Carbon–carbon supercapacitors: Beyond the average pore size or how electrolyte confinement and inaccessible pores affect the capacitance

Carbon–carbon supercapacitors are high power electrochemical energy storage systems, which store energy through reversible ion adsorption at the electrode–electrolyte interface. Due to the complex structure of the porous carbons used as electrodes, extracting structure–property relationships in these systems remains a challenge. In this work, we conduct molecular simulations of two model supercapacitors based on nanoporous electrodes with the same average pore size, a property often used when comparing porous materials, but different morphologies. We show that the carbon with the more ordered structure, and a well defined pore size, has a much higher capacitance than the carbon with the more disordered structure and a broader pore size distribution. We analyze the structure of the confined electrolyte and show that the ions adsorbed in the ordered carbon are present in larger quantities and are also more confined than for the disordered carbon. Both aspects favor a better charge separation and thus a larger capacitance. In addition, the disordered electrodes contain a significant amount of carbon atoms, which are never in contact with the electrolyte, carry a close to zero charge, and are thus not involved in the charge storage. The total quantities of adsorbed ions and degrees of confinement do not change much with the applied potential, and as such, this work opens the door to computationally tractable screening strategies

Ionic Liquids under Confinement: From Systematic Variations of the Ion and Pore Sizes toward an Understanding of the Structure and Dynamics in Complex Porous Carbons

We use molecular simulations of an ionic liquid in contact with a range of nanoporous carbons to investigate correlations between the ion size, pore size, pore topology, and properties of the adsorbed ions. We show that diffusion coefficients increase with the anion size and, surprisingly, with the quantity of adsorbed ions. Both findings are interpreted in terms of confinement: when the in-pore population increases, additional ions are located in less-confined sites and diffuse faster. Simulations in which the pores are enlarged while keeping the topology constant support these observations. The interpretation of properties across structures is more challenging. An interesting point is that smaller pores do not necessarily lead to a larger confinement. In this work, the highest degrees of confinement are observed for intermediate pore sizes. We also show a correlation between the quantity of adsorbed ions and the ratio between the maximum pore diameter and the pore limiting diamete

On the development of an original mesoscopic model to predict the capacitive properties of carbon-carbon supercapacitors

We report on the development of a lattice model to predict structural, dynamical and capacitive properties of electrochemical double layer capacitors. The model uses input from molecular simulations, such as free energy profiles to describe the ion adsorption, and experiments, such as energy barriers for transitions between lattice sites. The model developed is approximately 10,000 times faster than common molecular simulations. We apply this model to a set of carbon structures with well-defined pore sizes and investigate the solvation effect by doing simulations with neat ionic liquids as well as acetonitrile-based electrolytes. We show that our model is able to predict quantities of adsorbed ions and capacitances in a range compatible with experimental values. We show that there is a strong dependency of the calculated properties on the pore size and on the presence or absence of solvent. In particular, for neat ionic liquids, larger capacitances are obtained for smaller pores, while the opposite trend is observed for organic electrolytes

Simulations of ion adsorption in nanoporous carbons to investigate the relationship between structure-performance in supercapacitors

