Effects of hydrophobic side chains in poly(fluorenyl-co-aryl piperidinium) ionomers for durable anion exchange membrane fuel cells

Abstract

Durable and conductive catalyst layers are critical for anion exchange membrane fuel cells (AEMFCs) to achieve high power density and sufficient lifespan. Great efforts have been devoted to improving the quality of anion exchange membranes (AEMs), while research on ionomer design has rarely been reported. Here, we propose a convenient strategy to improve the stability of fuel cells by introducing hydrophobic side chains in poly(fluorenyl-co-aryl piperidinium) (PFAP) ionomers to promote dimensional stability and electrochemical stability of the catalyst layers. We systemically studied the effects of hydrophobic side chain length, grafting position, and polymer backbone on the physical and electrochemical properties of ionomers. Specifically, ionomers with hydrophobic side chains naturally possess decreased water adsorption and improved dimensional stability. The polymers with hydrophobic side chains grafted on the piperidinium group (s-PFAP-Pip-Cx, x denotes the side chain carbon length) possess limited alkaline stability and hydroxide conductivity (<90 mS cm⁻¹ at 80 °C). Conversely, polymers with hydrophobic side chains grafted onto the fluorene monomer (s-PFAP-FLN-Cx) possess improved dimensional stability (swelling ratio <50% vs. reference poly(fluorenyl-co-biphenyl piperidinium) of 90%) and conductivity (>134 mS cm⁻¹ at 80 °C) and good mechanical properties (tensile strength >60 MPa, Young's modulus >1000 MPa) and electrochemical stability. Importantly, s-PFBP-FLN-C8 ionomer-based fuel cells showed a remarkable peak power density of 2.607 W cm⁻² at 80 °C. Moreover, the s-PFBP-FLN-C8 ionomer-based fuel cell exhibited a 26 times lower voltage decay rate than the benchmark fuel cell with PFBP ionomer after operating at a constant current density of 0.6 A cm⁻² at 70 °C.