text of DOI:10.1039/c4ra15950k

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Influence of electrode assembly on catalytic
activation and deactivation of a Pt film immobilized
Cite this: RSC Adv., 2015, 5, 9912
H+ conducting solid electrolyte in electrocatalytic
reduction reactions
a
Mohammad A. Hasnat, Muhammad Amirul Islamab and M. A. Rashedac
*
Symmetric (Cu–Pt|Nafion|Pt–Cu) and asymmetric (Pt|Nafion|Pt–Cu) assemblies were fabricated to study
the nitrate reduction processes at the cathode. The electrocatalytic nitrate reduction reactions were
performed in these assemblies in order to investigate the prerequisite for the enhanced catalytic
activity, electrochemical cell durability as well as preferable product selectivity resulting from the
reduction of nitrate at the cathode. It has been observed for the symmetric assembly that Cu particles
were oxidized on the anode surface under an applied potential and the resulting copper ions migrated
to the cathode surface through the Nafion membrane, which deposited as copper oxide on the
cathode surface. The formation of this copper oxide covering layer on the Pt–Cu cathode surface is
attributed as the reason for the deactivation of the cathode that governed the reduced nitrate
reduction along with increasing nitrite selectivity. These problems were addressed and resolved with
the asymmetric design of the electrocatalytic reactor, where enhanced hydrogen evolution activates
the surface by eroding the CuO over layer as well as speeding up the slow rate determining
hydrogenation reactions.
Received 8th December 2014
Accepted 19th December 2014
DOI: 10.1039/c4ra15950k
www.rsc.org/advances
indenitely high resistivity of the medium, will eliminate the
need for a supporting electrolyte and therefore enables the easy
Introduction
separation of the synthesized products.
The tuning of a catalytic material's properties, such as size,
shape and elemental composition leads to modied reactivity
and selectivity through the well known processes of electronic
and geometric effects.1–10 Besides the geometry of the elements
in the catalytic materials, the geometry of the electrode mate-
rials in the electrochemical reactor are pivotal in directing the
reactivity and selectivity.11–14 In this regard, sandwich type
M|conductive membrane|M reactor (M ¼ Pt, Pd, Pt–Pd etc.) cells
have been successfully employed for the selective oxidation of
various organic compounds.9,10,15–19 In these electrocatalytic
cells, the conductive membrane serves as the electrolytic junc-
tion between the anode and the cathode. In a conventional
reactor, ample supporting electrolyte is required to compensate
for high medium resistivity towards the ion mobility through
the membrane. Therefore, a supplementary process is obliga-
tory to separate the synthesized products from the supporting
electrolytes. The use of specially designed M|conductive mem-
brane|M type reactors, which are seamlessly operational at the
The nitrate reduction reaction is very important in terms of
environmentalpollutioncontrol aswell asthe synthesisofseveral
nitrogenous compounds.3,20–29 In the previous works, we reported
the use of sandwich type reactors (M|Naon|M–X; X ¼ Ag,26 Rh,27
Cu (ref. 28 and 29)) to study the nitrate reduction reactions (NRR)
in an asymmetric type reactor, where the promoter Cu metal is
immobilized only on the cathode surface. To our knowledge,
there is no literature precedent of investigating NRR using a
symmetric type assembly of Cu–Pt|Naon|Pt–Cu.
In this context, we investigated the comparative electro-
catalytic activities obtained by a symmetric Cu–Pt|Naon|Pt–Cu
(Fig. 1a) and an asymmetric Pt|Naon|Pt–Cu (Fig. 1b) assem-
blies. The catalytic comparisons have been demonstrated based
on reactivity and selectivity pertaining to NRR as a model study,
which in addition has led us to unveil the reasons for the
catalytic deactivation of symmetric type assemblies. These
interesting ndings behind the deactivation of symmetric type
reactors offers a way to identify the parameters that need to be
addressed while designing a new electrochemical reactor.
