Periodic Reporting for period 3 - AMPERE (Accounting for Metallicity, Polarization of the Electrolyte, and Redox reactions in computational Electrochemistry)
Período documentado: 2021-04-01 hasta 2022-09-30
Applied electrochemistry plays a key role in many technologies, such as batteries, fuel cells, supercapacitors or solar cells. It is therefore at the core of many research programs all over the world. Yet, fundamental electrochemical investigations remain scarce. In particular, electrochemistry is among the fields for which the gap between theory and experiment is the largest. From the computational point of view, there is no molecular dynamics (MD) software devoted to the simulation of electrochemical systems while other fields such as biochemistry (GROMACS) or material science (LAMMPS) have dedicated tools.
This is due to the difficulty of accounting for complex effects arising from (i) the degree of metallicity of the electrode (i.e. from semimetals to perfect conductors), (ii) the mutual polarization occurring at the electrode/electrolyte interface and (iii) the redox reactivity through explicit electron transfers. Current understanding therefore relies on standard theories that derive from an inaccurate molecular-scale picture.
My objective is to fill this gap by introducing a whole set of new methods for simulating electrochemical systems. They will be provided to the computational electrochemistry community as a cutting-edge MD software adapted to supercomputers. First applications will aim at the discovery of new electrolytes for energy storage. Here I will focus on:
(1) ‘‘water-in-salts’’ to understand why these revolutionary liquids enable much higher voltage than conventional solutions
(2) redox reactions inside a nanoporous electrode to support the development of future capacitive energy storage devices.
These selected applications are timely and rely on collaborations with leading experimental partners. The results are expected to shed an unprecedented light on the importance of polarization effects on the structure and the reactivity of electrode/electrolyte interfaces, establishing MD as a prominent tool for solving complex electrochemistry problems.
This is due to the difficulty of accounting for complex effects arising from (i) the degree of metallicity of the electrode (i.e. from semimetals to perfect conductors), (ii) the mutual polarization occurring at the electrode/electrolyte interface and (iii) the redox reactivity through explicit electron transfers. Current understanding therefore relies on standard theories that derive from an inaccurate molecular-scale picture.
My objective is to fill this gap by introducing a whole set of new methods for simulating electrochemical systems. They will be provided to the computational electrochemistry community as a cutting-edge MD software adapted to supercomputers. First applications will aim at the discovery of new electrolytes for energy storage. Here I will focus on:
(1) ‘‘water-in-salts’’ to understand why these revolutionary liquids enable much higher voltage than conventional solutions
(2) redox reactions inside a nanoporous electrode to support the development of future capacitive energy storage devices.
These selected applications are timely and rely on collaborations with leading experimental partners. The results are expected to shed an unprecedented light on the importance of polarization effects on the structure and the reactivity of electrode/electrolyte interfaces, establishing MD as a prominent tool for solving complex electrochemistry problems.
In a first step we have focused on the development of an approach in which a finite field is applied to an electrochemical cell made of a single electrode in contact with an electrolyte (Dufils. et al., Phys. Rev. Lett., 123, 195501, 2019).The polarization by the field yields two electrochemical interfaces on opposite sides of the metal slab. It has two interesting features: i) it allows to switch from 2D to 3D periodic boundary conditions, which will accelerate the simulations. ii) the method is readily extended to perform finite electric displacement simulations, that is to simulate an electrochemical system under open circuit conditions, which was not possible before. Another important methodological advance is the introduction of a mass-zero constrained molecular dynamics algorithm, which has the advantage to be symplectic and time reversible. This will allow long term stability for the simulations and to systematically introduce additional kinematic conditions (Coretti et al., J. Chem. Phys., 152, 194701, 2020). FInally, we have developed the theoretical framework of a semiclassical Thomas-Fermi model to tune the metallicity of electrodes in molecular simulations. By systematically varyiing the corresponding parameters in the simulation of a typical capacitor, we have shown that all the interfacial properties are modified by screening within the metal: the capacitance decreases significantly and both the structure and dynamics of the adsorbed electrolyte are affected (preprint: Scalfi et al., arXiv:1910.13341 2020). Our molecular dynamics code, MetalWalls, include all these developments. It is available under an Open Source license (https://gitlab.com/ampere2/metalwalls) and we have just published a manuscript describing its capabilities (Marin-Laflèche et al., J. Open Source Softw., 5, 2373, 2020).
