Energy Storage Capacitor – An Important Part Of Energy Storage Systems

1. Theoretical calculation methods commonly used in energy storage capacitor
2. Calculation method of constant potential and constant charge
3. Research on energy storage capacitor in nanoporous electrodes
   
   1. Ion arrangement in carbon nanopores
   2. Ion dynamics in carbon nanopores
   3. Ion arrangement and dynamics in MOF pores
4. Research on energy storage mechanism in 2D layered MXene electrodes
   
   1. Aqueous electrolyte
   2. Intercalation of various cations
5. Summary and outlook

Electrochemical energy storage devices, such as supercapacitors and secondary batteries, can store clean energy such as wind energy and solar energy to power electronic/electrical equipment without generating greenhouse gases.

This advantage enables it to play an even more critical role in the increasing worldwide challenges in energy, environment and climate change. The electrochemical performance of electrochemical energy storage devices such as energy storage capacitor mainly depends on their active components, namely electrodes and electrolyte.

Studies in recent years have shown that two-dimensional materials: including traditional two-dimensional materials (graphene) and a series of new two-dimensional transition metal layered materials: two-dimensional transition metal carbon/nitride, two-dimensional transition metal sulfides.

They have great research potential to fill the gap between the performance of existing energy storage devices such as energy storage capacitor and the needs of modern energy storage devices.

Theoretical calculation methods commonly used in energy storage capacitor

Supercapacitor electrochemical energy storage devices mainly store energy at the interface between the electrode material and the electrolyte. The current mainstream view on the energy storage mechanism divides it into electric double layer capacitors represented by carbon-based materials and transition metal oxides/ Nitride is represented by two types of faradaic pseudocapacitors.

Electric double layer capacitance stores charges through electrostatic adsorption without chemical changes, which makes it very suitable for classical molecular dynamics that can accurately describe electrostatic interactions and non-bonding interactions between atoms/molecules method to simulate it.

In addition, first-principles molecular dynamics methods and reactive molecular dynamics methods can effectively simulate some key interfacial reactions and interactions in electrochemical processes. Compared with classical non-reactive force fields, reactive molecular dynamics methods have the advantage of more accurate prediction of transition states and reaction kinetics.

Compared with static quantum computing, it has the advantages of simulating the service environment of materials (such as temperature, pressure and other external conditions) and dynamic processes, so it is very suitable for the study of dynamic electrochemical cycles in the process of electrochemical energy storage.

Calculation method of constant potential and constant charge

The classical molecular dynamics method has the unique advantage of being able to simulate a controllable applied potential, so it can simulate the electrochemical real-time charge and discharge state, and obtain data that can be directly compared with some in situ experimental analysis results.

In molecular dynamics simulations, the dynamic charging/discharging process can be simulated by applying an external bias voltage to the electric double layer computational model, which usually consists of two distant electrode models and an interelectrode electrolyte, by means of a potentiostatic or constant charge calculation method.

Research on energy storage capacitor in nanoporous electrodes

Ion arrangement in carbon nanopores

Using computational simulation methods, it is possible to gain a good understanding of the charge storage mechanism in nanometer-confined environments, which is a key factor for designing supercapacitors with high energy density and high power density. When electrolyte ions are confined in carbon nanopores, they adopt a monolayer or bilayer structure depending on the average pore size.

When the average pore size matches the size of ions in RTIL, such as when EMIM-TFSI ions are confined to pores with a diameter of 0.7 nm, monolayers with partially disrupted Coulombic order can be observed.

This effect will facilitate charge separation, that is, the separation of hetero-charged ions from like-charged ions, which facilitates the increase of electrolyte charge in the electrode and is compensated by the electrode charge, thus providing higher capacitance. This phenomenon explains the experimentally found increase in capacitance on an atomic scale.

Ion dynamics in carbon nanopores

The ion kinetics and adsorption kinetics under confined conditions are another key factor affecting the characteristics of energy storage supercapacitor, which directly affects the power density of energy storage capacitor.

The charging dynamics simulation results of slit-shaped subnanometer carbon pores and ionic liquids using molecular dynamics show that the charging dynamics of ions show a non-monotonic relationship between the pore size and the charging rate. In particular, the charging process can be significantly improved in subnanopores with sizes of 0.45 and 0.75 nm, respectively, which breaks the conventional view that small pore sizes lead to slow charging rates.

The mechanism of this unusually enhanced charging kinetics may be attributed to the transformation of the ionic structure within the pores. The layered arrangement of ions in the pores will facilitate the transport of ions from one layer to another and accelerate the diffusion of ions along the length of the pores.

