Molecular simulation of aqueous electrolytes in nanoporous carbons : blue energy and water desalination

Lors du mélange de l'eau douce des rivières avec l'eau salée de la mer, une quantité considérable d'énergie est dissipée. Plusieurs procédés sont actuellement à l'étude pour parvenir à exploiter cette énergie bleue (Blue Energy). Inversement, la désalinisation de l'eau de mer pour la production d'eau potable nécessite de très grandes quantités d'énergie. Depuis la proposition en 2009 d'une nouvelle approche pour parvenir à ces objectifs, grâce à des cycles thermodynamiques reposant sur la charge/décharge d'électrodes à forte/faible concentration en sel, expérimentateurs et ingénieurs ont essayé d'améliorer le procédé. Dans ce contexte, l'utilisation d'électrodes nanoporeuses de carbone semble une piste très prometteuse. Un défi de taille reste à relever pour déterminer les quantités pertinentes (capacité électrique et quantité de sel adsorbé en fonction de la composition de l'électrolyte et de sa concentration). En effet, les modèles traditionnels (Poisson-Boltzmann, etc) ne peuvent pas être utilisés dans ce cas où les interactions au niveau moléculaire jouent un rôle essentiel. Nous surmontons cette difficulté grâce aux simulations de dynamique moléculaire, qui permettent également de comprendre les mécanismes microscopiques à l'origine des propriétés observées. Nous étudions également l'influence de la structure microporeuse de l'électrode de carbone ainsi que l'effet de la nature du sel chimique.When fresh river water mixes with salty sea water, a large amount of energy is lost. Conversely, the desalination of seawater for the production of drinking water requires very large amounts of energy. A new approach has been proposed in 2009 to harvest this "blue energy", thanks to the charge/discharge of electrodes in electrolytes with high/low salt concentration. The use of nanoporous carbon electrodes seems promising, but the traditional models (such as Poisson-Boltzmann) used to determine the relevant quantities do not apply in this case where molecular interactions play an essential role. We overcome this difficulty by performing molecular dynamics simulations of nanoporous carbon electrodes in the presence of an aqueous electrolyte. We evaluate the electrical capacity and the amount of ions adsorbed inside the electrodes as a function of the electrolyte composition and its concentration. In addition, these simulations allow us to understand the microscopic mechanisms leading to the storage of the charge, the effect of the structure of the carbon electrode, the salt concentration in the electrolyte and the chemical nature of the salt

Simulation moléculaire d'électrolytes aqueux dans les carbones nanoporeux : Energie bleue et désalinisation de l'eau

When fresh river water mixes with salty sea water, a large amount of energy is lost. Conversely, the desalination of seawater for the production of drinking water requires very large amounts of energy. A new approach has been proposed in 2009 to harvest this "blue energy", thanks to the charge/discharge of electrodes in electrolytes with high/low salt concentration. The use of nanoporous carbon electrodes seems promising, but the traditional models (such as Poisson-Boltzmann) used to determine the relevant quantities do not apply in this case where molecular interactions play an essential role. We overcome this difficulty by performing molecular dynamics simulations of nanoporous carbon electrodes in the presence of an aqueous electrolyte. We evaluate the electrical capacity and the amount of ions adsorbed inside the electrodes as a function of the electrolyte composition and its concentration. In addition, these simulations allow us to understand the microscopic mechanisms leading to the storage of the charge, the effect of the structure of the carbon electrode, the salt concentration in the electrolyte and the chemical nature of the salt.Lors du mélange de l'eau douce des rivières avec l'eau salée de la mer, une quantité considérable d'énergie est dissipée. Plusieurs procédés sont actuellement à l'étude pour parvenir à exploiter cette énergie bleue (Blue Energy). Inversement, la désalinisation de l'eau de mer pour la production d'eau potable nécessite de très grandes quantités d'énergie. Depuis la proposition en 2009 d'une nouvelle approche pour parvenir à ces objectifs, grâce à des cycles thermodynamiques reposant sur la charge/décharge d'électrodes à forte/faible concentration en sel, expérimentateurs et ingénieurs ont essayé d'améliorer le procédé. Dans ce contexte, l'utilisation d'électrodes nanoporeuses de carbone semble une piste très prometteuse. Un défi de taille reste à relever pour déterminer les quantités pertinentes (capacité électrique et quantité de sel adsorbé en fonction de la composition de l'électrolyte et de sa concentration). En effet, les modèles traditionnels (Poisson-Boltzmann, etc) ne peuvent pas être utilisés dans ce cas où les interactions au niveau moléculaire jouent un rôle essentiel. Nous surmontons cette difficulté grâce aux simulations de dynamique moléculaire, qui permettent également de comprendre les mécanismes microscopiques à l'origine des propriétés observées. Nous étudions également l'influence de la structure microporeuse de l'électrode de carbone ainsi que l'effet de la nature du sel chimique