Les supercondensateurs sont des systèmes de stockage d'énergie non faradiques. Leur processus de charge-décharge, de nature électrostatique, est basé sur l'adsorption-désorption d'ions de l'électrolyte à la surface des électrodes. Ceci permet aux supercondensateurs d'avoir une excellente performance en terme de puissance par rapport aux systèmes faradiques comme les batteries. En revanche, ils se caractérisent par une relativement faible densité d'énergie. Les supercondensateurs de type carbone - carbone, étudiés ici, utilisent des carbones poreux comme matériaux d'électrodes. L'examen des relations entre les caractéristiques de ces systèmes et leurs propriétés électrochimiques peuvent permettre de mieux les optimiser. Dans le passé, il a été montré que les ions pouvaient entrer dans des pores de taille sub-nanométrique, conduisant à une forte augmentation de capacité. Cette observation, depuis assez bien comprise, ne permet cependant pas de savoir s'il est possible d'obtenir des capacités encore plus grandes. En effet, en raison de la structure complexe des électrodes et de la nature de l'électrolyte (solution concentrée), il est difficile de prédire les performances électrochimiques de ces systèmes. Pour progresser, nous avons besoin d'une meilleure connaissance fondamentale du transport ionique et de la structure locale de l'électrolyte dans les pores. Dans cette thèse, nous avons réalisé des simulations de dynamique moléculaire classique pour déterminer les propriétés dynamiques et structurales de deux types de systèmes avec soit un carbone neutre soit deux électrodes de carbones en contact avec un liquide ionique. Nous nous sommes d'abord concentrés sur des carbones poreux de structure ordonnée. Ceci nous a permis de faire varier systématiquement certains descripteurs géométriques, tels que la taille des pores et la taille des ions. Nous avons déterminé les propriétés structurales et dynamiques et montré que les coefficients de diffusion des ions augmentent quand la taille de l'anion diminue et, de manière surprenante, quand la quantité d'ions adsorbés dans les pores augmente. Ces résultats ont pu être interprétés en termes de degrés de confinement. Nous avons également étudié l'influence de la présence de groupes fonctionnels de surface (-O et -H) sur les propriétés du liquide ionique confiné. Nous avons effectué des simulations avec des répliques de zéolite, de structure ordonnée, avec et sans groupes fonctionnels. Nos simulations montrent que les coefficients de diffusion des ions sont plus grands pour les carbones fonctionnalisés, en accord avec les résultats expérimentaux. L'influence relative du changement de structure et de la nature chimique du matériau a été étudiée en remplaçant les groupes fonctionnels par des atomes de carbones et a montré que, pour la quantité d'ions adsorbés, la structure a plus d'impact que les groupements fonctionnels, mais que pour les coefficients de diffusion, la nature chimique semble avoir une certaine importance. Enfin, nous avons réalisé des simulations pour deux supercondensateurs modèles, avec des électrodes de structures différentes mais ayant une même densité et une même taille de pore moyenne. L'un des supercondensateurs a des électrodes de structure régulière et l'autre a des électrodes de structure désordonnée. Les simulations montrent que le carbone régulier a une capacité plus grande que le carbone désordonné. L'analyse des charges portées par les atomes des électrodes et de la localisation des ions adsorbés indiquent que la faible capacité du carbone désordonné est liée à l'existence de nombreux carbones inaccessibles au liquide.Supercapacitors are non-faradaic energy storage systems. Their electrostatic charge-discharge process is based on the adsorption-desorption of electrolyte ions on the surface of the electrodes. This allows supercapacitors to have an excellent performance in terms of power compared to faradaic systems such as batteries. However, they are characterized by a relatively low energy density. Carbon-carbon supercapacitors, studied here, use porous carbons as electrode materials. Examining the relationships between the characteristics of these systems and their electrochemical properties can allow for their optimization. In the past, it has been shown that ions can enter the pores of sub-nanometric size, leading to a large increase in capacitance. This observation, which has been fairly well understood since then, does not allow us to conclude whether it is possible to obtain even greater capacitances. Indeed, due to the complex structure of the electrodes and the nature of the electrolyte (concentrated solution), it is difficult to predict the electrochemical performance of these systems. To progress, we need a better fundamental understanding of ion transport and local structure of the electrolyte in the pores. In this thesis, we carried out classical molecular dynamics simulations to determine the dynamic and structural properties of two types of systems with either a neutral carbon or two carbon electrodes in contact with an ionic liquid. We first focused on porous carbons with an ordered structure. This allowed us to systematically vary certain geometric descriptors, such as the pore size and the ion size. We determined the quantity of ions confined in the pores and their diffusion coefficients and showed that the diffusion coefficients of the ions increase when the anion size decreases and, surprisingly, when the quantity of ions adsorbed in the pores increases. These results could be interpreted in terms of degrees of confinement. We also studied the influence of the presence of functional surface groups (-O and -H) on the properties of the confined ionic liquid. We carried out simulations with zeolite templated carbons, which have an ordered structure, with and without functional groups. Our simulations show that the ion diffusion coefficients are greater for the functionalized carbons, in agreement with the experimental results. The relative influence of the change in structure and the chemical nature of the material has been studied by replacing functional groups with carbon atoms and has shown that, for quantities of adsorbed ions, the structure has more impact than functional groups but, for diffusion, the chemical nature of the surface groups has some impact. Finally, we carried out simulations for two model supercapacitors, with electrodes having different structures but the same density and the same average pore size. One of the supercapacitors has electrodes of regular structure and the other has disordered electrodes. The simulations show that the regular carbon has a greater capacity than the disordered carbon. The analysis of the charges carried by the electrode atoms and the localisation of the adsorbed ions suggest that the lower capacity of the disordered carbon is linked to carbons inaccessible to the liquid

