Rationally designing anti-poisoning polymer electrolyte by electronegativity modulation: Towards efficient ammonia-cracked hydrogen fuel cells

Ammonia as a hydrogen enrichment medium has tremendous advantages over pure hydrogen in terms of transportation and utilization. The high-efficiency anti-ammonia-poisoning membrane electrode assembly (MEA) is pivotal for the practical implementation of ammonia-resistant proton exchange membrane fuel cells (PEMFCs). As for the polymer electrolyte, which is an integral part of the proton exchange membrane (PEM) and catalyst layer (CL) adhesion agents in the MEA, structural design is an effective way to improve its resistance to ammonia poisoning. Herein, an in-depth study of sulfonimides with superacidic structures reveals that the proton dissociation energies can be altered by tuning the terminal groups with different electronegativity and enhancing the competitive adsorption of protons on Pt. Four perfluorinated backbone sulfonimides (PFSI-x (x = TSM, MSA, ESA, and iPSA)) with distinct charge delocalization were fabricated by grafting different electronegative monomers onto the perfluorosulfonyl fluoride (PFSF). The results show that PFSI-TSM possesses a higher electrostatic potential (2.69 eV) and the lowest activation energy (12.1 kJ/mol). Meanwhile, the performance of the PFSI-TSM PEMFC is 40.6% higher than the perfluorosulfonic acid (PFSA) at 80 °C, 100% relative humidity (RH) under 100 ppm NH3, with a maximum power density of 635.2 mW/cm2. The underlying reason for enhanced performance is in virtue of the more electronegative end groups render the superacid electrolyte with higher acidity and pKa values, which change the direction of the acid dissociation equilibrium reaction on the Pt surface, enhanced competitive adsorption of protons in MEAs elevated oxygen reduction reaction (ORR) activity and proton concentration in PEM and reduced the probability of ammonia adsorption poisoning. The insights in charge delocalization and proton ionization construct the structure-property relationship, which would guide a rational design of PEM for ammonia-resistant fuel cells.