These schematic illustrations and corresponding transmission electron
microscope images show the evolution of platinum/nickel from polyhedra
to dodecahedron nanoframes with platinum-enriched skin. Credit: Image courtesy of DOE/Lawrence Berkeley National Laboratory
A big step in the development of next-generation fuel cells and
water-alkali electrolyzers has been achieved with the discovery of a new
class of bimetallic nanocatalysts that are an order of magnitude higher
in activity than the target set by the U.S. Department of Energy (DOE)
for 2017. The new catalysts, hollow polyhedral nanoframes of platinum
and nickel, feature a three-dimensional catalytic surface activity that
makes them significantly more efficient and far less expensive than the
best platinum catalysts used in today's fuel cells and alkaline
electrolyzers. This research was a collaborative effort between DOE's Lawrence Berkeley
National Laboratory (Berkeley Lab) and Argonne National Laboratory
(ANL)."We report the synthesis of a highly active and durable class of
electrocatalysts by exploiting the structural evolution of
platinum/nickel bimetallic nanocrystals," says Peidong Yang, a chemist
with Berkeley Lab's Materials Sciences Division, who led the discovery
of these new catalysts. "Our catalysts feature a unique hollow nanoframe
structure with three-dimensional platinum-rich surfaces accessible for
catalytic reactions. By greatly reducing the amount of platinum needed
for oxygen reduction and hydrogen evolution reactions, our new class of
nanocatalysts should lead to the design of next-generation catalysts
with greatly reduced cost but significantly enhanced activities."
Yang, who also holds appointments with the University of California
(UC) Berkeley and the Kavli Energy NanoSciences Institute at Berkeley,
is one of the corresponding authors of a paper in Science that
describes this research. The paper is titled "Highly Crystalline
Multimetallic Nanoframes with Three-Dimensional Electrocatalytic
Surfaces." The other corresponding author is Vojislav Stamenkovic, a
chemist with ANL's Materials Science Division, who led the testing of
this new class of electrocatalysts.
Fuel cells and electrolyzers
can help meet the ever-increasing demands for electrical power while
substantially reducing the emission of carbon and other atmospheric
pollutants. These technologies are based on either the oxygen reduction
reaction (fuel cells), or the hydrogen evolution reaction
(electrolyzers). Currently, the best electrocatalyst for both reactions
consists of platinum nanoparticles dispersed on carbon. Though quite
effective, the high cost and limited availability of platinum makes
large-scale use of this approach a major challenge for both stationary
and portable electrochemical applications.
"Intense research
efforts have been focused on developing high-performance
electrocatalysts with minimal precious metal content and cost," Yang
says. "In an earlier study, the ANL scientists showed that forming a
nano-segregated platinum skin over a bulk single-crystal platinum/nickel
alloy enhances catalytic activity but the materials cannot be easily
integrated into electrochemical devices. We needed to be able to
reproduce the outstanding catalytic performance of these materials in
nanoparticulates that offered high surface areas."
Yang and his
colleagues at Berkeley accomplished this by transforming solid
polyhedral bimetallic nanoparticles of platinum and nickel into hollow
nanoframes. The solid polyhedral nanoparticles are synthesized in the
reagent oleylamine, then soaked in a solvent, such as hexane or
chloroform, for either two weeks at room temperature, or for 12 hours at
120 degrees Celsius. The solvent, with its dissolved oxygen, causes a
natural interior erosion to take place that results in a hollow
dodecahedron nanoframe. Annealing these dodecahedron nanoframes in argon
gas creates a platinum skin on the nanoframe surfaces.
"In
contrast to other synthesis procedures for hollow nanostructures that
involve corrosion induced by harsh oxidizing agents or applied
potential, our method proceeds spontaneously in air," Yang says. "The
open structure of our platinum/nickel nanoframes addresses some of the
major design criteria for advanced nanoscale electrocatalysts,
including, high surface-to-volume ratio, 3-D surface molecular
accessibility, and significantly reduced precious metal utilization."
In
electrocatalytic performance tests at ANL, the platinum/nickel
nanoframes when encapsulated in an ionic liquid exhibited a 36-fold
enhancement in mass activity and 22-fold enhancement in specific
activity compared with platinum nanoparticles dispersed on carbon for
the oxygen reduction reaction. These nanoframe electrocatalysts,
modified by electrochemically deposited nickel hydroxide, were also
tested for the hydrogen evolution reaction and showed that catalytic
activity was enhanced by an order-of-magnitude over platinum/carbon
catalysts.
"Our results demonstrate the beneficial effects of the
hollow nanoframe's open architecture and surface compositional
profile," Yang says. "Our technique for making these hollow nanoframes
can be readily applied to other multimetallic electrocatalysts or gas
phase catalysts. I am quite optimistic about its commercial viability."
Other co-authors of the Science paper
in addition to Yang and Stamenkovic are Chen Chen, Yijin Kang, Ziyang
Huo, Zhongwei Zhu, Wenyu Huang, Huolin Xin, Joshua Snyder, Dongguo Li,
Jeffrey Herron, Manos Mavrikakis, Miaofang Chi, Karren More, Yadong Li,
Nenad Markovic and Gabor Somorjai.
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