Simulations de l'adsorption des ions dans des carbones poreux modèles pour étudier les relations structure-performance dans les supercondensateurs

Supercapacitors are non-faradaic energy storage systems. Their electrostatic charge-discharge process is based on the adsorption-desorption of electrolyte ions on the surface of the electrodes. This allows supercapacitors to have an excellent performance in terms of power compared to faradaic systems such as batteries. However, they are characterized by a relatively low energy density. Carbon-carbon supercapacitors, studied here, use porous carbons as electrode materials. Examining the relationships between the characteristics of these systems and their electrochemical properties can allow for their optimization. In the past, it has been shown that ions can enter the pores of sub-nanometric size, leading to a large increase in capacitance. This observation, which has been fairly well understood since then, does not allow us to conclude whether it is possible to obtain even greater capacitances. Indeed, due to the complex structure of the electrodes and the nature of the electrolyte (concentrated solution), it is difficult to predict the electrochemical performance of these systems. To progress, we need a better fundamental understanding of ion transport and local structure of the electrolyte in the pores. In this thesis, we carried out classical molecular dynamics simulations to determine the dynamic and structural properties of two types of systems with either a neutral carbon or two carbon electrodes in contact with an ionic liquid. We first focused on porous carbons with an ordered structure. This allowed us to systematically vary certain geometric descriptors, such as the pore size and the ion size. We determined the quantity of ions confined in the pores and their diffusion coefficients and showed that the diffusion coefficients of the ions increase when the anion size decreases and, surprisingly, when the quantity of ions adsorbed in the pores increases. These results could be interpreted in terms of degrees of confinement. We also studied the influence of the presence of functional surface groups (-O and -H) on the properties of the confined ionic liquid. We carried out simulations with zeolite templated carbons, which have an ordered structure, with and without functional groups. Our simulations show that the ion diffusion coefficients are greater for the functionalized carbons, in agreement with the experimental results. The relative influence of the change in structure and the chemical nature of the material has been studied by replacing functional groups with carbon atoms and has shown that, for quantities of adsorbed ions, the structure has more impact than functional groups but, for diffusion, the chemical nature of the surface groups has some impact. Finally, we carried out simulations for two model supercapacitors, with electrodes having different structures but the same density and the same average pore size. One of the supercapacitors has electrodes of regular structure and the other has disordered electrodes. The simulations show that the regular carbon has a greater capacity than the disordered carbon. The analysis of the charges carried by the electrode atoms and the localisation of the adsorbed ions suggest that the lower capacity of the disordered carbon is linked to carbons inaccessible to the liquid.Les supercondensateurs sont des systèmes de stockage d'énergie non faradiques. Leur processus de charge-décharge, de nature électrostatique, est basé sur l'adsorption-désorption d'ions de l'électrolyte à la surface des électrodes. Ceci permet aux supercondensateurs d'avoir une excellente performance en terme de puissance par rapport aux systèmes faradiques comme les batteries. En revanche, ils se caractérisent par une relativement faible densité d'énergie. Les supercondensateurs de type carbone - carbone, étudiés ici, utilisent des carbones poreux comme matériaux d'électrodes. L'examen des relations entre les caractéristiques de ces systèmes et leurs propriétés électrochimiques peuvent permettre de mieux les optimiser. Dans le passé, il a été montré que les ions pouvaient entrer dans des pores de taille sub-nanométrique, conduisant à une forte augmentation de capacité. Cette observation, depuis assez bien comprise, ne permet cependant pas de savoir s'il est possible d'obtenir des capacités encore plus grandes. En effet, en raison de la structure complexe des électrodes et de la nature de l'électrolyte (solution concentrée), il est difficile de prédire les performances électrochimiques de ces systèmes. Pour progresser, nous avons besoin d'une meilleure connaissance fondamentale du transport ionique et de la structure locale de l'électrolyte dans les pores. Dans cette thèse, nous avons réalisé des simulations de dynamique moléculaire classique pour déterminer les propriétés dynamiques et structurales de deux types de systèmes avec soit un carbone neutre soit deux électrodes de carbones en contact avec un liquide ionique. Nous nous sommes d'abord concentrés sur des carbones poreux de structure ordonnée. Ceci nous a permis de faire varier systématiquement certains descripteurs géométriques, tels que la taille des pores et la taille des ions. Nous avons déterminé les propriétés structurales et dynamiques et montré que les coefficients de diffusion des ions augmentent quand la taille de l'anion diminue et, de manière surprenante, quand la quantité d'ions adsorbés dans les pores augmente. Ces résultats ont pu être interprétés en termes de degrés de confinement. Nous avons également étudié l'influence de la présence de groupes fonctionnels de surface (-O et -H) sur les propriétés du liquide ionique confiné. Nous avons effectué des simulations avec des répliques de zéolite, de structure ordonnée, avec et sans groupes fonctionnels. Nos simulations montrent que les coefficients de diffusion des ions sont plus grands pour les carbones fonctionnalisés, en accord avec les résultats expérimentaux. L'influence relative du changement de structure et de la nature chimique du matériau a été étudiée en remplaçant les groupes fonctionnels par des atomes de carbones et a montré que, pour la quantité d'ions adsorbés, la structure a plus d'impact que les groupements fonctionnels, mais que pour les coefficients de diffusion, la nature chimique semble avoir une certaine importance. Enfin, nous avons réalisé des simulations pour deux supercondensateurs modèles, avec des électrodes de structures différentes mais ayant une même densité et une même taille de pore moyenne. L'un des supercondensateurs a des électrodes de structure régulière et l'autre a des électrodes de structure désordonnée. Les simulations montrent que le carbone régulier a une capacité plus grande que le carbone désordonné. L'analyse des charges portées par les atomes des électrodes et de la localisation des ions adsorbés indiquent que la faible capacité du carbone désordonné est liée à l'existence de nombreux carbones inaccessibles au liquide

