High-precision electrochemistry: The new gold standard in fuel cell
catalyst development

Date:
September 10, 2020
Source:
DOE/Argonne National Laboratory
Summary:
Scientists have made a pivotal discovery that could extend the
lifetime of fuel cells that power electric vehicles by eliminating
the dissolution of platinum catalysts.



FULL STORY ========================================================================== Vehicles powered by polymer electrolyte membrane fuel cells (PEMFCs) are energy-efficient and eco-friendly, but despite increasing public interest
in PEMFC-powered transportation, current performance of materials that
are used in fuel cells limits their widespread commercialization.


========================================================================== Scientists at the U.S. Department of Energy's (DOE) Argonne National
Laboratory led a team to investigate reactions in PEMFCs, and their
discoveries informed the creation of technology that could bring fuel
cells one step closer to realizing their full market potential.

PEMFCs rely on hydrogen as a fuel, which is oxidized on the cell's anode
side through a hydrogen oxidation reaction, while oxygen from the air
is used for an oxygen reduction reaction (ORR) at the cathode. Through
these processes, fuel cells produce electricity to power electric motors
in vehicles and other applications, emitting water as the only by-product.

Platinum-based, nano-sized particles are the most effective materials
for promoting reactions in fuel cells, including the ORR in the
cathode. However, in addition to their high cost, platinum nanoparticles
suffer from gradual degradation, especially in the cathode, which limits catalytic performance and reduces the lifetime of the fuel cell.

The research team, which included DOE's Oak Ridge National Laboratory and several university partners, used a novel approach to examine dissolution processes of platinum at the atomic and molecular level. The investigation enabled them to identify the degradation mechanism during the cathodic
ORR, and the insights guided the design of a nanocatalyst that uses gold
to eliminate platinum dissolution.

"The dissolution of platinum occurs at the atomic and molecular scale
during exposure to the highly corrosive environment in fuel cells," said Vojislav Stamenkovic, a senior scientist and group leader for the Energy Conversion and Storage group in Argonne's Materials Science Division
(MSD). "This material degradation affects the fuel cell's long-term
operations, presenting an obstacle for fuel cell implementation
in transportation, specifically in heavy duty applications such as
long-haul trucks." Starting small


==========================================================================
The scientists used a range of customized characterization tools to
investigate the dissolution of well-defined platinum structures in single-crystal surfaces, thin films and nanoparticles.

"We have developed capabilities to observe processes at the atomic
scale to understand the mechanisms responsible for dissolution and to
identify the conditions under which it occurs," said Pietro Papa Lopes,
a scientist in Argonne's MSD and first author on the study. "Then we implemented this knowledge into material design to mitigate dissolution
and increase durability." The team studied the nature of dissolution
at the fundamental level using surface-specific tools, electrochemical
methods, inductively coupled plasma mass spectrometry, computational
modeling and atomic force, scanning tunneling and high-resolution
transmission microscopies.

In addition, the scientists relied on a high-precision synthesis approach
to create structures with well-defined physical and chemical properties, ensuring that the relationships between structure and stability discovered
from studying 2D surfaces were carried over to the 3D nanoparticles
they produced.

"We performed these studies -- from single crystals, to thin films, to nanoparticles -- which showed us how to synthesize platinum catalysts
to increase durability," said Lopes, "and by looking at these different materials, we also identified strategies for using gold to protect
the platinum." Going for gold


==========================================================================
As the scientists uncovered the fundamental nature of dissolution by
observing its occurrence in several testbed scenarios, the team used
the knowledge to mitigate dissolution with the addition of gold.

The researchers used transmission electron microscopy capabilities at
Argonne's Center for Nanoscale Materials and at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory -- both DOE Office
of Science User Facilities -- to image platinum nanoparticles after
synthesis and before and after operation. This technique allowed the
scientists to compare the stability of the nanoparticles with and without incorporated gold.

The team found that controlled placement of gold in the core promotes the arrangement of platinum in an optimal surface structure that grants high stability. In addition, gold was selectively deposited on the surface to protect specific sites that the team identified as particularly vulnerable
for dissolution. This strategy eliminates dissolution of platinum from
even the smallest nanoparticles used in this study by keeping platinum
atoms attached to the sites where they can still effectively catalyze
the ORR.

Atomic-level understanding Understanding the mechanisms behind
dissolution at the atomic level is essential to uncovering the correlation between platinum loss, surface structure and size and ratio of platinum nanoparticles, and determining how these relationships affect long-term operation.

"The novel part of this research is resolving the mechanisms and
fully mitigating platinum dissolution by material design at different
scales, from single crystals and thin films to nanoparticles," said Stamenkovic. "It's the insights we gained in conjunction with the design
and synthesis of a nanomaterial that addresses durability issues in fuel
cells, as well as the ability to delineate and quantify dissolution of
platinum catalyst from other processes that contribute to fuel cell
performance decay." The team is also developing a predictive aging
algorithm to assess the long- term durability of the platinum-based nanoparticles and found a 30-fold improvement in durability compared to nanoparticles without gold.


========================================================================== Story Source: Materials provided by
DOE/Argonne_National_Laboratory. Original written by Savannah
Mitchem. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Pietro P. Lopes, Dongguo Li, Haifeng Lv, Chao Wang, Dusan Tripkovic,
Yisi
Zhu, Roberto Schimmenti, Hideo Daimon, Yijin Kang, Joshua Snyder,
Nigel Becknell, Karren L. More, Dusan Strmcnik, Nenad M. Markovic,
Manos Mavrikakis, Vojislav R. Stamenkovic. Eliminating dissolution
of platinum- based electrocatalysts at the atomic scale. Nature
Materials, 2020; DOI: 10.1038/s41563-020-0735-3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200910150340.htm

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