Comb-shaped proton exchange membranes with dangling polystyrene grafted onto PVDF for PEM fuel cells and water electrolysis

Abstract

Herein, oxy-tethered polystyrene grafted poly(vinylidene fluoride) (PVDF)-based comb-shaped proton exchange membranes (PEMs) are developed using ozone-induced grafting, followed by post-sulfonation of the PVDF-g-polystyrene graft copolymer. A series of PEMs (sPSt-x) with different –SO₃H densities were fabricated by varying the sulfonation temperature for their employment in PEMFC and PEMWE devices. Structural characterization revealed an enhanced electroactive β-phase and nanophase-separated morphology with sub-nanometer proton transport channels, driven by increased sulfonation. sPSt-60, with the highest –SO₃H density (ion exchange capacity, IEC = 1.48 meq g⁻¹), exhibited superior proton conductivity (K^m = 18.1 mS cm⁻¹ at 80 °C, hydrated), and with extensive hydrogen-bonding networks and thermally induced sulfone crosslinks, showed robust mechanical, oxidative, and hydrolytic stability. In PEMFCs, sPSt-60 achieved a peak power density of 112.4 mW cm⁻² at 80 °C and 100% RH, constrained by the inherent hydrophobicity of PVDF and low dry-state K^m (1.9 mS cm⁻¹), indicating its unsuitability for PEMFCs. In contrast, sPSt-60 excelled in PEMWEs at 80 °C, surpassing Nafion 117 by 6.5% (423.6 vs. 395.7 mA cm⁻² at 1.8 V) due to reduced ohmic losses (high-frequency resistance, HFR = 0.74 Ω cm²) and enhanced proton mobility. A durability test for 72 h showed a modest 5% decline in its performance (402.5 mA cm⁻² at 1.8 V) and a minimal 5.4% HFR increase (0.78 vs. 0.74 Ω cm²), highlighting the exceptional gas barrier properties and stability of sPSt-60 for PEMWEs.