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117 | 2019
Annuaire du Collège de France 2016-2017
Résumé des cours et travaux 117e année
Autres enseignements et recherches
IV. Conférenciers invités

The renaissance of electrochemistry

Nenad M. Markovic
p. 678-680

Notes de la rédaction

Cycle de quatre conférences prononcées les 18, 21, 25 et 28 avril 2017 (invitation sur proposition des Pr Marc Fontecave et Jean-Marie Tarascon). Ces conférences sont disponibles en audio et vidéo sur le site internet du Collège de France : https://www.college-de-france.fr/site/jean-marie-tarascon/guestlecturer-2017-04-18-11h00.htm.

Texte intégral

1. Electrochemical interfaces

1Developing and deploying renewable energy technologies will require the application of knowledge, concepts, and tools from a variety of fields including chemistry, materials science, physics and, in particular, electrochemistry. Electrochemistry is, in the broadest sense, the study of relationships between the transformation of electrical energy in chemical bonds and, in the reverse process, the energy stored in chemical bonds back to electrons that can power electrochemical energy storage and conversion systems. For decades, advances in electrochemistry parallel fundamental understanding of electrochemical interfaces that represents a junction between the electrode material and electrolyte. Central to this presentation will be to introduce —at atomic and molecular levels— electrochemical interfaces in aqueous environment. We first describe the conventionally accepted picture of the double layer, focusing on substrate-adsorbate interactions that involve the sharing of electrons and orbital overlap (covalent bonds) as well as non-covalent electrostatic interactions (e.g., van der Waals forces) between hydrated ions and covalently bonded adsorbents. Examples of covalent-type of interactions will include adsorption of hydrogen, oxygenated species, carbon monoxide, and ions on metal single crystal surfaces in electrolytes with various pH values. Examples of the role of non-covalent interactions will be limited to interaction between hydrated cations and covalently bonded adsorbates. While discussing various types of forces that control interfacial properties we will introduce ex-situ and in-situ experimental/computational probes that have been developed for determining the relationships between the energy of adsorption and relaxation/reconstruction, adsorbate structures and corresponding adsorption isotherms as well as the position of cations in the double layer. We conclude the presentation by announcing the topic of the second lecture: how fundamental understanding of the synergy between covalent and noncovalent interactions can form the basis for any predictive ability in tailor making active, stable and selective electrochemical interfaces for efficient energy conversion and storage.

2. Electrochemical interfaces and electrocatalysis

2The second lecture will introduce contemporary electrocatalysis as the fundamental study of electrode reactions where the kinetics depends strongly on the physicochemical properties of electrochemical interfaces; encompassing both the nature of electrode materials and the rarely discussed structure of the double layer. At least for judiciously chosen systems, a key fundamental issue addressed in this presentation is the degree to which the fundamental understanding of the structure/nature of electrochemical interface (that is introduced in the first lecture) determines the efficiency of the water cycle—that is controlled by the making and breaking of HH, HO and OO bonds and the associated production of electrons (electricity) and H2O in fuel cells; or on the utilization of electrons for the H2O splitting reaction to regenerate H2 and O2 in electrolyzers. For decades, the design of catalysts for the water cycle has been guided by energetic factors, whereby the binding energy of intermediates to elusive “active sites” is assumed to control the reaction kinetics. This presentation will go beyond a singular “kinetics-designing” principle, providing atomic-/molecular-level insights that will open some new avenues in designing novel electrochemical interfaces; in particular, for improving the kinetics of the hydrogen evolution (HER) and hydrogen oxidation (HOR) reactions. Key correlations will be discussed, including structure-function relationships, the role of pH values, the role of temperature and the nature of adsorbed spectator species that are formed either from adsorption of components present in supporting electrolytes or formed during the course of the reaction. We also show how the nature of electrochemical interfaces controls the reaction kinetics. The second archetypical example will include discussions about the kinetics of the CO oxidation reaction, which is one key molecule that determines the efficiency of the carbon cycle—that is in general based either on C-H and C-O bond making/breaking events and the concomitant production of CO2 in fuel cells or on the reduction of CO2 to produce hydrocarbon fuels and O2 in electrolyzers. At the end of the second we provide atomic-/molecular-level insights needed to simultaneously control activity and stability of electrode materials such as metals, oxides and sulfides. Indeed this will be used as a prelude for the third lecture which will be focusing on the oxygen electrochemistry.

3. Electrocatalysis and dioxygen electrochemistry

3This lecture is entirely dedicated to the oxygen electrochemistry, encompassing both the oxygen reduction reaction in fuel cells (ORR) and the oxygen evolution reaction (OER) in electrolyzers on wide range of well-characterized materials; ranging from metals, meta/metal-oxides, and complex oxides. Of central importance is to show how the knowledge gained from well-characterized single crystal materials can be used to design real-world nanoscale materials at the atomic and molecular level. First we show that both reactions are structure sensitive processes and that the rates are strongly dependent on the pH environment. For the ORR we will argue that structure sensitivity arises via structure sensitive adsorption of spectator species rather than reaction intimidate. To demonstrate this we show a specific example for the ORR on platinum and gold single crystal surfaces first and then on the corresponding bimetallic surfaces that are obtained by alloying the host metal with 3d-TM elements. Then we introduce the OER, emphasizing that the most active materials are less stable—indicating that the active sites are defects which are created during the reaction. We will argue that instead of characterizing catalytic performance by (mass or specific) activity, which does not capture any information about the stability (and ultimate commercial viability) of a given material, the design of catalyst materials for the OER should instead be guided by a new, quantitative descriptor—the Activity-Stability Factor; ASF—which is expressed as the ratio between activity and stability for a given catalyst. In closing the lecture, we will point out that the importance of oxygen electrochemistry goes well beyond the success of hydrogen economy—in fact; it serves as a unique bridge between electrochemistry in aqueous and organic environments.

4. The renaissance of electrochemistry

4The key massage of this lecture is that we are witnessing the renaissance of electrochemistry and that fundamental understanding of critical electrochemical processes at interfaces in aqueous and organic environments will provide ample opportunities (and challenges) to further improve the current landscape of sustainable energy production and utilization. Thus far, interfaces in aqueous and organic solutions have been studied by many groups, albeit usually as independent research areas. In this presentation we will discuss that this circumstance has led to an artificial partitioning of the two environments that under well-defined experimental conditions may, in fact, be governed by similar fundamental principles. For some specific processes, we show that activated-water may serve as unifying descriptor that can control both H+-O2 and Li+-O2 electrochemistry. We will conclude by asking us what we don’t know but we would like to know about electrochemical interfaces.

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Annuaire du Collège de France 2016-2017. Résumé des cours et travaux 117e année, Paris, Collège de France, septembre 2019, p. 678-680. ISBN 978-2-7226-0489-6

Référence électronique

Nenad M. Markovic, « The renaissance of electrochemistry », L’annuaire du Collège de France [En ligne], 117 | 2019, mis en ligne le 04 juin 2020, consulté le 18 juin 2021. URL : http://journals.openedition.org/annuaire-cdf/15017 ; DOI : https://doi.org/10.4000/annuaire-cdf.15017

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Auteur

Nenad M. Markovic

Chercheur, Materials Science Division Argonne National Laboratory (États-Unis)

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Collège de France

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