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Article
Highly efficient dual active palladium nanonetwork
electrocatalyst for ethanol oxidation and hydrogen evolution
Halima Begum, Mohammad Shamsuddin Ahmed, and Seungwon Jeon
ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09855 • Publication Date (Web): 25 Oct 2017
Downloaded from http://pubs.acs.org on October 26, 2017
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1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
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ACS Applied Materials & Interfaces
Highly efficient dual active palladium nanonetwork electrocatalyst for ethanol
oxidation and hydrogen evolution
Halima Begum, Mohammad Shamsuddin Ahmed, Seungwon Jeon*
Department of chemistry and institute of basic science, Chonnam National University, Gwangju
500-757, Republic of Korea. Tel.: +82 62 530 0064; fax: +82 62 530 3389.
*Corresponding author: swjeon3380@naver.com
Abstract
Tunable palladium nanonetwork (PdNN) has been developed for catalyzing ethanol oxidation
reaction (EOR) and hydrogen evolution reaction (HER) in alkaline electrolyte. 3D PdNN is
regarded as a dual active electrocatalyst for both EOR and HER for energy conversion
application. The PdNN has been synthesized by the simple chemical route with the assistance of
zinc precursor and a surfactant (i.e. cetyltrimethylammonium bromide, CTAB). The thickness of
the network can be tuned by simply adjusting the concentration of CTAB. Both EOR and HER
have been performed in an alkaline electrolyte, and characterized by different voltammetric
methods. The 3D PdNN has shown 2.2-fold higher electrochemical surface area than the
commercially available Pt/C including other tested catalysts with minimal Pd loading. As a result,
it provides a higher density of EOR and HER active sites and facilitated the electron transport.
For example, it shows 2.6-fold higher mass activity with significantly lower CO2 production for
EOR and the similar overpotential (110 mV @ 10 mA cm−2) for HER compared to Pt/C with
better reaction kinetics for both reactions. Thus, the PdNN is proved as an efficient
electrocatalyst with better electrocatalytic activity and stability than state-of-the-art Pt/C for both
EOR and HER because of the crystalline, monodispersed and support-free porous nanonetwork.
Keywords: Dual active electrocatalyst, Ethanol oxidation, Hydrogen evolution, nanonetworks,
Palladium.
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1. Introduction
Searching for the sustainable and renewable clean energy sources has been expended due to the
fast expanding demand and environmental impact of traditional energy resources.1,2 In this
regards, renewable electrochemical energy conversion devices are regarded as clean energy
sources.3,4 Ethanol oxidation reaction (EOR) and hydrogen evolution reaction (HER) are two
important electrochemical processes that take place in the energy conversion devices such as fuel
cells (FCs) and water splitting cells.5,6
The electrocatalytic EOR is greatly highlighted due to ethanol has less toxicity, sustainability,
and higher energy density among different alcohol fuels, and thus, it is regarded as the superior
fuel for direct alcohol fuel cells (DEFCs).7,8 Particularly, DEFCs has superior merits such as
nearly zero pollution, renewable, easy transportable, and relatively low operating temperature.9,10
At the same time, the electrolysis of water is a cost-effective and eco-friendly technique for
hydrogen gas (H2) production for hydrogen fuel cells.11 For increasing H2 production, nowadays,
intensive research have been focused on finding higher-active as well as low cost catalysts for
electrochemical decomposition of water.12,13
However, the overall efficiency of these energy conversion devices are rigorously hindered by
the sluggish kinetics of those electrocatalytic reactions.14–16 For boosting the sluggish kinetics,
Pt-based materials are widely used for electrocatalysis of those reactions.17,18 Unfortunately, the
usages of Pt-based catalysts are susceptible due to the scarcity, high cost, inferior durability and
typical poisoning.19,20 However, to solve the problem of Pt dependence fully, relatively more
abundant and cheaper Pd-based catalysts are more likely to be put into practical use for DMFCs
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in alkaline media, owing to its good tolerance to the poisoning effects from carbonaceous
intermediates21–23 and stability.24,25
However, the efficiency and activity of Pd-based catalysts are still the main challenges for
DEFCs applications.26 Thus, many efforts such as, nanoparticles (NPs), nanonetwork (NN),
nanoflowers, nanotubes, or nanodendrites have been explored to develop more electroactive and
efficient Pd-based nanocatalysts for FCs application.8,27–32 Among them, metallic (i.e. Pd)-NN
can not only maximize the Pd utilization but also enhance the catalytic activity due to its
structural effects such as, self-support, large surface area, low density, and special electrical
properties, resulting in an increase in electrochemically active surface area (ECSA) and
associated with the higher performance.33,34 It is worth mentioning that the electrocatalytic
performances of nanocatalysts are strongly dependent on their size and shape.35
Recently, bifunctional (dual active)36 or even trifunctional37 catalysts system have been reported,
although it is still difficult to develop a bifunctional catalysts system for several electrochemical
reactions due to the variation of mechanism among reactions. However, there is no single Pdcatalyst that can perform as a bifunctional catalyst for both EOR and HER. Therefore, new
strategies to develop a cost-effective and high-performance EOR and HER bifunctional
electrocatalysts as the replacement of Pt catalysts are still anticipated.