Consequently, the design of an asymmetric type reactor is
shown to be effective for the electrocatalytic reduction of
potential environmental pollutant nitrate ions to the valuable
fertilizer precursor ammonia.
aDepartment of Chemistry, Shahajalal University of Science and Technology,
Sylhet-3114, Bangladesh. E-mail: mahtazim@yahoo.com; mah–che@sust.edu
bDepartment of Chemistry, University of Alberta, 11227 Saskatchewan Drive,
Edmonton, Alberta, T6G 2G2, Canada
cSchool of Materials Science, Japan Advanced Institute of Science and Technology, 1-1
Asahidai, Nomi, Ishikawa 923-1292, Japan
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chambers were lled with 8 ml aqueous solution of KNO3 and
water, respectively. To accomplish electrolysis, the reactor was
connected with a DC supplier. Inꢂ order to evaluate the
+
concentration changes of NO3ꢂ, NO2 and NH4 with time, a
portion of 20 ml was collected from the solution in the cathode
compartment at a constant interval and diluted to 5 ml. The
concentration changes of NO3ꢂ, NO2ꢂ and NH4+ were measured
by an ICA-2000 model (DKK-TOA Corporation) ion chromato-
graph. In order to compare the reactivity differences among
different types of assemblies, the rate constant (k) was evaluated
on the supposition that the concentration of NO3ꢂ (C) decreases
following rst order kinetics, i.e., C ¼ C0 exp(ꢂk1t), where C0 is
the initial concentration and t is time (min). The metallic
composition of each MEAs was determined by X-ray uores-
cence analysis. The interior (inside the membrane) distribution
of solid particles was analyzed with a Simudzu model electron
probe micro analyzer. The electrochemical experiments were
carried out using a Metrohm 797VA automatic electrochemical
polarization system. The scanning electron microscope (SEM)
image of the surface was taken with a JEOL model JSM-6060LV
instrument. X-ray absorption near-edge structure (XANES)
spectra were recorded at the BL47XU beam-line at SPring-8.
Fig. 1 Schematic representations of (a) symmetric and (b) asymmetric
reactors.
Experimental
Results and discussion
Relative NRR efficiency
In order to prepare a membrane-electrode assembly (MEA),
platinum was rst chemically deposited from H2PtCl6 on the H+
conducting Naon-117 membrane (Du Pont) surfaces supplied
by Wako Incorp. Japan. A Naon membrane of 2 cm ꢀ 3 cm was
rst sand blasted, cleaned with deionized water, dried at 110 ꢁC
and immersed into 200 ml of a 7.5 mM H2PtCl6 solution. Next, a
mixed solution of 2.0 M NaBH4 and 4.0 M NaOH was added to
When a Pt lm was alone used as a cathode in a membrane
reactor, the reaction rate was very slow (k1 < 2 ꢀ 10ꢂ3 minꢂ1).
Distinct catalytic activities were noticed when Cu particles were
immobilized onto the Pt surface with an enhanced NRR rates.
Nitrate ions are reduced using a consecutive pathway, where
nitrate ions are converted into intermediate nitrite ions, which
are then converted into other products as per the following trail
(1).28
the membrane containing system at a rate of 2.0 ml hꢂ1
.
Meanwhile, the reaction mixture was heated from 35 ꢁC to 60 ꢁC
at a rate of 5 ꢁC hꢂ1. The Pt plating on both sides of the Naon
membrane was completed within 12 hours. A second plating of
Cu metal was deposited from 0.01 M CuSO4 solution to
construct symmetric (Cu–Pt|Naon|Pt–Cu) type assemblies
(Fig. 1a). To prepare the asymmetric type (Pt|Naon|Pt–Cu)
k1
ƒƒƒ!
k2
ƒƒƒ!
ꢂ
ꢂ
NO3
NO
NH ; N etc.
