10 research outputs found
In Situ Transformation of Hydrogen-Evolving CoP Nanoparticles: Toward Efficient Oxygen Evolution Catalysts Bearing Dispersed Morphologies with Co-oxo/hydroxo Molecular Units
Reported herein is elucidation of
a novel Co-based oxygen evolution
catalyst generated in situ from cobalt phosphide (CoP) nanoparticles.
The present CoP nanoparticles, efficient alkaline hydrogen-evolving
materials at the cathode, are revealed to experience unique metamorphosis
upon anodic potential cycling in an alkaline electrolyte, engendering
efficient and robust catalytic environments toward the oxygen evolution
reaction (OER). Our extensive ex situ characterization shows that
the transformed catalyst bears porous and nanoweb-like dispersed morphologies
along with unique microscopic environments mainly consisting of discrete
cobalt-oxo/hydroxo molecular units within a phosphate-enriched amorphous
network. Outstanding OER efficiency is achievable with the activated
catalyst, which is favorably comparable to even a precious iridium
catalyst. A more remarkable feature is its outstanding long-term stability,
superior to iridium and conventional cobalt oxide-based materials.
Twelve-hour bulk electrolysis continuously operating at high current
density is completely tolerable with the present catalyst
RhodiumāTin Binary NanoparticleīøA Strategy to Develop an Alternative Electrocatalyst for Oxygen Reduction
A RhāSn nanoparticle
is achieved by combinatorial approaches
for application as an active and stable electrocatalyst in the oxygen
reduction reaction. Both metallic Rh and metallic Sn exhibit activities
too low to be utilized for electrocatalytic reduction of oxygen. However,
a clean and active Rh surface can be activated by incorporation of
Sn into a Rh nanoparticle through the combined effects of lateral
repulsion, bifunctional mechanism, and electronic modification. The
corrosion-resistant property of Rh contributes to the construction
of a stable catalyst that can be used under harsh fuel cell conditions.
Based on both theoretical and experimental research, RhāSn
nanoparticle designs with inexpensive materials can be a potential
alternative catalyst in terms of the economic feasibility of commercialization
and its facile and simple surfactant-free microwave-assisted synthesis
Origin of the Enhanced Electrocatalysis for Thermally Controlled Nanostructure of Bimetallic Nanoparticles
The thermal annealing process is
a common treatment used after
the preparation step to enhance the electrocatalytic properties of
the oxygen reduction reaction (ORR). The structure of a Pt-based bimetallic
nanoparticle, which is significantly affected by the catalytic properties,
is reconstructed by thermal energy. We investigated the effect of
structural reconstruction induced by thermal annealing on the improvement
of the ORR using various physical and electrochemical methods. We
found that the structural evolution of PtNi nanoparticles, i.e., the
PtāNi ordering with the Pt shell and the surface reorientation
into the (111) facet, is the source of the enhanced ORR activity as
well as electrochemical stability through the thermal annealing. This
result confirms the crucial factors for the ORR properties by the
thermal annealing process and proposes a way to design advanced electrocatalysts
Surface Structures and Electrochemical Activities of PtRu Overlayers on Ir Nanoparticles
PtRu overlayers were deposited on carbon-supported Ir
nanoparticles
with various Pt:Ru compositions. Structural and electrochemical characterizations
were performed using transmission electron microscopy (TEM), X-ray
diffraction, high-resolution powder diffraction (HRPD), X-ray photoelectron
spectroscopy (XPS), cyclic voltammetry (CV), and CO stripping voltammetry.
The PtRu overlayers were selectively deposited on the Ir nanoparticles
with good uniformity of distribution. As a result, the PtRu utilization
of the present samples was higher than that of PtRu/C. The mass-specific
activities for methanol oxidation were also significantly higher.
Single-cell performance using the Pt<sub>2</sub>Ru<sub>1</sub> overlayer
sample as an anode catalyst was slightly higher than that obtained
using commercial PtRu/C despite the fact that the PtRu anode loading
for Pt<sub>2</sub>Ru<sub>1</sub>/Ir/C was only 42% of that of PtRu/C
Design of an Advanced Membrane Electrode Assembly Employing a Double-Layered Cathode for a PEM Fuel Cell
The membrane electrolyte assembly
(MEA) designed in this study utilizes a double-layered cathode: an
inner catalyst layer prepared by a conventional decal transfer method
and an outer catalyst layer directly coated on a gas diffusion layer.
The double-layered structure was used to improve the interfacial contact
between the catalyst layer and membrane, to increase catalyst utilization
and to modify the removal of product water from the cathode. Based
on a series of MEAs with double-layered cathodes with an overall Pt
loading fixed at 0.4 mg cm<sup>ā2</sup> and different ratios
of inner-to-outer Pt loading, the MEA with an inner layer of 0.3 mg
Pt cm<sup>ā2</sup> and an outer layer of 0.1 mg Pt cm<sup>ā2</sup> exhibited the best performance. This performance was better than
that of the conventional single-layered electrode by 13.5% at a current
density of 1.4 A cm<sup>ā2</sup>
Role of Electronic Perturbation in Stability and Activity of Pt-Based Alloy Nanocatalysts for Oxygen Reduction
The design of electrocatalysts for polymer electrolyte
membrane
fuel cells must satsify two equally important fundamental principles:
optimization of electrocatalytic activity and long-term stability
in acid media (pH <1) at high potential (0.8 V). We report here
a solution-based approach to the preparation of Pt-based alloy with
early transition metals and realistic parameters for the stability
and activity of Pt<sub>3</sub>M (M = Y, Zr, Ti, Ni, and Co) nanocatalysts
for oxygen reduction reaction (ORR). The enhanced stability and activity
of Pt-based alloy nanocatalysts in ORR and the relationship between
electronic structure modification and stability were studied by experiment
and DFT calculations. Stability correlates with the d-band fillings
and the heat of alloy formation of Pt<sub>3</sub>M alloys, which in
turn depends on the degree of the electronic perturbation due to alloying.