In parallel, we have also started to simulate practical systems. We mostly focused on the water-in-salts electrolytes. In a first step we have studied the transport and interfacial properties of the most common water-in-salt, namely the one based on the LiTFSI salt (Li et al., J. Phys. Chem. C, 122, 23917, 2018 & J. Phys. Chem. B, 123, 10514, 2019). We have shown that the nanostructuration of the liquid impacts the diffusivities of all the species and the ionic correlations inside the melts. Then we have used our simulations to interpret a series of experimental results: On the one hand, we have shown the occurence of aqueous biphasic systems when mixing water-in-salts with simpler aqueous electrolytes (Dubouis et al., ACS Central Sci., 5, 640, 2019). On the other hand we have provided further understanding on the formation of a stable solid electrolyte interphase in water-in-salt-based Li-ion batteries by establishing a competitive salt precipitation/dissolution during the reduction of water at the interface (Bouchal et al., Angew. Chem., Int. Ed., 59, 15913, 2020). As an extension to this work, we have started a study on the effect of confining water in organic electrolytes (instead of ionic ones as in water-in-salts) (Dubouis et al., J. Phys. Chem. Lett., 9, 6683, 2018). We have shown that these systems provide very important information on the influence of the chemical speciation of water over its interfacial reactivity, with potential impact in the field of electrocatalysis (Dubouis et al., Nature Catalysis, 3, 656, 2020).
The second series of systems that we target, biredox ionic liquids, needed more developments. In particular, there was no force field available to simulate them so we started by performing an (electronic) Density Functional Theory study of their electrochemical properties (Reeves et al., Phys. Chem. Chem. Phys., 22, 10561, 2020). The obtained data are now being used to develop polarizable force fields; and we expect to have finished this step by the end of 2020 in order to concentrate on the simulation of redox properties of these systems using classical molecular dynamics.
It is worth noting that another lead was followed, which was not anticipated upon writing the project. In order to develop a multi-scale approach, we have started to perform simulations in which the molecular dynamics step are replaced by molecular DFT calculations, both to determine capacitive properties (Jeanmairet et al., J. Chem. Phys., 151, 124111, 2019) and electron transfer reactions (Jeanmairet et al., Chem. Sci., 10, 2130, 2019). This approach allows a much more efficient sampling since it consists in computing the minimum of free energy of the system. The gains in computer time could be huge, but many difficulties have to be overcome for simulating complex systems such as water-in-salts or biredox ionic liquids.
In parallel, we have also started to simulate practical systems. We mostly focused on the water-in-salts electrolytes. In a first step we have studied the transport and interfacial properties of the most common water-in-salt, namely the one based on the LiTFSI salt (Li et al., J. Phys. Chem. C, 122, 23917, 2018 & J. Phys. Chem. B, 123, 10514, 2019). We have shown that the nanostructuration of the liquid impacts the diffusivities of all the species and the ionic correlations inside the melts. Then we have used our simulations to interpret a series of experimental results: On the one hand, we have shown the occurence of aqueous biphasic systems when mixing water-in-salts with simpler aqueous electrolytes (Dubouis et al., ACS Central Sci., 5, 640, 2019). On the other hand we have provided further understanding on the formation of a stable solid electrolyte interphase in water-in-salt-based Li-ion batteries by establishing a competitive salt precipitation/dissolution during the reduction of water at the interface (Bouchal et al., Angew. Chem., Int. Ed., 59, 15913, 2020). As an extension to this work, we have started a study on the effect of confining water in organic electrolytes (instead of ionic ones as in water-in-salts) (Dubouis et al., J. Phys. Chem. Lett., 9, 6683, 2018). We have shown that these systems provide very important information on the influence of the chemical speciation of water over its interfacial reactivity, with potential impact in the field of electrocatalysis (Dubouis et al., Nature Catalysis, 3, 656, 2020).
The second series of systems that we target, biredox ionic liquids, needed more developments. In particular, there was no force field available to simulate them so we started by performing an (electronic) Density Functional Theory study of their electrochemical properties (Reeves et al., Phys. Chem. Chem. Phys., 22, 10561, 2020). The obtained data are now being used to develop polarizable force fields; and we expect to have finished this step by the end of 2020 in order to concentrate on the simulation of redox properties of these systems using classical molecular dynamics.
It is worth noting that another lead was followed, which was not anticipated upon writing the project. In order to develop a multi-scale approach, we have started to perform simulations in which the molecular dynamics step are replaced by molecular DFT calculations, both to determine capacitive properties (Jeanmairet et al., J. Chem. Phys., 151, 124111, 2019) and electron transfer reactions (Jeanmairet et al., Chem. Sci., 10, 2130, 2019). This approach allows a much more efficient sampling since it consists in computing the minimum of free energy of the system. The gains in computer time could be huge, but many difficulties have to be overcome for simulating complex systems such as water-in-salts or biredox ionic liquids.
The next methodological steps will consist in (i) introducing mutual polarization effects occurring at the electrode/electrolyte interface, both at equilibrium and during electron transfer events. (ii) to perform classical simulations of explicit electron transfer reactions. To this end, we will introduce a mixed Monte-Carlo/MD scheme in which reactive trajectories are generated for the redox species.
Once these developments are finished, it will be possible to study much more deeply biredox ionic liquids, in order to understand their charging mechanism in supercapacitor applications. In particular our objective is to study the interplay between diffusion and electron transfer kinetics, and how it impacts the power performance of the devices.
Once these developments are finished, it will be possible to study much more deeply biredox ionic liquids, in order to understand their charging mechanism in supercapacitor applications. In particular our objective is to study the interplay between diffusion and electron transfer kinetics, and how it impacts the power performance of the devices.