From Methanol concentration to thermal performance: insight into Nafion/Lignosulfonate-Hydroxyapatite composite membranes for direct Methanol fuel cell

Abstract

This study evaluates the recast Nafion (rN)/lignosulfonate (LS)-hydroxyapatite (HAP) composite membranes in passive direct methanol fuel cell (DMFC) across range of methanol concentration and thermal operation. The rN/LS-HAP membrane developed by solution casting showed remarkable improvement compared to both the commercial N117 and recast rN117 membrane. The rN/LS-HAP membrane achieved a peak power density of 22.45 mW cm^‒2 at an optimal methanol concentration of 2 M due to its improved proton conductivity and minimised methanol crossover. Electrochemical evaluations, which included linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS), showed a 46.85 % decrease in methanol permeability and improved charge transfer resistance compared to N117 membrane. In addition, the rN/LS-HAP membrane showed remarkable Faradaic efficiency (81.01 %) and energy efficiency (20.09 %) as well as turn over number (TON) 14.11 s^‒1. Durability test assessment was conducted over 96 h demonstrated remarkable stability, characterized by membrane thinning and ruthenium (Ru) crossover. These findings were corroborated through scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). Composite rN/LS-HAP membrane exhibited the highest power density of 29.82 mW cm^‒2 at 80 °C, surpassing that of N117. This superior performance can be attributed to the improved mass transport properties, the lower activation energy and the improved interfacial contact between the membrane and the electrodes. In summary, the rN/LS-HAP membrane exhibited outstanding promise as a proton exchange membrane for DMFC, offering superior performance, durability, and efficiency. These findings suggest its viability for large-scale applications in fuel cell technology, making it a promising solution for next-generation energy systems.