Abstract
Simultaneous transport of an electrolyte and dissolved oxygen is analyzed with Newman’s concentrated-solution theory to assess how nonuniform oxygen distributions might impact the voltages of metal/air batteries. For a solution comprising a neutral solvent, a simple salt, and oxygen, the Onsager–Stefan–Maxwell transport equations are inverted, yielding flux-explicit laws for oxygen, anion, and cation transport that distinguish the effects of individual diffusion and migration driving forces. Along with the ionic conductivity, electrolyte diffusivity, oxygen diffusivity, and cation transference number, a migration coefficient and a cross-diffusion coefficient are identified, which respectively account for the effects of electro-osmotic drag on oxygen and diffusional drag between salt and oxygen. A derived current/voltage relation reveals how oxygen gradients can in principle affect the cell potential; significant diffusion potential can arise from oxygen if it experiences electro-osmotic drag. Prior models are proven to follow from an assumption that cross-diffusion and electro-osmosis are both negligible, or, equivalently, that oxygen/ion interactions are weak. Experiments to quantify the novel transport properties are discussed, along with quantitative estimates of the cross diffusivity and migration coefficient.
Highlights
A standard liquid electrolyte for a metal/oxygen battery is made up of a neutral solvent, dissolved oxygen, and a salt with a single type of anion and a single type of cation
Given that four electrochemical species comprise these electrolytes, one expects on the basis of the Onsager mass-transport theory[1] that six independent transport coefficients are needed to describe all the possible pairwise interactions associated with diffusion and migration
We set out here to address this issue by proposing convenient flux laws that account for dissipative interactions between oxygen flow and ion flow
Summary
A standard liquid electrolyte for a metal/oxygen battery is made up of a neutral solvent, dissolved oxygen, and a salt with a single type of anion and a single type of cation. Considering the complete set of proper mass-transport driving forces from irreversible thermodynamics suggests that diffusion potential might arise from differences in oxygen concentration, as well as from differences in salt concentration.
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