Recent Developments in the Electrocatalysis of the O<sub>2</sub>Reduction Reaction

Xu, Ye; Shao, Minhua; Mavrikakis, Manos; Adžić, Radoslav R.

doi:10.1002/9780470463772.ch9

Cited by 25 publications

(31 citation statements)

References 155 publications

(168 reference statements)

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“…The single biggest challenge for ORR catalysis in acidic electrolytes is in enabling the initial adsorption of molecular O 2 at an optimal binding energy with or without the electron transfer. This critical requirement narrows down the choice of catalytic materials to the precious metals group with optimal d- band electronic structure and geometric features that provides a sufficiently high free energy of adsorption for O 2 and other related reaction intermediates. − On the other hand, the lack of necessity for O 2 adsorption prior to the first electron transfer at high pH conditions liberates the entire ORR process from this critical requirement thus enabling a wide range of electrode materials at high pH conditions. Such a scenario arises due to the possibility of outer-sphere electron transfer mechanism under alkaline conditions, which is discussed briefly here in this section. ,, …”

Section: Oxygen Reduction Reaction In Alkaline Electrolytementioning

confidence: 99%

Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer

2019

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Alkaline anion-exchange membrane (AAEM) fuel cells have attracted significant interest in the past decade, thanks to the recent developments in hydroxideanion conductive membranes. In this article, we compare the performance of current state of the art AAEM fuel cells to proton-exchange membrane (PEM) fuel cells and elucidate the sources of various overpotentials. While the continued development of highly conductive and thermally stable anion-exchange membranes is unambiguously a principal requirement, we attempt to put the focus on the challenges in electrocatalysis and interfacial charge transfer at an alkaline electrode/electrolyte interface. Specifically, a critical analysis presented here details the (i) fundamental causes for higher overpotential in hydrogen oxidation reaction, (ii) mechanistic aspects of oxygen reduction reaction, (iii) carbonate anion poisoning, (iv) unique challenges arising from the specific adsorption of alkaline ionomer cation-exchange head groups on electrocatalysts surfaces, and (v) the potential of alternative small molecule fuel oxidation. This review and analysis encompasses both the precious and nonprecious group metal based electrocatalysts from the perspective of various interfacial charge-transfer phenomena and reaction mechanisms. Finally, a research roadmap for further improvement in AAEM fuel cell performance is delineated here within the purview of electrocatalysis and interfacial charge transfer.

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Section: Oxygen Reduction Reaction In Alkaline Electrolytementioning

confidence: 99%

Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer

2019

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“…The oxygen reduction reaction (ORR) on Pt is among the current research focuses in energy science, due to the seemingly indispensable role of Pt in catalyzing the ORR in fuel cells and the considerable activity and durability gap of Pt remaining to the requirements for fuel cells to be a cost-effective electricity generation technology. − As the best metal catalyst, Pt-based materials still exhibit ORR onset potentials nearly 200 mV less than the thermodynamic equilibrium potential (∼1.23 V vs RHE). The origin of this large overpotential remains largely elusive, which is not only a fundamental embarrassment in electrochemistry but also hampers the electrocatalyst progress.…”

Section: Introductionmentioning

confidence: 99%

Electrocatalytic O₂ Reduction on Pt: Multiple Roles of Oxygenated Adsorbates, Nature of Active Sites, and Origin of Overpotential

et al. 2017

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This work aims to reveal the origin of the large onset overpotential and sluggish kinetics of the oxygen reduction reaction (ORR) on Pt, a standing problem that hinders the progress of fuel cell technology. By investigating various possible reaction steps and pathways through detailed DFT calculations on Pt(111) surface covered by rationalized phase structures of oxygenated adsorbates, we show that the ORR overpotential and Tafel kinetics originate from the potential-dependent formation of a site-blocking spectator phase, √3 × √3-structured oxygen adatoms (O*), which coexists with a relatively weak blocking phase, (√3 × √3)R30°-patterned adsorption network of hydroxyl group (OH*) and water molecule (H2O*) at ORR relevant potentials. The OH*/H2O* phase provides sites for ORR to proceed through a dissociative pathway consisting of four proton/electron transfer (PET) steps. The first step, PET-coupled O2 adsorption, is identified as the activity-determining step. Different from the usual beliefs, we found the O2 and O* do not directly accept proton during the reduction steps; rather, OH* and H2O* act as PET mediators to facilitate the O2 adsorption and dissociation and the O* reduction. These findings unveil the distinctly multiple roles of various oxygenated adsorbates as intermediates, spectators, and PET mediators in ORR. The implications of these findings on designing Pt-based catalysts are discussed. It is concluded that the binding strength of O* impacts the ORR activity of Pt-based surface predominantly by modulating the number of the available active sites, rather than the activation barriers for the rate-determining step.

