We present a method for calculating the stability of reaction intermediates of electrochemical processes on the basis of electronic structure calculations. We used that method in combination with detailed density functional calculations to develop a detailed description of the free-energy landscape of the electrochemical oxygen reduction reaction over Pt(111) as a function of applied bias. This allowed us to identify the origin of the overpotential found for this reaction. Adsorbed oxygen and hydroxyl are found to be very stable intermediates at potentials close to equilibrium, and the calculated rate constant for the activated proton/electron transfer to adsorbed oxygen or hydroxyl can account quantitatively for the observed kinetics. On the basis of a database of calculated oxygen and hydroxyl adsorption energies, the trends in the oxygen reduction rate for a large number of different transition and noble metals can be accounted for. Alternative reaction mechanisms involving proton/electron transfer to ...

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Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode

The mass production of proton exchange membrane (PEM) fuel-cell-powered light-duty vehicles requires a reduction in the amount of Pt presently used in fuel cells. This paper quantifies the activities and voltage loss modes for state-of-the-art MEAs (membrane electrode assemblies), specifies performance goals needed for automotive application, and provides benchmark oxygen reduction activities for state-of-the-art platinum electrocatalysts using two different testing procedures to clearly establish the relative merit of candidate catalysts. A pathway to meet the automotive goals is charted, involving the further development of durable, high-activity Pt-alloy catalysts. The history, status in recent experiments, and prospects for Pt-alloy cathode catalysts are reviewed. The performance that would be needed for a cost-free non-Pt catalyst is defined quantitatively, and the behaviors of several published non-Pt catalyst systems (and logical extensions thereof), are compared to these requirements. Critical research topics are listed for the Pt-alloy catalysts, which appear to represent the most likely route to automotive fuel cells.

Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs

The slow rate of the oxygen reduction reaction (ORR) in the polymer electrolyte membrane fuel cell (PEMFC) is the main limitation for automotive applications. We demonstrated that the Pt3Ni(111) surface is 10-fold more active for the ORR than the corresponding Pt(111) surface and 90-fold more active than the current state-of-the-art Pt/C catalysts for PEMFC. The Pt3Ni(111) surface has an unusual electronic structure (d-band center position) and arrangement of surface atoms in the near-surface region. Under operating conditions relevant to fuel cells, its near-surface layer exhibits a highly structured compositional oscillation in the outermost and third layers, which are Pt-rich, and in the second atomic layer, which is Ni-rich. The weak interaction between the Pt surface atoms and nonreactive oxygenated species increases the number of active sites for O2 adsorption.

/pdf/improved-oxygen-reduction-activity-on-pt3ni-111-via-42v5n6il8a.pdf

Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability

The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core–shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

http://repository.ust.hk/ir/bitstream/1783.1-77094/1/acs.chemrev.5b00462_withlink.pdf

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

One of the key objectives in fuel-cell technology is to improve and reduce Pt loading as the oxygen-reduction catalyst. Here, we show a fundamental relationship in electrocatalytic trends on Pt(3)M (M=Ni, Co, Fe, Ti, V) surfaces between the experimentally determined surface electronic structure (the d-band centre) and activity for the oxygen-reduction reaction. This relationship exhibits 'volcano-type' behaviour, where the maximum catalytic activity is governed by a balance between adsorption energies of reactive intermediates and surface coverage by spectator (blocking) species. The electrocatalytic trends established for extended surfaces are used to explain the activity pattern of Pt(3)M nanocatalysts as well as to provide a fundamental basis for the catalytic enhancement of cathode catalysts. By combining simulations with experiments in the quest for surfaces with desired activity, an advanced concept in nanoscale catalyst engineering has been developed.