Simulations of Ionic Liquids Confined in Surface Functionalized Nanoporous Carbons: Implications for Energy Storage

International audiencePorous carbons are used in a wide range of applications, including electrochemical double layer capacitors for energy storage, in which electrolyte ion properties under confinement are crucial for the performance of the systems. While many synthesis techniques lead to the presence of surface functional groups, their effect on the adsorption and diffusion of electrolyte ions is still poorly understood. In this study we investigate the effect of surface chemistry on dynamical and structural properties of adsorbed ions through molecular dynamics simulations of a neat ionic liquid in contact with several zeolite templated carbons, used here as model materials for the larger range of existing ordered microporous carbons. The steric and specific influence of functional groups is explored using three structures: a structure without any functional groups; a structure with ester, hydroxyl, anhydride acid and carboxyl functional groups; and a structure where the oxygen and hydrogen atoms are replaced by carbon atoms. We calculate quantities of adsorbed ions and diffusion coefficients which show variations with the addition of functional groups in agreement with experimental data for similar systems. We observe that the functionalization has a limited impact on the structure of the confined electrolyte but affects the ion mobility more markedly, and that the relative importance of the structure and chemical nature of the functional groups is not the same depending on the ion type and property considered

Ab Initio Screening of Divalent Cations for CH4, CO2, H2, and N2 Separations in Chabazite Zeolite

International audienceThe efficient separation and adsorption of critical gases are, more than ever, a major focus point in important energy processes, such as CH4 enrichment of biogas or natural gas, CO2 separation and capture, and H2 purification and storage. Thanks to its physicochemical properties, cation-exchanged chabazite is a potent zeolite for such applications. Previous computational screening investigations have mostly examined chabazites exchanged with monovalent cations. Therefore, in this contribution, periodic density functional theory (DFT) calculations in combination with dispersion corrections have been used for a systematic screening of divalent cation exchanged chabazite zeolites. The work focuses on cheap and readily available divalent cations, Ca(II), Mg(II), and Zn(II), Fe(II), Sn(II), and Cu(II) and investigates the effect of the cation nature and location within the framework on the adsorption selectivity of chabazite for specific gas separations, namely, CO2/CH4, N2/CH4, and N2/H2. All the cationic adsorption sites were explored to describe the diversity of sites in a typical experimental chabazite with a Si/Al ratio close to 2 or 3. The results revealed that Mg-CHA is the most promising cation for the selective adsorption of CO2. These predictions were further supported by ab initio molecular dynamics simulations performed at 300 K, which demonstrated that the presence of CH4 has a negligible impact on the adsorption of CO2 on Mg-CHA. Ca(II) was found to be the most favorable cation for the selective adsorption of H2 and CO2. Finally, none of the investigated cations were suitable for the preferential capture of N2 and H2 in the purification of CH4 rich mixtures. These findings provide valuable insights into the factors influencing the adsorption behavior of N2, H2, CH4, and CO2 and highlight the crucial role played by theoretical calculations and simulations for the optimal design of efficient adsorbents