A first principles study of electric double layer capacitors

Electric double layer capacitors (also called supercapacitors) are a type of energy storage devices with balanced energy and power densities, filling the gap between conventional capacitors and batteries. The energy storage mechanism is through electrostatic interaction between charged electrodes and counter-ions in electrolytes, forming EDLs at interfaces. Graphitic carbon is widely used as an electrode material for EDLCs because it satisfies all the requirements for this application including high porosity, good electric conductivity, electrochemical stability and low cost. It has been long thought that sub-nanometer pores are inactive for charge storage in carbon electrodes because they are inaccessible to solvated ions. This widely-accepted axiom, however, has been challenged by the recent discovery of anomalous increase in the capacitance inside carbon micropores.

This discovery has spurred a great deal of fundamental research, aiming at understanding this intriguing phenomenon. Particularly, theoretical modeling has provided a wealth of microscopic information on the EDLs at carbon electrodes. However, the majority of the theory studies use classical models, omitting the electronic structures of ions and carbon electrodes. In this proposal, the state-of-the-art ab initio molecular dynamics (AIMD) will be employed to investigate the EDLCs, which is the first attempt to offer a full atomistic and dynamical description of the EDLs at carbon electrodes at the electronic structure level. AIMD simulations are computationally demanding, but recent advance in computing algorithm and availability of the UK supercomputing facility (ARCHER) have made it possible to model the EDLs at electrochemical interfaces.

The proposed research will closely collaborate with the experimental group of Prof. Clare Grey at Cambridge. Combining theoretical modelling and analytic techniques (e.g. in-situ NMR), we aim at unraveling the microscopic structures of the EDLs at carbon electrodes, and how the EDL structures affect the capacitances. Another objective is to quantify the electronic charges of the ions adsorbed on the electrode surface using a finite field approach developed in computational solid state physics. This will help obtain a fundamental understanding of the EDL capacitances at the electronic structure level.

Simulating electrochemical interfaces at an atomistic quantum mechanical level is one of the grand challenges in computational science, attracting lots of interests at present due to its importance in energy and environment relates issues. Therefore, the proposed research will be of interest to the wide computational communities. Furthermore, this project will bridge the gap between the fields of solid-liquid interface and solid-solid hetero-junction by connecting the concepts familiar to the individual fields. This conceptual link will have impact on both electrochemists and solid state physicists. Finally, to establish the relation between the EDL structures and capacitances, as intended in this proposal, will provide useful chemical insight into designing more efficient electrode materials for energy storage.

The proposal involves theoretical investigation of electric double layers (EDLs) at electrochemical interfaces, with the focus on the application to energy storage, i.e. carbon EDL capacitors (EDLCs). Not only will the proposed study directly contribute to the UK's academic excellence, but also have indirect impact on our economy.

1. Knowledge
EDLs are ubiquitous at solid-liquid interfaces, having great influence on interfacial processes of significance in many disciplines including electrochemistry, colloid science and geochemistry. Due to the intrinsic complexity, microscopic level understanding is largely lacking. The proposed research will enhance our knowledge of this important phenomenon at interfaces. On the other hand, simulating interfacial EDLs from first principles is a challenging task, and the proposed theoretical study will contribute to the computational development of modeling electrochemical interfaces in general.

2. Economy
Attainment of the principles of rational design of materials can have beneficial effects on the UK economy. Our route to achieving this impact is to understand the fundamental mechanisms and the activity of the materials in terms of their structures (both geometric and electronic) by establishing the so-called structure activity relations. Electric double layer capacitors (EDLCs) have the potential to impact a variety of commercial markets in energy storage. EDLCs play an important role in complementing or replacing batteries, and the applications include uninterruptible power supplies and load-levelling devices in vehicles, airplanes, photographic flashes, portable electric devices, etc. Our simulations, in collaboration with experimental work, can help connect the EDL capacitances with their underlying atomic and electronic structures. This acquired understanding can help synthetic chemists to make electrode materials with higher capacitances.

3. People
This project will provide excellent professional training for the Postdoctoral researcher and undergraduate project students. They will acquire both research specific skills and knowledge as well as generic transferable skills. The project will closely collaborate with an experimental group, and regular meetings will be held to exchange results and ideas. This will provide a good opportunity for the researcher and the students to learn how to work within a team, to develop effective communication skills, especially with the researchers having a different knowledge background, and to gain broad knowledge and deep understanding on the project. They will also obtain valuable experience in writing up research papers and presenting their research at national and international conferences.

In addition, this project addresses one of the areas in need of emphasis and encouragement highlighted in Chemistry for the Next Decade and Beyond - 2009 International Review of UK Chemistry Research, namely, Integration of Computational Chemistry (Need to enhance the participation of theory and computation especially in areas that involve energy, materials and health applications).