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“…The most extensive and successful work in the field is due to Adzic and co-workers, who prepared finely tuned Pt monolayer catalysts with very high ORR intrinsic catalytic activity and the highest reported mass specific activity [65,66,79,[81][82][83]86,[88][89][90][91][92]96,[163][164][165][166][167][168][169][170][171][172][173][174][175][176][177][178][179][180]. In more detail, they have predicted theoretically (by means of Density Functional Theory (DFT) calculations) and confirmed experimentally that the most active catalyst for ORR is based on a Pt skin and a Pd-based core [81] which can be further improved by the use of a mixed Pt-noble metal skin and can reach an enhancement of mass activity in the 3-20-fold range [82].…”

Section: Oxygen Reductionmentioning

confidence: 99%

Electrocatalysts Prepared by Galvanic Replacement

et al. 2017

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Galvanic replacement is the spontaneous replacement of surface layers of a metal, M, by a more noble metal, M noble , when the former is treated with a solution containing the latter in ionic form, according to the general replacement reaction: nM + mM noble n+ → nM m+ + mM noble. The reaction is driven by the difference in the equilibrium potential of the two metal/metal ion redox couples and, to avoid parasitic cathodic processes such as oxygen reduction and (in some cases) hydrogen evolution too, both oxygen levels and the pH must be optimized. The resulting bimetallic material can in principle have a M noble-rich shell and M-rich core (denoted as M noble (M)) leading to a possible decrease in noble metal loading and the modification of its properties by the underlying metal M. This paper reviews a number of bimetallic or ternary electrocatalytic materials prepared by galvanic replacement for fuel cell, electrolysis and electrosynthesis reactions. These include oxygen reduction, methanol, formic acid and ethanol oxidation, hydrogen evolution and oxidation, oxygen evolution, borohydride oxidation, and halide reduction. Methods for depositing the precursor metal M on the support material (electrodeposition, electroless deposition, photodeposition) as well as the various options for the support are also reviewed. 1. Principle of Galvanic Replacement/Deposition 1.1. Thermodynamic Considerations When a metal, M (e.g., Cu, Fe, Co, Ni, Al, etc.), is immersed in a solution containing ions of a more noble metal, M noble n+ (e.g., Pt, Au, Pd, Ag, Ru, Ir, Rh, Os, etc.-i.e., a metal with a higher standard potential) then, due to the difference in their standard electrochemical potentials, E 0 noble − E 0 > 0, and provided that the ionic form of M is stable under the given experimental conditions (of temperatue, pH, complexing agents etc.), the following reaction is thermodynamically favored and can take place spontaneously: M noble n+ + n/m M → M noble + n/m M m+ (1) This reaction, which bears similarities with transmetalation reactions between metal complexes [1] and is also known as immersion plating [2] in the plating industry and galvanic replacement [3] in materials chemistry, can be considered to originate from the coupling of the following two half-reactions: M noble n+ + ne − ↔ M noble (E 0 noble) (2) M m+ + me − ↔ M (E 0) (3)

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Recent Developments in the Electrocatalysis of the O₂Reduction Reaction

Cited by 25 publications

References 155 publications

Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer

Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer

Electrocatalytic O₂ Reduction on Pt: Multiple Roles of Oxygenated Adsorbates, Nature of Active Sites, and Origin of Overpotential

Electrocatalysts Prepared by Galvanic Replacement

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Recent Developments in the Electrocatalysis of the O2Reduction Reaction

Cited by 25 publications

References 155 publications

Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer

Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer

Electrocatalytic O2 Reduction on Pt: Multiple Roles of Oxygenated Adsorbates, Nature of Active Sites, and Origin of Overpotential

Electrocatalysts Prepared by Galvanic Replacement

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Recent Developments in the Electrocatalysis of the O₂Reduction Reaction

Electrocatalytic O₂ Reduction on Pt: Multiple Roles of Oxygenated Adsorbates, Nature of Active Sites, and Origin of Overpotential