Model of porous redox active solids with the redox electrolytes was implemented in COMSOL Multiphysics. Thermodynamics and kinetics of such systems were formulated, resulting in one article. The basis of the system was earlier models of Li-ion batteries employing redox active solids, so the implementation was verified with modelling of Li-ion battery, including a sensitivity analysis. General finite element models for solid boosters on the electrode in the presence and absence of redox mediators was developed to understand the electrochemical response of the different techniques (cyclic voltammetry, potential step, current step etc. at both macro- and microelectrodes). This will allow determining what techniques are useful to characterize the soluble redox mediator-redox active solid –couples. The model was utilized in understanding scanning electrochemical microscopy experiments to gain insight on how charge transfer between redox electrolyte and redox solid takes place. This information will help to better design the system.
For the experimental part, first reactor set-up with different redox sensors has been constructed and the measurements have been performed with model redox couples. We have also focused on developing experimental set-ups for reproducible flow battery testing. As major achievement, testing capability for up to 16 batteries simultaneously has been achieved. Initial testing of bipolar membranes has been performed, but results for utilizing differential pH batteries are not promising. Unfortunately, the idea of pH gradient generation with another cell was published by another group. Development of lignosulfonate based polymeric redox species has started, focusing on demethylation of OMe groups.
Biphasic systems for flow batteries have been explored. We have shown that it is possible to increase the cell voltage of biphasic flow battery significantly, by more than 0.5 V by utilizing Galvani potential difference between the two phases. However, interfacial charge transfer results in significant self-discharge, and efforts to realize low resistance cell have been challenging. Porous membranes tend to allow one phase to pass through, resulting in cross-over. Best results have been achieved with conventional ion exchange membranes, indicating that it will be difficult to realize a membraneless system.