(1)
2
3
2
In order to evaluate the electrocatalytic nitrate reduction
assembly (Fig. 1b), one surface of the Pt membrane assembly kinetics, the change in the concentrations of the reactant
(Pt|Naon|Pt) was sealed with masking tape so that Cu particles nitrate along with that of intermediate nitrite and product
were deposited only on the other surface. Aer Cu deposition, ammonia were monitored and quantied at given time inter-
the masking tape was removed; as a consequence an asym- vals. Fig. 2a shows the concentration prole of the nitrate
metric assembly was fabricated. The as prepared MEAs were reduction reaction (0.05 M KNO3, 0.1 A, 180 min) with the
rst washed with dilute HCl, then were sonicated for 30 s to
course of time while electrolysis was carried out using Pt|Na-
remove unreacted NaBH4 and NaOH from the surfaces, and on|Pt–Cu (16 atom% Cu deposits) asymmetric assembly
nally dried at 110 ꢁC in the air. The resistivity of the surfaces of (Fig. 1b). The concentration of reactant nitrate decreased
the as prepared assemblies was less than 10 U cmꢂ1. The elec- exponentially with time, which is consistent with 1st order
ꢂ
trocatalytic reduction of NO3 ions, in the absence of any sup- kinetics. On the other hand, the concentration of nitrite
porting electrolyte, was carried out using both of the symmetric increased fairly rapidly leading to a plateau and followed by a
and asymmetric type assemblies by installing them in the micro slow decrease. Consequently, the ammonia concentration
reactors as shown in Fig. 1. The cathode surface was separated increased slowly due to the slow conversion of intermediate
from the anode surface by the Naon membrane. This opera-
nitrite into ammonia.
tion created a sandwiched type reactor where the geometric area
Recyclability and reusability are the important aspects of any
of each surface was 6 cm2. The true aloofness between the catalytic processes. In order to assess the possibility of recy-
anode and the cathode was 180 mm, which corresponds to the clability and/or reusability of these two different types of cata-
thickness of the membrane. The cathode and the anode lytic assemblies, four repetitive experiments were conducted
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design of the catalytic assembly provides not only a way to
overcome the limitation of deactivation associated with
symmetric type reactors, but also offers increased reactivity on
long term repeated use.
Having the evidence of longer time sustainability for the
asymmetric assembly, we endeavoured to investigate product
selectivities of the catalytic assemblies. In the case of symmetric
assembly, the nitrite selectivity is seen to increase remarkably
from 45% to 68% in the second experiment, which is followed
by a steady increase in the subsequent experiments (see Fig. 2b).
Conversely, a gradual decrease in nitrite selectivity was noticed
for asymmetric assembly, numerically, from 54% in the 1st
experiment to 34% in the 4th experiment (see Fig. 2b). However,
a decreasing trend for ammonia selectivity was observed for the
repeated experiments irrespective of geometry of the assembly
used. The most remarkable feature for the asymmetric assembly
is its consistently higher selectivity for ammonia than for the
symmetric analogue, which implies that the former assembly
converts the intermediate nitrite to ammonia more rapidly and
efficiently. The instantaneous increase in the reaction rate and
decrease of nitrite selectivity suggest that Cu–Pt surface at the
asymmetric assembly was activated during the electrolysis
experiments to enhance step-2 of the reactions given in eqn (1).
The increase in the rate of conversion of nitrite to ammonia was
balanced by the faster conversion of nitrate to nitrite and so on.
This enhanced reactivity of the asymmetric assembly will offer
at least two benecial impacts. First, it will help to minimize the
environmental pollution problem by effectively reducing the
pollutants nitrate and nitrite. Second, it is expected to facilitate
the food production by providing alternative way to synthesize
the valuable fertilizer precursor ammonia.
Inuence of copper oxides on NRR activities
Aer the recognition of the superior reactivity and preferable
product selectivity for asymmetric design over the symmetric
ones, we were motivated to inspect the reasons behind the
activation or deactivation of the respective reactor types. Before
switching into the deeper aspects of the catalytic activation–
deactivation processes, it is required to discuss briey about the
catalytic nitrate reduction processes taking place on the cathode
of the membrane reactor. It is established that water molecules
are oxidized on the anode surface producing molecular oxygen
and protons, which under the applied potential migrate
towards the cathode surface through the conducting Naon
membrane. These protons are next reduced on the cathode
surface to form adsorbed hydrogen.26
Fig. 2 Nitrate reduction reaction profiles at Cu–Pt catalytic assem-
blies. (a) Concentration profile of nitrate reduction reaction in an
asymmetric reactor. (C NO3ꢂ concentration, - NO2ꢂ concentration,
+
: NH4 concentration) Cathode: 16 atom% Cu + Pt, 0.1 A, 180 min,
295 K, reactor capacity 8 ml, initial nitrate concentration 0.05 M. (b and
c) Dependence of the rate constant and product selectivity on the
repetition of experiments in Cu–Pt|Nafion|Pt–Cu symmetric and
Pt|Nafion|Pt–Cu asymmetric assemblies respectively.