Based on the above discussions, we have developed a simple strategy to synthesize selfsupported PdNN with interconnected porous nanoarchitecture. For the first time, the PdNN has
been employed successfully as dual active Pd-based electrocatalyst for EOR and HER. The asprepared PdNN showed better electrocatalytic activities for both EOR and HER with superior
stability than the commercially available Pt/C. The porous PdNN ensured a high density of
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active sites with outstanding mass and charge transport for EOR and HER. Therefore, the PdNN
is demonstrated to be a promising low-cost and efficient alternative for Pt catalysts because of
the ~40% less amount of Pd had been used than Pt during catalysis in this study (Table 1).
2. Experimental
2.1 Synthesis of PdNN
The synthesis of all catalysts has been simplified by the schematic diagram in Figure 1. As can
be explained, the 1 mL of 10 mM K2PdCl4 and 1 mL of 10 mM ZnCl2 were mixed in a 20 mL
vial, followed by the addition of 1 mL of 80 mM cetyltrimethylammonium bromide (CTAB) into
the vial under magnetic stirring continued to ensure adequate mixing. Under continuous stirring,
1 mL of 100 mM NaBH4 was added to the solution drop wise. Afterwards, the above solution
kept in an oven for steady-state heating at 60 °C for 12h. After completion of the reaction, the
solution was centrifuged thoroughly with excess ethanol for two times. Subsequently, for
removing Zn by etching treatment, centrifuged with 100 mM HCl and finally followed by the
centrifuging with ethanol for another two times. The as-obtained black solid, PdNN, was dried at
40 °C for 12 h. For comparison, PdNPs and PdZnNN catalysts were prepared with same protocol
but no addition of CTAB solution and no wash by 100 mM HCl, respectively. Also, carbonbased 20% Pt-Ru (atomic ratio of 1:1) catalyst was prepared with the same protocol described
earlier.18 The Physical and electrochemical characterizations were described in the supporting
information.
3. Result and discussions
3.1 Catalysts characterization
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The surface morphology of as synthesized PdNN, PdNPs and PdZnNN materials was first
characterized by the TEM technique (Figure 2). The TEM image confirms the formation of 3D
porous nanonetworks-like structure of PdNN sample (Figure 2a). According to the corresponding
SAED pattern in Figure 2a inset, PdNN sample displays a good crystalline nature. The magnified
image of PdNN nanocatalyst (Figure 2b) clearly presenting that the PdNN appeared
monodispersed and interconnected with 1D net-like structure which finally appear as 3D network.
The average diameter of 1D net is 4.9 nm. From high resolution TEM (HRTEM) image (Figure
2b inset), it was found that the lattice planes with d-spacing of 0.222 and 0.195 nm, which
attributed to the face centered cubic (fcc) structure of Pd (111) and Pd (200) planes,
respectively.38,39 The lattice fringes were coherently extended over the entire nanonetwork,
signifying highly crystalline PdNN. TEM image of PdNPs shows an aggregated NPs
morphology through the whole sample (Figure 2c), where CTAB was not been used. Although
the size distribution was not uniformed in this sample, an average diameter size was calculated as
~8 nm. The HRTEM image (Figure 2c inset) of single PdNP shows the calculated lattice dspacing are to be 0.222 and 0.195 nm, in accordance with their own interplanar distance of (111)
and (200) lattice planes of Pd which is same with PdNN. However, the TEM image of PdZnNN
also shows the formation of porous networks-like structure of PdZnNN sample (Figure 2d). The
HRTEM image (Figure 2d inset) shows the lattice d-spacing which are calculated as 0.222 and
0.28 nm, which are corresponding to their own interplanar distance of (111) lattice plane of Pd
and (100) lattice plane of ZnO, respectively.40 Therefore, it has confirmed that the CTAB has a
great influence to prepare 3D nanonetwork-like structure. The numerical analysis of Pd and Zn
elements in all prepared catalysts was also observed by EDX patterns (Figure S1). EDX patterns
show Pd and Zn peaks along with the Cu and O peaks. Cu peak is due to the TEM grid which is
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made by cupper and O is representing the state of Zn as ZnO. The amount of Pd and Zn (as wt%)
is listed on the corresponding EDX patterns and Pd amount is 1.2, 1.6 and 1.8 wt% for PdNN,
PdNPs and PdZnNN, respectively.
