Alexis Grimaud

Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis

In this Review, we discuss the state-of-the-art understanding of non-precious transition metal oxides that catalyze the oxygen reduction and evolution reactions. Understanding and mastering the kinetics of oxygen electrocatalysis is instrumental to making use of photosynthesis, advancing solar fuels, fuel cells, electrolyzers, and metal–air batteries. We first present key insights, assumptions and limitations of well-known activity descriptors and reaction mechanisms in the past four decades. The turnover frequency of crystalline oxides as promising catalysts is also put into perspective with amorphous oxides and photosystem II. Particular attention is paid to electronic structure parameters that can potentially govern the adsorbate binding strength and thus provide simple rationales and design principles to predict new catalyst chemistries with enhanced activity. We share new perspective synthesizing mechanism and electronic descriptors developed from both molecular orbital and solid state band structure principles. We conclude with an outlook on the opportunities in future research within this rapidly developing field.

Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution

The electronic structure of transition metal oxides governs the catalysis of many central reactions for energy storage applications such as oxygen electrocatalysis. Here we exploit the versatility of the perovskite structure to search for oxide catalysts that are both active and stable. We report double perovskites (Ln₀.₅Ba₀.₅)CoO(₃-δ) (Ln=Pr, Sm, Gd and Ho) as a family of highly active catalysts for the oxygen evolution reaction upon water oxidation in alkaline solution. These double perovskites are stable unlike pseudocubic perovskites with comparable activities such as Ba₀.₅Sr₀.₅Co₀.₈Fe₀.₂O(₃-δ) which readily amorphize during the oxygen evolution reaction. The high activity and stability of these double perovskites can be explained by having the O p-band centre neither too close nor too far from the Fermi level, which is computed from ab initio studies.

Electrode–Electrolyte Interface in Li-Ion Batteries: Current Understanding and New Insights

Understanding reactions at the electrode/electrolyte interface (EEI) is essential to developing strategies to enhance cycle life and safety of lithium batteries. Despite research in the past four decades, there is still limited understanding by what means different components are formed at the EEI and how they influence EEI layer properties. We review findings used to establish the well-known mosaic structure model for the EEI (often referred to as solid electrolyte interphase or SEI) on negative electrodes including lithium, graphite, tin, and silicon. Much less understanding exists for EEI layers for positive electrodes. High-capacity Li-rich layered oxides yLi2-xMnO3·(1-y)Li1-xMO2, which can generate highly reactive species toward the electrolyte via oxygen anion redox, highlight the critical need to understand reactions with the electrolyte and EEI layers for advanced positive electrodes. Recent advances in in situ characterization of well-defined electrode surfaces can provide mechanistic insights and strategies to tailor EEI layer composition and properties.

Anionic redox processes for electrochemical devices

Understanding and controlling anionic redox processes is pivotal for the design of new Li-ion battery and water-splitting materials.

Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution

Understanding how materials that catalyse the oxygen evolution reaction (OER) function is essential for the development of efficient energy-storage technologies. The traditional understanding of the OER mechanism on metal oxides involves four concerted proton-electron transfer steps on metal-ion centres at their surface and product oxygen molecules derived from water. Here, using in situ 18O isotope labelling mass spectrometry, we provide direct experimental evidence that the O2 generated during the OER on some highly active oxides can come from lattice oxygen. The oxides capable of lattice-oxygen oxidation also exhibit pH-dependent OER activity on the reversible hydrogen electrode scale, indicating non-concerted proton-electron transfers in the OER mechanism. Based on our experimental data and density functional theory calculations, we discuss mechanisms that are fundamentally different from the conventional scheme and show that increasing the covalency of metal-oxygen bonds is critical to trigger lattice-oxygen oxidation and enable non-concerted proton-electron transfers during OER.

Solid-state activation of Li2O2 oxidation kinetics and implications for Li–O2 batteries

As one of the most theoretically promising next-generation chemistries, Li–O2 batteries are the subject of intense research to address their stability, cycling, and efficiency issues. The recharge kinetics of Li–O2 are especially sluggish, prompting the use of metal nanoparticles as reaction promoters. In this work, we probe the underlying pathway of kinetics enhancement by transition metal and oxide particles using a combination of electrochemistry, X-ray absorption spectroscopy, and thermochemical analysis in carbon-free and carbon-containing electrodes. We highlight the high activity of the group VI transition metals Mo and Cr, which are comparable to noble metal Ru and coincide with XAS measured changes in surface oxidation state matched to the formation of Li2MoO4 and Li2CrO4. A strong correlation between conversion enthalpies of Li2O2 with the promoter surface (Li2O2 + MaOb ± O2 → LixMyOz) and electrochemical activity is found that unifies the behaviour of solid-state promoters. In the absence of soluble species on charge and the decomposition of Li2O2 proceeding through solid solution, enhancement of Li2O2 oxidation is mediated by chemical conversion of Li2O2 with slow oxidation kinetics to a lithium metal oxide. Our mechanistic findings provide new insights into the selection and/or employment of electrode chemistry in Li–O2 batteries.