Anode (Pt/Cu–Pt):
H2Oads / 2H+ + 1/2O2 + 2eꢂ
(2)
(3)
under same experimental condition. While the rate constant for
the symmetric assembly (Fig. 1a) was decreased from 24.1 ꢀ
10ꢂ3 minꢂ1 to 3 ꢀ 10ꢂ3 minꢂ1 (see Fig. 2b), an increase of rate
was observed from 22.4 ꢀ 10ꢂ3 minꢂ1 to 30.7 ꢀ 10ꢂ3 minꢂ1 (see
Fig. 2c) in the fourth experiment for an asymmetric assembly
(Fig. 1b). This observation suggests that the asymmetric type
H+(anode) / H+(cathode) (migration)
Cathode (Cu–Pt):
2H+ + 2eꢂ / 2Hads /H2,ads
(4)
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30
´
Considering the study reported by N. Barrabes, et al. and
our observations, the catalytic nitrate reduction reactions at the
cathode surface may be summarized as shown in Scheme 1. The
noble metal atoms (Pt) adsorb hydrogen atoms to form Pt–H
species on the cathode surface. At the same time, metallic Cu
atoms are oxidized to Cu2+ ions by reducing nitrate ions. The
Cu2+ species is then reverted to its metallic state (Cuo) by
receiving electrons from the Pt–H species. In this way, catalytic
cycles are completed and new cycles are started. Due to enor-
mous stability, Cu2O and CuO are not expected to participate in
the catalytic cycle shown in Scheme 1. As a result, the presence
of Cu2O and or CuO may deactivate the catalytic performance by
blocking active NRR sites.
Besides the catalytic hydrogenation process, heterogeneous
electron transfer (ET) reactions are also possible, which can
reduce nitrate ions. Therefore, in order to investigate the
inuence of copper oxide on ET reactions, cyclic voltammetric
experiments were performed. In this respect, Cu particles were
electrochemically deposited on a Pt disk (2 mm) electrode from
a 0.01 M solution of CuSO4 under de-aerated and aerated
conditions separately by cycling the electrode four times
between 0 and ꢂ1 V vs. Ag/AgCl reference electrode at a scan
rate of 100 mV sꢂ1. This yielded formation of Cu over layers on
the Pt disk. Thus, electrode (AE) prepared under aerated
condition contained higher amount of copper oxides than that
of the electrode (DE) prepared under de-aerated conditions. The
DE electrode was kept under an inert atmosphere so that the
formation of CuO due to air oxidation could be minimized
before application; whereas for the AE electrode, no precaution
Fig. 3 Cyclic voltammograms of 5 mM KNO3 in 0.5 M KCl using Cu–Pt
electrode at a scan rate of 50 mV sꢂ1 under nitrogen atmosphere. Solid
line: Cu–Pt (DE) electrode prepared under de-aerated conditions, and
dotted line: Cu–Pt (AE) electrode prepared under aerated conditions.
potentials i.e., ꢂ0.83 V (ꢂ4.5 mA) and ꢂ1.21 V (ꢂ15.5 mA),
respectively. The less NRR currents generated by the AE elec-
trode at relatively positive potentials provide concrete evidence
that the presence of copper oxides in the matrix of Cu–Pt elec-
trode inhibited both of the steps (eqn (1)) on the way to
complete NRR. In other words, it is reasonable to claim that the
lesser is the CuO content, the higher is the NRR efficiency.
