Synthesis of Pd-Pt Ultrathin Assembled Nanosheets as Highly Efficient Electrocatalysts for Ethanol Oxidation

Article abstract of DOI:10.1002/asia.202000041

Control over composition and morphology of nanocrystals (NCs) is significant to develop advanced catalysts applicable to polymer electrolyte membrane fuel cells and further overcome the performance limitations. Here, we present a facile synthesis of Pd?Pt alloy ultrathin assembled nanosheets (UANs) by regulating the growth behavior of Pd?Pt nanostructures. Iodide ions supplied from KI play as capping agents for the {111} plane to promote 2-dimensional (2D) growth of Pd and Pt, and the optimal concentrations of cetyltrimethylammonium chloride and ascorbic acid result in the generation of Pd?Pt alloy UANs in high yield. The prepared Pd?Pt alloy UANs exhibited the remarkable enhancement of the catalytic activity and stability toward ethanol oxidation reaction compared to irregular-shaped Pd?Pt alloy NCs, commercial Pd/C, and commercial Pt/C. Our results confirm that the Pd?Pt alloy composition and ultrathin 2D morphology offer high accessible active sites and favorable electronic structure for enhancing electrocatalytic activity.

Full text of DOI:10.1002/asia.202000041

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Synthesis of Pd-Pt Ultrathin Assembled Nanosheets as Highly
Efficient Electrocatalysts for Ethanol Oxidation
Authors: Yeji Han, Jeonghyeon Kim, Su-Un Lee, Sang-Il Choi, and
Jong Wook Hong
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10.1002/asia.202000041
Chemistry - An Asian Journal
Synthesis of Pd-Pt Ultrathin Assembled Nanosheets as Highly Efficient
Electrocatalysts for Ethanol Oxidation
Yeji Han,[a] Jeonghyeon Kim,[b] Su-Un Lee*,[c] Sang-Il Choi,*[b] and Jong Wook Hong*[a]
[a]Department of Chemistry, University of Ulsan, Ulsan 44610, South Korea.
[b]Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National
University, Daegu 41566, South Korea.
[c]Carbon Resources Institute, Korea Research Institute of Chemical Technology, 141,
Gajeong-ro, Yuseong-gu, Daejeon 34114, South Korea.
*Correspondence and requests for materials should be addressed to J.W.H., S.-I.C., and S.-U.L.
(email: jwhong@ulsan.ac.kr; sichoi@knu.ac.kr; sulee@krict.re.kr)
Abstract
Control over composition and morphology of nanocrystals (NCs) is significant to develop
advanced catalysts applicable to polymer electrolyte membrane fuel cells (PEMFCs) and
further overcome the performance limitations. Here, we present a facile synthesis of Pd-Pt alloy
ultrathin assembled nanosheets (UANs) by regulating the growth behavior of Pd-Pt
nanostructures. Iodide (I-) ions supplied from KI play as capping agents for the {111} plane to
promote 2-dimensional (2D) growth of Pd and Pt, and the optimal concentrations of
cetyltrimethylammonium chloride (CTAC) and ascorbic acid (AA) result in the generation of
Pd-Pt alloy UANs in high yield. The prepared Pd-Pt alloy UANs exhibited the remarkable
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10.1002/asia.202000041
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enhancement of the catalytic activity and stability toward ethanol oxidation reaction (EOR)
compared to irregular-shaped Pd-Pt alloy NCs (IS NCs), commercial Pd/C, and commercial
Pt/C. Our results confirm that the Pd-Pt alloy composition and ultrathin 2D morphology offer
high accessible active sites and favorable electronic structure for enhancing electrocatalytic
activity.
