Understanding the Impact of Ligand and State-of-charge on Electrolyte, Redox Reaction and Electrode Stability for the Cerium-based Flow Battery

Project: Research

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Redox flow batteries (RFBs) promise to solve one of the critical energy challenges towardsustainable development – efficient storage of renewable energy. Improving theconcentration of the electrolyte is the most important approach to promote the energy densityand capacity in a finite volume of the RFB, which can be obtained effectively by using anappropriate supporting electrolyte. Single and mixed acid media have been investigated invarious RFBs, including zinc-cerium RFB, a battery with higher storage capacity than allvanadium RFB (VRFB). Methanesulfonic acid (MSA) is the most commonly used andfavorable acid medium for cerium electrolyte. We have investigated mixed acid media ofMSA with other inorganic acids and found that mixed acids of MSA and HCl can facilitatethe redox reaction and contribute to better reversibility. However, there is absence ofknowledge on the mechanism of cerium reaction in HCl solution. Whilst researchers havestudied the cerium reaction in MSA, the improved solubility has not been explained, withoutsuch knowledge, systematic improvement of supporting electrolytes remains unattainable.Of particular interest here is the cerium ion with methanesulfonate ligand and mixed ligandsof methanesulfonate and chloride, studied at glassy carbon surface (as a standard carbonmaterial). It is known that many aspects of the reaction are improved in mixed ligands, buteven at the well-studied glassy carbon there is no clear understanding of the interactionbetween electrode and electrolyte, which forms the foundation for further development.We propose therefore to gain a comprehensive understanding of the impact of ligands on theelectrolyte and redox reaction, as well as the stability of electrode. Saturation ion product willbe calculated, combining with the recorded time before precipitation, to assess the solubilityand stability. Formal potential, reversibility, diffusion coefficient and reaction kinetics will bemeasured to study the role of ligands and concentrations, under a range of state-of-charge.Stability of the carbon material can be affected by the ligand, which will also be investigatedby charge-discharge cycling at different temperatures. Conclusion drawn from the experimentwill be explained in conjunction with density functional theory (DFT) calculations within theframework of electrocatalysis. Using the state-of-the-art DFT techniques, the complexationand activation energies of the large metal ion, Ce(III)/Ce(IV) will be gained. Each of thefactors affecting the electrochemical reaction will be accounted for – the approach of the ionto the electrode, the reorganization of ligands and solvents, the double layer, the transfer ofcharge and the relaxation to the new equilibrium.The study presents a broad and accurate investigation of the RFB, which would explain theimpact of ligand on the system. The approach taken here has the potential to be applied notonly to other RFB but also to more general electrochemical situations. The study thereforewould be a prototype of great benefit to the electrochemical field, such as application to otherelectrode materials, ions and other multi-step reactions.


Project number9042553
Grant typeGRF
Effective start/end date1/01/183/12/21