HOR Activity of Pt-TiO_2-Y at Unconventionally High Potentials Explained: The Influence of SMSI on the Electrochemical Behavior of Pt

Abstract

The formation of strong metal support interactions (SMSI) is known for many metal/metal oxide systems and its consequences are well established in the field of heterogeneous catalysis, but this knowledge has only been recently transferred to the field of electrocatalysis. In this study, Pt was deposited via atomic layer deposition (ALD) onto TiO2−Y, which allowed a good control of the particle size through the number of ALD cycles. During the ALD process, a thin-film of reduced titania is formed on the Pt surface, which leads to SMSI effects. With increasing Pt particle size, the fraction of the titania-covered Pt surface decreases. As a result, the extent of platinum oxide formation in cyclic voltammetry (CV) measurements scales with the size of the Pt particles. The influence of these thin titanium oxide films, which cover the Pt surface, on the catalytic behavior with respect to oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), CO oxidation and oxygen evolution reaction (OER) is investigated by using an RDE setup. The covering TiOX thin-films reduce the ability to catalyze ORR, OER and CO oxidation, while it does not influence the HOR and Pt H-UPD formation. These findings indicate that proton and hydrogen transport are possible through the thin TiOX film, while oxygenated species suffer from transport limitations through the thin-film. Due to this selective permeability, these materials are able to oxidize hydrogen well beyond 1.2 VRHE.

Catalyst overcoating engineering towards high-performance electrocatalysis

“…Depending on the molecular size of reactants and/or spectator species, the coating overlayer can be rationally constructed to control their transport on/to the metal surface. As such, extremely high catalytic selectivity toward the HOR vs. ORR (under shut-down/start-up conditions in H 2 -O 2 fuel cells) 63,64,260,261,265 and the ORR vs. MOR (the case associated with methanol crossover in DMFCs) 51,54,56,75,[219][220][221] has been achieved. ORR and HOR catalytic activity can be additionally enhanced because of the abundant active sites free from blocking by poisoning species.…”

“…Stu ¨hmeier et al 264 reported that partial-encapsulation of Pt NPs with TiO x overlayers through reductive heating treatment could hinder the process of Pt oxidation and oxygen reduction, while allowing superior HOR selectivity and activity at high potentials. The influence of SMSIs on the electrochemical behavior of Pt was investigated by Geppert et al 265 Coating with a TiO x thin-film could reduce the ability of Pt to catalyze the ORR and OER, while no influence was observed on the HOR even above 1.2 V RHE . These findings were attributed to the selective permeability of the TiO x film, allowing proton and hydrogen diffusion but impairing the transport of oxygenated species.…”

Electrocatalytic Properties of Mixed-Oxide-Containing Composite-Supported Platinum for Polymer Electrolyte Membrane (PEM) Fuel Cells

“…Furthermore, the literature suggests that a smaller limiting current density can be originated from reversible oxide formation/reduction on Pt, which leads to the growth of the Pt particles and, consequently, to the reduction of the actual active surface area [ 66 ]. Marginally lower diffusion-limited current density in the composite-supported Pt catalysts can also be caused by a slower diffusion of oxygen through the oxide layer covering the Pt nanoparticles [ 40 , 67 ].…”

Strong Metal–Support Interaction in Heterogeneous Catalysts

“…recently. [ 101 ] Interestingly, the covering TiO x thin‐film significantly increased the overpotential of ORR and OER, and decreased the electrochemical activity towards CO oxidation reaction. In contrast, high electrocatalytic activity towards HOR was retained even up to the onset of OER.…”