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.

Platinum (Pt) is a highly efficient electrocatalyst for both the cathodic oxygen reduction reaction (ORR) in fuel cells (and metal-air batteries) and the hydrogen evolution reaction (HER) in alkaline electrolyzers. However, the high cost and scarcity of Pt are key obstacles for its broad deployment in fuel cells and metal-air batteries for both stationary and portable applications. Intense research efforts have been focused on developing high-performance electrocatalysts with minimal precious metal content and cost. Specifically, alloying Pt with non-noble metals can reduce the Pt content of electrocatalysts by increasing their intrinsic activity.

… However, these materials cannot be easily integrated into electro- chemical devices but their outstanding catalytic performance needs to be mimicked in nanoparticulate materials that offer high surface areas. Caged, hollow or porous nanoparticles offer a promising approach to meeting these performance goals.

… In this report, we present a novel class of electrocatalysts exploiting structural evolution of bimetallic nanoparticles during which PtNi₃ solid polyhedra were transformed into hollow Pt₃Ni nanoframes with surfaces that have three-dimensional (3D) molecular accessibility. Controlled thermal treatment of the resulting nanoframes formed the desired Pt-Skin surface structure. Synthesis of Pt₃Ni nanoframes can be readily scaled up to produce high-performance electrocatalysts at gram-scale, and importantly our protocol can be generalized toward the design of other multimetallic nanoframe systems.

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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 °C. 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.

These schematic illustrations and corresponding transmission electron microscope images show the evolution of platinum/nickel from polyhedra to dodecahedron nanoframes with platinum-enriched skin. Source: Berkeley Lab. Click to enlarge.

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. 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.

—Peidong Yang, with Berkeley Lab’s Materials Sciences Division, who led the discovery

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. 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.

—Peidong Yang

This research was funded by the DOE Office of Science.

Resources

• Chen Chen, Yijin Kang, Ziyang Huo, Zhongwei Zhu, Wenyu Huang, Huolin L. Xin, Joshua D. Snyder, Dongguo Li, Jeffrey A. Herron, Manos Mavrikakis, Miaofang Chi, Karren L. More, Yadong Li, Nenad M. Markovic, Gabor A. Somorjai, Peidong Yang, and Vojislav R. Stamenkovic (2014) “Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces,” Science doi: 10.1126/science.1249061