This concept provides realistic parameters for rational catalyst design
in Pt-based alloy systems
Reversible Surface Segregation of Pt in a Pt<sub>3</sub>Au/C Catalyst and Its Effect on the Oxygen Reduction Reaction
Reversible
surface segregation of Pt in Pt<sub>3</sub>Au/C catalysts
was accomplished through a heat treatment under a CO or Ar atmosphere,
which resulted in surface Pt segregation and reversed segregation,
respectively. The Pt-segregated Pt<sub>3</sub>Au/C exhibited a significantly
improved oxygen reduction reaction (ORR) activity (227 mA/mg<sub>metal</sub>) compared to that of commercial Pt/C (59 mA/mg<sub>metal</sub>).
For the Pt-segregated Pt<sub>3</sub>Au/C, the increased OH-repulsive
properties were validated by a CO bulk oxidation analysis and also
by density functional theory (DFT) calculations. Interestingly, the
DFT calculations revealed that the binding energy for Pt-segregated
Pt<sub>3</sub>Au (111) surfaces was 0.1 eV lower than that for Pt
(111) surfaces, which has been previously reported to exhibit the
optimum OH binding energy for the ORR. Therefore, the reversible surface
segregation is expected to provide a practical way to control the
surface states of PtāAu bimetallic catalysts to enhance ORR
activity. In addition, the Pt-segregated Pt<sub>3</sub>Au/C showed
excellent electrochemical stability, as evidenced by its high-performance
retention (96.4%) after 10ā000 potential cycles, in comparison
to that of Pt/C (55.3%)
Electrocatalytic Effects of Carbon Dissolution in Pd Nanoparticles
Highly dispersed Pd nanoparticles were prepared by borohydride
reduction of PdĀ(acac)<sub>2</sub> in 1,2-propanediol at an elevated
temperature. They were uniformly dispersed on carbon black without
significant aggregation. X-ray diffraction showed that carbons from
the Pd precursor dissolved in Pd, increasing its lattice parameter.
A modified reduction process was tested to remove the carbon impurities.
Carbon removal greatly enhanced catalytic activity toward the oxygen
reduction reaction. It also generated an inconsistency between the
electronic modifications obtained from X-ray photoelectron spectroscopy
and the electrochemical method. CO displacement measurements showed
that the formation of PdāC bonds decreased the
work function of the surface Pd atoms
Understanding Interface between Electrode and Electrolyte: Organic/Inorganic Hybrid Design for Fast Ion Conductivity
Ion transport is an important issue
in electrochemical-based energy conversion and storage devices. Ion
transport at the interface of the electrode and electrolyte is critical
for performance. However, there is little understanding of the interface
phenomena based on ion transport properties. Here, the proton transport
behavior in a Nafion membrane (electrolyte) and that of an ionomer
in the catalyst layer (electrode/electrolyte interface) was investigated
simultaneously by electrochemical impedance spectroscopy. Our study
indicates that the proton transport behavior in the catalyst layer
is different from that in membrane. To elucidate the interface phenomena,
we analyzed the Nafion electrolyte and proton behavior by molecular
dynamics (MD). On the basis of the MD results, we modified the catalyst
with a hybrid of inorganic Pt catalyst and organic 3-mercaptopropionic
acid to promote a positive interfacial reaction between the electrolyte
and electrode, which resulted in improved proton transport and performance
Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction
Demand on the practical synthetic
approach to the high performance
electrocatalyst is rapidly increasing for fuel cell commercialization.
Here we present a synthesis of highly durable and active intermetallic
ordered face-centered tetragonal (fct)-PtFe nanoparticles (NPs) coated
with a ādual purposeā N-doped carbon shell. Ordered
fct-PtFe NPs with the size of only a few nanometers are obtained by
thermal annealing of polydopamine-coated PtFe NPs, and the N-doped
carbon shell that is <i>in situ</i> formed from dopamine
coating could effectively prevent the coalescence of NPs. This carbon
shell also protects the NPs from detachment and agglomeration as well
as dissolution throughout the harsh fuel cell operating conditions.
By controlling the thickness of the shell below 1 nm, we achieved
excellent protection of the NPs as well as high catalytic activity,
as the thin carbon shell is highly permeable for the reactant molecules.
Our ordered fct-PtFe/C nanocatalyst coated with an N-doped carbon
shell shows 11.4 times-higher mass activity and 10.5 times-higher
specific activity than commercial Pt/C catalyst. Moreover, we accomplished
the long-term stability in membrane electrode assembly (MEA) for 100
h without significant activity loss. From <i>in situ</i> XANES, EDS, and first-principles calculations, we confirmed that
an ordered fct-PtFe structure is critical for the long-term stability
of our nanocatalyst. This strategy utilizing an N-doped carbon shell
for obtaining a small ordered-fct PtFe nanocatalyst as well as protecting
the catalyst during fuel cell cycling is expected to open a new simple
and effective route for the commercialization of fuel cells