Theoretical Modelling and Facile Synthesis of a Highly Active Boron-Doped Palladium Catalyst for the Oxygen Reduction Reaction

Article abstract of DOI:10.1002/anie.201601727

A highly active alternative to Pt electrocatalysts for the oxygen reduction reaction (ORR), which is the cathode-electrode reaction of fuel cells, is sought for higher fuel-cell performance. Our theoretical modelling reveals that B-doped Pd (Pd-B) weakens the absorption of ORR intermediates with nearly optimal binding energy by lowering the barrier associated with O₂dissociation, suggesting Pd-B should be highly active for ORR. In fact, Pd-B, facile synthesized by an electroless deposition process, exhibits 2.2times and 8.8times higher specific activity and 14times and 35times less costly than commercial pure Pd and Pt catalysts, respectively. Another computational result is that the surface core level of Pd is negatively shifted by B doping, as confirmed by XPS, and implies that filling the density of states related to the anti-bonding of oxygen to Pd surfaces with excess electrons from B doping, weakens the O bonding to Pd and boosts the catalytic activity. Better with a B in its bonnet: Theoretical modelling shows that B doping negatively shifts the surface core level of Pd and lowers the barrier to O₂dissociation for the oxygen reduction reaction (ORR). A B-doped Pd nanoparticle catalyst was then rationally designed, synthesized in a facile manner by electroless deposition, and shown to be a highly active ORR catalyst compared to commercial Pd and Pt catalysts.

Full text of DOI:10.1002/anie.201601727

Angewandte
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International Edition: DOI: 10.1002/anie.201601727
German Edition: DOI: 10.1002/ange.201601727
Oxygen Reduction Reaction
Theoretical Modelling and Facile Synthesis of a Highly
Active Boron-Doped Palladium Catalyst for the Oxygen
Reduction Reaction
Tat Thang Vo Doan+, Jingbo Wang+, Kee Chun Poon, Desmond C. L. Tan,
Bahareh Khezri, Richard D. Webster, Haibin Su,* and Hirotaka Sato*
Angewandte
Chemie
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Abstract: A highly active alternative to Pt electrocatalysts for
gen.[1d–f, 6] According to these curves, the pure metals Pt and
the oxygen reduction reaction (ORR), which is the cathode-
electrode reaction of fuel cells, is sought for higher fuel-cell
performance. Our theoretical modelling reveals that B-doped
Pd (Pd-B) weakens the absorption of ORR intermediates with
nearly optimal binding energy by lowering the barrier asso-
ciated with O2 dissociation, suggesting Pd-B should be highly
active for ORR. In fact, Pd-B, facile synthesized by an
electroless deposition process, exhibits 2.2 times and 8.8 times
higher specific activity and 14 times and 35 times less costly
than commercial pure Pd and Pt catalysts, respectively.
Another computational result is that the surface core level of
Pd is negatively shifted by B doping, as confirmed by XPS, and
implies that filling the density of states related to the anti-
bonding of oxygen to Pd surfaces with excess electrons from B
doping, weakens the O bonding to Pd and boosts the catalytic
activity.
Pd exhibit superior catalytic activities for oxygen reduction,
which can be further improved by alloying them with other
elements to weaken the oxygen binding strength. This notion
has been confirmed by the enhanced ORR activity of Pt3M
alloys (M = Ni, Co, Fe, V, Ti, Sc, Y) in which the d-band center
of the Pt atoms is lower than that of pure Pt.[1e,f, 6] Pt-
alternative catalysts that will lower the cost while maintaining
the high ORR activity have also been researched.[7] The
activities of Pd-alloy catalysts (Pd–Co and Pd–Fe) are
comparable with that of Pt.[7b,c,f] Despite a recent report on
P-doped Pd that exhibited very high catalytic activity owing to
its amorphous structure,[8] much less work has been devoted
to doping non-metallic elements into Pd. In the present study,
we seek to understand how the ORR is enhanced in an
alloyed catalyst (e.g. B-doped Pd). The investigation com-
bines density functional theory (DFT) calculations[1e, 6b,9] with
experimental studies. We first investigated the adsorption of
ORR intermediates onto the Pd material, constructed the
ORR mechanism, and identify the preferential mechanism on
the pure Pd and B-doped Pd catalysts. We then synthesized
a real B-doped Pd and pure Pd catalyst by our previously
developed stepwise electroless deposition process.[8] In this
method, the electrode is sequentially dipped into two differ-
ent solutions, the first containing a reducing agent (in this
study, dimethylaminoborane (DMAB) or hydrazine (N2H4))
and the second containing a metal salt (metal ion source,
PdCl2). During the second dipping, Pd nanoparticles are
electrochemically deposited onto the electrode surface. The
Pd nanoparticles become doped with B atoms released by the
decomposition of DMAB (herein, the B-doped Pd is denoted
as Pd–B) while pure Pd is deposited (no doping) when
hydrazine is used (the synthesized pure Pd is denoted as Pd–
N2H4). As predicted by the theoretical modelling, the
synthesized Pd–B exhibits superior catalytic activity to the
Pd–N2H4 and a commercial Pd-loaded carbon (denoted as Pd/
C). Finally, to further understand the mechanism of the ORR
enhancement in Pd–B, we compute the surface core level shift
(SCLS), defined as the difference in core level binding energy
between the surface and bulk, for the pure Pd and Pd–B
catalysts, and experimentally confirm the computations by X-
ray photoelectron spectroscopy (XPS).
