Transport models for non-isobaric electrolytes

Thesis

Conventionally, transport models employed for describing processes in electrochemical devices have been developed for liquid electrolytes under isobaric conditions. The assumption of constant pressure starts to break down for solid or other viscoelastic electrolytes, which respond to internal or external stress and strain changes in a fundamentally different way. Mechanical effects and their coupling with charge and mass transfer have, therefore, become a key area of research. The present work focuses on the Nafion ionomers used in polymer electrolyte membrane (PEM) fuel cells. The tendency of these membranes to swell upon solvent sorption in spatially constrained environments, when viewed in conjunction with the delicate water balance necessary for optimum operation and the durability requirements from the cell stacks, makes for a perfect case study of non-isobaric transport in electrolytes.

The thermodynamics underlying the transport model for a viscoelastic electrolyte such as a hydrated Nafion membrane was recently revisited by the authors. A membrane (or more generally, a solid) can store energy in the form of elastic work obtained from deformation of the material, in addition to the pV compressive work, heat work, and electrochemical work; this introduces deformation stress as an additional intensive variable. Conservation laws for material, momentum, and energy, local equilibrium conditions, as well as material and momentum flux constitutive laws, have been developed using basic principles of classical and irreversible thermodynamics to maintain congruency with this modified basis of thermodynamic intensive variables.

Electrochemical impedance spectroscopy (EIS) response of the model has been studied within the new framework to gain insight into electrochemical/mechanical coupling. EIS concentrates on the first-order effects and identifies the various time-scales, length-scales, and the dominant ranges of the modelled physical phenomena. For instance, current-induced pressure variations at high frequencies (˜ MHz) were found to manifest as resonance peaks on Bode plots, which can partially explain the high-frequency inductance observed in Nafion systems. Further, a parameter study was performed to assess which material and mechanical properties are necessary to enhance or downplay stress effects in the model.

Authors: Priyamvada

• DOI: 10.5287/ora-n6pnr2yz4
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford