Theoretical methodology for calculating water uptake and ionic exchange capacity parameters of ionic exchange membranes with applications in fuel cells

• We built molecular computer models of sulfonated poly(ether imide) (SPEI).
• We obtained properties of water uptake and ion exchange capacity of each polymer SPEI.
• The water uptake is one of the important properties of sulfonated polymers.
• The theoretical values agree with the experimental values.

The water uptake (WU) and ionic exchange capacity (IEC) are important parameters for proton exchange membranes (PEMs), WU represents their total water content, while IEC is the measure of the ability of the membrane to facilitate movement of H30+ ions, located in the aqueous environment, through the −SO3− sulfonyl groups anchored in their structure. IEC is also the measure of relative concentration of acid groups within polymer electrolyte membrane. According to the vehicle mechanism of proton transport, an appropriate amount of water content and a suitable value of IEC in PEMs are necessary for achieving high proton conductivity. In this work, studies based on density functional theory (DFT), molecular mechanics (MM) and dynamics molecular (DS) were realized in order to develop a theoretical methodology to obtain these parameters. Our proposal considers the ionomer, the solvation medium and the interactions between the ionomer and water molecules. The methodology is applied to structures of sulfonated poly(ether-imide) (SPEI) with (–SO3H)n (n = 1, … 6) groups. These structures were built and optimized aiming to obtain above properties as a function of the number of sulfonyl groups. The comparative study demonstrates the SPEI with four sulfonyl groups is the polymer having better properties for successful operation in Fuel Cell.