I
n fuel cells, the oxygen reduction reaction (ORR) plays an
important role, because oxygen is a common fuel for cathodic
=
reduction. However, the ORR is non-facile because the O O
bond is very strong.[1] Developing highly active oxygen-
reduction electrocatalysts at low cost remains a great chal-
lenge, although intensive efforts have been expended on the
development of cathode materials. Non-precious metal-based
catalysts, such as transition-metal nitrogen carbon (M-N-C,
M = Co, Fe, Ni, Mn)[2] and transition-metal oxides (MOx, M =
Mn, Fe, Co, Cu,)[3] exhibit high ORR catalytic activity and
have drawn much attention owing to their abundance and low
cost. Metal-free materials ,such as nitrogen-doped carbon
have shown significant improvement in ORR catalytic
activity that reaches the level of commercial Pt/C in term of
onset potential and half-wave potential.[3d,4] They are very
promising catalysts but need further improvement in terms of
durability, synthesis process (mostly high vacuum, high
temperature, or long synthesis time is required), or electric
conductivity.[5] Platinum is thus still the practically used ORR
catalyst and platinum-alternative precious-metal catalysts
have been intensively investigated.
Nørskov and co-workers established the so-called volcano
curve, which relates the oxygen reduction activity of different
metals and alloys to the binding energy of atomic oxy-
The ORR intermediate species (especially the radical
species) adsorb more weakly on the surface of Pd–B
than on Pd as summarized from Table S1 in the Supporting
Information. On the pure Pd surface, O prefers the fcc
hollow, that is, the f site with a binding energy of À4.44 eV,
whereas on B doped Pd surface (Pd–B), the f2 site is pre-
ferred with a binding energy of À4.12 eV: the B doping
weakens the O adsorption by 0.32 eV. According to the work
by Stamenkovic et al.,[6b] the highest ORR catalyst is pre-
dicted to bind O more weakly than pure Pt by 0.20 eV. Since
O adsorbs 0.06 eV more strongly to Pd(111) than to Pt-
(111),[7b, 10] the downshift should be 0.26 eV (relative to pure
Pd) to reach the maximum activity. Therefore, Pd–B exhibits
nearly optimal O adsorption and should have high catalytic
activity for ORR.
[*] T. T. Vo Doan,[+] K. C. Poon, D. C. L. Tan, Prof. H. Sato
School of Mechanical and Aerospace Engineering
Nanyang Technological University
50 Nanyang Avenue, Singapore 639798 (Singapore)
E-mail: hirosato@ntu.edu.sg
J. Wang,[+] Prof. H. Su
School of Materials Science & Engineering
Nanyang Technological University
50 Nanyang Avenue, Singapore 639798 (Singapore)
E-mail: hbsu@ntu.edu.sg
B. Khezri, Prof. R. D. Webster
Division of Chemistry & Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University
21 Nanyang Link, Singapore 637371 (Singapore)
[+] These authors contributed equally to this work.
OH and O2 preferentially bind to the f sites on the pure Pd
surface with binding energies of À2.58 eV and À0.99 eV,
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201601727.
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respectively. On Pd–B, these binding energies decrease down
to À2.55 eV and À0.67 eV, respectively. H2O always prefers
the top site of the Pd surface regardless of B-doping; the
directions of the H atoms have little effect on the binding
strength. All the above binding energies on the Pd surface are
consistent with previous results by DFT calculations.[7a,11]
Furthermore, the binding energies show stronger site-
dependence on Pd–B than on pure Pd. Depending on the
number of underlying B atoms, the binding strength of O
varies from À3.62 eV to À4.12 eV. The variation (0.50 eV) is
much wider than the variation on Pd(111) surfaces (0.26 eV).