KEYWORD: Pd-Pt bimetallic, Nanosheet assembly, Ethanol oxidation reaction,
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Polymer electrolyte membrane fuel cells (PEMFCs) are efficient energy converting systems
that operate based on the electrooxidation of fuels.[1-4] PEMFCs potentially have high energy
and power densities, but fall short of expectations due to the lack of high-performance
electrocatalysts.[5] Pt has been accepted as the most efficient catalyst for fuel oxidation
reactions,[6-9] but has the disadvantages of high cost and low-reserves.[9-11] In addition, its
insufficient activity and poor stability due to the poisoning by reaction intermediates and
deformation of catalysts are also considered as limitations for practical application.[12-14] An
effective strategy for improving the activity and stability of Pt is alloying it with other metals,
such as Pd, Ir, Ru, Au, and Pb.[15-19] Introducing the secondary metal component can modulate
the electronic structure of Pt and thus weaken the affinity of poisoning species.[20] In particular,
Pd is a promising candidate to form a strong coupling with Pt and synergistically enhance the
catalytic properties due to the same face-centered cubic (fcc) structure with a highly similar
lattice constant of nominal mismatch of 0.77%.[6,21,22]
As an another approach to enhance the electrocatalytic performance, preparing nanocrystals
(NCs) can offer an advantage for increasing high accessible active sites participating in the
electrocatalytic reaction due to their large surface area to volume ratio as compared with bulk
materials.[23] Notably, metal ultrathin assembled nanosheets (UANs) composed of multiple
ultrathin nanosheet subunits are one of efficient nanostructures that can maximize surface area
to volume ratio.[24-27] For example, Pi et al. reported that Ir UANs exhibited larger catalytic
activity compared with irregular-shaped 3-dimensional (3D) Ir NCs for oxygen evolution
reaction (OER).[27] However, synthetic conditions for achieving the UANs are strict because
the in-plane growth rate should be sufficiently fast by effectively suppressing the out-of-plane
growth of the metal. Since it is difficult to simultaneously control in-plane and out-of-plane
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growth kinetics, typical 3D metal nanostructures are produced under common synthetic
conditions. Therefore, the synthesis of metal UANs has attracted significant interest in recent
years for the preparation of highly challenging and efficient electrocatalysts.
Herein, we report the facile synthesis of Pd-Pt alloy UANs by co-reduction of Pd and Pt
precursors. In our synthetic route, the iodide (I-) ions as the stabilizing agent play a key
regulator for developing the UAN structure. In addition, growth rate of the UANs can be
modulated by adjusting the amounts of cetyltrimethylammonium chloride (CATC) and
ascorbic acid (AA) used as stabilizer and reductant, respectively. The Pd-Pt alloy UANs
exhibited highly enhanced electrocatalytic activity and stability toward ethanol oxidation
reaction (EOR) compared with Pd-Pt alloy irregular-shaped NCs (IS NCs), commercial Pd/C,
and Pt/C owing to the synergistic advantages by Pd-Pt alloy component and unique morphology.
The Pd-Pt alloy UANs were synthesized by reducing K2PdCl4 and K2PtCl4 in an aqueous
solution containing KI, CTAC, and AA as a capping agent, a stabilizer, and a reductant,
respectively. Figure 1a,b and Figure S1 display representative scanning electron microscopy
(SEM) and transmission electron microscopy (TEM) images of the products, demonstrating
that they are composed of several 2D nanosheets and each nanosheet subunit is interconnected
by sharing edges. Weak contrasts at subunit parts in the UAN indicate the ultrathin thickness.
The small spherical NCs with an average diameter of 4.2 nm were loaded on the nanosheet
subunits (Figure 1c). The fast Fourier transform (FFT) pattern obtained from a nanosheet
subunit confirms the highly crystalline nature of the products (the inset of Figure 1c). The
thickness of the nanosheet subunits of UANs was estimated to be approximately 6.0 ± 0.5 nm
measured by high resolution TEM (HRTEM) image taken from the side plane (Figure 1d).
For an investigation of the detailed compositional characteristics of the UANs, elemental
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mapping images were obtained by using high angle annular dark field scanning TEM−energy-
dispersive X-ray spectroscopy (HAADF−STEM−EDS) measurement (Figure 2), which clearly
confirms the formation of a homogeneous Pd-Pt alloy structure. Furthermore, HRTEM image
in the inset of Figure 1c reveals the lattice spacing of 2.24 Å , which corresponds as the (111)
planes of fcc Pd-Pt alloy.[6,28] The Pd/Pt atomic ratio of the Pd-Pt UANs was determined as
10:1 by inductive coupled plasma-optical emission spectroscopy (ICP−OES). For X-ray
diffraction (XRD) pattern of the Pd-Pt alloy UANs (Figure S2), the representative diffraction
peaks of the (111), (200), (220), and (311) planes were detected at 40.0, 46.5, 68.1, and 82.1°
between the positions expected to pure Pd and Pt, indicating the formation of Pd–Pt bimetallic
feature.
