In this study, we build off our recent development of a database of data describing the composition of polymer electrolytes in contact with various acids, bases and salts. This situation is common in practical electrochemical systems such as redox flow batteries and electrochemical reactors. For example, the vanadium redox flow battery (VRFB) electrolyte solutions consist of several molar solutions of aqueous sulfuric acid (with other acids used in certain cases) and up to 2M vanadium salts. As we have shown, these concentrations cause the general phenomenon of breakdown of the Donnan exclusion effect in a variety of polymer electrolyte membranes, both cation and anion exchanging membrane types.^1,2,3 A description of these processes would be useful in helping to separate the effects of various modifications of the membrane composition and structure on its behavior.

We develop the groundwork for a thermodynamic description of uptake of ionizable species by polymer electrolyte membranes in contact with concentrated solution electrolyte solutions using the VRFB membrane-electrolyte combination as our test case.

We note that all such descriptions of the behavior of polymer systems entails the assumption of a ‘local’ equilibrium, relevant for short times. Here the adjective short refers to times long enough for uptake processes to occur but short enough that the polymer itself does not have time to change from its initial state, set by its thermal history. This allows us to assess membranes that might be thermally ‘re-arranged’ as separate entities, thus isolating any effects of the treatment.

We first develop a description of the local equilibrium of the acid solution with the membrane. As Tang et al showed,¹ this has several interesting features that are generic to acid uptake into Nafion and related polymer electrolytes. A convenience of this approach is the broad availability of descriptions of the acid solution properties.

Once a description of the acid-membrane equilibrium is available, we proceed to consider the situation with added vanadium. For simplicity, we will begin by developing the case for VOSO₄ solutions in acid. In this case, we need to reassess the solution thermodynamics, adding some measurements specific to a given ternary composition. We will describe these measurements in the oral presentation.

Acknowledgements

We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk) and by the Office of Naval Research. We also thank 3M for providing membranes.