The hydrogen evolution reaction: from material to interfacial descriptors

International audienceThe production of sustainable hydrogen with water electrolyzers is envisaged as one of the most promising ways to match the continuously growing demand for renewable electricity storage. While so far regarded as fast when compared to the oxygen evolution reaction (OER), the hydrogen evolution reaction (HER) regained interest in the last few years owing to its poor kinetics in alkaline electrolytes. Indeed, this slow kinetics not only may hinder the foreseen development of the anionic exchange membrane water electrolyzer (AEMWE), but also raises fundamental questions regarding the parameters governing the reaction. In this perspective, we first briefly review the fundamentals of the HER, emphasizing how studies performed on model electrodes allowed for achieving a good understanding of its mechanism under acidic conditions. Then, we discuss how the use of physical descriptors capturing the sole properties of the catalyst is not sufficient to describe the HER kinetics under alkaline conditions, thus forcing the catalysis community to adopt a more complex picture taking into account the electrolyte structure at the electrochemical interface. This work also outlines new techniques, such as spectroscopies, molecular simulations, or chemical approaches that could be employed to tackle these new fundamental challenges, and potentially guide the future design of practical and cheap catalysts while also being useful to a wider community dealing with electrochemical energy storage devices using aqueous electrolytes

Importance of Water Structure and Catalyst–Electrolyte Interface on the Design of Water Splitting Catalysts

International audienceHydrogen production technologies have attracted intensive attention for their potential to cope with future challenges related to renewable energy storage and conversion. However, the significant kinetic barriers associated with the oxygen evolution reaction (OER), one of the two half reactions at the heart of water electrolysis, greatly hinder the sustainable production of hydrogen at a large scale. A wide variety of materials have thus been designed and explored as OER catalysts. In this perspective, we briefly review the development of Ir-based OER catalysts in acidic conditions and discuss the limitations of a design strategy solely based on the physical and electronic properties of OER catalysts, highlighting the importance of understanding the catalyst-electrolyte interface which affects the stability and activity of the catalyst. We then share our perspective on a group of crystalline, bulk protonated iridates obtained via cation exchange in acidic solutions to be used as promising stable and active OER catalysts. Finally, we discuss the advances recently made in understanding the impact of the active sites environment on the OER kinetics, emphasizing the influence of the water structure and/or solvation properties of ions in the electrolyte. We highlight the importance of developing a better understanding of these influencing factors and incorporate them into our design of OER catalysts with enhanced properties

Crystallographic and magnetic structures of the VI3_3 and LiVI3_3 van der Waals compounds

Two-dimensional (2D) layered magnetic materials are generating a great amount of interest for the next generation of electronic devices thanks to their remarkable properties associated to spin dynamics. The recently discovered layered VI$_3$ ferromagnetic phase belongs to this family, although a full understanding of its properties is limited by an ill-defined crystallographic structure. This is not any longer true. Here, we investigate the VI$_3$ crystal structure upon cooling using both synchrotron X-ray and neutron powder diffraction and provide structural models for the two structural transitions occurring at 76 K and 32 K. Moreover, we confirm by magnetic measurements that VI$_3$ becomes ferromagnetic at 50 K and discuss the difficulty of solving its full magnetic structure by neutrons. We equally determined the magnetic properties of our recently reported LiVI$_3$ phase, which is alike the well-known CrI$_3$ ferromagnetic phase in terms of electronic and crystallographic structures and found to our surprise an antiferromagnetic behavior with a N\'eel temperature of 12 K. Such a finding provides extra clues for a better understanding of magnetism in these low dimension compounds. Finally, the easiness of preparing novel Li-based 2D magnetic materials by chemical/electrochemical means opens wide the opportunity to design materials with exotic properties

The Effect of Water on Quinone Redox Mediators in Nonaqueous Li-O2 Batteries.

