Non-standard state thermodynamics of metal electrodeposition

Abstract

The development of new high temperature electrolytes is hindered by lack of information about their thermodynamic solution properties, which must be determined through experiments or modeling. Current models, however, are unable to accurately predict the behavior of the complex multicomponent liquids that make up such electrolytes, and gathering sufficient experimental data for a full analysis is lengthy and expensive. Even if the properties of an electrolyte are well-determined, the link between their thermodynamics and the extent of codeposition that will occur during electrolysis remains unclear. Previous endeavors aimed at linking the difference in deposition potential 𝛥Ε of two elements to their codeposition behavior focused on binary cathode alloys that formed ideal solutions. Herein, this approach is generalized to multicomponent cathodes exhibiting real solution (ɑ[subscript i] ≠ ᵡ[subscript i]) behavior. Through this methodology, targeted experimental data and classical Gibbs energy curves can be used in combination to map out the thermodynamic nature of complex electrolytes. To facilitate this effort, a new thermodynamic reference state for activity is derived that allows one to determine electrolyte activities directly from 𝛥Ε. The merits of this approach are tested against experimental case studies and compared to traditional standard state assumptions of two elements to their codeposition behavior focused on binary cathode alloys that formed ideal solutions.