(Invited) Chemical-Mechanical Interactions in Ion-Conductive Polymers
Authors: Kusoglu Ahmet , Weber Adam Z 
Published: February 2020

Research activities and interests in electrochemical energy-storage and conversion devices (e.g., fuel cells, solar-fuel generators, batteries, etc.) have been continuously increasing due to their great potential to provide clean and renewable sustainable-energy technologies for stationary, transportation, and solid-state applications. Common to all these devices is the optimization of multiple functionalities in the solid-electrolyte polymer, such as transport of ions, gases and water in a mechanically stable matrix. However, improving the transport functionality might undermine the polymer stability, and device longevity. Thus, a challenge in these devices is achieving sustainable performance in solid-state electrolytes which requires a concerted effort to meet the performance and durability demands, i.e., to improve transport without compromising mechanical integrity. This requires an understanding of how the ionomer’s transport and mechanical properties are interrelated through the interactions between chemical structure, electrochemical state, solvent uptake, deformation, and morphology. In such a complex structure, it becomes critical, yet challenging, to optimize and balance the mechanical properties that provide stability and the physiochemical properties that are critical for the performance. Understanding the origins of such chemical-mechanical interactions would have significant impact on technologies that exploit electrochemical-mechanical phenomena, from shape-memory applications, to electro-active polymers and bio-inspired ionomer-based materials, to solid-state batteries and electrochemical devices, to flexible electronics to soft-active materials. Thus, mechanical and electrochemical phenomena exhibit many interesting multidirectional couplings in ion-conductive polymers due to their intrinsic physio-chemical states and responses to environmental stressors.

In this talk, such chemical-mechanical coupling occurring in ionomers will be explored within the context of performance-and durability of polymer-electrolyte fuel-cell membranes. First, the role of chemical-mechanical energy balance in controlling the phase-separated nanostructure of the perfluorosulfonic acid (PFSA)-based ionomer membranes will be discussed, along with the impact of key environmental stressors on altering this balance, including degradation effects. Then, the interplay between the transport properties and mechanical stresses generated by the ionomer’s hydration behavior will be elucidated and modeled. The morphology of PFSA ionomers of different equivalent weights and side-chain chemistries, as well as reinforced composite membranes, will be discussed to explore material solutions that could provide new avenues for tuning chemical-mechanical interactions and improving the transport-stability interplay. In addition, (electro)chemical-mechanical coupling of ionomers in other applications will also be discussed, as there is need to elucidate these interactions to optimize and exploit material functionality in emerging technologies and applications.