Simulation moléculaire d'électrolytes aqueux dans les carbones nanoporeux : énergie bleue et désalinisation de l'eau

When fresh river water mixes with salty sea water, a large amount of energy is lost. Conversely, the desalination of seawater for the production of drinking water requires very large amounts of energy. A new approach has been proposed in 2009 to harvest this "blue energy", thanks to the charge/discharge of electrodes in electrolytes with high/low salt concentration. The use of nanoporous carbon electrodes seems promising, but the traditional models (such as Poisson-Boltzmann) used to determine the relevant quantities do not apply in this case where molecular interactions play an essential role. We overcome this difficulty by performing molecular dynamics simulations of nanoporous carbon electrodes in the presence of an aqueous electrolyte. We evaluate the electrical capacity and the amount of ions adsorbed inside the electrodes as a function of the electrolyte composition and its concentration. In addition, these simulations allow us to understand the microscopic mechanisms leading to the storage of the charge, the effect of the structure of the carbon electrode, the salt concentration in the electrolyte and the chemical nature of the salt.Lors du mélange de l'eau douce des rivières avec l'eau salée de la mer, une quantité considérable d'énergie est dissipée. Plusieurs procédés sont actuellement à l'étude pour parvenir à exploiter cette énergie bleue (Blue Energy). Inversement, la désalinisation de l'eau de mer pour la production d'eau potable nécessite de très grandes quantités d'énergie. Depuis la proposition en 2009 d'une nouvelle approche pour parvenir à ces objectifs, grâce à des cycles thermodynamiques reposant sur la charge/décharge d'électrodes à forte/faible concentration en sel, expérimentateurs et ingénieurs ont essayé d'améliorer le procédé. Dans ce contexte, l'utilisation d'électrodes nanoporeuses de carbone semble une piste très prometteuse. Un défi de taille reste à relever pour déterminer les quantités pertinentes (capacité électrique et quantité de sel adsorbé en fonction de la composition de l'électrolyte et de sa concentration). En effet, les modèles traditionnels (Poisson-Boltzmann, etc) ne peuvent pas être utilisés dans ce cas où les interactions au niveau moléculaire jouent un rôle essentiel. Nous surmontons cette difficulté grâce aux simulations de dynamique moléculaire, qui permettent également de comprendre les mécanismes microscopiques à l'origine des propriétés observées. Nous étudions également l'influence de la structure microporeuse de l'électrode de carbone ainsi que l'effet de la nature du sel chimique

Performance of Microporous Carbon Electrodes for Supercapacitors: Comparing Graphene with Disordered Materials

Over the past decades, the specific surface area and the pore size distribution have been identified as the main structural features that govern the performance of carbon-based supercapacitors. As a consequence, graphene nanostructures have been identified as strong candidates for maximizing their capacitance. However, this hypothesis could not be thoroughly tested so far due to the difficulty of synthesizing perfect materials with high pore accesibility and a sufficiently large density. Here we perform molecular simulations of a series of perforated graphene electrodes with single pore sizes ranging from 7 to 10 Angstroms in contact with an adsorbed ionic liquid, and compare the capacitances (using various metrics) to the one obtained with a typical disordered nanoporous carbon. The latter displays better performances, an observation that we explain by analyzing the structure of the liquid inside the pores. It appears that although the smaller pores are responsible for the largest surface charges, larger ones are also necessary to store the counter-ions and avoid the formation of detrimental opposite charges on the carbon. These results rationalize the need for disordered or activated carbon materials to design efficient supercapacitors