The parasitic reactions associated with reduced oxygen species and the difficulty in achieving the high theoretical capacity have been major issues plaguing development of practical nonaqueous Li-O2 batteries. We hereby address the above issues by exploring the synergistic effect of 2,5-di-tert-butyl-1,4-benzoquinone and H2O on the oxygen chemistry in a nonaqueous Li-O2 battery. Water stabilizes the quinone monoanion and dianion, shifting the reduction potentials of the quinone and monoanion to more positive values (vs Li/Li+). When water and the quinone are used together in a (largely) nonaqueous Li-O2 battery, the cell discharge operates via a two-electron oxygen reduction reaction to form Li2O2, with the battery discharge voltage, rate, and capacity all being considerably increased and fewer side reactions being detected. Li2O2 crystals can grow up to 30 μm, more than an order of magnitude larger than cases with the quinone alone or without any additives, suggesting that water is essential to promoting a solution dominated process with the quinone on discharging. The catalytic reduction of O2 by the quinone monoanion is predominantly responsible for the attractive features mentioned above. Water stabilizes the quinone monoanion via hydrogen-bond formation and by coordination of the Li+ ions, and it also helps increase the solvation, concentration, lifetime, and diffusion length of reduced oxygen species that dictate the discharge voltage, rate, and capacity of the battery. When a redox mediator is also used to aid the charging process, a high-power, high energy density, rechargeable Li-O2 battery is obtained.The authors thank EPSRC-EP/M009521/1 (T.L., G.K., C.P.G.), Innovate UK (T.L.), Darwin Schlumberger Fellowship (T.L.), EU Horizon 2020 GrapheneCore1-No.696656 (G.K., C.P.G.), EPSRC - EP/N024303/1, EP/L019469/1 (N.G.-A., J.T.F.), Royal Society - RG130523 (N.G.-A.), and the European Commission FP7-MC–CIG Funlab, 630162 (N.G.-A.) for research funding

Impact de la structure de solvatation des électrolytes pour le stockage électrochimique de l’énergie

Cette thèse s’intéresse au rôle de l’électrolyte et la structure de solvatation des espèces qui le composent pour des dispositifs de stockage électrochimique de l’énergie. Plus particulièrement, le début de ce manuscrit décrit les récentes avancées dans la compréhension du rôle de la molécule l’eau pour la génération d’hydrogène dans des électrolyseurs. Nous proposons ensuite une stratégie basée sur le confinement de l’eau au sein d’une matrice organique inerte afin de mieux comprendre comment les interactions non-covalentes modifient la cinétique de la réaction de réduction de l’eau. Des simulations moléculaires et l’emploi de techniques spectroscopiques couplées à des mesures électrochimiques permettent d’obtenir une image « moléculaire » de l’environnement de l’eau dans ces électrolytes et de dessiner une corrélation entre structure et réactivité de l’eau. Dans un second temps, nous étudions la stabilité cathodique de l’eau dans des électrolytes aqueux superconcentrés employés dans certaines batteries aqueuses. En étudiant ces systèmes superconcentrés, nous mettons en évidence qu’en présence de deux sels avec des anions aux propriétés (taille et géométrie) opposées, la formation de système aqueux biphasiques est observée. Finalement, nous exploitons cette incompatibilité intrinsèque entre petits anions inorganiques et gros anions organiques. En effet, nous montrons que l’utilisation d’électrolytes superconcentrés permet de drastiquement atténuer la solubilité de composés de type VX3 (X = Cl, Br, I) habituellement observée dans des électrolytes dilués, ce qui nous permet d’étendre les réactions d’intercalations électrochimiques aux halogénures lamellaires.This thesis aims at understanding the role of the electrolyte, and more specifically the role of solvation structures, for electrochemical storage applications. First, this manuscript reviews the most recent advances on the comprehension on the role of the water molecule for the generation of hydrogen in electrolyzers. Then, we set up a strategy, which consists in studying the reactivity of water in an organic inert matrice in order to better understand how non-covalent interactions can tune the water reactivity. Coupling electrochemical measurements with molecular simulations and spectroscopies, we sketch a “molecular” picture of the water environment in these electrolytes and draw a structure-reactivity correlation. Then, we study the cathodic stability of water in aqueous superconcentrated electrolytes employed for aqueous batteries. Studying these systems, we describe that when two salts sharing the same cation but anions with very different properties (size, geometry) are mixed with water, aqueous biphasic systems can form. Finally, we use this intrinsic non-compatibility between small inorganic anions with large organic anions. Indeed, we demonstrate that the utilization of organic superconcentrated electrolytes can nearly suppress the dissolution of VX3 (X = Cl, Br, I) compounds usually observed in diluted electrolytes, allowing us to expand intercalation electrochemistry to the layered halides family

