Proton exchange membranes based on sulfonated polybutylene fumarate: Preparation and characterization

Based on a novel unsaturated polyester, polybutylene fumarate (PBF), a series of sulfonated proton exchange membranes were prepared and characterized for direct methanol fuel cell application. In this work, PBF underwent sulfonation at different degrees, and the optimum degree of sulfonation (DS) was identified by assessing the hydrolytic stability. In the second step, an effective strategy to enhance the membranes' proton conductivity was adopted by the addition of the 2-(1H-Imidazol-2-yl) acetic acid (Im-COOH) compound as a proton-conducting agent to the sulfonated PBF (SPBF) matrix with the optimum DS. The results unveiled significant alterations in the physicochemical properties of PBF due to sulfonation, including an elevation in water uptake and the glass transition temperature (T_g), accompanied by the disappearance of the crystallinity within SPBF. In terms of electrochemical characteristics, the SPBF-17.5 PEM demonstrated a proton conductivity of 3.36 × 10⁻³ S cm⁻¹ and a methanol permeability of 1.068 × 10⁻⁷ cm² s⁻¹, yielding a selectivity of approximately 31,460 S s cm⁻³. Furthermore, the attractive interactions between -SO₃H and -NH groups further elevated proton conductivity to 4.12 × 10⁻³ S cm⁻¹, accompanied by a decrease in methanol permeability to 0.942 × 10⁻⁷ cm² s⁻¹, enhancing the selectivity parameter to 43,736 S s cm⁻³.