Novel chemometric Raman approach for spatially resolved quantification of graft distribution in anion exchange membranes

Abstract

Precise control of the graft distribution, represented by the degree of grafting (DoG), in anion exchange membranes (AEMs) prepared by radiation-induced grafting (RIG), is critical for alkaline fuel cells' performance, especially aiming to improve water management. However, current methods offer only bulk or qualitative assessments of DoG, limiting the ability to understand and optimize local membrane properties. In this work, we present a novel Raman-based chemometric method for the spatially resolved quantification of DoG using micro-Raman spectroscopy. By applying classical least squares (CLS) fitting to decompose Raman spectra into contributions from the polymer base and grafted side chains, we establish a direct correlation between CLS scores and the local DoG. This approach enables, for the first time to our knowledge, the use of a multivariate technique for quantitative mapping of grafting profiles across the membrane cross-section using a widely accessible and non-destructive technique. The method is validated on membranes with known grafting levels and applied to asymmetric DoG AEMs, revealing detailed insights into spatial variations in functionalization. Moreover, the approach is broadly applicable to any grafted copolymer system with side-chain functionalization, beyond the specific membranes studied here. By combining spatially resolved measurement with rigorous chemometric analysis, this technique offers a robust tool for the design and optimization of next-generation ion-conducting membranes in electrochemical energy systems.