Atomic layer deposition synthesis of platinum-tungsten carbide core-shell catalysts for the hydrogen evolution reaction

Article abstract of DOI:10.1039/c1cc15812k

Pt was deposited onto tungsten carbide powders using atomic layer deposition to produce core-shell catalysts for the hydrogen evolution reaction (HER). The Pt loading on these catalysts was reduced nearly ten-fold compared to a bulk Pt catalyst while equivalent HER activities were observed.

Full text of DOI:10.1039/c1cc15812k

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COMMUNICATION
Atomic layer deposition synthesis of platinum–tungsten carbide core–shell
catalysts for the hydrogen evolution reactionw
Irene J. Hsu,a Yannick C. Kimmel,a Xiaoqiang Jiang,b Brian G. Willisb and
Jingguang G. Chen*a
Received 20th September 2011, Accepted 17th November 2011
DOI: 10.1039/c1cc15812k
Pt was deposited onto tungsten carbide powders using atomic
layer deposition to produce core–shell catalysts for the hydrogen
evolution reaction (HER). The Pt loading on these catalysts was
reduced nearly ten-fold compared to a bulk Pt catalyst while
equivalent HER activities were observed.
methanol oxidation reaction and the hydrogen evolution
reaction (HER). For example, one ML of Pt deposited onto
a WC foil substrate by physical vapour deposition (PVD) was
previously shown to perform as well as a bulk Pt catalyst for
the HER.7 These exciting results demonstrate that the Pt
loading can be reduced considerably while the surface is still
able to produce activities comparable to bulk Pt catalysts.
These results, however, were produced on thin films, which
are impractical for real world applications that require catalysts
to be in powder form.
Despite the promise of fuel cells, photoelectrochemical cells,
and electrolyzers as clean alternative energy sources, the high
loadings of precious metal electrocatalysts drive up the cost of
these devices and affect their competitiveness with conventional
power sources. Precious metals, notably Pt, are necessary for
the operation of fuel cells and photoelectrochemical cells
because they facilitate the electrochemical reactions at the
electrodes. However, Pt is expensive and scarce. A reduction
in its use is essential to ensure that these clean energy devices
are economically viable. Therefore, there is a need to develop
electrocatalysts with lower Pt loadings that can perform as well
as conventional bulk Pt catalysts.
Synthesis of ML Pt–WC powder catalysts is challenging,
and to date no established methods exist. PVD has been shown
to successfully produce monolayers on planar substrates, but
PVD is a line-of-sight technique, and therefore, not applicable
to particle substrates. Previously, Pt-modified WC particle
catalysts were made using a wet impregnation method, but
were unable to exhibit the same activities for the methanol
oxidation reaction that were observed on thin film catalysts
produced with PVD.10 This confirms that wet impregnation
methods, which are industrially the most common way to
produce catalysts, do not provide sufficient control to produce
a complex system such as ML Pt–WC powder catalysts.
Core–shell particles have been produced for other systems,
but to our knowledge no synthesis method for producing
Pt–WC core–shell particles has been reported. For Pt–Ni or
Pt–Pd, a method using ion exchange of an underpotentially
deposited Cu adlayer has shown some success in producing
Pt monolayers.11,12 However, this is not applicable to WC
because Cu does not form an underpotentially deposited
adlayer.13,14 Other synthesis methods must then be explored
to produce thin Pt layers on WC particles. Atomic layer
deposition (ALD) was determined to be one possibility because
it can produce ultra-thin films with nanometre-level control on
a number of substrates. Furthermore, ALD is not a line-of-sight
technique and has been demonstrated on different substrate
geometries such as thin films, nanotubes, and microspheres.
The objectives of the current communication are (1) to utilize
ALD to produce Pt–WC core–shell catalysts, and (2) to
evaluate whether they show promising HER performance as
predicted from ML Pt–WC thin films. The WC particles were
synthesized using a solid state method detailed previously where
WO3 and carbon black were heated to 1300 1C in the presence
of KCl and a Ni catalyst to promote WC synthesis.15,16
One strategy to lower Pt loadings is to use tungsten mono-
carbide (WC) as a bulk replacement to support one monolayer
(ML) of Pt in a core–shell structure. WC has been found to
possess Pt-like catalytic properties and has been identified as a
promising support material for Pt in fuel cells since Pt has
shown good adhesion on WC.1–6 Additionally, WC is much less
expensive than Pt. However, for electrochemical applications,
the similarities between WC and Pt often deviate at the surface,
making it difficult to use WC as a direct replacement for Pt. It
has been observed that modifying the WC surface with Pt can
produce Pt-like activity while minimizing the amount of Pt
used.2,7–9 An optimal lower limit to the Pt loading is a single
ML that fully covers the WC surface. The ML Pt–WC system
has previously been demonstrated with thin films as model
catalysts for a few electrochemical reactions, such as the
a Department of Chemical Engineering, Center for Catalytic Science
and Technology, University of Delaware, 150 Academy Street,
Newark, DE, USA. E-mail: jgchen@udel.edu;
Fax: +1 302-831-1048; Tel: +1 302-831-0642
b Department of Chemical, Materials, and Biomolecular Engineering,
University of Connecticut, Unit 3222, 191 Auditorium Road, Storrs,
CT, USA. E-mail: bgwillis@engr.uconn.edu; Fax: +1 860-486-2959;
Tel: +1 860-486-9429
w Electronic supplementary information (ESI) available: Detailed
experimental procedures, powder XRD patterns, and electrochemical
characterization. See DOI: 10.1039/c1cc15812k
c
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Chem. Commun., 2012, 48, 1063–1065 1063