However, the tunable thickness of 1D net of PdNN was also observed by TEM analysis. The
Figure 3 is displaying TEM images of three different PdNN samples which were synthesized
with 40 mM (a), 80 mM (b) and 120 mM (c) CTAB concentrations during the synthesis
procedure. The uniformly distributed PdNN were formed upon CTAB usage. Upon low
concentration of CTAB applied, the thickness was much higher and seems the coexistence of
PdNPs and PdNN; while PdNPs was formed upon no addition of CTAB as observed in Figure 2c
earlier, indicating low PdNN formation with thick in diameter size (8.8±0.1 nm). At 80 mM
concentration of CTAB, the PdNN was formed uniformly with aggregation-free distribution and
the average diameter size was 4.9±0.1 nm. At 120 mM concentration of CTAB, the PdNN was
also formed uniformly with lower diameter size (4.5±0.1 nm). The corresponding diameter size
histograms for all samples can be seen in Figure S2.
The N2 absorption/desorption isotherm is presented for better understanding the porous structure
of PdNN, PdNPs and PdZnNN catalysts in Figure 4a. The presented isotherms are of type IV
(according to the Brunauer–Deming–Deming–Teller classification), signifying the presence of
nano- and/or meso-pores in those samples. 41,42 An important parameter, specific surface area
(SSA), can be determined from these isotherms profile.42 The SSA of PdNN, PdZnNN and
PdNPs samples was determined as 454.1, 226.9, and 103.2 m2 g−1, respectively, by BrunauerEmmett-Teller (BET) method. The SSA of PdNN was 4.4 magnitudes higher than that of PdNPs.
Pore size distribution curves of all samples are presented in Figure 4a inset. All curves are
showing a prominent pore size around 4 nm, which could be due to the nanopores in between
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nanoparticles. Particularly, PdNN showed another two large peaks at 16 and 50 nm, which could
be ascribed to the empty spaces occurred from etching treatment. Therefore, confirming the
hierarchical porous structure of PdNN, PdZnNN than PdNPs. This finding corresponded with the
morphological analysis.
The XRD patterns of all catalysts are shown in Figure 4b which characterizes their crystalline
nature. The XRD patterns of PdNN and PdNPs reveal their fcc structures with five common
peaks. The typical peaks around the 2θ values of 40.1°, 46.1°, 68.1°, 81.7° and 85.9° are
allocated to the crystalline Pd(111), Pd(200), Pd(220), Pd(311) and Pd(222) facets,
respectively.43,44 All peaks in the XRD pattern are assigned to fcc crystalline Pd (JCPDS 05–
0681). On the contrary, for the PdZnNN catalyst, there are many additional peaks for ZnO which
indicating Zn existence in PdZnNN catalyst along with above mentioned Pd indication peaks.45
The diffraction peaks at 2θ of 31.3°, 33.9°, 37.0°, 44.3°, 53.4°, 57.6°, 62.6°, and 28.8° were
ascribed to the diffractions from the following ZnO planes: (100), (002), (101), (102), (110),
(103), (200), and (201), respectively.4 However, no significant peak shift [i.e. Pd(220)] was
observed from any catalyst compared to the fcc Pd (JCPDS 05–0681) (Figure S3), which
suggests that Pd and Zn are in composite form rather than alloy form47,48 with this preparation
method. Based on XRD result, however, the alloy formation with novel metals may be hampered
due to poor metallic nature of Zn. Other group also observed that there was no any alloy
formation between Au and ZnO in their nanoarchitectures preparation.48 It should be mentioned
that co-metals were found to be well alloyed with Pd during the synthesis of networks catalysts
(PdAg, PdPbPt) via previously reported method,27,49 probably due to the good metallic nature of
those co-metals (Ag and Pt).