Deactivation of the symmetric assembly
was taken to avoid CuO formation. Both of the electrodes were It is evident from the CV experiments as well as from the liter-
next used to record cyclic voltammograms (CVs) of 5 mM KNO3 ature reports that existence of copper oxides must inhibit both
(in 0.5 M KCl, N2 environment) at a scan rate of 50 mV sꢂ1 as of the catalytic hydrogenation reactions and ET reactions per-
shown in Fig. 3. In both cases, the CVs show two well-dened taining to NRR. From this point of view, we evaluated the rela-
waves (E1 and E2) and a broad cathodic peak (E3). In a tive abundances of Cu2O and/or CuO in catalytic matrixes
previous report,28 E1 has been resolved as the reduction of before and aer use of the catalytic surfaces in the case of the
surface copper oxides, E2 as the reduction of NO3ꢂ to NO2ꢂ, and symmetric assembly.
E3 as the formation of ammonia. The E1 wave of the DE elec-
Fig. 4 exhibits the EPMA maps of symmetric MEA before and
trode appeared at ꢂ0.52 V with a peak current of ꢂ1.52 mA in aer use for NRR. These maps do not provide quantitative
contrast to AE electrode where this wave appeared at a more compositional data, but they are useful for determining the
negative potential (ꢂ0.64 V) with an increased peak current of relative Cu/Pt distribution and the change in the distribution
ꢂ3.35 mA. This shi in reduction potential is expected as the AE that occurred during the course of electrocatalysis. The corro-
electrode contained more copper oxides that required more sion resistant Pt particles were deposited only on the surfaces of
driving potential for the reduction. Conversely, the E2 and E3 the membrane, and it was found that the Pt particles did not
diffusive waves by the AE electrode appeared at ꢂ0.80 V (ꢂ2.64 penetrate the cross sectional area even aer electrolysis. More-
mA) and ꢂ1.13 V (ꢂ8.21 mA), respectively. In the case of the DE over, copper species were distributed across the membrane
electrode, these waves were generated at more negative during electrolysis, because Cu2+ ions can easily be exchanged,
and they migrated deeply inside the membrane under an applied
potential. Aer nitrate reduction reaction (0.05 M NO3ꢂ, 0.1 A, 3
h), the concentration of Cu species was decreased near the
anode–membrane interface, and they were populated adjacent to
the membrane–cathode interface (see Fig. 4c and d).
Having the evidence of migration of Cu(II) species, we
attempted to evaluate the relative abundance of copper oxides
in the Cu–Pt matrix of the cathode assembly before and aer
use. Because every single element has its distinct core level
energy, X-ray Absorption Near Edge Structure (XANES) spec-
troscopy permits the extraction of the signal from a surface
Scheme 1 Catalytic nitrate reduction reaction mechanism at Cu
modified Pt surface.
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Fig. 4 Electron probe micro analysis of membrane cross section
samples from symmetric electrode-membrane assembly. (a) Sampling
arrangement (b) Cu dispersion in the fresh membrane (c) Cu dispersion
adjacent to the membrane–cathode interface after use, and (d) Cu
dispersion near the anode–membrane interface after use. The dark
areas indicate epoxy of the sample carrier.
monolayer or even a single buried layer, such as dopants below
an electrode surface in the presence of huge background
signals. The edge position, dened as the maximum point of
the rst derivative function in the rapidly rising edge step of the
absorbance vs. energy plot, have been found to be 8978.6, and
8993.0 eV for Cu2O, and CuO, respectively as shown in Fig. 5a.
This shows the increase in edge energy as the oxidation number
increases, which is a well known Phenomenon of XANES that
allowed its uses to detect oxidation state and chemical envi-
ronment of an element. In order to quantify the relative exis-
tence of Cu2O and CuO, XANES spectra of fresh and used Cu–Pt
cathodes were taken as shown in Fig. 5b. It has been noticed
that the existence of Cu2O was almost removed from the
cathode matrix aer 3 h use and the absorbance vs. energy peak
area between 8989.0 eV and 9000.0 eV was enlarged by 11.5%.
These observations clearly indicate the increase of CuO content
in the cathode matrix aer electrolysis.
