2 research outputs found
Understanding and Controlling Nanoporosity Formation for Improving the Stability of Bimetallic Fuel Cell Catalysts
Nanoporosity is a frequently reported phenomenon in bimetallic
particle ensembles used as electrocatalysts for the oxygen reduction
reaction (ORR) in fuel cells. It is generally considered a favorable
characteristic, because it increases the catalytically active surface
area. However, the effect of nanoporosity on the intrinsic activity
and stability of a nanoparticle electrocatalyst has remained unclear.
Here, we present a facile atmosphere-controlled acid leaching technique
to control the formation of nanoporosity in Pt–Ni bimetallic
nanoparticles. By statistical analysis of particle size, composition,
nanoporosity, and atomic-scale core–shell fine structures before
and after electrochemical stability test, we uncover that nanoporosity
formation in particles larger than ca. 10 nm is intrinsically tied
to a drastic dissolution of Ni and, as a result of this, a rapid drop
in intrinsic catalytic activity during ORR testing, translating into
severe catalyst performance degradation. In contrast, O<sub>2</sub>-free acid leaching enabled the suppression of nanoporosity resulting
in more solid core–shell particle architectures with thin Pt-enriched
shells; surprisingly, such particles maintained high intrinsic activity
and improved catalytic durability under otherwise identical ORR tests.
On the basis of these findings, we suggest that catalytic stability
could further improve by controlling the particle size below ca. 10
nm to avoid nanoporosity. Our findings provide an explanation for
the degradation of bimetallic particle ensembles and show an easy
to implement pathway toward more durable fuel cell cathode catalysts
Evolution of Structure and Activity of Alloy Electrocatalysts during Electrochemical Cycles: Combined Activity, Stability, and Modeling Analysis of PtIrCo(7:1:7) and Comparison with PtCo(1:1)
This
study explores the changes in bulk composition/structure and
oxygen reduction activity of two alloys, Pt<sub>7</sub>IrCo<sub>7</sub> and PtCo, caused by Co leaching during electrochemical cycles and
as a result of membrane electrode assembly (MEA) fabrication procedures.
Exposure to liquid electrolyte and electrochemical cycles in a rotating
disc electrode (RDE) environment resulted in substantial Co loss and
no stabilization from the low levels of Ir used in the ternary material.
The true composition of the ternary material was determined as Pt<sub>8</sub>IrCo<sub>3</sub> following initial exposure to 0.1 M HClO<sub>4</sub> (before cycling) and Pt<sub>11</sub>IrCo<sub>4</sub> after
5000 cycles. Density functional theory (DFT) modeling of the cycled
catalyst compositions indicated that structures with Pt-rich upper
layers would show the highest stability; however, addition of 0.25
ML oxygen adsorption favored Co segregation from second and third
atomic layers. The high initial activities (>0.44A/mgPt) achieved
in the RDE environment decreased with cycles and were not reproduced
in MEAs. X-ray diffraction (XRD) analysis revealed a measurable increase
in lattice parameter caused by the MEA preparation procedure, consistent
with Co (and some Ir) leaching into the ionomer phase and relaxation
of the lattice. MEA fabrication procedures and cycling in 1 M H<sub>2</sub>SO<sub>4</sub> at 80<sup>â—¦</sup>C showed greater changes
to catalyst structure and increased Ir and Co loss compared to exposing
the catalyst to RDE like conditions (0.1 M HClO<sub>4</sub>, RT) explaining
the observed discrepancy in activity between RDE and MEA