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The details of the structural features with oxidation state of metals in PdNN, PdNPs, and
PdZnNN samples were elucidated through the XPS analysis. Surface composition of all samples
is analyzed, and the Pd/Zn ratios (wt%) are found as 1:0, 1:0.1 and 1:0.9 for PdNN, PdNPs and
PdZnNN, respectively, which is the consistent of EDX analysis. The core level of the Pd3d and
Zn2p spectra are shown in Figure 4c and 4d, respectively. In Figure 4c, the Pd3d spectra present
a doublet consisting with two bands at 340.1 eV and 334.9 eV which are assigned to the (Pd3d3/2)
and (Pd3d5/2), respectively, for all catalysts. The metallic states of Pd in those samples are
investigated as in Figure S4. After close observation, the Pd peak positions for PdZnNN and
PdNPs are as same as the peak position of PdNN. Such phenomenon is suggesting that no
change transfer (or very weak) in the electronic structure of Pd from ZnO, 47 which is consistent
with the XRD analyses. The Zn2p spectra show two symmetric peaks in Figure 4d, the peaks at
1021.85 eV and 1045.05 eV are assigned to the Zn2p3/2 and Zn2p1/2, respectively, which
indicating that the Zn presence in the oxide form.48 No difference was observed in the peak
position between PdNPs and PdZnNN Zn2p spectra even though higher amount of ZnO
existence in PdZnNN sample, indicating no charge transfer between ZnO and Pd. Similar
behavior was also observed in core level of Pd3d spectrum.
3.2 Electrochemical measurements
We evaluated the catalytic activities of PdNPs, PdNN, and PdZnNN including Pt/C toward EOR
and HER. Prior to electrochemical measurements, all the catalysts were placed on GCE with
same mass loading and investigated by CVs in Ar-purged 1 M KOH electrolyte which are
displayed in Figure 5a. Figure 5a shows CVs recorded from the four different catalysts in the
potential range of −1.2 to 0.2 V (vs. Ag/AgCl) at 50 mV s−1. The metal oxide (PdO or PtO)
reduction peak located around −0.4 V in the backward CV scan is appeared in all curves. The
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ECSA of all Pd-catalysts was derived from the Coulombic charge (Q) for the reduction of
PdO8,50 and the Q associated with hydrogen adsorption for Pt/C.2,50 For comparison, the PdO or
PtO peaks at around −0.4 V and corresponding charge variation can be seen in Figure S5. The
ECSA of all catalysts was obtained from equation (1) by normalizing against the Pd (or Pt) mass
2 2 2 and the values were 85.6 m2 g 23.9 m g 12.2 m g and 38.6 m g for the PdNN, PdNPs,
PdZnNN and Pt/C catalysts, respectively (Table 1). The obtained ECSA for PdNN was 2.2 and
3.6 magnitudes higher than that of Pt/C and PdNPs, respectively. The Pd/Pt loading and the
corresponding ECSA can be compared in Figure S6a.
ECSA = ×
(1)
where, Qo is the charge for oxide-reduction, Qr is the charge required for the monolayer
adsorption of oxygen on Pd surface (405 µC cm−2)50 or hydrogen on Pt surface (210 µC cm−2)2
and m is the metal loading (in µg cm−2) onto the GCE.
The durability of electrocatalysts is an important issue for widespread and commercial
application of FCs. The stability experiment was performed for evaluating longterm
electrochemical application and the ECSA was calculated for all catalysts up to 3000 CV cycles
(Figure 5b). As shown in Figure 5b, the ECSA of Pt/C dropped almost 53% of its initial ECSA
after 1000th cycle, and the final ECSA after 3000 cycles dropped to 43%. The PdNN displays a
great ECSA stability upto 87.6% after the 1000th cycle and remaining 81.7% after completing the
durability test. On the contrary, the ECSA of PdNPs and PdZnNN catalysts dropped at 36.5%
and 20.5%, respectively, of their initial value after finishing the test. Thus, the durability test
demonstrating that PdNN has significantly higher stability than the traditional carbon supported
Pt/C and other tested catalysts. It is worth to mention that the ECSA value is an indicator of the
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precious metals utilization and this result suggests a better utilization of Pd active sits in PdNN
catalyst probably due to the higher SSA, porous structure and support free catalysis. For PdNPs
and PdZnNN catalysts, however, under practical operating conditions, PdNPs are prone to
migration and agglomeration,51 and Pd active site blocked by the Zn due to coexistence as less
conductive oxide form without alloy formation,8,47 respectively, resulting in a decrease in ECSA
and associated loss of performance on PdNPs and PdZnNN catalysts.
