Rhombohedral Pd-Sb Nanoplates with Pd-Terminated Surface: An Efficient Bifunctional Fuel-Cell Catalyst

Developing high-performance electrocatalysts for the ethanol oxidation reaction (EOR) and the oxygen reduction reaction (ORR) is essential for the commercialization of direct ethanol fuel cells, but it is still formidably challenging. In this work, a novel Pd-Sb hexagonal nanoplate for boosting both cathodic and anodic fuel cell reactions is prepared. Detailed characterizations reveal that the nanoplates have ordered rhombohedral phase of Pd₈ Sb₃ (denoted as Pd₈ Sb₃ HPs). The Pd₈ Sb₃ HPs exhibit much enhanced activity toward the oxidation of various alcohols. Particularly, Pd₈ Sb₃ HPs/C displays superior specific and mass activities of 29.3 mA cm^-2 and 4.5 A mg_Pd ^-1 toward the EOR, which are 7.0 and 11.3 times higher than those of commercial Pd/C, and 9.8 and 3.8 times higher than those of commercial Pt/C, respectively, representing one of the best EOR catalysts reported to date. In situ electrochemical attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) measurements reveal that Pd₈ Sb₃ HPs/C can effectively promote the C₂ pathway of the EOR. As revealed by density functional theory calculations, the high EOR activity of the Pd₈ Sb₃ HPs can be ascribed to the reduced energy barrier of ethanol dehydrogenation. Additionally, Pd₈ Sb₃ HPs/C also shows superior performance in the ORR. This work advances the controllable synthesis of the Pd-Sb nanostructure, giving huge impetus for the design of high-efficiency electrocatalysts for energy conversion and beyond.

Golden Palladium Zinc Ordered Intermetallics as Oxygen Reduction Electrocatalysts

Exploring Pt-free electrocatalysts with high activity and long durability for the oxygen reduction reaction (ORR) has been long pursued by the renewable energy materials community. In this work, we have designed an ordered intermetallic PdZn/C (O-PdZn) with a several atomic-layer Pd shell, which achieved a 3-fold enhancement in ORR mass activities (MA) in alkaline media, relative to Pd/C and Pt/C. Further Au incorporation in O-PdZn/C (Au-O-PdZn/C) yielded a catalyst with superior durability with less than 10% loss in MA after 30000 potential cycles. These effects have attributed to the rationally designed ordered structure and stabilizing effect of Au atoms. Aberration-corrected scanning transmission electron microscopy and synchrotron-based X-ray fluorescence spectroscopy were employed to show that Au not only galvanically replaced Pd and Zn on the surface but also penetrated through the PdZn lattice and distributed uniformly within the particles. Au-O-PdZn/C was also tested as an effective oxygen cathode in broad applications in rechargeable Li-air and Zn-air batteries.

Revealing the atomic ordering of binary intermetallics using in situ heating techniques at multilength scales

Ordered intermetallic nanoparticles are promising electrocatalysts with enhanced activity and durability for the oxygen-reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). The ordered phase is generally identified based on the existence of superlattice ordering peaks in powder X-ray diffraction (PXRD). However, after employing a widely used postsynthesis annealing treatment, we have found that claims of "ordered" catalysts were possibly/likely mixed phases of ordered intermetallics and disordered solid solutions. Here, we employed in situ heating, synchrotron-based, X-ray diffraction to quantitatively investigate the impact of a variety of annealing conditions on the degree of ordering of large ensembles of Pt₃Co nanoparticles. Monte Carlo simulations suggest that Pt₃Co nanoparticles have a lower order-disorder phase transition (ODPT) temperature relative to the bulk counterpart. Furthermore, we employed microscopic-level in situ heating electron microscopy to directly visualize the morphological changes and the formation of both fully and partially ordered nanoparticles at the atomic scale. In general, a higher degree of ordering leads to more active and durable electrocatalysts. The annealed Pt₃Co/C with an optimal degree of ordering exhibited significantly enhanced durability, relative to the disordered counterpart, in practical membrane electrode assembly (MEA) measurements. The results highlight the importance of understanding the annealing process to maximize the degree of ordering in intermetallics to optimize electrocatalytic activity.