Multiphysics modelling for rechargeable zinc-air flow batteries – Part I: A physico-chemical and mathematical reassessment of the model

Mathematical modelling of zinc-air flow batteries is a crucial tool for the design of prototypes and their scaleup: these are highly strategic tasks in the current development of this technology. A limited amount of efforts has been devoted to this research and, notwithstanding results of high scientific standard, often the emphasis on applications dominates the methodological one. In this work, we have first reassessed and clarified some physico-chemical complexities aspects of a standard multiphysics model for a zinc-air flow battery with a porous GDE, a metallic Zn anode and a flowing electrolyte, and extended it to the rechargeable case. The model couples: (i) material conservation for oxygen, zincates and hydroxide with (ii) electrodic electrochemistry in the porous cathode and the compact anode, (iii) charge conservation in the electronically conductive region of the GDE and in the electrolyte, (iv) fluid dynamics for both gas and electrolyte. Then, we have employed the model to: (i) predict polarization curves as a function of operating conditions; (ii) map the limiting current density distribution over the Zn anode during charge, as an estimate of shape change. Some of the functionally relevant computed trends are confirmed by a selection of experimental results.