3.3 Electrochemical EOR
The electrocatalytic EOR on PdNPs, PdNN, PdZnNN and Pt/C in Ar-saturated 1 M KOH
electrolyte with containing 1 M ethanol was performed by CVs at a scan rate of 50 mV s−1. Two
well-defined peaks are observed in each CV curve which denoted as f-peak in the forward scan
and b-peak in the backward scan, respectively, in Figure 5c. This feature are signifying the EOR
at all tested catalysts, that are similar to other reports.27,28,30,47 The magnitude of f-peak is as a
result of the oxidation of chemisorbed species that come from ethanol and b-peak is due to the
carbonaceous species that not completely oxidized during forward scan.52 The onset potential
(Eonset) was −0.75, −0.71, −0.65 and −0.63 V and the area normalized current density at f-peak (jf)
was 34.8, 16.4, 7.4, and 4.1 mA cm−2, for the PdNN, Pt/C, PdNPs and PdZnNN catalysts,
respectively. The highest jf was found at PdNN, which is 2.1, and 4.7 magnitudes superior to that
of Pt/C and PdNPs, respectively. Therefore, the PdNN catalyst shows the highest electrocatalytic
activity toward EOR in terms of higher Eonset and jf among all tested catalysts. The metal mass
normalized forward scan of corresponding CVs curves are also shown in Figure 5d. Among all
catalysts, PdNN delivers the best catalytic performance towards EOR with the highest mass
activity (MA, 2.04 A g−1
) at jf which is 3.5, 6.2 and 12.7 magnitudes higher than that on Pt/C
(0.58 A g−1
), PdNPs (0.33 A g−1
) and PdZnNN (0.16 A g−1
), respectively. For better
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understanding, the comparison of specific activity (SA, ECSA normalized jf) and MA at jf for all
catalysts are presented in Figure S6b. However, the MA calculated from PdNN is much higher
even than that of other reported nanonetwork-like catalysts.27−29 Better comparison can be seen
in Table 2.
As mentioned earlier, the b-peak current density (jb) is associated with the oxidation of
carbonaceous intermediates of ethanol dissociative adsorption. The magnitude of jb is caused by
the accumulation of these intermediates and which is an indicator of catalyst poisoning.7,8,27,53
Typically, the ratio of both peaks (jf/jb) is used to evaluate the catalyst poisoning exposure or
tolerance to accumulation of carbonaceous intermediates. The Figure 5d inset shows that the jf/jb
values of the PdNN (1.8) is larger than that of all tested catalysts including Pt/C (0.74) (Table 1),
indicating the better tolerance to carbonaceous intermediates accumulation of as-prepared porous
PdNN. Therefore, PdNN shows best electrochemical performance towards EOR in respect to the
more negative Eonset, higher jf, better utilization of Pd mass and poisoning tolerance ability.
3.4 Kinetics of EOR
To understand the kinetics and final products of electrochemical EOR, however, the faradaic
efficiency (η ) can be evaluated and derived from the stoichiometry (average number of
electrons transferred per ethanol molecule, ). As in equation 2, the CO2 can be the major
product through C−C bond breaking and 12 electron (e−) involved EOR which must shows
higher η . On the contrary, in practice low yields of CO2 have generally been reported, with the
major products being acetic acid (equation 3) and acetaldehyde (equation 4) through 4e− and 2e−
involved EOR, respectively which shows lower η .18,54
CH3−CH2OH + 3H2O → 2CO2 + 12e− + 12H+
(2)
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CH3−CH2OH + H2O → CH3−COOH + 4e− + 4H+
(3)
CH3−CH2OH → CH3−CHO + 2e− + 2H+
(4)
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The η was calculated from the stoichiometry-based equation (5)54 and plotted as a function of
potential for the PdNN, PdNPs, PdZnNN and Pt/C catalysts in Figure 6a. As can be seen in
Figure 6a, the η was much lower at the PdNN catalyst than at all catalysts. In contrast, the
maximum η was obtained for Pt/C and PdNPs. Thus, indicating the higher yield of CH3–CHO
and/or CH3–COOH at the PdNN catalyst and the higher yields of CO2 that have generally been
reported for ethanol oxidation at Pt/C.18
η =
=∑
(5)
Where, is the number of electrons transferred to form product i and
is the fraction of ethanol
converted to product i.