Fig. 5 XANES spectra of (a) Cu foil and CuO, and (b) Cu species
sampled from the cathode of symmetric assembly before and after
3 h use.
possible by minimizing the concentration of the copper
oxides. On the cathode surface of the asymmetric assembly
(Pt|Naon|Pt–Cu), the only possible source of oxides is the
air oxidation of metallic copper atoms because copper
metal is slowly oxidized to CuO in presence of air according to
eqn (5).30
2Cu (s) + O2 (g) / 2CuO (s)
(5)
Consequently, oxidation of metallic Cu at the anode surface
followed by the subsequent appearance of Cu(II) species at the
cathode surface entails us to speculate the migration of copper
(as Cu2+) from the anode to the cathode surface where they
deposited as oxides. Parts of the Cu2+ ions are also assumed to
be trapped within the membrane pores. This might have caused
a signicant increase of anode to cathode impedance during
electrolysis causing an inhibition towards the NRR catalytic
process as shown in Fig. 2b.
In order to estimate the relative abundance of the oxides in
the Cu–Pt matrix of the asymmetric assembly before and
aer long term electrolysis, voltammetric experiments were
performed in the presence of 0.5 M KCl, and at a scan rate of
50 mV sꢂ1 as shown in Fig. 6. In this case, a circular (2 mm in
diameter) area of a fresh and a used (aer electrolysis) Cu–Pt/
Naon surface specimens from Pt|Naon|Pt–Cu assembly were
used as working electrodes. It was found that the area under the
Cu(II) reduction peak was decreased by ca. 40% in the case of the
used electrode assembly. These observations therefore suggests
that the reduction and/or removal of CuO content from the
Activation of the asymmetric assembly
Because the copper oxides inhibit the catalytic process, the cathode surface of Pt|Naon|Pt–Cu assembly during the period
catalytic activation (as shown in Fig. 2c) is speculated to be of electrolysis increased NRR efficiency as shown in Fig. 2c.
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Although vigorous H2 is evolved on the cathode surface of
the reactor (see Fig. 1) under the present experimental condi-
tions (25 ꢁC), the existence of copper oxides in the cathode
matrix should remain unaltered because the reduction of
copper oxide to metallic copper by evolved H2 at this ambient
temperature is not possible. Thus, nally we assumed that the
erosion of CuO due to continuous ow of H2 gas evolved at the
Cu–Pt cathode surface was the way of decreasing CuO from
the Cu–Pt surface. Therefore, we measured copper species in
the catholyte by means of Inductively Coupled Plasma (ICP)
technique. Aer each of the four experiments reported in
Fig. 2c, we determined 62, 40, 21 and 18 ppb of copper species
in the catholyte. This successive removal trend of CuO is
consistent with the upward trend of the rate constant in the
repetitive experiments as shown in Fig. 2c.
The removal of CuO from the cathode surface increased the
metallic luster and decreased the sizes of the deposited Cu
particles, which is apparent from the SEM images (Fig. 8) of
fresh and used surfaces. The decrease of particle size increased
the specic surface area of the used cathode, which played vital
roles of surface activation of Pt|Naon|Pt–Cu assembly con-
cerning nitrate reduction reactions. In addition, as the cathode
surface was negatively charged and H2 evolution reaction
creates a reduction environment, further slow air oxidation of
Cu was returded. This phenomenon ensured the stability of the
cathode surface for long term use.
Fig. 6 Cu(II) reduction waves of fresh (solid line), and used (after 9 h,
dotted line) Cu–Pt/Nafion in 0.5 M KCl at a scan rate of 50 mV sꢂ1. The
circular working electrode (f ¼ 2 mm) was collected from the cathode
of the Pt|Nafion|Pt–Cu assembly.
Now, the question arises why CuO content differed on the
cathode surface of Pt|Naon|Pt–Cu assembly before and aer
use. Because the E1 peak in Fig. 3 denotes the reduction of
CuO to Cu, the content of the oxides might be decreased due to
its possible Cu(II) reduction during electrolysis. Considering
this assumption, repetitive CVs (20 cycles) of 5 mM KNO3 in
0.5 M KCl using Cu–Pt (AE) electrode was recorded consecu-
tively as shown in Fig. 7. From this gure, it is seen that the
shapes and sizes of all of the three E1, E2 and E3 peaks Inuence of H+ migration on activation–deactivation
remained unaltered irrespective of cycle numbers. This processes
observation suggests that the electron transfer reactions could
not alter the Cu to CuO balance. Consequently, it can be
Finally, knowing the benecial impacts of hydrogen evolution
on the catalytic performances, we compared the relative
ascribed that the Cu–Pt surface in the Pt|Naon|Pt–Cu
hydrogen evolution efficiency of the symmetric and asymmetric
assembly was not activated due to direct electrochemical
reactors with a view to constructing the rationale for deactiva-
activities. The most acceptable process of reduction of copper
tion of symmetric and activation of asymmetric assemblies.