A similar trend is seen for OH adsorption. The energy range
of OH adsorption reduces from 0.39 eV on Pd–B to 0.21 eV
on Pd(111). Although the adsorption of the saturated species
(O2 and H2O) is relatively insensitive to site configuration, it
remains more variable on Pd–B than on the Pd(111) surface.
Since the Pd–B surface contains two distinct preferable
adsorption sites for O2 and O with lower binding energies
than for Pd(111), this optimizes both dissociative adsorption
of O2 and further reduction of O similar to the scenario
suggested by the simple thermodynamic model proposed by
Bard and co-workers.[7f]
The primary factor determining which mechanism pre-
vailing is the relative feasibility of the rate-determining step
among above mechanisms. For each type of mechanism, the
initiation reaction corresponds to the rate-determining step.
In mechanisms I, II, and III, the initiation reactions are
step (R1), the O2 dissociation, step (R2), the hydration of O2
to HO2 and step (R7), direct dissociation of H2O, respectively.
Because water dissociation must overcome a higher barrier
(> 1 eV) than the other steps, mechanism III cannot occur on
both surfaces. The barriers of R8 and R9 on Pd(111) are 0.68
and 0.43 eV, respectively, consistent with those of previous
results.[10,11] On Pd(111), the barrier of R1 is slightly higher
than that of R2 (0.72 eV vs. 0.68 eV). Because these two rate-
determining steps are very similar on the Pd(111) surface, the
dissociative and associative mechanisms compete, although
the associative mechanism is slightly more dominant. This
situation alters on the Pd–B surface, where the barrier
difference between R1 and R2 (0.63 eV vs. 0.71 eV) becomes
sufficiently wide that the dissociative mechanism dominates.
From the rate-determining step of the preferential mecha-
nism, we can estimate the ORR activity of Pd-B and compare
it with that of pure Pd. The comparison predicts a seven-fold
Considering the binding energies of the ORR intermedi-
ates in the Table S1, we can briefly understand the ORR
activity enhancement of the Pd-alloyed B catalysts and
implement the reaction barrier calculations for additional
insight. The ORR mechanism has been extensively studied on
Pt-alloyed and transition-metal surfaces in acidic solu-
tions.[1d,10–12] However, it appears that the reaction scheme in
alkaline electrolytes has been largely neglected.[13]
Given that H2O is a stronger proton donor than OH in
alkaline solutions,[12c,13,14] and assuming that all intermediates
are adsorbed to the surface, the elementary steps (R1)–(R9)
described in Table 1 can be categorized into three possible
mechanisms:
I. The dissociative mechanism involves the dissociation of
O2 (R1), followed by the hydration of adsorbed O to OH
(R3).
II. The associative mechanism is initiated by the hydration
of O2 to form HO2 (R2), followed by HO2 dissociation
into OH (R4) or its hydration to H2O2 (R5). Finally, OH
is formed by H2O2 decomposition (R6).
III. Reactions related to direct water dissociation into H and
OH (R7), followed by H insertion into O2 (R8) and HO2
(R9).
Table 1: Reaction barriers Ea (eV) and reaction energies DE (eV) of ORR
reactions on B-doped Pd and Pd(111) surfaces.
Reactions
Pd(111)
B-doped
Pd (111)
Ea
Ea
DE
DE
(R1)
(R2)
(R3)
(R4)
(R5)
(R6)
(R7)
(R8)
(R9)
O2Ð2O
0.72
0.68
0.37
0.02
0.65
0.20
1.03
0.68
0.43
À1.46
0.65
0.27
0.63
0.71
0.17
0.08
0.56
0.19
1.22
0.55
0.62
À1.18
0.64
0.02
O2 +H2OÐHO2 +OH
O+H2OÐOH+OH
HO2ÐOH+O
Figure 1. B-doped Pd surface and the reaction path of O2 dissociation.
a),b) Side view (a) and top view (b) of the Pd–B surface. The config-
urations of the Pd atoms and binding site notations are explained in
the footnote of Table S1. c)–f) Two adjacent images represent a path
on a surface, c),d) on Pd(111) and e),f) on Pd–B. c) and e) show the
structure of the reactant and d) and f) show the geometry of the
transition state. Pd cyan, B pink, O red.
À1.84
À1.58
HO2 +H2OÐH2O2 +OH
H2O2Ð2OH
0.56
0.41
À2.32
0.35
0.01
À0.25
À1.74
0.42
H2OÐH+OH
O2 +HÐHO2
À0.26
HO2 +HÐH2O2
À0.29
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increase in the catalytic activity of Pd–B relative
0:63e
kT
0:68e V
kT
to pure Pd at 258C (eÀ V=eÀ
¼ 7). Consider-
ing the slowdown on Pd(111) by the competitive
mechanism I, B-doping of the Pd should acceler-
ate the reaction kinetics by an order of magnitude.
