Vera Beermann

Rh-Doped Pt–Ni Octahedral Nanoparticles: Understanding the Correlation between Elemental Distribution, Oxygen Reduction Reaction, and Shape Stability

Thanks to their remarkably high activity toward oxygen reduction reaction (ORR), platinum-based octahedrally shaped nanoparticles have attracted ever increasing attention in last years. Although high activities for ORR catalysts have been attained, the practical use is still limited by their long-term stability. In this work, we present Rh-doped Pt-Ni octahedral nanoparticles with high activities up to 1.14 A mgPt(-1) combined with improved performance and shape stability compared to previous bimetallic Pt-Ni octahedral particles. The synthesis, the electrocatalytic performance of the particles toward ORR, and atomic degradation mechanisms are investigated with a major focus on a deeper understanding of strategies to stabilize morphological particle shape and consequently their performance. Rh surface-doped octahedral Pt-Ni particles were prepared at various Rh levels. At and above about 3 atom %, the nanoparticles maintained their octahedral shape even past 30,000 potential cycles, while undoped bimetallic reference nanoparticles show a complete loss in octahedral shape already after 8000 cycles in the same potential window. Detailed atomic insight in these observations is obtained from aberration-corrected scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) analysis. Our analysis shows that it is the migration of Pt surface atoms and not, as commonly thought, the dissolution of Ni that constitutes the primary origin of the octahedral shape loss for Pt-Ni nanoparticles. Using small amounts of Rh we were able to suppress the migration rate of platinum atoms and consequently suppress the octahedral shape loss of Pt-Ni nanoparticles.

Tuning the Electrocatalytic Oxygen Reduction Reaction Activity and Stability of Shape-Controlled Pt–Ni Nanoparticles by Thermal Annealing − Elucidating the Surface Atomic Structural and Compositional Changes

Shape-controlled octahedral Pt-Ni alloy nanoparticles exhibit remarkably high activities for the electroreduction of molecular oxygen (oxygen reduction reaction, ORR), which makes them fuel-cell cathode catalysts with exceptional potential. To unfold their full and optimized catalytic activity and stability, however, the nano-octahedra require post-synthesis thermal treatments, which alter the surface atomic structure and composition of the crystal facets. Here, we address and strive to elucidate the underlying surface chemical processes using a combination of ex situ analytical techniques with in situ transmission electron microscopy (TEM), in situ X-ray diffraction (XRD), and in situ electrochemical Fourier transformed infrared (FTIR) experiments. We present a robust fundamental correlation between annealing temperature and catalytic activity, where a ∼25 times higher ORR activity than for commercial Pt/C (2.7 A mgPt-1 at 0.9 VRHE) was reproducibly observed upon annealing at 300 °C. The electrochemical stability, however, peaked out at the most severe heat treatments at 500 °C. Aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy (EDX) in combination with in situ electrochemical CO stripping/FTIR data revealed subtle, but important, differences in the formation and chemical nature of Pt-rich and Ni-rich surface domains in the octahedral (111) facets. Estimating trends in surface chemisorption energies from in situ electrochemical CO/FTIR investigations suggested that balanced annealing generates an optimal degree of Pt surface enrichment, while the others exhibited mostly Ni-rich facets. The insights from our study are quite generally valid and aid in developing suitable post-synthesis thermal treatments for other alloy nanocatalysts as well.

The Effect of Surface Site Ensembles on the Activity and Selectivity of Ethanol Electrooxidation by Octahedral PtNiRh Nanoparticles

Direct ethanol fuel cells are attractive power sources based on a biorenewable, high energy-density fuel. Their efficiency is limited by the lack of active anode materials which catalyze the breaking of the C-C bond coupled to the 12-electron oxidation to CO2 . We report shape-controlled PtNiRh octahedral ethanol oxidation electrocatalysts with excellent activity and previously unachieved low onset potentials as low as 0.1 V vs. RHE, while being highly selective to complete oxidation to CO2 . Our comprehensive characterization and in situ electrochemical ATR studies suggest that the formation of a ternary surface site ensemble around the octahedral Pt3 Ni1 Rhx nanoparticles plays a crucial mechanistic role for this behavior.

Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis

Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose ‘surface distortion’ as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory experimental observations in regards to the electrocatalytic oxygen reduction reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalize the electrocatalytic properties of state-of-the-art of PtNi/C nanocatalysts with distinct atomic composition, size, shape and degree of surface defectiveness under a simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and highlights strategies to design more durable ORR nanocatalysts.Tuning surface structure is key for electrocatalytic performance and stability of proton-exchange membrane fuel cells. Surface distortion as a structural descriptor can help to clarify the role of surface defects and to design enhanced nanocatalysts.