Chemical vs Electrochemical Formation of Li2CO3 as a Discharge Product in Li–O2/CO2 Batteries by Controlling the Superoxide Intermediate

The Li-O2/CO2 battery with high capacity has recently been proposed as a new protocol to convert CO2. However, the fundamental mechanism for the reaction still remains hazy. Here, we investigated the discharge processes of Li-O2/CO2 (70%/30%) batteries in two solvents, dimethyl sulfoxide (DMSO) and 1,2-dimethoxyethane (DME). During discharge, both solvents initially show the reduction of oxygen. However, afterward, the solvent affects the reaction pathways of superoxide species by solvating Li+ with different strength, depending on the so-called donor number. More precisely, the initial formation of CO4•- is favored in DMSO at the expense of lithium superoxide formation that we observed in DME. Despite the different intermediate processes, X-ray diffraction showed that Li2CO3 was the final discharge product in both solvents. Moreover, we observed that CO2 cannot be reduced within the electrochemical stability window of DMSO and DME.

Charge-transfer-energy-dependent oxygen evolution reaction mechanisms for perovskite oxides

Numerous studies have reported electronic activity descriptors of oxygen evolution reaction (OER) for oxide catalysts under a single reaction mechanism. However, recent works have revealed that a single mechanism is not at play across oxide chemistries. These works underscore a need to deeply investigate the electronic structure details of active oxide catalysts and how they align with the OER potential, which is critical to understanding the interfacial charge-transfer kinetics that dictate catalytic mechanisms. In this work, we use soft X-ray emission and absorption spectroscopy of perovskites to analyze the partial density of states on an absolute energy scale, from which energetic barriers for electron transfer and surface deprotonation were estimated and correlated with OER activity. Through this lens, we identify that decreasing the solid-state charge-transfer energy of perovskites can change the mechanisms of the OER from electron-transfer-limited to proton–electron-coupled, to proton-transfer-limited reactions. This concept is supported by the analysis of potential energy surfaces for sequential and concerted proton–electron transfer pathways using a Marcus model. Our work highlights the importance of understanding the physical origin of experimental OER activity trends with electronic descriptors and the need to promote surface deprotonation from oxides to discover new catalysts with enhanced activity.

Strontium influence on the oxygen electrocatalysis of La2−xSrxNiO4±δ (0.0 ≤ xSr ≤ 1.0) thin films

Substitution of lanthanum by strontium (Sr) in the A-site of cobalt-containing perovskites can greatly promote oxygen surface exchange kinetics at elevated temperatures. Little is known about the effect of A-site substitution on the oxygen electrocatalysis of Ruddlesden–Popper (RP) oxides. In this study, we report, for the first time, the growth and oxygen surface exchange kinetics of La2−xSrxNiO4±δ (LSNO, 0.0 ≤ xSr ≤ 1.0) thin films grown on (001)cubic-Y2O3-stabilized ZrO2 (YSZ) by pulsed laser deposition. High-resolution X-ray diffraction analysis revealed that the LSNO film orientation was changed gradually from the (100)tetra. (in-plane) to the (001)tetra. (out-of-plane) orientation in the RP structure with increasing Sr from La2NiO4+δ (xSr = 0) to LaSrNiO4±δ (xSr = 1.0). Such a change in the LSNO film orientation was accompanied by reduction in the oxygen surface exchange kinetics by two orders of magnitude as shown from electrochemical impedance spectroscopy results. Density functional theory (DFT) calculations showed that Sr substitution could stabilize the (001)tetra. surface relative to the (100)tetra. surface and both Sr substitution and increasing (001)tetra. surface could greatly weaken adsorption of molecular oxygen in the La–La bridge sites in the RP structure, which can reduce oxygen surface exchange kinetics.

The role of hydrogen evolution reaction on the solid electrolyte interphase formation mechanism for “Water-in-Salt” electrolytes

Aqueous Li-ion batteries have long been envisioned as safe and green energy storage technology, but have never been commercially realized owing to the limited electrochemical stability window of water, which drastically hampers their energy density. Recently, Water-in-Salt electrolytes (WiSEs) in which a large amount of organic salt is dissolved into water were proposed to allow for assembling 3 V Li-ion batteries. Hereby, our attention focused on the fate of water at the electrochemical interface under negative polarization and the potential reactivity of TFSI anions with products originating from the water reduction. Hence, combining analysis of bulk electrolytes with electrochemical measurements on model electrodes and operando characterization, we were able to demonstrate that hydroxides generated during the hydrogen evolution reaction can chemically react with TFSI and catalyze the formation of a fluorinated solid–electrolyte interphase (SEI) that prevents further water reduction. Mastering this new SEI formation path with the chemical degradation of TFSI anions mediated by the electrochemical reduction of water can therefore open new avenues for the future development of not only WiSEs but also Li batteries functioning in organic electrolytes.