From a thermodynamic viewpoint, the higher
binding energies of the reactant should lower the
dissociation barrier.[10,16] However, O2 dissociation
proceeds with a smaller barrier on Pd–B than on
Pd (0.63 eV vs. 0.72 eV), for reasons that require
further investigation. In the initial state, the O–O
˚
˚
distance on Pd–B is 1.370 A (Figure 1e), 0.003 A
shorter than on Pd(111) (Figure 1c). This implies
that O2 interacts more strongly with pure Pd than
with Pd–B.
À
From the reactant to transition states, the O
O bond is elongated by 0.630 A on the Pd surface
˚
but by only 0.462 Š on the Pd–B surface. This
shorter elongation implies that the transition state
occurs earlier on Pd–B than that on Pd, thus the
barrier is reduced on Pd–B and kinetically facil-
itating the dissociative adsorption of O2.
According to the above-described theoretical
model, B-doped Pd nanoparticles should exhibit
higher catalytic ORR activity than pure Pd nano-
particles. We now evaluate this activity using B-
doped Pd nanoparticles synthesized by a previ-
ously developed stepwise electroless deposition
process.[8] In this process, a glassy carbon electrode
is sequentially dipped into two separate solutions,
one containing DMAB (reducing agent, the
source of B), and the other containing PdCl2 (Pd
ion source). Each dipping is held for a specified
time (see the supporting information for details).
Figure 2. Physical properties of B-doped Pd (Pd–B) nanoparticles. a) Field emission
scanning electron microscopy image of Pd–B nanoparticles. b) X-ray diffraction
(XRD) patterns, and c) X-ray photoelectron spectra (XPS) of Pd–B, Pd–N2H4,
commercial Pd/C, and Pd plate. The calibration for XPS was carried out using C1s
binding energy at 284.5 eV.[15] Gray dotted lines in (b) and (c) indicate the position/
change in position of a peak relative to its position in the trace of Pd plate.
This deposition process formed well-dispersed crystalline Pd
nanoparticles (Figure 2a and Figure S1). Boron was actually
co-deposited on the Pd nanoparticles during the electroless
deposition, as evidenced by the Inductively coupled plasma
mass spectrometry (ICP-MS) data of the obtained Pd nano-
particles (B content = 11.8 Æ 0.7 at%). Furthermore, the
XRD profile supports the fact that the co-deposited B is
indeed embedded in the Pd lattice (Figure 2b). Every peak in
the Pd–B profile shifts to a lower angle than in the Pd–N2H4,
Pd/C, and pure Pd bulk profiles, suggesting that the Pd lattice
has expanded due to the B doping as also confirmed by HR-
TEM images (Figure S1).
As predicted by the theoretical model, the synthesized
Pd–B nanoparticles catalyst indeed exhibits higher catalytic
activity than Pd/C (Figure 3a; Table 2). The positive half-
wave potential (0.86 V) is higher in the B-doped Pd particle
catalyst than in Pd–N2H4 (0.83 V) and Pd/C (0.83 V). The
specific activity of the B-doped Pd particles (4.13 mAcmÀ2),
defined as the kinetic current normalized by the electro-
chemical active surface area (ECSA), is over 1.4 times,
2.5 times, and 8.8 times higher than those of Pd–N2H4, Pd/C
and Pt/C, respectively (Table 2). Moreover, the mass activity,
defined as the kinetic current normalized by the Pd load, is
1.7 times, 14 times, and 35 times higher in the B-doped Pd
nanoparticles than those of Pd–N2H4, Pd/C, and Pt/C,
Figure 3. Electrochemical performances of ORR catalysts, Pd–B, Pd–
N2H4, and Pd/C nanoparticles. a) ORR polarization curves were
measured in 0.1m O2-saturated KOH solution at a scan rate of
10 mVsÀ1. Rotational speed: 1600 rpm.[8,19] b) Chronoamperometry
responses were evaluated as percentage of initial current density. The
measurements were performed in 0.1m O2-saturated KOH solution at
a potential of 0.67 V versus the reversible hydrogen electrode (RHE).
