Research proposal: Towards Molecular Understanding of Electrochemical Double Layers for Energy Storage

Background and motivation: Moving towards net zero will require the implementation of a variety of renewable power sources. However, the common drawback of many renewable sources, variable power output, will require the introduction of large-scale energy storage to offset. Which will in turn require significant improvements in battery and supercapacitor technology to implement.

The most scope for this improvement is in designing efficient system setups (e.g. porous electrodes), and intelligently making choices to achieve desired properties while using sustainable materials. Due to both technologies making use of an electrochemical cell for charge storage, clearly progress in both areas will require an improved understanding of the electrode-electrolyte interface, where this charge is stored.

The importance of a molecular description in this understanding is illustrated by the fact that, in the a traditional view of the double layer, the Helmholtz layer is somewhat independent of electrolyte, while the diffuse layer shows large variation.

Methodology and Objectives: Computer simulation is already widely used as a powerful tool for gaining molecular level insight in electrolyte systems. Recently, advances in performing molecular dynamics in a "Constant Potential Ensemble", where two simulated electrodes can be held at a constant potential difference and the charges of individual atoms are allowed to vary, have led to a renewed interest in its use to describe electrochemical systems.

Easy access to system properties such as capacitance, one of the key properties for supercapacitor design, and the ability to directly link these to a molecular model of the electrolyte presents a promising prospect for theoretical study. For applications in electrochemical double layer energy storage, answering the following questions should be prioritised:

What order is seen in the electric double layer structures present in these systems? What properties of the electrolyte are responsible, and can they be tuned to give desired results? Do predictions from computer simulations reliably agree with experimental measurement, and can therefore be used to design better electrolytes?

However, the approximations required in simulation often trade accuracy in favour of computational cost, particularly when imposing a constant potential. Therefore, this project will aim to compare results from simulation to those from experimental groups, and explore methods to improve this agreement. This could potentially lead to using electronic structure methods to assess configurations of atoms from simulation, likely taking a similar approach to those already adopted for studying adsorption in catalysis.

Recent developments in machine learning potentials also present a possible route to improved accuracy, but application electrochemical systems is not yet well established. Impact: The primary impact of this project will be to improve molecular understanding of the structure and dynamics of electrolytes at electrochemical solid-liquid interfaces. Improved understanding of which will be crucial in theory driven design of the energy storage devices required for the transition to a sustainable energy grid.