Ph.D Thesis


Ph.D StudentRonen-Manukovsky Rona
SubjectMembraneless Bromine-Based Redox Flow Batteries
DepartmentDepartment of Energy
Supervisor ASSOCIATE PROF. Matthew E. Suss
Full Thesis textFull thesis text - English Version


Abstract

Redox flow batteries are an emerging technology for stationary, grid-scale storage of energy generated by intermittent renewable sources, such as solar and wind. Aqueous redox flow batteries have the potential to provide safe and scalable energy storage, but the high cost of storage, particularly the membrane, has inhibited commercialization. Membraneless redox flow batteries with slender channel geometry are promising for low-cost storage, as they combine a low-cost system architecture with inexpensive and abundant reactants such as bromine.

This thesis is concerned with the investigation of membraneless bromine-based redox flow batteries through theoretical analysis and experimental demonstration, with the aim of realizing a robust model for improving battery performance.

In the first part of the thesis, we extended the model of halogen-based batteries to capture the homogeneous chemical reactions with infinitely fast equilibria in the bulk electrolyte simultaneously with the heterogeneous electrochemical reactions occurring at electrode surface. The model simplifies the governing equations of species conservation by reducing the reaction term by gathering the species into “families”, an approach not commonly used in electrochemistry. We apply this approach to a specific example of a hydrogen-bromine membraneless flow battery. We benchmark the model neglecting homogeneous reaction against a previous report, and then incorporate the homogeneous reaction into the model to study its effect on battery performance, which provides an explanation for the higher open-circuit voltage observed in some experimental studies

In the second part, we present a novel mathematical model for a membraneless electrochemical cell employing a single laminar flow between electrodes, consisting of a multiphase flow of a continuous, reactant-poor aqueous phase and a dispersed, reactant-rich fused-salt phase. We derive analytical approximations and numerical solutions for the concentration profile and current-voltage relation via boundary layer analysis. We investigate regimes of slow and fast reactant transport between phases, and apply the theory to a multiphase zinc-bromine flow battery. 

In the third part, we compare our theory for the single-flow multiphase flow battery to a dedicated experimental set to unravel the interplay between interphase mass transfer, multiphase flow phenomena and battery performance during discharge. We characterize the parameters of the model for this battery, assess assumptions, and refine the model by including activation overpotentials. We show the battery operates in a regime characterized by a high Stanton number, and demonstrate that the analytical solutions yield excellent predictions in several operational regimes when using the interphase mass transfer coefficient as a single-valued fitting parameter. In other regimes, such as at low electrolyte velocity, we show that gravity acting on the denser fused-salt phase plays a significant role. We also describe the exploration of a potentially attractive configuration for a cell with a permeable electrode and an adjacent secondary channel to improve the current capability of the battery, demonstrating high mass transport to the permeable electrode can be achieved through gradually thickening the adjacent channel. The analysis presented in this study provides a physical understanding of this flow battery and the limitations associated with its future development.