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Fig. 2 Tafel plots of WC powder; 5, 10, and 30 ALD Pt–WC
catalysts; and 10 wt% Pt/C produced from HER measurements.
Fig. 1 TEM images of ALD Pt–WC samples. (a) Low-magnification
HAADF-STEM image of ALD Pt–WC. (b) High resolution BF TEM
image of the 10 ALD cycle sample. (c) 5 Pt ALD cycles on WC and
(d) 30 Pt ALD cycles on WC.
electrocatalysts. In the current study, HER measurements for
samples of WC powder; 5, 10 and 30 ALD cycles Pt–WC; and
10 wt% Pt supported on high surface area carbon (Pt/C) are
compared in the form of Tafel plots shown in Fig. 2. The
current is normalized with respect to the geometric surface
area of the electrodes. The trend displayed here is in agreement
with what has previously been observed on thin films. Based
on the previous study, WC is expected to have poor HER
activity.7 This is reflected in the location of its Tafel plot
shown in Fig. 2. On the other hand, Pt is known to have very
good HER activity. When Pt is deposited onto the WC
powder substrates, the HER activity increases.
The resulting WC particles are about 1 micron in diameter on
average, as seen in the high annular angular dark field
(HAADF) STEM image shown in Fig. 1(a). ALD of Pt was
achieved in a flow-through type reactor using a design by
George et al.17 in a quartz tube furnace fitted with a frit to
prevent solids from entering the vacuum foreline. The Pt
precursor molecule used for ALD was trimethyl-methyl-cyclo-
pentadienylplatinum (MeCpPtMe3), which has been studied
extensively for Pt ALD on a variety of oxide surfaces and on
carbon.18–20 Oxygen was used as the co-reactant. Pt ALD has
been demonstrated on WC thin films in a previous article,21
showing that Pt grows by forming small particles on the
WC surface, which eventually coalesce to form a film with
increasing number of ALD cycles.21
As few as 5 Pt ALD cycles on WC increases the HER
activity considerably from unmodified WC, as seen in the
upward shift in its Tafel plot. Furthermore, after only 10 ALD
cycles, the HER activity is nearly equivalent to the Pt/C
catalyst, indicated by their nearly overlapping Tafel plots.
The 30 ALD cycle Tafel plot also falls on the Pt Tafel plot,
showing that the low loading Pt reaches a limiting HER
activity which is equivalent to that of Pt/C. Fig. 2 shows
how little Pt is needed on WC to achieve the same activity as
the Pt/C catalyst.
A similar growth mechanism was observed on the WC
powders as seen in the TEM images shown in Fig. 1. Pt
particles are observed on the WC surface. An example is seen
in Fig. 1(b) showing a high magnification image of the ALD
Pt–WC after 10 cycles. The particles are cubooctahedral
shaped, which is consistent with an fcc metal such as Pt.
Lattice fringes observed on the cubooctahedral particles were
measured and matched with Pt(111) and Pt(200) d-spacings,
identifying these particles as Pt. Fig. 1(c) and (d) show bright
field (BF) TEM images of the ALD Pt–WC samples after 5
and 30 ALD cycles, respectively. The BF TEM images show
that the Pt particle size increases with increasing Pt ALD
cycles. After 5 ALD cycles, the Pt particles are B2 nm in
diameter and well dispersed throughout the WC surface. The
particles grow to be slightly wider after 10 ALD cycles.
Fig. 1(d) shows that the WC surface is essentially covered
with Pt after 30 ALD cycles.
The HER is one of the most fundamental and extensively
studied electrochemical reactions, and is often used as a probe
reaction to evaluate the catalytic activity of Pt and Pt-based
Fig. 3 Log of exchange current density plotted against (a) the
number of Pt ALD cycles and (b) Pt surface area on WC determined
from Cu stripping voltammetry.
c
1064 Chem. Commun., 2012, 48, 1063–1065
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Table 1 Pt wt% loadings for each catalyst and total Pt mass on
electrodes for each HER measurement
should be performed on the ALD Pt/WC and Pt/C powder
catalysts to further understand the electrochemical stability of
these catalysts.