We have confirmed the final product of EOR at the PdNN using the single potential alteration
infrared reflectance spectroscopy (SPAIRS) technique. The SPAIRS spectrum showed several
peaks which indicate the final product was mainly CH3–CHO and/or CH3–COOH at the PdNN
catalyst in Figure S7. Briefly, the peak at 915 cm−1 is assigned to the C−C−O bond.55 The peak at
1250 cm−1 is due to the C−O stretching. The peak at 1350 cm−1 is for −CH3, and at 1412 cm−1 is
for O−C−O bond. The peak at 1710 cm−1 can be assigned to the C=O bond.55 Therefore, it has
been confirmed that the CH3–CHO and/or CH3–COOH was the main product at the PdNN
catalyst.
For comparing kinetic activities, the EOR CVs derived Tafel polarization plots of PdNN, PdNPs,
PdZnNN and Pt/C catalysts toward EOR are plotted in Figure 6b. The Tafel slope was derived
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by plotting the overpotential vs. the log of j and the obtained slope values were 122, 128, 132
and 139 mV dec−1 for PdNN, PdNPs, PdZnNN and Pt/C, respectively. It is noteworthy that at
lower Tafel slopes, the EOR charge-transfer kinetics on a catalyst can be faster than higher one
which facile to the EOR process.8
Also, the potentiodynamic measurements of EOR at PdNN, PdNPs, and PdZnNN catalysts were
studied by CVs upon addition of various concentrations of ethanol (Figure S8). In this
experiment, however, nearly constant jf/jb was observed at PdNN compared to the PdNPs, and
PdZnNN catalysts as the function of ethanol concentration which indicating stable and sensibly
same amounts of intermediates were formed and the peak ratio was concentration independent at
the PdNN site (Figure S9a). Also, nearly same f-peak potential can be observed at PdNN
compared to PdNPs, and PdZnNN catalysts (Figure S9b), indicating uncompensated resistance
may not be occurred in the test system.7 Moreover, the relationship between jf/jb and square root
of scan rate is linearly proportional (Figure S9c), which suggests the EOR was a diffusion
controlled process on PdNN catalyst.56,57 Furthermore, the f-peak position was shifted negatively
with increasing ln of scan rates (Figure S9d), followed by the correlation coefficient of 0.999,
suggesting the EOR was an irreversible electrode process on PdNN catalyst.8,56
The durability of the all tested catalysts towards EOR was monitored by the chronoamperometry
technique. The i vs. t curves were recorded on all tested catalysts at an applied potential of −0.3
V for 30,000 s in Ar-saturated 1 M KOH with 1 M ethanol solution, as displayed in Figure 6c.
Initially, all curves are showing a rapid decay in current density, this is caused by the formation
of poisonous intermediate species during EOR process in alkaline electrolyte. The slow decrease
in the current density and reached at pseudo-steady state was observed under the subsequent
measurements.7,27,58 As expected, however, the PdNN shows higher current density
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(normalization to metal mass) at the start and the end of the test, and the stability remaining at
43.8 % which is 4.8 magnitude greater than the Pt/C (9.1%) indicating its better catalytic activity
and stability against the poisoning.54,59,60 The surface morphology was also investigated after the
EOR stability test. The PdNN (Figure 6c inset, i) was much stable and nonaggregated in nature
than that of PdNPs (Figure 6c inset, ii) and PdZnNN (Figure 6c inset, iii) catalysts. Furthermore,
the PdNN catalyst showed a good reproducibility for EOR which investigated by five different
PdNN-modified electrodes (Figure S10).