Trends in electrocatalysis on extended and nanoscale Pt-bimetallic alloy surfaces

Electrocatalytic activity of Pt alloys with Ni, Co, and Fe, formed by sputtering, was investigated with regard to the oxygen reduction reaction (ORR) in perchloric acid solution. Hydrodynamic voltammograms with rotated electrodes were used to measure the electrocatalytic activity. Maximum activity was observed at ca. 30, 40, and 50% content of Ni, Co, and Fe, respectively, by which 10, 15, and 20 times larger kinetic current densities than that of pure Pt were attained. X‐ray photoelectron spectroscopy analysis of the surfaces after the reaction indicated that the active surfaces were covered by a few monolayers of Pt. We present an enhancing mechanism for the ORR based on an increased d‐electron vacancy of the thin Pt surface layer caused by underlying alloy. Results of this work contribute in development of new active cathode catalysts for fuel cells used as power sources of electric vehicles, etc. © 1999 The Electrochemical Society. All rights reserved.

https://iopscience.iop.org/article/10.1149/1.1392544/pdf

Enhancement of the Electroreduction of Oxygen on Pt Alloys with Fe, Ni, and Co

Enhancement of the electrocatalytic O2 reduction on Pt–Fe alloys

The electrocatalytic activity for H2 oxidation in the presence of 100 ppm CO has been investigated on a
 series of binary Pt alloy electrocatalysts with non-precious metals of various compositions. Regardless of
 the composition, Pt–Fe, Pt–Ni, Pt–Co and Pt–Mo alloys have been found to exhibit excellent CO tolerance in H2
 oxidation, similar to that of the Pt–Ru alloy. At these CO-tolerant electrodes, the equilibrium coverage of CO
 was suppressed to values less than ca. 0.6. Based on X-ray photoelectron spectroscopy (XPS) data, it was found that
 the surfaces of all non-precious metal alloys are composed of a thin Pt layer with an electronic structure different
 from that of pure Pt, indicating an increased 5d vacancy of Pt in the layers of the CO-tolerant alloys.
 The CO coverage, particularly with multi-bonding, was lowered due to decreased electron donation from the
 Pt band to the 2π* orbital of CO. A weakening of bond strength between the Pt skin layer and CO was also indicated
 by in situ FTIR, suggesting that the H2 oxidation sites are not blocked by CO due to its enhanced mobility.
 Thus, the mechanism of CO tolerance described above
 at the Pt skin on alloy
 surfaces is proposed as a “detoxification mechanism”.

CO Tolerance of Pt alloy electrocatalysts for polymer electrolyte fuel cells and the detoxification mechanism

Special catalysts — Pt supported zeolites — for the selective oxidation of carbon monoxide in reformed fuels from methanol or natural gas were proposed. They can be applied for the application to polymer electrolyte fuel membrane cells of which anode Pt catalysts suffer serious poisoning by the presence of trace carbon monoxide. The proposed Pt-supported zeolite catalysts can oxidize carbon monoxide much more selectively in a large excess of hydrogen with the addition of a low concentration of oxygen than a conventional Pt-supported alumina catalyst. The selectivity was affected by supports, in the order A type zeolite mordenite X type zeolite alumina. With decreasing oxygen content, an enhanced selectivity was obtained on Pt-zeolite catalysts (approaching 100%). Pt supported on mordenite showed the highest selectivity, with high conversion from carbon monoxide to carbon dioxide among the catalysts examined. It was demonstrated that the oxygen addition can be minimized almost to the stoichiometric amount required for the complete oxidation of carbon monoxide in a large excess of hydrogen.

Hiroshi Igarashi

Papers

Enhancement of the Electroreduction of Oxygen on Pt Alloys with Fe, Ni, and Co

Enhancement of the electrocatalytic O2 reduction on Pt–Fe alloys

CO Tolerance of Pt alloy electrocatalysts for polymer electrolyte fuel cells and the detoxification mechanism

Removal of carbon monoxide from hydrogen-rich fuels by selective oxidation over platinum catalyst supported on zeolite

Hydrogen electro-oxidation on platinum catalysts in the presence of trace carbon monoxide