For the formation of the Pd-Pt alloy UANs, the concentrations of I- ions, CTAC, and AA are
critical factors to control the growth behavior of Pd and Pt. To investigate the influence of
concentration of I- ion, different amounts of KI ranging from 0 to 36.50 mM were used under
otherwise identical conditions for standard UANs. When 0, 3.29, 6.17, and 10.99 mM of KI
were introduced instead of standard KI concentration (18.02 mM), irregular-shaped
nanostructures were obtained (Figure S3a-d), while introduction of KI from 18.02 to 22.90 mM
produced the nanostructures with assembled nanosheet shapes (Figure S3e,f). It is noted that
the shape and size of the NCs formed in 3.29 mM of KI were similar with those of spherical
NCs (4.2 nm) on the Pd-Pt alloy UANs produced by 18.02 mM KI. Based on this finding, we
believe that the formations of spherical NCs and nanosheets are competitive in our synthesis
condition. The 18.02 mM of KI can dominantly lead to the formation of nanosheets by
passivating the {111} facets of Pt-Pd. However, the small spheres were still formed on the
surface of Pt-Pd UANs, indicating that suppressing the formation of Pt-Pd spheres were not
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perfectly achieved.
However, the addition of 26.49 and 36.50 mM of KI induced the aggregated nanostructure
(Figure S3g,h) because the excessive I- ions in the reaction mixtures can cause the oxidative
etching by halide ion/oxygen pairs.[29,30] These results unambiguously demonstrate that
introducing the optimal amount of I- ions, which can effectively promote the 2D growth of Pd
and Pt by suppressing the growth of [111] orientation and avoid unnecessary oxidative etching,
is the primary prerequisite for the formation of the Pd-Pt alloy UANs. It is in good agreement
with the previously reports that I- ions are preferentially bound to the {111} crystal facet and
facilitate the formation of anisotropic structures such as plates and prisms.[31] However, we
found that I- ion is not the only factor to determine the growth behavior of Pd and Pt, which
was confirmed through a set of control experiments using different amounts of CTAC and AA.
At lower amounts of CTAC (0.45 mM) or AA (0, 0.49, 1.21, and 2.36 mM) compared with
those used in the standard synthesis nanosheets, highly similar shapes of nanostructures with
standard Pt-Pd alloy UANs were developed (Figure S4a and Figure S5a-d); 2.27 mM of CTAC
(Figure S4b) and 4.50 mM of AA (Figure S5e) were employed in the standard synthetic
procedure for the Pd-Pt ultrathin assembled nanosheets, respectively. On the other hand,
increasing the concentration of CTAC (4.55, 9.09, 22.73, and 45.45 mM) or AA (9.92, 16.56,
and 24.88 mM) produced the 3D nanostructures with random sizes and shapes (Figure S4c-f
and Figure S5f-h). These results can be interpreted that the capping effect of I- ion is attenuated
in the high concentration of CTAC and AA. The excess concentrations of CTAC and AA may
disturb the adsorption of I- ion on the surface of Pd-Pt nanostructures, and thus trigger the
growth of 3D nanostructures with irregular shapes. On the basis of these results and findings,
we found that I- ions primarily facilitate the preferential 2D growth of Pd and Pt by being
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passivated {111} plane and the adjustment of CTAC and AA concentration allows the
production of Pd-Pt alloy UANs in high yield.
To investigate the morphological and compositional benefits from Pd-Pt alloy UANs, the
EOR activity of the Pd-Pt alloy UANs supported on Ketjen black used as carbon support was
evaluated in comparison with Pd-Pt alloy IS NCs loaded on the Ketjen black, commercial Pd/C,
and commercial Pt/C catalysts. For the preparation of the Pd-Pt alloy IS NCs, two-times higher
concentration of CTAC (4.55 mM) than that used for the synthesis of the Pd-Pt alloy UANs
was introduced while other conditions kept the same as the preparation of the Pd-Pt alloy UANs.
