Structure-performance relationships in porous carbons for energy storage

Our society currently relies mostly on fossil fuels, harmful to the environment, for its energy consumption. Due to their damaging nature, there is an urging need to move towards more sustainable forms of energy production and transportation. In addition to large-scale energy storage systems, necessary to store energy from intermittent renewable energies such as solar and wind energies, energy storage systems can be used to reduce greenhouse gas emissions in applications such as regenerative energy braking or start-stop systems.
In this context, supercapacitors are of great interest as they exhibit very high rates of charge/discharge, long cycle lifes, and they are made of cheap and light materials. These attractive properties arise from the electrostatic nature of the charge storage which results from ion adsorption in the electrode pores. In 2006, it was demonstrated that ions can enter pores of sub-nanometer sizes leading to a huge increase of capacitance. This was an important breakthrough as the energy density of supercapacitors, relatively low compared to batteries, is what currently limits their application.
While this finding has generated a great deal of technological activity to refine potential devices and fundamental research to examine the underlying molecular phenomena, no major improvements of the energy density were observed. The progress towards more powerful supercapacitors is limited by our incomplete understanding of the relation between their performance, in particular their capacitance and charging rate, and the complex structure of the porous carbon electrodes.
The aim of SuPERPORES was to carry out a systematic multi-scale simulation study of supercapacitors. We have used a combination of molecular and mesoscopic simulations to calculate the capacitive properties of a wide range of systems. Molecular simulations were used to model ordered three-dimensional porous carbons. The systematic study of the effects of ion and pore size on the carbon-electrolyte interfacial properties have allowed us to extract key trends. In parallel, we have developed very efficient lattice simulations to calculate capacitive properties. The simulations can also provide other macroscopic properties, e.g. Nuclear Magnetic Resonance (NMR) spectra, which can potentially be used to propose a new method to determine accurately the structure of disordered porous carbons.

One of the main issues when simulating supercapacitors is the generation of realistic three-dimensional porous carbon models. Indeed, due to their disordered nature, it is a challenge to determine their structure in a precise manner. The models we use are of two types: atomistic structures (for molecular simulations) and pore network models (for mesoscopic simulations). The most common method to generate atomistic models is to conduct quench molecular dynamics simulations using interaction potentials, parameterised on experimental data, which sometimes exaggerate certain characteristics (e.g. bonds or angles too rigid). With the progress of machine learning, it is possible to generate interaction potentials with a precision equivalent to Density Functional Theory (DFT). Following such methodology, we generated porous carbon structures of various densities. For the pore network models, we followed a strategy to extract pore networks directly from tomography images.
Regarding the systematic study at the molecular scale, simulations of several electrolytes in contact with a set of ordered and disordered porous carbons were conducted. In total, around 50 carbon/electrolyte systems were simulated to investigate the effects of pore size, topology and ion size on various properties. We demonstrated that diffusion coefficients increase with the anion size and, surprisingly, with the quantity of adsorbed ions. Both findings were interpreted in terms of confinement: when the in-pore population increases, additional ions are located in less-confined sites and diffuse faster. The interpretation of properties across structures is more challenging. The effects of the carbon structure flexibility and the presence of functional groups were also explored. The most promising carbons were used in simulations of model supercapacitors showing that porous carbons with equal average pore sizes but different topologies can have very different capacities and related this observation to the amount of carbon atoms isolated from the liquid and the quantity of adsorbed ions in the pores.
Regarding mesoscopic simulations, we adapted an existing program suitable for the calculation of NMR spectra of adsorbed species to the calculation of properties relevant to the project. The mesoscopic model, available publicly, is approximately 10,000 times faster than classical molecular simulations and is able to predict i) quantities of adsorbed ions and capacitances compatible with experiments and ii) a strong dependency of the properties on the pore size and the solvation. Future implementations are needed to improve the description of the diffusion before realising an extensive screening of porous carbons.
Regarding the development of a new method to determine accurately the structure of disordered carbons, we focused on Nuclear Magnetic Resonance (NMR) parameters as a constraint to improve the accuracy of existing methods. We built a database of chemical shifts for molecules and solids having similarities with the porous carbons of interest and for species in the vicinity of such structures. We proposed a tight-binding model which is very successful in predicting chemical shifts for molecules close to polycylic aromatic hydrocarbons and is orders of magnitude faster than conventional DFT. This model, too simplistic for disordered solids, is still being refined. Other parameters which can impact the measured NMR spectra (e.g. ion concentration, particle size) can also be explored using the mesoscopic model.

The molecular simulations were conducted on a large set of original porous carbons in a systematic fashion and the effects of ion size, presence of functional groups and flexibility of the carbon, unexplored in the past, were investigated. The mesoscopic model developed is original because it is intermediate between molecular simulations and classical theories (e.g. Poisson-Boltzmann, Stern). The comparison between experimental and modeled in situ NMR results are excellent for organic electrolytes and refinements are ongoing for aqueous electrolytes with additional complexity. The inclusion of exchange between the interior of carbon particles of different sizes and the bulk electrolyte cannot be explored by any other technique. Regarding magnetic properties, previous works have mainly focused on ideal aromatic molecules. Here, we included defects (e.g. holes in the molecules) and did calculations on periodic amorphous solids.

Periodic Reporting for period 4 - SuPERPORES (Structure-performance relationships in porous carbons for energy storage)

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