Origin of Enhanced Activity in Palladium Alloy Electrocatalysts for Oxygen Reduction Reaction

Abstract

We explored the origin of the enhanced activity of Pd-alloy electrocatalysts for the O2 reduction reaction by correlating the electrocatalytic activity of intrinsic Pd and Pt surfaces and Pd and Pt overlayers on several substrates with their electronic properties, and established the volcano-type dependence of O2 reduction activity on the binding energy of oxygen and the d-band center of the top metal layer. Intrinsic Pd and Pt surfaces bind oxygen too firmly to allow efficient removal of the adsorbed reaction intermediates. Therefore, they do not have the highest activity and are not on the top of the volcano plot. A Pd overlayer on a Pd3Fe(111) alloy, was predicted to lie on top of the volcano plot, and thus, it appears to be the most active catalyst among investigated ones because of its moderate interaction with oxygen. The results can help in designing better electrocatalysts for fuel cells and other applications.

Size and alloying induced shift in core and valence bands of Pd-Ag and Pd-Cu nanoparticles

“…7,8 The origin of the enhanced activity has been linked to modification of the electronic structure of Pd upon bonding with the alloying metal. 9 In addition to alloying, the structural and electronic properties of the Pd can also be tailored by reducing its dimensions. The Pd nanoparticles have shown better hydrogenation properties in comparison to their thin film and bulk counterparts.…”

Platinum Monolayer Electrocatalysts for the Oxygen Reduction Reaction: Improvements Induced by Surface and Subsurface Modifications of Cores

“…The high ORR kinetics and significantly decreased cost of materials along with their satisfactory stability make some Pd-M alloys very promising candidates to replace conventional Pt catalysts for the ORR. The enhanced ORR activity observed with bulk and high-surfacearea Pd-M alloys was ascribed either to the modification of the electronic property of Pd on the segregated surface layer [16,48,49] or to the synergistic effect between Pd and the second metal [49,50]. This segregated Pd surface layer that has different electronic properties from pure Pd [16] was considered to be the key factor responsible for the enhanced ORR activity on Pd-M alloys [51].…”

“…D-band center shift induced by the surface strain has been demonstrated to be a major factor determining the catalysts' activity [15]. Moreover, electronic (ligand) effect from the electronic coupling between Pt ML and its supporting substrate play an additional role in determining the catalytic activity [16]. Density functional theory (DFT) calculations have shown that the binding energies and reactivity of small adsorbates have a great correlation with the position of d-band center on strained surfaces and metal overlayers [17,18].…”

“…In the ORR kinetics, the reaction rate is either limited by the dissociation of O 2 or the protonation of O 2 for metal surface that binds oxygen too weakly, or by the removal of adsorbed O and OH species if the metal binds oxygen too strong [21]. In the case of Pd, the oxygen binding energy is still slightly higher than the optimum value for the ORR kinetics [16], therefore in the design of the new generation Pt ML electrocatalysts, we are targeting at fine-tuning the core materials to further weaker the oxygen binding on Pd substrate. In this review, we will describe several examples demonstrating the improvements of the Pt monolayer electrocatalysts for the ORR via the modifications of surface and subsurface of the core materials.…”

Enhanced methanol oxidation and CO tolerance using CeO2-added eggshell membrane-templated Pd network electrocatalyst

“…), but with lower cost. [6][7][8] Unfortunately, like Pt, Pd-based electrocatalysts also suffer CO poisoning (although to a lesser degree), resulting in a rapid deterioration in performance of DMFCs. The CO poisoning mechanism can be understood in that the surface of Pd is preferentially covered by CO in the form of an intermediate compound Pd-CO, causing the loss of catalytic activity on methanol.…”