The elemental mapping images of the IS NCs exhibited that Pd and Pt signals are
homogeneously dispersed through entire the NCs, demonstrating that compositional structure
of the NCs is Pd-Pt alloy (Figure S6). The Pd/Pt atomic ratio of the Pd-Pt alloy IS NCs was
determined as 10:1 by ICP-OES, similar to that of the Pd/Pt alloy UANs. Cyclic
voltammograms (CVs) of catalysts are demonstrated in Figure 3a. Based on the integrated
charges associated with cathodic peak between -0.54 and -0.10 V, the electrochemically active
surface areas (ECSAs) of the Pd-Pt alloy UANs, Pd-Pt alloy IS NCs, Pd/C, and Pt/C catalysts
were measured to be 27.7, 14.9, 30.4, and 31.2 m2 g-1, respectively (Figure S7). The Pd-Pt alloy
UANs showed roughly 1.9 times larger ECSA than the Pd-Pt alloy IS NCs due to their
morphological benefit. Figure 3b shows the CVs obtained from the EOR of different catalysts
in 0.5 M KOH and 1.0 M ethanol. The mass activities of the Pd-Pt alloy UANs, Pd-Pt alloy IS
NCs, Pd/C, and Pt/C catalysts were 3369, 1510, 1430 and 1243 mA mg-1, respectively (Figure
3b,d). Moreover, the analogous trend across the various catalysts was also obtained in the
specific activity. The Pd-Pt alloy UANs, Pd-Pt alloy IS NCs, Pd/C, and Pt/C catalysts exhibited
12.16, 11.86, 4.71, and 3.99 mA cm-2, respectively (Figure 3c,d). For both mass and specific
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activities, the Pd-Pt alloy UANs and IS NCs showed the enhanced performances than
commercial Pd/C, and Pt/C catalysts. This indicates that Pd-Pt alloy can effectively promote
EOR than pure Pd and Pt surfaces. On the other hand, the higher activity of the Pd-Pt alloy
UANs than that of the Pd-Pt alloy IS NCs obviously shows the larger ECSA of the Pd-Pt alloy
UANs. Therefore, the significantly enhanced catalytic activity of the Pd-Pt alloy UANs is
ascribed to the synergistic effects of Pd-Pt alloy and ultrathin 2D morphology, simultaneously
leading to promote the intrinsic electrocatalytic properties and increasing the active sites.
Electrooxidation of ethanol in alkaline medium proceeds via the following ⅰ) reactive
intermediate and ⅱ) poisoning intermediate pathway.[32, 33]
ⅰ) reactive intermediate pathway:
M + OH- → M-OHads + e-
M + CH3CH2OH → M-(CH3CH2OH)ads
M-(CH3CH2OH)ads + 3OH- → M-(COCH3)ads + 3H2O + 3e-
M-(COCH3)ads + M-OHads → M-CH3COOH + M: rate determining step
M-CH3COOH + OH- → M + CH3COO- + H2O
ⅱ) poisoning intermediate pathway
M + OH- → M-OHads + e-
M + CH3CH2OH → M-(CH3CH2OH)ads
M-(CH3CH2OH)ads + 3OH- → M-(COCH3)ads + 3H2O + 3e-
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For reactive intermediate pathway, electrooxidation of (COCH3)ads intermediate on the metal
surface by reacting with OHads in the alkaline medium is the rate-determining step.[3,20] It has
been previously reported that the up-shift of d-band center of Pd-Pt alloy surfaces compared
with that of pristine Pd surface can promote the formation of OHads with optimal binding
strength[34] and thus enhancement of EOR activity can be achieved on Pd-Pt alloy surface due
to accelerated rate of (COCH3)ads oxidation. To investigate the change in the electronic structure
of the Pd-Pt alloy UANs, X-ray photoelectron spectroscopy (XPS) spectra of valence band
regions were obtained and corresponding d-band center positions relative to the Fermi level
were calculated. Indeed, the d-band center of the Pd-Pt alloy UANs was at -4.66 eV, which was
-1.38 eV higher than that of Pd/C (Figure 4), which implies the up-shifted d-band center of the
Pd-Pt alloy UANs. This can facilitate the formation of OHads on the Pd-Pt alloy than Pd surface,
and thus transformation of COCH3 to CH3COOH on the catalysts (rate determining step) can
be accelerated, boosting the EOR activity. Conclusively, the large surface area to volume by
ultrathin assembled morphology and modified electronic structure of the UANs by alloying Pd
and Pt can account for their enhanced electrocatalytic activity than pristine Pt and Pd catalysts.