Performance of Microporous Carbon Electrodes for Supercapacitors: Comparing Graphene with Disordered Materials

International audienceOver the past decades, the specific surface area and the pore size distribution have been identified as the main structural features that govern the performance of carbon-based supercapacitors. As a consequence, graphene nanostructures have been identified as strong candidates for maximizing their capacitance. However, this hypothesis could not be thoroughly tested so far due to the difficulty of synthesizing perfect materials with high pore accesibility and a sufficiently large density. Here we perform molecular simulations of a series of perforated graphene electrodes with single pore sizes ranging from 7 to 10 Angstroms in contact with an adsorbed ionic liquid, and compare the capacitances (using various metrics) to the one obtained with a typical disordered nanoporous carbon. The latter displays better performances, an observation that we explain by analyzing the structure of the liquid inside the pores. It appears that although the smaller pores are responsible for the largest surface charges, larger ones are also necessary to store the counter-ions and avoid the formation of detrimental opposite charges on the carbon. These results rationalize the need for disordered or activated carbon materials to design efficient supercapacitors

Blue Energy and Desalination with Nanoporous Carbon Electrodes: Capacitance from Molecular Simulations to Continuous Models

Capacitive mixing (CapMix) and capacitive deionization (CDI) are currently developed as alternatives to membrane-based processes to harvest blue energy—from salinity gradients between river and sea water—and to desalinate water—using charge-discharge cycles of capacitors. Nanoporous electrodes increase the contact area with the electrolyte and hence, in principle, also the performance of the process. However, models to design and optimize devices should be used with caution when the size of the pores becomes comparable to that of ions and water molecules. Here, we address this issue by simulating realistic capacitors based on aqueous electrolytes and nanoporous carbide-derived carbon (CDC) electrodes, accounting for both their complex structure and their polarization by the electrolyte under applied voltage. We compute the capacitance for two salt concentrations and validate our simulations by comparison with cyclic voltammetry experiments. We discuss the predictions of Debye-Hückel and Poisson-Boltzmann theories, as well as modified Donnan models, and we show that the latter can be parametrized using the molecular simulation results at high concentration. This then allows us to extrapolate the capacitance and salt adsorption capacity at lower concentrations, which cannot be simulated, finding a reasonable agreement with the experimental capacitance. We analyze the solvation of ions and their confinement within the electrodes—microscopic properties that are much more difficult to obtain experimentally than the electrochemical response but very important to understand the mechanisms at play. We finally discuss the implications of our findings for CapMix and CDI, both from the modeling point of view and from the use of CDCs in these contexts

Effect of the carbon microporous structure on the capacitance of aqueous supercapacitors

International audienceUsing a combination of cyclic voltammetry experiments and molecular dynamics simulations, we study the effect of microporous carbon structure on the performance of aqueous supercapacitors using carbide derived carbon (CDC) electrodes. The structures investigated by molecular simulations are compatible with the experimental results for CDC synthesized at 800°C, but not with the other two materials (CDC-1100 and YP-50F), which are more graphitic. In fact, the specific capacitance obtained for the latter two are in good agreement with molecular simulations of graphite electrodes, assuming that all the charge is localized in the first plane in contact with the electrode (a very good approximation). Our molecular simulations further allow to examine the solvation of ions inside the electrodes. Unlike what was observed for large organic ions dissolved in acetonitrile, we find that most Na+ cations remain fully solvated. Overall, microporous carbons such as CDCs are good candidates for applications involving aqueous supercapacitors, in particular the harvesting of blue energy or desalination, but their performance remains to be optimized by tailoring their microstructure