Rotational speed: 200 rpm.[8,20]
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Table 2: Half-wave potentials and specific and mass activities of Pd and
this catalyst is nearly optimal. The ORR activity on the B-
doped Pd catalyst is enhanced by the large site dependence of
the binding energy. According to the mechanistic analysis, the
B-doped Pd surface presents a lower barrier to O2 dissocia-
tion than pure Pd, suggesting better catalytic activity of Pd–B
than pure Pd. In fact, in terms of the half-wave potential,
specific and mass activities, and durability, the catalytic
activity of the synthesized Pd–B nanoparticles is superior to
the pure Pd. In addition, as determined from the modelled B-
doped Pd surface and the experimental XPS data of the
synthesized Pd–B, the SCLS becomes positive when Pd atoms
bond to B, implying electron transfer from Pd to B atoms.
Such additional electrons on the Pd–B surface would improve
the catalytic activity. Notably, the stepwise electroless depo-
sition (by which we synthesized the Pd–B) is remarkably
facile, requires no harsh conditions (high temperature and
vacuum)- and is completed within a short time (in the present
study, the total synthesis time was less than 5 min). In
addition, by changing the metal-ion source and reducing
agent, we can synthesize a variety of pure metal and alloy
catalysts with different microstructures.[8] The stepwise elec-
troless deposition is suitable for rational tailoring and design
of catalyst materials.
Pt catalysts.[a]
Catalyst
Half-wave
potential
[V]
Specific
activity
Mass
activity
[mAcmÀ2
]
[mAmgÀ1
]
Pd–B
0.86
0.83
0.83
0.74
4.13
2.87
1.58
0.47
2.38
1.33
0.17
0.068
[8]
Pd–N2H4
Pd/C[8]
Pt/C[8]
[a] Specific and mass activities were calculated as described elsewhere.
The current was measured at 0.85 V vs. RHE at a rotational speed of
1600 rpm. The values are averaged from several experiments.[8,19a]
respectively. Considering the market price of Pd is lower than
that of Pt,[17] Pd–B catalyst is more than 35 times less costly
than Pt. The high catalytic activity of Pd–B is also maintained
over a long time. In the durability test, Pd–B maintained over
75% of its initial catalytic activity, whereas the catalytic
activity of Pd–N2H4 and Pd/C rapidly reduced to 53% and
47%, respectively, within 10000 s (Figure 3b).
On the modelled Pd–B surface, Pd atoms are either
directly bonded to B atoms or have no B neighbors. In
Figure 1a,b, these two situations are labelled PdB and PdnB,
respectively. Table S2 lists the surface core level shifts
(SCLSs) of the Pd atoms in each case for the upper two
layers of Pd–B together with the results for Pd(111). Boron-
doping dramatically changes the SCLSs of the PdB state in
both layers. In fact, the SCLSs switch sign, changing to 0.07
and 0.16 eV from À0.46 and À0.28 eV in the top and second
layers of Pd(111), respectively. In contrast, the SCLS of PdnB
scarcely changes from that of Pd(111) in the first layer but
becomes more negative in the second layer. Considering the
connection between the core level shift and charge transfer,[18]
we conclude that the PdB atoms in the top two layers lose
electrons. Consequently, the electron density increases
around B and PdnB atoms in the second layer. This conclusion
is supported by the experimental XPS results of the synthe-
sized Pd–B nanoparticles (Figure 2c). Since the B composi-
tion in the nanoparticles is 12 at%, most of the Pd atoms in
the nanoparticles are the type of PdnB, as reflected in the XPS
profile of Pd–B. The binding energy of Pd–B is lowered
relative to pure Pd (Figure 2c), consistent with the computa-
tional results. Again, most of the surface atoms of the
synthesized Pd–B nanoparticles exhibit higher electron
density than those observed in the pure Pd nanoparticles.
Such excess electrons would effectively weaken the bonding
between O and Pd atoms, reducing the binding energy
(Table S1) and boosting the catalytic activity of Pd–B
(Table 1, Figure 3).
Acknowledgements
This study was financially supported by the Nanyang Assis-
tant Professorship (NAP, M4080740) and Singapore Ministry
of Education (MOE2013-T2-2-049). We appreciate Ms.
Koh Joo Luang, Ms. Yong Mei Yoke, Mr. Leong Kwok
Phui, and Ms. Lee Pui Mun at Material/Biological & Chem-
ical Laboratories at MAE, NTU, for their continuous support
and effort to set up and maintain an excellent experimental
environment. We thank Dr. Ming Lin at IMRE, A*STAR, for
his help in obtaining the HR-TEM images.
Keywords: density functional theory · electrocatalysts ·
nanoparticles · oxygen reduction reaction · palladium
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6842–6847
Angew. Chem. 2016, 128, 6956–6961
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