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Design of Three-Dimensional Nanostructured Catalysts for the Oxygen Reduction Reaction


Anthropogenic CO2 emissions from human dependence on fossil fuel are causing rising concentrations of CO2 in the atmosphere and oceans, increasing the average temperature of the planet and acidifying the oceans, among other negative effects. Reliance on carbon-based fuels is not sustainable and must be replaced by alternative, renewable energy sources, including solar, wind, hydroelectric, and geothermal. The water cycle, splitting water into hydrogen and oxygen and electrochemically oxidizing the hydrogen to regenerate water, can be an efficient and sustainable energy storage cycle. An electrolyser powered by renewable electricity splits water, and a proton exchange membrane fuel cell uses hydrogen fuel to produce electricity on demand. The performance of proton exchange membrane fuel cells is significantly limited by the platinum-based oxygen reduction catalyst, which must be improved in activity and durability in order to commercialize applications such as fuel cell vehicles.

Bimetallic nanoframes have great potential for achieving new levels of catalytic activity in various heterogeneous reactions due to their high surface area dispersion of expensive noble metals on the exterior and interior surfaces of the structure. Pt-based nanoframes such as Pt-Ni and Pt-Co can be specifically tuned for excellent oxygen reduction activity and application in the cathode of proton exchange membrane fuel cells. Nanoframes were made by first synthesizing polyhedral nanoparticles with composition such as PtCo3 or PtNi3 by a hot-injection method. Scanning transmission electron microscopy combined with energy dispersive x-ray spectroscopy showed that these nanoparticles demonstrate elemental segregation of platinum to the edges of the polyhedron, forming the basis for a framework nanostructure. The process of preferential oxidative leaching was used to remove the non-noble metal from the interior of the framework, leaving the platinum-rich edges intact to form Pt3Co or Pt3Ni nanoframes.

Understanding the atomic structure of the nanoframe is crucial to exposing the source of its performance characteristics. It is highly unlikely that a catalyst remains the same under reaction conditions when compared to as-synthesized. Hence, the ideal experiment to study the catalyst structure should be performed in situ. X-ray absorption spectroscopy is an ideal in situ technique to study Pt3Ni nanoframes, which are an excellent electrocatalyst for the oxygen reduction reaction. First, the surface characteristics of the nanoframes were probed through electrochemical hydrogen underpotential deposition and carbon monoxide electrooxidation, which proved that nanoframe surfaces with different structure exhibit varying levels of binding strength to adsorbate molecules. It is well-known that Pt-skin formation on Pt–Ni catalysts will enhance oxygen reduction activity by weakening the binding energy between the surface and adsorbates. Ex situ and in situ X-ray absorption spectroscopy revealed that nanoframes which bind adsorbates more strongly have a rougher Pt surface caused by insufficient segregation of Pt to the surface and consequent Ni dissolution. In contrast, nanoframes which exhibit extremely high oxygen reduction activity demonstrated more significant segregation of Pt over Ni-rich subsurface layers, allowing better formation of the critical Pt-skin.

The nanoframe structure is made possible by the compositional heterogeneity in polyhedral Pt-Ni or Pt-Co nanocrystals, which offers additional variables to maneuver the functionality of the nanocrystal. However, understanding how to manipulate anisotropic elemental distributions in a nanocrystal is a great challenge in reaching higher tiers of nanocatalyst design. The evolutionary trajectory of phase segregation in Pt–Ni rhombic dodecahedra was studied in order to elucidate the mechanisms that enable synthesis of the Pt3Ni nanoframe. The anisotropic growth of a Pt-rich phase along the <111> and <200> directions at the initial growth stage resulted in Pt segregation to the 14 axes of a rhombic dodecahedron, forming a highly branched, Pt-rich tetradecapod structure embedded in a Ni-rich shell. With longer growth time, the Pt-rich phase selectively migrated outwards through the 14 axes to the 24 edges such that the rhombic dodecahedron became a Pt-rich frame enclosing a Ni-rich interior phase. The Ni-rich interior was then selectively oxidized and corroded to form the hollow Pt3Ni nanoframe that demonstrated exceptional catalytic activity for the oxygen reduction reaction.

The anisotropic phase segregation and migration mechanism offers a radically different approach to synthesis of nanocatalysts with desired compositional distributions and performance. The newly acquired understanding of these mechanisms was applied to manipulate the morphology of Pt-Ni rhombic dodecahedral nanoframes into excavated nanoframes that also exhibit high catalytic activity for the oxygen reduction reaction without need for Pt-skin formation. Controlling the ratio of platinum and nickel precursors within the nanoframe synthetic system adjusted the three-dimensional platinum anisotropy in the rhombic dodecahedron, resulting in a transition from the previous hollow nanoframe to a unique excavated structure. Excavated nanoframes showed ~10 times higher specific and ~6 times higher mass activity than commercial Pt/C, and twice the mass activity of the hollow nanoframe. The high specific activity of the excavated nanoframe is attributed to enhanced Ni content in the near-surface region and the extended two-dimensional sheet structure within the nanoframe that minimizes the number of low-coordinated Pt sites.

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