5 research outputs found
Tailoring the Electronic Structure of Nanoelectrocatalysts Induced by a Surface-Capping Organic Molecule for the Oxygen Reduction Reaction
Capping organic molecules, including
oleylamine, strongly adsorbed
onto Pt nanoparticles during preparation steps are considered undesirable
species for the oxygen reduction reaction due to decreasing electrochemical
active sites. However, we found that a small amount of oleylamine
modified platinum nanoparticles showed significant enhancement of
the electrochemical activity of the oxygen reduction reaction, even
with the loss of the electrochemically active surface area. The enhancement
was correlated with the downshift of the frontier d-band structure
of platinum and the retardation of competitively adsorbed species.
These results suggest that a capping organic molecule modified electrode
can be a strategy to design an advanced electrocatalyst by modification
of electronic structures
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
Pt–Ni 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
High-Performance Hybrid Catalyst with Selectively Functionalized Carbon by Temperature-Directed Switchable Polymer
Carbon-supported Pt (Pt/C) catalyst
was selectively functionalized
with thermally responsive poly(<i>N</i>-isopropylacrylamide)
(PNIPAM) to improve water transport in the cathode of proton exchange
membrane fuel cell (PEMFC). Amine-terminated PNIPAM selectively reacted
with the functional group of −COOH on carbon surfaces of Pt/C
via the amide reaction by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
(EDC) as a catalyst. Pt surfaces of Pt/C were intact throughout the
carbon surface functionalization, and the carbon surface property
could be thermally changed. The PNIPAM-functionalized Pt/C was well-dispersed,
because of its hydrophilic surface property at room temperature during
the catalyst ink preparation. In sharp contrast, when PEMFC was operated
at 70 °C, PNIPAM-coated carbon surface of Pt/C became hydrophobic,
which resulted in a decrease in water flooding in the cathode electrode.
Because of the switched wetting property of the carbon surface, PEMFC
with PNIPAM-functionalized Pt/C catalyst in the cathode showed high
performance in the high current density region. To explain the enhanced
water transport, we proposed a simple index as the ratio of systematic
pressure (driving force) and retention force. The synthetic method
presented here will provide a new insight into various energy device
applications using organic and inorganic composite materials and functional
polymers
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 “dual purpose” 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