oxides into metallic copper is heating the oxides under H2
Here, the results have been presented in terms of volume of the
atmosphere at 280 C.31
ꢁ
H2 gas collected from the cathode chamber at certain time
intervals as shown in Fig. 9. With the passage of time, the
volume of the evolved hydrogen gas was increased linearly while
0.1 A current was passed through the asymmetric reactor due to
its considerable stability. On the other hand, the volume vs.
time curve with respect to the symmetric reactor gradually
deviated negatively from linearity implying that the gas evolu-
tion rate decreased with the passage of time. This observation
CuO + H2 / Cu + H2O
(6)
Fig. 7 Repetitive cyclic voltammograms (20 cycles) of 5 mM KNO3 in
0.5 M KCl using a Cu–Pt (AE) electrode at a scan rate of 50 mV sꢂ1 Fig. 8 SEM Images of Cu–Pt surface. (a) As prepared, and (b) after
under nitrogen atmosphere.
extensive electrolysis (Pt|Nafion|Pt–Cu).
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2 M. Cao, D. Wu and R. Cao, ChemCatChem, 2014, 6, 26–45.
3 W. Siriwatcharapiboon, Y. Kwon, J. Yang, R. L. Chantry, Z. Li,
S. L. Horswell and M. T. M. Koper, ChemElectroChem, 2014, 1,
172–179.
4 C. H. Cui and S. H. Yu, Acc. Chem. Res., 2013, 46, 142–
1437.
5 J. Lai, L. Zhang, W. Qi, J. Zhao, M. Xu, W. Gao and G. Xu,
ChemCatChem, 2014, 6, 2253–2257.
6 Y. Nakayama and M. Oyama, Chem. Commun., 2013, 49,
5228–5230.
7 M. Shand and J. A. Anderson, Catal. Sci. Technol., 2013, 3,
879–899.
´
´
8 D. Reyter, D. Belanger and L. Roue, Electrochim. Acta, 2008,
53, 5977–5984.
Fig. 9 Cathodic hydrogen evolution due to water hydrolysis. (C) Cu–
Pt|Nafion|Pt–Cu, and (B) Pt|Nafion|Pt–Cu electrode assemblies.
Current passed: 0.1 A.
9 Z. Zhang, L. Xin and W. Li, Appl. Catal., B, 2012, 119–120, 40–
48.
10 S. K. Green, G. A. Tompsett, H. Ju Kim, W. Bae Kim and
G. W. Huber, ChemSusChem, 2012, 5, 2410–2420.
clearly supports that H+ migration through the membrane (eqn
(3)) was hindered due to presence of Cu(II) species (trapped 11 S. P. Jiang, J. Appl. Electrochem., 2004, 34, 1045–1055.
inside the membrane) and reduction of H+ ions to molecular 12 L. Y. Woo, R. S. Glass, R. F. Novak and J. H. Visserb, J.
hydrogen (eqn (4)) were inhibited as a consequence. Overall, the
Electrochem. Soc., 2010, 157, J81–J87.
electrochemical redox reactions of an ideal electrolysis cell was 13 C. Setevich, F. Prado and D. Z. de Florio, Int. J. Hydrogen
perturbed, hence, the performance of the symmetric reactor
was lost as reported in Fig. 2b.
Energy, 2014, 39, 8738–8743.
14 E. A. B. Ramirez and S. P. McGarry, Org. Electron., 2014, 15,
1131–1137.
15 Z. Ogumi, S. Ohashi and Z. Takehara, Electrochim. Acta, 1985,
30, 121–124.