Ion arrangement and dynamics in MOF pores

Conductive MOFs are another promising class of nanoporous electrodes due to their high surface area far exceeding that of conventional porous carbons, high compressibility, and tunable structure. In the neutral state, ions form a layer adsorbed near the pore surface and follow a hexagonal pattern in planar cross-section.

Under charged polarization, dissimilarly charged ions separate and reside between the centerline and the surface adsorption layer, while likely charged ions are located between these two regions. In terms of capacitive performance, the simulation results predict that Ni3(HITP)2 MOF can provide a capacitance of ~9 μFcm-2, which is comparable to the reported RTIL-based porous carbon EDLC.

Research on energy storage mechanism in 2D layered MXene electrodes

2D transition metal carbides and nitrides (also known as MXenes), benefiting from their inherent hierarchical structures, functional-group-rich surfaces, and multiple oxidation states, are typical representatives of transition-metal-based low-dimensional systems and have been investigated. Electrode materials have been extensively studied for energy storage applications.

Aqueous electrolyte

Combining DFT calculations and in situ XRD, studies on the charge storage mechanism of Ti3C2Tx MXene in 1 M H2SO4 aqueous solution revealed that the charge storage mechanism in H2SO4 solution can be divided into three stages:

• Electric double layer energy storage capacitor mechanism, MXene structure between -0.25 V and 0 V (compared to the Ag electrode) there is no noticeable lattice parameter change; ii) below -0.25 V, protons intercalate between the layers accompanied by electrostatic contraction, and the redox reaction begins to shift part = O groups are converted to -OH).
• Below -0.5 V, the second redox reaction upon further proton insertion converts more =O groups into -OH groups and increases the lattice parameters due to the same charge repulsion. The conversion of =O to -OH resulted in a change in the oxidation state of Ti from +2.33 to +2.43.
• By comparing pristine Ti3C2Tx MXenes (named P-MXene) and Ti3C2Tx MXenes after annealing at 500 °C (named 500-MXene), MD simulation results show that the confined water molecules inside the MXene layer will provide proton transport pathways To activate the redox reaction on Ti atoms, thereby affecting its pseudocapacitive behavior.

A well-organized hydrogen bond network will facilitate fast proton transfer during electrochemical processes. The probability distribution of dipole orientations of water molecules inside the layers of P-MXene and 500-MXene.

Intercalation of various cations

Combining QENS, SANS and XRD experiments, MD simulations using the ReaxFF force field found that the diffusion coefficient of water confined in the interlayer gap is about half of that of bulk water. When K+ ions are intercalated between MXene layers, larger and/or more ordered domains are formed, and the MXene layer becomes more uniform.

Intercalating metal cations reduce the strength of hydrogen bonds between intercalated water and enhance the order of water molecules. In K+-intercalated MXenes, these more stable and less mobile water molecules can enhance the stability of 2D layered materials, thereby limiting their structural changes during charge and discharge.

Summary and outlook

Understanding the intrinsic properties of the electrode/electrolyte interface and the various physicochemical changes that occur during charging and discharging is one of the hotspots in current theoretical calculations and experimental studies of electrochemical energy storage systems.

Although a series of theoretical and experimental methods have been developed to reveal the electrochemical energy storage mechanism of nanoscale low-dimensional materials, there are still many potential scientific problems and key technical challenges.

In the development and design of next-generation electrochemical energy storage capacitor, the following key points need to be explored by combining theoretical calculation methods and advanced experimental analysis techniques:

1. Real-time dynamics of electrolyte ions during charge and discharge in electrode materials with special internal morphology (irregular nanopores, two-dimensional layered pores, etc.). This migration/diffusion property is directly related to the power characteristics of energy storage capacitor, which is the key to designing high-power energy storage capacitor.
2. Spatial distribution of atoms and charges at the electrode/electrolyte interface, electronic interactions between electrolytes, surface active sites of electrode materials and their relationship to surface potential. These factors are directly related to the energy storage mechanism and energy storage capacity of the electrode/electrolyte interface, which is the key to designing energy storage capacitor with high energy density.
3. Changes in the structural parameters of electrode materials during electrochemical charging and discharging, such as changes in the interlayer spacing of two-dimensional materials, changes in the pore size of porous materials, changes in the electronic structure characteristics of composite electrode materials, etc.
4. The changes in the geometric structure parameters and internal electronic structure of electrode materials are directly related to the structure/conduction cycle stability of electrode materials, which is the key to design electrode materials with high conductivity and stability.

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