3.5 Electrochemical HER
To demonstrate the dual active electrocatalytic performance, we also tested HER activities for
the PdNN, PdNPs, PdZnNN and Pt/C using RDE measurements in 1 M KOH (Figure 7a). As can
be seen, the PdNN is highly efficient for the HER with an Eonset of −1.1 V, beyond which the
cathodic current density increases promptly. For comparing the electrocatalytic HER from
different catalysts, the overpotential (η10) required to drive a current density of 10 mA cm−2 (j10)
for the PdNN is 110 mV, which is considerably better than the PdNPs (170 mV), and similar to
the Pt/C. The turnover frequencies (TOF) of HER were calculated using BET surface area.11 The
PdNN catalyzed HER in the KOH electrolyte was with TOF of 3.1 s−1 at 110 mV (at j10) which is
better than recently reported Pd-based HER catalysts.13,61 The calculated TOF of HER at PdNPs
catalyst was 1.9 s−1 at 170 mV at j10, and the HER at PdZnNN was at below level (at j10). Thus,
the remarkable HER activity at PdNN catalyst was higher than those of other tested catalysts.
3.6 Kinetic of HER
Tafel slope analysis of above described LSV curves for HER on PdNN, PdNPs, PdZnNN and
Pt/C electrodes is given in Figure 7b. The Tafel slope values can be used to determine the rate
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determining step (RDS) of HER process.62 It is also noteworthy that in alkaline medium, the
Tafel slopes for the Volmer, Heyrovsky and Tafel steps as RDS are 120, 40 and 30 mV dec−1,
respectively.63 However, at higher overpotentials, the slope values were 121, 133, 142 and 119
mV dec−1 for PdNN, PdNPs, PdZnNN and Pt/C catalysts, respectively. The Tafel slopes obtained
in this work suggest that for all electrodes the RDS was the Volmer step as in equation (6).63 The
lowest Tafel slope was found at PdNN among all Pd-based catalysts which indicating a fastest
RDS and the best reaction kinetics than that on other tested catalysts in the alkaline medium.64
The similar values to those previously reported for other Pd-based materials.63,65
H2O + e− + Pds ↔ Pd-Had + OH−
(6)
where Had is adsorbed hydrogen and Pds is the hydrogen adsorption Pd-site. In this step, the
reduction of water molecules with the formation of Had as an intermediate.
For practical application, it is also equally important for PdNN catalyst to work stably for HER
without any undesirable side-reactions. Figure 7c shows that the PdNN catalyst has good
stability in alkaline electrolyte with only a slight negative shift overpotential (13 mV) after 1000
CV cycles, whereas, the overpotential required to achieve at j10 on the Pt/C catalyst increased by
55 mV after the same CV cycles, which signifies the superior durability of PdNN for HER.66
Furthermore, the PdNN (Figure 7d) showed an about 92.3% current retention after 15,000s
investigated by chronoamperometry at an applied potential of 110 mV, which was much better
than that of the homemade Pt-Ru/C (89%) and Pt/C (86.4%) electrodes. The poor stability was
observed at Pt/C due to the typical poisoning during long run operation. However, this
experiment is clearly indicating the remarkable long term operational stability of PdNN.
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Therefore, the outstanding features of high activity, favorable kinetics, and strong durability
indicate that PdNN is a suitable catalyst for HER.
4. Conclusion
In this study, we have prepared a tunable Pd nanonetwork with the assistance of CTAB and Zn
precursor which employed as an electrocatalyst for both EOR and HER. The presence of
hierarchical structure ensures a high surface area with abundant interfacial active sites for
electrochemical reactions with minimal Pd usage. Owing to its crystalline nanonetwork porous
structure and properties such as large surface area, and rich active sites, the newly developed
PdNN was demonstrated to be an effective dual active electrocatalyst with excellent activities
and stability. Therefore, our study on the facile synthesized PdNN has provided new application
for dual active catalysts particularly for EOR and HER, which could disclose new possibilities
for the development of sustainable energy conversion technology based on scalable, durable and
superior electrocatalysis.
Acknowledgments
This research was supported by the National Research Foundation of Korea (NRF) funded by the
Ministry of Education, Science, and Technology (NRF-2015R1D1A1A09059344).
Supporting Information
Physical parameters and electrochemical characterization; EDX spectra; the diameter size
histograms; enlarged XRD patterns; the core level of Pd3d XPS spectra; backward scan of CVs
and their corresponding charge density variation; plots of metal loading with corresponding
ECSA and mass activity with specific activity; SPAIRS spectrum; CVs for EOR in various
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concentration of ethanol; plots of jf/jb, and jf variation vs. ethanol concentration, the plot of jf/jb
vs. square root of scan rate, jf shift vs. ln of scan rates.
