Ab Initio Calculations of Intermediates of Oxygen Reduction on Low-Index Platinum Surfaces

Abstract

Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Platinum catalysts are involved in a vast variety of redox reactions, and O 2 reduction is one of the most important of them. Therefore, considerable efforts are undertaken by electrochemists to understand the fundamental aspects of Pt catalysts and to improve the properties of applied catalytic systems in general. The fundamental relevance of Pt catalytic properties has become even more important because carbon-supported Pt catalysts have so far been the best choice for the oxygen reduction at the cathode of low-temperature polymer electrolyte membrane fuel cells ͑PEMFCs͒. 1The oxygen reduction reaction ͑ORR͒ on Pt surfaces has been widely studied in electrochemistry, but the details of the mechanism remain elusive. The overall ORR is a multistep process involving four electron transfers during which bonds are broken and formed. It is still not clear whether the process starts from the oxygen molecule dissociation on Pt electrodes, this being followed by electron and proton transfer, or whether the first reduction step happens before O-O bond cleavage. Damjanovic proposed that the first step is an electron transfer to the O 2 moleculeThis step is rate-determining and is either accompanied by or followed by a fast proton transfer. A theoretical study by Sidik et al. has proven the first electron transfer to be rate-determining on dual adsorption sites. Proton transfer is involved in this step, because the electron affinity of the reactant complex is increased significantly by t...

Electrocatalysis of Oxygen Reduction in Polymer Electrolyte Fuel Cells: A Brief History and a Critical Examination of Present Theory and Diagnostics

“…OH ads , rather than O ads , seems the likely next intermediate on the way to the final H 2 O product, as can be concluded, again, from the deep energy sink associated with O ads on Pt. The recent DFT calculations further support the O 2 -to-OOH ads -to-OH ads route by showing that OOH is indeed an intermediate of significant bonding energy to Pt, at least on some Pt crystal surfaces [Panchenko et al, 2004]. Another important contribution of the theoretical calculations has been in providing an explanation for the higher measured ORR activity of Pt-alloy catalysts versus unalloyed Pt.…”

“…1.2) [Nørskov et al, 2004] (rather than a single metal atom or a pair of metal atoms as was done in pioneering contributions made somewhat earlier [Anderson et al, 2000]), as well as the aqueous molecular environment adjacent to the metal catalyst surface. Last, but not least, the effects of variations in the electrode-electrolyte potential difference on interfacial thermodynamics and dynamics have also been included in these models [Nørskov et al, 2004;Panchenko et al, 2004;Desai and Neurock, 2003]. Figure 1.2 Free energy map for the "associative mechanism" of ORR at Pt metal and the steps considered in this mechanism [Nørskov et al, 2004].…”

“…For example, the probability that the first step in the ORR process is direct, dissociative chemisorption of O 2 to form two O ads species on the Pt metal surface appears to be low, not only in light of the low rates of this process at the Pt/oxygen gas interface at such low temperatures, but also in light of the deep energy sink associated with chemisorption of an oxygen atom on Pt [Nørskov et al, 2004]. Accordingly, there seems to be an advantage for the ORR route starting with a single electron and proton transfer to O 2 to generate the HOO ads intermediate and thereby facilitate subsequent breaking of the O2 2O bond, which in HOO ads has an order significantly lower than 2 [Panchenko et al, 2004]. OH ads , rather than O ads , seems the likely next intermediate on the way to the final H 2 O product, as can be concluded, again, from the deep energy sink associated with O ads on Pt.…”

Theoretical Study on Current-Voltage Characteristics of Oxygen Reduction Reaction near Platinum Surfaces

“…A previous study of ORR on Pt 9 (111) concluded that the favorable position of oxygen adsorption were the hollow and bridge site [5]. Furthermore, the other previous calculation using a slab model also stated that O 2 molecule tended to be adsorbed on these two sites [18].…”

“…Although their work provided the useful results of the dissociative adsorption process considering the one-electron transfer, it was limited to describe the ionization potential (IP) and electron affinity (EA) Due to time-consuming computations for large-sized electronic structures, effectively smaller scale models are thus required for more improvements. Panchenko et al [5] carried out a density functional theory (DFT) calculation using a cluster model of Pt 9 (111) and showed the capability of computing the adsorption energy, geometry, and the charge transfer in such a small-scale system.…”