Angewandte
Communications
Chemie
Electrocatalysis
The Effect of Surface Site Ensembles on the Activity and Selectivity of
Ethanol Electrooxidation by Octahedral PtNiRh Nanoparticles
Nina Erini, Vera Beermann, Martin Gocyla, Manuel Gliech, Marc Heggen, Rafal E. Dunin-
Borkowski, and Peter Strasser*
Abstract: Direct ethanol fuel cells are attractive power sources
based on a biorenewable, high energy-density fuel. Their
efficiency is limited by the lack of active anode materials which
catalyze the breaking of the CꢀC bond coupled to the 12-
The combination of Pt with Rh and Ni in a single-phased
spherical nanoscaled electrocatalyst proved to be highly
[
5]
promising for the EOR. Even higher electrocatalytic
activities and stabilities are to be expected for polyhedral
shapes in comparison to their unshaped counterparts, since
polyhedral surface facets provide better preconditions for the
electron oxidation to CO . We report shape-controlled PtNiRh
2
octahedral ethanol oxidation electrocatalysts with excellent
activity and previously unachieved low onset potentials as low
as 0.1 V vs. RHE, while being highly selective to complete
[
6]
formation of ordered active surface site ensembles. Electro-
catalytic properties and the reactivity of nanoparticles are
directly related to their surface structure and shape. Perfect Pt
(111) planes are known to show no formation of poisonous
CO species, unless they show defects or steps, but earlier
oxidation to CO . Our comprehensive characterization and in
2
situ electrochemical ATR studies suggest that the formation of
a ternary surface site ensemble around the octahedral
Pt Ni Rh nanoparticles plays a crucial mechanistic role for
onset potentials for CO formation in comparison with other
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1
x
2
[
7]
this behavior.
Pt basal planes. In order to elucidate the beneficial shape-
controlling effects for electrocatalysts, a novel electrocatalytic
system comprising octahedral Pt-Ni-Rh with fixed Pt:Ni
ratios and different Rh contents (Pt Ni Rh -oct/C) in the form
T
he conversion of ethanol in direct ethanol fuel cells
(
DEFCs) directly into electrical energy has been widely
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x
[1]
examined in recent years. Ethanol is considered a green fuel
since it can be produced from biomass and offers high
volumetric and gravimetric energy density (8 kWhkg ), good
energy efficiency, and easy handling, storage and transporta-
of supported nanoparticles with {111} facets were prepared
using a wet-chemical approach with metal carbonyls present
during the reduction process and a mixture of oleylamine and
oleic acid as solvents, where x was varied between the
equivalent of 1 and 6 atomic %. The bulk composition of the
resulting PtNiRh-1/C, PtNiRh-3/C and PtNiRh-6/C was
controlled by adjusting the initial Pt, Ni and Rh precursor
ratios. The high Pt content was chosen to ensure sufficient
active surface site formation and to determine the influence
of changes in the minimal amounts of Rh in terms of
selectivity to different ethanol oxidation pathways. The bulk
composition and metal loading, which were determined by
inductively coupled plasma (ICP) optical spectroscopy,
proved to be close to the desired nominal values (see
Table S1 in the Supporting Information). Transmission elec-
tron microscopy (TEM) images of the electrocatalysts (Fig-
ure S1) show that in all three samples the nanoparticles were
well distributed across the carbon support and largely regular
octahedral in shape, enclosed by eight {111} facets. Mean edge
lengths, which were estimated from TEM and derived edge
length histograms inserted in the TEM images in Figure S1
and listed in Table S1, vary in a narrow range from 7.2 ꢁ
1.1 nm to 8.0 ꢁ 1.0 nm. The higher magnification images
reveal a lattice spacing of 0.23 nm, which can be attributed
to Pt fcc (111) in all samples. An additional lattice spacing of
ꢀ
1
[2]
tion, in contrast to gaseous fuels. The ethanol oxidation
reaction (EOR), however, is often incomplete due to
difficulties in CꢀC bond cleavage, resulting in a number of
byproducts other than CO . Alkaline medium in the fuel cell
2
presents several advantages when compared to acidic
medium, like faster kinetics for the oxygen reduction on the
cathodic counterpart and a broader range of non-noble co-
[
3]
catalysts. Alloying Pt with highly oxophilic transition metals
has been a promising strategy to modify the electrocatalytic
surface properties of Pt in order to supply active oxygen-
containing species, such as OH, which readily oxidize
adsorbed molecular fragments while reducing the cost of
[4]
the catalyst considerably.
[
*] Dr. N. Erini, V. Beermann, M. Gliech, Prof. P. Strasser
The Electrochemical Energy, Catalysis, and Materials Science Labo-
ratory, Department of Chemistry, Chemical Engineering Division,
Technical University Berlin
10623 Berlin (Germany)
E-mail: pstrasser@tu-berlin.de
M. Gocyla, Dr. M. Heggen, Dr. R. E. Dunin-Borkowski
Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons,
Forschungszentrum Juelich GmbH
0
.19 nm in samples PtNiRh-1/C and PtNiRh-6/C can be
attributed to Pt fcc (200).
Figure 1a–c shows atomic-scale high angle annular dark
field scanning TEM (HAADF/STEM) images of the different
52425 Juelich (Germany)
Prof. P. Strasser
Ertl Center for Electrochemistry and Catalysis, Gwangju Institute of
Science and Technology
Pt Ni Rh -oct/C catalyst nanoparticles after the first EOR
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Gwangju 500-712 (South Korea)
cycle, approximately oriented along h110i, with octahedral
morphologies. Figure 1d–i shows corresponding energy dis-
persive X-ray spectroscopy (EDX) composition maps. For all
Angew. Chem. Int. Ed. 2017, 56, 1 – 8
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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