Rigid-flexible synergistic alginate-based composite membranes reinforced with modified hydrothermal carbon for direct methanol fuel cells

Abstract

Conventional perfluorinated PEMs hinder sustainable energy transitions due to fossil-fuel dependence and environmental concerns. Herein, we develop rigid-flexible alginate-based crosslinked biomass membranes using sulfonated alginate and acid-base bifunctionalized hydrothermal carbon from corn stalks. l-cysteine grafting creates covalent/amide linkages, Ca²⁺ forms "egg-box" coordination, and thermal treatment reinforces the network. Unlike most biomass-based PEMs that rely on a single crosslinking mode or conventional inorganic fillers, this multilevel crosslinking strategy enhances dimensional stability while providing plentiful proton-hopping sites and channels. The eco-friendly hydrothermal carbon bears functional groups analogous to those of alginate, ensuring its homogeneous dispersion throughout the polymer matrix. Compared with the sulfonated alginate membrane, the optimized membrane containing 6 wt% additive achieves a 68% enhancement in tensile strength (30.87 MPa), a 75% increase in proton conductivity (0.0556 S cm-1), and a threefold improvement in selectivity (16.8 × 104s cm-3). Methanol diffusion is suppressed by 56% to 3.3 × 10–7 cm2 s-1, contributing to a 48% higher maximum power density (59.74 mW cm-2). By employing renewable agricultural waste as an additive precursor and combining it with biodegradable alginate, this membrane avoids toxic and expensive synthetic components, rendering it both eco-friendly and cost-effective. This work demonstrates that functional additives derived from agricultural waste can be used to construct high-performance biomass PEMs, offering a promising alternative to conventional perfluorinated membranes with potential for cost reduction and environmental sustainability.