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New core-shell bi-layer nanocatalyst tolerant to CO; potential for low-temperature fuel cells with reformates
21 September 2013
Researchers at Brookhaven National Laboratory have created a high-performing bi-layer durable nanocatalyst that is tolerant to carbon monoxide, a catalyst-poisoning impurity in hydrogen derived from natural gas. The novel core-shell structure—ruthenium coated with platinum—resists damage from carbon monoxide as it drives the energetic reactions central to electric vehicle fuel cells and similar technologies.
The single crystalline Ru cores with well-defined Pt bilayer shells address the issues in using a dissolution-prone metal, such as ruthenium, to alleviate carbon monoxide poisoning, and thereby open the door for commercialization of low-temperature fuel cells that can use inexpensive reformates (H2 with CO impurity) as the fuel, the authors noted. Their paper is published in the journal Nature Communications.
These nanoparticles exhibit perfect atomic ordering in both the ruthenium and platinum, overcoming structural defects that previously crippled carbon monoxide-tolerant catalysts. Our highly scalable, green synthesis method, as revealed by atomic-scale imaging techniques, opens new and exciting possibilities for catalysis and sustainability.—study coauthor Jia Wang
Platinum performs exceptionally well with pure hydrogen fuel as a fuel cell catalyst, but the high cost and rarity of the metal impedes its widespread deployment. By coating less expensive metals with thin layers of platinum atoms, however, scientists can retain reactivity while driving down costs and creating core-shell structures with superior performance parameters.
The carbon monoxide impurities in hydrogen formed from natural gas present a challenge because they deactivate most platinum catalysts. Ruthenium—less expensive than platinum—promotes carbon monoxide tolerance, but is more prone to dissolution during fuel cells’ startup/shutdowns, causing gradual performance decay.
The researchers set out to protect ruthenium cores from dissolution with complete platinum shells just one or two atoms thick. Doubts existed about whether or not a highly ordered ruthenium core was even possible with a platinum shell—previously synthesized nanoparticles exhibited a weakened crystal structure in the ruthenium.
Luckily, we found that the loss of ruthenium structure was due to defect-mediated interlayer diffusion, which is avoidable. By eliminating any lattice defects in ruthenium nanoparticles before adding platinum, we preserved the crucial, discrete atomic structure of each element.—Jia Wang
The scalable and inexpensive synthesis method uses ethanol as the reductant to fabricate the nanoparticle core and shell. The process requires no other organic agents or metal templates.
Simply adjusting temperature, water, and acidity of the solutions gave us complete control over the process and yielded remarkably consistent ruthenium nanoparticle size and uniform platinum coating. This simplicity offers high reproducibility and scalability, and it demonstrates the clear commercial potential of our method.—Radoslav Adzic, co-author
Scientists at Brookhaven Lab’s National Synchrotron Light Source (NSLS) revealed the atomic density, distribution, and uniformity of the metals in the nanocatalysts using x-ray diffraction, a technique in which high-frequency light scatters and bends after interacting with individual atoms. The collaboration also used a scanning transmission electron microscope (STEM) at Brookhaven’s Center for Functional Nanomaterials (CFN) to pinpoint the different sub-nanometer atomic patterns. With this instrument, a focused beam of electrons bombarded the particles, creating a map of both the core and shell structures.
Determining the ideal functional configuration for the core and shell also required the use of the CFN’s expertise in computational science. With density functional theory (DFT) calculations, the computer helps identify the most energetically stable platinum-ruthenium structure.
Ballard Power Systems independently evaluated the performance of the new core-shell nanocatalysts. Beyond testing the low-platinum catalysts’ high activity in pure hydrogen, Ballard looked specifically at the resistance to carbon monoxide present in impure hydrogen gas and the dissolution resistance during startup/shutdown cycles. The bilayer nanocatalyst exhibited high durability and enhanced carbon monoxide tolerance—the combination enables the use of impure hydrogen without much loss in efficiency or increase in catalyst cost.
The nanocatalyst also performed well in producing hydrogen gas through the hydrogen evolution reaction, leading to another industrial partnership. Proton Onsite, a company specializing in splitting hydrogen from water and other similar processes, has completed feasibility tests for deploying the technology in their production of water electrolyzers, which will now require about 98% less platinum.
Water electrolyzers are already on the market, so this nanocatalyst can deploy quickly. When hydrogen fuel cell vehicles roll out in the coming years, this new structure may accelerate development by driving down costs for both metal catalysts and fuel.—Jia Wang
The Center for Functional Nanomaterials at Brookhaven National Laboratory is one of the five DOE Nanoscale Science Research Centers (NSRCs), premier national user facilities for interdisciplinary research at the nanoscale. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge and Sandia and Los Alamos national laboratories.
The National Synchrotron Light Source (NSLS) provides intense beams of infrared, ultraviolet, and x-ray light for basic and applied research in physics, chemistry, medicine, geophysics, and environmental and materials sciences. Supported by the Office of Basic Energy Sciences within the US Department of Energy, the NSLS is one of the world’s most widely used scientific facilities.
Yu-Chi Hsieh, Yu Zhang, Dong Su, Vyacheslav Volkov, Rui Si, Lijun Wu, Yimei Zhu, Wei An, Ping Liu, Ping He, Siyu Ye, Radoslav R. Adzic & Jia X Wang (2013) Ordered bilayer ruthenium–platinum core-shell nanoparticles as carbon monoxide-tolerant fuel cell catalysts. Nature Communications 4, Article number: 2466 doi: 10.1038/ncomms3466
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