Synergistic radical scavenging and multifaceted membrane properties: comparative evaluation of CeMOx (M = Zr, Mn, Zn) bimetallic oxide embedded sPEEK membranes for hydrogen fuel cells

Abstract

The degradation of proton exchange membranes (PEMs) caused by radical attacks during real-time fuel cell operations poses a significant challenge to their performance and durability. To address this, cerium-based bimetallic oxides were developed by chemically incorporating a secondary transition metal (Zr, Mn, and Zn) into the ceria crystal lattice, to make it more efficient in mitigating radical degradation. This study presents a comparative analysis of the hydrogen fuel cell performance and durability of sulfonated poly(ether ether ketone) (sPEEK)-based composite PEMs (SPZr, SPZn, and SPMn) containing a fixed filler loading of 2 % (w/w) each. The reversible Ce³⁺/Ce⁴⁺ redox cycle scavenges harmful radicals, which is further enhanced by the secondary metal promoting oxygen vacancies and increasing Ce³⁺ content. However, the performance and stability of the prepared PEMs varied depending on the secondary metal present in the bimetallic oxides. Among the composite PEMs, SPZr outperformed the others due to the enhanced synergistic interaction (combined effect of electrostatic, ionic, interfacial, and hydrogen bonding interactions) between CeZrO_x and the polymer electrolyte, and the surplus oxidative tolerance provided by Zr. Representative SPZr showed only a 17 % and 15 % loss in mass and conductivity after its prolonged exposure to Fenton's solution. Additionally, SPZr achieved an impressive peak power density of 589.6 mW cm⁻² and a current density of 788 mA cm⁻² at 0.6 V at 80 °C and 100 % RH, a H₂ crossover current density of 1.62 mA cm⁻², and proton conductivity of 34.9 mS cm⁻¹ at 80 °C. The fuel cell performance of SPZr was found to be 52 % higher than that of the pristine sPEEK membrane, with 85.2 % retention in its OCV with a decay rate of 1.4 mV h⁻¹, after the durability test.