Special tailoring of a copper–pyromellitic acid complex with sulfonated poly(ether ether ketone) to form a composite membrane for polymer electrolyte fuel cells

Abstract

Alternative solid polymer membrane electrolytes have garnered significant attention due to their extensive utilization in electrochemical power sources. Among these, sulfonated poly(ether ether ketone) (sPEEK) based membranes stand out as promising candidates due to their cost-effectiveness, robustness, and proton conducting properties, making them a potentially viable alternative to Nafion® in polymer electrolyte fuel cells (PEFCs). In this study, we have enhanced the proton conductivity of sPEEK through a novel approach of tailoring a hydrophilic Cu–pyromellitic acid complex (Cu–PMA) to form sPEEK/Cu–PMA composite membranes. In this study, various sPEEK/Cu–PMA composite membranes with different Cu–PMA wt% (0.25, 0.5, 0.75 and 1) were prepared. The structural morphologies along with the physicochemical and mechanical properties of sPEEK, and sPEEK/Cu–PMA composite membranes were analyzed by scanning electron microscopy, atomic force microscopy (AFM), X-ray diffraction (XRD), ¹H-nuclear magnetic resonance (NMR), infra-red spectroscopy, universal testing machine (UTM), simultaneous thermal analysis (TG-DSC), dynamic mechanical analysis (DMA) etc. Moreover, the incorporation of Cu–PMA into the sPEEK matrix facilitated the formation of hydrophilic domains, leading to improved ionic conductivity. Notably, the sPEEK/Cu–PMA composite membrane with an optimal Cu–PMA loading of 0.5 wt% exhibited an impressive proton conductivity of 0.162 S cm⁻¹, surpassing both the sPEEK membrane and other sPEEK/Cu–PMA composite membranes. This represents a remarkable >60% enhancement in proton conductivity for the sPEEK/Cu–PMA-0.5 wt% composite membrane in comparison with sPEEK (0.10 S cm⁻¹) under high temperature and fully humid conditions. Furthermore, the sPEEK/Cu–PMA-0.5 wt% hybrid composite membrane demonstrated superior performance retention during cycling, as evidenced by heating–cooling–heating (HCH) measurements. Additionally, in a PEFC, the membrane electrode assembly (MEA) of the sPEEK/Cu–PMA-0.5 wt% composite membrane delivered an impressive current density of 960.14 mA cm⁻² at an operating cell voltage of 0.6 V and a peak power density of 748 mW cm⁻², which is ∼2 times higher than that of the MEA prepared with sPEEK (a current density of 488.5 mA cm⁻² at a cell voltage of 0.6 V, and a peak power density of 404.11 mW cm⁻²). Notably, the sPEEK/Cu–PMA-0.5 wt% hybrid composite membrane exhibited 86% retention of open circuit voltage (OCV) even after a stability test of >100 h at 30% relative humidity and a cell temperature of 80 °C, confirming its viability for energy conversion and storage devices.