Impact de la structure de solvatation des électrolytes pour le stockage électrochimique de l’énergie

This thesis aims at understanding the role of the electrolyte, and more specifically the role of solvation structures, for electrochemical storage applications. First, this manuscript reviews the most recent advances on the comprehension on the role of the water molecule for the generation of hydrogen in electrolyzers. Then, we set up a strategy, which consists in studying the reactivity of water in an organic inert matrice in order to better understand how non-covalent interactions can tune the water reactivity. Coupling electrochemical measurements with molecular simulations and spectroscopies, we sketch a “molecular” picture of the water environment in these electrolytes and draw a structure-reactivity correlation. Then, we study the cathodic stability of water in aqueous superconcentrated electrolytes employed for aqueous batteries. Studying these systems, we describe that when two salts sharing the same cation but anions with very different properties (size, geometry) are mixed with water, aqueous biphasic systems can form. Finally, we use this intrinsic non-compatibility between small inorganic anions with large organic anions. Indeed, we demonstrate that the utilization of organic superconcentrated electrolytes can nearly suppress the dissolution of VX3 (X = Cl, Br, I) compounds usually observed in diluted electrolytes, allowing us to expand intercalation electrochemistry to the layered halides family.Cette thèse s’intéresse au rôle de l’électrolyte et la structure de solvatation des espèces qui le composent pour des dispositifs de stockage électrochimique de l’énergie. Plus particulièrement, le début de ce manuscrit décrit les récentes avancées dans la compréhension du rôle de la molécule l’eau pour la génération d’hydrogène dans des électrolyseurs. Nous proposons ensuite une stratégie basée sur le confinement de l’eau au sein d’une matrice organique inerte afin de mieux comprendre comment les interactions non-covalentes modifient la cinétique de la réaction de réduction de l’eau. Des simulations moléculaires et l’emploi de techniques spectroscopiques couplées à des mesures électrochimiques permettent d’obtenir une image « moléculaire » de l’environnement de l’eau dans ces électrolytes et de dessiner une corrélation entre structure et réactivité de l’eau. Dans un second temps, nous étudions la stabilité cathodique de l’eau dans des électrolytes aqueux superconcentrés employés dans certaines batteries aqueuses. En étudiant ces systèmes superconcentrés, nous mettons en évidence qu’en présence de deux sels avec des anions aux propriétés (taille et géométrie) opposées, la formation de système aqueux biphasiques est observée. Finalement, nous exploitons cette incompatibilité intrinsèque entre petits anions inorganiques et gros anions organiques. En effet, nous montrons que l’utilisation d’électrolytes superconcentrés permet de drastiquement atténuer la solubilité de composés de type VX3 (X = Cl, Br, I) habituellement observée dans des électrolytes dilués, ce qui nous permet d’étendre les réactions d’intercalations électrochimiques aux halogénures lamellaires

Towards the Understanding of Water-in-Salt Electrolytes: Individual Ion Activities and Liquid Junction Potentials in Highly Concentrated Aqueous Solutions