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Figure captions:
Figure 1: The schematic synthesis of PdNN, PdZnNN and PdNPs.
Figure 2: TEM images of PdNN (a and b), PdNPs (c) and PdZnNN (d); insets: the SAED (a)
and HRTEM image (b) of PdNN, the HRTEM images of PdNPs (c) and PdZnNN (d).
Figure 3: TEM images of PdNN prepared with CTAB concentrations of 40 mM (a), 80 mM (b)
and 120 mM (c).
Figure 4: Nitrogen adsorption/desorption isotherms (a), the XRD patterns (b), core level of Pd3d
(c) and Zn2p XPS spectra (d) of PdNN, PdNPs, and PdZnNN; inset: the corresponding pore-size
distribution of those catalysts (a).
Figure 5: CV curves recorded in Ar-saturated 1 M KOH electrolyte at a scan rate of 50 mV s−1
with scale bar of 3 mA cm−2 (a), the plot of ECSA value from durability test for 3000 CV cycles
(b), the CV curves for EOR in presence of 1 M EtOH with current density normalized to
electrode area with scale bar of 10 mA cm−2 (c) and forward scan of same CV curves are shown
with current density normalized to Pd or Pt mass for PdNN, PdNPs, PdZnNN and Pt/C
electrodes (d); inset: the plot of jf/jb (d).
Figure 6: The calculated faradaic efficiency (a), Tafel plots (b) and chronoamperometric curves
(i vs. t) recorded in 1 M KOH electrolyte in presence of 1 M ethanol on PdNN, PdNPs, PdZnNN
and Pt/C-modified electrodes (c); insets: the TEM images of PdNN (i), PdNPs (ii), PdZnNN (iii)
after i vs. t test (c).
Figure 7: LSV curves for HER recorded in Ar-saturated 1 M KOH at a rotation speed of 1600
rpm and scan rate of 10 mV s−1 (a), the corresponding Tafel plots of PdNN, PdNPs, PdZnNN and
Pt/C (b), the LSV curves for HER before and after stability test (c) and the i vs. t curves on
PdNN, Pt/C and homemade Pt-Ru/C electrodes at an applied potential of 110 mV (d).
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Figure 1:
Figure 2:
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Figure 4:
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Figure 5:
Figure 6:
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Table 1: The electrochemical performances comparison among all tested catalysts
Name
PdNN
PdNPs
PdZnNN
Pt/C*
Metal loading (µg cm−2)
16.9
22.6
25.5
28.3
ECSA (m2 g )
85.6
21.3
13.7
38.6
MA (Ag ) @ jf
2.04
0.29
0.18
0.58
EOR
−0.75
−0.65
−0.63
−0.71
HER
−1.1
−1.1
−1.11
−1.1
jf/jb
1.8
0.8
1.5
0.74
η (%) @ -0.2V
22.5
61
37.5
79
η10 for HER (mV)
110
170
-
110
ECSA (%)
81.8
36.5
20.5
43.5
EOR (%)
43.8
22.1
7.8
9.1
HER (%)
92.3
-
-
86.4
Eonset (V)
Stability
*normalized to the Pt mass.
Table 2: The electrochemical performances of PdNN on EOR compared with other catalysts
including nanonetwork-like catalysts.
Name
Electrolyte
ECSA (m2 g )
MA (Ag )
PdNN [this work]
1 M KOH
85.6
2.04
Pd1Ni1/C14
1 M NaOH
67.3
2.37
Pd50Ag5027a
1 M KOH
32.81
1.97
Au1Pt128a
0.5 M H2SO4
101.14b
0.567b
Pt53Cu4729a
0.5 M H2SO4
61.3b
0.171b
Pd1Pb149a
1 M KOH
15.43
6.936
Pd40Ni43P1753
1 M NaOH
63.22
4.42
Pd1Au3 NSFs59
0.5 M KOH
4.89
0.317
1 M KOH
58.3
~3
alkaline
18
10.8
FePd−Fe2O3/MWNTs69
1 M KOH
120.4
1.191
PdCu2-270
1 M KOH
59.27
0.163
Pd73Cu2767a
Pd1Au168a
a
network-like catalyst; bECSA and MA were normalized to the Pt mass.
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