Electrode
Pt wt%
Mass of Pt on electrode/mg
In conclusion, Pt–WC core–shell particles were synthesized
using ALD to deposit thin Pt layers on WC powder substrates
in order to scale up the ML Pt–WC thin film system to a more
industrially relevant powder catalyst. In terms of reducing the
Pt loading while maintaining bulk-like performance, the
Pt–WC core–shell powder catalysts produced HER results
that are equivalent to the ML Pt–WC thin films reported
previously. To our knowledge, no other method has been
demonstrated to produce core–shell Pt–WC catalysts. While
improvements to the synthesis of these catalysts can be made
in making even thinner Pt films and using smaller WC
particles, the positive results observed here are a significant
step towards more efficiently utilizing precious metals in
powder catalysts in electrochemical devices.
5 ALD cycles Pt–WC
10 ALD cycles Pt–WC
30 ALD cycles Pt–WC
Pt/C
0.20
0.49
0.85
10
0.00055
0.0031
0.0055
0.025
The log of the exchange current densities were calculated
from the Tafel parameters and are plotted against Pt ALD
cycles shown in Fig. 3(a) and Pt surface area as determined by
Cu stripping measurements shown in Fig. 3(b). These figures
show that a limiting value for HER activity is reached, which
corresponds to the Pt/C value. The exchange current densities
of the 10 wt% Pt/C catalyst and the WC powders are indicated
by dashed lines. As shown in Fig. 3(a) and (b), increasing the
Pt deposition brings the Pt/WC exchange current density to
the Pt/C value, which is in agreement with the previous study
on planar thin film catalysts.7
We acknowledge financial support from the Department of
Energy (DE-FG02-00ER15104). We thank Ying Liu and
Dr William Mustain for synthesizing and providing the
WC particles.
Furthermore, similar to what was observed with ML
Pt–WC thin film catalysts, Pt/C HER activity is reached as
the Pt coverage on WC increases to an equivalent of 1 ML.
Although an ideal single ML of Pt is not produced on the
WC powder using ALD, the presence of a thin Pt layer on the
WC substrate is sufficient to bring about activities that are
comparable to conventional Pt catalysts. The Pt loadings are
also considerably less than what is typically used in electro-
chemical experiments. The Pt wt% for each of the above-
mentioned ALD Pt–WC catalysts were determined from
atomic absorption spectroscopy (AAS) measurements after
digestion of the powder samples in aqua regia, and are
displayed in Table 1. By comparison, the literature typically
uses Pt loadings of 10–40 wt%. Table 1 also shows the total mass
of Pt loaded onto the electrode for each HER measurement. The
mass of Pt for each of the ALD Pt–WC electrodes is a fraction of
the mass of Pt used for the Pt/C electrode, clearly demonstrating
how the ALD Pt-coated WC catalysts utilize Pt more efficiently
than the bulk Pt catalyst.
Notes and references
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Previous work with ML Pt–WC thin films postulated that
the reasons for the similar HER activities are due to the
similarities in the electronic properties between bulk Pt and
ML Pt on WC. This can be observed when the hydrogen
binding energy (HBE) is used as a chemical descriptor for the
HER reaction. The HBE of WC is very strong, but when a ML
Pt is added on top of it, the HBE value decreases to be very
similar to that of Pt.7 Although more than the ideal one single
ML of Pt is deposited onto WC in this work (Fig. 1), ultra-low
loading Pt thin films are achieved. Additional work should be
devoted to determine ways to make the Pt nucleation more
efficient, which in turn will create smaller particles and thinner
Pt layers on the WC powder. WC was shown here to be a good
core material for Pt for this particular electrochemical reaction.
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before and after HER measurements, which revealed that
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c
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Chem. Commun., 2012, 48, 1063–1065 1065