Conclusions
Presence of oxidizable metallic species on the anode surface 16 Z. Ogumi, H. Yamashita, K. Nishio, Z. I. Takehara and
deactivates the catalytic performance of the cathode surface of S. Yoshizawa, Electrochim. Acta, 1983, 28, 1687–1693.
the Cu–Pt|Naon|Pt–Cu type symmetric assemblies. The 17 K. Otuska and I. Yamanaka, Appl. Catal., B, 1986, 26, 401–
asymmetric design of the assembly as in Pt|Naon|Pt–Cu not 404.
only alleviates the problem of deactivation, but also increases 18 G. R. Dieckmann and S. H. Langer, J. Appl. Electrochem.,
the nitrate reduction rate and provides preferable product
1997, 27, 1–8.
(ammonia) selectivity. More precisely, vigorous hydrogen 19 G. R. Dieckmann and S. H. Langer, Electrochim. Acta, 1998,
evolution reaction in the rst place improves NRR reactivity of a 44, 437–444.
cathode surface of Pt|Naon|Pt–Cu assembly by removing CuO 20 Y. Y. Birdja, J. Yang and M. T. M. Koper, Electrochim. Acta,
particles; second, it drives faster reduction by facilitating the 2014, 140, 518–524.
rates of hydrogenation of the nitrate and nitrite species; third, it 21 A. Anastasopoulos, L. Hannah and B. E. Hayden, J. Catal.,
protects the further formation of CuO by creating reducing 2013, 305, 27–35.
environment; and nally, when the content of CuO is mini- 22 J. Yang, P. Sebastian, M. Duca, T. Hoogenboom and
mized on the cathode surface, the cathode assembly exhibited
M. T. M. Koper, Chem. Commun., 2014, 50, 2148–2151.
enhanced performance with respect to nitrate reduction 23 A. Miyazaki, T. Asakawa, Y. Nakano and I. Balint, Chem.
reactions.
Commun., 2005, 3730–3732.
24 S. Kuwabata, S. Uezumi, K. Tanaka and T. Tanaka, Inorg.
Chem., 1986, 25, 3018–3022.
25 Y.-M. Fang, J.-J. Sun, A.-H. Wu, X.-L. Su and G.-N. Chen,
Langmuir, 2009, 25, 555–560.
Acknowledgements
The authors acknowledge Prof. Masato Machida, Department of
Nano Science and Technology, Kumamoto University, Japan for 26 M. A. Hasnat, M. Saiful Alam, M. H. Mahbub-ul Karim,
providing required materials and experimental setups. Shahja-
lal University of Science and Technology research center and
M. A. Rashed and M. Machida, Appl. Catal., B, 2011, 107,
294–301.
UGC of Bangladesh is acknowledged for nancial supports 27 M. A. Hasnat, M. A. Islam, S. M. Borhanuddin,
(2014–15).
M. R. U. Chowdhury and M. Machida, J. Mol. Catal. A:
Chem., 2010, 317, 61–67.
28 M. A. Hasnat, S. Ben Aoun, S. M. Nizam Uddin, M. Mahbubul
Alam, P. P. Koay, S. Amertharaj, M. A. Rashed,
M. M. Rahman and N. Mohamed, Appl. Catal., A, 2014,
478, 259–266.
Notes and references
1 F. F. Tao, S. Zhang, L. Nguyen and X. Zhang, Chem. Soc. Rev.,
2012, 41, 7980–7993.
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´
29 M. A. Hasnat, M. A. Rashed, S. Ben Aoun, S. M. Nizam Uddin, 30 N. Barrabes, J. Just, A. Danov, F. Medina, J. L. G. Fierro,
M. Saiful Alam, S. Amertharaj, R. K. Majumder and
N. Mohamed, J. Mol. Catal. A: Chem., 2014, 383–384, 243–
248.
J. E. Sueiras, P. Salagre and Y. Cesteros, Appl. Catal., B,
2006, 62, 77–85.
31 J. Y. Kim, J. A. Rodriguez, J. C. Hanson, A. I. Frenkel and
P. L. Lee, J. Am. Chem. Soc., 2003, 125, 10684–10692.
This journal is © The Royal Society of Chemistry 2015
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