The electrochemical stability of Pd-Pt alloy UANs was estimated by repeating CV and
compared with Pd-Pt alloy IS NCs, Pd/C and Pt/C catalysts. Glassy carbon electrode (GCE)
containing catalysts was applied between -0.9 and 0.3 V in N2-saturated 0.5 M KOH and 1.0
M ethanol. After 300 cycles, the Pd-Pt alloy UANs, Pd-Pt alloy IS NCs, Pd/C, and Pt/C
catalysts showed 2355, 1057, 763, and 772 mAmg-1, respectively, which were decrease of 28.8,
30.0, 44.5, and 41.3% in mass activity, respectively (Figure 5). The higher stabilities of the Pd-
Pt alloy UANs and IS NCs are attributed to the Pd-Pt alloy effect, which can effectively
suppress the formation of poisoning intermediates and thus maintain clean surfaces during the
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Chemistry - An Asian Journal
reactions for long-term.
In summary, the Pd-Pt alloy UANs have been prepared through a facile wet-chemical
synthesis. For the formation of UANs, the growth behavior of Pd-Pt nanostructures was
modulated by adding optimal concentrations of KI, CTAC, and AA. The I- ions generated from
KI served as a primary ionic capping agent for the preferential 2D growth of Pd and Pt with
the passivated {111} plane, and the concentrations of CTAC and AA were adjusted to allow
the generation of Pd-Pt alloy UANs in high yield. The Pd-Pt alloy UANs exhibited enhanced
EOR catalytic activity and stability compared to Pd-Pt alloy IS NCs, Pd/C, and Pt/C catalysts
due to increased active sites by their assembled nanosheet morphology and Pd-Pt alloy
composition. This work demonstrates the significance of composition and morphology
engineering of nanostructures for potential applications, especially in electrocatalysis.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the 2019 Research Fund of University of Ulsan.
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Figure 1. a) SEM, b) TEM, and c) HRTEM images of the Pd-Pt alloy UANs. Insets in (c)
showing high-magnification HRTEM image and FFT pattern obtained from a subunit of Pd-Pt
alloy UAN. d) HRTEM image of the side face of an ultrathin nanosheet subunit.
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Figure 2. HAADF-STEM image and corresponding EDS elemental maps of a) Pd-Pt alloy
UANs and b) ultrathin nanosheet subunit.
Figure 3. a) CVs of the various catalysts on GCE in 0.5 M KOH and b, c) in 0.5 M KOH + 1.0
M ethanol. The current values were normalized with respect to the b) total metal mass and c)
ECSA, respectively and d) corresponding mass activities and specific activities of the various
catalysts.
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Figure 4. a) XPS spectra for the valence band and b) d-band center positions relative to the
Fermi level of Pd-Pt alloy UANs and Pd/C.
Figure 5. CVs obtained before and after stability test for a) Pd-Pt alloy UANs, b) Pd-Pt alloy
IS NCs, c) Pd/C, and d) Pt/C catalysts in 0.5 M KOH + 1.0 M ethanol at a scan rate of 50 mV
s-1.
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TOC
: Pd-Pt alloy ultrathin assembled nanosheets (UANs) were synthesized by regulating the
growth behavior of Pd-Pt nanostructures. The prepared Pd-Pt alloy UANs exhibited the
remarkable enhancement of the catalytic activity and stability toward ethanol oxidation
reaction (EOR) compared to irregular-shaped Pd-Pt alloy NCs (IS NCs), commercial Pd/C, and
commercial Pt/C. This confirms that the Pd-Pt alloy composition and ultrathin 2D morphology
offer high accessible active sites and favorable electronic structure for enhancing
electrocatalytic activity.
16
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