Swelling Behaviour of Static Seals in Redox Flow Batteries

This paper analyses the implications of swelling in perspective of sealing technology for redox flow batteries (RFB). Mechanical failure due to swell in static sealing can occur on the elastomeric seal or on sealing flanges. The mechanical failure is in this case a fractured or deformed RFB component because of the swelling pressure. Swelling of such an elastomeric seal is a coupling of different physics phenomena, namely diffusion and solid mechanics. In an assembled RFB, seal swelling increases the contact pressure with the sealing flange, while making the components softer. It shifts the mechanical material properties of the components and also the stress tolerances. Graphite-polymer bipolar plates or PTFE / PP flowframes are common RFB sealing flanges and can become softer due to diffusion of the electrolyte fluid, making the flanges more prone to creep. These are time dependent effects that can create sealing failure or mechanical break of the sealing flanges and must be taken into account in RFB design. The presented experimental and modelling results validate the swelling behaviour for an assembled RFB. The compatibility tables of sealing materials in catalogues do not often differentiate swelling compatibility from chemical compatibility of a fluid. In addition, this paper proposes a basic selection procedure for swelling compatibility based on group experiments on Hansen solubility parameters (HSP).