Highly concentrated electrolytes were recently proposed to improve the performances of aqueous electrochemical systems by delaying the water splitting and increasing the operating voltage for battery applications. While advances were made regarding their implementation in practical devices, debate exists regarding the physical origin for the delayed water reduction occurring at the electrode/electrolyte interface. Evidently, one difficulty resides in our lack of knowledge regarding ions activity arising from this novel class of electrolyte, it being necessary to estimate the Nernst potential of associated redox reactions such as Li+ intercalation or the hydrogen evolution reaction. In this work, we first measured the potential shift of electrodes selective to either Li+, H+ or Zn2+ ions from diluted to highly concentrated regimes in LiCl or LiTFSI solutions. Observing similar shifts for these different cations and environments, we establish that shifts in redox potentials from diluted to highly concentrated regime originates in large from an increase junction potential, it being dependent on the ions activity coefficients that increase with concentration. While our study shows that single ion activity coefficients, unlike mean ion activity coefficients, cannot be captured by any electrochemical means, we demonstrate that protons concentration increases by approximatively two orders of magnitude from 1 mol.kg-1 to 15-20 mol.kg-1 solutions. Combined with the increased activity coefficients, this increases the activity of protons and thus the pH of highly concentrated solutions which appears acidic

Toward the understanding of water-in-salt electrolytes: Individual ion activities and liquid junction potentials in highly concentrated aqueous solutions

International audienceHighly concentrated electrolytes were recently proposed to improve the performances of aqueous electrochemical systems by delaying the water splitting and increasing the operating voltage for battery applications. While advances were made regarding their implementation in practical devices, debate exists regarding the physical origin for the delayed water reduction occurring at the electrode/electrolyte interface. Evidently, one difficulty resides in our lack of knowledge regarding ion activity arising from this novel class of electrolytes, which is necessary to estimate the Nernst potential of associated redox reactions, such as Li+ intercalation or the hydrogen evolution reaction. In this work, we first measured the potential shift of electrodes selective to Li+, H+, or Zn2+ ions from diluted to highly concentrated regimes in LiCl or LiTFSI solutions. Observing similar shifts for these different cations and environments, we establish that shifts in redox potentials from diluted to highly concentrated regimes originate in large from an increased junction potential, which is dependent on the ion activity coefficients that increase with the concentration. While our study shows that single ion activity coefficients, unlike mean ion activity coefficients, cannot be captured by any electrochemical means, we demonstrate that the proton concentration increases by one to two orders of magnitude from 1 to 15–20 mol kg−1 solutions. Combined with the increased activity coefficients, this phenomenon increases the activity of protons and thus increases the pH of highly concentrated solutions which appears acidi

Confining Water in Ionic and Organic Solvents to Tune Its Adsorption and Reactivity at Electrified Interfaces

International audienceConspectusThe recent discovery of "water-in-salt" electrolytes has spurred a rebirth of research on aqueous batteries. Most of the attention has been focused on the formulation of salts enabling the electrochemical window to be expanded as much as possible, well beyond the 1.23 V allowed by thermodynamics in water. This approach has led to critical successes, with devices operating at voltages of up to 4 V. These efforts were accompanied by fundamental studies aiming at understanding water speciation and its link with the bulk and interfacial properties of water-in-salt electrolytes. This speciation was found to differ markedly from that in conventional aqueous solutions since most water molecules are involved in the solvation of the cationic species (in general Li+) and thus cannot form their usual hydrogen-bonding network. Instead, it is the anions that tend to self-aggregate in nanodomains and dictate the interfacial and transport properties of the electrolyte. This particular speciation drastically alters the presence and reactivity of the water molecules at electrified interfaces, which enlarges the electrochemical windows of these aqueous electrolytes.Thanks to this fundamental understanding, a second very active lead was recently followed, which consists of using a scarce amount of water in nonaqueous electrolytes in order to control the interfacial properties. Following this path, it was proposed to use an organic solvent such as acetonitrile as a confinement matrix for water. Tuning the salt/water ratio in such systems leads to a whole family of systems that can be used to determine the reactivity of water and control the potential at which the hydrogen evolution reaction occurs. Put together, all of these efforts allow a shift of our view of the water molecule from a passive solvent to a reactant involved in many distinct fields ranging from electrochemical energy storage to (electro)catalysis.Combining spectroscopic and electrochemical techniques with molecular dynamics simulations, we have observed very interesting chemical phenomena such as immiscibility between two aqueous phases, specific adsorption properties of water molecules that strongly affect their reactivity, and complex diffusive mechanisms due to the formation of anionic and aqueous nanodomains